Dynamic Torsion Testing of Nanocrystalline Coatings Using High-Speed Photography and Digital Image Correlation

The strength and ductility of microcrystalline and nanocrystalline tungstsen carbide–cobalt (WC–Co) cermets have been evaluated by employing a stored energy Kolsky bar apparatus, high-speed photography and digital image correlation. The test specimens were thin-walled tubular Al7075-T6 substrates 250 μm thick, coated with a 300 μm thick microcrystalline or nanocrystalline WC–Co layer with an average grain size of about 3 μm and 100 nm, respectively. Dynamic torsion experiments reported in this paper reveal a shear modulus of 50 GPa and a shear strength of about 50 MPa for both microcrystalline and nanocrystalline WC–Co coatings. The use of high-speed photography along with digital image correlation has shown that damage to the coating coincides with a significant softening on the stress–strain curve. The coating failed in mode III, and strong interactions between the crack faces were probably responsible for the increase in load after failure of the coating. The overall failure of the coating–substrate system was not brittle but rather progressive and controlled by the ductility of the aluminum substrate. A methodology for investigating damage kinetics and failure has been established. This methodology can be applied to examine the behavior of other advanced materials that can be manufactured as coatings on ductile substrates. Manufacturing coatings free of initial microcracks remains a significant challenge. Research on optimization of the spray deposition parameters should be pursued to produce high-quality nanostructured coatings that can fully exploit the benefits of nanosize grains. KEY WORDS—Nanostructured materials, WC–Co, dynamic properties, wave propagation, stress–strain relationship measurements