Experimental and analytical studies of dynamic friction at the head/disk interface

Friction is an important parameter that critically impacts the tribological performance of a head/disk interface. The head/disk interface with laser zone texture affords a model system for the study of dynamic friction by virtue of its precisely-controlled contact geometry. By using two types of head sliders, i.e. the conventional slider and the padded slider, and a matrix of hard disks with a wide range of laser zone texture parameters, head/disk contacts involving a small number as well as a large number of bumps are realized. A rich variety of dynamic friction behaviors are observed with respect to bump height and bump density variations. To shed new light on the nature of HDI dynamic friction, an analytical model that treats both the deformational and the adhesive friction components on equal footings is formulated. It is shown that, based on the model analysis, the friction is deformation-dominated for HDIs involving a small number of contacting bumps and adhesion-dominated for HDIs involving a large number of contacting bumps. In the former case the friction decreases with bump density, whereas in the latter the friction increases with bump density.     

Experimental investigation of AE and friction signals related to the durability of head/disk interface

The durability of a hard disk drive is one of the most critical issues that must be optimized for best performance. Especially as the flying height of the head slider of a hard disk drive decreases over the years, the concern for surface damage and head contamination continues to grow. In this paper the characteristics of AE and friction signals for various operating conditions using CSS and drag tests were investigated from the durability point of view. Also, the wear characteristics of the laser bumps on a magnetic disk were compared between the CSS and drag tests. The general shapes of the AE and friction signals during a single CSS test were quite similar even under less than ideal operating conditions. However, it was found that the AE signal was more sensitive than the friction signal in assessing the damage of the slider/disk interface. Finally, a correlation was established between the CSS and drag testing methods with respect to the laser bump wear. This outcome suggests that the drag test may be used to accelerate the surface damage effect of head/disk system.     

Microwear mechanism of head carbon film during head disk interface sliding contact

Thermomechanical sliding contact of head disk interface (HDI) causes critical wear on the carbon film of a head slider. An improved contact model accounting for both asperity and substrate deformation is applied to analyze the HDI contact behavior, while theories of frictional heat generation and heat transfer are used to investigate the change in HDI temperature. Based on actual HDI design and operation parameters, parametric study of thermomechanical HDI contact has been performed. It was found that severe wear of head carbon film would be significantly attributed to thermal degradation of carbon material during its sliding contact.     

The decomposition mechanisms of a perfluoropolyether at the head/disk interface of hard disk drives

The decomposition mechanisms of a perfluoropolyether (ZDOL) at the head/disk interface under sliding friction conditions were studied using an ultra‐high vacuum tribometer equipped with a mass spectrometer. Chemical bonding theory was applied to analyze the decomposition process. For a carbon coated slider/CN_x disk interface, the primary decomposed fragments are CFO and CF₂O, caused by the friction decomposition and electron bombardment in the mass spectrometer. For an uncoated Al₂O₃–TiC slider/CN_x contact, CF₃ and C₂F₃ fragments appear in addition to CFO and CF₂O, resulting from the catalytic reactions and friction decomposition, indicating that the decomposition mechanism associated with friction leads to the breaking of the main chain of ZDOL and forms CF₂=O, which reacts with Al₂O₃ to produce AlF₃, and the rapid catalytic decomposition of ZDOL on the AlF₃ surface follows. Moreover, the effects of frictional heat, tribocharge, mechanical scission and Lewis acid catalytic action, generated in friction process, on the decomposition of ZDOL are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sliding-pin shapes and lubricant thickness change on a thin-film magnetic disk

In developing hard disk drives, it is necessary to keep lubricant as thick as possible during operations. For this purpose, we studied lubricant loss under different-shape contact-sliders on thin-film magnetic disks by using transparent-pin sliding tests with a built-in ellipsometer. We compared sliding pins with spherical, flat circle, flat square, and double-flat-rail surfaces.We found that lubricant loss was smaller under flat pins than under the spherical pin, and the smallest under the double-flat-rail pin among flat sliders. The results show that the horizontal and vertical shapes of sliders must carefully be selected for contact recording systems.     

The Effect of Phosphazene additives to passivate and stabilize lubricants at the head/disk interface

There have been a number of applications for lubricant additives in the disk drive media area, the first of which was for pseudo-contact recording with inductive heads (tri-pad sliders) in an effort to stabilize the head/disk interface and minimize lube decomposition under hot/wet conditions. A number of additives have been tried which include antioxidants as well as Lewis bases, the latter in an effort to passivate the catalytic activity of the Lewis Acid sites on the slider which results in the decomposition of the perfluoropolyether (PFPE) lubricants such as Z-Dol, AM and Z-Tetraol. In addition to this passivation action of the phosphazene toward catalytic decomposition of the lubricant, it has recently been reported that the use of X-1P (a cyclic phosphazene) also enhances reflow of the lube, increasing the durability of the head disk interface. In this regard there are still a number of unanswered questions that pertain to the mechanism of the interaction of the X-1P with the lubricant and/or carbon to cause this increase in mobility of the lubricant resulting in the enhanced durability.There are numerous technical issues associated with the use of the various additives with the main one being compatibility between the additive and the PFPEs as well as the carbon surfaces on which they are coated. These issues include bonding, phase separation of the components, and the transfer mechanism for the additive to the slider where the passivation is required.In this paper, we will look at the interaction of the X-1P with the carbon overcoat on the media in an effort to try to better understand the mechanism of such an interaction and its effect on the mobility of the lubricant as well as the amount of bonded lube on the disks.     

