Coatings 2018 , 8, x doi: FOR PEER REVIEW www.mdpi.comjournal coatings [631861]

Coatings 2018 , 8, x; doi: FOR PEER REVIEW www.mdpi.com/journal/ coatings
Article 1
Torsional Fretting Wear Properties of Thermal 2
Oxidation -Treated Ti 3SiC 2 Coatings 3
Wang Jian, Luo Xiaohui, Sun Yanhua* 4
State Key Laboratory of Digital Manufacturing Equipment and Technology, School of Mechanical Science 5
and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China 6
* corresponding author: [anonimizat] 7
Abstract: In the present work, efforts have been made to oxidize the Ti 3SiC 2 coating surface to 8
improve its wear resistance by producing oxide layers and healing microcracks that initiated from 9
the thermal sprayed process. Tribological behaviors of the thermal oxidation -treated Ti 3SiC 2 10
coatings subjected to various temperatures (200, 300 and 400 ℃) and durations (1, 3 and 5 h) are 11
investigated comparatively by fretting wear. The results show that the thickness of the oxide layer 12
and the average content of element O on the surface are gradually increase d with increasing 13
temperature. Lower friction coefficients are observed in coatings at 200 -400℃ for 1h. Better 14
performance of crack -healing features is demonstrated under 400 ℃ while fresh microcracks are 15
formed under the frett ing condition due to the fragility of oxides at the same time. The tribological 16
behavior of thermal oxidation -treated Ti 3SiC 2 coatings is mainly controlled by abrasive wear and 17
delamination. The volume losses induced by wear scars decrease with the increas e of oxidation time 18
under oxidation treatment of 200 ℃ and increase with increasing oxidation time under oxidation 19
temperature of 300 and 400 ℃. 20
Keywords: Fretting wear ; Thermal -sprayed coating; Friction coefficient; Wear mechanism 21
22
1. Introduction 23
The ternary carbide Ti 3SiC 2, one of the MAX phases, is of significant interest for its potential in 24
tribological applications due to its remarkable combination of properties in damage tolerance, 25
machinability, electric and thermal conductivity, self -healing ca pability and lubrication [1 -5]. 26
However, the conditions allowing excellent tribological properties are limited to either high sliding 27
speed or high temperature , which cause the formation of tribo -induced oxides on the contact 28
interfaces [6 -8]. The synthesi zed Ti 3SiC 2 bulks restrict the potential for tribological applications in 29
engineering components while plasma spraying with a high spray temperature and a prominent 30
deposition rate provides an efficient way to achieve these wear -resistant coatings of the M AX phase. 31
In our previous study [9], the plasma -sprayed Ti 3SiC 2 coatings exhibited improved fretting wear 32
properties because of the healed cracks by selective oxidation of Ti 3SiC 2 and the lubrication of tribo – 33
induced oxides. However, the friction coefficie nt maintained at a relatively higher level compared 34
with that in high temperature or high sliding speed. 35
It has been observed that the lubricious oxide layer on the frictional surface of Ti 3SiC 2 was 36
composed of amorphous titanium and silicon oxides [10]. Similar oxidation could occur in thermal 37
treated Ti 3SiC 2 coating surfaces. The oxidation process was controlled by the inward diffusion of 38
oxygen and the outward diffusion of titanium, resulting in a stratified structure of an outer rutile and 39
an inner rut ile/silica layer [11 -13]. Lubricious rutile with tribo -oxidation or pre -oxidation treatment 40
has been proved to be effectively in reducing the friction and wear of Ti -based alloys [14, 15] as well 41
as Ti 3SiC 2 [16]. Besides, it has been demonstrated that crac k damage in MAX phase, which could be 42
easily observed in thermal sprayed coatings [17, 18], could be healed by filling the crack gap with 43

