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A review of two-body abrasive wear
Wear, 27 (1974) 1-17 10 Elsevier Sequoia S.A., Lausanne
1 - Printed
A REVIEW OF TWO-BODY
in The Netherlands
ABRASIVE WEAR
M. A. MOORE
of Agricultural Engineering,
National Institute
(Received
Silsoe, Beds. (Gt. Britain)
July 19, 1973)
SUMMARY
The present state of the art of two-body abrasive wear has been reviewed, emphasising the wide range of variables which can influence the abrasive wear of materials. The basic mechanism of abrasive wear, properties of the abrasive, variables in the wear environment and the relation between abrasive wear, mechanical properties and metallurgical structure have been considered.
1. INTRObUCTION
In the engineering industry abrasive wear is probably the most common cause of mechanical damage. It manifests itself in several forms: in the wear of equipment which engages an abrasive medium, in the wear of seals or machine parts between which abrasive particles can penetrate and wear by abrasives entrained in fluids. The first case in which abrasive particles simply rub against a surface is referred to as two-body abrasive wear. The second and last cases in which the abrasive can become trapped between two sliding surfaces are referred to as three-body abrasive wear. The exact conditions of abrasive wear and the wear environment may vary widely, and in considering any particular wear system attention must be paid to several variables. The variables affecting wear in two-body situations are discussed in the following review of the literature. 2. THE BASIC MECHANISM
OF ABRASIVE
WEAR
The abrasive wear referred to in this review is defined as the removal of solid material from a surface by the unidirectional sliding action of discrete particles of another material. The surfaces are maintained in a constant orientation relative to one another for the period of contact. The basic mechanism of abrasive wear has been the subject of many investigations. Khrushchev and Babichev’ identified two processes taking place when abrasive grains made contact with the wearing surface: (1) the formation of plastically impressed grooves which did not involve metal removal, and (2) the separation of metal particles in the form of microchips.
2
M. A. MOORE
Other investigations 24 have shown that some contacting abrasive grains merely make elastic contact with the surface, and Aghan and Samuels’ found that the extruded fins at the edges of grooves produced by rubbing can sometimes become detached forming secondary chips, although they concluded that primary chips dominate in terms of metal loss. In a comprehensive study Mulhearn and Samuels6 determined that although the number of contact points per unit area varied with the size of the abrasive the proportion of contacts producing a chip was approximately constant and as low as 12%. More recently, Larsen-Badse3 has estimated that 50 - 60% of the contacting points produce chips. Mulhearn and Samuels6 argued that chips are only produced when the contacting face of an abrasive grain makes an angle greater than or equal to the “critical attack angle” with the wearing surface. Some later work7.8 showed that the orientation as well as the inclination of the cutting face are critical in determining whether or not a chip is cut, and that the critical attack angle varies with the material under wear and is determined by the coefficient of friction between the contacting surfaces. Kragelskii’, Khrushchev and Babic,hev’, Aghan and Samuels’, and Graham and Bau14 have also found that chip cutting and rubbing depend upon the shape of the indenting particle. In particular spherical indenters have been observed to show a change over from rubbing to at least partial chip formation when the indentation strain (defined as the depth of indentation divided by the diameter of the indenter) exceeds a certain value. Goddard and Wilman’ have calculated that the coefficient of friction depends upon the particle shape, which, in the light of Sedriks and Mulhearn’s findings’, might account for the observed particle shape dependence. The combined effect of the chip cutting and rubbing processes was shown by Stroud and Wilman” who estimated that less than 409; of the total groove volume is removed as wear debris, the remainder being displaced by plastic flow. They concluded that much of the energy used in abrasion is expended in plastic deformation, while Larsen-Badse I1 has estimated that 45% of the minimum work to remove material is plastic shear to form chips and ridges, and Buttery and Archard” have associated the efficiency of abrasion with the pile-up of material, i.e. plastic flow. In view of this the mechanism of abrasive wear of brittle solids may be modified by the onset of fracture13. 3. THE EFFECT
OF PROPERTIES
OF THE ABRASIVE
3.1. Abrasive type and relative hardness Several investigations 13P18 have shown that relative wear resistance is not independent of the hardness of the abrasive. Khrushchev and Babichev14 concluded that wear was independent of abrasive hardness when this was very much greater than the hardness of the wearing material, but as the hardness of the wearing material approaches that of the abrasive, wear decreases and if the hardness of the abrasive is less than that of the wearing material wear decreases rapidly as the difference increases. However, Nathan and Jones l5 found that even with relatively very hard abrasives the volumetric wear still depends on the abrasive hardness. Richardson’3.‘6-‘8 has defined hard abrasives as those whose hardness exceeds
TWO-BODY
ABRASIVE
3
WEAR
500
200
loo
“/a
=O
20
IO
5
2
I
0:4
0.6
048 %a
“O
Is2
)A
Fig. 1. The increase in wear resistance of martensitic steels on glass abrasive, showing the effect of relative abrasive hardness; b is the relative wear resistance on glass, fi is the relative wear resistance on 180 grit Sic, H is the hardness of the test material, Ha is the hardness of the abrasive. (Richardson)
that of the worn surface material, and soft abrasives as those whose hardness is less than that of the worn surface. Richardson showed that both hard and soft abrasive wear are very sensitive to the characteristics of the individual abrasives, the wearing action of soft abrasives is similar to that of hard abrasives but modified by increasing damage to the grit, and that true scratching ceases when the yield stresses of the wearing material and the abrasive are approximately equal. Figure 1 shows the effect of the relative abrasive hardness on wear resistance. The different strength properties of abrasives” and their mode of deterioration may account for the importance of their relative hardness. Duwell and McDonald” identified two abrasive deterioration mechanisms: attritious wear caused by chemical degradation of the surface, and fragmentation due to relief of internal stresses. They found that oxide structures wear more by fragmentation and carbides and borides wear more by attrition, and they suggest that this is because of the chemical inertness of the oxide structures. Johnson” and Patterson and Mulhearn” have found that abrasive fracture usually commences at favourably situated cracks which are inherent from manufacture. Grit failure may produce
M. A. MOORE
Mean
o
ALUMINIUM
A
BRASS
0
COPPER
0
BRONZE
A
SWEDISH IRON
diameter
Fig. 2. The effect of abrasive
of
abrasive
particles
01)
grit size on volume wear. (Nathan
and Jones).
grit fragments which can act as fresh abrasive, but a cutting point is only regenerated if the original cutting angle of the grit is between the critical angle and 90”. The overall tendency is, therefore, a reduction of cutting points but probably at a slower rate than by attritious wear. Richardson’s field results23 show that particle fracture in the weaker ironstones may render them more effective in cutting than the stronger flint. 3.2. Abrasive yrir size It has been established3.“.‘8.24 t h at volume wear increases steeply with grit size to some critical size and then increases at a reduced rate with increasing grit size. (Fig. 2). With decreasing wear resistance the gradient of both linear portions of the wear/grit size curves increases and so does the critical grit size. Mulhearn and Samuels’ account for their results in terms of the geometry of abrasive particles which they found to vary with grit size. Larsen-Badsej.” has suggested that the proportion of the load carried by elastic contacts varies with grit size and. more recently2’. that specimen size may account for the critical size effect since the length of contact of an abrasive particle has a different effect on the deterioration has also accounted for his results by the different of different grit sizes. Richardson”
TWO-BODY
ABRASIVE
5
WEAR
modes of deterioration of the different grit sizes. Rabinowicz and Mutisz6 concluded that the critical size effect is due to the interference between adhesive and abrasive particles which increases as the grit size decreases. Avient, Goddard and Wilman” have suggested that abrasive particles which become embedded in the wearing surface might cause an increase in wear resistance on the smaller grit sizes, and Johnson2’ has confirmed that the pick up of abrasive increases rapidly as the grit size decreases. 3.3 Abrasice shape Mention has already been made (Section 2) of the effect that the abrasive particle shape has on chip cutting and rubbing. Burwell29 cites the results of R.D. Haworth who found that angular soft particles produced more wear than rounded hard particles. Goddard and Wilman’ have calculated the friction coefficient in abrasion for various idealised particle shapesand since the friction coefficient is related to plastic deformation in wear, particle shape is expected to contribute to the ratio of material removed to material which flows into ridges, etc. Hata and Muro3’ have found experimentally using a quadrahedral diamond tool that resistance to abrasion increases as the cone angle decreases and if a facet leads as opposed to an edge. In a study of abrasion by loose graphite and molybdenum disulphide particles Giltrow and Groszek 3’ found that the rate of abrasion increases as the particles become less plate like and suggest that this is because plates are more likely to lie flat at the sliding interface. 4. THE EFFECT
OF VARIABLES
OTHER
THAN
THE ABRASIVE
4.1. Speed
Khrushchev and Babichev32 and Nathan and Jones24 have established that the volume wear increases slightly as sliding speed increases in the range O-2.5 m/s, (Fig. 3). The increase is more marked for larger grit sizes over the lower end of the range and. also, for more wear resistant materials. Khrushchev and Babichev attributed a 13”” increase to a change in the properties of the abrasive cloth, while Nathan and Jones have suggested that frictional heating may account for the increase, especially as the rate of heating in abrasion is greater for the larger grit sizes. Both Khrushchev and Babichev and Richardsonj3 have stated that changes arising from strain rate and frictional heating are expected with increasing sliding speeds, but discarded frictional heating as a serious problem since no evidence of surface heating could be found. 4.2. Load Several investigations24.2’.30.32 have shown that volumetric wear is directly proportional to the nominal load up to a critical load which is determined by the onset of massive deformation of the specimen or instability of the abrasive surface. Nathan and Jones24 found that deviation from linearity occurred at lower loads for the smaller grit sizes. Larsen-Badse3. l l suggests that failure of the abrasive commences when the applied load on a grain reaches a value corresponding to a groove width of about 0.17 of the grit diameter, since groove widths are fairly constant with load while the number of contact points increases roughly linearly
6
M. A. MOORE
35
00
3oolJ 0
7o)r l
ALLJMNIUM
0
l
BRASS
D
A
SWEDISH
c
IRON 3
”
5
0
0.50
1.00
2.00
1.50
Velocity
Fig. 3. The effect of velocity
IZ.O-
of abrasion
@ Specimen diameter 0.100 in I! x 11 0.180 in
0 ll.O_ $! lO.O,o -: 9.0Lu i
2.50
(m/set) on volume wear. (Nathan and Jones)
Material 2% C . I4 yo Cr Dicsteel Hv = 902 Abrasive
40 grit flint paper
8.0-
: 7.00 ’
6.0I
I
IO0 Contact
so Fig. 4. The effect of specimen
diameter
I IS0 stress, g/mm2
and nominal
contact
I 200
1 250
stress on relative wear resistance.
