Interactions between nano-spacing flying head sliders and ultra-thin liquid lubricant films with non-uniform distribution in hard disk drives

This paper describes the effect of ultra-thin liquid lubricant films on air bearing dynamics and flyability of less than 10 nm spacing flying head sliders in hard disk drives. In particular, the effect of non-uniform lubricant film distributions on head/disk interface dynamics are studied. The disks with lubricant on one half of disk surface thicker than the other half were used in this study. The dynamics of sliders is monitored using acoustic emission (AE) and the interactions between the slider and disk are investigated experimentally. The disks were also examined with a scanning micro-ellipsometer before and after each test. Complicated slider responses were observed and clarified. In addition, it was found that the periodic lubricant film thickness modulations or non-uniformity caused by the slider-disk contact interactions could be observed. It is suggested that this lubricant film thickness non-uniformity will be one of the technical issues in order to achieve ultra-low head/disk contact interface of less than 10 nm.     

A flyability and durability dilemma in hard disk drives

A magnetic hard disk drive lubrication dilemma was demonstrated through the touch down, wearability and burnish rate tests. It was found that attempts to improve durability and flyability through changes in lubricant film thicknesses, the addition of additives, changes in lubricant molecular weight and irradiating lubricant with deep UV rays (185 nm) have been futile with loss in either flyability or durability of magnetic hard disk. Three key types of head disk interface testing methods were introduced and the results from each parameter change were shown. The touch down test was used to check the fly height of the magnetic heads. The wearability test was used to check the wear resistance of the magnetic hard disks when in contact with the magnetic heads and the burnish rate test was employed to determine the amount of wear of the magnetic heads when in contact with magnetic hard disks. These three techniques may be used for the feasibility study for any newly designed lubricant or technique to reduce the spacing between the magnetic head and magnetic disk. We demonstrate the capability of the three techniques to discriminate different process treatments. The experiments were conducted in a class 100 cleanroom.     

Lubricant transfer caused by breakage of liquid meniscus bridge

In recent magnetic storage systems, the spacing between the flying head and the disk has been decreased remarkably to less than 5?nm in order to realize ultra-high density recording. Lubricant on the disk is picked up by the flying head slider as a result of intermittent contact between the slider and the disk, or due to condensation of the lubricant vapor. In the present study, the basic characteristics of lubricant transfer (lubricant pick-up) caused by the breakage of a liquid meniscus bridge are investigated experimentally and theoretically. An experimental method by which to measure the volume fraction of the lubricant pick-up has been established. The theoretical results obtained from a simple model proposed by the authors showed good agreement with the experimental results. The validity of the theoretical model was verified based on the experimental results.     

Air-bearing surface chemical modification for low-friction head–disk interface

As a chemical modification of slider air bearing surface (ABS), fluorine ion implantation (FII) was conducted. FII treated ABS was fabricated to reduce slider surface energy, which is expected to reduce adhesion and friction force between the slider and disk. Tribological performance, such as slider flyability, was experimentally investigated. Touchdown speed, corresponds to a flying height of about 0.4 nm, was reduced by FII treatment. Slider touch-down and take-off hysteresis was also improved by reducing the adhesive force of the lubricant by fluorine-ion implantation. AE output and friction force were also reduced in the drag test. FII treated slider showed a 45% reduction of AE amplitude and 14% reduction of friction force. FII treatment was confirmed to be effective in improving slider flyability.     

Numerical simulation of molecularly thin lubricant film flow due to the air bearing slider in hard disk drives

In this paper we numerically study the evolution of depletion tracks on molecularly thin lubricant films due to a flying head slider in a hard disk drive. Here the lubricant thickness evolution model is based on continuum thin film lubrication theory with inter-molecular forces. Our numerical simulation involves air bearing pressure, air bearing shear stress, Laplace pressure, the dispersive component of surface free energy and disjoining pressure, a polynomial modeled polar component of surface free energy and disjoining pressure and shear stress caused by the surface free energy gradient. Using these models we perform the lubricant thickness evolution on the disk under a two-rail taper flat slider. The results illustrate the forming process of two depletion tracks of the thin lubricant film on the disk. We also quantify the relative contributions of the various components of the physical models. We find that the polar components of surface free energy and disjoining pressure and the shear stress due to the surface free energy gradient, as well as other physical models, play important rolls in thin lubricant film thickness change.     

