Micropillar Tests Research Articles

Measuring Plasticity in Confined Cu Thin Films at Different Geometries

We report quantitative measurement of plasticity in confined Cu thin films with a new micro-pillar testing protocol. Polycrystalline Cu and CrN thin films were sequentially sputter deposited onto Si (100) substrates, forming thin film assemblies in which polycrystalline Cu thin films of various thicknesses were confined between non-deforming Si and CrN. Cylindrical micro-pillars of CrN/Cu/Si were fabricated through scripted focused Ga+ ion beam milling, with the interfaces either normal to the axial direction or at a 45° inclination. The CrN/Cu/Si micro-pillars were compression loaded in the axial direction with a flat diamond punch on an instrumented nanoindenter. The axial compression loading caused extensive plasticity within the thin Cu interlayers with the interfaces in both the normal and inclined orientations, but with distinctly different responses. We show that significant plastic flow occurs within the confined Cu thin films in both normal compression and shear loading. The flow stress of the confined Cu films is dependent on the Cu layer thickness and the deformation geometry. The presently described micro-pillar testing protocol offers quantitative evaluation of the plastic response of thin metal films under different deformation geometries. The present results offer new experimental examples of scale-dependent plasticity in thin metal films, and new experimental test cases for non-local plasticity theories.

Specimen size effects on the weakening of a bulk metastable austenitic alloy

In this work, we are investigating the scaling effects on an austenitic stainless steel with changing grain size using microcompression testing. It is our aim to evaluate at what sample-to-grain size ratio the mechanical properties such as yield stress deviate from the macroscopically determined data. It was found that decreased yield stresses with decreasing specimen size (weakening) occur during the microcompression. The weakening was observed when the diameter-to-grain size ratio (D/d) dropped below a critical value. The effect of the grain size on the critical value, above which the bulk property can be obtained, was systematically investigated utilizing a wide range of D/d values (0.5–30) for specimens with a grain size ranging from 0.3 to 2 μm. It was found that the critical D/d value decreases with increasing grain size. A simple analytical model was developed, which is applicable to both micropillar and tensile tests. Comparison of the model equations with experimental data showed that the reduction in yield stress with sample size can be associated with the weaker near-surface zone, which has a reduced strength compared to the specimen interior zone. Furthermore, this study suggests that the size of the near-surface zone may be related to the dislocation structures in near-surface grains, and therefore the overall behavior is governed by the grain size and stacking fault energy.

Analytical determination of the constitutive behavior from micro-pillar testing: Application to a tempered martensitic steel

Micro-compression experiments were carried out on micro-pillars, with nominal diameter of 10 μm and height 20 μm, and fabricated from a high-chromium tempered martensitic steels developed for nuclear applications. An analytical method was proposed to calculate the imposed compressive displacement on the pillar that allows determining the constitutive behavior up to about 5% of plastic strain. The experimental micro-compression curves were showed to be adequately modeled by the finite element simulations, which were used to validate the main assumption considered for the analytical model development.

Development and testing of microcompression for post irradiation characterization of ODS steels

For decades, the nuclear materials community has been interested in smaller tests for obvious benefits like in limited space or reduction in activity. In this work, we are presenting micropillar tests on ODS steel with a grain size of ∼450nm and finely distributed oxides as a part of an attempt to use this method in evaluating the change in mechanical properties of nuclear materials after irradiation. Systematic works combining microcompression tests and TEM observations showed that there is no significant size effect on the measured strengths and the dislocation structures of the ODS alloy down to a pillar size of 1μm. We used both circular and square pillars and discuss the effect of pillar geometry with the evaluated strength and errors. The measured strengths at half the pillar height were found to be in good agreement with the bulk strength measured from a miniature tensile test. The cross-sectional TEM observation of the deformed pillar showed that oxide particles act as strong internal constraining sources for dislocation movement and the formation of dislocation structure.

Effects of machine stiffness on the loading–displacement curve during spherical nano-indentation

Abstract

Elastoplastic buckling as source of misinterpretation of micropillar tests

Microscopic compression tests (micropillar tests) are typically used to obtain stiffness and strength properties of materials at small length scales. In this work it is shown that structural effects, in particular instabilities, have implications on the resulting load–displacement diagram. Care has to be taken when the measured load–displacement path of a micropillar is interpreted as a stress–strain path of the material. Several structural effects are discussed by means of computational analysis.

