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 (Cf/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. Cf/C-TaC composites show a complex relationship of mass gain/loss with oxidation temperature. The mass loss increases at 900–1300 °C; and begins to loss at 1400 °C, when the temperature reaches to 1500 °C, the mass losses acutely. XRD results show that the residual oxide products on the surface of Cf/C-TaC composites at 900–1500 °C are all of Ta2O5 phase but with different crystal structures and morphologies, including hexagonal (δ) structure with an amorphous morphology at 900 °C, orthorhombic (β) structure with a hexagonal-block or oval-shaped crystallite morphology at 1000–1300 °C, tetragonal (α) structure with square rectangular shape morphology, monoclinic (α’) structure with triangular shape morphology and triclinic (α’’) structure with rhombic/long-strip-shape morphology at 1400–1500 °C, which may have great influence on the oxidation-resistance of Cf/C-TaC composites. The formation mechanism of β-Ta2O5 phase had been analyzed by TEM. The oxide contains an amorphous matrix, in which crystallized β-Ta2O5 phase was nucleated and grown. The nucleation and growth process of crystallized β-Ta2O5 are both controlled by the rearrangement of lattice at the crystallite/amorphous matrix interface.