Effect of volatile organic contamination on head-disk interface tribology and a method for its reduction

Volatile organic contamination is known to be one of the factors to cause the failure of head-disk interface (HDI). Therefore, reduction of its harmful effects and improvement of the stability and reliability of HDI is becoming an important issue. In this study, the effects of some model compounds of volatile organic contamination on the tribological characteristics of HDI were systematically investigated using a contact start/stop (CSS) tester. The slider surface after the CSS tests was analyzed using Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS). Transfer of lubricating oil onto the slider surface was detected after the CSS tests. The organic contaminants promoted the transfer and resulted in high and unstable friction force. Fluorinated self-assembled monolayers (SAMs) were applied on the slider surface for reducing the transferred amount of the lubricating oil. Tribological performance of the slider coated with the SAMs and the transfer amount of lubricating oil onto the slider surface in the presence of contaminant was investigated. The friction force was low and stable in the case of the SAMs coated slider even under environmental contaminant. This result could be explained by the reduction of the transferred lubricating oil because the SAMs that coated on the slider surface were low surface energy.     

Tribological Performance and Transfer Behavior of Lubricating Oils at Head-Disk Interface under Volatile Organic Contamination

Tribological performance of head-disk interface (HDI) under volatile organic contamination was investigated using a contact start/stop (CSS) tester. Slider and disk surfaces were analyzed using Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) after CSS tests. The CSS test results indicated that the friction forces were high and unstable under contamination. Transfer of lubricating oil onto the slider surface was detected after the CSS tests. The transfer amount of lubricating oil was revealed to be dependent on the chemical structure of the terminal group in the lubricating oil. Piperonyl (–CH₂−phe=(O)₂=CH₂) terminated AM3001 lubricating oil was lost more easily than two hydroxyl (–OH) terminated Tetraol lubricating oil, probably because of the weak attractive force of the piperonyl groups with carbon overcoat. TOF-SIMS chemical images indicated that the transferring behavior of the lubricating oil onto the slider surface during CSS tests was dependent on the chemical structure of volatile organic contaminants. The lubricating oil became built up on the slider surface when the dioctyl sebacate (DOS) pollutant used. In contrast, the lubricating oil distribution on the slider surface was uniform under a polydimethylsiloxane (PDMS) vapor. The different transfer behavior of lubricating oil onto the slider surface may be resulted from the changeable surface properties of slider and disk because of the coexistence with gaseous contaminants.     

Chemical analysis of wear tracks on magnetic disks by TOF-SIMS

Surface reactions on magnetic recording disks have been studied during sliding with ceramic sliders in the main chamber of TOF-SIMS. Chemical change of lubricant oil in the wear track was observed by the chemical image of TOF-SIMS. The magnetic disk surface was covered with perfluoroalkyl polyether lubricant (Fomblin Zdol). The Si tip slider surface was covered with Al₂O₃, DLC, TiN or c-BN coating. Experimental conditions were as follows: 0.8 mN of load and a sliding speed of 0.01 m/s. Lubricant oils were decomposed with Al₂O₃ and TiN slider surfaces. Metal (Al, Ti) fluorides were detected by TOF-SIMS in the sliding track. Material transfer occurred by chemical wear of slider material. From TOF-SIMS observation, the decomposition of lubricant molecules was initiated at the end group of molecules (-CF₂CH₂OH). On the other hand, DLC and c-BN sliders suppressed the decomposition reaction of PFPE oils. In conclusion, hard and chemical inert materials such as DLC and c-BN are suitable for a long-life HDI.     

微量供油条件下润滑油液滴的生长与脱附

润滑油液滴的生长与脱附性能对于微量供油过程和微量润滑效果有重要影响。采用试验和数值仿真相结合的方法,研究微量供油条件下、在重力环境中的润滑油液滴在毛细管出口端的生长与脱附行为,考察毛细管管径和表面润湿特性变化对润滑油液滴脱附性能的影响。结果表明,润滑油液滴的生长与脱附是毛细力、黏性力、表面张力和重力等共同作用的结果;减小毛细管管径或增大润滑油液滴在毛细管表面的接触角,均可有效减弱毛细效应,降低润滑油液的爬移高度和脱附粒径,改善液滴脱附性能;毛细管管径由1.2 mm减小至0.7 mm过程中,液滴脱附粒径减小了4.5%;接触角由5°逐渐增加至90°的过程中,液滴脱附粒径减小了9.3%;通过选用低表面能材料制作微量供油的毛细管可以显著增大接触角。     

