**3.1. Microstructure analysis**

10

**2.3. Sample preparation and characterization**

mined by the weight gain after the plating process.

of volume loss per unit sliding distance.

(a) (b)

**Figure 1.** (a) Specimen size of wear resistance test sample and (b) electroplating equipment and setup.

Figure 1. (a) Specimen size of wear resistance test sample and (b) electroplating equipment and setup.

Coated samples were evaluated for hardness using a Vickers Mitutoyo HM-122 hardness tester with a load of 200 g. Two body abrasive wear tests were conducted using Plint Multi-station Block-on-Ring tester under a load of 20 N at a fixed sliding speed of 3.35 m/s for a sliding distance of 5000 m against steel disc of hardness 500 HV. During sliding, the load is applied on the specimen through cantilever mechanism, and the specimens brought in intimate contact with the rotating disc at a track radius of 100 mm. The samples were cleaned with acetone and weighed (up to an accuracy of 0.01 mg using a Sartorius microbalance) before and after each test. The wear rate was calculated from the weight loss measurement and expressed in terms

tester with a load of 200 g. Two body abrasive wear tests were conducted using Plint Multi-station

Block-on-Ring tester under a load of 20 N at a fixed sliding speed of 3.35 m/s for a sliding distance of

5000 m against steel disc of hardness 500 HV. During sliding, the load is applied on the specimen

through cantilever mechanism, and the specimens brought in intimate contact with the rotating disc at

a track radius of 100 mm. The samples were cleaned with acetone and weighed (up to an accuracy of

0.01 mg using a Sartorius microbalance) before and after each test. The wear rate was calculated from

determine the wear resistance. Examination of the joints microstructure was performed using a Ziess

optical microscope, an ASPEX 309 scanning electron microscope (SEM), and a transmission electron

For each coated sample, three specimens were tested and the average value was used to

the weight loss measurement and expressed in terms of volume loss per unit sliding distance.

Coated samples were evaluated for hardness using a Vickers Mitutoyo HM-122 hardness

cathode in the plating solution.

192 Electrodeposition of Composite Materials

The mild steel samples of dimensions 0 × 20 × 15 mm were cut, prepared to 800-grit abrasive paper, and polished to 1-μm diamond suspension, after which they were cleaned in an acetone bath (see Figure 1a). Acid pickling took place in a solution of 15 wt.% HNO3 and 2 wt.% HF at 50°C for 2 minutes and then rinsed in distilled water. These samples were then used as the

The electrodeposition of an Ni/Al2O3 coating was carried out in a 250-mL glass beaker, as shown in Figure 1(b). 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 ceramic particles were added separately to the nickel bath to produce the composite coating. The particles were thoroughly mixed into the solution for 2 hours and kept in suspension in the bath with a magnetic stirrer. The following parameters were adjusted: cathode current density, agitation, and stir rate (A); the pH level of the solution (B); concentration of composite particles (C); and bath temperature (D), as shown in Table 1. The thickness of Ni/Al2O3p coatings was controlled by the plating time. The actual amount of Ni/Al2O3p electroplated onto a surface was deter‐

> The TEM micrograph of the Ni/Al2O3 coating, as shown in Figure 2(a), revealed the presence of nanosized Al2O3 particles embedded in the nickel matrix. The deposition of particle reinforcement during the coating process can be attributed to parameters such as current densities, bath temperature, stir rate, bath pH, and particle concentration [29]. TEM analysis of the as-received powder shown in Figure 2(b) indicated the presence of agglomerated particle clusters. These particle clusters are believed to have been subsequently embedded in the coating during the electrodeposition process. In situations where particle clustering is present, surfactants such as saccharin, hexadecylpyridinium bromide (HPB), and cetyltrimethylam‐ monium bromide (CTAB) are used to improve particle distribution and reduce clustering [4, 30, 31]. In this study, the surfactant saccharin was used to reduce particle clustering; however, particle agglomeration was still present, as shown in Figure 2(a).

> An optical micrograph of the nano-composite Ni/Al2O3 coating deposited using current density 3.2 × 10−4 A/mm2 , stir rate 440 rpm, bath temperature 50°C, pH 4.45, and particle concentration 20 g/L is shown in Figure 3(a). Analysis of the image revealed Al2O3 particle clusters consistent with the particle agglomeration observed in the as-received Al2O3 powder is shown in Figure 3(b). Similar surface morphology obtained for coatings deposited using current density 5 × 10−4A/mm2 , stir rate 820 rpm, bath temperature 60°C, pH 4.45, and particle concentration 10 g/L was observed to posses large globules that are believed to be agglomer‐ ated Al2O3 particles embedded into the Ni matrix.

