Engineering Tribology 2E Episode 11 pdf

TABLE 9.1 Available techniques for modifying the surface to improve its tribological Thin discrete coating; no limitations on materials Ion implantation Thin diffuse coating; mixing with

reduction of the coefficient of friction from 0.25 to 0.18 and a small reduction in wear [53] Onthe other hand, it has also been shown that molybdenum disulphide when added to nylonoxidizes during wear and does not develop an effective transfer film [54] Under theseconditions, the friction performance of nylon/molybdenum disulphide blend was found to

be inferior to plain nylon [54]

Adhesion blocked

by adsorbed films

Adsorbed film of surfactants

MoS2 present below desorption temperature MoS2 present above desorption temperature

Adhesion of lamella Inter-lamellar sliding

Desorbed surfactants

Inter-lamellar sliding inhibited

FIGURE 9.16 Conceptual model of the mechanism of lubrication by molybdenum disulphide

suspended in oil

In polyimides the addition of the same amount of graphite reduced the coefficient of friction

to less than half of pure polyimide and significantly reduced wear Although molybdenumdisulphide showed the same reduction of coefficient of friction as graphite/polyimide blendits reduction in wear rate was inferior to that of graphite/polyimide blend [53]

Improvements achieved by adding molybdenum disulphide and graphite topolytetrafluoroethylene (PTFE) are very limited [55,56] The coefficients of friction for PTFEfilled with graphite and molybdenum disulphide are very similar to that of unfilled PTFEand slightly lower than those obtained with most other fillers [55]

Interest in graphite has recently been extended by the incorporation of carbon fibres intopolymers Carbon fibres offer a unique combination of mechanical reinforcement andlubricity [57] It has been shown that a carefully formulated polyimide/carbon fibre compositecan sustain high contact loads and maintain a friction coefficient close to 0.2 at temperaturesreaching 300°C with very low wear rates [58,59]

9.3 WEAR RESISTANT COATINGS AND SURFACE TREATMENTS

Wear resistant coatings consist of carefully applied layers of usually hard materials which areintended to give prolonged protection against wear Abrasive wear, adhesive wear andfretting are often reduced by wear resistant coatings There are numerous methods ofapplying hard materials For example, sputtering and ion-plating are used in a similarmanner as in the deposition of solid lubricants to generate thin coatings Other methods areused to deposit very thick layers of hard material Applications of wear resistant coatings arefound in every industry, and for example, include mining excavator shovels and crushers[60], cutting and forming tools in the manufacturing industries [61], rolling bearings inliquefied natural gas pumps [62], etc In most of these applications, wear rather than friction isthe critical problem Another benefit of hard-coating technology is that a cheap substratematerial can be improved by a coating of an exotic, high-performance material Mostengineering items are made of steel and it is often found that some material other than steel

is needed to fulfil the wear and friction requirements Many wear resistant materials arebrittle or expensive and can only be used as a coating, so improved coating technology hasextended the control of wear to many previously unprotected engineering components

9.3.1 TECHNIQUES OF PRODUCING WEAR RESISTANT COATINGS

There are many different methods of applying wear-resistant or hard coatings to a metalsubstrate currently in use [e.g 63-65] New techniques continue to appear as every availabletechnology is adapted to deposit a wear resistant coating more efficiently The wear resistance

of a surface can also be improved by localized heat treatment, i.e thermal hardening, or byintroducing alloying elements, e.g nitriding or carburizing Many of these methods havebeen in use for many years but unfortunately suffer from the disadvantage that the substrateneeds to be heated to a high temperature Carburizing, nitriding and carbonitriding inparticular suffer from this problem Various coating techniques available with their principalmerits and demerits are listed in Table 9.1

TABLE 9.1 Available techniques for modifying the surface to improve its tribological

Thin discrete coating; no limitations on materials

Ion implantation Thin diffuse coating; mixing with substrate inevitable

Thick coatings; coating material must be able to melt Laser glazing and alloying

Electroplating

Friction surfacing Simple technology but limited to planar surfaces; produces thick

metal coating Explosive cladding Rapid coating of large areas possible and bonding to substrate is

good Can give a tougher and thicker coating than many other methods

Very thick coatings possible but control of coating purity is difficult Thermal spraying

Suitable for very thick coatings only; limited to materials stable at high temperatures; coated surfaces may need further preparation Surface welding

The thinner coatings are usually suitable for precision components while the thicker coatingsare appropriate for large clearance components

Coating Techniques Dependent on Vacuum or Gas at Very Low Pressure

Plasma based coating methods are used to generate high quality coatings without anylimitation on the coating or substrate material The basic types of coating processes currently

in use are: physical vapour deposition (PVD), chemical vapour deposition (CVD) and ionimplantation These coating technologies are suitable for thin coatings for precision

components The thickness of these coatings usually varies between 0.1 - 10 [µm] These

processes require enclosure in a vacuum or a low pressure gas from which atmosphericoxygen and water have been removed As mentioned already the use of a vacuum during acoating process has some important advantages over coating in air The exclusion ofcontaminants results in strong adhesion between the applied coating and substrate andgreatly improves the durability of the coating

· Physical Vapour Deposition

This process is used to apply coatings by condensation of vapours in a vacuum Theextremely clean conditions created by vacuum and glow discharge result in near perfect

adhesion between the atoms of coating material and the atoms of the substrate Porosity isalso suppressed by the absence of dirt inclusions PVD technology is extremely versatile.Virtually any metal, ceramic, intermetallic or other compounds which do not undergodissociation can be easily deposited onto substrates of virtually any material, i.e metals,ceramics, plastics or even paper Therefore the applications of this technology range from thedecorative to microelectronics, over a significant segment of the engineering, chemical,nuclear and related industries In recent years, a number of specialized PVD techniques havebeen developed and extensively used Each of these techniques has its own advantages andrange of preferred applications Physical vapour deposition consists of three majortechniques: evaporation, ion-plating and sputtering.

