Wear and friction characteristics of PVD-coated roller bearings

Wear and friction characteristics of PVD-coated roller bearings

Surface and Coatings Technology 177 – 178 (2004) 469–476 Wear and friction characteristics of PVD-coated roller bearings M. Kuhn*, P.W. Gold, J. Loos...

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Surface and Coatings Technology 177 – 178 (2004) 469–476

Wear and friction characteristics of PVD-coated roller bearings M. Kuhn*, P.W. Gold, J. Loos ¨ Maschinenelemente und Maschinengestaltung der RWTH Aachen, Schinkelstr. 10, 52062 Aachen, Germany Institut fur

Abstract On the basis of cylindrical roller thrust bearings it was systematically examined to what extent physical vapour deposition (PVD)-coatings are able to take over the function of EPyAW-additives. The bearings were tested under heavy-duty conditions in order to distinguish very fast the efficiency of different coating–substrate-systems. Four Me-C:H-coatings showed the best performance of the investigated coatings and fulfilled the required criterion for roller bearings in the boundary friction. Least wear was produced, if only the bearing washers were coated. Two different ZrCg coatings were varied in their design of gradient and carbon top layer. It could be shown that a homogenous transient from the Zr-bonding layer to the graded ZrC coating and to the carbon top layer reveals a better wear protection than sudden transients, which built a inadvertent breaking point in the coating system. The different hardness and thickness of the carbon top layers could not be differentiated in relation to their wear protection. The decreasing friction coefficient in the tests correlates with increasing values of wear protection. The carbon containing coatings showed a positive effect on friction properties with the lowest friction coefficients. As a consequence, firstly, the amorphous carbon coatings have an inert character and consequently, the adhesive reactions between the friction partners are very low. And secondly, high contact pressures might transform amorphous carbon into graphite, which is able to work as a solid lubricant. Material carryover from the carbonaceous coating to the steel surface was developed, more or less, by the Me-C:Hcoatings during the tests. This mechanism was able to protect the un-coated rollers. Closer investigations were done with an ESMA analysis (electron beam micro range analysis) on ZrCg coatings. It could be seen that a reaction layer was formed on the rollers. This layer contained remarkable masses of zirconium and oxygen. As zirconium has a higher affinity to oxygen than to carbon, a chemical reaction of zirconium carbide with the oxygen to zirconium dioxide and carbon dioxide might be possible. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Roller bearings; PVD-coatings; Zirconium carbide coatings; Friction properties; Mixed friction; Material carry over

1. Introduction

ible tribological systems research has been done on three main topics:

If roller bearings run under unfavourable or mixed lubrication conditions, extreme pressure- and antiwearadditives in the lubricant protect the bearings against wear in order to reach long working lives. A separating tribolayer is produced between the friction partners rollers and washers. Disadvantages of the additives are partially environmental toxicology and the cause of pollution w2x. The ability of physical vapour deposition (PVD)coatings is investigated in one subproject as a part of the German Collaborative Research Centre ‘SFB 442’ in order to replace the toxic additives found in lubricating oils. For the development of environmental compat-

● environmental compatibility of wear resistance, ● compatibility or long life suitability of the tribological elements, ● suitability of additive-free biodegradable lubricants.

*Corresponding author. Tel.: q49-241-809-5645; fax: q49-241809-2256. E-mail address: [email protected] (M. Kuhn).

In the present paper several PVD-coatings were checked for their wear resistance in roller bearings, their friction characteristics and their ability to protect an uncoated counterpart. Therefore, the washers of cylindrical roller thrust bearings (81212) were coated, and the whole bearing was tested with its high slippage under very unfavourable conditions in order to distinguish very fast their efficiency. Particular attention is given to the tests of two different ZrCg coating systems and their abilities concerning wear protection. As some kind of material carryover from carbonaceous coated rollers to un-coated bearing washers was noticed during several

0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0257-8972Ž03.00914-9

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Fig. 1. Axial cylindrical roller thrust bearings 81212.

