TOUGHENING OF SIC CERAMICS BY A BIMODAL GRAIN-SIZE DISTRIBUTION PRODUCED BY HIPING ULTRAFINE AND COARSE-GRAINED SIC POWDERS

Toughening of ceramic materials by using a bimodal grain size distribu tion has already been mentioned early in literature. This kind of toug hening can be explained by an interlocking of the large grains during crack propagation. This so called bridging can operate very effectivel y for large grain size ratios and large aspect ratios of the coarse gr ained phase. In this approach ultrafine particles have been used to pr oduce the bimodal grain size distribution. The main advantages over co nventional powders is that large coarse grained powders can be densifi ed. Then the established microstructure reveals a large grain size rat io. Furthermore the defect size which is correlated to the size of the largest grains is still quite low and hence high strength values can be expected. As starting powders mixtures of ultrafine SiC powder (par ticle size below 20 nm) and 10 or 20% coarse grained alpha-SiC powder with a particle size of 10 mu m were used. These mixtures and pure ult rafine powders were Hot Isostatically Pressed (HIPed) at low temperatu res (below 1700 degrees C) and high pressures (350 MPa). At 1500 degre es C densities above 90%TD and at 1600 degrees C above 95%TD could be achieved. A microstructural evaluation shows that the ultrafine powder particles exhibit large grain growth but still remain in the submicro n range. The large particles show only a negligible coarsening. The re sultant large grain size ratio larger than 10 leads to a significant i ncrease in fracture toughness. While the fine grained SiC without addi tion of coarse grained SiC shows fracture toughness values of about 4. 5 MPam(1/2), the addition of 10% coarse grained powders leads to incre ase of fracture toughness of about 10%. SiC samples with 20% of coarse grained powders even reach K-1c values of 6 MPa(1/2).