ICMCTF2013 Session B5-2: Hard and Multifunctional Nanostructured Coatings

Protective hard coatings make use of the outstanding mechanical and thermal stability of early transition metal nitrides (TMN) and their alloys with Al. Modern applications require sophisticated designs of the protective thin films in which modelling aims to play an important role, in particular in predicting otherwise hardly accessible material properties. Quantum mechanical calculations of elastic constants of compounds as well as alloys have become a well established and widely used tool.

Voigt, Reuss, Hill and Hershey methods allow for calculation of the polycrystalline elastic constants. However, the commonly used formulae yield these estimates for isotropic aggregates of grains (i.e., all grain orientations are present with the same probability), which is hardly the case for highly orientated polycrystalline thin films. To overcome this shortcoming, we generated a wide range of orientation distribution functions (ODFs) representing various common textures, and performed the polycrystalline averaging with respect to these ODFs. The compositional dependence of single crystal elastic constants for the ternary Zr_1–xAl_xN system is obtained from first principles calculations. This allows us to discuss the dependence of the elastic response on both, the composition and the particular texture at the same time.

Understanding the physical mechanisms controlling thin film growth is of vital importance to obtain the desired microstructures and related specific properties. Subtle structural changes may occur in the early growth stages, driven by surface/interface effects, epitaxy, chemical driving forces or energetic conditions intrinsic to PVD techniques like sputtering. Atomic-scale sensitive and real-time diagnostics are therefore required to address such issues.

In the present work, we demonstrate the unique potential offered by real-time stress diagnostics, combined with structural investigation, to understand not only the stress development during thin film growth but more generally to study dynamic microstructural evolution processes. It is shown that stress measurements using a multiple-beam optical stress sensor (MOSS) implemented in the deposition chamber offer an efficacious and accurate way to identify structural changes with sub-monolayer sensitivity.

The paper reports on structure, mechanical and optical properties of Zr-Al-O films with enhanced resistance to cracking in bending. The Zr-Al-O films with Zr/Al > 1 and Zr/Al < 1 were prepared by reactive sputtering using ac pulse dual magnetron. The magnetrons were equipped with a target composed of Al plate (ϕ = 50 mm ) fixed to the magnetron cathode by a Zr fixing ring with inner diameter ϕ_in. The content of Al in the Zr-Al-O film was controlled by ϕ_in.. This way it was possible to control the ratio of the crystalline ZrO₂ phase and the X-ray amorphous Al₂O₃ phase in the Zr-Al-O film and thereby its structure. The Zr-Al-O films were deposited on (i) Si(100) substrates for measurement of (a)the mechanical properties (hardness H, effective Young’s modulus E^* and elastic recovery W_e) and (b) the film structure, (ii) glass substrate for measurement of the optical transparency of film and (iii) on the thin metallic strip for measurement of resistance of the film to cracking in bending. It was found that (i) the Zr-Al-O films with Zr/Al < 1 are X-ray amorphous and exhibit a low hardness (H ≤ 13 GPa), effective Young’s modulus E^* satisfying a low H/E^* < 0.1 ratio and low elastic recovery W_e ≤ 60%, (ii) the Zr-Al-O films with Zr/Al > 1 are crystalline and exhibit a high hardness (H » 18 to 19 GPa), effective Young’s modulus E^* satisfying a high H/E* ≥ 0.1 ratio and high W_e up to 75% and (iii) the highly elastic hard Zr-Al-O films with H » 18 -19 GPa, low Young’s modulus E^* satisfying the ratio H/E^* > 0.1 and high value of elastic recovery W_e ≥ 70% exhibit strongly enhanced resistance to cracking; here E^* = E/(1-n²), E is the Young’s modulus and n is the Poisson’s ratio. The last finding is the most important result of this investigation.

