From mirco-mechanical properties to tribological performance

Abrasive contacts are still too complex to allow reliable predictions of part performance. There is not a single material model with satisfying generality, yet. However, it should be a combination of the contact body’s relative elastic and plastic properties as well as the loading condition.1 In industry carbide- and boride-reinforced, metal matrix composites are often cladded on a base part with welding processes to counter abrasive wear. These composites generally pose structural features at a multitude of length scales. A millimeter thick coating is reinforced with carbides of 100 µm diameter. Precipitates of 1 to 10 µm decorate the matrix and during the major solidification, a metal-stable carbide-metal eutectic forms with domains widths of 1 µm and 100 – 200 nm lamella spacing. Please click Additional Files below to see the full abstract

Rrevealing plastic deformation mechanisms in polycrystalline thin films with synchrotron XRD

Understanding the fundamentals of plastic deformation mechanisms in polycrystalline thin metal films and the associated size effects is crucial to the design and fabrication of microelectronic devices. A combination of in-situ synchrotron diffraction experiments was conducted to investigate two cooperative plastic deformation mechanisms in polycrystalline face-centered cubic thin metal films: conjugate deformation twinning in uniaxially strained polycrystalline thin gold films and subgrain structure rotations in biaxially strained polycrystalline thin silver films. The experimental results demonstrate an increase in the total coverage of (115) oriented deformation twins in the thin gold films upon uniaxial deformation to 2% strain at a macroscopic yield stress of 250 MP

Brittle-to-ductile transition in ultrathin Ta/Cu film systems

Current semiconductor technology demands the use of compliant substrates for flexible integrated circuits. However, the maximum total strain of such devices is often limited by the extensibility of the metallic components. Although cracking in thin films is extensively studied theoretically, little experimental work has been carried out thus far. Here, we present a systematic study of the cracking behavior of 34- to 506-nm-thick Cu films on polyamide with 3.5-to 19-nm-thick Ta interlayers. The film systems have been investigated by a synchrotron-based tensile testing technique and in situ tensile tests in a scanning electron microscope. By relating the energy release during cracking obtained from the stress-strain curves to the crack area, the fracture toughness of the Cu films can be obtained. It increases with Cu film thickness and decreases with increasing Ta film thickness. Films thinner than 70 nm exhibit brittle fracture, indicating an increasing inherent brittleness of the Cu film

Portevin‐Le Chatelier effect studied at small scale

Portevin-Le Chatelier (PLC) effect [1] manifests itself as a serrated flow in the stress-strain curve associated with the phenomenon of dynamic strain aging (DSA), which arises from the interaction between solute atoms and matrix dislocations. The overwhelming majority of the data available in the literature about PLC effect is conducted at macro scale, often with a large and complicated microstructure. The PLC effect studies at small scale, the fundamental studies, could offer great insights to the dislocation theory of plasticity. Here we study the PLC effect in an Al-Cu diffusion couple using in situ strain rate jump micro-pillar-compression technique [2] facilitated with focused ion beam (FIB) machining. The deformed microstructures are characterize using high-resolution SEM images. Transmission electron microscopy (TEM) is used to study the atomistic origin of the DSA. References [1] A. Portevin, F. Le Chatelier, Comptes Rendus de l\u27Académie des Sciences Paris, 176 (1923) 507-510. [2] G. Mohanty, J.M. Wheeler, R. Raghavan, J. Wehrs, M. Hasegawa, S. Mischler, L. Philippe, J. Michler, Philosophical Magazine, 95 (2015) 1878-1895

Nanoscale electrochemical 3D deposition of cobalt with nanosecond voltage pulses in an STM

To explore a minimal feature size of <100 nm with electrochemical additive manufacturing, we use a strategy originally applied to microscale electrochemical machining for the nanoscale deposition of Co on Au. The concept's essence is the localization of electrochemical reactions below a probe during polarization with ns-long voltage pulses. As shown, a confinement that exceeds that predicted by a simple model based on the time constant for one-dimensional double layer charging enables a feature size of <100 nm for 2D patterning. We further indirectly verify the potential for out-of-plane deposition by tracking growth curves of high-aspect-ratio deposits. Importantly, we report a lack of anodic stability of Au tips used for patterning. As an inert probe is the prerequisite for controlled structuring, we experimentally verify an increased resistance of Pt probes against degradation. Consequently, the developed setup and processes show a path towards reproducible direct 2D and 3D patterning of metals at the nanoscale.ISSN:2040-3364ISSN:2040-337

Probing the limits of strength in diamonds: From single- and nano-crystalline to diamond-like-carbon (DLC)

