The Evolution Of Pores In Thermal Barrier Coatings Under Volcanic Ash Corrosion Using X-Ray Computed Tomography

The development of aeroengine is an important embodiment of national defense science and technology.Thermal barrier coatings(TBCs)can effectively protect the turbine blades of aero-engine and reduce the impact of heat flow due to thermal insulation performance.TBCs contain a certain amount of pores naturally,which can help improve the insulation efficiency.However,the pores of TBCs change during the service process,especially the high temperature corrosion of volcanic ash(VA)can have a great influence on the pore structure.VA melted and infiltrated in TBCs after ingested by engine,changing the coatings’ pore structure,which affected the coatings’insulating performance and mechanical performance.Analysis of the pores evolution induced by VA corrosion is an important approach to understand the TBCs’ premature spalling failure.X-ray computed tomography(CT)is a method of microstructure exploration,which has the characteristics of nondestructive,three-dimensional,high resolution.Hence,it is expected to become the effective means of TBCs’internal pore structure analysis.This topic analyzed the atmospheric plasma sprayed TBCs’pore distribution and evolution rule caused by VA corrosion using image morphology processing method based on CT characterization means.Besides,several kinds of pore detection method were applied to compare with CT results.The main research contents include:(1)An image segmentation method is proposed.TBCs have small and complex pores.These pores do not have obvious boundary with the sample.The commonly used single threshold segmentation method cannot separate the pores from the sample completely.Top-Hat based on image morphology has an advantage on the recognition of image details,which can identify most of pores in TBCs.For individual missing part,region growing method is used as supplement.By the combination of these two methods,pores can be fully extracted from the TBCs.In this way,the internal pore structure of TBCs was reconstructed.(2)The influence of VA corrosion on the porosity,pore size and shape was investigated.In terms of pore size,VA mostly fills the pores of radius under 3μm in the process of melting and permeating;From the shape,VA corrosion can make pore shape irregular.The higher the corrosion temperature is,the greater the irregularity of pore shape is.In contrast,the effect of VA coated amount on the pore shape is small;The VA corrosion leads to a decrease in the porosity of TBCs in the field of porosity.Specifically,the porosity decreases with the increase of the VA coating amount or the corrosion temperature.(3)Several kinds of pore detection methods including mercury intrusion method(MIP),scanning electron microscope(SEM)and scanning acoustic microscope(SAM)were applied as comparison with CT.The experimental results of MIP and SEM were consistent with the CT method.However,the MIP can only obtain the cumulative pore distribution value,and the SEM method analyzes the pore distribution information of a certain section merely.In addition,both methods will cause irreversible damage to the sample.The SAM method cannot meet the need of pore detection in TBCs because of the severe scattering of ultrasonic wave in the porous sample.