ICMCTF2015 Session E2-2: Mechanical Properties and Adhesion

Environmental barrier coating (EBC) coated ceramic matrix composite (CMC) systems are currently being investigated for use as turbine engine hot-section components in extreme environments. It therefore becomes critical to understand their response to environmental exposure and performance under thermo-mechanical loading conditions. Electrical resistance (ER) monitoring has recently been correlated to tensile damage accumulation in SiC_f/SiC CMCs, and the focus of this study is to extend the use of ER to evaluate high-temperature, thermal gradient fracture of EBC/CMC systems. Tensile strength tests were performed at high temperature (1200° C) using a laser-based heat flux technique. Specimens included as-produced SiC_f/SiC CMCs and coated SiC_f/SiC substrates that had been exposed to simulated combustion environments in a high-pressure burner rig. Localized stress-dependent damage was determined using acoustic emission (AE) monitoring and compared to full-field strain mapping using a high-temperature digital image correlation (DIC) technique. The results of which are compared with in-situ ER monitoring, and post-test inspection of the samples in order to correlate ER response to damage evolution.

This is a test of the sub/superscript combination:

J_Ti²⁺/J_Ti⁺

Nanoscale multilayers, made up of alternating layers of two materials with layer thickness in the nm range, have been the subject of an increasing number of studies in the past 10 years due to their exceptional high strength at room temperature. Their unique properties are mainly a result of the high density of interfaces, which change the standard mechanisms of plastic deformation and fracture, when the layer thickness is below ~100 nm. However, little is known about their anisotropic mechanical behavior due to the lack of appropriate testing techniques adequate for thin films up to date. With the development of novel nanomechanical testing techniques, like micropillar compression, it is not feasible to test the mechanical response of nanolaminated thin-films under uniaxial testing as a function of the orientation between the loading axis and the layers. In this talk, we will show the very different mechanical response displayed by Al/SiC multilayers, when compression is applied parallel, perpendicular and at 45º with respect to the layer direction, at temperatures between RT and 150ºC. The results reveal that the flow stress of the Al layers, which is layer thickness dependent, and the interface strength play a major role on the overall strength of the nanolaminate, when compression is applied perpendicular to the layers. However, the development of significant compressive stresses parallel to the layers triggers the formation of shear bands. The role of layer waviness on the shear band formation will be discussed.

Metal thin films are using as capacitance switches in microelectromechanical systems (MEMS). Problems with long-term reliability, however, set limits on the lifetimes of MEMS capacitance switch applications. The better the thin films can resist relaxation, the longer the lifetime of the MEMS device. Al thin films have been used as capacitance switches, but they have only a weak resistance to relaxation and therefore a shorter lifetime. Solid strengthening can improve the mechanism of the material without decreasing other mechanisms. We added Mg to Al thin films in order to increase the stress relaxation resistance. The viscoelastic behavior of pure aluminum thin film in comparison with thin films of 12.63 ％ Mg and 16.30 ％ Mg are investigated by using bulge testing. Adding Mg in pure Al thin films significantly decrease the relaxation behavior and increase the mechanism of thin film. The result shows the more Mg content in Al thin films the more resistance of thin films. The results of experiments show the normalize modulus decreasing less with a greater content of Mg. This result proves that adding more different atoms can obstruct the movement of dislocation and enhance the mechanism of Al thin films. It shows that Al-Mg thin films have better relaxation resistance than Al thin films and thus serve as a better material for capacitance switches.

It is impossible to determine the intrinsic stresses of a coated/uncoated surface by normal indentation measurements without a reference sample having known intrinsic stresses, because such experiments do not provide enough linear independent information to calculate several unknowns. Therefore, multi-axial indentation measurements, namely a lateral-force indentation which registers load and displacement in normal and tangential direction to the sample surface, in combination with a physical analysis of these experiments (lateral-force indentation analysis) [1] taking the layered structure into account [2] will be used to determine the intrinsic stresses in the coatings (regardless of their origin like thermally or mechanically induced) in terms of biaxial normal stresses. In this work, the experimental and the analysis procedure of this combined method will not only be introduced, but also demonstrated taking two samples of ta-C coatings on Si with different stress states.

The superhard tetrahedral amorphous carbon (ta-C) films with low internal stress were prepared by a combination of pulsed laser deposition [3] and pulsed laser annealing [4] on silicon (100) substrates. The 500 nm thick ta-C films were found to have up to 85% sp3 bonds and a hardness of up to 83 GPa.

By means of the laser pulse annealing (tempering), developed at the laser institute of the University of Applied Sciences Mittweida, it is feasible to set the intrinsic stress of the resulting layers to a defined value or even zero. The intrinsic stress results of the ta-C films of both samples determined by lateral-force indentation analysis have been verified by comparison with results derived from substrate bending using Stoney’s formula. Finally, a critical discussion of both methods for determination of intrinsic stress will be part of this work.

[1] N. Schwarzer, J. Mater. Res., vol. 24, no. 03, pp. 1032–1036, 2009.

