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1. Original Article
Tribological behavior of spark plasma
sintered ultrafine-grained WC-cobalt
cemented carbides in dry sliding
Zhenhua Wang , Boxiang Wang, Zengbin Yin and Kui Liu
Abstract
Ultrafine-grained tungsten carbide-cobalt (WC-Co) cemented carbides with different Co content were fabricated by
spark plasma sintering. Then, tribological behavior of the ultrafine-grained WC-Co cemented carbides were studied by
performing sliding wear tests with grinding balls made of GCr15 bearing steel (HRC 6266) and Ti-6Al-4V titanium alloy
(HRC 36). When the grinding ball is composed of GCr15 steel, the time required to reach the stable friction stage
decreases with the increase in cobalt content, whereas the friction coefficient and wear rate tend to increase, and the
wear type is abrasive wear. When the grinding ball is made of titanium alloy, the friction coefficient and adhesive wear
volume first increase and then decrease with the increase in Co content, both reaching maximum values at 6 wt.% Co,
and the wear type is adhesive wear.
Keywords
Cemented carbide, ultrafine-grained, friction coefficient, wear mechanism, spark plasma sintering
Date received: 23 October 2019; accepted: 7 February 2020
Introduction
Nowadays, advances in high-speed machining and dry
cutting have placed higher requirements on the wear
resistance of tool materials.1
High wear resistance of
tool materials can improve the cutting speed and tool
life and improve machining accuracy, particularly in
micro hole drilling processes.2
Owing to their high
hardness, good wear resistance, and stable chemical
performance, tungsten carbide (WC)-based cemented
carbides are widely recognized as ideal tool materials
for machining oil exploration equipment, aircraft
gears,3
and bearings,4
as well as die cutting.5
Numerous factors can influence the tribological
properties of WC cemented carbides, such as sintering
conditions, WC grain size, cobalt (Co) content, mech-
anical properties, and microstructure.6–10
Modern
cemented carbides are usually composed of 0.1 lm–
10 lm WC and 4–30 wt.% Co.11
Wear resistance of
the material gradually decreases with increasing Co
content but increases with decreasing grain
size.8,10,12,13
Therefore, ultrafine WC grains are an
effective way to improve the tribological properties
of cemented carbides. Additives such as chromium
(II) carbide (Cr3C2) and vanadium carbide (VC) can
effectively inhibit WC grain growth.14–16
The effect of
adding grain growth inhibitors on the friction and
wear characteristics of 12 wt.% Co cemented carbides
has been studied. The results show that the wear
resistance of cemented carbides is greatly enhanced
by adding grain growth inhibitors, in particular, VC
was found to increase wear resistance by approxi-
mately 90%.17
Jia et al.13
studied the wear mechanism
of nano-crystal cemented carbides. They found that
nano-crystalline cemented carbides offer superior
wear resistance compared with conventional cemented
carbides because of their higher hardness.
Wear resistance of cemented carbides can be fur-
ther improved by reducing the Co content. Moreover,
the sintering method can also affect the wear resist-
ance of materials. In the case of cemented carbides,
sintering methods include hot-press sintering,18,19
microwave sintering,20,21
and spark plasma sintering
(SPS).16,22
Comparing these methods, smaller grains
were obtained with SPS due to its fast heating rate
and short retention time.2,8,16
Furthermore, ultrafine-
School of Mechanical Engineering, Nanjing University of Science and
Technology, Nanjing, China
Corresponding author:
Zhenhua Wang, Nanjing University of Science and Technology, School
of Mechanical Engineering, Nanjing 210094, PR China.
Email: niatwzh17@163.com
Proc IMechE Part C:
J Mechanical Engineering Science
0(0) 1–9
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DOI: 10.1177/0954406220909849
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2. grained cemented carbides can produce superior
mechanical properties, thereby effectively improving
wear resistance.23
In our previous work, high-perfor-
mance ultrafine WC-based cemented carbide series
tool materials were prepared by SPS, and the density,
microstructure, and mechanical properties of the
materials were analyzed.24
In this paper, tribological
behavior of the ultrafine WC-based cemented carbides
is studied, which is of great practical significance for
the application.