Development of flying type head/slider for optical recording technology

Flying-type sliders with the integration of optical components were developed for Near-Field Recording (NFR) and Far-Field Recording (FFR) technologies. The key design issue was the integration of the optical components with the slider. Due to the size of the lenses mounted on it, the slider had to be relatively large, corresponding to the size of a micro-slider. Also, the non-uniform distribution of the slider body density was incorporated in the design. As for the optical disk substrate, a plastic material such as polycarbonate was investigated because of its manufacturing convenience and cost effectiveness. The flying and tribological performance of the prototype optical sliders on various media were assessed. The results showed that the tribological characteristics of the slider/disk interface were sensitive to several factors including the properties of the disk. Adequate flying characteristics of the optical sliders on glass (NFR) and plastic (FFR) disks could be attained by optimizing these parameters.     

Study on the cyclotriphosphazene film on magnetic head surface

Stable lubrication is very important to the slider/disk interface with the increasing demand on the life of computer hard disk drive (HDD). The inert lubricant perfluoropolyether (PFPE) on the surface of magnetic hard disk is still prone to be catalyzed to decomposition by the slider material Al₂O₃. The properties of a partial fluorinated hexaphenoxy cyclotriphosphazene, X-1P, are investigated and its function to reduce the catalytic decomposition of PFPE is discussed. The results of contact start–stop (CSS) tester indicate that the thermal stability of the lubricant was greatly improved in the presence of X-1P, and its film thickness has a great influence on the lubrication properties of the HDD.     

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.     

A finite element simulation of the two-dimensional head/tape interface for head contours with longitudinal bleed slots

The two-dimensional (2D) steady-state linear head/tape interface with surface asperity contacts and tape shear deformations is analyzed using the finite-element method. A non-uniform mesh is utilized to improve the computational efficiency and to simulate cylindrical heads with longitudinal slots. Numerical results for the head/tape spacing and contact pressure are obtained for 2D head/tape interfaces using cylindrical heads with longitudinal slots. In addition, numerical examples are presented for the head/tape spacing and the pressure distribution using heads consisting of triple arc elements with and without longitudinal slots.     

Simultaneous five-wavelength interferometry for head/tape spacing measurement

An improved five-wavelength interferometer with high-speed shutters in the light path was designed and implemented. The interferometer allows switching between two sets of three wavelengths, keeping one wavelength in each measurement in common. The set-up allows nearly simultaneous acquisition of fringe intensities and can be used to measure the head/tape spacing in a moving linear tape drive. The precision of the new five-wavelength interferometer was investigated and was found to be superior to the precision obtained with a three-wavelength interferometer.     

Roughness effects on head–disk interface durability and reliability

This paper deals with the tribological reliability and durability issues in modern magnetic recording hard disk drives, where the slider flies at typically less than 10 nm. Specifically, we investigate the effect of disk surface roughness on the above performance characteristics. The durability of the interface is investigated using low-pressure on-track tests. The drive-level reliability data are presented to confirm general conclusions reached from the component-level testing. Component-level slider–disk clearance measurements are also used to better understand how the roughness affects both durability and reliability. Finally, we discuss possible explanations for the experimentally observed relationships.     

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.     

Effect of slot edge defects on the head/tape spacing

The effect of slot edge defects on the performance of the head/tape interface is studied for single and double module heads. A number of typical edge defects was created artificially on a glass replica of an actual head, and three-wavelength interferometry was used to study spacing changes caused by these defects. The results show that head edge defects have a small influence on the flying behavior of tape with the effects being localized to the immediate defect area.     

Ultra-low-flying-height design from the viewpoint of contact vibration

The design of a head-disk interface for ultra-low flying height has been studied from the viewpoint of contact vibration. It is known that a super-smooth disk is necessary for a slider to fly at an ultra-low flying height; however, such a disk increases the friction force, which potentially increases the vibration of the slider. To solve this problem, the head-disk interface must be optimized to reduce this increased vibration. It has been shown that a large pitch angle and center-pad-mounted read/write elements have advantages in terms of slider/disk contact. It has also been found that a micro-texture on the air bearing surface can prevent contact vibration. Moreover, a frequency-shift-damping slider was found to damp the vibration effectively. To further investigate these findings, numerical simulation and modeling of slider dynamics during contact have been performed. Their results revealed two zones of contact vibration: a stable zone and an unstable zone.     

