Surface modification to improve friction and galling

Journal of Materials Processing Technology 174 (2006) 334–341

Surface modification to improve friction and gallingproperties of forming tools

B. Podgornik a,∗, S. Hogmark b

a Centre for Tribology and Technical Diagnostics, University of Ljubljana, Bogisiceva 8, SI-1000 Ljubljana, Sloveniab The Tribomaterials Group, Angstrom Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden

Received 23 September 2003; accepted 20 January 2006

Abstract

The aim of the present work was to investigate and compare different surface modification techniques in terms of galling properties when appliedon forming tools. The surface modification techniques included were different grades of polishing, plasma nitriding and DLC coating, all appliedto cold work tool steel, respectively. Tribological evaluation against austenitic stainless steel was carried out in a load-scanning test rig. A specialtest configuration makes it possible to gradually increase the normal load during testing, with the load being in the range between 100 and 1300 N(1–3.5 GPa).

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It is concluded that the galling tendency can be greatly reduced by proper polishing of the tool surface. However, selection of a carbon-basedultilayer WC/C coating leads to substantially reduced probability of work material transfer even at a relatively high surface roughness, and also

nder starved lubrication. If polished after treatment, a plasma nitrided surface will provide improved friction and wear properties of the tool steelnd to some extend also reduce the galling tendency in sliding against austenitic stainless steel. A combination of nitriding, surface polishing andLC coating is suggested as the optimum surface treatment of tools aimed at forming austenitic stainless steel.2006 Elsevier B.V. All rights reserved.

eywords: Forming tools; Surface engineering; Friction; Galling

. Introduction

Manufacturing of parts is confronted with ever-increasingemands on higher productivity, which put increased require-ents on wear and fatigue resistance of the tools. In the case

f cutting tools, deposition of hard PVD and CVD coatingsi.e. TiN, TiAlN, CrN, . . .) has proven to provide consider-ble enhancement in wear resistance and therefore tool life inumerous cases [1–3]. However, in competitive environmentnly increased production at maintained product quality can beuccessful because this directly influencing the profitability.

The surface finish is a critical quality parameter in thease of sheet metal parts. A smooth surface often results inarketing advantages, as well as improved performance. Theajor obstacles to generation of superior smooth surfaces

re smudge, orange peel and galling [4], with galling alsoeing the dominant cause for tool failure in sheet forming

∗ Corresponding author. Tel.: +386 1 4771463; fax: +386 1 4771469.E-mail address: [email protected] (B. Podgornik).

processes for austenitic stainless steel, aluminium-, nickel- andtitanium-based alloys. Typically, galling develops graduallyduring forming operations as an accumulation of sheet materialpick-ups on the tool surface. This causes aesthetic problems forthe work-peace and generates unstable friction conditions forthe continuing forming process [5].

The main focus on improving the tribological properties insheet forming has been on modifying lubricants for better reten-tion and permeability at the tool/sheet contact area. Nevertheless,galling can successfully be hindered also by modification of thetool surface [6,7]. One way of modifying the surface, alreadyeffectively used in the forming industry to improve wear resis-tance of the tools, is thermo-chemical surface treatment, i.e.hardening, nitriding, carbonitriding, etc. [7–9]. In the last coupleof years hard coatings have started to compete successfully withthe traditional thermo-chemical treatments. However, althoughhard and corrosion-resistant coatings are frequently used to pro-tect and enhance the lifetime of cutting tools, the majority ofthe industrial forming tools is still uncoated. Besides a complexshape that often makes the forming tools difficult to coat, com-mercial hard ceramic coatings used in cutting tool applications

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B. Podgornik, S. Hogmark / Journal of Materials Processing Technology 174 (2006) 334–341 335

often give a relatively high friction coefficient and a high ten-dency to galling when in contact with soft metals [3,10,11]. Withthe introduction of carbon-based coatings such as diamond anddiamond like carbon (DLC), which show high wear resistanceand excellent frictional properties [12–14], regular use of coatedforming tools can be expected for the near future.

Another way to improve the galling properties of formingtools is to optimise the surface topography. Investigations onthe influence of surface roughness showed that it has a greatinfluence on the tribological behaviour of contacting surfaces[15–17], thus playing an important role in the process of improv-ing the galling properties of forming tools.

The aim of the present work was to investigate and com-pare different surface modification techniques aimed to reducethe galling tendency of forming tools. A tribological evaluationof plasma nitrided, DLC coated, and polished cold work toolsteel was carried out in a load-scanning test rig, using austeniticstainless steel as the counter material.

