- Email: [email protected]
The effect of oil temperature and additive concentration on the wear of non-hydrogenated DLC coating
Tribology International 77 (2014) 65–71
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
The effect of oil temperature and additive concentration on the wear of non-hydrogenated DLC coating H. Abdullah Tasdemir a,n, Masaharu Wakayama a, Takayuki Tokoroyama a, Hiroyuki Kousaka a, Noritsugu Umehara a, Yutaka Mabuchi b, Tsuyoshi Higuchi b a Department of Mechanical Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan b Nissan Motor Co., Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 8 November 2013 Received in revised form 21 March 2014 Accepted 13 April 2014 Available online 26 April 2014
Diamond-like carbon (DLC) coatings are amorphous hard carbon films which offer excellent mechanical and tribological properties. Recently, these coatings have been applied to engine and powertrain components in passenger cars, including those under boundary lubricated conditions. Tribological performance of DLC coatings in oil lubricated conditions and their interaction with various lubricant additives are very complex due to the variety of DLC coatings and lubricants. In this study, the effects of oil temperature and additive concentration on the wear and friction performance of non-hydrogenated tetrahedral amorphous carbon (ta-C DLC) coating have been investigated. Tribological tests were conducted using a pin-on-disc tribotester with ta-C DLC coated steel pins sliding against non-coated steel discs. Tribological tests were also carried out between DLC coated pin against germanium disc in base oil for clarification of wear mechanism. The ta-C coating gave very low boundary friction with base oil at 50 1C but the coating experienced limited durability with increased wear rate at higher temperature against steel disc. Organic friction modifier glycerol-mono-oleate was able to preserve low friction behavior of the coating at higher temperature with enhanced durability. The anti-wear additive Zinc Dialkyldithiophosphate (ZnDTP) provided excellent wear protection for the ta-C coating due to the formation of thick pad-like tribofilm on steel counterpart at all temperature ranges but rubbing in ZnDTP additivated oil lead to higher friction coefficient. The results obtained at high temperature show the significant and beneficial influence of oil additives on the wear performance of the coating. Based on the obtained results and analysis, the wear performance of non-hydrogenated DLC was found to have a clear dependence on the concentration of the lubricant additives, test temperature and counter material. & 2014 Elsevier Ltd. All rights reserved.
Keywords: ta-C DLC Wear GMO ZnDTP
1. Introduction Diamond-like carbon (DLC) coatings are becoming attractive protective films for many automotive parts as they offer high hardness, ultra-low friction and good wear resistance under dry or lubricated contacts [1–3]. Tribological performance of DLC coatings under lubricated conditions and their interaction with lubricant additives strongly depend on the intrinsic factors of DLC coatings such as hydrogen content, doping elements and sp2/sp3 ratio. Therefore, more research in their tribological performance and their tribochemical interactions with conventional lubricant and lubricant additives are still needed for successful operation of these coatings under lubricated conditions.
n
Corresponding author. E-mail address: [email protected] (H. Abdullah Tasdemir).
http://dx.doi.org/10.1016/j.triboint.2014.04.015 0301-679X/& 2014 Elsevier Ltd. All rights reserved.
Systematic studies on the friction and wear behavior of DLC coatings under lubricated conditions were intensified in the last decade [4–10]. Based on these studies, it has been shown that hydrogenated form of DLC coatings reach ultra-low friction values in the lubricant containing friction modifier Molybdenum Dithiocarbamates (MoDTC) due to the formation of self-lubricating MoS2 sheets [4,11]. However, it has also recognized that the wear rates of hydrogenated DLC coatings in MoDTC containing oils were higher than pure base oil lubrication [12–14]. It has shown that MoDTC and Zinc Dialkyldithiophosphate (ZDDP/ZnDTP) additivated oils further improve the friction and wear performance of DLC coatings under boundary lubricated conditions [15,16]. ZDDP is a widely used anti-wear additive in engine oils which forms wear protective tribofilms on ferrous surfaces through tribochemical reactions [17]. Even though some studies have reported that ZDDP do not have anti-wear performance on DLC surfaces and no tribochemical reaction occurs between ZDDP additives and DLC
66
H. Abdullah Tasdemir et al. / Tribology International 77 (2014) 65–71
surfaces [5,18,19], there are numerous studies which report the formation of weak ZDDP tribofilm on DLC coatings [20–23]. On the other hand, many studies have confirmed that ultra-low friction and even super-low friction can also be achievable with hydrogen-free DLC or ta-C coatings under boundary lubricated contacts when lubricated with ester containing lubricants [24–26]. Surface-sensitive analysis using X-ray photoelectron spectroscopy (XPS) [24,27,28] and time-of-flight secondary ion mass spectrometer (ToF-SIMS) [29] have confirmed a thin absorbed OH layer on the surface of ta-C coating. Based on these findings, it is suggested that mechanically activated surface dangling bonds of ta-C surfaces are terminated by alcohol function groups of ester which resulted in ultra-low friction. Mabuchi et al. reported that dangling bonds in ta-C surfaces are key factor for the ultra-low friction mechanism of ta-C coating, when lubricated in glycerol-mono-oleate (GMO) containing oil [30]. Recently, it was observed that the wear rate of ta-C coating under the lubricated condition tented to decrease with increasing surface hardness [31] and it was shown that the addition of GMO greatly enhanced the wear resistance of ta-C coating [32]. Many DLC coated mechanical parts have to operate against steel counterpart in actual working environment due to the physical and economical limitations of coating both the contact surfaces. Lubrication of such a DLC/steel contact is a complex system with many influencing operation parameters. Therefore, in order to better understand the friction and wear mechanism of ta-C coating in lubricated condition, in this study we present the effect of oil temperature and additive concentrations on the lubrication performance of ta-C coating when rubbed against steel counterpart.
2. Experimental details 2.1. Material characterization and lubricants The tested ta-C coatings were deposited on cylindrical JIS SUJ2 (equivalent to AISI 52100 or DIN 100Cr6) bearing steel substrate by filtered cathodic vacuum arc (FCVA) method. The cylindrical steel pins were commercially available, standard pins, measuring 5 mm in diameter and 5 mm in length, with a hardness of 63 HRC and a surface roughness (Ra) of 0.05 μm. The ta-C coatings were deposited in 0.7 μm thickness with a adhesion-promoting interlayer. The average hardness and Young's modulus of the ta-C coatings were 75 75 GPa and 900 750 GPa, respectively. Hardness and Young's modulus characterizations of DLC films were made using nano-indentation method with a Berkovich indenter (Elionix ENT-1100a), while the surface roughness was evaluated with atomic force microscopy (Nanopics 1000, SII instruments, Japan). The coatings were supplied by Nippon ITF Inc., Japan. Commercially available synthetic poly-alphaolefin (PAO4) was used as a base oil in this study having 19 mm2/s viscosity and 17.08 GPa 1 pressure–viscosity coefficient at 40 1C. Glycerol mono-oleate (GMO) was added to the base oil as an organic friction modifier. Anti-wear additive zinc dithiophosphate (ZnDTP) was also used to enhance the wear performance of tribosystem. The ZnDTP was a secondary type which is generally used for commercial engine oils. Incorporating the additives did not result in significant change of base oil viscosity. The lubricants used are defined in Table 1. The initial tests were conducted with Group A lubricants in order to obtain oil temperature effect. The tests were repeated at 80 and 110 1C with Group B lubricants which contain half the additives concentration of Group A lubricants to analyze the effect of additive concentration. For the clarification of wear mode and failure mechanism of the ta-C coating in PAO, the ta-C coating was also rubbed against single-crystalline, pure germanium disc which is hard metalloid with a diamond-like crystalline structure. Pure germanium is a semiconductor material which has less reactivity than ferrous
Table 1 Boundary lubricant components and additive composition. Lubricants
Base oil (PAO) (wt%)
GMO (wt%)
ZnDTP (wt%)
Group A PAO PAOþ GMO PAOþ ZnDTP PAOþ GMO þ ZnDTP
100 99 99.82 98.82
– 1 – 1
– – 0.08 0.08
Group B PAOþ GMO PAOþ ZnDTP PAOþ GMO þ ZnDTP
99.5 99.86 98.86
0.5 – 0.5
– 0.04 0.04
surfaces with carbon materials. The Ge–C phase diagram indicates that the solid solubility of carbon in germanium is extremely low [33]. The surface roughness of the tested germanium disc was 6 72 nm. Surface hardness of the germanium disc was 11 72 GPa which was measured by nano-indentation method with a Berkovich indenter (Elionix ENT-1100a).
