Flame Spray Powder And Process

Abstract

The flame spraying of the flame spray powder which has been formed by spray drying of a slip or slurry of fine particles of a flame spray material. This flame spray powder has individual particles which are of substantially spheroid shape, of a size between about 20 mesh and 1 micron, and are formed of multiple subparticles bound together without fusion by a spray-dried binder and having a crush resistance of at least 0.7 grams. For the spray drying, fine particles of any known or conventional flame spray material or combination thereof, such as metals, ceramics, carbides, etc., suspended in a slip or slurry of a liquid, preferably water, with a suitable binder and preferable auxiliary agents, is atomized into a hot drying gas stream forming the spheroid larger composite particles.

Description

This invention relates to an improved flame spray powder, to a process for its production, and to a flame spray process utilizing the same.

Flame spraying involves the feeding of a heat-fusible material into a heating zone wherein the same is melted or at least heat-softened and then propelled from the heating zone in a finely divided form, generally onto a surface to be coated. The spraying is effected utilizing a device known as a flame spray gun. The material may initially be in the form of a powder which is designated as a "flame spray powder," and the guns utilized for spraying the material initially fed in this powder form are known as "powder-type flame spray guns." In powder type flame spray guns, the flame spray powder, usually entrained in a carrier gas, is fed into the heating zone of the gun which is most commonly formed by a flame of some type. The powder is either melted or at least the surface of the grains heat-softened in this zone, and the thus thermally conditioned particles are propelled onto a surface to provide the coating. While not required, a blast gas may be provided in order to aid in accelerating the particles and propelling them toward the surface to be coated and/or to cool the workpiece and the coating being formed thereafter. The heat for the heating zone while most commonly produced from a flame caused by the combustion of a fuel, such as acetylene, propane, natural gas or the like, using oxygen or air as the oxidizing agent, may also be produced by an electric arc flame or plasma flame, or any other known heating device.

Flame spraying in the initial stages of its commercial development was mostly used for spraying metals and still is often referred to as "metallizing," though the same is now used for spraying a much wider group of materials, including higher melting point or refractory metals, ceramics, cermets, carbides, and other metal compounds.

A powdered heat-fusible material in order to be satisfactorily sprayed and thus considered a "flame spray powder" must have certain physical characteristics with respect to size, physical strength, flowability, etc. The powder must be sufficiently flowable to be passed through the flame spray gun without difficulty or clogging, and this flowability not only depends on size, absence of caking materials, such as excess moisture, but on shape and surface characteristics. In order to produce a satisfactory coating the powder must have a specific size range, as for example, between 20 mesh and 1 micron, and preferably between 100 mesh and 3 microns, and the individual particles should not vary too greatly in their size distribution, i.e., the powder should be free of excessive fines and larger particles. The uniformity and quality of the coating formed by flame spraying is often dependent on the uniformity of size of the individual particles in the powder. During spraying the kinetic propelling of the particles and its contact with fluid, including combustion fluids and propelling fluids, often causes a classifying effect which adversely affects the homogeneity and uniformity of the coating, and the presence of excess fines may change the nature of the coating, producing for example excessive oxides or the like.

In obtaining suitable commercial flame spray powders extensive screening and classifying techniques are generally required and only a relatively small cut of the available powder material is generally suitable for marketing as a flame spray powder. In most instances powders are formed by reducing, as for example, milling a larger mass which results in a rather wide range of particle sizes which must then be screened to obtain operative size range cuts. This is generally true, irrespective of the source of the mass which is milled, including pressed, cast, calendered or extruded masses.

One object of this invention is the production of an improved flame spray powder. This and still further objects will become apparent from the following description read in conjunction with the drawing which is a flow sheet illustrating the process for producing powders in accordance with the invention.

In accordance with the invention I have discovered that a superior and improved flame spray powder may be produced utilizing spray drying techniques. In accordance with the invention finely divided flame spray material suspended in a slip or slurry of liquid, preferably water, with a suitable binder and preferable auxiliary agents, is atomized and the atomized suspension dried in a hot gas stream, forming the coarser flame spray powder, the individual particles of which are of substantially spheroid shape, have a size between about 20 mesh and 1 micron, and are formed of a multiple number of subparticles bound together without fusion by the spray dried binder, and which have a crush resistance of at least 0.7 grams.

As the starting finely divided material utilized in the formation of the slip, any of the known or conventional flame spray materials, or any known combination thereof, may be used. In addition to the conventional metals or alloys or metal mixtures, which will ultimately form alloys or semialloys when flame sprayed, there may be mentioned oxides, as for example refractory oxides, such as alumina A1.sub.2 O.sub.3, Beryllia BeO, Ceria CeO.sub.2, Chromia CR.sub.2 O.sub.3, cobalt oxide CoO, gallum oxide Ga.sub.2 O.sub.3, hafnia HfO.sub.2, magnesia MgO, nickel oxide NiO, tantalum oxide TA.sub.2 O.sub.5, thoria ThO.sub.2, Titania TiO.sub.2, yttrium oxide Y.sub.2 O.sub.3, zirconia ZrO.sub.2, vanadium oxide VO, niobium oxide NbO, manganese oxide MnO, iron oxides Fe.sub.2 O.sub.3, zinc oxide ZnO; complex aluminates such as BaO.sup.. Al.sub.2 O.sub.3, i.e. BaO.sup.. A1.sub.2 O.sub.3, CeO.sup.. A1.sub.2 O.sub.3, CoO.sup.. A1.sub.2 O.sub.3, Gd.sub.2 O.sub.3 .sup.. A1.sub.2 O.sub.3, K.sub.2 O.sup.. A1.sub.2 O.sub.3, Li.sub.2 O.sup.. A1.sub.2 O.sub.3, Li.sub.2 O.sup.. 5 A1.sub.2 O.sub.3, MgO.sup.. A1.sub.2 O.sub.3, NiO.sup.. A1.sub.2 O.sub.3, Sr.sub.2 O.sub.3 .sup.. A1.sub.2 O.sub.3, SrO.sup.. A1.sub.2 O.sub.3, SrO.sup.. 2A1.sub.2 O.sub.3, 2Y.sub.2 O.sub.3 .sup.. A1.sub.2 O.sub.3, ZnO.sup.. A1.sub.2 O.sub.3 ; zirconates such as CaO.sup.. ZrO.sub.2, SrO.sup.. ZrO.sub.2, BaO.sup.. ZrO.sub.2 ; titanates such as A1.sub.2 O.sub.3 .sup.. TiO.sub.2, 2BaO.sup.. TiO.sub.2, CaO.sup.. TiO.sub.2, HfO.sub.2 .sup.. TiO.sub.2, 2MgO.sup.. TiO.sub.2, SrO.sup.. TiO.sub.2 ; chromites, such as CaO.sup.. Cr.sub.2 O.sub.3, CeO.sup.. Cr.sub.2 O.sub.3, MgO.sup.. Cr.sub.2 O.sub.3, FeO.sup.. Cr.sub.2 O.sub.3 ; phosphates such as A1.sub.2 O.sub.3 .sup.. P.sub.2 O.sub.5, 3BaO.sup.. P.sub.2 O.sub.5, 3CaO.sup.. P.sub.2 O.sub.5, 3SrO.sup.. P.sub.2 O.sub.5 ; and other mixed oxides, such as La.sub.2 O.sub.3 .sup.. Fe.sub.2 O.sub.3, MgO.sup.. Fe.sub.2 O.sub.3, 2MgO.sup.. GeO.sub.2, CaO.sup.. HfO.sub. 2, La.sub.2 O.sub.3 .sup.. 2HfO.sub.2, Nd.sub.2 O.sub.3 .sup.. 2HfO.sub.2, 6BaO.sup.. Nb.sub.2 O.sub.5, Dy.sub.2 O.sub.3 .sup.. Nb.sub.2 O.sub.5, 2MgO.sup.. SnO.sub.2, BaO.sup.. ThO.sub.2, SrO.sup.. UO.sub.3, CaO.sup.. UO.sub.3, CeO.sub.2 .sup.. Cr.sub.2 O.sub.3 ; silicates such as 3A1.sub.2 O.sub.3 .sup.. 2SiO.sub.2 (mullite), BaO.sup.. 2SiO.sub.2, BaO.sup.. A1.sub.2 O.sub.3 .sup.. 2SiO.sub.2 , BaO.sup.. TiO.sub.2 .sup.. SiO.sub.2, 2CaO.sup.. SiO.sub.2, Dy.sub.2 O.sub.3 .sup.. SiO.sub.2, Er.sub.2 O.sub.3 .sup.. SiO.sub.2, ZrO.sub.2 .sup.. SiO.sub.2 (zircon), 2MgO.sup.. SiO.sub.2 , ZrO.sup.. ZrO.sub.2 .sup.. SiO.sub.2 ; carbides, such as titanium carbide TiC, zirconium carbide ZrC, hafnium carbide HfC, vanadium carbide VC, niobium carbide NbC, tantalum carbides TaC, Ta.sub.2 C, chromium carbides Cr.sub.3 C.sub.2, Cr.sub.1 C.sub.3, Cr.sub.23 C.sub.6, molybdenum carbides Mo.sub.2 C, MoC, tungsten carbides WC, W.sub.2 C, thorium carbides ThC, THC.sub.2 ; complex carbides, such as WC+ W.sub.2 C, ZrC+ TiC, HfC, NbC, TaC, or VC, TiC+ HfC, TaC, NbC, or VC; VC+ NbC, TaC, or HfC; HfC+ TaC or NbC; HbC+ TaC; W.sub.2 WC+ TaC, NbC, ZrC, TiC; WC+ TiC or ZrC; TiC+ Cr.sub.3 C.sub.2 ; TiC+ mo.sub.2 C.

