Tribological Behavior of Ni-P Electroless Coating of Inconel 625 with Multiwall Nano Carbon Tubes

An attempt was taken to study the wear rate of coated Inconel 625 using 0.3gm of multiwall carbon tubes (MWCNT).Te coating was carried out by the Ni-P electroless coating method. Te Ni-P-MWCNT coating was prepared by using nickel phosphorous solution. Te sliding wear test was conducted using pin on discs tribometer. Te wear rate behavior was investigated at various levels of pin on discs tribometer factors, and a predictive model was developed using regression equations. Te wear test ex-periment was carried out based on the L 27 orthogonal array. Te wear process parameters load, sliding velocity, and sliding distance were chosen. It was observed that the rate of wear increased as the load increases, whereas increase in sliding velocity and sliding distance reduces the rate of wear. Te developed regression model was validated with the measured wear rate. Te percentage error was observed within 0.99%.


Introduction
Inconel alloy is used in automotive, aerospace, and defense industries due to its high-strength and corrosion resistance [1,2]. Inconel 625 alloy is widely utilized in diferent area owing to its features such as high work hardening rate and low thermal conductivity. Tis alloy is particularly used to make turbine blades, aero engine parts, and heat exchangers parts. Inconel 625 is subjected to high temperature environment, and its alloy wear and corrosion resistivity need to be improved in order to withstand the impact of high temperature environment. Naji et al. [3] have studied the corrosion resistivity, biocompatibity of HA/TiO 2 , and ZrO 2 / HA coating material. Titanium alloy (Ti-6Al-7Nb) was used as substrate material. Te anodic microarc oxidizing (MAO) coating technique was used. Tey have reported that the rutile and anatase phases were present by MAO coating. Furthermore, they have said that the metaphosphates were seen by mixing ZrO 2 to the electrolyte. Guo et al. [4] have studied the efect of nano tubes-coated Ti6Al4V alloy on wear and corrosion resistivity and microstructure. Tey have carried out coating process on this alloy using various concentric level of sodium chloride solution, and the coating was done by microarc oxidation. Tey have suggested that 0.15 Gram/liter CNT has shown low porosity and roughness. Te corrosion resistivity was enhanced on increasing the CNT concentration up to 0.15 Gram/liter.
Li et al. [5] have done using microarc oxidation coating on Zr alloy substrate using composite coat Al 2 O 3 /MoS 2 / CeO 2 /graphene oxide. Tey have investigated the surface status, cross section status, composition, and structure of phase of various composite microarc oxidation coating. Tey have suggested that the composite coating consisting ZrO 2 s hashown better corrosion resistance and the MAO coating-Al 2 O 3 has shown the good resistivity of corrosion.
Askarnia et al. [6] have coated the surface of AZ91 magnesium alloys using various amounts of graphene oxide and they have used microarc oxidation process in the presence of alkaline electrolyte. Te test of scratch, behavior of corrosion, antibacterial and bioactivity properties of coated AZ91 magnesium alloys were carried out. Tey have observed that the MgO and Mg 2 SiO 4 ceramics were existed as a result of oxidation and electrolyte reaction. Te surface of AZ91 magnesium alloy coated with 20 mg/L of GO leads to minimum pores on the surface. Te crashed width of the scratch reduced from about 137 µm to 87 µm for sample coated with 20 mg/GO owing to strengthening of the coating by graphene oxide reinforcement. Küçükosman et al. [7] have deposited composite coat on AZ91 alloy using MAO (microarc oxidation) and the MAO-hydrothermal treatment. Si/ph-based electrolyte/graphite particles size of 5-10 and 75 µm were used. Te surface morphology with good appearance was observed in the MAO coating, and also the resistance of wear of the graphite-doped MAO-HT composite coatings was improved identically.
Tanu et al. [8] have used electro deposition of Ni-Ta coating on Nickel-based alloy. Teir study revealed that the scanning electron microscopic shows evidently that constant electro deposition generates smooth and uniform coatings. Higher hardness was achieved in the coating done using Ni/ Ta: 4 : 1 and it increases to 270 VHN. Reis et al. [9] stated that copper-beryllium is the materials which are widely utilized to make mold in the injection moulding process. Tough it has great abrasion resistance, still its resistivity needs to be improved. Terefore, this material is coated using Ni-P coating. Tey have investigated the hardness, wear test, XRD, and SEM analysis, and the improvement of the coated alloy behavior is reported.
Corona-Gomez et al. [10] stated that radio-frequency magnetron sputtering is used to coat TaZrN on CoCrMo biomedical substrate. Te wear and corrosion behaviour in addition to mechanical test of the coated substrate is evaluated. Tey have reported that the coated specimen has proved lower wear rate and higher corrosion resistivity. Finally, they have said that the coated specimen would improve the hip joint implant's lifetime. Wei et al. [11] reported the wear and corrosion resistivity of coated AZ31 magnesium alloy. Te alloy was coated with Al using the magnetron sputtered method after that the coated specimen is treated using plasma electrolytic oxidation (PEO). Te aluminate and silicate electrolytes are used for PEO. Tey have resulted that the aluminate produces better wear resistivity than silicate coat of the specimen. Furthermore, the PEO/Al has improved the corrosion resistivity of AZ31 alloy.
Ma et al. [12] have studied the corrosion and wear resistance including morphology of Mg-Li-coated alloy by adding Al-Y. Te microarc oxidation coatings are used. Te best corrosion resistance, wear resistance enhanced hardness, and minimum porosity were obtained for Mg-14Li-3Al-1Y alloy. Jiang et al. [13] have investigated the behaviour of microstructure, microhardness, and wear resistance of WC-Co coating on Ti-6Al-4V substrate. Te laser cladded coating method is used to coat Ti-6Al-4V substrate. Tey have seen an improvement in microhardness and wear resistivity of WC-Co coating on Ti-6Al-4V substrate. Qin et al. [14] have deposited SiO 2 -Ni on AZ 91 D substrate using electroplating technique, characterized by AFM, SEM, and mechanical and tribological test. It has found that the corrosion resistivity was more improved owing to composite coating than pure Ni coating. Furthermore, it was noticed that the SiO 2 -Ni composite coat has shown great wear resistance. Divya Sadhana et al. investigated the wear characteristic-Specifc Wear Rate-is optimized using the Taguchi approach (SWR) [15]. Physical Vapor Deposition (PVD)-based AlTiN can be applied to coat titanium alloy and SS 316LVM materials for biomedical applications since it has greater wear resistance qualities [16,17]. An innovative multistage micromechanical approach is proposed to explore the function of carbon nanotube (CNT) coating on carbon fbre (CF) surfaces in the efective thermal conductivities of the unidirectional polymer hybrid nanocomposites [18].
Te polarisation efciency of poly (vinylidene fuoride)/ barium titanate (PVDF/BaTiO 3 ) composites was increased by coating carbon nanotubes [19]. Titanium alloy was coated with carbon nanotubes (CNTs)-reinforced composites using a laser cladding. Due to the self-lubricating efect, the coating's COF was drastically lowered [20]. Analysis of wear characteristics of titanium nitride had been deposited by DC magnetron sputtering over 6061 aluminium alloy. Te Vickers hardness tester was used to assess the hardness of untreated and coated aluminium specimens, and the fndings showed that coated specimens had a 26% improvement in comparison to uncoated samples [21].
From the literature review, it is evidently seen that there was no adequate investigation on coated Inconel alloy. Terefore, an attempt is taken to study the MWCNT-coated Inconel 625 using electroless coating technique. Te wear rate was observed and the efect of pin on disc wear test on wear rate is analyzed.

