Thermal shock of porous silicon nitride with preferentially aligned grains
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Y. Inagakia) Synergy Ceramics Laboratory, Nagoya, 463-8687, Japan
W.J. Clegg Department of Materials, University of Cambridge, Cambridge CB2 3QZ, United Kingdom (Received 23 April 2003; accepted 29 August 2003)
We studied the thermal shock resistance of a silicon nitride containing elongated and preferentially aligned grains with different volume fractions of pores, ranging from 0 to 0.27. It was found that an increase in the volume fraction of pores decreased both the strength and the temperature through which the sample must be quenched to cause cracking. However, at intermediate values of the porosity (0.07), the temperature change required to cause cracking was much smaller than predicted. Observations of the resulting damage suggested that this had occurred because of the formation of cracks just underneath and parallel to the cooled surface of the sample that were able to change the direction of their growth. The extent of cracking was found to be only very weakly dependent on the volume fraction of pores, consistent with calculations of the variation of crack driving force within the sample.
I. INTRODUCTION
Silicon nitride has a better resistance to thermal shock compared to most oxides due to its relatively low coefficient of thermal expansion and relatively high values of the thermal conductivity and of the fracture energy.1 The latter arises from the elongated grains that tend to form, allowing grain bridging to occur.2–6 Aligning the grains enables the fracture energy to be increased still further, at least where the crack is growing perpendicular to the direction of the aligned grains.7,8 Techniques for incorporating porosity into these materials have also been developed,9–12 and the resulting reduction in Young’s modulus might be expected to further increase the resistance to thermal shock.11 The aim of this paper therefore is to study the thermal shock behavior of these materials and how this is influenced by their porosity. II. EXPERIMENTAL A. Sample fabrication
Silicon nitride samples containing preferentially aligned grains and with volume fractions of pores of 0, 0.07, and 0.27 were produced at the Synergy Ceramics Laboratory, Nagoya, Japan. They had been made by tape casting a slurry containing elongated -silicon nitride a)
Present address: Ishikawajima-Harima Heavy Industries Co., Ltd., Yokohama, 235-8501, Japan
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J. Mater. Res., Vol. 18, No. 11, Nov 2003 Downloaded: 14 Mar 2015
seed crystals, together with the sintering additives and a suitable binder, into sheets approximately 100 m thick. This aligned the seed crystals in the plane of the tape. Green bodies were then made by cutting, stacking (top to bottom), and pressing the tapes together. Fully dense material contained 5 wt.% alumina and 2 wt.% yttria as sintering additives and was pressureless sintered at 1850 °C for 6 h. A volume fraction of pores of 0.07 was produced using the same additives, but the time for which the samples were sintered was reduced to 3.5 h. The porosity of 0.27 was obtai
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