- Email: [email protected]
Microstructure developed in the surface layer of Ti-6Al-4V alloy after sliding wear in vacuum
Materials Characterization 50 (2003) 275 – 279
Microstructure developed in the surface layer of Ti-6Al-4V alloy after sliding wear in vacuum Y. Liu *, D.Z. Yang, S.Y. He, W.L. Wu Space Materials and Environment Engineering Laboratory, Harbin Institute of Technology, P.O. Box 432, 92 Xidazhi Street, Harbin 150001, People’s Republic of China Received 6 December 2002; accepted 6 July 2003
Abstract Microstructural changes in the surface layer of Ti-6Al-4V alloy after sliding wear in vacuum have been studied by means of scanning and transmission electron microscopy (SEM and TEM). The wear rates of Ti-6Al-4V alloy in vacuum were measured under different sliding velocities and loads. The experimental results showed that a severely deformed layer with a grain size of 50 – 100 nm and thickness about 70 Am was formed underneath the worn surface. Under the slower sliding velocities, the substructure of the layer had a high dislocation density, while under higher sliding velocities, twins were found to exist in the substructure. A process by which the deformed layer formed has been proposed and the deformation of materials at the contacting spots of the Ti-6Al-4V sample is discussed. D 2003 Elsevier Inc. All rights reserved. Keywords: Ti-6Al-4V alloy; Wear; Microstructure; Deformation
1. Introduction Sliding wear processes of ductile materials are often accompanied by severe plastic deformation [1]. Shear instability can occur soon after sliding begins and can result in nucleation and propagation of microcracks on the worn surface and the formation of flat pieces of debris [2 –4]. It is known that the contact spots play a load-bearing role during the sliding contact. If the combined stresses induced by mechanical impacts and the adhesive forces of seizure
* Corresponding author. Tel.: +86-4516412462; fax: +864516415168. E-mail address: [email protected] (Y. Liu). 1044-5803/$ - see front matter D 2003 Elsevier Inc. All rights reserved. doi:10.1016/S1044-5803(03)00125-6
bridges at microcontact spots exceed the yield strength of the materials, plastic deformation can occur. However, the friction and wear process is more complicated. The distribution of stresses at the contact spots changes dynamically every second, and deformation and fracture of the sliding surface layer occurs in a very short time. It is difficult to evaluate the subsurface strain and stress distribution in sliding wear specimens using a static method of mechanics or metallurgy. Studies on surface deformation [5 –7] have shown that a plastic flow zone exists in the contact spot material and a special friction-induced deformed layer can be formed in the surface layer during the sliding contact [8,9]. The surface layer is characterized by the formation of a gradient structure due to the occurrence during friction of deformation at
276
Y. Liu et al. / Materials Characterization 50 (2003) 275–279
different levels. The physical mesomechanics illustrate that a vortex motion in the surface layer is generated, and microcracks initiate at its boundaries [10,11]. On the other hand, metallographic observations of the surface layers have shown that the deformation can result in the formation of a nanocrystalline structure with a grain size of 10– 50 nm in some alloys, usually with a second phase [12,13]. All of the above results suggest that the structural changes in the worn surface layer present a beneficial trend to deformation and energy dissipation. Titanium and its alloys have poor wear resistance, although they exhibit high specific strength. The dry wear mechanism of Ti-6Al-4V alloy against hardened steel in an air environment is controlled by delamination [14 – 16] and oxidation [15,16]. During reciprocating-sliding motion of Ti-6Al-4V alloy, extensive plastic deformation [15,17], material transfer [15 – 17], and ultrafine microstructure [18,19] are observed. Usually, the close-packed hexagonal structure is considered to have good wear characteristics. The crystalline characteristics of titanium alloys make it interesting to study their sliding wear behavior. To explore the intrinsic characteristics of sliding wear of these alloys, it was determined to carry out wear tests in vacuum because of the great effects that atmospheric composition and pressure have on the sliding wear. The particular focus of this study was the change in microstructure of the Ti-6Al-4V alloy surface layer after sliding wear in vacuum.
