A Theoretical Study of Vapor Lubrication for Heat Assisted Magnetic Recording

Heat assisted magnetic recording (HAMR) is a promising technique to overcome the superparamagnetic limit to further increase the areal recording density of hard disk drives. However, HAMR brings about serious problems to the slider-disk interface, such as lubricant depletion on disk surface caused by laser heating. It is proposed to overcome the lubricant depletion problem by using vapor lubrication. The lubricant film formation process on disk surface in vapor lubrication is studied theoretically based on fundamental adsorption and desorption theories. The controlling parameters of lubricant film thickness and film formation time are identified. It is found that the lubricant film thickness is controlled mainly by lubricant vapor pressure and molecular weight. The film formation time can be shortened by using low molecular weight lubricant and high temperature lubricant vapor.     

Material Transfer Inside Head Disk Interface for Heat Assisted Magnetic Recording

Heat assisted magnetic recording (HAMR) promises to deliver higher storage areal density than the current perpendicular magnetic recording products. Laser heating is implemented in HAMR to achieve magnetic writing of the very high coercivity media. However, the high temperature environment creates several reliability challenges for the head disk interface (HDI). In this paper, material transfer within the HDI under HAMR recording conditions is studied. The mechanisms of material transfer are explored via experiments and modeling. This study revealed that temperature difference and mechanical interaction between the head and media are the main mechanisms for material transfer inside the HDI. Possible methods to remove the material are also discussed in this paper.     

Effect of Viscoelasticity on Lubricant Behavior Under Heat-Assisted Magnetic Recording Conditions

In Heat-Assisted Magnetic Recording (HAMR) technology, the lubricant layer coating on the disk is exposed to severe thermal conditions, leading to evaporation, depletion, and chemical degradation. In general, those studying the effects of laser exposure on lubricant depletion and recovery have assumed the lubricant to be a viscous fluid and have modeled its behavior using lubrication theory. However, PFPE lubricant depletion and recovery behavior at the timescale of HAMR conditions (microsecond to millisecond) is known to be that of a viscoelastic fluid. In this paper, we introduce a modification to the traditional lubrication equation that accommodates viscoelastic effects. The results suggest that this method is numerically unstable for small laser spot sizes, close to the target of HAMR. Accordingly, we developed a novel approach to model the viscoelastic depletion and recovery behavior of PFPE ultra-thin films using a Finite Element Analysis. We show that this new method is able to model the entire range of material viscoelasticity, from purely viscous to purely elastic extremes. The results show that the viscoelastic effects become remarkably pronounced with a decrease in laser spot size. For the micron-size laser spots, close to typical experimental conditions, the lubricant behaves like a viscous fluid. For the laser spot size of 20 nm, close to the target of HAMR, it behaves like an elastic material. In exposing the consequences of this viscoelastic behavior, this study predicts that lubricant flow due to thermo-capillary effects will not be a significant issue in the development of the HAMR technology. Rather, future efforts should concentrate on the thermal degradation and evaporation effects upon HAMR lubricants.     

Experimental Study of Lubricant Depletion in Heat Assisted Magnetic Recording over the Lifetime of the Drive

Heat assisted magnetic recording (HAMR) is a promising choice to overcome the superparamagnetic limit in magnetic recording and further increase the areal recoding density of hard disk drive. However, it is expected that HAMR causes lubricant depletion problem on disk surface under the high temperature in the heating assisted writing process. Experimental studies of the lubricant depletion under HAMR conditions are still very limited so far. Lubricant depletion over the lifetime of the drive still remains unaddressed. In this study, a self-developed HAMR tester is introduced. The methods to control the repeatability of laser heating temperature, to determine laser heating temperature, and to adjust laser heating temperature are explained. Laser heating time in the test is correlated with that in the drive. Lubricant depletion is determined quantitatively and the relationship between lubricant depletion depth and laser heating time is established. Then, lubricant depletion depth over the lifetime of the drive is predicated. It is found that almost all lubricant on the disk surface will be depleted over the lifetime of the drive.     

