**2. ESD process description**

The technology can be defined as a micro-pulsed welding technique that allows deposition of one "electrode material" on a metallic sublayer; mass transport is made in high current intensity and in short time periods. ESD method implies heat and mass transfer phenomena. Basically, multiple phenomena involve such as heating to incandescence, melting, and eventually evaporating of the electrode material. There is a stream of electrons, ions, and neutral fast atoms in the electric field between the electrodes. Electric field is concentrated on a microportion of the piece where metal melting bath forms. Microalloying between base material and electrode melt generates the formation of nitrides, carbides, carbonitrides, and plasma through air decomposition (nitrogen and active oxygen). All these phenomena and others successively describe the formation of the hardened superficial layer through deposition.

the variety of filler materials and basic materials used. There are also a multitude of purposes of use: in obtaining amorphous layers, in obtaining epitaxial layers, layers with special physical, chemical, and mechanical properties (corrosion resistance, wear resistance, fatigue

It is noted that the main reasons that cause system need to replace some parts in the industry are the destruction of surface layer by corrosion, wear, fatigue, and mechanical shock. The properties of metallic materials have been improved [5, 6], in order to increase the adhesion of deposits to the base material layer using the layer of composite material by means of electro-

Currently, electro-spark deposition process is gaining more ground to thin film deposition with various metallic materials, which gives the surface characteristics of hardness and high wear resistance [7–16]. Most studies have focused on the analysis of the mechanical properties of the material base-surface multilayer system and mass transfer behavior to obtain required properties and to control as much as possible the thickness of the deposited layer. But there are unsolved issues that deserve the attention of a detailed study. Going to these relevant issues in industrial practice, research conducted in different countries, like Japan, China, [17–19], and Ireland [20] had highlighted a number of concern technologies knowledge and fundamental character and experimental studies leading to improve properties of surface layers obtained using the method with pulsed electrical discharges. The wear, fatigue, and corrosion phenomena are the most aggressive factors that lead to scrapping one part. From this, situation results the high importance for the physical-chemical research of the phenomenon that takes place into these superficial layers in connection with coating technologies [21, 22]. One of the methods to process the materials is electro-spark deposition method that can allow to coat complex shaped and reduced dimension parts by extracting material from the surface of the part or developing coating layers by applying the compact material electrodes or by introducing into working area of powders or mix of powders. It is necessary that the surface layers have a good adherence to the part and especially a good connection to the sublayer. In some surface areas, high hardness layers are formed. Most studies are focused on chemical, physical, and mechanical properties of deposited layer and on mass transfer behavior, to obtain the properties imposed and to control as much as possible the coating layer thickness. Compared with laser ablation technology, CVD, PVD, etc., the ESD method is technologically much easier to execute and requires no vacuum chamber or additional devices. The deposition can be done both automatically and manually, allowing a wide range of surface

The technology can be defined as a micro-pulsed welding technique that allows deposition of one "electrode material" on a metallic sublayer; mass transport is made in high current intensity and in short time periods. ESD method implies heat and mass transfer phenomena. Basically, multiple phenomena involve such as heating to incandescence, melting, and eventually evaporating of the electrode material. There is a stream of electrons, ions, and neutral fast atoms in the electric field between the electrodes. Electric field is concentrated on a

resistance, and shock resistance) [1–4].

adjustments and ease of correcting surface quality.

**2. ESD process description**

spark deposition process.

46 Advanced Surface Engineering Research

The process of electro-spark deposition on the material surfaces is based on the electro-erosion phenomenon and the polar transfer of the anode (electrode) material to the cathode (metallic part) during the electrical discharge in pulses between the anode and the cathode, discharge which occurs in a gaseous medium. Unlike conventional electro-erosion machining, a pulsed rectifying current with reversed polarity is used at pulse electrical discharge. In this case, the impulse electric discharge process has the air as the gaseous medium. In the deposition technology, the electrode performs a vibratory motion. Due to the polar effect, the predominant transfer of the anode material (electrode) to the cathode (the piece) ensures the formation of the superficial layer with well-defined physic and chemical properties. Upon completion of the discharge, at a very low temperature range, cathode anode removal begins, an action that ends with the electrical circuit break, after which the process is resumed. As a result of the transfer of material and thermal changes from the discharge zone, in the process of superficial discharge of metallic materials with electric sparks, the surface layer of the cathode changes its structure and chemical composition. The characteristics of this layer may vary greatly depending on the electrode material, the composition of the medium between the electrodes, the impulse discharge parameters, and other conditions of the cathode layer formation [23]. Between two sparks, the low quantity of melted metal solidifies, developing a protective layer.

