Kinetic modelling of a N₂flowing microwave discharge with CH₄addition in the post-discharge for nitrocarburizing treatments

A kinetic study is conducted to investigate the elementary processes scheme that produces methane decomposition as a small percentage of this gas is introduced downstream in the flowing afterglow of a nitrogen microwave discharge. For this purpose a self-consistent kinetic model is used for the discharge and corresponding afterglow including, besides the species associated with active nitrogen, such as vibrationally excited molecules, and metastables and N( 4 S) atoms, various hydrocarbons formed from methane decomposition, and other species produced in nitrogen–methane reactions. It is observed that CH 4 is primarily dissociated in CH 3 and CH 2 in collisions with N 2 ( A ), the stable hydrogen cyanide molecule HCN is formed at an intermediate stage of the process, HCN and CH 2 give place to the formation of CN( X 2 Σ + ), and ultimately C atoms are produced by collisions of CN( X ) with N atoms. The predicted concentrations so obtained are compared with experimental determinations of N and C atoms and of N 2 ( B 3 Π g ) and CN( B 2 Σ + ) states, these latter obtained from spectroscopic measurements.