Tribological performance of nano-thin fluoropolymer coated sliders

With the increase in recording density, any accumulation on slider surface can cause serious problems, such as high fly–stiction and extensive slider–disk interaction. Therefore, how to mitigate the accumulation on slider surface and thus, improve the stability and reliability of head–disk interface is becoming an important issue. In this work, a nanothin fluoropolymer overcoat with very good oleophobic and hydrophobic properties was applied on slider surface with a dipping process. Tribological performance, such as fly–stiction, normal stiction, and takeoff and landing processes, of the coated sliders was studied. Test results show that although normal stiction is not lowered, normal stiction modulation is reduced obviously by the overcoat. Fly–stiction and its modulation of coated sliders are much smaller than those of uncoated sliders. Coated sliders show much better takeoff and landing performance during contact start stop tests. After tests, the surfaces of tested sliders and disks were examined with an optical microscope, surface reflectance analyzer, and TOF–SIMS to interpret the tribological performance of the coated sliders. It can be concluded that the fluoropolymer overcoat reduces the amount of accumulation on slider surface and thus, improves the tribological performance of the coated sliders.     

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.     

重型清障车吊臂结构设计与优化

在对某一托吊分离式重型清障车吊臂结构设计的基础上,利用ANSYS软件对一定滑块尺寸的设计结构进行静力分析,对局部强度不足部位提出了改进措施。以滑块的长度尺寸和宽度尺寸为参数,以吊臂与滑块接触部位的最大应力满足强度要求为约束条件,应用ANSYS软件中提供的参数化设计语言(APDL)对滑块的结构尺寸进行参数化建模与求解。得到了滑块两个几何尺寸单独变化和联合变化时对应的应力变化曲线,找到了如何通过增大滑块尺寸来减少吊臂接触部位应力的最有效办法。     

A Computational Study of Contact Mechanics between Magnetic Recording Heads and Thin-Film Disks

The tribological phenomena between magnetic head sliders and thin-film disks are studied during rest, during start-up and shutdown, and during regular operation using numerical simulations. A theoretical model of stiction is first derived based on the surface tension of liquid and capillary condensation. Next, a hybrid method of FEM and BEM is developed in order to carry out high-precision microscopic contact stress analysis of a disk covered by multilayer thin films. An impact simulator based on FEM is then presented which considers the air lubricant film to be an elastic spring system determined from the solution of a compressible Reynolds equation, ft is found, that these simulators can be used to effectively study the tribology of slider/disk interactions and that the results can be used to establish qualitative design criteria for slider/disk mechanisms.     

Effect of peak radius on design of W-type donut shaped laser textured surfaces

Friction/stiction behavior of ultra high density magnetic disk drives can be controlled by controlling the size and shape of the laser bumps using a laser texturing tool. Different laser bump parameters like rim radius, rim height, peak radius and number of bumps (under the slider) play an important role in the design of laser textured disk surfaces. In the present study, an algorithm is developed for generation of W-type laser bumps on the computer. W-type laser surfaces with a Gaussian height distribution have been generated and contact analyses of these surfaces have been carried out. Design curves have been generated to calculate critical number of asperities required to minimize wear and stiction. Effect of the coefficient of friction on the number of bumps has also been studied.     

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).     

Acoustic emission studies of thin film rigid disks and proximity recording sliders and its applications to tribology

This work investigates the flying and contact phenomena of proximity contact recording sliders and their effects on the tribological performance of thin film media by the use of acoustic emission analysis. Proximity contact recording sliders included negative pressure and tri‐rail types of tripad sliders. Mechanical and laser‐induced textured magnetic rigid disks were used in this study. The laser bump heights of laser textured disks were controlled to achieve a desired tribological performance and the relationship between the laser bump heights and acoustic emission energy during drag and start/stop testing for different slider designs was studied. Emphasis was also placed on identifying the critical parameters of media and slider designs for tribological performance improvement. An analysis of the slider body natural frequencies during flying at the operational speed has been demonstrated to be well correlated to the contact behavior at the head‐disk interface. Moreover, the environmental and lubrication effects on the fixed tracking flyability performance were investigated and the acoustic emission data also agreed fairly well with the observed degree of contamination on sliders and degradation on the textured media. This revised version was published online in July 2006 with corrections to the Cover Date.     

Improving the performances of ultrasonic motors using intermittent contact scheme

Most ultrasonic motors operate in intermittent contact scheme. Their stators drive the rotors (or sliders) when the stators contact the rotors, and the rotors (or sliders) move under an inertia force when the stators and the rotors are separated. The duty cycle of the contact and the “flight” manages motors’ output performance. To obtain a large output force or output velocity, this paper proposes a concept using the alternative work of a multistator or the multi-driving end of a single stator. The method can avoid larger noise, poor efficiency, and lifetime of motors. A novel linear ultrasonic motor using the alternative work of the multi-driving end of a single stator was fabricated and investigated experimentally. The traveling speed without load of the slider is 88 mm/s, and the maximum load is 0.32 N.     