Electrical and working principle of the pulse electric discharge process is shown in **Figure 1**.

The electrical scheme represents a generator, the cathode coupled part, and the electrode as the anode, which is used to form the superficial layers on the part. The surface processing begins with the proximity of the sample electrode and at the critical breaking distance triggering the electrical discharge by impulses, which is often continuous and ends only at the contact of the electrodes. After penetrating the gap between the electrodes, due to the energy accumulated in the Cp capacitor, at the contact surface of the electrodes, strong heated area (local melting and evaporation outbreaks) occurs that causes the electrical erosion of the electrodes (the piece and the electrode itself). The essence deposition using the pulsed electrical discharge method lies in the complexity of physical phenomena which are occurring during the technological process. Discharge parameter regimes (voltage, current, and pulse time)

**Figure 1.** Surface processing using electro-spark deposition method: wiring diagram for the processing device: Cp, condenser; Rb , variable resistance; E-S, electrode connected to the anode; and P, part connected to the cathode.

depend on the physical and chemical properties of the electric and working circuit (deviceelectrode-base material). In this context, we can say that the parameters depend on the type of electrode deposition, and its melting temperature, the thermal conductivity, chemical reactivity of the anode elements, diffusivity, density, electrical resistance, thermal inertia, flow ability, and parameter temperature dependence.

on the atomic number, and on the value of the elements that are in the cathode, anode, and work environment composition. The electro-spark deposition can be viewed evolutionarily, over time, by the steps required to form the layer, that is, the proximity of the electrodes, the penetration of the gaseous zone between the electrode and the base material, and also the creation of a thermal flux that has the effect of melting the areas limiting the flow to both the cathode and the anode; creation of a plasma area surrounding the energy and mass transfer field, an area that favors chemical activities, due to the formation of strong reactivity ions [24]. But, the same deposition can also be seen in terms of evolution in space by studying the structure of the areas that make up the deposition system. The deposition areas can be divided according to their position and role in the operation, as follows: anode electrode area (area of the filler material); deposition flow area; part deposited material area; and cathodic area of the base material. *Electrode anodic area***.** Priming arc between the two electrodes has the first effect, a thermal

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Because of the temperature parabolic variations, the electrode material will melt, so the top of the electrode will take an interior shape like the thermal field. During coating, the initial thermal flux separates, and warm small areas are developed inside the cone. Inside the cone appears small bumps because small warm areas are migrating inside, and the material melting from inside is done depending on local thermal peaks. Tungsten electrode peak pictures show that the cone peak coating is deeper (**Figure 2b**). Electro-erosion energetic flux is creating a small top cone, with high differentiated and with a visible and deep crater. Crater side surface is soft, less involved in coating and plasmatic cinematic processes of the treatment.

*Flux area***.** It is the area with the higher dynamism, here is forming the electrical, thermal, and plasmatic flux, with active elements development: atoms, ions, and electrons. This is influencing

**Figure 2.** (a) Maximum and minimum of local mass transfer; (1) electrode; (2) attack zone; (3) enlarged attack zone for highlighting the formations at the interior cone base; (4) maximum and minimum local thermal areas developed during

flux that leads to electrode top melting (**Figure 2a**).

mass transfer; (b) tungsten electrode peak used in this experiment.

