Rolling contact fatigue behavior of Ti/TiN coated ADI by cathodic arc deposition

Abstract

This work studies the rolling contact fatigue (RCF) behavior of austempered ductile iron samples coated with Ti/TiN by cathodic arc deposition. The influence of the TiN layer thickness on the characteristics and RCF life of the coated samples is analyzed. RCF tests were performed in a flat washer type testing rig, using a thrust ball bearing as a counterpart. The maximum contact pressure was 1400 MPa. RCF tests results were analyzed using the two-parameter Weibull distribution. The rolling track of the tested samples was examined by using SEM and EDS. The results show that the surface hardness, residual stresses and coating adhesion strength of the samples increase as TiN layer thickness increases while surface roughness, coating hardness and elastic modulus do not vary significantly with the TiN layer thickness. Regarding the RCF tests, two failure mechanisms were observed in the coated samples, coating delamination and substrate spalling. Coating delamination turned out to be the most predominant mechanism and occurred at fewer loading cycles than substrate spalling. The EDS analysis revealed that inside the delaminated areas the Ti interlayer remained adhered to the substrates in all the cases. Extended tests on delaminated samples show growing delamination until final failure by substrate spalling occurred. The Weibull analysis indicates that the RCF life of the coated samples seems to have a tendency to increase with the TiN layer thickness and, in the case of the thicker TiN layer, to exceed the life of uncoated ADI after the extended tests. However, due to the number of samples employed, the statistic analysis does not show significant differences in the RCF behavior of the uncoated and coated samples.