ICMCTF2016 Session B6: Coating Design and Architectures

It is well understood that TiAlN age-hardens by spinodal decomposition into TiN- and AlN-rich regions [1]. Here, gas phase combinatorics was used to deposit Ti1-xAlxNz with continuous variation of the metal to non-metal ratio. Thermal stability and mechanical properties after annealing treatments at 850°C and 900°C of these films were investigated by ERDA, XRD, nanoindentation and atom probe tomography. A significant influence of the metal to non-metal ratio on thermal stability is reported where off-stoichiometry decreases thermal stability extremely. It is suggested that the reported thermal stability data in the last decade should be re-evaluated by taking the nitrogen stoichiometry into account. Finally, it is discussed how point-defect engineering can be used to tailor thermal stability and hence age-hardening of TiAlN for application.

References:

[1] Mayrhofer et al., Self-organized nanostructures in the Ti-Al-N system, Appl. Phys. Let. 83 (2003) 2049.

Superlattice films composed of two alternating materials with a periodicity length in the nanometer range evoke much scientific interest due to their superior properties (e.g. high hardness) in comparison with their monolithic counterparts. With decreasing layer thickness, the interfaces become the key building elements of the multilayer films predetermining their properties.

The first part of this talk addresses the question how the volume content of coherent interfaces in CrN/TiN superlattice films and the bilayer period affect microstructure, hardness and residual stresses of the films. By using different substrate materials within the same deposition runs additionally allowed to directly compare “polycrystalline” superlattice films grown on the native oxide of the Si (100) substrates and “single crystalline” superlattice films adapting their growth to the template of the underlying single crystalline MgO (100) substrates.

In the second part the influence of the interface type (coherent, semicoherent and incoherent) on the microstructure evolution is studied on a nanolayered ~4 µm thick multilayer CrN/AlN film, where the growth mode of the individual nanolayers was intentionally changed from coherent to incoherent in the course of the deposition process by continuously increasing the AlN layer thicknesses while keeping the CrN template layer thicknesses constant. Synchrotron based wide angle X-ray nano-diffraction and transmission electron microscopy reveal that AlN grows coherently in its metastable face-centered cubic (B1) structure on lattice matched B1-CrN up to ~4 nm layer thickness. Above ~4 nm, AlN crystallizes in its stable wurtzite-type (B4) variant accompanied by coherency loss. While the maximum grain size in film growth direction is limited to the layer thickness for the incoherently grown layers, coherently grown layers form large elongated grains extending over several bi-layers. The grain size in in-plane direction is (at least partly) influenced by the layer undulations formed in the course of the film growth. Finally, implications of the interface-type-related microstructure formation on the film properties are briefly discussed.

The combinatorial approach, combining combinatorial materials synthesis of thin film composition-spreads with high-throughput property characterization has proven to be a powerful tool to delineate composition-structure-property relationships, and hence to efficiently identify composition windows with enhanced properties. [1] The combination of modern electronic structure calculations with the highly efficient combinatorial thin film composition-spread method constitutes an effective tool for knowledge based materials design of hard and wear resistant coatings. [1-3] Besides the elastic property and phase stability also the interaction of the coating with the ambient can be described based on quantum mechanics. In the talk predictions of the interaction of coated tool surfaces with gases [4-6] contained in the atmosphere as well as materials to be formed are discussed. Coatings used for forming operations of Al [7] and Polymers [8] are investigated and initial experimental data characterizing these interactions will be discussed.

1. Th. Gebhardt, D. Music, T. Takahashi, J.M. Schneider, Combinatorial thin film materials science: From alloy discovery and optimization to alloy design, Thin Solid Films 520, 5491-5499 (2012)

2. K.P. Shaha, H. Rueß, S. Rotert, M. to Baben, D. Music, J.M. Schneider, Nonmetal sublattice population induced defect structure in transition metal aluminum oxynitrides, Applied Physics Letters 103, 221905 (2013)

3. H. Bolvardi, J. Emmerlich, S. Mràz, M. Arndt, H. Rudigier, J.M. Schneider, Low Temperature synthesis of Mo₂BC thin films, Thin Solid Films 542, 5-7 (2013)

4. D. Music, J.M. Schneider, Ab initio study of Ti_0.5Al_0.5N(001)-residual and environmental gas interactions

New Journal of Physics 15, 073004, (2013)

5. Ch. Kunze, D. Music, M. to Baben, J.M. Schneider, G. Grundmeier, Temporal evolution of oxygen chemisorption of TiAlN, Applied Surface Science 290, 504-508 (2014)

6. C. Gnoth, C. Kunze, M. Hans, M. to Baben, J. Emmerlich, J.M. Schneider, G. Grundmeier, Surface chemistry of TiAlN andTiAlNO coatings deposited by means of high power pulsed magnetron sputtering, Journal of Physics D: Applied Physics 46 (8), 084003-1 (2013) D

7. H. Bolvardi, D. Music, and J.M. Schneider, Interaction of Al with O2 exposed Mo2BC, Applied Surface Science 332, 699-703 (2015)

8. D. Music, D. Lange, L. Raumann, M. to Baben, F. von Fragstein, J.M. Schneider, Polypropylene-MAlN (M=Ti, Cr) interface interactions, Surface Science 606, 986-989 (2012)

AlTiN hard coatings prepared by PVD technologies are among the most widely used wear-resistant materials in the cutting tool industry. Requirements for good application performance include high hardness and thermal stability in ambient air, as well as forgiving wear behavior.

Recently LPCVD was used to synthesize a new kind of highly Al-rich Al_0.95Ti_0.05N coating with a self-organized nano-composite microstructure comprised of alternating hard fcc-Ti(Al)N and soft w-Al(Ti)N lamellae with a bi-layer period of 13 nm [1]. This coating exhibited excellent oxidation resistance, as well as good hardness and wear properties [2]. However, it was recognized that changing the microstructure to purely cubic lamellae should improve mechanical properties significantly.

In order to attain this goal, an iterative deductive approach was chosen, for which several graded coatings were deposited and then characterized using cross-sectional scanning X-ray nanodiffraction [3] and cross-sectional micromechanical testing. Thus it was possible to identify the deposition parameters that lead to optimized hardness and the resultant coating was further investigated with respect to its microstructure, thermal stability, high temperature hardness and oxidation resistance.

TEM analysis confirmed a purely cubic nano-lamellar microstructure and revealed an overall composition of Al_0.8Ti_0.2N. Due to the somewhat lower Al content, also the oxidation resistance suffered a little, but phase stability remained similarly high.

Compared to the original Al_0.95Ti_0.05N coating, the arrangement and architecture of lamellar packets was shown to be notably different, as lamellar grains are much larger and show a preferential orientation. Also, a coating hardness increase in the order of 30% was observed, which remained stable up to 950°C and at 1100°C hardness still remained at approximately 30 Gpa.

[1] Keckes J, Daniel R, Mitterer C, Matko I, Sartory B, Köpf A, et al. (2013), Self-organized periodic soft-hard nanolamellae in polycrystalline TiAlN thin films, Thin Solid Films 545, 29-32.

[2] Todt J, Pitonak R, Köpf A, Weißenbacher R, Sartory B, Burghammer M, et al. (2014), Superior oxidation resistance, mechanical properties and residual stresses of an Al-rich nanolamellar Ti_0.05Al_0.95N coating prepared by CVD, Surf Coat Techn 258, 1119-1127.