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Tribotechnical Characteristics of Commercially Pure Titanium with Different
Grain Sizes and TiC and TiO2 Coatings
Article  in  Journal of Friction and Wear · July 2019
DOI: 10.3103/S1068366619040111
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ISSN 1068-3666, Journal of Friction and Wear, 2019, Vol. 40, No. 4, pp. 349–354. © Allerton Press, Inc., 2019.
Russian Text © The Author(s), 2019, published in Trenie i Iznos, 2019, Vol. 40, No. 4, pp. 446–453.
Tribotechnical Characteristics of Commercially Pure Titanium
with Different Grain Sizes and TiC and TiO2 Coatings
V. I. Semenova,
*, D. B. Alemayehub
, L. Sh. Schustera
, G. I. Raaba
, S. V. Chertovskikha
,
V. V. Astanina
, S.-J. Huangb
, and I. N. Chernyakc
aUfa State Aviation Technical University, Ufa, 450008 Russia
b
National Taiwan University of Science and Technology, Taipei, 106 Taiwan, R.O.C.
c
Institute of Powder Metallurgy, Minsk, 220005 Belarus
*e-mail: semenov-vi@rambler.ru
Received August 20, 2018; revised May 3, 2019; accepted May 4, 2019
Abstract—The paper presents experimental data on the determination of the tribological properties of com-
mercially pure titanium with different microstructures, with and without a coating. As a result of the per-
formed experiments, it was established that the integral value of the friction coefficient, as well as its adhesive
component, are structurally sensitive parameters. It is noted that the ultrafine-grained structure obtained as
a result of severe plastic deformation contributes to a reduction in the friction coefficient, as well as to an
increase in the load-bearing capacity of the triboconjugation.
Keywords: friction coefficient, strength of adhesive bonds, hardness, severe plastic deformation, microarc
oxidation, ion-plasma spraying
DOI: 10.3103/S1068366619040111
INTRODUCTION
According to experts’ estimates, it is promising to
use in medicine (for implants, instruments, fasteners,
etc.) strain-hardened commercially pure (CP) Ti [1].
To a great extent, this is conditioned by the fact that
CP Ti has a high biocompatibility, bioinertness, hypo-
allergenicity, and is also non-toxic [2, 3]. The disad-
vantage of CP Ti is its relatively low strength as com-
pared to doped Ti alloys, such as the Grade 5 Ti alloy.
The applied technologies of deformation processing
enable producing a high-strength state due to the for-
mation of an ultrafine-grained (UFG) microstructure
contributing to the enhancement of mechanical and
fuctional properties [4].
There are works reporting the fabrication of semi-
products from CP Ti for medical applications with a
UFG structure, using high-pressure torsion (HPT)
[5, 6] with a subsequent application of a coating from
titanium nitride [5] and diamond-like carbon with Zr
[6]. This produces a high strength in the material,
together with improved tribological properties. How-
ever, one of the disadvantages of this type of process-
ing is the small size of samples—diameter up to 5–
8 mm and thickness below 1 mm, which creates signif-
icant limitations for their further use.
Thus, very interesting for structural applications
are technologies that enable producing bulk (large-
sized) UFG materials with the required physico-
mechanical, performance and fuctional properties
[1, 2, 7, 8]. These technologies are based on the equal-
channel angular pressing (ECAP) processes [7, 9], in
particular, on the ECAP-Conform process used to
produce UFG structures in large-sized long-length
metallic materials.
In addition, of practical interest are comparative
tribological studies performed on the material with
different microstructures, with and without coatings.
The present study is useful due to the fact that prac-
tically all products from strain-hardened CP Ti for
medical applications are exposed to frictional contact
to some extent.
The aim of this study is a comparative evaluation of
the tribological properties of CP Ti depending on the
microstructure and the type of coating applied on the
surface.
