Applications of Zirconia

**Chapter 5**

## Zirconia in Restorative Dentistry

*Hanumantha Murali Rao, Mamtha Kumaraswamy, Dhanu Thomas, Shivakumar Boraiah and Kuldeep Singh Rana*

#### **Abstract**

Advancements in dental material science and technology have improved over the past decade tremendously. The demand for tissue-friendly esthetic materials has been fulfilled to a certain extent on account of the development of new materials. Most materials meet the requirements of esthetics, function and biocompatibility. They exhibit the properties of color stability, improved resistance to wear, dimensional stability and they are tissue-friendly. These new materials are able to provide desirable and optimum treatment outcomes on a long-term basis on account of their nature and advances in manufacturing and fabrication. Reinforced ceramic restorations are now completed within a very short time from start to finish—from digital impression to bonding of the restoration. Zirconia-reinforced ceramics share the top choice in materials along with lithium disilicate. The most recent zirconia has improved optical properties and has the potential to overcome its problem of increased opacity. These zirconia-based ceramics have successfully replaced even precious metal alloys and porcelain-fused-to-metal prostheses due to the above-mentioned qualities. This chapter throws light on zirconia and the different types used in dentistry, applications, methods of fabrication and clinically relevant properties.

**Keywords:** zirconia, yttria stabilized tetragonal zirconia, zirconia-based ceramics, dental porcelain, CAD-CAM

#### **1. Introduction**

Ceramics are inorganic substances produced by firing metallic and non-metallic constituents at a high temperature [1]. Ceramics have been widely used in dentistry for more than 100 years [2], and have since been advancing in terms of their physical, mechanical and optical properties [3]. As described by Kelly and Benetti [4], ceramics have been classified into three groups based on the glass content.


iii.Polycrystalline ceramics: no glass is present and is much stronger than glass-based ceramics. They can either be pressed into oversized molds (compensates for shrinkage during firing) under pressure or blocks may be processed into restorations using CAD-CAM (Computer-Aided-Designing Computer-Aided-Machining).

The drawback is that there is no quantifying the amount of glass phase required for the ceramic to be included in either the predominantly glassy or the particlefilled glasses category. Also, polycrystalline ceramics do not contain glass, hence the classification lacks clarity. Another classification was outlined by Gracis et al. [5] who proposed a new grouping based on the formulation of the ceramics.


Zirconia is presently the most studied and researched dental material. The name "zirconium" originates from the Arabic term "Zarcon", which translates to "golden in colour." The dioxide form of zirconium (ZrO2), known as zirconia, was first identified by a German chemist Martin Heinrich Klaproth in 1789 in a reaction product of heating some gems [6]. It was used along with other rare earth oxides as pigments in ceramics. Zirconia was first used as a biomaterial in 1969 for hip head replacement in orthopedics [3]. Crystalline zirconia occurs in three forms: monoclinic (M), tetragonal (T) and cubic (C). At room temperature, pure zirconia is monoclinic and remains stable in this phase up to 1170°C. Monoclinic zirconia is usually associated with surface microcracks, higher susceptibility to low temperature degradation (LTD) and lower reliability for use in dentistry [7]. Above this temperature, it transforms first into a tetragonal and then into the cubic phase. They transform from one phase into another, induced by a combination of different factors such as, temperature, humidity and stress [8].

It is crucial to know that all three phases differ in their properties. One that is useful in dentistry is tetragonal zirconia. The tetragonal form is stable between temperatures 1170° and 2370°C. The transformation from tetragonal to the monoclinic occurs upon cooling, and results in a volumetric *increase* of 4%. Pure unalloyed zirconia is unstable at room temperature and would shatter spontaneously and catastrophically on cooling due to t → m. Hence, the tetragonal form must be stabilized to room temperature to overcome stress cracks and transformation volumetric changes on cooling [1, 7, 9]. For this reason, zirconia is doped with various oxides, such as yttrium oxide (Y2O3) or cerium oxide (CeO2) or other metallic oxides such as magnesium oxide (MgO) and calcium oxide (CaO). Significant molecular stability can be obtained with ZrO2 doped with Y2O3. It has superior mechanical properties than other combinations; although sintering is much more difficult, this is the main kind of zirconia considered for biomedical use (**Figure 1**) [9].

The concept of fracture toughness and tensile stress is central to the excellent mechanical properties of yttria-stabilized zirconia. Ceramics are fired at such high *Zirconia in Restorative Dentistry DOI: http://dx.doi.org/10.5772/intechopen.111601*

**Figure 1.** *Yttria-stabilized zirconia.*

temperatures that upon immediate cooling, a natural crack forms, weakening the material. Fracture toughness characterizes the potential stress directed through the developed crack or flaw. When stress occurs on a zirconia surface, there is a t → m transition associated with cracking, followed by a volumetric expansion [9]. By inducing external stress or a higher tensile stress, there is an increase in volumetric expansion resulting in compressive stress around the crack tips [2] and there is an increased resistance to crack propagation. Hence, these cracks are shielded by controlled transformation from t → m and result in a gradual stabilization. The strength and fracture toughness is dependent on grain size and also the dopant concentration (yttria) [10]. This concept is called "transformation toughening", first reported by Garvie, Hannink, and Pascoe in a paper titled "Ceramic Steel?" [11]. Zirconia-based ceramics used in dentistry mostly consist of tetragonal zirconia polycrystals, partially stabilized with 3 mole % yttrium (3Y-TZP) [1].

Initially zirconia was used in root posts, orthodontic brackets dental implants. With the development of CAD/CAM, traditional metal-based crowns, prosthesis and FPD's have been replaced by improvised esthetics and enhanced tissue compatibility obtained using tooth-colored metal-free systems [3].

#### **2. Types of zirconia in dentistry**

#### **2.1 Yttrium tetragonal zirconia polycrystals (Y-TZP)**

As mentioned previously, zirconia used in dentistry usually contains 3 mol% yttria (Y2O3) as a stabilizer (3Y-TZP) is indicated for the fabrication of crowns and fixed partial dentures. The restorations are processed either by soft machining of pre-sintered blanks followed by sintering at high temperature, or by hard machining of fully sintered blocks. Yttria stabilized zirconia was first introduced in the dental market in the year 2002 via soft machining [12].

The mechanical properties of 3Y-TZP are strongly influenced by its grain size. Optimum t → m transformation occurs only in a limited range of grain size of 0.2 μm to 0.8 μm [13]. Above a critical grain size of >1 μm, 3Y-TZP is less stable and more susceptible to spontaneous t → m transformation whereas, grain sizes of <1 μm have a lower transformation rate. Below a grain size about 0.2 μm transformation is not possible, leading to reduced fracture toughness. Grain size is controlled through sintering and the sintering conditions have a strong impact on both the stability and mechanical properties of the final product. Higher sintering temperatures and longer sintering times lead to larger grain sizes [12].

Currently, available 3Y-TZP for soft machining of dental restorations employs final sintering temperatures between 1350°C and 1550°C, depending on the manufacturer. Restorations produced by soft machining are sintered after milling the restoration. This process prevents the stress-induced transformation from tetragonal to monoclinic and leads to a dense restoration and a surface virtually free of the monoclinic phase unless grinding adjustments are needed or sandblasting is performed [14].

Most manufacturers of 3Y-TZP blanks do not recommend grinding or sandblasting to avoid both the t → m transformation and the formation of surface flaws that could be detrimental to long-term performance. In contrast, restorations produced by hard machining contain a significant amount of monoclinic zirconia, usually associated with surface microcracks, higher susceptibility to LTD and lower longterm reliability [7, 12].

Other less commonly used TZPs contain 4% or 5% mol concentrations of yttria. Increasing yttria content increases the cubic phase which does not undergo transformation to the monoclinic phase. This results in a ceramic that is highly translucent but with weaker mechanical properties as transformation toughening does not occur [15, 16].

#### **2.2 Glass-infiltrated zirconia: Toughened alumina (ZTA)**

Alumina matrix is added to zirconia in order to utilize the stress-induced transformation capability of zirconia. In-Ceram Zirconia (VidentTM, Brea, CA) was introduced as a core ceramic by the addition of 33 vol% of 12 mol% ceria stabilized zirconia (12Ce-TZP) to In-Ceram Alumina [12]. It is processed by either slip casting or soft machining. In slip-casting, initial sintering takes place at 1100°C for 2 h, followed by lanthanum glass infiltration of this porous ceramic. The glass phase represents approximately 23% of the final product. The amount of porosity is greater than that of sintered 3Y-TZP and comprises between 8 and 11%, explaining the lower mechanical properties of In-Ceram® Zirconia® when compared to 3Y-TZP dental ceramics [12]. In-Ceram Zirconia is stronger and more opaque than In-Ceram Alumina and In-Ceram Spinell, therefore its use is limited to posterior crowns and fixed partial dentures [17].

#### **2.3 Partially stabilized zirconia (PSZ)**

There was a considerable amount of research on magnesia-stabilized zirconia (Mg-PSZ) for biomedical use but it was stopped in the 1990s to many causes. Mg-PSZ had higher residual porosity, which resulted in a less dense and weak final structure. The large grain sizes (30–60 μm) led to wear of the opposing structure. It also requires a higher sintering temperature of 1800°C as opposed to 1400°C of TZP, and a strictly controlled cooling cycle, especially in the aging stage. Partially stabilized zirconia usually contains a cubic matrix as the major phase and monoclinic and tetragonal zirconia as the minor phases [6]. The amount of MgO in the composition of commercial materials usually ranges between 8 and 10 mol% [18]. Precipitation of the transformable t-phase occurs during this stage, in which volume fraction is a critical factor in controlling the fracture toughness of the material [7]. Denzir-M® (Dentronic AB) is the sole Mg-PSZ ceramic available for dental restorations fabricated via hard machining [19].

#### *Zirconia in Restorative Dentistry DOI: http://dx.doi.org/10.5772/intechopen.111601*

Another oxide that is used to stabilize zirconia is ceria (CeO2). Ceria provides better thermal stability and resistance to LTD than yttria-stabilized zirconia although the amount of ceria required to maintain the same stability is greater [20]. Ce4+ itself is unstable as such and reduces to Ce3+ which does not have a very good stabilizing effect on zirconia [19]. It also has a lower flexural strength but this could be improved by adding nanosized alumina in the matrix, called ceria-stabilized zirconia/alumina nanocomposite available commercially as Nanozir (Hint-Els, Griesheim, Germany) [20].

#### **2.4 Zirconia containing lithium disilicate (ZLS)**

Lithium disilicate has gained popularity as a monolith ceramic for single crowns and partial coverage restorations due to its excellent optical properties and strength and is available as machinable blanks [21]. To this technology, tetragonal zirconium oxide with a mean grit size of approximately 0.5 to 0.7 μm is added as fillers. Zirconia crystals act as a nucleating agent but remain in solution in the glassy matrix. The flexural and fracture strengths are higher than lithium disilicate glass ceramics. The mechanical properties are approximately three times higher than those determined for leucite-reinforced glass ceramics. ZLS ceramics offer an excellent combination of high strength and outstanding optical properties. Thus, these materials are interesting for the fabrication of monolithic restorations. ZLS ceramics are marketed as Vita Suprinity and Celtra Duo developed by Vita (Vita Zahnfabrik, H. Rauter GmbH & Co., Bad Säckingen, Germany) and Dentsply (Dentsply Sirona, DeguDent, GmbH, Hanau-Wolfgang, Germany), respectively [22, 23]. These materials are available as industrially prefabricated blanks for various CAD/CAM systems [7].

