Effect of crystal growth of HiPIMS deposited Cr₂O₃ interlayer on the growth of Al₂O₃ coatings

Abstract

α-phase Al₂O₃has been used as a hard coating film for cutting tools because of its excellent wear and oxidation resistance. Currently, commercially available α-phase Al₂O₃ films are mainly formed by thermal CVD. However, the high deposition temperature of over 1000°C by thermal CVD limits the selectivity of the substrate materials, and thus, PVD processes has been attracted as an alternative deposition process to realize low deposition temperature. In addition, roles of nucleation layers on the growth of α-phase Al₂O₃ films have been studied, e.g., as magnetron sputtered α-phase Al2O3 coatings were achieved at deposition temperatures of approximately 700°C by introducing CrN or TiAlN as interlayer between the substrate^[1].In this study, we focused on α-phase Cr₂O₃ films as it has a same corundum crystal structure as nucleation layers and aimed to clarify the effect of its crystal texture on the Al₂O₃ film growth. To realize the evolutions of the crystal textures of the α- Cr₂O₃, high-power impulse magnetron sputtering (HiPIMS) was used, as deposition process which enables flexible control of the film structure through the formation of high-density plasma. Feasibility on the crystal texture evolutions of α- Cr₂O₃ and its effect on the growth of Al₂O₃ coatings has been thoroughly discussed.

The target materials were 3-inch pure Cr and pure Al. Oxide formation was achieved by introducing O₂ gas into an Ar atmosphere. During the growth of Cr₂O₃ films the substrate potential was set at ground, substrate bias voltage U_s: -50 V and -100 V. All experiments were performed in a high vacuum stainless-steel chamber with a base pressure of ∼10⁻⁴ Pa. A planar circular unbalanced magnetron with a Cr and Al disk with a diameter of 75 mm was used as the sputtering target. Ar gas was introduced into the chamber through a mass-flow controller at a constant flow rate of 100 sccm and was maintained at a constant working pressure of 1.0 Pa. O₂ gas of 2.3 sccm was introduced as reactive gas to grow films in the transition sputtering modes. HiPIMS pulses were supplied by a HiPSTER 1 pulsing unit fed by a 1 kW HiPSTER 1-DCPSU DC power supply (Ionautics AB, Sweden). During the growth of Cr₂O₃ films, the substrate bias voltage (U_s) was set at ground potential, U_s: -50 V and -100 V. The crystal structures of the films were characterized by using X-ray diffraction (XRD) and its microstructures were analyzed by using scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

As results, α- phase crystals could successfully obtained for the Cr₂O₃ films and its crystal texture with α(006) became preferentially oriented as the substrate bias voltage is increased, while the randomly oriented crystal structure was obtained at grounded substrate bias configuration. As the (006) plane is known as a thermodynamically stable crystal plane, the higher kinetic energy of incident ions could contribute to the thermodynamic events, leading to exhibit the more preferential orientation.

Moreover, effects of these crystal textures on the growth of Al₂O₃ coatings are clearly observed, showing the strong η-phase Al₂O₃ growth on the highly textured Cr₂O₃ with biased configuration.

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