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Surface topography effect on galling resistance of coated and uncoated tool steel
Surface & Coatings Technology 206 (2012) 2792–2800
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Surface topography effect on galling resistance of coated and uncoated tool steel B. Podgornik a,⁎, J. Jerina b a b
Institute of Metals and Technology, Lepi pot 11, SI-1000 Ljubljana, Slovenia University of Ljubljana, Centre for tribology and technical diagnostics, Bogisiceva 8, SI-1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 16 June 2011 Accepted in revised form 26 November 2011 Available online 3 December 2011 Keywords: Cold forming Roughness Surface modification Galling Friction
a b s t r a c t Tribological evaluation of the tool steel, focused on determining coefficient of friction and critical load for galling initiation against austenitic stainless steel as a function of surface topography was carried out in a load-scanning test rig. Surfaces investigated included turned, ground, polished, shot penned and laser surface textured cold work tool steel. Additionally, effect of surface roughness and post-polishing on galling resistance of TiN and DLC coated surfaces was investigated. Results of this investigation show that by removing sharp peaks polishing of the bearing surface gives plateau-like topography and improves galling properties of forming tool steel. This becomes even more important when using hard ceramic coatings (i.e. TiN). When superbly polished contact surfaces can give similar galling resistance as otherwise obtained through contact lubrication. However, application of low friction coatings or introduction of micro-dimples with proper density and contact lubrication will result in superior galling resistance of the surface. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In metal forming industry tools can be exposed to very complex and surface demanding conditions [1], which are the result of different effects (mechanical, thermal, chemical or tribological loading) and require well defined mechanical and tribological properties of the tool material. Besides that the surface finish of the formed parts is a critical quality parameter in many forming operations. A smooth surface, requiring smooth and defect free surface of the tool results in marketing advantages, as well as improved load-carrying capacity and tribological performance of the component [2]. The major obstacles to generation of smooth and defects free components are tool wear and adhesion of work material to the tool surface, known as galling [3]. Galling develops gradually as an adhesion of work material on the tool surface, with repeated forming cycles causing successive buildup of patches of transferred work material. During adhesion transferred work material may experience several hardening mechanisms [4] thus damaging the subsequently formed part by scratching and indentation, and leading to unstable friction [5,6]. In deep drawing and sheet metal forming processes galling is also the dominant cause for tool failure [7]. Generally, lubricants are used in order to decrease friction and adhesion in the contact between work material and tool [8]. However, ever increasing demands for “green” or environmentally friendly production require reduced lubrication and use of less toxic bio-degradable
⁎ Corresponding author. Tel.: + 386 1 4701930; fax: + 386 1 4701939. E-mail address: [email protected] (B. Podgornik). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.11.041
lubricants [9]. This and high degree of plastic deformation experienced in forming inevitably lead to metal-to-metal contact and galling initiation. But, wear resistance and tribological properties of forming tools can be further improved through selection of high quality tool steels or application of proper surface engineering technique [6,10,11]. In recent years the main focus in improving galling resistance of forming tools has been on use of hard coatings [12,13]. While ceramic coatings used in cutting applications normally show a relatively high friction and high tendency to galling low-friction DLC coatings can give very stable friction and excellent galling resistance [5,6]. However, coating selection is greatly influenced by the type of work material, with surface treatments, i.e. plasma nitriding and deep-cryogenic treatment providing similar or even better results when forming Ti and Al alloys [11,14,15]. Anyway, in order to reduce the effect of galling polishing of the tool surface is still required in many forming operations. It is known that among other things the risk of work material adhesion and galling increases with tool roughness [16,17]. On the other hand, surface topography investigations indicate that even with “rougher” surfaces, which are cheaper to produce selection of proper topography can lead to improved tribological performance [18]. Especially surface texturing, involving micro-dimples on the bearing surface shows great potential in lubricated sliding [19]. In general, generation of the beneficial topography should be decisive for obtaining desired anti-galling properties of forming tools, especially those equipped with hard, wear resistant coatings [20]. Therefore, the aim of the present work was to investigate the effect of surface preparation on roughness parameters and topography, and consequently on galling properties of coated and uncoated forming tool steel.
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ground specimens was performed also with 150 μm steel balls at 6 bar (FG-SPS2), and 150 (FG-SPC2) and 230 μm ceramic balls at the pressure of 4 bar (FG-SPC1). Final group of specimens (V; FG-P2 + T) included fine ground and double polished surfaces (FG-P2) subsequently laser surface textured (LST) with Nd:YAG laser [19], producing dimples with a diameter of 60 μm and depth of 15 μm. Textured surfaces, being re-polished after LST in order to remove bumps around the dimples (Fig. 1) comprised three different texturing densities of 1 (FG-T1), 5 (FG-T5) and 20% (FG-T20), obtained by changing distance between the dimples from 500 to 250 and 125 μm. Texturing parameters, showing the best potential of reducing friction under boundary lubrication were selected on the basis of previous investigations [21,22]. The effect of surface roughness for coated surfaces was evaluated on coarse and fine ground samples (CG-P1, CG-P2, FG and FG-P2), coated with commercial monolayer PVD TiN hard coating (28 GPa) and “softer” multilayer PACVD W-doped DLC coating (12 GPa) [5], and tested under dry sliding conditions.
