Impact of Microstructure and Surface Treatment on Thermal Properties of Gray Cast Iron Brake Rotors 2021-01-5106

Friction interaction between brake materials sees a rise in temperatures of over 1000°C contributing to thermal fade of brakes and deterioration/cracking of rotors. Various microstructural features like graphite, ferrite, and pearlite could influence the thermal properties and related friction performance of the brake materials. Even more relevant impact on thermal properties of rotors can be expected after coatings or surface treatments. The primary purpose of this research is to identify the impact of microstructure and surface treatment on the thermal properties of four types of gray cast irons subjected to modified (when compared to their current industrial production) manufacturing processes. These rotors were marked as A (ASTM A48, C30), B (ASTM A48, C20), C (ASTM A48, C30), and D (JIS G5501, FC150), respectively [1, 2]. Complete chemical and material characterization of the brake rotors using optical emission spectrometer (OES), carbon-sulfur combustion analyzer, laser flash apparatus, polarized light microscopy (PLM), and density (analytical balance and Archimedes principle). The gray cast iron rotors are typified for a fully pearlitic gray cast iron with about 2-4 vol.% of “free” ferrite. Graphite can be further classified as type VII-C5 of superimposed flake size and random orientation for rotors B, C, and D, and type VII-D5 of interdendritic segregation and random orientation for Rotor D. Thermal properties were recorded at room temperature (25°C) and between 50°C and 500°C, with a step size of 50°C. Thermal diffusivity and conductivities decreased with increasing temperature, while specific heat capacities increased with increasing temperature for all studied rotors. Initial mathematical models show the impact of surface treatment and graphite content to be dominant over observed thermal properties. Further scrutiny identifies the influence of applied surface treatment to be dominant over microstructure for thermal diffusivity when the combined effect of microstructure and surface treatment was studied. However, none of these factors were found to be contributing well to thermal conductivity and specific heat capacity models when the combined effect was considered.