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.     

Determination of the elastic moduli of silicone rubber coatings and films using depth-sensing indentation

Depth-sensing indentation (DSI) has been used to determine the elastic moduli of silicone rubber coatings on substrates and freestanding films at micro-penetration depth. Complete elastic behavior for rubber-like films was observed for the first time. Substrate effects are hardly observed when the indentation displacement is less than 10% of the total coating thickness. The calculated elastic moduli of the silicone rubber films from the DSI measurements are in good agreement with those measured by dynamic mechanical analysis (DMA), showing that the DSI technique is a reliable and convenient tool for an accurate estimation of the elastic modulus of a rubber-like coating/film.     

Mechanical properties of polymer‐ceramic nanocomposite coatings by depth‐sensing indentation

The mechanical properties of antimony‐doped tin oxide (ATO) nanoparticle/poly (vinyl acetate‐co‐acrylic) (PVAc‐co‐acrylic) coatings with various ATO contents were investigated using depth‐sensing indentation. These coatings were prepared from aqueous dispersions of ATO and PVAc‐co‐acrylic latex. Three types of methods, including a prolonged load holding time, analysis of the pull‐off portion of the unloading curve, and dynamic indentation, were used to characterize the mechanical properties of these composite coatings. As compared to dynamic indentation, quasistatic conventional indentation even with a prolonged load holding time and analysis of the pull‐off portion of unloading curves generate more scattered coating modulus data. This is due to the effect of creep deformation and inconsistency of the pull‐off portion dimension, respectively. The results obtained using dynamic indentation are more reliable because the technique minimizes the effect of creep deformation using a combination load including static and dynamic components. The dynamic indentation results indicate that the addition of the ATO nanoparticles made the composite coatings stiffer and more elastic solid–like. For example, the storage indentation modulus of the PVAc‐co‐acrylic coating is ～1 GPa and tan δ is ～1.6; the addition of 0.50 volume fraction of ATO increased the modulus to ～5 GPa and reduced the tan δ to ～0.01. POLYM. ENG. Sci. 45:207–216, 2005. © 2005 Society of Plastics Engineers.     

Mechanical properties of thermally sprayed porous alumina coating by Vickers and Knoop indentation

Depending on the thermal spraying conditions, coatings obtained can present different defects, like pores, cracks and/or unmelted particles, and different surface roughnesses, that can affect the determination of the hardness and elastic modulus. The present work investigates the mechanical properties, determined by means of Knoop and Vickers indentations, of a plasma as-sprayed alumina coating, obtained with a nano-agglomerated powder sprayed using a PTF4 torch, in order to highlight how the surface defects interfere into the indentation process. As a main result, Knoop indentation compared to Vickers one gives less dispersive results (15% and 33%, respectively), that are, in addition, more representative of the coating properties. The mean values obtained are 110 ± 40 GPa for the elastic modulus and 1.75 ± 0.42 GPa for the hardness. In addition, and for the two indenter types used, multicyclic indentation has been performed because it allows a more appropriate characterization of such heterogeneous coatings due to the representation of the mechanical properties as a function of the indentation load and/or the penetration depth, leading to more reliable results according to the depth-variability of the coating microstructure.     

Effect of carbon content on structural and mechanical properties of SiCN by atomistic simulations

Silicon carbonitride (SiCN) presents good performance on thermal stability and mechanical properties at high temperature. However, experiments still have problems to investigate the chemical structure of nanodomains and high temperature mechanical properties for SiCN. In this paper, atomistic simulations were used to generate amorphous SiCN with different carbon contents, the resulting structures show a tendency to include a “free carbon” phase when the carbon content increases. The calculated pair distributions, angular distributions and structure factor are comparable with experiments. Particularly, the first peaks of C-C and Si-C distributions become more significant when C content decreases, this is related to the variations of Si-C bonds near the graphene regions when the sizes of carbon phases change. The calculated Young's moduli are close to the experimental data and increase with increasing carbon content. The proposed atomic model can be used to predict the structural and mechanical properties of SiCN at different compositions.     

Predicting the mechanical properties of Cu–Al2O3 nanocomposites using machine learning and finite element simulation of indentation experiments

Micromechanics model, finite element (FE) simulation of microindentation and machine learning were deployed to predict the mechanical properties of Cu–Al₂O₃ nanocomposites. The micromechanical model was developed based on the rule of mixture and grain and grain boundary sizes evolution to predict the elastic modulus of the produced nanocomposites. Then, a FE model was developed to simulate the microindentation test. The input for the FE model was the elastic modulus that was computed using the micromechanics model and wide range of yield and tangent stresses values. Finally, the output load-displacement response from the FE model, the elastic modulus, the yield and tangent strengths used for the FE simulations, and the residual indentation depth were used to train the machine learning model (Random vector functional link network) for the prediction of the yield and tangent stresses of the produced nanocomposites. Cu–Al₂O₃ nanocomposites with different Al₂O₃ concentration were manufactured using insitu chemical method to validate the proposed model. After training the model, the microindentation experimental load-displacement curve for Cu–Al₂O₃ nanocomposites was fed to the machine learning model and the mechanical properties were obtained. The obtained mechanical properties were in very good agreement with the experimental ones achieving 0.99 coefficient of determination R2 for the yield strength.     