Evaporation is one of the oldest and most commonly used vacuum deposition techniques

This is a relatively simple and cheap process and is used to deposit coatings up to 1 [mm]

thick During the process of evaporation the coating material is vaporized by heating to a

temperature of about 1000 - 2000°C in a vacuum typically 10 -6 to 1 [Pa] [64] The source

material can be heated by electrical resistance, eddy currents, electron beam, laser beam or arcdischarge Electric resistance heating usually applies to metallic materials having a lowmelting point while materials with a high melting point, e.g refractory materials, needhigher power density methods, e.g electron beam heating Since the coating material is inthe electrically neutral state it is expelled from the surface of the source The substrate is also

pre-heated to a temperature of about 200 - 1600°C [64] Atoms in the form of vapour travel in

straight lines from the coating source towards the substrate where condensation takes place.The collisions between the source material atoms and the ambient gas atoms reduce theirkinetic energy To minimize these collisions the source to substrate distance is adjusted so

that it is less than the free path of gas atoms, e.g about 0.15 - 0.45 [m] Because of the low

kinetic energy of the vapour the coatings produced during the evaporation exhibit lowadhesion and therefore are less desirable for tribological applications compared to othervacuum based deposition processes Furthermore, because the atoms of vapour travel instraight lines to the substrate, this results in a ‘shadowing effect’ for surfaces which do notdirectly face the coating source and common engineering components such as spheres, gears,moulds and valve bodies are difficult to coat uniformly The evaporation process isschematically illustrated in Figure 9.17

Vacuum pump

Resistance heater

Coating

Substrate

Vapour Coating material (molten)

FIGURE 9.17 Schematic diagram of the evaporation process

Ion-plating is a process in which a phenomenon known as ‘glow discharge’ is utilized If anelectric potential is applied between two electrodes immersed in gas at reduced pressure, astable passage of current is possible The gas between the electrodes becomes luminescenthence the term ‘glow discharge’ When sufficient voltage is applied the coating material can

be transferred from the ‘source’ electrode to the ‘target’ electrode which contains thesubstrate The process of ion-plating therefore involves thermal evaporation of the coatingmaterial in a manner similar to that used in the evaporation process and ionization of thevapour due to the presence of a strong electric field and previously ionized low pressure gas,usually argon The argon and metal vapour ions are rapidly accelerated towards the substratesurface, impacting it with a considerable energy Under these conditions, the coating materialbecomes embedded in the substrate with no clear boundary between film and substrate.Usually prior to ion-plating the substrate is subjected to high-energy inert gas (argon) ionbombardment causing a removal of surface impurities which is beneficial since it results inbetter adhesion The actual coating process takes place after the surface of the substrate hasbeen cleaned However, the inert gas ion bombardment is continued without interruptions.This causes an undesirable effect of decreasing deposition rates since some of the depositedmaterial is removed in the process Therefore for the coating to form the deposition ratemust exceed the sputtering rate The heating of the substrate by intense gas bombardmentmay also cause some problems The most important aspect of ion-plating whichdistinguishes this process from the others is the modification of the microstructure andcomposition of the deposit caused by ion bombardment [65] Ion plating processes can beclassified into two general categories: glow discharge (plasma) ion plating conducted in a low

vacuum of 0.5 to 10 [Pa] and ion beam ion plating (using an external ionization source) performed in a high vacuum of 10 -5 to 10 -2 [Pa] [64] The ion-plating process is schematicallyillustrated in Figure 9.18

High voltage power supply

− +

Vacuum pump

Resistance heater

≈0.1 Pa argon gas

Coating

Substrate

Plasma Coating material (molten)

FIGURE 9.18 Schematic diagram of the ion-plating process

Sputtering is based on dislodging and ejecting the atoms from the coating material bybombardment of high-energy ions of heavy inert or reactive gases, usually argon Insputtering the coating material is not evaporated and instead, ionized argon gas is used todislodge individual atoms of the coating substance For example, in glow-discharge

sputtering a coating material is placed in a vacuum chamber which is evacuated to 10 -5 to 10 -3

[Pa] and then back-filled with a working gas, e.g argon, to a pressure of 0.5 to 10 [Pa] [64] The

substrate is positioned in front of the target so that it intercepts the flux of dislodged atoms.Therefore the coating material arrives at the substrate with far less energy than in ion-plating

so that a distinct boundary between film and substrate is formed When atoms reach thesubstrate, a process of very rapid condensation occurs The condensation process is critical tocoating quality and unless optimized by the appropriate selection of coating rate, argon gaspressure and bias voltage, it may result in a porous crystal structure with poor wearresistance

The most characteristic feature of the sputtering process is its universality Since the coatingmaterial is transformed into the vapour phase by mechanical (momentum exchange) rather

than a chemical or thermal process, virtually any material can be coated Therefore the mainadvantage of sputtering is that substances which decompose at elevated temperatures can besputtered and substrate heating during the coating process is usually negligible Althoughion-plating produces an extremely well bonded film, it is limited to metals and thuscompounds such as molybdenum disulphide which dissociate at high temperatures cannot

be ion-plated Sputtering is further subdivided into direct current sputtering, which is onlyapplicable to conductors, and radio-frequency sputtering, which permits coating of non-conducting materials, for example, electrical insulators In the latter case, a high frequencyalternating electric potential is applied to the substrate and to the ‘source’ material Thesputtering process is schematically illustrated in Figure 9.19

+

−

Vacuum pump

≈1 Pa argon gas

Deposition of dislodged atoms

FIGURE 9.19 Schematic diagram of the sputtering process

· Chemical Vapour Deposition

In this process the coating material, if not already in the vapour state, is formed byvolatilization from either a liquid or a solid feed The vapour is forced to flow by a pressuredifference or the action of the carrier gas toward the substrate surface Frequently reactant gas

or other material in vapour phase is added to produce a metallic compound coating Forexample, if nitrogen is introduced during titanium evaporation then a titanium nitridecoating is produced The coating is obtained either by thermal decomposition or chemicalreaction (with gas or vapour) near the atmospheric pressure The chemical reactions usually

take place in the temperature range between 150 - 2200°C at pressures ranging from 50 [Pa] to

atmospheric pressure [64] Since the vapour will condense on any relatively cool surface that

it contacts, all parts of the deposition system must be at least as hot as the vapour source Thereaction portion of the system is generally much hotter than the vapour source butconsiderably below the melting temperature of the coating The substrate is usually heated byelectric resistance, inductance or infrared heating During the process the coating material isdeposited, atom by atom, on the hot substrate Although CVD coatings usually exhibitexcellent adhesion, the requirements of high substrate temperature limit their applications tosubstrates which can withstand these high temperatures The CVD process at low pressureallows the deposition of coatings with superior quality and uniformity over a large substratearea at high deposition rates [64] The CVD process is schematically illustrated in Figure 9.20

· Physical-Chemical Vapour Deposition

This is a hybrid process which utilizes glow discharge to activate the CVD process It isbroadly referred to as ‘plasma enhanced chemical vapour deposition’ (PECVD) or ‘plasmaassisted chemical vapour deposition’ (PACVD) In this process the techniques of forming

solid deposits by initiating chemical reactions in a gas with an electrical discharge are utilized.Many of the phenomena characteristic to conventional high temperature CVD are employed

in this process Similarly the same principles that apply to glow discharge plasma insputtering apply to CVD In this process the coating can be applied at significantly lower

substrate temperatures, of about 100 - 600°C, because of the ability of high-energy electrons produced by glow discharge, at pressures ranging from 1 to 500 [Pa], to break chemical bonds

and thus promote chemical reactions Virtually any gas or vapour, including polymers, can

be used as source material [64] For example, during this process a diamond coating can beproduced from carbon in methane or in acetylene [88] Amorphous diamond-like coatings invacuum can attain a coefficient of friction as low as 0.006 [96] Although contamination by airand moisture tends to raise this coefficient of friction to about 0.02-0.07, the diamond-likecoating still offers useful wear resistance under these conditions [97-99] The mechanismresponsible for such low friction is still not fully understood The PECVD process isschematically illustrated in Figure 9.21

