Machinable CMC Surface Coating With Density Gradient
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Solution Overview
Problem
Machining of ceramic matrix composites (CMCs) is challenging due to their hardness, leading to tool wear and potential structural degradation, and the application of environmental barrier coatings (EBCs) is costly and prone to cracking during machining.
Innovation Solution
A method involving the formation of a porous ceramic multilayer on a fiber preform with a porosity or low-wettability particle gradient, followed by melt infiltration and machining, to create a density-gradient surface coating that enhances machinability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If machining is performed on densified CMC to achieve desired surface finish and dimensional tolerances, then manufacturing precision is improved, but tool wear increases and structural degradation occurs
Solution Approach 1:
The patent applies preliminary machining to the fiber preform before melt infiltration, when the material is in a softer, more machinable green state. This preliminary machining establishes near-final dimensions and geometry, reducing the amount of machining required after densification. The preform is machined to within close tolerances of the final part dimensions, and then melt infiltration densifies the structure while maintaining those dimensions with minimal additional machining.
Solution Approach 2:
The patent changes the physical state and mechanical properties of the CMC material by performing machining at different stages of the manufacturing process. The fiber preform in the green state has different hardness and machinability characteristics compared to the densified CMC. By exploiting this parameter change through staged processing, the patent achieves both good surface finish and reduced tool wear.
2Manufacturing precision
If machining is performed on densified CMC to achieve desired surface finish, then manufacturing precision is improved, but productivity decreases due to slow machining speed
Solution Approach 1:
The patent performs the majority of machining operations on the fiber preform before melt infiltration, when the material is softer and removes more efficiently. This preliminary machining establishes near-final dimensions and geometry, reducing the amount of time-consuming machining required after densification. The preform is machined to within close tolerances of the final part dimensions.
3Reliability
If environmental barrier coating is applied to CMC before machining to protect the surface, then reliability is improved, but device complexity and cost increase
Solution Approach 1:
The patent performs preliminary machining on the fiber preform before melt infiltration and coating application. By establishing near-final dimensions and geometry in the green state, the patent minimizes subsequent machining operations that would require protective coatings. This eliminates or reduces the need for complex EBC applications and subsequent machining protection strategies.
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 allows for efficient machining of CMCs with reduced tool wear and improved surface finish, while minimizing structural degradation and reducing the need for costly EBCs.
Implementation Method 1
a SiC fiber preform is exposed to molten silicon, which is drawn into the (porous) fiber preform via capillary forces
Implementation Method 2
the impregnated fiber preform and the porous ceramic layer are infiltrated with a molten material comprising silicon
Data Source
Figure 1A~1B
Figure 2A~2B
Figure 3A~3C
AI summary
A method to form a machinable ceramic matrix composite comprises forming a porous ceramic multilayer on a surface of a fiber preform. In one example, the porous ceramic multilayer comprises a gradient in porosity in a direction normal to the surface. In another example, the porous ceramic multilayer includes low-wettability particles having a high contact angle with molten silicon, where an amount of the low-wettability particles in the porous ceramic multilayer varies in a direction normal to the surface. After forming the porous ceramic multilayer, the fiber preform is infiltrated with a melt, and the melt is cooled to form a ceramic matrix composite with a surface coating thereon. An outer portion of the surface coating is more readily machinable than an inner portion of the surface coating. The outer portion of the surface coating is machined to form a ceramic matrix composite having a machined surface with a predetermined surface finish and/or dimensional tolerance.