E L S E V I E R Journal of Materials Processing Technology 48 (1995) 503--515

Journal of Materials Processing Technology

T R I B O L O G Y I N M A T E R I A L S P R O C E S S I N G

Andrew W. Batchelor 1 and Gwidon W. Stachowiak 2

1 Department of Mechanical and Production Engineering Nanyang Technological University; 2 University of Western Australia, Department of Mechanical and Materials Engineering.

Friction and wear affect all processes involved in the extraction of materials and their convers ion into f inished products. Physical contact be tween tool, die, c lamp or any other device that contacts the processed material is the basic cause of wear. Excessive friction imposes limits on the efficiency of cutting tools, dies and many other equipment. Wear is a severe problem in the extraction and pr imary processing of raw materials and even the conveyance of raw materials from mine site to refinery imposes addit ional problems of wear. Therefore research and development into means of controlling friction and wear in materials processing is actively pursued by many research groups. The most well established method to control friction and wear is by the application of lubricants. Al though the deve lopment of solid and liquid lubricants has greatly advanced materials processing it still do not give an ideal performance. Lubricants also br ing pol lut ion and heal th hazards . Two types of substitutes for lubricants are being developed: advanced materials such as ceramics to replace metals for the construction of tools, dies etc.; surface coatings to provide wear resistant and low friction coat ings wi thout the need for lubricants. Projected benefi ts from these newer technologies are low levels of friction and wear, economy in the use of expensive hard metals, less pollution and toxicity hazards. In this paper current developments into friction and wear control in materials processing are reviewed.

INTRODUCTION

In mos t s tages of mater ia ls processing, contact occurs between the tool or processing m a c h i n e r y a n d the p rocessed mater ia l . Exceptions to this rule include, the use of high pressure water jets to ei ther drill holes in a material or in a larger form strip ore from a mine wall. Conveyance of components by jets of air or by flotation is another situation where contact is avoided. The majority of processing ope ra t i ons h o w e v e r involve a solid tool con tac t ing a solid process mater ia l wi th a t tendant wear and friction. Wear and friction are usually a hindrance to materials processing operations as they result in (i) damage to tools, (ii) i nc reased e n e r g y c o n s u m p t i o n , (iii) contaminat ion of processed material by wear particles and (iv) problems associated with technologies to control friction and wear. Examples of such problems are; (i) destruction of cu t t ing tools and dri l ls by wear , (ii) frictional energy losses in cutting, drawing and stamping, (iii) contaminat ion of molten metal by wear from stirr ing blades [1], damage to silicon wafers in semiconductor manufacture by

wear ing contact wi th sawblades [2] and (iv) heal th hazards to factory personnel caused by coolants and oil mist lubrication. The costs associated with friction and wear begin with the extraction of ore from the ground and only terminate wi th delivery of the product to the consumer. Al though each specific cause of wear and friction may impose only a small cost to the materials processor but there are so many friction and wear events in any materials process that the cumulat ive cost is very large. Even if the task invo lved is not direct ly r e l a t ed to m a t e r i a l s p rocess ing , severe penalties may be imposed by friction and wear. For instance, i ron ore is usual ly hauled by railway from the minesite to the nearest port. The axle loads on iron ore trains are usually very high to ensure economic transport and this causes severe wear of the rai lway track and vehicles [3]. The handl ing of minerals in bulk also causes wear to silos and conveyor belts while excessive friction between ore particles or with the hand l ing equ ipment can prevent flow of the minera l s [4]. For this reason, friction and wear control is critical to the success of materials processing. The purpose of

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504 A.W. Batchelor, G.W. Stachowiak I Journal of Materials Processing Technology 48 (1995) 503-515

this review is to describe the basic forms of wear and frict ion as found in materials processing and to suggest how they can be better controlled.

MECHANISMS OF FRICTION AND WEAR

Wear and friction are caused by any physical or chemical phenomenon that can occur be tween contacting surfaces. The most common interaction is probably mechanical deformation but there are many others that are significant. These are brittle fracture, thermal deg rada t ion , chemical reactions, solid state bonding and solution transfer to name a few [5]. All of these phenomena occur on a microscopic scale within a dynamic contact (a contact which involves some form of movement , e.g. sl iding or rolling) and the overall levels of friction and wear depend on the random interaction between many different events inside the contact. Wear and friction are there fore chaotic processes [6] but prediction of chaotic processes is still not fully developed so that an analytical approach to w e a r r e m a i n s i m p o s s i b l e [6]. A phenomenological approach is still the most ef fect ive m e t h o d of u n d e r s t a n d i n g and controlling tribological problems.

