Local Seminar

7/30/2019 Local Seminar

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Thermal Spray Coatings

Presented by:

Swarup Satyajit Nanda

Roll no-27628 1

Guided by:

Mrs Swarnalata

Behera


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Flame Spraying . Flame spraying uses the heat of combustion of a fuel

gas (usually acetylene or propane) and oxygen mixture

to melt the coating material, which can be fed into thespraying gun in two forms, either powder or solidwire/rod.

Its of two types

powder flame spraying

powder is fed directly into the flame by a stream ofcompressed air or inert gas (argon or nitrogen).

Alternatively, in some basic systems, powder is drawninto the flame with air by a venturi effect, which issustained by the fuel gas flow..

wire flame sprayingprocess, the wire feed rate andflame settings must be balanced to produce continuousmelting of the wire and a fine particulate spray. Theannular compressed air flow atomises and acceleratesthe particles towards the substrate.

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The Plasma Spraying Processis basically the sprayingof molten or heat softened material onto a surface toprovide a coating. Material in the form of powder isinjected into a very high temperature plasma flame,where it is rapidly heated and accelerated to a highvelocity.

The plasma gun comprises a copper anode andtungsten cathode, both of which are water cooled.Plasma gas (argon, nitrogen, hydrogen, helium) flows

around the cathode and through the anode which isshaped as a constricting nozzle.

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Schematic of Flame Spraying System

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Schematic of Arc Spraying System

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Schematic of Plasma Spraying System

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Benefits of Thermal Spraying Comprehensive choice of coating materials: metals,

alloys, ceramics, cermets and carbides.

Thick coatings can be applied at high deposition rates. Coatings are mechanically bonded to the substratecan

often spray coating materials which are metallurgicallyincompatible with the substrate, e.g., materials with ahigher melting point than the substrate.

Components can be sprayed with little or no pre- or post-heat treatment, and component distortion is minimal.

Parts can be rebuilt quickly and at low cost, and usuallyat a fraction of the price of a replacement.

By using a premium material for the thermal spraycoating, coated components can outlive new parts.

Thermal spray coatings may be applied both manuallyand automatically.

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Fusion hard facing is a process by which weldmaterials, with superior properties than the substrate,are applied to the substrate. Often, two layerstotal 3 - 6mm (1/4 - 1/2 in) thick are applied to reduce the surfacehardness dilution of the relatively low-cost steelsubstrates in the expansive cobalt-base alloys.

Hard facing processes are very useful for improvingwear and corrosion resistance to selected areas ofmachinery, such as cutting edges of earth-movingmachinery. Common hard facing techniques include arc,

torch, and other processes.

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Adhesion

With the lower energy processes of flame and arcspraying, adhesion to the substrate is considered to belargely mechanical and is dependent on the work piecebeing very clean and suitably rough. Roughening iscarried out by grit blasting and occasionally, rough

machining. With the higher energy processes, bond strengths are

higher because of the higher impact velocities. Adhesionis improved through disruption of oxide layers on thesubstrate and embedding of the particles into the

substrate. There is some evidence to suggest that acertain proportion of diffusion bonding takes place.Surface preparation by cleaning and grit blasting isextremely important.

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Examples of applications

Corrosion protection of structures and components (e.g.,bridges, offshore platforms, L.P.G. bottles) with aluminum orzinc coatings

Reclamation of worn shafts.

Spraying of self fluxing hard facing alloys e.g., NiCrBSi alloysto improve wear resistance at very high temperature.

A good application example is glass plungers, where the

abrasive medium is molten silica glass at temperaturesapproaching 700C.

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Oxyfuel (HVOF) Spraying . HVOF spraying differs from conventional flame

spraying in that the combustion process is internal, and

the gas flow fates and delivery pressures are much. thanthose in the atmospheric burning flame sprayingprocesses

The HVOF process produces exceptionally high qualitycermet coatings (e.g., WC-Co), but it is now also used to

produce coatings of metals, alloys and ceramics. Not allHVOF systems are capable of producing coatings fromhigher melting point materials, e.g., refractory metals andceramics. The capability of the gun is dependent uponthe range of fuel gases used and the combustionchamber design.

A liquid fuel (kerosene) HVOF system, has just beenlaunched, which is capable of much higher depositionrates than the conventional gas-fuelled units.

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Examples of Applications HVOFspraying.

Tungsten carbide-cobalt coatings for fretting wearresistance on aeroengine turbine components.

Wear resistant cobalt alloys onto fluid control valveseating areas.

Tungsten carbide-cobalt coatings on gate valves.

Various coatings for printing rolls, including copper,alumina, chromia.

NiCrBSi coatings (unfused) for glass plungers.

NiCr coatings for high temperature oxidation/corrosion

resistance. Alumina and alumina-titania dielectric coatings.

Biocompatible hydroxylapatite coatings for prostheses.

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Schematic of High Velocity Oxyfuel (HVOF)

Spraying System

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Comparison of Thermal Spraying Processes and Coating Characteristics

Process

Particle

Velocity

(m/s)

Adhesion

(MPa)

Oxide

Content

(%)

Porosity

(%)

Deposition

Rate

(kg/hr)

Typical

Deposit

Thickness

(mm)

Flame 40 70 12 12 15 0.22

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Process Fuels that can be used Other gases

HVOF Acetylene, hydrogen,

propylene, propane, orliquid kerosene

depending on material

type

Oxygen and argon

Arc spraying Normally

compressed air but

can use nitrogen orargon

Flame spraying Mainly acetylene, but

sometimes propane

depending on material

Oxygen

Plasma spraying Argon andhydrogen

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Detonation Thermal Spray Coating Process

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Factors Effecting The Thermal Spray Coating Process

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Arc Spraying - Electric Arc Wire Thermal Spray Process

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PROPERTIES

Compare coatings to their wrought or cast equivalents:

PROPERTY..COATING.CAST/WROUGHT Strengthlow (5-30%)...100%

Ductilityvery low (l-10%)100%

Impact..low..high

Porosityyes (not if fused).in some

castings Hardness..slightly higher (microhardness)

Wear resistancehigh..low

Corrosion.low resistance..high resistance

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Porosity This is present in most thermally sprayed coatings (except VPS,

post heat treated coatings or fused coatings).

1 to 25% porosity is normal but can be further manipulated bychanges in process and materials.

Porosity can be detrimental in coatings with respect to:

Corrosion - (sealing of coatings advised). Machined finish.

Strength, macrohardness and wear characteristics.

Porosity can be important with respect to: Lubrication - porosity acts as reservoir for lubricants.

Increasing thermal barrier properties.

Reducing stress levels and increasing thickness limitations.

Increasing shock resisting properties.

Abradability in clearance control coatings.

Applications in prosthetic devices and nucleate boiling etc.

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Some Properties Thermally SprayedCoatings can Provide: Tribological (wear, resistance).

Corrosion resistance.

Heat resistance. Thermal barrier.

Electrical conductivity or resistivity

Abradable or abrasive.

Textured surfaces. Catalyst and prosthetic properties,

Restoration of dimension.

Copying of intricate surfaces.

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HVOF sprayed Tungsten Carbide / Chromium NickelCoating

(WC/20Cr7Ni )

HVOF sprayed Tungsten Carbide / Chromium Nickel

Coating (WC/20Cr7Ni ) 30


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thank u

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