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.     

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.     

Investigation on Contact Mechanics Between Soil and Metal

Soil-metal contact interaction is common with machinery operations in agricultural production. In this paper, a method has been proposed to determine the stress-strain relation of soil at soil-metal contact surface with a modified triaxial apparatus, and a constitutive model is developed for numerical analyses of such kind of contact problems.     

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.     

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.     

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.     

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.     

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.     

A Critique of Rolling Contact Fatigue Criteria for Bearing Rings with Hoop Stresses

Tensile residual and interference fit stresses not treated in classical bearing formulations are known to reduce bearing rolling contact fatigue (RCF) life. Recent modifications of such theory to account for these stresses have simply included them in the computation of a single yield stress type criterion—either maximum shear or equivalent stress. An alternative modification is proposed and demonstrated for fatigue crack initiation that recognizes the primary influence of the maximum range of shear stress but includes the effect of normal stress on the critical planes, as in other successful bulk fatigue criteria for multiaxial nonproportional stress cycle fatigue.     

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.     

On the Modeling of Elastic Contact between Rough Surfaces

The contact force and the real contact area between rough surfaces are important in the prediction of friction, wear, adhesion, and electrical and thermal contact resistance. Over the last four decades various mathematical models have been developed. Built on very different assumptions and underlying mathematical frameworks, model agreement or effectiveness has never been thoroughly investigated. This work uses several measured profiles of real surfaces having vastly different roughness characteristics to predict contact areas and forces from various elastic contact models and contrast them to a deterministic fast Fourier transform (FFT)-based contact model. The latter is considered “exact” because surfaces are analyzed as they are measured, accounting for all peaks and valleys without compromise. Though measurement uncertainties and resolution issues prevail, the same surfaces are kept constant (i.e., are identical) for all models considered. Nonetheless, the effect of the data resolution of measured surface profiles will be investigated as well. An exact closed-form solution is offered for the widely used Greenwood and Williamson (GW) model (Greenwood and Williamson, Proceedings of the Royal Society of London A, vol. 295, pp. 300–319), along with an alternative definition of the plasticity index that is based on a multiscale approach. The results reveal that several of the theoretical models show good quantitative and qualitative agreement among themselves, but though most models produce a nominally linear relationship between the real contact area and load, the deterministic model suggests otherwise in some cases. Regardless, all of the said models reduce the complicated surface profiles to only a few key parameters and it is therefore unrealistic to expect them to make precise predictions for all cases.     

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.     

电传动推土机履带-地面接触力学仿真分析

采用多体动力学仿真软件RecurDyn的履带车辆子系统Track(LM)建立履带式电传动推土机多体动力学模型,通过Ground模块建立地面模型,并对其在干砂路面和黏土路面工况下直线推土作业进行了动力学仿真,研究分析了推土机在不同作业工况下整机的质心、履带接地长度、履带接地比压和滑转率随切土深度的变化规律,以及推土机在推...     

Effects of Friction on the Contact and Deformation Behavior in Sliding Asperity Contacts

Finite-element analyses are carried out to study the effects of friction on the contact and deformation behavior of sliding asperity contacts. In the analysis, on elastic-perfectly-plastic asperity is brought in contact with a rigid flat at a given normal approach. Two critical values of the normal approach are used to describe the asperity deformation. One is the approach corresponding to the point of initial plastic yielding, and the other at the point of full plastic flow. Additional variables used to characterize the deformation behavior include the shape and size of the plastic zone and the asperity contact size, pressure, and load capacity. Results from the finite-element analysis show that the two values of critical normal approach decrease significantly as the friction in the contact increases, particularly the approach that causes plastic flow of the asperity. The size of the plastically deformed zone is reduced by the friction when the contact becomes fully plastic. The reduction is very considerable with a high friction coefficient, and the plastic deformation is largely confined to a small thin surface layer. For a low friction coefficient, the contact size, pressure and load capacity of the asperity are not very sensitive to the friction coefficient. For a moderate friction coefficient, the contact pressure is reduced and the junction size increased; the load capacity of the asperity is not significantly affected due to the compensating effects of the pressure reduction and the junction growth. For a high friction coefficient, the pressure-junction compensation is not longer sufficient and the asperity load capacity is reduced. The degree of the friction effects on these contact variables depends on the applied force or the normal approach. Although the analyses are conducted using a line-contact model, the authors believe that the effects of friction in sliding asperity contacts of three-dimensional geometry are essentially the same and the same conclusions would have been reached. These results may provide some guidance to the modeling of rough surfaces in boundary lubrication, in which the asperity friction coefficient can be high and vary significantly both in time and from one micro-contact to another.     