tests, the rollers were analysed by an ESMA analysis (electron beam micro range analysis) to get information about the formed reaction layer. 2. Experimental 2.1. Wear mechanism of cylindrical roller thrust bearings Axial cylindrical roller bearings (bearing type: 81212) consist of a housing and a shaft washer, which are honed as surface finishing. Fifteen rollers, mounted in a brass cage, are forced on a circular running track (Fig. 1). Consequently, pure rolling motion only takes place in the centre of the cylindrical rollers. Their drill motion causes a rising slippage (up to 14%) towards their ends. If a fully separating lubricating film is missing, sliding wear arises in the areas of positive and negative slippage as shown in Fig. 2. In the centre of the rolling contact the wear is small because of the low glide ratio. In the area of large wear, the height of rolling elements and washers is reduced. Simultaneously the deformation as well as the pressure decrease. In addition to the pressure the rate of wear is reduced in these areas as well. As a consequence, the wear process in the cylindrical roller thrust bearings has a degressive character over the time. Apart from the adhesive and abrasive wear, fatigue damages can also arise early in coated bearings. From

Fig. 2. Slippage situation in axial cylindrical roller thrust bearings drawn to the washer.

Fig. 3. Sectional drawing of the FE8 test rig.

the point in time of first fatigue damage the wear process becomes progressive. 2.2. Test rig and test conditions The tests were performed on the FE8 test rig, which is illustrated in Fig. 3. Two cylindrical roller thrust bearings can be examined at the same time. The load is applied by two plate springs and can be varied by different distance plates. The friction torque is measured by a rope wheel, its rope is connected to a load cell w10x. The test conditions are summarised in Table 1. An additive-free mineral oil (FVA 3 reference oil, ISO VG 100) was used as lubricant in order to avoid wear protection mechanisms of additives. This lubricant also has the advantage that its composition is known. Furthermore, it is available in the same quality for a long time. The main criterion of evaluation is the loss of mass of the rolling elements. To measure this loss of mass the bearings were dissembled after accurate time periods Table 1 FE8 test system and test conditions Test system Test rig Test sample Bearing type

FAG-FE8 Cylindrical roller thrust bearings 81212 MPB 801865

Lubrication Method of lubrication Lubricant Circulating oil volume Volume flow Degree of filtration

Oil circulation Additive-free mineral oil FVA 3 2l 14 cm3ymin 5 mm

Test conditions Rotational speed Axial load Bearing temperature

7.5 rpm 80 kN 70 8C

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and the rollers and the washers were weighed separately. If the additional wear of all rollers is less than 10 mg after 80-h operation time and no fatigue damages arose, the wear protection of the coating can be evaluated as very good according to the experience of the German collaborative research project ‘SFB 442’ and other research projects w6x. For another criterion of wear the surface shapes of the bearing elements are measured to indicate partial delamination of the coating or cohesive failure w3,4x. Apart from these main criterions, pictures, scanning electron microscope shots and ESMA analysis can give further information about coating performance and surface conditions. In order to prove and quantify a material carryover process, the ESMA analysis was used. It is able to detect elements and their masses in a tribolayer of a thickness of 0.1 nm up to 1 mm. Fig. 4. Design for the ZrCg coatings with different trends of carbon concentration.

2.3. Examined PVD-coatings Table 2 shows the properties of the coatings, which were deposited on the roller bearings (measurements by the Material Science Institute in Aachen). PVD processes were used for all coatings, which guarantee a temperature smaller than the tempering temperature of the substrate (100Cr6). The loss of hardness is here less important than the change of accurate dimensions w8x. The bearings were coated by Lugscheider w8x, the companies Balzers w9x and Metaplas Ionon w7x. The thicknesses of the examined coatings are between 2 and 3 mm. In detail the influence of two different gradients of ZrCg-coated bearings on wear resistance were investigated. The carbon gradient in the coatings 1006.ZrCg and 1009.ZrCg (for the mechanical properties see Table 2) was separated in different ways. A linear inclination of the carbon gradient was produced in the 1009.ZrCg coating. The top layer of pure carbon was separated out of the reaction gas during the PVD-process. 1006.ZrCg became a non-linear carbon gradient with more homogenous transition to the Zr-bonding layer and the carbon top layer. Here, the carbon top layer was a bit thicker. Both coating designs are shown in Fig. 4.

