A study of rarefied and contact air bearing modeling for hard disk drives using the direct simulation Monte Carlo method

Slider Air Bearings are part of the head-disk interfaces (HDI) in the hard disk drives. An accurate mathematical model of the HDI is very important to the magnetic recording industry because it controls the performance and durability of a disk drive. The conventional Reynolds lubrication equation is of questionable validity when the slider/disk spacing is of the order of magnitude of the gas molecular mean free path. Then the rarefactional effect is expected to be very significant.; This dissertation focuses on the modeling of several aspects of the slider air bearing. The widely used Molecular Gas-film Lubrication (MGL) air bearing model is based on the Reynolds equation with slip velocity correction. At a very low flying height, this model needs to be verified by a more fundamental theory, such as the Direct Simulation Monte Carlo (DSMC) method. The DSMC results were compared with the MGL results. The comparison shows that both calculations agree well with each other. In addition, the MGL model and DSMC model converge at very large bearing numbers due to decreasing influence of the Knudsen number.; Prediction of the slider behavior when an isolated asperity contact occurs is important to the evaluation of the slider air bearing design. Previous studies found that the conventional Reynolds equation predicts unbounded pressure at the contact point. Even the resultant force obtained by the integration of the pressure is unbounded. The DSMC method required an additional boundary condition, called the non-fly-zone condition, in order to solve the problem effectively. The bounded pressure was obtained and the pressure structure around the contact point was found. Comparison of the contact results obtained from DSMC versus those obtained from MGL (with use of the non-fly-zone condition in both cases) shows good agreement. Therefore a contact air bearing force database was created using the modified MGL model. Then a new contact air bearing model was created using the combination of the above database and the Greenwood-Williamson model to solve for the multiple asperity contact problems. The effect of the contact air bearing force was studied; it increases the flying height and decreases the pitch angle by about 10%, which can not be ignored.; Both the MGL and DSMC models require an accommodation coefficient as part of the input. The effect of the accommodation coefficient on the slider air bearing was investigated. When the slider and the disk have different accommodation coefficients, the Couette flow term in the Reynolds equation also needs to be corrected, because the velocity profile loses its symmetry. It is found that, in general, the smaller the accommodation coefficient, the lower the flying height and pitch angle. The positive pressure sliders are more sensitive to the accommodation coefficient than the so-called negative pressure sliders. Also, it is shown that for positive pressure sliders, the lower the flying height, the larger the discrepancy percentage between the calculations with the accommodation coefficients being unity and non-unity.