Effects of substrate roughness on the tribological properties of duplex plasma nitrided and MoS2 coated Ti6Al4V alloy

The poor surface shear resistance of Ti6Al4V alloy limits its applications involving moving components. To improve its tribological properties, a duplex treatment method involving plasma nitriding and MoS2 coating was proposed. The objective was to enhance load-bearing capacity, optimize surface roughness, and extend the lifetime of the MoS2 coating. Plasma nitriding significantly increased surface hardness by 2.3 times compared to untreated alloy. MoS2 coating on the hardened subsurface resulted in low friction coefficients of 0.05 at 25 % relative humidity. The duplex treated sample with a rough subsurface showed a lifetime of 11.90 h, six times better than the smooth subsurface sample. Higher subsurface roughness facilitated lubricant storage, release, and the formation of a continuous tribofilm, resulting in longer lifetime.