Magnetorheological fluids are magnetic field responsive fluids containing a field polarizable particle component and a liquid carrier component.  Magnetorheological fluids are useful in devices or systems for controlling vibration and/or noise.  Magnetorheological fluids have been proposed for controlling damping in various devices, such as dampers, shock absorbers, and elastomeric mounts.  They have also been proposed for use in controlling pressure and/or torque in brakes, clutches, and valves.  Magnetorheological fluids are considered superior to electrorheological fluids in many applications because they exhibit higher yield strengths and can create greater damping forces. 
The particle component compositions typically include micron-sized magnetic-responsive particles.  In the presence of a magnetic field, the magnetic-responsive particles become polarized and are thereby organized into chains of particles or particle fibrils.  The particle chains increase the apparent viscosity (flow resistance) of the fluid, resulting in the development of a solid mass having a yield stress that must be exceeded to induce onset of flow of the magnetorheological fluid.  The particles return to an unorganized state when the magnetic field is removed, which lowers the viscosity of the fluid. 
Magnetorheological (MR) fluids are comprised of small spherical ferromagnetic or paramagnetic particles dispersed within a carrier fluid.  Small magnetic particle size permits easy suspension and the design of devices having small gaps.  Standard carbonyl iron (CI), a commonly used iron, is derived from iron pentacarbonyl vapor by a gas-phase decomposition process, resulting in a spherical particle with a relatively high carbon content.  Reduced CI, prepared by reduction of standard CI and having very low carbon content, can also be used.  However, standard and reduced CI are somewhat expensive compared to other iron types.  Moreover, the use of carbonyl iron limits the range of metallurgy that can be used due to the process used to obtain such CI particles. 
Development of lower-cost MR fluids for use in primary automotive suspension dampers has been an ongoing effort for several years.  The main focus has been to use lower-cost water-atomized iron (WAI) particles to replace the current higher-cost carbonyl iron (CI) used in prior art fluids.  The production of WAI particles is known to the art and to the literature and generally relates to melting iron or an iron alloy, allowing it to flow from a small orifice to form a thin stream, and subjecting the molten stream to high-pressure water spray to form metal particles.  Much effort has been devoted to determining whether larger water-atomized iron (WAI) powder can meet durability requirements, with little success to date.  Simply substituting larger WAI for CI in an MR fluid specially formulated for good durability produced a fluid with unacceptable durability.  Attempts to optimize the MR fluid formulation with larger WAI were not successful. 
The root cause of durability failure in fluids with larger WAI is degradation of the iron powder through mechanical working such that a large amount of fine particles (less than 1 micron diameter) are produced. 
It would therefore be desirable to provide a magnetorheological (MR) fluid employing water-atomized particles which meet durability criteria and produce fewer degraded fine particles after periods of mechanical working.