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well -adhering oxides and other reaction products to restore the material integrity [19 -23]. Thus, it is 44
valuable to ex plore the fretting wear behavior of Ti 3SiC 2 coatings with oxidation treatment for a 45
further improvement of their tribological properties. 46
In this paper, plasma -sprayed Ti 3SiC 2 coatings are thermal oxidation treated at 200, 300 and 47
400° C for various durations to improve the fretting wear properties. Previous works on the oxidation 48
behavior of Ti 3SiC 2 have been conducted at temperatures above 900° C [24, 25] and intermediate 49
tempe rature from 500 to 900° C [26]. However, considering the operating temperature of the coating 50
substrates (CuNiAl), the maximum temperature in this paper is limited within 400° C. The 51
morphologies of oxidized Ti 3SiC 2 are investigated by the X -ray diffraction (XRD) and an energy – 52
dispersive X -ray spectroscopy analyzer (EDS). The tribological behaviors of oxidized Ti 3SiC 2 against 53
42CrMo4 are comparatively studied under a flat -on-flat contact. Furthermore, the oxidation of Ti 3SiC 2 54
as well as the wear mechanism und er different conditions are analyzed and discussed in detail. 55
2. Materials and experimental methodology 56
2.1. Materials and Thermal Oxidation of Ti 3SiC 2 coatings 57
The Ti 3SiC 2 coatings were deposited by an air plasma spray system (APS) on grit -blasted (3 bar, 58
corundum EKF 30) nickel -aluminum bronze substrates with a dimension of 30×50×7 mm3. 59
Commercial grade Ti 3SiC 2 powders (purity ≥98.0 wt.%) with a particle size of 40 μm were served as 60
starting materials and details of components have been reported in our re cent work [ 9].The plasma 61
spray parameters were listed as follows: the input power supplied to the gun was 33 kW, primary 62
gas (Ar) flow rate was 33 standard liters per minute (slpm), secondary gas (H 2) flow rate was 2 slpm, 63
and the spray distance was 85 mm. 64
Thermal oxidation of Ti 3SiC 2 coatings was carried out in an air furnace at various temperatures 65
ranging from 200 to 400 ℃ for a period of 1 –5 h. The rate of heating process was about 2 ℃/min and 66
the specimen was laid in the furnace to cool itself after the oxidation treatment. All the specimens 67
were grounded, polished and degreased in ethanol before experiments. 68
2.2. Torsional fretting test 69
Torsional fretting wear tests were performed on a torsional fretting tester with a flat -on-flat 70
contact configuration as shown in Fig. 1a. The test rig has been described in detail in our previous 71
studies [27 -29]. Briefly, the normal loads (Fn) were applied by dead weights from the top. The angular 72
displacement amplitude (θ), the frequency and the total fretting cycles we re controlled by the motor 73
impulse and measured by an encoder. The lower flat specimen was fixed on the lower holder, which 74
was driven by a reduced speed stepping motor. The upper counter -body (42CrMo4, Fig. 1b) was 75
designed in a partial annulus with two r aised 45° sectors and fixed on the upper holder, which was 76
connected to a torque sensor [30, 31]. 77
The test parameters were set as follows: the angular displacement amplitude ( θ) was 1.5° and 78
the normal load ( Fn) was 106 N. The frequency was 2 Hz and the t otal number of cycles was 40,000. 79
All tests were carried out at 23° C in laboratory ambient conditions with a relative humidity of 40 – 80
45% and all specimens were cleaned with ethanol and dried with cold air after tests. 81

