Bangalore Plasmatek can, at present, deposit following coatings
on cutting tools and other components.
Aluminium, Titanium, Nickel, Copper, Stainless steel, Gold,
Silver, Special alloys, Titanium Nitride (TiN), Titanium
Carbide, Titanium Carbonitride (TiCN), Multilayer coatings,
RESEARCH & DEVELOPMENT
The coating system at Bangalore Plasmatek, which includes vacuum
arc and magnetron sources, was developed indigenously. The
processes for various coatings are also developed in house.
Bangalore Plasmatek enjoys taking up challenging research and
Vacuum brazing of ceramics and graphite to metals
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Vacuum brazing of ceramics and graphite to
Vacuum brazing of ceramics and
graphite to metals
Bangalore Plasmatek Pvt.Ltd,
129, Block –14, Jeevanmitra Colony I-Phase, Bangalore 560 078
This work was
carried out under a works contract from the Institute for Plasma
Research , Bhat, Gandhinagar, Gujarat.
JOINING OF METALS TO CERAMICS AND GRAPHITE IS IMPORTANT IN MANY APPLICATIONS
SUCH AS X-RAY TUBES, MICROWAVE DEVICES, NUCLEAR FUSION REACTORS AND SO ON.
SOME OF THE BASIC REQUIREMENTS OF SUCH JOINTS ARE ELECTRICAL INSULATION,
THERMAL CONDUCTION, VACUUM COMPATIBILITY, MECHANICAL STRENGTH ETC. SEVERAL
TECHNIQUES ARE IN USE AND NEW ONES ARE BEING DEVELOPED TO MEET STRINGENT
REQUIREMENTS. MOLY- MANGANESE METALLISATION IS PROVEN TECHNIQUE FOR BRAZING
CERAMICS TO METALS. RECENTLY THE SO-CALLED ACTIVE BRAZING ALLOYS HAVE BEEN
INTRODUCED IN ORDER TO REDUCE THE NUMBER OF STEPS INVOLVED IN BRAZING.
ANOTHER TECHNIQUE, WHICH IS USED IN THE PRESENT WORK, IS TO COAT CERAMIC
WITH TITANIUM AND THEN USE CONVENTIONAL BRAZING ALLOYS. TITANIUM IS
DEPOSITED ON CERAMICS USING A CATHODIC ARC PLASMA SOURCE IN VACUUM.
INDIGENOUSLY PREPARED COPPER SILVER EUTECTIC ALLOY FOILS ARE USED AS FILLER
MATERIAL. BRAZING IS CARRIED OUT UNDER HIGH VACUUM AT ABOUT 9000C. THESE
TECHNIQUES HAVE BEEN USED TO BRAZE STAINLESS STEEL-ALUMINA, TITANIUM-
ALUMINA, AS WELL AS COPPER-GRAPHITE. IN ALL THESE CASES MECHANICALLY STURDY
JOINTS HAVE BEEN OBTAINED. THESE TECHNIQUES CAN ALSO BE USED TO BRAZE METALS
TO METALS AND CERAMICS TO CERAMICS. SOME OF THE RESULTS OF THE WORK IN
PROGRESS WOULD BE PRESENTED.
Keyword: Vacuum; Brazing; Ceramics; Graphite.
Brazing is a technique of joining two materials using a
filler material whose melting point is below the melting
points of the materials to be brazed. A typical brazing
procedure involves cleaning of the surfaces to be joined,
interposing a filler material between the two surfaces,
holding the parts to be brazed with suitable fixtures and
heating to a temperature slightly above the melting point of
the filler material. Heating cycle should ensure that
thermal equilibrium is maintained between the filler
material and the components to be brazed. The components
react with atmospheric gases like nitrogen, oxygen, moisture
etc. When this is not acceptable brazing will have to be
carried out under a protective environments like inert gas,
hydrogen gas or vacuum. Brazing is possible only if the
molten filler material wets the surfaces and flows properly.
This depends on the properties of the filler material as
well as the surfaces to be brazed. Commercially a number of
filler materials are available in the form of wires, foils,
powders and pastes. Choice depends on the properties of the
materials to be brazed and the temperature and other
environmental factors in which the brazed components are to
Brazing of ceramics
Ceramics are widely used in industries due to their
electrical, thermal and mechanical properties. But joining
of ceramics to metals and to themselves is not straight
forward. There are basically two problems. First, the usual
brazing fillers do not wet the surfaces of ceramics. Second,
there is a big difference in the thermal expansion
coefficients of metals and ceramics. This induces tremendous
stresses in the brazing process which can lead to cracking.
Special techniques have been developed for brazing ceramics.
Moly-manganese metallisation is the standard practice for
brazing ceramics. Here a paint of the refractory metal
molybdenum with 10% manganese is applied to the ceramic and
sintered around 1400oC. In this process manganese oxidises
and diffuses into ceramic forming transition layer between
the ceramic and the molybdenum layer. This reduces the
thermal mismatch between ceramic and molybdenum . It is
then protected from oxidation by plating with nickel.
Brazing is then carried out using conventional filler
materials either in vacuum or in an inert atmosphere.
