## 3 Results and Discussion
### Characterization of the Mechanical Behavior  
Figure 7 demonstrates nanoindentation load-depth curves that were conducted with a peak force of 200 mN of as-plated Ni-P, Ni-P/SiC nanocomposite coatings, and those heat-treated at 400 \({}^{\circ}\)C. The diagram corresponds to the plot of the moving average values (from 20 measurements) of the indentation load and the depth. There were not any visible cracks formed during the nanoindentation, since there are no discontinuities that were noticed by the loading-unloading curves. The maximum indentation depths at the peak load were varied from around 1.1 to 1.5 \(\upmu\)m. The results clearly show the stiffening effect that SiC introduces into the Ni-P. Furthermore, there is an obvious stiffening effect between the heat-treated and non-heat treated coatings. The lowest values of indentation depths were  
Figure 5: Average surface roughness and root mean square (RMS) roughness of the produced coatings.  
detected for the heat-treated Ni-P/SiC nanocomposites, while the highest depths were observed for the non-heat treated Ni-P coating.  
The hardness (H) and the elastic modulus (E) of as-plated Ni-P, Ni-P/SiC, and heat-treated coatings were calculated from the load-displacement curves. Figure 8 demonstrates the calculated mechanical properties, as elastic moduli and hardness, for all of the under study samples. The as-plated Ni-P coating shows the lowest hardness (Figure 8a). The incorporation of SiC nanoparticles increased the hardness of Ni-P, from 4.7, up to 6.9 GPa. The heat-treated coatings exhibited a higher average hardness, 8.9 and 9.6 GPa. The hardness increase is attributed to the formation of a hard Ni\({}_{3}\)P phase,  
Figure 6: Typical two-dimensional (2D) and three-dimensional (3D) surface reconstructions for (**a**) non treated Ni-P/SiC and (**b**) heat treated Ni-P/SiC coatings.  
Figure 7: Nanoindentation load–depth curves of as-plated Ni-P, Ni-P/SiC nanocomposites, and the heat treated coatings.  
as seen from the XRD results and as reported elsewhere [51]. In the case of the elastic modulus as shown in Figure 8b the effect of SiC codeposition is higher than heat treatment as Ni/SiC has higher elastic modulus values than both as-plated and heat treated Ni-P.  
The data that are reported in Figure 8 are in a good agreement with the results of other experimental efforts, since the literature [3; 47; 48; 49; 50; 51; 52] reports values for the as plated, untreated condition of plain Ni-P coatings for hardness, ranging from 4 to 8 GPa, depending mainly on the P content and the processing parameters that were adopted. These values can rise to 12 GPa after heat treatment depending also on the P content and the temperature of annealing. When various reinforcing particles are introduced, the as-plated values seem to lay between 6 and 10 GPa, depending on particle type, morphology, and content and processing parameters. Subsequent heat treatments raise these values up 12 GPa, additionally depending on the temperature of the process.  
As far as the modulus of elasticity is concerned, the values for the as-plated Ni-P coatings of 100-150 GPa are reported and can be slightly increased after heat treatment [49; 50; 51; 52]. P content and processing parameters are also the key factors that affect the values of elastic modulus. The values of the present effort are somehow lower, which is most likely associated with the P content and the different synthesis route adopted. The introduction reinforcing particles leads to elastic modulus values of 150-200 GPa for the as plated condition that may rise to 220 GPa after heat treatment [47; 48; 49; 50; 51; 52; 57]. As in the case of the plain Ni-P coatings, in the present effort the values are lower, most likely due to same reasons, as previously stated. However, the tendency that the particle insertion improves elastic modulus persists.