ICMCTF 2024 Session CM2-1-MoA: Advanced Mechanical Testing of Surfaces, Thin Films, Coatings and Small Volumes I

Understanding the effects of hydrogen in materials became a pressing topic with the imminent shift towards green technologies, and the use of hydrogen as energy carrier. It is expected that the use of hydrogen will increase in all industries, together with the need for safe transport and storage and consequently the development of new materials and technologies to cope with it. A critical challenge is hydrogen-induced damage, or hydrogen embrittlement, that can cause the sudden failure materials. Hydrogen barrier coatings represent, in this regard, an appealing option to prevent and/or slow down the hydrogen ingress into structural alloys that are susceptible to embrittlement.

To characterize hydrogen and its effects in materials, at the relevant small-scale dimensions where embrittlement initiates, is a substantial yet demanding task. Current studies on hydrogen effects are in their majority limited to post-mortem probes and ex-situ charging, which neglect diffusible hydrogen, its migration and desorption at the analysis time. To rise above these constraints, we designed a novel “back-side” charging approach, to perform micromechanical testing during hydrogen charging [1]. Hydrogen is generated electrochemically at the back-side and diffuses towards the testing (front-side) surface. This unique method allows differentiating between the effects of trapped and mobile hydrogen, and performing well controlled measurements with different hydrogen levels monitored over time to consider hydrogen absorption, diffusion and release.

Using this new method, we unraveled dynamic effects of hydrogen on the mechanical properties of bulk alloys [2], and recently, we successfully applied it to study of hydrogen barrier coatings [3,4]. In this talk, I will present an overview of the technique, together with the case study of an Al₂O₃ hydrogen barrier coating. The hydrogen diffusion on Al₂O₃, ~9 orders of magnitude slower with respect to the used substrate, was measured by Kelvin probe. The mechanical stability of the coating was tested by nanoindentation and nanoscratching during hydrogen loading. The accumulation of hydrogen at the substrate-coating interface reduces the critical load required for cracking and leads to local delamination. Mechanical tests were complemented by atom probe tomography, confirming the presence of hydrogen close to the substrate/coating interface, and transmission electron microscopy, revealing the underlying microstructural changes.

[1] M.J. Duarte, et al., J. Mat. Sci. 56 (2021) 8732.

[2] J. Rao, et al., Mater. Des. 232 (2023) 112143.

[3] M. Wetegrove, et al., Hydrogen 4(2) (2023) 307.

[4] S.W Hieke, et al., Adv. Eng. Mater. (2023), Accepted.

Damage tolerance is a prerequisite for using protective coatings in components subject to long-term stress. Physical Vapor Deposition offers possibilities in coating architectures using combinations of ductile and hard materials, even on the nm scale. However, ductility through dislocation motion is often suppressed on the micro-scale due to geometric limitations, resulting in unusually brittle behavior. In this work, we show a linear dependence between the necessary shear stress for dislocation motion in Nb layers and the overall plastic behavior of micropillar samples.

Computational pre-screening identifies fcc-TiN/bcc-Nb as a promising system providing stable, sharp, and strong interfaces with essentially different elastic moduli. Using a TiN compound target enables a sharp interface without nitrogen cross-contamination. Layer variation and changing the TiN to Nb ratio offer insights into the small-scale plastic behavior using the micropillar compression test. These show a fluent transition from ductile deformation for thick Nb layers to a brittle behavior similar to monolithic TiN upon decreasing the Nb layer thickness.

Combining micromechanical data with TEM analysis of fractured micropillars, we correlate these observations with increased stresses necessary for dislocation motion within the confined layer slip model. Furthermore, we will show the results of unique experiments combining micromechanics with synchrotron nanodiffraction to understand the stress situation in a pillar and describe deformation mechanisms.

