Investigations of Different-based Coatings Obtained by Plasma-powder

Spraying

Dr. Hristo Skulev, Dr. Savko Malinov

v b

Technical University of Varna Bulgaria

Queen’s University of Belfast UK

History of thermal spraying

• The basic priority in the technology today is to provide new qualities for different kinds of structural materials. One of the methods used in this direction is thermal-spraying.

• Spraying of coatings is a method for surface treatment, which isconstantly being developed. It is generally accepted that the first thermal-spraying process was invented by M.U. Shoop in 1911 in Switzerland and is known as flame spraying.

• As a method of surface treatment plasma spraying was invented later on – in 1962 and was patented by R.M. Gage, O.H. Nestor and D.M. Yenii.

• The interest of thermal spraying technologies began from the midsixties and is mainly related to the cosmic programs.

• Due to the contemporary technologies and upgrading of equipment, plasma spraying is regarded as a method of high quality level, which is constantly increasing its application.

Sale of equipment in U.S.A.

050

100150200250300350

1900 1920 1940 1960 1980 2000

Година

Продажби

на материали

и

оборудване

[млн

.$]

Shoopprocess

Ballard publication–the first on this topic

Foundation of MetcoDetonation gun

Low energy plasma

Introduction in aircraft industry

High speed flame

Robotization

Water flow removing of thermal coatings

High energyplasma

Introduction of new materials

Sale

of E

quip

men

t (m

illio

ns $

)

Year

15%

15%

70%

плазменоелектродъговогазопламъчно

55%

6%

28%11%

плазмено

електродъгово

газопламъчнопрахово

газопламъчно стелов добавъченматериал

48%

15%8%4%

25%

плазмено

електродъгово

газопламъчнопраховогазопламъчно с теловдобавъчен материалвисокоскоростногоривокислородно

2000

1960

1980

пламъчно

Gas dust welding

Gas welding with

Plasma

Additional were

material

Arc welding

PlasmaArc weldingGas welding

Plasma

Arc welding

Gas dust welding

Gas welding with additional wire material

Thermal-spraying application

The sale of equipment and materials for thermal sprayingaround the world in the year 2000 is about 2,5 milliard $. Leading countries are U.S.A, Japan, France and Germany.

Equipment for plasma spraying

• The plasmotron is a basic component of the plasma spraying equipment.

• The transformation of the power from the electric arc to the thermal power of the plasma jet is very important for the plasmaspraying process. It depends on the plasmotron.

• The plasmotron for plasma spraying consists of two basic parts-nozzle-anode and arc cathode. There is an ionisation chamber between them.

• The materials that can be used for plasma spraying are wires and powders.

Equipment for Plasma Spraying inPlazma Ltd Bulgaria

• Spraying is performed using computerized equipment set PPS–800 with direct current plasmotron PN - 80.

• The regulation of the powder quantity for spraying is performed and controlled according to a preliminary given program.

• The spraying is conducted in a chamber set with rotator alternation of the velocity of rotation within a range from 30 to 1200 rpm.

• The plasmotron is moving along the three axis by manipulator with three degrees of freedom and variable motion velocity ranging from 0 to 20 m/min.

Schematic Layout of the Equipment for Plasma Spraying PPS - 800

1

2

3 104 5

6

8

9

À V

À V

7

1-power source2-central computer

block of the system

3-cooling system 4-gas supplying

device 5-powder supplying

device 6-plasma jet 7-mixer and

regulator of plasma forming gases

8-sample 9-plasmotron PN-80

10-chamber for plasma-spraying

Plasmotron PN 80 for plasma-powder spraying

1. Nozzle.2. External case.3. Cathode unit.4. Internal case.5. Split bush6. Gaseous bush.7. Gas cell.8. Cathode holder.9. Cu-W nozzle.10. Acceleration nozzle11. Ionisation chamber.

