CVD diamond water lubricated tribosystems for high load planar sliding

Water lubricated tribological systems involving chemical vapour deposited (CVD) diamond coatings are appealing to many practical situations, namely in metalworking and fluid handling applications. The motivation behind the present study lies in insufficient knowledge on the behaviour of these water-based lubricated tribosystems especially under high loads. The microwave plasma chemical vapour deposition technique was used to coat dense Si3N4 substrates from CH4–H2 gas mixtures, setting adequate deposition parameters during 2.5 h, resulting in a film thickness around 15 μm. Self-mated planar reciprocating sliding ball-on-flat wear tests were performed up to 16 h (690 m of sliding distance) with normal applied load ranging from 70 N to 160 N. SEM and AFM characterisation techniques were used to identify the prevailing wear mechanisms. The worn surfaces exhibit a polished appearance resulting from diamond asperities abrasion on the counterface combined with the action of nanometric debris. Some longitudinal wear grooves along the sliding direction are the result of two-body abrasive action of larger debris spawn from the truncation of aggregates of crystals in the first steps of sliding. A steady-state sliding regime sets for almost the test full duration, with a very low friction coefficient in the range 0.04–0.05 and a wear coefficient of about 10−8 mm3 N−1 m−1. Partial film wear-out was found at a threshold load of 160 N, however without seriously affecting the tribosystem performance.