Synthesis of niobium carbonitride by self-propagating combustion of Nb-C system in nitrogen

An experimental investigation on the preparation of niobium carbonitrides by self-propagating high-temperature synthesis (SHS) was conducted using compacted samples of niobium and carbon powders ignited in gaseous nitrogen. Effects of the carbon content, nitrogen pressure, and NbN addition on the combustion behavior and the degree of conversion were studied. Due to the nature of reaction in the combustion front, steady propagation of the combustion wave is observed in the sample of Nb + 0.3C, whereas the flame front associated with the samples of Nb + 0.5C and Nb + 0.7C travels in an oscillatory manner. Moreover, the flame-front propagation velocity was found to decrease with an increase in the carbon content of the sample. For the samples of Nb + 0.3C and Nb + 0.5C, after the passage of the combustion front the reaction continues lengthily in an afterburning stage, resulting in the nitridation percentage as high as about 80% that is nearly unaffected by the variation of nitrogen pressure. However, due to the lack of afterburning combustion the degree of nitridation in the sample of Nb + 0.7C was decreased to between 53 and 67%. The XRD analysis of final products indicates that in addition to the dominant phase of Nb(C,N), there exist small amounts of Nb and carbon left unreacted.