Phase formation in cathodic arc synthesized Al–Hf and Al–Hf–O coatings during high temperature annealing in ambient air

Al–Hf and Al–Hf–O coatings were synthesized by cathodic arc evaporation utilizing composite targets with composition of Al75Hf25. The reaction at the target surface and the composition of the film were analyzed by energy-dispersive X-ray spectroscopy (EDX), Rutherford backscattering spectrometry (RBS) and X-ray diffraction (XRD) techniques. The Al–Hf coatings deviate distinctively from target composition showing a deficiency in Al. XRD phase analysis shows the synthesis of Al3Hf intermetallic compound for the Al–Hf coatings while the oxide coatings indicate an amorphous matrix and monoclinic HfO2. The coatings were annealed in ambient air up to 1290 °C to study the oxidation process. In-situ XRD analysis of the coatings was performed during annealing. The intermetallic phase undergoes phase transformations at temperatures between 700 °C and 1100 °C, with the formation of a metastable face-centered cubic (fcc) HfO2 simultaneously with the monoclinic polymorph. After this transition period, the cubic phase disappears and the coating stabilizes with two oxide phases: corundum (α-Al2O3) and monoclinic HfO2 (m-HfO2). Applying the same treatments to the Al–Hf–O coatings results in a similar evolution with an improvement of crystallinity for the monoclinic HfO2 phase and the crystallization of the fcc HfO2 starting at about 860 °C. This phase disappears above 1200 °C, where the coating stabilizes with corundum and m-HfO2 phases. The formation of the metastable fcc HfO2 phase is discussed and associated with a deficiency in oxygen during the oxidation process.