U.S. patent application number 11/581698 was filed with the patent office on 2007-04-19 for method of producing a tubular target.
This patent application is currently assigned to Plansee SE. Invention is credited to Peter Abenthung, Karl Huber, Harald Lackner, Gerhard Leichtfried, Peter Polcik, Christian Weratschnig.
Application Number | 20070086909 11/581698 |
Document ID | / |
Family ID | 36952522 |
Filed Date | 2007-04-19 |
United States Patent
Application |
20070086909 |
Kind Code |
A1 |
Abenthung; Peter ; et
al. |
April 19, 2007 |
Method of producing a tubular target
Abstract
A method for producing a tubular target formed of molybdenum or
a molybdenum alloy which has an oxygen content of less than 50
.mu.g/g, a density of greater than 99% of the theoretical density
and an average grain size of less than 100 .mu.m and which is
connected to a supporting tube. The molybdenum or molybdenum alloy
tube is produced by extrusion.
Inventors: |
Abenthung; Peter; (Reutte,
AT) ; Huber; Karl; (Reutte, AT) ; Lackner;
Harald; (Reutte, AT) ; Leichtfried; Gerhard;
(Reutte, AT) ; Polcik; Peter; (Reutte, AT)
; Weratschnig; Christian; (Reutte, AT) |
Correspondence
Address: |
LERNER GREENBERG STEMER LLP
P O BOX 2480
HOLLYWOOD
FL
33022-2480
US
|
Assignee: |
Plansee SE
|
Family ID: |
36952522 |
Appl. No.: |
11/581698 |
Filed: |
October 16, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/AT06/00406 |
Oct 5, 2006 |
|
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11581698 |
Oct 16, 2006 |
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Current U.S.
Class: |
419/29 |
Current CPC
Class: |
B22F 7/08 20130101; B22F
3/20 20130101; C23C 14/3414 20130101; B22F 2998/10 20130101; C22C
1/045 20130101; B22F 5/106 20130101; B22F 2998/10 20130101; B22F
3/04 20130101; B22F 3/1007 20130101; B22F 2003/208 20130101; B22F
2003/248 20130101; B22F 7/08 20130101; B22F 3/17 20130101 |
Class at
Publication: |
419/029 |
International
Class: |
B22F 5/12 20060101
B22F005/12 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 14, 2005 |
AT |
GM 699/2005 |
Claims
1. A method of producing a tubular target, the method which
comprises the following steps: providing a metal powder of Mo or an
Mo alloy with an average particle size according to Fisher of from
0.5 to 10 .mu.m; cold-isostatic pressing the metal powder in a
flexible mold using a core at a pressure p, where 100
MPa<p<500 MPa, for producing a green compact in the form of a
tube blank; producing a tube blank by sintering the green compact
at a temperature T, where 1600.degree. C.<T<2500.degree. C.,
in a reducing atmosphere or a vacuum; producing a tube by heating
the tube blank to a forming temperature T, where
DBTT<T<(T.sub.S minus 800.degree. C.) and extruding over a
mandrel; joining the tube to a supporting tube of non-magnetic
material and mechanically processing, to form a tubular target of a
tube of molybdenum or a molybdenum alloy with an oxygen content of
less than 50 .mu.g/g, a density of greater than 99% of the
theoretical density and an average grain size transversely to the
axial direction of less than 100 .mu.m, and the supporting
tube.
2. The method according to claim 1, which comprises machining the
sintered tube blank.
3. The method according to claim 1, which comprises fastening a
steel tube end piece with an outer diameter and an inner diameter
approximately equal to an outer diameter and and inner diameter,
respectively, of the tube blank to at least one end of the tube
blank.
4. The method according to claim 1, which comprises annealing the
extruded tube in the reducing atmosphere or a vacuum at an
annealing temperature T of 800.degree. C.<T<1600.degree.
C.
5. The method according to claim 1, wherein the supporting tube
consists of a copper alloy, austenitic steel, titanium, or a
titanium alloy.
6. The method according to claim 1, wherein the supporting tube
consists of Cu--Cr--Zr.
7. The method according to claim 1, which comprises connecting the
supporting tube by a joining process to the tube of molybdenum or
molybdenum alloy, to thereby cause a plastic deformation of the
supporting tube.
8. The method according to claim 7, which comprises connecting the
supporting tube to the tube of molybdenum or a molybdenum alloy by
a forming process.
