Nitriding phase transformations

Riter J R Jr 1973 Shock-induced graphite.far.wurtzite phase transformation in boron nitride and implications for stacking graphitic boron nitride J. Chem. Phys. 59 1538... 

Ca3(BN2)2 is readily formed when (distilled) calcium metal is melted in the presence of (layer-type) boron nitride. This reaction provides some insight on how alkaline-earth metals like calcium may act as a catalyst in the phase transformation of layered a-BN into its cubic modification. Instead of metals, nowadays alkaline-earth (Ca, Sr, Ba) nitridoborates can be used as a flux catalyst in high-pressure and high-temperature transformation reactions to produce cubic boron nitride [15]. 

The c-BN phase was first obtained in 1957 [525] by exposing hexagonal boron nitride phase (h-BN) to high pressures and low temperatures. A pressure of more than 11 GPa is necessary to induce the hexagonal to cubic transformation, and these experimental conditions prevent any practical application for industrial purposes. Subsequently, it has been found that the transition pressure can be reduced to approximately 5 GPa at very high temperature (1300-1800°C) by using catalysts such as alkali metals, alkali metal nitrides, and Fe-Al or Ag-Cd alloys [526-528]. In addition, water, urea, and boric acid have been successfully used for synthesis of cubic boron nitride from hexagonal phase at 5-6 GPa and temperature above 800-1000°C [529]. It has been... 

Nevertheless, the microstructure cannot be directly correlated to the initial /1-content in the starting powder. The experimentally determined particle density in sintered samples indicates that only a part of the initial /I particles are able to grow [283]. The number of growing /1-particles depends on a critical particle diameter dcrit (Fig. 18). Particles below dcrit will dissolve in the oxide nitride liquid during phase transformation and reprecipitate on the overcritical /1-particles according to an anisotropic Ostwald ripening process [284, 292, 293]. 

Turan, S. and Knowles, K.M., (1996a), a — (3 reverse phase transformation in silicon carbide in silicon nitride-particulate-reinforced-silicon carbide composites ,, /. Am. Ceram. Soc., 79 (11), 2892-2896. 

P. F. Becher, G. S. Painter, N. Shibata, S. B. Waters, H-T. Lin, Effects of rare-earth (RE) intergranular adsorption on the phase transformation, microstructure evolution, and mechanical properties in silicon nitride with RE203 4- MgO additives RE=La, Gd, and Lu, 7. Amer. Ceram. Soc., 91 [7], 2328-2336, (2008). 

Both metals have sufficiently high energy of formation of the nitride Gibbs phase however, only one nitride phase is easily transformed into the amorphous one under the radiation, for example, nc-MeN/a-Si3N4 (Me=Ti, W, V) [33,34,36,39-41]. 

A. V. Kurdyumov and A. N. Pilyankevich, Phase Transformations in Carbon and Boron Nitride, Naukova Dumka, Kiev, 1979. 

The most important commercial silicon nitride powders have a-Si3N4, i.e. the phase which is metastable at high temperatures, as the main constituent [1]. The consequence of this is that during densiflcation, a phase transformation takes place, having a great influence on the subsequent formation of the microstructure. 

H. Mandal, D. P. Thompson and K. H. Jack, a -> Phase Transformations in Silicon Nitride and Sialons , preprint. 

Bowen LJ, Weston RJ, Carmthers TG, Brook RJ (1978) Hot-pressing and alpha-beta phase-transformation in sUicon nitride. J Mater Sci 13 341-350... 

FIGURE 7. Relative density and weight percentage of a to J-Si3N4 phase transformation of as-sintered samples, as a function of sintering temperature (with a constant sintering time for 30 min, relative density and p sihcon nitride content in green body are -55% and -5% respectively)... 

A low cost-high performance silicon nitride-BAS composite can be fabricated via pressureless sintering. In this composite, the BAS glass-ceramic serves as an effective liquid-phase-sintering aid, to attain full densification and complete the a-(3 Si,iN4 phase transformation, and remains as a structural matrix that is reinforced by the silicon nitride whiskers, Si3N4 whiskers grow in random directions in a near completely crystallized matrix of hex-acelsian BAS. 

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