Kinetic modelling of a N2 flowing microwave discharge with CH4 addition 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 N-2(X (1)Sigma(+)(g), v) molecules, N-2(A(3)Sigma(+)(u)) and N-2(a' (1)Sigma(-)(u)) metastables and N(S-4) atoms, various hydrocarbons formed from methane decomposition, and other species produced in nitrogen-methane reactions. It is observed that CH4 is primarily dissociated in CH3 and CH2 in collisions with N-2(A), the stable hydrogen cyanide molecule HCN is formed at an intermediate stage of the process, HCN and CH2 give place to the formation of CN(X (2)Sigma(+)), 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)Pi(g)) and CN(B (2)Sigma(+)) states, these latter obtained from spectroscopic measurements.