9. The method according to claim 1, which comprises using glass
powder for lubrication during the extrusion.
10. The method according to claim 1, which comprises extruding with
a degree of forming of 40 to 80%.
11. The method according to claim 1, which comprises straightening
the extruded tube on a mandrel by a forging process.
12. The method according to claim 1, which comprises deforming the
extruded tube on a mandrel by a forging process in such a way that
a wall thickness differs over a length of the tube.
13. The method according to claim 12, which comprises deforming the
extruded tube in such a way that a wall thickness of the tube
increases towards the ends of the tube.
14. The method according to claim 1, wherein the molybdenum tube
consists of pure molybdenum with an metallic purity, exclusive of
tungsten, of greater than 99.99% by weight.
15. The method according to claim 1, wherein the molybdenum tube
consists of a molybdenum alloy containing 0.5 to 30% by weight of
at least one of V, Nb, Ta, Cr, and W.
16. A deposition process, which comprises forming a tubular target
according to the method of claim 1 and utilizing the tubular target
in the production of LCD-TFT flat screens.
17. A deposition process, which comprises forming a tubular target
according to the method of claim 1 and utilizing the tubular target
for glass coating.
18. A method of producing a tubular target, which comprises the
following method steps: providing a metal powder of Mo or an Mo
alloy with an average particle size according to Fisher of from 0.5
to 10 .mu.m; cold-isostatic pressing the metal powder in a flexible
mold using a core at a pressure p, where 100 MPa<p<500 MPa,
for producing a green compact in the form of a tube blank;
sintering the green compact at a temperature T, where 1600.degree.
C.<T<2500.degree. C., in a reducing atmosphere or a vacuum
for producing a tube blank; working the tube blank and joining at
least one steel tube end piece for fixing a supporting tube of
austenitic steel blank inside the tube blank; producing a composite
tube by heating to a forming temperature T, where
900<T<1350.degree. C. and co-extruding over a mandrel; and
machining the composite tube for forming a tubular target with a
tube of molybdenum or a molybdenum alloy with an oxygen content of
less than 50 .mu.g/g, a density of greater than 99% of theoretical
density, and an average grain size transversely to an axial
direction of less than 100 .mu.m, and the supporting tube.
19. The method according to claim 18, which comprises annealing the
composite tube in a reducing atmosphere or a vacuum at an annealing
temperature T of 800.degree. C.<T<1300.degree. C.
20. The method according to claim 18, which comprises forming the
assembly with a gap of 0.2 to 1 mm between the tube blank and the
supporting tube blank.
21. The method according to claim 18, which comprises forming the
assembly with gap of from 3 mm to 20 mm between the tube blank and
the supporting tube blank and filling the gap with steel
powder.
22. The method according to claim 18, which comprises using glass
powder for lubrication during the extrusion.
23. The method according to claim 18, which comprises extruding
with a degree of forming of 40 to 80%.
24. The method according to claim 18, which comprises straightening
the composite tube on a mandrel by a forging process.
25. The method according to claim 18, which comprises deforming the
composite tube on a mandrel by a forging process in such a way that
a wall thickness differs over a length of the tube.
26. The method according to claim 25, which comprises deforming the
composite tube in such a way that the a wall thickness of the tube
increases towards the ends of the tube.
27. The method according to claim 18, wherein the molybdenum tube
consists of pure molybdenum with an metallic purity, exclusive of
tungsten, of greater than 99.99% by weight.
28. The method according to claim 18, wherein the molybdenum tube
consists of a molybdenum alloy containing 0.5 to 30% by weight of
at least one of V, Nb, Ta, Cr, and W.
29. A deposition process, which comprises forming a tubular target
according to the method of claim 18 and utilizing the tubular
target in the production of LCD-TFT flat screens.
30. A deposition process, which comprises forming a tubular target
according to the method of claim 18 and utilizing the tubular
target for glass coating.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This is a continuing application, under 35 U.S.C. .sctn.120,
of copending international application No. PCT/AT2006/000406, filed
Oct. 5, 2006, which designated the United States; this application
also claims the priority, under 35 U.S.C. .sctn.119, of Austrian
patent application No. GM 699/2005, filed Oct. 14, 2005; the prior
applications are herewith incorporated by reference in their
entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] The invention relates to a method for producing a tubular
target, which comprises a tube of molybdenum or a molybdenum alloy
with an oxygen content of less than 50 .mu.g/g, a density of
greater than 99% of the theoretical density, and an average grain
size transversely to the axial direction of less than 100 .mu.m as
well as a supporting tube of a non-magnetic material.
