Self-propagating high-temperature synthesis and spark plasma sintering of high-entropy (Hf,Ta,Nb)(C,N) carbonitride

Abstract

In this research, we combined mechanical activation (MA), self-propagating high-temperature synthesis (SHS), and spark plasma sintering (SPS) methods to obtain a dense high-entropy (Hf,Ta,Nb)(C,N) carbonitride and studied its properties. To implement the SHS process, a mixture of initial metals and carbon was subjected to pre-treatment in a planetary mill in the low-energy mode, in which the jar rotation speed reached 350 rpm. We studied the evolution of microstructure and phase composition during the MA process. It has been established that after 60 min of treatment, Hf/Ta/Nb/C layered composite particles consisting of Hf, Ta, Nb and C submicron layers, with an average size of about 15 μm, were formed. However, according to the X-ray diffraction analysis, the components in the jar did not interact. SHS of Hf/Ta/Nb/C reactive mixtures was performed in a nitrogen atmosphere (P = 0.8 MPa); after synthesis, two isomorphic (Hf,Ta,Nb)(C,N) phases of the Fm-3m (225) space group with lattice parameters of a = 0.4476 nm (71 wt. %) and a = 0.4469 nm (22 wt. %) were revealed in the powder. After SHS, the average size of agglomerates was 10 μm and their morphology resembled that of composite particles after MA. The agglomerates formed during SHS consisted of pores and round-shaped particles ranging in size from 0.5 to 2 μm, which was caused by the melting of metal components in the combustion zone and rapid crystallization of product grains from the melt, followed by subsequent recrystallization. Spark plasma sintering at a temperature of 2000 °C, a pressure of 50 MPa and a holding time of 20 min enabled to obtain a single-phase high-entropy (Hf0.33Ta0.33Nb0.33 )C0.5N0.3 material with a lattice parameter of 0.4482 nm characterized by a high relative density of 98 %, a hardness of 21.5 ± 0.4 GPa, a Young’s modulus of 458 ± 10 GPa, and a fracture toughness value of 3.7 ± 0.3 MPa∙m1/2.