Microhardness testing of plastics

Microhardness measurements have been carried out on polymethyl methacrylate, unplasticised polyvinyl chloride, polypropylene and acetal coplymer using two different test methods. In one case the depth of penetration of a Vickers indenter is measured during application of the load and in the other case the size of the Vickers indent has been measured after removal of the load. It has been found that the microhardness value for a plastic is dependent on the indenter load and the time for which the load is applied. After load removal there is time-dependent recovery of the size of the indent although the vast majority of this occurs in the depth of the indent rather than the diagonals. From the results in these tests it has been possible to obtain an equation which predicts the microhardness value in each material for any combination of loading time and applied load.