2. Experimental

The material used in this investigation was a powder metallurgy cold worktool steel, VANADIS 6, from Uddeholm Tooling AB, Sweden. It has a nominalchemical composition (wt.%) of 2.1 C, 1.0 Si, 0.4 Mn, 6.8 Cr, 1.5 Mo, 5.4 V.The cylindrical test rods (Ø 10 mm, 100 mm long) were hardened and tempered

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to 850 HV, and ground to an average Ra value of ≈0.25 �m. Three groups oftest specimens were prepared in terms of surface modification.

The first group contained two plasma nitrided steels, obtained with differentgas mixtures, see Table 1. These treatments resulted in nitrided layers with andwithout a compound layer, respectively. After nitriding all samples were re-

Fig. 2. Typical appearance of the contact surface of hardened steel specimensfrom the dry, single stroke sliding test against austenitic stainless steel. (a) Atabout 200 N load where the material transfer begins (light contrast). (b) At about250 N load where a layer of transferred material has formed. The arrows indicatethe direction of sliding.

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(h) Case depth (�m) Compound layer (�m) Surface hardness (HV0.1)

A 60 2-Fe4N (�′) 1300B 55 0 1130C × 2 Through – 850

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C

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ig. 1. Representative friction coefficient vs. normal load curves for hardenednd plasma nitrided tool steel, recorder during dry, single stroke sliding againstustenitic stainless steel. All test rods had a surface roughness of Ra = 0.25 �m.

able 1etails of substrate treatment processes and obtained surface hardness values

Treatment Atmosphere Temperature (◦C) Time

Plasma nitriding 75% H2–25% N2 500 6Plasma nitriding 95% H2–5% N2 500 9Hardening Air quenched 1060/525 0.5/2

able 2eposition parameters and resulting coating properties

oating Process Temp. (◦C) Substrate bias (V) Interlayer (

iN Reactive e-beam 320–420 −110 Ti-0.1C/C Reactive sputtering 230 NA Cr-0.1

A, not available.

�m) Hardness (GPa) Young’s modulus (GPa) Residual stress (GPa)

30 ± 2 500 ± 50 −3.8 ± 0.412 ± 1 130 ± 7 −0.3 ± 0.1

336 B. Podgornik, S. Hogmark / Journal of Materials Processing Technology 174 (2006) 334–341

ground in order to re-establish the original surface roughness of Ra ≈ 0.25 �m.The second set of specimens was hard coated. Two commercial PVD coatingswere included in this investigation; TiN as the most commonly used coatingin cutting tool applications and WC doped hydrogenated diamond like carbonwith a multilayer structure of WC and C (WC/C), representing the class of hardlow-friction coatings, see Table 2. The third group of specimens was preparedusing different grades of grinding and polishing, which gave average surfaceroughness values Ra of 0.05, 0.1, 0.15, 0.25 and 0.4 �m, respectively.

The tribological evaluation included comparison between surface engineeredsteel samples and hardened and ground reference ones. It was performed in anew type of load-scanning test rig [18,19]. An austenitic stainless steel (AISI304; 350 HV) was used as counter material. The test configuration, involves twocrossed cylinders, which are forced to slide reciprocally against each other undera constant speed. The normal load increases gradually during forward strokesand decreases correspondingly during reversed strokes. Thus, each point alongthe contact path of both specimens will experience a unique load and display aunique tribological history after test completion.

In the first phase of this investigation, the test equipment was set to a single,forward stroke mode, and operated under dry conditions with the sliding speedfixed to 0.01 m/s. Some additional tests were performed dry or under starvedlubricated conditions, with the test rig set to a multicycle mode. An approxi-mately 10 �m thick film of pure poly-alpha-olefin (PAO) oil (v40 = 46.6 mm2/s)was applied on the work material sample before each starved lubricated test. Thesliding speed was set to 0.1 m/s, and the highest number of test cycles was 50. Inboth types of test, the normal load was gradually increased from 100 to 1300 N(3.5 GPa maximum Hertzian contact pressure), reached at the end of each stroke.

Results of this test were displayed as friction versus load, and by imaginingthe worn surfaces by optical (OM) or scanning electron microscopy (SEM).Monitoring of the friction coefficient as a function of load and time has been

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utilised for the lubricated multicycle tests to prepare friction maps, which showany transition point in the tribological behaviour. From the OM micrographs,critical loads for the initiation of galling (Lc1) and for development of a wholelayer of transferred material (Lc2) could be established.

