The ta-C hardness effect on wear properties under MoDTC contained boundary lubrication

doi:10.21203/rs.3.rs-3150434/v1

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The ta-C hardness effect on wear properties under MoDTC contained boundary lubrication

https://doi.org/10.21203/rs.3.rs-3150434/v1

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published 06 Dec, 2023

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The wear acceleration of hydrogenated amorphous carbon under lubrication with molybdenum dithiocarbamate (MoDTC) is one of the problems to apply it to engine components. It is assumed that using high hardness tetrahedral amorphous carbon (ta-C) can easily solve the wear problem regardless of chemical wear by Mo degradable materials, however, we still do not know the effect of ta-C coating hardness on wear of itself with MoDTC. Because, amount and structure of Mo degradable material is assumed to relate contact pressure. It suggested that even if ta-C coating has high hardness, the wear amount changes with normal load. In this study, we prepared four different hardness ta-C coatings (such as approximately 18.2, 22.1, 46.6, and 53.6 GPa) and conducted friction tests to clarify the effect of ta-C hardness on wear characteristics under boundary lubrication with MoDTC. Eventually, chemical analysis (Raman, EDS and XPS) was conducted to reveal the chemical spices existence of tribofilm for three hardness ta-Cs, especially for 22.1 GPa as a representative low hardness ta-C coating. As a result, the wear suppression was achieved when the contact pressure was higher than 200 MPa regardless of the coating hardness.

tetrahedral amorphous carbon

boundary lubrication

MoDTC

wear acceleration

It is believed that greenhouse gases are one of the causes of global warming. Carbon dioxide has a high percentage of these gases, and transport emissions from vehicles account for approximately 20% of the world's carbon dioxide emissions. It was reported in 2012 that 17% of the fuel energy in vehicles is consumed by friction loss [1, 2]. Therefore, to save fuel consumption, reducing friction and wear loss in the mechanical sliding parts of automobiles is a serious issue in tribology engineering.

In general, one approach to control friction and wear loss is coating material for surface. It is expected that Diamond-Like Carbon (DLC) coatings would be applied to automotive systems. DLC coatings are well known for offering excellent mechanical and tribological properties of low friction, high hardness, high wear resistance and chemical stability, because of amorphous carbon consisting of sp² and sp³ structures. They have been shown to offer low friction and outstanding wear performance in both dry and lubricating conditions. Application of DLC coatings for automotive components is considered as a promising strategy to cope with the emerging requirements regarding fuel economy and durability [3–9]. DLC coatings are mainly divided into three types; tetrahedral amorphous carbon (ta-C) coating [10–16], hydrogenated amorphous carbon (a-C:H) coating [17–19], and others [20–25]. Those were subjected to friction by authors. The differences of these different types DLC coatings are containing hydrogen or not and the ratio of sp² and sp³ structures of carbon. Because of these differences, friction and wear properties of ta-C coating and a-C:H coating are a little different. Compared to a-C:H coating, ta-C coating has higher hardness and wear resistance. Wear suppression can be achieved by reducing friction and forming a sheared transfer layer on the friction surface. Thus, the bond change in the DLC coating has a great influence on the friction and wear characteristics.

The other approach to control friction and wear loss is using lubricating oil containing additives. As a good kind of friction modifier, molybdenum dithiocarbamate (MoDTC) is widely used because molybdenum disulfide (MoS₂) is highly effective in reducing friction and wear of sliding surfaces under boundary lubrication, which is one of the decomposition products of MoDTC. In the research of Grossiord et al., when a friction test between carbon steels is performed using MoDTC containing lubricating oil, MoS₂ adhere to the friction surface like a film (tribofilm) [26, 27], and the formation of such tribofilm was observed via in-situ observation using reflection spectroscopy [28, 29]. Therefore, the friction reduction by MoDTC decomposes MoDTC at the friction interface.

