Homogeneity Region and Thermal Stability of Neodymium-Doped α-Sialon Ceramics |
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Authors: | Zhijian Shen Thommy Ekström Mats Nygren |
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Affiliation: | Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China;Department of Inorganic Chemistry, Arrhenius Laboratory, University of Stockholm, S-106 92 Stockholm, Sweden |
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Abstract: | Dense sialon ceramics along the tie line between Si3N4 and Nd2O3·9AlN were prepared by hot-pressing at 1800°C. The materials were subsequently heat-treated in the temperature range 1300–1750°C and cooled either by turning off the furnace (yielding a cooling rate (Tcool) of ~50°C/min) or quenching (Tcool≥ 400°C/min). It was found necessary to use the quenching technique to reveal the true phase relationships at high temperature, and it was established that single-phase α-sialon forms for 0.30 x 0. 51 in the formula NdxSi12–4S x Al4.5 x O1. 5 x , N16–1.5 x . The α-sialon is stable only at temperatures above 1650°C, and it transforms at lower temperatures by two slightly different diffusion-controlled processes. Firstly, an α-sialon phase with lower Nd content is formed together with an Al-containing Nd-melilite phase, and upon prolonged heat treatment thus-formed α-sialon decomposes to the more stable β-sialon and either the melilite phase or a new phase of the composition NdAl(Si6-zAlz)N10-zOz. Nd-doped α-sialon ceramics containing no crystalline intergranular phase show very high hardness (HV10 = 22. 5 GPa) and a fracture toughness ( K lc= 4.4 MPa·m1/2) at room temperature. The presence of the melilite phase, which easily formed when slow cooling rates were applied or by post-heat-treatment, reduced both the fracture toughness and hardness of the materials. |
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