2. Experimental details 2.1. Material and surface preparation Material used in this investigation was a standard AISI D2 cold work tool steel with the nominal composition (wt.%) 1.55 C, 0.35 Si, 0.40 Mn, 12.0 Cr, 1.0 Mo and 0.85 V. The steel samples in the shape of cylinders (ϕ10 × 100 mm) were cut, machined and ground from rolled and soft annealed bars. Cylindrical specimens were then heated to 1040 °C and cooled in air to reach surface hardness of ~ 900 HV1. In order to produce different surface roughness and topography five groups of specimens were prepared. Specimens' description and preparation procedure with corresponding roughness parameters are given in Table 1. First or base group (I), produced by standard machining and circumferential grinding operations included turned (T), coarse ground (CG) and fine ground (FG) samples. Through subsequent polishing, using polishing wheel and industrial polishing paste with the particle size of ~20 μm, second group of specimens (II; I + P1) denoted T-P1 (turned and post polished), CG-P1 (coarse ground and post polished) and FG-P1 (fine ground and post polished) was prepared. For the third group (III; I + P1 + P2) two step polishing of the base sample group was applied, combining polishing with 20 μm polishing paste and followed by 10 μm polishing paste (T-P2, CG-P2 and FG-P2). Fourth group of specimens (IV; I + SP) included shot peening performed by different balls on three base samples. Shot peening with 230 μm diameter steel balls at a pressure of 6 bar was applied on turned (T-SPS1), coarse ground (CG-SPS1) and fine ground (FG-SPS1) specimens. Additionally, shot peening of fine
2.2. Surface roughness measurement Surface roughness measurements were performed with the stylus profilometer Hommelwerke T8000. For each specimen three measurements were performed along the main cylinder axis using stylus speed of 0.5 mm/s and measurement length of 4.8 mm. Each measurement involved 5 successive profiles taken at a cylinder rotation step of about 15°. Average values of five roughness parameters,
Table 1 Specimens description and corresponding roughness parameters. Sample
Description
Roughness parameters [μm] Ra
Rq
Rz
Rsk
Turning Coarse grinding Fine grinding
3.42 ± 0.24 0.80 ± 0.15 0.13 ± 0.01
4.09 ± 0.23 1.02 ± 0.19 0.17 ± 0.01
16.46 ± 0.3 5.24 ± 0.93 1.10 ± 0.06
0.37 ± 0.06 0.19 ± 0.11 − 0.39 ± 0.24
2.32 ± 0.02 3.21 ± 0.38 3.78 ± 0.68
Group II T-P1 CG-P1 FG-P1
T + polishing (20 μm) CG + polishing (20 μm) FG + polishing (20 μm)
2.65 ± 0.24 0.49 ± 0.07 0.10 ± 0.01
3.11 ± 0.25 0.61 ± 0.06 0.13 ± 0.01
12.07 ± 1.0 2.57 ± 0.31 0.79 ± 0.08
0.13 ± 0.08 0.16 ± 0.12 − 0.47 ± 0.13
2.07 ± 0.06 3.31 ± 0.17 4.18 ± 0.14
Group III T-P2 CG-P2 FG-P2
T-P1 + polishing (10 μm) CG-P1 + polishing (10 μm) FG-P1 + polishing (10 μm)
1.69 ± 0.07 0.29 ± 0.04 0.09 ± 0.00
1.96 ± 0.05 0.37 ± 0.06 0.12 ± 0.01
7.54 ± 0.41 1.85 ± 0.24 0.64 ± 0.02
0.07 ± 0.08 − 0.09 ± 0.06 − 0.46 ± 0.21
2.11 ± 0.21 3.28 ± 0.31 4.29 ± 0.77
T + shot peening (steel balls ϕ230 μm) CG + shot peening (steel balls ϕ230 μm) FG + shot peening (steel balls ϕ230 μm) FG + shot peening (steel balls ϕ150 μm) FG + shot peening (ceramic balls ϕ230 μm) FG + shot peening (ceramic balls ϕ150 μm)
3.40 ± 0.57
4.15 ± 0.72
18.28 ± 2.6
0.01 ± 0.05
2.63 ± 0.05
2.25 ± 0.04
2.96 ± 0.12
14.86 ± 0.1
− 0.77 ± 0.36
4.76 ± 0.64
1.31 ± 0.07
1.97 ± 0.09
11.71 ± 0.6
− 1.72 ± 0.07
8.68 ± 0.52
0.66 ± 0.03
1.00 ± 0.08
2.25 ± 0.21
2.82 ± 0.28
14.08 ± 1.2
− 0.29 ± 0.10
3.26 ± 0.36
2.06 ± 0.03
2.59 ± 0.01
12.98 ± 0.7
− 0.26 ± 0.05
3.17 ± 0.33
FG-P2 + LST (1% density, 500 μm) FG-P2 + LST (5% density, 250 μm) FG-P2 + LST (20% density, 125 μm)
0.06 ± 0.00
0.07 ± 0.00
0.44 ± 0.06
− 0.18 ± 0.04
3.91 ± 1.44
0.07 ± 0.00
0.09 ± 0.01
0.48 ± 0.02
− 0.10 ± 0.14
3.16 ± 0.35
0.08 ± 0.01
0.10 ± 0.01
0.55 ± 0.02
− 0.08 ± 0.06
3.19 ± 0.42
Group I T CG FG
Group IV T-SPS1 CG-SPS1 FG-SPS1 FG-SPS2 FG-SPC1 FG-SPC2
Group V FG-T1 FG-T2 FG-T3
6.38 ± 0.49
− 2.11 ± 0.15
Rku
12.78 ± 0.9
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Fig. 1. (a) Textured - T, (b) fine ground - FG, (c) shot peened - FG-SPC1 and (d) textured - FG-T3 surfaces.
namely Ra, Rq, Rz, Rsk and Rku were then calculated taking into account all 15 individual profiles. 2.3. Galling evaluation In practice different test methods are used to evaluate galling resistance of forming tool materials, ranging from a very simple pinon-disc test rig to the ASTM standard G98-91 button-on-block test method. In metal forming, especially in sheet metal forming at every stroke new raw material is coming into contact with the tool surface, which is not the case for pin-on-disc configuration. On the other hand, ranking of tool materials is commonly based on the critical load when galling occurs, thus requiring a lot of testing. As shown in Ref. [23] load-scanning test rig has been found as one of the most suitable methods for comparing tool materials in terms of galling resistance. Although it doesn't directly correspond to forming operations its cross-cylinder configuration allows gradual increase in load during a single stroke, with each point along the contact path of both specimens corresponding to a specific load and displaying a unique contact history [23,24].
Therefore, galling resistance and the ability of D2 cold work tool steel to prevent transfer of austenitic stainless steel (ASS) was evaluated in a load-scanning test rig (Fig. 2). The test configuration involved tempered ASS cylinder (AISI 304, 335 HV1, Ra ≈ 0.2 μm, ϕ 10 mm), which was forced to slide against stationary tool steel cylinder of interest. Single pass galling tests were performed under dry and boundary lubricated sliding conditions at a constant sliding speed of 0.01 m/s and normal load in a range of 20–1300 N, corresponding to a nominal contact pressure between 1.3 and 5.2 GPa. In the case of lubricated tests ~ 5 μm thick film of PAO 8 oil (v40 = 46.8 mm 2/s), as estimated by optical interferometry was applied by painting foam roller on the stationary D2 tool steel specimen prior testing. Influence of surface roughness and topography on galling resistance was determined by monitoring coefficient of friction as a function of load and by examining contact surfaces after sliding. Through microscopy examination of wear tracks locations for galling initiation, indicating the first signs of material transfer and ASS transfer layer formation, where a layer of transferred material starts to build on the tool surface were identified and correlated to critical loads [23]. Prior to testing, performed at room temperature (22 ± 1 °C) and relative humidity of ~40%, all specimens were ultrasonically cleaned in ethanol and dried in air. To provide proper reliability of results each galling test was repeated at least three times. 3. Results 3.1. Surface roughness and topography
Fig. 2. Load-scanner.
