ICMCTF2010 Session B6-1: Hard and Multifunctional Nano-Structured Coatings

Monday, April 26, 2010 1:30 PM in Room Town & Country
Monday Afternoon

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Start Invited? Item
1:30 PM B6-1-1 Morphological and Structural Development of AlTiN Coatings Deposited by Cathodic Vacuum Arc
Joerg Vetter, Juergen Mueller (Sulzer Metaplas GmbH, Germany); Jon Andersson, Jacob Jolen, Lennart Karlsson (Seco Tools AB, Germany); Jones Alami, Georg Erkens (Sulzer Metaplas GmbH, Germany)

Coating structure and morphology often play a dominating rule in applications (i.e. cutting tools, components). The morphological and structural development of AlTiN coatings deposited by cathodic vacuum arc are high lightened. The observed morphologies include the fine columnar growth and the super fine growth. Deposition conditions (pressure, bias, voltage, temperature) are influencing both the coating morphology and the phase composition, e.g. the existence of a minor hcp phase for coatings with Al/Ti-content larger than 1. It will be shown that the Al content in the coating and the type of evaporators influence the coating growth. Experimental results will be shown for different Al/Ti compositions at different growth conditions. Experimental methods used were X-ray investigations and SEM. The possibility to generate super fine growth will be discussed.

1:50 PM Invited B6-1-3 New Generation Nanscale Multilayer Coatings to Serve High Temperature, Corrosion and Tribological Applications Deposited by HIPIMS
Papken Hovsepian, Arutiun Ehiasarian (Sheffield Hallam University, United Kingdom); Reinhold Braun (DLR-German Aerospace Centre, Germany)

Oxidation resistant and wear resistant coatings for environmental protection of light weight materials such as gama-TiAl alloys for future applications in aero and automotive engines are in high demand. CrAlYN/CrN coatings utilising nanoscale multilayer structure with a typical bi-layer thickness of 4.2nm were successfully produced on large scale by utilising the High Power Impulse Magnetron Sputtering (HIPIMS) technology. HIPIMS was employed for surface pre-treatment as well as for coating deposition. The surface pre-treatment was carried out by bombardment with Cr+ ions. For comparison CrAlYN/CrN was deposited by Unbalanced Magnetron Sputtering, (UBM) as well.

Scanning Transmission Electron Microscopy, (STEM) revealed that the coating/substrate interface was extremely clean and sharp. Large areas of coating grown epitaxially were observed. STEM-Energy Dispersive Spectroscopy (EDS) profile analysis further showed that during the HIPIMS ion bombardment Cr had been implanted into the substrate to a depth of 5 nm. HIPIMS deposited coatings showed extremely sharp interfaces between the individual layers in the nanolaminated material and almost no layer waviness. This improved structure resulted in further enhancement of the coatings barrier properties.

CrAlYN/CrN showed potential for reliable protection of gama-TiAl alloys against wear and aggressive environmental attack. For coated gama-TiAl alloys, thermo gravimetric quasi-isothermal oxidation tests in air at 7500 C after 2000 hours exposure showed four times smaller weight gain compared to the uncoated material. HIPIMS coatings were superior to UBM deposited coatings. Tested even at higher, 8500 C temperature the HIPIMS coatings showed by factor of 2 lower mass gain as compared to the UBM deposited coatings.

In sulphidation tests after 1000 hours exposure to aggressive H2 /H2S/H2O atmosphere the CrAlYN/CrN protected gama-TiAl alloys showed reduced weigh gain by factor of four as compared to the uncoated substrate. High temperature pin-on-disc tests revealed that CrAlYN/CrN reduces its friction coefficient from 0.56 at room temperature to 0.4 at 6500C, which demonstrates the excellent high temperature tribological behaviour of the coating. HIPIMS deposited coatings showed extremely low wear coefficient of KC= 1.83 E-17 m3 N-1m-1 at this temperature. Importantly HIPIMS deposited coatings retained the ultimate tensile strength of the gama-TiAl and reduced the fatigue strength only by 9%, compared to 20 % measured for the UBM deposited coatings, which opens perspectives for turbine blade application.

