**4.1 Hot Isostatic Pressing (HIP)**

Dense ceramic-ceramic composites are usually obtained by pressing and conventional sintering of powders using pressure assisted methods, such as hot pressing, hot isostatic pressing, etc.

The high sintering temperatures and long sintering times required for the consolidation of HAP/ZrO2 powders often result in excessive grain coarsening and decomposition of the HAP into second phases including phase transformation of *t*-ZrO2 to cubic phase during sintering in air, which are characteristic for conventional sintering methods and results in the deterioration of the mechanical properties of such ceramics.

The advantages of the hot pressing technique are the enhancement of the densification kinetics and the limiting of grain growth, while the disadvantages are the limited geometry of the end product and the expensive equipment required. Ahn et al., 2005, studied the effect of nano ZrO2 reinforcement on the strength of hot pressed HA/ZrO2 composites containing 1.5 to 8 wt% ZrO2. They observed the highest strength and hardness at 3wt% ZrO2 which subsequently decreased on higher ZrO2 loading. They also reported that lower volume fraction of ZrO2 addition will help to retain both HA and *t*- ZrO2 phase, which may be due to the combined effect of matrix constraint and uniform dispersion of fine ZrO2 particles. However, there are still controversial results in the literature about the phase stability of these composites prepared by pressure sintering methods (e.g HIP, hot pressing). Indeed, some reaction has been reported between HA and ZrO2 by some authors (Veljovic et al., 2007) but not by others (Li et al., 1996). However, there is a general conformity that HA/ZrO2 composites were considerably stable especially after HIP (Wu & Yen, 1988), even though traces of reaction may be expected due to the increased contact area between HA matrix and dispersed ZrO2 particles (Adolfsson et al., 2000). On the other hand, a partial reaction between HA and ZrO2 was observed in the composites prepared by hot pressing but still much less than observed in sintering in air (Evis et al., 2005, 2007). Therefore, the sintering environment and statistical effects on the ZrO2 distribution within matrix are important parameters that affect the thermal stability of HA and ZrO2 in the composites. Hot pressing of HA was found to allow the occurrence of densification at temperatures much lower than during conventional sintering (Halouani et al., 1994; Veljovic et al., 2009)

New Challenges in the Sintering of HA/ZrO2 Composites 189

electrical energy and effectively applying high temperature spark plasma generated briefly. It is regarded as a rapid sintering method similar to microwave sintering. Therefore, it is capable of sintering ceramic powders quickly to a high density at a relatively lower temperature, compared to the conventional sintering method. Due to the applied electric field, the diffusion rate increases and therefore, powder can be densified at much lower temperature with shorter holding time (only few minutes). However, expensive equipment required limits its generalized employ. Nevertheless, Hap ZrO2 (Kumar et al., 2005) composites have already been prepared by SPS technique and the results showed improved properties compared to their pressureless sintered composites. For example, HA–40 wt.% ZrO2 composites were sintered at different temperatures by SPS. After sintering at 1200 ºC for 5 min, the relative density of the composite increased up to 93% (Miao et al., 2004). The respective microstructure is presented in Fig.5. One can see that the t-ZrO2 phase (white spots) is uniformly dispersed in the HA matrix, and the composite is also very dense, since

Fig. 5. SEM micrograph of the HA-YSZ composite prepared by spark plasma sintering at

A currently paradigm of synthesizing and processing of powder compacts emphasizes the tailored assembly of particles, from the atomic scale to the macroscopic scale. One of the trends is prepare finer powder for the ultimate processing and better sintering to achieve dense matrices, with uniform microstructures and high degree of dispersion of the ZrO2 phase. More is the fineness, more is the surface area, which increases the "reactivity" of powders; The sol-gel process is a very efficient method to synthesize a large variety of compositions carefully doped with additional phases, allowing obtain HA nanosized particles and a greater control of the morphology and microstructure of the following HA/ZrO2 compacts. Thus, while suitably uniform nanophase powder materials are becoming increasingly available, challenges remain in the fabrication of fully dense nanostructured products. Another important challenge is the production of nanostructures

**4.4 Sol-gel synthesis and powder consolidation on the sinterability of HA/ZrO2** 

large pores are not found.

1200 ºC for 5 min (Source: Miao et al., 2004).

with excellent intergranular distribution of ZrO2 phase.

**composites** 

### **4.2 Microwave sintering developments**

It is usually accepted that an important issue in ceramics microstructural development is the interaction between densification and coarsening. To control it, parameters such as temperature, pressure, sintering time, and heating rate must be optimized. Rapid heating (RH) has produced high density compacts when compared with slow heating, for similar densities (D. J. Chen & Mayo, 1996). However, differential sintering that causes differential densification is one of the problems most often encountered in RH.

In this context, microwave sintering has emerged in recent years as an alternative technique to overcome the problems of RH. Because it is a noncontact technique, and the heat is transferred to the body via electromagnetic waves, large amounts of heat can be transferred to material's interior, minimizing the effects of differential sintering. Conventional furnaces heat samples by surface heating transference and, thus depending on the rate of heating, a large thermal gradient from the surface to the centre can be generated within a powder compact. However, volumetric heating via microwave radiation ensures an uniform heating and almost no temperature gradients, which yield higher heating rates and must lower sintering time. Therefore, this technique provides a series of benefits, such as great microstructure control, no limit of the geometry of the bodies, improved mechanical properties and reduced manufacturing costs due to energy savings, lower temperatures of sintering and shorter processing times.

Microwave sintering of ceramic materials with significant time and energy savings has been widely investigated. Microwave sintering of HA was first reported in the 90's. Fang et al., 1995, showed that for transparent HA, the total processing time from start to finish of the sintering process was ~ 20 min for microwave sintering while the same was about 4 h in the case of conventional sintering. Hydroxyapatite ceramics with tailored mechanical properties have also been fabricated by this technique (Rodriguez-Lorenzo et al., 2003). Further it has been shown that HA samples microwave sintered showed better densification, higher density and certainly higher hardness and fracture toughness than samples conventionally sintered at the same temperature (Veljovic et al., 2010). The addition of ZrO2, however, can reduce the sinterability of the composite and therefore does not reach a high density. At ~20 years ago, only ~50%-78% of relative density was achieved in conventional sintering of HA/ZrO2 at 1100-1400º C for 3 h (Wu & Yeh, 1988). Then, a few years later, it was reported an achieved relative density of 93% in microwave sintered HA/10%ZrO2 composites at 1200ºC, for only half hour (Fang et al., 1993). Very recently, a compared study between microwave and conventional sintering was performed on HA/ZrO2 composites (Declan et al., 2010 ). The effect of microwave heating on green bodies has been investigated in order to understand how microwave energy may affect the physical and mechanical properties of the resultant densified composites. The main difference between the two methods is that materials with different microstructures are formed. The higher levels of interconnected open porosity in microwave sinterized HA–ZrO2 composites are considered to be useful in promoting osteo-integration.

