Neutron Reflectometry under High Shear in Narrow Gap for Tribology Study

doi:10.21203/rs.3.rs-3126825/v1

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Neutron Reflectometry under High Shear in Narrow Gap for Tribology Study

https://doi.org/10.21203/rs.3.rs-3126825/v1

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published 25 Oct, 2023

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A parallel-face narrow gap viscometer was installed in the energy-resolved neutron reflectometer to investigate the change in interfacial structure under high shear. The viscometer was designed to operate with high shear rate while maintaining a µm-order constant gap between two parallel faces. When the base oil without any additives was sandwiched in the gap of viscometer, the neutron reflectivity profiles without and with the rotation of viscometer was the same. This result shows that correct measurements were performed regardless of whether the upper specimen was rotated or not. On the other hand, when the base oil containing polymethacrylate-type additive was sandwiched in the gap, both the thickness and density of adsorbed additive layer in shear field became lower than that in stay condition. The results demonstrate that the proposed method can be used to analyse the structural changes in the adsorbed layer formed by the additive on the surface. Such approaches with the combination of neutron reflectometer and narrow gap viscometer are promising for near-future tribology studies.

Physical sciences/Engineering/Mechanical engineering

Physical sciences/Physics/Applied physics

Neutron reflectometer

Narrow gap viscometer

High shear rate

Tribology study

Fluid lubrication

Adsorbed additive layer

Machine components are desirable to be operated under oil lubricated condition to prevent damage due to direct contact between the two opposing sliding surfaces. Many studies of mechanical sliding surfaces under lubricated condition have been conducted in the field of tribology. Friction behaviours of sliding surfaces under lubricated condition can be divided into three states: boundary lubrication, mixed lubrication, and fluid lubrication. Under fluid lubrication, the shear resistance of the oil in the gap is the dominant factor in friction force because the two surfaces are completely separated by the oil film. Friction loss can be suppressed by using a low-viscosity oil to reduce fluid resistance. However, since low-viscosity oils make the oil film thinner and reduce the gap between two sliding surfaces, the risk of seizure is increased as well as friction and wear of sliding surfaces. The trend toward friction loss reduction using low-viscosity oil has made the role of lubricating additives in protecting surfaces particularly important. It is thus essential to evaluate the characteristics of the adsorbed layer formed in narrow gaps by additives in fluid lubricated condition.

Many studies of the friction characteristics of lubricant additives between two surfaces separated by the oil film have generally been performed under elastohydrodynamic lubricated condition using a glass disk and a steel ball. The entrainment speed dependence of the friction coefficient and the oil film thickness of lubricants containing various additives such as fatty acid and polymer additives have been investigated in several studies ^1–5. In contrast, few studies have investigated the effect of the adsorbed layer formed on metal surfaces by lubricant additives under fluid lubricated conditions ^6,7 and the influence of the shear on the adsorbed additive layer structure remains unclear.

In this study, neutron reflectometry was used to investigate the structure of adsorbed additive layers under fluid lubricated conditions. Neutron reflectometry is becoming widely used in the field of tribology, but it is mainly used to evaluate the thickness and density of adsorbed additive layers in a static state ^8–10. For example, fatty acids such as palmitic and oleic acids were found to form a nanoscale adsorbed layer on Fe, Cu, and diamond-like carbon, with thicknesses roughly equivalent to their molecular chain lengths ^11–14. Regarding for polymer additives, the temperature dependence of the thickness and density of the adsorbed layer formed under static states has been evaluated ¹. In contrast, few operando analyses have been conducted in the shear field ¹⁵, and the change in the structure of the adsorbed layer formed by additives at the sliding surfaces has not been clarified.

To obtain a clear neutron reflectivity profile, it is essential to use a substrate coated with a smooth thin metal film. It is thus common to deposit a metal film with a thickness of 30–50 nm on a mirror-polished silicon block using a metal deposition system ^16,17. However, damage to the metal film due to friction during neutron reflectometry causes operando analysis to fail. Therefore, when Armstrong et al. analyzed the structure of an adsorbed layer of glycerol monooleate (GMO) under shear conditions, they set the gap between the roller and flat disk to as much as 200 µm ¹⁵. Reflectometry experiment have also been performed with a commercially available rheometer installed in a reflectometer, but the minimum gap between the cone and plate is typically on the order of 100 µm, which does not correspond to the typical oil film thickness in tribological conditions, and the shear rate was only around 10³ s^{− 1 15,18–21}.