#### **2.5 Resin nanoceramics**

Resin-matrix ceramics also called hybrid ceramics, were specially formulated for CAD/CAM and mostly contain zirconia as fillers (5). Lava Ultimate (3 M ESPE) is a highly cross-linked polymeric matrix that contains a proprietary blend of three types of fillers: silica nanoparticles, zirconia nanoparticles and zirconia-silica nanoclusters in 80% wt. (65% vol.) [24]. This material may offer higher flexural strength and fracture toughness, which results in better long-term durability and polish over time. It is available in eight shades, in both high and low translucencies. The material is indicated for single-tooth restorations, including implant abutments [7].

Novel zirconia materials have also been chronologically divided into the following three generations:


grain size of 3Y-PSZ is increased to 0.5 to 0.7 μm, and the cubic phase content is increased from 6 to 12% to 20–30%. As a consequence, the translucency is increased marginally, and the biaxial strength is decreased to 900 to 1150 MPa. Although translucency is improved, it was still insufficient for use in the anterior esthetic zone, especially single tooth restorations [16].

iii.The third generation aimed at reducing the opacity, incorporates more optically isotropic cubic zirconia (50–80%), has a grain size of 1.5 μm and is produced by increasing the yttria dopants to 4–5 mol% and increasing the sintering temperature and/or duration more than that of the second generation. This is known as 5Y-PSZ. However, cubic zirconia is weaker and more brittle than its tetragonal counterpart, which jeopardizes the strength and toughness of the zirconia. The translucency is increased significantly, but the biaxial strength is decreased to 450 to 740 MPa.

Additional experimental novel zirconia types with improved translucency have been developed, including graded zirconia and nanostructured zirconia [14, 16]. Zhang and Kim developed graded zirconia in which feldspathic glass was infiltrated into Y-TZP with improved hardness and esthetics. The graded zirconia cross-sectional structure consists of an outermost glass layer, a glass-Y-TZP layer, and a Y-TZP interior. This type of gradation occurs to have eliminated delamination of the glass [25, 26].

#### **3. Applications of zirconia in dentistry**

#### **3.1 Dowels or posts**

Generally, endodontically treated teeth have undergone significant tooth destruction, both coronal and radicular, which compromises the mechanical integrity of the tooth [27]. All endodontically treated teeth also require a build-up to replace the tooth structure that had been lost due to caries or other pathologies, and access cavity preparations [28]. An assessment is made after the treatment to determine if the prospective build-up requires additional support to be retained on the tooth in the long term. This support comes in the form of a post that is inserted from ½ to 2/3rds the length of the root [29]. Posts are available in different materials such as stainless steel, nickel chromium alloys, carbon fiber and glass fiber [30]. Zirconia esthetic posts are used in the anterior teeth to overcome the black color and shadow of metal posts under translucent crowns. They have high strength and fracture toughness and are extremely hard and stiff materials which makes their removal difficult in an event that necessitates retreatment [3]. Although zirconia posts are esthetically acceptable, their clinical use is limited due to a lack of retreatability and higher stiffness than dentin which may cause functional stresses to be transferred to the dentin [31].

#### **3.2 Crown and bridge**

Missing teeth cause not only functional and structural disturbance but also influence a person's psychology and social interactions. All-ceramic restorations were limited to anterior or single-tooth restorations due to their weak mechanical strength. Porcelain-fused-to-metal has been considered as the gold standard for load-bearing

*Zirconia in Restorative Dentistry DOI: http://dx.doi.org/10.5772/intechopen.111601*

restorations and multiple units till recent research focused on reinforcing ceramics which resulted in the development of lithium disilicate and oxide ceramics (alumina and zirconia) [32]. In fixed partial dentures, zirconia is used as the framework (or matrix) over which the veneering ceramic is fired. It has been found that fabricating the framework in the anatomic design rather than an arbitrary form contributes to increased strength and bonding to the veneering ceramic [33]. The cumulative survival rate for zirconia single tooth crowns was 92.7% after 3 years and for fixed partial dentures is 73.9–100% after 2–5 years [34].

#### **3.3 Implant abutments**

Dental implants have shown a high success rate for single tooth rehabilitation. Titanium, stainless steel, gold alloy, zirconia and polyether ether ketone (PEEK) are the most commonly used abutment materials in implant dentistry [35]. Titanium abutments have excellent biocompatibility and mechanical strength and are considered the gold standard for posterior regions; nonetheless, they cause a grayish discolouration of the soft tissues around the abutment [36]. In its stabilized form, zirconia ceramics show better tissue adaptation and lower plaque retention as compared to alumina and titanium when used as implant abutments. Four-year survival rate is 100%, although long-term studies show that zirconia is prone to delamination and degradation in the oral environment [3].

#### **4. Fabrication of restorations**

Zirconia restorations are fabricated using CAD/CAM by either soft machining of pre-sintered blanks or hard machining of fully sintered blanks. The conventional CAD/CAM procedure involved purely laboratory works followed by first generation developed by Duret and colleagues combining intraoral scanning and final crown produced by controlled machining and milling. As it was not widely used due to the lack of accuracy, another approach called networked CAD/CAM processing was introduced into dental technology. While computer-aided designing (or CAD) allows one to determine the 2-D geometry, computer-aided machining (or CAM) allows the processing of the proposed design to calculate the path of cutting using various tools. CAM is nothing but a machine language used by copy-milling to fabricate a ceramic prosthesis. In spite of CAD/CAM having a high success rate in terms of design and fitting, final crown fabrication and veneering are done in a dental lab [37].

Normally, there are three different approaches by which ceramic restorations are processed in dentistry: green stage processing, white stage processing and processing through hot isostatic pressing (HIP). The difference between these approaches lies in the heat treatment of the raw material used for processing. The green stage of ceramic is only ceramic powder and binder pressed into a blank. It is very soft as it is extremely porous and is not used for processing zirconia. The heat-treated (or pre-sintered) green stage gives the white stage of ceramic which is processed via soft machining. Further heat treatment of this white body results in an extremely dense (~99% theoretical density), fully sintered blank which requires custom coloring [7, 38, 39]. Sintering is a thermal treatment for bonding particles into a coherent, predominantly solid structure via mass transport events that often occur on the atomic

scale. Bonding leads to improved strength and lower system energy. Sintered blanks are characterized by enhanced density and improved properties [40].

#### **4.1 Soft machining**

Soft machining of pre-sintered blanks is the most common method of processing zirconia restorations. Pre-sintered zirconia blanks are manufactured by cold-isostatic pressing (CIP) of a mixture of zirconia powder, stabilizing oxides and binding agents (the latter removed during the pre-sintering process) [21]. The prosthesis is milled from this pre-sintered block but with bigger dimensions so as to compensate for the sintering shrinkage [39]. The most common sintering method for zirconia uses conventional furnaces at temperatures between 1350°C and 1400°C and holding times ranging from 2 to 4 hours. An alternative protocol that is recommended by manufacturers using conventional ovens is a short "speed" sintering protocol which uses temperatures of 1500–1600°C and a holding time of 30 minutes that is supposed to save time and be more economical [14]. The zirconia framework attains its final mechanical properties at the end of the sintering process when it undergoes a contraction at about 25%, and reaches its correct dimensions. In order to optimize the fitting of the restoration it's imperative to know the exact volume shrinkage for every zirconia blank. The vast majority of blocks have barcodes that give information fed into the computer regarding the density of the milling block so that the framework is milled adequately oversized [39].

#### **4.2 Hard machining**

Fully sintered blocks are processed through hot isostatic pressing (HIP) at temperatures between 1400 and 1500°C. Unlike cold isostatic pressing which uses room temperature fluid under pressure to process zirconia blanks, HIP uses heat and high pressure in an argon atmosphere. This is done using special furnaces, which in addition to heavy milling procedures makes the prosthesis fabrication an expensive task [41]. Although fully dense blanks have better mechanical properties, they lack the popularity of partially sintered blanks owing to their long milling times and the hardness of the dense blanks, especially in the fabrication of fixed partial denture frameworks [38].

#### **5. Clinical aspects**

#### **5.1 Mechanical and physical properties**

Mechanical properties of the final restoration are influenced by the very first step of synthesis of the Y-TZP nanopowder. Starting powders must be crystalline, homogeneous, with high purity and narrow particle size ranges [42]. The powder is then compacted most commonly by cold isostatic pressing, to achieve a certain level of densification, followed by sintering. Many studies have been done to assess the effect of sintering temperature on the mechanical properties of zirconia and it was found that monolithic zirconia retained its biaxial flexural strength at a sintering temperature of 1550°C whereas, the biaxial flexural strength of core zirconia decreased significantly. It is established that monolithic zirconia has higher flexural strength and fracture resistance than not only conventional glass ceramics but also lithium disilicate [15]. Zirconia shows similar mechanical properties to stainless steel [6] and

#### *Zirconia in Restorative Dentistry DOI: http://dx.doi.org/10.5772/intechopen.111601*


**Table 1.**

*Mechanical properties of types of zirconia in dentistry.*

is the highest one among ceramics used in dentistry. As the strongest and toughest of all dental ceramics, zirconia has 900–1200 MPa flexural strength, and 9–10 MPa.m1/2 fracture toughness [43]. Some mechanical properties comparing the different types of zirconia ceramics are summarized in **Table 1**.

#### **5.2 Optical properties of zirconia**

In an all-ceramic restoration, the ceramic material may be monolithic consisting of a single ceramic material, or a ceramic core material that is covered with a ceramic veneer and is known as a bi-layered all-ceramic restoration. In the bi-layered all-ceramic restoration, the core supports the restoration and gives it strength, and the veneer provides the restoration with its final shape, shade and esthetic. However, the core may also play a part in the development of the final restoration's shade [44]. An inherent drawback of the multi-layered restoration is the chipping or delamination of the veneering ceramic, which was overcome by introducing the monolith zirconia restoration. The color of monolithic zirconia restorations is initially affected by the original shade and optical properties of zirconia ceramics determined by the manufacturing processes. Various laboratory procedures conducted to fabricate monolithic zirconia restorations may influence the color. Clinical factors such as dental background, cement and zirconia restoration features can impact the resulting color. Shade reproduction of monolithic zirconia restorations may be affected during the long process from the production of zirconia ceramic to restoration delivery. The final color of restoration is the result of manufacturing processes, laboratory procedures and clinical factors [45].

The most significant disadvantage of zirconia crowns is their relatively opaque appearance. The least translucent zirconia has 42.1% translucency of a typical glass ceramic and 72% translucency of a lithium disilicate ceramic. Two factors determine the appearance of a restoration: first, the intrinsic character and color of the material and the second is extrinsic parameters like cement layer, restoration thickness and low thermal degradation. The gloss and translucency of zirconia are brand dependent and are greatly affected by grain size and content, yttria content and the amount of impurities. Light scattering and thickness are two important factors determining the translucency of ceramic [46].

As natural zirconia is white, an immediate advantage over PFM restorations is the absence of the metallic collar usually placed on the margin, and identified by patients as a "black line" as seen with gingival recession. The high opacity of zirconia can be used as an advantage to a certain degree. The presence of discolored tooth stumps, amalgam or other heavily colored restorative materials, metallic posts and cores or

carbon fiber posts may be masked up to an extent to achieve a natural appearing restoration [47]. Manufacturers have presented zirconia ceramics with different visible light transmission percentages (VLTPs) ranging from 20–50%. Five types of translucency are available for zirconia including low, medium, high, super and ultratranslucency. Low and medium translucency zirconia are commonly indicated for zirconia frameworks, whereas high, super and ultra-translucency zirconia are mostly designated for monolithic restorations [45].

Much research and development has been done to improve the optical properties of zirconia. The opacity is said to be a result of the interaction of grain sizes with the wavelength of light, refractive index mismatch between zirconia grains and the matrix, and refractive indices of the monoclinic, cubic and tetragonal phases. These factors cause light to be scattered instead of being transmitted through zirconia, leading to an opaque appearance [46]. Therefore, light scattering must be reduced to increase translucency.

