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Surface and Coatings Technology 184 (2004) 338–348 0257-8972/04/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.11.002 Influence of surface roughness and coating type on the galling properties of coated forming tool steel B. Podgornik *, S. Hogmark , O. Sandberg a, a b The Tribomaterials Group, Angstrom Laboratory, Uppsala University, Box 534, SE- 751 21 Uppsala, Sweden a ˚ ¨ Uddeholm Tooling AB, SE-683 85 Hagfors, Sweden b Received 1 October 2003; accepted in revised form 7 November 2003 Abstract The aim of the present work is to elucidate the influence of surface roughness on the galling properties of coated forming tool steel. The tribological evaluation included TiN, TiB , TaC and WCyC coatings deposited on cold work tool steel. Representing a 2 material difficult to form, austenitic stainless steel was used as a counter-material. A special test configuration made it possible to gradually increase the normal load during forward sliding strokes, and to correspondingly decrease the load during reversed ones. In this investigation, the load was varied between 100 and 1300 N, corresponding to a contact pressure between 2 and 5 GPa. The main observation is that the galling and anti-sticking properties of the tool surface dramatically improve by reducing the surface topography. Consequently, reduced substrate roughness or polishing of the contact surface after coating is highly recommended. However, selection of a carbon-based low-friction coating leads to reduced probability of worked material adhesion even at high surface roughness values and under starved lubrication. 2003 Elsevier B.V. All rights reserved. Keywords: Surface roughness; Forming tools; PVD coatings; Galling; Friction 1. Introduction Hard and corrosion-resistant coatings are frequently used to protect and enhance the lifetime of cutting tools under high loads w1x. Although introduced more than two decades ago, TiN still dominates among the hard coatings employed in the industry. However, further requirements to withstand aggressive environments and to improve oxidation and wear resistance of contact surfaces exposed to extreme conditions constantly lead to the development and introduction of new coatings, i.e. TiB , diamond and diamond-like carbon (DLC) 2 coatings, etc. w2,3x. Cutting tools almost always show a high increase in performance after coating w4–6x, and the majority of them are coated today. In contrast, most of the forming tools are still uncoated. This is primarily due to the *Corresponding author. Permanent address: University of Ljublja- na, Centre for Tribology and Technical Diagnostics, Bogisiceva 8, SI-1000 Ljubljana, Slovenia, Tel.: q386-1-477-1463; fax: q386-1- 477-1469. E-mail address: [email protected] (B. Podgornik). larger size and a complex shape of most forming tools, which makes it difficult to apply a coating and to obtain a good adhesion between the coating and the substrate material w3x. If a coating fails due to adhesion problems, coating fragments can lead to impairment in the product surface quality and destruction of a very expensive tool. In addition, many of the commercial hard ceramic coatings used in cutting tool applications w4x show a relatively high friction coefficient and galling tendency when sliding against the work materials w3,6x. This is highly undesirable in forming operations. However, in the last couple of years, tremendous progress has been seen in the field of coating deposition as well as in introducing new coatings with excellent frictional prop- erties w7–11x. Especially carbon-based coatings, like diamond and diamond-like carbon (DLC) coatings, were found to give very promising results. Besides improved wear resistance, they also give considerable reduction in friction, even under dry sliding. In general, the surface topography has a great influ- ence on the tribological behaviour w12–14x, and should also be decisive for obtaining desired anti-galling prop-

Transcript of resistência 1

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Surface and Coatings Technology 184(2004) 338–348

0257-8972/04/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2003.11.002

Influence of surface roughness and coating type on the galling propertiesof coated forming tool steel

B. Podgornik *, S. Hogmark , O. Sandberga, a b

The Tribomaterials Group, Angstrom Laboratory, Uppsala University, Box 534, SE- 751 21 Uppsala, Swedena ˚ ¨Uddeholm Tooling AB, SE-683 85 Hagfors, Swedenb

