Slider-disk Interface Research Articles

Effect of relative humidity and disk acceleration on tribocharge build-up at a slider–disk interface

High performance disk drives require high spindle speed. The spindle speed of typical hard disk drives has increased in recent years from 5400 to 15000 rpm and even higher speeds are anticipated in the near future. The increasing disk velocity leads to increasing disk acceleration and slider–disk interaction. As the head-to-disk spacing continues to decrease to facilitate increasing recording densities in disk drives, the slider–disk interaction has become much more severe due to the direct contact of head and disk surfaces in both start/stop and flying cases. The slider–disk interaction in contact-start-stop (CSS) mode is an important source of particle generation and tribocharge. Charge build-up in the slider–disk interface can cause electrostatic discharge (ESD) damage and lubricant decomposition. In turn, ESD can cause severe melting damage to MR or GMR heads. We measured the tribocurrent/voltage build-up generated at increasing disk acceleration. In addition, we examined the effects of relative humidity on the tribocharge build-up. We found that the tribocurrent/voltage was generated during pico-slider/disk interaction and that its level was below 250 pA and 0.5 V, respectively. Tribocurrent/voltage build-up was reduced with increasing disk acceleration. Higher humidity conditions (75–80%) yielded lower levels of tribovoltage/current. Therefore, a higher tribocharge is expected at a lower disk acceleration and lower relative humidity condition.

Intermolecular force and surface roughness models for air bearing simulations for sub-5 nm flying height sliders

When the spacing between the slider and the disk is less than 5 nm, the intermolecular forces between the two solid surfaces can no longer be assumed to be zero. The model proposed by Wu and Bogy (ASME J Trib 124:562–567, 2002) can be view as a flat slider–disk intermolecular force model. The contact distance between the slider and disk needs to be considered in this model when the slider-disk spacing is in the contact regime. To get more accurate intermolecular force effects on the head disk interface, the slider and disk surface roughness need to be considered, when the flying height is comparable to the surface RMS roughness value or when contact occurs. With the intermolecular force model and asperity model implemented in the CML air bearing program, the effect of intermolecular adhesion stress on the slider at low flying height is analyzed in the static flying simulation. It is found that the intermolecular adhesion stress between the slider and the disk has slight effect on the slider-disk interface for a flying slider.

The pulse width effect on the shock response of the hard disk drive

A finite element model (FEM) of the ST drive from Seagate is developed in ANSYS to investigate the shock response of the hard disk drive (HDD). The FEM includes the pivot bearing, the head stack assembly (HSA) and the disk. The free state of the HSA is determined by an iterative procedure to produce the prescribed preloading force at the head–disk interface. The FE model is then verified by conducting a modal analysis over the HSA. The obtained mode shapes and resonant frequencies are compared with the modal testing results. An acceleration pulse is applied to the shaft and the whole disk surface at the same time to study the shock response of the HDD. The head slap behavior is examined at the slider–disk interface. The effect of the pulse width on the head slap behavior is studied. The duration of the acceleration pulse varies from 0.3 to 1 ms. It is found that the lift-up height of the slider reaches a peak value at different pulse widths when the amplitude of acceleration pulse changes. This is due to the nonlinear behavior introduced by the contact surfaces.

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 A 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.

Particle monitoring method using acoustic emission signal for analysis of slider/disk/particle interaction

One of the most important reliability issues in an information storage device is the contamination problem. The slider and disk can be damaged by the particles intruded into the slider/disk interface (SDI). In this work, in order to monitor the slider/disk interaction due to particle injection the acoustic emission (AE) method, which is typically utilized for the detection of slider contact, was used. The raw as well as frequency spectrum of the AE signal were obtained during the particle injection test. The particles were artificially injected inside the test apparatus to simulate the effect of contamination on the slider/disk interaction. SiC and polystyrene particles were used for the tests. As a result, the 1st torsional and bending mode frequencies of the nano-slider were observed when 1 μm SiC particles and 60 nm polystyrene particles were injected into the SDI. Also, it was shown that the particle behavior at the SDI can be predicted from the characteristics of the AE raw signal.

Tribocharge build-up and decay at a slider-disk interface

As the gap between the head and disk decreases and the sensitivity of recording transducers increases, the head and disk are more likely to come in contact, which may damage the recording transducer during start/stop and flying cases. One important effect associated with the intermittent head-disk contacts is the tribocharge/tribocurrent phenomenon. In this study, tribocharge and tribocurrent generation during a pico-slider/disk interaction were measured by using an electrometer. These triboelectrical properties were compared with friction force and acoustic emission signals. The electrical potential difference between the slider and the disk was caused by mechanical interactions between them. The tribocharge was generated during the slider-disk interaction and its saturation charge level was about 1 V. The tribocharge build-up level was independent of the slider-disk interaction time. However, the tribocharge decayed when there was no interaction between the slider and the disk. The decay of tribocharge was inversely proportional to the square root of time. Tribocurrent generation coincided with tribocharge generation. The tribocurrent was also independent of the slider-disk interaction time. The current level measured in this study may not reach the electrostatic discharge (ESD) damage level, but it can induce the dissociation of lubricant.

