# Several Aspects of Application of Nanodiamonds as Reinforcements for Metal Matrix Composites
## 3 Results and Discussion  
Mechanical alloying can lead to the fragmentation of big nanodiamond agglomerates up to single nanoparticles inside the metal matrix after treatment for a long time. However, in a big volume fraction of nano reinforcements, this fragmentation is complicated: it can be executed only up to nanogelomerates. Figure 2a demonstrates ND nanoagglomerates on a cross-section of \"aluminium + 25%vol. nanodiamonds\" composite granule (the cross-section was produced by ion beam). It is necessary to create additional stresses and micro deformations around these nanoagglomerates. The addition of a second metal in the matrix leads to the formation of new phases with different densities; this results in changes in material volume and the appearance of additional stresses and deformations. As a result, nanoagglomerates can be destroyed. For a demonstration of this scheme, at first, the composite \"Al + 30%vol. ND\" was prepared by mechanical alloying for two hours. After, the copper powder was added to the grinding jar, and a new mixture was also treated for two hours. Figure 2b shows an obtained non-agglomerated distribution of ND reinforcements inside a metal matrix. An XRD study determined the appearance of intermetallic phases in the matrix (Figure 3): CuAl\({}_{2}\) (density D = 4.37 g/cm\({}^{3}\)); Cu\({}_{4}\)Al  
Figure 1: Nanodiamonds of detonation synthesis: (**a**) primary nanodiamond particle without crystalline defects; (**b,c**) nanodiamonds with twins; and (**d**) agglomeration of nanodiamonds. Figure 3: X-ray diffraction patterns from the composite with the matrix from Al and Cu with nanodiamond reinforcements; as shown by the arrows. Figure 2: Nanodiamonds inside metal matrix: (**a**) nanoagglomerates; and (**b**) non-agglomerated distribution of nanodiamond-reinforcing particles in metal matrix from aluminium and copper. The non-agglomerated state of nanodiamond particles leads to a change of properties of the material. A vital change is the cardinally decreasing start temperature of the reaction between the aluminium matrix and diamond nano reinforcements with the formation of aluminium carbide Al\({}_{4}\)C\({}_{3}\): for macro-materials (bulk and big pieces), the start temperature is 1200 \({}^{\circ}\)C [30], for micro-materials (powders) it is 750 \({}^{\circ}\)C [31], but for nanodiamonds, this starting temperature is 450 \({}^{\circ}\)C as it is demonstrated by a DSC study (Figure 4). The investigation shows a possibility of obtaining the non-agglomerated distribution of nanodiamond reinforcements inside the metal matrix, i.e., the size of nano reinforcements equals approximately 4-6 nm. Furthermore, to determine the possibility of reducing the size of agglomerated nanodiamonds by annealing and cleaning from graphitic material, the process of ND transformation into carbon onions during annealing in a vacuum was studied. It is necessary to say that there are two hypotheses (or two variants) about the process by which nanodiamonds transform into carbon onions during annealing. According to the first [18], all ND particles simultaneously undergo a transformation, layer by layer. This hypothesis states that each ND particle consists of a diamond core coated with a graphitic material while in an intermediate state. According to the second hypothesis [19], while in the agglomerated state, the nanoparticles in the surface layers of ND agglomerates undergo a complete and rapid transformation first, followed by the transformation of nanoparticles in their deeper layers. While in its intermediate state, each ND agglomerate has a core of diamond nanoparticles and completely transformed carbon onions on the agglomerate's surface. The principal difference between these hypotheses is explained below. We suppose that the transformation of diamond nanopowders takes place according to the first scheme. In this case, it is possible to reduce the size of the nanoparticles by cleaning intermediately annealed nanodiamond powders of the graphitic substance.