2.2. Tribological experiments The tribological tests were carried out in the cylinder-on-disc tribotester using ta-C coated pins and non-coated SUJ2 steel plate for DLC/steel contacts. The geometry of the steel discs was 22.5 mm in diameter and 4 mm in thickness. Non-coated SUJ2 steel discs with a hardness of 63 HRC were polished in several steps to surface roughness Ra 5 72 nm. The pin was fixed to prevent from rotating to ensure pure sliding condition and loaded against the face of the disc. The pin and disc were immersed in a lubricant and all of the tribological experiments were performed at a normal load of 5 N, which corresponds to a maximum Hertzian line contact pressure of 150 MPa. The tests were performed at 25, 50, 80 and 110 1C with 0.1 m/s average linear speed. The average speed was calculated in accordance with the middle of the line contact. The oil temperature was controlled by thermocouple which is fitted to just below the holder. In each test 10,700 cycles were made which corresponds to a sliding distance of 405 m. Test duration was 1 h and each test was repeated three times under the same conditions with new set of samples in order to verify the repeatability of the measurements. Before each test the samples were cleaned in an ultrasonic bath of acetone to remove any contaminants and oil species. Fig. 1 displays the schematic of tribotester and pin-on-disc configuration. The theoretical minimum film thickness (hmin ) and dimensionless lambda (Λ) ratio were calculated using Eqs. (1) and (2), respectively. The minimum film thickness, hmin U η0 0:68 W 0:073 ðαE0 Þ0:49 0 02 ð1 e 0:68k Þ; 0 ¼ 3:63 0 0 R ER ER
ð1Þ
where R0 is the reduced radius of curvature, U is the entraining surface velocity, W is the normal load, E0 is the reduced Young's modulus, η0 is the dynamic viscosity, α is the pressure–viscosity coefficient h
min Λ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
R2q;1 þ R2q;2
ð2Þ
where Rq;1 is the surface roughness of pin and Rq;2 is the surface roughness of disc. The calculated lambda ratio was 1.2 for 20 1C and less than unity for 50, 80 and 110 1C which means that operating lubrication regime was mixed lubrication for 20 1C and boundary lubrication for 50, 80 and 110 1C.
H. Abdullah Tasdemir et al. / Tribology International 77 (2014) 65–71
Fig. 1. Schematic of tribotester and pin-on-disc configuration for line contact.
Fig. 2. Wear rate of ta-C coated pin for DLC/steel contact as a function of temperature. The curves are for guidance only.
2.3. Surface analysis Wear tracks of pins and discs were studied using optical microscopy, field emission scanning electron microscopy (JEOL, JSM-7000FK), and non-contact, three-dimensional, scanning white light interferometry (Zygo, Newview). Since the accurate wear volume measurements on discs were not possible, wear calculation was performed only on pin specimens. The wear rates were calculated using Archard wear equation: V ¼ kFs
ð3Þ 3
where k is the dimensional wear rate (m /Nm), F is the normal load (N), s is the sliding distance (m) and V is the wear volume loss (m3).
3. Results 3.1. Effect of oil temperature In Fig. 2, the wear rate of the ta-C coated pin as a function of oil temperature is shown for DLC/steel contact lubricated with Group A lubricants. Generally, increased oil temperature led to higher wear rate in all lubricants but PAO þZnDTP. When tested in pure PAO oil ta-C coated pin gave very low wear rate at 20 and 50 1C, but further increase in oil temperature to 80 and 110 1C led to
67
severe wear of the ta-C coating, resulting in total wear of the ta-C coated pin against steel sample. As can be seen in Fig. 3, the coating is totally worn out and thus the substrate material was exposed with a brighter wear scar when tested in PAO at 80 1C and above temperature. From the wear analysis, it was noted that total wear of the ta-C coating in PAO observed around 8000 cycles at 80 1C and around 6000 cycles at 110 1C. The additive GMO significantly improved the wear performance and greatly enhanced the durability of ta-C coating by eliminating the total wear at higher temperature. The wear observed in GMO containing oil was more than 10 times lower than pure PAO oil at 80 and 110 1C. The lowest pin wear occurred in ZnDTP containing oil in all temperature ranges. It is interesting to note that GMO þZnDTP showed similar wear performance with GMO alone. The average values of the steady-state coefficient of friction, over the last 5 min of test period, as a function of oil temperature are given in Fig. 4 for DLC/steel contact lubricated with Group A lubricants. The ta-C coating exhibited low coefficient of friction about 0.05–0.06 in all lubricants at 25 1C. When tested in pure PAO at 50 1C, the ta-C coating gave ultra-low boundary friction value of 0.018. As it is mentioned above, the durability of ta-C coating was poor at 80 1C and above in PAO. Therefore, although the friction initially decreased reaching the lowest levels of 0.03–0.04 before the total wear, it jumped to 0.08 levels due to the total wear of ta-C coating and resulted in direct steel/steel contacts at the end of the tests at 80 1C and above in PAO. The PAOþ GMO oil showed ultra-low friction coefficient with values around 0.025 at 50 and 80 1C. The lowest friction value in PAOþGMO observed at 110 1C, which was around 0.016. For PAOþZnDTP the friction consistently showed the relatively high friction at all temperature ranges, with a stable value slightly below 0.09. In the case of PAOþ GMOþZnDTP oil, friction coefficient reduced to around 0.05 at 50 1C which was higher than GMO alone and lower than ZnDTP alone. At higher temperature, GMOþZnDTP oil exhibited similar friction performance with GMO alone. 3.2. Effect of additive concentration The effect of additive concentration on the friction coefficients was compared in Fig. 5. In all tests, a very small change in friction was seen when the additive concentration was reduced by half. In the case of PAO þGMO, low concentration of GMO additive resulted in lower friction coefficient. This effect was more pronounced at 80 1C tests. It is clear that reducing the concentration of ZnDTP additive in PAOþ ZnDTP lubricant did not change the friction performance. In the PAO þGMOþ ZnDTP lubricants, low concentration of additives provided a significantly lower coefficient of friction at 80 1C than high concentration of additives. However, this effect was not observed when the temperature increased to 110 1C. The change of wear rate as a function of the additive concentration is presented in Fig. 6. In contrast to the friction behavior, reduction of GMO additive had significant effect on the wear performance of ta-C coating. It is clear from the results that the wear rate of ta-C coating was increased substantially when the use of GMO additive reduced by half. In the PAO þZnDTP lubricants, the amount of wear was very similar for low and high concentration of ZnDTP additive. GMO þZnDTP additivated lubricants exhibited similar wear behaviour with GMO alone additivated lubricants in terms of the effect of additive concentration. 3.3. DLC vs. germanium disc DLC vs. germanium disc tests were performed only in pure PAO lubricant. Fig. 7 compares the average values of the steady-state
68
H. Abdullah Tasdemir et al. / Tribology International 77 (2014) 65–71
Fig. 3. Microscopic images of the wear scar on the ta-C coated pin when tested in pure PAO at (a) 50 1C and (b) 80 1C.
