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1 Tribology Online Japanese Society of Tribologists Vol. 12, No. 3 (2017) ISSN DOI /trol Article Advanced Control of Frictional Properties on Paper Clutch Materials by a Combination of Friction Modifiers Go Tatsumi *, Shinji Hasegawa and Yasushi Onumata Lubricant Research Laboratory, JXTG Nippon Oil & Energy Corporation, 8 Chidoricho, Naka ku, Yokohama , Japan *Corresponding author: Go Tatsumi (tatsumi.go@jxtg.com) Manuscript received 28 February 2017; accepted 21 April 2017; published 31 July 2017 Abstract This study proposes a novel approach for controlling frictional performances at paper clutch systems by a combination of organic friction modifiers (OFMs). The OFMs are commonly used for modifying the frictional properties at the paper clutch. Although the effect of the OFMs tends to be dependent on temperature due to their working mechanism based on the surface adsorption, the frictional properties are preferable to be stable in all the operating temperature for the consistent and precise clutch control. Aiming to modify the temperature dependence of the OFM effect, an OFM with an Advanced Concept (FMAC) was newly developed, and the impact on the clutch frictional performance was investigated using Low Velocity Friction Apparatus (LVFA). Only with the conventional OFM, the friction values experienced excessive reduction at 80 or 120 C, while an optimal property was achieved at 40 C. In the presence of the FMAC, it was possible to inhibit the conventional OFM selectively at the high temperature conditions preventing the friction reduction, leading to the ideal frictional property in all the temperature conditions. The surface analysis revealed that the FMACs were capable of adsorbing on the substrate more intensively at high temperature, which should be the reason of the temperature dependent competitive effect of the FMACs. Keywords lubricant, automatic transmission fluid (ATF), friction modifier (FM), low velocity friction apparatus (LVFA) 1 Introduction Fuel efficiency has been one of the most important performances for automobiles. Car manufacturers put a priority for dealing with legislations on CO2 emissions, while it is also a motivation for the customers to cut the fuel cost off. In terms of the transmission systems, there have been a wide range of challenges for improving fuel efficiency. There are two major approaches; to minimize the efficiency loss at the current system or to apply a new fuel efficient control. For the former one, low viscosity lubricants are effective for reducing the churning losses caused by the lubricants inside the transmissions [1], and there are various commercial products which applied this concept. However, further viscosity reduction appears to be difficult because the lowering viscosity leads to a thinner oil film formation on the substrate, which possibly has a negative influence on anti wear or anti fatigue performance. The second approach, the fuel efficient control should be represented by the operation of a lock up clutch inside a torque converter. It was initially installed for the lock up operation, which directly connects the engine input to the gear shift systems skipping the torque amplification fluid coupling mechanism [2,3]. This operation is capable of reducing the churning loss at the torque converter, resulting in a better fuel efficiency. In addition, a sliding control of the lock up clutch was developed for the purpose of further fuel economy. On the sliding control, the engine power can be transferred partially thorough the sliding clutch, so that it is possible to achieve the best balance between the churning loss reduction and the torque amplification effect. This control requires an appropriate frictional property for the sliding surfaces between the clutch and the counter steel plate in order to prevent noise or vibration issues, so called shudder. There have been several attempts to mathematically analyze the sliding control, and it was clarified that a relationship between the sliding speed and the friction coefficient, μ V characteristics, is essential [4]. A smooth clutch sliding control can be achieved when the μ V relationship shows a positive slope (the friction value becomes higher as the sliding speed increases), while the negative slope possibly causes shudder. Furthermore, the overall friction at the clutch system needs to be maintained at the appropriate values in order to control the clutch engagement Copyright 2017 Japanese Society of Tribologists This article is distributed under the terms of the latest version of CC BY NC ND defined by the Creative Commons Attribution License. 103

