2.5.1(h) Graphite Fibers
The graphite fibers consists of >99% carbon. They are very similar to the carbon fibers with only the difference being that carbon fibers consist of 91-94% carbon [11]. This difference arises is due to the different processing temperature for both the fibers. The poly-acrylo-nitrile (PAN)-based carbon fiber is produced at 1320 °C, while graphite fibers are graphitized at 1950 to 3000 °C. The physico-chemical properties of graphite are unaffected at elevated temperatures; yet, it can readily react with the metals during its processing, e.g. aluminium matrices produces the carbides at their interface [8]. The stiffness of the fiber increases with the graphite content. However, with the increase in stiffness, the strength decreases. The graphitic fibers are expensive and in PAN based fibers, other raw materials are equally expensive.
2.5.2 Whiskers
Whiskers are the single crystals which are grown with nearly zero defects. They are made from several materials like graphite, silicon carbide, copper, iron etc and exist in the form of short and discontinuous fibers having varied cross sections. Unlike particles, the whiskers have a definite length to width ratio (>1), which allow them to possess extraordinary strengths up to 7000 MPa. Whiskers of ceramic materials exhibit high strengths, moduli and low densities [4]. Due to their high specific strength and modulus, ceramic whiskers are excellently fit for low weight structure composites. Ceramic whiskers are more resist to temperature, mechanical damage and oxidation than metallic whiskers. However, they are more prone to damage while handling.
2.5.3 Laminar Composites
Laminar composites comprises of layers of materials bonded together and can exists in numerous combinations of layers. In laminar composites, the layers of materials occur either alternately or in an orderly fashion as per the application need [21]. Clad metals are more suitable in intensive environments which require denser faces. Different metallurgical processes such as diffusion bonding, hot pressing, roll bonding and brazing are used for processing different alloys of powder, foil, sheet or sprayed materials, wherein, the sheets and foils can be easily designed in 2-D geometry more easily than fibers [19]. The thin continuous film of pre-coated metals can be synthesized by forming a layer on a substrate by using hot dipping, chemical plating and electroplating processes.
2.5.4 Flake Composites
Flakes composites have densely packed structures and are much cheaper than fibers. When using the metal flakes in polymer matrices, electricity or heat conductivity is observed, which is not in the case of mica and glass flakes [14]. More often, the flakes fall short of expectations while controlling the size, shape and hence leads to defective product. Flakes composed of glass materials have notches and cracks around their edges, which weaken the final product. When held together in a matrix with a glue-type binder, it is difficult to align them parallel to each other thus leading to an uneven strength. Flakes offer various advantages to fibers in structural applications. Angle-plying is easier in flakes than in continuous fibers [17]. Flake composites possess higher theoretical modulus of elasticity and much cheaper to produce and be handled in small quantities than fiber reinforced composites.
2.5.5 Filled Composites
Filled composites are processed by adding the filer materials to plastic matrices which enhances the strength and reduces the weight of the composites. The filler particles can be irregular or polyhedrons, short fibers or spheres in shape and they acts as the primary or additional ingredient in a composite. The base matrix possesses networks of open pores resembling to that of a group of cells or honeycomb structures. The infiltrate binds the components like powders or fibers in the matrix [13]. The matrix is not naturally formed in the honeycomb structure, but is specifically processed to fit to a predetermined shape. They are seldom used to impart color or opacity to the composite in which they are utilized. In their role as inert additives, they can change matrix characteristic in all directions. The properties of the final composite are greatly dependent on the blend of particle types, surface treatment, shape and size of the particle in the filler material. The advantages of fillers included increase stability, stiffness, porosity, strength, thermal resistance, coefficient of thermal expansion and abrasion resistance [21].
2.5.6 Particulate Reinforced Composites
Particle reinforced composites are the microstructures of metal and ceramics composites, in which particles of one phase are deliberately strewn in one other. Reinforcement can be of the square, triangular and round shapes, wherein their side dimensions are more or less equal [22]. The dispersion size in these composites is in microns range while the volume concentration is more than 28%. The reinforcements in the particulate composites reinforce the matrix alloy by imposing a check on the motion of dislocations. The 3-D reinforcement in composites owns isotropic properties devised owing to the three systematical orthogonal planes including diameter of the particles, the inter-particle spacing, and the volume fraction of the reinforcement.
2.5.7 Cermets
Cermets are particle strengthened composites comprises of ceramic grains of oxides, carbides or borides. The grains which constitute about 20 to 85% of the total volume are dispersed in a refractory ductile metal matrix. The ceramic and metal constituents are bonded together due to the presence of mutual solutions. Power metallurgy techniques are generally needed to produce cermet structures. Their potential properties are based on the relative volumes of the metal and ceramic constituents [2]. On impregnating the porous ceramic structure with a metallic matrix binder a cermet is obtained. Cermets are used as coating in a powder form where the powder is sprayed through a gas flame and fused to a base material. Although, a wide variety of cermets are available, yet only a few are used commercially.
2.5.8 Microspheres
Microspheres are one among the most useful and widely utilized fillers. Their strength, specific gravity, stable particle size and controlled density without compromising the profitability or physical properties are their main features. Generally two types of microspheres are available:
2.5.8(a) Solid Glass Microspheres (SGM)
The SGM prepared from glass is widely utilized for reinforcing plastics. When coated with a binding agent, it bonded itself along with the sphere’s surface to the resin. This process enhances the bonding strength and removes absorption of liquids around the spheres. SGM, because of its low density influences the commercial value and weight of the finished product. Their inherent strength is transferred to the finished part to form a constituent.
2.5.8(b) Hollow Microspheres (HM)
The HM are made from silica and generally prepared at controlled specific gravity. They are larger than solid glass spheres and available in a wider range of particle sizes. Generally, the microspheres are less sensitive to moisture which reduces the attraction between particles. Hollow spheres were mostly used for thermosetting resin systems, however, strong spheres have been designed which are >5 times and >4 times respectively stronger than HM in static crush strength and times long lasting in shear [23]. The specific gravity of HM is lower than pure resin which makes them to use in lightning resin dominant compounds. Due to their weight reducing features, they are widely utilized in aerospace and automotive industries