ACTIVE SPECIES IN MICROWAVE POSTDISCHARGE FOR STEEL-SURFACE NITRIDING

Production of nitrogen atoms has been studied in a 2.45-GHz flowing postdischarge in N2 and N2-H2 gas mixtures with Ar as a buffer gas in the high-pressure regime (5 × 103 to 6.5 × 104 Pa). N atom densities have been measured by NO titration in the 1014-1015 cm3 range and monitored by the first positive emission resulting from the N atom recombination. The rate coefficient of the N + N + N2 recombination has been found to be k = 6 × 10-33 cm6 atom-2 s-1 at T0 = 300 K, which agrees with previously published data. The N atom production (or degree of N/N2 dissociation) in front of an Fe-0.1%C substrate correlates well with the thickness of a γ′ Fe4N layer produced by the postdischarge treatment. The H2 gas was first introduced in the initial phase of treatment to remove surface oxidizing and then was cut off to keep high densities of N atoms. It is deduced that N atoms are more active nitriding species then NH-type radicals.