The Role of Asperity Geometry and Roughness Orientation for the Friction-Reducing Effect of Adsorbed Molecular Films

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1 Tribology Online, 11, 2 (2016) ISSN DOI /trol Article The Role of Asperity Geometry and Roughness Orientation for the Friction-Reducing Effect of Adsorbed Molecular Films Masaki Tsuchiko *, Saiko Aoki and Masabumi Masuko Department of Chemical Engineering, Graduate School of Science and Engineering, Tokyo Institute of Technology S1-31, 12-1 O-okayama 2-Chome, Meguro-ku, Tokyo , Japan * Corresponding author: tsuchiko.m.aa@m.titech.ac.jp ( Manuscript received 31 August 2015; accepted 05 January 2016; published 30 April 2016 ) ( Presented at the International Tribology Conference Tokyo 2015, September, 2015 ) The influence of roughness orientation on the friction-speed characteristics of adsorbed films was investigated by changing the angle of the anisotropically striated roughness to the sliding direction of the counter surface. As the angle of the striated roughness becomes perpendicular to the sliding direction, the friction coefficient is decreased. The friction-speed characteristics of the adsorbed film in the transverse direction of the striated roughness having different height or shape of asperities were also investigated. The friction coefficient obtained in the low speed region decreased with increasing arithmetic mean peak curvature (S pc ) when stearic acid-formulated oil was used as a lubricant oil. The results demonstrated that the roughness orientation and the asperity shape of the transverse roughness were key factors for generating additional load-bearing pressure so that the adsorbed films could exert their intrinsic friction-reducing effects. Keywords: boundary lubrication, surface roughness, striated roughness, adsorbed film, micro-ehl 1. Introduction Fatty acids, which are widely used as oiliness additives, have long played a key role in reducing undesirable friction loss between rubbing surfaces of mechanical components under a boundary lubrication regime [1-3]. Fatty acids such as stearic acid show an excellent friction-reducing performance because the molecules can adsorb onto the rubbed surfaces and then form an adsorbed molecular film. However, the load-bearing capability of the adsorbed film is suddenly lost under severe conditions such as high temperature, high contact pressure, and low speed conditions, once the orientation of the adsorbed film is disordered and then the molecules are released from the surface [4,5]. Therefore, some additional load-bearing pressure is required to support the load-bearing capability of the adsorbed film under such severe conditions. Previous studies by the authors have reported that the friction-reducing performance of an adsorbed film could be improved by a combination with surface roughness and texture; in particular, a remarkable reduction in friction was observed in a transverse direction of an anisotropic roughness in a low speed region in which a hydrodynamic lubrication effect of a fluid is believed to be ineffective [5-7]. This result was probably attributable to the generation of additional pressures on the fluids in the transverse direction of the anisotropic roughness, a phenomenon known as the microscopic elastohydrodynamic lubrication (micro-ehl) effect, to support the load-bearing capability of the adsorbed film. Although it is probable that the generation of the additional pressure, that is, the micro-ehl effect, was premised on the existence of the adsorbed film, fundamental knowledge regarding a relationship between the friction-reducing performance of the adsorbed film and the roughness orientation remains still limited. Organic polymers such as polymethacrylate (PMA) have been widely used in engine oils as a viscosity index improver to prevent a reduction in oil viscosity with an increase in oil temperature. Recent studies have indicated that functionalized polymers were effective for reducing friction under low speed conditions due to the formation of an adsorption film by the polymer molecules [8-11]. A functionalized PMA, copolymerized with alkylmethacrylates and functionalized methacrylate monomers, may have formed thicker and more viscous boundary films by adsorption of the functional groups. The generation of thicker viscous film resulted in enlargement of the elastohydrodynamic lubrication (EHL) effect down to a much lower speed region. On the other hand, the authors have also reported that the functionalized PMA Copyright 2016 Japanese Society of Tribologists 140