Disk shape and its effect on flyability

The static spacing loss of slider-disk interface is investigated through careful characterization of disk shape for disks of different form factors with different peak-to-valley flatness values. Measured radial profiles are normalized and found to be confined within a relatively narrow region. The representation of these shapes is reduced to a simple set of quadratic equations. Static flyability models associated with the disk radial slope (crown effect) and radial curvature (camber effect) are presented. Spacing loss is calculated for a negative pressure pico-slider and a positive pressure catamaran nano-slider. Results show that slider geometries and crown/camber sensitivity, as well as disk flatness and shape, all affect static slider-disk spacing loss. The analysis is focused on the effect of disk shape on the static spacing loss and its relationship to slider design.

Nonlinear aspects of air-bearing modeling and dynamic spacing modulation in sub-5-nm air bearings for hard disk drives

A new analytical model and method of analysis are proposed for understanding the dynamical behavior of ultralow flying height air-bearing sliders in proximity based on nonlinear dynamics. It is found that for sub-5-nm flying height air-bearing sliders, the nonlinear effects cannot be neglected. These nonlinear effects cause a slider's response to become highly nonstationary, making frequency domain analysis by fast Fourier transforms (FFTs) an insufficient means of analysis. Joint time-frequency analysis is applied for accurately analyzing the nonstationary slider responses and to verify the nonlinear nature of the air bearing for both experimental and simulation results. One degree-of-freedom (DOF) and 2-DOF nonlinear lumped parameter models are proposed showing the effect of nonlinearities on the FFT representations of air-bearing slider responses. The 2-DOF model is used to further investigate the nonlinear coupling effect, and it shows high correlation with experimental results from two different slider designs when in unsteady proximity. These findings suggest that the nonlinearities of the air-bearing slider must be considered when modeling slider-disk interface dynamics. Also, complex frequency domain representations of slider responses in proximity can be explained by the nonlinear nature of the air-bearing slider without contact between the slider and disk as previously proposed by other models.

Investigation of contamination particle's trajectory in a slider disk interface

Design of slider rail for contamination control becomes an important issue in magnetic recording. In this paper, trajectory of particles in a slider disk interface was simulated with considerations on various forces, such as drag force, gravitational force, Saffman lift force, and electrostatic force, all of which affect the motion of the particles. It was found that the charged particles could easily adhere to the slider or the disk surface if an electric field existed in the slider disk interface. It was presumed that the vertical motion of the particles would not only be related to the Saffman force but also to the electrostatic force.

In situ quantitative analysis of nano-scale lubricant migration at the slider–disk interface

A thin layer of lubricant is a critical element of the head/disk interface needed to improve its tribological durability and to prevent media corrosion. Local thinning of lubricant with its subsequent breakdown often leads to the immediate failure of the interface. This paper is devoted to the in situ quantitative analysis of nano-scale lubricant migration on the surface of a thin-film disk using the Optical Surface Analyzer (OSA). The calibration procedure, which enables quantitative measurements, is discussed and the technique's capabilities are demonstrated using specially prepared samples. Two cases of slider/disk interaction are analyzed: low-speed, when the slider is dragged over the disk surface, and high-speed, when the slider is flown over the same track for several days. Lubricant migration phenomena, such as depletion and pooling, are investigated quantitatively to analyze the origination of carbon wear and the mechanisms of interfacial failure. A model of high-speed slider/disk interaction involving the dynamic formation of a liquid bridge at the interface is proposed.

Strength and stability of a meniscus in a slider-disk interface

The behaviour of a meniscus in a slider-disk interface is analyzed in three cases. In a quasi-steady state, normal force due to the meniscus pressure and surface tension are compared. Bimodal distribution of the fly/stiction is explained by the availability of a wick. In the absence of slider-disk movement, the tendency of meniscus growth is studied. A constant volume is sustainable only in a concave gap. In the onset of slider-disk motion, the change in stiction is analyzed. A meniscus is more resilient against a steady force than against a short pulse.

An experimental study of slider vibration in nanometer spaced head-disk interface

This paper reports results of experimental investigations into slider-disk interaction and slider vibration in nanometer spaced head-disk interface. The investigations were based on an slider-disk interface (HDI) configuration which retains the tribological features of the interface formed by the functional head slider and disk media but make the optical monitoring and quantitative measuring of the slider disk spacing possible. The effects of the lubricant and its thickness on the HDI dynamics were also studied. The results indicate that the slider vibration is dependent not only on slider air-bearing surface design but also strongly on lubricant and other interface conditions.

Slider-disk interaction and its effect on the flying performance of slider

This paper reports a novel optical head-disk interaction tester setup and experimental investigations into the effects of slider-disk interaction in nanometer spaced head-disk interface. The setup revealed quantitatively the lubricant induced change of slider-disk spacing before the slider enters its fully flying status. The setup was also applied to investigate the slider sinking process which is caused by the meniscus force when the slider starts to rest on disk surface. Results shows that such a setup has the advantage of quantitative characterization of slider-disk interface compared with acoustic emission method. Furthermore, the results also demonstrate its advantages in reflecting the actual interaction between slider, lube, and disk surface, compared with the current optical flying height testers.

