Morphology, tensile and fracture characteristics of epoxy-alumina nanocomposites

Abstract

The paper presents studies of the morphology, mechanical properties and fracture behavior of epoxy-alumina nanocomposites; the influences of alumina particle's shapes and sizes are discussed. Alumina particles (pre-treated with surface modifier) of platelet- and rod-shapes ranging from 10-40 nm in size are introduced into epoxy resin. It is found that the dispersion of the nano-sized alumina particle within the epoxy matrix mainly depends on the geometry of the particle. Mechanical characterization and fracture mechanics tests show that the tensile modulus, tensile strength and fracture toughness are affected by the geometry of the particles. The fracture surface investigation shows that several toughening mechanisms, including particles pull-out, crack pinning, plastic yielding and deformation are the main factors for the increments of the fracture toughness of the epoxy-Alumina nanocomposites.

Introduction

Recently, polymer matrix nanocomposites have become the main research interest among the research scientists in the field of composites. Many works have showed that the nanofillers can improve the mechanical properties of composites more efficiently compared to that of the conventional micron-sized filler [1], [2], [3]. The significant reinforcement effects is found to be related to the higher surface area to volume ratio of the nanofillers, in which results in large amount of interactions between the nanofillers and polymer matrix. Therefore, significant improvements in mechanical, thermal, optical, and electrical properties can be obtained even with low percentage loading of the nanofiller (such as 1–5%) [4], [5]. Nevertheless, the effects of the nanofillers also depend strongly on the filler's shape, size, surface characteristic and dispersion homogeneity. For example, it has been reported that the mechanical properties of the polymer nanocomposites are highly depending on the filler dispersion homogeneity. Significant improvements on the mechanical properties can be obtained in well dispersed system compared to the nanocomposites systems with the high filler agglomeration [6], [7], [8], [9].

Epoxy resins are widely use as adhesives; coatings; electronic encapsulates; structural composites and aerospace structures. These resins are not only excellent in mechanical, chemical, thermal and yet there are usually light in total weight and low cost compared to other materials. However, the brittle nature of the cured epoxy usually leads to low fracture resistant and therefore limits the applications of the epoxy in many areas [9], [10]. Hence, in recent years, introducing rigid nanofillers, such as carbon nanotubes, organoclay, alumina, and silica into epoxy resin to form polymer-matrix nanocomposites has become a popular method for toughening the epoxy-based materials [11], [12], [13]. For example, Johnson and colleagues found that the fracture toughness of epoxy resin was improved by almost 140% when 1.34 vol% of silica nanoparticles were added into epoxy resin, and the debonding of the nanoparticle from matrix and subsequent plastic void growth were found to be the major toughening mechanisms [9]. On the other hand, in recent work by Zhao et al. [14], significant increases in fracture toughness were observed when 10 vol% of nano-sized silane coated and uncoated alumina filler were added into epoxy resins. Several fracture mechanisms, such as particle pull-out, spalling and tearing of matrix, crack pinning, crack bridging and crack blunting, were contributed to the increment of the fracture toughness. Similarly, Wetzel and colleagues also discovered the significant toughening effect of nano-sized alumina filler in epoxy [15]. At the filler content of 5 vol%, the fracture toughness increases by 60%, and with 10 vol% of filler, the improvement are further increased to 120%. The SEM investigation showed that toughening mechanisms, such as crack deflection, crack pinning and plastic deformation of matrix, and debonding were presented to improve the fracture toughness.

This work aims to study the effects of the shapes and sizes of the nanofiller to the mechanical properties and fracture behavior of the epoxy-alumina nanocomposites. Alumina particles with different shapes and sizes have been selected as reinforcement fillers, i.e., ranging from 10–40 nm with rod- and platelet-shapes and pre-treated with para-toluenesulfonic acid (PTSA) surface modifier are incorporated into epoxy resin via solvent-assisted method. The morphology, mechanical properties, and fracture toughness of the nanocomposites are characterized using Transmission Electron Microscope (TEM), tensile test and fracture mechanics tests, respectively. The fracture surfaces of the nanocomposites are studied using Scanning Electron Microscope (SEM) to understand the toughening mechanisms in these materials.