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In compar ison wi th metals ,

ceramic mater ia l coat ings

have high mel t ing temperatures

with poor thermal conductivi ty

an d up to now could only be made

economically using plasma spray-

ing.

Coating characteristics

The most frequent ly used ceramic

spraying mater ia ls are Cr 2O3

(Fig . 1), Al2O3 and Al2O3-TiO2. I n

v iew of their h igh hardness and

wear resis tance, they are em-

ployed a s w ear-protection coatings

in the pr in t ing and the text i le

industry, and likewise in hydra ulic

engineering a nd for the ma nufa c-

ture of pla stic foi l. The ha rdness

and wear resista nce of the plasma-

sprayed ceramic coatings, how-

ever, are limited by the level of

porosity w hich, depending on t he

material , the employed type of

powder , the spraying system a nd

the operat ing para meters , is usu-

a lly betw een 2 and 5%. In other

words, a ny reduction in the level of

porosity leads to a n improvement

in qua lity.

Profitability

Feed ra tes of 80 g/min ca n be set

with conventional plasma-spray-

ing systems wi th ra t ings o fbetween 40 and 80 kW a nd oxide-

ceramic powders. Depending on

the spraying parameters , the

employed powd er an d th e dimen-

sions of the component , th e deposi-

tion efficiency with Al2O3 is

betw een 40 an d 70%. With Cr 2O3,

how ever, it is only 35 t o 45%.

Improvements in the deposit ion

eff iciency lead to a sa ving of pow-

der and short er coating t imes.

32 SULZ ER T ECHN I CA L REV I EW 4 / 99 3912

GRARD BARBEZATSULZER METCOKLAUS LANDES

UNIVERSITT DERBUNDESWEHR, MUNICH

Plasma Torch-System Triplex:

Increased Productionand a More Stable Process

I n many r espects, the existi ng sin gle-cath ode pl asmatorches have r eached t he li m it s of t echn ical feasibi l i ty.Sul zer M etco and th e team of Kl au s L and es (Uni ver sit tder Bu nd eswehr, Mun ich), whi ch is acti ve in the fieldof pl asma ph ysi cs, have devel oped a t or ch wi th th r eecath odes th at h as produ ced ver y pr omi sin g resul ts intests cond ucted in the ind ustri a l envi r onm ent.

1 Th e coati ngs produ ced by th e new Tri plex system w it h i ts thr ee

cathodes are appr eciably denser and har der th an those achieved

wi th convent ional single-cath ode torches. Left: a typical cer am ic

coati ng (Cr2O3), r i gh t : a typ i cal bond i ng coat i ng (N iCr ); the l at ter

is pr ovided to bond surf aces wit h top coati ngs.

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such as the Sulzer Metco F4, the

a verage service life of such a cath -

ode under production conditions

and para meters is between 40 and50 hours.

HIGH AIMS

The following requirements there-

fore form t he ba sis for the develop-

ment of a new plasma-torch sys-

tem: Longer service life of the cat h-

odes to improve the process stabil-

ity a nd th us reduce the possibility

of metal l ic inclusions and, in the

ideal case, to prevent them com-

pletely. Further goals were the

reduction of th e porosity content t o

enha nce the qua lity of the coat ing,

increased prof i tabil i ty of the

spraying process through better

ef f iciency and, simultaneously,

improved flow of th e powder. To

fulfil these requirements , the con-

vent ional p lasma torch ha d to be

completely redesigned.

TRIPLEX: THREE CATHODES

INSTEAD OF ONE

With a convent ional torch such a s

the F4, an arc i s es tab l i shed

through an electr ic gas discharge

betw een a single tungsten cat hode

and a one-piece, nozzle-shaped

copper anode, and maintained

thr ough the supply of an electrical

power. During th e t ime the a rc is

af f ixed to the t ip of the tungsten

cathode with i ts cat hodic root, thea nodic root on the extended inner

wall of the anode can move in an

axia l and tangent ia l d i rec t ion

(Fig . 2). The migration of the

anodic root means a change in the

length of the a rc which with a con-

stant ly mainta ined current leads

to f luctuations in the arc voltage

and performance. On the other

hand, t he heating a nd a cceleration

var iat ions on th e powd er injected

SULZ ER T ECHN I CA L REV I EW 4 / 99 33

2 With conventi onal singl e-cath ode torches, th e arc root on

the anode side (arr ow) can m igrat e in both the axial and tan-

gent ial di r ection. Th is causes a chan ge in th e voltage and per-

forman ce, and l eads, in tur n, to undesir ed fl uctuations in t he

effi ciency and noise em issions.

