Effect of Various Process Parameters of EDC on Mild Steel Substrate and their Properties


 In the present study, A layer of the modified composite coated surface is made using Copper, molybdenum disulfide, and hexagonal boron nitride. For this process, Electrical discharge machine (EDM) is used but in reverse polarity. Various factors of the machine influenced the thickness of the deposited layer like the amplitude of peak current, duty factor, powder mixing ratio, used etc. For the deposition process, green compacted electrodes which was made after mixing the powder material in mortar for approximately 2.5 hour and post processing in Hot mount press moulding machine. After that the mixture powder was put in the Hot mounting press machine to made green compact electrode with specific parameters. Experiment was performed on EDM and many analysis were carried out to study the morphology of the coated surface. To get the morphology of the coated layer, FESEM images were examined and found the satisfying uniform distribution of deposited layer constituents with material powder mixing ratio of (Cu/HBN/MoS2) at (20/40/40) got with 50% duty factor and 10 Ampere peak current amplitude. Also done XRD, for the evidence of Cu, MoS2 and HBN.


Introduction
Coating is a process of covering the surface of substrate with the help of another material. Coating material usually different from substrate elements and it modi es the surface behaviour of substrate without changing the bulk properties of substrate [1][2]. Coating the surface now days a popular method to utter the substrate's surface speci cations like hardness, Anti-friction properties like tear and wear resistance, low coe cient of friction, score resistance, corrosion and oxidation resistance etc. [3].
For surface coating there are various known methods like Electro-less plating [4][5], Electro-Plating, conversion coating [5], hot-dipping [6], chemical vapour deposition method, Roll-to-Roll coating process, thermal spraying and many more. These processes are e cient in their own area but with advantage they have some disadvantages [7] such as costly machine involvement, complexity during operation, complexity during setup of work-surface, speci c (high/low) temperature and speci c working environment requirement like inert environment or vacuums that somehow creates hindrance the easiness for some speci c use and conditions. Also, with the help of some of the methods we get thin coating and some other provide thick coating.
Electrical discharge coating (EDC) is a reverse method of electrical discharge machining [8]. In this technique, we interchange the polarity of the operation. Unlike EDM, in EDC we connect the work piece to the positive terminal and the electrode to the negative terminal. Also, the substrate is dipped into the dielectric medium with the electrode. The dielectric medium is nothing just a hydro-carbon chemical compound like kerosine. This dielectric medium helps in the coating process just as in electrical discharge machine process. This process is somehow, easy to coat for at surface and small area once at a time. Although all methods have their own advantage, so EDC also have its own advantage. For coating in the substrate surface using EDC, we need only dielectric medium which is at atmospheric pressure without any special vacuum or inert environment [9]. Surface coating with the help of EDC doesn't require special arrangement, it needs only a Electro discharge machine with reverse polarity [3].
The other advantages are, it doesn't require special arrangement to form layer material. Only by mixing [10] required layer material powder we can ready our layer forming material for coating.
The main application of coating of the surface is to provide the protection from environmental deterioration, provide smooth surface with different properties like low coe cient of friction and high hardness etc [11] [12]. One of the applications of electro discharge coating used to increase tool-life of any cutting tool by decreasing the coe cient of friction and by increasing the hardness after coating with the help of EDC. Also, it may help in making the rubbing part smooth for low power loss in some speci c applications [13].
Also, above liquid lubricants, solid lubricants have some speci c advantages like rmly tackiness to the substrate area, high temperature stability as compared to liquid or air lubrication, no change in properties in variable pressure, also inert to radiation and for reactive environment etc. In complicated areas where applying lubrication all the time is possible can be encountered by using solid surface lubrication, just by coating the rubbing surface by special lubricating property materials. Somehow the coating process needs some processing costs but it also minimises the breakdown probability of machine which also can help to minimise the overall running cost. Also, in traditional machining process we are using metal cutters which rubs and the surface deteriorates. This rubbing vanishes the surface nish of the product.
So, if tool tips would able to coat then it is possible that the surface nish of nal product can be enhanced. Hence, for industrial and research perspective Electric discharge coating is feasible and a perfect choice. Mostly Hexagonal boron nitride, graphite, molybdenum disul de etc. are used for surface modi cations where lubrication effect needed [14], [15].
Here, are some technical terms associated with the process like Duty factor. It's a term de ne that for how much time current ows from the set-up over the period of one cycle as shown in gure.1.
Here; T high = Time for which current ows with its peak amplitude.
T period = Time period of one cycle.

