AbstractZirconium alloys, which are used as cladding materials in third‐generation light‐water reactors (LWRs), fulfil the features to work as cladding materials in nuclear power plants, but during Loss of Coolant Accidents (LOCAs) they perform poorly as it occurred in Fukushima (2011). In addition, new materials or coatings on present cladding materials are required to withstand increased working span and operating temperatures of nuclear reactors for future fission nuclear reactors (e.g., Gen III+, Gen. IV). One of the proposed materials are MAX phases, as coatings on zirconium alloy claddings due to their radiation resistance at high temperatures, superior oxidation, and corrosion resistance.
This thesis reports the fabrication of stochiometric MAX phase and non-stochiometric ceramic coatings on silicon and superalloy substrates. In addition, assessing their radiation resistance and nano hardness was the original theme of this thesis. The fabrication of MAX phase thin films on different substrates was done using magnetron sputtering at room, and high temperatures. In addition, the irradiation of stochiometric MAX phase and non-stochiometric ceramic coatings were evaluated by using heavy ion and doses ranging between 3 to 135 displacements per atom at temperatures of 550 °C, 350 °C and room temperature, environments relevant to application in light water reactors and next generation nuclear reactors.
Cr2AlC thin films were synthesised by dc magnetron sputtering via layer-by-layer deposition from elemental targets onto silicon and In-718 superalloy substrates at 375 °C and 585 °C. In addition, amorphous Cr2AlxC (x = 0.9, 0.75 and 1.2) thin films in near, under, and over-stoichiometric compositions were deposited on silicon substrates without intentional heating using dc magnetron sputtering. Post synthesis the crystalline Cr2AlC thin films deposited on silicon substrate at 375 °C consisted of nanocrystalline Cr2AlC MAX phase with stochiometric composition confirmed by energy dispersive X-ray (EDX) in field emission scanning electron microscope (FESEM). Characterisation of dual phase Cr2AlC thin films deposited on silicon substrate at 375 °C revealed presence of nanocrystalline Cr2AlC MAX phase and nano amorphous zones confirmed by transmission electron microscope (TEM), scanning transmission electron microscope (STEM), and grazing incidence X-ray diffraction (GIXRD). In the case of amorphous thin films with different stochiometries, lamellas were prepared using focused ion beam milling (FIB) which were then crystallised by heating in a TEM heating holder. The crystallisation of near-stoichiometric Cr2AlxC (x = 0.9) thin films resulted in nano MAX phase after crystallisation at 600 °C. In the case of under-stoichiometric Cr2AlxC (x = 0.75) thin films consisted of nanocrystalline Cr2AlC MAX phase along with chromium aluminides (Cr5Al8) after crystallisation at 700 °C. Finally, the over stoichiometric Cr2AlxC (x = 1.2) thin films consisted of nanocrystalline Cr2AlC MAX phase along with chromium aluminides (Cr5Al8) and chromium carbides (Cr7C3) after crystallization at 923 K. Cr2AlC thin films deposited on Inconel-718® superalloy substrates at 515 °C were found to be partially amorphous as confirmed by X-ray diffraction (XRD). In the case of Cr2AlC thin films deposited on In-718 substrate at 585 °C were found to be nanocrystalline Cr2AlC MAX phase as confirmed by XRD and TEM. The hardness of stochiometric Cr2AlC thin films deposited at 585 °C were around 15 GPa, reduced Young's modulus at around 260 GPa. The thickness of all the thin films in this study were in the range of 0.8 to 1.2 μm for silicon and In-718 substrates as shown by TEM.
Ion irradiations of all the thin films in this study was achieved using an in-situ ion irradiation microscope and ion accelerator for materials investigations (MIAMI) facility, available at the University of Huddersfield. The stochiometric Cr2AlC thin films deposited on silicon substrate at 375 °C were irradiated with 320 keV Xe+ ions up to a fluence 1 × 1016 ions·cm-2 at room temperature and 350 °C. In the case of stochiometric Cr2AlC irradiated at room temperature started to amorphise at around 0.3 dpa. The stochiometric Cr2AlC transformed from crystalline structure to amorphous at displacement levels of 3.3 dpa. However, stochiometric Cr2AlC irradiations at 350 and 550 °C and doses up to 90 dpa showed no recordable amorphisation. Low defect recombination energy barriers and presence of many grain boundaries attributed for high radiation hardness of Cr2AlC MAX phase at 350 and 550 °C. Annealed thin films of near-stoichiometric Cr2AlxC (x = 0.9) irradiated with 320 keV Xe+ ions up to 83 displacements per atom (dpa) at temperature of 350 °C showed no observable changes. Also, irradiation of nanocrystalline under-stoichiometric Cr2AlxC (x = 0.75) thin films with 320 keV Xe+ ions up to 138 dpa at temperature of 350 °C did not show any observable amorphization, and transformation of fine grains to nano grains was observed. Furthermore, ion irradiation of dual-phase Cr2AlC thin films exposed to ion irradiation using a 300 keV Xe ion to a dose of up to 40 dpa at 350 °C showed resistance to radiation. However, irradiation of over stoichiometric Cr2AlxC (x = 1.2) thin films at 350 °C resulted into partial amorphisation at dose of 60 dpa, is believed to be due to presence of chromium carbides.
These in-situ ion irradiation results of different Cr2AlC thin films irradiated at 350 and 550 °C suggest they are resistance to radiation and this is attributed to the known self-healing property of Cr2AlC MAX phase further enhanced by nanocrystallinity.
|Date of Award||21 Jul 2022|
|Supervisor||Vladimir Vishnyakov (Main Supervisor), Jonathan Hinks (Co-Supervisor) & Anamul Haq Mir (Co-Supervisor)|