Concert and Blocking Effects of Polar Compounds on the Friction Reduction and Tribofilm Formation of Zinc Dialkyldithiophosphate

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1 Tribology nline, 11, 2 (216) IN DI /trol Article Concert and Blocking Effects of olar Compounds on the Friction Reduction and Tribofilm Formation of Zinc Dialkyldithiophosphate Yuji Matsui 1,2), aiko Aoki 1), samu Kurosawa 2) and Masabumi Masuko 1)* 1) Department of Chemical Engineering, Tokyo Institute of Technology 1-11, okayama 2-chome, Meguro-ku, Tokyo , Japan 2) Lubricants Research Laboratory, JX Nippon il & Energy Corporation * Corresponding author: mmasuko@chemeng.titech.ac.jp ( Manuscript received 3 August 215; accepted 15 March 216; published 3 April 216 ) ( resented at the International Tribology Conference Tokyo 215, 16-2 eptember, 215 ) The final target of this study is to discover the concert effect between DT and ashless FMs (polar compounds) that improve friction reducing performance together with keeping the good antiwear performance of DT. Evaluation of tribological performance and DT tribofilm formation were performed with a ball-on-disk type rolling-sliding tribometer. The tribofilm generated on the disk was analyzed by FT-IR analysis, Auger Electron pectroscopy (AE) and Electron robe Micro Analysis (EMA). It was found that amino compounds showed a blocking effect, in which the DT tribofilm formation was prevented and the friction coefficient increased, whereas carboxyl compounds showed a concert effect, in which the friction coefficient was reduced with maintaining DT tribofilm formation. Keywords: lubrication, tribofilms, DT, surface analysis, EMA, AE, FT-IR 1. Introduction ince zinc dialkyldithiophosphate (DT) has been well-known to show supreme antioxidant and antiwear performances, it is widely used in many lubricating oils such as automotive engine oils, hydraulic oils, etc [1,2]. DT is known to prevent direct contact of surfaces by forming polyphosphate tribofilm resulting in good antiwear or antiscuffing action [3-8]. The phosphate tribofilm does not cover uniformly the rubbed surface, and shows patchy island-like morphology [8]. In addition, boundary friction of this film is known to be high [2]. Although it was reported that DT with long alkyl chain could reduce friction coefficient [9,1], the reduction is not enough and further reduction of the friction coefficient is still required. In case of the automotive engine oils, strong requirement of reducing DT concentration due to the exhaust catalyst poisoning is also a big issue to keep the good antiwear performance. However, since demand of the usage of lower viscosity engine oils is increasing to attain new fuel efficiency specifications, the antiwear performance of DT should be ensured, because lowering of engine oil viscosity expands boundary lubrication region [11]. DT produces sulfuric acid in engine oil during use that promotes the consumption of metal detergent in engine oil [1,12]. Generally, various additives are formulated together with DT to fulfill the requirements in engine oils. Typical additives are antioxidant (A), viscosity index improver (VII), detergent and dispersant (DD), and friction modifier (FM). Each formulated additive mixed together in the lubricating oils is expected to show its own good performance. However, it has been already pointed out that DT suffered interference from coexistenting dispersant [13-16]. Although molybdenum dithiocarbamate (MoDTC) formulated together with DT gives extremely low friction, seeking alternative ashless FM is also required from the durability aspect [17-2]. A viscosity index improver (VII) is an indispensable additive for the engine oils to obtain a good temperature-viscosity characteristic, and attracts further attention in case of using lower viscosity base oil. olyalkylmethacrylates (MA) known to be a good VII were found to possess to reduce boundary friction coefficient by forming adsorbed molecular layers on the friction surface [21,22]. If the formulated additives can decrease the friction coefficient and does not interfere or hopefully promote the antiwear performance of DT, the dose level of DT is possible to be reduced. In this study, influence of the coexisting additives in engine oil on the DT-performance was investigated. The final target of this study is to discover the concert Copyright 216 Japanese ociety of Tribologists 417

