Tribology Online. Japanese Society of Tribologists Article

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1 Tribology Online Japanese Society of Tribologists Vol. 12, No. 3 (2017) ISSN DOI /trol Article Friction and Wear Properties of Tetrahedral Si Containing Hydrogenated Diamond Like Carbon Coating under Lubricated Condition with Engine Oil Containing ZnDTP and MoDTC Kentaro Komori 1)* and Noritsugu Umehara 2) 1)Honda R&D Co., Ltd. Automobile R&D Center, 4630 Shimotakanezawa, Haga machi, Haga gun, Tochigi Japan 2)Department of Mechanical Science and Engineering, Graduate School of Engineering, Nagoya University, Furo cho, Chikusa ku, Nagoya, Aichi Japan *Corresponding author: Kentaro Komori (Kentaro_Komori@n.t.rd.honda.co.jp) Manuscript received 03 May 2017; accepted 08 June 2017; published 31 July 2017 Abstract In this study, we investigated tribological properties of tetrahedral Si containing hydrogenated DLC coating (TMS coating) during sliding against steel or cast iron lubricated with engine oil containing MoDTC and ZnDTP additives. TMS coatings derived from only tetramethylsilane were prepared using PACVD with high bias process. TMS coating had the highly carbon sp 3 bonded structure induced by Si (over 20 at.%) with high hydrogen content (30 at.%). TOF SIMS analysis showed that TMS coating could form the additive derived tribofilm on both of the non ferrous coating surface and the ferrous counter surface to promote Mo sulfide formation effectively, leading a low friction coefficient. The tribo chemical reactions were related not only to Mo sulfides formation, but also reduction of Mo oxides. Mo oxides could react with DLC material and induce hydrogen evolution and transformation into further weak carbon structure, causing an increasing wear of hydrogenated DLCs. TMS coating showed no significant wear in spite of plenty hydrogen content. Si induced sp 3 bond and could act so as not to induce clustering of the sp 2 phase. This could inhibit increasing wear of the DLC in the presence of MoDTC. The effectiveness of TMS coating in friction and wear behavior was shown to depend on tribo chemical reactions and transformation of carbon bond structure. Keywords diamond like carbon, tribochemistry, lubricant additives, engine oil 1 Introduction In recent years, tribological investigations on mechanical components for internal combustion engine have received increasing demands for friction reduction and long term durability responding to address environmental issues [1]. Lubricants and coating technologies played important roles for improvement of performance of internal combustion engine. Zinc dialkyldithiophosphate (ZnDTP or ZDDP) has been used for many years as an antiwear additive. It is probably the most widely used in formulated engine oils, and it also acts as an antioxidant and corrosion inhibitor. Molybdenum dithiocarbamate (MoDTC) was also applied to automotive lubricants and greases as a friction modifier to improve the fuel efficiency. These have been widely spread to automotive lubrication blends [2 4]. Automotive lubrication trend is directed toward the application of low viscosity oils for further improving fuel efficiency [5,6]. These low viscosity oils can decrease hydrodynamic pressure and increase contact pressure of solid contact in mixed lubrication for mechanical components. This tendency causes severe tribological conditions to provide the issue on durability of mechanical components. The application of low friction and wear resistant coatings has a possibility to respond to these demands. Diamond like carbon (DLC) coatings are well known for their excellent sliding performance and wear resistance [7]. Recent tribological investigations on DLCs are going toward understanding the effect of lubricants under boundary lubricated condition. In particular, the interaction between DLC coatings and additives has been investigated [8,9]. Mo based friction modifiers interact with DLC coating to form MoS2 and MoO3. It is observed that the higher the ratio of MoS2/MoO3, the lower the coefficient of friction [9,10]. It is also reported that hydrogenated DLC coatings can exhibit considerably low friction performance under 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. 123

2 Kentaro Komori and Noritsugu Umehara lubricated condition in the presence of MoDTC and ZnDTP additives, since MoS2 tribofilm can be formed effectively [11]. However, it is reported that the wear of DLC coatings increase in the presence of MoDTC in the oil [12]. MoO3 observed on the worn surface increases with an increasing hydrogen content of DLC coatings during DLC/steel sliding contact with MoDTCcontaining oil [10]. MoO3 comes from decomposition of MoDTC and reacts with hydrogen and carbon in DLC material. This can cause significant wear of hydrogenated DLC [10,11]. On the other hand, a hydrogen free DLC shows no significant wear in the presence of MoDTC [10,13]. However, it does not present sufficient lubrication with MoDTC containing oil [14], suggesting appropriate lubricant or additive is necessary for favorable lubrication of hydrogen free DLCs [14,15]. Hydrogenated carbon materials preferentially interact with Mo 4+ involved in the formation of MoS2, while hydrogen free carbon materials also react with Mo 6+ involved in the formation of MoO3. The chemical hardness approach (HSAB principle) may explain why hydrogencontaining carbon materials promote the MoS2 formation [11]. We have already reported from the viewpoint of surface morphology that appropriate rough surface could promote the MoS2 formation in tribofilm effectively, leading low friction with no significant wear of hydrogenated DLCs [10]. However, the reduction of MoO3 does not necessarily correspond to the wear amount of the DLC coating by one to one [10,13], and effect of carbon bond structure of DLC coatings has not been made abundantly clear. In this research, we investigated friction and wear behavior of DLCs from the further intrinsic viewpoint of carbon bond structure of DLCs involving tribo chemical reactions. Hydrogen free DLCs show out standing mechanical properties due to their high hardness and high elasticity. In particular, tetrahedral amorphous carbon (ta C) is the diamond like amorphous carbon and it has the highest content of carbon sp 3 bond. In order to inhibit an increasing wear in the presence of MoDTC, the highly sp 3 bonded structure in DLC material is