Process development for ZrO2-ZrSiO4/NiCr functionally graded materials

Summary of the work Metal-ceramic functionally graded materials (FGMs) are promising candidates for future high-temperature applications, with the ceramic component offering thermal barrier effects and protecting the metal from melting and oxidation, while the FGMs are strengthened by the metallic component. In previous works, it has been proved that ZrO2-ZrSiO4/NiCr FGMs of thermal barrier type possess significantly higher oxidation resistance as compared to conventional ZrO2/NiCr FGMs. Moreover, it was also found that thermal mismatch stresses in pressurelessly sintered ZrO2-ZrSiO4/NiCr FGMs vanish at high temperatures. In the present study, further efforts were given with the aim to develop pressureless sintered ZrO2-ZrSiO4/NiCr FGMs based on two different microstructural concepts. On the one hand, five-layered ZrO2-ZrSiO4/NiCr FGMs with a stepwise gradient were prepared via slip casting by pressureless sintering with a shrinkage matching process. Shrinkage behavior of the constituent layers of FGM was matched based on the strategy of reducing the mismatches of shrinkage rate between the adjacent layers by a systematic adjustment of the ZrSiO4 inclusion content and the particle size of ZrO2 matrix. By ultilizing this strategy, ZrO2-ZrSiO4/NiCr FGMs without any sintering defects were obtained. The prapared FGMs were further investigated with respect to their microstructures, mechanical, thermal properties and high temperature oxidation behavior. Bending strength and Young’s modulus of the individual layers of FGM depend not only upon relative density, but also upon phase connectivity of metal and the amount of ZrO2/NiCr interface. Coefficients of thermal expansion of the individual layers are lower than calculated theoretical values, due mainly to the relatively low phase connectivity of metal in the metal-ceramic composite layers. The prepared FGMs exhibit superior thermal barrier effects due to the existence of a huge amount of interfacial microstructural defects as thermal resistance in the ceramic-rich side of FGM. In addition, oxidation kinetics of the individual layers decreases with increasing volume fraction of metal, due mainly to the increasing phase connectivity of metal and corresponding increasing fracture toughness of the layers, which also governed the crack pattern caused by the volume expansion of NiCr particles during oxidation. On the other hand, attempts were made to integrate an isotropic NiCr foam with hollow struts as an interpenetrating metallic preform into the five-layered ZrO2-ZrSiO4/NiCr FGM. By integrating the NiCr foams, several property improvements for the ZrO2-ZrSiO4/NiCr FGMs were expected: (1) Improvement of fracture toughness of FGM, especially for the relatively brittle ceramic-rich side of FGM; (2) Reductions in mismatch of thermal expansion between the ceramic- and the metal-rich sides of FGM and resulting thermal mismatch stresses generated during thermal cycling; (3) Improvement of strain tolerance and thermal shock resistance of FGM by introducing the hollow space inside foam struts. From the processed FGMs with an integrated NiCr foam, however, partially debonded metal-ceramic interfaces were observed, due mainly to the low bonding strength between ZrO2 and NiCr phases.