Note: Descriptions are shown in the official language in which they were submitted.
Lo
SURFACE MODIFIED POWDER METAL PARTS AND
METHODS FOR MAKING SAME
Mark. F. Mouser and Bruce Go McMordie
This invention relates generally to powder metal (P/M)
parts, more specifically to a part consisting of a powder metal
core coated with a sistered metal surface layer possessing
different properties (i.e. density, hardness) and/or
composition than that core. The invention also relates to
methods of forming such parts. These parts combine the
advantage of powder metal technology with those of machined or
cast parts. The products of the invention make an important
contribution to the field of powder metallurgy.'
In powder metallurgy, it is well known to compress
metal particles (e.g. powders) into a coherent mass having the
desired shape of the part to be formed, and then to fuse these
particles together by heating the compact in a reducing
atmosphere at some temperature below the melting point of the
powders. This technique, known as pressing and sistering,
produces a strong metal part and utilizes less time, raw
material and energy than do conventional casting and machining
processes.
Powder metallurgy is well known in the art. For
reference, see, for instance, Powder Metallurgy, TV Level,
Metal Powder Industries Federation (1980); Handbook of Powder
metallurgy, Henry H. Hausner, Chemical Publishing Co. (1973);
Technology of Metal Powders, Recent Developments 1980, Edited
by LO ~averbaum, r~Toyes Data Corp. (1980); Powder Metallurgy
Processing, Jew Techniques and Analyses, Edited by HA. Kuhn
-
and A. Hawley, Academic Press (1978); Particulate Science and
Tech , Jo Bud, Chemical Publishing Co. (1980), Source
Hook on Powder Metallurgy, Samuel Brad bury, American Society
I
for Metals (19~9), Sistering, MOB. Waldron and BLUE. Daniel],
.,
2 --
Hayden & Son (1978); Terms Used in Powder Metallurgy, Intel
Puns Sock for P/M (1975); Powdered Metals Technology, J.
McDermott, Lucy Wright Corp. (1974); Powder Metallurgy for Leigh
Performance Applications, edited by J. Burke and V. Weiss,
Syracuse University Press (1972); and Handbook of Metal
Powders, AIR. Poster, Reinhold-Litton (1966).
Not withstanding the manufacturing advantages of P/M
parts, they have a potentially serious drawback. All P/M parts
contain some degree of porosity. The metal powders that are
the raw material for P/M parts never liquefy during sistering
and the voids which exist between the deformed particles in the
compacted shape are retained in the finished product. The
resulting unique structure of rigid metal encompassing a
network of interconnected voids renders P/M products ideal for
applications where parts must be permeable to fluids, such as
filters or self-lubricating bearings. However, in applications
in which the Pi parts are designed to be strong and durable
(as in "structural" parts), the porosity inherent in the
pressed and sistered product makes these parts more susceptible
to corrosion damage than are their cast or machined
counterparts. Owing to the presence of the open network of
voids, internal as well as external, surfaces are exposed to
the debilitating effects of the environment. These extensive
surface areas also render these parts vulnerable to
deterioration by chemicals.
Additionally, P/M parts exhibit lower surface hardness
than do cast or machined items of identical composition,
because some proportion of the P/M surface is open space.
Furthermore it is extremely difficult to produce a continuous
metal plating or to achieve a uniform finish of any kind on the
porous surface of a pressed and sistered part.
The techniques required to make P/M products with low
bull porosity and hence low surface porosity are well Known but
are also expensive. The interconnected porosity of -the product
can be significantly reduce by pressing and sistering a par-t
more than once or by sistering compacted powders at
temperatures very near their melting temperature, and perfectly
dense Purl parts have been produced by pressing the metal powder
at the sistering temperature in special autoclave equipment.
Unfortunately repressing and recentering Pi parts nearly
doubles equipment and die wear as it halves production rates,
high temperature sistering requires more energy and unique
furnace designs, and the equipment required for "hot pressing"
is expensive and its production rates low. Consequently, high
density P/M parts are used only in those applications in which
economics allow for use of one of these capital intensive
production techniques.