2:30 PM B6-1-5 Application Oriented Characterization of Nitride Coatings for Cutting Tools
Markus Lechthaler, Dominik De Gregorio, Michaela Raschke (OC Oerlikon Balzers AG, Liechtenstein)

The tribomechanical environment during cutting operations differs highly between machining techniques and cutting conditions. One important circumstance regards the temperature at the cutting edge, which varies between values close to room temperature for applications such as thread tapping and values higher than 1000°C for continuous cutting applications.

Generally, temperature dependent coating properties have to be considered in the characterization and later on in the selection of coatings for cutting tools. Mechanisms such as oxidation, phase transformation, recovery and chemical reactions in the contact area can occur. The measurement of the hardness at different temperatures is a characterization method which indicates presence of mentioned effects in form of changing the coating properties.

Hence, the hardness in relation to the temperature is discussed based on measurements which were conducted on TiAlN, AlCrN and alloyed AlCrN coatings deposited on polished cemented carbide (WC / 6% Co) substrates. The micro-hardness indents were performed under a non-oxidizing gas atmosphere at temperatures between 25°C and 1000°C. Furthermore, residual hardness after the annealing sequence was determined to distinguish between permanent and reversible transformations.

Finally, basic cutting tests serve to support the findings obtained in the hardness investigations and the results are discussed in combination with data from the literature.
2:50 PM B6-1-6 Theoretical Thermodynamics of Hard Coatings Materials - Beyond the Mean Field Approximation
Björn Alling (Linköping University, Sweden)
During the past few year, theoretical calculations from first-principles has provided important insights, increased understanding and fundamental physical explanations to structural, mechanical and thermodynamics properties of hard coatings materials. However, in order for theory to go beyond explanations, towards predictions, quantitative methods needs to be applied to account for temperature effects. In this work we present both difficulties and solutions to the application of state-of-the-art alloy theory to hard coatings materials. We show results for important systems such as TiAlN and discuss prospects and limitations of theoretical design of hard coatings materials in the future.
3:10 PM B6-1-7 Controllably Manipulating AlN Incorporation in HfxAl1-xN(001) Single-Crystal Thin films During Magnetically–Unbalanced Reactive Magnetron Sputter Deposition by Low-Energy (10 to 80 eV) Ion-Bombardment
Brandon Howe, Ernie Sammann, Jian-Guo Wen, Mauro Sardela, Timothy Spila, Joseph Greene (University of Illinois at Urbana-Champaign); Andrey Voevodin (Air Force Research Laboratory); Lars Hultman (Linköping University, Sweden); Ivan Petrov (University of Illinois at Urbana-Champaign)

We show that the AlN incorporation in single crystal Hf1-xAlxN(001) films can be controllably manipulated between ~ 0 and 100% by varying the ion energy (Ei) incident at the growing film over a narrow range, 10 – 40 eV. The layers are grown on MgO(001) at 450°C using ultrahigh-vacuum reactive magnetically-unbalanced magnetron sputtering from a single Hf/Al 70/30 (at. %) alloy target in 5% N2/Ar mixtures at a total pressure of 20 mTorr. The ion-to-metal flux ratio incident at the growing film is maintained constant at 8, while Ei is varied from 10 to 80eV. Film compositions vary from x = 0.3 with Ei = 10 eV to 0.27 with Ei = 20 eV, 0.17 with Ei = 30 eV, and ≤ 0.02 with Ei ≥ 40 eV. Thus, the AlN incorporation probability decreases by greater than two orders of magnitude! This extraordinary range in real-time manipulation of film chemistry during film deposition is due to the efficient resputtering of Al atoms (27 amu) by Ar ions (and fast Ar atoms, both 40 amu) scattered off heavy Hf atoms (178.5 amu) in the film. This effect can be used to grow planar heterostructures and superlattices with abrupt interfaces at high deposition rates from a single target by controllably switching Ei. The choice of Ei values determines the layer compositions and the switching speed controls the layer thickness. Here, we present the effects of film micro- and nanostructural properties on film hardness in Hf0.70Al0.30N/HfN superlattices with bilayer thicknesses ranging from 1 to 7 nm using high-resolution transmission electron microscopy (HR-TEM), HR-STEM, HR-XRD and nanoindentation.