#### *5.2.1 Factors that affect zirconia translucency*


translucency can be done by increasing the sintering temperature and time, though speed-sintered Y-TZP can acquire less desirable wear properties than normal sintered zirconia [48].


Grain size: The large grain size of tetragonal zirconia polycrystals has been correlated to reduced translucency and increased light scattering. According to classical physics, materials containing grain sizes of less than 1 μm appear less opaque due to the reflection and absorption of visible light. On the other hand, particles larger than 10 μm scatter more light and appear more opaque. Due to the birefringence (double refraction) and polycrystalline nature of zirconia, there is more scattering than transmission. Reducing the grain size of TZP zirconia improves translucency. The higher sintering temperature is associated with increased grain size but reduced strength of the ceramic. A grain size of less than 100 nm is necessary for acceptable translucency. Currently, the mean grain size of contemporary zirconia ceramics lies between 0.2 and 0.8 μm [13, 48]. To achieve a translucency comparable to dental porcelains, the mean grain size of 3Y-TZP should be about 82 nm for 1.3 mm restorative thickness, 77 nm for 1.5 mm and 70 nm for 2 mm [25].

Restoration thickness: Translucency decreases with increased restoration thickness and seems to be brand-dependent [45]. A restoration as thick as 0.5 mm has better tooth-like translucency. A minor increase or decrease in the restoration thickness can alter the translucency significantly. A minimum thickness of 0.9 mm is required for acceptable shade matching of HT zirconia. For color masking, a minimum thickness of 1 mm is required or 1.6 mm thickness for ideal masking [45, 46].

#### **5.3 Low temperature degradation (LTD)**

LTD is also called aging and is defined as a spontaneous t → m transformation occurring over time at low temperatures, and the transformation is not triggered by local stresses or an advancing crack [50]. This occurs in the presence of water and starts in isolated grains on the zirconia surface leading to an increase in volume [51]. Due to this, there are stresses created in the neighboring grains and resulting in microcrack

formation which allows the water to penetrate and ultimately there is a significant decrease in the strength [20]. This transformation may be accelerated by water humidity [52], and slowed using smaller grains and higher amounts of stabilizing oxides [39]. CeO2 in 12 and 14 mol% is more resistant to LTD than Y-TZP, however, it appears more yellow. When Ce4+ is reduced to Ce3+, the Ce-TZP becomes dark gray due to the high concentration of oxygen vacancies. LTD in Ce-TZP may be accelerated on intake of reducing foods like glucose and lactose [50]. Other factors that influence LTD are tensile stresses, grain size and residual stressed post sintering [20]. Tensile stresses as low as 400 MPa can induce LTD over a course of 5 years. A crack tip forms on the surface of zirconia which are exposed to water (or any other fluids) and the grain transforms to monoclinic. As t → m is associated with a volume expansion, there is an uplift of the surface at the site of transformation. Under this grain, there is a large compression that subjects the underlying grains to small tensile stresses. These grains transform to monoclinic and as a cycle, continue to crack through the depth of the zirconia, thus weakening the entire structure. Reducing the grain size is said to have a restraining effect on LTD. Critical grain size for pure zirconia at room temperature falls in the range of 5–10 nm, although this number was calculated at above 100°C in various experiments [50].

#### **5.4 Other considerations**

Technical problems associated with the clinical performance of zirconia crowns and fixed dental prostheses have been reported, in particular, chipping of the veneering porcelain when applied to zirconia framework structures and loss of retention. Attempts to minimize the chipping of veneering porcelain by milling the veneers and frameworks separately and subsequently luting them with either a luting agent or using fusing firing (CAD on) have not been quite sufficient to address the chipping concerns. Another attempt to overcome the veneer chipping problem was the introduction of zirconia in the form of fully anatomical contoured monolithic prostheses intended to be used without veneering porcelain [14]. The problems of layering in zirconia-based restorations, such as veneering ceramic delamination/chipping and veneering ceramic zirconia ceramic incompatibility, do not exist in monolithic zirconia restorations [45]. Although monolithic zirconia ceramics have lower flexural strength than the framework zirconia of zirconia-based restorations of equal thicknesses, monolithic zirconia crowns have a higher fracture resistance than zirconia-based crowns due to increased zirconia thicknesses and lack of veneering ceramics [45].

Ideally, chair-side grinding and adjustment of anatomically contoured zirconia restorations should be avoided as they can produce rough surfaces. However, for many restorations, it is necessary to optimize occlusion, proximal contacts and axial contour. Grinding has two counteracting effects on zirconia: either it produces surface compressive stress that can positively enhance crack healing (by transformation toughening) and increase the material strength by transformation toughening or it can induce surface flaws that may exceed the depth of the compressive layer and negatively influence the strength of the material. Microcracks can be triggered by a number of stimuli such as thermal changes, humidity, airborne-particle abrasion and grinding, as mentioned previously [53]. There are a number of finishing and polishing systems available for zirconia restorations in different grades of diamond grits.

Aging of anatomically contoured Y-TZP restorations could be more crucial because restorations are in direct contact with oral fluid. Hydrothermal aging of zirconia, known as low-temperature degradation, can occur over time within the temperature range of 65–500°C in the presence of water and other solvents. Although this

*Zirconia in Restorative Dentistry DOI: http://dx.doi.org/10.5772/intechopen.111601*

mechanism is very slow in oral temperatures, zirconia restorations are exposed to other factors such as constant humidity, thermal changes, pH fluctuation, and repeated high occlusal loads due to mastication and parafunctional habits that can accelerate the aging process and reduce the material's fracture resistance. Also, the chemical composition, the microstructure of various brands of high-translucency zirconia, the thickness of the restoration, and the processing can influence resistance to aging [53].

There are few long-term clinical trials to assess the longevity of zirconia restorations. Most studies concluded that the most common cause of failure was the chipping of the veneering ceramic. Moreover, secondary caries and marginal gaps were found in 56 to 59% of fixed partial denture abutments, although periodontal health was maintained as zirconia is shown to be highly biocompatible with gingival tissues. Zirconia implants were well-tolerated with healthy peri-implant bone and no bleeding on probing in a 2-year clinical study [54].

#### **6. Conclusion**

The innovation of newer materials in the field of dentistry has been expanding over the past decade. Most research is focused on eliminating the inherent opaque nature and susceptibility to LTD. With single-day, chairside fabrication of prosthesis through CAD/CAM, zirconia is still in its early stages and has a long way ahead to meet various criteria that make it perfectly fit for use as long-term restorations.

#### **Author details**

Hanumantha Murali Rao1 \*, Mamtha Kumaraswamy2 , Dhanu Thomas3 , Shivakumar Boraiah4 and Kuldeep Singh Rana5

1 Department of Conservative Dentistry and Endodontics, D.A.P.M.R.V. Dental College and Hospital, Bengaluru, Karnataka, India

2 Private Practitioner, Bengaluru, Karnataka, India

3 Conservative Dentistry and Endodontics, D A Pandu Memorial R V Dental College, Bengaluru, India

4 Government Dental College and Research Institute, Bengaluru, India

5 Conservative Dentistry and Endodontics, College of Dental Surgery, Indore, MP, India

\*Address all correspondence to: drmuralihrao@yahoo.com

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 6**

## Recent Modifications of Zirconia in Dentistry

### *Ghassan Albarghouti and Haneen Sadi*

#### **Abstract**

In restorative dentistry, there are basically two requirements aspired to be fulfilled by the material of choice to be the main constituent of the restorations, those include superior mechanical characteristics and outstanding esthetic properties. Zirconia (ZrO2) attains great popularity nowadays and is considered a promising material in dental applications. The excellent tensile strength, high thermal stability, relatively low thermal conductivity, wear resistance, corrosion resistance, chemical stability, low cytotoxicity, minimal bacterial adhesion, and biocompatibility properties of zirconia adding to them its tooth-like color and esthetic appearance have promoted its introduction as a successive dental substance. It was found to be a potential alternative and favorable material in dental restorations competing with many of the previously known and employed ceramics and metals, such as titanium. Despite the excellent properties and wide use of titanium in dental applications, it still suffers from unfavorable drawbacks. However, some problems in zirconia diminish its mechanical properties, such as phase transformation and aging, which could be overcome *via* the utilization of dopants within the zirconia's structure. This chapter discussed the main stabilized zirconia types, properties, dental components, manufacturing, and treatment techniques. Further modifications on zirconia with the maintenance of both mechanical and esthetic properties are still under investigation.

**Keywords:** zirconia, ceramics, dental applications, properties, surface modifications

#### **1. Introduction**

Zirconium, a strong solid group (IV) transition metal, was isolated for the first time as an impure metal in 1824 by Berzelius and produced as a pure metal by van Arkel and de Boer in 1925 [1]. It has a hexagonal close-packed (HCP) crystalline structure at ambient conditions [2]. Zirconium is well-known for its ductility, malleability, and ease of forming stable compounds. Therefore, it occurs in nature in conjunction with other components as ores, not as a pure metal. The most common zirconium ores include the more abundant but less pure zirconium(IV) silicate (ZrSiO4), commonly known as zircon, and the relatively pure baddeleyite which contains 96.5 to 98.5 weight percent zirconium dioxide also named zirconia [3]. Some of the main chemical and physical characteristics of zirconium are presented in **Table 1**. The importance of zirconium manifests clearly through its compounds,


#### **Table 1.**

*Some physical properties of zirconium.*

which have several interesting applications. Some of these applications are associated with the nuclear field in the form of zircaloys. Moreover, zirconium in organometallic compounds, such as zirconocenes, could act as an intermediate in the synthesis of several biological organic compounds; and zirconia ceramics, which will be discussed in depth in this chapter [4, 5].

Zirconia (ZrO2) is a white polycrystalline ceramic material; it is one of the oxide ceramics with superior mechanical and biomedical properties. The crystallographic form of zirconia is pressure and temperature-dependent, which means it can assume one of three phases: monoclinic (m), tetragonal (t), or cubic form (c). At ambient pressure, the monoclinic structure, with the shape of a deformed rectangular prism, is the stable form at room temperature, however, it has inferior mechanical performance when compared with other phases. Crystals assume the monoclinic structure until 1170°C, where the unusual performance occurs: shrinkage upon heating, resulting in a transition to the metastable tetragonal shape, a regular rectangular prism with improved mechanical properties. This structure is stable between 1170 and 2370°C; above this temperature, further shrinkage occurs to form the stable cubic structure, a form with moderate mechanical properties [6]. Such phase transitions are summarized in **Figure 1**.

The unusual performance of temperature-dependent crystalline structural transitions presents a considerable problem upon cooling. A phase transformation from the metastable tetragonal into the monoclinic phase (t → m) occurs upon cooling with volume expansion of about (3–5%), the lattice becomes rigid and unable to accommodate the rapid volume expansion. Such considerable stresses through the lattice result in a hysteresis behavior of zirconia, where catastrophic fracture and propagation of any lattice cracks ensue over time. Therefore, pure zirconia is not easily produced and employed due to spontaneous structural failures [7, 8]. In the case of zirconia-based biomaterials, this problem could be catastrophic.

For the sake of inhibiting (t → m) transformation or at least reducing the transformation rate and maintaining the metastable tetragonal lattice at room temperature, which in turn inhibits crack propagation, some significant factors could be controlled [7, 8]. The addition of stabilizers in sufficient concentrations allows the tetragonal lattice to maintain at ambient conditions.

Zirconia stabilization is a process that aims to suppress its crystalline structure during cooling; by adding specific amounts of dopants to zirconia to avoid the conversion to the monoclinic form during cooling and stabilize the tetragonal form at room temperature to a great degree. Dopants, also called stabilizers, are metallic oxides such as magnesium, calcium, cerium, and yttrium oxides.

