In-situ carbon content adjustment in polysilazane derived amorphous SiCN bulk ceramics

The present paper is concerned with the in-situ carbon content adjustment in amorphous bulk silicon carbonitride (SiCN) ceramic matrices prepared by the polymer to ceramic transformation of crosslinked and compacted poly (hydridomethyl) silazane powders. Heat treatment in inert (Ar) or reactive atmosphere (ammonia, or mixed Ar/NH3 with different volume ratio of ammonia) was used for carbon content adjustment. Isothermal annealing steps in Ar and/or mixed atmospheres at various intermediate temperatures were also included into the pyrolysis schedule (i) to adjust the final carbon content, (ii) to control outgassing of low molecular reaction products like methane or hydrogen from the matrix during polysilazane decomposition and thus (iii) to avoid cracking of the pressed polymer powders. Optimal annealing temperature for carbon content adjustment was found to be in the range between 500 and 550 degrees C. Increasing NH3 contents from 10 to 50 vol% in the pyrolysis atmosphere as well as enhanced transient annealing temperature and time promote carbon reduction. In contrast intermediate isothermal annealing in Ar at 500 up to 600 degrees C results in pronounced formation of Si-C bonds and in increased carbon contents after the final pyrolysis process. Depending on the pyrolysis conditions, flawless bulk specimens with carbon contents ranging from 0.3 up to 16.2 wt% were obtained. Different possible chemical reactions are considered to explain the generation of the particular Si(C)N compositions found. (C) 1999 Elsevier Science Limited. All rights reserved.