- Email: [email protected]
Duplex treatment for surface improvement of 2024 Al
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Vacuum 62 (2001) 337}343
Duplex treatment for surface improvement of 2024 Al Jiayi Sun*, Lijun Weng, Qunji Xue State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People's Republic of China Received 8 November 2000; received in revised form 4 January 2001; accepted 4 January 2001
Abstract Duplex treatment, a newly developed surface modi"cation method, was used to improve the tribological properties of an aluminum alloy. In this paper, the 2024 Al was anodically oxidized "rst and then coated with REMF}MoS }Au nanocomposite "lm and Ti/Ag dual layer "lm by rf-sputtering and multi-arc ion-plating, respectively. It is shown that duplex treatment, especially sputtered REMF}MoS }Au nanocomposite "lm, can decrease the friction coe$cient and improve the endurance life of 2024 Al in vacuum. The lubrication failure mechanism of the duplex treated 2024 Al is attributed to the generation of the cracks in the anodic oxide coating induced by normal load combined with friction force (shearing force) and the crack propagation mainly caused by friction force. It is proposed that pores of several tens of nanometers in diameter in the anodic oxide coating can postpone the propagation of the cracks. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Duplex treatment; Aluminum alloy; Tribological properties
1. Introduction Aluminum and its alloys such as 2024 Al are of high speci"c strength and good chemical stability and so they are good candidate materials for aerospace applications. Hence they are increasingly used in aerospace as constructional or optical elements [1]. When used as tribological parts, however, they show some serious defects including high friction coe$cient, serious wear and a tendency for cold-welding when mated with other metals, especially, steel [2,3]. * Corresponding author. 604 Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People's Republic of China. Tel.: #86-0931-8274335; fax: #86-0931-8274335. E-mail address: [email protected] (J. Sun).
The tribological surface modi"cation of aluminum alloy used in vacuum has been investigated by many researchers in recent years. This research focuses on the hardening of the aluminum surface by ion implanting, depositing hard "lms and anodic oxidation [4}7]. These methods can produce a hard coating on the aluminum surface and so e!ectively enhance its wear resistance. For instance, anodic oxidation is an e!ective method to enhance the wear resistance of aluminum alloy, e.g. an aluminum oxide coating of a few to several hundred micrometers thick can be formed to increase the hardness of the surface and to improve the wear-resistant properties. But the hard surfaces have high friction coe$cients when sliding against metals. On the other hand the endurance life cannot meet the increasing demands.
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J. Sun et al. / Vacuum 62 (2001) 337}343
Duplex treatment, a recently developed surface modi"cation method, is an established method to improve tribological properties of materials [8}11]. The advantages of two conventional surface treatment methods are combined and tribological performance can be much improved compared to a single treatment method. In this paper, 2024 Al was anodic oxidized "rstly and then two lubricating "lms have been deposited [12,13] onto the anodized 2024 Al to reduce its friction coe$cient and improve wear resistance. In previous works we have investigated the mechanism of lubrication degradation and failure of MoS -sputtered "lms stored in moist air, and found a way to improve the storage resistance to the moist air for MoS -based "lms by including small amount of rare earth metal #uorinated compound (REMF) [14]. Furthermore, a new type of MoS -based co-sputtered nanocomposite "lm, REMF}MoS }Au, was prepared and investigated recently. The results show that the new "lm has a preferential crystal orientation and more dense stable structure, it also has a more stable and lower friction coe$cient, longer endurance life, higher load-bearing ability and high reliability in vacuum. The "lm was "rst published here and used for improving tribological performance of 2024 Al.
