Nanoindentation Studies in a Liquid Environment

Abstract

Surface and chemomechanical effects are of considerable importance in tribology, wear, and friction. Recently there has been substantial interest in the use of nanoindentation techniques to investigate these phenomena for asperity size indentations. In this paper we report a new type of nanoindentation experiment where tip and sample are immersed. We show that with due care the difficulties due to surface tension when testing in liquid can be overcome, and well-controlled nanoindentation experiments can be conducted for even the shallowest indentation depths. This type of testing, under model environmental conditions, has potential utility in the examination of several key mechanisms involved in tribology. This is demonstrated by experimental results for GaAs in distilled water and single-crystal tungsten in aqueous HCl and distilled water. When GaAs is indented in conditions of high atmospheric humidity, the area around the indentation exhibits substantial bulging, reminiscent of lateral cracking. Testing of the same sample under distilled water does not give this result. The implication is that capillary condensation present in atmospheric ambient has a quite different effect to complete immersion in water. This is probably due to the modified forces acting when water has condensed at the tip-sample interface. Nanoindentation curves for electropolished, single-crystal tungsten are almost perfectly elastic for shallow indentations. We have assessed the effects of the passivating, surface oxide film on the elastic behavior by nanoindentation tests in air and under aqueous HCl and distilled water. The results for HCl, which is known to remove the oxide film, indicate that the elastic behavior in BCC metals is modified by the passivating layer, but is not wholly dependent on it.