Engineering Tribology Episode 2 Part 9 ppsx

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

various test machines were also increased by the addition of suspended molybdenum

disulphide [49,50] Although in most cases 1% concentration by weight of molybdenum

disulphide in oil is sufficient, improvements were still obtained at higher concentrations

reaching 5% [49].

0 50 100 150

Sputtering

100V bias

200V bias Ion Plating

FIGURE 9.15 Comparison of the durability of a gold lubricant film produced by different

coating techniques under fretting conditions [46]

However, an increase in wear when molybdenum disulphide is added to oil has also beenreported [51] Under moderate conditions of sliding speed and load where molybdenumdisulphide is not expected to improve lubrication, abrasive impurities in the solid lubricantcan cause rapid wear [51] Silica in particular accentuates wear when in concentrations above

0.01%, and pyrites (iron sulphide) are also destructive [51] The quality, i.e cleanliness, of the

solid lubricant added to oil is therefore critical Although solid lubricant additives aresuitable for extremes of loads and speeds, they are not suitable for reducing wear undermoderate conditions Molybdenum disulphide suspensions provide a limited reduction infriction and wear when added to an oil containing sulphur based additives or zincdialkyldithiophosphate On the other hand, the presence of detergents or dispersants in theoil, such as calcium sulphonate, inhibits the lubricating action of molybdenum disulphide[48,50]

The mechanism of lubrication by molybdenum disulphide dispersed in oil has unfortunatelyreceived very little attention It is widely believed, however, that molybdenum disulphideprovides a complimentary role to surfactants Where there is a worn surface devoid ofsurfactant, it is hypothesized that molybdenum disulphide particles adhere to form alubricating film A conceptual model of solid lubrication by molybdenum disulphide whichoccurs only when there are no surfactants to block adhesion by lamellae of solid lubricant tothe worn surface is illustrated schematically in Figure 9.16

It has been found that molybdenum disulphide lubricates by film formation on a wornsurface at high temperatures where all surfactants, both natural and artificial, are unlikely toadsorb on worn surfaces [52] However, evidence which confirms that molybdenumdisulphide is only effective beyond the desorption temperature of the specific surfactants isabsent from the published literature

Solid lubricants are also used to improve the frictional characteristics of polymers [33] Ingeneral they do offer some improvement but the effectiveness of solid lubricants added topolymers depends on the type of polymer used The greatest improvements in polymerfriction and wear characteristics are achieved with polymers of moderate lubricity such asnylon and polyimide [53] For example, the addition of graphite to nylon results in a

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]

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