Effect of Nanoadditives with Surfactant on the Surface Characteristics of Electroless Nickel Coating on Magnesium-Based Composites Reinforced with MWCNT

An experimental investigation has been carried out on optimizing process parameters of electroless nickel-phosphorous coatings on magnesium composite reinforced with carbon nanotube. A comprehensive experimental study of electroless Ni–P coatings on magnesium composite reinforced with multiwalled carbon nanotube under specific coating conditions was performed. The electroless coating bath consists of nickel sulphate (26 g/L), sodium hypo-phosphite (30 g/L) as reducing agent, sodium acetate (16 g/L) as stabilizer, and ammonium hydrogen difluoride (8 g/L) as the complexing agent. The surfactant SLS was added in the solution for better wetting and spreading of coating on substrate. The stabilizer thiourea (1 ppm) was added in the bath to prevent decomposition of bath.Different nanoadditives such as ZnO,Al 2 O 3 , SiOwith various concentrationswere used in the bath and their influence on coating process characteristics were studiedThe nano additives such as ZnO, Al 2 O 3 , SiO were added at concentrations of 0.1%, 0.5%, 1%, and 2% in the EN bath.The output parameters such as surface roughness, microhardness, specific wear rate, and surface morphology were measured. Surface morphology was studied using scanning electronmicroscope.The results showed that the proposed method resulted in significant improvement on the quality of the coatings produced.


Introduction
Electroless nickel coating has received widespread acceptance as it provides a uniform deposit on irregular surfaces, direct deposition on surface-activated nonconductors, formation of less porous deposits, and high hardness and excellent resistance to wear, abrasion, and corrosion [1,2].All smooth surfaces possess some degree of roughness, even if only at the atomic level.Correct function of the fabricated component often is critically dependent on its degree of roughness.Every machining operation bequeaths some characteristic on the machined surface.This characteristic microirregularities left by the cutting tool are termed as surface irregularity or surface roughness [3].Roughness is sometimes an undesirable property, as it may cause friction, wear, drag, and fatigue, but it is sometimes beneficial, as it allows surfaces to trap lubricants and prevents them from welding together.Magnesium composites have promising properties for several industrial applications because of their low density [4].Magnesium composite with metallic (electroless/electroplating) deposits are being used, in new light-weight engines which are less in weight and hence consume less energy.However, metallic coatings in magnesium are having multitudinous problems caused by surface roughness.Example of mechanical malfunction can be found in high-performance engine machine parts which are required to move or rotate at high speed without wear.Excess surface roughness can lead to unacceptably high levels of frictional heating, causing damage and even failure [5].Surfactants are specifically added into the electrolyte bath to reduce the vertical component of surface tension forces, which binds the nickel particles to the hydrogen gas bubbles generated during the plating reaction.Due to this, uniform and pit-free coating can be obtained.Smooth and pit-free electroless Ni-P deposits were obtained by adding 150 ppm of sodium dodecyl sulfate (SDS) to the electroless nickel bath [5].Similarly, a very brief conclusion was derived by Hagiwara et al. [6] as well, who studied the effect of three different surfactants added in the Ni-P electroless bath on the morphology of the resulting Ni-P particles.Many attempts have been made to find out the effect of surfactants on the roughness of electrodeposited Ni-P coatings.Tripathy et al. [7] and Wheeler et al. [8] studied a numerical model to explain the influence of catalytic surfactant on roughness evaluation.Alsari et al. 's [9] research studied the SDS effect on the electroplating deposition.Several researchers had carried out investigations on the influence of surfactants on coatings of ferrous substrate [10][11][12].Elansezhian et al. [13] investigated the influence of SDS on quality of electroless Ni-P coatings and reported that there is a possibility of significant improvement in the average surface finish of electroless Ni-P deposit on mild steel.However, there was no such investigation on magnesium composite with coating of nanoadditives and moreover it is complicated because of the corrosive nature of magnesium substrate in the electrolyte bath.Hence, in this investigation, three types of nanoadditives such as Al 2 O 3 , SiO, and ZnO were used in the electrolyte bath and their influence on electroless Ni-P deposit of magnesium composite was studied.The quality of obtaining electroless Ni-P deposit on the substrate depends on many factors such as temperature, pH of bath, bath loading, concentrations of nickel and the reducing agent, and the surface properties of the substrates [14][15][16].Wetting agents, such as ionic and nonionic surfactants, are often added to increase the wettability of coated surfaces [17].Despite the complicated behaviour of the deposition reactions, qualitative discussions on the effects of added nanoadditivesss such as Al 2 O 3 , SiO, and ZnO in the presence of surfactant (SLS) the surface roughness, surface morphology, microhardness, specific wear rate, and wear morphology are investigated and reported in this paper.

