The Head/Tape Interface: A Critical Review and Recent Results

Modeling and measurement techniques for the head/tape interface are discussed. With the advent of contact between the head and tape, both the numeric models and experimental procedures need to be modified to account for the contact. In the field of head/tape interface modeling, the basic Green's function, finite difference, and finite element solution techniques are presented. The asperity compliance curve is introduced to calculate the contact pressure between the head and tape resulting from compression of surface asperities. Modern experimental measurements of head/tape spacing use monochromatic interferometry. The basic theory is presented, and the need for multi-wavelength interferometry is described to improve measurement resolution at extremely close spacings.     

Effect of Interchanging Tapes and Head Contour on the Durability of Metal Evaporated,Metal Particle and Barium Ferrite Magnetic Tapes

Interchanging metal evaporated in 120-minule cassettes (ME-120) and particulate [metal particle (MP) and barium ferrite (BaFeO)] tapes in a tape recorder caused excessive head and tape wear while a tape formed its preferred head shape in these experiments. The fundamental cause of this phenomenon was that the tapes formed different head shapes during tape recorder operation. To extend the usable lifetime of a rotary head recorder, and to minimize the extent of tape damage, as well, as to preserve the integrity of the recorded information, the authors recommend avoiding the practice of interchanging ME-120 and particulate tapes in a tape recorder. The interchanging of ME-180 (180-minute cassette) tape with ME-120 and particulate tapes is not recommended either, but apparently does not lead to excessive head and tape wear. Thin ME-180 tape was inefficient in significantly changing the head contour. This study suggests that the head contour is a determining factor for the performance of a tape in a tape drive. In the authors study of tape compliance using ME tapes, the highly compliant ME-180 tape gave more uniform head wear, lower fiction and slightly better durability than the lower compliance ME-120 tape.     

Effects of Gas Composition,Humidity and Temperature on the Tribology of the Head/Disk Interface—Part II: Model and Analysis

A qualitative model for the effect of water condensation on the frictional behavior of unlubricated and lubricated carbon-overcoated disks is presented. The model suggests that for unlubricated disks adsorbed water acts as a lubricant, protecting the unlubricated disk surface from direct solid/solid contact and direct exposure to the environment. For lubricated disks, the interaction between adsorbed water and lubricant molecules seems to be responsible for the effect of humidity on the frictional behavior of lubricated disks. The effect of temperature on the frictional behavior of the head/disk interface is discussed in terms of surface energy, lubricant viscosity and mobility.     

The Effects of Viscosity and Clearance on the Performance of Hydrodynamic Journal Bearings

In the present work, the effects of viscosity and clearance ratios have been analyzed on the reliable performance and design of a steady-loaded, pressure-fed, hydrodynamic cylindrical bearing. The technique for bearing performance evaluation has been developed based on the maximum oil temperature in the region of load carrying oil film, and variation of oil viscosity with temperature.

In this paper viscosity coefficients have been determined using iterative procedures for different oils. And also, a viscosity integral has been evaluated by method of splines as a function of inlet oil temperature to the load carrying oil film and exit oil temperature from the oil film. The viscosity integral has also been evaluated for a bearing operating under different conditions. Thus, by comparison of the viscosity integrals, exit oil temperature from the load carrying oil film and safe load carrying capacity of a bearing with different clearance ratio and using different oils have been examined. Nomographs have also been plotted for easy assessment of bearing performance.

The developed technique gives a more realistic approach for design and performance evaluation of a bearing as compared to conventional procedures. This may have potential as an effective tool to assess performance of a hydrodynamic journal bearing.     

Metal Core Recession and Head Stain Studies of MIG Heads Sliding against Cobalt-Doped Gamma Iron Oxide and Metal Particle Tapes

Metal-in-gap (MIG) heads are commonly used for high-density magnetic recording. Metal core recession and head stains increase the gap between the tape and the head, resulting in signal loss. In this study, accelerated sliding wear tests of Co-γFe₂O₃, and metal particle (MP) tapes against MIC heads made of three different amorphous and, nanocrystalline metals were conducted under various operating conditions. Metal core recess and propensity for head staining were measured. The degree of tape contact with a recessed core was also measured by pressing the tape against a glass slide with Cr grids and using an optical interference technique. The authors found that the core recess was about the same for all three core metals. Core recess by MP tape was larger than that, by oxide tape. The tape speed appeared to have little effect on the recess value. The authors also found that significant recess may occur during initial contouring of the head surface; however, after sliding for about 250 km, core recess reaches a steady-stale value which may be either higher or lower than the initial values. The mechanism of core recession was studied. The authors believe that core recession occurs as a consequence of the debris trapped, between the tape and the core, in addition to that caused by some tape contact. Formation of head stains was observed in all combinations. The stains fanned on the metal cores were heaviest for CoNbZr metal, followed by CoTaZr and FeTaC. Co-γFe₂O₃ tape produced a more severe stain than the MP tape. The apparent roughening of head rubbing surface observed for CoNbZr heads sliding against Co-γFe₂O₃, tape was due to the formation of stains.     

