The role of droplets on the cavitation erosion damage of TiN coatings produced with cathodic arc physical vapor deposition

Abstract

In this study, the role of droplet-related defects on the initiation and propagation of cavitation erosion damage of TiN coatings produced by Cathodic Arc Physical Vapor Deposition (CA-PVD) was investigated. Ultrasonic-aided cavitation erosion tests were conducted in distilled water, using a specially designed specimen holder. By using this holder, following of the defect-related damages on the same region of the sample became possible by scanning electron microscopy (SEM) during a total test duration of 12h. Focused ion beam (FIB) investigations were also carried out on cross-sections of some selected defects to understand the damage mechanisms. Results of the study revealed the important role of droplet-related defects on the cavitation erosion damage. Deep cavities formed by detaching of conical droplets were determined as the most detrimental type of defects. At these sites, large impact craters were formed with sizes extending to 100μm with a substrate-reached area at their center after 3.5h of cavitation erosion test. However, the damage created by buried droplets was very limited when compared to the cavities. Their role on the initiation and propagation of the cavitation damage depends on their shape and position in the coating. Conical droplets embedded in the coating resulted in the capping of a coating layer above them. These damages were not deepened or extended during the test duration. On the other hand, wide droplets with a flat top that sat on the substrate did not result in the formation of neither capping of the coating nor impact craters after 12h of testing.