Tribological Behavior of NbSe₂ and MoS₂ Film by Magnetron Sputtering at Different Humidity

Molybdenum disulfide suffers from tribological degradation in wet and oxygen-containing environments due to oxidation, which limits its application in atmospheric environments. Therefore, improving the tribological properties of MoS₂ at high humidity or finding alternatives to it is a focus of tribological research. Molybdenum disulphide, as a transition metal dichalcogenides (TMDs) compound, is ionically bonded within its layers and can be regarded as an ionic compound in a sense. As ionic bond strength increases, the chemical activity of the ionic compounds decreases, while stronger ionic bonds also mean that the layers are more resistant to damage. Thus, we wanted to improve the oxidation resistance and lubricating humidity adaptability of TMDs compounds in wet environments by enhancing the ionic bond strength. Niobium disselenide, a TMDs compound as MoS₂ with more stronger ionic bond than MoS₂, was selected as a substitute to compare with molybdenum disulfide, and explore the possibility of improvement of the tribological performance in wet environments based on the enhancement of ionic bond strength. In this study, the NbSe₂ and MoS₂ films were deposited by magnetron sputtering technique. The friction performances of the two films were studied in different humidity of atmospheric environment (20% RH, 35% RH, 55% RH, 75% RH). Then the crystal structures and chemical compositions of two films were compared in stable wear stage, for finding the mechanism of NbSe₂ in the environment of high atmospheric humidity. The friction tests results showed that NbSe₂ film had better friction properties and humidity adaptability in atmospheric environment, the coefficients of friction were stabilized at around 0.08 in 20% RH, 35% RH, 55% RH, and the friction coefficient was not increased until 75% RH, while MoS₂ films suffered from rapid lubrication failure just in 35% RH. Then, the Raman analysis results showed that MoO₃ peaks started to appear on the surface of the wear track of MoS₂ films in 35% RH, and MoS₂ peaks started to break down of MoS₂ films in 55% RH, while NbSe₂ peaks did not change significantly with changes in humidity. The XPS analysis showed that the oxidation of the MoS₂ film showed a monotonic increase with increasing humidity (from 20% RH to 75% RH), with the proportion of MoO₃ increasing from 14.12% to 38.64%; while for the NbSe₂ film, the oxidation of the surfaces of the wear tracks showed a stable or even slowly decreasing trend. Further XRD analysis showed that the MoS₂ film started to show significant MoO₃ (300) peaks in 35% RH, while the NbSe₂ film showed no significant changes in the XRD pattern in 20% RH, 35% RH and 55% RH, and the NbSe₂ peaks only started to change in 75% RH. The above analysis results illustrated that MoS₂ films were susceptible to oxidation under humidity conditions, which started to occur in 35% RH, while NbSe₂ films were more stable under humidity conditions, and the structure did not start to change until 75% RH. The structural characterization results were consistent with the trends in the tribological behaviour of NbSe₂ and MoS₂ films under humid conditions. It was initially verified that NbSe₂ films were oxidation-resistant under high humidity and had good humidity adaptability because of increased ionic bond strength, which made NbSe₂ with more stable interlayers and had little effect on sliding mechanism, then conducive to good lubrication and long wear life. Finally, this paper provided a new idea for the subsequent development of atmospheric lubricating materials.