Borides, such as TiB.sub.2, ZrB.sub.2, HfB or HfB.sub.2, borides of V, borides of Nb, borides of Ta, borides of Cr, borides of Mo, borides of W, borides of the rare earth metals;

Silicides, such as silicides of Ti eg Ti.sub.5 Si.sub.3

silicides of Zr eg Zr.sub.6 Sr.sub.5

silicides of Hf eg Hf.sub.5 Si.sub.3

silicides of V eg V.sub.3 Si or VSi.sub.2

silicides of Nb eg Nb.sub.5 Sr.sub.3 or NbSi.sub.2

silicides of TA eg Ta.sub.5 Si or TaSi.sub. 2

silicides of Mo eg MoSi.sub.2

silicides of W eg WSi.sub.2

silicides of Cr eg Cr.sub.3 Si or Cr.sub.3 Si.sub.2

silicides of B eg B.sub.4 Si or B.sub.6 Si

silicides of the rare earth metals.

Nitrides such as boron nitrides and silicon nitrides.

Sulfides such as MgS, BaS, GrS, TiS ZrS, ZrS.sub.2, HfS, VS, V.sub.2 S.sub.3, CrS, MoS.sub.2, WS.sub.2, the various rare earth sulfides;

Metalloid elements, such as boron, silicon, germanium.

Cermets, such as WC/Co, W.sub.2 C/Co, WC+ W.sub.2 C/Co, Cr/A1.sub.2 O.sub.3, Ni/A1.sub.2 O.sub.3, NiA1/A1.sub.2 O.sub.3, NiA1/ZrO.sub.2, Co/ZrO.sub.2, Cr/Cr.sub.3 C.sub.2, Cr/Cr.sub.2 O.sub.3, Co/TiC, Ni/TiC, Co/WC+TiC, Cr+Mo/A1.sub.2 O.sub.3, Ni, Fe and/or their alloys, Cu and/or its alloys such as aluminum bronze, phosphor bronze, etc., with the disulfides or deselenides of Mo, W, Nb, Ta, Ti, or V, or boron nitride for "self-lubricating" coatings with very low friction coefficient.

Cermets which contain an active metal from the group composed of Ti, Zr, Ta, Cr, etc., or hydrides or other compounds or alloys of these active metals, which will alloy with the metal phase of the cermet and promote adhesion of the metal phase to the refractory phase by promoting "wetting" of the surface of the refractory phase.

Cermets, for instance those containing a metal and a carbide as the refractory phase, which also contain free carbon, such as high purity graphite or the like, which will effectively reduce or prevent oxidation of the carbide phase and reduce solutioning of the carbide phase in the metal binder phase.

Mixtures of any desired combinations of these or any known flame spray material may be used for any purpose, including for the formation of synergistic composites of the type mentioned in U.S. Pat. No. 3,254,970 or combinations which will exothermically react to form an intermetallic compound as disclosed in the aforesaid patent and U.S. Pat. No. 3,322,515. In addition, combinations which when flame sprayed, will endothermically react, or combinations or components which will decompose to form desired coating materials, as for example carbonates, oxalates, nitrates or oxychlorides which will decompose to form oxide coatings, as for instance those of thorium, zirconium, magnesium or yttrium may be used. Furthermore, mixtures of oxides and metals which react in a redox-type of reaction, converting a metal to an oxide and an oxide to a metal, forming metal-oxide mixtures into metal-oxide or intermetallic-oxide or cermets or the like, as for instance

3NiO+ 2A1 3Ni+A1.sub.2 O.sub.3 or

3NiO+ 5A1 3NiAl+ A1.sub.2 O.sub.3

Cr.sub.2 O.sub.3 + 2A1 2Cr+ A1.sub.2 O.sub.3 or

Cr.sub.2 O.sub.3 + 4A1 2CrAl+ A1.sub.2 O.sub.3

Fe.sub.2 O.sub.3 + 2A1 2Fe+ A1.sub.2 O.sub.3

may be used.

There also may be mentioned mixtures of metal oxides and reducing agents, metals and nonmetals, such as boron, silicon, nitrogen, sulfur, phosphorus or the like. Still further, there may be mentioned metal hydrides alone or in mixture with other materials, such as metal oxides and the like.

These finely divided components should preferably be in the form of fine or superfine particles having, for example, a particle size below 200 mesh and preferably below 325 mesh, and most preferably below 15 microns.

These fine or superfine particles are then mechanically mixed with water or another liquid and the binder forming a suspension which is termed a "slip." The concentration of the fine or superfine particles in the slip may vary between about 40 weight percent and 99 weight percent, and preferably between 50 weight percent and 98 weight percent.

While water is the preferable liquid used to form the slip, due to its ready availability, low cost, nonflammability, high evaporation rate at relatively low temperature, relative inertness and ability to dissolve or suspend useful binders, it is also possible to use other liquids, such as hydrocarbon solvents, alcohols or other organic liquids, alone or in admixture. When using, however, flammable liquids, care must be taken to avoid combustion or explosion in the drying.

The slip must additionally contain a binder which is capable of ultimately binding the subparticles together into the flame spray particles of the required strength and crush resistance.

As a binder any material which can be dissolved or suspended in the mother liquid of the slip and which when dried will form a film and/or adhere to the material being agglomerated, can be used as a binder providing that the same is sufficiently hard and tenacious to form an agglomerate of the required strength and hardness. In general, film-forming organic resins which are soluble in the liquid of the slip, may be used. Examples of these include polyvinyl alcohol, gum arabic and other natural gums, carboxy methyl cellulose salts, polyvinyl acetate, methyl cellulose, ethyl cellulose, polyvinyl butyral dispersions, protein colloids, acrylic resin emulsions, ethylene oxide polymers, water-soluble phenolics, wood extracts such as sodium, ammonium, or calcium lignin sulfonates, sodium, ammonium, potassium or propylene glycol alginates, various flour and starches.

Empirically, a potential binder-material combination can be selected and a small quantity of a test slip formulated, including in the slip any additive required for specific purpose and compatible with the binder, i.e., wetting agent, suspending agent, deflocculents, etc., the need for which is determined by observation during the mixing and evaluation of the slip. The binder must not be precipitated from solution by any additive or the solid; the solids must remain reasonably well suspended and completely dispersed in the liquid; the slip must not gel nor should the solids precipitate out as a solid cake; nor should there by any unusual chemical reaction between ingredients in the slip such as to result in the evolution of a gas.

A qualitative measure of the effectiveness of the binder in cementing the particles to each other can be made by drying a film of the slip on a glass microscope slide and judging the hardness abrasion resistance of the composite film, the adhesion of the binder to the solid particles, and, by destructively abrading the dried film gross segregation of the binder from the solids in drying. Relative film hardness for various binder concentrations is very simply determined in this manner.

In addition to organic binders, inorganic binders, such as sodium silicate, boric acid, borax, magnesium or other soluble carbonates, nitrates, oxalates, or oxychlorides may be used.

In addition to serving strictly a binding function, binders may be chosen to perform auxiliary functions, or to impart additional desirable characteristics to the flame spray powder. Thus, for example, pigments or dyes may be added to the binder or to the slip for ultimate incorporation in the dried binder in order to permit color coding of the flame spray powder. If the flame spray material is prone to undesirable oxidation when flame sprayed, hydrocarbon binders may be chosen which will produce a protective inert coating or reducing atmosphere adjacent the melting or reacting particles during the flame spraying in order to suppress such oxidation. If it is desirable to add a further element or prevent loss of an element in the flame sprayed coating, a binder may be selected which will perform this function. Thus, a carbon-containing binder, such as an organic binder, may be used in order to introduce carbon to form a carbide or to prevent carbon depletion in the spraying of the carbide. Binders which will decompose to form a reducing atmosphere or containing reduction agents may be used in connection with practically all metal or alloy components in order to reduce the oxide films inherently present on the subparticle surfaces and thus improve consolidation, bonding, alloying, or reaction between constituents as the case may be.

