Growth and thermal stability of (V,Al) 2 C x thin films
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Riza Iskandar Central Facility for Electron Microscopy, RWTH Aachen University, 52074 Aachen, Germany
Moritz to Baben, Tetsuya Takahashi, Jie Zhang, and Jens Emmerlich Materials Chemistry, RWTH Aachen University, 52074 Aachen, Germany
Joachim Mayer Central Facility for Electron Microscopy, RWTH Aachen University, 52074 Aachen, Germany
Conrad Polzer and Peter Polcik Plansee Composite Materials GmbH, 86983 Lechbruck am See, Germany
Jochen M. Schneider Materials Chemistry, RWTH Aachen University, 52074 Aachen, Germany (Received 30 January 2012; accepted 16 May 2012)
substrates at Vanadium (V)–aluminum (Al)–carbon (C) thin films were deposited on Al2O3ð1120Þ 500 °C by direct current magnetron sputtering using a powder metallurgical composite target with 2:1:1 MAX phase stoichiometry. Transmission electron microscopy (TEM) and x-ray diffraction results suggest that a hexagonal Al-containing vanadium carbide solid solution (V,Al)2Cx was formed. The films exhibited a strong basal plane texture. The lattice parameter of the hexagonal solid solution was dependent on the annealing temperature: the c lattice parameter decreased by 3.45% after annealing for 1 h at 750 °C compared to the as-deposited film. Based on the comparison between experimental and theoretical lattice parameter data, it is reasonable to assume that this annealinginduced change in lattice parameter is a consequence of atomic ordering. Meanwhile, the formation of V2AlC MAX phase was observed at 650 °C and phase-pure V2AlC was obtained at 850 °C. TEM images support the notion that V2AlC forms by nucleation and growth.
I. INTRODUCTION
Mn 1 1AXn phases, where M is an early transition metal, A is an A-group element, X is either carbon (C) or nitrogen (N), and n 5 1–3,1 have aroused great interest in the last decade. The MAX phase structure can be described as nanolaminates of strongly bonded M-X layers interleaved with weakly bonded M-A layers. Also, MAX phases possess both metallic and ceramic characteristics, such as high electrical and thermal conductivities but also high thermal stability,2–5 and high stiffness.6–9 Moreover, some MAX phases are oxidation-resistant,10–15 due to the formation of a protective layer composed of the oxides of the A-element10,12,15 and/or the M-element.13,14 Recently, the oxidation resistance of Cr2AlC coatings has been reported to exhibit a similar activation energy as bulk Cr2AlC and NiAl.15 The self-healing ability of MAX phases is currently generating significant interest16 and a model for the oxide/MAX phase interface has recently been communicated.17 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.202 J. Mater. Res., Vol. 27, No. 19, Oct 14, 2012
The bonding character of V2AlC (space group P63/mmc) may be described as a mixture of covalent-ionic and, due to the d resonance in the vicinity of the Fermi level, metallic.18 The density of V2AlC is 4.87 g/cm3 19 and the lattice parameters are a 5 2.91 Å and c 5 13.10 Å.3,18 Furthermore, bulk modulus values were report
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