Tribological behaviour of alumina sliding against Ti6Al4V in unlubricated contact

Tribological behaviour of alumina sliding against Ti6Al4V in unlubricated contact

Wear 225–229 Ž1999. 874–884 Tribological behaviour of alumina sliding against Ti6Al4V in unlubricated contact H. Dong ) , T. Bell School of Metallurg...

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Wear 225–229 Ž1999. 874–884

Tribological behaviour of alumina sliding against Ti6Al4V in unlubricated contact H. Dong ) , T. Bell School of Metallurgy and Materials, The UniÕersity of Birmingham, Birmingham B15 2TT, UK

Abstract Tribological behaviour of alumina balls Ž99.5%. sliding against a Ti6Al4V disc over a range of loads Ž5–80 N. and speeds Ž0.0625–1 m sy1 . has been investigated using a pin-on-disc tribometer under unlubricated conditions. The maximum wear coefficient was observed to be several orders of magnitude higher than the reported value for alumina against alumina or alumina against steel counterfaces. When the load or speed increased, the wear rate of the alumina ball increased initially and then decreased, showing typical transition features. On the other hand, the friction coefficients for the Ti6Al4Vralumina tribosystem were found to increase inversely with the applied loads or the sliding speeds. The wear mechanisms and the wear transition were investigated based on examinations of worn surfaces as well as debris using SEM, XRD and XPS, and it was revealed that a tribochemical mechanism accounted for the observed high wear rate of alumina sliding against the titanium alloy. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Ti6Al4V; Alumina; Unlubricated sliding; Tribochemical wear

1. Introduction The tribological behaviour of alumina has been the topic of many investigations. On one hand, alumina has been widely used in such tribological applications as mechanical seal, cutting tools, guides, extrusion dies and medical prostheses owing to its remarkable tribological characteristics w1x; on the other hand, the tribological behavior of alumina is greatly influenced by the doping of other elements w2x, contact load and speed w3–5x, temperature w6x, counterface materials w7,8x, the presence of lubrication w9,10x and environment w11–13x. Correspondingly, investigations into the fundamental wear mechanisms of alumina have made great progress, predominantly involving plastic deformation, brittle micro-fracture and tribochemical processes w14,15x. In spite of the significant achievements made in the study of tribology of alumina, it is noted that most of these studies were focused on the tribological behaviour of alumina sliding against itself or steel. Although the tribological behaviour of the aluminarTi6Al4V system has been investigated by Kailas

)

Corresponding author. Tel.: q44-121-4145197; fax: q44-1214147373; e-mail: [email protected]

and Biswas w16x and Hong and Winer w17x, attention was paid more to the wear of Ti6Al4V rather than alumina. Indeed, it has long been reported that when turning titanium alloys, tools made of alumina showed one to two orders of magnitude higher wear rate than most other tool materials, indicating that alumina tools are not suitable for machining titanium and its alloys w18,19x. Recent tribological characterisation work using alumina and steel balls as counterparts also revealed that alumina balls wore much faster than steel balls when sliding against Ti6Al4V titanium alloy discs w20,21x. Preliminary observations revealed that severe wear occurred not only to the Ti6Al4V disc but also to the alumina balls, the latter showing much higher wear rates than expected. Whilst it has been frequently quoted that unfavourable tribochemical reactions may be responsible for the observed unusual high wear rate of alumina, the relevant mechanisms remain far from well understood and thus need to be further explored. Therefore, advancing our understanding of the above phenomenon is an important task from both a scientific and technological point-of-view. This is mainly because the aluminarTi6Al4V pair has received substantial attention due to their potential bio-medical applications in total joint replacement. Accordingly, the main objectives of the present work were to investigate the tribological behaviour of alumina sliding against Ti6Al4V and explore the wear mechanisms involved.

0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 8 . 0 0 4 0 7 - 4

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2. Experimental procedure 2.1. Materials The wear tests were performed using a ball-on-disc configuration. High purity Ž99.5%. commercially available a-alumina balls Ž8 mm diameter. with a surface roughness of 0.01 mm Ž R a . were used as riders, sliding against a steel or Ti6Al4V disc. The impurities consisted of mainly silicon oxide and other oxide sintering aids. The alumina balls have a mean grain size of about 5 mm and a hardness of 16.5 GPa. For the purpose of comparison, commercial AISI 52100 balls of 8 mm diameter with a hardness of 7.2 GPa were also used. Ti6Al4V discs of 100 mm diameter were cut from hot rolled, annealed bar Žwith a hardness of 3.5 GPa. supplied by Timet UK. 709M40 steel discs with a hardness of 6.1 GPa were also used for comparison. Both the Ti6Al4V and 709M40 disc surfaces were ground to 0.8 mm Ž R a ..

ture about 258C. Prior to testing, all the specimens were carefully degreased with suitable solvents, dried in hot air and stored in a desiccator over night.

