Cyclic degradation in flexible electronic inks remains a key challenge while their deployment in life critical applications is ongoing. The origin of electrical degradation of a screen-printed stretchable conductive ink with silver flakes embedded in a polyurethane binder is investigated under uniaxial monotonic and cyclic stretching, using in-situ confocal microscopy and scanning electron microscopy experiments, for varying ink thickness (1, 2, and 3 layers, each layer around 8–10 μm) and trace width (0.5, 1, and 2 mm). Cracks form under monotonic stretching, and the evolution of crack pattern (density, length and width) with applied strain is affected by ink thickness such that the 3-layer ink exhibits larger normalized resistance but slightly lower resistance than the 1-layer ink up to strains of 125%. For cyclic stretching, the crack density and length do not evolve with cycling. However, the cracks widen and deepen, leading to an increase in resistance with cycling. There exists a strong correlation between fatigue life, i.e. the number of cycles until a normalized resistance of 100 is reached, and the strain amplitude. The normalized resistance increase rate with respect to cycling is also found to scale with strain amplitude. The rate of change in resistance with cycling decreases with ink thickness and trace width. For practical applications, thicker ( ⩾ 25 μm) and wider (⩾2 mm) inks should be used to lower resistance increases with repeated deformation.

Understanding resistance increase in composite inks under monotonic and cyclic stretching

A series of Al and Al/Al2O3 thin‐film multilayer structures on flexible polymer substrates are fabricated with a unique deposition chamber combining magnetron sputtering (Al) and atomic layer deposition (ALD, Al2O3, nominal thickness 2.4–9.4 nm) without breaking vacuum and thoroughly characterized using transmission electron microscopy (TEM). The electromechanical behavior of the multilayers and Al reference films is investigated in tension with in situ X‐ray diffraction (XRD) and four‐point probe resistance measurements. All films exhibit excellent interfacial adhesion, with no delamination in the investigated strain range (12%). For the first time, an adhesion‐promoting naturally forming amorphous interlayer is confirmed for thin films sputter deposited onto polymers under laboratory conditions. The evolution of Al film stresses and electrical resistance reveal changes in the deformation behavior as a function of oxide thickness. Strengthening of Al is observed with increasing oxide thickness. Significant embrittlement can be avoided for oxide layer thicknesses ≤2.4 nm.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/adem.202200951

Electromechanical Behavior of Al/Al2O3 Multilayers on Flexible Substrates: Insights from In Situ Film Stress and Resistance Measurements

Internal nitriding behavior of Mo-15 at.% Ti and Mo- 5/15 at.% V alloys were compared in a temperature range from 700 °C to 1100 °C using Forming gas (95% N2 and 5% H2) for nitriding times of 10 h and 15 h. The variations of specimen weight, microstructure and hardness of nitrided layer as well as the developed nitriding products were investigated as a function of treatment temperature and time. VN and TiN nitrides with a minor amount of TiO2 oxide are formed on the surface of Mo-V and Mo-Ti alloys, respectively. The maximum nitrided depths of around 14 μm are achieved for both Mo-15 Ti/V alloys after nitriding at 1100 °C for 10 h. Larger hardness values are measured on nitrided layer of Mo-Ti in comparison to that of Mo-V at the same treatment temperature. According to X-ray diffraction assessment, at moderate temperature of 900 °C, the diffraction of Mo matrix and VN nitrides is recognized as an incoherent diffraction, whereas coherent diffraction by matrix and nitrides is realized for Mo and TiN. In general, higher affinity of nitride formation for V is observed at lower temperature of 700 °C as compared to Ti in a Mo matrix, which is attributed to the stronger interaction of V and N in Mo matrix given by calculating corresponding interaction parameters. The difference in stress relieving and coarsening behavior were also observed in matrices containing VN and TiN, which is argued with regards to the dissimilar impacts of VN and TiN on precipitation hardening of Mo matrix at investigated temperatures. These results imply that the previously reported brittleness of Mo-based alloys containing Ti can be improved at moderate temperatures by replacing Ti with other nitride forming elements such as V, regardless of N concertation in Mo matrix. 