The steel was water quenched from 840 jC then tempered at 150 jC to give a hardness of 62 HRC. The disc itself was 10 mm thick and had a diameter of 70 mm. The surfaces of the pin and the disc were ground to obtain surface roughness values, Ra, of about 0.42 and 3.20 Am, respectively. The wear tests were performed using loads of 30, 50, 70, and 90 N and sliding velocities in the range 0.2 –1.2 m/s. Each test involved a total sliding distance of 1000 m. Wear was characterized by the mass loss of the pin, measured using an electronic balance with the precision of F10 5 g. Three tests were performed at each load/velocity combination, with the wear rate being calculated as the average of the three tests. The wear teats were carried out under an environmental pressure of 10 5 Pa using a YTEI TB 100 type of vacuum friction-and-wear tester made in Ukraine. Scanning electron microscope (SEM) observations were made using a Hitachi type S-570 SEM. Before examination in the SEM, the worn surfaces of the samples were prepared by ultrasonic washing in acetone and drying. Transmission electron microscope (TEM) observations were made using a Philips type 22 TEM. To examine the change in microstructure in the surface layer after wear in vacuum, thin foils were cut along the plane parallel to the worn surface then polished to a distance of about 80 Am from the worn surface. Finally, the foils were thinned by ion milling. According to the procedures used in the preparation of the thin foils, the zone for TEM observation was about 40 Am from the surface.
2. Experimental details The wear tests were carried out using a pin-on-disc scheme in which the wear couple was composed of a Ti-6Al-4V alloy pin and a steel disc. The nominal chemical composition of the pin material was (in wt.%): Al 5.5 – 6.5, V 3.5 – 4.5, Fe 0.03, O 0.15, balance Ti). The Ti-6Al-4V, received in the form of hot-rolled rod, was annealed at 760 jC for 1 h and furnace cooled to obtain an average hardness of 345 HV (35 HRC). The rotating pin was cylindrical in shape, 9.0 mm in diameter, and 20 mm in length and with a spherical end of radius of 40 mm. The disc counter material was the high-carbon chrome steel GCr15 (equivalent to AISI 52100 and containing around 1% carbon and 1.5% chromium by weight).
3. Results and discussion Fig. 1 shows wear rate as a function of sliding velocity for different applied loads. At low loads (30 and 50 N), the wear rate increased slightly as the sliding velocity increased. However, under higher loads (70 and 90 N), this dependence of wear rate on sliding velocity was more pronounced. Over the range of loads and velocities investigated, the wear rates were much lower than those reported in Refs. [14,15] for Ti-6Al-4V alloy sliding against hardened steel in air. SEM analyses showed that the worn surfaces of all the Ti-6Al-4V alloy samples exhibited similar mor-
Y. Liu et al. / Materials Characterization 50 (2003) 275–279
277
Fig. 1. Wear rate of Ti-6Al-4V alloy as a function of sliding velocity for different applied loads in vacuum (10 5 Pa).
phologies. Fig. 2 shows the surface morphology of the Ti-6Al-4V alloy after wear under a sliding velocity of 0.6 m/s and load of 50 N. Typical characteristics of plowing along the sliding direction and tongue-shaped wedges were observed, forming a laminar microstructure similar to a fracture morphology. However, there are some differences between the morphologies caused by wear and by fracture. Some plowing traces can be seen, indicating the original friction surface. The wedges show a plastic deformation characteristic. Fig. 3 shows the microstructure in the surface layer (f40 Am from the surface) of Ti-6Al-4V alloy samples before and after wear under a load 70 N at two different sliding velocities. The original grain size of the sample after annealing was about 100 Am (see Fig. 3a), while a very fine microstructure with a grain size of 50 – 100 nm was formed after the wear in
Fig. 3. TEM micrographs showing microstructure of the surface layer of Ti-6Al-4V alloy (a) before and after wear in vacuum (10 5 Pa) under the load of 70 N and sliding velocity of (b) 0.2 m/s and (c) 0.8 m/s, respectively.