Temperature and velocity of the plasma jet

• The temperature and the velocity of the plasma spraying process are basic processing parameters. With the increase of the temperature and the velocity, the adhesive strength of the coating increases and the porosity decreases.

• The thermal gradient within spraying distance of 60-120 mm, varies between 1800-2000 ºC. It depends on the spraying power.

• The velocity gradient of the plasma jet at the same distance varies between 950-1200 m/s. It depends on the spraying power and the quantity of the plasma gases.

• The developed new plasmotron (PN80) gives higher values of the temperature (25-26% increase) and the velocity (80-88% increase) as compared to the regular plasmotron at same processing parameters. This is a result of the modification of the new plasmotron.

Changing of plasmajet temperature depending on the spraying distance

Changing of plasma jet velocity depending on the spraying distance

02000400060008000

100001200014000160001800020000

0 20 40 60 80 100 120 140

L ,mm

T, oC ПН50щатенPN 80regular

T ºC

0200400600800

1000120014001600180020002200

40 60 80 100 120 140

L ,mm

T ,oCПН50щатенPN 80regular

V m/s

Temperature and velocity of the plasma jet

Microstructure of the plasma-powder sprayed coatings

• The plasma thermal coatings have laminated microstructure;• The technology of plasma spraying allows the regulation and

control of the composition of the material and the coating microstructure during the process.

• There are boundaries between the particles. The adhesive strength between them determine some of the properties of the coatings.

• The adhesive strength at the boundary between the coating and the base material is determined by the adhesive and cohesive strength at the surface.

• The formation of the coating leads to pores appearance.

А B

А – coating and base; B - coating RegimeI = 350 [A]Q (Ar/N2) = 20/2 [l/min]P = 18 [kW]

Microstructure of Cu-based coatings

Sprayed layers

Base material

Sprayed layers 20 μm200 μm

Transition zone

А - coating and base ; B - coatingRegime : I = 550 [A]Q (Ar/N2) = 30/6 [l/min]P = 33 [kW]

Microstructure of Fe-based coatings

Base materialА B

20 μm200 μm

Sprayed layers

Transitionzone

Sprayed layers

А – coating and base; B - coatingRegime: I = 450 [A]Q (Ar/N2)= 30/6 [l/min]P = 26 [kW]

Sprayed layers

Transition zone Base material

20 μm200 μm

А B

Sprayed layers

Microstructure of Ni-based coatings

Microstructure of Plasma-powder Sprayed Ni - based Coatings on Steel Specimens at Different Magnification

27.042560450SteelN 738.563070550SteelN 424.563070350SteelN 218.222052350SteelN 1

N2Ar

P, kW

Gas flow l/min

Voltage, V

Current, ABaseSample

N 1 N 2 N 4

N 7

• The melted powder particles are deposited in the form of lighter and darker layers. The non - melted and partly melted particles have spherical or round prolate form.

• The results show the microstructure change with power increase from 18 to 38 kW. The non-homogeneous distribution at power from 18 to 24 kW is with clear boundaries between the separate layers. At power from 24 to 38 kW the boundaries are blown over and turned to homogeneous layers.

• The results from the analysis of the microstructure show that during all regimes of spraying the bounding steel base-coating is very good, without pores, cracks or other defects.

Analyses of the Microstructure of Plasma-powder Sprayed Ni-based Coatings

X-ray Diffraction Pattern and Profile fits (Dotted Lines) in the Range of 30 to 100 2θ in Ni-base as Sprayed Condition

The analysis of the X-ray diffraction patterns show that, at room temperature, amorphous state and γ (Ni, Cr, Fe, Cu, C) are visible.

40 50 60 70 80 900

50

100

150

200

250

Two theta (°)

Counts

Sample 7

fcc

{111

}

fcc {

200}

fcc {

220}

fcc{

311}

fcc{

222}

Distribution of the chemical elements along the depth of the layer

• The distribution of the chemical elements along the depth of the layer is very important for the coatings obtained after plasma spraying.