Fig. 4. Variation of friction coefficients as a function of temperature using Group A lubricants. The curves are for guidance only.
Fig. 6. Effect of additive concentration on the wear rate of ta-C DLC.
Fig. 5. Effect of additive concentration on the friction coefficient of ta-C DLC. Fig. 7. Friction coefficients of ta-C DLC when rubbed against Germanium as a function of temperature in PAO oil.
friction coefficients for DLC/germanium and DLC/steel contacts as a function of the oil temperature. The DLC/germanium system in general gave very low coefficient of friction. At room temperature (25 1C), the coefficient of friction was 0.035, lower than what was observed for DLC/steel contact. When tested at 50, 80 and 110 1C, the average friction of the DLC/germanium contacts was noticeably lower than room temperature, reaching around 0.016 levels. The wear rates of the ta-C coated pins are compared in Fig. 8 for DLC/steel and DLC/germanium contacts when tested in PAO oil. The results showed that rubbing against germanium disc significantly reduced the wear of ta-C coated pins at higher temperature. At 25 and 50 1C tests, the wear rate was similar for both contacts.
However, the wear rates obtained in DLC/steel contacts at higher temperature were approximately 10 times higher than DLC/germanium contacts.
4. Discussion The results showed a clear effect of oil temperature and additive concentrations on the wear behavior of ta-C coated pins. Firstly, the ta-C coating was able to provide ultra-low friction with
H. Abdullah Tasdemir et al. / Tribology International 77 (2014) 65–71
Fig. 8. Wear rate comparison of ta-C DLC when rubbed against Germanium and Steel discs as a function of temperature in PAO oil. The curves are for guidance only.
Fig. 9. Worn surface of ta-C coated pin rubbed against steel disc in PAO oil at 80 1C (SEM image is taken after 2000 running cycles and arrow shows the sliding direction).
decent durability at 50 1C in pure PAO. However, when the oil temperature was increased over 50 1C, the ta-C coating exhibited limited lifetime against steel disc in pure PAO because of the accelerated wear rate. It is evident from Fig. 2 that the wear rate of ta-C coating is substantially lowered when additivated lubricants were used for higher temperature (80 and 110 1C). Moreover, the results are exhibited that rubbing against germanium disc caused an order of magnitude lower wear rate than rubbing against the SUJ2 steel disc at 80 and 110 1C (Fig. 8). The surface analysis of worn surfaces showed smooth polishing wear on the ta-C coating in PAO (Fig. 9). The much lower wear rate of ta-C coating sliding against germanium than those against steel could be attributed to the extremely low solubility of carbon in germanium [33]. Therefore, it was thought that accelerated wear of the ta-C coating against steel at higher temperature in PAO could be explained by the tribo-chemical wear. It was hypothesized that, with increasing temperature, thermally activated carbon atoms on the topmost surfaces of ta-C coating diffused into the steel surface and thermo-chemical interaction occurs between iron and carbon atoms which cause higher wear rate [34]. From these results, it was suggested that the wear of ta-C coating in DLC/Germanium contact was caused by polishing wear. On the other hand, the wear of ta-C coating in DLC/steel contact caused by
69
polishing mechanical wear was associated with thermally activated tribo-chemical wear at higher temperature. The poor durability of the ta-C coating in PAO at higher temperature was eliminated with the addition of GMO. The effectiveness of GMO for friction reduction of ta-C coating has been reported by several authors [24,28,29]. Some authors reported that dangling bonds of carbon atoms on the ta-C surfaces were terminated by hydroxyl –OH group of GMO additive and that the formation of a hydrogen network leads to ultralow friction [24,27]. Besides this model, some other authors have proposed that the GMO molecules were assembled and bonded with C atoms on the hydrogen-free DLC surfaces through the agent of O atoms of hydroxyl –OH to form a monomolecular layer tribofilm (Fig. 10) [28,29]. Molecular dynamics simulation by Morita et al. has also proposed that H- and OH-terminated surfaces repulsively interact with Fe surfaces and thus weaken the covalent interaction between the ferrous surface and the diamond surface [35]. In the present study, enhanced wear resistance was also achieved along with the friction reduction in the presence of GMO. It is thought that the formation of OH-terminated tribofilm prevents direct contact of the carbon atoms with the ferrous surface which
Fig. 10. Schematic presentation of GMO tribofilm formation on ta-C coating. Termination of dangling bond of C atoms by GMO additive and its decomposed products [24,28].
Fig. 11. FESEM images of the ZnDTP tribofilm on ta-C coated pin.
70
H. Abdullah Tasdemir et al. / Tribology International 77 (2014) 65–71
Fig. 12. EDS analysis and element distribution on the ta-C coated pin tested in PAOþZnDTP. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
eliminates the thermo-chemical interaction between iron and carbon atoms and reduces the wear rate. The friction coefficient in 1 wt% GMO containing oil at 50 and 80 1C is slightly higher than in pure PAO at 50 1C (Fig. 4). This friction behavior can be associated with the efficient passivation of the ta-C surface by GMO additive. However, when tested in 1 wt% GMO containing oil at 110 1C, the friction coefficient is reduced to similar level with pure PAO at 50 1C and the wear rate of ta-C coating increased substantially (Fig. 2). This behavior can be explained by reduced adsorption/desorption rate resulting in the inefficiency of the GMO additive and its passivation mechanism at 110 1C. Moreover, the reduction of GMO percentage by half led to increased wear rate both at 80 and 110 1C (Fig. 6). In PAO þZnDTP lubricant, the ta-C coated pin was found to have excellent wear performance with very low wear rate and relatively high friction coefficient, around 0.09, in all temperature ranges (Figs. 2 and 4). Surface analysis showed the ZnDTP tribofilm formation both on steel disc and ta-C coated pin (Fig. 11). Typical element distributions on ta-C coated pin tested in PAOþ ZnDTP are shown in Fig. 12. In EDS picture, black color indicates the absence of probed elements, while, white color reflects the most intense signal of the probed elements. Different colors between the black and white suggest different intensity of the elements signal on the corresponding region. ZnDTP tribofilm formation on the steel was due to the chemical reaction between ferrous surface and ZnDTP molecules [17]. However, as has been noted in our previous research, ZnDTP tribofilm formation on ta-C coated pin was due to the transfer of ferrous molecules from steel disc to ta-C pin instead of chemical reaction between ZnDTP molecules and ta-C coating [32,34]. These ZnDTP tribofilm formed on both steel and ta-C surfaces prevented the direct ta-C and steel contact which offered superior wear protection. This excellent wear protection is achieved even with 0.04 wt% ZnDTP additive in 80 and 110 1C (Fig. 6). It is interesting to note that ZnDTP tribofilm found neither on steel disc nor on the ta-C coated pin when ZnDTP was used together with GMO. In all tests, GMO þZnDTP containing oil behaved very similar to only GMO containing oil. It is suggested
that the GMO additive suppresses the ZnDTP tribofilm formation by reacting with ZnDTP in oil or adsorbing and blocking the solid surfaces [36].
5. Conclusions In this study we investigate the effect of oil temperature and additive concentrations on the tribological behavior of hydrogenfree ta-C DLC coated pin rubbed against steel disc. It has been found that:
The ta-C coated pin can provide ultra-low friction in pure PAO,
but the wear resistance of ta-C coated pin is very poor against steel disc at 80 1C and above temperature which can be attributed to the thermally activated tribo-chemical interaction between carbon and ferrous atoms. The use of additives enhances the wear resistance of ta-C coated pin at high temperature by eliminating the tribochemical wear due to the termination of surfaces. Effectiveness of GMO on friction and wear is significantly affected by temperature and additive concentration. GMO is found to be more effective at 50 and 80 1C. Reducing the concentration of GMO by half causes almost one order of magnitude higher wear rate at 80 and 110 1C. The ZnDTP additive is very effective for the reduction of wear even with reduced concentration in all temperature ranges, but at the same time it causes relatively high friction.
