The Structure, Morphology, and Mechanical Properties of Ta-Hf-C Coatings Deposited by Pulsed Direct Current Reactive Magnetron Sputtering

Ta, Hf, TaCx, HfCx, and TaxHf1-xCy coatings were deposited by reactive pulsed Direct Current (DC) magnetron sputtering of Ta or Hf pure metallic targets in Ar plus CH4 gas mixtures. The properties have been investigated as a function of the carbon content, which is tuned via the CH4 flow rate. The discharge was characterized by means of Optical Emission Spectroscopy and, in our conditions, both Ta-C and Hf-C systems seem to be weakly reactive. The structure of the as-deposited pure tantalum film is metastable tetragonal beta-Ta. The fcc-MeCx carbide phases (Me = Ta or Hf) are {111} textured at low carbon concentrations and then lose their preferred orientation for higher carbon concentrations. Transmission Electron Microscopy (TEM) analysis has highlighted the presence of an amorphous phase at higher carbon concentrations. When the carbon content increases, the coating's morphology is first compact-columnar and becomes glassy because of the nano-sized grains and then returns to an open columnar morphology for the higher carbon concentrations. The hardness and Young's modulus of TaCx coatings reach 36 and 405 GPa, respectively. For HfCx coatings, these values are 29 and 318 GPa. The MeCx coating residual stresses increase with the addition of carbon (from one-hundredth of 1 MPa to 1.5 GPa approximately). Nevertheless, the columnar morphology at a high carbon content allows the residual stresses to decrease. Concerning TaxHf1-xCy coatings, the structure and the microstructure analyses have revealed the creation of a nanostructured coating, with the formation of an fcc superlattice. The hardness is relatively constant independently of the chemical composition (22 GPa). The residual stress was strongly reduced compared to that of binary carbides coatings, due to the rotation of substrates.