**2. Experimental results of electrospark alloying**

The following sets of experiments were carried out in order to have a broad idea about the mass transfer behavior from the treating electrode to the substrate. Pulse amplitudes ranging from 100 A to 1000 A and pulse duration ranging from 25 µs to 4000 µs were used (Rybalko et al., 2003d, 2004a, 2004b). The aim is to maximize mass gain of substrate at the initial stage of ESA, i.e. first layer of deposition, by increasing pulse energy. Because, as explained by Lazarenko (1976), when chemical composition of substrate surface becomes same as treating electrode after a single or multiple layer of deposition, mass transfer ceases down. Therefore, erosion of treating electrode should be as high as possible for the first layer of deposition. Experimental results revealed that one of the reasons limiting the thickness of deposit is the destruction of the layer already deposited during processing (Rybalko et al., 2003e). This was due to local evaporation of material underneath the outer surface of deposit. Evaporation was caused by the heat provided locally by spark discharges during second layer of deposition. Upon further processing evaporation intensifies and even cavities form. The point is to find out a common criterion for coating destruction during processing.

The mass loss of the treating electrode, the mass gain of the substrate, the calculation of mass transfer coefficient, the measurement of thickness and roughness (parameter Ra) of deposited layers, and some characteristics of the ESA process were studied for every 30 seconds for a total 3 minutes of processing. The amount of mass loss of the treating electrode and the amount of mass gain of the substrate at each time interval of 30 seconds for various pulse parameters (amplitude and duration) as a function of processing time were given in Figs 2-7. Usually in this type of ESA studies, not the mass difference between two successive measurements, but the cumulative mass change of the treating electrode and substrate versus processing time has been used to describe the change in the mass of the electrodes. The treating electrode and substrate were WC92-Co8 with cross section of 8 mm2 and steel 35 with an area of 1 cm2, respectively.

Mass loss curves of treating electrode and mass gain curves of substrate were not linear from the very beginning of alloying, for all investigated range of pulse parameters. Nonlinearity of the mass loss curves of treating electrode at the initial stage of the electrospark alloying was attributed to the nature of ESA method itself. After approximately 2 minutes of processing, mass loss of treating electrode leveled off and became stationary for the deposition with pulse amplitudes of 100 A, 200 A and 400 A. In case of deposition with pulse amplitude of 600 A, 800 A and 1000 A, this happened after approximately 1 minute of processing. Any increase in pulse duration caused: an increase in erosion of treating electrode at the initial stage of alloying, i.e. the first layer of deposition, an increase in the amount of stationary mass loss of the treating electrode, stationary level to emerge in shorter processing time.

Substrate mass gain curves were not linear either. After gradually decreasing they eventually became asymptotic to the horizontal axis. When substrate mass gain curves became asymptotic, mass loss of treating electrode became stationary. Upon further alloying the mass increment of substrate became negative. That is, instead of gaining mass, substrate started to lose mass. This is a sign for the beginning of destruction of deposit already formed. Moreover, for pulse amplitudes of 600 A, 800 A and 1000 A, mass gain of substrate sharply decreased down from the very beginning of processing, (Fig. 5-7), and almost for all experimental conditions, they were negative. These curves were indicated by dotted lines.

The total electricity through the inter-electrode gap was kept constant at 3 Coulomb as a

The following sets of experiments were carried out in order to have a broad idea about the mass transfer behavior from the treating electrode to the substrate. Pulse amplitudes ranging from 100 A to 1000 A and pulse duration ranging from 25 µs to 4000 µs were used (Rybalko et al., 2003d, 2004a, 2004b). The aim is to maximize mass gain of substrate at the initial stage of ESA, i.e. first layer of deposition, by increasing pulse energy. Because, as explained by Lazarenko (1976), when chemical composition of substrate surface becomes same as treating electrode after a single or multiple layer of deposition, mass transfer ceases down. Therefore, erosion of treating electrode should be as high as possible for the first layer of deposition. Experimental results revealed that one of the reasons limiting the thickness of deposit is the destruction of the layer already deposited during processing (Rybalko et al., 2003e). This was due to local evaporation of material underneath the outer surface of deposit. Evaporation was caused by the heat provided locally by spark discharges during second layer of deposition. Upon further processing evaporation intensifies and even cavities form. The point is to find

The mass loss of the treating electrode, the mass gain of the substrate, the calculation of mass transfer coefficient, the measurement of thickness and roughness (parameter Ra) of deposited layers, and some characteristics of the ESA process were studied for every 30 seconds for a total 3 minutes of processing. The amount of mass loss of the treating electrode and the amount of mass gain of the substrate at each time interval of 30 seconds for various pulse parameters (amplitude and duration) as a function of processing time were given in Figs 2-7. Usually in this type of ESA studies, not the mass difference between two successive measurements, but the cumulative mass change of the treating electrode and substrate versus processing time has been used to describe the change in the mass of the electrodes. The treating electrode and substrate were WC92-Co8 with cross section of 8 mm2 and steel

