Effect of ZDDP on friction in fretting contacts

Effect of ZDDP on friction in fretting contacts

Wear 273 (2011) 70–74 Contents lists available at ScienceDirect Wear journal homepage: www.elsevier.com/locate/wear Effect of ZDDP on friction in f...

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Wear 273 (2011) 70–74

Contents lists available at ScienceDirect

Wear journal homepage: www.elsevier.com/locate/wear

Effect of ZDDP on friction in fretting contacts Mattias Grahn a,b,∗ , Aldara Naveira-Suarez a,c , Rihard Pasaribu a a

SKF Engineering & Research Center, Nieuwegein, The Netherlands Luleå University of Technology, Division of Chemical Engineering, 971 87 Luleå, Sweden c Luleå University of Technology, Division of Machine Elements, 971 87 Luleå, Sweden b

a r t i c l e

i n f o

Article history: Received 29 September 2010 Received in revised form 30 March 2011 Accepted 2 May 2011 Available online 8 May 2011 Keywords: Fretting ZDDP Friction

a b s t r a c t The effect of ZDDP on fretting wear was investigated in a ball on flat machine. The results confirm previous work that anti-wear agents may reduce friction and wear in fretting contacts. It was further found that temperature, adsorption time, base oil polarity as well as the presence of other surface active additives in the oil were all important parameters affecting the ability of ZDDP to protect the surfaces against fretting wear. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The term fretting is used for describing contact conditions where two surfaces in mechanical contact are subjected to low amplitude oscillatory movements, thus vibrations in machine components is a classical source for fretting conditions. Fretting causes damage to the contacting surfaces in the form of wear, corrosion and fatigue crack initiation. The damaged surfaces may eventually result in failure of the components. The low amplitudes typical for fretting conditions results in such contact conditions where the central parts of the contact are not directly exposed to the surrounding atmosphere. Further, any wear debris created during the wear process may because of the low amplitudes get trapped inside the contact. Several different factors are affecting fretting wear, viz. contact pressure, temperature, amplitude of motion, frequency, number of cycles, surface hardness, friction coefficient, lubrication, type of base oil, humidity as well as other environmental parameters [1–3]. From the literature, three means of attacking the problem of fretting may be identified viz. (1) reducing relative slip or vibration, (2) increase the surface strength of the contacting surfaces by changing material or coating the surfaces, and (3) reduce friction. The latter mean, i.e., reducing friction, can frequently be obtained by the introduction of a lubricant. Despite the wide use of liquid lubricants, surprisingly little work is reported on the effect of these on fretting, and in particular, reports on the effect of lubricant additives such as anti-wear

or extreme pressure agents are scarce. However, some previous work indicates that these additives may be effective in preventing fretting wear. Godfrey [4] found that certain phosphorous based additives, e.g., tricresyl phosphate and triethyl phosphorothioate were capable of reducing fretting wear by forming reaction films on the contacting surfaces. Law and Rowe [5] investigated the effect of additives commonly used in transmission and gear oils on fretting and found that the result was highly dependent on the additive system. Qiu and Roylance [6] investigated the effect of various sulfur containing anti-wear additives on friction and wear in fretting contacts. The results show that these additives were effective in reducing friction and wear under the conditions studied. In this work, the effect of iso-C4 -ZDDP (zinc dialkyl dithiophosphate) on friction in fretting contacts was investigated, and in particular the effect of adsorption time and base oil polarity was addressed. A polar base oil is expected to compete with ZDDP for a place on the surface to a larger extent than a non-polar base oil [7], thus reducing any effect of ZDDP. The present work was part of a larger project on the influence of AW/EP additives, like ZDDP, under contact conditions of relevance to rolling element bearings. The fitting between the inner ring-shaft and outer ring-housing could experience fretting during bearing operation, e.g., industrial gearbox application. The oil used to lubricate the gear train may influence the fretting between inner ring-shaft and outer ring-housing during operation.

