Active screen plasma nitriding of Al–Si eutectic alloy and evaluation of compound coatings

Abstract

Active screen plasma nitriding (ASPN) of steel has been extensively studied during the past few years. ASPN is a new technique with many advantages and can be used for treating non-electrical conducting materials like oxides and polymers. Recently, ASPN technique has been used for aluminum and aluminum alloys for surface treatment. The current study reports the growth and characterization of the compound aluminum and iron nitride coating on aluminum–silicon (Al–11Si) eutectic alloy using the active screen plasma nitriding setup to improve the surface properties of the alloy. After treatment in low-pressure plasma environment for five-hours, samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), atomic force microscopy, and Vickers's Micro Hardness testing to investigate the changes in surface morphology, elemental composition, and mechanical properties before and after the plasma treatment. XRD peaks (111), (200), (220), (311) of AlN were observed while that of FeXN peaks were also observed. Spherical, highly crystalline, agglomerated aluminum, and iron nitrides were observed in SEM micrographs. Elemental analysis using EDS and laser-induced breakdown spectroscopy confirmed the presence of nitrogen at the surface of the low pressure plasma treated sample. The presence of nitrides on the surface of the alloy resulted in improvement of the hardness as observed from the Vickers' hardness testing. It is concluded that ASPN is a useful technique for the improvement of surface hardness of Al–11Si alloy.