Coatings for the prevention of localized corrosion of M-50 bearing steel

Abstract

In this study we deal with the improvements in localized corrosion resistance and wear reduction of type M-50 steel used for engineering components such as bearings by the use of overlay coatings. The overlay coating materials selected to resist localized pitting corrosion were titanium, chromium, Nichrome (Ni20Cr) and molybdenum deposited by physical vapor deposition techniques of direct evaporation, activated evaporation ( i.e. evaporation in the presence of a plasma) and ion plating, and TiC, TiN and HfN deposited by the activated reactive evaporation process. Coated and control (uncoated) samples of M-50 steel were corrosion tested, the test having been designed to stimulate the bearing environment inside an engine with a specified roller-bearing-on-race geometry. The results showed that the control (uncoated) samples were severley pitted along the contact line and that general corrosion occurred outside this region, whereas overlay coatings of titanium, chromium, Nichrome, TiC and TiN completely eliminated the localized pitting corrosion. Coatings of these materials deposited by different techniques showed equally good results. Wear test samples with TiC and other hard coatings were prepared and submitted to the Naval Engineering Support Office, San Diego, for rolling contact fatigue wear tests. The results showed that a TiC coating produces no decrease in the fatigue life of the M-50 alloy. It appears that overlay coatings are a viable way to prevent localized pitting corrosion whichh not only is a cheaper alternative to ion implantation but also has a significant advantage in that hard coatings such as TiN and TiC can provide corrosion and wear resistance simultaneously.