A review of tribological properties and deposition methods for selected hard protective coatings

Surface modification is widely adopted as a viable solution for surface damage under sliding or impacting conditions. Various coatings can be manufactured using different techniques and it has been applied on various substrates in previous studies which results in different characteristics. This review presents the anti-wear techniques with a focus on the most recent coatings of superior properties. Hard protective coatings may consist of various compositions; however, based on the hardness and the resistance to wear, some of the frequently used elements are Titanium (Ti), Nickel (Ni), carbides, Zirconium (Zr), Boron (B) and the Diamond Like Carbon (DLC). The deposition methods such as Chemical Vapor Deposition (CVD), Physical Vapor Depo-sition (PVD) as well as the cutting-edge coating hybrid-deposition methods such as Pulsed DC Magnetron Sputtering (PDCMS), High Power impulse (HiPIMS) and Arc Ion Plating (AIP) are reviewed and the resulted significant differences in the coating properties are discussed. In this review, listing the most important findings from previous studies in a table for each group of coatings make it easy to compare between various techniques and coatings. Some of the listed factors affecting the coating properties, such as the substrate, and others affecting the coefficient of friction, such as the counter body, were not been sufficiently highlighted before despite their importance in analyzing the tribological behavior for the coatings investigated. Although the focus was on the most recent studies, a wide range of studies was covered in order to accumulate and integrate the knowledge toward a complete analysis and discussion. Coating hardness is essential for better wear resistance; however, coating bonding, thickness and roughness may be equally important. Various techniques are used to improve the bonding. For some coatings, the Coefficient of Friction (CoF) increases with increasing the wear resistance where the counter body plays a major role. For most of the deposition techniques, the controlling parameters are the cleaning treatment, arc current, pressure, time, and gas supply. It was found that diamond-like carbon coatings are among the hardest coatings and the Plasma-Assisted Chemical Vapor Deposition (PACVD) is frequently reported as one of the most promising deposition techniques. Many coatings including the diamond-like carbon coating were not been investigated for their erosion wear resistance by solid particles despite the importance of such investigations for many applications.