Mechanical properties of Ni-nano-Al₂O₃ composite coatings on AISI 304 stainless steel by pulsed electrodeposition

2.1 Design of experiments (Taguchi’s techniques)

Experiments are conducted based on Taguchi’s method with three factors at three levels each. The values taken by a factor are termed to be levels. The factors to be studied and their levels are chosen in detail in Table 3.

2.2 Signal-to-noise (S/N) ratio

The S/N ratio is calculated using the-higher-the-better criterion for maximizing the hardness, which is given by Taguchi [15] as depicted in Eq. (1):

where y is the observed data and n is the number of observations. From the orthogonal array used, it is possible to get the effects of each factor at different levels. For instance, the average S/N ratio for factor A at levels 1–3 can be obtained by calculating the mean of the S/N ratios for the trials 1–9, 10–18, and 19–27, respectively. The mean S/N ratio for each level of all other factors is computed in a similar fashion. Here, L₂₇ orthogonal array is used for experimental investigations.

Two trials were conducted over the specimen and each trail comprises 27 experiments. The average values are taken as the final microhardness value and are listed in Table 2. The corresponding S/N ratio results are calculated as per “the-higher-the-better” formula [15]. Table 2 clearly shows the optimum level of the experiment, that is, the maximum hardness at frequency 10 Hz, duty cycle 20%, and peak current density 0.6 A/cm². Furthermore, the results of analysis of variance (ANOVA) for the Ni-nano-Al₂O₃ composite coatings under different pulse parameters are shown in Table 4, according to which frequency is found to be a more dominant source (40.40%) in affecting hardness, whereas the lowest contribution is the current density (11.80%). The percentage of error in this experiment is obtained within the optimal level [15]. The results of percentage contributions are plotted in Figure 1. The main effect of Ni-nano-Al₂O₃ composite coatings are shown in Figure 2. Each level has three factors, that is, frequency A1-A2-A3, duty cycle B1-B2-B3, and current density C1 C2 C3 as shown in Figure 2. The maximum values are selected for each level. The optimum values are A1-B2-C3, that is, frequency 10 Hz, duty cycle 20%, and peak current density 0.6 A/cm².

Figure 1

Percentage contribution of pulse plating parameters of Ni-nano-Al₂O₃ composite coatings.

Figure 2

S/N graph for microhardness of Ni-nano-Al₂O₃ composite coatings.

3.1 Surface morphology and microstructural characterization

The surface morphology of the optimized parameters having frequency 10 Hz, duty cycle 20%, and peak current density 0.6 A/cm² of Ni-nano-Al₂O₃ composite coatings is observed under SEM. Jung et al. [11] reported that the composite coating deposited without any additive is made of regular pyramidal grains. Moreover, the regular “pyramidal” structures with uniform grain size are formed throughout the bright surface of the coated area of the optimized specimen, whereas the smallest irregular-shaped Al₂O₃ partials are embedded between the Ni matrix (Figure 3C). In the PC composite coatings, the surface morphology and distribution of the Al₂O₃ particles in the matrix are fairly uniform [2, 16]. The coating surface composition was determined by EDX as shown in Figure 4. The examinations showed that the Ni is the dominant element (66.52 at.%), whereas Al₂O₃ composites are 12.23 at.%.

Figure 3

Microstructure of Ni-nano-Al₂O₃ composite coatings of optimized stainless steel specimen of AISI 304.

Figure 4

EDX of Ni-nano-Al₂O₃ composites of AISI 304 stainless steel specimen.

3.2 XRD

The XRD patterns of the Ni-nano-Al₂O₃ composites are prepared by optimum AISI 304 stainless specimen at optimal parameters under pulse electrodeposition methods, as shown in Figure 5. The graph shows intensity (cps) against wavelength (2θ). The composite coatings exhibited face-centered cubic (FCC) lattice with different orientations, which were influenced by the pulse frequency. The composite coatings are obvious (111) preferred orientation with Ni atoms τ at low pulse frequency, which means that the lower pulse frequency with constant duty cycle gives more time to the Ni atoms to migrate to a more stable position during the intervals than under higher frequency. Moreover, a greater crystal growth rate was obtained in the pulse electrodepositing process, the increased fresh atoms were incapable of migrating to (111) plane, and some of them would rest on (200) plane that has higher surface energy [10]. Borkar and Harimkar [8] have reported the intensity of (111) peaks in the XRD patterns of Ni composite coatings for PC, indicating more random crystallographic texture in the coatings. Moreover, the atomic density of (111) plane is higher than that of (200) plane in FCC crystal structure of Ni, so surface energy of (111) plane is lower than that of (200) plane. The widths of the diffraction peaks (111), (200), and (220) of pulse coatings are significantly identical with the PRC coatings, indicating the fine-grained texture of pulse coatings (PC) [16]. The peak width of the Ni diffraction peaks increased due to both a decrease in the current density and the codeposition of Al₂O₃ nanoparticles [17]. The present investigation revealed that the PC technique is used to achieve the fine-grained texture, smooth surface, and compact microstructure of the coatings.

