The Effect of Laser Texturing of Steel Surfaces and Speed-Load Parameters on the Transition of Lubrication Regime from Boundary to Hydrodynamic

Laser surface texturing (LST) is an emerging, effective method for improving the tribological performance of friction units lubricated with oil. In LST technology, a pulsating laser beam is used to create thousands of arranged microdimples on a surface by a material ablation process. These dimples generate hydrodynamic pressure between oil-lubricated parallel sliding surfaces. The impact of LST on lubricating-regime transitions was investigated in this study. Tribological experiments were carried out on pin-on-disk test apparatus at sliding speeds that ranged from 0.15 to 0.75 m/s and nominal contact pressures that ranged from 0.16 to 1.6 MPa. Two types of oil with different viscosities (54.8 cSt and 124.7 cSt at 40°C) were evaluated as lubricants. Electrical resistance between flat-pin and laser-textured disks was used to determine the operating lubrication regime. The test results showed that laser texturing expanded the range of speed-load parameters for hydrodynamic lubrication. LST also reduced the measured friction coefficients of contacts that operated under the hydrodynamic regime. The beneficial effects of laser surface texturing are more pronounced at higher speeds and loads and with higher viscosity oil.     

磨料粒度对表面微织构纯钛干摩擦性能的影响

为了提高钛及钛合金钻具在超深钻探、深海钻探和外太空钻探工程中的减摩抗磨性能。利用激光表面加工技术在工业纯钛（TA2）表面制备了不同参数的点阵微织构。采用MS-T3000摩擦磨损试验机测试了微织构钛合金在不同粒度模拟月壤作用下的摩擦学性能。利用扫描电子显微镜和能谱分析仪分析磨痕形貌及元素含量。研究结果表明：当磨料粒度小于微织构点阵的直径时,磨料压入微织构点阵里,磨料具有滚动和滑动两种运动方式。当粒度大于微织构点阵的直径时,磨料不能完全压入微织构点阵里,磨料对微织构TA2表面产生了滑动犁削作用。由于两种磨料磨损的作用机理不同,同等条件下,小粒度的磨料作用下的微织构TA2的摩擦因数和磨损率较大粒度磨粒作用下的最大减少量分别为50%和53%。考虑磨料粒度与微结构的匹配性,可以大大降低摩擦减少磨损。     

A Novel Surface Texture Shape for Directional Friction Control

An experimental study is presented to evaluate the influence of anisotropically shaped textures on the behaviour of sliding friction and sensitivity to sliding direction. The plate samples were textured with triangular sloped dimples using an ultrafast laser surface texturing technique. Reciprocating cylinder-on-plate tests were conducted with steel sliding pairs using mineral base oil as a lubricant to compare the tribological performance of reference non-textured specimen and dimpled samples. The dimples were designed with varying converging angles in the transverse y–z plane and top-view x–y plane. In this study, no dimple was fully covered in the contact area since the dimples size is much larger than the Hertzian line contact width. Stribeck style dynamic friction curves across boundary, mixed and hydrodynamic lubrication regimes were used to determine the benefit or antagonism of texturing. Observation of the directional friction effect of the anisotropic textures indicated that the converging shapes are beneficial for friction reduction, and the dimpled specimens have a lower friction coefficient particular under prevailing boundary lubrication conditions. It was also found that the real contact length variation rate is a major factor controlling the local friction response. The sloped bottoms of the textures produce effective converging wedge action to generate hydrodynamic pressure and contribute to the overall directional friction effects.     

Grooved surface texturing by electrical discharge machining (EDM) under different lubrication regimes

The aim of the present research work was to investigate the effectiveness of grooved surface texturing with a rhombic geometry under different lubrication regimes. Tribological investigation under unidirectional sliding was focused on the effect of texturing parameters including pattern area density on the coefficient of friction under different lubrication regimes, achieved by varying sliding speed and lubricant viscosity. Grooved patterns with different textured area densities were produced on steel samples by electrical discharge machining. Results of this investigation showed that under boundary lubrication, textures resist sliding thus resulting in increased friction. The largest improvement of friction reduction was observed under hydrodynamic lubrication, for low‐viscosity oil when using the textured disc with 21% pattern area density. The reduction of the coefficient of friction if compared with the untextured surface was of approximately 24%. Examination of the sliding surfaces has not shown any quantifiable wear for the contact conditions studied.     

