Slider-disk Contact Research Articles

Investigation of Bias Voltage and Relative Humidity on Wear and Pit Formation at the Head–Disk Interface

Wear and pit formation of a thermal-flying height-control (TFC) slider was studied as a function of bias voltage and environmental relative humidity (RH; 5%–85%). Burnishing tests were performed by applying an “over-push” heater power to the heater element, causing slider–disk contacts. The experimental results showed that both bias voltage and RH have a large effect on the power-to-contact change and the wear of the slider. Atomic force microscopy (AFM) was used to study the wear and pit formation as a function of bias voltage and RH. The wear of the slider surface was found to be less at the positive disk bias voltage compared with the negative disk bias voltage at all RH conditions. Pit formation was observed on the slider in the vicinity of the read–write transducer area when a positive bias voltage was applied to the disk at high RH conditions (85%). Pit formation was not observed at low RH.

Analysis of the Loading Process of a Slider With Reflowed Lubricant in a Hard Disk Drive

As the recording density of hard disk drives has increased exponentially recently, the load/unload process can be affected severely by the nanolevel changes in the air-bearing surface (ABS) induced by the reflow behavior of perfluoropolyether molecules. In this paper, clean-up time and reflow time were first measured by using thermal flying controller measurements to check whether reflowed lubricant can affect loading performance. The measured clean-up time and reflow time were 120 s and 40 min, respectively. Because the clean-up time is so much longer than the loading process, almost all the reflowed lubricant remained on the bottom of the ABS during the loading process. Next, the lubricant behavior at rest was modeled and simulated theoretically by using a diffusion process. At rest, the time-dependent profiles of the reflowed lubricant were investigated. In addition, the ABS was modified with reflowed profiles at various time intervals and loading process was carried out with the modified ABS for various parameters such as reflow time and loading velocity. As a result, the slider–disk contact distinctly occurred during the loading process in the case with reflowed lubricant profiles. The simulation results were verified with the experimental results. Consequently, the effects of reflowed lubricant should be considered when designing the loading process of a slider to improve head–disk interface reliability.

Dynamics of head-disk interface in hard disk drives during operational shock

Dynamic responses of the head-disk interface (HDI) and other mechanical components, such as suspension, disk, ramp and base are critical to the mechanical robustness of a hard disk drive (HDD) during the operational shock (op-shock). The conventional op-shock simulations use the simplified linear spring model to decouple the structural and air bearing analyses. This paper presents a coupled field analysis method to integrate the air bearing model with the reduced structural model based on the finite element model of the full HDD, including the all major mechanical components for op-shock simulations. The dynamics of HDI and mechanical components in HDDs during the op-shock are then investigated by simulations. The effects of air bearing nonlinearity and the disk-ramp contact on the HDI responses to op-shocks are evaluated and understood by comparisons to the simplified linear model. The results reveal that conventional de-coupled simulation methods are no longer valid for the complicated nonlinear op-shock events, especially when the disk ramp contacts or slider-disk contacts are considered. The newly developed coupled-field method is suitable for such op-shock simulations by considering the complexity of the system nonlinearity.

Accurate detection of light contact of thermal fly-height control sliders using advanced singular value decomposition

The dynamic contact and nonlinear dynamics of TFC slider has been widely characterized by harmonics in FFT of vibrational signal. The accurate determination of the harmonics have been challenging as the small peaks of the harmonics in FFT are frequently hidden by varied deterministic vibrations and noise. In this paper, advanced singular value decomposition method is employed to enhance the accuracy of the detection of the multi-harmonics. The track matrix of attractor of TFC slider vibration time series in light contact with disk is reconstructed in phase space. By using singular value decomposition theory, the singular spectrum from reconstructed attractor track matrix is estimated in order to increase the signal-to-noise ratio of the LDV response signal, which is further used for the accurate detection of uncertain, abrupt information and for early slider-disk light contact detection. This approach enables signal noise attenuation, which is critical to the accurate detection of weak harmonics, and enables the accurate extraction of light contact information, particularly for the transient abrupt cases, as well as improves the accuracy of harmonics-based system identification.