expected. However, it is also necessary to form the additivederived tribofilm effectively for realizing low friction. Therefore, it is essential to introduce the highly sp 3 bonded structure into hydrogenated DLC material for achieving both of low friction and wear resistance in the engine oil containing MoDTC. It is reported that the incorporation of Si into carbon network structures promoted the formation of sp 3 carbon clusters [16]. Many researches have been reported on introduction of Si into DLC [17 20], but the friction and wear properties in the presence of MoDTC are not clear as to the highly sp 3 bonded structure like ta C induced by Si. In this study, to introduce the highly sp 3 bonded tetrahedral structure in the hydrogenated DLC, Si containing DLC coatings derived from only tetramethylsilane (TMS coating) were prepared using plasma assisted chemical vapor deposition (PACVD). Tribological properties of TMS coatings sliding against steel under lubricated condition with the engine oil containing MoDTC and ZnDTP additives were investigated in comparison with hydrogenated and hydrogen free DLCs. Surface analysis was employed to gain an understanding of chemical compositions in the tribofilm derived lubricant additives. Additional tribological test was conducted with more severe thermal load on the DLC material in order to find the difference in wear behavior depending on carbon bond structure of DLC. From these results, we discuss that the effectiveness of TMS coating in friction and wear behavior is shown to depend on the tribo chemical reactions and the transformation of carbon bond structure in the presence of MoDTC. 2 Experimental 2.1 DLC coatings TMS coatings which were Si containing diamond like amorphous carbon coatings derived from tetramethylsilane (TMS) were prepared. TMS coatings were deposited on the polished steel substrates (JIS G 4805/SUJ2) by means of plasma assisted chemical vapor deposition (PACVD) using only the tetramethylsilane vapor as a precursor. For the TMS coatings, two different coating samples in mechanical properties and surface roughness were obtained by varying a deposition condition of precursor gas pressure (0.1 or 0.4 Pa). The depositions for TMS coatings were conducted with the high bias voltage (2000 V) applied to substrates. By way of comparison, hydrogenated DLC coatings (a C:H) were prepared using acetylene (C2H2) or toluene vapor (C7H8) as a pure hydrocarbon precursor not containing silicon, and TMS/C7H8 coating was also prepared using the mixed precursor gas of tetramethylsilane vapor and toluene vapor. The deposition condition was varied with the combination of gas pressure ( Pa), substrate bias ( V), and plasma power ( W). Furthermore, three different coating samples consisting of lower hydrogen content and higher fraction of carbon sp 3 bond were additionally prepared by means of physical vapor deposition (PVD) method. A hydrogen free amorphous carbon (G DLC sample) was obtained using arc ion plating (AIP) process. And then, two different tetrahedral amorphous carbon (ta C) coatings were obtained using pulsed cathodic arc plasma source. One was deposited by applying the lower arc voltage, 80 V, with a tiny hydrocarbon gas (H DLC sample). The other was deposited by applying the higher arc voltage, 200 V, under a high vacuum condition (ta C sample). Thickness of the coatings was regulated to approximately 1 4 μm basically. It should be noted that the H DLC sample was rather thin of 0.5 μm in thickness, in order to avoid forming an excessive rough surface attributed to droplet during the arc PVD process. Surface finishing treatment was not applied to the coated surface. 2.2 Characterization of DLC coatings Atomic compositions of the coating samples were characterized by Rutherford backscattering spectrometry and hydrogen forward scattering spectrometry (RBS/HFS) to determine content of hydrogen and the other elements. The measurements were performed using Pelletron 3SDH (National Electrostatics Corporation). DLC coating samples were irradiated with a helium ion beam (incident beam diameter of 2 mm) at an incident angle of 75 degrees. Incident energy was of 2.3 MeV and energy resolution per was of 19 kev. Sample current was 30 na, and irradiation dose was arranged within the range of μc. Chamber was evacuated to Pa. Mechanical properties of the coatings were measured by means of nano indentation method, based on ISO The measurements were performed using Nano Indenter XP (MTS Systems Corporation) to estimate nano indentation hardness and elastic modulus at 10% depth of the coating thickness. The microindenter of Berkovich type was used for the measurements. Microscopic surface morphology on the DLC coatings was observed by means of atomic force microscopy (AFM) using SPI4000 NanoNavi/E sweep (SII Nano Technology Inc.). The scanning area was of 20 μm in square. Surface roughness parameter for the root mean square height (Sq) was estimated based on ISO Japanese Society of Tribologists ( Tribology Online, Vol. 12, No. 3 (2017) / 124

3 Friction and Wear Properties of Tetrahedral Si Containing Hydrogenated Diamond Like Carbon Coating under Lubricated Condition with Engine Oil Containing ZnDTP and MoDTC 2.3 Carbon bond structure Raman spectroscopy analysis Carbon bonding structures of the DLC coatings were characterized by Raman spectroscopy. Raman is widely used as a non destructive way to characterize the structural quality of carbon materials [21]. Raman spectra of the DLC coatings were obtained within the range of cm 1 by the laser irradiation (Nd YAG, wave length: 532 nm) using LabRAM ARAMIS (HORIBA JOBIN YVON). In order to estimate the Raman parameters to characterize carbon bond structure, the peak decomposition of the Raman spectra were performed. The spectra line was fitted with a Gaussian Lorentzian line for the so called G and D peaks lie at around cm 1 and around 1350 cm 1 respectively. The G peak is due to the bond stretching of all pairs of sp 2 atoms in both six fold rings and carbon chains, and the D peak is due to the breathing modes of sp 2 atoms in rings [22]. G peak position was characterized as the Raman parameter. The G peak position corresponds to the sp 2 configuration and the sp 3 fraction with hydrogen, in hydrogenated DLC. The intensity ratio of the D and G peaks, I(D)/I(G), and the full width at half maximum (FWHM) of G peak, G width, were also characterized as the Raman parameters. In