Comments on the sliding wear of metals

Comments on the sliding wear of metals

Tribology Internarional Vol. Printed SCIENCE: PII: Colmments of metals on the 30, No. 5 pp. 361-367, 1997 0 1997 Elsevier Science Ltd in Great ...

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Tribology

Internarional

Vol.

Printed SCIENCE:

PII:

Colmments of metals

on the

30, No. 5 pp. 361-367, 1997 0 1997 Elsevier Science Ltd in Great Britain. All rights reserved 0301479x/97/$17.00 + 0.00

SO301-679X(96)00065-5

sliding

wear

D. A. Rigney The author reviews selected experimental results which have contributed to improved understanding of sliding wear processes. The emphasis is on the chemical and structural changes which occur at and near the surface of metallic materials during sliding in diff’erent environments. The importance of plastic deformation, fracture, transfer, mechanical mixing, phase transformations and oxidation is discussed. Examples of transitions are described, and interesting correlations noted. In selecting the content of this paper, the author includes controversial results and conclusions and raises questions about the development of wear equations, interpretations of the wear coefficient, the importance of adhesiion, the roles of hardness, the causes of transitions and the location of debris-producing cracks. 0 1997 Elsevier Science Ltd Keywords: and and

sliding microstructure, transfer

wear, near-surface material, wear wear rate transitions, subsurface

It is clear from archeological studies that humans have recognized the importance of the wear of metals for many centuries’. Although many advances in technology have allowed some degree of wear control, understanding of wear processes has come slowly. True, certain correlations have been noted, such as the resistance of harder materials to wear in some situations, but the exceptions demonstrate that many pieces of the puzzle are missing. LucretiJs, a careful observer of natural phenomena, showed remarkable insight concerning wear. He suggested ::hat wear typically involves processes on such a smal. scale that our limited powers of observation and the technology available do not reveal some of the important details’. Fortunately, our ability to observe the details has improved dramatically. The optical microscope, combined with improved metallographic techniques, provided inportant new information about the importance of local differences in materials and the relation of these to properties. It became clear that microstructure could affect the behaviour of materials in service. X-rays, various electron and ion beam instruments, and, more recently, sensitive surface probes, have provided Maferials Science and Engineering, umbus, Ohio 43210-1179, USA

The Ohio

State

University,

Col-

Tribology

mechanisms, wear fracture, adhesion

further information about microstructure and nanostructure. This wealth of new information demonstrates that many of our assumptions about friction and wear have been greatly oversimplified, and some are clearly wrong. Yet, as in many fields, old paradigms are persistent, or, as Samuels has noted, ‘Myths die hard, even in science.‘3 There are many ways to judge the degree of understanding of a subject. In science and engineering, mathematical models, based on well established principles and on the results obtained from carefully done experiments, should describe observations quantitatively and should have some predictive capability. In the case of wear, it would be desirable to have equations which would allow prediction of wear rate for a given combination of materials, environment, geometry, load, speed, coatings, etc. Many wear equations have been proposed, but only the simple linear wear equatior? is widely used, and none is reliable for the wide range of tribo-systems which are important. Perhaps it should not be surprising that a universal wear equation is elusive. In this paper, some of the reasons will be discussed. They include effects of microstructure and also dramatic changes in microstructure which occur as a result of tribochemical processes. The literature on tribology includes many reviews of the wear of metals9-r5. This paper is not intended to be a comprehensive review. Instead, the author has selected a limited number of topics which illustrate International

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important aspects of this subject. Certain common themes emerge. These should be considered by anyone evaluating existing wear models or attempting to develop new models. In selecting the content of this paper, the author has included controversial results and conclusions and has raised questions which need to be addressed. It is hoped that this will stimulate discussion and help to achieve the goals of the editor. Near-surface

