8th International DAAAM Baltic Conference
"INDUSTRIAL ENGINEERING
19-21 April 2012, Tallinn, Estonia
TWO-BODY ABRASIVE WEAR OF WC-CO HARDMETALS IN WET
AND DRY ENVIRONMENTS
Pirso,J., Juhani, K., Viljus, M., Letunovitš,S.
Abstract: A comparison of two-body
abrasive wear behaviour of WC-Co
hardmetals in both wet and dry
environments is presented. Five different
composites were studied. Two-body
abrasive wear tests were conducted on a
block-on-ring tester, described in the
ASTM B611-85. The steel wheel was
replaced with an alumina grinding wheel.
The wet environment promoted lower wear
rate compared to the dry conditions. The
wear volume decreases with the increase in
bulk hardness. SEM examination of the
wear tracks in the worn blocks suggests
that abrasive wear mechanisms are similar
in dry and wet environment and occur
through surface elastic-plastic and plastic
deformation.
Key words: WC-Co, Hardmetals, Dry
abrasion, Wet abrasion, Wear mechanism.
1. INTRODUCTION
Abrasive wear behaviour of a material is
dependent on a number of factors, such as
chemical content and structure of the
material, contact geometry, surface
roughness, speed, load, temperature,
environment and lubrication [1]. Two-body
wear occurs when the grits, or hard
particles, are rigidly mounted or adhere to a
surface, where they remove the material
from the opposite surface. The common
analogy is that of material being removed
with sand paper or abrasive wheel. In the
conditions of abrasion usually multiphase
materials – cemented carbides or
hardmetals – are used in which extremely
hard carbide grains are dispersed
throughout a softer matrix. WC-Co
hardmetals are well-known high wear
resistant materials [2,3].
There is a long history of examination of
the abrasive wear behaviour of WC based
hardmetals
[4-14].
It
has
been
demonstrated that the abrasive wear rate of
WC-Co hardmetals mainly depends on the
carbide/cobalt ratio and the size of the
carbide grains. The wear rate increases in
proportion with increase in the cobalt
content [4-12] and size of the carbide
grains [10-19].
It is found that the abrasive wear resistance
of the hardmetals increases with increasing
of the carbide content, which also causes a
gain in the hardness of the composite [812]. These studies have proved the
existence of a direct connection between
abrasive wear resistance and the hardness
of material.
The effect of carbide grain size on the wear
rate of hardmetals can be different. Usually
fine-grained hardmetals are more wear
resistant than coarse-grained ones. Jia and
Fisher [12] also found that WC–Co
nanocomposites possess an abrasion
resistance approximately double that of the
most resistant conventional material. At the
same time Engqvist et al. [13] have found
that coarse-grained hardmetals show a
lower abrasive wear rate than the finegrained ones.
O’Quigley et al. [14] also found that the
coarser grades have higher abrasion
resistance in the 1000–1600HV hardness
range while the finer grades are expected to
have a higher abrasion resistance at
hardness values higher than 1600 HV.
The main objective of this work was to
compare abrasion behavior of WC-Co
composites in dry and wet conditions.
2. MATERIALS AND
EXPERIMENTAL PROCEDURE
The WC-Co samples were fabricated at
Tallinn University of Technology using a
conventional cemented carbide production
route [2]. The structure of cermets consists
of tungsten carbide grains with mean grain
size 1-2 µm in a metal binder.
Two-body abrasive wear tests were
conducted on a modified block-on-ring
tester, described in our former work [9].
Steel wheel was replaced by abrasive
grinding wheel. Specimens of different
WC-Co composites with dimensions of
23x14x5 mm were clamped in a holder and
held rigidly against a rotating 225 mm
diameter abrasive wheel under normal load
of 20 N. Alumina used in these tests as
abrasive has Knoop hardness of 1900[15].
The structure of a vitrified grinding wheel
is composed of sharp abrasive grits, a
bonding system, and a large number of
pores. The abrasive grits average size was
0,3 mm. The rotation speed of the abrasive
wheel was 235 RPM, which gave a linear
speed of 2.8 m s-1. Sliding distance was 50
m. Prior to each wear testing, the abrasive
wheel was sharpened, and each specimen
ran on fresh surface of the abrasive wheel.
The blocks were ground to a surface
roughness (R a ) of about 1 µm prior to
testing.
Each specimen was weighed
before and after testing to an accuracy of
0.1 mg. Weight loss was converted into the
volume loss. The abrasion results were
averaged over three samples for each
material.
The surface of the specimens after wear
tests was observed with scanning electron
microscope (JEOL JSM 840A). The
hardness of the samples was measured
using a Vickers pyramid indenter.
Measurements were made under a load of
10 kgf using a load time of 30 s. An
average hardness value was determined,
based on 5 indentations.
3. RESULTS AND DISCUSSION
3.1. Volume loss
The wear behaviour of hardmetals in
dry and wet environment is different. As
seen in Fig.1, the volume wear of WC-Co
hardmetals in dry conditions increased
approximately linearly with increasing of
the binder content.
Fig. 1. Two-body abrasive volume wear of
cermets depends on binder content
In wet condition the volume wear
increases also linearly up to 15 wt% binder
content and above that boundary decreases.
The reason for this is not completely clear.
Factors which may contribute are cooling
of the abrasive contacts by the water and
consequent reduction in temperature at the
surface. Such reason of different wear rate
was noticed by Grant et al [20]. The second
reason may be that extruded cobalt filled
the pores in the abrasive wheel and by such
way the water wedge formed and behaved
as a lubricant. It leads to decrease specific
loads in the contact area and stress
redistribution into the bulk, causing a drop
in the material removal rate. The both
reasons are significant for high binder
compositions.
Fig.2 shows that the volume loss of
hardmetals appears to increase with the
increase in the sliding distance.
contact with the specimen and becomes
less effective in removing material from
the sample.
As seen in Fig.3 the wear rate of the
coarse-grained
WC–20
wt.%
Co
hardmetals is approximately twice lower
than that of the alloys with medium-size
grains. These results are similar with
Engqvist et al. [13] and Okamoto et al. [16]
showing that the abrasion resistance of the
alloys with wide carbide grain size
distribution is higher than that of the
conventional ones. They suggest that
materials with smaller WC grains are
brittle, whereas those with larger grains are
ductile.
a)
Fig.3.Volume wear of WC-20wt.%
hardmetals with medium and coarse
carbide grain size in dry and wet
environment
b)
Fig.2.Volume loss of WC-Co hardmetals in
dry and wet environment vs. sliding
distance. a – dry; b- wet
In dry condition the volume of wear varies
in an approximately linear manner during
all 150 m sliding distance. In wet
conditions after 50 m run the water layer
was formed between the surfaces and the
volume wear rate stabilized or decreased.
The abrasive surface deteriorates during its
As seen from Fig.4, the volume wear in dry
conditions depends on the bulk hardness of
the composites and decreases with increase
in the bulk hardness.
The penetration depth of the abrasive
particles is determined by the hardness of
the wearing material. In general, the
abrasion damage is inversely proportional
to the material hardness, which affects the
penetration of the abrasive particles into
the target surface.
a)
Fig. 4. Wear volume loss after 50 m run vs.
bulk hardness of hardmetals.
A small penetration depth results in less
subsurface deformation, and thus, less
abrasive wear. As seen in Fig.4 such
behavior of hardmetals is not applied for
wet environment. It may be explained by
wedge effect of high binder content alloys
as shown before.
3.2. Wear mechanism
The two-body abrasive wear is most
undesirable, due to its dramatic surface
damage. Figs. 5 and 6 illustrates the
typical behavior of WC-5wt% Co and WC20 wt% Co hardmetals sliding against an
alumina wheel in dry and wet environment.
Fig. 5 shows a typical behaviour of the
hardmetals after dry sliding of 1 m against
an abrasive wheel. In Figs. 5a and 5b, the
worn surface of WC–5 wt.% Co hardmetal
is shown. The abraded surface is relatively
smooth and featureless. A small plastic
deformation of the surface by the alumina
particles can be observed. As seen in Fig.
5b, some extrusion of the binder phase has
taken place, followed by pullout of small
carbide grains from the surface.
Figs. 5c and 5d illustrates the behaviour of
WC–20wt.%Co hardmetal in dry condition.
Significant plastic deformation has
occurred and deep grooves were formed
parallel to the sliding direction.
b)
c)
d)
Fig. 5. Wear damages on the WC-Co
hardmetal surface after 1 m run on the
abrasive wheel in dry environment .
a, b) WC-5wt%Co; c,d) WC-20wt%Co.
The surface is filled with deep grooves and
lateral ridges, parallel to the sliding
direction. The passage of the abrasive
particles causes plastic deformation of the
surface which results in the formation of
grooves with material pile up at the groove
edges.
Examination at high magnification showed
significant damage on the wear surface
(Fig. 5d). Considerable fracture of the WC
grains can be seen. Many small fragments
of carbide grains had been entered into the
binder phase regions of the worn surface.
The carbide grains thus lose their binder
phase support and fall out of the surface.
abraded surface of the low binder cermets
(WC-5wt%Co) in wet environment is also
relatively
smooth
and
featureless,
indicating that the binder phase and carbide
framework
were
worn
down
simultaneously (Fig.6a). Several pits can be
observed on the worn surfaces.
In high binder content alloys (15wt% and
more Co) significant plastic deformation
occurs (Fig.6b) with corresponding fracture
and fragmentation of WC grains and
delamination of the material from the
surface. Although some of these small
fragments of WC grains are removed from
the wear surface, many still remain in the
materials structure.
Surface shearing and grooving displaces
the carbide grains, leading to an extrusion
of the Co binder phase towards the surface.
The binder phase is partly removed from
between the tungsten carbide grains by a
combination of plastic deformation and
micro-abrasion.
4. CONCLUSION
a
b
Fig. 6. Wear damages on the WC-Co
hardmetal surface after 1 m run on the
abrasive wheel in wet environment.
a) WC-5wt%Co; b) WC-20wt%Co.
The deformation in the surface changes
from elastic to elastic-plastic or plastic. The
Two-body abrasive wear behaviour of
WC–Co in dry and wet environment was
studied.
1. The wear resistance of the hardmetals in
both cases depends on the carbide/binder
ratio. The wear rate was low and increased
with an increase in the binder content,
corresponding to a decrease in the bulk
hardness.
2. The volume wear of hardmetal samples
in wet condition was approximately twice
lower than that in dry conditions. The
reason for this is not completely clear.
3. The volume wear of hardmetals
increases approximately linearly with the
sliding distance up to the first 50 m in dry
and in wet environment. After that distance
the wear volume in wet environment
stabilized.
4. The material removal mechanism during
wear is similar. Wear of the low binder
cermets (up to 15 wt. % binder phase) is
elastic-plastic deformation of the surface,
followed by fracture and fragmentation of
carbide grains and carbide framework after
multiple deformations. In the hardmetals
with high binder content (more than 15 wt.
% Co), significant plastic deformation
occurs with displacing of the material to
groove edges without direct material
removal, binder phase extrusion and brittle
cracking of the carbide grains.
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6. CORRESPONDING ADDRESS
DTech. Juri Pirso
TUT, Institute of Material Technology
Ehitajate tee 5, 19086 Tallinn, Estonia
Phone: +372 620 3356,
E-mail: [email protected]