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82
Fig. 1. (a) Schematic of torsional fretting wear test rig for flat -on-flat configuration: (1) Torque sensor; 83
(2) Upper holder; (3) Lower holder; (4) Stepping motor; (5) Encoder; (6) Lower specimen; (7) Upper 84
specimen; (b) Upper counter -body designed with two raised pa rtial annulus . 85
2.3. Analysis 86
The phase compositions of oxidized Ti 3SiC 2 coatings were identified by X -ray diffraction (XRD, 87
X’Pert Pro/Empyrean) with Cu -Kα radiation. The surface morphologies and the wear volume were 88
measured by a three -dimensional (3D) op tical microscope (OLYMPUS -DSX510). The morphologies 89
of worn surfaces as well as initial surfaces and cross -section of the oxidized samples were conducted 90
using a field emission scanning electron microscope (FESEM, Helios G3 CX) equipped with an 91
energy -disp ersive X -ray spectroscopy analyzer (EDS). 92
3. Experimental results and discussion 93
3.1. Composition and characterization of thermal -oxidation treated coatings 94
The XRD patterns of the oxide scales on the Ti 3SiC 2 coatings after thermal -oxidation treatment 95
at 200-400℃ for 1h are exhibited in Fig. 2, using the XRD patterns of as -sprayed Ti 3SiC 2 coatings as a 96
contrast and only 2θ with the range of 22 -42° is shown for clarity. XRD results of samples for 3 -5h are 97
similar to those as shown in Fig 2, so we choose the results of coatings with 1h treatment to show the 98
differences at various temperatures. No diffraction peaks of any oxides is detected at the oxidation 99
temperature of 200 ℃. With increasing temperatures, both anatase TiO 2 and rutile TiO 2 are identified 100
at 30 0-400℃ and still no peaks of oxides of Si is found in the XRD pattern. The intensity of the 101
diffraction peaks associated with rutile TiO 2 are more significant at 400 ℃ than that of anatase TiO 2. 102
Furthermore, it should be noted that the intensity of Ti 3SiC 2 peaks exhibits an evident decrease after 103
thermal -oxidation treatment corresponding to the variation of the intensity of TiC peaks, implying 104
that the oxide scales that formed during oxidation were transformed mostly by the oxidation reaction 105
of Ti 3SiC 2 phas es. 106
107
Fig. 2. XRD patterns of thermal -oxidation treated Ti 3SiC 2 coatings at 200 -400 ℃ for 1h . 108

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The chemical compositions of the initial surface and the cross -section taken from the sample 109
oxidized at 400 ℃ for 1h are measured by the EDS elemental scans of Ti, Al, Si, C, and O, as shown 110
in Fig.3. It can be observed that the oxide should be a mixture of titanium oxide and silicon oxide. It 111
is in good agreement with the result of previous publications [1 6, 26]. The contents of oxides as well 112
as the thickness of the oxide layers could be estimate according to cross -section EDS analysis . Noted 113
that the thickness value is defined as the mean value of the maximum and the minimum values 114
shown in Fig. 4b. The maximum and minimum values refer to the depths in which the intensity of 115
element O and Ti gradually reached to the average value of coatings. The measurements of oxidation 116
at 200 -400℃ for 1 -5h are observed in the same manner. Fig. 4a shows the average composition of 117
element O of the surface and Fig. 4b shows the thickness of the oxide layer as a function of oxidation 118
temperature. The peaks of element O could be justified under the oxidation temperature of 200 ℃ in 119
Fig.4a while the thickness of oxide layers maintain at a relatively limited value in Fig.4b. Besides, it 120
can be seen that for both the curves, the average composition as well as the thickness are increased 121
with increasing temperature but remain a similar value under various oxidation time. The oxidation 122
is mainly a function of oxidation temperature from the macro view. 123
124
Fig. 3. EDS elemental scans taken from (a) the surface and (b) the cross -section on a sample oxidized 125
at 400 ℃ for 1h . 126
127
Fig. 4. (a) Average composition of element O in oxidized surfaces and (b) thickness of oxide layers 128
under various thermal -oxidation treatments . 129
Typical morphologies of microcracks in Ti 3SiC 2 specimens after the oxidation treatment at 200 – 130
400℃ are exhibited in Fig.5. The cracks of Ti 3SiC 2 coatings, which were easily formed during the 131
plasma spraying process [17, 18], are quite distinct after the oxidation treatment at 200 ℃ (Fig. 5a). 132
While the cracks after oxidation treatment at 300 ℃ (Fig. 5b) are partially filled with oxides due to the 133
self-healing capability of Ti 3SiC 2 phases. The well -adhering phases of TiO 2 and some other 134
productions formed by the selective oxidation of Ti 3SiC 2 are even more significant in the crack gap at 135
a higher temperature of 400 ℃ (Fig. 5c). Meanwhile, microcracks occur in the oxide layers during 136
Composition O (wt% )
Temperature ( ℃) Temperature ( ℃)

