Note: Descriptions are shown in the official language in which they were submitted.
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WEAR RESISTANT ALLOY POWDERS AND COATINGS
Field of the Invention
This invention relates to chromium-molybdenum
alloys and wear resistant alloy powders useful for
deposition through thermal spray devices. The wear
resistant alloy powders are useful for forming coatings
having the same composition.
Background of the Invention
Hard surface coating metals and alloys are known
in the art. For example, chromium metal has been used
as an electroplated coating for many years to restore
worn or damaged parts to their original dimensions, to
increase wear and corrosion resistance, and to reduce
friction. Hard chromium electroplate, however, has a
number of limitations. When the configuration of the
part becomes complex, obtaining a uniform coating
thickness by electro-deposition is difficult. A non-
uniform coating thickness necessitates grinding to a
finished surface configuration, which is both difficult
and expensive with electroplated chromium because of
its inherent brittleness and hardness. The rate of
deposition by electroplating is relatively low, and
thus a substantial capital investment in plating
equipment is required. It is often necessary to apply
one or more undercoats, or to use expensive surface
cleaning and etching procedures to prepare substrates.
Disposal of spent plating baths also adds significantly
to the cost of the process.
An alternative method of depositing chromium metal
is by metal spraying such as with a plasma, high
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velocity oxygen fuel (HVOF) or detonation gun. This
method allows the coating to be applied to almost any
metallic substrate without using undercoats. The
coating thickness can be controlled very closely so
that any subsequent finishing can be kept to a minimum.
However, considerable finishing may be required for
certain coatings with wear resistance tailored for
specific applications.
U.S. Patent No. 3,846,084 discloses coatings made
by the plasma or detonation gun process that are
superior to hard chromium electroplate in
compatibility, frictional characteristics and wear
resistance by incorporating a dispersion of chromium
carbide particles in a chromium matrix. Coatings of
this type can be made from mechanical mixtures of
powders. However, there are certain limitations to the
quality of coatings made from them. Plasma, HVOF and
detonation-gun coatings result in a multilayer
structure of overlapping lamellae or "splats." Each
splat is derived from a single particle of the powder
used to produce the coating. There appears to be
little, if any, combining or alloying of two or more
powder particles during the coating deposition process.
U.S. Patent No. 6,562,480 Bl discloses a wear
resistant coating for protecting a surface undergoing
sliding contact with another surface such as piston
rings and cylinder liners of internal combustion
engines. The wear resistant coating is applied by HVOF
deposition of a powder which comprises a blend of about
13 weight percent to about 43 weight percent of a
nickel-chromium alloy, about 25 weight percent to about
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64 weight percent chromium carbide, and about 15 weight
percent to about 50 weight percent molybdenum.
U.S. Patent No. 6,503,290 Bl discloses a corrosion
resistant powder useful for deposition through thermal
spray devices. The powder comprises about 30 to 60
weight percent tungsten, about 27 to 60 weight percent
chromium, about 1.5 to 6 weight percent carbon, a total
of about 10 to 40 weight percent cobalt plus nickel and
incidental impurities plus melting point suppressants.
The corrosion resistant powder is useful for forming
coatings having the same composition.
A need continues to exist for powders and
coatings that can be deposited by thermal spray
devices and that exhibit excellent wear and/or
corrosion resistance. Therefore, a need continues to
exist for developing new powders and for exploring
their potential for thermal spray deposition of wear
and corrosion resistant coatings. It would therefore
be desirable in the art to provide powders and
coatings that can be deposited by thermal spray
devices and that exhibit excellent wear and corrosion
resistance.
Summary of the Invention
This invention relates to alloys comprising about
20 to-65 weight percent chromium, about 20 to 65.weight
percent molybdenum, about 0.5 to 3 weight percent
carbon, and about 10 to 45 weight percent nickel. The
alloys include precipitated carbides (and optionally
nitrides) of chromium and molybdenum interspersed
throughout. This invention also relates to wear
resistant alloy powders useful for deposition through
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thermal spray devices. The powders comprise an alloy
of about 20 to 65 weight percent chromium, about 20 to
65 weight percent molybdenum, about 0.5 to 3 weight
percent carbon, and about 10 to 45 weight percent
nickel. The wear resistant alloy powders are useful
for forming coatings having the same composition.
