- Email: [email protected]
An investigation of fretting behavior of ion-plated TiN, magnetron-sputtered MoS2 and their composite coatings
Wear 225–229 Ž1999. 46–52
An investigation of fretting behavior of ion-plated TiN, magnetron-sputtered MoS 2 and their composite coatings Guizhen Xu b
a,b
, Zhongrong Zhou b, Jiajun Liu
a,)
, Xiaohua Ma
a
a Tribology Research Institute, Tsinghua UniÕersity, Beijing, China Tribology Research Institute, Southwest Jiaotong UniÕersity, Chengdu, China
Abstract All surface techniques can be roughly divided into two categories: anti-wear coatings characterized by high hardness and anti-frictions coatings with low friction coefficient. However, it is still unclear which kind of coatings is more effective for preventing the fretting damage so far. So it is of important meaning to conduct a comparative study on fretting-resistant behavior of these coatings. This paper analyzes the fretting properties and damage mechanism of ion-plated TiN, magnetron sputtered MoS 2 and their composite coatings on a SRV testing machine through a systematic examinations of wear scar and wear debris under SEM. The main results obtained in the slip regime showed that the fretting resistance of TiN coating was superior to that of MoS 2 coating, especially at high load and large number of cycles. Detachment of particles was the main reasons of degradation for both TiN and MoS 2 coatings, while the shape of the detached particles were quite different. The composite coating of TiN q MoS 2 showed excellent fretting-resistant performance under the same testing conditions, as the solid lubrication property of MoS 2 coating could be fully developed by a strong support from the underneath TiN coating, i.e., the advantages and disadvantages of two kinds of surface techniques could be mutually supplemented. In addition, the formation and effect of the third-body is discussed in detail as well. q 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Fretting wear; Ion plating TiN; Magnetron sputtering MoS 2 ; Composite coating
1. Introduction Crack nucleation and propagation and wear induced by fretting have been widely encountered in machinery industries w1,2x, especially for fasteners subjected to structure oscillating and alternating loading w3,4x. In order to palliate the contact damage resulted from fretting and to extend the lifetime of components, many investigators have been doing their best to improve the fretting wear-resistant properties of materials by trying various methods w5x. Surface modification techniques, which are regarded as effective methods to improve the wear-resistant properties of materials, have been successfully applied to enhance the ability of components to resist the fretting damage in recent years w6–8x. Roughly speaking, all surface techniques could be divided into two categories: antiwear coatings characterized by high hardness and anti-friction coatings with low friction coefficient. However, it is still unclear which kind of coatings is more effective for pre-
)
Corresponding author. [email protected]
Fax:
q 861062781346;
e-m ail:
venting the fretting wear so far, because each kind of coatings possesses intrinsic advantages and disadvantages. So it is of important meaning to conduct a comparative study on fretting-resistant behavior of these coatings. This paper attempts to analyze the fretting properties and damage mechanism of ion plated TiN, magnetron sputtered MoS 2 and their composite coatings in order to find an effective way for preventing the fretting damage. Owing to the important role of third bodies emerged in the contact region during fretting process, their formation characteristics and effects for three different coatings were deeply examined and discussed in detail as well.
2. Experimental procedures All fretting tests were carried out on a SRV testing machine. A ball-on-disc contact was used. The upper specimens were 52100 steel balls with diameter of 10 mm. The roughness of ball surface was Ra s 0.08 mm. The lower disc specimens were made from 1045 steel with the dimensions of f 24 = 7.8 mm. They were heated at 8608C for 20 min, then water-quenched and tempered at 6008C
0043-1648r99r$ - see front matter q 1999 Published by Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 9 . 0 0 0 0 5 - 8
G. Xu et al.r Wear 225–229 (1999) 46–52
47
Table 1 Chemical composition Žwt.%.
52100 Steel 1045 Steel
C
Si
Mn
S
P
Cr
Ni
1.00 0.45
0.25 0.27
0.30 0.65
0.02 0.04
0.02 0.04
1.45 0.25
0.20 0.25
for 30 min. The roughness of their surface was also about Ra s 0.08 mm. The compositions and mechanical properties of the balls and discs are listed in Tables 1 and 2. Three kinds of coatings were made on the cleaned disc specimens— Ž1. ion-plated TiN with thickness of 2 mm; Ž2. magnetron-sputtered MoS 2 with thickness of 1 mm, Ž3. at first, TiN was deposited with thickness of 1.8 mm by ion-plating and then a MoS 2 coating was sputtered onto TiN coating with thickness of 1.5 mm. The main parameters of fretting tests were as follows: temperature. 258C; relative humidity, 60% " 10%; slip amplitude, 50 to 100 mm, normal load; 10 to 20 N; frequency, 10 Hz; number of cycles, up to 15 000 cycles. Before each test, the ball and disc were thoroughly cleaned in an ultrasonic bath of alcohol. The variation of friction coefficient with number of cycles was recorded automatically. After each fretting test, the wear debris were collected and the wear volumes were calculated through measurements of wear scar. In addition, the compositions and the morphologies of fretting, wear scar and wear debris were examined by scanning electron microscopy.