Mechanisms of friction Although friction has several causes, the

most comm on cause is elastic and plastic deformation be tween opposing asperities of surfaces [7]. Asper i t ies are peaks or high points of a typically rough surface. Asperity de fo rma t ion is associated with modera te levels of friction and is usually found in lubricated contacts where the lubricating film is very thin. Examples of this type of friction can be found where lubricant additives have been successful in preventing scuffing or scoring between surfaces.

The second most common mechanism of friction is adhesion in particular solid state adhes ion . W h e n metals are cleaned ot superficial contaminant layers then strong spontaneous adhesion during metallic contact

becomes possible [8]. The adhesion is a result of electron transfer between metals or between a metal and non-metal . Severe conditions of sliding contact which sweep away any surface layers of metal oxide and lubricant films render metal surfaces sufficiently clean for significant adhesion. A classic example of this phenomenon is the adhesion of workpiece material to a cutting tool. A lump of material often remains stuck to the edge of the cutting tool but when a piece of the same material is pressed against the surface of the tool no adhesion occurs. The reason for this is that the oxide films have not been disrupted by wear or if they were, the oxide films would have rapidly re fo rmed on the worn surface. Adhesion dependent friction is often a cause of very high friction coefficients or frictional seizure. The main objective of lubrication is to prevent this form of friction.

The third cause of friction is viscous drag within any in te rven ing material between contacting surfaces. Viscous drag is usually generated by a thick film of liquid lubricant and in some cases, a very low friction coefficient can be obtained. This low friction c o n d i t i o n is u s u a l l y r e f e r r ed to as h y d r o d y n a m i c lub r i ca t ion [9]. Careful selection of lubricant can allow hydrodynamic lubrication to be init iated in almost any dynamic contact. For example, wire drawing dies fitted with a pressurized lubricant supply and metal pressings lubricated by grease and oil will have a friction characteristic at least partly controlled by viscous drag. Viscous drag or solid deformation of solid lubricant films between contacting surfaces will also control friction. The rheotogy of solid lubricant films can vary from nearly Newtonian to near solid state depending on condi t ions and type of lubricant so that in some cases viscous drag prevails but in other instances, plastic or elastic deformation of the film material is the controlling factor. The three basic mechanisms of friction are i l lustrated schematically in Figure 1.

A.W. Batchelor, (7.. I,E. Stachowiak /Journal of Materials Processing Technology 48 (1995) 503-515 505

Asperity ot harder sudace or trapped wear particle

PLOUGHING// ~ . ( ~ , - - J BODY 1/motio n

~ t e r i a l Plastically deformed .layer

ADHESION Adhesive bonding

deformed a s p e r ~

Figure 1.

VISCOUS DRAG

~ g of film material

:..~_ -~;;J~(.-~ :.: )'/~~;.~i~ ' :: Ji 'i: -~------- film material

Schematic illustration of the mechanisms of friction.

Mechanisms of wear A wide range of mechanisms is usually

i nvo lved in wear process d e p e n d i n g on operat ing condi t ions and external agents, e.g. lubricant or process fluid. In terms of scale and number of si tuations where it occurs, abrasive wear is the mos t s ignif icant type of wear. Almost all minera l processing equ ipment is subject to abras ive wear by silica as this is present in virtually all rocks and soils [10, 11]. Silica has a hardness of 1100 Vickers which is harder than any known steel so that a special coating is required to prevent rapid abrasion of steel components by rocks and soil. The early mode l s of ab ras ive wear env i s ioned the abras ive gr i ts ac t ing as small cut t ing tools producing and removing wear particles when t ravers ing the surface. However , this view has been proved to be inaccurate and in most i n s t a n c e s w e a r p r o c e e d s by r e p e a t e d deformat ion or fatigue of the worn material [5]. In some cases, where cohesion between g ra ins is weak, wea r is con t ro l l ed by intergranular separation. In other words entire gra ins are extracted or pul led out by the abrasive grits. This is the reason why alumina

can be a b r a d e d e v e n t h o u g h it is c o m p a r a t i v e l y hard . Abra s ive wear has t radi t ional ly been seen as a problem of the mining indus t ry but is becoming relevant in ups t ream mater ia ls process ing of composite mater ia ls . Compos i t e mater ia ls very often contain hard re inforcement phases, e.g. glass fibres and ceramic particles in polymers and metals. Abras ive wear of dies is prevalent du r ing mou ld ing of composi te components . Abrasive wear is often a very rapid form of wear but alone among wear mechanisms it can be re la t ively easily suppres sed by s imply rais ing the ha rdness of the material . When rais ing the ha rdness of a m~--~erial to resist abrasive wear, the toughness of the material should be main ta ined if possible to prevent abrasive wear by brittle fracture which can be severe.