Detailed Description of the Invention
As indicated above, this invention relates to wear
resistant alloy powders useful for deposition through
thermal spray devices such as plasma, HVOF or
detonation gun. The powders are made from alloys
comprising about 20 to 65 weight percent chromium,
about 20 to 65 weight percent molybdenum, about 0.5 to
3 weight percent carbon, and about 10 to 45 weight
percent nickel. The alloys.include precipitated
carbides and optionally nitrides of chromium and
molybdenum interspersed throughout. The alloys are
useful for forming wear resistant powders and coatings
having the same composition.
The alloys herein rely upon a large concentration
of chromium and molybdenum for excellent wear
resistance. Advantageously, the alloys contain at
least about 20 weight percent chromium, preferably at
least about 30 weight percent chromium, and more
preferably at least about 35 weight percent chromium.
Powders containing less than about 20 weight percent
chromium may exhibit inadequate wear resistance for
many applications. Chromium levels in excess of about
65 weight percent may tend to detract from the wear
resistance of the coating because the coating may
become too brittle.
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Similarly, the alloys contain at least about 20
weight percent molybdenum, preferably at least about 25
weight percent molybdenum, and more preferably about 30
or 35 weight percent molybdenum. Powders containing
less than about 20 weight percent molybdenum may
exhibit inadequate wear resistance for many
applications. Molybdenum levels in excess of about 65
weight percent may tend to detract from the wear
resistance of the coating because the coating may
become too brittle.
In an embodiment of this invention, the alloys
comprise about 20 to 65, preferably about 30 to 60, and
more preferably about 35 to 55, weight percent
chromium; about 20 to 65, preferably about 25 to 60,
and more preferably about 30 to 55, weight percent
molybdenum; about 0.5 to 3, preferably about 1 to 2.5,
and more preferably about 1.5 to 2, weight percent
carbon; and about 10 to 45, preferably about 15 to 35,
and more preferably about 20 to 35, weight percent
nickel. These alloys are useful for forming wear
resistant powders and coatings having the same
composition.
In another embodiment of this invention, the
alloys comprise about 50 to 90, preferably about 60 to
80, and more preferably about 65 to 75, weight percent
chromium and molybdenum; about 0.5 to 3, preferably
about 1 to 2.5, and more preferably about 1.5 to 2,
weight percent carbon; and about 10 to 45, preferably
about 15 to 35, and more preferably about 20 to 35,
weight percent nickel. These alloys are useful for
forming wear resistant powders and coatings having the
same composition.
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The carbon concentration controls the hardness and
wear properties of coatings formed with the powders. A
minimum of about 0.5 weight percent carbon may be
necessary to impart adequate hardness into the
coatings. If the carbon exceeds about 3 weight
percent, the melting temperature of the powder may
become too high and it may become too difficult to
atomize the powder.
In another embodiment of this invention, cobalt
may be included in the alloys, powders and coatings.
The powders may contain about 10 to 45, preferably
about 15 to 35, and more preferably about 20 to 35,
weight percent nickel plus cobalt. This may facilitate
the melting of the chromium/molybdenum/carbon
combination that, if left alone, would form carbides
having too high of melting temperatures for
atomization. Increasing the concentration of cobalt
and nickel may also tend to increase the deposition
efficiency for thermal spraying the powder. Because
total nickel plus cobalt levels above about 45 weight
percent may tend to soften the coating and limit the
wear resistance of the coating, the total concentration
of nickel plus cobalt may best be maintained below
about 45 weight percent. In addition, the alloys may
contain only nickel or cobalt since coatings with only
nickel (e.g., about 10 to 45 weight percent nickel) or
only cobalt (e.g., about 10 to 45 weight percent
cobalt) may form powders with wear resistance tailored
for specific applications. But for most applications,
cobalt and nickel appear to be interchangeable.
