# State of the Art in Metal Matrix Composites Research: A Bibliometric Analysis
## 1 Introduction
### Metal Matrix Composites  
Metal matrix composites refer to a class of new age materials consisting of base metal alloys primarily reinforced with ceramic particles for enhanced functional properties. Figure 1 shows a pictorial representation of metal matrix composite structure reinforced with (a) single and (b) multiple types of particles. The composites with multiple types of reinforcement particles are called hybrid metal matrix composites. Composites with nano sized inclusions are termed nano metal matrix composites, or simply, nano-composites. Hybrid metal matrix composites can also consist combinations of metal matrices in the form of alloys such as aluminum-magnesium, aluminum-copper and titanium based compositions [7]. These materials offer superior mechanical characteristics as compared to monolithic alloys. These characteristics include strength to weight ratio, stiffness to weight ratio, wear resistance, corrosion resistance, strength, hardness and more [8]. However, these materials are difficult to prepare and to machine. One of the challenges involving these materials is to obtain uniform dispersion of the reinforcement particles in the base alloy matrix. Hence, researchers have explored various processing techniques such as stir casting and powder metallurgy to ensure the same [9]. Investigators have also studied various machining methods such as turning, milling, drilling and grinding to process these materials efficiently [10]. Others have tried different simulation and control methods towards accurate predictive modeling [11; 12] and automation of the MMC machining process outcomes such as surface  
Figure 1: Pictorial representations of a metal matrix composite structure with (**a**) single and (**b**) multiple types of reinforcement particles. roughness [13; 14; 15; 16], tool chip interface friction [17] and more. Aluminium is a commonly preferred base metal alloy for most MMC compositions because of its good strength, light weight machinability and aerospace applications [18; 19; 20; 21]. Common reinforcement materials include silicon carbide, boron carbide, alumina and others. Recently, nano scale reinforcement particles such as carbon nanotubes and graphene are also becoming popular among MMC researchers [13]. Metal matrix composites exhibit very unique strengthening and fracture mechanisms that make them difficult to machine. Researchers have conducted detailed experiments to understand the behavior of MMCs under quasi static as well as high strain rate processing conditions [22; 23; 24]. For instance, particle reinforcements tend to enhance the sensitivity of strain hardening to strain rate effects of machining [25], resulting in higher strain hardening rates at smaller increments in machining speeds. The hard ceramic reinforcements tend to generate higher tool wear during machining of composites [26]. Higher volume fractions of reinforcements lead to greater tool wear and damage [27]. These enhanced tool wear modes include edge chipping, grooving and notch wear [28; 29; 30]. The tool wear modes are generated during composite machining include two body and three body abrasion mechanisms caused by the mutual rubbing of the hard ceramic particles with the work surface and the tool flank face [31; 32; 33; 34]. The abrasions between the ceramic particles and the work/tool surfaces is enhanced because of completely/partially pulled out, debonded and fractured particles leading to greater tool wear [35; 36; 37; 38]. Additionally, the particle reinforcements improve the yield strength of the base metal alloy leading to high cutting force generation during machining. Alloy yield strength gets enhanced due to the numerous dislocations created by the particle inclusions in the matrix microstructure [26; 39; 40; 41; 42; 43; 44; 45; 46]. During machining, the dislodged and broken ceramic particles plough through the machined surface to generate higher machined surface roughness as compared to monolithic alloys [47; 48; 49; 50].