Operational shock response of ultrathin hard disk drives

Operational shock is one of the key challenges for designing ultrathin mobile hard disk drives (HDDs) due to the reduced thickness and stiffness of their mechanical components. The conventional operational shock simulations use the simplified models and methods that decouple the structural and air bearing dynamic analyses. These simplifications are not accurate as the coupling effect of air bearing and structural dynamics are ignored, and are even not valid in predicting the slider-disk contacts. In this paper, a method for the system level modelling and simulation of operational shock response of HDDs is proposed by integrating the structural finite element model of the HDD and the air bearing model, and coupling the structural dynamics analysis with the dynamic air bearing simulations. The dynamic shock response of the head-disk system in a 5 mm ultrathin HDD design is investigated. The effects of the drive base deformation together with the disk-ramp contact have been studied. The results reveal that the drive base deformation and disk-ramp contact are critical for the operational shock resistance performance of ultrathin drives.

Quantitative research into the influence of slider-disk contact force on the information intensity of the magnetic recording layer

In order to achieve the requirement of rapid growth of the magnetic storage density, the slider-disk spacing needs to be reduced to less than 2 nm. However, the slider-disk contact can easily occur within such a narrow spacing, and eventually result in the loss of the stored data in the magnetic recording film, i.e., demagnetization of the magnetic disk. Therefore, research into the magnetomechanical relationship related to the slider-disk contact demagnetization is significantly important to identify the demagnetization mechanism and further improve the anti-demagnetization performance of the magnetic disk. In this study, the nanoscratch experiment and the magnetic force microscope technology are used to investigate the magnetomechanical behavior induced by the slider-disk contact. And according to the phase imaging principle of the magnetic force microscope, the relationship between the information intensity of the magnetic recording layer and the magnetic contrast measured by the magnetic force microscope is found. Thus, a quantitative analysis method is proposed, which is different from the previous qualitative observation of the magnetic domain change. Experimental results show that the critical demagnetization load during the slider-disk contact is 120 up N. When the slider-disk contact force exceeds the critical demagnetization load, the increase of slider-disk contact force can lead to the decrease of the information intensity of the magnetic recording layer. And the decay rate of the information intensity will be rapidly enhanced after the slider-disk contact force reaches 380 up N. Moreover, the variation trend of the information intensity with the depth of the residual scratch is the same as that of the information intensity with the slider-disk contact force. Specially, before the slider penetrates the hard carbon layer of the magnetic disk, the slider-disk contact demagnetization still may occur, corresponding to the load cases from 120 up N to 200 up N. In addition, for any slider-disk contact force, the area of the surface damage of the hard carbon layer is always greater than that of the demagnetization of the magnetic recording layer. This phenomenon is related to the elasto-plastic force fields in the hard carbon layer and the magnetic recording layer. Moreover, when the slider repeatedly scratches the same location on the surface of the magnetic disk, the information intensity of the magnetic recording layer will decrease with the increase of scratching number. After the scratching number is beyond 20, the elastic shakedown status may occur in the magnetic recording layer, and correspondingly, the information intensity of the magnetic recording layer can be close to a constant value. This result is derived from the work hardening process during the slider-disk repeatedly scratching.

Thermal Protrusion-Induced Air Bearing Slider Instability at Disk Proximity and Light Contact

As the head-disk spacing is locally reduced to a few nanometers by the introduction of thermal fly-height control sliders, the instability of the flying slider at disk proximity and the detection of slider-disk contacts are of fundamental importance. Slider-disk contacts have been reported in several component-level experimental studies by observing an increase in averaged acoustic emission or laser Doppler vibrometry signals. In this paper, we investigate the transient slider dynamics at multiple locations on a slider body to study the instability from the conditions of disk proximity to light contacts. We define the onset of contact using the pattern transitions of acoustic emission signals, and we study separately the instabilities at the slider's trailing edge center (TEC), leading edge center (LEC), and down-track direction (DTD). The results show there is an evolution of slider instability that occurs during the transition from flying to contacts. Slider dynamics modulation in the DTD are more sensitive to the onset of contacts than at the LEC, whereas the dynamics at the TEC show the least instability. The results suggest that the down-track modulation, which happens prior to a severe TEC modulation, provides a preferred early contact detection criterion that can be used to prevent a more engaged slider-disk contact and mechanical wear.