#### **4.3 Spark Plasma Sintering (SPS)**

Spark plasma sintering is a newly developed process which makes possible sintering at low temperatures and short periods by charging the voids between powder particles with

It is usually accepted that an important issue in ceramics microstructural development is the interaction between densification and coarsening. To control it, parameters such as temperature, pressure, sintering time, and heating rate must be optimized. Rapid heating (RH) has produced high density compacts when compared with slow heating, for similar densities (D. J. Chen & Mayo, 1996). However, differential sintering that causes differential

In this context, microwave sintering has emerged in recent years as an alternative technique to overcome the problems of RH. Because it is a noncontact technique, and the heat is transferred to the body via electromagnetic waves, large amounts of heat can be transferred to material's interior, minimizing the effects of differential sintering. Conventional furnaces heat samples by surface heating transference and, thus depending on the rate of heating, a large thermal gradient from the surface to the centre can be generated within a powder compact. However, volumetric heating via microwave radiation ensures an uniform heating and almost no temperature gradients, which yield higher heating rates and must lower sintering time. Therefore, this technique provides a series of benefits, such as great microstructure control, no limit of the geometry of the bodies, improved mechanical properties and reduced manufacturing costs due to energy savings, lower temperatures of

Microwave sintering of ceramic materials with significant time and energy savings has been widely investigated. Microwave sintering of HA was first reported in the 90's. Fang et al., 1995, showed that for transparent HA, the total processing time from start to finish of the sintering process was ~ 20 min for microwave sintering while the same was about 4 h in the case of conventional sintering. Hydroxyapatite ceramics with tailored mechanical properties have also been fabricated by this technique (Rodriguez-Lorenzo et al., 2003). Further it has been shown that HA samples microwave sintered showed better densification, higher density and certainly higher hardness and fracture toughness than samples conventionally sintered at the same temperature (Veljovic et al., 2010). The addition of ZrO2, however, can reduce the sinterability of the composite and therefore does not reach a high density. At ~20 years ago, only ~50%-78% of relative density was achieved in conventional sintering of HA/ZrO2 at 1100-1400º C for 3 h (Wu & Yeh, 1988). Then, a few years later, it was reported an achieved relative density of 93% in microwave sintered HA/10%ZrO2 composites at 1200ºC, for only half hour (Fang et al., 1993). Very recently, a compared study between microwave and conventional sintering was performed on HA/ZrO2 composites (Declan et al., 2010 ). The effect of microwave heating on green bodies has been investigated in order to understand how microwave energy may affect the physical and mechanical properties of the resultant densified composites. The main difference between the two methods is that materials with different microstructures are formed. The higher levels of interconnected open porosity in microwave sinterized HA–ZrO2 composites are considered to be useful in

Spark plasma sintering is a newly developed process which makes possible sintering at low temperatures and short periods by charging the voids between powder particles with

densification is one of the problems most often encountered in RH.

**4.2 Microwave sintering developments** 

sintering and shorter processing times.

promoting osteo-integration.

**4.3 Spark Plasma Sintering (SPS)** 

electrical energy and effectively applying high temperature spark plasma generated briefly. It is regarded as a rapid sintering method similar to microwave sintering. Therefore, it is capable of sintering ceramic powders quickly to a high density at a relatively lower temperature, compared to the conventional sintering method. Due to the applied electric field, the diffusion rate increases and therefore, powder can be densified at much lower temperature with shorter holding time (only few minutes). However, expensive equipment required limits its generalized employ. Nevertheless, Hap ZrO2 (Kumar et al., 2005) composites have already been prepared by SPS technique and the results showed improved properties compared to their pressureless sintered composites. For example, HA–40 wt.% ZrO2 composites were sintered at different temperatures by SPS. After sintering at 1200 ºC for 5 min, the relative density of the composite increased up to 93% (Miao et al., 2004). The respective microstructure is presented in Fig.5. One can see that the t-ZrO2 phase (white spots) is uniformly dispersed in the HA matrix, and the composite is also very dense, since large pores are not found.

Fig. 5. SEM micrograph of the HA-YSZ composite prepared by spark plasma sintering at 1200 ºC for 5 min (Source: Miao et al., 2004).

#### **4.4 Sol-gel synthesis and powder consolidation on the sinterability of HA/ZrO2 composites**

A currently paradigm of synthesizing and processing of powder compacts emphasizes the tailored assembly of particles, from the atomic scale to the macroscopic scale. One of the trends is prepare finer powder for the ultimate processing and better sintering to achieve dense matrices, with uniform microstructures and high degree of dispersion of the ZrO2 phase. More is the fineness, more is the surface area, which increases the "reactivity" of powders; The sol-gel process is a very efficient method to synthesize a large variety of compositions carefully doped with additional phases, allowing obtain HA nanosized particles and a greater control of the morphology and microstructure of the following HA/ZrO2 compacts. Thus, while suitably uniform nanophase powder materials are becoming increasingly available, challenges remain in the fabrication of fully dense nanostructured products. Another important challenge is the production of nanostructures with excellent intergranular distribution of ZrO2 phase.

New Challenges in the Sintering of HA/ZrO2 Composites 191

particles, from the atomic scale to the macroscopic scale. Thus, nanosized particles with high surface area and improved reactivity can be easily obtained by the sol-gel route. Technologically, there are significant benefits from the lower sintering temperatures of HA nanosized particles: possibility of avoiding sintering aids, phase decomposition, deleterious interfacial interactions, and undesirable phase transformations. Compared to the conventional methods, the most attractive features of sol-gel process include *(a)* molecularlevel homogeneity can be easily achieved through the mixing of two liquids; *(b)* the homogeneous mixture containing all the components in the correct stoichiometry and ensures a much higher purity; and, *(c)* a lower heat treatment temperature to form polycrystalline ceramics is usually achieved without resorting to a high excessive temperature. As a result, several synthesis routes have been proposed for sol-gel synthesis of HA and HA/YSZ as well as different mixing conditions with various molar ratios among

A number of calcium and phosphorous precursor combinations were employed for sol-gel synthesis of HA, namely Ca(NO3)2, CaO, Ca(OH)2 and Ca alkoxide as a Ca sources, triethyl phosphite [P(OC2H5)3] (TEP) and triethyl phosphate [PO(OC2H5)3], (P2O5) and H3PO4 as a Psource. Once more, chemical activity and the temperature required forming the HA structure depends largely on the chemical nature of the precursors. One of most promising combination for the precursors yielding stoicheometric HA with Ca/P ratio of 1.67, is calcium nitrate and TEP using alcohol at the hydrolysis stage (Vasconcelos & Barreto, 2011). In the development of ceramic sol-gel composites, the different components of the sol may be tailored so that they do not react with each other to form new components. In particular, in HA/ZrO2 (PSZ; Y2O3-doped) composites, ZrO2 particles can be dispersed into a HA sol before gelation, leading to a composite with good homogeneity and intimate contact