In a review of the prior art describing methods of
modifying the structure and composition of P/M parts, the
following patents have been noted. US. Patent No. 3,320,05~ 1
to Knock et at relates to tungsten structures having high
density outer surfaces and a core of controlled porosity. Such .
"armored" structures are achieved by dusting the surfaces of .
compacted tungsten powders with nickel particles before
sistering at 1100C to 1400 C. In this temperature range,
the nickel diffuses into the compacted core along the
boundaries of the tungsten particles. The nickel activates
sistering of the tungsten by lowering activation energy for
diffusion. Hence the inwardly diffusing nickel leads to
complete densification of the surface of the tungsten compact.
The resultant tungsten structures are disclosed to be useful as
ion emitters, permeable membranes, conduit means for fluids
such as gases and liquids, and fluid filters. They are
proposed as replacements for fibrous materials, such as paper
filters and others.
US. Patent No. 2,644,656 to aquaria deals with
porous plates for alkaline storage batteries. The plates are
constituted of porous particles of nickel or of nickel-coated
iron subsequently sistered or fused. Particles are fused
together to form the consolidated or integral porous mass. The
nickel coating on the iron particles is fairly considerable in
that it exceeds 20~ of the total amount of nickel and iron.
US. Patent No. 3,682,062 to Jackson discloses a
centrical ferrous metal length with chromium penetrating its
thickness.
US. Patent No. 3,9~9,558 to Maynard et at discloses
carbides sistered with cobalt, like tungsten carbide coated
with osmium and ruthenium. The coating is made to diffuse into
the cobalt.
In addition to the above patents, the following US.
patents were considered in the preparation of this
application: Nos. 1,29:L,352 to Allen; 3,520,680 to Orlemann;
2,357,269 to Russet; 2,753,859 to Bartlett; 1,263,959 to
Swartley; 2,033,240 to Hardy; 2,679,6~3 to Luther; 2,933,~15 to
Lamar; and 3,395,027 to Isolates.
The invention described herein provides a means to
modify the surface of a pressed and sistered part without use
of the extensive processing or -the high temperature equipment
described above. Surface modification is instead accomplished
by sistering onto the surface of the part, a metal layer that
is distinct from the body of the part in composition and/or
structure. This unique metallic surface layer is produced by
coating zither an unsintered ("green") or sistered P/M part
with a slurry of metal pigments in a high temperature binder,
and then sinterin~ the coated part in accordance with typical
industry practice. During sinterincl, -the metal pigments in the
coating fuse to form a distinct metal layer, most preferably 10
to 50 microns thick, on the surface of the centric P/M part.
The composition and structure of this layer are controlled by
the composition of the coating slurry and, to a lesser degree,
by sistering time and temperature-
The invention provides powder metal parts which comprise a modified surface layer. The surface layer has
characteristics which are different from those of the core in
one or more respects. The modified surface may have different
I
hardness characteristics (typically increased hardness) -than
that of the uncoated part, different resistance to exposure to
chemicals, different porosity, and other properties as will
become apparent from the detailed description ox the invention.
Thus, the invention provides new metal parts not known
heretofore.
The invention will be further described, by way of example
only, with reference to the accompanying drawings, in which:
Plate I shows the structure of an unsintered part;
Plate II shows the structure of a coated compact;
Plate III shows the structure of a sistered coating and
sistered art-
a p
Figure 1 shows a coating "bridging" a pore on a sinteredP/M part;
Figure 2 shows compressed metal compact prior to
sistering;
Figure 3 shows a sistered coating on a sistered P/M disc;
Figure 4 shows a cured coating on a sauntered P/M part;
Figure 5 shows a sistered coating on a sistered P/M part;
Figure 6 shows a sistered coating on a sistered P/M part;
Figure 7 shows a steel gear with a coating of nickel;
Figure 8 shows a steel P/M part with a porous coating of
sistered nickel;
Figure 9 shows a cured coating on a spur gear;
Figure 10 shows sistered WE particles in the nickel
matrix undercoating;
Figure 11 shows an iron disc coated and sistered;
Figure 12 shows carbon steel coated with sistered nickel;
Figure 13 is a diagram of an unsintered compacted mass of
the metal powder;
Figure 14 is a diagram of a particulate coating on the
surface of an unsintered compact; and
Figure 15 is a diagram of coating and part after
sistering.
