Abrasive and Erosive Wear Tests for Thin Coatings a Unified Approach

Tribology InternationalVol. 31, Nos 1–3, pp. 5–15, 1998 1998 Elsevier Science Ltd. All rights reserved

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

PII: S0301–679X(98)00004–8

Abrasive and erosive wear testsfor thin coatings: a unifiedapproach

I. M. Hutchings

Abrasive and erosive wear tests are increasingly applied tosurface engineered components, since they have the potential foruse as quality assurance methods as well as for morefundamental tribological characterisation. A simple theoreticaltreatment of such tests is outlined, in which the concept of the‘tribological intensity’ of the test conditions is introduced.Available test methods are reviewed and their suitability for thinlycoated samples is discussed. There is considerable scope forfurther development of tests since only a few are satisfactory forthese important applications. 1998 Elsevier Science Ltd. Allrights reserved.

Keywords: coatings, abrasive wear, erosive wear, testing method

Introduction

In many engineering applications, surface coatings andother methods of surface treatment are used to increasethe lives of components exposed to abrasion or erosion.Reproducible and well-characterised methods are there-fore needed to determine the resistance of surfaceengineered materials to these types of wear1.

Reliable methods are also required to assess the mech-anical properties of engineered surfaces, in order toprovide quality assurance in the manufacturing processand quality control in finished products.

For both these purposes, there is thus a considerableincentive to develop innovative methods for abrasionand erosion testing, and much progress has been madein this area over the past two decades. It is the aimof this paper to review these advances, to highlightproblem areas which remain to be addressed, and toprovide some pointers to future progress.

Coatings and surface treatments vary greatly in thedepth to which they affect material properties. Depthsor coating thicknesses range from less than 0·1mm inthe case of ion implantation, to tens or even hundredsof millimetres for some weld hardfacings2–4. In terms

Department of Materials Science and Metallurgy, University of Cam-bridge, Pembroke Street, Cambridge CB2 3QZ, UK

Tribology International Volume 31 Numbers 1–3 1998 5

of wear testing, and also service performance, a thickhomogeneous coating can be treated as if it wereeffectively a bulk material. Thin coatings, in contrast,pose particular problems since an accelerated wear testis likely to lead to coating penetration. The terms‘thick’ and ‘thin’ used here must be defined. A thickcoating (or deep surface treatment) is one which is notpenetrated during the service life of the component orduring the wear test, and in which the properties of thesubstrate do not significantly influence the tribologicalperformance. A thin coating, or shallow surface treat-ment, will often be penetrated in testing even if not inservice, and the tribologist will then be faced with theproblem of extracting information about the perform-ance of the coating from data which may relate at thebeginning of the test to the coating alone, but later, asthe coating wears, to some composite performance ofboth coating and substrate. This problem is central tothe test methods we shall consider below.

A further problem results from the very small masschanges associated with removal of a thin surfacelayer. Complete removal of a typical coating a fewmicrometres thick over an area of 1 cm2 will cause amass change only of the order of 1 mg. Many wear testmethods intended for bulk samples rely on weighing tomeasure the extent of wear, but for thin coatings thereare clear advantages in designing a test method inwhich wear is detected in some other, potentially moreaccurate, way.

Abrasive and erosive wear tests for thin coatings: a unified approach: I. M. Hutchings

Notation rc radius of eroded area from which coatinghas been removed

A Sapparent area of contact in abrasive wear total sliding distance in abrasion testb tchordal radius of spherical wear scar (see time

Fig 8) T test durationE Tcmass removed by erosion per unit mass of time elapsed at which coating is

erodent particles penetratedm Utotal mass of erodent particles passing relative sliding velocity in abrasive wear

along nozzle v impact velocity of erodent particlesm Vmass flow per unit time along erosion volume of material removed by abrasive

nozzle wearN Vcnormal load applied to sliding contact volume worn from coatingNc Vsnormal load carried by coating volume worn from substrateNs xnormal load carried by substrate displacement of surface due to wear (seeNc Fig 1)mean load carried by coating over test

duration a defines divergence of particle flux inNs erosion testmean load carried by substrate over test

duration u impact angle of erodent particlesP nominal contact pressure (= N/A) f flux of erodent particles (mass per unitq area per unit time)volume lost by wear per unit sliding

distance k specific wear rate or dimensional wearQc coefficient‘erosion durability’ of coating (see

Equation (18)) kc k for coatingR ksradius of spherical counterbody k for substrater rposition vector (see Fig 1) density of material removed by erosion

c ‘tribological intensity’ of test conditions

Theoretical background

In comparing the various types of abrasion and erosiontest which can be applied to thin coatings, it is usefulto start from a common theoretical basis, which willbe developed here.