(Richardson)
TWO-BODY
ABRASIVE
7
WEAR
with load. More recentlyz5 he concluded that if grit wear can be eliminated the volumetric wear is directly proportional to the applied load. Richardsoni found that, although the volumetric wear increased linearly with load, the relationship was such that the wear/unit load decreases as the load increases, and that the sensitivity of relative wear resistance to increase with decreasing contact stress varies for different materials’s, (Fig. 4). 4.3. Specimen size Specimen size has been found to affect abrasive wear in various ways. In abrasive wear testing it is the usual practice to choose a specimen size such that deterioration of an abrasive grain in contact with the specimen is minimal. Avient, Goddard and Wilmanz7 have pointed out that it is important to use a short enough specimen such that the amount of metal worn off by an indenting grain is not enough to fill interstices between it and adjacent particles, which would reduce the effectiveness of the abrasive. Richardson’ 6 realised that the specimen diameter/groove depth ratio might affect relative wear resistance since losses at the edge of the specimen due to plastic flow would be large compared with losses from the surface if this
7
6
180
w
A_--'-A---A----A_
320
Lo Ii1
,__o----o--0--600
0
2
4 Specimen
Fig. 5. The effect of specimen
diameter
6 Diameter,
8
IO mm
on wear rate. (Larsen-Badse)
12
8
M. A. MOORE
ratio was small. He also found’* that if deterioration of the grit is important variation in specimen diameter has a large effect on the relative wear resistance, (Fig. 4). In studying the influence of grit diameter Larsen-Badse” realised that the critical grit diameter was proportional to specimen size. In a further study25 he concluded that the variation of wear rate with load and grit size are not linear because of the effect of specimen size, (Fig. 5). Larsen-Badse has suggested that a peak occurs in wear rate versus specimen length due to conditioning and deterioration of the abrasive, and that the critical specimen length corresponding to maximum wear depends on the applied conditions. Therefore, if the grit life is much longer or shorter than specimen contact, the effect on wear data will be small, but it may be appreciable when the two values are about the same. 4.4. Length of wear path Nathan and Jones24 have found a linear relationship between volumetric wear on hard abrasive and the length of the abrasive path, but it is likely that their experiments were not sensitive to the non-linear portion of the curve which occurs over a short sliding distance. This non-linear portion corresponds to the surface coming to equilibrium and Richardson I7 has shown that the shape of the curve depends on the relative hardness of the wearing material and the abrasive, (Fig. 6) because with soft abrasives the wearing material takes much longer to strain harden to equilibrium conditions. Avient, Goddard and Wilman 27 found that the coefficient of friction in abrasion rises to an equilibrium value after a sliding distance which is larger the harder the metal, and they attributed this to pick up of the abrasive. Johnson2* has observed an increase of abrasive pick up with track length. Wear-Distance
Run on 30 Material
Grit
Glass
0.74 Ok C steel
Hv 503 J*o-
Run I X Run 2
Paper
Hv 650
0
0 Run I OJ-
J.
x
Run2
X 0.6 -
,-2.0-
a’
0.5-
/
a I
Y; B 2
0.4 -
_s 3 1.0.
/ Initial gradient = 0.0069
mqhm
t
0.3 0.2-
/
o
lnitlol qradient
’
I
-s’ CI
$ 0
100
200
300
400
500
600
0 Track
Fig. 6. The effect of relative
material
hardness
/ ;
* 0.0026
mq/cm
1 100200300400500600
, cm on the wear-distance
run curve. (Richardson)
TWO-BODY
ABRASIVE
9
WEAR
5. FRICTIONAL
HEATING
5.1. Frictional
heating
AND HUMIDITY
Khrushchev and Babichev3’ and Richardson34 have concluded that during slow speed abrasive wear changes due to frictional heating have little affect on the wear rate. Nathan and Jones24 attributed the increase in wear with increasing sliding speed to frictional heating. Richardson33 does, however, point out that temperature rises may occur on a scale commensurate with the size of asperities on the grit particle surface and affect the friction at the gritchip interface. Moore3’ has found temperature rises of 32%900°C in the grit&chip contact zone and concludes that the physical, chemical and mechanical properties of the contact zone may be modified although the overall effect on abrasive wear is likely to be. small. In the grinding of metals, where relative speeds are much higher, Ragon et a1.36 have concluded that temperature rises in the order of 1500°C occur. 5.2. Humidity Rabinowicz3’ states that high humidity increases the wear rate in abrasion by about 15% and that the action of water vapour is similar to other lubricants which increase the effectiveness of abrasion. In field and laboratory tests, Khrushchev and Babichev3* and Richardson34 have found that the effect of moisture on the wear rate and relative wear resistance is small and only becomes of importance at low wear intensities. 6. ABRASIVE
WEAR IN RELATION
TO MECHANICAL
PROPERTIES
6.1. Elastic modulus and elastic limit of strain
In 1951 Oberle39 suggested that a measure of wear resistance is the amount of elastic deformation that the wearing surface can sustain because a material can deform elastically to accommodate the abrasive grit with no plastic deformation or metal removal. Oberle equates a high wear resistance with a high limit of elastic strain which was given approximately by the ratio Brine11 hardness/Young’s modulus. Khrushchev and Babichev’, however, found Oberle’s model unsuitable for assessing abrasive wear resistance. Spurr and Newcomb 4o have suggested that at equilibrium an abrasive particle displaces metal ahead of it plastically but that material behind recovers elastically so that the volume wear depends on the elastic recovery of the surface and is inversely proportional to the elastic modulus. Khrushchev and Babichev41 have also found that wear resistance of pure metals can be related to the modulus of elasticity, (Fig. 7) but that the relationship does not hold for heat treated steels42. They conclude that the relationship for the pure metals cannot be due to specific features of the abrasive wear process. 6.2. Bulk hardness The relationship between wear resistance and bulk hardness is often used to present abrasive wear data 16.4345, (Fig. 8). Khrushchev and Babichev43*44 showed that the wear resistance of pure metals is directly proportional to their hardness and Rubenstein46, in a simplified analysis of metal removal during wear, has shown that
10
M. A. MOORE
Fig. 7. The dependence of relative wear resistance, (Khrushchev and Babichev)
R, of some pure metals on the modulus
of elasticity
E.
relative wear resistance is related to hardness by the reciprocal of the hardness of the standard material. Rubenstein concludes that a better presentation of the data would be given by plotting the wear resistance against the ratio hardness of the test material/hardness of the standard material. Khrushchev and Babichev43 also found linear relationships between wear resistance and hardness for heat treated steels and some metal carbides. For the steels the data fitted an equation of the type: p=/?o+C’(HV+Hu,)
6.2.1 where fl is the relative wear resistance of the steel, B,, is the.relative wear resistance in the annealed condition, C is a constant, Hu is the Vickers diamond pyramid bulk hardness of the steel, and Ha, is the hardness of the steel in the annealed condition. Khrushchev and Babichev43 and Richardson l6 found that the constant C varied systematically with carbon and alloy content of the steel, and that structurally heterogeneous materials in general show a lower wear resistance than expected from their hardness compared with pure metals. Richardson also showed that impurities and alloying elements in the pure metals do not produce increases in wear resistance proportionate to increases in hardness. In a study of wear resistance of metals, ceramics and plastics, Selwood 45 found that hardness does not describe the wear resistance of very ductile, extendable or brittle materials.
