U.S. patent application number 12/470360 was filed with the patent office on 2010-01-28 for ultra-fast boriding of metal surfaces for improved properties.
This patent application is currently assigned to UChicago Argonne LLC. Invention is credited to Ali Erdemir, Osman L. Eryilmaz, Guldem Kartal, Servet TIMUR.
Application Number | 20100018611 12/470360 |
Document ID | / |
Family ID | 41567565 |
Filed Date | 2010-01-28 |
United States Patent
Application |
20100018611 |
Kind Code |
A1 |
TIMUR; Servet ; et
al. |
January 28, 2010 |
ULTRA-FAST BORIDING OF METAL SURFACES FOR IMPROVED PROPERTIES
Abstract
A method of ultra-fast boriding of a metal surface. The method
includes the step of providing a metal component, providing a
molten electrolyte having boron components therein, providing an
electrochemical boriding system including an induction furnace,
operating the induction furnace to establish a high temperature for
the molten electrolyte, and boriding the metal surface to achieve a
boride layer on the metal surface.
Inventors: |
TIMUR; Servet; (Istanbul,
TR) ; Kartal; Guldem; (Istanbul, TR) ;
Eryilmaz; Osman L.; (Plainfield, IL) ; Erdemir;
Ali; (Naperville, IL) |
Correspondence
Address: |
FOLEY & LARDNER LLP
321 NORTH CLARK STREET, SUITE 2800
CHICAGO
IL
60654-5313
US
|
Assignee: |
UChicago Argonne LLC
|
Family ID: |
41567565 |
Appl. No.: |
12/470360 |
Filed: |
May 21, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61059177 |
Jun 5, 2008 |
|
|
|
Current U.S.
Class: |
148/241 ;
148/400; 148/421 |
Current CPC
Class: |
C23C 8/42 20130101 |
Class at
Publication: |
148/241 ;
148/421; 148/400 |
International
Class: |
C23C 8/68 20060101
C23C008/68; C22C 14/00 20060101 C22C014/00 |
Goverment Interests
STATEMENT OF GOVERNMENT INTEREST
[0002] The United States Government has certain rights in this
invention pursuant to Contract No. W-31-109-ENG-38 between the
United States Government and The University of Chicago and/or
pursuant to Contract No. DE-AC02-06CH11357 between the United
States Government and UChicago Argonne, LLC representing Argonne
National Laboratory.
Claims
1. A method of boriding a surface of a metal comprising: providing
a metal component; providing a molten electrolyte having boron
components therein; providing an electrochemical boriding system
including one of an induction furnace and a resistively-heated
furnace; providing a current to the metal component; operating the
furnace to establish a high temperature for the molten electrolyte;
and boriding the metal surface to achieve a boride layer on the
metal surface, wherein a boriding rate of at least about 2
.mu.m/minute of the metal component is achieved for a boriding
period of less than about 30 minutes.
2. The method as defined in claim 1, wherein the boron components
comprise a borax.
3. The method as defined in claim 1, wherein the molten electrolyte
further includes an additive selected from the group consisting of
inorganic sodium, potassium, and lithium compounds.
4. The method as defined in claim 2, wherein the borax ranges from
about 30 to about 95 weight percent.
5. The method as defined in claim 4, further including sodium
carbonate ranging between about 5-70 weight percent.
6. The method as defined in claim 2, further including an additive
selected from the group consisting of an alkaline halide and an
alkaline earth halide.
7. The method as defined in claim 6, wherein the additive consists
essentially of at least one of CaCl.sub.2 and NaCl.
8. The method as defined in claim 7, wherein the additive ranges
from about 0.1-25 weight percent.
9. The method as defined in claim 7, wherein a ratio of
Na.sub.2O/B.sub.2O.sub.3 is controlled in the molten electrolyte to
reduce at least one of Na.sup.+ and Ca.sup.++ ions present on the
metal surface.
10. The method as defined in claim 9, wherein the boron
concentration in the molten electrolyte is further increased to
accelerate diffusion of boron ions, thereby accelerating boride
layer growth on the metal surface.
11. The method as defined in claim 1, wherein the electrochemical
boriding system includes an anode and cathode and a separation
distance between the anode and cathode is adjusted to accelerate
the rate of forming a boride on the metal surface.
12. The method as defined in claim 1, further including the step of
agitating the electrolyte to increase the rate of boriding the
metal surface.
13. The method as defined in claim 1, wherein the metal component
comprises a steel and the method further includes the step of
quenching the metal from the molten electrolyte, thereby forming
martensitic phases along with the boride layer proximate the metal
surface.
14. A method of boriding a surface of a metal comprising: providing
a metal component comprising a transition metal electrically
coupled to a cathode; providing a molten electrolyte having boron
components comprising about 30 to about 95 weight percent borax
therein; providing one or more additives to the molten electrolyte
to modify at least one of the electrochemical properties of the
molten electrolyte, the viscosity of the molten electrolyte, and
the melting temperature of the electrolyte; providing an
electrochemical boriding system including one of an induction and a
resistively-heated furnace; providing an electrical charge to the
metal component; maintaining the molten electrolyte between about
700.degree. C. and about 1000.degree. C.; and boriding the metal
surface to achieve a boride layer on the metal surface.
15. The method of claim 14, further including the step of agitating
the electrolyte to enhance the boriding rate of the metal
component.
16. The method of claim 14, wherein at least one of the one or more
additives is selected from the group consisting of an alkaline
halide and an alkaline earth halide.
17. The method of claim 14, further including the step of reversing
the polarization of the charge to the metal component.
18. A treated metal with a surface having a boride layer thereon
formed by the process of: providing a metal component; providing a
molten electrolyte having boron components therein; providing one
or more additives to the molten electrolyte to modify at least one
of the electrochemical properties of the molten electrolyte, the
viscosity of the molten electrolyte, and the melting temperature of
the electrolyte; providing an electrochemical boriding system
including one of an induction and a resistively heated furnace;
operating the furnace to establish a high temperature for the
molten electrolyte; and boriding the metal surface to achieve a
boride layer on the metal surface, wherein the surface of the
boride layer has a hardness value of at least about ten times the
hardness of the untreated metal component.
19. The treated metal as defined in claim 18, further including a
step of masking a portion of the metal component to selectively
boride a portion of the metal component.
Description
CROSS REFERENCE TO RELATED PATENT APPLICATIONS
[0001] The present application claims priority to U.S. Provisional
Patent Application No. 61/059,177, filed Jun. 5, 2008 incorporated
herein by reference in its entirety.
FIELD OF THE INVENTION
[0003] This invention is directed to an ultra-fast surface
treatment method that results in hard, wear, corrosion and erosion
resistant, and low-friction surface layers on metallic substrates.
More particularly, the present invention relates to an ultra fast
electrochemical boriding technique which can lead to dramatic
improvements in the mechanical and tribological properties of
treated metal surfaces, ferrous and non-ferrous.
