Design of transition metal carbide/nitride superlattices with bilayer period-dependent mechanical and thermal properties

Transition metal carbides are valued for high hardness, thermal and mechanical stability, but fall short in fracture toughness. Contrarily, their less hard transition metal nitride counterparts offer more favorable fracture characteristics. Here, we use magnetron-sputtering to synthesize nitrides and carbides—TiC/TaN, TiN/TaC—in a nanolaminate superlattice (SL) architecture and compare their properties (hardness, fracture toughness, thermal stability) with that of their layer materials, as well as of carbide SLs, TiC/TaC. Except for the monolithically grown TaN and TiC/TaN SLs with nominal bilayer periods above 14 nm, all other coatings are purely fcc-structured and feature close-to-stoichiometric compositions, revealed by EBS-ERDA and XRF measurements. In-situ X-ray diffraction investigations indicate that the monolithically grown coatings have poor thermal stability compared to the SLs, which remain stable up until well over 1000-°C. While the TiC/TaC superlattices retain the highest hardness of all three systems, with 44.1 ± 3.4 GPa at a bilayer period (Λ) of 2 nm, the TiN/TaC system exhibits significantly higher fracture toughness values with up to 4.75 ± 0.33 MPa√m for the Λ = 14 nm coating. The TiC/TaN system exhibits neither hardness nor fracture toughness enhancement, as explained by the formation of a secondary hexagonal Ta₂N phase.