TWO-BODY
ABRASIVE
11
WEAR
60
Ni
IO
cu
i Zn
Ii
i
Pb
loo
200
Fig. 8. The dependence hardness. (Khrushchev
100 400 Hordnrss,
500
600
IiD-Kg/mm’
700
of relative wear resistance, and Babichev)
800
900
E, of some pure metals and steels on the bulk material
6.3. Surface hardness
In the absence of a single relationship between bulk hardness and wear resistance and from results showing that the wear resistance was little affected by the initial degree of cold-work, Khrushchev and Babichev’ concluded that a high degree of strain hardening occurs at the surface of worn metals, and that wear resistance depends on the strength of the material in its “maximum work-hardened state”. Avient, Goddard and Wilman” argued that the abrasion process is to a considerable extent dependent upon the indentation of the abrasive grains into the metal surface, and they showed that a relationship existed between the surface hardness after wear and wear resistance. Larsen-Badse47T48 has pointed out that in a group of materials for which the rate of strain hardening is a function of the bulk hardness, the hardness of the abraded surface will be a function of the bulk hardness, and so abrasive wear resistance may be proportional to both of these properties. Richardson33 studied the surface hardness of materials strained by shot peening, trepanning and abrasive wear. He found that the whole of the effective surface does
12
M. A. MOORE
SO(160
pit
corundum cloth)
2.5-
QZ4 0.3790C lowalloysteel mort*nsitr
k
Al .
0
/)Al Alloy
“‘I 200 JO0 400 500 ” 600 700 ’ 800 ” 903 1000 i II00 f 1200 ” I300 I400 ” IS00 100
M&mumhordnrss, Xu kg/mm* Fig. 9. The relationship between relative weaP resistance and surface (maximum) hardness, Hu, after wear. (Richardson)
not strain harden to the same level, although limited regions reach a “maximum hardness”, Hu. Abrasion resistance was correlated with maximum hardness values (Fig. 9) and Richardson suggests that within limited classes of materials, the relative wear resistance may be taken as proportional to the maximum hardness. In a recent analysis of Richardson’s and further results, Moore et a1.49 have shown that the limiting strength attained at worn surfaces can be understood in terms of metallurgical structure and suggest that the limiting strength is a measure of the maximum dislocation density that can be stored by the material, under the stress system developed in abrasive wear, before fracture occurs. Although surface hardness gives a much more systematic correlation with abrasive wear resistance than bulk hardness, it seems that abrasive wear has to be defined more in terms of the flow and fracture properties of the materials and their metallurgical structures. 6.4. Flow and fracture properties In a theoretical analysis of the stress/strain system developed in the wear process Larsen-Badse 47.48 has used a power law to describe the stress/strain curve of pure metals: a=A&”
6.4.1
where c is the flow stress, A is a constant, E is true strain, and $2is the strain hardening
TWO-BODY
ABRASIVE
13
WEAR
exponent, and shows that abrasion resistance is proportional to either bulk or surface hardness and exponential (n). Therefore, abrasion resistance plotted against surface or bulk hardness should give a straight line through the origin with some scatter due to the spread in values of the strain hardening exponent, n. For instance, Alison and Wilman5 Oattributed the different wear properties of hexagonal and cubic metals to the difference in their slip processes which causes their strain hardening exponents to be different. Larson-Badse 47 also points out that variations in n will cause less scatter in wear resistance uersus surface hardness plots, and Richardson33 has found the relative wear resistance of hexagonal metals about the same as cubic metals in relation to their surface hardness. For interstitial and substitutional solid solutions, precipitation and dispersion hardened materials a single relationship between wear resistance and bulk or surface hardness does not exist because of the wide variation in their strain hardening exponents. Moore et al. 49 have shown that the limiting strength of worn surfaces can be understood in terms of metallurgical structure and the way it influences the strain hardening exponent. In studies of the abrasion of hardened and tempered steels, Larsen-Badse4’ et aL4* considered various models for dispersion hardening and concluded that finely dispersed hard particles influence the flow stress and increase abrasion resistance according to a Hall-Petch relation (i.e. abrasion resistance inversely proportional to the square root of cementite particle spacing), (Fig. 10). Larsen-Badse 51 has also investigated sintered-aluminium-powder-type alloys and found that abrasion resistance is directly related to the fineness of the dislocation SAE
1040
STEEL
SPHEROIDITE ‘i 8 z
-6.5
”
c
BULK
HARDNESS
5.0
I
too
’ 0.6
I
I
I 0.7 I/&,
0.8
_&
0.9
[MICRONS]
Fig. 10. The dependence of the wear resistance (Larsen-Badse and Mathew)
of spheroidal
cementite
steel on the particle
spacing.
14
M. A. MOORE
substructure formed in the abraded surface, and inversely proportional to the square root of the particle spacing, and that the flow stress responsible for abrasion resistance is governed by the Ansell-Lenel mechanism (i.e. flow occurs when the shear stress on particles due to dislocation pile-ups deforms or fractures the particles). Lin and Wilman 52 have related wear resistance to the structural changes occurring during age hardening an aluminium
WEAR
IN RELATION
TO METALLURGICAL
STRUCTURE
Metallurgical structure, such as grain size, dispersed phases, lamellae etc. have a considerable influence on the mechanical properties of a material and, therefore, since abrasive wear involves plastic flow, must also influence abrasive wear resistance. This has been well established in several investigations’3.‘6,18.33. 34,43.4749.51,s2, and discussed in the previous section. Of equal importance, however, is the effect that relatively large structural components have in obstructing the path of abrasive grit during wear. Khrushchev and Babichevs3 studied the wear of structurally heterogeneous materials and concluded that their wear resistance was equal to the sum of the products of the wear resistances of the individual components and their corresponding volumetric quantities, e.g. Fig. 11 shows wear resistance of some irons and steels as a function of cementite content. Richardsonl‘j states that an additivity law of this sort may need to be modified to take account of scale effects. He found that the intermediate hard abrasives, such as corundum, are not necessarily hard
s,:-_
N
2 ;.;:; p to-
s 2
I%C
IO
20
2%c 30
3%C 40
50
Cementite %
Fig. 11. The dependence of relative wear _resistance, E, and bulk hardness, of some steels and irons. (Khrushchev and Babichev)
H, on the cementite
content
TWO-BODY
ABRASIVE
15
WEAR
with respect to all carbides found in alloy steels and cast irons, and that, even if the abrasive is very much harder, hard phases become less effective as the scratch size increases. In studies on soft abrasives, Richardson’* concluded that hard phases are effective as discrete components of the structure when their size is the same or larger than the wear debris formed during abrasion, because they represent a powerful obstruction to the abrasive grit. When the size of the hard particles is less than the wear debris they are effective only as a means of strengthening the material as a whole. Heterogeneous materials are therefore much more sensitive to changes in the grit size and hardness of the abrasive, and may even change their “ranking” order under different wear conditions, Table I. Popovs4 has pointed out that the shape and coherency of hard phases contribute to their fracture characteristics under abrasive wear, and Larsen-Badse and Mathew suggested that regions of pearlite in a medium carbon steel can obstruct abrasive cutting as well as confine the ferrite grains. TABLE
I
WEAR RESISTANCE OF SOME HETEROGENEOUS TO EN 24 (Hv = 500 kg/mm’), (Richardson) Material
Hardness ( kglmm)
MATERIALS,
RELATIVE
Relative wear resistance Corundum 180 grit Hv=2180 kg/mm2
Flint Flint 180 grit 40 grit Hv= 1060 kgJmm’
29, c, 14% Cr die steel 3.6% C pearlitic white iron
708 902 676
2.67 3.10 2.33
17.5 48.6 6.44
2.81 5.53 2.51
3% C, 1.7% Cr, 3% Ni Ni-Hard, martensitic
710-870
2.3 1
8.87
2.37
3% C, 30% Cr, Fe Delcrome, cast
613
3.23
2.50,/, C, 33”/ Cr 9 13”/0 W 3 Co, Stelli;e, cast
632
2.60
192 40.0
3.56 3.61
ACKNOWLEDGEMENTS
Acknowledgement is made to the Agricultural Research Council and to the Director, National Institute of Agricultural Engineering for permission to publish, and to the Editor of Wear for permission to reproduce Figs. 2, 3, 5 and 6. Thanks are due to R. M. Douthwaite of the Department of Metallurgy, University of Newcastle-upon-Tyne for commenting on the work in draft. REFERENCES 1 M. M. Khrushchev and M. A. Babichev, Research on Wear of Metals, 1960, Ch.. 8; NEL Translation No. 893, National Engineering Laboratory, East Kilbride. 2 I. V. Kragelskii, Friction and Wear, Butterworths, London, 1965, pp. 2&26, 88-110.
