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.     

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.     

Evaluation of head/disk pseudo-impact contact byacoustic emission technique

Pseudo-impact contact at the interface between proximity slidersand magnetic rigid disks were investigated by acoustic emission(AE) technique. Under a hot/wet condition of 60°C/80% RH andan operation speed of 19.15 m/s, AE responses were collected inreal time to monitor interaction events at the head/diskinterface (HDI). The profile of AE responses shows various eventsat the HDI which could be correlated to the micro-wear of thecarbon overcoat andlubricant degradation. These events were also detected by opticalsurface analysis (OSA). These wear and degradation eventsindicate the need for choosing the proper lubricant anddemonstrate that AE is a suitable tool to detect these events atthe head/disk interface.     

Effect of volatile organic contamination on head-disk interface tribology and a method for its reduction

Volatile organic contamination is known to be one of the factors to cause the failure of head-disk interface (HDI). Therefore, reduction of its harmful effects and improvement of the stability and reliability of HDI is becoming an important issue. In this study, the effects of some model compounds of volatile organic contamination on the tribological characteristics of HDI were systematically investigated using a contact start/stop (CSS) tester. The slider surface after the CSS tests was analyzed using Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS). Transfer of lubricating oil onto the slider surface was detected after the CSS tests. The organic contaminants promoted the transfer and resulted in high and unstable friction force. Fluorinated self-assembled monolayers (SAMs) were applied on the slider surface for reducing the transferred amount of the lubricating oil. Tribological performance of the slider coated with the SAMs and the transfer amount of lubricating oil onto the slider surface in the presence of contaminant was investigated. The friction force was low and stable in the case of the SAMs coated slider even under environmental contaminant. This result could be explained by the reduction of the transferred lubricating oil because the SAMs that coated on the slider surface were low surface energy.     

Contact analysis of impact in magnetic head disk interfaces

Three-dimensional finite element analysis of head and disk contact effects induced by impact in magnetic head disk interface (HDI) are presented. Elastic–plastic contact simulations are performed using . The entire contact–impact procedures during head disk collision under the dynamic loading of half-sine pulse acceleration with profiles of 300 and 500 g amplitude and 1.0 ms in duration are described in detail. Simulation results for the contact pressure distribution at HDI, von Mises equivalent stress, and equivalent plastic strain fields are examined and interpreted in terms of impact history. A comprehensive history of head disk relative displacement and von Mises equivalent stresses within contact region are provided and the evolution of plasticity are discussed. It is shown that finite element method can provide the simulation of the contact behavior resulting from the dynamic loading.     

Effect of self-assembled monolayers modified slider on head-disk tribology under volatile organic contamination

Self-assembled monolayers (SAMs) with different endgroups were established on slider surface. The effect of the SAMs coated slider on head-disk tribology under volatile organic contamination (VOC) of octamethylcyclotetrasiloxane (D₄) was investigated using a contact start/stop (CSS) tester. The slider surfaces before and after the CSS tests were analyzed using Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS). The contact angle measurement and TOF-SIMS analysis proved that the SAMs were successfully formed on the slider surface. All the SAMs reduced the friction under the pollutant vapor. The transfer of lubricating oil onto the slider surface was detected after the CSS tests. It was found that the slider with a low surface free energy associated with small amount of lubricating oil transfer. The little the lubricating oil transfer was, the low the frictions were. These results indicate that a slider with low surface free energy can reduce the loss of lubricating oil from the disk surface, and hence improve the tribological properties of hard-disk interface (HDI) under VOC.     

Tribological Performance and Transfer Behavior of Lubricating Oils at Head-Disk Interface under Volatile Organic Contamination

Tribological performance of head-disk interface (HDI) under volatile organic contamination was investigated using a contact start/stop (CSS) tester. Slider and disk surfaces were analyzed using Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) after CSS tests. The CSS test results indicated that the friction forces were high and unstable under contamination. Transfer of lubricating oil onto the slider surface was detected after the CSS tests. The transfer amount of lubricating oil was revealed to be dependent on the chemical structure of the terminal group in the lubricating oil. Piperonyl (–CH₂−phe=(O)₂=CH₂) terminated AM3001 lubricating oil was lost more easily than two hydroxyl (–OH) terminated Tetraol lubricating oil, probably because of the weak attractive force of the piperonyl groups with carbon overcoat. TOF-SIMS chemical images indicated that the transferring behavior of the lubricating oil onto the slider surface during CSS tests was dependent on the chemical structure of volatile organic contaminants. The lubricating oil became built up on the slider surface when the dioctyl sebacate (DOS) pollutant used. In contrast, the lubricating oil distribution on the slider surface was uniform under a polydimethylsiloxane (PDMS) vapor. The different transfer behavior of lubricating oil onto the slider surface may be resulted from the changeable surface properties of slider and disk because of the coexistence with gaseous contaminants.     