Erosive wear is caused by the impact of solid or liquid particles. The typical examples found in manufactur ing are sand blasting and wa te r jet dr i l l ing . The m e c h a n i s m s and characteristics of erosive wear depend on the angle of impingement of the eroding particles

506 A.W, Batchelor, G.W, Stachowiak I Journal o f Materials Processing Technology 48 (1995) 503-515

to the worn surface [5]. At shallow angles of impingement , erosive wear resembles abrasive wear but at near normal impingement angles, the nature of erosive wear is fundamenta l ly different. Fatigue and fracture based processes are dominan t at large impingement angles as wear par t ic les are r emoved by what are effectively a succession of hammering blows as each part icle impacts. Brittle materials such as ceramics wear more rap id ly at large imp ingemen t angles than at shallow angles. The converse is true for ductile metals which usually reveal a maximum in wear around 30 ° i m p i n g e m e n t angle. Cavi ta t iona l wear is related to liquid droplet erosion as it is caused by large t rans ient pressures dur ing bubble

a) Abrasive wear

collapse. This wear process is milder than erosion and appears to be controlled by the fatigue resistance of materials. Components such as stirrer blades in process fluids would be v u l n e r a b l e to cav i ta t iona l wear on the d o w n s t r e a m faces of each blade . The m e c h a n i s m s of ab ra s ive , e ros ive and c a v i t a t i o n a l w e a r a r e i l l u s t r a t e d schematically in Figure 2.

High friction coefficients and wear rates in s l id ing componen t s are usual ly caused by adhes ive wear. When in sl iding contact the i n t e r v e n i n g f i lms b e t w e e n c o n t a c t i n g components have been removed, the s trong adhesive bonds that form across the sliding interface can cause severe wear as well as high

Fracture Cutting direction o! abrasive grit direction of abrasive 9rit

g ~ ~.~; c~'acks

Grain pull-out ~ direction of abrasive grit direction of abrasive grit

Fatigue ~ repeated ~ / ~ _ ~_\\\\\\\\~k~\\\\~ deformations

by subsequent grits

HIGH ANGLE OF IMPINGEMENT Erosive wear, ~ 3

c) Cavitation wear Movement of liquid

~ collapsing bubble

~ ~ Impact of solid and liquid

deformation or fracture of solids resulting in wear

Figure 2. Schematic illustration of the mechanisms of abrasive, erosive and cavitat ional wear.

A.W. Batchelor, (7..IV.. Stachowiak / Journal of Materials Processing Technology 48 (1995) 503-515 507

friction. In effect, the opposing surfaces tear fragments from each other dur ing adhesive wear.

This form of wear is very severe, almost never associated with 'mild wear' regime, and should be suppressed if at all possible. Unfortunately, most metalworking operations such as cutting, pressing and extrusion generate sufficient levels of contact stress and slidin~ speed to provoke the occurrence of adhesive wear. Most metalworking lubricants and wear res i s tan t f i lms are in tended to p reven t adhes ive wear which is why they are t o l e r a t ed in sp i t e of the i r o b v i o u s disadvantages. The mechanism of adhesive wear is illustrated schematically in Figure 3.

In materials processing impact very often occurs be tween two surfaces. Impact ing hammers in a forge are a simple example of this. Wear part icles can be released by mechanical fatigue resulting from repetitive contact be tween asperities of the opposing surfaces. Very often, the cracks developed that are invisible from the surface and large lamellar particles can be suddenly released from an apparently undamaged surface [12]. Cracks formed by sliding or rolling contact tend to grow parallel to the surface at a small depth below the surface where the shear stress is highest. This form of wear is known as delaminat ion and most often found in the rolling bearings, gears and cams which drive material processing machinery. A schematic i l lus t ra t ion of the pit fo rmat ion f rom subsurface cracking is shown in Figure 4.

