Abstract
In this work, we used mass spectrometry to investigate ammonia (NH3) formed during the nitrocarburizing and nitriding processes of α-Fe sintered and low alloy steel by using d.c. glow discharges in different N2/H2/CH4 gas concentration mixtures. Experiments were conducted using two different configurations: one to collect and analyze species probed primarily in the bulk of the plasma, at a position distant from the heated sample; and the other to collect and analyze species very close to the reactive surface of the heated sample in such a way as to allow the analysis of lower concentration or residence time species, formed at the plasma/sample surface interface. Use of these configurations contributed to the investigation of the physical chemistry involved in the above plasma processes. An exchange the expected stoichiometry in ammonia formation in N2/H2/CH4 mixtures, with a shift towards higher nitrogen consumption, is observed and is interpreted an indication of nitrogen removal from the surface associated with ammonia and carbonitrides/nitrides formation. The present results point to the existence of a correlation between the nitrocarburizing and nitriding processes with respect to the formation of non-stoichiometric ammonia as a product.
Highlights
Plasma nitrocarburizing and nitriding are thermochemical processes
This paper is devoted to the study of ammonia synthesis during the nitrocarburizing and nitriding processes in d.c. glow discharges sustained in different N2-H2-CH4 and N2H2 gas mixtures, respectively, by means of mass spectrometry
When using the surface configuration, we observed a departure from the expected stoichiometry of ammonia formation, with a well- defined shift towards higher nitrogen consumption, and resulting in a corresponding non-stoichiometric ratio N2:H2 ≈ 3:1, in both plasma processes of this study
Summary
Plasma nitrocarburizing and nitriding are thermochemical processes. In the former, nitrogen and carbon atoms diffuse into the surface of a ferrous substrate heated below the lower critical temperature, forming a compound layer and a subsurface diffusion layer, whereas in nitriding only nitrogen atoms are introduced into the subsurface matrix. A large number of chemical species and a considerable number of reactions continuously taking place within it and which involve different physic-chemical processes, such as ionization, neutralization, excitation, charge transfer, decomposition, Nitrogen and carbon atoms are amongst the species efficiently formed in the plasma used for nitrocarburizing. These species can be very reactive and their chemistry leads to an improvement in the diffusion of atoms into the ferritic matrix, even at temperatures lower than those used in conventional treatments. In addition to their reactivity, the ion bombardment onto the substrate surface along with the fast neutrals is responsible for heating the substrate to the processing temperature
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