Nanomechanical Characterization Research Articles

Nanomechanical Characterization of Organic Surface Passivation Films on 50 nm Patterns during Area-Selective Deposition

Area-selective deposition (ASD) is a “bottom-up” substrate-selective material deposition process, considered as a promising alternative to current “top-down” pattering techniques. The most studied and successful ASD strategies envisage a combination of atomic layer deposition (ALD) and a passivation layer, which prevents material deposition on the non-growth areas. As ASD targets increasingly smaller dimensions, metrology challenges are prominent along with preserving confined film growth. For patterned substrates with nanometric critical dimensions, only a few characterization techniques can be employed to assess the ASD performance. However, these techniques provide no or little insight into the passivation layer. This is a crucial limitation as the blocking film plays a key role in the ASD process. In this work, pulsed force mode atomic force microscopy (AFM) is used to characterize and monitor the quality of the passivation films by measuring the surface energy fluctuations occurring on the patterned substrate undergoing ASD. As the evolution of the relative adhesion force distribution of the sample under ALD conditions is recorded, the octadecanethiol (ODT) coverage on non-growth areas is accurately estimated. The heavily temperature-dependent self-assembled monolayer degradation revealed by the nanomechanical characterization is supported by X-ray photoelectron spectroscopy. As Hf3N4 ALD is performed, the top-down scanning electron microscopy investigation is employed to show the strong relationship between ASD quality upon ALD and pulsed force AFM-derived ODT coverage.

Viscoelastic Properties and Sulfur Distribution at the Nanoscale in Binary Elastomeric Blends: Toward Phase-Specific Cross-Link Density Estimations

Immiscible blends of elastomers present high technological interest, and the selection of the vulcanization system is important for the optimization of properties for different technical applications. In particular, the effect of the curing agents on the distribution of cross-links in each phase is key for the full comprehension of the structure–property relationships. Aiming at the understanding of the phase-specific network structure in rubber blends, this work presents an innovative strategy for the quantitative characterization of the local viscoelastic properties of immiscible rubber blends by atomic force microscopy (AFM) measurements. A systematic study on the quantitative nanomechanical characterization by AFM of unfilled single natural rubber (NR) matrixes with different degrees of cross-link densities ultimately allows for the estimation of the phase-specific cross-link density of the NR phase in NR/butadiene rubber (BR) blends, prepared with varying vulcanization systems. Complementary chemical information by high-resolution secondary ion mass spectrometry imaging is able to reveal differences in sulfur contents in each elastomeric phase of the blends.

Laser deposition of graded γ-TiAl/Ti2AlNb alloys: Microstructure and nanomechanical characterization of the transition zone

Additive manufacturing (AM) is a transformative technology to the aerospace industry. As one of the AM techniques, laser metal deposition (LMD) enables the fabrication of engine blades and a disk, as a single component, known as blisk. In this study, we use the LMD technique to fabricate a γ-TiAl/Ti2AlNb graded metallic alloy by depositing γ-TiAl powder on a Ti2AlNb alloy substrate. High-resolution scanning electron microscope (SEM) and high-speed nanoindentation are employed to characterize the microstructure and mechanical properties of the transition zone from the Ti2AlNb substrate (disk) to the γ-TiAl alloy (blade). The results show that the transition zone includes three layers (I, II and III) with gradient compositions and phases: (I) mainly β/B2 matrix with randomly distributed α2 and γ phases, (II) (α2 + γ) lamella with γ and β/B2 phases, and (III) similar microstructure with Layer II but finer γ and β/B2 phases. The results of nanoindentation mapping show good correlations between the mechanical properties (nanohardness and elastic modulus) and microstructure in the transition zone. Attributing to the rule of mixtures, the nanohardness and elastic modulus gradually increase from the substrate Ti2AlNb to Layer I, and gradually decrease from Layer I to γ-TiAl. This work demonstrates that the microstructure and phase analysis in combination with high-speed nanoindentation offers a new opportunity to study graded materials made using LMD.