MATERIALS AND METHODS
The material under study, Grade 4 CP Ti, having a
UFG structure was produced by a severe plastic defor-
mation (SPD) technique, ECAP-Conform, shown in
Fig. 1. The SPD processing for 6 cycles was performed
at room temperature via route Вс – with rotation of
the workpiece around its axis by 90° after each cycle.
The average grain size of the processed material was
about 300 nm. Samples with a coarse-grained (CG)
microstructure were produced by annealing at 600°С

350
JOURNAL OF FRICTION AND WEAR Vol. 40 No. 4 2019
SEMENOV et al.
for 1.5 h, and the average grain size of the as-annealed
material was about 20 μm.
The methods for the evaluation of the friction coef-
ficient and its adhesive component are described
below. For the tribological studies, we used two proce-
dures shown in Fig. 2.
The first procedure involving reciprocating motion
(Fig. 2a) was used to evaluate the friction coefficient in
the pairs “Grade 4 CP Ti–Fe-1.5Cr-1.0C chromium
bearing steel”.
We used for the tests parallelepiped-shaped sam-
ples with a length of 25 mm and a section of 9.5 ×
9.5 mm. We used an indenter from the Fe–1.5Cr–
1.0C bearing steel, having a spherical contact surface
with a diamteter of 3 mm. The test conditions were as
follows: room temperature; the motion amplitude
under a normal load of 5 N was 20 mm under
5000 cycles. The motion rate was 30 cycles/min.
The second procedure (Fig. 2b) was used to evalu-
ate the shear strength of adhesive bonds and the
molecular component of the friction coefficient. We
used for the tests parallelepiped-shaped samples with
a section of 9.5 × 9.5 mm and a thickness of 5 mm. A
spherical indenter with a sphere radius of 2.5 mm was
made of the Fe–1.5Cr–1.0C bearting steel. The tests
for the evaluation of the shear strength of adhesive
bonds were performed using a one-ball adhesion tester
according to the procedure shown in Fig. 2b.
All the tribological tests were performed at room
temperature, without any lubricants.
Underlying the method for the evaluation of the
shear strength of adhesive bonds is the physical model
that in a first approximation reflects the actual friction
conditions in a local contact [10]. According to this
model, a spherical indenter 1, compressed by two
plane-parallel samples 2 and 3, is rotated under a load
around its axis. The force F expended on the intenter’s
rotation is mainly related to the shear strength of adhe-
sive bonds, τn.
For the sake of comparison, in the tests performed
according to both procedures, one group of samples
was uncoated, a TiC coating was applied on the sur-
face of the second group of samples using ion-plasma
spraying (IPS), and the surface of the third group of
samples was treated by the microarc oxidation (MAO)
technology producing TiO2 titanium dioxide.
The shear strength of adhesive bonds, (MPa),
was determined from the relation:
(1)
where d1,2 are the diameters of the impressions on the
tested samples, m; М is the moment of the indenter’s
rotation, N mm.
The adhesive (molecular) component of the fric-
tion coefficient was determined as:
(2)
where pr is the rated pressure, MPa
(3)
where Р is the compression force of the samples, N.
RESEARCH RESULTS
AND THEIR DISCUSSION
The results of the tribological tests according to the
first procedure (Fig. 2a) in the following friction pairs:
“Grade 4 CP Ti without a coating – Fe–1.5Cr–1.0C
chromium bearing steel”; “Grade 4 CP Ti with an IPS
nτ
3
1,2
0.75 ,
2
n
M
d
τ =
⎛ ⎞
π⎜ ⎟
⎝ ⎠
,п
M
r
f
р
τ
=
2
1,2
2
r
Рр
d
=
⎛ ⎞
π⎜ ⎟
⎝ ⎠
Fig. 1. Principle of the ECAP-Conform process for the
fabrication of long-length semi-products with an ultraf-
ine-grained structure. (1) movable die; (2) stationary die;
(3) workpiece.