Figure 5

XRD of Ni-nano-Al₂O₃ composites of AISI 304 stainless steel specimen.

3.3 Microhardness

Microhardness measurements were performed on the surface of the pulse-plated Ni-nano-Al₂O₃ coatings using Taguchi’s techniques. The hardness values are optimized for this method of deposition. The optimized values are found in Table 2. The reported values are obtained at frequency 10 Hz, duty cycle 20%, and peak current density 0.6 A/cm². Compared with DC plating, pulse plating produces harder coatings. This indicates that the bath chemistry has influenced the hardness of the coating. The Watts-type bath produced more microhardness in the Taguchi’s techniques (370.2 HV) compared with the Bund et al. [1] baths of sulfate bath (50–105 HV) and alkaline pyrophosphate bath (135–275 HV). The improved hardness of nanocomposite coatings is almost impossible to achieve by using simple rules of mixtures. In nanocomposite films, two or more crystalline phases may be present and strongly bonded to each other at the grain boundaries so that grain boundary sliding is very difficult. Therefore, these coatings could exhibit hardness values two or three times higher than the uncoated substrate or even the composite matrix alone [2]. Jung et al. [11] have studied that the influence of the duty cycle and amount of suspended Al₂O₃ on the content of Al₂O₃ in the deposit shows the result of microhardness (285 HV) in the Ni-Al₂O₃+Na-saccharine solution. Furthermore, Thiemig at al. [17] have produced the maximum hardness value of 85 HV with the copper-Al₂O₃ composite coatings and also stated that the hardness of metal films as well as MMC is known to be related to the structure of the matrix and the amount and distribution of the reinforcing metal oxide particles. Moreover, Surender et al. [18] have produced maximum microhardness value of 324 (kg/mm²) in Ni-WC composite coatings through Julabo constant temperature bath at 0.1 A/cm² current densities. Furthermore, Marikkannu et al. [19] have reported that the effect of pH 4 of the Ni-Al₂O₃ composite coatings of the microhardness value is significantly identical to this study. From the data, it can be observed that the hardness of Ni-nano-Al₂O₃ composites is affected by different types of bath, pulse parameters, and bath composition.

3.4 Coating thickness

The cross-sectional image of Ni-nano-Al₂O₃ composite coatings of optimized pulsed electrodeposition parameters such as frequency 10 Hz, duty cycle 20%, and current density of 0.6 A/cm² is shown in Figure 6. The coatings were uniform and homogeneous, as per ASM (Handbook Volume 9) standard. Moreover, the Metscope-1, a metallurgical microscope with a magnification factor of ×200, is being used to identify the coating thickness and the thickness was found to be 15 μm, which is significantly identical to the results of Feng et al. [20, 21]. Furthermore, Gheorghies et al. [22] have reported that the coating thickness depends on the current density, the deposition time, and all the other variables of the electrodeposition process. Additionally, the concentration of the dispersed particles in the electrolyte has an effect on the thickness.

Figure 6

Metallurgical microscope (Metscope-1) image of coating thickness of Ni-nano-Al₂O₃ composite coatings of AISI 304 stainless steel specimen.

3.5 Corrosion studies

Salt spray tests have long been used to determine the corrodibility of metals and the degree of protection provided by inorganic or organic coatings. However, they are easily performed acceptable standards for comparing the behavior of materials and coatings [7]. In general, low carbon steel, particularly stainless steel, has more chromium content (about 18–20% of the total composition) and better corrosive resistance (resist more oxidizing acids and salt spray) than type 302 steel [23]. Salt spray test practice provides a controlled corrosive environment that has been utilized to produce a relative corrosion resistance information for specimens of metals and coated metals exposed in a given test chamber [24]. Moreover, Ganesan et al. [25] have reported the salt spray test techniques revealed excellent corrosion production properties. The salt spray method was used to study the corrosion effects. The experimentations were carried out by using ASTM B117-07 standards. The sample was checked periodically with the time period of 0–408 h. During the whole duration, no corrosion was evidenced over the specimen as depicted in Figure 7. First check of the sample was done between 28 and 35 h, and the second check was done after 77–105 h. Checks were repeated at 168, 224, 343, and 408 h, respectively.

Figure 7

Corrosion test-salt spray test (ASTM B 117-07) of the optimized AISI 304 stainless steel.

3.6 Confirmation test

The confirmation experiment was conducted at the optimum setting of process parameters. In this study, after identifying the optimum process parameters (frequency 10 Hz, duty cycle 20%, and peak current density 0.6 A/cm²), the settings were used again to produce a new specimen with coating, and its corresponding hardness value is evaluated. The magnitude of hardness was found to be 375.6 HV (Vickers hardness), which means the improved S/N ratio with a value of 0.78. Thus, the experiment results confirm the prior design and analysis for optimizing the hardness parameters. Also, it is clear that pulse frequency is predominant in deciding the hardness value.