- Email: [email protected]
Effects of friction on topography and vice versa
Wear 261 (2006) 101–106
Effects of friction on topography and vice versa E. Santner ∗ , D. Klaffke, K. Meine, Ch. Polaczyk, D. Spaltmann Federal Institute for Materials Research and Testing (BAM), D-12200 Berlin, Germany Accepted 13 September 2005 Available online 16 November 2005
Abstract Experimental results of friction and topography measurements are presented which demonstrate the mutual modification of friction and contact topography. The effect of topographical ‘landmarks’ on friction was tested by Al2 O3 -balls sliding over Ti-ridges on Ti-surface and by Si3 N4 -balls sliding over grooves in SiO2 -surfaces. However, experiments of 100Cr6-balls sliding against 100Cr6-substrates in ultrahigh vacuum, Al2 O3 -balls sliding on DLC coated 100Cr6 and Si3 N4 -balls sliding on SiO2 -surfaces reveal that the formation of triboreaction layers and moreover the creation of wear particles can screen the effect of the topographical ‘landmarks’ completely. Wear particles and their exact behaviour in the contact area can affect friction in a stochastical and hence unpredictable way. Most modern friction theories have difficulties in coping with this problem. © 2005 Elsevier B.V. All rights reserved. Keywords: Topography; Wear particles; Triboreaction layers; Structure; Al2 O3 ; Si3 N4
1. Introduction Unanimously, topography formation and its development in friction contacts are regarded of highest importance for understanding and modelling friction processes. For example, according to the Bowden–Tabor model in dry sliding contacts between flat surfaces friction forces can be modelled as elastic and plastic deformation forces of microscopic asperities in contact [1]. However, in this simple model the friction coefficient is neither dependent on the normal load nor on the velocity or on the topography. This model predicts unrealistic friction coefficients for metals sliding in contact of about 0.2. Experimentally, values of µ ∼ 0.5–0.6 are found for most dry sliding metal contacts and µ > 1 is found for sliding metal contacts in vacuum. Some friction models try to include the effects of the topography by considering the real contact area Ar , which depends on the radii of the contacting roughness hills [2]. Based on Ref. [2], Engel [3] published for the elastic, the plastic, and the elastic–plastic contact equations for the real contact area Ar depending linearly on the radius R of the asperity. Zum Gahr [4] deduced friction coefficients for elastic contact, plastic or ploughing contact, and for micro-fracture, which regard the radius of a contacting roughness with different exponents (R2/3 , R−1 , and R1 ). Further ∗
Corresponding author. E-mail address: [email protected] (E. Santner).
0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.09.028
models of friction also state an inverse dependency of the coefficient of friction on the hardness and include some other material properties, like fracture toughness or a critical crack opening factor, but none includes topography or roughness parameters. Further experimental data should help to clarify and finally model the influence of the topography on friction. However, a systematic experimental investigation of the correlation between topography or roughness and friction is very difficult, because many other influences can hide this relationship. The two most important types of such processes are the friction induced production of triboreaction layers (they can alter the friction without changing topography) and the production of wear particles in the sliding contacts. Chapters 1 and 2 show, examples, for the influence of triboreaction layers and of wear particles on friction. The third chapter shows the effect of an artificial topography on the friction behaviour and how this effect can be screened by the production of wear particles. For a more detailed study of the influence of roughness on friction without the influence of wear particles, the reader is referred to the literature [5,6]. 2. Influence of triboreaction layers on friction The influence of triboreaction layers on friction can be demonstrated with the results of the sliding tests, in which various steel and ceramic balls of diameter 10 mm slid on identically prepared DLC-coatings on 100Cr6 substrates at a relative
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Fig. 1. Friction coefficient development with the number of fretting cycles for an alumina ball on various production batches of DLC-coatings on steel (100Cr6) substrates.