Due to the excellent wear resistance, seizure, and friction characteristics of DLC coating, applying DLC to the sliding parts in the automobile engine has a great improvement of engine efficiency. However, many problems have been reported in which the wear amount of DLC increases sharply under MoDTC-containing lubrication [30–47]. Despite necessity of developing DLC or lubricating oil which could suppress wear acceleration, the details of the wear mechanism have not been clarified yet. Therefore, when applying DLC coatings at present, DLC is partially applied to a few parts, and it is necessary to use a specific coating type and lubricating oil. So, it is necessary to clarify the mechanism in order to further expand the DLC application in future. The wear mechanism of DLC by applying MoDTC is reported as Mo₂C generation during friction. Mo₂C is a harder material than MoS₂, and it is believed that it scratches DLC surface. It is assumed that using high hardness ta-C with MoDTC can easily solve the wear problem regardless of chemical wear by Mo degradable materials. The performance of nano-crystalline diamond (NCD: which was high sp³ and the hardness was approximately 74 GPa) under lubrication with MoDTC was reported in literature [48], approximately 7000 Hv ta-C showed low wear [49], 60–70 GPa hardness showed no acceleration of wear [50], however, we still do not know the effect of ta-C coating hardness on wear of itself with MoDTC. Because, amount and structure of Mo degradable material is assumed to relate contact pressure. It suggested that even if ta-C coating has high hardness, the wear amount changes with normal load. In this study, we prepared four different hardness ta-C coatings (such as approximately 18.2, 22.1, 46.6, and 53.6 GPa) and conducted friction tests to clarify the effect of ta-C hardness on wear characteristics under boundary lubrication with MoDTC. Eventually, chemical analysis was conducted to reveal the chemical spices existence of tribofilm for three hardness ta-Cs, especially for 22.1 GPa as a representative low hardness ta-C coating.

2.1. Friction test

To clarify the effect of ta-C hardness on wear under boundary lubrication with MoDTC, we conducted a roller-on-disk friction tests by using rotating sliding friction tester. Figure 1 shows the front and top view of the friction tester. The roller was fixed to specimen holder, therefore it did not roll. The friction tester equips a load cell as friction force sensor to measure during friction tests. A normal load is applied by dead weights.

$$\text{w=}\frac{\text{V}}{\text{L }\text{×}\text{ N}}$$

where w is the specific wear rate of a ta-C roller, V is the wear volume, L is the sliding distance and N is the normal load. The wear volume of ta-C rollers in each 400 cycles till 4800 cycles were obtained to clarify the wear process. The wear volume of initial 400 cycles showed higher amount rather than end of the friction test as shown in section 3.2. So, we conducted short friction tests as 400 cycles to obtain initial wear volume to clarify wear mode under PAO or PAO + MoDTC.

$${P}_{max}=\frac{W}{\pi bl}$$

$$b={\left(\frac{4WR}{\pi lE}\right)}^{0.5}$$

$$\frac{2}{E}=\frac{1-{{v}_{1}}^{2}}{{E}_{1}}+\frac{1-{{v}_{2}}^{2}}{{E}_{2}}$$

$$\frac{1}{R}=\frac{1}{{R}_{1}}+\frac{1}{{R}_{2}}$$

where, P_max is maximum contact pressure, W is a normal load, b is a half of contact width which is assumed to be an ellipse, l is a half–length of contact area which is perpendicular to the length b, E is equivalent elastic modulus, and R is equivalent radius, which could be calculated by roller’s elastic modulus E₁ and Poisson’s ratio v₁, disk’s elastic modulus E₂ and Poisson’s ratio v₂, and roller’s radius R₁, disk’s radius R₂, respectively [51].

$$\varLambda =\frac{{h}_{min}}{\sqrt{{{R}_{q1}}^{2}+{{R}_{q2}}^{2}}}$$

$${h}_{min}=2.65{\left(\frac{{\eta }_{0}u}{ER}\right)}^{0.7}{\left(\alpha E\right)}^{0.54}{\left(\frac{W}{E{R}^{2}}\right)}^{-0.13}$$

where h_min is the minimum oil film thickness, R_q1 is root mean square roughness of roller, R_q2 is the roughness of disk, η₀ is viscosity of oil under atmospheric pressure, u is average rotating speed, α is pressure coefficient of viscosity, W is load per unit width, E is equivalent elastic modulus and R is equivalent radius, which could be calculated by roller’s elastic modulus E₁ and Poisson’s ratio ν₁, disks are shown as E₂ and ν₂. According to the above estimation, the initial of the friction test to the end of the friction test were all under Λ < 1. Therefore, it was assumed that the lubrication conditions were under boundary lubrication.

