1967 Richardson THE WEAR OF METALS BY RELATIVELY SOFT ABRASIVES

Prévia do material em texto

Wear - Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands 245 
THE WEAR OF METALS BY RELATIVELY SOFT ABRASIVES 
R. C. D. RICHARDSON 
National Institute of Agricultural Engineering, Silsoe, Bedford (Gt. Britain) 
(Received April 14,Ig67; accepted May 5,1g67) 
SUMMARY 
In previous work, values were established for the relative wear resistance t3 
of a range of metals and alloys on hard abrasive, and the maximum hardness Hu 
of the surface material when strained plastically. The wear of these materials is now 
considered on abrasives of moderate hardness, Ha, such as flint and glass, with the 
object of explaining their wear characteristics under abrasion in soil. 
It is found that the relative wear resistance b increases, compared with the 
value /l applicable to hard abrasive, when Hu exceeds about 0.8 Ha. A secondary in- 
crease in b occurs when the hardness H of the bulk of the material exceeds about 0.8 
Ha. It is concluded from this and other evidence that both metals and alloys reach a 
maximum hardness Hu only locally. Significant relationships between b//l and HujHa 
or H/Ha, show that the wearing action is similar to that of hard abrasive, 
modified by damage to the abrasive grit. Scratching and abrasive wear cease under 
some conditions when the yield stress of the material slightly exceeds that of the 
abrasive. 
The increases in b//3 are much larger on fine than on coarse grit abrasive. This 
is associated with wear and plastic flow of the fine grit and shattering of the coarse 
grit. For heterogeneous materials this effect is augmented due to hard particles in the 
metals becoming more effective when they are large compared with the scale of cutting. 
This compares well with results in field soils. The similarity to types of wear arising 
in industrial practice and classed as scratching and grinding abrasion is noted. 
SYMBOLS 
H Vickers hardness of the test specimen, unstrained by wear. 
Hu Maximum hardness of the test material when strained plastically (or the hardness 
of a hard phase present in massive form), Vickers indentor. 
Ha Hardness of the abrasive material, Vickers indentor. 
Y Yield or flow stress calculated from H, Hu or Ha with appropriate suffix. 
E Young’s modulus of elasticity for the wearing material. Suffix a refers to the 
abrasive. 
A metallic material is tentatively termed soft with respect to an abrasive 
(or the abrasive is termed hard) when Hu for the material is less than Ha, and the wear 
is substantially independent of Ha. It is shown later in this report that a more correct 
criterion for this condition is Hu< 0.8 Ha. 
Wear, II (1968) 
246 R. c‘. D. RICHBRDSOX 
The relative wear resistance is the ratio, volume wear of the reference material 
(Armco iron)/volume wear of the test material, under identical conditions. 
@ The wear resistance of the test material relative to a soft metal (Armco iron), 
established under hard abrasive conditions chosen for reference. 
b The wear resistance of the test material relative to a soft metal (Armco iron) 
established under conditions being investigated, for instance when the test 
material may be hard relative to the abrasive. 
bo The value of b at the commencement of abrasion, when the length of the wear 
path is zero. 
I. INTRODUCTION 
The work to be reported forms part of an investigation of the wear of metallic 
materials by soill. Wear results in the field resembled in several ways the results 
obtained by KHRUSCHOV AND BABICHEV~ using bonded abrasive cloths and papers 
on a rotating disc abrasion tester. The wear of various metallic materials, including 
those tested in the field, was measured on several hard abrasives3 by a similar method. 
This was followed by an experimental study of the maximum hardness Hu of the 
metals and single phase alloys at high plastic straina. The mechanism of wear by 
hard abrasives was discussed. In the present account, the wear of the materials by 
relatively soft abrasives is considered. 
The abrasive is tentatively defined as soft when its hardness Ha is equalled or 
exceeded by the maximum hardness Ha of the wearing material or its constituents. 
As in previous work the wear results are expressed principally in terms of the relative 
wear resistance b or p taken as the ratio volume wear of the reference material/volume 
wear of the test material measured under nearly identical conditions. The symbol B 
refers to a hard abrasive condition taken as normal, and b relates to other conditions, 
notably to tests on potentially soft abrasives. A soft metal is used as reference in both 
cases. 
All hardness values refer to the Vickers diamond pyramid indenter with an 
included angle of 136” between opposite faces. For brevity, materials are described 
in the text by their designation in Tables I and III and their Vickers hardness H 
prior to abrasion. 
IS. Previozts work 
Using a pearlitic steel for control and specimens of a steel hardened and tem- 
pered to different levels of hardness, H, KHRUSCHOV AND BABICHEV~ (Chap. 5) found 
that the relative wear resistance, b, of the steel on 170 grit glass exceeded that on 170 
or 180 grit corundum, /3, when H/Ha was greater than 0.8, where Ha is the hardness 
of the abrasive. Progressively larger increases in b//l were observed as H/Ha exceeded 
0.8 but appreciable wear still occurred at unity and over. For comparison they also 
analysed data obtained by WELLINGER AND UETZ on 80 grit corundum and quartz 
and found a similar increase in wear resistance, this time when H/Ha for the steel 
exceeded about 0.5. TENENBAUM5, testing steels on IOO grit quartz with a modified type 
of rotating disc machine, also found b//3 > I for steels of hardness 600 kg/mm2 and over, 
again corresponding approximately to H/Ha >0.5. 
The discrepancy in the results on glass and on quartz was noted and requires 
Wear, II (1968) 
WEAR OF METALS BY RELATIVELY SOFT ABRASIVES 247 
explanation, together with the ability of the glass to produce a considerable amount of 
wear on steel of greater hardness. 
1.2. The present enquiry 
The principal object was to explore the soft abrasive wear regime, particularly 
with a view to explaining changes in the relative wear resistance of certain materials 
in different soils. 
In the first instance tests were carried out on flint and glass papers of 180 grit 
on a rotating disc machine and the relative wear resistance results were compared 
with those obtained on hard abrasives such as corundum and silicon carbide3 and 
with those obtained in the fieldl.6. Large increases in the relative wear resistance of 
various steels and particularly of the metals MO and W on glass grit (Ha = 5go 
kgjmmz), and of W on flint grit (Ha = 1060 kg/mm2) showed that the hardness 
levels reached under cold work applied in the process of abrasion were probably much 
higher than had previously been realized. This led to the investigation of maximum 
hardness already reported*. 
All the steels, cast irons and hardfacing alloys, containing sizeable particles of 
carbide, had a relative wear resistance on 180 grit flint much higher than the values 
recorded in field soils containing abrasive of similar hardness. Following the conclu- 
sions from the field trialsl, tests were carried out on coarse abrasive grits in order to 
increase the particle loading and the scale of abrasion relative to the size of the carbide 
particles. A significant resemblance was then revealed between the field and labora- 
tory test resultsl,6,7. Having thus established the similarity of the wear mechanism 
to that applying in soil cultivation, an exploratory study was undertaken of the wear 
phenomena that occur when the abrasive is not necessarily much harder than the 