2 Go Tatsumi, Shinji Hasegawa and Yasushi Onumata correctly. Taking into account a recent trend of the clutch sliding control region expanding for the fuel economy [5], it is necessary to manage the μ V properties precisely suitable for a wide range of the driving conditions. Automatic transmission fluids (ATFs) play an important role for the friction modification at the lock up clutch system. Among the additives, organic friction modifiers (OFMs) have a significant influence on the μ V characteristics. It generally consists of a polar group capable of adsorbing on the substrate and a hydrocarbon chain structure [6,7]. Although various types of the OFMs are commercially used depending on the frictional requirements, the issue is that their friction modification effect is often influenced by test conditions, such as temperature. It is known that the film formation of the OFMs onto the substrate is controlled by both the polar group and the hydrocarbon chain structures, and temperature is a factor which affects the film stability [8]. Tasdemir et al. [9] investigated the frictional and wear prevention performance of glycerol mono oleate (GMO) on ta C DLC. It was concluded that the effect of GMO is significantly affected by the test temperature, and it appears to be more effective between 50 and 80 C. Moreover, Zhao et al. [10] reported the influence of temperature on the μ V characteristics of the paper clutch material. The effect of an ester and amine type OFMs on the anti shudder property was examined using the clutch friction tester, and it was found that the performance is dependent on the temperature conditions as well as the polar group structure of the OFMs. In the study of Ingram et al. [11], the similar results regarding the influence of temperature on the clutch friction are also reported. This temperature dependence of the OFMs is considered to be a disadvantage for achieving the target clutch frictional properties in a wide range of the driving conditions, and it is ideal to show a stable performance independent on temperature. One of the solutions should be the use of competitive effect of the OFMs against the other additives. It is commonly known that an interaction between the OFMs and the additives may change the frictional properties. Ratoi et al. [12] reported the interaction between OFMs and ZDDP. The friction reduction effect of three types of the OFMs were inhibited in the presence of ZDDP, while the reaction film thickness derived by ZDDP became thinner when the OFMs were added. It was also mentioned that the chemical structure of the OFMs affected the competitive effect against ZDDP. Derevjanik [13] investigated the friction properties of some combinations of OFMs and metal detergents using metal or paper clutch specimens, and both synergetic and competitive effects were observed dependent on the combination. Interestingly, the effective combination was different among the test temperature conditions. In addition, Vengudusamy et al. [14] reported the influence of the gear oil additives on the friction reduction effect of the OFMs. This study aims to utilize these interactions actively in order to achieve the appropriate frictional properties in a wide range of the temperature conditions. A new type of the OFM named FM with an Advanced Concept (FMAC) was developed, which consists of a polar group and hydrocarbon chains similar to the conventional OFMs. The difference is the structure of the hydrocarbon chains that is branched to prevent the friction modification effect of the OFMs [15]. In addition, taking into account that the OFMs generally work more intensively at higher temperature, the polar group of the FMACs was designed to compete with the OFMs only at the high temperature condition. This is a novel approach for controlling the effect of the OFMs. The advantage is the selective interaction with the OFMs, which may realize an ideal friction control without changing the other additives. It should be essential to satisfy the complicated frictional properties that will be required for the advanced clutch systems in future. 