3. Results and discussions 3.1. Wear protection behaviour of the coating systems Fig. 5 shows the wear of several bearings with coated washers and un-coated rollers, related to the reference test with non-coated bearings after 1, 8, 80 and 400 h. This means, not only the bonding of the coating to the substrate is tested here, but also the ability of the coating system to protect an un-coated counterpart. On the right side of the diagram the wear is classified after 80 h running time in excellent wear protection (wear -10 mg), good wear protection (wear between 10 and 30 mg) and average wear protection (wear between 30 and 100 mg). This classification in wear is adopted from the FE8 oil tests, where the bearings are dismounted just once after 80 h. If lots of wear were produced in the bearings and so the wear protection scale for the whole bearing indicated just average or worse abilities, the test had been already stopped at this early stage. As visible in Fig. 5 it is remarkable that after 1 h of operation time, the wear of some un-coated rollers,

Table 2 Mechanical properties of the examined coatings; numbers are identity numbers of the German Collaborative Research Centre ‘SFB 442’; measurements were done by the Material Science Institute in Aachen with an experimental scatter of 5% Coating

Hardness (HV), in GPa

Young’s modulus, in GPa

Critical load LC2, in N

Adhesion strength class (VDI 3198)

121.TiAlN 136.CrAlN 208.ZrN 214.ZrCg 249.WCyC 363.W-C:H 1006.ZrCg 1009.ZrCg

25 19 14 15 12 12 11 11

340 320 320 200 150 169 77 182

25 40 25 30 70 40 50 30

5 2 3 2–3 1 1 1 2

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Fig. 5. Wear of bearing rollers run on different coating systems after different time periods and evaluation grades after 80 h; the tests were discontinued, if there was more roller wear than 10 mg.

which run on coatings like 121.TiAlN or 136.CrAlN, is larger than that of the fully non-coated reference. The reason for this phenomenon is that the coatings are extremely hard in contrast to the 100Cr6 rollers. The softer metal surface grinds off. Some coatings on the washers failed probably due to an unsatisfactory adhesion of the coatings on the substrate. The coatings 249.WCyC, 363.W-C:H, 1006.ZrCg and 1009.ZrCg do not show comparable signs of wear after even 400-h testing time under the heavy load conditions in the boundary lubrication regime. Moreover, they reduce the wear on rollers as well as on the washers very strongly compared to the un-coated reference. As shown in Table 2 these four coatings have quite low values in hardness and Young’s modulus and the best results in the coating’s adhesion class. Following these test results, the mechanical properties of the coating can

already give an estimation of the coating’s performance on axial cylindrical roller thrust bearings. By fulfilling these constrains, coatings on roller bearings should be able to resist high loads and stresses and its bonding to the substrate seems to be ideal. 3.2. Friction properties of the coating systems The friction coefficient for the non-coated bearing is approximately 0.0095. Any tested coating reduced the friction in the roller bearings (Fig. 6). It is not surprising that a decreasing friction coefficient correlates with increasing values for wear protection. Quite interesting is the fact that especially the carbon containing coatings show a positive effect on friction properties. An explanation is given by Enke w5x. He claims that, firstly, amorphous carbon coatings have an inert character and

Fig. 6. Friction coefficients of different coating systems.

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Fig. 7. Surface profile of the 1009.ZrCg-coated shaft washer after 0, 160 and 400 h.

consequently, the adhesive reactions between the friction partners are very low. Secondly, there are flash temperatures on the roughness tips over 625 K, which can transform the face-centred cubic lattice structure into graphite. The graphite lattice structure is able to slide easily comparable to a solid lubricant. 3.3. Influence of two different gradients of ZrCg-coated bearings on wear resistance During the full testing period of 400 h the two coatings could not be differentiated by their loss of mass. The sum wear, which means the wear of the rollers and that of the two washers, was approximately 11 mg for the 1009.ZrCg coating and 12 mg for the 1006.ZrCg coating. The wear protection of the un-coated counterpart was excellent because of their extremely low loss of mass after 400 h. The ZrCg coating system was the best performing coating in the full test series. After 8 h the optical examination revealed for the 1009.ZrCg coating with the linear carbon gradient a small area of cohesive wear on the running track of the shaft washer. This zone grew with the operation time and after 400 h the groove was measured to a depth of 1.2 mm with the help of a surface groping-procedure (Fig. 7). This fact in combination with the coating design indicates that the coating failed by fatigue on the transition between Zr-bonding layer and the starting point of the carbon gradient. The sudden addition of carbon built an inadvertent breaking point. Furthermore, microscope analysis showed that the starting point of cohesive wear built a honing mark. Also the form of the wear area followed the honing marks produced during the last surface finishing procedure for the bearing washers. But nevertheless, it has to be mentioned that the coating system did not fail entirely. The Zrbonding layer was still intact and able to protect the substrate with constant low friction properties.

