**5. Advanced applications of FGC ceramics**

The use of FGCs has rapidly gained popularity in recent years, especially in high temperature environments and aggressive media, as illustrated in Figure 4. The FGCs concept is applicable to almost all material fields. Examples of a variety of real and potential applications of FGCs in the field of engineering are cutting tools, machine parts, and engine components, while incompatible properties such as heat, wear, and corrosion resistance, plus toughness and machinability are incorporated into a single part. For example, throwaway chips for cutting tools made of graded tungsten carbide/cobalt **(WC/Co)** and titanium carbonitride **(TiCN)-WC/ Co** that incorporate the desirable properties of high machining speed, high feed rates, and a long life have been developed and commercialized. Various combinations of these ordinarily incompatible functions can be applied to create new materials for the aerospace industry, chemical plants, optoelectronic applications, bio-medical applications, solar cells, and nuclear energy reactors.

#### **5.1. FG Ceramics for aerospace, military and automotive applications**

Thermal barrier coating FGCs are used for military and commercial aero engines as well as in gas turbine engines for automobiles, helicopters, marine vehicles, and electric power genera‐ tors. They are also used in augmentor components, e.g. tail cones, flame holders, heat shields and duct liners, and in the nozzle section they are being used experimentally in the verging/ diverging flaps and on seals where hot gases exit the engine [58, 59].

Space vehicles flying at hypersonic speeds experience extremely high temperatures from aerodynamic heating due to friction between the vehicle surface and the atmosphere. One of the main objectives of investigating FGCs deposited by chemical vapor deposition (CVD-FGCs) was the development of thermal barrier coatings (TBCs) for a space plane. It was found that sheets of **SiC/C FGCs** produced by CVD provide excellent thermal stability and thermal insulation at 1227°C, as well as excellent thermal fatigue properties and resistance to thermal shock [60]. A combustion chamber with a protective layer of SiC/C FGC has been developed for the reaction control system engine of HOPE, a Japanese space shuttle. These FGCs produced for rocket combustors have undergone critical tests with nitrogen tetroxide and monomethyl hydrazine propellants at firing cycles of 55 seconds with subsequent quenching by liquid nitrogen. The maximum outer wall temperature of these model combustors was 1376°C to 1527°C, while the inner wall temperature reached 1677°C to 2027°C. No damage to the Advances in Functionally Graded Ceramics – Processing, Sintering Properties and Applications http://dx.doi.org/10.5772/62612 13

**Figure 4.** Areas of potential application of FGCs.

and physical vapor deposition (PVD). C-based materials that have an excessive chemical sputtering which yields at 600 to 1000 K and exhibits irradiation with enhanced sublimation at >1200 K when exposed to plasma erosion conditions, were successfully manufactured via the CVD method in 2002. The problem of serious C-contamination of the plasma was solved by using chemically deposited SiC coatings on the surface of the C-substrate. C-based FGCs such as **SiC/C, B4C/Cu, SiC/Cu and B4C/C bulk FGC** were also successfully manufactured

The use of FGCs has rapidly gained popularity in recent years, especially in high temperature environments and aggressive media, as illustrated in Figure 4. The FGCs concept is applicable to almost all material fields. Examples of a variety of real and potential applications of FGCs in the field of engineering are cutting tools, machine parts, and engine components, while incompatible properties such as heat, wear, and corrosion resistance, plus toughness and machinability are incorporated into a single part. For example, throwaway chips for cutting tools made of graded tungsten carbide/cobalt **(WC/Co)** and titanium carbonitride **(TiCN)-WC/ Co** that incorporate the desirable properties of high machining speed, high feed rates, and a long life have been developed and commercialized. Various combinations of these ordinarily incompatible functions can be applied to create new materials for the aerospace industry, chemical plants, optoelectronic applications, bio-medical applications, solar cells, and nuclear

Thermal barrier coating FGCs are used for military and commercial aero engines as well as in gas turbine engines for automobiles, helicopters, marine vehicles, and electric power genera‐ tors. They are also used in augmentor components, e.g. tail cones, flame holders, heat shields and duct liners, and in the nozzle section they are being used experimentally in the verging/

Space vehicles flying at hypersonic speeds experience extremely high temperatures from aerodynamic heating due to friction between the vehicle surface and the atmosphere. One of the main objectives of investigating FGCs deposited by chemical vapor deposition (CVD-FGCs) was the development of thermal barrier coatings (TBCs) for a space plane. It was found that sheets of **SiC/C FGCs** produced by CVD provide excellent thermal stability and thermal insulation at 1227°C, as well as excellent thermal fatigue properties and resistance to thermal shock [60]. A combustion chamber with a protective layer of SiC/C FGC has been developed for the reaction control system engine of HOPE, a Japanese space shuttle. These FGCs produced for rocket combustors have undergone critical tests with nitrogen tetroxide and monomethyl hydrazine propellants at firing cycles of 55 seconds with subsequent quenching by liquid nitrogen. The maximum outer wall temperature of these model combustors was 1376°C to 1527°C, while the inner wall temperature reached 1677°C to 2027°C. No damage to the

using this method [57].

energy reactors.

