African Fusion March 2015

SAIW: Thermal spray coatings seminar

March 2015

AFRICAN FUSION

up to 300 to 800 m/s; and the modern

cold spray process, which, at tempera-

tures below300 °C, accelerates particles

towards a substrate at up to 1 000 m/s.

Each process type has its own niche in

the market.

“From left to right, we can see

these processes as first, second, third

and fourth generation thermal spray

developments. The higher the particle

velocity, the better the layer quality

and bond strengths. And the lower the

temperature can be kept, the less the

chance of oxidisation occurring while

a particle is travelling to the surface.

The combination of high velocity and

low temperature characteristics make

cold spray processes very attractive

for sensitive and expensive coatings,”

she adds.

In terms of the coatings themselves,

since substrate temperature can be

maintained below 200 °C with good

thermal management, there is no heat-

affected zone (HAZ) or dilution between

the substrate and coating. Due to the

mechanical bondand the layerednature

of the coating structure, the material

properties of the coating are not the

same as bulk cast/wrought coating ma-

terial properties would be. And while a

vast range of coating properties can be

achieved, it is generally not a good idea

to subject coatings to high point or line

impact loading.

“Components can be coated with

metals, alloys, carbides/cermets, oxides,

polymers, blends and graded coatings.

Deposition rates of between 1.0 and

20 kg/hr can be achieved and, with the

addition of the cold spray process, the

thickness range is almost unlimited –

from 20



m to > 30 mm.

“The greatest limitation is that it is

a line of sight process. If you have a thin

narrowborewith limitedaccess, it is very

difficult to coat the inside of it,” Lovelock

informs. She also advises caution in

some aqueous corrosive environments,

because “thermal spray coatings are not

generally recommended for corrosion

barrier protection in the as-sprayed

condition, because they do not seal.

While penetration could take years, the

coating itself cannot be guaranteed to

last forever, even if it has been success-

fully subjected to laboratory corrosion

tests and appears to be fully dense. This

does not apply to sacrificial corrosion

protection coatings, however, because

this protection does not depend on a

sealed barrier,” she explains.

Overview of thermal spray

coating technologies

Flame spray processes

Powder flame spray is a subsonic

flame-based process. Powder is blown

or drawn into an annular combustion

chamber where an aspirating gas and

a fuel gas are mixed and combusted,

creating a high velocity (subsonic) ex-

haust stream. A second outer annular

gas nozzle feeds a streamof compressed

air around the combustion flame, which

accelerates the spray particles towards

the substrate and focuses the flame.

The powder can be fed through a

small hopper on the gun, or via larger

freestanding powder feeders. There are

also several fuel gas options, depending

on the application and temperatures

required. “It is a low velocity, low tem-

perature process and, because the par-

ticles are projected through air, a surface

layer with relatively high proportions of

oxides, porosity and unmelted particles

is produced,” says Lovelock.

Wire flame spraying is a similar

process, except that the coating mate-

rial is fed into the flame spray gun as

solid wire. The flame melts the wire,

which is then atomised and accelerated

towards the substrate by the annular

compressed air flow. Also a subsonic

process, Lovelock says that the wire

feed rate and flame settings have to be

balanced toproduce continuousmelting

of thewire anda continuous spray of fine

particulate. Showing amicrograph of an

aluminium coated sample she points

out the black lines of oxide stringers

and says, “If you consider how easily

aluminiumoxidises, this is not too bad”.

The flame spray processes have low

capital and running costs. They can be

done on-site and by hand, if appropriate

safety precautions are adopted, and

they offer adequate surface properties

for low-stress build-up applications and

sacrificial coatings. “They are suitable

for non-demanding applications such as

repairing wear or corrosion damage on

non critical parts that are not intended

for demanding engineering applica-

tions. A variety of iron- and nickel-based

powders are available for repair and

salvage,” she adds.

Avariationof theseprocesses are the

spray and fuse processes, also known

as spray-brazing, which involve remelt-

ing the fused layer – with a flame, in a

furnace in by induction heating – after

it has been deposited. “The fusingmelts

and seals the surface and can produce

ametallurgical bondwith the substrate.

The process is commonly used to coat

glass formingplungers andglassmoulds

with a NiCrBSiFe alloy, a ‘self-fluxing’

alloy with low melting point. This coat-

ing is known to produce a favourable

surface interaction with molten glass

(silica) which at 700 °C is very abrasive,”

says Lovelock.

Arc wire spray

The key difference between these pro-

cesses and flame-based process is that

an electric arc is used to generate the

requiredheat. Themost commonly used

variation is twinwire arc spraying, where

a dc electric arc is struck between two

continuously fedwires. Thewire speeds

are set to balance the melt-off rate of

the wires and to keep the arc stable.

On melting, droplets are propelled by

compressed air or an inert gas jet onto

the surface being coated.

Arc spraying is a very high produc-

tivity thermal spraying process with de-

position rates for steel at 10 to 14 kg/hr

and up to 5.0 to 8.0 kg/hr for aluminium.

“With flame spray, it is possible to spray

unmelted powders onto a surface, but

with twin wire arc spray, the coating

material has to be melted before it is

sprayed,” Lovelock notes. The process is

simple to operate; canbe usedmanually

or automated and a wide range of met-

als, alloys andmetal matrix composites

(MMCs) can be deposited, including a

limited rangeof carbide-basedmaterials

that are available in cored-wire form.

Generally speaking, the coating

quality is better than flame sprayed

coatings, with less porosity, fewer ox-

ides and higher bond strengths. The

Void

Oxide inclusion

Unmelted particle

Substrate

Features of thermal spray coatings: “The bonding

mechanisms at the interface and between the coating

‘splats’ is still subject to some speculation, but while

both mechanical interlocking and diffusion bonding

may occur, mechanical bonding predominates,” says

Lovelock.