Nanoindentation Loading Curves Research Articles

Micro-mechanical properties characterization of shot peening strengthened layer with nanoindentation

In this work, the influences of shot peening (SP) on ultra-high strength steel were investigated. For the characterization of the strengthened layer, an inclined specimen was proposed for X-ray diffraction and nanoindentation, the micro-mechanical properties in the strengthened layer were studied with both experimental and crystal plasticity finite element (CPFE) analysis. It's noted that SP introduced grain refinement and compressive residual stress were highly concentrated in the surface layer due to the high strength and hardness, and the maximum depth influenced was only 0.1 mm. Based on simulation results, it's found that the differences in nanoindentation loading curves were introduced by the work done by residual stresses. To estimate the residual stress in the strengthened layer, a modified model was proposed with nanoindentation. The newly proposed model gives improved estimation accuracy compared with Wang's model, and it could give more precise results in contrast to the X-ray diffraction method for strengthened layers.

Production and characterization of the Cr35Fe35V16.5Mo6Ti7.5 high entropy alloy

The microstructure, thermal stability, and mechanical properties of a novel Cr35Fe35V16.5Mo6Ti7.5 high-entropy alloy were studied. The mechanical properties were mapped by nanoindentation, and the results correlated with the microstructure and the Vickers microhardness measurements. The alloy was produced by arc melting in a low pressure He atmosphere. Thermal treatments were performed to study the thermal stability of the alloy. The as-cast microstructure of the alloy exhibited a body-centered cubic phase with morphology of dendrites, outlined by a very thin interdendritic phase with a crystallographic structure compatible with Fe2Ti. The presence of the intermetallic particles was predicted by a free-energy based model, in contrast with the single solid solution alloy predicted by a parameter-based model. The volume fraction of the dendrites in the alloy is ∼ 94 % after arc melting. A small fraction of sparse Ti-rich particles, ∼0.4 vol%, was observed. The thermal treatments produced an increase of the population of Ti-rich particles, the formation of a σ-phase and nucleation of precipitates enriched with Fe and Ti into the previous dendrites. The material in as-cast condition exhibited a microhardness value of 6.2 ± 0.3 GPa, while the alloy aged at 960 °C resulted in 7.1 ± 0.4 GPa. Nanoindentations maps showed an excellent correlation with the microstructure, and their statistical analyses yielded a nanohardness mean value of 8.2 ± 0.4 GPa in the dendritic BCC regions of the as-cast and thermal treated samples and 14.1 ± 0.6 GPa for the σ-phase. The onset of the plastic behavior has been studied by analyzing the pop-in phenomenon observed in the nanoindentation loading curves. For the as-cast alloy, this analysis showed that the elastic-to-plastic transition seems to be triggered by dislocation nucleation. The alloy has a low thermal diffusivity in the measured temperature range that increases on increasing temperature.

Deformation and fracture behaviours of a YAG single crystal characterized using nanoindentation method

In this work, nanoindentation tests were carried out to understand the deformation and fracture behaviours of yttrium aluminium garnet (YAG) single crystal. The elastic modulus, hardness and fracture toughness of the YAG crystal were 267.8 ± 1.3 GPa, 24.66 ± 0.37 GPa and 1.82 ± 0.23 MPa m1/2, respectively. A pop-in was observed on the nanoindentation loading curve at approximately 1 mN load, associated with the occurrence of the elastic-plastic transition. When the indentation load was in the range of 1–6 mN, YAG deformed both elastically and plastically. Evident slip band along (−110) was observed beneath the indent impression. Dislocations, stacking faults, lattice bending and kinking were found in the plastically deformed subsurface. Radial cracks formed when the indentation load exceeded 6 mN, with the half-penny cracks initiated below the surface during unloading. The knowledge of the fracture toughness of YAG and the insights into its deformation behaviour under quasi-static contact attained from this study would greatly benefit the process optimization of ultra-precision machining of YAG single crystal.

