Electro-Spark Coating with Special Materials - nonconventional ...

ELECTRO-SPARK COATING WITH SPECIAL MATERIALS
Victor VERBITCHI*; Cristian CIUCA*; Radu COJOCARU*
* National Research and Development Institute for Welding and Material Testing - ISIM Timisoara,
Bd. Mihai Viteazul, Nr.30, Postal code 300222, Timisoara, Romania;
e-mail: vverbitchi@isim.ro; cciuca@isim.ro; rcojocaru@isim.ro
ABSTRACT: The electro-spark coating is a deposition process. Consumable electrodes are used, made of
stainless steel, nickel, other metals, sintered tungsten carbide, metal-ceramics or ceramics. Specific
characteristics and applications possibilities are presented, such as small area reclaiming of tools, dies and
valves. An experimental model made by ISIM of Timisoara is described. It generates electric sparks between
a rotating electrode and a base metal. Some experiments and results are also presented and discussed. The
thickness of the coating is 0.01…0.5 mm. There is no distortion of the base metal. The operator has full
control on the process. The deposition rate depends on the power of the device (0.160...1.6 kVA).
Keywords: electro-spark coating, deposition, electrodes, special materials, spark generating,
material transfer, small reclaiming, reduced distortion.
1. INTRODUCTION
In the most cases, in the manufacturing
process of the moulds and dies, the final
operation consists in applying some thermal
treatments, as quenching, annealing, etc. But
the industrial practise has demonstrated the
increasing need for harder and more ductile
materials, that have a higher resistance in
time and are easier for processing [1-4].
There are several thermal-chemical
treatments widely used, as nitriding, physical
vapor deposition (PVD), chemical vapor
deposition (CVD), electroplating, thermal
diffusion (TD), etc., that improve the wear or
corrosoin resistance. The equipment cost,
process duration and pollution of the
environment are only a few of the
disadvantages of these methods. Instead, the
electro-spark coating and deposition
processes can eliminate these difficulties.
2. ELECTRO-SPARK DEPOSITION
As the instantaneous temperature of the
spark is about 25,000 ˚C, fusion of some fine
drops of the electrode material takes place,
that are ejected upon the base metal and
form a compact deposition, only with local
heating, so that no transformations and
distortions of the base metal occur. This
technological process was contrived to
perform electro-spark coating, having the
thickness 0.01 ... 0.5 mm [1,3], respectively
electrospark depositions, with the thickness
Nonconventional Technologies Review – no. 1/2011
57
0.5 ... 3.0 mm [2,4], for improving the some
functional proprieties or repairing some parts.
These represent functional variants, with
applicative character of a non-conventional
technology [1-5].
The consumable electrode is in a relative
movement against the base metal, by rotation
or vibration. The electric sparks are produced
by an electric circuit, that consists generally
of a spark generator and a capacitor, that
charges from the generator and discharges
periodically in a secondary circuit, as it closes
by means of the contact between the
electrode and the workpiece.
This process replaced the detonation-gun
or HVOF processes and provided orders of
magnitude increase in wear and damage
resistance, a five-fold improvement in
corrosion performance, lower friction, and
more than a 50% saving in cost, with the
same material. The process was in
production of nuclear components for 10
years, with no failure or reject [3].
Electro-spark Deposition (ESD) is a microwelding
process, which uses electrical pulses
frequencies in the 0.1... 2 kHz range and thus
allow substrate heat dissipation over ~99% of
the duty cycle, while heating only ~1%. The
result is cooling rates of 10 5 to 10 6 ○ C/sec,
and generation of nano-structures,
amorphous for some alloys, with corrosion
and tribological benefits. This also eliminates
thermal distortions.
The ESD are among the most damageresistant
coatings, suitable for high stresses,

high temperatures, thermal cycling,
irradiation, wear, corrosion, and erosion. An
applications is the three-times increase in life
of high-speed steel milling cutters after ESD
with tungsten carbide. Other application are:
ship propeller components, casting moulds,
fuel supply system parts, exhaust system
components, implants or cutting tools [3].
The ESD coating is realized in an
environment of local high temperature and
pressure [3]: shock wave pressure from the
electric spark (2...7)•10 3 GPa; temperature
(5...40)•10 3 °Celsius. Deposition times are
several minutes per cm 2 .
A generator for the electro-spark coating
and depositing is presented in fig.1.
Fig. 1. Generator for the electro-spark
coating and deposition [1]
The deposits are realized with various filler
materials, depending on the characteristics
intended to be obtained: high hardness, good
wear resistance, anti-corrosive proprieties,
decorative objects, thermal barriers, etc.