Fig. 2. SEM micrographs showing the worn surface morphology of Ti-6Al-4V alloy under 70 N and 0.6 m/s in vacuum (10 5 Pa).
vacuum (Fig. 3b and c). Following wear under lower sliding velocities, the substructure of the layer comprised a high density of dislocations (Fig. 3b). After wear at higher sliding velocities, many microtwins
278
Y. Liu et al. / Materials Characterization 50 (2003) 275–279
Fig. 4. SEM micrographs showing (a) microstructure prior to wear test and (b) cross-sectional laminar microstructure of the Ti-6Al-4V alloy sample after wear test under 70 N and 0.6 m/s in vacuum (10 5 Pa).
existed in the substructure (Fig. 3c). According to the contact spot deformation theory [12], the strain rate increases with increases in the sliding velocity. Thus, the above difference in the substructure might be related to a different response to strain rate. The deformation mechanism would change from slip under low strain rates to twinning under high strain rates. On the other hand, the very fine microstructure appearing after sliding wear in vacuum indicates that an intense plastic deformation occurred on a large scale. Refinement of the microstructure and the formation of high dislocation densities and twins can dissipate a large amount of the energy produced by sliding friction and also resist the formation of cracks on the worn surface. Considering the deformation depth range (> 40 Am) and the laminar structure characteristic on the worn surface, a special deformation process could occur
during the sliding wear. Because the Ti-6Al-4V alloy has low tear resistance, the material at the contact spots can be easily deformed due to the mechanical and adhesive forces applied by the counter disc. According to the shear model of wear [10], the material at the contact spots is bunched up and pushed forward, forming tongue-shaped wedges. Furthermore, the wedges can be compressed and rolled onto the front or side surface of the Ti-6Al-4V pins. Because the test is being carried out in vacuum, there will be no environmental effect on the interface between the compressed material and the original surface. Thus, there will be good adhesion at the interface and no cracking will occur. As the process is repeated, the wedges continue to develop and a stacked layer, in which the wedges overlap each other, can be formed on the worn surface of the Ti-6Al-4V alloy. The SEM micrographs in Fig. 4 depict the crosssectional microstructure of the Ti-6Al-4V alloy before and after wear testing. The acicular form of the microstructure before testing can be seen in Fig. 4a. Fig. 4b shows the morphology of the deformedstacked layer in a worn specimen (load 70 N, sliding velocity 0.6 m/s). It is apparent that the thickness of the layer is about 70 Am. A pronounced deformed and laminar characteristic can be seen clearly at the tail area where the deformed-stacked layer would be squeezed out of the worn interface and formed into wear debris, as shown in Fig. 5.
Fig. 5. SEM micrograph showing a piece of layered debris formed at the end of the friction interface of the Ti-6Al-4V alloy under 70 N and 0.6 m/s in vacuum (10 5 Pa).
Y. Liu et al. / Materials Characterization 50 (2003) 275–279
The thermal effect induced by friction is also important to the wear behavior of Ti-6Al-4V alloy. Straffelini and Molinari [15] demonstrate that the surface temperature of Ti-6Al-4V alloy samples rubbing against steel increases as the sliding velocity increases. Thermal softening ensues and the above deforming and stacking process of the material at the contacting spots is easily developed as the temperature is increased [14]. This can result in an increase in the wear rate of the Ti-6Al-4V alloy as the sliding velocity (and the load) increase, especially in the range of the higher sliding velocities and applied loads, as shown in Fig. 1.
4. Conclusions (1) A 70-Am-thick surface layer with a grain size of 50 – 100 nm was formed after the sliding wear of Ti-6Al-4V alloy in vacuum. The substructure in the surface layer of the worn sample comprised mainly of a high density of dislocations at the slower sliding velocities. In the case of higher sliding velocities, twins were found to exist in the substructure. (2) The wear rate of the Ti-6Al-4V alloy in vacuum increased with increasing the sliding velocity and applied load. (3) A special plastic deformation process in which the material at the contacting spots is pushed, rolled, and stacked was proposed to describe the formation of the severely deformed surface layer.
Acknowledgements The authors are grateful to G.D. Gumulya for his interest in and encouragement of this work. This research is supported by National Basis Research Foundation of China.
