International Journal of Refractory Metals & Hard Materials 25 (2007) 171–178

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EVect of microstructure on edge wear mechanisms in WC–Co

P.V. Krakhmalev ¤, J. Sukumaran, A. Gåård
Department of Materials Engineering, Karlstad University, Universitetsgatan 2, SE-651 88 Karlstad, Sweden

Received 9 January 2006; accepted 11 April 2006

Abstract

Edge wear of nano to coarse WC–Co grades was investigated under two-body abrasion conditions using SiC abrasive at 2 and 15 N
loads. With the Wne abrasive utilized, a transition from homogeneous plowing in submicron grades to inhomogeneous grains pullout in
coarse hardmetals was observed at the beginning stages. At Wnal stages, plowing was detected in all the grades. When using the coarse
abrasive, for all grades, Xaking at the beginning and plowing at the Wnal stages occurred. The performed numerical Wnite element simula-
tion of the microstructural mechanical response showed good agreement to experimental results and was discussed in relation to the oper-
ative wear mechanisms observed. The found results demonstrated a remarkable inXuence of microstructure and abrasive size on the
heterogeneous to homogeneous wear mechanisms transition at initial stages.
© 2006 Elsevier Ltd. All rights reserved.

Keywords: Hardmetal; Nanocomposite; Abrasive wear; Edge wear mechanisms; Microstructure

1. Introduction The mechanical wear is subdivided in abrasion, erosion,

adhesion and surface fatigue wear processes. The impor-
The WC–Co hardmetals were introduced into the Euro- tance of each wear process depends on the particular tribo-
pean market in 1920–1930 as materials possessing a favor- system in action and varies in quite a broad range.
able combination of high hardness, reasonable fracture However, the abrasive wear is the most undesirable, due to
toughness and, high wear resistance [1]. Nowadays, hard- high wear rates, dramatic surface damage, and activation of
metals have found a broad application as mining, masonry, other wear mechanisms. Initiation of the abrasive wear in a
wood and metal cutting and drilling tools. Recent progress tribosystem occurs, usually, in presence of particles or
in hardmetals is based on reWnement of the WC phase and asperities that are harder than the tool material. Neverthe-
manufacturing of ultraWne and nanoscale composites pos- less, abrasive wear components have been found even in
sessing superior combination of mechanical properties [2,3]. tribosystems where a workpiece has lower hardness than
Wear damage is the major factor limiting lifetime of cut- the tool, for instance in woodcutting (e.g. [4] and related ref-
ting and drilling tools. Generally, wear can be classiWed in erences) and explained by high cutting speeds, and inhomo-
two huge groups depending on the wear mechanisms geneity of the workpiece structure.
involved. The Wrst group is mechanical wear, which is origi- Abrasive wear of WC–Co hardmetals has been exten-
nated only by the mechanical interaction between counter- sively investigated the last decades, with particular atten-
faces. The second involves chemical interaction, like tion to novel nano and ultraWne grades. Commonly, a
oxidation or tribochemical reaction between counterfaces, positive eVect of decreasing the WC grain size has been
as an active wear component. reported. Thus, submicron grades have showed of about
double wear resistance compared to the conventional
grades [5–7]. Besides, the observed wear mechanisms
*
Corresponding author. Tel.: +46 54 700 20 36; fax +46 54 700 14 49. depend on the nature of the abrasive and the size of WC
E-mail address: Pavel.Krakhmalev@kau.se (P.V. Krakhmalev). grains. Hard diamond abrasive caused plastic deformation

0263-4368/$ - see front matter © 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijrmhm.2006.04.004
172 P.V. Krakhmalev et al. / International Journal of Refractory Metals & Hard Materials 25 (2007) 171–178

