Strength and toughness: The challenging case of TaC-based composites

This work aims at studying the relationships between strength and toughness of tantalum carbide (TaC) ceramics, a refractory ceramic used in aerospace and energy production sectors. The effect of different secondary phases was explored: (I) the addition of a transition metal silicide with suited thermo-elastic properties, TaSi 2 , (II) the addition of SiC particles, platelets or fibers, and (III) chopped carbon fibers. Microstructural analyses, performed by scanning and transmission electron microscopy, were essential in revealing at nanoscale level the morphological changes occurred during sintering in the reinforcing phase and its interaction with matrix and sintering additive. Mechanisms of reinforcement evolution are suggested accordingly. Fracture toughness and flexural strength were measured and the values were compared to unreinforced materials and discussed in agreement to the microstructural features. Strength approaching 1 GPa was obtained upon addition of SiC particles, but residual thermal stresses prevented from notable increase of toughness, which fluctuated around 4 MPam1/2. A good compromise between strength and toughness was found for addition of Hi-Nicalon SiC fiber, 550 MPa and 5.3 MPam1/2, respec- tively. More refractory SiC fibers resulted not effective, owing to the rising of tensional state in the matrix. On the other hand, TaSi 2 led to a toughness of 4.7 MPam1/2 and strength around 680 MPa. Conversely, car- bon fiber led to poor toughness due to unfavorable combination of coefficient of thermal expansion with the matrix.