amorphous carbons, I(D)/I(G) is a measure of the size of the sp 2 phase organized in rings [22] NEXAFS In order to investigate carbon bond structure further more in detail, near edge X ray absorption fine structure (NEXAFS) analysis was applied for the representative coating samples. NEXAFS analysis was conducted at the BL 12 in the SAGA Light Source (SAGA LS; Kyushu Synchrotron Light Research Center, Saga, Japan). The absorption spectra of C K edge (photon energy: ev) were obtained by the total electron yield method. The photon energy calibration was conducted using the C K edge peak position of highly oriented pyrolytic graphite (HOPG). The background level was determined for the pre edge region of the absorption edge. Normalization of the spectrum intensity was carried out with the post edge region in each spectrum. The X ray incident angle was 90 degrees to surface of the coating samples, and the slit size was 10 μm for the both incidence slit and emission slit. The resolution of the photon energy at C K edge region (around 280 ev) was approximately 0.1 ev. 2.4 Tribological tests and surface analysis Ball on disk test The tribological behaviors of DLC coated disks sliding against steel balls were investigated under lubrication condition with a full formulated engine oil using Tribometer CSEM (CSM Instruments). Diameter of the steel ball (JISG4805/SUJ2) was 6 mm. Diameter and thickness of the DLC coated disk were 20 mm and 3 mm, respectively. The ball was pressed against the rotating disk with a load of 5 N. The rotation diameter was 10 mm. The ball on disk test was performed at a sliding speed of 100 mm/s for 50 min. Lubricant was a low viscosity engine oil (SAE 0W 20) containing MoDTC and ZnDTP additives. Concentration of elements corresponding to the additives was 900 ppm of zinc (Zn), 800 ppm of phosphorous (P), 1900 ppm of calcium (Ca), 700 ppm of molybdenum (Mo), and 0.24% of sulfur (S), respectively. Viscosity of the engine oil was 35.0 mm 2 /s at 40 C and 8.29 mm 2 /s at 100 C. Viscosity index was 225. The ball on disk tests were conducted intentionally at the lower temperature as 40 C. It was reported that MoDTC and ZnDDP additives became active on steel surface to form tribofilm at the higher temperature more than 80 C [23]. In this ball on disk test, we investigated how the chemical reactions induced by thermal and strain effects during sliding contact varied essentially due to difference in characteristics of DLC coatings. After the ball on disk tests, the worn surfaces of the steel balls and the DLC coated disks were observed using an optical microscope. The wear amount of the DLC coating samples was estimated with width and depth of the wear tracks measured using a stylus based profilometer TOF SIMS analysis The worn surfaces of the both balls and disks were analyzed by means of time of flight secondary ion mass spectrometry (TOF SIMS) after the ball on disk tests. The worn surfaces were degreased by the organic solvents to eliminate the contaminants and residual lubricant before measurements. The measurements were performed using TOF.SIMS 5 (ION TOF GmbH). TOF SIMS spectra were obtained using bunched Bi3 2+ primary ion with acceleration energy of 25 kv, and the total ion dose density was approximately ions/cm 2. The analyzed areas were μm square for the balls, and μm square for the disks, depending on the size of wear scars or wear tracks. The spectra were extracted from only worn surfaces of measured area Reciprocating pin on plate test For the coating samples which showed no significant wear in the ball on disk test, further tribological test was conducted with reciprocating pin on plate configuration under more severe sliding condition on DLC wear. The DLC coatings were deposited on polished spherical surface of steel pins (spherical radius: 16 mm). This was the opposite configuration of the ball on disk test. This configuration could rather induce localized heat and a higher thermal load on the DLC coated pin side continuously, to accelerate wear of DLC. The pin substrate was made of chromium steel (JIS G 4053/SCr415). The DLC coated chromium steel pin was pressed against the reciprocating plate at the center position of plate, and the initial contact pressure was 650 MPa. The plate was made of cast iron (JIS G5501/FC250). The cast iron plate was cut from a cylinder bore of actual engine. Sliding surface of the plate was inner side of the cylinder bore. The reciprocating motion of the plate was paid attention to be the axial direction perpendicular to the circumferential direction of cylindrical surface. An inner peripheral surface of the cylinder bore was finished by machining and plateau honing, so that cross hatching grooves were formed on the sliding surface of the plate. The reciprocation movement of the plate was of 50 mm at frequency of 10 Hz. The sliding test was conducted at 80 C for 30 min under lubricated condition. Lubricant was the engine oil containing MoDTC and ZnDTP additives same as that used in ball on disk test. After the tests, the worn surfaces of the DLCcoated pins and cast iron plates were observed by means of an optical microscope. 3 Results and discussion 3.1 Characteristics of DLC coatings Table 1 shows the characteristics of the coating samples. The different coatings in chemical composition, carbon bond structure, mechanical properties and microscopic surface roughness were obtained by varying the deposition method and condition. TMS coatings had the high Si content over 20 at.% with Japanese Society of Tribologists ( Tribology Online, Vol. 12, No. 3 (2017) / 125