2.2. Tribological tests

2.3. Post-test characterisation

Tribological characterisation was conducted on a pinon-disc tribometer over a range of loads Ž5–80 N. and speeds Ž0.0625–1 m sy1 .. The diameter of circular wear tracks varied from 50 to 90 mm. A schematic drawing of the apparatus is shown in Fig. 1. Frictional force was measured using a load cell. Two to three repeated tests were conducted under the same test conditions, and the typical of friction curves was reported. Wear volume on the ball rider was quantitatively assessed after a total sliding distance of 500 m by measuring the diameter of the wear scar using the measuring system of a Leitz mini load hardness tester. The volume loss was obtained after simple geometrical calculation, assuming that the worn volume is a flat segment of a sphere. The wear rate is defined as the total volume loss per unit sliding distance. At least three repeated tests were carried out and the average value of the volume loss reported. The wear tests were carried out under ambient conditions. The relative humidity of the surrounding atmosphere was about 35% with the tempera-

In order to elucidate the wear processes and thus the fundamental mechanisms, the wear tracks on the discs and the wear scars on the ball riders were examined by scanning electron microscopy ŽSEM, JEOL6300. equipped with an energy-dispersive spectroscopy ŽEDX.. Where appropriate, an ultra thin window ŽUTW. was employed which facilitates the measurement of such light elements as oxygen, nitrogen and carbon. Wear debris was carefully collected and analysed using X-ray diffractometry ŽXRD, Philips Analytical X-Ray. with CuK a radiation. Peak positions were defined by minimum of second derivative. The chemical states of the elements in the wear debris were analysed using X-ray photoelectron spectroscopy ŽXPS or ESCA, Kratos XSAM 800.. A profilometer ŽTaylor-Hobson Talysurf 6. was used to measure the surface profile traces across the wear track on the disc perpendicular to the sliding direction.

Fig. 2. Wear coefficients for balls in different tribological systems.

3. Results and discussion 3.1. Friction pair effect

Fig. 1. Schematic of the pin-on-disc tribometer.

The wear coefficients, in terms of volume loss per unit of the total sliding distance per unit force Žmm3 Ny1 my1 ., are shown in Fig. 2 for aluminarTi6Al4V and aluminarsteel as well as steelrTi6Al4V tribosystems under a force of 20 N at a speed of 0.25 m sy1 . It can be clearly seen that the counterface played an important role in determining the wear of the ball rider. The wear coefficient of alumina was higher by three orders of magnitude when sliding against Ti6Al4V than against hardened steel although the latter was harder than the former by about 100%. Similarly, in spite of the fact that the alumina balls

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are much harder than the steel balls, when sliding against a Ti6Al4V, the wear coefficient of the former was about 100% lower than that of the latter. As shown in Fig. 3, typical wear scars produced on the as-tested alumina balls ŽFig. 3a. against a steel disc, or steel balls ŽFig. 3b. against a Ti6Al4V disc have an oval shape with the long-axis being perpendicular to the sliding direction. A typical wear scar on the alumina ball after

Fig. 4. Surface profiles of the wear track on Ža. 709M40 steel disc against alumina ball Ž20 N, 0.25 m sy1 , 500 m., Žb. Ti6Al4V disc against steel Ž20 N, 0.25 m sy1 , 500 m., Žc. Ti6Al4V disc against alumina ball Ž10 N, 1 m sy1 , 500 m., and Žd. Ti6Al4V disc against alumina ball Ž20 N, 0.25 m sy1 , 500 m..

