Studies on Head/Disc Contact Area Increase During Contact Start/Stop and Continuous Drag Testing of Thin Film Discs

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Abstract
The relationship between friction and surface roughness during start/stop and continuous drag testing of thin film discs and heads was studied. Roughness measurements were made in wear tracks on several types of discs using a stylus profiler. Two methods were used to relate surface topography to the real area of contact. For the first method the roughness data was used to generate Z-height distributions and bearing ratio curves. Using average mean-peak roughness as the height at which the head and disc make contact, the bearing ratio at that height was used as the relative contact area. In the second method, analysis of the highest peaks was used to find the standard deviation of heights and average curvature. From this the contact area was calculated using the Greenwood-Williamson model of elastic contact. Analysis of the contact area vs. coefficient of friction yielded fairly good linear correlation for both methods. Formation of ultra fine wear debris in the wear track of one disc was found to reduce significantly the effect of contact area. The shape of the Z-height distribution and bearing ratio curves were unique for each type of disc.

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