Dense silicon carbonitride ceramics by pyrolysis of cross-linked and warm pressed polysilazane powders

This study reports on the pyrolysis and densifaction behavior of cross-link ed poly (hydridomethylsilazane) powders. The influence of the cross-linking procedure such as temperature and annealing time of the polymer powders on the compaction behavior under cold and warm pressing conditions is discuss ed. The degree of cross-linking is determined by thermal mechanical analysi s (TMA). In addition to particle sliding which is assumed to be the compact ion mechanism obtained by cold-pressing, the polymer powder consolidates by plastic deformation applying warm-pressing. A continuous 3-dimensional pol ysilazane network is formed after a dwelling time under these conditions. P yrolysis of the cross-linked and compacted polysilazane powder in argon at 1100 degrees C gives crack-free amorphous silicon carbonitride Si3+xCx+yN4 with compositions ranging from x = 1.47 and y = 0.88 for cold pressed sampl es to x= 1.47 and y= 1.86 for warm pressed materials. The residual open por osity is significantly reduced from 10-15 vol% in the cold pressed specimen s to 1.3-5 vol% by the warm pressing procedure. The weight loss during pyro lysis between room temperature and 1300 degrees C is about 5 wt% lower than that for cold pressed specimens. This result is explained by a reduced met hane evolution during the polymer-to-ceramic conversion and is in accordanc e with the enhanced carbon content of the warm pressed material. (C) 1999 E lsevier Science Ltd. All rights reserved.