Debris growth and migration in the slider-disk interface

A debris causing serious wear and spacing deviation is often attached to, or slipping slowly on the slider. Its motion is dominated by contact forces, especially by the angle of the contact force with respect to the slider surface. The contact forces can be high if the debris is compressed as it slips. This occurs most often when the pitch (or wall-angle) is about 10/spl deg/. The debris may be dislodged when the angles of contact forces change, especially due to skew. A simple model is developed to predict the tendency of debris motion, given the coefficient of friction, the skew, and the pitch angle. The model is programmed in Excel spreadsheet. The results are verified by experiments.

Contact analysis of three-dimensional rough surfaces under frictionless and frictional contact

A methodology for surface and sub-surface stress calculation of nominally flat on flat rough surface contact has been developed. This methodology is applicable for both large area contact (Hertzian contact) and small area of asperity contact (point load contact) with and without surface friction. A total of nine rough surfaces are generated by the computer with specified standard deviation of surface heights, σ, of 0.3, 1.0 and 3.0 nm, and correlation length, β ∗ , of 0.1, 0.5 and 0.9 μm. Under the typical applied load at the magnetic head slider-disk interface, small numbers of contact points are obtained and the deformation is purely elastic. Since these contact points are scattered and isolated, asperity contact behaves like point load contact. As β ∗ becomes larger, more adjacent points will be in contact at a certain contact spot and this is especially true at small σ. All the cases of flat on flat rough surface contact yield maximum von Mises stress on and near the surface at both frictionless and frictional contacts; no local maximum occurs in the sub-surface. In general, the friction effect in the vicinity of contact point is to increase the stress magnitude, while outside this region it also alters the stress distribution. For a surface of small β ∗ and large σ at high load of 1000 times of the nominal pressure at the head-disk interface, the contact pressure reaches the hardness at a few contact points and plastic deformation takes place in the near surface.

Slider/Disk Interaction During the Landing Process

In rigid disk drives, slider/disk interface (SDI) wear should ideally occur only during the start/stop (S/S) process. Good S/S wear performance is crucial, but often difficult to achieve. This is due in part to a lack of understanding of slider/disk interaction during the S/S process. The intent of this study is to provide a thorough theoretical model to aid in our understanding of the slider/disk interaction during the landing process. In contrast to assessments in the literature, this model includes more rigorous surface characterization, the statistical nature of summit interaction, and the nonlinear rarefaction effects in the modified Reynolds equation. In this paper, we present results of a baseline slider/disk configuration, together with a crown sensitivity study.

Head material effects on interface tribochemistry

Sliding experiments between sliders made of different materials and magnetic thin-film disks were performed in a vacuum and an ambient in order to shed light on material effects in tribology. Al/sub 2/O/sub 3/-TiC, amorphous silicon, amorphous carbon, MnZn ferrite, and sapphire were chosen for their current or potential use as head substrate materials. In ambient, the amorphous carbon performed the best tribologically while Al/sub 2/O/sub 3/-TiC the worst. The wear mechanism of the slider-disk interface was studied in-situ by mass spectrometry and frictional measurements. For unlubricated interfaces, decreasing evolution of C, CO and CO/sub 2/ during initial sliding was accompanied by decreasing friction. For lubricated interfaces, fluorocarbon fragments with a wide range of masses were generated at a decreasing rate while friction was low and relatively constant. When the magnitude of gaseous, signals increased, frictional variations rose dramatically, hydrogen evolution followed with some delay, and a wear track was formed. With amorphous silicon sliders, a significant amount of SiF/sub 4/ was detected which indicates a tribochemical reaction between SiO/sub 2/ surface and HF that was produced by the scissioning of lubricants. Degradation mechanisms are dominated by removal of surface groups and adsorbates from solid surfaces and by scission, losses and alteration of fluorocarbon polymers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Ultra low flying height measurements using monochromatic and phase demodulated laser interferometry

Phase demodulated laser interferometry and three wavelength monochromatic interferometry are employed to measure the flying height at the slider-disk interface in the sub-100 nanometer spacing range. Phase demodulated laser interferometry measures the relative displacement of the slider using the actual slider and disk, while three wavelength monochromatic interferometry measures the absolute flying height of the slider using a transparent disk and an actual slider. The results from both techniques are found to agree down to the "landing height", i.e., the height when the slider is "resting" on the top of the disk asperities. Landing heights are found to follow the peak-to-valley disk surface roughness.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Motion pictures on in-situ air bearing dynamics

Knowledge of the detailed motion of an air bearing slider as it "flys" over the surface of a recording disk is essential for understanding and improving the slider-disk interface. The minute spacing between these surfaces (0.2 µM) permits the use of electrical capacitance for obtaining absolute flying height, pitch and roll of the slider relative to the disk surface. Employing special multi-sensor air bearing sliders in an assembled disk file, along with high bandwidth electronics and microsecond data acquisition, its motion can be captured and reconstructed. The results are visualized in a motion picture format, showing the behavior of the slider in slow motion during an entire full data band access stroke. No large excursions of the slider's motion are found even when it experiences a large radial acceleration during accessing.