|Title of Invention||
"METALLIC THERMAL BARRIER COATING "
|Abstract||Metal alloys having low electrical and thermal conductivity including relatively large fractions of P-Group element additions. The P-Group elements may be selected from the group including phosphorous, carbon, boron, and silicon. The resultant alloys do not exhibit significantly increased brittleness, and are applied as a coating that provides a metallic thermal barrier coating.|
|Full Text||Field of the Invention
The invention relates to a metallic thermal barrier coating. This invention is more particularly directed at metallic alloys, and more particularly at unique metallic alloys having low electrical and thermal conductivity. In coating form, when applied, such alloys present the ability to provide thermal barrier characteristics to a selected substrate.
Background of the Invention
Metals and metallic alloys have metallic bonds consisting of metal ion cores surrounded by a sea of electrons. These free electrons which arise from an unfilled outer energy band allow the metal to have high electrical and thermal conductivity which makes this c lass of m aterials c onductors. D ue to the nature o f the metallic bonds, metals and metallic alloys may exhibit a characteristic range of properties such as electrical and thermal conductivity. Typical metallic materials may exhibit values of electrical resistivity that generally fall in a range of between about 1.5 to 145 10-8 Qm, with iron having an electrical resistivity of about 8.6 10-8 Qm. Typical values of thermal conductivity for metallic materials may be in a range of between about 0.2 to 4.3 watts/cm°C, with iron exhibiting a thermal conductivity of about 0.8 watts/cm°C.
By contrast, ceramics are a class of materials which typically contain positive ions and negative ions resulting from electron transfer from a cation atom to an anion atom. All of the electron density in ceramics is strongly bonded resulting in a filled outer energy band. Ceramic alloys, due to the nature of their ionic bonding, will t- exhibit a different characteristic range of properties such as electrical and thermal conductivity. Because of the lack of free electrons, ceramics generally have poor electrical and thermal conductivity and are considered insulators. Thus, ceramics may be suitable for use in applications such as thermal barrier coatings while metals are not.
Designing a metal alloy to exhibit ceramic like electrical and thermal
conductivities is unique- The only area where this has been utilized in material
science is in the design of soft magnetic materials for transformer core applications.
In such applications, exit* silicon is added to iron in order to specifically reduce the
electrical conductivity to minimize eddy current losses. However, iron-silicon alloys
utilized for transformer cores typically contain a maximum of 2.5 at% (atomic
percent) silicon because any additional silicon embrittles the alloy. Additionally,
attempts to reduce electrical conductivity of iron transformer cores have not addressed
reduced thermal conductivity.
Summary of the Invention
A metal alloy comprising an alloy metal and greater than about 4 atomic % of
at least one P-group alloying element fa method form, a method of reducing the
thermal and/or electrical conductivity of a metal alloy composition comprising
supplying a base metal with a free electron density, supplying a P-group alloying
element and combining said P-group alloying element with said base metal and
decreasing the free electron density of the base metal.
Description of the Preferred Embodiment Of The Invention
A metallic alloy is provided which exhibits relatively low thermal conductivity
and a 1ow eloctrical conductivity. T he alloy may include primary alloying metals,
such as iron, nickel, cobalt, aluminum, copper, zinc, titanium, zirconium, niobium,
molybdenum, tantalum, vanadium, hafinium, tungsten, manganose, and combinations
thereof, and increased fractions of P-Group elemental additions in the alloy
composition. P-group elements are the non-metal and semi-metal constituents of
groups IDA, IVA, VA, VIA, and VIIA found in the periodic table, including but not
limited to phosphorous, carbon, boron, silicon, sulfur, and nitrogen. The metallic
alloy exhibiting relatively low thermal conductivity and electrical conductivity may
be provided aa a coating suitable for thermal and/or electrical barrier applications on a
variety of substrates.
Consistent with the present invention, metallic alloys are provided that exhibit
relatively low thermal and electrical conductivity. The alloys according to the present
invention may include relatively high fractions of P-group elemental alloying
additions in admixture with a metal The added P-group elements may include, but
are not limited to, carbon, nitrogen, phosphorus, silicon, sulfur and boron. The Pgroup
elements may be alloyed with the metal according to such methods as by the
addition of the P-group elements to the metal in a melt state.