Comparison between nitriding behavior of Mo Ti and Mo V alloys

The fatigue properties of metallizations used as electrical conductors in flexible electronic devices have been thoroughly studied over the years. Most studies use time-intensive characterization methods to evaluate mechanical damage. For their ease of access, in situ electrical resistance measurements are often performed along with other characterization methods. However, the data are mostly used as an indicator of failure and a thorough analysis is usually missing. This work presents some deeper analysis methods of such datasets, using gold films on polyimide, with and without a chromium interlayer, revealing that grain growth, through-thickness cracking, and more general fatigue behavior can be determined from electrical resistance data alone. A case is made for increased utilization of such easily obtained data, reducing the time required for the evaluation of experiments.

https://avs.scitation.org/doi/pdf/10.1116/6.0002362

Describing mechanical damage evolution through in situ electrical resistance measurements

The electrical performance of stretchable electronic inks degrades as they undergo cyclic deformation during use, posing a major challenge to their reliability. The experimental characterization of ink fatigue behavior can be a time-consuming process, and models allowing accurate resistance evolution and life estimates are needed. Here, a model is proposed for determining the electrical resistance evolution during cyclic loading of a screen-printed composite conductive ink. The model relies on two input specimen-characteristic curves, assumes a constant rate of normalized resistance increase for a given strain amplitude, and incorporates the effects of both mean strain and strain amplitude. The model predicts the normalized resistance evolution of a cyclic test with reasonable accuracy. The mean strain effects are secondary compared to strain amplitude, except for large strain amplitudes (>10%) and mean strains (>30%). A trace width effect is found for the fatigue behavior of 1 mm vs 2 mm wide specimens. The input specimen-characteristic curves are trace-width dependent, and the model predicts a decrease in N f by a factor of up to 2 for the narrower trace width, in agreement with the experimental results. Two different methods are investigated to generate the rate of normalized resistance increase curves: uninterrupted fatigue tests (requiring ∼6–7 cyclic tests), and a single interrupted cyclic test (requiring only one specimen tested at progressively higher strain amplitude values). The results suggest that the initial decrease in normalized resistance rate only occurs for specimens with no prior loading. The minimum-rate curve is therefore recommended for more accurate fatigue estimates.

https://iopscience.iop.org/article/10.1088/2058-8585/acbaab/pdf

Modeling resistance increase in a composite ink under cyclic loading

Linking through-thickness cracks in metallic thin films to in-situ electrical resistance peak broadening

Gaseous nitriding of Co-10 at.% and -15 at.% Cr alloys at 400 °C and 450 °C

Polymer components fabricated by additive manufacturing typically show only moderate strength and low temperature stability, possibly leading to severe wear and short lifetimes especially under dry tribological sliding. To tackle these shortcomings, we investigated the combination of single- and multi-scale textures directly fabricated by digital light processing with amorphous diamond-like carbon (DLC) coatings. The topography of the samples and conformity of the coatings on the textures are assessed and their tribological behaviour under dry conditions is studied. We demonstrate that the surface textures have a detrimental tribological effect on the uncoated samples. This changes with the application of DLC coatings since friction substantially reduces and wear of the textures is not observed anymore. These trends are attributed to the protection of the underlying polymer substrate by the coatings and a reduced contact area. The best tribological performance is found for a coating with highest hardness and hardness-to-elasticity ratios. Moreover, multi-scale textures perform slightly better than single-scale textures due to a smaller real contact area. Summarizing, we verified that the high flexibility and low production costs of 3D printing combined with the excellent mechanical and tribological properties of DLC results in synergistic effects with an excellent performance under dry sliding conditions. 

Combining multi-scale surface texturing and DLC coatings for improved tribological performance of 3D printed polymers

By increasing the density of interfaces in NiAl-CrMo in-situ composites, the mechanical properties can be significantly improved compared to conventionally cast material. The refined microstructure is achieved by manufacturing through electron beam powder bed fusion (PBF-EB). By varying the process parameters, an equiaxed or columnar cell morphology can be obtained, exhibiting a plate-like or an interconnected network of the (Cr,Mo) reinforcement phase which is embedded in a NiAl matrix. The microstructure of the different cell morphologies is investigated in detail using scanning electron microscope (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT). For both morphologies, the mechanical properties at elevated temperatures are analyzed by compression and creep experiments parallel and perpendicular to the building direction. In comparison to cast NiAl and NiAl-(Cr,Mo), the yield strength of the PBF-EB fabricated specimens is significantly improved at temperatures up to 1027 °C. While the columnar morphology exhibits the best improved mechanical properties at high temperatures, the equiaxial morphology shows nearly ideal isotropic mechanical behavior, which is a substantial advantage over directionally solidified material. This article is protected by copyright. All rights reserved. 

Anna Krapf

Papers

Linking through-thickness cracks in metallic thin films to in-situ electrical resistance peak broadening

Gaseous nitriding of Co-10 at.% and -15 at.% Cr alloys at 400 °C and 450 °C

Combining multi-scale surface texturing and DLC coatings for improved tribological performance of 3D printed polymers

Describing mechanical damage evolution through in situ electrical resistance measurements

Using selective electron beam melting to enhance the high‐temperature strength and creep resistance of NiAl‐28Cr‐6Mo in‐situ composites