Table 1. Electrodes of special materials
for electro-spark coating/depositing [1-3]
Electrode
type
LT-01
LT-02
LT-03
LT-04
LT-05
LT-06
Sizes
Φ x L
[mm]
2,4 x
100
3,3 x
100
3,3 x
100
2,4 x
100
2,2 x
100
2,2 x
100
Material
Ni alloy
Cr-Co
alloy,
stellite
Cr-Co
alloy,
stellite
alloy
steel
alloy
steel
alloy
steel
Nonconventional Technologies Review – no. 1/2011
Hardness
[HRC]
40
62
55-58
45-50
35
34
Frequen
cy
[Hz]
C / D
50-60
90-110
50-60
90-110
50-60
90-110
50-60
90-100
50-60
90-100
50-60
80-90
Thickness
max.
[mm]
3
1
1
3
3
3
58
Electrode
type
LT-07
LT-08
A1-01
TC=01
TC-02
Sizes
Φ x L
[mm]
2,2 x
100
2,2 x
100
2,0 x
100
3,3 x
100
2,2 x
100
Material
Hardness
[HRC]
inco- nel 29
stain
less
steel
alumi
nium
tungsten
car-bide
tungsten
car-bide
21
-
Frequen
cy
[Hz]
C / D
50-60
90-110
50-60
90-110
50-60
130-150
Thickness
max.
[mm]
3
3
3
on cast
Al
90-95 40-100 0.05
90-65
40-80
0.05
Note. C is electro-spark coating. D is electrospark
depositing.
3. EXPERIMENTAL MODEL
ISIM of Timisoara has contrived and realized
an experimental model device for electrospark
coating, shown in fig. 2 [6].
Fig. 2. Device for electro-spark coating [6]
The device has the following functions,
that asure the specific technology
requirements of the process [6]:
a. Generating electric discharges, as sparks,
over a gap (approximately 0.1...0.5 mm)
between the electrode of the device and the
base metal workpiece, as well as the transfer
of the filler material of the electrode, as small
drops, unto the base metal. The spark-based
material transfer is the key of the process.
b. Rotating movement of the electrode,
necessary for avoiding the accidental
welding of the consumable electrode on the
workpiece, on which the coating or the
depositing is made. The rotating motion is

performed by a servomotor, by means of a
flexible, torsion-resistant cable, about
1.0...1.15 m long; the cable rotates in a
flexible guiding tube; by this cable the electric
connection is also made for the depositing
current of 2...3 A; the inlet connection of the
power current is realized by means of two
graphite contact brushes, having a section of
9mm x 20mm, placed on both sides of the
electrode rotating axle.
3.1. Generator for electro-spark
coating
A reduced-power spark generator was
realized, having the following characteristics:
installed power S= 160 VA; open-circuit
voltage U0 = 55 V; frequency f= 50 Hz; shortcircuit
current Isc= 4 A; duty factor DA 10%.
3.2. Servomotor drive block
This block has the following components:
- rotational speed regulator;
- servomotor (24 VDC; 120 W; 5000 rot/min);
- worm reducer (i= 1/40);
- mechanical drive unit.
The rotational speed of the electrode can
be set in the continuous range n= 0... 60
rot/min. The pre-set speed is realized by a
speed regulator. This is a pulse width
modulation (PWM) electronic-set, with
thyristors, operating at the mains frequency.
It keeps the pre-set speed by compensating
the voltage drop over the rotor coils, during
the load operation, for any extent of the
torque, respectively of the load current, in the
operation range. In a certain connection, this
regulator can utilize the electro-motive force
of the rotor armature for stabilizing the speed.
The regulator can also operate at higher
frequencies, if necessary.
Fig. 3. Sparks aplicator (device tool) [6]
In fig.3 the sparks aplicator (tool of the
electro-sparks device) is presented.
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59
4. EXPERIMENTS
A. Electro-spark coating with a
stainless steel electrode
On the experimental model, some
experiments have been done, with the
purposes of verifying the model and the
possibilities of the process, concerning the
main characteristics of the coating [6].
A sample of steel S235JR, according to
EN10027, was prepared, with the sizes
60mm x 50mm x 25 mm, on which local
coatings were made.
In fig. 4 an image of the electro-spark coating
process , with stainless steel on steel
S235JR is presented.
Fig. 4. Experiments with the electro-spark
coating process [6]
Electro-spark coatings were performed
using a stainless steel electrode Φ2,5mm.