2. Experimental ∗ Corresponding author at: Luleå University of Technology, Division of Chemical Engineering, 971 87 Luleå, Sweden. Tel.: +46 920 491928; fax: +46 920 491199. E-mail address: [email protected] (M. Grahn). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.05.003

The fretting tests were conducted on an in-house designed fretting rig with a ball-on-flat configuration with a sample cell facilitating immersion of the contact in the oil as well as control of the

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Table 1 Contact conditions. Pressure (Hertzian maximum) Frequency Amplitude Ball diameter Temperature (bulk)

1 GPa 20 Hz 25 ␮m 12.7 mm Room temperature or 80 ◦ C

oil temperature. The fretting motion is induced by a piezoelectric actuator which in the same time measures the normal load and friction. The force exerted by the piezoelectric element is adjusted to give the desired displacement and the displacement is measured using a capacitance based displacement sensor. Hexadecane (Acros chemicals, purity 99%) and diethyleneglycol dibuthyl ether (DEGB) (Acros chemicals, 99%) were used as non-polar and polar low-viscosity model base oils, respectively. The iso-C4-ZDDP used in the experiments was supplied from A&S Chemie. Oleic acid (Aldrich, 90%) was used as a model friction modifier. The test samples (ball and flat) were of 52100 hardened steel with a surface roughness (Ra) of 10–20 nm, the test conditions are summarized in Table 1. The initial contact radius (Hertzian contact) was calculated to ca 87 ␮m. Before mounting the samples in the rig, the samples were cleaned in petroleum ether and subsequently allowed to dry before mounting the samples. After mounting the samples in the rig, the lower (flat) sample was immersed in the lubricant and the ball was lowered into the lubricant so that the lower portion of the ball was immersed in the oil but the ball was not allowed to touch the flat specimen leaving both surfaces exposed to the lubricant. Thereafter the first test started (0 h adsorption time), after the first test the flat sample was shifted slightly to the side and the ball was slightly rotated to allow fresh surfaces to be tested in the next experiment. During the waiting time (hereafter referred to as the adsorption time) to the next experiment both samples were immersed in the lubricant allowing the components in the lubricant to adsorb on the surfaces. The tested surfaces were subsequently characterized by optical microscopy, scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) and with a Wyko surface profiler. 3. Results and discussion Firstly, the fretting behavior of the pure model base oils were evaluated in the fretting rig. Measurements were performed at different adsorption times, i.e., the surfaces were exposed to the lubricant at different times before the measurements were executed. Fig. 1 shows the evolution of friction coefficient with number of cycles for DEGB at room temperature. As can be seen in the figure, the friction coefficient is low (∼0.2) for the first 170 cycles implying that the DEGB is protecting the surfaces, probably by forming an adsorbed layer on the surfaces.

Fig. 1. Friction coefficient as a function of fretting cycles for DEGB at room temperature and at four different adsorption times.

Fig. 2. Friction coefficient as a function of fretting cycles for DEGB 80 ◦ C and at four different adsorption times.

It is expected that DEGB will adsorb on the polar steel surfaces as it also is quite polar. It is desirable to have a low friction as this helps minimize the damage of the surface and the first period of low friction likely corresponds to adsorbed DEGB being able to protect the surfaces against wear, as this protective layer finally, after ca 170 cycles, breaks down, friction increases and wear of the metal surfaces are observed (the correlation between friction and wear will be shown later). The fact that the protecting layer breaks down at ca 170 cycles for all adsorption times implies that at these conditions there is no obvious effect of adsorption time. At higher cycles, the friction decreases to ca 0.25. This behavior has been observed previously in many studies on oil lubricated fretting contacts [8–11] and the large decrease in friction (larger than for unlubricated contacts) was attributed to the oil lowering the shear and adhesive stresses between the contacting surfaces [10,11]. As the temperature is raised to 80 ◦ C, the induction time more or less vanishes for DEGB, see Fig. 2. This behavior is most likely due to a weaker adsorption of DEGB at the higher temperature, consistent with physisorption of the lubricant molecules to the surface. Again, no effect of adsorption time is observed as the number of cycles run before the friction coefficient reaches high values are similar at all adsorption times. Fig. 3 shows the friction curves determined for the pure nonpolar base oil (hexadecane) at room temperature. The curves show no induction period as the curves immediately goes to high friction coefficients, i.e., the hexadecane is not able to protect the surfaces even at very few cycles. This is expected as the non-polar hexadecane is expected to interact weakly with the polar steel surfaces,

Fig. 3. Friction coefficient as a function of the number of fretting cycles determined at different adsorption times for pure hexadecane at room temperature.