Resistance heater

Substrate Exhaust

FIGURE 9.20 Schematic diagram of the CVD process

RF generator or

DC power supply

− +

is known as ion implantation During the process of ion implantation, ions of elements, e.g.nitrogen, carbon or boron, are propelled with high energy at the specimen surface andpenetrate the surface of the substrate This is done by means of high-energy ion beams

containing the coating material in a vacuum typically in the range 10 -3 to 10 -4 [Pa] Aspecialized non-equilibrium microstructure results which is very often amorphous as theoriginal crystal structure is destroyed by the implanted ions [66] The modified near-surface

layer consists of the remnants of a crystal structure and interstitial implanted atoms Themass of implanted ions is limited by time, therefore compared to other surfaces, the layers of

ion-implanted surfaces are very shallow, about 0.01 to 0.5 [µm] The thickness limitation of

the implanted layer is the major disadvantage of this method The coatings generated by ionimplantation are only useful in lightly loaded contacts The technique allows for theimplantation of metallic and non-metallic coating materials into metals, cermets, ceramics oreven polymers The ion implantation is carried out at low temperatures Despite thethinness of the modified layer, a long lasting reduction in friction and wear can be obtained,for example, when nitrogen is implanted into steel The main advantage of the ionimplantation process is that the treatment is very clean and the deposited layers very thin,hence the tolerances are maintained and the precision of the component is not distorted Ionimplantation is an expensive process since the cost of the equipment and running costs arehigh [64] The ion implantation process is schematically illustrated in Figure 9.22

Ions Current

Filament:

coating element

Non-ionized material retained

Ion accelerator Ion separator

Electrostatic flow controller

Raster on substrate

Vacuum pump Magnets

Ionization

FIGURE 9.22 Schematic diagram of the ion implantation process

More detailed information about surface coating techniques can be found in [45,64,65]

Coating Processes Requiring Localized Sources of Intense Heat

A localized intense source of heat, e.g a flame, can provide a very convenient means ofdepositing coating material or producing a surface layer of altered microstructure Coatingmethods in common use that apply this principle are surface welding, thermal spraying andlaser hardening or surface melting

· Surface Welding

In this technique the coating is deposited by melting of the coating material onto thesubstrate by a gas flame, plasma arc or electric arc welding process A large variety of materialsthat can be melted and cast can be deposited by this technique During the welding process aportion of the substrate surface is melted and mixed together with the coating material in thefusion zone resulting in good bonding of the coating to the substrate Welding is used in avariety of industrial applications requiring relatively thick, wear resistant coatings ranging

from about 750 [µm] to a few millimetres [64] Welding processes can be easily automated and

are capable of depositing coatings on both small components of intricate shape and large flatsurfaces

There is a variety of specialized welding processes, e.g oxyfuel gas welding (OGW), shieldedmetal arc welding (SMAW), submerged arc welding (SAW), gas metal arc welding (GMAW),gas tungsten arc welding (GTAW), etc., which are described in detail in [e.g 64] A schematicdiagram of the typical welding process is shown in Figure 9.23.

Completed weld

Parent metal

Products of combustion protect weld pool

Filler wire

FIGURE 9.23 Schematic diagram of the welding process

· Thermal Spraying

This is the most versatile process of deposition of coating materials During this process thecoating material is fed to a heating zone where it becomes molten and then is propelled tothe pre-heated substrate Coating material can be supplied in the form of rod, wire or powder(most commonly used) The distance from the spraying gun to the substrate is in the range of

0.15 to 0.3 [m] [64] The molten particles accelerated towards the substrate are cooled to a

semimolten condition They splatter on the substrate surface and are instantly bondedprimarily by mechanical interlocking [64] Since during the process a substantial amount ofheat is transmitted to the substrate it is therefore water cooled There are a number oftechniques used to melt and propel the coating material and the most commonly applied are:flame spraying, plasma spraying, detonation-gun spraying, electric arc spraying and others.Flame Spraying utilizes the flame produced from combustion gases, e.g oxyacetylene andoxyhydrogen, to melt the coating material Coating material is fed at a controlled rate into the

flame where it melts The flame temperature is in the range of 3000 to 3500°C Compressed

air is fed through the annulus around the outside of the nozzle and accelerates the molten orsemimolten particles onto the substrate The process is relatively cheap, and is characterized

by high deposition rates and efficiency The flame sprayed coatings, in general, exhibit lowerbond strength and higher porosity than the other thermally sprayed coatings The process iswidely used in industry, i.e for corrosion resistant coatings A schematic diagram of thisprocess is shown in Figure 9.24

Plasma Spraying is different from the plasma-based coating methods described previouslysince the coating metal is deposited as molten droplets rather than as individual atoms orions The technique utilizes an electric arc to melt the coating material and to propel it as ahigh-velocity spray onto the substrate In this process gases passing through the nozzle areionized by an electric arc producing a high temperature stream of plasma The coatingmaterial is fed to the plasma flame where it melts and is propelled to the substrate The

temperature of the plasma flame is very high, e.g up to 30,000°C and can melt any coating

material, e.g ceramics [89] The highest temperatures are achieved with a monoatomic carriergas such as argon and helium Molecular gases such as hydrogen and nitrogen produce lowerplasma temperatures because of their higher heat capacity Therefore plasma spraying issuitable for the rapid deposition of refractory compounds which are usually hard in order toform thick hard surface coatings The very high particle velocity in plasma sprayingcompared to flame spraying results in very good adhesion of the coating to the substrate and

a high coating density The application of an inert gas in plasma spraying gives high purity,oxides free deposits Although it is possible to plasma spray in open air the oxidation of theheated metal powder is appreciable and the application of inert gas atmosphere isadvantageous The quality of coating is critical to the wear resistance of the coating, i.e.adhesion of the coating to the substrate and cohesion or bonding between powder particles inthe coating must be strong These conditions often remain unfulfilled when the coatingmaterial is deposited as partially molten particles or where the shrinkage stress on cooling isallowed to become excessive [67] Plasma spraying is commonly used in applicationsrequiring wear and corrosion resistant surfaces, i.e bearings, valve seats, aircraft engines,mining machinery and farm equipment A schematic diagram of the plasma spraying process

