Figure 1 shows the XRD pattern and SEM micrographs of surface and fracture surface of pretreated Si substrate. The θ–2θ scan data exhibited strong 2θ peaks at 28.40°, 46.67°, 56.25° and 76.59°, respectively, corresponding to the (111), (220), (311) and (331) peaks compared with the standard d-values taken from JCPDS (43-0144). The XRD pattern revealed that the Si substrate is a polycrystalline structure (Fig. 1a). After reactive ion etching process, the substrate surface became relatively rough and consisted of many pyramid and pit structures (Fig. 1b). It can be observed from Fig. 1c that the fracture surface was composed of nanometer- to micrometer-sized pores, and the depth of these pores was about 4–5 μm. The surface morphology of the substrate could affect the microstructure and adherence of electroless Ni–P film.
Characterization of Ni–P film
Figure 2 shows the SEM micrographs of the film surface deposited at plating temperature of 60 °C. It can be seen from a low-magnification image (Fig. 2a) that the dense surface was composed of some large particles. No micro-cracks were found on the surface. Many flower patterns and large particles were present in Fig. 2b. Furthermore, the surface of the film was dense, observed by high-magnification SEM micrograph (Fig. 2c, d). A mesoscale colony structure can be observed on the top surface of the deposit. The mesoscale colony and nodules characters were present on the top surface due to the internal stress accumulation during the depositing process (Ashassi-Sorkhabi and Rafizadeh 2004). The film was composed of agglomerated mesoscale colony and nodules, which met and coalesced each other to form a compact layer.
Figure 3 shows the SEM micrographs of the top surface of the film deposited at 70 °C. After mechanical fracture, the Ni–P film exhibited relative softness. The film could be bended to be more than 90° angle, which can be observed by visible eyes. In Fig. 3a, the film was broken due to the effect of the mechanical fracture. However, it was evidence of delamination for the film on Si substrate. There seems to be a curved film on the surface. The mechanical property of the Ni–P film deposited at 70 °C would be studied in future work. Furthermore, the surface was dense and smooth, observed by high-magnification SEM micrograph. It seems to be many small particles, but no visible micro-cracks. A mesoscale nodular structure is evidence, with clear intercolony boundaries and surface grooves. These features indicate the film growth is a Volmer-Weber growth mode. The formation of mesoscale nodules resulted from nucleation extending laterally from an existing structural defects (Wu et al. 2015). As shown in Fig. 4, the chemical composition of the Ni–P film presents a uniform distribution. Furthermore, the Ni element was still present on the under layer (Fig. 4a). However, there were no signals of P element on the map scanning image (Fig. 4b).
Some micro-cracks were present on the upper layer of the film around the edge of sample (See Fig. 3b), where release of the hydrogen bubbles caused the microcrackings formation. The under layer was composed of nano-sized globular particles. Further, the under layer was composed of a sparse particle distribution. Some regions were not completely deposited by Ni–P particles. It could be influenced by the effect of the competitive of the initial nucleation stage. A mechanism of the electroless Ni–P plating on Si substrates is the nucleation reactions and then film growth. In the initial nucleation stage, the nucleation reactions could occur in the neighborhood of the original Ni nodules to form an irregular nodular structure (see Fig. 3b). The deposition mechanism of the electroless Ni–P plating on Si substrates will be addressed in “Discussions”.
When the plating temperature was increased to 80 °C, the surface of the film became relatively rough after deposition time of 40 min, at the same time the dendritic structure was formed on the surface, and lots of bubbles were released from the electrolyte due to the release of hydrogen. Figure 5 shows the SEM micrographs of the top surface of the film deposited at 80 °C. In Fig. 5a, b, the observed surface was rough. The dendritic structure was present, which was constituted with many small globular particles. The hump and nodules characters on the surface of the deposits were obviously found. No micro-cracking on the surface was observed.
Figure 6 shows the optical images of cross section of the Ni–P films on Si substrates. At 60 °C temperature, the film was uniform and the thickness was 1.47 ± 0.12 μm. It was indicated that the deposition rate was ~0.037 μm min−1. In Fig. 6a, the film bonded to the substrate has no evidence of delamination. With increasing bath temperature, the deposition rate remarkably increased. At plating temperature of 70 °C, the film thickness was increased to 3.7 ± 0.35 μm (Fig. 6b). It is depicted that the deposition rate was approximately 0.082 μm min−1. The adhesion of the film with the substrate was poor with evidence of delamination. However, the Ni–P film was also found at the untreated surface of Si substrate and the adhesion was also poor. It can be inferred that the Ni–P film could be produced on Si substrate surface without any pretreatment steps in an alkaline environment. With increasing bath temperature (Fig. 6c), the thickness was about 3.64 ± 0.14 μm and the deposition rate was approximately 0.091 μm min−1. The result clearly indicated that the deposition rate increased with increasing temperature up to 80 °C, which was consistent with the result of Ref. (Lobanova et al. 2011). This behavior could be due to the increase in the driving force of the reducing agent as a result of the increase in plating temperature (Abdel Hamid 2003; Hung 1985). However, the film had a delamination phenomenon. The adhesion of the electroless deposits to the substrate is primarily mechanical in nature (Wei et al. 2015). In the study, electroless Ni–P films were plated on Si substrates. The adhesion is dependent upon the surface morphology of the Si surface and the residual stress of the deposits. Mechanical and chemical roughening could provide enough areas and sites into which the Ni–P films became anchored and adhered to the substrates. The Ni–P films deposited at 70 and 80 °C adhered poor to Si substrates, which might result from the slow nucleation rate and the release of hydrogen bubbles. Lots of bubbles were observed during deposition, which flew through the void space at the interface between film and substrate, and then built-in pressure could break the integrity of the Ni–P film on the surface of the Si substrates (Zhang et al. 2006).
Figure 7 presents the chemical composition of the film deposited at different plating temperatures. The P-content of the deposits was not changed significantly with increasing bath temperature, and the value was kept stable at about 12 wt%. As shown in EDS pattern, the as-prepared deposit was composed only of elemental Ni and P. The content of the Ni in the deposit was more than that of the P. It can be depicted that the films should be a high-P content alloy film. Therefore, the plating temperature had little effect on the chemical composition of the film. High-P deposits have amorphous structure and excellent performances in wear and corrosion resistances (Gil et al. 2008; Keong et al. 2002; Afroukhteh et al. 2012).
The effect of plating temperature on the crystallographic structure of the deposits is shown in Fig. 8. In Fig. 8a, the deposition temperature was 60 °C, a board diffraction peak at ~45° indicates an amorphous phase formation. The diffraction peaks for polycrystalline Si substrate were also observed. Therefore, the thickness of Ni–P film was thin. With increasing deposition temperature, the structure of the film observed was still amorphous (Fig. 8b and c). The diffraction peak of polycrystalline Si substrate almost disappeared due to the increase in film thickness. The crystallographic structure of amorphous Ni–P alloy film is in agreement with the result of high-P deposits.