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.     

A study of electrical charge at head-disk interface

The electrical charge at head-disk interface (HDI) of disk drives becomes increasingly important as head-disk spacing drops below 10 nm range. In this study, a new method of measuring electrical charge at HDI is presented. It involves measuring magnetic read-back signals (i.e. PW50), while the flying height (FH) is lowered by electrostatic force. Typical HDI charges are in the range of –0.2 to –0.9 V, depending on individual head–disk combination. Experiments were also conducted to eliminate the HDI charge by using an ionizer and surface treatment of magnetic heads. It was found that the HDI charge can be effectively eliminated by treating the magnetic head with a fluorinated carbon coating. The mechanisms of HDI charge generation and elimination are discussed.     

Laplace pressure measurement on laser textured thin-film disk

To increase the recording density of hard disk drives (HDD), head and disk surfaces must be very flat. This will make the friction between them large when liquid bridges are formed. This is a result of Laplace pressure in the liquid bridge. Therefore, the study of Laplace pressure in real HDD interface is of an interest for head-disk interface engineers. However, Laplace pressure of perfluoropolyether (PFPE) lubricant on carbon coated thin-film disk surface was not clear until now.We measured Laplace pressure between transparent flat pins and carbon coated thin-film disks with laser texturing. Using laser textured disks, we could control the distance between two surfaces precisely by the bump height. The friction coefficient between the pin and the disk surfaces was determined when the interface was fully wet by liquids. It was 0.16 and 0.1 for water and a PFPE lubricant. The Laplace pressure was then calculated using the friction force and liquid wet area when the interface was partially wet by a liquid. The liquid wet area was measured by the observation of the contact point through the transparent pins.The results showed that the Laplace pressure at the lowest bump height (11 nm) was about 2.8 MPa for the PFPE lubricant. Results agreed well with calculated curves. We consider that PFPE acts as liquid down to 11 nm.     

Particle Contamination on a Thermal Flying-Height Control Slider

Particle contamination on a slider in a hard disk drive (HDD) affects the HDD’s reliability. With the introduction of the thermal flying-height control (TFC) slider, the temperature in the head–disk interface (HDI) becomes non-uniform, which induces a temperature-gradient dependent force on particles moving in the HDI. The present article investigates the effect of this force, the so called thermophoretic force, on a particle’s motion in the HDI as well as its effect on particle contamination on the TFC slider. By numerical simulation of the particle’s trajectory together with an analytical analysis, we show that the thermophoretic force is always negligible compared to the Saffman lift force, which points to a direction parallel to the thermophoretic force. We conclude that the current particle contamination simulator without any thermophoretic forces included would not be significantly altered by the inclusion of these forces.     

Investigation of Flying-Height Stability of Thermal Fly-Height Control Sliders in Lubricant or Solid Contact with Roughness

When the magnetic spacing in hard disk drives is reduced to sub-3 nm, contact between the slider and disk becomes inevitable. Stability analysis is used in this study to investigate the head–disk interface (HDI) stability of thermal fly-height control (TFC) sliders in light contact with the disk lubricant or solid roughness. We implement an improved DMT model with sub-boundary lubrication into the CML air bearing program and analyze the stability of equilibrium states of a TFC slider under different thermal actuations. It is found that stability is lost when the slider penetrates deeper into the lubricant layer, due to a fast growth in the adhesion force, and it is restored when the solid roughness contact develops. In addition, the critical point for the onset of this instability and the range of this instability region is found to vary with lubricant thickness and protrusion surface steepness, while keeping the air bearing design the same.     