The Effect of Solvents on the Perfluoropolyether Lubricants Used on Rigid Magnetic Recording Media

The adsorption characteristics and tribological properties of the perfluoropolyether (PFPE) lubricants Zdol and Z-Tetraol on amorphous nitrogenated CN_x carbon are investigated as a function of solvent used to apply the lubricants. The solvents used in these studies include perfluorohexane, CF₃CHFCHFCF₂CF₃ and C₄F₉OCH₃. Deposition studies indicate that the applied thickness of PFPE films is strongly solvent-dependent that can be related to differences in the solubility parameters between the various lubricants and solvents. The results of ab initio computations on the molecular electronic structure of the solvent molecules show that their solvent power is correlated to their polarity and in particular to the acidity of the protons on the CF₃CHFCHFCF₂CF₃ and C₄F₉OCH₃ molecules. Tribological reliability, as measured by contact start-stop testing, slider-disk clearance, lubricant pickup by the slider, lubricant smearing on the disk surface, etc., is independent of solvent and is limited to the physical properties of the adsorbed lubricant film. The kinetics of lubricant mobility are charateristic of confined liquids that are independent of solvent as shown by lubricant flow profiles, bonding kinetics, and contact angle goniometry.     

Simulation of Lubricant Recovery After Heat-Assisted Magnetic Recording Writing

The lubricant in a heat-assisted magnetic recording (HAMR) hard disk drive must be able to withstand the writing process in which the disk is locally heated a few hundred degrees Celsius within a few nanoseconds and be able to sufficiently recover the lubricant depletion and accumulation zones so as to allow for stable flying heights and reliable read/write performance. In a previous publication, we simulated the distortion of thin Zdol films due to a thermal spot during HAMR writing and predicted several Angstroms of depletion. In this paper, we continue these simulations into recovery. Our simulation results indicate that lubricant deformation caused by small thermal spots of 20-nm full-width half-maximum (FWHM) recover on the order of 100–1,000 times faster than larger 1-μm FWHM spots. However, the lubricant is unable to recover from sufficiently high writing temperatures. An optimal thickness at which HAMR writing deformation recovers fastest is apparent for sub-100-nm FWHM thermal spots. Our simulations show that simple scaling of experimental observations using optical laser spots of diameters close to 1 μm to predict lubricant phenomena induced by thermal spots close to 20-nm FWHM may not be valid. Researchers should be aware of the possibility of different lubricant behavior at small scales when designing and developing the HAMR head-disk interface.     

不同工况下涡旋泵空化与性能的数值模拟

针对涡旋泵建立了含有实际间隙的三维流体域模型,采用CFD技术对其进行了三维非定常模拟,研究了不同转角下的内部流场,分析了不同工况下的泵内空化现象及其性能.结果 表明:由于涡旋泵结构特点和运动方式,导致泵的2个工作腔内的流动具有不对称性;在吸液末期和排液初期,工作腔内产生较高的压力脉动,严重影响泵的稳定性;在高压差的作用...     

Lubricants and Lubricant Additives Under Shear Studied Under Operating Conditions by Optical and Infrared Spectroscopic Methods

A physical basis was sought for differences in fuel economy with a typical petroleum lube base stock when various additives were present. The kinematic viscosity was maintained at 7.1 cS at 100°C by blending. Film thickness, temperature, and traction were determined for a ball/plate contact under elastohydrodynamic conditions. Different additives produced considerably different results. Invariably, small film thicknesses paralleled low temperatures in the conjunction region, low traction and high fuel economy. Zinc dithiophosphate (ZDDP) was unique in that it always increased film thickness, but increased traction only at high loads and low speeds while decreasing it otherwise. Other additives, e.g. friction modifier (FM) and viscosity index (VI) improver and combinations decreased film thickness and traction. To help determine the reasons for these behaviors—whether surface or bulk effects—polarized infrared emission spectra were collected from an operating mock journal bearing. Adherent layers do not change polarization with increase of shear rate and could therefore be distinguished from the bulk material. ZDDP did change polarization with shear rate us did VI improver, but FM did not. The formation and breakup in the bulk phase of insoluble particles or globules—similar to grease thickeners held together by ZDDP is proposed as a novel possible explanation for its behavior. The established theories about the action of the other additives were confirmed.     

Data Loss and Demagnetization of Perpendicular Magnetic Recording Disk Under Sliding Contact

Data loss and demagnetization of perpendicular magnetic recording disk under sliding contact were investigated experimentally. The data loss tests of the disk against a diamond tip under normal forces (0.005–0.05 mN) and the scan of the disk with the magnetic head were sequentially carried out. Then, the demagnetization tests under normal forces (6–10 mN) were performed on the disk to examine the demagnetization behavior. After the tests, the sliding contact areas in the disk samples were observed by atomic force microscopy and magnetic force microscopy. The results showed that data loss occurred without any scratch damages on the disk and demagnetization of the magnetic medium did not occur in the data loss area. The demagnetization occurred only when the scratch depth in the disk exceeded the thickness of the diamond-like-carbon top layer. Finally, a method to study the relationship between data loss and demagnetization of the perpendicular magnetic recording disk under sliding contact was proposed and the conclusion was given that data loss of the disk was not induced by demagnetization of the magnetic medium.     

