**3. Powder metallurgy process**

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,

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

race car bodies, aerospace structural materials, space crafts and more.

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

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

46 Powder Technology

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 preprocessing stage.

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

**Figure 3.** Powder to bonded structure during sintering.

With the several advantages of powder technology it become highly sustainable processing method over many conventional metal forming methods in producing complex shape, effective raw material utilization and the high tolerance. In the recent years, powder metallurgy process has been upgraded to consolidate pores or fully dense structures. Out of several new methods like hot isostatic pressing (HIP), metal injection molding (MIM), composites produced using powder forging (PF) and metal additive manufacturing (AM) have gained much popularity. **Figure 4** shows some of the structures produced through powder metallurgy route. Most of the powder metallurgy parts include filtration systems, magnetic assemblies, automobile components and structural parts. Gears, bushes and bearings produced through powder metallurgy process exhibit the more porous but they naturally reduce the noise. Powder metallurgy is also a very feasible technique for producing parts with magnetic properties. Further, magnetism can be enhanced by varying the sintering parameters.

wear resistance, strength and rusting proof. Repressing is carried out to improve the various mechanical properties, surface roughness and physical properties. In order to improve the product performance, sintered components are heated to a certain temperature and cooled with controlled temperature. Thermomechanical treatment, chemical treatment and heat hardening are the commonly used methods. Heat treated parts exhibits the well refined grain structure, high strength and high fracture toughness. Surface heat treatment methods such as steam treatment, galvanizing, plating, etc., are performed to make pores free and dense surface. In addition, extrusion, forging, welding and special processing method are used to obtain a desired shape, improved mechanical properties and high tolerance sintered struc-

Processing of Graphene/CNT-Metal Powder http://dx.doi.org/10.5772/intechopen.76897 49

tures of the final requirement to improve the product quality and its performance.

Carbon (C) is a chemical element with atomic number 6 and having [He] 2s22p2 electron configuration [1]. It is the fourth most abundant chemical element on the earth by mass. Diamond, amorphous carbon, graphite, and fullerenes are the well-known allotropes of the carbon. **Figure 5** shows the various crystallographic physical structures of these allotropes. Diamond is well known for its high hardness, which consist of pure sp3 hybridized carbon atoms with the strong covalent bonding among carbon atoms (**Figure 5**). Diamond is frequently used as cutting and polishing tools [2]. Graphite is made up of layers of carbon atoms in a planar structure (**Figure 5b**). The carbon atoms are organized in a hexagonal lattice. Graphite is the softest structure in which the carbon atoms are sp2 hybridized and the layers are hold by van der Waals force of attraction. Graphite is mainly used in industrial lubrication purposes. Other allotropic form is the amorphous carbon. It is soot and black carbon which does not have any crystalline structure (**Figure 5**). Amorphous carbon can be used as inks, paints, and industrial rubber filler [3]. Fullerene is the fourth allotrope of carbon at nanoscale. **Figure 5d**–**f** demonstrates the structures of fullerene family members. It includes the ellipsoidal fullerenes, spherical fullerene (buckyball), cylindrical carbon nanotubes, and planar graphene. In the buckyball (C60) all the carbon atoms are arranged in the three adjacent carbon atoms

**4. Allotropes of carbon**

**Figure 4.** Complex structures produced through powder metallurgy.

#### **3.1. Advantages of powder metallurgy process**

The main advantage of powder metallurgy process is its ability to compress the powder into final size of closed dimensions and there is no need of any other subsequent forming process. Further, the process utilizes the 100% raw materials to get final component there by reducing the production cost compared to other conventional process (5–10% wastage). In the powder metallurgy process metal or alloy will not melt completely. So, there will not be any impurities by oxidation or deoxidizing or impurities from the crucible. Also, the process enables the production of high purity materials where, sintering is carried out in vacuum or gas atmosphere which will remain as the unique atmosphere throughout the process. Powder metallurgy enables the correctness of the material composition/weight/volume ratio and its homogeneity and it is suitable for mass production of same shape components.

#### **3.2. Post processing of powder metallurgy**

According to the specific densification requirements, sintered metal or alloy compacts are subjected to post processing treatments which includes impregnation, repressing, heat treatment, surface treatment and extrusion. Impregnation is performed by dipping oil or plastic into molten metal. The specific purpose of this process is to improve the self- lubrication,

**Figure 4.** Complex structures produced through powder metallurgy.

wear resistance, strength and rusting proof. Repressing is carried out to improve the various mechanical properties, surface roughness and physical properties. In order to improve the product performance, sintered components are heated to a certain temperature and cooled with controlled temperature. Thermomechanical treatment, chemical treatment and heat hardening are the commonly used methods. Heat treated parts exhibits the well refined grain structure, high strength and high fracture toughness. Surface heat treatment methods such as steam treatment, galvanizing, plating, etc., are performed to make pores free and dense surface. In addition, extrusion, forging, welding and special processing method are used to obtain a desired shape, improved mechanical properties and high tolerance sintered structures of the final requirement to improve the product quality and its performance.