Thin films with enhanced hardness based on Al-A-N with additions of A = Si, Ge or Sn are attractive candidate materials, as they can be prepared to be optically transparent. Using reactive unbalanced magnetron sputtering from elemental targets, series of films from Al-Si-N, Al-Ge-N and Al-Sn-N were deposited and characterized. Enhanced hardness of more than 30 GPa is observed in Al-Si-N coatings as a consequence of their nanostructure, but instead of a typically sharp hardness maximum observed in transition metal nitrides/silicon nitride nanocomposites, a broad hardness maximum exists as a function of the silicon content in the layers. The choice of the additional A element allows depositing highly transparent coatings for the case of Si and for the case of Ge and Sn the control of color in the range from yellow to red by the tuning of the UV absorption edge, which can be tuned in the range 200 to 500 nm (corresponding to optical band gaps of 6.2 to 2.5 eV). Also the index of refraction could be controlled in the range between 2.0 and 2.5. In these systems enhanced hardness can be obtained at certain concentrations of Si, Ge or Sn, respectively. Generally hardness values between 16 and 30 GPa were attained. Depending on the content of the third element AlN-based solid solutions phases and the formation of nanocomposites is observed. Trends in the materials' properties of these materials will be presented and discussed.

Deep oscillation magnetron sputtering (DOMS) is an alternative high power pulsed magnetron sputtering technique which offers virtually arc free deposition for reactively deposition of insulating films. TiAlSiN nanocomposite coatings have been deposited with different nitrogen flow rates (fN2) in a close field unbalance magnetron sputtering system by sputtering a Ti40Al50Si10 target using the DOMS technique. A -60 V substrate bias voltage was used during the depositions. The microstructure and properties of the TiAlSiN coatings were investigated by means of electron probe microanalysis, X-ray diffraction, transmission electron microscopy, nanoindentation, and ball-on-disc wear test. It was found that the TiAlSi coating possessed an fcc-TiAl structure with a (111) preferred orientation. As the fN2 was increased from 0% to 20%, the TiAlSiN coatings gradually transformed from nc-TiAl/a-Si3N4 nanocomposite structure to nc-AlN/a-Si3N4 nanocomposite structure. The coatings exhibited a dense, uniform and smooth surface. TiAlSi coating showed a low hardness of 10.9 GPa and a H/E ratio of 0.062, whereas the TiAlSiN coatings exhibited improved mechanical properties, wear resistance and oxidation resistance with the incorporation of N, in which the TiAlSiN coating deposited at a fN2 of 10 % exhibited the highest hardness of 25 GPa, a high H/E ratio of 0.95 and excellent wear and oxidation resistances.

Key words: Deep oscillation magnetron sputtering (DOMS), High power pulsed magnetron sputtering (HPPMS), TiAlSiN coating, Hardness, Wear resistance

Coatings of Me-Si-C (Me = early transition metal) are interesting due to their multifunctional properties. A general observation is that the Si content is strongly correlated to the microstructure and thereby the properties of the Me-Si-C coatings. Typically, an increased Si concentration leads to a reduction in carbide grain size and in many systems to completely X-ray amorphous coatings. However, the amount of Si required to form an amorphous structure is dependent on the Me type. These observations raise two questions: first, how is the tendency for amorphous growth varying with different Me? Secondly, is it possible to tune the properties of an amorphous material by the choice of Me? We have chosen to study the previously not investigated Nb-Si-C system. Critical Si concentrations for amorphous growth is determined and compared with Zr-Si-C to identify possible trends. The Nb-Si-C coatings have been deposited by dc-magnetron sputtering using elemental targets. Structure and composition of the coatings have been characterized with X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Nano-indentation were used to determine the mechanical properties of the coatings while the electrical resistivity has been measured using the four point probe technique.

Our results show a transition from a nanocomposite (nc-NbC/a-SiC) structure to a X-ray amorphous structure when the Si content reaches above 25 at.% (15 at.% for Zr). Electron beam induced crystallization is observed during TEM analysis, obstructing the characterization of the amorphous structures. However, bonding structures analysed by XPS indicates an amorphous network structure for the Nb-Si-C coatings reminding of the one described by Kádas et al. for dc magnetron sputtered Zr-Si-C coatings [1]. The transition in microstructure is reflected in the properties of the coatings with an increase in electrical resistivity (from 211 μΩcm to 3215 μΩcm) and a change in the mechanical behavior of the coatings. Hardness values of 19 GPa are achieved both for coatings exhibiting a nanocomposite and an amorphous structure. Comparison with dc magnetron sputtered Zr-Si-C films indicates a direct dependency for the hardness to the amount of C-Si bonds rather than type of transition metal for these types of amorphous Me-Si-C films.