Powder Metallurgy Progress, Vol.13 (2013), No 3-4
109
EFFECT OF STATIC PARTNER MATERIAL ON WEAR
CHARACTERISTICS OF YTTRIA STABILIZED ZIRCONIA
J. Balko, A. Kovalčíková, P. Hvizdoš
Abstract
Tribological testing and wear damage analysis of yttria stabilized
tetragonal zirconia was carried out using the ball-on-disk method. The
testing was performed at room temperature in air for dry sliding with
different types of ball material (Al2O3, ZrO2, WC-Co, and Si3N4). The
coefficient of friction was measured with a load 8 N. Material loss was
evaluated using confocal microscopy in terms of volume removed, and
specific wear rates were calculated. The measured values of the
coefficient of friction were in good agreement with the values in literature
for corresponding tribological material pairs. They were lower (0.65) for
WC-Co balls, medium (0.73) for the Si3N4 and Al2O3, and higher (0.8) for
ZrO2 balls. The highest wear rates were found for the ZrO2/ZrO2 pair
which is attributed to the chemical similarity of both tribological
partners.
Keywords: wear, friction coefficient, static partner, sliding distance,
ball-on-disk
INTRODUCTION
Tribology is the science concerned with studying the processes of friction and
wear. In practical circumstances the friction of materials leads to their surface damage and
wear. By studying and understanding the mechanisms of wear it is possible to predict the
behavior of materials in various applications. Structural ceramic materials have
useful tribological properties, such as low density, high corrosion
resistance, low coefficient of friction, low thermal expansion and high
hardness over a wide range of temperature. Because of these advantages of
ceramic materials, their usage is increasing in many of the engineering
applications where strong mechanical contact takes place [1-3].
Modern mechanical components and tribosystems must possess the ability to
withstand their severe operating conditions, they have to have higher reliability and an
excellent performance of their functions. These requirements are related to wear and
surface damage of interacting surfaces. Therefore, wear behavior of ceramics have been
studied for many years [4].
Polycrystallic zirconia is a ceramic material characterized by high values of
fracture toughness, strength and chemical resistance which makes it an attractive material
for its use in extreme conditions. It is used as a refractory material for the lining of melting
furnaces, the matrix wire drawing, cutting material and as a biomaterial for joint, bone and
dental replacements.
Ján Balko, Alexandra Kovalčíková, Pavol Hvizdoš, Institute of Materials Research, Slovak Academy of Sciences,
Watsonova 47, Košice, Slovak Republic
Powder Metallurgy Progress, Vol.13 (2013), No 3-4
110
ZrO2 occurs in three allotropic modifications: monoclinic (m-ZrO2) lowtemperature phase existing at temperatures below 1060C, tetragonal (t-ZrO2) mediumtemperature phase occurring at temperatures from 1060C to 2370C, and cubic (c-ZrO2)
high-temperature phase stable at temperatures above 2370C. The phase transformation
from t-ZrO2 to m-ZrO2 is accompanied by an increase of volume by 4-5% [5]. This causes
local microscopic stresses which can be accompanied by the formation of tension cracks.
ZrO2 can be stabilized by adding various oxides (e.g. Y2O3, MgO, CaO). In this way, a
metastable microstructure where residual compressive stresses occur is formed. Such
material can achieve KIC up to 14 – 20 MPa·m-1/2 which is the highest fracture toughness
among monolithic ceramic materials [6].
However, research on the tribological behavior of structural ceramic materials and
optimal operating conditions for ZrO2 are hard to find. Therefore, this study was made to
collect data on operating conditions of wear against ZrO2 and to investigate the wear
behavior and tribofilm formation among four different structural ceramic couples at room
temperature in unlubricated sliding [1].
There are many types of experimental set-ups used to more or less closely mimic
the real practical situations which differ in the used geometry and loading conditions. In
laboratories, one of the most frequently used techniques is a pin-on-disk. If the pin has a
spherical shape of the contact part, it is called also ball-on-disk. The measurement is based
on the friction of the ball on a sample on a circle track. By measuring the geometry and
volume of the wear track, we can study specific wear rate. The linear dependence of wear
rate on volume loss can be found.
The aim of this work is to measure the basic tribological characteristics
(coefficient of friction, wear rate) of zirconia using different ball materials and different
loads. An additional goal was to verify whether the volume loss can really be considered to
be a linear function of sliding distance, i.e. the wear rate remains constant over the duration
of the test.
