High temperature chromium coating cracking investigation during tensile tests monitored by acoustic emission

Abstract

The present study focuses on in-situ measurements of crack initiation and propagation in first-generation PVD-HiPIMS chromium coatings on M5_Framatome¹ cladding substrates using an acoustic emission (AE) device and a tensile test machine. A key novelty of this work is the implementation of a temperature-controlled cracking monitoring system adapted to the cladding geometry under tensile loading. Post-mortem examinations (after different interrupted tensile tests) provide an evaluation of the in-situ method for determining the crack initiation threshold and crack density evolution. The critical strain to crack initiation increases exponentially from 0.4 % at room temperature to 3 % at 350 °C. Above 410 °C, the coating no longer exhibits brittle cracking until reaching high macroscopic imposed strain (up to 30-50 %). Additionally, the crack density decreases more or less linearly with the increasing testing temperature. At higher temperatures, the coating becomes highly ductile, consistently with the increased plasticity of pure chromium. SEM observations of the coating cross-section confirm that cracks do not propagate beyond the coating and that no delamination occurs. Thus, after rapidly reaching crack density saturation, the residual uncracked chromium coating exhibits significant plasticity and widening of the existing Cr cracks while providing slight mechanical reinforcement to the Zr-based cladding up to at least 400 °C.