2 Yuji Matsui, aiko Aoki, samu Kurosawa and Masabumi Masuko effect between DT and ashless FMs (polar compounds) that improve friction reducing performance together with keeping the good antiwear performance of DT. To provide the pathway towards the final target, effect of polar groups such as amino, hydroxyl, and carboxyl groups on the friction reducing performance and tribofilm formation of DT was studied. The quantity of DT in engine oil can be reduced if the coexisting additives have concert effect. 2. Experimental procedure 2.1. Experimental methods A ball-on-disk type rolling-sliding tribometer was used. Both a ball (dia mm) made of high carbon-chromium bearing steel (JI-UJ2), and a disk (D mm, R a =.2 µm) made of carbon tool steel (K85) were rotated independently to give the entrainment speed of 8 mm/s and a slide-to-role ratio of 1% that is the tribocondition of this study. Figure 1 shows a schematic illustration and an external view of the sliding parts of the tribometer. The applied load was 63.4 N (mean Hertzian pressure of 667 Ma) and oil temperature was 1 C. olyalphaolefin (3.88 mm/s 1 C) was used as a base oil. Friction time was 24 h because the tribofilm produced by DT had been considered in a steady state by the observed friction coefficient trace ample oil and additives As a representative of DT additive, primary-c8 DT (R = 2-ethylhexyl) was used. As coexisting polar compounds, followings were used. Amino compounds: polyamino polyisobutenyl-succinimides [Dispersant], amino-functionalized polyalkylmethacrylates (MA-N [VII]), dimethyloctadecylamine (DMDA [FM]). Hydroxyl compounds: hydroxy-functionalized polyalkylmethacrylates (MA-H [VII]), glycerinmono-oleate (GM [FM]). Carboxyl compounds: carboxyl-functionalized polyalkylmethacrylates (MA-CH [VII]), stearic acid (ta [FM]), oleic acid (la [FM]). Mean molecular weight of all MAs was 2,. Concentration of the MA-N, H, and CH groups was approximately 2 mol%. Figure 2 shows chemical structures of all polar compounds and DT used in this study. Concentration of additives at each tests are shown in Table 1. After the friction tests, the tribofilm generated on the disk was analyzed by micro FT-IRRA analysis, Auger Electron pectroscopy (AE) and Electron robe Micro Analysis (EMA) FT-IRRA analysis (FT-IR) FT-IRRA analysis was used to measure the quantity of the phosphate-based tribofilm generated on the disk from DT. Each disk was analyzed at four different positions with the aperture size of 3 μm 3 µm. The vibration spectra of the phosphate chemical bond appear in the range between 1 cm -1 and 13 cm -1 ; and the Fig. 1 chematic illustration and an external view of the sliding parts of the tribometer Fig. 2 Chemical structures of all additives Japanese ociety of Tribologists ( Tribology nline, Vol. 11, No. 2 (216) / 418

3 Concert and Blocking Effects of olar Compounds on the Friction Reduction and Tribofilm Formation of Zinc Dialkyldithiophosphate Table 1 Lubricants tested DT olar compounds 1 DT (.1 mass% ) Dispersant (2 mass%) 2 DT (.1 mass% ) MA-N (2 mass%) 3 DT (.1 mass% ) DMDA (2 mass%) 4 DT (.1 mass% ) MA-H (1 mass%) 5 DT (.1 mass% ) GM (1 mass%) 6 DT (.1 mass% ) MA-CH (2 mass%) 7 DT (.1 mass% ) ta (.17 mass%) 8 DT (.1 mass% ) leic Acid (1 mass%) quantity of phosphate formed on the friction surface was compared relatively with the peak area in this region by averaging the peak area obtained at the four positions Auger electron spectroscopy (AE) AE is analysis technique to understand the