Materials > Composites
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When we build a skyscraper, to make a building that tall and strong, we first make a metal frame which we then wrap in concrete. We do this because although the concrete is strong, it is brittle and so doesn’t handle changing forces well (i.e. things like the wind, or a couple of thousand people showing up for work at the same time). Steel on the other hand has a bit of give, aka ductility. This means a building using both materials can handle everything it needs to. When we look on a small scale, we can actually do the same process. We build a frame that transmits the forces, and wrap it in a matrix. We call this type of material a composite and a common example of it is carbon fibre. This is where carbon fibres are woven together into a ‘frame’, and then fixed into place by a resin. Having fibres means we can weave them in different ways to make one direction stronger than another, and weave it in ways that make complex structures stronger.
There are many different types of composites as there is an infinite number of matrix and fibre material combinations. In aerospace, we focus on two major types of composites called Metal Matrix Composites (MMCs), and Ceramic Matrix Composites (CMCs). If we think back to our materials in general, we know ceramics are strong, tough, oxidation/corrosion resistant but brittle. Metals on the other hand are strong, ductile and have good conductivity. So we can already roughly guess how these two types of materials will differ in properties.
The move towards using composites in the aerospace industry is influenced by a number of factors. A composite aims to do the same thing as every other material, and that is to combine different things together to try and get the best properties. With a composite we can mix metals and ceramics, or different ceramics together to make a material with a combination of properties. The other advantage of composites is that they are about half the weight of current nickel superalloys. Unfortunately these materials are not easy to make, and given the complex shapes needed to make a jet engine, manufacturing is difficult. This makes the very expensive too.
Metal Matrix Composites
When we talk about a MMC, we are generally talking about silicon carbide (SiC) fibres being held together by a metal (acting as the matrix). SiC can be made into a variety of crystal structures and therefore have different properties, but it is a ceramic. This means by itself SiC is tough and strong but very brittle. Having a metal matrix, we want the metal to support the SiC fibres and deal with the forces that would break the fibres. One of the important things about a composite is how it deals with specific situations and whether it is the fibre or the matrix that is dealing with the forces. It is also important to make sure they thermally expand and contract at the same rate, and basically stick together through all situations.
So when we look at MMCs what metals would we use? Well, we already have a bunch of high performance metals we already use. We know that titanium alloys have good high temperature performance and luckily work well with SiC. This means we can make MMCs using Ti6-4, Ti 6242 etc that can handle up to 760°C. We can then go a step further and use other titanium alloys such as Ti3Al and TiAl as the matrix holding together SiC fibres. This gives us materials that are very lightweight that can handle up to 1000°C.
I know what you are thinking, why not superalloy matrix composites (SMCs)? Well, quite a few people are trying to make it a reality but it isn’t simple, so its going to take some time before we see any SMCs in jet engines.
Ceramic Matrix Composites
Now we know ceramics are tough, corrosion resistant but brittle. By using fibres we can redirect forces through a component and so try and prevent situations where they would snap. This means if we make materials using ceramic fibres and ceramic matrix we can get some components to perform better than if they were a solid ceramic. Looking into the future we can see that at some point jet engines will run so hot that only ceramics will be able to handle the temperatures, but they also need to handle the forces/stresses. Currently, the ISM is looking into CMCs for this exact purpose.
So starting again with fibres of SiC which are heat resistant, corrosion/oxidation resistant, strong and have good mechanical properties. So what if we made the matrix of the same material? This is a SiC/SiC CMC. So a SiC/SiC CMC has good mechanical properties thanks to the fibres/matrix structure so much so that when it breaks it breaks almost like a metal rather than like a ceramic (i.e. bends before it breaks, no sudden failure). This means we have a strong, material able to cope with very high temperatures (~1000°C) which is also lightweight.
Another CMC we have investigated is AlO/AlO. This is aluminium oxide fibres and matrix. AlO is a ceramic which is light weight, able to handle very high temperatures and is very hard. By changing the structure from a solid ceramic to a material which has fibres and a matrix, then this material can deal with forces in a different way, meaning it is less brittle and is more able to handle changing forces seen in a jet engine.
So why aren’t we making engine parts out of CMCs? The difficulty of CMCs is that they are not simple to make, are expensive, and need to be made with extreme precision. This means that when it comes to the decision between a superalloy and a CMC, although the CMC is lighter, we can make the superalloy for cheaper, on a larger scale, and, at the moment know more about how they perform in the engine. Arguably this situation won’t last forever and science will figure out ways of solving these problems, but until then superalloys beat CMCs.
Fibres, Particle and Whiskers
So far we have described composites as having fibres held together by a matrix. Although this is the most common type of composite used for aerospace, there are other versions of composites. Fibres are long strands of material that can be woven in different ways to make certain directions of a material stronger than others. A particle is almost like a superalloy where there are globs of one material inside another. Whiskers, are somewhere between the two and look like short fibres. Each of these arrangements means the forces are transferred throughout a component differently and so affects how well it will perform in the extreme conditions inside the engine.