Materials and Methods
Inconel 625 is selected as the substrate owing to its usage in aerospace industries and heat exchangers manufacturers. Electroless coating technique is chosen to coat on the substrate to improve the wear resistivity. Te following steps are about the preparation of the electroless solution: Te specimens are machined equal to 20 mm and cleaned using acetone and diluted HCL, and the specimens are dried. Figure 1 shows the wear test assembly and specimen. Te sample size ranges with 20 mm diameters and 15 mm length. Te specifcations of Pin on disc are as follows: Load (N):5-200 N; disc speed: 100-2000 rpm; motor and drive: AC motor with drive; sliding velocity: 0.5 to 10 m/ s; specimen holders: universal holders; wear measurement (µ): ±5000 microns or ±5 mm.

Taguchi Analysis
Taguchi analysis is used to design the experiment and it gives reduced variance for the experiment with optimum settings of process factors. Te orthogonal array would provide a complete set of very well balanced design of experiments. Taguchi analysis consist experiments' plan, model determination, and adequacy check of the predictive model. Furthermore, it comprises of planning of experiments, determination of model, and checking adequacy of the developed model. Te parameters selected to study the wear behavior of the coated work material are sliding distance, velocity, and load. Experiment's plan is carried out by Taguchi analysis. Te L 27 experimental trails are done for the chosen number of factors. Table 1 shows the factors and their levels. Figure 2 shows the SEM images of the coated specimen, in which the CNTs (Carbon Nano Tubes) are agglomerated on the substrate.