2.2. Surface analysis

After finishing friction test under boundary lubrication and calculating the specific wear rate of the ta-C rollers, the wear track of SUJ2 disks and wear scar of ta-C rollers were observed via optical microscope. Furthermore, scanning electron microscopy (SEM) coupled with energy dispersive x-ray spectroscopy (EDS) and x-ray photoelectron spectroscopy (XPS) analyses were both conducted to find out what kind of elements and chemical compounds on the wear track of SUJ2 disks and ta-C rollers. From SEM-EDS analysis, we could obtain the images of elements composition and distribution on the wear track of SUJ2 disks and ta-C rollers. And from the information of elements composition and distribution by those SEM-EDS images, we could determine the specific spot area to conduct XPS analysis. The spot size of XPS analysis was 100 µm square. The depth of XPS analysis is assumed several nm–10 nm. Before the measurement, a natural contamination layer was removed by Ar ion beam sputtering with a sputtering range of 2 mm square; using an acceleration voltage of 4 kV for 15 seconds. The entire measured photoelectron spectrum was shifted by several eV due to measurement error. So, we confirm a little energy shift at each measurement by Ar peak position which appears stably at 241.9 eV. Shirley algorithm and Gaussian-Lorentzian function were used to separate the peak. In this paper, both inside and outside of the wear track of SUJ2 disks and ta-C rollers were selected as spot area to exclude the effect of other factors like contamination and oxidation of the metal surface, and compare the differences caused by friction test. Both C peak and Mo peak were separated to characterize how amorphous carbon structures changed and composition of different Mo compounds decomposed from MoDTC. To clarify the bonding state of the Mo compound derived from MoDTC adhering to the wear scars, the narrow spectrum of Mo was obtained by XPS. The peak separation conditions are determined by the measurement results of the powder standard samples [53–59]. Due to the reduction effect of etching, low-valence peaks were also confirmed from tetravalent and hexavalent Mo compounds. The hexavalent peak can be confirmed from the tetravalent MoS₂ because the oxide on the surface has not been completely removed. Since the positions of the peaks of 0-valent and 2-valent Mo are very close and cannot be separated, we consider four types of peaks with the same value. Since Mo occupies the 3d orbital, the peak splits into 3d_5/2 and 3d_3/2. Therefore, there are totally 9 peaks, including a total of 8 peaks and nearby 2s orbital peaks, the peak position of S. Mo3d_3/2 is 3d_5/2 + 3.13 eV, and the area ratio is 3d_5/2:3d_3/2 = 3:2, and the half-width is the same. Limit 80% of the Gaussian function ratio at each peak, and perform peak separation under the condition of a half-value width of 1.0–3.0 eV. The 0-valent, 4-valent, and 6-valent Mo compounds are considered to be produced by the decomposition of MoDTC. The divalent Mo and Mo₂C can be produced by bonding with C atoms in the ta-C coating. It is believed that pentavalent Mo is mainly generated when MoO₃ is sputtered. In addition, it is reported that when Mo is supported on a metal oxide such as alumina, there are both pentavalent and stable tetravalent, hexavalent, and an intermediate layer of oxysulfide may also exist [60–62].

3.1 Specific wear rate of 4 different hardness ta-C rollers under boundary lubrication

The wear volume of 4 different hardness ta-C rollers were measured via confocal laser scanning microscope and specific wear rate of each ta-C rollers was calculated from Eq. (1). In the conditions of 85°C and increasing normal load from 5–15 N, the results of specific wear rate are shown in Fig. 3(a) and (b). The specific wear rate of 4 different hardness ta-C rollers under PAO changed from approximately 0.81 × 10^− 7 to 0.35 × 10^− 7 mm³/Nm with increasing hardness. The specific wear rate of each hardness ta-C roller shows almost same value which means that specific wear rate of ta-C rollers was independent of normal load when using PAO. However, the specific wear rate of the same hardness ta-C roller shows different values under PAO + MoDTC lubrication when changing the normal load from 5–15 N. In addition, the softer ta-C roller used, the greater gap in the specific wear rate occurred when changing normal load (specific wear rate of 18.2 GPa ta-C roller changed from approximately 1.35 × 10^− 7 to 3.41 × 10^− 7 mm³/Nm and that of 53.6 GPa ta-C roller changed from approximately 0.82 × 10^− 7 to 1.39 × 10^− 7 mm³/Nm). It was also clarified that the high wear was more pronounced at the lighter load of 5 N than that at 10 and 15 N. It was shown that the wear of the ta-C coating was not always constant and that the amount of wear differs only when the coatings of different hardness are prepared.

