In Situ Observation of Lubricant Film of Semi-Solid Lubricants at EHL Contact Using Micro-FTIR

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1 Tribology Online, 11, 2 (2016) ISSN DOI /trol Article In Situ Observation of Lubricant Film of Semi-Solid Lubricants at EHL Contact Using Micro-FTIR Koji Takiwatari 1)*, Yasushi Hoshi 2), Hidetaka Nanao 2), Tomie Hojo 2) and Shigeyuki Mori 2) 1) National Institute of Technology, Ichinoseki College Takanashi, Hagisho, Ichinoseki, Iwate , Japan 2) Faculty of Engineering, Iwate University Ueda, Morioka, Iwate , Japan * Corresponding author: takiwatari@ichinoseki.ac.jp ( Manuscript received 31 October 2015; accepted 22 February 2016; published 30 April 2016 ) ( Presented at the International Tribology Conference Tokyo 2015, September, 2015 ) Semi-solid lubricants, such as grease and gels, are mixtures of oil, thickeners, and gel agents. In this study, the concentration of the urea thickener and gel agent at the point of elastohydrodynamic lubrication (EHL) contact was investigated by in situ observation with a micro-fourier transform infrared spectrometer (micro-ftir). Micro-FTIR was used in conjunction with a ball-on-disk lubrication tester. The lubricant film was analyzed by micro-ftir under dynamic conditions. Characteristic peaks correlating to the C-H stretching mode at cm -1 and the N-H stretching mode at cm -1 were found to originate from the oil and the urea thickener and gel agent, respectively. The concentration of the thickener and gel agent were estimated from these characteristic IR peaks. The results showed that the concentration of urea thickener and gel agent was increased drastically at the Hertzian contact region. Meanwhile, infrared spectra under hydrostatic pressure were obtained using a diamond-anvil cell. It was confirmed that the hydrogen bonding of the urea thickener and gel agent was stabilized under high-pressure conditions. Keywords: in situ observation, FTIR, grease, thickener, gel lubricant, low friction 1. Introduction Better lubricants provide longer lifetimes for tribomaterials and conserve energy by decreasing wear and friction. Tribomaterials can be supported by the soft and protective surface layers formed by lubricant components. Under sliding contact, lubricants are exposed to high pressure, temperature, and shear. It is important to clarify the structure of the lubricant film formed at the area of contact [1]. In situ observation of a lubricant film was performed to understand the tribological phenomena at the contact point. Micro-Fourier transform infrared spectrometry (micro-ftir) is useful for the in situ observation of a lubricant film under elastohydrodynamic lubrication (EHL) conditions. In previous studies, several tribological phenomena at the contact point were investigated, such as the change in concentration of components [2], molecular orientation [3], and molecular interactions [4]. The results demonstrated that tribological performance was closely related to the structure of the lubricant film at the contact point. In this study, a lubricant film of a semi-solid lubricant was the focus. Semi-solid lubricants, such as urea grease and gel lubricants [5,6], are mixtures of oil, urea thickener, and gel agents. In a previous study, infrared spectroscopy data confirmed that urea thickener was present in the contact region [7,8]. The tribological properties of semi-solid lubricants may relate to the concentration and structure of the urea thickener and gel agent at the EHL contact point. In order to clarify the properties of extended bearing material lifetimes and decreased friction with semi-solid lubricants, the structure of semi-solid lubricant films under lubrication must be understood. Therefore, the objective of this study is to clarify the behavior of the thickener and the gel agent at the contact point by direct observation. The structures of semi-solid lubricants were investigated under dynamic conditions by in situ observation with micro-ftir. The concentrations of the thickener and the gel-agent at EHL contact were investigated. The concentrations were estimated from characteristic IR peaks. The molecular interactions between the urea thickener and gel agent molecules were explored. Infrared spectra at high hydrostatic pressure were obtained using a diamond-anvil cell (DAC). Copyright 2016 Japanese Society of Tribologists 376