Frictional heat and mechanical agitation of chemically reacting material often causes increased corrosive activity in a wear ing contact compared to the same unworn contact. Corrosive wear is a process where corrosion product films are removed and then reform in a cyclic process. In metals, the corrosion product is typically an oxide or sulphide film but for ceramics and polymers, the corrosion product could be a hydrated or chemically weakened layer [5]. The most common instance of corrosive wear in materials processing is where a lubricant of excessive chemical activity is

adhesion sliding

fracture

Figure 3. Schematic illustration of the mechanism of adhesive wear.

applied to metal tools. Sulphur and chlorine based lubricants are usually the cause of this p rob lem. Direct corrosive wear usual ly proceeds at a steady rate with a relatively small friction coefficient. For this reason it is usually considered as a benign form of wear. When corros ive and abras ive wear act together there is a synergistic effect resulting in a very rapid form of wear. Surface films p roduced by corrosion are very rapidly removed by abrasion and a very fast cycle of corrosion and corrosion product removal results [5]. The mechanisms of corrosive wear and corrosive-abrasive wear are shown in Figure 5.

maximum shear stress occurring some distance below the surface

\ inclusions or flaws

Branching of crack to surface

crack propagate along plane of maximum shear stress

Figure 4. Schematic illustration of pit formation during the rolling contact.

Mineral ore processing equipment which must process slurries of ore are subject to severe corrosive-abrasive wear which can rapidly

508 h.lK. Batchelor, G.I,V.. Stachowiak / Journal o f Materials Processing Technology 48 (1995) 503-515

destroy even a component made of the hardest mater ia l . In m a n y cases a soft po lymer componen t will outlast a steel componen t simply because it is non-corrodible.

Fre t t ing wear occurs at the interfaces between components which are in nominal ly stationary or static contact. Fretting is caused by e x t r e m e l y smal l , t yp ica l ly a few micrometres, reciprocating movement between two surfaces. The damage caused by frett ing belies the small sl iding distances and sl iding speeds involved . The l imi ted m o v e m e n t be tween contac t ing surfaces a l lows wear particles to be t rapped between the surfaces. The t rapped wear particles whether in their original form or subsequent ly modif ied by oxidation, accentuate the wear process to cause f re t t ing wear. In min ing e q u i p m e n t the interference fits between rotating shafts and bush are common sites of fretting. The lifetime of wire ropes is also determined by the fretting between wires during flexure of a rope [131.

Examples of impact, diffusive and thermal fatigue wear are widely found in materials processing equipment. Impact wear, as its name suggests, is a consequence of repeated collisions between hard objects. Impact wear is caused by fatigue of surface layers and crack formation to release wear particles, bulk fracture or plastic deformation at very high collision energies or a form of corrosive wear if the temperature is high enough and air is present. Hammers and anvils provide the classic example of this type of wear. Percussive rock drills also suffer from impact wear particularly when dri l l ing hard granite [14].

Diffusive wear occurs at the in terface be tween cu t t ing tool and chip. The h igh temperatures of cutting allow rapid diffusion of critical alloying elements from the tool to the chip. The tool mater ia l s ubs equen t l y weakens and wear particles are released. This form of wear was originally noticed dur ing the development of tungsten carbide cutting tools. The tungsten carbide tool originally showed rap id wear w h e n mach in ing s teels un t i l t i tanium carbide was added to the tungsten c a r b i d e c o m p o s i t e to s u p p r e s s the

solubilization effect. With the development of new ceramic cut t ing tools, diffusive wear is causing problems with silicon nitr ide cutt ing tools [151. Thermal fatigue wear is most ly found on the surface of metal forming rollers. The intense heat of deformation followed by drastic cooling from water sprays subjects the

corrosive reagent

initial rapid corrosion ~ ~ / _ Z ~ / / / / /_A

Formation of corrosion product film

Cyclic process ~

Film destruction by wear

Figure 5. Schematic illustration of the synergistic interaction between corrosive and abrasive wear.

metal surface to a cycle of oxidation, plastic deformat ion followed by thermal stress [16]. Thermal fatigue wear is a cause of severe damage to rolls.

