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An ultra-stable instrumented nano-indentation tester (UNHT, Anton Paar) was used to study extremely long (30,000 s) indentation creep of polymers. Total drift rate, measured on fused silica and sapphire samples, was less than 0.2 pm/s for up to 10 h, enabling collection of valid, low-uncertainty, long-term creep data - orders of magnitude longer than previously possible. Fits of the popular N-element Kelvin model to indentation creep data gave values of instant elastic modulus E 0 ∗ and infinite modulus E ∞ ∗ strongly dependent on the time-span of data fitted. Comparison with the long-term experimental data showed that the model was unable to use short term data to predict creep at the much longer times required by industry. A new analysis method is proposed to obtain a better estimate of the true value of infinite modulus, E ∞ ∗ , which is the simplest indicator of maximum dimensional change of polymeric material components subject to long-term stress.

Micro- and nanoindentation and Dynamic Mechanical Analysis (DMA) are different methods to determine mechanical and viscoelastic material properties. The aim of this study was to broaden the capabilities of their utilization by merging these methods. Three standard diamond indenters and a tungsten needle were implemented in a conventional DMA. Four types of standard polymers were investigated. Quasi-static microindentation was performed to determine local material static stiffness, and dynamic microindentation was performed to evaluate local material complex modulus. The results of the quasi-static microindentation showed that different static stiffnesses of the polymers can be distinguished. Even the smallest differences in local mechanical properties due to processing and annealing were distinguished. The complex moduli determined by dynamic microindentation were in good agreement with literature values and three-point bending results. It was shown that a conventional DMA is suitable to determine local and bulk mechanical viscoelastic material properties within one instrument.

Property variability in conjunction with morphological variability are important sources of uncertainty in composite modeling. While image processing of experimental microstructures has enabled accurate quantification of morphological variability, the characterization of material variability is not as well established. In this study, the local material properties of epoxy extracted from a prepreg sheet was determined using nanoindentation with a spherical indenter tip with a radius of 50 μm. Indentations were carried out at four different indentation depths to evaluate the change in the variability of epoxy modulus with the sampling volume. For each length scale studied, 40 indentations were carried out to determine the variability in epoxy modulus. A significant decrease was observed in the coefficient of variation as the indentation depth increased. The corresponding modulus distributions were quantified. The results suggest that, similar to morphological variability, material variability is length-scale dependent and the appropriate variability associated with the selected length scale must be considered for stochastic modeling of composite structures.

Quasi-static nanoindentation has been used to characterize the mechanical properties of polycarbonate reinforced with graphite nanoplatelets (GNPs). Poor dispersion or low quality interfacial interactions of GNPs in a polymer matrix can significantly decrease the relative improvement in the material's mechanical strength and stiffness. In this study, the surfaces of GNPs were modified to achieve better dispersion and interfacial interaction between fillers and matrix. The GNP/PC nanocomposite has a heterogeneous microstructure, and the original mechanical properties between filler and matrix have large differences. Using a spatially sensitive probe method leads to measured values of modulus and hardness that correlate with the indentation sampled volume. A grid indentation procedure was performed with variable sampling volumes to provide a statistical measurement of modulus and hardness for the nanocomposite materials. The surface treatment leads to a significant increase in both stiffness and hardness for GNP reinforced composites.

Force-deformation curves have been acquired using nanoindentation and atomic force microscopy on two amorphous polymer samples. The shape and size of the indenter tip was characterized using a white light interferometer and AFM. The measured nanoindentation curves were fitted with the Hertz equation to calculate the Young's modulus of the polymers. Once the Young's moduli of the polymers were known, AFM was used to acquire force-distance-curves on the same samples. We also used the Hertz theory for the analysis in this case. As a result, the tip radius of the AFM cantilever tip could be measured. This procedure is proposed as a method to determine the shape and size of AFM tips for the quantitative characterization of surface forces through force-distance curves.

This paper presents a convenient method which is referred to as “Revised Step-Hold” method for determination of shear creep compliance of linear viscoelastic-plastic solids using instrumented indentation. Its main idea is using the revised load-depth curve which does not contain plastic deformation to determine the shear creep compliance instead of using the measured load-depth curve. The revised load-depth curve can be obtained through a three-step procedure. Applications are illustrated on two typical viscoelasticplastic solids, namely PMMA and UPVC. The shear creep compliance of PMMA and UPVC determined by the revised step-hold method has satisfactorily good agreement with the corresponding uniaxial tensile result. Therefore, it demonstrates that the revised step-hold method is a reliable method for determination of shear creep compliance of linear viscoelastic-plastic solids.

The mechanical and tribological properties of PMMA-based bone cement (CEMEX RX, TECRES Company, Italy) were measured using nano-indentation and nano-scratch tests. The polymer samples were prepared using two different mixing methods i.e., hand mixing and vacuum mixing. Cubic CEMEX RX specimens were produced and the nano-indentation and nano-scratch tests were performed on both the hand-mixed and the vacuum-mixed specimens by a nano-indenter setup and atomic force microscopy (AFM) observation. The values of Young's modulus, hardness and wear resistance obtained on the hand-mixed and the vacuum-mixed cements were compared. The results indicate that the vacuum-mixed cement has better mechanical and tribological properties compared with the hand-mixed one. Since the nano-indentation and the nano-scratch test methods need less sample material, decrease costs and obtain reliable results, they can be considered as suitable techniques for determination of the mechanical and tribological properties of bone cements.