16
M. A. MOORE
3 .I. Larsen-Badse. influence of grit size on groove formation during sliding abrasion, Wear. 1 I (1968) 213-222. 4 D. Graham and R. M. Baul. An investigation into the mode of metal removal in the grinding process. Weur, 19 (1972) 301-314. 5 R. L. Aghan and L. E. Samuels, Mechanism of abrasive polishing, Wear, 16 (1970) 293-301. 6 T. 0. Mulhearn and L. E. Samuels, The abrasion of metals: a model of the process, Weur, 5 (1962) 478498. 7 A. J. Sedriks and T. 0. Mulhearn. Mechanics of cutting and rubbing in simulated abrasive processes. Wear, 6 (1963) 457466. 8 A. J. Sedriks and T. 0. Mulhearn, The effect of work hardening on the mechanics of cutting in simulated abrasive processes, Weur, 7 (1964) 451459. 9 J. Goddard and H. Wilman, A theory of friction and wear during the abrasion of metals. Wear, 5 (1962) 114-135. 10 M. F. Stroud and H. Wilman. The proportion of groove volume removed as wear in abrasion of metals, hit. J. Appl. P~Js., 13 (1962) 173-178. 11 J. Larsen-Badse. Influence of grit diameter and specimen size on wear during sliding abrasion, Weur. 12 (1968) 35-53. 12 T. C. Buttery and J. F. Archard. Grinding and abrasive wear. Proc. Inst. Mech. Etrgrs., 185 (197071) 537.-551. 13 R. C. D. Richardson, The abrasive wear of metals and alloys,. Pro<. Inst. Mech. Enyrs., 182 (3A) (1967-68) 410-415. 14 M. M. Khrushchev and M. A. Babichev. Investigation of the resistance of metals to abrasion as influenced by the hardness of the abrasive, Frichm und Wear ir7 Machinery~ 11 (1956) 19-26; NEL Translation No. 830, National Engineering Laboratory> East Kilbride. 15 G. K. Nathan and W. J. D. Jones, Influence of the hardness of the abrasive on the abrasive wear of metals. Proc. lm. Mech. Enyrs.. I8 I (30) (196&67) 215%221. 16 R. C. D. Richardson. The wear of metals by hard abrasives, Wear, 10 (1967) 219-230. 17 R. C. D. Richardson. Unidirectional wear tests on soft abrasives and the wear resistance of steel surfaces of nearly uniform hardness. N.I.A.E. Norr No. 12. 1967, National Institute of Agricultural Engineering, Silsoe. 18 R. C. D. Richardson, The wear of metals by relatively soft abrasives, Weur. I I (1968) 245-275. 19 D. G. Attwood. Elastic moduli, Poisson’s ratio and yield stress of abrasives. derived from crystal constant and microhardness measurements. Proc. It7af. Med. Enyrs.. 182 (3A) (1967-68) 369-373. 20 E. J. Duwell and W. J. McDonald, Some factors that affect the resistance of abrasive grits to wear, Wear, 4 (1961) 372-383. 21 R. W. Johnson, The use of the scanning electron microscope to study the deterioration of abrasive papers. Wetrr, 12 (1968) 213-216. 22 G. W. Patterson and T. 0. Mulhearn, The fracture of idealized abrasive particles. Wear. 13 (1969) 175-182. 23 R. C. D. Richardson. The wear of metallic materials by soil&practical phenomena, /. Agric. Enqnq. Rex. 12 ( 1967) 22-39. 24 G. K. Nathan and W. J. D. Jones, The empirical relationship between abrasive wear and the applied conditions. Weur. 9 (1966) 300-309. 25 J. Larsen-Badse, Some effects of specimen size on abrasive wear, Weur, 19 (1972) 27-35. 26 E. Rabinowicz and A. Mutis. Effect of abrasive particle size on wear, Wear, 8 (1965) 381-390. 27 B. W. E. Avient. J. Goddard and H. Wilman. An experimental study of friction and wear during abrasion. Proc. Roy. Sot. (London), A258 (1960) 159-180. 28 R. W. Johnson, A study of the pick up of abrasive particles during abrasion of annealed aluminium on silicon carbide abrasive papers. Wear, 16 (1960) 351-358. 29 J. T. Burwell, Survey of possible wear mechanisms, Weur. 1 (1957-58) 119~141. 30 S. Hata and T. Muro. Mechanism of friction and wear of steel plate against solidified sandy soil, Mern. Fat. E17yn. Kyoto Univ.. 31 (1969) 456489. 31 J. P. Giltrow and S. J. Groszek, The effect of particle shape on the abrasiveness of lamellar solids, R.A.E. Teclulicol Report 69024, 1969, Royal Aircraft Establishment. 32 M. M. Khrushchev and M. A. Babichev, Research on Wear of‘Metul,s, 1960, Ch. 2; NEL Translation No. 889. National Engineering Laboratory, East Kilbride.
TWO-BODY
ABRASIVE
WEAR
17
33 R. C. D. Richardson, The maximum hardness of strained surfaces and the abrasive wear of metals and alloys, Wear. 10 (1967) 353-382. The wear of metal shares in agricultural soils, Ph.D. Thesis. London, 1969. 34 R. C. D. Richardson, investigation of frictional heating during abrasive wear, Wear, 17 (1971) 35 M. A. Moore, A preliminary 51-58. of abrasion, Ren. Met., 36 C. Ragon, L. Belon and H. Forestier, A method of studying the mechanism 66 (1969) 471476. 37 E. Rabinowicz, Friction and Wear qf Materials, Wiley, New York, 1965. and M. A. Babichev, Research on Wear of Metals, 1960, Ch. 9; Friction and Wear 38 M. M. Khrushchev in Machinery, ASME Translation, 12 (1958) 25-38. 39 T. L. Oberle. Properties influencing the wear of metals, J. Metals, 3 (1951) 438439. 40 R. T. Spurr and T. P. Newcombe, The friction and wear of various materials sliding against unlubricated surfaces of different types and degrees of roughness. Inst. Mech. Engrs. Proc. Conf. Luhrication and Wear, (1957) 269-275. 41 M. M. Khrushchev and M. A. Babichev, Correspondence between the relative abrasive wear resistance of metals, alloys and some minerals. and their moduli of elasticity. Friction and Wear in Machinery, ASME Translation, 17 (1962) I-8. 42 M. M. Khrushchev and M. A. Babichev. Abrasive wear resistance and the modulus of elasticity of heat treated steels. Friction and Wear in Machinery, ASME Translation. 17 (1962) 9-18. 43 M. M. Khrushchev and M. A. Babichev, Investigation of the wear of metals and alloys during friction against an abrasive surface, Friction and Wear in Machinery, 11 (1956) 5-18; NEL Translation No. 83 1, National Engineering Laboratory, East Kilbride. Resistance of metals to wear by abrasion as related to hardness, Inst. Mech. 44 M. M. Khrushchev, Engrs. Proc. Conf Lubrication and Wear, (1957) 655459. 45 A. Selwood, The abrasion of materials by Carborundum paper, Wear, 4 (1961) 311-318. 46 C. Rubenstein, A note on the relation between the abrasion resistance and the hardness of metals, Wear, 8 (1965) 70-72. 47 J. Larsen-Badse. The abrasion resistance of some hardened and tempered carbon steels, Trans. AIME, 236 (1966) 1461-1466. 48 J. Larsen-Badse and K. G. Mathew. Influence of structure on the abrasion resistance of 1040 steel, Wear, 14 (1969) 199-206. 49 M. A. Moore, R. C. D. Richardson and D. G. Attwood, The limiting strength of worn metal surfaces. Met. Trans., 3 (1972) 2485-2491. 50 P. J. Alison and H. Wilman, The different behaviour of hexagonal and cubic metals in their friction, wear and work hardening during abrasion, Brit. J. Appl. Phys., 15 (1964) 281-289. 51 J. Larsen-Badse, Abrasion resistance of some SAP-type alloys at room temperature, Wear, 12 (1968) 357-368. 52 D. S. Lin and H. Wilman, The relation of the friction and wear in abrasion of Al-4 wt.?; Cu alloy, to the estimated precipitate particle size and separation during age hardening, Wear. 14 (1969) 337346. 53 M. M. Khrushchev and M. A. Babichev, Resistance to abrasive wear of structurally heterogeneous materials, Friction and Wear in Machinery, 12 (1958) 15-26; NEL Translation No. 828. National Engineering Laboratory, East Kilbride. 54 V. S. Popov, Fracture of carbides in the abrasive wear of alloy steels, Russ. Engng. J. 49 (1969) 78-81.
