# Several Aspects of Application of Nanodiamonds as Reinforcements for Metal Matrix Composites
## 1 Introduction  
Carbon nanoparticles, including nanodiamonds (NDs), evoke considerable interest among researchers because of their unique properties [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12]. The size of primary ND particles is generally 4-6 nm. Nevertheless, depending on synthesis conditions, particles of both smaller (up to 2 nm) and larger (up to 10 nm and more) [1] sizes also occur. A peculiarity of ND powders, which is valid for any powdery nanomaterials, is their agglomeration, i.e., their consolidation into larger aggregates. Aggregation is an issue typical for detonation nanodiamonds. ND agglomerates are divided into primary and secondary: (i) primary ND agglomerates (up to 100 nm) are constituted by primary ND particles with the creation of coherent and incoherent boundaries with C-C bonds (basically) between primary nanoparticles (created spontaneously under explosion conditions); (ii) secondary ND agglomerates are formed through the creation of bonds between oxygen-containing surface groups and Van der Waals forces during ND purification stages. The deagglomeration of detonation nanodiamonds was investigated by several groups of researchers [13; 14; 15].  
It was ascertained shortly after discovering NDs that heating in an oxygen-free environment to temperatures above 1000 \({}^{\circ}\)C resulted in NDs transforming into onion-like carbon nanoparticles (carbon onions), substantially being a graphitic material [16; 17; 18; 19; 20; 21; 22; 23]. Furthermore, the mechanism leading to the sp\({}^{3}\) to sp\({}^{2}\) transformation at the ND surface was previously reported [22].  
The use of carbon nanoparticles as reinforcements in metal matrix composites (MMC) is one of their future applications [24; 25; 26]. The presence of ND agglomerates inside the metal matrix can lead to the decomposition of material under complicated loading becausethe strength of agglomerates is weaker than the strength of the metal matrix. This is why it is necessary to develop a non-agglomerated state of nanodiamond-reinforcing particles inside the metal matrix. The most effective method for destroying agglomerates inside the metal matrix is mechanical alloying [27], i.e., treating composite components by balls (milling instruments) in a planetary mill. The purpose of the present study is to show a possibility of the complete crushing of ND agglomerates inside a metal matrix (to obtain a non-agglomerated distribution of nanodiamond-reinforcing particles inside the metal matrix in MMC) and check the workability of the developed material. This means that the size of the reinforcements will become a parameter for nanodiamonds after synthesis, namely 4-6 nm.  
The following question appears: is it possible to decrease the size of these reinforcing particles? It is well known that during heating by the electron beam in the transmission electron microscope, nanodiamond particles slowly, layer by layer (or small block by small block), transform into carbon onion. Based on this information, the hypothesis has appeared concerning the possibility of decreasing the nanodiamond size by partially annealing and purification from graphitic layers. However, temperature conditions in a microscope and the furnace are different: in the microscope, a particle receives energy from one side and loses it from the other side, but in the furnace, particles receive energy from all sides. In agglomerated state (in which the particles are used for annealing), the energetic state of particles on the surface of agglomerates and in central zones are different (different free surface and surface energy): it is the reason for a different way of transforming nanodiamonds from the surface and from deeper layers of agglomerates. Part of this paper is about the effect of the agglomerated state of NDs on such transformation processes.