Fig. 3. SEM BS images showing typical wear scar on Ža. alumina ball against steel, Žb. on steel ball against Ti6Al4V, and Žc. on alumina ball against Ti6Al4V.

rubbing against a Ti6Al4V disc ŽFig. 3c. showed a nearly perfect circular shape. The profiles across the corresponding wear tracks are shown in Fig. 4. It can be seen that the

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profile of the wear track on the steel disc after rubbing against an alumina ball ŽFig. 4a. seems quite smooth. However, the profiles on titanium discs ŽFig. 4b–d., whether produced by an alumina or a steel ball, fluctuate widely. Furthermore, material pile-up at both sides of the wear track on the titanium disc can be clearly seen. All these features are typical of the adhesive characteristics of titanium and its alloys, probably due to their intrinsic electron configuration and crystal structures w22x. 3.2. Aluminar steel tribosystem As shown in Fig. 3a, the wear scar produced on the alumina ball after rubbing against a steel disc was covered by other materials, as evidenced by the strong contrast of the composition-sensitive back scattering image. EDX analysis of the wear scar revealed the presence of O and Fe as well as the elements of the steel disc ŽCr, Mo, C and Mn., suggesting the formation of iron oxides. The morphologies of the mating track and the debris collected are depicted in Figs. 5 and 6, respectively. Visual observations of the wear debris revealed fine reddish brown particles and their platelet-like aggregate. The dark areas within and along the both sides of the wear track ŽFig. 5. were found to consist of iron oxides, and the wear debris collected ŽFig. 6. contained Fe and O as identified by EDX analysis. Figs. 7 and 8 show respectively the fragmentation and detachment of the oxidised region, and the removal of transferred tribolayer from the mating alumina ball. Based upon the above results, the wear process or mechanism involved in the aluminarsteel tribosystem can be qualitatively reconstructed as follows: when rubbed by an alumina ball, some small portion of the real contact areas on the steel disc surfaces would be oxidised under the combined action of the high contact stresses and frictional heating Žsee dark areas in Fig. 5.. Further stress cycling may cause fragmentation ŽFig. 7. and eventual detachment of the oxidised region as fine particles when a critical oxide thickness was reached. The effects of the

Fig. 5. Wear track on the steel disc after rubbing against an alumina ball.

Fig. 6. Wear debris collected during wear test of alumina ballrsteel disc.

formation of this debris on the wear are two-fold: on one hand, they act as third-bodies and cause abrasive wear of the alumina ball, although this was quite mild since the hardness of these iron oxides was lower than the alumina balls; on the other hand, this debris would fill the valleys on the ceramic surface and compact under traction on the alumina ball surface, thus protecting the surface from wear. Eventually, the transferred tribolayer would be removed ŽFig. 8. as wear debris by traction force through such processes as micro-cracking or micro-fracture, as reported by Komvopoulos and Li w23x. The morphology of the debris ŽFig. 6. is typical of tribochemical reaction products, consisting of compacted very fine particles. In addition to the abrasive effect arising from oxidational wear products of the steel counterpart, tribochemical reaction between the alumina ball and the steel counterface, as suggested by Smith and Pope w24x, may also contributed to the wear of the alumina slider as a result of spinal formation. However, in the present work the presence of Al was not identified by EDX analysis either in the scar on the alumina ball or in the wear track. Therefore, it

Fig. 7. SEM micrograph showing initiation of the fragmentation of the oxidised spot in the wear track on the steel disc against an alumina ball.

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above interpretation. In short, the low wear rate observed for both alumina rider and the steel disc can be mainly attributed to the oxidation wear mechanisms operating in the tribosystem. 3.3. Aluminar Ti6Al4V tribosystem

Fig. 8. SEM micrograph showing spallation of compacted tribolayer on the alumina ball against a steel disc.

seems unlikely that the above tribochemical mechanism would be responsible for the wear of the alumina rider in view of the fact that the sliding speed may not be high enough for the reaction, and that nearly all the contact surface of the alumina ball was covered by an iron oxide layer. Recent studies by Ravikiran et al. w4x support the

The worn surface on the alumina ball produced during rubbing against a Ti6Al4V disc under a load of 20 N at 0.25 m sy1 ŽFig. 3c. appeared relatively smooth with some streaks parallel to the sliding direction. Detailed topographies of the worn surface observed under higher magnification are given in Fig. 9. The topography of alumina ball was characterised by many island-like plateaus, and the inter-plateau areas were dispersed with much fine wear debris ŽFig. 9a.. Delamination of plates was observed in some plateaus, and Fig. 9b demonstrates the cracks in a large plateau and a small plate to be detached from the large plateau. Intergranular and transgranular fracture can be easily identified in the crater after the plate was detached under the traction of the mating surface ŽFig. 9c.. Most off-plateau regions ŽFig. 9d. showed almost the same features as those observed in the craters. Morphologies of

Fig. 9. SEM micrographs showing Ža. many island-like plateaus on the alumina ball after rubbing against a Ti6Al4V disc, Žb. delamination of the alumina, and Žd. morphology of the off-plateau region.

