Effects of different whiskers on the reinforcement of dental resin composites

Objective. Whiskers were recently used to reinforce dental composites to extend their use to large stress-bearing restorations. The aim of this study was to investigate the effects of different types of whiskers on composite properties. Methods. Silicon nitride and silicon carbide whiskers were each mixed with silica particles at whisker/silica mass ratios of 0:1, 1:5, 1:2, 1:1, 2:1, 5:1, and 1:0, and thermally treated. The composite was heat-cured at 140 °C. Strength and fracture toughness were measured in flexure, while elastic modulus and hardness were measured with nano-indentation. Results. Both whisker type and whisker/silica ratio had significant effects on composite properties (two-way ANOVA; p<0.001). Silicon nitride whiskers increased the composite strength and toughness more than did silicon carbide. Silicon carbide whiskers increased the modulus and hardness more than silicon nitride did. The silicon nitride whisker composite reached a strength (mean±SD; n=6) of 246±33 MPa at whisker/silica of 1:1, while the silicon carbide whisker composite reached 210±14 MPa at 5:1. Both were significantly higher than 114±18 MPa of a prosthetic control and 109±23 MPa of an inlay/onlay control (Tukeyʹs multiple comparison test; family confidence COEFFICIENT=0.95). Fracture toughness and work-of-fracture were also increased by a factor of two. Higher whisker/silica ratio reduced the composite brittleness to 1/3 that of the inlay/onlay control. Significance. Whisker type and whisker/silica ratio are key microstructural parameters that determine the composite properties. Reinforcement with silica-fused whiskers results in novel dental composites that possess substantially higher strength and fracture toughness, and lower brittleness than the non-whisker control composites.