Hindawi Publishing Corporation Advances in Materials Science and Engineering Volume 2010, Article ID 835018, 11 pages doi:10.1155/2010/835018 Research Article Sintering Behavior, Microstructure, and Mechanical Properties: A Comparison among Pressureless Sintered Ultra-Refractory Carbides Laura Silvestroni and Diletta Sciti CNR-ISTEC, Institute of Science and Technology for Ceramics, Via Granarolo 64, I-48018 Faenza, Italy Correspondence should be addressed to Laura Silvestroni, laura.silvestroni@istec.cnr.it Received 12 July 2010; Accepted 10 October 2010 Academic Editor: Paul Munroe Copyright © 2010 L. Silvestroni and D. Sciti. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Nearly fully dense carbides of zirconium, hafnium, and tantalum were obtained by pressureless sintering at 1950 ◦ C with the addition of 5–20 vol% of MoSi 2 . Increasing the amount of sintering aid, the final density increased too, thanks to the formation of small amounts of liquid phase constituted by M-Mo-Si-O-C, where M is either Zr, Hf, or Ta. The matrices of the composites obtained with the standard procedure showed faceted squared grains; when an ultrasonication step was introduced in the powder treatment, the grains were more rounded and no exaggerated grains growth occurred. Other secondary phases observed in the microstructure were SiC and mixed silicides of the transition metals. Among the three carbides prepared by pressurless sintering, TaC-based composites had the highest mechanical properties at room temperature (strength 590 MPa, Young’s modulus 480 GPa, toughness 3.8 MPa · m 1/2 ). HfC-based materials showed the highest sinterability (in terms of final density versus amount of sintering aid) and the highest high-temperature strength (300 MPa at 1500 ◦ C). 1. Introduction The carbides of the group IV–VI transition metals have extremely high melting points (3000–4000 ◦ C) and are commonly referred to as “refractory carbides”. Beside their stability at high temperatures, these compounds possess extremely high hardness, thus finding industrial use in cutting tools and wear-resistant parts [1–3]. They also have good corrosion resistance, as they are attacked only by concentrated acid or base in the presence of oxidizing agents, and retain good corrosion resistance to high temperatures. The refractory carbides are stiff, with Young’s modulus values competing with those of SiC. In addition, they have good thermal conductivity, permitting heat to be drawn away from the superheated surfaces. This gives them a benefit over other refractory materials, such as AlN, SiC, and Si 3 N 4 , which do not conduct heat so well [1–3]. For high-temperature applications, they outperform the “superalloys” in such applications as rocket nozzles and jet engine parts, where ablation resistance at temperatures of 2500 ◦ C and above is crucial. The electrical resistivity of the carbides is only slightly higher than that of the host metals, reflecting the metallic behaviour of these compounds and their strong metal-to-metal bond [1–3]. One drawback of these carbides is their poor oxidation resistance, as reported in the literature [1–4]. Among this class of materials, Zirconium carbide has found industrial importance as coating for atomic-fuel particle for nuclear-fission power plants, owing to its low activation under neutron irradiation [1–3]. Hafnium carbide is, with tantalum carbide, the most refractory compound available [1–3]. Hafnium carbide is considered a candidate material for high-temperature solar absorbers, because of its melting point above 3300 ◦ C and its intrinsic spectral selectivity [1–3]. Hafnium and Zirconium carbides can also be considered for thermoionic/thermoelectric converters at high temperature, by proper tuning of the grain boundary phases or carrier concentration and mobility. Tantalum carbide is produced industrially in appreciable quantity with a world production estimated at 500 tons annually (1994)