Accepted in revised form 26 October 2012
Arc evaporationTransmission electron microscopyX-ray photoelectron spectroscopyValence band structure-(Al1 xCrx)2+ O3fcc-(Al1 xCrx)2+ O3
oxpaoderate-temperature deposition by physical vapor deposition techniques. During
Surface & Coatings Technology 214 (2013) 4652
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Surface & Coatin
l setion coatings grown by physical vapor deposition (PVD) have foundan intense interest . The thermodynamically metastable corun-dum structure of this ternary oxide at an energetically preferred levelhas been reported to originate from the stable binary oxides, -Al2O3(corundum) and -Cr2O3 (eskolaite) at temperatures as high as1200 C . Further research effortswere lately directed to establish co-rundum phase formation by PVD methods at low temperatures (i.e.,below 700 C) under non-equilibrium conditions [1,2]. Ramm et al. and Pohler et al.  have carried out a comprehensive investigationon the formation of (Al,Cr)2O3 coatings deposited by reactive cathodic
vacancies on the metal sites and its transformation to corundum phaseduring annealing temperatures of 900 C and above. Moreover, Kurapov has also claimed an industrial synthesis of cubic (Al,Cr)2O3 lmsdeposited by cathodic arc evaporation. Furthermore, Stber et al. described the formation of AlCr-based oxynitride coatings in both corun-dum and fcc structures as a function of elemental compositions. In ourpreviouswork on AlCr-based oxynitride lms , we have furthermoreobserved the formation of fcc-(Al,Cr)OxN1x despite the large propor-tion of oxygen (i.e. O/(O+N)=97%) and the high content of metalvacancies.arc evaporation, relating the phase formatiostate on the one hand and substrate surfacehand. Considering the equilibrium diagramphase reections present in this systemspinel-structure of -alumina . It should
Corresponding author. Fax: +41 216934470.E-mail address: email@example.com (H. Naja).
0257-8972/$ see front matter 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.surfcoat.2012.10.062iding integrity and envi-C, (Al,Cr)2O3 solid solu-
equilibriummethods within the Al2O3Cr2O3 ternary system. However,recently Khatibi et al. [11,12] reported the PVD deposition of aface-centered cubic (fcc) (Al,Cr)2O3 solid solution thin lms with 33%Due to their pronounced capability for provronmental protection between 650 and 10001. Introductiongrowth stage, lms arc-deposited from Al0.55Cr0.45 targets form a rst zone, that was found to contain exclu-sively the metastable cubic fcc-(Al1xCrx)2+O3 phase. This kinetically favored phase is reproduciblyfollowed by the growth of a second zone made of the initially expected corundum phase, -(Al1xCrx)2+O3,as observed by TEM. This dynamic transition has a signicant effect on the lm properties. XPS studies andstructural data show that the formation of fcc-(Al
1xCrx)2+O3 with a (200) preferred orientation arisesfrom the initial presence of a metastable monoxide (M1xO) lm, which is stabilized by the incorporationof metal vacancies (31%) in the B1 structure. However, as the thickness of coating increases, the thermody-namic aspect becomes more important as compared to kinetics and leads to a loss of structural stability inthe cubic layer, which is a kinetically favored phase. As a result, the system will transform into the metastablecorundum -(Al1xCrx)2+O3, which is thermodynamically more stable than the cubic phase. In this paper,formation of fcc-(Al1xCrx)2+O3 and its transformation to corundum phase are discussed in detail with re-spect to the structural and electronic properties of the different phases.
