Slurry Coating Smoothing Thermal Barrier Surfaces
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Solution Overview
Problem
Thermal barrier coatings (TBCs) on gas turbine components exhibit high roughness due to atmospheric plasma spraying, leading to aerodynamic inefficiencies and thermo-mechanical stresses, which existing smoothing methods either damage the surface or are costly and ineffective in reducing roughness without compromising the underlying coating's lifetime.
Innovation Solution
A method involving a second slurry coating with a similar thermal expansion coefficient to the first coating, applied in multiple passes, partially sintered, and polished to cover only the valleys of the underlying TBC roughness, resulting in a dense, thin smooth layer that minimizes thermo-mechanical stresses and maintains the underlying coating's integrity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If mechanical smoothing methods (grinding, polishing, sandblasting) are used on the TBC surface, then surface roughness is reduced, but the coating surface is damaged and inherent porosity is exposed
Solution Approach 1:
The invention changes the chemical composition parameters of the slurry coating by incorporating glass materials and zinc titanate, and adjusts the thermal processing parameters (firing temperature and duration) to achieve a smooth surface without mechanical damage. The slurry composition is specifically designed to fill pores and create a dense, smooth surface layer that bonds well with the underlying TBC.
2Manufacturing precision
If a thick smooth coating layer is applied to cover the rough TBC surface, then surface smoothness is improved, but thermo-mechanical stresses increase and vertical cracks may form
Solution Approach 1:
The invention applies local quality by creating a smooth surface layer only where needed through the slurry coating process. The slurry preferentially fills the rough valleys and pores of the underlying TBC, creating a smooth surface locally without requiring a uniformly thick coating layer. This localized smoothing approach maintains coating strength while achieving the desired surface quality.
Solution Approach 2:
The invention uses composite materials in the slurry composition, combining glass materials and zinc titanate with other ceramic powders. This composite formulation allows the coating to accommodate thermal expansion differences and reduce thermo-mechanical stresses, preventing crack formation while maintaining surface smoothness.
3Manufacturing precision
If multiple TBC layers are deposited to create a smooth surface, then surface roughness is reduced, but manufacturing cost and process complexity increase
Solution Approach 1:
The invention merges the smoothing function with the protective coating function by applying a slurry coating that serves both purposes simultaneously. Instead of requiring separate TBC layers for protection and smoothing, the slurry coating is designed to provide both thermal barrier protection and surface smoothing in a single integrated process, reducing overall process complexity.
Solution Approach 2:
The invention applies preliminary action by using the slurry coating to pre-fill the rough surface features and pores before final curing. The slurry is applied in a state that allows it to flow into and fill the irregularities of the underlying TBC surface, creating a smooth substrate that is then cured to lock in the smooth surface morphology.
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 method effectively reduces surface roughness while maintaining the durability and aerodynamic performance of the thermal barrier coating, avoiding the costs and inefficiencies of previous methods by creating a smooth, thin layer that covers only the valleys of the underlying TBC, thus enhancing turbine efficiency.
Implementation Method 1
fully curing, but only partially sintering the slurry coating at a temperature in the range of 300 to 800 °C
Data Source
Figure 1~2c
Figure 3~4
AI summary
The present application refers to a method for reducing the roughness (Ra) of a first coating (1) with a defined coating composition and thermal expansion coefficient, said coating (1) covering the surface of a component (3) to be thermally high loaded, wherein in a first step the roughness (Ra) of said coating (1) is measured and in a following step a second slurry coating (2) is prepared for applying onto the surface of the first coating (1), whereby the coating composition of the second coating (2) is tailored to have a similar thermal expansion coefficient like the first coating (1). The method is characterized in calculating a minimum number of coating spray passes (N) necessary to provide the second slurry coating (2) with a thickness (T) that is at least two times of the roughness (Ra) of the first coating (1), then applying the slurry coating (2) with said calculated number of spray passes (N) onto the surface of the first coating (1), then fully curing, but only partially sintering the slurry coating (2) at a temperature in the range of 300 to 800°C and finally in polishing the second coating (2) to a reduced thickness (T') such that finally the second coating (2) does cover the first coating (1) only locally.