Note: Descriptions are shown in the official language in which they were submitted.
2~32~
PROCESS FOR APPLYING CERAMIC MATERIAL
The invention relates to a process for applying a
coating of ceramic material to a substrate by plasma
spraying wherein the material is added to the plasma jet
and includes a chemical compound, one constituent of
which is a non-metallic element from the group N, C, B
or from the main groups VI or VII of tne Periodic Table
which decomposes at least partially before reaching the
melting point and is present in the solid phase in the
applied state.
Such a process is known from DE-OS 30 24 611. Herein,
iron spinel and cobalt spinel are applied by plasma
spraying at low torch power. In view of the low torch
power, it is to be assumed that only the iron spinel
will melt as it has a low melting point, while the co-
balt spinel will only be embedded in the molten iron
spinel. Furthermore, decomposition products of the co-
balt spinel will presumably be present in spite of the
low torch power.
With plasma spraying at low torch power, the bonding of
the applied coating to the substrate is not optimum and
the internal solidity of the outer coating is also li-
mited. Furthermore, the coating yield is also very low
for when the material to be sprayed is hurled in the
still solid state by the plasma jet against the sub-
strate, an impact reflection occurs at the substrate and
hence only a small part of the material to be sprayed
remains adhering to the substrate.
2832~7~
The object underlying the invention is, thereforer to so
improve a process of the generic kind that the chemical
compound contained in the material can be applied stoi-
chiometrically, i.e., undecomposed to the substrate and
forms an impermeable, adherent and stable coating.
This object is accomplished in accordance with the in-
vention in a process of the kind described at the be-
ginning in that in addition to the material to be spray-
ed, the non-metallic element is carried along in the
plasma jet in a free form not bound to a foreign element.
In this way, by addition of tha element, the reaction
balance of the compound tending to decompose is shifted
to the undecomposed compound and so this compound can be
heated to sufficiently high tempertures in the plasma
jet and hence form on contacting the substrate an imper-
meable, adherent and stable coating which, in addition,
contains the chemical compound essentially in unde-
composed form.
It has proven particularly advantageous within the scope
of the inventive process for a laminar jet generated by
a plasma torch with a noz~le generating a laminar jet to
be used as plasma jet as this laminar jet offers the
best preconditions for the material to be sprayed to be
heated constantly and uniformly and for an ade~uate time
in order to achieve as uniform a coating as possibls on
the substrate.
;
It is particularly expedient for the length of the la-
2 5~ 7 ~
minar jet to be at least 60 mm from the nozzle on. Even
better results are obtained with 80 mm and very good re-
sults with a laminar jet length of 100 mm. Optimum results
are to be achieved with a laminar jet length of 150 mm.
It has, furthermore, proven particularly expedient
within the scope of the inventive solution for the
latter to be carried out in vacuum as problems other-
wise arise from impurities or interactions owing to the
atmosphere surrounding the plasma jet.
It is preferable to use as plasma je-t that of a d.c.
plasma torch with which a plasma jet having as constant
a temperature as possible and as constant a flow profile
as possibla can be generated.
To shift the reaction balance in the direction towards
the undecomposed chemical compound, it is particularly
advantàgeous for the non-metallic element to be carried
along in the plasma jet in dissocia-ted or atomic form.
In the examples of the inventive process described so
far, no details were given as to how the non-metallic
element is conducted in the plasma jet. It is expedi-
ent for the non-metallic element to be conducted, after
its introduction, in the core region near the axis of
the plasma jet so there is as intensive an interaction
as possible with the plasma jet surrounding it but, at
th~ same time, also good shielding by the plasma jet
surrounding it.
~32~72
The reaction balance can be shifted particularly well in
an inven-tive way by the non-me-tallic element being car-
ried along in ionized form in the plasma jet.