material

Experiments with various kinds of markers show clearly that sliding of metals produces very large plastic shear strains (y > 10) at the sliding interface and large strain gradients in the near-surface material. Thus, the regimes of deformation are stages IV and V, as indicated by the deformation substructures observed’6*‘7. Material transfer, aided by adhesion and shear instabilities, can occur soon after sliding begins. This begins a process of mechanical mixing which is similar to what happens in a ball mill, as in the early stages of mechanical alloying. This sequence commonly produces patches or a coating of nanocrystalline material, from which typical sliding wear debris are derived. Such debris have the same composition, structure and hardness as the mechanically mixed surface material produced by sliding. This sequence has many variations, including those involving environmental effects. Any one of the steps involved can be the rate controlling step for a given tribo-system. Therefore it is unrealistic to expect that a single wear equation can be developed to cover all cases- unless it contains so many parameters that it becomes impractical to use it. The sequence described above is not an efficient process for material removal. Plastic deformation, transfer and mechanical mixing involve much work. This provides a natural explanation for the small sizes often found for the wear coefficient li. Alternative interpretations of k, summarized in Reference 8, are based on assumptions which are not well supported by experimental observations. The nanocrystalline material commonly contains at least one second phase, the source of which could be the sample itself, a coating, the counterface or the environment. However, in some systems there is not enough of a second phase present to explain the stability of the small grain size. Perhaps adsorption or chemical reaction at the surface, combined with mechanical processes,allows incorporation in the boundaries of species which inhibit grain growth. The grain size of the nanocrystalline material produced during sliding is typically lo-50 nm, but grains as small as 3 nm have been observed using dark field TEM. Such small grain size is consistent with either a fracture model, based on a balance of fracture energy and surface energy, or a deformation model, based on dislocation sources in the boundaries and a limiting strength given by the theoretical shear strength of the crystal’6,17. Limited high resolution TEM indicates that the boundaries are well-defined high angle grain boundaries’8,‘9. 362

Tribology

The nanocrystalline material can be harder or softer than the underlying material, and this strongly influences the smoothness of sliding and the nature of the wear debris. It is difficult to predict the behaviour because both adhesion and mechanical properties are involved and these are interrelated. Adhesion influences transfer, the integrity of the mechanically mixed material and the chemical composition of both this material and the debris. These in turn affect the hardness, yield strength, ductility and fracture toughness of the surface material.

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Surface

vs. subsurface

fracture

Successful modelling of wear requires some knowledge about the origin of the cracks involved in generating debris. Unfortunately, the origin of these cracks has not been well established. Flake debris have been noted by many tribologists. In most cases they are ‘explained’ by stating that delamination was involved. The implication is that the process was similar to one of those described by Suh et n1”‘. A more recent treatment of damage accumulation leading to delamination has been given by Alpas2’. In each case, the fracture leading to delamination is assumed to originate below the sliding surface. However, flake debris can be produced by many different processes, as discussed in References 22-26, and some of these involve cracks which initiate at the surface rather than below it. Subsurface cracks may be responsible for generating some wear debris, but the evidence for them is not convincing. Some photographs show multiple cracks associated with particles, but the cracks seem to repel each other rather than link up. Other cracks appear to have subsurface origins, but they could have started at the surface and then spread laterally. This is difficult to determine from cross-sections. In some cases, crosssections show surface material which contains transfer material and components arising from interaction with the environment, yielding an apparent subsurface crack, but not in the base material. In still other cases, chemical etching has opened up cracks which may not have existed during sliding. There are practical problems associated with clarifying the question of surface vs. subsurface origins for debris-producing fracture. One is that deformation, defect accumulation or crack nucleation can be rate controlling, with crack propagation being so rapid that cross-sections will not show any cracks. Another is that cross-sections are two-dimensional slices which cannot give enough information on the complete threedimensional system. Still others involve transfer, mixing and environmental effects. These can be responsible for features which mimic subsurface cracks. Unless a non-destructive technique with adequate resolution and response time is developed, this problem is likely to be with us for some time. Effects

of environment

An emphasis on adhesion in discussing tribological processes would lead one to expect that friction and 5 1997

Sliding

wear would be higher in vacuum number of materials combinations, Severa! examples will be given.

than in air. With a the opposite is true.

The first example involves dual phase steels consisting of ferrite and martensiteZ7. These were selected to investigate a possible connection between fatigue and wear. Suzuki and McEvily2’ studied fatigue crack growth rate for ATSI 1018 steels heat treated to give two different dual phase microstructures. Type A material consisted of martensite islands within a continuous ferrite phase; type B material consisted of ferrite islands and the continuous phase was martensite. Normally, fatigue does not depend strongly on microstructure, but with these steels, the results were dramatic. The type B material had a higher threshold value of stress intensity and a much lower near-threshold fatigue crack propagation rate than type A. It was suggesed that when the continuous phase is hard, it acts as a barrier which stops cracks in the ferrite. Would these materials have different sliding behaviour? The results of sliding tests in vacuum were similar for the A :ults showed that boron had no apparent effect on the sliding behaviour of Ni3A1. For both materials the wear rate was appreciably higher in air than in vacuum and the debris particles were finer in air. Also, despite the well known oxidation resistance of nickel alumimdes, the debris particles were at least partially oxidized. In fact, the debris contained a ferromagnetic component (probably Ni) produced when the aluminium selectively oxidized. Increasing the test temperature to 400°C had very little effect on the sliding behaviour in vacuum, but the wear rate in air was reduced. Together, these results suggest that partial oxidation aided fracture at room temperature, as in the dual phase steel example described earlier. Recent work by Kennedy”” provides additional support for this interpretation. Further examples of the influence of environmental effects are provided by experiments with Pb-Sn and babbitt alloy~~‘.~~. In vacuum or in non-reactive Tribology