The gel composite slurry is typically dried at temperatures under 200°C, and sintered at temperatures lower than the corresponding calcined ceramics. Due to process synthesis, ZrO2 particles served as nucleation sites during HA precipitation, so HA crystals were formed on the surfaces of ZrO2 particles (Vasconcelos & Barreto, 2011). This phenomenon provided a more intimate mixing in these binary composites. Coarsening (i.e. growth of larger particles and pores, and shrinkage of finer ones), frequently observed during sintering of crystalline materials, results in overall grain growth which is in direct competition with the densification processes. However, in the case of sintering of dried gels derived amorphous powders, these competing processes are absent (there is no grains), therefore all the pores shrink during viscous sintering and the body densifies to its full

**4.5 Nanostructural design and improvement of the mechanical behavior in HA/ZrO2**

Fabrication of full dense ceramics composites is not easy. One of the main reasons lies to shrinkage heterogeneities and detrimental grain growth after the density of around 90% of theoretical density, when the continuous network of pores (in second stage of sintering) disintegrates to the closed ones (in final stage of sintering) and leads to an accelerated grain growth in conventionally sintered specimens, in special if the filler is not either

reactants (Han et al., 2004; Bogdanoviciene et al., 2006).

between the components.

density.

**ceramic composites** 

Sol-gel chemistry have gained much attention in glass and ceramics fields since the 60ths, as an alternative to the conventional melting techniques, when bulk inorganic gels were formed and converted through low-temperature heat treatments to porous and/or dense glasses or polycrystalline matrixes (Bouquin et al., 1987; Livage, 1998; Brinker & Scherer, 1990). A wide variety of glasses and ceramics with unique properties (materials "*à la carte*") can be prepared through sol-gel processing. The possibility to control a number of parameters of the final product such as homogeneity, (from the chemical composition to desired final characteristics, namely the mechanical properties, purity, microstructure (e.g. porosity, surface area, crystalline phases), and sintering temperature are among the advantages of sol-gel processing methods over the conventional ones. The resulting oxide materials have low melting and sintering temperatures, as opposed to the much higher melting temperatures required if they are produced via melting production routes. In addition, by controlling the rheological properties of the *sol* and *gel*, the final product could be shaped into various forms such as fibers, bulks, thin films coatings and powders. The most considerable constraint in the case of industrial sol-gel processing is the cost of raw materials, due to the high quality of the chemicals as precursors.

The expression sol-gel (from a liquid solution to a solid bulk gel) was applied to a colloidal solution (suspension), which is followed by a gel phase. This definition is now changed to the hydrolysis and polycondensation of a precursor and subsequent formation of a gel (Dimitriev et al., 2008). Upon destabilization of the colloidal suspension (sol), a gel which is a rigid network of the sol aggregated particles, is formed. In other words, gel is a transition state between the solid and liquid. The dispersed particles in the sol are small enough to form a stable suspension because of the Brownian motion. By varying the sol conditions, such as dehydration or pH, the colloidal dispersion (sol) transforms into the gel stage.

Although the concept of sol-gel processing covers a large variety of materials, it is specifically suitable to those obtained from hydrolysable metal alkoxides or salts. Hence, metal alkoxides M(OR)n are the most common precursors for the sol-gel synthesis of metal oxides. The precipitation of metal oxide particles from solutions was one of the earliest routes to produce ceramics through sol-gel processing.

Considering the numerous applications of HA in biomedical fields, development of various synthesis methods is currently a major issue. A number of synthesis techniques using various sources of Ca and P have been developed which includes wet chemical method (precipitation), hydrothermal synthesis procedure, continuous precipitation, thermal deposition, sol-gel and solid state reaction method (Feng et al., 2005; Thamaraiselvi et al., 2006). However, such techniques can lead to the formation of Ca-deficient apatite structure. Moreover, the presence of β-TCP, Ca4P2O9 or Ca4(PO4)2O along with the HA phase in the subsequent sintered material is indicative of a Ca/P ratio lower than the stoichiometric value of pure HA (Ca/P = 1.67). Ca/P ratios higher than 1.67 would indicate the presence of CaO with the HA phase (LeGeros et al., 1993). The final composition of dense HA after sintering depend on the sintering temperature, the experimental conditions and Ca/P molar ratio of the HA preparation prior to sintering (LeGeros et al., 1993). In some cases, a well-crystallized HA phase was only developed while approaching a sintering temperature of 1200 ºC.

The sol-gel approach provides significantly easier conditions for the synthesis of HA. A currently paradigm of synthesizing and processing emphasizes the tailored assembly of

Sol-gel chemistry have gained much attention in glass and ceramics fields since the 60ths, as an alternative to the conventional melting techniques, when bulk inorganic gels were formed and converted through low-temperature heat treatments to porous and/or dense glasses or polycrystalline matrixes (Bouquin et al., 1987; Livage, 1998; Brinker & Scherer, 1990). A wide variety of glasses and ceramics with unique properties (materials "*à la carte*") can be prepared through sol-gel processing. The possibility to control a number of parameters of the final product such as homogeneity, (from the chemical composition to desired final characteristics, namely the mechanical properties, purity, microstructure (e.g. porosity, surface area, crystalline phases), and sintering temperature are among the advantages of sol-gel processing methods over the conventional ones. The resulting oxide materials have low melting and sintering temperatures, as opposed to the much higher melting temperatures required if they are produced via melting production routes. In addition, by controlling the rheological properties of the *sol* and *gel*, the final product could be shaped into various forms such as fibers, bulks, thin films coatings and powders. The most considerable constraint in the case of industrial sol-gel processing is the cost of raw

The expression sol-gel (from a liquid solution to a solid bulk gel) was applied to a colloidal solution (suspension), which is followed by a gel phase. This definition is now changed to the hydrolysis and polycondensation of a precursor and subsequent formation of a gel (Dimitriev et al., 2008). Upon destabilization of the colloidal suspension (sol), a gel which is a rigid network of the sol aggregated particles, is formed. In other words, gel is a transition state between the solid and liquid. The dispersed particles in the sol are small enough to form a stable suspension because of the Brownian motion. By varying the sol conditions, such as dehydration or pH, the colloidal dispersion (sol) transforms into the gel stage.

