Elsevier

Composites Science and Technology

Volume 75, 11 February 2013, Pages 93-97
Composites Science and Technology

Anisotropic elastic moduli and internal friction of graphene nanoplatelets/silicon nitride composites

https://doi.org/10.1016/j.compscitech.2012.12.003Get rights and content

Abstract

Elasticity and internal friction of graphene nanoplatelets (3 wt.%)/Si3N4 composite is analyzed by ultrasonic methods. It is shown that the composite exhibits a degenerate (elliptic) form of transversal isotropy with the graphene nanoplatelets acting effectively as spheroidal voids and inducing significant softening in all directions. The shear internal friction is strongly anisotropic with the maximal value corresponding to the volume-preserving, ‘breathing’ vibrations of the nanoplatelets. Possible relations between the observed behaviors and the micromorphology of the composite are discussed.

Introduction

Graphene nanoplatelets (GNPs) were recently used as fillers for Si3N4 ceramic composites with the aim to obtain advanced materials with enhanced fracture toughness, or electric and thermal conductivity [1], [2], [3], [4], [5]. Due to the superior elastic properties of graphene (the in-plane Young’s modulus equal to 1.1 TPa [10]), the presence of such particles in the ceramic matrix can be also expected to have some significant impact on the macroscopic elastic moduli of the composite, especially if they are arranged along some preferred directions and form, thus, a spatially anisotropic microstructure. In this paper, we study the case of silicon nitride ceramics with 3 wt.% of GNPs prepared by spark plasma sintering (SPS), where the preferred orientation of the platelets is induced by uniaxial stress applied during the sintering process [3], [4], [5]. The knowledge of elastic constants of such material is important not only with respect to its possible mechanical applications, but, as we show in this paper, enables also a deeper insight into micromechanics of the composite. In addition, the determination of elastic constants reported here is complemented by analysis of the anisotropic internal friction in the examined material, which is a parameter never discussed for graphene/ceramic composites so far. While the mechanical, micromechanical and vibrational properties of graphene/polymer nanocomposites have been studied numerous times and from numerous points of view (e.g. [6], [7], [8]), similar analysis for graphene/ceramic composites is still lacking; this paper aims to present one of the possible approaches how this gap can be filled.

Section snippets

Examined material

The examined composite was prepared by SPS (Dr. Sinter, SPS-510CE, Japan) from the mixture of α-Si3N4 powders (plus sintering additives: 2 wt.% Al2O3 and 5 wt.% Y2O3) and 3 wt.% of GNPs. The details on mixing, homogenization and SPS conditions were reported in [4]. The prepared specimen was cylindrical, with 20 mm in diameter and 2.7 mm in thickness. The density of this material was 3.18 g cm−3, which equals to 99% of the theoretical one (calculated using values of 3.23 g cm−3 for Si3N4 and 2.2 g cm−3 for

Experimental determination of elastic constants

From the SPS specimen, two rectangular parallelepipeds of dimensions approximately 4  ×  3 ×  2 mm3 were prepared, oriented such that the shortest edge was always perpendicular to the preferred orientation of the platelets, i.e. that the largest face was always parallel to the expected isotropic plane. These two samples (denoted hereafter Sample 1 and Sample 2) were used for the determination of elastic coefficients by means of resonant ultrasound spectroscopy (RUS, [13], [14]). In particular, the

Discussion of elastic anisotropy

First of all, let us point out that the anisotropy induced by 3 wt.% of GNPs is surprisingly strong: the difference between the longitudinal moduli along the platelets (c11) and perpendicular to them (c33) is approximately 30% of c33. The overall character of the anisotropy can be more easily seen from the directional dependence of the Young’s modulus shown in Fig. 2. The Young’s modulus in any given direction n can be calculated from the coefficients cij asE=1/s11(n),where sij(n) is a matrix

Analysis of anisotropic internal friction

Besides the determination of the elastic coefficients, the obtained RUS spectrum also enables the estimation of anisotropy of internal friction in the examined material. If the damping is small, the internal friction coefficients can be directly extracted from the widths of the resonant peaks (more precisely from the values of FWHM/f, where FWHM denotes the full width at the half maximum of the peak and f is the resonant frequency) by using the perturbation theory [21], [22]. Since the RUS

Conclusions

This paper illustrates that the anisotropy of elastic moduli and of internal friction in the graphene/ceramic composites can be reliably analyzed by ultrasonic methods, in this case particularly by resonant ultrasound spectroscopy complemented by pulse-echo measurements. In summary, it can be concluded that the presence of just 3 wt.% of GNPs leads to significant anisotropization of the Young’s elastic modulus and the internal friction of SPSed Si3N4, while the anisotropy of the shear modulus is

Acknowledgements

This work was supported by Czech Science Foundation (Project No.GA 101/09/0702), Academy of Sciences of the Czech Republic (Project No. M100761203) and the Spanish Government (Project No. MAT2009-09600). C. Ramirez thanks the financial support of the JAE (CSIC).

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