OPTICAL EMISSION SPECTROSCOPY OF PULSED-DC GENERATED PLASMAS FOR SURFACE IONITRIDING OF AISI 304 STAINLESS STEEL

Abstract

Glow discharge-opti cal emission spectroscopy of different gases plasma such as I-h. N2, Nz-H2 and Ar-Nz generated by a cost effecti ve 50 Hz pulsed DC electrical power source is caJTi ed Ollt to investigate the differen t excitation and ioni zation processes involved in the production of active species, for surface ionitriding of materi als. Spectroscopic measurements of hydrogen Balmer-p and Baimcr-y line profiles arc perfonned in an abl1or 1 glow region of hyd rogen plasma for investigating the dependencies oftileir line shapes and intensities on discharge parameters. A technique based on dctclll1ining the relative intensities o f neutral atomic hydrogen emission lines is lIsed to evaluate the electron temperature wbereas the electron density is extracted from Stark broadening (FWHM) of the !-Ill emission pro fil e. It is found that both the emission intensity and the broadening of the Bahner-f.. and Bahner-y lines exhibit signifi cant dcpcndence on the filling pressure in thc same manner. However, both thc emission intcnsi ty and broadening of the HI' line exhi bit weak dependence on input power in contrast to the Hy emission line. The emission intensities of the band heads orsecolld positive (A. = 337.1 11m; 0-0) and first negative (A. = 39 1.4 run; 0-0) systems are lIsed to invcstigate the dependence of their radiative states [N2 (C)pu). Nt (B2ru+)] population dcnsi ty on discharge parameters in nitrogen plasma. Thc emission intensity of the N/ (B2rl'+) radiative statc is lIsed to detemline the relative dependence ofN2 + iOIl density on discharge conditions by consideri ng the fact that in low tcmperature plasmas. the dominant mcchanism of the popul ation of Nt (S 2ZIl+) state starts from grollnd state N2 +(X 2I:g+) mainly by clectron collisions. Trace rare gas-optical emission spectroscopy is used to characteri ze the nitrogen plasma as a function of discharge parameters. The excited stale population density ofN-atoms and Nrtllolecules. extracted from their optical emission is related to the ground state population density after nOl1n alizing the changes for excitation c ross-section and elec tron energy distribu tion function (EEDF) by optical actinometry. The electron temperature is determined from the plasma induced optical emission of trace rare gas by the line-to-line ratio method. The Ar-N2 plasma IS characterizcd to investi gate the diffcrent excit at ion and ioni zation processes of both atomic and 1ll0lectllar species under different' discharge - XV I - conditions. In particular, emission intensities of the band heads of second positive (A. = 337. 1 11m; 0-0) and first negati ve ().. = 39 1.4 nm; 0-0) systems arc used to determine the dependence of the ir radiative states [N z (CJn u), Nt (BZLu~») on argon frac tion in the mi xture. rt is observed that the addition of argon influences the radi ative slales diffe rently due to their different popUl ating mechanisms. The electron temperature and number density are determined from optical emission o fargollusing line-to-line ratio method. Optical emission spcctroscopy of the Nz-I-Iz mixture plasllla is carried out to monitor the differenl excitation and ioni zation mechanisms of plasma species involved in their optical emission. A major conccll1 of this work is to optimizc the plasma generat ion processes to produce a high concentration of active plasma species fo r surface treatments. AJS I 304 austenitic stainless stee l samples are nitrided for different time durations under these optimum discharge conditions. The treated samples nre ana lyzed by XRD to investi gate the changes induced in the crystallographic structure. Vickers mi crohardness tester is used to measure the surface hardness as a [uncti on o f indentation depth (~lm) to estimate the nitrogen content incorporated into the modifi ed layer. The XRD pattern COnlil111S the forma ti on of expanded austenite phase (rN) resulted by the incorporation o f nitrogen as interstitial solid solution in the iron lattice. Surface hardness value is found to increase by a factor of almost 2 after trealing the sample for time duration of 16 hours without any auxiliary heating. Based upon optical measurements reported here, plasma discharge parameters can be selected to optimi ze Ihe production of active species, electron temperature and density.