Structure of Composite Materials and Its Influencing Factors
What is the structure of composite materials?
Composite is composed of two or more different phases (matrix phase and dispersed phase) , and its bulk properties are significantly different from those of any one component. The structure of composite material determines its performance to a great extent. The structural factors that affect the performance of composite material are as follows:
The bonding strength between the dispersed phase and the Matrix Interface;
The shape of the dispersed phase (particle, fiber, Laminate) ;
Orientation of the dispersed phase (random or oriented).
interface integration
The good adhesion between the Matrix phase and the dispersed phase can transfer the load to the material and transfer it to the dispersed phase through the interface. Therefore, interfacial adhesion is necessary to achieve high mechanical properties of composites.
There are three types of combination between the Matrix phase and the dispersed phase:
Direct bonding without the Middle Layer. In this case, adhesion ("wetting") can be provided by covalent bonds or Johannes Diderik van der Waals forces.
The middle layer is composed of Matrix and dispersed phase in the form of solid and solution.An intermediate layer in the form of a third bonding phase (adhesive) .
2.shape and orientation of dispersed phase
particulate composites - The granular composite is composed of Matrix reinforced by dispersed phase in granular form. The effect of dispersed particles on the properties of composites depends on the particle size.
Fine particles (less than 0.25 microns in diameter) are finely dispersed in the Matrix, preventing dislocation movement and material deformation, a strengthening similar to precipitation hardening. In contrast to precipitation hardening, precipitation hardening disappears at elevated temperatures when the precipitation particles dissolve in the Matrix, while the dispersed phase of particulate composites (ceramic particles) is usually stable at elevated temperatures, thus retaining the strengthening effect. Many particulate composites are mainly used in the high temperature field.
Larger dispersed particles have a lower strengthening effect, but they can share the load applied to the material, resulting in an increase in stiffness and a decrease in ductility. Hard particles dispersed in a softer matrix increase wear resistance. Soft dispersible particles in a harder matrix improve machinability (lead particles in steel or copper) and reduce the coefficient of friction (lead in tin or copper in aluminum) .
Composites with high conductivity Matrix (copper, silver) and refractory dispersed phase (tungsten, Molybdenum) can be used in high temperature electrical applications. When the dispersed phases of these materials are composed of two-dimensional flat sheets (sheets) placed parallel to each other, the materials exhibit anisotropy. In the case where the sheet is parallel to a particular plane orientation, the material exhibits the same properties in all directions parallel to the plane and different properties perpendicular to the plane.
Fiber composite - The dispersed phase in the form of fiber (fiber composites) improves the strength, stiffness and fracture toughness of the material and prevents the crack propagation in the direction perpendicular to the fiber. The effect of increased strength becomes more pronounced when the fibers are aligned in a particular direction (preferred orientation) and apply stress in the same direction. Long Fiber (continuous fiber) reinforced composite material than short fiber (non-continuous fiber) reinforced composite material reinforcement effect is significant. The short fiber reinforced composite is composed of discrete phase reinforced Matrix (length < 100 * Diameter) in the form of discontinuous fiber, and its load-sharing capacity is limited.
The external load of continuous fiber reinforced composites is mainly borne by dispersed phase-fiber. The Matrix in this material is used only as an adhesive for the fiber to maintain its desired shape and to protect it from mechanical or chemical damage.
Laminated composite - The laminated composite material consists of layers with different anisotropic orientations or Lamellar substrates reinforced by dispersed phases. When a fiber-reinforced composite consists of several layers with different fiber orientations, it is called a multilayer (angle layer) composite.
The laminated composite provides enhanced mechanical strength in two directions perpendicular to the preferred orientation of the fiber or sheet and in only one direction, and the mechanical properties of the material are low.