4 Kentaro Komori and Noritsugu Umehara keeping higher hydrogen content of approximately 30 at.%. TMS coatings showed relatively higher hardness more than 20 GPa. In the DLC coating samples except the TMS coatings, hardness and elastic modulus increased with decreasing of hydrogen content, and ta C sample showed the highest hardness as 70 GPa. Microscopic surface roughness of DLC coatings varied depending on the precursor and coating growth process. TMS A sample deposited under the lower gas pressure showed the rougher surface. In contrast, TMS B sample deposited under the higher gas pressure showed the smoother surface. The DLC coating samples deposited by PVD method typically showed the larger surface roughness. 3.2 Carbon bond structure Raman spectroscopy Figure 1 shows Raman spectra of the DLC coating samples. The Raman parameters are also shown in Table 1. First of all, the Raman spectra of DLC coatings are mainly dominated by G mode and D mode attributed to the configuration and/or order of the carbon sp 2 sites, even when the carbons do not have particular graphitic ordering. The G mode is the stretching vibration of any pair of sp 2 sites, whether in C=C chains or in aromatic rings. G does not only mean graphite [21]. The D Intensity (cnt.) Raman Shift (cm -1 ) Fig. 1 ta-c G-DLC H-DLC C2H2 C7H8 TMS/C7H8 TMS-B TMS-A D Raman spectra of the DLC coating samples G mode labeled D for disorder is the breathing mode of those sp 2 sites only in rings, not in chains [21]. This is different from the diamond mode. The Raman spectra of C2H2 and C7H8 samples showed typical profiles of hydrogenated DLCs (a C:H) consisting of G peak and D peak, as shown in Fig. 1. In a C:H, hydrogen has an important effect that sp 3 content increases with an increasing hydrogen content [22]. Some aromatic ordering of sp 3 sites are preserved even if hydrogen content increases, so the increase of sp 3 content shows an alloying effect. This causes the G peak position to move down. As the hydrogen content further increases, carbon chains with hydrogen begins to become polyacetylene. There is also down shift in the G peak at olefinic sites [21]. This reduces the G frequency. The a C:H with higher hydrogen content has a high sp 3 content. The sp 2 sites are organized in very small clusters, the D peak disappears [21]. G peak position decreases to value around 1520 cm 1 in polymer like carbons (PLCs) [22]. In this study as shown in Table 1, G peak position in C2H2 sample was higher than that in C7H8 sample; furthermore, I(D)/I(G) in C2H2 sample was also higher than that in C7H8 sample. D peak in C2H2 spectra was more sizable in comparison with that in C7H8 spectra. It is suggested that the sp 2 sites in C2H2 sample are still mainly organized in aromatic rings, while the sp 2 sites in C7H8 sample are organized in smaller cluster with high hydrogen content. G width decreased due to the small cluster with increasing hydrogen content. C7H8 sample does not seem to reach PLC and significant overshadow by photoluminescence background was not found in C7H8 spectra, but C7H8 sample seems to consist of the structure with plenty hydrogen close to PLC. Thus, the G peak position moved down, and I(D)/I(G) decreased with increasing hydrogen content of a C:H. Similarly, G width decreased. As for hydrogen free DLC coating samples, these samples showed the higher G peak positions more than 1550 cm 1 and the different profile line shapes from the hydrogenated DLCs (a C:H). In particular, it should be noted that ta C sample showed a symmetry G peak; the D peak seems to be disappeared and the I(D)/I(G) indicated very small value as Furthermore, the G width showed the highest value in the G width of all samples in Table 1. As the change from sp 2 a C to sp 3 ta C, the sp 2 configuration is changed from rings to short chains (olefins) [21]. The G peak position rose due to high vibration frequency of shorter chains and reaches around 1570 cm 1 in ta C. I(D)/I(G) Table 1 Coating process and characteristics of the DLC coatings Japanese Society of Tribologists ( Tribology Online, Vol. 12, No. 3 (2017) / 126

5 Friction and Wear Properties of Tetrahedral Si Containing Hydrogenated Diamond Like Carbon Coating under Lubricated Condition with Engine Oil Containing ZnDTP and MoDTC decreased to nearly zero for ta C, because no sp 2 rings are present in ta C. The line shape of the G peak becomes more symmetric as the sp 3 reaches high values [21]. As for G DLC sample, the D peak rose remarkably and the I(D)/I(G) indicated much higher value, in comparison with ta C. It seems that G DLC sample does not reach the high sp 3 content as ta C and the disordered sp 2 rings still remain in G DLC sample. H DLC sample showed the similar line shape as a C:H, but the G peak position reached 1554 cm 1 which was higher than G peak position in C2H2 sample. The I(D)/I(G) showed the lower value than C2H2 sample as However this did not reach the zero intensity as ta C. H DLC sample had tiny hydrogen content as approximately 5 at.%. Therefore, H DLC sample is different from a category of hydrogenated tetrahedral amorphous carbon (ta C:H). In this region, the Raman parameters can be interpreted as in hydrogen free DLCs. If hydrogen is introduced in ta C, this increases the order and clustering in the structure; I(D)/I(G) increases [22]. H DLC sample has not so highly sp 3 bonded network, but the non hydrogenated sp 2 phase. As for TMS/C7H8 sample deposited using mixed precursor gas (TMS and C7H8), the Raman spectrum showed typical line shape of a C:Hs, as shown in Fig. 1. However, the G peak position downshifts to around 1500cm 1, while keeping the G width, in comparison with C7H8 sample as shown in Table 1. The variation of G peak position as a function of the Si/(Si + C) ratio for a series of a C1 xsix:h has been reported [20]. The G peak position linearly moves down with an increasing fraction of incorporated Si. This is expected from the reason as an increase in the Si content does not induce further clustering of the sp 2 phase [22]. Thus, Si can only bond to C as sp 3 bond. For the TMS/C7H8 sample, Si content was approximately 12 at.%, and the Raman spectrum showed that the I(D)/I(G) slightly smaller than C7H8 sample. It suggests that TMS/C7H8 sample has not only a silicon induced sp 3 phase, but also the network with similar sp 2 and sp 3 configurations to C7H8 sample. On the other hand, TMS coating samples (TMS A and TMS B) deposited using only TMS precursor showed the symmetry G peaks, as shown in Fig. 1. The G peak position downshifted and reached around 1460 cm 1. The D peak was almost disappeared and the I(D)/I(G) indicated very small value as around 0.1. The shape of spectrum of TMS coatings is very similar to ta C spectrum. The G width showed the lowest value in the all samples in contrast to ta C sample. TMS coating samples had the higher hydrogen content of all of DLC in this research as around 30 at.%. Their Si content reached over 20 at.