Azide self-propagating high-temperature synthesis of highly dispersed TiN–SiC ceramic nitride-carbide powder composites

Abstract

The study covers the method of azide self-propagating high-temperature synthesis (SHS-Az) to obtain a highly dispersed TiN–SiC ceramic composite with a theoretical ratio of nitride and carbide phases from 1 : 4 to 4 : 1 (in moles) using the combustion of the corresponding composition of powder reagent mixtures: NaN3 sodium azide, (NH4)2TiF6, (NH4)2SiF6 and Na2SiF6 halide salts, titanium, silicon and carbon in a nitrogen gas atmosphere. Thermodynamic calculations using the Thermo computer program showed that the optimum nitrogen pressure in the reactor is about 4 MPa, and the final composition of SHS-Az products can be completely different depending on the composition of reagents: it may include only target phases (TiN–SiC), contain silicon nitride and free carbon phases impurities (TiN–SiC–Si3N4–C) along with the target phases or consist only of nitride and free carbon phases (TiN–Si3N4–C). It was found that only target TiN and SiC phases are formed when using halide salt (NH4)2TiF6, at any ratio of nitride and carbide phases in the final powder composition. In cases where halide salts (NH4)2SiF6 and Na2SiF6 are used, target TiN and SiC phases are synthesized with an increased titanium content in reagents, i.e. only when composites of the 2TiN–SiC and 4TiN–SiC with an increased content nitride phase are obtained. Experimental studies of combustion products using scanning electron microscopy, energy dispersion analysis and X-ray phase analysis showed that they differ significantly from the theoretical compositions of products by the completely absent or significantly reduced SiC phase content in the final composition of powder composites synthesized during the combustion of bulk charge with carbon, and at the same time the absence of free carbon in the final composition of powder composites obtained. This difference is explained by the fact that when the combustion of a silicon and carbon powder mixture is initiated, silicon nitride is synthesized at the first stage with the temperature rising to high values of about over 1900 °C, at which the synthesized Si3N4 dissociates, and then at the second stage the resulting silicon reacts with carbon to form SiC that is more stable at high temperatures. But during combustion, very small light particles of carbon black (soot) may be removed (blown out) from a burning highly porous charge sample of bulk density by gases released at the first stage of combustion and not participate in the transformation of Si3N4 into SiC. In this regard, in case of low-carbon charge combustion, silicon carbide either does not form at all, or it is formed in small quantities compared to the theoretically possible amount, and Si3N4 silicon nitride remains the main component of the composite. A noticeable amount of SiC is formed only when burning high-carbon charges, but this amount is significantly less than the possible theoretical one, and the difference between them is replaced by the silicon nitride content. Therefore, it was experimentally shown for the first time that the SHS process can be used to obtain composites of highly dispersed ceramic powders TiN–Si3N4 and TiN–Si3N4–SiC consisting of a mixture of nanoscale (less than 100 nm) and submicron (100 to 500 nm) particles with a relatively low content of free silicon admixture (less than 1.4 %).