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Effects of Phase Equilibrium on the Oxidation Behavior of Rare-earth-doped a-sialon ceramics

Published online by Cambridge University Press:  31 January 2011

Zhijian Shen ,
Per-Olov Käll  and
Mats Nygren
Zhijian Shen
Affiliation:
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China
Per-Olov Käll
Affiliation:
Department of Physics and Measurements Technology, Linköping University, S-581 83 Linköping, Sweden
Mats Nygren
Affiliation:
Department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden
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Abstract

A series of rare-earth-(RE-)doped α-sialons (RExSi12–4.5xAl4.5xO1.5xN16–1.5x, with x = 0.40 for RE = Nd, Sm, Yb, and x = 0.48 for RE = Y) were prepared and heat-treated in air at 1350 °C for 66–727 h (3–30 days), and the variations in composition and structure with time of the formed oxide scales and matrix materials were investigated. In the oxide scales of the Nd-, Sm-, and Y-containing samples a liquid was formed, apparently in (quasi-)equilibrium with the crystalline phases cristobalite and mullite, while only crystalline Yb2Si2O7, cristobalite, and mullite were observed in the Yb sample. Apparently, the liquid plays an important role in the oxidation process. In the depleted zone, located between the scale and the matrix, the liquid attacks the matrix phases, and a process takes place in which the originally formed phases dissolve and reprecipitate as more oxygen-rich phases. In the Nd- and Sm-doped systems, where the α-sialon phase is inherently metastable at 1350 °C, an extensive α → β-sialon transformation takes place, creating still more liquid. As a consequence, the oxidation resistance of α-sialons containing Nd and Sm is much lower than those containing Y and, in particular, Yb.

Type
Articles
Information
Journal of Materials Research , Volume 14 , Issue 4 , April 1999 , pp. 1462 - 1470
Copyright
Copyright © Materials Research Society 1999

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