Thermal stability and mechanical properties of sputtered (Hf,Ta,V,W,Zr)-diborides

Non-reactive magnetron sputtering of a diboride target composed of HfB2, TaB2, VB2, W2B5, and ZrB2 with equimolar composition leads to the formation of crystalline single-phase solid solution diboride thin films, (Hf,Ta,V,W,Zr)B2, with a high-entropy metal-sublattice. Their growth morphology (dense and fine-fibrous), crystal structure (AlB2-type), as well as mechanical properties (indentation modulus E of ~580 GPa and hardness H of ~45 GPa), and chemical compositions are basically independent of the substrate bias potential applied (varied between -40 and -100 V) during the deposition at 450°C. Detailed X-ray diffraction (XRD) and atom probe tomography (APT) studies indicate that the (Hf,Ta,V,W,Zr)B2 thin films remain single-phase AlB2-structured (with randomly distributed elements at the metal-sublattice) during vacuum-annealing at temperatures up to 1200°C. Only when increasing the annealing temperature to 1400°C, the formation of small orthorhombic structured (V,W)B-based regions can be detected, indicating the onset of decomposition of (Hf,Ta,V,W,Zr)B2 thin films into (Hf,Ta,Zr)B2 and (V,W)B, accompanied by the formation of confined B-rich boundary regions between these phases. After annealing at 1400°C the hardness is still very high with ~44 GPa, as the volume fraction of the newly formed (V,W)B-rich domains is small and the majority of the coating is still solid-solution (Hf,Ta,V,W,Zr)B2 with severe lattice distortions. Only at even higher Ta of 1500 and 1600 °C, H decreased to ~39 GPa.