Lubricating action of MoS2 and HBN
Molybdenum disul de (MoS2 ) is a well-known solid lubricant used in industries such as in forging industries to lubricate dies to reduce friction between die and forged material. Molybdenum disul de which is same as graphite is heavily used in industrial lubricating oils to enhance the tribological properties of lubricating oil. MoS2 can used up-to 350 degrees Celsius in oxidising environment and 1100 degree Celsius in reducing environment. The lubrication behaviour of Molybdenum disul de is due the hexagonal structure of it like graphite. These hexagonal structure crystals are at and joint to each other and make a chain. Between each layer there is a gap. The gap between two hexagonal layer is high enough so that little external force can make them slide on each other. So as the external force is applied it overcomes the vender-walls force between the layers and each layers starts sliding on each other. This sliding creates a lubricating effect just like in viscous uids and hence it helps in reducing the frictional effect. This phenomenon helps to create the lubrication property of molybdenum disul de.
Hexagonal Boron Nitride (HBN) is also known by other name called "white graphite". It has similar crystal structure as molybdenum disul de and graphite. Although, HBN has superior properties than graphite, like low coe cient of friction, high thermal stability than graphite, high vacuum performance, high thermal conductance, low wet ability, chemical inertness and low thermal expansion. HBN is also used as an additive in lubricating oils, paints etc. to enhance tribological performance, Hence, it is a suitable material to choose as a coating material [16].
Algodi et al [1] performed electro discharge coating on Mild steel surface with the help of Ti-Fe cermets in Die sinking EDM machine and got hardness of the coated surface 4-8 times harder than the substrate surface. Mussada et al. [17] coated the mild steel surface with (W-Cu) :(75:25) with the help of powder mixed green compact compacted electrode in EDM machine. The input parameters selected was current(A), compaction load (Tons) to make green compact electrode and pulse on and off time (microseconds). As an output parameters material transfer rate (mg/min) and surface roughness were measured. Also, for con rmation of material in the coated surface XRD analysis were performed which con rmed the presence of Cu, W, WC and W2C. Krishna et al. [18] studied the effect of EDC on Mild steel surface with the help of (W-Cu) :(75:25) and got mass transfer rate of 281 mg/min and coated layer thickness of 1262.90 μm with 5-6 times harder than surface of substrate. Many studies have been carried for coating of Mild steel surface with the help of EDC with different materials and their after-process properties. However, there has not been a lot of attention given to Cu/HBN/MoS2 material with different mixing ratio and various process parameters. So, in our case we are using Mild steel substrate and Cu, HBN and MoS2 as a coating material. Input parameters are Powder mixing ratio, Current and Duty factor. As an output parameter, we studied mass deposition rate on the substrate surface, coating layer thickness with powder mixing ratio, FESEM analysis for uniformity and pores detection and kinematic coe cient of friction of coated surface.

Working principle of Electrical discharge coating:
This coating techniques is practically interchangeable to EDM. The only difference is here we are using reverse polarity to coat the surface alike straight polarity used in Electrical discharge machining. Basically, as shown in gure.2, in this method the substrate in which coating we want is being connected to the positive terminal of machine setup and the electrode used is being connected to the negative terminal, which is just opposite to the machining process. After that we dip the whole setup in the dielectric medium so that spark ignites under dielectric (inert medium) medium for die sinking effect.
After dipping, we start the EDM machine and reverse the polarity and set-up the required process parameters from the control console. After applying the appropriate voltage, there generates a voltage gap between substrate and electrode. This voltage gap created a dielectric breakdown and due to this spark generates between nearest part of the electrode surface and substrate surface. Due to this spark, heat generates and it melts down the loosely compacted electrodes surface particles. After melting, the melted particles move towards the substrate because of electromotive force generated due to applied voltage. Then these particles get settled down in the substrate surface and make uniform coating layer by layer [19] [20] if suitable process parameters are used. So, this process goes periodically and continuously for stipulated time and due to it layer by layer coating process happens and its thickness depends upon the timing of coating process and other parameters.
The thickness of coating depends upon various parameters. The chemical composition of substrate material and electrode material, electric conductivity of materials, green compact electrodes bond strength, applied various parameters like current, voltage, duty factor, peak current amplitude etc.