2. Experimental The 2024 Al specimens were machined to discs of 45 mm diameter and 8 mm thick, and working surface polished to R )1.25 m. Anodic oxide coating was prepared in the surface of the specimen by sulphuric acid anodizing. The conditions for anodic oxidation are listed in Table 1. Rf-sputterred REMF}MoS }Au composite "lms (1 m thick) and multi-arc ion-plated Ti/Ag soft metal dual layer "lms (1 m thick) were, respectively, deposited onto the anodized 2024 Al specimen. The latter "lms consisted of a 0.8 m thick upper layer of Ag and a 0.2 m thick Ti sublayer. The deposition conditions are listed in Table 2. The tribological properties of the "lms were investigated using a ball-on-disc tester in a vacuum about 5.0;10\ Pa. The ball was made of 440C steel (8 mm diameter) and it was pressed against the #at specimen under normal loads of 1, 2 and 5 N, respectively, at a sliding speed of 1000 r min\. The structures of the "lms were analyzed by X-ray diffraction (XRD). A scanning electron microscope (SEM) made by JEOL Corporation was used to monitor the surface morphology and wear track. Energy dispersive spectrometry (EDS) was used to
Table 1 The conditions of sulfuric acid anodizing for 2024 Al Electrolyte solution
Concentration (g l\)
Current density (A cm\)
Temperature (3C)
H SO solution
180
1.5
6}8
Anodic oxide coating thickness (m) 2.5 5 10
Table 2 Deposition conditions of PVD "lms Films
Pressure
Ion-bombardment power (mW mm\)
Ion-bombardment time (min)
Deposition arc current (A)
Deposition time (min)
REMF}MoS }Au Ag and Ti
2;10\ 6;10\
7.8 7.8
10 10
0.5 80
25 20
49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96
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investigate the elemental compositions of the wear tracks.
3. Results 3.1. Microstructure and topography of xlms The XRD and SEM results for an REMF} MoS }Au "lm are shown in Figs. 1 and 2. A Ti/Ag dual layer "lm (Ti/Ag "lm) was also studied. Ag "lms are good lubricants in vacuum. The Ti sublayer is used to improve the adhesion between the Ag "lm and the Al O coating. The XRD and SEM results of the "lm are shown in Figs. 3 and 4.
Fig. 2. SEM mirograph of REMF}MoS }Au nanocomposite "lms.
3.2. Tribological evaluation of duplex treated 2024 Al The results of ball-on-disc tests are shown in Figs. 5 and 6. It can be seen from Fig. 5 that friction coe$cients at a load of 1 N are about 0.19}0.20 for specimen deposited with Ti/Ag "lm on anodized oxide coatings with various thickness. However, at a load of 2 N the friction coe$cients are about 0.26$0.02; under a load of 5 N the friction responses become violent, accompanied with high friction noise for all the substrates, indicating that the Ti/Ag "lm is not suitable for higher load condi-
Fig. 3. XRD patterns of Ti/Ag "lms.
Fig. 1. XRD patterns of REMF}MoS }Au nanocomposite "lms.
tions. On the contrary, the specimens deposited with REMF}MoS }Au "lm show a very low fric tion coe$cient of about 0.05}0.06 for all substrates at a load of 5 N. The friction coe$cients are also about 0.05}0.06 under 1 and 2 N and not listed in the "gure. Fig. 6 shows obvious di!erence between the endurance lives of duplex treated 2024 Al with the two "lms. The endurance life increases along with the increasing thickness of anodic oxide coating and decrease along with the rising of loads in general. It is noted that for the REMF}MoS }Au coated specimen the endurance life is obviously
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Fig. 6. Endurance lives of duplex treated 2024 Al. Fig. 4. SEM micrograph of Ti/Ag "lms.
Fig. 5. Mean friction coe$cients of duplex treated 2024 Al.