Fig. 5. Critical loads for the beginning of transfer (Lc1) and formation of a layerof transfered stainless steel (Lc2) on uncoated and coated tool steel samples, asestimated from the dry, single stroke test.

ig. 3. Critical loads for the beginning of transfer (Lc1) and formation of aayer of transfered stainless steel (Lc2) on surface treated tool steel samples, asstimated from the dry, single stroke test.

ig. 4. Representative friction coefficient vs. normal load curves for coated toolteel, recorder during dry, single stroke sliding against austenitic stainless steel.

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ig. 6. Occasional spalling of WC/C coated steel after repeated dry slidinggainst austenitic stainless steel. (a) After 30, and (b) 50 loading cycles at 550 Noad. Note that the material transfer is associated to the spalling.


3. Results and discussion

3.1. Influence of plasma nitriding

In the single stroke test, hardened forming tool steel initiallydisplayed a friction coefficient between 0.3 and 0.4. An increasein friction, indicating transfer of work material to the tool steelsurface, was observed between 200 and 250 N load, as shown inFig. 1. This transfer is confirmed by the OM pictures in Fig. 2that shows beginning of work material transfer at about 200 N(Fig. 2a) and building-up of a relatively thick layer of transferredmaterial above 250 N load (Fig. 2b).

Plasma nitriding followed by polishing to the original sur-face roughness, reduced the initial friction coefficient againstaustenitic stainless steel to about 0.25 and improved the gallingproperties, with the critical load for the beginning of materialtransfer being in the range between 250 and 350 N load, seeFigs. 1 and 3. However, depending on the nitriding conditions,nitriding increases the surface roughness [20,21] and if not re-ground, nitrided surfaces will lead to an almost instantaneousstainless steel transfer, as indicated in Fig. 3.

A comparison between nitrided forming tool steel with andwithout a thin �′ compound layer showed somewhat improvedfriction and galling properties when using a combination ofdiffusion and compound layer (treatment A in Table 1), seeFigs. 1 and 3. However, previous investigations [21–23] haveshown that a compound layer may lead to an increased wearrate. Due to its brittleness and possible porosity, the compoundlayer may start to break up under high loads, leading to for-mation of hard abrasive particles that may accelerate the wearrate.

3.2. Influence of TiN and WC/C coating

As compared to uncoated hardened forming tool steel, depo-sition of a TiN coating, up till now one of the most successfullyused hard wear resistant coatings in cutting tool applications,gave similar friction but almost instantaneous transfer of stain-less steel to the tool steel surface, see Figs. 4 and 5. Althoughhard ceramic coatings show high wear resistance, their hightendency to pick up work material when in contact with softmetals [23,24] make them highly unsuitable for forming tools.

Fig. 7. Representative surface roughness profiles and average roughness parame
ters for the tool steel samples after: (a and b) grinding and (c–e) polishing.


Fig. 8. Representative friction coefficient vs. normal load curves for the groundand polished tool steel, recorder during dry, sinle stroeke sliding againstaustenitic stainless steel.

Fig. 9. Critical loads for the beginning of transferred stainless steel (Lc1) andformation of a layer of transfered material (Lc2) to ground and polished toolsteel samples.

Fig. 10. Proportion of the tool steel contact area covered by transferred stainlesssteel for the tool steels of different surface roughness.

On the other hand, the low-friction coating WC/C reducedthe friction against austenitic stainless steel dramatically, to alevel of about 0.15 in the whole load range investigated, seeFig. 4. Furthermore, the WC/C coating gave highly improvedgalling properties, with transfer of stainless steel being pre-vented almost up to the maximum load of 1300 N, as indicated inFig. 5.

These results clearly indicate, that carbon based low-frictioncoatings with excellent frictional properties [23,25] also have ahigh potential for improving the galling properties of formingtools. However, some coating spallation was observed for theWC/C coating in a dry multicycle test. This produced sharpedges, see Fig. 6a, which increased the amount of work materialtransfer, as shown in Fig. 6b.

3.3. Influence of surface topography

The investigation on galling properties of the VANADIS 6tool steel also included five different grades of grinding andpolishing, resulting in an average surface roughness Ra of 0.4,0.25, 0.15, 0.1 and 0.05 �m, respectively, see Fig. 7.

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ig. 11. Representative SEM micrographs of polished tool steel (Ra = 0.10 �m)ested in dry, single stroke sliding. (a) At a position corresponding to a normaload of 800 N. (b) At 1000 N load. The arrows indicate the direction of sliding.


