Surface force in slider air bearings of hard disks

Abstract

The clearance between the slider and the disk is now about 20∼30 nm for 10 Gbit/in 2 recording density, and it is predicted that the clearnace should be reduced to about 5 nm for 100 Gbit/in 2 , which is the next aim of the industry. The reduction in the clearance of the slider air bearing to such an ultimately small value will bring about the change in the physical phenomena occurring within the clearance. It has been well recognized that, instead of the continuous mechanics, molecular dynamics should be used to predict the behavior of the slider air bearing working under a clearance of less than about 100 nm, to take into consideration the effect of rarefied gas. One of other effects due to the reduction in the clearance is that two solid surfaces are so close to each other that the surface force will come into action. Until now only the effect of rarefied gas has been considered and the surface force has been, ignored without any evidence. In this paper, therefore, the effect of surface force in the air slider bearing was investigated in terms, of numerical simulation. In the simulation the rarefied gas lubrication theory gased on the first slip flow velocity was used, and the van der Waals force between two solid surfaces was considered. It is found that, because of the surface force, the stiffness of the slider air bearing will decrease to a negative value as the clearance is less than a critical value. The critical value of the clearance is called the lower limit clearance. Negative stiffness means that the slider air bearing loses its ability to sustain the clearance between the slider and the disk completely, and the slider and the disk will come into direct contact. Therefore a practical slider air bearing should be designed so that the clearance at operation is always greater than the lower limit clearance with a safe margin. The lower limit clearance is general of several nm in magnitude, and increases as the ratio of h max /h min and/or the slope of the slider decreases. Therefore it is of great importance to take into account the surface force in the design of the slider air bearing of such as 100 Gbit/in 2 hard disks.