Wear durability studies of ultra-thin perfluoropolyether lubricant on magnetic hard disks

This paper presents wear and friction studies on ultra-thin (~2 nm) film of perfluoropolyether (PFPE) coated on glass substrate magnetic hard disks. The lubricant was coated on the disk by the dip-coating method and the tribological tests were carried out by sliding a 3 mm diameter glass ball slider (normal load=20 mN) on the rotating disk surface. Lube thickness and lube wear profile were measured using an ellipsometer whereas the worn disk surface was studied using a surface reflectivity analyzer. The sliding speed and the lube bonding conditions were varied during the test. From the results, it is concluded that about 80% bonding of the lube to the disk surface leads to an increase in the wear durability of the lubricant by a factor of 2 when compared to the as-lubed condition. Lube bonding has an effect on increasing the coefficient of friction. Initially, increasing sliding speed increases both friction and wear but for very high sliding speed these values tend to decrease. The glass ball surface showed wear due to asperity interactions as well as lube transfer from the disk to the glass surface.     

Tribological Performance of UHMWPE and PFPE Coated Films on Aluminium Surface

A thin layer of Ultra High Molecular Weight Polyethylene (UHMWPE) or UHMWPE + PFPE is coated onto cylindrical aluminium (Al) pin (4.6 mm diametre) surface with the aim of providing wear resistant coating on this soft and tribologically poor metal. The coefficient of friction and wear life of the coated samples are investigated on a pin-on-disk tribometre under different normal loads (394–622 g) and two sliding speeds (0.1 and 0.31 m/s) against uncoated Al disk as the counterface. Both coatings provide coefficient of friction values in the range of 0.02–0.2 as compared to 0.4–1.0 for uncoated Al. There is tremendous improvement in the wear life of the pin, with UHMWPE + PFPE film giving wear life approximately twice to thrice higher than that with only UHMWPE film. A thin polymer film is transferred to the disk surface during sliding providing very long-term wear life (continuous low coefficient of friction) despite visual removal of the film from the pin surface. The present films will have applications in gears and bearings as solid or boundary lubricants for automotive and aerospace component.     

Contributions of Mobile and Bonded Molecules to Dynamic Friction of Nanometer-Thick Perfluoropolyether Films Coated on Magnetic Disk Surfaces

To protect the interface against intermittent head–disk contact in hard disk drives, nanometer-thick perfluoropolyether (PFPE) films consisting of both “bonded” and “mobile” molecules are applied on the disk surfaces. Because of their different adsorption states and mobility, the bonded and mobile molecules are supposed to contribute differently to friction properties, which directly impact the stability of ultra-low flying head–disk interfaces. By measuring the friction force at light loads and low to high speeds as a function of bonded and mobile film thicknesses, we studied the contributions of bonded and mobile molecules to the dynamic friction of nanometer-thick PFPE films. We found that the friction coefficient of lubricant films without or with less bonded molecules increased as a power function of sliding speed, whereas that of lubricant films with more bonded molecules increased logarithmically with sliding speed. We suggest that these results can be explained by the following mechanisms: the dynamic friction of lubricant films without and with less bonded molecules is dominated by shear thinning behavior of mobile molecules, while that of lubricant films with more bonded molecules is governed by bonded molecules which lead to boundary lubrication.     

Effect of Temperature on the Dry Sliding Friction and Wear of Rice Bran Ceramics against Different Counterpart Materials

In this study, we investigated the effect of temperature on the friction and wear of rice bran (RB) ceramics, a hard porous carbon material made from rice bran, sliding against alumina, stainless steel, and bearing steel balls under dry conditions. Friction tests were performed using a ball-on-disk-type friction tester wherein a ceramic heater was installed in the rotational stage. The surface temperature of the RB ceramic disk specimens was controlled at 20, 100, 150, or 200°C. The normal load was 1.96 N, sliding velocity was 0.1 m/s, and number of cycles was 20,000. The effect of surface temperature on the friction and wear of RB ceramics substantially differed among the ball material types. The friction coefficient for the RB ceramics sliding against an alumina ball decreased with increasing temperature and exhibited an extremely low value (0.045) at 200°C. The friction coefficient in the case of the RB ceramics sliding against a stainless steel ball exhibited a stable value as the temperature was increased to 150°C and slightly decreased as the temperature was increased further, reaching a low value of 0.122 at 200°C. The friction coefficient for the RB ceramics sliding against bearing steel ball drastically increased with increasing temperature, reaching 0.381 at 200°C. The specific wear rate of the RB ceramics increased with increasing temperature; it was lowest when sliding against alumina and highest when sliding against bearing steel. The wear of the alumina ball was the lowest and that of the bearing steel ball was the highest under all investigated temperature conditions. On the basis of these results, we concluded that alumina is a promising counterpart material for RB ceramics sliding at high temperatures (≤200°C).     