Received 1 October 2003; accepted in revised form 7 November 2003

Abstract

The aim of the present work is to elucidate the influence of surface roughness on the galling properties of coated forming toolsteel. The tribological evaluation included TiN, TiB , TaC and WCyC coatings deposited on cold work tool steel. Representing a2

material difficult to form, austenitic stainless steel was used as a counter-material. A special test configuration made it possibleto gradually increase the normal load during forward sliding strokes, and to correspondingly decrease the load during reversedones. In this investigation, the load was varied between 100 and 1300 N, corresponding to a contact pressure between 2 and 5GPa. The main observation is that the galling and anti-sticking properties of the tool surface dramatically improve by reducingthe surface topography. Consequently, reduced substrate roughness or polishing of the contact surface after coating is highlyrecommended. However, selection of a carbon-based low-friction coating leads to reduced probability of worked material adhesioneven at high surface roughness values and under starved lubrication.� 2003 Elsevier B.V. All rights reserved.

Keywords: Surface roughness; Forming tools; PVD coatings; Galling; Friction

1. Introduction

Hard and corrosion-resistant coatings are frequentlyused to protect and enhance the lifetime of cutting toolsunder high loadsw1x. Although introduced more thantwo decades ago, TiN still dominates among the hardcoatings employed in the industry. However, furtherrequirements to withstand aggressive environments andto improve oxidation and wear resistance of contactsurfaces exposed to extreme conditions constantly leadto the development and introduction of new coatings,i.e. TiB , diamond and diamond-like carbon(DLC)2

coatings, etc.w2,3x.Cutting tools almost always show a high increase in

performance after coatingw4–6x, and the majority ofthem are coated today. In contrast, most of the formingtools are still uncoated. This is primarily due to the

*Corresponding author. Permanent address: University of Ljublja-na, Centre for Tribology and Technical Diagnostics, Bogisiceva 8,SI-1000 Ljubljana, Slovenia, Tel.:q386-1-477-1463; fax:q386-1-477-1469.

E-mail address: [email protected](B. Podgornik).

larger size and a complex shape of most forming tools,which makes it difficult to apply a coating and to obtaina good adhesion between the coating and the substratematerialw3x. If a coating fails due to adhesion problems,coating fragments can lead to impairment in the productsurface quality and destruction of a very expensive tool.In addition, many of the commercial hard ceramiccoatings used in cutting tool applicationsw4x show arelatively high friction coefficient and galling tendencywhen sliding against the work materialsw3,6x. This ishighly undesirable in forming operations. However, inthe last couple of years, tremendous progress has beenseen in the field of coating deposition as well as inintroducing new coatings with excellent frictional prop-erties w7–11x. Especially carbon-based coatings, likediamond and diamond-like carbon(DLC) coatings, werefound to give very promising results. Besides improvedwear resistance, they also give considerable reductionin friction, even under dry sliding.In general, the surface topography has a great influ-

ence on the tribological behaviourw12–14x, and shouldalso be decisive for obtaining desired anti-galling prop-

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Table 1Deposition parameters and resulting coating properties

Coating Process Temp. Substrate bias Hardness Young’s modulus Residual stress(8C) (V) (GPa) (GPa) (GPa)

TiN Reactive e-beam 320–420 y110 30"2 500"50 y3.8"0.4TiB2 Sputtering 300 q50 54"9 600"85 y0.5"0.2 w15xTaC Sputtering 70 y50 15"2 230"20 NAWCyC Reactive sputtering 230 NA 12"1 130"7 y0.3"0.1

NA, not available.

Fig. 1. Scratch test results of investigated coatings.

erties of forming tools, especially those equipped withhard, wear resistant coatings.The aim of this work was to investigate the influence

of coating type and surface roughness before and afterthe coating deposition, respectively, on the galling prop-erties of coated forming tool steel. Four different PVDcoatings(TiN, TiB , TaC and WCyC deposited on cold2

work tool steel) were tested in a load-scanning test rig,using austenitic stainless steel as counter material.

