ICMCTF2014 Session E2-3: Mechanical Properties and Adhesion

MCrAlYs (where M = Ni/Co) are used throughout gas turbine engines, as oxidation resistant coatings, bond coats for thermal barrier coatings and more recently, to hold abrasive particles onto the tip of shroudless turbine blades. In the latter; the MCrAlY must have sufficient strength to hold the abrasive particles in place as the abrasive tip cuts a track in the abradable on the casing. The temperatures that these abrasive tips see are some of the hottest within the engine, and they are not protected by thermal barrier coatings. Existing work on MCrAlYs extends up to 1050 ^oC, although the abrasive tips may have to withstand temperatures higher than this.

In this paper we present a method to study the mechanical properties of thin free standing MCrAlYs at temperatures as high as 1200 ^oC. This method avoids the use of large specimens or substrate coatings couples, so that the results are representative of the materials that are seen within the engine. Coatings of different compositions and different fabrication techniques have been characterised and correlated with the microstructures of the MCrAlY.

In order to evaluate the performance of hard coating materials used in aggressive cutting operations such as end milling or high speed machining, it is necessary to investigate the materials properties at the high temperatures generated in service. However, the small length scale of the coatings limits the utility of conventional high temperature techniques. The recent extension of nanoscale techniques such as nanoindentation and nanoimpact to elevated temperatures has recently made this a possibility, and recent work using these techniques has demonstrated the utility of these techniques [1, 2]. The development of vacuum techniques for high temperature nanoindentation [3, 4] has recently also allowed measurement at temperatures where oxidation of the indenter or sample may be problematic.

Micropillar compression at high temperature offers several attractive advantages over hardness testing [5]. Indentation is highly dependent on tip geometry for accurate measurements, and indenter geometry variation due to erosion by oxidation and abrasive wear or plastic deformation of the tip is a serious concern at high temperatures. Using a flat punch indenter avoids any such geometric variation. Also, the micropillar geometry offers a uniaxial stress state in contrast to the triaxial stress state of indentation. This provides a direct measurement of the yield stress.

The elevated temperature performance of a wide range of Chromium Nitride-based hard coatings was evaluated using in situ micro-compression at temperatures up to 500°C. This allows the first direct measurement of the uniaxial high temperature yield strength, rather than the hardness, of such coatings. The microstructure of the coatings was analysed using X-ray diffraction, Raman spectroscopy and focused ion beam cross sectioning followed by electron microscopy. Micropillars were examined using electron microscopy before and after compression. Trends in deformation behaviour and yield stress with temperature are discussed in relation to the coatings’ microstructures.

References

[1] G.S. Fox-Rabinovich, J.L. Endrino, B.D. Beake, M.H. Aguirre, S.C. Veldhuis, D.T. Quinto, C.E. Bauer, A.I. Kovalev, A. Gray, Surface and Coatings Technology, 202 (2008) 2985-2992.

[2] B.D. Beake, V.M. Vishnyakov, J.S. Colligon, Journal of Physics D: Applied Physics, 44 (2011).

[3] S. Korte, R.J. Stearn, J.M. Wheeler, W.J. Clegg, Journal of Materials Research, 27 (2011) 167-176.

[4] J.M. Wheeler, J. Michler, Review of Scientific Instruments, 84 (2013) 064303.

[5] S. Korte, W.J. Clegg, Scripta Materialia, 60 (2009) 807-810.

This study uses a temperature controlled capacitance-based system to measure the mechanical behaviors associated with temperature dependent energy loss in ultra-thin metal films. Cu, Al and Ag thin film are widely used in electronic interconnections and MEMS structures; however, most studies have focused on their temperature dependent mechanical properties at larger scales. This study designed a paddle-like test specimen with metal films deposited on the upper surface in order to investigate the in-situ temperature dependent mechanical properties of metal thin films at elevated temperature up to 200C under high vacuum conditions at very small scales. In-Situ Energy loss was measured according to decay in the oscillation amplitude of a vibrating structure following resonant excitation. Film thickness and grain size were closely controlled with respect to the mechanical properties of the films. We also determined that the temperature dependent internal friction of thin and ultra-thin metal films is less strongly dependent on film thickness than on grain boundaries during annealing.