%. The Raman spectrum for the TMS coatings could be interpreted as that Si could induce highly sp 3 bonded structure from the downshift of G peak position. The increase of hydrogen content reduced the size of sp 2 ring structures significantly. The I(D)/I(G) was nearly zero similar to ta C or PLC. These results suggest that TMS coatings have the high carbon sp 3 content, and the sp 2 sites are organized in very small clusters with no rings of carbons NEXAFS NEXAFS was conducted for the selected coating samples to obtain sp 3 fraction of DLC structures. The sp 3 sites are invisible directly in present visible (laser) Raman spectroscopy. Figure 2(a) shows C K edge spectra. Figure 2(b) shows the spectra which are normalized on the basis of the peaks at 285 ev in photon energy from spectrums in Fig. 2(a). A peak at 285 ev is the contribution of 1s π * transition which is attributed to π electron in C C sp 2 bond. A peak around ev is the contribution of 1s σ * transition which is attributed to both of π electron in C C sp 2 bond and σ electron in C C sp 3 bond [24]. The sp 3 fraction in the DLC material could be estimated approximately from the intensity ratio of (σ * + π * continuum) / π * discrete, as (sp 3 + sp 2 ) / sp 2 ratio. C C sp 3 bond has the only σ electron. Therefore, the larger the intensity ratio of (σ * + π * continuum) / π * discrete is, the higher the content of carbon sp 3 bond in DLC material is. Figure 2(b) shows spectra normalized with π*discrete. The peak height of σ * in Fig. 2(b) can be interpreted as the the intensity ratio of (σ * + π * continuum) / π * discrete, since π * continuum exists continuously in the region above 295 ev. For the DLC samples excluding TMS coatings, order in the intensity ratio of (σ * + π * continuum) / π * discrete is described as follows: ta C >> H DLC >> C2H2 > C7H8 in descending order, as shown in Fig. 2(b). The intensity ratio of ta C sample was the largest, while the C7H8 sample exhibited the smallest ratio. It suggests that carbon sp 3 content of the DLC samples increases with the decreasing hydrogen content in the DLC materials excluding TMS coatings which corresponds to the results of Raman analysis Intensity (arb. units) (a) ta-c H-DLC C2H2 C7H8 TMS-A Intensity (arb. units) (b) π* σ* (C-H or C-C) σ* ta-c TMS-A H-DLC C2H2 C7H Photon Energy (ev) Photon Energy (ev) Fig. 2 NEXAFS spectra of the DLC coating samples: (a) C K edge spectra; and (b) normalized C K edge spectra on the basis of peaks at 285 ev in photon energy Japanese Society of Tribologists ( Tribology Online, Vol. 12, No. 3 (2017) / 127

6 Kentaro Komori and Noritsugu Umehara in this study. On the other hand, the spectrum of TMS A sample was clearly different from the hydrogenated DLCs deposited using CVD method. Intensity of the peak at 285 ev for TMS A sample was relatively weak; besides, the spectrum profile of TMS A sample around ev was similar to the spectrum of ta C sample. TMS A sample showed the large intensity ratio of (σ * + π * continuum) / π * discrete as much as ta C sample. Additional peaks were observed at ev and at ev. It is reported that the NEXAFS of hydrocarbon molecules show additional peaks at ev. However, it is not clear whether these features transfer to the solid state [21]. The peak at ev might be another π* resonance related to edge of the sp 2 cluster. The peak at ev seems to be attributed to σ σ * transition for the C H or C C bonds. Similar feature between TMS A sample and ta C sample also showed this peak. We noted that the shoulder of peak in TMS A sample slightly rose at the region below 285 ev compared with the other samples. This seems to be attributed to Si C bond. It can be considered from NEXAFS that TMS coatings have high fraction of the carbon sp 3 bond corresponding to the Raman spectroscopy. The results of Raman and NEXAFS allowed us to say that TMS coatings were tetrahedral Si containing hydrogenated amorphous carbon. Fig. 3 Friction coefficients in the final state of the ball on disk tests: DLC coated disks sliding against steel balls lubricated with engine oil 3.3 Ball on disk test Figure 3 shows the friction coefficients of the DLC coating samples in the final state of the ball on disk tests sliding against steel ball with the engine oil containing MoDTC and ZnDTP additives. It was noted that friction behavior was generally steady after running in period. The friction coefficients are the average values over the last 30 s of the each test. TMS A and C2H2 coating samples showed relatively low friction coefficients as , whereas the other coating samples exhibited the friction coefficients within the range of approximately Figure 4 shows the worn DLC disk surface after the ball on disk test. No significant wear was observed on the surface of both TMS coating samples as shown in Figs. 4(a,b). Similarly, the hydrogenated DLC with low hydrogen (C2H2) and the hydrogen free DLC (H DLC, G DLC and ta C) also showed no significant wear as shown in Figs. 4(e h). Wear amounts of these samples were undetected level in measurements by surface profilometer. The mark of transferred counter steel was slightly observed on the wear track of TMS B sample. On the other hand, TMS/C7H8 and C7H8 samples were worn more deeply as shown in Figs. 4(c,d). Cross sectional areas of wear tracks for TMS/C7H8 sample and C7H8 samples were mm 2 and mm 2 respectively. In particular, TMS/C7H8 sample showed remarkable wear. The wear track appeared to be deep and reached the steel substrate which was partially exposed on the wear track. Wear scar on the counter steel ball was also observed as shown in Fig. 5. Wear scars for G DLC and ta C coating samples as shown in Figs. 5(g,h) were remarkably large in comparison with the other samples. It can be considered that the higher hardness and the larger surface roughness in hydrogenfree DLC can cause abrasive wear on the steel ball. 3.4 Surface analysis by TOF SIMS Tribofilm compositions and friction behavior TOF SIMS analysis is employed to investigate the relation between tribological behavior and tribofilm formation derived from lubricant additives. We conducted TOF SIMS analysis for worn surface of both steel balls and DLC disks after ball on disk Fig.4 Fig. 5 Optical micrograph of wear tracks on the DLC coated disks after the ball on disk tests: (a) TMS A; (b) TMS B; (c) TMS/C7H8; (d) C7H8; (e) C2H2; (f) H DLC; (g) G DLC; and (h) ta C samples Optical micrograph of wear scars on the steel balls after the ball on disk tests: (a) TMS A; (b) TMS B; (c) TMS/C7H8; (d) C7H8; (e) C2H2; (f) H DLC; (g) G DLC; and (h) ta C samples tests with the engine oil containing MoDTC and ZnDTP additives excluding TMS/C7H8 coating sample because the TMS/C7H8 sample was worn remarkably and the substrate was partially exposed. MoDTC is decomposed during tribological contact to form MoS2 tribofilm on surfaces, leading friction reduction [10,25,26]. MoDTC decomposes into the core unit of MoDTC molecule, and this forms the oxysulfide [11]. The oxysulfide decomposes into MoS2 and MoO2, and these can be oxidized into MoO3 in the presence of O2 [11]. Figures 6 and 7 show Mo related negative fragment ions detected on the topmost surfaces by TOF SIMS analysis. On the surface of steel ball, MoO3 detected in C2H2 sample was smaller in quantity in comparison with C7H8 sample; a lot of MoO3 appeared to be present on the steel ball surface in C7H8 sample as shown in Fig. 6(a). In contrast, oxysulfides (MoSO2 ; MoS2O ; MoS3O3 ) and sulfide (MoS3 ) detected in C2H2 sample showed larger intensity than that in C7H8 sample as shown in Fig. 6(b). Japanese Society of Tribologists ( Tribology Online, Vol. 12, No. 3 (2017) / 128

