Accelerated wear testing of head-disc interfaces

To better assess wear characteristics of head and media combinations under conditions that approximate drive operation, we describe an accelerated wear test in which the z-height is increased so that the trailing edge of the air bearing surface makes contact. Using this test, we find that the wear rate for a particular head-media combination decreases as time^1/2 and we document the effect of media wear on the head wear rate. These experimental findings are combined with numerical modeling of head-disc interface wear that correlates well with our results and predicts a shift in wear mechanisms as surface deformation shifts from dominantly plastic to elastic.     

Air Bearing Design to Prevent Reverse Flow from the Trailing Edge of the Slider

A simulation approach that relies on an analysis of the flow patterns closest to an air bearing surface (ABS) was used to predict the lubricant accumulation on the ABS of a head slider. The lubricant accumulation patterns obtained through the simulation were in good agreement with experimental results and with our experimental apparatus. We used this method to study and analyze flow pattern droplets close to the trailing edge of a number of sliders and found that there was a reverse flow from the slider’s trailing edge on both sides of the trailing pad and behind the read/write element, which could result in a lubricant accumulation on the slider surface close to the trailing edge of a slider and thus lead a transient slider vibration and magnetic-signal loss in a hard disk drive. Further simulations and analyses revealed that the reverse flow is dependent on the depth of slider surface on adjacent to the trailing edge of the slider, and that if the depth is less than a critical depth, which is dependent on the velocity of the disk, the reverse flow could be eliminated. On the basis of these findings, we propose a new ABS design concept for effectively suppressing the reverse flow of lubricants from the trailing edge of the slider. In this concept, the slider has a “smooth flow pad” and the depths of outlet recesses are specified as being smaller than the critical depth. It was confirmed by both simulation and experiment that lube accumulation on the slider surface is obviously decreased and the reliability of a hard disk drive with this air bearing design is consequently improved.     

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.     

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.     

Roughness effects on head–disk interface durability and reliability

This paper deals with the tribological reliability and durability issues in modern magnetic recording hard disk drives, where the slider flies at typically less than 10 nm. Specifically, we investigate the effect of disk surface roughness on the above performance characteristics. The durability of the interface is investigated using low-pressure on-track tests. The drive-level reliability data are presented to confirm general conclusions reached from the component-level testing. Component-level slider–disk clearance measurements are also used to better understand how the roughness affects both durability and reliability. Finally, we discuss possible explanations for the experimentally observed relationships.     

Inert Gas Filled Head–Disk Interface for Future Extremely High Density Magnetic Recording

Inert gas filled head–disk interface (HDI) is a possible solution in reducing the magnetic spacing between the magnetic head and the magnetic media for achieving further increased recording density of a magnetic recording system. This article investigated the flying and thermal performances of a thermal actuated slider at inert gas filled HDI by using a couple-field analysis method which consists of a finite element model of the entire slider, an air bearing model based on the generalized lubrication equation and a heat transfer model which incorporates various molecular dynamics models and considers temperature effects. The simulation studies showed that the variation of gap flying height (FH) with the heater power in the inert gas is quite similar to that in air. It is also found that the slider’s thermal actuation efficiency in helium is slightly better than those in argon and air. However, the temperature effects in a fully sealed drive are totally different to those in an open drive. As a result, the inert gas filled HDI normally requires a larger thermal actuation stroke due to the temperature effects in a fully sealed drive.     

Probability Model for the intermolecular force with surface roughness considered

This paper studies the intermolecular force considering both the roughness of the air-bearing surface and the disk surface by simulation. A model is developed to deal with the intermolecular force, the contact force and the air-bearing force based on the probability distributions of the roughness of the surfaces. The intermolecular force is linked with the contact force when its repulsive term is stronger than its attractive term. In such a case, all the intermolecular force, the contact force and the air-bearing force can be extended to the various flying height regions. Some interesting results are observed and discussed. It is found that both the Hamaker constant and the surface roughness have significant influences on the intermolecular pressure. Compared with the intermolecular pressure with smooth surfaces, that with the surface roughness considered shows greater attractive pressure at the flying height higher than 0.7 nm approximately, but much smaller values between 0.26 and 0.7 nm approximately. A negative stiffness region exists when the minimum flying height is between −0.2 and 1.2 nm for the case studied in this paper. It shows that the Probability Model is suitable for the intermolecular force calculation with the surface roughness considered.     

