YSZ Thermal Barrier Coating with Controlled Porosity
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Solution Overview
Problem
Thermal barrier coatings in gas turbine engines face issues with increased thermal conductivity and reduced strain tolerance due to sintering, leading to spalling and reduced lifespan, as they densify over time, despite efforts to incorporate compliance through hollow spheres or surface grooving.
Innovation Solution
Thermal spraying of yttria stabilized zirconia particles with internal porosity, controlling the melting to maintain a partly melted state, resulting in larger inter-particle gaps and increased porosity, which reduces thermal conductivity and enhances compliance while minimizing sintering and spalling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If TBC is applied at less than full density to reduce thermal conductivity, then thermal insulation is improved, but the coating densifies during service due to sintering, increasing thermal conductivity and reducing strain tolerance
Solution Approach 1:
The patent applies porous materials by incorporating hollow ceramic spheres (porous structures) into the TBC matrix. These hollow spheres maintain permanent porosity that prevents sintering densification, thereby preserving both low thermal conductivity and strain tolerance during service. The porous structure acts as a buffer against thermal stress while maintaining thermal insulation.
Solution Approach 2:
The patent uses composite materials by combining hollow ceramic spheres with ceramic matrix material to create a composite TBC structure. This composite architecture provides both thermal insulation (from the porous hollow spheres) and mechanical compliance (from the composite structure's ability to accommodate strain), resolving the contradiction between thermal performance and reliability.
2Strength
If TBC material sintering is allowed to proceed, then coating density increases improving strength, but thermal conductivity increases and strain tolerance decreases leading to spalling
Solution Approach 1:
The hollow ceramic spheres create a stable porous architecture that prevents complete densification through sintering. The porosity maintains strain tolerance and resistance to spalling while the ceramic matrix provides structural strength, achieving both high strength and high reliability simultaneously.
3Reliability
If compliance features like hollow spheres or surface grooving are added to TBC, then strain tolerance is improved, but manufacturing complexity increases
Solution Approach 1:
The patent applies segmentation by incorporating discrete hollow ceramic spheres distributed throughout the TBC matrix. This segmented approach provides compliance and strain tolerance through the distributed porous structures, achieving improved reliability without requiring complex continuous surface grooving or intricate coating architectures.
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 provides a thermal barrier coating with reduced thermal conductivity, improved compliance, and extended lifespan at a lower cost, with elastic hysteresis and minimal sintering, effectively mitigating crack propagation and spalling.
Implementation Method 1
thermally spraying them onto a substrate using spray parameters that melt only an outer surface portion of each particle
Implementation Method 2
This retains the internal porosity of the particles. It also increases inter-particle gaps by reducing the average aspect ratio of the splats compared to fully melted splats
Implementation Method 3
reduces thermal conductivity
Implementation Method 4
present TBCs can densify during service asymptotically toward full density. This is due to tight conformance of ceramic splats to each other, resulting in small between-the-splat (inter-splat) gaps, which can close by sintering during service
Data Source
AI summary
Yttria stabilized zirconia (YSZ) particles (40) form a thermal barrier layer (58) on a metal substrate (24). The YSZ particles have a porous interior (52, 54) and a fully melted and solidified outer shell (50). The thermal barrier layer may have porosity greater than 12%, including porosity within the particles and inter-particle gap porosity. Inter-particle gaps may be greater than 5 microns. The thermal barrier layer may exhibit elastic hysteresis and an average modulus of elasticity of 15-25 GPa. A bond coat (44A, 44B) may be applied between the substrate and the thermal barrier layer. The bond coat may have a first dense MCrAlY layer (44A) on the substrate and a second rough, porous MCrAlY layer (44B) on the first MCrAlY layer, the bond layers diffusion bonded to each other and to the substrate.