EXPERIMENTAL MATERIALS
The experimental material was zirconia stabilized by 3 mol% yttria (3Y-TZP). It
was prepared using a commercial powder TZ-3Y- SB, by TOSOH Corp. The powder was
cold isostatically pressed at 200 MPa for 3–5 min, then pressurelessly sintered at 1450 °C
for 2 h in air. The resulting samples had forms of cylindrical bars with diameters of about 9
mm and length 60 mm. The cylinders were further cut by diamond saw, then ground and
polished down to 1 μm finish by diamond paste, so that final test specimens were formed of
disks of 1.5–2 mm thick with ~9 mm diameter [7]. As tribological static partners four ball
materials were used (Al2O3, ZrO2, WC-Co, Si3N4), all supplied by Spheric Trafalgar Ltd.,
Ashington UK.
EXPERIMENTAL METHODS
Vickers hardness was measured under a load of 49.05 N (5 kg). Fracture toughness
was calculated from indentation crack lengths using the Anstis equation [8].
 E  P 
(1)
K IC  0.016    3 / 2 
 H  c 
where E is modulus of elasticity, E is hardness, P is indentation load and c is the crack
length.
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Powder Metallurgy Progress, Vol.13 (2013), No 3-4
The wear behaviour of the experimental materials was studied in dry sliding in the
air at room temperature by means of tribometer THT (CSM Instruments, Switzerland) using
the pin-on-disk technique, against a highly polished (roughness Ra < 0.25 µm) Al2O3,
Si3N4, WC-Co and ZrO2 balls with 6 mm diameter. The applied load was 8 N, the sliding
speed 0.05 m/s and the total sliding distance was 400 m. Every 100 m the test was
interrupted and the volume loss was evaluated. The testing was then resumed while great
care was devoted to keeping the experimental set-up so that the geometry of the contact
before and after the interruption was precisely maintained. The tangential forces during the
test were measured and friction coefficients calculated. The material losses (volume of the
wear tracks) due to wear were measured by a high precision confocal microscope PLu neox
3D Optical Profiler, by SENSOFAR, and then specific wear rates (W) were calculated in
terms of the volume loss (V) per distance (L) and applied load (Fp) according to the
standard ISO 20808 [9]:
V  mm3 
(2)
W
L.Fp  m.N 
At the same time, the worn cap on the counter body was measured by a light microscope,
its volume and then its wear rate calculated [9, 10].
RESULTS AND DISCUSSION
The tetragonal microstructure of the material (t-ZrO2) consisted of two grain-size:
66% of the material consisted of grains with a size of 0.5 μm – 1.7 μm, the rest were grains
with size 2.5 μm – 5 μm. Microstructure of this material was studied and described in detail
elsewhere [7].
Basic mechanical properties (hardness, elastic modulus, fracture toughness) of the
balls and samples are shown in Table 1.
Tab.1. Basic mechanical properties of all ball materials and samples.
Material
HV 5 [GPa]
KIC [MPa·m-1/2]
E [MPa]
Ball Al2O3
16.8 ± 1.2
5.03 ± 0.12
370 ± 27
Ball Si3N4
17.5 ± 1.1
6.74 ± 0.14
320 ± 29
Ball WC-Co
16.3 ± 1.4
13.75 ± 0.27
450 ± 24
Ball ZrO2
13.2 ± 2.1
6.77 ± 0.14
220 ± 15
Sample ZrO2
12.55 ± 2.0
6.61 ± 0.19
200 ± 12
Coefficients of friction (COF) for all the experimental material pairs were obtained
as a function of sliding distance (or time). The tribological measurements showed the
unstable initial phase of friction curves, during which stabilization of the system took place,
Fig.1. This phase was relatively short, particularly for Si3N4 and ZrO2 balls, where less than
2 m was sufficient. In these two cases the COF was so stable that even the interruptions of
the testing run did not lead to changes in the friction and they did not have any observable
effect on the COF line. In the other two cases the run-up phases were longer, a few tens of
meters, and also after interrupting the test some changes in friction could be observed,
Fig.1. This was particularly pronounced in the case of WC–Co as the static partner. Here
the coefficient of friction was stabilizing over a longer distance than brittle materials,
probably because of the presence of ductile phase – cobalt.
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Powder Metallurgy Progress, Vol.13 (2013), No 3-4
1,0
0,9
0,8
0,7
COF [-]
0,6
0,5
0,4
0,3
Al2O3
Si3N4
WC-Co
ZrO2
0,2
0,1
0,0
0
100
200
300
400
Sliding distance [m]
Fig.1. Coefficient of friction curve of each material during the tests.
Table 2 shows the values of COF, averaged over the stable parts of COF lines.
