Microcantilever Bending Research Articles

Micro-mechanical evaluations of adhesion properties for Cr-coated accident tolerant fuel cladding

The development of accident tolerant fuel (ATF) cladding has been encouraged to satisfy increased nuclear safety demands under accident conditions since Fukushima accident. Coating techniques using oxidation-resistant materials, such as Cr and Al, on existing Zr-alloy claddings have been widely applied as short-term solutions. However, coating delamination due to poor adhesion may reduce its benefits, such as low waterside corrosion/oxidation rates and fretting resistance, which necessitates accurate evaluation of coating adhesion suitability. In this study, micro-mechanical tests, micro-cantilever bending test and surface and interfacial cutting analysis system test, were conducted to qualitatively and quantitatively evaluate adhesion properties on a coated ATF cladding. In situ scanning electron microscopy-based micro-cantilever bending tests were performed using a pico-indenter equipped with a diamond flat tip. We present the results for magnetron-sputtered Cr-coated ATF claddings in detail. Micro-cantilever tests on the Cr-Zr interface revealed that deformation and failure primarily occurred in the Zr substrate, rather than at the interface. And the Cr-coated cladding showed highly adherent mechanical bonds, as evidenced by the interfacial strength and elastic modulus. SAICAS tests confirmed that peeling of the coating layer occurred only when the applied force exceeded the cutting force of the coating layer. The experimental results suggest that the Cr-coated cladding exhibited a sufficiently adherent mechanical bond to prevent delamination of the coating.

Strain Rate Dependence of Slip Persistence in TiZrNbHfTa Investigated with Microcantilever Bending Tests

Strain Rate Dependence of Slip Persistence in TiZrNbHfTa Investigated with Microcantilever Bending Tests

Mechanical Characteristics of Al2O3, TiCN, and TiAlN Hard Coating Films Measured by Microcantilever Bending Test

Mechanical Characteristics of Al2O3, TiCN, and TiAlN Hard Coating Films Measured by Microcantilever Bending Test

Fracture of the C15 CaAl2 Laves phase at small length scales

The cubic C15 CaAl2 Laves phase is an important brittle intermetallic precipitate in ternary Mg–Al–Ca structural alloys. Although knowledge of the mechanical properties of the co-existing phases is essential for the design of improved alloys, the fracture toughness of the C15 CaAl2 intermetallic has not yet been studied experimentally due to limitations posed by macroscale testing of defect-free specimens. Here, miniaturised testing techniques like micropillar splitting and microcantilever bending methods are used to experimentally determine the fracture toughness of the CaAl2 Laves phase. It is found that the toughness value of ~ 1 MPa·√m obtained from pillar splitting with a sharp cube corner geometry is largely insensitive to sample heat treatment, the ion beam used during fabrication, micropillar diameter, and surface orientation. From correlative nanoindentation and electron channelling contrast imaging supported by electron backscatter diffraction, fracture is observed to take place mostly on {011} planes. Atomistic fracture simulations on a model C15 NbCr2 Laves phase showed that the preference of {011} cleavage planes over the more energetically favourable {111} planes is due to lattice trapping and kinetics controlling fracture planes in complex crystal structures, which may provide insights into the experimental results for CaAl2. Using rectangular microcantilever bending tests where the notch plane was misoriented to the closest possible {112} cleavage plane by ~ 8° and the closest {001}, {011}, and {111} planes by > 20°, a toughness of ~ 2 MPa·√m was determined along with the electron microscopy observation of significant deviations of the crack path, demonstrating that preferential crystallographic cleavage planes determine the toughness in this material. Further investigation using pentagonal microcantilevers with precise alignment of the notch with the cleavage planes revealed similar fracture toughness values for different low-index planes. The results presented here are the first detailed experimental study of fracture toughness of the C15 CaAl2 Laves phase and can be understood in terms of crack plane and crack front-dependent fracture toughness.Graphical