wearing material. 
Some principal features of soft abrasive wear are discussed in Section 3. In 
Section 4 the wearproperties of brittle solids are considered briefly, the results as a 
whole are reviewed, and suggestions are made for further work. 
2. PROCEDURE AND WEAR RESULTS 
Particulars of the test materials, the testing machines, the abrasive discs and 
the procedure have already been given 3. Two rotating disc machines were used. The 
initial work was carried out on an early N.E.L. machine subject to a small inherent 
error. Later tests were made, using a different batch of abrasives, on a machine 
designed at N.I.A.E. The run lengths were 315 cm and 516 cm respectively. The speci- 
men diameter 0.100 in., load 500 g and table speed 20 rev.jmin, previously adopted as 
normal were used except where stated otherwise. The results on the two machines 
agreed satisfactorily for the present purpose. Where precise comparisons are needed 
they are made only within homogeneous sets of test data. The N.I.A.E. machine 
results have been preferred in the graphs. 
The choice of a hard abrasive condition, and of a soft metal, for use as reference 
involves errors of + IO or 20% and is to this extent arbitrarys. For the present purpose 
all wear resistances are expressed relative to Armco iron Fe as reference standard, 
(and control) having Hu = 400 kg/mm2 compared with Ha = 590 kg/mm2 for the softest 
abrasive, glass. The hard abrasive used as reference to determine #? was 180 grit corun- 
Wear, II (1968) 
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dum cloth except in a few cases where 18, grit silicon carbide was used because of the 
high hardness of carbide phases in the test material (Section 3.5). 
Glass and flint were chosen as the abrasives, the flint to represent abrasives in 
field soils, and the softer glass to enable higher levels of relative hardness to be at- 
tained with the existing materials. The main wear results are summarized in Tables 
I-III and in an appendixs. Specific gravities were given previously3. Weighing errors 
probably became dominant when wear resistances were very high (b >30 or so}. 
Before proceeding to discuss soft abrasive wear it may be remarked that, when 
relatively large volumes of the wearing material are harder than the abrasive, and the 
relative wear resistance is say IO or more, the wear becomes much more sensitive to 
the characteristics of individual papers or batches of papers than under hard abrasive 
conditions. Thus the change in wear on the specimen from paper to paper is proportion- 
ately greater than that on the iron and the latter becomes less effective as an experi- 
mental control. Relative wear resistances b measured on soft abrasive are therefore of 
lower repeatability, specially at high levels, than the value B applicable to hard abra- 
sive. For the present purpose it will be seen that this is of little importance. For an 
accurate series of comparisons, however, some process of preliminary grading or sclec- 
tion of papers, together with random sampling and generous replication, might be 
necessary. 
3. SOFT ABRASIVE WEAR 
Many of the harder materials in all classes (Tables I and III) had a relative 
wear resistance b on glass or flint grit paper much greater than that on hard abrasive 
1. When such increases occurred they were always larger on the 180 grit abrasive and, 
excepting a few marginal cases, b x80 grit >b 36 grit >@. 
The main problem, and one of the main purposes, in considering these results 
is to establish the degree of work hardening that occurs. Previous analysis (Section 
1.1) has been in terms of the hardness H of the unstrained test specimen, However, the 
effective hardness of a material relative to an abrasive must be that of the contacting 
surface region. It is generally supposed that metals and some alloys during abrasion 
reachs maximum hardness, Hw, w~c~determines the weara(Chap. 18)3*10. If thisis so, 
the ratio Ha/Ha should be significant, and the hardness H of the unstrained material 
should be of no direct concern once the specimen is run in. 
Tests can be made with progressively harder materials on an abrasive of only 
moderate hardness, using a soft metal as reference. At some threshhold value of Hu/Ha 
the relative wear resistance b should start to exceed the value /? measured on a hard 
abrasive. Since abrasion requires indentation and cutting of the wearing material, 
this threshhold value is expected to be near unity. TABOR’S study of single pass 
scratching11 suggests a ratio of about 0.8, corresponding to the state when the “tool” 
ceases to cut a chip on a flat surface. 
According to a simple interpretation the action of an abrasive that is soft 
(Ha < Hu) is modified only insofar as the grit suffers an additional degree of damage 
determined by the relative hardness HujHa. Thus the relative wear resistance b of a 
material on a soft abrasive is given by p.f (Hu/Ha), and a functional relationship is 
Weav, rr (1968) 
05- 0 
X 
-t 
A 
Flint 
Glass 
Glass. 
Garnet 
1 Mattensitic Steels with 
- 
Ma~t~sltic 
Steels with 
H-Ho 
HeHa 
Curve 3 
1 I I I I I I I I I 
0.506 0.8 I.0 I.2 I.4 16 1.8 2.0 2.2 
Relative hardness H”/Ha (or H/Ha for curves 2 s 3> 
Fig. I. Wear resistance ratio b//3 and relative hardness, for zoo/180 grit abrasive papers. 
Martensitic 
Steels with H-Ho 
1 Martensitic 5 eels k with H* H xceptinq curve %I 
x Glass 
+ Glass Curve 3 
0 Flint 
I / I I I I I I I i J 
0.5 0.6 08 I.0 I.2 I.4 I.6 I.8 20 2.2 2.4 2.6 
Relative hardness H”!$a (or H%a for curves 2 a $ 
Fig. z. Wear resistance ratio b//3 and relative hardness, for 36/30 grit abrasive papers. 
Wear, IX (1968) 
expected between the wear resistance ratio b//3 and the relative hardness N~ciHtr. 
Results taken from Table I for the more homogeneous materials are plotted 
in this wav in Figs. I and z for zoo/180 and 36/3o grit abrasives respectively. It is seen 
that fairly good curves can be drawn through the points for the metals, curve I, 
whilst the results for the steels lie well below the metals curves when H < Ha, but ma) 
lie above when H&Ha. Curves 2 and 3 are derived curves to be discussed later. 
A point for W on 180 grit garnet (Ha= 1360 kgjmmx) is included in Fig. I. 
Comparison of the figurcsshows clearly the much greater increases in wear resist- 
ance that are attained on the finer grit abrasives. Bearing in mind that errors of 
about &200/o may be involved3 in using Armco iron and ISO grit corundum cIoth for 
reference, it appearsthat increasesin b//3 commence as H,~~Hu reaches avaluc of 0.7~0.9. 
This is in accordance with reasonable expectation and with the behaviour of materials 
in scratchingrr. 
3.2. Work hardcnimg 
The results for the steels in Figs. I and z may be compared with those for the 
nominally pure metals, curve I. When the hardness H of the bulk of the specimen is 
less than Ha, the rise in wear resistance b/Bis small compared with that of a “pure” 
metal with a similar maximum hardness. To check that this is due to failure to work 
harden fully, specimens of the austenitic steel 58 and of the martensitic steel 8 (344) 
were preworked by mounting them in a drill and burnishing them with a rotary and 
lateral sliding motion under water, against a rough-ground pad of tool steel, a pad of 
the specimen material, or a quartzite stone. At about 15 p diagonal, the surface hard- 
ness was very variable but ranged up to H.~J in both cases. The subsequent wear 
measurements on 180 grit glass gave wear resistance ratios b/p of 1.55 and up to 6.5 
for the two steels respectively, compared with 1.00 and 1.28 during anormal test. This 
shows that the differences in b/b for the different types of materials are due, among 
