Reactions during silicon nitriding

The influence of essential parameters (grain size, temperature, additives to the nitriding gas, impurities of the silicon powder) on silicon nitriding is under discussion. Reactions are explained by mechanisms of KD (nucleation diffusion) and VLS (vapour-liquid-solid), which take place simultaneously. The packing spaces between the grains of the silicon powder (primary pores) fill with fine Si3N4 crystals in the process, whereas new empty spaces (secondary pores) result in the silicon grains. During nitriding, iron impurities of the silicon lead to the formation of ferro-silicon melts. They allow the reaction to continue, even after the silicon grains have been fully enveloped by silicon nitride layers. The iron is not consumed in the process, and even smallest quantities will have catalytic effect as a consequence. (orig.)