7 Friction and Wear Properties of Tetrahedral Si Containing Hydrogenated Diamond Like Carbon Coating under Lubricated Condition with Engine Oil Containing ZnDTP and MoDTC Fig. 6 TOF SIMS analysis for molybdenum (Mo) related negative fragment ions on steel ball surfaces after the ball on disk tests: (a) Mo oxide and molybdate; (b) Mo oxysulfides and sulfides Fig. 7 TOF SIMS analysis for molybdenum (Mo) related negative fragment ions on DLC coated disk surfaces after the ball on disk tests: (a) Mo oxide and molybdate; (b) Mo oxysulfides and sulfides The thermal decomposition of MoS3 produces MoS2 [27]. It is reasonable to assume that this can show the potential presence of MoS2 in tribofilm. The result suggested that the ratio of Mo sulfides to Mo oxide was larger in C2H2 sample. It is also reported that MoS2 and MoS3 show catalytic activity for hydrogen evolution [28]. It is not yet clear, in this study, whether the Mo sulfides (MoS2 and MoS3) also interact with hydrogen in the DLCs due to the tribo chemical reaction during sliding contact. As for DLC disk side, MoO3 was present primarily on the DLC coated disk surfaces as shown in Fig. 7(a). Figure 7(b) showed that MoS2O and MoS3 were detected slightly, however, differences between the coating samples can be found. On the topmost of DLC coating surface, Mo oxysulfide and sulfide fragment ions detected in C2H2 sample showed larger intensity than that in C7H8 sample. C2H2 sample exhibited the lowest friction coefficient in the ball on disk test. It was considered that C2H2 coating sample could promote MoS2 tribofilm formation effectively leading to the low friction coefficient. We have already reported that MoS2 formation in the tribofilm increased with increasing the microscopic surface roughness of DLC coating during DLC/steel sliding contact lubricated with engine oil containing MoDTC and ZnDTP additives [10]. It seems that sulfuration of molybdenum needs the higher thermal energy [29]. C2H2 sample had the higher hardness and the larger surface roughness in comparison with C7H8 sample, which could bring about energetic contact and thermal effect to promote the tribo chemical reactions leading to sufficient sulfuration. As for hydrogen free DLC samples (H DLC, G DLC, ta C), these samples had the higher hardness and the larger surface roughness. High content of C C sp 3 bond with low hydrogen content contributes the high hardness, and the large surface roughness is attributed to high ion energy deposition process of arc PVD. Therefore, hydrogen free DLC samples were expected to promote the Mo sulfide formation. From the results of TOF SIMS analysis, Mo oxide and oxysulfides were detected on the steel ball as shown in Fig. 6. The oxysulfide and sulfide were also detected on the DLC coated disk side as shown in Fig. 7, but detected oxysulfide and sulfide were very tiny for H DLC sample. On the steel ball side, it should be noted that the intensities of detected fragment ions were generally decreased with increasing the hardness and roughness of hydrogen free DLC samples. In this research, ta C sample showed typically smaller intensity in all of detected fragment ions, while H DLC showed rather large intensity in the Mo oxysulfides and sulfide. This tiny detection of each ion fragments for hydrogen free DLC samples as even G DLC or ta C could be considered from the reason as large wear of steel balls against hydrogen free DLC samples. It can be considered that such aggressive contact could cause the excessive wear and scrape the tribofilm off immediately even if tribofilm were generated on the sliding surfaces for hydrogen free DLC Japanese Society of Tribologists ( Tribology Online, Vol. 12, No. 3 (2017) / 129

8 Kentaro Komori and Noritsugu Umehara samples. Therefore, as for G DLC and ta C samples, it appeared to be difficult to form the steady tribofilm continuously, resulting in not reaching low friction. As for TMS coatings (TMS A, TMS B), TOF SIMS result showed that a lot of oxysulfides were observed on the steel ball in TMS A sample, in comparison with TMS B sample as shown in Fig. 6(b). Microscopic surface roughness of TMS A sample was much larger than that of TMS B sample as mentioned above. The results suggested that TMS A sample could promote Mo sulfide tribofilm on the sliding surface leading the lower friction coefficient than TMS B sample, which is similar to the C2H2 sample of hydrogenated DLC. TOF SIMS results showed that oxysulfides, particularly MoSO2, observed on the steel ball in TMS A sample showed relatively large intensity, in comparison with the other samples in Fig. 6(b). However, a lot of Mo oxide as MoO3 was also detected on the counter steel in TMS coating samples in Fig. 6(a); so that, this suggested that TMS coatings showed relatively small ratio of Mo sulfide to Mo oxide. On the other hand, TOF SIMS results also showed the presence of Mo oxide, oxysulfide, and sulfide on the topmost surface of TMS coatings, as shown in Fig. 7. The oxysulfide (MoS2O ) and sulfide (MoS3 ) observed on the TMS coating surfaces showed larger intensity than the other coatings as shown in Fig. 7(b). In particular, TMS A sample shows smaller intensity of MoO3 than TMS B sample, which suggests TMS A can show the larger ratio of Mo sulfide to Mo oxide in Fig. 7(a). Figure 8 shows the intensity of Mo + positive fragment ion and S negative fragment ion detected on the steel balls and the DLC coated disks. On the DLC coated surfaces, the both Mo + ion and S ion in TMS coatings were obviously