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Tribology in materials processing
Journal of
Materials
ELSEVIER
Journal of Materials Processing Technology 48 (1995) 503--515 TRIBOLOGY
IN MATERIALS
Processing Technology
PROCESSING
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 a n d wear affect all processes involved in the extraction of materials a n d their conversion into finished products. Physical contact b e t w e e n tool, die, clamp or a n y other device that contacts the processed material is the basic cause of wear. Excessive friction imposes limits on the efficiency of cutting tools, dies a n d m a n y other equipment. Wear is a severe p r o b l e m in the extraction a n d primary processing of raw materials a n d even the conveyance of raw materials from mine site to refinery imposes additional problems of wear. Therefore research and development into means of controlling friction a n d wear in materials processing is actively pursued by many research groups. The most well established method to control friction a n d wear is by the application of lubricants. A l t h o u g h the d e v e l o p m e n t of solid and liquid lubricants has greatly advanced materials processing it still do not give an ideal performance. Lubricants also bring pollution a n d health 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 coatings w i t h o u t the need for lubricants. Projected benefits from these n e w e r 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 m o s t stages of materials processing, contact occurs between the tool or processing m a c h i n e r y a n d the p r o c e s s e d m a t e r i a l . Exceptions to this rule include, the use of high pressure water jets to either drill holes in a material or in a larger form strip ore from a mine wall. Conveyance of components by jets of air or b y flotation is another situation where contact is avoided. The majority of processing o p e r a t i o n s h o w e v e r i n v o l v e a solid tool c o n t a c t i n g a solid process m a t e r i a l with attendant wear a n d friction. Wear and friction are usually a hindrance to materials processing operations as they result in (i) damage to tools, (ii) i n c r e a s e d e n e r g y c o n s u m p t i o n , (iii) contamination of processed material b y wear particles a n d (iv) p r o b l e m s associated with technologies to control friction a n d wear. Examples of such problems are; (i) destruction of c u t t i n g tools a n d drills by wear, (ii) frictional energy losses in cutting, drawing and stamping, (iii) contamination of molten metal b y wear from stirring blades [1], damage to silicon wafers in semiconductor manufacture by
wearing contact with sawblades [2] and (iv) health h a z a r d s to factory personnel caused by coolants a n d oil mist lubrication. The costs associated with friction a n d wear begin with the extraction of ore from the ground and only terminate with delivery of the product to the consumer. Although each specific cause of wear a n d friction m a y impose only a small cost to the materials processor but there are so m a n y friction a n d wear e v e n t s in a n y materials process that the cumulative cost is very large. Even if the task i n v o l v e d is not directly r e l a t e d to m a t e r i a l s p r o c e s s i n g , s e v e r e penalties m a y be imposed by friction and wear. For instance, iron ore is usually 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 railway track and vehicles [3]. The h a n d l i n g of minerals in bulk also causes wear to silos and conveyor belts while excessive friction between ore particles or with the h a n d l i n g e q u i p m e n t can prevent flow of the m i n e r a l s [4]. For this reason, friction a n d w e a r 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 w e a r a n d friction as found in materials processing and to suggest how they can be better controlled.
MECHANISMS OF FRICTION AND WEAR Wear a n d friction are caused by any physical or chemical p h e n o m e n o n that can occur b e t w e e n contacting surfaces. The most c o m m o n interaction is probably mechanical d e f o r m a t i o n but there are many others that are significant. These are brittle fracture, t h e r m a l d e g r a d a t i o n , chemical reactions, solid state b o n d i n g and solution transfer to name a few [5]. All of these phenomena occur on a microscopic scale within a dynamic contact (a contact w h i c h involves some form of m o v e m e n t , e.g. sliding 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 t h e r e f o r e chaotic p r o c e s s e s [6] but prediction of chaotic processes is still not fully d e v e l o p e d so that an analytical approach to wear remains impossible [6]. A phenomenological approach is still the most e f f e c t i v e 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 A l t h o u g h friction has several causes, the most c o m m o n cause is elastic and plastic d e f o r m a t i o n b e t w e e n o p p o s i n g asperities of surfaces [7]. A s p e r i t i e s are peaks or high points of a typically rough surface. Asperity d e f o r m a t i o n is associated with m o d e r a t e levels of friction a n d is usually found in lubricated contacts where the lubricating film is very thin. Examples of this type of friction can be found w h e r e lubricant additives have been successful in preventing scuffing or scoring between surfaces. The second most c o m m o n mechanism of friction is a d h e s i o n in particular solid state a d h e s i o n . W h e n m e t a l s are cleaned ot superficial c o n t a m i n a n t 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 p h e n o m e n o n is the a d h e s i o n of workpiece material to a cutting tool. A lump of material often remains stuck to the edge of the cutting tool but w h e n 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 r a p i d l y r e f o r m e d on the w o r n 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 i n t e r v e n i n g material b e t w e e n contacting surfaces. Viscous drag is usually generated by a thick film of liquid lubricant and in s o m e 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 e d to as h y d r o d y n a m i c l u b r i c a t i o n [9]. Careful selection of lubricant can allow hydrodynamic lubrication to be initiated 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 d e p e n d i n g on conditions and type of lubricant so that in some cases viscous drag prevails but in o t h e r instances, plastic or elastic deformation of the film material is the controlling factor. The three basic mechanisms of friction are illustrated schematically in Figure 1.
A.W. Batchelor, (7..I,E. Stachowiak /Journal of Materials Processing Technology 48 (1995) 503-515
505
Asperityot hardersudace or trappedwearparticle PLOUGHING// ~ ( ~ , - - J BODY1/motio n
.
~ t e r i a l
VISCOUS DRAG
~
g of film material
:..~_-~;;J~(.-~ :.:)'/~~;.~i~' :: Ji'i:-~-------film material
Plasticallydeformed.layer ADHESION
Adhesivebonding
deformeda s p e r ~
Figure 1.
Schematic illustration of the mechanisms of friction.