3 Th e Tri plex system featu r es thr ee cath odes, whi ch ar e

suppli ed separ ately wi th power; th ank s to the counter-

in sulat ed n eut rodes (arr ows), the anode-sid e posit ion of th e

arc root r emai ns axial, and t angenti al stable thanks to the

recipr ocal r epul sion of the arcs.

Process stability

In t he printing industry, for exam-

ple, Cr2O3 is used for the coa ting of

anilox rollers, in the surface of

which a f ine, l inear-arranged dot

ras ter i s engraved by means o f

lasers . Apar t f rom t he h igh ha rd-

ness and w ear resis tance, a very

fine, uniform st ructure free of an y

impurit ies is the most importantrequirement here. Meta l l ic inclu-

sions a re the most frequent impu-

rities a nd ca used by t he burn-off of

the tungsten cathode dur ing the

spray ing process. A single metallic

inclusion is enough to the ma ke the

roller unsuitable for the laser

engra ving. The probabil i ty of the

formation of such inclusions

increases with the dut y cycle of the

cathode, because the tungsten

cath ode is thermally stressed and

can become dama ged. With a com-

mercia l ly a vai lab le p lasma torch ,

Cathode

+

Anode

AnodeCathode

C urrent s upply

+

C urrent s upply

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at the outlet of the nozzle alsoimpairs the quality of the process.

With th e new ly d eveloped Triplex

plasma torch, a more effective and

uniform thermal and f luid

mechanical influence is exercised

on the powder particles, namely

through the following characteris-

tics:

The Tr iplex torch has t hree

counter-insulated cathodes t ha t

are supplied by independent

power sources and from which

three ident ical par t ia l arcs

emanate .

A migrat ion of each of these par-

tia l arcs is prevented in both the

axia l and tangent ia l d i rec t ion

through appropriate design

measures (Fig. 3).

The par t i a l arcs , which are s ta -

bil ized in t his ma nner , combine

to form a stationary, full plasma

jet. Its three-fold symmetry can be

used advant ageously by means

of a th ree-fold injector (Fig. 4),

the f low of powder through i ts

nozzles is al igned exactly in t he

hottest parts of the plasma jet .

REDUCTION OF THE

THERMAL WALL STRESS

With a single arc and t he usual rat -

ings, th e fixation of the a nodic root

would cause excessive thermal

s t ress ing o f the wal l and thus

destroy the anode. With the

Triplex torch, t he electric power is

therefore distr ibuted to t hree par-

a llel-burning a rcs, so tha t the wa ll

stress on the anodic roots is also

markedly reduced. The resultant

adva nta ge is the increased service

life of the a node. The nozzle of the

Triplex plasm a torch consist s of

a number of counter-insulat ed, so-cal led neutrodes located bet ween

the ca thodic zone a nd a node. The

length and vol tage o f the arc as

well as t he enthalpy of the plasma

can be varied in a goal-oriented

manner through the respective

number of neutr odes.

PRACTICAL EXPERIENCE

The first Triplex syst em, wh ich ha s

a maximum electr ic rat ing of

25 kW, wa s put int o operat ion in

1997. Experience is being ga th ered

with a second system, which ha s a

maximum ra ting of 50kW and has

been operat ing in an industr ia l

environmen t since m id-1998. The

Triplex torch is operat ed wit h a ga s

mixture of argon and helium or

wit h pure a rgon. With Ar, th e flow

ra te is betw een 15 a nd 90 l/min,

and wi th He between 5 and

40 l/min. Tota l current densit y va l-ues, which distr ibute over the

thr ee ca thodes, ca n be set between

300 and ma ximum 600 A at vol t-

a ges of approximat ely 80 to 110 V.

