Composite Materials

The history of composite materials dates back to antiquity; however, the term "composite material" itself did not emerge until the 1940s. Materials such as straw- or wheat-stalk-reinforced clay—which have been utilized from ancient times to the present day—may be classified as *ancient composite materials*.

Reinforced concrete, which has been in use for over a century, is in fact a quintessential example of a composite material and may be termed an *early composite material*. Driven by the demands of the aviation industry, glass-fiber-reinforced unsaturated polyester resin (commonly known as "fiberglass") was subsequently developed. Characterized by its light weight and high strength, this material may be classified as a *modern composite material*; its defining characteristics can be summarized as being both light and strong. Following the 1960s, a succession of high-strength, high-modulus fibers—such as carbon fibers, graphite fibers, and boron fibers—were developed. Composite materials reinforced by these fibers possess not only light weight and high strength but also high stiffness (modulus); these are classified as *advanced composite materials*, and their defining characteristics can be summarized as being light, strong, and stiff. The 1970s witnessed the emergence of aramid fibers and silicon carbide fibers. These high-strength, high-modulus fibers can be combined with various matrix materials—including non-metallic matrices such as synthetic resins, carbon, graphite, ceramics, and rubber, as well as metallic matrices such as aluminum, magnesium, and titanium—to form composite materials with distinct and unique properties. These materials are characterized not only by their light weight, high strength, and high modulus but also by additional functionalities—such as electromagnetic wave transparency, stealth capabilities, and high-temperature resistance. They are classified as *multifunctional composite materials*, and their defining characteristics can be summarized as light weight, high strength, high modulus, and multifunctionality. Consequently, a "composite material" is defined as a multiphase new material system fabricated by humans using specific processing techniques to combine two or more constituent materials, resulting in comprehensive performance properties that surpass those of the individual constituent materials. The constituent material that serves the reinforcing function is generally referred to as the *reinforcing phase*, while the constituent material being reinforced is referred to as the *matrix phase*. Generally defined, composite materials must satisfy the following conditions:

1) Composite materials must be man-made—materials designed and manufactured by humans to meet specific needs;

2) Composite materials must consist of two or more constituent materials possessing distinct chemical and physical properties, combined in a specific designed form, proportion, and distribution, with distinct interfaces existing between the individual constituents;

3) They possess structural designability, allowing for the design of composite structures;

4) Composite materials not only retain the advantageous properties of their individual constituent materials but also achieve comprehensive performance capabilities—unattainable by any single constituent material alone—through the complementarity and interaction of the properties of their various components.

The definition of composite materials indicates that they are a type of mixture. Generally, composite materials can be classified into three major categories based on the composition of their matrix: First, organic polymer-matrix composites (typically resin-matrix or polymer composites), such as the aforementioned glass fiber-reinforced unsaturated polyester resin composites; second, inorganic non-metallic matrix composites (typically ceramic-matrix composites), such as carbon fiber-reinforced silicon carbide-matrix composites and silicon carbide fiber-reinforced silicon carbide-matrix composites; and third, metal-matrix composites (typically aluminum-matrix composites), such as carbon fiber-reinforced aluminum-matrix composites and silicon carbide fiber-reinforced aluminum-matrix composites.

Taking into account certain unique characteristics of carbon-based materials and their composite forms, some scholars propose adding two additional categories to the three mentioned above: one category is carbon-matrix composites, such as carbon fiber-reinforced carbon-matrix composites;

The other category consists of hybrid or super-hybrid composites—the former exemplified by carbon fiber- and aramid fiber-reinforced epoxy resin composites, and the latter by aramid fiber-reinforced aluminum foil composites.

It is particularly worth noting here that, with the exception of organic polymer-matrix composites, advanced composites—such as inorganic non-metallic composites and metal-matrix composites—typically require processing and manufacturing in high-temperature environments reaching hundreds or even up to two thousand degrees Celsius; furthermore, they are generally suitable for application in operational environments characterized by high temperatures ranging from hundreds to thousands of degrees Celsius.