"Physical Characterization of Niobium-Added Gray Irons for Brake Disc Applications "

"High carbon equivalent (CE) gray iron brake rotors exhibit desirable thermal diffusivity and dampening properties. High CE rotors suffer, however, from reduced mechanical and wear properties in comparison to low CE rotors. Niobium (Nb) is shown to increase the strength and wear resistance of gray iron, but the interaction of niobium with other common alloying elements, namely chromium (Cr) and molybdenum (Mo), is not well understood. Thirteen gray cast iron alloys were produced with varying levels of CE, Cr, Mo, and Nb. Bars with four different diameters (8, 14, 22, and 30 mm) were cast from each alloy and microstructural and physical properties such as graphite flake morphology, pearlite spacing, electrical resistivity, and thermal diffusivity were measured. Mechanical characterization, including tensile testing from ambient temperature to 680?C, ambient temperature pearlite microhardness, macro Brinell hardness, and ASTM G65 sand abrasion wear testing, was conducted. Fluidity during casting was measured using a purpose-built finger mold. It was found that niobium refined the graphite morphology, reduced pearlite spacing, and precipitated eutectic NbC. When compared to Mo, Nb was found to be 2-3 times more potent at increasing strength and wear resistance while minimally reducing thermal diffusivity. One of the Nb containing alloys (0.22 wt% Nb) is compared to test data from a modern Chevy Silverado 1500 rotor (similar alloy chemistry excluding Nb addition) and exhibited a 7.8% increase in thermal diffusivity, a 5.8% increase in tensile strength, and a 8.4% increase in hardness while exhibiting similar wear resistance. "