The binder may additionally be chosen to rapidly decompose in the flame generating the gas or vapor in order to, in effect, rapidly break up the agglomerated flame spray particles in the flame into a number of smaller consolidated, fused or reacted particles, which often are desirable for producing denser coatings. Binders may also be chosen which will decompose in the flame to form a protective atmosphere adjacent the melting particles in order to minimize or prevent hardening of susceptible metals, such as molybdenum, tungsten, tantalum, or niobium by contaminants such as oxygen, nitrogen, or carbon. Still further, binder materials which act as, or which contain, fluxes such as sodium silicate, boric acid, borax or the like, may be used to perform a fluxing function in order to aid interparticle cohesion, adhesion to the substrate, and produce a superior coating of lesser porosity and higher hardness.

In the case of spraying of oxides, such as ceramics, undesirable reduction may be prevented by including an oxidation agent, such as a nitrite, nitrate, or permangenate, in the binder or in the slip for depositing with the binder. In general, the binder material should be present in a concentration in the slip to form ultimate dried binder content in the particles of up to 10.0 weight percent, or preferably 0.1 to 5.0 weight percent. For this purpose concentrations of up to 10.0 weight percent, or preferably from 0.1 to 5.0 weight percent are generally required in the slip, based on the fine starting powder contained in the slip.

In addition to the fine starting powder material, the liquid and the binder, the slip may contain auxiliary agents, such as plasticizers, wetting agents, deflocculants, suspending agents, preservatives, corrosion inhibitors, antifoam agents or defoamers, deoxidants and/or oxidizing agents when required. The use of plasticizers is preferable in connection with binder materials which form hard, brittle films or which may tend to crack when drying, as for example sodium carboxymethylcellulose. Examples of plasticizers include glycerine, ethylene glycol, triethylene glycol, dibutyl phthalate, diglycerol, ethanolamines, propylene glycol, glycerol monochlorohydrin, polyoxyethylene aryl ether, etc. These plasticizers are generally used in amounts of 1 weight percent to 50 weight percent and preferably 5 weight percent to 30 weight percent, based on the dry binder materials.

Suspending agents may be desirable to prevent premature settling of the solids in the slip. For this purpose high molecular weight water-soluble synthetic resins or gums, as for example sodium carboxymethylcellulose of molecular weight 200,000, methyl cellulose of molecular weight 140,000, or polymers of ethylene oxide of molecular weight higher than around 125,000, may be used. In general, only relatively low concentrations ranging from a few parts per million to a few weight percent based on the fine starting powder contained in the slip, are required.

Deflocculating agents may be used to aid in the slip formation and to prevent agglomeration in the slip. Examples of these include sodium hexametaphosphate, sodium molybdate, tetrasodium pyrophosphate, ammonium citrate, ammonium oxalate, ammonium tartrate, ammonium chloride, monoethylamine, etc. Conventional amounts as are used in forming suspensions and colloids may be used which, for example, may range from zero to 1.0 weight percent, and preferably from 0.05 to 0.2 weight percent.

Wetting agents may also be used to aid in maintaining the solid suspension in the slip. These are the conventional synthetic detergents, such as alkylaryl sulfonates, sulfates, soaps, and the like, which may be used in the conventional quantities, for example ranging from 1 p.p.m. to 10.0 weight percent.

Certain of the binder materials may be susceptible to bacterial degradation or mold growth during storage, in which case it may be desirable to add a preservative to the binder material prior to incorporation in the slip, or the slip itself. Any of the known or conventional preservatives, such as sodium benzoate, phenol, or phenol derivatives, formaldehyde, merthiolate, etc. may be used in the conventional amounts and generally between about 0.1 and 0.5 weight percent of the initial binder solution. It may be preferable to use nontoxic preservatives due to the danger of decomposition in the flame.

In connection with fine powder materials which are susceptible to corrosion, or in connection with which the binders show a corrosive action, the binder should additionally contain conventional anticorrosion agents in conventional amounts.

If the slip tends to foam during its production or during handling, conventional antifoaming agents or defoamers, may be added in the conventional amounts, as for example from 0.1 p.p.m. to 200 p.p.m.

Other miscellaneous additives may be included in the slips for specific effects in the production or handling of the slip the slip or in the ultimate flame spraying of the powder produced, as for example chemical activators which will aid in the sintering of high melting refractory materials. Thus for example chlorine or a chlorine-generating compound may be added to enhance the sintering of the carbides. Hydrophobic binders may be used in connection with MgO as water vapor enhances sintering of this material. Conventional acids or bases may be added as buffering agents to control the pH of the slip.

The slip, as mentioned, is simply formed by mechanically mixing the liquid, fine powder and the additives, with sufficient agitation to form a uniform suspension.

The slip is then pumped into a conventional spray dryer where it is atomized and spray-dried. The heavier particles recovered from the bottom of the tower are used as the flame spray powder while the smaller particles which are also recovered from the spray drying may be reconstituted into the slip and again passed through the device.

Referring to the embodiment shown in the drawing, the slip is made up in the mixing tank 1, as described, and pumped by the metering pump 2 to the atomizing head 3 of the spray dryer tower 4. Atomizing air is passed into the atomizing head 3 from the compressor 5. The slip is atomized into the fine spray 6. Air is pumped by the fan or ventilator 7 through the heater 8, as for example a conventional combustion heater, into the top of the spray tower and passes downward as is indicated at 9, drying the atomized slip into the agglomerated flame spray particles which fall the bottom of the tower and are collected in the collector 10. The gas is exhausted at 11 through the cyclone separator 12, in which the finer suspended particles are separated and recovered in the collector 13. These finer particles may be reconstituted into the slip by the addition of further liquids, such as water, and repassed through the device. The spray dryer may be operated in any of the conventional elevated temperatures and gas flow rates, as for example drying gas inlet temperatures between about 400.degree. F. and 800.degree. F. and preferably between 500.degree. F. and 700.degree. F. In the equipment I have used, the liquid slip is generally evaporated at a rate between 2 and 12 gallons per hour, and preferably between 2.5 and 8 gallons per hour, based on a drying gas outlet temperature of 225.degree. F. to 400.degree. F. and preferably from 250.degree. F. to 350.degree. F.

The flame spray powder in accordance with the invention has a general overall spheroid shape. Some of the spheroids are somewhat collapsed, i.e. toroidal or donut shaped, and the term "spheroid" as used herein and in the claims includes this collapsed form of the spheroid, as well as other somewhat distorted spheroid shapes. These powders, as compared with the conventional flame spray powders, are unusually free flowing and may be handled in all of the conventional powder type flame spray equipment without difficulty. The powder may be produced at a substantially lower cost than was previously possible and quite surprisingly shows superior characteristics when sprayed, allowing for example a higher spray rate and a substantially improved deposit efficiency. The invention further allows almost unlimited possibilities of combining desired components into integral individual powder particles, which was not previously possible. The combined component particles in accordance with the invention show many advantages over the prior art mixtures or conventional aggregates or coated powders, having a uniform distribution of the constituents and very intimate and close contact with each other. In the spraying process this allows complete alloying, solutioning, or reacting of the components, allowing the formation of a much more homogeneous and uniform coating. When, for example, the constituents of the composite particles are materials which will form a a cermet, the ceramic and other phases of a very fine size are uniformly distributed. Furthermore, coatings produced from the spray dried powder particles in accordance with the invention often show higher density and abrasion resistance than those produced by the conventional powders of the same type. The unusual characteristics and flowability not only allow for better handling in the flame spray equipment but also allow for better screening and classification in order to obtain extremely uniform size cuts.

While the powders may generally be used as produced, with the binders remaining soluble in the particular liquid solvent of the slip from which they were formed, it is also possible to insolubilize same by a curing, cross-linking, or tanning treatment. Thus, for example the particles may initially be treated with a dilute alcoholic solution of chromic nitrate followed by removal of the excess solution and drying. Insolubilization may also be effected by treatment with concentrated solutions of dichromates, followed by exposure to actinic, such as ultraviolet light. The dichromates may, for example, by any of the alkaline or metal dichromates, such as ammonium, sodium, potassium or cupric. Insolubilization may also be effected by treatment with copper ammonium hydroxide, as for example prepared from copper sulfate, ammonium hydroxide, and sodium hydroxide.

