Joichi Sugimura

Characterization of wear particles and their relations with sliding conditions

Abstract Wear particles and worn surfaces generated in pin-on-disk steel sliding experiments are studied by microscope image analysis and two types of neural networks. Features of wear particles described by four particle descriptors depend strongly on sliding conditions. A multi-layer neural network successfully learns the relations between wear particle features and sliding conditions. If the network is trained with data representing typical features, it also recognizes the particles having similar features. This suggests that the network can be used as a tool for condition monitoring in which the network identifies wear particles produced under unknown sliding conditions to predict that conditions. A self-organizing neural network using the competitive learning rule classifies the wear particles based on their features without any supervisor data. Particle features are expressed by the position on a two-dimensional feature map. This type of network is useful in finding typical particle features, which in turn can be used as supervisor data for the multi-layer neural networks. In another application of the self-organizing network, microscopic images of both wear particles and worn surfaces are automatically classified, and characteristics of each surface are represented by the distribution of weights on the feature map.

Simple equation for elastohydrodynamic film thickness under acceleration

A simple approximation of EHD film thickness under varying speed conditions is proposed. The equation is based on continuity of flow, by which the film formed at the contact inlet moves downstream within the contact with little subsequent change in its thickness even though the boundary velocities are changing. The approximation is supported by experimental results of non-steady state film thickness measurement using ultra-thin film interferometry. It is also shown by numerical simulation that the approximation holds for film thickness in the rigid piezoviscous regime under line contact so long as the squeeze film effect is insignificant.

Technique for measuring EHD film thickness in non-steady state contact conditions

A technique has been developed for making film thickness measurements in rolling elastohydrodynamic contacts under non-steady state conditions. It is based upon ultrathin film interferometry and allows the central film thickness down to less than 5 nm, and also film profiles across the EHD contact, to be precisely measured every 0.02 seconds. The technique has been applied to experiments in which lubricated specimens alternately roll at a constant speed and stop. It is found that upon halting motion, both the central and the minimum film thicknesses fall rapidly within several hundredths of seconds to residual levels but that thereafter, film thickness falls further only slowly as lubricant is squeezed out from the contact. Upon sudden start of motion, there is a small overshoot in the film thickness followed by a small oscillation, after which film thickness stabilises to a constant level which corresponds to the thickness predicted by steady state theory. Results of constant acceleration/ deceleration tests are also presented.

Interferometry Study of Aqueous Lubrication on the Surface of Polyelectrolyte Brush

The water lubrication behavior of a polyelectrolyte brush was investigated by using double-spacer-layer ultra-thin-film interferometry to determine the thickness of the aqueous lubrication layer present at the interface between the brush and a spherical glass lens. A hydrophilic poly{[2-(methacryloyloxy)ethyl]trimethylammonium chloride} brush was prepared on an optical glass disk coated with layers of semireflective chromium and silica. The thickness of the hydrodynamic lubrication layer was estimated interferometrically. On increasing the sliding velocity from 10(-5) to 10(-1) m·s(-1), the gap between the rotating disk and loading sphere glass lens showed a marked increase to 130 nm at 2×10(-2) m·s(-1), and the friction coefficient simultaneously decreased to 0.01-0.02, indicating that the polyelectrolyte brush promoted the formation of a fluid lubrication layer that separates the rubbing surfaces, preventing direct contact and providing a low friction coefficient.

WS2 nanoadditized lubricant for applications affected by hydrogen embrittlement

Hydrogen is one of the cleanest available vehicle fuels but its small atomic size allows it to diffuse readily through the lattice of solid materials, which can cause catastrophic failure in high strength steels. Metal embrittlement has been identified as a major consequence of hydrogen uptake and represents an extra challenge for lubricated tribological parts in fuel cell vehicles, hydrogen compressors, storage tanks, dispensers and wind turbines that are normally subjected to high stresses. This study has found WS2 nanoparticles as an effective additive candidate to impede the permeation of hydrogen into rolling element bearings at high temperatures and pressures. Compared to the pure polyalphaolefin (PAO) base oil, WS2 nanoadditized oil reduced the concentration of permeated hydrogen in the bearing steel and led to controlled wear and smoother tracks. These effects are attributed to the formation of a chemical tribofilm on the wear track which reduces hydrogen embrittlement and extends the life of steel through several mechanisms: (1) its continuous generation impedes formation of nascent catalytic surfaces during rubbing and thus prevents the decomposition of oil/water molecules and generation of atomic hydrogen; (2) acts as a physical barrier to hydrogen permeation through the wear track; (3) the low coefficient of diffusion of hydrogen through the tungsten compounds found in the tribofilm further reduces hydrogen permeation; (4) some of the atomic hydrogen is used up in redox reactions during the formation of the tribofilm and (5) the tribofilm reduces the total amount of water in the steel formed by the reaction of hydrogen atoms with oxides and thus extends the fatigue life. WS2 nanoadditized lubricants can lead to improved profitability and sustainability of the emerging renewable energy industry.