Ana lyses of Cr 2O3, Al2O3 and Al2O3-

TiO 2 coat ings show t hat with com-

parable spray data appreciably

denser a nd ha rder coatings can be

obtained w ith t he Triplex syst em

tha n wi th t he F4 torch . Fur ther-

more, in a ddition to a tw o- or three-

34 SULZ ER T ECHN I CA L REV I EW 4 / 99

5 Th e per form ance of the Tr ipl ex cath odes rema in s stabl e for m ore th an

200 hour s. In compa r ison, the per form ance of a sin gle-cathode system decli nes

continu ously, and w ith crit i cal param eters it cannot be used for mu ch

longer t han 40 to 70 hours.

Processs

tability(V)

Triplex

Conventional torch

Operating time 200 h40 h

4 Section th r ough the nozzle ti p of the Tri plex tor ch: the

ma teria l t o be sprayed i s inj ected th rough one of the

th r ee openi ngs (one

for each arc) in to

th e hottest zone of

the plasma j et.

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fold powder flow ra te, a ppreciably

higher deposition efficiencies cana lso be a chieved. These tw o fa ctors

enable the t otal productivi ty t o be

increa sed b y u p t o 300%. Admit -

tedly, a thr ee-fold powd er qua ntit y

also means a three-fold thermal

str ess on the w orkpiece to be coa t-

ed. For th is rea son, th e Triplex sys-

tem is suitable, above al l , for the

coating of medium- to large-size

par ts .

Ca thode service lives of more than

200 hours have been a chieved with

the Triplex syst em, an d a lso by

very high performa nces (Fig. 5).

In t he f ir s t p lace, th is means tha t

the quali ty of coatings on cr i t ical

products, such a s a nilox rollers, is

clear ly enhanced, because the

cathode wea r is reduced a pprecia-

bly. Secondly, the profitability of

the spraying process improves,

because the downtime frequency

for cha nging th e ca thodes decreas-es. As a result of th e considerably

longer service l i fe and t he qua li ty

a ssura nce, costs a re reduced con-

siderably a s well . A further advan-

ta ge of the Triplex syst em is th e

lower noise nuisa nce level , w hich

is a t t r ibutable to the s table arc ,

in compar ison wi th s tandard

torches (Fig. 6 a n d 7) and the

a lmost la minar plasma f low. With

100 db(A), th e noise emission is

clea rly lower t ha n t he 120db(A) of

the F 4 torch.In comparison with conventional

single-cat hode t orches, th e Triplex

syst em improves the prof ita bil i ty

of the plasma spraying of ceramic

mat er ia l s qui te c lear ly , and wi th-

out having to increase the power

rating. The processing t imes are

also drastical ly reduced. Four

Triplex syst ems ha ve been put int o

operat ion in 1999 (three in E urope

a nd one in the US A).

F O R M O R E DE T A I L S

Sulzer Metco AG (Sw itzerland )G rard Ba rbezat

Rigackerstrasse 16

CH-5610 Wohlen

Switzerland

Telephone + 41 (0)56-618 81 81

Fa x + 41 (0)56-618 81 99

E -m a il g er a r d. ba r bez a t @s ulz er. ch

SULZ ER T ECHN I CA L REV I EW 4 / 99 35

100

80

60

40

20

0

0 1 2 3 4 Time (ms)

Arcvoltage(V)

6 Th e constant per form ance of the Tri plex tor ch is

al so shown by th e shape of th e pl asma j et (above);

i t shows a far m ore constant and stable form than the

F4 pl asma j et.

7 Whereas the perf ormance of convent ional torch systems fl uctua tes

ma rk edl y (above), the var iat ions in t he perf orm ance of the th r ee Tri plex

ar cs ar e no more th an m in im um (below). Th is also has a very positi ve

effect on t he noi se em i ssi ons.