Effect of relative humidity and disk acceleration on tribocharge build-up at a slider–disk interface

High performance disk drives require high spindle speed. The spindle speed of typical hard disk drives has increased in recent years from 5400 to 15000 rpm and even higher speeds are anticipated in the near future. The increasing disk velocity leads to increasing disk acceleration and slider–disk interaction. As the head-to-disk spacing continues to decrease to facilitate increasing recording densities in disk drives, the slider–disk interaction has become much more severe due to the direct contact of head and disk surfaces in both start/stop and flying cases. The slider–disk interaction in contact-start-stop (CSS) mode is an important source of particle generation and tribocharge. Charge build-up in the slider–disk interface can cause electrostatic discharge (ESD) damage and lubricant decomposition. In turn, ESD can cause severe melting damage to MR or GMR heads. We measured the tribocurrent/voltage build-up generated at increasing disk acceleration. In addition, we examined the effects of relative humidity on the tribocharge build-up. We found that the tribocurrent/voltage was generated during pico-slider/disk interaction and that its level was below 250 pA and 0.5 V, respectively. Tribocurrent/voltage build-up was reduced with increasing disk acceleration. Higher humidity conditions (75–80%) yielded lower levels of tribovoltage/current. Therefore, a higher tribocharge is expected at a lower disk acceleration and lower relative humidity condition.     

Dynamics of transient events at the head/disk interface

Slider/disk contacts of nano and pico sliders are investigated using an acoustic emission sensor and a high bandwidth laser Doppler vibrometer (LDV). The following cases are studied: (a) influence of scratch impact on the airbearing stiffness; (b) influence of lubricant thickness on slider dynamics for single bump impacts; (c) influence of lubricant thickness on slider vertical stick–slip vibrations; (d) dynamics of take-off and landing. Linear time frequency analysis is applied to study simultaneously the impact response of the airbearing and the slider torsional and bending modes. The contact dynamics of single bump impacts is examined as a function of disk velocity and lubricant thickness. Increased slider vibrations are found for thick lubricant films both for sliding contacts as well as for single bump impacts. During the transition from sliding to flying a change of the bending mode frequency is observed.     

Head slap simulation for linear and rotary shock impulses

A finite element model of a hard disk drive (HDD) is developed to investigate the response of the HDD to a shock impulse. Two types of shock are of interest, a linear shock and a rotary shock. The linear model corresponds to a HDD being dropped flat onto an impact surface. The rotary model is constrained to rotate about an axis and simulates a HDD standing on one edge that is allowed to drop and impact the opposite edge. The geometry is developed using Pro/E, a CAD package, and is then imported into Hypermesh, a pre- and post-processor. The transient solution is performed using LS-Dyna, a finite element solver. The three software packages are commercially available. Results are shown as animations and time series data. Comparison of the simulation results for the two models is used to develop a correlation between the linear and rotary shock tests.     

Dynamic Adhesion Characteristics of Spherical Sliders Colliding with Stationary Magnetic Disks with a Thin Lubricant Layer

