Influence of the etching process on the coating performance in dry tribological contacts

Physical vapor deposition (PVD) coatings are utilized for wear protection of gears. The use of PVD coatings in unlubricated contacts is also the subject of several research efforts. A promising approach to reduce wear in dry tribological contacts is the combination of MoS2 and WS2 embedded in CrAlN. One possible way to deposit these coatings is the application of High Power Pulse Magnetron Sputtering (HPPMS) process combined with direct current Magnetron Sputtering (dcMS). A challenge by depositing these coatings with an HPPMS/dcMS process is a sufficient adhesion between the coating and the substrate. The present study focuses on the influence of the etching process on the performance of CrAlN + Mo:W:Sg coatings in dry tribological contacts. Before coating deposition, the substrates were pretreated inside the vacuum chamber by different etching processes. During booster etching, which increases the Ar ionization, the substrate bias UB was varied between UB = −150 and UB = −250 V with steps of ΔUB = −50 V. Furthermore, an HPPMS metal ion etching process was used after booster etching at UB = −150 V for these processes. For HPPMS metal ion etching, the number of HPPMS cathodes was varied between one and two. With respect to the etching process, the morphology, surface roughness, compound adhesion by means of the scratch test as well as the coefficient of friction, and the wear volume of the subsequently deposited coatings were investigated. When increasing the substrate bias UB during booster etching, a reduced compound adhesion was found. Moreover, an increased wear volume was observed after pin-on-disk (PoD) tests. For metal ion etching with one activated HPPMS cathode, the wear volume after the PoD test was decreased compared to the pure booster etching processes. When increasing the number of HPPMS cathodes during metal ion etching, the compound adhesion decreased slightly. The wear volume after PoD tests, in contrast, increased. When only using one HPPMS cathode, the lowest wear volume of all investigated coatings was achieved. The present investigations show the potential of tailoring the etching process in order to increase the performance of PVD coatings. A customized HPPMS metal ion etching process can be used to improve the properties of well-established hard PVD coatings.