*Recent Modifications of Zirconia in Dentistry DOI: http://dx.doi.org/10.5772/intechopen.111891*

**Figure 1.** *Phase transformations of zirconia crystalline structure as temperature changes.*

The lattice after stabilization overcomes crack propagation through a mechanism known as "transformation toughening" in which the toughness is enhanced because of the dissipation of the energy when cracking initiates to overcome the effect of the stabilizer, a (t → m) transition occurs followed by volume expansion, which leads to close the crack and blunt any developing crack [6]. This phenomenon would increase the flexural strength of the lattice.

Stabilized zirconia takes a unique place among oxide ceramics due to its distinctive mechanical and biological properties. Its mechanical properties are comparable to those of stainless steel, for example, excellent tensile strength, high thermal stability, relatively low thermal conductivity, wear resistance, and corrosion resistance. Nevertheless, the excellent chemical stability, low cytotoxicity, minimal bacterial adhesion, biocompatibility, and tooth-like color "ivory" of zirconia make it a material of keen interest in several biomedical applications, substantially in dentistry.

Zirconia was used in orthopedic joint replacement, such as hip joint replacement and surgical implants. In dentistry, with its protruding mechanical properties, zirconia is successfully used in endosseous implants with the maintenance of the natural ivory look of the tooth. Its esthetic properties enhance its utilization in dental applications widely; since esthetic concerns become a priority, as well as medication aspects as in **Figure 2** [8].

Zirconia aging or low-temperature degradation (LTD) negatively affects its properties, it is a water-catalyzed phenomenon that results in a slow (t→m) transformation and does not require mechanical stress. The mechanism of zirconia aging is similar to that of transformation toughening where the transformation is prolonged

**Figure 2.** *Zirconia's esthetic properties in dentistry.*

with volume expansion and stress induction to the surrounding. Then, the surface could rise, allowing for water to move through the lattice and penetrate down, which would exacerbate surface degradation [9]. This problem could be triggered by surface roughness.

#### **2. Types of stabilized zirconia ceramics available for dental and biomedical purposes**

As a ceramic material, zirconia was found to be hard but brittle with inferior impact resistance; this constrained its application and dictated the modification of such material with promising characteristics to permit it to work as an alternative for several metal-based artificial joints and implants. Moreover, materials to be employed in dental aspects have to be durable and stable under harsh oral cavity conditions [10]. The stabilization processes of zirconia ceramics, in which oxides are added to zirconia, enhanced its phases' stability and retained the preferred tetragonal structure more than before at ambient temperature, resulting in partially stabilized zirconia (PSZ), zirconia toughened ceramics, or tetragonal zirconia polycrystals (TZP) with improved properties [11]. Even though there are many types of stabilized zirconia combinations, only three of them, yttrium tetragonal zirconia polycrystals (Y-TZP), magnesium partially stabilized zirconia (Mg-PSZ), and zirconia toughened alumina (ZTA), are intensively tested and introduced in dentistry to overcome (t → m) transformation. The mechanical properties of these types are summarized in **Table 2**.

#### **2.1 Yttrium tetragonal zirconia polycrystals (3Y-TZP)**

When Yttria (Y2O3) is the dopant, a monophasic tetragonal zirconia polycrystal (TZP) is obtained, usually containing 3 mol% of yttrium oxide (3Y-TZP). This type of low porosity and relatively high-density stabilized zirconia attracted considerable attention when compared with other combinations due to its chemical stability and better mechanical properties supporting its biocompatibility. Exhibiting the transformation toughening phenomenon makes it a favorable type in medical applications and particularly dental implementations, including implants, crowns, abutments, bridges, and fixed partial dentures. Production of dental restorations is carried out through hard or soft machining along with sintering processes (Processing techniques will be discussed in a separate section of this chapter). The key point of tetragonal phase stability at ambient temperature is the vacancies in the zirconia lattice appeared due to the presence of yttria.


#### **Table 2.**

*The main mechanical properties of the principal types of stabilized zirconia [8].*

#### *Recent Modifications of Zirconia in Dentistry DOI: http://dx.doi.org/10.5772/intechopen.111891*

Despite the features, (3Y-TZP) undergoes aging or LTD phenomenon under hydrothermal conditions at low temperatures as well as human body temperature, and as a result, its mechanical properties degrade. In fact, the vacancies in the stabilized zirconia could act as host sites for water molecules diffused from the surrounding under hydrothermal conditions, decreasing site numbers consequently, as well as the stability of the tetragonal phase. This phenomenon is induced by exposure to hydrothermal "intraoral" conditions, surface roughness, microcracking, and stresses. It is found to be sensitive to processing conditions, including mixing, distribution, and polishing techniques. Some factors can be controlled to impact the mechanical properties of stabilized zirconia, including the grain size and sintering conditions. Spontaneous (t → m) transformation is susceptible unless the grain size is small. The transformation rate is reduced when the grain size is less than 1 μm and prevented when it is less than about 0.2 μm, and thus the brittleness of zirconia will reduce. The grain size is directly affected by sintering conditions, particularly sintering time and temperature. Large grain size will be obtained after a long time or high sintering temperature affecting the phase stability [9]. To overcome the LTD problem and failure of zirconia and improve its mechanical properties, efforts focused on the production of new composites, alumina is introduced instead of yttria to have zirconia toughened alumina. But improved (3Y-TZP) is still an attractive composite especially when computer-aided manufacturing and computer-aided design (CAM/CAD) processing are applied. Some generations reduced the LTD by adding a small amount of alumina to the (3Y-TZP) composite [12].

#### **2.2 Zirconia-toughened alumina (ZTA)**

A biphasic ZTA is accomplished by combining zirconia with an alumina matrix by slip casting or soft machining processes for the sake of integration between alumina's high stiffness with zirconia's superior toughness [13]. This type of stabilized zirconia was found to have better fracture toughness and wear resistance, also when compared with (3Y-TZP) it has enhanced aging resistance [14]. There is no stabilizing dopant in this type. Thus, the stability of the tetragonal phase at ordinary temperatures depends mainly on the particles' morphology and size, in addition to whether the location is intergranular or intragranular particles [9]. By adding zirconia, the grain growth of alumina is inhibited, improving the fracture toughness of the composite. An advantage of this combination is that both of these components are naturally white and thus act as an efficient mask for teeth of dark colors. However, ZTA exhibits low translucency. Therefore, it is not ideal in cases where esthetic concerns are paramount. The opalescence of ZTA precluded it from dental restorations of anterior sites [12, 15].

For better outcomes, ZTA was incorporated into new composites using several additives. Many ceramic oxides are utilized within ZTA to modify their properties by influencing the lattice parameters. For example, a newly developed bio-safe ternary composite of zirconia, alumina, and titania (TiO2) facilitates sintering while processing and attains high mechanical properties with low manufacturing costs [13]. In several studies, alumina was used as an additive with other stabilized zirconia types. Alumina nanoparticles enhanced flexural strength and fracture toughness when incorporated with ceria-stabilized TZP [10]. The addition of a specific amount of silver as a dopant to ZTA enhances the antibacterial properties of the dental parts [16].

The two possible zirconia-alumina composites: ZTA, which is obtained when the alumina matrix is reinforced with ZrO2 particles (alumina is the main component in ZTA), and alumina-toughened zirconia (ATZ), which is obtained when the zirconia

matrix is reinforced with Al2O3 particles. Both composites show better toughness; ATZ displays improved mechanical stability and aging resistance, while ZTA exhibits much better-aging resistance [17–20].

#### **2.3 Magnesium partially stabilized zirconia (Mg-PSZ)**

Magnesia is the dopant of this type, usually using (8–10 mol%) of MgO to provide a biphasic composition of precipitates of tetragonal intragranular zirconia within the cubic matrix of stabilized zirconia through the introduction of magnesium cations within the tetragonal or cubic lattice of zirconia [9, 21]. Although (Mg-PSZ) exhibits transformation toughening with good chemical and thermal resistance properties, this material had precluded from several biomedical applications, it is considered an unstable material; due to its inferior properties resulting from the large grain size, which in turn increases the residual porosity and makes it susceptible to wear. Thus, it requires high sintering temperatures using special equipment for heating. Moreover, (Mg-PSZ) is almost impossible to be obtained purely free from alumina and silica, which diminishes magnesium content and allows (t → m) transformation [22–24]. The incorporation of 8 mol% (Mg-PSZ) was confirmed to enhance the fracture toughness of (3Y-TZP) [25]. The mechanical properties of (Mg-PSZ) can be controlled by treatment with isothermal heat, controlled cooling followed by temperature-controlled sub-eutectoid aging [26]. Spark plasma sintering before sub-eutectoid aging was found to contribute to finetuning (Mg-PSZ) properties [27]. Several methods were used for (Mg-PSZ) preparation, such as solid-state reaction, sol-gel, electrospinning, precipitation, microwave, and sugar techniques [28]. *In vitro* studies demonstrated the enhancement of the biocompatibility of (Mg-PSZ) when coated with functionally graded bioactive glass, such results promote the fixation of this type of stabilized zirconia-based dental parts [29].

#### **3. Investigation of zirconia in dental restorations**

#### **3.1 Zirconia dental posts**

If the root of the dentine is not healthy and cannot serve teeth stability, a dental post is an option for treatment of the readily existing tooth as an alternative to the root to support and strengthen the teeth *via* retention of the restorations. Stabilized zirconia ceramics were introduced in dental posts for the first time in 1995 with comparable mechanical properties to other metal-based posts [30]. Many limitations in metal-based and all ceramic restorations encourage the employment of zirconia in dental posts. Corrosion activity of some metal-based posts leads to sensitization, unfavorable metallic taste, oral burnings, and pains. Furthermore, the esthetic concern has been a critical factor in recent years since metal posts usually do not fulfill this concern. This is clear when employed in anterior restorations, where unfavorable metallic grayish-blue discoloration due to the complete opacity of metals could affect the root and gingiva.

Such health and esthetic concerns dictate the development of translucent white dental posts with high chemical stability and biocompatibility with very low toxicity besides their mechanical properties. Zirconia is the favored efficient choice and confirmed as a material that attained high clinical success rates with strength comparable to all ceramic posts. **Figure 3** represents the root translucency difference between

**Figure 3.** *Translucency of titanium (a) vs. TZP based posts (b) with composite build-up [30].*

titanium-based versus TZP-based posts with composite build-up. To prevent the most severe problem, restorations fractures, the root should uniformly distribute the stress caused by the occlusal load; this could be achieved through the engagement of materials of lowered Young's modulus analogous to that of dentine.

Upon studying the features and disadvantages of zirconia-based dental posts, some drawbacks limit their application. Retreatment of the zirconia posts in dentine is complex since it is nearly impossible to remove or grind away the posts from the root canal; likewise, the root temperature will increase if ultrasonic vibration is used to remove fractured posts. Moreover, some studies proved the poor resin bonding of zirconia posts after dynamic loading. Dental resin is a bonding material for integration between the restoration parts [31]. Also, the relatively high Young's modulus and stiffness of zirconia promote zirconia posts to transfer the applied stress to the surrounding less rigid tooth, resulting in root fractures [32, 33].

#### **3.2 Zirconia dental implants**

The dental implants are similar to posts in their function as an alternative tooth root, whilst the implant is a foundation for a missing tooth and acts as a base for other prosthetic components for a totally-artificial tooth in the mandible or maxilla, as shown in **Figure 4(a)**. This foundation should conduct a robust functional and pleasant esthetic role. In the current century, zirconia-based implants proved to be an efficient alternative to other prominent metal-based implants used widely in the past decades, including alumina and titanium alloys.