Research Objective
The main objective of this research paper is, to analyse the effect of various process parameters on the coating process of mild steel surface as a substrate with the help of EDC. Power form of Molybdenum disul de (MoS 2 ), Hexagonal Boron Nitride and copper were mixed extensively and were used as a green compact electrode formed using powder metallurgy method. Substrate surface was coated using EDC process which is reverse of EDM. To study the effect of various process parameters effect on EDC, we have performed several experiments by changing different parameters like peak current amplitude, duty factor and powder mixing ratio. After that, to analyse the morphology of the coated surface, we took the help of FESEM and also, to study the components present in to coated surface XRD were used. Apart from that, we calculated the mass deposition rate on substrate surface and tool wear rate with respect to current, duty factor and powder mixing ratio.

Speci cation of Material
As a work piece, mild steel plate is selected with material constituents of (Fe -96.8%, C -0.29 %, P -0.04 %, Cu -0.30 % and Mg -1.04 %,) with dimension of (measurement: 15 mm × 15 mm × 5 mm). Further cleaning and crushing of work piece have done with the assistance of cleaning paper of coarseness size of 900 μm.
MoS2, hBN and copper powder were bringing into play to create green compact cathode electrode comprising HBN nano particles of size 60 nm, Copper nano particles of size 10 μm and MoS2 nano particles of size 110 nm. The density of MoS2 used was 5.07 g/cm3, hBN particles was of 2.26 g/cm3 and Cu was of 8.97 g/cm3.

Experimental setup:
The set-up for Electro discharge machining is shown in g.4. In g.4. (a), it is showing the machine for the coating process and in g.4. (b), it is showing the arrangement of electrode and workpiece holder inside the EDM machine. The work piece is held in the magnetic vice with the help of clamp and the magnetic vice is stick to the machine base due to magnetic action so that during operation it doesn't move. The electrode is clamped to the machine head vertically upward to the work-piece arrangement. Special attention is given for parallel arrangement of electrode and substrate upper surface so that uniform spacing between both surface we get during operation. After that, by adjusting the machine head we setup the electrode just above the substrate so that substrate upper surface is fully covered by electrode for direct projection. After that, we close the side gates of the machine and start the pump which lls the arrangement space with dielectric uid. We made sure that the uid is continuously owing through the working area so that it takes out the debris particles and die sinking process proceeds. After that, we reversed the polarity of the process as we used in EDM process and setup current, duty-factor and voltage parameters from the controlling console of the machine. During experiment we can see the voltage and current uctuation with the help of controlling console and oscilloscope installed.

Preparation of green compact electrode
The MoS2, HBN [21] and Cu powder used have normal molecule size of 110 nm, 60 nm and 10 μm separately. For extremely proper mixing Cu, HBN and MoS2 were mixed in mortar for 2.5 hour continuously. The decision of 2.5 hour of mixing of powders has taken by trial experiments, 60-minutes of mixing time gives us improper coating. And due to improper mixing, it is almost di cult to get uniform distribution of material all over the substrate surface [22]. So, time greater than 2.5 hours for mixing the powder were chosen. After this process, using hot mounting press the green compact electrode was prepared by putting the mixed powder in the machine by applying appropriate pressure [23] [24]. Also, pressure plays an important role here. So, appropriate pressure was taken for further procedure. The pressure gauge shows the current pressure applied on the mixture continuously. And due to compacting the pressure gradually decreases all the time, we had maintained the appropriate pressure manually all the time. After that, cooling was done for 20 minutes. The rst 10-minute cooling was normal cooling and the next 10-minute cooling was done by running water outside the hot mould. To produce green compact electrode of size 5mm thickness and 15 mm diameter, we had chosen appropriate amount of power. As the coating process involves current, so all the constituents in this process should be conductive, hence the Copper bar and the green compact electrode were stick together by using silver paste [25], which is highly conductive in nature.
As the EDC process involves current, the materials used should be highly conductive so that current can pass easily and e ciently. As we know, HBN and MoS2 are not good conductor of electricity, so copper is blended with them to enhance the conductivity of the green compact electrode. Also, copper enhance the stickiness between HBN and MoS2 and helps to form a rm electrode structure. Also, copper helps to generate good spark. During spark, heat generates and as copper has low melting temperature it melts down earlier than HBN and MoS2 and allows the process to distribute the coating uniformly over the substrate surface.
Previously, during so many trial experiments we have tried to make appropriate mixing ratio for e cient electrode formation for uniform deposition of mass on substrate surface. But out of several mixing ratio of Cu, HBN and MoS2 only few were capable to provide satisfactory results. Mixing ratio (Cu/HBN/MoS2) of (20/40/40), (30/35/35) and (40/30/30) were used for further investigation.

