Tungsten nitrides properties

In devices with geometry below the 0.25 im level, titanium nitride will likely replace titanium-tungsten, since it has better barrier properties. The process of choice will likely be CVD, which offers improved uniformity and step coverage, although improvements in collimated sputtering may keep that process on line for a while. 

The carbides and nitrides of the early transition metals have attracted considerable attention. Much of this can be attributed to the catalytic properties of these materials. Levy and Boudart1 were the first to note the Pt-like catalytic behavior of WC, which for many reactions is more catalytically active than metallic tungsten.2 Others have noted that the early transition metal carbides and nitrides, in general, are excellent catalysts for reactions characteristic of the more expensive noble metals.3,4 For example, WC has been found to catalyze the isomerization of neopentane to isopentane,1 a reaction that previously had been known to be catalyzed only by iridium and platinum.5... 

The possibility of the incorporation of oxygen into the particle is particularly relevant for the carbides and nitrides of molybdenum and tungsten which possess a high affinity for this element.13,23 The oxygen may come from the carbonyl precursor, and result in oxycarbide or oxynitride formation in the core of the nanoparticle itself.16 Exposure to the ambient can also result in the formation of surface oxycarbides and oxynitrides with catalytic properties different from those of the pure nitride or carbide phase.15,24-26 However, heat treatment of these nanoparticles with a mixture of methane/hydrogen or ammonia/hydrogen should convert the surface to a pure nitride or carbide form. 

The crystal structures adopted by the binary carbides and nitrides are similar to those found in noble metals. The resemblance is not coincidental, and has been explained using Engel-Brewer valence bond theory [5]. Briefly, the main group elements C and N increase the metal s effective s-p electron count, so that structures and chemical properties of the early transition metals resemble those of the Group 8 metals. This idea was first introduced by Levy and Boudart [6] who noted that tungsten carbide had platinum-like properties. 

Tungsten carbide — WC, belongs to a class of Group IV B-VIB transition metal carbides and nitrides, often referred to as interstitial alloys, in which the carbon and nitrogen atoms occupy the interstitial lattice positions of the metal [i]. These compounds possess properties known from group VIII B precious metals like platinum and palladium [ii]. Thus, they show remarkable catalytic activities, attributed to a distinct electronic structure induced by the presence of carbon or nitrogen in the metal lattice. Tungsten carbide resembles platinum in its electrocatalytic oxidation activity (- electrocatalysis) and is therefore often considered as an inexpensive anode electrocatalyst for fuel cell [iii] and -> biofuel cell [iv] application. 

The metal nitrides of the transition metals of the subgroups of the IVth, Vth and Vlth groups are very similar to the corresponding carbides in their. structures (interstitial compounds of the MN type) and their properties such as hardness, melting point and electrical conductivity (see Table 5.6-3). Tungsten and molybdenum nitrides are exceptions with different structures and decompose with nitrogen loss above 800°C. 

Another material of this kind is tungsten disulphide, originally developed by NASA for aerospace applications, now also applicable to specialty industries [58]. Ti3SiC2 is a thermodynamically stable, nano-layered, ternary carbide and part of a family of over 50 ternary carbides and nitrides, the MAX phases [62]. These phases are a new class of solids possessing unique combinations of properties they are readily machinable, relatively soft for ceramics, but elastically stiff, and electrically and thermally conductive. They combine the good properties of both metals and ceramics that could lead to this technology contributing to future lubricant developments. 

Until recently, there were no tool materials that would stand up to the high stresses and temperatures necessary for FSW of materials with higher melting points, such as steels, stainless steels, and nickel-base alloys. In 1998, tungsten alloys and polycrystalline cubic boron nitride (PCBN) were developed to create FSW tools for use in steel, stainless steel, titanium alloys, and nickel-base alloys. Properties of the resultant welds have been shown to be outstanding. Although some issues remain (primarily limited tool life with tungsten-base tools), FSW has been demonstrated as a technically and eco-... 

Finally, two other characteristics of hard materials are important their density and their chemical stability. The importance of these properties depends on the application. Oxides are chemically more stable than nitrides, which are in turn more stable than borides or carbides. The susceptibility to oxidation of boron carbide prevents its application at high temperatures, but for ballistic protection, where hardness, rigidity and low density take precedence, boron carbide is unparalleled. Tungsten carbide, on the other hand, is the material of first choice for cutting tools, because of its high hardness and stiffness, and high-temperature oxidation resistance, but is far too dense for application in which weight or inertial forces are important. Its use for ballistic protection is therefore out of the question. 

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