Experimental Details
The substrate material used in the present study was magnesium composite synthesised with MWCNT.The specimen size was 26 × 8 × 7 mm.The magnesium composite was indigenously synthesised by using magnesium stir casting furnace.The output of casting was in rod form.The rod was cut into the desired shape by using wire cut EDM process.The % of elements present in magnesium composite was presented in Table 1 and the composition was confirmed with EDX.The bath composition and all the operation parameters for the electroless Ni-P deposit with chromiumfree pretreatment are reported in Table 2.In addition, anionic surface activator sodium lauryl sulphate (SLS) was used in this study as a surface activator and to enhance the properties of the deposits.No agitation was employed to the bath during the plating process.At critical micelle concentration (CMC) concentration, surface activator reduces the contact angle and this leads to the better wettability of Ni-P deposit.The SLS surfactant was used at its CMC value 1.2 g/L concentration.The samples were given thick nickel strike for about 20 minutes by using electroless bath itself and without any activator and then dipped into the bath having surface activator.The sizes of nanoparticles used in bath were ZnO (50 nm), Al 2 O 3 (40 nm) and SiO (25 nm), and all the nanopowders were imported from Alfa Aesar, USA, with a purity of 99.9%.After coatings all the samples were cleaned with deionized water for 2 minutes and dried.Wear studies were performed on a Ducom pin-on-disc model TR-201 friction and wear monitor with a computer interfaced data acquisition system.For all tests, sliding velocity was fixed at 0.5 m/s and sliding distance was 1000 m.The load applied was 30 N. No lubrication was done during the test.
(Linear variable differential transformer) LVDT was used to measure the linear displacement of the specimen and a load cell was used to measure the frictional force experienced by the specimen under load.The pins were coated with electroless Ni-P deposits with and without surfactant.The disc selected for wear test was high carbon-high chromium steel, fully hardened to 65 HRC and finished to 0.2   .All the experiments were conducted in an air-conditioned room at 20 ∘ C. Wear tracks on the electroless coated pins were examined using a scanning electron microscope (SEM).The magnesium samples prepared for EN-coatings are shown in Figure 1.The experimental setup used for ENcoating is shown in Figure 2. The EN bath prepared for the coating is presented in Figure 3.The coated samples are presented in Figure 4. Microhardness of the EN deposits was estimated using a Future-Tech microhardness tester with a diamond pyramid as an indenter, 200 gm load, and 15 seconds loading time.Surface roughness of EN deposits was measured using a stylus instrument.

Surface Morphology of Nanoadditivesss with and without
Surfactant in Electroless Ni-P Deposits on Magnesium.The SEM micrographs of EN-coated samples with nanoadditivesss are shown in Figures 6, 7, and 8. Without nanoadditivesss, the surface of the coating consists of relatively lower     amount of nickel particles on the matrix and nonuniform deposition of nickel resulted in higher surface roughness.The SEM micrograph presented in Figure 5 clearly showed the nonuniform deposition of nickel particles on the surface of substrate.After adding nanoadditivesss with surfactant, the surface morphology has changed from nonsmooth nodular appearance to a smooth surface resulting lower surface finish values.This similar trend was obtained by the earlier researcher's findings [18,19].The reason is that the amount of nickel particles deposited on the substrate surface is enhanced.This is due to the fact that the surfactant reduces the contact angle and this leads to the better wettability of Ni-P deposit on the substrate.

Variation of Al
On the EN-coated substrate surface, the traces of nano-Al 2 O 3 , nano-SiO and nano-ZnO particles are clearly seen over the Ni-P matrix and the nanoadditivesss are confirmed with EDX (see Figures 21-26).Among the three nanoadditivesss, addition of nano-SiO resulted in smooth surface finish and the surface finish is in the order of 0.26 m as compared to tha of nano-Al 2 O 3 (0.58 m) and ZnO (1.27 m).not influence the surface roughness vales very much.Furthermore, agglomeration of nanoparticles takes place over the ENi-P matrix and this leads to increased surface roughness.  of trend was obtained by earlier researchers after adding nanoadditivesss in their respective studies [20,21].nanoadditivesss, the EN-coatings show better wear resistance.Adhesive wear is characterized by the transfer of material from one surface to other which may later be removed as wear debris.The rate of adhesive wear is influenced by several factors such as hardness and adhesion between the interacting surfaces.Adhesive wear is related, though not directly to the hardness of the surface, which is an   indication of how much the tops of the asperities deform plastically.Greater the hardness less is the deformation, and consequently, less intimate is the contact.This leads to lower friction.The corresponding graphical values are shown in Figures 19 and 20.

