Spray coating

05/01/2023Dept. of Metallurgical Engineering

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THERMAL SPRAY COATINGSSubmitted byTHIRUNAVUKARASU.H13MY12Dept. Of Metallurgical Engineering


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Need For Surface Hardening

•Produce surfaces that ▫wear only a little,▫resistant to tarnishing and corrosion,


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Thermal Spray Coating•A group of coating processes where the coating is

deposited on a prepared substrate by applying a stream of particles, metallic or nonmetallic, which flatten more or less forming platelets, called splats, with several layers of these splats forming the coating

•Any material on almost any substrate


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Spray Coatings• Use either axial or radial consumable injection in a high energy

flow resulting from combustion or high-velocity gas streams.

• Coating thickness▫50 μm to a few mm

• Bonding▫Substrate surface

undercuts simple roughening


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Components• An energetic gas flow

▫ An appropriate gun Devices for feeding, accelerating, heating, and directing the flow of a thermal

spray material toward the substrate.

• Feedstock▫ Powder, wire, rod, or cord. ▫ Fed at a velocity allowing the spray gun to melt them

• Auxiliary gas feed ▫ To accelerate atomized molten material into the spray gun

• Controlled atmosphere or a soft vacuum▫ In air, coating oxidation occurs, increasing with the temperature


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Substrate•Should not be degraded by heat•Substrate should be roughened for good adhesion

▫Abrasive blasting – Aluminium Oxide▫2.5 µm roughness optimum

•Difficult to spray hardened steels•Thin sections prone to distortion during blasting and

heating


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Fig: Components of spray coating system


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Steps In Spray Coating1. Substrate preparation

2. Generation of the energetic gas flow

3. Particle or wire or rod or cord injection

4. Energetic gas particle or droplet interaction

5. Coating formation


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Thermal Spray Hardening•Gas Combustion

▫Oxy fuel process using Wire feed Powder feed Rod feed Jet / powder feed Detonation Gun process

•Arc process▫Plasma arc with powder feed▫Arc spray with wire feed


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Wire Processes

•Wire from reel fed to oxyacetylene flame•Metal droplets atomized by air jets•Atomized metal spray coats substrate•Gun to substrate distance

10 – 25mm

•Commonly sprayed materials• Zn, Al for corrosion resistance• Bronzes for wear resistance


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•Wear application: 1.25mm•Corrosion resistance: 25µm•Max thickness : 6mm

•Deposition rate: 93m2 per hour per 25µm

•Flame temperature: 27600C


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Powder Spray

•Powder feed instead of wire•Oxyacetylene torch modified for powder feed•No high pressure air to assist atomization; low deposition rate•Lower bond strength and higher porosity•Easy method for materials that cannot be made into wire


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More sophisticated equipment uses compressed air.Increased atomization.Higher deposition rate and bond strength. Flame temperature: 25000C

CoatingsCarbidesHigh alloy steelsCeramics


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Rod Consumable

•Ceramics cannot form flexible wire•Coatings made of powder; too friable•Newly designed ones use solid rod of ceramic•Impact velocity: 2.8m/s

•Rod consumables

• Al2O3

• Cr2O3

• Ceramic mixtures


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Detonation Gun (D- Gun)

•Powder fed under small gas pressure•Explosive mixture of O2 and acetylene detonated using spark•Temperature: 38700C•Detonation: 4 to 8 times per sec; 730 m/s•N2 gas for flushing detonated gas•Coating thickness: 75 to 125µm

•Noisy process; done in soundproof room•For• Carbides• Ceramics

•High bond strength and coating density•Good surface finish


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Combustion Jet Or High Velocity Oxygen Fuel (HVOF) Process

•Continuous gas combustion jet: heat source and carrier• O2 and fuel gas like propylene, H2

•Consumable sprayed as powder to center of jet stream•Temperature: 29800C and velocity:

1370 m/s

•45kg per hour deposition rate•Consumable: Tungsten carbide, cobalt•High bond strength•High cost and safety issues involved


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Electric Arc Spraying

•Uses electric arc as heating source•Uses two consumable wires: higher deposition rate•Wires on motor driven feed rolls and insulated from each other meet at tip of torch•After energizing the torch, wires on

contact produce arc•Arc melts metal and air jet carries it to substrate•Wires as large as 1.5mm•Spraying soft materials for corrosion resistance; Zn, Al


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Plasma Arc Deposition

• Consumable powder melted and atomized in plasma▫ Tungsten electrodes and Ar gas▫ Temperature: 28000oC


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Fig: Paper machine roll coated by NiCrBSi using two powder flame guns (Courtesy of Castoline)


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Fig: (a) PTA-coated tooth of excavator with Ni base coating + WC (25 kg/h) and (b) cross section of the coating (courtesy of Castolin)


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ComparisonProperties Electro/ electroless

platingCVD PVD Thermal spray

Equipment cost Low Moderate Moderate to high Moderate to high

Operating cost Low Low to moderate

Moderate to high Low to high

Coating thickness 10 μm–mm 10 μm–mm Very thin tomoderate

50 μm–mm

Adherence Moderate mechanical to good chemical bond

good chemical to excellent diffusion Bond

Moderate mechanicalto good Chemical Bond

Good mechanical bond

Coating materials Metals Metals, ceramics, Polymers

Metals, ceramics,polymers

Metals, cermets,ceramics,polymers

Surface finish Moderately coarseto glossy

Smooth to glossy

Smooth to glossy Coarse toSmooth (0.12 µm to 0.5 µm)


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Coating Evaluation• Destructive testing: Tensile shear tests

▫ Ends of two strips of desired substrate are sprayed with desired consumable

▫ Coated ends epoxied together▫ Uncoated ends put to tensile tester and pulled to failure▫ If epoxy fails and the coating is intact

Good coating Bond strength as “greater than X”, X- tensile strength of epoxy 82 to 138 MPa nominal

• Non-destructive testing▫ Visual inspection

Porosities, impurities, cracks▫ UT inspection, Thermal imaging


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Comparison Between Spray Processes• Wire gun

▫ Heavy deposits: upto 0.100 inch▫ For steels, brass, bronze

• Powder module▫ Minor shop repairs: upto 0.030

inch▫ For nickel base alloys

• Rod feed▫ Wear resistant coatings: upto

0.020 inch▫ For ceramics

• D-gun

▫ Premier coatings: upto 0.010 inch

▫ Of hardfacing alloys, carbides▫ Densest coating

• Electric arc▫ Rebuilding large areas with

steels: upto 0.100 inch▫ For Al and Zn

• Plasma arc▫ Applying hardfacing alloys,

repairs: upto 0.015 inch▫ For metals and ceramics


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Applications of Spray Coating•Wear-resistant coatings against abrasion, erosion•Corrosion-resistant coatings•Heat resistant coatings•Thermal insulation or conduction coatings•Electromagnetic shielding•Medical coatings


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Do’s And Don’t’s Of Thermal Spray Coating

•Do’s•Apply coating to

undercuts to avoid end chipping

•Hold gun normal while spraying

•Plug keyways when coating

•Don’t’s•Coat end of parts subject

to chipping•Coat faces subject to

impact•Spray at an angle < 600

•Coat cutting edges


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Reference1. Cartier M, Handbook of surface treatments and coatings.

ASME Press, New York, NY, 20032. Davis JR, Handbook of thermal spray technology. ASM

International, Materials Park, OH, 20043. Chattopadhyay R (2001), Surface wear. ASM International,

Materials Park, OH4. Kenneth G. Budinski, Surface Engineering for Wear

Resistance, Prentice Hall Inc., 1988, Pg: 221-240.5. ASM Handbook volume 4, Heat Treatment, 1991.