higher than that of Ti/Ag coated specimen, especially under 5 N load. The endurance life of the further under 1 and 2 N were not listed here since the values are too long ('3.2;10 revolutions (r)) to be drawn in "gure. Typical wear tracks were analyzed here. Fig. 7 shows the SEM morphologies of the wear tracks of the duplex treated 2024 Al. It can be seen that cracks appeared in the anodic oxide coating after 1;10 r, meanwhile the friction remains stable for the specimen deposited Ti/Ag "lm. The cracks are almost perpendicular to the sliding direction. As the number of revolutions increased, more and more cracks appeared and tended to propagate widely. At 4.2;10 r, pieces of anodic oxide coating with Ti/Ag "lm were peeled o! from the substrate
rapidly. Thus, the friction coe$cient rose sharply because of the lubrication failure. Fig. 7c shows the plough signs on the 2024 Al surface. When the REMF}MoS }Au "lm was deposited on the anodized 2024 Al, considerably improved friction and wear behaviors were obtained. This was especially so at relatively high loads. Fig. 7d shows the SEM morphologies of the worn surfaces of anodized 2024 Al deposited with REMF}MoS }Au "lms. It is seen that the worn surface appears to be much smoother, though some thin cracks are observed at higher revolution cycles.
4. Discussion The results of friction and wear tests show that the lubrication procedure of the duplex treated 2024 Al can be divided into three stages. Firstly, the steel ball rubbed on the PVD lubrication "lm and the friction coe$cient is thus lowered and kept stable. Secondly, with repetitive rubbing cracks begin to appear and propagate in the anodic oxide coatings under the combined action of normal stress and tangential frictional force (shearing force). Thirdly, pieces of anodic oxide coating are peeled o! from the substrate, thus the ball comes into direct contact with the 2024 Al substrate, and consequently the friction coe$cient and the wear rise signi"cantly.
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Fig. 7. The SEM photographs of wear tracks of the duplex treated 2024 Al (Arrows show the sliding direction). (a) Ti/Ag "lm, sliding under 2 N for 10 revolutions, (b) Ti/Ag "lm, sliding under 2 N for 3.9;10 revolutions, (c) Ti/Ag "lm, sliding under 2 N for 4.2;10 revolutions. (d) REMF}MoS }Au nanocomposite "lm, sliding under 5 N for 6;10 revolutions.
Based on the above results and materials fatigue fracture mechanism, a model of failure mechanism is proposed as follows. Fig. 8 shows the schematic diagram of the sliding system. The steel ball contacts with the specimen under a normal load and slides forward. The contact zone in the anodic oxide coating bore the normal load and shearing force (friction force). Fig. 9a shows the microstructure of anodic oxide coating. It is well known that there are many pores in a diameter range of 30}50 nm in the anodic oxide coating and the pores pass through the coating from the bottom to the surface. Most of the load is borne by the anodic oxide coating since the lubricating "lms are very thin. As we know that the anodic oxide coating is a fragile material and much harder than the Al
Fig. 8. Schematic diagram of the sliding system.
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Fig. 9. Schematic diagram of lubricating failure mechanism. (a) Micro-structure of anodic oxide coating. (b) Cracks generating and propagating. (c) Expend cracks meeting (d) Pieces of anodic oxide coating was peeled o!.
substrate. So most cracks will probably generate in the hard coating near the interface between the hard coating and the soft 2024 Al substrate since the strain is not continuous here (Fig. 9b). The cracks spread along the y direction toward the hard coating surface under the load action and at the same time spread perpendicularly to the sliding direction in the x}z plane under the action of shearing force. It is unavoidable that cracks spreading will meet the pores in x}z plane which act as plastic zone at the cracks tips and so the cracks spreading will be hindered (Fig. 9c). Thus cracks spreading rates are lowered. When the cracks spread to a certain degree, some cracks will meet together and probably produce some pieces separated from the hard coating. Then the pieces are peeled o! under the action of the sliding (Fig. 9d).
It needs to be pointed out that the generation and propagation of the cracks in anodic oxide coatings during friction process are directly related to the tribological properties of the PVD "lms deposited on them. The specimen deposited with Ti/Ag "lm is more liable to generate cracks, and the cracks are easier to propagate than that with REMF}MoS }Au "lm. According to the model mentioned above, the shearing force borne by the specimen deposited with Ti/Ag "lm is greater than that with REMF}MoS }Au "lm since the latter has lower friction. So the rates of cracks propagaton of the specimen coated with REMF}MoS }Au "lm caused by shearing force was much lowered and the specimen shows higher endurance life.