Nanomechanical Characterization of the Deformation Response of Orthotropic Ti–6Al–4V

The nanoindentation‐induced mechanical deformation response is applied to identify the orthotropic elastic moduli using the Delafargue and Ulm method as well as to validate the asymmetric orthotropic CPB06 nonlinear plasticity model required in simulations of nonuniform macroscopic mechanical response of the Ti–6Al–4V alloy. Scanning electron microscope (SEM) technique allows to select the maximum penetration depth for the indentation in the deformed alpha phase and alpha–beta interphase,αandα/β, respectively. The apparent macromechanical response can be successfully derived from several residual imprints conducted at micro‐ and/or submicrometric length scale and distributed throughout samples of the investigated bulk alloy, as demonstrated by correlation with finite element simulations based on the orthotropic elastoplastic model. The accurate numerical response obtained validates the material model and the Delafargue and Ulm approach, opening a window for next generation identification methods of macromechanical plasticity models with hybrid experimental–numerical method based on instrumented indentation and the use of SEM technique.

Nanomechanical characterization of ambient-cured fly ash geopolymers containing nanosilica

The elastic modulus, hardness, and volume fractions of different phases of ambient-cured fly ash (FA) geopolymers were determined using the nanoindentation method. A low-calcium FA was blended with either 15% GGBFS or 10% OPC to accelerate curing and the effect of 2% nanosilica (NS) was studied. It was found that the inclusion of NS increased the volume of reaction product from 70% to 79% and reduced the porous phase from 13% to 7% in the FA only geopolymer. Similar changes were also found in the OPC or GGBFS blended geopolymer specimens. The reduction of the porous phase correlated well with the porosity determined by mercury intrusion porosimetry tests. Thus, the improvement in the properties of ambient-cured geopolymers by the inclusion of NS was established using the nanoindentation characterization.

Evaluation of Local Mechanical and Chemical Properties via AFM as a Tool for Understanding the Formation Mechanism of Pulsed UV Laser-Nanoinduced Patterns on Azo-Naphthalene-Based Polyimide Films.

Aromatic polyimides containing side azo-naphthalene groups have been investigated regarding their capacity of generating surface relief gratings (SRGs) under pulsed UV laser irradiation through phase masks, using different fluencies and pulse numbers. The process of the material photo-fluidization and the supramolecular re-organization of the surface were investigated using atomic force microscopy (AFM). At first, an AFM nanoscale topographical analysis of the induced SRGs was performed in terms of morphology and tridimensional amplitude, spatial, hybrid, and functional parameters. Afterward, a nanomechanical characterization of SRGs using an advanced method, namely, AFM PinPoint mode, was performed, where the quantitative nanomechanical properties (i.e., modulus, adhesion, deformation) of the nanostructured azo-polyimide surfaces were acquired with a highly correlated topographic registration. This method proved to be very effective in understanding the formation mechanism of the surface modulations during pulsed UV laser irradiation. Additionally to AFM investigations, confocal Raman measurements and molecular simulations were performed to provide information about structured azo-polyimide chemical composition and macromolecular conformation induced by laser irradiation.

Influence of force volume indentation parameters and processing method in wood cell walls nanomechanical studies

Since the established correlations between mechanical properties of a piece of wood at the macroscopic scale and those of the cell wall at the submicron scale, techniques based on atomic force microscopy (AFM) have become widespread. In particular Peak Force tapping, allowing the differentiation of various layers, has become the new standard for wood cell wall’s nanomechanical characterization. However, its use requires fully elastic indentation, a good knowledge of stiffness of the probe and assumes a perfect tip shape of known radius (sphere) or angle (cone). Those strong hypotheses can result in large approximations in the extracted parameters for complex, nanostructured, and stiff and viscous materials such as wood. In this work, we propose a reliable and complementary alternative based on AFM force-volume indentation by refining the Oliver and Pharr nanoindentation processing and calibration procedure for AFM cantilever and tip. The introduced area-function calibration (AFC) method allows to considerably reduce these approximations and provides semi-quantitative measurements. No prior knowledge of the tip shape and cantilever stiffness are required and viscoplasticity is investigated through a qualitative index. Indentation parameters variations are shown to impact the resulting measurements, i.e., indentation modulus, viscoplasticity index, adhesion force and energy. AFC method, applied to map regions of tension wood, provides very stable mechanical parameters characteristic of each region, which makes this method of high interest for plant cell wall studies.