[0003] A target is understood as meaning the material to be
sputtered of a cathode atomization system. Rotating tubular targets
are known and described for example in U.S. Pat. Nos. 4,422,916 and
4,356,073. During the sputtering, the tubular target rotates about
a magnetron located in the tube. Tubular targets are predominantly
used for producing coatings over a large area. The rotation of the
tubular target achieves the effect of uniform erosion of the
sputtering material. Tubular targets therefore have a high
utilization rate of the target material and a long target lifetime,
which is of significance in particular in the case of expensive
coating materials, as is the case with molybdenum. The utilization
rate for planar targets is around 15 to 40% and for tubular targets
around 75 to 90%.
[0004] The target cooling performed in the space inside the tubular
target is much more effective than in the case of planar targets as
a result of the more favourable heat transfer in the tube, which
makes a higher coating rate possible. In order to ensure that no
cooling water flows out even with high target utilization, and also
to increase the mechanical load-bearing capacity and facilitate
fixing in the sputtering system, the tubular target is usually
connected to a supporting tube. The supporting tube must in this
case be of a non-magnetic material, in order not to interact with
the magnetic field which determines the erosion region.
[0005] As mentioned, the use of tubular targets is advantageous
whenever substrates of a large area are to be coated. In the case
of molybdenum as the target material, this is the case for example
in LCD-TFT production and glass coating.
[0006] A multitude of production methods are described for the
production of tubular targets. Many of those methods take a liquid
phase route, such as for example continuous and centrifugal
casting. The latter is described in German published patent
application DE 199 53 470 and its counterpart U.S. Pat. No.
6,793,784 B1. On account of the higher melting point of molybdenum
and the resultant problems with finding a suitable mold material,
these ways of carrying out production cannot be used for molybdenum
and its alloys.
[0007] Tubular targets may also be produced by winding a thick
strip around a core and welding the contact regions. However, the
weld seam has a much more coarse microstructure and pores, which
leads to non-uniform erosion and, as a consequence, different layer
thicknesses. Moreover, in the case of molybdenum, the welded region
is extremely brittle and consequently at risk of cracking.
[0008] A further tubular target is known from U.S. Pat. No.
4,356,073. Production takes place in this case by the sputtering
material being deposited on a backing tube by plasma spraying. Even
using the vacuum plasma spraying technique, however, completely
dense tubular targets cannot be produced with an adequately low gas
content. Electrochemical deposition, as is used for Cr and Sn, is
also not suitable for molybdenum and its alloys.
[0009] U.S. Pat. No. 5,435,965 and European patent publication EP 0
500 031 A1 describe the production of a tubular target by
hot-isostatic pressing. There, a backing tube is positioned in a
can, so as to produce between the backing tube and the mold an
intermediate space into which powder of the target material is
filled. After closing the can, it is subjected to a hot-isostatic
densification operation. The amount of powder used in relation to
the weight of the finished tubular target is in this case
unfavourably high.
[0010] U.S. Pat. No. 6,878,250 B2 and U.S. Pat. No. 6,946,039 B2
describe the use of ECAP (equal channel angular pressing) for the
production of sputtering targets. In the case of molybdenum alloys
with comparatively high k.sub.f values, this leads to a high level
of tool wear.
SUMMARY OF THE INVENTION
[0011] It is accordingly an object of the invention to provide a
method of producing a sputtering target which overcomes the
above-mentioned disadvantages of the heretofore-known devices and
methods of this general type and which on the one hand is
inexpensive and on the other hand produces a product which is
uniformly eroded in the sputtering process, does not tend to give a
locally increased sputtering rate and does not lead to any
contamination of the substrate or the deposited layer.
[0012] With the foregoing and other objects in view there is
provided, in accordance with the invention, a method of producing a
tubular target, the method which comprises the following steps:
[0013] providing a metal powder of Mo or an Mo alloy with an
average particle size according to Fisher of from 0.5 to 10 .mu.m;
[0014] cold-isostatic pressing the metal powder in a flexible mold
using a core at a pressure p, where 100 MPa<p<500 MPa, for
producing a green compact in the form of a tube blank; [0015]
producing a tube blank by sintering the green compact at a
temperature T, where 1600.degree. C.<T<2500.degree. C., in a
reducing atmosphere or a vacuum; [0016] producing a tube by heating
the tube blank to a forming temperature T, where
DBTT<T<(T.sub.S minus 800.degree. C.) and extruding over a
mandrel; [0017] joining the tube to a supporting tube of
non-magnetic material and mechanically processing, to form a
tubular target of a tube of molybdenum or a molybdenum alloy with
an oxygen content of less than 50 .mu.g/g, a density of greater
than 99% of the theoretical density and an average grain size
transversely to the axial direction of less than 100 .mu.m, and the
supporting tube.
