High temperature deformation of precursor-derived amorphous Si-B-C-N ceramics

The deformation of amorphous Si-B-C-N ceramics derived from polymeric boron -containing polysilylcarbodi-imide was investigated at temperatures between 1500 and 1700 degrees C in nitrogen atmosphere. The composition of the as- thermolysed ceramic was determined to be SiB0.41+/-0.11C3.39+/-0.44N2.42+/- 0.31 O0.22+/-0.14 and no remarkable change was observed after testing at 16 00 degrees C in nitrogen atmosphere. Under uniaxial compression a true stra in of more than 30% was achieved without visible damage. By compression at constant strain rate a maximum densification of 26% was achieved. Besides d ensification non-Newtonian viscous flow was observed with an apparent activ ation energy of 0.8 MJ mol(-1) and an apparent activation volume between 2. 5 and 0.2 nm(3). A free volume in the same order of a few cubic nanometers was assumed. At 1600 degrees C the amorphous ceramic starts to crystallize with a volume fraction of crystalline phase less than 10% after several hou rs. The apparent increase in stress during deformation was explained by cha nges in sample size, porosity and intrinsic structure due to a decrease in free volume. (C) 1999 Elsevier Science Ltd. All rights reserved.