The highest roughness and very periodic topography with high peaks and deep valleys was produced through turning operation. Post polishing of turned surface reduced its average roughness from 3.42 to 2.65 and 1.69 μm, depending on the post polishing procedure (Table 1). However, its periodic topography remained almost
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Fig. 3. Effect of post polishing on surface roughness profile for (a) turned and (b) ground surface.
unchanged, as shown by constant kurtosis parameter (Table 1) and Fig. 3a. In the case of ground surfaces post polishing had smaller effect on average roughness reduction, but it greatly affected surface topography, as shown in Fig. 3b and Table 1. By post polishing only top peaks are removed, resulting in plateau-like topography with negative skewness and high kurtosis values. Shot peening, on the other hand, led to surface hardness increase (~ 1050 HV1), higher average roughness values and formation of indent type topography with sharp peaks, especially when using ceramic balls (Fig. 4c and d). Finally, laser surface texturing of fine ground and double polished surface had almost no effect on its surface roughness parameters but produced very periodic topography with smooth plateau-like bearing surface and reservoir type pockets (Fig. 4b). 3.2. Galling resistance 3.2.1. Lubricated sliding Coefficient of friction curves recorded during lubricated sliding of Group I samples (Table 1) are shown in Fig. 5. The highest initial friction of ~0.35 was measured for the roughest surface obtained by turning (T), which also shows almost immediate galling initiation and transfer of ASS to D2 steel surface. However, as shown in Figs. 6 and 7, galling and ASS accumulation is concentrated around turning ridges, which also promote abrasive wear of the work material. Change in wear mechanism from adhesive to predominantly abrasive
type resulted in reduction in friction (Fig. 5) and made it very difficult to clearly identify critical load for transfer layer formation. For coarse (CG) and fine ground (FG) surfaces galling was found to be the prevailing wear mechanism (Fig. 8), resulting in friction increase as more and more ASS got transferred to D2 steel surface (Fig. 5). In the case of coarse ground surface initial friction was still 0.3, which indicates almost instantaneous galling initiation. However, at about 300 N load coefficient of friction showed sudden increase, corresponding to ASS transfer layer build-up, as shown in Fig. 8a. Similar trends can be observed for fine ground surface, with the lowest initial coefficient of friction of ~0.25 and the best resistance against ASS transfer layer formation in the range of 350 N (Fig. 9). Surface polishing (Group II and Group III specimens; Table 1) resulted in lower initial friction, smoother surface of the countersurface and improved galling resistance, as observed for all specimens (Fig. 9). However, initial surface preparation and topography determine the level of improvement. Lower the initial roughness larger gains were obtained through post-polishing. After first polishing step (FG-P1) initial friction of fine ground surface was reduced to ~0.2 and critical load for ASS transfer layer formation increased to ~650 N, and after second step (FG-P2) to ~ 0.15 and ~ 700 N, respectively (Fig. 9). Shot peening, on the other hand, led to higher initial coefficient of friction, which also for ground surfaces increased to over 0.35. Furthermore, shot peened surface characterized by sharp peaks caused
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Fig. 4. Roughness profiles of fine ground surface after (a) double polishing (FG-P2), (b) laser surface texturing (FG-T3), (c) shot peening with steel balls (FG-SPS1) and (d) shot peening with ceramic balls (FG-SPC1).
instantaneous galling initiation, greatly reduced resistance to ASS transfer layer formation (Fig. 10) and very rough counter-surface, as shown in Fig. 11a. The deterioration in galling resistance was of about 40%, if surface was shot peened with 230 μm diameter steel balls (Fig. 10). However, the level of deterioration depended on the shot peening conditions and topography produced. By reducing balls size topography with finer indents, lower roughness and less sharp peaks is formed, consequently resulting in smaller galling resistance reduction of less than 15%. On the other hand, with the use of ceramic balls, which produce topography with the sharpest and the most distinctive peaks, resistance of fine ground surface to ASS transfer layer formation has dropped even below 150 N (Fig. 10). Finally, for lubricated conditions the best galling resistance and the smoothest surface of the formed part (Fig. 11b) was obtained with laser textured surfaces, where dimples act as lubricant reservoirs. Through proper selection of LST parameters [21] critical load for ASS transfer layer formation was increased to over 800 N (Fig. 9).
Fig. 5. Coefficient of friction curves for Group I specimens tested under lubricated conditions.
Fig. 6. Micrographs of (a) machined surface (T) and (b) corresponding ASS counter surface exposed to 30 N load; lubricated sliding (arrows indicate direction of sliding).
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Fig. 7. Accumulation of ASS material in the valleys and around turning ridges of machined tool steel surface (T); lubricated sliding.
3.2.2. Dry sliding Similar behavior was observed in dry sliding experiments. However, critical loads for ASS transfer layer formation were much lower as compared to lubricated sliding. When comparing Group I samples all surfaces show similar critical loads of about 200 N, with turns surface again having the highest initial friction of ~ 0.4. In agreement with lubricated tests and Ref. [20] post-polishing resulted in reduced friction and improved galling resistance. When polished, machining and preparation of the basic surface turned out to be important. For turned surface (T) single or double polishing has only negligible effect while for fine ground surface (FG) up to 30% improvement can be achieved, as shown in Fig. 12. In contrast to lubricated sliding additional texturing of the surface doesn't give any further improvement. Contrarily, as shown in Fig. 13 dimples promote galling with ASS transfer starting around dimple edges thus resulting in higher friction (~0.5) and lower galling resistance (Fig. 12). As in the case of lubricated sliding shot peening of the contact surface resulted in greatly reduced galling resistance, with larger ceramic balls giving coefficient of friction of over 0.7, very rough countersurface and critical loads for ASS transfer layer formation as low as 100 N (Fig. 14). 3.2.3. Coated contact If surface is coated its galling resistance was found to depend also on the type of coating and material to be formed [14]. In the case of TiN coating, which generally shows lower galling resistance against ASS as compared to tool steels [6], substrate roughness played very important role. For rougher, coarse ground substrates (CG-P1 & CGP2) TiN coating gives higher friction (~ 0.4) and lower critical loads for ASS transfer layer formation as D2 tool steel (Fig. 15). However,
as substrate roughness is reduced (FG & FG-P2) TiN coating provided similar galling resistance of 250–300 N. Furthermore, if post-polished (P2) to an average roughness value of ~ 0.1, TiN coated surface can give up to 2 times better galling resistance against ASS, as shown in Fig. 15. On the other hand, W-doped DLC coating itself provided low friction and excellent galling resistance against ASS, with critical loads exceeding 1300 N even in dry sliding [6]. In this case substrate roughness as well as post-polishing had practically no effect on surface galling resistance (Fig. 15). 4. Discussion Results of this investigation clearly point out the importance of surface preparation, surface roughness and obtained topography on the galling resistance of forming tools. In the case of turning, even if post-polished surface topography consists of sharp periodic ridges, which represent galling initiators [20] and cause almost instantaneous galling and transfer of work material, which is concentrated around the ridges (Fig. 6a). However, sharp and rigid turning ridges also promote severe abrasive wear and very poor surface quality of the counter-material (Fig. 6b), which during sliding accumulates in the valleys between the ridges, as shown in Fig. 7. Overlapping of adhered and accumulated counter-material also made it very difficult to clearly identify critical loads for galling initiation and transfer layer build up and to properly evaluate galling resistance of turned surfaces. Grinding of the surface reduces surface roughness and by rounding sharp peaks leads to improved galling resistance of the surface. However, only polishing of the surface can give marked improvement in galling resistance. Through removal of asperities it forms plateau-
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Fig. 10. Effect of shot-peening on critical load for ASS transfer layer formation; lubricated sliding.
with very sharp edges and high roughness. Combined effect of high roughness, very sharp edges and increased surface hardness then leads to greatly reduced galling resistance as well as to abrasive wear and poor quality of the counter-surface. By varying material, size and velocity of shot peening balls negative effect on the surface galling resistance can be reduced. However, even in the best situation galling resistance of shot peened surface was found to be way below galling resistance of polished or even ground surface. Negative effect of sharp edges was confirmed by laser textured surfaces. Although providing plateau-like topography galling was very quickly initiated around the dimples (Fig. 13).