High temperature nano-indentation of tungsten: Modelling and experimental validation

It is very well known that tungsten is intrinsically brittle at room temperature, and the characterization of its ductile properties by conventional mechanical tests is possible only above the ductile-to-brittle transition temperature (DBTT), i.e. above 500–700 K. However, the design of tungsten-based components often requires the knowledge of constitutive laws below the DBTT. Here, we carried out instrumented hardness measurements in the temperature range of 300–691 K by nano-indentation. The obtained results are used to extend a set of constitutive laws for the plastic deformation of tungsten, developed earlier on the basis of tensile data, which now covers the temperature range of 300–1273 K. The validation of the constitutive laws was realized by the crystal plasticity finite element method (CPFEM) model, which was applied to simulate the nano-indentation loading curves. The distribution of stress and strain under the indenter was also studied by the CPFEM to bring an insight on the extension of the plastic zone in the process of the indentation, which is of crucial importance when nano-indentation is used to resolve the microstructural features generated by e.g. irradiation by energetic particles, plasma exposure or thermo-mechanical treatment.

Nanoindentation in Metallic Glasses with Different Plasticity

Nanoindentation and thermomechanical experiments on three types of metallic glasses with different glass forming ability were carried out. The nanoindentation behaviour at room temperature was associated with the creep at elevated temperatures. Different discontinuity populations and their shape observed on the nanoindentation loading curves were compared with morphology of plastic deformed indent regions. The influence of the differences in thermal stability of studied alloys on the nanoindentation in these alloys were studied as well.

Activation energy of the low-load NaCl transition from nanoindentation loading curves.

Access to activation energies E(a) of phase transitions is opened by unprecedented analyses of temperature dependent nanoindentation loading curves. It is based on kinks in linearized loading curves, with additional support by coincidence of kink and electrical conductivity of silicon loading curves. Physical properties of B1, B2, NaCl and further phases are discussed. The normalized low-load transition energy of NaCl (Wtrans/µN) increases with temperature and slightly decreases with load. Its semi-logarithmic plot versus T obtains activation energy E(a)/µN for calculation of the transition work for all interesting temperatures and pressures. Arrhenius-type activation energy (kJ/mol) is unavailable for indentation phase transitions. The E(a) per load normalization proves insensitive to creep-on-load, which excludes normalization to depth or volume for large temperature ranges. Such phase transition E(a)/µN is unprecedented material's property and will be of practical importance for the compatibility of composite materials under impact and further shearing interactions at elevated temperatures.

Experimental and theoretical confirmation of the scaling exponent 2 in pyramidal load displacement data for depth sensing indentation.

A series of articles by Kaupp et al. have recently been published in "Scanning," containing erroneous claims about the curvature of pyramidal nanoindentation loading curves. The present paper recalls the theoretical reasons why, for self-similar indenter shapes like pyramidal or conical indentations, the load scales with the indentation depth squared. Furthermore, experimental evidence for that behavior is provided for a wide variety of materials, ranging from ceramics to metals and polymers.

Effect of surface roughness in the determination of the mechanical properties of material using nanoindentation test

A quantitative model is proposed for the estimation of macro-hardness using nanoindentation tests. It decreases the effect of errors related to the non-reproducibility of the nanoindentation test on calculations of macro-hardness by taking into account the indentation size effect and the surface roughness. The most innovative feature of this model is the simultaneous statistical treatment of all the nanoindentation loading curves. The curve treatment mainly corrects errors in the zero depth determination by correlating their positions through the use of a relative reference. First, the experimental loading curves are described using the Bernhardt law. The fitted curves are then shifted, in order to simultaneously reduce the gaps between them that result from the scatter in the experimental curves. A set of shift depths, Δhc , is therefore identified. The proposed approach is applied to a large set of TiAl6V4 titanium-based samples with different roughness levels, polished by eleven silicon carbide sandpapers from grit paper 80 to 4,000. The result reveals that the scatter degree of the indentation curves is higher when the surface is rougher. The standard deviation of the shift Δhc is linearly connected to the standard deviation of the surface roughness, if the roughness is high-pass filtered in the scale of the indenter (15 µm). Using the proposed method, the estimated macro-hardness for eleven studied TiAl6V4 samples is in the range of 3.5-4.1 GPa, with the smallest deviation around 0.01 GPa, which is more accurate than the one given by the Nanoindentation MTS™ system, which uses an average value (around 4.3 ± 0.5 GPa). Moreover, the calculated Young's modulus of the material is around 136 ± 20 GPa, which is similar to the modulus in literature.

