What a composite material is

Two materials that stay two

The constituents of a composite do not dissolve into one another. The U.S. Department of Energy defines a composite as two or more materials that keep their macrostructure, and the FAA airframe handbook says the same thing from the workshop: the constituents do not dissolve or merge completely, they act together, and they can normally be told apart where they meet.^[1]

That is the point of the arrangement. Because the constituents stay distinct, each can be chosen for what it does well, and the combination can be designed to do better than either does alone. In structural composites the pair is almost always a reinforcement and a matrix.^[1]

What the matrix does and what the reinforcement does

In a fibre-reinforced composite the fibre is the load-carrying element, and the material is strong and stiff only in the direction the fibres run. The matrix holds the fibres in position and orientation, passes load into them, protects them from abrasion and the environment, and sets the highest temperature at which the composite can serve.^[1]

The division of labour is not the same in every family. In a ceramic-matrix composite what the matrix lacks is toughness, and the reinforcement is there chiefly to supply it.^[1]

The interface, and the interphase

Load gets from matrix to fibre across the interface between them, so the bond there governs how well the reinforcement works. Too weak, and the fibre is never properly loaded; too strong, and a crack can travel across the interface, lowering toughness and strength.^[1]

In natural fibre composites the problem is a weak bond: plant fibres are hydrophilic and most polymer matrices are not, so fibres are treated or coupling agents are added. In ceramic-matrix composites the answer is a deliberately applied interphase — in NASA’s SiC/SiC, a thin boron nitride coating on the fibres, chosen because it was compliant enough to toughen the composite and resisted oxidation better than carbon.^[1,2]

Particles, short fibres, continuous fibres and laminates

Reinforcement comes as particles, as short or long discontinuous fibres, or as continuous fibres. In metal-matrix composites the choice changes the character of the material: particles give more isotropic behaviour, fibres the largest gains in strength and stiffness.^[1,2]

Continuous fibres are usually laid in plies, and a laminate’s stiffness and strength then depend on the stacking sequence. A unidirectional ply is strong only along its fibres; a quasi-isotropic stack of plies at 0°, ±45° and 90° approximates the uniform behaviour of a metal. Architecture is chosen part by part, which is why Lithoen records it as a claim about a class rather than as one of the class’s defining attributes.^[1]

Four families, named by their matrix

Polymer-matrix composites combine a polymer resin with reinforcing fibres, most often glass or carbon. Metal-matrix composites carry particles, whiskers or fibres in a metal or alloy. Ceramic-matrix composites put fibres, whiskers or particles in a technical ceramic. Carbon–carbon composites are carbon fibres in a carbon matrix.^[1,2,3,4]

Each family is made differently, because in each the matrix reaches the reinforcement in a different state — a liquid resin, a molten or powdered metal, a gas or a ceramic precursor — and that is what the manufacturing articles in this domain follow.^[1,2,3]

Commonly misunderstood