Cube Corner Indenter Research Articles

Nano- and micro-scale impact testing of zirconia, alumina and zirconia-alumina duplex optical coatings on glass

Optimising the mechanical properties of optical coatings to improve their durability will be critical if they are to be used successfully in harsh environments where they may be subject to degradation by mechanical contact. In this study zirconia, zirconia-alumina duplex and alumina experimental coatings were deposited on soda lime and borosilicate glass and their resistance to repetitive impact under different experimental conditions evaluated in nano- and micro-scale impact tests. The influence of changing probe geometry (sharp and blunt contacts) and applied load on the deformation was studied. Spheroconical indenters were found to be more suitable to study the load sensitivity of the impact response than sharp cube corner indenters. Increased resistance to plastic deformation in the coating-substrate system (H3/E2) proved detrimental to the damage tolerance to the repetitive nano- and micro-impact tests. To compare the deformation behaviour in nano-impact and nano-scratch, tests were performed using the same spheroconical probe, revealing cracking and blistering of the glass substrate in both types of test. The change in probe depth after the first impact was found to be a very useful metric to effectively compare the evolution of surface damage on continued impact in nano- and micro-impact tests at different applied load and probe geometry.

On the governing fragmentation mechanism of primary intermetallics by induced cavitation

One of the main applications of ultrasonic melt treatment is the grain refinement of aluminium alloys. Among several suggested mechanisms, the fragmentation of primary intermetallics by acoustic cavitation is regarded as very efficient. However, the physical process causing this fragmentation has received little attention and is not yet well understood. In this study, we evaluate the mechanical properties of primary Al3Zr intermetallics by nano-indentation experiments and correlate those with in-situ high-speed imaging (of up to 1 Mfps) of their fragmentation process by laser-induced cavitation (single bubble) and by acoustic cavitation (cloud of bubbles) in water. Intermetallic crystals were chemically extracted from an Al-3 wt% Zr alloy matrix. Mechanical properties such as hardness, elastic modulus and fracture toughness of the extracted intermetallics were determined using a geometrically fixed Berkovich nano-diamond and cube corner indenter, under ambient temperature conditions. The studied crystals were then exposed to the two cavitation conditions mentioned. Results demonstrated for the first time that the governing fragmentation mechanism of the studied intermetallics was due to the emitted shock waves from the collapsing bubbles. The fragmentation caused by a single bubble collapse was found to be almost instantaneous. On the other hand, sono-fragmentation studies revealed that the intermetallic crystal initially underwent low cycle fatigue loading, followed by catastrophic brittle failure due to propagating shock waves. The observed fragmentation mechanism was supported by fracture mechanics and pressure measurements using a calibrated fibre optic hydrophone. Results showed that the acoustic pressures produced from shock wave emissions in the case of a single bubble collapse, and responsible for instantaneous fragmentation of the intermetallics, were in the range of 20–40 MPa. Whereas, the shock pressure generated from the acoustic cavitation cloud collapses surged up to 1.6 MPa inducing fatigue stresses within the crystal leading to eventual fragmentation.

Effect of Tip Roundness on the Nanoindentation of Fe Crystals

Indentation tips are never atomically sharp, but rounded at their end. We use atomistic simulation to study the effect of tip roundness for the particular case of a cube-corner pyramidal indenter by comparing the results of a spherical, a sharp cube-corner, and a rounded cube-corner tip during indention into bcc Fe. We find that as soon as the tip has indented so deeply that the spherical geometry does not hold any longer, strong deviations between the dislocation plasticity behavior show up. The rounded cube-corner tip produces less dislocations and a smaller plastic zone than the spherical indenter, when indented to the same depth. The results are better comparable, however, when the same displaced volume is considered. Finally, the dislocation nucleation mode is affected by the geometry, changing from homogeneous to heterogeneous nucleation as the tip changes from rounded to sharp. The cube-corner tips are found to produce more twinning and delay the formation of prismatic loops. For a penetration depth beyond the radius of the rounded cube-corner tip, atomic sharp pyramidal tips produce similar quantitative (hardness, dislocation density) and qualitative (pileup, dislocation arrangement) results compared to its rounded counterpart. Our results will prove important for understanding the differences between spherical indenter tips, as they are often used in simulation, and pyramidal tips, as they are used in experiment.

