Coating Design and Architectures

Tuesday, April 30, 2013 8:00 AM in Room Royal Palm 1-3

Start Invited? Item
8:00 AM B6-1-1 Flakey Stuff: Pushing the Limits of Engineering Coatings with Layered Atomic Structures
Christopher Muratore (Air Force Research Laboratory, Materials and Manufacturing Directorate, Nanoelectronic Materials Branch, US); Samir Aouadi (Southern Illinois University, US); Jianjun Hu (UDRI/Air Force Research Laboratory, Materials and Manufacturing Directorate, Nanoelectronic Materials Branch, US); Andrey Voevodin (Air Force Research Laboratory, Materials and Manufacturing Directorate, Nanoelectronic Materials Branch, US)
For over 70 years, the sensitivity of solid lubricant materials to extreme ambient environments has limited aerospace capability. Solutions to this problem have taken decades to emerge because there is such a big difference between the operating environment in which the material needs to perform (e.g., at high temperature in air) and the environment in which structural and compositional analysis is conducted (e.g., ultra high vacuum at room temperature). I will talk about novel in situ techniques to examine the mechanical response of contact interfaces in extreme environments , and show how we have used these techniques to develop environmentally adaptive materials to overcome decades-old aerospace challenges. In the course of designing these materials, we have integrated features such as temperature-actuated self-healing and integrated wear sensors for automatic structural health monitoring. These features are built upon basic principles of thin film design. For example, all lubricant coating materials we have developed are nanocomposites. For the temperature-adaptive materials, a hard matrix surrounds nanoinclusions of the active lubricant materials to maximize crack resistance and hardness. The phases comprising these nanoinclusions, and their geometry and concentration are selected based on modeling results, so that complex phases providing low shear at specific temperatures are present in the friction contact. The thermal conductivity of the materials also adapts with temperature by almost an order of magnitude for an added measure of thermal protection for the substrate. Multilayers are also used to selectively inhibit or guide diffusion-based self-healing or lubricant delivery mechanisms. These diffusion barriers can give the coatings additional multifunctionality, such as wear sensing capability, where rare-earth dopants displaying distinctive luminescence spectra are integrated into the material to provide a high-intensity light-actuated response indicating the lifetime of the solid lubricant coating. All of these mechanisms can be combined in a single coating material to protect components from friction, wear and extreme temperatures over multiple environmental cycles with automatic health monitoring.
8:40 AM B6-1-3 Ab Initio and Experimental Study on the Effect of Si Additives on the Phase Stability of γ - and α-Al2O3
Farwah Nahif, Denis Music, Stanislav Mráz, Moritz to Baben, Jochen Schneider (RWTH Aachen University, Germany)

Using density functional theory and filtered cathodic arc deposition experiments the effect of Si addition on the stability and electronic structure of γ- and α-Al2O3 has been investigated. The concentration range from 0 to 5 at.-% was probed and the additives were positioned at different substitutional sites in the γ-phase. The calculations for (Al,Si)2O3 predict a trend towards spontaneous decomposition into α-/γ-Al2O3 and SiO2. Therefore, the formation of the metastable γ-(Al,Si)2O3 phase can only be expected during non-equilibrium processing where the decomposition is kinetically hindered. The Si induced changes in stability of the metastable solid solution may be understood based on the electronic structure. Si additions clearly shift the relative stability towards the γ-phase which can be understood based on strong silicon-oxygen bonds in γ-(Al,Si)2O3. These predictions were critically appraised by diffraction experiments of as deposited and annealed Si-Al-O coatings deposited by filtered cathodic arc. The effect of Si additives on the amorphous to γ- and the γ- to α-phase transition temperatures was determined. It was found that the addition of Si significantly increases the amorphous to γ- and the γ- to α-transition temperatures in comparison to unalloyed Al2O3.

