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5

Laser-aidedThermal Coating

The metallic or ceramic coat-

ing of workpieces is part and

parcel of the industrial processes

that have become more and more

important in the last few years:namely as a functional coating to

enhance the wear and corrosion-

resistance and thus prolong the life

cycle or to change the physical

properties of the surface. With the

thermal coating of metallic sur-

faces, the preparation of the sur-

face is one of the decisive opera-

tions it is of prime importance for

the adhesion of the applied coat-

ing.

PREPARATION COSTS TIMEAND MONEY

With customary processes, the

substrate has to be degreased and

then blasted with corundum inorder to avoid reactions between

organic contaminants and the

melted particles of the coating

materials. This process gives rise

to various problems. The employed

solvents and blast media have to be

disposed of, and at no little ex-

pense; the surface of the substrate

is contaminated by particles of the

blast medium. With the notch-sen-

sitive surfaces of titanium alloys

and high-strength steels, for exam-

ple, the blasted part can exhibit a

tendency towards early fatigue.

The process may reach the limits of

its possibilities if very thin sub-strates have to be coated, because

the abrasion caused through blast-

ing with corundum can damage or

even destroy them.

Further difficulties are encoun-

tered with the coating of highly

reactive materials such as alu-

minium alloys: the activated sur-

face reoxidizes within 100ms. And,

last but not least, the degreasing

and blasting with corundum

24 S U L Z E R T E C H N I C A L R E V I E W 3 / 2 0 0 0 3918

GRARD BARBEZATSULZER METCOFRDRIC FOLIO

EPF LAUSANNE

Despite their undisputed advantages, thermalcoatings are time-intensive and, ecologically, notalways without problems because of the use of solventsand blast media when pretreating the surfaces tobe coated. The short reoxidation time of numerousmetals causes further difficulties. With the Sulzer

Metco laser-aided process Protal, the cleaning,activation and coating steps are combined in a singleworking operation. It is therefore quicker, more

economic and less harmful to the environment thanconventional processes.

1 Application principle of the Protal process. The

aser removes the oxide layer and the contaminants;

he cleaned surface is coated immediately after

reatment with the laser.

Laser beam

Plasma jet

Powder

Oxide layer

Substrate

Coating on thecleaned surface

1

2

3

4

5

6

2

46

1

3

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require valuable time, and the

respective equipment sufficient

space.

IN A SINGLE WORKING

OPERATIONThe laser-aided process Protal

(Projection thermique assiste par

Laser) combines the preparation of

the substrate and the thermal

spraying in a single operation

(Fig. 1). There is no need for the

customary treatment, namely

degreasing and blasting with

corundum. Moreover, we do not

have to cope any more with the dis-

advantages of the blasting process contamination of the surface

through residual blast medium

(penetration of particles), mechan-

ical stress of the surface, abrasion

and the disposal of blasting medi-

um.

The development of the process

was inspired by Prof. Christian

Coddet of the Institut Polytech-

nique in Sevenans (FR). Sulzer

Metco took over the central co-ordi-

nation of this research project,

which is sponsored by the EU in

1996, and proceeded to develop the

Protal process in co-operation with

the FIT Lausanne (CH) and other

partners. The first results were

presented already in 1998. Protal

is now ready for commercial appli-

cation (Fig.2), and Sulzer Metco

has secured the worldwide rights

for the process (the patents) and

the marketing of the respective

system.

With the Protal process, the sub-

strate is cleaned with a high-ener-

gy laser pulse. A laser beam with awavelength of 1.064 mm and a very

short pulse duration of only 10ns

evaporates the layer of grease,

detaches the oxide layers and

removes the uppermost atomic lay-

er by means of ablation, without

heating the base metal to any

appreciable degree. The impact

area of the laser is normally

18 8 mm. A surface area of 1 m2

can be treated with an energy den-

sity of 1.5 J/cm2 in just one minute.

A few nanometres of the substrate

are removed with this process. The

resultant activated surface is now

coated by means of a HVOF (High

Velocity Oxy-Fuel) or a plasma

torch with ceramic or metallic

materials.

2 The Protal unit: plasma torch F4 and a

high-energy laser, mounted on a robot arm.

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3 The comparison of the adhesive strength (EN 582) of plasma-sprayed coatings on

TiAl6V4-substrate with conventional blasting with corundum and Protal treatment

shows almost identical adhesion values.

Substrate Coating material(APS-F4)

Surfacepreparation

Adhesive strength(MPa)

TiAl6V4

TiAl6V4

TiAl6V4

TiAl6V4

TiAl6V4

TiAl6V4

Copper

Copper

Nickel chromium 20

Nickel chromium 20

Al2O3/13%TiO2

Al2O3/13%TiO2

Blasting with corundum

Laser 0,75 J/cm2

Blasting with corundum

Laser 0.75 J/cm2

Blasting with corundum

Laser 0.5 J/cm2

60

55

60

55

80

80

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4 A complete system: Experience has been acquired with the

Sulzer Metco Protal system in Wohlen (CH) since the end of 1998.

