Note: Descriptions are shown in the official language in which they were submitted.
CA 02330982 2001-O1-15
DEPOSITION AND THERMAL DIFFUSION OF BORIDES
AND CARBIDES OF REFRACTORY METALS
FIELD OF THE INVENTION
This invention relates to deposition and thermal diffusion to form a boride or
carbide of
a refractory metal on a substrate.
BACKGROUND OF THE INVENTION
Wear of metal surfaces, for example, in many types of machinery, is a problem
for which
many solutions have been proposed. Wear in many modern machines is aggravated
by their high
operational temperatures and loads, at which simultaneous metal oxidation,
fatigue, diffusion,
abrasion, or adhesion can occur. Refractory metal borides and carbides have
the necessary
hardness and resistance to high temperatures to resist wear.
Methods of electrodepositing refractory metals are known. For instance, such
methods
are disclosed in U.S. Patent No. 2,828,251 (Sibert et al.) and U.S. Patent No.
3,444,058 (Mellors
et al.). Mellors et al. disclose a method of electrodepositing refractory
metals from a solution of
the refractory metal fluoride in a molten alkali-fluoride eutectic mixture
onto a substrate. The
electrodeposited refractory metals form a coating which is essentially
unalloyed with the
substrate. The method disclosed, however, results only in a coating of the
refractory metal, and
not the refractory metal boride or carbide. Furthermore, the coating is not
metallurgicallybonded
to the substrate and it is therefore relatively less resistant to wear.
Sibert et al. disclose using a solid metalliferous form of the refractory
metal to be
deposited as an anode. However, Sibert et al. also disclose that, when the
electrodeposition is
earned out under high temperature and certain other conditions, the refractory
metal forms a
firmly adherent layer joined to a base metal substrate by a metal-to-metal
bond. The process
disclosed in Sibert et al. refers to alloying of the refractory metal and the
base metal substrate,
but not to forming a protective layer of a refractory metal boride or carbide.
U.S. Patent No. 2,950,233 (Steinberg et al.) discloses a method of, firstly,
forming a
"cladding" layer of certain transition metals on a base metal substrate
containing an amount of
a transition metal-hardening element, such as carbon, nitrogen, boron or
silicon, either
CA 02330982 2001-O1-15
interstitially or in solid solution. The cladding layer could be formed in
much the same manner
as disclosed by Senderoff et al. or Sibert et al., as referred to above.
Secondly, the method
requires the base metal substrate to be heated sufficiently to effect thermal
diffusion of the
transition metal-hardening element from the base metal substrate to the
cladding layer of the
S refractory metal. U.S. Patent No. 3,887,443 (Komatsu et al.) discloses a
similar approach, used,
however, with only V, Nb, and Ta.
The approaches disclosed in the Steinberg et al. patent and the Komatsu et al.
patent
suffer from the disadvantages that to form borides, carbides or silicides, the
substrate material
must contain boron, carbon, or silica. This means that this method is limited
to use where the
substrate includes alloys containing carbon, boron or silica as a component.
In addition to methods of electrodepositing refractory metals and the elements
boron or
carbon on a substrate, other methods of depositing refractory metals and such
elements on a
substrate are known.
Finally, various approaches have been taken to electrodeposit certain
refractory metals
and certain other elements simultaneously from a fused salt bath. Some of
these approaches are
described in U.S. Patents Nos. 3,697,390 (McCauley et a1.) (borides of Ti, Zr,
and Hf), 3,713,993
(Mellors et al.) (ZrBZ), 3,827,954 (McCauley et al.) (borides of Ti, Zr, and
Hf), 3,880,729
(Kellner) (TiBz), and 4,430,170 (Stern) (refractory metal carbides). In
general, these approaches
have been found to suffer from the disadvantage that their practical
applications were
problematic, as they do not generally involve stable processes. These
approaches are very
sensitive to impurities, and to minor variations in temperature and in the
composition of the salt
bath. Furthermore, these approaches do not provide a coating which is
metallurgically bonded
to the substrate.
There is therefore a need for a reliable method of forming a relatively
uniform and
metallurgically bonded layer of a boride or a carbide of a refractory metal on
a substrate.
