SI3N4 AND SI3N4-SIC PARTICULATE COMPOSITE CERAMICS

Studies of pressureless sintering of Si3N4 based matrices and Si3N4-Si C composites in the Y-Si-Al-O-N and Hf-Y-Si-Al-O-N additive systems ha ve been carried out. Mechanical testing indicated that matrices had hi gher hardness and higher flexural strength than composites. Composites had higher fracture toughness than matrices. The main toughening mech anisms are thought to be crack deflection and microcracking. Effective improvements in fracture toughness can be achieved by coarser SiC par ticles due to the fact that coarser dispersions enhance the amplitude of crack deflection and produce a greater number of microcracks.Using HfO2 as a sintering aid had little effect on densification but was fou nd to give improved flexural strength especially for composites. it is proposed that this may be due to the formation of HfO2-Y2O3 solid sol ution which had a beneficial influence on microstructural development in the composites.