Plating Powder feed

of coating material

Plasma flame Spark

Water cooling

Water cooling

Ar, He, H2 , N2

FIGURE 9.25 Schematic diagram of the plasma spraying process

Detonation-Gun Spraying is similar in some respects to flame spraying The mixture of ametered amount of coating material in a powder form with a controlled amount of oxygenand acetylene is injected into the chamber where it is ignited The powder particles are heated

and accelerated at extremely high velocities towards the substrate where they impinge Theprocess is repeated several times per second The coatings produced by this method exhibithigher hardness, density and adhesion (bonding strength) than can be achieved withconventional plasma or flame spraying processes The coating porosity is also very fine.Unfortunately very hard materials cannot be coated by this process because the high velocitygas can cause surface erosion Wear and corrosion resistant coatings capable of operating atelevated temperatures are produced by this method They are used in applications whereclose tolerances must be maintained, i.e valve components, pump plungers, compressorrods, etc A schematic diagram of this process is shown in Figure 9.26.

Semimolten spray stream

FIGURE 9.26 Schematic diagram of the detonation gun spraying process

Electric Arc Spraying differs from the other thermal spraying processes since there is noexternal heat source such as a gas flame or electrically induced plasma [64] In this process anelectric arc is produced by two converging wire electrodes Melting of the wires occurs at thehigh arc temperature and molten particles are atomized and accelerated onto the substrate bythe compressed air The use of an inert atomizing gas might result in improvedcharacteristics of some coatings by inhibiting oxidation The wires are continuously fed tobalance the sprayed material Since there is no flame touching the substrate like in the otherthermal spraying processes, the substrate heating is lower The adhesion achieved during thisprocess is higher than that of flame sprayed coatings under comparable conditions Duringthis process coatings of mixed metals, e.g copper and stainless steel, can be produced Aschematic diagram of this process is shown in Figure 9.27

Semimolten spray stream

Water-cooled substrate

Electric arc

· Laser Surface Hardening and Alloying

Laser hardening is a form of thermal hardening where a high power laser beam, such as

from a carbon dioxide laser (with the beam power up to 15 [kW]), is scanned over a surface to

cause melting to a limited depth Rapid cooling of the surface by the unheated substrateresults in a hard quenched microstructure with a fine grain size formed on re-solidification[68,69] Surface alloying is also possible if the surface of the substrate is pre-coated with thealloying element or the alloying element is fed into the path of the laser beam This process isalso known as laser cladding The coating material is mixed together with the melted toplayer of the substrate and subsequently solidifies Because of the very large temperaturegradients mixing of the molten material is intense A strong bond between the modifiedlayer and the substrate is formed since the substrate is never exposed to any atmosphericcontaminants For example, a stainless steel layer on a steel substrate can be produced by pre-coating steel with chromium and then melting the surface with the laser beam To produce a

500 [µm] thick layer of 1% stainless steel, a pre-coating of 5 [µm] thick chromium is required.

Although laser treatment can be performed in the open air the oxidation rate, e.g of steel,can be high and destructive Therefore it is often preferable to apply this process in an inertgas atmosphere The process is particularly useful in applications where the access to thesurface to be treated is more easily achieved by the laser than any other method, e.g a torch.The area coverage by this process is relatively slow and the overlap areas between successivelaser passes have inferior properties and microstructure [89] A schematic diagram of lasersurface alloying is shown in Figure 9.28

Molten pad

up to 0.5 mm deep Mixing

Substrate

Precoating Quenched alloyed layer

High power laser

FIGURE 9.28 Schematic diagram of the laser surface alloying process

Coating Processes Based on Deposition in the Solid State

It would be very convenient to directly join the coating material and substrate withoutintermediate processes such as plasma-based coating Under certain circumstances this ispossible although there are some comparatively severe limitations on the utility of suchmethods Two basic methods of direct joining or bonding are explosive bonding and frictionsurfacing These two methods do not require a carefully controlled environment or alocalized heat source and can be performed in the open air

Friction Surfacing is an adaptation of friction welding where a material from a rod is bonded

to a flat surface by a combination of rotation and high contact force It was discovered that ifthe flat surface was moved while the rod was pressed against it and simultaneously rotatedthen a layer of transferred material was deposited on the flat surface This constituted arelatively simple way of rapidly depositing a thick layer of metal [70] Friction surfacing hasbeen studied as a simple and robust way of re-surfacing worn military and agriculturalequipment in remote areas such as the interior of Australia [70] A major simplification ofthis coating technology compared to other coating methods is that there is no necessity forthe exclusion of atmospheric oxygen during the coating process However, the provision of

an inert gas atmosphere does improve adhesion or bonding between the coating and the

substrate [70] Shape limitations of the substrate, i.e that friction surfacing is only practicablefor plane surfaces or objects with axial symmetry, e.g metal extrusions, as opposed tocomplex surfaces, e.g gear teeth, restricts the application of this otherwise promising andsimple coating technology A schematic diagram of friction surfacing process is shown inFigure 9.29a.

Explosive Cladding, also known as explosive bonding or explosive welding, is essentially asolid-phase welding process during which bonding is produced by high velocity collisionbetween the substrate and coating material The high velocity is achieved by controlledexplosion In most cases, the coating material in the form of a sheet is placed at a small angle

of incidence to the substrate A protective buffer, usually in the form of rubber sheet, is placed

on top of the coating material When the explosives in the form of sheet or slurry aredetonated behind the buffer, contact between the sheet of coating material and the substratespreads out from the end of the sheet closest the substrate A front forms at the edge of thecontact where the sheet is momentarily folded Strong bonding of the cladding material isfacilitated by the expulsion of contaminants and oxide layers as a jet of fragmented or moltenmaterial in front of the impacting metal surfaces The removal of contaminants and oxides iscaused by the extremely high impact speed of the opposing surfaces during explosivebonding At the apex of the front, substrate and coating material melt Since the metal flowaround the collision point is unstable and oscillating it often produces a rippled or wavyinterface between the substrate and the coating material No external heat is required in thisprocess Virtually any combination of metals and alloys, which otherwise cannot be bonded,

e.g aluminium and steel, can be bonded by this process Very high pressures of about 3 [GPa]

generated during the process restrict the thickness of the coatings to the layers thicker than

0.3 [mm] as thinner layers could rupture The process is used in manufacturing, e.g corrosion

resistant coatings for chemical, marine and petrochemical industries The inconvenience ofexplosives, the limitation of this method to large flat surfaces and the requirement for thecoating material to be tough does, however, severely curtail the usefulness of this technique

A schematic diagram of the explosive bonding process is shown in Figure 9.29b

Detonator

Detonation Velocity

Explosive Coating metal Gap Substrate

Jet of molten metal oxides etc.