3. Experimental results 3.1. Friction coefficient The variations of friction coefficient of TiN, MoS 2 and their composite coatings with the number of cycles under same experimental conditions Žnormal load 20 N, slip amplitude 100 mm, frequency 10 Hz. are shown in Fig. 1 It can be seen that the friction coefficient of TiN coating rapidly increased to 0.3 at the beginning and slowly reached the maximum value 0.33 until 6000 cycles, and then it slowly diminished to the steady stage of 0.32. Compared with TiN coating, the friction coefficient of MoS 2 varied in a wide range of values. It showed that the friction coefficient kept a very low level of 0.055 before 1200 cycles, then suddenly increased to 0.33 at 2700 cycles
Fig. 1. Variation of friction coefficient of three kinds of coatings with number of cycles Žnormal loads 20 N, slip amplitudes100 mm and frequency s10 Hz..
However, at about 3000 cycles, the friction coefficient showed a sharp drop to 0.23, then slowly decreased from 0.34 to a steady state of 0.32. A great difference was observed in the variation of friction coefficient for TiN q MoS 2 composite coating. The friction coefficient was very low Ž- 0.04. during the incubation period Ž- 3500 cycles., then it increased sharply to 0.30 until 6000 cycles. During the following 1800 cycles, the friction coefficient diminished down to 0.068 and then it rose rapidly to 0.28 after 1200 cycles. Before it reached the steady stage of 0.32, the value displayed a sharp drop from 0.28 to 0.168. 3.2. Wear behaÕior The variation of wear volumes of TiN, MoS 2 , TiN q MoS 2 composite coatings and 1045 steel substrate with number of cycle under the same testing conditions Žnormal load 10 N, slip amplitude 100 mm, frequency 10 Hz and 15 000 fretting cycles. are displayed in Fig. 2. It indicated that the fretting wear-resistance of TiN q MoS 2 composite coating was the best and the anti-wear characteristic of TiN coating was superior to that of the single MoS 2 coating. The fretting wear resistance of 1045 steel was the worst among four kinds of materials. That means an
Table 2 Mechanical properties
52100 steel 1045 steel
E ŽGMPa.
R s ŽMPa.
R b ŽMPa.
HRC
210 74
1700 600
2000 800
55–63 29–33
Fig. 2. Variation of wear volumes of coatings and the substrate with number of cycles Žnormal loads10 N, slip amplitudes100 mm and frequency s10 Hz..
G. Xu et al.r Wear 225–229 (1999) 46–52
48
Fig. 3. Variation of wear volumes of coatings with slip amplitude.
appropriate surface coating is very effective for preventing the fretting wear, especially the duplex treatment, which can mutually supplement the advantages and disadvantages of the single coatings. The variation of wear volumes of coatings with slip amplitude is shown in Fig. 3. It seems that the growth rate of wear volumes of MoS 2 was higher than that of the other coatings, and the TiN q MoS 2 composite coating showed the smallest growth with the increase of amplitude. Fig. 4 shows the variation of wear volume of coatings with the number of cycles under different normal loads. It can be found that the normal load had little effect on TiN coating. For the composite and MoS 2 coatings, however, the wear volumes obviously increased with load, especially at larger load and higher fretting cycles. 3.3. Wear scar morphology Fig. 5 shows the variation of wear scars of TiN coatings during the fretting damage. It can be found that the TiN coating remained less worn even though there were a lot of abrasive grooves and a few TiN fine particles were present in the wear scar after 1800 fretting cycles ŽFig. 5a,b.. After 3000 cycles, TiN coating had been damaged in some area where the substrate was exposed, while the undamaged area displayed severe wear morphology ŽFig. 5c..
Fig. 5. Fretting wear scar morphologies of TiN coating.
Fig. 4. Variation of wear volumes of coatings with number of cycles under different normal load.