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the collected wear debris are shown in Fig. 10. Fig. 10a depicts the overall view of the wear debris. Observations under high magnification revealed some cutting-chip-like debris ŽFig. 10b., very fine particles ŽFig. 10c. and larger plates ŽFig. 10d.. Detailed observation of the detached plates and the corresponding EDX analysis results are given in Fig. 11. As can be clearly seen in Fig. 11a, some grains with white contrast were embedded in the plate, which were identified as alumina grains ŽFig. 11b.. Many hollows with grain-like shape can be also seen in Fig. 11a. It is of interest to find that most off-grain areas contained mainly aluminium and titanium ŽFig. 11c., with the Al to Ti ratio being much higher than that for Ti6Al4V obtained using the same measuring conditions ŽFig. 11d.. Although it is impossible to abstract quantitative evidence from these results, they still give some indication that the interface between the original alumina surface and the transferred Ti may contain higher Al content than what would be normally contained in transferred Ti6Al4V. Structural and crystallographic analyses of wear debris collected during the wear tests were conducted using XRD

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and XPS. The typical XRD profile of the wear debris had relatively high background and fluctuated largely, probably due to the high residual stresses in the debris. All the possibilities for indexing the typical diffraction patterns are illustrated in Table 1. XRD analysis indicated the presence of Ti, a-alumina, Ti 3 Al, TiO and possibly TiO 2 and TiAl. It can be seen from Table 1 that most peaks of Ti 3 Al at low angles were overlapped with those for Ti and aalumina. However, the measured d-spacing values for Ti 3 Al at high angles matched quite well with the standard values. Therefore, it seems that tribochemical reactions between the alumina ball and the mating Ti6Al4V disc occurred during wear process. Ti 2p doublets obtained by XPS analysis are shown in Fig. 12, and the eight synthetic components Žfour doublets. can be identified as Žfrom right to left. Ti, TiO, Ti 2 O 3 , and TiO 2 state w25x. XPS analysis results confirmed the presence of Ti, alumina, TiO 2 and TiO in the wear debris. No peaks of Ti 2 O 3 could be recognised from XRD analysis results ŽTable 1., indicating that the volume of this phase was too small to be identified by XRD. However, XPS could not give any evidence for the presence of Ti 3 Al in the wear debris largely because

Fig. 10. Morphologies of wear debris collected from the tribological system, alumina ball against Ti6Al4V disc: Ža. overall view, Žb. cutting-chip-like debris, Žc. thin, fine particles, and Žd. large plate.

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Fig. 11. SEM micrograph Ža. and the corresponding EDX analysis.

Ti 3 Al is a kind of intermetallic, and thus no chemical shift would occur when it formed. Despite its reputation for chemical inertness, tribochemical reactions may occur in the wear of alumina. For example, when sliding against a steel disc at a high speed, alumina reacted with the counterpart and thus produced FeAl 2 O4 or spinel w4x; when sliding in moisture or water, aluminium hydroxide was formed as tribochemical products w12x. It has been known that since titanium possesses the lowest value of d-bond character in its electron configuration, it is extremely active and ready to alloy with other materials w26x. Furthermore, titanium is characterised by poor thermal conductivity, about 1r6 that of most steels and 1r16 that of aluminium alloys. This low thermal conductivity promotes high flash temperatures, especially when titanium slides against such low thermal conductive materials as alumina. Consequently, such favourable conditions may promote the tribochemical reactions in the aluminarTi6Al4V tribosystem. Indeed, it has been reported that titanium can reduce alumina to form Ti 3 Al as well as TiAl intermetallics at

temperatures as low as 6508C w27x or 8008C w28x. These reactions may normally occur at a low rate under static conditions at the same bulk temperature. However, local high ‘flash’ temperatures at the asperity contacts, the exposure of atomically clean surfaces by the wear process, and direct mechanical stimulation Žhighly strained region. are all mechanisms which dramatically enhance tribochemical reactions w29x. Given high local stresses and temperatures generated at the asperity contacts between alumina and titanium, and the extremely active nature of titanium, it hereby seems likely that the observed abnormally high wear rate of alumina sliding against titanium can be attributed to the above tribochemical reactions. 3.4. Load and speed dependence Variation of the wear rate of the alumina balls as a function of the applied load and the sliding speed, and the coefficient of friction of the aluminarTi6Al4V tribosystem are shown in Figs. 13 and 14, respectively. Firstly, it can be seen from Fig. 13 that when sliding against a Ti6Al4V

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Table 1 d-spacing values obtained through XRD and all possibilities for indexing the typical patterns of the debris Measured d-spacing ŽInt..