Preferably, an alloy according to the present invention may include P-group
alloying constituents. Such constituents are preferably present at a level of at least 4
at % (atomic percent) of the alloy. Desirably, the alloy consistent with the present
invention may include more than one alloying component selected from P-group
elements, such that the collective content of all of me P-group elements is between
about 4 at % to 50 at %
Consistent with the present Invention, mo alloy may include relatively large
fractions of silicon in the alloy composition. For example, an iron/silicon coating
alloy can be prepared according to the present invention which coating may be
applied, e.g., to any given substrate. For example, it has been found mat 5.0 atomic %
of silicon, and greater, may be incorporated into the alloy without any measurable loss
of toughness when employed in a coating application.
As alluded to above, consistent with the present invention, the metal alloy may
be applied as coating using a thennal spray process. The resulting coating maybe
employed to provide a thermal and/or electrical barrier coating. The coating provides
thennal and/or electrical barrier properties exhibited similar to a ceramic material,
however without the associated brittleness of conventional ceramic coatings.
In addition to me use as a coating, the alloy of the present invention may also
be processed by any know means to process a liquid melt including conventional
casting (permanent mold, die, injection, sand, continuous casting, etc.) or higher
cooling rate, i.e. rapid solidification, processes including melt spinning, atomization
(centrifugal, gas, water, explosive), or splat quenching. One especially preferred
method is to utilize atomization to produce powder in die target size range for various
thermal spray coating application devices.
While not limiting me invention to any particular theory, it is believed at the
time of filing that by alloying metals with P-group elements, including but not limited
to carbon, nitrogen, phosphorus, and silicon, covalent bonds may be formed between
the electrons in the P-group alloying element and the free electrons in the base metal,
which base metal, as noted, may include iron. The interaction of the free electrons in
the base metal in covalent bonds with the P-group alloying elements apparently act to
reduce the free electron density of the base metal, and the outer electron energy band
of the base metal is progressively filled. Accordingly, by adding significant quantities
of P-group elements, the free electron density of the base metal can be continually
reduced and the outer electron energy band can be progressively filled. Because the
relatively high thermal conductively and electrical conductivity arise from the free
electrons in the unfilled outer energy bands of the metal, as me free electron density is
reduced, so are the electrical conductivity and the thermal conductivity. Therefore,
the present invention provides a metal alloy that behaves similar to a ceramic with
respect to electrical and thermal conductivity.
An exemplary alloy consistent with the present invention was prepared
containing a combination of several alloying elements present at a total level of 25.0
atomic % P-group alloying elements in combination with, e.g, iron. The experimental
alloy was produced by combining multiple P group elements according to the
following distribution: 16.0 atomic % boron, 4.0 atomic % carbon, and 5.0 atomic %
silicon with 54.5 atomic % iron, 15.0 atomic % chromium, 2.0 atomic % manganese,
2.0 atomic % molybdenum, and 1.5 atomic % tungsten.
The experimental alloy was prepared by mixing the alloying elements at the
disclosed ratios and then melting the alloying ingredients using radio frequency
induction in a ceramic crucible. The alloy was then process into a powder form by
first aspirating molten alloy to initiate flow, and men supplying high pressure argon
gas to the melt stream in a close coupled gas atomization nozzle. The power which
was produced exhibited a Gaussian size distribution with a mean particle size of
microns. The atomized powder was further air classified to yield preferred powder
sized rimer in the range of 10-45 microns or 22-53 microns. These preferred size
feed stock powders were men sprayed onto selected metal substrates using high
velocity oxy-fuel thermal spray systems to provide a coating on the selected
Reduced thermal behavior was observed for the exemplary alloy in a variety
of experiments. Specifically, a small 5 gram ingot of the exemplary alloy was arcmelted
on a water cooled copper hearth. It was observed mat the alloy ingots took
longer time for cooling baok to room temperature, relative to other alloys which did
not contain the P-group composition noted herein. More specifically, the increased
time for cooling was on the order of about 20 times longer.
Additionally, while conventional metals and alloys that have been healed to
high temperatures cool below their red hot radiance 1evel in a few seconds, it was
observed that when the exemplary alloy herein was heated to a temperature above the
red hot radiance level of the alloy, the red hot radiance persisted for several minutes
after removal of the heat source.
