Flame Spray Powder Mix

This invention relates to a flame spray powder mix or blend and, in particular, to a flame spray powder mix constituted of agglomerates of at least one metal silicide mixed with a coating metal powder, the mix being characterized when flame sprayed onto a metal substrate of providing an adherent bond coat exhibiting improved bond strength, said bond coat being capable of adhering to a subsequently applied overlay coating of substantial thickness.

It is known to coat metal substrates with a flame spray material to protect said metal substrates, such as a ferrous metal substrate, including steel and the like, and impart thereto improved properties, such as resistance to corrosion, and/or oxidation, and/or wear and the like. The metal sprayed may be in the form of a wire or a powder, powder spraying being a preferred method.

In order to provide a metal substrate with an adherent coating, it is the practice to clean the substrate and shot blast it with steel grit or thread the surface thereof on a lathe, if the shape is cylindrical, before depositing the metal coating thereon.

In U.S. Pat. No. 3,322,515, a method is disclosed for providing an adherent coating onto a metal substrate by first cleaning the substrate and flame spraying a metal bond coat therein using a flame spray powder in which elemental nickel and aluminum are combined together to form a composite particle. This type of powder which is referred to in the trade as bond coat powder provides a basis layer by means of which a sprayed overlayer of other metals and alloys of substantial thickness is adherently bonded to the metal substrate. With this technique, fairly thick overlayers can be produced.

The patent also states that ceramic deposits can be produced by mixing a ceramic with the nickel-aluminum composite powder, for example, 60% by weight of ceramic. Examples of ceramics are Al.sub.2 O.sub.3 and carbides and silicides of Cr, Mo, W, and other refractory metals.

It is known that heated aluminum powder reacts exothermically with air to release a large amount of heat. It is believed that this mechanism is responsible in large part for the production of an adherent bond using nickel-aluminum powder in which fine aluminum power is resin-bonded to a nickel core particle. The bond coat generally ranges in thickness from about 0.004 to 0.01 inch, as thicker coatings do not have satisfactory properties. It is also known to produce coatings of self-fluxing alloys, such as self-fluxing nickel-base alloys, in which aluminum power is simply mixed with the nickel-base powder and sprayed to produce a dense hard coating, reference being made to U.S. Pat. No. 4,031,278.

The bond coat produced from nickel-aluminum composite powder has not been adequate as a final coat due to its poor machinability. Moreover, it is difficult to obtain thick coatings of good quality as generally the thicker the sprayed coating, the more powdery is the deposit. Such deposit is not conducive for providing a smooth surface finish by grinding or turning in a lathe and thus this method has not been useful as a one-step coating technique.

One proposal for overcoming the foregoing problem and of providing a one-step bond coating is disclosed in U.S. Pat. No. 3,841,901. In this patent, it is proposed to add metallic molybdenum to the nickel-aluminum composite powder system or similar system (e.g. copper-aluminum, iron-aluminum or even the nickel-copper-aluminum system), the amount of aluminum ranging from about 2% to 18% and molybdenum from about 0.5 to 16% by weight. The patent states that the addition of molybdenum as a constituent of the composite particle enables the production of a one-step nickel-aluminum-molybdenum coating of thickness, e.g. 0.03 to 0.05 inch, capable of providing a machined surface of good quality.

However, one of the disadvantages of using molybdenum is that, during flame spraying, molybdenum tends to produce smoke, especially in the upper range of composition.

In U.S. Pat. No. 4,039,318, a metaliferous flame spray material is disclosed, formed of a plurality of ingredients physically combined together in the form of an agglomerate, the plurality of ingredients in the agglomerate comprising by weight of about 3% to 15% aluminum, about 2 to 15% refractory metal silicide and the balance of the agglomerate essentially a metal selected from the group consisting of nickel-base, cobalt-base, iron-base and copper-base metals. A preferred combination is at least one refractory metal disilicide, e.g. TiSi.sub.2, agglomerated with aluminum and nickel powder.

The foregoing combination of ingredients provides coatings e.g. one-step coatings, having improved bond strength and improved machinability.

A common ingredient of the two types of prior art agglomerated powders referred to hereinabove is aluminum. While the presence of aluminum is believed to be beneficial for improving bond strength, it has been noted that the bond coat produced tends to have a dispersion of oxide therein which hardens the coating. In the case of a nickel bond coat, the coating is generally characterized by a fine dispersion of oxides. It is believed that, because the aluminum and nickel are intimately combined in the agglomerate, the violent oxidation of aluminum during flame spraying causes a rushing in of air during spraying which oxidizes the aluminum and produces a coating with dispersed oxides therein.