us Jo
- pa -
There are several parameters which may be varied in
accordance with the invention to obtain the preferred metal
part. One important parameter affecting the quality of -the
metal layer formed from the "sinterable" coating described above
is the size of the metallic pigments in the coating liquid. The
metal particles in the coating may be as large as 150 microns;
however, it is most preferable that the size of the powders not
exceed 20 microns. Fine particles will enable the coating
liquid to "bridge" surface voids (Figure 1), forming a
continuous layer or skin on the porous surface. Use of such
powders also assures that any porosity retained in this
continuous coating layer after sistering will be orders of
magnitude smaller than -that in the body of -the P/M part which
is formed from metal particles averaging 50 to 100 microns in
diameter. Use of coarse particles or of particles that are
-themselves porous produces a sistered coating possessing a
greater proportion of porosity than that of the powder metal
substrate.
It is also within the contemplation of the invention that
the selection of coating pigments not be limited to spherical
powders. Flake and acicular powders also produce uniquely
dense metal skins on the P/M part. Irregardless of the
morphology of the pigment chosen for the coating, its
composition is such that during sistering a continuous skin
forms on the surface, and interior regions of the P/M par-t
remain substantially or totally free of the sistered metal
particles which form that coating.
The formation of a continuous sistered skin upon the
outer surface of the part is accomplished by formulating the
I
liquid coating to include any of a class of metal and alloy
pigments referred to hereafter as rapidly sistering materials.
These basic building blocs of the coating of this invention
are those elemental and alloy powders which are known to fuse
and coalesce without melting, at the -temperature at which the
coated part is to be sistered. The rapidly sistering component
provides the physical structure or cohesiveness of the sistered
coating. The pigmentation of the coating need not be limited
to these rapidly sistering pigments, though some must always be
present. In fact, as they must be present only in sufficient
quantity to provide the skeletal structure for the coating,
there are cases, described below, in which the proportion of
rapidly sistering pigments is only 5% of the total weight of
pigmentation.
The exact metals and alloys constituting the set of
rapidly sistering materials cannot be generally defined.
Instead each is determined by the range of sistering .
temperatures at which the coating is to be used. For example,
iron, nickel, cobalt, and their alloys stinter rapidly at
temperatures above about 1050C; therefore, these metals are
the building block components of sinterable coatings for iron
and steel (ferrous) P/M parts which are typically sistered
between 1100C and 1300C. These same metals are not
indispensable in coatings of the invention for use on brass or
bronze P/M parts because these parts are sistered below about
900C. Conversely, copper, which does not qualify as a
rapidly sistering component for use on ferrous parts because it .
melts at 1080C, qualifies as a rapidly sistering metal for
use on brass and bronze substrates.
It is within the contemplation of the invention that
many pigments can be used, in varying proportions in
conjunction with the basic rapidly sistering ones, to produce a
sistered skin possessing the desired properties. Pigments
which are liquids at the sinterinq temperature may be added -to
the crating to increase the sinterinq rate of the other
I
pigments as well as the density of the product. Generally, it
is preferable to limit the proportion of lower melting metals
to less than about 20~ of the total weight of pigment to
prevent the liquid metal from being totally absorbed into the
body of the part during sistering.
High melting elements or alloys which would not react
(stinter) with one antler at typical sistering temperatures can
nevertheless be used in coatings which also contain rapidly
sistering metals such as those described above. Even
refractory metals (e.g. tungsten) and metal compounds (e.g.
silicon carbides) can be incorporated as long as there is a
sufficient quantity of rapidly sistering metal in the slurry to
bind the unreactive particles to the substrate and each other
after sistering. One skilled in the art has no difficulty
determining the optimum amounts of metals required to produce a
coating possessing the desired properties.
It is also within the contemplation of the invention
that any metal which undergoes or causes exothermic or
stoichiometric reactions with either the base metal of the P/M
part or another metal in the coating (e.g. aluminum with iron)
should be avoided inasmuch as such a reaction will interfere
with sistering.