2 Experimental methods 2.1 Concept of FMACs and test oil formulations The OFMs generally consist of a polar group and linear chain hydrocarbons [7]. It is considered that the polar group is capable of adsorbing onto the substrate and the straight hydrocarbon structure can form a closely packed surface adsorption film which is effective for separating sliding surfaces [16]. The FMAC has a similar chemical structure to the OFMs except that the hydrocarbon chain is branched. The concept is to disrupt the excess film formation by the OFMs, which possibly causes too intensive friction reduction at the paper clutch system especially in a high temperature condition. The schematic images of the expected working mechanism of the FMAC are shown in Fig. 1. In case the film formation of the OFM is too excessive, it is expected that a competitive adsorption between the OFM and the FMAC adjusts the surface film condition appropriate for the required frictional property. The chemical structures of the OFM and the FMACs investigated in this study are shown in Fig. 2. Oleyl diethanolamine (ODEA) was applied as an OFM, which consists of two hydroxyl groups attached to the oleyl amine structure. Fig. 1 Fig. 2 Schematic images of a) OFM adsorption and b) expected surface condition with the FMAC Chemical structures of a) ODEA and b) the FMACs Japanese Society of Tribologists ( Tribology Online, Vol. 12, No. 3 (2017) / 104

3 Advanced Control of Frictional Properties on Paper Clutch Materials by a Combination of Friction Modifiers Three types of the FMACs, FMAC A, FMAC B and FMAC C, were designed for this study. They all have a polyamine polar group in common, while the structure of the alkyl chains was different. Firstly, FMAC A has a highly branched alkyl chain, which is expected to inhibit the OFM effect significantly. Then, the chain structure of FMAC B has only one branch, hence it may show a mild friction modification effect as well as competing with the OFM. Finally, FMAC C with a linear alkyl chain was prepared as a reference to estimate the working mechanism of the FMACs. Considering its structure similar to conventional OFMs, the friction characteristics of FMAC C should be helpful to estimate the temperature dependence of the polyamine group adsorption on the surface. The effect of the FMACs was investigated in the conventional ATF formulation as summarized in Table 1. The base ATF sample, shown as Base ATF in the table, consists of Group III base stock and typical ATF additives; phosphorous anti wear agent (P: 300 ppm), overbased calcium sulfonate (Ca: 100 ppm), polymethacrylate viscosity modifier, polybutenyl succinimide dispersant, anti oxidant, corrosion inhibitor and defoamer. OFMs were not included in this base ATF in order to evaluate a pure effect of the FMACs on the frictional properties. FMAC A, FMAC B and FMAC C were added in the base at 0.2 mass%, respectively. Furthermore, the interactive effect between the FMAC and ODEA was also investigated for FMAC A and FMAC B. 2.2 Tribotest Frictional properties of the FMACs were evaluated using Low Velocity Friction Apparatus (LVFA). A schematic layout of the LVFA tester is illustrated in Fig. 3. The test procedures were based on JASO M349:2012 [17], the test method for anti shudder performance of ATFs. A clutch plate (A795.D0AK) was attached at a shaft with a spline at the upper side of the apparatus, while a steel plate (FZ132 8 Y2) was placed in a reservoir at the lower side soaked in the test oil. The test conditions are summarized in Table 2. Following the 30 minutes of the breaking in process at 80 C, the μ V characteristics data were collected from low sliding speed (0.06 m/s) to high sliding speed (1.5 m/s) under constant Fig. 3 Schematic layout of LVFA pressure of 1.0 MPa. The influence of the test temperature on the frictional properties was investigated at three levels; 40, 80 and 120 C. The test at 80 or 120 C represents the transmission temperature at the cruise drive condition, while the 40 C measurement simulates the starting up. The frictional performance needs to be satisfied through all the driving conditions. 2.3 Surface analysis X ray Photoelectron Spectroscopy (XPS) was utilized using PHI Quantera SXM (Ulvac Phi, Inc.) to observe the surface chemistry of the FMAC on the steel plate. In order to detect clear peaks derived from the FMAC, simple formulations were used for the friction test; Poly α olefin (PAO) or PAO with 0.2 mass% of FMAC A. LVFA was also used to prepare the samples for XPS, but the condition was modified to be mild compared to that for the frictional performance test in order to prevent the damage on the specimen surface. The 30 minutes of the test was carried out at a constant speed of 0.3 m/s at 40 or 80 C. The post test samples were rinsed by toluene before the measurement. The XPS spectra were acquired using a monochromatic Al Kα source. The applied spot size was 100 μm, and C 1s detailed scan