279
References [1] Rosenfield AR. A shear instability model of sliding wear. Wear 1987;116:319 – 28. [2] Blau PJ, Doyle ED. Metallographic evidence for the nucleation of subsurface microcracks during unlubricated sliding of metals. Wear 1987;117:381 – 87. [3] Alpas AT, Hu H, Zhang J. Plastic deformation and damage accumulation below the worn surfaces. Wear 1993;162 – 164:188 – 95. [4] Mishina H. Surface deformation and formation of original element of wear particles in sliding friction. Wear 1998;215: 10 – 7. [5] Kato K. Micro-mechanisms of wear – wear modes. Wear 1992;153:277 – 95. [6] Kopalinsky EM, Oxley PLB. Explaining the mechanics of metallic sliding friction and wear in terms of slipline field models of asperity deformation. Wear 1995;190:145 – 54. [7] Torrance AA. The influence of surface deformation on mechanical wear. Wear 1996;200:45 – 54. [8] Karmakar S, Sethuramiah A. Response of dynamically stressed material in sliding. Wear 1991;151:269 – 78. [9] Chue CH, Chung HH, Lin JF, Chou CC. The effects of strain hardened layer on pitting formation during rolling contact. Wear 2001;249:109 – 16. [10] Kuhlmann-Wilsdorf D. What role for contact spots and dislocations in friction and wear? Wear 1997;200:8 – 29. [11] Panin V, Kolubaev A, Tarasov S, Popov V. Subsurface layer formation during sliding friction. Wear 2002;249:860 – 7. [12] Rigney DA. Some thoughts on sliding wear. Wear 1992;152: 187 – 92. [13] Rigney DA. Comments on the sliding wear of metals. Tribol Int 1997;30:361 – 7. [14] Molinari A, Tesi B, Bacci T. Dry sliding wear mechanisms of the Ti6Al4V alloy. Wear 1997;208:105 – 12. [15] Straffelini G, Molinari A. Dry sliding wear of Ti-6Al-4V alloy as influenced by the counterface and sliding condition. Wear 1999;236:328 – 38. [16] Alam MO, Haseeb ASA. Response of Ti-6Al-4V and Ti-24Al11Nb alloys to dry sliding wear against hardened steel. Tribol Int 2002;35:357 – 62. [17] Long M, Rack HJ. Friction and surface behavior of selected titanium alloys during reciprocating-sliding motion. Wear 2001;249:158 – 68. [18] Fayeulle S, Blanchard P, Vincent L. Fretting behavior of titanium alloys. Tribol Int 1993;36:267 – 75. [19] Nicolaou PD, Shell EB, Matikas TE. Microstructural and surface characterization of Ti-6Al-4V alloys after fretting fatigue. Mater Sci Eng A 1999;269:98 – 103.
Microstructure developed in the surface layer of Ti-6Al-4V alloy after sliding wear in vacuum Y. Liu *, D.Z. Yang, S.Y. He, W.L. Wu Space Materials and Environment Engineering Laboratory, Harbin Institute of Technology, P.O. Box 432, 92 Xidazhi Street, Harbin 150001, People’s Republic of China Received 6 December 2002; accepted 6 July 2003
Abstract Microstructural changes in the surface layer of Ti-6Al-4V alloy after sliding wear in vacuum have been studied by means of scanning and transmission electron microscopy (SEM and TEM). The wear rates of Ti-6Al-4V alloy in vacuum were measured under different sliding velocities and loads. The experimental results showed that a severely deformed layer with a grain size of 50 – 100 nm and thickness about 70 Am was formed underneath the worn surface. Under the slower sliding velocities, the substructure of the layer had a high dislocation density, while under higher sliding velocities, twins were found to exist in the substructure. A process by which the deformed layer formed has been proposed and the deformation of materials at the contacting spots of the Ti-6Al-4V sample is discussed. D 2003 Elsevier Inc. All rights reserved. Keywords: Ti-6Al-4V alloy; Wear; Microstructure; Deformation
1. Introduction Sliding wear processes of ductile materials are often accompanied by severe plastic deformation [1]. Shear instability can occur soon after sliding begins and can result in nucleation and propagation of microcracks on the worn surface and the formation of flat pieces of debris [2 –4]. It is known that the contact spots play a load-bearing role during the sliding contact. If the combined stresses induced by mechanical impacts and the adhesive forces of seizure
* Corresponding author. Tel.: +86-4516412462; fax: +864516415168. E-mail address: [email protected] (Y. Liu). 1044-5803/$ - see front matter D 2003 Elsevier Inc. All rights reserved. doi:10.1016/S1044-5803(03)00125-6
