Characterisation of Chromium Coated Zirconium Alloys for Accident Tolerant Fuel Claddings

Abstract

This thesis examines cold spray (CS) and physical vapor-deposited (PVD) chromium-coated zirconium alloy fuel claddings, chosen for their potential to produce coatings with different characteristics. Cold spraying results in heterogeneous microstructures with large prior powder particles and high interfacial roughness, while PVD coatings consist of highly textured fine columnar grains. Substrate bias variation in PVD affects grain size, compressive stress, hardness, and texture. Both CS and PVD coatings show promising characteristics at elevated temperatures, with minimal changes in microstructure and interfacial properties at reactor operating temperatures. At 300°C, the cold spray coating exhibits increased compressive stresses, but after 24 hours, these stresses show a small degree of relaxation. There's a risk of excessive relaxation at reactor conditions, eliminating compressive residual stresses, potentially impacting mechanical performance. After an 800°C heat treatment, the CS coatings experience significant recrystallization, while PVD coatings display grain growth at 800C followed by a breakdown of the columnar grain structure at 1000°C. Mechanical deformation at 300°C for both coatings demonstrates improved ductility compared to room temperature tests. Room temperature cracking correlates strongly with coating thickness, residual stress, and interfacial roughness. Coatings with greater thickness, like CS, exhibit reduced crack density at higher strains. FEA modelling has revealed that interfacial roughness localizes strains, leading to earlier coating fracture compared to a flat interface. Enhanced crack density typically corresponds to a higher proportion of intergranular to trans-granular cracks. The cracking mechanism is dependent on grain size, with CS coatings experiencing a combination of trans- and inter-granular cracking. PVD coatings show a transition in crack mechanism with increasing bias voltage. The multifaceted nature of cracking in chromium coatings is strongly associated with coating thickness, residual stress, interfacial roughness, grain size, and texture.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philipp Frankel (Supervisor) & Michael Preuss (Supervisor)