Sliding wear behavior of thermally sprayed 75/25 Cr3C2/NiCr wear resistant coatings

One of the foremost coating methods for combating wear is thermal spraying, however, despite its widespread industrial use, little is known about the basic friction behavior and the mechanisms by which such coatings wear. The manner of processing in thermal spraying inevitably leads to inhomogeneities, such as unmelted particles, oxide inclusions and porosity, in the sprayed deposits resulting in a structure markedly different from that of cast, wrought or even powder metallurgy materials. It is expected that thermal spray coating behavior would be even more complicated than that of homogeneous, uncoated bulk materials. Results of an investigation to determine the effects of some wear test variables on high velocity oxy-fuel (HVOF) sprayed Cr3C2/NiCr coatings using a pin-on-disk tribometer are presented. Room temperature sliding friction and wear behavior of coatings are discussed with respect to load, relative velocity and counterbody material. The present investigation shows that the tribological behavior of HVOF sprayed Cr3C2/NiCr coatings is significantly affected by its microstructural constituents such as splats, porosity and form and dispersion of second phases. It is also shown that changes in imposed sliding wear test conditions varied the friction and wear behavior of thermally sprayed coatings considerably. The break-in sliding coefficient of friction is found to be more significantly affected by load than other test parameters. Results also indicate that friction decreased with increasing velocity but wear decreased then increased with increasing velocity. By proper control of test conditions and by selected changes of those conditions, the physical wear mechanisms involved in thermally sprayed coatings could be understood. This study showed that the pin-on-disk is a well controlled test and can be used to understand certain basic relationships between the sliding friction and wear behavior of thermally sprayed coatings.