Hot-gas corrosion behavior of Si3N4 against high speed combustion gas was investigated by optical microscopy observation. A city gas (13A) with the composition of CH4: 88.60, C3H8: 3.20, C2H6: 5.09 and C4H10: 2.86 in vol%, was used as fuel. Three Si3N4 test bars with the size of 4 mm×4 mm×80 mm were used for each exposing run. The exposure was carried out in combustion gas temperature of around 1400°C with the current velocity of about 190 m/s. The exposing time was totally 60 h (7-8 h in a day). At the earlier stage, after 8-16 h exposure, thin and glassy oxide layer was formed on the surface of Si3N4. The oxide layer changed to a powdery substance with elapsing exposure time and the surface was fully covered by powdery oxide after ≈24 h. The powdery oxide was only attached on the surface very loosely and it was easily removed by soft touch. White spots on particles from the burner side which consisted of a larger amount of glassy phase, also changed to powder and fade away with elapsing time. At an earlier stage, after 8-16 h exposure, the specimens showed relatively lower weight loss compared to that detected after around 24 h exposure. It is considered that the decreasing weight may be divided into two stages according to the change of ratio in weight loss before and after each exposure. Thus, in the earlier stage, it can be assumed that vaporizing of Si, O and N2 from the surface grassy phase mainly takes place, while in the later stage, the main process is the removment of powdery oxide in addition to vaporization of Si, O and N2.
