Joining aluminum and ceramics - how to?

[bbtide]

Hi, materials engineering noob needs Your help.

I've got a problem. What i need to do, is to somehow join aluminum and ceramics in a rather dynamic application. Ideally, i would like do achieve it without using any other substance, i.e. glue.

What i need to work with: aluminum rod, about 100 inches long, 1" internal diameter, 1/8 inch thick wall; ceramics - let me just call it clay, i can either have it in 'soft form', before burning, or hardened, after burning; as far as equipment goes, i can use ceramics oven, as well as give it to metal shop, so all the metal working equipment is available - those guys do not work with ceramics, however.

What is the desired output: same aluminum rod, lined inside with a ceramic layer, about 1/4" thick. The remaining internal diameter would therefore be about 1/2".

My question: how to do it? Could i simply mold the internal ceramics "rod", slide it inside the aluminum one and then burn? Would there be any permanent bonding between the two substances? if not, is there any other way?

Thank You in advance, cheers.

 

[jselin]

The way to tackle this depends on much money you have to work with and what your performance requirements are. What you envision isn't likely to work for a number of reasons.

If you can tolerate a thin film internally I recommend a washcoat or CVD. If the ceramic needs to be thick you could form the ceramic tube alone then arc spray aluminum on top of that then machine it to spec.

What are you trying to do?

 

[Matcon]

Your main problem with getting aluminium to stick to anything is the oxide layer on the surface. This surface oxide is pretty unreactive but, if you can activate the metal under the surface the metal is reactive.

I presume that when you say you don't want to use glue, you mean substances that are not present in aluminium or clay.

For low stress joins in ceramics (and it works for aluminium to ceramic joins) I use hydrated aluminium oxide monomer. It is stable in acid solutions but polymerises as the pH rises, as when the solution reacts with the metal.

I like jselin's idea of arc spraying and would add that if you are looking at something that is going to undergo substantial thermal cycling, you will need a buffer layer that has an expansion coefficient somewhere between your metal and ceramic. Plasma sprayed molybdenum is a common buffer.

As jselin comments, the usefulness of the joint depends on what you are trying to achieve. If you need thermal contact, or just want to stop the clay from moving in the tube, an alumina layer will do. If you want a high strength bond, you almost certainly have to settle for a third component.

 

[pantaz]

Another potential problem to consider: How much will the clay grow and/or shrink during and after firing? Also, what is the required firing temperature of the clay? It may actually be higher than the melting point of the aluminum!

 

[Matcon]

As mentioned in the original post, the ceramic can be fired or unfired. Also, it is called 'clay' for convenience. A pure clay becomes plastic when wet because of water being adsorbed between the microscopic plates that constitute it, forcing them apart and allowing them to flow past one another. The amount of shrinkage on drying depends largely on the amount of water you add. Dry clay is usually less than 5% water but some, like bentonite, contain as much as 15%

In practical mouldable or castable ceramics, there is a cement phase (that sets by hydration) and a non-shrinking aggregate. Some, like phosphate-bonded materials derive bonding from inorganic chemical reactions. These materials harden well below the melting point of aluminium.

 

[hanbillion]

I suggest you may try to use some indium foil, which is ideal for creating vacuum seals.
For your reference, Indium creates effective and reliable helium tight hermetic seals between metals or non-metallics like glass and ceramics. For example, indium can be used to seal non-wettable surfaces in cryogenic applications, vacuum pumps and in heat-sensitive areas, etc..
When indium is used as the sealant, a chemical bond is formed between this silvery, semi-precious metal and the surfaces to be mated. Gaskets made from other materials only form a barrier to the medium being contained. Indium seals are far less sensitive to mechanical shock, vibration and low temperatures than other types of seals.
We are in a position to supply indium foil. If there is any interest, please feel free to contact me for more details. My email address is: tonyhantao@gmail.com