多种工况下液压支架底座的性能分析

由于液压支架的底座承载较大,针对不同工况下的底座的性能采用ANSYS进行有限元仿真分析.结果 显示,在底座的后连杆铰接孔位置处是底座较为危险的区域,需要进行结构的优化,提高底座的性能,更好地发挥液压支架的作用.     

Depletion of Monolayer Liquid Lubricant Films Induced by Laser Heating in Thermally Assisted Magnetic Recording

The white layer (WL) generally refers to an untempered martensite layer generated by severe sliding conditions, such as cutting, drilling, grinding, milling, and electron-discharge machining. Numerous tribologists have investigated the effect of the WL on wear behavior, but this phenomenon is not yet clearly understood because no attempt has been made to correlate the wear mechanism of the WL with its wear behavior. It is necessary to investigate the effects of the WL on wear comprehensively. Therefore, elasto-plastic properties, wear resistance, and wear mechanism of WL were studied with nanoindentation and wear tests. The results of this study show that the elastic moduli, yield strength, and hardness of the WL increased by approximately 170, 390, and 180%, respectively. Thus, it was found that the WL had distinct properties with respect to the substrate of AISI 52100 steel. It is possible that the increased surface hardness is an advantage in terms of wear resistance. However, the advantage diminishes, and the WL is subject to wear when delamination dominates the wear mechanism.     

The Contribution of Thin PFPE Lubricants to Slider-Disk Spacing. 3. Effect of Main Chain Flexibility

We have investigated the effect of main chain flexibility of a perfluoropolyether lubricant, Zdol 4000, on dynamic slider-disk spacing. The major conclusion of this work is that increasing the Zdol chain flexibility results in a decrease in the dynamic slider-disk spacing. The Zdol main chain flexibility is quantified by ab initio computations of the barriers to internal rotation about C-O and C-C bonds in model structures containing -CF₂O- and -CF₂CF₂O- monomer units. C-O bond rotations for -CF₂O- monomer units bordered by -CF₂O- monomer units are relatively unhindered with a barrier to internal rotation of 1.7 kcal/mol. C-O and C-C bond rotations in -CF₂OCF₂CF₂O- units have barriers to internal rotation of 8 and 5 kcal/mol, respectively. The presence of the C-C bond imparts considerable rigidity to the Zdol chain.     

The Change in Surface Properties of Magnetic Recording Media Under Pulsed Laser Application

The effects of laser heating on surface properties of magnetic recording media were systematically investigated through novel experiments and analytical simulations. When controlled laser pulses were applied onto the surface of a perpendicular magnetic recording (PMR) media, the measured values of surface roughness, surface free energy, and surface adhesive force were significantly increased with the number of the applied laser pulses. The heat transfer modeling and simulation was performed to evaluate the change in surface temperature of PMR media by the pulsed laser. The resulting temperature was not high enough to affect the carbon film and the underlying magnetic materials, but it could change the properties of the molecularly thin lubricant film on the media surface. Based on the thermal stability of the perfluoropolyether lubricant, it was found that the change of surface properties in experiments could be attributed to the thermal degradation of the lubricant through desorption process.     

The Sliding Friction of Hybrid Composite Journal Bearing Under Various Test Conditions

The friction and wear behaviour of SiC, Si₃N₄ and SiC/Si₃N₄ composite ceramics were investigated with oscillating sliding (gross slip fretting) at room temperature. The influence of counter body material and the humidity of the surrounding air was studied with a ball-on-disc configuration with different ball materials (1000Cr6, Al₂O₃ SiC and Si₃N₄). The effect of RH on friction is marginal with exception of SiC (low friction) as counter body material. The wear behaviour, however, is strongly affected by humidity, showing inverse trends for different counter body materials. Consequently, the wear behaviour of a tribo couple can be improved by selecting an adequate mating material. The results reveal the necessity to control RH in tribological tests. For estimation of the performance of tribo couples under varying environmental conditions, a variation of RH is required. In tribo couples with single phase SiC, either as ball or disc, the tribological behaviour of the system is dominated by SiC. The friction behaviour of the composite material is in between the behaviour of the two single phase materials, Si₃N₄ and SiC, whereas the wear behaviour is very similar to that of single phase Si₃N₄.     

