Hierarchically roughened microplatelets enhance the strength and ductility of nacre-inspired composites

Highlights

• Microplatelets with nanoroughness are created by nanoparticle surface adsorption.

• Smaller nanoparticles on platelets improve strength and stiffness of composites.

• Larger nanoparticles increase ductility at the expense of strength and stiffness.

• Hierarchically roughened platelets combine the benefits of both nanoparticle sizes.

Abstract

Rough interfaces featuring nanoscale asperities are known to play a major role in the mechanics of nacre. Transferring this concept to artificial bioinspired composites requires a detailed understanding about the effect of the surface topography of reinforcing elements on the mechanical performance of such materials. To gain further insights into the effect of asperity size, hierarchy and coverage on the mechanics of nacre-inspired composites, we decorate alumina microplatelets with silica nanoparticles of selected sizes and use the resulting roughened platelets as reinforcing elements (15 vol%) in a commercial epoxy matrix. For a single layer of silica nanoparticles on the platelet surface, increased ultimate strain and toughness are obtained with a large roughening particle size of 250 nm. On the contrary, strength and stiffness are enhanced by decreasing the size of asperities using 22 nm silica particles. By combining particles of two different sizes (100 nm and 22 nm) in a hierarchical fashion, we are able to improve stiffness and strength of platelet–reinforced polymers while maintaining high ultimate strain and toughness. Our results indicate that carefully designed hierarchically roughened interfaces lead to a more homogeneous stress distribution within the polymer matrix between the stiff reinforcing elements. By enabling the deformation of a larger fraction of the polymer matrix, this design concept improves the mechanical response of bioinspired composites and can possibly also be exploited to enhance the performance of conventional fiber–reinforced polymers.

Introduction

Emerging applications in transportation, aerospace and wind energy harvesting continue to drive an intensive search for strong, tough and lightweight composite materials (Mazumdar, 2015). The mechanical performance of composite materials depends not only on the intrinsic properties of its constituents but also on the way in which they are assembled and on the resulting interfaces (Hull and Clyne, 1996). A well designed interface is essential to control load transfer between the continuous matrix and the reinforcing phase and is thus a prerequisite for the optimization of stiffness, strength and toughness (Meyers and Chawla, 2009). Tailoring the surface chemistry of reinforcing elements is a common procedure in the fabrication of commercial composites to ensure optimum interfacial bonding (Hull and Clyne, 1996). Designing the surface topography of the reinforcement allows for additional mechanical interlocking and efficient stress-transfer mechanisms (Chawla and Chawla, 2013; Moon and Jang, 1999).

Nature is full of examples of composite materials with excellent mechanical performance due to a multiscale architecture exhibiting highly sophisticated interfaces (Dunlop et al., 2011; Meyers et al., 2008). Nacre for instance is well known for its brick-and-mortar structure consisting of 95 vol% aragonite (CaCO₃) microplatelets displaying nano-rough surfaces and occasionally slightly wavy shapes (Barthelat et al., 2007; Evans et al., 2001; Sun and Bhushan, 2012; Wang et al., 2001). Upon deformation of the composite, this morphology leads to energy dissipation by platelet-platelet interlocking and contact friction between the nanoasperities (Espinosa et al., 2011, Evans et al., 2001, Katti et al., 2005). These mechanisms are important contributors to the remarkable overall toughness of nacre, which is orders of magnitude higher than that of unstructured calcium carbonate (Espinosa et al., 2009; Wegst et al., 2015). Sutures are another example of intricate interfaces found in rigid biological materials as diverse as diatoms, turtle carapaces or mammalian skulls (Dunlop et al., 2011; Naleway et al., 2015). By combining stiff interlocking teeth with a compliant interphase, biological sutures provide a strong joint between two stiff parts while maintaining a finite controlled flexibility (Li et al., 2013; Naleway et al., 2015). Suture-like interfaces with a higher level of hierarchy can lead to improved and tunable mechanical properties (Li et al., 2012; Naleway et al., 2015).

There has been a major effort to replicate synthetically some of the architectural principles found in these fascinating natural materials (Studart, 2012; Studart, 2013; Wegst et al., 2015). An artificial suture structure at a single level of hierarchy prepared by laser engraving was shown to increase by a factor of 200 the toughness of glass (Mirkhalaf et al., 2014). The geometry dependent mechanics of suture structures with one or more hierarchical levels has been studied in detail both theoretically and experimentally on models produced by additive manufacturing (Li et al., 2013; Lin et al., 2014a,2014b).

In addition to sutures, attempts have also been made to replicate the nanoasperities of platelets in nacre-inspired materials (Brandt et al., 2013, Launey et al., 2009, Le Ferrand et al., 2015, Munch et al., 2008, Xia et al., 2015). Ice-templating, for example, can be used to generate nacre-like ceramic scaffolds consisting of aligned roughened lamella. In this approach, growing ice-crystals expel the solid constituents of a ceramic slurry to form a mineral lamellar scaffold that can be sintered and infiltrated with a compliant matrix phase (Deville et al., 2006). Depending on the slurry composition and freezing conditions the ice-crystals have different surface morphologies that can be used to tune the surface topography of the ceramic phase (Launey et al., 2010; Naglieri et al., 2013). More recently, polypeptide-templated biomineralization was used to prepare rough silica platelets that were incorporated into nacre-inspired composites with improved energy dissipation (Xia et al., 2015). Despite the proven beneficial effect of these approaches on the composite׳s mechanics, these methods to generate roughness during synthesis provide limited control over the size, shape and distribution of the final asperities (Naglieri et al., 2013; Xia et al., 2015).

Experimental studies on composites with roughened reinforcement have been complemented by computer simulations and modeling. By enabling exploration of a wider design space in terms of surface parameters, computational tools provide additional insights into the role of surface roughness on the mechanical behavior of biological and bio-inspired composites. Finite Element (FE) studies on the effect of shape, size and hierarchy of asperities on platelet surfaces predict an improved stress transfer for smaller asperities and for asperities with a fractal shape in nacre-inspired composites (Zhang et al., 2012). To verify such simulation results and to fully understand how the surface topography of reinforcing platelets can influence the mechanical behavior of the composite, it is highly desirable to create experimental systems that enable control of such structural parameters independently over a wide range.

A method to deliberately control size, density and level of hierarchy of surface asperities is the decoration of pre-existing platelets with selected colloidal particles before their assembly into nacre-inspired structures. Such a procedure was recently used to prepare roughened alumina platelets through the electrostatic adsorption of silica nanoparticles (Libanori et al., 2015). Nacre-inspired epoxy composites with improved strain to failure and toughness were obtained using such artificially roughened platelets. The improvement is explained by the more homogeneous deformation of the polymer matrix achieved through enhanced platelet–matrix interlocking (Libanori et al., 2015). Further exploration of this roughening procedure beyond this first study should enable a deeper understanding of the role of nanoasperities on the mechanics of nacre-inspired composites.

In the present study, we prepare alumina microplatelets with silica nanoasperities of different sizes and hierarchical designs. With the help of a magnetic field, these platelets are assembled into nacre-inspired composites containing 15 vol% of inorganic phase in an epoxy matrix (Erb et al., 2012, Libanori et al., 2013). By experimentally characterizing these structures, we systematically study the effects of asperity size in a single level of roughness and of different surface coverages of hierarchically roughened platelets on the mechanical behavior of nacre-inspired epoxy-based composites.