Tungsten carbide-cobalt flame spray powder and method

Tungsten carbide-cobalt agglomerated flame spray powder is produced by spray drying a slurry of particles in an aqueous cobalt nitrate solution. The agglomerates are classified according to size and the out-of-size agglomerates are recycled. The classified agglomerates are heated in flowing hydrogen, to reduce the nitrate to cobalt metal, and then sintered to strengthen the agglomerates for subsequent flame spraying.

BACKGROUND OF THE INVENTION 
This invention relates to flame spray powders, and more particularly 
relates to tungsten carbide-cobalt agglomerated powders utilizing cobalt 
nitrate as a binder, and also relates to a method for producing such 
powders. 
Generally speaking, powder for use with flame spray coating equipment must 
have a narrow size distribution and must be relatively free flowing. (As 
used herein the term "flame spray" is meant to refer generically to both 
flame spray and plasma spray). The narrow size range is necessary if all 
particles are to be heated uniformly. The flow is to enable a uniform and 
controllable feed through the small diameter tubes and orifices of the 
equipment. 
Tungsten carbide is commonly made by reacting tungsten powder or tungsten 
oxide with carbon at high temperatures. The result is a powder of average 
diameter less than about 10 micrometers and typically less than about 5 
micrometers which has very poor flow. To achieve good flow, such powders 
must be agglomerated by one of several processes well known to the art. 
Such processes typically use an organic binder of some sort, such as 
paraffin or one of the many organic waxes, to hold the agglomerates 
together. 
The organic binders have two main disadvantages. First, they must be 
removed prior to final processing or use of the powder or part made 
therefrom. Complete removal is difficult and time consuming. Second, the 
agglomerates are not very strong. When powders are blended or sifted they 
tend to deagglomerate, and to plug the sifting screens. Also, when stored 
in warm areas, the powder particles fuse together because of softening of 
the binders. 
When working with molybdenum and tungsten powders, it was recently 
discovered that ammonium molybdate and ammonium tungstate added to an 
aqueous slurry of molybdenum or tungsten powder would act as a binder of 
the powder being agglomerated. When the slurry is spray dried, the 
ammonium molybdate or ammonium tungstate is well distributed throughout 
the interstices of the dried agglomerate and provides a strong bond for 
subsequent operations. A subsequent reduction reaction permits conversion 
of the ammonium salt to pure metal which still acts as a good binder 
because of surface-to-surface bonding promoted between the metal particles 
at the reduction and/or heat treatment temperature. 
For hard, wear resistant surfaces, tungsten carbide, WC, is usually mixed 
intimately with about 4 to 20 weight percent of cobalt powder and flame 
sprayed to form a coating. Normally, the cobalt is agglomerated along with 
the WC as previously described prior to flame spraying. 
Replacing the organic binders normally used in spray drying would be 
desirable for the reasons already stated. However, ammonium tungstate 
reacts with cobalt to form a gel or large particle size precipitate, which 
would hinder the spray drying operation. Ammonium complexes of WC and/or 
cobalt are either nonexistent, not readily available commercially or 
chemically unstable. Many other soluble salts, either tend to evolve large 
amounts of gases during the decomposition heat treatment (carbonates, 
oxylates and oxychlorides), or leave contaminating residues (sulfates, 
silicates and boron containing compounds such as borax or boric acid), or 
are corrosive (chlorides, oxychlorides). 
Furthermore, any soluble salt used as a binder must be compatible with the 
processing of the tungsten carbide-cobalt powder, that is, it must be 
capable of being removed or decomposed without promoting substantial 
decarburization of the WC. 
SUMMARY OF THE INVENTION 
In accordance with the invention, it has now been discovered that in the 
agglomeration of WC particles and particles of cobalt or reducible cobalt 
compounds such as cobalt oxide to form flame spray powders, part of the 
cobalt can be added as cobalt nitrate, which acts as a binder when a 
slurry of the particles in an aqueous solution of the cobalt nitrate is 
spray dried, and the cobalt nitrate may be reduced to metallic cobalt 
without significant decarburization of the WC particles. 
