Quasi‐static and dynamic nanoindentation of particle‐reinforced soft composites

The mechanical properties of ferromagnetic particle‐reinforced silicone–rubber matrix composites are examined with quasi‐static and dynamic nanoindentation measurements using Berkovich and flat punch indenters, respectively. Quasi‐static nanoindentation is performed to examine primary factors such as the loading and holding time, particle volume fraction, and indentation depth for these particle‐reinforced soft composites (PRSCs). The Einstein–Guth–Smallwood equation based on macroscopically mechanical property testing is utilized to describe the relationship between elastic modulus and particle content of PRSCs in quasi‐static tests. A good agreement between the nanoindentation and simulation prediction is obtained. To characterize the storage modulus and loss factors of PRSCs, the dynamic nanoindentation is then conducted over the force frequency range of 0–45 Hz to show that the dynamic properties are dominated by the particle content and the force frequency, and independent of indentation depth and oscillation amplitude. It is indicated that the nanoindentation is a versatile methodology to assess mechanical properties of microsized particulate soft composites. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44559.     

Measuring the Surface and Bulk Modulus of Polished Polymers with AFM and Nanoindentation

Abstract

A new method to determine the elastic modulus of a material using the atomic force microscope (AFM) was proposed by Tang et al. [Nanotechnology 19, 495713 (2008)] and is used in this study. This method models the cantilever and the sample as two springs in a series. The properties of both the spring and cantilever are determined on two reference samples with known mechanical properties and these properties are then used to find the elastic modulus of an unknown sample. The indentation depth achieved with AFM is in the nanometer range (30–130 nm in this study); and hence when this technique is performed on polymers, whose surface structure is different from their bulk structure, AFM gives a measure of the surface elastic modulus. In the present study, after employing AFM to measure the surface modulus of five polymers, traditional depth-sensing nanoindentation, with penetration depths of about 1 μm, was used to determine the elastic modulus in the bulk. The mean values for elastic modulus from the AFM were within 5–50% of the nanoindentation results, suggesting the existence of a surface modulus for polished polymers.     

Elastic properties of adhesive polymers. II. Polymer films and bond lines by means of nanoindentation

Seven different polymers used frequently as adhesives and/or matrix polymers in wood, wood composites, and natural fiber‐reinforced composites were studied by uniaxial tensile tests and nanoindentation. It was shown that the elastic modulus, the hardness, the creep factor, and the elastic‐, plastic‐, and viscoelastic work of indentation of the seven different polymers is essentially the same regardless whether the polymers were tested in the form of pure films or in situ, i.e., in an adhesive bond line with spruce wood. An excellent correlation was found between the elastic modulus measured by tensile tests and the elastic modulus measured by nanoindentation. In spite of the good correlation, the elastic modulus measured by nanoindentation is significantly higher than the elastic modulus measured by tensile tests. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:1234–1239, 2006     

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.     

Relating morphology to nanoscale mechanical properties: from crystalline to mesomorphic iPP

A nanoindentation technique using an atomic force microscope (AFM) was applied to characterize the mechanical behaviour of several isotactic polypropylene (iPP) samples. The samples were solidified from the melt with a CCT (continuous cooling transformation) procedure spanning a wide range of cooling rates thanks to a fast quenching apparatus developed by the authors. The influence of instrumental parameters on the nanoscale mechanical properties (indentation depth, Young's modulus) shows that for modulus determination one has to rely on simpler methods of force curve analysis based on trace curve alone. Structure homogeneity up to the scale of macroscopic samples used to evaluate elastic moduli allowed a successful comparison of these values with those determined by AFM, which showed that an increase in cooling rate leads to a significant decrease in the material's mechanical response. AFM can thus provide correlations between operating conditions and mechanical properties and can be used for analysing the structure distribution and for mapping properties on a sub-micron scale.     

Nanoscale mechanical properties of polymers irradiated by UV

The application of depth sensing nanoindentation to determine mechanical properties of three different polymers is described in this work using three different techniques to calibrate the measurement system. The nano-hardness and the elastic indentation modulus of polyvinyl chloride, polyethylene oxide and polyacrylic acid were inferred from nanomechanical tests, and the influence of ultraviolet irradiation on the mechanical properties of measured polymers is studied. A multicycling test—a sequence of several loading and unloading procedures—allowed the measurement of changes in the sample viscoelasticity. The nano-hardness of the polymers is shown to increase with radiation dose while the viscoelasticity decreases.     

