# Comprehensive Study of the Mechanical and Tribological Properties of NiCr-Al Detonation Coatings
## 3 Results and Discussion
### Mechanical Properties  
Figure 8 presents the results of adhesion strength testing of NiCr-Al coatings. The tests were carried out by bonding the coated surface to a counterface using a high-strength  
Figure 6: Results of X-ray phase structural analysis of gradient (**a**) and homogeneous (**b**) NiCr-Al coatings. Figure 7: Microstructure of the cross-section of NiCr-Al detonation coatings. (**a**) Gradient; (**b**) Homogeneous. epoxy-based adhesive (Epidian 53, rated at 70 MPa), followed by tensile pull-off testing (Figure 4). The fracture morphology (Figure 8a,d) indicates a predominantly adhesive failure, as confirmed by elemental analysis showing the dominance of Fe in the peeled regions (Figure 8c,f), implying that separation occurred at the coating-substrate interface. Interestingly, partial retention of coating elements in the GC sample (Figure 8c) suggests stronger interfacial bonding in the gradient structure. Quantitative results showed that the average adhesion strength of the gradient coating (GC) was 38.7 \(\pm\) 6.9 MPa, whereas that of the homogeneous coating (HC) was 25.4 \(\pm\) 3.1 MPa. This demonstrates a significant enhancement in adhesion due to the gradient structure, which may be attributed to the gradual variation in aluminum content, contributing to better stress distribution and interfacial compatibility. When compared to literature data, HVOF-sprayed NiCr-Al coatings typically exhibit higher adhesion strength, often in the range of 50-60 MPa, depending on substrate preparation and process parameters. For example, [PERSON] et al. reported a bond strength of approximately 59 MPa for HVOF-sprayed NiCr-Al coatings on superalloy substrates [53]. This is likely due to the extremely high particle velocities and intense plastic deformation during HVOF deposition, which promote mechanical interlocking and metallurgical bonding at the interface. In contrast, the lower adhesion strength observed in the current detonation-sprayed (DS) coatings may be related to the cyclic nature of the detonation wave. Nevertheless, the adhesion performance of the GC coating remains notable, especially when compared to conventional homogeneous structures. The results of measuring the microhardness of NiCr-Al HC and GC using the Vickers method are shown in Figure 9. The microhardness of the GC of NiCr-Al coating is approximately 30% higher compared to the HC. It can be concluded that the difference in the microhardness of the NiCr-Al coating is related to its microstructure. The porosity of the HC based on element distribution is higher compared to the GC NiCr-Al (Figure 7). Figure 10 shows the values of nanohardness (H, GPa) and elastic modulus (E, GPa) along the cross-section of different structured NiCr-Al coatings based on element distribution. The distribution of nanohardness and elastic modulus values of NiCr-Al coating shown in Figure 10a corresponds to the gradient structure based on element distribution. The microstructure of the cross-section of the NiCr-Al coating, as shown in Figure 7a, reveals that due to the high amount of Al in the surface region, the values of hardness and  
Figure 8: Gradient (**a**–**c**) and homogeneous (**d**–**f**) structured NiCr-Al coatings after adhesion strength testing: (**a,d**) surface images of the samples; (**b,e**) morphology of the coating and peeled areas; (**c,f**) results of elemental analysis. elastic modulus are low, while they increase towards the substrate due to the higher presence of the CrNi\({}_{3}\) phase. Additionally, the elastic modulus and hardness of the substrate are lower compared to the coating. According to reference data, the elastic modulus of the 12 Kh1 MF steel used as the substrate is approximately 198 GPa, and its hardness is between 2.8 and 3.0 GPa. Figure 10b shows the results of the study of the HC NiCr-Al based on element distribution. The elastic modulus and hardness are approximately two times lower compared to the GC based on element distribution.