Oxidation-induced degradation of strength of tungsten carbide-cobalt

Abstract

Mixed aluminum–chromium oxide coatings in the form of (Al,Cr)2O3 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 Al0.55Cr0.45 targets form a first zone, that was found to contain exclusively the metastable cubic fcc-(Al1 − xCrx)2 + δO3 phase. This kinetically favored phase is reproducibly followed by the growth of a second zone made of the initially expected corundum phase, α-(Al1 − xCrx)2 + δO3, 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 + δO3 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 α-(Al1 − xCrx)2 + δO3, which is thermodynamically more stable than the cubic phase. In this paper, formation of fcc-(Al1 − xCrx)2 + δO3 and its transformation to corundum phase are discussed in detail with respect to the structural and electronic properties of the different phases.