bridges at microcontact spots exceed the yield strength of the materials, plastic deformation can occur. However, the friction and wear process is more complicated. The distribution of stresses at the contact spots changes dynamically every second, and deformation and fracture of the sliding surface layer occurs in a very short time. It is difficult to evaluate the subsurface strain and stress distribution in sliding wear specimens using a static method of mechanics or metallurgy. Studies on surface deformation [5 –7] have shown that a plastic flow zone exists in the contact spot material and a special friction-induced deformed layer can be formed in the surface layer during the sliding contact [8,9]. The surface layer is characterized by the formation of a gradient structure due to the occurrence during friction of deformation at
276
Y. Liu et al. / Materials Characterization 50 (2003) 275–279
different levels. The physical mesomechanics illustrate that a vortex motion in the surface layer is generated, and microcracks initiate at its boundaries [10,11]. On the other hand, metallographic observations of the surface layers have shown that the deformation can result in the formation of a nanocrystalline structure with a grain size of 10– 50 nm in some alloys, usually with a second phase [12,13]. All of the above results suggest that the structural changes in the worn surface layer present a beneficial trend to deformation and energy dissipation. Titanium and its alloys have poor wear resistance, although they exhibit high specific strength. The dry wear mechanism of Ti-6Al-4V alloy against hardened steel in an air environment is controlled by delamination [14 – 16] and oxidation [15,16]. During reciprocating-sliding motion of Ti-6Al-4V alloy, extensive plastic deformation [15,17], material transfer [15 – 17], and ultrafine microstructure [18,19] are observed. Usually, the close-packed hexagonal structure is considered to have good wear characteristics. The crystalline characteristics of titanium alloys make it interesting to study their sliding wear behavior. To explore the intrinsic characteristics of sliding wear of these alloys, it was determined to carry out wear tests in vacuum because of the great effects that atmospheric composition and pressure have on the sliding wear. The particular focus of this study was the change in microstructure of the Ti-6Al-4V alloy surface layer after sliding wear in vacuum.
The steel was water quenched from 840 jC then tempered at 150 jC to give a hardness of 62 HRC. The disc itself was 10 mm thick and had a diameter of 70 mm. The surfaces of the pin and the disc were ground to obtain surface roughness values, Ra, of about 0.42 and 3.20 Am, respectively. The wear tests were performed using loads of 30, 50, 70, and 90 N and sliding velocities in the range 0.2 –1.2 m/s. Each test involved a total sliding distance of 1000 m. Wear was characterized by the mass loss of the pin, measured using an electronic balance with the precision of F10 5 g. Three tests were performed at each load/velocity combination, with the wear rate being calculated as the average of the three tests. The wear teats were carried out under an environmental pressure of 10 5 Pa using a YTEI TB 100 type of vacuum friction-and-wear tester made in Ukraine. Scanning electron microscope (SEM) observations were made using a Hitachi type S-570 SEM. Before examination in the SEM, the worn surfaces of the samples were prepared by ultrasonic washing in acetone and drying. Transmission electron microscope (TEM) observations were made using a Philips type 22 TEM. To examine the change in microstructure in the surface layer after wear in vacuum, thin foils were cut along the plane parallel to the worn surface then polished to a distance of about 80 Am from the worn surface. Finally, the foils were thinned by ion milling. According to the procedures used in the preparation of the thin foils, the zone for TEM observation was about 40 Am from the surface.
2. Experimental details The wear tests were carried out using a pin-on-disc scheme in which the wear couple was composed of a Ti-6Al-4V alloy pin and a steel disc. The nominal chemical composition of the pin material was (in wt.%): Al 5.5 – 6.5, V 3.5 – 4.5, Fe 0.03, O 0.15, balance Ti). The Ti-6Al-4V, received in the form of hot-rolled rod, was annealed at 760 jC for 1 h and furnace cooled to obtain an average hardness of 345 HV (35 HRC). The rotating pin was cylindrical in shape, 9.0 mm in diameter, and 20 mm in length and with a spherical end of radius of 40 mm. The disc counter material was the high-carbon chrome steel GCr15 (equivalent to AISI 52100 and containing around 1% carbon and 1.5% chromium by weight).
3. Results and discussion Fig. 1 shows wear rate as a function of sliding velocity for different applied loads. At low loads (30 and 50 N), the wear rate increased slightly as the sliding velocity increased. However, under higher loads (70 and 90 N), this dependence of wear rate on sliding velocity was more pronounced. Over the range of loads and velocities investigated, the wear rates were much lower than those reported in Refs. [14,15] for Ti-6Al-4V alloy sliding against hardened steel in air. SEM analyses showed that the worn surfaces of all the Ti-6Al-4V alloy samples exhibited similar mor-
Y. Liu et al. / Materials Characterization 50 (2003) 275–279
277
Fig. 1. Wear rate of Ti-6Al-4V alloy as a function of sliding velocity for different applied loads in vacuum (10 5 Pa).
phologies. Fig. 2 shows the surface morphology of the Ti-6Al-4V alloy after wear under a sliding velocity of 0.6 m/s and load of 50 N. Typical characteristics of plowing along the sliding direction and tongue-shaped wedges were observed, forming a laminar microstructure similar to a fracture morphology. However, there are some differences between the morphologies caused by wear and by fracture. Some plowing traces can be seen, indicating the original friction surface. The wedges show a plastic deformation characteristic. Fig. 3 shows the microstructure in the surface layer (f40 Am from the surface) of Ti-6Al-4V alloy samples before and after wear under a load 70 N at two different sliding velocities. The original grain size of the sample after annealing was about 100 Am (see Fig. 3a), while a very fine microstructure with a grain size of 50 – 100 nm was formed after the wear in
Fig. 3. TEM micrographs showing microstructure of the surface layer of Ti-6Al-4V alloy (a) before and after wear in vacuum (10 5 Pa) under the load of 70 N and sliding velocity of (b) 0.2 m/s and (c) 0.8 m/s, respectively.
Fig. 2. SEM micrographs showing the worn surface morphology of Ti-6Al-4V alloy under 70 N and 0.6 m/s in vacuum (10 5 Pa).
vacuum (Fig. 3b and c). Following wear under lower sliding velocities, the substructure of the layer comprised a high density of dislocations (Fig. 3b). After wear at higher sliding velocities, many microtwins
278
Y. Liu et al. / Materials Characterization 50 (2003) 275–279
Fig. 4. SEM micrographs showing (a) microstructure prior to wear test and (b) cross-sectional laminar microstructure of the Ti-6Al-4V alloy sample after wear test under 70 N and 0.6 m/s in vacuum (10 5 Pa).
existed in the substructure (Fig. 3c). According to the contact spot deformation theory [12], the strain rate increases with increases in the sliding velocity. Thus, the above difference in the substructure might be related to a different response to strain rate. The deformation mechanism would change from slip under low strain rates to twinning under high strain rates. On the other hand, the very fine microstructure appearing after sliding wear in vacuum indicates that an intense plastic deformation occurred on a large scale. Refinement of the microstructure and the formation of high dislocation densities and twins can dissipate a large amount of the energy produced by sliding friction and also resist the formation of cracks on the worn surface. Considering the deformation depth range (> 40 Am) and the laminar structure characteristic on the worn surface, a special deformation process could occur
during the sliding wear. Because the Ti-6Al-4V alloy has low tear resistance, the material at the contact spots can be easily deformed due to the mechanical and adhesive forces applied by the counter disc. According to the shear model of wear [10], the material at the contact spots is bunched up and pushed forward, forming tongue-shaped wedges. Furthermore, the wedges can be compressed and rolled onto the front or side surface of the Ti-6Al-4V pins. Because the test is being carried out in vacuum, there will be no environmental effect on the interface between the compressed material and the original surface. Thus, there will be good adhesion at the interface and no cracking will occur. As the process is repeated, the wedges continue to develop and a stacked layer, in which the wedges overlap each other, can be formed on the worn surface of the Ti-6Al-4V alloy. The SEM micrographs in Fig. 4 depict the crosssectional microstructure of the Ti-6Al-4V alloy before and after wear testing. The acicular form of the microstructure before testing can be seen in Fig. 4a. Fig. 4b shows the morphology of the deformedstacked layer in a worn specimen (load 70 N, sliding velocity 0.6 m/s). It is apparent that the thickness of the layer is about 70 Am. A pronounced deformed and laminar characteristic can be seen clearly at the tail area where the deformed-stacked layer would be squeezed out of the worn interface and formed into wear debris, as shown in Fig. 5.