It is critical that the individual spray-dried powder particles in accordance with the invention have a crush resistance of at least 0.7 grams. This crush resistance is simply measure of the weight that individual particles will support before the same are broken, crushed or destroyed. This crush strength is most simply determined by placing an individual particle on anvil and determine the maximum weight that the same will support while remaining intact. I have found that this crush resistance can be most accurately determined with the use of an analytical balance as follows:

The compressive strength tester is a modified analytical balance on which the pan on one side was replaced by an anvil atop the horizontal beam and a counterweight, the sum weight of which exactly equalled the weight of the pan removed. A shallow depression on the upper surface of the anvil allows precise orientation of the particle to be tested. An adjustable platen is mounted above and closely adjacent to the anvil surface; the height is adjusted such that, with the particle to be tested in position, the zero-indicating arm of the balance is at zero on the reference scale when the particle is just contacting the platen face.

The load is then applied gradually and without shock by unwinding a fine chain from a calibrated rotating cylinder into the other pan of the balance, the calibrations showing the weight of chain deposited on the pan.

Compressive failure of the particle is indicated by movement of the zero-indicating arm relative to the reference scale. The weight of chain required to do this is directly read from the calibrated cylinder.

The particles must thus be substantially stronger and have a higher crush resistance than powders produced for most powder metallurgy purposes where the same are to be pressed into shapes or forms. Particles, such as ceramic particles intended for initial press forming, must have a relatively low crush resistance in order to be pressed into a green form of uniform consistency.

In addition to being sprayed per se in any of the known or conventional manners for flame spraying using any of the known or conventional powder-type flame spray equipment, the powder in accordance with the invention may, of course, be sprayed in any desired mixture or combination with other powder produced in accordance with the invention or any known or conventional flame spray powder.

The following examples are given by way of illustration and not limitation:

EXAMPLE 1

Tungsten Carbide-cobalt Cermet Powder

Tungsten carbide (WC) powder of 1.3-1.6 micron average Fisher subsieve Size (FSS) particle size and metallic cobalt powder of 2 micron average (FSS) particle size were blended in the proportion 88 weight percent WC: 12 weight percent Co. The blend of materials was then dry ball milled according to standard practice in the industry, so that the cobalt was smeared onto the WC particles and each WC particle was, in effect, clad with cobalt.

A gum arabic binder was dissolved in water to form a concentrated solution containing 30 weight percent gum arabic and 70 weight percent water. Phenol in the proportion of 0.05 weight percent, based on the total weight of the solution, was added as a preservative for the binder concentrate.

Sodium carboxy methyl cellulose of very high (approximately 200,000) molecular weight was used as a suspending agent. A concentrated solution, 1.4 weight percent of CMC and 98.6 weight percent of water, was prepared in advance.

Sodium hexametaphosphate (Calgon) was used as a dispersing and deflocculating agent. A concentrated solution, 25 weight percent of the solid in 75 weight percent of water, was prepared in advance.

Sodium lauryl sulfate (Proctor and Gamble Orvus WA Paste, 34 weight percent solids in H.sub.2 O) was used as a wetting agent. Because of its high efficiency and the need in p.p.m. only, dilute solution was prepared by dissolving 0.3 g. of the commercial paste in 100 g. of water, resulting in a solids concentration of 0.1 g. per 100 grams of water.

A slip was formulated according to the following table, using the prepared concentrates described above, where applicable, and in the proportions indicated. --------------------------------------------------------------------------- TABLE

Total Wt. Wt. Wt. Added Addition Solids Liquid % __________________________________________________________________________ 9000 g. Ball milled WC/Co blend 9000 g. 90 300 g. Binder at 30% solids 90 g. 210 g. 1 360 g. CMC at 1.4 wt.% solids 5 g. 355 g. 36 g. Calgon at 25 wt.% solids 9 g. 27 g. 0.1 65 g. Wetting agent at 0.1 wt.% solids 0.065 65 g. 9104.07 657 360 g. Water 1017 360 10 10121 1017 __________________________________________________________________________

In blending the ingredients to form the slip, all liquids and solutions were first weighted into the mixing tank with the mixer running. The dry powder was then fed into the mixing tank such that deflocculation occurred immediately, and after a short mixing time, the slip was uniform in consistency. At this point, pH was measured and adjusted to pH 7.4 by buffering with phosphoric acid, and samples were taken for viscosity and specific gravity measurements. Specific gravity was 5.5 g./m. Deflocculation of the powder was complete so that screening of the slip was not required. The slip was spray dried in a Laboratory Tower Spray Dryer (LT-04-1/2) as manufactured by Bowen Engineering Inc., North Branch, New Jersey 08856. The rated capacity of this dryer was approximately 20 lb./hr. of chamber product based on drying an A1.sub.2 O.sub.3 slip containing 60 percent to 70 percent by weight of solids together with a suitable binder system; the chamber product consists of approximately 75 percent of the total product, the remaining 25 percent being deposited in the cyclone collector and usually consists of fines. Heated air was introduced in a cyclonic flow pattern at the top of a vertical straight-cylindrical drying chamber. The slip is atomized into droplets near the bottom of the drying chamber and directed upwards along the vertical centerline by a blast of compressed air. The particles travel twice through the drying chamber - upwards against the flow of heated air and then downward to the bottom, and then settle by gravity into a collecting receptacle.

Approximately 10,000 grams of slip were fed by pumping into the atomizing nozzle from which the atomized slip was propelled through the drying chamber, to be finally collected in the chamber and cyclone collectors as a dry powder. The following machine parameters were used:

Slip Feed Rate Approximately 180 ml./minute Inlet Gas Temp. 550.degree. F. Outlet Gas Temp. 273.degree. F. Type Heat Direct Gas Atomizer Type Countercurrent SW Nozzle Atomizer Description 9-02B Atomizing Air Pressure 40 p.s.i. Atomizing Air Flow approximately 15 SCFM

approximately 6,500 g. of the 9000 g. of powder blended in the slip was collected as finished product in the chamber and cyclone collectors. The other 2500 g. was loss in the mixing tank, feed tank, feed lines, density test, etc. which proportionately high losses are peculiar to the Laboratory feed unit and the small quantities of material processed, and could have been recovered for reuse. The result was a free flowing powder having essentially spheroid particles. The chamber product was 91 percent of the total collected and had a particle size distribution as follows:

Screen Size Weight Percent __________________________________________________________________________ + 140 6.0 -140 + 170 4.9 -170 + 200 9.9 -200 + 230 6.9 -230 + 270 3.2 -270 + 325 14.5 -325 54.3 __________________________________________________________________________

The cyclone product was 9 percent of the total collected and was essentially -325 mesh size. The Hall (ASTM B-213-48 (1965)) Flow Rate of the -140 +325 mesh size cut of the chamber product was 2.96 g./second and the apparent density (not vibrated) was 3.94 g./ml. The Hall Flow Rate of the -325 mesh cut of the chamber product was 2.99 g./second and the apparent density (not vibrated) was 3.80 g./ml. Compressive strength of -60 +80 mesh particles was 10.0 grams.

A -325 mesh cut from the chamber product was flame sprayed, using a Metco Type 2M plasma flame spray gun, using argon plasma gas at 100p.s.i., 100 SCFH, hydrogen plasma gas at 50 p.s.i., 2.5 SCFH, and argon carrier gas at 100p.s.i., 15 SCFH. With a Type ES nozzle, input power was 500 amperes at 43 volts, spray distance was 3 inches, and the spray rate was 8.2 lb/hour.

The same powder was flame sprayed with the same equipment except using a Type E nozzle and using nitrogen plasma gas at 50 p.s.i., 150 SCFH flow, hydrogen plasma gas at 50p.s.i., 10 SCFH flow, and nitrogen carrier gas at 50 p.s.i., 15 SCFH flow. Input power was 300 amperes at 73 volts, spray distance was 3 inches, and the spray rate was 9.4 lb./hr.

The same powder was flame sprayed using a Metco Type 5 P ThermoSpray gun with a type P7G nozzle, -12 powder flow meter valve, at 4-5 spray distance, using hydrogen at 31 p.s.i., 315 SCFH, and oxygen as the combustion supporting and carrier gas at 31 p.s.i., 54 SCHFH. The spray rate was 8.7 lb./hr. of powder.

In all 3 cases above, excellent, hard, dense, adherent, and wear-resistant coatings were deposited.