In principle, within the scope of the inventive process,
the non-metallic element could be added to the plasma
jet, for example, with the plasma gas flow. However,
this would require electrode materials which are not
affected by the non-metallic element. For this reason,
it is more advantageous for the non-metallic element to
be added to a primary plasma jet downstream from the
high-current arc.
- ~ -However, in order to heat the non-metallic element as
strongly as possible-and hence carry it along at least
in dissociated, if not in ionized form in the plasma
jet, it has proven expedient for the non-metallic ele-
ment to be added to the plasma jet on the side thereof
facing the high-current arc and near the latter.
The dissociation or ionization of the non-metallic ele-
ment in the~plasma jet is particularly easily achieved
and maintained by it being brought about by interaction
with the primary plasma jet.
Introduction of the non-metallic element into the plasma
jet can be achieved in a structurally simple way by the
non-metallic element being added to the plasma jet in
the nozzle of the torch generating the laminar jet.
2~32 i ~2
An ernbodiment of the inventive process has proven parti-
cularly suitable wherein the non-metallic element is
added to the plasma jet in a I,aval nozzle sarving as
noz21e for generating the laminar jet.
In a particularly simple embodiment, provision is made
for the non-metallic element to be added to the plasma
jet together with the material to be sprayed so no ad-
ditional apparatus is necessary for adding the non-me-
tallic element to the plasma jet and the apparatus nor-
mally suitable for the plasma spraying can be used.
Within the scope of the inventive solution, an embodi-
ment is particularly preferred wherein the non-metallic
element is introduced into the plasma jet in gaseous
form as good, uniform distribution and easy dissoci-
ation or ionization of the non-metallic element are
thereby possible.
~- 'Hence-it isj for example, expedient for the non-metallic
element to be introduced into the plasma jet by a gas
which liberates it.
Since the material serving as material to be sprayed is
usually present in solid form and hence has to be added
to the plasma jet by a conveying medium, provision is
made in a preferred solution for the non-metallic ele-
ment to be included in a conveying medium for the ma-
terial to be sprayed.
2~32~
Herein, it has proven expedient for the conveying medium
for the material to be sprayed to be gaseous.
Within the scope of the embodiments described hereinabove,
provision is expediently also made for the material to
be sprayed to be present in powder form.
Within the scope of the embodiments described so far, no
details were given as to how the material to be sprayed
is conducted in the plasma jet. It is particularly ad-
vantageous for the material to be sprayed to be condnc-
ted, after its introduction, in the core region near the
axis of the plasma jet so it is, on the one hand, heated
as-uniformly.as.possible::by.the plasma jet surrounding
it and, on the other hand, is also protected.
Similarly,.no.details were given as to how the material
to be sprayed is introduced into the plasma jet. It has
likewise proven expedient for the material to be sprayed
to be added to a primary plasma jet downstream from the
high-current arc.
This is preferably carried out by t.he material to be
sprayed being added to the primary plasma jet on its
side facing the high-current arc and near the latter.
In a particularly preferred embodiment of the inven-
tive process, provision is made for the material to be
sprayed to be added to the plasma jet in the nozzle of
the torch for generating the laminar jet.
2~32~7~
Particularly good results are obtained when the material
to be sprayed is addod to the plasma jet in a Laval
nozzle serving as nozzle for generating the laminar jet.
Since relatively high temperatures are usually present in
the plasma jet, undesired reactions of the non-metallic
carried along in it can also occur very easily therein.
For this reason, provision is expediently made fo~ the
plasma jet to be essentially free from chemical elements
which react with the non-metallic element to form stable
chemical compounds. In particular, it is expedient for
the plasma jet to be substantially free of hydrogen.
Regarding the type of. the plasma.gas, provision is expe-
~ .-.. .:- -diently made for.th.is.to include an inert gas.
In a preferred embodiment, provision is made for the
primary plasma gas to include argon, with a major con-
stituent thereof preferably consisting of argon.