Fig. 5. SEM micrograph showing a piece of layered debris formed at the end of the friction interface of the Ti-6Al-4V alloy under 70 N and 0.6 m/s in vacuum (10 5 Pa).
Y. Liu et al. / Materials Characterization 50 (2003) 275–279
The thermal effect induced by friction is also important to the wear behavior of Ti-6Al-4V alloy. Straffelini and Molinari [15] demonstrate that the surface temperature of Ti-6Al-4V alloy samples rubbing against steel increases as the sliding velocity increases. Thermal softening ensues and the above deforming and stacking process of the material at the contacting spots is easily developed as the temperature is increased [14]. This can result in an increase in the wear rate of the Ti-6Al-4V alloy as the sliding velocity (and the load) increase, especially in the range of the higher sliding velocities and applied loads, as shown in Fig. 1.
4. Conclusions (1) A 70-Am-thick surface layer with a grain size of 50 – 100 nm was formed after the sliding wear of Ti-6Al-4V alloy in vacuum. The substructure in the surface layer of the worn sample comprised mainly of a high density of dislocations at the slower sliding velocities. In the case of higher sliding velocities, twins were found to exist in the substructure. (2) The wear rate of the Ti-6Al-4V alloy in vacuum increased with increasing the sliding velocity and applied load. (3) A special plastic deformation process in which the material at the contacting spots is pushed, rolled, and stacked was proposed to describe the formation of the severely deformed surface layer.
Acknowledgements The authors are grateful to G.D. Gumulya for his interest in and encouragement of this work. This research is supported by National Basis Research Foundation of China.
279
References [1] Rosenfield AR. A shear instability model of sliding wear. Wear 1987;116:319 – 28. [2] Blau PJ, Doyle ED. Metallographic evidence for the nucleation of subsurface microcracks during unlubricated sliding of metals. Wear 1987;117:381 – 87. [3] Alpas AT, Hu H, Zhang J. Plastic deformation and damage accumulation below the worn surfaces. Wear 1993;162 – 164:188 – 95. [4] Mishina H. Surface deformation and formation of original element of wear particles in sliding friction. Wear 1998;215: 10 – 7. [5] Kato K. Micro-mechanisms of wear – wear modes. Wear 1992;153:277 – 95. [6] Kopalinsky EM, Oxley PLB. Explaining the mechanics of metallic sliding friction and wear in terms of slipline field models of asperity deformation. Wear 1995;190:145 – 54. [7] Torrance AA. The influence of surface deformation on mechanical wear. Wear 1996;200:45 – 54. [8] Karmakar S, Sethuramiah A. Response of dynamically stressed material in sliding. Wear 1991;151:269 – 78. [9] Chue CH, Chung HH, Lin JF, Chou CC. The effects of strain hardened layer on pitting formation during rolling contact. Wear 2001;249:109 – 16. [10] Kuhlmann-Wilsdorf D. What role for contact spots and dislocations in friction and wear? Wear 1997;200:8 – 29. [11] Panin V, Kolubaev A, Tarasov S, Popov V. Subsurface layer formation during sliding friction. Wear 2002;249:860 – 7. [12] Rigney DA. Some thoughts on sliding wear. Wear 1992;152: 187 – 92. [13] Rigney DA. Comments on the sliding wear of metals. Tribol Int 1997;30:361 – 7. [14] Molinari A, Tesi B, Bacci T. Dry sliding wear mechanisms of the Ti6Al4V alloy. Wear 1997;208:105 – 12. [15] Straffelini G, Molinari A. Dry sliding wear of Ti-6Al-4V alloy as influenced by the counterface and sliding condition. Wear 1999;236:328 – 38. [16] Alam MO, Haseeb ASA. Response of Ti-6Al-4V and Ti-24Al11Nb alloys to dry sliding wear against hardened steel. Tribol Int 2002;35:357 – 62. [17] Long M, Rack HJ. Friction and surface behavior of selected titanium alloys during reciprocating-sliding motion. Wear 2001;249:158 – 68. [18] Fayeulle S, Blanchard P, Vincent L. Fretting behavior of titanium alloys. Tribol Int 1993;36:267 – 75. [19] Nicolaou PD, Shell EB, Matikas TE. Microstructural and surface characterization of Ti-6Al-4V alloys after fretting fatigue. Mater Sci Eng A 1999;269:98 – 103.