Quantitative in situ measurement of transfer film thickness by a Newton's rings method

We present a method, based on a Newton's rings analysis, to monitor and quantify the thickness of a solid lubricant transfer film during in situ tribological studies. The evolution of transfer film buildup was monitored by optical microscopy and recorded by videography through a stationary transparent hemisphere during reciprocating sliding. By monitoring the relative position of interference fringes (Newton's rings) outside the contact zone, the relative separation of the ball and track surface can be measured. The implementation of this technique is demonstrated through examples of sliding experiments with sapphire against a solid lubricant coating. Ex situ measurements of transfer film thickness using non-contact interferometry were consistent with in situ measurements from the Newton's rings method.