Despite their high success rates, titanium alloys in dental implants had some unfavorable aspects that limited their applications, including the esthetic requirements and high wear properties, resulting in metallic particles causing allergic and toxic consequences. Alumina-based implants were limited with their high fracture susceptibility and inferior osseointegration. Stabilized zirconia, and (Y-TZP) implants in particular, are favored for their high flexural strength, tooth-like color, low-temperature conductivity, and masticatory forces bearing.

Several studies proved the biocompatibility of zirconia implants which represent a critical property as a component placed directly in contact with alveolar bone and connective tissues. The low toxicity of such implants was also confirmed

**Figure 4.**

*Elucidative image of endosseous implant, abutment, and restoration (a). Bi-layer crowns of zirconia cores and translucent veneers (b) [8].*

when tested using the immune system cells, this is supported by zirconia's low plaque affinity, avoiding surrounding tissues' inflammatory risks, and lower bacterial colonization compared to titanium implants. Zirconia osseointegration was directly influenced by surface roughness, which can be controlled *via* manufacturing processes, other than the machining process such as coating, sandblasting, and acid etching [8, 34].

#### **3.3 Zirconia dental abutments**

Zirconia abutment, the connection between the implant and the prosthesis as illustrated in **Figure 4(a)**, was introduced in 1996 [35]. It was developed to overcome titanium abutment esthetic limitations, with the maintenance of its mechanical strength. Zirconia could be sintered in titanium abutments to form titanium-reinforced zirconia with improved properties.

Previously, one-piece implants were designed without the introduction of the abutment as a connection, for concerns related to wear, fitting precision, and the colonization potential of bacteria and microbes in the micro-gap between the abutment and the implant from one side and the prosthetic (crown or bridge) from the other side. Such a one-piece implant system limited the versatility of the prosthesis compatibility with the implants' position. Furthermore, preparation of this type and grinding, in particular, may trigger (t → m) transformation. Thus, a two-piece system was developed, including an implant and a separated abutment. This combination proved to achieve better osseointegration when the implant is not protruding, furthermore, better fracture resistance would result if the stress distributes over the two separate components.

An important point to be taken into account is that when a zirconia implant is fitted directly to the zirconia abutment, restoration fracture is already feasible; due to its inherent strength. Durable, nontoxic, and good adherence bonding cement is mandatory. Many types of cement were introduced, such as zinc phosphate, zinc polycarboxylate, glass, and resin ionomer. The last type is well-known for its ease of use and high retention [31]. Zirconia abutments are favored over the brittle alumina ones, on the other hand, zirconia abutments are more susceptible to fractures when compared to titanium-reinforced zirconia [12, 33, 36–38].

#### **3.4 Zirconia in dental prosthesis**

Dental prosthetics, including single prosthetic crowns (fixed dental prosthesis), bridges (multiple joined prosthesis), and dentures (Full jaw prosthesis), are used as an alternative for missing dentine. It is a major part of the restoration in which functional and esthetic considerations must be satisfied; due to its exposure to harsh conditions and bearing direct pressure and masticatory loads. Moreover, as an external part of the restoration, the esthetic appearance is a priority, especially for the anterior dentine [12]. Among the versatile types of ceramic restorations; zirconia and lithium disilicate-based prostheses showed intrinsic promising functional capability as in **Figure 5**. Zirconia prosthetics are preferred for their superior mechanical properties and have become a popular trend among dentists as a material that fulfills the prementioned requirements. CAD/CAM technologies' advancement in zirconia production also facilitates its selection as the approved type.

Prosthesis preparation is a critical factor for its fitting with both the foundation and the veneering components [9]. Transformation toughening is crucial in making its mechanical properties exceed other ceramic prosthetics. (3Y-TZP) is widely preferred for its high flexural strength and fracture toughness. Versatile types could be obtained with different properties *via* variations of stabilized zirconia concentration, grain size, sintering conditions, coloration technique, and surface processing and treatment [39].

Despite the favored clinical performance of zirconia crowns and other prosthetics, it has some drawbacks in bi-layer prosthesis, as shown in **Figure 4**(b), which consists of veneering porcelain that works as a translucent mask over the opaque white zirconia, mainly for esthetic concerns, and particularly in anterior regions. The veneering porcelain in bi-layer prosthetics is susceptible to chipping and aging problems in the presence of water. Chipping is the cohesive frailer between the zirconia substructure and the veneer.

The veneering problem is attributed to many reasons, including a mismatch of thermal expansion coefficient between the two layers, vacant sites due to porosity,

**Figure 5.** *Zirconia and lithium disilicate-based prostheses.*

lower fracture toughness of the veneer structure, overloading, and inappropriate design of zirconia frameworks with insufficient support of porcelain over the framework. Chipping could be solved in some cases simply by polishing the rough sites, using composite resin for fracture treatment, or total replacement in some complicated cases [40]. Monolithic zirconia recently is preferred, in which a one-piece component is introduced instead of two separated components core and veneer.

#### **3.5 Zirconia in esthetic brackets**

Several types of brackets have been used for orthodontic treatment, such as stainless steel, titanium composites, and alumina. Zirconia brackets were introduced as the type of greatest toughness among other ceramic-based types. Its frictional coefficient is lower than that of alumina, which is a feature. Nevertheless, zirconia's high opacity inhibits the esthetic appearance, especially with the growing trends for translucent brackets [41]. Recent advances work on developing the zirconia bracket's structure to enhance its properties and shorten the treatment period. A treatment using zirconia brackets fabricated in a specific process with three slots within its structure, , connected with nickel-titanium arch-wires was successful, in which the tooth movement occurs at a higher rate. A shorter treatment period was observed when compared to edge-wise appliances [42].

#### **3.6 Zirconia properties**

Understanding zirconia properties helps in a better conception of its performance and applications. One of the important phenomena in zirconia is low-temperature degradation (LTD) or aging described previously in the second section. Other properties will be described in this section:

#### **4. Biocompatibility**

Biocompatibility could be described as the ability of a biomaterial or medical component to carry out a medical therapeutic function usually for long-term contact with human body tissues, with relatively no pathogenic harmful side effects, such as inflammation, cytotoxicity, allergy, or carcinogenic effects [43]. Many researchers tested zirconia biocompatibility in dental restoration. It is proven that partially stabilized zirconia performed favorable initial fibroblast adhesion on its surface, which enhances the growth of the connective tissues when compared to titanium, polystyrene, and fully stabilized zirconia. Increasing the concentration of yttria in partially stabilized zirconia produces fully stabilized zirconia with a rougher surface which affects the adhesion behavior [44]. *In vivo* and *in vitro* studies proved that zirconia is an osseoconductive biomaterial in addition to being safe and stable without reported carcinogenic or mutagenic impacts [45]. Some studies worked on modifying zirconia surfaces, such as incorporating calcium ions, which enhanced the biocompatibility of such restorations without affecting their mechanical properties [46]. In general, no cytotoxic or damaging side reactions manifest from the introduction of zirconia in dental applications, in addition to

the good bone response with relatively no inflammation or bacterial growth within acceptable levels [47].

#### **4.1 Optical properties and translucency**

Translucency is an intermediate property between transparency and total opaque. Translucent material allows the transmission of light with dispersion which obstacles a clear seen of objects through it [48]. It is a requirement for esthetic aspects in dental restorations. Zirconia opacity is a drawback despite its naturally white color, but zirconia's translucency is required since color compatibility is essential. This material also showed relatively high X-Ray opacity obstructing diagnosis [47]. Therefore, efforts worked on the enhancement of zirconia translucency in several ways and attempted to control its optical properties and refractive index, to tong the top choice for restorative components. Many factors could affect zirconia's translucency, such as impurities, porosity, grain size, restoration thickness [49], and processing conditions. Controlling these factors could enhance translucency [50].

#### **4.2 Radioactivity**

Zirconia was found to contain small portions of radionuclides from uranium, radium, and thorium series type. It is possible to obtain zirconia of radioactivity within acceptable limits *via* purification processes [9]. To understand the effect of composition on zirconia radioactivity, an *in vitro* study represents the radioactivity of three common types of dental zirconia composites using gamma spectrometry, Vita In-Ceram YZ, Zirkonzahn, and Zirkonzahn Prettau (**Table 3**). Zirkonzahn Prettaud had the highest radioactivity as **Figure 6** represents. Even though, all results were within acceptable limits of 1000 Bq/kg according to the International Atomic Energy Agency (IAEA) [51].

#### **4.3 Wear behavior**

Surface roughness influences the abrasion of material and the wear with the adjacent teeth. Enamel wear is affected directly by the surface roughness and surface microstructures, in addition to the surrounding environmental conditions. The surface roughness could be raised by grinding and decreased by polishing [52].


#### **Table 3.**

*Chemical composition of three common zirconia types [51].*

**Figure 6.** *Total radioactivity for each type [51].*

#### **5. Manufacturing of dental zirconia**

Zirconia-based dental restorations nowadays are designed and manufactured using CAD/CAM technology. Two techniques are mainly used for the manufacturing process; soft and hard machining. Soft machining is based on the milling process of pre-sintered blanks produced by cold isostatic pressing of compact zirconia powder in the presence of a binder. Later on, those blanks will be fully sintered. High temperature is utilized for the sintering step. This technique is usually used for (3Y-TZP) manufacturing.

On the other hand, the hard machining technique uses fully sintered stabilized zirconia blocks. The pre-sintered zirconia is obtained at a temperature of less than 1500°C to maintain the required density. Then, hot isostatic pressing is obtained at temperatures between 1400 and 1500°C under inert conditions and high pressure to maintain high density, hardness, and homogeneity of the blocks. A special milling system then performs machining to obtain the required dimensions. The milling system has to be strong and robust because the fully sintered zirconia is very hard with low machinability [9, 15].

#### **6. Surface modifications for zirconia dental implants**

Many factors could affect the nature and properties of the fabricated zirconia and its quality and biocompatibility, such as its chemical composition, morphology, and surface roughness [53]. In this section, the main surface treatment methods of zirconia-based dental restorations are discussed, which are found to have a direct influence on its osseointegration as well as the mechanical properties. Various classifications for the treatment methods were used; herein the classification of these methods is presented within three categories; physical treatment, chemical treatment, and coating processes.

#### **6.1 Physical treatment**

#### *6.1.1 Surface sandblasting*

It is a surface abrasion process in which the pressure of the compressed air ejects particles strongly to acquire a surface with micro-roughness. In this process, the abrasion is performed as a homogeneous and anisotropic abrasion on the hard material [54]. Many studies confirm the advantageous role of increasing zirconia-based implant roughness with sandblasting on the amount of integration and contact with the bone tissues. The roughness is detected by a parameter known as bone-implant contact (BIC), in addition to improving the strength of the bond, detected by the removal torque parameter (RTQ ) [55].

Sandblasting of zirconia with alumina was confirmed to increase its surface roughness. Thus, highly efficient initial adhesion of human osteoblasts cells was achieved [56]. However, this process has some drawbacks; sandblasting with alumina will affect the elemental chemical composition of the treated surface, in which alumina act as a contaminant. The problem could be solved by utilizing the acid etching treatment [54]. Moreover, LTD is susceptible to occurring with increased sandblasting pressure due to the impact of the mechanical forces on the surface [57].

#### *6.1.2 Laser*

Laser is a promising treatment for zirconia surface to improve its osseointegration. Unlike sandblasting, zirconia phase transformation in laser treatment is uncommon and surface contamination is avoided since there is no direct contact between the laser and the surface [54]. This type of treatment is fast, easy to operate, clean, and highly accurate; it promotes micro-grooved implant surface and thus enhances the surface roughness. Laser treatment promotes surface properties changes, including roughness, topography, and wettability. This treatment type was confirmed to improve surface micro and nano-scaled roughness; this resulted in enhanced wettability, which directly influences biocompatibility, cell adhesion, and proliferation.

