**1. Introduction**

Combinations of two or more physically and chemically distinct materials that results in improved properties compared to individual materials are termed as "composites." Due to the adequate combination of such distinct materials, the properties of composites can be enhanced due to the presence of various materials. Composite materials bring the additional strength, stiffness apart from reducing the overall density compared to monolithic and nonaggregates allowing considerable reduction in the weight. Additionally, composites possess unidirectional properties (specific tensile strength) and increased fatigue endurance. The reinforcement plays an important role by providing the additional strength, stiffness and tribological properties in the composite. In many cases, the important properties of the reinforcement are strength, hardness and stiffness which are normally higher than the matrix

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and thermal expansion, MMCs are very good candidate for precision machinery, lasers and electronic packaging. In order to make use of MMCs in all the production areas and commercially attractive, prioritized research and development should be made on highly reliable manufacturing process with lowering the processing costs. Also, advanced processing techniques in powder metallurgy, plasma spraying, liquid metal infiltration, innovative casting methods and innovative combination of advanced reinforcing materials is needed very much. Further, functionalization and coating on the matrix/reinforcement are very much necessary which can

Processing of Graphene/CNT-Metal Powder http://dx.doi.org/10.5772/intechopen.76897 47

prevent the deleterious chemical reaction at higher operating temperature conditions.

Powder metallurgy consists of a sequence of activities where, a feedstock in powder form μm to nm is used for fabricating the components of several shape and structures. **Figure 2** shows the general sequence of operations involved in a typical powder metallurgy production technology to obtain a finished component. Mechanical alloying, milling, electrolytic decomposition and gas atomization are the few metal powder techniques. Metal or alloy powder comes in various shapes and sizes which are dependent on the production method and parameters. Mixing of powders involves the introduction of various metal/alloy powders along with calculated quantity of reinforcement materials. Thus obtained powder mixtures are subjected to consolidation using rigid tool set comprising of die and punches. Thus obtained green compacts are sintered to make the particle bonding, (**Figure 3**) enhance the strength and the integrity which is usually done in protective atmosphere. The powder metallurgy process exists for the past 100 years, over the past years it has become a superior method to produce high-quality realistic industrial components with integration of novel reinforcements during

**3. Powder metallurgy process**

preprocessing stage.

**Figure 2.** Conventional powder metallurgy process.

**Figure 1.** Type of reinforcements used for the composites.

materials. Today, the most man made engineered composites includes mortar; concrete; reinforced plastics; ceramic composites and metal composites. Particulates, whiskers/short fibers are the common type of reinforcements (**Figure 1**) used successfully for the fabrication of composite. Particulates are available in platelets, spherical and various regular or irregular shapes which may be having equal geometry in all directions. The particulate reinforcement was limited to 30–40 vol% in the composite due to its brittleness and fabrication difficulties. Fiber glasses were the first modern composites and are used for sports materials, car bodies, ship and other structural applications. But, due to the advancement in the composite technology, carbon fibers replaced the glass reinforcement in the composite and were used for many expensive sporting equipment and aircrafts structures. Carbon nanotube is being used successfully in these days for making of stronger and lighter composites. Another advantage of any composite material is that their properties are tailorable to certain extent along any direction. Further, these developed composite materials have design—flexibility, close tolerance, high durable, chemical inert and corrosive resistance. Also, the innovation in the fabrication techniques and combination of advanced materials resulted in superior thermal stability, high temperature retention and outstanding electrical properties. Composite materials are used for various applications such as building blocks, structures, bridges, automobile components, race car bodies, aerospace structural materials, space crafts and more.

#### **2. Metal matrix composites (MMCs)**

Metal matrix composites (MMCs) comprises lightweight and low-density materials (aluminum, magnesium, copper, etc.) reinforced with fiber or particulate of ceramic (silicon carbide, alumina, graphite, etc.). MMCs gave the opportunity to tailor the desire properties for specific applications. The important properties of metal matrix composites are stiffness, specific strength at elevated operating temperature and high tribological performance. On the other hand, fabrication cost of MMCs found to be higher for high performance application such as space and military and conceding the ductility and toughness. Also, MMCs have wide applications and are used in jet engines, aircrafts, satellite materials, and piston materials, cutting tools and space shuttle (NASA). MMCs with high strength and specific stiffness could be used in high speed machinery tools, robots, ships and rotating shaft where weight is an important criterion. MMCs also exhibit good wear resistance with high specific strength which is favorable for brake and engine components. Further, flexibility in tailorable thermal conductivity and thermal expansion, MMCs are very good candidate for precision machinery, lasers and electronic packaging. In order to make use of MMCs in all the production areas and commercially attractive, prioritized research and development should be made on highly reliable manufacturing process with lowering the processing costs. Also, advanced processing techniques in powder metallurgy, plasma spraying, liquid metal infiltration, innovative casting methods and innovative combination of advanced reinforcing materials is needed very much. Further, functionalization and coating on the matrix/reinforcement are very much necessary which can prevent the deleterious chemical reaction at higher operating temperature conditions.