other things, to the hardness of the wearing surface, and that in the steels the whole 
of the effective surface does not normally reach maximum hardness. The rapid rise 
in b//l for martensitic steels as H approaches Ha is further evidence of this. 
It is noticeable that the ratio b/p =6.5 for the pre-worked specimen 8 (344) 
lies above the curve for the metals in Fig. I. Some of the results for steels whose bulk 
hardness N 2 Ha also lie above curve I in Figs. I and 2. This suggests that curve I 
for the metals (all with H < Ha) does not represent alimiting condition of work harden- 
ing. micro-hardness measurements taken directly in the worn surfaces also lead to this 
conclusion. 
Results for the martensitic steels are replotted in curve z of Figs. I and z 
against the ratio H/Ha. Curves I and z now represent in turn, a condition of incomplete 
work hardening, Hu being an overestimate of the effective hardness (curve I), and a 
condition where some excess work hardening has occurred, H being an underestimate 
of the effective hardness (curve 2). A curve representing the true b//?-H/Ha relationis expected to lie between these curves as boundaries. 
Such a curve may be estimated by making use of the fact that martensitic 
steels with Ha Ha on glass grit, are found to need several runs during which the speci- 
men surface work hardens very slowly, before the wear reaches asteady value. For 
these materials and also for 42(503) and A&In, it proved possible to establish the wear- 
distance curves on the glass abrasives starting with polished or finely ground speci- 
wear, II (rgfx) 
WEAR OF METALS BY RELATIVELY SOFT ABRASIVES 253 
mcns in which work hardening was negligible. The initial gradient of the wear-track 
curves then gave an estimate of the wear rate and hence the relative wear resistance 
BO of surfaces of fixed hardness H, A brief account of these tests is given elsewhere* 
and two curves are shown in Fig. 3 as examples. The results 60jp-HIHa are plotted 
Material 0.74% C steel. 42 
H =503 kqhr? H= 650 kq/mm* 
3~0 0 Run I 
x Run 2 0.7 :: “,::,: 
/ 
,/ 
$/ 
06 
r/ 
g2.0 
1 
0.5 
2 
s / 
04 
i/ 
h 
‘. 
x 
.a 
3 I.0 
/ 
o-3 
_ 
/ 
1 $initioi qtadirnt=O-0069mqh O’* fx 
IRote for Fe = 0~0116mq/cm “’ u 
Initial gradient =0.0026 m&n 
& I I I I 
Rata for Fe = 0.0116 mqlcm 
i) I’ioO 2bO 300 400 MO 6CG 0 I00200300400sco~0 
Track , cm 
Fig. 3. Wear vs. distance for fine ground specimens on 30 grit glass paper. 
as curve 3 in Figs. I and 2 to represent the performance of an ideal metallic surface 
with H=Hzt. It should, however, be noted that the values of /I apply to an effective 
hardness greater than H and may therefore be rather high, making boj#? rather lower 
than it should be. This error is not important because compared with b, the value of B 
is much less sensitive to hardness. Since the steel 42 (813) is not scratched on 30 grit 
glass (Section 3,3. I), curve 3 in Fig. z must reach infinity at or before H/Ha = 8r3jsgo 
= 1.38. The experimental relationship of the three curves is shown by a chain dotted 
line between the points for the specimen 8 @go) in Figs. I and 2. 
It is seen that curve 3 occupies the expected region in Figs. I and 2 and lies 
well above curve I for the metals and all the other results except those of curve 2. 
This provides further confirmation that none of the materials developed a fully work- 
hardened surface during abrasion. 
Examining the results for the martensitic steels, and curve 2 in each of the 
figures, it is seen that a very steep rise in b//i occurs, as the bulk of the material becomes 
significantly hard between H/Ha=0235 and 1.00. This establishes that a hardness 
ratio of about 0.8 marks the start of transition to soft abrasive wear. This value agrees 
with expectation, and the hardness H is a conventional Vickers hardness measured 
on material in a homogeneous condition. The coincidence of the value HulHa ~0.8, at 
which the primary increases in wear resistance occur, therefore confirms that esti- 
mates of Hu obtained by the “trepanning” techniquedand used in this work are roughly 
correct, and that limited regions of the surface do in general reach this level during 
wear. 
Because of their slow progress to a steady wear rate during running-in, the 
practice of turning specimens through go” between successive runs3 had to be dropped 
for the steels with H 2 Ha. When tests were repeated with abrasion always along one 
direction, much higher wear resistances were recorded at the final steady state. These 
are the values employed in Table I and accepted for the present work. Checks on other 
material@, with H < Ha, showed that errors were insi~ificant and results by the 
Wear, II (1968) 
_* “” 
.- - 
lifica- 
WEAR OF METALS BY RELATIVELY SOFT ABRASIVES 255 
normal technique were therefore accepted, although unidirectional tests would be 
preferable in future.The long run needed to bring the steels with H>Ha toequilibrium 
was at least partly due to their high wear resistance (compare curves in Fig. 3). In 
contrast with the steel 42 (503) , no transient stage could be detected in the metal MO. 
3.3. Limiting conditions for soft abrasive zveay 
3.3.I. Conditions for resisting abrasive wear OY scratct%ng 
The type of abrasion discussed occurs through the removal of material as chips 
or slugs from grooves formed in the surface by slow unidirectional sliding contact 
with sharp particles disposed at random. Most of the material is removed by a minority 
of grains that cut a chip. This is termed scratching or cutting and is taken as being 
characteristic of abrasive wear. The results discussed below were obtained under 
normal testing conditions. 
Figure 4 shows an initially polished specimen of 0.74%C steel 42 at a hardness 
H = 650 kg/mm2 after sliding 15 m unidirectionally on 180 grit glass paper, Ha = 5go 
kg/mmz. Another IO m or so would be required for this specimen to reach equilibrium 
and for abrasive wear virtually to cease. The same specimen after a unidirectional 
wear test to an equilibrium condition(b=70) on 30 grit glass paper is shown in Fig. 5. 
The similarity with the appearance of Armco iron after testing under the same con- 
ditions, Figs. 6 and7 suggests that the wear mechanisms are closelyrelatedeven when 
H exceeds Ha. The high reflectivity of Figs. 4 and 5 is typical of steels having H $ Ha 
as equilibrium is approached. The light areas in Fig. 7 are powdered glass debris. 
Fig. 6. Armco Iron after sliding on zoo/180 grit glass paper. (Magnification x 48; the arrow denotes 
the direction of abrasion.) 
Wear, II (1968) 
250 ii. I I). lil(:H.~1<I)~O\ 
Limiting conditions can most con\cniently. be examined by tesiing ini&lly. 
flat surfaces on which scratching can easily, be observed. On zoo,‘180 grit glass a 
polished specimen of 8 (688) was not measurably worn during a 516 cm run, but grooves 
were formed by plastic flow. The same specimen was considerably worn by 30 grit 
glassx. Here H/Ha = 1.17, close to the ratio 1.2 at which the metal should scratch the 
glassii. On the finer grit, specimens of steel 42 experiencedlimited scratching even at 
H =S13 and 880 kg/mm” (Fig. 8). At higher magnification chips and flashes of dis- 
placed metal could be seen (Fig. g) and a secondary wear texture. 
In contrast with this, virtually no scratching occurs when the same material 
is tested on 30 grit glass paper (Fig. IO), but a very smooth and shallow texture is 
visible along the direction of sliding Fig. II, similar to that of Fig. (1. This difference in 
the ability to scratch hard material may be due to the greater resistance of fine lightly 
loaded grit to fracture (Section 3.4), combined with the increasing plastic flow stress 
of glass at higher strain rate 1%. In view of the confinement of the fine grit, it may be 