was performed. The peak positions were corrected by fitting the binding energy of saturated hydrocarbon (SHC) detected in the C 1s spectra at ev [18]. Since a sampling depth of XPS is generally a few nm [19], the results can reflect the chemical nature on the uppermost Table 1 Test oil formulations FMAC-A FMAC-B Base ATF FMAC-A FMAC-B FMAC-C ODEA + ODEA + ODEA Base ATF FMAC-A mass % FMAC-B mass % FMAC-C mass % 0.2 ODEA mass % Table 2 LVFA test condition Configuration Test specimens Breaking-in condition μ-v measurement Paper/steel pure sliding Steel plate (JASO M349) Paper clutch plate (JASO M349) 80 C, 1.0 MPa, 0.6 m/s, 30 minutes Temperature; 40, 80, 120 C Pressure; 1.0 MPa Velocity; 0.06, 0.12, 0.3, 0.6, 0.9, 1.2, 1.5 m/s Japanese Society of Tribologists ( Tribology Online, Vol. 12, No. 3 (2017) / 105

4 Go Tatsumi, Shinji Hasegawa and Yasushi Onumata substrate surface. Therefore, it should be suitable to observe the adsorption film of the FMACs formed on the steel surface. 3 Results and discussion 3.1 Frictional property of the FMACs The clutch frictional performances of the ATF base and the FMAC formulations are shown in Fig. 4. The graphs present μ V characteristics, a relationship between sliding speed and friction coefficient, at three different temperature conditions (40, 80 and 120 C) measured by LVFA. The base ATF (Fig. 4(a)) showed the friction values between 0.15 and 0.17, which was relatively higher compared to typical ATFs probably due to the absence of the OFMs. The μ V curve appeared to be flat at 120 C, while it was negative both at 40 and 80 C. The reason should be that the ATF additive, such as the dispersant or the calcium detergent, presented a weak friction modification effect at the high temperature condition [20]. FMAC A did not affect the μ V slope of the base ATF significantly (Fig. 4(b)), except that the curve at 120 C became slightly negative increasing the friction values at all the sliding speed. This result implies that FMAC A itself did not have a friction modification effect, but potentially inhibited the adsorption of the other additives at the high temperature condition. When FMAC B was added into the ATF (Fig. 4(c)), the relationship at 120 C turned to be positive, while that of 40 or 80 C did not change from the base ATF. It is assumed that FMAC B showed a mild friction modification effect, but it was not significant due to the branched hydrocarbon structure. In addition, it is considered that the polyamine polar group of the FMACs supposed to adsorb on the substrate intensively at the higher temperature condition. As expected from the molecular structure, FMAC C showed an obvious friction modification effect (Fig. 4(d)), leading to a positive μ V relationship at 80 and 120 C decreasing the friction values at low sliding speed. The effect was dependent on the temperature comparable to FMAC B, showing an intensive friction reduction at 120 C in contrast to the mild effect at 40 C. This result again indicates that the polyamine polar group is capable of affecting on the surfaces selectively at high temperature. 3.2 Interactive effect between the FMAC and the OFM The concept of this study is to manage the OFM effect, which tends to be dependent on the operating temperature, by the use of the FMACs. In order to observe the typical frictional performance of the OFMs, the μ V characteristics of ODEA in the base ATF is shown in Fig. 5(a). Compared to the property without ODEA (Fig. 4(a)), it reduced the friction values at all the sliding speed conditions, especially at the low speed region, resulting in a positive μ V relationship at all the temperature conditions. The issue is the effect appeared to be too intensive at 80 or 120 C for Fig. 4 μ V characteristics of a) base ATF, and the base ATF with b) FMAC A, c) FMAC B and d) FMAC C Fig. 5 μ V characteristics of the base ATF with a) ODEA, b) FMAC A + ODEA and c) FMAC B + ODEA Japanese Society of Tribologists ( Tribology Online, Vol. 12, No. 3 (2017) / 106