BACKGROUND OF THE INVENTION
[0004] Most mechanical components used in a variety of rolling,
rotating, or sliding bearing applications, as well as those that
are used in metal-cutting and--forming operations, rely strongly on
high hardness and low friction surface properties of base metals
for high performance and durability during use. In a dusty, sandy,
and corrosive environment, high resistance to erosion and corrosion
becomes important. There are numerous surface treatment methods
that are currently used to enhance the near-surface properties of
engineering components. Some of these methods (such as nitriding,
carburizing, carbonitriding, and boriding) are theremo-chemical in
nature and based on thermal diffusion of carbon, nitrogen, and
boron atoms into the near surface regions of these components at
high temperatures. It typically takes about 8 to 10 hours to
achieve case depths of 50 to 100 micrometers in the cases of
nitriding and carburizing processes; and as for boriding, the case
depths are much shallower (typically 10 to 15 micrometers for the
same processing time). Despite its ability to produce much harder
surface layers than carburizing and nitriding, boriding is not used
as extensively as the other surface treatment techniques
mentioned.
[0005] There are several other surface treatment methods based on
the uses of laser beams such as laser shot-peening, -glazing,
-cladding, as well as ion and electron beam processes such as
ion-beam deposition, electron-beam cladding, and hardening that can
also be used to achieve superior surface mechanical and
tribological properties. Besides these methods, there are
plasma-based physical and chemical vapor deposition techniques that
can also produce hard surface coatings (such as TiN, TiC, etc.) on
mechanical components for improved mechanical and tribological
properties. Unfortunately, all of these methods require long
processing times and consume large amounts of energy.
[0006] Among the many thermal diffusion-based surface treatment
processes mentioned above, nitriding and carburizing are used
extensively by industry to achieve greater mechanical and
tribological properties on all kinds of steel components. In the
case of boriding though, progress has been rather slow and at the
moment, this technique has limited uses. Just like nitriding and
carburizing, boriding is a surface hardening process in which boron
atoms diffuse into the near surface region of a work piece and
react with the metallic constituents to form hard borides. A deep
diffusion layer also exists beneath the boride layers. At present,
there are several kinds of boriding methods available (such as
salt-bath boriding, fluidized bed boriding, pack boriding, paste
boriding, gas-phase and plasma boriding) for the production of
borided surface layers. These methods are based on the uses of a
variety of boron-rich solid, liquid, or gaseous media. Fluidized
bed-, pack-, and paste-boriding methods use solid boron containing
powders (such as B4C, amorphous boron, ferro-boron, etc.) and other
compounds during the boriding process, while plasma boriding uses
gaseous boron compounds in a plasma environment.
[0007] All of the boriding methods mentioned above involve a high
processing temperature (typically ranging from 700 to 1000.degree.
C.). These boriding methods are most appropriate for the treatment
of ferrous alloys, but nonferrous and cermet-based materials can
also be treated. For example, salt-bath boriding of steel
substrates can be done in a complex salt bath typically consisting
of 60 to 70 wt % borax, 10 to 15 wt % boric acid, and 10-20 wt %
ferro-silicon or boron at temperatures ranging from 800 to
1000.degree. C. Boriding of a low carbon steel substrate for 5 to 7
hours in such a salt-bath may result in 7 to 10 micrometer thick
borided surface layers.
[0008] During boriding of steel and other metallic and alloy
surfaces, boron atoms diffuse into the material and form various
types of metal borides. In the case of ferrous alloys, most
prominent borides are: Fe.sub.2B and FeB. (Fe.sub.3B may also form
depending on the process parameters). Some of the boron atoms may
dissolve in the structure interstitially without triggering any
chemical reaction that can lead to boride formation. Iron borides
(i.e., Fe.sub.2B and FeB) are chemically stable and mechanically
hard and hence can substantially increase the resistant of base
alloys to corrosion, oxidation, adhesive, erosive, or abrasive
wear. Process conditions (such as duration of boriding, ambient
temperature, type of substrate material and boriding media) may
affect the chemistry and thickness of the borided surface layers.
Due to the much harder nature of borided layers, boriding has the
potential to replace some of the other surface treatment methods
like carburizing, nitriding and nitrocarburizing.
[0009] Boride layers may achieve hardness values of more than 20
GPa depending on the chemical nature of the base materials. TiB2
that forms on the surface of borided titanium substrates may
achieve hardness values as high as 30 GPa; ReB.sub.2 that forms on
the surface of rhenium and its alloys may achieve hardness values
as high as 50 GPa, while the hardness of boride layers forming on
steel or iron-based alloys may vary between 14 GPa to 20 GPa. Such
high hardness values provided by the boride layers are retained up
to 650.degree. C. Since there is no discrete or sharp interface
between the boride layer and base material, adhesion strengths of
boride layers to base metals are excellent. With the traditional
methods mentioned above, boride layer thicknesses of up to 20
micrometer can be achieved after long periods of boriding time at
much elevated temperatures. In addition to their excellent
resistance to abrasive, erosive, and adhesive wear, the boride
layers can also resist oxidation and corrosion even at fairly
elevated temperatures and in highly acidic or saline aqueous
media.
[0010] Materials that are most suitable for boriding include all
types of ferrous metals and alloys like low- and high-carbon
steels, low- and high-alloy steels, tool steels, bearing steels,
stainless steels, precipitation hardening steels, carburized,
nitrided, and carbonitrided steels, and cast irons. Non-ferrous
metals and their alloys like aluminum, hafnium, vanadium, nickel,
chromium, cobalt, titanium, tantalum, zirconium, tungsten, niobium,
molybdenum, rhenium, magnesium, and their alloys in particular
nickel-based and cobalt-based superalloys, cobalt-chrome alloys,
tungsten and sintered carbides and/or cermets can also be
borided.
[0011] Because of their impressive mechanical, tribological,
chemical and corrosion properties, borided surface layers can be
used in a large variety of industrial applications. In
metal-forming dies, they can be used to protect the critical
surface finish or profiles of all kinds of dies (such as punching
dies, drawing dies, bending dies, hot forming, and injection
moulding dies, forging dies, die-casting, extrusion dies, embossing
dies, deep drawing and impact extrusion dies). They can also be
used in insertion pins, rods, plungers, bushings, botts, nozzles,
pipe bending devices, guide rings, sleeves, mandrels, swirl
elements, clamping, chucks, guide box, metal casting inserts,
orifices, springs, balls, rollers, discs, valve components and
fittings, plugs, chain components, etc. They will be extremely
well-suited for stainless steel and other metallic-based mechanical
shaft seals used in pumping all kinds of fluids, pulps, powders,
slurries, in chemical and mining industries. In the automotive or
transportation fields, they can prevent seizure, galling and
scuffing-related failures under severe operating conditions, and
eliminate oxidative and corrosive degradation of a large variety of
engine components. They can also be used in a variety of gear
drives (such as bevel gears, screw and wheel gears, helical gear
wheels), including gears, ball and roller bearings, tappets, valves
and valve guides, power train components, piston pins, rings and
liners, fuel injector components for liquid (gasoline and diesel)
and gaseous fuels (like hydrogen, propane, natural gas) and other
types of mechanical components in all classes of moving mechanical
systems that experience heavy loading, high speeds, erosive,
corrosive, and oxidative media and elevated temperatures. Other
potential applications include cold and hot forging tools,
extrusion tools, press tools, glass industry tools, metal-casting
tools, cutters, razor blades and shaving machines, impellers used
in pumps, mixers used in chemical and mining industries, ball
joints, turbine and helicopter blades that are subject to sand
erosion, compressor parts and foil bearings, invasive and
implantable medical devices such as hip and knee joints made out of
titanium, zirconium, cobalt-chrome, stainless steel, and other
specialty metals and their alloys. Because of the high boron
content of their near surfaces, borided surfaces can also provide
an excellent substrate for the deposition of diamond and
diamondlike carbon films on metallic substrates. In most cases,
diamond is difficult to deposit on steel substrates; but after the
boriding process such surfaces could be ideal for the nucleation
and growth of crystalline diamond and amorphous diamondlike carbon
films. Such duplex boride-diamond or diamondlike carbon treated
surfaces can be ideal in many machining, metal forming, sealing,
and biomedical implant applications.
