Quantifying cracking and strain localisation in a cold spray chromium coating on a zirconium alloy substrate under tensile loading at room temperature

Abstract

The mechanical and cracking behaviour of a Cr coating, deposited on an Optimized ZIRLOTM substrate using the Cold Spraying (CS) technique, was investigated by in-situ scanning electron microscopy (SEM) tensile testing at room temperature. To achieve this, curved micro-tensile samples were machined from cladding tubes, and a styrene-assisted gold remodelled pattern was applied on the coating surface. The macroscopic crack density of the coating with applied axial strain was quantified through automated SEM image processing, whilst the localisation of strain was monitored with High Resolution Digital Image Correlation (HRDIC). Like with most brittle thin films deposited on ductile substrates, under applied external stress, parallel crack channelling appeared on the coating surface, perpendicular to the loading direction. The first crack was captured at 0.5% axial strain, whilst saturation was observed at 1.5%. Single crack formation in the coating was found to cause relaxation of local strains and redistribution towards the uncracked segment's midpoint. The local fracture strain of the coating was found to increase with the decreasing crack spacing, whilst shear bands appeared on the coating's surface at 35° and 145° with respect to the loading direction, which is evidence of plastic deformation. The crack density of the curved sample geometry was correlated with the tubes using a modified shear-lag model, considering the residual stresses in the coating measured by X-ray Diffraction (XRD). The CS Cr coating showed high levels of bonding and mechanical interlocking with the substrate, even after 3% axial strain.