Effect of Interfacial Properties on EHL Under Pure Sliding Conditions

This paper presents an experimental study of the effect of boundary slip on the lubricating film shape and friction of an elastohydrodynamic lubrication (EHL) contact under isothermal conditions. Ball and disc pure sliding experiments were carried out with a high viscosity polybutene oil using a conventional optical EHL test rig. The film shape and friction were measured simultaneously. The results obtained from two discs with different coatings were compared. One disc was coated only with Cr, the partially reflective layer, and the other had an extra layer of SiO₂ coating on top. When running under mild conditions of low load and speed, there was no evidence of any boundary slip effect. However, when the load increased, the Cr-coated disc produced lower film thickness and friction than the SiO₂-coated disc. The Cr-coated surface had a larger contact angle, i.e., smaller surface energy, than the SiO₂ surface, which reflects the weak bonding between the molecules of the surface and the lubricant. The study concludes that surfaces with low surface energy promote boundary slip at the EHL contact, leading to a reduction in friction and film thickness.     

Experimental investigation of AE and friction signals related to the durability of head/disk interface

The durability of a hard disk drive is one of the most critical issues that must be optimized for best performance. Especially as the flying height of the head slider of a hard disk drive decreases over the years, the concern for surface damage and head contamination continues to grow. In this paper the characteristics of AE and friction signals for various operating conditions using CSS and drag tests were investigated from the durability point of view. Also, the wear characteristics of the laser bumps on a magnetic disk were compared between the CSS and drag tests. The general shapes of the AE and friction signals during a single CSS test were quite similar even under less than ideal operating conditions. However, it was found that the AE signal was more sensitive than the friction signal in assessing the damage of the slider/disk interface. Finally, a correlation was established between the CSS and drag testing methods with respect to the laser bump wear. This outcome suggests that the drag test may be used to accelerate the surface damage effect of head/disk system.     

Experimental and direct numerical analysis of hard-disk drive

The purpose of this paper is to study methods for enhancing the reliability and performance of hard-disk drives (HDD) because it is essential for improving recording density, speed of data access, and output signal. This study also investigates various techniques that can be used for head/disk contact detection. The acoustic emission (AE) and friction signal characteristics were observed with respect to the durability of the head/disk interface (HDI) under various operating conditions using a contact start-stop (CSS) test. In addition, to study the influence of surface topography on the stiction performance of the HDI, a modified and polished laser pump was proposed and CSS investigations were accomplished. Moreover, the static and dynamic properties of an HDD air slider were studied using a finite element method (FEM).     

The effects of texture height and thickness of amorphous carbon nitride coating of a hard disk slider on the friction and wear of the slider against a disk

In order to minimize the stiction force caused by contact of the extremely smooth surfaces of head sliders and disks in hard disk drives, texture is usually applied on the disk surface. For future contact/near-contact recording, the stiction-induced high friction between slider and disk will become a problem. Texture on the slider/disk interface will still be an expected method to reduce friction. Recently, it was suggested to texture the slider surface. A protective coating is usually required on the textured slider surface to reduce wear of the texture. The results showed that texture on the slider surface was effective in reducing the friction between head sliders and disks. On the other hand, the texture and coating on the slider surface increase the spacing between the read/write element and the magnetic layer of the disk. The necessary and effective texture height and coating thickness are still not clear. In the present research, island-type textures with different heights (3–18 mn) were formed on slider surfaces by ion-beam etching. Amorphous carbon nitride (a-CN_x) coatings of different thicknesses (0–50 nm) were coated on the textured slider surfaces as a protective overcoat. The friction and wear properties of these sliders were evaluated by constant-speed drag tests against hard disks coated with diamond-like carbon (DLC). The results show that 2 nm texture on a slider surface is sufficient for low (0.3–0.5) and stable friction of the slider against the disk in a drag test, and coatings thicker than 5 nm show similar wear resistances of the texture on slider surfaces.     

Silicon Nitride Boundary Lubrication: Lubrication Mechanism of Alcohols

This paper describes the lubrication mechanism of alcohols with silicon nitride under boundary lubrication conditions. Dynamic wear tests and static chemical reaction studies were conducted to study the chemical interaction between alcohols and silicon nitride. Direct evidence of chemical reactions occurring between alcohols and silicon nitride was collected. Gel-permeation-chromatography-graphite-furnace-atomic-absorption (GPC-GFAA) analysis detected the presence of high molecular weight (HMW), silicon-containing, metallo-organic compounds in the wearing contact. Secondary ion mass spectrometry (SIMS) analysis of the reaction products from wear tests revealed the formation of silicon alkoxides. These alkoxides subsequently reacted to form HMW products which had been independently verified as capable of lubricating silicon nitride surfaces. A two-ball collision test was used to verify the lubricating quality of the film generated from the wear test. A lubrication mechanism is proposed in which alcohols adsorb and react with the oxide/hydroxide layer of Si₃N₄ to produce a bonded surface silicon alkoxide. Subsequent tribochemical reactions prompted by the surface disruption from the wearing contact cause the formation of free silicon alkoxides. These species then proceed to form a variety of silicon-containing high molecular weight products that have demonstrable lubricating ability. This mechanistic understanding provides a framework of Si₃N₄ lubrication.     