To clarify wear mechanism of ta-C, representative ta-C (22.1 GPa) under PAO and PAO + MoDTC friction tests were conducted. The wear volume measurement in each 400 cycles till 4800 cycles were obtained. The representative data of 15 N under PAO, PAO + MoDTC is shown in Fig. 4. The wear volume in PAO showed

proportional along with number of cycles. On the other hand, under PAO + MoDTC showed higher amount of ta-C wear during first 400 cycles. It was suggested that the wear volume increment decreased after the initial friction cycles due to the reduction of contact pressure because the initial wear for the roller made the contact width increase. The wear scar width on the roller specimens were observed optical microscope and SEM as shown later. Under the PAO + MoDTC situation showed larger wear scar width than only PAO situation. It caused reduction of contact pressure then it was assumed that the generation of Mo related degradable material reduced because of low contact pressure. The specific wear rate measured in PAO + MoDTC shows higher than in PAO for all the tests. Generally, it is assumed that wear mechanism is mechanical wear if specific wear rate and inverse value of hardness are in liner proportion. Figure 5 shows the relation between the specific wear rate and inverse value of ta-C hardness under PAO, and PAO + MoDTC as a representative of 15 N result. Under the PAO situation, the specific wear rate depends on the inverse value of hardness which indicates the wear mechanism is mechanical wear. The specific wear rate of PAO + MoDTC shows monotonic increment, and it shows an intercept of specific wear rate axis and the liner line of PAO + MoDTC. This intercept means that wear have to occur even if the hardness of the ta-C becomes maximum.

The maximum Hertzian contact pressures of different hardness ta-C under different normal load are calculated from Eq. (2)–(5). The relation between maximum contact pressure and specific wear rate under PAO, PAO + MoDTC are shown in Fig. 6(a) and (b). The specific wear rate of ta-C slid under PAO shows almost same values under different contact pressure. On the other hand, the specific wear rate of low P_max shows scattering values when ta-C slid under PAO + MoDTC. The scattering of specific wear rate saturates under around 200 MPa or over. The low hardness ta-C (18.2 and 22.1 GPa) shows wear amount reduction with contact pressure. The high hardness ta-C (46.6 and 53.6 GPa) shows inflection point around 200 MPa, then the specific wear rate reduces. Those results indicated that there are optimum contact pressure to reduce wear amount.

3.2 Friction coefficient of 4 different hardness ta-C rollers under boundary lubrication

Friction tests were conducted by using 4 different hardness ta-C rollers against SUJ2 disk under boundary lubrication when using only PAO as base oil and PAO with MoDTC for comparison. Besides, there were two groups friction tests conducted. One group friction test was conducted when changing normal load and the other group friction test was conducted when changing temperature. Thus, the results of friction coefficient increasing normal load from 5 N to 15 N are respectively shown in Fig. 7(a)–(f). In the series of friction coefficient under PAO, friction coefficient was almost stable from the initial of the test. At the very beginning of the test, high friction was detected in 53.6 GPa ta-C roller. It was assumed that the 53.6 GPa ta-C roller had the highest surface roughness among the rollers, so the asperities had damaged as shearing or fracture at the beginning of the test. After the short cycles of friction, the surface roughness decreased as same level as other rollers.

