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Influence of surface preparation on roughness parameters, friction and wear
Wear 266 (2009) 482–487
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Influence of surface preparation on roughness parameters, friction and wear M. Sedlaˇcek ∗ , B. Podgornik, J. Viˇzintin University of Ljubljana, Centre for Tribology and Technical Diagnostics, Bogiˇsiˇceva 8, 1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 1 October 2007 Received in revised form 29 February 2008 Accepted 7 April 2008 Available online 20 June 2008 Keywords: Roughness Roughness parameters Friction Wear Machining
a b s t r a c t The aim of the present research was to investigate influence of surface preparation on roughness parameters and correlation between roughness parameters and friction and wear. First the correlation between different surface preparation techniques and roughness parameters was investigated. For this purpose 100Cr6 steel plate samples were prepared in terms of different average surface roughness, using different grades of grinding, polishing, turning and milling. Different surface preparation techniques resulted in different Ra values from 0.02 to 7 m. After this, correlation between surface roughness parameters and friction and wear was investigated. For this reason dry and lubricated pin-on-disc tests, using different contact conditions, were carried out, where Al2 O3 ball was used as counter-body. It was observed that parameters Rku , Rsk , Rpk and Rvk tend to have influence on coefficient of friction. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Continuously increasing demands for improved reliability and effectiveness of mechanical parts, reduction of friction loses, and above all for greater power density, lead to increased tribological loads of contact surfaces. Because of that, properties of contact surfaces are becoming more and more important. Two of the most important surface properties are surface roughness and topography. For proper design of contact surfaces it is very important to understand the influence of surface roughness parameters on friction and wear. Unfortunately, standard surface roughness parameters do not describe contact surfaces sufficiently, with completely different surfaces showing similar or even the same values of standard roughness parameters and the other way round—similar surfaces having much different standard roughness parameters. In addition to that different standards use different parameters. In practice the most used parameters for surface roughness description are Ra , Rq , Rsk and Rku . Average surface roughness (Ra ) gives very good overall description of height variations, but does not give any information on waviness and it is not sensitive on small changes in profile. Root mean square deviation of the assessed profile (Rq ) is more sensitive to deviations from main line than Ra . Rsk is defined as skewness and is sensitive on occasional deep valleys or high peaks. Zero skewness reflects in symmetrical height distribution, while positive and negative skewness describe surfaces with high peaks or filled valleys,
∗ Corresponding author. Tel.: +386 1 4771 465; fax: +386 1 4771 469. E-mail address: [email protected] (M. Sedlaˇcek). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.04.017
and with deep scratches or loss of peaks, respectively. On the other hand kurtosis (Rku ) describes the probability density sharpness of the profile. For surfaces with lower peaks and low valleys, Rku is less than 3, and more than 3 for surfaces with higher peaks and low valleys [1]. The load bearing ratio, as well as maximum contact pressure increase with the increase of skewness and kurtosis [2]. There are also other surface roughness parameters, defined with different standards (ISO, DIN, BS, ANSI, . . .). However their influence is not well known or defined. These parameters include Rmax , R3z , R3zmax , Rt , Rpm , Rp , Rvm , Rv , Wt , Pt , Pc , etc. [3]. Although a lot of experimental work has been done in the field of surface roughness and topography of contact surfaces correlation between surface roughness and friction is not yet clearly defined [4–8]. And the aim of the present work was to investigate the influence of surface preparation on roughness parameters and correlation between roughness parameters and friction and wear. 2. Experimental For the purpose of this investigation, aimed at investigating influence of surface preparation on roughness parameters and correlation between roughness parameters and friction and wear, 100Cr6 (AISI 52100) steel samples were used. Disc type steel samples were prepared in terms of different average surface roughness, using different grades of grinding, polishing, turning and milling, resulting in Ra values from 0.005 to 7.3 m. To assure topography repeatability and analogy between surfaces prepared by grinding and polishing, all samples of the same roughness level were prepared simultaneously using grinding machine RotoPol-21. All samples were first grinded, under water
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Fig. 1. Surface roughness parameters (a) Ra , Rq and Rsk ; (b) Rku for grind samples.
