9 - Metal-ceramic functionally graded materials (FGMs)

Abstract

A functionally graded material (FGM) is a composite material whose properties change over a gradient across one or more axes of the composite material. This is most commonly a compositional gradient, but it can also be a microstructural gradient, such as a gradation in porosity or fiber reinforcement. It can also be graded atom order, or other property gradient. The most common type of FGM is the compositionally graded FGM, and of these, the most relevant to the present book is the metal–ceramic continuous bulk FGM. The concept of the FGM as an engineering material was first proposed in 1972, but it did not rise to prominence until it was proposed in 1984 at the National Aerospace Laboratory in Japan in response to a demand for a new material for the hypersonic space plane. The engineering objective was to develop a thermal barrier spaceplane skin capable of withstanding a surface temperature of 2000K and a temperature gradient of 1000K across a cross-sectional thickness of less than 10mm. The material was also required to have corrosion and high temperature resistance on the outer face. The only material capable of this would be a metal–ceramic continuous bulk FGM. Metal–ceramic FGM thin-film coatings and thin-film interface layers are now a well-established technology. Metal–ceramic continuous bulk FGMs remain an experimental technology. They show significant potential for extreme applications for which few alternatives exist, such as spaceplane heat shields, plasma facings for nuclear reactors, ballistic armour, and load-bearing implantable medical devices. While many methods have been published for manufacturing continuous bulk metal–ceramic FGMs (mm to cm thick), few are capable of producing broad regular continuous gradients. A case study in this chapter demonstrated that thixotropic casting successfully produced regular gradients in hydroxyapatite-316L stainless steel biomaterial FGMs. An impeller dry blending (IDB) case study in this chapter showed the potential of IDB for producing excellent linear gradients in metal–ceramic FGMs. A hydrostatic shock forming case study showed its potential as a densification method for continuous bulk FGMs for which the metal and ceramic components have greatly differing melting points. Metal infiltration in combination with IDB forming of pore-graded ceramics was shown to be the most viable densification for FGMs for which the metal and ceramic components have greatly differing melting points. This chapter overviews the FGM concept, the manufacturing methods, and has three research case studies: continuous bulk FGM forming by thixotropic casing, continuous bulk FGM forming by impeller dry blending, and densification by hydrostatic shock forming.