**3.2. Mechanical property of sol-enhanced coatings**

Several different kind of sol-enhanced coatings including Ni [8-10, 16-17], Ni-P [7, 14, 18], Ni-B [19-21], Ag [15] and Au-Ni [12-13, 22-24] nano-composite coatings were developed. Their mechanical property and microstructure were systematically studied. The mechanical properties of these sol-enhanced coatings are summarized in Table 1.

Based on the experimental results of mechanical properties and related microstructure of coatings, the corresponding strengthening mechanisms were suggested. A clear model of particle nano-dispersion, grain size and mechanical properties was established. We select solenhanced Au-Ni-TiO2 nano-composite coating as an example to demonstrate the strengthen‐ ing mechanism as below.


**Table 1.** The mechanical properties of sol-enhanced coatings

hanced Ag-12.5 mL/LTiO2 composite coatings shows a clear decrease from 38.5 nm of pure

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

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

Several different kind of sol-enhanced coatings including Ni [8-10, 16-17], Ni-P [7, 14, 18], Ni-B [19-21], Ag [15] and Au-Ni [12-13, 22-24] nano-composite coatings were developed. Their mechanical property and microstructure were systematically studied. The mechanical

Based on the experimental results of mechanical properties and related microstructure of coatings, the corresponding strengthening mechanisms were suggested. A clear model of particle nano-dispersion, grain size and mechanical properties was established. We select solenhanced Au-Ni-TiO2 nano-composite coating as an example to demonstrate the strengthen‐

matrix, finally causing a porous structure and deteriorating the property of coatings.

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

**3.2. Mechanical property of sol-enhanced coatings**

properties of these sol-enhanced coatings are summarized in Table 1.

Ag coating to 25.7 nm [15].

112 Electrodeposition of Composite Materials

electron microscope.

ing mechanism as below.

**Figure 7.** Nano-hardness and nano-scratch displacement results of sol-enhanced Au-Ni-TiO2 nano-composite coatings

Fig. 7 shows the nano-hardness and scratch displacement of Au-Ni-TiO2 coatings with different sol additions. The nano-indentation hardness and scratch tests were conducted on a Nanoindentor (Hystron, USA). Nano-scratch resistance tests were performed by using a conical tip with a 1000 μN constant load to 10 μm distance. The deeper of the scratch dis‐ placement, the better wear resistance of coating. The nano-hardness and scratch displacement of Au-Ni coating was 2.55 ± 0.13 GPa and 58.8 ± 3.7 nm, respectively. At a low sol concentration, nano-hardness increases and scratch displacement decreases gradually with increasing TiO2 content. 12.5 mL/L TiO2 sol enhanced composite coating to the highest nano-hardness of 3.20 ± 0.15 GPa (26% increase) and the lowest scratch displacement of 22.5 ± 4.3 nm (reduced to 38%). However, further increasing the concentration of TiO2 to 50 mL/L led to a decrease of nano-hardness to 2.66 ± 0.12 GPa, although it was still higher than that of the un-doped Au-Ni coatings. Meanwhile, the scratch displacement increases to 32.3±2.1 nm.

**Figure 8.** Schematic drawings of the enhancement mechanism of TiO2 sol in the composite coating

The enhancement mechanism of TiO2 sol on the composite coating can be elaborated in Fig. 8. The improved nano-hardness of sol-enhanced Au-Ni-TiO2 coating could be attributed to the combined effects of grain refinement and dispersion strengthening. The highly dispersed reinforced phase should play a more important role as the grain size change is rather small. When proper TiO2 sol was added into electrolyte, a good dispersion strengthening and grain refinement can be achieved, resulting in a significant improvement of mechanical property for the coating. However, when excessive TiO2 sol was added into electrolyte, TiO2 nanoparticles start to agglomerate and tend to cause porous structure in the grain boundaries, which reduced the effect of dispersion strengthening; finally lead to a deterioration of the mechanical property although the grain size was continuously decreased [12-13].