5 Advanced Control of Frictional Properties on Paper Clutch Materials by a Combination of Friction Modifiers Fig. 6 μ V characteristics of a) PAO and b) PAO + FMAC A Fig. 7 XPS C 1s spectra of the post test LVFA steel plate tested with PAO + FMAC A at a) 40 C and b) 80 C an optimal clutch control, because it possibly causes a clutch slip when the friction value is below the lower limit. Although it might be possible to adjust the friction at the high temperature conditions by reducing the ODEA concentration, the μ V slope at 40 C would become negative in this case due to the temperature dependent effect of ODEA. The ideal approach should be to take advantage of the additive interaction that is possible to decrease the ODEA effect only at high temperature, and the FMAC is the candidate. The interactive effect between the FMAC and ODEA was evaluated using FMAC A + ODEA and FMAC B + ODEA formulations shown in Table 1. The combination with FMAC C was not investigated because FMAC C itself indicated a sufficient friction modification effect, and the frictional properties were similar to those of ODEA. The μ V characteristics of FMAC A and FMAC B with ODEA are shown in Fig. 5(b,c), respectively. FMAC A had a significant impact on the frictional property of ODEA. The friction curve at 120 C pushed upward through all the speed range, and the increase of the friction value was larger at lower speed; for example, it was increased at approximately 0.04 at 0.06 m/s, while 0.03 at 1.5 m/s. Interestingly, the effect of FMAC A was negligible at 40 C, showing the μ V curve similar to that of ODEA. The influence at 80 C was intermediate between 120 and 40 C. The friction values became higher in all the speed conditions, but the increase was smaller compared to 120 C; for example, approximately at 0.06 m/s and 0.01 at 1.5 m/s. These results of FMAC A with ODEA indicate that FMAC A was able to disrupt the adsorption of ODEA, and the competitive effect became more intensive at the higher temperature condition. It agrees with the adsorption property of the FMACs, whose polyamine polar group appeared to form an effective surface film at high temperature as shown in Fig. 4. The effect of FMAC B with ODEA was similar to that of FMAC A, but the influence on the friction was milder (Fig. 5(c)). The increase in the friction value compared to ODEA at 0.06 m/s was approximately 0.02 and 0.01, respectively, at 80 and 120 C. In contrast, the impact on the friction curve at 40 C was very small similar to FMAC A. These trends were reasonable considering the individual effect of the FMACs shown in Fig. 4(b,c), which was derived from the hydrocarbon chain structure of the FMACs. Furthermore, the μ V characteristics of FMAC B + ODEA achieved almost the equivalent shape in all the temperature condition, which should be ideal for managing the frictional control at the clutch system. 3.3 XPS analysis The amount of the FMAC adsorbed onto the substrate was evaluated using XPS to confirm the working mechanism. The simple formulations without the ATF additives, PAO or PAO with 0.2 mass% of FMAC A, were prepared in order to clearly identify the XPS peak derived from FMAC A, since those from the ATF additives might overlap with it. Firstly, the μ V characteristics were measured following the conditions in Table 2. The 120 C condition was skipped because the specimens were damaged due to the absence of the ATF additives as well as the severe thermal condition. PAO had a negative curve at both 40 and 80 C as shown in Fig. 6(a) comparable to the previous study [21]. The overall frictional trend was the same when FMAC A was added (Fig. 6(b)), which confirms that FMAC A does not show a specific friction modification effect. For the XPS analysis, the post test steel plate was prepared by 30 minutes of a constant speed LVFA test at 0.3 m/s, 1.0 MPa, and 40 or 80 C. The selected speed at 0.3 m/s is where the temperature dependent effect of FMAC A was obviously observed as shown in Fig. 5, so that it is expected that the surface Japanese Society of Tribologists ( Tribology Online, Vol. 12, No. 3 (2017) / 107

6 Go Tatsumi, Shinji Hasegawa and Yasushi Onumata Table 3 XPS peak intensities in the C 1s spectra of PAO + FMAC A SHC C-O / C-N C-O-C O=C-N C-OH O-C-O / C=O O=C-O Peak position ev Peak intensity 40 C % C % Fig. 8 Assumed working mechanism of ODEA and the FMAC at the paper clutch system; a) ODEA at 40 C, b) ODEA with the FMAC at 40 C, c) ODEA at 120 C and d) ODEA with FMAC at 120 C chemistry of the FMAC would be different between 40 and 80 C. The XPS spectra of PAO + FMAC A with respect to C 1s detail scans are shown in Fig. 7 and Table 3. The peak positions identified in this study presented a good agreement with the report by Zhao et al. [22], which investigated the behavior of the OFMs with the nitrogen containing polar group on the steel plate after the clutch performance test. The C 1s spectra of the post test specimens after the 40 C test (Fig. 7(a)) and the 80 C test (Fig. 7(b)) had following peaks in common; ev derived from SHC, ev from C O / CN, ev from C O C / C OH, ev from O=C N / O C O / C=O and ev from O=C O. The difference between 40 and 80 C was found in the peak at ev, where the intensity at 80 C was almost doubled from that at 40 C. The peak position of ev is attributed to the adsorption of the amide groups contained in the polar group structure of FMAC A, it therefore implies that FMAC A formed more stable adsorption film at 80 C compared to 40 C. This result is consistent with the frictional properties shown in Figs. (4,5) which indicates that the FMACs worked more intensively at higher temperature. 