3:30 PM Invited B6-1-8 Design Concepts for Superhard Nitride Thin Films; Superlattices, Solid Solutions, and Nanocomposites
Lars Hultman (Linköping University, Sweden)
Transition metal nitrides processed by PVD methods are strategic materials in advanced surface engineering applications. Recent findings of nanostructuring by design and self-organization in different compounds will be presented herein. Analysis of the materials (nanoindentation, XRD, TEM, 3D Atom Probe, et.c.) is coupled with ab initio calculations. The latter is used to assess the miscibility gap in the phase diagram of the MeAlN (Me = Ti, Sc, Zr, Hf) systems. This allows for the investigation of different explanations for phase stabilities or decomposition behavior such as lattice mismatch and electronic band structure effects. [1]

For TiN/NbN(001) superlattices, dislocation glide within the layers is the dominant deformation mechanism. That confirms the present models for superhardening that presumes plasticity and postulates dislocation hindering at interfaces between layers of different shear modulus. In consequence, these coatings also exhibit crystal rotation during deformation. [2]

Superhardening occurs in TiN/Si3N4 nanocomposites due to Si segregation forming a few-monolayer (ML)-thick SiNx tissue phase, which can be amorphous or crystalline. In the latter case, we show the existence of a cubic-SiNx layer that is epitaxially stabilized to TiN. A hardness maximum at 34 GPa is observed in TiN/SiNx(001) superlattices at the epitaxial break-down limit for the epitaxial SiNx layers of 2-5 ML, well above the percolation limit. [3]

Our concept of age hardening in supersaturated cubic-phase transition metal nitride alloy systems is presented for the pseudobinary MeAlN systems. Secondary phase transformation including spinodal decomposition of TiAlN into isostructural (cubic-phase) nm-size domains of TiN and AlN is thus demonstrated. [4] The as-formed domains hinder dislocation glide in films annealed to temperatures corresponding to cutting tool operations. The effects of pressure [5], temperature, composition, disorder, and stoichiometry (vacancies) will also be discussed.

[1] B. Alling, et al., Surf. Coat. Technol. 203 (2008) p. 883.

[2] L. Hultman, in Nanostructured Coatings, ed. A. Cavaleiro, et al, Plenum 2006, Ch. 13, pp. 539-552.

[3] L. Hultman et al., Phys. Rev. B75 (2007) p. 155437

[4] P.H. Mayrhofer, C. Mitterer, L. Hultman and H. Clemens, Progress in Mater. Sci., 51 (2006) pp. 1033-1114.

[5] B. Alling et al., Appl Phys. Lett. 95 (2009)p. 181906

4:10 PM B6-1-10 Epitaxial Growth of ZrAlN Alloy and Superlattice Thin Films: Mechanical Properties and Structural Characterization
Naureen Ghafoor, Jens Birch, Jens Jensen, Lars Hultman, Magnus Odén (Linköping University, Sweden)
Ternary transition metal nitrides are commonly used as wear resistant coatings on cutting tools. In particular, the metastable TiAlN has proven to be both oxidation resistance and exhibit age-hardening due to spinodal decomposition. The purpose of this work is to study the related, but less explored ZrAlN system for which ab initio calculations [1,2] and experiments [3,4] show a relatively large miscibility gap. In order to elucidate the solid-solubility limit and any phase transformation occurring during growth, we propose to study single-crystal ZrAlN films. In this work, Zr1-xAlxN and Zr1-xAxN/ZrN superlattice films with 0 ≤ x ≤ 1 were deposited on MgO(001), MgO(111), Al2O3(0001), and WC-Co substrates at Ts = 700-800°C using high vacuum dual cathode unbalanced magnetron sputtering in a mixture of Ar and N2. The substrates were kept at floating potential providing an ion assistance of ~18-25 eV. Ion beam analysis (ERDA and RBS) revealed that all films were stoichiometric with N/(Zr+Al)=1±0.02. High resolution X-ray diffractometry shows that high-quality B1-NaCl structure ZrN(001) films form on MgO(001) substrates. Breakdown of the epitaxial crystal into nanosize crystallites takes place at x>0.2 accompanied by a shift in the 200 diffraction peak towards low 2θ values along with a progressive broadening. Alloying with Al yields films with increasing hardness from~22 Gpa (x=0) to 27 Gpa (x=0.7). Zr1-xAlxN/ZrN superlattices with varying composition modulation periodicities show higher hardness compared to monolithic alloy films. Preliminary tests show hardness as high as 38 Gpa for a 1.5 μm thick multilayer film with a compositional modulation period of 30 nm.