[0012] Despite their abilities to produce much harder surface
layers and superior components over other methods, conventional
boriding methods mentioned above are not used extensively by
industry at the moment. There are substantial problems that hinder
their wider use. Some of these problems include: high-cost, long
processing time, toxic emissions/byproducts, mechanical and
structural degradations, and poor surface condition or finish after
the boriding process. For all of these reasons, it would be
desirable to develop a new and improved boriding method that is
fast, inexpensive, safe, and applicable to a wide range of
materials.
SUMMARY OF THE INVENTION
[0013] The present invention provides a method for producing
metallic products with hard boride layers for a variety of
mechanical and erosion resistant applications. The preferred method
involves preparation of a molten electrolyte consisting of about 90
wt. % borates of alkaline and alkaline earth elements (such as
borax) and about 10 wt. % carbonates of alkaline and alkaline-earth
elements (such as sodium and/or calcium carbonate) or sodium
chloride. Addition of small amounts (0.1 to 5 wt. %) of other
halides (chlorides, fluorides, and iodides, etc.) of alkaline
and/or alkaline-earth elements (like, LiCl, NaCl, CaCl.sub.2) can
have positive effects as electrolyte enhancers. Oxides, hydroxides,
and carbonates of such elements may also be used to control the
viscosity and melting point of the electrolyte. Furthermore, using
at least one of a high frequency induction furnace, external
agitation, mixing of electrolyte or vibrating/shaking of the work
piece holder can help overcome diffusion barriers in the
electrochemical process and thus help achieve fast boriding and
thick boride layers (about 100 micrometers or more in the case of
low carbon steels) with desirable mechanical properties in short
processing times (for example, less than an hour). Such a procedure
can also result in a more uniform boride layer thickness on the
surfaces of odd-shaped or intricate work pieces.
[0014] In electrochemical boriding, graphite is often used as the
crucible material. The same graphite crucible can also serve as the
anode of the electrochemical cell. Due to the high temperature
nature of the boriding process, the graphite crucible or anode may
undergo oxidation and hence thin down or wear out after repeated
uses. As an alternative approach, in our process, we can also use
the metallic and/or borided forms of titanium, aluminum, zirconium,
hafnium, vanadium, niobium, tantalum, nickel, molybdenum, chromium,
tungsten, cobalt, iron and their alloys as anodes and/or crucible
materials. Specifically, we can form a thin boride layer on the
surface of these metals by reverse polarization (i.e., by making
the crucible a cathode) and then switch back to the regular
boriding practice by changing the polarity, switching the cathode
with the anode again. In particular, the boride layers that form on
titanium (and its alloys) have excellent resistance to high
temperature corrosion and oxidation. They are also electrically
conductive, hence they can be an ideal choice for the
industrial-scale boriding operations. Alternatively, iron and its
alloys can also be borided first and then used as crucibles and/or
anodes. Iron borides are also electrically conductive (this is why
they form thick boride layers during our boriding process). In
fact, reverse polarization of anodes and/or crucibles can be done
as needed if the boride layer thickness on the crucible or the
anode surface is reduced or there is a need for repair of a thinned
down or worn area. Such a practice will ensure long durability and
hence low cost.
[0015] The thickness and composition (e.g., type of boride, such as
FeB or Fe.sub.2B, or Fe.sub.3B, diffusion layer) of borided surface
layers can be controlled to achieve performance and durability
requirements of a given application. For certain applications,
Fe.sub.2B could be a preferred phase due to its superior strength
and toughness. During the boriding process, the boriding
temperature and/or current density may be maintained low to achieve
only this phase over the other. Alternatively, one can also keep
the boriding duration short but leave the work pieces in the molten
electrolyte for a longer duration to allow excess boron to diffuse
or distribute evenly within the structure and hence stabilize the
Fe.sub.2B phase over the FeB phase. Nano-to-micro scale boride
phases can also be produced in a given surface region by
selectively reacting diffusing boron atoms with secondary phases
and/or alloying elements within that region. This allows achieving
multiple objectives, such as improved mechanical properties without
degrading thermal and/or electrical properties of the base
material. It is also possible to partially or selectively boride
the surface or a region of a work piece by various masking methods
as will be discussed in Examples.