As the hardest known material, diamond represents the benchmark for the ultimate strength of materials. It is thus a very attractive material for a number of mechanical applications. Recent advances in synthesis techniques have enabled the fabrication of diamond in thin film form with various microstructures: single- and nano-crystalline and tetrahedral-amorphous or diamond-like carbon (DLC) [1, 2]. Microcompression has been demonstrated to enable the interrogation of even the strongest form of diamond - a -oriented single crystal - achieving the strength limit predicted by simulations (Figure 1) [3, 4]. Nowadays, these allotropes of carbon with high strength and low friction are used in microelectronics and micro-electromechanical systems (MEMS) as structure components [5]. However, the effects of these new nanostructures on the mechanical properties of these allotropes is mostly unknown especially at different service temperatures. In this study, the mechanical properties of single crystalline, nanocrystalline, and amorphous forms of diamond are systematically studied by conducting in situ microcompression at various temperatures in scanning electron microscope (SEM). This allows the investigation of thermally-activated defect behavior and activation energy for several different nanostructures of diamond. This is then correlated with the deformed structures using high resolution transmission electron microscope (HRTEM) and Raman spectroscopy to interpret the deformation mechanisms. Please click Additional Files below to see the full abstract

Optomechanics of small-scale structures

While size effects in mechanical properties have been a core focus of this community, size effects in optical properties exhibit different mechanisms. They range from interference in transparent materials over strong interference in lossy dielectrics or semiconductors to plasmonics in metal nanostructures. This paper will attempt to demonstrate these effects by examples of titania/silica Bragg reflectors, strong interference from Ge, Si, GST and AlN to plasmonics in Au, Ag and Cu alloys. Please click Download on the upper right corner to see the full abstract

Plasticity and size effects in germanium: From cryogenic to elevated temperatures

Germanium is extensively used as a substrate in functional components of devices and microelectromechanical systems (MEMS) because of its tunable band structure and carrier mobility via. strain engineering [1]. The mechanical properties of Ge with a diamond-cubic structure at such small scales, i.e. in range of micro/nano-meter, are expected to be extraordinary since the improved strength and ductility of brittle crystals with minimized geometries [2]. Recent advances in nano-mechanical testing systems enable the investigation of the size- and temperature-dependent deformation behaviors and relevant parameters [3]. In the present study, micro-compression of FIB-machined micropillars is conducted to obtain a thorough understanding of the plasticity and size effects of Ge from cryogenic to elevated temperatures, i.e. in the range of -100°C to 600°C, shown in Figure 1(a). Dislocation motion in Ge is quantitatively evaluated as a function of sample size and crystalline orientation at the low temperature regime. Furthermore, the brittle-to-ductile transition is investigated to study the transition of deformation mechanisms, i.e. full to partial dislocation motion on the glide set, at the elevated temperatures regime [4]. Deformed regions in micropillars are subsequently characterized using HRTEM to track dislocations and microtwins. An unambiguous interpretation of dislocation processes in the diamond-cubic structure will be presented. Please click Additional Files below to see the full abstract

Seed-mediated synthesis of gold nanorods: control of the aspect ratio by variation of the reducing agent

Seed-mediated growth methods involving reduction of tetrachloroaurate(III) with ascorbic acid are common for the synthesis of gold nanorods. This study shows, however, that simply by appropriate choice of the reducing agent a drastic influence on the aspect ratio can be attained. Weaker reducing agents, such as dihydroxybenzene isomers (hydroquinone, catechol or resorcinol) or glucose can increase the aspect ratio of the nanorods by an order of magnitude, up to values as high as 100 (nanowires). The increase in aspect ratio is mainly a consequence of an increase in length of the particles (up to 1-3μm). This effect is probably associated with a decrease in the reduction rate of gold(III) species by dihydroxybenzenes or glucose compared to ascorbic acid. The reduction potential of the reducing agents strongly depends on the pH value, and related effects on the dimensions of the nanoparticles are also reflected in this study. The nanorods exhibited penta-twinned nature without noteworthy defects (e.g. stacking faults and dislocations

Designing micro-patterned Ti films that survive up to 10% applied tensile strain

Reducing the strain in brittle device layers is critical in the fabrication of robust flexible electronic devices. In this study, the cracking behavior of micro-patterned 500-nm-thick Ti films was investigated via uniaxial tensile testing by in situ SEM and 4-point probe measurements. Both visual observations by SEM and 4-pt resistance measurements showed that strategically patterned oval holes, off-set and rotated by 45°, had a significant effect on limiting the extent of cracking, specifically, in preventing cracks from converging. Failure with regard to electrical conduction was delayed from less than 2% to more than 10% strai