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.     

Experimental Investigation into Friction Performance of Dimples Journal Bearing with Phyllotactic Pattern

In this paper, we discuss the developments of a novel dimples journal bearing and the experimental results on friction performance. Based on the phyllotaxis theory of biology, the lubricating dimples were positioned in the internal circle surface of the journal bearing. The effects of phyllotactic pattern parameters on the distribution of these dimples were discussed in detail and simulated. A series of experimental results was presented to examine the effect of phyllotactic coefficients and dimple diameters on the friction characteristics. It was shown that the friction performance of journal bearings can be evidently improved under the proper tested parameters. Meanwhile, the friction coefficients, which are obtained in the dimples journal bearings with phyllotactic pattern and linear pattern, were presented to reveal their differences, and lower values of friction coefficients were achieved for the proposed dimples journal bearing under the tested radial load and rotational speed.     

微织构光固化填充h-BN的巴氏合金表面摩擦学性能

为了提高巴氏合金在油润滑条件下的摩擦学性能,在巴氏合金表面加工凹坑微织构并利用光固化填充方法填充六方氮化硼(h-BN)固体润滑剂,制备出h-BN与表面微织构相结合的复合润滑结构。研究复合润滑结构在油润滑条件下的摩擦学性能及其减摩润滑机制。结果表明：复合润滑结构的摩擦学性能远高于未织构面和纯织构面;当凹坑微织构直径较小时,织构密度为10%～20%时,复合润滑结构摩擦因数较小,而凹坑直径较大时,随着织构密度的增加,复合润滑结构摩擦因数逐渐减小;当织构密度小于20%时,凹坑直径较小的复合润滑结构摩擦因数小,当织构密度达到30%时,随着凹坑直径的增加,复合润滑结构摩擦因数减小。复合润滑结构能够改善巴氏合金表面摩擦学性能,是因为h-BN固体润滑剂的释放在巴氏合金表面形成了固体润滑薄膜,避免了润滑油膜较薄处的巴氏合金表面直接与45钢表面接触,且释放h-BN固体润滑剂后的微织构凹坑可以起到收集磨粒,储存润滑油的作用。     

镶嵌MoS2的Al2O3/TiC基陶瓷刀具材料微孔表面摩擦磨损性能研究

采用Nd:YAG激光器在Al2O3/TiC陶瓷刀具材料表面加工出不同密度的微孔,并涂覆填装MoS2固体润滑剂,在UMT-2摩擦磨损试验机上进行往复摩擦试验,研究其在不同载荷和速度下的摩擦磨损性能,通过白光干涉仪、光学显微镜和扫描电镜观察激光织构化后表面特征和磨损后表面形貌。结果表明:激光织构化后,陶瓷材料表面发生了氧化;在相同的实验条件下,与光滑表面相比较,填装MoS2固体润滑剂的微孔表面能够有效地降低摩擦因数,减小磨损率。这主要是由于填装在微孔中的润滑剂在摩擦作用下涂覆到基体表面,形成润滑膜,起到减摩降磨作用,同时激光加工后微孔周围凸起及氧化后形成的摩擦特性优良的TiO2也能起到良好的减摩降磨效果。通过对磨损形貌分析,光滑表面磨损较为严重,有大量的犁沟产生,主要的磨损形式为磨粒磨损及脆性断裂;微孔表面磨损较为轻微,其主要磨损发生在微孔周围。     

Frictional Performance of a Textured Surface with Elliptical Dimples: Geometric and Distribution Effects

Geometric and distribution effects of inclined dimples on the friction and wear performance of a textured surface were experimentally investigated. The frictional coefficients and face temperatures of four different kinds of elliptical surfaces were measured under various conditions of rotation speed, load, and viscosity. Experimental results showed that distribution parameters significantly affected frictional performance. From the experiments it was found that surfaces with double-row elliptical dimples have smaller frictional coefficients and lower face temperatures than that of the single-row elliptical dimples and had a 59% decrease in frictional coefficient and a 58% decrease in face temperature rise. This means that double-row elliptical dimples can lead to a more profound hydrodynamic effect as well as possess a lower chance of surface contact and wear. Moreover, a larger dimple diameter and slender ratio further improved the frictional performance of the textured surface with double-row elliptical dimples.     

Drag Reduction by Dimples on Surfaces in Plane–Plane Contact Lubrication

An experimental system was designed to evaluate the drag reduction by dimples on surfaces in plane–plane contact lubrication. Because of the dimples on the surfaces, in some of the experiments, it was observed that stable gas bubbles existed in the lubricant film. The results of the experiments showed that the existence of gas was decided by the depth of dimples and the volume of lubricant supply; the deeper the dimples were and the less the lubricant supply, the higher the probability of gas existence. The reason for the existence of gas, based on a model analysis, was the meniscus force of the gas–liquid interface, which caused the gas to be enclosed in the dimples. When a small amount of lubricant was supplied, the friction force was reduced because of a mixed lubrication film of oil and gas. For different amounts of lubricant supply, an optimal depth of dimples existed in plane–plane contact lubrication, which could obviously reduce the friction force. An optimization model for full film lubrication was presented to determine the optimal depth of dimples. Based on the model, the optimal dimple depth is approximately proportional to the distance between the two surfaces. The theoretical analysis agreed with the experiments.     

