DESIGN AND STUDY OF PROTECTIVE COATINGS ON ALLOYS IN HIGH TEMPERATURE OXIDIZING ENVIRONMENT

Abstract

One critical safety issue of Zirconium (Zr) alloy cladding in light water reactors (LWRs) is its severe degradation due to the rapid oxidation of cladding upon exposure to high-temperature water vapour in a loss-of-coolant accident (LOCA) scenario. In this thesis, new designs of protective coatings on Zr alloy against water vapour oxidation are proposed and studied. In addition, a new protective coating on Ti alloy against high temperature air oxidation for aerospace application is also investigated. Zirconium nitride (ZrN) coating has been deposited on Zr alloy and comparison of the oxidation behaviour of ZrN coating in water vapour and air at high temperature has been investigated. The parabolic rate constant of ZrN oxidizing in water vapour environment at 600 degree celsius is approximately 100 times faster than that in air, due to the larger pores and greater number of cracks that are formed across zirconium oxide (ZrO2) layer formed during the water vapour oxidation process than during the air oxidation process. A bilayer-structure ZrO2 with tetragonal ZrO2 near the ZrN/ZrO2 interface and monoclinic ZrO2 approaching the outer ZrO2 surface are formed in both cases. Oxidation behaviour of zirconium nitride (ZrN), titanium nitride (TiN), titanium silicon nitride (TiSiN) and silicon aluminium nitride (SiAlN) coatings in high temperature water vapour environment has been studied and compared. The oxidation resistance of designed four nitride coatings in water vapour follows the order: SiAlN>TiSiN>TiN>ZrN. The SiAlN coating with a relative thick Mo interlayer (750nm) can provide excellent protection for Zr alloy and even no oxide forms on coating upon exposure to water vapour at 1000 degree celsius for 1 h. This is due to the good oxidation resistance of amorphous SiAlN coating and the synergetic effects of Mo interlayer on adhesion, spallation resistance and phase stability of SiAlN coating on Zr alloy at high temperature. The Mo interlayer not only improves spallation resistance of SiAlN coating owing to relaxation of thermal mismatch, but also mitigates the crystallization of amorphous SiAlN. Oxidation behaviour of a NiCrAl bulk alloy and a thermally sprayed NiCrAl coating (with the same composition as that of the NiCrAl bulk alloy) on a Zr alloy in water vapour at high temperature (900-1200 degree celsius) has been investigated. The NiCrAl bulk alloy exhibits good oxidation resistance by forming a slow-growing, protective alumina scale on the surface. However, a non-protective ZrO2 scale forms on the NiCrAl coating surface at temperatures above 1000 degree celsius due to the severe interdiffusion between the NiCrAl coating and Zr alloy. With the addition of the WC-CoCr interlayer, the outward diffusion of Zr is prevented and the NiCrAl coating provides excellent protection for the Zr alloy against water vapour oxidation at 1200 degree celsius. A multilayer nitride coating with self-bonding and adaptive deformability on the Ti and its alloy has been propose and studied. The multilayer nitride coating on titanium and its alloy is designed by magnetron sputtering a SiAlN coating (1.2 um thick) with molybdenum (Mo) interlayer, followed by cyclic oxidation at 800 degree celsius. Interdiffusion and interaction during cyclic oxidation between the Ti and SiAlN layers lead to the formation of a TiN0.26 interlayer which shows remarkably adaptive deformability via mechanical twinning, and thereby largely alleviating the thermal mismatch strain between the coating and substrate. Such adaptive deformability plus excellent bonding strength equip this coating system with excellent thermal cycling life. For instance, no cracking, spallation and oxidation of the coating are observed after hundreds of hours of cyclic oxidation in air at 800 degree celsius.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Cernik (Supervisor) & Ping Xiao (Supervisor)