Role of Al additions in wear control of nanocrystalline Mo(Si 1−x Al x ) 2 coatings prepared by double cathode glow discharge technique

Abstract

Four kinds of nanocrystalline Mo(Si_1−xAl_x)₂ coatings with differing Al contents are prepared onto a Ti–6Al–4V substrates by a double cathode glow discharge apparatus. The microstructural features of the deposited coatings were characterised by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. These coatings are composed of the equiaxed C40–MoSi₂ grains with the average grain size of ∼5 nm. Nano-indentation measurements indicated that the hardness H, elastic modulus E and the H/E or H³/E² ratio of the nanocrystalline Mo(Si_1−xAl_x)₂ coatings slightly increase with the increase in Al content. The tribological behaviour of the nanocrystalline Mo(Si_1−xAl_x)₂ coating sliding against a ZrO₂ ceramic ball at room temperature has been compared using a ball-on-disc type tribometer under unlubricated conditions. Experimental results showed that the specific wear rates of the nanocrystalline Mo(Si_1−xAl_x)₂ coatings decrease with increasing Al content and are dramatically reduced by more than 1–2 orders of magnitude over the uncoated Ti–6A1–4V. The enhancement of wear resistance of the nanocrystalline Mo(Si_1−xAl_x)₂ coatings by Al additions is correlated with the increased mechanical properties and the forming oxide layer by tribochemical reactions.