Non-Reactive Magnetron Sputtering of Ti-Al-N Coatings

Abstract

Hard protective coatings allow for increased lifetime of machining tools and more versatile applications. Although AlN-based coatings have a rich history in material science with various improvements for their production, little is known about non-reactive deposition using ceramic AlN compound targets. Aluminium nitride in its hexagonal close packed (hcp) wurtzite-type structure has the highest thermal conductivity among ceramic materials, a large electromechanical coupling factor and temperature stability, as well as a high acoustic velocity. Reactive deposition of such AlN coatings is studied in-depth, showing that especially for sputtering the resulting microstructure and consequently properties (next to deposition rate) hugely depend on the N2-partial pressure used. Alternatively, such nitrides can also be prepared nonreactively using nitride compound targets. Here, we use powder metallurgically prepared AlN compound targets to prepare coatings with pulsed DC magnetron sputtering with a 3” target and a 6” target. The primary investigations focused on how the mechanical properties such as hardness and indentation modulus depend on various deposition conditions, such as sputtering power density, pulse frequency, substrate temperature, substrate-to-target distance and plasma condition. Additionally, several experiments were conducted by adding H2 to Ar to study the effect of a reducing agent during the ion-etching of the substrate as well as during the deposition of the AlN film. To counteract understoichiometry, we added sometimes N2 as well. Detailed investigations by X-ray diffraction reveal that all coatings were single-phase hcp-structured, with various amounts of an amorphous phase and/or a metallic Al, depending on the deposition conditions. The highest hardness obtained for such films is 26.9 GPa. With the addition of H2 to the working gas Ar, the discharge became more stable even for high power densities, allowing for a deposition rate of up to 1 μm/h.