The rate of recession of the specimen surface due towear, dx/dt as defined in Fig 1, can be expected todepend on two factors: the tribological conditions towhich the surface is subjected, and the response of thematerial to these conditions. Although it is recognisedthat the rate of wear in general depends on all aspectsof the tribological system5,6, under the well-definedconditions of a wear test it may be justifiable to treatthe influences of the test conditions and the materialas independent, and write

Fig. 1 Schematic illustration of the recession of thespecimen surface during a wear test, in which it issubjected to abrasion or erosion with tribologicalintensity c

6 Tribology International Volume 31 Numbers 1–3 1998

dxdt

= c(r )k(r ) (1)

Here c(r ) describes the conditions causing wear at thepoint on the surface defined by the position vectorr(Fig 1), andk(r ) describes the response of the materialat that point.

For consistency with previous treatments of wear thequantity k, which is associated with the behaviour ofthe material under the test conditions, will be takento be the ‘specific wear rate’ or ‘dimensional wearcoefficient’, commonly expressed in the context ofsliding abrasion as volume loss per unit sliding dis-tance, per unit normal load on the contact2. k thus hasdimensions [mass]−1 × [length] × [time]2, and its valueis often quoted in units of mm3 m−1 N−1. The ‘wearresistance’ of the material under these test conditionscan conveniently be described by 1/k.

The quantityc describes the severity of the mechanicalstimulus applied to the surface which results in wear;we will call it the ‘tribological intensity’ of the test.c has dimensions [mass]× [time]−3.

The assumption implicit in Equation (1), that thematerial’s ‘wear resistance’ and the ‘tribological inten-sity’ of the test can be combined as independent factorsin producing wear, is of course a gross simplification,but is nevertheless commonly made in theoretical mod-els of wear.k is often effectively constant over quitelarge ranges of experimental conditions, although anychange in wear mechanism will usually cause a sub-stantial change ink and invalidate the assumptions ofEquation (1). We shall examine below how thisequation can be further developed and used to analysevarious abrasive and erosive wear tests.


The most straightforward type of test to interpretinvolves subjecting the whole of the specimen surface,or a well-defined part of it, to uniform abrasion orerosion under conditions of constant tribological inten-sity. c is then constant over the whole wearing area,and for a homogeneous specimen Equation (1) can beintegrated to give the depth of material removed aftera test of durationT:

x(T) = cET

0

kdt = ckT (2)

This depth, which in an idealised case will be uniformover the whole worn area, can in principle either bemeasured directly or deduced from a measurement ofmass loss, and the specific wear ratek can then bereadily calculated.

If the test is performed on a sample carrying a coatingof thicknessxc and the coating is penetrated duringthe test, then the total wear depthx at the end of thetest will be related to the wear coefficients of thecoating kc and the substrateks by

x(T) = c1ETc

0

kcdt + ET

Tc

ksdt2 = xc + cET

Tc

ksdt (3)

where Tc is the time elapsed at the moment at whichthe coating has just been worn through. Interpretationof the test results is then more complex than in theformer case, since the wear coefficient of either thecoating kc, or the substrateks, or the time at whichthe coating is penetratedTc must be known indepen-dently in order to determinekc from measurement ofthe final depth of wear or the mass loss.

