Gradient Mullite-Silicon Interlayer for CMC Corrosion Resistance
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Solution Overview
Problem
Ceramic matrix composite (CMC) materials with a silicon carbide matrix experience surface recession and degradation due to volatilization of silica at high temperatures, leading to cracking and decohesion of environmental barriers in corrosive atmospheres, particularly in gas turbine combustion chambers.
Innovation Solution
A composition-gradient bonding sub-layer transitioning from pure silicon to stoichiometric mullite between the substrate and anti-corrosion protection layer, with a thin mullite layer on the external face for chemical barrier function and a thin silicon layer on the internal face for bonding, reduces sensitivity to thermal shocks and chemical interactions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a BSAS layer is used as environmental barrier on SiC substrate, then corrosion resistance is improved, but at high temperatures above 1300°C silica volatilization causes surface recession and degradation
Solution Approach 1:
A mullite interlayer is introduced between the BSAS environmental barrier layer and the SiC substrate. This intermediary layer prevents direct contact between BSAS and SiC, blocking the chemical reaction pathway that leads to silica volatilization and surface recession at high temperatures above 1300°C.
Solution Approach 2:
The environmental barrier is structured as a composite system with multiple layers: an outer BSAS layer for corrosion protection, a middle mullite interlayer for chemical stability, and an inner Si bonding layer for adhesion. This composite structure combines the advantages of each material while mitigating their individual weaknesses.
2Strength
If a Si bonding layer is formed to facilitate bonding of the chemical barrier layer, then adhesion is improved, but thermal shocks cause cracking and decohesion of the environmental barrier
Solution Approach 1:
The mullite interlayer serves as a mediator between the Si bonding layer and the BSAS environmental barrier. It absorbs and distributes thermal stresses during thermal shocks, preventing crack propagation that would otherwise occur in the brittle Si-BSAS interface, thus maintaining adhesion while improving thermal shock resistance.
Solution Approach 2:
The mullite interlayer has intermediate thermal expansion and mechanical properties between Si and BSAS, creating a gradual transition in material parameters. This reduces thermal stress concentration and prevents cracking during thermal cycling while maintaining strong bonding.
3Duration of action of stationary object
If the thickness of the BSAS layer is increased to achieve desired lifetime, then durability is improved, but chemical interaction between BSAS and Si causes degradation at high temperatures
Solution Approach 1:
The mullite interlayer acts as a chemical barrier between BSAS and Si, preventing the harmful chemical interaction that occurs when these two materials are in direct contact. This allows the BSAS layer to maintain its thickness for durability without suffering from chemical degradation.
Solution Approach 2:
The harmful chemical interaction pathway is extracted from the system by removing the direct BSAS-Si contact interface. The mullite interlayer isolates the two materials, eliminating the source of chemical degradation while preserving the protective function of the BSAS layer.
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 the durability and adhesion of the environmental barrier, minimizing cracking and degradation, allowing CMC parts to withstand high temperatures and corrosive environments effectively, as demonstrated by thermal shock and corrosion tests.
Implementation Method 1
The formation of a composition-gradient mullite coating on a silicon-containing substrate... The mullite coating is formed by chemical vapor deposition, or CVD
Implementation Method 2
a layer 2 of an alkaline-earth metal aluminosilicate type compound... The anti-corrosion function is provided by a layer of BSAS
Implementation Method 3
a phenomenon of surface recession is observed with CMC materials with a SiC matrix due to the volatilization of silica (SiO2) which is formed by oxidation on the surface
Data Source
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AI summary
The invention concerns an environmental barrier formed on a substrate (10) made of composite material with ceramic matrix and comprising an outer anti-corrosion protective layer (12) containing a compound of the alkaline, alkaline-earth or rare earth aluminosilicate. Between the substrate (10) and the protective layer (12) an adhesion sublayer (14) is formed, the composition of which develops from substantially pure silicon into substantially pure mullite between an inner surface on the substrate side, and an outer surface, decreasing in silicon content and increasing in mullite content.