2 The Role of Asperity Geometry and Roughness Orientation forthe Friction-Reducing Effect of Adsorbed Molecular Films caused multiple-point adsorptions by a number of the functional groups, and then this prevented a drastic increase in friction with decreasing sliding speed [11,12]. Since the multiple-point adsorptions of the functional groups onto the surfaces promoted the formation of more homogeneous and hard-to-remove boundary films, the adsorbed film from the polymer probably maintained the load-bearing function under low speed conditions. However, there are still limitations to fully understanding the friction-reducing mechanism of the multiple-point adsorbed film from the PMA under low speed conditions. The present study addressed the limitation described above by focusing on the effects of the orientation and asperity shape of the anisotropic striated roughness on the friction-reducing performance of the adsorbed film. Not only stearic acid but also PMA functionalized with carboxy groups (PMA-COOH) were used as lubricating additives for comparing differences in the adsorption state between the single-point and multiple-point adsorbed films. To create variations in the orientation of the anisotropic striated roughness, test specimens were placed in the tribometer so that the angle of the striated roughness against the sliding direction of the counter surface was in the range of 0 (longitudinal) to 90 (transverse). In addition, several kinds of surface finishes were introduced to prepare the anisotropic striated roughness having different asperity shape and curvature. Friction measurement was carried out with a cylinder-on-disk tribometer developed in our laboratory under a varying sliding speed at a constant load and temperature. The results of the study demonstrated that the reduction in the friction coefficient was observed even under ultra-low speed conditions below 10-6 m/s as the roughness orientation became perpendicular to the sliding direction of the counter surface, and that the transverse roughness with higher slope and curvature radius of the asperity caused a larger reduction in the friction coefficient. 2. Test materials and methods 2.1. Experimental procedures Friction measurements were carried out using a laboratory-made cylinder-on-disk tribometer [6]. An outer ring of a commercially available tapered roller bearing was used as a disk specimen. The centerline circle of the disk specimen was 33.5 mm and the contact width was 1.0 mm. A roller of a commercially available roller bearing, with a diameter of 10 mm and length of 31.5 mm, was applied as a cylinder specimen. A schematic illustration of the contact configuration is shown in Fig. 1. Before beginning the friction tests, all specimens were ultrasonically cleaned in a toluene bath for 30 min, followed by drying and UV-Ozone cleaning for 30 min to remove contamination on the test specimens. The disk was attached to a rotating pulse-controlled servomotor, and the cylinder specimen was fixed to an oil cup on which a heating element with a temperature controller was installed to maintain a constant sample oil temperature. After the sample lubricating oil was heated up to the test temperature, an axial load was added to the disk against the cylinder by using a dead weight, and then the rotation of the upper disk was started. First, a run-in trial was carried out by rotating the upper disk continuously against the cylinder at a constant speed of 5.2 cm/s for a sliding distance of approximately 12 m. After the run-in, a friction measurement was conducted by changing the sliding speed at four measurement positions in a stepwise fashion from 4.9 μm/s to 8.5 cm/s. The friction torque and axial load between the rotating disk and the stationary cylinder were measured individually using two piezoelectric sensors installed under the cup holder. As shown in Fig. 2, four positions at 90 intervals on the disk specimen (0, 90, 180, and 270 ) were chosen for the measurement positions. At each of the four measurement positions, the friction data were obtained within a range of 5 (0 ± 2.5, 90 ± 2.5, 180 ± 2.5, Fig. 2 Fig. 1 Test specimen Data acquisition area of the cylinder-ondisk tribometer [7] Japanese Society of Tribologists ( Tribology Online, Vol. 11, No. 2 (2016) / 141