The variation of wear morphologies of MoS 2 coating is shown in Fig. 6. Fig. 6a reveals that on the wear scar the MoS 2 coating had been almost worn away completely after 1800 cycles. In the following 1200 cycles, the wear scar further increased and a large quantity of debris containing MoS 2 spread over the wear surface ŽFig. 6b,c,d.. When the fretting process reached 6000 cycles, there was less MoS 2 contained in the debris which were remained in the interface ŽFig. 6e,f.. Fig. 7 shows the variation of wear scar of the composite coating, which is obviously different from that of single
G. Xu et al.r Wear 225–229 (1999) 46–52
49
Fig. 6. Fretting wear scar morphologies of MoS 2 coating.
coatings. It can be found that the damage of coating had been postponed to larger fretting cycles. Before 1800 cycles, the MoS 2 coating remained perfectly ŽFig. 7a..
From 3000 cycles to 6000 cycles, the MoS 2 coating was gradually eliminated from the contact area, and the TiN coating had emerged at large portion of wear scar. The
Fig. 7. Fretting wear scar morphologies of TiN q MoS 2 composite coating.
G. Xu et al.r Wear 225–229 (1999) 46–52
50
feature of it was similar to that of the single TiN coating ŽFig. 7b,c.. When the fretting cycle was up to 9000, the wear scar was full of large quantity of debris which contained a large proportion of TiN particles and a small amount of MoS 2 debris ŽFig. 7d,e,f..
4. Discussion 4.1. Fretting damage mechanism of three kinds of coatings All fretting tests conducted here were in gross slip regime to the higher amplitude and the lower load. From the previous analyses, it can be considered that the basic fretting damage model of coatings is particle detachment. However, the procedure and mechanism of debris detachment differed greatly because of the differences in mechanical and chemico-physical properties of coatings. 4.1.1. Fretting wear mechanism of TiN and MoS2 coatings Titanium nitride as a kind of ceramics shows very high hardness of more than Hv s 2000, lower friction coefficient and smaller tendency of adhesion, which are beneficial to increase its wear-resistance. However, the common disadvantage of ceramics is their brittleness, which makes the microcracks easily propagated in the TiN coating under the same contact stress. So at the beginning of fretting wear, small pittings and some detached fine wear particles, which are the products of contact fatigue, could be found on the wear scar as shown in Fig. 5b. These hard wear particles remained in the contact area as abrasive resulted in the abrasive wear of TiN coating and increase of its friction coefficient. When this abrasive wear process reached a certain degree, the TiN coating became thinner and it would be no longer strong enough to bear the normal load, that easily induced the formation of cracks and at last the breakdown of TiN coating Žsee Figs. 5c and 8.. Therefore, the nucleation, propagation of microcracks and the consequent fracture of coating could be considered the main mechanism of fretting wear of TiN coating. Related with that, the abrasive wear could also be devel-
Fig. 8. Brittle rupture of TiN coating.
Fig. 9. Debris morphology of MoS 2 coating.
oped due to the continuous formation of hard abrasives from the detached TiN coating. That would further increase the friction coefficient. As soon as the hard particles were gradually squeezed away from the contact area, the friction coefficient approached to the steady state. MoS 2 is a widely used solid lubricant characterized by its hexagonal structure and low shearing strength. However, its low hardness usually results in poor wear resistance and short endurance, especially when its substrate is not strong enough. The morphology of wear debris ŽFig. 9. indicated that MoS 2 coating detached in platelike shape. These MoS 2 debris were easily trapped in the contact area as a third body and could well play the lubricating role, that resulted in the sudden decrease of friction coefficient. The following rapid increase of friction coefficient probably could be explained by the contact of upper specimen with the substrate when the MoS 2 debris were not enough in the contact area. At last the friction coefficient approached to the steady state when the MoS 2 debris were squeezed out completely. One important phenomenon should be noticed, i.e., the MoS 2 and its oxides like MoO 3 could be detected as a boundary film by the EDX and XPS analyses as shown in Fig. 6d and Fig. 10. That means, in the fretting process the MoS 2 coating could be not only transferred to the wear debris, but also to a boundary film.
Fig. 10. XPS analyses of the boundary film on the wear scar of MoS 2 coating.
