CMAS-Resistant Thermal Barrier Coating via Layer Segmentation
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Solution Overview
Problem
Thermal barrier coatings (TBCs) used in gas turbine engines face challenges in resisting infiltration and damage from CMAS contaminants, which can lead to spallation and degradation due to their low melting temperature and ability to infiltrate porosity, causing compliance issues and chemical reactions that result in component failure.
Innovation Solution
A coating system comprising a bond coat and inner and outer ceramic layers, where the inner layer is zirconia stabilized with 6-9 weight percent yttria and optionally hafnium oxide, and the outer layer is zirconia stabilized with 25-75 weight percent yttria and additional hafnium and tantalum oxides, with controlled thickness and porosity to enhance spallation resistance and react with CMAS to form protective layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional TBC system with single-layer ceramic coating is used, then the coating provides thermal insulation, but it is susceptible to CMAS infiltration and spallation due to porosity and low melting temperature of contaminants
Solution Approach 1:
The ceramic coating is divided into two distinct layers: an inner layer with lower porosity and higher melting point to resist CMAS infiltration, and an outer layer with optimized porosity for thermal insulation. This segmentation allows each layer to perform its specific function, resolving the contradiction between reliability and complexity.
Solution Approach 2:
The coating system uses composite material structure with different ceramic compositions in each layer. The inner layer contains materials with higher melting points for CMAS resistance, while the outer layer has optimized composition for thermal barrier performance. This composite approach enables simultaneous achievement of both protection and insulation functions.
2Temperature
If the ceramic coating porosity is increased to reduce thermal conductivity, then thermal insulation improves, but CMAS infiltration increases leading to spallation
Solution Approach 1:
Different porosity levels are assigned to different locations within the coating system. The inner layer has lower porosity to prevent CMAS infiltration, while the outer layer has higher porosity for optimal thermal insulation. This local differentiation resolves the contradiction between thermal performance and contamination resistance.
Solution Approach 2:
The solution moves from a single-dimensional porosity control to a two-dimensional approach by introducing layer depth as an additional dimension. Porosity is controlled differently through the thickness of the coating, with the inner layer having restricted porosity and the outer layer having enhanced porosity, thus resolving the contradiction.
3Temperature
If the coating thickness is increased to improve thermal barrier performance, then thermal insulation improves, but stress and spallation resistance deteriorate
Solution Approach 1:
The thick coating is segmented into two layers with different thicknesses and properties. The inner layer is thinner and denser to maintain adhesion and resist spallation, while the outer layer is thicker and more porous to provide thermal insulation. This segmentation allows the system to achieve both thermal performance and mechanical strength.
Solution Approach 2:
The coating system changes parameters (porosity, composition, thickness) through the depth of the coating. By varying these parameters between the inner and outer layers, the system achieves optimal balance between thermal insulation (requiring thickness) and spallation resistance (requiring adhesion).
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The coating system effectively withstands thermal cycling and CMAS infiltration, reducing thermal conductivity, increasing melting point, and promoting crystalline CMAS precipitation, thereby enhancing spallation resistance and extending the operational life of gas turbine engine components.
Implementation Method 1
the outer layer is zirconia stabilized with 25-75 weight percent yttria and further contains greater than 0.5 to 10 weight percent hafnium oxide and optionally 1 to 10 weight percent tantalum oxide... reacting with CMAS to form protective layers
Implementation Method 2
promoting crystalline CMAS precipitation, thereby enhancing spallation resistance
Implementation Method 3
TBCs formed by the various methods noted above generally have a lower thermal conductivity than a dense ceramic of the same composition as a result of the presence of microstructural defects and pores at and between grain boundaries
Implementation Method 4
Bond coat materials are typically selected to be capable of forming a continuous and adherent oxide scale on their surface to promote the adhesion of the ceramic coating to the bond coat. The oxide scale can be formed by subjecting the bond coat to an oxidizing environment
Data Source
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AI summary
Coating systems and processes by which the coating systems can be deposited to be resistant to contaminants, and particularly resistant to infiltration and damage caused by CMAS. The coating systems include inner and outer ceramic layers. The inner ceramic layer consists essentially of zirconia stabilized by about 6 to about 9 weight percent yttria and optionally contains greater than 0.5 to 10 weight percent hafnium oxide. The outer ceramic layer overlies and contacts the inner ceramic layer to define the outermost surface of the coating system. The outer ceramic layer consists essentially of zirconia stabilized by about 25 to about 75 weight percent yttria, has a thickness that is less than the thickness of the inner ceramic layer and further contains greater than 0.5 to 10 weight percent hafnium oxide and optionally 1 to 10 weight percent tantalum oxide. The outer ceramic layer has a porosity level that is lower than that of the inner ceramic layer.