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BACKGROUND OF THE INVENTION
This invention relates to a process of producing composite powder which
consists of metal sulfide particles coated with copper in a thin film.
More particularly, the invention is concerned with a process of producing
such powder by coating the particles of a metal sulfide, such as
molybdenum disulfide or tungsten disulfide, with copper in the form of a
thin layer made by a cementation reaction between copper ions and a metal
and/or an alloy baser than copper.
The copper-coated metal sulfide powder thus obtained is exceedingly
desirable as solid lubricant for use in the self-lubricating powder
materials to be shaped and sintered to oilless bearings, sliding parts,
and the like.
The self-lubricating parts such as those of oilless bearings have usually
been prepared by mixing the powder of copper, tin, or other metal, as the
base, with the powder of molybdenum disulfide, graphite or other solid
lubricant, pressing the mixture in dies to produce compressed shapes,
sintering the shapes, and finally impregnating the products with oil.
However, even if such solid lubricating powder of molybdenum disulfide,
graphite or the like, is directly added to the base metal powder, a good
self-lubricating product is not obtainable for a number of reasons
including the infeasibility of achieving uniform mixing and low strength
attained on sintering. Therefore, it is desirable that the powder of solid
lubricant, as of molybdenum disulfide or graphite, be not employed
directly as it is but in the form of a composite powder in which the
individual particles are thinly coated with copper, nickel, or other
second metal.
Such composite powders are available in a number of known and possible
combinations of solid lubricant particles as cores and metals in the form
of coating films. Typical of them are the combinations of molybdenum
disulfide, tungsten disulfide or other metal sulfide powder and a copper
coating.
Methods of producing the composite powder have heretofore depended on
thermal decomposition, reduction, vapor-phase reaction, plating reaction,
and vacuum evaporation. However, they have a variety of disadvantages in
the process of manufacture, quality of the product, and in respect of the
equipment required. No method has been established yet which will produce
a composite powder of a high, stable quality in a simplified process on an
industrial scale. More recently, some proposals have been made for the
manufacture of composite powder. One of them is a method, disclosed by
Japanese Patent Application Public Disclosure No. 32436/1976, for coating
core particles of a non-metal, alloy, or metal with copper. The method
consists of dispersing the core particles in an ammoniacal ammonium salt
solution containing dissolved nickel, reacting the resulting slurry with a
reducing gas at a high temperature and a high pressure so that the core
particles can be coated with nickel, taking out the nickel-coated powder
from the solution, redispersing the particles in a solution of copper
adjusted to pH 7 or less, and obtaining copper-coated powder by replacing
nickel with copper through a cementation reaction. According to the
method, it is not until the nickel once applied on the core particles has
been replaced by copper that the eventual copper coating is attained. The
double procedure necessitates an accordingly large number of steps,
calling for much time and labor.
Another example is Japanese Patent Application Public Disclosure No.
82871/1976. It teaches a technique aimed at coating the particles of a
powdery lubricant, e.g., graphite, molybdenum disulfide, or boron nitride,
with one of various metals, e.g., copper, silver, nickel, iron, or
aluminum. In this method, the powder of a lubricant ranging in particle
size from 1 to 2000.mu. is mixed into a dispersion of aforementioned metal
powder, in the range of 0.05-500.mu., in a solvent, thereby allowing the
metal particles to adhere to the lubricant particles, and after the
filtering, in order to attain more strong and stable adhesion, the coated
particles obtained are baked with heat in a hydrogen stream or a mixed
stream of hydrogen and nitrogen. This method again requires much time and
labor because of the very cumbersome steps of mixing, separation by
filtration, and heating following the filtration. Apparently the adhesion
of the metal particles upon the lubricant particles is weak and they have
to be handled with the greatest possible care prior to the heating.
