Slider Wear on Disks Lubricated by Ultra-Thin Perfluoropolyether Lubricants with Different Molecular Weights

In this study, the wear properties of a magnetic head slider on disks lubricated by ultra-thin perfluoropolyether (PFPE) lubricants with different molecular weights were evaluated by the continuous sliding of magnetic head sliders using the slider contact by the dynamic flying height control. Two types of PFPE lubricants (Z-tetraol and D-4OH) with different molecular weights were evaluated. Results show that the slider wear depended on the coverage of the lubricant film; i.e., the lubricant film with sufficient coverage reduced slider wear. The lubricant film with a low molecular weight (low-Mw), including a lubricant material with a Fomblin and Demnum main chain, exhibited better coverage on a diamond-like carbon surface. Sliders with a low-Mw lubricant film showed less wear than those of a high molecular weight (high-Mw), and the depletion of the low-Mw lubricant film was less than that of the high-Mw lubricant film.     

环境湿度对浸金属碳滑板磨损及温升的影响

为探究环境湿度对弓网摩擦副载流滑动过程中电弧放电能量、浸金属碳滑板温升及滑板磨损量的影响,采用环-块式高速载流摩擦磨损试验台,对比不同湿度条件下,电弧能量、滑板温升及滑板磨损量随滑动速度、电流强度、法向力的变化情况。试验结果表明：不同环境湿度下,滑动速度和电流强度的增大均会导致电弧能量及滑板温升急剧增大;电弧热是导致温升的主要热源;增大法向力对于抑制电弧放电、降低滑板温升均有显著效果,而对于滑板磨损量变化的影响,不同湿度情况则截然相反;高湿度环境下接触副附着的水膜改善了接触状况和散热情况,电弧能量及滑板温升都小于低湿度环境;低湿度环境下滑板表面受到更严重的机械摩擦,其表面状态相比高湿度更差;在平均湿度较高的夏季适当增加升弓压力,在平均湿度较低的冬季适当降低列车行驶速度可以减少浸金属碳滑板磨损。     

Evaluation of head/disk pseudo-impact contact byacoustic emission technique

Pseudo-impact contact at the interface between proximity slidersand magnetic rigid disks were investigated by acoustic emission(AE) technique. Under a hot/wet condition of 60°C/80% RH andan operation speed of 19.15 m/s, AE responses were collected inreal time to monitor interaction events at the head/diskinterface (HDI). The profile of AE responses shows various eventsat the HDI which could be correlated to the micro-wear of thecarbon overcoat andlubricant degradation. These events were also detected by opticalsurface analysis (OSA). These wear and degradation eventsindicate the need for choosing the proper lubricant anddemonstrate that AE is a suitable tool to detect these events atthe head/disk interface.     

激光织构和碳基薄膜复合处理提高钛合金摩擦学性能研究

在重载滑动干摩擦条件下，对比不同织构密度的钛合金表面的摩擦学性能；在耐磨性最好的织构密度钛合金表面再制备碳基薄膜，并与直接在钛合金表面制备的碳基薄膜的摩擦学性能进行对比。结果表明：3种低织构密度条件下，TC4钛合金的摩擦因数减小、磨损率降低；随着织构密度的增大，钛合金材料的摩擦因数变化极小，磨损率有所增加；在织构密度5.95%的钛合金表面制备的碳基薄膜，因织构微凹处产生的小微湍流，减少了摩擦阻力，使得其摩擦因数相比直接在钛合金表面制备的碳基薄膜的摩擦因数有所减小。织构化碳基薄膜的磨损率比钛合金的磨损率降低了99.31%，比直接在钛合金表面制备碳基薄膜的磨损率也降低了约60%，这是因为高接触应力摩擦过程中触发石墨化转变，被磨损的石墨化颗粒碎片嵌入织构微凹中，抑制了摩擦接触界面的磨损行为。     

Role of contact frequency on the wear rate of steel in discontinuous sliding contact conditions

