Analysis of Application Progress of Tungsten Disulfide Solid Lubrication

- Aug 04, 2020 -

Lubricating properties of tungsten disulfide

Tungsten disulfide (WS2) has excellent thermal stability and oxidation resistance, sizeable compressive strength, strong load-bearing capacity, good wetting with the substrate, and is used in high-tech fields such as aviation, aerospace, military, and automotive industries. The working temperature range of tungsten disulfide in the atmosphere is -270-650 ℃. The friction coefficients of the three types of solid lubricants with the same parts added in the grease are: graphite> molybdenum disulfide> tungsten disulfide; and the maximum non-seizure load PB is tungsten disulfide> molybdenum disulfide> graphite. Molybdenum disulfide is thermally oxidized and degraded in the air at 360 ℃ to form MO3 and SO2, while tungsten disulfide only begins to oxidize and degrade at about 45 thermally. The resulting WO3 is also a high-temperature lubricant.

Lubricating mechanism of tungsten disulfide film

In the hard and soft metal friction pair, the hard metal is pressed into the soft metal under the load, and the furrow causes the contact area to increase, thereby increasing the friction force. When the hard metals slide relative to each other, the yield strength is enormous, and the friction force is large. The friction surface heats up and causes bite. A layer of film with low shear strength is coated on the surface of the hard metal substrate, which neither increases the contact area of the friction pair and reduces the shear strength, reducing friction and friction coefficient. Furrows will occur. Therefore, a thin film with a low shear strength is adhered to the friction surface of the hard metal substrate for lubrication, reducing the friction coefficient and reducing wear.

Dispersion of nano tungsten disulfide

The smaller the particle size of WS2, the better the adsorption and the better the lubricating effect. However, particle agglomeration affects its lubricating performance. Physical or chemical methods can disperse the modification. Physical methods are used to disperse the nanoparticles in the liquid medium, and the particles will re-aggregate due to the intermolecular forces after the external forces are canceled. The chemical dispersion changes the surface properties of the particles, improves the interaction between the particles and the liquid medium, and the particles, enhances the repulsion between the particles, and permanently inhibits flocculation and agglomeration. Combining physical dispersion and chemical dispersion, using physical means to deagglomerate, chemical methods to maintain stable dispersion, to achieve a better dispersion effect. Besides, the surface charge distribution of the particles can be changed by the adsorption of the dispersant, resulting in electrostatic stability and steric stabilization to enhance the dispersion effect.

When untreated, the average particle size of the suspended particles in the lubricating oil reaches ten μm, and the nano tungsten disulfide particles are obviously agglomerated. Still, with the increase of the ultrasonic dispersion and mechanical stirring treatment time, the particle size of the suspended particles decreases, 5.5 h. After that, the average particle size reached below one μm. After treatment, the agglomeration of nano tungsten disulfide particles changes from large to small, and the minimum agglomeration particle size is close to the size of the primary particles. According to Stokes' law, the sedimentation speed of ultrafine particles in the liquid medium becomes Proportionally. The particle size of nano tungsten disulfide agglomerates is significantly reduced, which improves their dispersion stability in lubricating oil. With the increase of the mass ratio of surface modifiers and nano tungsten disulfide particles, the stabilization time of the particles in the lubricating oil increases first and then decreases. When the mass ratio reaches 0.4, the stabilization time is the longest. With the increase of the amount of surface modifier, the surface of nano tungsten disulfide particles is gradually modified, preventing the agglomeration between particles, so that the dispersion stability of particles in lubricating oil is improved; when the amount of surface modifier exceeds a specific value, the surface Excess modifier causes flocculation and agglomeration of particles. The mass ratio of surface modifier to nano tungsten disulfide is 0.4-0.5. After nano-tungsten disulfide microencapsulation, it can also significantly improve dispersion stability and wear resistance in dispersion media. Measures for microencapsulation include the use of sodium carboxymethyl cellulose, PMMA in situ, or epoxy resin.

The two methods of using WS2 powder are:

1) Combine WS2 powder with moist lubricant (such as oil, grease and other synthetic lubricants)

Most lubricating oils adopt organic or inorganic, liquid or solid lubricating oil additives to improve the lubricating performance. Green engine oil is environmentally friendly and can be used to replace mineral-based engine oil that is prone to environmental pollution. Still, its base oil is susceptible to oxidation failure at high temperatures. Nano tungsten disulfide is surface-modified under the action of ultrasonic waves to reduce friction and anti-wear in lubricating oils, even under high-temperature oxidation conditions. The green engine oil containing nano tungsten disulfide has more excellent extreme pressure anti-wear performance and viscosity-temperature characteristics.

Add quality score 3% of the nano-WS2 added to the finishing oil has higher oil film strength and lower friction coefficient than the unadded lubricating oil. Add quality score 0. 5% of WS2 powder is added to the emulsified oil cutting fluid, and the surface quality of the workpiece is increased by 35% on average. Superfine WS2 particles have undergone surface chemical modification and adsorption modification surface modification, and are added to synthetic oil and mineral oil. The friction factor decreases with time. The surface of the wear spot is smooth, and the diameter is small, indicating anti-wear and anti-friction performance.

2) Coat WS2 powder on the substrate that needs (dry) lubricity:

The powder can be applied by spraying (120 psi) the substrate with dry (and cooled) pneumatic air. It does not require any adhesive and can be sprayed at an average room temperature. The coating film will be 0.5 microns thick. In an alternative application method, the powder can also be mixed with isopropyl alcohol, and the paste can be polished onto the substrate. Coating applications have been established in many areas, such as automotive parts, racing engines and other parts, aerospace parts, bearings (linear, ball, roller, etc.), shafts, marine parts, cutting tools, blades, cutting machines, knives, peeling Molding agents, precision gears, valve components, pistons, chains, mechanical components, and many other fields.

The application of tungsten disulfide in reliable lubrication has expanded from powder filling and dispersion in lubricating grease to the high-end field of film application. The lubrication mechanism of tungsten disulfide solid lubrication film still needs to be explored. Improving the performance of tungsten disulfide solid lubricating film by constructing the crystal structure and grain boundary characteristics of the film layer will become the leading research direction in the future.

Infomak 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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