2012 Elsevier B.V. All rights reserved.
metastable fcc phase of (Al1xCrx)2O3 has never been synthesized byKeywords:our research into the reactive cathodic arc deposition of this type of coating, we found a previouslyunobserved transition between two crystalline aluminumchromium oxide structures. During the earlyAvailable online 3 November 2012comparative ease of their mFormation of cubic structured (Al1xCrx)2corundum phase during cathodic arc evap
H. Naja a,, A. Karimi a, P. Dessarzin b, M. Morstein b
a Institute of Condensed Matter Physics (ICMP), Ecole Polytechnique Fdraqle de Lausanneb PLATIT AG, Advanced Coating Systems, CH-2545 Selzach, Switzerland
a b s t r a c ta r t i c l e i n f o
Article history:Received 6 September 2012
Mixed aluminumchromiumresearch interest during the
j ourna l homepage: www.en to the target surfaceprocesses on the other
, the only known cubicare assigned to thebe emphasized that a
rights reserved.O3 and its dynamic transition toration
FL), CH-1015, Lausanne, Switzerland
ide coatings in the form of (Al,Cr)2O3 solid solutions have attracted extensivest years due to their successful use for challenging wear applications and the
v ie r .com/ locate /sur fcoatIn this work, using xed, stable deposition conditions such as arccurrent, oxygen ow, process temperature and a high substrate bias,we found that the growth of oxide coating reproducibly starts by theformation of fcc-(Al1xCrx)2+O3 at an early stage of deposition,which at a certain point (thickness; 2 m) reverts to corundum struc-ture -(Al1xCrx)2+O3 growth, as schematically illustrated by Fig. 1.Interestingly, there is an extremely fast transition from cubic to corun-dum growth for which no information is available. Due to the similar
line-of-sight to the arc sources (with diameter of 150 mm) and target
To explore the chemical environment on the atomic scale, X-rayphotoelectron spectroscopy (XPS) was employed (Kratos analytical,Manchester, UK). The system was operated using monochromatizedAl K X-ray source (1486.6 eV). The electronic properties in termsof core-level electron binding energy (BE) and valence band (VB)electronic structure were considered. In order to remove possible sur-face contaminations, sputter-cleaning was employed solely for 1 minusing 4 keV Ar+ ions. The binding energy scale was referenced to theC 1s peak position at 284.8 eV to be independent of any possible elec-trostatic sample charging effects. The microstructure of samples wasobserved by means of transmission electron microscopy (TEM),using a Philips CM-20 equipment working at 200 kV. To prepareTEM foils, the coated specimens were rst cut by a diamond wiresaw to obtain slices with thickness of around 600 m. The sliceswere subsequently thinned using mechanical polishing on diamond
Table 1The nature of investigated coatings and their elemental composition calculated by Elec-tron Probe Micro Analysis (EPMA, systematical error for the main elements1 at.%).
Sample Interlayer Target
Al Cr O (0.03) [%]
A CrN Al55Cr45 22.5 20 57.5 0.74 53B (Al0.97Si0.03)N Al55Cr45 23 18.5 58.5 0.71 55
47H. Naja et al. / Surface & Coatings Technology 214 (2013) 4652surface. The substrate temperature was set to 550 C10, and thetotal coating thickness was kept at around 3 m. For all deposition,a high multi-frequency pulsed substrate bias was selected. The stabletotal pressure was obtained within 23 min. The ion current densityat the substrate was in the range of 1050 mA cm2 and ions havea directed kinetic energy of 2030 eV. The coatings were depositedwith a deposition rate of around 2 m/h. In order to improve the ad-hesion of the oxide coating on the substrate, a thin nitride buffer layerwith a thickness (t) of 100200 nmwas deposited. To investigate anypotential epitaxial effect of this layer, both cubic (CrN) and hexagonalcrystal structure and lattice parameter, rst, a cubic nitride interlayer(B1-structured CrN, a=4.13 ) used for promoting adhesion of theoxide coating had been suspected to generate pseudomorphic growthof fcc-(Al1xCrx)2+O3. In order to refute this hypothesis, an interlayerwith hexagonal structure, i.e. (Al0.97Si0.03)N,1 was deposited and,according to our results, the same growth behavior has been observed.The possible causes for this dynamic transition from cubic phase to co-rundum structure growth are discussed in the following,with respect toboth kinetic and thermodynamic aspects.
2. Experimental details
The solid solution AlCrO coatings were deposited using a Platit300 rotating cathodes arc system from powder metallurgical AlCrtargets (PLANSEE) with a composition of 55 at.% Al and 45 at.% Cr.The mean grain size of the powders was b80 m. In the experiments,the residual base pressure was below 103 Pa and the oxygen owwas controlled by a ow controller at a total working pressure of 23 Pa. The arc sources were operated with an arc current of up to200 A. Polished WC-10% Co disks (Extramet, grade EMT210) with adiameter of 32 mm were used as substrates for coating deposition.The substrate holders had two-fold rotation and were positioned in
Fig. 1. A schematic view of the growth of (Al,Cr)2O3 coating on the WCCo substrate.((Al0.97Si0.03)N) nitride buffer layers were applied in situ, immediate-ly prior to oxide deposition using metallic Cr and Al0.97Si0.03 targetsand a nitrogen-containing plasma. The chemical composition of thelms was quantitatively analyzed by Electron Probe Micro Analysis(EPMA) as well as energy dispersive X-ray (EDX) microanalysis inthe TEM. The architecture of the investigated coatings and theirchemical composition are presented in Table 1. The crystal structurewas determined by X-ray diffraction (XRD) using a Rigaku X-ray dif-fractometer (Cu K radiation, 40 kV, 30 mA) operating in eitherBraggBrentano or grazing incidence (=4) geometries. Theobtained coating diffraction patterns were Rietveld rened byJana-crystallographic computing system , which allows investi-gating the crystallographic details for all present phases such as unitcell const