On-tread braking generates high heat and often results in temperatures on the tread surface in excess of 400C (750F). Temperatures this high result in rapid oxidation of the tread surface. High temperature oxidation for steel generally follows a parabolic rate law: the time to generate a given weight or thickness of an oxide layer increases proportionately to the square of the weight or thickness of that oxide layer. The decrease in the rate of generation of new scale thickness results from the longer time it takes for oxygen to diffuse through an increasingly thicker layer of oxide. However, wheel/rail contact and abrasive action from the brake pad often erodes the oxide layer as it forms, accelerating the rate of formation of the oxide. Further, as the oxide forms, cold worked tread material from rolling contact forces is removed thereby reducing the beneficial residual stresses and higher hardness layer that may have protected the tread surface from further plastic deformation. Environmental factors may also aggravate oxidation losses sufficiently to make it a primary rather than a secondary cause of wear.
High temperature oxidation can contribute to an accelerated rate of shelling, which can result from oxide forming within tread cracks. Tensile stresses at the crack tip are caused by expansion when the oxide forms.
Compressive stresses in the oxide are usually minimal, as expansion is not constrained in the scale growth direction. Volume constraints within a crack when the oxide forms generates compressive stresses in the newly formed oxide, which results in wedging forces at the crack tip.
Oxidation of the tread surface may permit debris to adhere to the tread surface creating a defect called “built-up tread”. Although built-up tread defects require other factors in order to become a problem, oxidation of the tread surface appears to be an initiating mechanism.
Results from experimentation and Finite Element Analyses are used to support this work.