cooling for about 10 min, using grinding paper Piano 120, force 250 N, and rotation speed of 300 rpm. Samples with Ra roughness of about 0.4 m (denote G4), were then grinded in water for 7 s with SiC paper and grain number 80, force 50 N and rotation speed of 150 rpm. Ra roughness value of about 0.16 m (denote G3) was obtained with Piano 120, water lubricated grinding for 10 s, 50 N load and 150 rpm. Grinding in water for 10 s, using SiC paper with grain number 220, force 50 N and rotation speed 150 rpm, resulted in surface roughness Ra of about 0.07 m (denote G2). Samples with surface roughness of about 0.02 m (denote G1) were first polished for 10 min with 9 m DP-plan disc, using force of 250N and 150 rpm, and followed by 10 min polishing with 6 m DP-plan disc, at 200 N force and 150 rpm. Surfaces on all grinded samples were randomly orientated. Average surface roughness (Ra ), root square (Rq ), skewness (Rsk ), kurtosis (Rku ), core peak-to-valley height (Rk ), reduced peak height (Rpk ) and reduced valley depth (Rvk ). Those parameters were chosen because they showed the biggest differences in the values in dependence of the type of preparation. Results of first four parameters, for samples obtained by grinding, are gathered in Fig. 1. To produce different samples manufactured by turning, feeding and rotation speed were varied. To achieve samples denoted as T1 to T3, rotation speed was 750 rpm, and feeding speeds were 0.06, 0.2 and 0.4 mm/min, respectively. For samples denoted as T4–T6, feeding speed was 0.2 mm/min and rotation speeds were 212, 750 and 1400 rpm, respectively. Surface pattern was unidirectional. Values of surface roughness parameters are gathered in Fig. 3. For samples produced by milling, ration speed was 160 rpm and feeding speed 9, 35 and 90 mm/min for samples denotes as M1–M3, respectively. For samples denoted as M4–M6, feeding speed was 56 mm/min and rotation speeds were 40, 160 and 400 rpm, respectively. Surface pattern was unidirectional. Obtained values of surface roughness parameters are gathered in Fig. 5. Surface topography measurements were performed by stylus profilemeter. For every sample, 25 measurements on three different locations were made in order to get statistically representative data of surface roughness parameters. Influence on surface roughness parameters values, regarding the orientation of the measurement, was also studied. It was found out that for samples prepared by grinding and turning there was no significant change in roughness parameters values with direction of measurement or location of the measurement. Surface pattern on grinded samples was not orientated, while on sample produced with turning, the pattern done by a cutting knife was virtually concentric around the centre of the specimen. Samples produced by milling, had pattern eccentric of the centre of the specimens. Direction of measurement showed some influence on surface roughness parameters. Regardless if direction of measurement was perpendicular or longitudinal to the direction of movement of the working tool, we cannot see significant differences in Ra and Rq values. Difference can be seen for parameter Rp with values shifting from 1.3 to 6 m. The difference was
highest when measurement was perpendicular to the direction of movement of the working tool. Rsk values were also ranging from 0.03 to 0.49, and were even shifting from negative to positive. In general values of roughness parameters measured perpendicular to the direction of movement of the working tool were higher. When calculating the statistical values of parameter, measurements longitudinal to the direction of movement of the working tool were used. Tribological testing under different contact conditions was made on CSEM Pin-on-disc tribometer with ball on disc contact. During testing, coefficient of friction was monitored as a function of time and wear of contact surface determined after the test by means of topography analysis. Al2 O3 ball (Ø10 mm) was used as counterpart in order to concentrate all the wear and surface topography change on the steel disc. Ball had very smoothly polished surface. Dry and lubricated tests were carried out, with pure PolyAlpha-olefin (PAO) oil used as a lubricant in lubricated tests. All tests were made at a room temperature (22 ± 2 ◦ C) and the relative humidity of 50 ± 10%. Tests were done at sliding speeds of 0.05, 0.1 and 0.2 m/s and a normal load of 1 N, which corresponds to a contact pressure of 0.56 GPa. 3. Results 3.1. Dry sliding 3.1.1. Samples manufactured by grinding With the increase of roughness, surface parameters as Ra , Rq and Rku are increasing (Fig. 1a and b), while on the other hand Rsk values are decreasing (Fig. 1a). As it can be seen on Fig. 2a rougher surfaces have lover coefficient of friction, but sliding distance when steady-state conditions are reached, was longer for rougher surfaces (Fig. 2b). Wear of the surfaces was also monitored, but shows great scatter. Main wear mechanism was abrasion wear, which completely changed initial roughness after the test. Because of that, it is hard to draw some conclusions about correlation between surface roughness parameters (Fig. 1a and b) and tribological behaviour. It could be also noticed that sliding speed does not have an influence on the coefficient of friction, except for the roughest surface, where higher sliding speed tends to lower friction. 3.1.2. Samples manufactured by turning In the case or turning, increase of feeding speed results in increase of roughness and roughness parameters (T1–T3), while on the other hand, increase of rotation speed leads to lower roughness (T4–T6). As shown in Fig. 4a, coefficient of friction reduces with contact surface becoming smoother. At lover sliding speeds, friction tends to be lover than for higher sliding speeds. When comparing trends
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Fig. 2. (a) Coefficient of friction; (b) sliding distance to steady-state conditions for grind samples at different sliding speeds.
