Synthesis of niobium carbide at low temperature and its use in hardmetal F.F.P. Medeiros, A.G.P. da Silva * , C.P. de Souza Department of Chemical Engineering, Universidade Federal do Rio Grande do Norte, 59072-970, Natal, RN, Brazil Received 1 May 2001; received in revised form 1 January 2002; accepted 31 January 2002 Abstract A new method of synthesizing niobium monocarbide at a low temperature (950 jC) and in a short time (2 h) is described. Conventionally, niobium monocarbide (NbC) can only be produced at 1600 – 1800 jC for longer periods. The method consists of the carburization of a niobium complex by a gaseous (H 2 + CH 4 ) mixture. The method of preparation of the precursor and its carburization are presented. Precursor and carbide are characterized by XRD, SEM, TG/DTG and granulometry. Shape, mean size and size distribution of the NbC particles are similar to those of the niobium precursor. Reactions during carburization do not alter these characteristics. NbC particles are very large and porous. NbC powder is used in a hardmetal alloy as a WC grain growth inhibitor. Its performance is compared to that of a commercial NbC powder. NbC synthesized by the new technique exhibited a slightly higher efficiency in inhibiting the WC coarsening, evidenced by the SEM of the hardmetal structures and hardness measurements. D 2002 Published by Elsevier Science B.V. Keywords: Chemical synthesis; Carbides; Sintering; Grain growth 1. Introduction Carbides of refractory metals have properties such as high hardness, high melting or decomposition temperature and good thermal conductivity. In addition, some of them exhibit catalytic activity. Hardmetals and cermets are composite materials that contain WC, TiC, VC, MoC, TaC and niobium monocarbide (NbC) in their compositions. These materials are used in cutting tools, drills, abrasives and wear-resistant pieces. These carbides are industrially produced by heating either a mixture of pure metal with carbon black or a mixture of the metal oxide with carbon black. In both cases, carburiza- tion reaction proceeds via solid state diffusion. This mech- anism requires a high temperature to shorten the reaction time. Niobium carbide is industrially produced from the mixture of Nb 2 O 5 with carbon black. The reaction temper- ature lies in the range of 1600–1800 jC. This high temper- ature is also necessary to eliminate oxygen from the produced NbC. New synthesis methods have been developed that both lower the temperature and time of reaction. Furthermore, very fine powders, which are suitable for catalysis, can be obtained. The new methods are based on varying the reactants and the kind of carburization reaction. Li et al. [1] produced nearly stoichiometric NbC whiskers by heating a mixture of Nb 2 O 5 with carbon black at 1120 jC for 30 min. The carburization reaction proceeds via a vapor transport mech- anism. This explains the whisker crystals obtained. Dal et al. [2] produced NbC by heating Nb–C-containing composite material at 1000 jC for 4 h under argon. The composite contains the layered oxides (C 8 H 17 NH 3 )Nb 3 O 8 and (C 6 H 13 NH 3 )Nb 3 O 8 and the polymer polyacrylonitrile (PAN). How- ever, a large amount of amorphous carbon is found (around 50 wt.% C) in the product. This is unsuitable for using in cemented carbides. Moreover, a nearly stoichiometric NbC x (x = 0.95) is achieved only after a 48-h heating. Kim et al. [3] produced niobium carbide and niobium carbonitride by reacting commercial Nb 2 O 5 with a mixture of CH 4 and H 2 . Very fine, nearly stoichiometric NbC with a low amount of free carbon contamination (0.5 wt.%) was obtained at 850 jC for 8 h. The same reactants were used by Teixeira da Silva et al. [4] but nearly stoichiometric NbC was obtained under different conditions (higher tem- perature—1100 jC, and shorter reaction time—42 min). In contrast to the powder produced by Kim et al., Teixeira da Silva’s NbC had a low specific surface area and a high carbon contamination. The particles were coarse and porous and have the same shape as the starting Nb 2 O 5 particles. 0032-5910/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII:S0032-5910(02)00042-6 * Corresponding author. E-mail address: carlson@ufrnet.ufrn.br (A.G.P. da Silva). www.elsevier.com/locate/powtec Powder Technology 126 (2002) 155– 160