Metal-Ceramic Brazed Joint Using Stacked Spacers
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Solution Overview
Problem
The challenge lies in effectively assembling ceramic matrix composite (CMC) parts with metal parts in aeronautical applications, where conventional methods fail due to significant differences in thermal expansion coefficients and reactivity, leading to stress-induced breakage and poor joint performance.
Innovation Solution
A brazing assembly technique using a stacked structure with a metallic spacer to compensate for thermal expansion differences and a chemically less reactive ceramic spacer to prevent chemical compound formation, employing specific solder compositions and intermediate ceramic materials like AlN or mullite to ensure mechanical and chemical protection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If brazing assembly technique is used to join metal part and ceramic part, then strong joint is achieved, but stress-induced breakage occurs due to significant difference in thermal expansion coefficients
Solution Approach 1:
A metallic spacer is introduced as an intermediary element between the metal part and ceramic part. This spacer has a coefficient of thermal expansion that is lower than the metal part but higher than the ceramic part, allowing it to act as a buffer that compensates for the differential thermal expansion. During thermal cycling, the spacer deforms elastically to absorb expansion mismatches, preventing stress concentration and potential breakage of the ceramic component while maintaining strong joint integrity.
Solution Approach 2:
The invention changes the physical parameters of the joint system by selecting a spacer material with specific mechanical properties (lower elastic modulus, controlled thermal expansion coefficient) that differ from both the metal and ceramic parts. This parameter adjustment allows the spacer to deform under thermal stress, compensating for expansion differences and maintaining joint reliability across temperature variations.
2Ease of manufacture
If conventional brazing is used to assemble ceramic and metal parts, then assembly is achieved, but fragile chemical compounds form due to high reactivity between C/SiC and metals
Solution Approach 1:
The metallic spacer serves as a chemical barrier between the reactive ceramic (C/SiC) and the metal part. This intermediary layer prevents direct contact between the reactive surfaces, thereby avoiding the formation of fragile carbide or silicide compounds. The spacer is selected to be chemically stable with both materials, ensuring assembly ease while maintaining joint strength by preventing detrimental chemical reactions.
Solution Approach 2:
The invention converts the potential harm of high chemical reactivity into a benefit by using the reactive characteristics to control the brazing process. The metallic spacer, when properly selected, can form stable, non-fragile intermetallic layers that actually enhance joint strength while preventing the formation of harmful compounds. The controlled chemical interaction at the spacer interfaces creates a more reliable joint than direct metal-to-ceramic bonding.
3Reliability
If metallic spacer is used to compensate thermal expansion, then expansion difference is compensated, but device complexity increases
Solution Approach 1:
The metallic spacer is designed to perform multiple functions simultaneously: it compensates for thermal expansion differences, acts as a mechanical support structure, provides a bonding surface for brazing, and serves as a barrier against chemical reactions. By consolidating these multiple functions into a single component, the invention achieves improved joint reliability without proportionally increasing assembly complexity, as the spacer integrates several protective and functional roles.
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
This approach effectively compensates for thermal expansion differences and reduces chemical reactivity, enhancing the mechanical strength and reliability of the ceramic-metal assembly, thereby improving the durability and performance of the joint.
Implementation Method 1
the first spacer is metallic and capable of deforming to compensate for the difference in expansion between said metal part and the second spacer
Implementation Method 2
the first spacer is metallic and capable of deforming to compensate for the difference in expansion
Implementation Method 3
assembled two by two, in this order, by brazing
Implementation Method 4
during the cooling following the melting of the solder composition
Data Source
Figure 1~2
Figure 3
AI summary
The invention relates to a joint between a metal part (1) and a ceramic part (7) which is made from a ceramic material that is based on SiC and/or C. The inventive joint is characterised in that it comprises a stacked structure consisting of the following elements which are assembled in pairs, in the following order, by means of brazing, namely: the metal part (1), a first spacer (3), a second spacer (5), and the ceramic part (7). According to the invention, the second spacer (5) is made from another ceramic material which is less chemically reactive with metals than SiC or C and which has a lower expansion coefficient than that of the material forming the metal part (1). Moreover, the first spacer (3) is metal and can deform in order to compensate for the expansion differential between the metal part (1) and the second spacer (5). The invention also relates to the use of said joint in a turbomachine.