The ISO 6743 standard “Lubricants, industrial oils and related products (class L) — Classification” divides lubricant products into 18 groups, which are arranged according to the letters A to Z.
A：Total loss systems
B：Mould release
C：Gears
D：Compressors (including refrigeration and vacuum pumps)
E：Internal combustion engine
F：Spindle bearings,bearings and associated clutches
G：Slideways
H：Hydraulic systems
M：Metal working
N：Electrical insulation
P：Pneumatic tools
Q：Heat transfer
R：Temporary protection against corrosion
S：Applications of particular lubricants
T：Turbines
U：Heart treatment
X：Applications requiring grease
Y：Other applications
Z：Steam cylinders
Solid lubricants, although not historically long-established, offer good economic benefits, wide adaptability, and rapid development. They are particularly suited for extreme conditions such as high temperatures, high pressure, low speeds, high vacuum, and intense radiation. They are especially useful in situations where oiling is inconvenient or where assembly and disassembly are difficult. However, solid lubricants have drawbacks, such as higher friction coefficients and poor heat dissipation. Solid lubricants are typically categorized into inorganic and organic substances. The inorganic category includes graphite, molybdenum disulfide, oxides, fluorides, soft metals, and others, while the organic category includes polytetrafluoroethylene (PTFE), nylon, polyethylene, and polyimide. Based on their form, solid lubricants can be classified into powders, films, and self-lubricating composite materials.
Solid powders can be used dispersed in gases, liquids, or colloids. Films can be applied through spraying, vacuum deposition, flame spraying, ion plating, electrophoresis, and sintering, among other methods. Composite materials are produced using a variety of processes and represent an emerging class of important lubricants. Graphite, both natural and synthetic, has a hexagonal crystal layer structure. It is a black, soft material with high chemical stability, resistant to almost all organic solvents and corrosive chemicals, and it is not easily wetted by many molten metals or glass. As a result, graphite retains its properties when mixed with water, solvents, greases, rubber, resins, and some metals. Graphite has a low thermal expansion coefficient and elastic modulus, giving it resistance to thermal shock.
The operating temperature range for graphite is between -270°C and 1000°C in the atmosphere, with a melting point of 3500°C. It oxidizes at 450–500°C. Additionally, graphite has good electrical conductivity and thermal conductivity. Its crystallinity, impurities, particle size, and particle shape all significantly affect its lubricating properties, as do external conditions like ambient temperature, operating temperature, speed, and load. Graphite lubricants are often used in composite materials or in combination with other solid lubricants. Pure graphite is rarely used as a lubricant on its own, though water-based and oil-based graphite lubricants do exist.
Molybdenum disulfide (MoS₂) has a metallic gray-black sheen and a hexagonal layered crystal structure, similar to graphite. It has a very low friction coefficient, reaching as low as 0.04, and is thermally and chemically stable. MoS₂ operates at temperatures between -270°C and 350°C in atmospheric conditions, with a melting point of 1250°C, and it begins to oxidize at 380–450°C. It also resists corrosion from most acids. At room temperature and in humid air, MoS₂ undergoes slight oxidation, but this can result in a significant acid value. Generally, molybdenum disulfide is effective in heavy-load, low- to medium-speed, and high- (or low-) temperature sliding friction components. The purity of commercially available MoS₂ powder ranges from 98% to 99.8%. While pure MoS₂ powder is rarely used alone, it is often mixed with other substances. Common applications include MoS₂ paste lubricants and MoS₂-based greases.
Plastics like polytetrafluoroethylene (PTFE) exhibit excellent lubricating properties, shock absorption, impact resistance, corrosion resistance, and insulation. PTFE’s usable temperature range is between -270°C and 260°C.
Soft metals such as gold, silver, zinc, lead, and tin are also used as solid lubricants. They can be applied in two ways: one as thin films, particularly with low-melting-point metals like lead, zinc, and tin; and the other includes metals like copper and bronze, which, although not low-melting-point, are occasionally used in similar applications.
Liquid lubricants is the most widely used and diverse category of lubricants, including mineral oils, synthetic oils, animal and vegetable oils, and water-based liquids. Due to the wide range of viscosities available, liquid lubricants offer broad selection options for moving parts under various load, speed, and temperature conditions. Fluid lubricants provide low and stable friction coefficients, low compressibility, and can effectively carry away heat from friction surfaces, ensuring dimensional stability and precision of moving parts. Additionally, most of these products are inexpensive, leading to widespread use, with mineral oil currently being the most widely used liquid lubricant.
Water has excellent thermal conductivity, is abundant, and inexpensive, but its viscosity is too low, so thickeners or oiliness agents must be added. Water-based cutting fluids and water-ethylene glycol hydraulic fluids are widely used today and are considered promising lubricants. In the future, when global oil resources are depleted, these water-based fluids may become important alternatives to mineral oils.
Among animal and vegetable oils, vegetable oils such as rapeseed oil, tea seed oil, castor oil, peanut oil, and sunflower oil are the main types used. Their advantages include good oiliness and biodegradability, but they suffer from poor oxidative stability and thermal stability, as well as inadequate low-temperature performance. Currently, they remain important components of certain cutting fluids. As petroleum resources become scarcer and environmental regulations become stricter, there is renewed interest in developing and using animal and vegetable oils as lubricants. Researchers hope to improve their thermal oxidative stability and low-temperature performance through chemical means, making them a viable replacement for mineral oils in the future.