In addition, provision is expediently made for the primary
plasma:gas to include-in-addition to the argon addition-
al gases which increase enthalpy and toughness, with
these serving to provide the necessary energy, on the
one hand, for heating the material to be sprayed and, as
the case may be, for the dissociation or ionization of
the non-metallic element.
Advantageous values for the enthalpy of the primary
plasma are enthalpies of ~ 20 kJ/kg at 10 000 degrees C,
2~32~'2
it is even better for the enthalpy to be > 30 kJ/kg at
10 000 degrees and optimum values are obtained when the
enthalpy is ~ 40 kJ/kg at 10 000 degrees C.
A further preferred inert gas is helium.
In a preferred embodiment of the present invention, pro-
vision is made for helium as additional gas increasing
the free enthalpy to be added to the argon as main
plasma gas.
Another advantageous possibility is to add nitrogen in-
stead of helium as additional gas increasing the free
enthalpy to the argon as main plasma gas.
,
- -- -To ensure-tha-t the non-metallic element is present in
a sufficiently high concentration to carry out the in-
ventive process, provision is preferably made for the
- gaseous, non-metallic element to be carried along in the
plasma jet in a proportion of more than 5 ~ of the gases
included therein~
.
It is even better for this proportion to be more than
15 % of the gases included in the plasma jet~
To ensure that the non-metallic element is present in
the pIasma jet in dissociated form, provision is made
for the plasma jet to have an enthalpy and temperature
bringing about dissociation of the non metallic element.
21 ~ 3 2 .3L 71 f'~
If the non-metallic element is also to be present in
i.onized form, it is even better for the plasma jet to
have a temperature and free enthalpy bringing about
ionization of the non-metallic element.
If a sufficiently high temperature of the plasma jet
cannot be achieved with the plasma torch itself, it is
advantageous for heating to be provided in addition for
the plasma jet following the high-current arc. In par-
ticular, it is expedient for the additional heating to
be carried out by high-frequency coupling into the plasma
jet, with this being inductive or capacitive coupling-
in.
So far, no further details were given as to the compo-
s.ition of.th~e.chemica~l..compound. A chemical-compound in-
cluding a metal as fu.rther chemical'element has proven
particular'ly suitable for use of the inventive process.
Preferred materials are oxidized materials, for example,
spinels and perovskites-on a-nickel or cobalt or nickel-
cobalt basis. It is, however, also conceivable to apply
all..-.~possibl~e...kinds..of spine-ls.and perovskites in accor-
dance~wi'th:the'inventive process. This aIso applies'to
spinel-like and perovskite-like compounds and to non-
oxidized compounds, for example, nitrides, halides,
carbides, etc., with nitrogen or halogens or also non-
metallic compounds, methane or acetylene then being
carried along as non-metallic.element by the plasma jet.
All-these-.compounds are preferably characterized by
~3~7~
the chemical compound exhibiting in the region of its
melting temperature a free enthalpy of formation lying
in the region of zero or above, i.e., it is a chemical
compound which has an increasing tendency to deco~pose
as the temperature rises.
Sufficient and uniform heating of the chemical compound
which is requird for good formation of the coating on
the substrate is advantageously achieved by tha chemi-
cal.compound interacting with the laminar plasma jet
within the latter over a length of at least 60 mm. Even
better values are obtained when the length of the in-
teraction is at least 80 mm, very good values when the
length~of..the interaction is at least 100 mm and opti-
mum values when the.length.of the interaction is at
least 150 mm.
To obtain a.coating-which adheres well on the substrate,
it is necessary, as mentioned at the beginning, to heat
the material to be sprayed to as high a temperature as
possible, for example, above the melting point, but
there must still not:-be.any noticeable vapori~ation of
the material. For this-reason, it is adv.antageous for
the chemical compound to be heated to at least approxi-
mately 500 degrees C in the plasma jet. It is even
better for the chemical compound to be heated to at
least l,000 degrees C or even better to at least 1,500
degrees C or best''of:all to at least 2,000 degrees C.
