Acoustic emission monitoring of diamond-like carbon coating degradation during sliding wear

Abstract

Coatings of diamond-like carbon (DLC) were deposited by direct ion-beam deposition of ionized methane onto O1 tool steel blocks for sliding wear tests against non-coated 440C steel rings. Acoustic emission (AE) was used to evaluate the early stages of wear failure of the DLC coating and to compare AE response with frictional force output from a load cell. An incrementally increased load test showed that AE and fractional force are linearly proportional to each other. Significantly increased activity for both parameters was observed when the DLC coating was removed during the wear test, exposing steel surfaces in contact. Additional tests terminated at shorter sliding durations showed an increased activity of AE and increased frictional force which was correlated with incipient DLC delamination. However, AE appears to be a more sensitive early time indicator of coating failure than resisting frictional force. It appears the DLC coatings failed by either a gradual coating thickness reduction to a limiting thickness which could not adequately support the load, or by complete wear-through of the coating, thereby exposing bare steel surfaces in sliding contact.