Fiber reinforced polymer composites are two component material systems in which fibers are embedded in a polymer matrix. Such a system inherently has an interface region where the two components meet. The interface region around the fiber is also referred to as the ‘interphase’. Properties at the fiber-matrix interphase change by changing the chemistry of the composite system. Study of property variations with changing chemistry will help in better understanding and tailoring of the composite properties. The present work concentrates on the investigation of nanomechanical properties at the fiber-matrix interphase of glass fibers embedded in a polyester matrix. The glass fibers were coated with two types of silanes to produce a strong and a weak bond at the fiber-matrix interphase. Nanoindentation techniques coupled with atomic force microscopy imaging capabilities have been used for this investigation. Two different tips were employed for indenting, one being a Berkovich diamond tip supplied by Hysitron, Inc., and another a parabolic tungsten tip, which was made in the laboratory. Indentations were performed in the fiber-matrix interphase region and also in the bulk matrix. Mechanical properties such as modulus, stiffness, hardness and penetration depth were obtained in the interphase and their variation on moving away from the fiber. Variations of the elastic modulus in the interphase region and its relation to the chemistry are presented. The results obtained using two different tip shapes have been compared.

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