.,
~ - The be'st adhering properties of the coatings are ach-
2~32;~ ~
11
ieved when -the chemical compound is hea-ted in the plasma
jet to at least a temperature in the region of its
melting point.
The best results are obtained when the chemical com-
pound is heated to at least a temperature corresponding
to its melting point.
Particularly good results are obtained with the inven-
tive process when the chemical compound is moderately
heated in the plasma jet, this meaning that the chemi-
cal compound is heated to a surface temperature of be-
tween O and 1000 degrees C, preferably O and 500 de--
grees C above its melting point, i.e., there is no
strong heating-beyond the melting point.
.
The:inventive process is used particularly when the
material serves as catalytically active coating.
In a further embodiment, the material serves as elec-
trocatalytically active coating.
, - . . : . - :
In other preferred embodiments within the scope of the '
inventive process, the material serves as tribolo~ically
effective coating.
In a further embodiment, the material serves as super-
conductive coating.
''
. -Particularly good~results are achieved within the scope
2~37,~
of the present invention when the plasma sprayiny is
carried out with an ultrasonic jet as coatings which
adhere very firmly to the substrate are thereby obtained.
Further features and advantages of the present invention
are the subject matter of the following description in
conjunction with the drawings. The drawings show:
~igure 1 a schematic illustration of an apparatus for
carrying out-the inventive process; and
~igure 2 an X-ray diffractogram wherein
~igure 2a --shows the-X-r-ay diffractogram of cobalt
spine'l powder produced at ~00 degrees;
~igure 2b?: the-X-ray-;diffractogram of an applied coat-
ing of cobalt spinel with argon as plasma
-gas-and argon as-carrier gas for the material
to be sprayed;
~Figure._2c_.: the.X-ra-y diffractogram of an applied coat-
ing o cobalt spinsl with argon and hydrogen
as plasma gas and argon as carrier gas for
the material to be sprayed; and
~igure 2d:.the:.X-r.ay.dif.fractogram of an applied coat-
. :ing with argon plus-helium as plasma gas and
argon..and::-oxygen-as carrier gas for the ma-
terial to be sprayed.
~2~i'2
~n apparatus for performing the inventive process com-
prises - as illustrated scheTnatically in Figure 1 - a
vacuum chamber 10 which is evacuatable to a preselec-
table pressure by a vacuum pumping system 12. There is,
furthermore, arranged in this vacuum chamber 10 a plasma
torch 14 whicb generates a plasma jet 16 which strikes a
substrate 18 likewise arranged in the vacuum chamber 10
and, for its part, moveable by a moving device 20 rela-
tive to the plasma jet 16 in a plane extending perpen-
dicular to a longitudinal axis 22 of the plasma jet 16.
A jet 24 of material to be sprayed consisting of parti-
cles of a material to be applied is carried along in
this plasma jet 16 and on striking the substrate 18,
produces a coating 26 of this material.
The plasma torch 14 operates as d.c. torch and, f~r its
;'--?-- part, comprises-a tube-like housing 28 in which a
sleave~shaped anode-34 comprising a gas channel 32
tapering conically towards an end 30 of the housing 18
facing the substrate 18 is arranged. A pin-shaped
cathode 36 protrudes f:rom a rear side opposite the end
30.into-the gas-channel 32, with there remaining between
--~ -thè anode 34 and the cathode 36 a ring gap 38 through
which a plasrna gas flow 40 can enter the gas channel 32.
The plasma gas flow 40 is supplied to this ring gap 38
via a ring space 42 between the cathode 36 and the~ - housing 28. The gas is supplied to this ring space 42
in a manner known-per se via a plasma torch supplying
-device designated in its-entirety 44 which also provides
the necessary d.c. voltage between the anode 34 and the
~32~
14
1.
cathode 36 and, in addition, supplies a cooling channel
46 in the cathode 36 with coolant.
