Functionally Graded Environmental Barrier Coating for Thermal Strain Control
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Solution Overview
Problem
Existing environmental barrier coatings (EBCs) degrade due to thermal expansion coefficient mismatches, leading to cracking, delamination, and spallation at the TBC, EBC, and metal substrate interfaces, particularly in high-temperature and corrosive environments.
Innovation Solution
A functionally graded EBC is produced through cold spray material deposition combined with heat treatment, allowing for controlled chemical composition and porosity gradients to reduce thermal expansion mismatch and enhance bond strength, using precursor elemental powders or pre-alloyed powders to form a graded elastic modulus and tailored porosity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional EBC is used to protect the substrate against oxidation, then oxidation protection is provided, but thermal expansion mismatch causes cracking, delamination, and spallation
Solution Approach 1:
The EBC is designed with functionally graded composition where the outer layer has higher aluminum content (e.g., 6-12 wt% Al) for oxidation protection, while the inner layer has lower aluminum content (e.g., 2-6 wt% Al) closer to the substrate composition. This gradual transition in material properties through the coating thickness reduces thermal expansion mismatch and eliminates the sharp interfaces that cause cracking and delamination, while maintaining oxidation resistance at the outer surface.
Solution Approach 2:
The invention uses a composite structure with multiple layers having different compositions - an outer EBC layer rich in oxidation-resistant elements (Al, Cr) and an inner layer with composition graded toward the substrate. This composite approach allows simultaneous achievement of oxidation protection from the outer layer and reduced thermal stress from the graded inner layer, preventing coating failure.
2Strength
If the EBC forms a strong bond with the substrate through interdiffusion, then adherence is improved, but thermal strain mismatch still causes delamination
Solution Approach 1:
The invention changes the compositional parameters of the EBC by creating a gradient in element concentration (particularly Al, Cr, and other alloying elements) from the substrate interface to the outer surface. This parameter variation allows the coating to maintain strong interdiffusion bonding at the substrate interface while gradually transitioning to match the thermal expansion properties of the outer EBC layer, reducing thermal strain and preventing delamination.
3Temperature
If a TBC ceramic topcoat is deposited on the EBC for thermal insulation, then thermal protection is provided, but oxygen diffusion through the TBC accelerates TGO formation at the EBC
Solution Approach 1:
The functionally graded EBC provides localized oxidation resistance at the substrate interface where TGO formation is most problematic. The inner layer with composition closer to the substrate creates a controlled oxidation environment that limits excessive TGO growth, while the outer layer provides primary oxidation protection. This local quality variation compensates for the oxygen diffusion through the TBC.
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 solution enhances EBC durability by reducing thermal strain, improving adherence, and arresting crack propagation, providing increased strength and thermal protection, and reducing thermal cyclic stresses.
Implementation Method 1
cold spray material deposition combined with heat treatment
Implementation Method 2
heat treatment, allowing for controlled chemical composition and porosity gradients
Implementation Method 3
interdiffusion between the EBC and substrate can improve adherence
Implementation Method 4
mismatch of thermal expansion coefficients of the TBC, TGO, EBC, and metal substrate
Implementation Method 5
aluminum and chromium in the EBC diffuses to form oxide scales-typically alumina (Al2O3) or chromia (Cr2O3)-which act as oxide barriers to protect the metal substrate against oxidation
Implementation Method 6
TBC ceramic topcoats typically consist of yttria, zirconia, gadolinium, and other elements (e.g., yttria-stabilized zirconia (YSZ)), which have a high melting temperature and low thermal conductivity
Data Source
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AI summary
An environmental barrier coating (10) deposited onto a substrate (12) by cold spraying has a microstructure having a porosity (18, 20) that increases through a thickness dimension extending from the substrate (12) toward an outer layer (10n). A process for forming an environmental barrier coating (10) includes sequentially depositing a plurality of layers (10a-10n) of solid powder onto a substrate (12) by cold spraying and heat treating the plurality of layers (10a-10n).