New instrumentation and analysis methodology for nano-impact testing

Nanoindentation testing has become increasingly popular for mechanical characterization of materials. This is motivated by the high versatility of the technique that allows testing of small volumes that could not be tested otherwise by macroscopic techniques, with minimal test preparation. The interest on nano-/microscale characterization of materials has been also extended to the study of high strain rate mechanical behaviour. One of the available techniques is nano-impact testing. It is carried out on a pendulum-based force-actuated, displacement-sensing device with the ability of performing energy-controlled impacts. The combination of conventional nanoindentation, for which a range of strain rates from 10-3 to 10-1 s-1 can be tested, with nano-impact provides a tool for materials characterization at the nano/microscale from 10-3 to 103 s-1. Regarding the analysis of nano-impact test results, there has been no consensus in literature over what material metrics to extract from the test. Several authors base the analysis of nano-impact test on the calculation of a dynamic hardness defined as change in kinetic energy throughout the impact divided by the residual volume of indentation [1-4]. However, there are two issues with the assumptions in which this equation is based. First, it only considers the change in kinetic energy and it neglects other important contributions like the work of impulse force. Then, it assumes that hardness is constant throughout the entire impact period. While for self-similar indenters this is true in the loading part, Cheng’s dimensional analysis shows that this is not the case in the unloading [5]. Therefore, the hardness calculated from this definition is not necessarily equal to the hardness under load commonly used in the instrumented indentation literature. To this end, an alternative analysis methodology is proposed. The analysis is based on the same definition of hardness under load commonly used in the instrumented indentation literature, computed as force divided by contact area. This way, the nano-impact hardness is directly comparable with results of conventional nanoindentation that use this definition. The instrumentation of the nanoindentation device with force-sensing capability was found crucial for the implementation of the analysis methodology. In addition, and in line with the nano-impact hardness definition in literature, an energy-based hardness is presented. The technique is assessed using finite element simulations and by testing six materials covering a wide range of mechanical behaviours. The FE simulations are used to assess the two energy-based definitions of hardness, the one in literature and the one proposed in this work. It was found that the literature definition leads to values that differ significantly from the ones obtained as force divided by contact area. On the other hand, the proposed energy-based definition provides values that match the ones obtained by force-approach. The experimental results are also in line with this conclusions. The literature energy-based hardness presents significant differences compared to the force-based hardness, which are higher for the more elastic materials. Furthermore, the force-based hardness computed from nano-impact results was compared with the hardness from conventional nanoindentation. A close match is found between both set of results. References [1] J.R. Trelewicz, C.A. Schuh, The Hall–Petch breakdown at high strain rates: Optimizing nanocrystalline grain size for impact applications, Appl. Phys. Lett. 93 (2008) 171916. [2] H. Somekawa, C.A. Schuh, High-strain-rate nanoindentation behavior of fine-grained magnesium alloys, Journal of Materials Research. 27 (2012) 1295–1302. [3] J.M. Wheeler, A.G. Gunner, Analysis of failure modes under nano-impact fatigue of coatings via high-speed sampling, Surface and Coatings Technology. 232 (2013) 264–268. [4] C. Zehnder, J.-N. Peltzer, J.S.K.-L. Gibson, S. Korte-Kerzel, High strain rate testing at the nano-scale: A proposed methodology for impact nanoindentation, Materials & Design. 151 (2018) 17–28. [5] Y.-T. Cheng, C.-M. Cheng, Scaling, dimensional analysis, and indentation measurements, Materials Science and Engineering: R: Reports. 44 (2004) 91–149. doi:10.1016/j.mser.2004.05.001

Fabrication of HfO2 patterns by laser interference nanolithography and selective dry etching for III-V CMOS application

Nanostructuring of ultrathin HfO2 films deposited on GaAs (001) substrates by high-resolution Lloyd's mirror laser interference nanolithography is described. Pattern transfer to the HfO2 film was carried out by reactive ion beam etching using CF4 and O2 plasmas. A combination of atomic force microscopy, high-resolution scanning electron microscopy, high-resolution transmission electron microscopy, and energy-dispersive X-ray spectroscopy microanalysis was used to characterise the various etching steps of the process and the resulting HfO2/GaAs pattern morphology, structure, and chemical composition. We show that the patterning process can be applied to fabricate uniform arrays of HfO2 mesa stripes with tapered sidewalls and linewidths of 100 nm. The exposed GaAs trenches were found to be residue-free and atomically smooth with a root-mean-square line roughness of 0.18 nm after plasma etching

An XFEM/CZM implementation for massively parallel simulations of composites fracture

Because of their widely generalized use in many industries, composites are the subject of many research campaigns. More particularly, the development of both accurate and flexible numerical models able to capture their intrinsically multiscale modes of failure is still a challenge. The standard finite element method typically requires intensive remeshing to adequately capture the geometry of the cracks and high accuracy is thus often sacrificed in favor of scalability, and vice versa. In an effort to preserve both properties, we present here an extended finite element method (XFEM) for large scale composite fracture simulations. In this formulation, the standard FEM formulation is partially enriched by use of shifted Heaviside functions with special attention paid to the scalability of the scheme. This enrichment technique offers several benefits since the interpolation property of the standard shape function still holds at the nodes. Those benefits include (i) no extra boundary condition for the enrichment degree of freedom, and (ii) no need for transition/blending regions; both of which contribute to maintaining the scalability of the code. Two different cohesive zone models (CZM) are then adopted to capture the physics of the crack propagation mechanisms. At the intralaminar level, an extrinsic CZM embedded in the XFEM formulation is used. At the interlaminar level, an intrinsic CZM is adopted for predicting the failure. The overall framework is implemented in ALYA, a mechanics code specifically developed for large scale, massively parallel simulations of coupled multi-physics problems. The implementation of both intrinsic and extrinsic CZM models within the code is such that it conserves the extremely efficient scalability of ALYA while providing accurate physical simulations of computationally expensive phenomena. The strong scalability provided by the proposed implementation is demonstrated. The model is ultimately validated against a full experimental campaign of loading tests and X-ray tomography analyzes.A.J., A.M., D.T., L.N. and L.W. acknowledge funding through the SIMUCOMP ERA-NET MATERA + project financed by the Fonds National de la Recherche (FNR) of Luxembourg, the Consejería de Educación y Empleo of the Comunidad de Madrid, the Walloon region (agreement no 1017232, CT-EUC 2010–10-12), and by the European Unions Seventh Framework Programme (FP7/2007–2013).Peer ReviewedPostprint (author's final draft

