Short communication
On the use of peak-force tapping atomic force microscopy for quantification of the local elastic modulus in hardened cement paste

https://doi.org/10.1016/j.cemconres.2011.08.009Get rights and content

Abstract

A surface of epoxy-impregnated hardened cement paste was investigated using a novel atomic force microscopy (AFM) imaging mode that allows for the quantitative mapping of the local elastic modulus. The analyzed surface was previously prepared using focussed ion beam milling. The same surface was also characterized by electron microscopy and energy-dispersive X-ray spectroscopy.

We demonstrate the capability of this quantitative nanomechanical mapping to provide information on the local distribution of the elastic modulus (from about 1 to about 100 GPa) with a spatial resolution in the range of decananometers, that corresponds to that of low-keV back-scattered electron imaging. Despite some surface roughness which affects the measured nanomechanical properties it is shown that topography, adhesion and Young's modulus can be clearly distinguished.

The quantitative mapping of the local elastic modulus is able to discriminate between phases in the cement paste microstructure that cannot be distinguished from the corresponding back-scattered electron images.

Introduction

Within the last two decades, a number of publications on assessment of the nanomechanical properties of cement based materials have been published. The objective of this nanomechanical testing is to assess the mechanical properties of the individual constituents of heterogeneous materials and in this way provide a meaningful experimental input for a (multiscale) modeling. Nanoindentation has been a widely used technique hitherto utilized for the purpose. It was recently shown that statistical nanoindentation might not be the optimum testing technique for the elucidation of mechanical properties of microstructured porous multiphase materials. In particular, the issues of the surface roughness due to the intrinsic porosity of cementitious materials [1], the partial volume effects within the nanoindentation interaction volumes [2], [3] and the statistical evaluation of the elastic modulus histograms [4] were shown to pose serious obstacles for the provision of reproducible and unambiguous results.

Despite all the above-mentioned problems, the statistical nanoindentation remains a popular technique for assessment of mechanical properties on micro- and nanoscale [e.g. [5], [6], [7], [8], [9]. In the search of a different technique for the purpose that would circumvent at least some of the problems inherent to the statistical nanoindentation, we identified peak-force tapping atomic force microscopy (further only ‘peak-force tapping AFM’) as one of the possible alternatives. This technique so far was applied to quantify the elastic properties of materials in a lower GPa (0–10 GPa) range [10]. Here we present the results of a pilot test that was performed on a sample of epoxy-impregnated hardened cement paste and show that this range can be extended to higher values of elastic modulus (<~100 GPa).

Section snippets

Peak-force tapping atomic force microscopy

Atomic force microscopes can be also utilized for (statistical) nanoindentation testing. Even though AFM-based nanoindenters have a minor advantage over the standard nanoindentation instruments in the ability of imaging the sample surface before the actual indentation is performed, the nanomechanical properties are derived according to the same principles [11] and therefore AFM-nanoindentation [12] does not significantly differ from the above-mentioned statistical technique.

Interestingly,

Materials and methods

The 20 × 20 × 10 mm block of hardened cement paste (w/c = 0.5) was produced from white Portland cement (Aalborg white) and water cured for 21 days. After that, the block was impregnated using common methods for epoxy-impregnation. After the epoxy impregnation, a small cylindrical sample originally residing inside the block was prepared by high-current FIB milling and fixed at the end of a stainless steel sample holder (see Fig. 2).

The cylindrical sample was fixed on the stainless steel holder. The

Results and discussion

Fig. 3 shows the 512 × 512 maps of the topography, adhesion, sample deformation and Young's modulus of a square region of the top plane of the sample as acquired by peak-force tapping AFM.

Even though the sample preparation should result in a very low surface roughness of the investigated surface, the root-mean-square surface roughness based on the topography map (see Fig. 3a) equaled 38.5 nm. Apart from the general waviness of the surface, waterfall artifacts of FIB milling are observed. The

Conclusions

In this communication, we show that the novel AFM-based imaging modality (peak force tapping AFM with quantitative nanomechanical analysis) provides a tool for assessment of nanomechanical properties of multiphase materials that is superior to the currently rather popular statistical nanoindentation technique. We demonstrate that the technique readily provides images (maps) of elastic modulus of heterogeneous microstructures, such as those of the hardened cement pastes, with the lateral

Acknowledgments

We would like to acknowledge Messrs Luigi Brunetti and Boris Ingold (both EMPA) for the assistance with the sample preparation. We gladly acknowledge that the FIB sample preparation was performed at EMPA, Electronics/Metrology/Reliability Laboratory and Electron Microscopy Centre of ETH Zurich. We thank Mr Daniel Schreier (EMPA) for assistance with EDX analysis. We thank Dr Pietro Lura and Dr Beat Münch (both EMPA) for the critical reading of the manuscript.

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