Bonded Explosive cladding

Friction surfacing

Coating metal

Hot plastic metal

Substrate

Contact force Rotation

Frictional heat

Deposited coating Heat affected zone

Expelled contaminants

Miscellaneous Coating Processes

There is a wide range of coating processes which are extensively used for applicationsrequiring resistance to corrosion and mild wear These coating processes are very muchsimpler and cheaper than the processes already described For example, coatings can bedeposited by dipping the substrate in a coating material, spraying the coating material in anatomized liquid form, e.g the technique commonly used for paint applications, by utilizingbrush pad or roller, by chemical deposition or electroplating Although the adhesion of thesecoatings is sometimes not adequate for severe tribological applications they can be used ascorrosion resistant coatings and as soft, low shear strength solid lubricant and metalliccoatings for sliding wear applications

Electroplating is a well established process with proven benefits in controlling corrosion andwear resistance This process is a convenient way of applying coatings of metals with highmelting points such as chromium, nickel, copper, silver, gold, platinum, etc., onto thesubstrate The electroplating system consists of an electrolytic bath, two electrodes and a DCpower source A conducting solution which contains a salt or other compound of the metals

to be deposited is placed in the bath When an electrical potential is applied to the electrodes,i.e one is the material to be coated the other is the donor electrode, the metal is deposited onthe substrate by electrochemical dissolution from the donor electrode as schematicallyillustrated in Figure 9.30 Since, in general, the process is conducted under atmosphericconditions and material is deposited with low energy, the coating-substrate adhesion is poor.Coatings can be applied by this method to most metallic surfaces

Electron flow

Electrolyte e.g.MSO4

Coating metal (anode)

Coated metal

(cathode)

Dashed line shows original shape

FIGURE 9.30 Schematic diagram of the electroplating process

Application of Coatings and Surface Treatments in Wear and Friction Control

There is a wide range of coating techniques and careful selection of the appropriate coatingmaterial and method is a pre-requisite for an effective coating Prior to selecting the coatingmaterial and method the first question to be asked is whether wear or friction is of greaterconcern If the prime objective is to reduce friction then a solid lubricant coating should beselected and the coating method will, in most cases, be either sputtering or a combination ofpainting and baking

To suppress wear by the application of coatings, it is first necessary to determine themechanism of wear occurring, e.g whether abrasive wear or some other form of wear ispresent Although most coatings can suppress several forms of wear, each type of coating ismost effective at preventing a few specific wear mechanisms Therefore during the selectionprocess of the most effective coating to suppress wear in a particular situation, i.e coatingoptimization, the prevailing wear mechanism must first be recognized and assessed Thebasic characteristics of the coatings which can be achieved by the methods described in theprevious section in terms of wear control are summarized in Figure 9.31

It can be seen from Figure 9.31 that while the optimization of a coating to resist abrasive wear

is relatively simple, i.e it is sufficient to produce a thick hard surface layer with toughnesshigh enough to prevent coating fracture, other wear mechanisms require much greater care

FIGURE 9.31 Basic characteristics of coatings in terms of wear control

Characteristics of Wear Resistant Coatings

Studies of wear resistant coatings reveal that hard coatings are most effective in suppressingabrasive wear An example of this finding is illustrated in Figure 9.32 which shows the wearrate of a pump rotor as a function of the hardness of the coating applied to the surface It can

be seen from Figure 9.32 that the abrasive wear rate declines to a negligible value once a PVDcoating of titanium nitride, which is characterized by extremely high hardness, is employed

In this example abrasive wear was caused by very fine contaminants present in the pumpedfluid and the size of the abrasives was sufficiently small for a thin PVD coating to be effective

In other applications where the abrasive particles are much larger, thicker coatings are moreappropriate

0.5 1 2 5 10 20

Vickers hardness of rotor [kg/mm2]

Nitriding

TiN PVD Tuftride treatment

FIGURE 9.32 Example of the resistance of a hard coating, TiN, to abrasion [60]

It was also found that thin films of ceramics such as titanium nitride are quite effective insuppressing adhesive wear in poorly lubricated and high stress contacts For example, when acutting tool is coated with titanium nitride, adhesion and seizure between the tool and metalchip does not occur even when cutting is performed in a vacuum [71] Titanium nitridecoatings were also applied to gears and the scuffing tests on coated and uncoated gearsrevealed that the critical load and scuffing resistance for coated gears is much higher [71,72].This coating also reduces the coefficient of friction in unlubricated sliding as well as wearrates, e.g coefficients of friction close to 0.1 between titanium nitride and zirconium nitridecoatings on hardened bearing steel have been observed [73] Unfortunately titanium nitridecoatings do not provide corrosion resistance [74] Since zirconium and hafnium belong to thesame IVB group of the periodic table of chemical elements as titanium, some similarity inwear properties of their compounds can be expected In fact, hafnium nitride was found togive the best wear resistance performance in tests on cutting tools [75] Zirconium nitride isalso extremely useful as a coating [73,76] It should also be mentioned that for hard coatings to

be effective an adequate substrate hardness is essential [63] Therefore hardened steels andmaterials such as stellite are generally used as a substrate for this type of coating

Fretting wear can be mitigated by the use of hard coatings, e.g carbides, especially at smallamplitudes of fretting movement [77] However, at higher fretting amplitudes, spalling of thecarbide coatings renders them ineffective

Coatings produced by ion implantation, in certain applications, can also provide largereductions in wear Since the coatings produced by this technique are very thin they are onlyeffective in reducing wear at low load levels as illustrated in Figure 9.33

It is, however, difficult to give general rules for the applicability of the ion implantationtechnique since the results are only specific to a particular combination of substrate andimplanted material Since, there are about ten substrate metals in common use, e.g steel, castiron, aluminium, copper, titanium, etc., and theoretically the entire periodic table of

elements is available for implantation, the number of combinations of test materials farexceeds available resources and time Currently the most commonly implanted elements are:nitrogen, carbon and boron and the conclusions about the usefulness of ion implantationmay change as ‘new’ implantation elements are discovered, e.g combined yttrium andnitrogen implantation [81].

