An accelerated wear test method to evaluate lubricant thin films on magnetic hard disks

Abstract

A high speed ball-on-inclined-plane test method has been developed to evaluate the lubrication effectiveness of Z-Dol on magnetic hard disks. The test evaluates the combined durability of the lubricant film and the carbon overcoat under sliding conditions. A polished ruby (Al2O3) ball without suspension is used to simulate the head material. The ball slides over an inclined (at an angle of 0.055°±0.005°) section of the disk surface at 2.0 m/s linear velocity. The load is controlled by the geometric interference of the preloaded ball and the inclined plane. The contact forces are sampled periodically at 2 rpm and the frictional coefficients calculated. Repeated sliding between the ball and the disk sample leads to an increase in friction approaching that of the unlubricated case. Post test analysis using atomic force microscopy (AFM) suggests that the increase in friction is due to the loss of lubricant effectiveness of the lubricant and the wear of the carbon overcoat. X-ray photoemission microscopy (PEEM) results suggest progressive oxidation of Z-Dol as one of the degradation mechanisms leading to wear. The durability of the lubricating thin films is defined by the number of cycles to failure. Test repeatability is about 10%, depending on lubricant, film thickness, and surface roughness. The test can be used to evaluate different lubricant chemistries as well as different carbon overcoats. Compared to other pin-on-disk tests and step loading ball-on-disk methods, this test introduces two additional factors: high speed impact and wear acceleration by the inclined angle. The high speed impact simulates potential thermal stresses associated with head–disk contact. With an inclined angle, the load increases evenly for each contact cycle, hence simulates the ability of the lubricant layer to react to dynamic loads. The test is intended as a basic research tool to measure the fundamental resistance of the lubricant layer to resist repeated high speed contacts.