The sliding wear mechanisms of metals, mainly steels

The sliding wear mechanisms of metals, mainly steels

The sliding wear mechanisms of metals, mainly steels T.H.C. Childs* Early studies of the dry and boundary lubricated wear of metals concentrated on t...

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The sliding wear mechanisms of metals, mainly steels T.H.C. Childs*

Early studies of the dry and boundary lubricated wear of metals concentrated on the most catastrophic wear conditions, severe wear and scuffing. It was established how these states could be avoided by protective film formation. Subsequently studies away from these conditions have shown wear either by the removal of surface reaction films or by metal surface fatigue. This review traces these developments and places them in perspective

Our knowledge of the mechanisms of the wear of metals sliding on metals stems mainly from research since 1950. This brief review compares and contrasts some of these studies. It is limited to wear which might occur in machine elements such as gears, bearings, slides and linkages and thus is not primarily concerned with die wear in metal forming and cutting; nor, despite its occurrence in machine elements, is wear produced by abrasives discussed. Further it is mainly concerned with qualitative descriptions of mechanisms rather than with quantitative theories of wear rates. Wherever possible, examples have been taken from the wear of steels. Early measurements of sliding wear were carried out mainly in dry conditions, partly because the interpretation of results was easier in the absence of hydrodynamic lubrication but also because of the direct need of the nuclear power programme, for example, for this information. In the ten years from 1955 to 1965, Archard, Hirst, Kerridge, Lancaster and Welsh ~-1° demonstrated two conditions of dry wear which they called severe and mild. In the former, rubbing caused surface roughening and the wear debris was metallic with linear dimensions from 10 to I00 btm, occasionally increasing to 1 mm. In the latter, surfaces were smoothed by rubbing and debris was mainly flakes of oxide with dimensions from 0.01 to 1.0/am. In the conditions of their tests, adhesive transfer of metal from one surface to the other was often a preliminary to its degradation to wear debris. For this reason these forms of wear have since frequently been called adhesive wear, following Burwell's H earlier classification of wear mechanisms, although these authors never used that term and recognised that failure of adhesion was required to produce a wear particle 8 . Subsequently Sub and his colleagues ~2- ~9 have studied conditions of dry wear in which metallic debris has been produced with surface smoothing and without transfer and Quinn 2°-24 has led work in which oxidised debris has been produced without transfer. The theories of these groups have become known respectively as delamination and oxidational wear. Expectations from the earlier studies ~- 1o that wear can occur rapidly by removal of metal or more slowly by removal of surface reaction films have been carried over *School o f Mechanical and Manufacturing S y s t e m s Engineering, University o f Bradford, Bradford, West Yorkshire B D 7 1DP, UK

0301-679X/80/060285-09 $02.00 © 1980 IPC Business Press

into studies of wear in oil-lubricated conditions and much effort has been put into the study of the chemistry of metallic reactions with oils, additives, and contaminants to produce protective long-lasting surface films 2s-34 . Monitoring wear debris in oils from bench tests and successfully-running machinery has shown, however, that the debris frequently contains fine metal plates only a few microns in size 3s-36 , similar to but smaller in size than those observed by Suh in dry conditions ~3-14. The chemically inert conditions in which these are formed are beginning to be studied in laboratory experiments ~7-39 . The experiments carried out by Archard and Hirst 1 established that after initial running-in periods, the rate of change of wear volume with sliding distance often became constant and proportional to the load pressing the sliding surfaces together. The constant of proportionality k, called the dimensional wear coefficient, is a useful measure of the harshness of a wear process. The ranges of k which may be calculated from wear measurements in the dry and lubricated conditions just mentioned are shown for the sliding of steels on steels in Fig 1

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Fig 1 Steel on steel wear coefficients for dry and lubricated sliding

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Childs - The sliding wear mechanisms o f metals, mainly steels

(k values in the presence of lubricant have been obtained from tests in which only a small proportion of the total load was supported hydrodynamically). The decreasing intensities of wear down the left-hand column suggest different rate controlling mechanisms for severe, delamination and lubricated metallic wear. The wear rates of surface reaction films, covering three orders of magnitude, also suggest a variety of rate controlling conditions. As a preliminary to considering these in more detail, a brief account will be given of the mechanical conditions in which wear occurs.

Mechanical regimes of wear When two metal surfaces are pressed together the occurrence of plastic flow at the contacting high spots depends on the roughness of the surfaces and on (E'/H), the ratio of their effective Young's modulus to the hardness of the softer surface. Contact theories, reviewed by Archard 41 , show that the appropriate roughness parameter may be interpreted as the mean slope e of the high spots flattened by the contact and that for plastic flow the plasticity index (E'/H)e should be greater than 1.0. For common combinations of metals 10 < (E'/H) < 100 while for ground surfaces e ~ 0.1 rads. It is clear, therefore, that for many surfaces initial plastic flow of the high spots is usual. If sliding occurs between the surfaces, plastic flow depends additionally on (r/k), the ratio of the interfacial shear strength at a contact to the shear flow stress of the metal. For contact regions deformed plastically in the absence of sliding, sliding can cause them either to become smoother or rougher. If they become rougher, the roughness can take the form of grooving or tearing. Examples of all three possibilities are shown in Fig 2. Analyses of the flow of model contacts, recently discussed by Challen and Oxley 43 , suggest a criterion for smoothing to be (r/k) < cos 2e; for roughening (r/k)/> cos 2e is suggested, but whether grooving or tearmg predominates depends additionally on the deformed metal's ductility and work-hardening characteristics 44 . If the surfaces are smooth enough for their high spots to remain elastic in the absence of sliding, other effects of (r/k) on flow need to be considered. Studies of the stresses beneath the contact between elastic spheres or cylinders show the maximum shear stress to lie below the surface in the absence of sliding; sliding shifts the position of the maximum shear stress closer to the surface but provided the sliding friction coefficient/a ~ 0.25 it does not influence its magnitude; when/a > 0.25, the maximum shear stress increases with/a and is at the surface 4s . Assuming slightly conservatively that the maximum pressure a high spot can withstand elastically is 2k, it can be in~rred from this work that, if (r/k)~< 0.5, the criterion for asperity plastic flow is not influenced by sliding although the stress distribution beneath the surface is altered; for ( r / k ) ~ 0.5, however, sliding will induce superficial plastic flow for values of the plasticity index less than 1.0. The different regimes of response of asperities to loading are summarised in Fig 3 in which (r/k)is plotted against asperity slope, the latter on a logarithmic scale. The boundary between elastic deformation and plastic burnishing regimes has been drawn broadly as its exact position depends on (E'/H).

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Fig 2 Tracks on a surface showing (a) smoothing o f a hard steel after many sliding cycles, (b) tearing o f a soft iron after one pass o f a slider and (c) the formation o f grooves at the end o f another track on soft steel

Childs - The sliding wear mechanisms o f metals, mainly steels

Sliding can modify not only surface roughness but also (r/k). Various possibilities of surface change with time, all of which might be expected to influence the nature of the wear process, are represented by the paths l to 5 superimposed on Fig 3.

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Breakdown of a protective surface film at the start of sliding is represented by path 1, leading to surface tearing and transfer, the perpetuation of plastic flow and to rapid wear. In some cases sliding may, over a longer-than-initial time scale, cause surface modifications which decrease (r/k)and a transition to burnishing can occur, continuing until the surface is smoothed to an elastic, low-wear, state (path 2). Cyclic conditions (path 3) can be imagined in which removal of the run-in layer leads to renewed transfer, modification and running-in. These are the conditions studied in the main by Hirst and his co-workers 1- i0. Alternatively (paths 4 and 5) protective surface films may prevent tearing and transfer and running-in may occur smoothly by a mixture of burnishing and film removal to establish elastic or nearly elastic conditions at various stress levels depending on the nature of the surface films. In air, oxide films may give protection with fairly high (r/k)values; in lubricated sliding a variety of friction polymer, organo-metallic or inorganic films may additionally influence ( r / k ) . Wear in the run-in state will then occur by metal fatigue or by the removal of surface films (both without transfer) the dominant mode depending on the balance between the stress level causing fatigue and the metal content and stability of the surface films. Fig 1 suggests that, for steels, metal fatigue wear occurs at high and low wear rates, presumably associated with high and low stress but the intermediate wear rates ( 1 0 -7 t o 10 -1° mm 3 mm -1 N -a) associated with intermediate stress levels are controlled by the removal of the surface films which give those stress levelst. Patterns of wear

Hirst and his colleagues used pin-on-ring and crossedcylinders machines to study the wear of metals at sliding speeds from 10 mm/s to 10 m/s, at loads from 0.5 to 100 N, mostly in dry air. Apart from two studies h2 which estabfished the general occurrence in these conditions of mild and severe wear they confined their detailed studies to the wear of steel on steels 7-~° and to the wear of a 60•40 leaded brass on either a hard tool steel, stellite, or ceramic s u r f a c e 3-6 . In this section the patterns of wear they observed are summarised and in the following sections the wear mechanisms they recorded are described and compared with the additional mechanisms studied since. Fig 4, which shows the variation of wear coefficient with load for a 0.63%C steel pin sliding on a cylinder of the

f a n alternative wear index to k is the well established dimensionless coeJ]icient K = k H , where H is the hardness o f the sliding

surface, The range o f H Jbr steels is approximately 103 to 104 MPa, much narrower than the range o f t c l f Fig i were redrawn to show the ranges o f K it would be only slightly changed; K would vary from 10 -2 for severe wear to 10 - ~ for lubricated metallic wear. k is mainly preferred in this review as it gives a direct appreciation o f the wear for a given load although K may have more fundamental importance, relating wear to the amount o f contact between two surfaces, even though there may be difficulties in selecting the appropriate value o f H, because o f surface hardening, or arguments that non-dimensionalising by multiplying k by an elastic contact stress might be more sensible in run-in conditions.

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same material at a speed of 1 m/s, illustrates the general behaviour observed with steels 7 . When both surfaces were annealed there existed, when sliding in air, two loads marked T1 and T2 in Fig 4 between which sliding continually caused surface roughening and metallic wear debris. Below Tt and above T2, although initial sliding produced metallic debris, the surfaces were eventually smoothed by rubbing, a visible oxide film developed and wear debris was finally produced from this film. Above a higher load, T 3 , s o m e change of the mild wear mechanism

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Childs - The sliding wear mechanisms o f metals, mainly steels

appeared to occur. Severe wear between T~ and T2 could be eliminated by hardening both surfaces or the ring alone or its range could be altered by changing the sliding speed or extended by reducing the amount of oxygen in the atmosphere. These observations were shown to be in accord with a number of earlier studies of the wear of steels.

transfer of material between the surfaces almost ceased, wear occurred by surface oxidation followed by direct abrasion of the oxides from the surfaces without any transfer. For soft steel, however, the wear process was cyclic on a local scale (path 3, Fig 3). Removal of a hardened transferred particle by oxidation was followed by fresh metallic transfer, hardening and oxidation.

The pattern of wear of 60/40 brass sliding against a hard steel surface was similar in many respects to the wear of soft steel on soft steel. Fig 5 shows the variation of wear coefficient with speed under a load of 30N and at an ambient temperature of 300°C. A region of severe wear at intermediate speeds separates regions of mild wear at high and low speeds. The sliding atmosphere was important: Fig 5 shows that an oxygen enriched atmosphere reduces the range of severe wear.

The cyclic behaviour with the soft steel indicates a precarious existence of the protective skin. Welsh7 thought that this arose from the difficulty of hardening the transferred layer. This has been recently confirmed by a detailed analysis by Archard and Rowntree 46 of the friction heating conditions between the T2 and T 3 transitions. They showed that the temperature/time variations in the transferred layer would have been such that many cycles of rubbing would have been needed to produce transformation hardening.

Wear by oxidation The most detailed studies of the mechanisms of oxidative wear of steels have been carried out in the neighbourhood of the T2 transition (Fig 4), and certainly below the T3 transition: Kerridge 8 and Archard and Hirst 9 used radiotracer techniques to follow the history of the wear process. In both these studies it was shown, in agreement with Welsh's observations 7 , that initial sliding was severe, causing the adhesive transfer of thin slivers of metal from one surface to the other. The transferred fragments built-up with further sliding, losing their individual identities by agglomeration and plastic shearing until they eventually became detached as wear particles. All the metallic wear debris produced in the initial period was formed in this way. With time, however, hardening by transformation (for initially soft steels) and surface oxidation of the transferred layer occurred to produce a surface skin with a protective (r/k)value (path 2, Fig 3), the surface became smoother until finally wear was by removal of oxide (Fe 20 3) only. Welsh demonstrated that both hardening and oxidation were essential for protection in these conditions: removing the oxide or tempering the hardened layer caused a reversion to severe wear. There was a difference between the run-in wear of soft 8 and hard 9 steels. For hard steels, the run-in state was stable, I I I

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288

A series of detailed pin-on-ring wear studies has been carried out by Quinn 2°-24 in conditions which may be interpreted as being above T 3 (although this point has not been made explicitly by Quinn). The basis for this interpretation is the similar variation of wear coefficient with load observed by Quinn (Fig 6) and by Welsh above T 3 (Fig 4) and Quinn's detection of the oxides F e 3 0 4 and FeO in his wear debris, oxides which are formed at higher temperatures than the Fe 203 observed in Kerridge's and Archard and Hirst's studies below T 38,9

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Such detailed studies of the wear mechanism were not carried out above the T3 transition although Welsh noted that T 3 coincided with the generation of surface temperatures sufficient to transform the surface layers rapidly to a hard state. It is conceivable, therefore, that wear would then proceed by oxidation without transfer as found for hard steels under less taxing conditions 9 . Such a single change, however, would not explain the wear minimum observed slightly above T 3 (Fig 4) or the rapid increase in wear coefficient at even higher loads. Welsh speculated that the minimum might be associated with improved oxide protection and the rapid increase by a thermal softening of the pin hinterland, noting that at loads and speeds only slightly greater than considered in his work the pin wear coefficient increased dramatically and another regime of severe metallic transfer of the pin to the ring commenced.

TRIBOLOGY international uecember 1980

Quinn has observed (Fig 6) that the range of minimum wear coefficient coincides with the appearance of Fe304 in the wear debris. Furthermore 2° , the amount of initial severe wear is much less than when Fe203 is the main oxide formed. The low wear and transfer-reducing properties of Fe304 have also been noted by Clarke, Pritchard and Midgley 47. Quinn also noted much thicker and cracked oxides (Fig 7) in his work than Archard and Hirst observed and he has proposed that in the conditions of his experiments the mechanism of wear is simply the growth of oxide (without metal transfer) until it reaches a thickness such that it breaks up, allowing the process to start again. Quinn has attempted to make his model of wear quantitative. There are two main difficulties. The oxides he obtains indicate that oxidation occurs at the friction hot spot temperature rather than at the average surface temperature (other workers have found that this is not always the case s'47) and this temperature is difficult to measure or calculate. The more fundamental difficulty, however, is lack of knowledge of how thick oxide films are formed

Childs - The sliding wear mechanisms o f metals, mainly steels

in the high stress conditions existing at hot spots. In static conditions the oxidation rate is controlled by the diffusion of metal or oxygen ions through the oxide. Theories, such as Quinn's and others 48'49 , which assume film growth to be diffusion controlled in friction conditions always conclude that the activation energies for diffusion are substantially less than in static conditions, or the Arrhenius constants are greater or both. While some of these conclusions may be invalidated by the difficulty mentioned earlier of estimating the temperature at which oxidation occurs, attempts of my own to estimate activation energies from Clarke, Pritchard and Midgley's work 47 or from the'variation of T2 (Fig 4) transition loads with temperature 7 or the variation of similar transitions with temperature for the wear of brass s support these observations. Krause s° has demonstrated that a small amount of slip in a rolling contact can cause the oxide film formed to be some two hundred times thicker than in the absence of slip. It is possible that oxide cracking during sliding (or metallic plastic shearing when it occurs) provides more rapid transport of metal and oxygen ions to one another, so that diffusion could become irrelevant to film growth. A variant of this theme is Stott's and Wood's s~ recent observation of thick oxide films in a friction contact which are in fact compacted agglomerations of finer particles. Clearly further studies of tribooxidation would be beneficial. The wear mechanisms of steels above the T2 transition is thus seen to depend on which oxide is formed and on the hardening response of the surface to temperature. Such detailed studies of wear by oxidation have not been carried out below the T~ transition. However, Welsh 7 noted that running-in occurred more rapidly than between T2 and T3. Both surface hardening and oxidation were necessary for a protective film but for annealed steels the hardening could be produced by straining. It was not ascertained whether transfer was a feature of the run-in

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Fig 7 The wear o f a thick oxide film on steel, from reference 21 (reproduced by permission o f the Council o f the Institution o f Mechanical Engineers) wear process but it seems likely from the rapidity of running-in and the likely depth of the work-hardened layer relative to the size of the real areas of contact that stabilised elastic contact conditions would have suppressed metal transfer, at least at levels well below T~. Additional factors enter into the generation of protective surface layers when unlike materials are slid over one another. When a leaded-brass pin was slid on a ceramic ring at high speeds s , the mild wear was cyclic (path 3, Fig 3). Initial transfer of brass to the ceramic was followed by its oxidation, wearing away and the repetition of the cycle. A feature additional to that found sliding steel on steel was back-transfer of the oxidised brass from the ceramic to the pin before its final breakdown to wear debris. When, however, the same brass was slid on a hard tool-steel surface, although initial transfer and backtransfer of the brass occurred, as with the ceramic tests, the back-transferred oxidised brass was found to include iron and tungsten from the tool steel which increased the layer hardness and stabilised its protection (path 2). Subsequently wear of the brass occurred by oxidation from the film, the iron and tungsten content of the film was maintained by transfer of oxides from the steel surface and, indeed, the wear rate of the tool steel was similar to that of the brass. Thus, in mild oxidative wear conditions it is the relative mechanical properties of the oxide film and the substrate on which it is formed which are important, but the composition of the oxide and hence its properties can depend on both the sliding surfaces.

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Fig 6 Variations o f oxidational wear coefficient with load fi)r 0.4%C steel24 ; O. 1%C, l%Cr steel23; and 0.4%C, 2%Ni-Cr-Mo steel 2°

Although the severe wear mechanism of steels between T1 and T2 (Fig 4) has not been studied in detail, the severe wear during running-in to mild conditions between T2 and T3 has been and, as already recorded a'9 , plastic shearing led to transfer of metal from one surface to the other, subsequent transfer led to agglomeration and growth of the transferred layer, and finally break-up of this layer resulted in wear debris. The rate determining step in severe wear is thought to be the transfer stage 6 . The break-up of the layer probably occurs once a critical accumulation of recoverable strain-energy per unit area is exceeded, in a manner similar to that suggested by Rabinowicz s2 . More details of the transfer and subsequent stages of severe wear have been obtained for the sliding of a brass on

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Childs - The sliding wear mechanisms o f metals, mainly steels

steel 3'6 . In one set of tests (at a load of 225N) individual transfer particles typically had an area of 0.05 mm 2 and a thickness of 0.005 mm but the area and thickness of the final wear particles was about 1.0 mm 2 and 0.015 mm respectively: each wear particle therefore contained about 60 transfer fragments; no detail of the individual fragments could be seen in the wear particles. During initial sliding on roughened surfaces transfer occurred immediately but on smooth surfaces a critical strain (or roughness) had to be generated first. Once transfer commenced almost every contact produced a transter particle. This can be deduced from data in reference 3 which shows that at the load of 225N referred to above, 10 transfer fragments were produced per mm of sliding, giving a total area of transfer of 0.5 mm 2 mm -~ ; this is close to 0.4 mm 2 m m - ~, the area swept out by the real area of contact per mm of sliding. The dimensionless wear coefficient K for this experiment was calculated to be approximately 10-2. The conclusion that every contact produced a transfer particle is, therefore, at odds with the interpretation of K as the probability of forming such a particle and supports the view that it can also depend on the shape of the transfer particle: in this case transfer particles typically had a thickness to diameter ratio of 1 0 - 2 . Delamination wear Wear below T1 which was neglected in the earlier studies of steels 7-9, was later studied by Suh and his c0-workers, also using pin-on-ring and crossed-cylinders machines. Loads from 10 to 20N and sliding speeds from 10 to 30 mm/s were used but protection and wear by oxidation observed in the earlier studies were suppressed by sliding in an argon atmosphere. In these conditions the wear debris was metal flakes formed by fatigue failure of the sliding surfaces. The clearest photographs of subsurface fatigue cracks interacting to form wear particles may be found in reference 13. There is a difficulty about calling this wear mild or severe. Surfaces were smoothed by sliding; wear coefficients between 4 x 10 -7 and 4 x 10 -s mm 3 mm -1 N -1 were measured, one to two orders of magnitude less than observed in severe conditions (Fig 1): in these respects the wear would be classed as mild. However the wear particles of area 0.01 mm z and thickness 1 to 10/am, although smaller than the transfer fragments and very much smaller than the wear particles previously described for the severe wear of brass on steel 3'6 , are clearly large enough to be classified as severe. This justifies the coining of a new term, delamination wear, to describe these observations. In a later series of papers ls-17 Suh and his co-workers, still working at low loads and speeds and with an argon atmosphere, demonstrated delamination wear for a wide variety of sliding metal alloys other than steels and showed a correlation between wear resistance and fatigue resistance in these conditions. The mechanics of delamination have been considered in two theoretical papers ls'~9 . It was shown that in the conditions of the tests and with the materials studied, crack nucleation would occur in the first ten cycles of surface stressing. Crack propagation rates were analysed in terms of elastic fracture mechanics and it was concluded that the formation of a wear particle could be expected from 10 z to 104 stress cycles. It was

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TRIBOLOGY international December 1980

shown that crack propagation rate was strongly dependent on friction coefficient and in this respect it is interesting to note Rabinowicz's recent conclusion that at slow speeds and low loads the sliding wear rates of a range of noble metals correlate with friction coefficient to the fourth power s3 . There is little doubt that at low sliding speeds and loads, if severe wear and wear by oxidation can be suppressed, wear by primarily elastic fatigue or delaminatior controls the wear rate. It has been suggested earlier that the sub-surface stress system leading to fatigue will be markedly different for ( r / k ) > 0.5 than for ( r / k ) < 0.5. The friction coefficients for steels of 0.5 to 1.013'14 measured in delamination conditions suggest (r/k) > 0.5 and that wear is associated with path 4 in Fig 2. Metal fatigue wear in run-in elastic contact, high traction, conditions clearly deserves a name, delamination, to distinguish it from the plastic severe wear studied earlier. However, Suh has also labelled other forms of metallic wear as delamination and this serves only to confuse. Fig 8 shows, as the solid line, the wear coefficient measured by Saka, Eleiche and Suh s4 for a 0.2%C steel sliding in air at speeds from 0.5 to 10 m/s under a load of 50N. Below 1 m/s wear occurred by oxidation (Fe203 was formed) but between 1 and 5 m/s the wear particleswere formed as metal plates and these authors have described this as delamination, despite recording that the surface was roughened by sliding and that large scale plastic flow occurred and despite commenting that the elastic crack growth theory developed earlier might not be applicable. They could not explain the decrease of wear coefficient above 5 m/s. These results are clearly but a small portion of a broader pattern of changing wear behaviour with speed, part of which has already been established by W e l s h 7 . In Fig 8 the wear coefficients measured by Welsh at lower speeds and by Kinsella and Childs ss at higher speeds are included as the dashed lines. Welsh recorded that at speeds only

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Fig 8 Variations o f wear coefficient with speed f o r low carbon steels measured by Ieelsh 7, Saka et al s4 , and Kinsella and Childs ss at loads close to 5 0 N

Chi/ds - The sliding wear mechanisms o f metals, mainly, steels

slightly above those at which he made his detailed studies a further regime of severe metallic wear with transfer set in, triggered by bulk thermal softening of the rubbing pin. It is thought that this is what was observed by Suh s4 between 1 and 5 m/s. Kinsella and Childs, in studies of friction cutting under differ~ent geometrical conditions to those used by Suh and by Welsh, observed severe metallic wear with transfer at sliding speeds' of 35 to 45 m/s but at higher speeds, at loads of 50N, yet a further regime of wear by oxidation commenced (FeO was formed). Sub's observations of decreased wear above 5 m/s could have been caused by such a change of wear mechanism.