Fig. 8. Micrographs of (a) coarse ground surface (CG) and (b) corresponding ASS counter surface exposed to 300 N load; lubricated sliding (arrows indicate direction of sliding).
like topography with higher load-carrying capacity and up to 20% better galling resistance. Consequently, more intense polishing procedure results in smoother plateau-like topography with negative skewness and high kurtosis [18,21] and better galling resistance. However, the level of improvement, obtained by surface polishing greatly depends on the initial surface preparation. Lower the initial surface roughness and more rounded peaks and asperities are larger gains in galling resistance improvement can be expected through surface post-polishing. Although shot peening is known to have beneficial effect on the surface fatigue properties [25] it produces indent type topography
Fig. 9. Effect of surface preparation, post polishing and texturing on critical load for ASS transfer layer formation; lubricated sliding.
Fig. 11. Micrographs of ASS counter-surface sliding against (a) shot-peened (FG-SPS1; FN = 30 N) and (b) laser textured surface (FG-T3; FN = 300 N); lubricated sliding (arrows indicate direction of sliding).
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Fig. 12. Effect of surface preparation, post polishing and texturing on critical load for ASS transfer layer formation; dry sliding.
Fig. 14. Effect of shot-peening on critical load for ASS transfer layer formation; dry sliding.
Traditionally galling resistance of forming tools is improved by lubricating the contact, which was true for all surfaces investigated. However, in the case of lubricated contact surface roughness and topography have even higher importance. Smoother and more uniform the surface higher gain in galling resistance is obtained through lubrication. Smooth surface provides faster and more efficient lubrication film formation, which then carries the load and separates metallic surfaces. Furthermore, experiments with laser textured surfaces showed that formation of plateau-like topography with sufficient cavities or dimples has the most beneficial effect in terms of galling resistance improvement. Dimples, if properly designed and prepared [21,22] will act as oil reservoirs, which then effectively supply lubricant into the contact and postpone galling initiation and transfer layer formation. On the other hand, any bulge or irregularity on the surface causes local increase in contact pressure and lubrication film breakdown, thus leading to metallic contact and galling initiation. If surfaces are coated, their galling resistance and influence of surface roughness and topography depend also on the type of coating. In the case of hard ceramic coatings, i.e. TiN, which in general show high friction against steel surface roughness has even more pronounced effect as observed for the uncoated tool steel. Due to combined effect of high shear strength and very hard surface any increase in surface roughness leads to substantial decrease in galling resistance of the coated surface. Beside that any droplet formed during coating deposition will have further detrimental effect. Therefore, for hard ceramic coatings post-polishing of the coated surface is required in order to provide acceptable galling resistance. On the other hand, when using carbon based low friction coatings (DLC) coating anti-galling properties against stainless steel start to prevail over the negative effect of surface roughness. In this case smoothening of the substrate or
even post-polishing of the coated surface has no or very limited effect on the improvement of galling resistance. Present results of surface roughness analysis and galling resistance evaluation indicate that Rz, Rsk and Rku roughness parameters could also be used to classify surfaces in terms of galling resistance. The main ranking parameters are still Rz and Ra, and the higher the values the higher is the likelihood for galling initiation and material transfer. However, for surfaces displaying similar topography and average roughness Rsk and Rku become important, with more negative Rsk and higher Rku in general resulting in better galling resistance. Importance of Rsk and Rku were found to increase when contact is lubricated or as the surfaces becomes harder and diminishes with the application of low friction coatings. On the other hand, introduction of 3D surface topographies (texturing, shot peening) requires 3D topography characterization which would emphasize sharp edges around the dimples or indents.
Results of this investigation confirm that surface roughness as well as surface topography has considerable influence on galling resistance of forming tools. Although surfaces produced by turning may show similar galling resistance against ASS as ground ones their periodic topography with sharp ridges will lead to higher friction and very poor quality of the formed part. In the case of ground surfaces friction and galling resistance can be greatly improved through post-polishing, which forms plateau-like topography. A degree of improvement depends on post-polishing procedure as well as initial surface preparation.
Fig. 13. Micrograph of dry tested laser textured surface (FG-T3; FN = 250 N; arrow indicates direction of sliding).
Fig. 15. Effect of substrate roughness and post-polishing of coated surfaces on critical load for ASS transfer layer formation; dry sliding.
5. Conclusions
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Formation of well-defined plateau-like topography with dimples, produced through LST improves galling resistance of contact surfaces, but only if lubricated. When lubricated, dimples act as oil reservoirs and postpone adhesion of work material to tool surface while for dry sliding adhesion is accelerated. Shot-peening, on the other hand, has negative effect on friction and galling resistance of contact surfaces, regardless if running dry or lubricated. Although producing surfaces with very high kurtosis and negative skewness, which should be beneficial, very sharp peaks cause almost instantaneous galling. When coated, effect of surface roughness and topography depends on coating type. In the case of hard ceramic coating of TiN smoother substrate and especially post-polishing of the coated surface can give up to 2 times better galling resistance against ASS. However, for softer low friction coatings (DLC) substrate roughness and postpolishing have very minor effect on their galling resistance. References [1] B. Podgornik, V. Leskovšek, in: J. Vižintin, I. Velkavrh, B. Podgornik (Eds.), SLOTRIB'10 – Conference on Tribology, Lubricants and Alternative Fuels, Slovenian Society for Tribology, Ljubljana, 2010, p. 29. [2] C. Mitterer, F. Holler, C. Lugmair, R. Nobauer, R. Kullmer, C. Teichert, Surf. Coat.Technol. 142–144 (2001) 1005. [3] S.R. Schmid, W.R.D. Wilson, in: B. Bhushan (Ed.), Modern Tribology Handbook, CRC Press, Columbus, 2000, p. 1385. [4] S. Hogmark, S. Jacobson, E. Coronel, Tribologia 26 (2007) 3.