Tribological behaviors and mechanisms of Ti<Subscript>3</Subscript>AlC<Subscript>2</Subscript>

Tribological behaviors and the relevant mechanism of a highly pure polycrystalline bulk Ti₃AlC₂ sliding dryly against a low carbon steel disk were investigated. The tribological tests were carried out using a block-on-disk type high-speed friction tester, at the sliding speeds of 20–60 m/s under a normal pressure of 0.8 MPa. The results showed that the friction coefficient is as low as 0.1∼0.14 and the wear rate of Ti₃AlC₂ is only (2.3–2.5) × 10⁻⁶ mm³/Nm in the sliding speed range of 20–60 m/s. Such unusual friction and wear properties were confirmed to be dependant dominantly upon the presence of a frictional oxide film consisting of amorphous Ti, Al, and Fe oxides on the friction surfaces. The oxide film is in a fused state during the sliding friction at a fused temperature of 238–324 °C, so it takes a significant self-lubricating effect.     

Friction and wear durability studies on the 3D negative fingerprint and honeycomb textured SU-8 surfaces

The effects of two different textures (a 3D negative fingerprint texture and a honeycomb texture) on the tribological performance of SU-8 polymer surface have been investigated with a ball-on-disc tribometer. Friction and wear behaviors of the textured surfaces are conducted against a 4 mm diameter silicon nitride (Si₃N₄) ball counterface. The coefficient of friction for the negative fingerprint textured surface (μ=∼0.08) is much lower than that of the untextured surface (∼0.2) and the honeycomb textured surface (∼0.41) under a normal load of 100 mN and a rotational speed of 2 rpm. The coefficients of friction of the textured surfaces decrease with increasing normal loads between 100 mN and 300 mN. Above the normal load of 300 mN, the coefficient of friction of the negative fingerprint textured surface increases due to the occurrence of plastic deformation. The honeycomb textured surface has shown the highest coefficient of friction. The wear durability tests are also conducted at a normal load of 100 mN and a rotational speed of 500 rpm on the untextured/textured surfaces on SU-8 in the presence of an overcoat of a nano-lubricant, perfluoropolyether(PFPE). Six samples i.e. the untextured surface (Si/SU-8 and Si/SU-8/PFPE), the 3D negative fingerprint textured surface (Si/SU-8/FP and Si/SU-8/FP/PFPE) and the honeycomb textured surface (Si/SU-8/HC and Si/SU-8/HC/PFPE), each with and without PFPE nano-lubricant, have been investigated for their tribological behaviours. The negative fingerprint pattern on SU-8 with PFPE coating has shown the highest wear life of 60,000 cycles under a normal load of 100 mN. The reasons for excellent tribological performance of 3D fingerprinted SU-8 surface are analyzed using the Hertzian contact area calculation.     

Velocity-Dependent Adhesion with Lubricants on Thin-Film Disks

The well-known problem of stiction in a magnetic disk drive largely depends on the forces induced by the presence of a thin liquid film. It is commonly recognized that both adhesive and viscous effects contribute to the magnitude of the stiction force, but is is not known what relative roles the two effects have in a lubricated contact. In the present work, the nature of adhesive and viscous effects is investigated for the slider/disk interface under conditions of constant-speed sliding.

Friction measurements are conducted over a range of sliding speeds, 0.25-250 mm/s, with eight perfluoropolyether (PFPE) lubricants applied in various thicknesses, 0-6.6 nm, to carbon-coated magnetic thin-film disks. The lubricants were selected to cover a broad range of viscosities. For several sliding speeds and lubricant film thicknesses, the friction force is found to decrease significantly with increasing sliding speed for all lubricants. In several instances, large friction forces are observed at the lowest sliding speeds, indicating stiction-like behavior, whereas, at higher speeds, the friction is reduced to even below unlubricated friction levels. At the highest film thickness and sliding speed, the friction was found to increase with speed for some lubricants. The implications of these results on current models of lubricant-mediated adhesion are discussed.     