LUBRICATION AND TRIBO-COATINGS

The basic purpose of lubrication and tribo- coatings is to control wear and friction at the i n t e r f ace b e t w e e n i n t e r a c t i n g sur faces . Lubrication is the act of supplying either gas, l iquid or a solid powder to the wearing contact which functions as a film material or sustains chemical t ransformat ion to become a film mater ia l [5]. Typical examples are p la in mineral oil which directly generates a liquid film, a solid lubricant such as m o l y b d e n u m disu lphide and addi t ives in a lubricating oil which chemically react wi th the surface to form a film material . The oil addi t ives are specifically des ign to undergo a chemical t r a n s f o r m a t i o n w h e n in contac t wi th a metall ic surface. For example, su lphur and chlorine based lubricants react with metallic

A.W. Batchelor, G.W. Stachowiak / Journal of Materials Processing Technology 48 (1995) 503-515 509

surfaces to form sulphide and chloride films respectively.

Tr ibo-coa t ings are def ined as coat ings which are in tended to reduce friction and wear. These coatings function by possessing the mechanical s t rength sufficient to survive in a sliding, rolling or impacting contact and are also able to provide a minute amount of film material which effectively controls friction and wear . Tr ibo-coa t ings are of ten non- metallic so that adhesion between metal and a coated tool can be lower than that be tween metal and an uncoa ted meta l tool. This proper ty of tribo-coatings is very effective in suppress ing adhes ive wear and associated high friction. The classical example of non- adhesion between tribo-coatings and processed material is that occurring between steel chip and tungs t en n i t r ide coated tool d u r i n g machining process. It has been observed with the aid of video camera that the chip slid over the tool wi th negligible adhes ion [17]. This s l id ing character is t ic is in comple te contrast to an uncoated tool where the chip bonds to the tool and is forced to flow over the tool in a pseudo-liquid manner.

Lubrication The mode of lubrication with probably the

m o s t w i d e s p r e a d a p p l i c a t i o n is t he h y d r o d y n a m i c l u b r i c a t i o n [9]. In h y d r o d y n a m i c lubr ica t ion l iquid or gas is en t r a ined in to the contact b e t w e e n two converging surfaces by viscous drag forces fo rming a very effect ive lubr ica t ing film. Because of the converging geometry a larger quant i ty of gas or l iquid is ent ra ined at the contact inlet than is expelled at the contact outlet. Cont inui ty of flow is achieved by a pressure field which forms in response to the apparen t loss of lubricant. The pressure field tends to oppose the en t ra inment flow at the contact inlet and boost the expulsion flow at the contact outlet. Side flow from the contact is also dr iven by this pressure field. The same pressure field supports the imposed load and prevents contact be tween oppos ing surfaces. H y d r o d y n a m i c lubr ica t ion can in theory

enable complete prevent ion of wear and it is cons ide red to be the o p t i m u m form of lubr ica t ion . U n f o r t u n a t e l y h y d r o d y n a m i c lubricat ion is a velocity dr iven phenomenon and certain level of velocity is required for its successful operation. In this lubrication regime the level of contact s t resses that can be sus ta ined is also restricted. Hydrodynamic l u b r i c a t i o n f inds app l i c a t i on in l iqu id lubr icated rolls and metal press ings where plasto-hydrodynamic lubrication is promoted. When the contact stresses are very high such as, for example, encountered in rolling contact bearings and gears, a more specialized form of l u b r i c a t i o n , i.e. e l a s t o h y d r o d y n a m i c l u b r i c a t i o n t a k e s p l a c e . T h e e l a s t o h y d r o d y n a m i c l u b r i c a t i o n is a synergistic interaction between hydrodynamic effects, elastic de format ion of the contact materials and a pressure induced viscosity rise in lubricating oil [18, 19]. Films of lubricant are maintained under enormous contact pressures, often exceeding 1 GPa by elastohydrodynamic lubricat ion. Elas tohydrodynamic lubrication f inds appl ica t ion in the n u m e r o u s rol l ing bearings, gears and cams which drive material p r o c e s s i n g m a c h i n e r y . Some fo rm of e l a s t o h y d r o d y n a m i c l u b r i c a t i o n is also believed to occur between oil lubricated rolls and pressed material [20].

Solid lubr ican t s such as g raph i t e and m o l y b d e n u m disulphide function as lamellar c rys t a l l i ne ma te r i a l s wh ich a l low easy sl iding between lamellae. The lameilae bond to the s l id ing surfaces and are resistant to penetrat ion so that it is not easy to remove a f i lm of sol id lub r i can t by wear . Solid l ub r i can t s are essent ia l for hot wear ing contacts where the temperatures would cause decomposi t ion of a liquid lubricant. Forging, extrus ion and hot pressing all rely on solid lubricants to prevent adhes ion and adhesive wear tool and mater ia l to occur. At the t e m p e r a t u r e s above 500°C, g raph i t e and m o l y b d e n u m d isu lph ide become thermal ly unstable or are oxidized and therefore they are unsui table above this temperature range. Low m e l t i n g po in t oxides are of ten used as

510 A.W. Batchelor, G.W. Stachowiak /Journal o f Materials Processing Technology 48 (1995) 503-515

subs t i tu tes and these appea r to work by forming a viscous sticky paste on the worn surface.