[0018] With the above and other objects in view there is also
provided, in accordance with the invention, a method of producing a
tubular target which comprises the following method steps: [0019]
providing a metal powder of Mo or an Mo alloy with an average
particle size according to Fisher of from 0.5 to 10 .mu.m; [0020]
cold-isostatic pressing the metal powder in a flexible mold using a
core at a pressure p, where 100 MPa<p<500 MPa, for producing
a green compact in the form of a tube blank; [0021] sintering the
green compact at a temperature T, where 1600.degree.
C.<T<2500.degree. C., in a reducing atmosphere or a vacuum
for producing a tube blank; [0022] working the tube blank and
joining at least one steel tube end piece for fixing a supporting
tube of austenitic steel blank inside the tube blank; [0023]
producing a composite tube by heating to a forming temperature T,
where 900<T<1350.degree. C. and co-extruding over a mandrel;
and [0024] machining the composite tube for forming a tubular
target with a tube of molybdenum or a molybdenum alloy with an
oxygen content of less than 50 .mu.g/g, a density of greater than
99% of theoretical density, and an average grain size transversely
to an axial direction of less than 100 .mu.m, and the supporting
tube.
[0025] In order to achieve an adequate fine grained structure,
sintering activity, and consequently density, a metal powder with a
particle size according to Fisher of 0.5 to 10 .mu.m is used. For
the production of pure Mo tubular targets, Mo powder with a metal
purity of greater than 99.9% by weight is advantageously used. If a
tubular target is produced from an Mo alloy, either powder mixtures
or prealloyed powders are used, the particle size however likewise
lying in the range from 0.5 to 10 .mu.m. The powder is filled into
a flexible mold, in which the core is already positioned. The core
determines the inner diameter of the tube blank, with allowance for
the compaction during the pressing operation and the sintering
shrinkage. Customary tool steels are suitable as the material for
the core. After filling the flexible mold with the metal powder and
liquid-tight closing of the flexible mold, it is positioned in a
pressure vessel of a cold-isostatic press. The compaction takes
place at pressures between 100 and 500 MPa. After that, the green
compact is taken out of the flexible mold and the core is removed.
Following that, the green compact is sintered at a temperature in
the range from 1600.degree. C. to 2500.degree. C. in a reducing
atmosphere or a vacuum. Below 1600.degree. C., adequate
densification is not achieved. Above 2500.degree. C., undesired
grain coarsening begins. The sintering temperature to be chosen
depends on the particle size of the powder. Green compacts produced
from a powder with a particle size of 0.5 .mu.m according to Fisher
can be sintered at a sintering temperature of as low as
1600.degree. C. to a density of greater than 95% of the theoretical
density, whereas for green compacts which are produced from a
powder with a particle size of 10 .mu.m according to Fisher a
sintering temperature of approximately 2500.degree. C. is required.
If the dimensional accuracy of the pressing process is not
adequate, which is usually the case, the sintered blank is
machined. The outer diameter of the sintered blank is in this case
determined by the inner diameter of the container of the extrusion
press. To make it possible for the extruded blank to be positioned
unproblematically in the container of the extrusion press, the
outer diameter of the sintered blank is somewhat smaller than the
inner diameter of the container. The inner diameter is in turn
determined by the diameter of the mandrel.
[0026] In order to reduce the discharge loss of molybdenum during
the extrusion, it is advantageous to mechanically fasten a steel
end piece to one end of the molybdenum tube blank. This mechanical
fastening may be performed for example by means of a screwed or
bolted connection. The outer diameter and the inner diameter of the
steel end piece of the tube blank in this case correspond to the
outer diameter and the inner diameter of the molybdenum tube
blank.
[0027] For the extrusion, the tube blank is heated to a temperature
T, where DBTT<T<(T.sub.S-800.degree. C.). DBTT is to be
understood here as meaning the ductile brittle transition
temperature. At lower temperatures, cracking increasingly occurs.