Mechanisms of wear A wide range of mechanisms is usually i n v o l v e d in w e a r p r o c e s s d e p e n d i n g on operating conditions a n d external agents, e.g. lubricant or process fluid. In terms of scale and n u m b e r of situations where it occurs, abrasive wear is the m o s t significant type of wear. A l m o s t all m i n e r a l processing e q u i p m e n t is subject to a b r a s i v e wear b y silica as this is present in virtually all rocks a n d soils [10, 11]. Silica has a h a r d n e s s of 1100 Vickers which is harder than a n y k n o w n steel so that a special coating is required to prevent rapid abrasion of steel c o m p o n e n t s b y rocks a n d soil. The early m o d e l s of a b r a s i v e w e a r e n v i s i o n e d the a b r a s i v e grits acting as small cutting tools producing a n d r e m o v i n g wear particles w h e n t r a v e r s i n g the surface. However, this view has been p r o v e d to b e inaccurate a n d 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 d e f o r m a t i o n or fatigue of the w o r n material [5]. In some cases, w h e r e cohesion between g r a i n s is weak, w e a r is c o n t r o l l e d b y intergranular separation. In other words entire g r a i n s are extracted or pulled out by the abrasive grits. This is the reason w h y alumina
can be abraded even though it is c o m p a r a t i v e l y h a r d . A b r a s i v e w e a r has traditionally b e e n seen as a p r o b l e m of the m i n i n g i n d u s t r y but is b e c o m i n g relevant in u p s t r e a m materials processing of composite materials. C o m p o s i t e materials v e r y often contain hard reinforcement phases, e.g. glass fibres a n d ceramic particles in polymers and metals. A b r a s i v e w e a r of dies is prevalent d u r i n g m o u l d i n g of composite components. Abrasive wear is often a very rapid form of wear but alone a m o n g wear mechanisms it can be relatively easily s u p p r e s s e d b y simply raising the h a r d n e s s of the material. When raising the h a r d n e s s of a m~--~erial to resist abrasive wear, the toughness of the material should be m a i n t a i n e d if possible to prevent abrasive wear b y brittle fracture which can be severe. Erosive wear is caused by the impact of solid or liquid particles. The typical examples found in manufacturing are sand blasting and w a t e r jet drilling. The m e c h a n i s m s a n d characteristics of erosive wear d e p e n d on the angle of i m p i n g e m e n t of the eroding particles
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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 fundamentally different. Fatigue and fracture based processes are d o m i n a n t at large i m p i n g e m e n t angles as w e a r particles are r e m o v e d b y w h a t are effectively a succession of hammering blows as each particle impacts. Brittle materials such as ceramics w e a r m o r e r a p i d l y at large i m p i n g e m e n t angles than at shallow angles. The converse is true for ductile metals which usually reveal a m a x i m u m in wear around 30 ° i m p i n g e m e n t angle. C a v i t a t i o n a l wear is related to liquid droplet erosion as it is caused b y large transient pressures d u r i n g b u b b l e a) Abrasive wear
collapse. This wear process is milder than erosion a n d appears to be controlled by the fatigue resistance of materials. C o m p o n e n t s such as stirrer blades in process fluids would be v u l n e r a b l e to c a v i t a t i o n a 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 a b r a s i v e , e r o s i v e a n d cavitational wear are illustrated schematically in Figure 2. High friction coefficients and wear rates in sliding c o m p o n e n t s are usually caused by a d h e s i v e wear. W h e n in sliding contact the intervening films between contacting c o m p o n e n t s have been removed, the strong adhesive b o n d s that form across the sliding interface can cause severe wear as well as high
Cutting direction o! abrasive grit g
Fracture direction of abrasive9rit
~
~.~; c~'acks
Grain pull-out~ direction of abrasive grit direction of abrasive grit Fatigue ~ repeated ~ / ~ _ ~_\\\\\\\\~k~\\\\~deformations by subsequent grits
HIGHANGLEOF IMPINGEMENT Erosive wear,
~ 3
c) Cavitation wear
Movementof 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 cavitational wear.
A.W. Batchelor, (7..IV.. Stachowiak / Journal of Materials Processing Technology 48 (1995) 503-515
friction. In effect, the opposing surfaces tear fragments from each other d u r i n g adhesive wear. This form of wear is very severe, almost never associated with 'mild wear' regime, and s h o u l d be s u p p r e s s e d 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 r e s i s t a n t films are i n t e n d e d to p r e v e n t a d h e s i v e w e a r w h i c h is w h y they are t o l e r a t e d in s p i t e of t h e i r o b v i o u s disadvantages. The m e c h a n i s m of adhesive wear is illustrated schematically in Figure 3. In materials processing impact very often occurs b e t w e e n t w o surfaces. I m p a c t i n g h a m m e r s in a forge are a simple example of this. Wear particles can be released by mechanical fatigue resulting from repetitive contact b e t w e e n asperities of the o p p o s i n g surfaces. Very often, the cracks developed that are invisible from the surface and large lamellar particles can be s u d d e n l y released from an apparently u n d a m a g e d surface [12]. Cracks formed by sliding or rolling contact tend to g r o w parallel to the surface at a small d e p t h below the surface where the shear stress is highest. This form of wear is k n o w n as delamination and m o s t often found in the rolling bearings, gears and cams which drive material processing machinery. A schematic i l l u s t r a t i o n of the pit f o r m a t i o n f r o m subsurface cracking is shown in Figure 4. Frictional heat a n d mechanical agitation of chemically reacting material often causes increased corrosive activity in a w e a r i n g 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 m o s t c o m m o n instance of corrosive wear in materials processing is where a lubricant of excessive chemical activity is
adhesion
507
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 r o b l e m . Direct corrosive w e a r u s u a l l y p r o c e e d s at a steady rate with a relatively small friction coefficient. For this reason it is usually considered as a benign form of wear. W h e n c o r r o s i v e and a b r a s i v e w e a r act together there is a synergistic effect resulting in a very rapid form of wear. Surface films p r o d u c e d by corrosion are very r a p i d l y 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 propagatealongplane of maximum shear stress Figure 4.
Schematic illustration of
pit formation during the rolling contact. Mineral ore processing e q u i p m e n t which must process slurries of ore are subject to severe corrosive-abrasive wear which can rapidly
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destroy even a component made of the hardest material. In m a n y cases a soft p o l y m e r c o m p o n e n t will outlast a steel c o m p o n e n t simply because it is non-corrodible. Fretting wear occurs at the interfaces between components which are in nominally stationary or static contact. Fretting is caused b y e x t r e m e l y s m a l l , t y p i c a l l y a few micrometres, reciprocating movement between two surfaces. The damage caused by fretting belies the small sliding distances and sliding s p e e d s i n v o l v e d . The limited m o v e m e n t b e t w e e n c o n t a c t i n g surfaces a l l o w s w e a r particles to be trapped between the surfaces. The trapped wear particles w h e t h e r in their original form or subsequently modified b y oxidation, accentuate the wear process to cause fretting wear. In m i n i n g e q u i p m e n t the interference fits between rotating shafts a n d b u s h are common sites of fretting. The lifetime of wire ropes is also determined b y the fretting between wires during flexure of a rope [131. Examples of impact, diffusive a n d t h e r m a l 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. H a m m e r s a n d anvils provide the classic example of this type of wear. Percussive rock drills also suffer from impact wear particularly w h e n drilling h a r d granite [14]. Diffusive wear occurs at the interface b e t w e e n c u t t i n g tool a n d chip. The h i g h temperatures of cutting allow rapid diffusion of critical alloying elements from the tool to the chip. The tool material s u b s e q u e n t l y weakens and wear particles are released. This form of wear was originally noticed d u r i n g the development of tungsten carbide cutting tools. The tungsten carbide tool originally showed r a p i d wear w h e n m a c h i n i n g steels u n t i l titanium carbide was a d d e d to the tungsten carbide composite to s u p p r e s s the
solubilization effect. With the development of n e w ceramic cutting tools, diffusive wear is causing problems with silicon nitride cutting tools [151. Thermal fatigue wear is mostly 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 deformation 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 a n d tribocoatings is to control wear and friction at the interface between interacting surfaces. Lubrication is the act of supplying either gas, liquid or a solid powder to the wearing contact which functions as a film material or sustains chemical t r a n s f o r m a t i o n to b e c o m e a film m a t e r i a l [5]. Typical e x a m p l e s are p l a i n mineral oil which directly generates a liquid film, a solid lubricant such as m o l y b d e n u m d i s u l p h i d e and additives in a lubricating oil w h i c h chemically react with the surface to form a film material. The oil additives are specifically d e s i g n to u n d e r g o a chemical t r a n s f o r m a t i o n w h e n in contact w i t h a metallic surface. For example, s u l p h u r a n d chlorine based lubricants react with metallic