The properties of the treated surface depend on the irradiation conditions, which could be optimized by controlling the irradiation frequency, intensity, and time [58]. The femtosecond laser established a consistent roughness between the surface and the bonding resin, increasing the bond strength. A fiber laser could achieve wider adequate grooves; this advances the (BIC) and (RTQ ) parameters [4, 55].

#### *6.1.3 Ultraviolet (UV) light*

As mentioned before, wettability is correlated with biocompatibility and integration with bone tissues. Many studies proved that surface treatment with UV light increases wettability by lowering the surface contact angle below 20° to obtain a hydrophilic and even super-hydrophilic surface. The hydrophilicity surface character is mainly obtained due to decreasing the superficial hydrocarbons *via* UV light [59], and increasing the oxygen character. When the treated hydrophilic oxide binds to water in the wet environment near the tissues, the surface will be in the form of hydroxylated oxide, which induces surface reactivity toward the surrounding amino

acids and proteins [54]. Thus, zirconia treated with UV radiation exhibits very good osteoblast response and proliferation. However, the influence of UV light on zirconia aging still needs further effort and study. Many studies indicated that UV treatment reduces the ability of zirconia to age, while some controversial studies found that UV light triggers crystalline transformation [55].

#### **6.2 Chemical treatment**

#### *6.2.1 Acid etching*

Acids, such as HF, HNO3, or H2SO4 usually used for zirconia surface treatment in the acid etching process to increase its roughness homogeneously, even in the case of irregular surfaces, without destroying its morphology. Acid etching is effective in overcoming the prementioned sandblasting contamination problem by removing excessive residues. Thus, acid etching is usually employed in conjunction with a previous surface sandblasting. Sandblasting of zirconia implants using large grits followed by surface etching with a strong acid provides SLA implants, an abbreviation for sandblasted, large grit, acid-etched implant surface, with an increased surface area, promoting surface bio-adhesion [60, 61]. A comparative animal study tested three different types of sandblasted zirconia implants, the three types are acidetched (SLA), alkaline-etched, and sandblasted zirconia without etching. The results indicated the highest BIC values were attributed to the acid-etched SLA type, while the alkaline-etched implants achieved the lowest BIC values [62].

#### *6.2.2 Electrochemical treatment*

Despite the nonconductive character of zirconia, many electrochemical treatment methods were proven to enhance the properties of zirconia dental restorations. For example, electrochemical deoxidation of zirconia (ECD) improved its biocompatibility by developing a micro-porous surface and thus lowering the contact angle. This method is known to enhance surface wettability through oxygen removal from the surface of the solid zirconia using molten salt electrolysis [63].

Recent electrochemical techniques focused on producing nanostructures on the surface of zirconia such as nanotubes [64]. The electrochemical anodization technique (EA) is extensively used for the fabrication of zirconia nanopores or nanotubes since it is a cost-effective technique and can control the physical and chemical properties of the prepared nanostructures. Zirconia nanotubes were found to promote the stability of zirconia implants with better initial cell adhesion. The fabricated nanotubes could be modified *via* annealing to improve their corrosion resistance [65].

#### **6.3 Coating**

Several types of bioactive coatings are employed to augment the function of the osteoblasts. Some of these coating materials, such as calcium phosphate, polydopamine, bioactive glass, and biomolecular coatings are discussed in this section.

#### *6.3.1 Calcium phosphate*

Calcium phosphate is a mineral component in bones; this critical point accelerates and supports the osseointegration when zirconia implants are coated with these *Recent Modifications of Zirconia in Dentistry DOI: http://dx.doi.org/10.5772/intechopen.111891*

compounds. Furthermore, calcium deposition on such coated zirconia implants and the adhesion of proteins will be enhanced [61]. This type of coating is affected by the properties of the utilized compound since several compounds are implicated as members of the calcium phosphate family, such as β-tricalcium phosphate (β-TCP) and the most stable hydroxyapatite (HA) with the chemical formula Ca10(PO4)6(OH)2.

Several methods are employed for coatings. The plasma spraying method is favored over sol-gel, wet powder spraying, and aerosol deposition techniques since it is inexpensive and its deposition rate is high, but not suitable for complex morphologies [55, 66]. However, calcium phosphate bonding on the zirconia surface is relatively weak; several studies attempted to reinforce bonding in different ways, such as laser treatment before coating, or the application of coating along with (HA) and hydrothermal sintering after the coating process [65].

#### *6.3.2 Polydopamine (PDA)*

The good adhesion properties of marine mussels' proteins attracted the interest to take advantage of the components in these proteins. Upon studying this property, the unusual amino acid 3,4-dihydroxy-L-phenylalanine referred to as (L-DOPA) is confirmed as the liable component. Dopamine is considered an (L-DOPA) precursor [67]. The introduction of dopamine and polydopamine was approved to augment the bioactivity of the zirconia surface and enhance cell adhesion over the surface. The enhancement is attributed to the easy adsorption processes taking place on the surfaces due to the strong anchoring of the catechol functionality [68]. This type of coating also reduces bacterial adhesion on the coated surface and improves antimicrobial activity, which aids in the rapid healing and regeneration of the soft tissues surrounding the dental implants. Moreover, PDA coating is simple and nontoxic [67].

#### *6.3.3 Coating with biomacromolecules*

Arginine-glycine-aspartate, referred to as RGD is a tripeptide biomolecule known for its proteins' adhesion properties. It is found that RGD plays a key role in the adhesion of osteogenic cells and thus holds significant promise to achieve efficient bioactivity if incorporated with dental implants [69]. Zirconia surface coating with RGD was accomplished through the immobilization of the coating material on the zirconia implant surface by a process called surface biofunctionalization or biomimetic modification. This process aid in the enhancement of its biological character. RGD coated (Y-TZP). The ability of successful chemical bonding between RGD and (Y-TZP) was proven, and this combination performed better biocompatibility [54, 70].

#### *6.3.4 Bioactive glass*

Bio-glass, a composition of sodium, calcium, silicon, and phosphorous oxides, is considered a bioactive material, which promotes its introduction in zirconia surface coating. It forms hydroxyapatite between the implant's surface and biological tissues when present within a biological environment [55]. This type of coating is proposed to enhance the biocompatibility of the implants and reduce the healing period. However, the success of these attempts was limited because of several drawbacks in bio-glass. Its mechanical properties are insufficient, making it a fragile component. Furthermore, its thermal expansion coefficient is relatively high, so it is unsuitable for zirconia thermal coating, and cracking of the coating is susceptible [61].

#### **7. Challenges and future developments of zirconia application**

To overcome the dental veneer drawbacks, chipping, and delamination, in addition to thickness considerations, attention has focused on the development and further modifications of the monolithic zirconia. To enhance monolithic zirconia's properties, chemical dopants are utilized. Increasing the yttria content with a lowering in alumina content and the incorporation of (0.2 mol%) La2O as a dopant resulted in improved (3Y-TZP) translucency and aging resistance enhanced. Increased yttria concentration to (5Y-TZP) resulted in superior translucency and aging resistance properties, but the toughness was scarified. Zirconia opacity is still a drawback that prevents its participation in the anterior sites. Dopants incorporation is considered a promising technique. The opacity of stabilized zirconia could be attributed to the light scattering performed by its grain boundaries and microstructural defects. Alumina was found to increase light scattering, thus its content in zirconia should decrease, but not eliminate, to a level below (0.25 wt%). The transmittance was enhanced when the grain size of zirconia was reduced to a diameter of less than 100 nm [71].

As known, nanotechnology is spreading every day through most of the manufacturing processes and applications; due to its distinct properties and the superior development of the material's character. For example, nano-powders with a regulated composition of stabilizing material or additives could be used for the fabrication of zirconia and has found to improve grains development during sintering and decreases porosity. But for this technique, new processing methods should be developed; since the normal fabrication method used for processing zirconia with nano-powders was very hard [52]. Some studies confirmed the promising properties and positive role of alumina-zirconia nanocomposite with better toughness and high capability for aging resistance in addition to crack propagation inhibition, which increases its reliability in medical applications [33].

The future of stabilized zirconia in the material science field is promising and requires intensive efforts and searching for new strategies to withstand the challenge of enhancing the esthetic properties while maintaining the mechanical properties of zirconia at the same time. This is a game of advancements and compromises.

#### **8. Conclusion**

Zirconia has attracted significant attention recently, especially in dental applications for mechanical and esthetic considerations. Herein, we demonstrate a general view of zirconium, zircon, and zirconia. This chapter discussed the main types of stabilized zirconia incorporated in dental restorations: Y-TZP, ZTA, and Mg-PSZ. Zirconia-based dental restorations, including posts, implants, abutments, fixed denture prostheses, and orthodontic brackets, were explained. Then, an illustration of some of the substantial zirconia properties that directly affect its mechanical and esthetic properties, such as (LTD) or aging, zirconia biocompatibility, optical properties, translucency, and radioactivity. Surface modifications of dental zirconia are also presented. The physical treatment techniques: sandblasting, laser, and UV light, in addition to the chemical treatment, including acid etching and electrochemical treatment, were discussed. Different coatings utilizing calcium phosphate, polydopamine, bio-macromolecules, and bioactive glass were introduced. Finally, we demonstrated recent developments, challenges, and directions for future research to enhance the survival rates of different zirconia-based dental restorations.

*Recent Modifications of Zirconia in Dentistry DOI: http://dx.doi.org/10.5772/intechopen.111891*

#### **Conflict of interest**

The authors declare no conflict of interest.

### **Author details**

Ghassan Albarghouti\* and Haneen Sadi Birzeit University, Ramallah, Palestine

\*Address all correspondence to: gdaghra@birzeit.edu

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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[56] WMS AQ, Schille C, Spintzyk S, MSA AQ, Engel E, Geis-Gerstorfer J, et al. Effect of surface modification of zirconia on cell adhesion, metabolic activity and proliferation of human osteoblasts. Biomedizinische Technik. 2017;**62**(1):75-87

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[58] Cunha W, Carvalho O, Henriques B, Silva FS, Özcan M, Souza JCM. Surface modification of zirconia dental implants by laser texturing. Lasers in Medical Science. 2022;**37**:77-93

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#### **Chapter 7**

### Perspective Chapter: The Application of Yttria-Stabilized Zirconia (YSZ)

*Alexander Chee Hon Cheong and SivaKumar Sivanesan*

#### **Abstract**

Pure zirconia will transform into different phases, which include monoclinic, tetragonal, and cubic, at different high temperature levels. Specific phases can be retained at room temperature by adding stabilizer and yttria is one of the most common stabilizers for zirconia, commonly formed yttria stabilizer zirconia (YSZ). To utilize YSZ in various industry applications, the amount of yttria and sintering temperature played a vital role. Thus far, YSZ has received a warm welcome in the industries of thermal barrier coating (TBC), solid oxide fuel cell (SOFC), and biomaterial. However, the limitations and challenges still occur, and this opens up the room and possibility of enhancing and improving the material properties of YSZ for a better performance in the mentioned area. This chapter explained the working principles of YSZ in the industries respectively and the research been conducted to improve the materials accordingly.

**Keywords:** zirconia, application, biomaterial, SOFC, TBC

#### **1. Introduction**

Zirconia (ZrO2) possessed attractive material properties (mechanical, thermal, and electrical properties) when additional conditions are included. However, the material will encounter phase transformation at different levels of temperature [1]. As shown in **Figure 1**, the crystal structure of zirconia will transform from monoclinic to tetragonal and to cubic as the temperature is increasing, and the transformation is reversible.