Among the basic features of the impulse electric discharge method, which also include advantages, can be mentioned: lack of special preparation of the surface to be processed; the deposited layer, due to the high solidification/cooling speed, obtains amorphous-specific properties; the deposited layer does not usually require subsequent finishing; apply easily to the surface of complex parts; the deposition can be done in strictly indicated places; the deposited layer has good adhesion to the support; the possibility of using as electrode both pure metals and their alloys; the possibility of deposition using metal-ceramic materials and hard fusible compounds; lack of heating of the sample in the process of deposition and implicitly of the collateral effects; the effect of pollution is minimal and completely eliminates the use of toxic nonmetallic compounds such as cyanide in the coating process; the necessary equipment is relatively simple, and the technology has costs mainly generated by the quality of the additive materials [23].

Procedure's advantage is that heat density of the piece is minimum maintaining chemical composition and the properties of basic material. The thin layer system hardened through impulse electric discharge method is splitted into: an exterior layer with a strongly modified structure at the surface and an interior layer (diffusion layer) with the properties corresponding to basic material and added material. The hardened layer presents cracks that are advantageous to the lubrication process (during exploitation) by protecting basic material of excessive wear.

An inadequate diffusion is a risk factor for obtaining thin films, their long-term stability, and security in service. This can be solved by applying a heat treatment or thermochemical treatment afterwards. This may cause high electrode consumption due to its elongation and tear, due to the rapid dissipation of heat developed to part contact area, while the heat dissipated in the electrode is made slower, this being due to the large mass difference between the piece and the electrode. This problem can be solved by choosing an optimum working regime using appropriate parameters. Deposited layer characteristics are controlled process parameters: spark energy, discharge voltage, spark duration, inductance, frequency, temperature, number of passes, pressure on the end electrode (filler) on the surface of the base material, linear speed horizontal electrode, etc. The difficulty of the problem, in some cases, to the incompatibility between the filler material is characterized by special properties and support.

For this reason, there is a need for multiple layers: the first layer deposited actually providing the support anchoring surface layer.

### **2.1. Mass transfer**

The mass transfer intensity and coating parameters can vary depending on the electrode material nature and basic material nature. Energetic transfer is depending on the physical and chemical characteristics (specific heat, density conductivity, and thermal transfer coefficient), on the atomic number, and on the value of the elements that are in the cathode, anode, and work environment composition. The electro-spark deposition can be viewed evolutionarily, over time, by the steps required to form the layer, that is, the proximity of the electrodes, the penetration of the gaseous zone between the electrode and the base material, and also the creation of a thermal flux that has the effect of melting the areas limiting the flow to both the cathode and the anode; creation of a plasma area surrounding the energy and mass transfer field, an area that favors chemical activities, due to the formation of strong reactivity ions [24]. But, the same deposition can also be seen in terms of evolution in space by studying the structure of the areas that make up the deposition system. The deposition areas can be divided according to their position and role in the operation, as follows: anode electrode area (area of the filler material); deposition flow area; part deposited material area; and cathodic area of the base material.

depend on the physical and chemical properties of the electric and working circuit (deviceelectrode-base material). In this context, we can say that the parameters depend on the type of electrode deposition, and its melting temperature, the thermal conductivity, chemical reactivity of the anode elements, diffusivity, density, electrical resistance, thermal inertia, flow

Among the basic features of the impulse electric discharge method, which also include advantages, can be mentioned: lack of special preparation of the surface to be processed; the deposited layer, due to the high solidification/cooling speed, obtains amorphous-specific properties; the deposited layer does not usually require subsequent finishing; apply easily to the surface of complex parts; the deposition can be done in strictly indicated places; the deposited layer has good adhesion to the support; the possibility of using as electrode both pure metals and their alloys; the possibility of deposition using metal-ceramic materials and hard fusible compounds; lack of heating of the sample in the process of deposition and implicitly of the collateral effects; the effect of pollution is minimal and completely eliminates the use of toxic nonmetallic compounds such as cyanide in the coating process; the necessary equipment is relatively simple, and the technology has costs mainly generated by the quality

Procedure's advantage is that heat density of the piece is minimum maintaining chemical composition and the properties of basic material. The thin layer system hardened through impulse electric discharge method is splitted into: an exterior layer with a strongly modified structure at the surface and an interior layer (diffusion layer) with the properties corresponding to basic material and added material. The hardened layer presents cracks that are advantageous to the lubrication process (during exploitation) by protecting basic material of excessive wear.

