Deformation and fracture mechanisms in nanocomposite and nanolaminate thin films revealed through combinatorial design and nanomechanical testing

We’ve integrated an atomic layer deposition (ALD), a physical vapor deposition (PVD) and a nanoparticle inert gas condensation (NP) deposition system into a single vacuum chamber. This combined system allows for PVD sputtering of micrometer thick films and incorporation of size filtered nanoparticles and/or controlled deposition of mono-layer highly conformal film coatings within a multilayer structure. In this way, unique model thin film microstructures can be architectured. We designed three thin films to understand the basic mechanism of plasticity and fracture in thin films: a) Al2O3 oxide films were deposited on combinatorial libraries of the ternary noble metal alloys with full compositional range to understand interfacial adhesion between oxide and noble metal alloys b) monosized tungsten nanoparticles were deposited at the interface of Cu/Ni multilayers to understand how thin film hardness and thermal stability can be engineered, c) ultrathin monolayers of Al2O3 layers were sandwiched between sputtered Al layers and micropillar compression was used to understand dislocation transmission and fracture across ultrathin ceramic layers. Please click Additional Files below to see the full abstract

Experimental design for uniaxial tensile measurements at the microscale

Bone’s unique combination of mechanical properties like strength, stiffness, toughness and low weight are the result of its complex hierarchical structure spanning several length scales. Despite extensive research on bone, there is still a lack of understanding on how its micromechanical behavior relates to its macroscopic failure behavior. While recent research has mostly utilized microcompression and nanoindentation, pure tensile testing at the lamellar level has not yet been reported. Nevertheless, critical failure events in bone are often attributed to tensile stresses. In this study a tensile experiment is designed using an in-situ micromechanical testing platform (Alemnis AG, Switzerland). The setup consists of a translatable silicon gripper driven by a piezo motor and a movable sample stage mounted atop a load cell. The setup is compact and can be installed within an electron microscope for in-situ testing. Please click Additional Files below to see the full abstract

A new push‐pull sample design for microscale mode 1 fracture toughness measurements under uniaxial tension

The miniaturization of microelectronic devices and the use of thin hard coatings have led to an increased demand for knowledge on the fracture behaviour of microscopic structures. A new geometry called Micro-SENT is proposed in this work that allows performing experiments in uniaxial tension on the microscale using a standard flat punch indenter by making use of a symmetric push-pull sample design. This enables the measurement of mode 1 fracture toughness under uniform tensional far-field loading as opposed to current state of the art approaches based on cantilever bending or micropillar splitting. Please click Additional Files below to see the full abstract

Resolution in Focused Electron- and Ion-Beam Induced Chemical Vapor Deposition

The key physical processes governing resolution of focused electron-beam and ion-beam-assisted chemical vapor deposition are analyzed via an adsorption rate model. We quantify for the first time how the balance of molecule depletion and replenishment determines the resolution inside the locally irradiated area. Scaling laws are derived relating the resolution of the deposits to molecule dissociation, surface diffusion, adsorption, and desorption. Supporting results from deposition experiments with a copper metalorganic precursor gas on a silicon substrate are presented and discussed.Comment: 4 pages, 4 figures, 1 tabl

In situ micromechanical testing inside the scanning electron microscope at subambient temperatures

In material science, the measurement of mechanical material properties as a function of temperature is of great interest, as it allows determining the activation parameters of the underlying deformation mechanism. In the case of nanostructured materials or MEMS devices, it is interesting to probe local properties by means of micromechanical experiments. However, in the case of nanograined metals, testing at elevated temperatures is not possible due to heat induced grain growth and thus changes in the microstructure during testing. We developed a device that allows performing micromechanical tests at low temperatures down to -150°C inside a scanning electron microscope. A cold finger connected to the sample and tip holders by copper braids is cooled by circulating nitrogen. Independent thermal management of the indenter and the sample allows minimizing temperature differences and thereby drift. Local cooling, thermal isolation of the cold regions, and a closed loop frame temperature control reduce frame drift, noise, and the need for a temperature-dependent calibration. A symmetric design of the cooling bodies was chosen in order to minimize bending moments on the indenter and the sample, which increases the accuracy of the measurements. The high vacuum environment minimizes condensation of water vapor and hydrocarbons on the indenter and sample. Positioning as well as in situ observation is made possible by the use inside a scanning electron microscope. For validation of the system, indentations in Cu were performed down to -150°C. Please click Additional Files below to see the full abstract

Analysis of onset of dislocation nucleation during nanoindentation and nanoscratching of InP