**5. Advanced applications of FGC ceramics**

12 Advances in Functionally Graded Materials and Structures

**5.1. FG Ceramics for aerospace, military and automotive applications**

diverging flaps and on seals where hot gases exit the engine [58, 59].

combustors was observed after two test cycles [61]. It is expected that the Si-based ceramics, **SiC** and **Si3N4**, will be introduced in the hot-sections of the next generation of gas turbines operating at higher temperature. **Mullite/SiC** TBC FGC exhibited excellent adhesion and corrosion resistance as shown in the study by [62].

Graded zirconia/nickel **ZrO2/Ni** and **Al2O3/ZrO2 FGC** TBCs have also been considered for other rocket engines, such as in the small regeneratively cooled thrust chambers in orbital maneuvering systems [63, 64]. These chambers are prepared using a combination of galvanoforming and plasma spraying. No delamination of ZrO2 was observed after 550 seconds of combustion.

Nowadays it is necessary to reduce the weight of army systems in order to cope with the rapidly developing requirements of military contingencies. Ultralight weapons will be the cornerstone of future battlefield domination. Military strategists have asked for radical weight reductions in future military equipment, which will need new materials in new structures and designs. The concept of FGCs is one of the material technologies identified for this purpose [65].

Stealth missiles are now a required component of a modern weapons system. Parts made from specific materials can be used to absorb the electromagnetic energy emitted in order to minimize waves reflected in the direction of the enemy radar receiver. The most promising new materials for use in these applications are ceramic matrix composites reinforced with ceramic woven fabrics. The use of long, continuous ceramic fibers embedded in a refractory ceramic matrix creates a composite material with much greater toughness than basic (mono‐ lithic) ceramics, and which has an intrinsic inability to tolerate mechanical damage without brittle fracture. **Nicalon SiC fibers**, which have semiconducting properties, and **Nextel mullite (3Al2O3- 2SiO2- 0.1 B2O3) fibers**, which are completely dielectric, are used in the preparation of graded oxide matrix ceramic composites [66].

Some structural ceramics such as **B4C, SiC, Al2O3**, AlN, TiB2 and Syndie (synthetic diamond) FGCs [67–70] are viewed as potential materials for use in armor applications for both personnel and vehicle protection, owing to their low density, reliability, superior hardness, compressive strength and greater energy absorption capacity, which enable effective protection from projectiles.

Moreover, spark plasma sintered **Ti/TiB2, TiB2 /MoSi2** [71] and Ni/Al2O3 [4], FGCs are used as lightweight armor materials with high ballistic efficiency.

**Figure 5.** Radical weight reduction for future ground vehicles [65].

At present, the braking system is one of the most important part of the world's transportation systems. The traditional disc brake rotors in use today are manufactured from gray cast iron [72]. Up until very recently, the best candidate material for the future generation replacement of car brake rotors in terms of the relationship between high speed and lower coefficients of friction had not been identified.

The new advances in functionally graded ceramics allows them to be utilized in car braking systems as brake discs. It is anticipated that aluminum titanate **(Al2TiO5) FGCs** may replace

conventional gray cast iron as a result of its better thermal performance when used in car brake rotors. Moreover, due to its low density compared to gray cast iron, **Al2TiO5,** it is a fuel saving option for use in car brake rotors [73].

Nowadays, [74] it is known that functionally graded **Al2O3/ Al2TiO5** ceramics can be used successfully in car brake rotor systems due to the excellent properties and behaviors they exhibit.