References [1] Erdemir A, Donnet C. Tribology of diamond-like carbon film: recent progress and future prospects. J Phys D: Appl Phys 2006;39:311–27. [2] Bewilogua K, Brauer G, Dietz A, Gabler J, Goch G, Karpuschewshi B, et al. Surface technology for automotive engineering. CIRP Ann – Manuf Technol 2009;58:608–27.
H. Abdullah Tasdemir et al. / Tribology International 77 (2014) 65–71
[3] Kalin M, Velkavrh I, Vižintin J, Ožbolt L. Review of boundary lubrication mechanisms of DLC coatings used in mechanical applications. Meccanica 2008;43:623–37. [4] de Barros Bouchet MI, Martin JM, Le-Mogne T, Vacher B. Boundary lubrication mechanism of carbon coatings by MoDTC and ZDDP additives. Tribol Int 2005;38:257–64. [5] Podgornik B, Jacobson S, Hogmark S. DLC coating of boundary lubricated components – advantages of coating one of the contact surfaces rather than both or none. Tribol Int 2003;36:843–9. [6] Neville A, Morina A, Haque T, Voong M. Compatibility between tribological surfaces and lubricant additives – how friction and wear reduction can be controlled by surface/lube synergies. Tribol Int 2007;40:1680–95. [7] Forsberg P, Gustavsson F, Renman V, Hieke A, Jacobson S. Performance of DLC coatings in heated commercial engine oils. Wear 2013;304:211–22. [8] Kalin M, Velkavrh I. Non-conventional inverse-Stribeck-curve behavior and other characteristics of DLC coatings in all lubrication regimes. Wear 2013;297:911–8. [9] Topolovec-Miklozic K, Lockwood F, Spikes H. Behaviour of boundary lubricating additives on DLC coatings. Wear 2008;265:1893–901. [10] Kalin M, Kogovšek J, Remškar M. Nanoparticles as novel lubricating additives in green, physically based lubrication technology for DLC coatings. Wear 2013; 303:480–5. [11] Yue W, Liu C, Fu Z, Wang C, Huang H, Liu J. Synergistic effects between sulfurized W-DLC coating and MoDTC lubricating additive for improvement of tribological performance. Tribol Int 2013;62:117–23. [12] Shinyoshi T, Fuwa Y, Ozaki Y. Wear analysis of DLC coating in oil containing Mo-DTC. JSAE 20077103 SAE 2007-01-1969; 2007. [13] Vengudusamy B, Green JH, Lamb GD, Spikes HA. Behaviour of MoDTC in DLC/ DLC and DLC/steel contacts. Tribol Int 2012;54:68–76. [14] Sugimoto I, Honda F, Inoue K. Analysis of wear behavior and graphitization of hydrogenated DLC under boundary lubricant with MoDTC. Wear 2013;305:124–8. [15] Haque T, Morina A, Neville A, Kapadia R, Arrowsmith S. Effect of oil additives on the durability of hydrogenated DLC coating under boundary lubrication conditions. Wear 2009;266:147–57. [16] Kosarieh S, Morina A, Laině E, Flemming J, Neville A. The effect of MoDTC-type friction modifier on the wear performance of hydrogenated DLC coating. Wear 2013;302:890–8. [17] Spikes H. The history and mechanisms of ZDDP. Tribol Lett 2004;17 (3):469–89. [18] Haque T, Morina A, Neville A, Kapadia R, Arrowsmith S. Non-ferrous coating/ lubricant interactions in the tribological contacts: assessment of tribofilms. Tribol Int 2007;40:1603–12. [19] Kalin M, Vižintin J. Difference in the tribological mechanisms when using nondoped, metal-doped (Ti, WC) and non-metal doped (Si) diamond-like carbon against steel under boundary lubrication, with and without oil additives. Thin Solid Films 2006;515:2734–47.
71
[20] Equey S, Roos S, Mueller U, Hauert R, Spencer ND, Crockett R. Tribofilm formation from ZnDTP on the diamond-like carbon. Wear 2008;264:316–21. [21] Vengudusamy B, Green JH, Lamb GD, Spikes HA. Tribological properties of tribofilms formed from ZDDP in DLC/DLC contacts. Tribol Int 2011;44:165–74. [22] Haque T, Morina A, Neville A. Influence of friction modifier and antiwear additives on the tribological performance of a non-hydrogenated DLC coating. Surf Coat Technol 2010;204:4001–11. [23] Vengudusamy B, Green JH, Lamb GD, Spikes HA. Influence of hydrogen and tungsten concentration on the tribological properties of DLC/DLC contacts with ZDDP. Wear 2013;298–299:109–19. [24] Kano M, Yasuda Y, Okamoto Y, Mabuchi Y, Hamada T, Ueno T, et al. Ultralow friction of DLC in the presence of glycerol mono-oleate (GMO). Tribol Lett 2005;18:245–51. [25] Kano M. Super low friction of DLC applied to engine cam follower lubricated with ester-containing oil. Tribol Int 2006;39:1682–5. [26] Joly-Pottuz L, Matta C, De Barros Bouchet MI, Vachet B, Martin JM, Sagawa T. Superlow friction of ta-C lubricated by glycerol: an electron energy loss spectroscopy study. J Appl Phys 2007;102:064912. [27] Matta C, De Barros Bouchet MI, Le-Mogne T, Vachet B, Martin JM, Sagawa T. Tribochemistry of tetrahedral hydrogen-free amorphous carbon coatings in the presence of OH-containing lubricants. Lubr. Sci 2008;20:137–49. [28] Ye J, Okamoto Y, Yasuda Y. Direct insight into near-frictionless behavior displayed by diamond-like carbon coatings in lubricants. Tribol Lett 2008; 29:53–6. [29] Minami I, Kubo T, Nanao H, Mori S, Sagawa T, Okuda S. Investigation of tribochemistry by means of stable isotopic tracers. Part 2. Lubrication mechanism of friction modifiers on diamond-like carbon. Tribol Trans 2007;50:477–87. [30] Mabuchi Y, Higuchi T, Weihnacht V. Effect of sp2/sp3 bonding ratio and nitrogen content on friction properties of hydrogen-free DLC coatings. Tribol Int 2013;62:130–40. [31] Mabuchi Y, Higuchi T, Inagaki Y, Kousaka H, Umehara N. Wear analysis of hydrogen-free diamond-like carbon coatings under a lubricated condition. Wear 2013;298–299:48–56. [32] Tasdemir HA, Wakayama M, Tokoroyama T, Kousaka H, Umehara N, Mabuchi Y, et al. Ultra-low friction of tetrahedral amorphous diamond-like carbon (taC DLC) under boundary lubrication in poly alpha-olefin (PAO) with additives. Tribol Int 2013;65:286–94. [33] Olesinski RW, Abbaschian GJ. Bulletin of Alloy Phase Diagram 1984;5:484. [34] Tasdemir HA, Wakayama M, Tokoroyama T, Kousaka H, Umehara N, Mabuchi Y, et al. Wear behaviour of tetrahedral amorphous diamond-like carbon (ta-C DLC) in additive containing lubricants. Wear 2013;307:1–9. [35] Morita Y, Shibata T, Onodera T, Sahnoun R, Koyama M, Tsuboi H, et al. Effect of surface termination on superlow friction of diamond film: a theoretical study. Jpn J Appl Phys 2008;47(4). [36] Topolovec Miklozic Ksenija, Reg Forbus T, Spikes Hugh A. Performance of friction modifiers on zddp-generated surfaces. Tribol Trans 2007;50:328–35.