Selection of process parameters
For satisfactory results various experiments were carried out with different parameters and mixing ratio.
The substrate was associated with positive terminal and the apparatus was associated with negative terminal. To check the spark event the oscilloscope was associated over the terminals, which is nothing but an appropriate condition for covering process. Several trial experiments were carried out and after that, mixing ratio of (Cu/HBN/MoS 2 ) -(20/40/40), (30/35/35), (40/30/30), duty factor (50%) and peak current (5,7 and 10 ampere) were giving good results without sparking and considered as suitable parameters. Parameters used beyond these values created unusual arcing before proper coating time and due to that an island type structure formed in the substrate surface which is the closest point. The process was carried out for 4 minute each to stop further arcing for the selected process parameters.

Chemical composition and micro-structure
To study the chemical composition and morphology a small-scale basic investigation was carried out with the assistance of FESEM. Fig.6. and   Fig.6. (b) and shows minor cracks between the coating and substrate. This may happen because of non-compatibility between the mixture and the substrate surface, also process parameters have an important role. In g.6. (c), there is a good diffusion between the substrate and coating material. Also, there is a difference in the coating thickness w.r.t mixing ration [26]. Also, the average coating thickness is shown in the Fig.6. d. The coating thickness decreases with copper content in the mixing ratio. This may be because of high binding of particles as the copper content is increasing.
5.1.2. Surface morphology of coated surface FESEM images of coated surface is shown in Fig.7. (a-c) to study the surface morphology, separately with (Cu/HBN/MoS 2) :(20:40:40). In Electrical discharge machining, the pinnacle ow perhaps demonstrates the capacity to get an ideal spark temperature to move the anode material into the workpiece. To deposit uniform material with high density with high material deposition rate at substrate surface, we need to take appropriate high peak current that an appropriate high peak current. Some voids or pores with non-uniform coating may create with low current because it will not provide enough spark energy to melt down the green compact electrode. So appropriate parameters should have to melt down the anode material for uniform testimony of the melted material on the substrate surface layer.
In g.7. (a), we can see that with mixing ratio of (20/40/40) of (Cu/HBN/MoS 2 ) with duty factor of 50% and 7A peak current have uniform mass deposition rate but some big pores are found. In g.7. (b), we can see that, with same mixing ratio and duty factor but with high peak current (10 A), the coating material deposition is uniform as well less very less big voids are noticed. This may occur due to excessive energy produced due to high current ow that helped to generate good spark, also results in easily melt-down and deposit the materials uniformly.

5.1.3.x-ray diffraction analysis
The chemical composition of the coated surface during EDC is examined with the help of XRD analysis. The result is showing in g.8 and X'PERT programming was used to examine the X-ray diffraction to get the diffraction tops. The diffraction angle (2θ) was used between 20° to 80°. With this result, we found various constituents like Fe 2 B, MoS 2 , BN, Cu, CuS and BC.

Effect of peak current amplitude to the mass deposition rate
The material deposition rate is shown in the g.9. This graph is drawn using observation data for Table.2.
Here we found that for mixing ratio (Cu/HBN/MoS2) :(20/40/40), the material deposition rate is increasing with respect to temperature. Also, for other mixing ratios the phenomena are same.
Hence, we can conclude that the mass deposition rate is directly proportional to the peak current used. Also, from the graph we found that the rate of change of mass deposition rate is high from 5 to 7 Ampere current, after that we found that the rate of increase of mass deposition rate decreases from 7 to 10 A. May be, this happens due to excessive heat generation which cause more mass removal rate of substrate than the mass deposition rate on substrate.

