Structural Design and processing of Laminated Fiber-reinforced Ceramic Matrix Composites

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A sol-gel-processed mullite powder and Si-Ti-C-O fiber woven fabrics were formed into laminated composites by filtration or doctor blade method using aqueous mullite suspensions containing polyacrylic ammonium and methyl cellulose at pH 8.5. The laminate green composites with 6 and 10 vol% fabrics were hot-pressed to near full density at 1500ﾟ-1650ﾟC for 1h under a pressure of 39 MPa in a N2 atmosphere. Graphite layrs of about 10 nm thickness and TiC particles of 0.1-0.2 mum were produced at the interfaces between fibers and matrix after hot-pressing at 1650ﾟC.beta-SiC and TiC particles of 10-30 nm and graphite phase with short cycle structure were recognized in the fibers. These results indicate that the crystallization of fibers progresses with formation of the amorphous phase in fiber. The EDS quantification of the fibers indicated that the fibers pyrolyzed during hot-pressing at 1650ﾟC by 13.7 % of oxygen content.A linear relation of stress-displacement in fracture behavior of monolithic mullite changed to a non-linear relation in the composites with 10-30 vol% of Si-Ti-C-O fabrics and hot-pressed at 1650ﾟC.Addition of Si-Ti-C-O fabrics decreased the four point flexural strength of monolithic mullite (328 MPa) to 292 MPa at 6 vol%, 271 MPa at 10 vol% and 59-78 MPa at 20-30 vol% of fabrice. The decrease of flexural strength for the composite was due to the residual tensile stress caused in the mullite layrs along the direction of length of composites. It is important to study fracture behavior of composite under multiaxial stress state for structural components in practical applications. The tension-compression biaxial strengh of composite materials was lower than that of monolithic mullite ceramics.