金属碳化物(English).doc

Chapter Eight Introduction and Background on Transition Metal Carbides 8.1 Technological Uses of Transition Metal Carbides8.2 Structures of Transition Metal Carbides 8.3 Synthesis and Characterization8.4 Tour of the Group IV-VI Carbides 8.5 Goals of the Part II Project 8.6 References 8.1 Technological Uses of Transition Metal Carbides 8.1.1 Structural/refractory uses1,2 The carbides of the transition metals in Groups IV - VI have extremely high melting points (Table 8.1) and are therefore referred to collectively as the “refractory carbides.” In addition to their stability at high temperatures, these compounds are extremely hard (Table 8.2), finding industrial use in cutting tools and wear-resistant parts. Their hardness is retained to very high temperatures, and they have low chemical reactivity – they are attacked only by concentrated acid or base in the presence of oxidizing agents at room temperature, and retain good corrosion resistance to high temperatures. The refractory carbides are strong, with Young’s modulus values – a measure of elastic deformation resistance – rivaling those of SiC at room temperature. In addition, they have good thermal shock resistance and good thermal conductivity, permitting heat to be drawn away from the working surface of the tool. This gives them a benefit over other refractory materials, which do not conduct heat so well. (Table 8.3) Tungsten carbide, WC, is the most commonly used for fabrication as “cemented carbide” tools for cutting steel, in which the carbide is bonded in a metal matrix, usually cobalt. Cobalt is used because it wets the carbide particles and therefore behaves as a good binder without having significant ability to dissolve the carbide, so that the carbide is left pure in the bound form. However, pure WC-Co cemented carbides tend to weld locally with the steel being cut. TiC, TaC, and NbC are often used in conjunction with WC because TiC locally forms a layer of TiO2 or TiO23 which protects the tool fro