Processing-structure-microscale properties of silicon nitride

This paper presents an overview of recent works on microscale mechanical and other properties of silicon nitride (Si3N4) determined by microcantilever bending tests and their relationships with the processing and microstructures. We first focus on deformation behaviors and fracture strength of Si3N4 single crystals. β-Si3N4 single crystals are plastically deformed at room temperature under high bending stress, and the yield stress depends on the crystal orientation. The critical resolved share stress of the primary slip system is determined to be below 1.5 GPa from the yield stress. Next, we address microscale mechanical properties of Si3N4 polycrystalline ceramics. Emphasis is placed on their grain-boundary strength or toughness in conjunction with intergranular glassy film (IGF) which is determined by processing parameters such as sintering additives. Assessment is made on two cases of fractures at the IGF-grain interface and that within the IGF, and in each of the cases, effects of rare earth oxide additives are discussed. In Si3N4 ceramics doped with Al2O3, β-SiAlON layer forming on β-Si3N4 grains enhances the microscale grain-boundary strength. Finally, we shed light on microscale property-degradation behaviors of Si3N4 ceramics, including the deterioration due to the contact with molten Al and the corrosion in sulfuric acid solution. The variation of the microscale properties appears in very short periods compared to the macroscale approaches, demonstrating the advantage in terms of rapid assessment of time-dependent degradation behaviors.

Two scale models for fracture behaviours of cementitious materials subjected to static and cyclic loadings

This study employs a lattice fracture model to simulate static and fatigue fracture behaviour of Interfacial Transition Zone (ITZ) at microscale and mortar at mesoscale. The heterogeneous microstructure of ITZ and mesostructure of mortar are explicitly considered in the models. The initial step involves calibrating and validating the microscopic model of the ITZ through micro-cantilever bending tests. Subsequently, this validated ITZ model serves as a constitutive law to simulate the fracture behavior of mortar at the mesoscale using an uncoupled upscaling method. The influence of microstructural features, such as w/c ratio and microscopic roughness, on the fracture behaviour of ITZ is investigated. Moreover, the effect of ITZ properties and stress level on the fracture performance and fatigue damage evolution of mortar is also studied. The simulation results for both the ITZ and mortar demonstrate good agreement with experimental results. The proposed two models provide insights into the fracture mechanisms and fatigue damage evolution in cementitious materials subjected to static and cyclic loadings.

Deformation Behavior and Fracture Strength of Single‐Crystal 4H‐SiC Determined by Microcantilever Bending Tests

In this study, the deformation behaviors and mechanical properties of 4H‐SiC single crystals are investigated using microcantilever beam specimens with two different sizes, A and B (A < B). Tensile stress is applied along <20> direction. Plastic deformation, or nonlinearity, is observed in the stress–strain curves, and yield stress, or proportional limit, coincides between the two specimens at ≈25 ± 2 GPa. Scanning transmission electron microscopy and transmission electron microscopy studies show that the plastic deformation is due to dislocation activities; multiple‐dislocation pileup areas are observed in both the specimens. Assuming {100}/<110> prismatic slip which most plausibly occurs in the <20> stress application, the critical resolved shar stress is estimated to be 10.9 GPa, which agrees well with the previous studies. Measured fracture strength is 41.9 ± 2.8 and 33.5 ± 2.4 GPa for the A and B, respectively. Dislocation–fracture relationship is discussed on the basis of dislocation‐based fracture mechanics, etc. It is suggested that cracks form within the multiple‐dislocation pileup area, by interaction with dislocation pileups, and act as fracture origins. A's strength is close to an ideal tensile strength of 4H‐SiC in the <110> direction, 47–55 GPa.

Hydrogen enhanced localised plasticity of single grain α titanium verified by in-situ hydrogen microcantilever bending

The effect of hydrogen on the micromechanical characteristics of pure grade 2 titanium was investigated by comparing prenotched microcantilever bending in air versus in-situ electrochemical hydrogen charging. The nanoscale interaction between hydrogen and the metal's defects was analysed in three different grain orientations, i.e. the top surface parallel to {101‾2}, {0001}, and {202‾1}. The subsequent high resolution post-mortem characterisation enabled to characterise the interplay between hydrogen in solid solution and the dislocations in the local deformation process. The results showed that hydrogen facilitated a localised plasticity mechanism, causing hydrogen assisted degradation at the applied low hydrogen concentration below the hydride formation threshold. Meanwhile, hydrogen provoked a reduction in defect formation energy, following the defactant concept, which was responsible for a softening effect on the yield stress and flow stress. Furthermore, due to the anisotropic characteristics of the α titanium hexagonal close packed crystal lattice, the susceptibility to hydrogen effects also depended on the grain orientation. This single grain in-situ micromechanical testing is complementing the existing macroscaled identification of hydrogen effects in α titanium.