50 weight percent of the -140 +325 mesh cut of the chamber product was blended with 50 percent of a -140 +325 mesh cut of a conventional spheroid powder of the self-fluxing, hard-facing, alloy type, to make a powder blend equal in proportion and chemistry to Metco 31C, which uses conventional cobalt-bonded tungsten carbide powder of the same chemistry and particle size range as the spray dried material. The blended material was flame sprayed, using a Metco Type 2 P ThermoSpray gun with a Type P7 nozzle, 2 powder flow meter valve, using acetylene at 10 p.s.i., 25 SCFH, and oxygen at 12 p.s.i., 35 SCFH, with acetylene as the carrier gas, and at 9.5 lb./hr. After post deposition fusing, the resultant coating was a fully fused, pore-free, homogeneous mixture of the coating ingredients and fully fused to the substrate.

A powder similar to that of the 31C (previous example) except containing 80 weight percent of the spray dried WC/Co powder and 20 weight percent of the self-fluxing, hard-facing powder, was flame sprayed in the same manner as in the previous example except that, after deposition of the coating, a subsequent overcoating of the self-fluxing, hard-facing alloy alone was deposited in thickness equal to 20 percent to 25 percent of the first coating. The coating system was then fused, the overcoat material being absorbed by the first coating during the fusing, to effectively fill all of the pores and weld the whole to the substrate. The result was a homogenous mixture of the coating ingredients, very high in WC content, and fully fused to the base.

In each of the last two spraying examples cited, the result was a coating which showed a superior grind finish, lower porosity, and equal wear-resistance and other characteristics to its conventional counterpart.

EXAMPLE 2

Tungsten Carbide - Cobalt Cermet Powder

Tungsten carbide powder of 1.3-1.6 micron average (FSS) particle size and metallic cobalt powder of 2 microns average (FSS) particle size were blended in a simple mixture, in the proportion 88 weight percent WC: 12 weight percent Co., for incorporation in a slip as a simple mixture of powders. The preblending was accomplished as a convenience only in preparing powders for a number of experimental batches; but could be added to the slip without prior mixing.

The binders, suspending agents, deflocculent (dispersing agent) agent) and wetting agent, etc. were prepared for use in concentrated solutions, the same as in example 1.

A slip was formulated according to the following table, using the prepared concentrates where applicable, and in the proportions indicated. --------------------------------------------------------------------------- TABLE

Total Wt. Wt. Wt. Added Addition Solids Liquid % __________________________________________________________________________ 4700 g. Mixed WC/Co powder 4700 g. 157 g. Binder at 30% solids 47 g. 110 g. 1 20 g. Calgon at 25 % solids 5 g. 15 g. 190 g. CMC at 1.4 % solids 2.7 187 g. 30 g. Wetting agent at 0.1 weight percent solids 30 g. 4754.7 g. 342 g. 491 g. Water 833 491 14.9 5587.7 g. 833 __________________________________________________________________________

The slip was blended in the same manner as described in example 1. Specific gravity of the slip was 3.84 g./ml.

The slip was spray dried in the same equipment and in the same manner as described in example 1.

The following machine parameters were used:

Slip Feed Rate Approximately 120 ml./min. Inlet Gas Temp. 465.degree. F. Outlet Gas Temp. 275.degree. F. Type Heat Direct Gas Atomizer Type Countercurrent SW Nozzle Atomizer Description 9-02B Atomizing Air Pressure 30 p.s.i. Atomizing Air Flow Approximately 15 SCFM

approximately 3,900 g. of the 4,700 g. of powder blended in the slip was recovered as finished product in the chamber and cyclone collectors. The result was a free-flowing powder having essentially spheroid particles. The chamber product comprised 84 percent of the total collected and had a particle size distribution as follows:

Screen Size Weight Percent __________________________________________________________________________ +140 15.7 -140 + 170 6.8 170 + 200 12.5 -200 + 230 5.5 -230 + 270 3.5 -270 + 325 14.5 -325 41.5 __________________________________________________________________________

The cyclone product comprised 16 percent of the total collected and was essentially -325 mesh size. The Hall Flow Rate of the -140+ 325 mesh size cut of the chamber product was 1.95 g./second and the apparent density (not vibrated) was 2.62 g./ml. Compressive strength of - 60+ 80 mesh particles was 17.0 grams.

A -325 mesh cut from the chamber product was flame sprayed with the Metco Type 2M plasma flame spray gun, using the argon/hydrogen and nitrogen/hydrogen plasma gases, and with the Metco Type 5P ThermoSpray gun as described in example 1. In all three cases excellent hard, dense, adherent, and wear-resistant coatings were deposited.

50/50 and 80/20 weight percent mixtures of the spray dried WC/Co powder and conventional self-fluxing, hard-facing alloy powders were blended and flame sprayed according to the procedure described in example 1. The results were essentially identical. The sprayed coatings as compared to conventional sprayed coatings of the same material had smaller pores and more uniform distribution of deposited particles and crystallites.

EXAMPLE 3

Self-Fluxing, Hard-Facing Alloy Powder

The binders, suspending agents, wetting agents, and deflocculent were prepared for use in concentrated solutions and/or dispersions as described in example 1, or used dry. The plasticizer, glycerin, was a liquid as received.

A slip was formulated according to the following table, using the prepared constituents where applicable, and in the proportions indicated. The "powder" was of the following composition:

Ferrosilicon 8.0 weight percent Chromium Boron 18.2 weight percent Blocking Chrome 1.5 weight percent Electrolytic Chrome 4.0 weight percent Graphite 0.94 weight percent Nickel Balance

TABLE

Total Wt. Wt. Wt. Added Addition Solid Liquid % __________________________________________________________________________ 2100 g. "Powder" 2100 g. 78 330 g. CMC at 10% solids 33 g. 297 g. 20 g. Glycerin 20 g. 15 g. Wetting agent at 0.1 weight percent solids 15 g. 2 g. Ammonium Tartrate, dry 2 g. 2135 332 270 g. Water 602 270 22 2737 602 __________________________________________________________________________

The ingredients for the slip were blended together in the manner described in example 1. Specific gravity of the slip was 3.03 g./ml.

The slip was spray dried in the same equipment and in the same manner as described in example 1. The following machine parameters were used:

Slip Feed Rate 90 ml./minute Inlet Gas Temp. 510.degree. F. Outlet Gas Temp. 300.degree. F. Type Heat Direct Gas Atomizer Type Countercurrent SW Nozzle Atomizer Description 9-02B Atomizing Air Pressure 30 p.s.i. Atomizing Air Flow Approximately 15 SCFM

approximately 1,100 g. of the 2,100 g. of powder blended in the slip were recovered as finished product in the chamber and cyclone collectors. The result was a free-flowing powder having essentially spheroid particles. The chamber product comprised 81 percent of the total product and had a particle size distribution as follows:

Particle Size Weight Percent __________________________________________________________________________ +140 29.8 -140 +170 7.0 -170 +200 8.1 -200 +230 5.7 -230 +270 3.9 -270 +325 10.5 -325 36.2 __________________________________________________________________________

The cyclone product comprised 19 percent of the total product and was essentially -325 mesh. The Hall Flow Rate of the -140+ 325 particle size cut of the chamber product was 1.24 g./second and the apparent density (not vibrated) was 1.66 g./ml. The Hall Flow Rate of the -325 mesh cut of the chamber product was 0.86 g./second and the apparent density (not vibrated) was 1.68 g./ml. Compressive strength of -60+ 80 mesh particles was 6.0 grams.

The - 140+325 cut of the chamber product was flame sprayed, using a Metco Type 5P ThermoSpray gun with a Type P7G nozzle, -11 powder flow meter valve, at 7 inches spray distance using acetylene as the combustible and carrier gas at 12 p.s.i., 33 SCFH, and oxygen at 21 p.s.i., 60 SCFH. The spray rate was 9.2 lb./hr. After deposition of the coating on the mild steel substrate, which previously had been grit-blasted to improve adhesion of the as-sprayed coating, the whole was heated to around 1,900-2,000.degree. F. to fuse the particles in the coating to each other and the coating to the substrate. Melting and coalescence of the coating was apparent by the formation of a layer of slag on the surface. Upon and during cooling to room temperature, the slag layer spalled off exposing the bright, smooth surface of the hard, wear-resistant overlay which was welded to the substrate.

The -325 cut of the chamber product was flame sprayed the same as the previous - 140+325 cut except that a Type P7B nozzle was used and cooling air surrounded the flame of the gun. Spray rate was 7 lb./hr. Upon heating the deposited layer to around 1,900-2,000.degree. F. to fuse the coating particles to each other and to the substrate, melting and coalescence was apparent by the formation of a thin layer of slag on the surface which coalesced into beads and permitted an excellent "shine" to be observed. The result was a smooth, even layer of a hard, wear-resistant overlay which was welded to the substrate.