Effect of Low-Frequency Vibration in $Z$-Direction (Out-of-Plane) on Slider Dynamics

To achieve the areal density of 1 TB/in2 in hard disk drives (HDDs), the magnetic spacing between the slider and disk needs to reduce to 1 nm or smaller. At such small clearance, the contact between the slider and disk is inevitable. Slider-disk contact may induce a drastic slider response, which has a significant negative impact on the reading/writing performance. In this paper, the effect of a low-frequency vibration in the Z-direction (out-of-plane) brought by the tip of gimbal (TG) on slider dynamics is studied. The impacts of three different excitations on the TG resonance are explored. Under such excitations, the bending mode of TG can be easily excited, which consequently induces the vigorous slider vibration. The influences of the air-bearing stiffness on slider dynamics subject to TG mode are also studied. A slider designed with weaker air-bearing stiffness will suffer a stronger TG resonance than that of stiffer air bearing. These experimental and simulation results show that better TG design and more strict environmental or excitations control are required to achieve a robust head disk interface (HDI).

A numerical investigation of different touchdown patterns of thermal-flying-height-control sliders

In this study we employ a numerical approach to explore the touchdown patterns of a thermal-flying-height-control (TFC) slider. Depending on the roughness of the head disk interface and thickness of the lubricant layer a TFC slider can experience different stages during touchdown. Three different touchdown patterns are shown. With a rougher interface profile the slider smoothly transfers from a flying stage to a sliding stage. With an intermediate smooth interface profile the slider experiences a flying-bouncing-sliding transition. With the smoothest interface the slider goes through a flying-bouncing-surfing-sliding transition. Different stages are characterized by different slider dynamics and slider-disk contact status. The different touchdown dynamic patterns shown here can result in a significant difference in the easiness of successful touchdown detection. The general approach proposed here may also be applied to investigate the effects of other important head disk interface factors, e.g., air bearing surface design, heater, suspension, etc. on the slider’s touchdown dynamic behaviors.

Effect of Mechanical Parameters for Loading Contact and Instability in HDD

Ramp load/unload (L/UL) technology becomes core technology in mobile hard disk drive (HDD) because it enables higher areal density and stronger shock robustness. However, ramp L/UL has some instability because of the structural limit that a head should repeatedly move between a ramp and a disk. Among much instability, the slider-disk contact generated during L/UL process mainly affects the drive reliability. In particular, the slider-disk contact during the loading process has a critical effect on head-disk interface (HDI) failure due to difference of impact force by much higher height level despite of low frequency. Therefore, this paper mainly focused on the loading contact and instability. At first, we defined the loading contact, scratch and instability generated by the slider-disk contact. To do so, we verified the contact location considering the interaction of the air bearing surface (ABS) and the disk, and we analyzed the relations for the loading contact between the ABS and the slider motions. After definition for the loading contact and instability, we evaluated the effects of the head stack assembly (HSA) height, the ramp height and the disk height based on the ABS performance. The relations among the HSA, ramp, and disk height affected the main parameters related to the loading contact and instability such as the Z-height, the pitch/roll slider attitude, and the gramload. We focused on the effect of the mechanical parameters according to the loading contact and instability. Finally, we measured the height differences for passed and failed drive samples. We could find out how the height differences cause the loading contact and instability through the relation among mechanical parameters such as the height of the HSA, the ramp and the disk. Based on these results, this paper proposed that the mechanical parameters such as the height relations should also be considered in order to avoid the loading contact and instability and to improve the loading performance.

