Correlations Between Active Plasma Species and Steel Surface Nitriding in Microwave Post-Discharge Reactors

Abstract

Thermochemical treatments with N₂-H₂ and CH₄ plasmas have led to nitrided and carburized layers^1,2 on metal surfaces connected as a cathode of glow discharges at low gas pressures (1-10 Torr). The metal surface is surrounding by a cathode fall where plasma ions are accelerated to impinge cathode surfaces. Then positive ions as in N₂ plasmas act to heat and partly to nitride the sample surface. To separate the two heating and nitriding processes, post-discharge treatments have been recently performed³ where active species are produced in flowing discharges and work pieces are settled downstream inside a separated heating device. First in this lecture spectroscopic emission of active species in Ar — N₂ and Ar — CH₄ flowing discharges is analysed. Emphasis is given on quantitative measurements using a titration method with NO for N and C atom densities in flowing post-discharges. Then correlations are given between active specie densities in the plasma flow and chemical composition and thickness of nitrided layers on steel surfaces. The effect of H₂ molecules in N₂ plasmas is specially discussed.