Abrasive wear

Tribological intensity in abrasive wear

The simplest model for abrasive wear under slidingconditions, as derived for example by Rabinowicz7,suggests that the volume lost from the surface per unitsliding distance,q, is linearly proportional to the nor-mal load,N:

q = kN (4)

where the constant of proportionalityk has the samemeaning ask in Equation (1). Simple manipulationleads to an expression for the rate of linear recessionof the surface, dx/dt:

dxdt

= kUNA

(5)

where A is the (apparent) area of contact between thesliding surfaces andU is the relative sliding velocity.Equation (5) can be compared with Equation (1) toshow that

c = UP (6)

where P is the nominal contact pressure (P = N/A).For sliding abrasion, the ‘tribological intensity’c can


thus be interpreted in physical terms as the product ofsliding speed and contact pressure.

Several practical abrasion tests are available in whichboth c and the worn area are nominally constant duringthe whole test. These include the Taber abraser methodand various polishing and grinding tests. In othermethods, such as the dry sand or wet sand rubberwheel abrasion tests and the ball-cratering method,both c and the worn area change continuously as wearprogresses. We shall examine these classes of testin turn.

Abrasion tests with nominally constant c

Various polishing and grinding tests constitute a groupof methods in which the nominal tribological intensityc remains constant over the worn area. In these tests,illustrated schematically in Fig 2, a plane specimen isloaded against the flat surface of a much larger coun-terface in the presence of abrasive particles, and issubjected to a polishing or grinding action. The methodhas been used as a wear test for bulk materials andalso for coated samples for at least 20 years8,9. Inmost cases, the test uses a conventional metallographicpolishing machine, and has been promoted by at leastone manufacturer of such equipment10. Both rotary8,11–13

and vibratory polishing equipment11,14,15 have beenused. Abrasive particles have included silica sand12,silicon carbide and alumina abrasives13,14 and diamondpolishing grits with a wide range of sizes11,16.

The progress of wear is usually determined by period-ically interrupting the test and weighing the sample,and with sufficiently gentle abrasion and accurateweighing this method can detect the removal of verysmall depths of material. For example, the use of amicrobalance with a reproducibility of 5mg can resultin measurement errors corresponding to a depth ofonly 5 nm in steel, although the depth of penetrationof the abrasive particles may be considerably greaterthan this and provide the practical limit to sensitivity11.Nevertheless, changes in wear resistance with depthover a few tens of nanometres have been reliablyreported in polishing wear studies on ion-implantedmetals, in which peak wear resistance was achieved ata depth of about 100 nm11,17,18.

Ideally, a polishing wear test carried out on an evenlycoated (or surface-modified) sample should produce

Fig. 2 Polishing wear test, in which the specimenmoves with velocity U over a plane counterface undernormal load N in the presence of abrasive particles


progressive wear of the coating, leading eventually tocomplete and even removal of the coating over thewhole specimen area. Several critical studies of thepolishing method for ion-implanted metals17,18, hardcoatings16 and bulk metals15 have revealed that this isunfortunately not the case, and that wear is generallynot uniform over the specimen surface. In some cases,wear is most rapid at the specimen edge, while inother work most wear has been observed at the centre.These variations in wear rate may have several origins.It has been suggested that in the case of a coatedspecimen, residual stress variation across the surfacemay influence the local wear resistance16, but the factthat non-uniform wear is also seen in ion-implantedand bulk samples suggests that it must also be due toa variation in the tribological intensity over the speci-men. This in turn may be ascribed to uneven contactpressure on the sample16,18, while the complexities offluid–particle flow through the gap between polishingdisc and specimen are also likely to play a role.

When the wear is uneven, the mass loss after thecoating has been first penetrated will contain contri-butions from both substrate and coating, as illustratedschematically in Fig 3. Interpretation of the resultingdata is then difficult, unless the distribution of contactpressure or wear rate between the coating and substrateis known19.

The Taber abraser, shown schematically in Fig 4,provides a further test method in which the tribologicalintensity c and the worn area both remain nominallyconstant during the test. In this procedure a planespecimen is rotated at a speed of 1 revolution persecond about a vertical axis beneath a pair of compositeabrasive wheels which rotate freely and independentlyabout a horizontal axis which does not intersect theaxis about which the specimen is driven. Various typesof wheels are available, containing abrasive grit par-ticles (usually silicon carbide) dispersed in (mostcommonly) an elastomeric binder; the different grades

Fig. 3 Comparison of the response of a coated speci-men, and an uncoated substrate, to an abrasive weartest. The coating is penetrated unevenly and the sub-sequent mass loss contains contributions from bothsubstrate and coating


Fig. 4 The proprietary Taber abraser tests in which acoated specimen is rotated about a vertical axisbeneath two counter-rotating abrasive wheels. Thespecimen is a sheet typically 100 mm across (not toscale)

of wheel contain different binder materials and sizesof grit. The abrasive wheels, which are pressed againstthe specimen surface under a dead load (typically inthe range 1–10 N on each wheel), roll and slide againstthe specimen and produce an abraded annular weartrack some 12 mm wide20.