3 Masaki Tsuchiko, Saiko Aoki and Masabumi Masuko Table 1 Conditions of the friction tests and 270 ± 2.5 ). The coefficient of friction was calculated using values for the friction torque and the normal load simultaneously obtained by the two piezoelectric sensors. Table 1 summarizes the test conditions conducted by the cylinder-on-disk tribometer Sample oils and additives Poly-alpha-olefin (PAO) was used as an additive-free base oil to prepare sample lubricant oils. The kinematic viscosity of PAO was 16.9 mm 2 /s at 40 C and 3.88 mm 2 /s at 100 C. Two types of lubricating additives were used in this series of experiments. One was stearic acid (StA), which is composed of a straight-chained hydrocarbon having a single polar group at the end. StA was dissolved in the base oil at a concentration of 5 mmol/kg. The other was a functionalized dispersant polymethacrylate with carboxy groups (PMA-COOH), which was copolymerized with tetradecyl methacrylate monomers and methacrylic acid monomers. The average molecular weight of PMA-COOH was 20,000 and the percentage of carboxy groups was 19 mol%. The polymer was added to PAO at a concentration of 3 mass%. The chemical structures of these two additives are shown in Fig Materials and surface preparation of the specimens Both the disk and cylinder specimens were parts of a commercially available bearing made of heat-treated high carbon chromium bearing steel corresponding to JIS-SUJ2 steel. The end surface of the as-received disk specimen was polished with a lapping machine by changing the particle size of oil-based diamond slurries successively from 9 to 3 to 1 μm. The three-dimensional arithmetic average roughness S a of the polished surface was approximately 0.01 μm. Fig. 3 Chemical structure of additives used in this paper: (a) StA, (b) PMA-COOH A surface grinding machine with reciprocating motion was employed to apply an anisotropic striated roughness with the texture of a ground scar in one direction on the polished surface after the polishing process. Three kinds of the anisotropic striated roughness having different S a values ranging from 0.1 μm to 0.4 μm were made on the disk surface with the surface grinding machine by changing the grid size of the grindstone. The three kinds of disk surfaces were designated ANI-0.1, ANI-0.3, ANI-0.4 based on the size of S a. In other words, ANI-0.1 represented the anisotropic striated roughness having the smallest S a and ANI-0.4 indicated that having the largest S a. In order to vary the asperity shape or curvature of the anisotropic striated roughness, the disk surface having even greater S a which was designated ANI-0.45 was prepared and further surface finishes were applied on the disk surface of ANI-0.45 after the grinding process. ANI-0.45 was polished by using a lapping machine with an oil-based diamond slurry having a particle size of 0.25 μm. Three kinds of striated roughness having grind scars with different textures and curvature radii were prepared by changing the polishing time in the polishing process. The three kinds of roughness were referred to as ANI-0.45-PO1, ANI-0.45-PO2, and ANI-0.45-PO3. ANI-0.45-PO1 required the shortest polishing time, followed by ANI-0.45-PO2, and ANI-0.45-PO3 had the longest polishing time. Micrographs and profiles of each disk surface were measured by a confocal leaser scanning microscope (Keyence VK-8500 or VK-X250) before and after the friction tests. The measurements of the series of ANIs were conducted by VK-8500 and those of ANI-0.45 and POs were conducted by VK-X250. The measurement conditions of each microscope were summarized in Table 2. Figures 4 and 5 show the micrographs and profiles obtained from the series of roughness ANIs and ANI-0.45-POs. Three kinds of three-dimensional surface roughness parameters - the arithmetic average roughness (S a ), root mean square gradient (S dq ) and arithmetic mean peak curvature (S pc )- were calculated from data of the surface profiles. Since S a is one of the most basic roughness parameter and commonly used for evaluating surface property, S a was employed to evaluate asperity property in this study. In Japanese Society of Tribologists ( Tribology Online, Vol. 11, No. 2 (2016) / 142