An inadequate diffusion is a risk factor for obtaining thin films, their long-term stability, and security in service. This can be solved by applying a heat treatment or thermochemical treatment afterwards. This may cause high electrode consumption due to its elongation and tear, due to the rapid dissipation of heat developed to part contact area, while the heat dissipated in the electrode is made slower, this being due to the large mass difference between the piece and the electrode. This problem can be solved by choosing an optimum working regime using appropriate parameters. Deposited layer characteristics are controlled process parameters: spark energy, discharge voltage, spark duration, inductance, frequency, temperature, number of passes, pressure on the end electrode (filler) on the surface of the base material, linear speed horizontal electrode, etc. The difficulty of the problem, in some cases, to the incompat-

ibility between the filler material is characterized by special properties and support.

For this reason, there is a need for multiple layers: the first layer deposited actually providing

The mass transfer intensity and coating parameters can vary depending on the electrode material nature and basic material nature. Energetic transfer is depending on the physical and chemical characteristics (specific heat, density conductivity, and thermal transfer coefficient),

ability, and parameter temperature dependence.

of the additive materials [23].

48 Advanced Surface Engineering Research

the support anchoring surface layer.

**2.1. Mass transfer**

*Electrode anodic area***.** Priming arc between the two electrodes has the first effect, a thermal flux that leads to electrode top melting (**Figure 2a**).

Because of the temperature parabolic variations, the electrode material will melt, so the top of the electrode will take an interior shape like the thermal field. During coating, the initial thermal flux separates, and warm small areas are developed inside the cone. Inside the cone appears small bumps because small warm areas are migrating inside, and the material melting from inside is done depending on local thermal peaks. Tungsten electrode peak pictures show that the cone peak coating is deeper (**Figure 2b**). Electro-erosion energetic flux is creating a small top cone, with high differentiated and with a visible and deep crater. Crater side surface is soft, less involved in coating and plasmatic cinematic processes of the treatment.

*Flux area***.** It is the area with the higher dynamism, here is forming the electrical, thermal, and plasmatic flux, with active elements development: atoms, ions, and electrons. This is influencing

**Figure 2.** (a) Maximum and minimum of local mass transfer; (1) electrode; (2) attack zone; (3) enlarged attack zone for highlighting the formations at the interior cone base; (4) maximum and minimum local thermal areas developed during mass transfer; (b) tungsten electrode peak used in this experiment.

the character and the quality of the coating. In the flux area (**Figure 3**), are developing, beside high thermal, electrical fields and added material witch strikes with high force. This material is solid (particles) or liquid (drops). Depending on the electrode type, we will have mainly one of the two up-mentioned forms: nickel and titanium have in their flux, mainly, melted material drops, and tungsten, because of high melting temperature created inside the flux, specially extracted particles.

*Material deposition area***.** The area of the deposited material is important because it gives information about the surface characteristics. The amount of melted and deposited material on the surface depends on various parameters, such as: pulse current frequency, gap size, electrode material properties, average intensity, pulse power, etc. Studying the part surface microgeometry modification using the development of the waves on the surface of the liquid metal in the impulse discharge conditions, author Topală (**Figure 4**) writes that the phenomenon is accompanied by the appearance of craters on the surfaces of electrodes [25].

In fact, three types of craters have been recorded, all of them having the shape of a spherical calotte (**Figure 5**): (i) a smooth profile with good flow properties (low relative melting temperature such as Ni and Ti); (ii) a rugged profile, specific for the fragile material electrodes, which

break out during deposition and do not melt easily (due to very high melting temperature, for example, tungsten); and (iii) a middle meniscus, characteristic for materials that solidify at a high rate (high melting temperature, when the "drop" touches the cold part, solidification begins).

craters together with the wave; dc1, dc2, and dc3—the diameters of three types of craters; hc1, hc2, and hc3—The depth of the

, D2 , and D3

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—the diameter of the

51

**Figure 5.** (a) Types of craters obtained by dimensional processing by electroerosion (D1

craters; M—meniscus; and V—wave); (b) the waveform of the deposition surface.