PCrNiMo材料摩擦磨损特性研究

采用自制的销盘式干滑动摩擦磨损试验机,研究了不同硬度炮钢材料PCrNiMo自配副时的摩擦磨损特性。研究结果表明：材料的磨损率随着速度、载荷的增加而增大,摩擦因数随着速度、载荷的增加而减小;低速低载条件下,接触表面以磨粒磨损为主,而在高速、高载条件下,接触表面以粘着磨损形式为主。     

Tribological Behavior of Ceramics at High Sliding Speeds in Steam

Three sets of tests were conducted using a pin-on-disk tribometer to determine the tribological behavior of ceramics at high sliding speeds in steam. In the first set, the speed was increased from 4000rpm to 10,000rpm in 1000 rpm increments. Constant rotational speeds of 4000rpm, 6000rpm, 8000 rpm and 10,000 rpm were used in the second test series. In the third series of tests, the rotational speed was slowly increased to 10,000rpm and allowed to coast down to zero. While the coefficient of friction for silicon nitride/YTZP pair varied between 0.2 and 0.4 without a clear pattern as the speed was increased in the first two test series, it decreased from about 0.6 to 0.2 when the speed was raised to 10,000 rpm in the third test series. This behavior is attributed to the general phenomena of powder lubrication as the wear debris provides an interfacial layer leading to reduced friction at high speeds. The coefficient of friction for silicon nitride/silicon carbide pair was substantially reduced to about 0.02 as the speed was raised. The low coefficient of friction, however, increased to a high level as the speed was further increased. The drop in friction is explained based on analysis of elasto-hydrodynamic lubrication assuming that a water film containing solid particles exists at the interface. Several possible mechanisms are suggested for the transition to a higher friction as the speed is raised: thermal effects at high flash temperatures, low residence times (for water adsorption on surface), collapse of the lubricant film and starvation effects.     

An Accelerated Wear Test Method to Evaluate Lubricant Thin Films on Magnetic Hard Disks

A high speed ball-on-inclined-plane test method has been developed to evaluate the lubrication effectiveness of Z-Dol on magnetic hard disks. The test evaluates the combined durability of the lubricant film and the carbon overcoat under sliding conditions. A polished ruby (Al₂O₃) ball without suspension is used to simulate the head material. The ball slides over an inclined (at an angle of 0.055°±0.005°) section of the disk surface at 2.0 m/s linear velocity. The load is controlled by the geometric interference of the preloaded ball and the inclined plane. The contact forces are sampled periodically at 2 rpm and the frictional coefficients calculated. Repeated sliding between the ball and the disk sample leads to an increase in friction approaching that of the unlubricated case. Post test analysis using atomic force microscopy (AFM) suggests that the increase in friction is due to the loss of lubricant effectiveness of the lubricant and the wear of the carbon overcoat. X-ray photoemission microscopy (PEEM) results suggest progressive oxidation of Z-Dol as one of the degradation mechanisms leading to wear. The durability of the lubricating thin films is defined by the number of cycles to failure. Test repeatability is about 10%, depending on lubricant, film thickness, and surface roughness. The test can be used to evaluate different lubricant chemistries as well as different carbon overcoats. Compared to other pin-on-disk tests and step loading ball-on-disk methods, this test introduces two additional factors: high speed impact and wear acceleration by the inclined angle. The high speed impact simulates potential thermal stresses associated with head–disk contact. With an inclined angle, the load increases evenly for each contact cycle, hence simulates the ability of the lubricant layer to react to dynamic loads. The test is intended as a basic research tool to measure the fundamental resistance of the lubricant layer to resist repeated high speed contacts.     

Measurement and Characterization of “Resonance Friction” at High Sliding Speeds in a Model Automotive Wet Clutch

The friction forces between various lubricated “friction materials” and sapphire disks were measured using a new “high-speed” rotating disk attachment to the surface forces apparatus (SFA). Two different clutch lubricants and two different friction materials were tested at sliding speeds and normal loads from 5 to 25 m/s, and 0.2 to 1 N (nominal pressures ~1 MPa), respectively. The results show that “resonance friction”—characterized by large amplitude oscillatory (i.e., sinusoidal) vibrations, also known as shudder or chatter—dominates dynamical considerations at high sliding speed, replacing the smooth sliding or low-amplitude stick–slip that is characteristic of low speed/low load sliding. The characteristic (rotational) speeds or frequencies at which resonance friction occurs depend only on the coupled/uncoupled mechanical resonance frequencies of the loading and friction-sensing mechanisms. In contrast, the intensity of and time to enter/exit shudder depends strongly on the lubricating oil and, to a lesser extent, on the friction material. Physical–chemical analyses of the friction materials before and after testing showed that the samples undergo primarily structural rather than chemical changes. Our results provide new fundamental insights into the resonance friction phenomenon and suggest means for its control.     