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CA 02330982 2001-O1-15
SUMMARY OF THE INVENTION
In one of its aspects, the present invention provides a method of providing a
boride or a
carbide of a refractory metal on a substrate of a workpiece. Included are the
steps of depositing
a layer of a refractory metal on the substrate, depositing at least one of the
elements boron and
carbon from a source other than the workpiece on the workpiece having the
refractory metal
layer, and heating the workpiece at a temperature and for a time period
sufficient to diffuse at
least a portion of the deposited refractory metal into the substrate and at
least a portion of the
deposited boron or carbon into the refractory metal layer and the substrate to
form a substantially
uniform and metallurgically bonded layer of the boride or the carbide of the
refractory metal on
the substrate. The refractory metal i s selected from the group consisting of
Ti, V, Cr, Zr, Nb, Mo,
Hf, Ta, and W.
In another aspect of the present invention, there is provided a method of
providing a
boride or a carbide of a refractory metal on a substrate of a workpiece. The
method includes the
steps of depositing a layer of a refractory metal on the substrate, heating
the substrate at a first
temperature and for a first time period sufficient to diffuse at least a
portion of the refractory
metal layer into the substrate, and depositing at least one of the elements
boron and carbon from
a source other than the workpiece on the refractory metal layer. The substrate
is heated at a
second temperature and for a second time period sufficient to diffuse at least
a portion of the
deposited boron or carbon into the refractory metal layer and the substrate to
form a boride or
carbide of the refractory metal and to provide a substantially uniform and
metallurgically bonded
layer of the boride or the carbide of the refractory metal on the substrate.
It is preferred that the
layer of the refractory metal is deposited by electrodeposition.
In accordance with another aspect of the present invention, there is provided
a method
of providing a boride or carbide of a refractory metal on a substrate of a
workpiece. The method
includes the steps of providing a first molten salt bath of an anhydrous fused
salt electrolyte in
an inert container, the molten salt bath comprising a substantially eutectic
mixture of at least one
halide from the group consisting of alkali metal halides and alkaline earth
metal halides and a
reducing agent for a refractory metal, immersing an anode comprising the
refractory metal in the
first molten salt bath, immersing a cathode comprising the workpiece in the
first molten salt bath,
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the workpiece being electrically conductive, and electrodepositing a layer of
the refractory metal
on the workpiece.
The workpiece with the electrodeposited refractory metal thereon is heated to
a first
temperature in a range of about 700 °C to about 900 °C for a
first time period sufficient to diffuse
at least a portion of the electrodeposited refractory metal into the substrate
such that a refractory
metal layer is metallurgically bonded to the substrate. Subsequently, a second
molten salt bath
is provided in an inert crucible, the second molten salt bath comprising an
anhydrous fused salt
electrolyte comprising at least one halide from the group consisting of alkali
metal halides and
alkaline earth metal halides and a compound containing at least one second
element from the
group consisting of B and C. The cathode comprising the workpiece having at
least a portion of
the electrodeposited refractory metal diffused therein is immersed in the
second molten salt bath.
A layer of the second element is then electrodeposited from the second molten
salt bath on the
workpiece having the refractory metal layer. The workpiece having the second
element
electrodeposited on the layer of the refractory metal is heated to a second
temperature in the
range of about 700°C to about 900°C for a second time period
sufficient to diffuse at least a
portion of the boron or carbon into the refractory metal layer and the
substrate to form a boride
or carbide of the refractory metal and to provide a substantially uniform and
metallurgically
bonded layer of the boride or carbide of the refractory metal on the
substrate.
It is preferred that the inert container contains a protective atmosphere of
argon, for
preventing contaminants from entering the container.
It is also preferred that the reducing agent is selected from the group
consisting of a
fluoride of the refractory metal and a chloride of the refractory metal.
Preferably, electrodepositing of the refractory metal is effected by passing
direct current
at a current density in the range of between about 5 mA per square centimetre
to about 100 mA
per square centimetre through the first molten salt bath. Also,
electrodepositing of the second
element is effected by passing direct current at a current density in the
range of between about
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200 mA per square centimetre to about 300 mA per square centimetre through the
second molten
salt bath.
Where B is the second element, the second molten salt bath additionally
comprises a
second reducing agent.
In another alternative embodiment, there is provided a substantially uniform
layer of a
compound comprising a refractory metal and at least one of the elements B and
C metallurgically
bonded on an electrically conductive substrate formed by electrodepositing and
thermally
diffusing the refractory metal on the substrate to form a refractory metal
layer, and
electrodepositing from a source other than the workpiece and thermally
diffusing at least one of
the elements boron and carbon on the refractory metal layer and into the
refractory metal layer
and the substrate to form the compound.
