Head-disk Contact Research Articles

Experimental study of smear formation and removal in heat-assisted magnetic recording

The head-disk interface (HDI) in heat-assisted magnetic recording (HAMR) hard disk drives is a system where a laser beam is launched from a recording head and focused on a recording disk to facilitate data writing. During the laser exposure, material transfers from the disk to the head surface due to the high temperature field and steep thermal gradient. The material accumulation on the head surface, also known as smear, is a challenging reliability issue for HAMR. In this paper, we experimentally investigated the effect of disk temperature and laser exposure time on the smear formation and studied the smear removal by frictional interactions between the head and the disk. In the experiments, the disk temperature and the laser exposure time were controlled separately to generate the smear, which was later characterized by atomic force microscopy (AFM). The AFM images show that the smear forms when the lubricant evaporation occurs for a certain time, and that the smear amount increases with the disk temperature and the laser exposure time. Furthermore, touchdown experiments were performed using the heads with smear. The results indicate that the smear is mostly removed by friction from the head-disk contact. This study reveals the mechanism of the smear formation in HAMR and presents a mechanical approach to mitigate the smear without damaging the head.

Sensing and Modeling Heat Transfer Between a Recording Head and Medium for Clearance Setting in HAMR

A thermistor-based proximity sensor enables identification of head-disk contact in a hard disk drive, allowing the setting of head-disk clearance. This sensor also provides thermal information, permitting validation of computational models used to design recording heads. We describe here a set of models, and a related set of experiments to probe thermal and mechanical characteristics at the head-disk interface. Example results are shown, leading to the conclusion that models with diffusive-ballistic equations are capable of simulating heat transfer through the gas bearing.

Wear Study for Dual-Heater Slider Under High Temperature

With the continuous increase of a hard disk drive (HDD) areal density, the required clearance between a magnetic head and a disk is lower than 1 nm. The HDD’s reliability becomes a significant concern due to the inevitable contact of an intermittent/continuous head–disk contact. Slider wear is one of the most important topics. This paper investigated the slider wear at a near-contact/contact region under high temperature based on a drive-based tester. Based on the slider wear identification results using the scanning electron microscope and atomic force microscope (AFM), it is concluded that a slider with higher temperature is prone to produce more wear to slider. Furthermore, a modified wear model, which implies that wear occurs through an atom–atom attrition process (bond breaking) at nanoscale, was used to fit the wear data measured using AFM. Such bond breaking can be described by a thermally activated process that indicates wear amount is highly related to temperature and it agrees well with the experiments.

Effect of Air and Helium on the Head–Disk Interface During Load–Unload

The tribological characteristics of the head–disk interface are investigated during load–unload for air and helium-filled drives as a function of the pitch static angle and the roll static angle between slider and disk. A custom-made experimental tester inside a sealed environmental chamber was used to determine the regions of “safe” pitch static angle and “safe” roll static angle in air and helium environment during the load–unload process. The presence of head–disk contacts during load–unload were evaluated by measuring the acoustic emission signal and the decrease in rotational speed of the spindle. Scanning electron microscopy and optical surface analysis were used to investigate wear of the slider and the redistribution of lubricant on the disk surface after 10,000 load–unload cycles. The results indicate that the tribological performance of the head–disk interface is improved in helium environment compared to air environment.

Repeatable and Non-repeatable FH Modulation Study Near Head Contact

The lower head–media spacing (HMS) is always expected for achieving better recoding performance at high linear density. Besides the efforts to reduce the overcoat thickness of head and media, the low flying height (FH) and smaller FH modulation (FHM) are necessary to ensure the low HMS and good interface reliability. In the current HDD, the FH change is measured from the readback signal by the Wallace spacing loss equation. The real-time dynamic FH signals consist of repeatable and non-repeatable FH changes, or FHM. The repeatable FHM (RFHM) correlates with the disk morphology variations. It usually has the variation of transition parameter $a$ inside. Because the change of $a$ is the noise of the RFHM, it has to be very careful for the compensation of the RFHM by thermal FH control. Otherwise, it may cause unexpected head–disk contact. The non-RFHM (NRFHM) measurement is able to remove the media-related noise, which has a good testing accuracy of around 1 A. This paper studies both RFHM and NRFHM characteristics of four types of sliders with different air-bearing surface designs.

Dual track Wallace fly height measurement method to monitor slider motion in 2 dimension at near contact regime

Head media spacing of hard disk drive (HDD) is expected to continue its reduction in order to support areal density growth. At sub-nm clearance between the head and the disk, the intermittent head disk contact and interactions may happen. This does not only cause the fly height modulation but also induces the off-track motions as well. It is desirable to understand the characteristics of the 2D motions in the near contact regime, so as to enable further reduction of the clearance and to improve the reliability of HDD. This paper presents a method to measure instantaneous fly height (FH) motion and cross track motion concurrently by using read back signal from the dual data tracks written at different frequencies. The method is able to separate the FH motion and cross track motion of the head.