Contact and temperature rise of thermal flying height control sliders in hard disk drives

Contact and interfacial temperature rise upon slider-disk contact in hard disk drives is investigated using thermal flying height control (TFC) sliders. To achieve contact, the heater element of the TFC slider is energized with constant and square wave voltage input profiles with increasing bias. The temperature rise during slider-disk contact is estimated from the resistance change at the read element using auxiliary calibration measurements. Laser Doppler vibrometer measurements show that vertical gimbal vibrations occur if the power input to the heater exceeds a critical level, seemingly related to slider-disk contact. During contact, the same frequencies occur in the spectrum of the resistance of the read element and the vertical gimbal velocity. Furthermore, it is found that the gimbal is excited at additional frequencies for a square wave heater input profile compared to a constant heater input profile.

Analysis of the unloading performance considering dimple–flexure behavior in HDD

Currently, almost all hard disk drives (HDDs) have adopted a loading/unloading mechanism that increases their recording capacity and improves their reliability. However, these mechanisms still create a few scratches or defects in the loading/unloading zone. Slipping at the dimple–flexure interface was recently reported as one of the causes. In this research, we first analyzed the relative behavior of dimple–flexure based on fretting wear marks. We determined that the dimple–flexure behavior included both slipping and rotating motion simultaneously. We then verified the distinct slipping and rolling phenomenon at the moment of ramp contact using finite-element method (FEM) analysis. An experimental setup was constructed, and an unloading experiment was carried out to obtain the ramp contact characteristics corresponding to various unloading velocities. Based on the verified FEM, the characteristics of dimple–flexure relative behavior were investigated for various suspension design parameters, ramp contact characteristics, and unloading velocities. A higher ramp contact force and shorter contact duration resulted in larger slip displacements and roll angles between the dimple and the flexure. Finally, we analyzed the unloading performance of an HDD for various design parameters using quasi-static approximation while considering the relative behavior between the dimple and flexure. The quasi-static analysis indicated a change of approximately 15 % in the flying height at an unloading velocity of 40 in. per second when the relative motion between the dimple and flexure was considered during the unloading process. Even, slider–disk contact occurred at an unloading velocity of 50 ISP.

Dynamic instability of thermal-flying-height-control sliders at touchdown

With the wide application of thermal flying-height control (TFC) technology in the hard disk drive industry, the head-disk clearance can be controlled to as low as ~1 nm. At this ultra-low clearance, the air bearing slider is subject to relatively large interfacial forces, and it experiences more complicated dynamics, compared with the flying case. In this study we conduct a numerical analysis to investigate the dynamics of TFC sliders during touchdown. The general trend of the slider’s motion predicted by the numerical simulation qualitatively agrees with experimental findings. The touchdown process begins with a slight intermittent contact between the slider’s trailing edge and the disk, followed by a partial slider-disk contact at the trailing edge accompanied by a large pitch motion at the 1st air bearing mode; this pitch motion gets suppressed and the slider comes into stable sliding on the disk as the protrusion is further increased.

Three-DOF dynamic model with lubricant contact for thermal fly-height control nanotechnology

A three-degree-of-freedom dynamic contact model with friction and lubricant contact is introduced for thermal fly-height control of a near-contact flying slider. The proposed model addresses the issue of contact with a molecularly thin lubricant layer and includes a third degree-of-freedom, roll. Neither lubricant contact nor roll has been accounted for in previous models of the head–disk interface. Roll angle rotations allow for possible contact at other slider features such as the contact pads, which may induce destabilizing moments to the slider motion and lead to unwanted slider–disk contact. While roll is important primarily during operational shock, the inclusion of lubricant contact is found to be an important determinant of the severity of steady contact while lubricant properties are shown to affect the transient properties of the jump-to- and out-of-contact slider behaviour for near- and light-contact operation. The model's validity is examined using available experimental and simulation data that predict flying height gain due to air bearing stiffening as well as significant vibration zones before and after steady-state contact.

