Influence of Crystallinity of Matrix on Mechanical Properties of Fiber-Reinforced Composites Prepared by Polysilazanes as Matrix Precursor

Fiber-reinforced ceramics were fabricated by using pehrydropolysilzane (PHS), polyborosilazane added PHS (PHS-B) or methylhydropolysilazane (MHS) as matrix precursors, and their mechanical properties were investigated with respect to the crystallinity of the matrix. By firing at 1523 K, MHS and PHS-B formed amorphous matrix and gave composites with the flexural strength of 820 and 920 MPa. PHS formed a crystalline matrix and gave a composite with a flexural strength of 450 MPa. The high strength composites may be brought by the lower elastic modulus of amorphous matrix as compared to the crystalline one. Because the low-elastic matrix decreases the stress concentration on fiber due to matrix-cracks. The heat resistance of the composite was tested upon heat exposure at 1523 K for 720 ks. The amorphous composite, fabricated by MHS at 1623 K, endured the heat exposure and retained 77% of its initial strength.