4 The Role of Asperity Geometry and Roughness Orientation forthe Friction-Reducing Effect of Adsorbed Molecular Films Table 2 Conditions of the surface measurement by leaser scanning microscope Fig. 4 Images and profiles in the crossing direction of anisotropic surface roughness made by different grindstones Fig. 5 Images and profiles in the crossing direction of anisotropic surface roughness made by polishing ANI-0.45 addition to S a, other roughness parameters were investigated on the relation between asperity shapes and the friction-speed characteristics. Relatively good correlations were observed between roughness parameter and friction coefficient in case of S dq, S pc, and then S dq and S pc were also chosen for evaluating asperity property. Tables 3, 4 and 5 summarize S a, S dq and S pc of each disk surface before and after friction tests. S a of the cylinder specimen was 0.03 μm before friction tests, and the S a values of the cylinder after friction tests were Japanese Society of Tribologists ( Tribology Online, Vol. 11, No. 2 (2016) / 143

5 Masaki Tsuchiko, Saiko Aoki and Masabumi Masuko Table 3 summarized in Tables 4 and 5. These three parameters S a, S dq, S pc are defined in the equations (1) to (3), respectively. S z x, y dxdy (1) a A Surface roughness parameters of disk specimens before the friction test (σ means standard deviation) zxy (, ) zxy (, ) Sdq dd x y A x (2) y A n 11 2 zx, y 2 zx, y S pc 2 2 (3) 2 n k 1 x y Here, A is the measurement area of the surface profile, z (x, y) is the height of the surface and n is the number of tips on the surface. 3. Results and discussion 3.1. Influence of roughness orientation on the friction characteristics of the adsorbed films For clarifying the influence of roughness orientation on the friction characteristics of the adsorbed films, the disk having the anisotropic striated roughness was attached to the servomotor so that the striated roughness of the disk surface was placed in any direction against the sliding direction of the counter cylinder surface. As shown in Fig. 6, the angle of the striated roughness against the sliding direction of the counter surface at the measurement positions was denoted as θ, and θ was varied from 0 (longitudinal) to 90 (transverse) at an interval of approximately 15. In the friction test, for example, when the value of θ was 30 at both the measurement positions of 0 and 180, the angle of the striated roughness became 60 at both 90 and 270, by using the anisotropic striated roughness. The friction data obtained at both 0 and 180 (or both 90 and 270 ) were almost identical due to the use of commercially available bearing parts made of well-defined materials and due to the carefully operated surface finish. Therefore, the data at both 0 to 180 were averaged to get the mean value at the angle of θ, and then the data at both 90 to 270 were also averaged as a mean value at the angle of 90 θ. The mean value was used as a representative value at the angle of θ. Figure 7 shows the friction-characteristics of additive-free oil obtained at the angle of the striated roughness θ of 0, 15, 30, and 90. From Fig. 7, the friction-characteristics measured at different θ were almost same regardless the value of θ. Figure 8 shows the friction-speed characteristics of StA-formulated oil and PMA-COOH-formulated oil obtained at the angle Table 4 Surface roughness parameters of disk specimens after the friction test with StA (σ means standard deviation) Japanese Society of Tribologists ( Tribology Online, Vol. 11, No. 2 (2016) / 144

6 The Role of Asperity Geometry and Roughness Orientation forthe Friction-Reducing Effect of Adsorbed Molecular Films Table 5 Surface roughness parameters of disk specimens after the friction test with PMA-COOH (σ means standard deviation) Fig. 6 Anisotropic striated roughness with angle θ in the sliding direction Fig. 7 Friction-speed characteristics of additivefree oil with several angles in the sliding direction of the striated roughness θ of 0, 15, 30, and 90. From Fig. 8, as θ varied from 0 to 90, the friction coefficient decreased gradually in the lower speed region below 10-3 m/s in the case of both StA and PMA-COOH. The result indicated that the coefficient of friction decreased even in the low speed region as the roughness orientation was close to the transverse direction and this relation between the orientation and the friction coefficient was not observed in the case of the additive-free oil. Previously, the authors have reported on the friction characteristics of StA-formulated oil in both the longitudinal (θ = 0 ) and the transverse directions (θ = 90 ) of the anisotropic roughness [7]. As with the results in Fig. 8(a), a remarkable difference in the friction characteristics between the two directions was observed in the lower speed region below 10-3 m/s. The friction coefficient moderately decreased in the transverse direction as the sliding speed was decreased, while the friction coefficient in the longitudinal