2. Experimental

A powder metallurgy cold work tool steel, VANADIS4, from Uddeholm Tooling AB, Sweden, was used assubstrate material throughout this investigation. It hasthe nominal composition(wt.%) 1.5 C, 1.0 Si, 0.4 Mn,8.0 Cr, 1.5 Mo, 4.0 V. The steel samples in the shapeof cylinders were hardened and tempered to 850 HV.They were ground and sputter-cleaned prior to coatingdeposition, in order to obtain optimum adhesion betweencoating and substrate. Four different PVD coatings(TiN,TiB , TaC and WCyC), each approximately 2mm in2

thickness, were deposited using commercial PVD pro-cesses(see Table 1). The WCyC coating, deposited ata substrate temperature of;230 8C was a WC-dopedhydrogenated diamond-like carbon coating(DLC), witha multilayered structure of WC and C. For the refractoryhard coatings of TiN, TiB and TaC, the deposition2

temperature was in the range between 70 and 4208C.To improve adhesion of the coatings, a thin(;0.1mm)intermediate layer of Ti was deposited for the TiN,TiB and TaC coatings, and of Cr for the WCyC coating.2

The influence of surface roughness was investigatedusing TiN coated samples of differentR roughnessa

values, obtained by polishing(-0.05, 0.1 and 0.15mm), grinding (0.25mm) and shot peening(0.32mm),respectively. For comparison, post-polishing of a coatingdeposited on ground substrate was included.The coating adhesion was evaluated with a Scratch

tester equipped with a 200mm radius Rockwell-Cdiamond stylus, using coatings deposited on polishedflat samples(R f0.02 mm). The loading rate was 10a

Nymm and the maximum load 100 N. The critical loadfor coating failure as cracking or spalling was deter-mined by post-test optical microscopy(OM).

A load-scanning test rig was used to investigate thetribological properties of the coated tool steel, and anaustenitic stainless steel(AISI 304; 350 HV) was usedas a counter material. In the test two crossed, elongatedcylindrical test specimens(� 10 mm, 100 mm long)are forced to slide against each other under a constantspeed, with the normal load gradually increasing duringforward strokes and correspondingly decreasing duringreversed strokesw16,17x. Thus, each point along thecontact path of both specimens will experience a uniqueload and display a unique tribological history after testcompletion.For the major part of this investigation the test

equipment was set to a single, forward stroke mode,and operated under dry conditions, with the slidingspeed fixed to 0.01 mys. The normal load was graduallyincreased from 100 to 1300 N. Additionally, some testswere performed under starved lubricated conditions,where an approximately 10mm thick film of pure poly-alpha-olefin oil (PAO, v s46.6 mmys) was applied2

40

on the worked material sample before the test. Here, thesliding speed was fixed to 0.1 mys, with the normalload changing as before.Results of the study were extracted from the recorded

friction vs. load history and by imaging the wornsurfaces by optical microscopy(OM) or scanning elec-tron microscopy(SEM), and analysing them by energydispersive X-ray spectroscopy(EDS). Optical profilo-metry was used to assess the surface roughness.

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Fig. 2. Coating failure mechanisms observed in scratch testing of(a) TiN and (b) TaC coatings deposited on tool steel.

Fig. 3. Friction coefficient vs. normal load for the different coatings on tool steel(R f0.25 mm), recorder during sliding against austenitica

stainless steel.

3. Results and discussion

3.1. Scratch testing

Fig. 1 shows critical loads for the investigated coat-ings, corresponding to the appearance of the first visiblefailure of the coating during scratching as determinedby OM. In the case of the TiN, TiB and WCyC2

coatings, the first failure of the coating by cracking andspallation on either side of the scratch(Fig. 2a), wasdetected in the load range 10–25 N. The TiN coatingdisplayed the best results, followed by the much softerWCyC, and the very hard and brittle TiB coating,2

which started to fail at;10 N load, as shown in Fig.1. However, the TaC coatings flaked instantaneously at

loads below 5 N(Figs. 1 and 2b), which indicates pooradhesion.

