Improvement of chemical corrosion resistance of silicon nitride (Si 3 N 4 ) ceramics is strongly desired from the industry side. However, there is presently not enough information on chemical corrosion. This paper investigates corrosion resistance of two kinds of commercialized Si 3 N 4 -based ceramics with different additives developed for attaining high wear-resistance. One of the specimens is composed of Si 3 N 3 -Y 2 O 3 -Al 2 O 3 -AlN-TiO 2 (specimen A), the other of Si 3 N 4 -MgAl 2 O 4 -SiO 2 -TiO 2 (specimen B). We adapted a corrosion test at 30 and 80°C in acid, basic and salt solutions. The corrosion at 30°C in acid and basic solutions in both specimens was very low in comparison with that at 80°C. Specimen A with additives of Y 2 O 3 and Al 2 O 3 was easily corroded by both acids and bases. in particular the specimen was strongly corroded by 5 mass% H 2 SO 4 , 5 mass% HNO 3 and 35 mass% NaOH solutions at 80°C. Specimen B, containing MgAl 2 O 4 as an additive, was better corrosion resistant than specimen A, although specimen B was slightly corroded by a high concantration of basic solutions. The extent of corrosion by NaCl and CaCl 2 solutions was almost negligible. Solution analysis after soaking the specimens in the solutions indicated that the grain boundary phase, mainly a glassy phase, was reacting with acid. The existence of Y 2 O 3 and Al 2 O 3 in the glassy phase is considered to be mainly responsible for the lowered chemical corrosion resistance. Si 3 N 4 grains easily reacted with bases, although the grain boundary phase was also corroded by them. These results were also confirmed by scanning electron microscopy (SEM) observation. It was concluded that an improvement of corrosion resistance against basic solutions at higher temperatures such as 80°C seems to be hardly achievable. The above data on corrosion resistance of typically commercialized Si 3 N 4 ceramics are considered to be very useful to expand the application areas.
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