Transient Thermomechanical Contact of an Impacting Sphere on a Moving Flat

Contact between a slider and a magnetic recording disk is modeled as transient contact of a sphere on a moving flat. The sphere is assumed to be rigid, and the flat is treated as an elastic-plastic body with isotropic hardening. Heat generation is related to friction at the contact interface. Dimensionless solutions are obtained for maximum temperature rise, maximum contact force, maximum contact area, and maximum penetration as a function of dimensionless vertical initial velocity of the sphere. It is observed that transient thermomechanical contact with elastic-plastic deformation deviates from “classical theories” for dynamic elastic and quasi-static elastic-plastic contacts as the dimensionless vertical initial velocity of the sphere increases. The results are applied to optimize the slider-disk interface in a hard disk drive with respect to slider-disk contacts.

Tolerance Analysis of the Pitch Static Attitude of a Thermally Assisted Magnetic Recording System With an Optical Fiber and Prism

In this paper, we propose a thermally assisted magnetic recording (TAMR) system with an optical fiber and specially designed prism attachment. The TAMR system utilizes an optical fiber and prism to facilitate the delivery of light to the slider and media. However, problems such as increasing suspension stiffness were encountered. Using an optical fiber increases the TAMR suspension stiffness and affects the pitch torque of the system. The pitch torque affects the flying height (FH), which can lead to slider-disk contact. Therefore, we investigated the stiffness of the TAMR suspension and the relationship between FH and the pitch static attitude (PSA). The various factors affecting the FH loss and the environmental conditions were also investigated. Finally, to compensate the large pitch stiffness and prevent slider-disk contact, the air bearing surface (ABS) of the TAMR system was optimized for robust slider performance, and the tolerance of the TAMR system was analyzed for the PSA considering various parameters such as temperature, relative humidity, operating conditions and manufacturing errors. The final PSA tolerances of the conventional and TAMR suspension were calculated. These values were 1.58° and 0.37°, respectively. The sensitivity of the optimal PSA design was decreased by about 21.5%. The PSA tolerance using the optimal design was 0.47°. This indicates that the tolerance of the optimal model is larger than that of original model, which was about 21.3%.

Transient slider-disk contacts in the presence of spherical contamination particles

Transient thermal–mechanical contacts between a slider and a rotating disk in the presence of contamination particles are investigated using finite element analysis. The maximum temperature rise due to frictional heating, the maximum plastically deformed area, and the maximum residual penetration into the disk surface are determined as a function of particle properties for aluminum and glass disk substrate. It is found that contact with soft and deformable particles can cause a large temperature rise, sufficient for thermal erasure, even though physical damage of the media is not observed. Contact with hard and rigid particles can lead to severe plastic deformation as well as high temperature rise of the media, representing the worst case for tribological integrity of the slider-disk interface.

Effect of Impact Conditions and Slider Corner Radius on the Thermal-Mechanical Response During Slider-Disk Contacts

We investigated the transient contact between a magnetic recording slider and a rotating disk using finite-element analysis. We determined the maximum temperature and the maximum plastic strain in the contact region as a function of impact conditions and the corner radius of the impacting slider. We observed a strong dependence of the maximum temperature and the maximum plastic strain on the impact conditions and slider corner radius. We examined the effect of substrate properties for both glass and aluminum disks.

Slider Dynamics in the Lubricant-Contact Regime

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Thermal fly-height control sliders that are capable of subnanometer level actuation are used to investigate the vertical, down-track, and off-track slider dynamics in slider-disk lubricant contact. Slider-lubricant contact introduces significant excitation in all three directions, and the slider dynamics is dependent on the degree of lubricant-contact. The lubricant surface has a significant role in determining the physical clearance and slider fly-height. While slider-lubricant contact may be successfully achieved by carefully controlling the heater power, improved slider designs and associated heater induced protrusion profiles are necessary to successfully mitigate contact induced vibrations and meet the challenges for future hard disk drives. </para>

Simulation of Magnetic Erasure Due to Transient Slider-Disk Contacts

Erasure of magnetic information due to transient slider-disk contacts is investigated. A three-dimensional thermal-mechanical model of a transient contact between a slider and a multilayered thin film disk is developed. Temperature and plastic strain distributions in the contact zone are evaluated and erasure of information is determined based on a simplified erasure criterion. The effect of material properties and impact conditions on erasure is determined for disks with glass and aluminum substrate. Design recommendations for minimization of erasure are discussed.