The Effect of Scale-Dependent Hardness on Elasto-Plastic Asperity Contact between Rough Surfaces

Statistical methods are used to model elasto-plastic contact between two rough surfaces using a recent finite element model of elasto-plastic hemispherical contact and also recent advances in strain gradient modeling. The elasto-plastic hemispherical contact model used to model individual asperities accounts for a varying hardness effect due to deformation of the contact geometry that has been documented by other works. The strain gradient model accounts for changes in hardness due to scaling effects. The contact between surfaces with hypothetical material and surface properties, such as the elastic modulus, yield strength, and roughness are modeled. A model is also constructed to consider a variable asperity contact radius to evaluate if the strain gradient model will affect it differently. The models produce predictions for contact area, contact force, and surface separation. The strain gradient effects decrease the real area of contact and increase the average contact load in comparison to the model without these effects. The strain gradient model seems to have a larger influence on the predictions of contact load and area than does considering a variable asperity contact radius for the cases considered in this work.     

FEM Comparison of Ball and Roller Bullgears

A structural finite element model has been developed for calculating the forces transmitted through the rolling elements (load distribution) in a bullgear assembly. The elastic structural model consists of 3-D beam elements used to approximate the global race deflection and non-linear springs that approximate the combined rolling element/raceway contact deflections. For rollers, an upperbound on the contact stress (assuming linear variation of force along the length of the roller) is estimated by modeling the rollers as pairs of nonlinear springs. The finite element approach iteratively solves the contact forces at each, rolling element. Contact stresses are then calculated from the contact, forces using Hertz contact theory. This approach is applied to analyze two proposed designs of ball and crossed roller bearing, bullgear assemblies used for rotating the radar antenna on top of a ship's mast. The loads analyzed include those arising from wind loading and from out-of-flatness of the inner race of the bearings due to deflection of the mast. The distribution of the load and the maximum contact stresses for the proposed bullgear assemblies are estimated and compared. It is found that the maximum contact stress in the crossed roller bearing is less than that in the ball bearing for both types of loads. Furthermore, the analysis shows that the out-of-flatness loading produces significantly higher stresses than wind loading.     

A Simulation Scheme with Coupled Dynamic Models for the Slider and the Suspension Lift-Tab during the Unloading Process of a Hard Disk Drive

A numerical scheme was developed to simulate the interaction between the suspension lift-tab and the ramp of a load/unload-type hard disk drive with consideration of the contact and separation states. The suspension stiffnesses and effective masses were determined based on experiments and finite element analysis. The slider motion was simulated with a degenerated two-degree-of-freedom model, and the results were used as input to a single-degree-of-freedom model for the lift-tab motion. The ramp profile was converted, based on the lateral velocity of the suspension, to a vertical displacement versus time. Computational efficiency was achieved by using a head–disk constraint, instead of a full air-bearing solution, based on dual-scale considerations. The simulation results show that the maximum indentation depth at the tab–ramp engagement increases with an increase in the effective masses, lateral velocity, or ramp angle or with a decrease in the contact stiffness. The bouncing height and bouncing distance of the lift-tab increase with an increase in the contact stiffness, effective masses, lateral velocity, or with a decrease in the suspension stiffnesses. The air-bearing separation time decreases as the suspension stiffnesses, lateral velocity, or ramp angle increases. The coefficient of friction of the tab–ramp interface was found to have a slight influence on the lift-tab behavior on the inclined portion of the ramp.     

A Model for the Effect of Humidity on Stiction of the Head/Disk Interface

A model has been developed for the effect of humidity on stiction of the head/disk interface. The model combines the meniscus force of the adsorbed water film at the interface with the Greenwood-Williamson surface model, and takes into account the effects of suspension load as well as surface roughness and material properties of both the head and the disk. It correctly predicts the trend for stiction vs. relative humidity as obtained from experimental data.     

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.