A/mm2

the Ni matrix.

density 3.2 × 10<sup>−</sup>4 A/mm2

situations where particle clustering is present, surfactants such as saccharin, hexadecylpyridinium bromide (HPB), and cetyltrimethylammonium bromide (CTAB) are used to improve particle distribution and reduce clustering [4, 30, 31]. In this study, the surfactant saccharin was used to reduce particle clustering; however, particle agglomeration was still present, as shown in Figure 2(a).

An optical micrograph of the nano-composite Ni/Al2O3 coating deposited using current

concentration 20 g/L is shown in Figure 3(a). Analysis of the image revealed Al2O3 particle clusters consistent with the particle agglomeration observed in the as-received Al2O3 powder is shown in Figure 3(b). Similar surface morphology obtained for coatings deposited using current density 5 ×

, stir rate 440 rpm, bath temperature 50°C, pH 4.45, and particle

, stir rate 820 rpm, bath temperature 60°C, pH 4.45, and particle concentration 10 g/L was

Figure 2. TEM image of the Ni/Al2O3coating. **Figure 2.** TEM image of the Ni/Al2O3coating.

Figure 3. Optical micrograph of coatings deposited with the parameters outlined in experiment: (a) 2 **Figure 3.** Optical micrograph of coatings deposited with the parameters outlined in experiment: (a) 2 and (b) 17, as shown in Table 2.

and (b) 17, as shown in Table 2.

#### **3.2. Micro-hardness**

**3.2. Micro-hardness**  The influence of the process parameters was assessed to maximize the coating hardness. Analysis of the surface hardness revealed that if the parameters were set to current density at level 1 (3.2 × 10<sup>−</sup><sup>4</sup> A/mm2 ), revolutions per minute (RPM) at level 2 (630 rpm), pH at level 2 (4.45), Al2O3 concentration at level 3 (30 g/L), and temperature at level 2 (50°C), a surface hardness of 501.5 HV can be achieved. When compared with pure nickel, a surface hardness of 278.82 HV was achieved. The differences observed were attributed to dispersion hardening effects caused by the presence of The influence of the process parameters was assessed to maximize the coating hardness. Analysis of the surface hardness revealed that if the parameters were set to current density at level 1 (3.2 × 10−4 A/mm2 ), revolutions per minute (RPM) at level 2 (630 rpm), pH at level 2 (4.45), Al2O3 concentration at level 3 (30 g/L), and temperature at level 2 (50°C), a surface hardness of 501.5 HV can be achieved. When compared with pure nickel, a surface hardness of 278.82 HV was achieved. The differences observed were attributed to dispersion hardening effects caused by the presence of nanosized Al2O3 in the composite coating. According to Lehman et al. [32], the nanosized particles act to restrict/reduce dislocation motion in the nickel matrix, which causes an increase in the surface hardness. The ANOVA test showed that stir rate (RPM), particle concentration, and bath pH had the greatest impact on the hardness of the coatings.

nanosized Al2O3 in the composite coating. According to Lehman et al. [32], the nanosized particles

13

Stir rate (RPM) is used to disperse the nano-particles during the coating process, thus control‐ ling the microstructure produced by keeping particles suspended in the bath solution during coating [33, 34]. It is believed that stirring increases the amount of nano-particles embedded in the coating up to 630 rpm beyond which a reduction in surface hardness is observed.

Similar effects were observed when the concentration of Al2O3 particles suspended in the solution increased. The results indicated that particle incorporated into the coating increased with increasing particle concentration in the bath solution until 30 g/L was reached. The microstructure of the coating may also be attributed to the pH of the bath which is believed to control the nucleation and morphology of the coating; as the pH decreases, the grain size of the crystallite also decreases, resulting in an increase in the hardness of the material [35, 36]. The estimated effect of each parameter is shown graphically in Figure 4. Analysis of the data using the larger-is-better characteristics indicated that optimum coating hardness can be achieved if concentration is set to level 3, while all other parameters are set to level 2.