1 - Printed
A REVIEW OF TWO-BODY
in The Netherlands
ABRASIVE WEAR
M. A. MOORE
of Agricultural Engineering,
National Institute
(Received
Silsoe, Beds. (Gt. Britain)
July 19, 1973)
SUMMARY
The present state of the art of two-body abrasive wear has been reviewed, emphasising the wide range of variables which can influence the abrasive wear of materials. The basic mechanism of abrasive wear, properties of the abrasive, variables in the wear environment and the relation between abrasive wear, mechanical properties and metallurgical structure have been considered.
1. INTRObUCTION
In the engineering industry abrasive wear is probably the most common cause of mechanical damage. It manifests itself in several forms: in the wear of equipment which engages an abrasive medium, in the wear of seals or machine parts between which abrasive particles can penetrate and wear by abrasives entrained in fluids. The first case in which abrasive particles simply rub against a surface is referred to as two-body abrasive wear. The second and last cases in which the abrasive can become trapped between two sliding surfaces are referred to as three-body abrasive wear. The exact conditions of abrasive wear and the wear environment may vary widely, and in considering any particular wear system attention must be paid to several variables. The variables affecting wear in two-body situations are discussed in the following review of the literature. 2. THE BASIC MECHANISM
OF ABRASIVE
WEAR
The abrasive wear referred to in this review is defined as the removal of solid material from a surface by the unidirectional sliding action of discrete particles of another material. The surfaces are maintained in a constant orientation relative to one another for the period of contact. The basic mechanism of abrasive wear has been the subject of many investigations. Khrushchev and Babichev’ identified two processes taking place when abrasive grains made contact with the wearing surface: (1) the formation of plastically impressed grooves which did not involve metal removal, and (2) the separation of metal particles in the form of microchips.
2
M. A. MOORE
Other investigations 24 have shown that some contacting abrasive grains merely make elastic contact with the surface, and Aghan and Samuels’ found that the extruded fins at the edges of grooves produced by rubbing can sometimes become detached forming secondary chips, although they concluded that primary chips dominate in terms of metal loss. In a comprehensive study Mulhearn and Samuels6 determined that although the number of contact points per unit area varied with the size of the abrasive the proportion of contacts producing a chip was approximately constant and as low as 12%. More recently, Larsen-Badse3 has estimated that 50 - 60% of the contacting points produce chips. Mulhearn and Samuels6 argued that chips are only produced when the contacting face of an abrasive grain makes an angle greater than or equal to the “critical attack angle” with the wearing surface. Some later work7.8 showed that the orientation as well as the inclination of the cutting face are critical in determining whether or not a chip is cut, and that the critical attack angle varies with the material under wear and is determined by the coefficient of friction between the contacting surfaces. Kragelskii’, Khrushchev and Babic,hev’, Aghan and Samuels’, and Graham and Bau14 have also found that chip cutting and rubbing depend upon the shape of the indenting particle. In particular spherical indenters have been observed to show a change over from rubbing to at least partial chip formation when the indentation strain (defined as the depth of indentation divided by the diameter of the indenter) exceeds a certain value. Goddard and Wilman’ have calculated that the coefficient of friction depends upon the particle shape, which, in the light of Sedriks and Mulhearn’s findings’, might account for the observed particle shape dependence. The combined effect of the chip cutting and rubbing processes was shown by Stroud and Wilman” who estimated that less than 409; of the total groove volume is removed as wear debris, the remainder being displaced by plastic flow. They concluded that much of the energy used in abrasion is expended in plastic deformation, while Larsen-Badse I1 has estimated that 45% of the minimum work to remove material is plastic shear to form chips and ridges, and Buttery and Archard” have associated the efficiency of abrasion with the pile-up of material, i.e. plastic flow. In view of this the mechanism of abrasive wear of brittle solids may be modified by the onset of fracture13. 3. THE EFFECT
OF PROPERTIES
OF THE ABRASIVE
3.1. Abrasive type and relative hardness Several investigations 13P18 have shown that relative wear resistance is not independent of the hardness of the abrasive. Khrushchev and Babichev14 concluded that wear was independent of abrasive hardness when this was very much greater than the hardness of the wearing material, but as the hardness of the wearing material approaches that of the abrasive, wear decreases and if the hardness of the abrasive is less than that of the wearing material wear decreases rapidly as the difference increases. However, Nathan and Jones l5 found that even with relatively very hard abrasives the volumetric wear still depends on the abrasive hardness. Richardson’3.‘6-‘8 has defined hard abrasives as those whose hardness exceeds
TWO-BODY
ABRASIVE
3
WEAR
500
200
loo
“/a
=O
20
IO
5
2
I
0:4
0.6
048 %a
“O
Is2
)A
Fig. 1. The increase in wear resistance of martensitic steels on glass abrasive, showing the effect of relative abrasive hardness; b is the relative wear resistance on glass, fi is the relative wear resistance on 180 grit Sic, H is the hardness of the test material, Ha is the hardness of the abrasive. (Richardson)
that of the worn surface material, and soft abrasives as those whose hardness is less than that of the worn surface. Richardson showed that both hard and soft abrasive wear are very sensitive to the characteristics of the individual abrasives, the wearing action of soft abrasives is similar to that of hard abrasives but modified by increasing damage to the grit, and that true scratching ceases when the yield stresses of the wearing material and the abrasive are approximately equal. Figure 1 shows the effect of the relative abrasive hardness on wear resistance. The different strength properties of abrasives” and their mode of deterioration may account for the importance of their relative hardness. Duwell and McDonald” identified two abrasive deterioration mechanisms: attritious wear caused by chemical degradation of the surface, and fragmentation due to relief of internal stresses. They found that oxide structures wear more by fragmentation and carbides and borides wear more by attrition, and they suggest that this is because of the chemical inertness of the oxide structures. Johnson” and Patterson and Mulhearn” have found that abrasive fracture usually commences at favourably situated cracks which are inherent from manufacture. Grit failure may produce
M. A. MOORE
Mean
o
ALUMINIUM
A
BRASS
0
COPPER
0
BRONZE
A
SWEDISH IRON
diameter
Fig. 2. The effect of abrasive
of
abrasive
particles
01)
grit size on volume wear. (Nathan
and Jones).
grit fragments which can act as fresh abrasive, but a cutting point is only regenerated if the original cutting angle of the grit is between the critical angle and 90”. The overall tendency is, therefore, a reduction of cutting points but probably at a slower rate than by attritious wear. Richardson’s field results23 show that particle fracture in the weaker ironstones may render them more effective in cutting than the stronger flint. 3.2. Abrasive yrir size It has been established3.“.‘8.24 t h at volume wear increases steeply with grit size to some critical size and then increases at a reduced rate with increasing grit size. (Fig. 2). With decreasing wear resistance the gradient of both linear portions of the wear/grit size curves increases and so does the critical grit size. Mulhearn and Samuels’ account for their results in terms of the geometry of abrasive particles which they found to vary with grit size. Larsen-Badsej.” has suggested that the proportion of the load carried by elastic contacts varies with grit size and. more recently2’. that specimen size may account for the critical size effect since the length of contact of an abrasive particle has a different effect on the deterioration has also accounted for his results by the different of different grit sizes. Richardson”
TWO-BODY
ABRASIVE
5
WEAR
modes of deterioration of the different grit sizes. Rabinowicz and Mutisz6 concluded that the critical size effect is due to the interference between adhesive and abrasive particles which increases as the grit size decreases. Avient, Goddard and Wilman” have suggested that abrasive particles which become embedded in the wearing surface might cause an increase in wear resistance on the smaller grit sizes, and Johnson2’ has confirmed that the pick up of abrasive increases rapidly as the grit size decreases. 3.3 Abrasice shape Mention has already been made (Section 2) of the effect that the abrasive particle shape has on chip cutting and rubbing. Burwell29 cites the results of R.D. Haworth who found that angular soft particles produced more wear than rounded hard particles. Goddard and Wilman’ have calculated the friction coefficient in abrasion for various idealised particle shapesand since the friction coefficient is related to plastic deformation in wear, particle shape is expected to contribute to the ratio of material removed to material which flows into ridges, etc. Hata and Muro3’ have found experimentally using a quadrahedral diamond tool that resistance to abrasion increases as the cone angle decreases and if a facet leads as opposed to an edge. In a study of abrasion by loose graphite and molybdenum disulphide particles Giltrow and Groszek 3’ found that the rate of abrasion increases as the particles become less plate like and suggest that this is because plates are more likely to lie flat at the sliding interface. 4. THE EFFECT
OF VARIABLES
OTHER
THAN
THE ABRASIVE
4.1. Speed
Khrushchev and Babichev32 and Nathan and Jones24 have established that the volume wear increases slightly as sliding speed increases in the range O-2.5 m/s, (Fig. 3). The increase is more marked for larger grit sizes over the lower end of the range and. also, for more wear resistant materials. Khrushchev and Babichev attributed a 13”” increase to a change in the properties of the abrasive cloth, while Nathan and Jones have suggested that frictional heating may account for the increase, especially as the rate of heating in abrasion is greater for the larger grit sizes. Both Khrushchev and Babichev and Richardsonj3 have stated that changes arising from strain rate and frictional heating are expected with increasing sliding speeds, but discarded frictional heating as a serious problem since no evidence of surface heating could be found. 4.2. Load Several investigations24.2’.30.32 have shown that volumetric wear is directly proportional to the nominal load up to a critical load which is determined by the onset of massive deformation of the specimen or instability of the abrasive surface. Nathan and Jones24 found that deviation from linearity occurred at lower loads for the smaller grit sizes. Larsen-Badse3. l l suggests that failure of the abrasive commences when the applied load on a grain reaches a value corresponding to a groove width of about 0.17 of the grit diameter, since groove widths are fairly constant with load while the number of contact points increases roughly linearly
6
M. A. MOORE
35
00
3oolJ 0
7o)r l
ALLJMNIUM
0
l
BRASS
D
A
SWEDISH
c
IRON 3
”
5
0
0.50
1.00
2.00
1.50
Velocity
Fig. 3. The effect of velocity
IZ.O-
of abrasion
@ Specimen diameter 0.100 in I! x 11 0.180 in
0 ll.O_ $! lO.O,o -: 9.0Lu i
2.50
(m/set) on volume wear. (Nathan and Jones)
Material 2% C . I4 yo Cr Dicsteel Hv = 902 Abrasive
40 grit flint paper
8.0-
: 7.00 ’
6.0I
I
IO0 Contact
so Fig. 4. The effect of specimen
diameter
I IS0 stress, g/mm2
and nominal
contact
I 200
1 250
stress on relative wear resistance.
