Thermal spray

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Thermal spray / Termopihustus / ???????????
?????????Loengukonspekt

• MTX0100 Pulbermetallurgia ja pindamine
• Koostasid Andrei Surženkov, Heikki Sarjas,
• Arkadi Žikin

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Process definition

• Thermal spray is a coating process, where the
  coating material (metallic / nonmetallic in the
  form of powder, wire or rod) is melted by an
  energy source (flame, electric arc, plasma arc)
  to a molten or semimolten state and is after that
  accelerated and propelled towards the substrate
  by process gas / atomization jet. Subsequent
  sprayed particles build up to form the coating
  with the lamellaer structure.

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Process illustration
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Process features

• high cooling rates (106 K/s)
• low heat input to the substrate
• virtually all materials
• thickness from tenths of mm-s to tens of mm-s
  (depending on the process)
• Porosity 0 10 (depends on the process).

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Process applications.

• (sliding, abrasive, erosive) wear protection
• corrosion (all types) protection
• thermal insulation
• electrical conduction / insulation
• medica purposes
• radiation protection
• restoration of worn parts.

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Comparison of coating processes
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Development of the thermal spray technology 1
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Thermal spray technology market 2
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Thermal spray in Estonia 3

• AS Reneko (plasma, flame spraying)
• AS Korund (plasma, flame spraying)
• OÜ Gold plating (cold spraying)
• Tallinna Tehnikakõrgkool (plasma, flame, cold
  spraying)
• Tallinna Tehnikaülikool (high velocity oxy-fuel
  spraying, hopefully soon plasma transferred arc
  spraying).

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Classification of spraying methods 1
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Substrate preparation. Cleaning

• Purpose removal of contamintants, such as oil,
  rust, etc.
• chemical cleaning
• vapor degreasing
• baking (porous materials 315 345 ºC)
• ultrasonic cleaning
• wet / dry abrasive blasting.

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Surface preparation. Roughening.

• Purpose to obtain clean in-plane (longtitudal)
  stress free surface with an increased area,
  providing bigger possibility for the sprayed
  material to form bonds and/or be mechanically
  interlocked with the surface.
• dry abrasive grit blasting (angular chilled
  iron, crushed slag, flint, garnet, silica sand,
  Al2O3, SiC)
• machining or macroroughening.
• Rule of thumb roughening must be done no longer
  than 2 h before spraying 3.

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Surface preparation. Preparation of sprayed
material.

• Powders drying at 100 ºC at least 8 h 1,
  polymer powders 50 ºC 130 150 ºC 3 5 h,
  ceramic powders 600 700 ºC 3 5 h 4
  thickness of powder layer no bigger than 20 mm
  4. Purpose to eliminate moisture, in order to
  avoid the coalescence of powder during spraying
  and to avoid degradation of coating's properties
  through hydroxides' formation.
• Wires cleaning using washing liquids, bronze
  wires using acids 4. Purpose to remove
  lubricants / oxides.

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Instant before spraying. Heating of the surface

• Purpose moisture removal, pre-expansion of the
  substrate.
• gas torch / spray torch without the sprayed
  material
• 100 ºC for stainless steels, lt 150 ºC for carbon
  steels no longer than 60 s
• aluminium, copper, titanium, manganese alloys
  should NOT be pre-heated due to oxide film
  formation.
• Rule of thumb it is better to underheat than to
  overheat the surface 3.

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Spraying process. General rules.

• Longer dwell times bigger heat input, but also
  more intensive oxidation ? good for ceramics,
  NOT good for cermets, pure metals, polymers. To
  increase dwell time, increase the spraying
  distance, use more calorific fuels.
• Spraying torch must be perpendicular to the
  surface or close to that otherwise porosity
  increases.
• Sharp edges, narrow holes must be avoided.

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Flame spraying / Leekpihustus / ?????????????
?????????

• Combustion gases heat the sprayed material and
  carry it on the surface
• Ignition outside the torch
• Axial feed of materials
• Powder (10 1000 µm / wire (1,5 2 mm) / rod
• Feed rate 0,5 10 kg/h (powder), 1,0 7,5
  m/min (wire).
• Spraying distance 100 200 mm

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Advantages / disadvantages 4

• Relatively low surface heating (350 450 ºC)
• Relatively high production rate
• High deposition rate (60 ... 95 )
• Flexibility
• Simplicity.
• Low adhesion
• High porocity
• Low heating efficiency (2 12 of gas jet
  energy is consumed to heat the powder)
• Not possible to spray materials with melting
  point over 2800 ºC.