chemical composition of the tribofilm. The thickness of the tribofilm generated on the steel surface was determined using AE by performing Ar sputtering. rior to AE, the disk was degreased by immersion several times in toluene with ultrasonic cleaning and photographed by scanning electron microscope (EM). The size of the EM image was 25 µm 25 µm. ince DT produces non-uniform island-liked tribofilm on the steel surface [2-6], three positions whose each size was 3 µm 3 µm were selected in each EM image for the analysis of chemical composition. The analyzed positions were determined by the appearance of EM images that might have different thicknesses. Analyzed elements were phosphorous, oxygen, sulfur, iron and carbon, under 3 kev of accelerating voltage and 5 na of illuminating current. In addition, depth direction analysis was performed every 4.2 nm by Ar sputtering until the tribofilm disappeared Electron probe micro analysis (EMA) Chemical compositions and 2-dimensional distribution of elements of the tribofilm on the disk specimen were analyzed by EMA. rior to the EMA, the disk specimens were washed in the same manner as for the AE analysis. The tribo-tested disks were photographed by an optical microscope beforehand, and EMA was performed at the same position. Analyzed elements were phosphorus, oxygen, sulfur, iron, and carbon. The size of the measured area was 1 μm 2 μm that was pixelated into 5 (5 1). For the quantitative comparison of the elements among specimens, a histogram of output signals of each chemical element was made at each chemical image of 5 pixels. From the histogram, obtained mode value was used as a representative intensity of the elements at the observed tribofilm. 3. Results 3.1. Friction coefficient Figure 3 shows an example of the time course of friction coefficient for the test with DT alone. Average friction coefficient is obtained by using the data in the final 5 minutes before the end of the test. Figure 4 compares average friction coefficient at 24 h friction time. Friction coefficients of DT + Dispersant (denoted as +Dispersant) and DT + MA-N Friction coefficient Fig. 3 Friction coefficient Fig sliding distance, m Friction coefficient of DT alone DT + MA-H + MA-CH + MA-N + Dispersant + GM + DMDA + ta + la Average friction coefficient at 24 h friction time Japanese ociety of Tribologists ( Tribology nline, Vol. 11, No. 2 (216) / 419

4 Yuji Matsui, aiko Aoki, samu Kurosawa and Masabumi Masuko (denoted as +MA-N) were higher than that of DT alone. In contrast, DT + MA-CH (denoted as +MA-CH) and DT + FMs (+GM, +DMDA, +ta, +la) effectively reduced friction coefficient. MA-H did not show significant effect on friction coefficient AE results Figure 5 shows depth analyses of AE data of DT alone. Upper picture shows EM image of the tribosurface and three regions were selected for the depth analyses as indicated as colored rectangle. Those positions were selected as thick tribofilm, medium tribofilm, and thin tribofilm based on the appearance of the images. Bottom three figures show the results of the depth analysis, and the depth data shown in vertical axis denoted depth from the outermost surface. ince it was difficult to compare all chemical elements, we focused on the phosphorus element that was considered one of major key element of the tribofilm for further discussion. Analyzed data of thick tribofilm in Fig. 5 shows characteristic feature near the outermost surface that had higher than 15 at% of phosphorus concentration and the concentration was almost constant. This part prescribes deposited tribofilm layer, and the thickness of the tribofilm was determined with this film. In contrast, underneath the tribofilm, phosphorus concentration reduced gradually along the depth. This part prescribes diffusion layer. Film thickness of the tribofilm was obtained by this method and two analyzed data (thick tribofilm, medium tribofilm) were averaged in each EM image. Figure 6 shows