direction showed a drastic increase in the lower speed region. It was also found that the reduction in friction in the transverse direction was probably premised on the presence of the adsorbed film. From Fig. 8(b), as in the case of StA, there were marked differences in the friction characteristics of PMA-COOH between the two directions. Since PMA-COOH has a number of carboxy groups in one molecule and these functional groups act as adsorption-sites, multiple-point adsorption can occur at the carboxy groups [8,9,11]. Multiple-point adsorption can lead to the formation of an adsorbed film with a density as high as that of the film formed with StA, resulting in a higher friction-reducing performance similar to that of StA. Figure 9 shows the friction coefficient at three different sliding speeds of , , and m/s as a function of the angle of striated roughness. At a high speed of m/s, the friction coefficients obtained from StA remained low at approximately 0.1 regardless of the value of θ. The friction coefficients obtained from PMA-COOH also remained constant, and these values were higher than those for StA. Since the macroscopic elastohydrodynamic lubrication (macro-ehl) action of Japanese Society of Tribologists ( Tribology Online, Vol. 11, No. 2 (2016) / 145

7 Masaki Tsuchiko, Saiko Aoki and Masabumi Masuko Fig. 8 Friction-speed characteristics of StA and PMA-COOH on anisotropic roughness with several angles in the sliding direction Fig. 9 Relation between the angle of striated roughness and friction coefficient at three sliding speeds a fluid seems to remain dominant in the higher speed region, the load-bearing pressure is sufficient to supplement the adsorbed films and exert the intrinsic load-bearing function. Therefore, the friction coefficients maintained low values regardless of the orientation of the anisotropic roughness. As the sliding speed was decreased below m/s, the friction coefficient varied with the angle of the striated roughness in both cases. In particular, at a low speed of m/s, as θ varied from 0 to 30, the friction coefficient drastically decreased, and then it showed an almost constant low value at a range of θ from 45 to 90 both in the case of StA and PMA-COOH. On the other hand, the friction-reducing effect was not observed in the case of the additive-free base oil. These results indicate that the friction-reducing effect at the range of θ from 45 to 90 became prominent only when the adsorption film existed on the surface. It was thus inferred that the friction-reducing effect of the adsorbed film was affected by the angle of the striated roughness. As θ varied from 45 to 90, lubricating oil was trapped between asperities in the contact area because the sliding direction of the counter surface was nearly perpendicular to the direction of the striated roughness. As the contact pressure between asperities increased with decreasing speed, the high contact pressure increased the oil viscosity, leading to a kind of wedge effect by the oil. This effect, referred to Japanese Society of Tribologists ( Tribology Online, Vol. 11, No. 2 (2016) / 146

8 The Role of Asperity Geometry and Roughness Orientation forthe Friction-Reducing Effect of Adsorbed Molecular Films as a microscopic elastohydrodynamic lubrication (micro-ehl) effect in previous studies [13-14], probably acts as an additional pressure that keeps the intrinsic load-bearing function of the adsorbed film in the low speed region where the load-bearing function of the adsorbed film cannot be expected. On the other hand, as θ varied from 0 to 45, since the sliding direction of the counter surface was nearly parallel to the direction of the striated roughness, the lubricating oil could easily flow out along with the asperities, and thus an additional pressure sufficient to support the load-bearing function of the adsorbed film was not generated by the micro-ehl effect Influence of the asperity shape and curvature of the anisotropic roughness on the friction characteristics of the adsorbed films As mentioned above in section 3.1, it was necessary to trap a sufficient amount of lubricating oil between the asperities at the contact area in order to exert the micro-ehl effect as an additional load-bearing pressure. Although the lubricating oil was sufficiently supplied to the asperity at the contact area until the angle of the striated roughness against the sliding direction of the counter surface reached from 90 to approximately 45, the supplemental supply of oil was gradually decreased as