October 4-9, 2015
The environmental sensitivity of the surface mechanical properties of ceramics has long been known and properties such as plasticity, creep, fracture and fatigue have been shown to be susceptible to changes in the environment. These effects are often called Rehbinder effects after his pioneering work in the 1930s. Most relevant to indentation testing is “anomalous indentation creep” which has been observed by many workers in ceramics using both conventional microhardness and nanoindentation testing. Whereas a constant, time-independent hardness was found in microhardness measurements by testing in dry toluene the hardness was found to drop as the indentation time increased when testing in moist air. In nanoindentation testing it is often observed that the surface hardness of a ceramic material is reduced after exposure to water or moist air and this can be reversed by solvent or vacuum treating the material. However, the mechanism for this change is not generally well understood. It is usually accepted that it is not due to the formation of a measureable reaction layer on the surface, rather mechanisms arising from the effect of adsorbates on friction, surface energy and dislocation core energy are often postulated. In this study the chemomechanical effects arising from water exposure on a range of oxide ceramics and ceramic coatings have been investigated by nanoindentation and the potential mechanisms which explain the observed effects have been assessed. It is concluded that adsorption results in the modification of the energy levels of a crystalline ceramic near the surface and this will affect the energy levels in the core of a dislocation. In this way changes in dislocation mobility can be induced. The importance of these observations in engineering applications will be emphasised for ceramic femoral heads and solar control coatings on architectural glass.
Steve Bull and Noushin Moharrani, "Chemomechanical effects in thin film and bulk oxides" in "Nanomechanical Testing in Materials Research and Development V", Dr. Marc Legros, CEMES-CNRS, France Eds, ECI Symposium Series, (2015). http://dc.engconfintl.org/nanomechtest_v/90