Effects of additive and impurity species on the oxide morphology of silicon nitride
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Si3N4 sintered with Lu2O3 was exposed to 1 atm of oxygen and steam environments at 1200–1400 °C. The effects of additive and impurity species on the morphological development of the oxide layer were examined. Oxide layers grown on as-received samples in oxygen generally contained bubbles and cracks and underwent spallation due to the presence of an initial impurity-laden oxide layer. Oxide layers grown on as-received samples in steam exhibited layered morphology: a glassy outer layer and a cristobalite inner layer with a high population density of Lu2Si2O7 particles between. The Lu2Si2O7 particles accumulated at the interface led to extensive spallation of the upper oxide layer. Removal of the initial oxide by polishing resulted in improved oxidation resistance and oxide morphology in oxygen and in steam.
I. INTRODUCTION
High-temperature oxidation resistance of silicon-based ceramics is dictated largely by the quality of oxide formed. It is well established that oxidation of pure silicon nitride in clean and dry oxygen results in a highly protective oxide scale,1–5 which gives this material excellent intrinsic oxidation stability. Oxides formed on commercial silicon nitride ceramics in actual service environments are far from ideal. Additive and impurity species in the ceramic bulk6–9 as well as reactive components in the environment10–13 significantly diminish the protective role of the oxide layers. Silicon nitride sintered with rare-earth (RE) oxides (RE refers to Sc, Y, and metals of lanthanide group from La to Lu) is reported to possess relatively high oxidation resistance,14–19 due to their incorporation into stable crystalline disilicates in the grain boundaries. Lange et al.20 showed that Y2Si2O7 is the most oxidationresistant secondary phase for Si3N4 ceramics sintered with yttria because it is the only Y-containing phase in equilibrium with SiO2. Phase relations in the Si3N4– SiO2–RE2O3 systems are not yet well established for the majority of the lanthanide oxides. They can be regarded, nevertheless, to be analogous in general to those in the Si3N4–SiO2–Y2O3 system.21–23 Disilicates of all lanthanides may not contribute significantly to additiveaccelerated oxidation of Si3N4. There is sparse literature a)
On leave from Metal-Polymer Research Institute, Belarus. Present address: Metal-Polymer Research Institute, 32a Kirov Street, 246652 Gomel, Belarus.
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http://journals.cambridge.org
J. Mater. Res., Vol. 18, No. 4, Apr 2003 Downloaded: 16 Mar 2015
dealing with comparative studies of oxidation of Si3N4 sintered with different oxides of lanthanide metals such as the lighter La, Ce, Nd, and Sm,14–17 and the heavier Sm, Gd, Dy, Er, and Yb.19 Nevertheless, it has been found that oxidation resistance of these materials correlates reasonably well with the refractoriness of the RE2O3–SiO2 system. Parabolic oxidation law is followed for majority of RE-sintered Si3N4.19 It is generally concluded that rate-limiting step during oxidation of these materials is outward diffusion of RE3+ and impurity cations fro
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