the angle became 45 or less. This suggested that a roughness orientation which prevented the oil from flowing out of the contact area was one of the key factors for realizing the superior friction-reducing performance of the adsorbed films in the lower speed region. This, in turn, raised the question of which factors were responsible for the superior frictionreducing performance of the adsorbed films when the lubricating oil was sufficiently supplied at the contact-in other words, when the striated roughness was perpendicular to the sliding direction of the counter surface. As shown in Figs. 4 and 5, several kinds of anisotropic striated roughness having different asperity shapes and curvatures were prepared to investigate the influence of the asperity geometry on the friction characteristics of the adsorbed films in the transverse direction of the striated roughness (θ = 90 ). For quantification of the asperity geometry, several kinds of three-dimensional roughness parameters were applied. The arithmetic average roughness S a, defined as Eq. (1), was employed to reveal the influence of the asperity height on the friction-reducing performance of each adsorbed film. The composite S a was calculated by summing the squared roughnesses measured from worn surfaces of both the disk and the cylinder after the friction tests. Figure 10 shows plots of the friction coefficients obtained from StA- and PMA-COOH-formulated oils in the transverse direction of the anisotropic striated roughness at three different sliding speeds of , , and m/s against the composite S a. As shown in Fig. 10(a), the friction coefficients with the StA-formulated oil showed almost constant values Fig. 10 Relation between composite S a and the friction-coefficient of StA and PMA-COOH at three sliding speeds Japanese Society of Tribologists ( Tribology Online, Vol. 11, No. 2 (2016) / 147

9 Masaki Tsuchiko, Saiko Aoki and Masabumi Masuko regardless of composite S a at the high speed of m/s. In the lower speed region below m/s, the friction coefficient gradually decreased as S a took on a larger value, and then a scattering of the plots was observed at the lowest speed of m/s. Since the friction coefficient at ANI-0.1 was similar to that at ANI-0.45-PO2 although there was a big difference in S a between ANI-0.1 and ANI-0.45-PO2, the parameters for the quantification of roughness height, such as S a, did not seem to be suitable for evaluating the effect of the surfaces geometry on the friction characteristics of the adsorbed film from StA. On the other hand, it is found from Fig. 10(b) that the friction coefficient obtained from PMA-COOH increased with increasing S a at three different speeds. This means that the greater the height of the roughness, the more the frequency of direct contacts between the asperities could be increased, resulting in high friction. Since the polymers such as PMA-COOH can form thicker and more viscous boundary films that could provide an enhancement of viscosity at contact region between the asperities, the hydrodynamic lubrication action of the fluids may have been remarkable even in the lower speed region. From Fig. 10(a), there was a less pronounced relationship between the friction coefficients with StA and the amplitude parameters such as the arithmetic average roughness. It is thus required that other hybrid parameters be introduced which can express both the height and the shape of the asperities. A root mean square gradient (S dq ) defined as shown in Eq.(2) was applied to quantify the asperity geometry in addition to S a. S dq can be provided by calculating all slopes between two successive points of the profiles over the assessed scale-limited surface and then averaging these slopes. A higher value of S dq means that the projection edge of the asperity becomes sharper. Figure 11 shows the relation between the friction coefficient and S dq at three sliding speeds. As shown in Fig. 11(a), the friction coefficient of StA-formulated oil at m/s was almost constant irrespective of the S dq value, while the friction coefficient was gradually decreased with increasing S dq in the lower speed region below m/s. On the other hand, as shown in Fig. 11(b), when PMA-COOH-formulated oil was used as a lubricant oil, the friction coefficient was increased with increasing S dq value. In addition to S dq, another hybrid parameter-namely, the arithmetic mean peak curvature S pc defined as in Eq. (3)-was applied to evaluate the asperity shape. The curvature of tips was calculated as the summation of partial second order differentials in both the x- and y-axials of a surface profile around the tips. S pc is calculated by averaging the curvature of all tips. A higher value of S pc means that the curvature of tips is higher, so there are many sharp tips. Figure 12 shows plots of the friction coefficients obtained in the transverse direction as a function of S pc measured after Fig. 11 Relation between S dq (root mean square gradient) and the friction-coefficient of StA and PMA-COOH at three sliding speeds Japanese Society of Tribologists ( Tribology Online, Vol. 11, No. 2 (2016) / 148

