Physico-Tribo-Mechanical and Adhesion Behaviour of Plasma Treated Steel and Its Alloys: A Critical Review

Steel and its alloys are the most commonly used materials in industries such as bearing, automotive, aerospace, manufacturing including medical sector for fabrication of different types of components and products because of their light weight. But these materials have restricted use in extreme conditions environments because of their poor properties such as adhesion, corrosion resistance, wear resistance and tribological properties. The researchers have tried to improve these properties by applying coatings and surface treatments using plasma. Most of the researchers have used plasma nitriding for surface treatment of different grades of steel in either N2 -H2 or O2 environment. They have varied the temperature between 300 to 700 °C and treatment time from 20 min to 20 h according to the grade of steel. They observed an improvement in adhesion property and corrosion resistance with plasma treatment but the higher plasma exposure time and increment in temperature caused degradation in adhesion property due to formation of brittle layer followed by its destruction due to mechanical stresses. Therefore, researchers have further developed duplex plasma treatment of steel and its alloys. The duplex plasma treatment consisted of plasma nitriding followed by either plasma oxidation in most of the cases or surface coatings. For surface coating, diamond-like coating (DLC) was widely used by researchers. DLC coating is carried out by using physical vapour deposition method. They observed drastic improvement in adhesion, corrosion and tribological properties of steel and its alloys as compared to a single treatment, due to the formation of S-phase and CrN layer on the surface. Further, they also observed the decomposition of CrN into expanded austenitic phase which further transformed into martensite with increase in temperature and treatment time, thus resulted in reduction of corrosion and wear resistance properties. Hence, optimum treatment time and temperature are required for surface treatment of steel and its alloys using plasma process. In this review, the positive impacts of various plasma treatments of steel and its alloys on physico-tribo-mechanical and adhesion properties are systematically and critically reviewed.