thermal spray coatings for industrial applications · a thermal spray process can be divided into...
TRANSCRIPT
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Thermal Spray Coatings
For Industrial Applications
INSTM Research Unit “Roma La Sapienza”
Dept. of Chemical and Materials Engineering (ICMA)
Sapienza University of Rome
Giovanni Pulci
INSTM - Italian Consortium
on Materials Science
and Technology
Le nuove frontiere delle tecnologie al plasma - Esempi di applicazioni industriali
Consorzio Innova FVG - 24 Settembre 2018
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Coatings for severe operating environment
Heavy combination of mechanical stresses and aggressive environment
Temperature
Mechanicalstress
OxidationCorrosion
Wear
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Coatings for severe operating environment
Aim of presentation:
Description of case studies related to innovations and R&D in thermal spray field
R&D plays a key role for component working in critical operating conditions
R&D
Improvementof performance
Costs reduction
R&D
Introductioninto the market
Design
Existing product New product
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energy
heat/energy source
material
material feed/flow
atomizing
gas or air
material spray
substrate
coating
molten particlesspray equipment
A thermal spray process can be divided into four sections:
heat/energy source (thermal energy for heating and melting)
material feed/flow
material spray (kinetic energy for propelling dispersion)
material deposition
Surface engineering by thermal spray process
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Surface engineering by thermal spray processes
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Surface engineering by thermal spray processes
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• An inert gas is passed through a potential differential.
• An arc is formed between the two dipoles.
• The gas is ionized and recombines after going through a free expansion.
• The recombination effect releases heat and creates a plasma flame (“plume”).
• Powder is inserted into the flame and is propelled towards the substrate.
Plasma spraying
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9
Plasma spraying
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10
• Max. plume temperature ≈ 12000 °C
• Impact speed of particles 200-400 m/s
• In-flight time of particles ≈ 10-3 s
• Heating/cooling rate ≈ 107 K/s
Plasma spraying
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• Plasma (Primary) gas flow
• Secondary gas flow
• Plasma power
• Spray distance
• Injector angle, length and size
• Carrier gas flow
• Gun spray angle
powder injection
molten particle stream
plasma
spray gun
plasma
flame or
plume
Plasma spraying
Process parameters
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• Primary gases (Ar, N2): plasma
stability and transport properties
• Secondary lighter gases (He, H2),
enthalpy adjustment.
• N2 and H2 are diatomic gases.
These plasmas have higher
energy contents for a given
temperature than argon and
helium because of the energy
associated with dissociation of
molecules.
gas e
nerg
y c
on
ten
t [W
h/S
CF
]
N2 H2Ar
He
gas temperature [°C]
0
100
200
300
400
500
600
0 2200 4400 6600 8800 11100 13300 15500 17800 20000
dissociation
H2 2 H
dissociation
N2 2 N
Plasma spraying
Plasma gases
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13
pow
de
r p
art
icle
tem
pera
ture
big particles,
high powder density
small particles,
low powder
density
flow
te
mp
era
ture
part
icle
ve
locity
Ar/H2
N2/H2 N2/H2
flo
w v
elo
city
Plasma spraying
Spray distance
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part
icle
ve
locity
N2/H2
29 kW
20 kW(same gas comp.)
oxid
e c
on
ten
tsu
rfa
ce
ro
ugh
ne
ss
cold particles
Plasma spraying
Spray distance
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Controlled Atmosphere Plasma Spray
chamber is backfilled after evacuation, either at
• near vacuum (1 - 10 mbar) VPS
• reduced pressure (> 50 mbar) LPPS
• standard pressure APS
• elevated pressure (up to 4 bar) HPPS
• with a substitute atmosphere (reactive (CxHy), inert (Ar)) RPS / IPS
VPS / LPPS / LVPS
➢ spraying in near vacuum or under reduced pressure conditions
➢ spray particles are unimpeded by frictional forces of the atmosphere
Plasma spraying
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• Ceramics (high melting point)
• Al2O3, Al2O3/TiO2, Cr2O3, ZrO2 -Y2O3, La2Zr2O7, ecc.