Fig. 3. Surface roughness parameters (a) Ra , Rq and Rsk ; (b) Rku for samples obtained by turning.
Fig. 4. (a) Coefficient of friction and (b) sliding distance steady-state conditions for surfaces obtained by turning at different sliding speeds.
of surface parameters and coefficient of friction, we can see similarities in trends for parameter Rsk and Rku (Figs. 3a and b and 4a). Distance to steady-state conditions is increasing with surface roughness, as shown in Fig. 4b. 3.1.3. Samples manufactured by milling For samples prepared by milling, trend of surface parameters value change is similar as for turning. With increase of feeding speed, roughness and roughness parameters are increasing (M1–M3). On the other hand, increase of rotation speed tends to decrease roughness values (M4–M6).
In the case of surface preparation by milling, coefficient of friction tends to be lover at lover sliding speeds and increases with increasing sliding speed. Distance to steady-state conditions is longer for rougher surfaces (Fig. 6b). When comparing trends of surface parameters and coefficient of friction, it can be seen that only Rku parameter shows some similarities. At this point we can also compare samples M2 and M5, which reflects similar Ra and Rq values, but different Rsk and Rku values. When comparing sliding distance to steady-state conditions, it can be seen that for sample M2 values are decreasing with increased sliding speed, and the opposite for sample M5, as shown in Fig. 6b.
Fig. 5. Surface roughness parameters (a) Ra , Rq and Rsk ; (b) Rku for samples obtained by milling.
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Fig. 6. (a) Coefficient of friction and (b) sliding distance to steady-state conditions for samples prepared by milling at different sliding speeds.
Fig. 7. (a) Coefficient of friction and (b) sliding distance to steady-state conditions for grind samples for lubricated contact.
Fig. 8. (a) Coefficient of friction and (b) sliding distance to steady-state conditions for samples prepared by turning and tested at under lubricated conditions.
3.2. Lubricated sliding At lubricated tests, wear of all surfaces was minimal, and original topography was not altered.
3.2.1. Samples manufactured by grinding As shown in Fig. 7a, higher sliding speeds and/or smother surfaces give lower friction under lubricated sliding. Sliding distances to steady-state conditions were in principle longer for rougher surfaces (Fig. 7b).
When comparing surface parameters with coefficient of friction it can be seen that in general friction increases with increase in roughness parameter values. Comparing trends of Rsk and Rku parameters with friction, we can see some resemblance in the trend (Figs. 1a and 7a).
3.2.2. Samples manufactured by turning Generally, increase in sliding speed leads to lower friction under lubricated conditions (Fig. 8a), but sliding distance to steady-state conditions tends to get longer for smother surceases prepared by
Fig. 9. (a) Coefficient of friction and (b) sliding distance to steady-state conditions for milled samples tested under lubricated conditions.
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Fig. 10. Surface roughness parameters (a) Ra , Rq and Rsk ; (b) Rku , Rk , Rpk , Rvk for samples obtained by grinding, turning and milling.
turning (Fig. 8b). When comparing samples with similar Ra and Rsk , but different Rku values (T5 and T6), we can see that sample with higher Rku value (T5) gives lower friction (Figs. 3b and 7a). Rk values for these samples are almost the same. For sample T5 and T6, Rpk is higher than Rvk . The highest difference between parameters Rpk and Rvk (Rvk > Rpk ) is for sample T4, which also shows the lowest friction. For this sample, Rsk parameter has the lowest vales of all samples prepared by turning (Fig. 3a). 3.2.3. Samples manufactured by milling Also for milled samples lower surface roughness will give lower friction. In general with higher sliding speeds we also get lover friction. Sliding distance to steady-state conditions is the lowest for the rougher surfaces (Figs. 5a and 9b). If we compare samples M3 and M4 which have similar Ra and Rq values, and almost the same Rku and lower value of Rsk for M4, we can see that sample M4 is showing lower friction. This can be related to the difference in Rk parameter value, with M3 sample showing approximately 7 m higher Rk values as compared to sample M4. On the other hand for sample M3 Rpk value is higher than Rvk , and for sample M4 this is the opposite. The same relations can be observed for samples M1 and M2, with sample M2 showing higher Rvk than Rpk and lower friction as compared to M1 (Rpk > Rvk ) although M1 displaying lower roughness (Figs. 5a and 9a). 4. Discussion It is hard to draw some direct conclusions between roughness parameters and tribological behavior, because roughnesses of different surfaces are not the same. As it can be seen from Figs. 1, 3 and 8, roughness is the lowest for grinded samples and the highest for milled ones. To verify the influence of specific parameter, only one parameter should be varied. In that manner samples with similar Ra and Rq values, but different Rsk and Rku values should be tested. Although in dry tests, the main wear mechanism was abrasion, and consequently initial surface topography was destroyed, we can draw some conclusions. Sliding distance to steady-state conditions is getting longer with increase in roughness. It can be also noticed