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the oxidation treatment at 400 ℃ (Fig. 5d) are still partially filled with oxides. The formation of 137
microcracks was presumably caused by the stress that produced by a phase change in the oxide 138
products [26]. It is supported that both the oxidation temperature and time have an influence on the 139
microcracks. 140
141
142
Fig. 5. Typical morphologies of microcracks of the samples oxidized at (a) 200 ℃, (b) 300 ℃, (c) and 143
(d) 400 ℃ for 1h . 144
3.2. Friction kinetics behavior 145
The fretting running kinetics behaviors can be described by the friction torque vs. angular 146
displacement amplitude loops ( T-θ curves) [32 -34]. As shown in Fig 6, the T-θ curves of all cycles 147
under the oxidation temperature of 400 ℃ show th e parallelogram shape. Similarly, the torsional 148
fretting wear under various temperature should be running in the gross slip regime of fretting maps. 149
For the further discussion of the effect of oxidation treatment on the fretting running behaviors, the 150
friction coefficient is depicted as a function of the number of cycles with logarithmic coordinates as 151
shown in Fig. 7, calculating by the average amplitude Ti of a T -θ curve of each fretting cycle [28,30]. 152
Usually, there are three typical stages occurred in t he fretting running progress [35 -37]. In the initial 153
stage (stage Ⅰ), the friction coefficients are relatively small, which could due to two reasons: one was 154
because of the protection and lubrication of fresh films on contacting surfaces; the other one was due 155
to relative high roughness at very beginning of contact that cause a higher contact pressure, resulting 156
low friction [38,39] . After approximately 10 or 100 cycles, the curves increase quickly and entered the 157
ascent stage (stage Ⅱ), due to the adhesion a nd abrasion between the contact interfaces that are more 158
flattened with bigger contact area. Then the curves achieve a steady value in the steady stage (stage 159
Ⅲ), corresponding to the fluctuation within a narrow range. Besides, the friction coefficients ar e 160
shown to have a relationship to the oxidation temperature and time. Due to the lubrication of 161
titanium oxide formed in the oxidation treatment, for the oxidation temperature of 200 ℃, as seen in 162
Fig. 7a, friction coefficient values of coatings in the stea dy stage shows a relatively lower level with 163
the oxidation time of 1 and 3h compared to those without oxidation treatment. While the friction 164
coefficient rises with longer oxidation time of 5h. Moreover, when the oxidation temperature 165
increas ed to 300 (Fig. 7b) or 400 ℃ (Fig. 7c), with 1h oxidation, the friction coefficient at steady stage 166
maintain ed a significantly lower value. The increase in oxidation time is corresponding to an elevated 167
value in the steady stage of friction coefficient. It is suggested that the friction coefficient is expected 168
to have a relatively lower value with decreased oxidation time. The 1h oxidation treatment is 169
supposed to have a better lubrication effect. 170

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171
Fig. 6. T-θ curves of thermal -oxidation treated samples unde r oxidation temperature of 400 ℃ as a 172
function of the number of cycles . 173
174
175
Fig. 7. Friction coefficients of thermal -oxidation treated samples under oxidation temperature of (a) 176
200 ℃, (b) 300 ℃ and (c) 400 ℃ as a function of the number of cycles. 177
3.3. Wear scar observation 178
Detailed analyses of wear scars are carried out by measuring the wear volume with a 3D optical 179
microscope and typical morphologies with a scanning electron microscope (SEM). As indicated in 180
Fig.8, the wear volume of oxidation -treated Ti3SiC 2 coatings are compared to evaluate the wear 181
resistance of those coatings with different oxidation temperatures and durations. It is noted that all 182
the wear volume exhibits an appreciable decrease after oxidation treatment under fretting wear, due 183
to the lubrication of TiO 2 and the healing of microcracks formed by the oxidizing reaction of Ti 3SiC 2 184
coatings during the oxidation treatment. Furthermore, the volume losses of coatings under oxidation 185
treatment of 200 ℃ decrease with the increase of oxidat ion time. As mentioned previously, the 186
thickness of oxide layers as well as the content of element O is quite low under the oxidation 187
temperature of 200 ℃. The healing performance of microcracks still can ’t be identified at 200 ℃. 188
Presumably, the increasing oxidation time could provide more titanium oxide to form a protective 189