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Process, microstructure
Powder flame spraying 5
Wire flame spraying 6
Mullite/ ZrO2 coating a, c stainless steel, b,
d stoneware substrate 7
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Equipment, price

• Castolin, Sulzer Metco, GTV MBH, Metallisation
  Inc., Haiams Inc., etc.
• Approximate price of a new flame spray gun
  10,000 EUR EXW.

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Detonation gun spraying / Detonatsioonpihustus /
????????????? ?????????

• An explosive mixture of gas fuel, oxygen and
  powder is loaded into the gun's barrel and is
  ignited. The mixture explodes, the resulting
  detonation wave heats up and 'shots' the powder
  particles toward the substrate at a supersonic
  speed.
• Nitrogen purges the barrel between detonations.
• Frequency of 'shots' 3 6 Hz.
• Powder particles' size 10 50 µm.
• Feed rate 0,5 12 kg/h.
• Spraying distance 50 200 mm.

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Advantages / disadvantages 4

• High adhesion ( 100 MPa)
• Low porosity (lt 1 )
• High feed rate (up to 12 kg/h)
• Higher amount of retained carbides (in comparison
  with flame and plasma spraying)
• Non-sensitivity to the condition of the surface.
• Hard to spray materials with low density (e.g.
  TiC)
• High level of noise (gt 140 dB)
• Need for sealed boxes
• High price for the obtained coatings.

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Equipment, microstructure
Detonation gun spraying machine PT-100 8
Microstructure of the detonation gun sprayed
coatings a) WC-Co, b)Al2O3-TiO2 1
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Equipment, price

• Praxair Surface Technologies Inc. (D-gun),
  Beijing Do-Talent Sci. Tech. Development Co.,
  Ltd. (PT-100), etc.
• Price higher than for the flame spraying
  equipment.

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High velocity oxy (air) fuel spraying (HVOF/HVAF)
/ Kiirleekpihustus / ???????????????? ?????????

• An oxygen / air -fuel mixture is burned inside a
  combustion chamber. Resulting combustion gases
  are lead through a water- or air-cooled Laval
  nozzle outside the gun , accelerating to the
  velocities of 1525 1825 m/s.
• Powder is fed by the carrier gas into the
  nozzle, carriear gas to the nozzle, becomoing
  entrained into a high velocity gas jet, and
  becomes sprayed onto the surface.
• Powder particles' size (45 20) µm
• Feed rate 6 9 cm3/min
• Spraying distance 380 400 mm
• Note HVAF gives better results for spraying of
  cermets (lower oxidation), HVOF at metals'
  spraying.

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Advantages / disadvantages 4

• High adhesion
• Low porosity (lt 1 )
• Thicker coatings (up to several mm-s)
• Higher amount of retained carbides (in comparison
  with flame and plasma spraying).
• High level of noise (gt 130 dB)
• Low deposition rate (35 50 )
• Relatively high price of the equipment.

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Process, microstructure
HVOF spraying process 9
Microstructure of HVOF sprayed coating (WC-17Co)
(produced at TUT)
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Equipment, price

• HVOF Sulzer Metco, Castolin, Metallisation,
  etc. HVAF AcuKote, Solid Spray, Uniquecoat
  Technologies, LLC, etc.
• Price for a HVOF spraying system may vary from
  15,300 EUR EXW (Metallizing Equipment Co. Pvt.
  Ltd.) to 66,000 EUR EXW (Metallisation) and
  more.
• HVAF systems are more expensive than HVOF ones.

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Cold spray / Külmpihustus / ????????
???????????????? ?????????