(Richardson)
TWO-BODY
ABRASIVE
7
WEAR
with load. More recentlyz5 he concluded that if grit wear can be eliminated the volumetric wear is directly proportional to the applied load. Richardsoni found that, although the volumetric wear increased linearly with load, the relationship was such that the wear/unit load decreases as the load increases, and that the sensitivity of relative wear resistance to increase with decreasing contact stress varies for different materials’s, (Fig. 4). 4.3. Specimen size Specimen size has been found to affect abrasive wear in various ways. In abrasive wear testing it is the usual practice to choose a specimen size such that deterioration of an abrasive grain in contact with the specimen is minimal. Avient, Goddard and Wilmanz7 have pointed out that it is important to use a short enough specimen such that the amount of metal worn off by an indenting grain is not enough to fill interstices between it and adjacent particles, which would reduce the effectiveness of the abrasive. Richardson’ 6 realised that the specimen diameter/groove depth ratio might affect relative wear resistance since losses at the edge of the specimen due to plastic flow would be large compared with losses from the surface if this
7
6
180
w
A_--'-A---A----A_
320
Lo Ii1
,__o----o--0--600
0
2
4 Specimen
Fig. 5. The effect of specimen
diameter
6 Diameter,
8
IO mm
on wear rate. (Larsen-Badse)
12
8
M. A. MOORE
ratio was small. He also found’* that if deterioration of the grit is important variation in specimen diameter has a large effect on the relative wear resistance, (Fig. 4). In studying the influence of grit diameter Larsen-Badse” realised that the critical grit diameter was proportional to specimen size. In a further study25 he concluded that the variation of wear rate with load and grit size are not linear because of the effect of specimen size, (Fig. 5). Larsen-Badse has suggested that a peak occurs in wear rate versus specimen length due to conditioning and deterioration of the abrasive, and that the critical specimen length corresponding to maximum wear depends on the applied conditions. Therefore, if the grit life is much longer or shorter than specimen contact, the effect on wear data will be small, but it may be appreciable when the two values are about the same. 4.4. Length of wear path Nathan and Jones24 have found a linear relationship between volumetric wear on hard abrasive and the length of the abrasive path, but it is likely that their experiments were not sensitive to the non-linear portion of the curve which occurs over a short sliding distance. This non-linear portion corresponds to the surface coming to equilibrium and Richardson I7 has shown that the shape of the curve depends on the relative hardness of the wearing material and the abrasive, (Fig. 6) because with soft abrasives the wearing material takes much longer to strain harden to equilibrium conditions. Avient, Goddard and Wilman 27 found that the coefficient of friction in abrasion rises to an equilibrium value after a sliding distance which is larger the harder the metal, and they attributed this to pick up of the abrasive. Johnson2* has observed an increase of abrasive pick up with track length. Wear-Distance
Run on 30 Material
Grit
Glass
0.74 Ok C steel
Hv 503 J*o-
Run I X Run 2
Paper
Hv 650
0
0 Run I OJ-
J.
x
Run2
X 0.6 -
,-2.0-
a’
0.5-
/
a I
Y; B 2
0.4 -
_s 3 1.0.
/ Initial gradient = 0.0069
mqhm
t
0.3 0.2-
/
o
lnitlol qradient
’
I
-s’ CI
$ 0
100
200
300
400
500
600
0 Track
Fig. 6. The effect of relative
material
hardness
/ ;
* 0.0026
mq/cm
1 100200300400500600
, cm on the wear-distance
run curve. (Richardson)
TWO-BODY
ABRASIVE
9
WEAR
5. FRICTIONAL
HEATING
5.1. Frictional
heating
AND HUMIDITY
Khrushchev and Babichev3’ and Richardson34 have concluded that during slow speed abrasive wear changes due to frictional heating have little affect on the wear rate. Nathan and Jones24 attributed the increase in wear with increasing sliding speed to frictional heating. Richardson33 does, however, point out that temperature rises may occur on a scale commensurate with the size of asperities on the grit particle surface and affect the friction at the gritchip interface. Moore3’ has found temperature rises of 32%900°C in the grit&chip contact zone and concludes that the physical, chemical and mechanical properties of the contact zone may be modified although the overall effect on abrasive wear is likely to be. small. In the grinding of metals, where relative speeds are much higher, Ragon et a1.36 have concluded that temperature rises in the order of 1500°C occur. 5.2. Humidity Rabinowicz3’ states that high humidity increases the wear rate in abrasion by about 15% and that the action of water vapour is similar to other lubricants which increase the effectiveness of abrasion. In field and laboratory tests, Khrushchev and Babichev3* and Richardson34 have found that the effect of moisture on the wear rate and relative wear resistance is small and only becomes of importance at low wear intensities. 6. ABRASIVE
WEAR IN RELATION
TO MECHANICAL
PROPERTIES
6.1. Elastic modulus and elastic limit of strain
In 1951 Oberle39 suggested that a measure of wear resistance is the amount of elastic deformation that the wearing surface can sustain because a material can deform elastically to accommodate the abrasive grit with no plastic deformation or metal removal. Oberle equates a high wear resistance with a high limit of elastic strain which was given approximately by the ratio Brine11 hardness/Young’s modulus. Khrushchev and Babichev’, however, found Oberle’s model unsuitable for assessing abrasive wear resistance. Spurr and Newcomb 4o have suggested that at equilibrium an abrasive particle displaces metal ahead of it plastically but that material behind recovers elastically so that the volume wear depends on the elastic recovery of the surface and is inversely proportional to the elastic modulus. Khrushchev and Babichev41 have also found that wear resistance of pure metals can be related to the modulus of elasticity, (Fig. 7) but that the relationship does not hold for heat treated steels42. They conclude that the relationship for the pure metals cannot be due to specific features of the abrasive wear process. 6.2. Bulk hardness The relationship between wear resistance and bulk hardness is often used to present abrasive wear data 16.4345, (Fig. 8). Khrushchev and Babichev43*44 showed that the wear resistance of pure metals is directly proportional to their hardness and Rubenstein46, in a simplified analysis of metal removal during wear, has shown that
10
M. A. MOORE
Fig. 7. The dependence of relative wear resistance, (Khrushchev and Babichev)
R, of some pure metals on the modulus
of elasticity
E.
relative wear resistance is related to hardness by the reciprocal of the hardness of the standard material. Rubenstein concludes that a better presentation of the data would be given by plotting the wear resistance against the ratio hardness of the test material/hardness of the standard material. Khrushchev and Babichev43 also found linear relationships between wear resistance and hardness for heat treated steels and some metal carbides. For the steels the data fitted an equation of the type: p=/?o+C’(HV+Hu,)
6.2.1 where fl is the relative wear resistance of the steel, B,, is the.relative wear resistance in the annealed condition, C is a constant, Hu is the Vickers diamond pyramid bulk hardness of the steel, and Ha, is the hardness of the steel in the annealed condition. Khrushchev and Babichev43 and Richardson l6 found that the constant C varied systematically with carbon and alloy content of the steel, and that structurally heterogeneous materials in general show a lower wear resistance than expected from their hardness compared with pure metals. Richardson also showed that impurities and alloying elements in the pure metals do not produce increases in wear resistance proportionate to increases in hardness. In a study of wear resistance of metals, ceramics and plastics, Selwood 45 found that hardness does not describe the wear resistance of very ductile, extendable or brittle materials.