In another aspect of the present invention, there is provided a method of
providing a
carbide of a refractory metal on a substrate of a workpiece. Included are the
steps of providing
a first molten salt bath of an anhydrous fused salt electrolyte in an inert
container, the molten salt
bath comprising a substantially eutectic mixture of at least one halide from
the group consisting
of alkali metal halides and alkaline earth metal halides and a reducing agent
for a refractory
metal, irmnersing an anode comprising the refractory metal in the first molten
salt bath,
immersing a cathode comprising the workpiece in the first molten salt bath,
the workpiece being
electrically conductive, and electrodepositing a layer of the refractory metal
on the workpiece.
Also included are the steps of heating the workpiece with the electrodeposited
refractory metal
thereon to a first temperature in a range of about 700 ° C to about 900
° C for a first time period
sufficient to diffuse at least a portion of the deposited refractory metal
into the substrate such
that a refractory metal layer is metallurgically bonded to the substrate, and
providing a second
molten salt bath in an inert crucible, the second molten salt bath comprising
a substantially
eutectic mixture of at least one of the fluorides of Li, Na, or K, including
about two percent to
about ten percent by weight the reducing agent for the refractory metal and
about two percent to
about ten percent by weight crystalline powder graphite. The cathode
comprising the workpiece
having at least a portion of the electrodeposited refractory metal diffused
therein is immersed in
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CA 02330982 2001-O1-15
the second molten bath, after which a layer of carbon is electrodeposited from
the second molten
salt bath on the workpiece having the refractory metal layer. The workpiece
having the carbon
electrodeposited on the layer of the refractory metal is heated to a second
temperature in the
range of about 850°C to 900°C for a second time period
sufficient to diffuse at least a portion
of the carbon into the refractory metal layer and the substrate to form a
carbide of the refractory
metal and to provide a substantially uniform and metallurgically bonded layer
of the carbide of
the refractory metal on the substrate.
Preferably, the refractory metal is Ta, and the second molten salt bath
comprises about
five percent by weight potassium heptafluorotantalate and about five percent
by weight
crystalline powder graphite.
In accordance with another aspect of the present invention, there is provided
a method
ofproviding a carbide of a refractory metal on a substrate of a workpiece. The
method comprises
the steps of providing a first molten salt bath of an anhydrous fused salt
electrolyte in an inert
container, the molten salt bath comprising a substantially eutectic mixture of
at least one halide
from the group consisting of alkali metal halides and alkaline earth metal
halides and a reducing
agent for a refractory metal, immersing an anode comprising the refractory
metal in the first
molten salt bath, immersing a cathode comprising the workpiece in the first
molten salt bath, the
workpiece being electrically conductive, and electrodepositing a layer of the
refractory metal on
the workpiece. The method also comprises heating the workpiece with the
electrodeposited
refractory metal thereon to a first temperature in a range of about
700°C to about 900°C for a
first time period sufficient to diffuse at least a portion of the
electrodeposited refractory metal
into the substrate such that a refractory metal layer is metallurgically
bonded to the substrate,
providing a crystalline graphite powder in an inert crucible, burying the
workpiece having at least
a portion of the electrodeposited refractory metal diffused therein in the
crystalline graphite
powder, and compressing the crystalline graphite powder with pressure in the
range of up to
about 5,000 grams per square centimetre. Air is evacuated from the inert
crucible. A layer of
carbon is deposited from the crystalline graphite powder on the workpiece
having the refractory
metal layer. The workpiece is heated to a second temperature in the range of
1,000 ° C to 1,200 ° C
for a second time period sufficient to diffuse at least a portion of the
carbon in the refractory
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CA 02330982 2001-O1-15
metal layer and the substrate to form a carbide of the refractory metal and to
provide a
substantially uniform and metallurgically bonded layer of the carbide of the
refractory metal on
the substrate.