2 In Situ Observation of Lubricant Film of Semi-Solid Lubricants at EHL Contact Using Micro-FTIR Table 1 Chemical structure of sample oil Sample Base oil Urea grease Urea thickener Poly-α-olefin (PAO) Gel lubricant Gel agent npoly-α-olefin (PAO) 2. Experimental methods 2.1. Samples Urea greases and gel lubricant were used as the sample materials. The urea grease as commercially available grease containing 12 mass% of urea thickener was used as received. The consistency of the urea grease is 287 (60W). The gel lubricant was adjusted by heating the mixture of gel-agent in 12 mass% and base oil. Synthetic poly-α-olefin (PAO) hydrocarbon oil, with a viscosity of 30 mm 2 /s at 40 C, was used as the base oil. (Table 1) Apparatus and procedure In order to obtain IR spectra of the lubricant film at the EHL contact region, in situ observation with a micro-ftir spectroscope was used, as shown in Figure 1. In these dynamic tests, the micro-ftir is used in conjunction with a ball-on-disk lubrication tester. Hertzian contact is formed by a point contact between the steel ball and the IR-transparent sapphire disk. The steel ball is lubricated against the disk with the semi-solid lubricant, and the lubricant film formed between ball and disk is analyzed by micro-ftir under dynamic conditions. The entrainment speed is 0.13 m/s, and the applied loads were controlled from 4.9 to 14.7 N. All tests were performed at a controlled temperature of 25 C. Spectra were obtained from the inlet to outlet of the Hertzian contact point. The sampling area was a 50-μm square, and the sampling interval was 50 μm, as shown in Figure 2. For the measurement of IR spectra under conditions of high pressure and temperature, a DAC with heater was used, as shown in Figure 3. The sample oil is compressed with ruby chips in the sample chamber. The opposing anvils are brilliant-cut diamonds. Pressure measurements were performed by measuring the peak shift of the ruby chip fluorescence, collected with Raman spectroscopy. Fig. 2 Sampling positions of IR spectra around EHL contact region Fig. 1 Schematic view of in situ observation with micro FTIR under EHL condition Fig. 3 DAC for IR spectra at high pressure and high temperature Japanese Society of Tribologists ( Tribology Online, Vol. 11, No. 2 (2016) / 377

3 Koji Takiwatari, Yasushi Hoshi, Hidetaka Nanao, Tomie Hojo and Shigeyuki Mori To control the sample temperature, the DAC is set in a thermal cell consisting of an electric resistive heater and a thermocouple. Transmission IR spectra of the sample oils were recorded with a micro-ftir spectroscope. In this study, the test conditions used a pressure range from ambient pressure to 2 GPa and temperature up to 200 C. 3. Results and discussion 3.1. IR spectra of semi-solid lubricant at EHL contact Figure 4 shows the IR spectra of urea grease and gel lubricant at the center of EHL contact and under conditions of 9.8 N load, 0.13 m/s entrainment speed, and a slide-roll ratio of 0. Two characteristic peaks corresponding to the C-H stretching mode at cm -1 and the N-H stretching mode at cm -1 originate from the base oil, urea thickener, and gel agent. The concentrations of the urea thickener and gel agent Fig. 4 IR spectra of grease and gel lubricant at EHL contact during lubrication Load: 9.8 N, Entrainment speed: 0.13 m/s, SRR: 0, Measuring point: 0 μm are estimated from the intensity ratio of the N-H and C-H peaks. In order to obtain the calibration curve of the concentration, transmission IR spectra of the samples adjusted to predetermined concentrations were measured using a sandwich cell consists of two IR transparent window Lubricant film of semi-solid lubricant at EHL contact Figure 5 shows the concentrations of urea thickener and gel agent around the Hertzian contact region. The results show that the concentrations of urea thickener and gel-agent are increased at the Hertzian contact region [9]. The broken lines in the figures indicate the size of the Hertzian contact zone depending on the contact pressure. The results indicate that urea thickener and gel-agent are introduced to the Hertzian contact region. Although the original concentrations of the additives were 12 mass%, the concentration is increased to more than 40 mass% in the contact region. The increased concentration of additives should affect the lubricating properties of the semi-solid lubricants, such as the friction coefficient and the lifetime of bearing materials. After the lubrication tests with the semi-solid lubricants, the tracks on the ball were observed to be covered with thickener and gel agent. Therefore, much of the concentrated additives between the ball and the disk become adhered to the ball surface, covering the friction track with concentrated additives, which affect the friction coefficient and the life of the tribomaterials [8,10]. In addition, the peak position of the N-H stretching mode was found to shift to a lower wavenumber at the center of the Hertzian contact region, as shown in Figure 6. The peak position of the C-H stretching mode related to hydrocarbon oil is known to shift to higher wavenumbers at higher pressures [11-13]. The peak shifts to lower wavenumbers as the applied load is increased. Interestingly, the peak shift of the N-H stretching mode opposes that of the C-H stretching mode. The N-H stretching mode at cm -1 is related to Fig. 5 Concentration of urea thickener and gel agent around EHL contact Entrainment speed: 0.13 m/s, SRR: 0 Japanese Society of Tribologists ( Tribology Online, Vol. 11, No. 2 (2016) / 378