G. Xu et al.r Wear 225–229 (1999) 46–52
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This thin film might be helpful to lengthen the role of MoS 2 coating as well. 4.1.2. Fretting wear mechanism of TiN q MoS2 composite coating As expected, the composite coating comprehensively played the role of two single coatings and showed desirable synergetic effect, i.e., the hard TiN coating offered a very strong support to the MoS 2 coating, so that the latter could fully play the role of solid lubricant and showed much longer endurance. In contrast, the MoS 2 coating reduced the friction coefficient of TiN coating and also lengthened its fretting life. The fretting wear mechanism of composite coating was not new, compared with that of TiN and MoS 2 coatings separately, but the damage process was considerably postponed, and more fluctuations of friction coefficient were present before the complete damage of the composite coating. At 1800 cycles Ž3 min. only small portion of MoS 2 coating detached ŽFigs. 4a and 11; see also Fig. 12.. At 3000 cycles Ž5 min., most part of MoS 2 coating still remained ŽFig. 4b.. Only after 6000 cycles Ž10 min., the MoS 2 coating fully detached ŽFig. 4c., which was correspondent to the first peak of friction coefficient. The damage of TiN coating was also postponed to 9000 cycles Ž15 min. ŽFig. 4d,e.. The second fall of friction coefficient was slighter than the previous one, that might be due to the combined effect of TiN, MoS 2 and substrate wear debris in the contact area. 4.2. Formation and effect of third-body Many investigators had demonstrated that in most cases; the formation of third-body can reduce fretting damage by the following reasons w9–11x. Ž1. It can separate the contact surfaces and prevent serious adhesion. Ž2. It can accommodate the velocity and absorb a part of displacement. Ž3. It can accommodate the imposed slip amplitude. Ž4. It can decrease local stress field and resist fretting fatigue. Fig. 12. The scheme diagram of fretting damage mechanism of TiNqMoS composite coating.
Fig. 11. Debris detachment of MoS 2 coating.
They are maybe true based on the researches of fretting damage on the homogeneous materials. The formation and effect of the third-body for materials with coatings show a completely different picture. They are dependent on a series of factors, including the mechanical, physical and chemical properties of first bodies Žin our case the surface coatings are regarded as the first bodies., experimental parameters, environmental conditions etc. Especially, there are a lot of different surface coatings, and they can produce the third-body of different characteristics and behaviors.
52
G. Xu et al.r Wear 225–229 (1999) 46–52
In this research, the TiN coating as a brittle material can easily yield the particle-shape third-body by the stress fatigue. These hard particles can induce abrasive wear and increase the friction coefficient. However, they are also easily ejected from the contact area. In contrast, the MoS 2 coating as a soft material usually produces platelike wear debris, they are easily trapped and remained in the interface; and can effectively separate the contact surfaces and reduce the friction and wear as a solid lubricant. For the composite coating, the behavior and effect of third-body became more complicated: because the TiN, MoS 2 and substrate particles are present at the same time, it is quite difficult to understand how they behave and transform separately in the fretting process. MoS 2 debris is a good third-body, but how can we remain it longer in the contact area. How we can make use of the boundary film formed from MoS 2 is still unclear. It needs further investigation.
normal load, and the composite coating is insensitive to the variation of slip amplitude. Ž4. The formation and effect of third-body in the fretting process play important role to the fretting behavior and damage mechanism. The soft and plate-like debris can release the friction and wear of first bodies and reduce the friction coefficient. However, the harder debris can lead to abrasive on the contact surfaces and increase the friction coefficient. Acknowledgements The authors would like to thank National Natural Science Foundation ŽProject number 59725513. and Solid Lubrication Lab. of Lanzhou Institute of Chemical Physics, Academy of Sciences for their financial support to this research. References
5. Conclusion Ž1. The fretting wear-resistance of TiN, MoS 2 and TiN q MoS 2 composite coatings are much better than that of 1045 steel without coatings. Appropriate surface techniques are effective for preventing the fretting damage. Ž2. The TiN q MoS 2 composite coating showed much better tribological properties than single TiN and MoS 2 coatings. The duplex treatment can create significant improved results through the synergetic effect of single coatings. Ž3. The wear volumes of three coatings increase with slip amplitude and normal load. The growth rate of wear volumes of MoS 2 coating is higher than that of the other coatings. TiN coating is not sensitive to the variation of
w1x R.B. Waterhouse, Fretting Corrosion, Pergamon, Oxford, 1972. w2x R.B. Waterhouse, Fretting Fatigue, Applied Science, London, 1981. w3x L. Dong-zi, Fretting Wear and Protective Techniques, Xi’an, Shiyan Science Technique Press, 1992. w4x J.J. Liu, Wear Mechanism of Materials and Wear Resistance, Beijing, TsingHua Univ. Press, 1993. w5x J. Beard, Fretting Fatigue, ESIS 18, in: R.B. Waterhous, T.C. Lindley ŽEds.., Mechanical Engineering Publications, London, 1994, 419–436. w6x T.C. Chivers, Wear 106 Ž1985. 63–76. w7x S.J. Harris, M.P. Overs, A.J. Gould, Wear 106 Ž1985. 35–52. w8x R.C. Bill, Wear 106 Ž1985. 283–301. w9x M. Godet, Wear 100 Ž1984. 437–452. w10x Y. Berthier, L. Vincent, M. Godet, Journal of Tribology 106 Ž1984. 194–201. w11x Y. Berthier, L. Vincent, M. Godet, Tribology International 22 Ž1989. 235–241.