Similarly, conventional metals and metallic alloys typically cool rapidly from
a melt state on a conventional water cooled copper aro-melter, and can be safely
handled in a matter of a few minutes. The experimental alloy prepared M described
above required in excess of 30 minutes to cool from a melt state down to a safe
handling temperature after being melted on a water cooled copper hearth arc-melter.
Finally, when thermally sprayed the experimental alloy powder does not
transfer heat sufficiently using conventional operating parameters due to its relatively
low conductivity and inability to absorb heat When using high velocity oxy-fuel
thermal spray system, conventional alloys can be sprayed with equivalence ratios
(kerosene fuel/oxygen fuel flow rate) equal to 0.8. Because of the low thermal
conductivity of the modified experimental alloys, much higher equivalence ratios, in
the range of 0.9-1.2, are necessary in order to provide sufficient heating of ttie power.
Additionally, when deposited via the LENS (Laser Engineered Net Shape) process, in
which a high powered laser is used to melt metal powder supplied to the focus of fhe
laser by a deposition head, the very thin deposit (22S um thick weld) took excessive
tune before another layer can be deposited since it glows red hot for an extended time.
In fhe broad context of the present invention alloy compositions of fhe
following are to be noted, with fhe numbers reflecting atomic %: SHS717 Powder,
with an alloy composition of Fe (52.3), Cr (19.0), Mo (2.5), W (1.7), B (16.0), C
(4.0), Si (2.S) and Mn (2.0); SHS717 win, with an alloy composition of Fe (55.9), Cr
(22.0), Mo (0.6), W (0.4), B (15.6), C (3.5), Si (1.2) and Mn (05).
The thermal conductivity data for fhe SHS717 coatings was measured by a
Laser Flash method and fhe results are given in Table 1. Note that the wire-arc
conductivity is generally lower man fhe HVOF due to the higher porosity hi the wirearc
coating. Note that fhe conductivity of the coatings is lower than mat of titanium
which is the lowest thermal conductivity metal and at room temperature are even
lower than alumina ceramic (see Table 2).(Table Removed)
1. A metallic thermal barrier alloy coating comprising iron wherein the coating comprises a base alloy metal that is present at a level of at least about 52 atomic percent in combination with at least one of chrome, molybdenum, tungsten, manganese, cobalt, nickel, copper, and combination as herein described thereof, and at least one P-group alloying element wherein the P-group alloying element is present at a level of 5 atomic % to 50 atomic %, wherein said one or more additional P-group alloying elements are selected from the group consisting of carbon, nitrogen, phosphorous, boron, and combination as herein described thereof, wherein said metallic thermal barrier coating has a thermal conductivity equal to or less than about 10 W/m-K at 400 degrees Celsius.
2. A metallic thermal barrier coating as claimed in claim 1 wherein the base alloy metal is an iron base alloy metal.
3. The coating as claimed in claim 1 wherein the base metal is with a free electron density.
4. The coating as claimed in claim 1, wherein said at least one P-group alloying element comprises 16.0 atomic % B, 4.0 atomic % C, and 5.0 atomic % Si.
5. The coating as claimed in claims 1 or 3 or 4 wherein the free electron density of the base metal is reduced from its base metal value, and wherein said free electron density is generally representative of a fully filled outer shell after combination with said P- group alloying element.
6. The coating as claimed in claims 1 or 3 or 4 wherein the base metal is selected from the group consisting of iron, nickel, cobalt, aluminum, copper, zinc, titanium, zirconium, niobium, molybdenum, tantalum, vanadium, hafnium, tungsten, manganese, and combinations thereof.
7. A metallic thermal barrier coating as claimed in any preceding claims as and used for reducing the thermal and/or electrical conductivity of a metal alloy composition.
|Indian Patent Application Number||3568/DELNP/2005|
|PG Journal Number||31/2010|
|Date of Filing||11-Aug-2005|
|Name of Patentee||THE NANOSTEEL COMPANY ,|
|Applicant Address||MAITLAND PROMENADE,485 NORTH KELLER ROAD, SUITE 100, MAITLAND, FLORIDA 32751, U.S.A.|
|PCT International Classification Number||C22C|
|PCT International Application Number||PCT/US2004/004026|
|PCT International Filing date||2004-02-11|