Polishing of the surface reduced the friction and improved thegalling properties of the tool steel when sliding against austeniticstainless steel, see Figs. 8 and 9. The two roughest surfaces(Ra = 0.4 and 0.25 �m, respectively) gave the highest initial fric-tion coefficient, about 0.4, and an almost instantaneous transferof stainless steel. Reducing the roughness leads to a substantialreduction in friction and a corresponding significant increase incritical load for the beginning of stainless steel transfer. Notethat the finest polishing (Ra = 0.05 �m) gave a low and very uni-form friction coefficient of about 0.15, and an almost completeprotection against work material transfer in the entire tested loadinterval, see Fig. 9. Besides an increased critical load, polishingof the surface also reduces the amount of stainless steel beingtransferred at loads higher than the critical, as shown in Fig. 10.

SEM observations of the contact surfaces showed that thetransfer of work material to the tool steel surface preferen-tially starts at the edge of grinding grooves and roughnessasperities, see Fig. 11a. At further increasing load, the trans-

formation gradually creates a layer of transferred material, seeFig. 11b. Obviously, any irregularity in the tool surface repre-sents a potential source for initiation of work material transfer.Therefore, by removing groves and roughness asperities, pol-ishing of the surface eliminates potential sources for materialtransfer, which leads to improved galling properties of formingtools.

3.4. Friction and galling at starved lubrication

Friction maps for surface engineered tool steel loaded againstaustenitic stainless steel under starved lubrication conditions areshown in Fig. 12. For the hardened tool steel (Ra = 0.25) anabrupt increase in friction was detected already during the sec-ond stroke, see Fig. 12a. The test had to be stopped after the thirdstroke due to extensive transfer of stainless steel. These resultsindicate that, as the reciprocal sliding proceeds, the initial regimeof boundary lubrication moves towards a mixture of boundary

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ig. 12. Representative friction maps for surface engineered tool steel, sliding agRa = 0.25 �m) (b), hardened and polished steel (Ra = 0.1 �m), (c) TiN coated and (d)

ainst PAO lubricated austenitic stainless steel. (a) Hardened reference steelWC/C coated.


lubrication and dry sliding. Similar results, with the initial fric-tion in the range of 0.15 and 0.20 and transfer of work materialstarting already during the second stroke, were observed for theplasma nitrided samples.

Polishing of the surface improved the friction properties of thetool steel under boundary lubrication, reducing the initial frictionto about 0.1 and increasing the number of strokes needed for thelubrication conditions to deteriorate, see Fig. 12b. Five to 10strokes were needed to substantially increase the friction for thesteel polished to Ra = 0.10 �m. However, the increase in friction,indicating a less effective lubrication, was load dependant, withextensive adhesion of work material being restricted to loadsabove 500 N.

In the case of TiN coated steel a rapid increase in friction,corresponding to a rapid transfer from boundary lubricated to drysliding, started at approximately 1000 N load already during thefirst stroke, see Fig. 12c. On the other hand, the WC/C coatedsteel displayed a uniform frictional behaviour with a frictioncoefficient of ∼0.1 during the whole 50-cycle test, see Fig. 12d.In addition, deposition of the WC/C coating gave a completeprotection against transfer of austenitic stainless steel duringthe whole test. Similar conditions of low friction and no gallinghave only been obtained through the use of fully formulatedforming oil [26].

4. Conclusions

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Acknowledgements

Uddeholm Tooling AB, The Swedish Research Council andCarl Trygger Foundation are greatly acknowledged for financialsupport. The supply of test materials and coatings from Udde-holm Tooling and Balzers Sandvik Coating AB, respectively, ismuch appreciated.

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The major observation in this investigation is that both frictionand tendency to galling reduces with the surface roughness.Polishing of the surface removes irregularities and asperitiesfrom the contact surface, thus eliminating potential sourcesfor material transfer.Carbon based low friction coatings of the DLC type givea certain tolerance to surface roughness and can maintainlow friction and galling resistance at high loads in drysliding.Provided that sufficient coating to substrate cohesion, adhe-sion and load-carrying capacity of the substrate is met, DLCcoatings also have the potential to give excellent protectionagainst work material pick-up and transfer layer formation inlubricated contacts.However, fine polishing seems to be competitive alternativeto DLC coating.Plasma nitriding, a commonly used surface treatment toimprove wear resistance and durability of forming tools, cangive up to 40% improvement in galling properties, but only ifthe surface is polished after the treatment.Hard TiN coating shows high friction and high affinity to pick-up work material when in contact with austenitic stainlesssteel, and does not improve the galling properties of formingtools.An optimum solution to providing superior friction andgalling properties of forming tools should therefore includethe following steps. Plasma nitriding to improve the load-carrying capacity of forming tool steel. Post-polishing of thecontact surface to eliminate any surface asperities. Depositionof a hard low-friction coating.

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Surface modification to improve friction and galling

Engineering

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