Proceedings of the Institution of Mechanical Engineers,Part J: Journal of Engineering Tribology

By combining shear rate range data in engine components with measured viscosity shear rate curves on lubricants (at different temperatures), useful insights have been obtained on how the viscosity shear rate curve of a lubricant should be “designed” to give low friction (and hence improved fuel economy). A brief review is carried out of typical shear rates in key engine components, which is backed up by the authors’ own calculations (using in-house lubrication software that includes realistic viscosity/temperature/shear rate data). It is found that shear rates in journal bearings are typically in the range of 10⁵ to 5 × 10⁶s⁻¹, whilst peak shear rates for the piston rings can be as high as 2 × 10⁷s⁻¹, and for the valve train, peak shear rates can reach 2 × 10⁸s⁻¹. Accurate viscosity shear rate curves have been measured for different temperatures using a range of viscometers, including a novel mid-shear capillary viscometer that is capable of measuring viscosities in the shear rate range of 10⁴ to 10⁶s⁻¹. The use of such a viscometer is crucial to obtain good fits to the measured data (since usually, viscosity data are only usually available at low shear rates, 10²–10³s⁻¹, and extremely high shear rates, > 10⁶s⁻¹, and such data are difficult to use for accurate viscosity/temperature/shear rate fits). The implications of the above data are then discussed for the design of low friction lubricants which give improved fuel economy. It is highlighted that the traditional high temperature high shear viscosity, HTHS150, measured at 150 ℃ and a shear rate of 10⁶s⁻¹, although adequate for bearing durability purposes, is probably not the ideal parameter to use for estimating the fuel economy potential of an oil (since shear rates in engines are usually much greater than 10⁶s⁻¹, and also because oil temperatures in fuel economy engine tests are usually much lower than 150 ℃). Other possible high shear viscosity parameters are discussed which may be an improvement on HTHS150. The work also highlights that the choice of viscosity modifier, and the amount used, can have a substantial impact on the fuel economy performance of a lubricant.

Tribological machine components such as engine crankshaft bearings operating at high load or rotational speeds often experience very thin oil films leading to asperity contact between the bearing and crankshaft. When the oil film thickness drops below the peak asperity height there is an increase in contact, wear and asperity power loss leading to worsening severity factors and seizure risk. Key factors influencing the bearing lubrication performance are the surface characteristics of the materials at the contact interface. SABRE-TEHL is a software simulation tool used for performing thermo-elastohydrodynamic analysis of bearing applications. In this current study, the development of a combined contact and wear model is demonstrated. This includes a hard contact model giving a direct surface contact pressure in areas of zero oil film thickness, a surface yield model with yield limits and plastic gradient from measured stress–strain data, an asperity wear model using measured surface roughness data and an asperity contact model based on measured roughness, asperity density and tip radius. Input surface roughness data are measured using a white light interferometry method. The effects of surface characterization in terms of roughness and stiffness properties are demonstrated using simulation results comparing an aluminium bimetal bearing with a polymer coated bearing, showing a significant reduction in asperity friction for the bearing with a polymer coating. Simulation results are compared with experimental test results for a 1.5 l diesel engine, a 2.0 l diesel engine and for a bearing fatigue test rig. Generally good agreement is seen when comparing the wear contours and wear depth on the bearing surface.

Traction in highly loaded elastohydrodynamic contacts is of great importance to reduce losses. The calculation of traction in these EHL contacts relies on the rheological models used. In this paper, results from traction experiments, which form an integration over Hertzian contacts with strongly inhomogeneous conditions, are presented. They are compared to data from laboratory measurements with homogeneous conditions. Due to the fact that the integral data do not directly represent local rheological fluid properties, further investigations are presented. Here, thermographic measurements are used to discern the contact temperature locally. Furthermore, a model for the maximum shear stress depending on pressure is proposed and compared to existing models.

The current study uses Reynolds equation and the cross-film flow velocity profile to analytically determine pertinent locations for texture feature positioning in sliding hydrodynamic contacts. The position of textures is shown to have a significant effect on the lubricant film thickness, thus the load carrying capacity and generated friction and power loss. It is shown that textures, residing after the inlet lubricant recirculation boundary and prior to the position of maximum contact pressure enhance film thickness and reduce friction in the contact of real rough sliding surfaces. The methodology is applied to partial surface texturing of a thin compression ring of a high performance race engine, with the predicted results confirming the utility of the expounded analytical technique and its conformance to the findings of others reported in literature. The time–efficient analytical and fundamental approach constitutes the main contribution of the paper to furtherance of knowledge.

The friction and wear properties of supramolecular gel lubricant impregnated into the laser surface texturing steel fabricated by laser micromachining were investigated, with that impregnated with commercial oil PAO10 as a comparison. Scanning electron microscopy characterization shows the gel lubricant have been successfully pumped into the pores of laser surface texturing steel from liquid state and solidified afterwards via supermolecular assembly. The friction was measured on an Optimal SRV-IV oscillating reciprocating friction and wear tester. It is shown that laser surface texturing steel impregnated with gel lubricant exhibited outstanding friction reduction and antiwear performance than impregnated with PAO10. The gelator molecules act not only to solidify lubricating oil, but also contribute to boundary lubrication by strong adsorption on substrate surface. The instant self-assembly of gelator to solidify lubricating oils makes impregnation of lubricants easier and helps to keep lubricating oils retention for long-term functioning.

