Dry lubricant is a kind of material that reduces the friction coefficient and reduces wear by preventing the friction parts from directly contacting under regular use or high load. Besides, the dry lubricant is also a key additive material for high-performance anti-seize agents and anti-wear coatings. It is often mixed in the form of powder particles in grease and lubricating oil to play a sliding role. The additives will fill up the rough surface of the accessories when the friction pair slides relatively. Therefore, under extreme operating conditions, dry lubricants provide sufficient boundary lubrication conditions, thereby achieving a reduced friction coefficient and reduced wear. Generally speaking, the advantages of solid lubricants are more evident than liquid lubricants under the terms of high vacuum below 10-2 Pa and near atmospheric pressure above 104 Pa, the low temperature below 0℃, and high temperature above 177℃.
Typical dry lubricant additives
Dry lubricant additives mainly fall into four categories: (1) carbon-based materials (such as graphite, DLCs, and nanocrystalline diamond); (2) transition metal disulfides (such as Mo S2 and WS2); (3) polymers (such as polytetrafluoroethylene) Fluoroethylene PTFE); (4) Ceramic high-temperature lubricating materials (such as metal oxides, metal fluorides, and sulfates). The first and second types of solid lubricating materials belong to the typical layered structure dry lubricant.
(1) Carbon-based lubricating material
Graphite is a typical carbon-based dry lubricant, which has the characteristics of high-temperature resistance and corrosion resistance.
And is famous for its reliable lubrication characteristics. The atoms in the carbon-based planes are bonded together by strong covalent bonds, and the base planes are combined by weak van der Waals forces, resulting in weak mechanical bonding between the aircraft. The presence of water vapor and oxygen in the environment can promote graphite.
Shear movement between crystal layers. When the base surface is worn, these active edges are neutralized (passivated) by adsorbing water or other steam. Graphite can also maintain low friction. This is for other carbon-based solid lubricants. Carbon-based lubricating materials also include various types of DLC coatings. Unlike graphite, DLCs usually present a typical short-range ordered amorphous phase, which is a mixture of sp3 type tetrahedral structure (diamond hybrid) and sp2 type trigonal structure ( Graphite hybrid), this feature reflects the material has excellent friction and mechanical properties such as low friction coefficient, flat wear rate, high hardness, and high elastic modulus. Another type of DLC coating, the so-called low-friction carbon coating (NFC), can reach the lowest coefficient of friction (0.005) among known materials. In the sliding resistance of the NFC-NFC friction pair, the NFC coating is No adhesion transfer film is formed on the part, but the passivation of the contact surface by hydrogenated carbon atoms results in an ultra-low friction coefficient.
(2) Transition metal disulfide (TMDs) lubricating materials
In the TMD family, MoS2 and WS2 are widely used for their reliable lubricating properties. The primary mechanism for achieving low shear resistance at the interface is parallel to sliding The direction-oriented base surface and the transfer film formed on the friction coupler.
MoS2 and WS2 coatings will not oxidize or react with water vapor in dry gas or ultra-high vacuum, thus maintaining their reliable internal lubrication. However, in humid air, friction oxides such as MoO3 and WO3 may be formed due to the dangling edges of the base surface or the reaction of unsaturated bonds 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) Polymer lubricating materials
Polymer dry lubricant is usually deposited on the surface of the substrate in the form of a coating (film) (such as polymer-based engine bearing caps). The molecular structure of these materials is composed of long chains parallel to each other, and the intermolecular bonding strength is weak. It may slip under stress.
PTFE is typical of this type of dry lubricant. Unlike other dry lubricants, PTFE does not have a layered structure, but it is easy to slide between PTFE macromolecules, similar to a layered structure. The minimum static and dynamic friction coefficient of PTFE can be reduced to 0.04. The metal treated with PTFE coating can be used under harsh conditions of temperature, pressure, and media, and can achieve an extended protection period.
（4）High temperature resistant dry lubricant
Some oxides, such as B2O3, MoO2, MoO3, ZnO, Re2O7, TiO2, V2O5, and PbO, have a low melting point and soften at high temperatures, and have better properties The anti-friction ability. PbO has better lubricating performance than MoS2 in the temperature range of 480℃～850℃. Many metal fluorides and their compounds have excellent lubricating properties at high temperatures, such as CaF2, BaF2, LiF, NaF, and CeF3, and LaF3 have excellent lubricating properties at 500-1000℃.
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