Formation of cubic structured (Al1-xCrx)(2+delta)O-3 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.
Record created on 2013-03-28, modified on 2016-08-09