TWO-BODY
ABRASIVE
11
WEAR
60
Ni
IO
cu
i Zn
Ii
i
Pb
loo
200
Fig. 8. The dependence hardness. (Khrushchev
100 400 Hordnrss,
500
600
IiD-Kg/mm’
700
of relative wear resistance, and Babichev)
800
900
E, of some pure metals and steels on the bulk material
6.3. Surface hardness
In the absence of a single relationship between bulk hardness and wear resistance and from results showing that the wear resistance was little affected by the initial degree of cold-work, Khrushchev and Babichev’ concluded that a high degree of strain hardening occurs at the surface of worn metals, and that wear resistance depends on the strength of the material in its “maximum work-hardened state”. Avient, Goddard and Wilman” argued that the abrasion process is to a considerable extent dependent upon the indentation of the abrasive grains into the metal surface, and they showed that a relationship existed between the surface hardness after wear and wear resistance. Larsen-Badse47T48 has pointed out that in a group of materials for which the rate of strain hardening is a function of the bulk hardness, the hardness of the abraded surface will be a function of the bulk hardness, and so abrasive wear resistance may be proportional to both of these properties. Richardson33 studied the surface hardness of materials strained by shot peening, trepanning and abrasive wear. He found that the whole of the effective surface does
12
M. A. MOORE
SO(160
pit
corundum cloth)
2.5-
QZ4 0.3790C lowalloysteel mort*nsitr
k
Al .
0
/)Al Alloy
“‘I 200 JO0 400 500 ” 600 700 ’ 800 ” 903 1000 i II00 f 1200 ” I300 I400 ” IS00 100
M&mumhordnrss, Xu kg/mm* Fig. 9. The relationship between relative weaP resistance and surface (maximum) hardness, Hu, after wear. (Richardson)
not strain harden to the same level, although limited regions reach a “maximum hardness”, Hu. Abrasion resistance was correlated with maximum hardness values (Fig. 9) and Richardson suggests that within limited classes of materials, the relative wear resistance may be taken as proportional to the maximum hardness. In a recent analysis of Richardson’s and further results, Moore et a1.49 have shown that the limiting strength attained at worn surfaces can be understood in terms of metallurgical structure and suggest that the limiting strength is a measure of the maximum dislocation density that can be stored by the material, under the stress system developed in abrasive wear, before fracture occurs. Although surface hardness gives a much more systematic correlation with abrasive wear resistance than bulk hardness, it seems that abrasive wear has to be defined more in terms of the flow and fracture properties of the materials and their metallurgical structures. 6.4. Flow and fracture properties In a theoretical analysis of the stress/strain system developed in the wear process Larsen-Badse 47.48 has used a power law to describe the stress/strain curve of pure metals: a=A&”
6.4.1
where c is the flow stress, A is a constant, E is true strain, and $2is the strain hardening
TWO-BODY
ABRASIVE
13
WEAR
exponent, and shows that abrasion resistance is proportional to either bulk or surface hardness and exponential (n). Therefore, abrasion resistance plotted against surface or bulk hardness should give a straight line through the origin with some scatter due to the spread in values of the strain hardening exponent, n. For instance, Alison and Wilman5 Oattributed the different wear properties of hexagonal and cubic metals to the difference in their slip processes which causes their strain hardening exponents to be different. Larson-Badse 47 also points out that variations in n will cause less scatter in wear resistance uersus surface hardness plots, and Richardson33 has found the relative wear resistance of hexagonal metals about the same as cubic metals in relation to their surface hardness. For interstitial and substitutional solid solutions, precipitation and dispersion hardened materials a single relationship between wear resistance and bulk or surface hardness does not exist because of the wide variation in their strain hardening exponents. Moore et al. 49 have shown that the limiting strength of worn surfaces can be understood in terms of metallurgical structure and the way it influences the strain hardening exponent. In studies of the abrasion of hardened and tempered steels, Larsen-Badse4’ et aL4* considered various models for dispersion hardening and concluded that finely dispersed hard particles influence the flow stress and increase abrasion resistance according to a Hall-Petch relation (i.e. abrasion resistance inversely proportional to the square root of cementite particle spacing), (Fig. 10). Larsen-Badse 51 has also investigated sintered-aluminium-powder-type alloys and found that abrasion resistance is directly related to the fineness of the dislocation SAE
1040
STEEL
SPHEROIDITE ‘i 8 z
-6.5
”
c
BULK
HARDNESS
5.0
I
too
’ 0.6
I
I
I 0.7 I/&,
0.8
_&
0.9
[MICRONS]
Fig. 10. The dependence of the wear resistance (Larsen-Badse and Mathew)
of spheroidal
cementite
steel on the particle
spacing.
14
M. A. MOORE
substructure formed in the abraded surface, and inversely proportional to the square root of the particle spacing, and that the flow stress responsible for abrasion resistance is governed by the Ansell-Lenel mechanism (i.e. flow occurs when the shear stress on particles due to dislocation pile-ups deforms or fractures the particles). Lin and Wilman 52 have related wear resistance to the structural changes occurring during age hardening an aluminium
WEAR
IN RELATION
TO METALLURGICAL
STRUCTURE
Metallurgical structure, such as grain size, dispersed phases, lamellae etc. have a considerable influence on the mechanical properties of a material and, therefore, since abrasive wear involves plastic flow, must also influence abrasive wear resistance. This has been well established in several investigations’3.‘6,18.33. 34,43.4749.51,s2, and discussed in the previous section. Of equal importance, however, is the effect that relatively large structural components have in obstructing the path of abrasive grit during wear. Khrushchev and Babichevs3 studied the wear of structurally heterogeneous materials and concluded that their wear resistance was equal to the sum of the products of the wear resistances of the individual components and their corresponding volumetric quantities, e.g. Fig. 11 shows wear resistance of some irons and steels as a function of cementite content. Richardsonl‘j states that an additivity law of this sort may need to be modified to take account of scale effects. He found that the intermediate hard abrasives, such as corundum, are not necessarily hard
s,:-_
N
2 ;.;:; p to-
s 2
I%C
IO
20
2%c 30
3%C 40
50
Cementite %
Fig. 11. The dependence of relative wear _resistance, E, and bulk hardness, of some steels and irons. (Khrushchev and Babichev)
H, on the cementite
content
TWO-BODY
ABRASIVE
15
WEAR
with respect to all carbides found in alloy steels and cast irons, and that, even if the abrasive is very much harder, hard phases become less effective as the scratch size increases. In studies on soft abrasives, Richardson’* concluded that hard phases are effective as discrete components of the structure when their size is the same or larger than the wear debris formed during abrasion, because they represent a powerful obstruction to the abrasive grit. When the size of the hard particles is less than the wear debris they are effective only as a means of strengthening the material as a whole. Heterogeneous materials are therefore much more sensitive to changes in the grit size and hardness of the abrasive, and may even change their “ranking” order under different wear conditions, Table I. Popovs4 has pointed out that the shape and coherency of hard phases contribute to their fracture characteristics under abrasive wear, and Larsen-Badse and Mathew suggested that regions of pearlite in a medium carbon steel can obstruct abrasive cutting as well as confine the ferrite grains. TABLE
I
WEAR RESISTANCE OF SOME HETEROGENEOUS TO EN 24 (Hv = 500 kg/mm’), (Richardson) Material
Hardness ( kglmm)
MATERIALS,
RELATIVE
Relative wear resistance Corundum 180 grit Hv=2180 kg/mm2
Flint Flint 180 grit 40 grit Hv= 1060 kgJmm’
29, c, 14% Cr die steel 3.6% C pearlitic white iron
708 902 676
2.67 3.10 2.33
17.5 48.6 6.44
2.81 5.53 2.51
3% C, 1.7% Cr, 3% Ni Ni-Hard, martensitic
710-870
2.3 1
8.87
2.37
3% C, 30% Cr, Fe Delcrome, cast
613
3.23
2.50,/, C, 33”/ Cr 9 13”/0 W 3 Co, Stelli;e, cast
632
2.60
192 40.0
3.56 3.61
ACKNOWLEDGEMENTS
Acknowledgement is made to the Agricultural Research Council and to the Director, National Institute of Agricultural Engineering for permission to publish, and to the Editor of Wear for permission to reproduce Figs. 2, 3, 5 and 6. Thanks are due to R. M. Douthwaite of the Department of Metallurgy, University of Newcastle-upon-Tyne for commenting on the work in draft. REFERENCES 1 M. M. Khrushchev and M. A. Babichev, Research on Wear of Metals, 1960, Ch.. 8; NEL Translation No. 893, National Engineering Laboratory, East Kilbride. 2 I. V. Kragelskii, Friction and Wear, Butterworths, London, 1965, pp. 2&26, 88-110.
