Modelling of the removal of multi-layer coatings using high pressure water jet and the study of its material removal mechanisms
Publication Date
December 14, 2023
Creator
Abstract
Aerospace coatings (e.g. Thermal Barrier Coatings (TBC), abradable coatings…) have been utilised in the industry for decades with the aim of protecting the substrate materials from the harsh environment and the elevate temperature of the engines as well as to improve their performance. However, due to the thermal mismatch between materials, coatings are damaged and in consequence spalled. As their durability is four or five times lower than the service life of the engine, these parts need to be replaced. The goal is to strip the coatings efficiently and without damaging the substrate material in order to recoat and hence reassemble them in the engine.
For this Plain Water Jet (PWJ) milling has been proposed, which is a nonconventional machining technique that benefits from the kinetic energy of the water released at high pressure. This technique erodes the target material and generates an accurate cut or milled trench with negligible processing temperatures and forces. Hence, it is ideal to remove damaged multi-layer coatings efficiently without damaging the substrate material at a controlled, clean and economical way. Nevertheless, coating removal with PWJ technology presents its challenges: each coating layers as well as the substrate have distinct mechanical properties that need to be addressed and the characteristic time dependency of the WJ machining process, makes the operation difficult to predict (e.g. jet penetration).
This research aims to develop a mathematical model that calculates the jet footprint to remove multi-material coatings efficiently using a rotational multiple plain water jet nozzle accurately. The proposed model is based on the understanding of the etching rate considering the mechanical properties of each individual material layer that compounds a coating and the energy of a PWJ system. This interaction has been obtained for any specific pressure, considering the energy variations along the height of the jet, for one or multiple jets, which follows a defined arbitrary trajectory in a 3D space, during a period of time. Subsequently, the model has been validated and the optimal milling conditions have been defined. Moreover, a software with a user’s friendly interface (UI) has been developed to easily integrate de mathematical model in an industrial environment.
Finally, to understand the effect of the high energy impact of water jet droplets on the coating materials (i.e. in this case YSZ) a water jet and high strain rate nanoindentations have been used to understand how the behaviour of 6-8YSZ changes upon different strain rates (from 10-3 to 108 s-1). This study shows that at extremely high strain rates (106 - 108 s-1) the material suffers from a brittle-dominated failure which induces localized phase transformation even if the material is in its partially stabilized form in where limited amount of transformation toughening is expected, phenomenon that is not observed at lower strain rates (10-3 - 1 s-1).
Item Type
ethesis
Thesis Type
PhD
Supervisors
Subjects (LC)
Associated Schools / Departments
Faculty of Engineering
Department of Mechanical, Materials and Manufacturing Engineering (UK)
eprints ID
74635
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