significant to realize that even a pure hydrostatic pressure can cause plastic flow around 
a cavity. 
When MARSH’S results are employed to relate hardness and yield stress4, it is 
noticeable that the yieldstress of the steel in Fig. IO, Y = 350 kg/mm” (H =813 and E = 
ZIOOO kg/mmz) approximately equals that of the glass Ya=345 kg/mm” (Ha=5go, 
E = 7000 kg/mm”). A plausible account of this limiting condition may be built up from 
work reported by BOWDEN ANI) T.~BOR’~ (Chap. I) when discussing the area of con- 
tact between solids. In interpreting the results from the deformation of copper cones, 
WEAR OF METALS BY RELATIYELY SOFT ABRASIVES 257 
Fig. 8. Steel 42 at H= 813 kg/mm2 after sliding 4 m on 200/180 grit glass paper Ha= 590 kg/mm2. 
(%Iagnification x 48; the arrow der otcs the direction of abrasion.) 
,P-- ., 
Fig. 9, Steel 42 at H= 880 kg/mm2 after sliding 5 m on 200/180 grit glass paper, showing in 
scratching and secondary wear texture. (Interference contrast; magnification x IOO; the 
denotes the direction of abrasion.) 
cipient 
: arrow 
Weav,rr (7968) 
WEAR OF METALS BY RELATIVELY SOFT ABRASIVES 259 
L_____.._l I 
* bY - I-IH 
/ ""% Hard Motc;iol 
Fig. 12. Mean stresses on the projected area of contact when cones are deformed by hard flat sur- 
faces, and when flat surfaces are penetrated by hard cones. 
it seems fair to substitute H for 3Y. With this assumption, Fig. 12 shows the mean 
stress on the projected area of contact, when flat surfaces are penetrated by hard 
cones, and when cones are deformed by hard flat surfaces. Taking the cones to represent 
the abrasive grit, it is seen that plastic flow may occur in geometrically weak grit even 
when Y is much less than Ya, and H < Ha. As soon as H exceeds Ha, plastic flow can 
occur in even the geometrically strongest grit. The condition at the contact surface is 
then analogous to that of a sphere, representing the flattened tip of the grit, in contact 
with a flat surface, the undeformed metal. Here, owing to the large influence of elastic 
strains, plastic flow can occur in the metal when the mean contact stress reaches 1.1 Y. 
If the surface of the grit is nearly flat, asimilar condition applies in the grit when the 
mean stress reaches 1.1 Ya. Thusif Ya is still somewhat greater than Y, plastic flow 
can be induced in the metal by successive grains of abrasive until its surface is no 
longer flat and it is then susceptible to cutting. It may be noted here that TABOR’S 
study of scratchingii related to a flat surface. 
Near the limit of abrasion the wear is small and an element of surface material 
becomes subject to a large number of strain cycles. If the elastic range is exceeded, 
materials such as martensitic steels and work-hardenedmetals maysoftend. Within the 
elastic range, transition to a new wear mode would be expected, involving surface 
fatigue. The secondary wear in Figs. g and II is probably of this type. Since many 
mineral abrasives have high H/E ratios compared with metals, the metals will normal- 
ly have to be considerably harder than the abrasive before the condition Y > Ya is 
reached. 
The foregoing condition applies to a flat surface of uniform hardness and a 
material that softens under alternating plastic strain. The alternative case, the cessa- 
tion of scratching in grooved surfaces work hardened by the abrasive process itself, is 
difficult to observe. In the present work a weight loss of < 0.01 mg per 516 cm run has 
been taken as negligible and corresponds to an increase in wear resistance of about 
x 200 for most of the materials. Nevertheless, some scratching may continue beyond 
this stage and the cessation of abrasive wear has not been properly established. 
3.3.2. Conditions for transition from hard to soft abrasive WeaY 
Pearlitic steels are significantly hard relative to glass abrasive both because 
they contain a cementite phase harder than glass and because the steel 8 (174) for 
instance, has a relative hardness HtilHa = 0.91 with glass as abrasive. The wear resist- 
Wear, II (1968) 
260 H. (‘. I). KIC‘HhIII)SOS 
ante ratio for this steel was hip= I.ZO on 200~1Ho grit (Table I). This accounts for the 
anomalous results obtained by. KHR~JSCH~~ AND BARICHEV on glass grit (Section x.11 
using a pearlitic steel as control. The rise in bj,!l for martensitic steels starting at H/Ha 
about 0.5 in curve z of Figs. I and z agrees well with the results of TESE~RA~I~ and 
WELLINGER _~SD UETZ (see KHRUSCHO~ AND BABICHE~” (Chap. 5)), but the value of 
H in this case is significant only as a measure of Hu, or of the surface hardness, for the 
particular class of material. 
With silicon carbide as abrasive, MULHEARX AND SAMUELS~~ found that much 
of the grit was notched and cracked, and fractured in use, and that the usedgrit became 
worn in sliding against the much softer steel. It is also clear from Fig. 12 that quite 
soft material can induce plastic flow in geometrically weak abrasive even if it is perfect. 
The onset of effects due to the relative hardness of the abrasive may therefore be 
gradual, but for the present conditions a significant threshold occurs when regions of 
the wearing surface become harder than about 0.8 Ha. This may be attributed to 
plastic flow occurring even in perfect regions of the grit that are geometrically strong 
(Fig. 12)) and perhaps also to accelerated wear of the grit. Large increases in b//?,possibly 
starting from a threshold less than Hu=o.8 Ha, may be expected under extreme 
fine grit conditions (Section 3.4), with largely blunted grains remaining in contact and 
working a large proportion of the surface to near Hu. 
3.4. The effect ofgrit size 
The most notable features of abrasion on coarse grit are the larger scratches 
formed in the wearing material and the prevalence of grit particle fracture. More of 
the scratches are interrupted. Alimited number of tests were performed on the N.I.A.E. 
machine with a range of glass papers of increasing grit size. Sliding was unidirec- 
tional and tests were repeated until equilibrium was reached. The results given in 
Table II show that, excepting W, the transition from the “fine grit” condition is ab- 
rupt This is accompanied by an audible change and by a change in the appearance of 
the track, from a smooth texture, marked by a coloured metal deposit in the case of the 
faster wearing materials, to a whitish track of rupturedgrit. This change occured with 
the soft metal Armco iron as well as with hard test materials. A somewhat similar 
TABLE 11 
RELATIVE WEAR RESISTANCE b OF SOME MATERIALS WORN UNIDIRECTIONALLY ON A RANGE OF GLASS PAPERS 
(N.I.A.E. machine) 
-.--~ 
Material H B Helatme weay resista.zce b on glass paper 
jkg/mm’2j ~___._______~. _~_ _ _~_ _...^ .___. ._. ~~_. -.- ~. ..-. 
Grade: EACOO Oakey oh 0ake.y Fz Oakey M2 Oakey S2 Oakey 24 Oakcy3 
Grit: r80/2oo I20 SOlI 60/80 40 36 39 -__-._--. ___I___ _~ ~___ ..~_. __._._~. __ .___-_.__-.. ..-. .___.” ~. 
AMn 220 2.12 39.5 7.67 9.47 6.46 6.23 6.42 7.43 
42 503 1.08 5.06 2.64 2.90 2.44 2.35 2.50 2.7X 
659 I.87 *co 41.6 =jL.O 55.1 48.0 37.5 69.4 
8 599 1.81 _ 139 9.4’ 1r.9 12.7 10.8 1X.9 16.4 
24 626 I.91 N 200 16.2 31.1 16.3 18.1 18.3 26.0 
Cr 147 I.70 4.60* 2.33 2.02 1.80 1.76 x.89* 
W 471 2.70 Nsu N 400 N 160 76.2 80.0 49.3 50 5 
Mean loss on Fe, mg/gI6 cm 3.85 5.04 4.91 4.09 4.55 5.50 5.74 
* Not a unidirectional test result. 
Wear, II (1968) 
WEAROFMETALS BY RELATIVELYSOFTABRASIVES 261 
distinction is apparent between the tracks left on Sic papers of 500 grit, and Sic or 
corundum cloths of x80 grit. 
In the tests reported in Table II only the 2ooj18o grit paper was backed with 
cardboard. As a check, values of b for the tempered martensitic steel 8 (590) and the 
initially austenitic steel AMn (220) were remeasured on the 120 grit paper, backed 
