Batchelor, A.W. - Tribology

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Journal of

ELSEVIER

Journal of Materials Processing Technology 48 (1995) 503--515 TRIBOLOGY IN MATERIALS PROCESSING

Processing Technology

Materials

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

0924-0136/95/$09.50 1995 Elsevier Science S.A. All rights reserved SSD! 0924-0136(94)01689-X

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 BODY 1/motio n .VISCOUS DRAG

~

g of film material

~ t e r i a lPlasticallydeformed.layer ADHESION Adhesivebonding

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

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