Optimization of electroless bath process parameter for improving the tribology behavior of Ni-P/CaBr₂ composite coating against the hardened EN-31 steel

A circular low carbon steel sample with the diameter of 55 mm and thickness of 8 mm were used as substrate material for the electroless deposition of Ni–P and Ni–P/CaBr₂ coating.

All chemical reagents were dissolved in the 1000 ml of pure double distilled (DD) water and stirred for 1 h duration using the magnetic stirrer. The schematic representation of the pretreatment and coating process is shown in figure 1. Typical weight combination of nickel sulphate and CaBr₂ particles were shown in table 1 while maintaining the other chemical agents and parameters constant (Sodium hypophosphite—23 g l⁻¹, Tri-sodium citrate—13 g l⁻¹, lead acetate—0.001 g l⁻¹, and pH—7). The weight combination of nickel sulphate and CaBr₂ particle was selected based on the trial and error method. After the preparation of Ni–P solution, CaBr₂ particles were dispersed in the bath and stirred for 1 h duration at the speed of 350 rpm (Rotation per Minute). Non-homogeneous anhydrous CaBr₂ particle with the size of 10–18 μm was used as in received condition. After a continuous stirring of CaBr₂ particles, heat is supplied to the bath at a rate of 1.6 °C min.⁻¹ to reach a temperature of 97 °C [17]. The pretreated samples were immersed immediately in the alkaline bath after the temperature reached 97 °C and deposition process were carried out for the duration of 1 h. The pretreatment process carried out on the mild steel prior to the deposition process is detailed in the author's recent article [17]. Fabricated coatings are then post heat treated at 400 °C for 1 h at the rate of 30 °C min.⁻¹ using the tubular furnace [20].

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Morphology and elemental analysis of Ni–P and Ni–P/CaBr₂ composite coatings were analyzed using scanning electron microscope-EDS (Hitachi S-3400N) at different magnifications. The phase structure change of the Ni–P/CaBr₂ composite coatings with respect to the reinforcement of CaBr₂ particle and heat treatment process were analyzed using XRD (X' per PRO, CuKα (1.54 Å)) in the step size of 0.05°. Micro-hardness was analyzed in the Vickers hardness tester for a dwell period of 10 s. The average value of the hardness for eight samples was measured and reported. The average surface roughness (Ra) of the coatings was obtained using 3D surface profilometer. Friction and wear test is done against the hardened EN-31 counter-face using the pin on disc (DUCOM) 15 N loading condition at dry sliding mode using ASTM G99-05. Duration of the wear test was 10 min at a sliding velocity of 0.75 m s⁻¹. Mass of the sample was measured using the digital weighing machine and wear rate is calculated using the equation referred to the recent article [21]. Wear track characterization was done using SEM and EDS analyzer to discuss the wear mechanism influencing the material removal during the sliding of coated samples against the En-31 counter-face.

For optimization of the process parameter levels, RSM was employed through the Design-Expert software (Ver.7). RSM is a statistical approach used for analysing the problems on which desired response is affected by several input parameters [22, 23]. Two input parameters (Nickel sulphate and CaBr₂ particle) were used to analyze the output responses like roughness, hardness, COF, and wear rate. Design of experiment was done using the central composite face centered design. Process parameters and their levels (Low, medium, and High) used in this research work are shown in table 2. Mathematical model was generated from the experimental results. Response surface plots, residual plots, prediction values, and desirable graphs were used to validate the mathematical model generated for the taken composite coating.

