Note: Descriptions are shown in the official language in which they were submitted.
CA 02645287 2008-11-27
COMPOSITE ARTICLE AND METHOD
FIELD OF THE INVENTION
[0001] The present invention relates to methods for forming a protective dark
oxide layer or
coating upon an article comprising zirconium, and to articles formed thereby.
More
specifically, the invention relates to the formation of a protective dark
oxide layer upon an
article consisting of certain binary zirconium titanium alloys, and to the
articles formed
thereby.
BACKGROUND OF THE INVENTION
[0002] With the increase in civil use of what was considered during the "cold
war" years as
"strategic" or "restricted" metals, such as zirconium and titanium, and the
relatively recent
drop in their prices, an increasing number of consumer goods, medical, dental
and
orthopedic devices, civil engineering and architectural structural and
decorative
components, and other industrial as well as civil and military uses have been
made of these
metals and alloys thereof. With this increase in use, there has been a growing
interest in the
unique metallurgical properties of these metals and advantages as employed in
known and
new applications. These unique metallurgic properties include very high
tensile and yield
strength, light weight, and chemical inertness together with its corollary
hypoallergenic
property, which makes these metals and alloys suitable for dental, orthopedic
and other
prostheses such as joint replacements, arterial stents, and cardiac valves, as
well as for
consumer fashion accessories that benefit from the same properties, such as
body-piercings,
wrist watches, sunglass frames, and the like.
[0003] Increased interest in these metals and their uses has been accompanied
by demand for
methods for providing hardened surfaces, for providing surfaces exhibiting
reduced friction,
and for improving surface appearance. At the same time, their strength, low
elasticity and
ductility has rendered them materials suitable for night-stealth activities,
from hunting to
law enforcement and the military, for which dark colours are preferred, once a
method for
the generation and control of those colours and their shades of darkness is
provided.
[0004] Anodizing is known for altering the colour and surface appearance of
titanium and
niobium. Anodizing of these metals and certain of their alloys generates a
thin, colourful
outer layer on the metal, which wears off readily and is easily scratched,
chipped, or
otherwise removed.
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[0005] US Patent Number 6,093,259 to Watanabe et al. teaches methods for
providing
various coloured surfaces on titanium by treatment with aqueous alkaline
solutions of KOH,
NaOH and Li0H, applied singly or as a mixture, optionally accompanied by
thermal
treatment at moderate temperatures and optionally comprising a nitriding
process.
[0006] US Patent Number 5,037,438 to Davidson, and US Patent Number 5,169,597
to
Davidson et al., disclose surface treatment of another cold war metal,
zirconium, by thermal
or salt bath oxidation within temperature ranges readily achievable by
conventional kilns,
for improving mechanical and metallurgical properties. The resulting smooth
and very hard
blackened surface reportedly reduced friction, increased scratch resistance,
enhanced the
strength of the metal immediately beneath the surface coating and provided a
blue/blackness coloured surface. These enhancements were attributed to oxygen
diffusion
into the substrate metal, which also improved the fatigue properties of the
metal.
[0007] In attempting to produce articles that require or would benefit from
the combination
of high tensile strength, hardness, scratch and wear resistance, and colour
control from dark
gray to blackness, light weight, and hypoallergenicity, it is known that
zirconium and
titanium provide these benefits to varying degrees.
[0008] However, unalloyed titanium coloured according to the method taught by
Watanabe
et al. does not exhibit enhanced resistance to wear and generally retains the
properties of
untreated titanium. Also, the method requires the use of hazardous materials,
personal
safety equipment such as gas masks, impermeable gloves, complete skin
coverage, and the
like.
[0009] The use of unalloyed zirconium to the extent taught by Davidson is
limited to
unalloyed zirconium or alloys containing at least 80% zirconium, and
preferably from about
95% to about 100%, by weight. In contrast, Davidson et al. teaches the use of
a ternary alloy
including niobium, adding cost and complexity compared to binary alloys.
Davidson and
Davidson et al. are primarily directed to weight bearing prosthetic implants
for which colour
control is relatively unimportant.
[0010] While unalloyed zirconium displays high tensile strength,
hypoallergenicity, and a
beneficial surface coating when oxidized, it is known that alloys containing
both zirconium
and titanium offer superiour metallurgical properties compared to each metal
alone. (See
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Yoshiaki, I. et al. "Improved Biocompatibility of Titanium-Zirconium (Ti-Zr)
Alloy: Tissue
Reaction and Sensitization to Ti-Zr Alloy Compared with Pure Ti and Zr in Rat
Implantation" Mater. Trans. 46(10) : 2260-2267 (2005) (teaching superiour
biocompatibility of
Ti-Zr alloys compared to each metal alone)).