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5. Conclusions (a) The duplex treated 2024 Al has improved tribological performance in vacuum. The specimen deposited with REMF}MoS }Au "lm shows both much lower friction and longer endurance life than that deposited with Ti/Ag dual layer "lm. (b) The lubrication failure of the duplex treated 2024 Al is attributed to the generation and propagating of the cracks in the anodic oxide coating during the friction process. When the crack propagation causes some pieces of the anodic oxide coating to peel o! from the aluminum substrate, lubrication would fail. (c) A model of failure mechanism was proposed. Most cracks probably generate in the hard coating near the interface between the hard coating and the soft Al substrate, and they propagate under the action of shearing force. The pores in the anodic oxide coating can lower the rates of cracks spreading. When the cracks spread to a certain degree, some cracks will meet together and probably produce some pieces separated from the anodic oxide coating. Then the pieces are peeled o! under the action of the sliding.
Acknowledgements The "nancial support from the National Natural Science Foundation of China (Grant
343
No.59825116 and 59805019) is gratefully acknowledged.
References [1] Sheng XY, Callahan DL, Barrera EV. Mater Manuf Process 1997;12(2):215}28. [2] Zhang W, Bay N. Welding J 1997;76(8):326}30. [3] Nautiyal PC, Schey JA. J Tribology (China) 1990;11(2):282}7. [4] Yoon H, Sheiretov T, Cusano C, 1998;218(1):54}65. [5] Maejima M, Saruwatari K, Isawa K, Takaya M. Metal Finishing 1998;96(36):38}41. [6] Maejima M, Saruwatari K, Isawa K. Metal Finishing 1997;95(12):10}3. [7] Zhang A, Chen J, Shi W, Liu Z. Nucl Instr and Meth B 2000;169(1}4)2:43}7. [8] Zlatanovic MD, Kunosic AM, Belosevac RB, Popovic NV. Mater Sci Forum 1996;214:179}88. [9] Quaeyhaegens C, Kerkhofs M, Stals LM, Van Stappen M. Surf Coat Technol 1996;80(1}2):181}4. [10] Spies H-J, Larisch B, Hoeck K, Broszeit E, Chroeder H-J. Surf Coat Technol 1995;74}75(1}3):178}82. [11] Lee SY, Chung JW, Kim CW, Han JG, Kim SS, Lee JH. Surf Coat Technol 1997;94}95(1}3):272}8. [12] Wahi KJ, Dunn DN, Singer IL. Wear 2000;237:1}12. [13] Fu Y, Loh NL, Wei J, Yan B, Hing P. Wear 2000;237:12}20. [14] Yu D-Y, Wang J-A, Ou Yang J-L. Thin Solid Films 1997;293:1}5.
29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52
Vacuum 62 (2001) 337}343
Duplex treatment for surface improvement of 2024 Al Jiayi Sun*, Lijun Weng, Qunji Xue State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People's Republic of China Received 8 November 2000; received in revised form 4 January 2001; accepted 4 January 2001
Abstract Duplex treatment, a newly developed surface modi"cation method, was used to improve the tribological properties of an aluminum alloy. In this paper, the 2024 Al was anodically oxidized "rst and then coated with REMF}MoS }Au nanocomposite "lm and Ti/Ag dual layer "lm by rf-sputtering and multi-arc ion-plating, respectively. It is shown that duplex treatment, especially sputtered REMF}MoS }Au nanocomposite "lm, can decrease the friction coe$cient and improve the endurance life of 2024 Al in vacuum. The lubrication failure mechanism of the duplex treated 2024 Al is attributed to the generation of the cracks in the anodic oxide coating induced by normal load combined with friction force (shearing force) and the crack propagation mainly caused by friction force. It is proposed that pores of several tens of nanometers in diameter in the anodic oxide coating can postpone the propagation of the cracks. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Duplex treatment; Aluminum alloy; Tribological properties
1. Introduction Aluminum and its alloys such as 2024 Al are of high speci"c strength and good chemical stability and so they are good candidate materials for aerospace applications. Hence they are increasingly used in aerospace as constructional or optical elements [1]. When used as tribological parts, however, they show some serious defects including high friction coe$cient, serious wear and a tendency for cold-welding when mated with other metals, especially, steel [2,3]. * Corresponding author. 604 Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People's Republic of China. Tel.: #86-0931-8274335; fax: #86-0931-8274335. E-mail address: [email protected] (J. Sun).