The parameters of the process were:
- Electrode tilt angle, related to horizontal: 20º
- Electrode rotative-speed 40-50 rev/min,
- Open-circuit voltage 55 V,
- Load voltage 25...45 V,
- Average current 2.0...3.5 A,
- Travel speed 2...4 mm / min,
- Cross oscillation amplitude 10...12mm ,
- Cross oscillation frequency 0.2...1.0 Hz.
In 10 min, a coating of 10mm x 12mm and
0.05...0.10 mm thick can be made.
In order to obtain uniform coatings,
referring to thickness and aspect, constant
speed of the electrode travel and oscillating
movements during the process are
necessary. These requirements depend on
the professional skills and physical status of
the welding operator, that are not stable. This
inconvenient can be taken away by
implementing a mechanized, automated or
robot process, with reproducible parameters.

The operator has full control over the
process, during its run time. It is a cold
coating process, without heating the base
metal, so that no distortion occurs. The
coating rate is 17,4... 18,6 g / h.
B. Electro-spark coating with an
aluminium electrode
On a sample with the sizes 60 mm x 50
mm x 25 mm of the same base metal, some
coatings were also performed, using an 1.6
mm diameter aluminium electrode [6]. The
applied parameters were in the ranges
mentioned in the previous example. In 20
min, a coating of approximately 8mm x 12mm
with continuous aspect is made on the base
metal sample. The thickness of the coating is
estimated at 0.05...0.15 mm. The deposition
rate is 18... 25 g / h.
C. Electro-spark depositing with tungsten
carbides
In an experiment [7,8], the coatings were
realized by the electro-spark process, using a
WC-Co (97% WC and 3% Co) electrode with
a cross-section of 3 x 4 mm (connected to the
anode), onto rings made of carbon steel C45
(connected to the cathode) [7,8].
The EIL-8A model was used as equipment
for ESD, with the following parameters:
· voltage U = 230 V,
· capacitor C = 300 μF,
· current intensity I = 2.4 A.
The quality of electro-spark deposition
depends mainly on the shape, duration, and
average value of current or pulse power.
Then, the coatings were treated with an
Nd:YAG laser (impulse mode), model BLS
720, with the following parameters:
· laser spot diameter: d = 0.7 mm,
· laser power: P = 20 W,
· traverse speed: v = 250 mm/min,
· nozzle-workpiece distance: Df = 1 mm,
· pulse duration: ti = 0.4 ms,
· pulse repetition frequency: f = 50 Hz,
· laser beam shift jump: S = 0.4 mm.
5. RESULTS AND DISCUSSION
5.1. Analysis of the coating morphology
A. In the case of the electro-spark coating
with a stainless steel electrode, Fig.5
presents the structure of the stainless steel
coating (magnification 250 X). The structure
is relatively uniform. The structural
constituents are austenite and delta ferrite
Nonconventional Technologies Review – no. 1/2011
60
(4...6% content), spread relative uniformly in
the coating. There are white and black zones,
coresponding to these structural constituents.
No defects, as cracks, lack of material,
inclusions of other materials. This means, at
this magnification of the microscope, that the
material is uniformely overlayed on the base
metal surface.
Fig. 5. Electro-spark coating with
stainless steel on carbon steel (250 X) [6]
Fig. 6. Electro-spark coating with
aluminium on carbon steel (250 X) [6]
B. In the case of the electro-spark coating
with an aluminium electrode, as presented in
the microstructure of fig.6, the following
structural costituents are detected: metal
matrix of poly-crystalline aluminium, with
poly-crystalline silicon grains and aluminiumsilicon
eutectoid at the grain boundary.
Inclusions of silica (silicon oxide) and alumina
(aluminium oxide) are observed, which arised
from the elaboration of the aluminium batch,
in the continuous casting of the wire, that
then was cold drawn to 1.6 mm diameter
wire, of which the electrode applied here was

delivered. No defects were seen, of the
categories cracks, lack of metal, macroscopic
non-metalic inclusions with elongated or
round shape, etc.
By informative scratch tests, under
experimental conditions, it was estimated that
the adherence of the coating is satisfactory.
C. A micro-structure analysis was conducted
for the tungsten carbides (WC-Co) coatings
[7], using a scanning electron microscope
Jeol JSM-5400, before and after the laser
treatment.
Fig. 7. WC-Co coating microstructure after
the electro-spark coating process [7]
Fig. 8. Microstructure of the electro-spark
WC-Co coating after treatment with a
Nd:YAG laser [7].
The thickness of the ESD obtained layers
was 20 to 30 µm, whereas the heat affected
zone (HAZ) ranged 15 to 20 µm into the
substrate. There is a clear boundary between
the coating and the substrate, where pores
within micro-cracks are observed, as shown
in the fig. 7.