In some cases the incorrect choice of implantation material for coating may actually result inincreased wear rates It has been shown that implantation of stainless steel by argon increasedthe coefficient of friction from 0.8 to 1.0 in dry unlubricated sliding at room temperaturewhile implantation by boron reduced the coefficient of friction to about 0.15 [82] The causes

of wear reduction by ion implantation are largely unknown

It has been hypothesized that ion implantation causes surface hardening, passivation andloss of adhesion [79] Although the hardening is effective for pure metals, hardened alloyssuch as martensitic steel are not hardened by ion implantation [79]

An interesting feature of nitrogen ion implantation is that the effect of implantation persistsafter wear has exceeded the depth of implantation, in some cases by a factor of 10 or more [79]

It was observed that nitrogen migrated inwards with the wear surface [83] As with othercharacteristics of ion implantation this phenomenon is also poorly understood

Surface hardening by high power lasers also results in reductions in wear for a wide range ofapplications For example, the flank and rake faces of a cutting tool made of high speed steelshowed less wear after laser hardening [84] However, the reductions in wear achieved by theapplication of laser hardening are not as dramatic as those obtained by nitride coatings Onthe other hand, laser hardening has been found to be very beneficial in the unlubricatedsliding of cast irons [85,86] This effect is illustrated in Figure 9.34 where laser hardened castiron exhibits superior wear behaviour to untreated cast iron

Eutectic flake

Hypoeutectic flake

Nodular cast iron

Severe wear

Mild wear

As-cast Laser melted

FIGURE 9.34 Effect of laser hardening on the wear rates of various cast irons [85]

It can be seen from Figure 9.34, that the transition from mild to severe wear is suppressed bylaser hardening and the mitigation of wear at high loads is clear

Laser surface alloying has also been found to effectively reduce wear under fretting Forexample, a zirconium alloyed layer formed on the surface of carbon steel caused a reduction

in the volume of fretting wear by at least a factor of 5 [87].

The performance of non-metallic coatings such as tungsten carbide used for rolling elements

is related to the operating conditions For example, it was found that 100-200 [µm] thick

plasma-sprayed coatings on steel and ceramic balls fail by surface wear when lubrication ispoor or by sub-surface delamination when lubrication is effective [93].

Wear-resistant coatings can be as vulnerable to oxidative wear as monolithic metalsubstrates For example, copper causes rapid wear of cutting tools coated with titaniumnitride, titanium carbide or a combination of both compounds It was found that the primarycause of rapid wear of the titanium nitride and carbide coatings is a catalytic effect of copper

on the oxidation of the nitride and carbide to titanium oxide which is then rapidly wornaway In contrast, the oxidation of chromium nitride in air is much slower than titaniumnitride [94], thus permitting the chromium nitride to effectively protect machining toolsfrom wear by copper [95]

Solid lubricants and surface treatments have rapidly evolved in recent decades from simpleand traditional methods to extremely sophisticated technologies These developments arepart of an effort to eliminate the limitations imposed by oil-based lubrication and in theprocess are changing the general perception of the limits of wearing contacts Knowledge ofthe mechanisms behind these improvements in lubrication and wear resistance is, in mostcases, very limited The methods employed in most studies on surface coatings are empiricaland there is relatively little information available on which surface treatment is the mostsuitable for a particular application This is a new area subjected to extensive research and thenumber of new surface treatments and coating technologies available to control friction andwear is rapidly increasing

REFERENCES

1 W.L Bragg, Introduction to Crystal Analysis, G Bell and Sons, London, 1928.

2 F.P Bowden and D Tabor, Friction and Lubrication of Solids, Part 1, Oxford University Press, 1954.

3 A.I Bailey and J.S Courtney-Pratt, Real Contact and Shear Strength of Monomolecular Layers, Proc Roy Soc., London, Series A, Vol 227, 1955, pp 500-515.

4 F.P Bowden and D Tabor, Friction and Lubrication of Solids, Part 2, Oxford University Press, 1964.

5 R.F Deacon and J.F Goodman, Lubrication by Lamellar Solids, Proc Roy Soc., London, Series A, Vol 243,

11 M.N Gardos, The Synergistic Effects of Graphite on the Friction and Wear of Molybdenum Disulphide Films

in Air, Tribology Transactions, Vol 31, 1988, pp 214-227.

12 J.W McCain, A Theory and Tester Measurement Correlation About Molybdenum Disulphide Dry Film

Lubricant Wear, Journal of Society of Aerospace, Material and Process Engineers, Vol 6, 1970, pp 17-28.

13 J.K Lancaster, Lubrication by Transferred Films of Solid Lubricants, ASLE Transactions, Vol 8, 1965, pp

146-155.

14 A.J Haltner and C.S Oliver, Frictional Properties of Some Solid Lubricant Films Under High Load, Journal

of Chemical Engineering Data, Vol 6, 1961, pp 128-130.

15 W.J Bartz and J Xu, Wear Behaviour and Failure Mechanism of Bonded Solid Lubricants, Lubrication Engineering, Vol 43, 1987, pp 514-521.

16 G Salomon, A.W.J de Gee and J.H Zaat, Mechano-Chemical Factors in Molybdenum Disulphide Film

Lubrication, Wear, Vol 7, 1964, pp 87-101.

17 J.W Midgley and D.G Teer, An Investigation of the Mechanism of the Friction and Wear of Carbon,

Transactions ASME, Journal of Basic Engineering, Vol 85, 1963, pp 488-494.

18 W.J Bartz, Tribological Behaviour of Three Component Bonded Solid Lubricant Films, Proc JSLE Int Tribology Conference, 8-10 July, 1985, Tokyo, Japan, Elsevier, pp 419-424.

19 M Campbell and V Hopkins, Development of Polyimide Bonded Solid Lubricants, Lubrication Engineering,

Vol 23, 1967, pp 288-294.

20 R.D Hubbel and B.D McConnell, Wear Behaviour of Polybenzimidazole Bonded Solid-Film Lubricants,

Transactions ASME, Journal of Lubrication Technology, Vol 92, 1970, pp 252-257.

21 P.W Centers, The Role of Oxide and Sulfide Additions in Solid Lubricant Compacts, Tribology Transactions,

27 P.A Grattan and J.K Lancaster, Abrasion by Lamellar Solid Lubricants, Wear, Vol 10, 1967, pp 453-468.

28 R.L Fusaro and H.E Sliney, Graphite Fluoride, a New Solid Lubricant, ASLE Transactions, Vol 13, 1970, pp.

56-65.

29 R.L Fusaro, Graphite Fluoride Lubrication: the Effect of Fluorine Content, Atmosphere and Burnishing

Technique, ASLE Transactions, Vol 22, 1977, pp 15-24.

30 P.M Magie, A Review of the Properties and Potentials of the New Heavy Metal Derivative Solid

Lubricants, Lubrication Engineering, Vol 22, 1966, pp 262-269.

31 W.E Jamison and S.L Cosgrove, Friction Characteristics of Transition-Metal Disulfides and Di-Selenides,

ASLE Transactions, Vol 14, 1971, pp 62-72.

32 S Hironaka, M Wakihara, H Einode, M Taniguchi, T Moriuchi and T Hanzawa, Lubricity of Synthetic Niobium Sulfides With Layer Structure, Proc JSLE International Tribology Conf., 8-10 July 1985, Tokyo, Japan, Elsevier, pp 389-394.