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and lubricated layers on the contact interfaces. With the increasing temperature, the oxidation is 190
conspicuous at 300 and 400 ℃ (as revealed in Fig. 4) while the friction coefficients ten d to increase 191
with increasing oxidation time. The wear volume under oxidation temperature of 300 and 400 ℃ 192
shows an increase tendency with increasing oxidation time. Moreover, the crack is filled partially at 193
300℃ while the crack -healing feature is more pro nounced at 400 ℃. The highest volume loss is 194
observed at coatings under the oxidation temperature of 300 ℃ and the oxidation time of 5h. 195
Considering the microcracks that formed during the oxidation treatment at 400 ℃, the lowest volume 196
loss presents the oxida tion temperature of 300 ℃ and the oxidation time of 1h. 197
198
Fig. 8. Wear volume of oxidized coatings under various thermal -oxidation treatments. 199
Typical SEM morphologies of wear scars under various oxidation treatments of 200 ℃ are 200
presented in Fig.9, respectively. As indicated in Fig.9a and b, coatings after oxidation treatment of 201
1and 3h are mainly controlled by wear mechanisms of delamination and abrasive wear. With the 202
increase of oxidation time to 5h, the main wear mechanis ms remains to be delamination and abrasive, 203
but visible deformation (Fig.9c) and cracks under a higher magnification in Fig 9d are observed at 204
the same time. Furthermore, the oxide layers are justified by the EDX patterns shown in Fig.9e. 205
Combined with the results in our previous study [9], the oxide layers formed by the fretting wear 206
process are composed of TiO 2, SiO 2 and Fe 2O3, Fe 3O4 debris transferred from the counterpart. It is 207
noted that the composition of element O has been significantly increased due to the pre -oxidation 208
treatment and the transformation from the counter faces. The generated oxide layers could act as a 209
lubrication function and prevent the wear surface from severe damage, corresponding to the 210
decreasing friction coefficient and volume l osses compared to the Ti 3SiC 2 coatings without oxidation 211
treatment. Since all the specimen were cleaned with ethanol in an ultrasonic bath and the loose debris 212
have been cleared, the remaining debris should not be considered as a source of error, but as an effect 213
of various testing conditions [ 40]. The oxide layers on the worn surface of 200 ℃ and 5h (Fig.9c) are 214
revealed to be rougher than that with the oxidation time of 1 and 3h (Fig.9 a and b), due to the 215
deformation during fretting wear. That is in good agreement with the fact of increasing friction 216
coefficient in Fig.7a. 217
218
219

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220
221
222
Fig. 9. SEM morphologies of the wear scars of coatings after oxidation temperature of 200 ℃: (a) 1h 223
and 5000x magnification; (b) 2h and 5000x magnification; (c) 3h and 5000x magnification; (d) 3h and 224
50000x magnification. (e) EDS analysis of the wear scars under oxidation time of 1h . 225
Fig. 10 presents the SEM worn surface morphologies of oxidation treated Ti 3SiC 2 coatings at 300 226
℃. It is found that there is only a slight damage with oxidation treatment of 1h, as presented in Fig 227
10a, caused by the delamination and abrasive wear. Deformation is revealed under the oxidation time 228
of 3 and 5h in Fig.10b and c, with an increasing friction coefficient (Fig. 7b). The cracks on the wear 229
scars are distinct even in the morphology with a low magnification, as shown in Fig. 10b and c. These 230
cracks are mainly caused by reciprocating loading and unloading during the fretting wear procedure. 231
On the one hand, the increase of oxidation temperatur e associates with the microcracks that is just 232
partially filled in the original coating surfaces (Fig. 5b). On the other hand, it results in the increase 233
of thickness of the oxide layers (Fig. 4b), accompanied by the brittle oxide which is highly susceptib le 234
to cracks under fretting. The cracks propagate and encounter on worn surface, refined between the 235
contact interfaces and then lead to further abrasive wear. In this case, the damage of the worn surface 236
is quite severe with the oxidation temperature of 3 and 5 h. 237

