Nanoindentation with spherical tips of single crystals of YBCO textured by the Bridgman technique: Determination of indentation stress–strain curves

The mechanical properties of superconductor ceramics are of interest in the manufacture of superconducting devices. The current trend is to produce smaller devices (using, e.g., thin films), and the correct characterization of small volumes of material is critical. Nanoindentation is used to assess mechanical parameters, and several studies determine hardness and Young's modulus by sharp indentation. However, studies on the elasto-plastic transition with spherical indentation are scare. Here we used, spherical diamond tip indenter experiments to explore the elasto-plastic transition and to measure the yield strength of the orthorhombic phase of YBa₂Cu₃O_7?δ (YBCO or Y-123) at room temperature. The study was carried out for a range of monodomains on the (1 0 1)-plane for Bridgman samples. Inspection of the load–unload curves for penetration depths lower than 200 nm allows for observation of the elasto-plastic transitions. Focused ion beam (FIB) trenches showed no cracking due to the indentation, although oxygenation cracks were apparent. The mean pressure for the onset of elasto-plastic deformation is 3.5 GPa, and the elastic modulus, E, calculated using the Hertzian equations is 123.5 ± 3.4 GPa.     

Hertzian contact damage in a highly porous silicon nitride ceramic

Hertzian indentation tests were performed to evaluate the contact damage behavior of a highly porous Si₃N₄ ceramic. Using a bonded-interface technique, the Hertzian contact damage patterns were examined. As a result of intragranular microfracture under Hertzian contact, a distributed subsurface damage region is developed beneath the indenter. It was found that the damage region extends progressively with increasing contact load. In strength tests, failures were observed to originate from Hertzian indentation sites, giving rise to a gradual strength degradation.     

Measurement of nanoindentation properties of polymers considering adhesion effects between AFM sharp indenter and material

Abstract

Nanomechanical properties of polymer samples were calculated using an adhesive contact model appropriate for AFM indentation problems. A series of Polydimethylsiloxane (PDMS) samples were indented by the sharp indenter in the air by using an AFM, and dozens of the force–displacement curves of each sample were obtained. An adhesive contact model suitable for sharp indentation with adhesion was established based on the same assumptions of the JKR model which is only suitable for spherical indentation at small penetration depth. Differences between sharp indentation problems with and without adhesion were discussed, and the limitations of the traditional adhesion model were given. The elastic modulus was obtained by fitting experimental force–displacement curves with theoretical ones, and results were compared to those macroscopic values in literature. The adhesion energy between the indenter and the sample surface was accurately calculated using the adhesion model based on the calculated elastic modulus. The influence of the indenter tip angle on the calculation results of the elastic modulus was also discussed theoretically. In this study, the mechanical properties of polymer samples were calculated at the nanoscale considering the adhesion effect.     

Process and quasi-static compressive properties of highly porous palladium bulks

Highly porous palladium bulks with open porosities from 77.8% to 82.0% are prepared by powder metallurgical process with Na₂CO₃ as the filler material. Compressive properties of the prepared porous Pd bulks have been investigated at strain rates of 10⁻³–10⁻¹ s⁻¹. It has been found that the porous Pd bulks first show a short elastic region, then a long and oblique stress yield region, and finally, a densification region where the stress increase rapidly in the nominal stress–nominal strain curves. The effect of strain rate on the compressive properties of the porous Pd bulks is also discussed.     

Preparation of conductive polyaniline/nanosilica particle composites through ultrasonic irradiation