The forces that compete to create surface geometry are the ion beam pressure force and the metal vapor reaction force, due to the static pressure of the metal vapors in the crates. It has been noticed that the height of the meniscus depends on the following factors: discharge energy, discharge time, electrode material, and application of the electric field to the gap.

*Base material area***.** This area is important to the necessary consistency with the addition material, so that the small bonding area that ensures the adhesion of the layer is as strong as possible.

The hardening of the surface of a piece by the ESD method requires knowledge of the properties of both the basic material (physical and chemical properties) and the deposition material. When heterogeneous multilayer layers are depositing, the base layer must be metallurgically compatible with the adduct and have good adhesion properties and near by physical properties (coefficient of expansion, conductivity, diffusivity, electrical resistance, melting point, and vaporization). The exterior layers must have the characteristics required to operate the part (wear resistance, dynamic shock, corrosion resistance, abrasion resistance, hardness, etc.). In the case of an intermediate layer, the deposited layer and the surface layer must be

Tests were made with the Elitron 22A type machine (**Figure 6**). The device parameters listed in the machine's technical manual are: the power consumed (kVA)—0.5; productivi-

(A)—(A1 = 0.04 mm, A2 = 0.06 mm, A3 = 0.08 mm, A4 = 0.1 mm, A5 = 0.12 mm, A6 = 0.14 mm, A7 = 0.16 mm, A8 = 0.18 mm, and A9 = 0.2 mm); mass (kg)—21 [26]. The Elitron 22A instru-

ment used in the experiments has nine amplitude and six work modes.

/min)—4; working voltage (V)—220; working regimes (r), amplitude of vibration

**2.2. Surface morphology**

connected.

ties (cm2

**Figure 3.** Flux elements; (1) liquid added material (drops); (2) extracted added material (particles); (3) thermal flux—heat jet; and (4) plasma (developed by gases decomposition and nitrogen, oxygen, and carbon active atoms.

**Figure 4.** (a) General view of a deposition erosion crater; (b) the spherical calotte is an idealized form of the crater.

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**Figure 5.** (a) Types of craters obtained by dimensional processing by electroerosion (D1 , D2 , and D3 —the diameter of the craters together with the wave; dc1, dc2, and dc3—the diameters of three types of craters; hc1, hc2, and hc3—The depth of the craters; M—meniscus; and V—wave); (b) the waveform of the deposition surface.

break out during deposition and do not melt easily (due to very high melting temperature, for example, tungsten); and (iii) a middle meniscus, characteristic for materials that solidify at a high rate (high melting temperature, when the "drop" touches the cold part, solidification begins).

The forces that compete to create surface geometry are the ion beam pressure force and the metal vapor reaction force, due to the static pressure of the metal vapors in the crates. It has been noticed that the height of the meniscus depends on the following factors: discharge energy, discharge time, electrode material, and application of the electric field to the gap.

*Base material area***.** This area is important to the necessary consistency with the addition material, so that the small bonding area that ensures the adhesion of the layer is as strong as possible.

#### **2.2. Surface morphology**

**Figure 4.** (a) General view of a deposition erosion crater; (b) the spherical calotte is an idealized form of the crater.

the character and the quality of the coating. In the flux area (**Figure 3**), are developing, beside high thermal, electrical fields and added material witch strikes with high force. This material is solid (particles) or liquid (drops). Depending on the electrode type, we will have mainly one of the two up-mentioned forms: nickel and titanium have in their flux, mainly, melted material drops, and tungsten, because of high melting temperature created inside the flux, specially

*Material deposition area***.** The area of the deposited material is important because it gives information about the surface characteristics. The amount of melted and deposited material on the surface depends on various parameters, such as: pulse current frequency, gap size, electrode material properties, average intensity, pulse power, etc. Studying the part surface microgeometry modification using the development of the waves on the surface of the liquid metal in the impulse discharge conditions, author Topală (**Figure 4**) writes that the phenom-

In fact, three types of craters have been recorded, all of them having the shape of a spherical calotte (**Figure 5**): (i) a smooth profile with good flow properties (low relative melting temperature such as Ni and Ti); (ii) a rugged profile, specific for the fragile material electrodes, which

**Figure 3.** Flux elements; (1) liquid added material (drops); (2) extracted added material (particles); (3) thermal flux—heat

jet; and (4) plasma (developed by gases decomposition and nitrogen, oxygen, and carbon active atoms.

enon is accompanied by the appearance of craters on the surfaces of electrodes [25].

extracted particles.