The dynamic indentation characteristics of 1- and 2-mm-radius hemispherical glass sliders when colliding with stationary magnetic disks under various lubricant conditions were investigated to clarify the dynamic interfacial forces between flying head sliders and magnetic disks. The collision times were ~15 and ~30 μs, respectively, and independent of the impact velocity. For a 1-mm-radius slider (Ra roughness = 1.71 nm), a clear adhesion force nearly equal to the static pull-off force was observed at the instant of separation when the lubricant thickness was from 1 nm without UV (0.69 nm mobile lubricant thickness) to 3 nm with UV (1.89 nm mobile lubricant thickness). The dynamic adhesion force was maximum when the slider had separated from the disk surface by about 2 nm and dropped from the maximum to zero when the separation reached more than 5 nm. When the mobile lubricant thickness was 0.43 nm, a clear adhesion force was not observed. For a 2-mm-radius slider (Ra roughness = 0.34 nm), a clear adhesion force, similar to the static pull-off force, was observed at the instant of separation at almost all lubricant thicknesses and impact velocities tested except at a small mobile lubricant thickness of 0.43 nm with impact velocities greater than 1.1 mm/s. The dynamic adhesion force dropped from the maximum to zero when the distance traveled from the maximum reached more than 5 nm. These results suggest that the dynamic adhesion force of 1- and 2-mm-radius sliders originates from meniscus formation rather than van der Waals force.     

Head and media design considerations for reducing thermal asperity

Recent use of magnetoresistive (MR)/giant-magnetoresistive (GMR) heads in disk drives requires tighter control on particle contamination that may generate thermal asperities at the head/disk interface. In this study, the effects of slider air bearing surface (ABS) design and media on TA reduction capability are investigated. The motion of particles at the head/media interface is simulated numerically. Drive level TA tests are performed using a particle injection chamber. It is observed that a new ABS design, that has an aerodynamic U-shaped rail and a central airflow passage, is beneficial in reducing the particle contamination on the slider. Scratch-resistance of the media surface is correlated to TA reduction capability of the media; more scratch-resistant media produced less TA. On the other hand, the adhesion properties of the media have minor effects on TA reduction, as the particles that usually generate TA in a drive are too small to spin off the media.     

Design of Carbon Surface Functional Groups on the Viewpoint of Lubricant Layer Structure

Chemical modification technique with X-ray photoelectron spectroscopy has been used for a quantitative analysis of surface functional groups of the carbon overcoats. As the nitrogen content in the carbon overcoat layer increases, the total abundance of functional groups on the carbon overcoat surface increases. The increase of the functional groups leads to an increase of a physisorbed lubricant layer ratio.

Increase of the physisorbed layer ratio on the carbon overcoat surface showed a higher durability on the head/disk interface, and decreased the spin-off of the lubricant on a high rotation of the disk. The increase of the physisorbed layer leads to a higher recovery of the lubricant defect by the wear, and the increase of functional groups on the carbon overcoat surface leads to a stronger interaction of the lubricant with the carbon overcoat surface is estimated.     

脂润滑轴承静置状态下漏油机理及对策

脂润滑轴承在静置状态下的漏油问题一直是一个难题。枯文旨在地润滑脂的分油及基础油在固体壁面的迁移分析,来探求脂润滑轴承润滑剂泄漏机理;同时介绍了在润滑脂中加入氟表面活性剂、改善轴承壁面结构设计两项措施来降低脂润滑轴承在静置状态下的漏油。     

Study of the Adsorption of Siloxane and Hydrocarbon Contaminants onto the Surfaces at the Head/Disk Interface of a Hard Disk Drive by Thermal Desorption Spectroscopy

The adsorption of siloxanes and hydrocarbons, which are common harmful contaminants found on hard disk drives (HDDs), has been investigated by thermal desorption spectroscopy (TDS). The desorption energy of each molecule from various modeled surfaces at the head/disk interface (HDI) was determined. The surfaces studied in this literature were (a) hydrogenated amorphous carbon (a-C:H), (b) a-C:H with perfluoropolyether (PFPE) and (c) a nascent surface of a-C:H. Considerable variations in the monolayer desorption energy of a siloxane model compound were observed. The monolayer desorption energy of a siloxane model compound from a plain a-C:H surface is higher than that from an a-C:H surface with perfluoropolyether, while that from a nascent a-C:H surface is far higher than that from an a-C:H surface. Meanwhile, the monolayer desorption energy of a hydrocarbon model compound from a-C:H is a little bit higher than that from a-C:H with perfluoropolyether. These results suggest that the adsorption of these harmful contaminants can be decreased by the control of HDI surfaces, and therefore we can reduce the risk of failure of HDDs.     