the average film thickness of the tribofilm at 24 h friction time. Film thicknesses of the tribofilms obtained with VII, Dispersant, and ta shown in Fig. 6 were thicker than that of DT alone. In contrast, film thicknesses of the tribofilms obtained with DMDA and GM shown in Fig. 6 were thinner than that of DT alone. Figure 7 shows EM images and analyzed positions by AE analysis. Difference in morphology of tribofilms among additives in Fig. 7 is recognized. ize of the island structure of tribofilms obtained with DT + Dispersant, DT + MA-N, DT + GM and DT + DMDA were smaller than that from DT alone. DT with amino-functionalized additives, GM or DMDA produced small patchy island-like tribofilm, which was thicker than DT tribofilm. n the other hand, size of the island structure of the tribofilms from DT + MA-H, DT + MA-CH and DT + ta were film thickness, nm Fig. 6 DT +MA-CH +MA-H +MA-N +Dispersant +DMDA +ta +GM Film thickness of tribofilm at 24 h friction test Thick tribofilm Medium tribofilm 3 μm Thin tribofilm 25 μm Depth,nm 5 1 Fe Concentration,at % Depth,nm 5 1 Fe Concentration,at% Fig. 5 AE data of DT alone Depth,nm 5 1 Fe Concentration,at% Japanese ociety of Tribologists ( Tribology nline, Vol. 11, No. 2 (216) / 42

5 Concert and Blocking Effects of olar Compounds on the Friction Reduction and Tribofilm Formation of Zinc Dialkyldithiophosphate DT +Dispersant +MA-H +MA-N +MA-CH +GM +DMDA +ta Fig. 7 EM images and analyzed positions by AE analysis at 24 h friction test ptical DT 24h + Disp 24h Microscope + MA-H 24h + MA-N 24h + MA-CH 24h ptical Microscope + GM 24h + DMDA 24h + ta 24h ptical Microscope Fig. 8 Images by optical microscope and EMA analysis at 24 h friction test Japanese ociety of Tribologists ( Tribology nline, Vol. 11, No. 2 (216) / 421

6 Yuji Matsui, aiko Aoki, samu Kurosawa and Masabumi Masuko similar to that from DT alone. These compounds promoted thickness of tribofilm without reduction of the size of the island in the tribofilm EMA results The analysis that can sweep over the whole area of the tribofilm is also important. Figure 8 shows images of 2-dimensional distribution of each element by EMA and images by optical microscope. DT produced island-like tribofilm regardless of a kind of polar compounds but all images showed some difference in distribution of chemical elements in the tribofilm. For the focus on the distribution of tribofilm, phosphorous that is the main constituent of the tribofilm was analyzed in more detail by the methods explained in 2.5. Figure 9 shows an example of histogram at 24h with DT alone. The horizontal axis shows signal intensity of phosphorus, and the vertical axis is the surface area. The signal intensity that the surface area at which the surface area is largest is the mode value. Figure 1 shows the mode intensity of phosphorus at 24h; and Fig. 11 shows the mode intensity of phosphorus in each friction time (1 h, 4 h, 7 h, and 24 h). The quantity of phosphorus in the tribofilm generated with DT + Dispersant, DT + MA-N and DT + FMs was lower than that with DT alone in Fig. 1. This result shows that the amino-functionalized additives and FMs produce non-uniform island-liked tribofilm. In contrast, the quantity of phosphorus in the tribofilm generated with DT + MA-CH was very similar to DT alone. MA-CH did not obstruct the formation of DT-derived tribofilm. The quantity of phosphorus generated with DT + MA-H was lower than that with DT alone at 1 h, but it was very similar to that with DT alone at 24 h in Fig. 11. DT + MA-H had the potential for an increasing wear because it did not produce the tribofilm in early period of friction FT-IR results As explained in 3.2 and 3.3, DT + MA-CH produced the tribofilm like DT alone, but, in contrast, DT + ta did not (Fig. 1). In order to study the influence of the carboxyl group in detail, oleic acid was also used. The tribofilms formed by DT + FMs at 24 h friction time were analyzed by FT-IR. Figure 12 shows peak