EXCIMER-LASER SURFACE-TREATMENT OF A SPHEROIDAL GRAPHITE CAST-IRON - POSSIBLE APPLICATION FOR RUNNING-IN PARTS OF AUTOMOTIVE MOTORS

After the final machining on spheroidal graphite cast iron, a thin lay er of the matrix material covers the graphite spherulites. This layer has been selectively removed from the graphite nodules using a KrF exc imer laser. The plasma created on the surface generates shock waves, w hich harden the bulk material to a depth of nearly 10 mu m and which b y reflecting on the spherulites, pushing away the molten metal, form c raters. The molten layer resulting from the treatment has an extremely fine microstructure and is harder than the matrix, with a roughness s lightly higher than that of the initial surface. Preliminary wear test s show that the friction coefficient appears to be lower than that of the untreated material, because of the improvement in surface properti es and because of the opened spherulites which are acting like oil res ervoirs. A mathematical model is presented in order to calculate the r oughness of the treated surfaces. It is in rather good accordance with experimental measurements, taking into account the laser processing p arameters.