Question

Manufacturing process

What are the main functions of the matrix and reinforcing phases in general composites?

Answer 1

Matrix

There are three main types of matrices: polymer, metal, and ceramic. Matrices hold and protect the fibers (or other reinforcing material geometries-e.g., particles, platelets, short fibers, or whiskers, etc.) from environmental and physical damage. Keeping the fibers separated decreases cracking and redistributes the load equally among all fibers. Thus, the matrix contributes greatly to the properties of the composites. The ability of composities to withstand heat, or to conduct heat or electricity depends primarily on the matrix properties since this is the continuous phase. Therefore, the matrix selection depends on the desired properties of the composite being constructed.

Reinforcements

The strength, stiffness, and density of the composite material is very dependent on the reinforcing material. The ultimate tensile strength of a composite is a result of the synergy between the reinforcement and the matrix. The matrix forces load sharing among all the fibers, strengthening the material. The main types of reinforcements are continuous fibers, discontinuous (short fibers), whiskers, and particulates. The elements selected as reinforcements are usually the lower atomic numbered elements of groups 2, 13, 14, 15, and 16 (i.e. Ba, Mg, B, Al, Si, C, N, O)because of the desirable engineering traits of light weight, high strength, and stiffness. These elements, or compounds of these elements, can be made into various kinds of reinforcements. The stable covalent bonding contributes to the strength and stiffness of the compounds.

Some of the common reinforcing fibers include:

• carbon or graphite (C)-high stiffness, strength, and cost-unidirectional
• glass (Si02 )-amorphous, isotropic, low cost-often used with polymers
• organic fibers-high stiffness, strength, low cost
• silicon carbide fibers (SiC)-high temperature stability-used with metals and ceramics
• boron fibers (B)-used to reinforce metals
• metallic fibers (Mg, Ti)-used with metal matrices

Types of reinforcements:

Continuous Fibers

Continuous fibers are long strands of fibers having a small cross sectional area. The fibers are placed in bundles containing between 1,000 to 12,000 fibers each depending on the parameters of the load the composite must carry. The bundles are then placed in the matrix in geometrical patterns. If aligned in a single direction, the resulting composite will have anisotropic properties. Anisotropic materials have different properties in different directions. When a load is applied in the direction of the fibers, the fibers become the principle load carrying constituent of the composite. Anisotropic composites are extremely strong in the fiber direction but are generally weak in a direction perpendicular to the fiber.

Short Fibers / Whiskers Discontinuous

discontinuous (short) fibers (0.3 cm or less), which are placed randomly into a matrix. The advantage of discontinuous fibers lies in the fact that the resulting composite tends to be more isotropic than continuous fibers and they are generally easier for industry to fabricate particularly for complex irregularly-shaped component geometries. Whiskers can also be used as reinforcement. These are very short fibers (less than 1 mm in length), usually of silicon carbide (SiC). Whiskers are used to make brittle ceramic matrixes more crack -resistant and to increase the specific stiffness and strength of a metal. Although whiskers don't have the same reinforcing capability as continuous fibers, composites with whiskers are easy to fabricate.

Particulates

Particle composites contain ceramic (or metal) powders with very small particle dimensions. Particles are not commonly used for fracture resistance. They are used when improved performance is desired at elevated temperatures. Particle composites reduce friction, increase wear resistance, increase abrasion resistance, improve machinability, increase surface hardness, and reduce shrinkage. Their main advantage is that they are extremely cost effective, since lay-up of fibers is not required.