Mass loss curves of treating electrode and mass gain curves of substrate were not linear from the very beginning of alloying, for all investigated range of pulse parameters. Nonlinearity of the mass loss curves of treating electrode at the initial stage of the electrospark alloying was attributed to the nature of ESA method itself. After approximately 2 minutes of processing, mass loss of treating electrode leveled off and became stationary for the deposition with pulse amplitudes of 100 A, 200 A and 400 A. In case of deposition with pulse amplitude of 600 A, 800 A and 1000 A, this happened after approximately 1 minute of processing. Any increase in pulse duration caused: an increase in erosion of treating electrode at the initial stage of alloying, i.e. the first layer of deposition, an increase in the amount of stationary mass loss of the treating electrode, stationary level to emerge in shorter processing time. Substrate mass gain curves were not linear either. After gradually decreasing they eventually became asymptotic to the horizontal axis. When substrate mass gain curves became asymptotic, mass loss of treating electrode became stationary. Upon further alloying the mass increment of substrate became negative. That is, instead of gaining mass, substrate started to lose mass. This is a sign for the beginning of destruction of deposit already formed. Moreover, for pulse amplitudes of 600 A, 800 A and 1000 A, mass gain of substrate sharply decreased down from the very beginning of processing, (Fig. 5-7), and almost for all experimental conditions, they were negative. These curves were indicated by dotted lines.

base for comparison of experimental results.

35 with an area of 1 cm2, respectively.

**2. Experimental results of electrospark alloying** 

out a common criterion for coating destruction during processing.

Fig. 2. Mass loss of treating electrode and mass gain of substrate (solid dots) as a function of pulse energy for a period of 3 minutes of processing. Pulse amplitude was 100 A. Pulse duration was variable

Fig. 3. Mass loss of treating electrode and mass gain of substrate (solid dots) as a function of pulse energy for a period of 3 minutes of processing. Pulse amplitude was 200 A. Pulse duration was variable

Electrospark Deposition: Mass Transfer 487

0,5 1,0 1,5 2,0 2,5 3,0 3,5

0,5 1,0 1,5 2,0 2,5 3,0 3,5

Fig. 7. Mass loss of treating electrode and mass gain of substrate (solid dots) as a function of pulse energy for a period of 3 minutes of processing. Pulse amplitude was 1000 A. Pulse

Fig. 6. Mass loss of treating electrode and mass gain of substrate (solid dots) as a function of pulse energy for a period of 3 minutes of processing. Pulse amplitude was 800 A. Pulse

t, min

 300 μs 300 μs 400 μs 400 μs 600 μs 600 μs 800 μs 800 μs 1000 μs 1000 μs

t, min

 200 μs 200 μs 300 μs 300 μs 400 μs 400 μs 600 μs 600 μs 800 μs 800 μs 1000 μs 1000 μs


duration was variable


duration was variable






0

5

10

15 ΔM, mg





0

4

8

12 ΔM, mg

Fig. 4. Mass loss of treating electrode and mass gain of substrate (solid dots) as a function of pulse energy for a period of 3 minutes of processing. Pulse amplitude was 400 A. Pulse duration was variable

Fig. 5. Mass loss of treating electrode and mass gain of substrate (solid dots) as a function of pulse energy for a period of 3 minutes of processing. Pulse amplitude was 600 A. Pulse duration was variable

0,5 1,0 1,5 2,0 2,5 3,0 3,5

Fig. 4. Mass loss of treating electrode and mass gain of substrate (solid dots) as a function of pulse energy for a period of 3 minutes of processing. Pulse amplitude was 400 A. Pulse

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5

Fig. 5. Mass loss of treating electrode and mass gain of substrate (solid dots) as a function of pulse energy for a period of 3 minutes of processing. Pulse amplitude was 600 A. Pulse

t, min

 200 μs 200 μs 300 μs 300 μs 400 μs 400 μs 600 μs 600 μs 800 μs 800 μs 1000 μs 1000 μs

t, min

 100 μs 100 μs 150 μs 150 μs 200 μs 200 μs 300 μs 300 μs 400 μs 400 μs 600 μs 600 μs 800 μs 800 μs 1000 μs 1000 μs 2000 μs 2000 μs


duration was variable


duration was variable





0

4

8

12 ΔM, mg

Fig. 6. Mass loss of treating electrode and mass gain of substrate (solid dots) as a function of pulse energy for a period of 3 minutes of processing. Pulse amplitude was 800 A. Pulse duration was variable

Fig. 7. Mass loss of treating electrode and mass gain of substrate (solid dots) as a function of pulse energy for a period of 3 minutes of processing. Pulse amplitude was 1000 A. Pulse duration was variable

