Abstract
The structural evolution from the (inter)metallic to the oxide-stoichiometric regime within oxygen-gradient Al0.25Cr0.75-based coatings was investigated by transmission electron microscopy (TEM). Grain refinement of the (inter)metallic Cr(Al) structure — induced by the addition of oxygen to the growth process — is less significant than for the higher Al-containing Al0.50Cr0.50-based coatings. Our detailed TEM studies clearly show that between 110 and 120 sccm O2 per active Al0.25Cr0.75-source (corresponding to 0.7–0.9 Pa), the coating structure changes from partially oxidised — where (inter)metallic Cr(Al) particles are encapsulated by amorphous-like Al-rich oxides — to a columnar, corundum-type sesquioxide structure. Contrary to the higher Al-containing Al0.50Cr0.50-based coatings, this transformation takes place without the formation of metastable cubic (Al1-xCrx)2O3 structures.
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