The results of each average friction coefficient were calculated as the stable friction coefficient of 1600–2400 cycles, which are summarized in Fig. 8(a) as under PAO and (b) as under PAO + MoDTC. Almost friction coefficients under PAO show approximately 0.07–0.08 regardless of the ta-C coating hardness without 5 N of 22.1 GPa ta-C. The lowest friction coefficient was obtained at 5 N of 22.1 GPa, the value was approximately 0.056. The friction tests under PAO did not make any tribofilm on ta-C rollers and SUJ2 disks, so it was ta-C and SUJ2 friction coefficient under boundary lubrication. On the other hand, friction coefficient measured under MoDTC was assumed to be affected by tribofilm. The friction coefficients under 5 N shows lower than approximately 0.06 regardless of the hardness, and the lowest one was 0.049 of 22.1 GPa ta-C. Almost all average friction coefficient under MoDTC containing lubrication at each normal load showed lower values rather than under PAO. This results implied that the reduction of friction coefficient was due to MoS₂ generated on the surface. To compare with environmental temperature on friction coefficient, friction tests of each hardness ta-C slid against SUJ2 disk under PAO, PAO + MoDTC under 20°C were conducted. The summary of results is shown in Fig. 9. At 20°C, there were nearly no changes of friction coefficient between PAO, and PAO + MoDTC. The friction coefficient of PAO + MoDTC under 85°C showed decrease without 18.2 GPa of ta-C roller. This friction coefficient reduction is assumed formation of MoS₂ tribofilm. All results of 20°C show no friction reduction. It implied that MoDTC did not act as friction modifier under 20°C.

3.3 Surface analysis of sp²/sp³ structure changes and Mo-tribofilm

From the images taken via optical microscope are shown in Fig. 10(a)–(c) as (a) 400 cycles, (b) 1600 cycles, and (c) 4800 cycles under PAO + MoDTC, Fig. 10(d)–(f) under PAO at 85°C and 15 N of normal load; 22.1 GPa hardness ta-C as a representative data. Figure 11(a)–(c) shows SEM-EDS of same specimen tested in PAO + MoDTC as shown in Fig. 10. It was clear that the wear scar on ta-C roller after friction test, the element of Mo, S, C and O could be detected, which means that molybdenum tribofilm derived from MoDTC was generated during friction test. Generally, it is difficult to distinguish Mo and S by EDS because the peak positions at kinetic energy are too close. Therefore, it is necessary to obtain other evidence. We conducted XPS and Raman analysis to distinguish what chemical bonds were existed as shown later. The wear scar was generated at the initial 400 cycles as shown in Figs. 10 and 11, especially for MoDTC included situation shows wider wear scar. The EDS map analysis shows that Mo and S were on the wear scar at 400 cycles. Then, Mo and S intensity in the wear scar decreased with friction cycles and the high intensity area moved toward front edge and back edge of wear scar. The results indicated that contact area between ta-C roller and SUJ2 disk were covered by Mo and S during initial friction cycles.

In order to clarify the bonding state of the Mo compound derived from MoDTC and structure changes of amorphous carbon, Raman and XPS analysis was conducted. The representative surface of 15 N normal load, 400 cycles and 85°C with MoDTC is shown in the paper. Figure 11 shows Raman analysis results. The measurement positions are shown in Fig. 12 (a) as spot1–spot3. Those spots are molybdenum spices which formed from MoDTC during friction. The all spots show almost same peak around D peak (1330 cm^–1) to G peak (1580 cm^–1) as shown in Figs. 12(b)–(d). The enlargement Raman data of three spots from 100–1100 cm^–1 are shown in Figs. 12 (e)–(g). The peaks are divided several molybdenum spices. It is clearly exhibited that MoS₂ existed at the sopt1, MoO₂, MoO₃, and Mo₂C also existed in several spots. This result indicated that MoDTC was degraded during friction, several molybdenum spices attached on ta-C roller as deposited material. Furthermore, Raman analysis for wear scars were conducted. The typical analysis results of before and after friction of 22.1 GPa ta-C roller surface is shown in Fig. 13. The as-deposited surface is shown in Fig. 13(a) and the wear scar surface is shown in (b). The results show that as-deposited ta-C surface shows smaller fluctuation of measured Raman shift rather than after friction surface, although the measurement procedure and same incident laser light irradiation condition. It indicated wear scar of ta-C changed as damaged ta-C. The summary of I_D/I_G ratio and G peak position change of 400 cycles’ surfaces of different hardness is shown in Fig. 14. The I_D/I_G ratio of each hardness ta-C increased by friction as shown in Fig. 14(a). It generally interprets as graphitization or disruption of carbon covalent bonds. Figure 14 (b) shows G peak position change. It shows a little G peak center position change. However, it does not show same tendency as increase or decrease. From the results of Raman measurement, the intensity became small by friction as shown in Fig. 13 and I_D/I_G ratio increase, the ta-C coating surface had some damage and it is interpreted that inherent hardness is not maintained.

