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 homogeneity, density and fracture toughness of silicon carbide whisker reinforced silicon nitride (SiCw/Si3N4). Further improvements in the mechanical properties are realized by combining ball milling with ultrasonic dispersion of the composite suspension. Ball milling reduces the SiCw aspect ratio from 25 to 15 which in turn increases the dispersion of the whiskers within the suspension, resulting in increases in the green and sintered density, along with the fracture toughness. In a binderless process, 20 volume% reduced aspect ratio (r = 15) SiCw/Si3N4 can be densified to 95% theoretical density by pressureless sintering using 8% Y2O3 and 2% Al2O3 by weight as sintering 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.