Hard Disk Drive Introduction
The future of computer technology lies not only in increasing the computing power but also in providing the maximum amount of information at the lowest cost. For these reasons, there is a large demand for high capacity storage devices with low access times. As of today, hard disk drives are the only high capacity storage devices that can meet these requirements at the lowest cost.
Figures 1, 2 and 3 show schematic views of the slider-disk interface. The binary information is written and read from the disk using a magnetic head. This head is situated at the trailing edge of the slider (see Figure 2) which is in turn supported above the disk by the suspension arm which moves the slider around the disk (see Figure 1). The most commonly used heads are the inductive type wherein the binary information (1s and 0s) on the disk is distinguished by measuring the induced voltage at the transition between two adjeccent bits in a given track (see Figure 3). In the past few years there has been a great interest in using magetoresistive (MR) heads. The MR heads detect the 1s and 0s by measuring the change in the resistance of the magnetic element. Thus with MR heads the readback voltage is independent of the rotational speed of the disk.
Courtesy Ed Grochowski, IBM
The separation between the magnetic read/write element and the hard disk during operation, known as the flying height, is one of the important parameters that controls the performance and durability of a hard drive. In order to increase the recording density it is necessary to decrease the flying height so that the signal to noise ratio obtained from the read element is within an acceptable range. Ideally, zero spacing is preferred. However, zero spacing or contact recording would lead to higher friction and wear at the head-disk interface, hence degrading the performance of the hard drive.
Decreasing the flying height is not the only way to improve the performance of a hard disk drive. Better magnetic materials and signal processing allow more bits per inch (linear density) while better servo techniques and more sensitive heads allow more tracks per inch (track density).
In present-day hard drives the flying height is between 5 and 20 nanometers (nm) and lower with thermal flying height control. Hence, there is partial contact between the head and the disk during the operation of the hard drive. In the partial or semi- contact regime an ideal head-disk interface would consist of a slider where 99.999% of the weight is supported by the air bearing force that is developed beneath the slider as the disk spins. In other words, the normal load on the disk due to the slider is only 0.001% of the total weight of the slider, resulting in lower friction and wear at the head-disk interface. However, control over the interactions between the head and the disk during flying is limited by tolerances in the slider and suspension geometry and the materials and processing parameters used in the industry.
Figures 4-7 show various types of sliders that are used in hard drives. The first two types of sliders (Figs. 4 and 5) have flying heights ranging from 60 to 120 nanometers. This flying height is much greater than the peak to valley surface roughness of typical disks. Hence these sliders operate in the flying or non- contact regime. Sliders shown in figure 6 and 7 operate in semi-contact regime. The major difference between the sets of sliders is the presence of a third pad at the trailing edge and shortening of the two side rails. Shortening of the side rails helps in reducing the flying height and the addition of third pad decreases the contact area between the slider and the disk. The sliders shown in figures 5 and 7 have a cavity at the leading edge which creates negative (below atmospheric) pressure in the cavity and which reduces the pitch of the slider. Another advantage of using negative pressure sliders is the uniformity in the flying height from the inner to the outer diameter of the disk.
Professor Talke's group is involved in studying the various ways to improve the tribological and mechanical performance of the head-disk interface. Equal emphasis is given to conduct both numerical and experimental work in the field of mechanics and tribology of the head-disk interface.