The average surface roughness measured using 3D surface profilometer (table 3) was plotted in the figure 2. High surface roughness of 1.505 μm was exhibited for the Ni–P/CaBr₂ composite coating with the combination of 1 g l⁻¹ of CaBr₂ and 20 g l⁻¹ of nickel sulphate. The primary reason for high surface roughness was due to the impingement and solubility of CaBr₂ particle in the bath. Solubility of CaBr₂ particle leads to two phenomenon; one is high rate of nickel deposition and doping of Ca elements between the nickel and phosphorus atoms. High rate of nickel deposition was due to the increased oxidation reaction created by the CaBr₂ particle in the bath. To discuss the role of CaBr₂ particle on the rate of Nickel and phosphorus deposition, the obtained EDS analysis of the composite coating is shown in table 4. The increase in weight percentage of Nickel is clearly evidenced for the composite coatings. Generally in the electroless composite deposition, the nickel and phosphorus were replaced with second phase particles. Hence, the weight percentage of nickel and phosphorus generally decreases with the increase in the second phase particles. For ceramic composite coatings, only the impingement of second phase particle could take place on the Ni–P matrix and it do not have much influence on redox reaction. This impingement of particles [24] on the surface of the matrix is the main reason for increase in the surface roughness of ceramic reinforced composite coatings. However, this accounts only for the insoluble ceramic particle [25]. In the recent article [26], it is quoted that the reinforcement of sea shell and nano zinc oxide particle could increase the roughness of the composite deposit to 2.21 μm. The obtained surface roughness (1.505 μm) of Ni–P/CaBr₂ composite coating was low as compared to the composite deposit seen in the recent literature [26]. This clearly states that the roughness of Ni–P composite deposit could be increased even for the nano particles reinforcement. Also, the low surface value obtained for the Ni–P/CaBr₂ composite coating justifies that the surface roughness of the Ni–P deposits can be controlled by using the soluble reinforcement particles [17].

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For soluble particle, the atom of second phase particle tends to locate between the nickel and phosphorus atoms and increase the growth of the nodules. The formation of nodules mainly depends upon the rate of nickel deposition. For example, the size of the nodule decreases with increase in the nickel deposition [27] and vice versa. Similarly for the reinforcement of CaBr₂, the high rate of nickel deposition has affected the nodular formation. The doping of high Ca elements has enhanced the growth of the nodular structure. This growth in the nodular structure created an uneven profile on the surface, which induced a high surface roughness for Ni–P/CaBr₂ composite coating (figure 3(b)) as compared with the nodular structure of Ni–P coating (figure 3(a)).

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It is clear from the table 3 that the increases in the addition of CaBr₂ particles have decreased the surface roughness. However, the surface roughness increases gradually on increasing the addition of nickel sulphate from 16 g l⁻¹ to 20 g l⁻¹. This is because of the increase in weight percentage of nickel in the coating. During the deposition process, the particle reinforcement mainly depends on the nickel—phosphorus matrix. Deposition of more nickel atoms could reduce the impingement of second phase particles and vice versa. The deposition of more nickel atoms was due to two reasons, one is presence of high source of nickel, and other is increased oxidation reaction as discussed earlier. Hence, on comparison, the weight combination of CaBr₂ (3 g l⁻¹) and nickel sulphate (16 g l⁻¹) has presented low surface roughness of 0.345 μm. It is also concluded that the low weight percentage of nickel sulphate could play a vital role on reducing the surface roughness with high weight combination of CaBr₂ particle. The accumulation of un-dissolved CaBr₂ particles was clearly mapped on the surface of NiP-CaBr₂ composite coating using the EDS analysis (figure 4), which clearly states that the CaBr₂ could also remain as a compound in the Ni–P/CaBr₂ composite deposit.

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Figure 5 depicts the cross section image of Ni–P and Ni–P/CaBr₂ coatings. Low coating thickness of 18 μm was presented for Ni–P, which is comparatively lower than that of 23 μm of Ni–P/CaBr₂ composite coating. There was an increase in the coating thickness with the initial dispersion of CaBr₂ particle (1.0 g l⁻¹) (figure 5(b)), which was related to impingement of CaBr₂ in the Ni–P matrix. There was not much difference in the thickness of Ni–P/CaBr₂ composite coating after increasing the weight dispersion of CaBr₂ particle in the Ni matrix. The existence of Ni, P, Fe, Ca, and Br elements were also confirmed in the EDS mapping, which evidencing the constituents of mild steel and Ni–P/CaBr₂ composite coating (figure 6).

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Figure 7 shows the XRD pattern of Ni–P and Ni–P/CaBr₂ composite coatings for the weight combination of CaBr₂ (1 g l⁻¹, 2 g l⁻¹ and 3 g l⁻¹) and nickel sulphate (16 g l⁻¹, 18 g l⁻¹ and 20 g l⁻¹). It was noticed that the Ni phase peaked at an angle 2theta of 50° was the dominant phase for the Ni–P coating along with the Meta stable phases such as Ni₃P and Ni₁₂P₅ phases. These results were almost matches with the result obtained in the various studies [27, 28]. Reinforcement of CaBr₂ particles had greatly influenced the crystallographic change of the Ni–P/CaBr₂ composite coatings. The FWHM width of the major Ni (111) peak was found to be increasing after the reinforcement of CaBr₂ in Ni–P matrix. The increase in the width of Ni (111) plane indicates the growth of the Ni crystal after the reinforcement of the CaBr₂ particles. The growth of the crystal size was related to an increase in the volume of the Ni space lattice due to the doping of Ca and Br elements. The dissolved Ca and Br ions could locate between the Ni atoms during the deposition process. This allows the increase in the volume of the space lattice of Ni, which caused the increase of crystal size.

