Fracture Behavior of Epoxy-Based Hybrid Composites

Abstract Crosslinked epoxy resins have become popular materials in advanced engineering applications such as the aerospace, packaging, coatings, electrical, and adhesive industries. These resins offer a high strength to weight ratio, but have a drawback of being inherently brittle and exhibit poor flaw tolerance. In microelectronic packaging applications, the thermal expansion coefficients (CTEs) of neat epoxy resins are too high. The addition of inorganic fillers is a traditional solution for increasing fracture toughness and lowering CTE. Such a solution is routinely utilized in epoxy-based underfills used in flip chip packaging. Of particular interest in this study, is the incorporation of soft, compliant rubbery particles and stiff, rigid silica particles into a model epoxy system. Rubber particle addition has proven to be extremely effective at improving fracture toughness of epoxy composite resins through rubber particle cavitation, matrix void growth, and massive shear yielding mechanisms. The addition of silica particles can also provide increases in fracture toughness, and do so without the compromises of decreased yield strength or increased CTE. Hybrid composites comprised of both rubber and silica particles have shown synergistic toughening benefits by some researchers. In this particular study, an aromatic amine cured bisphenol-A based epoxy resin is used as the model epoxy matrix. The fracture behavior of epoxies filled with silica micro-spheres and/or rubber nano-particles at various concentrations in the matrix have been studied. Micron-size silica particles successfully showed decreases in the CTE of the epoxy matrix, but only provided minimal toughening benefits. Nanometer-size core-shell-rubber particles provided exceptional toughening benefits, but resulted in increased CTE of the epoxy matrix. Interestingly, some hybrid epoxy composite formulations exhibited excellent fracture toughness and reasonable CTE behavior. It is the ultimate goal of this study to provide an understanding of the toughening mechanisms produced for each additive and their interactive effects on each other. It has been hypothesized that the use of rubber nanoparticles in a highly silica filled epoxy may result in significant improvements in toughness since these nanoparticle may be able to grow shear bands around the micron-size silica particles. The results from this study support this hypothesis.