Effect of annealing and silylation on the strength of melt-spun Ni–Mn–Ga fibres and their adhesion to epoxy

Single crystals of ferromagnetic Ni–Mn–Ga shape memory alloys show magnetic-field and stress induced twinning, leading to shape memory. Adaptive composites can thus be produced by embedding single crystalline particles or bamboo-structured Ni–Mn–Ga fibres into a polymer matrix. To guarantee a durable performance of these composites, adhesion between reinforcement phase and matrix should be quantified and optimised. The influence of annealing and surface treatment with an aminosilane of melt-spun Ni–Mn–Ga fibres on their strength and adhesion to an epoxy matrix was investigated using single fibre tension and fragmentation tests. Annealing of the melt-spun Ni–Mn–Ga fibres changed the surface from a “pimpled” to a smooth microstructure. This resulted in a reduced adhesion of the annealed fibres in comparison to the as-spun fibres embedded in an epoxy matrix. As-spun fibres exhibited an interfacial shear strength (IFSS) comparable to the shear strength of the epoxy matrix so that the silylation did not change the adhesion significantly. For the annealed fibres, the silane treatment improved the IFSS by 67%. Furthermore, the silylation increased the fracture strength of the Ni–Mn–Ga fibres due to surface flaw healing or forming of a protective surface coating.