Elsevier

Progress in Materials Science

Volume 67, January 2015, Pages 1-94
Progress in Materials Science

Nanoindentation in polymer nanocomposites

https://doi.org/10.1016/j.pmatsci.2014.06.002Get rights and content

Abstract

This article reviews recent literature on polymer nanocomposites using advanced indentation techniques to evaluate the surface mechanical properties down to the nanoscale level. Special emphasis is placed on nanocomposites incorporating carbon-based (nanotubes, graphene, nanodiamond) or inorganic (nanoclays, spherical nanoparticles) nanofillers. The current literature on instrumented indentation provides apparently conflicting information on the synergistic effect of polymer nanocomposites on mechanical properties. An effort has been done to gather information from different sources to offer a clear picture of the state-of-the-art in the field. Nanoindentation is a most valuable tool for the evaluation of the modulus, hardness and creep enhancements upon incorporation of the filler. It is shown that thermoset, glassy and semicrystalline matrices can exhibit distinct reinforcing mechanisms. The improvement of mechanical properties is found to mainly depend on the nature of the filler and the dispersion and interaction with the matrix. Other factors such as shape, dimensions and degree of orientation of the nanofiller, as well as matrix morphology are discussed. A comparison between nanoindentation results and macroscopic properties is offered. Finally, indentation size effects are also critically examined. Challenges and future perspectives in the application of depth-sensing instrumentation to characterize mechanical properties of polymer nanocomposite materials are suggested.

Introduction

Depth-sensing indentation (DSI) represents nowadays one of the principal techniques for the mechanical characterization of materials. The method monitors the penetration of an indenter into the material surface during the application and release of a load [1]. At the present time, the most advanced DSI instruments can produce indentations with depths of only a few tens of nanometers, most of them also offering the possibility of approaching an upper limit in the micron regime [2]. The terminology “DSI” defines the principle of measurement to attain indentation data. Usually, this technique is also referred to as “nanoindentation”, even when penetration depths of a few microns are involved.

A typical DSI test includes a loading-hold-unloading cycle (see Fig. 1). Elastic contact considerations are usually adopted to analyse load-depth curves. Most commonly, hardness, H, and quasi-static elastic modulus, E, values are derived assuming linear elastic behaviour at the onset of unloading [3], [4]. Because the new generation of DSI testers involves the most advanced engineering approaches to accurately record penetration depths in the nanoscale range, special care should be paid to calibration procedures. These are usually friendly integrated in the software developed for collecting the indentation data. In addition, as DSI devices have sophisticated over the years to widen the range of applications, important considerations related to instrumental artefacts and to data analysis are matters of active discussion within the indentation community. This is reflected in the number of papers fully or partially dedicated to the problems encountered in obtaining meaningful DSI data [5], [6], [7]. The application of DSI to polymers raises additional concerns [6], [8], [9], [10], [11], [12], [13]. On the one hand, linear elasticity cannot be straightforward assumed and particular strategies need to be taken into account. On the other hand, the calibration procedures should be carefully examined because some of them require pure elastic–plastic response. Finally, a comprehensive characterization of polymer materials by means of DSI requires specific approaches to extract information on their time dependence. A step forward in this direction has been produced with the introduction of dynamic DSI, or continuous stiffness measurements (CSM) [14]. CSM superimposes an oscillatory force to the quasi-static loading opening up the possibility of using DSI devices as micro or nanoscale dynamic mechanical analysers.

DSI data are most frequently acquired using purpose-designed instrumentation. Alternatively, Atomic Force Microscopy (AFM) can be used as a depth-sensing instrument provided that careful calibrations to achieve meaningful load and depth data are carried out [15], [16]. The advantage of using atomic force microscopy for indentation purposes is the higher displacement sensitivity and superior imaging facilities. In contrast, compared to instrumented indentation, the technique is more liable to instrumental errors and miscalibrations, especially in force modulation experiments [16].

DSI is being progressively incorporated as a routine technique for mechanical characterization of polymer materials and yet a number of decisions concerning the device calibration, the test procedure and the test parameters are of great importance and should be critically examined by an experienced researcher. The attractiveness of instrumented indentation relies on the ability to extract mechanical properties from a small local deformation. This is extremely valuable for systems that are only available in small amounts, or those with limited dimensionality (thin films and coatings). In addition, DSI allows spatially resolving the mechanical properties and this is of great importance for heterogeneous materials such as polymer nanocomposites.

