Roughness Effects on the Dynamic Coefficients of Ultra-Thin Gas Film in Magnetic Recording

The stiffness and damping coefficients of a slider gas bearing operating under arbitrary Knudsen number are calculated. A perfect gas is used as the lubricant, and its behavior is described by the modified average Reynolds equation proposed by Makino et al. (1993). The effects of molecular mean free-path on the roughness-induced flow factors are included. The effects of the nondimensional film thickness, H_s0, the surface characteristics, (γ₁, γ₂). the inverse Knudsen number, D₀, the nondimensional frequency, Ω, and the modified bearing number, Λ_b, on the dynamic coefficients are discussed in this paper. As expected, the values of dynamic coefficients for various roughness orientations approach the smooth value as the ratio, H_s0, becomes greater and greater, and thus the roughness effect is getting smaller and smaller. The air film of two-sided longitudinal oriented roughness is stiffer than the other two-sided oriented cases. The effect on translational damping coefficients for various two-sided oriented roughnesses is reversed as D₀ greater than some value, and this value is affected significantly by Λ_b. Transversely rough stationary case has the lowest critical value of Λ_b, at which the negative translational damping appears. The results show that the roughness effect and rarefaction effect on dynamic coefficients are significant, and they cannot be ignored for stability analysis.