On Thermal Diffusivity of Selected Gray Cast Irons and its Impact on Friction Performance of Automotive Brakes

Increased temperature resulting from friction between brake pads and rotors results in formation of friction layers on the friction surfaces and could lead to the bulk material degradation impacting the braking performance. The most often discussed phenomenon is the thermal fade, but there are additional phenomena like thermal shock, crack formations, and increased residual stresses in brake rotors which occur due to local heating. Formation of friction layers is also strongly influenced by temperature on the friction surface, as it defines thermodynamics and kinetics of processes occurring during friction. Gray cast iron is a metal matrix composite comprising of ferrite, pearlite, graphite, and additional inclusions. Morphology, quality, and quantity of these phases can change as temperature varies during and after friction process. Thermal diffusivity characterizes how quickly a material could dissipate heat through it. Grey cast irons exhibit a reasonably high thermal diffusivity and an excellent capacity to dissipate heat. But this characteristic varies in dependence on composition and microstructure of cast irons. The volume content and morphology of graphite flakes found in gray cast irons have the most relevant impact on their thermal diffusivity values. The current study compares the graphite flake morphology of three commercially available gray cast iron rotors, named A, B, and C respectively, manufactured according to the ASTM A48 standard. These rotors are subjected to a complete currently available standardized SAE J2522 friction test on a bench top tester using scaled-down approach, and a commercially available non asbestos organic (NAO) brake pad. Complete material characterization of the friction material using laser flash apparatus (NETZSCH LFA 467), polarized light microscopy (Nikon Microphot FX), scanning electron microscopy (FEI Quanta FEG 450), energy dispersive X-Ray microanalysis (Oxford detector, Inca Systems), topography (NPFLEX 3D Optical Microscopy), and density (analytical balance and Archimedes principle). The polarized light microscopy results of the three commercially available brake rotors reveal the presence of flake-like graphite with average flake sizes as 55 µm, 33 µm, and 60 µm and area fraction as 28%, 26%, and 30%, respectively. Thermal diffusivity values of the studied rotors when measured in temperature range between 25 oC and 500 oC show a decrease by 52.4%, 53.6%, and 54.8% respectively. Commercial brake rotor C exhibited the presence of increased content of oxides in the friction layer formed during elevated temperatures, which helped in the observed improved friction performance.