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Fig. 4. Optical micrographs of typical wear scars after fretting tests obtained with (a) hexadecane and (b) DEGB. The diameter of the scars were ca 160 ␮m.

i.e., the hexadecane molecules are less strongly adsorbed to the surfaces than the polar DEGB molecules. At 80 ◦ C, very similar curves as in Fig. 3 were obtained (not shown), again suggesting a weak interaction between hexadecane and the steel surfaces. Optical micrographs of the wear scar after fretting revealed that the base oil caused differences in the appearances of the scars. Fig. 4 shows typical optical micrographs of wear scars obtained with pure hexadecane and DEGB as lubricants. The wear scar obtained with hexadecane had a metallic appearance whereas the wear scar obtained with DEGB showed a black and matt appearance. An EDS analysis of the wear scars showed that the black surface obtained with DEGB as a lubricant was iron oxide, possible Fe3 O4 , as the oxygen levels were elevated whereas the carbon levels were in the same range as outside the scar. It has been reported that one of the functionalities of liquid lubricants in fretting contacts is that they may regulate the availability of oxygen which may react with the surfaces to create iron oxides during fretting, see [1] and references therein. The solubility of oxygen in lubricant is one important parameter determining the availability of oxygen in the contact. However, for DEGB oxygen might possibly also be supplied directly from the molecule since one molecule of DEGB contain 3 oxygen atoms. To elucidate if the oxygen originates from dissolved oxygen or directly from the lubricant molecules, more experiments are needed. The wear scars showed the same appearance irregardless if an additive was present or not under the conditions studied. When the behavior of the pure base oils had been established, the effect of ZDDP could be addressed. Fig. 5 shows the friction coefficient as a function of the number of cycles for a 1% solution of ZDDP in DEGB at room temperature. At shorter adsorption times the “induction period”, where a low friction is obtained, is about the same as for the pure DEGB, i.e., ca 170 cycles (see Fig. 1). At longer adsorption times the induction period becomes substantially longer. This is substantially different than the curves for pure DEGB shown in Fig. 1 and indicates that

Fig. 5. Friction coefficient as a function of fretting cycles for a 1% solution of ZDDP in DEGB at room temperature and at four different adsorption times.

Fig. 6. Friction coefficient as a function of fretting cycles for a 1% solution of ZDDP in hexadecane at room temperature and at four different adsorption times.

the ZDDP may protect the surface if given time to adsorb on the surface. Fig. 6 shows the results obtained for a solution of 1% ZDDP in hexadecane at room temperature. The trend is similar as for ZDDP in DEGB viz. at short adsorption times (0 h) the result is similar as for the pure base oil (no induction time was observed for pure hexadecane) but at longer adsorption times a clear induction period where the ZDDP protects the surface emerges, so clearly there is an effect of adding ZDDP. By comparing Figs. 5 and 6 it may be noticed that the induction period develops faster when ZDDP is adsorbed from hexadecane than from DEGB which is expected since ZDDP will have larger problems replacing DEGB on the surface since DEGB is adsorbed stronger on the steel surface than hexadecane. Energy dispersive spectroscopy (EDS) of the wear scars revealed traces of ZDDP in the form of zinc, phosphorus and sulfur, especially at the perimeter of the scars. Fig. 7

Fig. 7. Typical EDS spectrum recorded from the edge of a wear scar. The fretting experiment was conducted at room temperature with 1% ZDDP in hexadecane as lubricant and after 4 h adsorption time.

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Fig. 8. Wear scars as measured with the Wyko surface profiler (top) and optical microscopy (bottom) on the flat specimen after 350 fretting cycles at different adsorption times. The lubricant used was a 1% solution of ZDDP in hexadecane and the experiment was performed at room temperature.

shows a typical EDS spectrum recorded at the perimeter of a wear scar. To illustrate the protective power of ZDDP in fretting wear, an experiment at the same conditions as in Fig. 6 (1% ZDDP in hexadecane, room temperature) was carried out but with the difference that the test was suspended after 350 cycles and the surfaces of the flat specimen were examined with optical microscopy and the Wyko surface profiler. Fig. 8 shows the surface profiles and the corresponding optical micrographs at four different adsorption times. As can be seen both from the surface profiles and the optical micrographs, the wear scars get less severe with increasing adsorption time. This indeed shows that ZDDP protects the surface from fretting wear under the conditions studied, in agreement with previously reported findings for phosphor and sulfur containing EP/AW additives in fretting contacts [4–6]. At 80 ◦ C, both the 1% ZDDP in hexadecane and the 1% ZDDP in DEGB showed the same trend viz. at this higher temperature the curves were shifted to the left as compared to at room temperature, i.e., the number of cycles that ZDDP was able to protect the surface decreased. Fig. 9 shows the friction curves for 1% ZDDP in hexadecane at 80 ◦ C. The effect of adding oleic acid to the ZDDP formulated oils was also investigated. Oleic acid is a friction modifier that works by adsorbing to the steel surfaces and thereby forming easily sheared surface films which may result in low friction under moderate severe conditions. When oleic acid was added to the pure base oils (hexadecane and DEGB) the result was very similar to when the two pure base oils were used, thus the oleic acid was not able to create a low friction under the conditions studied. Fig. 10 shows the friction