An investigation of fretting behavior of ion-plated TiN, magnetron-sputtered MoS 2 and their composite coatings Guizhen Xu b
a,b
, Zhongrong Zhou b, Jiajun Liu
a,)
, Xiaohua Ma
a
a Tribology Research Institute, Tsinghua UniÕersity, Beijing, China Tribology Research Institute, Southwest Jiaotong UniÕersity, Chengdu, China
Abstract All surface techniques can be roughly divided into two categories: anti-wear coatings characterized by high hardness and anti-frictions coatings with low friction coefficient. However, it is still unclear which kind of coatings is more effective for preventing the fretting damage so far. So it is of important meaning to conduct a comparative study on fretting-resistant behavior of these coatings. This paper analyzes the fretting properties and damage mechanism of ion-plated TiN, magnetron sputtered MoS 2 and their composite coatings on a SRV testing machine through a systematic examinations of wear scar and wear debris under SEM. The main results obtained in the slip regime showed that the fretting resistance of TiN coating was superior to that of MoS 2 coating, especially at high load and large number of cycles. Detachment of particles was the main reasons of degradation for both TiN and MoS 2 coatings, while the shape of the detached particles were quite different. The composite coating of TiN q MoS 2 showed excellent fretting-resistant performance under the same testing conditions, as the solid lubrication property of MoS 2 coating could be fully developed by a strong support from the underneath TiN coating, i.e., the advantages and disadvantages of two kinds of surface techniques could be mutually supplemented. In addition, the formation and effect of the third-body is discussed in detail as well. q 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Fretting wear; Ion plating TiN; Magnetron sputtering MoS 2 ; Composite coating
1. Introduction Crack nucleation and propagation and wear induced by fretting have been widely encountered in machinery industries w1,2x, especially for fasteners subjected to structure oscillating and alternating loading w3,4x. In order to palliate the contact damage resulted from fretting and to extend the lifetime of components, many investigators have been doing their best to improve the fretting wear-resistant properties of materials by trying various methods w5x. Surface modification techniques, which are regarded as effective methods to improve the wear-resistant properties of materials, have been successfully applied to enhance the ability of components to resist the fretting damage in recent years w6–8x. Roughly speaking, all surface techniques could be divided into two categories: antiwear coatings characterized by high hardness and anti-friction coatings with low friction coefficient. However, it is still unclear which kind of coatings is more effective for pre-
)
Corresponding author. [email protected]
Fax:
q 861062781346;
e-m ail:
venting the fretting wear so far, because each kind of coatings possesses intrinsic advantages and disadvantages. So it is of important meaning to conduct a comparative study on fretting-resistant behavior of these coatings. This paper attempts to analyze the fretting properties and damage mechanism of ion plated TiN, magnetron sputtered MoS 2 and their composite coatings in order to find an effective way for preventing the fretting damage. Owing to the important role of third bodies emerged in the contact region during fretting process, their formation characteristics and effects for three different coatings were deeply examined and discussed in detail as well.
2. Experimental procedures All fretting tests were carried out on a SRV testing machine. A ball-on-disc contact was used. The upper specimens were 52100 steel balls with diameter of 10 mm. The roughness of ball surface was Ra s 0.08 mm. The lower disc specimens were made from 1045 steel with the dimensions of f 24 = 7.8 mm. They were heated at 8608C for 20 min, then water-quenched and tempered at 6008C
0043-1648r99r$ - see front matter q 1999 Published by Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 9 . 0 0 0 0 5 - 8
G. Xu et al.r Wear 225–229 (1999) 46–52
47
Table 1 Chemical composition Žwt.%.