observed more than the other coatings. In particular, Mo + showed very small intensity on the other coating surfaces. As for the TMS coating samples, Mo + ion and S ion on the coating surface showed the values close to the intensity of those on the steel ball surface, whereas for the other coating samples, Mo + and S ion ion were predominantly observed on the steel ball. Furthermore, Fig. 9 shows Si related fragment ions detected on the DLC coated surfaces. It could be found that the fragment ions indicate the presence of Si O bond such as SiOH +, SiO2 and SiHO2 on the coating surfaces of TMS coating samples. These were scarcely detected on the surfaces of the other coated samples, even on the counter steel surfaces of TMS coating samples. This corresponds to the variation trends of S ion and Mo + ion (in Fig. 8). The result implies that the Si O bond can induce adsorption of lubricant additives bonded to Fig. 9 Silicon (Si) related fragment ions detected on the DLC coated disk surfaces by TOF SIMS analysis after the ball on disk tests oxygen of the Si O, and tribofilm formation on the TMS coating surface, unlike the other DLC coating samples. Figure 10 shows phosphorus (P) related negative fragment ions detected on the steel balls and the DLC disks by TOF SIMS; phosphorus oxides (PO2, PO3 ) attributed to poly phosphate come from ZnDTP are mostly observed. It is well known that degradation of ZnDTP produces poly phosphate tribofilm to inhibit metal to metal contact of sliding parts [30,31]. The intensity variation of the phosphorus oxides due to different coating samples almost corresponded to the variation trends of Mo oxide in Figs. 6 and 7. These results could show that ZnDTP and MoDTC interacted strongly to form the tribofilm. It was reported that the ZnDTP derived tribofilm typically could be form even on the non ferrous DLC surfaces [10], and ZnDTP had the important role to provide the sulfur atoms to promote the sulfuration of the oxysulphide [11]. It has been also reported that ZnDTP can increase the tribo chemical formation of MoS2 on the steel surfaces in the combination with MoDTC [2 4,11,23,30]. However, TMS A sample showed small intensities of P oxides (PO2 and PO3 ) on the counter steel ball surface, as shown in Fig. 10, similarly for C2H2 and H DLC samples. Figure 11 shows the Fe + fragment ion detected on the counter steel ball surface by TOF SIMS. TMS A sample showed small intensity of Fe + on the steel ball surface. This suggests that the tribofilms on the steel ball for these samples are rather thick. Poly phosphate tribofilm can Fig. 8 TOF SIMS analysis for (a) Mo + positive fragment ion and (b) S negative fragment ion detected on the steel balls and the DLC coated disks Japanese Society of Tribologists ( Tribology Online, Vol. 12, No. 3 (2017) / 130

9 Friction and Wear Properties of Tetrahedral Si Containing Hydrogenated Diamond Like Carbon Coating under Lubricated Condition with Engine Oil Containing ZnDTP and MoDTC Fig. 10 Phosphorus (P) related negative fragment ions on (a) steel ball surfaces; and (b) DLC coated disk surfaces detected by TOF SIMS after the ball on disk tests be formed on the steel bonded to Fe constituents. TMS A sample showed a lot of Mo oxysulfides and Mo oxide on the steel ball surface, as mentioned above. Since the other compositions, such as Mo sulfide and/or Mo oxide rather formed on the polyphosphate film, the P oxides seem to be hardly detected on the topmost surface by TOF SIMS. TMS A sample showed smaller intensity of P oxides on the DLC coated disk surface in compared with TMS B sample in Fig. 10. This seems to be attributed to transferring of the tribofilm from the coating surface to the counter steel surface. Figure 12 shows the ratio of total intensity of Mo oxysulfides and sulfide to MoO3 detected by TOF SIMS. TMS A sample showed the higher ratio on both of the steel ball and DLC coated disk surfaces than TMS B sample. TMS A sample showed a lower friction coefficient close to C2H2 sample. TMS A sample could promote sulfuration to form MoS2 on the counter steel ball surface due to the larger surface roughness of TMS A sample, as mentioned above. TMS A sample has a possibility to hold the MoS2 formed on the outermost surface of the counter steel on the coating surface due to the Si O bond of TMS coating surface. The results suggest that MoS2 formation on both the steel surface and the DLC surface can lead the friction reduction in TMS A sample Tribofilm compositions and wear behavior The tribo chemical reactions are also related to the wear of DLC coatings. We have already reported that MoO3 in the tribofilm increased with an increasing hydrogen content of DLC coatings during the DLC/steel sliding contact in the presence of MoDTC, and reduction of the Mo oxides proceeded remarkably in the DLC coating with hydrogen content of at.% [10]. Additionally, it was also reported that the wear of DLC coating was dominated by oxidation of DLC material in pressurized hot water [32]. These suggest that the chemical reactions involving hydrogen and carbon in the DLC coating material can lead to significant wear of hydrogenated DLC. As reported in this study, C7H8 sample had high hydrogen content (36 at.%) with small surface roughness; furthermore, the higher the hydrogen content of DLC coating, the lower the hardness of that. C7H8 sample showed the weak sulfuration and a lot of Mo oxide particularly on the counter steel surface. It should be noted that the C7H8 sample shows remarkable wear. In contrast, C2H2 sample had the lower hydrogen content (18 at.%), which could induce the sulfuration on the sliding surfaces. No significant wear was observed in the C2H2 sample. As for the Fig. 11 Fe + positive fragment ion detected on steel ball surfaces by TOF SIMS after the ball on disk tests Fig. 12 Intensity ratio of Mo oxysulfides and sulfide to Mo oxide detected by TOF SIMS analysis hydrogen free DLC samples (H DLC, G DLC, and ta C), significant wear was not observed, either. TOF SIMS results show that MoHO4 fragment ion is observed on the topmost surfaces as shown in Figs. 6(a) and 7(a). It is interesting to note that intensity variation of MoHO4 depending on different coating samples almost corresponds to the variation trend of MoO3. MoO3 shows catalytic activity and is known to interact with hydrogen, oxygen, Japanese Society of Tribologists ( Tribology Online, Vol. 12, No. 3 (2017) / 131

