Insights into the microstructure characteristics, mechanical properties and tribological behaviour of gas-phase chromized coating on GCr15 bearing steel

In this study, a high quality chromized coating was produced on GCr15 steel substrate by vapor phase chromizing technique for sprag clutch application. The microstructure characteristics, mechanical properties and sliding friction behaviour were systematically investigated. The results show that the chromized coating consists of 1) an outmost layer of columnar Cr7C3 grains with excess carbon atoms (55 at.%) and considerable amount of iron atoms (6.2 at.%); and 2) an inner layer of coarse Fe3C grains. The carbide phase formation can be well explained via the thermodynamics evaluation for the Fe-Cr-C system, while the origin of carbon contamination is likely originated from the side reactions of precursor gases. The high surface hardness (16.6 GPa) and interfacial adhesion (~50 N) endow the chromized coatings with superior wear resistance against steel counterface under ambient conditions. The wear rates are found to decrease with the applied load (10-25 N). Scanning electron microscopy (SEM) analysis of the worn surfaces indicates that a chromium oxide tribolayer tends to be formed under a higher loading which displays lubrication and hence protects the contacting material during the friction process.