[1] S. H. Sheng, R. F. Zhang, S. Veprek, Acta Mater. 56, 968 (2008).

[2] B. Alling, A. Karimi, I. A. Abrikosov. Surf. Coat. Technol., 203:883-886 (2008).

[3] R. Sanjinés, C. S. Sandu, R. Lamni, F. Lévy. Surf. Coat. Technol., 200:6308 (2006).

[4] L. Rogström, L. Johnson, M.P. Johansson, M. Ahlgren, L. Hultman, M. Odén, Age hardening in arc evaporated ZrAlN films, Submitted (2009).

4:30 PM B6-1-11 Effect of the Aluminium Contents and the Bias Voltage on the Microstructure Formation in the Ti1-xAlxN Protective Coatings
Christina Wüstefeld, David Rafaja, Volker Klemm (TU Bergakademie Freiberg, Germany); Martin Kathrein (CERATIZIT Austria GmbH, Austria); Claude Michotte (CERATIZIT Luxembourt S.a.r.l., Austria); Carsten Baehtz (Forschungszentrum Rossendorf, Germany)

Microstructure features like phase composition, crystallite size and lattice strain are usually related to the formation and mobility of microstructure defects and influence always the properties of protective hard coatings. Among the parameters of the deposition process, which are employed to adjust the microstructure of the Ti-Al-N coatings deposited by cathodic arc evaporation (CAE), the aluminium contents and the bias voltage play a crucial role. In this study, the influence of both parameters on selected microstructure parameters of the CAE Ti1-xAlxN coatings deposited from the Ti-Al cathodes was investigated by using X-ray diffraction and transmission electron microscopy.

The microstructure of the coatings was described in terms of the chemical and phase composition, local distribution of the phases, crystallite size and lattice strain, which results from the interaction between neighbouring crystallites and from the presence of the dominant microstructure defects. The microstructure model based on the knowledge of the above microstructure parameters allowed the microstructure formation during the deposition process to be explained and correlated to the parameters of the deposition process. The correlation between the microstructure formation and the parameters of the deposition process contributed substantially to the understanding of the effect of the aluminium contents and the bias voltage on the microstructure formation in CAE Ti1-xAlxN coatings.

This study was performed on coatings with the chemical compositions Ti0.60Al0.40N, Ti0.50Al0.50N and Ti0.40Al0.60N, which were deposited at the bias voltages (UB) ranging between -20 V to -120 V. Disregard the chemical composition, the coatings deposited at UB ≤-40 V contained face centred cubic (Ti,Al)N as a single phase. With increasing UB, Al segregated from the (Ti,Al)N and created AlN. The segregation of Al contributed to the reduction of the crystallite size and to the increase of the lattice strains in the coatings. Both these phenomena are related to the increase of the hardness. A departure from the monotonous reduction of the crystallite size and from the monotonous increase of the lattice strain was observed at the highest bias voltage (UB = -120 V).

The non-monotonous effect of the bias voltage on the phase composition, on the crystallite size and on the kind and density of the microstructure defects will be discussed with respect to several competing processes, which are related to the energy of impinging particles, to the mobility of the deposited species, to the formation, arrangement and mobility of microstructure defects and to the mechanical interaction of neighbouring crystallites.