Nano and Nanocomposite Superhard Coatings of Silicon Carbonitride and Titanium Diboride by Magnetron Sputtering

Superhard coatings (hardness >20 GPa) are being widely researched, driven by the need to use the components under different combinations of environments. Complex properties such as low friction and low wear, increased life time, and increased toughness are required in the same coating. Nanocomposite and nanocoatings are promising materials to achieve them. In the present communication, the author presents research findings on hard nanocomposite and nanocoatings of silicon carbonitride (SiCN) and titanium diborides by magnetron sputtering on Si and steel substrates. XPS, atomic force microscopy, and nanoindentation studies on both the films showed that they possessed high hardness, modulus, and significant elastic recoveries after unloading. These properties can be attributed to nanocrystalline dispersions of carbon nitride and silicon nitride phases in the amorphous SiCN matrix in nanocomposite SiCN thin films, whereas in the case of the TiB₂ film, nanosize grains led to higher hardness.     

Methods for studying the mechanical and tribological properties of hard and soft coatings with a nano-indenter

This study illustrates the capabilities of a nanoindentation/nanoscratch tester to assess mechanical and tribological properties of coating films. Properties such as hardness, elastic modulus, mar and scratch resistance, and critical force for cracking can be accurately measured. Operation of the Nano-Indenter is described in detail. A scanning probe microscope (SPM) is shown to be a valuable supplement to the Nano-Indenter. Well-characterized thermoset acrylic clearcoats and thermoplastic latex films were studied. For the first time, operating parameters are described for measurement of relatively soft coatings, such as films cast from a latex with a glass transition temperature (T_g) of 8°C. Thus, the method is made available for study of most types of coatings. The method can easily discriminate between coatings with different T_gs and crosslink densities. Once operating parameters are established, it takes about 10 minutes for an indentation test and 10 minutes for a scratch test with the Nano-Indenter, and with further automation this time could be reduced. Each indentation test accurately measures hardness and elastic modulus as a function of depth within the coating, and each scratch test provides additional insight into the material’s behavior. The method is sensitive to small changes in polymer composition and formulation, and results are highly reproducible. Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology, November 12–14, 2003, in Philadelphia, PA.     

Investigation of cohesive FE modeling to predict crack depth during deep-scratching on optical glasses

Optical glass scratching can induce various types of cracks, among which median cracks are extremely detrimental and penetrate deeply under the surface. Due to deep-scratching process complexity, it is challenging to devise a method to predict median crack depth. Indentation testing has been examined comprehensively in prior research works. It has been found that using the correlation between scratch and indentation testing can simplify predictive method development. In this research, a numerical method based on indentation testing is proposed to determine median crack depth during deep scratching. In the first step, an FE model is configured to simulate the indentation testing process and the Cohesive Zone Method is applied to describe median crack behavior. The cohesive parameters calibrated through experimental indentation testing are implemented in the FE scratch model, and the results are compared with the experimental scratch test results. According to the results, the FE scratch model was enhanced by mode II fracture energy and the modeled friction coefficient. The indentation and scratch experiments were conducted with BK7, F2, Fused silica, K5, Pyrex, Quartz, SF6, and SF19. The experimental results prove that the nonlinearity of the median crack depth curve correlates with K_Ic. A comparison of the experimental and numerical results demonstrates the model is virtually functional for materials with K_Ic below 1000?kPa?m^1/2. Comparisons between the current findings and other studies infer the model and experimental results are accurate and reliable.     

Mechanical surface characterization: A promising procedure to screen organic coatings

Two ductile coating ma-terials were subjected to a combined indentation and scratch test procedure de-signed to screen a predetermined pattern of many small sample surfaces in a limited time. The screening of 50 surface spots ordered in a matrix pattern on the surface was carried out in 4.5 hr. The test provides reproducible data in terms of indentation modulus, elastic recovery, scratch penetration depth, and scratch residual depth, and also offers the possibility of detecting critical mechanical transitions such as rupture. The presented procedure produces sufficient data in a limited time scale to fulfill the requirements for a fast method to screen coating compositions. Dept. Polymer Technology, SE-100 44 Stockholm, Sweden. Jaquet-Droz 1, CH-2007 Neuchatel, Switzerland.     

