Crystal carbonitrides

An extensive compilation of the crystal structures of transition metal carbides is found in Pearson s Handbook. Ward has discussed the structures of carbides extensively. Epicier and de Novion have suimnarized the results of investigations on ordering in transition metal carbides. Lengauer recently reviewed the knowledge on transition metal carbides and carbonitrides. For a comparison of close-packed transition metal carbides with close-packed transition nitrides, see Nitrides Transition Metal Solid-state Chem istry. 

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. 

However, in many cases the pyrolysis products are amorphous. For instance, the polysilazane-derived silicon carbonitride mentioned above crystallizes only when it is heated above 1450 C, and its characterization is difficult. Elemental analysis poses problems, in part because the pyrolysis product is very porous. As a result of its high surface area, the material adsorbs moisture and volatiles very readily, and improper prior handling and preparation for analysis can result in misleading results. 

All precursors are amorphous up to calcination temperatures of around 600°C. At higher temperatures, in most cases powders with extremely small crystallite sizes of around 20-40 nm are formed (Fig. 7). A further increase in calcination temperature promotes crystal growth. With aluminum nitride, a white powder with a low oxygen and carbon content is obtained [97]. Other main group element precursors exhibit fairly different behaviors Mg and Ca precursors yield metal cyanamide [99]. Calcination of the transition element precursors (Fig. 8) results in the formation of nitrides, carbonitrides, or carbides. For the titanium-containing precursors, TiN/TiC solid solutions can be obtained [96] the quantity of carbon strongly depends on the calcination atmosphere applied (argon, 31 wt% ammonia, 5.1 wt%). 

Initial explanations of the extraordinary high-temperature stability of precursor-derived Si-B-C-N ceramics were presented previously by Jalowiecki et al. The authors investigated the microstmcture of boron-doped silicon carbonitride composites by HR-TEM and fotmd turbostratic BN(C) segregation occuring along grain boundaries of nano-sized silicon carbide and silicon nitride crystals. We suppose... 

Amineboranes were thermally decomposed at 1273 K in Ar to boron carbonitrides [262]. These materials were reported to show a turbostratic structure where atoms in a carbon-like structure are partially substituted by boron and nitrogen, respectively. At temperatures higher than 2073 K (Ar-atmosphere) the single phase material of compositions BC4N and BC2N, respectively, started to crystallize and a-BN, B4C and elemental C (graphite) were observed. 

For the carbonitrides, the stoichiometry of the amorphous product is Si3+vN4C + [167] and thus in terms of possible crystallization products there is always C in addition to Si3N4 and SiC. In precursor-derived Si-C-N-ceramics,... 

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]. 

Because fonnation of cubic boron carbonitride is of great fundamental interest with respect to superhard materials much additional effort is needed to succeed in the preferential synthesis of the cubic B-C-N phase. As follows from the above results the most promising way would be synthesis under nonequilibrium conditions such as flash-heating at static pressures or shock-wave compression [140]. Successful synthesis of cubic BC2.5N solid solution in 18% yield by shock-compression of hexagonal BC2.5N has been reported [143]. The material obtained was a single cubic BC2.5N phase with a diamond-like structure and crystals between 5 and 20 nm in size. 

Non-oxide preceramic polymers which are expected to yield, under convenient thermal and chemical conditions, boron-containing amorphous or crystallized ceramics including boron nitride (BN), boron carbide (B C), boron carbonitride (B-C-N), and boron silicon carbonitride Si-B-C-N. 

The powder of silicon nitride of the SHS-Az type can contain in the structure from 40% up to 95% (a-nitride phase as anisotropic whiskers by a diameter 1 pm forming wave structure (Figure 8.2)). The particles of the boron nitride powder of the SHS-Az type are ultra fine and have the disc form, and the diameter of disks at 10-15 of time exceeds their thickness having the linear size about 20 nm. The crystal lattice BN has deformations of turbostrate kind with a degree of three-dimensional order 0.40-0.50. The powder of titanium carbonitride with composition TiCggNog of the SHS-Az type differs from similar powders of traditional technologies of synthesis of more branched structure, typical for formation of... 

The data available on the structure of the electronic states in more complicated systems are extremely limited. The MO LCAO calculations for the clusters [MoN Xi Mi2] (n = 0, 1,..., 5 M = Ti, V X = C, O) were performed by Ivanovsky (1988). Such clusters serve as models for Ti and V carbonitrides and oxynitrides, when the solid solution contains vacancies (Chapter 4). On the emergence of vacancies the occupied part of the metal subband broadens while the 2s- and 2p-nonmetal subbands (whose atoms are removed from the crystal) become narrower. The localised bonds between the metal and second nonmetal component (X) change in this case also. For example, the Ti-C bond population in the [TiN5CTii2] cluster is 16% less, than its counterpart in the [TiN4DCTii2] cluster, corresponding to nonstoichiometric carbonitride. 

Titanium carbonitride that may be found in 4340 steel or 18 Ni maraging, are so brittle that break under very low deformation of the matrix crystals in which they are contained. Once broken or debonded, an inclusion nucleates a micro crack. Manganese-sulhde inclusions, MnS, are quite common in low-medium strength carbon steel, but they are less brittle. They may be long and sharp representing points of high stress concentration that may also activate a slip band. Figure 3.20... 

The decomposition and crystallization of silicon carbonitride-h tA ceramics was associated with the chemical composition, architecture and chemical homogeneity of the amorphous SiCN network. (Colombo, 2010a) Polysilazane derived SiCN ceramics generally lay within the compositional range of the ternary phase... 

Silicon carbonitride ceramics were also studied with respect to their creep behavior at high temperatures (An, 1998 Shah, 2001 Zimmerman, 2002). Si-C-N samples show similar creep behavior and shear viscosity values with those determined for Si-O-C materials. Also in the case of SiCN, a creep hardening behavior was observed and was claimed to rely not necessarily on crystallization processes (as SiCN exhibits an improved crystallization resistance at high temperatures if compared to SiOC), but to a nanoscale densification creep mechanism (Shah, 2001). 

Polymer derived ceramics have been known for the last four decades and are prepared via solid-state thermolysis of preceramic polymers. They exhibit a unique combination of remarkable properties due to their covalent bonding and amorphous nature. Thus, silicon oxycarbide (SiOC) and silicon carbonitride (SiCN) based ternary PDCs have been shown to possess outstanding high-temperature properties such as stability with respect to crystallization and decomposition, oxidation and corrosion resistance as well as excellent thermomechanical properties (e.g., near zero steady state creep resistance up to temperatures far beyond 1000 °C). Their properties are directly influenced by the chemistry and the architecture of the preceramic precursors, thus there is an enormous potential in tuning the microstructure and properties of the PDCs by using tailored polymers. Furthermore, suitable chemical modification of the preceramic precursors leads to quaternary and multinary ceramics, as it has been shown for instance for silicon boron carbonitride ceramics in the last 25 years, which in some cases exhibit improved properties as compared to those of the ternary materials. 

Iwamoto, Y, Volger, W., Kroke, E., Riedel, R., Saitou, T., Matsunaga, K. (2001). Crystallization behavior of amorphous silicon carbonitride ceramics derived from organometallic precursors. Journal of the American Ceramic Society, 54(10), 2170-2178. doi 10.1111/j.l 151-2916.2001.tb00983.x. 

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