Ki Myung Lee

Relationship between scratch hardness and yield strength of elastic perfectly plastic materials using finite element analysis

With thin solid film usage expanding in numerous technologies, reliable measurements of material properties such as yield strength become important. However, for thin solid films the measurement of yield strength is not readily available, and an alternative method to obtain this property is to measure hardness and convert it to yield strength. Tabor suggested dividing hardness by ∼3 to obtain yield strength, which has been used extensively, despite its shortcomings. Since the pioneering work of Tabor, researchers have performed numerical and experimental studies to investigate the relationships between hardness, yield strength, and elastic modulus, using the indentation technique. In this study, finite element analysis was performed to simulate the nanoscratch technique. Specifically, the nanoscratch finite element analysis was used to validate a previously developed analytical scratch hardness model. A full-factorial design-of-experiments was performed to determine the significant variables for the ratio of calculated scratch hardness to yield strength and a simple analytical prediction model for the ratio of hardness to yield strength was proposed.

Dynamic microwaviness measurements of supersmooth disk media used in magnetic hard disk drives

Recent technological advances in magnetic storage suggest the feasibility of extremely high-density magnetic recording up to 1 terabit per square inch (1 Tbit = 10-12 bits) areal densities. Modeling indicates that approximately 3 nanometers (nm) of physical head-disk spacing is required for such high recording densities. When the recording slider is flying at such ultra low spacing over a high-speed rotating disk, it is experiencing disturbances from various different sources and of a wide frequency range. These disturbances may cause the recording slider to vibrate significantly, a condition that is known as fly-height modulation (FHM), which may result in data loss. A significant source of excitation is from the surface irregularities of the rotating disk and is termed dynamic microwaviness. The term dynamic microwaviness has been introduced recently to differentiate from regular topographical microwaviness that is measured statically. In this paper, the procedure for making reliable dynamic microwaviness measurements of disk media used in hard disk drive (HDD) systems is described. Furthermore, such measurements are made on different magnetic disks that are intended for extremely high recording densities using non-contact laser vibrometry. The source of the dynamic microwaviness and its interaction with system dynamics are also investigated

Mechanical property measurements of thin-film carbon overcoat on recording media towards 1Tbit∕in2

In the pursuit of extremely high areal density recording of 1Tbit∕in2, very thin protective carbon overcoats are needed, and reliable measurement of their mechanical properties is critical. Measuring mechanical properties of thin films has been challenging especially as their thicknesses have shrunk to only few nanometer thick, and one is faced with both instrument and technique limitations in extracting reliable information about such films. Two common techniques utilized for the characterization of submicron thin films are the nanoindentation and nanoscratch techniques. In this paper, shallow sub-10-nm nanoindentation and nanoscratch experiments were performed to extract the mechanical properties of reduced elastic modulus, hardness, and shear strength of a 10-nm-thick magnetic medium carbon overcoat.

Crystallite coalescence during film growth based on improved contact mechanics adhesion models

Intrinsic tensile stress, which can lead to problems in deposited thin films such as cracking, peeling, and delamination, often develops during the early stages of thin film growth. Many attempts have been made to estimate the tensile stress during crystallite coalescence, both experimentally and analytically. Most recently, using a combination of Hertzian contact mechanics and elasticity theory, Freund and Chason applied the Johnson-Kendall-Roberts (JKR) theory to account for adhesion between crystallites under specific conditions. Other existing contact mechanics models that naturally account for adhesion include the improved Derjaguin-Muller-Toporotov and Maugis-Dugdale theories. The objective of this study is to provide useful analytical and numerical techniques based on these contact mechanics theories for a wide range of conditions that accurately approximate the intrinsic tensile stress that develops during crystallite coalescence. As an analytical method, the Maugis-Dugdale model is proposed as a ...

Noninvasive vibration diagnostics to detect head-disk contact in hard disk drives

Reliability of modern hard disk drives (HDDs) is a major concern especially with decreasing head-to-disk spacing in order to attain higher recording densities. One of the reasons for HDD failure is the occurrence of intermittent or sustained head–disk contacts. It is therefore important to be able to detect such head–disk contacts early on so that backup safety schemes can be implemented before catastrophic HDD failure and permanent data loss. In the study reported here, we used noninvasive vibration diagnostics and multiaxial vibration measurements to assess the condition and functionality of HDDs.We chose a miniature triaxial accelerometer for the diagnostic study, given that it is relatively inexpensive and easy to use.We measured contact vibration, induced by a scratch on the top surface of the rotating disk, on the outside of the HDD by attaching an accelerometer on the actuator shaft.We found that contact vibrations between the head and disk inside the HDD can reliably be measured on the outside of the HDD with an accelerometer. We also found that a miniature accelerometer does not cause significant mass loading, as verified by noncontacting laser vibrometry

Transient and steady state dynamic vibration measurements of HDD

Vibration diagnostics is based on the detection of change from a reference condition, which is generally defined through a reference characterization signature. In the present study, the reference characterizations have been obtained with the HDD operating in normal condition. Once the reference signatures are established under normal operating conditions, malfunctions on the HDD are imposed using a sharp scratch on the top surface of the top disk inside the drive and by exposing the HDD on multi-directional shocks. The vibration signatures are again measured, and it is found that vibration measurements can reliably detect malfunctioning HDD.

MICRO/NANO SCALE WEAR BEHAVIOR OF PEARLITIC AND BAINITIC RAIL STEELS

To develop rails with higher hardness and thus better durability and longer life, alternative microstructures have been suggested, since conventional pearlitic rail steels have reached their hardness limit. Such a newly developed material has a fine bainite microstructure (coded J6 bainitic steel) and showed higher initial hardness but poorer on-site wear performance, compared to conventional pearlitic steels. This was explained by the fact that pearlitic steels show significant work hardening under severe stress conditions, even though their initial hardness was lower. In this work, the wear behavior of pearlitic and J6 bainitic rail steels was investigated at the micro/nano scale, using the nanoscratch technique. It was found that pearlitic steel shows better wear performance at the micro scale as well, in agreement with large scale rail field tests.Copyright