Issue 31

M. Merlin et alii, Frattura ed Integrità Strutturale, 31 (2015) 127-137; DOI: 10.3221/IGF-ESIS.31.10 133 action caused by the silane coupling agent. The silane is able to covalently attach itself to both a metal oxide surface through a silicon-chlorine or silicon-methoxy bond and the propagating polymer chain. Smith et al. [36] proved that the chemical binding to the matrix rather than polymer chain entanglement is the cause of the roughly 100% relative increased adhesion and the higher shear stress strength. Another way to enhance interfacial adhesion between the actuator and the resin is by physical adhesion through mechanical interaction. If the surface of the NiTi wire is made rougher by sandblasting or chemical etching, for example, a polymer of low viscosity can fill the crevices and form mechanical projection that hinders the pull-out of the wire. Fig. 4 shows SEM micrographs of the surfaces of the wires after the different surface treatment conditions. The SEM micrograph for the functionalised wires is not reported since the modification of the surface chemistry does not alter the surface topology of the constituent. Based on the peak load levels in Fig. 2a and b, the more viscous VE resin suffers in comparison with the PE resin in this aspect of adhesion as well. For rough surfaces (Fig. 4b and c) the mechanical interaction with the polymeric matrix is increased as well as the interfacial adhesion and the peak load levels (VE_A, PE_A and much more VE_AB plus PE_AB in Fig. 2a and b); in contrast, for smooth surfaces (Fig. 4a), if poor mechanical interaction is present the bond strength is completely dependent on the intermolecular attraction based on hydrogen or Van der Waals bonds, which could be occurred [32]. Figure 4 : SEM micrographs of the surfaces of the wires for the different surface treatment conditions: (a) NT, (b) A and (c) AB samples, respectively. (Red arrow indicates the main direction of the wire). For the same four different surface treatment conditions, the SEM analysis of the PE/wire interfaces before the strain recovery tests highlighted around a 10 μm-thick native oxide layer interposed between the polymer and metal surfaces (Fig. 5). In Fig. 5a the native oxide resulting from the oxidation reactions occurring during the heating of the wire is evident. According to Rossi et al. [44] the EDS analyses performed at the native oxide/metal alloy interface (yellow arrow in Fig. 5a) confirmed the titanium depletion in the outer surface of the alloy and the consequent nickel enrichment due to the outward migration of the titanium to form the oxide layer. The increased roughness of the native oxide caused by the chemical etching treatments should be noted in Fig. 5b and c. The SEM observations are consistent with the demonstrated improvement in both physical adhesion and interfacial adhesion due to the mechanical interaction between the wire and the polymer matrix. It is well known that native oxides usually enhance the interfacial adhesion because of their higher roughness; however, several studies highlighted that they