TRANSITION-METAL CARBIDES AND NITRIDES

Transcription

1 TRANSITION-METAL CARBIDES AND NITRIDES 3.1 INTRODUCTORY REMARKS Titanium carbide (TiC) and zirconium carbide (ZrC) are extremely hard refractory metal compounds. Hafnium carbide (HfC) with a melting point over 3890 C is a refractory binary compound. Vanadium carbide (VC) and niobium carbide (NbC) are also extremely hard refractory materials. VC can be used as an additive to tungsten carbide (WC) to refine the carbide crystals and to improve the property of the cermet. TiC, ZrC, VC and NbC are commercially used in tool bits for cutting tools. ZrC is used extensively as coating of UO 2 and ThO 2 particles of nuclear fuel. HfC and NbC can also be used as refractory coatings in nuclear reactors. The mixed carbide Ta 4 HfC 5 possesses the highest melting point of any currently known compound at 4215 C. Titanium nitride (TiN) is an extremely hard ceramic material, often used as a coating on Ti alloys, steel, carbide and Al components to improve the substrate s surface properties. Applied as a thin coating (less than 5 µm), TiN is used to harden and protect cutting and sliding surfaces, for decorative purposes due to its gold appearance and as a non-toxic exterior for medical implants. Zirconium nitride (ZrN) is a hard ceramic material similar to TiN. It is commonly used for coating medical devices, industrial parts (e.g., notably drill bits), automotive and aerospace components and other parts subject to high wear and corrosive environments. ZrN is light gold in color, similar to elemental gold. Hafnium nitride (HfN) is one of the refractory metallic compounds having a melting point of 3310 C. Vanadium nitride (VN) and TiN exhibit a relatively high critical temperature in superconducting 509

2 510 Transition-Metal Carbides and Nitrides phenomena. Niobium nitride (NbN) thin films were noticed at an early stage because of their critical transition temperatures as low as K which could find wide applications in superconducting microelectronics. Because of its complicated phase diagram, NbN is difficult to prepare in the pure face-centered cubic, rocksalt structure. Hard-coating materials range from ultra-hard materials such as diamond-like carbon through the refractory compounds to alloys. However, the transition-metal carbides and nitrides have achieved by far the highest level of commercial success. Perhaps, the most important property of this group of carbides and nitrides is their defect structure. Ideal stoichiometry is generally not found in these phases; deviations from the stoichiometry are found to be far more common. The transition-metal carbides and nitrides are typically metallic in their electrical, magnetic and optical properties. Table 3.1 summarizes some physical properties of transition-metal carbides and nitrides. Table 3.1 Crystal structure (space group), melting point T m, crystal density g and color of some transition-metal carbides and nitrides. rs = rocksalt. Compound Crystal structure T m ( C) g (g/cm 3 ) Color TiC rs gray ZrC rs gray HfC rs > gray TiN rs golden-yellow ZrN rs golden HfN rs golden-green VC rs gray NbC rs lavender VN rs gray-red-brown NbN rs yellow-silver 3.2 IVA-METAL CARBIDES Titanium Carbide (TiC) Table 3.2 Optical constants of TiC at 300 K [1,2]. ev ev ε 1 ε 2 n k α (cm (cm 1 ) R E E E E E E E E E E

3 3.2 IVa-Metal Carbides E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E

4 512 Transition-Metal Carbides and Nitrides E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-06

5 3.2 IVa-Metal Carbides E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-08

6 514 Transition-Metal Carbides and Nitrides E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-11

7 3.2 IVa-Metal Carbides E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-11 Figure 3.1 ε 1 (E) and ε 2 (E) spectra of TiC at 300 K. ε ε 2 ε 1 TiC ε E (ev) E (ev) Figure 3.2 n(e) and k(e) spectra of TiC at 300 K. n, k n TiC k E (ev) References [1] T. Koide, T. Shidara, H. Fukutani, A. Fujimori, S. Otani, and Y. Ishizawa, Jpn. J. Appl. Phys. 32, 1130 (1993). [2] B. L. Henke, E. M. Gullikson, and J. C. Davis, constants.