that friction tends to be lower at lower sliding speeds. For smoother grinded samples sliding speed does not have an influence on the coefficient of friction. When analyzing influence of Rku parameter it was noticed that when value is lower, but still greater than 3, friction tends to decrease. Friction also tends to decrease when parameter Rsk is becoming more negative. In general for lubricated contact friction gets lower with lower roughness, what can be explained with better EHD lubrication. Higher sliding speeds also improve hydrodynamic lubrication, so coefficient of friction is also lower. Sliding distance to steadystate conditions, for turned and milled samples, is reduced when using rougher surfaces. However, for grinded samples sliding distance to steady-state conditions is getting longer with increase in roughness. In the case of grinded samples friction gets higher with increase in roughness. We can also observe that with increase of parameter Rku , friction is decreasing. When parameter Rsk value is getting more negative, friction also tends to get lower. It was also noticed that parameters Rvk and Rpk could have an influence on friction. When Rvk is greater than Rpk friction tends to be lower. Because different machining procedures do not produce surfaces with the same Ra values, direct comparison of different machining procedures is very difficult. However for comparison purpose we selected milled sample M6, turned sample T1 and grinded sample G4. Turned and milled samples achieved roughness degree of N6, and grind one of N5. If we compare different surface roughness parameters, we can see that the greatest difference is in Rsk parameter (Fig. 10a). The lowest value was measured for grinded sample, followed by turned and milled one. Parameter Rku shows values greater than 3, for all samples, but the lowest value for milling and the highest for turning operation. For all samples parameter Rvk was greater than Rpk (Fig. 10b). Furthermore Rk parameter is high for milled sample and lowest for grind one. Others parameters do not show any significant difference between different machining procedures. The most significant parameters are therefore Rsk and Rku . When compared to friction, sample with lowest value of Rsk and highest Rku (T1) will result in the lowest friction for lubricated tests. Because of abrasion as dominant wear mechanism for dry tests, friction is lowest for sample with highest roughness (M6) (Fig. 11a and b).
Fig. 11. Coefficient of friction for grinded, turned and milled samples tested (a) dry and (b) lubricated.
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5. Conclusions Pin-on-disc tests with 100Cr6 steel discs with Al2 O3 ball were made to identify the effect of surface roughness on coefficient of friction. General conclusions based on the experimental results are: • For dry test coefficient of friction is lower when roughness high. • Coefficient of friction is lower when roughness is low for lubricated test. • With higher sliding speed friction gets reduced for dry and lubricated sliding conditions. • Increase in parameter Rku , leads to decrease in friction for lubricated tests and increase in friction for dry tests. • Friction also tends to get lower when parameter Rsk is getting more negative for lubricated tests. • For dry contact sliding distance to steady-state friction conditions, tends to get longer with increase in roughness. • At lubricated contact, sliding distance to steady-state conditions depends on the type of surface preparation. For grinded samples sliding distance gets longer with increased surface roughness and shorter for milled and turned samples.
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• Rvk and Rpk parameters could have an influence on friction. When Rvk is greater than Rpk friction tends to be lower. References [1] E.S. Gadelmawla, M.M. Koura, T.M.A. Maksoud, I.M. Elewa, H.H. Soliman, Roughness parameters, Journal of Materials Processing Technology 123 (2002) 133–145. [2] Wen-Zhong Wang, Hui Chen, Yuan-Zhong Hu, Hui Wang, Effect of surface roughness parameters on mixed lubrication characteristics, Tribology International 39 (2006) 522–527. [3] Operating Manual Turbo Roughness for Windows, Hommelwerke GmgH, February 1999. [4] Fredrik Svahn, Asa Kassaman-Rudolphi, Erik Wallen, The influence of surface roughness on friction and wear of machine element coatings, Wear 254 (2003) 1092–1098. [5] Noureddine Tayebi, Andreas A. Polycarpou, Modeling the effect of skewness and kurtosis on the static friction coefficient of rough surfaces, Tribology International 37 (2004) 491–505. [6] K. Meine, T. Scheider, D. Spaltmann, E. Santner, The influence of roughness on friction. Part I: The Influence of a single step, Wear 253 (2002) 725–732. [7] Bharat Bhushan, Contact mechanics of rough surfaces in tribology: multiple asperity contact, Tribology Letters 4 (1998) 1–35. [8] Wen-Ruey Chang, Mikko Hirvonen, Raoul Gronqvist, The effects of cut-off length on surface roughness parameters and their correlation with transition friction, Safety Science 42 (2004) 755–769.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Influence of surface preparation on roughness parameters, friction and wear M. Sedlaˇcek ∗ , B. Podgornik, J. Viˇzintin University of Ljubljana, Centre for Tribology and Technical Diagnostics, Bogiˇsiˇceva 8, 1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 1 October 2007 Received in revised form 29 February 2008 Accepted 7 April 2008 Available online 20 June 2008 Keywords: Roughness Roughness parameters Friction Wear Machining