Electrospark Deposition: Mass Transfer 489

duration (μs) 50 100 150 200 300 400 600 800 1000 2000

energy (J) 0,097 0,257 0,427 0,602 0,927 1,28 1,9 2,72 3,31 6,74

(Hz) 528 198 119 92 55 39 26 19 16 8

electricity (C) 2,99 2,997 2,99 3,25 3 2,95 2,91 3,04 3,12 3,17

scan 1cm2 (sec) 50 40 40 40 40 50 60 65 65 60

anode (mg) 2,33 3,47 4,27 5,03 5,59 8,6 11,2 13,0 16,1 16,9

cathode (mg) 1,33 2,13 2,66 3,6 4,02 5,5 7,46 8,11 10,1 10

coefficient 0,57 0,61 0,62 0,72 0,72 0,64 0,67 0,62 0,63 059

roughness (μm) 3,07 3,12 4,1 4,61 5,01 6,8 7,42 8,83 9,98 11,67

to failure (μm) 12±7 17±5 32±5 37±10 41±10 50±20 55±20 55±20 55±25 60±25

duration (μs) 100 150 200 300 400 600 800 1000 2000

energy (J) 0,342 0,639 1,02 1,68 2,39 3,77 5,12 6,48 13,3

(Hz) 158 80 50 31 23 13 10 8 4

electricity (C) 3,002 3,008 3,009 3,066 3,23 3,108 3,01 3,048 3,13

scan 1cm2 (sec) 35 50 50 50 50 50 40 40 28

anode (mg) 2,96 4,2 5,17 10 11,5 13,15 10,92 11,56 17,01

cathode (mg) 0,35 2,27 1,67 5,17 5,52 6,47 5,38 6,8 10,9

coefficient 0,12 0,54 0,32 0,52 0,48 0,49 0,59 0,59 0,64

roughness (μm) 3,79 4,42 6,32 10,1 11,7 12,48 13,62 15,45 14,69

till to failure (μm) 16±5 35±5 50±20 60±30 65±30 65±30 75±40 65±40 100±80

number of layers 3 2 2-1 2-1 1 1 1 1 1

number of layers 3 3 3-2 2 2 2-1 1 1 1 1

Table 2. Some parameters of processing. Pulse amplitude is 200 A

Table 3. Some parameters of processing. Pulse amplitude is 400 A

Pulse

Pulse

Frequency

Processing

Time t1 to

Mass loss of

Mass gain of

Mass transfer

Surface

Recommended

Coating thickness till

Pulse

Pulse

Frequency

Processing

Time t1 to

Mass loss of

Mass gain of

Mass transfer

Surface

Recommended

Coating thickness

Longer pulse durations caused higher mass gain of substrate at the first layer of deposition and shorter processing time till to the beginning of mass loss by these substrates. Higher pulse amplitude also was accompanied by an increase in substrate mass gain at the initial stage of alloying. The only exceptional data is that, in case of ESA with pulse amplitude of 1000 А, comparing to the case of 800 А, any significant increase in mass gain of substrate was not obtained.

Since, environment and initial condition of electrode materials were same for all experiments, the difference in experimental results was due to the difference in pulse parameters (pulse energy) for only the processing time of 30 seconds (first layer of deposition). This is not true for second and third layers of deposition. The mass gain of substrates, in turn surface properties of substrates, after first layer of deposition was all different from each other. Therefore, only the experimental results of first layer of deposition including some characteristic of processing were used to form a base, Tables 1-6, in order to find out the effect of pulse energy on mass transfer. Mass transfer coefficients were calculated from the data of mass transfer during the first 30 seconds of alloying.


Table 1. Some parameters of processing. Pulse amplitude is 100 A

Longer pulse durations caused higher mass gain of substrate at the first layer of deposition and shorter processing time till to the beginning of mass loss by these substrates. Higher pulse amplitude also was accompanied by an increase in substrate mass gain at the initial stage of alloying. The only exceptional data is that, in case of ESA with pulse amplitude of 1000 А, comparing to the case of 800 А, any significant increase in mass gain of substrate

Since, environment and initial condition of electrode materials were same for all experiments, the difference in experimental results was due to the difference in pulse parameters (pulse energy) for only the processing time of 30 seconds (first layer of deposition). This is not true for second and third layers of deposition. The mass gain of substrates, in turn surface properties of substrates, after first layer of deposition was all different from each other. Therefore, only the experimental results of first layer of deposition including some characteristic of processing were used to form a base, Tables 1-6, in order to find out the effect of pulse energy on mass transfer. Mass transfer coefficients were calculated from the

duration (μs) 25 50 100 150 200 300 400 600 800 1000 2000 4000

energy (J) 0,032 0,08 0,17 0,233 0,297 0,529 0,72 1,08 1,44 1,7 3,4 6,79

(Hz) 1613 645 309 219 178 99 71 47 36 30 15 8

electricity (C) 3,00 3,03 2,99 3,00 3,11 3,08 3,01 2,99 3,05 3 3 3,2

1cm2 (sec) 50 50 50 45 45 45 55 50 55 55 70 60

anode (mg) 1,25 1,67 2,25 2,92 3,75 4,5 6,05 7 7,52 10,3 12,1 17,99

cathode (mg) 0,75 1,00 1,66 2,1 2,55 2,85 4,35 5,17 5,68 7,66 8,4 9,46

coefficient 0,6 0,6 0,74 0,68 0,68 0,63 0,72 0,74 0,75 0,74 0,69 0,76

roughness (μm) 2,65 2,77 3,0 3,25 3,35 3,71 4,35 4,9 5,36 7,2 8,97 12,03

Table 1. Some parameters of processing. Pulse amplitude is 100 A

3 3 3 2-3 2-3 2-3 2 2 1-2 1 1 1

10±4 13±7 13±5 16±6 17±5 20±6 22±6 23±6 35±10 35±20 45±35 60±50

data of mass transfer during the first 30 seconds of alloying.

was not obtained.