Nanoindentation and nanoscratching of an indium phosphide (InP) semiconductor surface was investigated via contact mechanics. Plastic deformation in InP is known to be caused by the nucleation, propagation, and multiplication of dislocations. Using selective electrochemical dissolution, which reveals dislocations at the semiconductor surface, the load needed to create the first dislocations in indentation and scratching can be determined. The experimental results showed that the load threshold to generate the first dislocations is twice lower in scratching compared to indentation. By modeling the elastic stress fields using contact mechanics based on Hertz's theory, the results during scratching can be related to the friction between the surface and the tip. Moreover, Hertz's model suggests that dislocations nucleate firstly at the surface and then propagate inside the bulk. The dislocation nucleation process explains the pop-in event which is characterized by a sudden extension of the indenter inside the surface during loadin

Studying grain boundary regions in polycrystalline materials using spherical nano-indentation and orientation imaging microscopy

In this article, we report on the application of our spherical nanoindentation data analysis protocols to study the mechanical response of grain boundary regions in as-cast and 30% deformed polycrystalline Fe-3%Si steel. In particular, we demonstrate that it is possible to investigate the role of grain boundaries in the mechanical deformation of polycrystalline samples by systematically studying the changes in the indentation stress-strain curves as a function of the distance from the grain boundary. Such datasets, when combined with the local crystal lattice orientation information obtained using orientation imaging microscopy, open new avenues for characterizing the mechanical behavior of grain boundaries based on their misorientation angle, dislocation density content near the boundary, and their propensity for dislocation source/sink behavio

Microtensile properties and failure mechanisms of cortical bone at the lamellar level.

Bone features a remarkable combination of toughness and strength which originates from its complex hierarchical structure and motivates its investigation on multiple length scales. Here, in situ microtensile experiments were performed on dry ovine osteonal bone for the first time at the length scale of a single lamella. The micromechanical response was brittle and revealed larger ultimate tensile strength compared to the macroscale (factor of 2.3). Ultimate tensile strength for axial and transverse specimens was 0.35 ± 0.05 GPa and 0.13 ± 0.02 GPa, respectively. A significantly greater strength anisotropy relative to compression was observed (axial to transverse strength ratio of 2.7:1 for tension, 1.3:1 for compression). Fracture surface and transmission electron microscopic analysis suggested that this may be rationalized by a change in failure mode from fibril-matrix interfacial shearing for axial specimens to fibril-matrix debonding in the transverse direction. An improved version of the classic Hashin's composite failure model was applied to describe lamellar bone strength as a function of fibril orientation. Together with our experimental observations, the model suggests that cortical bone strength at the lamellar level is remarkably tolerant to variations of fibrils orientation of about ±30°. This study highlights the importance of investigating bone's hierarchical organization at several length scales for gaining a deeper understanding of its macroscopic fracture behavior. STATEMENT OF SIGNIFICANCE: Understanding bone deformation and failure behavior at different length scales of its hierarchical structure is fundamental for the improvement of bone fracture prevention, as well as for the development of multifunctional bio-inspired materials combining toughness and strength. The experiments reported in this study shed light on the microtensile properties of dry primary osteonal bone and establish a baseline from which to start further investigations in more physiological conditions. Microtensile specimens were stronger than their macroscopic counterparts by a factor of 2.3. Lamellar bone strength seems remarkably tolerant to variations of the sub-lamellar fibril orientation with respect to the loading direction (±30°). This study underlines the importance of studying bone on all length scales for improving our understanding of bone's macroscopic mechanical response

Tension-compression strength asymmetry of bone extracellular matrix

Bone features a hierarchical architecture, as a result of which antagonistic properties like toughness and strength are achieved. On the macroscale, bone exhibits a distinct anisotropy and loading mode dependence, with a considerably lower strength in tension compared to compression. To better understand the mechanisms leading to this behavior, anisotropic tensile yield and failure properties of ovine bone were characterized on the length scale of a single lamella (3-7 μm) and then compared to compression data for the same scale [1]. In situ microtensile experiments were carried out using an improved testing methodology, developed to overcome typical issues encountered during small scale testing related to sample fabrication, sample handling and misalignment [2]. The methodology is based on self-aligning silicon grippers prepared by means of reactive ion etching and an optimized microtensile sample geometry that can be fabricated via focused ion beam (FIB) milling. The measured elastic modulus, strength, yield stress and strain at maximum stress are summarized in table 1. Please click Additional Files below to see the full abstract