Basics of Tribology


Basics of Tribology

Expenditure on machine condition monitoring and maintenance constitutes a significant cost in a steel plant. Tribology helps in reducing this expenditure. Tribology is a new word coined by Dr. H. Peter Jost in England in 1966. ‘The Jost Report’, provided to the British Parliament – Ministry for Education and Science, indicated ‘Potential savings of over £ 515 million per year for industry by better application of tribological principles and practices’. But tribology is not a new field.

Tribology comes from the Greek word, ‘tribos’, meaning rubbing or to rub. And from the suffix, “ology” means the study of. Therefore, tribology is the study of ‘rubbing’, or ‘the study of things that rub’.

Tribology is the science and technology of interacting surfaces in relative motion. It is the study (Fig 1) of (i) friction, (ii) wear, and (iii) lubrication.

Study of tribology

Fig 1 Study of tribology

Tribology is the science and technology of interacting surfaces in relative motion and is commonly known as the study of friction, wear and lubrication. It is the science and technology of friction lubrication and wear and is of considerable importance in material and energy conservation. It is an old knowledge of great importance when it comes to everything in movement but as a scientific discipline tribology is rather new.

Tribology, although one of the oldest engineering discipline, it is one of the least developed classical sciences to date. The reason is that tribology is neither truly a single discipline nor well represented by steady state processes. It involves all the complexities of materials.

Tribology is multidisciplinary in nature, and includes mechanical engineering (especially machine elements as journal and roller bearings and gears), materials science, with research into wear resistance, surface technology with surface topography analysis and coatings, and the chemistry of lubricants and additives. The relatively younger disciplines of tribology are namely (i) bio-tribology, which includes (among other topics) wear, friction, and the lubrication of total joint replacement, and (ii) nano-tribology, where friction and wear are studied on the micro- and nano-scales.

In any machine there are lots of component parts that operate by rubbing together (bearings, gears, cams, tyres, brakes, piston ring etc.). Sometimes it is desirable to have low friction, to save energy, or high friction, as in the case of brakes. Tribology finds applications in all industrial sectors including steel industry.

Due to the technological advancements, material and energy conservation is becoming increasingly important. Wear i s a major cause of material wastage, so any reduction of wear can affect considerable savings. Friction is a principal cause of energy dissipation and considerable savings are possible by improved frictional control. Lubrication is the most effective means of controlling wear and reducing friction.

Tribology is the introduction of a substance between the contact surfaces of moving parts to reduce friction and to dissipate heat. The selection of the best lubricant and understanding the mechanism by which it acts to separate surfaces in a bearing or other machine components is a major area for study in tribology.

Lubrication is done to minimize friction between two interacting surfaces in relative motion. Friction occurs because a solid surface never microscopically smooth. Even the best machined surface has peaks and valleys called ‘roughness’. When two such surfaces come into contact, it is only the peaks on the surfaces that make actual contact. These contacts support the normal load and deform plastically and get cold welded. Depending upon the magnitude of the normal load more and more high spots or peaks come into contact and the ‘real area’ of contact increases in contrast to the ‘apparent area’, which is the geometrical area of the surfaces in contact. This phenomenon is called adhesion.

Friction is believed to be caused by this adhesion. When two such surfaces have to be moved in relation to each other, some force is needed to sheer these contacts. This force is called frictional force. Tribology helps in better visualizing conceptually the problems of friction, wear and lubrication involved in relative motion between surfaces.

Tribology is a complex science with small possibilities to theoretical calculations of friction and wear. Therefore, tribology is strongly associated with practical applications which make elaborative work and empirical experience valuable. The tribological properties are of utmost importance for the materials in contact and the system is sensitive to operating conditions and environment. To understand the tribological behaviour, knowledge in physics, chemistry, metallurgy and mechanics is necessary which makes the science interdisciplinary. By optimizing the friction and wear in technological applications, such as machine components or in metal working systems, both environment and costs can be saved.