**Figure 4.** Effect of process parameters on micro-hardness.

#### **3.3. Wear testing**

12

13

**Al2O3**

situations where particle clustering is present, surfactants such as saccharin, hexadecylpyridinium bromide (HPB), and cetyltrimethylammonium bromide (CTAB) are used to improve particle distribution and reduce clustering [4, 30, 31]. In this study, the surfactant saccharin was used to reduce particle clustering; however, particle agglomeration was still present, as shown in Figure 2(a).

An optical micrograph of the nano-composite Ni/Al2O3 coating deposited using current

concentration 20 g/L is shown in Figure 3(a). Analysis of the image revealed Al2O3 particle clusters consistent with the particle agglomeration observed in the as-received Al2O3 powder is shown in Figure 3(b). Similar surface morphology obtained for coatings deposited using current density 5 ×

observed to posses large globules that are believed to be agglomerated Al2O3 particles embedded into

**Al2O3**

Figure 2. TEM image of the Ni/Al2O3coating.

**Al2O3 Al2O3**

(a) (b)

Figure 3. Optical micrograph of coatings deposited with the parameters outlined in experiment: (a) 2 and (b) 17, as shown in Table 2.

The influence of the process parameters was assessed to maximize the coating hardness. Analysis of the surface hardness revealed that if the parameters were set to current density at

**Figure 3.** Optical micrograph of coatings deposited with the parameters outlined in experiment: (a) 2 and (b) 17, as

The influence of the process parameters was assessed to maximize the coating hardness. Analysis of the surface hardness revealed that if the parameters were set to current density at level 1 (3.2 × 10<sup>−</sup><sup>4</sup>

(4.45), Al2O3 concentration at level 3 (30 g/L), and temperature at level 2 (50°C), a surface hardness of 501.5 HV can be achieved. When compared with pure nickel, a surface hardness of 278.82 HV was achieved. The differences observed were attributed to dispersion hardening effects caused by the presence of nanosized Al2O3 in the composite coating. According to Lehman et al. [32], the nanosized particles act to restrict/reduce dislocation motion in the nickel matrix, which causes an increase in the surface hardness. The ANOVA test showed that stir rate (RPM), particle concentration, and bath pH had the greatest impact on the hardness of the

), revolutions per minute (RPM) at level 2 (630 rpm), pH at level 2 (4.45), Al2O3 concentration at level 3 (30 g/L), and temperature at level 2 (50°C), a surface hardness of 501.5 HV can be achieved. When compared with pure nickel, a surface hardness of 278.82 HV was achieved. The differences observed were attributed to dispersion hardening effects caused by the presence of nanosized Al2O3 in the composite coating. According to Lehman et al. [32], the nanosized particles

), revolutions per minute (RPM) at level 2 (630 rpm), pH at level 2

, stir rate 440 rpm, bath temperature 50°C, pH 4.45, and particle

, stir rate 820 rpm, bath temperature 60°C, pH 4.45, and particle concentration 10 g/L was

density 3.2 × 10<sup>−</sup>4 A/mm2

10<sup>−</sup><sup>4</sup> A/mm2

194 Electrodeposition of Composite Materials

the Ni matrix.

**Figure 2.** TEM image of the Ni/Al2O3coating.

**3.2. Micro-hardness** 

A/mm2

level 1 (3.2 × 10−4 A/mm2

shown in Table 2.

coatings.

**3.2. Micro-hardness**

Two-body abrasive wear tests were carried out to evaluate the effects of electroplating parameters on the wear resistance of the deposited coating. Evaluation of the results presented in Table 3 indicated that the wear rate increased from 0.22 kg/s to 2.11 kg/s. The estimated effect of each parameter is shown graphically in Figure 5. Analysis of the data using the smalleris-better characteristics indicated that the S/N ratio decreased as the stir rate increased from 440 rpm to 630 rpm; however, increasing the stir rate to 820 rpm corresponded to an increase of the S/N ratio; this change can be attributed to achieving suitable particle suspension in the solution at 630 rpm; however, further increase in stir rate caused an increase turbulence in the solution, which can reduce particulate inclusion in the Ni matrix [34]. The impact of bath pH and temperature had similar effects on the S/N ratio [34]. The results indicated that minimal mass loss can be achieved if both pH and temperature are set to level 1 [37].