[0016] These and other objects, advantages, and features of the
invention, together with the organization and manner of operation
thereof, will become more apparent from the following detailed
descriptions and examples when taken in conjunction with the
accompanying figures described below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1A is a general photograph of an electrochemical cell
and FIG. 1B is a schematic of the cell used to perform boriding
operations in accord with a method of the invention;
[0018] FIG. 2A shows a cross sectional micrograph after 1 minute of
the boriding treatment; FIG. 2B shows after 5 minutes of boriding;
FIG. 2C shows after 15 minutes of boriding and FIG. 2D shows after
30 minutes of boriding; FIG. 2E shows the sample of FIG. 2C
following a 45 minute soak in the molten electrolyte;
[0019] FIG. 3 shows the X-ray diffraction spectrum of boride layers
formed on steel substrates for various boriding times, including 1,
5, 10 and 15 minutes, respectively, for FIGS. 2A, 2B and 2C, and
also the location of the diffraction peaks for FeB and Fe.sub.2B
phases (vertical lines);
[0020] FIG. 4 shows the variation of boride thickness with time in
minutes;
[0021] FIG. 5 shows the variation in boride layer thickness
(micrometers) with time (minutes) up to a boriding time of 120
minutes under the same boriding condition as in FIG. 2;
[0022] FIG. 6 shows rate of boriding of a low carbon steel
substrate for an anode to cathode distance of 1 cm;
[0023] FIG. 7 shows a comparison of boride layer thicknesses with
respect to anode to cathode distances as a function of time; for
thicker boride layers, shorter distances give a faster boriding or
higher boriding rates;
[0024] FIG. 8 shows the relationship of B.sub.2O.sub.3/Na.sub.2O
ratio to FeB and total borided layer thickness for a fixed boriding
time of 60 minutes;
[0025] FIG. 9 shows the relationship of boriding temperature to
boride layer thickness for a fixed boriding time of 1 hour;
[0026] FIG. 10A shows hardness profile as a function of distance
from the surface (beneath the boride layer is a boron diffusion
zone); and FIG. 10B is a micrograph cross-section at one
magnification for a steel specimen with different iron boride
phases borided for 60 minutes;
[0027] FIG. 11 shows the relationship between total borided layer
thickness and current density (20% NaCl+80% Na.sub.2B.sub.4O.sub.7,
1 hour, 900.degree. C.);
[0028] FIG. 12 shows the relationship between electrochemical cell
potential and the current density (20% NaCl+80%
Na.sub.2B.sub.4O.sub.7, 1 hour, 900.degree. C.);
[0029] FIG. 13A shows a cross-sectional micrograph (400.times.) of
boride layers produced by electrochemical boriding in 20% NaCl and
80% Na.sub.2B.sub.4O.sub.7 for 1 hour at 900.degree. C. and at 50
mA/cm.sup.2; FIG. 13B is for 100 mA cm.sup.2 ; FIG. 13C for 200
mA/cm.sup.2; FIG. 13D for 300 mA/cm.sup.2 and FIG. 13E for 700
mA/cm.sup.2;
[0030] FIG. 14 shows dependence of total boride and FeB layer
thicknesses to electrolyte temperature (10% NaCl+90%
Na.sub.2B.sub.4O.sub.7, 1 hour, 200 mA/cm.sup.2);
[0031] FIG. 15A shows a cross-sectional optical micrograph of a
steel sample borided at 800.degree. C. in a 10% NaCl plus 90%
Na.sub.2B4O.sub.7 for 1 hour at 200 mA/cm.sup.2; FIG. 15B is at
900.degree. C. and FIG. 15C at 1000.degree. C.;
[0032] FIG. 16 is cross-sectional optical micrograph of the corner
of a sample showing the morphology of boride layer formed at
1000.degree. C. (Magnification: 100.times.), (10% NaCl+90%
Na.sub.2B.sub.4O.sub.7, 1 hour at 200 mA/cm.sup.2);
[0033] FIG. 17A shows a cross-sectional micrograph of a steel
sample at .eta.=B.sub.2O.sub.3/Na.sub.2O ratios=1.25 with a
magnification of 100.times. treated at 900.degree. C., 200
mA/cm.sup.2 for 1 hour; FIG. 17B is for .eta.=1.5; FIG. 17C is for
.eta.=1.75; FIG. 17D is for .eta.=1.87 and FIG. 17E is for
.eta.=2.245;
[0034] FIG. 18 shows the effect of different additives in
electrolyte on thickness of borided and FeB layers (10%
Additive+90% Na.sub.2B.sub.4O.sub.7, 200 mA/cm.sup.2, 900.degree.
C., 1 hour);
[0035] FIG. 19A shows cross-sectional micrographs at 100.times. of
a steel sample borided by using a 10% NaCl additive treated at 200
mA/cm.sup.2, 900.degree. C. for 1 hour; FIG. 19B is for 10%
CaCl.sub.2; FIG. 19C is for 10% Na.sub.2CO.sub.3; FIG. 19D is for
10% NaOH; FIG. 19E is for 10% LiCl; and FIG. 19F is for 10%
BaCl.sub.2;
[0036] FIG. 20 shows X-ray diffraction spectra of low carbon steel
sample that has been borided with different additives (10%
Additive+90% Na.sub.2B.sub.4O.sub.7, 200 mA/cm.sup.2, 900.degree.
C., 1 hour);
[0037] FIG. 21 shows total boride and FeB layer thickness
dependence on NaCl concentration (Electrolyte composition: X %
NaCl+100-X % Na.sub.2B.sub.4O.sub.7; process time was 1 hour, and
the current density was 200 mA/cm.sup.2);
[0038] FIG. 22 shows the effect of bonding time on borided layer
thickness on a 99.7% pure titanium substrate treated at 950.degree.
C., 300 mA/cm.sup.2, 15% Na.sub.2CO.sub.3 and 85%
Na.sub.2B.sub.4O.sub.7;
[0039] FIG. 23 shows the relationship between diffusion layer
thickness and time for a Ti substrate (950.degree. C., 300
mA/cm.sup.2, 15% Na.sub.2CO.sub.3 ve 85%
Na.sub.2B.sub.4O.sub.7);
[0040] FIG. 24A shows a cross-sectional SEM micrograph at
1500.times. illustrating borided layers on titanium substrate after
a process time of 5 minutes of boriding at 950.degree. C., 300
mA/cm.sup.2 using 15% Na.sub.2CO.sub.3 and 85%
Na.sub.2B.sub.4O.sub.7; FIG. 24B is the same as FIG. 24A but at
2000.times.; FIG. 24C is for 10 m boriding at 3500.times.; FIG. 24D
is for 15 minutes boriding at 1500.times.; FIG. 24E is for 30 m of
boriding at 1500.times.; FIG. 24F is for 30 minutes of boriding at
3500.times.; FIG. 24G is for 1 hour of boriding at 3500.times. and
FIG. 24H is for 2 hours of boriding at 1500.times.; and
[0041] FIG. 25 shows a cross-sectional SEM micrograph with thin
nano-structured long whiskers of TiBx grown inside of the titanium
substrate (950.degree. C., 300 mA/cm.sup.2, 2 hours, 15%
Na.sub.2CO.sub.3 ve 85% Na.sub.2B.sub.4O.sub.7; and
[0042] FIG. 26 shows an optical micrograph of a selectively borided
1045 steel surfaces (950.degree. C., 300 mA/cm.sup.2, 1 hour, 50
rpm, 15% Na.sub.2CO.sub.3+85% Na.sub.2B.sub.4O.sub.7) and an area
which was not borided that was masked by a thin metal sheet before
immersing into electrolyte.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0043] In a preferred embodiment, ultra-fast boriding is carried
out in an electrochemical cell using high-temperature salt bath
electrolytes that typically consist of borax and a range of
inorganic sodium, potassium, lithium compounds (like
Na.sub.2CO.sub.3, CaCl.sub.2, NaOH, etc.). Borax is a preferred
source for boron in the electrolyte but other boron sources, such a
boron oxides, boric acids, potassium borofluoride (KBF.sub.4), and
the borates of alkaline and alkaline earth elements, as well as
various boron minerals (including ulexite
(NaCaB.sub.5O.sub.9.8H.sub.2O), colemanite
(Ca.sub.2B.sub.6O.sub.11.5H.sub.2O) and kernite
(Na.sub.2B.sub.4O.sub.6(OH).sub.2.3H.sub.2O)) may also be used in
the electrolyte. In a most preferred embodiment, the composition of
base electrolyte includes borax as the main ingredient with a
source for boron (most preferably between 30 to 95 wt. %) in
combination with sodium carbonate (between about 5 to 70 wt. %) as
the other ingredient. In addition, in a most preferred embodiment,
electrolytes enabling ultra-fast boriding include some small
amounts (0.1 to 5 wt. %) of alkaline and/or alkaline-earth halides
(such as CaCl.sub.2, NaCl, etc.). Other halides (chlorides,
fluorides, and iodides, etc.) of alkaline and/or alkaline-earth
elements can have positive effects as electrolyte enhancers.
