Nucleation control for hot-working of silicon oxynitride based ceramics
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Nucleation control for hot-working of silicon oxynitride based ceramics Masayoshi Ohashi and Yasuo Iida National Industrial Research Institute of Nagoya, Nagoya 462-8510, Japan
Stuart Hampshire Materials Research Centre, University of Limerick, Limerick, Ireland (Received 27 February 1988; accepted 12 May 1998)
An attempt was made to develop an engineering ceramic plastically deformable at high temperatures with low flow stress at high strain rates, and without strain hardening. Dense ceramic preforms were fabricated by pressureless sintering Si3 N4 1 SiO2 mixed powders with an addition of MgAl2 O4 at 1500 ±C. A transient liquid, which occurs during the reaction sintering of Si2 N2 O, was utilized for subsequent net-shape forming. The ceramic (6f 3 6 mm column) was deformed without any cracks and cavities in compression tests at high strain rates s1022 –1023 s21 d at 1500 ±C, but this was not achieved in a test at lower strain rates for a long time, because of the growth of elongated Si2 N2 O grains during the test. Potassium fluoride (KF) was used as a dopant for retardation of nucleation of Si2 N2 O during sintering and hot-working. The KF-doped preforms were successfully plastically deformed even in the test for a long time.
I. INTRODUCTION
Ceramics are usually machined to final shape and size after sintering because it is difficult to predict and design the firing shrinkage of ceramics precisely, which is generally large. The machining of hard ceramics is very costly, so it would be desirable to apply net-shape fabrication to the forming of ceramics as for metals, plastics, and glasses. The superplastic forming of finegrained ceramics sometimes with intergranular glassy phases is a novel approach toward this achievement. The superplasticity in structural ceramics with very fine grains such as tetragonal ZrO2 polycrystals,1 Si3 N4 /SiC composites,2 Al2 O3 ,3 a 0yb 0 -sialon,4,5 etc. has been reported. The intergranular glassy phase is utilized especially for the superplastic forming of covalent crystal composites such as in Si3 N4 -related ceramics, because it enhances grain-boundary sliding.2 A transient liquid, which occurs during the reaction sintering of a 0yb 0 -sialon, is also utilized for this type of forming.4,5 However, the deformation of these polycrystalline materials is usually accompanied by strain hardening, which is attributed to grain growth (especially of nonequiaxed grains) and crystallization of intergranular glassy phase. The strain hardening in these ceramics, which limits the working time of the plastic forming, restricts the size and design of products, because it is necessary to perform the deformation of ceramics at very low strain rates, compared with metals. Silicon oxynitride (Si2 N2 O) based ceramics are fabricated by sintering Si3 N4 and SiO2 powder mixtures 170
J. Mater. Res., Vol. 14, No. 1, Jan 1999
with metallic oxides. A large amount of transient liquid forms as a result of a reaction between all constituents above a eutectic t
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