A New Look at Micro Wear in Cast Iron Brake Rotors

Wear in cast iron is generally a macro phenomenon, measured in grams of weight loss or millimeters of thickness lost in a test specimen. Traditional wear test methods employ loads that are relatively high and of a short duration. In the automotive industry NVH (Noise, Vibration and Harshness) is a primary concern with automotive driveline and chassis components. Recent NVH issues on some specific vehicles were determined to be caused by brake rotor thickness variation as observed during actual on-road tests after a short period in service. Changes in brake rotor thickness were observed in isolated areas of the brake plates during routine dynamometer testing of disc brake rotors. A new test was developed to determine the cause of the thickness variations. Numerous studies were conducted to reduce what was first thought to be thermal growth. Thermal growth reduction was investigated by looking at casting residual stresses, pearlite decomposition reactions, elevating Martensite transition temperatures, pearlite spacing, matrix hardness, and alloy levels affecting all of these. Micro wear was determined to be the cause for brake rotor thickness variation. The micro wear was in turn found to be a function of the graphite density differences around the rotor brake plate diameter caused by localized cooling rates. Casting process simulation studies were conducted to determine the effects of gating systems on cooling profiles. Changes were made to the gating system, minimizing the cooling rate differences.