ICMCTF 2023 Session E2-1-ThM: Mechanical Properties and Adhesion I

Optical coatings are an enabling technology in the space industry for a variety of applications requiring high reflectance, transmittance, spectral selectivity, or thermal control. Traditional multilayered optical coatings can be designed to achieve virtually any reflectance or transmittance characteristics by tuning layer thickness, material selection, and layer material properties. For space applications, additional multifunctional performance requirements must be considered, including, but not limited to, low residual stress, good durability and adhesion in extreme environments, and tuning of surface structure. The space environment brings increasing demands on the performance of optical coatings and materials, and as such, requires an understanding of the mechanisms leading to maximized multifunctional properties.

In this talk, a discussion on adhesion, stress, and interfaces for optical films for space applications will be presented, with two case studies for residual stress and mechanical properties to follow. The first study will explore multifunctional properties of antireflection (AR) coatings and evaluation of the structure-property relationship in the context of interface character and layer microstructure in crystalline/amorphous and amorphous/amorphous multilayers. Characterization by spectrophotometry, transmission electron microscopy, and nanoindentation showed substantial variations in microstructure and film properties with tuning of layer configurations for improved transmittance. The second case study presents microstructural tailoring in highly reflective, mirrored films for improved surface scatter and residual stress performance. Thick (>1 µm) optical coatings with favorable optical properties are desired in space to protect sensitive substrates, such as radiation sensitive glass. However, traditional high reflectivity optical films, such as Ag, exhibit a degradation in optical properties due to increases in surface roughness with increasing film thickness. To circumvent these traditional limitations, the role of alloying, microstructural progression, and in-situ and ex-situ residual stress evolution is explored.

In summary, the design and fabrication of multifunctional optical coatings for space requires insight into the role of residual stresses, interfaces, delamination behavior, and microstructure, all of which inform the process-structure-property relationship in optical films.

Flexible and wearable electronics use metal films on polymer substrates. Here, the adhesion energy of these films to the substrates determines device performance. The interface between film and substrate is critical to adhesion energy. In this work, tensile induced delamination is used to quantify the adhesion energy of various Mo-based films on Polyimide, a common electrode material in thin film transistor displays. The method uses uniaxial straining to cause fracture of the film perpendicular and film delamination parallel to the tensile loading. Three different approaches to modify the interface to alter the adhesion energy will be compared, namely (i) the use of metallic interlayers between Mo and Polyimide (Al, Ta), (ii) the use of alloy interlayers between Mo and Polyimide (MoAl, MoTa), (iii) and the use of an alloyed Mo film (MoAl, MoTa) without an interlayer. Experiments on the different samples were performed with in-situ resistance measurements, confocal laser scanning microscopy (CLSM), and X-ray diffraction (XRD). CLSM allows for the quantification of crack density and delamination dimensions necessary for the adhesion measurement, while XRD provides film stress evolution to understand fracture and delamination mechanisms. The main aspects presented will be the adhesion energy along with the crack and stress evolution under uniaxial tensile loading. Results show that only the use of metallic interlayers between Mo and Polyimide (Al, Ta) provide a difference in the crack and delamination densities as well as a significant increase in the calculated adhesion energy compared to any of the other modification strategies.

3D printed artefacts are becoming more common and the effect of printing parameters on their properties is key to their performance in applications. Although parameters like build orientation and raster direction are well-studied the effect of layer thickness (and its introduction of sizeeffects into the properties of the material) is less well-known. This study determines the influence of layer thickness on the mechanical properties of polylactic acid (PLA), acrylonitrile butadiene styrene (ABS) and PETG (Polyethlene terephtalate Glycol) 3D printed specimens made with fused filament fabrication (FFF). Samples were printed with differing layer thickness and tensile tested according to ASTM D638. The study also found that when increasing the layer thickness the mechanical properties of the specimens for both ABS and PLA decreased. When it came to ultimate tensile strength, the effect of layer thickness on PLA was more significant than on ABS with PETG being in between. The differences were attributed to differences in additive layer adhesion and the effect of the structure and defects introduced by the additive layerprocess.

Nanoindentation and microbending testing were used to investigate the mechanical properties of Zr(–Hf)–Cu thin-film alloys prepared by nonreactive magnetron co-sputtering. A detailed analysis of nanoindentation data and microscopic images of indents allowed a more precise determination of the effective Young’s modulus of the films thanks to taking the pile-up effect into account.

Microbending testing in a scanning electron microscope was performed with microcantilevers fabricated by focused ion beam and the data were evaluated using a finite element method model. As outputs of this elasto-plastic model, Young’s modulus, yield strength, elastic strain, apparent yield point and approximate ultimate strength and strain of the films were determined.

From a material point of view, the effect of elemental composition (Cu content and Hf substitution) and structure (glassy and crystalline) was investigated and discussed. It was shown that the substitution of Hf for Zr has less pronounced effect on the mechanical properties than the increase in the Cu content in the films that leads to a pronounced increase in the hardness, Young’s modulus, elastic strain, yield strength, apparent yield point and ultimate strength but also to a decrease in the plastic parameter k and ultimate strain. Furthermore, a different atomic ordering in the crystalline and glassy Zr–Cu films of identical elemental composition results in differences in their mechanical properties and deformation behavior. The crystalline film was observed to be harder and stiffer with approximately the same elastic strain but higher yield strength and its plastic deformation was free of shear bands events.