with cardboard. In both cases moderate increases occurred, from 9.42 to 15.38 and 
from 7.67 to 8.50 for the two materials respectively. It is nevertheless clear that the 
abrupt reduction in the value of b on 120 grit abrasive is not due to the absence of the 
cardboard backing. 
The results in Table II, and comparison of other data obtained on 40 grit and 
36 grit flint papers, suggest that the relative wear resistance b does not continually 
reduce but may reach a minimum on an intermediate size of grit. 
If abrasive surfaces of differing grit size are taken as geometrically similar, 
all the linear characteristics of the wear process such as the scratch depth are propor- 
tional to grit diameter, whilst the volume wear is independent of grit size. As against 
this it is well known that for hard abrasive the volume wear increases fairly steeply 
with grit diameter up to about 180 grit and then, increases at a slower rate and not 
always monotonically (see Fe in Table II). MULHEARN AND SAMUELS~* working with 
600-150 grit Sic papers account for this as an effect of varying abrasivegeometry. 
They show fine gritsurfaces consisting of severallayers of mainly acicularparticles well 
embedded in glue. With respect to fine grit the resilient backing is relatively thick. 
At the other extreme the present 36/30 grit surfaces are substantially a single layer 
of mainly equiaxed particles set in a thin layer of glue on a relatively thin backing. 
The effect of grit size in soft abrasive wear may be explained fairly simply 
if “fine” grit abrasive deteriorates predominantly through wear and plastic flow at the 
contacting points, whilst in “coarse” grit this effect is reduced and modified by extcn- 
sivc fracture of the grit on both hard and soft metals and alloys. In discussing the 
indentation and scratching of non-metals BOWDEN AND TABoR call attention to the 
presence of a size effect in very brittle materials. Such materials may yield plastically 
until the deformed region exceeds a critical size beyond which they shatter. MARSHA” 
finds for glass that the most likely fracture criterion is a critical size of plastic zone. 
This accounts well for the abrupt transition from “fine” to “coarse” grit wear behav- 
iour, andsuch phenomena are here concludedto be of majorimportancein abrasive wear. 
The conditions may be compared briefly as follows. In fine grit, extensive 
fracture is inhibited due to low particle loading, the confinement of the well-supported 
grit, and a general hydrostatic pressure. In the absence of fracture the grit may strength- 
en through plastic flow (Section 4.1) andscratch very hardmaterials (Section 3.3.1). 
The dimensions of the engaged facets are small and the track length per particle is 
relatively very large. The resulting wear and plastic flow at the cutting facets has a 
profound effect on their shape. The loss of cutting efficiency as Hu or H exceeds Ha 
is therefore large, and is cumulative as progressively less of the heavily strained surface 
material is removed as debris. This is exaggerated when a smaller proportion of the 
contacting grains cut a chip. 
In coarse grit with higher loading of poorly supported particles, critical dimen- 
sions of plastic zones are commonly exceeded and widespread fracture occurs in both 
hard and soft abrasives. In hard abrasive this may occur following plastic flow due to 
geometric weakness (Fig. 12), faults or stress raisers in the grit, and contact with 
Wear, II (1968) 
262 
i 
f-0 
; 
c 
WEAR OF METALS BY RELATIVELY SOFT ABRASIVES 263 
neighbouring grit or grit fragments, either before or after rupture of the glue bond. 
High tangential forces may be important. The cushioning effects of a deep glue bond 
and relatively thick resilient backing are absent. Under the action of continually 
reformed very sharp cutting facets a greater proportion of heavily strained surface 
material is continually removed and the increases in wear resistance that occur as Hu 
or H exceeds Ha are relatively less marked. Embedment of shattered grit debris 
(Fig. 7) does not seem to have a great effect. 
Somewhat analogous conditions are well known in machine shop practice, the 
accumulation of a heavily worked layer in drilling austenitic steels if the drill feed is 
momentarily reduced, and the greater effectiveness of a coarse friable grit in grinding 
very hard material. 
3.5. Heterogeneous materials-containing large particles of a hard phase 
Materials of heterogeneous structure containing relatively large particles of a 
hard phase cannot be characterized by single values of H or Hu. However their wear 
behaviour may be compared visually with that of the metals by replacing Hu for the 
metals by/I, since for the metals,Hu cc /3 to a fair approximation4. Curves I fromFigs. I 
and 2 are shown in Fig. 13(a) and (b) plotting b/p-/l/Ha, together with values for a 
selection of hypereutectoid steels, white irons and hard facing alloys. The data are 
loo 
[ 50 
Mk 
IO 
5 
% 
2 
I 
(0) 
3 oD.Cr ,’ 
Motols 
& 
CCCr 
COO 
r 
44+ 
NHO sP+ 
WI0 
2750 J stxDG s +44 +44 py44D.Fe 
+ Glass 2001180qril 
I 0 Flint lEOgrit x Corundum lEOgrit 
(b) 
+ Gloss 30qril 
0 Flint 40qrit 
Oi-ii 
IdS 
v 4 5 0 I 2 
Ha lOJZ HO 
Fig. 13. Wear resistance ratios of some materials containing large carbides. (a) On zoo/180 grit 
abrasives, (b) on 40/30 grit abrasives. 
taken from Table III and may in some cases be subject to correction owing to the 
technique of turning specimens through 90” between successive runs. This method of 
plotting has no precise physical significance and is only used as a means of presenting 
the data visually. 
Although the metals are themselves very responsive to grit size of the abrasive, 
the figures show the remarkable way in which the wear resistance of these carbidc- 
bearing materials increases on fine grit soft abrasive and declines on coarse grit towards 
the hard abrasive value. In doing so it is noticeable that the “ranking” order of the 
materials changes. 
Wear, II (1968) 
WEAR OF METALS BY RELATIVELY SOFT ABRASIVES 265 
Fig. 16. Delcrome, iron base hardfacing alloy. (Magnification x 300) 
3.5.I. Hardness of the structural co?n+vae:tts 
Micrographs of CCr (709, WI and DCr are shown in Figs. 14-16. The hardness 
of the carbides and of the matrix in some of the materials was measured by micro- 
indentation (Table IV). The impressions in the carbides were generally about 1/5-1!3 
TABLE IV 
HARDNESS OF THE CONSTITUENTS OF SOME MATBRIALS CONTAINING CARBIDES 
Material Carbide M&ix 
Indentation Hardness (kglmmz) Identation Hardness (kglmm2) 
__- diagonal 
Mean (PI Aange Mean 
CCr(go2) 
WI (676) 
NHA(~Io-870) 
diagonal __ -~ 
(PI Range 
IO-12 1230-1360 
4 1090-1180 
6-7 757-995 
DCr(632) 7-10 1830-2130 
St4613) 5-7 1950-2350 
1280 
rr40 
870 
2060 
2130 
12-13 
9 
g-11 
8 
6-g 
7-9 
Martensite 
830-884 860 
Pearlite 
467-504 480 
Martensite 
368-467 410 
Carbide + y 
545-625 580 
Eutectic 
450-524 480 
Eutectic 
673-737 7oo 
Means refer to about 8 observations 
Wear. II (1968) 
the particle diameter. The results are in good agreement with values given I)!- Jltrrr: 
for carbides of the appropriate composition. The difference in the hardness of tlit 
cementite in h:HA and \\‘I may not be significant because these structures would 
only accommodate verv small impressions. The high hardness of the matrix in St ma\’ 
be due to the difficulty of avoiding carbides of significant size. 
As expected from the behaviour of the metals and single phase alloys, increases 
in wear resistance occur, as shown in Table III, when the carbides are harder than 
about 0.8 Ha. Sections from specimens tested in field soil, Fig. 15 show that the carbid- 
es flow and may therefore work harden. There is thus some doubt for instance whether 