9:00 AM B6-1-4 A Combinatorial Approach to the Synthesis of Cr-Zr Oxynitride Thin Films by Reactive r.f. Magnetron Sputter Deposition
Stefanie Spitz, Michael Stüber, Harald Leiste, Sven Ulrich (Karlsruhe Institute of Technology, Germany)

Cr-Zr-O-N thin films were deposited by reactive r.f. magnetron sputtering at 500 °C substrate temperature onto cemented carbide and silicon substrates. The depositions were realised by an experimental combinatorial approach: the samples were placed in a row below a segmented Cr-Zr target so that the coatings exhibit different elemental compositions (from Cr-rich to Zr-rich according to the position in relation to the target) and constitution. In order to investigate the influence of oxygen and nitrogen contents on the films’ structure, constitution and properties the O2/N2 gas flow ratios as well as the total reactive gas flows were varied systematically. The coatings were thoroughly characterised by electron probe micro analysis (EPMA), X-ray diffraction (XRD), transmission electron microscopy (TEM) and by microindentation.

A single-phase corundum-type structure was observed for Cr-rich coatings. The coatings` hardness could be increased up to 23.5 GPa by increasing the nitrogen flow while keeping the Ar and O2 flows constant (i.e. total pressure increase). Up to 5.2 at% nitrogen could be incorporated in these corundum-type coatings. By keeping the total pressure constant and varying the O2/N2 flow ratio, the increase in hardness was moderate; however, the deposition rate could be doubled. Up to 1.9 at% nitrogen could be incorporated into corundum type films. The conditions for the growth of single-phase corundum-type Cr-Zr-O-N coatings will be discussed in detail.

Furthermore, results of the Zr-rich coatings will be presented: a change in the microstructure with increasing N2 flow (to single-phase cubic or tetragonal structures) and hardness values up to 20 Gpa could be observed.

9:20 AM B6-1-5 Protective Coatings for Aerospace Applications: From Materials Architecture to Coating Removal
Jolanta Klemberg-Sapieha (École Polytechnique de Montreal, Canada)

Material damage caused by solid particle erosion remains a crucial problem in aeronautical engines. Different wear phenomena occur in various parts of the jet engine such as gas turbine components, compressor section, heat exchangers, pumps and piping systems. Good understanding of materials deterioration allows one to develop appropriate strategies to protect technologically relevant metal substrate materials. Advanced erosion-resistant coatings, ERC, call for an “ideal” combination of the mechanical elasto-plastic, tribological, corrosion, thermal and other characteristics that can only be satisfied by using specifically tailored coating architectures considering nanocomposite, nanolaminate, multilayer and graded layer systems.

In the first part of this presentation, we demonstrate that finite element modeling of the coating architecture, combined with the tailored mechanical properties of individual materials of the coating systems including appropriate stress management, opens new opportunities as a predictive tool for high performance ERCs. We then introduce and discuss the selection rules describing the overall film behaviour with respect to their microstructure and their basic elasto-plastic properties, namely their hardness, H, Young’s modulus, E, elastic strain-to-failure, resilience, and resistance to plastic deformation, expressed, respectively, by the H/E, H2/E, and H3/E2 ratios.

After many hours in service, certain areas of the ERCs may begin to deteriorate. Since the components of engine are generally very costly, it is desirable to remove the original coating, repair the parts if necessary, and then apply a new coating. In response to these needs, in the second part of this presentation, we will describe new processes for removing damaged ERCs including dry and wet chemical etching. Preliminary results will be discussed in relation to the technological, economic and environmental context in the field of surface engineering solutions for aerospace industry.