3.4 Assumed working mechanism of the FMACs The schematic images of the assumed working mechanism of the FMACs are summarized in Fig. 8. ODEA showed an ideal FM effect at 40 C in contrast to the excess friction reduction at 120 C, which might have a negative influence on the clutch control. It is considered that ODEA covered the substrate surface intensively at the high temperature conditions; hence it is necessary to inhibit the adsorption of ODEA only at high temperature without affecting the behavior at 40 C. This study developed the FMACs in order to solve this issue. In the presence of the FMAC together with ODEA, the friction values were improved at the high temperature conditions without changing those at 40 C as shown in Fig. 5. In addition, the XPS results indicated that the polar group structure of the FMACs was capable of adsorbing more intensively on the substrate intensively at 80 C than 40 C. Based on these results, it is assumed that the FMACs competed with ODEA at high temperature, partially covering the substrate surface. Since the FMACs themselves did not have the significant friction reduction effect due to their branched alkyl chain structure, the effect of ODEA was weakened, resulting in the increase in the friction values. In contrast, at the low temperature condition, the FMACs did not interact with ODEA due to the small adsorption power, resulting in the similar friction trend to that of ODEA without the FMACs. It is notable that the competitive effect between the FMAC and the FM can be controlled by the alkyl chain structure of the FMAC. While FMAC A with the highly branched hydrocarbon chain increased the friction values at 120 C significantly, FMAC B with the single branched chain caused a moderate improvement achieving the frictional properties independent on temperature. 4 Conclusions The Friction Modifier with an Advanced Concept (FMAC) was newly developed in order to control the temperature dependence of the frictional performance at the paper clutch systems. The molecular structure of the FMACs investigated in this study consisted of the branched hydrocarbon chains and the polyamine polar group. Two FMACs with the different alkyl chain structure were developed; FMAC A with the highly branched chain, and FMAC B with the single branched one. In addition, FMAC C with the straight chain was also prepared as a Japanese Society of Tribologists ( Tribology Online, Vol. 12, No. 3 (2017) / 108

7 Advanced Control of Frictional Properties on Paper Clutch Materials by a Combination of Friction Modifiers reference. The frictional performance of the FMACs in the base ATF was evaluated using the LVFA clutch tester. FMAC A did not show the friction reduction in contrast to FMAC C that worked like a typical FM, and FMAC B presented the intermediate effect. These results appeared to reflect the chemical structure of the alkyl chains. In the presence of ODEA, FMAC A and FMAC B were able to improve the friction values selectively at 80 or 120 C, without affecting those at 40 C, which was effective to relax the temperature dependence of ODEA on the μ V characteristics. The XPS analysis of the steel specimen tested with FMAC A revealed that the polyamine polar group of the FMACs interacts with the steel surface more intensively at higher temperature. Therefore, it can be concluded that the competitive adsorption between the FMAC and ODEA, which occurred at the high temperature conditions, could realize the selective friction modification dependent on temperature. The influence of FMAC A on the friction at high temperature was more intensive compared to that of FMAC B. The alkyl chain structure appears to be a key factor to determine the competitive effect, and it should be possible to design the FMAC structure appropriate for the required frictional properties. This is a noble approach to control the frictional properties at the paper clutch, which is able to deal with the complicated frictional requirement that will be essential for the advanced transmission systems. 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