This work presents a novel approach of sliding ball-on-disk wear tests where the disc material is investigated. Each part of the wear track on the disc is in discontinuous contact with the counterbody. The contact frequency at each part of the wear track on the disc with the counterbody is defined by the rotation frequency of the disc. The sliding speed is however a function of both the rotation frequency and wear track diameter. In this work, the effect of the contact frequency on friction and wear was investigated on carbon steel in discontinuous sliding contact with corundum balls. Various sliding speeds were used while maintaining the contact frequency at a fixed value, and various contact frequencies were applied at constant sliding speeds.The wear rate of the disk material is shown to depend not only on the usual wear test parameters, namely sliding speed and contact load, but also on contact frequency. Moreover, contact frequency is shown to be a key factor determining the wear mode even at constant sliding speed and load. At contact frequencies above 9 Hz, the dominant wear mechanism is oxidational wear, while at frequencies below 4 Hz the dominant wear mechanism is adhesive wear. This transition from adhesive to oxidational wear takes place together with a change in the type of debris generated and in the value of the coefficient of friction.The validity of the Garcia-Ramil-Celis model proposed earlier for discontinous sliding contact conditions, is demonstrated for the case of carbon steel disks sliding against a chemically inert counterbody.     

Carbon Nanotube Reinforced Polyimide Thin-film for High Wear Durability

In this paper, the influence of single walled carbon nano tubes (SWCNTs) addition on the tribological properties of the polyimide (PI) films on silicon substrate was studied. PI films, with and without SWCNTs, were spin coated onto the Si surface. Coefficient of friction and wear durability were characterized using a ball-on-disk tribometer by employing a 4 mm diameter Si₃N₄ ball sliding against the film, at a contact pressure of ∼370 MPa, and a sliding velocity of 0.042 ms⁻¹. Water contact angle, AFM topography, and nano-indentation tests were conducted to study the physical and mechanical properties of the films. SWCNTs marginally increased the water contact angle of PI film. The addition of SWCNTs to PI has increased the hardness and elastic modulus of pristine PI films by 60–70%. The coefficient of friction of PI films increased slightly (∼20%) after the addition of SWCNTs, whereas, there was at least two-fold increase in the wear life of the film based on the film failure condition of coefficient of friction higher than 0.3. However, the film did not show any sign of wear even after 100,000 cycles of rotation indicating its robustness. This increase in the wear durability due to the addition of the SWCNTs is believed to be because of the improvement in the load-bearing capacity of the composite film and sliding induced microstructural changes of the composite film.     

Role of the Transfer Film on the Friction and Wear of Metal Carbide Reinforced Amorphous Carbon Coatings During Run-in

The relationship between friction, wear, and transfer films of three metal carbide-reinforced amorphous carbon coatings (TiC/a:C, TiC/a:C–H, and WC/a:C–H), sometimes referred to as metal-doped diamond-like carbon coatings, has been investigated. Tribological tests were performed in an in situ tribometer with sapphire or steel hemispheres run against coated flats in dry or ambient air. The sliding contact interface was observed and recorded by optical microscopy during reciprocating sliding tests. The friction and wear behavior during run-in depended on the number of sliding cycles to form a stationary transfer film on the hemisphere. Stationary transfer films formed rapidly (within ten cycles) and the friction coefficient fell to 0.2 (ambient air) or 0.1 (dry air), except with sapphire against WC/a:C–H in dry air; with the latter, a stationary transfer film required nearly 100 cycles to form, during which the friction remained high and the wear rate was from 10 to 100 times higher than the other two coatings. For all coatings, three velocity accommodation modes (VAM) were observed from run-in to steady-state sliding and were correlated with the friction and wear behavior. The delayed adherence of the transfer film to sapphire from WC/a:C–H coatings in dry air is discussed in terms of equilibrium thermochemistry. Friction and wear behavior during run-in, therefore, depended on transfer film adherence to the hemisphere and the VAM between transfer films and the coating.     

