Experimental Study of Material Pick Up on Heat-Assisted Magnetic Recording (HAMR) Heads

Tribology Letters - We study the friction force during lubricated sliding of a rigid cylindrical indenter against a viscoelastic substrate in the iso-viscous visco-elasto-hydrodynamic lubrication...     

Influence of load and elastic properties on the rolling and sliding friction of lubricated compliant contacts

Lubricated “soft” contacts, where one or both contacting solids have a low elastic modulus, are present in many practical engineering and biological applications including windscreen wipers, wet tyres, elastomeric seals, contact lenses and the tongue/palate system. In such contacts, the prevailing lubrication mode is “isoviscous EHL” (elastohydrodynamic lubrication). Unlike in steel–steel contacts, rolling friction can be considerable and this originates in part from the viscoelastic properties of the compliant surfaces.In this paper the influence on friction of both applied load and the elastic properties of the solids is studied using a mini traction machine. In this machine, the rolling and sliding friction can be separately determined. The viscoelastic properties of the polymers employed are measured using a dynamic mechanical analysis apparatus. The measured friction is compared to theoretical models for soft EHL and the viscoelastic energy losses arising from the contact deformation. Consideration of the frequency dependence of the substrate viscoelasticity enables reasonably accurate predictions of the rolling friction coefficient, especially within the mixed and boundary lubrication regimes.     

Influence of temperature on the friction performance of gear oils in rolling–sliding and pure sliding contacts

The friction behaviour of five different gear oils in rolling–sliding and pure sliding contacts and how temperature influences their friction properties were investigated. It is found that increasing temperature decreases boundary friction with gear oils that contain friction modifiers while not for other gear oils, at all contact pressures investigated. In mixed lubrication region, temperature decreases friction at low contact pressures while increases friction at high contact pressures. The effect of slide–roll ratio on friction is significant in boundary lubrication region especially at higher temperature while less significant in mixed lubrication region at both low and high temperatures. The ranking of gear oils for friction in boundary and mixed lubrication regimes is similar both in rolling–sliding and pure sliding contacts, regardless of temperature. Copyright © 2013 John Wiley & Sons, Ltd.     

Subsurface Zones Created Under Lubrication Conditions Studied by Positron Annihilation

We present positron annihilation studies of the subsurface zones induced in the sliding contact of two metals under lubrication conditions. The surfaces of copper samples were exposed to the sliding of a small steel ball and the open volume defects induced under the surface were detected using both a positron beam experiment and a conventional positron experiment. From our studies we deduced that the total range of subsurface zones was almost independent of the lubrication conditions. For the highest load of the ball we observed that the subsurface zone created under lubrication condition extends deeper than for the case where the zone was induced in the dry sliding contact. We found that sliding with lubricant induces a layer on the surface whose properties, as detected by positrons, were different than those of other samples. It seems that this layer could contain open volume defects that are larger than those in deeper layers. Our studies have shown that the positron beam technique is a suitable tool for defect characterization in tribotested samples.     

Lubricant Effects in the Transition from Boundary to Microelastohydrodynamic Lubrication

Lubricant effects in the friction transition from boundary to microelastohydrodynamic lubrication were investigated by using a ballon-flat tribotester at sliding speeds from 0.02 to 0.88 mm/sec. Three lubricants—cyclophosphazine (X-IP), poly-alpha-olefin (PAO) and Z-DOL—were used, in this investigation. When X-IP was used at room temperature, a drop in friction coefficient from 0.22 to 0.12 at sliding speeds ≥0.10 mm/sec. (an unusually low speed) was observed, accompanied by a rise in the contact electrical resistance across the ball-fiat interface. The friction drop did not occur at temperatures ≥100°C. The friction transition was achieved at lower speeds when sliding perpendicular to the surface roughness texture. No transition occurred when PAO and Z-DOL were substituted as the lubricant. The latter him lubricants were working in the boundary lubrication regime as indicated by the contact resistance measurement.     

Comparison of frictional properties of gear oils in boundary and mixed lubricated rolling–sliding and pure sliding contacts

The friction responses of five fully formulated gear oils including mineral and synthetic oils were studied. This article examines the impact of contact motion types (rolling–sliding and pure sliding) and contact pressure on boundary and mixed friction properties of the selected gear oils in MTM (minitraction machine) and SRV (Schwing-Reib-Verschleiss tribometer). Mineral oils are found to be less affected by contact pressure compared to synthetic oils. Gear oils that show adsorption appear to be less sensitive to contact motion type in mixed lubrication while behave much more sensitive in boundary lubrication regimes. The ranking of gear oils for mixed friction was similar regardless of contact motion types at low contact pressures while differ at high contact pressures.     

