Coatings and Surface Modification of Alloys for Tribo-Corrosion Applications

This review of the tribocorrosion of coatings and surface modifications covers nearly 195 papers and reviews that have been published in the past 15 years, as compared to only 37 works published up to 2007, which were the subject of a previous review published in 2007. It shows that the research into the subject area is vibrant and growing, to cover emerging deposition, surface modification and testing techniques as well as environmental influences and modelling developments. This growth reflects the need for machines to operate in harsh environments coupled with requirements for increased service life, lower running costs and improved safety factors. Research has also reacted to the need for multifunctional coating surfaces as well as functionally graded systems with regard to depth. The review covers a range of coating types designed for a wide range of potential applications. The emerging technologies are seen to be molten-, solution-, PVD- and PEO-based coatings, with CVD coatings being a less popular solution. There is a growing research interest in duplex surface engineering and coating systems. Surface performance shows a strong playoff between wear, friction and corrosion rates, often with antagonistic relationships and complicated interactions between multiple mechanisms at different scale lengths within tribocorrosion contacts. The tribologically induced stresses are seen to drive damage propagation and accelerate corrosion either within the coating or at the coating coating–substrate interface. This places a focus on coating defect density. The environment (such as pH, DO2, CO2, salinity and temperature) is also shown to have a strong influence on tribocorrosion performance. Coating and surface modification solutions being developed for tribocorrosion applications include a whole range of electrodeposited coatings, hard and tough coatings and high-impedance coatings such as doped diamond-like carbon. Hybrid and multilayered coatings are also being used to control damage penetration into the coating (to increase toughness) and to manage stresses. A particular focus involves the combination of various treatment techniques. The review also shows the importance of the microstructure, the active phases that are dissolved and the critical role of surface films and their composition (oxide or passive) in tribocorrosion performance which, although discovered for bulk materials, is equally applicable to coating performance. New techniques show methods for revealing the response of surfaces to tribocorrosion (i.e., scanning electrochemical microscopy). Modelling tribocorrosion has yet to embrace the full range of coatings and the fact that some coatings/environments result in reduced wear and thus are antagonistic rather than synergistic. The actual synergistic/antagonistic mechanisms are not well understood, making them difficult to model.