STRAIN RATE AND TEMPERATURE EFFECTS ON MECHANICAL PROPERTIES OF ZINC DITHIOPHOSPHATE TRIBOFILMS MEASURED BY NANOINDENTATION

摘要

Nanoindentation tests have been performed on pads of a ZnDTP tribofilm at several temperatures, from ambient temperature up to 80℃, and at several strain-rates (ε = 0.003 s~(-1) to 0.1 s~(-1)). Contact pressure affects the measurement of hardness and elastic modulus (Anvil effect). The intensity of this effect was found to be temperature and strain-rate dependent.At low contact pressure, before Anvil effect occurs, the elastic modulus of the tribofilm appears to be neither dependent of temperature nor dependent of strain-rate. On the opposite, at constant strain-rate, the hardness is temperature dependent. It is constant until a threshold temperature of about 50℃, then it decreases with temperature. In the studied range, the initial hardness value also depends on strain-rate. The influence of strain-rate on hardness varies with the test temperature. For temperatures lower than 50℃, the pads exhibit a viscoplastic behavior: the hardness increases with increasing strain-rate. For temperatures higher than 50℃, the behavior looks like "shear thinning": the hardness decreases with increasing strain-rate. This point has to be further investigated, considering possible phenomena of true stress-true strain curve crossings due to strain-rate variation.