Carbonitride phases

Combustion of undiluted Ti or Ti + 0.5 C compacts in gaseous nitrogen at pressures up to 1.4 MPa resulted in incomplete conversion to the nitride or carbonitride. The product included TiN and solid solutions of nitrogen in titanium for the Ti samples and a single non-stoichiometric titanium carbonitride phase for the Ti + 0.5 C samples. 

The best results were obtained with compound 21 that exhibited high vapor pressure and low decomposition temperature (<523 K). Various CVD conditions were applied and gave in all cases shiny, dark-brown deposits.43 XRD and XPS analyses of the deposits indicated the presence of a vanadium carbonitride phase with little contamination from oxygen and free carbon. The films were less adherent on steel substrates than on silicon ones. The steel substrates seemed to suffer corrosion due to the presence of Cl-containing species. We had noticed the same feature in the case of Cl-containing precursors to vanadium carbide. Therefore, in order to increase the volatility of compound 23 and to reduce the Cl content of the molecule, we prepared compounds 24 and 25. Unfortunately, the yields obtained in their syntheses were much too low to permit TG and CVD experiments. 

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. 

Hagg carbide was the final phase up to 500°C., whereas cementite was formed at 700°C. Hagg carbide appeared when N had been decreased below 0.18, i.e., when about 64% of the nitrogen corresponding to the upper limit of the e-phase (Fe N) had been replaced by carbon. According to Jack s phase diagram (Fig. 2), the upper limit of the ("-carbonitride phase may be as high as C + N = 0.56. 

Hall, Dieter, Hofer, and Anderson (19) studied reactions of nitrides and carbonitrides in a reduced, fused iron catalyst (Bureau of Mines number D3001). The results of these experiments were in general similar to those of Jack, and most of the differences may be explained by the differences in the type of iron employed. The major discrepancy was that in the catalyst of large surface area and small crystallite size, the <-carbonitride phase was found under conditions under which massive iron is converted to the f-phase. Since the transformation of the - to the f-phase involves only slight changes in the lattice positions of iron atoms and small changes in the x-ray pattern, it is possible either that this transformation did not occur in the catalyst or that the pattern of the f-phase could not be distinguished from that of the e-phase in the diffuse diffraction patterns. 

Nitrided iron catalysts are remarkably stable toward oxidation and deposition of elemental carbon in the synthesis at both 7.8 and 21.4 atm. Shultz, Seligman, Lecky, and Anderson (23) described composition changes in nitrided fused, sintered, and precipitated catalysts in the synthesis. In a test (X218) with 1H2 + ICO synthesis gas at 7.8 atm., the catalyst was sampled frequently for analysis. X-ray diffraction patterns showed only the e-nitride or carbonitride phase, except for weak... 

Among the various compounds of carbon, the carbides of different elements have a special significance from the tribological point of view. Many of these materials, for instance WC, SiC, and TiC, are mechanically hard and have found use in the tribological industry. The carbonitride (or what can also be called the carbide of nitrogen) is an important constituent of this class of materials. In the last decade or so, the theoretical prediction of crystalline carbonitride phases that could be harder than diamond has generated much interest in the synthesis of this material. 

Table 11.3 presents the results of an equilibrium calculation relating to the precipitation of a number of phases, including the niobium carbonitride phases, from dilute solution in an austenitic steel at 1223 K. The calculation shows that at this temperature, the Nb (N, C) phase itself is stable, together with AIN, BN and MnS as other precipitated phases. By carrying out a series of such calculations for a number of temperatures for the given steel, the changes in the amounts of the precipitated phases can be determined, as well as the composition of the carbonitride phase. This is illustrated for the present steel in Fig. 11.1. 

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

A hquid-phase reaction in which TiCl is reacted with hquid ammonia at —35 C to form an adduct that is subsequendy calcined at 1000°C has also been proposed (35). Preparation of titanium nitride and titanium carbonitride by the pyrolysis of titanium-containing polymer precursors has also been reported (36). 

In essence two types of carbonitride are formed in a Ti,Nb-hardened micro-alloyed steel. At high temperatures a predominantly TiN-rich carbonitride is formed. However, on cooling to lower temperatures a predominantly NbC-iich carbonitride also precipitates. Both caibonitrides are based on the NaCl structure and form part of a continuum usually described by a formula such as (TixNb. xXCzNi.2). This can be expanded to include elements such as V and Ta, so the formula becomes (TazTiyNb Vi.,. z)(CzNi.z). The formation of two types of carbonitride can be consisted due to phase separation and Fig. 10.54 shows a projected miscibility... 

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). 

Carbides and nitrides can be prepared in many ways (chemical vapour deposition, physical vapour deposition, precipitation of salts containing metal, carbon and oxygen followed by reduction and annealing, reaction of a metal or its oxides with a gas or with solid carbon). Carbides and nitrides are often nonstoichiometric with complex phase diagrams.4-9 The compounds sometimes contain multiple phases and impurities, notably oxygen. This can lead to even more complex compounds, like oxycarbides, carbonitrides or oxycarbonitrides. 

Among the physical techniques, reactive sputtering is used most frequently because it produces films of high purity with relative ease and good reproducibility. In addition, many compound types can be prepared (carbides, nitrides, oxides, carbonitrides, oxicarbonitrides) including metastable phases. 

In amido derivatives used as precursors to nitrides, the presence (e.g. in V(NEt2)4) of alkyl groups that could self-eliminate to alkenes, was not sufficient to ensure nitrogen incorporation into the films, and addition of NH3 was necessary. More important was control of the molar fraction of V(NEt2)4 and NH3 in the gas phase, which allowed the formation of vanadium carbonitride films with variable and adjustable stoichiometry. 

Chemical and phase purity are not always desirable. For example, H- and N-doped silicon carbide films behave as high temperature semiconductors, while silicon carbonitride glasses offer properties akin to glassy carbon with room temperature conductivities of 103 2 cm-118. Additional reasons for targeting materials that are not chemically or phase pure stem from the desire to control microstructural properties. 

Chen, L.M., Lengauer, W., Ettmayer, P., Dreyer, K., Daub, H.W., Kassel, D., (2000a), Fundamentals of liquid phase sintering for modem cermets and functionally graded cemented carbonitrides (FGCC) , Int. J. Refract. Met. Hard Mat., 18(6), 307-322. 

Phases listed in order of decreasing intensity of diffraction pattern, e = c-phase (nitride or car bonitride). f f-phase (carbonitride), x M Hftgg carbide, a — a-iron, and M magnetite. 

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