In another aspect of the present invention, there is provided a method of
providing a
carbide of a refractory metal on a substrate of a workpiece. The method
includes the steps of
providing a first molten salt bath of an anhydrous fused salt electrolyte in
an inert container, the
molten salt bath comprising a substantially eutectic mixture of at least one
halide from the group
consisting of alkali metal halides and alkaline earth metal halides and a
reducing agent for a
refractory metal, immersing an anode comprising the refractory metal in the
first molten salt bath,
immersing a cathode comprising the workpiece in the first molten salt bath,
the workpiece being
electrically conductive, electrodepositing a layer of the refractory metal on
the workpiece, and
heating the workpiece with the electrodeposited refractory metal thereon to a
first temperature
in a range of about 700°C to about 900°C for a first time period
sufficient to diffuse at least a
portion of the electrodeposited refi-actory metal into the substrate such that
a refractory metal
layer is metallurgically bonded to the substrate. A layer of carbon is
deposited on said workpiece
having the refractory metal layer by gas carburizing. The workpiece is heated
to a second
temperature in the range of 1,000°C~ to 1,400°C for a second
time period sufficient to diffuse at
least a portion of the carbon in the refractory metal layer and the substrate
to form a carbide of
the refractory metal and to provide a substantially uniform and
metallurgically bonded layer of
the carbide of the refractory metal on the substrate.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS)
In a preferred embodiment of the method of the present invention, a layer of a
refractory
metal is deposited on a workpiece including a substrate. At least one of the
elements boron and
carbon is then deposited from a source other than the workpiece on the
refractory metal layer.
The workpiece is then heated at a temperature and for a time period sufficient
to diffuse at least
a portion of the deposited refractory metal and at least a portion of the
boron or carbon into the
refractory metal layer and the substrate to form a substantially uniform and
metallurgically
bonded layer of the boride or the carbide of the refractory metal on the
substrate.
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The refractory metal is selected from the group consisting of Ti, V, Cr, Zr,
Nb, Mo, Hf,
Ta, and W.
The refractory metal or the carbon and boron can be deposited by any suitable
method,
for example, electrodeposition, thermal spraymethods such as by flame spray or
by plasma spray,
or vapour-based methods such as pack cementation or physical vapour deposition
(PVD)
including cathodic arc, sputtering or electron beam evaporation.
In another preferred embodiment of the invention, the workpiece having the
refractory
metal deposited thereon is heated at a first temperature and for a first time
period sufficient to
diffuse a least a portion of the refractory metal layer into the substrate. A
second element, being
at least one of the elements boron and carbon, is then deposited from a source
other than the
workpiece on the refractory metal layer. The workpiece is then heated at a
second temperature
and for a second time period sufficient to diffuse at least a portion of the
deposited boron or
carbon into the refractory metal layer and the substrate to form a
substantially uniform and
metallurgically bonded layer of the boride or the carbide of the refractory
metal on the substrate.
Preferably, the layer of refractory metal is deposited by electrodeposition.
In yet another embodiment of the present invention, a first molten salt bath
of a first
anhydrous fused salt electrolyte is provided in an inert container. The first
molten salt bath
comprises a substantially eutectic mixture of at least one halide from the
group consisting of
alkali metal halides and alkaline earth metal halides and a reducing agent for
a refractory metal.
An anode comprising the refractory metal (being one of Ti, V, Cr, Zr, Nb, Mo,
Hf, Ta, and W)
is immersed in the first molten salt bath. After a cathode comprising a
workpiece which is
electrically conductive has been immersed in the first molten salt bath, a
layer of the refractory
metal is electrodeposited on the workpiece. The workpiece is heated to a first
temperature, in
a range of about 700°C to about 900°C, for a first time period
sufficient to cause thermal
diffusion of at least a portion of the electrodeposited refractory metal into
the substrate. As a
result of this thermal diffusion, arefractory metal layer is metallurgically
bonded to the substrate.
A second molten salt bath is then provided in an inert container. The second
molten salt bath
comprises an anhydrous fused salt electrolyte comprising at least one halide
from the group
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CA 02330982 2001-O1-15
consisting of alkali metal halides and alkaline earth metal halides and a
compound containing at
least one second element selected from the group consisting of boron and
carbon. A cathode
comprising the workpiece having at least a portion of the electrodeposited
refractory metal
diffused therein is immersed in the second molten salt bath. A layer of the
second element is
then electrodeposited from the second molten salt bath on the workpiece. The
workpiece is
heated to a second temperature, the second temperature being in the range of
about 700°C to
about 900°C, for a second time period sufficient to diffuse at least a
portion of the boron or
carbon into the refractory metal layer and the substrate to form a bonde or
carbide of the
refractory metal and to provide a substantially uniform and metallurgically
bonded layer of the
bonde or the carbide of the refractory metal on the substrate.