1 3
2
Fig. 2. Reciprocating test procedure: (a) (1) spherical
indenter; (2) tested sample; test procedure to evaluate the
shear strength of adhesive bonds and the molecular com-
ponent of the friction coefficient: (b) (1) spherical
indenter; (2 and 3) tested samples.
P
1
2
1
3
2
M
P
(a) (b)

TRIBOTECHNICAL CHARACTERISTICS OF COMMERCIALLY PURE TITANIUM 351
coating – Fe–1.5Cr–1.0C chromium bearing steel”;
“Grade 4 CP Ti with a MAO coating – Fe–1.5Cr–
1.0C chromium bearing steel” are shown in Fig. 3.
As it can be seen from the presented relationships,
the friction coefficient values for the as-annealed sam-
ples, both with and without a coating (curves 1–3), are
higher than those for the samples processed by SPD
for 6 cycles (curves 1'–3'). It should be noted that the
greatest effect, in terms of the relative reduction of the
friction coefficient, is observed on the uncoated sam-
ples having a UFG microstructure after deformation
processing (curves 1, 1'). For the sample having a CG
microstructure, after 5000 cycles of the sample’s sur-
face being exposed to testing under reciprocating
motion according to the procedure shown in Fig. 2a,
the friction coefficient is about 0.6. Meanwhile, for
the UFG sample processed by SPD for the same num-
ber of cycles, it decreased approximately by a factor of
1.5, amounting to 0.4, which is a rather significant
result.
We do not observe a sharp decrease in the friction
coefficient for the samples having different microstruc-
tures with applied coatings (curves 2, 2' and 3, 3').
However, a slight reduction is revealed. Apparently,
this is related to the fact that the tribological properties
of CP Ti are more heavily influenced by a coating itself
than by a change in the rheological properties of the
substrate that a coating is applied on. It has been found
that the TiO2 coating formed on the CP Ti surface by
the MAO technology is more preferable in terms of tri-
bological properties (curves 3, 3'). In addition, it
should be noted that when this coating is used, a
shorter run-in portion is observed, especially for the
material having a UFG microstructure. This property,
conditioned by the high tribological properties of a
MAO coating, is very important and attractive for med-
ical implants that are exposed to frictional contact.
The lowest values of the friction coefficient, as
noted above, are observed on the sample having its
sufrace treated by the MAO technology, resulting in
the formation of TiO2 titanium dioxide.
Analysis of Fig. 4 reveals a similarity in the mor-
phology of friction tracks on the samples from Grade
4 CP Ti without a coating (Fig. 4a) and with an ion-
plasma coating (TiC) (Fig. 4b). Apparently, this is due
to the fact that in the accepted conditions of the phys-
ical experiment there occurs an intensive abrasion of
the coating and baring of the substrate material—
Fig. 3. Dependence of the friction coefficient f on the
number of cycles, N (numbers without a prime designate a
material with a CG structure; numbers with a prime desig-
nate a material with a UFG structure): (1–1') Grade 4 CP
Ti; (2–2') Grade 4 Ti with an IPS coating (TiC);
(3‒3') Grade 4 CP Ti with a coating produced by the
MAO technology (TiO).
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
500 1500 25001000 2000 3000 3500 4000
1
2
3
3'
1'
2'
f
N, cycles
Fig. 4. Friction tracks produced during the tribological tests according to the reciprocating motion procedure: (a) uncoated Grade
4 CP Ti having a CG structure; (b) Grade 4 CP Ti having a CG structure with an IPS coating (TiC); (c) Grade 4 CP Ti having a CG
structure with a coating produced by the MAO technology (TiO); (d, e, f) similar, but with a UFG structure (×3 magnification).
(a) (b) (c)
(d) (e) (f)

352
SEMENOV et al.
Grade 4 CP Ti. Meanwhile, the friction track shown in
Fig. 4c, produced on the sample with a TiO2 coating
formed by the MAO technology, is an even track with-
out any ruptures. This indicates the preserved integrity
of the coating and its high strength, and consequently,
a higher load-bearing capacity under these experi-
mental conditions.
