nc-MeB2/a-MPEA structured (HfMoNbZr)xTi1-xBy films for enhanced hardness, toughness and tribological performance

This study demonstrated the composite concept to precisely control the grain boundary composition to address TiB2 films inherent brittleness. Such concept was realized by combining high-power impulse magnetron sputtering (HiPIMS) with direct current magnetron sputtering (dcMS). By employing an HfMoNbZr-HiPIMS/TiB2-dcMS co-sputtering configuration, we successfully replaced the B in the grain boundaries with a B-containing amorphous multi-principal element alloy (MPEA). The results showed that TiB2 film deposited by dcMS alone typically had an overstoichiometric composition, with a B/Ti ratio of 2.96, and a relatively low hardness (H) of 26.6 ± 1.2 GPa. In contrast, the formation of a nanocomposite structure (nc-MeB2/a-MPEA, i.e., nanocrystalline MeB2 (Me = HfMoNbZrTi) embedded in an amorphous MPEA matrix), with MeB2 in-plane grain sizes ranging from 2 to 5 nm, increased the hardness to 41.8 ± 3.7 GPa, while also enhancing indentation fracture toughness and reducing wear rate. This strengthening mechanism was attributed to lattice distortion within the MeB2 grains and the suppression of grain boundary sliding by the MPEA. In addition, density functional theory (DFT) calculations indicated that the higher bulk modulus (B) /shear modulus (G) ratio of the TM0.44Ti0.56B2 (TM = HfMoNbZr) solid solution compared to TiB2 suggested that the brittleness of the TM0.44Ti0.56B2 solid solution was lower. This study paves a way for the structure design of diborides for various applications.