and fragmentation of the surface, while softer ZrO2, SiC or excludes possible eVects of adhesion or chemical interac-
SiO2 abrasives caused preferable binder removal followed tions, which might complicate identiWcation of the abrasive
by detachment of WC grains [5,7]. It has also been demon- wear mechanisms. Wear mechanisms and volumetric wear,
strated that coarser materials exhibited predominantly brit- were investigated and discussed in dependence on the slid-
tle response whereas plastic deformation has been observed ing distance and normal force. The main attention was
in submicron materials [5,7]. It is important that in all those devoted to identiWcation of wear micromechanisms and the
research, the size of abrasive exceeded microstructural fea- correlation to microstructural response.
tures of WC–Co composite signiWcantly.
In a contradiction, if 1–10 m diamond, SiC and Al2O3 2. Materials and methods
abrasives have been used, a decrease in wear rates with
increase of the WC size has been observed recently, Totally, six WC–Co hardmetals of diVerent WC grain
although, the same wear micromechanisms have been size were investigated. The selected grades found industrial
detected [8,9]. With the hard diamond, binder and WC applications as cutting, drilling or milling tools of wide
grains are worn with the same rate, while binder phase materials range. The HM1-C and HM2-M grades are
removal followed by pullout of undermined WC particles is applied for woodcutting of soft and hard or frozen wood,
the main wear mechanisms if SiC or Al2O3 are utilized. the HM3-F grade is for sawing of laminated particle
Such a behavior has been explained by an increase in resis- boards, the HM4-EF grade is typical for medium or high
tance to carbide pullout as the size of carbide increases, due density Wberboard and, the HM5-UF and HM6-N grades
to increase in a depth of the binder needed to be removed. applied for drilling, milling and routing of printed circuit
For materials selection and cutting tool design, wear boards and glass reinforced plastics. Selected properties of
resistance is the vital property, determining tool perfor- the investigated materials are collected in Table 1.
mance and costs for manufacturing and exploitation. The specimens were ground and polished from two sides
Although there are the ASTM G65 and ASTM B611-85 with 3 m diamond paste to produce mirror-Wnished 60°
standards for the abrasive wear evaluation, these test meth- sharp edge. As an example, Wnal edge sharpness of the
ods usually employ coarse abrasive and do not represent HM2-M specimen is present in Fig. 1a. Polished specimens
the geometry and parameters of the real cutting or drilling were mounted onto the vertically movable holder, built on
operations. This disagreement resulted in recent interest in the drill machine. A normal force of 2 and 15 N was applied
tribological phenomena at the edge of a tool that is impor- by a static weight. The abrasive paper was Wxed on a hori-
tant practically, since sharpness loss inXuences dramatically zontal movable X–Y-table, which was drawn along the
the tool performance and Wnal quality of the product. loaded specimen manually, using worm screwfeed, at slid-
To characterize the edge fracture resistance and evaluate ing speed of about 0.5–1.0 cm/s. The speed was measured in
fracture toughness of hardmetals, a tribotester for investiga- advance and not currently controlled during the sliding. A
tion of the edge wear on prismatic specimens has been sketch of the specimen arrangement and sliding geometry is
designed by Scieszka [10,11]. It has been found that initial present in Fig. 1b. Long sliding distances were collected by
stages are controlled by brittle fracture whereas microabra- a number of strokes of about 15 cm each until required slid-
sive wear mechanisms dominated the steady-stage wear [10]. ing distance was reached. Every new stroke drew along the
Furthermore, the edge Xaking tendency has been examined fresh abrasive paper surface. The 4000 (»5 m grit size) and
using an indentation method. Good correlation between 80 (»200 m grit size) abrasive papers were used. The tests
fracture toughness and so-called edge toughness has been were performed without lubricant.
found for brittle materials including hardmetals [12–14]. In The tested specimens were examined in a GEMINI LEO
both approaches described above, the size of abrasive parti- 1530 scanning electron microscope (SEM) to explore the
cles or size of the indenter exceeded considerably features of operative wear mechanisms. The volumetric wear for speci-
the microstructure and therefore, micro-scale edge behavior mens slid along the Wne abrasive paper at 2 N load was cal-
still has not been completely discovered. culated from the mean of 10–15 measurements of wear
In the present research work, abrasive wear of the wedge width (the 2 parameter in Fig. 1b), measured in the SEM.
specimens of WC–Co hardmetals from nano to coarse The coarse abrasive produced very rough and uneven worn
grades, was investigated under two-body abrasion condi- surface so that calculations of volumetric wear were
tions. The specimens sharpened and polished to 60° edge assumed inaccurate.
sharpness slid along the SiC abrasive paper in a direction To analyze the eVect of microstructure on the strain dis-
perpendicular to the edge line. Such geometry was selected tribution in the wedge specimen at the Wne abrasive and 2 N
to model a situation when the edge meets abrasive particles normal load test conditions, Wnite element simulations were
embedded into the substrate (for instance inclusions in cut- conducted with Abaqus® software. The contact situation
ting timber, drilling of hard particle reinforced composite was interpreted as a simultaneous multiasperity contact
material etc.). The disadvantage of this test arrangement is between specimen side and abrasive and was simpliWed as a
that the specimen does not penetrate the substrate as in the distributed pressure applied from the side normally to the
real situation. Nevertheless, such a scheme describes con- side surface of the specimen. The applied pressure value of
tact between the edge and particles appropriately and 500 MPa was calculated so that a component normal to the
 P.V. Krakhmalev et al. / International Journal of Refractory Metals & Hard Materials 25 (2007) 171–178 173

