Plasma transferred arc composite coatings with self lubricating properties, based on Fe and Ti sulfides: Microstructure and tribological behavior

The plasma transferred arc (PTA) technique was used to produce composite coatings based on co-melting of MoS2, TiC and iron ingredients, in an attempt to obtain wear resistant layers with self lubricating properties. As a matter of fact, both molybdenum and titanium are known to form with sulfur either crystalline or amorphous compounds exhibiting special self lubricating properties. In this perspective three coatings containing the same quantity of TiC and increasing MoS2 contents were produced using plain carbon steel material as the substrate. Three other coatings containing only increasing contents of MoS2 without TiC were prepared under the same conditions for comparison.The experiments have shown that both TiC and MoS2 ingredients dissolve completely in the melt under the plasma arc, whereas their elements, Ti, C and S, are redistributed in the solidification products forming new compounds. The influence of the presence of sulfur in the solidification path of the Fe–Ti(Mo)–C system is studied. Although the original MoS2 compound is totally absent in the final microstructure, the composites containing both MoS2 and TiC have formed new complex sulfides, presumably endowed with self lubricating properties. As a result, the wear tests performed with a pin-on-disk apparatus have shown that the coatings which were produced using both TiC and MoS2 ingredients exhibited improved friction coefficients and lower wear rates in comparison to the steel substrate as well as to the composite containing TiC, but exempted from MoS2.