Oxides, hydroxides, and carbonates of such elements may also be
used to control the viscosity and melting point of electrolyte. The
addition of these halides into molten electrolytes results in a
significant increase in the boriding rates and also refines the
grain size and morphology of the borided surface layers. These
halides release sufficient amounts of halide ions like Cl- into the
electrolyte bath and hence increase electrical conductivity and
surface transport activities on metal surfaces and also increase
boron intake or diffusion. While not required for operation of the
invention, it is believed that Cl ions also insure uniform current
distribution across the electrolyte which can be helpful for
achieving uniform case depth on intricate or odd-shaped work
pieces. The halide additives also make it easy to clean the work
pieces after the boriding process, since they are all water
soluble. FIG. 1B shows a schematic illustration of the main
components of a typical electrochemical cell. The electrolysis
principles apply to larger systems than shown in this figure and
FIG. 1A as long as the anode, cathode, and other components are
designed and configured accordingly. The anode is made of graphite
(but can also be made of all kinds of other materials that are
electrically conductive and compatible with high temperature
electrolytes used in the boriding process). Cathodes can be made of
any type of electrically conductive materials to which the work
pieces to be borided are attached by an appropriate manner.
[0044] Using the system shown in FIG. 1B, a variety of
electrochemical boriding treatments were performed to demonstrate
the fast nature of the boriding technique. In these experiments,
square-shaped substrates fabricated from a low-carbon steel plate
(DIN EN 10130-99 DC04, 0.004 Carbon) were used. In one case, a
molten electrolyte was prepared (composed of 10 wt % NaCl+90 wt %
Na.sub.2B.sub.4O.sub.7) and a current density of 200 mA/cm.sup.2
applied to the steel substrates at 900.degree. C. for a duration
ranging from one minute to thirty minutes. Electron microscopy and
X-ray diffraction methods were used to analyze the structure and
chemical nature of the borided steel surfaces. Micro-hardness tests
across the borided cross-sections to determine the hardness profile
as a function of depth from the top surface. As shown in a series
of photomicrographs in FIGS. 2A-2E, even in one minute, a fairly
thick (more than 10 micrometer) boride layer is formed using the
technique. After 5 minutes, the thickness of the boride layer was
26 micrometers (see FIG. 2B), after 15 minutes, the layer thickness
was 54 micrometers (see FIG. 2C) and after 30 minutes, the layer
thickness reached 95 micrometers (see FIG. 2D). After the
electrochemical boriding treatment, the power to the electrodes may
be switched off and the borided sample left in molton electrolyte
for an additional time period (e.g., as short as 10 minutes and as
long as 2 hours), resulting in elimination of the top FeB layer as
shown in FIG. 2E.
[0045] X-ray diffraction analysis of the borided steel surfaces was
performed using a Phillips diffractometer (Model PW 3710). As can
be seen in FIG. 3, at shorter boriding durations, Fe.sub.2B is the
most dominant phase, but with longer boriding time, FeB phase
begins to dominate while Fe.sub.2B decreases in intensity. It is
possible that near the interface, the Fe.sub.2B phase is still
dominant but X-ray signals from such deep regions may have been
weak or not available. Overall, the thickness or proportion of each
boride phase in the borided surface layers is dependent on several
parameters. For example, the thickness of the Fe.sub.2B phase can
be controlled by the selection and manipulation of different
process parameters, including the temperature of the electrolyte,
current density and applied voltage, the duration of the boriding
process, the chemical composition of the electrolyte, and length of
soak time in the molten electrolyte following the electrochemical
boriding process. As shown in FIG. 2E, this layer can be totally
eliminated by leaving the borided samples in the electrolyte for an
additional time period.
[0046] As shown in FIG. 4, the boride layer grows almost linearly
with boriding time. Specifically, the thickness of borided layers
with respect to boriding time is: 12 .mu.m after 1 minute, 26 .mu.m
after 5 minutes, 54 .mu.m after 15 minutes and 95 .mu.m after 30
minutes. These data points can be represented in a formulae like:
d=2.8832*t+12.09 where d is the thickness in micrometers and t is
the boriding time or duration in minutes.
[0047] In this boriding process, the NaO/B.sub.2O.sub.3 ratio was
also optimized by using additional salts (such as Na.sub.2CO.sub.3
and CaCl.sub.2) in the electrolyte bath. The reduction of Na.sup.+
and/or Ca.sup.++ ions on the cathode surface may be a key step;
through this reduction, a significant amount of boron reduction in
the molten salt bath is achieved and hence the diffusion of boron
into the metal has been accelerated. Again, as is clear from FIG.
4, boriding depth increases linearly with time in the first 30
minutes. Similar results can be achieved on other metallic or alloy
systems by preparing and using highly optimized salt bath
compositions. In this example, we used a high frequency induction
furnace for not only heating but also mixing the electrolyte
continuously during the boriding process. Heating or melting of the
electrolyte can also be done by electrical resistance heating,
external gas fires or burners or any other means that can provide
necessary heating to the electrolyte. Mixing of molten electrolyte
can be done by vibration of the work piece holder or cathode, or by
ultrasonic, magnetic, or mechanical mixing of the electrolyte by an
appropriate mixer. The rotation of the work piece or cathode may
also provide sufficient mixing of the electrolyte and hence faster
diffusion of boron atoms into the work pieces. An induction furnace
can externally be used to provide mixing of the electrolyte as
well, while melting of the electrolyte is done by resistive or
flame heating. Mixing or agitation of the electrolyte during
boriding significantly influenced borided layer thickness and the
proportion of each boride phase in the borided layer. For example,
without any type of mixing or vibrating, after 2 hours of boriding,
a boride layer thickness of about 150 micrometers is achieved in a
low carbon steel sample. When the steel sample is rotated at a
speed of 50 rpm under the same process conditions and time, the
layer thickness reached 180 micrometers. If a high-frequency
induction furnace is used, the layer thickness became 215
micrometers under the same process condition and duration.
[0048] Increasing the boriding time further (say to 60, 90, and 120
min), the nearly linear relationship between borided layer
thickness and boriding time is lost. At such longer boriding times,
the boride layer thickness continues to increase, but not at a
linear rate as shown in FIG. 5.
[0049] It has also been confirmed that the distance between the
anode and cathode (specimen or specimen holder) is important.