A.W. Batchelor, G.W. Stachowiak / Journal of Materials Processing Technology 48 (1995) 503-515
surfaces to form sulphide and chloride films respectively. T r i b o - c o a t i n g s are defined as c o a t i n g s which are i n t e n d e d to reduce friction a n d wear. These coatings function by possessing the mechanical strength sufficient to survive in a sliding, rolling or impacting contact a n d are also able to provide a minute a m o u n t of film material w h i c h effectively controls friction a n d wear. T r i b o - c o a t i n g s are often n o n metallic so that adhesion between metal and a coated tool can be lower than that between metal a n d an u n c o a t e d m e t a l tool. This property of tribo-coatings is very effective in s u p p r e s s i n g a d h e s i v e wear a n d associated high friction. The classical example of nonadhesion between tribo-coatings and processed material is that occurring between steel chip a n d t u n g s t e n n i t r i d e 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 with negligible a d h e s i o n [17]. This sliding characteristic is in c o m p l e t e 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 most widespread application is t h e hydrodynamic lubrication [9]. In h y d r o d y n a m i c l u b r i c a t i o n liquid or gas is e n t r a i n e d into the contact b e t w e e n two converging surfaces b y viscous d r a g forces f o r m i n g a v e r y effective l u b r i c a t i n g film. Because of the converging geometry a larger quantity of gas or liquid is entrained at the contact inlet t h a n is expelled at the contact outlet. Continuity of flow is a c h i e v e d by a pressure field which forms in response to the a p p a r e n t loss of lubricant. The pressure field tends to oppose the e n t r a i n m e n t flow at the contact inlet a n d boost the expulsion flow at the contact outlet. Side flow from the contact is also driven by this pressure field. The same pressure field supports the imposed load and prevents contact between o p p o s i n g surfaces. H y d r o d y n a m i c l u b r i c a t i o n c a n in t h e o r y
509
enable complete prevention of wear and it is c o n s i d e r e d to be the o p t i m u m f o r m of lubrication. Unfortunately hydrodynamic lubrication is a velocity driven p h e n o m e n o n and certain level of velocity is required for its successful operation. In this lubrication regime the level of contact stresses that can be sustained is also restricted. H y d r o d y n a m i c l u b r i c a t i o n f i n d s a p p l i c a t i o n in l i q u i d lubricated rolls and metal pressings where plasto-hydrodynamic lubrication is promoted. W h e n the contact stresses are very high such as, for example, encountered in rolling contact bearings a n d gears, a more specialized form of lubrication, i.e. elastohydrodynamic lubrication takes place. The elastohydrodynamic lubrication is a synergistic interaction between hydrodynamic effects, elastic d e f o r m a t i o n of the contact materials a n d a pressure induced viscosity rise in lubricating oil [18, 19]. Films of lubricant are maintained u n d e r enormous contact pressures, often exceeding 1 GPa by elastohydrodynamic lubrication. E l a s t o h y d r o d y n a m i c lubrication finds a p p l i c a t i o n in the n u m e r o u s rolling 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 . S o m e f o r m 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 a n d pressed material [20]. Solid l u b r i c a n t s such as g r a p h i t e a n d m o l y b d e n u m disulphide function as lamellar c r y s t a l l i n e m a t e r i a l s w h i c h a l l o w easy sliding between lamellae. The lameilae b o n d to the sliding surfaces a n d are resistant to penetration so that it is not easy to remove a film of solid l u b r i c a n t b y wear. Solid l u b r i c a n t s are essential for hot w e a r i n g contacts where the temperatures would cause decomposition of a liquid lubricant. Forging, extrusion a n d hot pressing all rely o n solid lubricants to prevent a d h e s i o n a n d adhesive w e a r tool a n d material to occur. At the t e m p e r a t u r e s a b o v e 500°C, g r a p h i t e a n d m o l y b d e n u m d i s u l p h i d e b e c o m e thermally unstable or are oxidized and therefore they are unsuitable above this temperature range. Low m e l t i n g p o i n t oxides are often used as
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A.W. Batchelor, G.W. Stachowiak /Journal o f Materials Processing Technology 48 (1995) 503-515
s u b s t i t u t e s a n d these a p p e a r to work by forming a viscous sticky paste on the worn surface. Extreme-pressure additives, friction modifiers and anti-wear additives all function by chemical interaction with the worn surface to form a surface film. Extreme p r e s s u r e additives contain sulphur, chlorine and p h o s p h o r o u s as the active ingredient which reacts with a w o r n metal surface to form a friction reducing film of corrosion products [5]. Although the extreme pressure additives are ineffective at s u p p r e s s i n g w e a r they are extremely effective in suppressing unrestrained severe a d h e s i v e wear. Therefore e x t r e m e p r e s s u r e a d d i t i v e s a r e critical to tapthreading 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 w e a r will c a u s e u n a c c e p t a b l e damage to the surface of the d r a w n component. Friction modifiers a n d anti-wear a d d i t i v e s function by depositing a monomolecular layer of adsorbed lubricant on the worn surface [8]. Each molecule of adsorbed lubricant is linear in s h a p e a n d c o n t a i n s a p o l a r e n d that is attracted to the oxides on a metal surface and a non-polar end that repels any other molecule including itself. W h e n two surfaces covered with these a d s o r b e d films are b r o u g h t into contact, the m u t u a l repulsion b e t w e e n the exposed non-polar ends creates a low friction interface. The critical d i s a d v a n t a g e of a d s o r b e d films is that they d e s o r b a b o v e temperatures in the range 100-200°C a n d they cannot adsorb back onto exposed unoxidized metal. Since b o t h h i g h t e m p e r a t u r e s a n d e x p o s e d m e t a l surface are p r e v a l e n t in materials processing anti-wear additives 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 additives still find however some application in light m a c h i n i n g and pressing operations. The basic m e c h a n i s m s of l u b r i c a t i o n are illustrated schematically in Figure 6. Tribo-coatings a n d wear resistant m a t e r i a l s As discussed previously, there are m a n y
d i s a d v a n t a g e s associated w i t h lubrication w h i c h is increasingly seen as a n obsolescent t e c h n o l o g y . A l t h o u g h t r i b o - c o a t i n g s are currently viewed as the future successor to lubrication they may also become the subject of scepticism. There is a large, even bewildering r a n g e of coatings a n d coating techniques available since m a n y research g r o u p s a n d c o m p a n i e s i n d e p e n d e n t l y p u r s u e their o w n coating technology. Much more is k n o w n about the methods of producing tribo-coatings than is understood about how tribo-coatings function a n d w h i c h is the o p t i m u m coating for any specific application. Almost every feasible coating technique has been applied and tested for its potential in producing an effective tribocoatings. Despite the variety and differences between various tribo-coatings, there are some f u n d a m e n t a l similarities between almost all of them. ~9ating methods All t r i b o - c o a t i n g s d e p e n d o n s t r o n g a d h e s i o n to the substrate for their durability u n d e r wear so that the coating methods have to p r o v i d e a n e n v i r o n m e n t c o n d u c i v e to adhesion d u r i n g deposition of the coating. This r e q u i r e m e n t for strong adhesion involves the exclusion of oxygen and water from the site of c o a t i n g d e p o s i t i o n a n d also a mechanical r e q u i r e m e n t that there is contact between atoms of coating a n d the substrate as opposed to isolated contact b e t w e e n asperities. An interface between coating and substrate consisting of isolated contacts separated by v o i d s is mechanically weak and a cause of early coating failure. A successful method of producing tribo-coatings is based on deposition in a v a c u u m where a coating is developed on a s u b s t r a t e b y the accumulation of atoms or e x t r e m e l y s m a l l p a r t i c l e s of m a t e r i a l . Sputtering, ion plating, vacuum deposition [21] a n d arc coating [22] are the typical examples of the coating techniques. It is also possible to i n t r o d u c e a n electric potential b e t w e e n the substrate a n d source of coating material so that ionized material is projected at the substrate w i t h s u f f i c i e n t e n e r g y to p e n e t r a t e the substrate and e m b e d 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
~
Lubricantpressurereduces
/ ~ ~trained flow
c) Lubricationby adsorbedmonomolecularfilrns Intermolecular ~ ~ - _ ~ ~ ~°~t~:td '~ e.g. pad bearingsurfaces " ~ _ ~ , ~ support . t / , ~,omg Weakbondingor repulsionbetween opposing - CH 3 groups provides low interfaeial shearstress b) Elastohydrodynamiclubrication ~ Pressureprofile e.g. ballbearing / -- ~(//
~
Lub~/////////////~ pre:~uarm ~ n g boosts ed _._
ball r
~ a
rolling, ~, ~ ~
~" elastic deformationof the contactingsurfaces! Figure 6.