Lubricant bonding, chemical structure, and additive effects on tribological performances at head–disk interfaces

 The lubricant film for head/disk application consists of bonded fraction and unbonded (mobile) fraction. It is well known that the mobile fraction of the lubricant film can replenish the surface sites where the lubricant film was depleted, thus, the surface wear is postponed or alleviated. With a continuous decrease in the head disk spacing, however, too much mobile fraction of lubricant may cause head slider lubricant pick-up, and deteriorate the interface. Two perfluoropolyether (PFPE) lubricants of Z-tetraol and Z-DOL are discussed in this paper. Lubricant Z-tetraol is characteristic of stronger bonding to a carbon overcoat, lower vapor pressure, and higher thermal stability but less mobility than Z-DOL. It is found that, for CSS (contact-start-stop) durability, the interfaces with Z-tetraol show no worse in performance than those with Z-DOL, and less head slider lubricant pick-up on those with Z-tetraol. Based on the above-mentioned, it is possible that the interfaces relying more on the lubricant bonding strength and chemical structure stability are more beneficial to tribological performances than those relying more on the lubricant replenishment. The effects of lubricant additive X1P mixed to Z-tetraol, and Z-DOL, respectively are also studied on tribological performances. Stiction, CSS durability, and head slider lubricant pick-up are discussed among lubricants Z-tetraol, Z-DOL, Z-tetraol/X1P and Z-DOL/X1P. Statistical t-test, F-test, and Weibull analyses are applied to CSS data to differentiate CSS durability performances. Additive X1P is found to enhance CSS durability for both lubricants. Lubricant Z-tetraol/X1P is recommended for the best tribological performances, followed by Z-DOL/X1P, Z-tetraol, and Z-DOL. Received: 7 August 2001/Accepted: 11 December 2001 Authors would like to thank Vidya K. Gubbi, Youmin Liu, and Gunter P. Barth at Seagate, Fremont, CA, for media supplies, lubricant properties, and head slider lubricant pick-up measurements. Authors also thank Gunter P. Bath, Frank Chang, Roger Y. Shih, Hamid R. Saman, Caroline Tjengdrawira and Sam Liang at Seagate for fruitful discussions.     

Experimental study of lubricant depletion in heat assisted magnetic recording: different lubricants on HAMR media

Heat assisted magnetic recording (HAMR) is a promising choice to surpass the super-paramagnetic limit in magnetic recording and to allow continued increase in the areal recording density of hard disk drive. However, lubricant depletion on disk surface might be a problem in HAMR due to high temperature during writing process. In this work, depletion of four types of commercial lubricants on our HAMR media has been evaluated under equivalent HAMR conditions. Linear relationships between the lubricant depletion depth and the logarithm of the equivalent laser heating time have been established for these lubricants. The depletion depths of the lubricants at different laser heating durations are predicated. It is found that the performance of the four evaluated lubricants is acceptable in respect of lubricant depletion depth by hypothesizing the total heating duration for a given point on the media over the lifetime of the drive is 10⁶ ns.     

Experiments on slider lubricant interactions and lubricant transfer using TFC sliders

