Anisotropic properties in hot pressed silicon nitride-silicon carbide platelet reinforced composites

The anisotropic properties (microstructure, mechanical properties) of a hot -pressed platelet reinforced silicon nitride composite were compared with t hose of the monolithic material. The platelets appeared to be orientated wi th their basal plane in the compressive plane, and to be embedded in a sili con nitride matrix consisting of interlocked elongated beta-Si3N4 grains wi th their c axis orientated in this plane. TEM analysis showed an interface, consisting of glassy phase and graphite at the platelet-matrix grain bound ary. Moreover the interfacial tensile stresses are in favour of a crack def lection mechanism. It was shown by TEM analysis that crack deflection occur s not only at the silicon nitride-platelet interface, but also at silicon n itride-silicon nitride grain boundaries. The efficiency of this reinforcing mechanism is highly orientation dependent. Because of their two dimensiona l geometry compared to the one-dimensional beta-Si3N4 grains, platelets inc rease the toughness in two dimensions. (C) 1999 Elsevier Science Ltd. All r ights reserved.