Evaluation of the Effectiveness of Tungsten Disulfide and Molybdenum Disulfide as Dry Lubricants

- Aug 04, 2020 -

Dry lubricant is a method of coating or plating a substance on the surface of the friction pair to form a thin film to reduce friction. The technology of using dry lubricant for lubrication is called robust lubrication technology. Reliable lubricating technology was first used in the military industry and was gradually promoted in some high-tech fields, which solved some severe problems of liquid lubricants.

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Disulfide compounds that have application value as dry lubricants include tungsten, molybdenum, niobium, tantalum disulfide, diselenide, and ditelluride. MoS2 is currently widely used and representative, it is naturally formed, and other dichalcogenides are artificially synthesized. Among them, the representative products are tungsten disulfide (WS2), WS2 and other synthetic products. The impurity content is less than 0.5%, which is a costly and unique material. Where it is restricted, it will be introduced as a direct replacement for MoS2.

Both MoS2 and WS2 are excellent solid lubricating materials and have incredibly high application value in the field of ultra-solid lubrication. Since the late 1990s, major foreign developed countries have begun to apply friction components in the aerospace, precision instruments and ultra-high vacuum. At the same time, this technology is also referred to as the hard disk drive bearing of the computer, the rotation, sliding and other components in the ultra-high vacuum system.

MoS2 is a dark grey shiny powder, soluble in aqua regia and concentrated sulfuric acid, insoluble in water and dilute acid, can be used for maintenance and lubrication of processing and storage conditions, can form an efficient adhesion dry lubricant film, is to reduce wear and tear Special material for many problems in friction technology. MoS2 is the most common natural form of molybdenum, extracted and purified from the ore and used directly as a lubricant. Because MoS2 has a layered structure of hexagonal crystals, it is a beneficial oil additive. These layers can slide on each other, allowing free flow on steel and other metal surfaces, even under heavy pressure, such as bearing surfaces. Because MoS2 is formed by geothermal action, it has the chemical stability to withstand hot pressing. A small amount of sulfur and iron react and form a sulfide layer, which is compatible with molybdenum sulfide and maintains a lubricating film. MoS2 is inert to many chemicals and will complete its lubrication under vacuum, while graphite cannot. MoS2 has better thermal stability in an air environment lower than 400 ℃. When the temperature is higher than 400 ℃, MoS2 will be oxidized to MoS3 in a short time, and MoS3 is a kind of high-hardness particulate matter that adheres to friction Abrasive particles are formed on the secondary surface. But in a vacuum and inert gas environment, MoS2 can maintain a relatively stable structure at 1100 ℃.

WS2 is a new type of solid lubricant with similar characteristics to MOS2. Mohs hardness is low, it is easy to form a transfer film, the friction factor is slightly smaller than MOS2, and the thermal stability is higher; the air begins to decompose at 510 ° C, 650 ° C Completely decomposed, can be lubricated for a long time at 425 ° C; decomposed in vacuum at 1100 ° C, completely decomposed at 2000 ° C. WS2 has better lubrication performance than MOS2, and can completely replace MOS2.

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Tungsten disulfide WS2 also has a layered hexagonal structure, similar to MoS2. Tungsten atoms and sulfur atoms are covalently bonded, and there are only sulfur atoms on the surface of the molecular layer. Weaker molecular bonds connect the sulfur atoms between the sheets. By combining Van der Waals forces between layers, compared with MoS2, WS2 has a more substantial layer spacing and a lower friction coefficient. Moreover, WS2 is insoluble in almost all media, including acids, alkalis, oils and water, but is more sensitive to free gaseous fluorine, hydrofluoric acid and hot sulfuric acid. Compared with MoS2, WS2 has better thermal stability than MoS2. In the air, the decomposition temperature of WS2 is 510 ℃. It will be rapidly oxidized at 539 ℃, and the decomposition temperature in a vacuum and inert gas is 1150 ℃. Compared to MoS2, WS2 has a broader temperature range.

Several domestic and foreign lubricant manufacturers have compared the lubricating properties of WS2, MoS2 and graphene. Through experiments, it was found that tungsten disulfide is not only more reliable than MoS2 but also more durable than graphene. Also, in high-radiation operating environments, WS2 has higher radiation resistance and can adapt to more complex operating environments. Therefore, manufacturers have shown a more substantial production interest in tungsten disulfide lubricants.

Through friction and wear test, study the friction performance comparison of WS2 and MOS2 solid lubricants. The results show that under normal temperature conditions, the friction factor of WS2 solid lubricant is similar to that of MOS2, but the film formation state of WS2 coating on the metal substrate is not as good as MOS2 coating; under high-temperature conditions, the friction factor of WS2 based coating is stable and the friction performance Better than MOS2 solid lubricating surface.

In the United States, the aerospace industry mainly uses MoS2 as a solid lubricating material. This is because the United States is also a major producer of molybdenum and has the convenience of taking resources locally. Tungsten, compared to molybdenum, tungsten is superior to molybdenum in terms of heat resistance, physical and chemical stability, and radiation resistance so that WS2 lubricant will be a better choice.

 Luoyang Tongrun Info Technology Co., Ltd.is dedicated to the technology development of special oil additives, combined the Technology of nanomaterials developed dry lubricant and oil additives two series. Our products can significantly improve the performance of lubricating oil, improve energy efficiency, effectively protect the lubrication device and extend the oil change cycle, which can satisfy the lubrication oil constantly upgrading for high-end engine oil additives.

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