corundum is hard with respect to the die-steel CCr. The effectiveness of the carbides 
in the white irons, as well as in the high alloy materials, against wear by flint is oi)- 
viously of great practical concern. 
3.5.2. Discrete ejfectiveness of hard @articles-size effect 
The materials containing carbide in relatively massive form show b >/j on 
abrasives that give normal hard abrasive results for more uniform structures of equal 
bulk hardness (e.g. St on corundum and Sp on flint). This, together with the much 
greater sensitivity of the heterogeneous materials to grit size, and hence to the scale of 
scratching, suggests that the carbides are effective as a discrete component of the 
structure and not only as a means of strengthening the material as a whole. 
A very rough comparison was made of the size of carbides and wear debris, 
taking the steel CCr(To8) as an example. From a micro specimen, of which Fig. 14 
shows part, the size of the larger carbides was estimated as 0.01-0.02 mm. \VearWEAR OF METALS BY RELATIVELY SOFT ABRASIVES 267 
debris was collected from wear tracks onto a filter paper held flat on a pot magnet 
about I mm clear of the track. The debris was enclosed in the filter paper, demagnetized 
and transferred to a glass slide. On 40 grit flint of particle diameter typically 0.48 mm, 
the wear resistance ratio b//l was 1.08 and the debris, Fig. 17, ranged widely about an 
average of some 0.2 mm x 0.04 mm, i.e. larger than the carbides. The potential size of 
the debris on 180 grit flint in the absence of the carbides was estimated roughly by 
collecting debris from a test on 180 grit corundum, grit diameter 0.075 mm. Here the 
debris ranged about a mean size of about 0.03 mm x 0.01 mm, Fig. 18, commensurate 
with the carbides. 
Fig. 18. Wear debris from CCr at H = 708 kg/mm2 on 180 grit corundum cloth. (Magnification 
x I.501 
Thus very roughly, the carbide particles become very effective against 180 
grit flint, b[/l=6.76, because they are in the same size range, or larger, than the chips 
that would be cut in their absence, whilst they are only slightly effective, b//3 = 1.08, 
against 40 grit flint because they are small compared with the typical size of a cut chip, 
and may easily be dug out rather than worn. 
3.5.3. Properties affecting soft abrasive wear 
Precise analysis of the effectiveness of different factors such as the hardness, 
size, shape and proportion of carbide, the configuration of the structure and the prop- 
erties of other phases is precluded in most cases by the absence of control over these 
variables in the materials tested. A fairly satisfactory comparison can be made, how- 
ever, of the eutectoid steel 42, representing the matrix of the hypereutectoid steel 
44, on glass (Table V). This shows that the pro-eutectoid carbides are more effective 
Wear, II (1968) 
on the finer grit and become more dominant as the matrix material becomes mttrtx 
resistant to wear by the abrasive in question. The properties of the matrix appear to fb(. 
particularly important on coarse grit and this is also seen, Fig. 13(a) and (b). by com- 
paring CCr(yoa) with the other materials on 180 grit and40 grit flint. The wear resist- 
ance of the two carbon steels, 42 and 44 fell to about the corundum value on r80 grit 
flint although the annealed high speed steel Sp with larger harder carbides but a weaker 
matrix, and WI with more carbide, present as a partly continuous phase, showed 
significant response. 
TARLF: V 
COMPARISON OF THE WEAR RESISTANCE OF STEELS 42 AND 44 
_-. 
I’ickers Glass 
H~&MS~ 
Ii (kg/mm2) r80/200 grit 30 grit 
-. .-.. 
42 44 b4z b44 bu 
h brz 
2’2 1 191 I.90 1.21 
341 349 2.07 1.40 I.05 
503 499 4.98 I.70 1.41 
These and further comparisons lead to the general view that hard particles 
become specially effective against wear as their hardness approaches or exceeds Ha, 
as their size and proportion increases, and as the wear resistance of the other phases 
increases on the abrasive in question, the latter being specially important on coarse grit 
abrasive. The ranking orclerr6917 of these materials may change under different abra- 
sive conditions 
3.6. Effect of load and specimen diameter 
Since deterioration of the grit and the scale of cutting are of principal impor- 
tance in soft abrasive wear, variations in the relative wear resistance b might be expect- 
ed with specimen diameter and load. To test this, the specimen feed per table revolu- 
tion on the N.1 A.E. machine was increased to accommodate larger specimens. 
Preliminary results with the die-steel CCr(goz) on 40 grit flint (Table VI, 
Fig. 19) show that b is very sensitive to the nominal contact stress (load~s~ecimen area) 
CHANGE IN RELATIVE WEAR RESISTANCE b OF CCr(902) WITH SPECIMEN DIAMETER AND 
NOMINAL CONTACT STRESS 
(40 grit flint paper unbacked, unidirectional wear, N.I.A.E. machine) 
.spccirrz1w Load Helatzoe ~Nominal 
dam. ls) wenr contact 
(in.) resistance, stress 
b (gimm2) 
_^_ .__-... _ .--.- _ _ __ --. ._._. ~~_~_~_~~ 
0.100 1008 5.52 199 
500 6.60 99.0 
I54 70.5 30.4 
0.180 1008 6.05 61.2 
500 g.00 30.4 
Wear, II (1968) 
WEAR OF METALS BY RELATIVELY SOFT ABRASIVES 269 
a Specimen diameter O.lOOin 
Fig. 19. Change in relative wear resistance b with nominal contact stress and specimen diameter, 
CCr (902) on 40 grit flint paper. 
and also varies with the specimen diameter when the contact stress is constant. Similar 
tests on 180 grit glass with a steel 24(4gg) not containing a massive carbide phase, 
showed only slight variations over the range tested (diameter 0.18c~~070 in., 500 g 
load). In all cases the diameter of the iron standard was the same as that of the test 
specimen. 
These results may be very important practically and may enable test conditions 
to be chosen so as to simulate more closely the relative wear arising in particular 
service dutiesi9697. It is not known at present to what extent the rather critical transi- 
tion between coarse and fine grit conditions (Table II) is affected by specimen diame- 
ter and load, and this needs exploring. It is clear that many of the phenomena described 
here by reference to these grit conditions are similar to the types of wear met in 
engineering, classified by NORMANI* and by AvERY~~J~ as high stress (or grinding) 
and low stress (or scratching) abrasion. It is now seen that thereis, to some extent, a 
continuous merging of these groups. Whether the wear mode is better characterized 
by reference to the nominal contact stress, or to some other feature, will no doubt 
become clear as knowledge accumulates. At present the terms grinding and scratching 
abrasion seem preferable if suitably defined. 
4. FURTHER DISCUSSION 
4.1. Brittle solids 
The wear of some brittle solids was measured to see whether they could be con- 
sidered as materials of fixed hardness for use as standards of reference. KHRUSCHOV 
AND BABICHEV~ (Chap. 13) assumed that minerals would have a constant wear 
resistance b relative to a metal on different 180 grit abrasives so long as the abrasives 
were harder than the mineral. The few exploratory results in Table VII show that 
this is not so. While the wear of the metal Fe varied only moderately on the different 
180 grit abrasives, that of the brittle solids increased substantially in step with the 
hardness of the abrasive. This needs explanation. 
It seems likely that the type of chip formation characteristic of abrasive wear in 
metals 19,20 is drastically modified in these materials. Examination of initially polished 
Pyrex (Y > Ya) and soda glass (Y = Yu) after sliding on zoo/I8ogrit glass paper showed 
scratches with various plastic and brittle characteristics. This is not accounted for by 
Wear, II (1968) 
T
A
B
L
E
 