[2] N. Schwarzer, J. Tribol., vol. 122, no. 4, pp. 672–681, 2000.

[3] G. Reiße et al., patent DE10319205 A1, 11-Nov-2004.

[4] G. Reiße et al., patent DE10319206 B4, 14-Aug-2013.

Most engineering materials have complex microstructures that can affect their properties in various ways. Metals are usually polycrystalline, and their inherently heterogeneous crystallographic nature can produce strong variations in deformation behavior at the grain (i.e. micron) scale. In small components, or when deformations are localized by defects or intentional geometry (e.g. holes or fillets), the details of grain-scale deformation can dictate the material's performance. In this work, we used micron-scale digital image correlation (μDIC), electron backscatter diffraction (EBSD), and finite element analysis to measure and predict, respectively, the evolution of surface strains and crystallographic orientations during the tensile deformation of columnar tantalum multicrystals containing only a few grains in the gauge section. These measurements are compared to crystal plasticity finite element simulations of the subgrain surface strain fields, and the predictions provide an accurate estimate of the location of failure initiation. We will outline the μDIC, EBSD, and crystal plasticity finite element methods; describe the procedure by which large-grained tantalum multicrystals were fabricated; and discuss the validation of the simulations' predictions against the experimental data.

The analysis of deformation and failure mechanisms in small-scale devices and thin films is a critical issue, not yet solved.

In this presentation, we describe the recently proposed indentation pillar splitting test for the analysis of toughness in a series of thin film materials.

Cohesive finite element simulations are used for analysis and development of a simple relationship between the critical load at failure, pillar radius, and fracture toughness for a given material. A series of additional simulations were performed in order to investigate the effect of the ratio between the elastic modulus and the yield strength on the critical load for pillar splitting. An explanation for the load instability during pillar’s indentation and the correlation with indentation cracking on homogeneous materials are then given.

Micro-pillars are then produced by Focused Ion Beam (FIB) ring milling, being the pillar diameter approximately equal to its length; this ensures full relaxation of pre-existing residual stress in the upper portion of the specimen. Nanoindentation splitting tests are performed in-situ and the deformation mechanisms corresponding to each class of materials have been investigated.

Obtained results on ceramics compare well with the independent measurements obtained by other techniques on the same samples.

The limitations of the method are finally discussed. In particular, a minimum pillar’s diameter for the nucleation and growth of a crack during indentation is identified and quantified for a wide range of materials properties. The end result is that testing of a metallic or polymeric material would require loads and pillar diameters that are ~5 orders of magnitude greater than for ceramic materials. Finally, the influence of substrate’s stiffness of splitting load is described, and proper corrections are proposed.

In this presentation, we report on the plastic deformation mechanisms in lamellar nanocomposites processed via Severe Plastic Deformation as a function of decreasing layer thickness. We process bulk Cu-Nb nanolamellar composites from 1 mm thick polycrystalline sheet down to layer thicknesses of 10 nm using Accumulative Roll Bonding. This technique has the advantage of producing bulk quantities of nanocomposite material, and can result in rolling textures, interfacial defect structures, and deformation mechanisms very different from those seen in nanolamellar composites grown via Physical Vapor Deposition methods. Mechanical behavior as evaluated via micropillar compression, nanoindentation, and bulk tension/compression will be discussed in terms of the effects of interfacial structure and content on deformation processes at diminishing length scales, and defect/interface interactions at the atomic scale. This class of materials has also been shown to exhibit radiation damage tolerance under ion irradiation at elevated temperatures. Radiation damage tolerance of nanomaterial containing different types of interfaces (twin, grain boundaries, and heterophase interfaces) will be presented, as well as a novel spherical nanoindentation test technique for quantifying the effects of ion irradiation on the stress-strain response of metallic materials.

In indentation tests, the material response to the loading and unloading cycle is commonly used to evaluate the mechanical properties of single and multiple layer coatings. For example, in addition to hardness and elastic modulus, the failure behavior of these materials can be analyzed based on specific signatures on the load-displacement (P-h) indentation curves. In this work, experimental and numerical methods were applied to analyze the failure behavior of single and multi-layer coatings deposited on compliant substrates. A recent publication defined a technological parameter (F₁), which indicates the possibility to observe the nucleation and propagation of cohesive cracks in single-layer coatings under indentation loads. This work further explores this parameter with experimental nano-indentation tests and expands the parameter definition to include characteristics of multilayer coatings. Two sets of multilayer coatings (different number, mechanical properties and thicknesses of each layer) were produced and experimentally evaluated. The numerical model was based on a rigid indenter in contact with the coated compliant substrate. The mechanical behavior of the coating layers was based on the properties of brittle elastic materials, while the substrate was assumed as a ductile elastic-perfectly plastic material. Both cohesive and adhesive failure models were included in the analyses, allowing the evaluation of failure modes in the coating and/or at discrete interfaces (between layers and between the coating and the substrate). Results allowed not only an experimental analysis on the parameter F₁ for single layer systems, but also the identification of the relevant variables to define the threshold of cohesive failure in multilayer systems.