Study on the cyclotriphosphazene film on magnetic head surface

Stable lubrication is very important to the slider/disk interface with the increasing demand on the life of computer hard disk drive (HDD). The inert lubricant perfluoropolyether (PFPE) on the surface of magnetic hard disk is still prone to be catalyzed to decomposition by the slider material Al₂O₃. The properties of a partial fluorinated hexaphenoxy cyclotriphosphazene, X-1P, are investigated and its function to reduce the catalytic decomposition of PFPE is discussed. The results of contact start–stop (CSS) tester indicate that the thermal stability of the lubricant was greatly improved in the presence of X-1P, and its film thickness has a great influence on the lubrication properties of the HDD.     

Role of Head Parameters on Head-Disc Interface

The effect of a nanoslider's crown, camber, twist and suspension load are parametrically studied and compared from a tribological standpoint. In this paper, stiction, friction, and contact start-slop (hereafter CSS) were performed on a modified disc drive equipped with a special strain gauge. For convex (positive crowned and cambered) sliders, up to five to six times reduction in rest stiction can be easily attained compared to flat sliders. This also applies to drives which have been turned off and turned on after one week.

Flat sliders with much of the air bearing surfaces etched away are, also studied. The patterned sliders did not reduce stiction when tested on a lightly textured disc coated with a lube system consisting of mobile and bonded phases.

The normal suspension force and weight of the slider when bonded together determine the apparent slider contact between the disc surface asperities. The reduction in contact area from six to four grams suspension load on the asperities is about 24 percent.

The effect of convex slider improves the disc durability during CSS. It is believed the geometry allows the convex-shaped magnetic slider to take off from the disc surface sooner than flat ones. That was observed by Lee and Bolasna. The concave slider will reduce stiction also because of smaller contact area. However, its inability to take off early results in catastrophic disc wear. Once in flight, the sliders are insensitive to the convex or concave effect.     

不同润滑状态下游离水对点接触油润滑润滑性能的影响*

为研究水污染对润滑油性能的影响,通过点接触光干涉试验得到充分供油、轻微乏油及严重乏油3种润滑状态下的油膜光干涉图和膜厚形状曲线,研究游离水对接触区油润滑润滑性能的影响。结果表明:在充分供油条件下,游离水对点接触润滑油膜厚度的影响不大,主要影响中心膜厚;在乏油润滑状态下,游离水对点接触润滑油膜厚度有增益效果;但3种润滑状态下,游离水都会使气穴区边界变得模糊不清,提高了油润滑的弹流润滑性能。     

滚滑轴承滑块的油-气两相流润滑分析

为改善滚滑轴承的润滑,运用两相流理论对其滑块进行油气润滑设计,建立滑块的油-气两相流CFD模型,分析不同入口角度、进气速度、进油速度和润滑油黏度对流场油相分布的影响。结果表明:油-气混合润滑方式能在内外滚道接触区形成有效的润滑油膜;油气管道夹角影响油滴分布,角度过大时大量油滴会在滑块侧面上附着,角度过小时油滴会在外滚道入口处堆积,造成供油连续性不好,油膜稳定性下降;进气速度过大会降低油滴附着率,无法形成有效油膜,而进油速度过大会造成润滑油累积,出现搅油现象,因此选择合适的进气和进油速度,才能控制油滴的大小和保持润滑过程的连续性;润滑油黏度会影响油滴在滑块上的附着效果,合理地选择润滑油黏度,才能保证流场油相分布均匀。     

Wear of the magnetic head-carbon overcoated disk interface. A Raman spectroscopy study

