What is solid lubricant?

Solid lubricant is a kind of material that reduces the friction coefficient and wear by preventing direct contact of friction parts under regular use or high load. In exceptional cases where liquid lubricants such as oil and grease cannot be used, solid lubricants have an excellent application market. Besides, the solid lubricant is also an additive material for high-performance anti-bite and anti-wear coatings. It is often mixed with grease and lubricating oil in the form of powder particles to play a sliding role. This kind of special solid lubricant will slip when the friction pair slides relatively Fill out rough surfaces. Therefore, under extreme operating conditions, solid lubricant provides sufficient boundary lubrication conditions, thereby reducing the friction coefficient and reducing wear. Generally speaking, solid lubricants have a higher advantage than liquid oils under high vacuum conditions below 10^-2 Pa and near atmospheric pressure above 10⁴ Pa, and low temperatures below 0℃and high temperatures above 177℃.

There are five main types of solid lubricants: carbon-based materials, transition metal disulfides (such as MoS2 and WS2), polymers (such as PTFE lubricant), soft metal, and ceramic high-temperature lubricating materials. Among them, carbon-based materials and transition metal disulfides belong to the typical layered structure solid lubricant.

(1) Carbon-based lubricating material

Graphite is a typical carbon-based lubricating material with high-temperature resistance and corrosion resistance and is known for its reliable lubrication characteristics. Graphite has a layered solid with a hexagonal lattice. The atoms in the carbon base plane are bonded together by strong covalent bonds, while the base planes are combined by weak Van der Waals forces, resulting in a broken mechanical relationship between the aircraft. The presence of water vapor and oxygen in the environment can promote the shear movement between graphite crystal layers. When the base surface is worn, these active edges are neutralized (passivated) by adsorbing water or other vapors. Graphite can also maintain low friction. This is true for other carbon-based solid lubricants. Carbon-based lubricants also include various types of DLC coatings and low-friction carbon coatings (NFC).

2) Transition metal group disulfide (TMD) lubricating materials

In the TMD family, MoS2 and WS2 are widely used for their reliable lubrication performance. The primary mechanism for achieving low shear resistance at the interface is a base surface oriented parallel to the sliding direction and a transfer film formed on the friction couple. MoS2 and WS2 coatings do not oxidize or react with water vapor in a dry gas or ultra-high vacuum, thereby maintaining their reliable internal lubrication. However, in humid air, friction oxides such as MoO3 and WO3 may be formed due to the dangling or unsaturated bonds on the edge of the base surface reacting with water vapor or oxygen in the environment. At this time, the sliding friction coefficient is relatively high (0.15 ~ 0.2), and the wear life is also concise.

(3) PTFE lubricant:

Polymer solid lubricant is usually deposited in the form of a coating (film) on the surface of a substrate (such as polymer-based engine bearing cover). The molecular structure of these materials consists of long chains parallel to each other, and the bond strength between molecules Weak may slip under low shear stress. PTFE is typical of such solid lubricants. Unlike other solid lubricant, PTFE does not have a layered structure, but PTFE macro molecules easily slide, similar to a layered structure. The minimum static and dynamic coefficient of friction of PTFE can be reduced to 0.04. The PTFE-coated metal can be used under harsh temperature, pressure, and medium, and can achieve an extended protection period. Polymer-based nano composites, such as PTFE / nano Al2O3, also show excellent friction performance under specified test conditions.

(4) Soft metal lubricating materials

Due to low shear strength and high plasticity, some soft metals also have lubricating properties such as lead, tin, bismuth, indium, zinc, cadmium, and precious metals such as gold and silver. For example, Zn and Sn have been used in sliding bearings, while precious metals such as gold and silver have begun in the aerospace industry. In daily life, soft metals are widely used as solid lubricants in engine bearing materials.

Soft metals used as solid lubricant materials usually have a face-centered cubic lattice, so their crystals are all isotropic. Therefore, these soft metals do not have a high bearing capacity, and the external load can only be borne by the substrate. Another advantage of face-centered cubic crystals is that there is no low-temperature brittleness, and the lubricating properties will not be lost in low-temperature environments.

(5) High temperature resistant lubricating materials

Certain oxides, such as B2O3, MoO2, MoO3, ZnO, Re2O7, TiO2, V2O5, and PbO, etc., have lower melting points, soften at high temperatures, and have better properties. 

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