In another embodiment of this invention, boron,
silicon and/or manganese may be included in the alloys,
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powders and coatings. The alloys may contain about 0.5
to 3, preferably about 1 to 2.5, and more preferably
about 1.5 to 2, weight percent carbon plus boron,
silicon and/or manganese. To facilitate melting for
atomization, the alloys may optionally contain melting
point suppressants such as boron, silicon and
manganese. An excessive amount of melting point
suppressants may tend to decrease both corrosion and
wear resistance.
As indicated above, the alloys include
precipitated carbides (and optionally nitrides) of
chromium and molybdenum interspersed throughout. The
alloys may contain a volume fraction of the
precipitated carbides and optionally nitrides in excess
of 0.25. Preferably, the volume fraction of the
precipitated carbides and optionally nitrides dispersed
in the alloys may be 0.25 or greater and more
preferably between 0.35 and 0.80. Preferably, the
precipitated carbide and optionally nitride grains may
be of micrometer and submicrometer size, for example,
between 0.5 or less and 20 micrometers, more preferably
between 1 and 10 micrometers in its largest dimensions.
The size and volume fraction of the precipitated
carbides and optionally nitrides can be adjusted by
varying the chromium, molybdenum and carbon content.
The alloys of this invention may be blended with
molybdenum to form powders with wear resistance
tailored for specific applications. The amount of
molybdenum that may be blended with the alloys of this
invention is not narrowly critical and may range from
about 10 to 50, preferably about 15 to 45, and more
preferably about 20 to 40, weight percent of the total
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alloy/molybdenum blend composition. The amount of blended
molybdenum is in addition to the amount of alloy
molybdenum. The amount of blended molybdenum will depend
upon the desired application.
Advantageously, the powders of this invention may be
produced by means of inert gas atomization of a mixture
of elements in the proportions stated herein. Preferred
atomization methods that may be employed in making the
powders of this invention are described in U. S. Patent
5,863,618. The alloys of these powders are typically
melted at a temperature of about 16000 C and then
atomized in a protective atmosphere (e. g., argon, helium
or nitrogen). Most advantageously the atmosphere is
argon. A nitrogen atmosphere may be employed which may
result in the formation of additional hard phases
interspersed throughout the alloys, e. g., nitrides. As
indicated above, to facilitate melting for atomization,
the alloy may optionally contain melting point
suppressants like boron, silicon and manganese.
Alternatively, sintering and crushing, sintering and
spray drying, sintering and plasma densification are
possible methods for manufacturing the powders. Gas
atomization however represents the most effective method
for manufacturing the powder. Gas atomization techniques
typically produce a powder having a size distribution of
about 1 to 500 microns. For thermal spray applications,
the powder is classified to a size of about 1 to 100
microns.
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Coatings may be produced using the alloys of this
invention by a variety of methods well known in the
art. These methods include thermal spray (plasma,
HVOF, detonation gun, etc.), laser cladding; and plasma
transferred arc (PTA). Thermal spray is a preferred
method for deposition of powders to form the coatings
of this invention. The wear resistant alloy powders of
this invention are useful for forming coatings having
the same composition.
The alloy powders of this invention are useful for
forming coatings or objects having excellent wear
properties, for example, wear resistant coatings for
protecting surfaces undergoing sliding contact with
other surfaces such as piston rings and cylinder liners
of internal combustion engines.
The examples that follow are intended as an
illustration of certain preferred embodiments of the
invention, and no limitation of the invention is
implied.
Example 1
The alloy powders listed in Table I were made by
processes alike to those described in U.S. Patent
5,863,618.
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Table I
Alloy Cr Mo Ni C Other
A 45 20 32 2 1 B
B 42 23 33 2
C 40 35 22.5 2.5
D 39 20 40 1
E 35 40 23 2
F 34 22 43 1
G 30 30 39 1
H '50 21 3(+2OCo) 0.5 0.4B,
2.3Fe,
2.2 Si
I 51 22 13(+10Co 1.8 1.8
J 35 35 27.5 1.5 0.5 B
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Other variations and modifications of this
invention will be obvious to those skilled in the art.
This invention is not limited except as set forth in
the claims.
