Formation of cubic structured (Al1-xCrx)2+δO3 and its dynamic transition to corundum phase during cathodic arc evaporation

Mixed aluminum-chromium oxide coatings in the form of (Al,Cr)(2)O-3 solid solutions have attracted extensive research interest during the past years due to their successful use for challenging wear applications and the comparative ease of their moderate-temperature deposition by physical vapor deposition techniques. During our research into the reactive cathodic arc deposition of this type of coating, we found a previously unobserved transition between two crystalline aluminum-chromium oxide structures. During the early growth stage, films arc-deposited from Al10.55Cr0.45 targets form a first zone, that was found to contain exclusively the metastable cubic fcc-(Al1-xCrx)(2+delta)O-3 phase. This kinetically favored phase is reproducibly followed by the growth of a second zone made of the initially expected corundum phase, alpha-(Al1-xCrx)(2+delta)O-3, as observed by TEM. This dynamic transition has a significant effect on the film properties. XPS studies and structural data show that the formation of fcc-(Al1-xCrx)(2+delta)O-3 with a (200) preferred orientation arises from the initial presence of a metastable monoxide (M1-xO) film, which is stabilized by the incorporation of metal vacancies (31%) in the B1 structure. However, as the thickness of coating increases, the thermodynamic aspect becomes more important as compared to kinetics and leads to a loss of structural stability in the cubic layer, which is a kinetically favored phase. As a result, the system will transform into the metastable corundum alpha-(Al1-xCrx)(2+delta)O-3, which is thermodynamically more stable than the cubic phase. In this paper, formation of fcc-(Al1-xCrx)(2+delta)O-3 and its transformation to corundum phase are discussed in detail with respect to the structural and electronic properties of the different phases. (C) 2012 Elsevier B.V. All rights reserved.