[1] Haviar S., Kozák T. et al. Nanoindentation and microbending analyses of glassy and crystalline Zr(–Hf)–Cu thin-film alloys. Surf. and Coat. Technol. 399 (2020) doi: 10.1016/j.surfcoat.2020.126139

The medium-entropy alloys (MEAs) or high-entropy alloys (HEAs) have many superior properties, including high strength, high ductility, corrosion resistance, oxidation resistance, and high temperature resistance, which can be used in many applications. In addition, nanotwin (NT) structure plays an important role on strengthening mechanism in the microscopic scale because of its special properties, such as great thermal stability, high strain rate sensitivity, low electrical resistivity, and increased mechanical strength. In this study, we combined HEA and nanotwin structure to fabricate NT-MEA thin films and investigated its mechanical properties at different temperature ranges. At first, the NT CoCrFeNi MEA films were fabricated by pulsed DC magnetron sputtering system on Si substrate. The hardness of nanotwinned CoCrFeNi medium entropy alloy films was examined by nanoindentation at varied temperatures from -80 ℃ to 300 ℃. At the same time, the microstructures of the indentation regions were studied by transmission electron microscope (TEM). As-deposited CoCrFeNi medium entropy alloys had a columnar grain structure containing high-density nanoscale growth twins parallel to the substrate with an average twin thickness of 2.4 nm. The results show that NT-MEA was observed to have remarkably high strength, outperforming many other bulk HEAs, MEAs, nanocrystalline HEAs, and conventional NT-metals in the temperature range from -80 °C to 300 °C. The maximum hardness of NT-MEA was measured with a value of 11.3 GPa at cryogenic environment of -80°C and its hardness slightly decreased to 9.4 GPa when temperature increased from -80 ℃ to 300 ℃. Meanwhile, neither cracks nor fractures were observed in all temperatures, suggesting that the NT-MEA was still ductile material, especially in the cryogenic environment. The corresponding deformation mechanism at different temperature ranges would be discussed in details in this talk.

Amniotic eggs emerged over 300 million years ago in tetrapod vertebrates and are now the primary reproductive mode of all terrestrial amniotes. Unlike the simpler, shell-less eggs of amphibians, amniotic eggs are covered with shells, which allow the eggs to develop fully on land without drying out and thus open up new terrestrial habitats for amniotes, including birds and egg-laying reptiles. In birds, eggshells are multifunctional thin-walled structures that protect the embryo from excessive water loss, provide calcium, and sustain the weight of incubating birds. As a load-bearing structure, the eggshell must be strong enough to resist deformation and impact. Meanwhile, it must be breakable for the hatchling to emerge. Those are mechanical design trade-offs that must be adequately balanced. Here, we use combined experimental, numerical, and theoretical methods to analyze the morphology, material properties, and mechanics of 700 freshly-laid bird eggs from 58 species across three orders of magnitude in egg mass (from 1 g to 1459 g). We characterize the mineral content by acid-base titration, the crystallographic characteristics by electron backscatter diffraction (EBSD), and effective Young’s modulus E by compression test and finite-element analysis (FEA). We find that the mineral content is positively correlated with E, with the lowest of 83.1% and 23.28 GPa in Zebra finch and the highest of 96.5% and 47.76 GPa in ostrich in this study. The EBSD shows that eggshell is anisotropic and non-homogeneous, with different species exhibiting different degrees of crystal orientation and texture. The experimental results are consistent with the nanoindentation test and theoretical prediction of linear elasticity. This study provides new data and insights into the material properties and mechanics of eggshells and provides clues for the future development of bioinspired multifunctional thin-walled shells.

In this study, we fabricated the porous aluminum oxide (Al₂O₃) coatings on 6061 aluminum alloy by using plasma electrolytic oxidation (PEO) in alkaline sodium silicate solution under bipolar pulsed power mode with maxima anodic voltage at 500V, maxima cathodic voltage at 100V, duty cycle of 25%, pulse frequency in 1000 Hz, anodic current in 2A and cathodic current in 1~5A.The effects of cathodic current on microstructural, mechanical and corrosion behaviors of PEO coatings on 6061 aluminum alloy were investigated by X-ray diffractometer (XRD), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), scratch test, pin-on-disk wear test and potentiodynamic polarization measurement. The XRD results show that the PEO coatings are mainly composed of a-Al₂O₃ and γ-Al₂O₃. However, the ratios of a-Al₂O₃ and γ-Al₂O₃ in PEO coatings are identical with various cathodic currents applied. The SEM images display that the thickness of PEO coating is increased with increasing cathodic current from 11mm (1A) to 17mm (5A) and surface porosity of PEO coating is decreased with increasing cathodic current from 16.2% (1A) to 9.4% (5A). The potentiodynamic polarization curves show that the PEO coatings, as compared to 6061 aluminum alloy, exhibit higher corrosion resistances in 3.5 wt% NaCl solution. However, the corrosion current density (Icorr) and passivation current density (Ipass) for PEO coatings are decreasing with increasing cathodic current applied. In summary, the thickness, porosity and corrosion resistance of PEO coatings on 6061 aluminum alloy can be significantly improved by controlling cathodic current applied.