52100 Steel 1045 Steel
C
Si
Mn
S
P
Cr
Ni
1.00 0.45
0.25 0.27
0.30 0.65
0.02 0.04
0.02 0.04
1.45 0.25
0.20 0.25
for 30 min. The roughness of their surface was also about Ra s 0.08 mm. The compositions and mechanical properties of the balls and discs are listed in Tables 1 and 2. Three kinds of coatings were made on the cleaned disc specimens— Ž1. ion-plated TiN with thickness of 2 mm; Ž2. magnetron-sputtered MoS 2 with thickness of 1 mm, Ž3. at first, TiN was deposited with thickness of 1.8 mm by ion-plating and then a MoS 2 coating was sputtered onto TiN coating with thickness of 1.5 mm. The main parameters of fretting tests were as follows: temperature. 258C; relative humidity, 60% " 10%; slip amplitude, 50 to 100 mm, normal load; 10 to 20 N; frequency, 10 Hz; number of cycles, up to 15 000 cycles. Before each test, the ball and disc were thoroughly cleaned in an ultrasonic bath of alcohol. The variation of friction coefficient with number of cycles was recorded automatically. After each fretting test, the wear debris were collected and the wear volumes were calculated through measurements of wear scar. In addition, the compositions and the morphologies of fretting, wear scar and wear debris were examined by scanning electron microscopy.
3. Experimental results 3.1. Friction coefficient The variations of friction coefficient of TiN, MoS 2 and their composite coatings with the number of cycles under same experimental conditions Žnormal load 20 N, slip amplitude 100 mm, frequency 10 Hz. are shown in Fig. 1 It can be seen that the friction coefficient of TiN coating rapidly increased to 0.3 at the beginning and slowly reached the maximum value 0.33 until 6000 cycles, and then it slowly diminished to the steady stage of 0.32. Compared with TiN coating, the friction coefficient of MoS 2 varied in a wide range of values. It showed that the friction coefficient kept a very low level of 0.055 before 1200 cycles, then suddenly increased to 0.33 at 2700 cycles
Fig. 1. Variation of friction coefficient of three kinds of coatings with number of cycles Žnormal loads 20 N, slip amplitudes100 mm and frequency s10 Hz..
However, at about 3000 cycles, the friction coefficient showed a sharp drop to 0.23, then slowly decreased from 0.34 to a steady state of 0.32. A great difference was observed in the variation of friction coefficient for TiN q MoS 2 composite coating. The friction coefficient was very low Ž- 0.04. during the incubation period Ž- 3500 cycles., then it increased sharply to 0.30 until 6000 cycles. During the following 1800 cycles, the friction coefficient diminished down to 0.068 and then it rose rapidly to 0.28 after 1200 cycles. Before it reached the steady stage of 0.32, the value displayed a sharp drop from 0.28 to 0.168. 3.2. Wear behaÕior The variation of wear volumes of TiN, MoS 2 , TiN q MoS 2 composite coatings and 1045 steel substrate with number of cycle under the same testing conditions Žnormal load 10 N, slip amplitude 100 mm, frequency 10 Hz and 15 000 fretting cycles. are displayed in Fig. 2. It indicated that the fretting wear-resistance of TiN q MoS 2 composite coating was the best and the anti-wear characteristic of TiN coating was superior to that of the single MoS 2 coating. The fretting wear resistance of 1045 steel was the worst among four kinds of materials. That means an
Table 2 Mechanical properties
52100 steel 1045 steel
E ŽGMPa.
R s ŽMPa.
R b ŽMPa.
HRC
210 74
1700 600
2000 800
55–63 29–33
Fig. 2. Variation of wear volumes of coatings and the substrate with number of cycles Žnormal loads10 N, slip amplitudes100 mm and frequency s10 Hz..
G. Xu et al.r Wear 225–229 (1999) 46–52
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Fig. 3. Variation of wear volumes of coatings with slip amplitude.
appropriate surface coating is very effective for preventing the fretting wear, especially the duplex treatment, which can mutually supplement the advantages and disadvantages of the single coatings. The variation of wear volumes of coatings with slip amplitude is shown in Fig. 3. It seems that the growth rate of wear volumes of MoS 2 was higher than that of the other coatings, and the TiN q MoS 2 composite coating showed the smallest growth with the increase of amplitude. Fig. 4 shows the variation of wear volume of coatings with the number of cycles under different normal loads. It can be found that the normal load had little effect on TiN coating. For the composite and MoS 2 coatings, however, the wear volumes obviously increased with load, especially at larger load and higher fretting cycles. 3.3. Wear scar morphology Fig. 5 shows the variation of wear scars of TiN coatings during the fretting damage. It can be found that the TiN coating remained less worn even though there were a lot of abrasive grooves and a few TiN fine particles were present in the wear scar after 1800 fretting cycles ŽFig. 5a,b.. After 3000 cycles, TiN coating had been damaged in some area where the substrate was exposed, while the undamaged area displayed severe wear morphology ŽFig. 5c..