4:50 PM B6-1-12 Microstructure and Characterization of Ni-Alloy/CrN Nanolayered Coatings with Various Bilayer Periods
Hao-Hsiung Huang, Fan-Bean Wu (National United University, Taiwan); Jeh-Wei Lee (Mingchi University of Technology, Taiwan)
In this study, the Ni-alloy/CrN nanolayered coatings were fabricated on (100) silicon wafer and 420 stainless substrates by dual-gun sputtering technique. The influence of the interlayer alloy coatings on microstructure, morphology, and corrosion behavior of the nanolayered thin films were investigated. The bilayer thickness was controlled approximately 10-20 nm with a total coating thickness of 1μm . The microstructure evolution of the NiAl, NiP and Ni-alloy/CrN coatings under various process temperatures were evaluated. Through phase identification, precipitation or transformation was observed in the Ni alloy layers. The period stacking and theoretical values of bilayer period were analysis by low-angle XRD diffraction technique. The nanolayered coatings showed a higher corrosion resistance as compared to single layer coatings. The corrosion mechanisms of the coatings were compared through AC impedance and equivalent electrical circuit results in the frequency range of 0.1Hz -10kHz. Through Tafel curves analysis, the corrosion resistance improved effectively by nanolayered structure was evident. Through nanoindentation analysis, the hardness and Young’s modulus were also improved by multilayer configuration. The correlation between microstructure evolution, corrosion and indentation behaviors was discussed.
5:10 PM B5-2-4 3D-Atom Probe Investigations of Ti-Al-N Thin Films
Richard Rachbauer, Stefan Massl, Erich Stergar (Montanuniversität Leoben, Austria); Peter Felfer (University of Sydney, Australia); Paul Mayrhofer (Montanuniversität Leoben, Austria)

State of the art three-dimensional atom probe tomography (3D-APT) enables the simultaneous investigation of morphology and chemical composition with near atomic resolution. Since spinodal decomposition in Ti1-xAlxN thin films results in the formation of a nanostructure and an increase in hardness at high temperatures, this process has attracted increasing interest and is in the focus of many research activities on the design and modification of chemical and phase-modulated nanostructured thin films.

We employ a local electrode 3D-atom probe in laser-mode to study the morphological and chemical development of Ti1-xAlxN coatings. The results obtained clearly indicate compositional fluctuations of Ti and Al atoms of cubic Ti0.46Al0.54N coatings, prepared by dc magnetron sputtering at 500 °C, already in the as-deposited state [1]. This is in good agreement to results reported for epitaxially grown Ti0.50Al0.50N coatings [2]. Vacuum annealing to 900 °C induces an increase in the compositional amplitude of the fluctuations and the formation of a 3D-interconnected network of cubic Ti- and Al-rich domains. Detailed 3D-APT studies exhibit diffuse domain boundaries, which highlight the spinodal character of the decomposition process. During vacuum annealing to 1450 °C the aluminium enriched domains phase-transform in the stable modification, hexagonal (B4) AlN, whereas the titanium enriched domains maintain the cubic (B1) structure while transforming into TiN.

Based on our studies, we can conclude that starting from an almost randomly distributed input of 22.9 at.% Ti, 26.4 at.% Al and 50.5 at.% N in the as-deposited single-phase cubic Ti0.46Al0.54N, the complexity of the system increases upon annealing to temperatures above 900 °C. The decomposition process into TiN and AlN via spinodally formed Ti- and Al-rich domains also causes a rearrangement of the initially randomly incorporated 0.13 at.% oxygen impurities. After decomposition, the oxygen is preferentially incorporated in the AlN phases. Consequently, 3D-APT studies will have a strong impact on future developments of super-saturated, chemically and phase-modulated hard coatings and thin films, especially involving investigations on the effect of minority trace elements.

[1] R. Rachbauer, E. Stergar, S. Massl, M. Moser, P.H. Mayrhofer, Scripta Materialia 61 (2009) 725-728.

[2] F. Adibi, I. Petrov, L. Hultman, U. Wahlström, T. Shimizu, D. McIntyre, J.E. Greene, J.E. Sundgren, J. Appl. Phys. 69 (1991) 6437-6450.
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