Specifically, this distance has a dramatic effect on the boriding
rate especially when an induction or electrical heating system is
used. The results shown in FIG. 4 were obtained from a set-up where
the distance between the anode and cathode was 2 cm; and the
electrolyte consisted of NaCl. However, similar results are
obtained with other halides and or carbonates of the alkali and
alkaline-earth metals. Further, when this distance is reduced to 1
cm, much faster boriding rates and boride thicknesses as shown in
FIG. 6 were achieved in an electrolyte which consisted of
Na.sub.2CO.sub.3. In fact, if boriding is continued for a duration
of 4 hours, the boride layer thickness becomes 230 micrometers in
the same electrolyte. FIG. 7 compares the results shown in FIG. 5
(for greater anode-cathode distance) and to FIG. 6 (shorter
anode-cathode distance).
[0050] In another aspect of the method of the invention, stirring
or agitation of electrolyte or vibrating and rotating the work
pieces during the boriding process increased the boriding rate and
insured a uniform layer thickness in intricate or odd-shaped
samples.
[0051] In another feature of the invention the boriding rate was
determined to be a function of the ratio of
B.sub.2O.sub.3/Na.sub.2O. This ratio can be adjusted, and
appropriately optimized, by introducing additional compounds such
as Na.sub.2CO.sub.3 and CaCl.sub.2. Reduction of Na.sup.+ cations
on the cathode surface may be a key step for the release of boron
atoms. Specifically, through this reduction, a significant amount
of boron is reduced to elemental form which eventually diffuses
into the work piece and thus accelerates the boriding process. As
shown in FIG. 8, as the B.sub.2O.sub.3/Na.sub.2O ratio decreases,
the thickness of the total boride and FeB layers increases
substantially for the same duration (in this case, about 1
hour).
[0052] It was also determined that boriding temperature has a
strong influence in the rate of boriding. As shown in FIG. 9, for
steel substrates, the higher the boriding temperature, the thicker
the boride layer. Note that this trend may be different for
different metals and alloys. At 800.degree. C., the thickness of
the boride layer is about 50 micrometers, but when the temperature
is increased to 1000.degree. C., the boride layer thickness reaches
more than 150 micrometers for the same boriding time. A 700 to
1100.degree. C. temperature range is most appropriate for
iron-based alloys. At much higher temperatures, there may be some
complications related to process control/handling and safety.
Durability of anode materials may also degrade. Also, certain
steels may undergo undesirable structural/chemical changes at such
high temperatures. For high-carbon or carburized steels, borided
work pieces may be quenched directly from such boriding
temperatures to achieve a martensitic structure beneath the borided
surface layers. Such a combination of a hard martensite phase with
hard boride layer on top could be desirable for demanding
mechanical applications.
[0053] The variable or high-frequency induction furnaces can also
be used for achieving a faster boriding rate in steel and other
alloys. Such furnaces not only heat, but also vigorously agitate or
mix, the molten electrolyte and hence increase the chances for free
boron atoms to reach the surface of work pieces and hence diffusing
into the structure.
[0054] Another feature of the invention for improved boriding is to
maintain a clean surface (particularly free of organic contaminants
or oxide layers). A brief grinding with 200 to 800 grit emery
papers seems to be effective in removing such contaminants and
hence increasing the boriding rate. Sometimes, applying a reverse
polarity to work pieces (e.g., making them the anode for a short
period) also seems to be effective in cleaning the work piece
surfaces. Other important parameters that can influence the rate
and quality of boriding are: current density, type of anode
materials and their positions in the bath, roughness and
cleanliness of the work piece surfaces, and geometric shape of the
work piece.
[0055] Micro hardness testing of borided surfaces in cross-section
revealed a significant increase in their hardness. Specifically,
the typical measured hardness values of borided top layer were in
the range of 1500-1900 HV, whereas the hardness of un-borided steel
was 100 HV as shown in FIG. 10A.
[0056] These and other objects, advantages, and features of the
invention, together with the organization and manner of operation
thereof, will become more apparent from the following non-limiting
examples when taken in conjunction with the figures described
hereinbefore.
Example 1
The Effect of Boriding Time on Boron Layer Thickness
[0057] The following Tables I and II show the relationship between
total boride and FeB layer thickness and boriding time.
(Electrolyte composition: % 10 NaCl+% 90 Na.sub.2B.sub.4O.sub.7;
Current density: 200 mA/cm.sup.2; Temperature: 900.degree. C.).
After the electrochemical boriding treatment, by switching off the
power to electrodes and leaving the borided sample in the molton
electrolyte for an additional time period (e.g., as short as 10
minutes and as long as 2 hours), the top FeB layer may be
eliminated.
TABLE-US-00001 TABLE I Time 1 minute 5 minutes 10 minutes 15
minutes Total Total Total Total Borided FeB Borided FeB Borided FeB
Borided FeB Layer Layer Layer Layer Layer Layer Layer Layer
Thickness Thickness Thickness Thickness Thickness Thickness
Thickness Thickness (.mu.m) (.mu.m) (.mu.m) (.mu.m) (.mu.m) (.mu.m)
(.mu.m) (.mu.m) Maximum 18.80 N/A 39.77 23.60 45.11 20.89 62.30
34.25 Minimum 6.58 7.14 8.59 30.00 14.23 40.85 20.00 Measured 15.75
30.36 16.60 41.50 16.87 56.76 23.04 Thickness 7.20 26.05 21.58
40.13 17.69 51.36 37.22 Values 14.50 16.70 40.07 16.23 61.75 8.50
28.00 34.89 56.87 13.90 37.32 40.02 59.03 10.12 21.52 41.26 46.34
Average 11.92 0 25.86 17.59 39.12 17.18 54.41 28.62
TABLE-US-00002 TABLE II Time 30 minutes 60 minutes 90 minutes 120
minutes Total Total Total Total Borided Borided FeB Borided Borided
Layer FeB Layer Layer Layer Layer FeB Layer Layer FeB Layer
Thickness Thickness Thickness Thickness Thickness Thickness
Thickness Thickness (.mu.m) (.mu.m) (.mu.m) (.mu.m) (.mu.m) (.mu.m)
(.mu.m) (.mu.m) Maximum 108.80 96.00 124.08 57.24 142.00 90.96
160.45 113.94 Minimum 54.37 62.00 76.33 27.78 88.63 47.00 111.85
48.00 Measured 94.12 88.00 109.83 29.61 119.45 79.65 158.00 97.21
Thickness 105.37 92.50 83.42 48.92 134.83 79.34 155.35 71.20 Values
94.02 86.00 84.34 51.59 114.65 64.43 113.60 89.00 96.04 78.15 106.2
39.57 115.95 49.38 129.47 76.87 101.68 84.81 126.81 115.25 100.90
103.26 86.99 113.60 128.20 Average 94.70 58.77 94.75 42.45 119.49
68.46 134.02 85.30
Example 2
Effect of Current Density on Boride Layer Thickness
[0058] In another example, the relationship was determined between
current density and total borided and FeB layer thickness, as
described in Table III below. (Electrolyte composition: % 20 NaCl+%
80 Na.sub.2B.sub.4O.sub.7; Total process time: 1 hour; Temperature:
900.degree. C.). The graphical appearance of boride layer thickness
versus current density is shown in FIG. 11.