With recent progress of material technologies, the wear resistance of ultra-high molecular weight polyethylene for total joint prostheses has been improved, but under severe conditions friction and wear problems have not yet been completely solved. Therefore, the application of artificial hydrogel cartilage with similar properties to natural articular cartilage is expected to solve the friction and wear problems by improvement of lubrication mechanism with superior tribological functions. In this study, reciprocating tests of four kinds of poly(vinyl alcohol) hydrogels were carried out and the biphasic finite element analysis was conducted. As artificial cartilage specimens, four kinds of poly(vinyl alcohol) hydrogels were prepared using the repeated freeze–thawing (FT) method, the cast-drying (CD) method and the hybrid method with different layered structure as FT on CD or CD on FT. In reciprocating test of ellipsoidal poly(vinyl alcohol) hydrogel specimen against flat glass plate in saline solution, four kinds of hydrogels exhibited very different frictional levels as hybrid (CD on FT) < CD < FT < hybrid (FT on CD). It is noticed that hybrid (CD on FT) gel maintained extremely low friction and showed minimal wear. The effectiveness of biphasic lubrication was evaluated by biphasic finite element analysis. The importance of the load support by fluid phase at early stage and the surface lubricity after lowering of interstitial fluid pressure in poly(vinyl alcohol) hybrid (CD on FT) gel are discussed by comparison of experiment and finite element analysis.

Classical approach for elastohydrodynamic lubrication problems contains solution of Reynolds and elasticity equations simultaneously, where elasticity equation was derived based on semi-infinite solid assumption. Fluid–structure interaction method which uses finite element formulation is another alternative approach for elastohydrodynamic lubrication problems. Present study contains two sections: first finite element method was used to evaluate accuracy of semi-infinite assumption for deformation in an artificial joint cup for a verity of material and geometrical properties. Then fluid–structure interaction method was used to simulate an artificial hip joint lubrication under squeeze film motion and efficiency and accuracy of this method was speculated by comparing the results to a previously done work. In the first section, deformation of a cup under Hertzian contact was calculated by finite element software ADINA. Various combinations of cup thickness, material properties, and dimensions of contact ellipse were modeled and results compared to the answer of semi-infinite assumption and column method for the same problem. Then a ball-in socket configuration for an artificial hip joint was modeled by an ultra-high-molecular-weight polyethylene cup and a Stainless Steel ball in the same software. Fluid film pressure and thickness were extracted from the results and compared to a previous study. Results for deformation of cup show that semi-infinite assumption and column method do not lead to acceptable accuracy for geometrical conditions of artificial hip joint. For fluid–structure interaction analysis of squeeze film motion, at first time steps, pressure distribution shows differences with the previous work, but at last time steps, fluid film pressure matches the previous study. For all time steps, film thickness of fluid–structure interaction method is higher than what was reported by previous work. The main reason is that no mesh independency check was performed for previous work, while for this study mesh independency of answers was analyzed by creating two other models.

Double-glow plasma coatings are recommended for metallic components to mitigate the damage induced by complex loading conditions. In this paper, Cr–Nb alloyed layer was formed onto the TiAl substrate via a double-glow plasma process to enhance its anti-fretting wear performance. Nano-indentation and scratch tests were used to evaluate the mechanical properties of the coating. The fretting wear behaviour of the coating was investigated using a pin-on-plate fretting rig by rubbing against the Si₃N₄ ball. The 7 µm thick Cr–Nb coating was well bonded to the substrate with the 2 µm thick diffusion layer. The hardness of the coating was 9.5 GPa, which was 1.6 times greater than that of the uncoated TiAl substrate. Scratch tests showed that the critical load of Cr–Nb coating was 17.6 N. The fretting wear mechanism of the coatings was discussed in detail in this paper.

Experimental investigations were carried out to better understand the rolling contact fatigue mechanisms in nitrided layers of the 33CrMoV12-9 steel grade. Surface-initiated pitting failure mode was reproduced on a twin-disc machine to analyse crack growth and compressive residual stress behaviour within the nitrided layers. Metallographic examinations, 3D observations by means of high-resolution X-ray computed tomography and residual stress analysis were realised on nitrided 33CrMoV12-9 specimens before and after rolling contact fatigue tests. The study revealed that if the initial compressive residual stresses associated with the surface treatment are released during the process of rolling contact fatigue, pre-existing superficial cracks propagate in the nitrided layers along the intergranular carbides. These precipitates induced by the nitriding process therefore act as preferential crack propagation sites.

Accidents increase during the first rain after a long dry period due to the accumulation of fine particles, which contaminates the road surface and induces a friction loss. This paper presents new experimental evidences that help to understand how the particulate contaminants lubricate the tire–road interface. A laboratory test method is developed to reproduce the deposit of contaminant particles on the road surface and measure the friction coefficient on dry and wet contaminated surfaces. Test protocol is described with respect to the contaminant collection, the spreading of contaminant particles on the road specimen and their compaction to simulate the effect of the traffic, the wetting of the test surface to simulate precipitations, and the friction measurement. Friction tests are conducted in dry and wet conditions (by adding successive known quantities of water). Results show that dry particles can induce friction loss and the friction coefficient of dry contaminated surfaces is always lower than that of a dry and clean surface. The dry lubrication mechanism is attributed to shearing and sliding of the particle layer. Water induces first a significant friction drop, attributed to the viscosity of the particles–water mixture, then a friction increase due to a washing effect. Weight analysis shows a tight relationship between the friction coefficient and the mass of contaminants (particles in dry condition and particles–water mixture in wet conditions). There is a minimum in the friction versus water weight variation. Interpretation is made by considering fine particles as a wet granular material. Discussions are made with respect to the influence of particle concentrations and surface texture.