50 Advanced Surface Engineering Research

The hardening of the surface of a piece by the ESD method requires knowledge of the properties of both the basic material (physical and chemical properties) and the deposition material. When heterogeneous multilayer layers are depositing, the base layer must be metallurgically compatible with the adduct and have good adhesion properties and near by physical properties (coefficient of expansion, conductivity, diffusivity, electrical resistance, melting point, and vaporization). The exterior layers must have the characteristics required to operate the part (wear resistance, dynamic shock, corrosion resistance, abrasion resistance, hardness, etc.). In the case of an intermediate layer, the deposited layer and the surface layer must be connected.

Tests were made with the Elitron 22A type machine (**Figure 6**). The device parameters listed in the machine's technical manual are: the power consumed (kVA)—0.5; productivities (cm2 /min)—4; working voltage (V)—220; working regimes (r), amplitude of vibration (A)—(A1 = 0.04 mm, A2 = 0.06 mm, A3 = 0.08 mm, A4 = 0.1 mm, A5 = 0.12 mm, A6 = 0.14 mm, A7 = 0.16 mm, A8 = 0.18 mm, and A9 = 0.2 mm); mass (kg)—21 [26]. The Elitron 22A instrument used in the experiments has nine amplitude and six work modes.

formed in a dependent rotary movement order of the electrode is important to study the deposition "drops" generation and also the characteristics for the uni-impulse drops. At the same time, the interface chemical reactions lead to the formation of varied compounds, also influencing the coating properties. The drop represents the part exterior shape after the treatment, because the layer is formed by a lot of drops, some of them remelted by the subsequent crossing. The flattening of the "drops" sputtered on the substrate is one of the most important processes occurring in coatings using electric arc. The layer structure will determine the coating structure and therefore, adhesion and coating properties. Therefore, attention was paid to

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*Analysis of the "droplet" shape using a titanium electrode.* It is noticed that the Ti electrode does not only achieve a smooth deposition of the "droplet," but also the process is much more dynamic, the deposition takes place at a certain speed, and the strike impulse creates micro-depression surface, which subsequently, upon the fall of a new "droplet", leads to a

The map of the surface elements of the titanium shell (Ti) on the ferrite-perlite substrate is

Uniform distribution of the elements can be observed, indicating the complete alloying process between the base and the substrate. The Ti was distributed in the melting zone on the cast iron substrate and by diffusion in area interface. New phases have been formed in the coating layer due to the chemical reaction during the deposition process. Note the titanium

Thus, due to the high temperature conditions, the following reaction takes place: Ti + C = TiC. The "drop" striking energy on the metal surface is added to the thermal energy and plasma energy that is formed during the deposition. Thus, optimum conditions are created for the formation of highly intermetallic compounds or chemical compounds. Another particular aspect is that Ti compact droplets are distributed at the edge of the droplet rather than the center, this being possible due to the splashing dynamics. An advantage of the

**Figure 7.** Distribution of Fe and Ti elements; (a) SE\_Fe; (b) SE\_Ti; and (c) the "drop" by Ti, SE image giving information

microalloying with the subsequent in depth layer (it is at most 20 μm) (**Figure 7**).

the study of "drops" by experimental observations.

shown in **Figure 8**.

distribution in **Figure 8b**.

about surface relief.

**Figure 6.** Components of the Elitron 22A pulse electric pulse deposition device: (1) power supply cord; (2) electrode support; (3) piece support; (4) electrode support cable; (5) piece support cable; (6) and (7) connecting sockets; (8) adjustment knob (intensity); (9) vibration amplitude adjustment knob; and (10) stop-start button.

As a base material for the coating, ferritic-pearlitic cast iron was used, which chemical composition is presented in **Table 1**. The chemical composition determined by means of Foundry Master spectrometer.

