In situ indentation behavior of bulk multi-layer graphene flakes with respect to orientation Chris Rudolf, Benjamin Boesl, Arvind Agarwal ⇑ Plasma Forming Laboratory, Composites Laboratory, Department of Mechanical and Materials Engineering, Florida International University, 10555 West Flagler Street, Miami, FL 33174, United States article info Article history: Received 23 February 2015 Received in revised form 21 July 2015 Accepted 22 July 2015 Available online 23 July 2015 abstract In situ indentation is performed on bulk multi-layer graphene flakes (MLG) consolidated by spark plasma sintering to study the effect of orientation on the deformation behavior and associated energy dissipation capabilities of MLG. Spark plasma sintering of MLG aligns them into a uniformly oriented, densely packed pellet. With respect to the 2D surface of consolidated MLG, indentation is carried out on the surface (out-of-plane MLG orientation) and in the orthogonal direction (in-plane MLG orientation). The combina- tion of instrumented indentation and imaging provided evidence of deformation and failure mechanisms in real-time, as well as a quantitative comparison of energy dissipation. Indentation performed in the orthogonal direction resulted in a work of indentation 270% greater than indentation performed on the surface. The prevalent energy dissipation mechanisms observed when indenting in the orthogonal direc- tion are compressive reinforcement, bending, push-out, and pop-out while the prevalent mechanisms observed in the surface indent are sliding, bending, kinking, and MLG pull-out. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Graphene is a single layer of sp 2 bonded carbon that is known for its excellent thermal, mechanical, and electrical properties. The functional properties of graphene include having a high Young’s modulus (0.5–1 TPa) [1] and high tensile strength (130 GPa) [2], which has gained it considerable attention for use as reinforcement in polymer, metallic, and ceramic composite matrices [3–5]. Rafiee et al. [3] found that compared to other carbon-based nanostructures, such as single- and multi-walled carbon nanotubes, a low content of graphene addition offered bet- ter improvement in mechanical properties in a reinforced epoxy nanocomposite. While offering great reinforcement properties, the use of graphene was slow until the development of multi-layer graphene flakes (MLG) which are also called graphene nanosheets [6]. Multi-layer graphene flakes are particles consisting of multiple layers of stacked graphene [7]. MLG are becoming more widely used because they are easier and less expensive to form compared to single layer graphene while retaining much of the desirable properties [8,9]. Typically, MLG are made up of 10–30 sheets of graphene held together by weak van der Waals forces and have a thickness of 3–10 nm and a width of 1–25 lm, provid- ing large surface areas. The mechanical properties of graphene have been previously reported by Lee et al. [2] who performed atomic force microscopy on a suspended graphene flake and found that the bending rigidity is higher in the principal (in-plane) direction. Golkarian et al. [10] reported on the effect of the van der Waals forces when increasing the number of layers in graphite flakes and found by theoretical modeling that the Young’s modulus decreased about 13% with increasing the number of layers fourfold. Nanoindentation, previ- ously performed on fully dense samples of MLG consolidated by spark plasma sintering (SPS), showed a correlation of mechanical properties as a function of loading direction [11]. Energy dissipa- tion mechanisms were observed post-fracture with a goal of eval- uating future use of MLG in ceramic composites. Subsequent published reports on MLG-reinforced composites detail the various deformation or energy dissipation mechanisms. Nieto et al. [12] prepared a 5 vol.% graphene platelet reinforced tantalum carbide composite by spark plasma sintering and reported a fracture toughness increase of 99% over the monolith. Additionally, post fracture analysis of the graphene platelet fracture surfaces has shown evidence of energy dissipation mechanisms [6]. Hypotheses for increased toughness in composites reinforced with MLG can be attributed to three different regimes; (i) property changes during processing, (ii) increased load capacity prior to ini- tial crack propagation, and (iii) crack propagation suppression mechanisms, examples of toughening mechanisms occurring in regime (i) include increased densification and grain wrapping http://dx.doi.org/10.1016/j.carbon.2015.07.070 0008-6223/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: agarwala@fiu.edu (A. Agarwal). CARBON 94 (2015) 872–878 Contents lists available at ScienceDirect CARBON journal homepage: www.elsevier.com/locate/carbon