Synthesis and Tribology of Micro-Carbon Sphere Additives for Enhanced Lubrication

Poor or inefficient lubrication often gives rise to high friction and wear losses in machine components, which adversely affect their performance, efficiency, and durability. Many approaches are being explored to enhance the antifriction and antiwear properties of sliding machine components. In this study, the antifriction and antiwear properties of carbon spheres, synthesized from plastic waste by an autogenic process, were investigated as an additive to a poly-alpha-olefin (PAO-4 grade) oil. When dispersed at 1 wt% concentration, the carbon spheres reduced both friction and wear under boundary-lubricated sliding conditions. In particular, the reduction in wear was quite dramatic and appeared to be enabled by the formation of a fairly thick (≈200 nm) carbon-rich boundary film, the formation of which is attributed to tribochemical interactions between the carbon particles and sliding contact surfaces.     

Effects of carbon content and sliding ratio on wear behavior of high-vanadium high-speed steel (HVHSS) under high-stress rolling–sliding contact

This study developed a wear tester to investigate the wear properties of high-vanadium high-speed steel (HVHSS) with approximately 9% vanadium and different carbon contents under rolling–sliding condition. The carbon content significantly affected microstructure of matrix and mechanical properties of HVHSS, and therefore played an important role in wear resistance. Nevertheless, the wear failure mode was mainly related to sliding ratio, which varied from fatigue wear to sliding wear with increasing sliding ratio. The wear behavior was affected by the interaction of carbon content and sliding ratio. The high-stress rolling–sliding contact not only caused severe wear but transformed austenite to martensite.     

Amorphous hydrogenated carbon films for tribological applications I. Development of moisture insensitive films having reduced compressive stress

Although earlier investigations on the tribological behaviour of amcrphous hydrogenated carbon (AHC) films in sliding contact with steel showed encouraging results, four open issues were identified. They were: (a) dependence of friction and wear on humidity (i.e., the friction coefficient and the wear increased with humidity), (b) limitations on film thickness (i.e., films greater than 2 μm thick delaminated due to large compressive stress), (c) deposition of films on substrates other than silicon and (d) lubricant compatibility (i.e., formation of lubricant-derived antiwear films on AHC film surfaces). Steps were taken to address some of these open issues by incorporating silicon in AHC films. Friction and wear tests were conducted on AHC films containing various amounts of silicon. Incorporation of silicon in AHC films rendered the friction coefficients and the wear of a steel counterface insensitive to moisture. Silicon incorporation in AHC films also significantly reduced compressive stress. This allowed deposition of 10 μm thick films. These effects were achieved without any compromise with the friction coefficient and the film wear if the amount of silicon in the film was kept within a certain concentration range. In addition, silicon-containing AHC films were thermally more stable than silicon-free films. Experiments conducted with two lubricants resulted in significantly lower wear of the silicon-free AHC films than that obtained for unlubricated sliding. Similar friction coefficients were obtained for AHC film/steel and steel/steel combinations in lubricated sliding.     

Transmission Electron Microscopy Analysis of Mo–W–S–Se Film Sliding Contact Obtained by Using Focused Ion Beam Microscope and In Situ Microtribometer

To better understand the fundamentals of solid lubrication, microstructural analyses on the wear scar surface and contact interface of Mo–W–S–Se composite films produced by pulsed laser deposition were completed. Focused ion beam (FIB), transmission electron microscopy (TEM), and X-ray energy dispersive spectroscopy were employed to study the cross-sectional microstructure and chemistry of wear scars. In particular, a novel microtribometer was built for in situ tribological measurements within a FIB microscope. The sliding tip was welded in contact to the wear scar surface on the film under load by re-deposition of sputtering materials from the FIB cut of the tip. Using this technique, cross-sectional TEM specimens were prepared precisely at the contact point without tip/film separation. Here, the in situ FIB microtribometer is critically important for retaining the microstructure of lubricant films as formed at the sliding contact interface between the tip and film without separation. It provides the unique ability to stop sliding, section the contact, and reveal microstructural changes to that contact without disrupting the sliding interface. The cross-sectional TEM measurements were performed on the sliding contact interface for both the regions in contact and just past contact, and both the reorientation and recrystallization of lubricant films were revealed.     

Effectiveness of bubble structure in contact damage reduction of Au film

Au films have been utilized widely for electrical connectors and devices. The film may be damaged due to repeated contact sliding which may ultimately lead to failure of the electrical system. In this work, the effect of bubble structure on the contact damage characteristics of Au film was compared to that of a smooth film to understand the wear mechanism of thin Au films with the aim to improve its durability. Nano-scratch tests as well as sliding wear experiments were performed using smooth and bubble-structured Au film specimens. It was found that the bubble-structured film was more durable with respect to contact damage. The reason for the increased durability was attributed to the frictional energy dissipation function of the bubble structure.     

A study of contact-start-stop wear tracks by TOF SIMS

In magnetic hard disk drive system, an ultra thin layer of lubricant is coated to the thin film media surface to prevent wear. Under the condition of relative motion, the displacement and replenishment of the lubricant at the head and media contact area are the factors that control the friction and wear behavior of the system. In this study, we investigate the sliding wear disk surface prepared by contact-start-stop (CSS) test using TOF SIMS (Time of Flight Secondary Ion Mass Spectrometry). TOF SIMS is a power tool for surface analysis with both high spatial and high mass resolution. Our investigations show that the lubricant thickness variation of the disk media at the contact area can be captured by sharp ion map images of TOF SIMS, and the thickness can be inferred based on the relative ion fragment intensity. In addition, the composition variation of the slider material and the magnetic layer materials can also be monitored. Finally the sliding effect is analyzed.     

地铁刚性弓网系统接触线磨损特性试验研究

为研究地铁刚性接触网系统接触线磨损的规律,以地铁刚性接触网系统常用的浸金属碳滑板/铜银合金接触线作为摩擦副,通过模拟地铁弓网系统运行参数,使用载流摩擦磨损试验机研究有、无电火花放电情况下,浸金属碳滑板与铜银合金接触线直流电滑动过程中磨损量、摩擦因数、载流效率随滑动距离的变化。试验结果表明：电火花放电会使得接触线与浸金属碳滑板磨损量显著上升,出现电火花放电时摩擦因数较小,弓网系统载流效率会明显降低同时出现大幅波动。试验后对碳滑板和接触线表面形貌的观察可知：电火花放电会使得浸金属碳滑板表面烧蚀坑数量和尺寸大小增加,同时会出现滑板材料大面积剥落和表面裂纹增多的问题,接触线表面形貌变得更加粗糙。     

钢/玻璃的摩擦磨损性能动态观测研究

本实验在自行设计的摩擦磨损动态观测实验机进行,摩擦副之间的接触采用球一盘式接触。研究表明：当以较低速度滑动时,钢球表面的氧化物起到抗磨作用;滑动速度达到一定值时,氧化膜的生成速度小于氧化膜的磨损速度,摩擦表面为粘着磨损;当滑动速度继续升高时,摩擦表面的活化能增加,氧化加速．又出现氧化磨损;而滑动速度过高时,粘着磨损成为主要磨损形式,同时由于磨粒的作用,表面也发生疲劳磨损和磨料磨损．致使磨损急剧增加;表面层在摩擦热导致的高温条件下,氧化膜的生成速度又有所增加,氧化磨损为主要形式。     

Ultralow Friction Behaviors of Hydrogenated Fullerene-Like Carbon Films: Effect of Normal Load and Surface Tribochemistry

