Optimum strength of composites of ceramic particles in poly(methyl methacrylate)

Ceramic particles were sedimented in methyl methacrylate, which was then polymerized by exposre to γ‐rays. After annealing, specimens were cut and tested, and the ceramic content determined by ashing. Diamond indentation hardness was found to increase monotonically with increase in ceramic content. In contrast, both Young's modulus and, more markedly, flexural strength were found to give optimum (peak) values for a content of lithium aluminum silicate near 80 wt‐percent. The higher silicate contents were reached with use of a silane coupling agent and by centrifuging, prior to polymerization. Examination of fracture surfaces by scanning electron microscopy indicated that the decrease in mechanical properties was due to the presence of voids in the polymeric matrix. It is suggested that void formation becomes serious when ceramic particles are initially too close‐packed to respond to conraction, during polymerization, by shifting closer together. Consistent with this suggestion, both void formation and peaking in strength can be prevented by continuously centrifuging the composite throughout the polymerization reaction.