Furthermore, the cobalt nitrate is sufficiently strong and well distributed 
as a binder in the spray dried agglomerates to permit normal size 
classification such as by sieving or screening to obtain the desired size 
fraction. The out-of-size agglomerates can then be deagglomerated by 
reslurrying them in water, (resulting in dissolution of the cobalt 
nitrate) and repeating the spray drying cycle again, thereby avoiding the 
need for a separate binder removal step. 
The cobalt nitrate may be reduced to cobalt metal either by carrying out 
flame spraying under reducing conditions or carrying out a separate 
reducing heat treatment prior to flame spraying. 
In accordance with a preferred embodiment, the spray dried powder is 
subjected to a reducing step and a sintering step to further strengthen 
and densify the agglomerates prior to flame spraying. 
Elemental carbon and/or tungsten powder may be added to the slurry prior to 
spray drying, or to the spray dried agglomerates prior to reduction and/or 
sintering, in order to adjust or compensate for shifts in the 
stoichiometry of the WC during processing. Cobalt may be introduced as an 
insoluble chemically reducible compound of cobalt, such as cobalt oxide, 
which can then be reduced subsequently when the soluble cobalt nitrate 
binder is reduced. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
For a better understanding of the present invention, together with other 
and further objects, advantages and capabilities thereof, reference is 
made to the following disclosure and appended claims in connection with 
the above-description of some of the aspects of the invention. 
In the formation of WC-cobalt flame spray powder, cobalt is normally 
present in the amount of from about 4 to 20 weight percent of the total 
weight of the powder. From about 2 to about 50 percent by weight of the 
cobalt may be introduced as the soluble salt, cobalt nitrate, below which 
insufficient binding action occurs during spray drying and subsequent to 
spray drying, and above which amount the agglomerate density and strength 
are adversely affected. 
As already stated, the starting materials may additionally include 
elemental carbon and/or tungsten powder in order to compensate for shifts 
in stoichiometry during processing. For example, despite the precautions 
taken during processing, usually carbon is removed to a slight extent 
during the reduction step in which cobalt nitrate is reduced to cobalt. 
This usually amounts to at most about several tenths of one percent by 
weight and can be compensated by adding the appropriate amount of carbon 
at some point during the processing, preferably to the starting materials 
prior to slurrying or spray drying. 
The starting materials are intimately mixed, such as by ball milling or 
attritor milling, and slurried in the cobalt nitrate solution. Preferably 
the amount of powder particles in slurry and solution is from about 50 to 
85 weight percent, below which the removal of the excess water in the 
slurry is an additional expense and particle size control becomes 
difficult and above which the slurry becomes too viscous to easily pass 
through the spray nozzle. The concentration of cobalt nitrate in solution 
should be from about 10 grams per liter to 1000 grams per liter, below 
which insufficient binding action occurs during and subsequent to spray 
drying, and above which the agglomerate density and strength are adversely 
affected. 
Spray drying may be carried out using commercially available spray drying 
equipment. The inlet and outlet air temperatures should be maintained 
below 370.degree. C. and 190.degree. C. respectively, to prevent 
substantial oxidation or decarburization of the slurry constituents, or 
decomposition of the cobalt nitrate. 
The spray dried agglomerates may then be classified, usually by sieving or 
screening, in order to obtain a desired particle size distribution, 
typically within about 60 micrometers and preferably 80 percent within 30 
micrometers, for flame spraying application. 
Following classification by screening to obtain the desired size fraction, 
out-of-size material may be deagglomerated by reslurrying in water to 
dissolve the cobalt nitrate binder, and the spray drying cycle repeated. 