Pulse

Pulse

Frequency

Processing

Time t1 to scan

Mass loss of

Mass gain of

Mass transfer

Surface

Recommended number of layers

Coating thickness till to failure (μm)





Electrospark Deposition: Mass Transfer 491

duration (μs) 300 400 600 800 1000

energy (J) 3,02 4,54 7,87 11,33 14,52

(Hz) 17 11 6 4 3

(mg) 5,06 5,33 8,65 11,87 19,41

electricity (C) 3,009 2,937 2,772 2,668 2,562

scan 1cm2 (sec) 25 20 20 20 20

cathode (mg) 0,88 0,73 2,8 4,6 7,2

coefficient 0,17 0,14 0,32 0,39 0,37

number of layers 3 2-1 1 1 1

till to failure (μm) 55±45 60±45 65±50 100±85 100±<sup>100</sup>

The experimental results obtained under the condition of constant pulse amplitude and increasing pulse duration were given in Tables 1-6. As mentioned previously, processing electricity was kept constant at 3 Coulomb for all experiments. Since pulse amplitude was constant, pulse energy was increased by increasing pulse duration. Main objective of this investigation was to increase both mass loss of treating electrode and mass gain of the substrate, in turn coating thickness, especially for the first layer of deposition. Higher pulse energy causes higher mass gain of substrate and shorter processing time till to the beginning of mass loss (destruction of deposit already formed). Despite the mass gained from treating electrode, a decrease in weight of the substrate was observed for pulse duration of 4000 µs in case of 100 A pulse amplitude after 1.5 minutes of processing (Fig. 2), for pulse duration of 2000 μs in case of 200 A pulse amplitude after 1.5 minute processing (Fig. 3) and for pulse duration of 400 μs in case of 400 A pulse amplitude after 2.0 minute processing (Fig. 4). Further increase of pulse duration caused substrate to start to lose mass in shorter processing time. Upon further alloying, the mass loss, in other words, the destruction of layer already formed intensifies and the destruction could reach substrate by removing already formed layer totally. In Fig. 8, cross-sectional micrograph of a coating was given as an example in order to show the beginning of destruction. Part of the specimen was not purposely alloyed (see the upper part of micrograph) in order to be able to compare the level of coating thickness and

In case of 100 A pulse amplitude, during alloying with pulse duration longer than 200 µs, erosion of molten pool on substrate was observed from the very beginning of alloying due to the presence intensive gas dynamics phenomena in plasma channel. This type of erosion was named "washing away" which is the mass lost by substrate (Rybalko et al., 2008).

(μm) 5,49 8,28 14,84 16,38 17,62

Pulse

Pulse

Frequency

Processing

Time t1 to

Mass loss of anode

Mass gain of

Mass transfer

Surface roughness

Recommended

Coating thickness

Table 6. Some parameters of processing. Pulse amplitude is 1000 A

depth of destruction with the original level of the surface.


