Synergistic reinforcement of carbon nanotubes and silicon carbide for toughening tantalum carbide based ultrahigh temper
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Tantalum carbide (TaC) is an ultrahigh temperature ceramic, where low damage tolerance limits its potential application in propulsion sector. In this respect, current work focuses on enhancing the toughness of TaC based composites via synergistic reinforcement of SiC and carbon nanotubes (CNTs). Spark plasma sintering of TaC, reinforced with 15 vol% SiC and 15 vol% CNT (processed at 1850 °C, 40 MPa, 5 min), has shown enhanced densification from ;93% (for TaC) to ;98%. Potential damage of the tubular CNTs to flaky graphite was revealed using transmission electron microscopy, and was supplemented via Raman spectroscopy. SiC addition has enhanced the hardness to ;19.5 GPa while a decreases to 12.6 GPa was observed with CNT addition when compared to the hardness of TaC (;15.5 GPa). The increase in the indentation fracture toughness (from 3.1 MPa m1/2 for TaC to 11.4 MPa m1/2) and fracture strength (from ;23 MPa for TaC to ;183 MPa) via synergetic reinforcement of SiC and CNT is mainly attributed to energy dissipating mechanisms such as crack branching, CNT bridging, and crack-deflection. In addition, the reduction of interfacial residual tensile-stresses with SiC- and CNT-reinforcement, resulting an overall increase in the fracture energy and toughening, is also established.
I. INTRODUCTION
Nose-cones of hypersonic vehicles experience temperatures in excess of .2200 °C, and are associated with severe oxidation, extreme heat fluxes, and high mechanical stresses. Such demanding requirements mandate good chemical resistance, and high melting temperatures (.3300 K) toward use as an ultrahigh temperature ceramic (UHTC).1 Among the UHTCs, TaC is an important transition metal mono-carbide, possessing an extremely high melting point of ;4200 K (Ref. 2), making its consolidation into useful engineering shapes a major challenge. Since, solid state sintering occurs typically at temperatures .0.5Tm, the temperature required for consolidation of TaC exceeds .2500 K under pressureless conditions, whereas under pressure-assisted sintering conditions, i.e., spark plasma sintering (SPS) and hot pressing, sintering temperature can be brought down to as low as 1600 K.3–7 SPS is a relatively novel consolidation method for the synthesis of UHTCs composites, and provide an advantage of short sintering duration even to achieve nearly full densification (;100%) while showing superior mechanical properties
Contributing Editor: Yanchun Zhou a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.65
(hardness, elastic modulus, and fracture toughness) than hot pressing.8,9 In earlier publications, the unique capability of multistage SPS approach to develop ceramics with uniform and better mechanical properties has already been established.10,11 An alternative method to deal with the issue of sinterability and difficulty to reach full densification and the relatively low strength (;400 MPa) and toughness (;3.4 MPa m1/2) of TaC materials,4,12,13 secondary agent such as TaB2, B4C, MoSi2, and SiC have been utilized
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