The tribological surface modi"cation of aluminum alloy used in vacuum has been investigated by many researchers in recent years. This research focuses on the hardening of the aluminum surface by ion implanting, depositing hard "lms and anodic oxidation [4}7]. These methods can produce a hard coating on the aluminum surface and so e!ectively enhance its wear resistance. For instance, anodic oxidation is an e!ective method to enhance the wear resistance of aluminum alloy, e.g. an aluminum oxide coating of a few to several hundred micrometers thick can be formed to increase the hardness of the surface and to improve the wear-resistant properties. But the hard surfaces have high friction coe$cients when sliding against metals. On the other hand the endurance life cannot meet the increasing demands.
0042-207X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 2 8 7 - 1
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J. Sun et al. / Vacuum 62 (2001) 337}343
Duplex treatment, a recently developed surface modi"cation method, is an established method to improve tribological properties of materials [8}11]. The advantages of two conventional surface treatment methods are combined and tribological performance can be much improved compared to a single treatment method. In this paper, 2024 Al was anodic oxidized "rstly and then two lubricating "lms have been deposited [12,13] onto the anodized 2024 Al to reduce its friction coe$cient and improve wear resistance. In previous works we have investigated the mechanism of lubrication degradation and failure of MoS -sputtered "lms stored in moist air, and found a way to improve the storage resistance to the moist air for MoS -based "lms by including small amount of rare earth metal #uorinated compound (REMF) [14]. Furthermore, a new type of MoS -based co-sputtered nanocomposite "lm, REMF}MoS }Au, was prepared and investigated recently. The results show that the new "lm has a preferential crystal orientation and more dense stable structure, it also has a more stable and lower friction coe$cient, longer endurance life, higher load-bearing ability and high reliability in vacuum. The "lm was "rst published here and used for improving tribological performance of 2024 Al.
2. Experimental The 2024 Al specimens were machined to discs of 45 mm diameter and 8 mm thick, and working surface polished to R )1.25 m. Anodic oxide coating was prepared in the surface of the specimen by sulphuric acid anodizing. The conditions for anodic oxidation are listed in Table 1. Rf-sputterred REMF}MoS }Au composite "lms (1 m thick) and multi-arc ion-plated Ti/Ag soft metal dual layer "lms (1 m thick) were, respectively, deposited onto the anodized 2024 Al specimen. The latter "lms consisted of a 0.8 m thick upper layer of Ag and a 0.2 m thick Ti sublayer. The deposition conditions are listed in Table 2. The tribological properties of the "lms were investigated using a ball-on-disc tester in a vacuum about 5.0;10\ Pa. The ball was made of 440C steel (8 mm diameter) and it was pressed against the #at specimen under normal loads of 1, 2 and 5 N, respectively, at a sliding speed of 1000 r min\. The structures of the "lms were analyzed by X-ray diffraction (XRD). A scanning electron microscope (SEM) made by JEOL Corporation was used to monitor the surface morphology and wear track. Energy dispersive spectrometry (EDS) was used to
Table 1 The conditions of sulfuric acid anodizing for 2024 Al Electrolyte solution
Concentration (g l\)
Current density (A cm\)
Temperature (3C)
H SO solution
180
1.5
6}8
Anodic oxide coating thickness (m) 2.5 5 10
Table 2 Deposition conditions of PVD "lms Films
Pressure
Ion-bombardment power (mW mm\)
Ion-bombardment time (min)
Deposition arc current (A)
Deposition time (min)
REMF}MoS }Au Ag and Ti
2;10\ 6;10\
7.8 7.8
10 10
0.5 80
25 20
49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96
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339
investigate the elemental compositions of the wear tracks.