Fig. 5. Fretting wear scar morphologies of TiN coating.
Fig. 4. Variation of wear volumes of coatings with number of cycles under different normal load.
The variation of wear morphologies of MoS 2 coating is shown in Fig. 6. Fig. 6a reveals that on the wear scar the MoS 2 coating had been almost worn away completely after 1800 cycles. In the following 1200 cycles, the wear scar further increased and a large quantity of debris containing MoS 2 spread over the wear surface ŽFig. 6b,c,d.. When the fretting process reached 6000 cycles, there was less MoS 2 contained in the debris which were remained in the interface ŽFig. 6e,f.. Fig. 7 shows the variation of wear scar of the composite coating, which is obviously different from that of single
G. Xu et al.r Wear 225–229 (1999) 46–52
49
Fig. 6. Fretting wear scar morphologies of MoS 2 coating.
coatings. It can be found that the damage of coating had been postponed to larger fretting cycles. Before 1800 cycles, the MoS 2 coating remained perfectly ŽFig. 7a..
From 3000 cycles to 6000 cycles, the MoS 2 coating was gradually eliminated from the contact area, and the TiN coating had emerged at large portion of wear scar. The
Fig. 7. Fretting wear scar morphologies of TiN q MoS 2 composite coating.
G. Xu et al.r Wear 225–229 (1999) 46–52
50
feature of it was similar to that of the single TiN coating ŽFig. 7b,c.. When the fretting cycle was up to 9000, the wear scar was full of large quantity of debris which contained a large proportion of TiN particles and a small amount of MoS 2 debris ŽFig. 7d,e,f..
4. Discussion 4.1. Fretting damage mechanism of three kinds of coatings All fretting tests conducted here were in gross slip regime to the higher amplitude and the lower load. From the previous analyses, it can be considered that the basic fretting damage model of coatings is particle detachment. However, the procedure and mechanism of debris detachment differed greatly because of the differences in mechanical and chemico-physical properties of coatings. 4.1.1. Fretting wear mechanism of TiN and MoS2 coatings Titanium nitride as a kind of ceramics shows very high hardness of more than Hv s 2000, lower friction coefficient and smaller tendency of adhesion, which are beneficial to increase its wear-resistance. However, the common disadvantage of ceramics is their brittleness, which makes the microcracks easily propagated in the TiN coating under the same contact stress. So at the beginning of fretting wear, small pittings and some detached fine wear particles, which are the products of contact fatigue, could be found on the wear scar as shown in Fig. 5b. These hard wear particles remained in the contact area as abrasive resulted in the abrasive wear of TiN coating and increase of its friction coefficient. When this abrasive wear process reached a certain degree, the TiN coating became thinner and it would be no longer strong enough to bear the normal load, that easily induced the formation of cracks and at last the breakdown of TiN coating Žsee Figs. 5c and 8.. Therefore, the nucleation, propagation of microcracks and the consequent fracture of coating could be considered the main mechanism of fretting wear of TiN coating. Related with that, the abrasive wear could also be devel-
Fig. 8. Brittle rupture of TiN coating.
Fig. 9. Debris morphology of MoS 2 coating.
oped due to the continuous formation of hard abrasives from the detached TiN coating. That would further increase the friction coefficient. As soon as the hard particles were gradually squeezed away from the contact area, the friction coefficient approached to the steady state. MoS 2 is a widely used solid lubricant characterized by its hexagonal structure and low shearing strength. However, its low hardness usually results in poor wear resistance and short endurance, especially when its substrate is not strong enough. The morphology of wear debris ŽFig. 9. indicated that MoS 2 coating detached in platelike shape. These MoS 2 debris were easily trapped in the contact area as a third body and could well play the lubricating role, that resulted in the sudden decrease of friction coefficient. The following rapid increase of friction coefficient probably could be explained by the contact of upper specimen with the substrate when the MoS 2 debris were not enough in the contact area. At last the friction coefficient approached to the steady state when the MoS 2 debris were squeezed out completely. One important phenomenon should be noticed, i.e., the MoS 2 and its oxides like MoO 3 could be detected as a boundary film by the EDX and XPS analyses as shown in Fig. 6d and Fig. 10. That means, in the fretting process the MoS 2 coating could be not only transferred to the wear debris, but also to a boundary film.