[0011] Certain ratio ranges of zirconium to titanium exhibit superiour
mechanical properties
compared to the component metals in the unalloyed state. (See Kobayashi, E.
"Mechanical
properties of the binary titanium-zirconium alloys and their properties for
biomedical
purposes" J. Biomed Materials Research 29(8) (1995)). Alloys in the range of
1:1 zirconium to
titanium by weight, disclosed for use as dental implants, exhibit hardness and
tensile
strength about 2.5 times as high as the unalloyed components. These results
were reported
for both cast and homogenized specimens.
[0012] Ternary alloys containing zirconium, titanium and a third metal are
also known for
applications including prostheses. US Patent Numbers 5,820,707 to Amick et al.
teaches
ternary alloys including a third metal selected from niobium, tantalum and
vanadium. The
third metal is taught as passivating the tendency of the zirconium and
titanium to ignite and
combust. Amick et al. teaches very high temperatures and long duration for
complete or
near complete oxidation of the alloy workpiece which, therefore, requires
passivation
through the inclusion of the third metal in the alloy. The method reportedly
provides
smooth and hard surfaces which for some alloys are described as being
"blue/blackness".
[0013] US Patent Number 6,759,134 to Rosenberg discloses ternary alloys
containing
titanium, niobium, and a third metal from the group consisting of zirconium,
tantalum,
molybdenum, hafnium, zirconium, chromium and mixtures thereof, with emphasis
on
alloys containing from 3% to 17% by weight niobium for its passivating
properties and for
the creation of a smooth and hard surface layer of niobium containing oxide
with an
aesthetic chromatic value.
[0014] However, Amick et al. and Rosenberg require at least a ternary alloy
and do not teach
control of the surface shade on a scale from dark gray to blackness. Amick et
al. and
Rosenberg also do not teach the benefits of enhanced tensile strength of the
treated alloy.
[0015] In summary, Yoshiaki et al. and Kobayashi et al. teach binary zirconium
titanium
alloys of specified weight ratio that possess good metallurgical, mechanical
and
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hypoallergenic properties. The ternary alloys of Amick et al. and Rosenberg
are more
intricate and costly to produce and have not been shown to possess the
additional strength
and hypoallergenic benefits of the binary alloy. Amick et al. furthermore,
requires the
application of very high temperatures requiring costlier equipment and
production
equipment, and/or longer process duration a parameter that adds to the costs
too.
Davidson and Davidson et al. teach the benefits of zirconium based alloys
comprising a
zirconium oxide coating, focusing predominantly on very high zirconium
contents in the
alloys, and the suitability to certain prosthetic tasks, while Rosenberg and
Amick et al. offer
combinations that rely upon the presence of niobium oxide in the coating which
form of the
oxide was not shown to possess the same enhanced strength and fatigue
resistance as the
primarily zirconium oxide coating disclosed by Davidson
[0016] While the prior art provides a subset of the group of properties
required by and
benefiting various articles, namely, high tensile strength, high hardness, low
ductility and
elasticity, enhanced fatigue resistance, and biocompatability, the prior art
does not teach the
capability to combine the full scope of all of these benefits and the
advantages in the
capability to have controllable shades of dark gray to blackness nor does it
offer the benefits
of simplicity and cost reduction to be gained through the use of a binary
alloy.
[0017] Therefore, there is a need in the art for alloys and surface coatings
capable of
providing articles exhibiting all of the potential beneficial properties
available from
zirconium titanium binary alloys. All this and more will become apparent to
one of ordinary
skill upon reading the following disclosure and claims.
SUMMARY OF THE INVENTION
[0018] The present invention is directed in one aspect to a method for
overcoming the
aforementioned disadvantages and limitations of the prior art by providing a
method for
darkening and hardening the surface of an article consisting of a binary
zirconium titanium
alloy of specified compositions. The inventor has found a synergistic
combination, within
articles produced by the method, of the metallurgical, mechanical, and
hypoallergenic
advantages of certain binary zirconium titanium alloys, combined with a
hardened,
darkened surface that resists abrasion, and has a colour from gray to
blackness that is
selectable according to the parameters of the method.