• Deformable particles are introduced to the
  supersonic (300 1200 m/s) preheated gas stream,
  directed onto the substrate and foem there a
  coating or deposit by the impaction process.
• No heating of particles (gas is heated to
  achieve higher sonic flow speeds).
• Only plastic materials can be used.
• Particle size 1 50 µm.
• Powde feed range 2 8 kg/h.
• Spraying distance 0,01 0,05 m

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Advantages / disadvantages 4

• Easy to apply
• Relatively safe technology
• Low or no oxidation of the sprayed material
• High adhesion
• Low porosity
• Thick coatings available.
• Restricted range of sprayed materials (metals,
  alloys, polymers, composites with plastic matrix,
  cermets)
• Relatively low mechanical properties of sprayed
  coatings (soft materials).

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Process, microstructure
a
b
Cold spray process 10
Microstructure of cold sprayed coating 1 a
copper coating on aluminium substrate, b
titanium-aluminium coating on aluminium substrate
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Equipment, price

• ASB Industries, Rus Sonic Technology Inc.,
  Obninsk Center for Powder Spraying, etc.
• An approximate price for the cold spraying unit
  is 20,000 EURO (Dymet equipment, 2009).

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Arc spray / Kaarpihustus / ??????????????
?????????

• An electric arc is ignited between the
• two consumable electrode wires, what causes
• their melting at the tips.
• A high-velocity air jet, situated behind the
  arc, flows
• the melted material away and carries it onto the
  substrate.
• Only electrically conductive materials can be
  sprayed.
• Way to spray cermets and amorphous materials
  cored wires.
• Wire diameter 1,0 3,5 mm.
• Feed rate 2 50 kg/h.
• Spraying distance 60 150 mm.
• Spraying angle 45 ... 90.
• Gun movement velocity 30 50 m/min.

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Advantages / disadvantages 4

• High production rate
• Maximal usage of energy among other methods
  (almost all electrical energy transforms into
  thermal one during melting of the material)
• Relatively low porosity
• Relatively low adhesion
• Restricted range of sprayed materials
  (electrically conductive)
• High oxidation rate of sprayed material.

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Process / microstructure
Arc spraying process 11
Microstructure of arc sprayed layer 1
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Equipment, price

• Equipment suppliers Metallisation Ltd.,
  Thermion, Haiams, etc.
• Price for the used arc spraying equipment is
  27,000 EURO EXW, for the new one must be in
  the range 30,000 35,000 EURO.

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Plasma spraying / Plasmapihustus / ??????????
?????????

• Plasma is generated inside the gun from the
  plasma forming gas by the means of direct
  current (DC), electric arcs or radio frequency
  (RF) discharge.
• Generated plasma expands radially and axially,
  simultaneously accelarating, and exits through
  the (supersonic) nozzle.
• Powder / wire is introduced to the plasma jet,
  melted and carried by it onto the substrate.
• Lots of modifications air and vacuum plasma
  spraying (respectively APS and VPS),
  inert-chamber, shrouded, Gator-Gard,
  radiofrequency, underwater (UPS), extended-arc,
  water-stabilized plasma spray.

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Parameters of plasma spraying

• Particle size 10 45 µm.
• Feed rate 2 8 kg/h for 20 60 kW plasmatrons,
  up to 50 80 kg/h for 150 200 kW plasmatrons.
• Spraying distances APS 0,075 0,12 m, VPS
  gt0,4 m, underwater plasma spraying 0,03 m.

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Advantages / disadvantages

• Easy to apply
• Flexibility
• Relatively cheap technology in comparison with
  detonation gun and HVOF/HVAF spraying (valid for
  APS)
• Low or no oxidation of the sprayed material
  (valid for VPS, UPS, inert-chamber, shrouded,
  Galor-Gard plasma spraying)
• Low porosity (valid for VPS, UPS, inert-chamber,
  extended-arc plasma spraying)
• High range of potentially sprayed materials
• High deposition efficiency (25 70 for wire,
  30 80 for powder).
• Low adhesion in comparison with detonation gun
  and HVOF/HVAF spraying (valid for APS)
• High porosity in comparison with detonation gun
  and HVOF/HVAF spraying (valid for APS)
• Low efficiency of power usage (1 5 for
  powders and 2 18 - for wires)
• High level of noise 60 120 dB (valid for
  APS).

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Process, microstructure
Air plasma spraying process 11
b
a
Microstructure of the plasma sprayed coatings
1 a 80Ni/20Cr, APS b MCrAlY, VPS
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Equipment, price

• Equipment suppliers Castolin, Sulzer Metco,
  Metallisation Ltd., etc.
• Price varies according to the supplier and the
  process in the large range APS is the cheapest
  process.