Friction and wear behaviors of hydrogenated fullerene-like (H-FLC) carbon films sliding against Si₃N₄ ceramic balls were performed at different contact loads from 1 to 20 N on a reciprocating sliding tribometer in air. It was found that the films exhibited non-Amontonian friction behaviors, the coefficient of friction (COF) decreased with normal contact load increasing: the COF was ~0.112 at 1 N contact load, and deceased to ultralow value (~0.009) at 20 N load. The main mechanism responsible for low friction and wear under varying contact pressure is governed by hydrogenated carbon transfer film that formed and resided at the sliding interfaces. In addition, the unique fullerene-like structures induce well elastic property of the H-FLC films (elastic recovery 78%), which benefits the high load tolerance and induces the low wear rate in air condition. For the film with an ultralow COF of 0.009 tested under 20 N load in air, time of flight secondary ion mass spectrometry (ToF-SIMS) signals collected inside and outside the wear tracks indicated the presence of C₂H₃ ⁻ and C₂H₅ ⁻ fragments after tribological tests on the H-FLC films surface. We think that the tribochemistry and elastic property of the H-FLC films is responsible for the observed friction behaviors, the high load tolerance, and chemical inertness of hydrogenated carbon-containing transfer films instead of the graphitization of transfer films is responsible for the steady-state low coefficients of friction, wear, and interfacial shear stress.     

Contact analysis of impact in magnetic head disk interfaces

Three-dimensional finite element analysis of head and disk contact effects induced by impact in magnetic head disk interface (HDI) are presented. Elastic–plastic contact simulations are performed using . The entire contact–impact procedures during head disk collision under the dynamic loading of half-sine pulse acceleration with profiles of 300 and 500 g amplitude and 1.0 ms in duration are described in detail. Simulation results for the contact pressure distribution at HDI, von Mises equivalent stress, and equivalent plastic strain fields are examined and interpreted in terms of impact history. A comprehensive history of head disk relative displacement and von Mises equivalent stresses within contact region are provided and the evolution of plasticity are discussed. It is shown that finite element method can provide the simulation of the contact behavior resulting from the dynamic loading.     

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.     

Tribochemical interactions of Si-doped DLC film against steel in sliding contact

This study concerns the effects of tribochemical interactions at the interface of Si-DLC (silicon-doped diamond-like carbon) film and steel ball in sliding contact on tribological properties of the film. The Si-DLC film was over-coated on pure DLC coating by radio frequency plasma-assisted chemical vapor deposition (r.f. PACVD) with different Si concentration. Friction tests against steel ball using a reciprocating type tribotester were performed in ambient environment. X-Ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES) were used to study the chemical characteristics and elemental composition of the films and mating balls after tests. Results showed a darkgray film consisting of carbon, oxygen and silicon on the worn steel ball surface with different thickness. On the contrary, such film was not observed on the surface of the ball slid against pure DLC coating. The oxidation of Si-DLC surface and steel ball was also found at particular regions of contact area. This demonstrates that tribochemical interactions occurred at the contact area of Si-DLC and steel ball during sliding to form a tribofilm (so called transfer film) on the ball specimen. While the pure DLC coating exhibited high coefficient of friction (∼0.06), the Si-DLC film showed a significant lower coefficient of friction (∼0.022) with the presence of tribofilm on mating ball surface. However, the Si-DLC film possesses a very high wear rate in comparison with the pure DLC. It was found that the tribochemical interactions strongly affected tribological properties of the Si-DLC film in sliding against steel.     

A New Roughness Parameter Including Hardness and Contact Frequency Effects on Lubricated Rolling/Sliding Wear

Surface roughness, roughness arrangement, film thickness, material hardness, and run-in process have significant effects on the lubricated rolling/sliding wear of mechanical components such as gears and bearings. In conventional analysis, a film thickness parameter is calculated by a geometric approach to study the wear resistance of a contact system without considering the effects of material hardness and run-in process. Although the conventional parameter is simple, it does not correlate with some experimental observations. In this work, a new roughness parameter is developed for the prediction of lubricated rolling/sliding wear. Surface roughness will be adjusted by its hardness and contact frequency. The calculation results are consistent with four groups of experimental data. It is proved that the conventional models can be derived as a special case of the new model when two contact surfaces have the same properties. The new model can be used in the optimal design and manufacturing of mechanical interfaces to reduce lubricated rolling/sliding wear.