humidity of 3%, 50%, and 100%, respectively. The whole set of experiments is described in detail elsewhere [7]. The present work concentrates on Al2 O3 -balls sliding against a DLC-coating at a load 10 N. The stroke was typically 200 m at a sliding frequency of 20 Hz. The experiments were carried out at a relative humidity of 3%. The friction development of an Al2 O3 -ball sliding in reciprocating motion against a DLC-coating is shown as an example in Fig. 1. At first, the friction increases from very low values to about 0.27 and drops suddenly after about 1000–2000 cycles to a very constant value of 0.02. Aside from the very low coefficient of friction for a non-lubricated contact, the reproducibility of this friction modification during the sliding process is most impressive. The explanation of this effect is the “transfer” of a carbon layer from the DLC-coating to the alumina ball [7]. For sliding couples with SiC or Si3 N4 we did not find such stable low friction behaviour [7]. The friction varies strongly and stochastically between short low friction periods and long high friction phases. Triboreaction of carbon with Si therefore plays a role in this case because the effect is not found for other counter bodies like steel or Al2 O3 . Another case with triboreaction induced friction modification is shown in Fig. 2. Here, the results of a 100Cr6 ball (diameter: 6 mm) sliding on a 100Cr6 plane are presented. The test procedure was: transfer of samples into the vacuum chamber (base
Fig. 2. Friction development with the number of cycles in reciprocating sliding of a 100Cr6/100Cr6 couple in UHV.
pressure p < 1 × 10−9 mbar) and start of the sliding experiments. After 150, 300, and 520 cycles (stroke: 3 mm; sliding frequency: 1 Hz) the test was stopped over 1, 3, 5 nights, respectively, while the samples remained in the UHV-chamber. The test was continued the next morning. The low friction at the start is due to oxide/hydroxide layers on the steel. After 10–20 cycles this layer begins to wear down and the friction becomes very high. Friction coefficients above 2 are normal for metal sliding in UHV. During the stops of sliding over night a thin oxide/hydroxide layer was built on the steel from the residual gases in the vacuum chamber. The layer must have been very thin as indicated by the very short period of low friction after the overnight stops. The total decrease of friction over the whole number of cycles is probably an effect of wear particle agglomeration in the contact. This effect is already a topography modification by friction and wear. Of course, it is also a change of other contact surface properties (such as chemical composition). 3. Modification of topography/friction by wear particles The wear particles produced in a sliding contact are not only of interest for the development of a “Third Body Model” but also influence the friction in an unpredictable and literally unpleasant way. This is because the impact of wear particles in a sliding contact on the friction behaviour seems to be of stochastic nature and leads to giant fluctuations of the friction force. These friction fluctuations can lead to mechanical vibrations of the sliding system and hence to noise and even failure [8]. Fig. 3 shows the friction force signals for a sliding Si3 N4 couple together with the signal for linear wear. The “sinusoidal” wear signal indicates the misalignment of the rotating disc (Body 2 rotates, Body 1 is fixed). The friction “noise” leads to vibrations of the whole tribometer including the wear sensor which
Fig. 3. Friction force and wear signal development for a sliding Si3 N4 /Si3 N4 couple: (a) after some running time; (b) after whipping away wear particles (FN = 10 N, v = 0.03 m/s, T = 23 ◦ C, relative humidity ∼50%).
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Fig. 4. Friction signal of a steel/steel sliding couple related to the surface position at the disc (optical microscope photo of the sliding path on the steel disc).
produces the noise in the wear signal exactly at the same time (Fig. 3a). This noisy friction signal could actually be heard as noise. The smoothed friction force and wear signals in Fig. 3b are the result of pressing a dry paper tissue onto the rotating disc (Body 2) during the test run without interruption of the rotation. The cleaning of the sliding path from wear particles by whipping with the tissue stopped the noise and smoothed the signals. Wear particles in sliding contacts produce an irregular modification of friction, depending on their fate. If they are continuously transported out of the contact, a relatively constant friction can be expected. But this happens very seldom. In most cases, wear particles remain in the contact for a varying amount of time. They may be transferred from one body surface to the other or adhere there and an unpredictable amount may agglomerate. Wear particles may also be detached after some sliding contacts and can undergo chemical reactions because of their often chemically very reactive surface. They are building a new contact with modified topography, chemistry, and mechanical properties. Until now, the understanding is that these processes seem to be stochastic ones and are hard to model. In Fig. 4 is presented the friction modification by “adhesive” material transfer from a sliding steel ball to the steel counter disc. The photo of the sliding path on the steel disc (lower part of Fig. 4) clearly shows the grinding marks and the horizontal sliding marks with material transfer, visible as dark points. At these positions the corresponding friction is always higher (arrows). The material transfer could be the result of an adhesive wear of the steel ball, but more probably of wear particle agglomeration. The increase of friction at these material transfer positions is probably caused by the change of topography: the material islands transferred act as high roughness peaks. An example for a massive wear particle agglomeration on a steel ball (100Cr6) after sliding over a steel disc (100Cr6) is presented in Fig. 5. It shows a wear particle pile-up of about 4 m as demonstrated by the line scan on the AFM picture. In such a situation a drastic modification of the topography is caused by friction produced wear and agglomeration of the wear particles. Such a contact modification is usually combined with a friction
Fig. 5. AFM picture of a wear particle agglomeration on a steel ball (100Cr6) after sliding on steel (100Cr6); after 50 cycles at 0.4 GPa.