Friction

Friction can be defined as the resistance to movement of a body against another and is of utmost importance in metal working operations. Friction is not a material parameter, but a system response in the form of a reaction force. It depends on e.g. temperature, moisture, load, mechanical properties and surface topography. Generally the law of friction, known as Amonton’s- Coulomb’s law, describes the friction coefficient (M) as the relationship between the frictional force Ft (tangential force) and the normal force Fn (load).

M = Ft/Fn

This law is assumed to be accurate in tribological contacts with ordinary contact pressures (as most of the contacts around are) and is often referred to as Coulomb’s friction. During the contact, friction can generally be divided into two components namely (i) the adhesive component (Ma) and the ploughing component (Mp).

M = Ma + Mp

The adhesive component is related to the materials in contact and is controlled by adhesive force acting at the areas of real contact, i.e. the asperities at the surfaces. The adhesive force originates from the force required to break the inter-surface bonds when the surfaces are sliding against each other. Hence, the adhesion of the two solids in contact is important and is depending on the chemistry of the tribo surfaces in the sliding interface.

The ploughing component originates from the deformation force acting during the ploughing of the softest material in contact by the surface asperities of the harder material and is related to the surface topography. Also, deformation hardened wear particles attached in the interface act in a ploughing way.

One additional part to the ploughing component is the asperity deformation which is related to the deformation of the asperities on micro level.  

Wear mechanisms

In tribological contacts, wear occurs due to the interaction between the two surfaces in contact and implies gradual removal of the surface materials, i.e. material loss. Wear of the materials in contact is, just as friction, a system parameter. The wear mechanisms of importance can be abrasive, adhesive, fatigue and tribo chemical wear. Typically there is a combination of wear mechanisms in a contact. An inter-relationship exists between friction and wear. Often a low friction results in low wear. However, this is not a general rule and there are numerous examples showing high wear rate despite low friction.

Adhesive wear 

Adhesive wear means damage resulting when two metallic bodies rub together without the deliberate presence of an abrasive agent.  Abrasive wear is characterized by damage to a surface by harder material introduced between two rubbing surfaces from outside. The severity of abrasive wear depends on size and angularity of abrasive particles and also the ratio between hardness of metal and the abrasive particles, more the tendency to wear.

Adhesive wear originates from the shearing contact between the severities of two solids in relative motion. During sliding elastic and plastic deformation of the asperities occur resulting in a contact area where the bonding forces give a strong adherence and the surfaces get welded together. The adhesive wear occurs when the tangential relative motion causes a separation in the bulk of the asperities in the softer material instead of in the interface and hence material is removed.

The real contact area consists of all the areas of welded asperities at the surfaces and during sliding the material removal results in wear that can be measured as a volume or weight decrease. However, it is more common to present the wear in a wear rate or wear coefficient. The wear rate is normally defined as the wear volume per sliding distance and load.

Abrasive wear

Abrasive wear provides significant plastic deformation of the surface material and occurs when one of the surfaces in contact is substantially harder than the other. This is known as two body abrasion. Abrasion also generally occurs when harder particles are introduced into the tribo system. This is known as three body abrasion when the particle is not attached to any surface and as two body abrasion when the particle is attached to one of the surfaces in contact. Consequently, the harder material of the two in contact can be abrasively worn. The sharp and hard asperities or particles are pressed into the softer surface which results in a plastic flow of the softer material around the harder. Due to the tangential movement the harder surface scratches the softer in a ploughing action, resulting in wear and remaining scratches or grooves. The abrasive wear can further be classified in different wear mechanisms such as micro cutting, micro fatigue and micro chipping. The abrasive wear rate is defined in the same way as for the adhesive wear.

Fatigue wear

Fatigue wear is essential in periodically loaded dies and tools, such as rolls. In loaded tools, the surface is in compression and shear stresses are generated below the surface. Repeated loading causes generation of micro cracks, usually below the surface, at a point of weakness, such as an inclusion or second phase particle. On subsequent loading and unloading, the micro crack propagates and voids coalesce. When the crack reaches a critical size, it changes direction to emerge the surface and a flat sheet-like particle is detached. This is also known as delamination wear or if the particle is relatively large it is known as spalling. When normal loading is combined with sliding, the location of maximum shear stress moves towards the surface and fatigue cracks may then originate from surface defects.