Although the concept of sol-gel processing covers a large variety of materials, it is specifically suitable to those obtained from hydrolysable metal alkoxides or salts. Hence, metal alkoxides M(OR)n are the most common precursors for the sol-gel synthesis of metal oxides. The precipitation of metal oxide particles from solutions was one of the earliest

Considering the numerous applications of HA in biomedical fields, development of various synthesis methods is currently a major issue. A number of synthesis techniques using various sources of Ca and P have been developed which includes wet chemical method (precipitation), hydrothermal synthesis procedure, continuous precipitation, thermal deposition, sol-gel and solid state reaction method (Feng et al., 2005; Thamaraiselvi et al., 2006). However, such techniques can lead to the formation of Ca-deficient apatite structure. Moreover, the presence of β-TCP, Ca4P2O9 or Ca4(PO4)2O along with the HA phase in the subsequent sintered material is indicative of a Ca/P ratio lower than the stoichiometric value of pure HA (Ca/P = 1.67). Ca/P ratios higher than 1.67 would indicate the presence of CaO with the HA phase (LeGeros et al., 1993). The final composition of dense HA after sintering depend on the sintering temperature, the experimental conditions and Ca/P molar ratio of the HA preparation prior to sintering (LeGeros et al., 1993). In some cases, a well-crystallized HA phase was only

The sol-gel approach provides significantly easier conditions for the synthesis of HA. A currently paradigm of synthesizing and processing emphasizes the tailored assembly of

materials, due to the high quality of the chemicals as precursors.

routes to produce ceramics through sol-gel processing.

developed while approaching a sintering temperature of 1200 ºC.

particles, from the atomic scale to the macroscopic scale. Thus, nanosized particles with high surface area and improved reactivity can be easily obtained by the sol-gel route. Technologically, there are significant benefits from the lower sintering temperatures of HA nanosized particles: possibility of avoiding sintering aids, phase decomposition, deleterious interfacial interactions, and undesirable phase transformations. Compared to the conventional methods, the most attractive features of sol-gel process include *(a)* molecularlevel homogeneity can be easily achieved through the mixing of two liquids; *(b)* the homogeneous mixture containing all the components in the correct stoichiometry and ensures a much higher purity; and, *(c)* a lower heat treatment temperature to form polycrystalline ceramics is usually achieved without resorting to a high excessive temperature. As a result, several synthesis routes have been proposed for sol-gel synthesis of HA and HA/YSZ as well as different mixing conditions with various molar ratios among reactants (Han et al., 2004; Bogdanoviciene et al., 2006).

A number of calcium and phosphorous precursor combinations were employed for sol-gel synthesis of HA, namely Ca(NO3)2, CaO, Ca(OH)2 and Ca alkoxide as a Ca sources, triethyl phosphite [P(OC2H5)3] (TEP) and triethyl phosphate [PO(OC2H5)3], (P2O5) and H3PO4 as a Psource. Once more, chemical activity and the temperature required forming the HA structure depends largely on the chemical nature of the precursors. One of most promising combination for the precursors yielding stoicheometric HA with Ca/P ratio of 1.67, is calcium nitrate and TEP using alcohol at the hydrolysis stage (Vasconcelos & Barreto, 2011).

In the development of ceramic sol-gel composites, the different components of the sol may be tailored so that they do not react with each other to form new components. In particular, in HA/ZrO2 (PSZ; Y2O3-doped) composites, ZrO2 particles can be dispersed into a HA sol before gelation, leading to a composite with good homogeneity and intimate contact between the components.

The gel composite slurry is typically dried at temperatures under 200°C, and sintered at temperatures lower than the corresponding calcined ceramics. Due to process synthesis, ZrO2 particles served as nucleation sites during HA precipitation, so HA crystals were formed on the surfaces of ZrO2 particles (Vasconcelos & Barreto, 2011). This phenomenon provided a more intimate mixing in these binary composites. Coarsening (i.e. growth of larger particles and pores, and shrinkage of finer ones), frequently observed during sintering of crystalline materials, results in overall grain growth which is in direct competition with the densification processes. However, in the case of sintering of dried gels derived amorphous powders, these competing processes are absent (there is no grains), therefore all the pores shrink during viscous sintering and the body densifies to its full density.

#### **4.5 Nanostructural design and improvement of the mechanical behavior in HA/ZrO2 ceramic composites**

Fabrication of full dense ceramics composites is not easy. One of the main reasons lies to shrinkage heterogeneities and detrimental grain growth after the density of around 90% of theoretical density, when the continuous network of pores (in second stage of sintering) disintegrates to the closed ones (in final stage of sintering) and leads to an accelerated grain growth in conventionally sintered specimens, in special if the filler is not either

New Challenges in the Sintering of HA/ZrO2 Composites 193

150 nm) and are trapped at grain boundaries between HA grains. This tailored microstructure provide a more intimate mixing in binary composites, yielding a higher dispersion, allowing ZrO2 particles to be present mostly at grain boundaries, without agglomerates. Thus, the prepared samples were fully densified with very small isolated

The principal merit of the microstructure observed in these particular composites, obtained by the sol–gel route, is the adequate relative grain size ratio and phase distribution between the both phases, allowing ZrO2 particles to be present mostly at grain boundaries without agglomerates. During the HA grain growth, residual pores were eliminated throughout the sinter of the HA matrix, and thus an equilibrium-point between densification and coarsening were achieved. Simultaneously, ZrO2 particles, trapped in the grain-boundaries

Other approaches were also used to obtain better mechanical properties. The properties of these materials are determined by their microstructures; therefore, to control their microstructural development and achieve fine microstructures, the sintering parameters must be optimized. For example, partially or fully fluorine-substituted hydroxyapatite (HAF) ceramics (Adolfsson & Hermansson, 2005) have shown a high thermal stability and some level of bioactivity, and they have been used to develop HAF-ZrO2 composites with aimed improved mechanical behavior (Chen et al, 2008). The HAF-ZrO2 microstructures exhibited ZrO2 grains distributed on the grain boundaries and also within the grains of the matrix (Fig.8). Furthermore, with the increase of the ZrO2 content, more ZrO2 grains were

For mechanical improvement is quite essential that ZrO2 particles are mainly as tetragonal phase. However, if *m*-ZrO2 is present on cracked surface, such presence indicates that the transformation toughening is active for toughness enhancement (Vasconcelos & Barreto, 2011). Transformation toughening is induced from the stress-induced transformation experienced by *t*-ZrO2 particles when interacting with an advancing crack (Fig.9a) (Stevens,

> Intragranular **ZrO2 particles**

Fig. 7. Tailored microstructure a HA–10 mol%ZrO2 sol-gel composite (adapted from

**pore** 

**HA** 

Intergranular **ZrO2 particles** 

acts as effective abnormal grain growth inhibitor of HA

voids.

trapped within the grains.

Vasconcelos & Barreto, 2011).

1986).

homogenously distributed. On the other hand, the retention of *t*-ZrO2 in HA derived composites has to be controlled carefully until is used for implant devices. It has been recognized that the level of toughening is complexly dependent on the microstructure of such composites (i.e., volume fraction, size, shape, location and size distribution of ZrO2 (Heuer et al., 1982). It can be seen (Fig.6) that toughness (KIc) of HA-based ZrO2 composites increased with the ZrO2 content up to 20wt% (Chen et al., 2008). The poor mechanical behavior of the composites with 40 to 60 wt% ZrO2 is correlated with the presence of high porosities (up to 16%) and phase distribution heterogeneities which tended to retard the densification.