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Comparison of HVOF and APS processes
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Comparison of thermal vs kinetic energy
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Comparison of thermal spray processes 1
Attribute Flame spray D-gun HVOF / HVAF Wire arc Air plasma Vacuum plasma RF plasma
Jet Tº, K 3500 5500 5500 gt 25,000 15,000 12,000 10,000
Jet v, m/s 50 100 gt 1000 50 1200 50 100 300 1000 200 600 20 80
Gas flow, sl.m 100 200 N/A 400 1100 500 - 3000 100 200 150 250 75 150
Gas types O2, C2H2 O2, C2H2 CH4, C3H6, H2, O2 Air, N2, Ar Ar, He, H2, N2 Ar, He, H2 Ar, He, H2
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Comparison of thermal spray processes 1
Attribute Flame spray D-gun HVOF / HVAF Wire arc Air plasma Vacuum plasma RF plasma
P, kW 20 N/A 150 300 2 5 40 200 40 120 40 200
Particle Tº, ºC 2500 N/A 3300 gt 3800 gt 3800 gt 3800 gt 3800
Particle v, m/s 50 100 N/A 200 1000 50 100 200 800 200 600 20 50
Feed rate, g/min 30 50 N/A 15 50 150 200 50 150 25 150 20 500
Density, 85 90 gt 95 gt 95 80 95 90 95 90 99 95 99
Adhesion, MPa 7 18 82 68 10 40 lt 68 gt 68 gt 68
Oxides high small moderate to dispersed moderate to high moderate to coarse none none
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Popularity of different thermal spray methods
over years 13
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Plasma transferred arc (PTA) welding /
Plasmasulatus / ?????????? ????????

• An electrical arc is ignited between the coated
  part and the plasmatron.
• Plasma gas is fed into the plasmatron, is heated
  and ionized by the electrical arc, forming plasma
  stream.
• Powder or wire is fed to the plasma stream, is
  melted and sprayed onto the surface.
• The spraying zone is protected by the shielding
  gas to avoid / decrease oxidation.
• Particle size 10 45 µm for metallic alloys,
  100 300 µm for cermets.
• Feed rate 1 13 kg/h.
• Spraying distance 0,09 0,1 m

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Advantages / disadvantages

• Easy to apply
• Bigger size of cermet particles higher wear
  resistance
• Low or no porosity
• Thick coatings (up to several tens of mm-s) can
  be welded.
• Low heating of the substrate in comparison with
  GTAW.
• High oxidation of sprayed material
• Impossible to obtain thin coatings (1 mm or
  thinner).

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Process, microstructure
Plasma transferred arc welding process 14
Microstructure of PTA welded coating NiCrSiB
WC-Co (AC2T, Austria)
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Equipment, price

• Equipment suppliers Castolin, Sulzer Metco,
  Deloro Stellite, Mogul Metallizing GmbH, etc.
• Equipment price starting from 37,500 EURO
  (Mogul Metallizing GmbH) to 100,000 EURO
  (Castolin).

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Laser cladding / Lasersulatus / ???????? ????????

• Powerful laser (CO2, NdYAG) beam melts the
  powder / wire material, supplied to the
  substrate, together with the substrate material.
• Due to highly localized quickly moved heat
  source, the melted material solidifies
  momentarily.

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Advantages / disadvantages

• Easy to apply
• Flexibility
• Higher wear resistance due to fine structure
• Virtually no porosity
• High adhesion
• Coatings with any thickness (up to several tens
  of mm-s) are available.
• Low heating of the substrate.
• High internal stresses in the cladded coating
  due to high cooling rates
• Slow process (in the case of large areas).

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Process, microstructure
Laser cladding process 15
Microstructure of laser cladded layer (Ni-CrWC)
16
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Equipment, price

• Equipment suppliers Trumpf, Rofin, Castolin,
  etc.
• Price over 65,000 EURO.

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Posttreatment of coatings

• Sintering
• Impregnation (resins, Ag-Cu, Cu-Zn, Ni-Mn,
  Ni-Mn-Cr soldering brazes, melted oxides for
  heat-resistant hermetic coatings)
• Remelting (especially self-fluxing alloys)
• Plastic deformation
• Annealing
• Mechanical treatment grinding, polishing.