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B. Podgornik, S. Hogmark, O. Sandberg, V. Leskovšek, Wear 254 (2003) 1113. B. Podgornik, S. Hogmark, J. Mater. Process. 174 (2006) 334. W.R.D. Wilson, J. Manuf. Sci. Eng. 119 (1997) 695. J.A. Schey, Tribology in metal working: friction, lubrication and wear, American Society for Metals, Metals Park, 1984. N. Bay, T. Nakamura, S. Schmid, in: E. Felder, P. Montmitonnet (Eds.), ICTMP 2010 – International Conference on Tribology in Manufacturing Processes, MINES, Nice, 2010, p. 5. B. Podgornik, J. Vizintin, S. Hogmark, Surf. Eng. 22 (2006) 235. B. Podgornik, J. Vizintin, in: A. Azushima (Ed.), ICTMP 2007 - International Conference on Tribology in Manufacturing Processes, Yokohama, 2007, p. 195. F. Clarysse, W. Lauwerens, M. Vermeulen, Wear 264 (2008) 400. W. Tillmann, E. Vogli, S. Momeni, Surf. Coat.Technol. 205 (2010) 1571. B. Podgornik, S. Hogmark, O. Sandberg, Wear 261 (2006) 15. B. Podgornik, V. Leskovšek, J. Vizintin, Mater. Manuf. Process. 24 (2009) 734. O. Mahrenholz, N. Botcheva, R. Iankov, J. Mater. Process. 159 (2005) 9. M. Hanson, S. Hogmark, S. Jacobson, Mater. Manuf. Process. 24 (2009) 913. M. Sedlaček, B. Podgornik, J. Vizintin, Wear 266 (2009) 482. L.M. Vilhena, B. Podgornik, J. Vizintin, J. Mozina, Meccanica 46 (2011) 567. B. Podgornik, S. Hogmark, O. Sandberg, Surf. Coat.Technol. 184 (2004) 338. M. Sedlaček, Influence of surface topography on tribological behavior of contact surfaces, PhD thesis, University of Ljubljana, Ljubljana, 2010. B. Podgornik, L.M. Vilhena, M. Sedlaček, M. Rodriguez Ripoll, Z. Rek, I. Žun, Nordtrib 2010 – 14th Nordic Symposium on Tribology, Luleå, 2010, 8 pp. B. Podgornik, S. Hogmark, J. Pezdirnik, Wear 257 (2004) 843. S. Hogmark, S. Jacobson, O. Wanstrand, Proceedings of the 22nd IRG-OECD Meeting, Cambridge, 2000, 3 pp. K.A. Soady, B.G. Mellor, J. Shackleton, A. Morris, P.A.S. Reed, Materials Science and Engineering: A 528 (2011) 8579–8588.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Surface topography effect on galling resistance of coated and uncoated tool steel B. Podgornik a,⁎, J. Jerina b a b
Institute of Metals and Technology, Lepi pot 11, SI-1000 Ljubljana, Slovenia University of Ljubljana, Centre for tribology and technical diagnostics, Bogisiceva 8, SI-1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 16 June 2011 Accepted in revised form 26 November 2011 Available online 3 December 2011 Keywords: Cold forming Roughness Surface modification Galling Friction
a b s t r a c t Tribological evaluation of the tool steel, focused on determining coefficient of friction and critical load for galling initiation against austenitic stainless steel as a function of surface topography was carried out in a load-scanning test rig. Surfaces investigated included turned, ground, polished, shot penned and laser surface textured cold work tool steel. Additionally, effect of surface roughness and post-polishing on galling resistance of TiN and DLC coated surfaces was investigated. Results of this investigation show that by removing sharp peaks polishing of the bearing surface gives plateau-like topography and improves galling properties of forming tool steel. This becomes even more important when using hard ceramic coatings (i.e. TiN). When superbly polished contact surfaces can give similar galling resistance as otherwise obtained through contact lubrication. However, application of low friction coatings or introduction of micro-dimples with proper density and contact lubrication will result in superior galling resistance of the surface. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In metal forming industry tools can be exposed to very complex and surface demanding conditions [1], which are the result of different effects (mechanical, thermal, chemical or tribological loading) and require well defined mechanical and tribological properties of the tool material. Besides that the surface finish of the formed parts is a critical quality parameter in many forming operations. A smooth surface, requiring smooth and defect free surface of the tool results in marketing advantages, as well as improved load-carrying capacity and tribological performance of the component [2]. The major obstacles to generation of smooth and defects free components are tool wear and adhesion of work material to the tool surface, known as galling [3]. Galling develops gradually as an adhesion of work material on the tool surface, with repeated forming cycles causing successive buildup of patches of transferred work material. During adhesion transferred work material may experience several hardening mechanisms [4] thus damaging the subsequently formed part by scratching and indentation, and leading to unstable friction [5,6]. In deep drawing and sheet metal forming processes galling is also the dominant cause for tool failure [7]. Generally, lubricants are used in order to decrease friction and adhesion in the contact between work material and tool [8]. However, ever increasing demands for “green” or environmentally friendly production require reduced lubrication and use of less toxic bio-degradable
⁎ Corresponding author. Tel.: + 386 1 4701930; fax: + 386 1 4701939. E-mail address: [email protected] (B. Podgornik). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.11.041
lubricants [9]. This and high degree of plastic deformation experienced in forming inevitably lead to metal-to-metal contact and galling initiation. But, wear resistance and tribological properties of forming tools can be further improved through selection of high quality tool steels or application of proper surface engineering technique [6,10,11]. In recent years the main focus in improving galling resistance of forming tools has been on use of hard coatings [12,13]. While ceramic coatings used in cutting applications normally show a relatively high friction and high tendency to galling low-friction DLC coatings can give very stable friction and excellent galling resistance [5,6]. However, coating selection is greatly influenced by the type of work material, with surface treatments, i.e. plasma nitriding and deep-cryogenic treatment providing similar or even better results when forming Ti and Al alloys [11,14,15]. Anyway, in order to reduce the effect of galling polishing of the tool surface is still required in many forming operations. It is known that among other things the risk of work material adhesion and galling increases with tool roughness [16,17]. On the other hand, surface topography investigations indicate that even with “rougher” surfaces, which are cheaper to produce selection of proper topography can lead to improved tribological performance [18]. Especially surface texturing, involving micro-dimples on the bearing surface shows great potential in lubricated sliding [19]. In general, generation of the beneficial topography should be decisive for obtaining desired anti-galling properties of forming tools, especially those equipped with hard, wear resistant coatings [20]. Therefore, the aim of the present work was to investigate the effect of surface preparation on roughness parameters and topography, and consequently on galling properties of coated and uncoated forming tool steel.
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ground specimens was performed also with 150 μm steel balls at 6 bar (FG-SPS2), and 150 (FG-SPC2) and 230 μm ceramic balls at the pressure of 4 bar (FG-SPC1). Final group of specimens (V; FG-P2 + T) included fine ground and double polished surfaces (FG-P2) subsequently laser surface textured (LST) with Nd:YAG laser [19], producing dimples with a diameter of 60 μm and depth of 15 μm. Textured surfaces, being re-polished after LST in order to remove bumps around the dimples (Fig. 1) comprised three different texturing densities of 1 (FG-T1), 5 (FG-T5) and 20% (FG-T20), obtained by changing distance between the dimples from 500 to 250 and 125 μm. Texturing parameters, showing the best potential of reducing friction under boundary lubrication were selected on the basis of previous investigations [21,22]. The effect of surface roughness for coated surfaces was evaluated on coarse and fine ground samples (CG-P1, CG-P2, FG and FG-P2), coated with commercial monolayer PVD TiN hard coating (28 GPa) and “softer” multilayer PACVD W-doped DLC coating (12 GPa) [5], and tested under dry sliding conditions.
