2.4 Matrix-Based Composites
The matrix serves two main purposes in composites which includes (a) binding the reinforcement phases in place and (b) uniformly distributing the stresses among the constituent reinforcement materials in the event of an applied force. The matrix offer weight advantages and ease of handling. The metals, polymers and inorganic materials possesses high elastic strength and exhibit minimum failure strain, when loaded in tension and compression [5]. The fabrication method is selected considering the matrix properties and its related effect on the properties of reinforcements. Chemical inertness and non-reactivity is the primary condition for the processing of composites. The various type of matrices materials are illustrated in Figure 2.2.
2.4.1 Polymer Matrix Materials
Polymers are the materials of choice for their easier fabrication, lightweight and tunable mechanical properties. Two types of polymers are available viz. thermosets and thermoplastics:
Figure 2.2 Classification of matrices.
2.4.1(a) Thermoplastics
Thermoplastics have both 1-D and 2-D molecular structure and they tend to soften at an elevated temperature and show exaggerated melting point [6]. This process of softening can be reversed to regain its properties during cooling, which facilitates the processing of composite materials. The presence of reinforcements can minimize the tendency of shape loosening for the composite material. The thermoplastic materials do not require any chemical reactions for their synthesis, which causes unnecessary release of gases and heat. Figure 2.3 shows different kinds of thermoplastics materials.
Thermoplastics resins are generally used as molding compounds. The fibers are randomly dispersed in thermoplastics, and so the reinforcement is isotropic but directionality can be achieved using molding processes [7]. The heat resistance in thermoplastics can be increased by the addition of filler, however, a loss in strength at elevated temperature can occur. Because of their fascinating physico-chemical properties such as toughness, rigidity, and resistance to creep, thermoplastics have evolved as a material of choice in the fabrication of automotive control panels and electronic products encasement. The reinforced thermoplastics are readily available and can be easily transformed into desired shape, thereby facilitating the fabrication of bulky components [8].
Figure 2.3 Types of thermoplastics.
2.4.1(b) Thermosets
Thermosets are 3-D molecular structure and which tends to decompose on hardening. The composition of resins can leads to a change in their properties. To induce flexibility, the thermosets are retained for long period of time in a partially cured condition [9]. Generally, the condition of fiber material (chopped, aspect ratio) in resins decides the final application of thermosets. The thermosets are generally fabricated direct condensation polymerization followed by rearrangement reactions to form heterocyclic entities. The water as reactant product hinders the development of voidfree composites [10]. Figure 2.4 represents important kinds of thermosets:
2.4.2 Metal Matrix Materials
Metal matrices materials are known for their high stiffness, toughness, strength and their ability to withstand elevated temperature in corrosive environment. Most of the metals and alloys can be used as matrices, however, they often require compatible reinforcement materials which are stable over a range of temperature and also non-reactive [11]. Light metals form the matrix for temperature application and the reinforcements, besides the aforementioned reasons are characterized by high module. The high strength in metallic matrix materials can be achieved by utilizing high modulus reinforcements. In such composites materials, the strength-to-weight ratios can be higher than most alloys. The physico-chemical properties of the matrix materials determine the service temperature of the composites and so the choice of reinforcements becomes more important in application demanding higher melting temperature [12].
Figure 2.4 Classification of thermosets.
2.4.3 Ceramic Matrix Materials
Ceramics are the solid materials which exhibit very strong ionic bonding with partial covalent bonding. Ceramic-based matrix materials exhibits good corrosion resistance, high melting points, high compressive strength and stability at elevated temperatures and therefore can be utilized in high temperatures applications (~1500 °C) [13]. The addition of reinforcements in ceramic overcomes the problems related with high modulus of elasticity and low tensile strain to obtain strength improvement. The addition of reinforcements in adequate amount causes the ceramics to effectively transfer the load to them which reduces the possibilities of ceramics rupture at high stress levels [6]. However, this does not mean that the addition of high-strength fiber will result in strengthening a weaker ceramic. In such a case, the reinforcement possessing high modulus of elasticity can solve the problem. Usually, if the thermal expansion coefficient of ceramics is higher than reinforcement materials, the resultant composite may not be possessing good strength. In such a case, the tendency of forming microcracks while cooling increases and these microcracks extends from fiber to fiber within the matrix [9].
2.4.4 Carbon Matrices
Carbon based materials are generally known for high temperature withstanding properties which remain unaffected up to 2300 °C. The carboncarbon composite can be synthesized using compaction of carbon or multiple impregnations of porous frames with liquid carboniser precursors and subsequent pyrolization or through chemical vapor deposition of pyrolytic carbon [14]. These composites can retain their temperature stability upto 2400 °C along with dimensional stability and thus make them the oblivious choice in space research, military and aeronautics. The components which are regularly exposed to higher temperature and responsible for demands of high standard performance generally utilize carbon-carbon composites [15].
2.4.5 Glass Matrices
Composites with glass matrix are more reinforcement-friendly as compared to ceramics based composites. The methods of processing of polymers can be implied for glass matrices also. Composite based on glass matrix possess high strength and modulus and can withstand temperature up to 650 °C. Composites have low thermal expansion behavior with glass matrices and so are possess better dimensional properties then polymer or metal system.
2.5 Reinforcements
In composites, the reinforcements can be a whisker, fabrics particles or fibers. The whiskers possess a definite shape but are shorter in diameter and length of the fiber. The particles do not have any fixed shape or orientation whereas the fibers consist of a long axis and the two other axes in either circular