Zirconium-based coatings in highly stressed rolling contacts as alternative solution to DLC and ta-C coatings

The slip-rolling resistance of DLC, a-C and ta-C thin film coatings was improved considerably in the last years, but there is still room for improvements in relation to (a) the temperature stability and (b) initial surface roughness after deposition. This paper presents a novel coating-substrate system in comparison to recently developed DLC coatings in a bench-mark test procedure exerting slip-rolling conditions in the presence of liquid lubricants. One of the Zr-based thin film coatings can withstand at least 1 million cycles under initial Hertzian contact pressures of up to P0max = 3500 MPa and oil temperatures of at least 120 °C associated with low coefficients of friction under mixed/boundary conditions or ten million of cycles under P0max = 2940 MPa. In comparison, some of the newly developed DLC coatings are slip-rolling resistant for at least up to 10 million cycles at RT (some of them also at 120 °C oil temperature) under Hertzian contact pressures of Pmax = 2600/2940 MPa. In general, Zr-based thin film coatings do not require special formulated oil formulations and Zr(C,N) bear on a straight coating architecture suited for mass production with nanosized layers.

Research highlights▶ Improvement of load carrying capacity of steel substrate under rolling contact fatigue under extreme conditions and very high Hertzian contact pressure (Pmax = 3.0 GPa) by application of thin film coatings which consists in a future step of improvement in mechanical applications and shows the capacity to increase the lifetime of steel. ▶ Identification of critical microstructural features in Zr(C,N) nanocrystalline coatings influencing the slip-rolling resistance and leading to the achievement of a better reproducibility of the coating deposition process. ▶ Tribological considerations with comparison to DLC coatings which have reached similar lifetime and tested under the same slip-rolling test procedure.