V
II
 
W
E
A
R
R
E
S
U
LT
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F
O
R
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M
E
B
R
IT
T
LE
 
S
O
LI
D
S
 
T
E
S
T
E
D
 
O
N
 
18
0 
G
R
IT
 
A
B
R
A
S
IV
E
S
 
(N
IA
E
 
m
ac
h
in
e)
 
M
a
te
ri
a
l 
H
ar
dn
es
s 
S
p
e
c
i
f
i
c
 si
c 
C
or
u
n
du
m
 
W
l
m
m
2
)
 
gr
av
it
y 
H
a 
=
30
00
 
kg
/
m
m
=
 
H
a 
=
 
2
1
8
0
 
kg
/
m
m
2 
b 
H
/
H
a 
b 
H
/H
a 
P
yr
ex
 
gl
as
s 
S
od
a 
gl
as
s 
F
u
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si
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ca
 
S
in
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al
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a 
79
0 
2.
23
 
59
0 
2.
47
 
10
50
 
2.
5 
1
1
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0
/2
1
8
0
* 
3.
58
 
M
ar
bl
e 
po
ly
cr
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ta
li
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W
t 
lo
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 o
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 F
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m
&
51
6 
cm
 
15
3 
2.
6 
0.
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8 
0.
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3 
0.
24
8 
0.
19
7 
0.
23
40.
35
0 
I.
16
 
0.
36
6/
0.
72
7 
0.
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64
 
0.
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99
 
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9
6
 
0.
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3 
0.
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3 
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98
 
0.
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1.
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0.
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5.
74
 
G
ar
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H
a 
=
 
13
60
 k
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m
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b 
H
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a 
0.
62
4 
0.
58
1 
0.
77
0 
0.
77
3 
54
 
0.
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5.
91
 
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* 
B
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lk
 
h
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 ,
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 d
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H
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of
 
al
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a 
21
80
 
k
g/
m
m
2.
 
F
li
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H
a 
=
 
10
60
 
kg
/
m
m
2 
b 
H
/
H
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0.
73
’ 
0.
74
5 
0.
59
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0.
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I.
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4.
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G
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H
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1.
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-3
0 
1.
78
 