A close regard with the help of a scanning electron microscope reveals a smoothing process on the washer’s surface. In the intact coating areas the honing marks are only hardly visible. This may be due to a flat rolling process of the relative thin carbon top layer in combination with a low hardness. In case of the 1006.ZrCg coating with non-linear carbon gradient only very small areas of cohesive wear and delamination of the full coating, mostly in the centre of the running track, could be seen (Fig. 8). The borders of these zones are formed by honing marks in the way already mentioned above. Here, the more homogenous transition from Zr-bonding layer to the gradient layer and from the gradient layer to the carbon top layer pays. The gradual wear verifies a rubbing off process of the carbon top layer with progressive testing time. This coating is more than two times softer than the one discussed before. As the carbon top layer has flaked off, it can no more fill the gaps produced by the honing process and the surface does not improve. After the carbon top layer has gone, the rollers are in touch with the ZrC-transition layer. The optical impression of this coating is much better than the one of the 1009.ZrCg coating. 3.4. Process of material carryover As mentioned above, comparing tests of different coating systems have shown that only the carbonaceous coatings are able to protect the steel rolling partner against wear in an acceptable way. For example the wear of non-coated rollers, which roll over CrAlNcoated washers, was nearly two times larger than the ones in the reference test with non-coated bearings. As Fig. 9 illustrates, a material carryover from the coating to the roller’s steel surface develops during the test. The rollers get in touch with the coating over their full length, so no unaffected reference could be observed. In

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Fig. 8. Surface profile of the 1006.ZrCg-coated housing washer after 0, 160 and 400 h.

earlier tests with coated rollers and un-coated washers, material carryover was detected on the washer’s surface along the running track. Besides this track no coating material was found. This material transfer, which is typical for carbonaceous coatings, causes a change in material pairing and a smoother contact with less contact stresses. The carbon performs probably like a solid lubricant, which reduces friction and separates the friction partners. However, the surface roughness of the coated element seems to be important and the roughness in the beginning has to be considered. With the help of the ESMA analysis on the surface of the bearing rollers a reaction layer was measured to a thickness of 5–6 nm, which was transferred by the coated washers. Furthermore, the analysis revealed for every roller a nearly constant carbon content on the surface. Just some single peaks can be found. Obviously, the desire to force a durable carbonaceous material carryover by a carbon top layer has failed for the ZrCg coatings, as the constant carbon content might due to the substrate material 100Cr6. Both coating systems lost

parts of their carbon top layers, either it flaked off in case of the 1006.ZrCg coating or it was rubbed off in case of the 1009.ZrCg coating (Fig. 10). As zirconium was detected, the elements found on the roller’s surfaces belonged to some coating regions under the carbon top layer. Quite interesting is the fact that a high amount of oxygen has been found on the rollers. This may derive from the corrosive processes of the roller surface with the oil or the surrounded air. As the oxygen is also a part of the coating system, it is more probable that it was transferred to the roller’s surface. The oxygen became part of the coating accidentally. The origin of the oxygen in the coating is not found in the coating process, because the coating chamber is totally evacuated and only filled with reactive gas. Arntz w1x found out by glow discharge optical emission spectroscopy analysis that the oxygen penetrates into the coating after the process by the surrounding air or more probable by the air humidity. The hydrogen contained in the air humidity cannot be detected. Evidence for this thesis is

Fig. 9. Mass on the un-coated roller surface, which run on 1009.ZrCg-coated washers.