Table 1
Selected properties of the WC–Co hardmetals investigated
HM1-C HM2-M HM3-F HM4-EF HM5-UF HM6-N
Coarse Medium Fine Extra Wne Ultra Wne Nano
Grain size, m 3.5–4.9 1.4–2.0 1.0–1.3 0.5–0.9 0.3–0.5 0.1–0.3
Co, wt.% 11 11 6 3 8 9
Hardness, HV30 1150 1280 1620 1900 1900 1950
Fracture toughness, MPa/m3/2 19.0 16.0 11.0 7.0 8.5 8.0
Transverse rupture strength, N/mm2 3200 3200 2800 2100 4300 4700
Width (1), mm 4.1 4.3 5.5 4.6 3.3 3.3

Fig. 1. Edge sharpness of the polished HM2-M specimen in front view (a), and a sketch of the specimen arrangement and sliding geometry (b).

abrasive surface would satisfy the 2 N normal load test con- 3. Results
ditions. The contact area was approximated as 1 m in
height. The plane strain conditions were assumed to prevail 3.1. Wear diagram
in the middle of the specimen, since the thickness of the
specimen exceeds other dimensions signiWcantly. As the A diagram summarizing results of worn volume in
boundary conditions, the nodes located at the top surface dependence on sliding distances using 4000 abrasive grit
of the specimen, Fig. 7, were restricted from movement dur- and 2 N normal load test conditions is present in Fig. 2.
ing the analysis. Generally, materials demonstrate similar behavior with the
Two model microstructures of WC–11%wt.Co compos- highest wear rate at the beginning, which further decreases
ites, one corresponding to the ultraWne and another to the with increase in sliding distance. Nevertheless, as seen in the
coarse grades, were used, Fig. 7. The microstructures used close view in Fig. 2b, if for the ultraWne HM5-UF and
in the analysis represented a 2D cross-section of the 3D nanoscale HM6-N composites the diagrams are described
specimen in this simulation. The Co and WC phases were with quite smooth curves, the diagrams for the HM1-C,
assumed to behave isotropic elastic with perfect phase HM2-M, HM3-F and HM4-EF materials look like a com-
interface, i.e. no separation along the Co/WC interface was bination of two regions with a transition after about 30–
allowed. 60 cm sliding. Such a behavior is well pronounced in the
Meshes and Abaqus® input Wles were created based on HM1-C, HM3-F and HM4-UF materials but not in the
micrographs of the materials through the “ppm2oof”and HM2-M grade. In the further discussion, the initial and
“oof2abaqus” software, respectively, developed at the the Wnal stage terms are used to describe the corresponding
National Institute of Standards Technology (NIST) [15]. portions of the wear diagram.
The diVerent phases were identiWed as separate element
groups, with signiWcantly reWned meshes in the Co phase, 3.2. Wear mechanisms
and phase boundaries. Totally, 3.0 £ 105 and 1.1 £ 105 plain
strain 6-node triangular elements were used to represent the Three regimes of the two-body abrasion were run: (i) 2 N
ultraWne and the coarse materials, respectively. The princi- load and Wne abrasive (4000 grit), (ii) 2 N load and coarse
pal strain distribution maps were calculated on the cross- abrasive (80 grit) and (iii) 15 N load and coarse abrasive (80
section parallel to the sliding direction and normal to the grit). Worn surfaces were examined in the SEM to identify
abrasive surface. The microstructures used for the calcula- wear mechanisms operative at each stage.
tions as well as strain distribution maps are present in The worn surfaces investigated after sliding under 2 N
Fig. 7. load and Wne abrasive test conditions in the coarse and
174 P.V. Krakhmalev et al. / International Journal of Refractory Metals & Hard Materials 25 (2007) 171–178