内燃机排气凸轮-滚轮副异常工况下热弹流润滑分析

以某型号内燃机排气凸轮-滚子副为研究对象，建立有限长线接触条件下瞬态热弹流润滑数学模型及完全数值求解方法。依据实际载荷谱等工作参数下，给出凸轮-滚子间的滑动模型，获得凸轮-滚子副在凸轮旋转周期内润滑特性，并分析凸轮与滚子之间的打滑、滚轮偏斜和滚轮凸度对接触副润滑的影响。结果表明：打滑导致接触区温度和摩擦因数明显升高，从而弱化接触区润滑状态；滚子偏斜时油膜厚度明显减小，增加润滑失效的可能性。研究表明，在设计滚子凸度时考虑滚子偏斜问题的影响，可以在一定程度上缓解其负面影响。研究结果为凸轮-滚子副的润滑设计提供了一定的理论依据。     

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.     

Determination of load carrying ability of chemical films developed in sliding point contact

It has already been known for many years that the use of some extreme-pressure (EP), antiwear or friction modifier (FM) additives in mineral oils can produce different kind of boundary or chemical reaction films on sliding contact surfaces of some kinds of steel in boundary lubrication conditions. Using a sliding ball-on-disc configuration lubricated with some kinds of EP or FM, the wear scars on the balls can always reach the same limit size at a specified applied load and sliding velocity. From the fact that the limit sizes of wear scars decrease as sliding speed is increased or applied load is decreased, the load carrying ability of a chemical film can be obtained by extrapolating the data to the condition of zero sliding speed and is so defined that if the contact pressure is greater than this load carrying ability, the contact surfaces will continuously be worn; if the contact pressure is smaller than it, no more wear will occur on the surfaces. Based on this load carrying ability, the hydrodynamic effect of sliding pairs can also be identified. Therefore, the limit size of wear scar at specified sliding speed and applied load can also be predicted in a mixed lubrication condition.     

Modeling of Dynamic Friction in Lubricated Line Contacts for Precise Motion Control

A simple dynamic friction model for an elastohydrodynamic lubrication sliding and rolling line contact has been developed. This model uses the technique introduced earlier by Harnoy and Friedland (1). The model includes low-velocity regions where friction is a combination of contact and elastohydrodynamic friction. The study shows that the lime-variable friction is not only a function of instantaneous sliding velocity, but is also a memory function of the velocity history. Simulation of the model for an oscillating velocity exhibits similar hysteresis effects in friction-velocity curves as observed earlier in several experimental studies. The model can be useful for friction compensation to enhance the precision of motion in control systems.     

In Situ Studies of Cartilage Microtribology: Roles of Speed and Contact Area

The progression of local cartilage surface damage toward early stage osteoarthritis (OA) likely depends on the severity of the damage and its impact on the local lubrication and stress distribution in the surrounding tissue. It is difficult to study the local responses using traditional methods; in situ microtribological methods are being pursued here as a means to elucidate the mechanical aspects of OA progression. While decades of research have been dedicated to the macrotribological properties of articular cartilage, the microscale response is unclear. An experimental study of healthy cartilage microtribology was undertaken to assess the physiological relevance of a microscale friction probe. Normal forces were on the order of 50 mN. Sliding speed varied from 0 to 5 mm/s, and two probes radii, 0.8 and 3.2 mm, were used in the study. In situ measurements of the indentation depth into the cartilage enabled calculations of contact area, effective elastic modulus, elastic and fluid normal force contributions, and the interfacial friction coefficient. This work resulted in the following findings: (1) at high sliding speed (V = 1–5 mm/s), the friction coefficient was low (μ = 0.025) and insensitive to probe radius (0.8–3.2 mm) despite the fourfold difference in the resulting contact areas; (2) the contact area was a strong function of the probe radius and sliding speed; (3) the friction coefficient was proportional to contact area when sliding speed varied from 0.05 to 5 mm/s; (4) the fluid load support was greater than 85% for all sliding conditions (0% fluid support when V = 0) and was insensitive to both probe radius and sliding speed. The findings were consistent with the adhesive theory of friction; as speed increased, increased effective hardness reduced the area of solid–solid contact which subsequently reduced the friction force. Where the severity of the sliding conditions dominates the wear and degradation of typical engineering tribomaterials, the results suggest that joint motion is actually beneficial for maintaining low matrix stresses, low contact areas, and effective lubrication for the fluid-saturated porous cartilage tissue. Further, the results demonstrated effective pressurization and lubrication beneath single asperity microscale contacts. With carefully designed experimental conditions, local friction probes can facilitate more fundamental studies of cartilage lubrication, friction and wear, and potentially add important insights into the mechanical mechanisms of OA.     