Electrostatic Force Manipulation Methodology: Principles, Mechanisms, and Setup for Head–Disk Interactions Monitoring

To achieve higher hard disk drive areal density, the head–disk clearance had been reduced to $F_{\mathrm {es}}$ ) across the head–disk interface were investigated. It was shown that the distribution of the applied $F_{\mathrm {es}}$ was different for the two setups. This led to further studies of the effects of the $F_{\mathrm {es}}$ distribution on interactions monitoring. The results showed that when an alternating $F_{\mathrm {es}}$ was distributed at the pole region, the magnitude of the electrodynamically excited first-harmonic vibration increased significantly as the gap fly height was reduced. It was very sensitive to spacing change and useful for the head–disk interactions monitoring at the near-contact regime. The second harmonic was found sensitive to head–disk contact. When the alternating $F_{\mathrm {es}}$ was distributed at the slider body, the magnitudes of the first and second harmonics were larger due to larger effective surface area. However, the fly height of the slider body was mainly affected by the protrusion push-up effect, and hence the first and second harmonics showed a reverse trend as compared with the case where $F_{\mathrm {es}}$ was distributed at the pole region. In addition, slider pitch motion was also significantly affected. The effective EFM setup was constructed to demonstrate its effectiveness in head–disk interactions monitoring and contact detection.

Low SNR signal processing for head/media contact detection in HDD

A new methodology for processing a grossly undersampled hard disk drive (HDD) position error signal (PES) is proposed to improve the head-disk contact detection process. The proposed method is a combination of compressive sampling (CS) and adaptive discrete wavelet transform (ADWT) techniques. Here, CS is employed to reconstruct the PES information from very few revolutions of data sampled with the HDD servo system while ADWT is utilized for extracting head-disk interface (HDI) modulation changes during thermal protrusion of the read/write element. An example of HDD level signal processing is provided where CS synthesized PES is processed using a discrete wavelet transform embedded into an entropy minimization and statistical characterization framework.

TuA-2-3 Experimental and Numerical Investigation of Hydrocarbon Transfer Mechanism at the Head-Disk Interface

In hard disk drives, hydrocarbon oils are used to lubricate the disk spindle and the actuator pivot. During disk drive usage, oil molecules can escape, resulting in contamination of the head-disk interface. In this study, experimental investigation and molecular dynamics simulations are used to study hydrocarbon contamination at the head-disk interface in hard disk drives. A "two-step" transfer mechanism is assumed: Hydrocarbon molecules first condense onto the disk surface from the vapor phase, and then transfer to the slider surface in the absence of head-disk contacts. Experimental results and molecular dynamics simulations verify the assumed transfer mechanism.

Optimization of molecularly thin lubricant to improve bearing capacity at the head-disk interface

A molecularly thin lubricant layer (of the order of 1---2 nm thick) has been shown to provide bearing forces at the interface between contacting solid surfaces under light loads and high shear rates. This phenomenon is important, for example, in the head-disk contact in magnetic storage hard disk drives to ensure that some of the contact is sustained by the lubricant layer and thus avoiding damage of the solid surfaces. The magnitude of the normal and tangential bearing forces that the lubricant layer can provide depends on temperature, viscosity of the lubricant, sliding velocity and radius of gyration of the lubricant molecules. This study shows that viscosity has the greatest effect on the load bearing capacity of the molecularly thin lubricant. Thus, by controlling the flash temperature and the ratio of molecularly thin lubricant-to-bulk viscosity, the bearing load carrying capacity of the layer can be controlled. This would allow for the contact to be sustained within the mobile lubricant layer, avoiding solid contact so as to protect the diamond-like carbon coating, and thus reduce wear and potential catastrophic failures.

Experimental and Simulation Study of Thermal Protrusion-Induced Head-Disk Contact Instabilities in Hard Disk Drives

Stability of the thermal fly height control slider has profound effects on the read-write performance and reliability of the head-disk interface. As magnetic storage technologies evolve aggressively toward higher areal density in hard disk drives, the fly height around the transducer area on the slider has achieved a read-write physical spacing of \({\sim }2\) nm, and is expected to be even lower to meet the magnetic storage road map in the near future. At such a low spacing, instabilities can occur in terms of slider vibration and prominent lubricant transfer from the disk to the slider, resulting in significant interface change that raises reliability concerns. In this paper, we experimentally investigate the thermal protrusion induced slider instability at contact proximities, and perform numerical simulations to explain the measured dynamics at head-disk contacts. The results suggest that as the slider's thermal protrusion area approaches the disk surface, instabilities mainly come from down-track force excitations. These excitations lead to more significant vibrations in the down-track direction and on the slider's leading edge rather than its trailing edge. The behavior of slider vibration at disk proximity is also dependent on the head-gimbal-assembly design.