-
constrictionat the exit limiting lubricantflow
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 along the surface while being rotated u n d e r 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 coating p a r a m e t e r s is n e e d e d to p r e v e n t a p o r o u s m i c r o s t r u c t u r e from d e v e l o p i n g . Thermal spraying and plasma spraying generate a porous microstructure which results in the formation of relatively brittle coatings. With p r e s e n t technology a c o m p r o m i s e is necessary b e t w e e n coating deposition speed a n d r e s u l t i n g microstructure of the coating. F l a m e a n d p l a s m a s p r a y i n g give r a p i d deposition rate while v a c u u m based coatings a l t h o u g h slower can produce coatings with a better microstructure.
times longer than uncoated moulds [26]. The life time of cutting tools is not only increased but also machining speeds and feed rates can be increased w h e n coated tools are used. Ionimplantation 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 a p p e a r to be limited to small scale precision machining tools and dies. Friction surfacing a n d explosive b o n d i n g are suitable for planar 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 vacuumbased coating m e t h o d s which were mostly developed subsequently to the D-gun. Some typical tribo-coatings for materials processing applications are listed in Table 1.
Avvlications of tribo-coatin~s Tribo-coatings are rapidly gaining e x t e n s i v e u s e in m a t e 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 i n i s h e d p r o d u c t . C o a t i n g s r e a c h i n g several m i l l i m e t r e s in thickness t h a t are produced b y thermal and plasma spraying are v e r y u s e f u l to the m i n i n g i n d u s t r i e s in particular. Ore extraction and processing tools, such as drills, crushers a n d 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 m o r e p r e c i s e l y d i m e n s i o n e d m a t e r i a l p r o c e s s i n g tools. V a c u u m - b a s e d coatings are being applied to cutting tools, dies and moulds to prevent wear. In the case of moulds, any d a m a g e by wear detracts from product quality so it is found that t h e u s e of c o a t i n g s e n a b l e s r e m a r k a b l e extensions of mould life before product quality becomes unacceptable [26]. Moulds represent a very dramatic example of the benefits brought b y tribo-coating. Any d a m a g e by wear to a mould such as scratches detracts from product quality so that the working life of uncoated m o u l d s is c o m p a r a t i v e l y short. It has been reported that in certain cases the lifetime of coated moulds was found to be more than five
Wear resistant materials Wear resistant materials are vital to the d u r a b i l i t y of material processing tools and equipment. Although the traditional materials used have been hardened steels, e.g. 'high speed steel', ceramics and ceramic-hard metal composites are b e c o m i n g increasingly important because of their advantages in terms of durability, wear resistance and productivity. Ceramic tools made of alumina, s i l i c o n c a r b i d e a n d silicon n i t r i d e are relatively well d e v e l o p e d a n d they are used commercially [15]. These materials are useful b e c a u s e of t h e i r e x t r e m e h a r d n e s s and resistance to a d h e s i v e wear. W h e n 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 l a t e s t idea in n e w n o n - m e t a l l i c materials is the d e v e l o p m e n t of ultra-hard materials w h i c h are as h a r d or harder than diamond. A composite of cubic boron nitride powder with aluminium nitride and a l u m i n i u m boride binder has been tested as a cutting tool material [28]. However, the main p r o b l e m associated w i t h the cubic boron
A.W.. Batchelor, G.W. Stachowiak /Journal of Mate~als Processing Technology 48 (1995) 503-515
Table 1.
513
Some typical tribo-coatings for materials processing applications
i Processing equipment Mineral Processing tools
Type of coating
Coating Process
Hard coating on softer metal
Plasma a n d thermal spraying,
D-g~.
Moulds, dies
Non-metallic coating on metal
PVD, CVD, plasma spraying
Cutting tools
Hard nonmetallic material on metal or transformed microstructure on metal.
PVD, CVD
Ion implantation Laser surface alloying
Coating characteristics Resistant to: abrasive wear, erosive wear, cavitational wear, rolling wear a n d 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 a n d cavitational w e a r Pressing tools, Punches
Rolls
Compressive sh'ess in surface layers. Amorphous material in surface Thick layer with hardness at high temperatures
Ion implantation useful for small tools
Resistant to impact wear, a d h e s i v e wear, fatigue wear
Weld overlays
Resistant to high temperature wear and contact fatigue
n i t r i d e composite is the wear of the binder w h i c h allows b o r o n nitride particles to be detached from the tool during cutting [28].
n e w l y d e v e l o p e d m e t h o d s of o p t i m i z i n g friction a n d w e a r for specific m a t e r i a l s processing applications, the following conclusions can be drawn:
Conclusions 1. Materials processing presents many varied a p p l i c a t i o n s of wear a n d friction control technology. From a review of established and
F r i c t i o n a n d w e a r i m p o s e severe limitations 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 w e a r p r o t e c t i o n only offer partial protection against wear and friction. Tribo-coatings, i.e. mechanically robust coatings to reduce friction and wear, can offer substantial i m p r o v e m e n t s in tool life a n d consistency of product quality. There is a very wide range of triboc o a t i n g s w h i c h necessitates detailed testing before the optimum coating for a particular application is found. The t e c h n o l o g y of friction a n d wear control is in a state of f u n d a m e n t a l change a n d most accepted m e t h o d s of lubrication or wear prevention may in future be superseded by entirely different technologies.
3.
4.
5.
Acknowledgements The a u t h o r s w o u l d like to t h a n k the D e p a r t m e n t of Mechanical a n d Production Engineering, Nanyang Technological University a n d the Department of Mechanical a n d Materials Engineering, the University of Western Australia for their s u p p o r t of this work. The efforts of the Centre for Educational Development, Nanayang Technological University to arrange the illustrations are also gratefully appreciated.
5.
