Mechanical Property Enhancement Of C/C-SiC Composites With Modified PIP Process

Carbon fiber reinforced silicon carbide(C/SiC)composites have attracted intensive research interests over the world in last 3 decades as a new class of ceramic matrix composite(CMC)due to their many merits like light weight,high potential strength,good resistance to friction,nuclear irritation,oxidization and ablation,which have made them widely applied in aeronautics,nuclear plant and brakes of airplane,high speed railway and racing cars.As one of the well developed fabrication methods for C/SiC composites,the precursor infiltration and pyrolysis(PIP)route has attracted much attention due to its simple equipments,near net shape formation of complex components,good composition homogeneity and so on.However,C/SiC composites fabricated by PIP method have some disadvantages such as low density,expensive manufacturing process and low mechanical property.In this dissertation,C/C-SiC composites with excellent mechanical properties were successfully prepared by combining the PIP method and chemical vapor infiltration(CVI)as well as the introduction of SiCN matrix.Carbon fiber preforms with different densities and pore structures were prepared by varying the CVI time.The effects of pyrolytic carbon(PyC)coating on the densification behavior,microstructure and mechanical property of composites were systematically investigated,and the fracture mechanism of C/C-SiC composites was also discussed.The results indicated that the sample prepared without PyC interface showed relatively high density,but flexural strength of only 39.72 MPa.The sample with CVI time of 20 h not only showed the highest density of 1.815 g/cm3 but also the highest flexural strength of 429.36 MPa.Further increasing the CVI time to 40 h slightly decreased the density and flexural strength of the C/C-SiC composite.The sample without PyC interface exhibited a brittle fracture behavior.There were no carbon fibers pulled-out.But the samples with PyC interface showed ductile fracture behavior with lots of carbon fibers being pulled-out.This indicated that the introduction of PyC interface had effectively conditioned the bond between carbon fibers and SiC matrix,which resulted in crack deflection and so that enhanced ductibility of the composites.The SiC matrix produced from pyrolysis of PCS have many cracks that limited the mechanical strength of the C/C-SiC composites.By introducing the low shrinkage SiCN matrix,the cracks were effectively reduced,which subsequently increased the density and mechanical strength of the composites.For examples,the C/C-SiC-SiCN composites prepared with 6 cycles of PCS infiltration-pyrolysis and 6 cycles of SiCN polymer infiltration-pyrolysis exhibited higher density as compared to the samples prepared only with 12 PIP cycles of PCS infiltration-pyrolysis.The flexural strengths were increased by 25%,2%and 30%,when the CVI time of 0,20 and 40 h,respectively.