High frequency acoustic emission monitoring in nano-impact of alumina and partially stabilised zirconia

High frequency acoustic emission monitoring is proving a useful technique in improving the understanding of deformation processes occurring in nano- and micro-scale mechanical contacts. In this study AE monitoring has been used to investigate the fracture of alumina and MgO-partially stabilised zirconia (PSZ) at high strain rate in repetitive nano-impact tests with cube-corner and 5 μm radius spherical diamond indenters. Focussed ion beam milling of impact craters revealed sub-surface intergranular cracking on alumina and sub-surface transgranular cracking on PSZ. The evolution of AE through the test was strongly dependent on the geometry of the indenter used to create the impacts. AE energy was generally much higher when impacts cause rapid increases in penetration depth, with single high energy hits observed on abrupt depth bursts in tests with the cube corner indenter. The damage progression in tests with the spherical indenter differed between the ceramics, with more AE events near the start of the test on alumina and later in the test for PSZ. Reasons for these differences are discussed.

Dynamic fracture of CrN coating by highly-resolved nano-impact

Nano-impact testing has provided localized high strain rate deformation information in either (i) repetitive impact for fatigue of brittle materials or (ii) high-resolution data acquisition of single impacts for energy dissipation and dynamic hardness in ductile materials. The current study focusses on whether high-resolution data acquisition can be used to determine fracture occurring during a single impact event. A PVD CrN coating on 42CrMo4 high speed steel (HSS) was impacted with a cube corner indenter over a range of different acceleration forces. By analyzing the high precision depth-time curves during the first contact phase of the impact, an abrupt depth change was observed at and above 30 mN acceleration force. Focused ion beam technology confirmed through-thickness coating fracture without delamination on tests where the step was observed. The step was absent at lower force or on the more ductile HSS throughout the load range. The fracture dissipated energy and fracture toughness of the coating was estimated under 50 mN acceleration force based on the analysis of impact energy. The calculated dynamic fracture toughness of CrN coating is in the range of 2.75–7.74 MPa·m1/2, depended on the indenter geometry constant, which is comparable with the reported value of about 3.13 MPa·m1/2.

Effective method to simultaneously release residual stress and promote planarization of surface indentation achieved by secondary indentation

Regarding the surface residual stress and accompanied pile-up phenomenon produced during indentation, a novel method was proposed to effectively release residual stress and promote surface planarization. The procedures included an initiation indentation by using Vickers indenter and a secondary indentation at nano-scale on already formed surface by using a cube-corner indenter. Both finite element analysis and indentation experiments revealed significant stress release and surface planarization, which were closely dependent with the indentation depth and location. A 33.03% stress reduction and an internal energy reduction of 6.04 × 10−8 J were obtained through finite element analysis. Meanwhile, due to the effect of secondary indentation, the average residual stress and crystallinity of AlSi7Mg0.3 alloy specimen were reduced by 64.09% and 49.72%, respectively, which was calculated by X-ray microdiffraction (μXRD), the atomic force microscopy (AFM) micrographs of initiation and secondary indentation edges also indicated a pile-up reduction with 368.7 ± 46.1 nm, which verified the planarization through the comparison of pile-up displacements between symmetrical paths.

Fracture toughness determination of fused silica by cube corner indentation cracking and pillar splitting

In this paper the applicability of the pillar splitting technique for fracture toughness determination on anomalous behaving bulk fused silica glass is explored. The results are compared to conventional cube corner indentation cracking analyzed using the Lawn, Evans and Marshall model (JACerS, 63 (1980) 574). The experimental analysis is supported by constitutive Finite Element Analysis with cohesive zones to determine adequate gauge factors to correlate the load instability upon splitting to the fracture toughness Kc. The role of densification on pillar splitting was critically examined.The results show a fragmentation of the micro pillar into three parts, a failure pattern as proposed by Sebastiani et al. (Philos. Mag., 95 (2014) 1928). Therefore, the applicability of pillar splitting to (anomalous) glasses is confirmed. Cohesive zone FEA delivered the gauge factors required for fracture toughness calculation. The influence of densification on those factors, however, was found to be small for indentation cracking and negligible for pillar splitting. With the corresponding set of gauge factors fracture toughness values in good accordance with literature could be determined. Inside the SEM, moreover, electron beam irradiation has been found to enhance the fracture properties of fused silica.

Effects of testing conditions on the deformation behaviour of a Ti-based bulk metallic glass

This study investigated the deformation behaviour of a Ti32.8Zr30.2Ni5.3Cu9Be22.7 bulk metallic glass via indentation and compression tests with loading rate ranging from 30 μN/s to 30000 μN/s. The load-displacement (P-h) curves show evident displacement bursts (pop-in events). The statistic analysis reveals that the pop-in events are correlated to the loading rate, i.e., the pop-in size increasing linearly with the indentation depth and decreasing exponentially with the loading rate. Cube corner indenter with much sharper geometry favours the promoting of pronounced pop-in events, compared to Berkovich indenter. Similar loading rate effect can be observed in the micro-pillar compression tests. SEM observation of the deformed pillars reveals that the shear bands (along a ~50.7° plane with respect to the loading axis) initiate from the side wall and run across the entire pillars. The free-volume theory and shear transformation zone (STZ) are used to explain these observations quantitatively. The serration to smooth shape of P-h curves is most likely due to the simultaneous operation of multiple shear bands at high loading rate while the single shear band contribution could not be resolved.

