Chemical conversion during transient liquid-phase hot pressing of TaSi2–TaC–SiC SHS-powder

Abstract

This article presents a study on the manufacturing of three-phase TaSi2–TaC–SiC ceramics through self-propagating high-temperature synthesis (SHS) and their subsequent chemical conversion to TaC–SiC carbide composites during transient liquid-phase hot pressing (HP). The effect of carbon black doping, ranging from 0% to 7%, on the degree of chemical conversion, structure, mechanical, and thermophysical properties of the ceramics was investigated. Our results showed that the proportionate increase of carbide content and decrease of TaSi2 content in hot-pressed samples was achieved through carbon black doping. The increase of TaSi2 content during hot pressing led to an increase in porosity from 4.3% to 23.8%, while the density decreased from 6.3 to 4.6 g/cm3. Superior mechanical properties were obtained when SHS-powder was doped with 1.5% carbon black (HV = 15.2 GPa, KIC = 4.8 MPa × m1/2, and σbend = 331 MPa). The structure of the ceramics was characterized by a TaSi2–SiC matrix and highly dispersed TaC grains predominantly residing inside TaSi2, with the TaC–TaSi2 and TaSi2–SiC interface being incoherent, as demonstrated through TEM studies. Complete conversion of TaSi2 to TaC and SiC was achieved through 7% carbon black doping, resulting in the hot-pressed sample consisting solely of carbide grains. Two-stage hot pressing was employed to enhance the relative density of the two-phase TaC–SiC sample, resulting in ceramics characterized by HV up to 22.3 GPa, KIC up to 6.1 MPa × m1/2, σbend up to 256 MPa, and λ up to 36 W/(m × K).