In accordance with the invention, the structure and
properties of the finished metal part can be varied
considerably. The particular elements in the coating slurry
wit] determine the structure and properties of the finished
product For example, on iron or steel Ply parts, coatings
containing nickel or stainless steel will produce sistered
films with good corrosion resistance. The optimum performance
will be achieved when the coatings contain 60 to 100% by weight
of those metals. The surface porosity of ferrous parts will be
decreased or eliminated by coatings containing copper pigments
because these pigments will be liquid at the sistering
temperature of the ferrous parts. Preferably the amount of low
melting pigment (i.e. copper, tin, etc.) will be only about 10
- 8 -
to 20% by weight in the coating. The porosity of the ferrous
P/M surface could also be increased if so desired by sistering
to it a coating containing between 60 and 95~ sponge iron
powder
Coatings of the invention which contain in addition
hard metals such as chromium, or interstitial hardening
elements, such as carbon, will increase the superficial
hardness of iron P/M parts, and those blending tungsten carbide
or boron nitride with iron or nickel will produce durable, wear
resistant surfaces. These hard facing and wear resistant
sistered coatings can contain as much as 95~ by weight of the
hard species (e.g. silicon carbide).
In accordance with the invention the metal particles
which comprise the coating are applied to the part in a liquid
binder. While any binder known in the art may be used, certain
considerations should be taken into account in the selection of
the preferred binders for use in the invention. Isle binder not
only facilitates applications of the chosen metal pigments to
the P/M parts ho spraying, brush or dip techniques, but also
holds the metal pigments to the part surface as the temperature
increases toward that of sistering. The binder preferably
should not impede or hinder diffusion between particles in the- ¦
coating and those constituting the P/M compact. Preferably,
the binder should evaporate at high temperatures or at least
generate loosely adhering residues that can be easily removed
from the sistered part. Ever, it is by far preferable that
the binder be such as to allow a high concentration of pigment
in the cured film, most preferably 75~ or more by volume. I
One class of binders which has been proven especially
suitable is comprised of phosphate anions and chromates (or
dichromate) and/or molybdate anions. A variety of such
solutions is known -for treatment of metal surfaces. or
instance, Kirk and Other, Encyclopedia of Chemical Technology,
end Ed., Vol. 1~3, Intrusions Publishers, John Wiley & Sons,
Inc., 1969 pages 292-303), describes phosphate and chromates
12~
coatings. The united States patent literature describes
coating solutions or dispersions for protective coating of
metals, which compositions are suitable for supplying the metal
particles to the porous part Such compositions are disclosed
by Allen (No. 3,248,2~1); Braumbaugh (No. 3,869,293); Collins
(No. 3,248,249); worry no. 3,~57,717); Boles (No. 3,081,146);
Romig (No. 2,245,609); Helwig (No. 3,967,984); Bunch (No.
3,443,977); Hurst (No. 3;562,011) and others.
It is noteworthy that, in accordance with the
invention, a greater latitude is provided in the type of
phosphate compositions which can be used with the specified
metal additives. For instance, with respect to the above
mentioned Allen patent (US. Patent No. 3,248,251), it it not
necessary that the phosphate binder be confined to the various
concentrations and other molar relationships disclosed by that
patent. It is desirable but not necessary that there be
present at least about 0.5 mole of phosphate and about 0.2 mole
of chromates and/or molybdate. The Allen patent also discloses
supplying a metal ion, as by way of a metal salt like a metal
oxide, hydroxide or carbonate. (See, for instance, column 7.)
In accordance with this invention such addition is optional.
The pi of the aqueous binder used herein is preferably
but not necessarily in the range of about 1.0 to about 3Ø
Often when using chromate/phosphate binders of the
type described in the literature it may be necessary to modify
the rheology the solution to produce thixotropic, stable
suspensions of heavy metal pigments such as nickel, chromium or
tungsten.
Another class of suitable binders is silica-containing
organic and inorganic liquids, especially water-soluble alkali
silicates, like potassium and sodium silicate. Also included
are those squids which generate silicates, such as alkali (e.g.
ethyl) silicates. It is preferable that those having low
rather than high alkalinity be used, e.g. those having a high
-- 10 --
Siam mole ratio. Other useful binders include
synthetic organic binders such as silicones and finlike resins
and inorganic glasses such as borate and other frets.
