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Research on Molybdenum disulfide coating lubrication

2020-08-10 15:23:31  Knowledge

MoS2 is a material found in the extraction of granite veins and is highly refined to a purity that can be used as a lubricant. MoS2 has a Mohs hardness of 1.0 to 1.5 and a density of 4.80 g/cm3. It starts to oxidize when heated to 315℃ in the air. The temperature increases, the oxidation reaction accelerates, and it is insoluble in water.
MoS2 as a solid lubricant has the advantages of low friction coefficient, high bearing capacity, full use temperature range, excellent adhesion, anti-friction, and corrosion resistance, but the moisture in the humid environment will cause the MoS2 friction coefficient to increase.


1.Preparation technology of MoS2 coating
 Surface engineering can be divided into two categories: surface modification technology (no cover) and coating technology (protection). With the development of surface engineering technology, the preparation of MoS2 coating has developed supersonic flame spraying technology, non-equilibrium magnetron sputtering, plasma-enhanced chemical vapor deposition, and laser melting based on single techniques such as electroplating, thermal spraying, and vapor deposition. Coating and other new composite preparation processes. However, the conventional MoS2 coating preparation technology is mainly vaporing deposition. Still, due to the high temperature and difficult to handle gases in the CVD process, PVD technology is by far the most commonly used coating preparation method.

2.Molybdenum disulfide coating performance study
The friction properties of molybdenum disulfide coating are related to many factors. First, the coefficient of friction is related to the hardness of the substrate and the surface roughness of the parts. Molybdenum disulfide coating exerts a lubricating effect through the substrate to bear the load. Generally, the higher the hardness of the substrate, the smaller the coating friction coefficient, and the better the abrasion resistance. The surface roughness of the parts directly affects the formation of the transfer film. The smaller the surface roughness, the lower the chance of mechanical fit of the lubricating coating thereon, and the lower the adhesive strength. Secondly, it is related to the basic physical properties of molybdenum disulfide coating. The particle size of the sheet, the crystal orientation parallel to the base surface, and the coating density and impurities will directly affect the lubricating performance. The most important thing is that MoS2 coating has the most significant impact on environmental conditions. Most MoS2 coatings have defects such as high wear rates under high temperature and high-pressure environments and performance degradation under high temperature and humid climates. Tribochemical reactions are natural to occur on friction surfaces in the atmospheric environment, resulting in oxide particles. Leading to accelerated wear, the existence of these factors makes the practical application of MoS2 coating is minimal.

Research on Molybdenum disulfide coating lubrication 

3.MoS2 coating lubrication mechanism
Although the mechanism of MoS2 as a lubricating material has been studied for a long time, the theory explaining the excellent friction properties of MoS2 coating has not been unified.
One of the accepted theories is the theory of intracrystalline displacement. The lone pair of electrons in the lower S atoms of MoS2 penetrates the hole region composed of three S atoms in the upper layer and is negatively charged. Due to electrostatic repulsion, it is easy to be separated to obtain better lubrication.

The second MoS2 friction mechanism believes that low friction is due to the adsorption of some foreign substances on the surface of MoS2, which weakens the structure of MoS2. According to this theory, the friction coefficient of MoS2 should be increased in a high vacuum environment. At that time, tests confirmed the adsorption layer. It does affect the friction performance, but it just causes an increase in the friction coefficient.
The third is that the high-energy edge surface of the MoS2 crystal reacts rapidly with oxygen during the friction process to form a stable oxide. This has a weak attraction to the cleavage surface with low surface energy and other edge surfaces, leading to intergranularity: weak adsorption and low friction.

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