Silicon carbonitride ceramics produced by pyrolysis of polymer ceramic precursor
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Two processing routes were explored to produce crack-free amorphous Si–N–C ceramics by pyrolysis of polyureasilazane ceramic precursor. Using a warm-pressing/ pyrolysis route, a ceramic body with certain amount of open porosity was produced; densification behavior during pyrolysis was examined. A prepyrolysis/binding/pyrolysis route was also developed. Ceramics formed using this route were characterized by higher density, lower volume shrinkage during consolidation, and larger viable material size. Open porosity was essentially absent in consolidated amorphous materials produced by this second route. Recrystallization of the consolidated amorphous ceramics resulted in a Si3N4/SiC nanocomposite with both silicon nitride and silicon carbide grains in the nanometric size range.
Polymer precursor pyrolysis provides a way of consolidating covalent ceramics such as silicon nitride and silicon carbide, at temperatures lower than conventional sintering techniques and usually without the incorporation of oxide sintering additives.1,2 This technique has become a common industrial practice in fabricating ceramic fibers and coatings. Although the pioneering effort in making bulk ceramics by polymer pyrolysis dates back to the 1970s,3,4 high density crack-free ceramics have only been made possible by the recent advances in the polymer-to-ceramic conversion rate.5 The general procedure of pyrolysis consolidation includes compacting of polymer precursor powder, followed by controlled pyrolysis to a temperature around 1000 °C. The polymerto-ceramic conversion is accompanied by weight loss, volume shrinkage, and release of gaseous pyrolysis products. In order to prevent the compact from cracking, the density of the green compact is usually controlled such that there is enough open porosity to allow for the escape of gaseous phases.6 A porous structure is retained in the amorphous ceramic, which on one hand provides an opportunity for infiltration by liquid metal or polymer to produce a denser product. On the other hand, retained porosity may deteriorate the chemical inertness of a specimen if the amorphous material is placed in service application in corrosive environments. The interest in pyrolysis consolidation of silicon carbonitride was initiated in part by the prospect of crystallizing the consolidated amorphous ceramic into nanocrystalline materials. However, since there exists significant lattice shrinkage during the crystallization process, cracking usually occurs during crystallization.6 J. Mater. Res., Vol. 15, No. 8, Aug 2000
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The present work is dedicated to finding a processing route that might solve some of the problems that hinder the current processing and consolidation of amorphous silicon carbonitride materials as described above. This work also looks into the possibility of obtaining crystallized nanoceramics by directly crystallizing a consolidated amorphous compact. The polymer precursor used in this work is a commercially available poly(ureasilazane), Ce
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