Rolling contact fatigue of ion nitrided and TiN coated AISI 4140 steel under pure rolling condition

•The use of ion nitriding as the only surface treatment greatly improved RCF life.•The use of a hard TiN coating did not improve the RCF performance of the steel.•When nitriding was combined with the TiN coating, premature delamination occurred.•The increase of the H/E ratio did not provide any beneficial effect on the RCF life. This work studies the influence of the ion nitriding pre-treatment on the surface characteristics and, particularly, the rolling contact fatigue behavior of PVD TiN coated AISI 4140 steel. In addition, single ion nitriding and PVD TiN coating processes were also studied for comparative purposes. Depending on the substrate characteristics, the TiN coating remained practically intact throughout the test or delaminated after hundreds of thousands of load cycles, but it did not provide a measurable improvement on RCF life. Although the solely nitrided specimens did not have the expected influence as pre-treatment, they presented the best RCF performance, and it is attributed to the absence of interfaces and the hardening of the subsurface.