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In addition to the crystal growth, there was also a decrease in the Meta stable phases (Ni₃P and Ni₁₂P₅) with increasing dispersion of CaBr₂ particles in the Ni–P bath. Formation of Meta stable phases is generally because of the solubility of phosphorus (P) element in the matrix. The presence of P element in terms of high or low content could play a crucial role in the formation of Ni₃P and Ni₁₂P₅ phase. So, a decrease in the phosphorus content due to the doping of Ca⁺ and Br⁻ has been the reason for the reduction in the Meta stable phase formation in Ni–P/CaBr₂ composite coatings. The presence of Ca⁺ and Br⁻ elements between the Ni and phosphorus atoms inhibits the formation of Meta stable phases (Ni₃P and Ni₁₂P₅) during heat treatment process. Hence, Ni₃P and Ni₁₂P₅ phase formation was affected for the Ni–P/CaBr₂ composite coatings. On comparison, Ni–P/CaBr₂ composite coating with the weight combination of 3 g l⁻¹ of CaBr₂ and 20 g l⁻¹ of nickel sulphate has shown a low Meta stable phase formation due to the high doping of Ca⁺ and Br⁻ elements in the NiP matrix. There was also a shift of Ni (111) peak from the angle of 50° to the angle of 45° for Ni–P/CaBr₂ coatings after the dispersion of CaBr₂ particle. The doping of Ca⁺ and Br⁻ impurities displaced the crystal planes and creates an internal stress in the coating. Hence, there is a slight shift in the Ni peak for Ni–P/CaBr₂ coatings with the initial weight dispersion (1 g l⁻¹) of CaBr₂ particle in the bath. Peak shift increases considerably to 44.5° and 44.2° with the dispersion of CaBr₂ particle (2 g l⁻¹, and 3 g l⁻¹). Maximum shift was obtained for the Ni–P/CaBr₂ coating with the high weight combination of nickel sulphate and CaBr₂ particle (20 g l⁻¹ and 3 g l⁻¹), which is due to the high stacking fault created by the Ca⁺ and Br⁻ elements in the Ni crystal lattice.

The hardness value measured for the low carbon steel and Ni–P deposit are reported as 125 HV_0.05 and 513 HV_0.05. Hardness of low carbon steel has been significantly improved after the Ni–P deposit. To discuss the influence of CaBr₂ and nickel sulphate, the hardness of the Ni–P/CaBr₂ composite coatings were measured and shown in figure 8. Figure 8 demonstrates the hardness plot of Ni–P/CaBr₂ coating with various weight combination of CaBr₂ and nickel sulphate.

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Hardness of Ni–P/CaBr₂ composite coating rises gradually with the nickel sulphate and dispersion of CaBr₂ in the bath. Maximum hardness of 793 HV_0.05 was achieved for the high weight combination of CaBr₂ (3 g l⁻¹) and nickel sulphate (20 g l⁻¹). This is due to the increase in Ni content with respect to the increased oxidation created by the CaBr₂ particle during the deposition process. Though, many articles reveal the decrease in hardness due to existence of Ca in Ni–P matrix [29], the presence of Ca elements in Ni–P/CaBr₂ composite coating has no role on decreasing the hardness. This is because of the increase in the Ni content caused by the high weight combination of Nickel sulphate and CaBr₂ in the Ni–P bath. As discussed earlier, solubility of CaBr₂ particle was the main cause for increase in the hardness. The dissolved CaBr₂ has promoted the increase of Ni deposition and simultaneously decreased the deposition of P. On account of that, Ni is the dominant phase in the composite coating, which is found to be a main reason for achieving the improvement in the hardness of Ni–P/CaBr₂ coating. Various article revealed that the hardness of composite coating depends on the hardness of the reinforcement particles [30]. However, in the case of CaBr₂ composite coating, the increased oxidation is the cause for the improved hardness of Ni–P/CaBr₂ composite coating.