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.     

Compatibilised LDPE/LLDPE/nanoclay nanocomposites: I. Structural,mechanical, and thermal properties

Nanocomposites of LDPE/LLDPE/nanoclay have been prepared using a lab‐scale co‐rotating twin screw extruder. Using XRD, tensile testing, AFM, TGA, effects of some material properties and one processing parameter on mechanical and thermal properties of the prepared nanocomposites were evaluated. Tensile properties indicated that all the prepared nanocomposites exhibited a significant improvement in elastic modulus and toughness compared to pristine LDPE/LLDPE blends of the same composition. Thermal stability of nanocomposites in the air and nitrogen atmosphere was improved. XRD patterns and AFM micrographs showed semi‐exfoliated and intercalated microstructures for the prepared nanocomposites with different orders of mixing.     

Indentation measurements using a dynamic mechanical analyzer

A dynamic mechanical analyzer equipped with a diamond indenter tip was used to measure the elastic modulus of polymeric coatings as well as various bulk materials. A fabricated indenter probe was used to indent bulk samples of aluminum and fused quartz, as well as gelatin and polystyrene films in order to compare the micron-level indentation measurements with sub-micron (nanoindentation) techniques. The measured moduli were in agreement for ductile materials and thick films (>20 μm), but limited displacement resolution, material cracking, and hydrostatic pressure effects led to diverging values for thinner coatings and more brittle materials.     

Nanoindentation,AFM and tribological properties of thin nc-WC/a-C Coatings

Instrumented indentation, AFM (atomic force microscopy) and tribological studies were performed on PE CVD (Plasma Enhanced Chemical Vapor Deposition) nanocomposite WC–C coatings to investigate the effects of surface roughness on the reliability of nanoindentation data and the possibilities of different AFM modes in nanomechanical testing, which can be used as a feedback to optimize deposition technology from the viewpoint of their mechanical properties. It was confirmed that surface roughness below 30 nm is necessary to keep the scatter of indentation modulus, E_IT, and hardness, H_IT, below 15%. PF QNM (Peak Force Quantitative NanoMechanical) mode was successfully applied for qualitative mapping of the elastic modulus of coatings with the stiffness above 300 GPa. LFM (lateral force microscopy) mode showed only weak contrast and quantitative measurements in both AFM modes require precise calibration. Coefficients of friction of the studied WC–C coatings were below 0.2 at RT, but increased to 0.7–0.8 at 450 °C due to the formation of a transfer film. Optimization of the deposition conditions based on nanoindentation resulted in the increase of E_IT from ～220 GPa to 350 GPa and H_IT from ～17 GPa to ～29 GPa.     

Morphology,nanomechanical and thermodynamic surface characteristics of nylon 6/feather keratin blend films: an atomic force microscopy investigation

The surface structure and nanomechanical properties of solution‐cast nylon 6 (NY6)/feather keratin (FK) blend films were investigated using a combination of tapping‐mode atomic force microscopy (AFM) phase imaging and nanoscale indentation. A tendency for a nanoscale phase separation between NY6 and FK in their various blends was judged based on the blend phase images. The surface topography and roughness analysis of the AFM height images revealed that FK‐rich blends had coarser surfaces than NY6‐rich ones, possibly due to the heterogeneous nature of the FK chemical structure. Amplitude–phase–distance measurements involving the assignment of the darker and brighter regions of the phase images to NY6‐rich and FK‐rich, respectively, or vice versa led to the recognition of a phase inversion in the blend containing 40 wt% FK. The occurrence of the phase inversion phenomenon was related to the significant difference between the molecular weights of the blend constituents. Analysis of nanoindentation data showed that blending FK and NY6 at various ratios resulted in mixtures with modified mechanical and adhesion features. On the one hand, the NY6 component was responsible for an enhanced elastic modulus and stiffness of the blends, and on the other hand, the FK component provided higher pull‐off force and work of adhesion for the samples. A new approach is also proposed to directly determine the surface energy (γ) values of samples from the nanoindentation data. The excellent consistency between the calculated γ values and the results obtained from contact angle measurements lends credence to the proposed approach. Copyright © 2012 Society of Chemical Industry     

