Evaluation of Polysilazanes-Perhydropolysilazane, Polyborosilazane and Methylhydropolysilazane-as Matrix Precursors of Ceramic-Matrix Composites

摘要

Preceramic polymer of Si-N (-C), perhydro-polysilazane (PHS), polyborosilazane (PBS) and methyl-hydropolysilazane (MHS), were synthesized by a pyridine-adduct method. The properties of polysilazanes were examined in relation to their potential application to the precursors of ceramic composites. PBS had 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 studied as the alternative of PBS. The char-yield of the respective raw polymers ranged from 60 to 80 mass percent, while that of the cured polymers was by 10-20 mass percent higher than the raw polymers. The polymers pyrolyzed below 1400 K were amorphous. However, #alpha#-Si_3N_4 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 heat-resistance of which was examined in nitrogen atmosphere for the exposing time span of 360 ks. The heat-resistance test at 1723 K significantly enhanced the crystallization, the test at 1623 K resulted in a subtle change in crystallization, and the test at 1523 K caused no microstructural change. It was concluded from these heat-resistance test results that the amorphous Si-N-C via MHS may be usable as a matrix of ceramic composites at temperatures below 1623 K.