Polyaniline/nano‐SiO₂ particle composites were prepared through ultrasonic irradiation. Polymerization of aniline was conducted under ultrasonic irradiation in the presence of two types of nano‐SiO₂: porous nanosilica and spherical nanosilica. The stability of the colloid dispersion, UV–vis spectra, composition, interaction, conductivity, and other characteristics of the composites were examined. It was found that the aggregation of nano‐SiO₂ could be reduced under ultrasonic irradiation and that nanoparticles were redispersed in the aqueous solution. The formed polyaniline deposited on the surface of the nanoparticle, which led to a core–shell structure. Two particle morphologies, threadlike aggregates with a few spherical nanoparticles for porous nanosilica and spherical particles with a few sandwichlike particles for spherical nanosilica, were observed. X‐ray photoelectron spectroscopy showed that for two types of composites the ratio of Si atoms to N atoms (Si:N) on the surface was much higher than that in the bulk. The UV–vis spectra of the diluted colloid dispersion of polyaniline/nano‐SiO₂ composite particles were similar to those of the polyaniline system. Fourier transform infrared spectroscopy suggested strong interaction between polyaniline and nano‐SiO₂. The conductivity of the polyaniline/porous nanosilica (23.1 wt % polyaniline) and polyaniline/spherical nanosilica (20.6 wt % polyaniline) composites was 2.9 and 0.2 S/cm, respectively. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1811–1817, 2003     

Spherical indentation for brittle fracture toughness evaluation by considering kinked-cone-crack

This work aims at evaluating the fracture toughness of brittle materials by spherical indentation. The cone-cracking is simulated by the extended finite element method (XFEM) in Abaqus. The formation of a kinked-cone-crack is observed when the indenter comes into (second) contact with the surface part outside the ring-crack. The effects of friction, Poisson’s ratio and cone-crack kinking on the Roesler’s constant κ_c are analyzed. Based on numerical results, the Roesler’s method for evaluating the fracture toughness is enhanced by considering kinked-cone-crack. By performing systematic XFE analyses, a database for enhanced Roesler’s constant κ_c | _kink is provided for the fracture toughness evaluation of brittle materials. Finally, the proposed method is verified by conducting spherical indentation tests on soda-lime glass specimens.     

Micromechanical properties of Dy3+ ion-doped (LuxY1-x)3Al5O12 (x = 0, 1/3, 1/2) single crystals by indentation and scratch tests

Three kinds of Dy³⁺ ion-doped (Lu_xY_1-x)₃Al₅O₁₂ (x = 0, 1/3, 1/2) single crystals fabricated by the Czochralski method with 4 at.% Dy³⁺ ion doping were investigated by indentation and scratch techniques under Vickers, Knoop, Berkovich, and spherical indenters to understand the influence of Lu ion on micromechanical properties and fracture behavior of Y₃Al₅O₁₂ (i.e. YAG for x = 0) single crystals. The largest (or smallest) values of hardness, elastic modulus, and fracture toughness were found for x = 1/3 (or 1/2). The indentation size effect was explained by four different models with the Hays-Kendall approach being the most suitable one to determine the true hardness. Fracture toughness values of YAG crystals obtained by the Vickers hardness method agreed with those obtained by scratching with a spherical indenter based on linear elastic fracture mechanics.     

Buckypaper-based catalytic electrodes for improving platinum utilization and PEMFC's performance

Platinum (Pt) catalytic electrode was developed by using carbon nanotube films (buckypaper) as supporting medium and electrodeposition method to deposit Pt catalyst. Buckypapers are free-standing thin films consisting of single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs) and/or carbon nanofibers (CNFs) held together by van der Waals forces without any chemical binders. Special mixed buckypapers was developed by layered microstructures with a dense and high-conducting SWNT networks at the surface, as well as large porous structures of CNF networks as back supports. This unique microstructure can lead to improve Pt catalyst accessibility and mass exchange properties. Pt particles of about 6 nm were uniformly deposited in porous buckypapers. A promising electrochemical surface area of ∼40 m²/g was obtained from these electrodes. A Pt utilization as low as 0.28 g_Pt/kW was achieved for the cathode electrode at 80 °C. Pt utilization efficiency can be further improved by optimization of the electrodeposition condition in order to reduce the Pt particle size.     

van der Waals effect on the nanoindentation response of free standing monolayer graphene

Using molecular mechanics simulations we investigate the elastic properties of monolayer graphene determined from free standing indentation and the effects of graphene size and indenter tip size are considered. In free standing indentation, the overall system potential energy includes two parts: the strain energy of graphene monolayer and the van der Waals (VDW) interaction energy between indenter tip and graphene. The VDW interaction will strongly affect the elastic properties of graphene monolayer determined from free standing indentation. Without considering the VDW interaction, the classical free standing indentation analysis is not able to accurately estimate the second-order elastic stiffness (E) and the third-order nonlinear elastic constant (c_m) (E will be underestimated and c_m will be overestimated). It is found that both E and c_m determined from free standing indentation are quite close to their counterparts determined from in-plane tension after properly considering the VDW interaction. The results can provide a useful guideline to understand the elastic properties of graphene monolayer determined from free standing indentation tests.     