The objective of this study was to establish an operando analysis method for evaluating the interfacial structure of an adsorbed layer formed by an additive on a metal surface under fluid lubrication conditions in which a non-contact condition was maintained with a few micrometers gap. To achieve this, an analytical method was developed that combines the use of a neutron reflectometer with that of a previously developed parallel-disk viscometer with a narrow gap that can measure the shear force of oil film between two parallel faces. Its use demonstrated that interfacial slip occurs on the adsorbed layer formed by fatty acids. It was shown that there was no direct contact between the two sliding surfaces and that the adsorbed layer reduces shear force even under fluid lubrication conditions when the gap is less than several µm ⁶.

In this study, the feasibility of interfacial analysis was verified by installing the narrow gap viscometer in a neutron reflectometer. A high shear force was applied while maintaining a narrow gap between the two parallel faces sandwiching lubricant between them and irradiating a neutron beam onto the interface. First, an experiment using base oil without any additive was conducted to confirm that using the viscometer and neutron reflectometer combination results in correct measurement. Next, the structure of the adsorbed layer of a polymer additive formed on Cu layer under a high shear field in the narrow gap was evaluated.

2.1 Apparatus Configuration

A schematic illustration of the previously designed narrow gap viscometer and its components are shown in Figs. 1(a)-(c) ⁶. The parallel-face configuration is marked by the solid line circle in Fig. 1(a), and more detailed information about it is shown in Figs. 1(b) and (c). The upper specimen is made of stainless steel (inner diameter: 24 mm; outer diameter: 30 mm) and contains an aerostatic bearing pad in the center, and sample lubricant was supplied only between the flat outer ring part of the upper specimen and the lower disk, as shown in Fig. 1(c). The upper specimen was rotated using a motorized pulley and timing belt arrangement. The aerostatic bearing enables the upper specimen to rotate while a virtually constant gap (~ several µm) is maintained between the upper specimen and the lower disk. The gap remains stable at a constant value depending on the pressure of the supplied air regardless of the rotational speed of the upper specimen.

The lower disk is a mirror-polished silicon block (40 × 40 × t4 mm) and mounted on a holder that is supported by two specially shaped leaf springs that are rigid in the axial direction while flexible in the inclined direction. The lower disk can thus follow the rotational deflection of the upper specimen, and the two surfaces can maintain parallel without any contact. In the previous study ⁶, we used a load cell to measure the rotational torque generated by the shearing force. However, in this study the load cell was removed due to the space limitation imposed by the installation of the neutron reflectometer.

2.2 Gap Control

Gap control is explained in detail in our previous report ⁶, so only a brief description is given here. As shown in Fig. 1(c), the central part of the upper specimen has an aerostatic bearing (outer diameter: 22 mm), whose surface was digged down by approximately 5 µm. The central bearing surface has fan-shaped surface restrictions and is supplied with air through a central hole (diameter: 2 mm). The supplied air is prevented from leaking through the gap by the lubricant sandwiched between the flat outer ring part of the upper specimen and the lower disk, and is eventually exhausted through holes on the side wall of the upper specimen. By adjusting the pressure of the air supplied to the aerostatic bearing, a downward force is generated, and the gap between two parallel faces is then determined by the equilibrium with the upward force by an air cylinder under the lower disk. The load capacity of the aerostatic bearing is calculated using the compressible Reynolds equation to theoretically predict the gap between the aerostatic bearing and the lower disk surface. The force generated by the pressure of the supplied air was measured beforehand by setting a load cell between the lower disk and the air cylinder to calibrate the relationship between the supplied air pressure and the gap. The gap length varies only with the supplied air pressure to the aerostatic bearing, and does not depend on the viscosity of the sample lubricant or the rotational speed, confirming that the gap remains constant even during rotation ⁶.

3.1 Overview of Neutron Reflectometry

In neutron reflectometry, the thickness and density of the adsorbed layer formed by an additive on metal film can be evaluated since the layer structure at the solid-liquid interface is clearly distinguished by replacing the hydrogen atom of either the base oil or the additive with deuterium. In our experiment, the target surface was the top of lower disk. A Cu layer with a thickness of 30 nm was previously formed on the lower disk surface by an ion beam sputtering system (KUR-IBS ²²). Cu is a common industrial material used in mechanical elements such as sliding bearings. Unlike Fe, Cu has no magnetism, which enables simple and accurate analysis. The surface roughness of the Cu layer measured by atomic force microscopy was Ra ~ 0.5 nm.