Sliding-pin shapes and lubricant thickness change on a thin-film magnetic disk

In developing hard disk drives, it is necessary to keep lubricant as thick as possible during operations. For this purpose, we studied lubricant loss under different-shape contact-sliders on thin-film magnetic disks by using transparent-pin sliding tests with a built-in ellipsometer. We compared sliding pins with spherical, flat circle, flat square, and double-flat-rail surfaces.We found that lubricant loss was smaller under flat pins than under the spherical pin, and the smallest under the double-flat-rail pin among flat sliders. The results show that the horizontal and vertical shapes of sliders must carefully be selected for contact recording systems.     

Effect of environment humidity and temperature on stationary and transient flying responses of air bearing slider

It is well known that the environment humidity and temperature have a significant influence on the flying height of an air bearing slider. However, not many research papers address this topic, especially when the transient flying response is considered. This paper studies the influences of the environment humidity and temperature on both the stationary and transient flying responses of slider by simulation. A slider design for the thermal protrusion application is addressed. The reason for causing the drop of the air bearing pressure is discussed, and the methods for decreasing the drop are proposed. It is observed that the environment humidity and temperature may determine whether the slider is in full flying state or in partial flying/partial dragging state, when the slider is released from a certain height. The reason may be due to the high humidity and temperature which weakens the air bearing. As a result, the air bearing becomes not strong enough to support well the full flying of slider when the influence of the intermolecular force is significant. Slider vibrations for the full flying case and the partial flying/partial dragging case are analyzed in frequency domain, and the slider vibration frequencies are discussed. It shows that the environment temperature and humidity have significant effects on both the stationary and transient flying responses of the slider.     

Analysis and measurement of torque hysteresis of pivot bearing in hard disk drive applications

During hard disk drive (HDD) operation in the track-following, short seek and seek settling modes, the motion of the actuator, which is supported by a pivot bearing cartridge, has very small amplitudes. The effect of hysteresis friction from the pivot bearing becomes significant, impacting servo performance. Often such effects are not well understood and thus, it is not being taken into consideration during servo design. A study of the hysteresis friction behaviour of pivot bearings is presented. An integrated test apparatus for pivot bearing analysis was designed and built where a Laser Doppler Vibrometer (LDV) was used to accurately measure the displacement and velocity of the actuator for feedback control. Using this test apparatus, the hysteresis friction behaviour in both frequency and time domain was analysed. The effects of pre-load, oscillating magnitude and oscillating frequency were investigated. The results obtained can now be used for servo design and pivot bearing evaluation.     

开放式数控系统的数字伺服接口和通讯协议

开放式数控系统及数字伺服通讯技术是９０年代数控技术的主要发展方向之一,重点介绍欧洲的ＳＥＲＣＯＳ协议和北京航空航天大学在其开放式数控系统上采用的ＡＤＤＰ协议。     

固体润滑涂层在干摩擦及有油条件下的摩擦磨损性能

采用MRH-3环块磨损试验机对FM-510二硫化钼润滑涂层在于摩擦及有油条件下进行了摩擦磨损性能的考察和评价,评价结果表明：该涂层在干摩擦条件下具有低的摩擦系数、高的承载能力和长的耐磨寿命,摩擦系数随负荷增高而降低,随速度提高也降低。摩擦偶对双面涂膜比单面涂膜有更长的耐磨寿命,速度低时涂层的磨耗小,寿命长,可满足特定条件下的干摩擦工作要求,在有油润滑条件下二硫化钼基的FM-510润滑涂层可显减轻对偶磨损程度,摩擦系数比单独使用油润滑时大大降低。在难以形成连续的流体润滑薄膜,亦即不能形成流体动力润滑的情况下。摩擦偶对涂敷固体润滑涂层是解决其润滑问题的有效方案。