10 The Role of Asperity Geometry and Roughness Orientation forthe Friction-Reducing Effect of Adsorbed Molecular Films Fig. 12 Relation between S pc (arithmetic mean peak curvature) and the friction-coefficient of StA and PMA-COOH at three sliding speeds the friction tests at three different sliding speeds. Although the friction coefficient of StA at the high speed of m/s was almost constant, the friction coefficient was decreased with increasing S pc as the sliding speed was decreased. On the other hand, in the case of PMA-COOH, the friction coefficient was slightly increased with increasing S pc for all speed regions just as S a and S dq provided gradual increases in friction coefficients of PMA-COOH. The results indicated that the friction-reducing performance of the adsorbed film from StA was enhanced not by the height but rather by the shape and the curvature of the asperity, whereas that from PMA-COOH showed less dependence on the height, the shape, and the curvature of the asperity. As mentioned in section 3.1, the friction coefficient of the adsorbed film varied with θ for the case of both StA and PMA-COOH, and this tendency became remarkable as sliding speed was decreased. It was considered that when θ was set to be nearly 0 the lubricating oil easily flew out from the asperities while as θ was set to be perpendicular to the sliding direction of the counter surface the lubricating oil was trapped by the asperities. Therefore, the friction characteristics of the adsorbed film in the low speed region depended on the microscopic flow state between asperities. Besides, when the shape of asperities changed in case of transverse direction (θ = 90 ), the friction characteristics of the adsorbed films formed by StA or PMA-COOH showed a different dependence on the asperity shape. In the case of StA, the friction coefficient drastically varied with the parameters- S a, S dq and S pc - in the lower speed region, especially the relationships between the friction coefficient and S dq, S pc were observed. In contrast, the friction coefficient of PMA-COOH did not varied with roughness parameters, compared with that of StA. The difference in the friction characteristics between StA and PMA-COOH was probably due to the difference in the morphology of the adsorbed molecular film formed by these two additives. StA is an organic polar compound having linear alkyl-chain carbon and one adsorption site at the end of the chain. StA can easily adsorb on a metal surface and then form a highly orientated and high-density adsorption film. On the other hand, since PMA-COOH is a functionalized organic polymer having an average molecular weight of 20,000 and a number of functional groups that act as adsorption sites, the multiple-point adsorption occurring at these functional groups probably promoted the formation of highly-dense, bulky film that was unlikely to desorb. This bulky film was capable of not only enhancing the viscosity of the oil at the contact but also providing a load-bearing performance that was not dependent on the orientation of the adsorbed molecules [8-11, 15-17]. The differences in morphology of the adsorbed film were reflected in their friction characteristics, as a result, there were significant differences in the friction characteristics between the additives. In addition, it is probable that the surface with Japanese Society of Tribologists ( Tribology Online, Vol. 11, No. 2 (2016) / 149