CHEMICAL BONDINGOF COATINGS

The corundum process results in

the mechanical interlocking of the

coatings, because the roughening

of the surface enables the coating

to mechanically interlock closely

with the substrate. With the Pro-

tal process, the substrate is acti-

vated by the laser. The smooth,

very clean surface facilitates theformation of a chemical, respec-

tively metallurgical bond between

the particles of the coating materi-

al and the substrate. This results

in an adhesive strength which is

comparable to or higher than that

of the conventional process.

between the preparation of the

surface and the application of the

coating to be reduced to 2040ms,

which is ideal for this application.

ECONOMIC AND ECOLOGICALADVANTAGES

Protal can be described as a

green process. The elimination of

solvent and blasting media makes

Protal especially compatible withthe environment. In comparison

with the traditional processes, it

also reduces the consumption of

energy: The Protal process oper-

ates with a power input of only

3 kW.

Since degreasing, surface prepara-

tion and coating are carried out

simultaneously in a single working

operation, it results in an astonish-

ing saving of time together with

the high speed of the process an

important factor for the total pro-

ductivity.

MEASURABLE ADVANTAGESUsing a strength test (EN 582), it

was established that the adhesive

strength of plasma coatings and

Protal-treated aluminium and

titanium substrates was similar to

that of comparative corundum-

blasted specimens, even thoughthe surface roughness with Protal

is clearly lower than that of sub-

strates blasted with corundum

(Fig.3).

The indentation test provides

information about the state of

toughness at the interface. If we

take the length of the cracks under

an identical load as a benchmark,

it reveals significant differences.

With the Protal pretreatment, the

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SIMPLE COATINGOF ALUMINIUM

A layer of oxide forms on alumini-

um or aluminium alloys within the

shortest possible time. Although

this is welcomed for surface protec-

tion reasons in many cases, it

reduces the adhesive strength of

thermal coatings.

With the Protal process, the differ-

ence in time t between the prepa-ration of the surface and the

impact of the particles can be kept

very short. Depending on the sub-

strate, we can expect a reduction in

the adhesive strength already by a

t of only 100 ms, and so the coat-

ing must be applied immediately

after the treatment with the laser.

For this reason, the laser optic is

coupled together with plasma

torch and mounted as one unit on

a robot arm. This enables the time

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RESEARCH WORK CONTINUESThe mechanisms which are

brought into play at the interface

substrate/coating with the Protal

process and result in high bond

strengths have not been complete-

ly clarified so far. Up to now, it

could be shown that metallurgical

reactions also play a part in the

adhesion. With the system Copper

Coating on an Aluminium Sub-

strate, for example, it has been ver-

ified that there is an enrichment of

copper down to a depth of about

200 nm. An in-depth knowledge of

this metallurgical process will play

a decisive role in the developmentof Protal with new industrial appli-

cations, and thus increase the

importance of thermal spraying in

industry even more.

F O R M O R E D E T A I L SSulzer Metco AG (Switzerland)

Grard Barbezat

Rigackerstrasse 16

CH-5610 Wohlen

Switzerland

Telephone +41 (0)56-618 81 79

Fax +41 (0)56-618 81 99

E-mail gerard.barbezat@sulzer.com

S U L Z E R T E C H N I C A L R E V I E W 3 / 2 0 0 0 27

cracks are much shorter than in

the case of the corundum-blasted

substrates.

COATING AND

COATING SYSTEMSA Protal coating unit has been in

operation at Sulzer Metco in

Wohlen (CH) since autumn 1998.

It is employed not only for research

purposes, but also for productive

assignments (Fig. 4). It executes

customer contracts and also

enables us to gain experience,

which is then used expediently for

working the market and the com-

mercialization of the completerange of coating systems. The

potential for Protal is immense;

whether it is in conjunction with

turbine blades or motor compo-

nents, applications in the printing,

textile or paper industries, in the

aircraft industry or the energy sec-

tor there are innumerable fields

of application in which the benefits

of Protal can be used to full advan-

tage (Figs. 5 and 6).

Al2O3/13%TiO2

Chemical nickel

Steel

5 A Protal application for the

printing industry: Anilox roller;

ceramic Cr2O3-coating on an

intermediate coating of NiCr20.

6 Example of a ceramic coating with the

Protal process: Al2O3/13%TIO2, applied to

chemical nickel.

Cr2O3

NiCr20

Anilox roller

20mm

50 m

50m