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

The present paper is concerned with the in-situ carbon content adjustment i n amorphous bulk silicon carbonitride (SiCN) ceramic matrices prepared by t he polymer to ceramic transformation of crosslinked and compacted poly (hyd ridomethyl) silazane powders. Heat treatment in inert (Ar) or reactive atmo sphere (ammonia, or mixed Ar/NH3 with different volume ratio of ammonia) wa s used for carbon content adjustment. Isothermal annealing steps in Ar and/ or mixed atmospheres at various intermediate temperatures were also include d into the pyrolysis schedule (i) to adjust the final carbon content, (ii) to control outgassing of low molecular reaction products like methane or hy drogen from the matrix during polysilazane decomposition and thus (iii) to avoid cracking of the pressed polymer powders. Optimal annealing temperatur e for carbon content adjustment was found to be in the range between 500 an d 550 degrees C. Increasing NH3 contents from 10 to 50 vol% in the pyrolysi s atmosphere as well as enhanced transient annealing temperature and time p romote carbon reduction. In contrast intermediate isothermal annealing in A r 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. Dependi ng on the pyrolysis conditions, flawless bulk specimens with carbon content s ranging from 0.3 up to 16.2 wt% were obtained. Different possible chemica l reactions are considered to explain the generation of the particular Si(C )N compositions found. (C) 1999 Elsevier Science Limited. All rights reserv ed.