Figure 14 The summary of each hardness ta-C coating (a) I_D/I_G ratio of as-deposited and in wear scar, (b) G peak position (400 cycles, 15 N, 85°C).

The representative data of ta-C roller (22.1 GPa) surface was analyzed via XPS is shown in Fig. 15 as (a) outside of wear scar, (b) inside of wear scar at 400 cycles (15 N, 85°C), and (c) inside of wear scar at 2400 cycles (15 N, 85°C). All figures show C1s peak and Mo3d peak. The outside of wear scar shows no O1s. The outside of wear scar was just immersed in lubricants. Therefore, the Mo3d peak derives from MoDTC additive which attached on the ta-C surface as intact. In the wear scars, they show O1s and Mo3d peak which clearly show two divided peaks of molybdenum. The results indicated that inside of wear scar had molybdenum oxide spices which was derived from MoDTC by friction.

The detail of C1s, O1s and Mo3d XPS data is shown in Fig. 16 as the representative of ta-C roller (22.1 GPa) after friction at 400 cycles with 15 N, 85°C. The outside of wear scar is interpreted as-deposited surface. It has carbon covalent bonds (Fig. 16(a)). After the friction test surface shows new bond of Mo-C at 283.3 eV in Fig. 16(b). According to the Figs. 16 (c) and (d) shows oxygen and molybdenum spices. Especially for molybdenum spices, Mo and Mo²⁺ at 228.3 and 231.5 eV is the main peaks and Mo⁴⁺ at 229.4 and 232.5 eV follows the 2nd high intensity. The peak of 228.3 and 231.5 eV is assumed Mo₂C which is verified from Fig. 16(a) as Mo-C bond and Raman analysis result shown in Fig. 12(f). The Mo⁴⁺ peak is assumed as MoO₂ which is intermediate degradated material of MoDTC or MoS₂, both is also revealed via Raman analysis (Fig. 12). Mo⁶⁺ and Mo⁵⁺ are decomvoluted from the Mo peak and it shows MoO₃ and MoDTC. This result indicated that the wear scar is consisted only carbon prior to the friction test as C-C and C = C as main peaks, then C-C bond intensity decreased and C-O and Mo-C increased by friction. Furthermore, Mo related spices attached on wear scar.

Figure 17 shows the summary of molybdenum spices’ chemical bonds existence percentage as decomvolution of Mo3d with different hardness of ta-C rollers. The typical difference among the different hardness rollers is shown that Mo and Mo₂C existed higher amount for low hardness ta-C rather than higher hardness one. The existence of Mo₂C is assumed wear acceleration because Mo₂C is hard material (approximately 15 GPa) as abrasive powder in polishing field [41, 46]. The second typical difference is MoS₂ and MoO₂ existence. The higher hardness ta-C shows higher amount of MoS₂ and MoO₂ rather than lower hardness ta-C. This result indicated the possibility of low friction coefficient for high hardness ta-C. Finally, MoDTC amount for higher hardness shows high percentage rather than low hardness, and MoO₃ amount shows high for low hardness ta-C. Those results indicated that low hardness ta-C consumed MoDTC during friction faster than high hardness ta-C although the friction test conditions were same. The differences of molybdenum spices generation are assumed to be determined by contact pressure. The contact pressure between different hardness ta-C roller and SUJ2 disk can be estimated from Hertzian contact related to the material properties and geometrical shape of specimens. The generation of MoS₂ is related to contact pressure [63, 64]. It is thought that the hardness of the ta-C exceeding a certain value facilitates the formation of MoS₂, and as a result, little intermediate products are formed and wear is not accelerated.

In hydrogen-containing DLC coating, Komori et al. reported that different wear rates were obtained depending on coating hardness, normal load, surface roughness and the amount of MoS₂ generation [65]. This study shows that in ta-C coating, there is a low contact pressure environment in which coatings of different hardness tend to wear, and that the higher the contact pressure, the lower the wear. In the case of low contact pressure, as shown by EDS and Raman analysis as shown in Figs. 11 and 12, intermediate products such as MoO₃ and Mo₂C are generated as tribofilm in the wear scar, which accelerate wear, however as the coating hardness increases, the amount of Mo₂C in the intermediate products decreases and wear is suppressed. These results indicate that excessive wear of the ta-C coating can be avoided by controlling the coating hardness and the amount of molybdenum intermediate product formed by the contact pressure.