The use of the Taber tester has been standardised bythe American Society for Testing and Materials(ASTM) for organic paint coatings21, but it is also quitewidely applied to other coated systems. For example, ithas been used for relatively thick plasma-sprayed alum-ina coatings20 and electroplated and electroless platedmetals22, as well as for much thinner PVD titaniumnitride23,24 and more complex nitride and carbide coat-ings25. The extent of wear is usually assessed by weigh-ing.

The Taber test results in two-body abrasion, and intheory at least, constant tribological intensity. In prac-tice the reproducibility of the test between laboratoriesis disappointing21. This appears to be largely becauseof variability in the properties of the abrasive wheels,the hardness of which changes with time as the bindercompound ages. Added variability may result from theneed to dress the surfaces of the wheels in a standardway before each test in order to expose fresh abrasive;from frictional heating of the wheels and specimenduring the test; and from the influence of the weardebris which is transferred to some extent to the wheelsurface, despite measures to remove it continually fromthe specimen. Although the abraded area remains con-stant, it is thus clear thatc will almost certainly changeas the test progresses, in a manner which depends tosome extent on the properties of the specimen material.

A final method in which c would be expected toremain effectively constant has been proposed recentlyby Axen et al.19, and is illustrated in Fig 5. A hollowcylinder is rotated with its axis normal to the specimensurface, in the presence of a slurry of abrasive particles.As in the Taber test, an annular wear scar results, butthe constant supply of fresh abrasive particles to thecontact zone should avoid some of the problems whichcausec to change in that method. The cylinder diam-eter can be small, so that the wear test can be per-


Fig. 5 Abrasive wear test proposed by Axe´n et al.19 inwhich a hollow flat-ended cylinder is pressed androtated against a plane specimen surface in the pres-ence of an abrasive slurry

formed on a specimen area only a few millimetresacross. Only preliminary results have so far been pub-lished, but the method appears to offer significantpotential for further development.

Abrasion tests with variable c and abraded area

Several practical abrasive wear tests involve continualand progressive changes in both the tribological inten-sity during the test and the area exposed to wear.

One important example is the rubber wheel abrasiontest, in which a rubber-rimmed wheel slides againstthe surface of a plane test sample in the presence ofabrasive particles. Although the applied load is heldconstant, as the sample wears the contact becomesmore conformal and the area of contact increases; thepressureP and the value ofc therefore fall. The test,which is shown schematically in Fig 6, has been

Fig. 6 The dry sand rubber wheel abrasion test (notto scale). A plane specimen is pressed against a rotat-ing rubber-rimmed wheel, 230 mm in diameter. Abras-ive particles (silica sand) are fed into the gap betweenthe specimen and wheel and are dragged through thecontact zone


standardised by ASTM with dry silica sand as theabrasive26, and is widely used as a standard methodfor evaluating low-stress abrasion in bulk materials. Ashort-duration test is specified for use on coatings(Procedure C), although non-standard conditions havealso been used by several investigators. A similar testcan be performed with an aqueous slurry of silicasand, and has also been standardised27,28.

When a thick coating is tested by rubber wheel abra-sion, wear may be confined to the coating alone. TheASTM test has been used, for example, to provideuseful information on the relative abrasion resistanceof various electroplated and electroless plated metalliccoatings, all at least 40mm thick, as well as anodizedaluminium29. When the test is applied to thin coatings,however, the coating is rapidly penetrated and thespecimen mass loss involves contributions from bothsubstrate and coating30. These contributions cannot beseparated unless the variation of tribological intensityc over the worn area is known. This in turn willdepend on the distribution of contact pressure over thewear scar. Although in principle this might be derivedfrom an analysis of the elastic deformation of therubber wheel in contact with the worn profile of thespecimen surface, such an analysis has not, as far asis known, been attempted.

