Determination of elastic moduli of polymeric materials using microhardness indentation

Abstract

Young's modulus of elasticity (or stiffness, E) is an important material property for many applications of polymers and polymer-matrix composites. The common methods of measuring E are by measuring the velocity of ultrasonic pulses through the material or by resistance to flexure, but it is difficult for ultrasound to penetrate polymers that contain filler particles, and flexural measurements require large specimens that may not mimic the clinical case. Thus, it may be difficult to determine E using conventional techniques. It would be useful to have a relatively rapid technique that could be applied to small specimens, highly filled materials, and even specimens cured in situ. We suggest using a microhardness indentation technique that was originally developed for ceramic materials. We tested two unfilled rigid polymers, four resin composites, and four unfilled polymers with lesser hardness for this study. The study found that greater Vickers hardness loads yielded more consistent results than lesser loads. We developed a modified equation for E based on Knoop microhardness indentations. We concluded that laboratories may use a microhardness indenter to estimate the elastic moduli of polymers and resin composites. The results support our initial hypotheses that the slope of the equation relating the indentation parameter and the hardness/elastic modulus ratio was different for polymers and resin composites than for ceramics; however, the intercept is the same irrespective of the material tested.