Fig. 9. Friction coefficient as a function of the number of fretting cycles determined at different adsorption times for hexadecane + 1% ZDDP at 80 ◦ C.

curves for pure hexadecane and hexadecane with 1% oleic acid at room temperature. Under these conditions, the adsorption of oleic acid on the surface should be very favourable, but the curves more or less coincide showing that under these conditions oleic acid is not capable of creating low friction and protecting the surface. For the DEGB base oil there was a weak tendency of oleic acid to reduce the weak protective effect of pure DEGB, possibly the oleic acid replaced some DEGB molecules on the surface and thereby disrupting the protective layer of adsorbed DEGB (not shown). When oleic acid (1%) was added to oils formulated with 1% ZDDP, this resulted in that the induction time was reduced, i.e., the surfaces were protected by ZDDP for fewer cycles, a representative example is presented in Fig. 11. The figure compares the friction curve of hexadecane + 1% ZDDP after 4 h of adsorption time at room temperature with the curves for hexadecane + 1% ZDDP + 1% oleic acid at 4 and 9 h adsorption time. As mentioned previously, as oleic acid is added the oils, the effect of ZDDP diminishes, most probable because oleic acid competes with ZDDP for a place on the surface and thus the surface concentration of ZDDP is diminished. The same trend was observed when the DEGB base oil was used, on the other hand, at 80◦ there was no clear effect of adding oleic acid, probably due to a combination of weak adsorption of oleic acid at this temperature and a small effect of ZDDP as well. This observed behavior with a decreased efficiency in protecting the surfaces towards wear at higher temperatures (80 ◦ C) was unexpected considering that higher temperatures generally favors

Fig. 10. Effect of adding 1% oleic acid (OA) to hexadecane on friction during fretting. The experiments were conducted at room temperature and after 4 h of adsorption time.

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presence of other polar molecules like friction modifiers, are important parameters as the polar molecules will tend to compete with ZDDP for a place on the surface. Acknowledgements The authors wish to thank SKF for permission to publish this work. The work presented in this paper has been supported by the EC, Sixth Framework Programme, Marie Curie Action (WEMESURF research network entitled: Characterization of wear mechanisms and surface functionalities with regard to life time prediction and quality criteria-from micro to nano range under contract MRTN CT 2006 035589). References Fig. 11. Effect on fretting friction of adding 1% oleic acid (OA) to an oil consisting of 1% ZDDP in hexadecane. The experiments were conducted at room temperature and after 4 or 9 h adsorption time.

the formation of protective anti-wear films in sliding contacts, see e.g., [12] and references therein, which may suggest that another mechanism could be active during fretting. The results, however, would be consistent with differences observed in the adsorption of ZDDP on iron oxide [13–17]. At low temperatures ZDDP is physisorbed whereas at higher temperatures, ZDDP is chemisorbed (or has started to degrade) on the surfaces. We speculate that either ZDDP is physisorbed in our measurements or, the fretting behavior of physisorbed (which should be dominating at room temperature) and chemisorbed (at higher temperatures) ZDDP is different and in the latter case, the physisorbed ZDDP shows a better capability in reducing fretting wear. We are currently trying to establish whether ZDDP is physisorbed or chemisorbed at higher temperatures in our system. 4. Conclusions The effect of ZDDP in fretting wear was investigated. Experiments were carried out at different temperatures, different adsorption times and in two different model base oils with different polarities. It was found that under the conditions studied that ZDDP could reduce fretting wear and longer adsorption times resulted in better protection. The effect of ZDDP decreased with increasing temperature, probably due to different adsorption conditions. Further, it was found that base oil polarity, as well as the

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