• ZrB2, TiB2, SiC (non standard)
• Metals (refractory, oxidation control is mandatory, controlled
atmosphere)
• MCrAlY, Ni-and Mo-based alloys
• Cermets
• Cr3C2 - based (high T, generally NiCr matrix)
• WC - based (better mechanical properties, Tmax 500 °C
(decarburation), Co - CoNiCr - based matrix)
Plasma spraying
Materials
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• Low porosity
• Lamellar microstructure, partially molten ceramic phase
Cermet Cr3C2 – Ni-Cr
Plasma spraying
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HVOF deposition
Plasma Spray HVOF
• Max. jet temperature ≈ 2800 °C
• Particles impact speed 400-700 m/s
• In-flight time ≈ 10-3 s
• Heating/cooling rate ≈ 107 K/s
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Applications
Wear and corrosion resistant coatings
(Mechanical, chemical, aerospace, textile industries)
- Cermet (WC, Cr3C2 or TiC in Co, Co-Cr or Ni-Cr metal matrix);
- Ceramics (Cr2O3, Al2O3-TiO2);
- Metallic (Ni or Co base, e.g. NiCrBSiC).
Thermal barrier coatings (TBC)
(Aerospace, energy industries)
- ZrO2 - Y2O3.
High temperature oxidation resistant coatings
(Aerospace, energy, chemical industries)
- Ni - Cr – Al base (e.g. NiCoCrAlY).
Biocompatible coatings
Porous Ti , hydroxy apatite (Ca10(PO4)6(OH)2
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Materials and Surface Engineering Lab
Research partners and clients
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Aircraft engines and power gas turbines
Max gas temperature in turbine:
900 - 1400 °C
Rise in gas temperature in turbine
Improvement of thermodynamic
efficiency
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Max service temperature
of superalloy!
Turbine blades protection
High temperature and combustion products (e.g. SO2, SO3, V2O5) cause
surface damages on the Ni-base superalloy
High temperature
oxidation (T > 1000°C)
Hot corrosion
(700 – 925 °C)
(SO3 attack, sulphides
formation)
Erosion
(Solid particles in
gas stream)
Protective ceramic overlay
Thermal Barrier Coating (TBC)
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Top Coat (ZrO2–6-8%Y2O3)• Thermal insulation• Erosion protection
Bond Coat (Ni/Co-CrAlY) •Enhanced adhesion•Hot corrosion resistance
TGO (a-Al2O3) • Oxidation barrier
Substrate (Ni–base superalloy) •Thermo-mechanical loading
ZrO2 permeable to O2(through open porosity & by ionic conduction)
Thermal Barrier Coatings (TBC)
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Innovative materials for gas turbines
Strategies to improve turbines performance:
use of CMC materials
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Modification of substrate-coating system
Innovative materials for gas turbines
Strategies to improve turbines performance:
use of CMC materials
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Modification of substrate-coating system
Innovative materials for gas turbines
Strategies to improve turbines performance:
use of CMC materials
GE Rolls-Royce – development of F136 engine (second choice for
F-35 aircraft)
3rd low pressure stage – stator in SiC/SiC
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Modification of substrate-coating system
Innovative materials for gas turbines
Strategies to improve turbines performance:
use of CMC materials
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Modification of substrate-coating system
Innovative materials for gas turbines
Strategies to improve turbines performance:
use of CMC materials
Problem:
- Active oxidation of SiC/SiC, expecially in presence of H2O
steam: Thermal & Environmental Barrier Coatings
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Environmental Barrier Coatings
EBC deposited by APS (examples)
- HfO2-Y2O3-Gd2O3-Yb2O3
- Barium-Strontium-Alumino-Silicate (BSAS)
Innovative materials for gas turbines
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Need for innovation in surface engineering
Case study: Hard chrome replacement
Hard Chrome plating is an electrolytic method of depositing chrome for
engineering applications, from a chromic acid solution.