In addition, the results further revealed that particle concentration had a significant effect on the wear rate, which corresponded to an increase in the hardness of the coatings deposited as the particle concentration increased from 10 g/L to 20 g/L. The reduction in wear rate was attributed to the increase in the Al2O3 particles embedded in the Ni matrix during the codeposition process. The Al2O3 nano-particulates co-deposited in the Ni matrix could restrain the Ni grains and the plastic deformation of the matrix under a loading due to dispersion strengthening. The effect is that the coating becomes stronger as the nano-Al2O3 particle content increases, thus increasing micro-hardness and wear resistance of the coating. Further increase in the particle content resulted in increase of the brittleness of the coating, which is subse‐ quently reflected as a reduction of the wear resistance of the coating. By utilizing the wear resistance values shown in Table 4, the optimum coating parameters were determined to be A2, B1, C2, and D1.


**Table 3.** Measured results of response variables and S/N ratios.


**Table 4.** Mean S/N ratio of individual levels.

In addition, the results further revealed that particle concentration had a significant effect on the wear rate, which corresponded to an increase in the hardness of the coatings deposited as the particle concentration increased from 10 g/L to 20 g/L. The reduction in wear rate was attributed to the increase in the Al2O3 particles embedded in the Ni matrix during the codeposition process. The Al2O3 nano-particulates co-deposited in the Ni matrix could restrain the Ni grains and the plastic deformation of the matrix under a loading due to dispersion strengthening. The effect is that the coating becomes stronger as the nano-Al2O3 particle content increases, thus increasing micro-hardness and wear resistance of the coating. Further increase in the particle content resulted in increase of the brittleness of the coating, which is subse‐ quently reflected as a reduction of the wear resistance of the coating. By utilizing the wear resistance values shown in Table 4, the optimum coating parameters were determined to be

A2, B1, C2, and D1.

**Current density**

196 Electrodeposition of Composite Materials

**(A/mm2 ;** **Stir rate (A)**

**Table 3.** Measured results of response variables and S/N ratios.

**pH (B)**

**Concentration (C)**

**Temperature (D)**

**× 10−4) (rpm) (g/L) (°C) (HV) (η; dB) (kg/s) (η; dB)**

 3.2 440 4.3 10 40 208.5 46.38 2.11 116.32 3.2 440 4.45 20 50 318.3 50.06 1.00 122.50 3.2 440 4.6 30 60 268.3 48.57 0.220 137.44 3.2 630 4.3 10 50 357.3 51.06 0.44 131.19 3.2 630 4.45 20 60 325.5 50.25 1.10 122.59 3.2 630 4.6 30 40 369.4 51.35 0.88 124.32 3.2 820 4.3 20 40 300 49.54 0.66 128.05 3.2 820 4.45 30 50 363.9 51.22 0.55 129.39 3.2 820 4.6 10 60 199.9 46.02 1.00 123.36 5 440 4.3 30 60 224.5 47.02 0.66 127.16 5 440 4.45 10 40 282.1 49.01 0.33 133.18 5 440 4.6 20 50 229.9 47.23 0.55 128.36 5 630 4.3 20 60 219 46.81 1.00 122.84 5 630 4.45 30 40 449.8 53.06 0.22 136.57 5 630 4.6 10 50 582.9 49.99 1.00 122.18 5 820 4.3 30 50 279.7 48.93 0.33 133.18 5 820 4.45 10 60 403.5 52.12 0.33 132.38 5 820 4.6 20 40 178.5 45.03 0.88 124.22

**Hardness S/N**

**Wear rate (×10−6)**

**S/N**

**Experiment**

**Figure 5.** Effect of process parameters on wear resistance.

#### **3.4. Analysis of the level averages**

A second analysis using the level averaging technique was also carried out by averaging the experimental results achieved at each level for the respective parameter. A summary of these calculations is shown in Table 5 and is graphically represented in Figure 6. When the effects of stir rate and concentration were evaluated, it was found that the wear rate decreased as both concentration and stir rate increased similar to the results obtained when the S/N ratio was calculated. The optimum wear resistance can be obtained by setting both the stir rate and concentration are set to level 3, whereas bath temperature and pH are set to level 2. Optimi‐ zation for hardness revealed that all the parameter settings were similar with the only exception being stir rate, which required a level 2 setting (Figure 7). These results are also consistent with literature [34, 37].


**Table 5.** Level averages for each parameter.

**Figure 6.** Mean wear rate of each parameter level.

**Figure 7.** Mean hardness of each parameter level.