3.2. Friction and galling during dry sliding

The galling properties of the coatings were determinedby the dry single-stroke sliding test, monitoring thecoefficient of friction as a function of load, see Fig. 3.In the case of uncoated steel against stainless steel theinitial friction varied between 0.3 and 0.35. The firstsign of adhesion of worked material to the tool steelsurface, as indicated by a sudden increase in frictionand confirmed by post-test microscopic observation wasdetected at approximately 200 N load. The TiN or theTiB coating displayed an increased friction coefficient,2

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Fig. 4. Typical appearance of the contact surfaces of the dry sliding test specimens at a load level where stainless steel transfer began(lightcontrast). (a) TiN coating at 200 N load, and(b) DLC coating at 1300 N load. The arrows indicate the direction of sliding.

Fig. 5. Friction coefficient vs. normal load for coated and uncoated polished steel, recorder during dry sliding against austenitic stainless steel.

which in the case of TiB even exceeded values of 0.8.2

Transfer of stainless steel to the surface of both thesecoatings occurred almost instantaneous, cp. Fig. 4a.However, the TaC and WCyC coatings gave an initialfriction coefficient down to 0.15 and showed the lowestability to material transfer. For the TaC coating, transferof stainless steel started at approximately 750 N load,while virtually no transfer at all could be detected forWCyC, even at the maximum load of 1300 N(see Fig.4b).Fig. 5 shows the dry friction curves for the uncoated

and coated steel samples that had been polished to anaverage surface roughness of 0.2mm, prior to testing.

In the case of uncoated steel and steel coated with TiNor TiB , polishing reduced the coefficient of friction2

approximately 50–100% and gave much higher criticalload for material pick up, see Fig. 6. However, in thecase of the carbon-based low-friction coatings TaC andWCyC polishing had practically no influence on theirfrictional behaviour, and gave only minor improvementin resistance to material transfer see Figs. 5 and 6.As shown in Fig. 7, polishing of the surface also

reduced the amount of worked material transferred totool material surface, especially in the case of hardceramic coatings of TiN and TiB . However, even2

polished, TiN and TiB coatings were unable to reach2

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Fig. 6. Critical load for beginning of transfer of stainless steel to coated and uncoated steel.

Fig. 7. Relative coverage of the contact area of coated and uncoated steel by transferred stainless steel.(a) Without and(b) with polishing ofthe tool surface before testing.

good protection against transfer of worked materialobtained by as-deposited TaC and WCyC coatings,respectively.The influence of substrate roughness(prior to coat-

ing) on the galling properties of coated tool steel wasinvestigated for TiN-coated steel. The substrate prepa-ration included shot-peening, grinding and differentgrades of polishing, respectively, which gave the surfaceroughness profiles and parameters shown in Fig. 8.Reducing the substrate roughness fromR f0.25mma

to R f0.05mm gave only a limited reduction in frictiona

for the TiN-coated polishing grades that showed initialvalues between 0.25 and 0.3(see Fig. 9). However,smoothing the substrate increased the ability of thecoated steel surface to resist transfer of worked material.

The critical load at which transfer of worked materialstarted raised from less than 200 N for the originalground substrate to;250 N,;300 N and;350 N forsubstrates polished toR f0.15 mm, R f0.10 mm anda a

R f0.05mm, respectively, see Fig. 10. Although almosta

100% improvement in critical load was achieved throughthe finest substrate polishing, it was not possible toreach as good galling resistance as that obtained bypost-polishing of the coating deposited on a groundsubstrate(see Fig. 10).Compared to grinding, shot peening of the substrate

has no influence on the friction behaviour of TiN coatedtool steel, see Fig. 9. However, it reduced the criticalload for transfer of worked material to below 200 N(see Fig. 10).

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Fig. 8. Surface roughness profiles and average roughness parameters for the tool steel substrate after(a) shot-penning,(b) grinding and(c–e)polishing.

Fig. 9. Friction coefficient vs. normal load for TiN coated tool steel, recorder during dry sliding against austenitic stainless steel.

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Fig. 10. Critical load for beginning of transfer of stainless steel to TiN coated tool steel.

