Micro-cantilever Tests 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.

The influence of crack and interface oxidation on dwell fatigue crack propagation behavior of the nickel-base superalloy ATI A718Plus

The crack propagation properties are an important and often limiting factor for the commercial application of superalloys in aerospace applications where the oxidation ahead of the crack tip plays an important role. Previous research on the Ni-base superalloy A718Plus revealed that the orientation and the volume fraction of the high temperature grain boundary phases δ and η have a major influence on the dwell fatigue crack propagation rate at 650 °C in ambient air but not in vacuum. In this work, the effect of internal oxidation at the η/δ–matrix interface and its effect on dwell fatigue crack propagation and mechanical properties are examined by advanced microscopic methods and micro-cantilever testing. During crack propagation, the crack tip is exposed to air where a Nb-rich oxide layer forms and embrittles the η/δ–matrix interface causing the cracks to deflect and propagate along the oxidized interfaces. Micro-cantilever tests on the oxidized interface show that these oxide layers also significantly reduce the local strength and fracture toughness of the material. This proves that interfacial oxide layers are the underlying reason for the reduction of the dwell fatigue crack growth resistance at 650 °C, particularly in microstructures whose η/δ–matrix interfaces are oriented parallel to the crack growth direction.Graphical

Micromechanical properties of TRISO coatings by in-situ high temperature nanoindentation and microcantilever fracture

Coated nuclear fuel particles, most commonly tri-structural isotropic (TRISO), are intended for use in advanced high temperature reactors. It is vital to understand the mechanical properties of each coating layer to accurately predict the performance of these fuel particles and how these might change at each stage of their lifecycle. This paper reports results of in-situ nanoindentation, with an emphasis on the structural SiC layer, along with microcantilever testing of the critical SiC-IPyC interface. At 1000 °C the hardness of the SiC layer is ~75% lower than at room temperature implying significantly more plasticity at the reactor operating temperature. The elastic modulus was slightly lower at 1000 °C than at room temperature. Microcantilever fracture at the SiC-IPyC interface shows that failure occurs within the pyrolytic carbon layer rather than an interfacial “debonding” with a strength similar to that of bulk pyrolytic carbon.

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.

Microstructure and mechanical properties of hydride blisters formed on Zircaloy-4 claddings

To assess the microstructural and mechanical characteristics of hydride blisters formed on Zircaloy-4 tubes, we employ the electron backscatter diffraction (EBSD) analyses and micro-cantilever failure tests directly on the hydride blister (HB) area. In the HB area, more than 70% of the blister area is characterized to be δ-hydride, and the HB area does not favor the orientation relation ((101¯1)α//(001)δ, [12¯10]α//[1¯10]δ) that is otherwise well-matched in the Zr matrix area. The geometrically necessary dislocation density is observed in the entire blister area, indicating a large amount of strain throughout the blister area. Also, the deformation strain is concentrated in both the hydride and α-Zr phases of HB areas; however, the hydride underwent intensive deformation in the Zr matrix. The average fracture toughness of the hydride blister area obtained through the micro-cantilever test is 1.81 MPam, indicating a brittle fracture that may threaten the integrity of spent nuclear fuel claddings.

Deformation and failure behavior of nanocrystalline WCu

The technical potential of WCu alloys is limited by the modest fracture characteristics of the material system in its coarse-grained condition. To provide a nanocrystalline microstructure and improve mechanical properties, a W-50 at.% Cu composite was processed using high-pressure torsion deformation at a temperature of 200 °C. Therefore, two specimens were subjected to 100% and 1000% shear strains, respectively. Scanning electron and scanning transmission electron microscopy, including nanoscale energy dispersive X-ray spectroscopy mappings, were used to quantify the resulting microstructures. The average grain sizes for the 100% and 1000% deformed specimens were determined to be 14.7 ± 6.6 nm and 10.5 ± 5.6 nm, with the amount of mechanically intermixed W in the Cu grains increasing from 15.4 at.% to 15.9 at.%. X-ray diffraction and selected area electron diffraction studies both revealed strained lattice parameters of the W and Cu phases, respectively. Mechanical properties were investigated using in-situ notched microcantilever tests. The mean conditional fracture toughness and J-integral values were comparable for both conditions, at 3.7 ± 0.4 MPa√m and 245 ± 58 J/m2, respectively. The related behavior could be attributed to the low fault tolerance of the highly deformed states and was substantiated by cleaved globular W grains along the fractured surfaces. In addition, the detailed relationship between the altered grain boundary conditions, the degree of mechanical intermixing and the influence of the different microstructures on the fracture properties were carefully evaluated and discussed to pave the way for future application of these high-strength nanocomposites.