16
M. A. MOORE
3 .I. Larsen-Badse. influence of grit size on groove formation during sliding abrasion, Wear. 1 I (1968) 213-222. 4 D. Graham and R. M. Baul. An investigation into the mode of metal removal in the grinding process. Weur, 19 (1972) 301-314. 5 R. L. Aghan and L. E. Samuels, Mechanism of abrasive polishing, Wear, 16 (1970) 293-301. 6 T. 0. Mulhearn and L. E. Samuels, The abrasion of metals: a model of the process, Weur, 5 (1962) 478498. 7 A. J. Sedriks and T. 0. Mulhearn. Mechanics of cutting and rubbing in simulated abrasive processes. Wear, 6 (1963) 457466. 8 A. J. Sedriks and T. 0. Mulhearn, The effect of work hardening on the mechanics of cutting in simulated abrasive processes, Weur, 7 (1964) 451459. 9 J. Goddard and H. Wilman, A theory of friction and wear during the abrasion of metals. Wear, 5 (1962) 114-135. 10 M. F. Stroud and H. Wilman. The proportion of groove volume removed as wear in abrasion of metals, hit. J. Appl. P~Js., 13 (1962) 173-178. 11 J. Larsen-Badse. Influence of grit diameter and specimen size on wear during sliding abrasion, Weur. 12 (1968) 35-53. 12 T. C. Buttery and J. F. Archard. Grinding and abrasive wear. Proc. Inst. Mech. Etrgrs., 185 (197071) 537.-551. 13 R. C. D. Richardson, The abrasive wear of metals and alloys,. Pro<. Inst. Mech. Enyrs., 182 (3A) (1967-68) 410-415. 14 M. M. Khrushchev and M. A. Babichev. Investigation of the resistance of metals to abrasion as influenced by the hardness of the abrasive, Frichm und Wear ir7 Machinery~ 11 (1956) 19-26; NEL Translation No. 830, National Engineering Laboratory> East Kilbride. 15 G. K. Nathan and W. J. D. Jones, Influence of the hardness of the abrasive on the abrasive wear of metals. Proc. lm. Mech. Enyrs.. I8 I (30) (196&67) 215%221. 16 R. C. D. Richardson. The wear of metals by hard abrasives, Wear, 10 (1967) 219-230. 17 R. C. D. Richardson. Unidirectional wear tests on soft abrasives and the wear resistance of steel surfaces of nearly uniform hardness. N.I.A.E. Norr No. 12. 1967, National Institute of Agricultural Engineering, Silsoe. 18 R. C. D. Richardson, The wear of metals by relatively soft abrasives, Weur. I I (1968) 245-275. 19 D. G. Attwood. Elastic moduli, Poisson’s ratio and yield stress of abrasives. derived from crystal constant and microhardness measurements. Proc. It7af. Med. Enyrs.. 182 (3A) (1967-68) 369-373. 20 E. J. Duwell and W. J. McDonald, Some factors that affect the resistance of abrasive grits to wear, Wear, 4 (1961) 372-383. 21 R. W. Johnson, The use of the scanning electron microscope to study the deterioration of abrasive papers. Wetrr, 12 (1968) 213-216. 22 G. W. Patterson and T. 0. Mulhearn, The fracture of idealized abrasive particles. Wear. 13 (1969) 175-182. 23 R. C. D. Richardson. The wear of metallic materials by soil&practical phenomena, /. Agric. Enqnq. Rex. 12 ( 1967) 22-39. 24 G. K. Nathan and W. J. D. Jones, The empirical relationship between abrasive wear and the applied conditions. Weur. 9 (1966) 300-309. 25 J. Larsen-Badse, Some effects of specimen size on abrasive wear, Weur, 19 (1972) 27-35. 26 E. Rabinowicz and A. Mutis. Effect of abrasive particle size on wear, Wear, 8 (1965) 381-390. 27 B. W. E. Avient. J. Goddard and H. Wilman. An experimental study of friction and wear during abrasion. Proc. Roy. Sot. (London), A258 (1960) 159-180. 28 R. W. Johnson, A study of the pick up of abrasive particles during abrasion of annealed aluminium on silicon carbide abrasive papers. Wear, 16 (1960) 351-358. 29 J. T. Burwell, Survey of possible wear mechanisms, Weur. 1 (1957-58) 119~141. 30 S. Hata and T. Muro. Mechanism of friction and wear of steel plate against solidified sandy soil, Mern. Fat. E17yn. Kyoto Univ.. 31 (1969) 456489. 31 J. P. Giltrow and S. J. Groszek, The effect of particle shape on the abrasiveness of lamellar solids, R.A.E. Teclulicol Report 69024, 1969, Royal Aircraft Establishment. 32 M. M. Khrushchev and M. A. Babichev, Research on Wear of‘Metul,s, 1960, Ch. 2; NEL Translation No. 889. National Engineering Laboratory, East Kilbride.
TWO-BODY
ABRASIVE
WEAR
17
33 R. C. D. Richardson, The maximum hardness of strained surfaces and the abrasive wear of metals and alloys, Wear. 10 (1967) 353-382. The wear of metal shares in agricultural soils, Ph.D. Thesis. London, 1969. 34 R. C. D. Richardson, investigation of frictional heating during abrasive wear, Wear, 17 (1971) 35 M. A. Moore, A preliminary 51-58. of abrasion, Ren. Met., 36 C. Ragon, L. Belon and H. Forestier, A method of studying the mechanism 66 (1969) 471476. 37 E. Rabinowicz, Friction and Wear qf Materials, Wiley, New York, 1965. and M. A. Babichev, Research on Wear of Metals, 1960, Ch. 9; Friction and Wear 38 M. M. Khrushchev in Machinery, ASME Translation, 12 (1958) 25-38. 39 T. L. Oberle. Properties influencing the wear of metals, J. Metals, 3 (1951) 438439. 40 R. T. Spurr and T. P. Newcombe, The friction and wear of various materials sliding against unlubricated surfaces of different types and degrees of roughness. Inst. Mech. Engrs. Proc. Conf. Luhrication and Wear, (1957) 269-275. 41 M. M. Khrushchev and M. A. Babichev, Correspondence between the relative abrasive wear resistance of metals, alloys and some minerals. and their moduli of elasticity. Friction and Wear in Machinery, ASME Translation, 17 (1962) I-8. 42 M. M. Khrushchev and M. A. Babichev. Abrasive wear resistance and the modulus of elasticity of heat treated steels. Friction and Wear in Machinery, ASME Translation. 17 (1962) 9-18. 43 M. M. Khrushchev and M. A. Babichev, Investigation of the wear of metals and alloys during friction against an abrasive surface, Friction and Wear in Machinery, 11 (1956) 5-18; NEL Translation No. 83 1, National Engineering Laboratory, East Kilbride. Resistance of metals to wear by abrasion as related to hardness, Inst. Mech. 44 M. M. Khrushchev, Engrs. Proc. Conf Lubrication and Wear, (1957) 655459. 45 A. Selwood, The abrasion of materials by Carborundum paper, Wear, 4 (1961) 311-318. 46 C. Rubenstein, A note on the relation between the abrasion resistance and the hardness of metals, Wear, 8 (1965) 70-72. 47 J. Larsen-Badse. The abrasion resistance of some hardened and tempered carbon steels, Trans. AIME, 236 (1966) 1461-1466. 48 J. Larsen-Badse and K. G. Mathew. Influence of structure on the abrasion resistance of 1040 steel, Wear, 14 (1969) 199-206. 49 M. A. Moore, R. C. D. Richardson and D. G. Attwood, The limiting strength of worn metal surfaces. Met. Trans., 3 (1972) 2485-2491. 50 P. J. Alison and H. Wilman, The different behaviour of hexagonal and cubic metals in their friction, wear and work hardening during abrasion, Brit. J. Appl. Phys., 15 (1964) 281-289. 51 J. Larsen-Badse, Abrasion resistance of some SAP-type alloys at room temperature, Wear, 12 (1968) 357-368. 52 D. S. Lin and H. Wilman, The relation of the friction and wear in abrasion of Al-4 wt.?; Cu alloy, to the estimated precipitate particle size and separation during age hardening, Wear. 14 (1969) 337346. 53 M. M. Khrushchev and M. A. Babichev, Resistance to abrasive wear of structurally heterogeneous materials, Friction and Wear in Machinery, 12 (1958) 15-26; NEL Translation No. 828. National Engineering Laboratory, East Kilbride. 54 V. S. Popov, Fracture of carbides in the abrasive wear of alloy steels, Russ. Engng. J. 49 (1969) 78-81.