Tribology of Textured Sliders in Near Contact Recording Situations

In order to increase the recording density of hard disk drives, the flying height of sliders must be reduced. It is estimated that in order to achieve a recording density of 100 Gb/in2, the flying height would have to be about 6 to 7 nm. This requires that the surface roughness of the disk must be decreased. However, a very smooth disk surface is undesirable since increased stiction between slider and disk can lead to failure of the head/disk interface. Slider surface texture was suggested to reduced the stiction in the presence of smooth disks. It has been shown that slider surface texture is effective to reduce friction, stiction, and contact-induced vibration of air-bearing modes in the near-contact regime.

To investigate the tribological properties of textured sliders at a low flying height and regular disk velocity, two kinds of pico-sliders were textured. Dynamics of the textured and the untextured sliders were analyzed using Laser-Doppler Vibrometry (LDV). The effect of slider surface texture on lubricant depletion on the disk surface is studied using scanning ellipsometry (Surface Reflectance Analyzer (SRA)).

The results show that the texture on the slider surfaces reduces slider in-plane and out-of-plane vibrations. Under "near contact" conditions the textured sliders with an optimum texture height were found to cause less lubricant depletion on the disk surface than the untextured sliders.

Lateral Tape Edge Motion and Tape Edge Wear

Research Abstract:

Sensitivity Analysis of Pico and Femto Slider

Research abstracts:

The flying height sensitivity of pico and femto sliders on air bearing contour design, operating conditions and manufacturing tolerances was studied numerically. A Monte Carlo analysis was performed to evaluate the distribution of flying height, pitch angle and roll angle, assuming the selected independent variables have normal distributions. Two types of pico sliders and one type of femto sliders were modeled and simulated. The results show that flying height is strongly influenced by the contour design of the air bearing slider. The resulting distributions of flying height, pitch angle and roll angle are near Gaussian.

Modeling of Tape Lateral Motion Control

Abstract

Lateral Tape Motion (LTM) is one of the main concerns in a tape drive. Lateral tape motion is the in-plane motion of the tape when it is moving along the guides and rollers of the tape drive making it move like a wave. These motions are sometimes in the range of more than 1 kHz. This high frequency lateral tape motion is our concern.

We have experimentally investigated high frequency lateral tape motion in a number of tape drives using optical edge probes. It is important to understand and explain the source of lateral tape motion in order to improve the design and performance of tape drives. Our research is directed towards understanding the cause of lateral tape motion and simulate lateral tape motion in a tape drive.

To model and simulate lateral tape motion, we have studied the various modes of vibrations of a magnetic tape in a tape drive. These modes are transverse vibrations of a string, as well as beam, and possibly plate bending modes. We have determined the natural frequencies of these modes for various lengths of the tape between supports. Frequencies in the 1 kHz to 10 kHz range were observed for lengths of tape between 150mm to 300mm between the supports. This range of length is close to the effective length of a tape between the supply reel and the take-up reel. Beam and string models for tape vibrations showed better resemblance to experimentally observed vibration frequencies than a plate model. A finite element analysis was also used for the beam model to determine the frequencies. Preliminary results for the various frequencies are presented and compared with experimental results.

The effect of rollers guides and the head in the tape path are being studied. Both the rollers and the head affect the boundary conditions and the frequencies of vibrations.

Numerical investigation of the contact force in the head/disk interface

Introduction

To increase the storage density of a hard disk drive the flying height of the read/write head has to become lower and lower. The distance between the slider and the disk is in the range of the surface roughness, therefore, contacts are unavoidable. These contacts cause an additional force that should be considered while computing the air bearing force and thus the actual position of the slider.

Contact model

The most common contact model for rough surfaces is the contact model developed by Greenwood and Williamson [1]. In this model the rough surfaces are assumed to be a collection of asperities where all their summits have the same radius and the heights follow Gaussian distribution. There is no interaction between the asperities. Contacts are elastic so Hertzian equations are applied. The load P is defined as:

P=4/3*eta*A*E'*beta^0.5*sigma^1.5*F(h)

where eta = the surface density of the asperities, A = area, E' = Young's modulus, beta = asperity radius, sigma = heights in terms of standard deviation and F(h) = scaled height distribution (with h = flying height).

To compute the contact force, the load for each element will be calculated, where eta, E', beta and sigma are given as constant. From the geometry of the mesh the area of each element can be calculated. Finally the function F(h) must be calculated for each element with the flying height computed by the simulator.

This force will be added to the airbearing force and will adjust the position of the slider.

[1] GREENWOOD, J.A., and WILLIAMSON, J.B.P., 1966: "Contact of nominally flat surfaces" Proceedings of the royal society of London, Vol. 295, pp.300-319

Shock Test Modelling of a Hard/Disk Drive Using Hypermesh and LS-DYNA

Finite element analysis is useful for predicting the behavior of a hard disk drive (HDD) subject to adverse conditions. Finite element models of a HDD have been developed for both operational and non-operational states. Simulations are useful in predicting a drives robustness to shock and vibration.

To perform the finite element analysis we use LS-Dyna, a commercially available transient finite element solver. LS-Dyna is used in contact problems, large deformation and dynamic simulations. To prepare the model for LS-Dyna and display the results we use Hypermesh, a commercially available pre- and post- processor.

Hard disk drives (HDD) must be designed to be resistant to shock. Two types of shock simulations are of particular interest, the linear drop test and the tilt drop test. In a linear drop test, all the components of the hard disk drive are given a vertical velocity component corresponding to a particular "drop height". In a tilt drop test, the hard disk drive is constrained to rotate about an axis such that one side of the HDD will contact the impact surface. The tilt drop model is given an initial angular velocity corresponding to a particular "drop angle".

We are interested in the correlation between a linear drop and a tilt drop test. The linear drop model is used to simulate an HDD being dropped from several different drop heights. The tilt drop model can simulate different drop angles. The comparison is made for several points of interest such as, duration of head slap, head slap amplitude loss, pitch and roll angles and lateral motion of the slider. The data is obtained for a variety of drop heights and angles for both operational and non-operational drives.

Hard disk drives must also be resistant to vibration. The effects of a vibration load on a hard disk drive is simulated for several frequency bandwidths and amplitudes. We investigate the effects on the head/disk interface using the results of the simulations, quantities such as lateral and vertical slider motion or stress in the suspension. Again, simulations are performed for both operational and non-operational drives.

The results of the simulations are compared with experimental results using Laser Doppler Vibrometry and high-speed video analysis. Initial results have shown good agreement between high speed video and animation of the finite element model. Modal analysis also shows agreement between measured and numeric results.