Standard d-spacing of potential phases and the intensity ŽInt.. Ti

3.4709Ž9.0. 2.7099Ž4.4. 2.5403Ž20.

2.557Ž30.

2.3332Ž90. 2.2221Ž100.

2.342Ž26. 2.244Ž100.

2.0835Ž24. 1.7158Ž15.

1.726Ž19.

1.5997Ž10. 1.4695Ž9.0. 1.4037Ž5.3. 1.3753Ž4.2. 1.3259Ž19. 1.2373Ž7.1. 1.1239Ž1.4. 0.8542Ž1.5. 0.8127Ž3.2. 0.8093Ž2.4. 0.7892Ž10. Mismatched strong peaksrlines

a-Al 2 O 3

Ti 3 Al

TiO

3.479Ž75.

2.320Ž70. 2.205Ž100.

2.391Ž80. 2.227Ž10.

1.700Ž60.

2.071Ž100. 1.701Ž30.

1.443Ž50.

1.591Ž10. 1.464Ž90.

2.085Ž100.

1.601Ž80. 1.404Ž30. 1.374Ž50. 1.332Ž16. 1.233Ž13.

No

1.239Ž15.

No

TiAl

3.47Ž80.B 2.729Ž4.B 2.476Ž25.B 2.487Ž50.R

2.552Ž90.

1.475Ž17.

TiO 2

1.315Ž60. 1.227Ž40. 1.102Ž20. 0.850Ž10. 0.834Ž20. 0.806Ž50. 0.784Ž40. No

1.231Ž10.

2.31Ž100. 2.188Ž25.R 2.244Ž18.B 2.04Ž20. 1.6874Ž60.R 1.691Ž20.B 1.4528Ž10.R 1.417Ž10.B 1.3598Ž20.R 1.364Ž10.B 1.3465Ž12.R 1.238Ž10.B

1.424Ž60. 1.407Ž20.

1.203Ž60.

0.817Ž60. 0.784Ž60. No

3.247Ž100.R 3.51Ž100.B 2.90Ž90.B

1.99Ž60.

Int.: intensity. R: rutile. B: brookite.

disc, the wear rate of the mating alumina balls increased with increasing applied load ŽFig. 13a. or sliding speed ŽFig. 13b. before it decreased when a certain value was reached. Secondly, although the wear rate of the alumina ball was clearly both load and speed dependent, the wear rate of the alumina balls rubbing against Ti6Al4V discs was still in the order of 10y2 –10y3 mm3 Ny1 my1 , which is about two orders of magnitude higher than that for alumina balls against steels w4x. Thirdly, both the load and speed dependence of the wear of alumina showed a similar transition feature.

Fig. 12. Decomposed Ti 2p peaks of the wear debris.

Fig. 13. Wear rate of alumina balls sliding against Ti6Al4V discs as a function of Ža. the sliding speed and Žb. the applied load.

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Fig. 16. Wear morphology of the track on the Ti6Al4V disc after rubbing against a alumina ball under 20 N at 0.25 m sy1 .

Fig. 14. Effect of sliding speed Ža. and the applied load Žb. on the friction coefficient of alumina balls sliding against Ti6Al4V discs.

On the other hand, as shown in Fig. 14, the coefficient of friction of the aluminarTi6Al4V tribosystem decreased with increasing the sliding speeds ŽFig. 14a. or applied loads ŽFig. 14b.. The topographies of the alumina balls

Fig. 15. SEM BS showing wear scar on the alumina ball after sliding against a Ti6Al4V disc at high sliding speed Ž1 m sy1 ..

rubbing against Ti6Al4V at the highest sliding speed Ž1 m sy1 . are shown in Fig. 15, with most areas being covered by larger plateaus. Whilst the wear track produced under 20 N at 0.25 1 m sy1 on the Ti6Al4V disc was relatively smooth with some grooves ŽFig. 16., many large oxidised plateaus as identified by EDX, were observed in the wear track formed under 10 N at the highest speed Ž1 m sy1 . ŽFig. 17.. The oxidised plateaus would significantly reduce the friction force between the alumina rider and the Ti6Al4V discs since titanium oxides usually exhibit lower friction than titanium dose w20x. Furthermore, the oxide layers would also act as a diffusion barrier between alumina and Ti6Al4V w30x. This may contributed to the lowest friction coefficient observed for alumina balls sliding against a Ti6Al4V disc at the highest speed Ž1 m sy1 . ŽFig. 14a. or under the highest load ŽFig. 14b.. On the other hand, when an alumina ball slide against a titanium disc at a low speed or under a low load, a low flash temperature would be expected. Consequently, the real

Fig. 17. SEM micrograph showing a typical large oxidised plateau in the wear track of a Ti6Al4V disc after rubbing against an alumina ball at a high speed Ž1 m sy1 ..