AFM nanoindentation was investigated as a method for determining the micromechanical properties of polymer materials. It is generally accepted that the shape of the tip of the cantilever undergoes a change in a standard AFM setup. The shape defines the projected contact area, so it is a parameter directly proportional to the elastic modulus; any change in the shape thus affects the accuracy of the results. The method suggested in this paper relies on the introduction of an experimentally determined tip-area function. Values for Young’s modulus were calculated for EPDM samples with different degrees of cure and crystallinity. The degree of crystallinity has a greater impact on the mechanical properties of the material than the degree of cure. Depending on the amplitude of the indentation, the E-moduli determined by AFM are systematically higher. When studying different regions of polymer materials, the values of the E-modulus determined by AFM become identical to those measured by means of DMA on extrapolation of the modulus at zero indentation.

This paper deals with the nanoindentation behaviour of spent polyamide-12 and spent polyamide-12/layered silicate filled nanocomposites. Layered silicates at 1 wt.%, 3 wt.% and 5 wt.% were used as filler in spent polyamide-12 and the nanocomposites were prepared by using a single screw injection moulding technique. The interlayer d-spacing, interlamellar structure and surface morphology were investigated by wide angle x-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. The effect of various concentrations of layered silicate reinforcement on the nanomechanical properties was investigated by a nanoindentation test method. The nanohardness increased from 0.156 GPa for neat spent PA-12 up to 0.338 GPa for 5 wt.% spent PA-12/nanocomposite. Similarly, reduced modulus of neat spent PA-12 increased by 73% with the incorporation of 5 wt.% layered silicate. Different levels of nanofiller dispersion characterised using XRD and TEM correlated strongly with improvements in nanohardness properties. The much improved nanohardness and modulus values are related to the intercalated and exfoliated structure of spent PA-12/nanocomposites. The reutilisation of spent materials such as spent PA-12 has great potential for economic and environmental benefits.

Nanoindentation was applied to determine the elastic moduli in the compression mode of films of fluorinated ethylene propylene copolymers (FEP) foamed by means of supercritical carbon dioxide. For this purpose, a flat punch indenter with a diameter larger than the average cell size of the foams was found to be appropriate. The dependence of the mechanical properties of the films on their cellular structure was investigated by varying material and process parameters: molar mass of the FEP, the magnitude of the pressure drop and the temperature during the foaming process. The elastic modulus showed a strong influence of the cellular structure of the films. A decrease of the molar mass of FEP as well as an increase of pressure drop and foaming temperature considerably reduced the elastic modulus of the foams generated. This lowering of the stiffness correlated with the decrease of foam density. In addition, a significant contribution of the cellular morphology and the thickness of the compact skin layers of the foams could be noted. A further reduction of the foam stiffness was achieved by a biaxial stretching of the foams, which deformed the cells into a lens shape.

This paper presents formulae for visco-elastic-plastic response to indentation for various indenter shapes and times of loading, and describes a procedure for obtaining parameters of creep compliance function from monotonic load. The application is illustrated on PMMA, whose properties are measured under constant load and in a load-unload test. A discussion follows on the influence of indenter shape, various forms of creep compliance function and the relation between the test duration and the model. Also, other information obtainable from nanoindentation tests is mentioned: apparent modulus, hardness and components of creep compliance function.

This work concerns the development of a technique for the nanoindentation of polymeric samples containing a graduated modulus region, and a method for the preparation of samples. The samples studied were prototype intervertebral disc prostheses, which were manufactured from polycarbonate urethanes using a single-step injection moulding procedure. Discs were sectioned, mounted and polished using a series of increasingly fine grades of silicon carbide paper. It was found that the smoothest samples exhibited a significant degree of polishing damage around the interface and that slightly rougher samples were in fact more uniform and more suitable for studying using nanoindentation. The variation of the measured Young's modulus value with surface roughness was also investigated using calibration samples, and a nanoindentation linescan method was developed, which allows the variation in modulus across a polymeric sample to be investigated.

A nanoindentation technique was used to investigate mechanical properties (i.e. hardness and modulus) of Nylon 11 (PA11) and its nanocomposites with different clay loading. It has been found that the hardness and modulus of the nanocomposites steadily increase with increasing clay content. With incorporation of 5 wt% clay into the PA11 matrix, the hardness and elastic modulus are improved by about 27% and 32%, respectively. The modulus data obtained by nanoindentation are compared with those obtained by tensile testing and dynamic mechanical analysis. The studies prove that nanoindentation is a simple but effective mechanical testing method for the polymer and the nanocomposites.

In the present study, a nanoindentation technique is used to probe the hardness and modulus of the two crystal forms (i.e. γ-phase and α-phase) of nylon 6, which were formed by isothermal annealing at 60 and 200 °C, respectively. The crystal structures of nylon 6 were evidenced by X-ray diffraction. By nanoindentation, the hardness and elastic modulus of the γ-form crystals were found to be 53 and 50%, respectively, lower than those of the α-form crystals of nylon 6. The much higher stiffness and hardness of the α-form crystals may be related to the higher packing density of polymer chains originated from the stronger hydrogen bonding interchain interactions in the α-phase crystals.