Performance Characteristics of Perfluoroalkylpolyether Synthetic Lubricants

An extensive test program was conducted to determine the performance characteristics (friction, wear, temperature) of a linear high temperature perfluoroalkylpolyether (PFPE) and two branched PFPE lubricants under normal and severe operating conditions. These include temperatures up to 200° C and Hertzian pressures as high as 2.8 GPa. A four-ball wear tester was used to measure wear scar and a two-disk machine was used to measure friction. The linear perfluoroalkylpolyether lubricant demonstrated low friction and wear at elevated temperatures under the test conditions considered. A bearing testing apparatus was used to measure friction and temperature of tapered roller bearings under various operating conditions for a branched PFPE and the results are reported.     

不同润滑液的电润湿性能研究

润湿性是指液体在固体表面铺展的倾向性,可以通过润湿角描述,是评价润滑液渗透性能的一个重要指标。为了研究润滑液的电润湿性,搭建电润湿润湿角测量装置,对施加负高电压的润滑液润湿角进行测量,得到不同电压下不同润滑液的润湿角变化情况,结合仿真分析施加负高压液滴润湿角变化机制。结果表明:润滑液润湿角受外加电场影响,在一定范围内,电压越大,润滑液润湿角越小;在润滑液中加入纳米颗粒可提高润滑液的电润湿性能;在同等电压下,与去离子水、去离子水基纳米流体、蓖麻油、蓖麻油基纳米流体、葵花油相比,葵花油基纳米流体的电润湿性能最好;对润滑液液滴施加电压后,液滴各位置压力场与速度场发生变化,其顶部压力场较大,速度指向液滴内部,边缘压力场较小,速度指向液滴外部,进而导致润湿角减小。     

Lubricating Performance of Organic Sulfides Under Repeated Rubbing Conditions

Repeated four-ball tests of squalane with or without organic sulfides have been carried out to investigate the relationship between the properties of surface film formed on the rubbing surfaces and the lubricating performance of several organic sulfides.

With dibenzyl disulfide and dibenzyl monosulfide, friction was remarkably reduced and load-carrying capacity raised in the second run with the additive after the first run without the additive, presumably in which only an oxide film had been formed. In contrast, for diphenyl disulfide, the repetition of rubbing tests, each with additive, was effective.

EPMA, XPS(ESCA) and electron diffraction techniques have been applied to examine surface films formed during rubbing process. It is confirmed that the formation of surface films with the optimum composition of oxide and sulfide has brought about the excellent lubricating performance of these organic sulfides.     

二氧化碳汽车空调器变工况性能分析

建立了超临界二氧化碳汽车空调器循环计算模型,结合美国Ｉｌｌｉｎｏｉｓ大学制冷空调中心（ＡＣＲＣ）二氧化碳汽车空调样机实验结果,对两种压缩机转速和两种气体冷却器空气进口温度的不同组合条件下的工况,进行了循环计算,并对计算结果作了分析。计算结果表明,压缩机转速越高,或者气体冷却器空气进口温度越高,二氧化碳汽车空调的工部越恶劣,ＣＯＰ值越低。     

热沉扩展表面积对换热性能影响的数值模拟研究

通过数值模拟的方式,研究了在冷却工质通过热沉通道消耗的分别泵功为0.04W和0.4W时,扩展表面热沉的扩展表面面积对热沉加热面的过余温度的影响。结果显示,在加热面面积一定,泵功为0.04W时,扩展表面面积的增大可以明显的降低热沉加热面的过余温度;在泵功为0.4W,扩展表面面积与加热面面积比值小于2.25时,扩展面积的增大可以降低热沉加热面的过余温度,当该面积比值大于2.25时,扩展面积的增大对热沉加热面的最高过余温度和最低温过余温度没有影响。     

Adsorbed Water Film and Heat Conduction from Disk to Slider in Heat-Assisted Magnetic Recording

In heat-assisted magnetic recording (HAMR), a tiny area of magnetic recording media has to be heated up to a high temperature with laser to lower the coercivity temporarily for information to be written on the area. In a humid environment, some of the water vapor molecules adsorb on the disk surface to form a water film. In HAMR writing, the adsorbed water film on the disk surface will desorb instantly from the high-temperature laser heating area to become high-temperature high-pressure water vapor. The water vapor molecules will transfer extra heat from the high-temperature laser heating area on the disk surface to the slider, which makes the temperature of the slider surface higher in a humid environment than that in dry air. The heat transfer increases dramatically with relative humidity and with the decrease in slider–disk spacing.