4 In Situ Observation of Lubricant Film of Semi-Solid Lubricants at EHL Contact Using Micro-FTIR Fig. 6 Peak shift of ν N-H of urea thickener and gel agent around EHL contact Entrainment speed: 0.13 m/s, SRR: 0 the associated structure of C=O in the urea thickener and the gel agent, formed by hydrogen bonding [14]. The peak shift of N-H results from the stabilization of the hydrogen bonding between N-H and C=O functional groups between the urea thickener and gel agent [15], which indicates that the hydrogen bonding between the urea thickener molecules and gel agent molecules is stabilized by high pressure in the Hertzian contact region. These results suggest that the stabilization of hydrogen bonding between the urea thickener and gel agent was caused by the contact pressure at the Hertzian contact region Molecular interaction of urea thickener and gel agent at high pressures and temperatures The effects of high pressure and temperature on the IR spectra were investigated in detail under hydrostatic high pressure. Three characteristic peaks correlating to N-H stretching, C-H stretching, and N-H bending modes were observed through a diamond window in the DAC. In this study, the N-H stretching mode was used as a representative to investigate molecular interaction. Figure 7 shows the temperature dependence of the peak shift of ν N-H using DAC. The peak position of bonds related to the urea grease gradually shift to higher wavenumbers with increasing temperature. This indicates that hydrogen bonding between N-H and C=O in the grease is weakened at higher temperatures. However, the N-H peak from the gel-agent shifts to lower wavenumbers at temperatures below the melting point of the gel-agent, then shifts drastically to higher wavenumbers above the melting point. Some hydrogen bonding within the gel agent is broken above the melting point of the agent [16]. Figure 8 shows the pressure dependence of the IR spectra using DAC. The results show that the N-H stretching mode shifts to lower wavenumbers with increasing pressure. This peak shift indicates that hydrogen bonding between molecules of urea thickener and gel agent is stabilized by high applied pressure. The peak shift of ν N-H is reversible as a function of pressure. Notably, the peak shift of ν N-H in the gel lubricant is larger than that of urea grease. This difference in peak shift between urea grease and gel lubricant implies that the aggregation structure of the urea thickener is rigid, while the structure of the gel agent is soft. The thickener molecules in grease form rigid crystalline fibrous structures, while gel agent molecules form random, Fig. 8 Fig. 7 Wavenumber of ν N-H as function of temperature Wavenumber of ν N-H as function of pressure Japanese Society of Tribologists ( Tribology Online, Vol. 11, No. 2 (2016) / 379

5 Koji Takiwatari, Yasushi Hoshi, Hidetaka Nanao, Tomie Hojo and Shigeyuki Mori flexible structures creating a soft network under the high pressure of Hertzian contacts. This is one reason for the more dramatic decrease in friction coefficient in the gel-like lubricant compared to that of urea grease [9]. From the IR spectra of the polar N-H functional group stretching mode as functions of hydrostatic pressure and temperature, the peak positions shift to lower wavenumbers with increasing pressure, whereas they are shifted slightly to higher wavenumbers with increasing temperature. This phenomenon indicates that the covalent N-H bond is influenced by pressure and temperature. The results indicate that pressure and temperature strengthen and weaken, respectively, the hydrogen bonding between molecules of urea thickener and gel agent. One explanation for the lubricating properties of semi-solid lubricant is addressed in detail here. The structure of the semi-solid lubricant film was observed under dynamic friction. At the point of Hertzian contact, both urea thickener and gel agent are present and concentrated. Therefore, the semi-solid lubricant film consists mainly of the urea thickener and the gel agent. At the Hertzian contact point, hydrogen bonding between molecules of urea thickener and gel agent is stabilized. However, the change in the hydrogen bonding of the urea thickener was small, while that of the gel-agent was large. Therefore, the lubricant film of urea grease may be rigid, while that of the gel lubricant is soft. The rigid film from the urea grease protects the surface, thus decreasing wear. The soft film from the gel lubricant works against frictional forces, thus decreasing the friction coefficient, as mentioned above. 4. Conclusions In this study, the concentrations of urea thickener and gel agent at an EHL contact point were measured by in situ observation with micro-ftir spectroscopy. The stabilization of hydrogen bonding between molecules of the urea thickener and gel agent was investigated. The results of the study can be summarized as follows. The concentrations of both the urea thickener and gel agent were increased drastically at the Hertzian contact region. It is clear that the urea thickener and gel-agent were introduced to the Hertzian contact region. The hydrogen bonding between the urea thickener and gel agent was stabilized by the high pressure at the Hertzian contact region. DAC observations revealed that the hydrogen bonding between molecules of urea thickener and gel agent was stabilized by increasing pressure, but destabilized by increasing temperature. The lubrication properties of semi-solid lubricants were discussed based on the structures of the lubricant films formed by different components at the lubricating contact. Acknowledgements A part of this study was supported by the Green Tribology Innovation Network.We would like to thank Kyodo Yushi Co., Ltd. for the supplying urea greases. References [1] Spikes H. A., In Situ Methods for Tribology Research, Tribology Letters, 14, 1, 2003, 1. 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6 In Situ Observation of Lubricant Film of Semi-Solid Lubricants at EHL Contact Using Micro-FTIR [14] Endo, T., Kimura, H., Moriuchi T., Yase, K. and Okada, M., Structure Analyses of Urea Grease Thickener OUDM, Journal of Japanese Society of Tribologists, 35, 5, 1990, (in Japanese). [15] Jeffrey, G. A., An Introduction to Hydrogen Bonding, Oxford University Press, 1997, 220. [16] Ohno, N., Mia, S., Tateishi, K., Morita, S. and Shitara, Y., Construction of Phase Diagram Up to 2 GPa and 200 C for Thermo-Reversible Gel Lubricants by Diamond Anvil Cell, Lubrication Science, 21, 5, 2009, Japanese Society of Tribologists ( Tribology Online, Vol. 11, No. 2 (2016) / 381