Evaluation of polysilazanes - Perhydropolysilazane, polyborosilazane and methylhydropolysilazane - as matrix precursors of ceramic-matrix composites

Preceramic polymer of SiN(-C), perhydro-polysilazane (PHS), polyborosilazan e (PBS) and methylhydropolysilazane (MHS), were synthesized by a pyridine-a dduct method. The properties of polysilazanes were examined in relation to their potential application to the precursors of ceramic composites. PBS ha d too high viscosity to be impregnated in a fibrous preform, Accordingly, a mixture of PBS and PHS with the mass ratio of PBS/PHS = 1/10 (PHS-B) was s tudied as the alternative of PBS, The char-yield of the respective raw poly mers ranged from 60 to 80 mass%, while that of the cured polymers was by 10 -20 mass% higher than the raw polymers. The polymers pyrolyzed below 1400 K were amorphous, However, alpha -Si3N4 crystallites were observed at higher temperatures exceeding 1473 K for PHS, 1673 K for PHS-B and 1723 K for MHS , The crystallization increased the density from 2.4 Mg/m(3) to 3.1 Mg/m(3) at the maximum, The MHS pyrolyzed at 1723 K, being nearly amorphous, the h eat-resistance of which was examined in nitrogen atmosphere for the exposin g time span of 360 ks, The heat-resistance test at 1723 K significantly enh anced the crystallization, the test at 1623 K resulted in a subtle change i n crystallization, and the test at 1523 K caused no microstructural change, It was concluded from these heat-resistance test results that the amorphou s Si-N-C cia MHS may be usable as a matrix of ceramic composites at tempera tures below 1623 K.