Balancing Wedge Action: A Contribution of Textured Surface to Hydrodynamic Pressure Generation

This paper suggests a new mechanism called ‘balancing wedge action’, which is based on the hydrodynamic lubrication theory for textured surfaces. While past studies have considered the local wedge film action produced by textured feature, this new mechanism is based on the promotion of a wedge film action between surfaces by the incorporation of a textured feature. The analytical model used in the current study is a one-dimensional centrally pivoted pad bearing having a single dimple on the pad, which considers the equilibrium of the moment applied to the surfaces. Analytical equations are derived for the pressure, shear stress, load, friction, and moment by integrating the Reynolds equation. A series of parametric simulations of the depth, width, and location of a dimple were conducted. The analytical results showed that the incorporation of a single dimple on the pad surface increases the convergence ratio between the surfaces, producing a load capacity and reducing the friction. No negative pressure can be found within the dimple, where a positive pressure with a greater positive gradient causes a higher shear stress than that outside the dimple. The trends for the load and friction in relation to the dimple depth and location are complex. The creation of the dimple closer to the centre results in a failure to obtain an equilibrium solution for the moment.

In Situ X-Ray Diffraction Study of Phase Transformation of Steel in Scuffing Process

Abstract We developed a novel in situ observation method associated with synchrotron radiation X-ray diffraction (XRD) that enables us to simultaneously monitor structural changes of materials, images at frictional interfaces, friction force and temperature with a time resolution on the order of tens of milliseconds. The in situ method was applied to study scuffing process of martensitic steel under a dry condition. The result shows that during scuffing, martensite to austenite phase transformation occurred with plastic flow. The generated austenite phase disappeared when the shear test was stopped. The austenite was present at a surface temperature lower than the nominal austenitisation temperature. After intermittent occurrences of the austenitisation with local plastic flow, the scuffing feature showed a larger amount of austenite, higher friction and greater plastic flow. The XRD spectra suggest that some metallurgical properties of the near-surface material of the steel may change at the scuffing-mode transition.

Estimation of Cavity Length in EHL Rolling Point Contact

Reciprocating lubricated contacts sometimes suffer from oil starvation due to cavitation at the reversal of motion. However, the behavior of cavities is not well understood such that starvation can be theoretically predicted. In this study, the length of cavity in a steady state elastohydrodynamic lubricated point contact was calculated. For numerical simulation, a modified Elrod algorithm was used. An equation was obtained for the cavity enclosed in the oil meniscus. The equation was constructed with Moes dimensionless parameters M and L, assumed pressure of cavity, and viscosity pressure index of the lubricant. The dimensionless cavity length (or the ratio of cavity length by Hertzian contact radius) is proportional to the product of M−a and Lb. Careful examination of the equation elucidated that the cavity length is dominated by the viscosity, sum velocity, cavity pressure, and geometry of the contact. Experimental measurements with a ball-on-disk apparatus have shown good agreement. The validity of the equation suggests that the algorithm is applicable for complete transient simulations. In practice, the estimated cavity length can be a parameter for starvation.

Bearing torque characteristics of lithium soap greases with some synthetic base oils

This article describes the influence of rheological properties on the bearing torque characteristics of the lithium soap greases with five types of base oils. The greases used had different yield stress depending on the base oils even with the same thickener concentration. Measurement of bearing torque was conducted for a deep-groove radial ball bearing by using a bearing test apparatus. The bearings filled with greases initially exhibited high torque but showed gradual decrease in the torque with prolonged rotation, where the greases with higher yield stress showed larger normalized torque decrease. Observation of bearing after the rotation revealed a tendency that the greases with larger normalized torque decrease had been pushed aside in the raceway by channeling. This implied that the greases with higher yield stress tended to show channeling. On the other hand, the greases with lower yield stress circulated within the bearing by churning and showed smaller normalized torque decrease. These behaviors were explained in terms of the yield stress of the greases and the shear stress to entrain the greases into the contacts. Observation of grease structure was made with atomic force microscopy showed that the greases whose thickener network structure was distributed more densely had higher yield stress.

Effect of high-pressure hydrogen exposure on wear of polytetrafluoroethylene sliding against stainless steel

Abstract Mechanical components in hydrogen energy systems, such as a fuel cell vehicle and related infrastructures, will operate in high-purity hydrogen. Especially, some seals and valves in fuel cell vehicles should articulate against metal counterface within a pressurized hydrogen gas. However, the effect of high-pressure hydrogen gas on tribological behaviour of materials used in sliding surfaces has not been identified yet. In this study, unfilled polytetrafluoroethylene (PTFE) pins and 316L austenitic stainless-steel discs were exposed to high-pressure hydrogen gas and then the chemical and physical changes in their surface and the tribological characteristics were investigated. The results of an X-ray photoelectron spectrometer analysis of the exposed stainless-steel surface indicated that metal oxides in the passive surface layer of stainless steel can be reduced significantly during the high-pressure hydrogen exposure. Increased metal contents of the stainless surface resulted in enhanced metal fluoride formation and subsequent development of a PTFE transfer film. Consequently, the exposed PTFE specimens showed lower specific wear rate when compared to the unexposed specimen.