Multi-resolution HDD contact detection technique for low SNR applications

The accurate calibration of active flying height in a hard disk drive (HDD) is an essential metrology for proper drive operation. Detecting head-disk contact of a head-disk-interface (HDI) is a critical step in calibrating the active fly height. Proposed is a head-disc contact detection technique that is an improved adaptive discrete wavelet transform (ADWT) methodology that can be integrated into an advanced HDD. The method is designed to process low SNR head/disc proximity probe data during a thermal protrusion pulse and extract HDI head modulation signatures correlating them with the contact event. The significance of the proposed ADWT implementation is in fact that thousands of DWT coefficients can be represented by a simple statistically derived data set of four parameters that can be tracked to declare the contact. This compact approach enables a practical, cost-effective ADWT implementation into the HDD hardware architecture using existing micro-processors and on-board memory.

Thermomechanical Contact Between Magnetic Recording Head and Disk Defect Accounting for Heat Partition Factor

The formation of defects on a magnetic recording disk is one of the critical issues in the reliability of a hard disk drive. When a magnetic recording head comes into contact with a disk defect, the resulting surface damage can cause significant degradation on read/write performance. Considering the carbon atoms in diamond like carbon (DLC) coating can experience phase change and oxidation under high temperature (i.e., mechanical softening process due to the thermal stability), the frictional heat generated by the sliding contact between a head slider and a disk defect can lead to severe damage on the head DLC film. In addition to the mechanical degradation of the head DLC film, the magnetic properties of underlying read/write elements and shield materials can also be deteriorated due to the mechanical stress and high temperature rise. In this paper, the thermomechanical contact between a head slider and a disk defect was systematically examined through analytical modeling and simulations. For the three different disk defects (i.e., Al2O3, SiO2, and AlTiC), the temperature rise and distribution on the contact area were determined from a theory of contact mechanics and heat transfer with the modified Francis heat partition factor. To obtain more physical insight into the thermomechanical head disk contact, parametric study was performed using a modified control parameter (ψ*), which was made of material properties of a disk defect and system operating conditions. According to the simulation results, it was observed that Al2O3 showed the worst thermomechanical contact performance among the three defects as it generated the highest flash temperature of 439.0°C on the head DLC film. The increased surface temperature was high enough to degrade the head DLC film as well as the underlying magnetic materials. Moreover, it was found that the surface temperature rise on a head slider was proportional to the values of ψ*.

Angstrom Scale Wear of the Air-Bearing Sliders in Hard Disk Drives

We utilize thermal fly-height control (TFC) technology to perform in situ measurements of carbon overcoat wear at the angstrom level at the read–write area of magnetic recording heads. We also study the durability of the molecularly thin lubricated disk surface. Experimental findings reveal a linear relationship between the quantified carbon wear depth on the flying head versus the head–disk contact level produced by the TFC power. It is demonstrated that this method can serve as a measurement and probing technique of wear resistance for different types of lubricants. Lubricants possessing more polar hydroxyl end-groups and less mobility tend to show a superior surface stability under head–disk contacts, but raise concerns on head carbon overcoat wear.

Contributions of Mobile and Bonded Molecules to Dynamic Friction of Nanometer-Thick Perfluoropolyether Films Coated on Magnetic Disk Surfaces

To protect the interface against intermittent head–disk contact in hard disk drives, nanometer-thick perfluoropolyether (PFPE) films consisting of both “bonded” and “mobile” molecules are applied on the disk surfaces. Because of their different adsorption states and mobility, the bonded and mobile molecules are supposed to contribute differently to friction properties, which directly impact the stability of ultra-low flying head–disk interfaces. By measuring the friction force at light loads and low to high speeds as a function of bonded and mobile film thicknesses, we studied the contributions of bonded and mobile molecules to the dynamic friction of nanometer-thick PFPE films. We found that the friction coefficient of lubricant films without or with less bonded molecules increased as a power function of sliding speed, whereas that of lubricant films with more bonded molecules increased logarithmically with sliding speed. We suggest that these results can be explained by the following mechanisms: the dynamic friction of lubricant films without and with less bonded molecules is dominated by shear thinning behavior of mobile molecules, while that of lubricant films with more bonded molecules is governed by bonded molecules which lead to boundary lubrication.