#### **5.2. FG ceramics for energy applications**

Stealth missiles are now a required component of a modern weapons system. Parts made from specific materials can be used to absorb the electromagnetic energy emitted in order to minimize waves reflected in the direction of the enemy radar receiver. The most promising new materials for use in these applications are ceramic matrix composites reinforced with ceramic woven fabrics. The use of long, continuous ceramic fibers embedded in a refractory ceramic matrix creates a composite material with much greater toughness than basic (mono‐ lithic) ceramics, and which has an intrinsic inability to tolerate mechanical damage without brittle fracture. **Nicalon SiC fibers**, which have semiconducting properties, and **Nextel mullite (3Al2O3- 2SiO2- 0.1 B2O3) fibers**, which are completely dielectric, are used in the

Some structural ceramics such as **B4C, SiC, Al2O3**, AlN, TiB2 and Syndie (synthetic diamond) FGCs [67–70] are viewed as potential materials for use in armor applications for both personnel and vehicle protection, owing to their low density, reliability, superior hardness, compressive strength and greater energy absorption capacity, which enable effective protection from

Moreover, spark plasma sintered **Ti/TiB2, TiB2 /MoSi2** [71] and Ni/Al2O3 [4], FGCs are used as

At present, the braking system is one of the most important part of the world's transportation systems. The traditional disc brake rotors in use today are manufactured from gray cast iron [72]. Up until very recently, the best candidate material for the future generation replacement of car brake rotors in terms of the relationship between high speed and lower coefficients of

The new advances in functionally graded ceramics allows them to be utilized in car braking systems as brake discs. It is anticipated that aluminum titanate **(Al2TiO5) FGCs** may replace

preparation of graded oxide matrix ceramic composites [66].

14 Advances in Functionally Graded Materials and Structures

lightweight armor materials with high ballistic efficiency.

**Figure 5.** Radical weight reduction for future ground vehicles [65].

friction had not been identified.

projectiles.

The majority of today's power stations still burn conventional fuels. By optimizing combustion techniques and combining stationary gas turbines with steam turbines, efficiencies close to 60 % have been achieved. The incorporation of advanced material concepts such as FGCs could further improve the efficiency of these systems [75].

Turbine blades made from **titanium aluminide** with gradients in Cr content have been produced by hot isostatic pressing. Measurement of the mechanical properties of machined pieces cut from tested **Ti48Al2Cr2Nb/Ti46Al3Cr5Nb2Ta FGC** turbine blades were evaluated after heat treatment at 1350°C for 2 hours, and confirm the presence of the expected microstructural and mechanical gradients [76].

Porous **SiC** FG ceramics are proving to be the most promising materials for use as liquid fuel evaporator tubes in gas turbine combustors with premix burners which can significantly reduce the probability of failure **[77, 78]**. FGCs can also be used as components for integrated thermionic/thermoelectric systems. Figure 6 shows a schematic of a hybrid direct energy conversion system proposed in the second Japanese FGC program [79]. Thermionic conversion is based on the principle that electrons discharged from a hot emitter will move to a low temperature collector located on the opposite side [80]. By applying the FGC concept **(TiC/Mo – MoW – WRe) FGCs**, the performance of the thermionic converter can be optimized by decreasing the energy loss between the emitter and the converter (the barrier index) [79].

Thermoelectric materials with a FGM structure show a higher performance than basic materials. FGC joining is also a useful technique for use in setting an electrode in order to relax thermal stress and suppress inter diffusion. SiGe is one of the materials under consideration for use in thermoelectric conversion at high temperatures. Dense graded SiGe units with electrodes have been manufactured by a one-step sintering process using hot isostatic pressing (HIP) with glass encapsulation, as shown in Figure 7 [81]. Materials with low electrical resistivity, including tungsten, molybdenum disilicide, and titanium diboride **(W, MoSi2, and TiB2)** were selected for the electrodes. They were blended with silicon nitride **(Si3N4)** in order to reduce the thermal expansion mismatch of the joints between the electrodes and the thermoelectric conversion unit.

It has recently been found that the tellurium compound**s Bi2Te3** and **Sb2Te3** having ZT > 2 and PbTe based FGCs are well established thermoelectric materials suitable for use in the future [82].

**Figure 6.** A hybrid direct energy conversion system consisting of thermionic and thermoelectric converters.

**Figure 7.** A dense, graded n-type (SiGe) conversion unit produced by HIP [81].

FGCs are also promising candidates for use in the manufacture of technological components in solid oxide fuel cells (SOFC). [83] has successfully manufactured nano-structured and functionally graded **LSM–LSC–GDC FGC** cathodes to have about 240 μm thick YSZ electro‐ lyte supports using a combustion CVD method. Moreover, FGCs are used as components in the fusion and nuclear reactor field. Chemical vapor deposited FGC coatings of 1 mm thick **TiC/C** were evaluated at a surface heat flux of up to 70 MW/m2 for several seconds. The FGC film sustained temperature differences as high as 1500°C without cracking or melting [84].