Quasi-static and dynamic nanomechanical characterization of PMMA/ZnO nanocomposite thick films synthesized by ultrasonication and spin-coating

Polymer nanocomposite films, comprising of polymethylmethacrylate (PMMA) as the matrix and zinc oxide (ZnO) nanoparticles as reinforcement, have been prepared using ultrasonication and spin-coating techniques, with ZnO content up to 20 wt.%. The effect of the processing on the microstructure and nanomechanical properties have been investigated. The nanocomposite film thickness is found to vary from 2.4 ± 0.2 µm for pristine PMMA to 33.1 ± 0.5 µm for PMMA/20 wt.% ZnO nanocomposite. Quasi-static nanoindentation showed that the indentation modulus varied from 4.68 ± 0.07 GPa for pristine PMMA to 5.04 ± 0.14 GPa for PMMA/20 wt.% ZnO nanocomposite, while the indentation hardness varied from 275.94 ± 5.67 MPa to 292.39 ± 10.88 MPa in the same range. However, the highest indentation modulus and the highest hardness are exhibited by PMMA/10 wt.% ZnO nanocomposite. Scanning electron microscopy of the synthesized films provided the evidence behind such variation in material properties. In addition, the experimentally obtained elastic moduli were compared with values predicted by using Eshelby-Mori-Tanaka micromechanics. Nanoindenter-based dynamic mechanical analysis of the PMMA nanocomposite thin films revealed the variation of storage modulus, loss modulus and loss factor of the films in the frequency range of 10 Hz to 201.5 Hz. For all PMMA/ZnO nanocomposites, the storage modulus is found to increase monotonically from 10 Hz to ∼100 Hz, beyond which the values reached a plateau. The loss modulus and loss factor for all PMMA/ZnO nanocomposites are found to decrease with increasing frequency. These results form an essential step toward establishing process-structure-nanomechanical property relationships for PMMA/ZnO nanocomposite films.

Nanomechanical and Fluorescence Characterizations of Weathered PV Module Encapsulation

Nanoindentation and fluorescence spectroscopy provide spatially resolved mechanical and chemical characterization for degradation of poly (ethylene-co-vinyl acetate) (EVA) encapsulants aged under accelerated weathering and in field deployed photovoltaic (PV) modules. For the accelerated weathering tests, two different EVA formulations in the glass/EVA/glass-laminated coupons were exposed to ultraviolet (UV) light for 180 days at three different temperatures (40, 60, and 80 °C) and UV intensities (0.4, 2.2, and 2.9 W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /nm at 340 nm). The fluorescence spectra and moduli were uniform through the cross section of both EVA formulations at the lowest temperature and UV intensity; however, under the highest temperature and UV intensity, the UV irradiated sides showed lower moduli and higher fluorescence compared to the opposite sides. For EVA obtained from PV modules deployed 27 years in Sacramento, CA, USA, changes in fluorescence intensity across the thickness were relatively greater than changes in modulus. On the other hand, for EVA obtained from the same type of module stored in the shed for 27 years, no fluorescence was observed, and a uniform modulus was seen through the EVA layer, but with substantially higher values than those obtained from the field-deployed modules. The different extent of changes between laboratory and field exposures suggests the importance of considering effects of temperature and UV intensity for designing a reliable accelerated testing. This study uniquely demonstrates the capability of microscale characterization using nanoindentation and fluorescence microscopy to monitor mechanical and chemical degradation of EVA encapsulants in PV modules.

Universal Theory of Dynamic Force Microscopy for Exact and Robust Force Reconstruction Using Multiharmonic Signal Analysis.