Mechanical properties of cellular ceramics obtained by gel casting: Characterization and modeling

Dense and cellular ceramics were produced from yttria partially stabilized zirconia powders by gel-casting, using agar as a gelling agent and polyethylene spheres (125–300 μm diameter) as volatile pore forming agent to create 50–65 vol.% spherical macropores, uniformly distributed in a microporous matrix.The mechanical properties of both dense and porous samples were investigated at the microscale by nanoindentation testing. The influence of micro-porosity on the mechanical properties of samples was evaluated by the analysis of hardness and modulus depth profiles, coupled with FIB-SEM section observations of selected indentation marks. The intrinsic elastic modulus of the zirconia phase resulted to be of the order of 220 GPa. Mechanical characterization at the macroscale consisted of uniaxial compression tests and four point bending tests. Elastic moduli of about 170 GPa were measured for about 93% dense ceramics, lowering down to 44 and 13 GPa with the addition 50 and 65 vol.% macropores, respectively. Digital image based finite element analysis (DIB-FEA) procedures were implemented in order to verify their applicability for the prediction of mechanical behavior of this type of cellular materials: results confirmed that a very good match between measured and calculated values of elastic modulus can be achieved, provided that the effects of micro-porosity are considered by the proper choice of the elastic properties to be assigned to each individual phase identified by Image Analysis.     

Elastic constants of lamellar and interlamellar regions in α and mesomorphic isotactic polypropylene by AFM indentation

A study of the nanoscale mechanical properties of isotactic mesomorphic and semi‐crystalline polypropylene (iPP) is presented. Two iPPs produced with metallocene and Ziegler‐Natta catalyst polymerization are used. The resulting fibers are characterized by wide angle X‐ray scattering, small‐angle X‐ray scattering (SAXS), and Raman spectroscopy. The spatial variability of the percentage crystallinity is evaluated based on the SAXS data. AFM indentation is performed to measure the elastic modulus of the fibers in the direction perpendicular and parallel to the fiber axis. Since the AFM probing is performed on a scale larger than the lamellar thickness, a statistical analysis of the AFM and SAXS data is necessary to infer the elastic moduli of the α crystals and of the inter‐lamellar regions. The elastic modulus of the crystalline lamellae in the direction perpendicular to the c‐axis of the α crystal probed in compression is estimated at approximately 3.3 GPa, while the effective modulus of the interlamellar regions ranges from 1.5 to 2.2 GPa. The method proposed can be applied to other material systems with similar layered structure to measure elastic moduli or hardness on length scales smaller than the resolution of the indentation test. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43649.     

The influence of dehydration on the interfacial bonding,microstructure and mechanical properties of poly(vinyl alcohol)/graphene oxide nanocomposites

The relationship between the interfacial bonding, microstructure and mechanical properties of the poly(vinyl alcohol)/graphene oxide nanocomposites (PVA/GO) has been investigated by controlling the water content through a dehydration process. The interfacial bonding in PVA/GO was predominantly by hydrogen bonds which were strongly affected by the dehydration process. Micro-voids in the microstructure formed after dehydration due to the shrinkage of the fibrils. A variety of hydrogen bonds including water–water, water–GO and water–PVA can be replaced with the strong PVA–GO interfacial bond resulting in a transition from ductile to brittle fracture. The tensile modulus and strength properties of the PVA and PVA/GO increased as the amount of residual water reduced, while the fracture strain was decreased. The surface mechanical properties of PVA/GO measured by nanoindentation showed broadly similar trends with water content as the bulk mechanical properties. However, there was a threshold value of approximately 3 wt.% water below which the surface mechanical properties decrease slightly. The indentation modulus was higher than the tensile modulus by a factor of at least three. The combined influence of the microstructure and the distribution of water in the nanocomposite is considered to be responsible for this.     

Quasi‐static and dynamic nanoindentation and scratch behavior of multifunctional titania/poly(methyl methacrylate) composite

Nanoindentation (NI) and nanoscratch testing was used to determine the dynamic viscoelastic properties of titania reinforced poly(methyl methacrylate) nanocomposites. It was observed that the dynamic NI have a significant effect on the measured indentation modulus and nano‐hardness of the polymer‐based composite. Agreement was found between quasi‐static and dynamic NI result of the nanocomposites. The sinus‐nanoindentation had a limited effect on the measured viscoelastic properties of the composite. However, tribological properties and scratch hardness confirmed that the titania nanofillers act as the friction coefficient modifier in polymer matrix. POLYM. COMPOS., 35:1372–1376, 2014. © 2013 Society of Plastics Engineers     

Nanoindentation study of polydimethylsiloxane elastic modulus using Berkovich and flat punch tips

This article explores polydimethylsiloxane (PDMS) mechanical properties, and presents nanoindentation experiments with Berkovich and flat punch indenters. In the Berkovich tip quasi‐static nanoindentation test, there are pull‐in and pull‐off events observed during the initial tip contact, and when withdrawing from the surface, respectively. The pull‐in interaction needs to be accounted for to properly determine the initial contact point, and thus the accurate contact area. Once accounted for the pull‐in event, the Berkovich and flat punch tips quasi‐static nanoindentation tests give comparable results of about 1.5 MPa for the PDMS elastic modulus (5 : 1 elastomer base to the curing agent ratio). However, PDMS unloading stiffness is higher than the loading stiffness, and dynamic PDMS testing yields higher elastic modulus of about 3.6 MPa. While these results are comparable with the large strain macroscopic compression test results, the difference underscores the complexity of elastomer mechanical characterization and illustrates the discrepancies typical of the reported values. This article describes nanoindentation methods and critical aspects of interpreting results to assess PDMS mechanical properties. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41384.     

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 responses in the length direction and thickness direction of quaternary 1,4-diazabicyclo-[2.2.2]-octane polysulfone alkaline anion-exchange fuel-cell membranes

A quaternary 1,4-diazabicyclo-[2.2.2]-octane polysulfone (QDPSU) alkaline anion-exchange fuel-cell membrane was synthesized. Nanoindentation and uniaxial tension methods were used to test the mechanical properties of this membrane in both the thickness direction and length direction at 26°C and 40% relative humidity. We found that the QDPSU membrane exhibited remarkable viscoelastic properties with significant loading rate-dependent behavior and time-dependent behavior (creep–relaxation). The value of compression creep rates and compression relaxation rates of the QDPSU membrane were measured to be about 0.2 by a spherical indenter and changed very little with different holding loads. Compared with Nafion, a most widely used fuel-cell membrane, the QDPSU alkaline anion-exchange membrane showed a better creep resistance. During the nanoindentation fatigue test, the variation of depths with the indentation cycles were divided into a rising primary region, a steady-state secondary region, and a continuous declining region. This was different from the uniaxial tension fatigue test. The cross-scale and cross-direction mechanical study from this work will help provide new evidence for bridging the gap between nanoindentation and uniaxial tensile testing. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47696.     

Nanoindentation and morphological studies on nylon 66/organoclay nanocomposites. II. Effect of strain rate

Strain rate effects on surface deformation behavior of exfoliated nylon 66 (PA66)/organoclay nanocomposites have been explored by nanoindentation in present study. Sharp indenter (Berkovich) has been used to indent on the surfaces of polymer/clay nanocomposite with different strain rates. Significant strain-rate hardening has been found consistently existing in both neat PA66 and its nanocomposite systems from surface to subsurface (a few micron deep into the bulk). However, strain rate shows almost no effect on the elastic moduli of the neat system and the nanocomposites. The elastic modulus and hardness increase with the indentation depth due to inhomogeneous distributions of the crystalline morphology as well as clay concentration for the case of the nanocomposites along the indentation direction. The mechanical properties observed are correlated with the inhomogeneous microstructures of the studied systems. The plastic index of PA66 and the nanocomposites have been evaluated as a function of strain rate.     

高分子纳米纤维表面压痕及其分形特征研究

采用静电纺丝法制备了纳米级高分子聚己酸内酯(PCL)纤维,通过扫描电子显微镜和原子力显微镜观察了聚己酸内酯纳米纤维的表面结构和微观特征,并通过原子力显微镜对纤维表面进行纳米压痕实验。实验发现随着压痕深度的变小,纤维的弹性模量呈现增大的趋势。利用统计分形理论对高分子纤维表面压痕过程进行建模,通过拟合实验曲线得到了材料的分形参数,该模型可以很好的模拟高分子纤维材料在纳米压痕实验中的尺寸效应。