Visual analysis of the friction tracks produced
during the tribological tests on the samples having a
UFG microstructure with applied coatings (see
Figs. 4e, 4f) yields approximately the same results as
for the samples having a CG microstructure (see
Figs. 4b, 4c). The friction tracks on the uncoated sam-
ples having CG and UFG microstructures are slightly
different. For instance, the friction track on the sam-
ple having a UFG microstructure (Fig. 4e) is more
“blurred” as compared to the friction track on the
sample having a CG microstructure (Fig. 4a). Appar-
ently, the “blurring” of the friction track on the UFG
sample is related to the higher strength of the contact
surface resulting from SPD processing by ECAP-
Conform.
Thus, in terms of the tribological efficiency, of
greatest interest are the TiO2 coating produced by the
microarc oxidation technology, and the material (in
this particular case, Grade 4 CP Ti) having a UFG
microstructure.
Evaluation of the Strength of Adhesive Bonds
and Finding the Adhesive Component
of the Friction Coefficient
It is very interesting to study the shear strength of
adhesive bonds depending on pressure. Knowing the
results of this study, one can calculate the adhesive
component of the friction coefficient according to for-
mula (2). Figures 5 and 6 show the results from the
evaluation of the shear strength of adhesive bonds for
the investigated material, having a CG microstructure
in the as-annealed state and a UFG microstructure,
with different types of surface treatment.
Figure 5 shows the impressions made on the tested
samples by the indenter.
The presented images demonstrate that in the
indentation cup on the surface of the uncoated sample
(Fig. 5а) there are bared surfaces due to the formation
of adhesive bridges with the indenter’s material. On
the surface of the sample with a TiC coating applied by
ion-plasma sparying (Fig. 5b) there are also bared
areas (bright regions in the photo), also caused by the
adhesive interaction between the sample’s material
and the indenter’s material. The most even and clean
impression is found on the sample with a TiO coating
(c) produced by the MAO technology. The samples
having a UFG microstructure (see Figs. 5d, 5e, 5f),
where the same types of surface treatment were used,
exhibit approximately the same effects as the samples
having the CG structure. The main difference between
these samples is the smaller diameters of the indenta-
tion cups under the same normal load, which is condi-
tioned by the higher hardness of the investigated mate-
rial after SPD processing by ECAP-Conform. Conse-
quently, the higher hardness determines the higher
load-bearing capacity of the triboconjugation.
As a result of the tribological tests and the process-
ing of the obtained data using formulae (1) and (3), we
built the dependencies between the shear strength of
Fig. 5. Impressions made by the indenter on the CG samples of Grade 4 CP Ti with different surface treatments: (a) without a
coating; (b) with a TiC coating applied by ion-plasma spraying; (c) with a TiO2 coating produced by the MAO technology; and
on the UFG samples: (d) without a coating; (e) with a TiC coating; (f) with a TiO2 coating (×10 magnification).
(a) (b) (c)
(d) (e) (f)

TRIBOTECHNICAL CHARACTERISTICS OF COMMERCIALLY PURE TITANIUM 353
adhesive bonds and the rated pressure, shown in
Fig. 6.
The table 1 below presents the results from the
comparative evaluation of the tribological characteris-
tics in the frictional contact “Fe–1.5Cr–1.0C chro-
mium bearing steel—CP Ti” with different micro-
structures and surface treatments.
Analyzing the table values and the graphic depen-
dencies shown in Fig. 6, it is found that the lowest
shear strength of adhesive bonds is observed on the
samples having a CG structure and a UFG structure,
coated with titanium oxide by the MAO technology.
The load-bearing capacity of the UFG material is
higher than that of the material having a CG micro-
structure (see Fig. 6, dependencies 5 and 6). It can be
seen from the presented results that the highest shear
strength of adhesive bonds is observed on the coarse-
grained material without a coating (dependence 1).