Fig. 2. Worn volume vs. sliding distance diagram under 2 N and Wne abrasive test conditions (a) and, close view of the beginning part of the diagram (b).

nano grades are illustrated in Fig. 3. It is seen that observed grooving indicated predominance of the plowing wear
wear mechanisms are not the same. In the coarse HM1-C mechanisms, although some indications of possible falling
material, at the initial stage, pullout and detachment of WC out of WC grain due to lost of the matrix support may be
grains were observed after 2 cm sliding, Fig. 3a. Further observed. Fig. 4b illustrates contact surface of the HM1-C
sliding led to the binder removal and some scratching of the grade after 30 cm sliding that corresponds to the deXection
WC grains, Fig. 3b. Surface plastic deformation and exten- on the wear diagram. A simultaneous operating of the WC
sive grooving wear were indicated in Fig. 3c and d illustrat- grain fracturing and detachment, binder deformation and
ing the worn surface observed after 2 and 200 cm sliding in removal and some grooving wear mechanisms observed
the nanoscale HM6-N grade. It is seen that plowing is the clearly.
main wear mechanism operative in the nanoscale grades at Fig. 5 illustrates worn surfaces after sliding at 2 N and
all sliding distances. coarse abrasive test conditions. It is seen that utilization of
The close view of the worn surface observed in the the coarse abrasive led to initiation of plowing and chip-
HM6-N grade after 2 cm is present in Fig. 4a. Extensive ping in both the coarse HM1-C and nano HM6-N materi-

Fig. 3. Worn surface observed at the initial and Wnal stages in the HM1-C (a) and (b), and in the HM6-N (c) and (d) grades under 2 N and Wne abrasive test
conditions, respectively.
 P.V. Krakhmalev et al. / International Journal of Refractory Metals & Hard Materials 25 (2007) 171–178 175

Fig. 4. High-magniWcation images of worn surface observed at the initial

stage in the HM6-N (a), and transition stage in the HM1-C (b), under 2 N
and Wne abrasive test conditions. Indications of possible falling out of WC
grain in the HM6-N grade due to lost of the matrix marked with black
arrows.

als at the beginning of sliding, Fig. 5a and b. Further sliding

did not promote chipping and after 100 cm sliding, plowing
was found to be the main wear mode, Fig. 5c.
Sliding at 15 N and coarse abrasive test conditions led to
extensive chipping at the beginning stages, Fig. 6a and b. Fig. 5. Worn surface observed after 10 cm sliding in the HM1-C (a), 2 cm
Sliding for longer distance, nevertheless, led to the situation sliding in the HM6-N (b), and 100 cm sliding in the HM6-N (c), under 2 N
similar to the low load and coarse abrasive test conditions. and coarse abrasive test conditions.
The extensive plowing was observed after 100 cm sliding in
the HM1-C specimen, Fig. 6c, and the submicron grades a critical strain, c, at which a crack starts to propagate into
demonstrated similar behavior. the bulk. If p < c, the wear rate is described by the
Archard’s equation and depends mostly on hardness. Oth-
4. Discussion erwise, when p > c, the wear rate depends on fracture
toughness of the material. Following this concept, the
As it is seen in Fig. 2a and b, the shape of the wear dia- beginning or unsteady stage corresponds to the case of
gram depends on the material grade. If for the HM6-N and p > c, while steady stage to the p < c case [10]. Assuming
HM5-UF materials the curves have quite a smooth para- that p depends only on the test conditions (size and shape
bolic character, for other grades, the diagrams look like a of abrasive, normal load, geometry of specimen, etc.), c
combination of two regions. Similar parabolic character of appeared to be a structure-sensitive parameter, related to
the wear diagram has recently been observed in [10,11] for the microstructure.
abrasive wear of specimens having similar shape as were Applying this approach to the edge wear by Wne abrasive
employed in the present research. As it has been reported in (the asperity size is comparable to the features of micro-
[10], such a behavior could be described in terms of structure), it might be concluded that a situation when the
unsteady and steady wear stages, which depend on correla- strain produced by asperity exceeds a certain value, which
tion between plastic strain, p, produced by the asperity and is critical for the failure, is rather likely at the beginning
176 P.V. Krakhmalev et al. / International Journal of Refractory Metals & Hard Materials 25 (2007) 171–178