#### **5.3. FG ceramics for electronic and optoelectronic applications**

**Figure 6.** A hybrid direct energy conversion system consisting of thermionic and thermoelectric converters.

16 Advances in Functionally Graded Materials and Structures

**Figure 7.** A dense, graded n-type (SiGe) conversion unit produced by HIP [81].

Ceramic/metal and ceramic/ceramic FGMs are showing great promise as both specialized electrical materials, and thermal barrier materials, due to their high temperature properties.

Functionally graded ceramics have become widely and commonly used in many advanced optical and electrical applications such as semi-conductor devices, anti-reflective layers, sensors, fibers, GRIN lenses and other energy applications [85]. In semi-conductors, concen‐ tration, carrier mobility, diffusion length, built-in electric field and other properties exert a strong influence on the parameters of electronic and optoelectronic devices. Functionally graded **AlN/GaN** ceramics can be used as a buffer layer for heteropitaxy that is able to distribute strain in the buffer layer and reduce cracking in the active layer [86].

In addition, in conventional edge lasers applied to fiber telecommunications, there are several factors that influence the quality of a device. Two most important are the low threshold current and the numerical aperture of the light beam. It is possible to decrease the numerical aperture, but also to increase the threshold current through increasing the thickness of the active region. One possible solution is the use of a graded-index separate-confinement heterostructure (GRINSCH). In such a structure, the FGC is used as a waveguide cladding layer, and as a barrier to carriers [87].

On the other hand, the substantial shortfall in the efficiency of silicon solar cells is due to the constant band gap width of the bulk material. In such cells, high radiation is absorbed in a shallow layer under the surface. As a result, it is important to initiate an electric field in close vicinity to the surface. A successful way to overcome this limitation is through the use of graded materials [88]. Functionally graded **AlxGa1-xN (n)/GaN (p)** ceramics can be used as high efficient photodetectors and in solar cells [89].

Piezoelectrics have been used extensively in the design of actuators and sensors in many fields due to their versatility and efficiency in the mutual transformation between mechanical and electrical energy. The piezoelectric actuator has many excellent properties, such as low energy consumption, a compact size, quick response and high resolution. Therefore, piezoelectric actuators and sensors are seen as promising candidates for use in microelectro-mechanical systems and smart material systems. Functionally graded piezoelectric ceramics are novel devices, which can successfully overcome the inherent structural defects in conventional piezoelectric bending-type actuators that result from the use of epoxy binder.

Functionally graded piezoelectric ceramics with a ceramic backing of **(1-x) Pb(Ni1/3Nb2/3)O/ xPb(Zr0.3Ti0.7)O3** are used as highly efficient ultrasonic transducers [90]. These ultrasonic transducers are widely used in ultrasonic measurement systems such as nondestructive testing and medical diagnosis.

Another advanced FGC is porous lead zirconate titanate **(PZT)**, which is manufactured by aqueous tape casting technology and is used in pyroelectric applications [91].

#### **5.4. FG ceramics in biomedical applications**

Over the past 30–40 years, there have been major advances in the development of medical materials and this has seen the innovation of ceramic materials for use in skeletal repair and reconstruction. Bioceramics are now used in a number of different applications throughout the body. However, the increase in biomedical applications of bioactive ceramics is occurring simultaneously with the growth of interest in tissue engineering.

The use of FGCs in biomaterial applications is growing in importance. Over 2500 surgical operations undertaken to incorporate graded hip prostheses have been successfully performed in Japan over the past twelve years. These graded hip implants enable a strong bond to develop between the titanium implant, bone cement, hydroxyapatite (HAp), and bone. The bone tissue penetrates HAP granules inserted between the implant and the bone forming a graded structure. Hence, FGCs have enabled the development of this promising approach to bone tissue repair [92].

Biomaterials must simultaneously satisfy various requirements and possess certain properties such as being non-toxic, having good mechanical strength, and they need to be biocompatible [93, 94]. Natural tissues often possess FGMs which enable them to satisfy multiple require‐ ments [95]. Human tissues have evolved to be best adapted to their multiple functional requirements. For instance, the perfect design of natural bone with a dense, stiff external structure (cortical bone) and a porous internal structure (cancellous bone) demonstrates that functional gradation has been utilized for biological adaptation [96].

