The effect of particle concentration in a magneto rheological suspension on the performance of a boundary lubricated contact

Abstract

The tribological effects of several different concentrations of the same solid particle in a magnetorheological suspension (MRS) were examined on an apparatus operating in the boundary lubricated regime, using two different base fluids. The choice of optimum magnetorheological (MR) particle concentration is nominally restricted by the provision of desirable rheological effects in the fluid mix, such as the yield strength responsiveness to an applied magnetic field and low zero excitation viscosity. However, the aim of the present work is to identify the desirable range of particle concentrations for magnetorheological suspensions from a tribological viewpoint alone for one readily available particle type and two smooth bearing surfaces, one relatively hard and the other soft. The results provide a useful reference towards the formulation of an MRS and the prediction of friction and wear when the MRS is used between slipping surfaces. Experimental studies were carried out in order to understand and uncover the boundary lubricated wear mechanisms of the MRS with different particle concentrations. The friction and wear that occur in seals and bearings that have to cope with such fluids can be a major impediment to application should they be operated in unduly adverse conditions. Generally these sliding surfaces operate outside of the area affected by the rheology controlling magnetic field, with one or two exceptions. Hence, the tests have been undertaken in unexcited conditions. Considering that the carbonyl iron powders used in the MRS are relatively high in concentration, their effect on a boundary lubricated contact of two smooth steel surfaces may or may not be adequately described by the typical three-body abrasive wear mechanism since the particles are also comparatively soft and pass through an extremely small gap (nominally smaller than the typical dimension of the MR particles). This scenario is not well-addressed by tribologists whose concerns traditionally involve low concentrations of particles of higher hardness than surfaces such as wear debris. Would the damaging effect be controlled by controlling the particle concentration of the MRS? Two different base oils were used to mix with carbonyl iron powders, which were extracted from a commercial MR fluid from Lord Corporation, as different specimen fluids. The two different base oils are a pure mineral oil and a hydraulic oil with extreme pressure (EP) additives.