Control of the structure and properties of aluminum oxide coatings deposited by pulsed magnetron sputtering

Abstract

Alumina coatings offer the potential to protect metallic components operating in hostile corrosive, or oxidative environments. Until recently, though, the high rate deposition of fully dense, defect-free oxide coatings has proved extremely difficult. Oxide coatings can be produced by the reactive direct current (dc) sputtering of a metallic target in an oxygen atmosphere, or by radio frequency sputtering of an oxide target. The latter process results in very low deposition rates and is difficult to scale up for commercial applications. Reactive dc sputtering of highly insulating materials, such as alumina, is also problematic. The build up of an insulating layer on the surface of the target causes arcing. Arc events affect the stability of the deposition process and can adversely effect the structure and properties of the growing film. However, a new technique, pulsed magnetron sputtering, has been developed which significantly increases the viability of producing this type of material. It has been shown that pulsing the magnetron discharge in the 10–200 kHz range prevents the formation of arcs at the target. The process is stable and high deposition rates can be achieved. This technique has, therefore, been used to deposit coatings with compositions ranging from pure aluminum to stoichiometric ${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}.$ The coatings were characterized by scanning and transmission electron microscopy, x-ray diffraction, four-point probe, and nano-indentation techniques. The observed variations in structure and properties are considered in terms of the composition of the coatings and expressed schematically in the form of a novel structure zone model.