A functionally graded carbon fiber (CF) reinforced poly-lacticacid (PLA)/nanometer hydrox‐ yapatite (HA) biomaterial has been prepared by [97]. CF was used as the reinforcement to improve mechanical properties, while at the same time the advantages of PLA and nano-HA were retained. [31] developed a dental implant with functionally graded titanium (Ti) and HA. [98, 99] developed a functional gradient HA composite containing glass-coated Ti and studied its microstructures, mechanical and thermal properties. [100] proposed a **HA–glass–titanium (HA–G–Ti)** composite and implanted it in the femur of a dog to evaluate its bonding strength. However, metal and polymer-based implants usually lead to stress shielding, wear debris, delayed osseointegration, resorption, degradability or other biological complications. There‐ fore, new bone tissue implants should aim to avoid these disadvantages and instead meet the multiple functional requirements of bone tissue [101, 102].

It was found that calcium phosphate ceramics, especially the bioactive nano-structured hydroxyapatite, have received considerable attention in recent years [103–105]. In vitro and in vivo experiments have demonstrated that the nano-HA has an excellent biological perform‐ ance when compared with conventional micro-grain HA [106, 107]. Nano-HA possesses exceptional biocompatibility and bioactivity with respect to bone cells and tissues. Hence, [108] prepared a successful nine layers of laminated and functionally graded HA/ yttria stabilized zirconia (Y-TZP) for orthopedic applications, using an SPS technique.

In addition, [92] presented a novel FGC with both micro-grain and nano-grain HA crystals that is able to satisfy the mechanical and biological property requirements of bone implants. It was concluded that a biologically functionalized nano-rough surface contributed better bioactive functionality to the HA ceramics. By applying the concept of FGM, bio-inspired multifunctional biomaterials open the door to a promising approach to bone tissue repair.

Other functionally graded ceramics that are used in biomedical applications are **ZrO2/ AISI316L** as artificial joints and hip prostheses, and **ZrO2/Al2O3** FGCs as teeth implants [109]. Nowadays, structure grading technology is also used in cancer prevention research. One of them, for instance, is a study on collagen structure reinforcement using grading technology. In such a type of graded structure, the graded material should not only possess excellent hardness, wear and corrosion resistance, but should also have high biological compatibility and harmlessness.

### **5.5. FG ceramics in structural and tribological applications**

transducers are widely used in ultrasonic measurement systems such as nondestructive testing

Another advanced FGC is porous lead zirconate titanate **(PZT)**, which is manufactured by

Over the past 30–40 years, there have been major advances in the development of medical materials and this has seen the innovation of ceramic materials for use in skeletal repair and reconstruction. Bioceramics are now used in a number of different applications throughout the body. However, the increase in biomedical applications of bioactive ceramics is occurring

The use of FGCs in biomaterial applications is growing in importance. Over 2500 surgical operations undertaken to incorporate graded hip prostheses have been successfully performed in Japan over the past twelve years. These graded hip implants enable a strong bond to develop between the titanium implant, bone cement, hydroxyapatite (HAp), and bone. The bone tissue penetrates HAP granules inserted between the implant and the bone forming a graded structure. Hence, FGCs have enabled the development of this promising approach to bone

Biomaterials must simultaneously satisfy various requirements and possess certain properties such as being non-toxic, having good mechanical strength, and they need to be biocompatible [93, 94]. Natural tissues often possess FGMs which enable them to satisfy multiple require‐ ments [95]. Human tissues have evolved to be best adapted to their multiple functional requirements. For instance, the perfect design of natural bone with a dense, stiff external structure (cortical bone) and a porous internal structure (cancellous bone) demonstrates that

A functionally graded carbon fiber (CF) reinforced poly-lacticacid (PLA)/nanometer hydrox‐ yapatite (HA) biomaterial has been prepared by [97]. CF was used as the reinforcement to improve mechanical properties, while at the same time the advantages of PLA and nano-HA were retained. [31] developed a dental implant with functionally graded titanium (Ti) and HA. [98, 99] developed a functional gradient HA composite containing glass-coated Ti and studied its microstructures, mechanical and thermal properties. [100] proposed a **HA–glass–titanium (HA–G–Ti)** composite and implanted it in the femur of a dog to evaluate its bonding strength. However, metal and polymer-based implants usually lead to stress shielding, wear debris, delayed osseointegration, resorption, degradability or other biological complications. There‐ fore, new bone tissue implants should aim to avoid these disadvantages and instead meet the

It was found that calcium phosphate ceramics, especially the bioactive nano-structured hydroxyapatite, have received considerable attention in recent years [103–105]. In vitro and in vivo experiments have demonstrated that the nano-HA has an excellent biological perform‐ ance when compared with conventional micro-grain HA [106, 107]. Nano-HA possesses exceptional biocompatibility and bioactivity with respect to bone cells and tissues. Hence,

aqueous tape casting technology and is used in pyroelectric applications [91].

simultaneously with the growth of interest in tissue engineering.

functional gradation has been utilized for biological adaptation [96].

multiple functional requirements of bone tissue [101, 102].

and medical diagnosis.

tissue repair [92].