基于声发射技术的点接触润滑状态研究

为研究点接触在部分弹流润滑条件下的润滑状态,使用球盘式摩擦磨损试验机进行试验,利用声发射技术监测不同工况下的润滑状态,并分析在不同工况下声发射信号特征参数的变化规律。结果表明:声发射信号特征参数计数、能量、信号强度对润滑状态的改变非常敏感,都随滑动速度的增加而减小,随载荷的增加而增加且在变化规律上几乎一致;利用声发射技术能够表征边界润滑向部分弹流润滑的过渡状态。     

Analysis of Thermal Stability of High-Performance Rolling–Sliding Contacts in Mixed Lubrication

Thermal instability has been considered by pioneer researchers to be one of the most promising lines for a fundamental investigation into the failure mechanisms of rolling–sliding contacts. This article uses a recently developed mixed lubrication model that integrates interrelated topographical, mechanical, thermal, and tribochemical aspects to study the thermal instability of high-performance rolling–sliding contacts. The effects of various system parameters on the relation between the system bulk temperature and the heat generation in the contact are analyzed. The parameters include surface roughness; contact component size; surface and lubricant mechanical, thermal, and tribochemical properties; and operating conditions. Key results and their implications to system design and operation considerations are summarized in the Conclusion section of the article in relation to enhancing the thermal stability of the contact, particularly under adverse lubrication conditions.     

含间隙曲柄滑块机构运动副动态磨损研究

含间隙曲柄滑块机构中运动副的润滑情况不同于滑动轴承,其相对速度不足以形成动压润滑而处于边界润滑状态下.为了分析此状态下的动态磨损问题,考虑到含间隙运动副边界润滑时轴套的切向弹性变形和切向阻尼,结合考虑间隙运动副碰撞接触的非线性弹簧阻尼模型,提出边界润滑条件下的间隙副接触力模型,进而在此基础上推导出间隙副的动态磨损模型,并对含间隙曲柄滑块机构的运动副动态磨损进行数值分析.计算结果表明副元素间呈现出连续弹性变形现象,在连续变形接触处,动态磨损量较大使磨损加剧,并出现非均匀磨损.     

The Critical Condition for the Transition from HL to ML in Water-Lubricated SiC

Silicon carbide (SiC) with water lubrication is being considered as the most promising combination to replace metals and oil for sliding bearings and mechanical seals of machines working in water. The basic properties of the Stribeck curves of water lubricated SiC in parallel contact, especially, the critical conditions for the transition from HL to ML were studied experimentally. The hydrodynamic lubrication regions and minimum friction coefficients of metal pair in oil and SiC pair in water are compared to give a quantitative value of the oil viscosity range, in which metal/oil can be directly replaced by SiC/water for triboelements.In order to improve the load-carrying capacity of SiC sliding bearings for the increasing strict demands from industry, a surface texture was introduced to one of the contact surfaces by means of reactive-ion etching. The effect of surface texture on the lubrication regimes and the minimum friction coefficient were evaluated experimentally.     

