Set a metal spoon, a plastic container, a ceramic mug, and a piece of fiberglass next to each other. Without listing their characteristics, look at the differences in weight, flexibility, scratchability, thermal conductivity, and sound on tapping. This may seem obvious but the question to address at this stage should be how structure, processing, and environmental conditions affect the properties of metals, polymers, ceramics and composites.
Metals contain crystalline structures made up of different grains and usually have a combination of some desirable properties: strength, stiffness, toughness, ductility and electrical conductivity. Many metals are easy to cast, form, machine, weld or heat treat. This still doesn’t define all of metals. There is a vast difference in the properties of steel, aluminum, copper and titanium, for example. Furthermore, within a particular group of metals, you can see a wide variation in properties if you change the alloying elements, grain size, defects, or the heat treatment method.
Polymers, however, do not usually have a crystal structure made up of metallic grains but are made up of long molecular chains. Polymers tend to have a lighter weight, are resistant to certain forms of corrosion, and often easier to produce as complex components. However, in general they have lower stiffness and lower strength than metals and their properties may change more dramatically at higher temperatures. A rigid polymer can soften when heated, whereas a flexible elastomeric polymer will exhibit deformation, or stretch and recover, as a result of molecular chains moving under load.
Ceramics comprise things like glass, ceramics, porcelain, brick and a variety of other materials. Ceramics tend to be hard, stiff and wear resistant and, in some cases, will also be useful at higher temperatures. They tend to be more fragile, though, which is to say brittle. Ceramics have the ability to resist surface indentation and resist a lot of compressive loading, for example, yet will fracture if a defect grows rapidly due to applied tension or impact load. They are quite sensitive to porosity and the presence of any defects because those conditions can lead to stress concentration and a reduction in the failure load.
Composites are made up of two or more dissimilar materials that act together in support of one another. Generally one of them (the matrix) acts as the primary material to shape the final component and transfer the load, while the other component (often called the reinforcement) is introduced to add another desired property to the material, such as stiffness or strength. Examples of composites include fiberglass, where you find glass reinforcement in a polymer matrix, and reinforced concrete, which combines concrete with steel reinforcement. Depending on the shape and arrangement of the reinforcing materials, the final composite may display directionality in the properties of the material, where the material will exhibit a greater property in the direction of the fibers. This makes the processing and arrangement of the composite reinforcement quite significant for its properties.
In order to make a fairer comparison between the four major material families, let’s look at each as it would perform in a single application, such as a lightweight protective panel. Each material will have a unique set of advantages and disadvantages as it relates to stiffness, toughness, density, temperature, moisture, processing, failure mode, and so on. A metallic material might exhibit the ability to deform prior to fracture, for example. A polymer component might have a lower density and be processed much easier than the metal or ceramic versions. A ceramic material might resist high temperature and exhibit a high degree of stiffness but be sensitive to shock loading. A composite component might have similar properties to the metal, yet offer the ability to tailor the properties of the material through directionality. The answer lies in the requirements of the service conditions.