It is preferred that the electrodeposition of the refractory metal layer is
earned out in a
protective atmosphere of argon, to prevent contaminants from entering the
container. Preferably,
the argon is maintained at a pressure slightly greater than atmospheric
pressure.
The reducing agent is selected from the group consisting of a fluoride of the
refractory
metal and a chloride of the refractory metal.
Electrodepositing of the refractory metal is effected by passing direct
current at a current
density in the range of between about 5 mA per square centimetre to about 100
mA per square
centimetre through the first molten salt bath between the anode and the
cathode. For Nb, a
suitable reducing agent is potassium heptafluoroniobate, and for Ta, a
suitable reducing agent is
potassium heptafluorotantalate. In this embodiment, the electrodepositing
ofthe second element
is effected by passing direct current at a current density in the range of
between about: 200 mA
per square centimetre to about 300 mA per square centimetre through the second
molten salt
bath. For boron, potassium tetrafluoroborate is a suitable reducing agent.
In general, the thicker the refractory metal boride or carbide coating
desired, the longer
the time required during which bonding or carburizing is carried out.
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CA 02330982 2001-O1-15
In the formation of the refractory metal layer, dendrites may form. This
appears to be the
result of the growth of the layer exceeding coherent disposition. When
dendrites start to form,
reverse current is applied to dissolve them. In the alternative, to discourage
the formation of
dendrites, a levelling compound can be added to the first molten salt bath.
For example, as is
known in the art, potassium heptafluorotantalate can be used as a levelling
agent for Nb
electrodeposition, and aluminum oxide can be used as a levelling agent for
electrodeposition of
Zr and Mo.
For the refractory metal Nb and the second element B, the first time period is
between
about five minutes and about three hours, depending on the desired thickness
of the refractory
metal boride or carbide coating. Preferably, the second temperature is about
800°C, and the
second time period is between about one hour and about nine hours. The thicker
the coating
desired, the longer the first and second time periods.
For the refractory metal Nb and the second element C, the electrodeposition
ofthe carbon
is effected by passing direct current through the second molten salt bath at a
current density of
about 100 mA per square centimetre. The second temperature is about
750°C and the second
time period is about four hours.
For the refractory metal Ta, and the second element B, the first time period
is between
about five minutes and about three hours. In this embodiment, the second
temperature is about
900°C and the second time period is about seven and one-half hours.
In another embodiment of the method of the present invention, a substantially
uniform
layer of a compound comprising a refractory metal of Ti, V, Cr, Zr, Nb, Mo,
Hf, Ta or W and at
least one of the elements boron and carbon is formed which is metallurgically
bonded to an
electrically conductive substrate of a workpiece. The layer of the compound is
formed by
electrodepositing and thernially diffusing the refractory metal on the
substrate to form a
refractory metal layer, and then electrodepositing from a source other than
the workpiece and
thermally diffusing at least one of the elements boron and carbon on the
refractory metal layer
and into the refractory metal layer and the substrate.
CA 02330982 2001-O1-15
In yet another preferred embodiment, the invention comprises another method of
providing a carbide of a refractory metal on a substrate of a workpiece. In
this preferred
embodiment, a refractory metal layer is deposited on the substrate, for
example, by
electrodeposition in the manner described above. The second molten salt bath
comprises a
substantially eutectic mixture of at least one of the fluorides of Li, Na, or
K, including about two
percent to about ten percent by weight the reducing agent of the refractory
metal and about two
percent to about ten percent by weight crystalline powder graphite. As in
other preferred
embodiments described above, the cathode which is immersed in the second
molten salt bath is
the workpiece, which has at least a portion of the electrodeposited refractory
metal diffused
therein. A layer of carbon is then electrodeposited from the second molten
salt bath on the
refractory metal layer, and the workpiece is heated to a second temperature in
the range of about
850°C to about 900°C for a second time period sufficient to
diffuse at least a portion of the
carbon into the refractory metal layer and the substrate to form a carbide of
the refractory metal
and to provide a substantially uniform and metallurgically bonded layer of the
carbide of the
refractory metal on the substrate.
The second molten salt bath includes about five percent by weight the reducing
agent of
the refractory metal and about five percent by weight crystalline graphite
powder. Also, it is
preferred that the refractory metal in this preferred embodiment is Ta. Where
the refractorymetal
is Ta, the reducing agent is preferably potassium heptafluorotantalate.