a b s t r a c t The aim of the present research was to investigate influence of surface preparation on roughness parameters and correlation between roughness parameters and friction and wear. First the correlation between different surface preparation techniques and roughness parameters was investigated. For this purpose 100Cr6 steel plate samples were prepared in terms of different average surface roughness, using different grades of grinding, polishing, turning and milling. Different surface preparation techniques resulted in different Ra values from 0.02 to 7 m. After this, correlation between surface roughness parameters and friction and wear was investigated. For this reason dry and lubricated pin-on-disc tests, using different contact conditions, were carried out, where Al2 O3 ball was used as counter-body. It was observed that parameters Rku , Rsk , Rpk and Rvk tend to have influence on coefficient of friction. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Continuously increasing demands for improved reliability and effectiveness of mechanical parts, reduction of friction loses, and above all for greater power density, lead to increased tribological loads of contact surfaces. Because of that, properties of contact surfaces are becoming more and more important. Two of the most important surface properties are surface roughness and topography. For proper design of contact surfaces it is very important to understand the influence of surface roughness parameters on friction and wear. Unfortunately, standard surface roughness parameters do not describe contact surfaces sufficiently, with completely different surfaces showing similar or even the same values of standard roughness parameters and the other way round—similar surfaces having much different standard roughness parameters. In addition to that different standards use different parameters. In practice the most used parameters for surface roughness description are Ra , Rq , Rsk and Rku . Average surface roughness (Ra ) gives very good overall description of height variations, but does not give any information on waviness and it is not sensitive on small changes in profile. Root mean square deviation of the assessed profile (Rq ) is more sensitive to deviations from main line than Ra . Rsk is defined as skewness and is sensitive on occasional deep valleys or high peaks. Zero skewness reflects in symmetrical height distribution, while positive and negative skewness describe surfaces with high peaks or filled valleys,
∗ Corresponding author. Tel.: +386 1 4771 465; fax: +386 1 4771 469. E-mail address: [email protected] (M. Sedlaˇcek). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.04.017
and with deep scratches or loss of peaks, respectively. On the other hand kurtosis (Rku ) describes the probability density sharpness of the profile. For surfaces with lower peaks and low valleys, Rku is less than 3, and more than 3 for surfaces with higher peaks and low valleys [1]. The load bearing ratio, as well as maximum contact pressure increase with the increase of skewness and kurtosis [2]. There are also other surface roughness parameters, defined with different standards (ISO, DIN, BS, ANSI, . . .). However their influence is not well known or defined. These parameters include Rmax , R3z , R3zmax , Rt , Rpm , Rp , Rvm , Rv , Wt , Pt , Pc , etc. [3]. Although a lot of experimental work has been done in the field of surface roughness and topography of contact surfaces correlation between surface roughness and friction is not yet clearly defined [4–8]. And the aim of the present work was to investigate the influence of surface preparation on roughness parameters and correlation between roughness parameters and friction and wear. 2. Experimental For the purpose of this investigation, aimed at investigating influence of surface preparation on roughness parameters and correlation between roughness parameters and friction and wear, 100Cr6 (AISI 52100) steel samples were used. Disc type steel samples were prepared in terms of different average surface roughness, using different grades of grinding, polishing, turning and milling, resulting in Ra values from 0.005 to 7.3 m. To assure topography repeatability and analogy between surfaces prepared by grinding and polishing, all samples of the same roughness level were prepared simultaneously using grinding machine RotoPol-21. All samples were first grinded, under water
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Fig. 1. Surface roughness parameters (a) Ra , Rq and Rsk ; (b) Rku for grind samples.
