Role of an oxygen atmosphere in high temperature sliding behaviour of W containing diamond-like carbon (W-DLC)

W containing diamond-like carbon (W-DLC) coating is of interest to manufacturing industry, as they exhibit low coefficient of friction (COF) values when sliding against aluminum, titanium and steel at elevated temperatures. The low COF of W-DLC observed at 400 degrees C and 500 degrees C could be attributed to the formation of tungsten oxide WO3 at the contact surfaces. However, at intermediate temperatures between 100 degrees C and 300 degrees C, high COF values of similar to 0.60 were recorded. In this work, the friction reduction mechanisms of W-DLC coatings were investigated in a dry oxygen atmosphere as a function of testing temperature up to 500 degrees C against an Al-6.5% Si alloy. The purpose of maintaining an oxygen rich environment was to increase the propensity of WO3 formation at the sliding surfaces at the intermediate temperatures. A steady state COF (mu(s)) of 0.11 was observed at 25 degrees C and the low friction values were maintained up to 500 degrees C including the 100-300 degrees C range. Micro-Raman spectroscopy revealed that at 25 degrees C the transfer layers were rich in carbon, whereas between 100 degrees C and 500 degrees C the transfer layers primarily consisted of monoclinic gamma-WO3, which could be responsible for the low ris of 0.09-0.15 observed in this temperature range.