wear

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D. A. Rigney

environments such as mineral oil, argon-5%H, or nitrogen, sliding was smooth and friction was low. No debris were produced in these very ductile materials. The material simply flowed plastically. In air, sliding was not smooth and both the friction and wear were high. Examination of cross-sections revealed that a fine-grained layer was produced on the surface only in the air tests. The debris had the same fine-grained structure as that surface layer. No oxide particles could be detected in the fine-grained material, but it is clear that this layer is produced and stabilized only in a reactive environment. The role of a lubricant for such materials would then seem to be indirect. It would not reduce friction, but would instead shield the material from the reactive environment and prevent the formation of surface layers which would increase friction. Additional analysis of these results has pointed to the importance of microstructure stability and the sign of the hardness gradient produced by sliding. However, it is more appropriate to discuss these effects below, in the section on hardness.

Examples

of transitions

It is well known that friction and wear rate can change suddenly as sliding progresses. A number of different reasons have been identified. Some of these will now be described. The first example is the sliding behaviour of austenitic stainless steels33m35. The most familiar of these, type 304, is known to have poor sliding characteristics, including poor resistance to galling and seizure. The key to understanding this behaviour is the degree of metastability of the austenite, or face-centred cubic phase. Type 304 stainless steels is a solid solution alloy containing significant amounts of both austenite stabilizing Ni and ferrite stabilizing Cr solutes. The composition is such that modest amounts of plastic strain can trigger a rapid martensitic transformation to the body-centred cubic phase, called (Y’ in this alloy. Therefore, the initial sliding behaviour is similar to that of typical FCC metals which exhibit work-hardening. A sudden transition occurs when (Y’ is formed at the surface. This, combined with transfer and mechanical mixing, leads to a surface layer which is hard enough to abrade tool steels. This kind of transition does not occur if the temperature of the interface is raised sufficiently, either by ambient heating or by using higher sliding speeds. It also does not occur in type 310 alloy, which has a composition which stabilizes austenite. Type 3 16 alloy has intermediate behaviour the kind of transition exhibited by 304 occurs only after sliding for a longer distance. The differences in sliding behaviour of 304 and 3 10 stainless steels extend to the type of debris produced. With 304 sliding against tool steel, the debris consist mainly of cutting chips from the tool steel. For 310 against the same tool steel, the debris are flakes consisting mainly of 310. They are produced by transfer to the counterface, further deformation, and ultimately delamination. Of course, martensite can also be formed by the more familiar austenitizing and quenching processesfamiliar International

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in ferrous metallurgy. Trilk36,37 and Venkatesan38 have used the wear maps of Lim and Ashby39.“0 and the results of Welsh4’ to select suitable loads and sliding speedsfor tests on steels. It was expected that martensite would be formed when frictional heating raised asperity temperatures into the austenite portion of the phase diagram and the heated material then cooled rapidly by conduction when it left the interface. Indeed, very hard surface material was formed, but it was much harder than would be expected from martensite itself, and the results were quite different in vacuum and in air. At most sliding speeds, local regions could be hard or soft. These variations developed from different combinations of plastic deformation, martensite formation, grain refinement and oxidation. It should be noted that published wear maps were useful in guiding the interpretation of the results for sliding of steels in air, but they did not provide much help in interpreting the results from tests in vacuum. The work of Sawa2’ has been cited earlier in the discussion of environmental effects. The same work provides interesting correlations of transfer with transitions in sliding behaviour. In both vacuum and air, and for both types of dual phase steel microstructures, the friction traces showed three distinct regions. The first stage exhibited relatively low friction with small fluctuations. Toward the end of this stage there were occasional brief spikes in the friction trace. These provided convenient early warnings that the first stage was about to end. The second stage began with a sudden transition to higher friction and larger fluctuations. The average friction then gradually decreased asymptotically toward a steady state level. In the third stage, the large fluctuations remained but the average friction remained nearly constant even after long sliding distances. Tests were interrupted after different sliding distances so that each stage could be investigated. Transfer material was found in each case. However, the amounts and the distribution were much different. In stage one the transfer patches were relatively small and randomly distributed on a plastically deformed surface. Immediately after the transition, the surface was somewhat rougher and the amount of transfer was much larger and was distributed in larger patches. In stage 3 the transfer material was evenly distributed and mechanically mixed with the substrate material. It is particularly interesting that the system could tolerate a certain amount of transfer, but a major change in sliding behaviour occurred when the amount of transfer exceeded a critical amount. What determines this critical amount is not yet clear. The last examples of transitions are provided by metal matrix composites. These are receiving increased attention for tribological applications”‘. Aluminium alloys containing hard particles such as SIC are particularly interesting. Larsen has demonstrated that the SIC particles near the surface tend to fracture during sliding43. Alpas et al 4448 have shown that such fractures are associated with a wear rate transition as load is increased. The authors suggest that at low loads the SIC particles help to support the load, resulting in reduced wear compared with unreinforced aluminium 364