Microscale-calibrated modeling of the deformation response of low-carbon martensite

Micropillar compression tests were used to determine the uniaxial compressive stress–strain response of martensite blocks extracted from a low-carbon, fully lath martensitic sheet steel, M190, with the nominal composition C=0.18, Mn=0.47, P=0.007, S=0.006, Si=0.18, Al=0.06, Ti=0.045, B=0.0014 and balance Fe (all in wt.%). Specimens with a diameter exceeding ∼1μm and consisting of a single martensite block showed elastic–nearly perfectly plastic behavior with a yield stress of the order 1200MPa. Similar specimens which contained multiple martensite blocks showed pronounced strain hardening, arising from the geometrical constraint produced by the interface(s). No size dependence of flow stress was observed in micropillars with diameters exceeding 1.0μm, but a significant scatter in strength and hardening rate was observed in micropillars with smaller diameters. Flow data for micropillars in the size-independent regime were used to determine parameters in a crystal-plasticity-based model of martensite. Full three-dimensional crystal plasticity simulations, with material properties determined from micropillar tests, were then used to predict the macroscopic uniaxial stress–strain behavior of a representative volume element of martensite. The predicted stress–strain behavior was in excellent agreement with experimental measurements, and demonstrates the potential for micropillar tests to determine material parameters for individual phases of a complex microstructure.

Determination of interfacial mechanical properties of ceramic composites by the compression of micro-pillar test specimens

A novel method to determine the fiber-matrix interfacial properties of ceramic matrix composites is proposed and evaluated; where micro-pillar samples containing inclined fiber/matrix interfaces were prepared from a SiC fiber-reinforced SiC matrix composites and then compression-tested using the nano-indentation technique. This new test method employs a simple geometry and mitigates the uncertainties associated with complex stress state in the conventional single-filament push-out method or tensile unloading–reloading hysteresis loop analysis method for the determination of interfacial properties. Based on the test results using samples with different interface orientations, the interfacial debond shear strength and the internal friction coefficient are explicitly determined and compared with values obtained by other test methods. SEM observation showed that micro compression caused an adhesive type of debonding between the fiber and the pyrolytic carbon interface. The results suggest that the debonding/failure behavior of the micro-pillars followed the Coulomb fracture criterion. The determined interfacial debond shear strength is ~100 MPa, which appears to be smaller than that determined from fiber push-out test for similar composite systems. The difference can be explained by the effect of normal stress (clamping stress) on the apparent interfacial debond shear strength.

Study of the Size Dependence of Time-dependent Plastic Deformation of Gold Micro-pillars and Micro-spheres

ABSTRACTIn this study the length scale dependence of the operative mechanisms of time-dependent plastic deformation was studied using room temperature compression tests performed on Au micro-pillars and micro-spheres of 1.0 to 5.0 µm diameter. All the samples tested displayed deformation that had a component of random strain jumps. In the case of the Au micro-pillars, the frequency of the strain jumps showed a bilinear dependence upon pillar diameter with the frequency being larger, and more sensitive to diameter, when the pillar diameter was small (and τR was high). We suggest that this indicates a transition from deformation occurring by deformation on multiple slip planes to deformation occurring predominantly by single-plane dislocation slip when the pillar diameter is less than 2 µm.The strain jump frequency during the constant-load micro-pillar creep tests showed a linear dependence upon τR. Creep tests performed on the micro-spheres of 5.0 µm diameter displayed displacement jump frequency that was essentially independent of the applied load while the jump frequency increased with increasing load for the smaller 2.5 µm diameter micro-spheres. We suggest that this difference is related to the volume of the micro-sphere. When the volume is small, the component of the deformation that occurs by a stochastic dislocation glide process is increased and becomes strongly dependent upon the magnitude of the local shear stress.