modification, because topography and material properties in the real contact area are changed. The later poses problems to an exact modelling. Wear particle agglomeration of that type occurs not only with metal contacts but also with ceramics. For an Al2 O3 -ball sliding on a TiN-coating without any lubrication, similar wear particle pile-up were formed on an Al2 O3 -ball. Fig. 6 reveals the effect of such wear particle transfer on the tribological behaviour in the case of an Al2 O3 -ball sliding on a TiN-coated steel disc. The friction force signal first increases for a short sliding period (0–200 s) and then decreases from about 8 N to 2 N during the sliding time period of 200–2000 s. During this decreasing period, the linear wear signal becomes more negative. This means that the distance between ball centre and disc becomes larger. This can only happen, if wear particles from the large area of the wear scar on the disc are collected at the “point”-contact on the ceramic ball. A sliding period of nearly constant friction and wear signal follows (2000–5000 s). The high “noise” on the signals indicates that the processes are of stochastic nature. After 5000 s sliding time (equals 500 m sliding distance) the friction signal drops suddenly to below 0.5 and becomes very smooth. At the same time, after 500 m sliding distance, the linear wear increases fast and the signal becomes also very smooth. The increase of linear wear after the wear particle agglomeration on the ball in combination with the drop in friction is the result of a retransfer of the agglomeration onto the disc. After this process, wear particle material is sliding on wear particle material. These layers of reaction products, which have been analysed as combinations of TiN, TiONx , and probably TiONx –H2 O, can lead to lower friction as compared to bulk TiN-material [9]. The friction behaviour of the system remains constant till 880 m sliding distance. Here again, a short period of wear particle transfer to the ball occurs. Such type of tribological behaviour is not found for all Al2 O3 /TiN-couples tested, but for the most.
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Fig. 6. Friction force, linear wear total displacement, and normal force development vs. sliding time for an Al2 O3 -ball on a TiN-coated steel disc.
4. Friction on artificial topographies A special experimental setup linking a linear tribometer with an AFM [10] allowed a correlation of the friction forces mea-
sured simultaneously with the contact position and the topography at this point. In this manner, friction force transitions and changes can be assigned to topography changes due to abrasion, adhesion, and wear particle agglomeration. The rough-
Fig. 7. Friction of Al2 O3 -ball on structured Ti-surface and corresponding AFM-picture of the sliding path (white color means elevated structure).
E. Santner et al. / Wear 261 (2006) 101–106
105
Fig. 8. Friction force curves of a silicon nitride ball sliding over artificial structures of silicon oxide on a silicon wafer.
ness of technical surfaces is of statistical nature. Therefore, the investigation of influences of single roughness elements on friction on such surfaces is very difficult. In order to study the effect of roughness on friction and vice versa, the roughness was simulated by etched ditches of defined width, depth, and distance on silicon surfaces. These artificial regular structures allowed a correlation of the mutual influence of topography and friction. Fig. 7 shows the results of a sliding test with an Al2 O3 -ball (diameter: 10 mm) on a Ti-surface with etched trenches, 0.4 m deep, 285 m wide, and 300 m in distance at load of 0.25 N. The friction force for one stroke is presented. After this single stroke, AFM images were taken at the positions labelled with the respective numbers in the diagram. It is interesting to note that the friction signal increases with sliding distance despite the nominally constant contact conditions. Furthermore, it is remarkable that the dips in the friction force correspond to the ridges of the structure. The AFM-pictures reveal that the Al2 O3 ball produced scratches, which became broader with sliding distance. The few small asperities on the ceramic ball plough the Ti-surface and the wear material agglomerates in front of the asperities. The scratches become broader and deeper with sliding distance (see increasing numbers of the AFM images). Finally, the shear stress becomes higher than the strength of the agglomeration and leads to the brake away of it (white structures