Study of the structure and mechanical properties of hexagonal BaTiO3 thin films prepared by sol–gel processing

Hexagonal barium titanate (HBT) thin films were prepared on borosilicate plate substrates via sol–gel method using the dip-coating process. The structure, texture and morphology of the thin film were analyzed by X-ray diffraction, atomic force microscopy, nanoindentation technique, and transmission electron microscopy. The results showed that the thin film annealed at 700 °C crystallized with BaTiO3 hexagonal phase and traces of Ba2TiO4 (secondary phase). The nanoparticles and the RMS roughness of the sample treated at 700 °C presented high values when compared with those thermally treated at lower temperatures. The hardness and Youngs’ modulus of the thin films increased with increasing in grain size, and the thin film annealed at 700 °C with crystallite size about 10 nm presented multiple “pop-in” events during nano-indentation loading curves. The annealing temperature, growth size and surface roughness were discussed in connection with the HBT mechanical properties.

Penetration Resistance and Penetrability in Pyramidal (Nano)Indentations

Pyramidal nanoindentation loading curves were linearly plotted, normal force versus (penetration depth)(3/2) . The slope is penetration resistance k, its inverse penetrability. Linear correlations verify. All contributions to the indentation are included in the penetrability. Dependencies and uses of the extrapolation tools are exemplified, identified, and discussed. In the case of phase transition including twinning within the loading range a sharp kink occurs, again with verifying correlation in both branches of the linear plot. The exponent 3/2 applies to all types of materials upon conical or pyramidal indentations onto normal flat surfaces, independent of the various mechanistic responses. While common curve fitting procedures of loading curves and finite element (FE) calculations miss phase transitions, gradients, surface effects, elbows, (nano)pores, and change from tip rounding to cone (at very low penetrations), these are recognized by the penetration resistance analysis. Also prominent undisturbed pyramidal or conical micro- and macroindentations provide linear plots with exponent 3/2. Numerous FE simulations create experimentally unsupported "loading curves." This is discussed with typical published examples. An explanation for the deviation from Sneddon's and Love's theory is given by correction for the shear-force part that does not participate in the penetration depth. The validation of the exponent 3/2 instead of previously assumed 2 requires adjustment of mechanical parameters that were defined by using the nonsupported exponent.

Quantification of first contact detection errors on hardness and indentation size effect measurements

A new method is proposed for the quantification of macroscopic hardness and indentation size effect. This method is based on the simultaneous treatment of several nanoindentation loading curves which are located according to the gap between their shape and the one predicted by the Bernhardt law. By applying this method on stainless steel loading curves, it is shown that hardness error can be reduced by a factor of 2 compared with usual treatment methods. It is shown that only the simultaneous treatment of all the curves enables the good prediction of pile-up in the material.

Plasticity enhancement of Zr-based bulk metallic glasses by direct current electropulsing

Direct current electropulsing was used to improve the plasticity of (Zr53Cu30Ni9Al8)99.5Si0.5 bulk metallic glasses. After the electropulsing treatment, the specimen showed reductions in both the glass transition and the crystallization temperatures while retaining its amorphous structure, and both Young's modulus and the hardness decreased while the nanoindentation loading curve became more serrated. Using the bond-interface method and Vickers indentation, the treated specimen showed more branching of semi-circular shear bands and less radial shear bands compared to its as-cast counterpart. The possible plasticity enhancement mechanism of the electropulsing treatment was also discussed.

Prediction of bluntness for pyramidal indenters from nanoindentation curves

A blunted height of a sharp indenter can be elucidated by fitting the indentation load versus the indentation depth data into Kick's law. Nanoindentations adapting two Berkovich indenters with different bluntnesses were performed on fused silica and then resulting nanoindentation loading curves were analyzed from a linear proportional fitting between the square root of the indentation load and the indentation depth modified with the blunted height; the blunted heights of 4.1±0.5 and 38.4±1.9nm were estimated for the new and the severely worn indenters, respectively. In addition, the validity of this bluntness prediction was confirmed by a direct observation with an atomic force microscope; the blunted height measured for the indenter B was 45nm comparable with the predicted value. The bluntness correction applied to the indentation data for a 200nm-thick SiO2 coating on Si substrate yielded not only roughly overlapped nanoindentation curves but also discrete discrepancies at low and peak load regimes. These phenomena are discussed from a geometrical difference of both indenters within the blunted regime and an extensive deformation of the hard substrate at deep indentations.