2. Experimental details 2.1. Material and surface preparation Material used in this investigation was a standard AISI D2 cold work tool steel with the nominal composition (wt.%) 1.55 C, 0.35 Si, 0.40 Mn, 12.0 Cr, 1.0 Mo and 0.85 V. The steel samples in the shape of cylinders (ϕ10 × 100 mm) were cut, machined and ground from rolled and soft annealed bars. Cylindrical specimens were then heated to 1040 °C and cooled in air to reach surface hardness of ~ 900 HV1. In order to produce different surface roughness and topography five groups of specimens were prepared. Specimens' description and preparation procedure with corresponding roughness parameters are given in Table 1. First or base group (I), produced by standard machining and circumferential grinding operations included turned (T), coarse ground (CG) and fine ground (FG) samples. Through subsequent polishing, using polishing wheel and industrial polishing paste with the particle size of ~20 μm, second group of specimens (II; I + P1) denoted T-P1 (turned and post polished), CG-P1 (coarse ground and post polished) and FG-P1 (fine ground and post polished) was prepared. For the third group (III; I + P1 + P2) two step polishing of the base sample group was applied, combining polishing with 20 μm polishing paste and followed by 10 μm polishing paste (T-P2, CG-P2 and FG-P2). Fourth group of specimens (IV; I + SP) included shot peening performed by different balls on three base samples. Shot peening with 230 μm diameter steel balls at a pressure of 6 bar was applied on turned (T-SPS1), coarse ground (CG-SPS1) and fine ground (FG-SPS1) specimens. Additionally, shot peening of fine
2.2. Surface roughness measurement Surface roughness measurements were performed with the stylus profilometer Hommelwerke T8000. For each specimen three measurements were performed along the main cylinder axis using stylus speed of 0.5 mm/s and measurement length of 4.8 mm. Each measurement involved 5 successive profiles taken at a cylinder rotation step of about 15°. Average values of five roughness parameters,
Table 1 Specimens description and corresponding roughness parameters. Sample
Description
Roughness parameters [μm] Ra
Rq
Rz
Rsk
Turning Coarse grinding Fine grinding
3.42 ± 0.24 0.80 ± 0.15 0.13 ± 0.01
4.09 ± 0.23 1.02 ± 0.19 0.17 ± 0.01
16.46 ± 0.3 5.24 ± 0.93 1.10 ± 0.06
0.37 ± 0.06 0.19 ± 0.11 − 0.39 ± 0.24
2.32 ± 0.02 3.21 ± 0.38 3.78 ± 0.68
Group II T-P1 CG-P1 FG-P1
T + polishing (20 μm) CG + polishing (20 μm) FG + polishing (20 μm)
2.65 ± 0.24 0.49 ± 0.07 0.10 ± 0.01
3.11 ± 0.25 0.61 ± 0.06 0.13 ± 0.01
12.07 ± 1.0 2.57 ± 0.31 0.79 ± 0.08
0.13 ± 0.08 0.16 ± 0.12 − 0.47 ± 0.13
2.07 ± 0.06 3.31 ± 0.17 4.18 ± 0.14
Group III T-P2 CG-P2 FG-P2
T-P1 + polishing (10 μm) CG-P1 + polishing (10 μm) FG-P1 + polishing (10 μm)
1.69 ± 0.07 0.29 ± 0.04 0.09 ± 0.00
1.96 ± 0.05 0.37 ± 0.06 0.12 ± 0.01
7.54 ± 0.41 1.85 ± 0.24 0.64 ± 0.02
0.07 ± 0.08 − 0.09 ± 0.06 − 0.46 ± 0.21
2.11 ± 0.21 3.28 ± 0.31 4.29 ± 0.77
T + shot peening (steel balls ϕ230 μm) CG + shot peening (steel balls ϕ230 μm) FG + shot peening (steel balls ϕ230 μm) FG + shot peening (steel balls ϕ150 μm) FG + shot peening (ceramic balls ϕ230 μm) FG + shot peening (ceramic balls ϕ150 μm)
3.40 ± 0.57
4.15 ± 0.72
18.28 ± 2.6
0.01 ± 0.05
2.63 ± 0.05
2.25 ± 0.04
2.96 ± 0.12
14.86 ± 0.1
− 0.77 ± 0.36
4.76 ± 0.64
1.31 ± 0.07
1.97 ± 0.09
11.71 ± 0.6
− 1.72 ± 0.07
8.68 ± 0.52
0.66 ± 0.03
1.00 ± 0.08
2.25 ± 0.21
2.82 ± 0.28
14.08 ± 1.2
− 0.29 ± 0.10
3.26 ± 0.36
2.06 ± 0.03
2.59 ± 0.01
12.98 ± 0.7
− 0.26 ± 0.05
3.17 ± 0.33
FG-P2 + LST (1% density, 500 μm) FG-P2 + LST (5% density, 250 μm) FG-P2 + LST (20% density, 125 μm)
0.06 ± 0.00
0.07 ± 0.00
0.44 ± 0.06
− 0.18 ± 0.04
3.91 ± 1.44
0.07 ± 0.00
0.09 ± 0.01
0.48 ± 0.02
− 0.10 ± 0.14
3.16 ± 0.35
0.08 ± 0.01
0.10 ± 0.01
0.55 ± 0.02
− 0.08 ± 0.06
3.19 ± 0.42
Group I T CG FG
Group IV T-SPS1 CG-SPS1 FG-SPS1 FG-SPS2 FG-SPC1 FG-SPC2
Group V FG-T1 FG-T2 FG-T3
6.38 ± 0.49
− 2.11 ± 0.15
Rku
12.78 ± 0.9
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Fig. 1. (a) Textured - T, (b) fine ground - FG, (c) shot peened - FG-SPC1 and (d) textured - FG-T3 surfaces.
namely Ra, Rq, Rz, Rsk and Rku were then calculated taking into account all 15 individual profiles. 2.3. Galling evaluation In practice different test methods are used to evaluate galling resistance of forming tool materials, ranging from a very simple pinon-disc test rig to the ASTM standard G98-91 button-on-block test method. In metal forming, especially in sheet metal forming at every stroke new raw material is coming into contact with the tool surface, which is not the case for pin-on-disc configuration. On the other hand, ranking of tool materials is commonly based on the critical load when galling occurs, thus requiring a lot of testing. As shown in Ref. [23] load-scanning test rig has been found as one of the most suitable methods for comparing tool materials in terms of galling resistance. Although it doesn't directly correspond to forming operations its cross-cylinder configuration allows gradual increase in load during a single stroke, with each point along the contact path of both specimens corresponding to a specific load and displaying a unique contact history [23,24].
Therefore, galling resistance and the ability of D2 cold work tool steel to prevent transfer of austenitic stainless steel (ASS) was evaluated in a load-scanning test rig (Fig. 2). The test configuration involved tempered ASS cylinder (AISI 304, 335 HV1, Ra ≈ 0.2 μm, ϕ 10 mm), which was forced to slide against stationary tool steel cylinder of interest. Single pass galling tests were performed under dry and boundary lubricated sliding conditions at a constant sliding speed of 0.01 m/s and normal load in a range of 20–1300 N, corresponding to a nominal contact pressure between 1.3 and 5.2 GPa. In the case of lubricated tests ~ 5 μm thick film of PAO 8 oil (v40 = 46.8 mm 2/s), as estimated by optical interferometry was applied by painting foam roller on the stationary D2 tool steel specimen prior testing. Influence of surface roughness and topography on galling resistance was determined by monitoring coefficient of friction as a function of load and by examining contact surfaces after sliding. Through microscopy examination of wear tracks locations for galling initiation, indicating the first signs of material transfer and ASS transfer layer formation, where a layer of transferred material starts to build on the tool surface were identified and correlated to critical loads [23]. Prior to testing, performed at room temperature (22 ± 1 °C) and relative humidity of ~40%, all specimens were ultrasonically cleaned in ethanol and dried in air. To provide proper reliability of results each galling test was repeated at least three times. 3. Results 3.1. Surface roughness and topography
Fig. 2. Load-scanner.
