Effect of functionally graded material (FGM) layers on the residual stress of polytypoidally joined Si3N4–Al2O3

A unique approach introducing sialon polytypoids as a functionally graded material (FGM) bonding has been used to join silicon nitride and alumina. The various multilayered FGM samples ranging from 3 to 20 layers were sintered to fabricate a crack-free joining of heterogeneous ceramics. To calculate thermal stresses for the various multilayered FGM samples, the finite element analysis program (FEAP) was used. These analyses results matched experimental results and showed why some samples had large residual stresses that resulted in fracture. Moreover, the electron probe X-ray microanalysis (EPMA) from a crack-free FGM sample had a smooth concentration profile, which verifies the interface diffusion during sintering at each graded layer and confirms a successful joining.