Effect of Surface Texturing on Friction and Film Thickness under Starved Lubrication Conditions

Tato disertační práce se zabývá vlivem mělkých mikro-textur na tření a tloušťku filmu v mazaných nekonformních kontaktech za extrémních podmínek a za podmínek hladovění kontaktu. Měření byla realizována na tribometru v konfiguraci ball-on-disk. Kontakt byl pozorován pomocí vysokorychlostní kamery. Pro stanovení součinitele tření byl využit snímač krouticího momentu. V této studii byly popsány dva typy mikrotextur – mikrovtisky a příčné mikrodrážky. Výsledky naznačují, že za podmínek hladovění vedou mikrovtisky ke snížení tření a to díky nárůstu tloušťky mazacího filmu. Mechanismus doplňování mikrovtisků čerstvým mazivem je pravděpodobně způsoben kapilárními jevy ve vstupní oblasti. Třecí plochy s příčnými mikrodrážkami, jejichž délka byla menší než průměr Hertzova kontaktu, potom obecně vykazovaly lepší tribologické parametry ve srovnání s hladkými povrchy. Příčné mikrodrážky vedly k výraznému nárůstu tloušťky mazacího filmu za podmínek hladovění i za extrémních provozních podmínek (protisměrný pohyb). Numerické simulace přechodových jevů příčných mikrodrážek ukázaly dobrou shodu s experimentálními výsledky.This PhD thesis focuses on studying the effects of shallow micro-textures on friction and film thickness of lubricated non-conformal contacts under extreme and starved conditions. Measurements were carried out using a ball-on-disc tribometer equipped with high speed camera and torque sensor. Two types of micro-textures have been assessed in this study, micro-dents and transverse micro-grooves. The results reveal that micro-dents are helpful in reducing friction under starved conditions due to the film thickness enhancement. The mechanism of filling the depleted micro-dents with fresh lubricant is probably attributed to the capillary effect in the inlet zone under starvation. On the other hand, the rubbing surfaces with transverse shallow micro-grooves with a length less than the diameter of the Hertzian contact have an improved tribological performance in comparison with smooth surfaces. Indeed, transverse shallow micro-grooves showed a significant enhancement of film thickness under starvation and under extreme operating condition (reverse motion). The numerical simulation of the transient behavior of transverse limited micro-grooves showed accepted agreement with experimental results.

A tribological study of the interaction between surface micro texturing and viscoelastic lubricants

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2004.Includes bibliographical references (p. 116-117).An experimental study is performed on micro textured surfaces using both elastic and Newtonian fluids in order to understand the effect of surface texturing and fluid rheology on sliding friction under lubricated conditions. Nickel micro textured surfaces containing 50 to 125 micron diameter pores and 1 to 20 percent area densities of pores are fabricated using novel techniques inspired by MEMS fabrication methods in order to obtain a metal surface that is corrosion and wear resistant. These surfaces are then friction tested using a torsional rheometer with a parallel plate configuration, and the resulting data is normalized to obtain a Stribeck diagram that spans the lubrication regimes from boundary lubrication to hydrodynamic lubrication. Each lubricant/surface pair results in a unique curve that is subsequently compared to other lubricant/surface pairs in order to study the effect of different surface texturing parameters as well as the effect of the particular lubricant rheology. Experimental results from studying the effect of different surface texturing parameters indicate that particular micro textured patterns can reduce friction by as much as seventy-five percent after a controlled break-in period. Data from the tribo-rheometer also indicates that along with a reduction in friction, micro texturing can result in shifting the entire Stribeck curve so that the transition points between lubrication regimes are delayed or induced by as much as an order of magnitude of the Gumbel number, depending on the lubricant and running conditions. Focusing next on the effect of lubricant rheology on sliding friction over micro textured surfaces, this research reveals that the Stribeck curve will experience a shift according to the lubricant elasticity. A new dimensionless number formed by dividing the Gumbel number by the elasticity factor of the fluid can accurately account for the Stribeck curve shift as a result of fluid elasticity.by Sara J. Hupp.S.M

Identification of Local Lubrication Regimes on Textured Surfaces by 3D Roughness Curvature Radius

This paper proposes a new method of 3D roughness peaks curvature radius calculation and its application to tribological contact analysis as a characteristic signature of tribological contact. This method is introduced through the classical approach of calculation of radius of asperity in 2D. Actually, the proposed approach provides a generalization of Nowicki's method [ ], depending on horizontal lines intercepting the studied profile. Here, the basic idea consists in intercepting the rough surface by a horizontal plane and to calculate the cross section area without including “islands into islands”, i.e. the small peaks enclosed in bigger ones. Then, taking into account the maximal value of the height amplitude of the roughness included into this area, an appropriate algorithm is proposed, without requiring the classical hypothesis of derivability, which may be unstable when applied to engineering surfaces. This methodology is validated on simulated surfaces, and applied to engineering surfaces created experimentally, with a laboratory aluminium strip drawing process. The regions of the textured and lubricated specimens surface are analysed, and the results gives interesting prospects to qualitatively identify the local lubrication regimes: regions with high curvature radii correspond to severe contact (boundary/mixed lubrication regime) while regions with low curvature radii correspond to hydrodynamic lubrication regime