• The interaction among Cr, B, Si, Fe, W, during the process of plasma spraying, leads to formation of borides, silicides, carbides, which improve the mechanical properties of the coating.

• The distribution of the chemical elements shows the existence of transition zone .

• The increase of the power from 17 to 36 kW leads to increase of the depth of the transition zone from 20 to 30 μm.

Regime:

I = 350 [A]

Q (Ar/N2) = 20/2 [l/min]

P = 17 [kW]

Distribution of chemical elements along the depth of the layer

0

20

40

60

80

100

120

0.02 0.06 0.1 0.14 0.18 0.22 0.26 0.3 0.34

Si FeCrAlNiCuW

%

mm

D1

D2

Dn

S

С1

С2

С3СnСD

Measurements scheme

Coating

Base

Properties of the plasma-powder sprayed coatings

The mechanical properties of the coatings are investigated and the results obtained are:

- adhesive strength – average 72MPa (27% higher than the regular plasmotron).

- wear resistance - average 3.35x10-2mm (20% higher than the regular plasmotron).

- tensile strength – average 188MPa (27% higher than the regular plasmotron).

- coating hardness – average 510HV0.5.

- efficiency – average 65% (20% higher than the regularplasmotron).

Adhesive strength

Adhesive strength at different values of the power

N I Ar/N2 U P σs

[A] [L/min] [V] [kW] MPa

25/4 53 16 96

37/6 61 18 100

123

350

50/8 65 20 89

25/4 53 21 96

37/6 61 24 103

456

450

50/8 68 27 72

25/4 55 28 97

37/6 61 31 110

789

550

50/8 70 35 68

0

30

40

50

60

70

80

90

100

120

16 18 20 21 24 27 28 31 35

Ad

hesi

ve s

tren

gth

, σ

MP

a

Power, kW

a - adhesive strength at different values of the current

b - adhesive strength using different amount of plasma forming gas

Adhesive strength

0102030405060708090

1 2 3

Q, l/min

I=300AI=400AI=500A

350450550

Ar25 N24 Ar37 N26 Ar50 N28

σ [MPa]

030405060708090100120

1 2 3Current, I, A

Ar25N4Ar37N6Ar50N8

Adhes

ive

stre

ngth

, σ

MPa

Coating hardness

200250300350400450500550600650

1 2 3 4 5 6 7 8 9

P , kW

HV

5

c

N I Q (Ar/N2) U P

[A] [l/min] [V] [kW]

HV5(average)

25/4 53 16 430

37/6 61 18 470

123

350

50/8 65 20 500

25/4 53 21 450

37/6 61 24 480

456

450

50/8 68 27 540

25/4 55 28 460

37/6 61 31 560

789

550

50/8 70 35 590

200

250

300

350

400

450

500

550

600

650

1 2 3

I , A

HV

5

Ar25N4Ar37N6Ar50N8

a

350 450 550

200

250

300

350

400

450

500

550

600

650

1 2 3

Q , l/min

HV

5I = 300 AI = 400 AI = 500 A

b

Ar25 N24 Ar37 N26 Ar50 N28

350450550

Wear resistanceN I Q (Ar/N2) U P Ip

[A] [l/min] [V] [kW] mm.10-2

25/4 53 16 6,7

37/6 61 18 4,5

123

350

50/8 65 20 3,1

25/4 53 21 5

37/6 61 24 3,9

456

450

50/8 68 27 3,4

25/4 55 28 4,1

37/6 61 31 2

789

550

50/8 70 35 1,6

0

1

2

3

4

5

6

7

8

1 2 3

I , A

Ar25N4Ar37N6Ar50N8

a

350 450 550

Ip mm.10-2

0

1

2

3

4

5

6

7

8

1 2 3

Q , l/min

I = 300 AI = 400 AI = 500 A

b

350450550

Ar25 N24 Ar37 N26 Ar50 N28

Ip mm.10-2

0

1

2

3

4

5

6

7

8

1 2 3 4 5 6 7 8 9

P , kW

cIp mm.10-2

Applications on different materials

• The method can be applied on different materials like metals,glass, polymers and ceramics because the temperature of the basematerial during plasma spraying can be controlled in the range of 50-250ºC.• The microstructure analysis shows that the coating is multilayered and inhomogeneous.• The measurements show that the soft Ni matrix has a hardness value of 350-450HV0.5.• The hardness readings in some points have values of 1700-1900HV0.05 confirming the presence of carbides, borides and silicides.

