Active screen plasma nitriding of AISI 316L austenitic stainless steel at different potentials

Abstract

AISI 316L austenitic stainless steel samples were nitrided by active screen plasma nitriding (ASPN) at anodic and floating potentials respectively. The microstructures, morphologies, phase structures, microhardness and corrosion resistance of the nitrided layers were characterised. Experimental results show that austenitic stainless steel treated at both potentials can obtain the same precipitation free nitrided layer of S phase structure on the surface, which exhibits not only high hardness but also good corrosion resistance. A great variety of particles in the active screen space play an important role in the ASPN of austenitic stainless steel. The neutral particles sputtered from the active screen by positive ions are the primary nitrogen carrier from atmosphere to matrix surface in nitriding process. In the atmosphere of little oxygen and more hydrogen in the vacuum chamber, the surface passive film can be cleaned out by many kinds of particles bumping its surface, such as the particles sputtered from the active screen, the energetic neutral species in the plasma flow through the screen towards the samples, and even the electrons moving towards the anode samples affected by the electrical field.