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238
239
Fig. 10. SEM morphologies of the wear scars of coatings after oxidation temperature of 300 ℃: (a) 1h 240
and 5000x magnification; (b) 2h and 5000x magnification; (c) 3h and 5000x magnification . 241
With the oxidation temeperature of 400 ℃, SEM micrographs of wear scars under various 242
oxidation time are exhibited in Fig.11. It could be seen from the morphologies in Fig. 11a, c and e that 243
the wear damage is still formed by delamination and abrasion wear and cracks are easily found in 244
the wear surface, as exhibited in Fig. 11b, d and f. The fretting wear runs in the same mechanism as 245
we presented in Fig. 10c. Similarly, the increasing temperatures of oxidation treatment further 246
increase the thickness of the oxide layers (Fig. 4c). However, the microcracks on the coating surface 247
are completely healed by the oxides filled in the crack gap (Fig. 5c). The overall effect is determined 248
by both and the propagation of cracks on the original coating surface could be markedly inhibited by 249
the healing proc ess, resulting in the reducing volume losses at 3 and 5h under oxidation temperature 250
of 400℃. Besides, the increasing composition of element O on the surface may lead to a decrease in 251
the friction coefficient (Fig. 7c), compared to that at 300 ℃, although t he wear scars are both rough in 252
the morphologies. 253
254
255
256
257
258
259
260

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261
262
263
Fig. 11. SEM morphologies of the wear scars of coatings after oxidation temperature of 200 ℃: (a) 1h 264
and 5000x magnification; (b) 1h and 50000x magnification; (c) 2h and 5000x magnification; (d) 2h and 265
50000x magnification; (e) 3h and 5000x magnification; (f) 3h and 50000x magnification. 266
4. Conclusion 267
Torsional fretting wear tests of oxidation -treated Ti 3SiC 2 coatings under various temperatures 268
and durations are investigated wit h a flat -on-flat contact configuration. The oxidative behaviors, 269
fretting running behaviors, wear mechanism as well as the effect of various oxidation temperatures 270
and time are discussed comparatively. The main conclusions obtained from the present study a re as 271
follows: 272
1. Combination of the XRD and EDX patterns have demonstrated that the products formed during 273
the oxidation treatment mainly consist of TiO 2, which are transformed mostly by the oxidation 274
reaction of Ti 3SiC 2 phases. The average composition of el ement O as well as the thickness of 275
oxide layers increase with increasing temperature. Well -adhering phases of TiO 2 and some other 276
productions heal the original cracks on the plasma -sprayed coatings with the increased 277
oxidation temperature of 400 ℃. 278
2. Accordi ng to the T-θ curves and damage morphologies, the torsional fretting is supposed to be 279
running in the gross slip regime and the main wear mechanisms are abrasive wear and 280
delamination. 281
3. The friction coefficient is expected to have a relatively lower value with decreased oxidation 282
time. The morphologies of wear scars with increased oxidation time are revealed to be rougher 283
due to the deformation in the fretting wear process. 284

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4. Compared with the as -deposited Ti 3SiC 2 coatings, the wear volume exhibits an appreci able 285
decrease after oxidation treatment due to the lubrication of TiO 2 and the healing of microcracks 286
generated by the oxidizing reaction of Ti 3SiC 2. The wear volume of coatings shows a decrease 287
with the increase of oxidation time under oxidation treatment of 200℃ while an increase with 288
increasing oxidation time under oxidation temperature of 300 and 400 ℃. Preferable tribological 289
performances are obtained under the oxidation temperature of 300 ℃ and the oxidation time of 290
1 h. 291
Acknowledgements : This work was supported by the National Basic Research Program of China 292
(Grant No. 2014CB046705) , the National Natural Science Foundation of China (Grant No s. 293
51705178) and Key Project of Hubei Province (Grant No. 2017AAA001 ). 294
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