Cr 6+ compounds
EU classification
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Need for innovation in surface engineering
Hard chrome replacement
- HIGH HARDNESS
Typical values of 850 - 1050 HV
- LOW COEFFICIENT OF FRICTION –
coefficient against steel of 0.16 (0.21 dry),
- WEAR RESISTANCE
extremely good resistance to abrasive
and erosive wear
- CORROSION RESISTANCE
extremely high resistance to atmospheric
oxidation, and a good resistance to most
oxidising and reducing agents- MACHINING
Can be successfully finished
Properties of hard chrome:
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Case study
Hard chrome replacement in marine engine components
Wärtsilä aims to be the leader in power
solutions for the global marine markets
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Case study
Hard chrome replacement in marine diesel engine components
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Case study
Hard chrome replacement in marine diesel engine components
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Case study
Hard chrome replacement in marine diesel engine components
Valves problems:
• High temperature
• Wear
• Cold and hot corrosion
Wear and cold corrosion
from sulphur compounds
Fatigue
Thermo-
mechanical fatigue
Hot-corrosion and oxidation
Creep
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Case study
Hard chrome replacement in marine diesel engine VALVES
Problem:
Wear and cold corrosion in
exhaust and intake valves stem
Hard chrome on stem has a short
service-life
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Case study
Hard chrome replacement in marine diesel engine valves
Ceramic-metal (cermet) HVOF coatings
Hard ceramic phase:
wear resistance
Metal matix:
toughness and corrosion
resistance
2 phases - microstructure
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Cr3C2 – 25 (Ni-Cr)✓
WC-CoCrNi✓
(WC-Co)-NiCrSiFeBC ✓
Case study
Hard chrome replacement in marine diesel engine valves
Materials (Sulzer powders):
Facility:
HVOF – JP5000 gun
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Wärtsilä requirements and constrain
Cermet HVOF coatings
• Thickness higher than150 µm
• Porosity as low as possible
• Hardness higher than 900 HV
• High deposition efficiency
Optimization of deposition
parameters by
Design of Experiment (DoE)
2 factors 3 levels
1) Kerosene - O2 flow
2) Spray distance
1) Low
2) Medium
3) High
32 parameter sets
Keros. (gph) – O2 (scfh)
5,5-1700 6-1850 6,5-2000
Dis
tan
ce
(mm
)
320
355
380
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Phase 1- HVOF coatings optimization (DoE)
Investigated properties
• Deposition efficiency
• Porosity as a function of total flow and spray distance
• Hardness
Weighted combination of these properties
provide the “desirability function”
DESIGN-EXPERT Plot
Actual Factors:
X = O2 mass flow
Y = Distance
0.096
0.281
0.466
0.651
0.836
Des
irabil
ity
1700
1775
1850
1925
2000
320
335
350
365
380
O2 mass flow
Distance
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DESIGN-EXPERT Plot
Actual Factors:
X = O2 mass flow
Y = Distance
0.096
0.281
0.466
0.651
0.836
Des
irability
1700
1775
1850
1925
2000
320
335
350
365
380
O2 mass flow
Distance
WC-CoCrNi
Properties Optimized value
Deposition efficiency
(um/pass)24,3
Hardness (HV100) 1610
Porosity (%) 3,04
Desirability 0,84
Keros. (gph) – O2 (scfh)
5,5-1700 6-1850 6,5-2000
Dis
tan
ce
(mm
)
320
355
380
Optimized
parameters
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DESIGN-EXPERT Plot
Actual Factors:
X = O2 mass flow
Y = Distance
0.000
0.206
0.412
0.619
0.825
Des
irabil
ity
1700.00
1775.00
1850.00
1925.00
2000.00
340.00
355.00
370.00
385.00
400.00
O2 mass flow
Distance
(WC-Co) - NiCrSiFeBC
Properties Optimized value
Deposition efficiency
(um/pass)10,8
Hardness (HV50) 1053,5
Porosity (%) 1,79
Desirability 0,82
Keros. (gph) – O2 (scfh)
5,5-1700 6-1850 6,5-2000
Dis
tan
ce
(mm
)
400
370
340Optimized
parameters
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DESIGN-EXPERT Plot
Actual Factor
X = O2 mass flow
1700.00 1775.00 1850.00 1925.00 2000.00
0.000
0.249
0.498
0.747
0.996
X: O2 mass flow
Y: Desirability
One Factor Plot
Cr3C2 – 25 (Ni-Cr)
Properties Optimized value
Deposition efficiency
(um/pass)34,3
Hardness (HV100) 1369
Porosity (%) 4,06
Desirability 0,996
Keros. (gph) – O2 (scfh)
5,5-1700 6-1850 6,5-2000
Dis
tan
ce
(mm
)
320
355
380
Optimized
parameters