Evolution of Tribological Properties of Reactor-Grade NiCr-B Coating

The tribological properties of reactor-grade NiCr-B hardfaced coating were studied at a constant load and sliding velocity. The objective of the present article is to investigate the changes in coefficient of friction that accompanies morphological and phase changes occurring in wear scars. Such changes result from the sliding with a spherical steel ball. The transition from the low to high coefficient of friction at higher sliding distance is attributed to severe cracking as well as fretting wear–induced deformation of surface oxide scales. Increased value of coefficient of friction arises from protracted sliding over longer distances continued to deform and detachment of weakly adhered oxides like Fe₂O₃ and Cr₂O₃. Such scales are tribochemically formed on the wear scars and contribute to alteration in the coefficient of friction. The evolution of oxide phases in wear scars is found to be one of the main mechanisms for dissipation of frictional force.     

Study of tribological performance of ECR-CVD diamond-like carbon coatings on steel substrates: Part 1. The effect of processing parameters and operating conditions

Diamond-like carbon (DLC) coatings were prepared on AISI 440C steel substrates at room temperature by electron cyclotron resonance chemical vapor deposition (ECR-CVD) process in C₂H₂/Ar plasma. Using the designed Ti/TiN/TiCN/TiC interfacial transition layers, relatively thick DLC coatings (1-2 μm) were successfully prepared on the steel substrates. The friction and wear performance of the DLC coatings was evaluated by ball-on-disk tribometry using a steel counterbody at various normal loads (1-10 N) and sliding speeds (2-15 cm/s). By optimizing the deposition parameters such as negative bias voltage, DLC coatings with hardness up to 30 GPa and friction coefficients lower than 0.15 against the 100Cr6 steel ball could be obtained. The friction coefficient was maintained for 100,000 cycles (∼2.2 km) of dry sliding in ambient environments. In addition, the specific wear rates of the coatings were found to be extremely low (∼10⁻⁸ mm³/Nm); at the same time, the ball wear rates were one order of magnitude lower. The influences of the processing parameters and the sliding conditions were determined, and the frictional behavior of the coatings was discussed. It has been found that higher normal loads or sliding speeds reduced the wear rates of the coatings. Therefore, it is feasible to prepare hard and highly adherent DLC coatings with low friction coefficient and low wear rate on engineering steel substrates by the ECR-CVD process. The excellent tribological performance of DLC coatings enables their industrial applications as wear-resistant solid lubricants on sliding parts.     

Friction and Wear at High Sliding Speeds

Sliding tests have been carried out using a variety of soft metal and nonmetal pins on a rotating steel disk at speeds up to 150 m/s. A new high-speed friction apparatus in which the normal force, the friction force and the friction coefficient are recorded, was used. In general, the wear rate increased drastically, and the friction coefficient decreased moderately as the sliding speed was raised, these changes being especially pronounced when pin materials of low melting temperature were used. The friction data are in good agreement with those obtained by others using the pin-on-disk geometry. However, although in many cases the interface reached the melting temperature of the lower melting sliding material, the very low friction coefficient values of under 0.05 reported by some investigators were not reached.     

Tribological Properties of Ultra-Thin Functionalized Polyethylene Film Chemisorbed on Si with an Intermediate Benzophenone Layer

In this study, an ultra-thin (~20 nm) functionalized polyethylene (fPE) film is successfully attached to Si substrate via a reactive benzophenone (Ph₂CO) layer. The presence of fPE promotes wear durability of Si/Ph₂CO/fPE to 1,000 cycles compared with 100 cycles for Si/Ph₂CO and nearly zero wear life for bare Si in a ball-on-disk (4-mm-diameter Si₃N₄ ball) wear test under 40 mN applied normal load and 500 rpm sliding speed. As an enhancement to the wear life, perfluoropolyether (PFPE) is applied as a top mobile lubricant layer coated onto Si/Ph₂CO and Si/Ph₂CO/fPE. A significant improvement in the wear durability is observed as Si/Ph₂CO/PFPE fails at 250,000 cycles and Si/Ph₂CO/fPE/PFPE does not fail until one million cycles. Si/Ph₂CO/fPE/PFPE can withstand a minimum applied load of 150 mN at a sliding speed of 0.052 ms⁻¹ without failure, providing a PV (pressure x velocity) limit of greater than 106.6 MPa ms⁻¹.     