The TiC coating on the substrate having a CG struc-
ture (dependence 3) exhibits a slightly lower shear
strength of adhesive bonds and a higher load-bearing
capacity.
In the considered conditions, the lowest shear
strength of adhesive bonds is in the contact pair “Fe–
1.5Cr–1.0C chromium bearing steel – Grade 4 CP Ti
with an oxidized surface treated by the MAO technol-
ogy” (dependencies 5 and 6). For all the variants of
tested samples, the material having a UFG micro-
structure (dependencies 2, 4 and 6) exhibit a higher
load-bearing capacity, as compared the samples hav-
ing a CG microstructure with similar surface treat-
ments (dependencies 1, 3 and 5). This is related to the
strain-induced strength enhancement of the material
under study – Grade 4 CP Ti.
The mutually correlating data were obtained when
we found the integral value of the friction coefficient,
and when we evaluated the adhesive component of the
friction coefficient.
In addition, from analysis of the table and Fig. 6 it
was established that the adhesive component of the
friction coefficient (for the used friction pair) is prac-
tically the same for the samples with a CG microstruc-
ture and with a UFG microstructure, in case their sur-
face has an oxide coating produced by the MAO tech-
nology. This observation indicates that it is the MAO
coating that has the predominant effect, and not the
rheological surfaces of the substrate conditioned by
the material’s different structural states. From this it
follows that the TiO2 coating produced by the MAO
technology on the surface of CP Ti can be efficiently
used under sliding contact, in both CG and UFG
materials.
Fig. 6. Effect of pressure on the shear strength of adhesive bonds for samples with different microstructures: (1) CG (without a
coating); (2) UFG (without a coating); (3) CG with a TiC coating; (4) UFG with a TiC coating; (5) CG with a TiO2 coating;
(6) UFG with a TiO2 coating.
200
400
0 400 800 1200 1600 2000 2400 2800
1 3
2 4
5
6
1
2
3
4
5
6
Wn, MPa
pr, MPa
Table 1. Effect of the structural state of CP Ti and coatings on the tribological characteristics under extreme loading con-
ditions
Here, β is the coefficient of strengthening of molecular bonds under compressive stresses, τ0 is the shear strength of adhesive bonds
in the absence of a normal load.
Structural state and surface treatment type
Tribological characteristics
pr, MPa τn, MPa τn/prn β τ0, MPa
1. CG microstructure (without a coating) 1605 376 0.235 0.208 43
2. UFG microstructure (without a coating) 1988 376 0.189 0.166 47
3. CG microstructure with TiC coating 2158 447 0.207 0.189 39
4. UFG microstructure with TiC coating 2311 266 0.115 0.112 8
5. CG microstructure with TiO2 coating 2480 189 0.076 0.064 31
6. UFG microstructure with TiO2 coating 2937 221 0.075 0.061 42

354
SEMENOV et al.
Thus, it can be concluded that from the considered
options, the most preferable type of surface treatment
for CP Ti, in terms of producing high tribological
properties together with relatively low values of the
shear strength of adhesive bonds and a high load-bear-
ing capacity of the triboconjugation, is the TiO2 oxide
coating applied by microarc oxidation in combination
with SPD processing by ECAP-Conform.
DESIGNATIONS
M moment
d1,2 diameters of the impressions on the tested
sample
fm adhesive component of the friction coeffi-
cient
τпп shear strength of adhesive bonds
pr rated pressure in the frictional contact
P compression force
UFG ultrafine-grained (structure)
CG coarse-grained (structure)
HPT high-pressure torsion
ECAP equal-channel angular pressing
IPS ion-plasma spraying
MAO microarc oxidation
ACKNOWLEDGMENTS
ThisresearchwassupportedbytheMinistryofScienceand
Higher Education under grant agreement no. 14.586.21.0059
(unique project identifyer RFMEFI58618X0059).
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