Analysis and optimization of dynamic response of air bearing sliders to disk waviness

Flying stability has been becoming more critical for air bearing sliders with extremely low flying height (FH). Therefore, the effects of disk waviness on flying height modulation (FHM) cannot be neglected. This paper presents an analytical study on the mechanism of FHM of air bearing sliders due to disk waviness, and a design optimization for increasing waviness following ability of sliders. An analytical three-degree-of-freedom (3-DOF) model is developed, where the air bearings are modeled as six lumped linear springs and dampers. The purpose of this model is to develop a quantitative understanding of how air bearing sliders respond to disk waviness. The dynamic characteristics of the slider-air bearing system are then analyzed, and the closed-form frequency resonance function (FRF) of FHM to disk waviness is derived. The impact of disk surface features and the positions of the trailing pad, the side pads, the leading pads and the negative pressure center on FHM are also investigated using parametric analysis. The analysis results show that the improvement of the roll-off characteristics of the disk surface waviness can also decrease the FHM. In addition, shortening the distance between the trailing pad pressure center and the head position, moving backward the side pads and leading pads and forward the negative pressure center can increase waviness following ability of the slider. Finally, an air bearing slider is designed according to the proposed design strategies for reducing the FHM due to disk waviness.     

Dynamic Adhesion Characteristics of Spherical Sliders Colliding with Stationary Magnetic Disks with a Thin Lubricant Layer

The dynamic indentation characteristics of 1- and 2-mm-radius hemispherical glass sliders when colliding with stationary magnetic disks under various lubricant conditions were investigated to clarify the dynamic interfacial forces between flying head sliders and magnetic disks. The collision times were ~15 and ~30 μs, respectively, and independent of the impact velocity. For a 1-mm-radius slider (Ra roughness = 1.71 nm), a clear adhesion force nearly equal to the static pull-off force was observed at the instant of separation when the lubricant thickness was from 1 nm without UV (0.69 nm mobile lubricant thickness) to 3 nm with UV (1.89 nm mobile lubricant thickness). The dynamic adhesion force was maximum when the slider had separated from the disk surface by about 2 nm and dropped from the maximum to zero when the separation reached more than 5 nm. When the mobile lubricant thickness was 0.43 nm, a clear adhesion force was not observed. For a 2-mm-radius slider (Ra roughness = 0.34 nm), a clear adhesion force, similar to the static pull-off force, was observed at the instant of separation at almost all lubricant thicknesses and impact velocities tested except at a small mobile lubricant thickness of 0.43 nm with impact velocities greater than 1.1 mm/s. The dynamic adhesion force dropped from the maximum to zero when the distance traveled from the maximum reached more than 5 nm. These results suggest that the dynamic adhesion force of 1- and 2-mm-radius sliders originates from meniscus formation rather than van der Waals force.     

Interface technology of ultra-low flying height and highly stable head–disk interface for perpendicular magnetic recording

This paper reports authors’ efforts in slider and interface technologies with extremely small and very high stability head–disk spacing. The dual shallow step strategy is proposed in the femto form-factor slider design. It is found that the dual shallow step design is very effective in reducing flying height modulation (FHM) caused by disk waviness and enhancing the cooling effects on the read/write elements. A simple geometric model is built to explain the schematic of the improvement in FHM.     

Probability Model for the intermolecular force with surface roughness considered

This paper studies the intermolecular force considering both the roughness of the air-bearing surface and the disk surface by simulation. A model is developed to deal with the intermolecular force, the contact force and the air-bearing force based on the probability distributions of the roughness of the surfaces. The intermolecular force is linked with the contact force when its repulsive term is stronger than its attractive term. In such a case, all the intermolecular force, the contact force and the air-bearing force can be extended to the various flying height regions. Some interesting results are observed and discussed. It is found that both the Hamaker constant and the surface roughness have significant influences on the intermolecular pressure. Compared with the intermolecular pressure with smooth surfaces, that with the surface roughness considered shows greater attractive pressure at the flying height higher than 0.7 nm approximately, but much smaller values between 0.26 and 0.7 nm approximately. A negative stiffness region exists when the minimum flying height is between −0.2 and 1.2 nm for the case studied in this paper. It shows that the Probability Model is suitable for the intermolecular force calculation with the surface roughness considered.     

Load/unload measurements using laser doppler vibrometry and acoustic emission

The dynamics of the load/unload process are studied using a so-called ‘periscope approach’ which allows us to follow the slider motion during load/unload (L/UL) with the beam of a Laser Doppler Vibrometer (LDV). LDV signals and acoustic emission signals are obtained for three different slider airbearing designs and for load/unload conditions with different vertical velocities and spindle speeds. The load process is investigated statistically using the acoustic emission signal in order to determine the effect of vertical load speed and spindle speed on the probability of contacts between slider and disk.The results indicate that small vertical load speeds decrease the number of head/disk contacts, and that slider designs with a cavity centered close to the trailing edge enable a smooth unloading process.