Microstructure, mechanical properties and oxidation behavior of carbon fiber reinforced PyC/C-TaC/PyC layered-structure ceramic matrix composites prepared by chemical vapor infiltration

A new kind of carbon fiber reinforced PyC/C-TaC/PyC layered-structure ceramic matrix composites (C-f/C-TaC composites) was prepared by chemical vapor infiltration. The middle C-TaC ceramic layer was a co-deposited layer of PyC and TaC phases, in which its structure can also be divided into three different zones, from amorphous near the inner PyC interface to equi-axial and then needle-like structures. Nano-indentation tests show that the hardness and elastic moduli of the C-TaC co-deposited ceramic layer are higher than the values of PyC layer, with the results of a higher flexural strength, fracture toughness of the as-prepared composites when compared with C/C composites. C-f/C-TaC composites show a complex relationship of mass gain/loss with oxidation temperature. The mass loss increases at 900-1300 degrees C; and begins to loss at 1400 degrees C, when the temperature reaches to 1500 degrees C, the mass losses acutely. XRD results show that the residual oxide products on the surface of C-f/C-TaC composites at 900-1500 degrees C are all of Ta2O5 phase but with different crystal structures and morphologies, including hexagonal (delta) structure with an amorphous morphology at 900 degrees C, orthorhombic (beta) structure with a hexagonal-block or oval-shaped crystallite morphology at 1000-1300 degrees C, tetragonal (alpha) structure with square rectangular shape morphology, monoclinic (alpha') structure with triangular shape morphology and triclinic (alpha '') structure with rhombic/long-strip-shape morphology at 1400-1500 degrees C, which may have great influence on the oxidation-resistance of C-f/C-TaC composites. The formation mechanism of beta-Ta2O5 phase had been analyzed by TEM. The oxide contains an amorphous matrix, in which crystallized beta-Ta2O5 phase was nucleated and grown. The nucleation and growth process of crystallized beta-Ta2O5 are both controlled by the rearrangement of lattice at the crystallite/amorphous matrix interface. (c) 2013 Elsevier B.V. All rights reserved.