Force reconstruction in dynamic force microscopy (DFM) is a nontrivial problem that requires the deconvolution of integrals. However, conventional reconstruction methods, which recover forces from single-frequency motion of the cantilever at its resonance, exhibit non-negligible error and reconstruction instability in the highly nonlinear force regime when the tip oscillates with its amplitude comparable to the decay length of the interaction. Here, we develop a theoretical platform of DFM based on multiharmonic signal analysis for exact and robust reconstruction of conservative and dissipative forces, valid for all oscillation amplitudes and entire tip-sample distances in both amplitude- and frequency-modulation atomic force microscopy. We achieve accuracy improvement by an order of magnitude for oscillation amplitudes comparable to or larger than the decay length, and by 2 orders of magnitude for smaller amplitudes at the force minimum, even in cases where conventional methods show poor accuracy (≳5%). Moreover, we obtain greater robustness with respect to the oscillation amplitude error, resulting in a fivefold increase in reconstruction precision. Our results demonstrate a fast and versatile reconstruction scheme for nanomechanical force characterization, with higher harmonics measured with sufficient signal-to-noise ratio, which provides unprecedented accuracy and stability beyond conventional methods.

Applicability of focused Ion beam (FIB) milling with gallium, neon, and xenon to the fracture toughness characterization of gold thin films

Focused ion beam (FIB) milling is an increasingly popular technique for fabricating micro-sized samples for nanomechanical characterization. Previous investigations have cautioned that exposure to a gallium ion beam can significantly alter the mechanical behavior of materials. In the present study, the effects of gallium, neon, and xenon ions are scrutinized. We demonstrate that fracture toughness measurements on freestanding gold thin films are unaffected by the choice of the ion species and milling parameters. This is likely because the crack initiation is controlled by the local microstructure and grain boundaries at the notch, rather than by the damaged area introduced by FIB milling. Additionally, gold is not susceptible to chemical embrittlement by common FIB ion species. This confirms the validity of microscale fracture measurements based on similar experimental designs.Graphical abstract

Nanomechanical Characterization of the Synergism of Antimicrobial Peptides PGLa and Mag2 for Lipid Membrane Disruption

Antimicrobial peptides (AMPs) are amphiphilic molecules used by eukaryotic organisms as first defensive mechanism against infections. There is an increasing interest in the study of these peptides, considered promising alternatives to antibiotics for the treatment of multi-resistance pathogens. AMPs target the bacterial membrane leading to membrane lysis by pore formation and bilayer disruption. Here we studied PGLa and Magainin 2 (Mag2), which are known for a particular synergistic behavior in their antimicrobial activity.

Microstructural and Finite Element Analysis - Assisted Nanomechanical Characterization of Maize Starch Nanocomposite Films

Biocomposite films were prepared using normal maize starch plasticized with glycerol and water and sodium montmorillonite clay particles employing the solution mixing procedure. Scanning electron microscopy (SEM), X-ray diffraction (XRD), three-dimensional profilometry and tensile along with nanoindentation tests assisted with a Finite Element Analysis (FEA) were used for the assessment of starch-based films with various percentages of nanoclay particles. XRD analysis revealed intercalation of the test specimens while their morphology was ascertained using SEM/EDX. The FEA results were compared with the experimental measurements from nanoindentation and tensile tests. A satisfactory correlation was obtained between the experimental measurements and the computational models, demonstrating FEA-assisted nanoindentation as a useful technique for assessment, showing the effect of the different nanoclay concentrations on the mechanical properties.

Macro-, Micro- and Nanomechanical Characterization of Crosslinked Polymers with Very Broad Range of Mechanical Properties.