Coatings constituted of a Metro of metal pigments in
a binder as described above are particularly well suited to be
applied to commercially produced P/M parts. The P/M part is
constituted, as is known, of a metal which comprises iron,
steel, nickel, cobalt, copper, aluminum, refractory oxides,
precious metals and alloys thereof. m e compositions and
properties of these substrate materials are described in the
Metal Powder Industries Federation Specification No. 35. The
Specification also prescribes the sistering time and
temperature required to achieve the desired property for a
particular composition. Said Specification is incorporated
herein by reference.
In forming the sistered metal parts of this invention,
metal powders are first compacted into the desired shape of the
part to be formed. The structure of the unsintered part is
illustrated in Plate I.
The compact can be sistered before applying the
coating, in which case a second sistering operation is employed
after applying the coating to form the fused, impervious or
porous coating. It is also quite satisfactory to apply the
coating to the compact when it is in a "green" (unsintered)
state, in which case only a single sistering operation needs to
be employed, this taking place after the coating is applied.
If an iron P/M part is to be infiltrated with copper particles
it is by far preferable to stinter the compact prior to applying
the coating. Otherwise, it is optional whether the compact is
sistered prior to or after applying the coating.
After applying the liquid coating to -the part, it is
dried and cured into a substantially water-insoluble fluorine most
preferably 15 to 100 microns thick with the particulate metal
particles of the coating filling or bridging voids on the
surface of the P/M core. Roy structure of the coated compact
is illustrated in Plate I
ye
The coated part is then placed into a sistering
furnace where it is heated in a vacuum or a reducing
atmosphere, usually consisting of nitrogen, hydrogen as well as
carbon monoxide, carbon dioxide and methane or propane, to a
temperature sufficient to fuse the coarse metal powders in the
compact to one another. Simultaneously, the fine metal
particulate in the coating is sistering into a continuous mass
as well as alloying itself with the metal in the compacted
shape. When the part is cooled, any binder residues are
removed from the surface by mechanical finishing techniques.
The structure of the sistered coating and the sistered part is
shown in Plate III.
It must be emphasized that, although the coated part
may be pressed and sistered again, or sistered at a very high
temperature, or even pressed and sistered simultaneously,
conventional press and stinter techniques are insufficient to
effect the structural and/or compositional modification of the
part surface accomplished by this invention. Therefore, this
invention is useful for commercially available P/M parts as a
post-treatment, or in the manufacture of P/M parts during which
the part coated with cured coating (but unsintered) can be
manufactured. These products are valuable intermediates in
accordance with the invention.
The objects and advantages of this invention will
become apparent by referring to the following examples of
sinterable coatings, taken in conjunction with the
microphotograph. These are not to be construed as limiting
the invention hut as merely illustrative.
EXAMPLE 1
Nickel powder was dispersed in a chromate/phosphate
birder of the following composition:
- 12 -
100.0 gym water
37.3 gym phosphoric acid (85%)
5.0 gym magnesium oxide
16.0 gym magnesium dichromate 6 hydrate
2.7 gym fumed silica (Cab-0-Sil M-5)
75.0 go nickel powder (-325 mesh)
0.3 ml non-ionic surfactant (Briton X-100)
The nickel/chromate/phosphate slurry was sprayed onto
a green compact of atomized iron and copper powders. The
structure of this stall disc, which had been pressed at 30
Tennyson to a green density of about 6.6 gm/cc, it shown in
Figure 2. After the nickel coating had been cured at 343C,
the compact was sistered in a vacuum at 1121C for one half
hour. The result was a sistered metal P/M disc with a sistered
nickel surface. Figure 3 shows the sistered iron disc with the
sistered nickel coating.
Although the coating formed on the sistered part was
porous, there was sufficient nickel on the surface to enable
the disc to survive 72 hours in 5% salt spray (ASTM B117)
without red rust. Another iron disc which had not been coated
before it was sistered rusted within 1 hour in the salt fog.
