In this paper, as part of a systematic study on the rolling contact fatigue of HVOF-sprayed hardmetal coatings, the behavior of WC-17%Co coatings on hardened and nonhardened substrates is compared. To obtain a meaningful assessment, the coatings were applied to roller specimens made from soft as well as case-hardened MnCr steel. Two coating thicknesses were used with the expectation that maximum stress will occur in the substrate, in one case, and in the other case, in the coating. Test results show that the durability of HVOF-sprayed WC-Co is significantly better on nonhardened (soft) substrates and that the limiting factor for endurable Hertz pressure is the fatigue strength of the coating.

A non-destructive experimental approach was adapted to investigate the variations in residual stress fields within thermal spray coatings. WC-Co coatings produced by a HVOF technique were considered for concentrated rolling sliding contacts in this study. These coatings were produced in various thicknesses on various substrates. Residual stress measurements were made using an x-ray diffraction technique, along and across the rolling direction. A modified four-ball machine was used to conduct rolling contact fatigue tests under various tribological conditions of contact stress, lubrication and contact configuration. Residual stress measurements were made before and after the tribological tests. Failed rolling elements were analyzed using scanning electron microscopy, electron probe microscopy and surface interferometry. Results indicate that the magnitude of compressive residual stress attenuates during fatigue failure. The magnitude of attenuated residual stress was dependent upon the type of tribological failure. This attenuation of residual stress was attributed to the microcracking of coating under the influence of contact stress.

Some applications of high velocity oxygen fuel (HVOF) flame sprayed WC cermet coatings in paper mills are summarized. The WC cermet sprayed coating is one of the most widely used in the thermal spray industry. HVOF sprayed WC cermet coatings have such excellent physical and chemical properties that they are extensively employed in the industrial field. Typical properties for example, are wear resistance, high stress rotating bending or rolling contact fatigue strength, frictional characteristics, wettability for aqueous agent, non picking or non adhesive property against foreign material, thermal conductivity, mirror finished surface property and so on. They are derived from the compact, denser microstructure, containing fine-sized hard WC particles, high bond strength onto substrate. These characteristics obtained by HVOF sprayed WC cermet coatings are utilized to improve the surface functions of machine parts which are installed mainly in the press department, drying department, size press, calender department and finishing department of paper mills. In this study, examples of some application of WC cermet coatings in several types of rolls in paper mills are introduced.