Satchowiak et al. [11] specifies the COF of ZrO2/ZrO2 pair as ~0.75 which is slightly lower
than in our case which can be caused using a lower load. The highest coefficient of friction
was observed with ZrO2 ball due to the chemical similarity with tested material, then Al2O3
and Si3N4. The lowest COF was in a test using WC-Co ball due to the presence of the softer
ductile metallic phase.
Tab.2. Friction coefficients of each sample/static partner pair.
Material
COF [-]
Ball Al2O3
0.72 ± 0.15
Ball Si3N4
0.73 ± 0.09
Ball WC-Co
0.65 ± 0.21
Ball ZrO2
0.81 ± 0.18
When calculating wear rates it is usually assumed that the wear rate is constant
over the sliding distance, as it is the case also in the standard [10]. Our interrupted testing
intended to assess how reliable such an assumption is. An example of evolution of the wear
track with the sliding distance is shown in Fig.2 where the steady growth, wear track width,
depth and area can be seen. Almost linear growth of the wear track profile area for all
experimental tribological pairs can be seen in Fig.3.
Evaluation of the volume loss showed its linear increase with the sliding distance.
Consequently, the wear rate (W ≈ V/L) is really basically constant, which means it is a
quantity useful for evaluation of the obtained results, as is stipulated in the ISO 20808
standard.
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2
0
Track depth [m]
-2
-4
100 m
-6
200 m
-8
- 10
300 m
- 12
- 14
400 m
- 16
- 18
0
200
400
600
800
1000 1200
1400
1600 1800 2000 2200
Track width [m]
Fig.2. Evolution of wear track in ZrO2/ZrO2 test.
24000
Al2O3
Si3N4
WC-Co
ZrO2
20000
Wear track profile [m]
16000
12000
8000
4000
800
600
400
200
0
100
150
200
250
300
350
400
Sliding distance [m]
Fig.3. Wear track profile area as a function of the test sliding distance for each experimental
tribological pair.
The highest wear rate (3.62 x 10-5 mm3/N·m) was observed for tribological
partners ZrO2/ZrO2 because of chemical similarity and therefore an active tribo-chemical
reaction can be assumed to occur. Chih-Chung [12] reported the wear rate of the
ZrO2/ZrO2 pair as 2.5 mm3/N·m which is in good agreement with our results. The wear
rate of zirconia/zirconia tribo-pair was about two orders of magnitude higher than the other
tribological pairs. The wear was less intense for Al2O3 (5.55 x 10-7 mm3/N·m), than the
Si3N4 ball (4.68 x 10-7 mm3/N·m), and the lowest one was observed for WC-Co (1.60 x 10-7
mm3/N·m) Fig.4.
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Powder Metallurgy Progress, Vol.13 (2013), No 3-4
1E-5
Al2O3
Si3N4
WCCo
ZrO2
3
Sample wear rate [mm /N.m]
1E-4
1E-6
1E-7
1E-8
100
150
200
250
300
350
400
Sliding distance [m]
Fig.4. Wear rates of ZrO2 disks in tests with different static partners in various testing
stages.
Ball wear rates show that the highest wear has the zirconia ball due to the chemical
similarity with the sample. The lowest values of wear were for the Al2O3 and WC-Co balls.
Fig.5. This is most probably connected to the very high chemical stability of the alumina on
one hand and to the lower friction (as shown by lower COF) of WC-Co on the other hand.
1
3
Ball wear rate [mm /N.m]
0,1
0,01
1E-3
1E-4
1E-5
Al2O3
Si3N4
WC-Co
Material
Fig.5. Ball wear rates of all static partners.
ZrO2
Powder Metallurgy Progress, Vol.13 (2013), No 3-4
115
CONCLUSION
The tribological characteristics were studied: the coefficient of friction, specific
wear rate. Tests were realized on ZrO2 disks with ZrO2, Si3N4, Al2O3 and WC-Co balls as
friction counterparts. The measured values of friction and the calculated values of specific
wear rate are in agreement with the literature data.
• The lowest COF had a ZrO2/WC-Co pair, 0.61, thanks to the presence of a metallic
phase.
• The highest values of both friction (0.81 ± 0.18) and wear were observed in tests with the
ZrO2 ball, because of chemical similarity and tribo-chemical reaction of the surfaces in
contact. In the other tribological pairs the wear rates of the ZrO2 disk were similar.
• Wear rates of the alumina and WC-Co balls were the lowest probably due to their high
chemical stability (Al2O3) and lower friction (WC-Co), respectively.
• Wear rate after each sliding period was calculated and it was shown that it can be
considered to be a constant over the test sliding distance.
Acknowledgements
This work was supported by the Slovak Grant Agency, projects VEGA 2/0075/13,
VEGA 2/0043/14 and APVV-0108-12.
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