*3.4.2. Conductivity of sol-enhanced coatings*

Both electrical conductivity and wear resistance are important properties for Au and Ag based coatings as these are required by applications of electric contacts. Traditional solid state alloying hardening techniques improve the wear resistance of Au and Ag coatings but severely reduce the conductivity due to the lattice distortion. The sol-enhanced strengthening technique improves the hardness of the coatings significantly but does not cause severe lattice distortion, therefore keeping the good conductivity for electrical applications.

Conductivity is frequently expressed in terms of IACS percent for convenience. An IACS value of 100% refers to a conductivity of 5.80 × 107 siemens per meter (58.0 MS/m) at 20°C. Fig. 11 shows the electrical resistivity and conductivity of sol-enhanced Au-Ni-TiO2 coatings as a function of TiO2 sol concentration. Comparing with the Au-Ni coating, the electrical conduc‐

**Figure 10.** ΔE-values of Au-Ni coatings with different dopants

The corrosion behaviors of coatings have a close relationship with the sol content due to its influence on the coatings microstructure. As it is well known that the corrosion resistance of a coating largely affected by its compactness, porosity is often the cause that a coating failure from corrosion. During the sol-enhanced electroplating process, the in-situ formed nanopar‐ ticles well distributed in the grain boundary areas can decrease the quantity of defects in the coating layer, making the coating more compact and less penetrable. Additionally, the nanoparticle itself is an inert compound, in the form of uniformly distributed nanoparticles in the coating, does not form micro galvanic cells. Instead, it may play a role of reducing the reactivity of matrix metal, therefore improving the corrosion resistance of the nano-composite coatings. However, when excessive sol was added into the electrolyte, the nanoparticles tend to agglomerate which increases the quantity of defects (voids) and lead to a porous structure

Metallic coatings and thin films have very wide applications which require not only mechan‐ ical and corrosion properties but also some functional properties. Here we use sol-enhanced Au-Ni coatings as an example to present the effect of sol addition on the surface gloss and

Surface gloss and color is important properties for gold and many other coatings as it dictates the quality and value of many products. Delta E (ΔE) is widely used to present the colour difference between samples being compared. It is generally accepted that if the difference of ΔE value between two samples is less than 1.0, they can be considered as the same colour, while 3.0 is the smallest colour difference that can be recognized by human naked eyes.

Fig. 10 presents the ΔE-values of Au-Ni coatings with different dopants. It can be seen that the sol addition impose a weak influence on the surface gloss and colors of coatings, which even cannot be detected by human naked eyes. The sol-enhanced coatings present almost identical surface gloss with the traditional coating due to the highly dispersed nano-structure [25].

Both electrical conductivity and wear resistance are important properties for Au and Ag based coatings as these are required by applications of electric contacts. Traditional solid state alloying hardening techniques improve the wear resistance of Au and Ag coatings but severely reduce the conductivity due to the lattice distortion. The sol-enhanced strengthening technique improves the hardness of the coatings significantly but does not cause severe lattice distortion,

Conductivity is frequently expressed in terms of IACS percent for convenience. An IACS value

shows the electrical resistivity and conductivity of sol-enhanced Au-Ni-TiO2 coatings as a function of TiO2 sol concentration. Comparing with the Au-Ni coating, the electrical conduc‐

siemens per meter (58.0 MS/m) at 20°C. Fig. 11

therefore keeping the good conductivity for electrical applications.

in the coating, resulting in significant deterioration of corrosion resistance [19].

**3.4. The other properties of sol-enhanced coatings**

*3.4.1. Surface gloss of sol-enhanced coatings*

*3.4.2. Conductivity of sol-enhanced coatings*

of 100% refers to a conductivity of 5.80 × 107

conductivity property.

116 Electrodeposition of Composite Materials

tivity of sol-enhanced coatings show a slight decrease (~4%) but keep at the same level with increasing sol content. After adding 50 mL/L TiO2 sol, the electrical conductivity of coating is still higher than 50% IACS.

**Figure 11.** Electrical resistivity and electrical conductivity of sol-enhanced Au-Ni-TiO2 nano-composite coatings

The same level conductivity of sol-enhanced coatings can be mainly attributed to the highly distribution of small nano-particles in the coating. The electron wave can bypass these small nano-particles and form a conductive network during the transportation process. Further‐ more, the sol addition does not change the alloy solubility and cause lattice distortion. The slight decrease of electrical conductivity can be attributed to the increment of scattering effect. The increase of grain boundaries and the scattering effect of electron wave strengthened by the second phase lead to the decrease of electrical conductivity [13].
