The influence of ball size on tribological behaviour of MoS2 coating tested on a ball-on-disk wear rig

The effect of ball diameter on wear and friction of a molybdenum disulphide (MoS2) coating deposited using the closed-field magnetron sputtering technique has been investigated on a ball-on-disk wear rig sliding against uncoated steel ball-bearing balls. It was observed that the wear rate of the coating decreased significantly with increase in ball diameter. Correspondingly, the average friction coefficient in the steady state of sliding increased with increase in ball diameter. Under the present experimental conditions, the effect of ball size on wear and friction of the coating vanished when the diameter of the ball exceeded 6.35 mm. This phenomenon is thought to be a result of wear mode transition as a function of contact pressure at the apparent area of contact from a low-rate ‘mild’ wear regime to a high-rate ‘severe’ wear regime when the contact pressure exceeds some critical value. Simulation results using a previously presented model based on the above assumption showed a good agreement with the experimental observations.