Irreversible evolution of dislocation pile-ups during cyclic microcantilever bending

In single crystals, plastic deformations are predominantly governed by dislocation movement and interactions. The group of dislocations that creates strain gradients, known as geometrically necessary dislocations (GNDs), also deterministically contributes to strain hardening, micron-scale size effects, fatigue, and Bauschinger effect. During bending large strain gradients naturally emerge which makes this deformation mode exceptionally suitable to study the evolution of GNDs. Here we present bi-directional bending experiment of a Cu single crystalline microcantilever with in situ characterisation of the dislocation microstructure in terms of high-resolution electron backscatter diffraction (HR-EBSD). The experiments are complemented with dislocation density modelling to provide physical understanding of the collective dislocation phenomena. We find that dislocation pile-ups form around the neutral zone during initial bending, however, these do not dissolve upon reversed loading, rather they contribute to the development of a much more complex GND dominated microstructure. This irreversible process is analysed in detail in terms of the involved Burgers vectors and slip systems. We conclude that at this scale the most dominant role in the Bauschinger effect and corresponding strain hardening is played by short-range dislocation interactions. The in-depth understanding of these phenomena will aid the design of microscopic metallic components with increased performance and reliability.

Micro-Mechanical Fracture Investigations on Grain Size Tailored Tungsten-Copper Nanocomposites

Tungsten-copper composites are used in harsh environments because of their superior material properties. This work addresses a tungsten-copper composite made of 20 wt.% copper, which was subjected to grain refinement by high-pressure torsion, whereby the deformation temperature was varied between room temperature and 400 °C to tailor the grain size. Deformation was performed up to microstructural saturation and verified by hardness measurement and scanning electron microscopy. From the refined nanostructured material, micro-cantilever bending beams with cross-sections spanning from 5 × 5 to 35 × 35 µm2 were cut to examine possible size effects and the grain size influence on the fracture behavior. Fracture experiments were performed in situ inside a scanning electron microscope by applying a quasi-static loading protocol with partial unloading steps. Inspection of the fracture surfaces showed that all cantilevers failed in an inter-crystalline fashion. Nevertheless, remaining coarser tungsten grains impacted the resultant fracture toughness and morphology. Cantilevers fabricated from the 400 °C specimen exhibited a fracture toughness of 220 ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm $$\\end{document} 50 Jm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\frac{{\ ext{J}}}{{{\ ext{m}}}^{2}}$$\\end{document}. For the room temperature cantilevers, a fracture toughness of 410 ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm $$\\end{document} 50 Jm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\frac{{\ ext{J}}}{{{\ ext{m}}}^{2}}$$\\end{document} was observed, which declined to 340 ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm $$\\end{document} 30 Jm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\frac{{\ ext{J}}}{{{\ ext{m}}}^{2}}$$\\end{document} for cantilevers < 10 × 10 µm2, confirming a size effect. The increased fracture toughness is attributed to the delamination-like structures formed in the room temperature sample.

Fracture toughness of AlTiN coatings investigated by nanoindentation and microcantilever bending

Metal nitride coatings are increasingly used in MEMS applications. The brittle nature of these coatings makes fracture one of the main failure mechanisms during operation. Therefore, quantifying the fracture toughness of nitride coatings in a reliable fashion is important to understand the failure behavior and to optimize device performance and reliability. This study investigated the fracture toughness of AlTiN coatings produced by cathodic arc evaporation, a widely used technique in the industry. Nanoindentation and microcantilever bending-based measurements indicated a fracture toughness of 6.6 and 4.8 MPa·m1/2, respectively. The lower toughness results of microcantilever bending were attributed to the columnar growth of the coating, which promoted crack propagation perpendicular to the film surface. The results provide useful data toward a better understanding of the fracture of hard coatings and give insight into the advantages and disadvantages of different measurement methods.

Unveiling the mechanical properties of near-surface microstructures in tribological contacts via in-situ micro-bending tests

This study demonstrates the application of the in-situ V-notched micro-cantilever bending method in tribology by analyzing three distinct cases of near-surface microstructures occurring in common tribological contacts. The brown etching layer region of a rail wheel from service shows predominant plastic material behavior. Significant variations in material response and crack initiation are revealed in distinctive regions of a roller bearing's near-surface microstructure and a self-lubricating laser cladding. Qualitative analysis and stress-displacement graphs highlight the method's potential for gaining high-resolution insights into material behavior, contributing to our understanding of microstructural changes affecting friction, wear, and component failure in tribosystems.

Phase-Specific Damage Tolerance of a Eutectic High Entropy Alloy.

