Grain size-wear rate relationship for titanium in liquid nitrogen environment

This paper presents the results of sliding wear experiments conducted on high-purity titanium (Ti) against bearing-steel in liquid nitrogen (LN2; boiling point: 77 K) environment. Ti samples of three different grain sizes (9, 17 and 37 mu m) were used to study the effect of hardness, derived from grain refinement as well as cryogenic test temperature, on the wear properties of Ti. In our experiments, a constant load of 10 N and sliding speeds of 0.67, 1.11 and 4.19 m s(-1) were used. The coefficient of friction (COF) for this tribo-couple varied between similar to 0.25 and similar to 0.50. While a steady state was always achieved, a peak in the COF was always noted in case of coarse-grain (37 mu m) Ti tested at a sliding speed of 4.19 m s(-1). Under the investigated sliding conditions, the wear rate was found to be of the order of 10(-3)-10(-4) mm(3) N-1 m(-1). The lowest wear rate was recorded in the fine-grain (9 mu m) Ti at the highest sliding speed of 4.19 m s(-1). The critical analysis of the worn surface topography reveals that the reduced wear rate was due to the formation of adherent and strain-hardened tribolayer. In order to show various dominant wear mechanisms of Ti, a qualitative map was developed in sliding speed-grain size space. Substructure evaluation revealed the formation of a dense array of deformation twins because of the plastic deformation, which often resulted in the subdivision of grains. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.