Single vs. Dual Stage Actuators: Why and When

Higher TPI HDD requires lower disturbance and higher error rejection capability. One of the limitations to achieve high error rejection capability is the dynamics of the actuator. Dual stage actuator (DSA) has been considered to replace single stage actuator (SSA) someday because of system dynamics difference and more freedom in servo design that may avoid the constraint of single stage actuator dynamics on servo. SSA and DSA were compared based on their dynamics, servo designs, and TMR benefits. The extendibility and limitations of both systems were studied. The criteria on when DSA would be implemented are also discussed.

Time-Frequency Analysis of Slider Dynamics and the Effect of Lubricant

The dynamics of pico sliders is investigated during scratch impact using laser Doppler vibrometry. Airbearing frequencies and damping of the pitch frequency are studied as a function of disk velocity for lubricant thickness values of 1.6nm, 1.0nm and 2.0nm. In addition, time-frequency analysis is used to study transient events during slider-scratch impact. The results indicate that the slider dynamics is not only a function of the design of the air-bearing, but also of the velocity and the characteristics of the lubricant film thickness on the disk surface.

Formation of Lubricant “Sierra” at Boundary Between PFPE Solution Dipped and Un-Dipped Zones Over DLC Coated Magnetic Disks

An unstable phenomenon arising at the boundary between PFPE solution dipped and un-dipped zones over DLC coated magnetic disks has been studied. The formation conditions of a ridge of lubricant, or “sierra,” at this boundary and the structure of the “sierra” were clarified. Lubricant film having a step-shaped boundary was first formed by the halfway dip-coating method, and film thickness distribution was then measured around the boundary area. It was found that the film thickness was uniform over the entire lubricated area for a higher speed of withdrawing the disk from the lubricant solution and decreased linearly when the withdrawing speed was lowered. However, the “sierra” structure of the lubricant suddenly formed along the boundary line when the withdrawing speed was further decreased to less than a specified value of around 1 mm/s. The “sierra” was less likely to form for a lower dilution and a longer elapsed time after making the lubricant solution. It was also revealed that, along the ridgeline of the “sierra,” peaks formed periodically and the peak feet propagated in the direction perpendicular to the boundary, forming convex fronts and leaving multiplex bead chains of lubricant accumulations inside the convex.

A Nano-Scale Multi-Asperity Model for Contact and Friction

As surfaces become smoother and loading forces decrease in applications such as MEMS and NEMS devices, the asperity contacts which comprise the real contact area will continue to decrease into the nano scale regime. Thus it becomes important to understand how the material and topographical properties of surfaces contribute to measured friction forces at this nano scale. We have incorporated the single asperity nano contact model of Hurtado and Kim into a multi-asperity model for contact and friction which includes the effect of asperity adhesion forces using the Maugis-Dugdale model. Our model spans the range from nano-scale to micro-scale to macro-scale contacts. We have identified three key dimensionless parameters representing combinations of surface roughness measures, Burgers vector length, surface energy, and elastic modulus. Results are given for the normal and friction forces vs. separation, and for the friction coefficient vs. normal force for various values of these key parameters.

Dynamic Analysis of Sliding Contacts at Head-Disk Interface

As magnetic data storage technology moves towards higher areal data density with higher rotational speeds and lower flying heights, the propensity of severe sliding contacts at the head-disk interface is bound to increase. The tribological performance of the head-disk interface will have significant impact on the durability and service life of the hard disk drive (HDD). A 3D finite element model is constructed to simulate the high speed impact event of a slider on the disk surface. For a given design of the disk with known layer thicknesses and properties, as well as that of the slider with its surface texture, the model predicts contact zone, depth force and duration as well as time-history of energy transfer and its partition, substrate stress and plastic zone for a given impact velocity. The effects of the material properties and layer thicknesses on the performance of the HDD are investigated.

Power Consumption and Temperature Considerations in Disk Drives

This work studies the contributions of various drive components to the overall drive temperature and temperature gradients within a drive. Data is presented from a variety of 3.5 inch disk drives under varying operating conditions. The average drive case temperature is a result of individual contributors like electronics components on the PCB, spindle motor and actuator. Each of these components dissipate power depending on their main function, the drive operation and in some cases depending on the current drive temperature. Due to the large variety of drive designs this work concentrates on the characteristics of different components in terms of individual temperatures and time constants rather than detailed analysis of one specific disk drive.

Simulation of Perfluoropolyether Lubricant Films

Thin-film lubricants play a key role in high-density data storage capabilities via increased reliability and performance of hard disk drive systems. The commercially-used perfluoropolyether (PFPE) lubricants are random copolymers with a linear, polymeric backbone chain structure: X–[(OCF2CF2)p–(OCF2)q]–O–X (p/q ≅ 2/3). Here, X (representing the endgroups) is CF3 in PFPE Z (non-reactive) and CF2CH2OH in PFPE Zdol (reactive), where the hydroxyl groups in PFPE Zdol exhibits strong polar interactions with solid surfaces and with other endgroups. Figure 1 illustrates the representative bead-spring models [1,2] for PFPE Zdol examined in this paper.

Precise Estimation of Mobility of PFPE Lubricants Combining the Experimental Method and Numerical Method

The diffusion coefficient has been used to estimate the mobility of PFPE lubricant on hard disk media. In former researches, initial lubricant thickness distribution is assumed to be step function. And similar solution was used to estimate the diffusion coefficient. But based on our experimentally measured data, initial distribution is not step function but has inclined slope. It leads the over estimation of the diffusion coefficient especially for low mobility lubricants. In this paper, instead of assuming initial distribution is step function and using similar solution, the diffusion coefficient is estimated by the method combining the experimentally measured results and the numerical solutions. The estimated diffusion coefficient is much lower for the low mobility lubricants. Calculated thickness distribution agrees well with the experimentally measured distribution. The mobility and spreading characteristics of PFPE lubricants can be estimated precisely using this method.

Reduction of Head Wear on Thin-Film Magnetic Disks

It is necessary to reduce head wear to develop future hard disk drives. For this purpose, we have been studying transparent pin-on-disk wear tests on thin-film magnetic disks. We reported that pin wear on thin-film magnetic disk showed running-in effects. The reason of the running-in was considered to be a result of disk surface flattening. This means that if we could introduce an efficient burnishing technique, we could reduce head wear in operation. We then introduced a burnishing technique using a hemispherical diamond slider and compared pin wear on disk surfaces with and without burnishing. The results showed that the pin wear was reduced by the introduction of the burnishing technique. We consider that burnishing with hard round slider is another way of reducing head wear on future disk surfaces.

Contact ABS Design Concepts

The design of a head-disk interface for an ultra-low flying height was 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. We found that the use of micro-texture on the air bearing surface can prevent contact vibration. Combining trimming with the use of a micro-texture was most effective in reducing contact vibration. A frequency-shift-damping slider was also found to damp vibration effectively.