33 F.J Clauss, Solid Lubricants and Self-Lubricating Solids, Academic Press, New York, 1972.

34 A.J Haltner, Sliding Behavior of Some Layer Lattice Compounds in Ultrahigh Vacuum, ASLE Transactions,

Vol 9, 1966, pp 136-148.

35 H.E Sliney, T.N Strom and G.P Allen, Fluoride Solid Lubricants for Extreme Temperatures and Corrosive

Environments, ASLE Transactions, Vol 8, 1965, pp 307-322.

36 R.A Burton and J.A Russell, Forces and Deformations of Lead Films in Frictional Processes, Lubrication Engineering, Vol 21, 1965, pp 227-233.

37 Y Tsuya and R Takagi, Lubricating Properties of Lead Films on Copper, Wear, Vol 7, 1964, pp 131-143.

38 S Miyake and S Takahashi, Small-Angle Oscillatory Performance of Solid-Lubricant Film-Coated Ball

Bearings for Vacuum Applications, ASLE Transactions, Vol 30, 1987, pp 248-253.

39 H Kondo, K Maeda and N Tsushima, Performance of Bearings With Various Solid Lubricants in High Vacuum and High Speed Conditions, Proc JSLE, International Tribology Conf., 8-10 July 1985, Tokyo, Japan, Elsevier, pp 787-792.

40 H.E.Sliney, The Use of Silver in Self-Lubricating Coatings for Extreme Temperatures, A S L E Transactions,Vol 29, 1986, pp 370-376.

41 C Della Corte and H.E Sliney, Composition Optimization of Self-Lubricating Chromium-Carbide Based

Composite Coatings for Use to 760°C, ASLE Transactions, Vol 30, 1987, pp 77-83.

42 M.B Petersen, S.F Murray and J.J Florek, Consideration of Lubricants for Temperatures Above 1000°F, ASLE Transactions, Vol 2, 1960, pp 225-234.

43 R.M Gresham, Solid-Film Lubricants: Unique Products for Unique Lubrication, Lubrication Engineering, Vol.

44, 1988, pp 143-145.

44 T Spalvins, Friction Characteristics of Sputtered Solid Film Lubricants, NASA TM X-52819, 1970.

45 D.H Buckley, Surface Effects in Adhesion, Friction, Wear and Lubrication, Elsevier, Amsterdam 1981.

46 N Ohmae, T Tsukizoe and T Nakai, Ion-Plated Thin Films for Anti-Wear Applications, Transactions ASME, Journal of Lubrication Technology, 1978, Vol 100, pp 129-135.

47 P.J Pacholke and K.M Marshek, Improved Worm Gear Performance With Colloidal Molybdenum Disulfide

Containing Lubricants, Lubrication Engineering, Vol 43, 1987, pp 623-628.

48 W.J Bartz and J Oppelt, Lubricating Effectiveness of Oil-Soluble Additives and Molybdenum Disulfide

Dispersed in Mineral Oil, Lubrication Engineering, Vol 36, 1980, pp 579-585.

49 J Gansheimer, Influence of Certain Vapours and Liquids on the Frictional Properties of Molybdenum

Disulphide, ASLE Transactions, Vol 10, 1967, pp 390-399.

50 W.J Bartz, Influence of Extreme-Pressure and Detergent-Dispersant Additives on the Lubricating

Effectiveness of Molybdenum Disulphide, Lubrication Engineering, Vol 33, 1977, pp 207-215.

51 J.P Giltrow, Abrasion by Impurities in MoS2 , Part 2 - Practical Tests and the Development of a Quality

Control, Tribology International, Vol 7, 1974, pp 161-168.

52 R.J Rolek , C Cusano and H.E Sliney, The Influence of Temperature on the Lubricating Effectiveness of MoS2

Dispersed in Mineral Oils, ASLE Transactions, Vol 28, 1985, pp 493-502.

53 J.K Lancaster, Dry Bearings: a Survey of Materials and Factors Affecting Their Performance, Tribology, Vol.

6, 1973, pp 219-251.

54 W Liu, C Huang, L Gao, J Wang and H Dang, Study of the Friction and Wear Properties of MoS2-filled

Nylon 6, Wear, Vol 151, 1991, pp 111-118.

55 K Tanaka and S Kawakami, Effect of Various Fillers on the Friction and Wear of

Polytetrafluoroethylene-Based Composites, Wear, Vol 79, 1982, pp 221-234.

56 M.B Low, The Effect of the Transfer Film on the Friction and Wear of Dry Bearing Materials for a Power

Plant Application, Wear, Vol 52, 1979, pp 347-363.

57 J.P Giltrow and J.K Lancaster, Friction and Wear of Polymers Reinforced with Carbon Fibres, Nature (London), Vol 214, 1967, pp 1106-1107.

58 R.L Fusaro, Polyimides Formulated from a Partially Fluorinated Diamine for Aerospace Tribological Applications, NASA, TM-83339, 1983.

59 R.L Fusaro and W.F Hady, Tribological Properties of Graphite-Fiber-Reinforced Partially Fluorinated

Polyimide Composites, ASLE Transactions, Vol 29, 1986, pp 214-222.

60 S Asanabe, Applications of Ceramics for Tribological Components, Tribology International, Vol 20, 1987, pp.

355-364.

61 E Bergmann, J Vogel and R Brink, Criteria for the Choice of a PVD Treatment for the Solution of Wear Problems, Proc Int Tribology Conference, Melbourne, The Institution of Engineers, Australia, National Conference Publication No 87/18, December, 1987, pp 65-74.

62 M Tomaru, Application of Surface Treatment on Rolling Bearings, Transactions JSLE, Vol 31, 1986, pp

66 A.E Berkowitz, W.G Johnston, A Mogro-Campero, J.L Walter and H Bakhru-Suny, Structure and Properties Changes During Ion Bombardment of Crystalline Fe75B25 , Metastable Materials Formation by IonImplantation, Proceedings of the Materials Research Society, Annual Meeting, November 1981, Boston, U.S.A., editors: S.T Picraux and W.J Choyke, Elsevier Science Publ Co New York, 1982, pp 195-202.

67 M.M Mayuram and R Krishnamurthy, Tribological Characteristics of Sprayed Surfaces, Proc Int Tribology Conference, Melbourne, The Institution of Engineers, Australia, National Conference Publication No 87/18, December, 1987, pp 203-207.

68 S.T Picraux and E.L Pope, Tailored Surface Modification by Ion Implantation and Laser Treatment, Science,

Vol 224, 1984, pp 615-622.

