Epitaxial growth of tantalum carbides by low carbon flow carburizing

The carburizing of tantalum samples under different carbon flow rates has highlighted the influence of the carbon flow rate on the structure of tantalum carbides. XRD analyses enabled the identification of surface structures that were revealed by optical micrographs. Four carbon flow rates were tested. The lowest carbon flow rate produced a Ta2C layer that grew epitaxially on the tantalum substrate according to the relationship: {101¯}Ta// {1¯101}Ta2C; 〈101¯〉Ta// 〈17¯17010〉Ta2C. An increase in the carbon flow rate resulted in the appearance of TaC nuclei through the carbon enrichment of the Ta2C layer. These nuclei appeared in the form of islets and highly oriented needles. EBSD (Electron Back Scattered Diffraction) analyses and pole figures revealed that the carbide layers grew epitaxially towards their substrates. The Ta2C layer grew according to the relationship {101¯}Ta// {1¯101}Ta2C while the TaC structures nucleated according to the following relationships: {111}TaC//{0001}Ta2C; 〈111〉TaC//〈0001〉Ta2C; {100}TaC// {202¯3}Ta2C; 〈100〉TaC// 〈909¯8〉Ta2C and {110}TaC// {101¯3}Ta2C; 〈110〉TaC// 〈909¯16〉Ta2C. The highest carbon flow produced stoichiometric TaC grains at the surface, which appeared to be equiaxed. The diversity of the TaC crystallographic orientations stems from the underlying Ta2C layer, the TaC grains having nucleated epitaxially from the Ta2C layer. This misorientation of the Ta2C grains with respect to Ta is governed by the exceeding of a carbon flow limit.