Brian D. Strom

Hard Disk Drive Reliability Modeling and Failure Prediction

A reliability model for the hard disk drive (HDD) is developed, focusing on head-disk separation as the primary independent variable. Among the many factors contributing to reliability, the role of water vapor is described for the first time. An experimental method based on magnetic spacing loss theory is used to characterize the head-media separation as a function of temperature, altitude, humidity, and HDD operating mode. A statistical model based on these empirical data is developed to predict HDD reliability for various operating conditions. The predictive capability of the model is verified experimentally through application to HDD product reliability test data.

Effects of Humid Air on Air-Bearing Flying Height

The pressure distribution in a squeeze film air bearing depends on the water vapor pressure and the saturation vapor pressure of water at ambient temperature. When the water vapor pressure is compressed beyond the saturation vapor pressure, a portion of the water vapor condenses, thus reducing the total gas pressure in the air bearing. This condensation mechanism thus reduces the flying height of the slider. Using a magnetic recording system for experiments, we quantified the effect of condensation on flying height by measuring the dependence of magnetic spacing on relative humidity. We developed a numerical model based on the proposed mechanism that matches the experimental data and provides a detailed view of the air-bearing response to humidity.

Stress Induced Permanent Magnetic Signal Degradation of Perpendicular Magnetic Recording System

Model scratches of the size found in hard disk drives are produced under controlled conditions at a series of applied loads on both longitudinal magnetic recording (LMR) media and perpendicular magnetic recording (PMR) media using a diamond tip. The scratches are created at low speed, eliminating thermal considerations from the interpretation of the media response. Nanoindentations are produced as well. The scratches and indentations are characterized by atomic force microscope (AFM), magnetic force microscope (MFM), and also by the same magnetic reader and writer used in an integrated hard disk drive (HDD). A comparison of the response of PMR and LMR media shows the PMR media to have larger scratches and greater magnetic signal degradation than LMR media for a given scratch load. The extent of magnetic damage, as measured by MFM, is greater than the extent of surface mechanical damage, as measured by AFM. Analysis of scratches using the HDD reveals that the magnetic damage is irreversible and permanent damage in magnetic layer, which is confirmed by cross section transmission electron microscope image. The experiments reveal the mechanism for magnetic scratch erasure in the absence of thermal effects. This understanding is expected to lead to improved designs for mechanical scratch robustness of next-generation PMR media.

Simulating the Air Bearing Pressure and Flying Height in a Humid Environment

For a hard disk drive operating in a humid environment, the water vapor in the sliders air bearing is typically compressed beyond its saturation vapor pressure, causing the vapor to condense. Consequently, the air bearing pressure decreases and the sliders flying attitude adjusts to balance the forces from the suspension. A method for calculating this air bearing response to humid air is presented. Using two air bearing designs, several test cases are analyzed to illustrate the air bearing response for various temperatures and humidity levels. The calculated flying heights agree with those measured in commercial hard disk drives.

Characterization of Thermally Actuated Pole Tip Protrusion for Head-Media Spacing Adjustment in Hard Disk Drives

As the areal density of magnetic disk storage continues to increase and head-disk spacing decreases, contact between the recording slider and the rotating media becomes imminent. In order to predict contact forces, fly-height modulations, and off-track motions, dynamic models are typically used. A critical element of these models is the contact stiffness and damping arising from the interfacial interaction between the slider and the disk. In this paper, we review different models for predicting contact stiffness based on roughness and layered media and then we report experimental data of both contact stiffness and contact damping of typical head-disk interfaces. It is found that the contact stiffness models (based on roughness alone) overpredict the contact stiffness of actual head-disk interfaces by as much as an order of magnitude. Also, it is found that the contact damping ratio is typically few percent and its behavior is substrate dependent. In addition, the effects of a molecularly thin lubricant and humidity on contact stiffness and damping were experimentally investigated and no significant effects were found.

Air Bearing Surface Designs in Consideration of Thermomechanical Actuation Efficiency

Thermomechanical actuation (TMA) at the transducer region of the air bearing surface (ABS) protrudes from the transducer toward the recording media. This actuation induces a change in the air bearing pressure and a concomitant lift of the slider. The actual actuation in flying height divided by the TMA protrusion, defined as the TMA efficiency, is intimately coupled to the ABS design. After introducing an expression describing the changes in the air bearing forces due to the TMA protrusion, three approaches are proposed that facilitate the optimization of the ABS design for improving the TMA efficiency. These approaches include (a) reducing the air bearing pressure, (b) reducing the size of the TMA affected area, and (c) decoupling the peak air bearing pressure area from the TMA affected area. To illustrate these approaches, several ABS designs are evaluated by comparing their TMA efficiencies.

Calculating Air Bearing Pressure and Flying Height in a Humid Environment

This paper proposes a model describing the adjustment of air bearing pressure in a humid environment. It is shown that the FH of a slider is strongly dependent on the partial pressure of water present in a hard disk drive. The FH loss induced by the humidity effect are calculated by introducing the adjusted pressure into the air bearing model. The model output is found to agree with the changes in fly height measured by experiment. Finally, the model is applied to analyze the FH sensitivity to temperature changes in a humid condition

Air Bearing Surface Design for Improving Efficiency of Thermomechanical Actuation

Thermomechanical actuation (TMA) at the transducer region of the air bearing surface (ABS) protrudes the transducers toward the recording media, yet also results in greater air bearing slider lift. The ratio of these two effects is defined as the TMA efficiency. An expression based on dimensional analysis is introduced to describe the changes of air bearing forces due to TMA protrusion. A framework is proposed that facilitates optimization of ABS design for improved TMA efficiency. On the basis of the theory presented, several ABSs are designed to have different TMA efficiency. Numerical solutions of these air bearings respect to the protrusion shows agreement with the proposed theory.Copyright

Simulating the Air Bearing Flying Height in a Humid Environment

For a hard disk drive operating in a humid environment, the water vapor in the slider’s air bearing is typically compressed beyond its saturation vapor pressure, causing the vapor to condense. Consequently, the air bearing pressure decreases and the slider’s flying attitude adjusts to balance the forces from the suspension. A method for calculating this air bearing response to humid air is presented. Using one particular air bearing design as an example, several test cases are analyzed to illustrate the air bearing response for various temperatures and humidity levels. The calculated flying heights agree with those measured in commercial hard disk drives.Copyright

Slider¢??s Unstable High Flying Height after Loading and Subsequent Stabilization

We present unstable high flying loading and subsequent stabilization of flying attitude, using as an example a Pemto slider (measuring 1.235x0.7x0.23mm3) design for micro-drive application. We then characterize the effects of various parameters such as PSA and loading speed on the phenomenon, using the U.C. Berkeley CML Load/Unload Code for simulation and analysis.stabilization air bearing