Effect of Nanoadditivesss on
Wear morphology of coated samples with different % of nanoadditivesss of nano Al 2 O 3 and nano SiO are presented in Figures 17 and 18.At lower concentrations (0.5%) of nanoadditivesss, the delamination of coatings and their debris are clearly visible in the wear morphology (Figure 17(a)).As the % of nanoadditivesss increased the wear tracks are smooth and exhibited a low wear rate.The reason for the low wear rate at higher concentration of nanoadditivesss is increased hardness of coatings.

Conclusions
A comprehensive experimental study under specific coating conditions on the influence of addition of various nanoadditivesss with SLS surfactant in the electroless nickel bath on the mechanical properties and tribological properties of the coatings produced has been carried out and the results are presented and analysed.In general, it has been observed that the surface finish, microhardness, specific wear rate and friction of the EN-coated layers improved significantly with the addition of nanoadditivesss.Based on the present investigations, the following specific conclusions could be drawn.
-P Deposits on Magnesium Composite.The variation of average surface roughness value (  ) of the coated layer with Al 2 O 3 , SiO, and ZnO are shown in Figures 9, 10

Figure 9 :
Figure 9: Variation in average surface roughness of EN-coatings as a function of Al 2 O 3 with surfactant (SLS) concentration in the EN bath.
Figures 12,13, and 14 showed the variation of microhardness of ENcoating with respect to Al 2 O 3 , SiO, and ZnO.At low % of nanoadditives the microhardness value is low; when there is

Figure 10 :Figure 11 :Figure 12 :Figure 13 :
Figure 10: Variation in average surface roughness of EN-coatings as a function of SiO with surfactant (SLS) concentration in the EN bath.

Figure 14 :Figure 15 :
Figure 14: Variation in microhardness of EN-coatings as a function of ZnO with surfactant (SLS) in EN bath.

Figure 16 :
Figure 16: Variation in specific wear rate of EN-coatings as a function of SiO with surfactant (SLS) in EN bath.

Figure 19 :
Figure 19: Variation in coefficient of friction of EN-coatings as a function of Al 2 O 3 with surfactant (SLS) in EN bath.

Figure 20 :
Figure 20: Variation in coefficient of friction of EN-coatings as a function of SiO with surfactant (SLS) in EN bath.

3. 5 .
Effect of Nanoadditivesss on Coefficient of Friction.The electroless Ni-P coatings produced with addition of nanoadditivesss with SLS surfactant in the EN bath lowered the friction coefficient upto 52.38% and 61.90% with the addition of nano Al 2 O 3 and nano SiO when compared to the coatings produced without nanoadditivesss.Due to increase in the hardness and the amorphous fraction in the coating the coefficient of friction was reduced.The smoother surface finish

BaseFigure 25 :
Figure 25: EDX diffractogram showing the % of elements present on the EN-coated Mg sample with 0.5% nano-ZnO.

Figure 26 :
Figure 26: EDX diffractogram showing the % of elements present on the EN-coated Mg sample with 2% nano-ZnO. .
(i) There was an improvement in the surface finish (upto 67.86%, 72.3%, 29.63% in   values) of the coatings due to addition of nanoadditivesss such as Al 2 O 3 , SiO and ZnO respectively to the EN bath.Addition of nanoadditivesss with surfactant concentration to the EN bath prevents the floatation of nickel particles generated during the chemical reaction of the coating process.Since the nickel particles do not float and move to the top surface of bath, more percentage of nickel particles get deposited as a fine layer thus improving the surface finish.(ii) Addition of nanoadditivesss with SLS surfactant in the EN bath significantly improved the microhardness (upto 46.21%, 50.8% and 42.64%) of the coatings due to addition of nanoadditivesss such as Al 2 O 3 , SiO and ZnO, respectively.(iii) The electroless Ni-P coatings produced with the addition of nanoadditivesss with SLS surfactant in the EN bath improved the wear resistance (upto 65.38% and 69.23%) and lowered the friction coefficient (up to 52.38% and 61.90%) when compared to the coatings produced without nanoadditivesss due to the increase in the hardness and the amorphous fraction in the coating.The smoother surface finish of EN-coatings produced reduced the friction coefficient.

Table 1 :
Percentage of elements present in magnesium composite.

Table 2 :
Compositions of coating bath for electroless Ni-P used in the experiments.
Figure 21: EDX diffractogram showing the % of elements present on the EN-coated Mg sample with 0.5% nano-Al 2 O 3 .Figure 22: EDX diffractogram showing the % of elements present on the EN-coated Mg sample with 2% nano-Al 2 O 3 .Figure 24: EDX diffractogram showing the % of elements present on the EN-coated Mg sample with 2% nano-SiO.
Figure 23: EDX diffractogram showing the % of elements present on the EN-coated Mg sample with 0.5% nano-SiO.