Table 4. Some parameters of processing. Pulse amplitude is 600 A


Table 5. Some parameters of processing. Pulse amplitude is 800 A

duration (μs) 200 300 400 600 800 1000

energy (J) 1,13 2,19 3,35 5,34 7,31 9,39

(Hz) 45 23 15 10 7 5

electricity (C) 2,997 2,967 2,955 3,14 3,01 2,763

scan 1cm2 (sec) 35 35 35 35 30 25

anode (mg) 4,9 5,95 7,43 11,12 11,1 11

cathode (mg) 1,67 2,53 3,77 4 4,1 4,39

coefficient 0,34 0,43 0,51 0,36 0,37 0,4

number of layers 3 2 1 1 1 1

till to failure (μm) 35±15 55±30 65±40 40±15 70±50 75±<sup>50</sup>

duration (μs) 200 300 400 600 800 1000

energy (J) 1,47 2,78 3,96 6,82 9,47 12,45

(Hz) 35 18 13 8 5 4

(mg) 5,25 5,77 8,67 9,32 14,7 16,33

electricity (C) 3,02 2,95 3,03 3,21 2,78 2,93

scan 1cm2 (sec) 35 35 35 25 25 25

cathode (mg) 0,23 0,91 1,95 5,21 6,83 8,63

coefficient 0,04 0,16 0,22 0,56 0,48 0,53

number of layers 3 2 2-1 1 1 1

till to failure (μm) 50±20 50±40 60±40 65±45 100±80 110±<sup>105</sup>

(μm) 4,63 9,59 10,31 13,59 17,7 17,75

(μm) 5,14 7,05 10,14 14,20 15,82 17,72

Pulse

Pulse

Frequency

Processing

Time t1 to

Mass loss of

Mass gain of

Mass transfer

Surface roughness

Recommended

Coating thickness

Pulse

Pulse

Frequency

Processing

Time t1 to

Mass loss of anode

Mass gain of

Mass transfer

Surface roughness

Recommended

Coating thickness

Table 4. Some parameters of processing. Pulse amplitude is 600 A

Table 5. Some parameters of processing. Pulse amplitude is 800 A


Table 6. Some parameters of processing. Pulse amplitude is 1000 A

The experimental results obtained under the condition of constant pulse amplitude and increasing pulse duration were given in Tables 1-6. As mentioned previously, processing electricity was kept constant at 3 Coulomb for all experiments. Since pulse amplitude was constant, pulse energy was increased by increasing pulse duration. Main objective of this investigation was to increase both mass loss of treating electrode and mass gain of the substrate, in turn coating thickness, especially for the first layer of deposition. Higher pulse energy causes higher mass gain of substrate and shorter processing time till to the beginning of mass loss (destruction of deposit already formed). Despite the mass gained from treating electrode, a decrease in weight of the substrate was observed for pulse duration of 4000 µs in case of 100 A pulse amplitude after 1.5 minutes of processing (Fig. 2), for pulse duration of 2000 μs in case of 200 A pulse amplitude after 1.5 minute processing (Fig. 3) and for pulse duration of 400 μs in case of 400 A pulse amplitude after 2.0 minute processing (Fig. 4). Further increase of pulse duration caused substrate to start to lose mass in shorter processing time.

Upon further alloying, the mass loss, in other words, the destruction of layer already formed intensifies and the destruction could reach substrate by removing already formed layer totally. In Fig. 8, cross-sectional micrograph of a coating was given as an example in order to show the beginning of destruction. Part of the specimen was not purposely alloyed (see the upper part of micrograph) in order to be able to compare the level of coating thickness and depth of destruction with the original level of the surface.

In case of 100 A pulse amplitude, during alloying with pulse duration longer than 200 µs, erosion of molten pool on substrate was observed from the very beginning of alloying due to the presence intensive gas dynamics phenomena in plasma channel. This type of erosion was named "washing away" which is the mass lost by substrate (Rybalko et al., 2008).

Electrospark Deposition: Mass Transfer 493

liquid material of the treating electrode. It is obvious that such a large volume of mass transfer warms up the substrate. For pulse duration of 4000 µs, superficial boiling in deposited layer was all over and pores were close to the substrate (Fig. 10). As in the case of pulse duration of 1000 µs, the essential role in forming a layer was determined by gas

Alloying in case of 200 A pulse amplitude was similar to the alloying in case of 100 A. Examination of the morphology and cross-section of deposited layer showed that high

It is noticed that in case of 400 A pulse amplitude, alloying till pulse duration of 300 μs could be carried out without any destruction. However, mass transfer was low. The alloying by pulse duration above 300 μs was already characterized by a decrease in mass transfer that marks the beginning of destruction of layer formed already. Further increase in pulse

Fig. 10. Cross-sectional micrograph of a coating. Pulse duration is 4000 μs. Pulse energy is

Morphology of the deposited layer was different than that of alloying by pulse amplitudes of 200 A and 100 А. This is due to the overheating of substrate at the interface of plasma channel and substrate. The longer the pulse durations, the higher the overheating. Concerning deposition under these conditions, the following could be assumed. The plasma channel of the discharge overheats the deposited layer. Its melting temperature is already higher than that of substrate and, as a results, molten zones of substrate form under deposited layer. Molten zones flow out through cavities in coating. After reaching the surface the molten substrate material covers and mixes with the erosive mass transferred from the treating electrode. The mixed mass solidifies on the surface of coating formed earlier. This would probably explain the presence of low microhardness at the coating surface, and also increase its surface roughness. In case of ESA with pulse durations more than 600 µs (pulse amplitude is 400 А), the process was characterized by an increase in the area of splash. Thus, rate of the first layer deposition increases, but the rate of thickness

quality deposit was limited by pulse duration of less than 400 μs.

durations leads to the evaporation of substrate through cavities in coating.

dynamics in plasma channel.

6.8 J

build up of this layer decreases.

Fig. 8. Cross-sectional micrograph of a coating showing the locations of failure

Therefore, the change in weight of the substrate is the difference between the liquid mass received from treating electrode and the liquid mass loss due to washing away of its molten pool at the beginning of a spark discharge. Intensive washing away was observed with pulse durations of 400 µs in case of 100 A pulse amplitude, 200 µs in case 200 A pulse amplitude, and 200 µs in case of 400 A pulse amplitude. The micrograph (Fig. 9) shows that, the liquid material ejected from treating electrode filled up the substrate pool after washing away was completed.

Fig. 9. Cross-sectional micrograph of a coating showing the location of pool on substrate

For the pulse duration of 1000 µs in case 100 A pulse amplitude, the morphology of coating changed essentially. There were pores at the upper part of the layer that were formed by superficial boiling of the transferred material. Pulse durations more than 1000 µs caused the formation of large pores not only at the surface, but also inside the deposited layer. Boiling was caused by significant amount of heat transferred to the substrate from overheated

Fig. 8. Cross-sectional micrograph of a coating showing the locations of failure

away was completed.