As all wear processes, fatigue wear is influenced by a large number of variables. To reduce fatigue wear, external and internal stress raisers are to be avoided and a strong interface between matrix and second phase particles is to be ensured. A further complication arises in hot working, where sudden heating results in surface expansion and generation of stresses between the surface and the bulk material. After contact, the cooling of the surface again induces stresses. In combination with stresses due to loading, thermal fatigue occurs resulting in a mosaic-like network of cracks called crazing or fire-cracking. Fatigue may also cause sudden catastrophic failure of the tool, such as complete failure of rolls.

Tribo chemical wear

In tribo chemical wear, the wear process is dominated by chemical reactions in the contact and the material is therefore consumed. Here, the environmental conditions in combination with mechanical contact mechanisms are of great importance. The chemical action, such as diffusion or solution, is not a wear mechanism on its own but is always in combination and interaction with other wear mechanisms. It may be more correct to talk about different mechanical wear mechanisms and consider the chemical effects as an additional influence parameter which changes the material properties of the surface in contact.

Tribo film formation

The high local temperatures and pressures obtained in the surface contact when two bodies are sliding against each other results in local shear deformation and fracture of the surfaces. The locally high temperatures may accelerate chemical reactions or melt the surfaces locally and wear occurs. However, these conditions do not necessarily have to be only destructive for the surfaces but may make it possible to form tribo film with new tribological properties. Usually tribo films are divided into two groups namely transformation type tribo film and deposition type tribo film. Both are changing the surface topography, chemistry and mechanical properties. In the formation of the transformation type tribo film, transformation of the original surface is obtained by plastic deformation, phase transformation, diffusion etc. without any material transfer. On the contrary the deposition type tribo film is only obtained by material transfer, i.e. by molecules from the counter surface, the environment or by wear debris. Accordingly, the surface topography, chemical reactivity and adherence may influence on the formation of a tribo film.

Lubrication

Lubricants are primarily used in order to lower both friction (and often consequently vibrations) and wear. They are agents introduced between two surfaces in relative motion to minimize friction. Selection and application of lubricants are determined by the functions they are expected to perform. The principal functions of lubricants are as follows.

  • To control friction
  • To control wear
  • To control temperature
  • To control corrosion
  • To remove contaminants
  • To form a seal (grease)

Lowering of friction can be made by two different mechanisms. If the lubricant completely separates the solid surfaces, the relative motion occur as a shear within the lubricant and the friction is consequently due to the shearing resistant of the lubricant. If the lubricant cannot completely separate the surfaces, the friction forces can be reduced when sliding occur between thin low friction films adsorbed to the surfaces. Lower friction also generates less heat, resulting in lower temperature.

Reduction of wear is also achieved by the separation – fully or partly – of the two solid surfaces. Wear is also reduced by the fact that the lubricant lowers the temperature, carries away possible wear particles and prevents contaminations from the surrounding.

Lubricant residues can be problematic in different production processes. For example, residues transferred from the tool onto a steel sheet can aggravate lacquering of car bodies. In some cases environmentally unfriendly detergents must be used to clean the surfaces. The lubricant itself can also be unsuitable in both health and environment aspect.

Lubricants can be either fluid or solid, but not necessarily an oil or grease. For example, also metals, oxides, sulphides, graphite etc. can act as lubricants. Following are the commonly known types.

  • Liquid Lubricants – Liquid minerals can be plain mineral oil, mineral oil plus additive, or synthetic lubricants
  • Quasi-solid lubricants (Grease)
  • Solid lubricants

Depending upon a typical application requirement a particular type of lubricant is chosen.

Liquid lubricants

Liquids are generally preferred as lubricants because they can be drawn between moving parts by hydraulic action. Apart from keeping the parts separated they also act as heat carriers. To select a liquid lubricant for a given application, primary consideration is normally the effect of temperature change on viscosity of the lubricant which is to  be minimum. Liquid Lubricants are in general be inert toward metal surfaces and other components.