Microarchitected Compliant Scaffolds of Pyrolytic Carbon for 3D Muscle Cell Growth

The integration of additive manufacturing technologies with the pyrolysis of polymeric precursors enables the design-controlled fabrication of architected 3D pyrolytic carbon (PyC) structures with complex architectural details. Despite great promise, their use in cellular interaction remains unexplored. This study pioneers the utilization of microarchitected 3D PyC structures as biocompatible scaffolds for the colonization of muscle cells in a 3D environment. PyC scaffolds are fabricated using micro-stereolithography, followed by pyrolysis. Furthermore, an innovative design strategy using revolute joints is employed to obtain novel, compliant structures of architected PyC. The pyrolysis process results in a pyrolysis temperature- and design-geometry-dependent shrinkage of up to 73%, enabling the geometrical features of microarchitected compatible with skeletal muscle cells. The stiffness of architected PyC varies with the pyrolysis temperature, with the highest value of 29.57 ± 0.78 GPa for 900 °C. The PyC scaffolds exhibit excellent biocompatibility and yield 3D cell colonization while culturing skeletal muscle C2C12 cells. They further induce good actin fiber alignment along the compliant PyC construction. However, no conclusive myogenic differentiation is observed here. Nevertheless, these results are highly promising for architected PyC scaffolds as multifunctional tissue implants and encourage more investigations in employing compliant architected PyC structures for high-performance tissue engineering applications

In situ small-scale mechanical testing under extreme environments

The high precision offered by small-scale mechanical testing has allowed the relationships between mechanical behavior and specific microstructural features to be determined to an unprecedented degree. However, of most interest to scientists and engineers is often the behavior of materials under service conditions in an extreme environment, such as high/low temperatures, high strain rates, hydrogen atmosphere, or radiation. In this article, we detail progress made to adapt nanomechanical testing systems and techniques to observe materials behavior in situ in extreme environments

Role of surface carbon nanolayer on the activation of flash sintering in tungsten carbide

Flash sintering has been recently successfully activated also in conductive ceramics like tungsten carbide (WC). The present work aims at understanding how the WC particles surface chemistry can influence the electrical properties of the material and play a fundamental role in the flash sintering phenomenon. An electrical contact resistance (ECR) model was developed to understand the role of resistive surface layers on the electrical behaviour of WC green compacts under different applied pressures during the initial stages of the processes. It is established that the large resistivity measured on green compacts can be attributed to the sole presence of an ultrathin carbon layer on the particles' surface. A carbon nanolayer with a thickness of about 1–2 nm, as detected by XPS and TEM analyses, is found to be responsible for the high resistance reached at the particles' contact points while evolving during the flash event. Flash sintering conditions can be achieved during the electrical resistance flash sintering (ERFS) process in WC nanoparticles covered by such carbon layer and independently of the presence of W oxides

Mechanical behavior and microstructure evolution of a quenched and partitioned steel during drop weight impact and punch testing

In this work, a Fe-0.25C-3.0Mn-1.5Si-0.023Al-0.015Cr (wt. %) steel was subjected to the Q&P treatment, and its mechanical behavior and microstructure evolution during drop weight impact testing and quasi-static punch testing were thoroughly analyzed. It is shown that the 1 mm thick Q&P steel sheet can withstand 110 J impact energy without any (micro) cracking, which is well above the impact resistance of DP 1180 steel. The local true plastic strain can reach 53.4% in biaxial stretching showing excellent formability of the material. The microstructure characterization shows that the volume fraction of retained austenite decreases exponentially with increasing plastic strain under dynamic biaxial stretching. Scanning electron microscopy analysis of cracks formed after drop weight impact testing with 120 J energy reveals a ductile fracture surface consisting of elongated dimples formed by MnS inclusions which are surrounded by fine chisel-point type dimples. The potential of Q&P treated steels for application in the automotive industry is discussed

The Evolution of Internal Damage Identified by Means of X-ray Computed Tomography in Two Steels and the Ensuing Relation with Gurson’s Numerical Modelling

This paper analyzes the evolution of the internal damage in two types of steel that show different fracture behaviors, with one of them being the initial material used for manufacturing prestressing steel wires, and the other one being a standard steel used in reinforced concrete structures. The first of them shows a flat fracture surface perpendicular to the loading direction while the second one shows the typical cup-cone surface. 3 mm-diameter cylindrical specimens are tested with a tensile test carried out in several loading steps and, after each of them, unloaded and analyzed with X-ray tomography, which allows detection of internal damage throughout the tensile test. In the steel used for reinforcement, damage is developed progressively in the whole specimen, as predicted by Gurson-type models, while in the steel used for manufacturing prestressing steel-wire, damage is developed only in the very last part of the test. In addition to the experimental study, a numerical analysis is carried out by means of the finite element method by using a Gurson model to reproduce the material behavior