Mechanical properties of Si₃N₄-Al₂O₃ FGM joints with 15 layers for high-temperature applications

"Crack-free" alumina-silicon nitride joints, comprised of 15 layers of gradually differing compositions of Al₂O₃/Si₃N₄, have been fabricated using sialon polytypoids as functionally graded materials (FGM) bonding layers for high-temperature applications. Using flexural strength tests conducted both at room and at elevated temperatures, the average fracture strength at room temperature was found to be 437 MPa; significantly, this value was unchanged at temperatures up to 1000 °C. Scanning electron microscopy (SEM) observations of fracture surfaces indicated the absence of any glassy phase at the triple points. This result was quite contrary to the previously reported 20-layer Al₂O₃/Si₃N₄ FGM samples where three-point bend testing revealed a severe strength degradation at high temperatures. Consequently, we believe that the joining of alumina to silicon nitride using polytypoidally functional gradients can markedly improve the suitability of these joints for high-temperature applications.