The Effect of Humidity on Low-Load Frictional Properties of a Bonded Solid Film Lubricant

Low-load slow-speed sliding friction tests were conducted on a MoS₂/graphite (90/10) bonded solid film lubricant in controlled-humidity environments. The average steady-state coefficient of friction increased from 0.23 to 0.28 as relative humidity (RH) was raised from 10 percent to 50 percent, and it increased to 0.6 when RH reached 90 percent.

At RH ≥80 percent, friction response was strongly influenced by chemically reactive metals used for slider and lubricant substrate. Thus, friction with carbon steel sliders was significantly lower than with stainless steel sliders, although corrosion of the former caused rapid destruction of the lubricant film. Also, continued sliding by stainless steel on lubricant applied to phosphor-bronze eventually resulted in a significant friction decrease and simultaneous formation of a dark red-brown film in the wear track. The proposed explanation for these effects is based on oxidation of MoS₂ at the sliding interface and reaction of the oxidation products with slider and substrate metals.     

Improving the performances of ultrasonic motors using intermittent contact scheme

Most ultrasonic motors operate in intermittent contact scheme. Their stators drive the rotors (or sliders) when the stators contact the rotors, and the rotors (or sliders) move under an inertia force when the stators and the rotors are separated. The duty cycle of the contact and the “flight” manages motors’ output performance. To obtain a large output force or output velocity, this paper proposes a concept using the alternative work of a multi-stator or the multi-driving end of a single stator. The method can avoid larger noise, poor efficiency, and lifetime of motors. A novel linear ultrasonic motor using the alternative work of the multi-driving end of a single stator was fabricated and investigated experimentally. The traveling speed without load of the slider is 88 mm/s, and the maximum load is 0.32 N.     

Wear of single crystal silicon as a function of surface roughness

With increasing activities on micro electro-mechanical systems (MEMS) type microactuators, there is a growing need in understanding the tribological properties of silicon, the most commonly used wafer material for those devices. In particular, it is of interest if single crystal silicon used in microactuators exposed to rather low vertical loads is subject to wear. Therefore, wear tests using monocrystalline silicon on both sides of the tribological interface were conducted.A classic way to measure wear on sliders in contact with a tape or a rotating disk is to create an imprint using a Berkovich diamond tip mounted on a picoindenter. However, in our case we used a different approach. We created three studs on a slider’s surface by recessing the material outside the studs through an ion milling process. During the wear tests, the studs wore off. By measuring the remaining stud height, the wear volume could be determined at any point in time. The tests were performed on a pin on disk tester with a gimbaled slider to realize a flat on flat contact and a rather low normal force of 30 mN.     

Influence of oxygen on the wear of silicon

The results of a series of wear tests on semiconductor silicon chips are presented. In these tests, hemispherical anodized aluminum sliders rubbed against flat silicon specimens with a small-amplitude oscillatory motion. The influence of several parameters, including the load, the slider roughness, the type of anodization, the contact geometry, the environment and the initial condition of the silicon surface, on the wear of the silicon was studied. Of these the influences of the environment and the initial condition of the silicon surface were most pronounced. It was concluded that oxide formation and growth on the silicon were important factors in the wear of the silicon and tended to increase the wear. It was also concluded that the primary wear mechanism of the silicon for low load is probably delamination or surface fatigue.     

Stiction problem of annular-shaped laser textured bump on a hard disk

The stiction force strongly depends on the bump parameters. Numerical simulation has been used to estimate the stiction force. Several schemes have been proposed. In these schemes, the relation between the elastic deformation of bumps and the load plays a very important part. To estimate the stiction force of annular shaped bumps, the solution of the cylindrical contact problem of Hertz theory is used. But this solution doesn't converge so that it may not give the correct solution for the annular contact problem. This paper presents an exact solution of elastic deformation of laser textured bumps for the annular type. The exact and precise solution is obtained numerically and analytically. The contact problem is reduced to a pair of integral equations of the Fredholm type. It can be solved by a simple numerical method. The solution can be used for the numerical estimation of the bump deformation during the CSS process. The results are compared with former researches.     

Large Bearing Number Stability Analysis for Tango Class Gas Bearing Sliders

Air bearing sliders in the Tango class use load bearing pads with inlet-throttled leading edges to control the mass flux and lift. The influence of leakage or diffusion effects is always present in real sliders. In some designs such as railed taper flat designs leakage is dominant. The behavior of such sliders must be determined with numerical methods that obscure deeper understanding. Many aspects of the behavior of Tango class sliders can be understood with the vast simplification allowed by inlet throttling. In this paper such a simplified analysis is applied to describe a complete air bearing slider composed of two pads. The conditions for static level flight are determined, as well as the linear stability of heaving and pitching oscillations. Both stable and unstable modes are identified. Either damping or amplification can result from convective effects in the absence of mechanical damping. In real implementations of Tango class sliders instability has not been observed thanks to diffusion. The present analysis can serve as a guide to select initial choices for the operating parameters that correspond to maximum convective damping.     

