High density plasma beam source for nitriding

Abstract

A high density plasma beam source has been developed for the purpose of surface treatment and study of low energy ions and neutrals with surfaces. The plasma is generated by pulse microwave power at 2.45 GHz. A pair of Helmholtz coils is used for generating axial magnetic field of 436 G which is also responsible for the production of plasma inside a stainless steel tube of 26 mm internal diameter. The plasma particles follow the axial magnetic field lines and hence, form a beam of same diameter as the internal diameter of the SS tube. The current measured by an interceptor plate (negatively biased) is about 60 mA while the plasma density is determined to be approximately 1.1X10¹² cm^-3 at the center and falling off to about 1X10¹¹ cm^-3 at 1 cm away from the center. The plasma source using a mixture of nitrogen and hydrogen (1:2) was used successfully to nitride a sample (diameter 16 mm) of SS201 at a low temperature of 400°C. The surface hardness achieved is rather high (1990 HV) which is more than five-fold increase compared to untreated surface (358 HV). The depth of the nitriding layer was found to be 26 microns. XRD results indicate efficient conversion of chromium to chromium nitride by the high plasma density of the present source accounting for the very efficient nitriding process.