A study of contact-start-stop wear tracks by TOF SIMS

In magnetic hard disk drive system, an ultra thin layer of lubricant is coated to the thin film media surface to prevent wear. Under the condition of relative motion, the displacement and replenishment of the lubricant at the head and media contact area are the factors that control the friction and wear behavior of the system. In this study, we investigate the sliding wear disk surface prepared by contact-start-stop (CSS) test using TOF SIMS (Time of Flight Secondary Ion Mass Spectrometry). TOF SIMS is a power tool for surface analysis with both high spatial and high mass resolution. Our investigations show that the lubricant thickness variation of the disk media at the contact area can be captured by sharp ion map images of TOF SIMS, and the thickness can be inferred based on the relative ion fragment intensity. In addition, the composition variation of the slider material and the magnetic layer materials can also be monitored. Finally the sliding effect is analyzed.     

Study on the Interaction between Talc and Perfluoropolyethers under Tribological Contact

Particle contamination at the head–disk interface (HDI) is a major concern for the long-term reliability of hard disk drives. In the current study, it is shown that, surprisingly, soft and lubricious talc particles have a significant damaging effect on the friction performance of media lubricants, nanometer-thick perfluoropolyethers. Thermogravimetric analysis (TGA) and the adsorption studies indicate that two mechanisms, Lewis acid–catalyzed decomposition and surface adsorption, are responsible for the observed tribological degradation. The potential impacts of the findings here on the design of next-generation media lubricant have been discussed.     

Thermomechanical analysis of elastoplastic medium in sliding contact with fractal surface

The effects of mechanical and thermal surface loadings on deformation of elastic–plastic semi-infinite medium were analyzed simultaneously by using the finite element method. Rigid rough surface of a magnetic head and smooth surface of an elastic–plastic hard disk were chosen to perform a comprehensive thermo-elastic–plastic contact analysis at the head–disk interface (HDI). A two-dimensional finite element model of a rigid rough surface characterized by fractal geometry sliding over an elastic–plastic medium was then developed. The evolution of deformation in the semi-infinite medium due to thermomechanical surface loading is interpreted in terms of temperature, von Mises equivalent stress, and equivalent plastic strain. In addition to this, the effects of friction coefficient, sliding, and interference distance on deformation behavior were also analyzed. It is shown that frictional heating increases not only the contact area but also the contact pressure and stresses.     

Flyability and Durability Test of Dynamic Fly-Height Sliders at 1-nm Clearance

The requirement of higher storage density in hard disk drives is pushing the head-to-media spacing(HMS) to become ever smaller. Currently, thermal protrusion at the transducer that is realized by the heating element in dynamic fly-height(DFH) sliders can be used to control the HMS by applying controllable electrical power. Thus, just how low the slider can fly stably and reliably using the DFH technology is a major concern for the hard-drive industry. This article describes a test for evaluating the flyability and durability of the head–disk interface(HDI) at desired and ultrasmall clearance using DFH sliders. Through such a test, the HDI flyability and durability at 1-nm clearance were examined using commercial DFH sliders with two specially designed air-bearing surfaces(ABSs). The possibility of stable and durable on-track flying at 1-nm clearance for optimized DFH sliders was demonstrated in an ambient environment.     

Material Transfer Inside Head Disk Interface for Heat Assisted Magnetic Recording

Heat assisted magnetic recording (HAMR) promises to deliver higher storage areal density than the current perpendicular magnetic recording products. Laser heating is implemented in HAMR to achieve magnetic writing of the very high coercivity media. However, the high temperature environment creates several reliability challenges for the head disk interface (HDI). In this paper, material transfer within the HDI under HAMR recording conditions is studied. The mechanisms of material transfer are explored via experiments and modeling. This study revealed that temperature difference and mechanical interaction between the head and media are the main mechanisms for material transfer inside the HDI. Possible methods to remove the material are also discussed in this paper.     

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.     

Simulation of a head slider considering a discrete track recording technology

Discrete track recording (DTR) is a new application technology that utilizes a separate physical storage disk with grooves and ridges comprised of radial and circumferential direction in order to achieve higher data transfer rates and storage densities on a hard disk drive (HDD). However, the grooves and ridges on the DTR media cause flying stability issues. Therefore, we analyze and compare the dynamic performance of a head slider with three types of DTR media in which different DTR parameters are defined, such as groove width and pitch. Prior to simulations, we propose the DTR flying height (FH) loss equation which estimates a loss of FH on the DTR media using the defined DTR parameters. The accuracy and creditability of this equation is then predicted by comparing the results of the equation to those of the simulation. Consequently, we propose a method for designing the air bearing surface (ABS) on the DTR media using the static performance of the ABS on a continuous track recording (CTR) along with the proposed DTR FH loss equation.