Fig. 10. XPS analyses of the boundary film on the wear scar of MoS 2 coating.
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This thin film might be helpful to lengthen the role of MoS 2 coating as well. 4.1.2. Fretting wear mechanism of TiN q MoS2 composite coating As expected, the composite coating comprehensively played the role of two single coatings and showed desirable synergetic effect, i.e., the hard TiN coating offered a very strong support to the MoS 2 coating, so that the latter could fully play the role of solid lubricant and showed much longer endurance. In contrast, the MoS 2 coating reduced the friction coefficient of TiN coating and also lengthened its fretting life. The fretting wear mechanism of composite coating was not new, compared with that of TiN and MoS 2 coatings separately, but the damage process was considerably postponed, and more fluctuations of friction coefficient were present before the complete damage of the composite coating. At 1800 cycles Ž3 min. only small portion of MoS 2 coating detached ŽFigs. 4a and 11; see also Fig. 12.. At 3000 cycles Ž5 min., most part of MoS 2 coating still remained ŽFig. 4b.. Only after 6000 cycles Ž10 min., the MoS 2 coating fully detached ŽFig. 4c., which was correspondent to the first peak of friction coefficient. The damage of TiN coating was also postponed to 9000 cycles Ž15 min. ŽFig. 4d,e.. The second fall of friction coefficient was slighter than the previous one, that might be due to the combined effect of TiN, MoS 2 and substrate wear debris in the contact area. 4.2. Formation and effect of third-body Many investigators had demonstrated that in most cases; the formation of third-body can reduce fretting damage by the following reasons w9–11x. Ž1. It can separate the contact surfaces and prevent serious adhesion. Ž2. It can accommodate the velocity and absorb a part of displacement. Ž3. It can accommodate the imposed slip amplitude. Ž4. It can decrease local stress field and resist fretting fatigue. Fig. 12. The scheme diagram of fretting damage mechanism of TiNqMoS composite coating.
Fig. 11. Debris detachment of MoS 2 coating.
They are maybe true based on the researches of fretting damage on the homogeneous materials. The formation and effect of the third-body for materials with coatings show a completely different picture. They are dependent on a series of factors, including the mechanical, physical and chemical properties of first bodies Žin our case the surface coatings are regarded as the first bodies., experimental parameters, environmental conditions etc. Especially, there are a lot of different surface coatings, and they can produce the third-body of different characteristics and behaviors.
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In this research, the TiN coating as a brittle material can easily yield the particle-shape third-body by the stress fatigue. These hard particles can induce abrasive wear and increase the friction coefficient. However, they are also easily ejected from the contact area. In contrast, the MoS 2 coating as a soft material usually produces platelike wear debris, they are easily trapped and remained in the interface; and can effectively separate the contact surfaces and reduce the friction and wear as a solid lubricant. For the composite coating, the behavior and effect of third-body became more complicated: because the TiN, MoS 2 and substrate particles are present at the same time, it is quite difficult to understand how they behave and transform separately in the fretting process. MoS 2 debris is a good third-body, but how can we remain it longer in the contact area. How we can make use of the boundary film formed from MoS 2 is still unclear. It needs further investigation.
normal load, and the composite coating is insensitive to the variation of slip amplitude. Ž4. The formation and effect of third-body in the fretting process play important role to the fretting behavior and damage mechanism. The soft and plate-like debris can release the friction and wear of first bodies and reduce the friction coefficient. However, the harder debris can lead to abrasive on the contact surfaces and increase the friction coefficient. Acknowledgements The authors would like to thank National Natural Science Foundation ŽProject number 59725513. and Solid Lubrication Lab. of Lanzhou Institute of Chemical Physics, Academy of Sciences for their financial support to this research. References
5. Conclusion Ž1. The fretting wear-resistance of TiN, MoS 2 and TiN q MoS 2 composite coatings are much better than that of 1045 steel without coatings. Appropriate surface techniques are effective for preventing the fretting damage. Ž2. The TiN q MoS 2 composite coating showed much better tribological properties than single TiN and MoS 2 coatings. The duplex treatment can create significant improved results through the synergetic effect of single coatings. Ž3. The wear volumes of three coatings increase with slip amplitude and normal load. The growth rate of wear volumes of MoS 2 coating is higher than that of the other coatings. TiN coating is not sensitive to the variation of
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