**2. Electrodeposition of the nano-composite Ni/Al2O3 coating**

Composite coatings can be produced by co-deposition of fine inert particles into a metal matrix from an electrolytic or an electroless bath. This technique is getting interesting due to its ability to produce films with excellent mechanical properties such as wear resistance, corrosion resistance, and lubrication.

The preparation of a composite coating is done in two steps. First, an effective dispersion of fine inert particles is produced in the electrolyte. Next, the preparation of the composite coating is made by the manipulation of electrochemical conditions. An effective dispersion of inert particles in the electrolyte promotes the adsorption opportunity of inert particles on the cathode. It causes a higher volume content of inert particles in the composite coating. The mechanical properties of the composite coating are also promoted with the enhancement of the volume content of inert particles in the coating. In electrodeposited composites, the particles to be dispersed in the metal matrix are maintained in suspension in the bath by agitation and they become incorporated in the coating by a process known as electrophoresis.

A number of coating parameters were studied to determine the effect of each on the coating thickness: pH of the electrolyte, current density, electrolyte temperature, agitation frequency and deposition time [33]. Samples were coated for time intervals of 2, 4, 6, 8, 10, 15, 20 and 30 minutes. After electrodeposition, coating thickness was determined using a light microscope. Figure 1(b) shows the relationship between coating thickness and deposition time when; the current density was set to 1.0A/dm2, pH 3.0, agitation frequency of 300 rpm and an electrolyte temperature of 50oC [25]. It was found that the coating thickness increased with increasing deposition time, in keeping with Faraday's law governing electrode reactions during electrolysis [26].

**Figure 1.** (a) TEM image of as-received nano-sized Al2O3 powder (b) Coating thickness as a function of electrodeposition time at 1.0 A/dm2 and 50oC [33].

In this study, Al -6061/Al2O3p samples of dimensions 10 × 10× 5mm were prepared to 800 grit abrasive paper and polished to 1 μm diamond suspension after which they were cleaned in an acetone bath. Acid pickling took place in a solution of 15 wt.% HNO3 and 2 wt.% HF at 50 oC for 2 minutes and then rinsed in distilled water. The plating solution was prepared by dissolving: 250 g/L NiSO4 6H2O, 45 g/L NiCl2.6H2O, 35 g/L H3BO3 and 1 g/L Saccharin in distilled water. The Ni- Al2O3 composite coating was produced by adding 50g/L of ceramic particles to a separately to the nickel bath. The particles were thoroughly mixed into the solution for two hours and kept in suspension in the bath with a magnetic stirrer rotating at 300 rpm. The coating solution was maintained at a temperature of 50oC and pH of 3.0 [33]. The thickness of Ni-Al2O3p coatings were controlled by the current density and plating time. The actual amount of Ni-Al2O3p electroplated onto a surface was determined by the weight gain after the plating process. The coating thickness was calculated by using the equations 2 and 3:

$$\rho\_{\mathbb{C}} = \frac{\text{mass of coating}}{\text{Area} \ge \text{ thickness}} = \frac{m}{A \,\text{at}} \,\text{s} \tag{2}$$

The density of the composite coating ( *<sup>C</sup>* ) can be calculated by using Equation 3 (rule of mixtures) where *<sup>v</sup> x* is the volume fraction of alumina particles in the Ni-Al2O3 coating.

$$
\rho\_{\mathbb{C}} = \rho\_{Al\_2O\_3} \mathbb{x}\_v + \rho\_{\mathrm{Ni}}(1 - \mathbb{x}\_v) \tag{3}
$$

#### **2.1. Spectroscopic analysis of the film structure and composition**

Wavelength dispersive spectroscopic (WDS) and X-ray diffraction (XRD) spectroscopic analyses of the coatings deposited were used to evaluate the distribution of the dispersion particles. Figure 2 shows an SEM micrographs of the coating produced by the coelectrodeposition of Ni + 18vol% (nano-Al2O3)p. Examination of the coatings using a light microscope revealed the absence of surface defects and interfacial void, however Al2O3 particle clusters were present in the coating. This was attributed to particle clustering in the powder prior to the coating process as indicated by the TEM image of the as-received Al2O3 powder shown in Figure 1(a). The volume fraction of Al2O3 present in the coating was studied by digital x-ray mapping and these results are shown in Figure 3. From Figure 3b and 3c, the areas which contains a high concentration of Al2O3 are easily identify and corresponds to Al and O which would confirm the compound to be Al2O3 since the base coating is Ni.

314 Advanced Aspects of Spectroscopy

the equations 2 and 3:

electrodeposition time at 1.0 A/dm2 and 50oC [33].

The density of the composite coating ( *<sup>C</sup>*

*C*

**2.1. Spectroscopic analysis of the film structure and composition** 

mixtures) where *<sup>v</sup> x* is the volume fraction of alumina particles in the Ni-Al2O3 coating.

2 3 (1 ) *<sup>C</sup> Al O v Ni v*

Wavelength dispersive spectroscopic (WDS) and X-ray diffraction (XRD) spectroscopic analyses of the coatings deposited were used to evaluate the distribution of the dispersion particles. Figure 2 shows an SEM micrographs of the coating produced by the coelectrodeposition of Ni + 18vol% (nano-Al2O3)p. Examination of the coatings using a light

**Figure 1.** (a) TEM image of as-received nano-sized Al2O3 powder (b) Coating thickness as a function of

In this study, Al -6061/Al2O3p samples of dimensions 10 × 10× 5mm were prepared to 800 grit abrasive paper and polished to 1 μm diamond suspension after which they were cleaned in an acetone bath. Acid pickling took place in a solution of 15 wt.% HNO3 and 2 wt.% HF at 50 oC for 2 minutes and then rinsed in distilled water. The plating solution was prepared by dissolving: 250 g/L NiSO4 6H2O, 45 g/L NiCl2.6H2O, 35 g/L H3BO3 and 1 g/L Saccharin in distilled water. The Ni- Al2O3 composite coating was produced by adding 50g/L of ceramic particles to a separately to the nickel bath. The particles were thoroughly mixed into the solution for two hours and kept in suspension in the bath with a magnetic stirrer rotating at 300 rpm. The coating solution was maintained at a temperature of 50oC and pH of 3.0 [33]. The thickness of Ni-Al2O3p coatings were controlled by the current density and plating time. The actual amount of Ni-Al2O3p electroplated onto a surface was determined by the weight gain after the plating process. The coating thickness was calculated by using

> *mass of coating m Area x thickness A xt*

> >

(2)

) can be calculated by using Equation 3 (rule of

*x x* (3)

**Figure 2.** (a) Surface of the Ni-Al2O3 coating and (b) Cross-section of the Ni-Al2O3 coating produced by electrodeposition [33].

**Figure 3.** X-ray digital composition maps taken from the Ni-Al2O3 coating surface for; (a) Ni, (b) O and (c) Al [33].