200μm

10μm 10μm

200μm

10μm

200μm

Microstructure of samples at power of 17 kW.

a- base glass b– base steel c- base cast iron

200μm 200μm

Microstructure of samples at power of 26 kW.

a- base glass b– base steel c- base cast iron

50μm

10μm 10μm10μm

Hardness of the coating on a glass base

L, μm

0100200300400500600700800900

10001100

0 100 200 300 400 500 600 700

HV

0,1 P = 15 kW"

P = 27 kW"P = 35 kW"

coating Base -glass

L, μm

HV0.5

172636

Hardness of coating on steel base

0

100

200

300

400

500

600

700

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200

HV

5

P = 15 kW "P = 27 kW "P = 35 kW "

HV0.5

L, μm

172636

0

200

400

600

800

1000

1200

0 200 400 600 800 1000 1200 1400 1600 1800

HV0,0

5 P = 15 kW"P = 27 kW"P = 35 kW"

coatingSteel

Transition zoneHV0.05

L, μm

172636

Hardness of the coating on cast iron base

0

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

1 2 0 0

1 4 0 0

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0

HV

0,1 P = 1 5 k W "

P = 2 7 k W "P = 3 5 k W "

coating cast iron

L ,μm

HV0.05

172636

0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

7 0 0

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0

HV

5 P = 1 5 k W "P = 2 7 k W "P = 3 5 k W "

HV0.5

L, μm

172636

Transition zone

Plasma Sprayed Ni-base coatings

• There is a particular interest towards spraying of thin-layer coatings using Ni and nickel alloys powders. This is due to the properties that these coating possess, namely wear resistance, corrosion resistance, etc.

• Plasma powder spraying method allows large number of technological parameters to be varied. This gives the opportunity for search of new combinations of properties of the Ni - based coatings obtained.

• As a result the coatings acquire new properties, not typical for the basic material. These properties are mostly due to the formation of metastable phases and structures. During the additional heating their metastable nature is often reversed to more stable state, which sometimes is undesirable.

Transition zone

coating

resin

base

Coating microstructure

200

400

600

800

1000

0 200 400 600 800 1000 1200 1400

coating baseTransition zone

HV0.5

L, μm

44

200

400

600

800

1000

0 200 400 600 800 1000 1200 1400 1600

coating С base

Transition zone

HV0.5

L,μm

P=17 kW

P=26 kW

P=35 kW

200

400

600

800

0 200 400 600 800 1000 1200 1400 1600h, μ m

HV 5

0

coating base

Transition zone

L,μm

HV0.5

Distribution of the Ni-based coatings hardness

5μm 5μm 5μm

a - coating on base of glass b - coating on base of steel c - coating on base of cast iron

Microstructure of samples at power of 36 kW

Microstructure of Ni-based coating at P=26 kW

5 μm

650

600

600

82010 μm

Results from DSC runs. Heating with rate 40 °C/min up to 1200°C and subsequent cooling with the same rate.

heating

cooling

X-ray Diffraction Pattern in the Range of 30 to 100 2θ after being Heated and Cooled from Different Temperatures

40 50 60 70 80 900

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Two theta (°)

Relative intensity

Sample 7

○ ○ ○

○

○ ○ ○

○

○

○ ○ ○

○

○

○ ○ ○

○

○

○ ○ ○ ○

○

■ ↓

◊ ■ ◊

■ ↓

■

■ ■ ↓

◊

◊ ■ ■

○ ■

■ ↓

As sprayed

700 °C

965 °C

1035 °C

1200 °C○ Ni ↓ Ni3Si ■ Ni3B ◊ Ni4Si

Heat treated up to 565, 700, 965, 1035 and 1200 °C in order to study the phase transformations.