Phase-specific damage tolerance was investigated for the AlCoCrFeNi2.1 high entropy alloy with a lamellar microstructure of L12 and B2 phases. A microcantilever bending technique was utilized with notches milled in each of the two phases as well as at the phase boundary. The L12 phase exhibited superior bending strength, strain hardening, and plastic deformation, while the B2 phase showed limited damage tolerance during bending due to micro-crack formation. The dimensionalized stiffness (DS) of the L12 phase cantilevers were relatively constant, indicating strain hardening followed by increase in stiffness at the later stages and, therefore, indicating plastic failure. In contrast, the B2 phase cantilevers showed a continuous drop in stiffness, indicating crack propagation. Distinct differences in micro-scale deformation mechanisms were reflected in post-compression fractography, with L12-phase cantilevers showing typical characteristics of ductile failure, including the activation of multiple slip planes, shear lips at the notch edge, and tearing inside the notch versus quasi-cleavage fracture with cleavage facets and a river pattern on the fracture surface for the B2-phase cantilevers.

Microscopic mechanical properties of silicon nitride ceramics corroded in sulfuric acid solution

The microscopic mechanical properties including the bending strength and Young’s modulus for Si3N4 ceramics doped with Y2O3 and Al2O3 which was immersed in 5 wt% sulfuric acid solution using microcantilever beam specimens was investigated in comparison with the macroscopic results obtained in conventional three-point bending tests. Both the properties sharply dropped in very short periods of the corrosion in the microcantilever tests, resulting from the dissolutions of yttrium and aluminum ions of grain boundary glass networks, followed by the hydration. After the drops at the beginning of the corrosion, they showed little degradation afterwards. The degradation behaviors of the mechanical properties are basically the same as those in conventional macroscopic approaches; however, they changes appear in very short periods of the corrosion in the microcantilever bending tests. This study also verified that a small difference of a ratio of sintering additives results in different grain boundary strengths of Si3N4 ceramics.

Effect of Strain Gradient on Elastic and Plastic Size Dependency in Polycrystalline Copper

This study unveils the presence of size effect not only in the plastic regime but also in the elastic regime under strain gradient. This discovery emerges from a comprehensive set of experiments encompassing micro-cantilever bending and micro-pillar compression tests. Subsequent finite element analysis serves to not only expose the limitation of classical continuum theory but also to effectively demonstrate the enhanced predictive capabilities of couple stress theory in both elastic and plastic size effects. To incorporate couple stress theory, a systematic and rigorous finite element analysis-based optimization was devised to determine length scale parameters, which are additional material properties in couple stress theory. The resulting length scale parameters for polycrystalline copper were found to be 0.536 and 0.669µm in the elastic and plastic regimes, respectively. The study also explores the physical interpretation of these length scale parameters in both regimes.

In situ mechanical testing of hard yet tough high entropy nitride coatings deposited on compliant steel substrates

High entropy nitrides are an interesting material class for wear resistant coatings. Their fracture properties and toughness are of particular interest for future application in tool coatings. Therefore, in situ tensile testing and micro-cantilever experiments were performed on (AlCrTaTiZr)N, (AlCrNbSiTiV)N, (HfNbTaTiVZr)N and (AlTi)N coatings inside a scanning electron microscope. Parallel cracks form during tensile testing from which the fracture toughness was determined via a modified Hsueh and Yanaka model. For this the misfit strain between coating and substrate was treated as an unknown parameter and was fitted to the beginning part of the underlying crack density – applied strain curves. The validity of the model was evaluated by comparing the results with fracture toughness values derived via micro-cantilever bending experiments. A good fit between the data could be observed for three out of four tested coatings. The limitations of the underlying analytical model are presented and discussed. The tested high entropy nitride coatings displayed a promising combination of high hardness and fracture toughness.