Influence of porosity on the mechanical properties of microporous β-TCP bioceramics by usual and instrumented Vickers microindentation

Beta-tricalcium phosphate [Ca₃(PO₄)₂, β-TCP] is a bioresorbable material showing an excellent biocompatibility. However, sintering of β-TCP is difficult and the material presents poor mechanical strength and a low resistance to crack-growth propagation. In this study, influence of the porosity on the hardness and the elastic modulus is studied by means of usual and instrumented microindentation tests. Nevertheless, indentation diagonals measurement by optical observations is not accurate due to the crack formation around the residual indent. That is why instrumented indentation test which allows deducing the hardness and the bulk modulus from the load-depth curve analysis is used as an alternative method. The corresponding hardness number can be calculated by using the maximum indentation depth (Martens Hardness) or the contact depth determined by Oliver and Pharr's method (Contact Hardness). But in order to give representative values when comparing classical and instrumented hardness measurements, Martens hardness is preferred because its value can be directly related to the value of the Vickers hardness number by simple geometrical considerations.In this work, bioceramics were produced by conventional sintering of β-TCP powders synthesized by aqueous precipitation. Different process conditions were chosen to obtain microporous ceramics with a porosity rate between 0 and 14% in volume. As main results, the elastic modulus is found decreasing between 166 GPa and 108 GPa and the hardness number from 4.4 GPa to 2.2 GPa when increasing the porosity rate. A model connecting mechanical properties to porosity rate and grain arrangement is validated for the elastic modulus whereas deviation is observed for the hardness number. However, we propose an original approach where the relative variation of the two mechanical properties can be expressed with a unique relation as a function of the porosity volume fraction.     

Evaluation of mechanical properties of a dense SiC/SiCN composite produced via PIP process

The material behavior of Polymer Infiltration and Pyrolysis based SiC/SiCN composites is studied and the characteristic thermal and mechanical properties in on- (0/90 °) and off-axis (±45 °) direction are summarized. The tensile properties are determined at room temperature and 1300 °C. Based on the ratio of Young’s modulus and strength between on- and off-axis loading, a new approach for the classification of Weak Matrix Composites (WMC) and Weak Interface Composites (WIC) is proposed, which seems to be reasonable for various CMCs. Even without fibre coating mechanical behavior of SiC/SiCN is similar to that of WIC. In order to explain this, a microstructure model is developed and confirmed by analysis of fracture surface. The effect of temperature on the tensile properties is investigated through analysis of residual thermal stresses. Even though at 1300 °C the strength is slightly lower, the fracture strain increased significantly from RT to 1300 °C.     

Nanoindentation and morphological studies on nylon 66 nanocomposites. I. Effect of clay loading

Nanoindentation technique has been used to investigate the mechanical properties of exfoliated nylon 66 (PA66)/clay nanocomposites in present study. The hardness, elastic modulus and creep behavior of the nanocomposites have been evaluated as a function of clay concentration. It indicates that incorporation of clay nanofiller enhances the hardness and elastic modulus of the matrix. The elastic modulus data calculated from indentation load-displacement experiments are comparable with those obtained from dynamic mechanical analysis and the tensile tests. However, the creep behavior of the nanocomposites shows an unexpected increasing trend as the clay loading increases (up to 5 wt%). The lowered creep resistance with increasing clay content is mainly due to the decrease of crystal size and degree of crystallinity as a result of clay addition into PA66 matrix, as evidenced by optical microscopy and X-ray diffraction. At lower clay concentration (here ≤5 wt%), morphological changes due to addition of clay plays the dominant role in creep behavior compared with the reinforcement effect from nanoclay.     

Nanoindentation-induced delamination of submicron polymeric coatings

An elastic model is developed to estimate the interfacial strength between a submicron surface coating and a compliant substrate. The analysis uses a shear-lag model and assumes the plane-stress state in the surface coating. The critical indentation load for the indentation-induced delamination of the coating from the substrate increases with the third power of the indentation depth and is a linear function of the reciprocal of the coating thickness. The indentation-induced delamination of SR399 ultrathin surface coatings over acrylic substrate has been evaluated, using the nanonindentation technique for coating thicknesses of 47, 125, 220 and 3000 nm. For the submicron coatings, the dependence of the critical indentation load on the coating thickness supports the elastic model. The interfacial strength is found to be 46.9 MPa. In contrast, the polymeric coating of 3000 nm displays multiple “excursions” in the loading curve, and the critical indentation load is a linear function of the indentation depth.     