E x t r e m e - p r e s s u r e a d d i t i v e s , f r ic t ion modifiers and anti-wear addit ives all function by chemical interaction with the worn surface to form a surface film. Extreme pressure add i t i ve s con ta in su lphur , ch lor ine and phosphorous as the active ingredient which reacts wi th a worn metal surface to form a friction reducing film of corrosion products [5]. Al though the extreme pressure addit ives are ineffective at suppres s ing wear they are extremely effective in suppressing unrestrained severe adhes ive wear. Therefore ex t reme p r e s s u r e a d d i t i v e s a re cr i t ical to tap- threading operations and the cutting of tough metals by uncoated tools. They are also used in deep drawing operations where even a trace of a d h e s i v e wear will cause unaccep t ab l e damage to the surface of the d rawn component. Friction modif iers and ant i -wear addi t ives function by deposit ing a monomolecular layer of adsorbed lubricant on the worn surface [8]. Each molecule of adsorbed lubricant is linear in shape and conta ins a polar end that is attracted to the oxides on a metal surface and a non-polar end that repels any other molecule including itself. When two surfaces covered wi th these adsorbed films are b rought into contact, the mutua l repuls ion be tween the exposed non-polar ends creates a low friction in ter face . The cr i t ical d i s a d v a n t a g e of adsorbed films is tha t they desorb above temperatures in the range 100-200°C and they cannot adsorb back onto exposed unoxidized metal. Since bo th h igh t empera tu res and exposed meta l surface are p r eva l en t in materials processing anti-wear addit ives and friction modifiers are more useful to material p r o c e s s i n g m a c h i n e r y r a t h e r t h a n the interface between tool and machinery. These addit ives still find however some application in light machin ing and press ing operations. The basic m e c h a n i s m s of lubr ica t ion are illustrated schematically in Figure 6.

Tribo-coatings and wear resistant mater ia l s As discussed previously, there are many

d i s a d v a n t a g e s associated wi th lubr ica t ion which is increasingly seen as an obsolescent t echno logy . A l t h o u g h t r ibo-coa t ings are current ly viewed as the future successor to lubrication they may also become the subject of scepticism. There is a large, even bewildering range of coat ings and coat ing techniques avai lable since many research g roups and companies independen t ly pursue their own coating technology. Much more is known about the methods of producing tribo-coatings than is unders tood about how tribo-coatings function and which is the op t imum coating for any specific applicat ion. Almost every feasible coating technique has been applied and tested for its potential in producing an effective tribo- coatings. Despite the variety and differences between various tribo-coatings, there are some fundamenta l similarities between almost all of them.

~9at ing methods All t r i bo -coa t ings d e p e n d on s t rong

adhes ion to the substrate for their durabil i ty unde r wear so that the coating methods have to p rov ide an e n v i r o n m e n t conduc ive to adhesion dur ing deposition of the coating. This requi rement for strong adhesion involves the exclusion of oxygen and water from the site of coa t ing depos i t ion and also a mechanical r equ i r emen t that there is contact be tween a toms of coating and the substrate as opposed to isolated contact be tween asperit ies. An in te r face b e t w e e n coa t ing and subs t ra te consis t ing of isolated contacts separated by voids is mechanical ly weak and a cause of early coating failure. A successful method of producing tribo-coatings is based on deposition in a vacuum where a coating is developed on a subs t ra te by the accumulat ion of atoms or e x t r e m e l y smal l pa r t i c les of mate r ia l . Sputtering, ion plating, vacuum deposition [21] and arc coating [22] are the typical examples of the coating techniques. It is also possible to in t roduce an electric potential be tween the substrate and source of coating material so that ionized material is projected at the substrate w i th suf f ic ien t ene rgy to pene t r a t e the subst ra te and embed itself into the surface.