0.
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86
 
0.
26
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3.
83
 
WEAROF METALS BYRELATIVELYSO~AB~S~ES 271 
the weakness of the specimen edges since plates of glass were also scratched by 200~180 
and 30 grit glass papers when fairly high local pressure was applied by hand. The glas- 
ses were tested at I cmjsec since the surface of soda glass (but significantly not 
Pyrex) melted locally at the standard speed on glass abrasive, and the wear was halved. 
Several aspects of brittle solids are of practical importance including their 
behaviour as abrasives, the wear of ceramics, the wear of brittle surface layerszl,zz 
and very hard solids2 (Chap. 10)33 and the wear of brittle hard phases in alloys or 
cermets, or for instance in plastics. 
Although the abrasives in the present analysis have been treated as of fixed 
strength Ha, crystalline minerals such as rocksaltg and magnesia24 work harden and 
are anisotropic while the strength of glass increases with strain raterz. These properties 
together with the fracture characteristics are expected to be important in wear whether 
the material is an abrasive or is itself worn. 
The relative wear resistance b of sintered alumina on flint paper was 61 on 180 
grit, and 15 on 40 grit. Since the coarser grit approximates wear conditions in cutting 
soill, material of this sort may be of practical use if fracture and chipping can be 
avoided. 
4.2. Review 
Work harden&g. In most of the materials tested, limited regions of the surface 
become fully worked, or nearly so, under the conditions of abrasion applied. The rela- 
tive wear resistance b exceeds the value ,f? applicable to hard abrasive as the maximum 
hardness Hzt reaches and exceeds about 0.8 Ha. The increases in b are larger when more 
extensive volumes reach higher hardness levels as for instance when the hardness of 
the bulk of the material equals Ha or more. The work hardening of wearing surfaces 
was discussed fully in Section 3 and in a previous report4. 
Wearing action. The wearing action of relatively soft abrasive appears to be 
similar to that of hard abrasive, but modified by a degree of damage depending on the 
relative strength of the wearing surface and the grit. Both these strengths are complex, 
and interact. Moreover the behaviour of the grit is profoundly affected by many other 
factors including the grit size, 
When variations in the extent of work hardening during wear are eliminated, 
a restricted group of materials such as the martensitic steels, with N=Hti and E 
constant, show an approximate single-valued relation between b;iB and H/Ha (Figs. I 
and 2, curve 3). Results for different abrasive materials appear reasonably compatible 
in the figures and suggest that the degree of damage to the grit is fairly well determin- 
ed by the relative hardness at moderate levels. At higher levels of b/p differences in 
elasticity YalEa and properties affecting fracture and wear of the grit, may render Ha 
inadequate as a parameter of the grit strength. Similarly, curves equivalent to curve 
3 for different groups of metals or alloys would not be expected to coincide precisely 
because of differencesin a variety of strength properties of which the hardness is not a 
perfect measure. Tests are required with different groups of alloys and different abra- 
sive materials covering a similar range of relative hardness. 
The variable extent of work hardening. However, the principal reason for the 
scatter of results in Figs. I and 2 {excepting curve 2) is evidently the variable extent 
of work hardening. On hard abrasive the relative wear I/#? is fixed by an average hard- 
ness of the surface, while on soft abrasive the effective hardness dete~ning 116 may 
Wear, rr (1968) 
be somewhat greater owing to less efficient cutting and is also biased towards hartl- 
ness levels exceeding 0.X Ha. /i,‘i? is therefore a different function of Hzi/Ha according 
to the proportion by volume of the surface reaching different fractions of the strength 
Hw. Thus, once again, restricted groups such as the metals, having a surface strength 
consistently related to Hzr, plot approximately on a single curve (curl-e I), though 
Hu is now a less direct measure of the effective strength of the wear surface than in 
curve 3. 
When 11 <Ha the poor wear resistance ratio of the martensitic steels compared 
with the metals is remarkable, and may be partly due to the greater effect of alternat- 
ing strain in reducing the work hardening of the steels”. Xear the limit of scratching, 
when the wear is small these steels may soften. 
S~~~~~~~~~ criferia. The poor response on glass grit of the austenitic stainless 
steel 58 compared with the austenitic manganese steel AMn, both of which transform 
during wear”, may be because the general level of surface hardness of the latter exceeds 
Ha, or because Yzb exeeds Ya. The martensitic steels gave an extremely high wear 
resistance on fine grit when H/Ha reached about 1.2, and on coarse grit scratching 
ceased when E’ slightly exceeded Ya. Relative strengths such as these should be 
established under various conditions, from which the expected level of b//3 may be 
estimated for different classes of material. 
In seeking criteria for changes in the magnitude of b/b, ratios that may be 
significant can be formed from H, Y, Hzi and Y?A for the metal or alloy, and Yn and 
Hn for the abrasive. Since stress-strain curves for metallic materials are often fairly 
flat at a simple natural strain about 2.5, the value of H and Y at this strain may be 
useful as a measure of the general hardness of the wear affected surface. In Figs. I 
and 2 the “effective” hardness of wearing surfaces may be estimated by reading 
across to curVe 3 to find the hardness of a surface fully hardened throughout, that 
would give the same rise in wear resistance, b//3. The effective hardness of the austeni- 
tic manganese steel AMn (220) estimated in this way is 655 and 690 kg/mm”, not far 
from the value 660 kg/mm” for this material when cold rolled to about 8oq& reduction 
in thicknesszs. Other relevant strength properties of metals and alloys were discussed 
previously4. 
Hard phases. In heterogeneous materials the presence of phases with a hardness 
exceeding about 0.8 Ha extensively modifies wear behaviour and introduces further 
powerful size scale effects (Section 3.5). With suitable ranges of test materials it 
should be possible to set up empirical relationships taking account of variables such 
as the particle size and proportion by volume of the hard phase for different types 
of structure. It may also be possible to derive relative wear resistances for the princi- 
pal constituents and a more realistic modificationof the additivity law proposed by 
KHRUXHOV AND BARICHEV" (Chap. 7)23. For surface-treated materials the thickness of 
the surface film necessary to make the wear independent of the core material should 
be established under various conditions. The behaviour of different extremely hard 
materials as a hard phase, or as wear resistant materials on their own account, is of 
particular interest. 
Properties uftke abrasive surface. The behaviour of the grit and hence the wear, 
is affected greatly according to whether the abrasive is fine and deteriorates primarily 
through wear and plastic flow (b//? large, Fig. I, and probably increasing as the grit 
gets finer) or whether it is coarse and deteriorates through blunting of continually 
IV&W, I.? (rg68) 
WEAR OF KETALS BY RELATIVELY SOFT ABRASIVES 273 
fracturing grit (bag smaller, Fig. 2, and reaching a minimum as the grit coarsens). 
The abrupt transition as the grit size increases is attributed to scale effects determining 
the strength of brittle solids. Many factors that may affect this transition such as the 
load and the size of the contacting surfaces, need investigation. The characteristics 
of the material underlying the grit are evidently important. Increased resilience favours 
fine grit behaviour perhaps because more asperities share the load and remain in 
contact as they slide over the uneven surface, and because the grit grains are more 
confined and are backed up by a more even pressure. Other abrasive surfaces such as 
cemented grit or grit embedded in metal may give rise to somewhat modified wear 
phenomena. 
General wear behaviour must be influenced by geometric properties such as 
the texture of the abrasive surface, and the shapes and size of the contacting surfaces, 
together with the load. The roughness and size affect the proportion of asperities that 
make contact under the imposed load, the scale of deformation and engagement, and 
the distance a contacting grain slides, while the shape and orientation of the engaged 
asperities decide the proportion of contacts that cut rather than rub. Special condi- 
tions exist at edges. The fracture, wear and plastic flow in the abrasive and the strength 
developed in the wearing surface are accordingly affected, and also the extent to 
which the abrasive becomes cloggedlo, or masked with wear debri9. 
Although for the purpose of analysis the strength of the abrasive has been 
represented by Ha, such material may be anisotropic, may harden when it flows plas- 
tically if it is crystalline, OI its strength may depend on the strain rate. Some of the 
results in the present work, using glass which is in the latter group, may not be typical 
of most mineral abrasives. In particular, conditions for the cessation of scratching 
may be affected by anisotropy in the hardness of the abrasive, as in lamellar solids”?. 
Properties of abrasive cloths in commercial use were investigated by DUWELL AND 
McDoNALD~*~~~, but at relatively high speeds. The method of calculating the typical 
elastic modulus and yield stress of an abrasive from the crystal constants and from 
hardness measurements is given by ATTWOOD~O. 
These various properties of the abrasive and of the contacting surfaces may 
have important effects on the magnitude of b//l, the conditions for the cessation of 
abrasive wear, and possibly on the transition region from hard to soft abrasive wear. 
Furtker work. The present investigation of a-body abrasive wear has been 
exploratory and more precisely planned studies of many aspects of the subject are 
needed. The effect of fluids, and especially water, should be explored. The conditions 
for transition to peripheral or associated forms of wear should be investigated further, 
including s-body abrasion”6.31, surface fatigue, erosion by abrasive entrained in 
fluids, wear of chemically affected material (surface films), and the wear of material 
affected by frictional heating. 
In the light of increasing knowledge of the wear process many possibilities 
exist for the active development of improved materials. It is most important that 
through associated “field” tests, phenomena arising in practical conditions of service 
should be brought under reviewza, and that if possible, ways should be found of 
simulating practical conditions of wear in the laboratorylZ637. 
~0~~~~~s~~~~. Formal conclusions to this paper are embodied in a separate 
reporta2. 
Wear, rr (1968) 
274 R. (:. I). RICRAKI)SOS 
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 Director, National Engineering Laboratory for the use of wear testingfacil- 
ities during two short visits. 
Thanks are due to Dr. K. H. K. WRIGHT of the National Engineering Labora- 
tory and to D. G. ATTWOOD for helpful discussion, and to Dr. D. TAROR of Cambridge 
University for reading the work in draft. 
D. G. ATTWOOD was responsible for the micro-hardness measurements and 
metallography. A. W. BARKER contributed to the practical work. 
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31 
32 
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