EFFECT OF DISPERSED PARTICLE-SIZE ON MECH ANICAL-PROPERTIES OF ALUMINA NON-OXIDES COMPOSITES

Alumina with SiC or TiC particles was hot-pressed at 1500-degrees-C fo r 2h in N2 under a pressure of 40 MPa, and the effect of the size and volume fraction of dispersed particles on mechanical properties of the composites was investigated. Compared with monolithic alumina, the fl exural strength of both composites increased when the dispersed partic le size was small. The fracture toughness of Al2O3/SiC composites incr eased with a decrease in particle size. However, a maximum fracture to ughness was observed for Al2O3/TiC composites when the particle size w as 4-6 mum. These results are explained as follows. The increase in fr acture toughness is caused by microcracks originated by thermal expans ion mismatch between the matrix and the dispersed particle. In the cas e where the thermal expansion mismatch and the size of dispersion part icles are both large, cracks develop spontaneously during cooling, rel easing thermal stress, and thus hindering the operation of the microcr acking mechanism that would increase the fracture toughness of the com posites. It is concluded that each system has a suitable size of dispe rsed particles. On the other hand, the increase in strength is caused by the increase in fracture toughness and by the decrease in the lengt h of pre-existing flaws due to suppression of abnormal grain growth of the matrix.