Microstructure-driven strengthening of TiB 2 coatings deposited by pulsed magnetron sputtering

Polyakov, M. N.; Morstein, M.; Maeder, X.; Nelis, T.; Lundin, D.; Wehrs, J.; Best, J. P.; Edwards, T. E.J.; Döbeli, M.; Michler, J.

Surface and Coatings Technology 368 (2019)

Titanium diboride (TiB 2 ) is a hard coating with a hexagonal crystal structure that maintains its high hardness and inertness at elevated temperatures, even in contact with aluminum and its alloys. Therefore, titanium diboride coatings are often used in cutting, forming, injection molding and tribological applications. While there are multiple deposition routes available for this coating, there are limitations in terms of control of the coating microstructure and internal stresses, which translate to limitations in the film properties and film integrity in industrial use. High-power impulse magnetron sputtering (HiPIMS) is a deposition technique which has recently shown promise for depositing titanium diboride films due to its intrinsic microstructural control capabilities. However, issues such as relatively low deposition rates and high compressive stresses have not been sufficiently addressed for this technique. This investigation reports on the implementation of a new HiPIMS-based TiB 2 process and a high average-power process optimization from a cylindrical rotating magnetron fitted with ceramic TiB 2 as the target material. By tuning the HiPIMS pulse conditions, high-hardness films were obtained at high deposition rates, while maintaining high ionization rates as determined by optical emission spectroscopy and Langmuir probe measurements. Micropillar compression and nanoindentation techniques were used to investigate the film deformation behavior. Tests at room temperature showed superior fracture strengths for the new HiPIMS coatings as compared to films deposited by traditional direct current magnetron sputtering under otherwise identical conditions. The differences in deformation are explained by a microstructural investigation by transmission electron microscopy.