R. Vassen et al., TOUGHENING OF SIC CERAMICS BY A BIMODAL GRAIN-SIZE DISTRIBUTION PRODUCED BY HIPING ULTRAFINE AND COARSE-GRAINED SIC POWDERS, Nanostructured materials, 6(5-8), 1995, pp. 889-892
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).