Effect of laser textured dimples on the lubrication performance of cylinder liner in diesel engine

Laser surface texturing (LST) technique was utilised on a cylinder liner in a diesel engine. In order to analyse the effect of LST micro‐dimples on the lubrication and friction properties of cylinder liner–piston ring (CL–PR), we developed a new mixed lubrication model on the basis of the average Reynolds equation and asperity contacts equation. The model can consider the coupling effects between the surface roughness of non‐texturing regions and micro‐dimples and the synergistic effects of multi‐micro‐dimples. The results show that cylinder liner surface by LST can form effective hydrodynamic lubrication effect in most regions of the strokes, only near the dead points, the friction pair is in mixed lubrication state, asperity contact plays a major role in balancing the external load and the asperity friction force is obvious. The micro‐dimple parameters were optimised to obtain a better lubrication effect with the following optimised results: r_p = 30–60 µm, S_p = 0.2–0.4 and e = 0.03–0.1. Copyright © 2012 John Wiley & Sons, Ltd.     

Friction reduction in low-load hydrodynamic lubrication with a hydrophobic surface

A novel tribometer capable of measuring low friction forces and low loads at high speeds has been employed to measure the friction coefficient in a pure sliding, ball-on-flat contact in hydrodynamic lubrication conditions. The tribometer was custom-built for measuring friction at low loads, to allow the authors to investigate the feasibility of using the liquid-slip phenomenon for the lubrication of high-sliding MEMS. The theory behind lubrication with liquid slip and its effect on friction is briefly discussed. Contacting surfaces were treated to create hydrophobic/hydrophilic or hydrophilic/hydrophilic pairs. Hydrophobic surfaces were made by coating mica with a self-assembled silane monolayer while the hydrophilic surfaces used were freshly cleaved mica and plasma-cleaned steel. Experiments were conducted at sliding speeds of up to 2 m/s and loads below 0.2 N. An aqueous glycerol solution was used as lubricant. Results obtained with hydrophilic/hydrophilic surfaces were in accord with hydrodynamic lubrication theory. Tests with hydrophobic/hydrophilic surfaces revealed a reduction in friction, which may be attributed to lubricant slip against the hydrophobic surface.     

The study of tribological properties of laser‐textured surface of 2024 aluminium alloy under boundary lubrication

The tribological property of aluminium alloy is critical for its reliable operation in practical applications. In this paper, the tribological performance of laser‐textured 2024 aluminium alloy is studied in unidirectional sliding tests under boundary lubrication. The dimples were produced on the aluminium alloy surface by using a pulse Nd : YAG laser. The topographical microstructures of these laser‐induced textures were characterised by optical and scanning electron microscopy. In comparison with untextured surfaces, a significant improvement in friction behaviour was observed for the textured surfaces. The influences of dimples density on the tribological properties were investigated. Two types of oil with different viscosities were evaluated as lubricants. It was found that the beneficial effects of laser surface texturing are more pronounced at higher speed and load with higher viscosity oil. The optimum dimples density of 8.5% was found to have a lower friction coefficient. On basis of the experimental results, the mechanism of friction reduction and anti‐wear is proposed. Copyright © 2012 John Wiley & Sons, Ltd.     

Impact of the Oil Temperature on the Frictional Behavior of Laser-Structured Surfaces

This study investigates how the beneficial effects of laser-manufactured surface microstructures on friction vary with varying oil temperature. Laser surface texturing (LST) is a favorable method to optimize friction and wear, which have always been considerable obstacles against the performance improvement of combustion engines. The aim of this study is to detect correlations between the dimensional parameters of the laser-textured surfaces and tribological performance at varying temperatures. Although the experiments are on a fundamental level, the obvious application of combustion engines has been selected to align the material, processing, and test parameters. A wide range of micro indentations—henceforth called “dimples”—were fabricated onto honed flat aluminum samples by means of ultrashort pulse laser texturing and the frictional behavior at different lubricating oil temperatures (30 and 90?°C) was analyzed.     

