The special characteristics of functionally graded materials (FGMs) is the continuous variation of material properties with respect to spatial coordinates. Bamboo, bone and human skin are examples of natural FGMs. In industry, FGMs can be manufactured as composite of at least two constituents, e.g. ceramics and metal. By special manufacturing process, the volume ratios of ceramics and metal vary continuously from 100% ceramics to 100% metal along one or two directions, enabling the continuous variation of the composite properties, see Figure 1 for illustration.
FGMs were originally developed to be used as thermal barrier in aerospace engineering. Nowadays, applications of FGMs can be found also in nuclear energy, electromagnetism, optics and bio-compatible materials.