10:00 AM B6-1-7 Transformation Toughening as Applied to Coatings
Chen Wang (Northwestern Polytechnical University, China); Jie Han (Northwestern University, US); Julio Pureza (Universidade do Estado de Santa Catarina, Brazil); Yip-Wah Chung (Northwestern University, US)

The objective of this research is to explore new ways to synthesize hard and tough materials. In this work, we synthesized multilayer coatings consisting of alternating nanolayers of TiB2 and FeMnx. Nanocrystalline TiB2 is hard but also quite brittle. At sufficiently large values of x, FeMnx has a metastable fcc structure at room temperature. In the presence of a microcrack, the stress field due to the microcrack can cause a phase transformation of FeMnx from fcc to bcc, resulting in a volume expansion and consequent arresting of the propagating crack. This phenomenon was explored by synthesizing TiB2/ FeMnx multilayer coatings using a dc magnetron sputter-deposition system. Using x-ray diffraction, we proved that FeMn0.35 films acquire the metastable FCC structure at room temperature. We used the method developed by Xia, Curtin, and Sheldon1 to measure the fracture toughness of TiB2/ FeMn0.35 and TiB2/Fe multilayer coatings. These measurements showed that replacement of bcc-Fe by fcc-FeMn0.35 increases the fracture toughness by roughly a factor of two, thus substantiating the concept of using phase transformation for the toughening of hard coatings.

1Z. Xia, W. A. Curtin, B. W. Sheldon, Acta Materialia 52, 3507-3517 (2004)

10:20 AM B6-1-8 Limits to the Preparation of Super- and Ultrahard Nanocomposites
Stan Veprek, Maritza Veprek-Heijman (Technical University Munich, Germany)
We shall discuss the conditions needed for achieving hardness in excess of 50 or even 80 to ≥ 100 GPa in nc-TiN/a-Si3N4 and related nanocomposites with high elastic limit, and high oxidation resistance at elevated temperatures. It will be shown that achieving such properties may be limited by several constraints, such as lack of long-term stability of quasi-ternary nc-TiN/a-Si3N4/TiSi2 nanocomposites, too low deposition temperature and impurities, which hinder the diffusion and formation of strong nanostructure in the quasi-binary, long-term stable nc-TiN/a-Si3N4 system. Moreover, achieving super- and ultrahardness in other nc-TmN/a-Si3N4 nanocomposites (Tm=transition metal) can be further limited by absence of spinodal mechanism of the decomposition of the Tm-Si-N solid solution.
10:40 AM B6-1-9 A Study of TiAl - powder Metallurgical Target Behaviour in Direct Current and High Power Impulse Magnetron Sputtering PVD Processes
Szilard Kolozsvari, Peter Polcik (PLANSEE Composite Materials GmbH, Germany)
Aluminum-based coatings produced by means of physical vapour deposition (PVD) that provide improved functionality on machine tools are well investigated. There are many studies dealing with the characterization of these TiAl- or AlCr-based high wear and oxidation-resistant thin films, deposited by both magnetron sputtering and cathodic arc-evaporation. Quality and cost optimization issues in machining drive the need to improve coating properties by, for example, alloying transition metals to standard compositions, as well as to develop targets with longer life time. In the present work we concentrate on the manufacturing of high performance sputtering targets and the relationship to the coatings produced by them. Two different target types - segmented and powder metallurgical - and thus two different manufacturing processes are compared. The performance of these will be examined using direct current (d.c.) and high power impulse magnetrons sputtering processes (HiPIMS) with the aim to achieve a defined TiAlN coating. The mechanical properties of the coatings - hardness, elastic modulus, adhesion, crystal orientation and chemical composition – will be examined with respect to the physical behaviour of the targets, e.g., voltage/current characteristics as the targets erode. These characteristics will be examined for a range of operating parameters (bias voltage, gas ratios, etc.) as well as at different power densities. The connection between coating quality, manufacturing costs and target functionality will always be a critical concern to those in the industry. The selection of a proper target and quality allows the coating manufacturer to decrease costs while simultaneously increasing the target and coating performance.
11:00 AM B6-1-10 The Effect of Droplets in Arc Evaporated Hard Coatings on the Wear Behavior
Michael Tkadletz (Materials Center Leoben Forschung GmbH, Austria); Christian Mitterer (Montanuniversität Leoben, Austria); Bernhard Sartory (Materials Center Leoben Forschung GmbH, Austria); Claude Michotte (CERATIZIT Luxembourg S.àr.l., Luxembourg)