Effect of self-assembled monolayers modified slider on head-disk tribology under volatile organic contamination

Self-assembled monolayers (SAMs) with different endgroups were established on slider surface. The effect of the SAMs coated slider on head-disk tribology under volatile organic contamination (VOC) of octamethylcyclotetrasiloxane (D₄) was investigated using a contact start/stop (CSS) tester. The slider surfaces before and after the CSS tests were analyzed using Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS). The contact angle measurement and TOF-SIMS analysis proved that the SAMs were successfully formed on the slider surface. All the SAMs reduced the friction under the pollutant vapor. The transfer of lubricating oil onto the slider surface was detected after the CSS tests. It was found that the slider with a low surface free energy associated with small amount of lubricating oil transfer. The little the lubricating oil transfer was, the low the frictions were. These results indicate that a slider with low surface free energy can reduce the loss of lubricating oil from the disk surface, and hence improve the tribological properties of hard-disk interface (HDI) under VOC.     

Experimental and theoretical evaluation of friction at contacting magnetic storage slider–disk interfaces

To understand better the friction force and wear processes at contacting slider–disk interfaces, we have developed an experimental method for measuring and a theoretical method for calculating the friction force. For this study, a slider with a 1500 μm² contact pad located at the recording head is burnished against a relatively rough disk (～12 Å rms), which ensures smooth sliding. In the experimental method, the friction force is measured as the disk is spun-down to bring the slider–disk interface into an increasing degree of contact. A modified air bearing code is used to determine the experimental normal contact force for each friction measurement. In the theoretical method, the friction force and other relevant interfacial forces are calculated using an improved sub-boundary lubrication (ISBL) rough surface model. The friction force calculation in this model is based on the force needed to induce yielding of the individual disk asperities contacting the flat surface of the contact pad without any assumption of the coefficient of friction. Good agreement is found between the measured and theoretical friction vs. normal contact force curves, indicating that the model is capturing the essential origins of friction at this interface. The model also provides valuable insights into how wear particles may be generated at this contacting slider–disk interface.     

Elastic contact with a “donut”-shaped asperity

The laser-textured surfaces used for the touchdown area of computer hard-disks are sometimes covered with asperities consisting of a crater surrounded by a raised rim; contact with the read-head takes place over the rim of the crater, colloquially referred to as a “donut”. In order to analyse the load/compliance relation or the stiction to be expected in contact of hard disks, a number of authors have proposed load/compliance relations for contact between such a single doughnut and a plane, usually as simple modifications of the Hertz line contact equations. In this note simple, asymptotically correct, relations for a ring asperity are derived and verified by direct solutions. In particular, the relation between elastic deflection and load is approximately δ=(W/π²RE^*)[ln(16R/b)+0.5)].     

Contact force studies of a burnishing slider

In order to design the flying height of a burnishing slider accurately, the contact force between the burnishing slider and the disk needs to be well evaluated. This paper studies the contact force of a burnishing slider by both experiment and simulation. The experiment is conducted by measuring the acoustic emission signals of the contact force avalanche, and the simulation is based on the self-developed air bearing surface simulation code applying the probability model for the contact force calculation. The influence of contact force on the burnishing effect is discussed. It is observed that the simulation results are well correlated with the experimental measurements. It is believed that the simulation code is capable to design burnishing sliders with reasonable accuracy.     

Shape of a worn slider

A slider can be expected to wear in due time to a shape that gives a uniform contact pressure and consequently uniform friction stress. Computation of the worn shape is discussed using the theory of elasticity. The results show that the trailing edge may be worn down more than the leading edge. The worn shape at a sharp corner may be either convex or concave depending on the elastic constants, the friction coefficient and the corner angle.     

About Joint Deformations of Asperities and the Destruction of Rough Surfaces During Friction

The mechanics of the deformations and the destruction of the asperities of rough surfaces has been considered, taking into account the mutual influence of the elastic constants of the materials participating in the friction couple.

The case when the destruction is taking place inside of the contact layer is assumed to have the highest probability. The tight bear of the peaks of one body against the valleys of the other and the adhesion phenomena are leading to a mutual deformation of the prevailing number of asperities of both surfaces. This allows one to consider the contact layer as a composite body consisting of two solid phases and voids.

It has been shown that such an approach does not exclude the possibility of shearing off only the soft material of the couple.

For the asperities subject to bending deformations, the mutual influence of the elastic constants is conditioned by the mutual angle of rotation of the cross sections.

In this case consideration of the tangential stresses in the destruction of the short bars (asperities) is essential.