The composition of the alloy fused to the substrate surface was typically:

C 0.7- 1.0 wt. % Cr 16- 18 Si 3.5-4.5 wt. % Ni+Co Balance B 2.75- 3.75 wt. % Others 1.0 Max. Fe 3.5-4.5 wt. %

EXAMPLE 4

Composite Mullite Powder

Fine Mullite, 3A1.sub.2 O.sub.3 .sup. . 2SiO.sub.2, can be formed into particles suitable for flame spraying by the spray drying method, by agglomerating fine particles of mullite per se. It can also be formed as a composite by combining available and cheap commodity raw materials, such as superfine molochite and high purity A1.sub.2 O.sub.3 in the correct proportion in the spray dried powder. Molochite is a naturally occurring mineral of the following typical composition:

SiO.sub.2 54-55 percent

A1.sub.2 O.sub.3 42-43 percent

Others 1.5- 2 percent

Mullite, theoretically, is 71.80 weight percent A1.sub.2 O.sub.3 and 28.20 weight percent SiO.sub.2. Therefore 50.8 weight percent molochite and 49.2 weight percent A1.sub.2 O.sub.3 should result in the theoretical chemistry of mullite.

The binders, suspending agents, deflocculants, wetting agents, etc. were prepared in concentrated solutions, the same as in example 1.

A slip was formulated according to the following table, using the prepared concentrates where applicable, and in the proportions indicated:

TABLE

Total Wt. Wt. Wt. Added Addition Solids Liquid % 1016 g. Superfine Molochite 1016 g. 70 984 g. Al.sub.2 O.sub.3 984 g. 67 g. Polyvinyl Alcohol at 30 % solids 20 g. 47 g. 1 12 g. Daxad - 30 at 25% solids 3 g. 9 g. 0.15 2023 56 808 g. Water 867 808 30 2890 864 __________________________________________________________________________

The slip was blended in the same manner as described in example 1. Specific gravity of the slip was 1.7 g./ml.

The slip was spray dried in the same equipment and in the same manner as described in example 1. The following machine parameters were used:

Slip Feed Rate Approximately 110 ml./minute Inlet Gas Temperature 600.degree. F. Outlet Gas Temperature 300.degree. F. Type Heat Direct Gas Atomizer Type Countercurrent SW Nozzle Atomizer Description 9-02 B Atomizing Air Pressure 50 p.s.i. Atomizing Air Flow Approximately 15 SCFM

approximately 1,400 g. of the 2,000 g. of powders blended in the slip were recovered as finished product in the chamber and cyclone collectors. The result was a free-flowing powder having essentially spheroid particles, each one of which was an homogenous mixture of molochite and A1.sub.2 O.sub.3 in the proportions blended in the slip. The chamber product comprised 79 percent of the total product and had a particle size distribution as follows:

Screen Size Weight % __________________________________________________________________________ +200 46 -200 +325 28 -325 26 __________________________________________________________________________

The cyclone product comprised 21 percent of the total product and had a particle size distribution as follows:

Screen Size Weight % __________________________________________________________________________ +200 9 -200 +325 14 -325 77 __________________________________________________________________________

Compressive strength of -60.degree. 80 mesh particles was 2.5 grams.

Various particle size cuts of the chamber product were flame sprayed with the Metco Type 2 P and Type 5 P ThermoSpray Guns, and with the Metco Type 2 M plasma flame gun. The following table lists the operating parameters: ##SPC1##

The following table compares spray rates and deposit efficiencies for spray dried composite mullite powder as compared with Metco XP1146 conventional mullite powder. The conventional material was heavily contaminated with metal, and the spray dried mullite coatings produced were vastly superior to the conventional mullite coatings. ##SPC2##

Optimum particle size based on the spray rates and deposit efficiencies for ThermoSpray equipment is either -325 or -270 mesh, and for plasma flame is -230 or -270 mesh. The poor flowability of the conventional powder (based on spray rate for equivalent feed conditions) is also readily apparent from the data shown in the above table. With plasma flame, the spray dry spray rate was 2.8 times that of the conventional material and deposit efficiency, even at the higher rate, is 1.46 times that of the conventional material. With the ThermoSpray 2 P, feed rate was 3.2 times that of the conventional material and deposit efficiency is slightly more than twice that of the conventional material. With ThermoSpray 5 P, feed rate was up to 5 times that of the conventional material and deposit efficiency is more than twice that of the conventional material, even at the vastly higher spray rate.

EXAMPLE 5

Nicket-Aluminum Exothermic Composites

Nickel-aluminum composites corresponding to the known Metco 404 (nominally aluminum clad with 80 weight percent Ni) and Metco 450 powder (nominally Ni clad with 5 weight percent Al) can be manufactured using this method. Ni-Al powders containing 5 weight percent Al and 7.5 weight percent Al have been manufactured by spray drying. The spray dried composites result in the formation of an homogenous reaction product by virtue of the homogenous mixture of very fine particles.

Carbonyl nickel, 3-5 microns average particle size, and high purity spheroid Al powder, 3.5-4.5 microns, were blended in the slips in the proportion required to produce the desired composites. While the 7.5 weight percent Al powder is used as a specific example, it is understood that the Ni:Al proportion can be anything between 99.5 weight percent Al and 0.5 weight percent Al, depending on the reaction product and the properties desired.

The binders, suspending agents, deflocculent, wetting agent, etc., were prepared for use in concentrated solutions, the same as in example 1.

A slip was formulated according to the following table, using the prepared concentrates where applicable, and in the proportions indicated:

TABLE

Total Wt. Wt. Wt. Added Addition Solids Liquid % __________________________________________________________________________ 3700 g. Carbonyl Nickel 3700 g. 300 g. Spheroid Al 300 g. 36 g. PVAc at 55 wt. % solids 20 g. 16 g. 0.5 37 g. PVAc at 55 wt. % solids 20 g. 17 g. 0.5 16 g. Calgon at 25 wt. % solids 4 g. 12 g. 40 g. CMC at 1.4 wt. % solids 0.5 g. 40 g. 4044. 85 g. 600 g. Water 685 600 14.5 4729 685 __________________________________________________________________________

The slip was blended in the same manner as described in example 1. Specific gravity of the slip was 2.93 g./ml.

The slip was spray dried in the same equipment and in the same manner as described in example 1.

The following machine parameters were used:

Slip Feed Rate Approximately 150 ml./minute Inlet gas temperature 550.degree. F. Outlet gas temperature 300.degree. F. Type Heat Direct gas Atomizer Type Countercurrent SW Nozzle Atomizer Description 9-02 B Atomizing Air Pressure 25 p.s.i. Atomizing Air Flow Approximately 15 SCFM

approximately 3,500 g. of the 4,000 g. of powder blended in the slip were recovered as finished product in the chamber and cyclone collectors. The result was a free-flowing powder having essentially spheroid particles. The chamber product comprised 92.5 percent of the total collected and had a particle size distribution as follows:

Mesh Size Weight Percent __________________________________________________________________________ +140 21.3 -140 +170 10 -170 +200 12.5 -200 +230 2 -230 +270 9 -270 +325 17 -325 28.5 __________________________________________________________________________

The cyclone product comprised 7.5 percent of the total and was essentially -325 mesh.

Compressive strength of -60+ 80 mesh particles was 3.6 grams.

The -170.degree. 325 cut of the chamber product was flame sprayed with a Metco Type 2 P ThermoSpray gun, using a Type P7 nozzle, acetylene as the combustible and carrier gas at 10 p.s.i., 25 SCFH, and oxygen at 12 p.s.i., 35 SCFH. Spray rate was approximately 6 lb./hr. The nickel and aluminum particles in the composite particle combined exothermically in the flame to produce an homogenous particle consisting of the nickel aluminides, the heat generated aiding in making the particles self-bonding to the clean, smooth surface of the steel substrate. There was practically no "smoke" produced in spraying the spray-dried powder. In the standarized *

EXAMPLE 6

Nickel-aluminum Exothermic Composites

Example 5 was repeated except that the Ni and Al were combined in the proportion 5 weight percent Al: 95 weight percent Ni, with identical result.

Compressive strength of -60+ 80 mesh particles was 3.1 grams.

EXAMPLE 7

Molybdenum Powder

Molybdenum powder of less than 8 microns maximum, approximately 5 microns average particle size, was agglomerated by spray drying into a powder, from which particle sizes desirable for spraying could be separated.

The binders, suspending agents, deflocculent, etc. were prepared in concentrated solutions for use the same as in example 1. A slip was formulated according to the following table, using the prepared concentrates, where applicable, and in the proportions indicated: --------------------------------------------------------------------------- TABLE

Total Wt. Wt. Wt. Added Addition Solids Liquid % __________________________________________________________________________ 4000 g. Molybdenum Powder 4000 g. 87.5 133 g. Gum Arabic at 30 % solids 40 g. 93 g. 1 16 g. Calgon at 25 % solids 4 g. 12 g. 0.1 40 g. Polyox at 1/2 wt. % solids 0.2 40 0.012 4044.2 145 g. 440 g. Water 585 440 g. 12.5 4629 585 g. __________________________________________________________________________

The slip was blended in the same manner as described in example 1. Specific gravity of the slip was 4.50 g./ml.