Fig. 11. Surface roughness of the sliding track of the austenitic stainless steel sample after sliding against uncoated and coated tool steel testedas deposited and after surface polishing.

Coating type and surface roughness do not influenceonly the galling properties of coated forming tool steel,but they also have an influence on the surface qualityof the formed part, as shown in Fig. 11. Hard wear-resistant coatings of TiN and TiB , which showed the2

highest friction and the lowest resistance against transferof stainless steel also gave the highest surface roughnessof the stainless steel counter-part, reaching averagesurface roughness values well above 0.15mm, as meas-ured for the uncoated tool steel. As expected, a bettersurface finish of the counter part, with the averagesurface roughness values in the range between 0.12 and0.14 mm, was obtained by using the low-friction coat-ings TaC and WCyC. Polishing of the coated surfaceimproved the counter-part surface quality, especially forTiN and TiB , which reached same finish as the non-2

polished low-friction coatings TaC and WCyC (see Fig.11). Similar results(R F0.1mm) were obtained by TiNa

deposited on the fine polished substrate.

3.3. Friction and galling during sliding under starvedlubrication

Friction maps, showing the friction coefficient as afunction of load and number of test strokes for coatedtool steel sliding against austenitic stainless steel understarved lubrication conditions are shown in Fig. 12. Foruncoated, and TiN and TiB coated tool steel, respec-2

tively, an increase in friction, indicating transfer froman initial boundary lubrication regime towards a mixtureof boundary lubrication and dry sliding was detectedalready during the first two strokes, as shown in

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Fig. 12. Friction maps for coated tool steel, sliding against austenitic stainless steel.(a) TiN coatingqPAO, (b) polished TiN coatingqPAO, (c)TiN coatingqfully formulated forming oil,(d) WCyC coatingqPAO.

Fig. 12a. Due to extensive transfer of stainless steel tothe tool surface the test had to be stopped after onlythree strokes. Polishing of the surface gave improvedfriction properties of the TiN coated forming tool steelunder boundary lubrication, reducing the initial frictioncoefficient to approximately 0.1 and increasing thenumber of strokes needed for the lubrication conditionsto change(see Fig. 12b). However, a change from purePAO oil to fully formulated forming oil (v s12040

mm ys) gave a very uniform friction behaviour of the2

TiN coated tool steel(mf0.1) and complete protectionagainst transfer of worked material for the whole 50strokes test(see Fig. 12c). The same behaviour wasobserved for uncoated and TiB -coated tool steel using2

formulated oil.However, TaC and WCyC coatings greatly improved

the friction properties of the tool steel when slidingagainst PAO lubricated stainless steel. Even non-pol-

ished coatings completely prevented transfer of workedmaterial. The WCyC coating showed the very bestresults in the test with PAO, results that were onlyobtained with the use of fully formulated forming oil,cp. Fig. 12c,d. The tests with the TaC coated sampleshad to be stopped after less than five strokes, but thiswas due to coating spallation rather than, adhesion ofworked material to the TaC surface.

3.4. General remarks on the influence of surface mate-rial and topography

Previous investigations have shown that hard ceramiccoatings of TiN and TiB have much higher wear2

resistance than carbon-based low-friction coatings ofTaC and WCyC w18,19x. However, in the case offorming applications, the ability of the surface to preventadhesion of worked material is often equally important

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Fig. 14. 3D-topography of TiN coated tool steel.(a) As deposited,(b) polished.

Fig. 13. SEM micrographs and EDS mapping of TiN coated tool steel after sliding against austenitic stainless steel at(a) 300 N and(b) 600 Nload. The lower images display the elemental content of Fe and Ti, respectively, in the surface as obtained by EDS.

as the wear resistance. Therefore, hard wear resistantcoatings of TiN and TiB that generate relatively high2

friction and a high tendency to material transfer(Figs.3 and 6) do not represent an optimum solution. However,the WCyC coating was found to give excellent frictionproperties and protection against stainless steel transfer,even under starved lubrication by non-additivated PAOoil. The same conditions are otherwise obtainable onlywith the use of highly chlorinated forming oil(cp. Fig.12).The poor adhesion of the TaC coating screened its

potential of galling protection. By improving its adhe-sion, it is believed that TaC coated tool would behaveclose to WCyC coated steels as to friction and galling.Furthermore, the high hardness and low friction natureof the TaC coating should make it a good candidate forforming abrasive materials, provided that the adhesionproblem could be solved.