next to the dark scratches in the AFM-pictures (numbers 7–11). The dips in the friction force curve at the ridges are a result of lifting many asperities of the ball during crossing the ridge by the ball. The results of another set of experiments carried out on an artificial structure are presented in Fig. 8 (see also Ref. [10]). Here, 130 nm deep and 1000 m wide grooves were etched in silicon oxide on a silicon wafer. The grooves were separated by planes of 1000 m in length. In these tests, a silicon nitride ball (diameter: 5 mm) slid over this structure with a load of 0.15 N. After every cycle, the sample position was changed so that every friction force curve can be assigned to a specific friction trace. However, the position of the contact area on the ball was always the same for all traces. The periodical appearance of the peaks in the first friction curves of Fig. 8 is clearly visible. The distance between the peaks of 1 mm corresponds to the etched structure, thus, indicating that the peaks are caused by the edges of the structure. Strong fluctuations occur in the friction signal from the forth curve onwards around edge (c). They result from the formation of wear particles, as is clearly visible in the AFM image taken at position (c) of cycle 5. The AFM image in the diagram of Fig. 8 is a magnification of a step edge which has just been passed by the ball. Wear particles were scratched off the ball and are now sticking to the step edge. These experiments clearly show that the influence of the topography (artificial
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steps) can be screened completely by the influence of wear particles. 5. Conclusions The effect of thin oxid/hydoxide layers on friction was demonstrated by sliding a 100Cr6 ball against a 100Cr6 plate in UHV. In removing wear particles out of the sliding contact of Si3 N4 couples, the friction signal could be “smoothed”, thus, proofing the influence of wear particles on friction. It could also be shown that material transfer from a steel ball to a steel disc, were responsible for high friction signals at the positions of material transfer. This demonstrates the importance of changing surfaces and surface topographies during sliding. Sliding an Al2 O3 -ball on a TiN-coating showed the influence of wear particle agglomeration on the topography of contact surfaces and in turn on friction. By etching regular structures into metal and SiO2 -surfaces and sliding on this artificial roughness, the influence of single topography structure elements on friction could clearly be demonstrated. The experimental results presented here in combination with microscopical topography analysis seem to demonstrate that the friction processes produce stochastic modifications of the contact properties, if wear particles are present and/or tribologically induced surface reactions take place. This may be the main reason why all friction models until now fail to simulate these complex phenomena properly, as they cannot describe the dynamic changes of the contact surfaces due to the friction processes.
References [1] F.B. Bowden, D. Tabor, The Friction and Lubrication of Solids, Oxford University Press, Oxford, 1950; F.B. Bowden, D. Tabor, The Friction and Lubrication of Solids, Part II, Oxford University Press, Oxford, 1964. [2] K.L. Johnson, Contact Mechanics, Cambridge University Press, Cambridge, 1985. [3] S. Engel, Reibungs-und Erm¨udungsverhalten des Rad-Schien-Systems mit und ohne Schmierung (Diss.), in: L. Deters, K.-H. Grote, S. Vajna (Eds.), Fortschritte in der Maschinenkonstruktion, Bd. 2002, 2 (219 Seiten), Shaker Verlag Aachen, 2002. [4] K.-H. Zum Gahr, Modeling and microstructural modification of alumina ceramic for improved tribological properties, Wear 200 (1996) 215– 224. [5] K. Meine, Th. Schneider, D. Spaltmann, E. Santner, The influence of roughness on friction. Part I: The influence of a single step, Wear 253 (2002) 725–732. [6] K. Meine, Th. Schneider, D. Spaltmann, E. Santner, The influence of roughness on friction. Part II: The influence of multiple steps, Wear 253 (2002) 733–738. [7] D. Klaffke, J. Brand, C. Brand, R. Wittorf, Tribological characterisation of a-C:H coatings at room temperature; effect of counterbody material, in: W.J. Bartz (Ed.), Proceedings of the 14th International Colloquium on Tribology, vol. I, TAE, Esslingen, 2000, pp. 605–614. [8] E. Santner, Reibkraftschwankungen – Quelle, Informationsquelle, Probleme, Tribologie und Schmierungstechnik 47 (2000) 19–24. [9] Th. Schneider, G. Meier zu K¨ocker, E. Santner, Topography changes on the surfaces of PVD coatings in humid air: an AFM/LFM study, Surf. Interface Anal. 24 (1996) 7–14. [10] K. Meine, Th. Schneider, D. Spaltmann, E. Santner, Correlation of friction and roughness, in: M., Dietzsch, H., Trumpold (Eds.), Proceedings of the X International Colloquium on Surfaces, Chemnitz, 2000, pp. 402–409.
