SiCN nanocomposite: creep behaviour

The compressive creep behaviour of a Si3N4/SiC nanocomposite densified with yttria and alumina is studied in the 1150–1350°C temperature range under stresses from 45 to 180 MPa. The stress exponent equals 1 and the apparent activation enthalpy is 580 kJ mol−1. Grain boundary sliding accommodated by diffusion through the intergranular glass phase is considered as the primary steady-state creep mechanism. It is argued that the diffusion step through the glass phase is the rate limiting step in the dissolution–diffusion–reprecipitation process. The results are compared with those obtained in previous studies for an α-Si3N4 rich monolith and a Si3N4/SiC micro-nanocomposite. The high creep deformation is explained by the nanometric size of the Si3N4 grains and the absence of grain growth in the temperature range explored. It is showed that a post-sintering heat-treatment consisting in 5 h at 1750°C followed by 3 h at 1400°C enhances the creep resistance to a fair level. It is concluded that the high ductility of the as-sintered material is suitable for hot-forging whereas the subsequent heat-treatment is able to provide shaped parts with a high creep resistance for structural applications at high temperature.