The laser treatment leads to the
homogenization of the chemical composition,
structure refinement, and crystallization of
supersaturated phases, due to the
occurrence of both increased temperature
gradients and high cooling rate. The laser-
Nonconventional Technologies Review – no. 1/2011
61
modified outer layer has no micro-cracks and
pores and no discontinuity of the coatingsubstrate
boundary, as presented in the fig.
8. Its thickness was in the range of 40 to 50
µm. The HAZ thickness was 30 to 40 µm. Its
content of carbon was higher.
5.2. Hardness tests
The average micro-hardness of the WC-
Co coating [7] was 617 HV0.04, that is a
335% increase compared to the substrate.
The HAZ micro-hardness after the laser
treatment has increased by 185% in relation
to the substrate. A slight decrease of 21% of
the coating micro-hardness after the laser
treatment occurred, that may cause an
improvement of the elastic properties. This is
important for operation under high loads, e.g.:
drilling tools in the extractive industry, press
elements for ceramics, etc. [7-10].
5.3. Tribological tests
The tribological tests [7-10] were carried
out with pin-on-disc testers. The pin of 4x20
mm was made of tool steel. The tests [8]
were conducted at the following parameters:
· rotational speed: n = 637 rev/min,
· number of revolutions: i = 5305 rev,
· test duration: t = 500 s (up to stabilization
determined in the initial tests),
· range of load changes from 5 to 15 N.
Dry friction and status stabilization of the
anti-wear layer was observed. The average
friction force was 35% lower after the laser
treatment. Seizure resistance was also
measured using a pin-on-disc tribo-tester.
The laser processing caused an increase in
the seizure of the coatings and C45 steel.
5.4. Adhesion tests
A scratch test was conducted to measure
the adhesion of the WC-Co coatings [7,8]
before and after laser treatment. A CSEM
Revetest scratch tester was used. The mean
value of the critical force was 5.99 N; after
laser treatment, it increased to 7.88 N. The
treatment caused a 32% improvement in the
adhesion of the WC-Co coating, probably due
to the lower porosity and higher sealing [7].
Other research on electro-spark deposition
is conducted by NASA and US Navy [3].
6. CONCLUSIONS
1. The electro-spark coating is a
technological proces for realizing 0.01 ... 0.5

mm thick overlays. Another variant of the
technology process is the electro-spark
deposition, that realizes overlays having a
thickness of 0.5 ... 3.0 mm.
2. Various filler materials are applied for the
electro-spark coating process: Ni alloy, Cr-Co
alloy (stellite), alloyed steel, inconel, stainless
steel, aluminium, tungsten carbide, etc.
3. The device for electro-spark coating
realized by ISIM Timisoara has the following
main functions: generating electric
discharges as sparks, for the transfer of the
filler material; rotating movement of the
electrode, for avoiding the accidental sticking
of the electrode to the base metal.
4. The experimental model device for electrospark
coating, realized by ISIM of Timisoara
has the following main technical
characteristics: power of the sparks generator
0.160 kVA; frequency of the sparks 50…100
Hz; servomotor power 100 W; rotating speed
of the electrode 0…60 rot/min; coating rate
17,4…25 g/h.
5. Electro-spark coating is a thermal spraying
process, at a microscopic scale, fully
controlled by the operator and with no
distortion of the base metal.
6. The laser treatment causes melting and
solidifying of the electro-spark depositions;
thus, both the refinement of structure and
disappearance of micro-cracks occur.
7. After laser treatment, the roughness of the
electro-spark coatings almost doubled; laser
smoothing by melting the micro-peaks of the
coating must be applied.
8. Laser treatment caused a 32% increase in
the adhesion of the electro-spark coatings
made with tungsten carbides (WC-Co).
9. The favorable changes in the properties of
electro-spark coatings after laser treatment
lead to higher abrasive wear resistance.
10. The electro-spark coating can be applied
for manufacturing and small area repairs of
various tools, dies, valves. fittings, etc.
REFERENCES
[1]. *** Ktech Industry Inc.: Mold Doctor.
Operational Manual. Electro - Spark Surface
Hardening and Micro Welding Machine.
[2]. *** Fasco International Inc.: Mold Doctor.
2003.
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Northwest National Laboratory, Richland,
WA. USA): Robust Coatings for Corrosion
and Wear: The Electrospark Deposition
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Process. Tri-Service Conference on
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19, 1999.
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Centre for Powder Metallurgy and New
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[6]. VERBIŢCHI V.* (*ISIM of Timisoara,
Romania); CIUCĂ C.*; COJOCARU R.*:
Technological process for electro-spark
coating with special-properties materials. (In
Romanian). Conference of the Welding
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