Segregation-enhanced grain boundary embrittlement of recrystallised tungsten evidenced by site-specific microcantilever fracture

Tungsten stands a prime candidate for plasma-facing applications in fusion reactors, attributed to its capacity to withstand high temperatures and intensive particle fluxes. The operational heat flux, however, can induce recrystallisation of the initial microstructure, increasing the brittle-to-ductile transition temperature. Although such a phenomenon is thought to result from impurity segregation to grain boundaries, direct evidence of impurity-induced grain boundary embrittlement has not yet been reported. Addressing this, our study employs microcantilever testing, coupled with local chemical analysis via atom probe tomography, to unveil the impact of impurity segregation on the fracture toughness of recrystallised tungsten with a purity of 99.98 at.%. The in situ fracture toughness measurements were performed with the notch placed directly at random high-angle grain boundaries, revealing brittle failure regardless of grain boundary misorientation or grain orientation. Notably, both single-crystalline microcantilevers and the as-received material exhibited significant plasticity before failure, with instances without crack propagation. In contrast, recrystallised grain boundaries displayed a fracture toughness of 4.7 ± 0.4 MPa·√m, determined using a linear elastic approach - notably lower than for cleavage plane fracture in tungsten microcantilevers. Local atom probe analysis of the high-angle grain boundaries exposed phosphorous segregation exceeding 2 at.% at the recrystallised interfaces, stemming from recrystallisation. Atomistic simulations confirmed the role of phosphorous in embrittling high-angle grain boundaries in tungsten, while additionally revealing mechanisms of crack-grain boundary interactions and their dependence on phosphorous segregation.

Characterization of the interface in a fiber-reinforced polymer composite using a novel single-fiber microcantilever test

The study of fiber to matrix interface properties is crucial in understanding the failure mechanisms of fiber-reinforced composite materials. In this paper, a novel in-situ experimental approach, using micro-cantilever tests, was proposed to characterize a carbon-epoxy unidirectional composite system interface. Computational models were implemented to support the experimental campaign and identify the interface properties. The computational model was validated based on the experimental results, and then it was used to investigate the interface debonding behavior under both damage modes I and II. The validated model allows to investigate the effectiveness of the proposed experimental approach in consistently assessing the interface characterization. Overall, the proposed approach provides a valuable tool for the study of fiber-reinforced polymers and contributes to the development of more accurate predictive models for their behavior.

Direct observation of crack arrest after bridge notch failure: A strategy to increase statistics and reduce FIB-artifacts in micro-cantilever testing

Focused ion beam (FIB) milling has been widely used to prepare micron-sized specimens for micromechanical testing, however, at the same time, unavoidable artifacts originating from the Ga+ ion beam might alter the obtained mechanical properties. Using a bridge notch geometry, which can promote the formation of a sharp natural crack after bridge-failure rather than creating a comparably blunt FIB notch was proposed as a strategy to reduce FIB-induced artifacts more than a decade ago. Even though bridge-failure is widely assumed and predicted by finite element method (FEM) simulations, it has never been observed and quantified experimentally. This study presents the first experimental observation of bridge notch failure and crack arrest before the entire through-thickness main notch (after crack arrest) propagates, which is possible by designing thin bridges and using a stiff experimental setup with superior load resolution. Consequently, we obtained up to three corresponding fracture toughness values from one bending cantilever and significantly less scattered data. Using previously reported geometry correction factors calculated by FEM, the fracture toughness estimated from the bridge-failure was compared with the one from the failure of the main through-thickness notch in CrN/AlN multi-layered and CrN hard coatings.