Multi-scale analysis of the roughness effect on lubricated rough contact

Determining friction is as equally essential as determining the film thickness in the lubricated contact, and is an important research subject. Indeed, reduction of friction in the automotive industry is important for both the minimization of fuel consumption as well as the decrease in the emissions of greenhouse gases. However, the progress in friction reduction has been limited by the difficulty in understanding the mechanism of roughness effects on friction. It was observed that micro-surface geometry or roughness was one of the major factors that affected the friction coefficient. In the present study, a new methodology coupling the multi-scale decomposition of the surface and the prediction of the friction coefficient by numerical simulation was developed to understand the influence of the scale of roughness in the friction coefficient. In particular, the real surface decomposed in different roughness scale by multi-scale decomposition, based on ridgelets transform was used as input into the model. This model predicts the effect of scale on mixed elastohydroynamic point contact. The results indicate a good influence of the fine scale of surface roughness on the friction coefficient for full-film lubrication as well as a beginning of improvement for mixed lubrication

Tribological Study of Textured Surfaces created using Modulation Assisted Machining for Steel-Aluminum Contact

One of the major problems the world faces currently is the loss of energy and mechanical efficiency in machine components due to friction. Also, surface damage due to friction and resulting part repairs and replacement has cost US industries a reportedly US $280 billion/year. Recent work in the field of surface modification and texturing has shown, that, artificially created micro textures significantly reduce friction and wear of lubricated surfaces. Micro-scale artificial textures in friction paring contacts acts as lubricant reservoirs and wear particle receptacles to boost hydrodynamic pressure, which influences the lubrication regimes. In this study, the Stribeck curve of untextured and textured surfaces created using vibration/modulation-assisted-machining are determined experimentally and numerically. Aluminum 6061-T6 disk mated with a high speed steel pin are studied in a pin-on-disk tribometer configuration for varying speeds and texture depths. Results show that at low speeds, textured surfaces accelerate the appearance of elasto-hydrodynamic lubrication regime. A 56% reduction in coefficient of friction was observed for 7 µm at 60 Hz, with a 90% wear reduction for similar conditions as compared to untextured surfaces. A numerical model based on the average Reynolds flow equation yielded similar results

A Review of Surface Texturing in Internal Combustion Engine Piston Assembly

This paper presents a brief review of surface texturing with a focus on piston assembly application. The paper begins with a general discussion on surface texturing and the manufacturing process of micro dimples.  Further, it discusses the theory of hydrodynamic lift generation and the effect of parameters of micro dimples texture on the surface-to-surface friction. Finally, the effect of surface texturing on heat transfer is briefly discussed. In pursuits to improve internal combustion engine (ICE) efficiency, tribological improvement of moving surfaces by means of micro surface texturing seems to be one of the way. However, texturing parameters have to be carefully designed as it can cause detrimental effect if the designs are wrong. Studies has shown micro surface texturing at piston ring could reduce friction around 20%-50% compare with un-textured piston ring and also reduce fuel consumption at 4%. Micro Surface texturing could also improve heat transfer between the surfaces to reduce piston slap and lubrication oil temperature. As reports on the surface texturing on friction reduction and heat transfer improvement in piston assembly are relatively scarce, it is suggested that optimization of micro dimple parameters for piston skirt application and its effect on engine tribology and heat transfer characteristics be further investigated

Laser textured surfaces for mixed lubrication: Influence of aspect ratio, textured area and dimple arrangement

Unidirectional sliding experiments with polished and laser textured steel surfaces were carried out to investigate the effects of different textured area densities, aspect ratios and dimple arrangements. The system was lubricated with Polyalphaolefin (PAO) at 100 °C and the contact pressure was 3 MPa. For measuring Stribeck curves, the sliding speed was controlled between 40 and 2000 mm/s. The textured area density was varied between 5% and 30%, with the lowest friction values found for 10%. Aspect ratios ranging from 0.02 to 0.2 were investigated and for 0.1 the lowest friction values were measured. The dimple arrangements tested were cubic, hexagonal and a random distribution for a textured area density of 10% and an aspect ratio of 0.1. Our results demonstrate that the dimple arrangement does affect friction values, hinting to the fact that individual texture elements do influence each other. The optimum dimple arrangement was found in a hexagonal packing. This systematic variation of these three key texturing parameters for the morphological texturing of a tribological surface with dimples will allow a strategic choice of texturing parameters. This makes the most of the tremendous potential that laser surface texturing has for reducing friction forces and thereby CO2 emissions

Optimised textured surfaces with application in piston-ring/cylinder liner contact

The application of textured surfaces in tribology has recently gained a huge momentum. In this chapter, a systematic approach to investigate the maximum outcomes from employing such surfaces is introduced with an insight into their application in internal combustion engines. A combination of various affecting parameters on the tribological performance of such surfaces is studied and the optimum results are introduced. The effect of employing such optimised textures in enhancing the lubrication condition in piston ring/cylinder liner contact is also studied