cooling for about 10 min, using grinding paper Piano 120, force 250 N, and rotation speed of 300 rpm. Samples with Ra roughness of about 0.4 m (denote G4), were then grinded in water for 7 s with SiC paper and grain number 80, force 50 N and rotation speed of 150 rpm. Ra roughness value of about 0.16 m (denote G3) was obtained with Piano 120, water lubricated grinding for 10 s, 50 N load and 150 rpm. Grinding in water for 10 s, using SiC paper with grain number 220, force 50 N and rotation speed 150 rpm, resulted in surface roughness Ra of about 0.07 m (denote G2). Samples with surface roughness of about 0.02 m (denote G1) were first polished for 10 min with 9 m DP-plan disc, using force of 250N and 150 rpm, and followed by 10 min polishing with 6 m DP-plan disc, at 200 N force and 150 rpm. Surfaces on all grinded samples were randomly orientated. Average surface roughness (Ra ), root square (Rq ), skewness (Rsk ), kurtosis (Rku ), core peak-to-valley height (Rk ), reduced peak height (Rpk ) and reduced valley depth (Rvk ). Those parameters were chosen because they showed the biggest differences in the values in dependence of the type of preparation. Results of first four parameters, for samples obtained by grinding, are gathered in Fig. 1. To produce different samples manufactured by turning, feeding and rotation speed were varied. To achieve samples denoted as T1 to T3, rotation speed was 750 rpm, and feeding speeds were 0.06, 0.2 and 0.4 mm/min, respectively. For samples denoted as T4–T6, feeding speed was 0.2 mm/min and rotation speeds were 212, 750 and 1400 rpm, respectively. Surface pattern was unidirectional. Values of surface roughness parameters are gathered in Fig. 3. For samples produced by milling, ration speed was 160 rpm and feeding speed 9, 35 and 90 mm/min for samples denotes as M1–M3, respectively. For samples denoted as M4–M6, feeding speed was 56 mm/min and rotation speeds were 40, 160 and 400 rpm, respectively. Surface pattern was unidirectional. Obtained values of surface roughness parameters are gathered in Fig. 5. Surface topography measurements were performed by stylus profilemeter. For every sample, 25 measurements on three different locations were made in order to get statistically representative data of surface roughness parameters. Influence on surface roughness parameters values, regarding the orientation of the measurement, was also studied. It was found out that for samples prepared by grinding and turning there was no significant change in roughness parameters values with direction of measurement or location of the measurement. Surface pattern on grinded samples was not orientated, while on sample produced with turning, the pattern done by a cutting knife was virtually concentric around the centre of the specimen. Samples produced by milling, had pattern eccentric of the centre of the specimens. Direction of measurement showed some influence on surface roughness parameters. Regardless if direction of measurement was perpendicular or longitudinal to the direction of movement of the working tool, we cannot see significant differences in Ra and Rq values. Difference can be seen for parameter Rp with values shifting from 1.3 to 6 m. The difference was
highest when measurement was perpendicular to the direction of movement of the working tool. Rsk values were also ranging from 0.03 to 0.49, and were even shifting from negative to positive. In general values of roughness parameters measured perpendicular to the direction of movement of the working tool were higher. When calculating the statistical values of parameter, measurements longitudinal to the direction of movement of the working tool were used. Tribological testing under different contact conditions was made on CSEM Pin-on-disc tribometer with ball on disc contact. During testing, coefficient of friction was monitored as a function of time and wear of contact surface determined after the test by means of topography analysis. Al2 O3 ball (Ø10 mm) was used as counterpart in order to concentrate all the wear and surface topography change on the steel disc. Ball had very smoothly polished surface. Dry and lubricated tests were carried out, with pure PolyAlpha-olefin (PAO) oil used as a lubricant in lubricated tests. All tests were made at a room temperature (22 ± 2 ◦ C) and the relative humidity of 50 ± 10%. Tests were done at sliding speeds of 0.05, 0.1 and 0.2 m/s and a normal load of 1 N, which corresponds to a contact pressure of 0.56 GPa. 3. Results 3.1. Dry sliding 3.1.1. Samples manufactured by grinding With the increase of roughness, surface parameters as Ra , Rq and Rku are increasing (Fig. 1a and b), while on the other hand Rsk values are decreasing (Fig. 1a). As it can be seen on Fig. 2a rougher surfaces have lover coefficient of friction, but sliding distance when steady-state conditions are reached, was longer for rougher surfaces (Fig. 2b). Wear of the surfaces was also monitored, but shows great scatter. Main wear mechanism was abrasion wear, which completely changed initial roughness after the test. Because of that, it is hard to draw some conclusions about correlation between surface roughness parameters (Fig. 1a and b) and tribological behaviour. It could be also noticed that sliding speed does not have an influence on the coefficient of friction, except for the roughest surface, where higher sliding speed tends to lower friction. 3.1.2. Samples manufactured by turning In the case or turning, increase of feeding speed results in increase of roughness and roughness parameters (T1–T3), while on the other hand, increase of rotation speed leads to lower roughness (T4–T6). As shown in Fig. 4a, coefficient of friction reduces with contact surface becoming smoother. At lover sliding speeds, friction tends to be lover than for higher sliding speeds. When comparing trends
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Fig. 2. (a) Coefficient of friction; (b) sliding distance to steady-state conditions for grind samples at different sliding speeds.