69 C.T Walters, A.H Clauer and B.P Fairand, Pulsed Laser Surface Melting of Fe-Base Alloys, Proc 2nd Int Conf on Rapid Solidification Processing: Principles and Technologies, 23-26 March 1980, editors: R Mehrabian, B.H Kear and M Cohen, Reston, Virginia, U.S.A., Baton Rouge, Claitor's Publ Division, L.A.,

72 Y Terauchi, H Nadano, M Kohno and Y Nakamoto, Scoring Resistance of TiC and TiN-Coated Gears,

Tribology International, Vol 20, 1987, pp 248-254.

73 S Ramalingam and S Kim, Tribological Characteristics of Arc Coated Hard Compound Films, Proc Int Tribology Conference, Melbourne, The Institution of Engineers, Australia, National Conference Publication

76 R.G Duckworth, Hot Zirconium Cathode Sputtered Layers for Useful Surface Modification, First

International Conference on Surface Engineering, Brighton U.K., 25-28 June 1985, Paper 42, pp 167-177, Publ.

Welding Institute, Abingdon, U.K.

77 R.C Bill, Fretting Wear and Fretting Fatigue, How are They Related?, Transactions ASME, Journal of Lubrication Technology, Vol 105, 1983, pp 230-238.

78 G Dearnaley, The Ion-implantation of Metals and Engineering Materials, Transactions Institute of Metal Finishing, Vol 56, 1978, pp 25-31.

79 C.J McHargeu, Ion Implantation in Metals and Ceramics, International Metals Reviews, Vol 31, 1986, pp

83 E Lo Russo, P Mazzoldi, I Scotoni, C Tosello and S Tosto, Effect of Nitrogen-Ion Implantation on the

Unlubricated Sliding Wear of Steel, Applied Physics Letters, Vol 34, 1979, pp 627-629.

84 Ming-Jen Hsu and P.A Mollian, Cutting Tool Wear of Laser-Surface-Melted High Speed Steels, Wear, Vol.

127, 1988, pp 253-268.

85 P.W Leech, Comparison of the Sliding Wear Process of Various Cast Irons in the Laser-Surface-Melted and

as-Cast Forms, Wear, Vol 113, 1986, pp 233-245.

86 W.J Tomlinson, R.F O'Connor and T.A Spedding, Running-in Wear of a Grey Cast Iron and the Effect of Laser

Transformation Hardening, Tribology International, Vol 21, 1988, pp 302-308.

87 A.W Batchelor, G.W Stachowiak, G.B Stachowiak, P.W Leech and O Reinhold, Control of Fretting Friction and Wear of Roping Wire by Laser Surface Alloying and Physical Vapour Deposition Coatings,

Wear, Vol 152, 1992, pp 127-150.

88 J.C Angus, Diamond and Diamond-Like Films, Thin Solid Films, Vol 216, 1992, pp 126-133.

89 B.C Oberlander and E Lugscheider, Comparison of Properties of Coatings Produced by Laser Cladding and

Conventional Methods, Materials Science and Technology, Vol 8, 1992, pp 657-665.

90 J.K Lancaster, Transfer Lubrication for High Temperatures, A Review, Trans ASME Journal of Tribology,

Vol 107, 1985, pp 437-443.

91 T Kayaba, K Kato and H Ohsaki, The Lubricating Properties of Friction-Coating Films of Pb-Sn Alloys in High Vacuum, Proc JSLE International Tribology Conference, Tokyo, 8-10, July 1985, Vol 1, Japan Society of Lubrication Engineers, Tokyo, 1985, pp 209-214.

92 A.W Batchelor, N.L Loh and M Chandrasekaran, Lubrication of Stellite at Ambient and Elevated

Temperatures by Transfer Films from a Graphite Slider, Wear, Vol 198, 1996, pp 208-215.

93 R Ahmed and M Hadfield, Rolling Contact Performance of Plasma Sprayed Coatings, Wear, Vol 220, 1998,

pp 80-91.

94 H Ichimura and A Kawana, High Temperature Oxidation of Ion-Plated CRN Films, Journal of Materials Research, Vol 9 (1), 1994, pp 151-155.

95 T Sato, T Besshi, D Sato and K Inouchi, Evaluation of Wear and Tribological Properties of Coatings

Rubbing Against Copper, Wear, Vol 220, 1998, pp 154-160.

96 C Donnett, M Belin, J.C Auge, J.M Martin, A Grill and V Patel, Tribochemistry of Diamond-Like Carbon

Coatings in Various Environments, Surface and Coatings Techn., Vol 68/69, 1994, pp 626-631.

97 K.-H Habig, Fundamentals of Tribological Behaviour of Diamond, Diamond-Like Carbon and Cubic Boron

Nitride Coatings, Surface and Coatings Techn., Vol 76/77, 1995, pp 540-547.

98 E.I Meletis, A Erdemir and G.R Fenske, Tribological Characteristics of DLC Films and Duplex plasma

Nitriding/DLC Coating Treatment, Surface and Coatings Techn., Vol 73, 1995, pp 39-45.

99 K Holmberg, J Koskinen, H Ronkainen, J Vihersalo, J.-P Hirvonen and J Likonen, Tribological

Characteristics of Hydrogenated and Hydrogen-Free Diamond-Like Carbon Coatings, Diamond Films and Techn., Vol 4, 1994, pp 113-129.

a strong influence on friction and wear The imperfections and features of a real surfaceinfluence the chemical reactions which occur with contacting liquids or lubricants while thevisible roughness of most surfaces controls the mechanics of contact between the solids andthe resulting wear The study of surfaces is relatively recent and the discoveries so far giverise to a wide range of questions for the technologist or tribologist, such as: what is theoptimum surface? Is there a particular type of optimum surface for any specific application?Why are sliding surfaces so prone to thermal damage? How can wear particles be formed byplastic deformation when the operating loads between contacting surfaces are relatively verylow? Although some of these questions can be answered with the current level ofknowledge, the others remain as fundamental research topics The characteristics of frictionare also of profound importance to engineering practice Seemingly mundane phenomena,such as the difference between static and kinetic friction, are still not properly understoodand their control to prevent technical problems remains imperfect The basic question: what

is the mechanism of ‘stick-slip’?, i.e the vibration of sliding elements caused by a largedifference between static and kinetic friction, has yet to be answered In this chapter, thenature of solid surfaces, contact between solids and its effects on wear and friction arediscussed

10.2 SURFACES OF SOLIDS

At all scales of size, surfaces of solids contain characteristic features which influence friction,wear and lubrication in a manner independent of the underlying material There are twofundamental types of features of special relevance to wear and friction:

· atomic-scale defects in a nominally plain surface which provide a catalytic effect forlubricant reactions with the worn surface;

· the surface roughness which confines contact between solids to a very smallfraction of the nominally available contact area