Increasing nanohardness and reducing friction of nitride steel by laser surface texturing

Laser surface texturing is often employed to improve tribological performances of mating surfaces. The principal aim of the present work is to study local modifications induced by laser beam during the texturing process (microdimpling), on the mechanical properties of a 30NiCrMo12 nitride steel.A brief tribological characterization was initially made in order to verify the improvement of tribological behaviour ensured by microdimpling. Friction measurements were carried out in lubricated flat-on-flat and round-on-flat configurations.A sub-surface characterization was performed by focused ion beam cross-section and imaging. Modified material layers were discovered in correspondence of laser-affected zones around dimples, where grain dimensions were observed to be reduced. To quantify this phenomenon, a new theoretical energetic model was developed.Hardness measurements were performed by atomic force microscope nanoindentation. This approach allowed to observe an hardening effect moving from bulk material to dimple edges. Therefore, the theoretical model was integrated with the Hall-Petch's law in order to quantify the observed hardening behaviour.