Synthesis and Mechanical Testing of Calcium Aluminosilicoferrite Crystals with High Alumina Content

Due to the gradual shift to less rich iron ores, the alumina content in the raw materials used for iron-making is progressively increasing, affecting the mineralogy and the properties of iron ore sinters. In this context, the effect of Al content on the mechanical properties of calcium aluminosilicoferrites Ca2(Ca,Mg,Fe)6(Fe,Si,Al)6O20 (SFCA), which is the most important bonding phase in iron ore sinters, is of particular interest. In this study, high-alumina calcium aluminosilicoferrites were synthesized and their mechanical properties were determined by nanoindentation using a cube-corner indenter. For synthesis, different raw materials were taken as proxies for the adhering layer in a sinter granule. Three mixtures were prepared, high-iron, high-silica, and high-alumina and heated in an alumina crucible, which was used to simulate the high-alumina nucleus in a granule. The different raw materials used for synthesis had only minor influence on the compositions of the synthesized ferrites. All ferrites showed similar mechanical behavior during indentation, indicating that neither the chemical nor the mechanical properties were affected by the different compositions of the adhering layer, when the sinter granule is dominated by a high-alumina nucleus. The crystallographic orientation of the tested grains had only minor influence on the results of the nanoindentation experiments.

Use of indentation to study the degradation of photovoltaic backsheets

The ability of electrical insulating materials within a module to act as insulators is a key safety requirement for photovoltaic (PV) technology. Presently, however, the durability of backsheets may not be readily assessed. For example, the mechanical tensile test continues to be developed, and its use has not been validated such that a technically based pass/fail criteria may be established. This study examines the use of simple indentation methods, including durometer hardness and instrumented indentation, as a means to quantitively assess the degradation of PV backsheets. Characteristics including: hardness, modulus, load/displacement profile, creep hold response, and residual impression are explored in an empirical study. Glass/encapsulant/backsheet mini-modules constructed using backsheets including: polyamide (specifically the AAA backsheet product), poly(ethylene terephthalate) (PET), polyvinyl fluoride (PVF) laminate (“TPE”), and polyvinylidene fluoride (PVDF) were examined. An M-type durometer as well as Berkovich and cube-corner tips were used in the indentation experiments. Additional characterizations were performed to interpret the indentation measurements including: surface roughness measurements using atomic force microscopy (AFM), a chemical structure study using Fourier-transform infrared spectroscopy (FTIR), and phase-transition measurements using differential scanning calorimetry (DSC). The results are analyzed in the context of the combined accelerated stress test (C-AST) also explored in this study. Instrumented indentation (i.e., using a Berkovich tip) was able to distinguish between backsheets and quantify the effects of accelerated testing (including up to 60%, 25%, and 20% change in hardness, modulus, and creep displacement, respectively). The embrittlement of the backsheets was not readily assessable using cube-corner indentation. Cracking of the known-bad polyamide backsheet was observed from the C-AST, which was not observed to result from steady state UV weathering.

Nanoindentation-based study of the mechanical behavior of bulk supercrystalline ceramic-organic nanocomposites

Bulk poly-supercrystalline ceramic-organic nanocomposites were produced and characterized with a nanoindentation-based study. These nanocomposites were processed using two different routines, to compare their properties with and without crosslinking the organic ligands interfacing the ceramic nanoparticles. Together with the expected material strengthening induced by crosslinking, a distinct response emerges when using Berkovich and cube-corner indenters. The supercrystalline materials are prone to compaction, cracking and chipping phenomena that become more severe when a sharper tip is employed, implying that a Berkovich indenter is more suitable for the evaluation of elastic modulus and hardness. The cube-corner tip, on the other hand, is employed for the investigation of fracture toughness, comparing two methods and multiple models available from the literature. The fracture toughness outcomes suggest that cracks evolve with a quarter-penny shaped profile at the indent’s corners, and that extrinsic toughening mechanisms, such as plastic-like deformation and crack deflection, play a significant role.