Raman spectroscopy was applied in the wear track region of carbon overcoated magnetic storage disks after drag and contact start/stop (CSS) tests with a manganese-zinc ferrite slider. The ratio of the relative intensities of the two major carbon bands, one centred near 1360 cm⁻¹ (the D band) and the other centred between 1560 cm⁻¹ and 1580 cm⁻¹ (the G band) increased with the number of cycles in the drag test and first increased and then decreased in the CSS test. The changed D/G spectral intensity ratio is attributed to temperature changes. Raman spectra of carbon layers sputtered to different thicknesses between 15 nm and 30 nm were essentially identical, indicating uniformity in overcoat composition. Raman spectra are consistent with generally higher temperatures in CSS tests than in drag tests. The intensity of a Raman band centred at 625 cm⁻¹ increased with number of drag cycles in the slider spectrum. Its frequency and shape corresponds to that reported for manganese ferrite in the literature. Its intensity increase with wear remains to be explained. Laser irradiation of carbon overcoats in a Raman spectrometer can contribute to changes of the carbon structure by heating.     

Self-assembled monolayer protective films for hybrid sliding contacts

Abstract

Silicon nitride as an energy efficient material is replacing conventional steels for new generation engineering components such as bearings, cutting tools, electronics and engine parts in automotive, aerospace and wind industries. Compared with steel bearings, silicon nitride bearings can be operated at much higher temperatures and speeds with >60% weight reduction and up to 80% friction reduction. These are all due to its unique material properties, including high wear and corrosion resistance, low density and heat generation. Current lubrication solutions for hybrid contacts, where silicon nitride balls and steel races are used, are mostly relying on the protection film formed on the metal surfaces. Self-assembled monolayers (SAMs) have been found very useful in modifying surfaces, especially for microelectromechanical system and nanoscale applications, e.g. atomic force microscopy tips, etc. This study aims to investigate the feasibility of forming a SAM protection film on industrial grade bearing material silicon nitride to reduce the friction for the oil lubricated hybrid contacts. Four silanes with different functional head groups, including octadecyltrichlorosilane (OTS), octyltrichlorosilane, chlorodimethyloctadecylsilane and octadecyltrimethoxysilane, were initially investigated to form SAMs on industrial grade silicon nitride surfaces. The effects of concentration and immersion time of the silanes on the formation of SAMs on the silicon nitride surface were evaluated using contact angle measurements. The preliminary results show that the wetting properties of the silicon nitride surface can be effectively modified by the formation of SAMs from the silane solutions. OTS can form an order and compact SAM on the silicon nitride surfaces within 2 min at the concentration of 2··5 mM in decane solution, while the other three alkylsilanes can also effectively modify silicon nitride surfaces given sufficient immersion time, e.g. over 1 h. Tribological tests were subsequently carried out on a ball on disc rig where a steel ball and a silicon nitride disc were used. The effect of the formation of alkylsilane SAMs on the friction between the sliding contacts has been evaluated in two different methods. The first method was to test preformed SAM films under dry conditions, and the second was to premix one of the surfactants with Shell Vitrea ISO 32 mineral base oil and then spray the mixture to the contacts during the ball on disc testing. The test results show that an average of over 40 and 30% friction reduction was achieved for the hybrid contact when lubricated with the base oil mixed with OTS (>2··5 mM) and octadecyltrimethoxysilane (5 mM) respectively compared with that of the sliding contact lubricated by the base oil only. Since OTS may produce corrosive byproducts during SAM formation, octadecyltrimethoxysilane may be a more suitable additive for the hybrid contacts.     

Experimental assessment of textured tools with nano-lubricants in orthogonal cutting of titanium alloy

In this study, we investigated the effects of composite nano-Cu/WS₂ lubricating oil and single-point diamond indentation-textures on improving the cutting performance of YG8 cemented carbide tools, which is crucial for textures tool applications. The aims of the study were to improve wear resistance and reduce chip adhesion at the tool’s rake face in cutting of titanium alloys. Dot textures with different spacings were fabricated on the surface of YG8 cemented carbide tools through the single-point diamond indentation method, and composite nano-Cu/WS₂ lubricating oil was prepared. Orthogonal cutting tests were carried out under dry cutting and minimal quantity lubricated (MQL) conditions. Investigate the effect of different texture spacing on the cutting performance in the light of cutting forces, friction coefficient, the deformed chip thickness, tool adhesions, and chip morphology. The results show that the dot texture effectively improved the lubrication conditions in machining titanium alloys under the MQL conditions. The dot texture is effective at low speed in the dry cutting conditions. With the increase of cutting speed, the friction coefficient of dot texture tool is affected by texture spacing, and the friction coefficient of DT-200 tool is the smallest. In addition, composite nano Cu/WS₂ lubricating oil forms a lubricating film on the wear path by atomizing the lubricating oil and stores it in the dot texture, which enhances the anti-wear performance in the cutting process and reduces the cutting force and friction coefficient at the tool chip interface. By evaluating cutting force, friction coefficient, chip and tool morphology, it is concluded that DT-100 tool is more effective in improving lubrication conditions.

