NANOINDENTATION STUDIES IN A LIQUID ENVIRONMENT

Surface and chemomechanical effects are of considerable importance in tribology, wear, and friction. Recently there has been substantial int erest in the use of nanoindentation techniques to investigate these ph enomena 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 whe n testing in Liquid can be overcome, and well-controlled nanoindentati on experiments can be conducted for even the shallowest indentation de pths. This type of testing, under ''model'' environmental conditions, has potential utility in the examination of several key mechanisms inv olved in tribology. This is demonstrated by experimental results for G aAs in distilled water and single-crystal tungsten in aqueous HCl and distilled water. When GaAs is indented in conditions of high atmospher ic humidity, the area around the indentation exhibits substantial bulg ing, reminiscent of lateral cracking. Testing of the same sample under distilled water does not give this result. The implication is that ca pillary condensation present in atmospheric ambient has a quite differ ent effect to complete immersion in water. This is probably due to the modified forces acting when water has condensed at the tip-sample int erface. Nanoindentation curves for electropolished, single-crystal tun gsten are almost perfectly elastic for shallow indentations. We have a ssessed the effects of the passivating, surface oxide film on the elas tic behavior by nanoindentation tests in air and under aqueous HCl and distilled water. The results for HCl, which is known to remove the ox ide film, indicate that the elastic behavior in BCC metals is modified by the passivating layer, but is not wholly dependent on it.