Effect of Normal Force on the Triboplasma Generation Under Oil Lubrication

Previously, triboplasma image was successfully taken under oil lubrication, and the mechanism of triboplasma generation was clarified to be caused by air discharge in the cavity or bubble produced in the divergent rear clearance gap of sliding contact. In the present study, the effect of normal force on the triboplasma generation was investigated in the tribosystem of a diamond pin sliding on a sapphire disk under perfluoropolyether oil lubrication. Two-dimensional images and the intensities of the photons emitted from the sliding contact and the rear divergent clearance of the plasma region were measured. UV photons are emitted mostly from the plasma region, whereas visible and IR photons are emitted from both the sliding contact and the plasma regions. The visible and IR photon emission intensity from the sliding contact increases linearly with the normal force, i.e., proportional to the real area of contact. On the other hand, photon emission intensity of the UV, visible and IR photons from the plasma region is nothing or negligibly small below a critical normal force, above which it increases greatly with the normal force. This means that there exists critical tribocharge-induced electric field determined by real area contact to accelerate the electrons passing through the thin oil film into the bubbles. Then, to avoid or utilize the plasma, the normal force should be applied below or above the critical normal force, which depend on the type of tribosystem, respectively.     

Numerical analysis of viscous elastohydrodynamic point contact under stationary conditions

A model is proposed for study of the effect of nonelastic qualities of contacting bodies separated by a fine lubricant film on the contact characteristics. The problem is studied of the movement of the fine lubricant film between the rigid spherical surface and the mobile viscoelastic-layer surface rigidly adhering to the base. A unidimensional Kelvin model of the viscoelastic medium serves as the rheological model of the viscoelastic layer. The calculation results show that the pressure is distributed in the viscous elastohydrodynamic point contact film very much differently from the pressure distribution in the UHD contact, particularly at slow sliding velocities. The friction coefficient is a nonmonotonous function of the sliding velocity. The friction coefficient drops at slow velocities to a minimum as the velocity accelerates, and then it grows.     

MoS2/Ti复合固体润滑膜的力学性能与摩擦特性

采用非平衡磁控溅射法在9Cr18轴承钢基底上制备了厚度约3μm的MoS2/Ti复合固体润滑膜,基于球形压头纳米压痕试验,采用连续刚度法对MoS2/Ti复合固体润滑膜的力学性能进行研究,探究MoS2/Ti复合固体润滑膜力学性能随压痕深度的变化规律;根据压痕试验载荷-位移曲线,采用Hertz接触理论计算MoS2/Ti复合固体润滑膜的弹性模量并与试验结果进行对比;利用CSM摩擦试验机对低速、低载下MoS2/Ti复合固体润滑膜的摩擦特性进行研究;基于压痕试验提出了一种能够更准确计算钢球加载时MoS2/Ti复合固体润滑膜接触应力的方法,并计算了摩擦试验不同载荷下的接触应力。结果表明:MoS2/Ti复合固体润滑膜的力学性能和摩擦特性都会受到表面形貌的影响;除表面初始压入阶段外,MoS2/Ti复合固体润滑膜的弹性模量和接触刚度都随着压痕深度的增大而增大;滑动速度和载荷共同影响MoS2/Ti复合固体润滑膜的摩擦特性。     

Effect of contact stress on friction and wear of ultra-high molecular weight polyethylene in total hip replacement

This paper studies the effect of contact stress on friction and wear of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups by means of friction and wear joint simulator testing under serum lubrication. For a given applied load, increasing the contact stress by increasing the ball/socket radial clearance decreased both the coefficient of friction and the wear rate. Friction and wear were highly correlated. The dependence of friction on contact stress for the UHMWPE socket under serum lubrication was similar to that of semi-crystalline polymers under dry sliding. This finding indicates the occurrence of partial dry contact at asperity levels for the metal-polyethylene ball-in-socket joint under serum lubrication.     

Effect of operational conditions and environment on lubricity of hydrophobins in water based lubrication systems

Abstract

In this study, the effect of operational conditions (normal load, sliding velocity) and environment (pH and ionic strength) on the lubrication properties of two different hydrophobin proteins were investigated using pin on disc tribometry and ellipsometry. The studied proteins were wild type HFBI and the glycosylated hydrophobin FpHYD5. It was observed that the friction of a stainless steel versus stainless steel contact lubricated with either of the hydrophobins did not depend on the normal load. However, increased sliding velocity occasionally led to a decrease in friction when the surfaces were lubricated with the glycosylated FpHYD5. The tribological behaviour of FpHYD5 was studied at pH values ranging from 3 to 9 and generally lowered friction by 31–38% and wear by 40–65% compared to the corresponding buffer solutions. An exception was pH 9, where FpHYD5 increased friction and wear compared to the buffer solution. Ionic strength affected both the amount of protein that was adsorbed and the lubrication properties of glycosylated hydrophobins.