Evaluation of Perfluorinated Polyalkylether Grease Rheology on Disk Drive Bearing Performance

Abstract

A matrix of specially formulated greases composed of perfluorinated polyalkylether (PFPE) oil and telomers of polytetrafluoroethylene (PTFE) particulate thickener were tested. The PFPE greases were chosen for this evaluation because their oils have a very low vapor pressure and they will not volatilize in the disk drive. Base oil viscosity, PTFE particle size and percentage of oil content were varied. The rheological properties of complex viscosity, storage and loss moduli, and loss factor were measured. Percent oil was determined using thermal gravimetric analysis. The thickener’s PTFE particle sizes were measured and their shapes were imaged using scanning electron microscopy. Bearing low speed torque and bearing noise tests were performed to evaluate the effect of the grease on bearing performance. The head settle track misregistration (TMR) was measured on disk drives to measure the effect of bearing lubrication on the servo performance. Generally, greases with high base oil viscosity had the lowest complex viscosity. Greases with highest viscosity PFPE oils had the highest torque and lowest noise. Greases with large PTFE particles had high loss factors and exhibited high torque and noise levels. High PTFE thickener to PFPE oil ratio (thicker grease) causes the bearings to have fluctuations in torque and noise levels. Actuator bearings lubricated with greases having higher viscosity oil had lower head settle TMR.