**5.4. FG ceramics in biomedical applications**

18 Advances in Functionally Graded Materials and Structures

FGCs offer great promise for use in applications where the operating conditions are severe, for example, in cutting tools and wear resistant linings for handling large heavy abrasive ore particles. These applications require graded ceramics with high corrosion and wear resistance. This type of FGC can also be used as protective coatings in the form of an **alumina/mullite FGC** that is used to protect SiC components from corrosion, and act as a thermal barrier coating, improving the efficiency of turbine engines by providing the capability to sustain a significant temperature gradient across the coating **ZrO2/Al2O3** FGC, which also improves thermal resistance and resistance to oxidation [110].

Moreover, a novel functionally graded **Al2O3/lanthanum hexaaluminate (LHA)** ceramic with a gradient in composition and porosity was developed using the PM method as a high temperature thermal barrier coating, protecting the components from a corrosive and severe thermal environment [111]. Graded **WC/Co FGCs** are used as abrasive cutting tools and in mining equipment, where a high wear resistance and toughness are both required [112].

In addition, the WC/Co FGC is coated with a layer of titanium nitride **(TiN)**, a layer of alumina **(AI2O3)**, and a layer of titanium carbonitride **(TiCN)** by chemical vapor deposition. These graded and multiple coated WC/Co FGC cutting tool chips are very resistant to flank wear. Furthermore, they have the advantage of a high machining speed combined with a high feed rate. Their graded composition can also control the internal stresses arising from the mismatch in thermal expansion. A simple, asymmetric gradient in composition such as in a ceramic/metal FGM can reduce thermal stress, while a symmetrical or radial gradient can induce a sizable compressive stress at the outer ceramic layer, resulting in stress reinforcement similar to that of tempered glass or pre-stressed concrete [113]. Graded cutting tools have also been made for interrupted cutting from cermets of **TiC-NiMo FGC** in which the percentage of TiC in the graded layer ranged from 95 wt. % at the top surface to 86 wt % at the site of transition to plain steel [114]. Recently, **Al2O3/TiC** and **Al2O3/(W-Ti) C** FG ceramics have been investigated as highly efficient ceramic tools with excellent thermal shock resistance [115].

FGCs are also used as engineering components, machine parts and in joints for gas and steam turbines as well as in coatings and wear resistant materials [116]. For example, **SiC/C** FGC acts as a structural part of the heat collector for an energy conversion system, and also provides thermal stress relaxation, heat conduction, and protection from oxidation.

Another FGC application that involves thermal stress relaxation and a low coefficient of friction, is in welding apparatus. For example, **Si3N4-Cu FGC** is used in automated electric arc welding of the large aluminum sheets used in building huge ships such as liquid natural gas (LNG) tankers [117]. Other suggested applications included use as filters, catalysts, mufflers, heat exchangers, self-lubricating bearings, silencers, vibration dampers, and shock absorbers [118].

**Silicon nitride Si3N4,** and silicon aluminum oxynitride **SiAlON** are a special class of high temperature ceramic and refractory materials. Moreover, they represent a vital and unique class of structural ceramics. They can be used in many industrial and structural applications that require chemical stability, high heat resistance and specific mechanical properties [119].

Previously, [120] developed graded in situ SiAlON ceramics by embedding **β-SiAlON** green compacts in **α-SiAlON** powder. The compositions, microstructures and properties of the graded SiAlON ceramic change gradually from the hard α-SiAlON with spherical morphology on the surface, to the tough and strong β-SiAlON with elongated grains in the core. [121] developed a technique for the in situ formation of an α-SiAlON layer on a β-SiAlON surface. In another study, [122] obtained a gradual change of α-SiAlON content from the surface through to the core using the rapid cooling method. Recently, [123] have manufactured a twin layer FGC of **α-SiAlON (100 wt%)/AlN-BN (50:50 wt%)** for advanced structural applications.

#### **5.6. Other applications of functionally graded ceramics**

In addition to the above mentioned applications, FGCs can be used in the lining of thermal furnaces and other ultra-high temperature applications:

