recent developments in wear-mechanism maps

Tribology InternationalVol. 31, Nos 1–3, pp. 87–97, 1998 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain0301–679X/98/$19.00+ 0.00

PII: S0301–679X(98)00011–5

Recent developments in wear-mechanism maps

S. C. Lim

This paper presents a summary of the author’s personal view ofthe development of wear-mechanism maps, culminating in thepresentation of some recently proposed maps. These maps, whichpresent wear data in a graphical manner, are able to provide amore global picture of how materials in relative motions behavewhen different sliding conditions are encountered; they alsoprovide the relationships between various dominant mechanismsof wear that are observed to occur under different slidingconditions as well as the anticipated rates of wear. Somethoughts on future directions for research in this area are alsopresented. 1998 Elsevier Science Ltd. All rights reserved.

Keywords: wear maps, tribological databases, tribological design

Introduction

Wear is a complex phenomenon. It occurs wheneversurfaces come into sliding contact, even in the presenceof a lubricant. To the designers and engineers whohave to make optimal decisions in situations wheretribological considerations are significant, it isimportant for them to have ready access to informationpertaining to the fundamental understanding of thewear processes of interest. Some kind of user-friendlydatabases would be most helpful here. These databasesshould be able to provide the appropriate informationfor materials selection and choice of the suitable(optimal) operating condition—such as contactgeometry, speed and environment—for a particular pairof materials in tribological contact.

There are many ways of presenting wear data. Themore common modes of presentation include the tabu-lation of wear rates and elucidation of the dominantmechanisms of wear observed under the sliding con-ditions of interest, the latter usually being accomplishedthrough the presentation of micrographs showing fea-tures on the worn surfaces. However, these presen-tations tend to be restrictive in the sense that theyusually cover a relatively narrow (localised) range ofsliding conditions. This can be inadequate and a morecomplete approach is perhaps through the linking ofthe wear rates and wear mechanisms over a muchwider range of sliding conditions in the form of a

Department of Mechanical and Production Engineering, NationalUniversity of Singapore, 10 Kent Ridge Crescent, Singapore 119260

Tribology International Volume 31 Numbers 1–3 1998 87

wear-mechanism map as Tabor had suggested earlier1.Such a map not only provides a multi- (most of thetime, two-) dimensional graphical presentation of weardata, it also provides an overall framework for thewear behaviour of a particular sliding system intowhich individual wear mechanisms observed undervarious sliding (operating) conditions may be fitted.

Many names have been given to diagrams whichdescribe the overall behaviour of wear, the more com-monly used ones include wear-mechanism map, wear-mode map, wear-transition map and wear-regime map;sometimes, the word ‘diagram’ is used instead of‘map’. Generally, wear-mode, wear-transition andwear-regime maps tend to focus on the description ofthe mode of wear, namely, mild wear, severe wearand the transition between them. In the case of wear-mechanism maps, details of the dominant wear mech-anisms are given and the regions of their dominanceare indicated; often, predicted rates of wear are alsoincluded in the maps. Names such as fretting map anderosion map have also been used for more specificwearing conditions.

In the following sections, the general methodology forthe construction of a wear map will first be described.This will be followed by an almost chronologicaldescription of the development of mostly sliding wearmaps for metals, ceramics, metal-matrix compositesand polymers. Maps describing the wear of coatings,tool wear, fretting wear, erosion and time-dependentwear transitions are then introduced. The paper willconclude by suggesting some future directions forresearch in wear mapping.

Recent developments in wear-mechanism maps: S. C. Lim

Methodology of wear mapping

Researchers involved in the construction of wear-mech-anism maps would have their individually favouredapproach; the choice of these slightly differentapproaches is almost always a personal one. The fol-lowing describe briefly the steps adopted by the author.

1. For the pair of materials of interest, their mode ofcontact (for example, unidirectional sliding), con-tact geometry (for example, pin-on-disc), theenvironment in which they are to interact in (forexample, atmospheric condition) and lubricationcondition will first have to be decided.

2. Gather experimental data from the literature onwear rates and wear mechanisms pertaining to thissliding pair measured in conditions exactly like orvery close to those specified in step (1). In-housetests will have to be carried out if data is lacking.Mathematical models describing wear behaviour ofthis pair should be gathered as well.

3. The parameters to be used as axes for the mapwill be decided. One can construct a two- or three-dimensional wear diagram; so far, the majority ofwear maps are of the two-dimensional type. Therange of sliding conditions to be included in themap will also have to be decided. It is desirableto select as wide as possible a range. For situationssuch as machining, the range should preferably besimilar to that recommended for that particulargroup of tools whose wear behaviour is to bemapped.

4. Construct the empirical wear maps. This is donefirst by grouping the wear data according to themode and mechanism of wear. The wear-rate andwear-mechanism data, appropriately classified, arethen plotted into the (usually) two-dimensionalspace defining the map. The field of dominanceof each mechanism is then demarcated using fieldboundaries and the approximate locations of thecontours of constant wear rate are located. At thisstage, the wear map is sufficiently informative andit should provide a summary of the global wearbehaviour of the sliding pair of interest.

5. The final step is to introduce the appropriate math-ematical models available to describe the wearbehaviour of this sliding system. When these arenot available, new models will have to bedeveloped. The calibrated model for each field isthen used to calculate the projected wear rates forconditions in the field where no experimental dataare available. These wear-rate contours are thensuperimposed onto the map. A complete wear-mechanism map is thus generated.

Developments in various groups of wearmaps

Wear maps for metals

The concept of creating wear maps of one form oranother for metals is not new: attempts were made asearly as 1941 to present wear data in this fashion2. Intheir work on the wear of cast iron and steel, Okoshiand Sakai2 presented wear rates as a surface in a three-

88 Tribology International Volume 31 Numbers 1–3 1998

dimensional space (Fig 1), with the sliding conditions(load and speed) as the two horizontal axes and thewear rate as the vertical (third) axis. It is clear fromthis wear-rate surface that wear rate depends on thetwo sliding conditions in a slightly non-linear fashion.As far as the author is aware of, the next wear diagramof some significance was to arrive more than twodecades later, when Welsh3 presented a diagram sum-marising the sliding conditions corresponding to themild-wear/severe-wear transitions observed in the wearof steels.

In the early 1980s, a series of diagrams, mostly forthe unlubricated wear of steels with different test con-figurations, was proposed. These include the works ofChilds4, Eyre5, Marciniak and Otimianowski6 andEgawa7. Apart from the diagram of Marciniak andOtimianowski which gives a wear-rate surface similarto the work of Okoshi and Sakai (Fig 1), the otherthree show the boundaries between mild- and severe-wear behaviour in their respective sliding systems andwithin the range of sliding conditions investigated. Theonly diagram that is significantly different in naturefrom the rest is the wear-regime map for soft steelssliding on soft steels presented by Childs4. In this map(shown in Fig 2) not only is the mode of weardescribed, in this case severe metallic wear, but theexpected dominant wear mechanisms in five differentregions (A to E) in the map are also described,although this was done in the text and not representedon the map. Later, the mode of mild-oxide-protectedwear was added in8. Furthermore, it is the only diagramthat has a wide range of sliding conditions (in thiscase, the sliding speed covers more than four decadesof values), fulfilling an important requirement of auseful wear map: i.e. to cover a wide range of slidingconditions. It undoubtedly suggested how subsequentwear maps were to be constructed.

Another line of approach to generate wear maps formetals was taken by Kato and Hokkirigawa9 whosummarised their SEM observations ofin situ abrasivewear into an abrasive-wear diagram, and this is shownin Fig 3. They found from tests carried out on brass,0.45%C–steel and 18-8 austenitic stainless steel

Fig. 1 The wear-rate surface for steel proposed byOkoshi and Sakai2


Fig. 2 The wear-regime map for soft steels proposedby Childs4

Fig. 3 The abrasive-wear diagram for three differentmetals proposed by Kato and Hokkirigawa9

samples that three wear types—namely, cutting, wedgeand ploughing—were operative under different con-ditions. They described the mechanisms of wearobserved rather than the wear mode as had been donemostly until that point in time. They introduced anindex, the degree of penetration (Dp), to describe theseverity of contact and this was used as one of theaxes of the diagram. The abrasive-wear diagram showsthe possible region for each type of wear defined byDp and the shearing strength at the contact interface.


Similar abrasive-wear diagrams for aluminium underboth dry and lubricated conditions were subsequentlypresented, showing the occurrence of another wearmode: cleaving10. More recently, in their investigationof the effect of tilted contact between sliding surfaces,simulating misaligned contacts found in machineelements which generally lead to abrasive wear, Hokki-rigawa et al.11 proposed a wear map describing thethree modes of wear of a tilted steel pin sliding withbase-oil lubrication, against a steel or aluminium-alloyplate, with the latter sometimes covered with a layerof hard anodic oxide film. The three modes includecutting, ploughing and plastic deformation, with thelast mode generally occurring when the plate is harder.

Turning back to sliding wear, the wear maps availableso far with the exception of Childs’ wear-regime map4

had two major limitations: namely, the limited rangeof operating conditions covered and the lack of infor-mation on the dominant mechanisms of wear. Theinformation provided was limited to whether mild wear,severe wear or a transition between them was observed.These limitations were addressed by the constructionof the wear-mechanism map for steels12. This map(Fig 4) describes the unlubricated pin-on-disc wearbehaviour of steels over a wide range of sliding con-ditions: some seven decades of values for speed andfive decades of values for contact pressure. The mappredicts the field of dominance of one wear mechanismand when its contribution becomes less important; con-tours of predicted normalised wear rates are superim-posed over these fields. For completeness, a companionwear-mode map and a wear-transition map were laterproposed13 and one of them (the wear-transition map)is shown in Fig 5. These two maps summarise thesliding conditions associated with mild and severe wearas well as how the various wear transitions reportedin the literature could be related: information which thewear-mechanism map for steels could not convenientlypresent. With such a wear-transition map, the operatingconditions under which a mild-wear condition existsfor a steel sliding component are clearly demarcated.Several refinements to make these maps more accuratehave also been suggested14. More recently, Katoetal.15, using the same methodology, constructed wear-mechanism maps to illustrate clearly the effects ofnitriding on the global wear behaviour of steels, pro-viding an additional dimension of information not poss-ible to be included in the wear-mechanism map forsteels (Fig 4). Following the same methodology usedin constructing the maps for steels, a wear-mechanismmap for the unlubricated sliding of aluminium andaluminium alloys on steel was later proposed by Liuet al.16. This map is a considerable improvement overthe earlier empirical wear map for the same group ofalloys presented by Antoniou and Subramanian17.

One of the important considerations in the constructionof wear maps is the temperature generated at thesliding interface. At the onset of the mapping of steelwear, it was recognised that the interfacial temperaturewould control the dominance of different mechanismsof wear in a significant manner. When the interfacialtemperature is high (and this occurs when the slidingspeed is higher than a certain critical value), mech-anisms involving oxidation, phase transformation and


Fig. 4 The wear-mechanism map for steels12

melting would be important; below this speed ofsliding, the sliding interface can be considered as ‘cold’and the dominant mechanisms are essentially controlledby the plasticity of the materials forming the interface.The mathematical models used in the prediction ofwear rates would then reflect how the wear processesare dependent on the interfacial temperature. Attemptswere therefore made to compute, based on one-dimen-sional equivalents of the more complex three-dimen-sional patterns of heat flow, both the flash and bulktemperatures. This gave rise to a temperature map forthe dry sliding of steels12. This method of temperaturecomputation has since been further extended andrefined18–21, resulting in the creation of the T-MAPSPC-based software22,23. Refinements of this method-ology to construct temperature maps for other materialpairs in sliding contacts have also been carried out,for example, by Wang and Rodkiewicz24 who proposedminor changes to the temperature maps for steels andsome ceramic materials.

There is another group of maps which one mightconsider as ‘failure maps’. The most notable example


is the transition diagram of the International ResearchGroup on Wear of Engineering Materials (IRG-OECD)which presents the critical load–velocity curves forthe failure of thin-film-lubricated sliding concentratedcontacts25,26. In this diagram, illustrated in Fig 6, thethree regimes of different tribological behaviour duringlubricated sliding are demarcated. Sliding contactswithin region I will theoretically suffer no wear, whilethose operating in region III will experience severewear. This is useful not only for the selection of propersliding conditions, it can also be very useful for failureanalysis. Actually, diagrams showing the ‘safe’operating conditions for various machine elements,such as bearings operated under different kinds oflubrication conditions, are already available in theliterature; for example, Neale27. These diagrams havehelped designers and engineers select the correctmachine elements and operating conditions to meet thedesign requirements. Such an approach was also usedduring the construction of wear maps for cutting toolswhere safety zones and least-wear regions are identifiedin which the rate of wear of tools would be a minimumor sufficiently low (more later).


Fig. 5 The wear-transition map for steels showing the regions of mild wear and severe wear. The sliding conditionscorresponding to the different types of wear transitions observed are also indicated13

This concept of defining regions of safe operation wasbrought one step further when Landheeret al.28 pro-posed a theoretical wear-mechanism map for plainjournal bearings based on the IRG transition diagram.They found that their diagram agreed well with datataken from actual bearing practices presented byNeale27. From such a map, it is possible to predict thestate of lubrication experienced by the journal bearingif the loading condition is known. The ‘safe zone’ ofbearing operation then corresponds to the region wherea no-wear condition is predicted. Qualitative infor-mation on bearing failure could also be obtained fromthis map. Having identified pitting, abrasive wear andscuffing as the three major process of tooth distress ingears, Tweedale29 proposed a map showing the variouszones of tooth distress: this is another form of a‘failure map’. It is only a qualitative one because novalues were suggested for the limits of the twooperating parameters—namely, load and speed—defin-


Fig. 6 The IRG transition map showing the criticalload–velocity curves for the failure of thin-film-lubri-cated sliding concentrated contacts, demarcating threeregimes of different tribological behaviour25,26


ing the various distress zones. This diagram would bean excellent tool for the diagnosis of gear teeth failureif details of the limiting values of load and speedthat would lead to the operation of undesirable wearmechanisms could be provided. It is the wish of theauthor to see more maps and diagrams of such diagnos-tic nature being generated to help designers and engin-eers in their continual combat against wear.

Wear maps for ceramics

The concept of wear mapping has also been extendedto include ceramic materials30. To date, substantialprogress has been achieved in developing wear mapsfor ceramics; see, for example, the works of Hsuetal.31,32, Kato 33, Lee et al.34, Hokkirigawa35, Kong andAshby36, Dong and Jahanmir37, Gautier and Kato38 andBlomberget al.39. In these diagrams, regimes of differ-ent dominant wear mechanisms are demarcated and, insome of them, the sliding conditions leading to tran-sitions between mild and severe wear are also indi-cated.

Fig 7 shows the wear-mechanism map for an aluminaball sliding on an alumina disc proposed by Kong andAshby36. In this map, the locations of the dominanceof seven different wear mechanisms are indicatedtogether with contours of constant flash and bulk tem-peratures generated during sliding. The map clearlyshows the inter-relationships between these mechanismsas well as their dependence on the temperatures gener-ated at the sliding interface. The different field bound-aries on the map suggest where transitions of onedominant wear mechanism to another may take place.Although the wear models developed were unable todescribe the wear rates accurately, they neverthelessprovided an overall framework for the wear character-istics of alumina. Because of the wide range of slidingvelocity and contact pressure covered by the map, itshould enable the designer to decide intelligentlywhether alumina will be able to meet the set of require-ments for a particular tribological application. To beuseful during the design process, wear maps shouldideally encompass as wide as possible a range of

Fig. 7 The wear-mechanism map for alumina proposedby Kong and Ashby36


sliding conditions so that global wear characteristicscan be clearly displayed.

Wear maps for metal-matrix composites

The emergence of composite materials, especiallymetal-matrix composites with different reinforcementphases, as a group of advanced tailor-made materialsfor tribological application has created the need tosummarise the wear mechanism and wear transitioninformation of some of them to optimise design. Unlikefor many monolithic metallic materials, understandingof the processes by which these composites wear dur-ing dry sliding are still limited, and in some cases,controversial40,41. Such a situation renders the construc-tion and calibration of wear-mechanism maps for com-posite materials much more difficult than for monolithicmetals such as steels or aluminium alloys. The wayto avoid the difficulties associated with the limitedunderstanding of the wear processes involved is toconstruct empirical wear-mechanism maps by carefullyintegrating the wear-rate and wear-mechanism dataobserved during sliding. This has been done for theAl(6061)/SiCw composite42 where, over a small rangeof normal load and speed, the transition boundariesseparating different modes of wear are drawn; an indi-cation of the dominant mechanism is also given ineach regime. This map is shown in Fig 8.

More recently, an attempt was made to examinewhether a map could be constructed for Al/SiCp com-posites over a larger range of sliding conditions. Thiswas done on the basis of extensive experimentation aswell as data from the literature, and the empirical wear-mechanism map is shown in Fig 943. The extensiveexperimentation enabled a sufficiently large amount ofdata, especially high-speed data, to be generated,thereby extending the range of sliding conditionscovered by this map. In the course of developing thismap, the amount of SiC reinforcement particles in thecomposites was considered to have a greater influenceon their wear behaviour than the matrix material (madeof different grades of aluminium alloy). As a result,this map was constructed based on data from different

Fig. 8 The empirical wear-mechanism map forAl(6061)/SiCw composite proposed by Wanget al.42.The range of sliding condition covered in this map is,however, rather limited


Fig. 9 The empirical wear-mechanism map for Al/SiCp

composites43. The regions of dominance of six differentwear mechanisms are demarcated with the contours ofconstant normalised wear rates superimposed overthem

Al/SiCp composites with nearly the same volume frac-tion of SiC particles (of about 20%). This map showsthat thermal effects play an increasingly important rolein the wear behaviour of this group of compositeswhen the sliding speed exceeds about 3 m s−1. Someattempts were made to develop physical models toexplain the observed responses of these composites interms of changes to their reinforcement phase underdifferent sliding conditions, especially during higher-speed sliding43. This map (Fig 9) provides the frame-work for future work to better understand the overallwear behaviour of these Al/SiCp composites.

Wear maps for polymers

It is interesting to note that diagrams were proposed along time ago for the purpose of design and theinitialselection of suitable bearing materials. In a 1973 paper,Crease44 commented that the data available then formost bearing materials on wear performance under theintended operating conditions were largely inadequate.He stressed that it is important to know how the wearfactor (a measure of the wear rate) varies with par-ameters such as bearing pressure and surface tempera-ture over the possible range of these variables likelyto be met in practice. Crease proposed diagrams, basedon available performance data of a series of polymeric-based bearings, relating the wear factors to some ofthe operating variables. An example of such a wear-factor diagram is given in Fig 10. In this diagram, therange of bearing pressure within which the three differ-ent polymeric-based bearing materials could safely


Fig. 10 The wear-factor diagram for three polymeric-based bearing materials (after Crease44)

operate and the corresponding expected values of wearfactor are given. The next significant wear map forpolymers came many years later in the form of thedeformation map for the unlubricated sliding of poly-tetrafluoroethylene (PTFE) on steel45. Much effort isneeded to address the paucity of polymeric wear mapsin the technical literature.

Wear maps for coatings

The usefulness of wear-mechanism maps is not restric-ted to bulk materials. Borelet al.46 have suggestedthat it is also meaningful to employ these maps tounderstand the wear of abradable coatings deposited oncertain gas-turbine components. They proposed wear-mechanism maps for two different AlSi–plastic coat-ings tested in a high-temperature environment. Theyconcluded that these maps can be used for modellingwear mechanisms and the design of coating systemsto provide enhanced performance of gas turbines atelevated temperatures. Further work along this linesupported the earlier findings that these wear mapsenabled the influence of coating microstructure vari-ations on abradability to be determined quickly, leadingto the formulation of a general abradability model foraero-engine coatings47. Wear characteristics of TiN andTiC coatings on tool inserts were also examined byusing mapping techniques48, and these will be detailedin the next section.


Wear maps for cutting tools

The originator of graphical representation of tool wearcan be traced to Trent who, in the late 1950s, produceda series of machining charts49. The idea of a diagramto describe tool wear did not catch on again until Yenand Wright50 proposed a qualitative wear map forcutting tools, and this was later taken up by Kendall51

who made an attempt to relate qualitatively theobserved tool wear with the wear-mechanism map forsteels12, and proposed that a safe zone exists withinwhich excessive tool wear would not occur: the sameconcept first suggested by Yen and Wright50. Withsuch a background, a proposition was made by Limet al.52 to construct a series of empirical wear mapsfor different cutting tools. These maps should enablemachinists to select the machining conditions in whichthe desired productivity (in terms of material removalrate) could be attained at an acceptable rate of toolwear. A similar qualitative approach was taken inde-pendently by Quinto and he recently presented a toolfailure mode diagram53 derived from Kendall’s qualitat-ive wear map51, Trent’s machining chart49 and thewear-mechanism map for steels12. In this map, Quintoproposed that the safe zone is a region of gradual wearassociated with predictable and reliable tool perform-ance. All these maps intend to inform the machinistof the machining conditions which would give rise tothe least amount of tool wear.

The effort to construct empirical wear maps for cuttingtools has resulted in several maps for two groups ofuncoated tools. These maps display the global charac-teristics of flank and crater wear (the two major formsof tool wear during turning operations) over the rec-ommended range of machining conditions for uncoatedhigh-speed-steel (HSS) and carbide tools54. One keydifference between these maps and the earlier qualitat-ive ones is that data (both wear rates and wearmechanisms) drawn from actual machining operationswere used, thereby allowing the optimisation of actualmachining operations. In parallel, the same method-ology was extended to coated tools, and the flank-wearmap first constructed for TiN-coated HSS tool insertsshows a significant enlargement of the safety zonefrom that found in the uncoated case55. Moreimportantly, this map shows that the amount of toolwear reduction which such a coating might provideis critically dependent on the machining conditionsemployed. Such a map will help end-users to employthese coated inserts in a cost-effective manner. Anexample is shown in Fig 1148, which superimposes thecrater-wear map of the uncoated HSS tools onto thecorresponding one for the TiN-coated HSS inserts. Theexpansion of the safety zone and least-wear regime asa result of the application of TiN coating is clear.

Fretting maps

When tribo surfaces come into oscillatory contactswith displacement of small amplitudes, they are oftendamaged by fretting wear. It will be useful from thedesign point of view to know when the transition toreciprocating sliding would take place. Addressing thisissue, Vingsbo and So¨derberg56 proposed fretting mapssummarising the fretting wear behaviour of three met-


allic materials, namely low-carbon steel, austeniticstainless steel and pure niobium. They believed thesemaps would help clarify some of the confusion con-cerning the distinction between different types of fret-ting and in practical service life prediction. A mapdetailing the various damage mechanisms sustained bysilver-plated copper contacts was later proposed57; sucha map is not only useful in design but in failureanalysis as well. Running condition fretting maps forWC–Co and TiN coatings have also been con-structed58,59.

Erosion maps

When hard particles, either carried by a gas stream orcontained in a flowing liquid, strike a surface, erosiontakes place. In the case of ceramics and brittlematerials, a relatively small change in conditions suchas impact velocity or angle past a certain threshold(critical) value can result in a significant change in themechanism of wear: a wear transition. Hutchings60 hasargued that such transitions are best understood througherosion maps and he has provided examples of suchmaps which display the regimes of particle size andimpact velocity over which different mechanisms oferosion dominate. When erosion takes place in a cor-rosive environment, Stacket al.61 suggested that thetotal wastage and degradation of material as a resultof the synergistic effects of erosion and corrosion mayin some cases be greater than that which would beobserved from the processes operating separately. Theyproposed an aqueous erosion–corrosion map showingthe transitions between the various regimes of aqueouserosion–corrosion in terms of erodent velocity andpotential. It is interesting to note that there is a regimein this schematic map within which no corrosion orerosion is expected to occur. This is the concept of‘safety zones’62. The ability to locate such safety zoneswhere damage to the surface or the rates of degradation(wastage) of materials would be at a minimum is oneof the major strengths of wear maps. A similarapproach has been adopted in the wear maps for cuttingtools discussed earlier.

Maps for time-dependent wear transitions

While addressing the durability issue of ceramicmachine components, Yust63 observed that because cer-amics are generally susceptible to fatigue effects, weardata obtained from short-term wear tests may be mis-leading when long-term service requirements are con-sidered. Fatigue effects may considerably influence theanticipated lifetime before the transition into severewear from the designed mild-wear condition. The samemay be true for other materials used as machineelements operating in an environment exposed to cyclicstresses. Yust63 argued that with the introduction of athird ‘time’ axis into a two-dimensional wear map, thewear-transition surface could provide a basis formaterials selection which includes anticipated lifetimeas a factor. He illustrated his concept by using theIRG transition map, and showed hypothetically thepossibility that wear testing at a constant load, forexample, may eventually result in a transition frommild to severe wear due to time-dependent failure of


Fig. 11 Map showing the expansion of safety zone and least-wear regime as a result of the application of TiNcoatings on the crater wear of HSS tools during dry turning operations48

the tribo surface. Similar comments were also madeby Luan et al.64, who observed from their experimentsin high-carbon steels under boundary lubrication thatboth the coefficient of friction and the mechanism ofwear changed with increasing testing time under thesame load and speed. They went on to propose athree-dimensional wear map for boundary-lubricatedsteels incorporating a third (time) axis.

What then are the implications of these findings? Twoquestions concerning materials selection for tribologicalapplications immediately follow. First, how importantis such a time-dependent wear behaviour? Second, ifit is important, how long will the components maintaintheir pre-transition mode of wear? This second questionpoints directly to the availability of relevant wear data.If time-dependent wear-transition maps for differentmaterials are available, designers will be able to incor-porate some form of safety factor into the designusing the ‘rate of progression’ towards the severe-weartransition provided in the relevant map. This mayensure mild wear for the components throughout theirexpected service life.

Future directions for wear mapping

The following are some personal views on future direc-tions for research in wear mapping.

1. Wear maps for different tribological conditionsshould be constructed; they should not be limited


to describe only the wear of unidirectional sliding.Effort to construct time-dependent wear-transitionmaps should be encouraged.

2. More wear maps designed to serve primarily asdiagnostic tools, such as ‘failure maps’ and‘operating-conditions maps’, should be prepared.

3. Wear maps should be constructed with an aim toserve the end-users.

4. Wear maps should have a sufficiently wide rangeof sliding (or operating) conditions. Even if thesemaps are of an empirical nature, they will still beable to provide an overall framework for the betterunderstanding of wear behaviour.

5. More wear map should use as axes (whether in atwo- or three-dimensional map) parameters that canbe easily controlled in practice. These parametersinclude contact pressure (which is often closelyrelated to the contact geometry) and speed; othermaterial-related parameters could be integrated intothem if necessary. Designers may find it difficultto use maps having parameters that they could notreadily link to other design parameters.

6. There is a need to address the paucity of wearmaps describing lubricated conditions as real-lifetribo systems almost always operate in a lubri-cated condition.

7. Companion friction maps may be useful for design-ers too. An earlier work on steels showed thatwhen the sliding becomes more severe, themeasured coefficient of friction depends on the


sliding condition much more than on the surfaceproperties (such as surface roughness), which werefound to be more important during slower sliding65.It may be possible to generate friction maps fromthe corresponding wear maps.

8. One pre-requisite as well as an outcome of a wearmapping exercise is the ordering of wear data.Often, much thought has to be put in to understandthe cause of scatter of wear data reported bydifferent research groups even though very similartest conditions and environments were used. Suchordering of data and the ensuing production ofphysical and mathematical models to describe theobserved wear behaviour will contribute to anincreased understanding of the underlying wearprocesses. These are often difficult and time-con-suming tasks. The challenge to tribologists remains.

Concluding remarks

This review is not intended to be an exhaustive oneand the omission of some pieces of work is thereforeunavoidable. The views expressed above are entirelypersonal; they are based on the author’s experiencein constructing wear-mechanism maps as well as hisunderstanding of this subject matter gained throughinteraction with colleagues engaged in similar endeav-ours.

In the author’s opinion, wear maps are useful to design-ers and engineers when they have to make engineeringdecisions where wear is one of the major consider-ations. Wear maps can also play the role of a diagnostictool during failure analysis. These maps combine wearrates and wear mechanisms observed under a certainset of sliding (or operating) conditions, and at the sametime provide a framework for the overall wear behav-iour of the materials in relative motions. Wear mappingis slowly gaining acceptance as a user-friendlyapproach to the presentation of wear-related infor-mation; this can be seen from the increasing numberof wear maps presented during recent years. Notwith-standing these achievements, a greater effort could bechannelled into the construction of new wear maps forthe many materials used in tribological applications aswell as the continual refinement of existing ones. Suchan endeavour may contribute towards fulfilling Lude-ma’s hope for the publication of a Complete Handbookfor Tribological Design66 in the not-too-distant future.

Acknowledgements

I would like to thank Dr J. K. M. Kwok and Dr C.Y. H. Lim for permission to use some of their results.

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