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Title: Microstructural evolution of MCrAlY coated Ni-based superalloy systems
Author: Yan, Xin J.
ISNI:       0000 0004 7971 0215
Awarding Body: Loughborough University
Current Institution: Loughborough University
Date of Award: 2014
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Industrial gas turbine blades are typically made from coated Ni-based superalloys for power generation plant. The operation environments for these blades are very aggressive, comprising high temperatures (> 900°C) and potential chemical contaminants leading to oxidation and corrosion. An MCrAlX (where M is Ni and/or Co) coating is usually applied directly on the top of the alloy surface and forms an oxide scale (typically alumina) that can protect the substrate against further oxidation. However, it is widely accepted that the current generation of MCrAlX coatings only provide limited protection and need significant improvement. The MCrAlX coatings are usually applied by using either low pressure plasma spraying (LPPS) or an HVOF spraying process is increasingly used to deposit MCrAlX coatings onto gas turbine blades as an alternative method, which is more economically efficient. The main aim of this project is to design a new composition MCrAlX coating for oxidation resistant applications as well as to demonstrate the compatibility of the HVOF method to the LPPS process. A detailed experimental study of typical commercial MCrAlX coatings was carried out, this included the examination of the as-received powder particles before spraying and partially and fully melted powder particles captured during the HVOF spraying process, through to the as-sprayed and finally the fully aged coating prior to entry into service. It has been shown that as-manufactured powders consist of dendritic structures. Powder particles were found to be deformed differently after the HVOF spraying process, and dendritic structures were only observed on the barely deformed particle surface and the average grain size within these differently deformed particles decreased as the deformation degree increases. Yttrium (Y) was found at the outer surface and internal boundaries within the powder particles both before and after the spraying process. Vacuum ageing homogenised the microstructure within the as-sprayed coating, and the phases started coarsening upon further ageing in the air, and a β depletion zone formed beneath the coating surface after a short term air ageing. A set of six coatings with very similar compositions sprayed either by LPPS or HVOF methods have been examined in three different ageing conditions: the as-received condition, and after short and long term ageing at high temperature, to determine the differences between the LPPS and HVOF spraying method and their effects on coated systems. The general microstructural evolution upon ageing for both LPPS and HVOF sprayed coatings were found to be similar to each other. Before pre-service heat treatments, the HVOF as-sprayed coating contained a lot of poorly melted particles, and pores were found around the splats, enriched in Y and O. The pre-service treatment carried out after the LPPS spraying process decreased the porosity level of the coating and promoted a fine grain structure across the coating. Upon ageing, the grain size and inter-diffusion zone (IDZ) were much bigger in the long-term aged HVOF coating than in the LPPS coating. Y was found to diffuse both outwards to the sub-surface and into the oxide scale, and into the IDZ area. It was typically associated with O, even in the as-sprayed coating. After ageing, Y-containing particles were found to grow and merge together in the sub-surface area whereas they only grew in size, rather than coalescing in the IDZ area. However, there were some differences in the particles found in the LPPS and HVOF sprayed coatings including particle morphology, distribution and the exactnature of the phase. Three new compositions of MCrAlX coatings were developed based on a systematic study of current coatings which including coating failure mechanisms, typical mechanical properties (Coefficient of Thermal Expansion, CTE and Ductile Brittle Transition Temperature, DBTT) and coating composition studies. A new algorithm for CTE calculation for coatings has also been developed. Coatings with these new compositions were subsequently produced by a commercial powder manufacturer and then sprayed onto CMSX4 substrates using an HVOF spray gun. Theses coatings were found to consist of a similar microstructure to that within the Praxair powders. There was no clear correlation between the oxide scale thicknesses with the addition of TBC. However, after 3,000 hours ageing at 1000°C, the TBC was found to spall off from coating C that contained 6 wt.% of Ta. Results suggested that the other two coatings retain a moderate amount of the beta phase after 10,000 hours ageing at 880°C.This project has established an understanding of current MCrAlX coatings in terms of microstructural evolution before and throughout the spraying process. In addition, an extensive study focused on the LPPS and HVOF sprayed coatings was carried out and it v has been demonstrated there is no significant difference between these two spraying processes. A least two of the three coatings designed in this project have shown potential for application in gas turbine blades.
Supervisor: Not available Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Materials Engineering not elsewhere classified ; MCrAlY ; HVOF ; LPPS ; Oxidation resistant coating