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Australia Tribology Conference, Brisbane, Australia, 3-5 Dec. 1990, Nat. Conf. publ. No. 90/14. pp. 1-11. G.W. Stachowiak a n d A.W. Batchelor, Engineering tribology, Elsevier, Amsterdam, 1993. T. Nakano, K. Hiratsuka and T. Sasada, Fractal analysis of worn surface a n d wear particles, Trans. Japan Society of Tribologists, Vol. 35, 1990, pp. 151-154. D.A. Rigney a n d J.P. Hirth, Plastic d e f o r m a t i o n a n d s l i d i n g friction of metals, Wear, Vol. 53, 1979, pp. 345-370. F.P. Bowden and D. Tabor, Friction and l u b r i c a t i o n of solids, Part 1, Oxford University Press, Oxford, 1954. O. R e y n o l d s , O n the t h e o r y of l u b r i c a t i o n a n d its application to Mr Beauchamp Tower's experiments including an experimental determination of the viscosity of olive oil, Phil. Trans. Roy. Soc. London, Vol. 177 (i), 1886, pp. 157-234. A.P. Mouritz and I.O. Smith, Present and future research in abrasive wear for the m i n i n g i n d u s t r y , The Institution of Engineers Australia Tribology Conference, Brisbane, Australia, 3-5 Dec. 1990, Nat. Conf. publ. No. 90/14. pp. 2832. K. Travers, Tribology of mining cutters, The Institution of Engineers Australia Tribology Conference, Brisbane, Australia, 3-5 Dec. 1990, Nat. Conf. publ. No. 90/14. pp. 85-90. N.P. Suh, The d e l a m i n a t i o n theory of wear, Wear, Vol. 25, pp. 111-124. R.B. W a t e r f i b u s e , F r e t t i n g fatigue, Applied Science Publishers Ltd., London, 1981. K.J. Swick, G.W. Stachowiak and A.W. Batchelor, Mechanism of wear of rotarypercussive drilling bits and the effect of rock type on wear, T r i b o l o g y International, Vol. 25, 1992, pp. 83-88. I.R. Pashby, J. W a l l b a n k and F. Boud, C e r a m i c tool w e a r w h e n m a c h i n i n g a u s t e m p e r e d ductile iron, Wear, Vol.
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Materials
ELSEVIER
Journal of Materials Processing Technology 48 (1995) 503--515 TRIBOLOGY
IN MATERIALS
Processing Technology
PROCESSING
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 a n d wear affect all processes involved in the extraction of materials a n d their conversion into finished products. Physical contact b e t w e e n tool, die, clamp or a n y other device that contacts the processed material is the basic cause of wear. Excessive friction imposes limits on the efficiency of cutting tools, dies a n d m a n y other equipment. Wear is a severe p r o b l e m in the extraction a n d primary processing of raw materials a n d even the conveyance of raw materials from mine site to refinery imposes additional problems of wear. Therefore research and development into means of controlling friction a n d wear in materials processing is actively pursued by many research groups. The most well established method to control friction a n d wear is by the application of lubricants. A l t h o u g h the d e v e l o p m e n t of solid and liquid lubricants has greatly advanced materials processing it still do not give an ideal performance. Lubricants also bring pollution a n d health 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 coatings w i t h o u t the need for lubricants. Projected benefits from these n e w e r 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 m o s t stages of materials processing, contact occurs between the tool or processing m a c h i n e r y a n d the p r o c e s s e d m a t e r i a l . Exceptions to this rule include, the use of high pressure water jets to either drill holes in a material or in a larger form strip ore from a mine wall. Conveyance of components by jets of air or b y flotation is another situation where contact is avoided. The majority of processing o p e r a t i o n s h o w e v e r i n v o l v e a solid tool c o n t a c t i n g a solid process m a t e r i a l with attendant wear a n d friction. Wear and friction are usually a hindrance to materials processing operations as they result in (i) damage to tools, (ii) i n c r e a s e d e n e r g y c o n s u m p t i o n , (iii) contamination of processed material b y wear particles a n d (iv) p r o b l e m s associated with technologies to control friction a n d wear. Examples of such problems are; (i) destruction of c u t t i n g tools a n d drills by wear, (ii) frictional energy losses in cutting, drawing and stamping, (iii) contamination of molten metal b y wear from stirring blades [1], damage to silicon wafers in semiconductor manufacture by
wearing contact with sawblades [2] and (iv) health h a z a r d s to factory personnel caused by coolants a n d oil mist lubrication. The costs associated with friction a n d wear begin with the extraction of ore from the ground and only terminate with delivery of the product to the consumer. Although each specific cause of wear a n d friction m a y impose only a small cost to the materials processor but there are so m a n y friction a n d wear e v e n t s in a n y materials process that the cumulative cost is very large. Even if the task i n v o l v e d is not directly r e l a t e d to m a t e r i a l s p r o c e s s i n g , s e v e r e penalties m a y be imposed by friction and wear. For instance, iron ore is usually 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 railway track and vehicles [3]. The h a n d l i n g of minerals in bulk also causes wear to silos and conveyor belts while excessive friction between ore particles or with the h a n d l i n g e q u i p m e n t can prevent flow of the m i n e r a l s [4]. For this reason, friction a n d w e a r 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 w e a r a n d friction as found in materials processing and to suggest how they can be better controlled.
MECHANISMS OF FRICTION AND WEAR Wear a n d friction are caused by any physical or chemical p h e n o m e n o n that can occur b e t w e e n contacting surfaces. The most c o m m o n interaction is probably mechanical d e f o r m a t i o n but there are many others that are significant. These are brittle fracture, t h e r m a l d e g r a d a t i o n , chemical reactions, solid state b o n d i n g and solution transfer to name a few [5]. All of these phenomena occur on a microscopic scale within a dynamic contact (a contact w h i c h involves some form of m o v e m e n t , e.g. sliding 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 t h e r e f o r e chaotic p r o c e s s e s [6] but prediction of chaotic processes is still not fully d e v e l o p e d so that an analytical approach to wear remains impossible [6]. A phenomenological approach is still the most e f f e c t i v e 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 A l t h o u g h friction has several causes, the most c o m m o n cause is elastic and plastic d e f o r m a t i o n b e t w e e n o p p o s i n g asperities of surfaces [7]. A s p e r i t i e s are peaks or high points of a typically rough surface. Asperity d e f o r m a t i o n is associated with m o d e r a t e levels of friction a n d is usually found in lubricated contacts where the lubricating film is very thin. Examples of this type of friction can be found w h e r e lubricant additives have been successful in preventing scuffing or scoring between surfaces. The second most c o m m o n mechanism of friction is a d h e s i o n in particular solid state a d h e s i o n . W h e n m e t a l s are cleaned ot superficial c o n t a m i n a n t 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 p h e n o m e n o n is the a d h e s i o n of workpiece material to a cutting tool. A lump of material often remains stuck to the edge of the cutting tool but w h e n 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 r a p i d l y r e f o r m e d on the w o r n 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 i n t e r v e n i n g material b e t w e e n contacting surfaces. Viscous drag is usually generated by a thick film of liquid lubricant and in s o m e 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 e d to as h y d r o d y n a m i c l u b r i c a t i o n [9]. Careful selection of lubricant can allow hydrodynamic lubrication to be initiated 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 d e p e n d i n g on conditions and type of lubricant so that in some cases viscous drag prevails but in o t h e r instances, plastic or elastic deformation of the film material is the controlling factor. The three basic mechanisms of friction are illustrated schematically in Figure 1.
A.W. Batchelor, (7..I,E. Stachowiak /Journal of Materials Processing Technology 48 (1995) 503-515
505
Asperityot hardersudace or trappedwearparticle PLOUGHING// ~ ( ~ , - - J BODY1/motio n
.
~ t e r i a l
VISCOUS DRAG
~
g of film material
:..~_-~;;J~(.-~ :.:)'/~~;.~i~' :: Ji'i:-~-------film material
Plasticallydeformed.layer ADHESION
Adhesivebonding
deformeda s p e r ~
Figure 1.
Schematic illustration of the mechanisms of friction.
