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APPLICATIONS
OF TECHNOLOGY:
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SAPPHIRE/COPPER/NIOBIUM INTERFACE CREATED USING THE BERKELEY LAB METHOD
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a) initial stage, looking through the sapphire onto the copper interlayer on the niobium core |
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b) intermediate stage |
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c) final stage, where the sapphire-niobium contact dominates the interface and therefore the thermomechanical characteristics of the joint |
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- Joining temperature-sensitive components and other chemically reactive and difficult to join materials
ADVANTAGES:
- Enables joining of an expanded range of materials, some of which have previously been difficult or impossible to join
- Increases flexibility in composition and properties of the interlayer systems
- High quality joints can be formed in five minutes
- Expands the joining temperature range of commercially available brazing materials as well as traditional interlayer materials
- Offers greater flexibility in final thermal capabilities of the interlayer, allowing use temperatures that approach or exceed the joining temperature
- Can be applied successfully to materials with as-ground surfaces
- Could be extended to allow joining of conformal curved surfaces and for wide-gap bonding
ABSTRACT:
Andreas Glaeser and colleagues at Berkeley Lab have developed joining methods that enable lower joining temperatures than more traditional joining approaches. The Berkeley Lab joining methods yield reliably strong interfaces using reduced joining temperatures while preserving the ability to use the assembly at temperatures that approach or exceed the original joining temperature. Reduced joining temperatures are especially valuable for preventing microstructural degradation in materials with temperature-sensitive microstructures, or chemical degradation when materials with temperature-sensitive reactivity are being joined.
The Berkeley Lab implementations of transient-liquid-phase (TLP) and liquid-film-assisted joining (LFAJ) methods extend and complement traditional joining processes. These new methods are based on multilayer, microdesigned interlayers that incorporate a limited amount of a low melting point layer that melts and forms a controlled thickness liquid film between the components to be joined and a solid, high melting point core layer. In TLP this liquid film disappears with time yielding a fully solid and refractory interlayer; in LFAJ the initially continuous liquid layer breaks up into discrete droplets and promotes bond formation between the material to be joined and the core layer.
The Berkeley Lab researchers have used a large number of interlayer systems to join alumina and silicon-based ceramics, but the method is not limited to these materials. The technology could be applied to a wide variety of metals, ceramics, and composites.
Conventional TLP bonding uses a melting point depressant (MPD) in the interlayer, and relies on adequate solid solubility and rapid diffusion of the MPD into the adjoining bulk materials to progressively increase the melting temperature of the joint region. These restrictive requirements have limited the materials used for interlayers and the materials that could be joined. To alleviate or eliminate these constraints on bulk and interlayer material choices and to expand the range of joining and ultimate use conditions, Glaeser and his team have extended the TLP concept and developed interlayers in which the counter diffusion of the MPD and the higher melting point material occurs within the interlayer itself. Alloy formation occurs at a significantly reduced length scale, widening the range of candidate interlayer constituents. As in traditional TLP joining, the melting temperatures of the joints in these systems exceed the joining temperatures.
Earlier implementations of the Berkeley Lab TLP method focused on core layers with melting points above 1300°C, bonding temperatures of 1150°C, and produced interlayer alloys with remelt temperatures as high as 1700°C. Recent implementations of the technology have used core layers with melting points between 1100°C and 715°C and cladding films 2–3 µm thick with melting temperatures below 200°C. Joining temperatures several hundred degrees below the core layer melting point have yielded robust joints with joint remelt temperatures similar to those of the core layer material. Further joining temperature reductions could be achieved by altering the chemistry and design of the interlayer.
In the Berkeley Lab LFAJ method, the liquid former does not have significant solubility in the solids to be joined or in the core layer. As a result, it does not disappear, but instead serves as a high transport rate pathway for dissolved core layer material. Diffusion of the core layer material at rates that are orders of magnitude higher than in solid-state bonding processes facilitates rapid bonding between the core layer and the components to be joined. Concurrently, this contact growth disrupts the liquid film and the dewetting of the film ultimately leads to a dispersion of discrete liquid droplets along the interface. Glaeser’s team has demonstrated this method in a copper-niobium interlayer system with joining temperatures as low as 1150°C, well below those required to melt niobium (2460°C) and below those typically used for ultrahigh vacuum solid-state bonding (>1400°C). The researchers have demonstrated that with this system a joint can be formed in five minutes that has properties equivalent to earlier joints that required a six hour treatment. Other interlayer systems may greatly reduce the temperatures involved in LFAJ. (Reference numbers IB-2095 and IB-2123.) |