The classified agglomerates may be reduced by a separate heat treating step 
prior to flame spraying, such as by heating in flowing hydrogen or other 
reducing gas at a temperature of at least about 400.degree. C., which is 
sufficient to reduce oxygen from the nitrate to low levels, for example 
0.1 weight percent, above which significant decarburization could occur 
during any subsequent sintering step or during flame spraying. The 
reduction temperature should not exceed about 900.degree. C., above which 
significant decarburization could occur in the presence of even trace 
amounts of water vapor and/or oxidizing contaminants. Reducing times may 
be from about 1/2 to 24 hours, the shorter times corresponding to higher 
temperatures. 
It has been found that the spray dried powders of the invention normally 
possess sufficient green strength to withstand such handling for size 
classification and reduction. However, it may be desired as an optional 
step to heat treat the agglomerates for purposes of further strengthening 
or densification. Of course, such treatment should be carried out under 
conditions to prevent formation of an unusable mass by substantial 
diffusion bonding of the agglomerates to one another. Such sintering is 
preferably carried out at a temperature within the range of about 
1100.degree. to 1350.degree. C. for about 5 to 120 minutes, in a neutral 
or nonoxidizing atmosphere in order to prevent decarburization of the WC. 
Following such sintering it may be desired to further screen the material 
to remove or breakup only cakes or chunks of material which may have 
formed. A 100 mesh screen has been found suitable for such purposes. 
Of course it is unnecessary that the spray dried agglomerates be subjected 
to separate reducing and sintering heat treatments. For certain 
applications, the spray dried agglomerates may be flame sprayed per se 
under reducing conditions in order to directly convert the spray dried 
agglomerates to a flame spray coating containing typically WC, W.sub.2 C, 
metallic cobalt, and several Co C-W compounds. 
The following examples are presented to further illustrate the practice of 
the invention:

EXAMPLE I 
44 pounds of tungsten carbide, 6.77 pounds of cobalt oxide and 8.2 liters 
of water were milled for one-half hour in a commercially available 
attritor mill. Tungsten carbide balls, 1/4 inch in diameter, were used as 
the milling medium. 3.34 pounds of cobalt nitrate were then dissolved in 
this milled slurry and the slurry was spray dried in a Proctor-Schwartz 
dryer using a two-fluid nozzle. The inlet air temperature was 600.degree. 
C. and the outlet air temperature was 360.degree. C. The resulting spray 
dried agglomerates were screened into two size fractions, the first being 
within the range of -325 mesh to +10 micrometers and the second within the 
range of -170 to +325 mesh. The oversize powder was reslurried for another 
spray drying cycle. 
The agglomerates within the desired size fractions were then heated in 
flowing hydrogen at 725.degree. C. for 31/2 hours to reduce the cobalt 
oxide and cobalt nitrate to metallic cobalt. The reduced agglomerates were 
then further heated in hydrogen at 1230.degree. C. for 1/4 hour to 
strengthen and densify the agglomerates by sintering. The powder was then 
passed through a 100 mesh screen to breakup cakes of agglomerates which 
had formed. Hall flow was measured of 50 grams samples for both fractions 
and was 32 seconds for the -325 mesh fraction and 15 seconds for the -170 
+325 mesh fraction. Such Hall flow values represent good to excellent 
flowability for such flame spray powders. 
EXAMPLE II 
The procedure of Example I was repeated except that the carbon level was 
lowered from 5.3 to 4 weight percent by adding tungsten powder to the 
starting material. Thus, the starting materials were 159 pounds of WC, 
35.5 pounds of cobalt oxide, 61 pounds of tungsten and 10.5 pounds of 
cobalt nitrate. Again the resulting powder exhibited good flowability as 
evidenced by Hall flow values on 50 gram samples of 14 seconds and 30 
seconds for the -325 and -170 +325 mesh fractions, respectively. 
While there has been shown and described what are at present considered the 
preferred embodiments of the invention, it will be obvious to those 
skilled in the art that various changes and modifications may be made 
therein without departing from the scope of the invention as defined by 
the appended claims.