intensity of - chemical bond-related absorption peak (peak area of 1-13 cm -1 ) on the disk that is a measure of the quantity of tribofilm formed. The tribofilms formed by DT + GM and DT + DMDA were less than that with DT alone, whereas DT + la maintained tribofilm formation. Tribofilm produced by DT + ta was nearly equal to that of DT alone, which shows that ta did not greatly obstruct tribofilm formation. It can be said from these results that carboxyl-functionalized additives produce enough tribofilm with reducing friction coefficient. surface area, μm 2 Fig. 9 Mode intensity of, AU signal intensity, AU Histogram of phosphate with EMA at 24 h friction test by DT alone DT +MA-CH +MA-H +MA-N +Dispersant +DMDA +ta +GM Fig. 1 Mode intensity of phosphorus by EMA at 24 h friction test Fig. 11 Mode intensity of phosphorus by EMA FT-IR peak intensity, AU DT + DMDA eak intensity Friction coefficient + GM + la + ta Friction coefficient Fig. 12 eak intensity of - bond and friction coefficient at 24 h friction test Japanese ociety of Tribologists ( Tribology nline, Vol. 11, No. 2 (216) / 422

7 Concert and Blocking Effects of olar Compounds on the Friction Reduction and Tribofilm Formation of Zinc Dialkyldithiophosphate 4. Discussion 4.1. Blocking effect by amino-functionalized additives Thickness of the tribofilm with DT + Dispersant was close to that with DT alone by AE (Fig. 6). This result shows that the quantity of tribofilm with DT + Dispersant was approximately equal to DT alone by the local observation of the tribofilm. From the results of EMA, it was found that DT + Dispersant partially produced the tribofilm compared with DT alone (Fig. 11). The distribution of the tribofilm that could not be analyzed by AE showed that Dispersant has blocking effect on the DT-derived tribofilm formation. It was difficult to grasp the tribofilm formation only by narrow local observation such as AE because DT produced non-uniform island-like tribofilm. It is necessary to pay attention to analyze the distribution of tribofilm. In addition, friction coefficient of DT + Dispersant was higher than that of DT alone (Fig. 4). From the observation, we thought that if Dispersant adsorbed at the polar groups in its molecule and formed molecular films on the friction surface, that might bring friction reduction [15]. Although we did not have any experimental evidences of the reason for increasing friction coefficient by DT + Dispersant, we thought that no tribofilm, which protected the steel surface, could be formed by the blocking effect of Dispersant. If some kind of adsorbed molecular films existed on the steel surface, the friction coefficient should be decreased. DT + MA-N increased friction coefficient and did not produce sufficient tribofilm. They are similar to DT + Dispersant. The amino group, which was the common group of two additives, brought a blocking effect Concert effect by MA-CH DT + MA-CH produced thick tribofilm (Fig. 6). The tribofilm formation process of DT + MA-CH resembles that of DT remarkably (Fig. 11). MA-CH did not block the formation of the tribofilm. Friction coefficient by DT + MA-CH is lower than that with DT alone (Fig. 4). It is thought that some kind of molecular film of MA-CH having friction reduction effect was formed on the contacted surface because the boundary friction of the DT-derived tribofilm itself is relatively high. Figure 13 shows a model of steel surface phenomenon by DT + MA-CH. MA-CH might have friction reduction effect to adsorb on tribofilm because MA has friction reduction effect by adsorption on the steel surface [21,22]. Inspection in detail will be necessary in future whether such a phenomenon is occurred FMs effects and concert effect by carboxylfunctionalized additives FMs showed large friction reduction (Fig. 4). FMs generally produced monomolecular adsorbed film on the Tribofilm teel MA-CH Fig. 13 Mechanism of film formation with DT+MA-CH steel surface [18]. Friction reduction by all FMs was provided by this film. DT + DMDA and DT + GM did not produce any DT-derived tribofilm (Figs. 6,1,12). This means that DMDA and GM molecules preferentially adsorbed on the steel surface so that DT-derived tribofilm could not be formed. n the other hand, tribofilm was produced with DT + ta or DT + la (Fig. 12). However, ta produced patchy tribofilm (Fig. 1). This means that la did not preferentially adsorbed on the steel surface, whereas ta did. It is thought that this effect of ta is weaker than that of GM and DMDA so that ta partially produced the tribofilm. It is thought that carboxyl group does not greatly obstruct the tribofilm formation. Both ta and la could form monomolecular adsorbed film on the DT-derived tribofilm to bring friction reduction. Carboxyl-functionalized additives, which were ta, la, and MA-CH, produced both DT-derived tribofilm and adsorbed molecular film. This effect of carboxyl-functionalized additives could be called a concert effect, in which the friction coefficient was reduced with maintaining DT-derived tribofilm formation Effect of hydroxyl-functionalized additives The quantity and thickness of the tribofilm by DT + MA-H were similar to these by DT alone at 24 h friction time (Figs. 6,1). As shown in Fig. 11, MA-H generated thin tribofilm in the early period of the friction test but gradually increased the quantity of tribofilm that was similar to DT alone and DT + MA-CH. In addition, as shown in Figs. 6,1,12, tribofilm was hardly generated at 24 h test with GM. Although GM has the same hydroxyl group as MA-H, the tribofilm formation of DT + GM at 24 h was different from that of DT + MA-H. robably, it was due to the structure of the adsorbed molecular film, since GM might produce denser adsorbed molecular layer than MA-H. Therefore, GM obstructed the formation of the DT-derived tribofilm unlike MA-H. 5. Conclusion This study compared coexisting effect of polar compounds with DT on the tribological performance. In particular, friction coefficient and the quantity of tribofilm were analyzed in detail. From the results of this Japanese ociety of Tribologists ( Tribology nline, Vol. 11, No. 2 (216) / 423

8 Yuji Matsui, aiko Aoki, samu Kurosawa and Masabumi Masuko study, the followings became clear. Amino-functionalized additives increased friction coefficient because any molecular adsorption film was not produced on frictional surface. In addition, the blocking effect of tribofilm became clear by investigating the distribution of tribofilm. Amino-functionalized additives showed a blocking effect, in which the DT-derived tribofilm formation was prevented and the friction coefficient increased. Carboxyl-functionalized additives like MA-CH and la showed a concert effect, in which the friction coefficient was reduced with maintaining DT tribofilm formation. In addition, ta did not greatly obstruct the tribofilm formation and reduced friction coefficient. Carboxyl-functionalized additives produced both DT-derived tribofilm and adsorbed molecular film. DT + MA-H produced thick tribofilm at 24 h of the friction test. However, DT + GM hardly produced tribofilm and reduced friction coefficient at 24 h of the friction test. Acknowledgements The authors would like to thank JX Nippon il & Energy Corporation (Funding and additive supply), ANY CHEMICAL INDUTRY (MA additives supply), Center for Advanced Materials Analysis at Tokyo Tech (urface Analyses) for supporting the study described in this article. References [1] Yagishita, K., New Evolution of the Engine il Additives, Journal of Japanese ociety of Tribologists, 53, 3, 28, (in Japanese). [2] pikes, H., The History and Mechanisms of ZDD, Tribology Letters, 17, 3, 24, [3] iras, F. M., Rossi, A. and pencer, N, D., Combined In itu (ATR FT-IR) and Ex itu (X) tudy of the DT-Iron urface Interaction, Tribology Letters, 15, 3, 23, [4] Willermet,. A., Dailey, D.., Carter III, R.., chmitz,. J. and Zhu, W., Mechanism of Formation