Metal Matrix Composites

Conventional monolithic materials have limitations in achieving good combination of strength, stiffness, toughness and density. To overcome these shortcomings and to meet the ever increasing demand of modern day technology, composites are most promising materials of recent interest. A composite is a structural material, which consist of combining two or more constituents in order to obtain a combination of properties that cannot be achieved with any of the constituents acting alone. The constituents are combined at a macroscopic level and or not soluble in each other. The constituents as well as the interface between them are recognizable and it is the behavior and properties of the interface that generally control the properties of the composite. The main difference between composite and an alloy is, in composites constituents materials are insoluble in each other and the individual constituents retain those properties, where as in alloys constituents materials are soluble in each other and form a new material which has different properties from their constituents.

Applications of metal matrix composites in defense, aerospace and light vehicles have been reported by Rittner (2001). She has concluded that the scope for MMC in all the above areas were optimistic and suggested further improvement in processes, selection of alloy, selection of reinforcement and selection of components to reduce the cost of end product. Robert (2001) has presented various forms of aluminium alloys and their applications. Based on his survey on the growth of aluminium alloys, he concluded that 32.2 % of the aluminum was consumed in transport industry in different forms. Foltz and Charles (1991) have presented various matrix alloys, reinforcements and their applications in space, defense, automotive and electronic packaging. They also presented the possible applications of MMCs in making automotive components like pistons, cylinder sleeve, connecting rod and brake disc

Of late, the automobile industry has been facing substantial technical challenges as it seeks to improve fuel economy, reduce vehicle emissions and enhance performance. It is important to reduce the overall weight of the vehicle for improving the fuel economy. Since the brake drum represents the unsprung rotating masses, the reduction in their weight is essential to increase the vehicle dynamics and acceleration. The possibility to substitute alternate materials, in order to improve brake performance and to reduce weight has made the development of advanced materials. Increased traffic density in cities requires brakes with more energy absorbing capability at more frequent intervals. Increased speed of automobiles with a demand of fuel economy, vehicle comfort and cost reduction envisages the suitable selection of materials for brake drums. Thus the need arises for searching a suitable material and designing comparatively smaller and light weight brakes.

Guner et al. (2004) have developed a cost effective approach to investigate brake system parameters. They have Investigated the dynamic and thermal behavior of the braking phenomenon by establishing a dynamic model. Roberts and Day (2000) have established a new systematic approach to design-evaluation-test product development cycle wherein the vehicle design and simulation environments are integrated. Limpert (1999) has developed models for determining the temperature rise during single stop, continuous and repeated braking. Ramachandra Rao et al. (1993) have simulated the temperature distribution and brake torque developed in a brake drum using finite element methods. They have observed a good agreement between the simulated results and the observations carried out using an inertia dynamometer. Ilinca et al. (2001) have determined the effect of contact area on the temperature distribution in brake drum which results in wear and deformation of brake drum.

The wear and friction measurements have been proposed by various researchers (Anderson et al. 1984). Deuis et al. (1997) have presented a review on the dry sliding wear of aluminium composites. The friction and wear mechanisms of aluminium composites and the influence of applied load, sliding speed, wearing surface hardness, model for wear volume and the role of reinforcement phase are also have been presented. Tjong et al. (1997) have conducted wear tests on compo- cast aluminum silicon alloys reinforced with low volume fraction of SiC. Based on the wear test conducted on the block on ring, they have concluded that the addition low volume fraction of SiC particles is an effective way of increasing the wear resistance of the matrix alloy. Kwok and Lim (1999) have investigated the friction and wear behavior of four Al/SiC_p composites over a wide range of sliding conditions by the use of a specially adapted high speed tester of the pin on disc configuration.

Metal Matrix Composites (MMC) owing to their increased specific strength and stiffness are replacing the conventionally used materials especially in the fields of automobile, aerospace and structural engineering. Due to their low cost, ceramic particles reinforced aluminium alloy are most popular among the MMC. The ceramic particles or reinforcements such as SiC and alumina make machining of composite tougher. This difficulty in machining opens a wide area of research in the processing of these materials (Teti 2002).