Mechanisms of Ceramic Whisker Reinforcement of Dental Composite Resins and Effect of Filler Level
Hockin D. Xu
Currently available composite resins are susceptible to fracture and hence are not recommended for large stress-bearing posterior applications involving cusps. This study investigated the strengthening of composite resins by using ceramic whisker reinforcement and the effect of varying whisker filler level. Silica particles were fused onto single-crystalline silicon nitride whiskers to roughen the whisker surfaces, thereby improving retention in the resin matrix. Flexural, compressive, hardness, and degree of conversion tests were conducted vs. whisker filler mass fraction, which ranged from 0% to 70%. Selected composites were clinically polished and the surface roughness was measured with profilometry. The whisker composite with a filler mass fraction of 55% had a flexural strength (mean + or - sd; n=6) of (196 + or - 10) MPa, significantly higher (family confidence coefficient=0.95; Tuckey's multiple comparison test) than (83 + or - 14) MPa of a conventional microfill and (120 + or - 16) MPa of a hybrid posterior composite. The composite modulus and hardness increased monotonically with the whisker filler level. The flexural and compressive strength first increased, then pateaued with further increasing the filler level. The degree of conversion decreased with increasing the filler level.The whisker reinforcement mechanisms were suggested by SEM observations to be the pinning of microcracks, and friction from whiskers being pulled out of the resin matrix during crack opening. The whisker composite had a polished surface roughness similar (p>0.1; Student t) to that of a conventional hybrid composite. To conclude, ceramic whisker reinforcement significantly improved the mechanical properties of composite resins; the whisker filler level played a key role in determining composite properties, and the reinforcement mechanisms appeared to be crack-pinning by whiskers and friction from whisker pullout.