This work is focused on the comparison of macro-, micro- and nanomechanical properties of a series of eleven highly homogeneous and chemically very similar polymer networks, consisting of diglycidyl ether of bisphenol A cured with diamine terminated polypropylene oxide. The main objective was to correlate the mechanical properties at multiple length scales, while using very well-defined polymeric materials. By means of synthesis parameters, the glass transition temperature (Tg) of the polymer networks was deliberately varied in a broad range and, as a result, the samples changed their mechanical behavior from very hard and stiff (elastic moduli 4 GPa), through semi-hard and ductile, to very soft and elastic (elastic moduli 0.006 GPa). The mechanical properties were characterized in macroscale (dynamic mechanical analysis; DMA), microscale (quasi-static microindentation hardness testing; MHI) and nanoscale (quasi-static and dynamic nanoindentation hardness testing; NHI). The stiffness-related properties (i.e., storage moduli, indentation moduli and indentation hardness at all length scales) showed strong and statistically significant mutual correlations (all Pearson′s correlation coefficients r > 0.9 and corresponding p-values < 0.001). Moreover, the relations among the stiffness-related properties were approximately linear, in agreement with the theoretical prediction. The viscosity-related properties (i.e., loss moduli, damping factors, indentation creep and elastic work of indentation at all length scales) reflected the stiff-ductile-elastic transitions. The fact that the macro-, micro- and nanomechanical properties exhibited the same trends and similar values indicated that not only dynamic, but also quasi-static indentation can be employed as an alternative to well-established DMA characterization of polymer networks.

Nano-mechanical characterization of asymmetric DLPC/DSPC supported lipid bilayers

Asymmetric distribution of lipid molecules in the inner and outer leaflets of the plasma membrane is a common occurrence in the membrane formation. Such asymmetric arrangement is a crucial parameter to manipulate the properties of the cell membrane. It controls signal transduction, endocytosis, exocytosis in the cells. The artificial membrane is often used to study the lateral and transverse arrangement of the lipid molecules in place of the cell membrane. Nano-mechanical characterization of the model membrane helps to understand the mechanical stability of the lipid bilayer. The stability is sensitive to the variations in the lipid composition and their local organization. In this article, we present both topographical and nano-mechanical properties of lipid bilayer characterized by atomic force microscopy (AFM). The results show that the asymmetric lipid bilayer formation is an intrinsic character. We have selected a bi-component fluid-gel phase 1,2-dilauroyl-sn-glycero-3-phosphocholine:1,2-disteroyl-sn-glycero-3-phosphocholine (DLPC: DSPC) system for our studies. We have observed domain formation and phase separation in the bilayer by increasing the composition of the gel phase DSPC. In force spectroscopy studies, we determine the mechanical strength of the bilayer for unique mixtures of DLPC: DSPC by measuring the breakthrough force. These results also show the effect of asymmetry in the lipid bilayer. Besides AFM studies, we have implemented a coarse-grained (CG) molecular dynamics (MD) simulation using the gromacs package at room temperature and 1 bar pressure. The results from the simulation study have been compared with AFM study. It was found that the simulation studies corroborated the findings from AFM such as an increase in the bilayer thickness, change in the phase state, asymmetric and symmetric domain formation in the lipid bilayer.

Nanomechanical Characterization of Iron Borides in Fe-Mn-B Ternary Alloys

Nanomechanical properties of iron borides, FeB and Fe2B were studied in Fe-Mn-B ternary alloys. The alloys were produced by arc melting method using high purity powders, which were subsequently annealed at temperature of 873 K and 1223 K, until fully equilibrated for a time period of 2160 hours and 1440 hours, respectively. Based on results obtained from experimental study and thermodynamic modeling of Fe-Mn-B system the solubility of Mn in these borides was determined. For the purpose of this study the influence of heat treatment temperature, as well as, the solubility of Mn in these borides on their nanomechanical properties is investigated. Nanomechanical properties, including determination of indentation modulus and hardness were measured using nanoindentation testing machine equipped with Berkovich type diamond indenter. The indentation process was carried out using an indentation depth controlled method, to a maximum depth of 500 nm.