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alloys. When particle fracture occurs above a critical load, the wear rates of the materials with and without Sic particles are similar. At still higher loads a transition to severe wear occurs in both materials, but the critical load is higher with SIC particles present. Alpas associates this transition with thermal softening, which is postponed in the composite material. Hardness There are good reasons for concentrating on hardness and its correlations with wear’. In the case of abrasion, addressed elsewhere in this set of papers, the basic reason is well understood. Effective abrasion involves microcutting, which is a relatively efficient way of removing material mechanically. Cutting in turn requires penetration into the abraded material, and harder materials resist such penetration. Cutting fluids provide cooling, but they can also change the critical angle for cutting by reducing friction. As expected, lubricants increase the abrasive wear rate. In contrast, a good lubricant reduces wear rate in sliding situations. The effects of hardness on sliding behaviour are much more varied and complex’. Transfer and mechanical mixing can modify the relative hardness values of the sliding components where they contact. The hardness of the mixed material can be greater or less than that of the underlying substrate, and this directly determines whether it pressesin or remains as patches or plateaus on the surface. The types of debris produced are different for these two cases49.The mixed material can have a wide range of hardness values, depending on the volume fractions of component phases and on the grain size. The material can also be heterogeneous, especially early in the mixing process, so its hardness can vary locally. Kato et nPom5’have examined the connections between the degree of penetration of an indenter, the shear strength of an interface in abrasion, the attack angle and the hardness ratio (HdisklHprn= H,IH,) in sliding wear. These ideas were later applied to help explain the results of pin-on-disk sliding tests for various pure metals in vacuum53. At first the results were extremely confusing, without any apparent pattern. The difficulty was related to the wide range of local hardness on each surface. Such variations are expected because of local differences in the amount of deformation, work hardening, transfer and mixing. A correlation with H,,/H, at the start of the test was not found. However, a good correlation was found if the variations in local hardness were analysed. Severe wear occurred when the range of hardnessratio HdIHp after the test included values below about 1.0. When It included only values above about 1.0, mild wear occurred. If the initial hardness ratio was unfavourable, severe wear appeared soon after the start of the test. If the initial hardness ratio was favourable, a transition to severe wear could still occur after a critical sliding distance if the sliding conditions allowed an unfavourable hardness ratio to develop locally. Thus, the observation54 that friction and wear behaviour change when pin and disk materials are interchanged (A on B; B on A) is readily explained. 5 1997

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This same correlation can be used for self-mated (A on A) sliding. The pin, or the tribo-element which is in mo:e nearly steady contact, work hardens faster, and this inevitably leads to an unfavourable hardness ratio, rough sliding and severe wear. This explanation is an alternative to the usual one, which is based on adhesion ideas.

ent as the surface is approached from the bulk material. Second, provision should be made to shield the material from any environmental component which would aid in the formation of a positive hardness gradient.

The critical hardness ratio may actually be about 0.8 rather than 1.0. The results would then be consistent with the ‘natural’ scratch hardness scale proposed by Bowden and TaborT5. Further experiments are needed to obtain enough data to decide which number is closer to the critical hardness ratio for sliding.

It is well known that conditions during early stages of sliding are different from those at much longer times. The early stage, or break-in period, is difficult to reproduce from one test to another because of its sensitivity to small changes in geometry and surface preparation. After long sliding times, all of the following tend to become constant: average friction coefficient, magnitude of friction fluctuations, surface roughness, depth of deformed layer, and the composition and microstructure of near-surface material. Fluctuations are thought to be a natural consequence of the distribution of asperity contacts and heterogeneities in the material and in other parts of the tribo-system. One would then expect the wear rate to have a steady state value also.

