In materials science, a composite material is a heterogeneous material, that is, made up of two or more phases with different physical properties, whose properties are much better than those of the phases that constitute it. Typically, the different phases in the composite are made up of different materials, as is the case with carbon fiber and epoxy resin composites. However, there are exceptions where the different phases are made of the same material, such as SiC / SiC and Self-Reinforced Polypropylene (SRPP). Composite materials can be both artificial and natural. Some examples of naturally occurring composite materials are wood, where cellulose fibers are dispersed in a lignin phase, and bones, where collagen is reinforced by mineral apatite.
The most primitive man-made composite materials were bricks, consisting of straw and mud combined together; the Biblical book of Exodus speaks of the Israelites oppressed by Pharaoh who were forced to prepare bricks without straw. The ancient brickworking process can still be seen on Egyptian tomb paintings in the Metropolitan Museum of Art.
Later other composite materials were developed, including plywood, concrete, and the combination of concrete and iron rods (called reinforced concrete).
Structure of composite materials
The individual materials that make up the composites are called constituents, and depending on their function they are called matrix and reinforcement (or filler). The combination of these two parts constitutes a product capable of guaranteeing extremely high mechanical properties (for this fundamental purpose is the care of the interfacial adhesion between reinforcement and matrix) and decidedly low density: for this reason composites are widely used in applications where lightness is crucial, aeronautics in the first place.
The matrix consists of a continuous homogeneous phase, which has the task of:
enclose the reinforcement, ensuring the cohesion of the composite material (and of any layers of which it is composed, in the case of laminated composite);
ensure that the reinforcing particles or fibers have the right dispersion within the composite and that no segregation takes place.Depending on the nature of the matrix, composite materials are divided into various categories, including:
PMC (Polymer-Matrix Composite): polymer matrix composites, for example thermoplastic (such as Nylon and ABS) or thermosetting (such as epoxy resins);
MMC (Metallic-Matrix Composite): metal matrix composites, generally aluminum, or titanium and their alloys, more rarely magnesium or others;
CMC (Ceramic-Matrix Composite): ceramic matrix composites, generally silicon carbide or alumina;
carbon-carbon composites: both the matrix and the reinforcement are made of carbon;
hybrid composites: they contain two or more types of fibers. In most cases the matrices are polymeric because they guarantee low density (and therefore lightness of the final material): however, they have the defect of drastically reducing performance as the temperature rises.
In composite materials with a polymeric matrix, for example, epoxy resins (the same used in some adhesives) or phenolic resins can be used as a matrix, possibly with additives with other polymers (e.g. PVB) which help to improve the mechanical characteristics (e.g. .es flexibility) of the composite material while maintaining adhesion to the reinforcement.
The reinforcement is represented by a dispersed phase, which is precisely dispersed in various ways inside the matrix and has the task of ensuring rigidity and mechanical resistance, taking on itself most of the external load.
Depending on the type of reinforcement, composite materials are divided into:
fiber reinforced composites;