Therefore, the change in weight of the substrate is the difference between the liquid mass received from treating electrode and the liquid mass loss due to washing away of its molten pool at the beginning of a spark discharge. Intensive washing away was observed with pulse durations of 400 µs in case of 100 A pulse amplitude, 200 µs in case 200 A pulse amplitude, and 200 µs in case of 400 A pulse amplitude. The micrograph (Fig. 9) shows that, the liquid material ejected from treating electrode filled up the substrate pool after washing

Fig. 9. Cross-sectional micrograph of a coating showing the location of pool on substrate

For the pulse duration of 1000 µs in case 100 A pulse amplitude, the morphology of coating changed essentially. There were pores at the upper part of the layer that were formed by superficial boiling of the transferred material. Pulse durations more than 1000 µs caused the formation of large pores not only at the surface, but also inside the deposited layer. Boiling was caused by significant amount of heat transferred to the substrate from overheated liquid material of the treating electrode. It is obvious that such a large volume of mass transfer warms up the substrate. For pulse duration of 4000 µs, superficial boiling in deposited layer was all over and pores were close to the substrate (Fig. 10). As in the case of pulse duration of 1000 µs, the essential role in forming a layer was determined by gas dynamics in plasma channel.

Alloying in case of 200 A pulse amplitude was similar to the alloying in case of 100 A. Examination of the morphology and cross-section of deposited layer showed that high quality deposit was limited by pulse duration of less than 400 μs.

It is noticed that in case of 400 A pulse amplitude, alloying till pulse duration of 300 μs could be carried out without any destruction. However, mass transfer was low. The alloying by pulse duration above 300 μs was already characterized by a decrease in mass transfer that marks the beginning of destruction of layer formed already. Further increase in pulse durations leads to the evaporation of substrate through cavities in coating.

Fig. 10. Cross-sectional micrograph of a coating. Pulse duration is 4000 μs. Pulse energy is 6.8 J

Morphology of the deposited layer was different than that of alloying by pulse amplitudes of 200 A and 100 А. This is due to the overheating of substrate at the interface of plasma channel and substrate. The longer the pulse durations, the higher the overheating. Concerning deposition under these conditions, the following could be assumed. The plasma channel of the discharge overheats the deposited layer. Its melting temperature is already higher than that of substrate and, as a results, molten zones of substrate form under deposited layer. Molten zones flow out through cavities in coating. After reaching the surface the molten substrate material covers and mixes with the erosive mass transferred from the treating electrode. The mixed mass solidifies on the surface of coating formed earlier. This would probably explain the presence of low microhardness at the coating surface, and also increase its surface roughness. In case of ESA with pulse durations more than 600 µs (pulse amplitude is 400 А), the process was characterized by an increase in the area of splash. Thus, rate of the first layer deposition increases, but the rate of thickness build up of this layer decreases.

Electrospark Deposition: Mass Transfer 495

radiation due to intensive evaporation in substrate was present (after scanning of the first layer), and use of pulse duration of 1000 µs formed a deposition with large pores. Alloying with pulse durations higher than 1000 µs could not been achieved due to strong sticking

Fig. 12. Cross-sectional micrograph of a coating. Pulse duration is 400 μs. Pulse amplitude is

In case of alloying with 1000 A pulse amplitude, when the pulse duration was 300 µs, erosive mass of electrodes was scattered all over because of intensive gas dynamics phenomena in plasma channel. Therefore, as same as the processing with long pulse duration, the coefficient of mass transfer was low and roughness of deposition was high because of the big differences in thickness in an individual spot. The average thickness of deposit was 55 ± 40 micron. Alloying was carried out without sticking of electrodes and

When the pulse duration was more than 300 µs, substrate evaporation began right after coating of the first layer and an oxide film formed on the surface of deposit. Above 800 µs electrodes stuck to each other and above 1000 µs formation of large pores was all over.

In case of alloying with pulse amplitudes of 600 A, 800 A and 1000 А, a porous deposition like foam was obtained by the application of 1000 µs pulse duration. Since, pulse energies were significantly different from each other for these three cases, pulse duration could be

The analysis of the experimental results shows that, the occurrence of destruction correlates with the time of decrease in mass transfer coefficient, calculated from data, given in Tables 4, 5 and 6. Proceeding from the data of the beginning of decrease in mass transfer coefficient, number of probable deposited layers (number of scanning), at which coating destruction will not be observed, was determined and recommended in these tables. Investigation shows that under the experimental conditions given above, it is possible to deposit only one

during the deposition of first layer.

400 A. Pulse energy is 2.39 J

oxidation of surface.

Coating thickness varied between 5 µs and 200 µs.

layer above the pulse duration of 400 µs.

responsible for the formation of similar kind of porous formation.

In case of alloying with pulse amplitudes of 100 А, 200 А and 400 А, if pulse duration is above 1000 µs, porous foam-like coatings were obtained. Since the amounts of pulse energies were significantly different for these three cases, one could conclude that the length of pulse duration is the reason of this kind of coating.