On the interplay between microstructure, residual stress and fracture toughness of (Hf-Nb-Ta-Zr)C multi-metal carbide hard coatings

The development of sputtered coatings with improved hardness-toughness property combination is widely sought after. Multi-element ceramic carbide (Hf-Nb-Ta-Zr)C coatings with excess carbon, synthesized by DC co-sputtering is presented in this study as a promising candidate to achieve this objective. The specific roles of microstructure and residual stress are decoupled in order to understand their influence on the mechanical properties. Extensive mechanical characterization through in situ testing of focused ion beam fabricated microcantilevers and nanoindentation based approaches are adopted to quantitatively separate the effect of residual stresses on the fracture toughness of the (Hf-Nb-Ta-Zr)C coatings. Residual stress free, microcantilever testing in notched and unnotched conditions, in combination with microstructural characterization unambiguously reveals the intrinsic mechanical behavior of coatings, which solely depend on the microstructure. On the other hand, nanoindentation based testing techniques probe the influence of residual stress and microstructure on the measured mechanical properties. The segregation and thickening of carbon-rich clusters, especially to the grain boundaries with increasing deposition temperatures is speculated to lead to substantial degradation in all mechanical properties measured. An easier fracture path through grain boundaries leads to a reduction in fracture resistance, which is possibly related to carbon enrichment.

Nanoindentation and micro-cantilever testing for understanding cold-dwell sensitivity of heat-resistant titanium alloys

The mechanism of cold-dwell fatigue of the Ti alloys Ti-6242S and IMI 834 was investigated by measuring their local strain-rate sensitivities using nanoindentation analysis, electron backscatter diffraction, and energy-dispersive X-ray spectroscopy. In addition, microcantilever bending tests involving a load-holding-induced change in plastic strain near grain boundaries were conducted to investigate the plastic behavior of the alloys. The difference in the strain-rate sensitivity exponent between soft and hard grains was large in Ti-6242S and small in IMI 834, although slips were generated in the hard grains of both IMI 834 and Ti-6242S by activated slips in the soft grains during the microcantilever bending tests. The hardness of IMI 834 varied with its Al concentration, whereas no Al-concentration-dependent changes in hardness were observed in Ti-6242S. These results suggest that the stress redistribution was caused by differences in the concentrations of solid elements and the crystal orientation of the hard and soft grains.

Erosion behavior of CrN, CrAlN and CrAlN/CrN multilayer coatings deposited on Ti6Al4V

CrN, CrAlN, CrAlN/CrN multilayer coatings were deposited on Ti6Al4V substrates with Multi-arc Ion Plating technique. Hardness and elastic modulus of the nitride coatings were characterized with nano-indentation. In addition, micro-cantilever test was also utilized to measure fracture toughness. Erosion performance of the nitride coatings were also tested with gas-solid erosion test. Moreover, the erosion performance of the tested coatings was analyzed. The interface in the multilayer coating could effectively hind the propagation of the crack increasing erosion resistance of the coating. Erosion resistance of coatings was positively related to hardness and fracture toughness. The CrAlN/CrN multilayer coating with the advantages of high hardness and high fracture toughness, showed the best erosion resistance.

Friction control by tailoring deformation mechanism of interfacial grains in metals

Micro-scale stick-slip motion of metals is closely related to interfacial plasticity. However, previous studies about the effect of interfacial plasticity on the stick-slip motion only focused on the role of asperity plasticity while ignored the role of grain size of interfacial grains. This study focuses on the stick-slip motion of coarse-grain (CG), ultrafine grain (UFG), nanograin (NG), and heterogeneous grain structure Ti–6Al–4V alloys. Micro-scale stick-slip motion controlled by tailoring grain size of interfacial grains was discovered for the first time. When interfacial materials are coarse-grains (CGs) and ultrafine grains (UFGs), the friction mode is mixed regime (stick-slip competed). When interfacial material is nanograins, the friction mode shows slip regime (slip dominated). When interfacial material is heterogeneous grain structure, the friction mode displays partial slip regime (stick dominated). Notched micro-cantilever tests show that dislocation slip in CGs contributes to large plasticity and high fracture toughness, and that grain boundary migration in NGs results in limited plasticity and low fracture toughness. The plasticity and fracture toughness affected by deformation mechanism play an important role in micro-scale stick-slip motion. The nature of the stick-slip motion control by tailoring grain size of interfacial grains is that the change of grain size can affect the deformation mechanism. This study may provide a new design strategy of micro-scale stick-slip motion control by tailoring deformation mechanism of interfacial grains in metals.

