Wear Analysis of Head-Disk Interface During Contact

Slider/disk, interface wear is inevitable for ultra low flying hard disk drives and is the central issue for proximity recording. While the disk wear has been addressed in the literature, slider wear has been largely considered to be trivial and is ignored. However, with the improvement of disk overcoat and introduction of diamond-like-carbon overcoat on slider air bearing surface, the surface hardnesses of the slider and disk are approaching each other and, therefore, the slider surface wear becomes significant or, in some conditions, even dominant. In this study, a theoretical model is developed which takes account of both the disk and slider wear. It includes the effects of the air bearing characteristics, pitch stiffness, material properties and surface topography of both the slider and disk. Numerical results are illustrated for typical proximity recording interface, where the evolutions of the slider, disk, contact force and pitch angle are evaluated for various air bearing stiffnesses, material properties and surface topographies. This model is intended to provide some fundamental understanding of the slider/disk interface evolution and be used as a guideline for designing a reliable slider/disk interface for proximity recording.

To increase the recording density of rigid magnetic disk files, it is important to understand and prevent the Head Crash problem. In Winchester type head-disk interfaces, Head Crash is most liable to occur during Contact Start / Stop (CS/S) o-erations. Head wear or disk wear after Head Crash were reported by past researchers. But the Head Crash process itself has not been made clear. In this study, friction force and acoustic emission(AE) signal during CS/S operations are measured until Head Crash. The results show that the piling up of disk binder on a head surfase is the main reason of Head Crash. It is triggered by the intrusion of wear particle into the head-disk interface. From these results, more durable magnetic coated disks will be developed in the future.

Understanding of dynamic adhesive, contact and friction interactions and associated instabilities at magnetic storage head-disk interfaces (HDIs) is critical as the physical distance between the recording slider and the high-speed rotating disk decreases to sub-five nanometers to achieve 155 Gbit/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (1 Tbit/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) recording densities. In this study, a two degree-of-freedom nonlinear dynamic model that includes realistic roughness, adhesion, friction and the dynamics of a flying and contacting HDI was developed to characterize a fully flying (6.8 nm) and a pseudo-contacting (3 nm) HDI utilizing different air-bearing designs. The effect of roughness on the slider flying performance and stability was also investigated through a comparison to a simple two flat parallel surface counterpart. The simulation results revealed that while adhesion has little influence in a fully flying interface when flying heights are above 5 nm, it leads to prolonged contact, higher bouncing vibrations and unstable slider behavior for 3 nm flying height. At such low nominal flying height, the inclusion of the electrostatic force also exacerbates HDI unstable behavior

Fly-contact head disk interface (HDI) is known as a promising approach for the further reduction of the magnetic spacing between the head and media for ultra high recording density. In this paper, we report a numerical simulation study on the dynamics of the fly-contact head disk interface. It is found that design of a stable and reliable fly-contact HDI needs to consider the balance of minimizing both the fly-height (FH) variation and the contact force. The critical contact force - the minimal contact force for a stable fly-contact HDI is explored and a critical contact force of as low as 3.1 mN is obtained. It is found that the critical contact force depends on the combined effect of air bearing force, adhesive force and suspension load, which can be optimized by a proper air bearing surface and suspension design.

With the introduction of thermal fly-height control sliders, the local head-disk clearance can be reduced to a range from $\sim10$ nm to contact. This actively controlled touchdown (TD) has been used as a way to study the head–disk interface (HDI), the characteristics of which greatly affect the areal density and reliability of drives. In this paper, a high spatial resolution TD study is performed at the drive level using TD sensor signals and head-media spacing signals. Different flying stages, including passive flying, TD transition, and over push modulation, are identified from the experimental results. A correlation between the aforementioned signals at the passive flying stage is found to represent the disk surface property. The passive flying stage and over push modulation stage of TD are further analyzed in both the time and frequency domains. By establishing a correlation between the fly height and the temperature response, we can provide insight into the heat transfer model for the HDI.

Magnetic head-disk interfaces (HDI) which are designed so that the mechanical spacing, between the magnetic recording head and spinning disk media, is so small that frequent intermittent contacts occur during normal steady state operation, can be said to be operating in the "proximity" recording regime. Mechanical spacing ("fly height") modulation, with characteristic frequency well above the estimated air bearing resonance frequencies, but well below slider ringing frequencies, has been observed in the proximity recording regime. In this paper, the effect of slider preload change, and the effect of atmospheric pressure change, on the frequency and amplitude of such fly height modulation, as measured using laser Doppler vibrometry, is reported. A simple analytical model was developed to explain the existence of the modulation and its observed dependence on applied gram load.

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.