Gold coatings that are ideally suited for low electrical contact resistance (ECR) applications are mechanically soft and exhibit unacceptable amounts of adhesion and friction. To mitigate these problems gold for ECR applications is typically alloyed with Ni, Co or Fe which increases the film hardness and wear resistance. A key limitation of hard gold coatings is the propensity for the non-noble alloying metal species to diffuse to the surface and form non-conductive oxide films that can severely impact the electrical contact behavior. These traditional hard gold films, which are fabricated via electro-deposition, have been limited to electrochemical compatible materials. Using co-deposition by physical vapor deposition (PVD) methods we have eliminated the material limitations and generated a new class of hard gold thin films. We have synthesized a thin film via PVD co-deposition comprised of gold and 0.1 – 2 vol% ZnO zinc oxide using a Triad e-beam evaporation system with capability for co-depositing three elements or compounds,. The ceramic phase is used to strengthen the composite via grain refinement. The resulting film can replace typical hard gold films and exhibits enhanced thermal stability as the ceramic phase has no thermodynamic driving force to migrate the surface. The results from accelerated aging studies and the role of ceramic inclusions on the stability of these nano-grain structures will be discussed.

* Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000

Complex in-plane strain paths are often applied to thin films during elaboration and use processes. Hence, the elastic-plastic-failure behaviour of the composite metallic film - polymeric substrate need to be investigated both under equi-biaxial and non-equi-biaxial loading conditions. This paper reports on the mechanical behaviour of nanostructured metallic thin films deposited on a polyimide substrate under controlled biaxial loadings thanks to a biaxial testing device developed on DiffAbs beamline at SOLEIL synchrotron (France). The in-situ tensile tests were carried out combining synchrotron X-ray diffraction (XRD) and digital-image correlation (DIC) techniques. The two techniques can accurately measure deformations at two different scales, namely the in- grain scale or lattice strain and the macroscopic scale. In the elastic domain, our results show that the two strain measurements, i.e. lattice strain in the crystalline part of the metallic components of the film measured by XRD and macroscopic strain in the substrate measured by DIC, match to within 1x10^-4. This result clearly demonstrates that the applied strain in the elastic domain is transmitted unchanged through the metal-polymer interface. The elastic limit of the nanostructured metallic composite (W/Cu) thin films was determined at the bifurcation point between the XRD lattice strain and the DIC macroscopic strain. Such an experimental combination allows to determine the yield surface of a polycrystalline thin film deposited on a stretchable substrate, and to scrutinize their mechanical behavior. As for example, the results show the brittle behavior of the W/Cu nanocomposite film.

In this study, tool steel surfaces are nitrided by primarily nitrogen atoms that maintained the as-finished surface conditions of the specimen. The nitriding of the tool steels was performed in electron beam excited plasma specifically geared to differentiate and use the neutral nitrogen atoms. For comparison, the charged nitrogen ions are used separately. Nitrogen concentrations of the treated specimen were varied in both neutral (atom) and ion nitriding to study the effect on the surface properties of the tool steels.

The results of our experiments show that in neutral nitriding the tool steel specimen for 6 hours, a mirror finish surface (Ra=14nm) with a diffusion layer of up to 80 µm and a surface hardness of more than two times (1300 Hv) that of the untreated specimen (600 Hv) were produced. On the other hand, in ion nitriding the tool steel specimen for 6 hours, surface finish of Ra=83 nm with a diffusion layer of up to 95 µm, a compound layer of about 7 µm and a surface hardness of 1350 Hv were produced. The obvious difference was that the mirror surface finish was retained on the specimen treated by neutral nitriding while a rough compound layer was formed on the specimen surfaces treated by ion nitriding. In consideration of the above results, neutral nitriding could be a good candidate for duplex treatment of cutting tools, punch and dies and precision mechanical components that require high hardness and wear resistance without altering the as-finished dimensional accuracy, surface roughness and appearance.