Effects of pyrolysis processes on microstructure and mechanical properties of Cf/Si-O-C composites fabricated by preceramic polymer pyrolysis

Three-dimensional braided carbon fiber reinforced silicon oxycarbide (3D-B C-f/Si-O-C) composites were fabricated by multiple polysiloxane impregnation and pyrolysis cycles, and the effects of pyrolysis processes on microstructures and mechanical properties were investigated. Increasing heating rate of each cycle could not weaken interfacial bonding and would lower flexural strength due to the pores resulting from rapid pyrolysis. Increasing the pyrolysis temperature of the first cycle from 1000 degreesC to 1600 degreesC could weaken interfacial bonding to a certain extent. At the same time, many pores derived from high temperature and rapid pyrolysis were created, decreasing flexural strength. The mechanical properties of 3D-B Cf/Si-O-C composites could be improved when the pyrolysis temperature of the seventh cycle was increased from 1000 degreesC to 1400 degreesC because of the weakened interfacial bonding. The mechanical properties and densities of 3D-B Cf/Si-O-C composites could be remarkably increased if the first pyrolysis was assisted by hot-pressing. When the first pyrolysis was conducted at 1600 degreesC for 5 min with a pressure of 10 MPa, the resultant composites showed 502 MPa in flexural strength and 23.7 MPa m(1/2) in fracture toughness while the composites fabricated without the assistance of hot-pressing showed only 246.2 MPa in flexural strength and 9.4 MPa m(1/2) in fracture toughness. The high mechanical properties were mainly attributed to desirable interfacial structure and high density. (C) 2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved.