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.     

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.     

Short glass fiber reinforced ABS and ABS/PA6 composites: Processing and characterization

In this study acrylonitrile‐butadiene‐styrene (ABS) terpolymer was reinforced with 3‐aminopropyltrimethoxysilane (APS)‐treated short glass fibers (SGFs). The effects of SGF concentration and extrusion process conditions, such as the screw speed and barrel temperature profile, on the mechanical properties of the composites were examined. Increasing the SGF concentration in the ABS matrix from 10 wt% to 30 wt% resulted in improved tensile strength, tensile modulus and flexural modulus, but drastically lowered the strain‐at‐break and the impact strength. The average fiber length decreased when the concentration of glass fibers increased. The increase in screw speed decreased the average fiber length, and therefore the tensile strength, tensile modulus, flexural modulus, and impact strength were affected negatively and the strain‐at‐break was affected positively. The increase in extrusion temperature decreased the fiber length degradation, and therefore the tensile strength, tensile modulus, flexural modulus, and impact strength increased. At higher temperatures the ABS matrix degraded and the mechanical strength of the composites decreased. To obtain a strong interaction at the interface, polyamide‐6 (PA6) at varying concentrations was introduced into the ABS/30 wt% SGF composite. The incorporation and increasing amount of PA6 in the composites broadened the fiber length distribution (FLD) owing to the low melt viscosity of PA6. Tensile strength, tensile modulus, flexural modulus, and impact strength values increased with an increase in the PA6 content of the ABS/PA6/SGF systems due to the improved adhesion at the interface, which was confirmed by the ratio of tensile strength to flexural strength as an adhesion parameter. These results were also supported by scanning electron micrographs of the ABS/PA6/SGF composites, which exhibited an improved adhesion between the SGFs and the ABS/PA6 matrix. POLYM. COMPOS. 26:745–755, 2005. © 2005 Society of Plastics Engineers     

Creep mechanisms in quasi-plastic silicon nitride by instrumented indentation

Quasi-plastic creep behavior of the commercial, fine-grained silicon nitride grade, ST 1, was investigated using variety of techniques with the focus on the analysis of instrumented indentation. Creep deformation in this material was characterized by high creep rates at temperatures above 1300 °C and failure strains around 20%. It was accompanied by strong oxidation, cracking of the oxide layers, excessive cavitation at multigrain junctions and slight texture formation. Instrumented indentation revealed degradation of indentation moduli in the oxide layers and enhancement of oxidation and elastic moduli degradation during creep. Because of the similarities between the mass transport processes in cavitation, diffusion processes involved in oxidation and similar activation energies, both creep and oxidation occur simultaneously, however, oxidation is enhanced by external stress. Texture formation implied from disappearance of -silicon nitride and anisotropy of indentation modulus contributes insignificantly (<5%) to total tensile strain. Creep processes in the studied material can be explained within the expanded cavitation creep model of Luecke and Wiederhorn assuming that cavitation is facilitated by low viscosity residual glass and small matrix grain size. Tertiary-like creep is attributed to the gradual increase of the applied stress resulting from the reduction of the effective cross section due to the formation of cracked oxide layers. Size and pre-oxidation effects were predicted and confirmed using creep samples with different gauge size.     

Quantifying the mechanical properties of polymeric tubing and scaffold using atomic force microscopy and nanoindentation

Measurement of mechanical parameters of polymeric scaffolds presents a significant challenge due to their intricate shape and small characteristics dimensions of their elements—around 100 μm. In this study, mechanical properties of polymeric tubing and scaffold, made of biodegradable poly(l ‐lactic) acid (PLLA), were characterized using atomic force microscopy (AFM) and nanoindentation, complemented with tensile testing. AFM was employed to assess the properties of the tube and scaffold locally, while nanoindentation produced results with a dependency on the depth of indentation. As a result, the AFM‐measured elastic modulus differs from the nanoindentation data due to a substantial difference in indentation depth between the two methods. With AFM, a modulus between 2 and 2.5 GPa was measured, while a wide range was obtained from nanoindentation on both the tube and scaffold, depending on the indentation scale. Changes in the elastic modulus with in‐vitro degradation and aging were observed over the 1‐year period. To complement the indentation measurements, tensile testing was used to study the structural behavior of the tube, demonstrating the yielding, hardening and fracture properties of the material. POLYM. ENG. SCI., 59:1084–1091, 2019. © 2019 Society of Plastics Engineers     

Macroindentation of polymers

Macroindentation tests were performed by means of a flat cylindrical indenter on high‐density polyethylene (HDPE) and polyamide 66 (PA66). Commercial sheets were acquired for both materials, and specimens were extracted for differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermal recovery tests. In this way, the fabrication effect was correctly identified in terms of degradation, orientation, and mechanical performance. Specimens were also prepared for tensile tests, which were performed for comparison with indentation experiments. Indentation tests were carried out on the external sheet surfaces at different indenter diameters (1 and 2 mm) and test rates (from 0.1 to 20 mm/min). A very high correlation was found between the tensile test and indentation results at the different test rates. In particular, the indentation stress at fixed depth was a linear function of the tensile yield stress, depending only on the indenter diameter and not on the material type. Moreover, a threshold value for the thickness for a correct comparison among samples having different dimensions is discussed. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

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     

Elastic Anisotropy of ß-Silicon Nitride Whiskers

The elastic anisotropy of ß-silicon nitride was examined using nanoindentation techniques. A coarse-grained polycrystalline sample composed of whiskers up to 15 µm in diameter and 100 µm in length allowed for the positioning of small indentations near the centers of grains, where neighboring grains minimally affect the property measurements. The crystallographic directions along which measurements were made were determined from the grain geometries in polished sections. The primary directions of interest were parallel to the c -axis (indenter penetration parallel to the whisker axis) and in the basal plane (indenter penetration perpendicular to the whisker axis), for which the indentation moduli were found to be 450 and 310 GPa, respectively. An indentation mechanics model used to estimate the single-crystal elastic constants from the nano-indentation results shows that Young's moduli for ß-silicon nitride, which are similar to but not exactly the same as the moduli measured by nanoindentation techniques, vary from 540 GPa parallel to the c -axis to 280 GPa perpendicular to it. Hardness data, which also exhibit a strong anisotropy, are also presented.     

The Elastic Modulus of β‐HMX Crystals Determined by Nanoindentation

This paper presents an investigation on the anisotropic modulus of energetic crystals of β‐HMX by using a Hysitron Triboindenter fitted with a Berkovich tip. High quality single crystals of β‐HMX, whose densities were determined by high precision density gradient tube (DGT), were used in order to extract the moduli of two orthogonal crystal faces: (010), the maximum growth habit face (termed face 1), and the face which is perpendicular to it (termed face 2). Contrary to common expectation, the moduli of face 1 and face 2 were found to be 23.4 and 26.1 GPa, respectively, indicating a negligible crystal anisotropy of the system. Nanoindentation measurement gave even narrower difference, with hardness values of 1.1 GPa for face 1 and 0.95 GPa for face 2, thus further supporting the above conclusion. The result obtained from nanoindentation contradicts with those from the impulse stimulated light scattering (ISLS) and the Brillouin scattering, but was close to the result predicted by molecular dynamics (MDs) simulation. It is also suggested that the modulus value of 31 GPa obtained from previous micro‐indentation measurement may be overestimated.     

Mechanical properties of polystyrene/polyamide 6 blends compatibilized with the ionomer poly(styrene‐co‐sodium acrylate)

The compatibilizing effect of the ionomer, poly(styrene‐co‐sodium acrylate) (PSSAc), on immiscible blends of polystyrene (PS)/polyamide 6 (PA6) was studied by mechanical tests and scanning electron microscopy. The PSSAc acts as an effective compatibilizer because both the deformation at break (%) obtained by tensile stress–strain tests and the impact rupture energy are larger in blends containing small amounts of PSSAc. The morphologies of the fractured surfaces produced by tensile stress–strain tests of blends with or without the ionomer confirm that PSSAc increases the interfacial adhesion between PS and PA6 phases. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2545–2551, 2004     

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     

Effect of Interfacial Compatibility on the Mechanical and Tribological Properties of Blending PTFE with PA6

Blending Polytetrafluorothylene (PTFE) to PA6 with and without maleic anhydride grafted Polytetrafluorothylene (PTFE-g-MA) was produced in a corotating twin screw extruder. The effect of PTFE-g-MA on the tensile properties and tribological properties of PTFE/PA6 polymer blends is studied. Results show that the presence of PTFE-g-PA6 in the PTFE continuous phase improves the interfacial adhesion, as a result of the creation of an interphase that was formed by the interaction between the formed PTFE-g-PA6 copolymer in situ and both phases. Compared with the PTFE/PA6 without PTFE-g-MA, the PTFE/PA6 with PTFE-g-MA had the lowest friction coefficient and specific wear rate under given applied load and reciprocating sliding frequency.     

Multiple cohesive cracking during nanoindentation in a hard W-C coating/steel substrate system by FEM

Stress evolution and subsequent cohesive cracking in the hard and stiff W-C coating on steel substrate during nanoindentation have been investigated using finite element modelling (FEM) and eXtended FEM (XFEM). The FEM simulations showed that the maximum principal stresses in the studied system were tensile and always located in the coating. They evolved in several stages. At indentation depths below 15% of the relative indentation depth, the maximum principal tensile stresses of ∼3 GPa developed at the top surface of the coating along the indenter/coating interface. At relative depths range 15–60%, the maximum tensile stresses of ∼6–8 GPa concentrated under the indenter tip in the coating along the interface with the substrate. At relative depths exceeding 60%, the maximum stresses gradually increased up to 10 GPa and they were located in the sink-in zone outside the indent as well as below the indenter tip. The first and subsequent cohesive cracks developed when the maximum tensile stresses in the sink-in zone at the top surface of the coating (and at the coating/substrate interface under the indenter) repeatedly reached the ultimate tensile strength of the coating. The hardness profile as well as cohesive cracking is controlled by the deformation of the substrate defined by the ration of the yield stresses of the coating and substrate. Very good correlation between the experimentally obtained cracks and multiple cracks predicted by XFEM confirmed the ability of the applied modelling in the prediction of fracture behavior of the studied coating/substrate system.     

Bulk high-entropy hexaborides

For the first time, a group of CaB₆-typed cubic rare earth high-entropy hexaborides have been successfully fabricated into dense bulk pellets (>98.5 % in relative densities). The specimens are prepared from elemental precursors via in-situ metal-boron reactive spark plasma sintering. The sintered bulk pellets are determined to be single-phase without any detectable oxides or other secondary phases. The homogenous elemental distributions have been confirmed at both microscale and nanoscale. The Vickers microhardness are measured to be 16?18 GPa at a standard indentation load of 9.8 N. The nanoindentation hardness and Young’s moduli have been measured to be 19?22 GPa and 190?250 GPa, respectively, by nanoindentation test using a maximum load of 500 mN. The material work functions are determined to be 3.7–4.0 eV by ultraviolet photoelectron spectroscopy characterizations, which are significantly higher than that of LaB₆.     

Determination of mechanical properties of polymeric microspheres used in controlled drug delivery systems by nanoindentation

In this study, we synthesized poly (vinyl acetate-co-divinyl benzene) microspheres with various monomer/cross-linker contents for oral/topical sustained drug release applications and determined the micromechanical properties by nanoindentation. Compression elastic moduli of materials were calculated by using the limited depth of indentation according to Hertz elastic deformation model and presented as the histogram of multiple data. In terms of drug release practices, poly (VAc-co-DVB) microspheres with a high DVB content, especially in topical applications, are expected to carry drugs under mechanical stresses of less than 1.0 GPa.     

Extension‐induced mechanical reinforcement in melt‐spun fibers of polyamide 66/multiwalled carbon nanotube composites

In this work, polyamide 66 (PA66) and its composites with multiwalled carbon nanotubes (MWNTs) were melt spun into fibers at different draw ratios. PA66 fibers at high draw ratio demonstrate a 40% increase in tensile strength, 66% increase in modulus and a considerable increase in toughness. It is demonstrated that this reinforcement can be mainly attributed to high‐draw‐ratio‐induced good dispersion and orientation of MWNTs, particularly the enhanced interfacial adhesion between MWNT and matrix thanks to interfacial crystallization. Our work provides a simple but efficient method to achieve good dispersion and strong interfacial interaction through melt spinning. Copyright © 2011 Society of Chemical Industry     

Morphology,thermal and mechanical properties of nylon 12/organoclay nanocomposites prepared by melt compounding

Nylon 12 (PA12) nanocomposites with different organoclay loadings were successfully prepared by melt compounding. X‐ray diffraction indicated the dominance of the exfoliated clay morphology throughout the matrix after mixing in a Brabender twin‐screw extruder, in accordance with transmission electron microscopy observations. Thermogravimetric analysis showed that the thermal stability of the PA12 matrix was improved by about 20 °C on incorporation of only 5 wt% clay. Tensile and nanoindentation tests indicated that the elastic modulus and the hardness steadily increased by about 52 % and 67 %, respectively, with a clay concentration up to 5 wt%, while improvements in tensile strength were limited. Impact strength decreased linearly by about 25 % as the clay loading increased (up to 5 wt%), indicating an embrittlement due to clay addition, as evidenced by SEM observation on the fracture surfaces. The embrittling effect may be due to the weak interfacial adhesion between the clay platelets and the polymer matrix. Copyright © 2004 Society of Chemical Industry     

Mechanical properties and morphology of PA6/EVA blends

The mechanical properties of blends of polyamide6 (PA6) and ethylene vinyl acetate (EVA) at a blending composition of 0–50 wt % EVA were studied. The notched Izod impact strength of PA6 increased with the incorporation of EVA, the increase being more than 100% compared to PA6 at 10% EVA. The tensile strength and the tensile modulus of the blends decreased steadily as the weight percent of EVA increased. Analysis of the tensile data using predictive theories indicated the extent of the interaction of the dispersed phase and the matrix up to 20 wt % EVA. SEM studies of the cryogenically fractured surfaces indicated increase in the dispersed phase domain size with EVA concentrations. On the other hand, impact fractured surfaces of PA6/EVA blends indicated debonding of EVA particles, leaving hemispherical bumps, indicating inadequate interfacial adhesion between PA6 and EVA. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1593–1606, 2002     

Morphological and nanomechanical characterization of anisotropic interfacial characteristic regions in CF/PA6 composites at different cooling rates

Macroscopic tensile tests on neat PA6 and CF/PA6 prepregs showed that the cooling rate significantly affects the mechanical properties of CF/PA6 composites because of their different crystallization behaviors both at the fiber surface and in the matrix. Polarizing optical microscopy, static nanoindentation (SNI), and dynamic mechanical imaging (DMI) tests were used to characterize the anisotropic morphologies and nanomechanical performances of the interfacial characteristic regions in CF/PA6 composites at five different cooling rates. As a result, the seven interfacial characteristic regions inside the CF/PA6 composites were clearly distinguished. The interphase thickness of the CF/PA6 composites decreased with a decrease in the cooling rate. On the contrary, the interphase modulus and transcrystallinity thickness and modulus showed significant increases with a decrease in the cooling rate. The DMI and SNI test results were in agreement with each other and with the macromechanical test results. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44106.     

The structure and properties of PA6/MMT nanocomposites prepared by melt compounding

Polyamide 6 (PA6)/organo‐montmorillonite nanocomposites prepared by melt compounding using a co‐rotating twin‐screw extruder were intercalated nanocomposites. The modulus, bending strength and heat distortion temperature of these nanocomposites increased with increasing clay content, and tensile strength increased up to 4.3 wt% and then decreased with further increases in montmorillonite content. The notched Izod impact strength decreased slightly with montmorillonite content. Nanocomposites based on the higher‐molecular‐weight PA6 yielded higher moduli, tensile strengths, and heat distortion temperatures at the same montmorillonite content. Polym. Eng. Sci. 44:2070–2074, 2004. © 2004 Society of Plastics Engineers.