It must be concluded that if the coating is penetratedduring the test, interpretation of the mass loss data froma rubber wheel abrasion test is not straightforward, andthat for this reason such methods are not suitable foruse on thin coatings.

An important group of tests consists of those in whichc and the worn area change during the test, but inwhich the geometry of the wear scar is imposed bythe test apparatus. An example of such a method isthe ‘ball-cratering’ test illustrated in Fig 7(a), in whicha hard sphere is pressed and rotated against the speci-men surface in the presence of an abrasive slurry. Ina typical implementation of the test a steel ball about25 mm in diameter rests against the surface of thespecimen and is driven by friction from a rotatingshaft. The normal force acting on the specimen derivesfrom the weight of the ball, and can be varied to alimited extent by moving the drive shaft relative tothe specimen. Other test methods can be devised whichemploy the same principle, such as the use of a rotatingcylindrical disc (Fig 7(b)). A rotating disc with aspherically domed rim can also be used, in combinationwith rotation of the specimen about an intersectingperpendicular axis (Fig 7(c)). With the correct rimprofile this then produces a wear crater in the form ofa spherical cap, of the same form as that produced bythe ball-cratering method. This method is often referredto as a ‘dimple grinder’ test, since it can be carriedout with commercially available equipment more com-monly used for the mechanical thinning (‘dimpling’)of specimens for transmission electron microscopy.

The important distinction between all these methodsand the rubber wheel abrasion test lies in their use ofa much more rigid counterbody. Elastic deformationof a rubber wheel results in displacements of the wheelrim which are comparable to the depth of the wearscar in the specimen. The shape of the scar is thus


Fig. 7 Examples of abrasion tests which produceimposed wear scar geometries. In each case a hardcounterbody is pressed and rotated against the speci-men surface in the presence of a slurry of abrasiveparticles. (a) Spherical counterbody: ‘ball cratering’test. (b) Cylindrical counterbody. (c) Domed cylindricalcounterbody combined with rotation of the specimenabout a perpendicular axis: ‘dimple grinder’ test

determined by both the distortion of the wheel and thewear resistance of the specimen. With a rigid coun-terbody, in contrast, any local reduction in wear resist-ance in the specimen will lead to a local increase inwear rate, which will however increase the gap betweenthe specimen and counterbody at that point, reducingthe contact pressure and thus reducing the wear rate.This mechanical feedback results in a wear scar witha conformal geometry which almost precisely replicatesthe shape of the counterbody, as illustrated in Fig 8.

The use of a cylindrical steel wheel for abrasion testing(Fig 7(b)) dates back to the work of Brinell in 192131,and the principle of an imposed wear scar geometry


Fig. 8 Geometry of an imposed spherical wear scaron a coated sample, as produced by the ball-crateringof dimple grinder test

has been used since at least 1956 to investigate depthprofiles in modified surfaces and to measure the thick-nesses of thin coatings32,33; it has been standardised byASTM for the latter purpose34. The use of a sphericalcounterbody was first reported in 197935, and commer-cial ball-cratering equipment is now widely availablefor the determination of coating thickness. It is onlyrecently, however, following the important work ofKassmanet al.36, that abrasive wear tests which utilisethe principle of imposed wear scars have become estab-lished.

For thick coatings it is possible to perform abrasivewear tests by the imposed wear scar method withoutpenetrating the coating. In that case, or for bulk un-coated samples, it is straightforward to measure thedimensions of the wear scar, and to note that Equation(5) can be recast in the following form:

Adxdt

=dVdt

= UNk (7)

where V is the volume of material removed by wear.With an imposed wear scar geometry,V can be calcu-lated from either the scar width or its depth, and thevalue of k simply derived provided thatN remainsconstant during the test and is known.