In recent years, metal multilayer composites have been the focus of research due to their exceptional mechanical properties. Recent experimental [1-3], theoretical, and modeling [4]studies on multilayers have indicated that the enhancement of both strength and ductility is related both to the structure and properties of the interfaces between the layers, as well as the thicknesses and properties of the individual layers [5]. In spite of the ample literature available on pure metallic nanolaminate structures, the experimental data on compositionally complex alloy multilayers is rather missing.

In this context, here we present recent experimental data on the fabrication and mechanical behavior of nanolaminate FCC-BCC high entropy alloy thin films with interlayer thicknesses 50 nm. The alternating FCC-BCC layers are separated by atomic layer deposition of 2 nm amorphous Al2O3 layer without breaking the vacuumin a new Cluster System combining both ALD and PVD in the same equipment (Swiss Cluster AG). As a model system, FCC-NiCoCrFe and BCC- NiCoCrFe-Al (with Al ~10 at. %) layers with a total thickness of 3 microns is deposited on Si (100) substrate by magnetron sputtering and subsequently tested by micro compression and nanoindentation experiments. The mechanical response of the multi-layered structures is also compared with FCC-BCC multilayer without ALD and single-layer FCC and BCC counterparts. The uniformity in composition and microstructure of interlayers is confirmed by performing S/TEM imaging along the cross-section samples prepared by FIB using a standard liftout procedure. The microcompression experimental results on micro-pillars show a clear effect of interfaces and interlayer size effect on the mechanical response of nanolaminates at different strain rates. The post-mortem electron microscopy investigation provides insight into deformation mechanisms and deformation-induced phase transformations in the individual layers. This first study on multilayer film of two HEA’s will help fundamental studies on high entropy alloys and transformative to other complex systems.

1. S.J. Zheng, I.J. Beyerlein, J. Wang, J.S. Carpenter, W.Z. Han, N.A. Mara, Acta Materialia, 60 (2012) 5858-5866.

2. N.A. Mara, I.J. Beyerlein, J.S. Carpenter, J. Wang, JOM, 64 (2012) 1218-1226.

3. N. Li, N.A. Mara, J. Wang, P. Dickerson, J.Y. Huang, A. Misra, Scripta Materialia, 67 (2012) 479-482.

4. J. Wang, K. Kang, R.F. Zhang, S.J. Zheng, I.J. Beyerlein, N.A. Mara, JOM, 64 (2012) 1208-1217.

5. N. Mara, I. Beyerlein, J Mater Sci, 49 (2014) 6497-6516.

In the transition to hydrogen as a future energy vector, thin films and coatings play a crucial role. Prominent current applications are coatings for bipolar plates in fuel cells and electrolyzers. In the future, many more applications are likely, such as diffusion barrier and hard coatings on high-strength or wear exposed components such as valves and hydrogen injectors. This is because many base materials of hard coatings have very low hydrogen diffusion coefficients. However, real hard coatings are not single crystals and thus a much better understanding of the interaction of hydrogen with real hard coatings is required. Especially interactions between hydrogen and crystal defects are important as they carry permeation and trigger failures.

To understand these interactions, we are developing new nanoscale characterisation methods. These include the development of an special atom probe with ultra-low hydrogen background for nanoscale imaging and a similar thermal desorption system for analysis of mobile and trapped hydrogen. Both of these systems are based on titanium ultra-high vacuum chambers, drastically reducing the amount of background hydrogen in the vacuum. As a result, very little to no spurious hydrogen appears in the analyses. For the titanium atom probe, this has been demonstrated in voltage pulsing already. This method of analysis is however not suitable for coatings. A laser for the analysis of non-conductive materials such as thin films and coatings is currently being fitted. This unlocks the ability to quantitatively image hydrogen in thin films at the nanoscale and thus shed light on the prermeation mechanisms and interactions with crystal defects. First results will be shown in the talk. In parallel, we are testing the titanium thermal desorption system on thin films, to complement the nanoscale imaging of the atom probe with qualitative and eventually quantitative results on the amounts of mobile and trapped hydrogen in thin films.