In this study, we investigated the effect of ta-C coatings’ hardness on wear and friction properties under boundary lubrication with/without MoDTC in PAO by conducting friction test using 4 different hardness ta-C (such as approximately 18.2, 22.1, 46.6, and 53.6 GPa) rollers against SUJ2 disk at 5, 10, and 15 N under 20 and 85°C. The preparation of 4 different hardness ta-C was able to reveal the effect of coating’s hardness on wear under different contact pressure. Then XPS, EDS as well as Raman analysis were conducted. Obtained main results are shown below.

The specific wear rate of each hardness ta-C roller shows almost same value (0.81 × 10^− 7 to 0.35 × 10^− 7 mm³/Nm) which means that specific wear rate of ta-C rollers was independent of normal load when using PAO. However, the specific wear rate of the same hardness ta-C roller shows different values under PAO + MoDTC lubrication when changing the normal load from 5–15 N. It was shown that the wear of the ta-C coating was not always constant and that the amount of wear differs only when the coatings of different hardness are prepared.

The specific wear rate of 4 different hardness ta-C showed scattered values when Herztian contact pressure was low (such as lower than 200 MPa). On the other hand, the specific wear rate decreased when the contact pressure was higher than 200 MPa, even if the ta-C coating hardness was low. It indicated that, wear amount suppression was able to be controlled by contact pressure.

The wear volume in PAO showed proportional along with number of cycles. On the other hand, under PAO + MoDTC showed higher amount of ta-C wear during first 400 cycles.

At 20°C, there were nearly no changes of friction coefficient between PAO, and PAO + MoDTC (approximately 0.07–0.08). The friction coefficient of PAO + MoDTC under 85°C showed decrease without 18.2 GPa of ta-C roller. This friction coefficient reduction is assumed formation of MoS₂ tribofilm. All results of 20°C show no friction reduction. It implied that MoDTC did not act as friction modifier under 20°C.

The reason of specific wear rate reduction for higher hardness ta-C was investigated by XPS, EDS and Raman analysis. The tribofilm generated on ta-C roller showed lower amount of MoO₃ and Mo₂C for the highest hardness ta-C (53.6 GPa) rather than lower hardness one (22.1 GPa). These results indicate that excessive wear of the ta-C coating can be avoided by controlling the coating hardness and the amount of molybdenum intermediate product formed by the contact pressure.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Takayuki TOKOROYAMA: Conceptualization, Methodology, Investigation, Project administration, Resources, Writing – original draft. Noritsugu UMEHARA: Supervision, Writing – review & editing. Shuai WANG: Investigation, Project administration, Resources, Methodology, Writing – review & editing. Motoyuki MURASHIMA: Methodology, Writing – review & editing. Woo Young LEE: Methodology, Writing – review & editing.

Data availability statement

The data used in this research is available upon request.

Acknowledgments

The authors would like to thank JSPS KAKENHI Grant Number 17K14577, The Hibi Science Foundation, Nagamori Foundation, Research Foundation for the Electrotechnology of Chubu and NSK Foundation for the Advancement of Mechatronics for their financial support.

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You are reading this latest preprint version

The ta-C hardness effect on wear properties under MoDTC contained boundary lubrication

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Experimental method and procedure

2.1. Friction test

2.2. Surface analysis

3. Results and discussion

3.1 Specific wear rate of 4 different hardness ta-C rollers under boundary lubrication

3.2 Friction coefficient of 4 different hardness ta-C rollers under boundary lubrication

3.3 Surface analysis of sp²/sp³ structure changes and Mo-tribofilm

Conclusion

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1

The ta-C hardness effect on wear properties under MoDTC contained boundary lubrication

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Experimental method and procedure

2.1. Friction test

2.2. Surface analysis

3. Results and discussion

3.1 Specific wear rate of 4 different hardness ta-C rollers under boundary lubrication

3.2 Friction coefficient of 4 different hardness ta-C rollers under boundary lubrication

3.3 Surface analysis of sp2/sp3 structure changes and Mo-tribofilm

Conclusion

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1

3.3 Surface analysis of sp²/sp³ structure changes and Mo-tribofilm