For example, the volume of a spherical scar withchordal diameter 2b (Fig 8) is given to a close approxi-mation by

V <pb4

64R(8)

for b¿R, where R is the radius of the sphericalcounterbody. If the total sliding distance during thetest is S, Equation (7) can be integrated over the testduration and combined with Equation (8) to give:

SN<1k Spb4

64RD for b¿R (9)


A plot of b4 againstS should therefore be linear, andthe value ofk can be readily deduced from its slope.Results which confirm the validity of this approachhave been reported from tests with both the dimple-grinder and the ball-cratering apparatus on a widerange of bulk specimens and thick coatings: materialshave included silicon single crystals, high speed steeland cemented carbide36; soda-lime glass, various engin-eering ceramics, copper and aluminium37; ultra-highmolecular weight polyethylene for orthopaedic pros-theses38; and relatively thick polymeric paint films39.

When thinly coated samples are tested, the substrateis often revealed at an early stage of the test unlessparticularly low loads and/or short test durations areemployed. The fact that the load applied by the rotatingcounterbody is then partitioned between coating andsubstrate, and that this distribution changes during thetest, would seem to suggest that derivation of the wearcoefficients for the coating and substrate via Equation(5) would not be straightforward. However, Equation(7) will apply to both the coating and the substrateseparately, so that we can write

dVs

dt= UNsks (10a)

dVc

dt= UNckc (10b)

which can be integrated over the test duration to yield:

Vs = SNsks (11a)

Vc = SNckc (11b)

where Vs,c are the total volumes removed by wearfrom the substrate and coating respectively, and

Ns,c

are the mean loads supported by each component overthe duration of the test. Since the total normal load,N is equal to

(Nc + Ns)

at all times, Equation (11)a Equation (11)b can becombined to give:

SN=Vc

kc

+Vs

ks

(12)

This important equation was first derived by Kassmanet al.36 who used it to deduce values ofkc and ks

from abrasive wear experiments performed with a dim-ple grinder (Fig 7(c)) on TiN and TiC coated steeland cemented carbide substrates. For a spherical wearscar geometry the volumesVc and Vs can be readilycomputed from measurements of scar diameter madeon the surface of the specimen. Although Kassmanetal. performed a supplementary test on an uncoatedsubstrate to measureks, an alternative method of datareduction can be used by which the values of bothkc

and ks can be determined from a single test on acoated sample37.

The ball-cratering and dimple-grinder methods havebeen further developed and used extensively over the


past 5 years to assess the abrasive wear resistance ofthin hard coatings40–46; it should be noted that thesemethods are not restricted to use on plane specimens,but can be applied successfully to any coated surface,even one with compound curvature47. Both methods,and recent refinements, have been comprehensivelyreviewed by Rutherford and Hutchings48 and Gåhlinetal.49. In particular, important advances have been madein understanding the influences of imperfections in thewear scar geometry, which are a disadvantage of thedimple-grinder method49, and in optimising the methodof data analysis to reduce errors37,48.

There is still further scope for development of thistype of abrasive test, however. For example, in themost common implementation of the ball-crateringmethod, the normal force acting on the specimen islinked to the weight of the ball and can be variedwithin only a small range; it must be measured duringthe test since it is influenced by the friction betweenthe specimen and the ball37. For some purposes it maybe useful to vary the load more widely, and alternativeimplementations of the test are possible in which theball is driven positively through a shaft and the loadis applied through a lever arm. Such an arrangementalso makes it possible to measure the depth of pen-etration of the ball continuously during the test50.Although throughout their original work on the dimplertest Kassmanet al. expressed their theoretical analysisin terms of wear scar depth, they and subsequentinvestigators of both the dimple-grinder and the ball-cratering test actually measured the surface diameterof the scar. There may be significant advantages inmodifying the test so that depth is the primarymeasurement, and recent work has shown that thepractical problems involved in making such a delicatemeasurement continuously during the test can besolved51.

Erosive wear

Tribological intensity in erosive wear

In order to derive an expression for the tribologicalintensity c associated with an erosive wear test, weshall assume a simple model forE, the mass lost froma surface due to the impact of unit mass of abrasiveparticles at velocityv (see Ref. 2):

E =k

2rv2 (13)

where r is the density of the surface material. Herekhas the same interpretation as in Equation (1). If theflux of erodent particlesf represents the mass ofparticles striking unit area in unit time, the rate oflinear recession of the surface, dx/dt, at any point onthe surface can be expressed as:

dxdt

=Ef

r(14)

On substituting forE from Equation (13), and com-paring the resulting expression for dx/dt with Equation(1), we deduce that for erosive wear the tribologicalintensity c is given by:


c =12

fv2 (15)

c is thus equal to the kinetic energy of the particlesstriking unit area in unit time.

