A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles. The property of reducing friction is known aslubricity. (Slipperiness)
A good lubricant possesses the following characteristics:
- High boiling point
- Low freezing point
- High viscosity index
- Thermal stability
- Hydraulic Stability
- Corrosion prevention
- High resistance to oxidation
One of the single largest applications for lubricants, in the form of motor oil, is protecting the internal combustion engines in motor vehicles and powered equipment.
Typically lubricants contain 90% base oil (most often petroleum fractions, called mineral oils) and less than 10% additives. Vegetable oils or synthetic liquids such as hydrogenated polyolefins,esters, silicones, fluorocarbons and many others are sometimes used as base oils. Additives deliver reduced friction and wear, increased viscosity, improved viscosity index, resistance tocorrosion and oxidation, aging or contamination, etc.
Lubricants such as 2-cycle oil are added to fuels like gasoline which has low lubricity. Sulfur impurities in fuels also provide some lubrication properties, which has to be taken in account when switching to a low-sulfur diesel; biodiesel is a popular diesel fuel additive providing additional lubricity.
Non-liquid lubricants include grease, powders (dry graphite, PTFE, Molybdenum disulfide, tungsten disulfide, etc.), PTFE tape used in plumbing, air cushion and others. Dry lubricants such as graphite, molybdenum disulfide and tungsten disulfide also offer lubrication at temperatures (up to 350 °C) higher than liquid and oil-based lubricants are able to operate. Limited interest has been shown in low friction properties of compacted oxide glaze layers formed at several hundred degrees Celsius in metallic sliding systems, however, practical use is still many years away due to their physically unstable nature.
Another approach to reducing friction and wear is to use bearings such as ball bearings, roller bearings or air bearings, which in turn require internal lubrication themselves, or to use sound, in the case of acoustic lubrication.
In addition to industrial applications, lubricants are used for many other purposes. Other uses include cooking (oils and fats in use in frying pans, in baking to prevent food sticking), bio-medicalapplications on humans (e.g. lubricants for artificial joints), ultrasound examination, internal examinations for males and females, and the use of personal lubricant for sexual purposes.
Base oil groups
Mineral oil term is used to encompass lubricating base oil derived from crude oil. The American Petroleum Institute (API) designates several types of lubricant base oil
- Group I – Saturates <90% and/or sulfur >0.03%, and Society of Automotive Engineers (SAE) viscosity index (VI) of 80 to 120 Manufactured by solvent extraction, solvent or catalytic dewaxing, and hydro-finishing processes. Common Group I base oil are 150SN (solvent neutral), 500SN, and 150BS (brightstock)
- Group II – Saturates over 90% and sulfur under 0.03%, and SAE viscosity index of 80 to 120 Manufactured by hydrocracking and solvent or catalytic dewaxing processes. Group II base oil has superior anti-oxidation properties since virtually all hydrocarbon molecules are saturated. It has water-white color.
- Group III – Saturates > 90%, sulfur <0.03%, and SAE viscosity index over 120 Manufactured by special processes such as isohydromerization. Can be manufactured from base oil or slax wax from dewaxing process.
- Group IV – Polyalphaolefins (PAO)
- Group V – All others not included above such as naphthenics, PAG, esters. The lubricant industry commonly extends this group terminology to include:
- Group I+ with a Viscosity Index of 103–108
- Group II+ with a Viscosity Index of 113–119
- Group III+ with a Viscosity Index of at least 140
Can also be classified into three categories depending on the prevailing compositions:
Lubricants for internal combustion engines contain additives to reduce oxidation and improve lubrication. The main constituent of such lubricant product is called the base oil, base stock. While it is advantageous to have a high-grade base oil in a lubricant, proper selection of the lubricant additives is equally as important. Thus some poorly selected formulation of PAO lubricant may not last as long as more expensive formulation of Group III+ lubricant.