Micromechanical Characterization of Electrophoretic-Deposited Green Films

Low-load indentation experiments have been performed on electrophoretic-deposited films of SiC particles on a graphite substrate. Films with thicknesses between 60 and 300 µm prepared at two current intensities and subsequently dried under different humidities were indented with spherical indenters with nominal radii of 10, 50, and 150 µm. Force-displacement data were analyzed to determine contact pressure and elastic modulus versus depth results. The modulus and contact pressure behavior with depth exhibited opposite trends with indenter radius: the modulus increase was least for the 10 µm and greatest for the 150 µm, whereas the contact pressure was the inverse. The results may be rationalized by plotting modulus normalized to the ratio "contact radius/film thickness" ( a / t ), whereas the contact pressure results at small a / t could be normalized when plotted versus contact strain, i.e., contact radius divided by indenter radius ( a / R ). These approaches enabled the properties of the variously deposited films to be compared. Additional interesting microstructural and cracking behavior patterns are also reported.     

Spherical instrumented indentation of porous nanocrystalline zirconia

Nanoindentation tests with a spherical tip were performed to analyze the stress-strain response on 3-mol%-yttria-doped tetragonal zirconia polycrystals produced by spark plasma sintering (SPS) with porosities in the range 2-21% and nanometric average grain sizes in the range 65-120 nm. Indentation stress-strain curves were obtained by using load (P)-displacement (h) data. Onset of elasto-plastic transition was determined by Hertz fits on P-h curves. Elastic modulus obtained from spherical indentation was similar to the values obtained by Berkovich indentation, with a slight difference with increasing porosity. Microstructural characterisation shows no cracks around and beneath the indentations. Raman spectroscopic analysis of the residual indentation imprints reveals tetragonal to monoclinic phase transformation in the most porous material, whereas no phase transformation was detected in the dense material in spite of its larger grain size. The results are discussed in terms of porosity, grain size and tetragonal-monoclinic transformability.     

A study on the elastic modulus of silicone duplex or bi-layer coatings using micro-indentation

A Fischerscope continuous microindenter with a spherical indenter was used to obtain maximum indentation load and depth data for a 2.2 mm sheet of RTV11 (a silicone elastomer), a 1.6 mm sheet of J501 (an elastomer containing 60% silicone and 40% butyl acrylate styrene) as well as six duplex elastomeric coatings. The duplex coatings consisted of RTV11 top coat and J501 bond coat. The Waters’s empirical relationship was used to determine the modulus of elasticity E for the RTV11 and J501 sheets. The Waters’s relationship was then used to determine the equivalent modulus, E_c, for duplex coatings from maximum indentation load versus elastic indentation depth data. The values of E_c as determined from the Waters’s model (and experimental data) were in good agreement with the values obtained by an equivalent stiffness method. By being able to determine E_c from the equivalent stiffness method and using this value in the Waters’s model, one may determine the load versus elastic depth of indentation for duplex coatings.     

Predictive model for spherical indentation on elastoplastic nanocomposites: Loading and unloading behavior

The response of elastoplastic nanocomposites in contact with rigid indenter has a lot of importance in engineering applications such as material characterization, Young modulus, Poisson's ratio and strain hardening parameters. With the aim of presenting a predictive model for the loading and unloading response, the problem of contact between elastoplastic composites with stiff spherical indenter was simulated using 2D axisymmetric model implemented in ANSYS. Parametric studies were performed numerically to investigate effects of geometrical and material parameters on the behavior of nanocomposites. Based on the parametric study observation, a normalization procedure is presented to express the loading and unloading responses in a nondimensional form. Three equations were numerically derived by fitting the FE normalized data to predict the loading and unloading responses and the residual indentation depth after unloading for elastoplastic nanocomposite with wide range strain hardening exponent. The predictions of proposed equations were in excellent agreement with experimental results for Al-Al₂O₃ nanocomposites and also with other experiments available in the literature for nanocomposites and pure metals. Moreover, derived equations were exploited to predict the Rockwell hardness of nanocomposites.     

