Circumventing boundary effects while characterizing epoxy/copper interphases using nanoindentation
(2017)
Characterization of the size and mechanical properties of interphases is essential when designing multicomponent materials. When nanoindentation is used to investigate the size and mechanical properties of an interphase, a common challenge is that the indenter or the stress zone formed around it are often restricted by the reinforcement, making it difficult to distinguish the mechanical property variations caused by the interphase itself from those caused by the boundary effect. In this work, a testing system was developed that allows determining the indent affected zone and accounting for it in the interphase measurements of an epoxy/Cu system. Using finite element analysis, we confirmed the validity of the proposed system. Nanoindentation was used to investigate the Interphase between copper and two different epoxy systems; amine-cured and anhydride-cured. Nanoindentation results showed that a copper layer that is only 10 nm thick still exhibits a constriction effect on the indentations in its vicinity. The amine-cured epoxy did not show any sign of interphase existence using the introduced method. However,
a soft interphase with a thickness of ~1.7 μm was measured on theanhydride-cured epoxy. Furthermore, we show that the proposed system can be used to determine the interphase thickness as well as its relative mechanical properties regardless of the indentation depth.
This system can be further used for investigating other polymer/metal interphases to better understand the factors influencing them, thus helping engineer the interphase size and properties to enhance composite performance.
The feasibility of reinforcing conventional carbon fiber composites by grafting carbon nanotubes (CNTs) onto the fiber surface has been investigated. Carbon nanotubes were grown on carbon fibers using the chemical vapor deposition (CVD) method. Iron was selected as the catalyst and predeposited using the incipient wetness technique before the growth reaction. The morphology of the products was characterized using scanning electron microscopy (SEM), which showed evidence of a uniform coating of CNTs on the fiber surface. Contact angle measurements on individual fibers, before and after the CNT growth, demonstrated a change in wettability that can be linked to a change of the polarity of the modified surface. Model composites based on CNT-grafted carbon fibers/epoxy were fabricated in order to examine apparent interfacial shear strength (IFSS). A dramatic improvement in IFSS over carbon fiber/epoxy composites was observed in the single fiber pull-out tests, but no significant change was shown in the push-out tests. The different IFSS results were provisionally attributed to a change of failure mechanism between the two types of tests, supported by fractographic analysis.
After a delamination process in a C-fiber reinforced polymer composite, the identification of the polymer residue of poly-(phenylene sulphide) on carbon fiber surfaces gives important information on the composite failure mechanism. Using scanning force microscopy (SFM) in its lateral force imaging mode or with the stiffness imaging mode, it is almost impossible to distinguish the two composite components owing to a very low material-based contrast of the local friction and compliance. The new technique using contrast based on a local AC current measurement allows a clear identification of the conducting carbon fiber and the non-conducting polymer coverage.
The strength of laminates distinctly depends on the transverse strength of the laminas. The transverse failure is dominated by the adhesion between fiber and matrix. Usual strength criteria however do not take into account the adhesive strength explicitly. The determination of the interface strength is performed on the micromechanical scale using single fiber specimens. The fibers are loaded under off-axis loading while the debonding is monitored under a microscope. The stresses acting in the Interface are calculated by finite element analyses. It is found that for off-axial angles up to 35° interfacial debonding is the dominant failure mode while fiber breakage takes place at lower angles. The occurrence of fiber breakage and debonding under off-axis loading shows that the restriction to two potential failure planes - perpendicular or parallel to the fibers - as applied in common failure criteria has to be put in question.