Structural regulation of hard NbB2 films by DC magnetron sputtering: From amorphous to crystalline

Abstract NbB2 with the AlB2-type structure exhibits a wide homogeneity range, providing more possibilities for studying the evolution of its structure and properties. Adjusting the growth conditions to induce the structural transformation, could be carried out to optimize its mechanical and tribological properties. Here, DC magnetron sputtering was used to deposit NbB2 thin films at different temperature, aiming to regulate the microstructure. Further investigations were carried out on chemical bonding states, mechanical and tribological properties by XPS, nanoindentation and tribometer. With the deposition temperature increasing, the crystallinity of the film is improved, while the amorphous B-rich phase is reduced, which is confirmed by the changing of chemical bonding states. The hardness increases first and reaches its maximum (29.90 ± 1.53 GPa) at 400 °C, and then decreases in the higher temperature. The tribochemical product (H3BO3), the hardness and the wear track morphology affect the tribological behavior of NbB2 films. It is worth noting that the samples deposited in middle temperature (200–600 °C) with the encapsulated structure exhibit the superior performance, reaching a higher hardness (>25 GPa) and a lower friction coefficient (~0.2).