Cleavage cracking of ductile-metal substrates induced by brittle coating fracture

Brittle coatings are often used to protect underlying ductile substrates from damage. Recent experimental observations show that the fracture of a brittle coating can cause the micro-cracking of the ductile metal substrates, threatening the safety and reliability of engineering structures. The cracking mode of the substrates is unclear and the corresponding mechanism remains poorly understood. In the present work, by performing room-temperature uniaxial tension experiments with a 10−4 s−1 straining rate, we have observed cleavage cracking in ductile-metal substrates (pure iron, AISI 1020 steel and brass) coated with WC-10%Co-4%Cr. Theoretical analysis reveals that the cleavage cracking is the result of brittle coating and its fracture, which synergically inhibit the local plastic deformation of the underlying substrate via two mechanisms. One is to restrain the dislocation nucleation and mobility near the interface of the substrate. The other is to bring a local high strain rate loading to the substrate, due to fast crack propagation in the brittle coating. The coupling of two effects leads to the nucleation of cleavage crack in the normally ductile substrates and causes significant loss in their ductility. This detrimental effect will be much more pronounced with the increase in the coating thickness. The findings shed new light on the failure mechanisms of brittle coating-metal substrate and provide guidelines in the material design of such systems.