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
The effect of oil temperature and additive concentration on the wear of non-hydrogenated DLC coating H. Abdullah Tasdemir a,n, Masaharu Wakayama a, Takayuki Tokoroyama a, Hiroyuki Kousaka a, Noritsugu Umehara a, Yutaka Mabuchi b, Tsuyoshi Higuchi b a Department of Mechanical Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan b Nissan Motor Co., Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 8 November 2013 Received in revised form 21 March 2014 Accepted 13 April 2014 Available online 26 April 2014
Diamond-like carbon (DLC) coatings are amorphous hard carbon films which offer excellent mechanical and tribological properties. Recently, these coatings have been applied to engine and powertrain components in passenger cars, including those under boundary lubricated conditions. Tribological performance of DLC coatings in oil lubricated conditions and their interaction with various lubricant additives are very complex due to the variety of DLC coatings and lubricants. In this study, the effects of oil temperature and additive concentration on the wear and friction performance of non-hydrogenated tetrahedral amorphous carbon (ta-C DLC) coating have been investigated. Tribological tests were conducted using a pin-on-disc tribotester with ta-C DLC coated steel pins sliding against non-coated steel discs. Tribological tests were also carried out between DLC coated pin against germanium disc in base oil for clarification of wear mechanism. The ta-C coating gave very low boundary friction with base oil at 50 1C but the coating experienced limited durability with increased wear rate at higher temperature against steel disc. Organic friction modifier glycerol-mono-oleate was able to preserve low friction behavior of the coating at higher temperature with enhanced durability. The anti-wear additive Zinc Dialkyldithiophosphate (ZnDTP) provided excellent wear protection for the ta-C coating due to the formation of thick pad-like tribofilm on steel counterpart at all temperature ranges but rubbing in ZnDTP additivated oil lead to higher friction coefficient. The results obtained at high temperature show the significant and beneficial influence of oil additives on the wear performance of the coating. Based on the obtained results and analysis, the wear performance of non-hydrogenated DLC was found to have a clear dependence on the concentration of the lubricant additives, test temperature and counter material. & 2014 Elsevier Ltd. All rights reserved.
Keywords: ta-C DLC Wear GMO ZnDTP
1. Introduction Diamond-like carbon (DLC) coatings are becoming attractive protective films for many automotive parts as they offer high hardness, ultra-low friction and good wear resistance under dry or lubricated contacts [1–3]. Tribological performance of DLC coatings under lubricated conditions and their interaction with lubricant additives strongly depend on the intrinsic factors of DLC coatings such as hydrogen content, doping elements and sp2/sp3 ratio. Therefore, more research in their tribological performance and their tribochemical interactions with conventional lubricant and lubricant additives are still needed for successful operation of these coatings under lubricated conditions.
n
Corresponding author. E-mail address: [email protected] (H. Abdullah Tasdemir).
http://dx.doi.org/10.1016/j.triboint.2014.04.015 0301-679X/& 2014 Elsevier Ltd. All rights reserved.
Systematic studies on the friction and wear behavior of DLC coatings under lubricated conditions were intensified in the last decade [4–10]. Based on these studies, it has been shown that hydrogenated form of DLC coatings reach ultra-low friction values in the lubricant containing friction modifier Molybdenum Dithiocarbamates (MoDTC) due to the formation of self-lubricating MoS2 sheets [4,11]. However, it has also recognized that the wear rates of hydrogenated DLC coatings in MoDTC containing oils were higher than pure base oil lubrication [12–14]. It has shown that MoDTC and Zinc Dialkyldithiophosphate (ZDDP/ZnDTP) additivated oils further improve the friction and wear performance of DLC coatings under boundary lubricated conditions [15,16]. ZDDP is a widely used anti-wear additive in engine oils which forms wear protective tribofilms on ferrous surfaces through tribochemical reactions [17]. Even though some studies have reported that ZDDP do not have anti-wear performance on DLC surfaces and no tribochemical reaction occurs between ZDDP additives and DLC
66
H. Abdullah Tasdemir et al. / Tribology International 77 (2014) 65–71
surfaces [5,18,19], there are numerous studies which report the formation of weak ZDDP tribofilm on DLC coatings [20–23]. On the other hand, many studies have confirmed that ultra-low friction and even super-low friction can also be achievable with hydrogen-free DLC or ta-C coatings under boundary lubricated contacts when lubricated with ester containing lubricants [24–26]. Surface-sensitive analysis using X-ray photoelectron spectroscopy (XPS) [24,27,28] and time-of-flight secondary ion mass spectrometer (ToF-SIMS) [29] have confirmed a thin absorbed OH layer on the surface of ta-C coating. Based on these findings, it is suggested that mechanically activated surface dangling bonds of ta-C surfaces are terminated by alcohol function groups of ester which resulted in ultra-low friction. Mabuchi et al. reported that dangling bonds in ta-C surfaces are key factor for the ultra-low friction mechanism of ta-C coating, when lubricated in glycerol-mono-oleate (GMO) containing oil [30]. Recently, it was observed that the wear rate of ta-C coating under the lubricated condition tented to decrease with increasing surface hardness [31] and it was shown that the addition of GMO greatly enhanced the wear resistance of ta-C coating [32]. Many DLC coated mechanical parts have to operate against steel counterpart in actual working environment due to the physical and economical limitations of coating both the contact surfaces. Lubrication of such a DLC/steel contact is a complex system with many influencing operation parameters. Therefore, in order to better understand the friction and wear mechanism of ta-C coating in lubricated condition, in this study we present the effect of oil temperature and additive concentrations on the lubrication performance of ta-C coating when rubbed against steel counterpart.
2. Experimental details 2.1. Material characterization and lubricants The tested ta-C coatings were deposited on cylindrical JIS SUJ2 (equivalent to AISI 52100 or DIN 100Cr6) bearing steel substrate by filtered cathodic vacuum arc (FCVA) method. The cylindrical steel pins were commercially available, standard pins, measuring 5 mm in diameter and 5 mm in length, with a hardness of 63 HRC and a surface roughness (Ra) of 0.05 μm. The ta-C coatings were deposited in 0.7 μm thickness with a adhesion-promoting interlayer. The average hardness and Young's modulus of the ta-C coatings were 75 75 GPa and 900 750 GPa, respectively. Hardness and Young's modulus characterizations of DLC films were made using nano-indentation method with a Berkovich indenter (Elionix ENT-1100a), while the surface roughness was evaluated with atomic force microscopy (Nanopics 1000, SII instruments, Japan). The coatings were supplied by Nippon ITF Inc., Japan. Commercially available synthetic poly-alphaolefin (PAO4) was used as a base oil in this study having 19 mm2/s viscosity and 17.08 GPa 1 pressure–viscosity coefficient at 40 1C. Glycerol mono-oleate (GMO) was added to the base oil as an organic friction modifier. Anti-wear additive zinc dithiophosphate (ZnDTP) was also used to enhance the wear performance of tribosystem. The ZnDTP was a secondary type which is generally used for commercial engine oils. Incorporating the additives did not result in significant change of base oil viscosity. The lubricants used are defined in Table 1. The initial tests were conducted with Group A lubricants in order to obtain oil temperature effect. The tests were repeated at 80 and 110 1C with Group B lubricants which contain half the additives concentration of Group A lubricants to analyze the effect of additive concentration. For the clarification of wear mode and failure mechanism of the ta-C coating in PAO, the ta-C coating was also rubbed against single-crystalline, pure germanium disc which is hard metalloid with a diamond-like crystalline structure. Pure germanium is a semiconductor material which has less reactivity than ferrous
Table 1 Boundary lubricant components and additive composition. Lubricants
Base oil (PAO) (wt%)
GMO (wt%)
ZnDTP (wt%)
Group A PAO PAOþ GMO PAOþ ZnDTP PAOþ GMO þ ZnDTP
100 99 99.82 98.82
– 1 – 1
– – 0.08 0.08
Group B PAOþ GMO PAOþ ZnDTP PAOþ GMO þ ZnDTP
99.5 99.86 98.86
0.5 – 0.5
– 0.04 0.04
surfaces with carbon materials. The Ge–C phase diagram indicates that the solid solubility of carbon in germanium is extremely low [33]. The surface roughness of the tested germanium disc was 6 72 nm. Surface hardness of the germanium disc was 11 72 GPa which was measured by nano-indentation method with a Berkovich indenter (Elionix ENT-1100a).