A time-of-flight energy-resolved neutron reflectometer (BL16 SOFIA ^23,24) installed at the Materials and Life Science Experimental Facility in the Japan Proton Accelerator Research Complex (J-PARC MLF) was used for the feasibility test and main experiments. The narrow gap viscometer was set on the reflectometer, and a neutron beam was irradiated and transmitted from the side wall of the lower disk to obtain the reflectivity profile at the interface at the top of lower disk with thin Cu layer.

A schematic illustration of the measurement is shown in Fig. 2. A neutron beam was irradiated on the sample at a small angle θ, and the ratio of all neutrons irradiated to those reflected at the interface was defined as reflectivity R. The scattering vector Q_z was used to make the reflectivity profiles, and the change in reflectivity with a change in Q_z was summarized.

Q _z = 4πsinθ/λ, (1)

where λ is the wavelength of the neutron beam. The profiles changed depending on the layer structure of the target interface. Fitting analysis was performed using MOTOFIT analysis software ²⁵. Each material has a specific scattering length density (SLD), which enables identification of the layer structure formed on the top of lower disk. The SLD values relevant to this study are listed in Table 1.

Table 1

Theoretical SLD values for materials related to this study.
Material	SLD [×10^–6 Å^–2]
Si	2.1
SiO₂	3.5
Cu	6.6
Cu₂O	5.4
CuO	6.5
Cu(OH)₂	2.5
PAO	–0.4
d-Sq	7.1
d-Sq + mineral oil	6.3
PMA	0.0

3.2 Neutron Reflectometry under High Shear Field for Base Oil

To determine the feasibility of the proposed analysis method, neutron reflectometry experiment was tried using the narrow gap viscometer. In this feasibility test, poly-alpha-olefin (PAO), as a typical base oil, was sandwiched between the flat outer ring part of the upper specimen and the lower disk. The air pressure supplied to the aerostatic bearing through the upper specimen was 0.27 MPa, and the air pressure to the air cylinder below the lower disk was 0.36 MPa, then the gap was kept 2.3 µm. The footprint that the neutron beam irradiates was set 2 × 10 mm so that it did not extend beyond the lubricant sandwiching area between the flat outer ring part of the upper specimen and the lower disk. It is noted that this footprint size was comparatively small, about 1/100 that in reflectivity measurements under static conditions using a conventional setup, which is described in Section 4.2. So, the reflected neutron count was about 150 per second, which is comparatively low, so the time required for getting each reflectivity profile was ~ 8 hours.

First, the refractivity profile was obtained under the stay condition, that is, where the upper specimen was not rotated. The reflectivity profile was obtained with two neutron incident angles, 0.4 and 0.8°, and they were combined to form a single reflectivity profile. Next, the same measurements were also conducted under shear condition, that is, where the upper specimen was rotated with a constant speed. The rotation speed was set to 100 rpm, so the shear rate was 6.1×10⁴ s^− 1.

3.3 Results of Feasibility Test

Figures 3 (a) and (b) show the reflectivity profiles obtained without and with the rotation of upper specimen while maintaining a 2.3 µm gap. Figure 3 (a) shows the profiles overlaid, and 3 (b) shows them with an offset in the vertical axis direction for better visibility. In Fig. 3 (a), the profiles are almost the same, and the interfacial fringes on the profiles does not change, indicating that the difference in reflectivity profiles between without and with the rotation is negligible.

Figure 3 (c) shows the SLD profiles obtained by fitting analysis for the reflectivity profiles shown in Fig. 3 (b). The range of scattering vector Q_z was smaller and the errors in each measurement plot were larger than those with the conventional setup, described in Section 4.2. Detailed structural analysis of the Cu₂O, CuO, and Cu(OH)₂ in the Cu thin layer on the lower disk (Si) that may form naturally after deposition of Cu layer could not be performed because reflectivity profiles with sufficient information could not be obtained due to insufficient measurement time resulting from the small footprint and low rate of neutron reflection. Fitting analysis was thus performed using a model in which a single layer of Cu oxide (CuO_x) is assumed to exist in a different state than the bulk Cu. The thicknesses of the SiO₂, Cu, and CuO_x layers in measurements without upper specimen rotation were estimated to be 4.7 nm, 26.0 nm, and 4.9 nm, respectively, whereas those with rotation were estimated to be 4.9 nm, 26.0 nm and 4.9 nm, respectively. The thicknesses of the Cu and CuO_x layers were about 31 nm, which corresponds to the target value for Cu deposition by sputtering. This indicates that the analysis was performed correctly.

These results confirm that rotation of the upper specimen had little effect on the reflectivity profile, which means that the interfacial structure on the lower disk can be analyzed precisely. In addition, these results demonstrate that the combination of the neutron reflectometer and the narrow gap viscometer, which does not damage thin Cu layer on the top of lower disk during long-term measurement, enables the measurement of neutron reflectivity at a higher shear rate than in previous studies.