Active brazing is a relatively new technique. A family of
brazing alloys called active brazing alloys are made by
adding a small percentage of titanium or vanadium to
conventional filler material compositions. Brazing is
carried out under high vacuum in clean conditions. During
brazing titanium is oxidised by the ceramic forming titanium
oxides and liberating some aluminium atoms. This interlayer
forms some kind of chemical bridge between ceramic and the
metal [2 ]
An alternate way is to have a titanium coating on the
ceramic and then carry out regular brazing. At high
temperatures titanium reacts well with ceramics as well as
other metals. Usual brazing alloys wet titanium surface well
leading to a good brazed joint.
Brazing of graphite
Graphite is an important material in nuclear industry - both
the conventional fission reactors and the fusion reactors
under development. In the latter it is used as a first wall
material to minimise impurities in the reactor. The graphite
tiles used there face enormous heat load. Unless there is an
efficient heat removal mechanism in place, temperatures rise
beyond tolerable levels. The practice is to attach water
cooled sheets of copper or copper alloys to graphite tiles.
It is imperative that thermal contact between graphite and
copper be very good in order to reduce thermal resistance.
This can be achieved only by brazing the two.
Brazing of graphite is as problematic as brazing ceramics.
It has very low thermal expansion coefficient. It reacts
with very few materials to form carbides.
Brazing of graphite to TZM – an alloy of molybdenum - has
been realised [3, 4]. Brazing filler materials like
Cu-Ti-Ag, Cu-Ti, Ti, Zr and Zr alloys have been used.
Russians  have developed several filler alloys in the
form of flexible ribbons for brazing copper and beryllium
based alloys with graphite. There are several groups engaged
in developing suitable technologies.
One can see from the above discussion that titanium promotes
good brazing in ceramics as well as graphite. This is due to
the fact that at high temperatures titanium easily reacts
with oxygen, carbon and many metals. It was therefore
decided to coat both ceramics and graphite with titanium and
then attempt brazing with usual brazing fillers.
The coating facility at Bangalore Plasmatek was used
for titanium coating. It consists of a rectangular
vacuum chamber of 1000x1000x600 mm (fig.1). A
magnetically steered titanium cathode is used for
producing intense titanium plasma in high vacuum.
Alumina ceramic blocks of >99% purity of size
70x13x7mm and high density graphite blocks of about
25x25x25mm and other sizes are used for trials. An
area of 68x5 of one of the 70x7 faces as well as all
the remaining faces were masked before coating. Thus
a rectangular frame of 1 mm width was coated on one
70 x 7 mm face of ceramic piece. Similarly, all the
faces except one face of the graphite block were
also masked. Alumina (95% purity) ceramic strips of
size 5x0.3x50 mm are also coated. Cleaned samples
are mounted inside the coating chamber with suitable
fixtures. Samples were further cleaned by argon glow
Titanium plasma was produced by striking the arc on
the cathode. An arc current of 90 A was used. The
arc consists of multiply charged titanium ions up to
about 150 ev energy. This results in very dense and
adhesive coatings. Coating thickness was between 10
to 15 microns.
Ceramic blocks were brazed to titanium blocks of
size 100x30x40mm with a through slot and a step to
place the titanium coated ceramic block. Eutectic
alloy (CuSil) of copper(28%) and silver was taken in
the shape of thin wires and placed between titanium
and coated ceramic surfaces.
Figure 1. A view of the coating chamber
The coated ceramic disc was brazed to stainless
steel (SS 304) disc while graphite block was brazed
to a copper block. The same eutectic alloy CuSil was
taken in the form of thin foils and interposed
between surfaces of the materials to be brazed.
Brazing was carried out in a vacuum furnace. A
pressure of 10-5 mb was maintained. The temperature
was raised at the rate of 100C per minute. It was
held constant at 7500C for ten minutes and then
raised to 8800C at 200C per minute. Temperature was
kept constant at 8800C for two minutes and then
All brazed samples showed evidence of good flow of the
filler material. Some are shown in Figs.2 and 3.
Mechanical strength of the joints is excellent. The
joint between the rectangular ceramic block and the
titanium block was meant to be vacuum tight. But it
showed leaks. This is attributed to rectangular geometry
and sharp corners rather than any inherent brazing
It has been clearly demonstrated that a coating of 10-15
m of titanium is sufficient to braze ceramics and
graphite to various metals using an eutectic alloy of
copper and silver. For vacuum joints special care may
have to be taken in the presence of sharp corners or
other special features. The same technique can also be
used to braze ceramic to ceramic, graphite to graphite,
ceramic to graphite and metals to metals which may be
Figure 2. Ceramic piece brazed to titanium block
(a) Graphite block brazed to copper
(b) Ceramic strip brazed to stainless steel
D.M.Mattox & H.D.Smith, Ceram Bull.
A.Murari, H.Albrecht, A.Barzon,
S.Curiotto and L.Lotto, Vacuum 68(2003)321-328].
I.Smid et.al. JNM171(1990)165
B.A.Kalin et.al. Fusion Eng. & Design
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Vacuum brazing of ceramics and graphite to