Enhancing the lifetime and vacuum tribological performance of PVD-MoS2 coatings by nitrogen modification

Abstract

As a solid lubricant, MoS2 is used for reducing friction and wear in machine elements under challenging operating conditions. In application, service life is thus a decisive factor that depends on the coating. Here, the effect of gaseous nitrogen modification on the performance of physical vapor deposited MoS2 coatings has been studied under vacuum conditions. The coatings were modified chemically or in their structural design by varying the bias voltage alternately. Tribologically induced changes were identified in short-running and long-running intermittent ball-on-disc tests, while the lifetime was evaluated in ball-on-disc tests until coating failure, with a coefficient of friction value exceeding 0.3 defined as the failure criterion. Furthermore, the microstructure was investigated before and after tribological loading. It appeared from scanning electronic microscopic imaging, that nitrogen modification reduced the initial degree of porosity and material compacting occurred after a short tribological loading. Nanoindentation measurements in the wear track of the compacted coating supported these results by a significant increase in indentation hardness and indentation modulus. Overall, the improvement in lubrication performance due to nitrogen modification led to a significant extension of service life, whereas the chemical and structural modification exhibited the highest service life of the coating.