凹坑织构对石墨材料水润滑性能的影响

为提高核主泵屏蔽电机用水润滑石墨轴承的摩擦润滑性能，采用高速雕铣机在石墨试样表面加工不同形状、深度和面积占有率的凹坑织构，通过水润滑条件下的销盘摩擦试验，测试分析凹坑织构结构参数对石墨材料水润滑性能的影响。结果表明：在水润滑条件下凹坑织构具有一定的减摩效果，随着转速的增加，凹坑织构的动压效应增强，石墨试样摩擦因数减小；随着载荷的增加，石墨试样摩擦因数减小；随着凹坑深径比、面积占有率的增加，石墨试样摩擦因数均呈现先减小后增大的变化趋势，当凹坑深径比为0.5、凹坑面积占有率为3%左右时，石墨试样摩擦因数最小；相较三角形凹坑织构，正方形凹坑和圆形凹坑织构的减摩效果更好。     

A Hydrodynamic Laser Surface-Textured Gas Mechanical Face Seal

A new type of hydrodynamic laser surface texturing gas seal with orientation ellipse dimples is introduced to improve hydrodynamic effect. Theoretical model is developed to study the hydrodynamic effect of this new gas seal. Then, a parametric investigation of the texturing parameters such as slender ratio, dimple inclination angle, dimple depth, and dimple area rate is presented for the presented gas seal under different operating parameters of rotation speed, seal pressure, and clearance. Results show that the orientation dimples can greatly improve hydrodynamic effect of laser surface texturing gas seals. Open force may be improved more than 20% greater by hydrodynamic effect in the analysis.     

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.     

Numerical Simulation of the Influence of the Bulges around Laser Surface Textures on the Tribological Performance

To improve the tribological performance of mating surfaces such as journal bearings, dimples with different shapes were introduced through laser surface texturing. In laser surface texturing processes, bulges are always induced around the dimples and may influence the tribological performance of the surface. In this article, the influence of the bulges on the tribological performance of a journal bearing is numerically investigated. Variations in both the friction coefficient and the eccentricity ratio of a journal bearing with the width and height of the bulges are studied. The influence of the radius of the dimples on the tribological performance of a journal bearing is also investigated. Results show that the friction coefficient increases with both the width and the height of bulges in the case of journal bearing under light and moderate loading conditions, which is in accord with the experimental results in previous works. The linear variation in the friction coefficient with the height of the bulges indicates that the wear rate of the bulges may be investigated by monitoring the friction coefficient. The abnormality of friction coefficient variation with the width of the bulges in the case of heavy loading condition indicates that introducing the bulges is an effective approach for reasonable tribological performance. From the results for the influence of the dimple radius, it can be concluded that although the introduction of dimples can improve the tribological performance, it also has some disadvantages such as decreasing the load-carrying capacity.     

Friction and Wear Properties of MoS<Subscript>2</Subscript>-Overcoated Laser Surface-Textured Silver-Containing Nickel-Based Alloy at Elevated Temperatures

The solid lubricant that is coated on a flat surface is easily removed during friction. Surface texture dimples, which act as reservoirs of solid lubricant, can prolong the wear life of solid lubricant films. We textured silver-containing nickel-based alloys by a pulse laser and filled the micro-dimples with molybdenum disulfide powders. The tribological properties of the alloys were tested by rubbing against alloyed steel on a ring-on-disk tribometer at temperatures ranging from room temperature to 600°C . After laser surface texturing, the friction coefficients of the silver-containing nickel-based alloy smeared with molybdenum disulfide powders were reduced at temperatures ranging from room temperature to 400°C. With increasing dimple density, the wear life of the MoS₂ film increased while the wear rate of the nickel-based alloy decreased. The wear life of the textured surface with a dimple density of 11.2% exceeded 10,000 m at room temperature. We conclude that molybdenum disulfide and its oxides stored in the micro-dimples play a role in lubrication at room temperature and high temperatures, respectively.     

Preliminary investigation of the effect of dimple size on friction in line contacts

The scale of surface texture is becoming an important issue of surface texture design, particularly for the condition of low speed and high load. Experiments were carried out to investigate the effect of dimple size on friction under line contact condition. The patterns of dimples distributed as square array were fabricated on the surface of brass disks. Each pattern has the same area density of 7%, the same depth over diameter ratio h/d of 0.03, and dimple diameter d varying from 20 to 60 μm. The frictional tests of the brass disk sliding against a stationary cylindrical surface of bearing roller were conducted. It was found that the pattern with dimple diameter of 20 μm presented the effect of friction reduction. For the further understanding of the effect of dimple size under line contact condition, numerical simulations were also carried out to evaluate the hydrodynamic pressure within the contact of cylindrical and plane surfaces. The effects of dimple size and radius of the cylinder on the load carrying capacity were evaluated and discussed.     

