Just as there are various states of matter, there are also different types of lubricants. The four main categories are gas, liquid, viscous (grease), and solid. In industrial lubrication, we deal with fluids and greases, but certain operating environments make them unsuitable—in such cases, we often turn to solid lubricants.
For oils and greases, we typically rely on the formation of a fluid film to separate two surfaces in relative motion. When the load is too high, or the speed is too low, we enter a mixed and boundary state and rely on additives such as anti-wear agents and extreme pressure agents to protect the machine surface.
Solid lubricants only work within boundary limits. Their lubrication mechanism is based on their ability to form a thin film on the surface of moving parts. This film (sometimes called a "tribofilm") acts as a barrier between surfaces and helps reduce adhesive wear. Since these particles have very low shear strength, they also reduce friction between surfaces.
Understanding the friction-reducing properties of solid lubricants helps to understand their underlying structure. Most solid lubricants are layered solid materials composed of atomically thin planes. It's like the word salad, so above is a picture to give you a better idea of what that means.
A single layer (in the case of MoS2, three atoms thick) is held together only by weak van der Waals forces and is easily sheared by relative motion. The movement is like a person running across a floor covered with playing cards.
The thickness of the friction film is a key factor in the lubrication process. Thin films provide a low coefficient of friction and reduce wear, whereas a film that is too thick can result in increased friction and wear.
The thickness of the friction film may be affected by various factors, including the type of solid lubricant used, the load on the moving parts, and the environment in which the lubrication occurs.
There are several common options when choosing a solid lubricant or a grease containing a solid lubricant. It is helpful to understand the properties of each method and their advantages and disadvantages:
Graphite is a naturally occurring form of carbon. It is a good lubricant for high-temperature applications and has a low coefficient of friction. It works like a low-friction contact between a graphite pencil and paper, as a single layer of graphite (technically now called "graphene") shears away.
Graphite is unusual in that it requires ambient gas to form a tribofilm. As Dr. Juan Preciado Flores discusses in Episode 9 of the Lubrication Expert Podcast, "Carbon requires a passive layer to have a low coefficient of friction.
This is why the CoF (Coefficient of Friction) might be 0.4 when doing a pin-on-disk test in a dry environment. If you add a little water or increase the humidity in the room, your coefficient of friction might be 0.2 because you're helping the friction film by adding reactive gases.
It is also highly conductive, which may be beneficial in slip ring contact applications but may be detrimental elsewhere.
Molybdenum disulfide (often referred to as "molybdenum") is a compound that is a good lubricant for high-temperature and high-pressure applications and has a low coefficient of friction. It is mined from sulfide-rich deposits and then refined to a purity suitable for lubricants.
MoS2's high-temperature performance is limited to 400 °C, which is the limit of excessive oxidation of the molecule. Water has the opposite effect on MoS2 compared to graphite, as it often reacts with molecules, increasing the shear strength between the layers and, thus, the friction coefficient.
BN is a compound composed of boron and nitrogen. It is an excellent lubricant for high-temperature and high-pressure applications. It has a low coefficient of friction and is highly resistant to thermal shock. BN is commonly used in applications such as furnace door lubrication and high-temperature chemical environments where MoS2 is not used.
Boron nitride comes in many forms (called "polytypes"). Hexagonal boron nitride (h-BN) has a layered structure like graphite, while cubic boron nitride (c-BN) is similar to diamond and is used in wear applications.
In each layer of h-BN, boron, and nitrogen atoms are held together by strong covalent bonds, but weak van der Waals forces mean the layers are not firmly bonded to each other. This gives it low shear strength, which is typical of other solid lubricants.
Fluoropolymers (usually PTFE in lubricants) are the exception to this rule. PTFE molecules do not have a low-shear layer but slide easily past each other because the outer fluorine atoms are packed very tightly together. PTFE can generally be used up to a temperature limit of 260°C and has good resistance to reactive chemicals.
Unmixed solid lubricants have a wide range of uses in industries where liquid lubricants and greases cannot be used. The aerospace industry has long relied on dry lubricant coatings for spacecraft, satellites and lunar/Mars landers because liquid lubricants can boil in the extreme space environment. Application methods vary, but solid lubricants are often ionized and sputtered onto equipment surfaces.
In industrial settings, we often see greases containing solid lubricants as additives. This is especially true in high-load, low-speed applications typical of the mining and construction industries, where lubricated gears, bushings and even rolling element bearings are unable to form a fluid film.
They provide excellent wear protection and can withstand the harsh operating conditions of these industries. In some high washdown scenarios, we may rely on residual solid lubricant that survived the spray water to lubricate the bucket pin before relubricating the element.
You may also have encountered the use of molybdenum and boron in liquid lubricants. They are usually present in soluble forms such as molybdenum dithiocarbamates or borate esters and act as multifunctional additives, providing some anti-wear protection and antioxidant properties.
Soluble molybdenum compounds provide their anti-wear function by decomposing into molybdenum disulfide, which then adheres to the load surface and performs the same function as a solid lubricant.
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