2.2. Tribological experiments The tribological tests were carried out in the cylinder-on-disc tribotester using ta-C coated pins and non-coated SUJ2 steel plate for DLC/steel contacts. The geometry of the steel discs was 22.5 mm in diameter and 4 mm in thickness. Non-coated SUJ2 steel discs with a hardness of 63 HRC were polished in several steps to surface roughness Ra 5 72 nm. The pin was fixed to prevent from rotating to ensure pure sliding condition and loaded against the face of the disc. The pin and disc were immersed in a lubricant and all of the tribological experiments were performed at a normal load of 5 N, which corresponds to a maximum Hertzian line contact pressure of 150 MPa. The tests were performed at 25, 50, 80 and 110 1C with 0.1 m/s average linear speed. The average speed was calculated in accordance with the middle of the line contact. The oil temperature was controlled by thermocouple which is fitted to just below the holder. In each test 10,700 cycles were made which corresponds to a sliding distance of 405 m. Test duration was 1 h and each test was repeated three times under the same conditions with new set of samples in order to verify the repeatability of the measurements. Before each test the samples were cleaned in an ultrasonic bath of acetone to remove any contaminants and oil species. Fig. 1 displays the schematic of tribotester and pin-on-disc configuration. The theoretical minimum film thickness (hmin ) and dimensionless lambda (Λ) ratio were calculated using Eqs. (1) and (2), respectively. The minimum film thickness, hmin U η0 0:68 W 0:073 ðαE0 Þ0:49 0 02 ð1 e 0:68k Þ; 0 ¼ 3:63 0 0 R ER ER
ð1Þ
where R0 is the reduced radius of curvature, U is the entraining surface velocity, W is the normal load, E0 is the reduced Young's modulus, η0 is the dynamic viscosity, α is the pressure–viscosity coefficient h
min Λ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
R2q;1 þ R2q;2
ð2Þ
where Rq;1 is the surface roughness of pin and Rq;2 is the surface roughness of disc. The calculated lambda ratio was 1.2 for 20 1C and less than unity for 50, 80 and 110 1C which means that operating lubrication regime was mixed lubrication for 20 1C and boundary lubrication for 50, 80 and 110 1C.
H. Abdullah Tasdemir et al. / Tribology International 77 (2014) 65–71
Fig. 1. Schematic of tribotester and pin-on-disc configuration for line contact.
Fig. 2. Wear rate of ta-C coated pin for DLC/steel contact as a function of temperature. The curves are for guidance only.
2.3. Surface analysis Wear tracks of pins and discs were studied using optical microscopy, field emission scanning electron microscopy (JEOL, JSM-7000FK), and non-contact, three-dimensional, scanning white light interferometry (Zygo, Newview). Since the accurate wear volume measurements on discs were not possible, wear calculation was performed only on pin specimens. The wear rates were calculated using Archard wear equation: V ¼ kFs
ð3Þ 3
where k is the dimensional wear rate (m /Nm), F is the normal load (N), s is the sliding distance (m) and V is the wear volume loss (m3).
3. Results 3.1. Effect of oil temperature In Fig. 2, the wear rate of the ta-C coated pin as a function of oil temperature is shown for DLC/steel contact lubricated with Group A lubricants. Generally, increased oil temperature led to higher wear rate in all lubricants but PAO þZnDTP. When tested in pure PAO oil ta-C coated pin gave very low wear rate at 20 and 50 1C, but further increase in oil temperature to 80 and 110 1C led to
67
severe wear of the ta-C coating, resulting in total wear of the ta-C coated pin against steel sample. As can be seen in Fig. 3, the coating is totally worn out and thus the substrate material was exposed with a brighter wear scar when tested in PAO at 80 1C and above temperature. From the wear analysis, it was noted that total wear of the ta-C coating in PAO observed around 8000 cycles at 80 1C and around 6000 cycles at 110 1C. The additive GMO significantly improved the wear performance and greatly enhanced the durability of ta-C coating by eliminating the total wear at higher temperature. The wear observed in GMO containing oil was more than 10 times lower than pure PAO oil at 80 and 110 1C. The lowest pin wear occurred in ZnDTP containing oil in all temperature ranges. It is interesting to note that GMO þZnDTP showed similar wear performance with GMO alone. The average values of the steady-state coefficient of friction, over the last 5 min of test period, as a function of oil temperature are given in Fig. 4 for DLC/steel contact lubricated with Group A lubricants. The ta-C coating exhibited low coefficient of friction about 0.05–0.06 in all lubricants at 25 1C. When tested in pure PAO at 50 1C, the ta-C coating gave ultra-low boundary friction value of 0.018. As it is mentioned above, the durability of ta-C coating was poor at 80 1C and above in PAO. Therefore, although the friction initially decreased reaching the lowest levels of 0.03–0.04 before the total wear, it jumped to 0.08 levels due to the total wear of ta-C coating and resulted in direct steel/steel contacts at the end of the tests at 80 1C and above in PAO. The PAOþ GMO oil showed ultra-low friction coefficient with values around 0.025 at 50 and 80 1C. The lowest friction value in PAOþGMO observed at 110 1C, which was around 0.016. For PAOþZnDTP the friction consistently showed the relatively high friction at all temperature ranges, with a stable value slightly below 0.09. In the case of PAOþ GMOþZnDTP oil, friction coefficient reduced to around 0.05 at 50 1C which was higher than GMO alone and lower than ZnDTP alone. At higher temperature, GMOþZnDTP oil exhibited similar friction performance with GMO alone. 3.2. Effect of additive concentration The effect of additive concentration on the friction coefficients was compared in Fig. 5. In all tests, a very small change in friction was seen when the additive concentration was reduced by half. In the case of PAO þGMO, low concentration of GMO additive resulted in lower friction coefficient. This effect was more pronounced at 80 1C tests. It is clear that reducing the concentration of ZnDTP additive in PAOþ ZnDTP lubricant did not change the friction performance. In the PAO þGMOþ ZnDTP lubricants, low concentration of additives provided a significantly lower coefficient of friction at 80 1C than high concentration of additives. However, this effect was not observed when the temperature increased to 110 1C. The change of wear rate as a function of the additive concentration is presented in Fig. 6. In contrast to the friction behavior, reduction of GMO additive had significant effect on the wear performance of ta-C coating. It is clear from the results that the wear rate of ta-C coating was increased substantially when the use of GMO additive reduced by half. In the PAO þZnDTP lubricants, the amount of wear was very similar for low and high concentration of ZnDTP additive. GMO þZnDTP additivated lubricants exhibited similar wear behaviour with GMO alone additivated lubricants in terms of the effect of additive concentration. 3.3. DLC vs. germanium disc DLC vs. germanium disc tests were performed only in pure PAO lubricant. Fig. 7 compares the average values of the steady-state
68
H. Abdullah Tasdemir et al. / Tribology International 77 (2014) 65–71
Fig. 3. Microscopic images of the wear scar on the ta-C coated pin when tested in pure PAO at (a) 50 1C and (b) 80 1C.
