Impact of Microstructure of Coated and Non-Coated Grey Cast Iron Rotors on their Thermal Properties

Abstract

Friction interaction between brake materials see a rise in temperatures of over 1000 oC contributing to thermal fade of brakes and deterioration/cracking of rotors. Various microstructural features like alloying, graphite morphology and size, morphology and contents of pearlite and ferrite 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 their coating by ceramics. The primary purpose of this research is to identify the impact of microstructure on thermal diffusivity of a commercially available ASTM A48 Class 30 gray cast iron brake rotor and an original equipment manufacturer (OEM) of Ford F150 combined with effects of ceramic coatings. Thermal diffusivity is a relevant property controlling the speed at which is heat dissipated from friction surfaces. Complete material characterization of the brake rotors using differential scanning calorimeter (TA Instruments Modulated DSC Q20), 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 commercially available ASTM A48 Class 30 gray cast iron rotors is typified for a fully pearlitic gray cast iron with about 5 vol.% of ?free? ferrite and type VII graphite flake morphology. Graphite can also be classified as type C of superimposed flake size and random orientation. On the other hand, the OEM Ford F150 brake rotors have almost fully pearlitic gray cast iron microstructure with less than 1 vol.% of free ferrite and type VII graphite flake morphology with a type E subclassification of interdendritic segregation and preferred orientation. Thermal diffusivity values were recorded between 50 oC and 500 oC with a step size of 50 oC. With increasing temperature, OEM, Uncoated C30 and Coated C30 brake rotors show a decrease in thermal diffusivity by 41.3%, 52.4%, and 53.6% respectively. Ceramic coatings helped in increasing the thermal diffusivity by an average of 6.8%. Additional studies are recommended to better assess these new generation coated gray cast iron rotors after friction tests, since the composition of friction layer developed on coated versus noncoated rotors may have a considerable impact on the thermal properties at real field applications.