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Materials for thermal spray
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Applications of materials and processes. Rules of
thumb

• The best results for cermets are found, when
  HVOF/HVAF processes are applied.
• Plasma processes are preferred, when ceramic
  materials should be sprayed, or when special
  properties of the coatings are required
  (porosity, extremely low oxidation etc.).
• Flame spraying is nowadays mostly applied for
  restoration works. It suits more for steels and
  Ni and Cr alloys.
• Materials with low melting point (Al, Zn, Cu,
  etc.), intended for corrosion protection, are
  preferrably sprayed by arc spraying.
• Cold spraying is the only way to spray pure
  metals and polymers without oxidation without
  application of special facilities (inert gas
  chambers, etc.).

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Area of industry Carbi-des Self-fluxing alloys Iron and steel Nickel alloys Super-alloys MCrAlY Cobalt alloys Non-ferrous
Aero gas turbine
Agriculture
Architecture
Automotive engines
Business equipment
Cement and structural clays
Chemical processing
Copper and brass mills
Defence and aerospace
Diesel engines
Electrical and electronics
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Area of industry Carbi-des Self-fluxing alloys Iron and steel Nickel alloys Super-alloys MCrAlY Cobalt alloys Non-ferrous
Electric utilities
Food processing
Forging
Glass manufacture
Hydro-steam turbines
Iron and steel casting
Iron and steel manufacture
Marine manufacture and repair
Medical
Mining, construction and dredging
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Area of industry Carbi-des Self-fluxing alloys Iron and steel Nickel alloys Super-alloys MCrAlY Cobalt alloys Non-ferrous
Nuclear
Oil and gas exploitation
Printing equipment
Pulp and paper
Railroad
Rock products
Rubber and plastics manufacture
Ship and boat manufacture and repair
Land-based gas turbine
Steel rolling mills
Textile
Transportation, non-engine
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Area of industry Flame HVOF/ HVAF Detona-tion gun APS VPS Shrou-ded plasma
Aero gas turbine
Agriculture
Architecture
Automotive engines
Business equipment
Cement and structural clays
Chemical processing
Copper and brass mills
Defence and aerospace
Diesel engines
Electrical and electronics
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Area of industry Flame HVOF/ HVAF Detona-tion gun APS VPS Shrou-ded plasma
Electric utilities
Food processing
Forging
Glass manufacture
Hydro-steam turbines
Iron and steel casting
Iron and steel manufacture
Marine manufacture and repair
Medical
Mining, construction and dredging
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Area of industry Flame HVOF/ HVAF Detona-tion gun APS VPS Shrou-ded plasma
Nuclear
Oil and gas exploitation
Printing equipment
Pulp and paper
Railroad
Rock products
Rubber and plastics manufacture
Ship and boat manufacture and repair
Land-based gas turbine
Steel rolling mills
Textile
Transportation, non-engine
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Application of materials and processes
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Application of materials and processes
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Application of materials and processes
Problem Material Spraying process Application
Abrasive wear Al2O3, CrO2, TiO2, ZrO2 Plasma spraying Slush-pump piston rods, polish rod liners, and sucker rod couplings (oil industry) concrete mixer screw conveyors grinding hammers (tobacco industry) core mandrels (dry-cell batteries) buffing and polishing fixtures fuelrod mandrels
Abrasive wear TiC, Cr2C3 Plasma spraying, HVOF Slush-pump piston rods, polish rod liners, and sucker rod couplings (oil industry) concrete mixer screw conveyors grinding hammers (tobacco industry) core mandrels (dry-cell batteries) buffing and polishing fixtures fuelrod mandrels
Abrasive wear Self-fluxing alloys (Ni/Fe/Co)CrSiB, (Ni/Fe/Co)CrSiB-WC/WC-Co Flame powder spraying, HVOF, PTA, laser cladding Slush-pump piston rods, polish rod liners, and sucker rod couplings (oil industry) concrete mixer screw conveyors grinding hammers (tobacco industry) core mandrels (dry-cell batteries) buffing and polishing fixtures fuelrod mandrels
WC/TiC/Cr2C3-Co/Ni Flame powder spraying, HVOF
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Application of materials and processes
Problem Material Process Application
Erosion Cr2C3, WC Plasma spraying, HVOF Exhaust fans, hydroelectric valves, cyclone dust collectors, dump valve plugs and seats, exhaust valve seats
Erosion CrO2 Plasma spraying Exhaust fans, hydroelectric valves, cyclone dust collectors, dump valve plugs and seats, exhaust valve seats
(Ni/Fe/Co)CrSiBWC/ Cr2C3/TiC(Co/No) Flame powder spraying, HVOF,. PTA, laser cladding Exhaust fans, hydroelectric valves, cyclone dust collectors, dump valve plugs and seats, exhaust valve seats
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Application of materials and processes
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Applications of materials and processes
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Case study 1. Tertiary knurled sorter rods 17