Mechanisms of wear A wide range of mechanisms is usually i n v o l v e d in w e a r p r o c e s s d e p e n d i n g on operating conditions a n d external agents, e.g. lubricant or process fluid. In terms of scale and n u m b e r of situations where it occurs, abrasive wear is the m o s t significant type of wear. A l m o s t all m i n e r a l processing e q u i p m e n t is subject to a b r a s i v e wear b y silica as this is present in virtually all rocks a n d soils [10, 11]. Silica has a h a r d n e s s of 1100 Vickers which is harder than a n y k n o w n steel so that a special coating is required to prevent rapid abrasion of steel c o m p o n e n t s b y rocks a n d soil. The early m o d e l s of a b r a s i v e w e a r e n v i s i o n e d the a b r a s i v e grits acting as small cutting tools producing a n d r e m o v i n g wear particles w h e n t r a v e r s i n g the surface. However, this view has been p r o v e d to b e inaccurate a n d 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 d e f o r m a t i o n or fatigue of the w o r n material [5]. In some cases, w h e r e cohesion between g r a i n s is weak, w e a r is c o n t r o l l e d b y intergranular separation. In other words entire g r a i n s are extracted or pulled out by the abrasive grits. This is the reason w h y alumina
can be abraded even though it is c o m p a r a t i v e l y h a r d . A b r a s i v e w e a r has traditionally b e e n seen as a p r o b l e m of the m i n i n g i n d u s t r y but is b e c o m i n g relevant in u p s t r e a m materials processing of composite materials. C o m p o s i t e materials v e r y often contain hard reinforcement phases, e.g. glass fibres a n d ceramic particles in polymers and metals. A b r a s i v e w e a r of dies is prevalent d u r i n g m o u l d i n g of composite components. Abrasive wear is often a very rapid form of wear but alone a m o n g wear mechanisms it can be relatively easily s u p p r e s s e d b y simply raising the h a r d n e s s of the material. When raising the h a r d n e s s of a m~--~erial to resist abrasive wear, the toughness of the material should be m a i n t a i n e d if possible to prevent abrasive wear b y brittle fracture which can be severe. Erosive wear is caused by the impact of solid or liquid particles. The typical examples found in manufacturing are sand blasting and w a t e r jet drilling. The m e c h a n i s m s a n d characteristics of erosive wear d e p e n d on the angle of i m p i n g e m e n t of the eroding particles
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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 fundamentally different. Fatigue and fracture based processes are d o m i n a n t at large i m p i n g e m e n t angles as w e a r particles are r e m o v e d b y w h a t are effectively a succession of hammering blows as each particle impacts. Brittle materials such as ceramics w e a r m o r e r a p i d l y at large i m p i n g e m e n t angles than at shallow angles. The converse is true for ductile metals which usually reveal a m a x i m u m in wear around 30 ° i m p i n g e m e n t angle. C a v i t a t i o n a l wear is related to liquid droplet erosion as it is caused b y large transient pressures d u r i n g b u b b l e a) Abrasive wear
collapse. This wear process is milder than erosion a n d appears to be controlled by the fatigue resistance of materials. C o m p o n e n t s such as stirrer blades in process fluids would be v u l n e r a b l e to c a v i t a t i o n a 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 a b r a s i v e , e r o s i v e a n d cavitational wear are illustrated schematically in Figure 2. High friction coefficients and wear rates in sliding c o m p o n e n t s are usually caused by a d h e s i v e wear. W h e n in sliding contact the intervening films between contacting c o m p o n e n t s have been removed, the strong adhesive b o n d s that form across the sliding interface can cause severe wear as well as high
Cutting direction o! abrasive grit g
Fracture direction of abrasive9rit
~
~.~; c~'acks
Grain pull-out~ direction of abrasive grit direction of abrasive grit Fatigue ~ repeated ~ / ~ _ ~_\\\\\\\\~k~\\\\~deformations by subsequent grits
HIGHANGLEOF IMPINGEMENT Erosive wear,
~ 3
c) Cavitation wear
Movementof 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 cavitational wear.
A.W. Batchelor, (7..IV.. Stachowiak / Journal of Materials Processing Technology 48 (1995) 503-515
friction. In effect, the opposing surfaces tear fragments from each other d u r i n g adhesive wear. This form of wear is very severe, almost never associated with 'mild wear' regime, and s h o u l d be s u p p r e s s e d 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 r e s i s t a n t films are i n t e n d e d to p r e v e n t a d h e s i v e w e a r w h i c h is w h y they are t o l e r a t e d in s p i t e of t h e i r o b v i o u s disadvantages. The m e c h a n i s m of adhesive wear is illustrated schematically in Figure 3. In materials processing impact very often occurs b e t w e e n t w o surfaces. I m p a c t i n g h a m m e r s in a forge are a simple example of this. Wear particles can be released by mechanical fatigue resulting from repetitive contact b e t w e e n asperities of the o p p o s i n g surfaces. Very often, the cracks developed that are invisible from the surface and large lamellar particles can be s u d d e n l y released from an apparently u n d a m a g e d surface [12]. Cracks formed by sliding or rolling contact tend to g r o w parallel to the surface at a small d e p t h below the surface where the shear stress is highest. This form of wear is k n o w n as delamination and m o s t often found in the rolling bearings, gears and cams which drive material processing machinery. A schematic i l l u s t r a t i o n of the pit f o r m a t i o n f r o m subsurface cracking is shown in Figure 4. Frictional heat a n d mechanical agitation of chemically reacting material often causes increased corrosive activity in a w e a r i n g 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 m o s t c o m m o n instance of corrosive wear in materials processing is where a lubricant of excessive chemical activity is
adhesion
507
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 r o b l e m . Direct corrosive w e a r u s u a l l y p r o c e e d s at a steady rate with a relatively small friction coefficient. For this reason it is usually considered as a benign form of wear. W h e n c o r r o s i v e and a b r a s i v e w e a r act together there is a synergistic effect resulting in a very rapid form of wear. Surface films p r o d u c e d by corrosion are very r a p i d l y 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 propagatealongplane of maximum shear stress Figure 4.
