Corrosion resistance of wear resistant silicon nitride ceramics in various aqueous solutions

Abstract

Improvement of chemical corrosion resistance of silicon nitride (Si3N4) 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 Si3N4-based ceramics with different additives developed for attaining high wear-resistance. One of the specimens is composed of Si3N4-Y2O3-Al2 O3-AlN-TiO2 (specimen A), the other of Si3N4-MgAl2O4-SiO2- TiO2 (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 Y2O3 and Al2O3 was easily corroded by both acids and bases. In particular the specimen was strongly corroded by 5 mass% H2SO4, 5 mass% HNO3 and 35 mass% NaOH solutions at 80°C. Specimen B, containing MgAl2O4 as an additive, was better corrosion resistant than specimen A, although specimen B was slightly corroded by a high concentration of basic solutions. The extent of corrosion by NaCl and CaCl2 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 Y2O3 and Al2O3 in the glassy phase is considered to be mainly responsible for the lowered chemical corrosion resistance. Si3N4 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 Si3N4 ceramics are considered to be very useful to expand the application areas.