Analytical method for the determination of indenter constants used in the analysis of nanoindentation loading curves

We present a simple analytical method for the determination of indenter constants, key parameters used in the computation of the elastic modulus (or hardness) from the nanoindentation loading curve. In both forward and reverse analyses of the loading curve the proposed set of values are found to be more effective than those obtained empirically for the materials considered herein. Further improvement is anticipated as the accuracy of these constants depends on how precisely mechanical properties and curve fitting parameters are computed.

Analysis of Berkovich Nanoindentation Loading Curves in SiC and SiO2 Materials

On two SiC ceramics and a standard fused silica specimen, depth-sensing indentation tests with a Berkovich indenter were carried out at several peak loads. The loading part of the load-displacement curves were statistically analyzed and it was shown that the loading exponent of the relation P ∝ hn was lower than the expected value of 2 in most of the cases. Factors affecting the loading exponent have been considered and evaluated. It was demonstrated that their effect was minimal on the fitting results.

On elastic nanoindentation of coated half-spaces by point indenters of non-idealshapes

The elastic contact of non-ideal conical and Berkovich indenters with bi-layer half-spaces isinvestigated. Blunted tips are simulated as smooth surfaces. The boundary elementmethod is employed to carry out the numerical simulations of nanoindentation.An analytical analysis of the influence of the coating thickness and the tip bluntnessmagnitude on the nanoindentation loading curve is realized. The dimensionless compressionforce is introduced in order to describe the nanoindentation at different approaches betweenthe indenter and the coated half-space. A practical technique for determining the Young’smodulus of coatings is proposed. The technique is based on the modelling of indentation ofthe blunted indenter tip into the coating/substrate composite. This technique is appliedto the nanoindentation study of nanocrystalline Cr coatings on silicon and glasssubstrates being tested by a diamond Berkovich indenter with a blunted tip.

Nanoindentation studies of shear banding in fully amorphous and partially devitrified metallic alloys

Nanoindentation loading curves on metallic glasses can show sharp, constant-load steps. The relationship of these steps to shear-band initiation is explored for a wide range of metallic glasses, from thin films to bulk. On a given loading curve, there is a well defined strain for initiation of shear bands. The steps may be blurred by instrumental effects or partial devitrification. The nanoindentation results do not suggest a transition in deformation mode as a function of strain rate.

Nanoindentation studies of shear banding in fully amorphous and partially devitrified metallic alloys

Nanoindentation loading curves on metallic glasses can show sharp, constant-load steps. The relationship of these steps to shear-band initiation is explored for a wide range of metallic glasses, from thin films to bulk. On a given loading curve, there is a well defined strain for initiation of shear bands. The steps may be blurred by instrumental effects or partial devitrification. The nanoindentation results do not suggest a transition in deformation mode as a function of strain rate.

Measurement of the mechanical properties of carbon nitride thin films from the nanoindentation loading curve

Mechanical properties such as Young's modulus of hard highly elastic carbon nitride films are difficult to determine by nanoindentation from the conventional analysis using the unloading curve as proposed by Oliver and Pharr due to the high elastic recovery of these materials, resulting in unexpectedly low modulus values. An analysis of the nanoindentation loading curve was used to determine the Young's modulus of carbon nitride thin films. The method also enables determination of the maximum penetration depth where the coating only properties can be achieved. Carbon nitride thin films were deposited by RF magnetron sputtering using graphite as a target material. Nanoindentation testing on the samples was carried out using a CSM™ nanohardness tester. The Young's modulus value of carbon nitride films, analysed from nanoindentation loading curves, was approximately 822 GPa, which is much higher than the value 170 GPa obtained from the conventional analysis using the unloading curve.

Evaluation of Thin-Film Residual Stress Using Nano-Indentation Combined with an Atomic Force Microscope

Thin-film stress changes the shape of nanoindentation loading curve. The change in the indentation load is treated as the effect of the residual stress on the indenting deformation. A residual-stress-induced normal load is defined as a multiple of contact area and plastic deformation-sensitive deviator stress component extracted from the equi-biaxial thin-film stress. A final equation for the residual stress is derived from the residual-stress-induced normal load by considering an integration along a depth-controlled stress-relaxation route. This proposed model is applied to the analyses of the nanoindentation curves for diamond-like carbon thin films. The residual stresses from the nanoindentation analyses were consistent with the values obtained by the conventional curvature method.