The presence of fine oxides in the coating tends adversely to affect the ductility of the bond coat. A ductile bond coat is desirable in that it has a greater resistance to fretting or spalling. Moreover, an applied overcoat to the bond coat will tend to resist spalling better where the underlying bond coat is more ductile and thus capable of withstanding thermal stresses better. The ductility of the bond coat is generally indicated by its hardness. Thus, the softer the coating, the greater is the tendency for it to be more ductile.

It would thus be desirable to provide an improved coating system capable of producing an adherent layer of substantial thickness on a metal substrate which is ductile, which is low in dispersed oxides and which can be machined to provide a good quality surface.

It is thus an object of the invention to provide a method for producing an adherent coating on a metal substrate having improved bond strength and which is capable of being machined to provide a good quality surface.

Another object is to provide a flame spray powder mix comprising agglomerates of at least one metal silicide mixed with a coating metal making up substantially the balance of the mixture.

A further object is to provide a flame spray powder capable of producing a bond coat of substantial thickness on metal substrates, e.g. ferrous metal substrates, in a one-step spray application and which is capable of being machined to a high finish.

Another object is to provide an improved flame spray method.

These and other objects will more clearly appear from the following disclosure and the accompanying drawing, wherein:

FIG. 1 is an illustration of one type of a flame spray torch which may be employed to spray the powder mix of the invention;

FIG. 2 is a representation of a photomicrograph taken at 160 times magnification of a cross section of a sprayed coating produced in accordance with the invention;

FIG. 3 is a representation of a photomicrograph taken at 160 times magnification produced from a powder agglomerate containing titanium disilicide, aluminum and nickel;

FIG. 4 is a representation of a photomicrograph taken at 160 times magnification of a cross section of a sprayed coating based on a powder agglomerate containing nickel and aluminum.

One embodiment of the invention is directed to a flame spray powder mix or blend formed of agglomerates of metal silicide mixed with a coating metal powder, the average size of said agglomerates and said coating metal powder falling in approximately the same size range, for example, from about 30 to 140 microns.

The term "average size" means that the average size of all the agglomerates with be such as to range from about 30 to 140 microns. For example, it will be appreciated that some of the agglomerates may have sizes below 30 microns so long as the overall average of the agglomerates is 30 microns and above. Similarly, some of the agglomerates could have sizes above 140 microns so long as the overall average of the agglomerates is 140 microns or less.

What has been said for the agglomerates applies equally for the average size of the coating metal powder. The terms "powder mix" or "powder mixture" employed herein are meant to cover simple mixtures or blends of the agglomerates and the coating metal powder, the agglomerates being discernible from the particles of coating metal under the miroscope. Where the powder mixture is a mixture of the agglomerates and particles of nickel, the nickel powder can be easily separated by a magnet from the agglomerates where the agglomerates are non-magnetic.

The composition of the powder mix with respect to the metal silicide content may vary by weight from about 2% to 15% metal silicide, with substantially the balance the coating metal.

In another embodiment the powder mix or blend may also contain agglomerates of silicon metal powder. Thus, the mixture may comprise agglomerates of at least one metal silicide and agglomerates of silicon mixed with particles of the coating metal, the composition of the mixture ranging by weight from about 2% to 15% metal silicide, 0 to 15% silicon and the balance essentially the coating metal powder.

The agglomerates comprise fine particles of the silicide bound together in a matrix of a fugitive binder. A fugitive binder is a thermally decomposable or vaporizable ingredient which serves as an adhesive in binding the particles together, such as a resin or an alkali metal silicate, but which will release the ingredients while passing through the flame.

The particle size of the ingredients in the agglomerate should be less than the particle size of coating metal. That is to say, the average particles size should be less than 20 microns, preferably less than 15 microns, and generally range from about 1 to 10 microns. This is the case for both the metal silicide agglomerate and the silicon agglomerate.

Thus, if the metal silicide has an average particle size of about 5 microns, the agglomerate of the silicide will have an average particle size ranging from about 30 to 140 microns similar to the coating metal powder.

Metal disilicides are preferred, such as disilicides of Ti, Zr, Hf, V Nb, Ta, Cr, Mo, W, Mn and Co. Boron silicide and magnesium silicide may also be employed. The coating metal mixed with the agglomerated silicide may be at least one metal from the group consisting of Ni, Co, Fe, Cu, nickel-base, cobalt-base, iron-base and copper-base alloys and aluminum bronze.

