The substrate of a catalytic converter fulfils a vital role in supporting the catalytic material, and at the same time must be capable of withstanding some extremely arduous conditions.
Operating temperatures may be in excess of 900 ℃ and the substrate will also be exposed to fast moving, corrosive exhaust gases, rapid changes in temperature and pressure and external factors such as shocks and vibration.
Another important attribute of the substrate, apart from durability, is that it must not cause an excessive pressure drop in the exhaust system.
The two most popular types of substrate are:
- ceramic extrusions
- metal versions made from ultra-thin foil.
Ceramic substrates dominate the automotive market, primarily because they are mass-produced and easy to coat. They are usually made from cordierite, which is a porous ceramic, and this porosity offers some advantages depending on the coating process used.
There are also several drawbacks to ceramic substrates, as follows:
They need to be ‘canned’ before they can be welded into an exhaust system. This refers to the process of installing them into a metal housing using a gasket material which protects them from the expansion and contraction of the metal. This stage adds cost whilst also introducing the risk that they might become loose in the can after a period of time.
The second drawback is that they need expensive tooling to produce, so a limited range of shapes and sizes is available.
The third drawback is that, being ceramic, they are brittle and may crack if they receive a shock (either physical or thermal).
Metal substrates are made from very thin metal foil, consisting of an alloy called fecralloy, which contains iron, chromium and aluminium. The reason that fecralloy is used is that, when heated, it forms a layer of aluminium oxide on the surface which protects the iron against oxidation and also gives a rough surface which acts as a key for the washcoat.
The main advantage of metal substrates is that they don’t require tooling, so they can be made to the customer’s requirements, even if the order quantity is small. They can also be made in larger sizes than ceramic substrates. Other advantages are that they are generally supplied with an integrated metal skin, which can be welded directly into an exhaust system without the need for ‘canning’. Finally, they are better than the ceramic variety at resisting thermal and mechanical shocks.