Recently, significant attention has been given to the
study of friction and wear of cemented carbides using
different tribosystem configurations, such as the wet
sand/rubber wheel abrasion (ASTM G65),25
pin-
on-plate,26
and wheel wear.11
Jia et al.13
tested the
wear resistance of YG6 cemented carbides with a
diamond grinding wheel using the scratch method.
The results show that finer WC particles improve
the wear resistance of cemented carbides. Saito
et al.10
also demonstrated that the unit wear rate of
cemented carbides can be increased by increasing the
WC particle size. They performed dry friction tests
using a no. 45 carbon steel wheel and YG16 cemented
carbide block. In another study, Quercia et al.27
tested
the wear rate of WC-Co cemented carbides using a
pin-on-disc system with a constant sliding speed and
different loads. Their results suggest that the wear rate
and friction coefficient increase with increasing load.
Bonny et al.28
conducted sliding wear experiments
using a high-frequency TE77 pin-on-plate system
and found that increasing the vibration speed can
improve the friction coefficient and degree of wear.
Although several studies have been conducted on
wear mechanisms of micro-grained cemented carbides,
as well as the factors influencing wear resistance, only
a few scholars have examined the wear mechanism of
ultrafine-grained nano-crystalline cemented carbides
sintered by SPS. The amount of Co will affect the
mechanical properties and grain size of WC-Co
cemented carbides, in turn, affecting the wear resist-
ance of the material. The effect of Co content on the
wear resistance of ultrafine-grained WC-Co cemented
carbides is analyzed in this paper. The ultrafine cemen-
ted carbide tool is widely used for cutting bearing steel
and titanium alloys. Therefore, this study focused on
the tribological behavior of the ultra-fine cemented
carbide tool material in reciprocating sliding contact
against GCr15 steel (HRC 6266) and Ti-6Al-4V
titanium alloy (HRC 36). Sliding wear tests were per-
formed under constant speed and variable loading.
The wear rate, friction coefficient, and wear mechan-
ism of ultrafine-grained WC-Co cemented carbides are
presented.
Experimental procedures
Preparation of WC-base cemented carbide
WC (purity: 99.9%, 60 nm) and Co (purity: 99.9%,
600 nm) powders used in the experiment were pur-
chased from Shanghai Chaowei Nanotechnology
Co., Ltd. WC-based ultrafine cemented carbides
with different cobalt contents (4, 6, 8, 10, 12,
14 wt.%) were fabricated by SPS.24
Figure 1 shows
scanning electron microscope (SEM) images of the
WC and Co powders. Ultrafine cemented carbide
materials were sintered using the SPS system
(LABOXTM-650F, Japan) at a heating rate of
100
C/min, sintering pressure of 30 MPa, and hold-
ing time of 5 min under vacuum. The sintering tem-
perature was 1300
C. After that, samples were cooled
to room temperature (25
C) inside the furnace. The
WC-Co cemented carbides were then cut into bulks
with dimensions of 10 mm 10 mm 5 mm using an
electrical discharge machining. Surface roughness (Rz)
of the bulks was 0.15 0.05 lm. Before each test,
Figure 1. Representative SEM images of: (a) WC; and (b) Co powders.
2 Proc IMechE Part C: J Mechanical Engineering Science 0(0)

3. the bulks and balls were cleaned ultrasonically in an
alcohol solution for 10 min.
In general, the wear resistance of cemented
carbides increases with decreasing grain size and
increasing hardness.1,2,13,17
Properties of cemented
carbides with different Co contents are presented in
Table 1. Average grain size, fracture toughness, and
relative density were measured according to previ-
ously described protocols.24
From Table 1, hardness
decreases with increasing Co content and fracture
toughness, whereas WC grain size gradually increases
with increasing Co content.
Experimental method
Wear experiments were carried out in reciprocating
dry sliding conditions using a ball with a diameter
of 4 mm made of either GCr15 steel (HRC 6266)
or Ti-6Al-4V titanium alloy (HRC 36) as the counter-
body. The sliding velocity was 80 mm/s under a 40 N
load, applied by the ball-on-block friction testing
machine (UMT-2, USA). Reciprocating sliding
length l was 8 mm. Cross sections of the wear tracks
were obtained using a stylus profiler (Dektak XT,
Bruker, USA). Figure 2 shows the friction testing
equipment and testing process. The wear experiment
was conducted in air at room temperature (25
C).