The preceding discussion was concerned with materials which work harden during sliding. The opposite trend would be expected if work softening occurred. This would be expected for certain materials at elevated temperature, and for some materials even at room temperature. This expectation has not yet been tested experimentally. Recenr work on the sliding behaviour of Pb-Sn and babbit: alloys has focussed renewed attention on the role cf hardness and hardness gradients in sliding behaviour”‘,3’. The microstructure of these materials consis:.s of at least two phases. Some of these alloys contain hard intermetallic particles which may contribute very little to their sliding behaviour5’. More important is the stability of the microstructures. Courtney et a15h-4’),Nakagawa et a16’ and Kitamura et a16’ have reported that coarsening and associated softening in PbSn alloys depend on the amount of plastic deformatior: at a given temperature. Morris et aZ62-65have noted coarsening and softening within shear bands in Pb-Sn solders subjected to thermal fatigue. The large plastic strains which accompany sliding should give rise tc similar effects which could influence sliding behavi our. Experiments with a hard bearing steel ball sliding in vacuum on various Pb-Sn and babbitt alloys indicated smooth sliding with low friction and wear. Crosssectior:s exhibited exactly the changes in microstructure expected, with coarsening and softening near the surface. Thus, these materials create the proper conditions for 10’~ friction - a soft surface layer supported by a harder base material. Such a materials system is selfrepairing, with continuing deformation localized near the surface, so sliding should remain smooth. As indicated earlier, sliding of these same materials in air gives very different results. A coarse, soft layer is still produced, but it becomes covered with a finegrained harder layer. The sign of the hardness gradient is now reversed compared with that which develops in vacuum or in a non-reactive environment. Very similar behavlour has recently been observed in selected aluminium alloys66. The cf,lrrelation with the sign of the hardness gradient which develops suggests simple guidelines which can be applied to design materials with desirable sliding characteristics. First, the material should work-soften at the intended use temperature and at plastic strains which are achieved during sliding, thus allowing development and maintenance of a negative hardness gradiTribology

‘Steady

state’

Zharin has developed a non-contact probe which can monitor a wear track continuously during sliding”7-70. It is sensitive to changes in the defect structure of material adjacent to the surface. Zharin has used his device to detect periodic changes in the wear rate of various metals. For a single phase metal such as brass, there is a single well-defined period, whereas for a two phase Cu-Sn alloy a second peak appears in the periodic signal. Thus, the probe seems to be sensitive to the alloy phase content. Zharin has also examined worn specimens for tests which were stopped at different stages (0, l/4, l/2, 3J4) in one of these periodic cycles. There were distinct differences in the appearances of the wear tracks for these different stages. Zharin suggested that the probe detects subsurface damage before it affects the surface in an obvious way. During each cycle, the damage accumulates until significant amounts of debris are generated in a correlated way over most of the wear track surface. That is, the wear debris are mainly generated during a relatively small number of specimen rotations compared with the number of rotations in one full period. While this seemsreasonable, it is quite different from the usual picture of wear by fatigue, as proposed by various tribologists, including Kragelski”, IIalling’*, Finkin and Kimura75. Their models assume that the distribution of asperities and asperity contacts are such that the fatigue of asperities is statistically random, leading to steady state conditions and a constant wear rate. It should be noted that the Zharin technique may be able to contribute new information to the long-standing debate in tribology concerning surface vs. sub-surface nucleation of debris-producing fracture, mentioned earlier in this paper. Summary The author has reviewed selected experimental results which have contributed to improved understanding of sliding wear processes. The emphasis has been on the chemical and structural changes which occur at and International

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near the surface of metallic materials during sliding in different environments. The importance of plastic deformation, fracture, transfer, mechanical mixing, phase transformations and oxidation have been discussed. Examples of transitions have been described, and interesting correlations have been noted. In the hope of stimulating discussion, the author has raised questions about the development of wear equations, interpretations of the wear coefficient, the importance of adhesion, the roles of hardness, the causes of transitions and the location of debris-producing cracks.

Acknowledgements The author is pleased to acknowledge the many contributions of his students and other colleagues and research support from the National Science Foundation (Surface Engineering and Tribology), Ohio Aerospace Institute, Packard Electric Div. of GMC, Goodyear Tire and Rubber Co., Nippon Steel Corp., the Office of Naval Research, and the Center for Materials Research at The Ohio State University.

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5 1997

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