**6. Future direction**

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

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

**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.

In addition to the above mentioned applications, FGCs can be used in the lining of thermal

**•** Novel **zirconia-mullite/alumina** FGC tailored by the reaction sintering method and used in refractory materials that line furnaces, and high temperature applications [6, 7].

**• ZrB2/ZrO2** FGC prepared using spark plasma sintering for ultra-high temperature applica‐

**• ZrO2/Fe** FGC with excellent thermal and mechanical properties, used for high temperature

**•** A crack-free **Si3N4/Al2O3** FGC suitable for high temperature structural applications [26].

**•** Multi-layered Zircon/yttria **(ZrO2.SiO2/Y2O3)** FGC with high thermal shock resistance, used as crucibles for the induction melting of TiAl based alloys with zero contamination [126].

thermal stress relaxation, heat conduction, and protection from oxidation.

20 Advances in Functionally Graded Materials and Structures

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

furnaces and other ultra-high temperature applications:

tions and in severe environments [124].

engineering applications [125].

[118].

Functionally graded ceramics are excellent advanced materials with unique properties and characteristics that have entered into the manufacturing world in the 21st century. The major success of FGCs is due to the fact that the irreconcilable properties on each side of a FGC can be fully utilized. FGCs can be tailored according to the application requirements by controlling the appropriate components in order to achieve some specific tailored applications and to overcome the problems of laminated composites. However, there are some obstacles to the realization of this success. The high costs that are entailed during the manufacturing process and powder processing are considered to be a crucial issue. The technology of powder metallurgy can offer a vital solution to this problem, however, there are a lot of issues relevant to this technology that need to be considered. In addition, an extra effort in different axes should be exerted in order to generate a predictive model for proper process control. This will improve the execution of the process and so reduce the cost of FGC production. Another issue that needs to be taken into consideration is that of determining the residual stresses resulting from the inhomogeneous cooling of the graded layers of the FGC body. The values of these residual stresses are an important indication to both the success of FGC preparation and to their subsequent properties. Because one of the main purposes when designing FGCs is to decrease or prevent the residual stress formed at the interface of the two dissimilar materials, and thereby prevent crack propagation and ultimately the delamination of these materials by having smooth transitions between layers.