The slip was spray dried in the same equipment and in the same manner as described in example 1. The following machine parameters were used:

Slip Feed Rate 120 ml./minute Inlet Gas Temperature 450.degree. F. Outlet Gas Temperature 275.degree. F. Type Heat Direct Gas Atomizer Type Countercurrent SW Nozzle Atomizer Description 9-02 B Atomizing Air Pressure 20 p.s.i. Atomizing Air Flow Approximately 15 SCFM

approximately 2,600 g. of the 4,000 g. of powder blended in the slip were recovered as finished product in the chamber and cyclone collectors. The result was a free-flowing powder having essentially spheroid particles. The chamber product comprised 90 percent of the total product collected and had a particle size distribution as follows:

Screen Size Weight Percent __________________________________________________________________________ +140 271/2 -140 +170 101/2 -170 +200 12 -200 +270 12 -270 +325 131/2 -325 241/2 __________________________________________________________________________

The cyclone product comprised 10 percent of the total product collected and had a particle size distribution as follows:

Screen Size Weight Percent __________________________________________________________________________ +140 Trace -140+ 170 Trace -170+ 200 Trace -200++270 Trace -270+ 325 64 -325 351/2

The Hall Flow Rate of the -170+ 325 cut of the chamber product was 2.25 g./second and the apparent density (not vibrated) was 2.8 g./ml. The -325 cut of the chamber product did not flow smoothly without vibration, so an accurate test of the flow rate could not be made; the apparent density (not vibrated) was 2.48 g./ml.

Compressive strength of the -60+ 80 mesh particles was 1.3 grams.

The -170+ 325 cut of this and other similar molybdenum powders were flame sprayed with the Metco 2P and Metco Type 5P ThermoSpray Gun and the Metco Type 2M plasma flame gun, using spray parameters previously described.

Some of the other Mo powders manufactured using the spray dry equipment included, in addition to the 1 weight percent gum arabic binder, another with 1.6 weight percent gum arabic binder, one-half weight percent, 1 weight percent, 2 weight percent, 3 weight percent polyvinyl alcohol binder, and 1 weight percent sugar binder.

The following table shows spray rates and deposit efficiencies achieved with several types of equipment flame spraying spray dried and conventional molybdenum powders: ##SPC3##

The following table shows pertinent data regarding the effect of binder proportion on the Hall Flow Rate, Apparent Density, Spray Rate and Deposit Efficiency for a group of powders. Spray tests were with the Type 2P ThermoSpray gun, -170+ 325 mesh powder, and deposit efficiencies have been corrected for binder burnout. ##SPC4##

EXAMPLE 8

Zirconia Powder

Lime stabilized zirconia (ZrO.sub.2) powder, containing approximately 5 weight percent of CaO to stabilize the crystal structure in thermal cycling, of less than 10 microns maximum particle size and approximately 3 microns average particle size, was agglomerated by spray drying into a powder from which particle sizes desirable for flame spraying could be separated. While the "prealloyed" powder is used in this example, it is understood that the spray dried particles could contain ZrO.sub.2 plus CaO in the form of one of its many compounds, including calcium zirconate in the correct proportion, such that the CaO content of the agglomerated and sprayed powder would be the desired amount.

The binders, suspending agents, deflocculents, etc. were prepared in concentrated solutions for use the same as in example 1. A slip was formulated according to the following table, using the prepared concentrates where applicable, and in the proportions indicated.

Total Wt. Wt. Wt. Added Addition Solids Liquid % __________________________________________________________________________ 3000 g. Zirconia Powder 3000 g. 72 100 g. PVA at 30% Solids 30 g. 70 g. 1 12 g. Calgon at 25% Solids 3 g. 9 g. 0.1 7.5 g. CMC (dry) 7.5 0.25 3040.5 79 g. 1103 g. Water 1182 1103 28 4222 1182 __________________________________________________________________________

The slip was blended in the same manner as described in example 1. Specific gravity of the slip was 2.04 g./ml.

The slip was spray dried in the same equipment and in the same manner as described in example 1. The following machine parameters were used:

Slip Feed Rate Approximately 150 ml./minute Inlet Gas Temperature 500.degree. F. Outlet Gas Temperature 275.degree. F. Type Heat Direct Gas Atomizer Type Countercurrent SW Nozzle Atomizer Description 9- 02B Atomizing Air Pressure 40 p.s.i. Atomizing Air Flow Approximately 15 SCFM

approximately 2,150 g. of the 3,000 g. of powder blended in the slip were recovered as finished product in the chamber and cyclone collectors. The result was a free flowing powder having essentially spheroid particles. The chamber product comprised 81 percent of the total product collected and had a particle size distribution as follows:

Screen Size Weight Percent __________________________________________________________________________ +140 14.5 -140 + 170 8 -170 + 200 10.5 -200 + 270 12.5 -270 + 325 20.5 -325 34 __________________________________________________________________________

The cyclone product comprised 19 percent of the total product collected and was essentially -325 mesh.

The Hall Flow Rate of the -200+ 325 cut of the chamber product was 1.08 g./second and the apparent density was 1.35 g./ml. (not vibrated). The -325 cut of the chamber product did not flow smoothly through the meter orifice of the Hall Flow Test Apparatus without vibration, so an accurate test of the flow rate could not be made; the apparent density (not vibrated) was 1.35 g./ml.

Compressive strength of the -60+ 80 mesh particles was 3.5 grams.

The -200+ 325 and the -325 cuts of the chamber product powder were flame sprayed, using the Metco Type 2P ThermoSpray gun and with the Metco Type 2M plasma flame system using spray parameters described in the previous examples. Spray rates and deposit efficiencies resulting from the test work and a comparison with tests using identical equipment and spray parameters with conventional Metco 201 (-325+ 15 microns) and Metco 201B (-200+ 325) zirconia powders are shown in the following table:

ThermoSpray Plasma Flame Type 2P Type 2M __________________________________________________________________________ Spray Deposit Spray Deposit Rate Efficiency Rate Efficiency lb./hr. % lb./hr. % __________________________________________________________________________ Spray Dried -325 2.4 93* 4.2 89* Metco 201 2 80 4.0 60 Spray Dried -200 + 325 1.7 90* 6.6 85* 2.5 81* Metco 201B Not normally con- 4.5 65 197 sidered sprayable __________________________________________________________________________

Spray rates and deposit efficiencies with spray dried powders have been consistently better than their conventional counterparts where direct comparisons have been made. In addition hardness and abrasion-resistance of the spray dry powder coatings has been consistently better.

One run each of zirconia powder using 1 weight percent and 2 weight percent of polyvinyl alcohol binder was made and flame spray tested in direct comparison with each other. When flame sprayed into water, dried and microscopically examined, the 2 weight percent PVA bonded powder was observed to have significantly more fully fused hollow particles than the 1 weight percent PVA bonded powder. In the preliminary coating evaluation, the coating produced with 1 weight percent PVA bonded powder was apparently denser and more abrasion-resistant. In addition, with identical ThermoSpray Type 2P gun and spray parameters, a higher deposit efficiency was achieved with the 1 weight percent PVA bonded powder:

Spray Spray Rate Deposit Rate Deposit lb./hr. Eff. % lb./hr. Eff. % __________________________________________________________________________ 1 wt.% PVA Binder 2.4 93* 4.2 89* 2 wt.% PVA Binder 2.4 92* 4.4 83*

EXAMPLE 9

The slip from example 1 was spray dried in a pilot plant size spray dryer as manufactured by Bowen Engineering Inc., North Branch, New Jersey 08856. The rated capacity of this dryer is 100 lbs./hour of chamber product based on drying an A1.sub.2 O.sub.3 slip containing 60 percent to 70 percent by weight of solids together with a suitable binder system. The results were the same.

EXAMPLE 10

Example 4 is repeated except that the subparticles of flame spray material suspended in the slip consisted of 70 weight percent of a mixture of MgO and 2 weight percent of TiO.sub.2 based on the MgO.

Compressive strength of the -60+ 80 mesh powder particles is greater than 0.7 grams.

The powder is flame sprayed in the same manner as in example 4. The result is a dense, adherent, abrasion-resistant coating consisting of essentially MgO but in which the TiO.sub.2 combined with the MgO in the flame, permitting the deposition by enhancing the melting and coalescence of the MgO subparticles.