Schematic illustration of
pit formation during the rolling contact. Mineral ore processing e q u i p m e n t which must process slurries of ore are subject to severe corrosive-abrasive wear which can rapidly
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destroy even a component made of the hardest material. In m a n y cases a soft p o l y m e r c o m p o n e n t will outlast a steel c o m p o n e n t simply because it is non-corrodible. Fretting wear occurs at the interfaces between components which are in nominally stationary or static contact. Fretting is caused b y e x t r e m e l y s m a l l , t y p i c a l l y a few micrometres, reciprocating movement between two surfaces. The damage caused by fretting belies the small sliding distances and sliding s p e e d s i n v o l v e d . The limited m o v e m e n t b e t w e e n c o n t a c t i n g surfaces a l l o w s w e a r particles to be trapped between the surfaces. The trapped wear particles w h e t h e r in their original form or subsequently modified b y oxidation, accentuate the wear process to cause fretting wear. In m i n i n g e q u i p m e n t the interference fits between rotating shafts a n d b u s h are common sites of fretting. The lifetime of wire ropes is also determined b y the fretting between wires during flexure of a rope [131. Examples of impact, diffusive a n d t h e r m a l 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. H a m m e r s a n d anvils provide the classic example of this type of wear. Percussive rock drills also suffer from impact wear particularly w h e n drilling h a r d granite [14]. Diffusive wear occurs at the interface b e t w e e n c u t t i n g tool a n d chip. The h i g h temperatures of cutting allow rapid diffusion of critical alloying elements from the tool to the chip. The tool material s u b s e q u e n t l y weakens and wear particles are released. This form of wear was originally noticed d u r i n g the development of tungsten carbide cutting tools. The tungsten carbide tool originally showed r a p i d wear w h e n m a c h i n i n g steels u n t i l titanium carbide was a d d e d to the tungsten carbide composite to s u p p r e s s the
solubilization effect. With the development of n e w ceramic cutting tools, diffusive wear is causing problems with silicon nitride cutting tools [151. Thermal fatigue wear is mostly 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 deformation 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 a n d tribocoatings is to control wear and friction at the interface between interacting surfaces. Lubrication is the act of supplying either gas, liquid or a solid powder to the wearing contact which functions as a film material or sustains chemical t r a n s f o r m a t i o n to b e c o m e a film m a t e r i a l [5]. Typical e x a m p l e s are p l a i n mineral oil which directly generates a liquid film, a solid lubricant such as m o l y b d e n u m d i s u l p h i d e and additives in a lubricating oil w h i c h chemically react with the surface to form a film material. The oil additives are specifically d e s i g n to u n d e r g o a chemical t r a n s f o r m a t i o n w h e n in contact w i t h a metallic surface. For example, s u l p h u r a n d chlorine based lubricants react with metallic
A.W. Batchelor, G.W. Stachowiak / Journal of Materials Processing Technology 48 (1995) 503-515
surfaces to form sulphide and chloride films respectively. T r i b o - c o a t i n g s are defined as c o a t i n g s which are i n t e n d e d to reduce friction a n d wear. These coatings function by possessing the mechanical strength sufficient to survive in a sliding, rolling or impacting contact a n d are also able to provide a minute a m o u n t of film material w h i c h effectively controls friction a n d wear. T r i b o - c o a t i n g s are often n o n metallic so that adhesion between metal and a coated tool can be lower than that between metal a n d an u n c o a t e d m e t a l tool. This property of tribo-coatings is very effective in s u p p r e s s i n g a d h e s i v e wear a n d associated high friction. The classical example of nonadhesion between tribo-coatings and processed material is that occurring between steel chip a n d t u n g s t e n n i t r i d e 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 with negligible a d h e s i o n [17]. This sliding characteristic is in c o m p l e t e 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 most widespread application is t h e hydrodynamic lubrication [9]. In h y d r o d y n a m i c l u b r i c a t i o n liquid or gas is e n t r a i n e d into the contact b e t w e e n two converging surfaces b y viscous d r a g forces f o r m i n g a v e r y effective l u b r i c a t i n g film. Because of the converging geometry a larger quantity of gas or liquid is entrained at the contact inlet t h a n is expelled at the contact outlet. Continuity of flow is a c h i e v e d by a pressure field which forms in response to the a p p a r e n t loss of lubricant. The pressure field tends to oppose the e n t r a i n m e n t flow at the contact inlet a n d boost the expulsion flow at the contact outlet. Side flow from the contact is also driven by this pressure field. The same pressure field supports the imposed load and prevents contact between o p p o s i n g surfaces. H y d r o d y n a m i c l u b r i c a t i o n c a n in t h e o r y
509
enable complete prevention of wear and it is c o n s i d e r e d to be the o p t i m u m f o r m of lubrication. Unfortunately hydrodynamic lubrication is a velocity driven p h e n o m e n o n and certain level of velocity is required for its successful operation. In this lubrication regime the level of contact stresses that can be sustained is also restricted. H y d r o d y n a m i c l u b r i c a t i o n f i n d s a p p l i c a t i o n in l i q u i d lubricated rolls and metal pressings where plasto-hydrodynamic lubrication is promoted. W h e n the contact stresses are very high such as, for example, encountered in rolling contact bearings a n d gears, a more specialized form of lubrication, i.e. elastohydrodynamic lubrication takes place. The elastohydrodynamic lubrication is a synergistic interaction between hydrodynamic effects, elastic d e f o r m a t i o n of the contact materials a n d a pressure induced viscosity rise in lubricating oil [18, 19]. Films of lubricant are maintained u n d e r enormous contact pressures, often exceeding 1 GPa by elastohydrodynamic lubrication. E l a s t o h y d r o d y n a m i c lubrication finds a p p l i c a t i o n in the n u m e r o u s rolling 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 . S o m e f o r m 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 a n d pressed material [20]. Solid l u b r i c a n t s such as g r a p h i t e a n d m o l y b d e n u m disulphide function as lamellar c r y s t a l l i n e m a t e r i a l s w h i c h a l l o w easy sliding between lamellae. The lameilae b o n d to the sliding surfaces a n d are resistant to penetration so that it is not easy to remove a film of solid l u b r i c a n t b y wear. Solid l u b r i c a n t s are essential for hot w e a r i n g contacts where the temperatures would cause decomposition of a liquid lubricant. Forging, extrusion a n d hot pressing all rely o n solid lubricants to prevent a d h e s i o n a n d adhesive w e a r tool a n d material to occur. At the t e m p e r a t u r e s a b o v e 500°C, g r a p h i t e a n d m o l y b d e n u m d i s u l p h i d e b e c o m e thermally unstable or are oxidized and therefore they are unsuitable above this temperature range. Low m e l t i n g p o i n t oxides are often used as
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s u b s t i t u t e s a n d these a p p e a r to work by forming a viscous sticky paste on the worn surface. Extreme-pressure additives, friction modifiers and anti-wear additives all function by chemical interaction with the worn surface to form a surface film. Extreme p r e s s u r e additives contain sulphur, chlorine and p h o s p h o r o u s as the active ingredient which reacts with a w o r n metal surface to form a friction reducing film of corrosion products [5]. Although the extreme pressure additives are ineffective at s u p p r e s s i n g w e a r they are extremely effective in suppressing unrestrained severe a d h e s i v e wear. Therefore e x t r e m e p r e s s u r e a d d i t i v e s a r e critical to tapthreading 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 w e a r will c a u s e u n a c c e p t a b l e damage to the surface of the d r a w n component. Friction modifiers a n d anti-wear a d d i t i v e s function by depositing a monomolecular layer of adsorbed lubricant on the worn surface [8]. Each molecule of adsorbed lubricant is linear in s h a p e a n d c o n t a i n s a p o l a r e n d that is attracted to the oxides on a metal surface and a non-polar end that repels any other molecule including itself. W h e n two surfaces covered with these a d s o r b e d films are b r o u g h t into contact, the m u t u a l repulsion b e t w e e n the exposed non-polar ends creates a low friction interface. The critical d i s a d v a n t a g e of a d s o r b e d films is that they d e s o r b a b o v e temperatures in the range 100-200°C a n d they cannot adsorb back onto exposed unoxidized metal. Since b o t h h i g h t e m p e r a t u r e s a n d e x p o s e d m e t a l surface are p r e v a l e n t in materials processing anti-wear additives 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 additives still find however some application in light m a c h i n i n g and pressing operations. The basic m e c h a n i s m s of l u b r i c a t i o n are illustrated schematically in Figure 6. Tribo-coatings a n d wear resistant m a t e r i a l s As discussed previously, there are m a n y
d i s a d v a n t a g e s associated w i t h lubrication w h i c h is increasingly seen as a n obsolescent t e c h n o l o g y . A l t h o u g h t r i b o - c o a t i n g s are currently viewed as the future successor to lubrication they may also become the subject of scepticism. There is a large, even bewildering r a n g e of coatings a n d coating techniques available since m a n y research g r o u p s a n d c o m p a n i e s i n d e p e n d e n t l y p u r s u e their o w n coating technology. Much more is k n o w n about the methods of producing tribo-coatings than is understood about how tribo-coatings function a n d w h i c h is the o p t i m u m coating for any specific application. Almost every feasible coating technique has been applied and tested for its potential in producing an effective tribocoatings. Despite the variety and differences between various tribo-coatings, there are some f u n d a m e n t a l similarities between almost all of them. ~9ating methods All t r i b o - c o a t i n g s d e p e n d o n s t r o n g a d h e s i o n to the substrate for their durability u n d e r wear so that the coating methods have to p r o v i d e a n e n v i r o n m e n t c o n d u c i v e to adhesion d u r i n g deposition of the coating. This r e q u i r e m e n t for strong adhesion involves the exclusion of oxygen and water from the site of c o a t i n g d e p o s i t i o n a n d also a mechanical r e q u i r e m e n t that there is contact between atoms of coating a n d the substrate as opposed to isolated contact b e t w e e n asperities. An interface between coating and substrate consisting of isolated contacts separated by v o i d s is mechanically weak and a cause of early coating failure. A successful method of producing tribo-coatings is based on deposition in a v a c u u m where a coating is developed on a s u b s t r a t e b y the accumulation of atoms or e x t r e m e l y s m a l l p a r t i c l e s of m a t e r i a l . Sputtering, ion plating, vacuum deposition [21] a n d arc coating [22] are the typical examples of the coating techniques. It is also possible to i n t r o d u c e a n electric potential b e t w e e n the substrate a n d source of coating material so that ionized material is projected at the substrate w i t h s u f f i c i e n t e n e r g y to p e n e t r a t e the substrate and e m b e d 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
~
Lubricantpressurereduces
/ ~ ~trained flow
c) Lubricationby adsorbedmonomolecularfilrns Intermolecular ~ ~ - _ ~ ~ ~°~t~:td '~ e.g. pad bearingsurfaces " ~ _ ~ , ~ support . t / , ~,omg Weakbondingor repulsionbetween opposing - CH 3 groups provides low interfaeial shearstress b) Elastohydrodynamiclubrication ~ Pressureprofile e.g. ballbearing / -- ~(//
~
Lub~/////////////~ pre:~uarm ~ n g boosts ed _._
ball r
~ a
rolling, ~, ~ ~
~" elastic deformationof the contactingsurfaces! Figure 6.
