Tribological properties and rolling-contact-fatigue lives of TiN/SiNx multilayer coatings

We reported in an earlier study the use of amorphous SiNx to periodically interrupt the growth of TiN in order to suppress the columnar structure. X-Ray diffraction and transmission electron microscopy studies confirmed that the columnar growth was effectively suppressed. In this paper, we showed that these TiN/SiNx multilayer coatings attain lower dry friction coefficient (half of TiN) and better nanoscratch resistance than TiN coatings. Block-on-ring wear tests showed that the wear rate of optimized TiN/SiNx multilayer coatings is three times better than that of TiN coatings. Most important, the rolling-contact-fatigue life of the M50 bearing rod, coated with a 0.75 μm thick TiN/SiNx multilayer coating, is more than 10 times (test stopped manually before failure) better than that of uncoated ones and five times better than rods coated with conventional TiN coatings. The optimum TiN/SiNx coating thickness appears to be approximately 0.75 μm. This remarkable fatigue performance appears to be due to a combination of improved microstructure, adhesion, hardness and surface topography.