The upper temperature element is given by the melting temperature
(T.sub.S) of the molybdenum alloy less 800.degree. C. This ensures
that no undesired grain coarsening takes place during the extruding
operation. The initial heating may in this case be performed in a
conventional gas or electrically heated furnace (for example a
rotary hearth kiln), allowance having to be made that the gas flow
control is chosen such that the lambda value is neutral or
negative. In order to obtain higher extrusion temperatures,
inductive reheating may be performed. After the initial heating
operation, the tube blank is rolled in a glass powder mixture.
After that, the tube blank is positioned in the container of the
extrusion press and pressed over a mandrel through a die to the
respective outer or inner diameter.
[0028] It is advantageous if the extruded tube is subjected to a
recovering or recrystallizing annealing process in a reducing
atmosphere or a vacuum at a temperature T of 700.degree.
C.<T<1600.degree. C. If the temperature goes below the lower
limit, the stress reduction is too little. At a temperature greater
than 1600.degree. C., grain coarsening occurs. The extruded tube is
machined on the outer side of the tube, the end faces and
advantageously the inner side of the tube.
[0029] The molybdenum tube produced in this way is connected to a
supporting tube of a non-magnetic material. The outer diameter of
the supporting tube corresponds approximately to the inner diameter
of the molybdenum tube. Furthermore, the supporting tube reaches
beyond the respective ends of the molybdenum tube. Copper alloys,
austenitic steels, titanium or titanium alloys are to be mentioned
as particularly suitable materials for the supporting tube.
[0030] Suitable as connecting methods are both those methods which
lead to a material bond and those which lead to a form-fitting
connection. One condition, however, is that the contact area
between the molybdenum tube and the supporting tube be at least 30%
of the theoretically possible area. If the area is smaller, the
heat removal is hindered too much. Also the low coefficient of
thermal expansion of molybdenum has to be considered. Therefore,
the joining temperature must be chosen to be as low as possible.
If, for example, the connection between the molybdenum tube and the
supporting tube is performed by a forging process, in that the
supporting tube is positioned in the molybdenum tube and forged
over a mandrel, lowest possible forming temperatures of about
500.degree. C. to 800.degree. C. must be chosen. Furthermore, it is
advantageous if the material of the supporting tube has a low yield
strength, in order that stresses occurring due to plastic flow can
be reduced.
[0031] In a further method according to the invention, the
molybdenum tube blank is co-extruded with a blank of the supporting
tube. The production of the molybdenum tube is in this case again
based on a metal powder with an average particle size according to
Fisher of 0.5-10 .mu.m. The tube blank is once again produced by
cold-isostatic pressing of the metal powder in a flexible mold
using a core and sintering in the range from 1600.degree. C. to
2500.degree. C.
[0032] Following the sintering, the tube blank is machined. Inside
the tube blank, a supporting tube blank of an austenitic steel is
positioned. At one or both end pieces of the tube blank, a steel
tube end piece is joined on by a mechanical connection (for example
a screwed or bolted connection). The tube end piece has in this
case approximately the inner diameter and outer diameter of the
tube blank. The thickness of the tube end piece preferably lies in
the range from 10 to 100 mm. Fastened in turn to the tube end piece
is the supporting tube blank. This fastening preferably takes place
by a welded connection.
[0033] The outer diameter of the supporting tube blank may
correspond approximately to the inner diameter of the molybdenum
tube blank or else be chosen such that a defined gap is produced
between the molybdenum tube blank and the supporting tube blank. A
steel powder, preferably of austenitic steel, is filled into this
defined gap. The composite tube body produced in this way is heated
to a forming temperature of from 900.degree. C. to 1350.degree. C.
Only tubular targets of molybdenum alloys which can be
appropriately deformed at this temperature can be produced in this
way. A higher extrusion temperature cannot be chosen on account of
the steel.
[0034] The composite tube blank produced in this way is extruded
over a mandrel (co-extrusion), whereby a composite tube is
produced. Optionally, this may be followed by carrying out an
annealing process, the annealing temperature preferably lying
around 800.degree. C. to 1300.degree. C.
[0035] Use of a gap with a steel powder fill lying in between has
the effect of improving the bond between the supporting tube and
the molybdenum tube during the co-extrusion. A gap width of from 3
mm to 20 mm proves to be advantageous.
