Concentrated suspensions undergo many types of complicated flow behavior, including shear thinning, shear thickening, and yielding. Shear thickening is of interest in industry as it can pose a problem in coating processes and can even damage process flow equipment. More recently, shear thickening fluids have been used in composites with protective fabrics such as KevlarŪ and Nylon to improve their response to ballistic and stab threats. Our goal is to develop a predictive understanding of how varied particle properties effect the onset, severity, and ultimate limit of shear thickening, which should be controlled only by the hydrodynamics of the system according to the hydrocluster theory. Literature reports of a second shear thinning regime after shear thickening in dense colloidal dispersions (Laun, 1984) are examined from the viewpoint of the material properties of the particle phase. We hypothesize that the particle modulus can limit the extent of shear thickening, and that the second shear thinning regime is a manifestation of particle softness. An elastohydrodynamic model (Meeker, PRL, 2004) is proposed that predicts a limiting scaling for this viscosity in the shear thickening regime.We test this model against a number of particle systems in literature of varied particle hardness. In addition, model poly (methyl methacrylate) (PMMA) colloidal dispersions are synthesized and purified. Stable suspensions are shown to exhibit strong shear thinning and thickening, followed by a second shear thinning regime. Using roughened plates, we determine that the second shear thinning regime is not an artifact of slip, or adhesive failure with the rheometer tooling. High strain-rate, compression-shear split Hopkinson pressure bar (CS-SHPB) measurements are also used to probe stresses and strain rates not measureable with conventional rheometers to probe this ultimate limit of shear thickening (Lim, JOR 2008). Finally, these shear thickening fluids (STF) are fabricated into composites with ballistic materials. Testing of these composites in quasistatic puncture and ballistic testing shows a correlation between the suspension rheology and STF-composite performance. These results demonstrate the important role of particle hardness in dense colloidal suspension rheology.