The friction coefficient was directly measured by the
friction testing machine. The wear rate was evaluated
using the following formula:
W ¼ V= FL
ð Þ ¼ 8bd=FL ð1Þ
where W is wear rate (mm3
/Nm), b is wear width
(mm), d is average wear depth (mm), V is wear
volume of the sample (V ¼ lbd ¼ 8bd, mm3
), F is the
load applied to the bulks (N), and L is the total wear
track distance at the end of the experiment (mm). The
wear track volume was measured six times, and the
arithmetic mean was calculated as the final wear
volume. The depth and width of the wear tracks
were measured with the surface profile instrument.
Microstructures of the different materials were
observed using a SEM (Quant 250FEG, USA) with
an integrated energy-dispersive X-ray spectroscopy
(EDS) function.
Figure 2. Friction testing equipment and testing process.
Table 1. Properties of cemented carbides with different Co content.18
Composition Hardness (GPa)
Fracture toughness
(MPa. m1/2
)
WC grain
size (nm)
Relative
density (%)
WC-4 wt.%Co 23.2 0.35 10.45 0.23 227 97.6 0.2
WC-6 wt.%Co 21.47 0.25 11.2 0.12 258 97.8 0.2
WC-8 wt.%Co 19.87 0.23 12.27 0.21 314 98.0 0.2
WC-10 wt.%Co 18.32 0.30 13.16 0.30 356 98.5 0.1
WC-12 wt.%Co 17.79 0.26 13.69 0.15 362 98.6 0.1
WC-14 wt.%Co 17.48 0.22 15.46 0.28 374 98.6 0.1
YG8a
14.3 0.22 10.3 0.12 900 99.5 0.3
a
YG8 represents a commercial WC-8 wt.%Co cemented carbide (Zhuzhou diamond cutting tool Co., Ltd.).
Wang et al. 3

4. Results and discussion
Tribological behavior of cemented carbides in sliding
wear tests using GCr15 steel ball
Figure 3 shows the evolution of the friction coefficient
over time for different cemented carbides when the
GCr15 steel ball was used as the counterbody. As
shown in Figure 3, the wear process can be divided
into two stages: initial running-in of the wear system
and the later period of stable wear. During the run-
ning-in stage, point contacts occur between the grind-
ing ball and the sample as the contact surfaces are still
rough. Therefore, the friction coefficient fluctuates
greatly. As the friction process proceeds, material on
the convex cemented carbide surface is smoothed or
pressed into the cemented carbide, therefore, the fric-
tion coefficient tends to become more stable.17
Subsequently, a stable friction layer forms and the
friction pair enters the stable wear stage. The wear
rate is high during the running-in stage, which
should be as short as possible in order to improve
the service life of the workpiece. Thus, it can be con-
cluded that a higher Co content reduces the time
required to reach the stable wear stage.
Figure 4 shows the relationship between the fric-
tion coefficient and Co content. The friction coeffi-
cient tends to increase with an increase in Co
content. This change may be attributed to the softer
Co phase, which easily adheres to the grinding ball
during sliding friction, whereas WC grains with high
hardness are left behind. This creates a rough friction
surface and the friction coefficient increases.13
As shown in Figure 5, the wear rate increases as the
Co content increases, which is consistent with the
results of Saito.10
Cross sections of the wear tracks
are presented in Figure 6. The depth and width of
the wear tracks increase with the increase in Co
Figure 3. Evolution of friction coefficient with sliding time for
different cemented carbides during reciprocating sliding tests
using GCr15 steel ball.
Figure 6. Cross sections of wear tracks formed after
reciprocating sliding tests using GCr15 steel ball against
cemented carbides with different Co contents.
4 6 8 10 12 14
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.19
0.492 0.499
0.533 0.537
0.66
Friction
coefficient
Co content (wt.%)
Figure 4. Relationship between friction coefficient and
Co content based on reciprocating sliding tests using GCr15
steel ball.
4 6 8 10 12 14
-1
0
1
2
3
4
5
6
7
Wear
rate
(10
-8
mm
3
/Nm)
Co content (wt.%)
Figure 5. Relationship between wear rate and Co content
based on reciprocating sliding tests using GCr15 steel ball.
