Measuring mechanical properties of fine-wire cross-sections used in medical devices

In this study, it was demonstrated that the nanoindentation technique can be used to measure the elastic modulus and hardness of fine-wire cross-sections with diameters ranging from 100-200 microm. Using miniature optics and a specially developed micrometer positioning system, measurements were successfully performed on the wire cross-sections with an accuracy of 1 microm. Homogeneous wire cross-sections, such as MP35N (high-strength nickel-cobalt alloy), composite wires such as MP35N with a silver core (MP35N/Ag-core), and platinum-clad tantalum (Pt-clad Ta) were studied. From the nanoindentation measurements, the elastic modulus and hardness at different positions and depths were calculated. It was found that both elastic modulus and hardness reached a constant value below the hardened surface, which resulted from mechanical polishing. Material properties obtained from the measurements were compared with the literature values of bulk materials as well as available results on fine wires. The data from the wire cross-sections, especially the uniform material samples, showed that both the elastic modulus and hardness were significantly lower near the outer 10-15 microm of the wire as compared to the rest of the wire, which is due to surface weakening during manufacturing. In addition, both modulus and hardness showed a small but consistent "sawtooth" variation across the diameter of the wires, excluding the outer 10-15 microm, that is postulated to be induced by the distribution of residual stress.