Transition metal disulfide compounds, especially molybdenum disulfide and tungsten disulfide, are two-dimensional layered materials due to the similarity of their structure and properties to graphene, and their multifunctional properties such as self-lubrication, high surface area, electrical resistivity, stability, good mechanical strength, ease of charge transfer, feasibility of synthesizing these nanoparticles, and customizable band gaps make them widely used in different applications such as catalysis, sensing, adsorbents, lubricants , lithium-ion batteries, supercapacitors, and solar light collectors, among other diverse fields. This paper briefly describes the differences in tribological properties of MoS2, WS2 and graphite.

01 Differences between MoS2, WS2 and graphene self-lubricating coatings #

At room temperature, WS2 self-lubricating coatings are not as good as MoS2 coatings for film formation on metal substrates; at high temperature, the friction performance of WS2 coatings is better than that of MoS2 solid lubrication coatings. Graphite is usually used in direct spraying, but it is not suitable for use in vacuum, and its coefficient of friction is twice as high as that in air, and the wear is even up to 100 times.

• Graphite: Its self-lubricating properties depend on the presence of air and water. As a lubricating material, graphite products, the critical value of moisture in the working environment is 5 g/m3. If it is lower than this value, the coefficient of friction of graphite products will increase. The coefficient of friction increases when graphite is strongly oxidized in an air medium at room temperature of 300 to 400 degrees. However, in neutral or reducing medium, 300~1000 degrees Celsius can also maintain good wear resistance. The higher the degree of crystallization of graphite, the more it protects the lubricated material from wear. In addition, the content of impurities must be controlled below 1%, otherwise significant abrasive wear will easily occur.
• Molybdenum disulfide: MoS2 is formed by geothermal action, it is chemically stable to withstand heat and pressure and therefore has good resistance to heavy pressure. In environments with harsh, heavily loaded gears and bearings, and components with shock loads, the use of molybdenum disulfide grease is more effective than the use of graphite grease.
• Tungsten disulfide: WS2 used to be 10 times more expensive than MOS2, and is only used as an alternative to MOS2 for applications requiring good oxidation resistance or low resistivity. The friction factor is slightly lower than that of MoS2, and the thermal stability is higher; for sliding friction parts under special working conditions, the application of WS2 can bring out its better effect than MoS2.

Molybdenum disulfide, tungsten disulfide coating and graphene performance differences #