Effects of powder bed conditions on the liquid-phase sintering of Si 3 N 4
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ung-Churl Choi and Keun-Ho Auh Department of Ceramic Engineering, Hanyang University, # 17, Haengdang-dong, Sungdong-Gu, Seoul 133-791, Korea (Received 20 June 2001; accepted 29 November 2001)
The effects of the active and passive protection mechanisms of powder beds on the sintering of Si3N4 were investigated. Shrinkage, density, and coloring behavior of sintered samples were analyzed using different compositions and packing conditions of powder beds based on BN and Si3N4 with different additives. Y2O3 additive in the powder bed influences the weight change and phase formation behavior of the samples, although it has a very low vapor pressure at the sintering temperature. When MgO/Y2O3 was used as sintering additives, the packing density and thickness of the powder bed had a much stronger effect than in the case of Al2O3/Y2O3. For the optimization of the powder bed conditions, the vapor pressure and chemical stability of sintering additives at the sintering temperature has to be considered.
I. INTRODUCTION
Liquid-phase sintered Si3N4 is one of the most intensively investigated ceramics due to its mechanical, thermal, and chemical properties. Above 1500 °C, it begins to decompose [Si3N4(s) → 3Si(s) + 2N2(g)],1 thus weight loss during sintering is a common phenomenon with Si3N4. To minimize the weight loss, the decomposition of Si3N4 and/or sintering additives should be suppressed. Using a powder bed improves the sintering properties of Si3N4, such as final density and weight loss, due to both an active and a passive protection mode.2,3 Active protection is based on the formation of the same vapor species in the powder bed as in the sintering sample, thus inhibiting the weight loss of the sample.4 The passive protection mode affects the diffusion and/or flow rate of gas species. The vapor pressure of decomposition products in the vicinity of the sample can be increased by suppressing the gas flow through the powder bed. The gas flow in porous media can be classified into the following: (i) pure diffusion (homogeneous pressure conditions); (ii) combined diffusion and hydrodynamic flow (inhomogeneous pressure conditions); (iii) special mechanisms like surface flow and electro-osmosis.5
a)
Delegated to Max-Planck-Institut fu¨ r Metallforschung by Hanyang University. J. Mater. Res., Vol. 17, No. 2, Feb 2002
Usually the gas flow in a powder bed follows the second mechanism. Low permeability of the powder bed can induce pressure differences between the surface of the sintering sample and the outside of the powder bed. Gas flow rates are related to the pressure difference, pore radius, and powder bed thickness. Small pore radii and a large powder bed thickness can increase the efficiency of this passive protection. Besides providing passive and active protection, powder beds can reduce the contamination of the sintering sample.6 The intention of this paper is to analyze the active and passive effects of powder beds having different compositions and packing conditions in order to improve the sintering properties of Si3N
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