Phase carbonitride 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... 

UFPs of the Fe-N system can be synthesized from iron pentacarbonyl Fe(CO)s] and NH3 as reactants by a IOOO-W continuous wave C02 laser irradiation. The NH, gas is the absorbent of the laser beam in this case. At the lower synthesis temperature, below 650°C, UFPs of y -Fe4N with particle size of 10-25 nm grew dominantly. Above 1150°C, however, the growth of y-Fe UFPs with larger particle size of 30-100 nm was predominant (73). Iron carbonitride (lCN) UFPs were also synthesized from the ternary reactants of Fe(CO)s, NH3, and C2H i. The structure oflCN UFPs was hexagonal with e-Fe3(N,C) phase. A large saturation magnetization up to 142 emu/g was obtained and was ascribed to the carbon layer on 1CN UFPs (74). 

The first detailed studies of carbonitrides of iron were published by-Jack (17) in 1948. The nitrogen-rich carbonitride phases are isomorphous with y -, e-, and ("-nitrides, i.e., nitrogen has simply been replaced partially by carbon. The compositions of phases in the Fe-N-C system as given by Jack (17) are shown in Fig. 2. 

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. 

Table 11.3 Calculation equilibrium in the Fe-B-C-Mn-N-Al-O-Nb system at 1223 K, including the niobium carbonitride phase...

By controlling the carbon-to-metal ratio in the precursor mixed sols, a broad range of compositions can be prepared. Materials such as carbides, nitrides, solid solutions of carbonitrides, and multiple-phase systems with excess carbon were obtained. By using heavy metal oxide sols of very small micelle size and high surface reactivity, it was possible to produce some of the most refractory solids known at relatively low temperatures. Carbides, nitrides, and carbonitrides of U, Th, Zr, Hf, Y, the lanthanides, and so forth were prepared. 

The boundary between hardmetals and cermets is not strict because many of these compacts resemble microstructure features of both type of materials [106] faceted WC crystals together with round-shaped titanium carbonitride-based hard particles. Generally, these titaniiun carbonitride hardmetals are comparable with respect to properties and microstructure to WC-based hardmetals. The powders of these materials are liquid phase sintered with Ni or Ni-Co binder metal alloys. The core-and-rim structure of the hard phase usually exhibit a molybdenum- and carbon-rich (Ti,Mo)C rim and a titanium- and nitrogen-rich Ti(C,N) but can also be inverted (compare Fig. 26). The metallurgy of the phase reactions is (because of the complexity of the multicomponent system) not yet fully understood [69]. 

Figure 31. Microstructures of functional-gradient cemented carbonitrides based on the system (Ti, W)(C, N)-Co which are surface modified by applying a reactive gas phase during sintering in a one step fabrication process [146].

The phase equilibria in this system between 550°C and 600°C are of relevance to nitrocarburizing of steels. From the point of tribological performance, it is desirable to have a layer of carbonitride as the outermost... 

Zaj] Zajac, S., Jansson, B., Thermodynamics of the Fe-Nb-C-N System and the Solubility of Niobium Carbonitrides in Austenite , Metall. Mater. Trans. B, 29B, 163-176 (1998) (Calculation, Phase Relations, Thermodyn., 33)... 

Pop] Popov, V. V, Gorbachev, I.I., Analysis of Solubility of Carbides, Nitrides, and Carbonitrides in Steels Using Methods of Computer Thermodynamics II. Solubility of Carbides, Nitrides, and Carbonitrides in the Fe-V-C, Fe-V-N, and Fe-V-C-N Systems , Phys. Met. Metallogr (Engl. Transl), 99(3), 286-299 (2005) (Phase Diagram, Phase Relations, Thermodyn., Calculation, 40)... 

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