4 Proc IMechE Part C: J Mechanical Engineering Science 0(0)

5. content. This indicates that the increase in Co content
reduces the wear resistance of the material. The influ-
ence of Co content on the wear track can also be
observed in Figure 7. No obvious adhered material
or furrows appear on the contact surface (Figure
7(a)). Moreover, the lines on the surface resemble
water waves. Figure 8 shows the EDS analysis of
the cemented carbide with 4 wt.% Co (region A of
Figure 7(a)). The results indicate that elements on
the friction surface are W, C, Co, and Fe.
Therefore, when the Co content is low, some plastic
deformation occurs on the scratched surface, but
there is no evidence of adhesive wear or abrasive
wear. The reason might be that the bonding strength
between WC and Co is very high when the Co content
is low, rendering WC grains difficult to pull out. Only
slight plastic deformation occurs on the friction sur-
face. The bonding strength of the WC-Co interface
cannot be measured. However, as the Co content
increased, the bonding strength between the WC
grains and matrix decreased, likely because more
low-strength cobalt is present between WC grains.
Therefore, WC particles are more likely to be pulled
out, leading to abrasive wear.10
On the other hand, Co
is typically removed by furrow formation or plastic
deformation.17
In Figure 7(b), an obvious furrow phe-
nomenon can be observed when the Co content is
14 wt.%. Furthermore, the wear type is abrasive,
and the degree of wear is highest. No obvious wear
tracks were observed on the GCr15 steel ball.
Tribological behavior of cemented carbides in sliding
wear tests using titanium alloy ball
Figure 9 shows evolution of the friction coefficient
over time for different cemented carbides when the
titanium alloy ball was used as the counterbody.
The friction coefficient first increased rapidly and
then became stable. An obvious running-in stage
cannot be observed, and the friction pair directly
enters the stable wear stage. During the stable wear
stage, the friction coefficients of all cemented carbides
continued to fluctuate slightly. This phenomenon may
be attributed to continuous shedding and bonding of
the shed material to the friction layer. A significant
amount of material was also found adhered to the
wear track, which further validates our hypothesis.
The relationship between the friction coefficient and
Co content is shown in Figure 10. The friction coeffi-
cient first increased and then decreased with increas-
ing Co content. The friction coefficient is largest when
the Co content is 6 wt.% and smallest when the Co
content is 12 wt.%.
There is no obvious wear on the friction surface of
the cemented carbides because the hardness of titan-
ium alloy is lower than that of the cemented carbides.
In contrast, adhered material was found on the
Figure 7. Representative SEM images of wear tracks formed after reciprocating sliding tests using GCr15 steel ball against the
cemented carbides: (a) 4 wt.%Co; and (b) 14 wt.%Co.
Figure 8. EDS analysis of cemented carbide with 4 wt.%Co
(area A of Figure 7(a)) after reciprocating sliding tests using the
GCr15 steel ball.
Wang et al. 5

6. friction surface of the cemented carbides. The adhe-
sive wear volume first increased and then decreased
with increasing Co content, as shown in Figure 11.
The volume of adhered material is largest when the
Co content is 6 wt.% and lowest when the Co content
is 14 wt.%. From Figures 10 and 11, it can be seen
that the friction coefficient and adhesive wear volume
both reach maximum values when of the Co content is
6 wt.%. Furthermore, the friction coefficient decreases
as the adhesive wear volume increases.
Cross sections of the wear tracks (Figure 12) show
that when the Co content is less than 14%, there is no
wear and tear on the friction contact surface, how-
ever, some adhered material can be observed.
Besides, some small peaks were observed and were
highest when the Co content was 6 wt.%, suggesting
that the adhesive wear volume is greatest for this par-
ticular cemented carbide. In contrast, the adhesive
wear volume is smallest when the Co content is
14 wt.%. In Figure 12, the formation of wear grooves
can be clearly observed. This phenomenon may be
attributed to excess Co, which reduces the hardness
of the cemented carbide, enhances plastic deform-
ation, and makes the material on the surface easier
to remove.
Representative SEM images of the wear tracks are
presented in Figure 13. When the Co content is
6 wt.%, a considerable amount of adhered material
Figure 9. Evolution of friction coefficient with sliding time for different cemented carbides during reciprocating sliding tests with
titanium alloy ball: (a) 4 wt.%Co; (b) 6 wt.%Co; (c) 8 wt.%Co; (d) 10 wt.%Co; (e) 12 wt.%Co; and (f) 14 wt.%Co.