Fig. 4. Variation of friction coefficients as a function of temperature using Group A lubricants. The curves are for guidance only.
Fig. 6. Effect of additive concentration on the wear rate of ta-C DLC.
Fig. 5. Effect of additive concentration on the friction coefficient of ta-C DLC. Fig. 7. Friction coefficients of ta-C DLC when rubbed against Germanium as a function of temperature in PAO oil.
friction coefficients for DLC/germanium and DLC/steel contacts as a function of the oil temperature. The DLC/germanium system in general gave very low coefficient of friction. At room temperature (25 1C), the coefficient of friction was 0.035, lower than what was observed for DLC/steel contact. When tested at 50, 80 and 110 1C, the average friction of the DLC/germanium contacts was noticeably lower than room temperature, reaching around 0.016 levels. The wear rates of the ta-C coated pins are compared in Fig. 8 for DLC/steel and DLC/germanium contacts when tested in PAO oil. The results showed that rubbing against germanium disc significantly reduced the wear of ta-C coated pins at higher temperature. At 25 and 50 1C tests, the wear rate was similar for both contacts.
However, the wear rates obtained in DLC/steel contacts at higher temperature were approximately 10 times higher than DLC/germanium contacts.
4. Discussion The results showed a clear effect of oil temperature and additive concentrations on the wear behavior of ta-C coated pins. Firstly, the ta-C coating was able to provide ultra-low friction with
H. Abdullah Tasdemir et al. / Tribology International 77 (2014) 65–71
Fig. 8. Wear rate comparison of ta-C DLC when rubbed against Germanium and Steel discs as a function of temperature in PAO oil. The curves are for guidance only.
Fig. 9. Worn surface of ta-C coated pin rubbed against steel disc in PAO oil at 80 1C (SEM image is taken after 2000 running cycles and arrow shows the sliding direction).
decent durability at 50 1C in pure PAO. However, when the oil temperature was increased over 50 1C, the ta-C coating exhibited limited lifetime against steel disc in pure PAO because of the accelerated wear rate. It is evident from Fig. 2 that the wear rate of ta-C coating is substantially lowered when additivated lubricants were used for higher temperature (80 and 110 1C). Moreover, the results are exhibited that rubbing against germanium disc caused an order of magnitude lower wear rate than rubbing against the SUJ2 steel disc at 80 and 110 1C (Fig. 8). The surface analysis of worn surfaces showed smooth polishing wear on the ta-C coating in PAO (Fig. 9). The much lower wear rate of ta-C coating sliding against germanium than those against steel could be attributed to the extremely low solubility of carbon in germanium [33]. Therefore, it was thought that accelerated wear of the ta-C coating against steel at higher temperature in PAO could be explained by the tribo-chemical wear. It was hypothesized that, with increasing temperature, thermally activated carbon atoms on the topmost surfaces of ta-C coating diffused into the steel surface and thermo-chemical interaction occurs between iron and carbon atoms which cause higher wear rate [34]. From these results, it was suggested that the wear of ta-C coating in DLC/Germanium contact was caused by polishing wear. On the other hand, the wear of ta-C coating in DLC/steel contact caused by
69
polishing mechanical wear was associated with thermally activated tribo-chemical wear at higher temperature. The poor durability of the ta-C coating in PAO at higher temperature was eliminated with the addition of GMO. The effectiveness of GMO for friction reduction of ta-C coating has been reported by several authors [24,28,29]. Some authors reported that dangling bonds of carbon atoms on the ta-C surfaces were terminated by hydroxyl –OH group of GMO additive and that the formation of a hydrogen network leads to ultralow friction [24,27]. Besides this model, some other authors have proposed that the GMO molecules were assembled and bonded with C atoms on the hydrogen-free DLC surfaces through the agent of O atoms of hydroxyl –OH to form a monomolecular layer tribofilm (Fig. 10) [28,29]. Molecular dynamics simulation by Morita et al. has also proposed that H- and OH-terminated surfaces repulsively interact with Fe surfaces and thus weaken the covalent interaction between the ferrous surface and the diamond surface [35]. In the present study, enhanced wear resistance was also achieved along with the friction reduction in the presence of GMO. It is thought that the formation of OH-terminated tribofilm prevents direct contact of the carbon atoms with the ferrous surface which
Fig. 10. Schematic presentation of GMO tribofilm formation on ta-C coating. Termination of dangling bond of C atoms by GMO additive and its decomposed products [24,28].
Fig. 11. FESEM images of the ZnDTP tribofilm on ta-C coated pin.
70
H. Abdullah Tasdemir et al. / Tribology International 77 (2014) 65–71
Fig. 12. EDS analysis and element distribution on the ta-C coated pin tested in PAOþZnDTP. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
eliminates the thermo-chemical interaction between iron and carbon atoms and reduces the wear rate. The friction coefficient in 1 wt% GMO containing oil at 50 and 80 1C is slightly higher than in pure PAO at 50 1C (Fig. 4). This friction behavior can be associated with the efficient passivation of the ta-C surface by GMO additive. However, when tested in 1 wt% GMO containing oil at 110 1C, the friction coefficient is reduced to similar level with pure PAO at 50 1C and the wear rate of ta-C coating increased substantially (Fig. 2). This behavior can be explained by reduced adsorption/desorption rate resulting in the inefficiency of the GMO additive and its passivation mechanism at 110 1C. Moreover, the reduction of GMO percentage by half led to increased wear rate both at 80 and 110 1C (Fig. 6). In PAO þZnDTP lubricant, the ta-C coated pin was found to have excellent wear performance with very low wear rate and relatively high friction coefficient, around 0.09, in all temperature ranges (Figs. 2 and 4). Surface analysis showed the ZnDTP tribofilm formation both on steel disc and ta-C coated pin (Fig. 11). Typical element distributions on ta-C coated pin tested in PAOþ ZnDTP are shown in Fig. 12. In EDS picture, black color indicates the absence of probed elements, while, white color reflects the most intense signal of the probed elements. Different colors between the black and white suggest different intensity of the elements signal on the corresponding region. ZnDTP tribofilm formation on the steel was due to the chemical reaction between ferrous surface and ZnDTP molecules [17]. However, as has been noted in our previous research, ZnDTP tribofilm formation on ta-C coated pin was due to the transfer of ferrous molecules from steel disc to ta-C pin instead of chemical reaction between ZnDTP molecules and ta-C coating [32,34]. These ZnDTP tribofilm formed on both steel and ta-C surfaces prevented the direct ta-C and steel contact which offered superior wear protection. This excellent wear protection is achieved even with 0.04 wt% ZnDTP additive in 80 and 110 1C (Fig. 6). It is interesting to note that ZnDTP tribofilm found neither on steel disc nor on the ta-C coated pin when ZnDTP was used together with GMO. In all tests, GMO þZnDTP containing oil behaved very similar to only GMO containing oil. It is suggested
that the GMO additive suppresses the ZnDTP tribofilm formation by reacting with ZnDTP in oil or adsorbing and blocking the solid surfaces [36].
5. Conclusions In this study we investigate the effect of oil temperature and additive concentrations on the tribological behavior of hydrogenfree ta-C DLC coated pin rubbed against steel disc. It has been found that:
The ta-C coated pin can provide ultra-low friction in pure PAO,
but the wear resistance of ta-C coated pin is very poor against steel disc at 80 1C and above temperature which can be attributed to the thermally activated tribo-chemical interaction between carbon and ferrous atoms. The use of additives enhances the wear resistance of ta-C coated pin at high temperature by eliminating the tribochemical wear due to the termination of surfaces. Effectiveness of GMO on friction and wear is significantly affected by temperature and additive concentration. GMO is found to be more effective at 50 and 80 1C. Reducing the concentration of GMO by half causes almost one order of magnitude higher wear rate at 80 and 110 1C. The ZnDTP additive is very effective for the reduction of wear even with reduced concentration in all temperature ranges, but at the same time it causes relatively high friction.