Improving fuel efficiency with laser surface textured piston rings

An experimental study is presented to evaluate the effect of partially laser surface textured piston rings on the fuel consumption and exhaust gas composition of a compression-ignition IC engine. Dynamometer tests were performed with a Ford Transit naturally aspirated 2500 cm³ engine at a wide range of engine speeds under near-half-load conditions. A comparison was made between the performance of reference non-textured conventional barrel-shaped rings and optimum partial laser surface texturing (LST) cylindrical-shape rings. It was found that the partial LST piston rings exhibited up to 4% lower fuel consumption, while no traceable change in the exhaust gas composition or smoke level was observed.     

Surface texturing by pulsed Nd:YAG laser

Introducing specific textures on a tribological surface can contribute to friction reduction in sliding contacts. In the present paper, a pulsed Nd:YAG laser emitting at 1064 nm, was used against 100Cr6 steel samples in order to produce well-defined surface micro-pores, which can act as lubricant reservoirs, micro-hydrodynamic bearings as well as traps for wear debris. Due to the high flexibility of the laser system, structural features such as shape, size, density and depth can be varied easily by changing the laser parameters. To optimize the parameters of the laser surface texturing process, an investigation was performed using different pulse numbers, various pulse energies and two different modes (single- and multi-mode). The microtextures were characterized with optical microscopy, scanning electron microscopy (SEM) and by topography techniques. The relationship between the laser processing parameters and qualitative and quantitative profile of the micro-pores was studied. Tribological testing of laser textured surfaces was performed in a low frequency–long displacement reciprocating sliding wear tester under boundary lubrication and results compared to un-textured case. Tribological comparison of textured, textured and lapped, and untextured surfaces shows only minimal influence of texturing for contact conditions investigated.     

Frictional performance of lithium 12‐hydroxystearate greases with different soap fibre structures in sliding contacts

The effect of the fibre structure of the grease on the frictional performance of lithium 12‐hydroxystearate greases with different fibre lengths was investigated in face, line, and point contact sliding tests. At high sliding speeds where the lubrication regime was practically hydrodynamic, the coefficient of friction of the base oil alone was lower than that of the greases. The coefficient of friction was roughly estimated as follows: base oil < long‐fibre grease < medium‐fibre grease < short‐fibre grease. The supply or replenishing capability of the grease played a critical role in maintaining hydrodynamic lubrication. At high contact pressures, the short‐fibre grease was superior in frictional performance to the long‐fibre grease due to the firm fibre network structure of the latter. In mixed and boundary lubrication regimes, the greases were superior in frictional performance to the base oil, since the soap fibres of the greases had a superior load‐carrying capacity. The long‐fibre grease, with a firm fibre structure, offered better frictional performance than the short‐fibre one.     

内燃机缸套激光珩磨技术及其性能试验研究

激光表面微造型技术是改善摩擦副表面摩擦学性能的有效途径之一。基于声光调Q技术的二极管泵浦固体光源（DPSS）Nd:YAG激光器,采用“单脉冲同点间隔多次”激光微加工工艺,对CA6DF2-26型柴油机气缸套内孔表面进行激光珩磨加工。加工出的缸套网纹参数具有较好的一致性,达到了主机厂的技术要求。进行了激光珩磨缸套的台架综合性能试验研究,结果表明,采用激光珩磨缸套的发动机,其功率、扭矩、燃油消耗指标保持稳定,与采用传统平台珩磨网纹缸套的发动机相比,柴油机的机油油耗降低53％,漏气量降低近50％。     

The Effect of Elastomer Surface Texturing in Soft Elasto-Hydrodynamic Lubrication

The present study focuses on modeling the effect of elastomer surface texturing in soft elasto-hydrodynamic lubrication. The model consists of an elastomer with spherical microdimples sliding at a constant velocity relative to a rigid and smooth counterpart in the presence of viscous lubricant. The hydrodynamic lubrication and the elasticity problems are solved simultaneously for the cases of full and partial texturing of the elastomer surface. Load carrying capacity and friction force are evaluated by integration of the pressure and shear stress fields in the viscous lubricant, respectively. A parametric analysis is performed to obtain the optimum surface texturing parameters for maximum load and minimum friction.     

