SiCN nanocomposite: creep behaviour

The compressive creep behaviour of a Si3N4/SiC nanocomposite densified with yttria and alumina is studied in the 1115-1350 degreesC temperature range under stresses from 45 to 180 MPa. The stress exponent equals 1 and the app arent activation enthalpy is 580 kJ mol(-1). Grain boundary sliding accommo dated by diffusion through the intergranular glass phase is considered as t he primary steady-state creep mechanism. It is argued that the diffusion st ep through the glass phase is the rats limiting step in the dissolution-dif fusion-reprecipitation process. The results are compared with those obtaine d in previous studies for an alpha -Si3N4 rich monolith and a Si3N4/SiC mic ro-nanocomposite. The high creep deformation is explained bq the nanometric size of the Si3N4 grains and the absence of grain growth in the temperatur e range explored. It is showed that a post-sintering hear-treatment consist ing in 5 h at 1750 degreesC followed by 3 h at 1400 degreesC enhances the c reep resistance to a fair level. It is concluded that the high ductility of the as-sintered material is suitable for hot-forging whereas the subsequen t heat-treatment is able to provide shaped parts with a high creep resistan ce for structural applications at high temperature. (C) 2001 Elsevier Scien ce Ltd. All rights reserved.