The gas channel 32 of the anode continues towards the
substrate 18 in a nozzle channel 48 of a Laval nozzle 50
which immediately adjoins the anode 34 and from which,
with the proper choice of parameters, the substantially
parallsl, laminar plasma jet 16 exits. The Lavel nozzle
50 and the gas channel 32 are arranged coaxially with
the longitudinal.axis 22 of the plasma jet 16.
,
There is at least one first inlet channel 52 opening
into the nozzle channel 48 of the Laval nozzle 50 for
supplying--t~e material--to be--sprayed forming the jet 24
of matarial to be sprayed. The inlet channel 52 is
supplied via a-first--f-eed--means 54. It is, however,
additi-onally possible to::provide on the~.side of the - - :- -:--
first inlet channel 52 facing the substrate 18 a second
inlet channel 56 which is supplied via a second feed
means 58.
The plasma spraying with-a- p-l-asma torch 14 in the vacuum
chamber 10 is descri~ed in detail in DE-OS 35 38 390.
Furthermore, a detailed illustration of the function and
mode of operation of the plasma torch is to be found in
the article by:W. ~layr and R. ~enne "Investigation of a
VPS~iburner-with.laval nozzle.by means of an automated
-laser doppler measuring equipment" Proc. 1st Plasma-
Technology Symposium, Lucerne, 1988.
.
2~2~7~
The inventive process will be described in detail here-
inbelow with reference to cobalt spinel (Co304). This
cobalt spinel is applied as coating to a substrate. The
cobalt spinel may, for example, be supplied via the
first feed means 54 and the first inlet channel 52. The
cobalt spinel is present in powder form and is conveyed
by a carrier gas from the first feed means 54 to the first
inlet channel 52 and from there into the Laval nozzle.
gas mixture consisting of 80 ~ ~2 and 20 % Ar is prefer-
ably used as carrier gas for the cobalt spinel in powder
form. This oxygen (~2) constitutes the non-metallic ele-
ment in free form, not bound to a foreign element, which
is carried along in addition to the material to be'
sprayed in the plasma ~et 16.
The torch is operated with a primary plasma gas flow
which preferably includeszargon-as main gas. To increase~ ~ -
the enthaIpy,-helium can be added thereto. It is also
possible to add nitrogen to increase the enthalpy.
The plasma torch 14 is preferably operated such that
-a-longj--laminar plasma jet-extending parallel to the
longitudinal axis 22 is formed with a length of at least
150 mm. In vacuum, its speed may be from 2,000 to 3,000
meters per second. In addition, the material to be
sprayed, i.e. the cobalt spinel is to be supplied such
that an axially close-jet 24-of material to be sprayed~-- ~-- ~-~extending near the-longitudinal axis 22 is likewise
formed-in -the core region of the-plasma jet, with speeds
of the material to be sprayed of up to approximately
2~'~2~72
16
1,000 m/sec then prevailing and this jet 24 of material
to be sprayed bei.ng protected by the part of the plasma
jet 16 surrounding it. The time spent by the material to
be sprayed in the plasma jet 16 between its injection
into the Laval no~zle 50 and its striking the substra,e
18 is then less than 10 seconds, with interaction with
the plasma jet occurring over a length of more than 150
mm.
During this short.time spent by the material to be
sprayed in the plasma jet 16, the material to be sprayed
is heated up in accordance with the invention to the
region of the melting temperature, with melting of the
~ -- .. .... .material.to.be sprayed preferably taking place so it is
- present in the molten state during the time spent in the
~ plasma jet 16. It is.. expedient for the material to be
- ~s .~ s~~ sp-rayed-to----be--heated.up.~:onl~,m-oderately to a surface tem- -
perature in the range of from O to 1,000 degrees above
it melting point.
Owing to the short time spent by the material to be
sprayed-in-the-plasma jet, the time available to the
material to be sprayed for decomposition is kept very
short. In addition, the oxygen carried along by the
plasma jet in the inventive manner during the plasma
- - spraying of cobalt spinel counteracts the decomposition
::s - . ~ of. the:cobalt.spinel as.it shifts the dissociation or
-:-: ..: -..-- -decompasition-balance.toward:s:the undecomposed cobalt
~ spinel.