of Antiwear Films from Zinc Dialkyldithiophosphates, Tribology International, 28, 3, 1995, [5] Komvopoulos, K., ennecot, G., Yamaguchi, E.. and Yeh,. W., Antiwear roperties of Blends Containing Mixtures of Zinc Dialkyl Dithiophosphate and Different Detergents, Tribology Transactions, 52, 1, 28, [6] Yamaguchi, E.., Roby,. H. and Yeh,. W., Time-Dependent Film Formation from DTs and Nonphosphorus Antiwear Agents, Tribology Transactions, 48, 1, 25, [7] Martin, J. M., Grossiord, C., Le Mogne, T., Bec,. and Tonck, A., The Two-Layer tructure of dtp Tribofilms: art I: AE, X and XANE Analyses, Tribology International, 34, 8, 21, [8] Yamaguchi, E.. and Ryason,. R., tructures of Adsorbed Zinc Dithiophosphates and their Relationship to Engine Wear, Tribotest, 3, 2, 1996, [9] Aoki,., uzuki, A. and Masuko, M., Comparison of liding peed Dependency of Friction between teel urfaces Lubricated with everal DTs with Different Hydrocarbon Moieties, roceedings of the Institution of Mechanical Engineers, art J: Journal of Engineering Tribology, 22, 4, 26, [1] Born, M., Hipeaux, J. C., Marchand,. and arc, G., The Relationship between Chemical tructure and Effectiveness of ome Metallic Dialkyl- and Diaryl-Dithiophosphates in Different Lubricated Mechanisms, Lubrication cience, 4, 2, 1992, [11] lumley, M. J., Wong, V., Molewyk, M. and ark,. Y., ptimizing Base il Viscosity Temperature Dependence For ower Cylinder Friction Reduction, AE Technical aper, 214, [12] Nagatomi, E., Long Life Technology for Automotive Engine il, Journal of Japanese ociety of Tribologists, 53, 4, 28, (in Japanese). [13] Martin, J. M., Grossiord, C., Le Mogne, T. and Igarashi, J., Role of Nitrogen in Tribochemical Interaction between dtp and uccinimide in Boundary Lubrication, Tribology International, 33, 7, 2, [14] Vengudusamy, B., Grafl, A., Novotny-Farkas, F., chimmel, T. and Adam, K., Tribological Behaviour of Antiwear Additives Used in Hydraulic Applications: ynergistic or Antagonistic with ther urface-active Additives?, Tribology International, 67, 213, [15] Fujita, H., Glovnea, R.. and pikes, H. A., tudy of Zinc Dialkydithiophosphate Antiwear Film Formation and Removal rocesses, art I: Experimental, Tribology Transactions, 48, 4, 25, [16] Roshan, R., riest, M., Neville, A., Morina, A., Xia, X., Warrens, C.. and. ayne, M. J., A Boundary Lubrication Friction Model ensitive to Detailed Engine il Formulation in an Automotive Cam/Follower Interface, Journal of Tribology, 133, 4, 211, [17] Komvopoulos, K., ernama,. A., Yamaguchi, E.. and Ryason,. R., Friction Reduction and Antiwear Capacity of Engine il Blends Containing Zinc Dialkyl Dithiophosphate and Japanese ociety of Tribologists ( Tribology nline, Vol. 11, No. 2 (216) / 424

9 Concert and Blocking Effects of olar Compounds on the Friction Reduction and Tribofilm Formation of Zinc Dialkyldithiophosphate Molybdenum-Complex Additives, Tribology Transactions, 49, 2, 26, [18] Ratoi, M., Niste, V. B., Alghawel, H., uen, Y. F. and Nelson, K., The Impact of rganic Friction Modifiers on Engine il Tribofilms, RC Advances, 4, 9, 214, [19] Lin,., Barber, G., Zou, Q., Anderson Jr, A. H., Tung,. and Quintana, A., Friction and Wear of Low-hosphorus Engine ils with Additional Molybdenum and Boron Compounds, Measured on a Reciprocating Lubricant Tester, Tribology Transactions, 51, 5, 28, [2] Muraki, M., Yanagi, Y. and akaguchi, K., ynergistic Effect on Frictional Characteristics under Rolling-liding Conditions due to a Combination of Molybdenum Dialkyldithiocarbamate and Zinc Dialkyldithiophosphate, Tribology International, 3, 1, 1997, [21] Tohyama, M., hmori, T., Murase, A. and Masuko, M., Friction Reducing Effect of Multiply Adsorptive rganic olymer, Tribology International, 42, 6, 29, [22] Fan, J., Müller, M., töhr, T. and pikes, H. A., Reduction of Friction by Functionalized Viscosity Index Improvers, Tribology Letters, 28, 3, 27, Japanese ociety of Tribologists ( Tribology nline, Vol. 11, No. 2 (216) / 425