3. Results 3.1. Microstructure and topography of xlms The XRD and SEM results for an REMF} MoS }Au "lm are shown in Figs. 1 and 2. A Ti/Ag dual layer "lm (Ti/Ag "lm) was also studied. Ag "lms are good lubricants in vacuum. The Ti sublayer is used to improve the adhesion between the Ag "lm and the Al O coating. The XRD and SEM results of the "lm are shown in Figs. 3 and 4.
Fig. 2. SEM mirograph of REMF}MoS }Au nanocomposite "lms.
3.2. Tribological evaluation of duplex treated 2024 Al The results of ball-on-disc tests are shown in Figs. 5 and 6. It can be seen from Fig. 5 that friction coe$cients at a load of 1 N are about 0.19}0.20 for specimen deposited with Ti/Ag "lm on anodized oxide coatings with various thickness. However, at a load of 2 N the friction coe$cients are about 0.26$0.02; under a load of 5 N the friction responses become violent, accompanied with high friction noise for all the substrates, indicating that the Ti/Ag "lm is not suitable for higher load condi-
Fig. 3. XRD patterns of Ti/Ag "lms.
Fig. 1. XRD patterns of REMF}MoS }Au nanocomposite "lms.
tions. On the contrary, the specimens deposited with REMF}MoS }Au "lm show a very low fric tion coe$cient of about 0.05}0.06 for all substrates at a load of 5 N. The friction coe$cients are also about 0.05}0.06 under 1 and 2 N and not listed in the "gure. Fig. 6 shows obvious di!erence between the endurance lives of duplex treated 2024 Al with the two "lms. The endurance life increases along with the increasing thickness of anodic oxide coating and decrease along with the rising of loads in general. It is noted that for the REMF}MoS }Au coated specimen the endurance life is obviously
49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96
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Fig. 6. Endurance lives of duplex treated 2024 Al. Fig. 4. SEM micrograph of Ti/Ag "lms.
Fig. 5. Mean friction coe$cients of duplex treated 2024 Al.
higher than that of Ti/Ag coated specimen, especially under 5 N load. The endurance life of the further under 1 and 2 N were not listed here since the values are too long ('3.2;10 revolutions (r)) to be drawn in "gure. Typical wear tracks were analyzed here. Fig. 7 shows the SEM morphologies of the wear tracks of the duplex treated 2024 Al. It can be seen that cracks appeared in the anodic oxide coating after 1;10 r, meanwhile the friction remains stable for the specimen deposited Ti/Ag "lm. The cracks are almost perpendicular to the sliding direction. As the number of revolutions increased, more and more cracks appeared and tended to propagate widely. At 4.2;10 r, pieces of anodic oxide coating with Ti/Ag "lm were peeled o! from the substrate
rapidly. Thus, the friction coe$cient rose sharply because of the lubrication failure. Fig. 7c shows the plough signs on the 2024 Al surface. When the REMF}MoS }Au "lm was deposited on the anodized 2024 Al, considerably improved friction and wear behaviors were obtained. This was especially so at relatively high loads. Fig. 7d shows the SEM morphologies of the worn surfaces of anodized 2024 Al deposited with REMF}MoS }Au "lms. It is seen that the worn surface appears to be much smoother, though some thin cracks are observed at higher revolution cycles.
4. Discussion The results of friction and wear tests show that the lubrication procedure of the duplex treated 2024 Al can be divided into three stages. Firstly, the steel ball rubbed on the PVD lubrication "lm and the friction coe$cient is thus lowered and kept stable. Secondly, with repetitive rubbing cracks begin to appear and propagate in the anodic oxide coatings under the combined action of normal stress and tangential frictional force (shearing force). Thirdly, pieces of anodic oxide coating are peeled o! from the substrate, thus the ball comes into direct contact with the 2024 Al substrate, and consequently the friction coe$cient and the wear rise signi"cantly.