Fig. 3. Surface roughness parameters (a) Ra , Rq and Rsk ; (b) Rku for samples obtained by turning.
Fig. 4. (a) Coefficient of friction and (b) sliding distance steady-state conditions for surfaces obtained by turning at different sliding speeds.
of surface parameters and coefficient of friction, we can see similarities in trends for parameter Rsk and Rku (Figs. 3a and b and 4a). Distance to steady-state conditions is increasing with surface roughness, as shown in Fig. 4b. 3.1.3. Samples manufactured by milling For samples prepared by milling, trend of surface parameters value change is similar as for turning. With increase of feeding speed, roughness and roughness parameters are increasing (M1–M3). On the other hand, increase of rotation speed tends to decrease roughness values (M4–M6).
In the case of surface preparation by milling, coefficient of friction tends to be lover at lover sliding speeds and increases with increasing sliding speed. Distance to steady-state conditions is longer for rougher surfaces (Fig. 6b). When comparing trends of surface parameters and coefficient of friction, it can be seen that only Rku parameter shows some similarities. At this point we can also compare samples M2 and M5, which reflects similar Ra and Rq values, but different Rsk and Rku values. When comparing sliding distance to steady-state conditions, it can be seen that for sample M2 values are decreasing with increased sliding speed, and the opposite for sample M5, as shown in Fig. 6b.
Fig. 5. Surface roughness parameters (a) Ra , Rq and Rsk ; (b) Rku for samples obtained by milling.
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Fig. 6. (a) Coefficient of friction and (b) sliding distance to steady-state conditions for samples prepared by milling at different sliding speeds.
Fig. 7. (a) Coefficient of friction and (b) sliding distance to steady-state conditions for grind samples for lubricated contact.
Fig. 8. (a) Coefficient of friction and (b) sliding distance to steady-state conditions for samples prepared by turning and tested at under lubricated conditions.
3.2. Lubricated sliding At lubricated tests, wear of all surfaces was minimal, and original topography was not altered.
3.2.1. Samples manufactured by grinding As shown in Fig. 7a, higher sliding speeds and/or smother surfaces give lower friction under lubricated sliding. Sliding distances to steady-state conditions were in principle longer for rougher surfaces (Fig. 7b).
When comparing surface parameters with coefficient of friction it can be seen that in general friction increases with increase in roughness parameter values. Comparing trends of Rsk and Rku parameters with friction, we can see some resemblance in the trend (Figs. 1a and 7a).
3.2.2. Samples manufactured by turning Generally, increase in sliding speed leads to lower friction under lubricated conditions (Fig. 8a), but sliding distance to steady-state conditions tends to get longer for smother surceases prepared by
Fig. 9. (a) Coefficient of friction and (b) sliding distance to steady-state conditions for milled samples tested under lubricated conditions.
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Fig. 10. Surface roughness parameters (a) Ra , Rq and Rsk ; (b) Rku , Rk , Rpk , Rvk for samples obtained by grinding, turning and milling.
turning (Fig. 8b). When comparing samples with similar Ra and Rsk , but different Rku values (T5 and T6), we can see that sample with higher Rku value (T5) gives lower friction (Figs. 3b and 7a). Rk values for these samples are almost the same. For sample T5 and T6, Rpk is higher than Rvk . The highest difference between parameters Rpk and Rvk (Rvk > Rpk ) is for sample T4, which also shows the lowest friction. For this sample, Rsk parameter has the lowest vales of all samples prepared by turning (Fig. 3a). 3.2.3. Samples manufactured by milling Also for milled samples lower surface roughness will give lower friction. In general with higher sliding speeds we also get lover friction. Sliding distance to steady-state conditions is the lowest for the rougher surfaces (Figs. 5a and 9b). If we compare samples M3 and M4 which have similar Ra and Rq values, and almost the same Rku and lower value of Rsk for M4, we can see that sample M4 is showing lower friction. This can be related to the difference in Rk parameter value, with M3 sample showing approximately 7 m higher Rk values as compared to sample M4. On the other hand for sample M3 Rpk value is higher than Rvk , and for sample M4 this is the opposite. The same relations can be observed for samples M1 and M2, with sample M2 showing higher Rvk than Rpk and lower friction as compared to M1 (Rpk > Rvk ) although M1 displaying lower roughness (Figs. 5a and 9a). 4. Discussion It is hard to draw some direct conclusions between roughness parameters and tribological behavior, because roughnesses of different surfaces are not the same. As it can be seen from Figs. 1, 3 and 8, roughness is the lowest for grinded samples and the highest for milled ones. To verify the influence of specific parameter, only one parameter should be varied. In that manner samples with similar Ra and Rq values, but different Rsk and Rku values should be tested. Although in dry tests, the main wear mechanism was abrasion, and consequently initial surface topography was destroyed, we can draw some conclusions. Sliding distance to steady-state conditions is getting longer with increase in roughness. It can be also noticed
