Verbindungsschichtbildung beim Nitrieren und Nitrocarburieren

Abstract

Only a limited understanding has been attained until now about the growth and constitution of the iron-(carbo)nitride compound layer at the surface of nitrided or nitrocarburized workpieces. Possible interpretations of well known phenomena are discussed. Equilibrium between nitriding/nitrocarburizing medium and the surface of the workpiece can be realized in practice only if certain conditions are obeyed. At relatively high temperatures and nitriding potentials stationary states can occur which are easily misinterpreted as states of equilibrium. The y' and E iron nitrides can be conceived as interstitial solid solutions exhibiting a more or less ordered arrangement of the interstitials, or might be considered as compounds, of a certain ideal stoichiometry, exhibiting a certain composition range. Diffusion in the iron nitrides was discussed in terms of diffusion of nitrogen over interstitial sites of the iron sublattice and of iron, too, if structural vacancies occur on the iron sublattice. Pore formation can always occur as consequence of the thermodynamic instability of the nitrides (N2 gas formation). Other, additional interpretations of pore formation are indicated: Kirkendall porosity and porosity induced by dissolution of iron (in salt baths). The enrichment of carbon in the bottom part of the compound layer is predominantly seen as a consequence of the gradient in carbon chemical potential in particular due to nitrogen-concentration depth profile.