Justifying the choice of the base material is given by the advantages of choosing this material in textile industry machines and installations (cams, friction skates, and gears in looms). Gray cast iron is a material with a good thermal shock resistance and does not require lubrication. Expanding and contraction coefficients are low and have the property to absorb easily apparent tensions during work by means that graphite from the matrix is creating discontinuities that can absorb vibrations and shock exposure.

Electrodes used for deposition are Ti, TiC, W, and WC. The surfaces of the deposition drops were analyzed by means of electron scanning microscope (SEM).

### **2.3. Analysis of deposition drops**

In the pulsed electric discharge deposition process, in addition to particular importance for the mass, transfer workflow parameters and physicochemical characteristics of the whole area, a great contribution to the achievement of appropriate structures is the type of deposition, and in particular adhesion and microalloying for the drop. As long as the connected metal layers are obtained using this technology, the coating properties are dependent by the chemical reactions at the interface. Since a coating obtained by depositing consists of multiple single drops,


**Table 1.** Chemical composition of base material.

formed in a dependent rotary movement order of the electrode is important to study the deposition "drops" generation and also the characteristics for the uni-impulse drops. At the same time, the interface chemical reactions lead to the formation of varied compounds, also influencing the coating properties. The drop represents the part exterior shape after the treatment, because the layer is formed by a lot of drops, some of them remelted by the subsequent crossing. The flattening of the "drops" sputtered on the substrate is one of the most important processes occurring in coatings using electric arc. The layer structure will determine the coating structure and therefore, adhesion and coating properties. Therefore, attention was paid to the study of "drops" by experimental observations.

*Analysis of the "droplet" shape using a titanium electrode.* It is noticed that the Ti electrode does not only achieve a smooth deposition of the "droplet," but also the process is much more dynamic, the deposition takes place at a certain speed, and the strike impulse creates micro-depression surface, which subsequently, upon the fall of a new "droplet", leads to a microalloying with the subsequent in depth layer (it is at most 20 μm) (**Figure 7**).

The map of the surface elements of the titanium shell (Ti) on the ferrite-perlite substrate is shown in **Figure 8**.

Uniform distribution of the elements can be observed, indicating the complete alloying process between the base and the substrate. The Ti was distributed in the melting zone on the cast iron substrate and by diffusion in area interface. New phases have been formed in the coating layer due to the chemical reaction during the deposition process. Note the titanium distribution in **Figure 8b**.

As a base material for the coating, ferritic-pearlitic cast iron was used, which chemical composition is presented in **Table 1**. The chemical composition determined by means of Foundry

**Figure 6.** Components of the Elitron 22A pulse electric pulse deposition device: (1) power supply cord; (2) electrode support; (3) piece support; (4) electrode support cable; (5) piece support cable; (6) and (7) connecting sockets; (8)

adjustment knob (intensity); (9) vibration amplitude adjustment knob; and (10) stop-start button.

Justifying the choice of the base material is given by the advantages of choosing this material in textile industry machines and installations (cams, friction skates, and gears in looms). Gray cast iron is a material with a good thermal shock resistance and does not require lubrication. Expanding and contraction coefficients are low and have the property to absorb easily apparent tensions during work by means that graphite from the matrix is creating discontinuities

Electrodes used for deposition are Ti, TiC, W, and WC. The surfaces of the deposition drops

In the pulsed electric discharge deposition process, in addition to particular importance for the mass, transfer workflow parameters and physicochemical characteristics of the whole area, a great contribution to the achievement of appropriate structures is the type of deposition, and in particular adhesion and microalloying for the drop. As long as the connected metal layers are obtained using this technology, the coating properties are dependent by the chemical reactions at the interface. Since a coating obtained by depositing consists of multiple single drops,

**Element C Si Mn P Cr Ni Cu** Percentage, % 3.97 2.87 0.25 0.06 0.28 0.12 0.17

Master spectrometer.

52 Advanced Surface Engineering Research

that can absorb vibrations and shock exposure.

**2.3. Analysis of deposition drops**

**Table 1.** Chemical composition of base material.

were analyzed by means of electron scanning microscope (SEM).