Cam and Tappet Lubrication III — Radioactive Study of Phosphorus in the EP Film

A radioactive tracer, phosphorus-32 synthesized into zinc dialkyl dithiophosphate molecules, was used to study the EP films formed by this motor oil additive on cast iron cams and tappets. The tightly bound films, which contain phosphorus and zinc, formed rapidly and then approached steady-state values in static and dynamic tests. The amount of film increased with oil temperature (200 to 300F) and with available surface area. Dynamic test conditions greatly increased the amount of film formed, indicating the marked effect of surface temperature, pressure or wear. The film is maintained on parts run in additive oil; moreover, the film was not easily worn off in non-additive oil. The chemical and physical properties of the films appeared to vary with the conditions of film formation. Radioactive counting was used to determine the film on tappets and the thermal stability of the compounded oil. Autoradiographs showed the concentration and distribution of the film on cams and tappets.     

Effect of Disk Flexibility on Rocking Mode Frequencies of a Disk Drive Spindle Motor System

The lowest natural frequency of a hard disk drive related to a spindle motor is the so-called “rocking mode,” the frequency at which the rotor of the motor is whirling conically. A traditional rotordynamics theory with thick/rigid disks is not able to predict the rocking mode frequencies correctly. In this paper, the rocking mode frequencies of a single-disk rigid-rotor motor were solved analytically, including the effects of disk flexibility and ball bearing contact angle. In addition, the disk flexibility model was integrated to a finite element-based (FEM) flexible-shaft rotordynamics computer program. The closed-form solution of the analytical model provides design engineers a tool to identify critical design parameters. It was found that the rocking mode frequency, which is mainly affected by the clamped condition at the inner radius of the disk, is the flexible disk one-nodal-diameter mode frequency reduced by the compliance of ball bearings. The results obtained from both the analytical model and the FEM computer program matched the experimental results with an error less than two percent. Neglecting the effect of ball bearing contact angle increases the error three times. The effects of design parameters were also investigated.     

Speed Dependence,Takeoff and Stiction of a New Class of Gas Bearing Sliders

While searching for a less-speed-sensitive programmable air bearing slider, the author noted that the primary reason for the sluggish takeoff and large dependence of fly height on speed is the very common 10 mrad taper. This taper is found on many sliders in the railed, taper/flat class. A single, plane wedge bearing operating at a large bearing number and a pitch angle considerably below that which gives maximum lift does not suffer from such speed sensitivity. The “inlet throttled” condition of the bearing inlet fixes the mass flux into the bearing gap. Because a single pad of this type has poor pitch stability, the author placed two of these pads in a tandem configuration, and abandoned rails. The result defines a new air bearing class, called Tango, which has front and rear pads, each having inlet throttled leading edges. Some members of this class offer takeoff velocities equalling and even improving more-complex designs using subambient pressure. In addition, numerical and hardware experiments have verified the considerably reduced sensitivity of fly height to speed, a reduced sensitivity to crown distortion, and improved stiction. Disk designers benefit by not having to accept a longer takeoff run when the fly height decreases.     

Contact Analysis of Non-Gaussian Surfaces for Minimum Static and Kinetic Friction and Wear

Most statistical contact analyses assume that surface heights and peak (summit) height distributions follow a Gaussian distribution. However, engineering surfaces are frequently non-Gaussian with a degree of non-Gaussian character dependent upon materials and surface finishing processes used. For example, magnetic rigid disk surfaces used in magnetic storage industry are highly non Gaussian. The use of a Gaussian analysis in such cases can lead to erroneous results. This study for the first time presents a method to carry out a statistical analysis of non-Gaussian surfaces. Real area of contact, number of contacts, contact pressure and meniscus force (in wet interfaces) are calculated for probability density functions having different skewness and kurtosis. From these curves, the optimum value of skewness and kurtosis can be predicted for minimum static/kinetic friction. It is found that a range of positive skewness (between 0.3–0.7) and a high kurtosis (greater than five) significantly lower the real area of contact and meniscus contribution implying low friction and wear. Also, sensitivity of film thickness to static friction goes down for a surface with a positive skewness and a high kurtosis.     

Stability of Gas Bearing Sliders for Large Bearing Number: Convective Instability of the Tapered Slider©

The dynamics and stability of tapered air bearing sliders used for computer hard disk drive magnetic recording heads is examined using analytical methods. Lubrication theory is applied to determine the lift on the slider from the Reynolds equation in the limit of large bearing number. The dynamics of the slider are given by a nonlinear integro-differential equation. Linear stability analysis of this model yields valuable insight into the behavior of the slider. Most significantly, it is found that convective effects can not be neglected and yield either damping or instability depending on the slider configuration. This analysis is also applied to determine the response of the slider motion due to deviations in the disk surface.     