11 Masaki Tsuchiko, Saiko Aoki and Masabumi Masuko higher S dq, indicating an asperity with a sharper projection edge, or higher S pc, indicating sharper projections, was capable of generating higher contact pressure, resulting in the generation of higher additional load-bearing pressure. In the case of the adsorbed film from StA, higher values of S dq and S pc possibly promoted generation of a sufficient load-bearing pressure to supplement the film and exert the intrinsic friction-reducing performance of the adsorbed film, resulting in a significant reduction in friction under severe conditions in which a breakdown of the film occurred due to direct contacts between the surfaces. On the other hand, as mentioned above, PMA-COOH formed non-oriented adsorbed film that load-bearing capability was not dependent on the orientation of the adsorbed molecule. This suggested that the frictionreducing performance of the adsorbed film was not affected by the presence of the additional pressure derived from the asperity shape, resulting in small reduction in the friction coefficient by varying asperity shape. Although we inferred that not only the roughness orientation and also the asperity shape affected the friction characteristics of the adsorbed film under boundary lubrication condition and their effects varied according to the type of the adsorbed films, the experimental evidences seem to be insufficient to demonstrate the mechanism in which reduction in friction occurred in lower speed region by synergistic effects between the roughness and the adsorbed film theoretically. Consequently, it is necessary to conduct further work in order to provide a theoretical explanation regarding the mechanism. 4. Conclusion The relationship between orientation of the anisotropic striated roughness and the friction-speed characteristics of adsorbed molecular films from StA and PMA-COOH was investigated. In addition, the influence of the asperity height and shape on the friction-speed characteristics of these adsorbed molecular films was examined by using several kinds of test specimens of the anisotropic striated surface having different shape and curvature. The major results obtained in this study were as follows: In the lower speed region, the friction characteristics of the adsorbed molecular films formed by stearic acid (StA) and the organic polymer (PMA-COOH) were affected by the surface roughness property. The friction coefficient was decreased as the striated roughness became perpendicular to the sliding direction of the counter surface for both StA and PMA-COOH, and in particular, this tendency was remarkable in the low speed region. When the asperity shape of transverse roughness changed, difference in the friction characteristics were observed between the adsorbed film formed by StA and PMA-COOH. The friction coefficient of the adsorbed film formed by StA showed lower values on the surface with higher S dq, S pc value. In contrast, the adsorbed film formed by PMA-COOH did not showed drastically change by the asperity shape compared with that by StA. Acknowledgments The authors would like to thank Sanyo Chemical Industries, Japan, for supplying polymers. References [1] Bowden, F. P. and Tabor, D., The Friction and Lubrication of Solids, Clarendon Press, Oxford, [2] Bowden, F. P., Gregory, J. N. and Tabor, D., Lubrication of Metal Surfaces by Fatty Acids, Nature, 156, 1945, [3] Levine, O. and Zisman, W. A., Physical Properties of Monolayers Adsorbed at the Solid-Air Interface. I. Friction and Wettability of Aliphatic Polar Compounds and Effect of Halogenation, The Journal of Physical Chemistry, 61, 8, 1957, [4] Campen, S., Green, J., Lamb, G., Atkinson, D. and Spikes, H. A., On the Increase in Boundary Friction with Sliding Speed, Tribology Letters, 48, 2, 2012, [5] Masuko, M., Aoki, S. and Suzuki, A., Influence of Lubricant Additive and Surface Texture on the Sliding Friction Characteristics of Steel Under Varying Speeds Ranging from Ultralow to Moderate, Tribology Transactions, 48, 3, 2005, [6] Aoki, S., Suzuki, A. and Masuko, M., Speed and Topography Dependent Boundary Friction Characteristics of Steel, Lubrication Science, 22, 8, 2010, [7] Aoki, S., Fukada, D., Yamada, Y., Suzuki, A. and Masuko, M., Synergistic Friction-Reducing Effects Between the Transverse Direction of Anisotropic Surface Roughness and the High-Density Adsorbed Films Under a Boundary Lubrication Regime, Tribology International, 58, 2013, [8] Cann, P. M. and Spikes, H. A., The Behavior of Polymer Solutions in Concentrated Contacts: Immobile Surface Layer Formation, Tribology Transactions, 37, 3, 1994, [9] Smeeth, M., Spikes, H. A. and Gunsel, S., Boundary Film Formation by Viscosity Index Improvers, Tribology Transactions, 39, 3, 1996, [10] Müller, M., Topolovec-Miklozic, K., Dardin, A. and Spikes, H. A., The Design of Boundary Film- Japanese Society of Tribologists ( Tribology Online, Vol. 11, No. 2 (2016) / 150

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