Effects of temperature dependent air properties on the performances of a thermal actuated slider

For increasing areal density in hard disk drives (HDDs), the physical clearance between the read/write element and the surface of the disk has been continuously decreasing to 1 nm or below [1]. At such a low clearance, the contact between the head and the disk is inevitable to occur, so head wear is becoming a critical issue in the development of HDD. The contact between the head and the disk induces a frictional heating, which may generate an additional thermal protrusion in the contact area of the head, and causes more wear. On the other hand, the target clearance in a HDD is generally determined by pulling back a setting TFC power from the touchdown point, accurately identifying the touchdown point is very significant for the clearance control in hard disk drive. A thermal protrusion is caused by friction-heating in the status of touchdown. Therefore, it is very necessary to quantitatively understand on friction induced thermal protrusion and clearance loss.

Heat-assisted magnetic recording (HAMR) is one of prospective high density recording technologies in current hard disk industry. In this paper, we incorporate HAMR optical structures into a finite element model of a thermal flying height control slider and study thermal effects of these structures on the HAMR head-disk interface. Our focus will be on the slider flying height (FH) changes due to the heat loss in the waveguide and near field transducer (NFT). According to our results, we find that large heat dissipation in NFT alone could affect the slider's FH due to the additional thermal protrusion induced around the writer. The heat dissipation in the waveguide could also influence on FH drop. In addition, the media back heating effect on the slider temperature and thermal deformation is also analysed in this paper. The numerical result shows that the respective thermal deformation caused by media back heating is about 0.09 nm with 72 nm of heat spot size. This amount could be significant when the slider's FH is below 1 nm. Therefore, it is very important to improve the efficiency of laser optical delivery system and reduce thermal effects on a HAMR head-disk interface.

The thermal flying height control (TFC), aka dynamic fly height (DFH), technique has been recently used in the head disk interface of hard disk drives to obtain a lower head-media spacing. The air bearing cooling effect, i.e., the heat conduction between the slider and the air film, has been incorporated in the numerical thermal–mechanical simulation of the slider’s static performance. However, the heating effect of the viscous dissipation of the air flow has not been considered yet. In this article, both effects are included in the simulation of a flying slider with its flying height controlled by thermal protrusion, and different models for the air bearing cooling are used to obtain the slider’s static flying attitudes. The simulation results directly show that the air bearing cooling is dominant compared with the viscous heating. All of the air bearing cooling models, including a recent one that considers the dependence of the air molecular mean free path on the air temperature, have simulation results close to each other. The largest relative difference in the simulated flying height is less than 9% even when the transducer flying height is lowered to below 2 nm.

The air bearing’s response to regions of elevated temperature on its bounding surfaces (the slider and disk) may be an important consideration in the head–disk interface design of heat-assisted magnetic recording (HAMR) systems. We implement the general non-isothermal molecular gas lubrication equation into an iterative static solver and dynamic air-bearing solver to evaluate the effect of localized heating of the air-bearing surface (ABS) due to the near-field transducer (NFT). The heat-dissipating components in our simplified HAMR design are the NFT, laser diode, and thermal flying height control (TFC) heater. We investigate the effect of each HAMR slider component on ABS temperature and thermal deformation and the slider’s flying height. The NFT induces a localized thermal spot and protrusion on the larger TFC bulge, and it is the location of maximum temperature. This ABS temperature profile alters the air-bearing pressure distribution, increasing the pressure at the hot NFT location compared with predictions of an isothermal air-bearing solver, so that the center of the pressure acting on the ABS is slightly closer to the trailing edge, thereby decreasing the pitch angle and increasing the minimum flying height. Other researchers have shown that the NFT’s thermal response time may be much faster than its protrusion response time (Xu et al. in IEEE Trans Magn 48:3280–3283, 2012). The slider’s dynamic response to a time-varying NFT thermal spot on the ABS while the combined TFC and NFT induced thermal protrusion remains constant is investigated with our dynamic air-bearing solver. We simulate the slider’s step response to a suddenly applied ABS temperature profile and a pulsed temperature profile that represents laser-on over data zones and laser-off over servo zones. The sudden (step) or rapid (pulse) increase in ABS temperature induces a sudden or rapid increase in pressure at the NFT location, thereby exciting the air bearing’s first pitch mode. For the slider design and simulation conditions used here, the result of the pitch mode excitation is to alter the position of the center of pressure in the slider’s length direction, thereby changing the pitch moment. In response, the pitch angle and minimum flying height change. The step response decays after approximately 0.15 ms. Because the laser duty cycle is much shorter than this response time, a periodic disturbance is predicted for the center of pressure coordinate, pitch angle, and minimum flying height. The peak-to-peak minimum flying height modulations are relatively small (only up to 0.126 nm); more significantly, the time-averaged minimum flying height increases 0.5 nm for the NFT that reached 208 °C compared to simulations of the isothermal ABS at ambient temperature.

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