The highest roughness and very periodic topography with high peaks and deep valleys was produced through turning operation. Post polishing of turned surface reduced its average roughness from 3.42 to 2.65 and 1.69 μm, depending on the post polishing procedure (Table 1). However, its periodic topography remained almost
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Fig. 3. Effect of post polishing on surface roughness profile for (a) turned and (b) ground surface.
unchanged, as shown by constant kurtosis parameter (Table 1) and Fig. 3a. In the case of ground surfaces post polishing had smaller effect on average roughness reduction, but it greatly affected surface topography, as shown in Fig. 3b and Table 1. By post polishing only top peaks are removed, resulting in plateau-like topography with negative skewness and high kurtosis values. Shot peening, on the other hand, led to surface hardness increase (~ 1050 HV1), higher average roughness values and formation of indent type topography with sharp peaks, especially when using ceramic balls (Fig. 4c and d). Finally, laser surface texturing of fine ground and double polished surface had almost no effect on its surface roughness parameters but produced very periodic topography with smooth plateau-like bearing surface and reservoir type pockets (Fig. 4b). 3.2. Galling resistance 3.2.1. Lubricated sliding Coefficient of friction curves recorded during lubricated sliding of Group I samples (Table 1) are shown in Fig. 5. The highest initial friction of ~0.35 was measured for the roughest surface obtained by turning (T), which also shows almost immediate galling initiation and transfer of ASS to D2 steel surface. However, as shown in Figs. 6 and 7, galling and ASS accumulation is concentrated around turning ridges, which also promote abrasive wear of the work material. Change in wear mechanism from adhesive to predominantly abrasive
type resulted in reduction in friction (Fig. 5) and made it very difficult to clearly identify critical load for transfer layer formation. For coarse (CG) and fine ground (FG) surfaces galling was found to be the prevailing wear mechanism (Fig. 8), resulting in friction increase as more and more ASS got transferred to D2 steel surface (Fig. 5). In the case of coarse ground surface initial friction was still 0.3, which indicates almost instantaneous galling initiation. However, at about 300 N load coefficient of friction showed sudden increase, corresponding to ASS transfer layer build-up, as shown in Fig. 8a. Similar trends can be observed for fine ground surface, with the lowest initial coefficient of friction of ~0.25 and the best resistance against ASS transfer layer formation in the range of 350 N (Fig. 9). Surface polishing (Group II and Group III specimens; Table 1) resulted in lower initial friction, smoother surface of the countersurface and improved galling resistance, as observed for all specimens (Fig. 9). However, initial surface preparation and topography determine the level of improvement. Lower the initial roughness larger gains were obtained through post-polishing. After first polishing step (FG-P1) initial friction of fine ground surface was reduced to ~0.2 and critical load for ASS transfer layer formation increased to ~650 N, and after second step (FG-P2) to ~ 0.15 and ~ 700 N, respectively (Fig. 9). Shot peening, on the other hand, led to higher initial coefficient of friction, which also for ground surfaces increased to over 0.35. Furthermore, shot peened surface characterized by sharp peaks caused
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Fig. 4. Roughness profiles of fine ground surface after (a) double polishing (FG-P2), (b) laser surface texturing (FG-T3), (c) shot peening with steel balls (FG-SPS1) and (d) shot peening with ceramic balls (FG-SPC1).
instantaneous galling initiation, greatly reduced resistance to ASS transfer layer formation (Fig. 10) and very rough counter-surface, as shown in Fig. 11a. The deterioration in galling resistance was of about 40%, if surface was shot peened with 230 μm diameter steel balls (Fig. 10). However, the level of deterioration depended on the shot peening conditions and topography produced. By reducing balls size topography with finer indents, lower roughness and less sharp peaks is formed, consequently resulting in smaller galling resistance reduction of less than 15%. On the other hand, with the use of ceramic balls, which produce topography with the sharpest and the most distinctive peaks, resistance of fine ground surface to ASS transfer layer formation has dropped even below 150 N (Fig. 10). Finally, for lubricated conditions the best galling resistance and the smoothest surface of the formed part (Fig. 11b) was obtained with laser textured surfaces, where dimples act as lubricant reservoirs. Through proper selection of LST parameters [21] critical load for ASS transfer layer formation was increased to over 800 N (Fig. 9).
Fig. 5. Coefficient of friction curves for Group I specimens tested under lubricated conditions.
Fig. 6. Micrographs of (a) machined surface (T) and (b) corresponding ASS counter surface exposed to 30 N load; lubricated sliding (arrows indicate direction of sliding).
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Fig. 7. Accumulation of ASS material in the valleys and around turning ridges of machined tool steel surface (T); lubricated sliding.
3.2.2. Dry sliding Similar behavior was observed in dry sliding experiments. However, critical loads for ASS transfer layer formation were much lower as compared to lubricated sliding. When comparing Group I samples all surfaces show similar critical loads of about 200 N, with turns surface again having the highest initial friction of ~ 0.4. In agreement with lubricated tests and Ref. [20] post-polishing resulted in reduced friction and improved galling resistance. When polished, machining and preparation of the basic surface turned out to be important. For turned surface (T) single or double polishing has only negligible effect while for fine ground surface (FG) up to 30% improvement can be achieved, as shown in Fig. 12. In contrast to lubricated sliding additional texturing of the surface doesn't give any further improvement. Contrarily, as shown in Fig. 13 dimples promote galling with ASS transfer starting around dimple edges thus resulting in higher friction (~0.5) and lower galling resistance (Fig. 12). As in the case of lubricated sliding shot peening of the contact surface resulted in greatly reduced galling resistance, with larger ceramic balls giving coefficient of friction of over 0.7, very rough countersurface and critical loads for ASS transfer layer formation as low as 100 N (Fig. 14). 3.2.3. Coated contact If surface is coated its galling resistance was found to depend also on the type of coating and material to be formed [14]. In the case of TiN coating, which generally shows lower galling resistance against ASS as compared to tool steels [6], substrate roughness played very important role. For rougher, coarse ground substrates (CG-P1 & CGP2) TiN coating gives higher friction (~ 0.4) and lower critical loads for ASS transfer layer formation as D2 tool steel (Fig. 15). However,
as substrate roughness is reduced (FG & FG-P2) TiN coating provided similar galling resistance of 250–300 N. Furthermore, if post-polished (P2) to an average roughness value of ~ 0.1, TiN coated surface can give up to 2 times better galling resistance against ASS, as shown in Fig. 15. On the other hand, W-doped DLC coating itself provided low friction and excellent galling resistance against ASS, with critical loads exceeding 1300 N even in dry sliding [6]. In this case substrate roughness as well as post-polishing had practically no effect on surface galling resistance (Fig. 15). 4. Discussion Results of this investigation clearly point out the importance of surface preparation, surface roughness and obtained topography on the galling resistance of forming tools. In the case of turning, even if post-polished surface topography consists of sharp periodic ridges, which represent galling initiators [20] and cause almost instantaneous galling and transfer of work material, which is concentrated around the ridges (Fig. 6a). However, sharp and rigid turning ridges also promote severe abrasive wear and very poor surface quality of the counter-material (Fig. 6b), which during sliding accumulates in the valleys between the ridges, as shown in Fig. 7. Overlapping of adhered and accumulated counter-material also made it very difficult to clearly identify critical loads for galling initiation and transfer layer build up and to properly evaluate galling resistance of turned surfaces. Grinding of the surface reduces surface roughness and by rounding sharp peaks leads to improved galling resistance of the surface. However, only polishing of the surface can give marked improvement in galling resistance. Through removal of asperities it forms plateau-
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Fig. 10. Effect of shot-peening on critical load for ASS transfer layer formation; lubricated sliding.