From the foregoing it is not too much to say that the two methods described
above admit of much improvement, especially in simplification of the
steps. The present invention has been arrived at after an extensive search
for a method of manufacturing composite powder on an industrial scale in a
more simplified and yet more positive way than the prior art techniques,
the method being primarily directed to the typical composite powder
consisting of metal-sulfide type solid lubricant core particles and copper
coating.
It has now been found that the coating of metal sulfide particles with
copper can be done in a simple way of mixing and succeeding cementation
treatment in a single vessel, by suitably choosing the particle size of
the metal sulfide, composition of the copper solution, kind and size of
the metal to be added, state of agitation and conditions for cementation.
The present method needs no step of preliminarily applying nickel on the
lubricant particles, transferring the powder to another vessel, or heating
the coated particles. Experiments have indicated that the composite powder
thus prepared, when added as a solid lubricating part, would give an
excellent self-lubricating powder material to be sintered. Thus, the
method of the invention is quite satisfactory not only because of the
simplicity but also from the viewpoint of quality of the product.
BRIEF SUMMARY OF THE INVENTION
In brief, this invention provides a process of producing metal sulfide
powder coated with copper, characterized by the steps of adding and mixing
powder of a metal or alloy baser than copper with powder of a metal
sulfide, and adding an acidic solution containing copper ions to the
mixture with stirring, whereby the metal sulfide particles are coated with
the metallic copper formed in a cementation reaction.
DETAILED DESCRIPTION
The present invention will now be described in detail.
Suitable metal sulfide powder for use in the invention is of a particle
size between -5 mesh and +400 mesh. Particles larger than 5 mesh in size
will have ununiform coating, whereas those finer than 400 mesh will give
copper-coated powder which poses the problems of low fluidity and surface
oxidation. The metal sulfide is typically represented by molybdenum
disulfide or tungsten disulfide, although others may be employed when
desired.
As regards the acidic solution containing copper ions, its copper
concentration is not definitely determined since it varies with the size
of the particular metal sulfide powder and the quantity of copper is to be
used in coating. Generally, the concentration may range from 0.5 g/l to a
saturation value. Where the quantity of copper for coating is 50% by
weight of that of the metal sulfide, a copper concentration in the range
of 30-60 g/l is desirable. As the copper source, the salts of sulfuric,
hydrochloric, nitric, and organic acids (hereinafter called "copper
salts") prove substantially equally effective, but coppr sulfate is the
most preferred. Another factor to be taken into consideration is the
concentration of the free acid. Among the useful acids are sulfuric,
hydrochloric, nitric, acetic, and oxalic acids. Their concentrations
cannot be definitely determined, either. In case of sulfuric acid, 0.5 g/l
or more will genrally have the same effect. If the acid concentration is
less than 0.5 g/l, the resulting copper-coated metal sulfide powder will
not only turn reddish brown but will show ununiform coating.
The powder of a metal and/or an alloy baser than copper, which is to react
with copper ions, may be the powder of zinc, iron, aluminum, magnesium,
calcium, or the like or of an alloy of such metals. While these powders
are similarly effective, economics dictates the use of iron powder,
especially reduced iron powder. Desirably the particle size ranges from
-100 to +400 mesh. If the powder contains many particles coarser than 100
mesh, the coating will become uneven. Conversely if the proportion of
particles finer than 400 mesh is high, the particles will be largely
dissolved by a reaction with the free acid in the process of cementation
and will be consumed without taking part in the reaction for precipitation
of copper. The proper amount to be added is believed to be slightly more,
say about 1.01 times larger, than the stoichiometric equivalent of the
intended amount of copper to be applied on the metal sulfide particles.