Ultrasonic plasticity of metallic glass near room temperature

Bulk metallic glasses (BMGs) are well-known for their superb strength (1–4 GPa) (Ashby and Greer, 2006) [1] but poor/localized plasticity when deformed at low temperatures or high strain rates (Inoue and Takeuchi, 2011; Kumar et al., 2009) [2,3]. Therefore, processing of BMGs, such as forming and shaping for various important applications, is usually performed above their glass transition temperatures (Tg) – also known as “thermo-plastic” forming (Geer, 1995) – for which the selection of alloy composition and the protocol for thermal treatment is demanding in order to promote extensive homogeneous plastic flows while avoiding crystallization (Geer, 1995). In stark contrast, here we demonstrate that homogeneous super-plasticity can occur rapidly in different BMGs below their Tg when subjected to ultrasonic agitations. Through atomistic simulations and nanomechanical characterization, we provide the compelling evidence to show that this super-plasticity is attributed to dynamic heterogeneity and cyclic induced atomic-scale dilations in BMGs, which leads to significant rejuvenation and final collapse of the solid-like amorphous structure, thereby leading to an overall fluid-like behavior. Our finding uncovers that BMGs can undergo substantial plastic flows through unusual liquefaction near room temperature and, more importantly, it leads to the development of a facile and cost-effective “ultrasonic-plastic” forming method to process a wide range of BMGs at low temperatures.

Vibrating FRET-Based Nanomechanical Sensor Preparation and Characterization for Environmental Monitoring Applications

In this study, the recently introduced graphene nanomechanical sensor design based on the vibrating Förster resonance energy transfer (VFRET) was experimentally demonstrated for the first time. VFRET was realized by the dynamic FRET between quantum dots on vibrating graphene layers. Graphene nanomechanical membranes were fabricated and experimentally analyzed with quantum dots donor and acceptor material loads. The loaded membranes were paired in the VFRET based sensor device structure. The FRET and the VFRET tests were applied to the prepared devices, experimental challenges and the device development solutions were utilized. Novel technology and the future applications of the VFRET based sensor examples were discussed with the advanced capabilities of sensing nano- and micro-scale particles around the device for the real-time point-of-care (PoC), and the environmental monitoring applications include imaging, monitoring and tracking of intra- or inter- cellular mechanical processes as the nanophotonic applications by exploiting the VFRET based nanoscale sensor design.

Nanomechanical Characterization for Cold Spray: From Feedstock to Consolidated Material Properties

Cold gas-dynamic spray is a solid-state materials consolidation technology that has experienced successful adoption within the coatings, remanufacturing and repair sectors of the advanced manufacturing community. As of late, cold spray has also emerged as a high deposition rate metal additive manufacturing method for structural and nonstructural applications. As cold spray enjoys wider recognition and adoption, the demand for versatile, high-throughput and significant methods of particulate feedstock as well consolidated materials characterization has also become more notable. In order to address the interest for such an instrument, nanoindentation is presented herein as a viable means of achieving the desired mechanical characterization abilities. In this work, conventionally static nanoindentation testing using both Berkovich and spherical indenter tips, as well as nanoindentation using the continuous stiffness measurement mode of testing, will be applied to a range of powder-based feedstocks and cold sprayed materials.

Tribological properties of PDA + PTFE coating in oil-lubricated condition

In this study, the tribological properties of 45 ± 2 μm-thick polytetrafluoroethylene (PTFE) and polydopamine (PDA) + PTFE coatings in oil-lubricated conditions were investigated under a normal load of 10 N and a linear reciprocating speed of 0.1 m/s. Both the PTFE and the PDA + PTFE coatings were deposited using an in-house spray-coating method. The PDA + PTFE coating was five times more durable compared to the PTFE coating in oil-lubricated conditions. Both PTFE and PDA + PTFE coatings showed lower COF in oil-lubricated conditions than in dry conditions. The nanomechanical properties measured by PeakForce Quantitative Nanomechanical (PFQNM) characterization method showed an increase in Young’s modulus and adhesion force for the PDA + PTFE coatings compared to the PTFE coatings. Chemical analysis showed that the cross-linking between PDA and PTFE polymer chains occurred during the high-temperature sintering procedure. The higher adhesion of PDA + PTFE coating to the cast iron substrate and the stronger cohesion due to the cross-linking between PDA and PTFE contributed to the higher durability of the PDA + PTFE composite coatings.