Modern refining technology has made it possible to produce lubricants of good quality from a wide variety of crude oils. An oil refinery makes only the base lube oil stocks of different viscosities. They are unsuitable for direct consumption. Therefore, oils are mixed to attain right viscosity and additives are added to improve other qualities.

Synthetic liquid lubricants can be characterized as oily and neutral liquids. They are not obtained from petroleum crude oils. But they have almost similar properties as petroleum lubricants. These find application in situations where petroleum oils cannot be used. Some specific chemical classes of synthetic lubricants are di-esters, organo-phosphate esters, silicone polymers etc.

Important lubricant characteristics are described below.

Specific gravity is the ratio of the weight of a given volume of substance at 15 deg C to that of water.

Viscosity is a measure of the resistance of the oil to flow. The more the viscosity of the oil more is its resistance to flow. As an example water is less viscous and hence flows freely as compared to molasses which has a high viscosity and flows sluggishly. An ideal oil film on a bearing depends on selecting oil with the right viscosity to maintain separation of two metal surfaces.

The speed of the journal and viscosity are closely allied in maintaining a good oil film in the bearing. The slower the journal speed, the higher viscosity or thicker oil is required. As journal speeds are increased, a thinner of lower viscosity oil is needed.

Bearing loads are also to be considered since the oil must have sufficient viscosity to maintain a good oil film to support the load. Technically speaking, it is defined as the force required for moving a plane surface of one square centimeter area over another plane surface at the rate of one centimeter per second, when the two surfaces are separated by a layer of liquid one centimeter in thickness. The unit of this force is ‘poise’ and is called absolute viscosity.

Kinematic viscosity is the ratio of absolute viscosity to the specific gravity of the oil at the temperature at which the viscosity is measured. Its unit is ‘stokes’. For practical purposes, viscosity of petroleum oils is expressed in time in seconds taken by a given quantity of oil to flow through a standard capillary tube. It is expressed as Saybolt universal seconds at 40 deg C or at 100 deg C.

Viscosity index (VI) is an expression of effect of change of temperature on the viscosity of oils. This change can be evaluated numerically and the result is expressed as VI.

Pour point of oil is an important quality. It is a temperature at which oil still remains fluid. It reflects on the capability of the oil to work at low temperatures.

Flash point is the temperature at which the oil gives off sufficient vapours which can be ignited. It reflects on the capability of the oil to work at higher temperature without any fire hazard.

The purification and manufacturing processes impact good qualities to lubricating oils. But still they cannot be used directly. Lubricating oils are prone to contamination and decomposition in the exacting working conditions. Hence certain chemical compounds and other agents which are termed as additives are added to the oils. Most modern lubricant additives can be classified as (i) those designed to protect the lubricant in service by maintaining deterioration, (ii) those that protect the lubricant from harmful fuel combustion products, and (iii) those which improve existing physical properties or impart new characteristics.

Use of chemical additives in lubricants is very wide. They are used in the lightest instrument and spindle oils to the thickest gear lubricants, automotive lubricants, cutting oils, and hydraulic fluids. There are over 50 characteristics of lubricating base oils which can be improved by the additives. Generally speaking the additives must have the properties namely (i) solubility in base petroleum oil, (ii) insolubility in and lack of reaction with aqueous solution, (iii) should not impart dark colour to the oil, (iv) have low volatility, (v)  must be stable in blending, storage and use, and (vi) should not impart unfavourable odour.

Various types of additives used along with their purposes are as given below.

  • Anti-oxidant used to increase oil and machine life and to prevent oxidation
  • Corrosion inhibitor to protect against chemical attack of alloy bearings and metal surfaces.
  • Detergents for cleanliness of lubricated surfaces.
  • Rust inhibitor for eliminating rusting in presence of water and moisture
  • Pour depressant for improving the low-temperature fluidity
  • Viscosity index improver for lowering rate of change of viscosity with temperature change
  • Anti-foam agent to prevent stable foam formation
  • Extreme pressure agent for improving film strength and load carrying capacity

There are over 300 different lubricating oils of industrial and automotive types. These are normally classified as (i) spindle oils, (ii) gear oils, (iii) general bearing oils, (iv) electric motor oils, (v) steam cylinder oils, (vi) turbine oils, (vii) air compressor oils, (viii) refrigeration compressor oils, (ix) hydraulic oils, (x) cutting oils, and (xi) automotive oils. Each type of these oils has certain characteristics that make it well adapted for the given application.

