Mechanism of active and passive oxidation of reaction-bonded Si3N4-SiC refractories

The mechanism of the active and passive oxidation of reaction-bonded Si3N4-SiC refractories used for six-month in silicon nitridation shuttle has been analyzed. The change of morphology and component of reaction bonded Si3N4-SiC block of saggar have been analyzed. The results show that in Si3N4-SiC, Si3N4 has a higher activity and is more easily oxidized than SiC in both active and passive oxidation modes. All the fibrous alpha-Si3N4 which exists in a substantial amount in Si3N4-SiC before use has transformed to beta-Si3N4 or was oxidized. The burning side of Si3N4-SiC block, which operates in high oxygen partial pressure, is oxidized in passive mode generating SiO2, Si2N2O and carbon. The working side of Si3N4-SiC block, who works in low oxygen partial pressure, is oxidized in the active mode generating gaseous SiO. In another stage of service, SiO generated by silicon nitridation in the saggar infiltrates into the block and reacts with nitrogen forming small rods of Si3N4 that fill pores, which leads to lower apparent porosity. In the middle section of the block, two different oxidation modes coexist, since there is a gradually variation of oxygen partial pressure.