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[0019] In a first aspect, a method having features of the present invention
includes a step of
providing an article consisting of between about 30.9% and about 65.6%
zirconium by atomic
weight and titanium. Without limitation, the articles can be formed into their
desired shapes by
machining, casting, die forging, stamping, or the like. The articles may
alternatively be formed
by laser cut forming or hot isostatic pressing (HIP), both techniques known in
the prior art. The
articles optionally comprise a polished, satin, or matte finish, which
influences the texture of the
finished blackened surface. The method further comprises heating the article
in an oxygen
containing atmosphere at a temperature of between about 250 and about 880
degrees Celsius for
between about 10 and about 110 minutes to produce the hardened, darkened
surface. In certain
preferred aspects, the alloy consists of between about 34.4% and about 65.6%
zirconium by
atomic weight.
[0020] In a second aspect, a method having features of the present invention
includes a step of
providing an article consisting of between about 18.496% and about 30.9%
zirconium by atomic
weight and titanium. The method further comprises heating the article in an
oxygen containing
atmosphere in a first heating step and a second heating step with a quenching
step interposed,
the heating steps being performed at a temperature of between about 250 and
about 880 degrees
Celsius for a total duration of between about 10 and about 110 minutes to
produce the
hardened, darkened surface. Optionally, the first heating step is performed at
a lower
temperature than said second heating step. For example, and without
limitation, in certain
embodiments, the first heating step is carried out at a temperature of between
about 250 and
about 480 degrees Celsius for between about 10 and about 40 minutes, and the
second heating
step is carried out at a temperature of between about 480 and about 880
degrees Celsius for
between about 10 and about 70 minutes.
[0021] In certain embodiments, the oxygen containing atmosphere is air.
[0022] In another aspect, an article having features according to the present
invention comprises
zirconium titanium binary alloy article consisting of between about 30.9% and
about 65.6%
zirconium by atomic weight and titanium further comprising a darkened oxide
containing
surface or portion thereof produced according to one of the foregoing methods.
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[0023] In certain preferred aspects, the article consists of between about
34.4% and about 65.6%
zirconium by atomic weight.
[0024] In another aspect, an article having features according to the present
invention
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comprises zirconium titanium binary alloy artide consisting of between about
18.4% and
about 30.9% zirconium by atomic weight and titanium further comprising a
darkened oxide
containing surface or portion thereof produced according to one of the
foregoing methods.
[0025] It is therefore an object of the present invention to provide articles
that require or
benefit from any combination of properties from within the group comprising,
without
limitation, high tensile strength, high hardness, resistance to fatigue or
wear or scratch, low
ductility and elasticity, hypoallergenicity, and shades of gray and blackness.
Heating
duration may be extended for larger articles.
[0026] It is a further object of the invention to provide articles comprising
an aesthetic outer
surface or coating that exhibits shades from gray to blackness.
[0027] It is a further object of the invention to provide articles comprising
a darkened surface
suitable for stealth goods, hunting and sporting equipment, and body
adornments that are
night stealthy such as a soldier's bracelet or ring.
[0028] It is a further object of the invention to provide a ceramic-like
coating that exhibits
low wear and low friction suitable for articles requiring extended periods of
mechanical
contact, such as for example butterfly valves.
[0029] It is a further object of the invention to provide a matte or satin
coating that has low
reflectivity and is suitable for nighttime stealth articles.
[0030] These and other features, aspects, and advantages of the present
invention will
become better understood with reference to the following description and
appended claims.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows a cross-section of an article according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0031] The composition of binary zirconium titanium alloys in the present
disclosure are
expressed by atomic weight ratio, in part to draw attention to the possibility
of using
unalloyed titanium and zirconium in preparing the alloys according to the
present
invention, despite the market's offerings of various popular alloys of
titanium and
zirconium. To convert from atomic weight ratio to actual weight ratio, the
product of a
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selected element's atomic weight and its atomic weight ratio in percentage is
divided by the
sum of such products for the alloy constituents. For example, a ratio of
34.42% zirconium to
65.58% titanium by atomic weight, given an atomic weight for Ti of 47.867 and
an atomic
weight for Zr of 91.224, provides the following for the titanium ratio by
weight:
(47.867 x 65.58%) : (47.867 x 65.58% + 91.224 x 34.42%) x 100 =
49.99%
[0032] The binary zirconium titanium alloys for use in the methods and
articles according to the
present invention consist of between about 18.4% and about 65.6% zirconium by
atomic weight
and titanium. Trace amounts of impurities, including other metals, may be
present to an art-
recognized degree. Certain alloys for use in the present invention can be
purchased from any of
several metal alloy-producing mills producing zirconium titanium alloys
worldwide, and in
particular in North America, and in Central and Eastern Europe.