EXAMPLE 11

Example 7 was repeated except that 0.2 weight percent ammonium alginate replaced the gum arabic as the binder.

The results were essentially the same except that the ammonium alginate being more protective and by providing a more reducing atmosphere, the particle hardness was less by approximately 100 Knoop hardness because a higher purity material was deposited and particle boundary oxides in the coating were significantly reduced.

EXAMPLE 12

Example 11 was repeated except that 0.1 weight percent of sodium nitrite based on the solids contained in the slip was added as an oxidizing agent. pH of the slip was buffered to 7.0, using sodium hydroxide before the addition of the nitrite to prevent decomposition of the nitrite and evolution of the toxic gas.

Compressive strength of the -60+ 80 mesh particles was greater than 0.7 grams.

The powder was flame sprayed in the same manner as example 11. The action of the oxygen supplied by the oxidizer on its decomposition in flame spraying was to harden the particles of molybdenum from KHN.sub.50 386 to KHN.sub.50 549 by virtue of intersticial containment in the molybdenum particles.

EXAMPLE 13

Example 4 is repeated except that 3 weight percent of Molybdate Orange YE-428-D*

The results are the same except that the powder produced is colored orange, which aided in identifying it.

EXAMPLE 14

Example 8 is repeated except that A1.sub.2 O.sub.3 with sodium silicate as the binder replaced the ZrO.sub.2 and the PVA BINDER.

The results are essentially the same except that the sodium silicate decomposed in the flame, the decomposition products including SiO.sub.2 acting to bind the A1.sub.2 O.sub.3 particles together.

EXAMPLE 15

Example 8 is repeated except that Cr.sub.2 O.sub.3 with sodium carboxymethyl cellulose as the binder replaced the ZrO.sub.2 and the PVA binder.

The compressive strength of the -60+ 80 mesh particles is greater than 0.7 grams.

The results are essentially the same.

EXAMPLE 16

Example 15 is repeated except that 15 weight percent of a borosilicate glass based on the Cr.sub.2 O.sub.3 was included in the slip.

The result is essentially the same except that the borosilicate glass effectively bonded the subparticles of Cr.sub.2 O.sub.3 to each other during flame spraying and aided in improving particle to particle cohesion in the coating, resulting in a harder, denser, more abrasion-resistant coating.

EXAMPLE 17

Example 8 is repeated except that NiO with methyl cellulose as the binder replaced the ZrO.sub.2 and its binder in the slip.

The results are essentially identical.

EXAMPLE 18

Example 8 is repeated except that CeO.sub.2 replaced the ZrO.sub.2 in the slip.

The results are essentially the same.

EXAMPLE 19

Example 8 is repeated except that TiO.sub.2 replaced the ZrO.sub.2 in the slip.

The results are essentially the same.

EXAMPLE 20

Example 2 is repeated except that boron carbide B.sub.4 C replaced the tungsten carbide and aluminum replaced the cobalt.

The results are essentially the same.

EXAMPLE 21

Example 2 is repeated except that chromium carbide Cr.sub.3 C.sub.2 replaced the tungsten carbide and a nickel-chrome alloy replaced the cobalt.

The results are essentially the same.

EXAMPLE 22

Example 5 is repeated except that chromium replaced the nickel incorporated in the slip in example 5.

The results are essentially the same.

EXAMPLE 23

Example 4 was repeated except that aluminum oxide A1.sub.2 O.sub.3 and Titania TiO.sub.2 replaced the solids in the slip in that example.

The results are essentially the same.

While the invention has been described in detail with reference to certain specific embodiments, various changes and modifications which fall within the spirit of the invention and scope of the appended claims will become apparent to the skilled artisan. The invention is therefore only intended to be limited by the appended claims or their equivalents wherein I have endeavored to claim all inherent novelty.

Claims

I claim:

1. A flame spray powder the individual particles of which are of a substantially spheroid shape having a size between about 1 micron and minus 20 mesh and formed of multiple subparticles bound together without fusion by a spray dried binder and having a crush resistance of at least 0.7 grams, substantially all of said subparticles having a size of the same order of magnitude below about 200 mesh.

2. Flame spray powder according to claim 1 having a size between about 100 mesh and 3 microns.

3. Flame spray powder according to claim 1 in which said binder is a water-soluble binder.

4. Flame spray powder according to claim 3 in which said binder is an organic polymer.

5. Flame spray powder according to claim 4 in which said binder is selected from the group consisting of polyvinyl alcohol, polyvinyl acetate, gum arabic, carboxy methyl cellulose salts, methyl cellulose, ethyl cellulose, and polyvinyl butyral dispersions.

6. Flame spray powder according to claim 1 in which said subparticles are of at least two different flame spray components.

7. Flame spray powder according to claim 6 having a size of 20 mesh and 1 micron and in which said subparticles have a size below about 200 mesh.

8. Flame spray powder according to claim 7 in which said binder is a water-soluble organic binder.

9. Flame spray powder according to claim 8 in which said binder is selected from the group consisting of polyvinyl alcohol, polyvinyl acetate, gum arabic, carboxy methyl cellulose salts, methyl cellulose, ethyl cellulose, and polyvinyl butyral dispersions.

10. Flame spray powder according to claim 1 in which said binder contains a fluxing material.

11. Flame spray powder according to claim 1 in which said binder contains a pigment.

12. Flame spray powder according to claim 1 in which said binder is capable of producing a reducing atmosphere upon thermal decomposition.

13. Flame spray powder according to claim 1 in which said binder contains an oxidizing agent.

14. Flame spray powder according to claim 1 in which said binder contains a material capable of thermally combining with said subparticles upon flame spraying to form a flame sprayed coating.

15. Flame spray powder according to claim 1 in which said subparticles are tungsten carbide subparticles.

16. Flame spray powder according to claim 1 in which said subparticles are tungsten carbide and cobalt subparticles.

17. Flame spray powder according to claim 1 in which said subparticles are components of a self-fluxing hard facing alloy.

18. Flame spray powder according to claim 1 in which said subparticles are components of mullite.

19. Flame spray powder according to claim 1 in which said subparticles are of at least two metals capable when melted together of exothermically reacting to form an intermetallic compound.

20. Flame spray powder according to claim 19 in which said subparticles are nickel and aluminum subparticles.

21. Flame spray powder according to claim 1 in which said subparticles are tungsten carbide and cobalt and said binder is sodium carboxy methyl cellulose.

22. In the flame spray process in which a flame spray powder is at least heat-softened in a heating zone and propelled onto a surface to be coated, the improvement which comprises utilizing a flame spray powder the individual particles of which are of a substantially spheroid shape having a size between about 1 micron and minus 20 mesh, and formed of multiple subparticles bound together without fusion by a spray-dried binder and having a crush resistance of at least 0.7 grams.

23. Improvement according to claim 22 in which said binder is an organic water-soluble binder.

24. Improvement according to claim 22 in which said subparticles are of at least two different flame spray components.

25. Improvement according to claim 24 in which said components are metal which exothermically react together at the temperature in the heating zone forming an intermetallic compound.

26. Improvement according to claim 22 in which said binder contains material capable of combining with the subparticles at the temperature in the heating zone to form the flame sprayed coating.

27. Improvement according to claim 22 in which said subparticles are selected from the group consisting of tungsten carbide with cobalt or molybdenum and in which said binder is selected from the group consisting of carboxy methyl cellulose and polyvinyl alcohol.

28. A process which comprises spray drying a slip containing fine particles of a flame spray material and a binder to form spray-dried aggregate particles having a crush resistance in excess of about 0.7 gram, passing these spray-dried particles into a heating zone, heating the particles to at least heat-softened condition in the zone and propelling the heated particles onto a surface to form a coating.

29. Process according to claim 28 in which said slip is an aqueous slip containing an organic water-soluble binder.

30. Process according to claim 29 in which said organic binder is a member selected from the group consisting of polyvinyl alcohol, polyvinyl acetate, gum arabic, carboxy methyl cellulose salts, methyl cellulose, ethyl cellulose, and polyvinyl butyral dispersions.

31. Process according to claim 28 in which said subparticles have a particle size below 200 mesh.

32. Process according to claim 28 in which said slip contains fine particles of at least two different flame spray materials.

33. Process according to claim 32 in which said two different flame spray materials are materials capable of exothermically reacting with each other at the temperature in the heating zone to form an intermetallic compound.

34. Process according to claim 28 in which the slip additionally contains a material capable of thermally combining with the flame spray material to form a flame sprayed coating.

35. Process according to claim 28 in which said fine particles of flame spray material are tungsten carbide and cobalt, and said binder is sodium carboxy methyl cellulose.