This example illustrates that the metal part to be coated need
not have been sistered when treated with the metal which will
form the alloyed sistered coating. The invention is thus
applicable to green parts.
EXAMPLE 2
.
Three coats of the slurry described in Example 1 were
sprayed onto a P/M spur gear which had been pressed and
sistered from a blend of iron (95~), nickel I copper (2%)
and carbon (1%) powders. Each layer of the coating was cured
at 343C for one half hour. Figure 4 shows the cured coating
on the surface of the gear. The coating was about 60 microns
thick.
The coated gears were heated to 1121C in a vacuum,
hot d for one half holly at that temperature and then rapidly
- 13 -
cooled by quenching in argon gas. At this temperature, the
nickel particles in the coating sistered to form a nickel-rich
layer on the surface of the gear. Figure 5 shows the sistered
coating on the porous part. The sistered coating was about 20
microns thick.
though this sistered coating contained some very
fine porosity, the additional nickel alloyed into the surface
of the part enabled the gear to survive 100 hours in 5% salt
spray (ASTM B117) without red rust. An uncoated gear rusted
within 5 hours in the salt fog. the presence of an alloy of
the sinterable petal of the coating (e.g. nickel, iron or
cobalt) at the interface with the P/M is a characteristic of
the finished porous part.
EXAMPLE 3
Some gears prepared as described in Example 2 were .
sistered at 1121C for one half hour in a sistering furnace
fueled by an endothermic gas (39% No, 39% ~12' 20% C0 and 2%
C02) instead of a vacuum. The sistered coating produced in
the furnace was very uniform and dense. The outer layer was
noticeably more dense than the P/M part. Generally the density
of the coating, i.e. outer layer, and alloyed interface is
about 2%. Figure 6 shows the sistered coating produced in the
production furnace atmosphere a-t a magnification of 400x.
EXPEL -
Nickel powder was dispersed in a silicate binder of
the following composition:
50 ml water
50 ml potassium silicate (Basil I
75 gym nickel powder (-325 mesh)
I Jo
This coating was misted onto a steel P/M gear,
identical to that used in Examples 2 and 3, to form a thick,
loosely packed coating layer. Potassium silicate binders cure
at room temperature. Figure 7 shows a steel P/M gear coated
with nickel. The coated part was allowed to sit at ambient
temperature for several days (I days) till cured. It was then
sistered in a vacuum at 1121C for one half hour. At this
temperature, the nickel powder in the coating sistered to form
a porous nickel "sponge" surface layer. Figure 8 shows the
sistered nickel/silicate coating on the steel P/M part.
The sistered coating is very compressible and is
useful as a corrosion resistant reservoir for liquids, e.g.
lubricants, resins, coolants, etc. This coated part may be
useful in applications requiring strong, dense parts with
lubricated surfaces such as gears or load bearing bushings.
When an identically coated gear was sistered at
1121C in the endothermic atmosphere described in Example 3,
the metal layer formed had an unusual "spiked" micro structure.
this layer was found to be harder than that formed from nickel
particle coatings employing chromate/phosphate binders.
A sodium silicate solution, such as PI "G" silicate
dissolved in water (18 gm/82 gym water) is a suitable substitute
or Basil #1.
EXAMPLE 5
In Examples 1 through I, nickel powder was substituted .
by a mixture of 80% nickel and 10% cobalt, powders by weight,
which -formed an alloy coating on the iron part. Excellent
resistance to salt spray was observed.
EXAMPLE 6
. '
in Examples 1 through I, the nickel powder in the
coating slurry was replaced by 316L stainless steel powder
which had been screened less than 325 mesh. The sistered
- 15 -
coating was more porous than that produced using nickel
powders; however, the salt corrosion resistance of the coated
parts was markedly better than than of hare iron P/M parts.