Future magnetic storage density targets (>4 Tb/in. ²) require subnanometer physical clearances that pose a tremendous challenge to the head disk interface (HDI) design. A detailed understanding of slider-lubricant interactions at small clearances and contact is important to not only address magnetic spacing calibration and long term HDI reliability but also to meet additional challenges imposed by future recording architectures such as heat assisted magnetic recording (HAMR). In this work, the behavior of the disk lubricant is investigated through controlled tests using TFC sliders which are actuated to proximity (i.e. backoff) and into contact (i.e. overpush) on one specific half of the disk per rotation by synchronization with the spindle index. Observations for lubricant distribution in contact tests (i.e. overpush) reveal an accumulation of lubricant on the disk near the onset of contact suggesting a migration of lubricant from the slider to the disk as the slider approaches the disk. Experiments also reveal that there is a similar deposition of lubricant even in the absence of contact for backoff tests. Furthermore, light contact tests result in significant lubricant rippling and depletion with associated slider dynamics. The lubricant rippling frequencies correlate well with the slider’s vibration frequencies. Interestingly, strong overpush may lead to stable slider dynamics (for certain air bearing designs) that is also associated with noticeably lower lubricant distribution (compared to the light contact case), and the greatest lubricant changes are observed only at the onset and the end of contact. This paper reveals the complex nature of slider-lubricant interactions under near-contact and contact conditions, and it highlights the need for further studies on the topic to help design a HDI for recording architectures of the future.     

Reduction in lubricant pickup by bias voltage between slider and disk surfaces

We studied the effect of bias voltage between slider and disk surfaces to reduce lubricant pickup by the slider. A perfluoropolyether (PFPE) lubricant film, which is coated on the disk surface, has been considered to be charged to a negative voltage by the airflow on the rotational disk surface. Because the PFPE lubricant film is negatively charged, the lubricant pickup should be reduced by a bias voltage with a negative voltage on the slider surface. We confirmed changes in the lubricant pickup in a lubricant pickup test conducted at different bias voltages. A positive voltage of the slider accelerated the lubricant pickup, whereas a negative voltage reduced it.     

Analysis of slip characteristics between dimple and flexure in hard disk drives

A mobile hard disk drive (HDD) is often faced with inevitable mechanical problems because of the portability of the drive. Then, almost HDD use much faster emergency parking system to protect the system from these problems. However, very fast emergency parking causes a large ramp contact, which could create dimple-flexure interactions such as a dimple-flexure slip, head-gimbal assembly vibration, unexpected slider motion. These dimple-flexure interactions largely affect the flyability of a slider. As such, dimple-flexure interactions are one of the most important factors in designing a mobile HDD. Therefore, in this study, we characterized the dimple-flexure slip among dimple-flexure interactions and analyzed it using a combination of experiments and finite element modeling. We evaluated the feasibility of dimple-flexure slip and verified the main cause of dimple-flexure slip. We also investigated the relationship between dimple-flexure slips and ramp contact during emergency parking. Finally, we designed the finite element ramp contact simulation, and analyzed dynamic characteristics for the dimple-flexure slip.     

Lubricant evaluation method for FDBs using TOF-SIMS

 With the increase in recording density and data transfer rate of hard disk drive (HDD), fluid dynamic bearing (FDB) motors have been introduced due to their silence and high rotation accuracy. Although lubricant plays a major role in the development of FDB motors, it is extremely difficult to perform thorough evaluation because the quantity of lubricant used in the motor is as small as several micro liters. This paper describes a lubricant evaluation method for FDBs using the time of flight secondary ion mass spectrometry (TOF-SIMS), which enables simultaneous analyses of elements and organic molecules at PPM levels in an extremely small quantity of sample. By using this method, we found that worn metal elements generated both from spindle and bearings have substantial influences on lubricant degradation. Received: 5 July 2001/Accepted: 1 November 2001     

Investigation of slider dynamics under electro-static force

 With the continual increase in the areal density of hard disk drives, the head/disk interface engineers are continuously challenged in designing an interface with a lower mechanical clearance between the slider and disk. Although, this has traditionally been achieved by making the disk smoother, some novel ideas have been discussed, where the slider generally flies high, and when necessary, a part of the slider is moved closer to the disk surface. Another method that has been discussed is applying a voltage across the slider and disk to bring the whole slider closer to the disk. However, rapid application of voltages across the interface may lead to undesirable slider oscillations. In this paper, we study the dynamics of the slider when a voltage is applied across the slider and disk. We show that one can easily excite the air-bearing frequency and if the voltage is not well controlled, the slider may also damage the disk. We also identify the most probable mechanisms of current flow and show that the disk lubricant plays a critical role in initiating current flow. Received: 26 June 2002 / Accepted: 9 September 2002     