Thus, due to the high temperature conditions, the following reaction takes place: Ti + C = TiC. The "drop" striking energy on the metal surface is added to the thermal energy and plasma energy that is formed during the deposition. Thus, optimum conditions are created for the formation of highly intermetallic compounds or chemical compounds. Another particular aspect is that Ti compact droplets are distributed at the edge of the droplet rather than the center, this being possible due to the splashing dynamics. An advantage of the

**Figure 7.** Distribution of Fe and Ti elements; (a) SE\_Fe; (b) SE\_Ti; and (c) the "drop" by Ti, SE image giving information about surface relief.

fluidity of the drop elements and the fact that the expansion coefficient of the substrate and

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The outside droplet pores found, the third scattering zone, are derived from the presence of chemically absorbed gases at the surface of the part, or gases from the metal bath reaction.

From micrographs made by scanning electron microscope shall be observed that Ti deposition on cast iron electrode creates a relatively compact layer with limited bumps and obvious cracks. Titanium shall be uniformly deposited both highlighted in **Table 2** by its high percentage and the EDX analysis (**Figure 10**), when is observed the distribution of the titanium

Titanium coatings are compact, as the atomic number of Ti (22) is much closer to that of Fe (26), which means that their atomic radius difference is very small. This makes it easy to transfer the titanium on the substrate. The outer layer of iron underwent a partial melting allowing for

Alloying titanium base material is achieved in a smaller extent because of high melting titanium temperature (1800°C) making titanium to quickly solidify once reached the surface. Thus, it explains the presence of areas with excess deposited material (compact undissolved titanium areas into "drop" metal bath and stuck to the surface). At Ti deposition on gray cast

**Figure 9.** Enlarged areas in the SEM image are for highlighting the extent of spreading of the melted area.

the drop is almost the same, no visible cracks appear, **Figure 9**.

titanium microalloying, but mainly the layer is thin, micro interrupted.

**2.4. Titanium deposition on a cast iron layer**

deposition on almost the entire surface.

**Figure 8.** (a) Area distribution of the drop; (b) elemental distribution of the "drop."

melting and solidification dynamics specific to Ti is that there are no large and visible cracks, and the layer is relatively compact.

A disadvantage of the dynamics of the phenomenon is that the roughness of the piece is quite high. Studying the BSE (Back Scattered Electrons) image shows the phase difference by the appearance of dark areas and bright areas, when the surface of the base material contains ferrite grains (light color) with darker vermicular areas (carbon), areas found in higher quantity in the center of the "droplet" (carbon forms titanium chemical compound due to chemical activity of carbon in the plasma). The black areas are made up of Ti droplets. The "droplet" area can be divided into three regions characterized by both the variation of the deposition form and the variation of the deposition element content.

The first region (**Figure 8a**) represents the center of the "droplet" that received the highest energy impulse in the deposition motion, where the temperature was the maximum, and a melting of the substrate took place. In this area, we do not actually encounter Ti in the form of a microdrop, but just accidentally by slashing. In this area, chemical compounds obtained through the reactions between Ti and the substrate elements predominate, facilitated by the energy released in the area.

The second region represents the immediate vicinity area, in the form of an annular shape, and which is a transition zone to the edges of the "droplet". In this area, Ti is found in the form of a compact micromelt, as well as microalloying compounds due to the dynamics of the melting bath and migrating from the center to the outside.

In the third zone, which is the periphery of the "droplet," melted and solidified splashes of Ti are predominant; also drops from the melted substrate pulled away the formation site due to the splashing motion. Also, due to the splashing, bumps are formed by microelevation formation right on the edge of the "drop," while in the center, there are flattened areas with a certain concavity.

The spreading zone is quite large because Ti and its chemical compounds melt at relatively low temperatures and are in a liquid form, a relatively long time (order of seconds). Due to the fluidity of the drop elements and the fact that the expansion coefficient of the substrate and the drop is almost the same, no visible cracks appear, **Figure 9**.

The outside droplet pores found, the third scattering zone, are derived from the presence of chemically absorbed gases at the surface of the part, or gases from the metal bath reaction.