that friction tends to be lower at lower sliding speeds. For smoother grinded samples sliding speed does not have an influence on the coefficient of friction. When analyzing influence of Rku parameter it was noticed that when value is lower, but still greater than 3, friction tends to decrease. Friction also tends to decrease when parameter Rsk is becoming more negative. In general for lubricated contact friction gets lower with lower roughness, what can be explained with better EHD lubrication. Higher sliding speeds also improve hydrodynamic lubrication, so coefficient of friction is also lower. Sliding distance to steadystate conditions, for turned and milled samples, is reduced when using rougher surfaces. However, for grinded samples sliding distance to steady-state conditions is getting longer with increase in roughness. In the case of grinded samples friction gets higher with increase in roughness. We can also observe that with increase of parameter Rku , friction is decreasing. When parameter Rsk value is getting more negative, friction also tends to get lower. It was also noticed that parameters Rvk and Rpk could have an influence on friction. When Rvk is greater than Rpk friction tends to be lower. Because different machining procedures do not produce surfaces with the same Ra values, direct comparison of different machining procedures is very difficult. However for comparison purpose we selected milled sample M6, turned sample T1 and grinded sample G4. Turned and milled samples achieved roughness degree of N6, and grind one of N5. If we compare different surface roughness parameters, we can see that the greatest difference is in Rsk parameter (Fig. 10a). The lowest value was measured for grinded sample, followed by turned and milled one. Parameter Rku shows values greater than 3, for all samples, but the lowest value for milling and the highest for turning operation. For all samples parameter Rvk was greater than Rpk (Fig. 10b). Furthermore Rk parameter is high for milled sample and lowest for grind one. Others parameters do not show any significant difference between different machining procedures. The most significant parameters are therefore Rsk and Rku . When compared to friction, sample with lowest value of Rsk and highest Rku (T1) will result in the lowest friction for lubricated tests. Because of abrasion as dominant wear mechanism for dry tests, friction is lowest for sample with highest roughness (M6) (Fig. 11a and b).
Fig. 11. Coefficient of friction for grinded, turned and milled samples tested (a) dry and (b) lubricated.
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5. Conclusions Pin-on-disc tests with 100Cr6 steel discs with Al2 O3 ball were made to identify the effect of surface roughness on coefficient of friction. General conclusions based on the experimental results are: • For dry test coefficient of friction is lower when roughness high. • Coefficient of friction is lower when roughness is low for lubricated test. • With higher sliding speed friction gets reduced for dry and lubricated sliding conditions. • Increase in parameter Rku , leads to decrease in friction for lubricated tests and increase in friction for dry tests. • Friction also tends to get lower when parameter Rsk is getting more negative for lubricated tests. • For dry contact sliding distance to steady-state friction conditions, tends to get longer with increase in roughness. • At lubricated contact, sliding distance to steady-state conditions depends on the type of surface preparation. For grinded samples sliding distance gets longer with increased surface roughness and shorter for milled and turned samples.
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• Rvk and Rpk parameters could have an influence on friction. When Rvk is greater than Rpk friction tends to be lower. References [1] E.S. Gadelmawla, M.M. Koura, T.M.A. Maksoud, I.M. Elewa, H.H. Soliman, Roughness parameters, Journal of Materials Processing Technology 123 (2002) 133–145. [2] Wen-Zhong Wang, Hui Chen, Yuan-Zhong Hu, Hui Wang, Effect of surface roughness parameters on mixed lubrication characteristics, Tribology International 39 (2006) 522–527. [3] Operating Manual Turbo Roughness for Windows, Hommelwerke GmgH, February 1999. [4] Fredrik Svahn, Asa Kassaman-Rudolphi, Erik Wallen, The influence of surface roughness on friction and wear of machine element coatings, Wear 254 (2003) 1092–1098. [5] Noureddine Tayebi, Andreas A. Polycarpou, Modeling the effect of skewness and kurtosis on the static friction coefficient of rough surfaces, Tribology International 37 (2004) 491–505. [6] K. Meine, T. Scheider, D. Spaltmann, E. Santner, The influence of roughness on friction. Part I: The Influence of a single step, Wear 253 (2002) 725–732. [7] Bharat Bhushan, Contact mechanics of rough surfaces in tribology: multiple asperity contact, Tribology Letters 4 (1998) 1–35. [8] Wen-Ruey Chang, Mikko Hirvonen, Raoul Gronqvist, The effects of cut-off length on surface roughness parameters and their correlation with transition friction, Safety Science 42 (2004) 755–769.