Hard coatings deposited by cathodic arc evaporation are often characterized by droplets, affecting their surface roughness and oxidation resistance. Within this work, the mechanical damage imposed by these droplets on the coating was investigated for dry sliding contacts. Ball-on-disk tests were done on TiAlN based coated cemented carbide discs at room temperature and 700°C. Surface as well as cross-sections of the wear tracks were investigated by scanning electron microscopy and focused ion beam (FIB) techniques. In wear tracks on areas without droplets, only evidence for mild abrasive wear of the coating could be found. The effects caused by droplets were examined by conventional FIB cuts as well as cut and slice techniques, energy dispersive X-ray spectroscopy mappings and transmission electron microscopy. While the area above these droplets is gradually worn away, tensile cracks are formed on the coating surface, tangentially surrounding the droplet in the area opposite to the sliding direction. In contrast, shear cracks are found in sliding direction, initiating at the droplet and propagating into the coating.

11:20 AM B6-1-11 In-situ Micro-fracture-test Investigations in the Influence of Structure and Phase Transformation of CrN/AlN Multilayer Coatings
Manfred Schloegl (Montanuniversität Leoben and Vienna university of Technology, Austria); Jörg Paulitsch (Vienna University of Technology, Austria); Jozef Keckes, Christoph Kirchlechner, Megan Chordill (Montanuniversität Leoben, Austria); Paul Mayrhofer (Vienna University of Technology, Austria)

Ceramic-like coatings are widely used for various industrial applications because of their outstanding properties like high thermal stability, oxidation resistance and abrasion resistance. However, the brittleness of such ceramic coatings often negatively influences their performance especially when used in conditions with an increased need for crack resistance. Therefore, this work is devoted to the study on fracture mechanisms of CrN based thin films with the aid of in-situ scanning electron microscopy (SEM) microbending, microcompression and microtension tests. The small test-specimens are prepared by focused ion beam milling. As generally monolithic coatings with their columnar structure provide low resistance against crack formation and propagation we perform our studies for CrN thin films and CrN/AlN multilayers. The latter offer alternating elastic constants and additional interfaces influencing crack propagation. Adjusting the AlN layer-thicknesses to ~3 and ~10 nm allows studying the impact of a cubic stabilized c-AlN layer and an AlN layer composed of cubic, amorphous and hexagonal fractions (for simplicity abbreviated with w-AlN) being sensitive to strain fields as suggested by ab initio calculations. The microtests clearly demonstrate that the monolithically grown CrN as well as the CrN/w-AlN multilayer coating with ~10 nm thin AlN layers (and hence a mixture of cubic, amorphous and hexagonal AlN phases) fail as soon as small cracks are initiated. Contrary, the CrN/c-AlN multilayer coatings composed of ~3 nm thin c-AlN layers are able to provide resistance against crack propagation. Hence, they allow for significantly higher loads during the tests. In-situ cross sectional scanning electron microscopy investigations during loading of our coatings clearly show the deflection of cracks within the CrN/c-AlN layers whereas no crack-deflection can be observed for the other coatings. Furthermore, in-situ micro-tensile-test investigations of coated polymer substrates exhibit the formation of a dense network of fine cracks within the CrN/c-AlN coatings. The other coatings exhibit fewer but more-open cracks. Consequently, the crack-propagation within the CrN/c-AlN coatings is more inhibited than within the CrN/w-AlN coatings. Additional studies on the structure and phase development of the CrN/c-AlN and CrN/w-AlN coatings ex-situ are conducted by HR-TEM studies. The study shows extensive in-situ fracture tests in a micro-scaled range providing necessary information on the fracture behavior of hard coatings.