Physical properties of La-doped NiO sprayed thin films for optoelectronic and sensor applications

Lanthanum-doped nickel oxide NiO:La thin films were deposited onto glass substrates at 450 °C, by the spray pyrolysis technique using nickel and lanthanum chlorides as precursors. These films belonging to cubic structure, crystallize preferentially along (111) plane. First, Raman study shows the presence of bands corresponding to NiO structure. The same study confirms the presence of both Ni(OH)₂ and LaNiO₃ as secondary phases. Moreover, using SEM observations, all samples exhibit porous microstructures with rough surfaces and spherical nanoparticles of about 40 nm as size. Second, NiO:La films present a direct band gap energy value lying in the range of 3.63–3.84 eV. Also, the effect of the La incorporation in NiO matrix on the disorder is studied in terms of Urbach energy. Some optical constants (refractive index, extinction coefficient, dielectric constants, and dispersion parameters) are reached. On the other hand, the photoluminescence spectroscopy reveals the presence of peaks related to the electronic transition of the Ni²⁺ ions and others confirming the presence of some defects in NiO matrix in terms of La content. Finally, it has been found that La doping allows the improvement of the electrical conductivity as well as Haacke’s figure of merit of NiO sprayed thin films by at least, three orders of magnitude.     

On the determination of the film hardness in hard film/substrate composites using depth-sensing indentation

The main difficulty in the mechanical characterisation of thin films using depth-sensing indentation is the determination of the relative substrate and film contributions to the measured properties of the film/substrate composite. In this study, a three-dimensional numerical simulation of the Vickers hardness test is used to study the influence of the substrate and film mechanical properties on the composite's behaviour under depth-sensing indentation. The particular case of hard films on soft substrates is analysed. In order to understand the behaviour of the composite, a study of the plastic strain distribution under indentation of several composites is performed. A methodology to determine the relative film hardness, i.e. H_F/H_S ratio, is proposed. The methodology is successfully verified using fictitious and real composite materials.     

Investigation of mechanical and creep properties of polypyrrole by depth-sensing indentation

In this study, we used depth-sensing indentation (DSI) technique to investigate some mechanical properties (reduced elastic modulus, indentation hardness, and creep) of polypyrrole (PPy) conducting polymer obtained with different support electrolyte concentration. The influence of support electrolyte concentration on these parameters was also determined. The order of doping degree of the samples was determined by cyclic voltammetry. The indentation load–displacement curves of the samples were obtained under different peak load levels with a 70 s holding time at maximum load. Reduced elastic modulus and hardness values were determined by analysis of these curves using the Feng–Ngan (F–N) and Tang–Ngan (T–N) methods, respectively. Both reduced elastic modulus (E _r) and indentation hardness (H) exhibited significant peak load dependence, i.e., indentation size effect (ISE). It was found that both E _r and H values decreased as the support electrolyte concentration was increased. This was explained by an increase in the free volume as the doping degree was raised. The creep behavior of the samples was monitored from the load holding segment of the load–unload curves. It was found that creep increases with the increasing support electrolyte concentration.     

