In a previous letter , we have demonstrated that corrosion of a physical vapor deposition (PVD) coating substrate system can be induced by coating defects, resulting in premature damage to the usually hard and corrosion resistant coatings. Shrinkage pin holes, one of the most common defects, allow access of solution to substrate to cause galvanic corrosion [2, 3], and this may be prevented by forming a pin-hole interrupting under or sandwich layers of passivating metals such as Al or Ti [4, 5]. Since the increasing application and development of plasma arc in various PVD techniques, owing to the capability of the arc technique to achieve relatively high metallic vapor ion valence states, surface and atom mobility and diffusivity, and consequently the greater coating-substrate adhesion , more attention has been paid to the study of the arc-related formation of droplet or macroparticle coating defects.Due to the nature of its formation, a macroparticle is the product of a droplet of cathode metal induced by plasma arc heating, which, after being ejected from the cathode, solidifies and becomes embedded within the coating after incomplete reaction (due to its size) within the deposition chamber gas (N-plasma, for example)during its flight to the substrate . That is why a macroparticle is different, in addition to its great size,in chemical composition (N-content) not only from the adjacent coating matrix, but also from the particle exterior to interior parts . The composition inconsistency of the droplets rendered the defects to galvanic coupling (anodic to the adjacent coating matrix) to corrode first upon contact with aqueous solutions, and with progression, this eventually led to the penetration of solutions to the substrate to cause more severe crevice corrosion . The severity of such droplet-induced corrosion depends on many factors but generally droplets are regarded to be detrimental as far as their effect on the overall coating's corrosion performance is concerned [10-12].
- materials science
Wang, H. W., Stack, M. M., Lyon, S. B., Hovsepian, P., & Munz, W. D. (2001). Preferential erosive wear of droplet particles for cathodic arc/unbalanced magnetron sputtering CrN.NbN superlattice PVD coatings. Journal of Materials Science Letters, 20(6), 547-550. https://doi.org/10.1023/A:1010984618231