Effects of friction on topography and vice versa E. Santner ∗ , D. Klaffke, K. Meine, Ch. Polaczyk, D. Spaltmann Federal Institute for Materials Research and Testing (BAM), D-12200 Berlin, Germany Accepted 13 September 2005 Available online 16 November 2005
Abstract Experimental results of friction and topography measurements are presented which demonstrate the mutual modification of friction and contact topography. The effect of topographical ‘landmarks’ on friction was tested by Al2 O3 -balls sliding over Ti-ridges on Ti-surface and by Si3 N4 -balls sliding over grooves in SiO2 -surfaces. However, experiments of 100Cr6-balls sliding against 100Cr6-substrates in ultrahigh vacuum, Al2 O3 -balls sliding on DLC coated 100Cr6 and Si3 N4 -balls sliding on SiO2 -surfaces reveal that the formation of triboreaction layers and moreover the creation of wear particles can screen the effect of the topographical ‘landmarks’ completely. Wear particles and their exact behaviour in the contact area can affect friction in a stochastical and hence unpredictable way. Most modern friction theories have difficulties in coping with this problem. © 2005 Elsevier B.V. All rights reserved. Keywords: Topography; Wear particles; Triboreaction layers; Structure; Al2 O3 ; Si3 N4
1. Introduction Unanimously, topography formation and its development in friction contacts are regarded of highest importance for understanding and modelling friction processes. For example, according to the Bowden–Tabor model in dry sliding contacts between flat surfaces friction forces can be modelled as elastic and plastic deformation forces of microscopic asperities in contact [1]. However, in this simple model the friction coefficient is neither dependent on the normal load nor on the velocity or on the topography. This model predicts unrealistic friction coefficients for metals sliding in contact of about 0.2. Experimentally, values of µ ∼ 0.5–0.6 are found for most dry sliding metal contacts and µ > 1 is found for sliding metal contacts in vacuum. Some friction models try to include the effects of the topography by considering the real contact area Ar , which depends on the radii of the contacting roughness hills [2]. Based on Ref. [2], Engel [3] published for the elastic, the plastic, and the elastic–plastic contact equations for the real contact area Ar depending linearly on the radius R of the asperity. Zum Gahr [4] deduced friction coefficients for elastic contact, plastic or ploughing contact, and for micro-fracture, which regard the radius of a contacting roughness with different exponents (R2/3 , R−1 , and R1 ). Further ∗
Corresponding author. E-mail address: [email protected] (E. Santner).
0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.09.028
models of friction also state an inverse dependency of the coefficient of friction on the hardness and include some other material properties, like fracture toughness or a critical crack opening factor, but none includes topography or roughness parameters. Further experimental data should help to clarify and finally model the influence of the topography on friction. However, a systematic experimental investigation of the correlation between topography or roughness and friction is very difficult, because many other influences can hide this relationship. The two most important types of such processes are the friction induced production of triboreaction layers (they can alter the friction without changing topography) and the production of wear particles in the sliding contacts. Chapters 1 and 2 show, examples, for the influence of triboreaction layers and of wear particles on friction. The third chapter shows the effect of an artificial topography on the friction behaviour and how this effect can be screened by the production of wear particles. For a more detailed study of the influence of roughness on friction without the influence of wear particles, the reader is referred to the literature [5,6]. 2. Influence of triboreaction layers on friction The influence of triboreaction layers on friction can be demonstrated with the results of the sliding tests, in which various steel and ceramic balls of diameter 10 mm slid on identically prepared DLC-coatings on 100Cr6 substrates at a relative
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E. Santner et al. / Wear 261 (2006) 101–106
Fig. 1. Friction coefficient development with the number of fretting cycles for an alumina ball on various production batches of DLC-coatings on steel (100Cr6) substrates.