Micro- and macro-scale strength properties of c-axis aligned hydroxyapatite ceramics

In this study we first investigated the strength of grains, i.e., single crystals, and grain boundaries (micro-scale strength) of hydroxyapatite ceramics in different crystal orientations, using microcantilever bending tests. The specimens were prepared from two types of hydroxyapatite ceramics; one is that with uniaxially aligned c-axes of grains and the other with randomly oriented ones. Both the grain and grain boundary strengths were slightly higher in tensile stress application parallel to c-axis than in the perpendicular in the aligned material. These strengths, however, were substantially higher than those of the randomly oriented one. Then, three-point bending tests were conducted to determine the polycrystalline strength (macro-scale strength) for the above two types of hydroxyapatite ceramics. While the strength values were comparable in tensile stress applications parallel and perpendicular to c-axis in the aligned material, that of the randomly oriented one is again significantly lower than them. In particular, the strength of the grain boundary was higher when the angle between the c-axis of the two hydroxyapatite grains divided by the boundary was close to 0 or 90°. The effects of the grain orientation and alignment on the mechanical properties were discussed on the basis of both the micro-scale and macro-scale results. These results indicate that selective formation of high-strength grain boundaries due to c-axis alignment of hydroxyapatite grains is effective in improving the mechanical properties of bulk ceramics, such as macroscopic strength and fracture toughness.

The effect of crystal anisotropy on fracture toughness and strength of ZrB2 microcantilevers

AbstractThe influence of crystal anisotropy on the micromechanical properties of ceramic grains plays an important role in the design of the macromechanical performance of bulk polycrystalline samples. To this end, the effect of crystal orientation on fracture toughness and strength was investigated by microcantilever bending experiments combined with finite element method (FEM) simulations in grains of a polycrystalline ZrB2 sample. The sample was prepared by hot pressing and the crystal orientations were determined by electron backscatter diffraction after careful surface preparation. The bending tests were carried out on notched and unnotched microcantilevers cut from specific grains along the prismatic (⊥ to c‐axis), basal (∥ to c‐axis), and intermediate (45° to c‐axis) directions of ZrB2 crystals using focused ion beam milling. The fracture toughness and strength were determined by FEM‐derived analytical solutions. The fracture strength was similar with values of about 11–12 GPa in every direction. Enhanced plasticity was found in the intermediate direction of unnotched beams as compared to the other two brittle orientations. The fracture toughness was found to be 30% higher in the intermediate direction (4.1 MPa m0.5) than those measured for the basal (3.1 MPa m0.5) and prismatic directions (3.3 MPa m0.5). These findings were explained by the orientation dependence of slip system activations which provide a new way in the design of textured polycrystalline ZrB2 ceramics toward enhanced damage tolerance.

The influence of microstructural orientation on fracture toughness in (V, Al)N and (V, Al)(O, N) coatings as measured by microcantilever bending

Since protective transition metal (oxy)nitride coatings are widely used, understanding of the mechanisms linking microstructure to their fracture behaviour is required to optimise wear resistance, while maintaining fracture toughness. To assess this interconnection, beam bending was performed using microcantilevers oriented parallel and at 90° to the growth direction. Furthermore, the tests were applied to favour normal bending and shear fracture. Coatings were synthesised by both direct current magnetron sputtering (DCMS) as well as high power pulsed magnetron sputtering (HPPMS). Here, we show that the fracture toughness depends on the alignment of the grains and loading directions. Furthermore, an improved fracture toughness was found in coatings produced by HPPMS, when microstructural defects, such as underdense regions in DCMS deposited coatings can be excluded. We propose indices based on fracture and mechanical properties to rank those coatings. Here, the HPPMS deposited oxynitride showed the best combination of mechanical properties and fracture toughness.Graphical abstractPrinciple of measuring the effects of microstructure and process route on the fracture toughness via microcantilever bending.

Measuring microscale mechanical properties of PVdF binder phase and the binder-particle interface using micromechanical testing

In this study, we developed a robust methodology for extracting the mechanical properties of individual components in complex systems such as Li-ion battery electrodes and provided quantitative values that can be used as input for modelling and lifetime estimation of Li-ion batteries. We employed micromechanical testing techniques, including micropillar compression, microcantilever bending, and nanoindentation, to measure the mechanical properties of the PVdF binder phase in the active layer. We discovered that nanoindentation tends to overestimate the modulus due to uncertainty associated with the test volume and initial large compression strains, while the micropillar compression technique provides more accurate modulus data with a narrower spread. Additionally, the yield stress of the binder phase can be evaluated using micropillar compression. Our obtained modulus values were in the range of 2.5–4.4 GPa, and the yield stress was in the range of 162–270 MPa. By microcantilever bending tests, we determined that the binder–particle interface often fails before the binder itself, suggesting that the interface significantly influences the failure mechanics. Overall, our results indicate that the microcantilever bending tests provide moduli estimates that agree with those obtained from micropillar compression tests. We also qualitatively examined the binder-particle and binder-current collector interfaces, further emphasising the significance of our methodology and the obtained quantitative values.