The model used to derive Equation (13) is very simple;in practice,E is usually found to depend on a higherpower of v (typically about 2·4), and also on theimpact angle u. A more general expression forcis therefore:

c = ff(v,u) (16)

For the purposes of the present discussion, it isimportant to note that the tribological intensityc islinearly proportional to the particle fluxf for constantv and u, and that it is a strong function of the particlevelocity v.

Erosion tests

Although several other types of erosive wear test havebeen devised, only two basic designs are in widespreaduse. In the gas-blast rig (Fig 9), erodent particles areaccelerated along a nozzle in a flowing stream of gas,usually air. The nozzle may be a parallel cylinder, asspecified for example in an ASTM standard52, or mayhave a more complex shape. The specimen is mountedat a fixed stand-off distance from the end of the nozzle.The centrifugal accelerator, shown schematically in Fig10, uses rotation to accelerate particles fed into thecentre of the rotor, and produce a stream of particlesin a (usually) horizontal plane. The apparatus can beused to test more than one specimen simultaneously,

Fig. 9 Gas-blast erosion test in which particles areaccelerated along a parallel cylindrical nozzle andstrike a plane coated specimen


Fig. 10 Centrifugal erosion accelerator in which par-ticles are fed into the centre of the rotor and moveoutwards along radial channels. On leaving the rimof the rotor they strike fixed specimens (of which fourare shown) mounted around the circumference

held at suitable angles around the periphery of therotor.

Both methods produce divergent streams of particles,so that the particle fluxf will in general vary overthe surface of the specimen. The impact velocity andangle may also vary over the specimen, so that signifi-cant variation of the tribological intensityc is to beexpected. Some investigators have masked the speci-men area so thatc varies little over the exposedarea23,53–55, and very few have attempted to quantifythe distribution of flux over the eroded area56,57; most,however, appear to have ignored the effect.

Shipway and Hutchings57 have recently reviewed theapplication of solid particle erosion testing to coatedsamples. As in abrasive wear testing, by far the mostcommon method of quantifying the extent of wear isby mass loss. Sufficiently thick coatings can be testedin the same way as bulk samples, but as for abrasion,problems arise in the interpretation of the results oncethe coating is penetrated. Attempts have been made toseparate the mass loss due to the coating from that ofthe substrate, but the method depends on detecting achange in the rate of mass loss as the substrate isrevealed, and can be used successfully only if theerosion rate of the substrate is significantly greaterthan that of the coating, or if there is little variationof c over the specimen surface58.

Some investigators have accepted penetration of thecoating as inevitable, and used the mass of erodentparticles, or the time of exposure to the particle stream,required to remove the coating completely, as ameasure of its erosion resistance59–61. Alternativeapproaches, which do not demand continuous obser-vation of the specimen during the test and are thusless prone to operator error, can also be used to assessthe durability of the coating in terms of the dose ofparticles required to penetrate it. Both energy-dispersiveX-ray microanalysis (EDX] and back-scattered SEM


imaging have been applied to a small area of theeroded specimen, over whichc can be assumed to beeffectively constant, to quantify the extent of coatingremoval56,62; the erosion resistance of a coating hasalso been described by the particle dose just requiredto expose 50% by area of the underlying substrate42.

If the durability of a coating under erosion conditionsis of interest, however, it is better to measure the doserequired to remove the coating completely; Hedenqvistand Olsson63 demonstrated in principle how this couldbe done by exposing a specimen to a spatially varyingflux of erodent particles, and noting the position on thesurface at which the coating has been fully penetrated.Hedenqvist and Olsson used a centrifugal acceleratorand inferred the distribution of particle flux over thespecimen surface by eroding glass specimens. Anessentially similar approach has been further developedby Shipway and Hutchings using a gas-blast appar-atus57.