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Fig. 10. Mass on the un-coated roller surface, which run on 1006.ZrCg-coated washers.

following the statements of Ref. w1x that the oxygen concentration decreases with the coating thickness. Remarkable in this context is that the zirconium content seems to correlate with the oxygen content. Due to the flash temperatures of approximately 625 K on the roughness tips, reactions between zirconium and the oxygen cannot be excluded. Additionally, zirconium has a higher affinity to oxygen than to carbon. A chemical reaction of zirconium carbide with the oxygen to zirconium dioxide and carbon dioxide might be possible. Another explanation could be that the full coating with its elements is transferred to the counterpart without any reaction. Maybe the oxygen reacts with the iron of the 100Cr6 substrate material similar to the EPyAW-additives. But for this mechanism the ESMA analysis would have detected about the same amount of oxygen everywhere on the surface.

roughness tips. Graphite is able to act like a solid lubricant and minimises the friction in the contacts. The wear protection behaviour of two ZrCg coatings, as the best performed system in the tests, was investigated concerning carbon gradient strategies. It could be shown that a homogenous transition between different coating layers resists better external stresses than sudden transitions, which built a breaking point in the coating system. ESMA analysis were implemented in order to clear the mechanism of material carryover in case of the ZrCg coatings. The theory that mainly carbon is transferred from the coating to the un-coated friction partner could not be confirmed in the present case. Moreover, it might be possible that the zirconium reacted with oxygen and built up a protecting reaction layer on the surface.

4. Conclusion

References

It was systematically examined on the basis of cylindrical roller thrust bearings to what extent PVD-coatings are able to take over the function of the lubricants. The bearings were tested under heavy-duty conditions in order to distinguish very fast their efficiency. It could be shown that wear is reduced significantly by suitable coatings. However, frequently to low durability of the coated bearings was caused by insufficient adhesion between substrate and coating. Four different Me-C:H-coating systems showed the best performance of the investigated coatings and fulfilled the required criterion for rolling bearings (low loss of mass and hardly deviation of the bearing surface’s components) in the boundary friction. Thus, PVD-coatings on rolling contacts offer an alternative to toxic additives for ecologically sensible regions. The best friction conditions were found for carbonaceous coatings. The positive effect might be that carbon is converted into graphite due to flash temperatures on

w1x K. Arntz, Entwicklung und Charakterisierung von gradierten PVD-Zirkonkarbidschichten mit Kohlenstoffdecklage (ZrCgq ¨ den Einsatz auf Axiallagern, Studienarbeit am LehrC) fur und Forschungsgebiet Werkstoffwissenschaften der RWTH Aachen, 2002. w2x Deutsche Forschungsgemeinschaft, Umweltvertragliche ¨ Tribosysteme durch geeignete Werkstoffverbunde und Zwischenstoffe am Beispiel der Werkzeugmaschine, SFB 442, RWTH Aachen, Arbeits- und Ergebnisbericht, 2000. w3x DIN 51819-2, Mechanisch-dynamische Prufung ¨ ¨ auf dem Wal¨ FE8, Teil 2, Dezember 1999. ¨ zlagerschmierstoffprufgerat w4x DIN 51819-3, Mechanisch-dynamische Prufung ¨ ¨ auf dem Wal¨ FE8, Teil 3, August 1999, Entwurf. ¨ zlagerschmierstoffprufgerat w5x K. Enke, Wie universell sind DLC-Schichten, Jahrbuch der ¨ ¨ Oberflachentechnik, Hurthing Verlag Heidelberg (1998) Band 54. w6x P.W. Gold, C. Aßmann, N. van de Sandt, FVA Forschungsvor¨ haben Nr. 327: Entwicklung experimenteller Grundlagen fur ¨ eine schmierstoffabhangige Verschleißlebensdauerberechnung ¨ ¨ Walzlager; fur Abschlussbericht, 1998–2001. w7x T. Kacsich, P.W. Gold, J. Loos, Low friction W-C:H coatings for wear resistance in roller bearings; 13th International Colloquium Tribology, TAE-Tagung, 2002.

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w8x E. Lugscheider, K. Bobzin, M. Burckhardt, H. Murrenhoff, D. van Bebber, Systematische Entwicklung gradierter ZrC- und ¨ tribologische Anwendungen am Beispiel HfC-Schichten fur hydraulischen Komponenten, Tribologie und Schmierungstechnik, 48. Jhg., 1 (2001) S 16–21.

w9x T. Michler, M. Laakmann, Antriebstechnik 38 (6) (1999) S67–S69. w10x H. Peeken, H.-G. Amort, F.-W.Stuber, ¨ Neßlinger, FVA Forschungsvorhaben Nr. 31 I-IV, Abschussberichte; 1979, 1983, 1986, 1992.