Microstructure and mechanical properties of a SiC containing advanced structural ceramics

Abstract

The fracture toughness of SiC-AlN structural ceramics can be improved by obtaining fine grain size distribution of both SiC and AlN phases and a weak SiC-AlN interfacial solid solution layer which helps in crack deflection. In order to achieve this, a SiC containing ceramics i.e. 55wt.%SiC+45wt.%AlN is prepared by spark plasma sintering (1850 °C/50 MPa/5 min.). High density (96.6%) with a very fine grain microstructure comprising of intimate mixture of both the SiC and AlN phases are obtained in the product. Vol% & wt% of SiC & AlN phases estimated through the EDS mapping and Rietveld analysis are found to be 54 vol% & 46 vol%, and 54.2 wt% & 45.8 wt% respectively. Formation of solid solution layer between hexagonal AlN (a = 3.1114 Å, c = 4.9792 Å) and hexagonal SiC (a = 3.081 Å, c = 5.031 Å) phase is confirmed indirectly by change in AlN lattice parameter and directly by EDS mapping. Electron probe micro analysis (EPMA) study reveals that, the solid solution between AlN and SiC phase is formed by mainly diffusion of SiC in AlN phase. Young modulus and hardness of the SPS product are similar to those reported in the literature. The product shows higher indentation fracture toughness (6.7 ± 0.9 MPa.m0.5 and 4.8 ± 0.6 MPa.m0.5 obtained using Median and Palmqvist crack model respectively) than the fracture toughness of a well known ultra-high temperature structural material i.e. ZrB2–20SiC (3.9 ± 1.1 MPa.m1/2). The indentation cracks are found to initiate at the AlN-SiC interface (Hv1263–1641) and are found to propagate along either the AlN phase (Hv1434–1720) or AlN-SiC interface. The cracks are found to stop when these encounter either SiC phase (Hv1896–2476) or porosity. The higher value of fracture toughness may be attributed to the mismatch in coefficient of thermal expansion (CTE) of SiC and AlN phase, fine grain size distribution, secondary crack generation and a weak AlN-SiC interfacial solid solution layer which helps in crack deflection. This product may find potential application as high temperature structural material since SiC containing materials exhibits outstanding high temperature oxidation and corrosion resistance.