Vacuum vapor deposition of PFPE molecules on CHxNy and CHxFy amorphous carbon surfaces

According to the demand of increasing storage density for the magnetic data storage, a contact recording system is proposed, in which the head constantly makes contact with the disk surface during read/write cycles. In this system a stronger lubricant film for head disk interface (HDI) is required. In a conventional dip-coating method, which is carried out under atmospheric conditions, however, a fresh diamond like carbon (DLC) surface adsorbs several contaminations from the air and a strong lubricant layer is difficult to form on the DLC surface. In this study, a DLC layer is coated on a Si substrate by a plasma chemical vapor deposition (PCVD) method, then the DLC-coated Si sample is transferred to another vacuum chamber without being exposed to the atmosphere, and a perfluoropolyether (PFPE) lubricant film is vapor-deposited on the sample. The sample is analyzed by XPS and compared to the surface on which the PFPE lubricant is dip-coated. It is noted that both samples are rinsed after the coatings and the loose PFPE molecules are washed away. A thicker bonded layer of PFPE is formed on the DLC surface treated by vapor deposition. A tribological wear test is also carried out using a pin-on-disk tribotester. Both results show that the tribological characteristics are improved by the vapor deposition of PFPE molecules compared to the dip-coating method.     

Optimization of thermal fly-height control slider geometry for Tbit/in2 recording

Magnetic storage advances including thermal fly-height control (TFC) technology were able to reduce the clearance between the read/write elements of the slider and the disk surface to increase the recording density of hard disk drives without compromising the stability of the head–disk interface (HDI). Sliders employing TFC technology are designed for flying recording and can yield clearances of few nanometers. However, it is estimated that TFC technology alone cannot provide the even smaller clearances necessary to achieve Tbit/in² recording densities primarily due to the presence of instability-inducing vibrations at the HDI. In this work we perform optimization of the geometry of TFC technology sliders to achieve extremely high-density recording. We propose a flyability parameter coupled with a dynamic, contact mechanics-based friction model of the HDI that accounts for TFC geometry and its influence on the HDI dynamics. Optimization results are analyzed and an operating actuation range is identified that can yield Tbit/in² recording densities with Angstrom-level clearance and minimized vibrations while also accounting for manufacturing and operational tolerances. This allows for light (lubricant) contact or ‘surfing’ recording. The proposed methodology can be used to reduce wear at the interface and investigate the feasibility of contact recoding.     

Experimental study of lubricant depletion in heat assisted magnetic recording: effects of laser heating duration and temperature

Heat assisted magnetic recording (HAMR) is a promising approach to overcome the superparamagnetic limit in magnetic recording and enable large increases in the storage density of hard disk drives. However, it is expected that HAMR causes lubricant depletion problem on disk surface under the high temperature in the heating assisted writing process. In this study, the effects of laser heating duration and laser heating temperature on lubricant depletion are studied experimentally. It is found that lubricant depletion depth is linearly proportional to the logarithm of laser heating duration. Lubricant depletion can be mitigated effectively by lowering laser heating temperature. Lubricant depletion depth over the life-time of the drive is also predicted.     

Dynamic conformations of fluorocarbon molecules on magnetic hard disk surfaces with charged sites

To meet the demand of extremely high recording density of magnetic storage device, magnetic head is expected to reduce its flying height to sub 5 nm. Lubricant films in such system become more important and the conformation characteristics of lubricant molecules, which receive attractive forces both from the disk and the head, must be clarified for the stable flying of the head. In this report molecular dynamics simulations are carried out to investigate the conformation of lubricant molecules. The model is composed of solid surface and polar-ended fluorocarbon molecules. The surface has several reactive sites, which interact with polar end groups of fluorocarbon molecules. Varying the number of reactive sites, the processes that the reactive sites attract molecules are simulated. Results from the present simulations indicate that lubricant molecules tend to gathered and piled up. It is difficult to achieve 100% coverage.     