In the operation, the reaction vessel is charged with a desired amount of
metal sulfide powder and an amount of metal and/or alloy powder calculated
on the basis of the aimed amount of copper for coating. The vessel must be
provided with means for producing an adequate stirring action. With this
in view, a vessel equipped with blades which will create a planetary
motion is employed. While the two components are being thoroughly mixed, a
copper salt solution is added. This is desirably done in such a way that
the addition up to realization of the funicular [II] region takes a
relatively long period, for example from 20 seconds to 10 minutes, and
then the slurry region is reached in a short period of from 5 to 10
seconds. If the time period held in the slurry region is unduly extended,
the iron powder and the powder to be coated would separate from each
other, making it unable for the precipitated copper to serve the coating
purpose. The terms "funicular region" and "slurry region" will be
explained later. The periods of time for those regions vary with the
powder quantities, agitation efficiency, and other factors. The copper
salt solution is desired to be added batchwise, because it adds to the
uniformity of the mixture. Following the completion of the addition of a
given amount of the copper salt solution sitrring of the mixture is
continued, e.g., for about 30 seconds. After the stirring, the resulting
composite powder is recovered. Under the invention the quantity of copper
to be applied can be controlled within the limits of the desired value
plus or minus 0.5%.
As a starting material the metal sulfide powder sometimes is too fine or
contains a large proportion of exceedingly flat particles or has an
excessively broad range of particle size, depending on the source from
which the material is derived. In such cases, preliminary granulation
and/or sizing of the granules or particles will prove highly beneficial.
By way of example, the metal sulfide powder may be ground and granulated
by a grinding-granulating mixer, e.g., a Henschel mixer, using a binder
prepared by diluting a resol and/or novalak type phenol resin with
alcohol. Following this, the excess alcohol is evaporated and collected
for reuse. After the alcohol has vaporized, the granules are classified by
sieving to obtain those in the desired range of size. Too coarse and fine
particles outside the desired range are sent back to the Henschel mixer,
where they are once again subjected to grinding and granulation, this time
with the addition of alcohol only. The second grinding and granulation
produces additional granules of the size in the intended range. The
product is sieved and too coarse and fine particles are returned to the
mixer again. Repetition of this procedure permits eventual granulation and
sizing of all the material powder to granules of a predetermined size. The
powder now takes the form of uniform spherical granules suited as the
metal sulfide particles to be coated with copper. Curing at
100.degree.-300.degree. C. for 0.5-2.0 hours stabilizes the particles. In
case of a novolak resin, hexamine or the like may be used for the
stabilization purpose. The quantities of the resin and alcohol required
depend upon the size and shape of the metal sulfide particles and the
desired size and shape of the final particles. Generally, however, a resin
amount in the range of 2-40% of the composition will not materially affect
the properties of the solid lubricant. With regard to alcohol, the amount
must be within the range enough for maintaining the funicular [I] and [II]
regions. When the capillary or even slurry region has been reached, it is
not objectionable to evaporate the excess alcohol down to either funicular
region.
By the procedure described immediately above, any material at hand, which
would otherwise be of no use in the process of the invention, is made
usable through granulation and sizing to a particle size within a
predetermined range. Thus, metal sulfide powders of particle sizes beyond
the ordinary limits can now be employed. In addition, the use of sized
particles brings uniformity of the reaction and hence evenness of copper
coating.
EXAMPLE 1
With the view to obtaining molybdenum disulfide powder coated with copper
in an amount of 50% by weight, the procedure now to be described was
followed. 200 g of natural molybdenum disulfide powder, ranging in
particle size from -80 to +400 mesh, and 177.2 g of reduced iron powder,
from -100 to +400 mesh in size, were charged into a cementation vessel
equipped with blades for planetary motion. The amount of reduced iron
powder was 1.01 times as much as the theoretical amount found necessary on
stoichiometric calculation. While the charge was being thoroughly stirred
by the blades, an acidic copper sulfate solution containing 48 g of copper
and 200 g of sulfuric acid per liter was added little by little, and the
funicular [II] region was reached in about 30 seconds. The amount of the
acidic copper sulfate solution added up to that point was 1.0 l (48 g in
terms of copper). Following this, a complete slurry region was arrived at
in about 3 seconds, and the remainder of the acidic copper sulfate
solution required was added over a period of about 10 seconds. The total
amount of the copper sulfate solution added was 3.2 l, or 154 g in terms
of copper. After the addition, stirring was continued for an additional
period of about 30 seconds, and the copper coating treatment was
concluded. The coated powder was recovered, washed, and dried, and finally
396g of copper-coated molybdenum disulfide powder was obtained. The
individual particles were evenly coated with copper, which was similar to
commercially available copper powder in the tone of color. The copper
coating accounted for 49.5% of the total weight. The composite powder thus
obtained was under 60 mesh in particle size.
