Phase transition metal-nitrogen systems

Figure 6 Phase diagrams of group 4 transition metal-nitrogen systems, (a) Zr-N. (Ref. 11. Reproduced by permission of Materials Information Society/ASM International), (b) Hf-N. (Reprinted from Ref 28. 1991, with permission from Elsevier)...

Ternary phases with structures different from those of the phases of the binary boundary systems are more the exception than the rule. Such phases have been reported in the systems Nb-Mo-N, Ta-Mo-N, Nb-Ta-N, Zr-V-N, Nb-Cr-N, and Ta-Cr-N. Information about ternary transition metal-nitrogen systems is often available for specific temperatmes only. This is even more the case for quaternary nitride systems, which play a role in the production of carbonitride cermets where quaternary compounds of the types (Ti,Mo)(C,N) and (Ti,W)(C,N) are of interest (see Carbides Transition Metal Solid-state Chemistry), as well as in layer technology where titanium nitride-based coatings of the type Ti(C,B,N) are prepared by magnetron sputtering. Layers consisting of ternary compounds of the type (Ti,Al)N and (Ti,V)N also have favorable properties with respect to abrasion resistance. 

Transition Metal-Nitrogen Systems and Structure of Phases... 

The main trends of phase formation in transition metal-carbon or transition metal-nitrogen systems are well known (see Goldschmidt (1967)). For example, ina-va subgroup metals form stable carbides, which are characterised by good thermomechanical properties, but on going to... 

Carbon does not stabilize the hep low-temperature modifications of these metals (in contrast to nitrogen) and the solnbility of C in the a-phases is much smaller as compared to the group 4 transition metal nitride systems. 

The present study made use of diffusion couples where metals were reacted with carbon or nitrogen with the purpose of determining both phase equilibria and nonmetal diffusivities for Group 4 and 5 transition metal carbide and nitride systems. 

Because of the evident structmal similarities between transition metal carbides and transition metal nitrides the carbon atoms in group 4 and 5 carbides can be replaced completely by nitrogen without changing the structme of the binary phases. So far only one distinct ternary phase Cr2 (C,N)2 has been reported. Intersolubihty between the binary nitrides and carbides in the group 6 carbonitride systems Cr-C-N and Mo-C-N is not complete because of the differences in the crystal structmes of the carbide and nitride phases. 

The reaction of ternary carbides with nitrogen has attracted interest because of a novel type of phase separation occurring in double pseudobinary solid solutions of transition-metal multicomponent carbonitrides. Rudy has investigated the system Ti-Mo-C-N and has found that the homogeneous solid solution (Ti, Mo) (C, N), formed at high temperatures, disproportionates into two phases, both of which are isostructural and have nearly the same lattice parameters. One of the two phases is a Ti carbonitride rich in N and poor in Mo, the other is a Ti-Mo carbide rich in Mo hut poor in N. This type of disproportionating is bound to occur in most of the double-pseudobinary carbonitride systems... 

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