humidity of 3%, 50%, and 100%, respectively. The whole set of experiments is described in detail elsewhere [7]. The present work concentrates on Al2 O3 -balls sliding against a DLC-coating at a load 10 N. The stroke was typically 200 m at a sliding frequency of 20 Hz. The experiments were carried out at a relative humidity of 3%. The friction development of an Al2 O3 -ball sliding in reciprocating motion against a DLC-coating is shown as an example in Fig. 1. At first, the friction increases from very low values to about 0.27 and drops suddenly after about 1000–2000 cycles to a very constant value of 0.02. Aside from the very low coefficient of friction for a non-lubricated contact, the reproducibility of this friction modification during the sliding process is most impressive. The explanation of this effect is the “transfer” of a carbon layer from the DLC-coating to the alumina ball [7]. For sliding couples with SiC or Si3 N4 we did not find such stable low friction behaviour [7]. The friction varies strongly and stochastically between short low friction periods and long high friction phases. Triboreaction of carbon with Si therefore plays a role in this case because the effect is not found for other counter bodies like steel or Al2 O3 . Another case with triboreaction induced friction modification is shown in Fig. 2. Here, the results of a 100Cr6 ball (diameter: 6 mm) sliding on a 100Cr6 plane are presented. The test procedure was: transfer of samples into the vacuum chamber (base
Fig. 2. Friction development with the number of cycles in reciprocating sliding of a 100Cr6/100Cr6 couple in UHV.
pressure p < 1 × 10−9 mbar) and start of the sliding experiments. After 150, 300, and 520 cycles (stroke: 3 mm; sliding frequency: 1 Hz) the test was stopped over 1, 3, 5 nights, respectively, while the samples remained in the UHV-chamber. The test was continued the next morning. The low friction at the start is due to oxide/hydroxide layers on the steel. After 10–20 cycles this layer begins to wear down and the friction becomes very high. Friction coefficients above 2 are normal for metal sliding in UHV. During the stops of sliding over night a thin oxide/hydroxide layer was built on the steel from the residual gases in the vacuum chamber. The layer must have been very thin as indicated by the very short period of low friction after the overnight stops. The total decrease of friction over the whole number of cycles is probably an effect of wear particle agglomeration in the contact. This effect is already a topography modification by friction and wear. Of course, it is also a change of other contact surface properties (such as chemical composition). 3. Modification of topography/friction by wear particles The wear particles produced in a sliding contact are not only of interest for the development of a “Third Body Model” but also influence the friction in an unpredictable and literally unpleasant way. This is because the impact of wear particles in a sliding contact on the friction behaviour seems to be of stochastic nature and leads to giant fluctuations of the friction force. These friction fluctuations can lead to mechanical vibrations of the sliding system and hence to noise and even failure [8]. Fig. 3 shows the friction force signals for a sliding Si3 N4 couple together with the signal for linear wear. The “sinusoidal” wear signal indicates the misalignment of the rotating disc (Body 2 rotates, Body 1 is fixed). The friction “noise” leads to vibrations of the whole tribometer including the wear sensor which
Fig. 3. Friction force and wear signal development for a sliding Si3 N4 /Si3 N4 couple: (a) after some running time; (b) after whipping away wear particles (FN = 10 N, v = 0.03 m/s, T = 23 ◦ C, relative humidity ∼50%).
E. Santner et al. / Wear 261 (2006) 101–106
103
Fig. 4. Friction signal of a steel/steel sliding couple related to the surface position at the disc (optical microscope photo of the sliding path on the steel disc).
produces the noise in the wear signal exactly at the same time (Fig. 3a). This noisy friction signal could actually be heard as noise. The smoothed friction force and wear signals in Fig. 3b are the result of pressing a dry paper tissue onto the rotating disc (Body 2) during the test run without interruption of the rotation. The cleaning of the sliding path from wear particles by whipping with the tissue stopped the noise and smoothed the signals. Wear particles in sliding contacts produce an irregular modification of friction, depending on their fate. If they are continuously transported out of the contact, a relatively constant friction can be expected. But this happens very seldom. In most cases, wear particles remain in the contact for a varying amount of time. They may be transferred from one body surface to the other or adhere there and an unpredictable amount may agglomerate. Wear particles may also be detached after some sliding contacts and can undergo chemical reactions because of their often chemically very reactive surface. They are building a new contact with modified topography, chemistry, and mechanical properties. Until now, the understanding is that these processes seem to be stochastic ones and are hard to model. In Fig. 4 is presented the friction modification by “adhesive” material transfer from a sliding steel ball to the steel counter disc. The photo of the sliding path on the steel disc (lower part of Fig. 4) clearly shows the grinding marks and the horizontal sliding marks with material transfer, visible as dark points. At these positions the corresponding friction is always higher (arrows). The material transfer could be the result of an adhesive wear of the steel ball, but more probably of wear particle agglomeration. The increase of friction at these material transfer positions is probably caused by the change of topography: the material islands transferred act as high roughness peaks. An example for a massive wear particle agglomeration on a steel ball (100Cr6) after sliding over a steel disc (100Cr6) is presented in Fig. 5. It shows a wear particle pile-up of about 4 m as demonstrated by the line scan on the AFM picture. In such a situation a drastic modification of the topography is caused by friction produced wear and agglomeration of the wear particles. Such a contact modification is usually combined with a friction
Fig. 5. AFM picture of a wear particle agglomeration on a steel ball (100Cr6) after sliding on steel (100Cr6); after 50 cycles at 0.4 GPa.
