Self Lubricating Composite Coatings Containing TiC–MnS or WC-MnS Compounds Prepared by the Plasma Transferred Arc (PTA) Technique

Abstract

Composite coatings containing manganese sulphide as lubricating addition and enhanced with hard carbide particles (TiC, WC) were synthesized on a plain steel substrate using the plasma transferred arc technique. The coatings are well bonded to the substrate, have a thickness of about 1 mm, and are free of any visual defects. They consist mainly of a martensitic or ferritic matrix enhanced with titanium or tungsten carbides and a dispersion of MnS particles. The tribological properties of the composites are assessed using a pin-on-disk device. Both composites possess self lubricating properties, due to the formation of a thin layer of manganese sulphide on their wear tracks. The corresponding friction coefficients vary between 0.25 and 0.28, compared with 0.50–0.60 obtained from similar hard coatings without MnS addition. The wear rates are of the order of 10−5 mm3/m N and are two orders of magnitude lower than those obtained from the substrate material with MnS addition, but without the presence of hard enhancing particles. The wear regime is mild abrasion due to the combined action of both lubricating (MnS) and hard (TiC or WC) particles.