# Comprehensive Study of the Mechanical and Tribological Properties of NiCr-Al Detonation Coatings
## 3 Results and Discussion  
### Microstructure and Phase Analysis  
Figure 6 presents the results of X-ray phase analysis of homogeneous (HC) and gradient (GC) structured NiCr-Al coatings obtained by the detonation spraying (DS) method. The HC coating consists primarily of the CrNi\({}_{3}\) phase, and no Al phases were detected due to the high-temperature conditions of the spraying process. In contrast, the GC sample contains both CrNi\({}_{3}\) and Al phases. The formation of the Al phase is attributed to the reduction in the filling volume of the explosive gas mixture in the detonation barrel during the coating process [41]. This demonstrates that the phase composition of the coating structure obtained by the DS method can be flexibly controlled by adjusting the spraying parameters. These results differ significantly from coatings obtained by plasma spraying techniques such as atmospheric plasma spray (APS) and high velocity oxy-fuel (HVOF). For example, [PERSON] et al. [52] reported that in two-layer NiCr/Al coatings obtained via APS, only Al phases were initially identified, while the XRD signals from the underlying NiCr layer were weak due to the overlying 20 \(\upmu\)m Al layer. This indicates that the Al topcoat formed a continuous but non-uniform structure. In the case of NiCr-Al coatings produced by HVOF, XRD analysis revealed that nickel (\(\gamma\)-phase) was dominant, while the diffraction peaks for Cr and Al were relatively weak [53]. In this context, the advantage of the DS method for producing gradient NiCr-Al coatings lies in its ability to control the phase composition--particularly the formation of Al phases--by modifying the spraying parameters. This allows for targeted manipulation of the elemental distribution and phase relationships across the surface and subsurface layers of the coating. The mechanical and tribological properties of the coating, as well as its phase composition and morphology (oxide, porous, and metallic lamellae), as seen in [54], altered the wear performance of the coating by manipulating its microstructure. Figure 7 presents the cross-sectional images of the homogeneous (HC) and gradient (GC) structures of the NiCr-Al coating. By gradually decreasing the detonation energy (from 50% to 25%), the amount of aluminum in the surface layer increased, forming a GC structure with an upward gradient in Al content from the substrate toward the coating surface. As a result, the coating exhibited a high density and low porosity (<1%) (Figure 7a). In contrast, it was found that the HC coating, obtained by filling the detonation barrel with explosive gases at 50%, contained a relatively low amount of aluminum. Consequently, the insufficient presence of molten Al between CrNi\({}_{3}\) particles limited the formation of a dense matrix, leading to an NiCr-Al coating with relatively higher porosity (2.22%) (Figure 7b). These results were compared with NiCr-Al coatings obtained using other spraying methods. For example, in coatings produced by the high-velocity oxy-fuel (HVOF) method, the particles are deposited onto the substrate in a semi-molten state at high velocities, forming a dense, layered structure through intense plastic deformation. Such coatings typically have low porosity levels (<1.7%), with aluminum predominantly distributed along the splat boundaries. Moreover, oxidation during HVOF spraying is limited due to the short in-flight time of the particles and the relatively low oxidizing potential of the environment [53]. In contrast, NiCr-Al coatings produced by the atmospheric plasma spray (APS) technique involve fully molten particles that impact the substrate at lower velocities. This results in coatings that are generally more porous, contain more oxides, and tend to form intermetallic phases (NiAl, Ni\({}_{2}\)Al\({}_{3}\)) as well as internal oxides (Al\({}_{2}\)O\({}_{3}\), Cr\({}_{2}\)O\({}_{3}\)) [52]. The main advantage of the detonation spraying (DS) method lies in the ability to tailor the coating structure by adjusting the fill ratio of the detonation barrel.