Fig. 6. Fracture toughness vs. ZrO2 content of HA-based ZrO2 composites conventionally sintered at 1400ºC for 2 hours (Source: Chen et al., 2008).

Homogeneous as well as fine dispersions of ZrO2 particles in the HA matrix can be obtained by chemical mixing of the constituents in the solution through the sol–gel processing (Vasconcelos & Barreto, 2011). The fine grain size and uniform microstructure of both phases within the composite satisfy the requirements for a toughness improvement; moreover, the toughness can be improved many folds due to the presence of tetragonal phase within the HA matrix, with a potential of 100% of transformation. However, such arguments can changes with the phase composition, crystallinity, crystallite size and pore morphology, specific surface area and subsequent shrinkage.

The sol-gel technology offers processing advantages and gives flexibility in tailoring the composite chemistry to obtain the desired properties. Moreover, the processing conditions, composition, retention of the t-phase of ZrO2 the calcination temperature and additives (H. W. Kim et al., 2002b) also control the morphology of the powders and their sintering behavior. An important challenge is the production of composite nanostructures with homogeneous microstructural distribution of ZrO2 phase. Fig.7. illustrates the SEM of a sintered (950 ºC for 1 h) HA/10 mol% ZrO2 composite (Vasconcelos & Barreto, 2011). As can be seen, the image reveals that ZrO2 particles are present as both intergranular and intragranular in the HA matrix. However, the presence of intragranular particles is much less compared to intergranular particles. The network of intergranular grain is thus the dominant in the toughening mechanism. The average grain size of HA varies from 0.5 mm to 2mm and is quite homogeneous in the entire matrix. ZrO2 particles are smaller in size (50-

homogenously distributed. On the other hand, the retention of *t*-ZrO2 in HA derived composites has to be controlled carefully until is used for implant devices. It has been recognized that the level of toughening is complexly dependent on the microstructure of such composites (i.e., volume fraction, size, shape, location and size distribution of ZrO2 (Heuer et al., 1982). It can be seen (Fig.6) that toughness (KIc) of HA-based ZrO2 composites increased with the ZrO2 content up to 20wt% (Chen et al., 2008). The poor mechanical behavior of the composites with 40 to 60 wt% ZrO2 is correlated with the presence of high porosities (up to 16%) and phase distribution heterogeneities which tended to retard the

Fig. 6. Fracture toughness vs. ZrO2 content of HA-based ZrO2 composites conventionally

Homogeneous as well as fine dispersions of ZrO2 particles in the HA matrix can be obtained by chemical mixing of the constituents in the solution through the sol–gel processing (Vasconcelos & Barreto, 2011). The fine grain size and uniform microstructure of both phases within the composite satisfy the requirements for a toughness improvement; moreover, the toughness can be improved many folds due to the presence of tetragonal phase within the HA matrix, with a potential of 100% of transformation. However, such arguments can changes with the phase composition, crystallinity, crystallite size and pore

The sol-gel technology offers processing advantages and gives flexibility in tailoring the composite chemistry to obtain the desired properties. Moreover, the processing conditions, composition, retention of the t-phase of ZrO2 the calcination temperature and additives (H. W. Kim et al., 2002b) also control the morphology of the powders and their sintering behavior. An important challenge is the production of composite nanostructures with homogeneous microstructural distribution of ZrO2 phase. Fig.7. illustrates the SEM of a sintered (950 ºC for 1 h) HA/10 mol% ZrO2 composite (Vasconcelos & Barreto, 2011). As can be seen, the image reveals that ZrO2 particles are present as both intergranular and intragranular in the HA matrix. However, the presence of intragranular particles is much less compared to intergranular particles. The network of intergranular grain is thus the dominant in the toughening mechanism. The average grain size of HA varies from 0.5 mm to 2mm and is quite homogeneous in the entire matrix. ZrO2 particles are smaller in size (50-

sintered at 1400ºC for 2 hours (Source: Chen et al., 2008).

morphology, specific surface area and subsequent shrinkage.

densification.

150 nm) and are trapped at grain boundaries between HA grains. This tailored microstructure provide a more intimate mixing in binary composites, yielding a higher dispersion, allowing ZrO2 particles to be present mostly at grain boundaries, without agglomerates. Thus, the prepared samples were fully densified with very small isolated voids.

The principal merit of the microstructure observed in these particular composites, obtained by the sol–gel route, is the adequate relative grain size ratio and phase distribution between the both phases, allowing ZrO2 particles to be present mostly at grain boundaries without agglomerates. During the HA grain growth, residual pores were eliminated throughout the sinter of the HA matrix, and thus an equilibrium-point between densification and coarsening were achieved. Simultaneously, ZrO2 particles, trapped in the grain-boundaries acts as effective abnormal grain growth inhibitor of HA

Other approaches were also used to obtain better mechanical properties. The properties of these materials are determined by their microstructures; therefore, to control their microstructural development and achieve fine microstructures, the sintering parameters must be optimized. For example, partially or fully fluorine-substituted hydroxyapatite (HAF) ceramics (Adolfsson & Hermansson, 2005) have shown a high thermal stability and some level of bioactivity, and they have been used to develop HAF-ZrO2 composites with aimed improved mechanical behavior (Chen et al, 2008). The HAF-ZrO2 microstructures exhibited ZrO2 grains distributed on the grain boundaries and also within the grains of the matrix (Fig.8). Furthermore, with the increase of the ZrO2 content, more ZrO2 grains were trapped within the grains.

For mechanical improvement is quite essential that ZrO2 particles are mainly as tetragonal phase. However, if *m*-ZrO2 is present on cracked surface, such presence indicates that the transformation toughening is active for toughness enhancement (Vasconcelos & Barreto, 2011). Transformation toughening is induced from the stress-induced transformation experienced by *t*-ZrO2 particles when interacting with an advancing crack (Fig.9a) (Stevens, 1986).

Fig. 7. Tailored microstructure a HA–10 mol%ZrO2 sol-gel composite (adapted from Vasconcelos & Barreto, 2011).

