High temperature tribological behavior of W containing diamond-like carbon (DLC) coating against titanium alloys

The current study has been undertaken to determine friction, adhesion and wear mechanisms of tribological coatings for elevated temperature applications of Ti-6Al-4V alloy. Sliding wear characteristics of diamond-like carbon (DLC) coatings and polycrystalline diamond (PCD) was studied and compared with those of conventional N-based coatings, which exhibited a high coefficient of friction (COF) due to titanium adhesion to the coating surface. Hydrogenated DLC (H-DLC) and W containing DLC (W-DLC) coatings offered low and stable COF values of 0.11-012 at 25 °C while the PCD had the lowest COF of 0.05 at 25 °C. At temperatures > 200 °C H-DLC's COF increased rapidly accompanied by high coating wear. At 100 °C W-DLC had a COF of 0.06 which increased to 0.46-0.54 between 200 °C and 300 °C similar to that of H-DLC. However, at 400 °C W-DLC's COF decreased to 0.07, and a low COF of 0.08 persisted at temperatures as high as 500 °C. The governing mechanisms of the low friction of W-DLC observed at elevated temperatures were revealed by studying the compositions of the coating surfaces and the transfer layers formed on Ti-6Al-4V. Raman spectroscopy indicated that at 25 °C the transfer layers were rich in carbon, whereas at 500 °C they consisted mainly of tungsten trioxide (WO3) that formed on W-DLC's surface.