Reaction-bonded silicon nitride is a promising candidate for high temperature dynamic engineering material. But this brittle material must be designed effectively, it is essential that it is fabricated with consistent properties that are reproducible from batch to batch of material.
Some of the processing factors influencing strength variability have been reported. In particular gas flow during nitriding has been shown to degrade strength. Recently the use of nitrogen/hydrogen gas mixtures eliminate the adverse effect of gas flow on strength when nitrogen alone is used.
The influence of hydrogen in a "static" and "flowing" state on the mechanism of formation, strength and reaction amount of reaction-bonded silicon nitride has been investigated.
For pure nitrogen gas flow encouraged formation of surface nitride layer and the increased flow rate markedly reduced the $α-Si_3N_4$ content because of the loss of silicon monoxide by gas flowing. The addition of hydrogen to nitriding gas made the microstructure of the silicon nitride more uniform and finer, and increased $α-Si_3N_4$ but decreased $β-Si_3N_4$.
Small percent of hydrogen was enough to increase amount of reaction and strength, but large amount of hydrogen was not significant because it seemed to inhibit the formation of α needle.
The effect of hydrogen at the early stage was similar to the effect of pretreatment with Ar or vacuum.
Hydrogen increased silicon monoxide partial pressure and the reaction rate, particularly at the early stage of nitriding, and therefore minimized the loss of vapor-transported species and favored the rapid formation of the skeletal silicon nitride network.
