**2. Origin of FG ceramics concept**

The FGCs concept originated in Japan in 1984 during the space plane project of Niino and coworkers [1] in the form of a proposed thermal barrier material capable of withstanding a surface temperature of 2000K and a temperature gradient of 1000K in a cross-section of <10 mm. It is difficult to find a single material able to withstand such severe conditions. The researchers used the FGM concept to manufacture the body of a space plane using material with high refractoriness and mechanical properties resulting from gradually changing compositions. They designed a ceramic material for the outer surface that is exposed to a high temperature environment and a thermally conductive metal for the inner surface. In 1987, the successful FGC research was accepted for use in a major project by the Ministry of Education and Science. During the period 1987–1991, a research project entitled "*Research on the generic technology of FGM development for thermal stress relaxation*" was conducted by Japanese scientists. In 1992, FGMs were selected as one of 10 most advanced technologies in Japan. Since then, FGM technology has grown in importance and has garnered the attention of many authors throughout the world. Although FGMs were invented fairly recently, these materials are not actually new. Gradual variations in the microstructure of materials have been explored for millions of years by the living organisms. FGMs have been long established in nature (biotissues of plants, bamboos, shells, coconut leaves and animals) and are even found in our bodies — such as in bones and teeth. [2].
