Elevated temperature tribology of cobalt and tantalum-based alloys

This paper describes the friction and wear behavior of a Co-Cr alloy sliding on a Ta-W alloy. Measurements were performed in a pin-on-flat configuration with a hemispherically tipped Co-base alloy pin sliding on a Ta-W alloy flat from ambient to 430 degrees C. Focused ion beam-scanning electron microscopy (FIB-SEM) and cross-sectional transmission electron microscopy (TEM) were used to identify the friction-induced changes to the chemistry and crystal structure in the subsurface regions of wear tracks. During sliding contact, transfer of material varied as a function of the test temperature, either from pin-to-flat, flat-to-pin, or both, resulting in either wear loss and/or volume gain. Friction coefficients (mu) and wear rates also varied as a function of test temperature. The lowest friction coefficient (mu=0.25) and wear rate (1 x 10(-4) mm(3)/N m) were observed at 430 degrees C in argon atmosphere. This was attributed to the formation of a Co-base metal oxide layer (glaze), predominantly (Co, Cr)O with Rocksalt crystal structure, on the pin surface. Part of this oxide film transferred to the wear track on Ta-W, providing a self-mated oxide-on-oxide contact. Once the oxide glaze is formed, it is able to provide friction reduction for the entire temperature range of this study; ambient to 430 degrees C. The results of this study indicate that glazing the surfaces of Haynes alloys with continuous layers of cobalt chrome oxide prior to wear could protect the cladded surfaces from damage. (C) 2015 Elsevier B.V. All rights reserved.