10 Kentaro Komori and Noritsugu Umehara or hydrocarbons to form various intermediate compositions. For example, it was reported that HxMoO3 can be generated during the reduction process from MoO3 to MoO2 [33]. MoHO4 was observed at a certain extent even for the DLC coating samples without hydrogen content; therefore, only hydrogen in DLC materials does not necessarily contribute to the MoHO4 production. Furthermore, if MoO3 interacts with the DLC materials, its influence should be reflected in the chemical state of tribofilm on the DLC coating surfaces originally. However, as shown in Fig. 7(a), the detected intensities of MoO3 and MoHO4 were remarkably small in the C7H8 sample which showed significant wear. This discrepancy can be due to carbon transferring from the DLC coating to the counter steel surface involving wear of the DLC coatings; besides, the wear behavior of DLC coatings are different depending on the samples, making direct observation of reaction process difficult. We reported similar issue that the presence of metallic Mo or Mo2C as a result of the reduction reaction of MoO3 was predominantly detected on counter steel surface by XPS analysis for hydrogenated DLCs which showed significant wear [10]. In this study, the detected intensity of MoHO4 varied depending on the different coating samples. Then, on the counter steel ball surfaces, TMS coatings and C7H8 sample with higher hydrogen content showed more intensity than C2H2 sample with a lower hydrogen content. Thus, MoO3 may possibly interact with the hydrogen in the hydrogenated DLC materials at least. In particular, TMS coatings exhibited the significant presence of MoO3 on the counter steel ball. Similarly, MoO3 detected on the DLC surfaces of TMS coatings also indicated larger intensity in TOF SIMS; besides, TMS coatings had higher hydrogen content (around 30 at.%). However, TMS coatings exhibited no significant wear. These results of wear were clearly different from cases for C7H8 or TMS/C7H8 samples which are typical hydrogenated DLCs with high hydrogen content. It should be noted that the TMS coatings have high content of carbon sp 3 bond, similar to the hydrogen free DLCs. Thus, accelerated wear of DLC coatings in the presence of MoDTC cannot be explained only by chemical reaction of hydrogen and carbon in DLC materials. Thermal load and strain effects during tribological contact can cause transformation from diamond like to graphite like carbon structure [34,35]. It was considered that the acceleration of wear in the presence of MoDTC should depend on the carbon structure transforms into a further weak structure involving chemical reaction with DLC. 3.5 Reciprocating pin on plate test Further tribological test was conducted with reciprocating DLC coated pin on cast iron plate configuration because the coating samples of disks did not show significant wear in the ball on disk test. The DLC coatings were coated on the pin side, which was opposite configuration of the ball on disk test. The DLC coated pin side was exposed to contact pressure at all times during tribotest. Therefore, it can be considered that the contact condition in this pin on plate test was more severe on DLC wear in terms of frequency and thermal stress during repeated sliding contact. G DLC, C2H2, and TMS B samples were applied to the pin on plate test. G DLC sample had rather graphitic structure in the hydrogen free samples. C2H2 sample was the only hydrogenated DLC which showed no significant wear in the ball on disk test. TMS B sample had the lower hardness, and Mo oxides were notably observed on the coating surface in the ball on disk test. Thus, G DLC and TMS B samples were intentionally selected from the each of hydrogen free DLC group and TMS coating group, as unfavorable samples to wear in the presence of MoDTC. This test was conducted to confirm the difference of wear behavior depending on carbon bond structure of DLC. Figure 13 shows the result of the pin on plate test in the engine oil containing MoDTC and ZnDTP additives same as that used in ball on disk test. As for G DLC sample, no significant wear was observed on the DLC pin surface, however, the counter cast iron plate exhibited remarkable wear. The cross hatching groove on the plate surface looked as if it is going to be disappeared, and a lot of scratched marks were observed on the worn track. G DLC sample did does not reach the highest sp 3 content as ta C, but it had the relatively higher sp 3 bond content than a C:H and the disordered sp 2 rings still remained in graphite like amorphous structure. Besides, almost no hydrogen was introduced in the G DLC sample, nearly hydrogen free. G DLC sample could cause an aggressive contact and excessive wear on the counter plate surface, due to the high hardness and the large surface roughness. This can pare off the tribofilm immediately. It appeared to be difficult to induce the tribofilm formation on the sliding surfaces. This is very similar to the result of ball on disk test. In contrast, C2H2 sample was worn remarkably and the substrate was clearly exposed as shown in Fig. 13, although the C2H2 sample showed no significant wear in the ball on disk test. Contact regime was no longer the DLC/cast iron sliding contact; it appeared to have completely changed into the metal to metal contact. The sp 2 sites in C2H2 sample were mainly organized in aromatic rings, but the C2H2 sample also had a certain amount of measurable C C sp 3 bond with relatively lower hydrogen content (18 at.%). This could endure the sliding contact stress and the thermal load in the ball on disk test. However, if thermal load on the DLC is so severe like this pin on disk test, it can cause to begin the hydrogen evolution and the destabilization of carbon bond structure. Simultaneously, the reduction of Mo oxides can cause the chemical reactions with hydrogen and carbon of DLC in the presence of MoDTC, as mentioned above. This can induce the transformation of DLC material to further weak sp 2 bonded structure, accelerating wear of DLC. Thus, the C2H2 sample seems to be the critical structure on the DLC wear in the presence of Fig. 13 Results of reciprocating pin on plate test: (a) optical micrograph of wear scars on DLC coated pin surfaces; (b) optical micrograph of wear tracks on cast iron plate surfaces; and (c) average coefficients of friction over the tests Japanese Society of Tribologists ( Tribology Online, Vol. 12, No. 3 (2017) / 132