Fig. 11. Cross-sectional micrograph of a coating. Pulse duration is 600 μs. Pulse energy is 1.1 J

Alloying with the pulse amplitude of 100 A leads to high quality coatings with an average thickness of 10-35 µs for the pulse duration up to 1000 μs (Fig. 11). Increasing pulse duration was accompanied by an increase in non-uniformity of thickness. For manual alloying with pulse amplitudes of 200 A and 400 А, pulse duration more than 400 µs would not be recommended. Below 400 µs, it is possible to obtain a high quality coating with an average thickness of 50 microns (200 A) and 65 microns (400 A) till the beginning of destruction (Fig 12). ESA with longer pulse durations allows only one layer of scanning without destruction. The thickness of a spot due to an individual discharge could be as much as 120 microns at the center and lower at the edges. In this condition, in order to obtain high coating thickness the thinner edges of the neighboring splashes should be overlapped during the deposition process. Therefore, it is necessary to carry out process with use of an automated installation. Scanning rate of automated installation could be adjusted to provide partial overlapping of subsequent splashes.

In case of alloying with 600 A pulse amplitude, if the pulse durations were more than 300 µs, intensive substrate overheating occurred at the plasma channel-substrate interface. When the pulse durations were above 400 µs, substrate evaporated during deposition of the second layer. ESA process was accompanied by unusually big plasma flame escaping from the interelectrode zone during spark discharge. The size of plasma flame was essentially wider than the cross-sectional area of treating electrode. Layer formation by spark discharges with pulse durations of 1000 µs, was characterized by the occurrence of large pores including open pores at the surface. Alloying with pulse duration of 2000 µs (pulse energy is 18,78 J) was tried. However, due to sticking of electrodes, alloying was not possible.

In case of processing with pulse amplitude of 800 A and pulse duration of 600 µs, electrodes stuck to each other. Moreover, during processing with pulse duration of 800 µs, strong

In case of alloying with pulse amplitudes of 100 А, 200 А and 400 А, if pulse duration is above 1000 µs, porous foam-like coatings were obtained. Since the amounts of pulse energies were significantly different for these three cases, one could conclude that the length

Fig. 11. Cross-sectional micrograph of a coating. Pulse duration is 600 μs. Pulse energy is 1.1 J Alloying with the pulse amplitude of 100 A leads to high quality coatings with an average thickness of 10-35 µs for the pulse duration up to 1000 μs (Fig. 11). Increasing pulse duration was accompanied by an increase in non-uniformity of thickness. For manual alloying with pulse amplitudes of 200 A and 400 А, pulse duration more than 400 µs would not be recommended. Below 400 µs, it is possible to obtain a high quality coating with an average thickness of 50 microns (200 A) and 65 microns (400 A) till the beginning of destruction (Fig 12). ESA with longer pulse durations allows only one layer of scanning without destruction. The thickness of a spot due to an individual discharge could be as much as 120 microns at the center and lower at the edges. In this condition, in order to obtain high coating thickness the thinner edges of the neighboring splashes should be overlapped during the deposition process. Therefore, it is necessary to carry out process with use of an automated installation. Scanning rate of automated installation could be adjusted to provide partial overlapping of

In case of alloying with 600 A pulse amplitude, if the pulse durations were more than 300 µs, intensive substrate overheating occurred at the plasma channel-substrate interface. When the pulse durations were above 400 µs, substrate evaporated during deposition of the second layer. ESA process was accompanied by unusually big plasma flame escaping from the interelectrode zone during spark discharge. The size of plasma flame was essentially wider than the cross-sectional area of treating electrode. Layer formation by spark discharges with pulse durations of 1000 µs, was characterized by the occurrence of large pores including open pores at the surface. Alloying with pulse duration of 2000 µs (pulse energy is 18,78 J)

In case of processing with pulse amplitude of 800 A and pulse duration of 600 µs, electrodes stuck to each other. Moreover, during processing with pulse duration of 800 µs, strong

was tried. However, due to sticking of electrodes, alloying was not possible.

of pulse duration is the reason of this kind of coating.

subsequent splashes.

radiation due to intensive evaporation in substrate was present (after scanning of the first layer), and use of pulse duration of 1000 µs formed a deposition with large pores. Alloying with pulse durations higher than 1000 µs could not been achieved due to strong sticking during the deposition of first layer.

Fig. 12. Cross-sectional micrograph of a coating. Pulse duration is 400 μs. Pulse amplitude is 400 A. Pulse energy is 2.39 J

In case of alloying with 1000 A pulse amplitude, when the pulse duration was 300 µs, erosive mass of electrodes was scattered all over because of intensive gas dynamics phenomena in plasma channel. Therefore, as same as the processing with long pulse duration, the coefficient of mass transfer was low and roughness of deposition was high because of the big differences in thickness in an individual spot. The average thickness of deposit was 55 ± 40 micron. Alloying was carried out without sticking of electrodes and oxidation of surface.

When the pulse duration was more than 300 µs, substrate evaporation began right after coating of the first layer and an oxide film formed on the surface of deposit. Above 800 µs electrodes stuck to each other and above 1000 µs formation of large pores was all over. Coating thickness varied between 5 µs and 200 µs.

