ICMCTF2013 Session B6-1: Coating Design and Architectures

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.

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.

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.

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 FeMn_x. Nanocrystalline TiB₂ is hard but also quite brittle. At sufficiently large values of x, FeMn_x 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 FeMn_x from fcc to bcc, resulting in a volume expansion and consequent arresting of the propagating crack. This phenomenon was explored by synthesizing TiB₂/ FeMn_x multilayer coatings using a dc magnetron sputter-deposition system. Using x-ray diffraction, we proved that FeMn_0.35 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/ FeMn_0.35 and TiB₂/Fe multilayer coatings. These measurements showed that replacement of bcc-Fe by fcc-FeMn_0.35 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)

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.

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.