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contact would be titanium against alumina or titanium against titanium owing to the severe titanium transfer. So the observed friction coefficients were high, typical of that for titanium against most other engineering materials w20,22x. With the increase in either loads or speeds, oxidation of titanium surface was enhanced. Thus low friction coefficients would be expected ŽFig. 14.. 3.5. Wear mechanism of alumina sliding against Ti6Al4V The above results and discussion strongly imply that different wear processes or mechanisms occurred for different tribosystems, and special tribological interaction might have occurred in the tribological pair of aluminarTi6Al4V. The wear process or wear model is suggested as follows. Although, there always is a layer of oxide on titanium surface, the oxide layer formed in air at room temperature is very thin Žabout 5–10 nm thick. and weak w31x. Under the action of a rider, this thin and weak oxide layer would be easily be broken and removed since the substrate was hardly strong enough to support it. Consequently, the titanium surface is extremely active and ready to alloy with other materials w26x. What is more, titanium has a low thermal conductivity. Therefore, under the high local contact stress and high flash temperature, titanium would reduce alumina to form such intermetallics as Ti 3 Al. With the continuation of this process, the thickness of the tribolayer bonded to the alumina ball increases. With the build-up of this tribolayer, micro-crack and micro-fracture may occur in the tribolayer ŽFig. 9b. and eventually results in rupture within the alumina ŽFig. 9c. under the strong traction force of the mating surface. This is probably because the bonding between the alumina surface and the tribolayer was strong as a result of the mutual diffusion and the formation of the titanium aluminide. However, the reduced areas in alumina ball may not be so strong as the normal alumina underneath. The detected Ti 3 Al in the wear debris ŽTable 1. and the observed alumina grains embedded in the plate-like debris ŽFig. 11. strongly support the above hypothesis. Therefore, the wear transition with either the applied load or the sliding speed may be explained by the tribochemical reactions observed in the tribosystem. Both the oxidation of the titanium surface and the reduction of alumina occurred in the tribosystem. The former would decrease the wear of the alumina ball as well as the friction of the tribosystem; however, the latter would increase the wear of the alumina ball. At relatively low speeds or under low loads, reduction of alumina by titanium may dominate the wear process. This is probably due to the fact that the growth rate of the titanium oxides under such conditions was so low that it would be easily removed by the rider, thus eliminating the potential of the oxide layers acting as a diffusion barrier. However, with increasing either the applied loads or the sliding speeds, the oxidation rate would increase accordingly. Once a

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critical load or speed was reached, the rate of the oxidation reaction would rapidly increase to such an extent that the oxide layer could effectively act as a diffusion barrier between the alumina ball and the Ti6Al4V disc, thus decreasing the reduction rate of the alumina ball. As a result, the overall wear rate of alumina balls would decrease with increasing either the applied load or the sliding speed, with the latter being much more effective than the former.

4. Conclusions Ž1. The counterface material plays an important role in determining the wear of alumina. The wear coefficient of alumina is about three orders of magnitude higher when sliding against a Ti6Al4V disc than against a hardened steel under the same conditions, even if the latter is two times as hard as the former. Ž2. The wear rate of alumina balls increased with increasing applied load or sliding speed before it decreased, showing typical transition characteristics. The coefficient of friction of the aluminarTi6Al4V couples decreased with increasing either the sliding speed or the applied load. Ž3. Titanium aluminides Žmainly Ti 3 Al. were found to form as products of tribochemical reactions between alumina and the Ti6Al4V mating surface, which is to a large extent responsible for the observed high wear rate of alumina rubbing against a titanium counterface.

Acknowledgements The authors would like to express their appreciation to Dr. I. Bertoti of the Hungarian Academy of Sciences for his technical assistance in XPS analysis and valuable discussion.

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w23x w24x w25x w26x w27x w28x

w29x w30x w31x

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