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 metal elements) 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. This decomposition is accompanied with the formation of confined B-rich boundary regions between the two phases. This leads to a decrease in H to ~39 GPa with Ta = 1500 and 1600 °C. 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.