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 suicide with suited thermo-elastic properties, TaSi2, (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 I GPa was obtained upon addition of SiC particles, but residual thermal stresses prevented from notable increase of toughness, which fluctuated around 4 MPa root m. A good compromise between strength and toughness was found for addition of Hi-Nicalon SiC fiber, 550 MPa and 5.3 MPa root m, respectively. More refractory SiC fibers resulted not effective, owing to the rising of tensional state in the matrix. On the other hand, TaSi2 led to a toughness of 4.7 MPa root m and strength around 680 MPa. Conversely, carbon fiber led to poor toughness due to unfavorable combination of coefficient of thermal expansion with the matrix. (C) 2014 Elsevier Ltd. All rights reserved.