IN ZIRCONIA CERAMIC COMPOSITES

Ceramic matrix composites consisting of partially stabilized zirconia with 5 % and 10 % by volume unstabilized zirconia particles (PSZ-ZrO 2) were tested for steady-state deformation behavior in tension and compression. While tensile experiments were inconclusive due to matrix cracking, compressive samples thermally cycled under stress about the allotropic temperature of the particulate phase (9500 C to 1250'C) deformed at faster strain rates than control samples. Up to 8 % strain has been created without macroscopic damage at strain rates of 0.85.10-7 to 5.310-7 S-1. The enhanced deformation rate is attributed to transformation-mismatch plasticity, caused by the biasing of cyclical internal stresses created by the 6 % volume change during the zirconia allotropic transformation. Dilatometry of a PSZ-5 % ZrO 2 composite shows a ± 37 MPa change in the internal mean level of stress, compared with the nominal value of ± 108 MPa calculated by the Eshelby technique. Microstructural observations of particle cracking can account for this discrepancy. These results are compared with three existing models for internal-stress plasticity and a numerical technique is developed to consider the relaxation o