Various methods of carburizing may be used for providing a carbide of a
refractory metal
on a substrate of a workpiece. 'the refractory metal is deposited on the
substrate by
electrodeposition in the manner described above. Instead of using a second
molten salt bath for
carburizing, however, pack carburizing is utilized. A crystalline graphite
powder is provided in
an inert crucible. The workpiece having at least a portion of the
electrodeposited refractory metal
deposited thereon is buried in the crystalline graphite powder. The
crystalline graphite powder
is then compressed by pressure of up to about 5,000 grams per square
centimetre, Air is
evacuated from the crucible. After a layer of carbon is deposited from the
crystalline graphite
powder on the workpiece, the workpiece is heated to a second temperature in
the range of about
1,000°C to about 1,200°C for a second time period sufficient to
diffuse at least a portion of the
11
CA 02330982 2001-O1-15
carbon in the refractory metal layer and the substrate to form a carbide of
the refractory metal and
to provide a substantially uniform and metallurgically bonded layer of the
carbide of the
refractory metal on the substrate.
In accordance with another embodiment, a workpiece having a refractory metal
layer
electrodeposited thereon and at least a portion of the electrodeposited
refractory metal diffused
therein is subjected to gas carburizing. The gas carburizing results in the
deposition of a layer
of carbon on the refractory metal layer. The workpiece is then heated to a
second temperature
in the range of about 1,000°C to about 1,400°C for a second time
period sufficient to diffuse at
least a portion of the carbon in the refractory metal layer and the substrate
to form a carbide of
the refractory metal and to provide a substantially uniform and
metallurgically bonded layer of
the carbide of the refractory metal on the substrate. Subsequent to gas
carburizing, the substrate
is heated at a temperature and for a time period sufficient to diffuse at
least a portion of the
refractory metal and the carbon into the substrate to form a substantially
uniform and
metallurgically bonded layer of the carbide of the refractory metal on the
substrate.
The second temperature could be higher than 1,400°C, where the melting
point of the
substrate is substantiallyhigher than 1,400 ° C. In this embodiment,
the second temperature range
is up to about 1,400 ° C because 1,400 °C is lower than the
melting point of many commonly used
substrates.
Gas carburizing is described, for example, in G.L. Zhunkovskii, "The Vacuum
Carbidization of Transition Metals of Groups IV and V", at 107-11 S in
Refractory Carbides
(1974), edited by Grigorii V. Samsonov, and in Tsutsumoto, "Improvement of Ta
filament for
diamond CVD", in Thin Solid Films 317 ( 1998) 371-375, each of which is
incorporated herein
by reference. In a preferred embodiment, a refractory metal layer is deposited
on a substrate of
a workpiece, and the step of carburizing the workpiece having the refractory
metal layer is
achieved by means of gas carburizing.
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Having fully described the present invention, the :following examples are
provided to
further illustrate the principles of the disclosed invention and are not
intended to limit the scope
of the invention in any manner.
Example I
A first molten salt bath was prepared and placed in a nickel container.
Samples of M4
tool steel, five centimetres long and in cross-section one-half centimetre by
one-half centimetre,
were coated with Nb. Electron microscopy of the interface between the
deposited layer of
refractory metal and the substrate revealed mutual diffusion of both material
of a substrate and
Nb for about 1 to 3 microns, proving that the refractory metal coat was
metallurgically bonded
to the substrate. The coatings of Nb on the samples were approximately 20 to
80 microns thick.
The first molten salt bath consisted of eutectic, or close to eutectic
mixtures of two or
more chlorides of Li, Na, and K. Temperatures varied from 700°C to
900°C. Current varied
also from 5 to 100 mA per square centimetre. The time required depended on the
required
thickness of the deposited layer of refractory metal. To enhance reduction of
the refractory metal
ions in the first molten salt bath, reducing agents were added. For Nb, a
suitable reducing agent
is potassium heptafluoroniobate.
To suppress formation of dendrites, and in order to have a uniform continuous
layer of
refractory metal electrodeposited, step current was applied in such a way that
the first step was
used to deposit the refractory metal layer. When the growth of the refractory
metal layer
exceeded coherent depositions and dendrites started to form, reverse current
was applied to
dissolve the dendrites. Subsequently, the process began again, until the
refractory metal layer
had achieved the predetermined required thickness.
The refractory metal electrodeposition was carried out in the nickel container
under the
protective atmosphere of argon. The argon was at a pressure s lightly above
atmospheric pressure.
