(The) influences of the various transport properties of nitriding atmoapheres on the formation of reaction-bonded $Si_3N_4$

Abstract

The diffusivities and viscosities of the binary gas mixtures, containing the different proportions of $N_2$ to second-component gases, are computed and how these different transport properties affect the formation of Reaction-Bonded $Si_3N_2$ are discussed. The computed data have shown that the additions of $H_2$ and He increase the diffusivity of $N_2$ gas no less than about 2.65 and 3.3 times in the range of 1200$^\circ$ to 1700$^\circ$K, almost independent of composition, whereas Ar has negligible effect. The viscosity of nitriding atmosphere decreases with the amount of added $H_2$, but increases with He or Ar. The discussion has been given to the infolouences these results have on the diffusion and flow of $N_2$ through the porous Si compact. It is suggested that $H_2$ enables $N_2$ gas to be distributed uniformly throughout Si compact during nitridation, thus making the reaction rates uniform throughout the compact, hence developing a uniform microstructure. And it is explained why the flow nitriding condition has the deleterious effects on structure and strength of RBSN, and how the addition of $H_2$ eliminates such effects. In addition, the thermal conductivities of $N_2$ gas containing the various second-component gases have been calculated, and an important role of gaseous phase in thermal conduction during nitridation has been described. And thermal segregations by Soret effect inside nitriding furnace containing the various binary gas mixtures, have been calculated and the influence this effect has on formation of Reaction-Bonded $Si_3N_4$ has been discussed. In $N_2-H_2$ and $N_2$-He gas mixtures, $N_2$ diffuse down the temperature gradient, leading to lower concentration of $N_2$ and higher concentration of $H_2$ or He around Si compact than expected from initial composition, whereas in $N_2$-Ar gas mixtures Ar diffuse down the temperature gradient, leading to the contrary results to above, and such phenomena affect the overall reaction rate and o...