modification, because topography and material properties in the real contact area are changed. The later poses problems to an exact modelling. Wear particle agglomeration of that type occurs not only with metal contacts but also with ceramics. For an Al2 O3 -ball sliding on a TiN-coating without any lubrication, similar wear particle pile-up were formed on an Al2 O3 -ball. Fig. 6 reveals the effect of such wear particle transfer on the tribological behaviour in the case of an Al2 O3 -ball sliding on a TiN-coated steel disc. The friction force signal first increases for a short sliding period (0–200 s) and then decreases from about 8 N to 2 N during the sliding time period of 200–2000 s. During this decreasing period, the linear wear signal becomes more negative. This means that the distance between ball centre and disc becomes larger. This can only happen, if wear particles from the large area of the wear scar on the disc are collected at the “point”-contact on the ceramic ball. A sliding period of nearly constant friction and wear signal follows (2000–5000 s). The high “noise” on the signals indicates that the processes are of stochastic nature. After 5000 s sliding time (equals 500 m sliding distance) the friction signal drops suddenly to below 0.5 and becomes very smooth. At the same time, after 500 m sliding distance, the linear wear increases fast and the signal becomes also very smooth. The increase of linear wear after the wear particle agglomeration on the ball in combination with the drop in friction is the result of a retransfer of the agglomeration onto the disc. After this process, wear particle material is sliding on wear particle material. These layers of reaction products, which have been analysed as combinations of TiN, TiONx , and probably TiONx –H2 O, can lead to lower friction as compared to bulk TiN-material [9]. The friction behaviour of the system remains constant till 880 m sliding distance. Here again, a short period of wear particle transfer to the ball occurs. Such type of tribological behaviour is not found for all Al2 O3 /TiN-couples tested, but for the most.
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Fig. 6. Friction force, linear wear total displacement, and normal force development vs. sliding time for an Al2 O3 -ball on a TiN-coated steel disc.
4. Friction on artificial topographies A special experimental setup linking a linear tribometer with an AFM [10] allowed a correlation of the friction forces mea-
sured simultaneously with the contact position and the topography at this point. In this manner, friction force transitions and changes can be assigned to topography changes due to abrasion, adhesion, and wear particle agglomeration. The rough-
Fig. 7. Friction of Al2 O3 -ball on structured Ti-surface and corresponding AFM-picture of the sliding path (white color means elevated structure).
E. Santner et al. / Wear 261 (2006) 101–106
105
Fig. 8. Friction force curves of a silicon nitride ball sliding over artificial structures of silicon oxide on a silicon wafer.
ness of technical surfaces is of statistical nature. Therefore, the investigation of influences of single roughness elements on friction on such surfaces is very difficult. In order to study the effect of roughness on friction and vice versa, the roughness was simulated by etched ditches of defined width, depth, and distance on silicon surfaces. These artificial regular structures allowed a correlation of the mutual influence of topography and friction. Fig. 7 shows the results of a sliding test with an Al2 O3 -ball (diameter: 10 mm) on a Ti-surface with etched trenches, 0.4 m deep, 285 m wide, and 300 m in distance at load of 0.25 N. The friction force for one stroke is presented. After this single stroke, AFM images were taken at the positions labelled with the respective numbers in the diagram. It is interesting to note that the friction signal increases with sliding distance despite the nominally constant contact conditions. Furthermore, it is remarkable that the dips in the friction force correspond to the ridges of the structure. The AFM-pictures reveal that the Al2 O3 ball produced scratches, which became broader with sliding distance. The few small asperities on the ceramic ball plough the Ti-surface and the wear material agglomerates in front of the asperities. The scratches become broader and deeper with sliding distance (see increasing numbers of the AFM images). Finally, the shear stress becomes higher than the strength of the agglomeration and leads to the brake away of it (white structures
