Nitriding reaction is complicated by many factors, one of which is the highly exothermic character of the reaction. This exothermic nature is related to the microstructure and the mechanical properties of RBSN.
The gas mixtures($H_2/N_2, He/N_2$) having a high thermal conductivity allows the heat generated by the nitriding exotherm to be dissipated from the compact into the nitriding atmosphere, permitting a more accurate control of temperature, and produces a more uniform microstructure and improved mechanical properties.
In the present work, in order to observe the effect of the mixed gas atmospheres on the microstructure of RBSN the specimen was nitrided in the mixed gas atmospheres which was contained up to 50 Vol.% $H_2$ or He for 0-12hr at 1350℃. Scanning electron microscope and optical microscope were used to observe the microstructure and X-ray diffraction analysis used to evaluate the α/β ratio.
The addition of hydrogen to nitrogen gas developed the growth of α-needle at the early stage of nitriding and increased the reaction rate and a finer and more uniform microstructure. This was due to the reduction of silica and the high thermal conductivity.
In case of the addition of helium, the behaviour of reaction was similar to the one with pure nitrogen, due to the inert nature of the two substances. As the amount of helium was increased, the partial pressure of nitrogen decreased and a coarser microstructure was formed. But because of the increased thermal conductivity, the microstructure obtained was more uniform than the one obtained by pure nitrogen.
This showed that a finer and more uniform microstructure produced improved mechanical properties.
