What are functionally graded composites?
Seth Price | March 30, 2024Functionally graded composites are a relatively new development in composites research. While many applications for composites focus on isotropy, meaning materials properties are uniform in all directions, functionally graded composites are deliberately anisotropic. Instead, they are used in applications where one material property is required for one side of the composite, and another material property is needed on the opposite side. Instead of simply sandwiching two materials together, the composition of the material transitions from 100% material A to 100% material B.
Properly engineered functionally graded composites can be created from virtually any combination of materials classes. Perhaps the most common are metal-ceramic composites, where one side is a metal and the opposite side is ceramic, though other compositions are possible.
Functionally graded composites give the best of both worlds to a material. They allow for the advantages of both materials to be combined without sacrificing one for the other, as traditional isotropic composites do. For example, a fiber-reinforced epoxy composite will be weaker than the fibers by themselves, but epoxy will make the composite stiffer than the fibers alone. Stiffness is contributed by the epoxy but strength by the fibers.
In a functionally graded material, maybe one surface has high tensile strength, and the other has high hardness for abrasion resistance. Each side of the composite is exposed to the environment where it will perform better than an isotropic composite.
When compared to “sandwich structure” composites, where multiple materials are stacked on top of each other, functionally graded composites can be much more resistant to delamination or spalling due to stresses or thermal cycling. In the sandwich structure composite, the weakest part and the part most susceptible to spallation is at the interface between the two materials. Functionally graded composites have no such sharp interface; it’s as though someone smeared the ink on a drawing, such that one material slowly transitions to the other. Without the sharp interface, spallation is less likely to occur.
Applications
Functionally graded composites are specialty items, which are often implemented in high value, niche markets. Aerospace components, sporting equipment and some performance automotive parts may have or could benefit from functionally graded composites. Furthermore, components, which will face extremely harsh environments, such as high temperatures, corrosive exposure, abrasive wear and others, also benefit from functionally graded composites.
One of the largest markets for functionally graded composites is in the coatings industry. Thermal barrier coatings (TBCs) are used to protect surfaces from high temperatures and can be made from functionally graded composites. TBCs may be sprayed or plated onto a component. However, with steep thermal gradients and thermal cycling, the coating can spall due to thermal stresses. This will leave the component exposed, as well as generate spallation particles, which can create other hazards.
For example, consider a jet turbine engine. Inside the turbine are numerous parts with a TBC. After a number of cycles, a TBC may begin to spall. Besides leaving parts exposed to high temperatures, the spallation components could get drawn into the moving turbine and destroy it. However, a TBC made from a functionally graded material removes the sharp interface between materials that is the origination site for most spallation particles.
Another example case is to use functionally graded materials as a coating for wear resistance. Dry materials transport of sand and grit, or wet materials, such as concrete or ceramic slurries lead to high erosion rates on the equipment handling the material. Nozzles for sandblasting or bead blasting systems are bombarded by millions of particles. For plated nozzles, the repeated shock of particle impact can cause cracks to form at the interface between the metal structure and a ceramic wear surface. Eventually, the stress will cause pieces of the ceramic component to fail. With a functionally graded nozzle, there is no sharp interface, and the nozzle can have a much longer surface life.
Shockwave and vibration dampening material can be improved with functionally graded composites. Waves at a given frequency pass through different materials at different speeds. If the wave hits a sharp interface, some of it is reflected backwards and some of it is transmitted through. It behaves the same way as an impedance mismatch in electronics. A shockwave can cause the layers of a sandwich composite to delaminate at the interface. However, if one material slowly transitions to another, there is no sharp impedance mismatch. The shockwave velocity is changed as it passes from different materials, but it happens much more gradually.
Current limitations
The biggest challenge is finding techniques to manufacture a functionally graded composite. In theory, these composites sound great, but there is no simple way to change the composition while processing. There is no simple dial that an operator can turn on a machine that will make one.
With the powder processing techniques available, achieving strong bonding between particles of different materials is a challenge. Sometimes, a sintering aid or binder can be added, but the designer must use extreme caution, as this additional material may weaken the whole composite.
Depending on the geometry and manufacturing technique, functionally graded composites can take a long time to produce. Often, they must be batch processed, and may require curing, annealing or sintering steps along the way. If the full deposition process is completed and thermal processing performed afterwards, some of the material can migrate in the higher temperature due to the concentration gradient. Alternatively, if there is a large density difference between components, they may settle before processing is complete, undermining the functionally graded effect.
What’s ahead?
With the drive to make aircraft and spacecraft components lighter, yet withstand high temperatures, make medical devices more biocompatible, yet strong, and many other niche applications, functionally graded composite research and development will see growth in the future. Advances in additive manufacturing techniques will continue to make new functionally graded composites and designs possible and make their manufacture more accessible to product engineers.