4 6 8 10 12 14
0.30
0.35
0.40
0.45
0.50
0.55
Friction
coefficient
Co content (wt.%)
Figure 10. Relationship between friction coefficient and Co
content based on reciprocating sliding tests with titanium alloy
ball.
Figure 11. Relationship between Co content and adhesive
wear volume based on reciprocating sliding tests with titanium
alloy ball.
6 Proc IMechE Part C: J Mechanical Engineering Science 0(0)

7. can be observed on the friction surface of the cemen-
ted carbides, but no furrows are present. Results of
the EDS analysis of adhered material in area B of
Figure 13(a) are presented in Figure 14. The adhered
material is assumed to be mainly titanium alloy since
the titanium content was up to 93.84%. As shown in
Figure 13(b), less adhered material can be observed on
the wear track when the Co content is 10 wt.%.
Figure 13. Representative SEM images of wear tracks formed after reciprocating sliding tests of titanium alloy against cemented
carbides: (a) 6 wt.% Co; (b) 10 wt.% Co; (c) 14 wt.% Co; and (d) Ti-6Al-4V titanium alloy ball.
0 200 400 600 800 1000 1200 1400 1600 1800 2000
-5
0
5
10
15
20
25
30
35
40
45
14 wt.% Co
12 wt.% Co
10 wt.% Co
8 wt.% Co
6 wt.% Co
Thickness
of
adhesive
wear
layer
(μm)
Distance from scratch boundary (μm)
4 wt.% Co
Figure 12. Cross sections of wear tracks formed after reci-
procating sliding tests with titanium alloy ball against cemented
carbides with different Co contents.
Figure 14. EDS analysis of cemented carbide with 6 wt.% Co
(area B of Figure 13(a)) after reciprocating sliding tests using
titanium alloy ball.
Wang et al. 7

8. In addition, wear grooves formed on the surface of
the cemented carbide with 14 wt.% Co (Figure 13(c)).
The morphology of the worn titanium alloy surface
was also examined (Figure 13(d)). Many wide
scratches can be observed on the surface of the titan-
ium alloy ball, which may have been caused by the
friction between material adhered to the surface and
the titanium alloy ball. Moreover, many peeling layers
can also be observed, mainly due to adhesive wear
which causes the bonded material on the worn titan-
ium alloy surface to peel off.
Wear properties of the cemented carbides in reci-
procating sliding against two different grinding balls
are summarized in Table 2. Comparing the wear
resistance of WC-8 wt.% Co cemented carbide with
commercially available YG8 cemented carbide, it
can be concluded that the cemented carbides prepared
for this study offer superior wear resistance.
Conclusions
Ultrafine-grained WC-Co cemented carbides with
desirable mechanical properties and excellent wear
resistance were fabricated by SPS. The effect of the
Co content on the mechanical properties and wear
mechanisms were studied. The main conclusions can
be summarized as follows:
1. When the counterbody is a GCr15 steel ball,
cemented carbides with higher Co content reach
the stable wear stage in less time. The friction coef-
ficient and wear rate tend to increase with an
increase in Co content. The wear type is abrasive
wear.
2. When the counterbody is a Ti-6Al-4V titanium
alloy ball, the wear process directly enters the
stable wear stage without any obvious running-in
stage. Continuous shedding of material and bond-
ing of the material to the friction surface cause
slight fluctuations of the friction coefficient in the
stable wear stage. The main wear type is adhesive
wear. The friction coefficient and adhesive wear
volume first increase and then decrease as the Co
content increases.
3. Compared with the commercially available YG8
cemented carbide, the ultrafine-grained WC-Co
cemented carbides prepared in this study offer
superior wear resistance.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of
this article.
Funding
The author(s) disclosed receipt of the following financial
support for the research, authorship, and/or publication
of this article: This study was funded by the National
Natural Science Foundation of China (Grant Number
51775280), Excellent Youth Foundation of Jiangsu
Province of China (BK20190070), and Jiangsu Provincial
Six Talent Peaks Project (Grant Number 2016-HKHT-019).
ORCID iD
Zhenhua wang https://orcid.org/0000-0002-0405-7165
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