The results of this investigation clearly show a stronginfluence of surface roughness on friction and gallingproperties of coated forming tool steel, especially in thecase of hard wear resistant TiN and TiB coatings. The2

friction and ability of the coated surface to preventtransfer of austenitic stainless steel, as well as the surfacequality of the workpiece improve with reduced substrateroughness(cp. Figs. 9–11). However, a simple polishingof the coating deposited on a ground substrate may leadto similar or even better galling protection than thefinest polishing of the substrate before applying thecoating(see Fig. 10). Polishing of the substrate removesits roughness asperities and makes the substrate smooth-er. However, depending on the PVD technique, deposi-tion of the coating will cause an increase in surfaceroughness by formation of new asperitiesw20x. Thisrepresents a potential source for the beginning of mate-rial transfer (cp. Fig. 13a). As soon as transfer of

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worked material starts it aggravates with increased loadand time, to form a thick layer of transferred material(see Fig. 13b). Polishing of as-deposited coatings, onthe other hand, removes the asperities in the workingsurface(see Fig. 14), thus greatly reducing the risk formaterial transfer. Be aware that incorrect polishing ofcoatings on a rough substrate can lead to excessiveremoval of the coating and in the worse case to substrateexposure. Therefore, polishing of a coated surface shouldbe carried out under well-controlled conditions, assuringthat only potential sources for the material transfer areeliminated.Although shot peening of the substrate removes sharp

edges and grooves caused by grinding it also causesformation of new edges and asperities, and does notgive any improvement in the galling properties, as seenin Figs. 9 and 10.In the case of the carbon-based coatings with excep-

tional properties as to low friction and ability to preventmaterial transfer in sliding against stainless steel, theinfluence of surface roughness is almost negligible.Polishing of the WCyC or TaC coatings only improvedtheir galling properties up to 15%.

4. Conclusions

Surface roughness has a crucial effect on the frictionlevel and the ability of a material to prevent pick-up ofcounter material, especially in the case of hard wear-resistant coatings known to generate high friction whensliding against austenitic stainless steel. Polishing of thesurface removes irregularities and asperities at the sur-face, thus eliminating potential sources for initiation ofmaterial transfer. The smoother the substrate, the higherthe critical load that the coated surface can withstandwithout stainless steel transfer.Polishing of the substrate prior to coating deposition

will lower the friction and increase the critical load formaterial transfer. However, a post-polishing of a groundand coated surface gives the same or even better gallingproperties as compared to the finest substrate polishing(R F0.05mm).a

Be aware that polishing of the coated surface has tobe carried out under appropriate conditions in order toprevent extensive removal of the coating and exposureof the substrate material, thus losing the protective actionof the coating in terms of wear andyor friction.On the condition that adequate coating-to-substrate

adhesion is obtained, carbon based low friction coatingsgreatly improve the friction and galling properties offorming tool steel. Furthermore, deposition of a DLCcoating of the WCyC type may lead to excellentprotection against transfer of stainless steel, even understarved lubrication by non-additivated PAO oil. Other-wise, such a good situation is only obtained by usinghighly chlorinated forming oil.