Friction characteristics of mechanically microtextured metal surface in dry sliding

Improving tribological properties by applying various textured surfaces have been reported by many researchers. In these studies, most utilized surfaces have been textured with lasers. However, such a texturing method contains several problems including thermal effects, formation of pileups, possible dimple shapes, and lower efficiency than mechanical methods. The traditional mechanical texturing methods also have some problems. In this study, an alternative method is developed by using vibration-assisted microcutting with a cube-corner diamond indenter which is vibrated in the depth direction with an amplitude of tens of microns or less. In this technique, the pileups generated around all the dimples in texturing can be removed by supplementally conducting a normal microcutting on the machine. By the developed method, textured surfaces of brass plate (c2680), in which micro dimples transferred tip shape of the indenter were regularly arranged at a period of tens of microns, were successfully manufactured. In order to verify their potentials, dry sliding tests using steel balls (534A99) were conducted on the surfaces textured by both the proposed and the normal cutting methods for comparison. Subsequently, the influence of the removing pileups and the dimple density were also examined through the dry sliding tests. A series of the sliding test results were analyzed through the monitored friction coefficients, wear rates, and observations of worn surfaces and generated debris. As a result, textured surface with a relatively high area density, such as 40% where pileups were completely removed showed the lowest friction coefficient and wear due to the trapping and reducing effects of the generated wear debris and real contact area by the periodically arranged micro dimples. Therefore, the proposed texturing method can be a new candidate to produce the surface textures for better tribological properties. Also, higher texturing efficiency could be realized by just applying higher sample rotation speed and/or vibration frequency.

TEM in situ cube-corner indentation analysis using ViBe motion detection algorithm

Transmission electron microscopic (TEM) in situ mechanical testing is a promising method for understanding plasticity in shallow ion irradiated layers and other volume-limited materials. One of the simplest TEM in situ experiments is cube-corner indentation of a lamella, but the subsequent analysis and interpretation of the experiment is challenging, especially in engineering materials with complex microstructures. In this work, we: (a) develop MicroViBE, a motion detection and background subtraction-based post-processing approach, and (b) demonstrate the ability of MicroViBe, in combination with post-mortem TEM imaging, to carry out an unbiased qualitative interpretation of TEM indentation videos. We focus this work around a Fe-9%Cr oxide dispersion strengthened (ODS) alloy, irradiated with Fe2+ ions to 3 dpa at 500 °C. MicroViBe identifies changes in Laue contrast that are induced by the indentation; these changes accumulate throughout the mechanical loading to generate a “heatmap” of features in the original TEM video that change the most during the loading. Dislocation loops with b = ½ <111> identified by post-mortem scanning TEM (STEM) imaging correspond to hotspots on the heatmap, whereas positions of dislocation loops with b = <100> do not correspond to hotspots. Further, MicroViBe enables consistent, objective quantitative approximation of the b = ½ <111> dislocation loop number density.

A critical review of FEM models to simulate the nano-impact test on PVD coatings

Nano-impact test is a reliable method for assessing the brittleness of PVD coatings with mono- or multi-layer structures. For the analytical description of this test, a 3D-FEM Finite Element Method (FEM) model and an axis-symmetrical one were developed using the ANSYS LS-DYNA software. The axis-symmetrical FEM simulation of the nano-impact test can lead to a significantly reduced computational time compared to a 3D-FEM model and increased result's accuracy due to the denser finite element discretization network. In order to create an axissymmetrical model, it was necessary to replace the cube corner indenter by an equivalent conical one with axis-symmetrical geometry. Results obtained by the developed FEM models simulating the nano-impact test on PVD coatings with various structures were compared with experimental ones. Taking into account the sufficient convergence between them as well as the reduced calculation time only in the case of an axis-symmetrical model, the latter introduced numerical procedure can be effectively employed to monitor the effect of various coating structures on their brittleness.

Characterization of the Mechanical Behavior of Colorado Mason Sand at Grain-Level by Nanoindentation

Colorado Mason sand has received increasing attention for use as a model granular material for the investigation of the soil response under a blast. The grain-level mechanical properties of Colorado Mason sand, which are important input for modeling and simulation of such granular material, are measured by nanoindentation. The as-received assorted sand is sorted into six grain sizes: 0.15 mm, 0.21 mm, 0.3 mm, 0.42 mm, 0.6 mm, and 0.84 mm. The sand chemical constituents are determined using energy dispersive X-ray spectroscopy, and three main types of sand grains are determined as silica, rock, and metal oxide. The Young’s modulus and hardness are measured using nanoindentation with a Berkovich tip. A cube-corner indenter tip is used to generate radial cracks, the lengths of which are used to determine the fracture toughness. The mechanical properties (Young’s modulus, hardness and fracture toughness) of the sand are statistically analyzed, and are found to follow a tri-modal Weibull distribution, corresponding to three main types of constituent grains. The grains show ductile behavior under Berkovich tip due to confinement by high pressure induced by the tip. An inverse method is used to determine the stress-strain relationship for individual sand grains using finite element analysis, with the consideration of Drucker-Prager yield criterion. The grain-level mechanical properties determined in this study can be used in microscale and mesoscale simulations.