In case of alloying with pulse amplitudes of 600 A, 800 A and 1000 А, a porous deposition like foam was obtained by the application of 1000 µs pulse duration. Since, pulse energies were significantly different from each other for these three cases, pulse duration could be responsible for the formation of similar kind of porous formation.

The analysis of the experimental results shows that, the occurrence of destruction correlates with the time of decrease in mass transfer coefficient, calculated from data, given in Tables 4, 5 and 6. Proceeding from the data of the beginning of decrease in mass transfer coefficient, number of probable deposited layers (number of scanning), at which coating destruction will not be observed, was determined and recommended in these tables. Investigation shows that under the experimental conditions given above, it is possible to deposit only one layer above the pulse duration of 400 µs.

Electrospark Deposition: Mass Transfer 497

As it was shown experimentally, the phenomenon of coating destruction by its evaporation occurred during alloying in all ranges of pulse parameters investigated. Thus, to obtain high quality depositions, it is necessary to limit the processing time by the moment of the signs, indicating destruction of the deposited layer. The beginning of coating destruction could be determined by some features of ESA, namely: when mass loss of treating electrode ceases down to a minimum stable level, when mass gain of substrate becomes zero or even

Most precisely, the beginning of coating destruction could be defined as the moment of sharp decrease in mass transfer coefficient, which is the ratio between the mass gain of substrate to the corresponding mass loss of treating electrode between two successive

The analysis of experimental results indicates that the number of superimposed layers prior to the beginning of coating failure, for the investigated range of pulse energies, depends on pulse energy and it could be between 1 and 3 (the latest one is for low pulse energy) (Fig. 14). A single-valued dependence of the number of these layers to the amount of pulse energy was not observed. The same number of layers could be obtained by the employment of different pulse energies which depend on pulse amplitude and pulse duration. As an example, for 3 layers of deposition, either a pulse amplitude of 400 A (pulse energy not more than 0.3 J), or a pulse amplitude of 800 A (pulse energy not more than 1.5 J) or pulse amplitude of 1000 A (pulse energy not more than 3 J) could be employed. It should be noticed that, although there is a ten fold increase in pulse energy, the maximum number of deposited layers prior to fracture does not change. This behavior was explained as follows. Despite of the constant amount of electricity for the ESA processing (3 Coulomb), deposition time of the first layer strongly varies, and it was, for example, 20 seconds for a high pulse energy case and 70 seconds for a low pulse energy case. Examination of the surface morphology of coatings shows that size, e.g. average diameter, of the solidified splashes due to mass transferred by a single pulse for the each energy significantly varies. For the low pulse energy of 0.032 J (100 А, 25 μs) the time to scan the substrate area of 1 cm2 requires 80650 pulses (then the average splash area as a result of each spark discharge is roughly 0.00124 mm2) and for the high pulse energy of 9.47 J (800 А, 800 μs), the same area was totally scanned by the application of only 125 pulses (thus the average splash area is nearly 0.8 mm2 per pulse). That is, although pulse energy was increased 259 times, the average splash area was increased 645 times. Therefore, the growth of splash size is not directly

The erosive processes on treating electrode and substrate are determined not only by the electrical parameters of discharge and the chemical properties of the electrodes, but also, substantially by the rate of heat flow received by the electrodes (Namitokov, 1978; Butkevich et al., 1978). The amount of heat which could cause evaporation of substrate under the first layer of coating is not reached by the employment of long duration pulses, as in the case of pulses which have same energy but shorter duration. To reach dense heat flow in electrodes high enough to cause same kind of fracture, it is necessary to raise the energy of a long

Since the number of superimposed layers prior to beginning the fracture of coating is limited, the amount of mass loss of the treating electrode and mass gain of the substrate are also limited. It is of interest to determine the amount of mass transfer, in turn coating thickness, quantitatively till fracture. As mentioned previously, it is a fact that the

negative and the moment of a sharp decrease in mass transfer coefficient

proportional to the increase in pulse energy.

duration pulse by increasing its amplitude.

measurements. The decision could be considered as a criterion to end the process.

In Tables 4, 5 and 6, average coating thickness obtained under recommended number of scanning is given. The analysis of this data shows that if pulse duration increases, the average thickness increases, in spite of the fact, that the number of recommended layers for alloying decreases.

In case, if it is necessary to deposit only one layer, irregular coating (non uniform thickness) is inevitable. For example, ESA with pulse duration of 800 µs and pulse amplitude of 1000 А, the minimum thickness of the deposited spot due to a single spark discharge was 10 microns at the edge and 200 microns at the center. Thus, a layer of 1 cm2 area could be formed by only 60 spark discharges: 3 pulses per second for a total processing of 20 seconds (Table 6). Morphology of coating is similar to the one illustrated in Fig. 13 (cross-sectional micrograph)

Fig. 13. A typical view of cross-section and surface of a coating could be obtained with high energy pulses. Pulse amplitudes and pulse durations could be from 600 A to 1000 A and 400 to 1000 µs respectively
