Testing of Diamond-Like Carbon Coatings under Slip-Rolling Friction Monitored by Acoustic Emission

Wear tests were carried out on different diamond-like carbon (DLC) coatings under lubricated and dry slip-rolling friction in a twin-disc testing rig. The analysis of acoustic emission (AE) provided an easy and comfortable tool for monitoring the wear life of DLC coatings. A DLC coating in-service was indicated by the appearance of no or just a single AE event due to single damage (crack, small spalling, etc.) to the coating and the counterbody. A high AE activity consisting of AE events of low energy was due to large spallings in the coating, which led to the undesirable contact between steel substrate and steel counterbody. However, a high AE activity caused by events of high energy indicated the end-of-service of the coating. In each and every test this AE activity was observed, the coating failed in the end. The AE responded much quicker and was much more sensitive than any changes in the coefficient of friction.