Synthetic oils were developed during World War II and include a wide range of compounds with different chemical structures and properties. They are often used under harsh conditions, initially for military applications, but have gradually been adopted in civilian industries. Synthetic lubricants are also seen as a key future alternative to mineral oils. In recent years, synthetic lubricants have gained wider recognition and use.
Gas Lubricants
Gases can act as lubricants just like oils, and the physical laws of fluid dynamic lubrication can also apply to gases. Since gases have very low viscosity, this means that the lubrication film is also very thin. Therefore, gas hydrodynamic bearings (gas dynamic bearings) are only suitable for high-speed, light-load, small-clearance, and highly precise tolerance conditions. For this reason, gas static bearings are more commonly used. They can support higher loads, have less stringent requirements for clearance and tolerance, and can be used at lower speeds or even at zero speed.
Gas lubrication can be used at much higher or lower temperatures than oil and grease, lubricating sliding bearings in the range of -200℃ to +2000℃. Its friction coefficient can be so low that it is difficult to measure, and bearing stability is very high. It provides high stiffness in high-speed precision bearings (such as medical dental drills, precision grinding machine spindles, and inertial navigation gyroscopes), without issues of sealing or contamination. However, the downside is that gas lubrication has a very low load-carrying capacity, and the design and manufacturing of bearings are extremely challenging. Bearings are also prone to surface damage during startup and shutdown. Common gas lubricants include air, helium, nitrogen, and hydrogen. These gases must be highly purified and carefully refined before use. Semi-solid lubricants are lubricants that are in a semi-fluid state under normal temperature and pressure, with a colloidal structure, known as greases. Greases are generally categorized into soap-based, hydrocarbon-based, inorganic, and organic greases. In addition to their anti-friction and wear-reducing properties, they also serve sealing and shock-absorbing functions, simplifying lubrication systems and reducing maintenance and operating costs. As a result, greases are widely used. However, their downsides include low fluidity, poor heat dissipation, and a tendency to undergo phase change or decomposition at high temperatures. Although grease production accounts for only about 2% of the total lubricant production, greases play a significant role in lubrication. Statistics show that about 90% of rolling bearings are lubricated with grease, and approximately 43% of rolling bearing failures are due to improper lubrication. Therefore, attention must be paid to grease quality and product composition.
Lithium-based greases are widely used due to their excellent properties. In advanced industrial countries, lithium-based greases typically make up more than 60% of their total grease production. In Taiwan,lithium-based grease production first reached over 60% of total production in 2000.
Comparison of four types of lubricants
1. Fluid Dynamic Lubrication Performance
Oil: Excellent
Grease: Average
Solid Lubricant: None
Gas: Good
2. Boundary Lubrication Performance
Oil: Poor to Excellent
Grease: Good to Excellent
Solid Lubricant: Good to Excellent
Gas: Poor
3. Cooling Performance
Oil: Very good
Grease: Poor
Solid Lubricant: None
Gas: Average
4. Low Friction
Oil: Average to Good
Grease: Average
Solid Lubricant: Poor
Gas: Excellent
5. Ease of Application to Bearings
Oil: Good
Grease: Average
Solid Lubricant: Poor
Gas: Good
6. Retention in Bearings
Oil: Poor
Grease: Good
Solid Lubricant: Excellent
Gas: Excellent
7. Sealing Capability
Oil: Poor
Grease: Excellent
Solid Lubricant: Average to Good
Gas: Excellent
8. Atmospheric Corrosion Resistance
Oil: Average to Excellent
Grease: Good to Excellent
Solid Lubricant: Poor to Average
Gas: Poor
9. Temperature Range
Oil: Average to Excellent
Grease: Good
Solid Lubricant: Excellent
Gas: Excellent
10. Volatility
Oil: Very high to low
Grease: Generally low
Solid Lubricant: Low
Gas: Very high
11. Flammability
Oil: Very high to very low
Grease: Generally low
Solid Lubricant: Generally low
Gas: Depends on gas type
12. Compatibility
Oil: Very high to average
Grease: Average
Solid Lubricant: Excellent
Gas: Generally good
13. Lubricant Price
Oil: Low to high
Grease: Quite high
Solid Lubricant: Quite high
Gas: Generally very low
14. Bearing Design Complexity
Oil: Quite low
Grease: Quite low
Solid Lubricant: Low to high
Gas: Very high
15. Life Determinant
Oil: Deterioration and contamination
Grease: Deterioration
Solid Lubricant: Wear
Gas: Maintenance of gas supply
Possible Solutions When Pure Mineral Oil Does Not Meet Bearing Requirements:
1、Excessive Load: Use higher viscosity oil, extreme pressure oil, grease, or solid lubricant.
2、High Speed (potentially causing high temperatures): Increase oil volume or circulation, use lower viscosity oil, or gas lubrication.
3、High Temperature: Use additives or synthetic oil, higher viscosity oil, increase oil volume or circulation, or use solid lubricant.
4、Low Temperature: Use lower viscosity oil, synthetic oil, solid lubricant, or gas lubrication.
5、Too Many Wear Particles: Increase oil volume or circulation.
6、Contamination: Use an oil circulation system, grease, or solid lubricant.
7、Longer Lifespan Requirement: Use higher viscosity oil, additives, or synthetic oil, with more oil or circulation lubrication.
The two main factors affecting the choice of lubricant type are speed and load.