TABLE-US-00003 TABLE III Current Density 50 mA/cm.sup.2 100
mA/cm.sup.2 200 mA/cm.sup.2 300 mA/cm.sup.2 700 mA/cm.sup.2 Total
Total Total Total Total Borided FeB Borided FeB Borided FeB Borided
FeB Borided FeB Layer Layer Layer Layer Layer Layer Layer Layer
Layer Layer Thickness Thickness Thickness Thickness Thickness
Thickness Thickness Thickness Thickness Thickness (.mu.m) (.mu.m)
(.mu.m) (.mu.m) (.mu.m) (.mu.m) (.mu.m) (.mu.m) (.mu.m) (.mu.m)
Maximum 52.68 N/A 110.96 60.12 124.08 57.24 112.80 68.31 142.66
98.624 Minimum 20.04 55.00 12.42 76.33 27.78 58.41 28.61 70.00
23.66 Measured 49.40 97.00 50.00 109.83 29.62 111.36 65.97 96.00
67.71 Thickness 50.82 103.00 35.80 83.42 48.92 69.67 123.42 85.24
Values 48.00 99.64 42.90 84.34 51.59 107.35 120.00 80.00 29.50
63.54 106.20 39.57 78.01 90.77 65.00 52.25 93.86 84.81 103.07 85.17
51.80 102.30 86.99 68.73 120.00 Average 44.31 0 90.66 40.24 94.75
42.45 88.67 54.29 106.00 70.03
Example 3
Relationship Between Electrochemical Cell Potential and Current
Density in Molten Electrolyte
[0059] The relationship between cell potential and the current
density (20% NaCl+80% Na.sub.2B.sub.4O.sub.7, 1 hour, 900.degree.
C.) is illustrated in FIG. 12 from a set of measurements and cross
sectional micrographs of the boride layers produced at different
current densities are shown in FIGS. 13A-13E for various current
densities for an electrolyte of 20% NaCl plus 80%
Na.sub.2B.sub.4O.sub.7 at 1 hour and 900.degree. C. Cell potential
directly related with resistivity of electrolyte, in general 1.5-6V
cell potential is the expected range for the working current
density applications. Depending on electrolyte resistance cell
potential can be as high as 20V. In addition to direct current
(DC), the cell potential may be applied in the radio-frequency (RF)
(MHz range), bi-polar pulse DC (Hz to kHz range, different wave
forms; e.g. square, sine, triangle sawtooth etc.), and high power
impulse modes, or any other modes available. In particular, the use
of pulse DC and RF may prevent any type of diffusion barriers
forming on work piece surfaces and hence slowing down the boriding
process.
Example 4
The Effect of Process (Boriding) Temperature on Borided Layer
Thickness
[0060] The relationships between total borided and FeB layer
thickness and electrolyte process temperature and micrographs
(Electrolyte: 10% NaCl+90% Na.sub.2B.sub.4O.sub.7; Process time: 1
hour, Current density: 200 mA/cm.sup.2) are shown in Table IV and
FIGS. 9, 14 and 15A-15C.
TABLE-US-00004 TABLE IV Electrolyte Temperature 800.degree. C.
900.degree. C. 1000.degree. C. Total Total Total Borided FeB
Borided Borided Layer Layer Layer FeB Layer Layer FeB Layer
Thickness Thickness Thickness Thickness Thickness Thickness (.mu.m)
(.mu.m) (.mu.m) (.mu.m) (.mu.m) (.mu.m) Maximum 61.80 32.55 124.08
57.24 206.00 115.75 Minimum 27.40 11.50 76.33 27.78 90.50 35.79
Measured 53.00 27.21 109.83 29.61 169.00 106.60 Thickness 58.00
27.20 83.42 48.92 93.30 65.61 Values 61.75 17.54 84.34 51.59 161.00
97.35 59.50 12.25 106.20 39.57 198.96 85.22 36.60 84.81 144.58
136.00 51.20 86.99 165.20 50.70 Average 51.15 21.37 94.75 42.45
153.56 86.63
Example 4
The Effect of B2O3/Na2O Ratio in Electrolyte on Total Boride and
FeB Layer Thicknesses
[0061] The relationship between .eta.=B.sub.2O.sub.3/Na.sub.2O
ratio and boride layer thickness (900.degree. C., 200 mA/cm.sup.2,
1 hour) is shown in Table V and FIGS. 8, 17A-17E.
TABLE-US-00005 TABLE V .eta. .eta. = 1.25 .eta. = 1.5 .eta. = 1.75
.eta. = 1.87 .eta. = 2.245 Total Total Total Total Total Borided
Borided FeB Borided FeB Borided FeB Borided FeB Layer FeB Layer
Layer Layer Layer Layer Layer Layer Layer Layer Thickness Thickness
Thickness Thickness Thickness Thickness Thickness Thickness
Thickness Thickness (.mu.m) (.mu.m) (.mu.m) (.mu.m) (.mu.m) (.mu.m)
(.mu.m) (.mu.m) (.mu.m) (.mu.m) Maximum 190.29 98.30 120.27 73.00
120.27 75.67 138.56 74.65 106.20 49.52 Minimum 113.00 36.00 88.00
45.00 86.71 35.00 81.25 26.24 72.21 21.26 Measured 166.79 60.80
119.63 72.318 103.87 66.50 112.58 48.70 73.00 48.18 Thickness
137.11 75.65 113.86 50.81 110.84 60.80 97.00 64.18 100.00 36.00
Values 166.78 72.50 95.90 68.85 110.74 38.00 119.20 31.00 105.13
24.03 153.38 77.30 119.11 49.00 105.63 49.52 91.70 30.50 103.00
36.00 169.56 50.00 110.00 108.00 61.11 110.00 57.30 96.00 173.65
118.12 116.97 106.00 97.00 Average 158.82 67.22 110.61 59.83 107.88
55.23 107.04 47.51 94.07 35.83
Example 5
The Effect of Electrolyte Composition on Boride Layer Thickness
[0062] Effect of different additives in electrolyte on thickness of
borided layer (10% Additive+90% Na.sub.2B.sub.4O.sub.7, 200
mA/cm.sup.2, 900.degree. C., 1 hour) and the results are shown in
Table VI and VII and FIGS. 18, 19A-19F and 20. Hardness values of
borided layers produced by using different additives, the hardness
values are measured on a cross-sectional surface.