[0033] In preferred embodiments, a binary alloy consisting of from about
65.58% titanium by
atomic weight (about 50% by weight) and about 34.42% zirconium by atomic
weight (about 50%
by weight) is used, or alloys are used that fall within about 4% of these
values. Kobayashi et al.
(supra) teaches superior strength and hardness, up to 2.5 fold, of these
alloys compared to pure
zirconium and titanium.
[00341 The articles of the present invention can be made by any means known in
the art for
shaping zirconium titanium alloys, including without limitation machining,
casting, stamping,
or die-forging. Laser cut forming or hot isostatic pressing (HIP), both known
in the prior art,
may alternatively be used. It is known that certain compositions of zirconium
titanium alloys
are ignitable (see, e.g. US Patent No. 5,820,707 to Amick et al.) and highly
reactive so due care
must be taken when working such alloys. Machining requires precautionary
measures as are
known in the art, including but not limited to slow speeds and liberal
lubrication and cooling.
Likewise, opening of a casting investment must be performed only after
complete cooling.
[00351 The article is heated, preferably by heating in a kiln providing an
oxygen containing
ambient gas, to within the range of about 250 degrees Celsius to about 880
degrees Celsius. In
certain embodiments, a single heating step is provided comprising a duration
of from
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about 10 to about 110 minutes, followed by air cooling, water quenching, or
the like.
[0036] The inventor has found that with an increased gas supply, oxidation
proceeds more
rapidly but is accompanied by an increased risk of combustion. In preferred
embodiments, a
kiln is selected to have a moderate and unforced air supply in the range of 4
to 6 square
inches per cubic foot of kiln volume.
[0037] Preferably, two heating steps are used, with a quenching step such as a
water or air
quenching interposed between the heating steps. The temperature and duration
of heating
are selected to provide a strongly adherent oxide-rich layer with the desired
shade from
gray to blackness and sufficient wear resistance. Outer layers or coatings
having a darker
appearance exhibit excellent resistance to wear and penetrate somewhat deeper
into the
substrate alloy. Where the surface of the article to be treated is polished, a
smooth coating is
obtained that is sufficiently hard and wear resistant to be particularly
suited for uses
involving frequent sliding contact with other surfaces, or impacts, or the
like. Articles to be
treated that have a brushed surface texture provide lighter shades.
[0038] Referring now to FIG. 1, there is shown an article according to the
present invention
comprising a zirconium titanium alloy 100 and a darkened surface 102.
[0039] Most preferably, a first heating step at a lower temperature is
followed by a second
heating step, with a quenching step interposed between the two. This process
has been
found beneficial to reduce ignition risk. In preferred embodiments, a first
heating step can
comprise heating to between about 250 degrees Celsius to about 480 degrees
Celsius for
between about 10 to about 40 minutes. Following an optional quenching step, a
second
heating step can be performed by heating to between about 480 degrees Celsius
to about 880
degrees Celsius for up to about 100 minutes or until a predetermined gray tone
or degree of
blacknessness is obtained.
[0040] Not to be thereby limited by theory, the thermal treatment of the
present invention
provides an oxide layer that is believed to comprise a high proportion of
zirconium oxide
and to further harden and strengthen the metal by the diffusion of oxygen
within the
partially oxidized surface layer, and in the deeper alloy substrate to which
it is adherent. In
embodiments comprising two heating steps, it is believed that a thicker final
oxide layer is
formed due to the possibility that oxygen penetrates more deeply into the
substrate metal
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during a first, lower temperature, step than it does if exposed to an initial
higher
temperature that produces a more rapid thickening of the oxide layer.
[0041] Articles treated according to the method of the present invention are
less susceptible
to subsequent ignition. Exposure of treated samples to direct flame in the
range of 1,300 to
1,400 degrees Celsius for up to ten minutes failed to combust or undergo
further oxidation.
This property usefully extends the range of applications of the present
invention to include,
for example, firearm parts, subject to proper testing and certification.
EXAMPLES
[0042] Alloys for use in the method and article of the present invention are
exemplified in
TABLE 1:
TABLE 1
ALLOY Zr by atomic wt. % Ti by atomic wt.
34.42 65.58
II 33.52 66.48
III 30.89 69.11
IV 40.05 59.95
V 67.37 32.63
[0043] In TABLE 2, it is demonstrated that the duration and temperature of
thermal
treatment can be adjusted to control the resulting shade of gray or
blacknessness in the
resulting article. A darker surface is obtained with longer and/or hotter
treatment, while a
lighter gray finish is obtained at lower temperatures and/or shorter duration.
