On the use of laser sources for microstructure modification, melting, and deposition of thermal barrier coatings
Publication Date
December 31, 2023
Creator
Abstract
One of the most innovative technologies in the area of non-conventional machining, which is now used in a wide range of sectors, is laser processing. For the machining of difficult-to-cut materials like nickel superalloy and ceramic thermal barrier coatings (TBC), laser processing and pulsed laser ablation have been acknowledged as a practical processing technique in the aerospace industry. Thanks to its non-contact nature and ease of miniaturisation and delivery, laser become a desirable tool for material processing in difficult to access areas. Lasers, combined with a specially designed robotic carrier, may allow on-wing maintenance of turbine aero-engines.
One of the main reasons for engine overhaul is TBC spallation and oxidation of the components over the acceptable limits. To date, this results in a full replacement of the part and engine downtime. As a result, significant efforts have been made to address durability of the coatings by alternating coating composition and deposition methods. While durability of the coatings address the frequency of necessary repairs, on-wing coating application contributes to the duration of maintenance, and the ability to apply TBC on-wing has been recently demonstrated with the aid of combustion flame spraying process at the Nottingham UTC in On-Wing and Manufacturing Technologies. This study continues the trend in on-wing engine maintenance and is dedicated to addressing the problems with engine downtime.
Firstly, this study is dedicated to the development of a TBC modification technology. Inspired by the durable coatings produced by vapour deposition, its microstructure was attempted to replicate on a thermally sprayed coating with the aid of pulsed lasers. Vapour-deposited coatings are known for their better thermal cycling performance and resistance to erosion. In this work, columnar microstructure has been replicated with an array of laser-machined grooves. The next step of optimisation addresses resistance to abrasive action and chemical interactions by providing the coating with a dense continuous surficial layer. In order to asses the durability characteristics, the coating was compared against the state-of-the-art thermally sprayed coating in simulated in-engine environment.
Secondly, particular attention was paid to the solidification phenomena in laser melted TBC. Unusual behaviour of crystal growth, here referred to as ”grain surface sealing” and ”grain bending”, has been identified. While this may present an advantageous microstructure for various materials, e.g. to locally change crystal orientation dependant properties or guide microcrack propagation by adjusting the ii grain boundary geometry, currently existing models fail to explain these phenomena. The responsible mechanism for both phenomena was identified as planar and dendritic grain growth modes. Therefore, by addressing both planar and dendritic solidification within a single model one can predict and control the grain bending and grain surface sealing phenomena. As a result, a predictive multi-scale model of solidification has been build with the example of laser-melted TBC. For this purpose, a three-dimensional finite-element thermo-capillary model has been developed, predicting the temperature distribution within the workpiece, which was coupled with a three-dimensional grain growth model based on cellular automata approach.
Thirdly, the possibilities of coating application with the aid of laser tool have been investigated. Current vapour deposition techniques are hardly suitable for on-wing coating application due to vacuum requirements, difficult to implement in-situ. In an effort to find a novel suitable deposition technology, various possibilities for coating generation in atmosphere have been identified and developed. While the properties of such coating are a subject for further optimisation, the main focus of this section is to study formation of the coating microstructure and interaction between the laser and absorbing target material. Two potential options for on-wing coating application with a laser source have been found and discussed. Resultant microstructures, possible formation mechanisms, and laser-material interaction in different deposition scenarios have been analysed and presented.
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
74155
UoN Repository URI
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ThesisDmitriiUshmaev-Final.pdf
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Full-text
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Examined. Thesis after corrections - with references
Size
32.13 MB
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Adobe PDF
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