[0036] The use of glass powder as a lubricant achieves the effect
of an outstanding surface of the tubular target in the case of both
extrusion and co-extrusion, whereby the machining can be reduced to
a minimum. Furthermore, this ensures that the tubular target is
free from pores and also free from grain boundary cracks. The range
from 40 to 80% has been found to be an advantageous degree of
forming during the extrusion process. The degree of forming is in
this case determined as follows: ((initial cross section before
extrusion less cross section after extrusion)/initial cross
section).times.100.
[0037] After the extrusion/co-extrusion process, it may be
advantageous for the extruded tube to be straightened. This can be
performed by a forging process over a mandrel.
[0038] Furthermore, the wall thickness over the length of the
molybdenum tube or composite tube can also be varied by a
subsequent forging process. The wall thickness can in this case be
advantageously made thicker in the region of the tube ends. The
region of the tube ends is also the region of greatest erosion
during use.
[0039] The surface quality and the dimensional tolerances are set
by appropriate machining.
[0040] It is ensured by the method according to the invention that
the oxygen content in the molybdenum alloy is <50 .mu.g/g,
preferably less than 20 .mu.g/g, the density is greater than 99% of
the theoretical density, preferably greater than 99.8% of the
theoretical density, and the average grain size transversely to the
axial direction is less than 100 .mu.m, preferably less than 50
.mu.m. The average grain size is determined transversely to the
axial direction because in the case of an deformed,
non-recrystallized microstructure, the grains are stretched in the
axial direction and consequently an exact determination of the
grain size in the axial direction is made more difficult. With both
methods described, it is possible to produce molybdenum tubular
targets with a metallic purity of greater than 99.99% by weight.
Metallic purity is to be understood in this case as meaning the
purity of the molybdenum tubular target without gases (O, H, N) and
C. Tungsten is also not considered in this value, which is
uncritical for the application.
[0041] For the tubular target according to the invention to be used
in the area of TFT-LCD production, molybdenum alloys which contain
0.5 to 30% by weight of V, Nb, Ta, Cr and/or W are also
particularly suitable.
[0042] Other features which are considered as characteristic for
the invention are set forth in the appended claims.
[0043] Although the invention is illustrated and described herein
as embodied in tubular target, it is nevertheless not intended to
be limited to the details shown, since various modifications and
structural changes may be made therein without departing from the
spirit of the invention and within the scope and range of
equivalents of the claims.
[0044] The construction and method of operation of the invention,
however, together with additional objects and advantages thereof
will be best understood from the following description of specific
embodiments and the following specific example.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
EXAMPLE
[0045] MoO.sub.3 powder was reduced in a two-stage reduction
process at 600 and 1000.degree. C. to give Mo metal powder with a
grain size of 3.9 .mu.m. In a rubber tube closed at one end, with a
diameter of 420 mm, a steel mandrel with a diameter of 141 mm was
positioned in the centre. The molybdenum metal powder was filled
into the intermediate space between the steel core and the rubber
wall.
[0046] This was followed by closing the rubber tube at its open end
by means of a rubber cap. The closed rubber tube was positioned in
a cold-isostatic press, and compacted at a pressure of 210 MPa.
[0047] The green compact had a density of 64% of the theoretical
density. The outer diameter was approximately 300 mm. The green
compact produced in this way was sintered in an indirect sintering
furnace at a temperature of 1900.degree. C. The sintered density
was 94.9% of the theoretical density.
[0048] After the sintering operation, the tube blank was machined
on all sides, the outer diameter being 243 mm, the inner diameter
123 mm and the length 1060 mm. The extrusion took place on a 2500 t
indirect extrusion press. The tube blank was heated to a
temperature of 1100.degree. C. in a gas-heated rotary hearth kiln.
The lambda value was in this case set such that the atmosphere was
slightly reducing, whereby oxidation of the molybdenum was
prevented. After the initial heating in the rotary kiln, the
extruded blank was inductively heated to a temperature of
1250.degree. C. and rolled in a loose fill of glass powder, so that
glass powder adhered to the outside on all sides.
[0049] This was followed by pressing over a mandrel, whereby an
extruded tube with a length of 2700 mm, an outer diameter of 170 mm
and inner diameter of 129 mm was obtained.
[0050] A supporting tube of an austenitic steel with a wall
thickness of 6 mm was positioned in the extruded tube. This
assembly was straightened over a mandrel on a three-jaw forging
machine at a temperature of 500.degree. C. and slightly deformed,
whereby a bond between the molybdenum tube and the supporting tube
was produced.
* * * * *