ζ‐Ta 4 C 3− x : A High Fracture Toughness Carbide with Rising‐Crack‐Growth‐Resistance (R‐Curve) Behavior

• Published in: Journal of the American Ceramic Society Vol. 98; no. 8; pp. 2601 - 2608
• Main Authors: Sygnatowicz, Michael, Cutler, Raymond A., Shetty, Dinesh K.
• Format: Journal Article
• Language: English
• Published: 01.08.2015
• ISSN: 0002-7820; 1551-2916
• DOI: 10.1111/jace.13635

The microstructures and mechanical properties of tantalum carbides containing predominantly the ζ‐Ta 4 C 3− x phase are compared with the properties of the monocarbide (γ‐TaC) and the hemicarbide (α‐Ta 2 C) and two‐phase composites. It is shown that a Ta and γ‐TaC powder mixture corresponding to a C/Ta at. ratio of 0.66 can be hot‐pressed (1800°C, 2 h) to obtain ~95 wt% of ζ‐Ta 4 C 3− x with a density of 98% of theoretical. This material has an attractive combination of high fracture toughness (13.8 ± 0.2 MPa√m) and fracture strength (759 ± 24 MPa) with modest hardness (5.6 ± 0.5 GPa). The fracture toughness and strength measured for this material were the highest among all the materials with C/Ta ratio ranging from 0.5 (hemicarbide) to 1.0 (monocarbide). It is also shown that a material containing 86 wt% ζ‐Ta 4 C 3− x can be consolidated by pressureless sintering of a hydrogenated Ta and γ‐TaC powder mixture without significant drop in density (97% of theoretical) or mechanical properties (13.4 ± 0.2 MPa√m, 700 ± 20 MPa, 6.0 ± 0.4 GPa). Materials containing high weight fraction of the ζ‐Ta 4 C 3− x phase exhibited rising crack‐growth‐resistance ( R ‐curve) behavior. Optical and scanning electron microscope observations suggested crack‐face bridging was the dominant toughening mechanism. The crack‐bridging ligaments were lamellae of the basal planes of the ζ‐Ta 4 C 3− x phase produced by their easy cleavage. The thickness of the lamellae ranged from 40 to 2000 nm, significantly less than the grain size.