**2.3. Sample preparation and characterization**

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 cathode in the plating solution.

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‐ mined by the weight gain after the plating process.

**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 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 of volume loss per unit sliding distance.

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.

10

For each coated sample, three specimens were tested and the average value was used to 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 trans‐ mission electron microscope (TEM). Quantitative compositional analyses were carried out using wavelength dispersive spectroscopy (WDS) and x-ray diffraction (XRD). Micro-hardness testing was performed on the cross section of the joints according to ASTM E92 standard test method for a Vickers micro-hardness testing. Indentations were made at 100 μm spacing using a diamond tip indenter to which a 0.2 kg load was applied for 15 seconds, after which the length of the diagonals was measured and the hardness number was recorded from tables.

For TEM analyses, sections of the coating were cut to 6 × 5 × 1.5 mm using a thin diamond tip cutter and subsequently mechanically grounded using 600 grit abrasive paper to a thickness of approximately 200 μm. Disc of 3 mm diameter was punched from the 200-μm coating and subsequently mechanically grinded to 25 μm. The grinded samples were thinned by electropolishing method. This was done in a solution containing 150 g/L Na2CO3 + 50 g/L Na3PO4 + 30 g/L. The solution was maintained at a temperature of 40°C and a voltage of 20 V. All the thinned coatings were examined with a JEOL TEM 2000FX TEM at an accelerating voltage of 200 kV.
