**3. Microstructure and property of sol-enhanced coatings**

#### **3.1. Microstructure of sol-enhanced coatings**

solution stable. Before TiO2 sol is added into the electrolyte, the sol can be regarded as a stable system without solid liquid interface. Under neutral and basic conditions, the condensation process of Ti macromolecule ions started before the completion of hydrolysis; and the

After TiO2 sol is added into the electrolyte, water in electrolyte aggravates the hydrolysis reaction and breaks up the dynamic balance. The sol system becomes unstable and the interface between solid and liquid emerges. Thus the amorphous TiO2 nano-particles formed in situ [5, 9-10]. Fig. 1 presents the size distribution of nano-particles in the sol and the Ni electrolyte after adding the sol. The particle size was characterized by a laser diffraction particle analyzer (Malvern Mastersizer Hydro 2000S). The particle size of TiO2 sol was distributed in the range of 1-10 nm, with a mean value of 2.5 nm as shown in Fig. 1a. The size distribution of sol added electrolyte keeps in a same level with the TiO2 sol. The mean value of the particle size was increased to 7.4 nm and the particle size distribution was in the range of 3-20 nm which means no significant agglomeration occurs (Fig. 1b). The size measurement results are consistent with the HRTEM observation as shown in Fig. 2. The TiO2 nano-particle got from electrolyte has an

**Figure 2.** TEM bright field image of TiO2 nano-particles separated from the Ni electrolyte with TiO2 sol addition

The overall sol-enhanced deposition process can be typically divided into several steps. These steps describe the process of particles from the solution to their incorporation in the metal matrix. The first step is the in-situ generation of nano-particles after adding the sol into the electrolyte. Once the nano-particles formed in the electrolyte, some of them are immediately

formation of an ordered structure was hindered.

108 Electrodeposition of Composite Materials

amorphous structure with a size of ~10 nm.

**2.3. Deposition process of sol-enhanced plating**

The microstructure of sol-enhanced coatings was studied by various characterization methods. We hereby elaborate the related characterization results of cross-section, surface morphology and intrinsic microstructure of sol-enhanced coatings.

#### *3.1.1. Cross-section images of sol-enhanced coatings*

Fig. 4 shows the cross-section morphologies of Ni-P and sol-enhanced Ni-P-TiO2 coatings. Solenhanced Ni-P-12.5 mL/L TiO2 nano-composite coating has a similar cross-section image with traditional Ni-P coating. No obvious TiO2 particles could be seen in the SEM cross-section, indicating their small size and relatively low content. However, with increasing TiO2 addition, the agglomerated TiO2 particles can be clearly seen in Ni-P-50 mL/L TiO2 composite coating. In addition, many voids were observed in the coatings [14]. When the sol concentration is relatively low (e.g., for sol-enhanced Ni-P-TiO2 coating, the TiO2 sol concentration is below 20 mL/L), the cross-section image of sol-enhanced coatings is similar with the traditional coating. However, when the sol concentration is relatively high (e.g., for sol-enhanced Ni-P-TiO2 coating, the TiO2 sol concentration is above 20 mL/L), a porous structure may form and large cluster area could be seen in the cross-section of sol-enhanced coatings due to the nano-particle agglomeration.

**Figure 4.** Cross-sectional morphologies: (a) traditional Ni-P coating, (b) sol-enhanced Ni-P-12.5 mL/L TiO2 coating, and (c) Ni-P-TiO2 coating with high concentration of sol (50 mL/L).