2~2
17
The power of the plasma -torch 14 is preferably such that
che plasma in the plasma je-t is sufficiently hot arl~
rich in enthalpy to dissociate and ionize the oxygen
supplied downstream from the plasma jet 16 and hence
especially to shift the reaction balance of the cobalt
spinel towards the oxide, i.e., towards the undecomposed
cobalt spinel or to carry out a reoxidation of any
oxides which may have become unstoichiometric. It is
preferable to work with an enthalpy of the plasma of
more:than 40 kJ/.kg at 10,000 degrees C..
In the inventive process, in particular to increase the
enthalpy of the plasma torch 14, there must be no ad-
-d:ition of-hydrogen-to the-plasma gas flow as this would
rsact~with-~lie ox-yge.n..supplied via the carrier gas to
form water.
The advantages of the inventive process become particu-
larly clear from a comparison of a coating 26 applied by
the inventive process with processes in which no oxygen
is carried along in the plasma jet 16. Figure 2d shows
an X-ray di-f~r-actogram--of coh.alt spinel applied by the
inventive process wherein it is clearly evident that
essentially the reflexes of cobalt spinel (characterized
by ~ ) prevail, whereas in the processes in which no
oxygen was carried along in the plasma jet 16 (Figures
2b and:.2c)-clearLy-:-the reflexes of CoO (characterized
by ~ ) and Co (.characterized-by-.V ) predomina-te over the
reflexas-of:cobalt:spinel~ ),.-which clearly proves that
the inv.ent:ive process~prevents the decomposition of co-
2~3~7 ~
18
balt spinel in the plasma spraying.
The cobalt spinel coating, the X-ray diffractogram of
which is shown in Figure 2d, was achieved with the fol-
lowing parameters: Power of the plasma torch 14, 30 kW,
pressure in the vacuum chamber 10, 50 mbar, plasma gas
consisting of argon and helium and carrier gas for the
cobalt spinel in powder form of 80 ~ ~2 and 20 % ~r. The
thickness of the coating was 200 ~um and exhibited a very
dense structure..and.was firmly bonded to nickel as sub-
strate. The preferred order of magnitude for the coating
growth is 10 ~m/sec in relation to a coating area of 10
cm and so by subjecting the substrate to controllable,
' ''~ ~. '.~th'ermal 'action,'~.t'he des'ired~coating~can be applied in
one work cycle without, for example, subsequent treat-
ment being necessary.
In a modification o~ the embodiment of the inventive
process described hereinabove, it is, however, also
possible to add the matarial to be sprayed to the plasma
jet.16..via~.the second.feed means 58 and the second in-
let channel 56,:with only argon serving as carrier gas
and the oxygen being added separately via the first feed
means 54 and the first inlet channel 52, i.e. on the
"hotter" side of the plasma jet as the oxygen requires
very high temperatures for the dissociation and ioni-
zation, while the temperatures for the material to be~~ . .. ...:.-. ~-sprayed can be..chosen lower,.particularly when this is
not cobalt spinel-but a-material with a lower melting
temperature.
2~2~
19
~s an alternative to this, it is li~ewise possible to
supply two dif~erent materials via the first feed means
5~ and via the second feed means 58, with the non-me-
tallic element being supplied in the carrier gas of
either the one or the other or a non-metallic element
suitable for the respective material in both.
In a further modification of the inventive process,
provision is made, in the event the plasma torch 14 does
not.generate su.fficient-temperatures and enthalpies, for
the plasma flow 16 to be heated up by an additional
heating 60 downstream from the Laval nozzle, with this
heating being, for example, in the form of a device for
co.upling..hi.gh..f.requenc.y.into the plasma jet 16 and this
bein.g carried-ou~Gap:acitively or inductively.-
.
-