Evaluation of Irradiation Effects on Fracture Strength of Silicon Carbide using Micropillar Compression Tests

Micro-scale material testing techniques (e.g., nanoindentation, micropillar compression, etc.) have been developed and applied for the evaluation of radiation effects on the mechanical properties of nuclear reactor materials. These techniques can measure the mechanical properties from small volumes, which can be beneficial in the investigation of both neutron and ion-irradiated materials. In this study, we evaluated the compressive fracture strength of ion-irradiated silicon carbide (SiC) using micropillar compression tests. SiC and SiC-based composites are considered as candidate materials for micro electronics, sensors, and various components in next-generation reactors. Micropillars with a nominal diameter of 0.65 μm were fabricated on a monolithic polycrystalline cubic SiC plate using a lithography and plasma etching technique. Half of the total micropillars were irradiated with Si2+ ions. The SiC micropillars were uniaxially compressed using a flat diamond punch. A finite element model was used to compute the elastic stress state of the pillar. The results showed irradiation-induced strengthening, which is consistent with other investigations. In addition, plastic deformation of SiC at room temperature was observed. A slip was found to occur on {111} planes from a cross-sectional transmission electron microscopy observation. The micropillar tests are expected to significantly contribute to the study of irradiation and size effect on the mechanical properties of ceramic materials to be used in a radiation environment.

Stress–strain analysis for evaluating the effect of the orientation of dentin tubules on their mechanical properties and deformation behavior

A model whose porosity does not vary with compression depth is developed for evaluating the mechanical properties of dentin tubules with various orientation angles from micro-pillar nanocompression tests. Experimental results for a range of loading rates indicate that the yielding parameters and the elastic modulus are little affected by the creep behavior. For a given compression depth, the hardness, elastic modulus, and yielding strength decrease with increasing orientation angle of dentin. The mechanical properties obtained using the proposed model are consistent with the reported data, and are actually more precise since they consider the orientation angle. The proposed testing method can be applied to materials that yield a negative value of the elastic modulus due to creep behavior.

Atomistic free-volume zones and inelastic deformation of metallic glasses

The amorphous nature of metallic glasses and their mechanical properties make them interesting for structural applications. However, the interplay between the nature of atomic structures in metallic glasses and mechanical properties remains poorly understood. In this study, high-frequency dynamic micropillar tests have been used to probe both atomic clusters and flow defects in metallic glasses. We show that loosely bonded atomistic free-volume zones that are enveloped elastically by tightly bonded atomic clusters show a deformation character similar to supercooled liquids. At room temperature, the effective viscosity of these free-volume zones is of the order of 1 x 10(8) Pa s before the occurrence of shear banding. The confined liquid-like deformation of free-volume zones leads to significant mechanical hysteresis in micropillars under dynamic loading, providing important insight into how atomistic structural features affect the deformation behaviours in metallic glasses in the apparent elastic regime. The inelastic behaviour also serves as the basis for the superior damping resistance of metallic glasses.

Measuring the Young’s Relaxation Modulus of PDMS Using Stress Relaxation Nanoindentation

AbstractThe Young’s relaxation modulus of Polydimethylsiloxane (PDMS) film specimens was measured by the nanoindenter with a flat punch indenter. In the stress relaxation test, the initial ramp part was carefully considered to develop an accurate viscoelastic contact model. This model was used to fit the load-time data from the experimental tests. The resulting relaxation function was expressed by a general Maxwell equation. In addition, a case study of PDMS micropillar bending tests was performed, and the viscoelastic constitutive law was applied to develop an analytical solution of the reaction force. The results show that the reaction force calculated from the corrected model is generally agreed well with the experimental data.

Microcharacterization of Damage in Materials for Advanced Nuclear Fission Plants

Miniature and sub-miniature samples were used for determination of mechanical properties of materials for advanced fission plants. Results from indentation and focused ion beam prepared micro-samples, punch tests and thin strip (irradiation) creep tests are shown. The results allow conclusions concerning materials damage. Irradiation damage profiles were determined with indentation. Results from micro-pillar tests showed a good agreement with results from conventional samples in case of oxide dispersion strengthened steels. Thin strip irradiation creep experiments revealed a negligible influence of dispersoid size/distribution on creep rates. Punch tests of fibre reinforced materials showed consistent results which still need quantitative analysis.

Effects of focused ion beam milling on the nanomechanical behavior of a molybdenum-alloy single crystal

Nanoindentation was performed on a Mo-alloy single crystal to investigate effects of focused ion beam (FIB) milling on mechanical behavior. On a non-FIB-milled surface, pop-ins were observed on all load-displacement curves corresponding to a transition from elastic to plastic deformation. Similar pop-ins were not detected on surfaces subjected to FIB milling. This difference indicates that FIB milling introduces damage that obviates the need for dislocation nucleation during subsequent deformation. A second effect of FIB milling is that it increased the surface hardness. Together, these effects could be the source of the size effects reported in the literature on micropillar tests.