Toughness measurement and toughening mechanisms of arc ion plating Cr2O3 films treated by annealing

Chromium sesquioxide (Cr₂O₃) films were deposited on Ni-based high-temperature alloy substrates by an arc ion plating technique and then annealed at different temperatures and heating rates. The influence of annealing conditions on the toughness of Cr₂O₃ films was calculated according to spherical indentation tests. The increase in grain size and compressive stress, variety of microstructure and surface morphology, and atom diffusion that resulted from annealing caused toughness variations. The increase in grain size closed micro-cracks along the direction of film growth. Compressive stress and a multi-crystal plane led to cracks caused by indentation that required more energy to break through the film. In the process of indentation, turning, bifurcating, and bridging of cracks on film surface was also able to dissipate energy. Atom diffusion in the process of 1000 °C annealing also played a role in grain boundary toughening. The toughness improvement of Cr₂O₃ film significantly improved friction life.     

Nanoindentation of diamond,graphite and fullerene films

The recently developed method of nanoindentation is applied to various forms of carbon materials with different mechanical properties, namely diamond, graphite and fullerite films. A diamond indenter was used and its actual shape determined by scanning force microscopy with a calibration grid. Nanoindentation performed on different surfaces of synthetic diamond turned out to be completely elastic with no plastic contributions. From the slope of the force–depth curve the Young's modulus as well as the hardness were obtained reflecting a very large hardness of 95 GPa and 117 GPa for the {100} and {111} crystal surfaces, respectively. Investigation of a layered material such as highly oriented pyrolytic graphite again showed elastic deformation for small indentation depths but as the load increased, the induced stress became sufficient to break the layers after which again an elastic deformation occurred. The Young’s modulus was calculated to be 10.5 GPa for indentation in a direction perpendicular to the layers. Plastic deformation of a thin fullerite film during the indentation process takes place in the softer material of a molecular crystalline solid formed by C₆₀ molecules. The hardness values of 0.24 GPa and 0.21 GPa for these films grown by layer epitaxy and island growth on mica and glass, respectively, vary with the morphology of the C₆₀ films. In addition to the experimental work, molecular dynamics simulations of the indentation process have been performed to see how the tip–crystal interaction turns into an elastic deformation of atomic layers, the creation of defects and nanocracks. The simulations are performed for both graphite and diamond but, because of computing power limitations, for indentation depths an order of magnitude smaller than the experiment and over indentation times several orders of magnitude smaller. The simulations capture the main experimental features of the nanoindentation process showing the elastic deformation that takes place in both materials. However, if the speed of indentation is increased, the simulations indicate that permanent displacements of atoms are possible and permanent deformation of the material takes place.     

Spherical and Vickers Indentation Damage in Yttria-Stabilized Tetragonal Zirconia Polycrystals

Damage induced in an yttria-stabilized tetragonal zirconia polycrystal by spherical and Vickers indentations was investigated. Scanning acoustic microscopy revealed that, as indentation stress increased, the spherical indentation gradually developed subsurface damage, until it experienced a transition to a fully plastic state, characterized by a highly anisotropic variation in the leaky Rayleigh wave velocity, v _R, and very similar to that for Vickers indentation. The transition was a result of the formation of a microcracked core beneath the contact. Indenter geometry had an appreciable effect only within the core; the distribution of microcracks differed depending on the indenter used, as confirmed by direct observations using a scanning electron microscope. In contrast, the residual stresses in the elastic-plastic zone were insensitive to indenter geometry. The resulting plastic zone was not hemispherical but rather cylindrical, irrespective of indenter geometry.     

Preparation of SnTe thin films on Au(1 1 1) by electrodeposition route

Tin telluride (SnTe) thin films were deposited onto Au(1 1 1) substrates from an aqueous solution containing SnCl₂, TeO₂, and C₆H₅Na₃ at room temperature (25 °C) for the first time via electrodeposition route. The electrodeposition of the thin films was studied using cyclic voltammetry, compositional, structural, optical measurements and surface morphology. It was found that the stoichiometric SnTe thin films could be obtained at −0.50 V. The as-deposited thin films were crystallized in the preferential orientation along the (2 2 0) plane. SEM investigations indicated that the shape of thin films could be altered from a spherical particle to a dendritic crystal by increasing the deposition potential. The growth of the dendritic films proceeds via formation of nanoparticles and growth of dendritic crystals on these nanoparticles. The optical absorption studies as a function of deposition time indicated that the band gap of the SnTe thin film increases as the deposition time decreases.