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.