By adding stabilizers to zirconia, it is one of the strategies to enhance the material properties of zirconia through retaining a specific phase, which the ceramics material did not come naturally at room temperature. Yttria (Y2O3) is one of the most common stabilizers for zirconia and formed yttria stabilizer zirconia (YSZ). The phase transition of YSZ is complicated but is well documented as the sintering temperature and the yttria concentration vary. The phase diagram, which is illustrated in **Figure 2**, clearly indicated the phase (or phases) of YSZ will be produced as the mole percentages (mol%) of yttria against the sintering temperature. For example, tetragonal (*t*) and cubic (*c*) will be produced with 5 mol% of yttria (5Y) and sintered at temperature 1000°C.

The Scanning Electron Microscope (SEM) samples images of different yttria amount is showed in **Figure 3**. Under the same level of close view observation, the images revealed the higher the amount of yttria, the higher the grain size of the

#### **Figure 1.**

*The phase transformation of zirconia.*

**Figure 2.** *Phase diagram of YSZ.*

**Figure 3.** *The example of SEM images of 2Y, 3Y, and 8Y sintered at 1500°C [2].*

ceramics material be produced. Grain size played a very important role in affecting the material properties which included hardness, fracture toughness, and elastic modulus. By sustaining a particular phase at room temperature, it allows YSZ to function specific role in the industry as specific material properties are induced. In

*Perspective Chapter: The Application of Yttria-Stabilized Zirconia (YSZ) DOI: http://dx.doi.org/10.5772/intechopen.110695*

this chapter, thermal barrier coatings (TBC), solid oxide fuel cells (SOFC) and biomaterial, which are the common industry applications, will be explained and discussed. Each application will highlight the advantages as well as the limitations respectively. All these are related with phase retention, which beyond the zirconia natural phase transformation toward the temperature.

#### **2. Thermal barrier coating (TBC)**

#### **2.1 The working principles**

Thermal barrier coating (TBC) refers to a thermal insulation layer for the purpose of protecting the original material (base material) at high service operation temperature and hot corrosion environment. The described situation normally happens in heavy duty industry like aircraft, automotive, offshore equipment (power generator). In order to protect and secure the base material from extreme scenario and further prolong the lifespan of the machine and system, TBC is one of the option to be used [3]. The common components will be applied the coating included but not limited, of diesel engine and gas turbine blade [4, 5].

**Figure 4** illustrated the concept, construction, and function of thermal barrier coating system. It basically consisted of a surface layer (or been called as "top-coat (TC)") to form a bond coat (BC) layer. In between these two layers, a thin layer called Thermal Grown Oxide (TGO) is formed due to high-temperature oxidation of the bond coat [6]. The three layers function as the coating to protect the base materials from extreme service environment.

YSZ is one of the common ceramic materials to form the TC layer and the BC layer is made of different types of metallic material. This is due to YSZ possessing thermal properties like low thermal conductivity and high coefficient of thermal expansion. This main role of BC layer is to generate an adhesion with TC, then to protect the base material from oxidation [7]. Many research showed that 6–8 mol% yttria stabilizer

**Figure 4.** *Illustration of thermal barrier coating.*

zirconia (6-8YSZ) exhibited excellent thermal properties for example, low thermal conductivity, and high thermal expansion coefficient [8]. The phases of this level normally consist of tetragonal and cubic, or both phases co-exist at the same time.

TBC can be constructed by two common methods—thermal spraying and electron beam-physical vapor deposited (EB-PVD). Thermal spraying methods included atmospheric-plasma spray (APS), plasma spray-physical vapor deposition (PS-PVD) and high-velocity oxy-fuel (HVOF) spraying [9]. The basic mechanism of thermal spraying is to melt the coating material and propel it into the substrate material with high and strong velocity. Plasma spraying basically created the scenario through plasma jet and HVOF is to use high-velocity jet instead.

It is more common to adopt thermal spraying method for made up of TBC for a few reasons. First, thermal spraying was widely accepted by the industry with the technique compatible with various materials, which included ceramics, metals, and alloys. Second, low operational cost and third, low equipment specification requirement. All these caused the mentioned technique received high welcoming small and medium-sized factory. However, many previous works had reported EB-PVD method did produce a higher quality result compared with plasma spray methods [10, 11]. Even though in recent years, new and advanced plasma spray techniques are emerging, the full exploration of the method is yet to be fully accepted by the industry. For example, new techniques like high-velocity air fuel (HVAF) and liquid feedstock thermal spraying, had claimed it able to produce a high quality of surface by increasing the adhesive strength of the coating [12]. The technique of HVAF spraying environment is showed in **Figure 5**.

#### **2.2 Limitation and solutions**

Even though YSZ is a proven material to form TC, degradation is still becoming one of the biggest challenges, and the major failure mechanisms included residual stress, hot corrosion, oxidation, and phase transformation [13]. All these became the factors of shortened the life span of TBC and hinder it continuing to perform in an optimum condition. The durability of TBC refers to the high quality of adhesive strength and low thermal conductivity (insulation) at high temperature [14]. Once these two capabilities been destroyed, TBC basically will fail under the tough operation and service environment, and the mentioned failure mechanisms are playing the role to degrade TBC.

Phase transformation can be related to degrading TBC through the tetragonal transformation back to monoclinic when thermal gradient occurs during the

**Figure 5.** *High-velocity air fuel (HVAF) technique [12].*

*Perspective Chapter: The Application of Yttria-Stabilized Zirconia (YSZ) DOI: http://dx.doi.org/10.5772/intechopen.110695*

operation. The happened due to the oxygen diffused to bond coating layer and allow chemical reaction with YSZ. This had greatly decreased the amount of oxygen of YSZ. Since oxygen ion is one of the factors to stabilize tetragonal phase by forming chemical bonding with zirconium ion, decreasing of the amount of the ion directly destabilized tetragonal phase and drove the phase transformation happen. Besides that, the cooling of TBC also contributed to the phase transformation, as that is the natural characteristic of zirconia. However, this mechanism does not occur immediately. Various research has been conducted to overcome the challenges of TBC to prolong the service life, and one of the common methods is by using dopant. Co-doping Yb2O3-Gd2O3- Y2O3 co-doped with ZrO2 (YGYZ), the outcome showed the tetragonal phase able to be retained 40% compared to the undoped YSZ [15].

The X-ray diffraction (XRD) result revealed that the doped sample consisted of stable tetragonal phase greater than the undoped YSZ, which is showed in **Figure 6**. Take note that YVO4 is the corrosion product, due to the experimental testing by using Na2SO4 + V2O5 molten salts to simulate the real hot corrosion environment. In another word, the doping technique also improved the hot corrosion resistance of YSZ. This research outcomes in **Figure 7** showed a similar trend of creating double layer of topcoats by using La2Zr2O7 under the similar experimental condition [16].

Slow cooling rate is another finding to avoid phase transformation, as well as to prolong the life span of TBC as showed in **Figure 8**. The research reported by reducing the cooling rate to 10 K/s, compared to 100 K/S, which is the common practice, also increased the operational temperature from 1200°C to 1500°C [17]. All the research revealed that doping (or co-doping) is an effective solution to minimize the phase transformation, which will avoid the mentioned failure mechanisms.

Besides avoiding the failure mechanism, the advantages of doping technique also bring benefits and improvement to YSZ as TBC. By doping TiO2 to YSZ, as one of the recent finding, showed the dopant able to increase the operational temperature of TBC from 1200°C to 1600°C [18]. In **Figure 9**, it also resulted the doped samples (TZ) had a lower thermal conductivity compared with the undoped sample (YSZ). The research claimed that low thermal conductivity was attributed by the lattice disorder, which restrict the movement of phono and lower the thermal conductivity.

Recent year, a method calls sol-gel method to fabricate YSZ aerogel had been developed. The research outcome reported the new coating successfully lower down

**Figure 6.** *XRD analysis after hot corrosion at 1100°C [15].*

**Figure 7.** *SEM image of double layer topcoat [16].*

**Figure 8.**

*Number of cycles to failure for YSZ as TBC [17].*

#### **Figure 9.**

*Thermal conductivity of doped and undoped YSZ [18].*

the thermal conductivity through a finer porosity in the aerogel structure [19]. The conceptual illustration of the method on gas turbine is illustrated in **Figure 10**.

This earlier work revealed that applying dopant to YSZ and using different sintering method are one of those effective strategies to overcome the limitation which been mentioned in TBC application. Some research work even showed that doping and sintering methods can affect the average grain size which further affected *Perspective Chapter: The Application of Yttria-Stabilized Zirconia (YSZ) DOI: http://dx.doi.org/10.5772/intechopen.110695*

**Figure 10.** *Conceptual illustration of the YSZ aerogel TBC coating on the gas turbine blade [19].*

the mechanical properties of the materials. However, the effect also depends very much on the substrates to be used.

#### **3. Solid-oxide fuel cells (SOFC)**

#### **3.1 The working principles**

Green technology and energy are becoming one of the major trends globally, and the Department of Economic and Social Affairs, United Nations has established 17 sustainable development goals (SDGs). Without any surprise, clean energy and a green environment are one of the major aspects.

Solid Oxide Fuels Cell (SOFC) received great attention for supply of environmental-friendly energy. One of the main advantages of SOFC is to generate power from hydrogen, natural gas, and other renewable fuels. Based on **Figure 11**, the SOFC reaction can be explained by the following chemical equation:

Anode:

$$\text{H}\_2 + \text{O}^{2-} \rightarrow \text{H}\_2\text{O} + 2\text{e} \tag{1}$$

$$\text{CO} + \text{O}^{2-} \rightarrow \text{CO}\_2 + 2\text{e} \tag{2}$$

**Figure 11.** *Illustration diagram of SOFC [20].*

Cathode:

$$\text{O}\_2 + 2\text{e} - \begin{array}{c} \text{O} \ \text{2O}^{2-} \end{array} \tag{3}$$

By burning the fuel (hydrogen or hydrocarbon) as the input on anode, the concentration of oxygen, from the surrounding environment (air), will be "consumed," as illustrated in Eqs. (1) and (2). The electrons will be transported through external circuits for industry applications. Meanwhile, oxygen on cathode side will react with the electron been transported and produced oxygen ion, as shown in Eq. (3). The porous character of electrolyte will allow the oxygen ions to diffuse to anode and continue the cycle. The whole operation needs to be happened at high operating temperature in between 800°C and 1000°C [21]. This operation of conversion greatly reduced the gas emission (heat), which is bringing air pollution to the environment, and the other product from the reaction will be water (H2O) [22].

YSZ is one of the promising materials to be utilized as solid electrolyte material for SOFC. A qualify material as the medium and play the role of solid electrolyte required characteristic of conducting ions effectively. 8YSZ possess characteristics of high concentration of oxygen ion vacancies, which allow an effectiveness and efficiency performance of SOFC, due to 8 mol% yttria able to retain cubic phase (c-YSZ) of zirconia, which allow highest oxygen ion vacancies concentration compare to the other phase of zirconia [23].

#### **3.2 Limitation and solution**

However, utilizing 8YSZ as the electrolyte of SOFC comes with restrictions and limitations which require the high operating temperature to "activate" SOFC to reach high ion conductivity. In such an operational environment, the components of SOFC may encounter thermal expansion and contraction and further consume the durability of the cell. To overcome this, it drives the scientist and engineers to study and develop different types of materials to widen the application of SOFC. At the same time, it also opened the possibility of continuously enhancing and improving 8YSZ without compromising the conductivity yet continues to lower the operational temperature as well as the mechanical properties [24]. 8YZP basically exhibited low mechanical properties, so several research and investigation had been conducted to reach a breakthrough of these limitations.