with very sharp edges and high roughness. Combined effect of high roughness, very sharp edges and increased surface hardness then leads to greatly reduced galling resistance as well as to abrasive wear and poor quality of the counter-surface. By varying material, size and velocity of shot peening balls negative effect on the surface galling resistance can be reduced. However, even in the best situation galling resistance of shot peened surface was found to be way below galling resistance of polished or even ground surface. Negative effect of sharp edges was confirmed by laser textured surfaces. Although providing plateau-like topography galling was very quickly initiated around the dimples (Fig. 13).
Fig. 8. Micrographs of (a) coarse ground surface (CG) and (b) corresponding ASS counter surface exposed to 300 N load; lubricated sliding (arrows indicate direction of sliding).
like topography with higher load-carrying capacity and up to 20% better galling resistance. Consequently, more intense polishing procedure results in smoother plateau-like topography with negative skewness and high kurtosis [18,21] and better galling resistance. However, the level of improvement, obtained by surface polishing greatly depends on the initial surface preparation. Lower the initial surface roughness and more rounded peaks and asperities are larger gains in galling resistance improvement can be expected through surface post-polishing. Although shot peening is known to have beneficial effect on the surface fatigue properties [25] it produces indent type topography
Fig. 9. Effect of surface preparation, post polishing and texturing on critical load for ASS transfer layer formation; lubricated sliding.
Fig. 11. Micrographs of ASS counter-surface sliding against (a) shot-peened (FG-SPS1; FN = 30 N) and (b) laser textured surface (FG-T3; FN = 300 N); lubricated sliding (arrows indicate direction of sliding).
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Fig. 12. Effect of surface preparation, post polishing and texturing on critical load for ASS transfer layer formation; dry sliding.
Fig. 14. Effect of shot-peening on critical load for ASS transfer layer formation; dry sliding.
Traditionally galling resistance of forming tools is improved by lubricating the contact, which was true for all surfaces investigated. However, in the case of lubricated contact surface roughness and topography have even higher importance. Smoother and more uniform the surface higher gain in galling resistance is obtained through lubrication. Smooth surface provides faster and more efficient lubrication film formation, which then carries the load and separates metallic surfaces. Furthermore, experiments with laser textured surfaces showed that formation of plateau-like topography with sufficient cavities or dimples has the most beneficial effect in terms of galling resistance improvement. Dimples, if properly designed and prepared [21,22] will act as oil reservoirs, which then effectively supply lubricant into the contact and postpone galling initiation and transfer layer formation. On the other hand, any bulge or irregularity on the surface causes local increase in contact pressure and lubrication film breakdown, thus leading to metallic contact and galling initiation. If surfaces are coated, their galling resistance and influence of surface roughness and topography depend also on the type of coating. In the case of hard ceramic coatings, i.e. TiN, which in general show high friction against steel surface roughness has even more pronounced effect as observed for the uncoated tool steel. Due to combined effect of high shear strength and very hard surface any increase in surface roughness leads to substantial decrease in galling resistance of the coated surface. Beside that any droplet formed during coating deposition will have further detrimental effect. Therefore, for hard ceramic coatings post-polishing of the coated surface is required in order to provide acceptable galling resistance. On the other hand, when using carbon based low friction coatings (DLC) coating anti-galling properties against stainless steel start to prevail over the negative effect of surface roughness. In this case smoothening of the substrate or
even post-polishing of the coated surface has no or very limited effect on the improvement of galling resistance. Present results of surface roughness analysis and galling resistance evaluation indicate that Rz, Rsk and Rku roughness parameters could also be used to classify surfaces in terms of galling resistance. The main ranking parameters are still Rz and Ra, and the higher the values the higher is the likelihood for galling initiation and material transfer. However, for surfaces displaying similar topography and average roughness Rsk and Rku become important, with more negative Rsk and higher Rku in general resulting in better galling resistance. Importance of Rsk and Rku were found to increase when contact is lubricated or as the surfaces becomes harder and diminishes with the application of low friction coatings. On the other hand, introduction of 3D surface topographies (texturing, shot peening) requires 3D topography characterization which would emphasize sharp edges around the dimples or indents.
Results of this investigation confirm that surface roughness as well as surface topography has considerable influence on galling resistance of forming tools. Although surfaces produced by turning may show similar galling resistance against ASS as ground ones their periodic topography with sharp ridges will lead to higher friction and very poor quality of the formed part. In the case of ground surfaces friction and galling resistance can be greatly improved through post-polishing, which forms plateau-like topography. A degree of improvement depends on post-polishing procedure as well as initial surface preparation.
Fig. 13. Micrograph of dry tested laser textured surface (FG-T3; FN = 250 N; arrow indicates direction of sliding).
Fig. 15. Effect of substrate roughness and post-polishing of coated surfaces on critical load for ASS transfer layer formation; dry sliding.
5. Conclusions
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Formation of well-defined plateau-like topography with dimples, produced through LST improves galling resistance of contact surfaces, but only if lubricated. When lubricated, dimples act as oil reservoirs and postpone adhesion of work material to tool surface while for dry sliding adhesion is accelerated. Shot-peening, on the other hand, has negative effect on friction and galling resistance of contact surfaces, regardless if running dry or lubricated. Although producing surfaces with very high kurtosis and negative skewness, which should be beneficial, very sharp peaks cause almost instantaneous galling. When coated, effect of surface roughness and topography depends on coating type. In the case of hard ceramic coating of TiN smoother substrate and especially post-polishing of the coated surface can give up to 2 times better galling resistance against ASS. However, for softer low friction coatings (DLC) substrate roughness and postpolishing have very minor effect on their galling resistance. References [1] B. Podgornik, V. Leskovšek, in: J. Vižintin, I. Velkavrh, B. Podgornik (Eds.), SLOTRIB'10 – Conference on Tribology, Lubricants and Alternative Fuels, Slovenian Society for Tribology, Ljubljana, 2010, p. 29. [2] C. Mitterer, F. Holler, C. Lugmair, R. Nobauer, R. Kullmer, C. Teichert, Surf. Coat.Technol. 142–144 (2001) 1005. [3] S.R. Schmid, W.R.D. Wilson, in: B. Bhushan (Ed.), Modern Tribology Handbook, CRC Press, Columbus, 2000, p. 1385. [4] S. Hogmark, S. Jacobson, E. Coronel, Tribologia 26 (2007) 3.
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