Local oxynitriding of AZ31 magnesium alloy by atmospheric-pressure plasma treatment at room temperature

A surface-hardening treatment for AZ31 magnesium alloy using an atmospheric-pressure plasma jet (APPJ) at room temperature was developed. Magnesium is a potential engineering material because it is lightweight; however, magnesium alloys are difficult to heat-treat because of their low flaming temperature. Magnesium alloy specimens were irradiated with a localized atmospheric-pressure plasma jet generated by dielectric-barrier discharge for 180 s in air. The APPJ excited oxygen and nitrogen molecules in the ambient air, resulting in the formation of an oxynitrided layer; oxygen and nitrogen diffusion layer, on the surface of the magnesium alloy. The hardness and elemental distribution for the treated surface were examined. The top surface of the APPJ-treated magnesium alloy achieved a maximum hardness of 108 HV, which was ∼1.7 times greater than that of the untreated surface. Elemental analysis using an electron-probe microanalyzer revealed strong oxygen and nitrogen signals corresponding to the hardened region of the magnesium alloy, meaning that the hardness increased as a result of the formation of the oxynitrided layer. The proposed APPJ treatment is a promising approach for locally hardening magnesium alloys without using a heat treatment.