Reduction in total surface area by the develpment of microdroplets during dewetting

To meet the demand of an increasing storage density, the lubricant film for the head disk interface (HDI) needs to be thinner and stronger. In recent years, a new head/disk system, such as the contact type, has been proposed. It is reported that PFPE Zdol coated on a magnetic disk is dewetted and microdroplets are formed due to polar interactions. This makes a flying magnetic head unstable, therefore, the physics and chemistry of the dewetting phenomenon are topics of current interest. We investigated the formation and development of microdroplets using an atomic force microscope (AFM) and an optical microscope. First, we observed the disk surface coated with PFPE Zdol by AFM. From the cross section images of the microdroplets, we found that the microdroplets had a shape similar to a sphere. With this finding, we estimated the contact angle of the microdroplets in each image. The results showed that the contact angle of the microdroplet gradually decreased with time, which indicated the existence of a PFPE thin film in the dewetted area. The thickness of the PFPE film in the dewetted area was then measured using an elliposometer. Next, we investigated the variation in the number and the average diameter of the microdroplets during dewetting using images observed by the optical microscope. The total surface area change was also calculated from the observed results, and it was found that the total surface area, namely the sum of the microdroplet surfaces and dewetted area, was reduced by the development of the microdroplets.     

Humidity effect on head-disk clearance

The mechanism of head-disk-clearance change due to a humidity effect was investigated experimentally. The head-disk clearance was measured by moving the head elements towards the disk by thermal flying-height-control technology until the head touched the disk while monitoring them with an acoustic-emission sensor in an environmentally controlled component tester. Measured clearance change from 3% RH to 80% RH reached about ?0.6?nm at 25°C and about ?1.6?nm at 60°C. Head-disk clearance change caused by humidity was classified as the slider-flying-height change and disk-touch-down-height (TDH) change to clarify the mechanism of the clearance change. Slider FH change was dominated by absolute humidity rather than relative humidity. On the other hand, the humidity effect on disk TDH change was classified as a ??water-film effect?? and a ??lubricant-mogul effect??, which both depend on relative humidity. Both effects caused clearance change of about 0.4?nm for lubricant A, which is a lubricant with two hydroxyl functional groups at the end of the main chain. These effects were assumed to be caused by water adsorption onto the surface of lubricant. Reduction of the number of free hydroxyl groups which attract water molecules could suppress the disk TDH change related to the humidity effect.     

Effect of thermal pole tip protrusion and disk roughness on slider disk contacts

Ultra-high areal density for hard disk drives requires a stable head disk interface at a flying height lower than 8 nm. At such a low flying height, small flying height variations may cause slider/disk contacts. Slider/disk contacts can also occur when a write-current is applied to the write coil since the flying height between slider and disk can be affected by the thermal expansion of the pole tip. In this paper, we investigate the vibration characteristics of sliders during thermally induced contacts using laser Doppler vibrometry. We perform a parametric study of contact events using disks with different surface roughness and lubricant thicknesses, and analyze the slider motion statistically. For a given write current, we observe that the slider vibrations increase with disk roughness and lubricant thickness.     

Study of the stiction free magnetic recording head with DLC pad

This paper presents the study of the stiction free magnetic recording head which has diamond-like-carbon (DLC) pad for use with laser zone texture disk and smooth disk. The magnetic recording head with DLC pad is one of the potential solutions to extend contact start stop (CSS) technology for further lower head-disk spacing, since it can decrease stiction and friction between magnetic recording head and magnetic recording disk during CSS. The optimization of air bearing surface (ABS) design for improving take off characteristics, the optimization of DLC pad design for improving tipping stability and the result of CSS reliability test under several environments are shown in this paper. It is found that the occurrence of tipping accelerates the wear of DLC pad, the degradation of the lubricant and the wear of carbon overcoat (COC) protective layer on the disk. Finally it is shown that the magnetic recording head, which has optimized ABS and optimized DLC pad, has small coefficients of stiction less than 2.0 and reliable CSS performance at 15 nm flying height (FH) on the laser zone texture disk having low bump height. Tipping and major wear of DLC pads are not observed under several environmental conditions with this optimized design.