#### *3.1.2. Surface morphology of sol-enhanced coatings*

Fig. 5 shows the surface morphology of Au-Ni coating and TiO2 sol enhanced Au-Ni-TiO2 coatings with different TiO2 sol concentrations. The Au-Ni coating shows typical granular morphology with the large protrusion size of ~400 nm. Some pores can be found on the Au-Ni coating surface, as shown by the white arrows in Fig. 4a, probably due to the formation of H2 during the electro-deposition process. The morphology of 12.5 mL/L TiO2 sol enhanced composite coating shows a uniform spherically nodular structure with a size of ~300 nm (Fig. 5b). A great number of black dots were seen on the surface of 50 mL/L TiO2 sol added composite coating, as shown by the white arrows in Fig. 5(c). The size of black dots ranges from ~50 nm to ~150 nm. Some of the black dots were attributed to the clusters formed by TiO2 nanoparticles, which confirmed by the EDS results. The Ti concentration in those locations was higher than other areas. As abundant TiO2 nano-particles agglomerate into clusters, porous structure formed in the nearby area. The other black dots are the voids that may come from the H2 release during the electro-deposition process [12-13].

the agglomerated TiO2 particles can be clearly seen in Ni-P-50 mL/L TiO2 composite coating. In addition, many voids were observed in the coatings [14]. When the sol concentration is relatively low (e.g., for sol-enhanced Ni-P-TiO2 coating, the TiO2 sol concentration is below 20 mL/L), the cross-section image of sol-enhanced coatings is similar with the traditional coating. However, when the sol concentration is relatively high (e.g., for sol-enhanced Ni-P-TiO2 coating, the TiO2 sol concentration is above 20 mL/L), a porous structure may form and large cluster area could be seen in the cross-section of sol-enhanced coatings due to the nano-particle

**Figure 4.** Cross-sectional morphologies: (a) traditional Ni-P coating, (b) sol-enhanced Ni-P-12.5 mL/L TiO2 coating, and

Fig. 5 shows the surface morphology of Au-Ni coating and TiO2 sol enhanced Au-Ni-TiO2 coatings with different TiO2 sol concentrations. The Au-Ni coating shows typical granular morphology with the large protrusion size of ~400 nm. Some pores can be found on the Au-Ni coating surface, as shown by the white arrows in Fig. 4a, probably due to the formation of H2 during the electro-deposition process. The morphology of 12.5 mL/L TiO2 sol enhanced

(c) Ni-P-TiO2 coating with high concentration of sol (50 mL/L).

*3.1.2. Surface morphology of sol-enhanced coatings*

agglomeration.

110 Electrodeposition of Composite Materials

**Figure 5.** Surface morphology of sol-enhanced Au-Ni-TiO2 nano-composite coatings: (a) Au-Ni as a comparison, (b) Au-Ni-12.5 mL/L TiO2, and (c) Au-Ni-50mL/L TiO2

#### *3.1.3. TEM microstructure of sol-enhanced nano-composite coatings*

Fig. 6 presents the bright field image TEM and HRTEM image of sol-enhanced Ag-TiO2 nanocomposite coating. It can be seen that many white small nano-particles with a size of 10-25 nm were distributed quite uniformly in the grain boundary areas and inside the coating matrix as shown in Fig. 6a. The nanoscale probe EDX analysis indicates the white nanoparticles contain Ti. HRTEM indicates that the small nano-particles have an amorphous microstructure, as shown in Fig. 6b. The grain size of the coatings can be calculated from the measured XRD patterns by using Scherrer's formula. The average grain size of sol-en‐ hanced Ag-12.5 mL/LTiO2 composite coatings shows a clear decrease from 38.5 nm of pure Ag coating to 25.7 nm [15].

**Figure 6.** Bright field image and HRTEM image of sol-enhanced Ag-TiO2 coating

Based on the results described above, we can concluded that when proper sol was added into the electrolyte, small nano-particles will be formed in-situ and co-deposited with the metal ions onto the substrate. These small amorphous nano-particles were distributed uniformly in the grain boundaries and inside the coating grains. Due to their small size, it is hard to detect them by using conventional microscopic tools such as optical microscope and scanning electron microscope.

These nano-particles incorporated into the coating matrix can increase the number of nuclea‐ tion sites, while the other nano-particles distributed in the grain boundary can act as the obstacles that restrict the grain growth. The increasing nucleation center and obstacles for grain growth finally lead to an obvious grain refinement. However, when excessive sol was added into electrolyte, the nanoparticles start to agglomerate and tend to form voids in the coating matrix, finally causing a porous structure and deteriorating the property of coatings.