• Application purpose to sort wooden chips by
  size during preparation of woodenchip board.
• Problem abrasive wear of the drop spots between
  the rods.
• Solutions galvanic hard chrome and HVOF
  spraying of WC-12Co.
• Results after 3 months HVOF sprayed rods
  demonstrated no wear, hard chrome plated ones had
  wear signs and had to be regauged due to
  irregularity of wear after 6 months HVOF sprayed
  rods had no wear signs, hard chrome plated had to
  be replaced.

Knurled sorter rods.
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Case study 2. Industrial land based turbines 1
Application of thermal spray coatings in turbines
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Case study 3. Medical implants
Bioimplants with APS sprayed coatings
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References (1)

• Handbook of Thermal Spray Technology. Ed. J.R.
  Davis. ASM International, 2004.
• www Powder coatings market summary,
  http//www.kuopont.com.tw/e_market.htm,
  07.10.2011.
• Personal contacts, 07.10.2011.
• ?.?. ???????, ?.?. ?????????. ??????????
  ????????? ?????????. ????????, ???, 2007.
• 5. Thermal spray powders (CastoDyn TeroDyn
  systems), http//www.castolin.com/wEutectic_usa/se
  rvices/CastoLab/Powder_cold_flame_spraying.php,
  21.10.2011
• 6. Thermal spray coatings Gordon England,
  http//www.gordonengland.co.uk/img/cws12e.jpg,
  21.10.2011.
• 7. C. Cano, E. Garsia, A.L. Fernandes, M.I.
  Osendi, P. Miranzo. Mullite/ZrO2 coating produced
  by flame spraying. Journal of the European
  Ceramic Society, 28, 2008, pp. 2191 2197.
• 8. Made-in-China.com connecting buyers with
  china suppliers, http//image.made-in-china.com/2f
  0j00MCyQtebEgRgN/Detonation-Gun-Spraying-Machine-P
  T-100-.jpg, 31.10.2011.
• 9. Hazmat Alternatives, http//www.hazmat-alternat
  ives.com/images/HVOFgun2-72dpi.jpg, 31.10.2011
• 10. Glass magazine the online resource for the
  glass industry, http//www.glassmagazine.com/files
  /u11/tech_0408c.jpg, 01.11.2011.

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References (2)

• 11. SurfAlloy Alloying Surfaces Inc.,
  http//www.surfalloy.com/images/worda/twinwirearc.
  jpg, 01.11.2011.
• 12. Altakitin Lda, http//www.altakitin.com/images
  /PlasmaGun.jpg, 01.11.2011.
• 13. E. Lugscheider, K. Bobzin, J. Zwick. Current
  situation and future of the thermal spraying
  industry in Europe and the world.
• 14. http//74.54.41.82/fusion/wallcolmonoy.co.uk/
  wp-content/uploads/2010/07/PTA2.jpg, 01.11.2011.
• 15. Vito, vision in technology Materials
  Technology bottom-up synthesis of
  multifunctional materials, http//www.vito.be/VITO
  /EN/HomepageAdmin/Home/WetenschappelijkOnderzoek/M
  ateriaaltechnologie/, 01.11.2011.
• 16. L. St-Georges. Development and
  characterization of composite Ni-Cr WC laser
  cladding. Wear, 263, 2007, 562 566.
• 17. K.W. David Hart, D.H. Harper, M.J. Gill, G.R.
  Heath. Case studies in wear resistance using
  HVOF, PTAW and spray fusion surfacing.

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Successful spraying!
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