Friction Modifier Behaviour in Lubricated MEMS Devices

Low viscosity fluids could provide reliable lubrication for certain microelectromechanical system’s (MEMS) applications where high-sliding speeds and/or high sliding distances occur. However, while the use of low viscosity fluids leads to reduced hydrodynamic friction, high boundary friction can be a significant issue at low entrainment speeds. This article describes a series of tests of low viscosity fluids, blended with a friction modifier additive so as to provide a combination of both low hydrodynamic and low boundary friction at MEMS scales. The low viscosity fluids tested were hexadecane, low viscosity silicone oil, toluene and water. With the exception of water, the addition of the organic friction modifier octadecylamine to all these lubricating fluids produced a significant reduction in boundary friction. For a MEMS contact lubricated with silicone oil for instance, boundary friction was reduced from 0.5 to close to 0.05. The presence of the amine dissolved in the toluene had the effect of reducing boundary friction from 0.75 to 0.55; this was further reduced to 0.25 after the specimens had been immersed in the toluene-additive blend for 48 h. A water-soluble additive, diethylamine, was added to de-ionized water, at 0.1% by weight concentration. Although an initial reduction in boundary friction was observed (0.45–0.25), under these conditions the rapid onset of severe wear negated these effects. It is suggested that corrosion of silicon by water, followed by abrasion, is the cause of this accelerated wear.     

Geometric Shape Effects of Surface Texture on the Generation of Hydrodynamic Pressure Between Conformal Contacting Surfaces

Micro-surface texturing design is becoming an important part of surface engineering since engineering practices and analyses have indicated that surface textures may significantly affect the tribological performance of contact interfaces. Advances in the manufacturing technologies of surface finishing and micro-machining, such as laser surface texturing, photolithography, and etching, LIGA process, have made it possible to fabricate different fine structures on various engineering surfaces. Though the influence of micro-surface texturing on hydrodynamic lubrication has been widely investigated over the last decade, such an influence may be complicated and difficult to characterize with only a few statistic surface parameters. Thus, very little attention has been paid to the effects of different textural shapes and orientations on hydrodynamic lubrication, which is the main topic of this article. A theoretical model based on a single dimple was established to investigate the geometric shape and orientation effects on the hydrodynamic pressure generated between conformal contacting surfaces. Using the Successive Over Relaxation method, the average hydrodynamic pressure generated by the texture pattern with the dimples in shapes of circle, ellipse, and triangle at different orientations to the direction of sliding are obtained. The results indicate that geometric shape and orientation have obvious influences on load-carrying capacity of contacting surfaces. With the same dimple area, area ratio and dimple depth given in this research, ellipse dimples perpendicular to the sliding direction showed the best load-carrying capacity. This result agrees with previous experimental results very well.     

Topography-related effects on the lubrication of nanostructured hard surfaces

This study reports the effect of nanoscaled surface structure of some hard coatings on the (micro-) frictional behaviour of systems under minimum lubrication conditions with modest contact pressures and low sliding speeds (below 1 mm/s). For this purpose, Cr-N coatings with a randomly crater-like topography and with varying dimensions of surface features as well as a smooth Cr-N surface were tested with a microtribometer. The friction on the samples was measured as a function of the viscosity of the applied mineral base oil and the sliding velocity. For all tests, the structured surfaces exhibited lower friction than the smooth surface. Furthermore, it was possible to detect variations in the lubrication-promoting effect of the structures depending on the oil viscosity and the sliding speed. Indications for the existence of an optimum topographic scale for this type of surface structure were found.     