EXAMPLE 7
A mixture of nickel powder and tungsten carbide powder
was dispersed in a chromate/phosphate binder of the following
composition:
90.0 gym water
20.0 gym 85~ H3PO~
7.3 gym Zoo
4.6 gym Crow
2.7 go fumed silica
80.0 gym nickel powder (-325 mesh)
80.0 gym tungsten carbide powder (-~00 mesh)
0~4 ml non-ionic surfactant (Briton X-100)
This coating was also sprayed onto a Fe-Ni-Cu-C P/M
spur gear and cured at 3~3C for one half hour. Figure 9
shows the cured coating on a spur gear.
m e coated gear was then heated in a vacuum at
1121C for one half hour. The resultant sistered coating
consisted of hard tungsten carbide particles dispersed in a
soft nickel matrix of limited or low porosity. Figure 10 shows
the structure of the sistered coating.
EXAMPLE 8
The binder of Example 7 was constituted without
surEactant and without fumed silica. Because this binder was
less viscous, the heavy particles settled from suspension
quickly Nevertheless, by constantly agitating the solution, a
coating could be sprayed onto the P/M gear. A coating of the
characteristics as described in Example 7 was obtained after
sistering.
- 16 -
EXAMPLE 9
Iron powder was dispersed in a chromate/phosphate
binder of the following composition:
80 gym water
40 gym 85% H3P0~
10 gym aluminum hydroxide, dried gel
9 gym Crow
3 gym fumed silica
150 gym atomized iron powder (-325 mesh)
This coating was sprayed onto a sistered iron P/M disc
with a density of only 5.8 gm/cc. The coated disc was
recentered at 1121C for one half hour in a vacuum. The
sistered coating was porous; however, the porosity was much
smaller than that of tile disc core. Figure 11 shows an iron
disc that had been coated and recentered.
The discs were then clamped in a device in which one
side was pressurized and the time measured until sufficient ax
leaked through -the disc to equalize the pressure on the two
sides of the disc. The iron disc contained so much large and
interconnected porosity that the pressure equalized within 3
seconds. The sistered iron coating did not completely seal the
part, however, it did take 30 seconds for the pressure to
equalize on the coated disc. This further establishes the
differences in porosity (in size and number of pores) between
the coating sauntered and alloyed onto the surface of the part)
and that of the part itself.
'
EXPEL 10
The coating described in the above example was sprayed
onto an unsintered compact of nickel powder. The density of
the compact was 7.2 gm/cc. When the part was vacuum sauntered
at 1200C, a uniform skin formed on the surface. The etching
behavior of this spin demonstrated that it was almost pure iron.
I
- 17 -
It is often highly desirable to provide the sistered
coating as a continuous layer about the entire outer periphery
of the part as in the examples cited above. However, in some
cases the coating need not be continuous. Certain regions of
the part may be deliberately masked so as not to receive -the
coating when this is required for the desired application.
The invention described herein provides a means to
economically produce a metal part from individual metal powders
that possesses surface properties similar to or superior to
that of the cast or machined item of the same base metal
composition. It is contemplated that many items routinely
produced by P/M technology (i.e. gears, bearings, levers, cams,
actuators, etc.) can now be produced with harder, more wear
resistant, more corrosion resistant surfaces than previously
possible and that some totally new products could be also
produced.
It is contemplated, for example, that the invention ,
provides a means to fabricate clad sheet steel directly from
iron and graphite powders. It is well known that iron and
graphite powders can be roll compacted into a continuous green
strip which can be fed directly into a sistering furnace to
produce a continuous P/M sheet steel product. However, the
economic savings of this technique over established billet hot
and cold rolling technologies are so small as to make the
process impractical. If, however, the green strip were sprayed
with a coating containing stainless steel or nickel pigments in
accordance with this invention, the sistered product is a
stainless steel alloy- or nickel clad carbon steel superior in
corrosion resistance to conventional rolled product and yet
just as ductile and economical to produce.
It is also contemplated that sinterable coatings need
not be limited to P/M products. The concept of the invention
is workable on any metal product (cast, wrought or P/M) that
can survive exposure to the temperatures required to stinter the lo
coating.
- 18 -
In fact, the coating described in example 1 produced a
very dense nickel film on a 1010 steel panel when sistered at
1121C for one half hour. The resultant coating is shown in
Figure 12.
It is evident from the above description and the
principles embodied in the invention that a significant
contribution to the art of metallurgy has been made. The
equivalents of the various embodiments of the invention are
considered to be within the contemplation and scope of the
invention.
.
