Effects of Phase Equilibrium on the Oxidation Behavior of Rare-earth-doped a-sialon ceramics
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Effects of phase equilibrium on the oxidation behavior of rare-earth-doped a-sialon ceramics Zhijian Shen Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
Per-Olov K¨all Department of Physics and Measurements Technology, Link¨oping University, S-581 83 Link¨oping, Sweden
Mats Nygren Department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden (Received 17 February 1998; accepted 2 September 1998)
A series of rare-earth-(RE-)doped a-sialons (REx Si1224.5x Al4.5x O1.5x N1621.5x , 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 Yb2 Si2 O7 , 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 a-sialon phase is inherently metastable at 1350 ±C, an extensive a ! b-sialon transformation takes place, creating still more liquid. As a consequence, the oxidation resistance of a-sialons containing Nd and Sm is much lower than those containing Y and, in particular, Yb.
I. INTRODUCTION
It is well-known that sintering additives usually undermine the refractory properties and oxidation resistance of Si3 N4 -based ceramics, but that the effects are strongly dependent both on the volume and the chemical composition of the intergranular phase(s) formed. One way of reducing the problems caused by the use of additives is to prepare a-sialon ceramics, or duplex a –b-sialon composites, because the additive metal oxides can be incorporated into the sialon crystals, and a material with a minimum volume fraction of residual grain boundary can be obtained.1–3 This method has proved successful for a- and duplex a –b-sialons prepared with Y2 O3 and Yb2 O3 as sintering aids. In fact, the oxidation resistance of Yb-doped a- and duplex a –bsialons is equally good as, or better than, that of pure Si3 N4 prepared by HIPing.4–6 In a previous article we have reported the oxidation kinetics of a series of RE a-sialon ceramics (RE Y, Nd, Sm, and Yb) in the temperature range 1250–1350 ±C.5 The oxidation performance of the Ybdoped samples was superior, with parabolic rate constants, Kp , as low as , 9 3 1028 mg2 cm24 s21 at 1462
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J. Mater. Res., Vol. 14, No. 4, April 1999
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1250 ±C, for both single-phase a-
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