Investigation of the tribological properties of the different textured DLC coatings under reciprocating lubricated conditions

In this study, we propose the use of laser surface texturing methods to improve the wear resistance of diamond-like-carbon (DLC) coatings. First, the application of dimples perpendicular to an engineering surface is introduced by using laser in a controlled manner. The solid DLC lubricant is subsequently deposited on the textured surface using the magnetron sputtering technique. In the experiments, the dimple densities were varied from 0 to 30%, and the dimple diameters were varied from 40 to 300 μm. The effect of the geometric parameters on the wear performance was studied using a reciprocating sliding-wear tester under oil lubrication conditions. The results showed that the DLC coatings with the appropriate dimple density (10%) and diameter (∼100 μm) demonstrated an obvious improvement in terms of the friction coefficient and wear rate compared with that of the un-textured DLC coatings. The experimental treatment produced respective reductions of 20% in friction and nearly 52% reduction of wear rate. This improvement could be explained by the action of reservoirs that enhance lubricant retention and trap the wear particles during sliding motions. In addition, the relationship between the dimple diameter, the contact width ratio and the wear performance is discussed.     

Friction Properties of Lubricated Laser-MicroTextured-Surfaces: An Experimental Study from Boundary- to Hydrodynamic-Lubrication

We present measurements of friction coefficient of lubricated laser surface textured (LST) microstructures with two different geometries. The former is made of a square lattice of microholes; the latter is constituted by a series of microgrooves. We analyze sliding velocities spanning more than two orders of magnitude to cover the entire range from the boundary to the hydrodynamic regime. In all cases, the interfacial pressure is limited to values (relevant to particular manufacturing processes) which allow to neglect macroscopic elastic deformations, piezo-viscosity and oil compressibility effects. The measured Stribeck curves data are compared with those obtained for the flat control surface and show that the regular array of microholes allows to reduce friction over the entire range of lubrication regimes with a decrease of about 50?% in the hydrodynamic regime. On the contrary, the parallel microgrooves lead to an increase of friction compared to the flat control surface with a maximum increase of about 80?C100?% in the mixed lubrication regime. These remarkably opposite friction results are then explained with the aid of numerical simulations. Our findings confirm that LST may have cutting edge applications in engineering, not only in classical applications (e.g., to reduce piston-ring friction losses in internal combustion engines) but also, in particular, in technological processes, such as hydroforming, superplastic forming, where the mapping of the frictional properties of the mold has a crucial role in determining the final properties of the mechanical component.     

线接触滑——滚条件下微凹坑表面摩擦特性

线接触副在工程中广泛存在,且大多工作在弹性流体动压润滑状态下。为研究微凹坑对线接触摩擦副摩擦学性能的影响及表面三维表征参数与摩擦学特性之间的联系,采用激光微造型技术通过控制微凹坑面积占有率、凹坑深度、间距等参数加工制造4个表面粗糙度相同,且表面微凹坑面积占有率分别为7%、14%、21%、28%的圆柱形试件;然后使用Talysurf CCI Lite非接触式三维光学轮廓仪对试样进行三维表面测量,且采用ISO25178定义的参数对三维表面形貌进行表征;并在电气化改造后的JPM-1型双盘摩擦磨损试验机上,针对不同滑滚比、不同载荷、不同转速等工况,完成一系列线接触弹流摩擦试验。结果表明,表面形貌的微观结构特性对线接触摩擦副的摩擦特性具有明显的影响,并给出表面体积参数以及特征参数与摩擦的关系;同时,在不同的工况条件及不同的滑滚比下表面结构特性对摩擦的影响效果也不同。     

Effect of surface laser texture on friction properties of nickel-based composite

Laser surface texturing (LST) was performed on the nickel-based composites by a Nd:YAG pulsed laser and the regular-arranged dimples with diameter of 150 μm were fabricated on their surfaces. The textured surfaces were smeared with molybdenum disulfide powder. The tribological properties of the textured and filled composites were investigated by carrying out sliding wear tests against an alumina ball as a counterface using a high temperature ball-on-disk tribometer. The tests were conducted at a sliding speed of 0.4 m/s and at normal loads ranging from 20–100 N and from room temperature to 600 °C. The friction coefficient of nickel-based composite textured and smeared with molybdenum disulfide was found to reduce from 0.18 to 0.1 at the temperature range from 200 to 400 °C. The texture with a dimple density of 7.1% was observed to prolong wear life of MoS2 film by more than four times in comparison to the texture with other dimple densities. The lubricious oxide particles stored in the dimples reduce friction coefficient at elevated temperatures and compensate for the extra lubricant owing to the degradation of MoS₂ caused by its oxidation at high temperatures.