Wear Test
Te sliding wear behaviour is studied on the coated Inconel 625 specimen to observe the impact of MWCNT on the improvement of wear resistance. Load applied on the specimen and the specimen adheres to the counterface. Te specimen is kept close to this counter face. Te disc is cleaned and polished using SiC emery sheets to have clean surface contact of coated specimen surface. Te parameters are set in the pin on disc setup, and tests are carried out based on L 27 orthogonal array. Te rate of wear is evaluated by weight loss way as shown in equation (1). Te weight of the specimen is calculated before and after the wear test.

Results and Discussions
Te impact of wear factors on the MWCNT-coated Inconel 625 substrate is analyzed with the help of ANOVA analysis. Te wear test based on 27 experimental trails was carried out and Table 2 shows wear test results. Te experimental results are analyzed through the Design Expert, and a predictive model is developed to predict the rate of wear. Table 3 shows the estimation of the coefcients for the development of a model and signifcance test of the model. Te wear rate is expressed in equation (2) using the regression coefcient as given in Table 3, and the average percentage error is found to be within 0.99%. Table 4 shows the diference among the experimental results and forecasted results. Te other new levels of factors are substituted in the developed model and verifed the model results with experimental results. Te comparison of actual with predicted results is shown in Figure 3.
(2)  Table 5 shows ANOVA analysis. R 2 and adjusted R 2 such as 91% and 89%, respectively, for signifcance test are evident that the model developed correlate very well the factors with the wear rate. Te ANOVA is used to estimate the impact of every factor. Te P-value indicates the statistical importance of every factor. Te particular factor can be said to be statically important, if the P-value is identifed to be lesser than 0.05. Te ANOVA is set with the signifcance level of 5%. Te F-value for every factor on wear rate is evaluated as Load (175.62), velocity (5.84) and sliding  Advances in Materials Science and Engineering distance (31.33). Te F-value of every factor on wear rate, the load, and sliding distance is identifed to be a more signifcant impact on wear rate followed by velocity as shown in Table 5.

Infuence of Factors on Wear
Rate. Te 3D plot developed to study the wear rate as the level of the wear test factors such as load, and sliding distance is shown in Figures 4(a)-4(c). Te changes in wear rate at the various level of load and velocity is shown in Figure 4(a). From this Figure 4(a), it is seen that the wear has increased with increase in the level of load and it would be owing to generation of higher pressure and higher stresses during the measurement. However, MWCNT coating on the substrate minimizes the deformation owing to good adherability, stability, and adequate wear resistivity. Lower wear is seen at 10 N load due to lower pressure via lever arm (tribometer/pin on disc). Te wear rate increases as the velocity level is reduced, and it would be owing to high contact/interaction time among the disc and the specimen. Te change in the wear rate due to change in the level of sliding distance is shown in Figures 4(a) and 4(b). Te wear rate is found to be high at lower level of sliding distance and the wear rate is reduced as the level of sliding distance is increased. At low sliding distance, the wear rate is increased. It would owe some irregularities presented in the coating process, and these irregularities would induce improper contact of disc with specimen surface. Te worn out surfaces of the coated Inconel 625 are examined with help of SEM as shown in Figure 5. Te coated Inconel surfaces worn out at various levels of pin on disc factors are examined. Te worn pot coated surface at various levels of the tribometer's factors is shown in Figures 5(a) and 5(b). Te minimum of scratches with merciful wear is seen at lower level of load, middle and higher level of velocity, and higher level of sliding distance as shown in Figures 5(a) and 5(b), and it would be owing to minimum pressure induced on the pin on disc.

Conclusion
MWCNT is deposited on the Inconel 625 through the Ni-P electroless coating method. Te following conclusions were observed: (i) A predictive model is developed to predict the wear rate and it facilitates a best relation among pin on disc wear process factors with wear rate of MCWNT coated specimen. (ii) ANOVA test provides and confrms the adequacy of the model with very minimum lack of ft. (iii) Load is infuenced signifcantly on wear rate followed by sliding distance and velocity. (iv) Te plot of wear rate at various levels of wear process factors shows that increase in load develops the wear rate. Te increase in velocity and sliding distance diminishes the rate of wear. (v) Te maximum wear rate observed at load � 50 (N), V � 3 (m/sec), SD � 500 (m) is found to be 1.92 times higher than the wear rate observed at load � 10 (N), V � 3 (m/sec), SD � 1500 (m). (vi) Te presence of coating has been ensured via scanning electron microscopy analysis.

Data Availability
Te data used to support the fndings of this study are included within the article.

Conflicts of Interest
Te authors declare that they have no conficts of interest.