Boundary lubricated wear When metals are slid in boundary lubricated conditions, reactions between the metal and the oil or impurities and additives in the oil greatly widen the possibilities of protective film formation on the metal surface. It was established early in the laboratory that boundary lubricants could form soaps on steel surfaces and extreme pressure additives could react to form chlorine, sulphur or phosphorus containing films 2s . Many early studies of the effects of oil additives on wear in machinery assumed these to be the films formed in practice 26,27. This is now known to be an over-simplification. Godfrey 2s recorded in 1962 that in sliding tests on steels lubricated with a sulphurised mineral oil the main film formed was iron oxide (FeaO4) while in a discussion of that paper Fein and Randall reported that iron sulphide was only formed in severely scuffed sliding conditions; in less severe conditions with a sulphurised oil Fe 3 04 was formed but in the absence of sulphur Fe2Oa was formed. Since then oxide film formation in lubricated conditions, the influence of oxygen, water and additives on that film formation and wear by removal of the film has been well documented2a-al; wear coefficients can be deduced to range from 5 x 10 -7 to 5 x 10 -1° mm a mm -1 N -1 . This reviewer has not found any reports of studies of the mechanism of film removal in these conditions; is it preceded by metal transfer during running-in as has been observed in laboratory tests s6, or does it occur by the fiaking away of thick films in severe scuffing conditions or by gently abrasion in more normal conditions? It is difficult to imagine, however, mechanisms other than those already discussed in previous sections. Two more recent observations are of greater interest. It has been found that the wear debris from lubricated sliding bench tests as and from operating machinery 36 , rather than being reaction film debris, can be in the form of fine metal plates, of area as small as 20/am 2 and thickness less than 1/am. It is claimed that the conditions leading to such debris are common in normally running machinery and that if wear is by oxidation something is amiss as . It has also been found that polymeric protective films (sometimes called friction polymer), rather than reaction films, can be formed when sliding in oils 29'32-34 The two observations may be coupled to suggest that when metal surfaces are sufficiently protected from inorganic film forming reactions, possibly by an organic coating, the possibility occurs that wear is controlled by metal fatigue on a micro-scale. The inert conditions leading to this fine metallic fatigue wear have most recently begun to be quantitatively studied a7-4°. Data in these papers enable wear coefficients between 10 -1° and 10 -11 mm 3 mm -1 N -1 to be estimated.

These rates and the sizes of the wear particles are much less than observed in dry delamination wear studies. It seems likely that wear is controlled by the initiation of fatigue cracks, rather than by crack growth, possibly by the exhaustion of ductility at grain boundaries as recently observed on a slightly finer scale still by Garbar and Skorinin s7 .

Discussion and summary This review outlines the development of wear studies since the 1950s. The earliest experiments in dry conditions concentrated on severe wear. It was demonstrated that conditions could occur in which almost every contact between two surfaces caused plastic transfer of metal from one surface to the other and in which the final wear particles were agglomerations of these transfer fragments 3'6 . A view of a continually sheared plastic interface emerged in which the stability of the transferred agglomerations determined the wear particle size s2 but their rate of build-up and hence the wear rate was determined by the plastic flow at the transfer stage. The requirements of surface hardness and oxide film formation necessary to prevent severe wear were demonstrated s'7. Studies of the oxidational wear of steels close to severe conditions s'9 showed that, with soft steels, wear could proceed by the cyclic transfer and oxidation of the surface but for hard steels a thin stable oxide layer could develop, its rate of abrasion determining the wear rate. Later studies 2°-24 of wear by oxidation in conditions further removed from severe showed that oxide films on steels could be generated by rubbing to a thickness where they flaked away. Then wear rate was controlled by the oxidation rate rather than by the abrasion of a stable oxide film. These studies also demonstrated the better protection and lower wear rate of steels when Fe3 04 rather than Fe2 O3 was the scale formed. However, careful studies do not seem to have been carried out to determine at what stage oxidation rather than abrasion rate takes over as the controlling mechanism. A contradiction seems to exist between Quinn's and Welsh's analyses of surface conditions during mild wear. Quinn 2a'24 calculates hot spot temperatures leading to oxidation typically between 250 and 650 ° whereas Welsh 7 calculated temperatures greater than 900°C and observed phase hardening of the metal beneath the oxide which would not have occurred at temperatures as low as 650°C. Perhaps Quinn's temperatures should be interpreted as the temperatures necessary at the metal/oxide interface, some 5/am beneath the surface. It would be interesting to know whether a phase hardened layer, believed by Welsh to be essential for the oxide to be protective, exists beneath Quinn's thick oxide film or, if not, whether one existed during initial sliding before the growth of the film to a thickness which is possibly protective even on a soft substrate. In low speed sliding experiments in which wear by oxidation has been inhibited by reducing the amount of oxygen available for reaction, it has been demonstrated that metallic wear debris can be produced by surface elastic fatigue rather than by the plastic shearing mode of severe wear1~-19. The difference between this form of wear, called delamination, and severe wear is clear in principle: severe wear is associated with roughening and increased