Quasi-solid lubricants (grease)

Lubricating grease is a semi-solid lubricant. It is usually a mineral oil to which special soap is added to produce a plastic mixture. The soap is called thickener. Certain additives are also added as in the case of oils to impart special characteristics. Advantages of using greases are given below.

  • Less frequent application necessary. This results in saving in the cost of the lubricant and maintenance.
  • It acts as a seal against entrance of dirt and dust.
  • Dripping and splattering is almost eliminated.
  • Less expensive seals are required for grease lubricated bearings.
  • Grease ensures some lubrication even when a bearing is neglected for a long period.
  • Due to clinging property of grease, chances of rusting are considerably reduced in the bearings even when the machine is idle.

Primary components of grease are soaps and mineral oils. Soaps may be derived from animal or vegetable fats or fatty acids. In addition certain additives are also present. Sometimes fillers are also added to impart special characteristics.

Greases are classified by the soap compound used in their manufacture. The properties of greases are influenced considerably by the type of soap compound used in making the grease. The following are the common types grease available are (i) calcium base grease, (ii) sodium base grease, (iii) lithium base grease, and (iv) barium base grease.

A calcium base in grease gives the grease a smooth battery appearance. This grease is highly resistant to water. Edible fats such as palm oil or cotton seed oil hydrated lime are used to make soap. This grease requires addition of water as stabilizer. This cannot withstand a temperature above 80 deg C. It breaks down oil and soap and gets separated. The separated soap particles become hard and abrasive and cause scoring of bearings. Sodium base greases on the other hand, can be used where higher temperatures up to 120 deg C are encountered. The sodium base grease is fibrous in structure. This enables the grease to withstand high loads on ball and roller bearings. However, sodium base grease is less resistant to water.  Barium base grease is good up to 175 deg C and above. This grease has good water resistance. Lithium base grease is also suitable for high temperature application and has excellent water resistant properties. For low temperature also this grease is suitable.

To withstand very high temperatures and load conditions certain special greases are used since the soap based greases are not able to withstand such conditions. These are called non-soap base greases. Modified bentonites clay and silica gels are used with synthetic fluids. Some soap base greases are used with synthetic fluids instead of mineral oils. As in the case of oils, additives also are added to grease to impart special characteristics. Commonly used additives are antioxidants, corrosion inhibitors, EP agents, rust inhibitors and tackiness additives.

The two most vital characteristics of grease are consistency and drop point. Consistency is expressed in numbers in tenths of millimeter. Standard ASTM D217-52T test method is used to determine this property. It is called penetration test. The National Lubricating Grease Institute (NLGI) USA has classified grease into various classes based on their penetration readings determined from the above test. Drop point is defined as temperature at which grease changes from quasi-solid to a liquid state under prescribed conditions of a test. ASTM D566-42 test is used to determine drop point. This is used as a qualitative indicator of resistance to heat.

Solid lubricants

Solid lubricants are thin films of a solid interposed between two rubbing surfaces to reduce friction and wear. The need for solid lubricants has grown rapidly with advance in technology. Solid lubricants have the characteristics of low sheer strength, low hardness, high adhesion to substrate material, continuity, self-healing ability (The film is to reform immediately if broken), freedom from abrasive impurities, thermal stability, and chemical inertness. Various inorganic compounds like graphite, molybdenum disulphide, tungsten disulphide, boron nitride, and organic compounds like aluminum, zinc, sodium, lithium stearate and waxes are used as solid lubricants. Solid lubricants have found wide application where conventional petroleum oils have failed to work at extreme working conditions.