Low carbon steel showed a high wear rate of 11.94 × 10⁻⁶ gm.m⁻¹ and a COF of 0.48 compared to the other deposit. Wear rate decreases considerably to 8.06 × 10⁻⁶ gm.m⁻¹ with the COF of 0.3 after the Ni–P deposit. The formation of a semi lubricating layer was found to be the reason for the improvement in the wear resistance of Ni–P. This lubricating layer resists the material removal and improves the friction resistance during the sliding. Formation of semi lubricating layer and its influence on COF and wear rate has been discussed in many articles [5]. To discuss the wear mechanism during the sliding, the SEM images of the wear track of Ni–P and Ni–P/CaBr₂ coating are shown in figure 9.

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Ni–P exhibited a severe adhesive wear in the wear track (figure 9(a)) compared to the Ni–P–CaBr₂ composite coating. In addition to adhesive, de-lamination wear is also spotted in the wear track of Ni–P coating. The formed lubricating layer was removed in the form of chips during the wear test. The removed lubricating layers from the wear track reports that the Ni–P coating starts to fail at the higher loading condition. The weak sustainment of tribolayer for Ni–P coating at high load condition was also been discussed in the recent article [29], which matches with the above obtained results. The detachment of chips with the crack formation was clearly observed in the track of Ni–P coating. Hence, the observed wear mechanism for Ni–P coating is adhesive and delamination wear. The 3D surface image of the wear track of Ni–P coating also confirms the removal of lubricating layer after the wear test (figure 9(a)). The measured mean wear depth of Ni–P coating (8.202 μm) is also high as compared to that of Ni–P/CaBr₂ coating (4.407 μm) with the high weight combination of nickel sulphate and CaBr₂ particle.

The wear rate and COF measured for the Ni–P/CaBr₂ composite coatings against the sliding with hardened En-31 counter-face is presented in table 3. From the table 3, it is clear that the COF and wear rate decreases considerably with the Ni–P/CaBr₂ as compared to the low carbon steel and Ni–P deposit. Wear rate and COF decreases considerably with the dispersion of CaBr₂ particle and nickel sulphate in the bath. The obtained wear rate (3.33 × 10⁻⁶ gm⁻¹) and COF (0.17) were found to be low for the high weight combination of CaBr₂ particle and Nickel sulphate (NS) as compared to the other weight combinations. The primary cause for low COF and wear rate is due to the improved hardness and lubrication created by the Ni and CaBr₂ particle in the coating. The above improved combination of hardness and lubrication had resisted the removal of lubricating layer during the sliding. The presence of lubricating layer is also clearly seen in the wear track of Ni–P/CaBr₂ composite coating, which is having the maximum weight combination of CaBr₂ particle and nickel sulphate (figure 9(b)). The elemental composition of the lubricating layer is shown in figure 10, which consists of nickel, oxide, phosphorus and iron. This indicates that, the nickel tends to adhere to the surface in the form of an oxide layer during the sliding, which is responsible for the improvement in the wear resistance. The adherence of a lubricating layer was found to increase in the wear track of Ni–P/CaBr₂ composite coating after the reinforcement of CaBr₂ in the matrix. Hence, restricting the delamination of lubricating layer from the wear track and subsequently led to the improvement in the wear resistance of Ni–P/CaBr₂ composite coating. Also, the reinforcement of CaBr₂ particle have resulted the considerable reduction in the size of the chips for the high weight combinations of CaBr₂ and nickel sulphate.

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The most influenced input parameters and optimum weight combination of nickel sulphate and CaBr₂ particle for obtaining the better hardness and wear resistance has been determined from the experimental results (table 5) using ANOVA. The ANOVA results (95% confidence level and 5% significance level and) for the output responses (Surface roughness, hardness, COF, and wear rate) are shown in tables 6–9. The mathematical model developed for the output response are shown in the equations (1)–(4).