A.W. Batchelor, (7,. W. Stachowiak / Journal o f Materials Processing Technology 48 (1995) 503-515 511

a) Hydrodynamic lubrication

Pressure [ ~ ~ . . ~ P r e s s u r e profile

~ Lubricant pressure reduces / ~ ~trained flow c) Lubrication by adsorbed monomolecular filrns

L u b ~ / / / / / / / / / / / / / ~ ~ Intermolecular pre:~ uarm ~ n g ~ ~ - _ ~ ~ ~°~t~:td '~ boosts e.g. pad bearing surfaces " ~ _ ~ , ~ support .

ed _._ t / , ~,omg Weak bonding or repulsion between opposing - CH 3 groups provides low interfaeial shear stress

b) Elastohydrodynamic lubrication ~ Pressure profile e.g. ball bearing / -- ~(//

ball ~ rolling, ~, ~

r a ~

~" - constriction at the exit elastic deformation of limiting lubricant flow

the contacting surfaces !

Figure 6. Schematic illustration of the basic mechanisms of lubrication. This technique allows the development of a very strongly bonded coatings. Ion-plating is an example of a high energy coating method. If a very high electric potential is used, the coating material does not really form a discrete layer but instead disrupts the crystalline structure of the substrate to form an amorphous surface layer. This technique produces hard amorphous surfaces layers with enhanced wear and corrosion resistance [23].

Another method of producing conditions favourable for the production of tribo-coatings is to use extreme heat to melt and burn away any contaminant films while delivering the tribo-coating material as liquid droplets which spontaneously bond to the substrate. Plasn~a and thermal spraying are examples of

this coating method [5]. It is also possible to hurl coating material with the force of an explosion to the substrate to achieve good bonding and interfacial contact. The D-gun or detonation-gun delivers small particles of coating material with the force of a gas explosion to the substrate. On the other hand explosive bonding can deposit thick sheets of material onto a substrate. During explosive bonding it is observed that the contaminant films of oxide and residual oils are expelled as a jet of debris ahead of the impacting sheet of coating material allowing strong adhesion between coating and substrate. Wear itself can be used to generate a clean surface in a process called 'friction surfacing' [24, 25]. Friction surfacing is a modification of friction welding

512 A.W. Batchelor, G.W. Stachowiak / Journal of Materials Processing Technology 48 (1995) 503-515

where the rod of coating material is moved a long the surface while being rotated under load. A coating material is smeared out onto the substrate surface during this process.

Another aspect of tribo-coating performance is the coating microstructure. Careful control of coat ing pa ramete r s is needed to prevent a p o r o u s m i c r o s t r u c t u r e f rom deve lop ing . The rma l s p r a y i n g and p lasma sp ray ing generate a porous microstructure which results in the format ion of relatively brittle coatings. With p re sen t technology a compromise is necessary be tween coating deposi t ion speed and resu l t ing microstructure of the coating. F lame and p l a sma sp ray ing give rap id deposi t ion rate while vacuum based coatings a l though slower can produce coatings with a better microstructure.

Avvlicat ions of tribo-coatin~s T r i b o - c o a t i n g s a re r a p i d l y g a i n i n g

e x t e n s i v e u se in ma te r i a l s p r o c e s s i n g technology at all levels from the minesite to the f in i shed produc t . Coa t ings reach ing several mi l l imet res in th ickness tha t are produced by thermal and plasma spraying are ve ry use fu l to the min ing indus t r i e s in particular. Ore extraction and processing tools, such as drills, crushers and shovels need thick hard coatings to suppress abrasive wear by the ore. Vacuum-based deposition methods produce thinner coatings of a few micrometres thickness t h a t a r e s u i t a b l e for more p rec i se ly d i m e n s i o n e d ma te r i a l p roces s ing tools. Vacuum-based coatings are being applied to cutting tools, dies and moulds to prevent wear. In the case of moulds , any damage by wear detracts from product quality so it is found that the use of coa t ings enables r emarkab le extensions of mould life before product quality becomes unacceptable [26]. Moulds represent a very dramatic example of the benefits brought by tribo-coating. Any damage by wear to a mould such as scratches detracts from product quali ty so that the working life of uncoated moulds is compara t ive ly short. It has been reported that in certain cases the lifetime of coated moulds was found to be more than five

t imes longer than uncoated moulds [26]. The life t ime of cutt ing tools is not only increased but also machining speeds and feed rates can be increased when coated tools are used. Ion- implantat ion produces very thin coatings less than I micrometre thick but with almost no dimensional change in the coated component. However the applications of this technique in materials processing appear to be limited to small scale precision machining tools and dies. Friction surfacing and explosive bonding are suitable for p lanar surfaces which precludes most material processing tools apart from the larger tools used in mining. The D-gun has largely been superseded by the other vacuum- based coat ing me thods which were mostly developed subsequent ly to the D-gun. Some typical tribo-coatings for materials processing applications are listed in Table 1.