Instead of using conventional sintering method, flash sintering method had be proven as the sintering method not only increase the ionic conductivity of 8YZP as

**Figure 12.** *Nyquist plot of 8YSZ with (a) conventional sintering (b) flash sintering method at 215°C [26].*

*Perspective Chapter: The Application of Yttria-Stabilized Zirconia (YSZ) DOI: http://dx.doi.org/10.5772/intechopen.110695*

electrolyte in SOFC, at the same time, produced 8YSZ to reach full densification at a lower sintering temperature [25].

In **Figure 12**, the Nyquist plot, which generated by Electrochemical Impedance Spectroscopy (EIS), indicated flash sintering produce higher conductivity, smaller grain size and porosity. In the plot, flash sintering showed the semicircle with smaller diameter, which referred to a smaller grain size. Smaller grain size offered higher surface area for more ion oxygen to move or transport. This factor attributed a higher conductivity of the ceramic material [26].

Besides sintering method, utilizing dopant also become another strategy to increase the ionic conductivity of 8YSZ. Dopant included iron(III) oxide (Fe2O3) reported an satisfactory result by increasing the ion conductivity [27].

The result in **Figure 13** was generated through molecular dynamics (MD) simulation, and it showed that the optimum result of the conductivity was 4 wt% of Fe2O3 dopant. The simulation result agreed with another research outcome by using the same dopant [28]. The research claimed that doping can stabilize cubic phase of 8YSZ, which is high of ion conductivity, through creating oxygen vacancies.

The result of XRD in **Figure 14** validated the claim, which showed the doped sample consist high volume of cubic phase regardless the sintering temperature. By using the cold sintering process (CSP), the study also showed the enhancement of ion conductivity compared with conventional sintering method.

**Figure 13.** *8YSZ doped with Fe2O3 of: (a) MD simulation diagram and (b) oxygen ion conductivity of [27].*

**Figure 14.** *XRD pattern of 4 wt% Fe2O3-doped 8YSZ sintered by using cold sintering process [28].*

**Figure 15.** *The illustration diagram of ion migration mechanism [29].*

**Figure 15** illustrated the how doping created oxygen vacancies and allow ion to migrate and increased the ion conductivity characteristic. Dopant will introduce lattice defects and distortions, which produced oxygen vacancies in the ceramic material. The vacancies allowed migration of oxygen ion, thus increasing the ionic conductivity of the material [29].

When entering millennium year, the doping method on 8YSZ started to focus on improving mechanical properties, which is important to enhance the durability of the solid electrolyte in the cell. A study had been conducted by using 8 mol% Lu2O3 doped 8YSZ and successfully improved the flexural strength of the materials without compromising the ion conductivity [30]. Other studies on the dopant like CuO, TiO2 and Bi2O3, also reported the sintering aids for 8YSZ effectively aided the ceramics material to reach full densification at a lower sintering temperature without compromising ion conductivity [31–33]. Lower sintering temperature for full densification, which is highly related to the mechanical properties like hardness and fracture toughness, means less energy required during processing.

Transition metal oxide doping strategy showed significantly of improving the mechanical properties of 8YSZ. However, an interesting study and investigation had been conducted. Instead of using transition metal oxide as the dopant, 3YZP (10, 25 and 35 wt.%) had been doped to 8YSZ and both mechanical and electrical properties had been evaluated. The study reported the doped material had an improved version in terms of Vickers' hardness and fracture toughness. At the same time, ionic conductivity also increased, but with only happen under the service temperature below 550°C [34].

Sintering methods and sintering aids (dopant) had been the main factor to enhance the ionic conductivity of the materials and elevate the performance of 8YSZ as the solid electrolyte in SOFC. While the mentioned investigation also revealed doping strategies able to improve the mechanical properties of the material by controlling the amount of the dopant.

#### **4. Biomaterials**

#### **4.1 The working principles**

The phase diagram (**Figure 2**) showed the amount of yttria stabilizer determine the phase of retention of zirconia (monoclinic, tetragonal, or cubic), and 3 mol% yttria tetragonal zirconia polycrystal (3Y-TZP) is one of the ceramic materials

*Perspective Chapter: The Application of Yttria-Stabilized Zirconia (YSZ) DOI: http://dx.doi.org/10.5772/intechopen.110695*

received great welcoming as biomaterial. Among all the three phases of zirconia, tetragonal phase exhibited high hardness strength and fracture toughness [35]. Besides that, zirconia also showed a friendly biocompatible characteristic, which is greatly benefit to bone and tissue implantation [36]. The high fracture toughness is attributed by a phenomenon called stress-induced transformation toughening. This mechanism happens when cracking happens on the surface of 3Y-TZP, metastable tetragonal will transform back to monoclinic, and this transformation will increase the toughness of the material, as illustrated in **Figure 16**.

Due to these attractive mechanical properties, ceramic material is usually utilized in biomaterial industry to enhance the mechanical structure of the product. Besides the mentioned material properties, 3Y-TZP also showed itself with the criteria as the bio-ceramic, which include high corrosion and wear resistance, and esthetics [37].

#### **4.2 Limitation and solution**

However, 3Y-TZP does come with limitations and the most common one is called low temperature degradation (LTD) or hydrothermal aging. It is a phenomenon when 3Y-TZP exposed to moisture or humidity environment (water or water vapor presence) at temperature between 65°C and 400°C, which commonly for biomaterial application [38]. Such condition will allow phase transformation, from tetragonal (*t*) to monoclinic (*m*), on the surface of the material then produced intergranular cracking [39]. This is because the tetragonal phase of 3Y-TZP is a metastable state at room temperature and transformed back to monoclinic phase, which is the most stable phase of zirconia at room temperature, will easily take place under such environment. Once monoclinic phase saturated and dominated the microstructure of the material, the mechanical properties will also degrade. It received a great welcome from the biomedical industry to study and investigate the factors to improve and overcome the mentioned challenge. However, the effect of LTD did not happen immediately but the degradation will take long period of time. In order to study LTD effectively, scientists and researchers simulated the condition under the environment of laboratory. The condition commonly by using conventional water-vapor autoclave operating at low temperature (98, 121 and 132°C) respectively and under adiabatic pressure for different exposure time [40]. In

**Figure 16.** *Illustration of transformation toughening of 3Y-TZP.*

**Figure 17.** *Monoclinic vs degradation time of 3Y-TZP under degradation exposure time (134°C, 4 bar) [41].*

**Figure 17** showed the example of the evaluation of the phase transformation (*t* - > *m*) when exposed the simulated degradation environment [41]. The method has been proven as an effective way to assess any modified and improved 3Y-TZP of the characteristic toward the resistance of LTD.

Several studies showed doped 3Y-TZP exhibited good LTD resistance by delaying the phase transformation then further minimized the degradation of mechanical properties. Co-doping CaO and CeO2 to 3Y-TZP exposed to the simulated LTD environment, the result in **Figure 18(a)**, reported that the no monoclinic phase exists in the microstructure of the samples regardless the amount of both the dopants [42].

Manganese dioxide (MnO2) (0.5 wt% and 1.0 wt% respectively) achieved a similar result under the same LTD laboratory condition but exposed to a longer holding time, 120 h, which showed in **Figure 18(b)**. The dopant effectively increase the LTD resistance of 3Y-TZP [43]. Both XRD diagrams also revealed a stable tetragonal phase in the microstructures, which enhanced the mechanical properties like hardness and fracture toughness of the doped 3Y-TZP as well.

Besides the mentioned dopant, other transition metal oxide like Copper Oxide (CuO), graphene oxide (GO), Flaysh also had been studied and investigated and

#### *Perspective Chapter: The Application of Yttria-Stabilized Zirconia (YSZ) DOI: http://dx.doi.org/10.5772/intechopen.110695*

achieved a similar outcome [44–46]. Some research even brings further insight by relating the LTD resistance of 3Y-TZP with grain size. It reported grain size less than 3 μm will improve the LTD resistance and beyond the mentioned size, the effect will not take place. And doping Ceria and Alumina to 3Y-TZP able to control the grain size below the mentioned level effectively [47].

Sintering condition, which included sintering temperature and sintering holding time, also played a vital role for improving the LTD resistance of 3-YZP. The sintering temperature, 1450–1650°C and holding time, 1, 2 and 4 h, became the study scope. The outcome revealed that the higher both the mentioned parameters, the higher sensitivity of LTD of 3Y-TZP [48]. In year 2018, a research showed that the level of LTD resistance as well as the microstructure of 3Y-TZP had been improved through different sintering cycle and conditions [49]. The effect of pressureless sintering with a range of temperature, 1400–1600°C, and to produce an optimum result between the resistance of LTD, hardness and fracture toughness also had been studied. The outcomes of the study showed that the optimum sintering temperature, 1500°C, is the best temperature to balance the three (3) mentioned material properties of 3Y-TZP [50].

There is a huge potential to continue to improve 3Y-TZP application for biomedical related industries. Both sintering and doping methods have shown promise in enhancing the aging resistance and mechanical properties of 3Y-TZP, making it more suitable for use in biomedical applications. Ongoing research in these areas is expected to further enhance 3Y-TZP's potential for biomedical use and lead to new innovations. Therefore, both methods are valuable tools in unlocking the potential of 3Y-TZP for biomedical applications.

#### **5. Conclusion**

YSZ is a versatile material that can be applied in various industries, which include TBC, SOFC, and biomaterials.

In the context of TBC, YSZ has become the promising material due to its material characteristic to withstand high-temperature environments and protect underlying components from thermal damage. This makes it ideal for use in gas turbine engines and other high-temperature applications.

In the field of SOFC, YSZ is commonly used as an electrolyte due to its high ionic conductivity and stability under operating conditions. The use of YSZ electrolytes has contributed to the development of high-performance SOFCs, which have the potential to play a significant role in the future of clean energy production.

YSZ's biocompatibility and high mechanical properties like hardness and fracture toughness make it a suitable material especially for implants, and other biomedical structural-related applications.

The use of different sintering methods and various dopants played an important role in to overcome the different challenges and able to enhance the material properties of YSZ, such as its crystal structure (phase) stability, ionic conductivity, and mechanical properties. Even though it seems that thermal properties, electrical properties, and mechanical properties are emphasized in TBC, SOFC and biomaterials respectively, it can be challenging to achieve significant improvements in all properties simultaneously. Therefore, it is important to prioritize specific properties based on the requirements of each application.

#### **Author details**

Alexander Chee Hon Cheong\* and SivaKumar Sivanesan Asia Pacific University Technology and Innovation, Kuala Lumpur, Malaysia

\*Address all correspondence to: alexander@apu.edu.my

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Perspective Chapter: The Application of Yttria-Stabilized Zirconia (YSZ) DOI: http://dx.doi.org/10.5772/intechopen.110695*

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### *Edited by Uday M. Basheer Al-Naib*

This book is a comprehensive resource for students, researchers, professionals, and enthusiasts eager to understand the science, technology, and applications of zirconia. Its in-depth chapters, authored by experts in the field, provide a holistic view of this extraordinary material. Whether you're a materials scientist, an engineer, a dentist, or simply intrigued by the wonders of advanced ceramics, *Zirconia - New Advances, Structure, Fabrication and Applications* will expand your knowledge and inspire your curiosity. Zirconia, a remarkable ceramic material, has taken the world of materials science by storm. In this book, you will explore the diverse facets of zirconia, from its intriguing structure to its innovative applications. Take a journey into the world of zirconia, where innovation knows no bounds. Uncover its secrets, explore its applications, and witness the future of materials science unfold before your eyes.

Published in London, UK © 2023 IntechOpen © Luda311 / iStock

Zirconia - New Advances, Structure, Fabrication and Applications

Zirconia

New Advances, Structure,

Fabrication and Applications

*Edited by Uday M. Basheer Al-Naib*