A Common Regularity of Stoichiometry-Induced Morphology Evolution of Transition Metal Carbides, Nitrides, and Diborides during Self-Propagating High-Temperature Synthesis

The morphologies of the transition metal carbide (TMC) (ZrC x , NbC x , and TaC x ), transition metal nitride (TMN) (TiN x ), and transition metal diboride (TMD) (NbB2x and TaB2x ) particles formed during the self-propagating high-temperature synthesis (SHS) were investigated. The results indicate that the ceramics with wide stoichiometric ranges all show a stoichiometry-induced morphology evolution, i.e., octahedron → truncated-octahedron → spherelike → sphere, for TMCs and TMNs, and hexagonal prism → polyhedron → spherelike, for TMDs. For TMCs and TMNs, the increase in the stoichiometry leads to the increase in the growth rate in the ⟨111⟩ crystalline direction. Hence, their morphologies show an evolution process of gradual exposure of the {100} surfaces and shrinkage of the {111} surfaces. When the exposed {100} surfaces are roughed because of the extremely high combustion temperatures during the SHS and thus turn round, the growth shapes of the TMC and TMN crystals change to spherelike. On the other hand, when the TMCs and TMNs are stoichiometric or near stoichiometric, the critical transition temperature for thermodynamic roughening of the {100} surfaces could be very high. Then, the rounded {100} will restore to the flat surfaces, and the cubic and truncated-cubic TMCs and TMNs particles are formed. For TMDs, the morphology evolution could be caused by the decrease in the stability of the {0001} and {101̅0} surfaces at high stoichiometries. With the increase in the stoichiometry, these two surfaces are less-exposed gradually while the {11̅01} surfaces are exposed and expand. The growth shapes of TMDs change from regular hexagonal prism to polyhedron. With the rounding (roughening) transition of the {11̅01} surfaces at high temperatures, the TMDs particles become spherical.