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stress severity during running-in while de]amination with smoothing and decreased stressing (paths ] and 4, Fig 3) In practice, however, the wear rates attributed to delamination form an almost continuous range, as shown in Fig ], with the rates attributed to severe wear. It would be interesting to carry out radio-tracer studies at the boundary between these two forms of wear to determine whether there is an abrupt change or a continuous one between the wear with transfer established in severe conditions and that without transfer typical of delamination. The influence of sliding speed and load on the wear mechanisms observed during the dry sliding of soft steels on soft steels might be mapped schematically as shown in Fig 9 which draws for its inspiration on the earlier Figs 4, 6 and 8. At low loads and speeds (region A) surface workhardening and smoothing can occur under a protective oxide t'dm, wear occurring either by removal of the film or by metal fatigue (delamination). At slightly higher loads and speeds (region B) oxide protection may break down leading to severe metallic transfer and wear but at higher speeds still (region C) enhanced oxidation caused by friction heating coupled with the possibility o f surface transformation hardening of steels can restore oxide protectiveness. In hotter conditions still (region D) gross surface softening can cause a reversion to severe wear but in even hotter conditions (region E) a further regime of oxide domination has been observed. Any observed variation of wear coefficient with load or speed will depend on what section is taken through Fig 9. The positions of the boundaries in Fig 9 depend strongly on the sliding materials. For example the tongue with which region B separates A and C when soft steels are slid can be retracted by hardening the steels. To the best of this reviewer's knowledge, however, experiments have not been performed to allow any complete map after Fig 9 to be quantitatively drawn for any steel. The development o f boundary lubricated sliding wear studies has paralleled that of dry wear. Early studies concentrated on the prevention of severe wear or scuffing by promoting protective film formation with additives in the oil 2s-27 . Later studies of wear in less severe conditions showed that the film forming reactions by which wear occurred differed from those which prevented scuffing 2s-3~ . Most recently, attention has turned to the study of conditions even further from severe in which

I0

>.-

Severe metallic wear

05

g

10-2

I0 -t

I Sliding speed,

I0 ms -t

Fig 9 Wear regimes mapped on a base o f load and sliding speed for soft steels sliding on soft steels

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fine metallic wear particles, formed by fatigue, rather than reaction film debris have been formed as-37 . As with studies in dry conditions, the conditions at which transitions occur between the various severities of wear have not been clearly established. The conditions which lead to fine metallic debris in lubricated sliding have been called normal 3s . They are certainly desirable, being associated with the lowest wear rates. It is not clear however whether this form of wear has recently received recognition as the result of a natural progression from studies of the fastest to the slowest wear generating mechanism or whether its importance has recently been enhanced by the development of a new class of oil additives which form organic protective films on sliding surfaces 32- 34. Successive laboratory studies have led to a broad qualitative understanding of the ways in which sliding metal surfaces can wear. However, the quantitative understanding which would enable prediction of which wear mechanism would be dominant in a particular set of conditions and which would hence enable predictions to be made of wear rates still awaits development.

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