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CA 02330982 2001-O1-15
After the desired thickness was reached, the sample was cleaned in water and
placed in
the second molten salt bath for bonding.
Bonding of samples coated with Nb was carried out in the protective argon
atmosphere
in the second molten salt bath. The second molten salt bath contained sodium
tetraborate as a
main boron carrier with up to 20% by weight potassium tetrafluoroborate as a
reducing agent.
The current applied was 224 mA per square centimetre, and a temperature of
800°C was
maintained for 6.5 hours. A continuous layer of brownish-gray color formed on
the surface was
determined, by means ofX-ray diffraction, to include approximately 80% niobium
dibonde,15%
niobium, and 5% unidentified amorphous substance.
Example 2
The method as described in Example 1 was repeated, except that instead of
chlorides, the
eutectic mixture of two or more fluorides of Li, Na, or K were used in the
first molten salt bath.
The bonding process lasted for about 7.5 hours, at 900°C. Formed on the
surface was a
continuous layer of brownish-gray color which was found, by using X-ray
diffraction, to include
approximately 80% niobium diboride and 20% niobium.
Example 3
The method as described in Example 1 was repeated, except that the bonding
process
lasted for about 8 hours and 20 minutes at 850°C. A continuous layer of
brownish-gray color
was formed on the surface. The brownish-gray layer was found, by using X-ray
diffraction, to
include approximately 80% niobium dibonde and 20% niobium.
Example 4
The method as described in Example 2 was repeated, except that the refractory
metal was
Ta and instead of potassium heptafluoroniobate, potassium heptafluorotantalate
was used as a
reducing agent. A continuous layer of brownish-gray color formed on the
surface was
determined, by means of X-ray diffraction, to include approximately 90%
tantalum diboride and
10% tantalum.
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Example 5
Workpiece samples of M4 tool steel were coated with Nb, as described in
Example 1.
The second molten salt bath consisted of an equimolar mixture of sodium
tetraborate and sodium
carbonate, with the addition of sodium fluoride and potassium fluoride, and
was placed into a
graphite crucible. The cathode was the workpiece covered with Nb, and the
anode was an ultra-
pure graphite rod. Carburizing was performed in the crucible for 4 hours. The
current density
was 100 mA per square centimetre at a working temperature of 750°C for
four hours. Formed
on the surface was a continuous layer of dark gray color which was found, by
using X-ray
diffraction, to include approximately 20% niobium carbide and 15% niobium, the
balance
comprising iron oxide, iron carbide, and an unidentified amorphous substance.
Example 6
The method described in Example 4 was repeated on a solid carbide insert. A
continuous
layer of brownish-gray color was formed on the surface. By means of X-ray
diffraction, the
brownish-gray color was found to include approximately 95% tantalum diboride,
and 5%
unidentified amorphous substance.
Example 7
The method of Ta deposition described in Example 4 was repeated on a solid
carbide
insert. The second molten salt bath consisted of an eutectic or close to
eutectic mixture of two
or more fluorides of Li, Na, or K with 5% by weight of potassium
heptafluorotantalate and 5%
by weight of crystalline powder graphite. The anode was a graphite electrode.
The applied
current was 254 mA per square centimetre for two hours at the temperature of
870°C. A
continuous layer of yellowish color formed on the surface was determined, by X-
ray diffraction,
to include approximately 97% tantalum carbide and 3% unidentified amorphous
substance.
Example 8
The method of Ta deposition described in Example 4 repeated on a solid carbide
insert.
The solid carbide insert, coated with Ta as described, was then placed into
crystalline graphite
power in a container and compressed at a pressure of 160 grams per square
centimetre. Air was
evacuated from the container. The sample, along with the graphite pack, was
heated to 1050°C
CA 02330982 2001-O1-15
and maintained at that temperature for approximately 2 hours. Formed on the
surface was a
continuous yellowish layer which was found, using X-ray diffraction, to
include approximately
20% tantalum carbide, 75% tantalum, and the balance tantalum oxides and other
oxides.
It will be evident to those skilled in the art that the invention can take
many forms and
that such forms are within the scope of the invention as claimed. For example,
any other method
of depositing a refractory metal layer on a substrate of a workpiece, and any
method of
carburizing or bonding the refractory metal layer in which the carbon or boron
is from a source
other than the workpiece, may be used with thermal diffusion of the refractory
metal and the
carbon or boron into the substrate.
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