AFM Nanoindentation To Quantify Mechanical Properties of Nano- and Micron-Sized Crystals of a Metal-Organic Framework Material.

The mechanical properties of individual nanocrystals and small micron-sized single crystals of metal-organic frameworks (MOFs), hitherto, cannot be measured directly by employing the conventional instrumented nanoindentation approach. Here we propose the application of atomic force microscopy (AFM)-based nanoindentation technique, equipped with a calibrated diamond cube-corner indenter tip to quantify the Young's modulus, hardness, adhesion energy, and interfacial and fracture strengths of a zeolitic imidazolate framework (ZIF-8) porous material. We use ZIF-8 as a model MOF system to develop AFM nanoindentation leveraging the concept of unloading strain rate, enabling us to critically assess the practicality and technical limitations of AFM to achieve quantitative measurements of fine-scale MOF crystals. We demonstrate the advantages of using a high unloading strain rate (ε̇ > 60 s-1) to yield reliable force-displacement data in the few μN load range, corresponding to a shallow indentation depth of ∼10s nm. We found that the Young's moduli (∼3-4 GPa) determined by AFM nanoindentation of the nanocrystals (<500 nm) and micron-sized crystals (∼2 μm) are in agreement with magnitudes derived previously from other techniques, namely instrumented nanoindentation and Brillouin spectroscopy (however, these methods requiring large 100-μm sized crystals) and also in line with density functional theory predictions of an idealized ZIF-8 crystal.

Slip activation controlled nanohardness anisotropy of ZrB2 ceramic grains

The orientation dependence of hardness and modulus of ZrB2 grains were determined on a polycrystalline specimen with low porosity using Berkovich nanoindentation at room temperature. Additionally, Cube corner indentation was carried out on differently oriented grains to generate visible slip patterns, which were used to analyze slip activation. The anisotropy ratio, which is the ratio of the hardest/stiffest to the softest/most compliant orientations, was higher for hardness (∼1.3) than for modulus (∼1.1). The orientation dependence of nanohardness was described using an analytical model based on the activation of multiple slip system families. Based on literature data and the present indentation results, only the {101¯0}〈112¯0〉, {101¯0}[0001] and {101¯0}〈112¯3〉 type slip system families were considered in the model. Simulations showed that none of the three slip systems by itself could describe the orientation dependence of the hardness of ZrB2. Analysis of the model in combination with the results of Cube corner indentation revealed that the experimental results can be appropriately described by the simultaneous activation of the {101¯0}[0001] and {101¯0}〈112¯3〉 slip systems. Further analysis of the model together with first principles calculations found in the literature were used to derive the hierarchy of slip activations for ZrB2 at room temperature.

Characterization and investigation of size effect in nano-impact indentations performed using cube-corner indenter tip

Abstract

Influence of loading rate on the mechanical performance of metallic glass

Amorphous metallic glass Cu47Ti35Zr11Ni6Si1 was investigated by load-control nanoindentation experiments using the cube corner indenter tip over a wide range of loading rates. The indentation hardness was calculated using different methods either from the loading curves or indent area. Pop-in events were observed on the loading part of the indentation curves mainly at lower rates of loading. Instantaneous plastic deformation decreases with increasing loading rate according to a power law. At high loading rate the instantaneous deformation is suppressed by continuous plastic deformation and no well-developed pop-ins are observed. The morphology of shear bands in the pile-up area of indents showed no correlation with the pop-in event population of the nanoindentation curves and the loading rate.

Shear Band Morphology of Indented Region in Cu-Based Metallic Glass

Plastic deformation after indentation of the metallic glass Cu47Ti35Zr11Ni6Si1 at different loading conditions was examined. Discontinuities on the loading curves were observed, the magnitude of which depends on the loading rate. The presence of these discontinuities is influenced by the precise shape of the indentation tip. At lower loading rates and using a cube corner indenter tip the discontinuities on the loading curves are more pronounced. An increase of the loading rate tends to diminish instantaneous plastic deformation as appear by pop-ins. Using a Berkovich type indenter tip the plastic deformation is more steady. It is concluded that the final morphology of the pile-up area strongly depends on the geometry of the indenter tip, whereas no correlation between discontinuities in the loading part of the indentation curve and the formation of shear band patterns was observed.