The particles leaving a long parallel nozzle in a gas-blast erosion apparatus form a divergent plume, theproperties of which can be deduced from erosionexperiments on coated specimens64. To a good approxi-mation, the flux of particlesf(r) striking the specimenat a radial distancer from the nozzle axis is:

f(r) <ma2 exp( − ar)

2p(17)

where m is the mass feed rate of particles along thenozzle, anda defines the divergence of the particleflux. The divergence of the particle stream is radiallysymmetrical, and as the coating is penetrated there isa circular boundary on the specimen between the cen-tral region over which the substrate is revealed andthe outer region over which coating material is stillpresent. It can be shown that the radius of this circularwear scar,rc, after a total massm of particles havepassed down the nozzle is given by57:

rc =1a

ln m −1a

lnS2pQc

a2 D (18)

where Qc is the dose of particles (mass per unit area)required to remove the coating. A plot ofrc againstthe logarithm of the mass of erodent particles shouldtherefore be linear, with slope 1/a, and Qc can bederived from the intercept of the line. Equation (18)has been found to be valid for many different coatingsystems tested by the gas-blast erosion method with aparallel cylindrical nozzle; these include thin hard PVDcoatings, anodized films on aluminium, diamond-likecarbon films and polymeric paint coatings39,45,57,65,66.The value ofQc, which has been termed the ‘erosiondurability’ of the coated system, has been shown tobe a sensitive indicator of coating adhesion for thinhard coatings, and to vary in a systematic way withimpact velocity, coating thickness and erodent par-ticle size67.

General discussion and conclusions

To be useful, a laboratory wear test method mustinvolve a reproducible and well-characterised means ofsubjecting the sample to abrasion or erosion, coupled


with an accurate method of quantifying the extent ofwear. In addition, tests which are to be applied tocoated or surface engineered materials should allowthe wear rate of the coating to be determined indepen-dently of the substrate, and this condition imposessignificant constraints on the test.

It is important also to acknowledge the purpose of thetest. As outlined above, samples may be subjected toabrasion or erosion testing in order to assess theirdurability when exposed to this type of wear. In thiscase, it is important that the conditions of the testreproduce, as far as possible, those of the practicalapplication for which the component is intended1. Inparticular, the influence on wear rate and wear mech-anism of the size, shape and most importantly, hardnessof the abrasive or erosive particles must be recognised2.In the case of brittle materials, transitions in wearmechanism can occur as the test conditions are varied,which in the extreme case may render the resultsfrom a particular wear test completely irrelevant to thebehaviour of a material in a given application68.

Wear tests may also be employed in a more generalway as part of the process of characterising a material;for example, tribological tests of various kinds com-bined with measurements of scratch response and infor-mation from indentation testing can be used to generatea ‘tribological profile’ which is particularly relevant tocoated systems41. Abrasion and erosion can also beused as controlled methods of mechanically loadingthe surface of a material, and can be used to generateinformation on the adhesion of a coating56,57,63,66. Inthis respect, such tests may have a valuable role toplay in the quality assurance of coated or other surfaceengineered components, although there is still muchprogress to be made in understanding the complexinteraction between test conditions and tribologicalresponse, and in relating this response to more funda-mental properties such as interfacial adhesion.

Any test which is intended for quality assurance onsurface engineered components should ideally beapplied directly to those components, and not tosamples specially produced for test purposes, even ifthese have been made from the same material as theproduction components and surface treated in the samebatch. Of the abrasion and erosion tests describedabove, only a few can readily be used to test suf-ficiently small areas to be applicable to most coatedcomponents; these include the imposed crater geometryabrasion tests (e.g. by ball-cratering), the rotating cylin-der abrasion test19 and the erosion test described byShipway and Hutchings57.

It is clear that there has been significant progressover the past 20 years in devising and developing newapproaches to the wear testing of surface engineeredmaterials, and that these can make a valuable contri-bution to our understanding and use of these economi-cally important technologies. There is, however, stillample opportunity for further development of versatileand robust wear test methods, and for building on ourcurrent understanding of material response to abrasionand erosion.


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Abrasive and Erosive Wear Tests for Thin Coatings a Unified Approach

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