$$\begin{eqnarray}&&\begin{array}{l}{\rm{Ra}}=-2.02805+1.21329\,* \,{{\rm{CaBr}}}_{2}+0.12933\,* \,{\rm{NS}}\\ \,\,\,\mbox{--}0.36324\,* \,{{\rm{CaBr}}}_{2}^{2}\end{array}\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&\begin{array}{l}{{\rm{HV}}}_{0.05}=+176.80952-38.44048\,* \,{{\rm{CaBr}}}_{2}\\ \,\,\,\,\,+\,14.83333\,* \,{\rm{NS}}+11.375\,* \,{{\rm{CaBr}}}_{2}\,* \,{\rm{NS}}\\ \,\,\,\,\,\mbox{--}\,26.78571\,* \,{{\rm{CaBr}}}_{2}^{2}\end{array}\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{\rm{COF}}=+0.47974-0.038333\,* \,{{\rm{CaBr}}}_{2}\mbox{--}0.01\,* \,{\rm{NS}}\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&\begin{array}{l}{\rm{Wear}}\,{\rm{rate}}\left({\rm{x}}{10}^{-6}\right)=+13.09821-2.02333\,* \,{{\rm{CaBr}}}_{2}\\ \,\,\,\,\,\,\,\,\mbox{--}\,0.18\,* \,{\rm{NS}}\end{array}\end{eqnarray} \tag{ 4 }$$

The model F-value for surface roughnes is 5.17 and found to be significant at 95% confidence level. Also it is noted that the obtained F-value for Nickel sulphate is greater than the F-value of CaBr₂ particle. This indicates that the nickel sulphate is more influential in controlling the roughness of the Ni–P/CaBr₂ coatings. Figure 11 presents the residual plots of the design model of Ni–P/CaBr₂ composite coating. It is clear that the residuals are distributed closer to the mean straight line, which confirms that the models are adequate for the Ni–P/CaBr₂ composite coatings. It is also clear that the predicted values were closer to the experimental ones (figure 12). This evidences that the formulated model is significant with the experimental result. Similary the obtained F-value (93.35, 84.94, and 292.96) for hardness, COF, and wear rate are found to be significant and hence, the formulated model can be considered. The accuracy of the formulated models were checked by using the residual analysis

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3.5.1. Influence of bath parameters on roughness, hardness, COF and wear rate

Figure 13 depicts the relationship between the input parameters and the output responses. Figure 13(a) represents the effect of nickel sulphate and CaBr₂ particles on surface roughness. When the dispersion of CaBr₂ particle is increased to 1 g l⁻¹, and 2 g l⁻¹, there is an increase in the surface roughness and decreases with the weight dispersion of 3 g l⁻¹. At the same time, when the nickel sulphate is increased with the CaBr₂, the surface roughness decreases considerabily (figure 13(a)). It means that the nickel sulphate particle dominates in reducing the roughness of Ni–P/CaBr₂ composite coating when compared to that of CaBr₂ particle.

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Figure 13(b) shows the relationship between the input parameters and hardness. It is clear that, increase in both CaBr₂ and Nickel sulphate has increased the hardness of the coatings. However, the contribution of nickel sulphate particle is found to be more in increasing the hardness of the Ni–P/CaBr₂ coating when compared to that of CaBr₂ particle. High hardness is achieved for the high weight combination of CaBr₂ and nickel sulphate, due to presence of more Ni atoms in the Ni–P/CaBr₂ coating. The relationship between the input parameters, COF, and wear rate is depicited in figures 13(c) and (d). COF and wear rate reduces gradually with increase in the dispersion of CaBr₂ and nickel sulphate in the bath. The increase in CaBr₂ particle has more influence in reducing the COF and wear rate when compared to that of Nickel sulphate.

Figure 14 shows the desirability chart of the formulated models for Ni–P/CaBr₂ coating. It is clear that the combination of 3 g l⁻¹ of CaBr₂ and 20 g l⁻¹ of nickel sulphate has the highest desirability of 1 for the COF of the Ni–P/CaBr₂ coating, followed by the hardness, wear rate, and surface roughness. There is a slight change in the desirability values for the nickel sulphate from the weight percentage of 16 g l⁻¹ to 18 g l⁻¹. This indicates that the influence of nickel sulphate from 16 g l⁻¹ to 18 g l⁻¹ on roughness, COF, hardness, and wear rate is very minimum. However, with increase of the weight percentage of nickel sulphate to 19 g l⁻¹ and 20 g l⁻¹, there is a gradual increase in the desirability values. This indicates that the nickel sulphate at the higher weight percentage will certainly have an impact on the mechanical and tribology property. In case of the weight percentage range of CaBr₂ particle, there is a tremendous increase in the desirability value. The above indicates that the minimum amount of CaBr₂ particle also influences the mechanical and tribology property.

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