Effect of whisker aspect ratio on the density and fracture toughness of SiC whisker-reinforced Si3N4

Controlling the suspension properties prior to slip casting optimizes the h omogeneity, density and fracture toughness of silicon carbide whisker reinf orced silicon nitride (SiCw/Si3N4). Further improvements in the mechanical properties are realized by combining ball milling with ultrasonic dispersio n of the composite suspension. Ball milling reduces the SiCw aspect ratio f rom 25 to 15 which in turn increases the dispersion of the whiskers within the suspension, resulting in increases in the green and sintered density, a long with the fracture toughness. In a binderless process, 20 volume% reduc ed aspect ratio (r = 15) SiCw/Si3N4 can be densified to 95% theoretical den sity by pressureless sintering using 8% Y2O3 and 2% Al2O3 by weight as sint ering aids. These composites had measured values of fracture toughness from 9-10.5 MPa . m(1/2), representing an average increase of approximately 30% over the fracture toughness for monolithic Si3N4 processed under identical conditions. (C) 2001 Kluwer Academic Publishers.