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 °C and 500 °C could be attributed to the formation of tungsten oxide WO3 at the contact surfaces. However, at intermediate temperatures between 100 °C and 300 °C, high COF values of ~ 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 °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 (μs) of 0.11 was observed at 25 °C and the low friction values were maintained up to 500 °C including the 100-300 °C range. Micro-Raman spectroscopy revealed that at 25 °C the transfer layers were rich in carbon, whereas between 100 °C and 500 °C the transfer layers primarily consisted of monoclinic γ-WO3, which could be responsible for the low μs of 0.09-0.15 observed in this temperature range.