Wear res is tant materials Wear res is tant mater ia ls are vital to the

durabi l i ty of mater ia l processing tools and e q u i p m e n t . A l t h o u g h the t r a d i t i o n a l materials used have been hardened steels, e.g. 'h igh speed steel', ceramics and ceramic-hard metal composi tes are becoming increasingly important because of their advantages in terms of d u r a b i l i t y , w e a r r e s i s t a n c e and productivity. Ceramic tools made of alumina, s i l i con ca rb ide and s i l icon n i t r i de are relatively well deve loped and they are used commercial ly [15]. These materials are useful because of the i r ex t r eme h a r d n e s s and resistance to adhes ive wear. When adhesive wear does occur, particularly between ceramics a n d metal, a metal transfer film forms on the surface of the ceramic [27]. This transfer film protects the ceramic surface and ensures that the ceramic wears far less than the metal.

The la tes t idea in new non-meta l l i c mater ia ls is the deve lopmen t of ul t ra-hard materials which are as hard or harder than diamond. A composite of cubic boron nitride p o w d e r w i t h a l u m i n i u m n i t r i d e and a lumin ium boride b inder has been tested as a cut t ing tool material [28]. However, the main p r ob l em associa ted wi th the cubic boron

A.W.. Batchelor, G.W. Stachowiak /Journal of Mate~als Processing Technology 48 (1995) 503-515 513

Table 1. Some typical tribo-coatings for materials processing applications

i Processing equipment Mineral Process- ing tools

Moulds, dies

Cutt ing tools

Pressing tools, Punches

Rolls

Type of coating Coating Process Coating character is t ics

Hard coating on softer metal

Non-metal l ic coating on metal

Hard non- metallic material on metal or transformed microstructure on metal.

Compressive sh'ess in surface layers. Amorphous material in surface

Thick layer with hardness at high temperatures

n i t r ide composi te is the wear of the b inder wh ich a l lows boron ni t r ide particles to be detached from the tool during cutting [28].

Conclusions

Materials processing presents many varied app l i ca t i ons of wear and frict ion control technology. From a review of established and

Plasma and thermal spraying, D-g~.

PVD, CVD, plasma spraying

PVD, CVD

Ion implantat ion Laser surface a l loying

Ion implantat ion useful for small tools

Weld overlays

Resistant to: abrasive wear, erosive wear, cavi ta t ional wear, rolling wear and contact fati~ue

i Resistant to: adhesive wear fatigue wear corrosive wear

Resistant to: adhesive wear, diffusive wear Reduces friction

also effective against : fretting wear and cavitat ional wear

Resistant to impact wear, adhes ive wear, fatigue wear

Resistant to h igh temperature wear and contact fa t igue

newly d e v e l o p e d m e t h o d s of op t imiz ing f r ic t ion a n d wear for specif ic mater ia l s p r o c e s s i n g a p p l i c a t i o n s , the fo l lowing conclusions can be drawn:

1. Fr ic t ion a n d wea r impose severe l imitat ions on materials processing not only in terms of destruction of tools but also through loss of product quality.

514 A.W. Batchelor, G.W. Stachowiak / Journal of Materials Processing Technology 48 (1995) 503-515

2. Existing techniques for lubrication and wea r p ro t ec t i on only offer par t ia l protection against wear and friction.

3. Tribo-coatings, i.e. mechanically robust coatings to reduce friction and wear, can offer substant ial improvements in tool life and consistency of product quality.

4. There is a very wide range of tribo- coa t ings which necessi tates detai led testing before the opt imum coating for a particular application is found.

5. The technology of friction and wear control is in a state of fundamenta l change and most accepted methods of lubricat ion or wear prevent ion may in f u t u r e be s u p e r s e d e d by en t i re ly different technologies.

Acknowledgements The au tho r s wou ld like to thank the

Depar tmen t of Mechanical and Product ion E n g i n e e r i n g , N a n y a n g T e c h n o l o g i c a l Universi ty and the Department of Mechanical and Materials Engineering, the University of Western Austral ia for their suppor t of this work. The efforts of the Centre for Educational D e v e l o p m e n t , N a n a y a n g Techno log i ca l University to arrange the illustrations are also grateful ly apprecia ted.

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