Acknowledgments

Uddeholm Tooling AB and The Swedish ResearchCouncil are greatly acknowledged for financial support.The supply of test materials, and TiN and DLC coatingsfrom Uddeholm Tooling and Balzers Sandvik CoatingAB, respectively, is much appreciated. Many thanks goalso to Urban Wiklund and Daniel Nilsson for preparingthe TaC and TiB coatings.2

References

w1x B. Bhushan, Modern Tribology Handbook, CRC Press, NewYork, 2000.

w2x V. Imbeni, C. Martini, E. Lanzoni, G. Poli, I.M. Hutchings,Tribological behaviour of multi-layered PVD nitride coatings,Wear 251(2001) 997–1002.

w3x S. Hogmark, S. Jacobson, M. Larsson, U. Wiklund, Mechanicaland tribological requirements and evaluation of coating com-posites, in: B. Bhushan(Ed.), Modern Tribology Handbook,CRC Press, New York, 2000.

w4x K.L. Rutherford, S.J. Bull, E.D. Doyle, I.M. Hutchings, Lab-oratory characterisation of the wear behaviour of PVD-coatedtool steels and correlation with cutting tool performance, Surf.Coat. Technol. 80(1996) 176–180.

w5x P.-Q. Wu, H. Chen, M. Van Stappen, L. Stals, J.P. Celis,Comparison of fretting wear of uncoated and PVD TiN coatedhigh-speed steel under different testing conditions, Surf. Coat.Technol. 127(2000) 114–119.

w6x K. Holmberg, A. Matthews, Coatings Tribology, Elsevier Tri-bology Series, 28,, Elsevier, Amsterdam, 1994.

w7x A. Erdemir, F.A. Nichols, X.Z. Pan, R. Wei, P. Wilbur, Frictionand wear performance of ion-beam-deposited diamond-likecarbon films on steel substrates, Diamond Relat. Mater. 3(1993) 119–125.

w8x P. Kodali, K.C. Walter, M. Nastasi, Investigation of mechanicaland tribological properties of amorphous diamond-like carboncoatings, Tribol. Int. 30(8) (1997) 591–598.

w9x O. Wanstrand, N. Axen, R. Fella, A tribological study of PVDcoatings with carbon-rich outer layers, Surf. Coat. Technol.94y95 (1997) 469–475.

w10x C. Rincon, G. Zambrano, A. Carvajal, P. Prieto, H. Galindo,E. Martinez, et al., Tungsten carbideydiamond-like carbonmultilayer coatings on steel for tribological applications, Surf.Coat. Technol. 148(2001) 277–283.

w11x B. Podgornik, Coated machine elements—fiction or reality?,Surf. Coat. Technol. 146y147 (2001) 318–323.

w12x J. Takadoum, H.H. Bennani, Influence of substrate roughnessand coating thickness on adhesion, friction and wear of TiNfilms, Surf. Coat. Technol. 96(1997) 272–282.

w13x J. Jiang, R.D. Arnell, The effect of substrate surface roughnesson the wear of DLC coatings, Wear 239(2000) 1–9.

w14x G.W. Stachowiak, A.W. Batchelor, Engineering Tribology, But-terworth Heinemann, Boston, 2001.

w15x M. Berger, L. Karlsson, M. Larsson, S. Hogmark, Low stressTiB coatings with improved tribological properties, Thin Solid2

Films 401(2001) 179–186.w16x S. Hogmark, S. Jacobson, O. Wanstrand, A new universal test

for tribological evaluation, Proceedings of the 21st IRG-OECDMeeting, Amsterdam, 1999.

w17x S. Hogmark, S. Jacobson, O. Wanstrand, The Uppsala Load-scanner—an Update, Proceedings of the 22st IRG-OECDMeeting, Cambridge, 2000.

Page 11: resistência 1

348 B. Podgornik et al. / Surface and Coatings Technology 184 (2004) 338–348

w18x B. Podgornik, S. Hogmark, O. Sandberg, Hard PVD coatingsand their perspectives in forming tool applications, Proceedingsof the 6th International Tooling Conference—The Use of ToolSteels: Experience and Research, Karlstad, 2002, pp. 881–891.

w19x B. Podgornik, S. Hogmark, O. Snadberg, V. Leskovsek, Wearˇ

resistance and anti-sticking properties of duplex treated formingtool steel, 10th Nordic Symposium on Tribology—Nordtrib ,Stockholm, 2002, p. 2002.

w20x M. Larsson, Deposition and evaluation of thin hard PVDcoatings, Uppsala University, Uppsala, 1996.