4.1 Sample

In recent years, the use of polymethacrylate (PMA)-based additives has become more prominent, especially in the trend toward the use of lower viscosity oils, as described in the background. As the sample lubricant for our study, a PMA-based additive was prepared and added to deuterated squalane (d-Sq), a non-polar solvent, at a concentration of 2%. This lubricant contains less than 10% mineral oil used for dilution to facilitate handling of the highly viscous PMA. The molecular weight of this PMA is approximately 430 000, and there are many polar groups in the molecule.

4.2 Analysis of Adsorbed Additive Layer in Static State

A precise analysis of the adsorbed layer structure formed by the PMA in the static state was carried out using a previously proven method ^1,3,10,13. A mirror-polished Si block (50 × 50 × 10 mm) covered with thin Cu layer deposited by spattering was used. First, the neutron reflectivity from the Cu layer was obtained after setting the Si block in a sample holder and immersing it in d-Sq without PMA. After the d-Sq was replaced with the lubricant containing the PMA, the neutron reflectivity was measured again at room temperature (~ 25°C) to evaluate the structure of the adsorbed layer formed by the PMA on the Cu layer surface. The temperature dependence of the structure of the adsorbed layer was also evaluated by additional measurements after heating at 50°C and 75°C for 30 minutes. Then, the footprint was 40 × 40 mm, and the reflectivity profile was obtained with three incident angles, 0.3, 0.6 and 1.2°, and they were combined to form a single reflectivity profile. The time required for getting one reflectivity profile was around 15 minutes.

The reflectivity profiles obtained in the static state are shown in Fig. 4 (a). The results are shifted in the vertical axis direction for better visibility, and the optimum solutions obtained by the fitting analysis are shown as solid lines. The SLD profiles obtained by the fitting analysis are shown in Figs. 4 (b) and (c). The range of scattering vector Q_z was wide, and the error in each plot was small since sufficient amount of reflection counts was obtained. This means that a precise analysis can be performed by modelling the formation of Cu₂O, CuO, and Cu(OH)₂ layers on the top surface of Cu layer.

The SLD of the PMA used in this research were almost 0.0, whereas that of the base oil (d-Sq + mineral oil for dilution) was 6.3 × 10^–6 Å^–2. Therefore, an adsorbed layer with a smaller SLD value contains a PMA with a higher density, and an adsorbed layer with a larger SLD value has more d-Sq penetration. Fitting analysis showed that the adsorbed PMA layer on Cu layer consisted of a higher-density layer with a thickness of around 2 nm and a lower-density layer with a thickness of around 20 nm on the higher-density layer (Fig. 4 (c)). There was a tendency for the thickness of the adsorbed layer on Cu layer to increase slightly with the temperature, which is consistent with the behavior observed in a previous study ¹. However, the differences between the reflectivity profiles obtained at the three temperatures were not significant, suggesting that the PMA used was in a swollen state even at 25°C due to its high compatibility with d-Sq.

4.3 Analysis of Adsorbed Additive Layer in High Shear Field

For the neutron reflectometry measurement for the adsorbed PMA layer in high shear field, a new lower disk (Si) with Cu layer on the top was prepared. The upper and lower supplied air pressures were again set to 0.27 and 0.36 MPa, respectively, to create a 2.3-µm gap between the two surfaces that was maintained during the measurement. 40 µL of d-Sq containing the PMA was supplied between the gap. First, the reflectivity profile was obtained without rotation to evaluate the structure of the polymer adsorbed layer that formed on the Cu layer surface in the stay condition. Next, the upper specimen was started to rotate, and the reflectivity profile was obtained again. The rotation speed was 100 rpm, and the 2.3-µm gap was maintained, so the shear rate was 6.1×10⁴ s^− 1. The footprint was 2 × 10 mm, and the reflectivity profile was obtained with the incident angle of 0.6°. The time required for getting one reflectivity profile was ~ 8 hours.

The obtained reflectivity profiles are shown in Fig. 5 (a). This graph shows the results in air (top), and with lubricant under stay (middle) and shear (bottom) conditions with fitting analysis and offsets in the vertical axis direction for better visibility. The appearance of the fitting line for in air in the small Q_z range (< 0.015 Å^–1) differs from the two in lubricant because total neutron reflection does not occur in air measurements.