next to the dark scratches in the AFM-pictures (numbers 7–11). The dips in the friction force curve at the ridges are a result of lifting many asperities of the ball during crossing the ridge by the ball. The results of another set of experiments carried out on an artificial structure are presented in Fig. 8 (see also Ref. [10]). Here, 130 nm deep and 1000 m wide grooves were etched in silicon oxide on a silicon wafer. The grooves were separated by planes of 1000 m in length. In these tests, a silicon nitride ball (diameter: 5 mm) slid over this structure with a load of 0.15 N. After every cycle, the sample position was changed so that every friction force curve can be assigned to a specific friction trace. However, the position of the contact area on the ball was always the same for all traces. The periodical appearance of the peaks in the first friction curves of Fig. 8 is clearly visible. The distance between the peaks of 1 mm corresponds to the etched structure, thus, indicating that the peaks are caused by the edges of the structure. Strong fluctuations occur in the friction signal from the forth curve onwards around edge (c). They result from the formation of wear particles, as is clearly visible in the AFM image taken at position (c) of cycle 5. The AFM image in the diagram of Fig. 8 is a magnification of a step edge which has just been passed by the ball. Wear particles were scratched off the ball and are now sticking to the step edge. These experiments clearly show that the influence of the topography (artificial
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steps) can be screened completely by the influence of wear particles. 5. Conclusions The effect of thin oxid/hydoxide layers on friction was demonstrated by sliding a 100Cr6 ball against a 100Cr6 plate in UHV. In removing wear particles out of the sliding contact of Si3 N4 couples, the friction signal could be “smoothed”, thus, proofing the influence of wear particles on friction. It could also be shown that material transfer from a steel ball to a steel disc, were responsible for high friction signals at the positions of material transfer. This demonstrates the importance of changing surfaces and surface topographies during sliding. Sliding an Al2 O3 -ball on a TiN-coating showed the influence of wear particle agglomeration on the topography of contact surfaces and in turn on friction. By etching regular structures into metal and SiO2 -surfaces and sliding on this artificial roughness, the influence of single topography structure elements on friction could clearly be demonstrated. The experimental results presented here in combination with microscopical topography analysis seem to demonstrate that the friction processes produce stochastic modifications of the contact properties, if wear particles are present and/or tribologically induced surface reactions take place. This may be the main reason why all friction models until now fail to simulate these complex phenomena properly, as they cannot describe the dynamic changes of the contact surfaces due to the friction processes.
References [1] F.B. Bowden, D. Tabor, The Friction and Lubrication of Solids, Oxford University Press, Oxford, 1950; F.B. Bowden, D. Tabor, The Friction and Lubrication of Solids, Part II, Oxford University Press, Oxford, 1964. [2] K.L. Johnson, Contact Mechanics, Cambridge University Press, Cambridge, 1985. [3] S. Engel, Reibungs-und Erm¨udungsverhalten des Rad-Schien-Systems mit und ohne Schmierung (Diss.), in: L. Deters, K.-H. Grote, S. Vajna (Eds.), Fortschritte in der Maschinenkonstruktion, Bd. 2002, 2 (219 Seiten), Shaker Verlag Aachen, 2002. [4] K.-H. Zum Gahr, Modeling and microstructural modification of alumina ceramic for improved tribological properties, Wear 200 (1996) 215– 224. [5] K. Meine, Th. Schneider, D. Spaltmann, E. Santner, The influence of roughness on friction. Part I: The influence of a single step, Wear 253 (2002) 725–732. [6] K. Meine, Th. Schneider, D. Spaltmann, E. Santner, The influence of roughness on friction. Part II: The influence of multiple steps, Wear 253 (2002) 733–738. [7] D. Klaffke, J. Brand, C. Brand, R. Wittorf, Tribological characterisation of a-C:H coatings at room temperature; effect of counterbody material, in: W.J. Bartz (Ed.), Proceedings of the 14th International Colloquium on Tribology, vol. I, TAE, Esslingen, 2000, pp. 605–614. [8] E. Santner, Reibkraftschwankungen – Quelle, Informationsquelle, Probleme, Tribologie und Schmierungstechnik 47 (2000) 19–24. [9] Th. Schneider, G. Meier zu K¨ocker, E. Santner, Topography changes on the surfaces of PVD coatings in humid air: an AFM/LFM study, Surf. Interface Anal. 24 (1996) 7–14. [10] K. Meine, Th. Schneider, D. Spaltmann, E. Santner, Correlation of friction and roughness, in: M., Dietzsch, H., Trumpold (Eds.), Proceedings of the X International Colloquium on Surfaces, Chemnitz, 2000, pp. 402–409.