-
constrictionat the exit limiting lubricantflow
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
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where the rod of coating material is moved along the surface while being rotated u n d e r 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 coating p a r a m e t e r s is n e e d e d to p r e v e n t a p o r o u s m i c r o s t r u c t u r e from d e v e l o p i n g . Thermal spraying and plasma spraying generate a porous microstructure which results in the formation of relatively brittle coatings. With p r e s e n t technology a c o m p r o m i s e is necessary b e t w e e n coating deposition speed a n d r e s u l t i n g microstructure of the coating. F l a m e a n d p l a s m a s p r a y i n g give r a p i d deposition rate while v a c u u m based coatings a l t h o u g h slower can produce coatings with a better microstructure.
times longer than uncoated moulds [26]. The life time of cutting tools is not only increased but also machining speeds and feed rates can be increased w h e n coated tools are used. Ionimplantation 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 a p p e a r to be limited to small scale precision machining tools and dies. Friction surfacing a n d explosive b o n d i n g are suitable for planar 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 vacuumbased coating m e t h o d s which were mostly developed subsequently to the D-gun. Some typical tribo-coatings for materials processing applications are listed in Table 1.
Avvlications of tribo-coatin~s Tribo-coatings are rapidly gaining e x t e n s i v e u s e in m a t e 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 i n i s h e d p r o d u c t . C o a t i n g s r e a c h i n g several m i l l i m e t r e s in thickness t h a t are produced b y thermal and plasma spraying are v e r y u s e f u l to the m i n i n g i n d u s t r i e s in particular. Ore extraction and processing tools, such as drills, crushers a n d 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 m o r e p r e c i s e l y d i m e n s i o n e d m a t e r i a l p r o c e s s i n g tools. V a c u u m - b a s e d coatings are being applied to cutting tools, dies and moulds to prevent wear. In the case of moulds, any d a m a g e by wear detracts from product quality so it is found that t h e u s e of c o a t i n g s e n a b l e s r e m a r k a b l e extensions of mould life before product quality becomes unacceptable [26]. Moulds represent a very dramatic example of the benefits brought b y tribo-coating. Any d a m a g e by wear to a mould such as scratches detracts from product quality so that the working life of uncoated m o u l d s is c o m p a r a t i v e l y short. It has been reported that in certain cases the lifetime of coated moulds was found to be more than five
Wear resistant materials Wear resistant materials are vital to the d u r a b i l i t y of material processing tools and equipment. Although the traditional materials used have been hardened steels, e.g. 'high speed steel', ceramics and ceramic-hard metal composites are b e c o m i n g increasingly important because of their advantages in terms of durability, wear resistance and productivity. Ceramic tools made of alumina, s i l i c o n c a r b i d e a n d silicon n i t r i d e are relatively well d e v e l o p e d a n d they are used commercially [15]. These materials are useful b e c a u s e of t h e i r e x t r e m e h a r d n e s s and resistance to a d h e s i v e wear. W h e n 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 l a t e s t idea in n e w n o n - m e t a l l i c materials is the d e v e l o p m e n t of ultra-hard materials w h i c h are as h a r d or harder than diamond. A composite of cubic boron nitride powder with aluminium nitride and a l u m i n i u m boride binder has been tested as a cutting tool material [28]. However, the main p r o b l e m associated w i t h the cubic boron
A.W.. Batchelor, G.W. Stachowiak /Journal of Mate~als Processing Technology 48 (1995) 503-515
Table 1.
513
Some typical tribo-coatings for materials processing applications
i Processing equipment Mineral Processing tools
Type of coating
Coating Process
Hard coating on softer metal
Plasma a n d thermal spraying,
D-g~.
Moulds, dies
Non-metallic coating on metal
PVD, CVD, plasma spraying
Cutting tools
Hard nonmetallic material on metal or transformed microstructure on metal.
PVD, CVD
Ion implantation Laser surface alloying
Coating characteristics Resistant to: abrasive wear, erosive wear, cavitational wear, rolling wear a n d 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 a n d cavitational w e a r Pressing tools, Punches
Rolls
Compressive sh'ess in surface layers. Amorphous material in surface Thick layer with hardness at high temperatures
Ion implantation useful for small tools
Resistant to impact wear, a d h e s i v e wear, fatigue wear
Weld overlays
Resistant to high temperature wear and contact fatigue
n i t r i d e composite is the wear of the binder w h i c h allows b o r o n nitride particles to be detached from the tool during cutting [28].
n e w l y d e v e l o p e d m e t h o d s of o p t i m i z i n g friction a n d w e a r for specific m a t e r i a l s processing applications, the following conclusions can be drawn:
Conclusions 1. Materials processing presents many varied a p p l i c a t i o n s of wear a n d friction control technology. From a review of established and
F r i c t i o n a n d w e a r i m p o s e severe limitations on materials processing not only in terms of destruction of tools but also through loss of product quality.
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2.
Existing techniques for lubrication and w e a r p r o t e c t i o n only offer partial protection against wear and friction. Tribo-coatings, i.e. mechanically robust coatings to reduce friction and wear, can offer substantial i m p r o v e m e n t s in tool life a n d consistency of product quality. There is a very wide range of triboc o a t i n g s w h i c h necessitates detailed testing before the optimum coating for a particular application is found. The t e c h n o l o g y of friction a n d wear control is in a state of f u n d a m e n t a l change a n d most accepted m e t h o d s of lubrication or wear prevention may in future be superseded by entirely different technologies.
3.
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5.
Acknowledgements The a u t h o r s w o u l d like to t h a n k the D e p a r t m e n t of Mechanical a n d Production Engineering, Nanyang Technological University a n d the Department of Mechanical a n d Materials Engineering, the University of Western Australia for their s u p p o r t of this work. The efforts of the Centre for Educational Development, Nanayang Technological University to arrange the illustrations are also gratefully appreciated.
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