[0044] Results obtained with the present invention are compared in TABLE 2
with unalloyed
zirconium (Zr702) and a zirconium alloy with low levels (2-3%) of niobium
(Zr705).
TABLE 2
(Formation of a darkened, hardened coating according to the method of the
present
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invention.)
Alloy 1st cycle 1st cycle 2nd cycle 2nd cycle
Resulting
(min) (C) (min) (C) surface*
1 II 25 250 50 750 Dark
blacknessness,
smooth*
2 VI 35 250 35 680 Medium
blacknessness,
smooth*
3 II 30 350 30 725 Pitch
blackness,
smooth*
4 VI 40 650 Light
blacknessness,
smooth*
5 II 65 600 Medium
blacknessness,
matte
6 VI 30 250 80 480 Medium gray,
smooth*
7 II 25 650 Medium
charcoal gray,
smooth*
8 VI 13 880 Light to
mediumcharcoal gray,
smooth*
9 11 880 Light to
o
medium gray,
smooth*
10 Zr702 35 620 Medium
charcoal,
matte
11 Zr702 70 700 Pitch
blackness,
smooth*
12 Zr705 40 650 Light
charcoal,
matte
13 Zr702 25 300 26 600 Medium to
dark charcoal,
matte
*articles polished prior to treatment
[0045] In a further example, a night-stealth automotive and/or stealth
automotive hubcap is
provided. The hubcap is cast into the desired shape and provided with a satin-
like low-
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polish. The hubcap is then heated to within the range of 250 to 350 degrees
for from 10 to 40
minutes. Next, the hubcap is heated by the same method for 20 to 40 minutes at
600 to 700
degrees Celsius. The hubcap is from charcoal gray to blacknessness in
appearance and has a
matte, wear resistant surface.
[0046] In use, the method of the present invention is used to produce articles
that are also
encompassed by the present invention. The articles can be any article
consisting of
zirconium titanium alloy within the composition range of the present invention
that requires
or may benefit from a hard, tough, gray to blackness outer surface layer.
Without limitation,
articles within the scope of the present invention can include articles that
comprise pivoting
or swiveling parts such as revolving disk and butterfly valves, cardiac
valves, and valves for
liquids and gases. In these applications, the swiveling parts can be
springingly retained
about their axis by insertion under tensile stress between mounting points, or
more loosely
retained. Other article embodiments can include dental implants and medical
prostheses
such as joint and bone replacements. Further, the present invention can
provide a tough and
attractive outer surface to sporting goods such as golf clubs, durable and
night-stealth
hunting goods such as knives, outdoor equipment such as binocular outer
casings, bow
coatings, water canteens, field-compasses and the like. The articles according
to the present
invention can be stealth items such as for law enforcement and armed forces,
such as
helmets, buckles, ID tags, night vision equipment, laptop and communications
and data
storage equipment casings, firearms and parts thereof such as sights,
triggers, cartridges,
magazines, barrels, and the like. Other articles within the scope of the
present invention are
night-stealth compatible jewellery items of low wear, for use by armed forces
and law
enforcement personnel, for example bands including bands that benefit from the
invention's
metallurgical attributes of excellent strength, tensile strength, low
elasticity and ductility,
and, therefore, strong springiness, wherein a stone may be set under pressure
and retained
between two connected portions of the band. Such stealth compatible and low
wear
jewellery items can further comprise wedding bands, buckles, bracelets,
chains, earrings,
watches, chains, sunglass frames, cuff links, tie-pins, money or document
clips, bracelets and
necklaces. The darkened surface of jewellery according to the present
invention can provide
an aesthetic and/or a utilitarian function. Yet further, marine and/or night-
stealth marine
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items such as boat masts, deck handles, steering wheels, throttles; automotive
and/or stealth
automotive parts such as gearshift levers, hubcaps, steering wheels; and
household items
such as door handles, cabinet handles, keys, cutlery, faucets, light fixtures
and kitchen
implements can all be provided within the scope of the present invention. Yet
further
examples can include musical instruments, such as brass instruments with
valves, for which
the hypoallergenic and excellent wear resisting properties of the present
invention are well
suited. Still further examples include architectural structural and surface
materials for which
a darkened surface prepared according to the method of the present invention
can be used
to alter the structural and surface material thermal properties and the
lightness of the alloy.
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