removal and scratching were found at the Wnal stage,

Fig. 3b.
Although the analysis above predicts wear mechanisms
of the micron grades properly, another behavior was
revealed for the nanoscale materials. It is seen in Fig. 3c and
d that in the HM6-N grade, plowing wear was the domi-
nant mechanism at the initial and Wnal stages. Conse-
quently, the wear diagram does not have pronounced
inXections, which might correspond to changes in wear
mechanisms.
To analyze a microstructure response at the beginning
stage in submicron and micron grades, a series of numerical
simulations was carried out. Model microstructures of
WC–11 wt.%Co corresponding to submicron and micron
grades were used to take into account inXuence of grain size
but eliminate contribution of Co content. Results of the cal-
culations are presented in Fig. 7 in form of principal strain
maps. It is seen that in the micron material, the highest
strains are localized in the binder phase on the tensed side
of a specimen. In case of the submicron microstructure,
besides the tensed side, there is a highly strained zone dis-
tributed into the bulk.
Results of the numerical simulation correlate well to the
approach described above and to experimental observa-
tions, although the cobalt content in the grades experimen-
tally tested is not the same. In a case of the micron
microstructure, the highest strains are localized in the
binder phase on the tensed side of a specimen and, if the
critical value is exceeded, failure of the binder phase fol-
lowed by detachment of WC from the bulk is initiated. Sim-
ilar behavior was detected in the micron grades
experimentally, Fig. 3a. When the edge is worn out, the con-
tact area is increased and a larger number of WC grains
appeared on the contact surface that leads to stress redistri-
bution and causes initiation of the plowing wear instead of
fracture. Changes in the wear mechanism aVected the wear
rate and, therefore, inXections were observed on the wear
diagrams for micron grades, Fig. 2.
In a case of the submicron microstructure, strains are
Fig. 6. Worn surface observed after 2 cm sliding in the HM1-C (a), 2 cm
sliding in the HM6-N (b), and 100 cm sliding in the HM1-C (c), under
not only localized on the tensed side, but also distributed
15 N and coarse abrasive test conditions. into the bulk. Therefore, volumetric plastic deformation
is activated immediately at the beginning stage and wear is
realized by plowing. Since the same wear mechanism is
stages when only few WC grains are in the contact zone. operative for all sliding distances, there were no inXections
Therefore, fracturing and detachment of WC grains or at the wear diagram. A parabolic shape of the wear dia-
fragments of microstructure is the most possible wear gram could be explained by increase in the contact area due
mechanism at this stage. Further wear must lead to increase to wear that led to drop in the wear rates.
in the contact area and stress redistribution into the bulk, DiVerence in the dominate wear mechanisms generally
causing initiation of plastic deformation. inXuenced the wear rates and wear resistance. As seen in
Such a prediction is in a good agreement to the experi- Fig. 2, the total worn volume at the Wnal stage observed for
mental results obtained for the micron grades at 2 N load the HM6-N is remarkably higher than that for the HM1-C.
and Wne abrasive test conditions. As seen in Fig. 2b, for the Although, the direct quantitative comparison of the total
HM1-C specimen, the diagram contains of two regions wear is not entirely accurate due to diVerent widths of spec-
with a transition point. Fig. 3a and b illustrates worn sur- imens, see Table 1, it is possible to conclude that the nano
faces corresponding to initial and Wnal stages in the HM1-C HM6-N has lower edge wear resistance than the coarse
material, respectively. Fracturing and detachment of WC HM1-C grade under the selected test conditions. Similar
grains are clearly observed at the initial stage, while binder behavior has been observed in [8] if Wne abrasive is utilized.
 P.V. Krakhmalev et al. / International Journal of Refractory Metals & Hard Materials 25 (2007) 171–178 177

Fig. 7. Microstructures of model WC–11 wt.%Co alloys used for simulation of the microstructure response in submicron (a) and micron (b) grades and,
maximum principal strain distributions calculated for the submicron (c) and micron (d) microstructures. Note strain localization in the binder phase in the
micron grade and bulk strain distribution in the submicron grade.

