Low Thermal Expansion Bondcoat for TBCs
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Solution Overview
Problem
Thermal expansion mismatch between metal and ceramic layers in thermal barrier coatings leads to interface stress and potential crack formation, which reduces the thermal cycle life of these coatings in high-temperature applications.
Innovation Solution
A low thermal expansion bondcoat comprising an inner and outer layer alloy of MCrAlM′, where M is nickel, cobalt, or iron, and M′ is yttrium, zirconium, or hafnium, with specific weight percent compositions and particle sizes, applied to minimize interface stress and crack formation by reducing thermal expansion to 6.5 mm/m between 25°C and 525°C.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a typical MCrAlY bondcoat (e.g., LCO-22) is used, then good oxidation resistance and adhesion are achieved, but thermal expansion mismatch with zirconia ceramic causes substantial interface stress and crack formation
Solution Approach 1:
The bondcoat composition is modified by adjusting the alloying elements (increasing Cr to 25-40 wt%, Al to 10-25 wt%, and adding M′ elements like Y, Zr, or Hf at 0.01-1.0 wt%) to change the thermal expansion coefficient from typical values to 6.5 mm/m or less between 25°C and 525°C, reducing mismatch with zirconia ceramic
Solution Approach 2:
The bondcoat is designed as a composite alloy system MCrAlM′ combining multiple elements with complementary functions: M (Ni, Co, Fe) provides base structure, Cr and Al provide oxidation resistance, and M′ elements fine-tune thermal expansion properties to match the ceramic layer
2Duration of action of stationary object
If the thermal expansion of the bondcoat is reduced to minimize interface stress, then thermal cycle life is improved, but the composition complexity and manufacturing difficulty increase
Solution Approach 1:
Specific compositional ranges are established (Cr: 25-40 wt%, Al: 10-25 wt%, M′: 0.01-1.0 wt%) that reliably produce the desired low thermal expansion coefficient (≤6.5 mm/m), providing manufacturing guidance while achieving the thermal match
Solution Approach 2:
The M′ elements (Y, Zr, Hf) act as intermediary additives in small quantities (0.01-1.0 wt%) that fine-tune the thermal expansion properties without significantly affecting the base MCrAlY alloy structure or oxidation resistance mechanisms
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 low thermal expansion bondcoat minimizes interface stress and crack formation, leading to longer thermal barrier coating cycle life and improved high-temperature oxidation resistance, surpassing typical Ni-based superalloys and stainless steels.
Implementation Method 1
The bondcoat has a thermal expansion of about 6.5 millimeters per meter or less between a temperature of from about 25° C. to about 525° C.
Data Source
AI summary
This invention relates to low thermal expansion bondcoats for thermal barrier coatings. The bondcoats comprise: (i) an inner layer comprising an inner layer alloy of MCrAlM′, and (ii) an outer layer comprising an outer layer alloy of MCrAlM′, wherein M is an element selected from nickel, cobalt, iron and mixtures thereof, and M′ is an element selected from yttrium, zirconium, hafnium, ytterbium and mixtures thereof. The inner layer alloy is thermally sprayed from a powder having a mean particle size of 50 percentile point in distribution of from about 5 microns to about 50 microns. The outer layer alloy is thermally sprayed from a powder having a mean particle size of 50 percentile point in distribution of from about 30 microns to about 100 microns. The bondcoat has a thermal expansion of about 6.5 millimeters per meter or less between a temperature of from about 25° C. to about 525° C.