Replenishment Speed of Depleted Scar in Submonolayer Lubricant

The evaluation of submonolayer lubricant mobility is becoming important in the field of nanotribology, in particular, in hard disk drives for realizing the near-contact or surfing–recording. This paper experimentally and theoretically investigates the replenishment speed of a depleted scar in a submonolayer lubricant caused by the head–disk contact. The theoretical analysis is based on continuum mechanics. The replenishment process of a submonolayer lubricant height profiles in a depleted scar caused by the head touchdown operation was experimentally measured for the Z-tetraol lubricant with a 0.24 nm mobile lubricant thickness and compared with the numerical simulation. It was found that the analytical replenishment process can fairly agree with the experimental one if the ratio of Hamaker constant to effective lubricant viscosity is properly determined. By using the validated basic equation, a simple but useful generalized formula is proposed to evaluate the replenishment speed in relation to the depleted scar width of the mobile lubricant, the lubricant thickness, and the ratio of Hamaker constant to effective lubricant viscosity.

Performance analysis of an integrated piezoelectric ZnO sensor for detection of head–disk contact

Integrated capability for detection of head–disk contact is desired for magnetic sliders with near-contact flying height. At the same time, fabrication of added features should be compatible with the existing slider micromachining process which is highly specialized and cost sensitive. Aimed at meeting the two requirements, a novel sensor configuration is explored in the present study. The new sensor configuration consists of a piezoelectric zinc oxide (ZnO) thin-film sensor sandwiched in the magnetic slider on its trailing side. Coupled structural and piezoelectric finite-element analysis for a sensor–slider–suspension assembly was performed to investigate the dynamic sensing performance. Output voltages on the millivolt level were obtained under typical head–disk interactions. The second in-plane bending mode of the slider was found to be the major contributor to the output voltage. Parametric study further showed that a thicker ZnO layer generally generated a larger output, while the thickness of the slider overcoat had only minimal effect. Simulation results from harmonic and transient analyses demonstrated that the piezoelectric thin-film ZnO sensor was sufficiently sensitive for detection of head–disk contact.

Simulation studies of contact sensor for disk defect mapping

The keep increasing areal density in hard disk drive demands reducing the clearance between the read/write element and the disk surface. A clearance of 2 nm could be achieved using thermal-flying-height technology (Kurita et al. IEEE Trans Magn 41(5):3007–3009, 2005). In such a low-clearance flying regime, it is important to ensure the reliability of the head-disk interface. Head carbon overcoat wear should be taken into account and it requires ultra-smooth disk surface. Conventional method for disk inspection is using glide tester (Nayak et al. IEEE Trans Magn 39(2):729–734, 2003). However, with lower and lower clearance, some small defects on the disk surface can no longer be neglected, and these small defects cannot be detected by glide test. Thus a thermal-contact sensor integrated in a magnetic-head slider was proposed by Shimizu et.al to detect defects on disk surface (Shimizu et al. IEEE Trans Magn 47(10):3426–3432, 2011). Their experimental results confirmed that the contact sensor has equivalent sensitivity for detecting head-disk contact to that of a conventional acoustic emission sensor. And the defect mapping with this method was more sensitive than measurements with an optical surface analyzer. All these results were obtained using experimental method. Thus, numerical simulation studies are necessary in order to investigate the contact mechanics between asperities and the contact sensor. In this paper, a simplified contact model was developed to study contact mechanics between contact sensor and the disk asperity using finite element method. Different parameters’ effect on the signal of a contact sensor was investigated. These results are of significance for the development of a thermal contact sensor in the hard disk drive for higher recording density.

Design and Analysis of a Slider-Level Piezoelectric Sensor Array for Head-Disk Contact Detection

To realize higher areal densities towards 10 Tbit/in2, the ever decreasing flying height of the slider approaches the near-contact regime. It is essential to monitor head-disk interactions to prevent instability and damage to the head-disk interface. In this study, a piezoelectric sensor array embedded on the slider was proposed for head-disk contact detection. Integrability of the sensor array with standard slider fabrication process was considered in the design. Dynamic response of the sensor array to typical contact scenarios was obtained by finite element analysis. Simulation results suggest that the sensor array is adequately sensitive and capable of identifying contact location under intermittent head-disk contact.

Thermal Asperity Sensor Application to Hard Disk Drive Operational Shock

Thermal asperity (TA) sensor is used to investigate head disk interface (HDI) dynamics during operational shock. It is revealed that the output of TA sensor as a function of shock acceleration and back-off level. Air bearing modes being excited during operational shock indicates head-disk contact happens. The mechanisms and exciting sources of head-disk contact were explored by analyzing the TA sensor signal. Both experiment and simulation results show that motor base mode and head stack assembly (HSA) bending mode are the two most important contributions to head-disk contact. From this study, TA sensor is found to be useful for in situ monitoring of operational shock dynamics and, thus providing the strategies to improve shock-resistant performance.