Lubricant Bonding Effects on Thin Film Disk Tribology

The magnetic recording industry predominantly uses Zdol to lubricate the carbon overcoat of magnetic recording disks. Zdol comprises a perfluoropolyether chain terminated with hydroxyl end groups that are capable of reversibly bonding to the carbon overcoat. Contact start/stop (CSS) tests were done to investigate the effects of Zdol lubricant bonding, thickness, and relative humidity on durability. The durability improved with increasing thickness of fully bonded or mobile Zdol. The durability decreased with increasing initial bonded fraction and with decreasing relative humidity. The bonded fraction increased with time during the tests at elevated temperature and low relative humidity.     

Carbon-Coated Sliders and Their Effect on Carbon Oxidation Wear

Alumina-titanium carbide composite sliders used in magnetic recording were coated with diamondlike carbon (DLC) to permit exploration of their effect on tribochemical wear. For comparison, testing was performed on both coated and uncoated sliders. Gases sampled directly from the sliding interface between a carbon-coated thin-film magnetic recording disk and the slider contained carbon dioxide in both dry nitrogen and dry oxygen environments. In the oxygen environment, uncoated sliders produce carbon dioxide at a rate 10 times greater than coated sliders. This suggests that catalysts in the slider composite material are necessary for carbon oxidation wear.     

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.     

Wear Measurements for Proximity Recording Heads

A focused ion beam is used to create wear fiduciary marks on subambient pressure air bearing sliders, and both atomic force microscopy and optical profilometry are used for localized wear measurement. Optical profilometry exhibits a σ_e (standard error) of 0.23-0.36 nm compared to 4.2-11.8 nm for the AFM, where the range results from differences in measurement location. Data from both measurement methods are used to examine wear behavior as a function of contact start/stop cycling. A group of test heads is oriented such that wear is expected to occur on an alumina surface at nominal operating speed. Wear results, evaluated by both AFM and optical profilometry, show patterns consistent with the flying attitude of the heads and excellent agreement between the two measurement techniques on the worn parts.     

Concepts and Limitations of Using Optical Profilometry for Air Bearing Simulations of Hard Disk Sliders

The topography of hard disk sliders is commonly measured with optical profilometers that produce a regularly spaced matrix of topography data corresponding to the pixel array of the CCD camera. This paper explores a new concept that combines optical profilometry and air bearing simulations to achieve an accurate method for predicting fly height early in the manufacturing process. Various sources of error in the topography measurement are analyzed with regards to their impact on the accuracy of air bearing simulations. The findings are illustrated by a feasibility study based on numerical simulations of a slider with a negative pressure air bearing design.     

摩擦制动系统摩擦表面的摩擦学研究

基于系统分析的方法,建立了摩擦制动系统的摩擦学模型,分析了系统结构随时间和外界条件变化的特牲。对摩擦表面的摩擦学行为作了系统的分析,揭示了制动过程中摩擦力在不同条件下的组成机制。     

Critical Operating Conditions For A Novel Flying Tester

Increased recording densities are often achieved through a reduction in the flying height over a thin film disk possessing diminishing surface roughness. Flying heights will continue to decrease until the head-disk interface (HDI) operates under quasi-contact conditions, i.e., ultra-low flying with intermittent slider-disk collisions. The failure mechanisms that occur in such quasi-contact devices may differ from those experienced in current, higher flying hard drive assemblies. In this paper, the authors will present the experimental, numerical, and theoretical tools that have been developed to study the behavior of the HDI under ultra-low flying conditions. These tools include an accelerated flyability tester and a numerical algorithm applicable to highly rarefied air bearings that possess large pressure gradients. Air bearing simulation results, as well as the results from a simple flying height scaling analysis, will be compared to flying test results in both air and helium to obtain insight into the stability of the HDI under accelerated testing conditions. A new concept introduced in this paper is that of critical conditions, i.e., the band of operating conditions which mark the transition from stable to erratic behavior, which can be determined both experimentally and theoretically. Such insight should provide design criteria for both quasi-contact storage devices, as well as novel accelerated wear testers.     

Numeric Simulation of Friction Phenomenon at Soil-Tool Interface

Tillage is an essential agricultural operation in which friction occurs at the soil-tool interface. The distribution of normal force, tangential force and tangential displacement at the soil-tool interface at different travel speeds was studied by using the finite element method. In addition, a method was proposed to evaluate the maximum potential savings of the tool draft.     

Externally Pressurized Journal Gas Bearings

Externally pressurized gas-lubricated bearings with multiple orifice feed are investigated. An analytical treatment is developed for a semicylindrical bearing with 9 orifices and for a cylindrical journal bearing with 192 radial and 24 axial orifices.

Experiments are described on models of the two bearing configurations with specially designed fixtures which incorporate pneumatic loading and means for determining pressure profiles, gas flow, and gap height. The correlation between theory and experiment is satisfactory.