Utilization of the coarse 80 grit abrasive paper resulted the Xaking wear mechanism, activated at the beginning, is
in initiation of chipping (Xaking) wear mechanism at the changed to the plowing at the Wnal stages, Figs. 5 and 6.
beginning stages. This wear mechanism caused formation At the Wnal stages, more than few grains of WC are in a
of the unevenly worn surface and, therefore, diYculties in contact to the abrasive paper. The wear behavior at this
quantitative analysis of the volumetric wear. Generally, as stage generally depends on WC grain size, abrasive size and
seen in Figs. 5 and 6, chipping was observed at 2 and 15 N abrasive hardness. It has been reported that for coarse
loads at the beginning stages and more pronounced for abrasives, homogeneous plastic deformation and plowing is
higher load test conditions. Further sliding led to predomi- the most common wear mechanisms, while if the size of the
nance of plowing wear and, only minor chipping was asperity is comparable to the microstructure, selective
observed after 100 cm sliding at 2 and 15 N load. Such a removal of the binder phase is observed [8,9,16].
behavior was not sensitive to the WC grain size because the Results obtained in the present research for Wnal stage
interaction zone exceeds signiWcantly features of the micro- wear mechanisms are in a good agreement to these consid-
structure and, was observed in the nano and coarse grades. erations. It is seen in Fig. 3b, that under 2 N and Wne abra-
The Xaking phenomena observed in the present research sive test conditions, removal of binder and some scratching
can be understood using a concept of the edge chipping ini- of the WC grains is observed in the HM1-C hardmetal,
tiation in brittle materials developed in [12–14]. It has been while evidences of plastic deformation and plowing are
shown that there is a linear dependence between an indenta- observed in the same material slid under 15 N and coarse
tion load critical for Xaking initiation and a distance from abrasive test conditions, Fig. 6. For the submicron HM6-N
the edge. Additionally, initiation of Xaking has been found of HM5-UF grades, even the Wne abrasive utilized was
dependent on the edge geometry – the critical Xaking loads remarkably larger than the features of microstructure and
have been found higher for specimens with a chamfer com- therefore, microstructure response was similar at all the test
pared to sharp specimens, even the edge toughness value conditions. The plowing was found the main wear mecha-
remained the same [12,13]. In the present research, the nism operative at the Wnal stage for 2 and 15 N normal
rounded surface occurred due to wear apparently plays a loads in submicron grades.
role of the chamfer. As the edge is worn, the critical load Discussing the overall wear mechanisms, it is diYcult to
value required for Xaking initiation increased and, therefore, link the edge wear resistance directly to the mechanical
178 P.V. Krakhmalev et al. / International Journal of Refractory Metals & Hard Materials 25 (2007) 171–178

properties (collected in Table 1). Nevertheless, obviously wear initiated at the beginning stages led to quick sharp-
the microstructure response vitally inXuenced the wear ness loss and formation of the rough and uneven edge.
mechanisms and wear rates at the initial stages. If the fea- The further sliding resulted in a transformation of Xak-
tures of microstructure are comparable to the size of abra- ing to plowing wear in submicron and micron grades.
sive particles, inhomogeneous pullout of the WC grains was
observed. In nanoscale materials, microstructure is remark- Acknowledgements
ably Wner than the abrasive and therefore, plowing was the
dominant wear mechanism at all sliding distances. Numeri- Authors would like to thank Mr. Mathias Tillman and
cal simulation showed good correlation to the experimental Mr. Stefan Ederyd, Sandvik Hard Materials, for materials
results although experimentally tested grades had diVerent supply and fruitful discussion and analysis of the obtained
cobalt content. Nevertheless, systematic investigations of results. Also, authors are grateful to Mr. Nils Hallbäck,
the edge wear in dependence on cobalt content and WC Karlstad University, for productive discussions on numeri-
grain size are required for better understanding of the edge cal simulation and Mr. Prasad Angaru, for active participa-
wear phenomena. The found results demonstrated a tion in the experimental part of the research.
remarkable inXuence of microstructure and abrasive size on
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