Abstract
(1) Oxidation behavior of Si3N4 ceramics is affected by the composition of sintering aids and the weight gain due to oxidation tends to decrease with increasing SiO2/Yb2O3 ratio. In specimens with SiO2/Yb2O3 ratio from about 0.07 to 0.14, the weight gain is small. In specimens with SiO2/Yb2O3 ratio larger than about 0.2, the weight loss due to active oxidation occurs.
(2) In Si3N4 ceramics with SiO2/Yb2O3 ratio smaller than about 0.2, the enrichment region of Yb and the depletion region of Yb are formed. At the initial stage of oxidation, the oxidation rate is controlled by the interface reaction. At the 2nd stage of oxidation, it is considered that the oxidation rate is affected by the rate of outward diffusion of Yb3+ ions. At the final stage of oxidation, oxidation rate becomes smaller than that of the parabolic behavior, which appears to be affected by the formation of Yb2Si2O7 during oxidation.
(3) In order to improve the oxidation resistance, it is necessary to decrease the amount of sintering aids and to use the sintering aids with an adequate composition.
