High Density Plasma Nitriding on Stainless Steel Alloys at PUPR-MC Plasma Machine

Abstract

A novel method of nitriding at high current levels at Polytechnic University of Puerto Rico Mirror Cusp plasma machine (PUPR-MC) has been performed. Nitrogen ions implantation or plasma nitriding increases the hardness at a surface level of stainless steel because of high plasma temperatures and high ion densities created in the chamber. Nitriding is the process of doping a crystalline metallic matrix with the presence of nitrogen ions; it was previously done in the past utilizing chemical processes with high acid densities at high temperatures. By using plasma nitriding procedure at PUPR-MC surface hardness is increased in 302/304 type stainless steel samples by exposing them to high ion density plasma at high vacuum. This method succesfully dopes the surface of the material with strengthening nitrogen ions, without the use of chemical procedures that sacrifice the resistance to corrosion of the given material. A 500 V negative bias is placed on the sample exposed to the nitrogen plasma, where high energy ions are therefore attracted and immersed into the metallic matrix microns of the stainless steel surface. This potential maintains a constant surface temperature at approximately 800°C. Plasma parameters including ion density and plasma temperature were diagnosed using Single Langmuir Probes. The stainless steel samples were the tested using scanning electron microscopy (SEM) and Vickers Micro-Hardness tests to determine the increment in the surface hardness of the material. The SEM showed a significant presence of nitrogen imbedded in the grains of the stainless steel surface.