Tensile properties of helical diamond fibres with platinum wire core

Abstract

Helical diamond fibres of 218 μm diameter and up to ∼100 mm long have been made by chemical vapour deposition on to a 50 μm diameter platinum wire in coil form. The fibres had 43 vol.-% diamond and a spring index diameter ratio, coil diameter (D)/fibre diameter (d), of 3·3. Tensile load versus extension curves were determined for the diamond fibres. An extension value of up to ∼20 times that for a straight fibre was obtained. Failure occurred by shear fracture of the diamond coating. This often occurred at defects in the deposit. The highest average surface shear stress measured was 0·54 GPa, equal to 0·82 GPa after the Wahl correction for the maximum shear stress, which is in the reported fracture strength range for diamond. Shorter coils deformed plastically with up to ∼1% elastic strain. Initial coil stiffness values (K) were in the range 242–1367 N m^-1.^a The K values were increased by prestrain to 2950 N m^-1. The effective coil shear modulus before correction was ∼300 GPa. Diamond in the form of a microcoil exhibited greater elastic extension by about a factor of 70 compared with a straight fibre, but this was obtained by sacrificing stiffness, which was about a factor of 70 greater for a straight fibre. The coil exhibited surprising damage tolerance with turns adjacent to fractured turns resisting fracture at higher loads. The results suggest helical fibres should enable free standing diamond microsensors to be produced that combine mechanical and physical properties and greater damage tolerance for use in severe environments.