Most surface-modification techniques used for solid materials are based on various chemical reactions that produce optimized structure and properties in order to improve the global performance of the materials. However, widespread uses of these techniques are frequently hindered by the reaction kinetics involved. For example, nitriding is a chemical treatment widely used to form surface nitrides. This technique is of great industrial interest, because it forms a unique composite structure with a hard surface (a layer of Fe-nitride compounds) and a tough interior, so that the global mechanical performance and wear/corrosion resistance of alloys and steels can be greatly improved (1). However, nitriding processes are performed at high temperatures (500°C)for along duration(20 to 80 hours) (1, 2) and may induce serious deterioration of the substrate in many families of materials. A change of the surface microstructure by means of grain refinement is one option that can be used to accelerate the chemical reaction of a
material surface.