Polymer-based composites incorporating nanoscaled fillers have attracted much attention over recent years owing to their unique mechanical, thermal and electrical properties that are difficult to achieve using conventional fillers [17], [18]. This superior performance combined with their low density make them suitable for a broad range of technological sectors such as telecommunications, electronics and transport industries, especially for aeronautic and aerospace applications where the reduction of weight is crucial to reduce fuel consumption [19]. The field of polymer nanocomposites has evolved very quickly since the last decade being layered silicates, carbon nanotubes (CNTs) and very recently graphene the most widely used and highly successful examples of nanofillers incorporated into polymer matrices. However, nanofiller aggregation has been found to hamper mechanical property improvements. Consequently, a great effort has been devoted to establish the most suitable conditions for the transfer of mechanical load from the matrix to the nano-reinforcement. A prerequisite for such an endeavour is the homogeneous distribution of the nanofillers and the establishment of a strong affinity with the surrounding polymer matrix.

The application of DSI to polymer nanocomposites has gained increasing interest in recent years. The technique has been proved to be sensitive to filler content, filler dispersion, as well as to the interfacial nanofiller-matrix adhesion [20], [21], [22]. Information on heterogeneities of the composite material, either across the thickness or along the surface arising as a consequence of changes in the matrix morphology or uneven distribution of the filler, can be readily detected by means of DSI [23], [24].

The present article reviews the most relevant contributions concerning the application of DSI to polymer-based nanocomposites, with special emphasis in those containing carbon-based (e.g. nanotubes, graphene, nanodiamond) or inorganic (e.g. nanoclays, spherical nanoparticles) nanofillers. The first part of the review introduces the DSI method and summarizes the most important approaches developed to derive mechanical data in case of materials with time dependent properties such as polymers. A number of specific recommendations for extracting meaningful mechanical data will be offered to the reader. The central part of the review presents the state-of-the-art in the application of DSI to polymer nanocomposites. This technique has been frequently used to evaluate the reinforcing effect of different kinds of nanofillers. Results from different laboratories in similar reinforced polymers often seem to be at variance. An effort has been done to present a comprehensive understanding of the influence of multiple factors to the mechanical enhancement. Finally, challenges and future perspectives in the application of DSI to polymer nanocomposites will be addressed.

Section snippets

Depth-sensing indentation

The usual aim of DSI experiments is to determine the material mechanical properties from load-indentation depth data. A characteristic DSI test includes a load-hold-unload sequence as that shown in Fig. 1. However, other methods including partial unloading, reloading, etc. are sometimes employed. In order to optimize the data collection, certain test parameters such as the approach distance, the allowable drift rate or the criteria to decide whether the indenter has contacted the surface should

General aspects on polymer nanocomposites

The optimization of the properties of polymer nanocomposites depends to a great extent on the interaction between the matrix and the filler [18]. The state of dispersion of the filler and the nature of the interface/interphase with the host matrix are the main two factors accounting for the interaction between filler and matrix, and in turn, compromising the performance of the nanocomposite. Other important parameters such as the aspect ratio and orientation of the filler or the changes

Synthesis methods and types of CNTs

One of the most efficient nanofillers to reinforce polymer matrices are carbon nanotubes (CNTs), one dimensional carbon-based nanomaterials discovered by Iijima in 1991 [83] that possess extraordinary high Young’s modulus (up to 1.2 TPa) and tensile strength (ca. 50–200 GPa), combined with very large aspect ratio (>1000), high flexibility and low density (∼1.8 g/cm3) [84], [85]. There are two main types of CNTs: those consisting of a single graphite sheet wrapped into a cylindrical tube with a

Polymer nanocomposites incorporating inorganic nanofillers

Inorganic–organic nanocomposites generally refer to polymer composites composed by nanoscale inorganic building blocks and a polymer matrix. These building blocks include: layered silicates (e.g., montmorillonite, hectorite, saponite), metal nanoparticles (e.g., Au, Ag), oxides (e.g., SiO2, TiO2, Al2O3), semiconductors (e.g., PbS, CdS) and so forth. They aim to combine the characteristic properties of polymers with those of an inorganic material. The small size of the nanofillers yields a very

Effect of the type of nanofiller

Fig. 26, Fig. 27, Fig. 28 compare the reinforcing effect of different carbon and inorganic nanofillers on the DSI modulus of epoxy, glassy and semicrystalline matrices, respectively. To provide a clearer comparison, Fig. 26 only includes the most representative data regarding epoxy/CNT composites, covering the whole range of modulus increments reported. The figure shows that limited EI improvements are observed at all nanofiller contents, most of them lying in the range 0–40%. Especially low

Conclusions and future perspectives

An effort has been done to gather indentation data on polymer nanocomposites. This is quite a challenging task. On the one hand, DSI devices are too often employed as routine instrumentation without a basic knowledge of the physics underlying. Moreover, the methods developed for the determination of the mechanical properties from DSI curves rely on a number of assumptions that are often overlooked. As a consequence, large errors in the determination of the modulus and hardness can be

Acknowledgments

The authors wish to thank the MICINN (Ministerio de Ciencia e Innovación), Spain, for financial support under the Grants MAT2010-21070-C02-01 and FIS2010-18069. AD would like to thank the CSIC for a JAE postdoctoral contract cofinanced by the EU.

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