References [1] Erdemir A, Donnet C. Tribology of diamond-like carbon film: recent progress and future prospects. J Phys D: Appl Phys 2006;39:311–27. [2] Bewilogua K, Brauer G, Dietz A, Gabler J, Goch G, Karpuschewshi B, et al. Surface technology for automotive engineering. CIRP Ann – Manuf Technol 2009;58:608–27.
H. Abdullah Tasdemir et al. / Tribology International 77 (2014) 65–71
[3] Kalin M, Velkavrh I, Vižintin J, Ožbolt L. Review of boundary lubrication mechanisms of DLC coatings used in mechanical applications. Meccanica 2008;43:623–37. [4] de Barros Bouchet MI, Martin JM, Le-Mogne T, Vacher B. Boundary lubrication mechanism of carbon coatings by MoDTC and ZDDP additives. Tribol Int 2005;38:257–64. [5] Podgornik B, Jacobson S, Hogmark S. DLC coating of boundary lubricated components – advantages of coating one of the contact surfaces rather than both or none. Tribol Int 2003;36:843–9. [6] Neville A, Morina A, Haque T, Voong M. Compatibility between tribological surfaces and lubricant additives – how friction and wear reduction can be controlled by surface/lube synergies. Tribol Int 2007;40:1680–95. [7] Forsberg P, Gustavsson F, Renman V, Hieke A, Jacobson S. Performance of DLC coatings in heated commercial engine oils. Wear 2013;304:211–22. [8] Kalin M, Velkavrh I. Non-conventional inverse-Stribeck-curve behavior and other characteristics of DLC coatings in all lubrication regimes. Wear 2013;297:911–8. [9] Topolovec-Miklozic K, Lockwood F, Spikes H. Behaviour of boundary lubricating additives on DLC coatings. Wear 2008;265:1893–901. [10] Kalin M, Kogovšek J, Remškar M. Nanoparticles as novel lubricating additives in green, physically based lubrication technology for DLC coatings. Wear 2013; 303:480–5. [11] Yue W, Liu C, Fu Z, Wang C, Huang H, Liu J. Synergistic effects between sulfurized W-DLC coating and MoDTC lubricating additive for improvement of tribological performance. Tribol Int 2013;62:117–23. [12] Shinyoshi T, Fuwa Y, Ozaki Y. Wear analysis of DLC coating in oil containing Mo-DTC. JSAE 20077103 SAE 2007-01-1969; 2007. [13] Vengudusamy B, Green JH, Lamb GD, Spikes HA. Behaviour of MoDTC in DLC/ DLC and DLC/steel contacts. Tribol Int 2012;54:68–76. [14] Sugimoto I, Honda F, Inoue K. Analysis of wear behavior and graphitization of hydrogenated DLC under boundary lubricant with MoDTC. Wear 2013;305:124–8. [15] Haque T, Morina A, Neville A, Kapadia R, Arrowsmith S. Effect of oil additives on the durability of hydrogenated DLC coating under boundary lubrication conditions. Wear 2009;266:147–57. [16] Kosarieh S, Morina A, Laině E, Flemming J, Neville A. The effect of MoDTC-type friction modifier on the wear performance of hydrogenated DLC coating. Wear 2013;302:890–8. [17] Spikes H. The history and mechanisms of ZDDP. Tribol Lett 2004;17 (3):469–89. [18] Haque T, Morina A, Neville A, Kapadia R, Arrowsmith S. Non-ferrous coating/ lubricant interactions in the tribological contacts: assessment of tribofilms. Tribol Int 2007;40:1603–12. [19] Kalin M, Vižintin J. Difference in the tribological mechanisms when using nondoped, metal-doped (Ti, WC) and non-metal doped (Si) diamond-like carbon against steel under boundary lubrication, with and without oil additives. Thin Solid Films 2006;515:2734–47.
71
[20] Equey S, Roos S, Mueller U, Hauert R, Spencer ND, Crockett R. Tribofilm formation from ZnDTP on the diamond-like carbon. Wear 2008;264:316–21. [21] Vengudusamy B, Green JH, Lamb GD, Spikes HA. Tribological properties of tribofilms formed from ZDDP in DLC/DLC contacts. Tribol Int 2011;44:165–74. [22] Haque T, Morina A, Neville A. Influence of friction modifier and antiwear additives on the tribological performance of a non-hydrogenated DLC coating. Surf Coat Technol 2010;204:4001–11. [23] Vengudusamy B, Green JH, Lamb GD, Spikes HA. Influence of hydrogen and tungsten concentration on the tribological properties of DLC/DLC contacts with ZDDP. Wear 2013;298–299:109–19. [24] Kano M, Yasuda Y, Okamoto Y, Mabuchi Y, Hamada T, Ueno T, et al. Ultralow friction of DLC in the presence of glycerol mono-oleate (GMO). Tribol Lett 2005;18:245–51. [25] Kano M. Super low friction of DLC applied to engine cam follower lubricated with ester-containing oil. Tribol Int 2006;39:1682–5. [26] Joly-Pottuz L, Matta C, De Barros Bouchet MI, Vachet B, Martin JM, Sagawa T. Superlow friction of ta-C lubricated by glycerol: an electron energy loss spectroscopy study. J Appl Phys 2007;102:064912. [27] Matta C, De Barros Bouchet MI, Le-Mogne T, Vachet B, Martin JM, Sagawa T. Tribochemistry of tetrahedral hydrogen-free amorphous carbon coatings in the presence of OH-containing lubricants. Lubr. Sci 2008;20:137–49. [28] Ye J, Okamoto Y, Yasuda Y. Direct insight into near-frictionless behavior displayed by diamond-like carbon coatings in lubricants. Tribol Lett 2008; 29:53–6. [29] Minami I, Kubo T, Nanao H, Mori S, Sagawa T, Okuda S. Investigation of tribochemistry by means of stable isotopic tracers. Part 2. Lubrication mechanism of friction modifiers on diamond-like carbon. Tribol Trans 2007;50:477–87. [30] Mabuchi Y, Higuchi T, Weihnacht V. Effect of sp2/sp3 bonding ratio and nitrogen content on friction properties of hydrogen-free DLC coatings. Tribol Int 2013;62:130–40. [31] Mabuchi Y, Higuchi T, Inagaki Y, Kousaka H, Umehara N. Wear analysis of hydrogen-free diamond-like carbon coatings under a lubricated condition. Wear 2013;298–299:48–56. [32] Tasdemir HA, Wakayama M, Tokoroyama T, Kousaka H, Umehara N, Mabuchi Y, et al. Ultra-low friction of tetrahedral amorphous diamond-like carbon (taC DLC) under boundary lubrication in poly alpha-olefin (PAO) with additives. Tribol Int 2013;65:286–94. [33] Olesinski RW, Abbaschian GJ. Bulletin of Alloy Phase Diagram 1984;5:484. [34] Tasdemir HA, Wakayama M, Tokoroyama T, Kousaka H, Umehara N, Mabuchi Y, et al. Wear behaviour of tetrahedral amorphous diamond-like carbon (ta-C DLC) in additive containing lubricants. Wear 2013;307:1–9. [35] Morita Y, Shibata T, Onodera T, Sahnoun R, Koyama M, Tsuboi H, et al. Effect of surface termination on superlow friction of diamond film: a theoretical study. Jpn J Appl Phys 2008;47(4). [36] Topolovec Miklozic Ksenija, Reg Forbus T, Spikes Hugh A. Performance of friction modifiers on zddp-generated surfaces. Tribol Trans 2007;50:328–35.