Micro-cantilever testing of diamond - silicon carbide interfaces in silicon carbide bonded diamond materials produced by reactive silicon infiltration

SiC-bonded diamond materials produced by pressureless reactive infiltration of diamond preforms with silicon show high hardness and wear resistance. These properties are due to the relatively high diamond volume content of approximately 50 vol% and the mechanically strong interface between diamond and SiC. To determine the bending strength of individual interfaces between diamond and SiC, micro-cantilevers were prepared by focused ion beam milling at 13 grain boundaries and in-situ bending tests were carried out in a scanning electron microscope. The determined strength of cantilevers showing interface fracture was 10.4 ± 4.0 GPa. Fracture surfaces were analyzed to verify the fracture behavior and initiation. In addition to fracture at the interface diamond/SiC, fracture occurred inside the SiC grains and at the SiC/silicon interface at comparable strength values. The results prove the high diamond/SiC-interface bonding strength.

Micro-cantilever testing of microstructural effects on plastic behavior of Ti–6Al–4V alloy

To evaluate the effects of microstructural factors on the plastic behavior of Ti–6Al–4V alloys, micro-cantilevers featuring several grains were machined using a focused ion beam. The equiaxed structure (a mixture of equiaxed α phase and intergranular β phase) and bimodal structure (a mixture of an equiaxed α phase and lamellar phase) were examined to verify the microstructural effects. To determine the slip plane and direction, the crystal structure was evaluated using electron backscattered diffraction, following bending tests of the microcantilever. In addition, the elastic modulus and hardness of each grain were determined via nanoindentation testing. In the equiaxed microstructure, the β-phase deformation often occurred first; then, slips were activated inside the equiaxed α phase. It is concluded that the β phase deforms more easily than the α phase because the elastic modulus and hardness of the former are lower, as indicated by the nanoindentation testing. In the bimodal microstructure, β-lath deformation occurred along the acicular direction in the lamellar-phase region, and slips occurred inside the α grain. The deformation of the lamellar structure was associated with β-phase deformation and affected by the high local stress at the grain boundary and phase interface, owing to the difference in grain orientation within the lamellar structure. The results of this study suggest that the plastic behavior of Ti–6Al–4V alloys depends not only on the slip system of the α grains but also on the deformability of the β phase and the misorientation of adjacent grains in polycrystalline Ti–6Al–4V alloy.

Assessing strain rate sensitivity of cement paste at the micro-scale through micro-cantilever testing

This study presents an experimental investigation of the rate-dependent mechanical properties of cement paste at the microscale. With the use of a nanoindenter, micro-cantilever beams with the size of 300 μm × 300 μm × 1650 μm were loaded at five different strain rates from around 10−6/s to 10−2/s until failure. It is found that with increasing strain rate, the stress-strain curves show less and delayed pre-peak nonlinearity. Both the flexural strength and the elastic modulus of beams increase with increasing strain rate, while the strain at peak stress exhibits an opposite trend. Examination of the fracture surface indicates that with increasing strain rate the possibility of a crack to pass through stronger components of the hydration products is increased. The experimental observations and possible mechanisms leading to changes in mechanical responses are discussed. It is suggested that at least two micromechanical processes, namely creep and Stéfan effect, are mainly responsible for the rate-dependent behaviour of cement paste within the investigated strain rate range and their dominances seem to vary with the strain rate. At lower strain rate, the strain rate sensitivity of cement paste is thought to be dominated by the creep effect, while at higher strain rate the Stéfan effect appears to be the governing factor.