The SLD profiles obtained by the fitting analysis are shown in Figs. 5(b) and (c). The surface structure of the Cu layer was first determined by the fitting analysis for the reflectivity profile measured in air without lubricant. The Cu oxide layer could not be precisely analyzed due to the small footprint, but the thicknesses of the Cu and CuO_x layers obtained by the fitting were close to the values described in Section 4.2, indicating that the analysis was generally correct.

Next, the structural changes in the adsorbed PMA layer without and with rotation of the upper specimen were evaluated using a model in which the state of the Cu layer determined by measurement in air was assumed to remain unchanged and the adsorbed PMA layer was assumed to form on the top of Cu layer. The SLD profile obtained by the fitting analysis of the reflectivity profile in stay condition showed that the adsorbed layer had a thin higher-density part and a thick lower-density part, similar to the findings obtained in Section 4.2. This demonstrates that the structure of adsorbed additive layer can be analyzed well in neutron reflectometry even with the narrow gap viscometer setup.

Figure 5(a) shows that the reflectivity profile with rotation is clearly changed from that without rotation, and 5(c) shows that the SLD of the adsorbed additive layer increased by rotation. This indicates that much d-Sq with a higher SLD penetrated the adsorbed PMA layer, meaning that the density of adsorbed layer became lower in high shear condition. The distance from the top surface of the Cu layer to the area where the SLD reached that of the bulk lubricant (6.3×10^–6 Å^–2) was shorter than that measured in stay condition, indicating that the thickness of the adsorbed layer also decreased in shear field. Although the actual temperature at the interface was not controlled, but this change is not temperature-dependent, as it was confirmed that the effect of temperature is negligible in Section 4.2. In general, certain polymers are well known to align in lubricant due to the shear thinning effect. If the adsorbed PMA layer in our experiment aligned by shear, the PMA layer thickness should decrease, whereas the density of the PMA layer should increase. However, the density of PMA layer also decreased in our result. Therefore, it is highly likely that some PMA molecules adsorbed onto the Cu surface desorbed due to the shear force. These results indicate that the adsorbed PMA layer in our study could not maintain a fully adsorbed state in shear condition and the number of adsorbed PMA molecules in shear field was less than that in stay condition.

To investigate the adsorbed additive layer structure under high shear rate, the parallel-face narrow gap viscometer was installed in the energy-resolved neutron reflectometer. Experimental results demonstrated that satisfactory reflectivity profiles can be obtained even with the viscometer regardless of its rotational operation. Particular, by the measurements with narrow gap viscometer sandwiching the lubricant containing the PMA additive, the obtained results show that both the thickness and density of the adsorbed PMA layer decreased, indicating that the polymer additive was likely to desorb from the top of Cu surface due to high shear force. This is a new finding in designing lubricant additives that can function under a wide range of lubricating conditions under high shear rate, and such approaches are promising for near-future tribology researches.

Data availability

The datasets in this study are available from the corresponding author on reasonable request.

Acknowledgments

This work was supported by JST, CREST Grant Number JPMJCR2191, and FOREST program Grant Number JPMJFR202D, Japan. The research was supported by Dr. Onodera and Ms. Watanabe in the EMG Lubricants Godo Kaisha; the polymer additive was supplied by Sanyo Chemical Industry Co., Ltd. The neutron reflectometry was carried out at the Materials and Life Science Experimental Facility in the Japan Proton Accelerator Research Complex (J-PARC) (Project No. 2019I1612). We thank the facility’s staff for their sincere supports.

Contributions

Conceptualization, N.Y., T.H.; Investigation, N. Y.; Methodology, N. Y., T. H., N. L. Y.; Formal analysis, N. Y.; Funding acquisition, T. H., Visualization, N. Y., Resources, M. H., M. L. Y.; Software, M. L. Y.; Project administration, T. H.; Supervision, T. H., Writing - original draft, N. Y.; Writing - review & editing, T. H., M. H., N. L. Y.

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Competing interests

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.

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No competing interests reported.

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Neutron Reflectometry under High Shear in Narrow Gap for Tribology Study

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Abstract

Figures

1. Introduction

2. Parallel-Face Narrow Gap Viscometer

2.1 Apparatus Configuration

2.2 Gap Control

3. Feasibility Evaluation

3.1 Overview of Neutron Reflectometry

3.2 Neutron Reflectometry under High Shear Field for Base Oil

3.3 Results of Feasibility Test

4. Interfacial Analysis of Adsorbed Polymer Layer

4.1 Sample

4.2 Analysis of Adsorbed Additive Layer in Static State

4.3 Analysis of Adsorbed Additive Layer in High Shear Field

5. Summary

Declarations

References

Additional Declarations

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