EXAMPLE 2
In order to obtain tungsten disulfide powder coated with 50% by weight of
copper, a copper sulfate solution was added to a mixture of tungsten
disulfide and reduced iron powder generally in the same manner as
described in Example 1. The amount of addition and the stirring conformed
to the conditions specified in the preceding Example. A composite powder
with 49.5% copper coating resulted. Its color tone was quite satisfactory.
EXAMPLE 3
To conform the effect of sizing by preliminary grinding and granulation of
metal sulfide powder, 1000 g of commercially available "Molykote" (the
trade designation of molybdenum disulfide sold by Amax Co.) was ground and
granulated. Molykote had a relatively wide range of particle size
distribution. Molykote was placed in a Henschel mixer, 167 g of phenol
resin (with 100 g of solid contents) diluted with 80 ml methanol was added
as a granulating binder, and the grinding and granulating treatment was
carried out for about 10 minutes. The resultant granules were dried at
70.degree. C. for 0.5 hour and sieved, when 400 g of granulated molybdenum
disulfide powder ranging in particle size from -150 to +250 mesh resulted.
The remainder of powder composed of +150 and -250 mesh particles was
returned to the Henschel mixer, where it was once again ground and
granulated with the addition of 40 ml of methanol. After sieving, the
above procedure was repeated until about 95% of the initial amount of
Molykote became granules of molybdenum disulfide ranging in size from -150
to +250 mesh. This granulated powder was cured at 120.degree. C. for one
hour and 300.degree. C. for further one hour. After resieving, 980 g of
molybdenum disulfide powder, granulated and sized to the range from -150
to +250 mesh, was obtained. The particles were spherical and uniform.
Using 200 g of this powder, 399 g of copper-coated molybdenum disulfide
powder was obtained by the same procedure as described in Example 1. The
composite powder thus prepared had uniform coating of copper, showing the
same color tone as that of commercially available copper powder. The total
amount of copper coating was 49.9% of the total weight, and all particles
were 80 mesh or finer in size.
From the foregoing description it will be understood that the process of
the invention permits continuous operation with a single vessel, in a very
simplified way without the necessity of extra time or labor as in the
conventional methods. Moreover, the product composite powder is impeccable
in external appearance. It is also of importance that the invention has
opened a way for utilization of the raw metal sulfide powder of particle
sizes outside the normally desirable range. With these advantages, the
present invention contributes greatly to the technical progress of the
art.
Lastly, the terms "funicular [I] and [II] regions" and the like herein used
will now be explained. The state of packing and fluidity of solid-liquid
systems, which is originally very difficult to define, is divided, in a
known practice, into five stages according to the degrees of packing and
fluidity and are designated respectively. In conformity with this
five-stage indication method, the conditions of powder-liquid mixtures are
herein defined as follows:
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Liquid Con-
Region Solid phase
phase dition
Fluidity
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(1) Pendular
Continuous
Dis- loose Dilatant
continuous dispersion
(2) Funicular
" Continuous
" Psuedoplastic
[I] dispersion
(3) Funicular
" " " Plastic
[II] dispersion
(4) Capillary
Dis- " Vis- Shear Hardened
continuous cous dispersion
(5) Slurry
Dis " Muddy False body
continuous dispersion
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