Effects of surface texturing on the frictional behavior of cast iron surfaces

The purpose of this study is to investigate the effects of the geometry and distribution of microdimples on the frictional behavior of surfaces for applications in automotive engines. A square array of microscale circular dimples was selected as the texture pattern. A laser beam was used to create microdimples with various dimensions on cast iron surfaces. Frictional tests were performed with selected loads and speeds to simulate the operation conditions of automotive engine parts. The effects of dimple distribution were also investigated. The aspect ratio of the dimples was found to be the most significant factor, while the effect of the surface density of the dimples on the coefficient of friction was found to be only marginal.     

Use of the Roughness Parameters Ssk and Sku to Control Friction—A Method for Designing Surface Texturing

The aim of this work was to show that with the use of the surface roughness parameters S_sk and S_ku we can predict tribological behavior of contact surfaces and use these parameters to plan surface texturing. This article presents a continuation of our research on virtual texturing and experimental work on surface textures in the form of channels. For this investigation, steel samples were laser surface textured in the shape of dimples with different spacings between the dimples and different dimple depths. The experimental results confirmed that the parameters S_sk and S_ku can be used to design the surface texturing, where a higher value of S_ku and more negative S_sk lead to lower friction.     

Micro/Nanotexture Design for Improving Tribological Properties of Cr/GLC Films in Seawater

Microdimples with different diameters and Cr/graphite-like carbon (GLC) films were fabricated on silicon by laser surface texturing and magnetron sputtering technology, respectively. The texturing effects on the microstructures and tribological performance in seawater were comparatively investigated. The results showed that both the friction coefficients and wear rates in seawater decreased with an increase in dimple diameter in the lower range and then increased with a further increase in diameter. The Cr/GLC film with an appropriate diameter of dimples (1.5 µm) is effective in enhancing the tribological properties due to entrapment of wear particles and seawater in the dimples. In addition, the roughness and graphitization contact area have obvious effects on the wear resistance. If the surface roughness is too high, the graphitization contact area will decrease, and the ratio of minimum aqueous film thickness to the surface roughness is so small that the load is almost totally borne by the boundary film.     

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.     

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.     

Experimental investigation of tribological performance of laser textured stainless steel rings

A series of experiments is conducted to examine the frictional characteristics of laser surface-textured, heat-treated 17-4 PH stainless steel specimens. Two dimple shapes are tested: circular and elliptical. The circular dimples are designed with different sizes, dimple densities and dimple depth-to-diameter ratios. It is found that the cavitation pressure (P_cav) in a circular dimple is strongly affected by the operational speed. The surface textured specimens provide low coefficient of friction compared with plain (dimple-free) surfaces. However, for the material used in the current experiments, the surface texture decreases the surface's resistance to wear.     

结构化凹坑减阻表面的拓扑磨削与仿真研究

为了磨削出凹坑型结构化减阻表面,基于磨削运动学关系与点集拓扑理论,提出拓扑磨削结构化减阻表面的方法。首先对凹坑型结构化表面进行拓扑特征分析,根据凹坑表面创成机制建立结构化表面与结构化砂轮间的拓扑映射矩阵,设计出磨料簇砂轮;建立拓扑磨削运动轨迹模型,并通过MatLab仿真磨削过程,分析速度比与磨削深度等参数对表面形貌的影响。仿真与分析结果表明:采用拓扑磨削的方法并合理选用速度比与磨削深度等参数,可以在工件表面磨削出特定形状的凹坑型结构化减阻表面,且随着速度比的增大,凹坑长度与长度方向排布周期均增大;随着磨削深度增大,凹坑的长度与宽度增大,排布周期不变。     

Localized Boundary Effect on Elastohydrodynamic Lubricated Film Shape

The study aimed to determine the formation of an adsorption film at elastohydrodynamic lubricated (EHL) contacts and its effects on EHL film shape and friction. Experiments were conducted on an optical EHL test rig with surfaces of different surface energies. A new type of “abnormal” EHL film shape characterized with three dimples in the inlet of the contact was obtained in pure ball sliding experiments with long-chain polybutene. The adsorption layer was inferred to be the main cause for the “tri-dimple” phenomenon. The “tri-dimple” formation was examined. Under a fixed speed, a single inlet dimple gradually broke into three dimples with increasing number of ball rotation, and it happened with slight increase in friction force. Three zones, namely a central and two lateral zones, of the contact were classified and characterized with different levels of influence on the adsorption layer.