TABLE-US-00006 TABLE VI Additives NaCl CaCl.sub.2 LiCl BaCl.sub.2
Na.sub.2CO.sub.3 NaOH Total Total Total Total Total FeB Total FeB
Borided Borided Borided Borided Borided Layer Borided Layer Layer
FeB Layer Layer FeB Layer Layer FeB Layer Layer FeB Layer Layer
Thick- Layer Thick- Thickness Thickness Thickness Thickness
Thickness Thickness Thickness Thickness Thickness ness Thickness
ness (.mu.m) (.mu.m) (.mu.m) (.mu.m) (.mu.m) (.mu.m) (.mu.m)
(.mu.m) (.mu.m) (.mu.m) (.mu.m) (.mu.m) Maximum 124.08 57.24 127.06
81.60 106.67 53.00 113.08 75.63 138.56 74.65 110.94 59.30 Minimum
78.33 27.78 70.00 38.00 66.56 21.30 66.00 15.30 81.25 26.24 99.43
44.00 Measured 109.83 29.61 115.60 78.00 77.76 46.66 104.43 71.49
112.58 48.70 104.35 50.50 Thickness 83.42 48.92 78.50 39.00 84.82
27.57 89.98 60.67 97.00 64.18 102.40 55.87 Values 84.34 51.59
124.13 65.32 78.40 36.00 90.54 56.52 119.20 31.00 105.68 48.00
106.20 39.57 120.50 79.70 69.96 40.30 81.96 62.04 91.70 30.50
107.02 84.81 110.90 86.00 77.29 52.68 75.26 34.05 110.00 57.30
110.67 86.99 86.60 82.12 30.00 100.49 46.37 106.00 114.37 Average
94.75 42.45 104.16 66.80 80.45 38.44 90.22 52.76 107.04 47.51
106.86 51.53
TABLE-US-00007 TABLE VII Additive FeB Fe.sub.2B Load Pure Borax
1460 1120 100 g NaCl 1750 1340 100 g CaCl.sub.2 2074 1580 100 g
Na.sub.2CO.sub.3 1890 1572 100 g NaOH 1885 1595 100 g LiCl 1350
1205 25* g. BaCl.sub.2 1800 1412 100 g *low load used.
Example 6
The Effect of NaCl as an Additive at Different Concentrations on
Thickness of Borided Layers
[0063] Borided and FeB layer thickness dependence on NaCl additive
concentration in electrolyte (Electrolyte composition: X %
NaCl+100-X % Na.sub.2B.sub.4O.sub.7; process time was 1 hour, and
the current density was 200 mA/cm.sup.2) and results shown in Table
VIII and FIG. 21.
TABLE-US-00008 TABLE VIII NaCl Ratio 10% NaCl + 90% 20% NaCl + 80%
100% Na.sub.2B.sub.4O.sub.7 Na.sub.2B.sub.4O.sub.7
Na.sub.2B.sub.4O.sub.7 Total Total Total Borided Borided Borided
Layer FeB Layer Layer Layer FeB Layer Thickness Thickness Thickness
FeB Layer Thickness Thickness (.mu.m) (.mu.m) (.mu.m) Thickness
(.mu.m) (.mu.m) (.mu.m) Maximum 106.19 49.51 124.08 57.24 145.00
96.67 Minimum 72.20 21.26 76.32 27.78 95.91 29.34 Measured 73.00
109.83 29.61 135.28 85.18 Thickness 100.00 48.18 83.42 48.92 125.97
83.02 Values 105.13 36.00 84.34 51.59 128.83 78.72 103.00 24.03
106.2 39.57 121.74 49.40 96.00 36.00 84.81 106.45 97.00 86.99
129.30 Average 94.07 35.83 94.75 42.45 123.56 70.4
Example 7
Boriding of Non-Ferrous Material (Titanium)
[0064] Effect of boriding time on borided layer thickness on a
99.7% pure titanium substrate is shown in FIGS. 22-25. Process
conditions and electrolyte compositions were: 950.degree. C., 300
mA/cm.sup.2, 15% Na.sub.2CO.sub.3 and 85% Na.sub.2B.sub.4O.sub.7.
The results with respect to borided time are shown in Table VIII
and FIGS. 22, 23, 24A-24H and 25.
Example 8
Selective Boriding of Metal Surfaces
[0065] As shown in FIG. 26, metal surfaces can be masked
selectively and only the areas that are not masked will be borided.
In another method, a burner flame can be used to melt an
electrolyte in a preferred area and then an electrochemical cell
can be established on that area to selectively boride the area
where the electrolyte is present. As shown, only the exposed
surface was borided. The transition from the borided to the
unborided surface is sharp (40 .mu.m). This sharp transition cannot
be achieved by traditional boriding processes and could be
important for some applications. Various coating methods (such as
electroless or electrochemical plating) can be used to deposit thin
layers of copper or other metals to mask the areas where boriding
is not desired.
[0066] As demonstrated from the Examples provided above, ultra-fast
boriding can be achieved in both the ferrous and non-ferrous metals
and alloys. These borided metals and alloys can be used in a
variety of manufacturing, earthmoving, agricultural, aerospace, and
transportation applications such as metal forming tools, fuel
injectors, gears, bearings and some of the power- and drive-train
applications in cars and tracks, blades and cutters used in
agricultural, forestry, and earthmoving applications. Turbine and
helicopter blades, impellers, mixers, and other components subject
to wind, sand, and solid particle slurry erosion or abrasion can
also be treated by the new method and protected. More specifically,
these borided surface layers can prevent wear and scuffing between
heavily loaded rolling, rotating, or sliding surfaces under
lubricated sliding conditions which are typical of these mechanical
components and others (like chain links used in conveyor belts and
other heavy machinery such as earth-moving equipments etc.). One of
the most important features of these borided surfaces is their
ability to function under severe loading conditions and provide low
friction and wear with and without lubrication.
[0067] This new ultra-fast boriding process can also be used to
boride the pre-carburized and nitrided surfaces. In the case of
pre-carburized steel surfaces, a compound layer consisting of not
only iron borides but also boron carbides, free boron and carbon
are also formed. In the case of pre-nitrided surfaces, a compound
layer consisting of not only iron borides but also boron nitride
and free boron are produced. Surfaces that are ion-implanted, or
laser-cladded, and alloyed with various elements may also be
borided by the new technique and the borides of such elements
formed during boriding can then provide greater hardness and other
desirable properties such as low friction and wear and greater
protection against corrosion and erosion as well a better
biocompatibility and/or reactivity.
[0068] Ultra-fast boriding is environmentally benign and there are
no toxic raw materials involved and by-products to discard or deal
with after the boriding process. The process also does not produce
any fumes or green-house gases. In the other boriding processes new
baths are needed and the old ones must be discarded properly and in
the case of gas-phase boriding, there are some toxic gases that
need to be handled carefully. One of the other advantages of the
new process is that the electrolyte can be re-used multiple times.
There is no need to discard and re-supply active ingredients
(except for the boron compounds). Again, the new process is
environmentally benign and there are no toxic by-products to
discard or deal with. In the new process, there is little deposit
to clean from the borided surface, remaining deposits (mainly
salts) are washed away in running water or removed by mechanical
brushes or tumbling in a sand box.
[0069] While several different features and embodiments are
described above, it is understood that changes and modifications
can be made to the invention without departing from the invention's
broader aspects. Therefore, the present invention is not limited to
the described and illustrated embodiments, but only by the scope
and spirit of the independent and dependent claims.
* * * * *