Transformations in the Plasma Coating during Secondary Heating

• The DSC curves show two exothermic peaks that lead to the conclusion that amorphous state of the coating is available at room temperature. These two exothermic peaks are not observed during cooling from 700º C.

• The X-ray and the DSC analysis demonstrates that amorphous state and γ (Ni, Cr, Fe, Cu, C) are evident before the first exothermic peak. The first exothermic peak is at 511°C and confirms the temperature at the beginning of Ni amorphous transformation towards its more stable phases Ni4Si (β1) and Ni3B. The appearance of the second exothermic peak at 630°C shows that the complete crystallization of amorphous Ni is achieved atabout 700°C.

• By increasing the temperature up to 1200°C, new exothermic peaks are not observed. Still, one can consider that there is a presence of γ (Ni, Cr, Fe, Cu, C) + Ni4Si(β1)+ Ni3B after heating up to 700°C.

DSC runs on heating with 40 °C/min for samples N1-N7 in the temperature ranges 450-700°C (a) and 940-1080°C (b)

Transformations in the Plasma Coating during Secondary Heating

a/

b/

Transformations in the Plasma Coating during Secondary Heating

• The DSC curves show that the regime parameters do not have influence on the phase transformations in the coatings during the heat treatment.

• The obtained exothermic and endothermic peaks keep their relative position without any influence from regime parameters change. The differences between the peaks are limited to the intensity of the peaks mainly due to the volume of the phase transformations.

Microstructure of Sample after Different Heat Treatment.

a/ after heating up to 700 °C

b/ after heating up to 1035 °C

a/ after heating up to 1200 °C

Hardness after Different Heat Treatment

The increase of the hardness in the range of 20-700°C confirms the conclusion about existence of phase transformation of the amorphous Ni to γ (Ni, Cr, Fe, Cu, C)+ Ni4Si(β1)+Ni3B. The measurements show that the hardness decreases in the temperature range of 700-1200°C. The hardness drop and the microstructure at 1200°C show that the coating has passed the melting point. After cooling γ (Ni, Cr, Fe, Cu, C) and Ni3Si(β3) are formed due to reaction (4).

0

200

400

600

800

1000

0 200 400 600 800 1000 1200 1400

Temperature, C

Hard

ness (

HV

1)

°

Ni-based coatings during heat treatment up to 1200°C.

1. The microstructure shows that the plasma sprayed Ni-based coatings are multilayered and irregular. With the increase of the power from 18.6 to 38kW, the non-homogeneous distribution with clear boundaries between the separate layers transforms to homogeneouslayer with a thickness of about 1-3μm. The porosity turns to fine, lamellar, with limited contact area.

2. The DSC and the X-ray analyses show that, after plasma spraying, the Ni-based coatings have amorphous state and crystal structure.

3. The amorphous transformation starts at 511ºC. New phases such as Ni4Si and Ni3B are formed. All phase transformations are temperature dependant.

4. The hardness changes during heat treatment. It increases in therange of 20-700°C and decreases in the range of 700-1200°C. The maximum hardness value of 785 HV1 measured after heating up to 680°C.

Plasma-powder spraying applications

• Technology for plasma-powder spraying for turbines restoration is developed and introduced. 692 different turbines have been restored.

Plasma-powder spraying applications

• All these components are working in different ships.

Conclusions

• Plasma spraying is a high-technology method for surface treatment of materials, which forms coatings with good corrosion and wear resistance properties.

• It is economical in terms of materials usage and processing time.

• Plasma spraying increases the quality of the parts and the equipment respectively.

• The high temperature of the plasma jet – 25,000-55,000 ºC allows spraying of wide range of materials from polymers to ceramics.