New Challenges in the Sintering of HA/ZrO2 Composites 195

Apart from the processing techniques to control the external sintering conditions, other approaches are also used to suppress the decomposition of the HA phase. To minimize the thermal decomposition and improve the densification of the composites, various atmospheres assisted sintering have been employed to produce HA/ZrO2 composites. However, if this chemical interaction exist, unfortunately would affect the phase purity and the microstructure of the HA/ZrO2 composites, leading to undesirable mechanical and

In the spike of that, it was observed that the release of water increases gradually with

by O2- ions and vacancies) (Adolfsson et al., 1999, 2000). This reaction occurs because HA is a hydrated calcium phosphate material, and begins to dehydroxylate at about 800°C to form oxyhydroxyapatite (Ca10(PO4)6(OH)2-2xOx x, where =vacancy). Further heating, and according to the equilibrium phase diagram of CaO/P2O5, HAP will decompose in β-TCP (β-Ca3(PO4)2), forming α-TCP when heated to temperatures in excess of 1350°C. It is well established that HA is thermally decomposed into mostly β-TCP, CaO and H2O(vapour) (Ahn et al., 2001; Shen et al., 2001; Heimann & Vu, 1997), according to the following reactions (2),

> <sup>2</sup> 10 4 6 34 2 2 49 *Ca (PO ) O Ca (PO ) Ca P O* ⇔− + β

() 3 10 4 6 2 22 3 4 *Ca (PO ) OH Ca (PO ) CaO H O* ⇔− + + ↑ β

Another process that removes CaO from HA is dissolution of CaO into *t*-ZrO2 as seen in

() 2( ) 2( ) *CaO ZrO CaO ZrO tetragonal cubic* + ⇔ (5)

Some authors (Wu & Yeh, 1998; Rao & Kannan, 2002; Evis et al., 2005;) have reported that the CaO release, due to HA decomposition to β-TCP, enhances the formation of CZ and

( )( ) 10 4 6 2 2( ) *Ca PO OH yZrO tetragonal* + ⇔

**5.1 Thermal stabilization of HA/ZrO2 composites with water vapor assisted sintering**  Since a gaseous species exists on the products side of the decomposition reactions, sintering atmosphere would be expected to influence the decomposition kinetics of HA. Consequently, sintering under vacuum conditions can induce decomposition at lower temperatures, favouring the formation of water vapour. On the other hand sintering in a

( ) 3 42 3 <sup>2</sup> <sup>2</sup> 3 2 2 *(cubic) y β* − ++ *Ca (PO ) CaZrO CaO(ZrO ) x H O* +↑ − (6)

CaO:ZrO2 solid solution, according to reaction (6):

10 4 6 2 10 4 6 2 2 2 *Ca (PO ) (OH) Ca (PO ) (OH) O xH O* ⇔ *( x) x* + ↑ − (2)

positions is replaced

(3)

(4)

increasing temperature creating vacancies in HA structure (former OH-

**5. Sintering advances to suppress the decomposition of the HA phase** 

biological properties.

(3) and (4):

reaction (5):

Moreover, crack deflection toughening by ZrO2 particles can also contributes to toughening of the composite through crack deflection around the dispersed ZrO2 particles (Fig.9b) (Vasconcelos & Barreto, 2011). Hence, KIc changes with the addition of ZrO2 may be rationalized by the relative predominance of the toughening mechanism, i.e., ZrO2 phase transformation and/or crack deflection.

Fig. 8. SEM micrographs of the thermally etched surfaces of the HAF-5wt%ZrO2 composites conventionally sintered at 1400ºC (Source: Chen et al, 2008).

In addition, a high degree of densification and homogeneous shrinkage of such composites can be yield if the formation of Ca-deficient HAP phases during sinterization is avoided. Therefore, through sol-gel process the T-onset of decomposition and the densification at low conventional temperatures of ZrO2-doped HA composites leads to more thermally stable HA–ZrO2 composite products than those obtained by other (e.g. coprecipitation) routes.

The improvements in sinterability and microstructure had a strong influence on the mechanical properties of the HA–ZrO2 composites.

Fig. 9. (a) Stress induced transformation of *t*-ZrO2 particles in the elastic stress field of a crack (Stevens, 1986); (b) Indentation crack propagation during indentation fracture of active crack deflection by the ZrO2 particles (adapted from Vasconcelos & Barreto, 2011).

Moreover, crack deflection toughening by ZrO2 particles can also contributes to toughening of the composite through crack deflection around the dispersed ZrO2 particles (Fig.9b) (Vasconcelos & Barreto, 2011). Hence, KIc changes with the addition of ZrO2 may be rationalized by the relative predominance of the toughening mechanism, i.e., ZrO2 phase

Fig. 8. SEM micrographs of the thermally etched surfaces of the HAF-5wt%ZrO2 composites

In addition, a high degree of densification and homogeneous shrinkage of such composites can be yield if the formation of Ca-deficient HAP phases during sinterization is avoided. Therefore, through sol-gel process the T-onset of decomposition and the densification at low conventional temperatures of ZrO2-doped HA composites leads to more thermally stable HA–ZrO2 composite products than those obtained by other (e.g. co-

The improvements in sinterability and microstructure had a strong influence on the

Fig. 9. (a) Stress induced transformation of *t*-ZrO2 particles in the elastic stress field of a crack (Stevens, 1986); (b) Indentation crack propagation during indentation fracture of active

crack deflection by the ZrO2 particles (adapted from Vasconcelos & Barreto, 2011).

conventionally sintered at 1400ºC (Source: Chen et al, 2008).

mechanical properties of the HA–ZrO2 composites.

transformation and/or crack deflection.

precipitation) routes.

#### **5. Sintering advances to suppress the decomposition of the HA phase**

Apart from the processing techniques to control the external sintering conditions, other approaches are also used to suppress the decomposition of the HA phase. To minimize the thermal decomposition and improve the densification of the composites, various atmospheres assisted sintering have been employed to produce HA/ZrO2 composites. However, if this chemical interaction exist, unfortunately would affect the phase purity and the microstructure of the HA/ZrO2 composites, leading to undesirable mechanical and biological properties.

In the spike of that, it was observed that the release of water increases gradually with increasing temperature creating vacancies in HA structure (former OH positions is replaced by O2- ions and vacancies) (Adolfsson et al., 1999, 2000). This reaction occurs because HA is a hydrated calcium phosphate material, and begins to dehydroxylate at about 800°C to form oxyhydroxyapatite (Ca10(PO4)6(OH)2-2xOx x, where =vacancy). Further heating, and according to the equilibrium phase diagram of CaO/P2O5, HAP will decompose in β-TCP (β-Ca3(PO4)2), forming α-TCP when heated to temperatures in excess of 1350°C. It is well established that HA is thermally decomposed into mostly β-TCP, CaO and H2O(vapour) (Ahn et al., 2001; Shen et al., 2001; Heimann & Vu, 1997), according to the following reactions (2), (3) and (4):

$$\text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{(OH)}\_2 \Leftrightarrow \text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{(OH)}\_{\text{(2-2x)}}\text{O}\_x + \text{xH}\_2\text{O}\uparrow\tag{2}$$

$$\rm Ca\_{10}(PO\_4)\_6O \Leftrightarrow 2\beta-Ca\_3(PO\_4)\_2 + Ca\_4P\_2O\_9 \tag{3}$$

$$\rm Ca\_{10}(PO\_4)\_6(OH)\_2 \Leftrightarrow 3\beta-Ca\_3(PO\_4)\_2 + CaCO + H\_2O \uparrow \tag{4}$$

