Impact of humidity on wear of automotive friction materials

Relevance of wear occurring in brakes increased with relation to the released chemicals and health and environmental hazards. Wear of brakes leads to formation of complex chemicals that are released into environment. Humidity can considerably modify the adhesion of surfaces by creating menisci and increasing the contact area. This paper addresses the impact of humidity on two commercial brake material types: "non-asbestos organic" and "semi-metallics" rubbed against pearlitic gray cast iron used in vehicles worldwide. The friction and wear tests were performed with the “UMT Lab Tester” manufactured by Bruker and the wear surfaces/mechanisms were studied by using of scanning electron microscopy, energy dispersive X-ray microanalysis, and optical topography methods. Both materials wore noticeably less at increased humidity. This was ascribed to a better capacity to form a protective friction layer. It was incorrectly assumed that humidity would be desorbed above the boiling point of water. The chemistry, morphology and phase composition of the friction layers formed on the surfaces during braking could play a determining role when amounts of adsorbed water are concerned. The wear mechanisms including abrasive, adhesive, fatigue, and corrosion wear were observed on both material types, irrespectively of humidity levels.