Elastic recovery and creep properties of waterborne two-component polyurethanes investigated by micro-indentation

Polyurethane (PUR) films were obtained by casting waterborne formulation of polyhydroxylated polyacrylate resin and hexamethylene 1,6-diisocyanate trimer hardener. Conversion of the polyaddition reaction was followed by FTIR spectroscopy and appears independent of the NCO/OH stoichiometric ratio between HDI trimer NCO groups and polyhydroxylated polyacrylate OH groups (resin), but thermally activated. The crosslinked networks were investigated by swelling experiments and dynamic mechanical analysis. Calculated values of the average mass between crosslinks allowed explaining the evolutions of the conservation modulus and loss factor with NCO/OH stoichiometric ratio. Elastic recovery and creep behavior of the PUR films were investigated by micro-indentation. A four-element viscoelastic model was used to fit the indentation depth evolution during micro-indentation creep experiments. Results show that creep instantaneous elasticity is fully controlled by the network elasticity and thus by the NCO/OH stoichiometric ratio and that the ability of the network to dissipate energy remains high for non-continuous (NCO/OH ratio < 1) networks. On the other hand, for high NCO/OH ratios, it was shown that hard PUR networks exhibit creep properties and significant retardation times. Finally the evolution of indentation springback factor vs. indentation creep factor was determined, showing that instantaneous elastic recovery behavior increases as creep behavior decreases.     

Mechanical response of novel SiOC glasses to high temperature exposition

The presented work describes mechanical properties of materials prepared by the pyrolysis of polysiloxane resins. The polymeric precursors have different chemical composition. Materials under investigation are predetermined for high temperature resistant applications usually in composite form, i.e. accompanied by some reinforcement. Instrumented hardness tests were employed for material characterisation. The Vickers hardness, Martens hardness and indentation elastic modulus were the key parameters, determined either from the standard optical technique or load–indentation depth curves. The influence of mechanical properties on the monomers ratio was established. The exposition to the temperature between 1200 °C and 1500 °C in air was applied to observe damage caused by these severe conditions.     

Point load-induced fracture behavior in zirconia plasma spray coating

Heat-resistant coatings prepared by two different spraying methods: atmospheric pressure plasma spraying (APS) and high-pressure plasma spraying (HPPS), were tested using tungsten carbide indenters of different diameters, for the purpose of proposing the best suited method of indentation testing. It was found that with the APS method, the indentation load–depth curve gave the indentation depth and the residual depth smaller than and the yield stress greater than those with the HPPS. On the basis of fracture morphology in the cross-section, it has been conjectured that the APS coating has greater elastic modulus than the HPPS coating, and exhibits high strength exceeding debonding force between bond coat and top coat.     

新型洛氏硬度测试方法探讨

将数码摄像技术与传统的洛氏硬度计相结合,通过对测试过程的实时监控,得到压痕深度随时间变化的曲线。在对聚酰胺、有机玻璃及环氧树脂3种材料的硬度测试中,通过分析在正确标尺下得到的压痕深度一时间曲线,比较了3种材料在不同阶段的弹性变形、塑性变形、蠕变及弹性后效等问题的不同。根据该曲线,拟合出保持总载荷阶段的3种材料的压痕深度-时间关系式。通过对同种材料、不同标尺下的压痕深度-时间曲线的比较,发现在保持总载荷阶段,同一种材料的蠕变趋势有所不同。     

Nanoindentation behavior of ultrathin polymeric films

Measurement of the mechanical properties of nanoscale polymeric films is important for the fabrication and design of nanoscale layered materials. Nanoindentation was used to study the viscoelastic deformation of low modulus, ultrathin polymeric films with thicknesses of 47, 125 and 3000 nm on a high modulus substrate. The nominal reduced contact modulus increases with the indentation load and penetration depth due to the effect of substrate, which is quantitatively in agreement with an elastic contact model. The flow of the nanoscale films subjected to constant indentation loads is shear-thinning and can be described by a linear relation between the indentation depth and time with the stress exponent of 1/2.     

Analytical estimation of fracture toughness for circumferential cracks in thin hard films on ductile substrates

In this study, the fracture toughness of circumferential crack caused by indentation effect of a rigid indenter on a thin and elastic coating deposited on the elastic substrate was calculated. In the coating and substrate, the analytical solution of displacement and stress field was used. The complete adhesion was considered for the coating on the substrate. The location of maximum circumferential stress was investigated using the analytical calculation of the stress and it was found that this place was located at a distance away from the center of the indenter. Then, the stress intensity factor and energy release rate for plane strain state was determined, and consequently, the energy release rate for a channel crack was calculated. Finally, the fracture toughness was calculated with energy release rate curves for plane strain crack and crack channeling. This method was used to calculate the fracture toughness of TiN/TiCrN ceramic multilayer coating which was deposited on the GTD450 substrate using the Cathodic Arc PVD method. To validate the results, the analytically calculated crack radius was compared with the experimental crack radius in the fracture load and the difference between the radiuses was in the acceptable range.