Another process that removes CaO from HA is dissolution of CaO into *t*-ZrO2 as seen in reaction (5):

$$\text{CaO} + \text{ZrO}\_2\text{(tertragonal)} \Leftrightarrow \text{CaO}(\text{ZrO}\_2\text{(cubic)}) \tag{5}$$

Some authors (Wu & Yeh, 1998; Rao & Kannan, 2002; Evis et al., 2005;) have reported that the CaO release, due to HA decomposition to β-TCP, enhances the formation of CZ and CaO:ZrO2 solid solution, according to reaction (6):

$$\text{Ca}\_{10}(\text{PO}\_4)\_6(\text{OH})\_2 + yZrO\_2(\text{tetraagonal}) \Leftrightarrow$$

$$\text{Ca}\beta-\text{Ca}\_3(\text{PO}\_4)\_2 + \text{CaZrO}\_3 + \text{CaO}(\text{ZrO}\_{2\{\text{cubic}\}})\_y + \uparrow(2-2x)H\_2O \tag{6}$$

#### **5.1 Thermal stabilization of HA/ZrO2 composites with water vapor assisted sintering**

Since a gaseous species exists on the products side of the decomposition reactions, sintering atmosphere would be expected to influence the decomposition kinetics of HA. Consequently, sintering under vacuum conditions can induce decomposition at lower temperatures, favouring the formation of water vapour. On the other hand sintering in a

New Challenges in the Sintering of HA/ZrO2 Composites 197

Similar approaches, like a flowing H2O(g)/O2 mixtures or hydrothermal sintering conditions have been successful to eliminate or reduce the decomposition of HA or increase the T-onset of decomposition (Ruys et al., 1995). On the other hand, if an intergranular distribution of ZrO2 particles can be achieved (Fig.4), the decomposition reactions of HA are avoided because diffusion of water from the reaction zone to the surfaces is retarded by the ZrO2 matrix (intergranular ZrO2 particles) in boundaries of HA grains, forming a continuous framework (Vasconcelos & Barreto, 2011). Besides, the morphological characteristics of the powders, the good stoichiometry, and their crystallinity are also factors

It was reported that one of the main disadvantages of the composite approach applied to HA is related to its processing, leading generally to poor densification. There is an agreement that the addition of ZrO2 to the HA causes it to decompose at lower temperatures in pressureless sintering, so that several workers have used hot aid pressing to reach higher density. However, it is possible improve densification by applying appropriate sintering additives for HA. The sintering additives should considerably improve composite densification without decomposition of HA. Moreover, the sintering additives could be used as structure stabilizers and/or to control grain size. Thus, several sintering additives (Suchanek et al., 1997) have been used in HA (e.g. sodium, lithium, magnesium, calcium and aluminium fluorides, lithium and sodium phosphates, among others.) Most of them (except NaF and AlF3) improved densification of HA, not only by influencing the processing conditions, but also by changing chemical properties of the powders (LeGeros, 1991). In a study of HA–ZrO2 composites sintered without pressure, small additions of CaF2 (H. W. Kim et al., 2002b) were added as a sintering aid to thermally stabilize the HA-ZrO2 composites. Thus, due to the substitution of OH- groups by F- ions, Fluorapatite (FHA) was obtained and thereby restrained the decomposition of HA to β-TCP. As a result, dense

Apart this, other approaches were also used to suppress the decomposition of the HA phase. For instance, (Kong et. al., 1999) added alumina into HA/ZrO2 composites to reduce the contact areas between the HA phase and the ZrO2 phase, and hence suppress the interactions between them. However, the introduction of another metal oxide phase might also result in a high level of residual thermal stresses and lead to microcracking of the sintered bodies due to the large difference in the thermal expansion coefficients of the various composite components. In addition, the presence of CaO may cause decohesion of the material due to stresses arriving from formation of Ca(OH)2 and related volume changes (Ababou et al., 1995). However, Heimann & Vu, 1997, have shown that addition of CaO to HA/ZrO2 composite shifts the chemical equilibrium of the product from β-TCP and tetra calcium phosphate (TTCP) towards HA making it more stable. Additional CaO will be effectively fixed by ZrO2 acting as

Bioceramics, used initially as alternatives to metals in order to increase the biocompatibility of implants, have become a diverse class of biomaterials. HA attracts major interest due to

to take into account in sinterability.

bodies with high strength and toughness were obtained.

a sink for Ca2+ ions resulting in the formation of either *t*-ZrO2 or CaZrO3.

**5.2 Other approaches** 

**6. Conclusion** 

moist atmosphere can neutralize this effect and avoid or delay the decomposition to some degree.

When a specific fraction of vacancies is created, HA will not be stable and it will decompose. However, if H2O loss is maintained at a low level, the equilibrium of these reactions is shifted to the left and decomposition into β-TCP, CaZrO3 and ZrO2(cubic) will not occur. In order to prevent water loss, it is recommended the use an atmosphere control for sintering (water vapor, for example), or incorporation of additives (e.g. incorporation of F- ions) (H. W. Kim et al., 2003) in HA structure. If fraction of vacancies formed is suppressed the temperature stability will be extended.

So far, it has been well recognized that, by pressureless sintering of HA/ZrO2 composites, it is very difficult to reach full densification in air (Li & Hermansson, 1996; Wu &Yeh, 1988). Therefore, only by introducing pressure water vapor it is possible to densify without any decomposition (Vasconcelos & Barreto, 2011). The obtained HA/ZrO2 compacts under Water Vapor Assisted Sintering did not contain any phases other than HA and the tetragonal modification of ZrO2, as revealed by their X-ray powder diffraction patterns (Fig. 10). The presence of an H2O atmosphere during sintering reaction causes a compensation of the partial vapour atmosphere of water inside the furnace. In this way, vacancies formation in the HA structure through reaction (2) could be effectively avoided by a left shift of the equilibrium of reactions (4) and (6), countering the HA decomposition by means of the Le Châtelier principle.

Fig. 10. XRD patterns of HA and HA /10%YSZ powder heated at 950ºC, 1 h in H2O vapour atmosphere (adapted from Vasconcelos & Barreto, 2011).

Similar approaches, like a flowing H2O(g)/O2 mixtures or hydrothermal sintering conditions have been successful to eliminate or reduce the decomposition of HA or increase the T-onset of decomposition (Ruys et al., 1995). On the other hand, if an intergranular distribution of ZrO2 particles can be achieved (Fig.4), the decomposition reactions of HA are avoided because diffusion of water from the reaction zone to the surfaces is retarded by the ZrO2 matrix (intergranular ZrO2 particles) in boundaries of HA grains, forming a continuous framework (Vasconcelos & Barreto, 2011). Besides, the morphological characteristics of the powders, the good stoichiometry, and their crystallinity are also factors to take into account in sinterability.

