Small Dopant Effect on Static Grain Growth and Flow Stress in Superplastic TZP

Abstract

Static grain growth behavior in 1 mol% of GeO2, TiO2, MgO or BaO-doped ZrO2-3 mol%Y2O3 (3Y-TZP) was examined at 1400 � C with a special interest in dopant effect on superplastic flow stress in fine-grained 3Y-TZP. The static grain growth can be described as normal grain growth in single-phase ceramics, and growth constant K for each material is in the order of 10% flow stress of the superplastic flow. The value of K in cation-doped TZP is correlated well with dopant cation's ionic radius. Assuming activation energy for diffusivity of constituent ion can be given as a linear function of strain caused by difference in the ionic size of dopant cation, the dependence of the growth constant and the flow stress on the ionic radius can be described as a function of the ionic radius of the dopant cation. The activation energy for the diffusivity in cation- doped TZP estimated from the calculation is in good agreement with the experimental data. The small dopant effect on the superplastic flow stress is well described by the activation energy as the function of the dopant cation's ionic size. is the grain growth exponent, and K is the growth constant. The reported data of the parameters are scattered, but in the case of normal grain growth in single-phase TZP, m ¼ 2 is often used for the phenomenological analysis. 13,15,17) The detailed mechanism of the grain growth in TZP has not been classified yet, but it is supposed that matter transport by diffusion in the vicinity of the grain boundaries is the rate- controlling mechanism for the normal grain growth. The growth constant K thus must be related to the diffusivity in TZP, and estimation of K value is expected to produce an important information about the dopant effect on the super- plastic flow stress in TZP. In the present study, the static grain growth behavior was investigated in ZrO2-3 mol%Y2O3 (3Y-TZP) and 1 mol% of cation-doped 3Y-TZP. The dopant effect on the grain growth behavior and the superplastic flow stress will be discussed from the viewpoint of ionic radius dependence of activation energy for the diffusivity. 2. Experimental Procedure The materials used in this study were tetragonal ZrO2- 3 mol%Y2O3 polycrystal (3Y-TZP) and 1 mol% of BaO, MgO, TiO2, or GeO2-doped 3Y-TZP. 3Y-TZPpowder (TZ3Y; Tosoh Co., Ltd.), barium oxide (BaO, Soekawa chemical Co., Ltd.), magnesium oxide (MgO, Ubekosan, Japan), titanium oxide (TiO2, Sumitomo Cement Co., Ltd.) and germanium oxide (GeO2, Rare Metallic Co., Ltd.) were used as starting materials. 3Y-TZP powders and the oxide powders were mixed, ball-milled in ethanol together with 5 mm diameter high-purity (>99:9%) zirconia balls for 24 h, dried and shifted through a 60 mesh sieve for granulation. The green compacts were prepared by pressing the mixed powders into bars with a cemented carbide die under a pressure of 33 MPa, and then isostatically-pressed under a pressure of 100 MPa. The green compacts were sintered at a temperature in the range 1400 or 1450 � C for 2 h in air to obtain an average grain size of about 0.4 mm and relative density of about or more than 99% for theoretical value in all samples. The sintering temperature is listed in Table 1. Heat treatment for investigation of grain growth behavior was conducted at 1400 � C for the annealing time in the range of 5-