     

不同润湿组合配流副的摩擦磨损试验研究

为调控配流副表面润湿性以提高摩擦学性能，采用低面能修饰法制备具有不同润湿性的上试样超级双相不锈钢SAF2507和下试样碳纤维增强聚醚醚酮(CFRPEEK)组成配流副，利用MMD-5A标准摩擦磨损试验机测试并分析其在蒸馏水、海水、L-HM46抗磨液压油3种润滑条件下的摩擦磨损特性。结果表明：低面能修饰使配流副上试样与下试样由亲水表面转变为疏水表面，上下试样表面虽仍保持亲油性但其油接触角明显增大；同种配流副在油、海水、水介质中摩擦因数依次减小；在水、海水、油介质下双疏水表面配流副较双亲水表面配流副摩擦因数分别减小8.2%、38.2%、24.4%；同种配流副在水、海水、油介质中磨痕深度依次减小。配流副的摩擦主要以犁沟效应和黏着磨损为主，双低表面能组合配流副能有效减小摩擦过程中的黏着磨损，从而表现出较好的摩擦学性能。     

Wear transitions and mechanisms in lubricated sliding of a molybdenum coating

Wear tests were performed for a Mo coating sliding against bearing steel specimen under boundary lubrication conditions. Results were compared with (i) hardened carbon steel sliding against bearing steel and (ii) Mo coating sliding against boron cast iron. Tests indicated that the wear resistance of the Mo coating was superior to that of the uncoated hardened steel. The initial surface topographies of the coatings were suitable to facilitate the transfer of the applied load directly onto the phases and prevented the softer phase directly involved in the wear process. The morphology of the transfer layer formed on the Mo coating was identified by X-ray diffractometry. And the layers were expected to supply an in situ lubrication effect. The wear rates of the coating against a steel slider were lower compared with those worn against a cast iron slider. With increasing applied load, the probability of the harder phases crack and fracture increased until the fraction of the unfragmented phases on the contact surfaces was no longer adequate to support the load. The dominant wear mechanisms in each wear regime were discussed.     

润湿性梯度集油表面制备及摩擦性能研究*

为了解决润滑油发生迁移，使接触面润滑油中断，导致润滑失效的问题，制备一种具有高集油性能的疏油-亲油-疏油的梯度表面。采用化学气相沉积的方法，在硅片表面沉积一层单分子膜，并采用接触角测量仪、UMT摩擦磨损试验机、共聚焦显微镜等对该样品进行表征，研究该类表面在限量供油条件下的润滑性能。结果表明：将油滴滴在疏油/亲油交界处，油滴能够迅速地从疏油区域向亲油区域运动；点接触往复运动摩擦实验结果表明，梯度表面硅片的摩擦因数明显低于原始的硅片，且梯度表面硅片的表面磨痕深度比原始硅片浅。各种实验结果表明，所制备的疏油-亲油-疏油梯度表面能够起到集油的作用，避免了润滑油中断的问题。     

The temperatures at scuffing and seizure in a four‐ball contact

The temperature of surface asperities affects lubricant‐surface tribochemical interactions. It is important to know the nature of this to identify ways of preventing scuffing and seizure under extreme‐pressure (EP) conditions. A new model for the determination of the temperature of contacting asperities is presented in this paper. It assumes the superposition of thermal processes occurring on the macroscale and thermal phenomena in the contact of asperity tips (microscale). Numerical results have been obtained for conditions of four‐ball testing of various lubricating oils — a mineral base oil with and without antiwear and EP additives. To calculate the scuffing and seizure temperatures, knowledge of the mechanical and physical properties of the test ball material (bearing steel) and lubricants, as well as the parameters describing the surface topography of the balls, was necessary. Friction coefficient curves were also needed; they were determined during four‐ball tests with a continuously increasing load. For the base oil with lubricating additives, the temperature of contacting surface asperities at the moment of scuffing initiation was calculated to be about 230°C and increased to over 1000°C at the highest loading of the four‐ball tribosystem. This suggests the possibility of tribochemical reactions of the lubricating additives with the steel surface, and diffusion of some elements, a modified surface layer having good antiseizure properties being produced. Such a layer prevents seizure of the tribosystem. For the base oil without lubricating additives, scuffing initiated at about 150°C and the temperature exceeded 1200°C at seizure. The temperature values obtained agree with results in the literature.