水润滑条件下转速对车轮钢滚动接触疲劳和磨损性能的影响

为探究水润滑条件下转速对车轮钢滚动接触疲劳和磨损性能的影响,利用滚动接触摩擦磨损试验测试不同转速下车轮试样的剥离寿命、摩擦因数和磨损率,并结合磨损形貌和裂纹扩展形貌观察,对比分析不同转速下摩擦磨损和剥离寿命的影响因素。结果表明:随转速提高,车轮材料氧化程度加剧,导致摩擦因数逐渐增加;当转速由250 r/min增至500 r/min时,摩擦因数增幅较小,应变速率增加导致磨损率下降,当转速由500 r/min增至1000 r/min时,摩擦因数急剧增加,导致材料磨损率增加;随转速提高,剖面塑性流动层厚度、裂纹扩展角度、裂纹分叉深度和最大扩展深度均呈现减小趋势。转速增加带来的摩擦因数的增加,一方面缩短裂纹萌生寿命,另一方面减小了裂纹发生向上转折的深度,最终导致滚动接触疲劳寿命随转速的增加而减小。     

On the role of sliding load and heat input conditions in friction stir processing on tribology of aluminium alloy–alumina surface composites

ABSTRACT

Aluminium (AA5083)-alumina surface composites are prepared by friction stir processing in two conditions of heat input. The low heat (LH) input conditions is achieved at a rotational speed of 710?rpm and a traverse speed of 100?mm/min, and high heat (HH) input conditions are achieved at a rotational speed of 1400?rpm and a traverse speed of 40?mm/min. The tribological characteristics of aluminium alloy, friction stir processed (FSPed) alloy and FSPed surface composites against steel ball are studied at 5, 10 and 20?N load. While no significant influence is found on frictional behaviour, wear resistance of FSPed composites is superior to FSPed alloys. FSPed composites fabricated at HH input conditions exhibited improved wear resistance as compared to LH input condition. Adhesion and delamination are dominant wear mechanisms at 20?N. Debris particles are reduced in size and hydroxidated in sliding of surface composites.     

The Tribological Behavior of a Ti-46Al-2Cr-2Nb Alloy Under Liquid Paraffine Lubrication

The tribological behavior of a Ti-46Al-2Cr-2Nb alloy prepared by hot-pressed sintering was investigated under liquid paraffine lubrication against AISI 52100 steel ball in ambient environment and at varying loads and sliding speeds. For comparison, the tribological behavior of a common Ti-6Al-4V alloy was also examined under the same testing conditions. The worn surfaces of the two alloys were analyzed using a scanning electron microscope. The friction coefficient of the Ti-46Al-2Cr-2Nb alloy in the range of 0.13–0.18 was significantly lower than that of the Ti-6Al-4V alloy (0.4–0.5), but comparable to that under dry sliding, which indicated that TiAl intermetallics could be more effectively lubricated by liquid paraffine than titanium alloys. Applied load and sliding speed have little effect on the friction coefficient of the Ti-46Al-2Cr-2Nb alloy. The wear rate of the Ti-46Al-2Cr-2Nb alloy was about 45–120 times lower than that of Ti-6Al-4V alloy owing to Ti-6Al-4V alloy could not be lubricated effectively. The wear rate of the Ti-46Al-2Cr-2Nb alloy increased with increasing applied load, but decreased slightly at first and then increased with increasing sliding speed. The wear mechanism of the Ti-46Al-2Cr-2Nb intermetallics under liquid paraffine lubrication was dominated by main plowing and slight flaking-off, but that of the Ti-6Al-4V alloy was plastic deformation and severe delamination.     

Tribological behavior of sialon ceramics sliding against steel lubricated by fluorine-containing oils

The friction and wear behavior of sialon ceramics sliding against steel and lubricated by perfluoropolyethers (PFPE), tetrakis (3-trifluoromethylphenoxy)-bis(4-fluoro-phenoxy)-cyclotriphosphazene (X-1P) and ionic liquid (1-methyl-3-octylimidazolium tetrafluoroborate, coded as L108) were investigated. It was found that the three fluorine-containing lubricants reduced friction coefficient and wear volume effectively. The effectiveness of the three lubricants in reducing wear volume could be ranked as L108>X-1P>PFPE. The antiwear films mainly consisting of organic oxyfluoride or carbonfluoride species and silicon fluoride are all observed for the three lubricants, while the degradation of PFPE during friction might account for the higher wear volume therewith. The lowest friction coefficient 0.065 was recorded for L108 under load of 0.5–400 N. This is dependent on the physically adsorbed ionic liquid on the rubbing surface and the formation of BN under the harsh conditions.