**3. Results**

*3.1. X-ray Diffraction*

> XRD patterns of the Ni–P and Ni–P/MWCNT composite coatings are shown in Figure 1.

**Figure 1.** XRD patterns of the Ni–P and Ni–P/MWCNT coatings.

The presence of Ni3P crystallites is clearly observed in the pattern of the conventional Ni–P film. These species are typical of electroless nickel coatings and are formed during annealing [39,40]. Strong reflections of the Ni (100) and (200) crystalline planes are also observed. The crystalline character of the composite coatings was affected by the carbon nanotubes. The strong Ni (200) reflection of the conventional Ni–P coating was markedly decreased for the Ni–P–CNT coatings, whereas the Ni (100) became the preferential orientation. Furthermore, the CNT-1.0 presented a steep reduction in the intensity of its diffraction peaks, indicating that the coating became less crystalline when CNT concentration was increased. This effect was observed by other authors [41], being attributed to a distortion of the crystalline nickel matrix by the incorporation of CNT particles.

#### *3.2. SURFACE and Cross-Section Morphology*

The morphology of the conventional Ni–P and Ni–P/MWCNT composite coatings was examined by scanning electron microscopy (SEM). Figure 2 shows the SEM micrographs (secondary electrons mode) of the top surfaces.

The binary Ni–P film displays a typical nodular morphology (Figure 2A). The nodular structure is associated with nucleation and growth of the deposit during electroless plating. A high nucleation rate is reported to enhance the number of nodules during deposition [42].

By adding the CNT filler into the plating, the nodules size became finer. The nodular morphology is less clearly perceived for CNT-0.25 (Figure 2B) due to their small size; this effect is enhanced for the coatings obtained in the baths with higher CNT loadings (Figure 2C,D). In this respect, the CNT particles tend to increase the nucleation rate during electroless deposition, as previously observed by Xu et al. [39]. As a consequence, the coating surface assumes a more compact character which, in turn, can affect the corrosion resistance in a positive way.

**Figure2.**SEMmicrographsofthetopsurfacesofthenickel-basedcoatings:(**A**)Ni–P;(**B**)CNT-0.25;(**C**)CNT-0.5;(**D**)CNT-1.0.

Coating thickness was examined by SEM/EDS analyses of the cross-sections. Figure 3 shows the results obtained for the CNT-0.25 film. The results for the other samples are very similar (not shown) and are provided as supplementary files.

The interface between film and substrate can be clearly distinguished by examining Figure 3B (Ni mapping), Figure 3C (P mapping), and Figure 3D (Fe mapping). Coating thickness was approximately 4.5 μm. The coating follows a continuous interface with the substrate with no signs of broken-off sites, suggesting it is well-adhered. Similar features were observed for the other Ni–P/MWCNT films (Supplementary Materials— Figure S1). Electroless nickel coatings are reported to present good adhesion to metallic substrates [40]. The results obtained in the present work point towards this direction.

Additional characterization of the adhesion properties of the composite coatings was undertaken by scratch tests. The results are shown in Section 3.3.

**Figure 3.** EDS mapping of the cross-sections for CNT-0.25: (**A**) SEM micrograph; (**B**) Ni; (**C**) P; (**D**) Fe.
