AVS 60th International Symposium and Exhibition
    Thin Film Friday Sessions
       Session TF+EM+NS+SS-FrM

Paper TF+EM+NS+SS-FrM8
Properties of Epitaxial VNx/MgO(001) (0.70 < x < 1.36) Layers Grown by Reactive Magnetron Sputter Deposition

Friday, November 1, 2013, 10:40 am, Room 104 A

Session: Thin Film: Growth and Characterization III
Presenter: A.R.B. Mei, University of Illinois at Urbana Champaign
Authors: A.R.B. Mei, University of Illinois at Urbana Champaign
D.G. Sangiovanni, Linköping University, Sweden
H. Kindlund, Linköping University, Sweden
B. Howe, Air Force Research Laboratory
E. Broitman, Linköping University, Sweden
V. Chirita, Linköping University, Sweden
L. Hultman, Linköping University, Sweden
A. Rockett, University of Illinois at Urbana Champaign
J.E. Greene, University of Illinois at Urbana Champaign
I.G. Petrov, University of Illinois at Urbana Champaign
Correspondent: Click to Email
Ceramic coatings are, despite their high hardness, often limited in use by their brittleness. Under high stress conditions, brittle hard coatings, such as transition metal (TM) nitride thin-films, fail prematurely through crack nucleation and growth. In order to extend the longevity and broaden the applications of ceramic hard coatings, they must be toughened: their capacity for absorbing elastic energy prior to fracture must be increased. Here, we report on the toughening of under-stoichiometric VNx, deposited on MgO(001) in ultra-high-vacuum by reactive magnetically-unbalanced magnetron-sputter deposition. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and selected area electron diffraction (SAED) indicate that the films are fully-dense single-phase epitaxial VNx(001). Hardness values of VNx(001), determined from nanoidentation measurements according to the Oliver and Pharr method, increase with off-stoichiometry from H = 14.0±0.8 GPa (x = 1) to 17±0.76 GPa (x = 0.77) and 16.7±0.1 GPa (x = 1.20). Scanning electron micrographs (SEM) of nanoidentations performed with a sharp cube-corner tip reveal the presence of cracks in VNx(001) overlayers with x > 0.95; four cracks were observed in each film with x > 1.23, two in films between 1.06 < x < 1.10, and one for 0.95 < x < 1.00. Cracks were not observed in VNx(001) with x < 0.88. The resilience of highly under-stoichiometric VNx(001) films to fracturing, as indicated by the absence of cracks, is evaluated via crystal orbital overlap populations (COOP) analysis using ab initio density functional theory (DFT) calculations. The results indicate that the toughening is of electronic origin and due to the strengthening of V-V bonds along the slip direction.