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Phase 1- HVOF coatings optimization (DoE)
Optimized coatings
WCN
WSF
CRC
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Phase 2- optimized coatings characterization
Corrosion tests Wear tests
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Phase 2- optimized coatings characterization
Corrosion tests
1 hour immersion test – boiling 5% H2SO4 solution
8,29 6,26 0,065
191,02
0
50
100
150
200
CRC WSF WCN HC
Mas
s lo
ss (
mg
/cm
2 )
Corrosion test - mass lost
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Not protective
Phase 2- corrosion tests
Hard chrome
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No surface damage
detectable
Phase 2- corrosion tests
WCN coatings
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0,42
0,49
0,14 0,13
0,02 0,030,07 0,08
0,230,27
0,16
0,22
0,01 0,01 0,01 n.c.0,00
0,10
0,20
0,30
0,40
0,50
0,60
Wear
(m
m3)
Senza attacco corrosivo
Con attacco corrosivo
As sprayed
Post corrosion attack
Phase 2- optimized coatings characterization
Wear tests - Load: 91 N- Sliding distance: 2000 m
- Speed: 1 m/s
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Phase 3- coating selection for valve deposition
WC-CoCrNi:
- Highest hardness
- Lowest wear rate
- Lowest corrosion rate
- Good density
- High deposition efficiency
- WC-CoCrNi coating was selected
- Coated valves tested for 3 years on a
Wartsila test engine
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Innovative coatings for hot corrosion
Case study: Hot corrosion on marine diesel engines valves
Aggressive environment produced by high impurity contents within
the fuel: V, Na, S.
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Innovative coatings for hot corrosion
Multiphase cermet coatings: CrystalCoat
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Innovative coatings for hot corrosion
Multiphase cermet coatings: CrystalCoat
Metal alloy SiO2 Basalt Oxide Other silicates
CRYSTALCOAT
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Innovative coatings for hot corrosion
Case study: Hot corrosion on marine diesel engines valves
Aim: development of innovative thermal spray coatings
alternative to Crystal Coat
Commercial
(non standard)
tested solutions
Development of
innovative
solutions
✓ Cr3C2 – NiCr
✓ Cr3C2 – NiCrAlY
✓ Cr3C2 – CoNiCrAlY
✓ Cr3C2 – self fusing alloy
✓ Mullite – nano SiO2 – NiCr
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Phase 1- coatings optimization (DoE)
Innovative coatings for hot corrosion
2 factors 3 levels
1) Kerosene - O2 flow
2) Spray distance
1) Low
2) Medium
3) High
32 parameter sets
Keros. (gph) – O2 (scfh)
5,5-1700 6-1850 6,5-2000
Dis
tan
ce
(mm
)
320
355
380
• Thickness higher than150 µm
• Porosity as low as possible
• High deposition efficiency
Optimization of deposition
parameters by
Design of Experiment (DoE)
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Very low porosity (< 1 %)
Ceramic phase dispersed in multiphase metallic matrix
Example: optimization of Cr3C2 - self fusing alloy
Optimized coating
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Innovative coatings for hot corrosion
✓ Mullite – nano SiO2 – NiCr
Powder agglomeration – spray drying
Nano-SiO2 cover completely
the spray dried agglomerates
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Innovative coatings for hot corrosion
✓ Mullite – nano SiO2 – NiCr
Coating deposition
Coatings retain the
nano-structure
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100 h
50 h
5 h
Innovative coatings for hot corrosion
Hot corrosion tests
• Temperature: 750 °C• Environment: Na2SO4 – V2O5 molten salt• Exposure time: up to 100 h
Oxide scale
evolution
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Innovative coatings for hot corrosion
Hot corrosion tests
Nanostructured coating
25 h
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Innovative coatings for hot corrosion
Hot corrosion tests
Comparison between nanostructured and self fusing coatings
Nano 25 hCr3C2 – self fusing alloy 25 h
Thinner oxide scale in nano-coating
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0
10
20
30
40
50
60
70
80
90
0 20 40 60 80 100 120
Thic
kne
ss [μ
m]
Time [h]
Scale growth kinetics
Carbide-NiCr Carbide-MCrAlY/2 Carbide-Self flux
Carbide-MCrAlY/1 Cer-nano/oxide-NiCr
Corrosion kinetics
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Faculty of EngineeringSapienza University of Rome