#### **5.2 Other approaches**

196 Sintering of Ceramics – New Emerging Techniques

moist atmosphere can neutralize this effect and avoid or delay the decomposition to some

When a specific fraction of vacancies is created, HA will not be stable and it will decompose. However, if H2O loss is maintained at a low level, the equilibrium of these reactions is shifted to the left and decomposition into β-TCP, CaZrO3 and ZrO2(cubic) will not occur. In order to prevent water loss, it is recommended the use an atmosphere control for sintering (water vapor, for example), or incorporation of additives (e.g. incorporation of F- ions) (H. W. Kim et al., 2003) in HA structure. If fraction of vacancies formed is suppressed the

So far, it has been well recognized that, by pressureless sintering of HA/ZrO2 composites, it is very difficult to reach full densification in air (Li & Hermansson, 1996; Wu &Yeh, 1988). Therefore, only by introducing pressure water vapor it is possible to densify without any decomposition (Vasconcelos & Barreto, 2011). The obtained HA/ZrO2 compacts under Water Vapor Assisted Sintering did not contain any phases other than HA and the tetragonal modification of ZrO2, as revealed by their X-ray powder diffraction patterns (Fig. 10). The presence of an H2O atmosphere during sintering reaction causes a compensation of the partial vapour atmosphere of water inside the furnace. In this way, vacancies formation in the HA structure through reaction (2) could be effectively avoided by a left shift of the equilibrium of reactions (4) and (6), countering the HA decomposition by means of the Le

Fig. 10. XRD patterns of HA and HA /10%YSZ powder heated at 950ºC, 1 h in H2O vapour

atmosphere (adapted from Vasconcelos & Barreto, 2011).

degree.

temperature stability will be extended.

Châtelier principle.

It was reported that one of the main disadvantages of the composite approach applied to HA is related to its processing, leading generally to poor densification. There is an agreement that the addition of ZrO2 to the HA causes it to decompose at lower temperatures in pressureless sintering, so that several workers have used hot aid pressing to reach higher density. However, it is possible improve densification by applying appropriate sintering additives for HA. The sintering additives should considerably improve composite densification without decomposition of HA. Moreover, the sintering additives could be used as structure stabilizers and/or to control grain size. Thus, several sintering additives (Suchanek et al., 1997) have been used in HA (e.g. sodium, lithium, magnesium, calcium and aluminium fluorides, lithium and sodium phosphates, among others.) Most of them (except NaF and AlF3) improved densification of HA, not only by influencing the processing conditions, but also by changing chemical properties of the powders (LeGeros, 1991). In a study of HA–ZrO2 composites sintered without pressure, small additions of CaF2 (H. W. Kim et al., 2002b) were added as a sintering aid to thermally stabilize the HA-ZrO2 composites. Thus, due to the substitution of OH- groups by F- ions, Fluorapatite (FHA) was obtained and thereby restrained the decomposition of HA to β-TCP. As a result, dense bodies with high strength and toughness were obtained.

Apart this, other approaches were also used to suppress the decomposition of the HA phase. For instance, (Kong et. al., 1999) added alumina into HA/ZrO2 composites to reduce the contact areas between the HA phase and the ZrO2 phase, and hence suppress the interactions between them. However, the introduction of another metal oxide phase might also result in a high level of residual thermal stresses and lead to microcracking of the sintered bodies due to the large difference in the thermal expansion coefficients of the various composite components. In addition, the presence of CaO may cause decohesion of the material due to stresses arriving from formation of Ca(OH)2 and related volume changes (Ababou et al., 1995). However, Heimann & Vu, 1997, have shown that addition of CaO to HA/ZrO2 composite shifts the chemical equilibrium of the product from β-TCP and tetra calcium phosphate (TTCP) towards HA making it more stable. Additional CaO will be effectively fixed by ZrO2 acting as a sink for Ca2+ ions resulting in the formation of either *t*-ZrO2 or CaZrO3.

### **6. Conclusion**

Bioceramics, used initially as alternatives to metals in order to increase the biocompatibility of implants, have become a diverse class of biomaterials. HA attracts major interest due to

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#### **7. References**


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**1. Introduction** 

**10** 

*India* 

**Ceramics in Dentistry** 

It is quite usual in dentistry to adopt a material from engineers and adapt it to clinical conditions. A good example of such an instance is dental ceramics. In Dental science, ceramics are referred to as nonmetallic, inorganic structures primarily containing compounds of oxygen with one or more metallic or semi-metallic elements. They are usually sodium, potassium,

As we peep into the dental history, a French dentist De Chemant patented the first porcelain tooth material in 1789. In 1808 Fonzi, an Italian dentist invented a "terrometallic" porcelain tooth that was held in place by a platinum pin or frame. Ash developed an improved version of the platinum tooth in 1837. Dr. Charles Land patented the first Ceramic crowns in

Structurally, dental ceramics contain a crystal phase and a glass phase based on the silica structure, characterized by a silica tetrahedra, containing central Si4+ ion with four O- ions. It is not closely packed, having both covalent and ionic characteristics. The usual dental ceramic, is glassy in nature, with short range crystallinity. The only true crystalline ceramic used at present in restorative dentistry is Alumina (Al2O3), which is one of the hardest and strongest oxides known. Ceramics composed of single element are rare. Diamond is a major ceramic of this type, hardest natural material used to cut tooth enamel. Ceramics are widely

Basically the inorganic composition of teeth and bones are ceramics – Hydroxyapatite. Hence ceramics like hydroxyapatite, wollastonite etc are used as bone graft materials. They have an entire plethora of synthetic techniques like wet chemical, sol-gel, hydrothermal methods etc. Also they are added as bioactive filler particles to other inert materials like polymers or coated over metallic implants. These ceramics are collectively called as bioceramics. There are basically two kinds of bioceramics-inert (e.g. Alumina) and bioactive (hydroxyapatite). They

Dental cements are basically glasses. Initially, silicate cements were introduced. They constitute the first dental cement to use glass as its component. The cement powder contains a glass of silica, alumina and fluorides. The liquid, is an aqueous solution of phosphoric acid with buffer salts. Fluoride ions leached out from the set cements are responsible for the anticariogenic property. But silicates are discontinued due to low pH during setting reaction

calcium, magnesium, aluminum, silicon, phosphorus, zirconium & titanium.

1903.Vita Zahnfabrik introduced the first commercial porcelain in 1963.

used in dentistry due to its dual role – strength and esthetics.

can be resorbable (Tricalciumphosphate) or non-resorbable (Zirconia).

that affects the dental pulp.

R. Narasimha Raghavan *Dental Surgeon, Chennai* 

