Papers

ICMCTF2007 Session B5-1: Properties and Characterization of Hard Coatings and Surfaces

Monday, April 23, 2007 10:00 AM in Room Golden West

Monday Morning

Start	Invited?	Item
10:00 AM		B5-1-1 Control of Micro- and Nanostructure in Transition Metal Nitrides I. Petrov (Frederick-Seitz Materials Research Laboratory and University of Illinois) Polycrystalline TiN and related transition-metal nitride thin films are typically deposited by reactive magnetron sputter deposition and employed as diffusion barriers in microelectronics as well as hard, wear-, and corrosion-resistant coatings in mechanical and optical applications. We use a combination of HR-XRD, TEM, HR-XTEM, AFM, and STM analyses to characterize micro- and nanostructures. We will review the fundamental film growth processes - nucleation, coalescence, competitive growth, and recrystallization - and their role in thin film microstructure evolution as a function of substrate temperature. Special attention will paid to in-situ substrate treatment by ion-irradiation and its effect on film microstructure and adhesion. Using spontaneous natural patterning processes, we show that self-organized nanostructures consisting of commensurate nanolamellae, nanocolumns, nanospheres, and nanopipes can be synthesized to further extend the range of achievable properties. All of these structures are a result of kinetic limitations and require low growth temperatures combined with low-energy (less than the lattice atom displacement potential), very high flux, ion irradiation during deposition. We extend these ideas to low-dimensional nanostructures such as periodic 1D wires and 2D domes, arcs, pyramids, and teardrops. Quantitative information of adatom transport and surface site energies required for the models are obtained from in-situ high-temperature STM and LEEM analyses. We will present in-situ real-time atomic-scale studies of energetic epitaxial film growth using a unique combination of an ion-beam deposition/etching system and low-energy electron microscopy.
10:40 AM		B5-1-3 Growth and Physical Properties of Epitaxial and Nanocrystalline Hf_1-xAl_xN Layers B. Howe (University of Illinois at Urbana-Champaign and Air Force Research Laboratory/MLBT); C. Muratore, A.A. Voevodin (Air Force Research Laboratory); I. Petrov (Frederick-Seitz Materials Research Laboratory and University of Illinois) Ti_1-xAl_xN thin films have been widely studied and proven to be effective multicomponent wear resistant tool coatings because of their high hardness, outstanding resistance to high-temperature oxidation, and beneficial age-hardening behavior. Analogous Hf_1-xAl_xN is likely to perform even better as a tribological hard coating however, because HfN is harder and more thermodynamically stable than TiN. Moreover, stronger driving forces for segregation during growth and aging are expected due to the increased lattice mismatch between HfN and AlN. Here, we present an initial investigation into the growth and physical properties of epitaxial and nanocrystalline Hf_1-xAl_xN. Hf_1-xAl_xN layers with 0≤x≤0.44 were grown on MgO (001) substrates at 550°C by UHV reactive magnetron sputter deposition in 90%Ar+10%N₂ discharges at 7mTorr. HRXRD and TEM results showed that B1-NaCl structure layers grow epitaxially with a cube-on-cube orientational relationship. The lattice parameter decreased linearly from 0.453 nm to 0.446 nm with x=0.44, compared to 0.435 nm expected from the linear Vegard's rule. We found a metastable single phase field that is remarkably broad given the large lattice mismatch between the two alloy components. Alloying HfN with AlN leads to an increase in hardness (~30% to 32GPa), as well as nanostructured compositional modulations due to the onset of spinodal decomposition. Hf_1-xAl_xN films were also grown in pulsed-DC discharges at frequencies ranging from 50-300 kHz, while maintaining a constant duty factor of 75 percent. Time-averaged ion energy distributions, measured with an electrostatic quadrupole mass spectrometer, extended to higher energies with increasing pulse frequency from 20eV in DC mode to over 100eV at 300 kHz pulsed-DC. Changes in coating microstructure and properties were observed using XRD and nanoindentation.
11:00 AM		B5-1-4 TEM Investigation of TiAlN/CrN Multilayer Hard Coatings Prepared by Magnetron Sputtering M. Panjan, S. Sturm, M. Cekada, P. Panjan (Jozef Stefan Institute, Slovenia) Multilayer hard coatings TiAlN/CrN were deposited by reactive magnetron sputtering in deposition system CC800 (CemeCon). Thickness of individual layers changed gradually from 3 nm to 25 nm, total thickness of coating was approximately 5 µm. Coatings in cross-section were investigated using conventional, scanning and high-resolution transmission electron microscopy (TEM). Conventional TEM studies revealed columnar microstructure of coating along the growth direction. Layers appeared to be dense with no cracks or pores between them. Chemical composition of layers was determined by energy-dispersive X-ray spectroscopy (EDXS). No substantial layer intermixing was observed. High resolution TEM (HRTEM) was used to study crystal structure of the coating and interfaces between layers, columnar grains and substrate/coating interface. Coating was crystalline with no amorphous regions. Layers of TiAlN and CrN grew coherently which can be attributed to the same crystal structure of TiAlN and CrN (B1 NaCl) and small mismatch between lattices. High resolution TEM image of the interface between the tool steel substrate and the coating showed a few nanometers thick deformation layer. Nanoindentation was used to measure hardness and indentation modulus. Hardness and indentation modulus were 24 GPa and 320 GPa, respectively.
11:20 AM		B5-1-5 Phase Stabilities of Ti_1-xAl_xN and Cr_1-xAl_xN P.H. Mayrhofer (Montanuniversität Leoben, Austria); D. Music, J.M. Schneider (RWTH Aachen University, Germany) Ti_1-xAl_xN and Cr_1-xAl_xN films and/or their alloys are employed in many industrial applications due to their excellent mechanical and thermal properties. Synthesized by plasma-assisted vapor deposition, coatings within these systems are known to crystallize in the cubic NaCl (c) structure or in the hexagonal ZnS-wurtzite (w) structure. Here we use ab initio calculations to analyze the effect of composition and Al distribution on the metal sublattice on phase stability, structure, and elastic properties of c-Ti_1-xAl_xN, w-Ti_1-xAl_xN and c-Cr_1-xAl_xN, w-Cr_1-xAl_xN. We show that the phase stability of supersaturated c-Ti_1-xAl_xN and c-Cr_1-xAl_xN not only depends on the chemical composition, but also on the Al distribution of the metal sublattice. An increase of the metastable solubility limit of AlN in c-Ti_1-xAl_xN or c-Cr_1-xAl_xN is obtained by decreasing the number of Ti-Al or Cr-Al bonds, respectively. This can be understood by considering the Al distribution induced changes of the electronic structure, bond energy, and configurational entropy. This may in part explain the large variation of the metastable solubility limit reported for Ti_1-xAl_xN in the literature. The metastable solubility limit for AlN in c-Ti_1-xAl_xN and c-Cr_1-xAl_xN is similar with a similar dependence on the Al distribution over the metal sublattice. However, the chemical driving force for decomposition is different, where the enthalpy change for decomposition of supersaturated c-Ti_1-xAl_xN into c-TiN and c-AlN is about four times larger than that for the decomposition of c-Cr_1-xAl_xN into c-CrN and c-AlN.