On the fracture behavior of Cr2AlC coatings

Bulk MAX phase materials were investigated heavily in the last decades due to their advantageous combination of metallic and ceramic properties. In recent years, MAX phases also gained the interest of the protective coatings community. Cr2AlC is a very promising material, since the crystalline MAX phase can be deposited at comparatively low (550 °C) substrate temperatures. Another advantage of the Cr2AlC MAX phase is its self-healing ability. The goal of this investigation was to characterize the fracture toughness of Cr2AlC protective coatings using in situ SEM micro-cantilever tests and to determine the influence of different microstructures on the fracture behavior. Surprisingly, the fracture toughness is only moderately affected by the microstructure of the crystalline samples investigated here, which reveal a fracture toughness ranging from 1.8 ± 0.1MPam1/2 to 2.4 ± 0.2MPam1/2.In contrast to that, it could be shown that there is a significant increase in fracture toughness for the amorphous coating with identical chemical composition (4.1 ± 0.5MPam1/2) of almost twice the fracture toughness compared to the crystalline coatings. The detrimental influence of grain boundaries in the crystalline coating and the lack of grain boundaries in the amorphous sample might explain the formidable fracture toughness.

Adhesion, structure, and mechanical properties of Cr HiPIMS and cathodic arc deposited coatings on SiC

Chromium deposited by high-power impulse magnetron sputtering (HIPIMS) versus cathodic arc (CA) processes exhibits very different mechanical properties. Combining the two can result in a single-phase with superior performance that can be tailored for use in coating SiC for advanced nuclear fuel cladding. Coating morphology, residual stress, elemental depth profiles, and mechanical testing by scratch, pull-off adhesion, and microcantilevers are shown for HiPIMS, CA, and combined coatings. CA coatings were likely to spall and had lower adhesion strength due to tensile residual stresses but had desirable material properties. By depositing an initial layer of Cr by HiPIMS followed by a layer of Cr by CA, a more adherent coating was achieved and some of the stress issues with CA morphology were resolved. Combined coatings withstood at least 80 MPa in pull-off adhesion tests and had maximum failure stress values of 5GPa in microcantilever tests. These results were better than either individual deposition method and point to a hybrid approach being a path forward for a more robust coating. • Cr coatings were deposited by HiPIMS and cathodic arc on model SiC substrates. • Properties of both individual and hybrid dual layer coatings were studied. • A hybrid coating benefitted both adhesion and mechanical properties.

Addressing Fracture Properties of Individual Constituents Within a Cu-WTi-SiOx-Si Multilayer

With modern materials applications continually decreasing in size, e.g., microelectronics, sensors, actuators, and medical implants, quantifying materials parameters becomes increasingly challenging. Specifically, addressing individual constituents of a system, such as interfaces or buried layers in a multilayer structure, emerges as a topic of great importance. We demonstrate herein a technique to assess fracture parameters of different interfaces of a Cu-WTi-SiOx-Si model system based on in situ microcantilever testing in a scanning electron microscope. Positioning the initial notch position with respect to the interface of interest enabled selection of different crack paths, while an additional overlaid sinusoidal signal permitted continuous measurement of stiffness changes and thereby experimental measurement of the actual crack extension. We thus achieved continuous J–Δa curve measurements for the interface between Cu and WTi, the bulk WTi, and the interface between WTi and SiOx. The localized nature of this novel approach makes it generally applicable to testing specific interfaces.

Micro-cantilever testing on the short-term creep behaviour of cement paste at micro-scale

This study proposes an experimental method for studying the short-term creep behaviour of cement paste at micro-scale. The micro-bending tests on miniaturized cantilever beams were used to characterize the viscoelastic properties of cement paste. The effects of w/b ratio, the type of binder and the stress level on the microscopic creep behaviour were investigated. It is found that the short-term creep of cement paste at microscale can be satisfactorily described by a power-law function. A linear viscoelastic behaviour has been observed in different cementitious systems at the microscale with the stress level up to 67.9%. When compared with the creep results in microindentation tests and conventional macroscopic tests, the obtained creep compliance function in this study is found to be both qualitatively and quantitatively representative of the macroscopic results. This experimental study underlines the importance of microstructural effect on the creep behaviours of cementitious materials at microscale.