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Fig. 7. The SEM photographs of wear tracks of the duplex treated 2024 Al (Arrows show the sliding direction). (a) Ti/Ag "lm, sliding under 2 N for 10 revolutions, (b) Ti/Ag "lm, sliding under 2 N for 3.9;10 revolutions, (c) Ti/Ag "lm, sliding under 2 N for 4.2;10 revolutions. (d) REMF}MoS }Au nanocomposite "lm, sliding under 5 N for 6;10 revolutions.
Based on the above results and materials fatigue fracture mechanism, a model of failure mechanism is proposed as follows. Fig. 8 shows the schematic diagram of the sliding system. The steel ball contacts with the specimen under a normal load and slides forward. The contact zone in the anodic oxide coating bore the normal load and shearing force (friction force). Fig. 9a shows the microstructure of anodic oxide coating. It is well known that there are many pores in a diameter range of 30}50 nm in the anodic oxide coating and the pores pass through the coating from the bottom to the surface. Most of the load is borne by the anodic oxide coating since the lubricating "lms are very thin. As we know that the anodic oxide coating is a fragile material and much harder than the Al
Fig. 8. Schematic diagram of the sliding system.
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Fig. 9. Schematic diagram of lubricating failure mechanism. (a) Micro-structure of anodic oxide coating. (b) Cracks generating and propagating. (c) Expend cracks meeting (d) Pieces of anodic oxide coating was peeled o!.
substrate. So most cracks will probably generate in the hard coating near the interface between the hard coating and the soft 2024 Al substrate since the strain is not continuous here (Fig. 9b). The cracks spread along the y direction toward the hard coating surface under the load action and at the same time spread perpendicularly to the sliding direction in the x}z plane under the action of shearing force. It is unavoidable that cracks spreading will meet the pores in x}z plane which act as plastic zone at the cracks tips and so the cracks spreading will be hindered (Fig. 9c). Thus cracks spreading rates are lowered. When the cracks spread to a certain degree, some cracks will meet together and probably produce some pieces separated from the hard coating. Then the pieces are peeled o! under the action of the sliding (Fig. 9d).
It needs to be pointed out that the generation and propagation of the cracks in anodic oxide coatings during friction process are directly related to the tribological properties of the PVD "lms deposited on them. The specimen deposited with Ti/Ag "lm is more liable to generate cracks, and the cracks are easier to propagate than that with REMF}MoS }Au "lm. According to the model mentioned above, the shearing force borne by the specimen deposited with Ti/Ag "lm is greater than that with REMF}MoS }Au "lm since the latter has lower friction. So the rates of cracks propagaton of the specimen coated with REMF}MoS }Au "lm caused by shearing force was much lowered and the specimen shows higher endurance life.
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5. Conclusions (a) The duplex treated 2024 Al has improved tribological performance in vacuum. The specimen deposited with REMF}MoS }Au "lm shows both much lower friction and longer endurance life than that deposited with Ti/Ag dual layer "lm. (b) The lubrication failure of the duplex treated 2024 Al is attributed to the generation and propagating of the cracks in the anodic oxide coating during the friction process. When the crack propagation causes some pieces of the anodic oxide coating to peel o! from the aluminum substrate, lubrication would fail. (c) A model of failure mechanism was proposed. Most cracks probably generate in the hard coating near the interface between the hard coating and the soft Al substrate, and they propagate under the action of shearing force. The pores in the anodic oxide coating can lower the rates of cracks spreading. When the cracks spread to a certain degree, some cracks will meet together and probably produce some pieces separated from the anodic oxide coating. Then the pieces are peeled o! under the action of the sliding.
Acknowledgements The "nancial support from the National Natural Science Foundation of China (Grant
343
No.59825116 and 59805019) is gratefully acknowledged.
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