coe cient of kinematic friction of the coating surface
The coe cient of kinematic friction of the coated surface was obtained with the help of a pin-on-disc test [1]. In g.10. (a) it is showing the uncoated workpiece and g. 10. (b) showing the coated wok-piece. This coated work-piece was used to analyse the tribological behaviour. Pin-on-disc test was performed between a Mild steel circular plate and the Coated surface sample as shown in g.10. (a). A Mild steel disc with of radius 120 mm and thickness 25 mm was used for the pin-on disc test. The test was carried out in the radius of 90 mm and the time duration was 5 minutes. The load applied for the test was 2 kgf (20 N). From the test, we found the variation in the coe cient of friction during the time because of shearing off of the layers of coating [27].
So, from the g.11 we can get that the average coe cient of friction is 0.290, standard deviation is 0.051 and variance of 0.003. although, Mild steel's coe cient of kinematic friction lie between 0.09 to 0.6. Also, in g.10. (c), we can see the FESEM image of wear track. So, we found that due to coating the coe cient of kinematic friction decreased by somehow due to presence of HBN and MoS 2 , as they have hexagonal packing structure and slides over each other layer easily which helps to decrease friction.

Conclusion
In this research work, to make the green compact electrode the powder material with mixing ratio of (Cu/HBN/MoS 2 ) is used with the help of hot mounting press machine in the mixing ratio of (20/40/40), 1. Bonding strength between HBN and MoS 2 is not good enough, so that they don't stick together rmly and so it is not possible to coat substrate. So, to make the powders stick together we used Cu as a binder which helps to stick all constituents together. It helps to form green compact electrode with ease and provides rmness to the green compact electrode. Also, HBN and MoS 2 are not a good conductor of electricity and for EDC process electricity needs to pass easily over it for e cient coating [12]. Hence, copper is added to increase the green compact electrodes conductivity. Apart from that, Cu has low melting temperature and during heat generation it melts down rst and allows HBN and MoS 2 to get detached from electrode and deposit in the substrate with ease. So, these are the main reason behind using of copper instead of other material 2. Substrate's coating which was made with the use of powder mixing ratio of (20/40/40), peak current amplitude of 10A and 50% duty factor has better surface morphology obtained with less voids or pores above all other samples. Although, to perform the experiment, binder selection with appropriate mixing ratio were obtained to enhance the better morphology of the coated surface after so many trial experiments.
3. The material fusion between substrate and coated surface is found better in mixing ratio of (40/30/30) with 50% duty factor and 5 A peak current.
4. Presence of Cu, BN and MoS 2 was found through XRD analysis, phases of Mo, BN and Cu-S are also showed with the inter-metallic compound formation. The results showed that there is a uniform distribution of compounds all over the coating surface.
5. The process parameters have high impact on mass deposition rate. From observation table.2, we can see that the mass deposition rate is increasing with peak current amplitude. Although, the maximum rate of mass deposition rate found between 5 to 7 Ampere.
. Peak current is the most important factor in electrical discharge coating to control the coating thickness. High amount of peak current amplitude is required to high amount of spark energy which means spark energy is directly proportional to peak current, so that more amount of electrode tool melts down and deposits on the substrate due to cumulative effect of plasma and gravity. So, peak current applied is directly proportional to coating thickness obtained.
7. The layer formed onto substrate's surface were analysed with the help of FESEM and XRD, and we found uniform coating onto the substrate with evidence of MoS2, BN and Cu compounds.
. Also, Wear test were performed on the pin-on disc to analyse the coating surface tribological behaviour. We found that the coe cient of kinematic friction between coated surface and mild steel is lowered due to coating.

Declarations
Funding-Not get any funding from any sources.
Data availability-There is no need to mention the availability of data and materials in the present study.
Ethical approval-Not applicable.
Consent to participate-There is no consent to participate needed in the present study.
Consent to publish-There is no consent to publish needed in the present study.         XRD peak graph showing analysis of (Cu/HBN/MoS2) with mixing ratio of (20/40/40) with 4-minute machining time, 50% duty factor and peak current of 10 A Figure 9 Showing the variation of mass deposition rate on substrate surface with respect to peak current at different powder mixing ratio.
Page 22/23 Figure 10 Showing images of (a) uncoated mild steel sample, (b) Coated sample, (c) FESEM image of Coated surface, (d) FESEM image of coated surface after pin-on-disc test.