Effect of fiber surface texture created from silane blends on the strength and energy absorption of the glass fiber/epoxy interphase

Most of the research to date has focused on tailoring the interphase adhesion by controlling the degree of chemical bonding between fiber and resin. The interfacial shear strength (IFSS) has been increased as much as 40% by modified chemical surface bonding [1-3]. However, it is well known that increasing the interfacial strength of the fiber reinforced polymeric composite material often leads to a reduction in the fracture toughness and vice versa [4-12]. In this study, the effects of mechanical interlocking, in addition to chemical bonding on the strength and energy absorption of glass fiber/epoxy interphase, were studied by creating texture on the fiber surface through the phase separation of silane blends. A series of tetraethoxysilane (TEOS)/3-glycidoxypropyltrimethoxysilane (GPS) blends in solutions of ethanol and water was selected to treat the glass fiber surface. The fiber coated with different surface treatments shows the change in fiber surface morphology due to the addition of TEOS. X-ray photoelectron spectroscopy (XPS) analysis showed that the GPS preferentially migrates to the coating surface which suggests that phase separation induced by the silane blend was the primary mechanism for the texture formation. Atomic force microscopy (AFM) was used to scan the fiber surface after the coating and the fiber surface texture was quantified by the roughness values. In addition, a single-fiber Microdroplet shear test was conducted to assess the interfacial properties between the textured glass surface and an epoxy matrix. Traditionally, interfacial shear strength is the only quantity that was determined from the load vs. displacement curve after microdroplet test. In this study, a new data-reduction scheme was developed to determine the energy absorption due to different failure mechanisms by taking into consideration both machine compliance and fiber stretching in the energy calculation. The results show as much as a three-fold increase in specific sliding energy absorption without sacrificing interfacial shear strength. The examination of failure surfaces shows that failure mode propagates through the textured interphase in a more tortuous path, which results in greater degree of energy absorption during fiber-matrix pullout. This study shows the potential for using chemical bonding and mechanical interlocking effects to improve both strength and energy absorption in fiber reinforced composites.