The powder mixture of the invention differs from the composite agglomerate of the prior art in that the metal silicide is not combined with the coating metal in the agglomerate but is simply a mixture of the silicide agglomerate with the coating metal.

Tests have shown that a system comprised of TiSi.sub.2 -Si-Ni in the form of agglomerates of 5% TiSi.sub.2 and agglomerates of 5% Si mixed with nickel when sprayed onto a steel substrate provided a surprisingly high bond strength of about 5,670 psi lbs/sq. in.). A binary system of 5% TiSi.sub.2 agglomerate and nickel alone provided a very good bond strength of about 3,920 psi. Both of these values are superior to the bond strength obtainable with a nickel-aluminum agglomerate in which both the nickel and aluminum particles are agglomerated together. In addition a cleaner bond coat is obtained with the powder composition of the invention.

An advantage of using metal silicide without aluminum in the agglomerate is that a narrow spray cone is obtained which provides better control of the flame spray reaction and greater heat concentration with the narrow cone where it is needed in the area being sprayed. The presence of aluminum tends to give a broader cone (an "overspray" cone configuration) in which the heat is not fully utilized where it is needed.

The metal silicide system appears to work on a controlled oxidation-reduction principle in contrast to when aluminum is present. With aluminum, a vigorous oxidation reaction occurs. It is believed that the vigorous oxidation reaction of the aluminum results in a deposited coating having dispersed oxides therein.

On the other hand, when nickel is the coating metal mixed with the metal silicide, e.g. TiSi.sub.2, the nickel deposited is very clean and exhibits a very low hardness of about 17 to 20 R.sub.B in contrast to the powder mix using aluminum combined with the nickel as an agglomerate in which the nickel deposited has dispersed oxides and has a hardness of about 60 to 70 R.sub.B. Dispersed oxides are also produced from a spray powder in which titanium disilicide, aluminum and nickel are agglomerated together.

The flame spray powder mix provided by the invention enables the production of a single bond coat capable of being machined to a good finish. The bond coat also enables the build-up of a top coat thereon of a desired coating composition which adheres strongly to the bond coat. The powder mix may be sprayed using various types of metal spray torches well known in the art, particularly oxyacetylene torches. However, plasma spray torches may also be employed. Thus, the term "flame spray" used herein is meant to cover the foregoing types of torches in which the powder mix is injected into the flame, be it an oxyacetylene flame or a plasma flame, and the heated powder than applied to the metal substrate.

A preferred torch is that disclosed in U.S. Pat. No. 3,620,454 which is adapted for gravity feed of the powder externally to the flame issuing from a nozzle, the torch being depicted in FIG. 1.

The powder mix of the invention enables the production of a one-step coating of substantial thickness ranging up to about 0.25 inch, for example, about 0.01 to 0.125 inch. Good bonding strengths are obtained. The bond coat which tends to be soft and ductile serves as an excellent base for the application of a top coat, such as a top coat of a nickel-base self-fluxing alloy or other alloy coatings of substantial thickness ranging up to 0.25 inch.

In producing the metal silicide agglomerate, the finely divided silicide powder is mixed in the proper amount with a fugitive bonding agent, such as a resin, or other adhesive, e.g. alkali metal silicate. One example of a fugitive bonding agent is methyl methacrylate dissolved in methyl ethyl ketone. The amount of resin employed corresponds on a dry basis with respect to the metal silicide content of about 2% to 3% by weight following evaporation of the solvent. Broadly speaking, the amount of binder on the dry basis may range from about 1% to 5% of the total weight of the ingredients being agglomerated.

Examples of resins which may be employed are the acrylates, e.g. methyl methacrylate, polyvinyl chloride, polyurethane, polyvinyl alcohol, and the like. The resins are employed as solutions, that is, dissolved in a compatible volatile organic solvent, such as alcohols, methyl ethyl ketone (MEK), xylol, and the like, and the solution in predetermined amounts mixed with the powdered ingredients and solvent evaporated to leave behind bonded agglomerates which are sized by passing the agglomerates through a screen of 100 mesh and preferably through 140 mesh. Examples of alkali metal silicates are sodium silicate and potassium silicate which are soluble in water. The mixing and agglomeration may be carried out in a Hobart mixer manufactured by the Hobart Manufacturing Company of Troy, Ohio.

As stated hereinbefore, while various flame spray torches may be employed for producing the coating on a metal substrate, a preferred torch is that shown in FIG. 1.

The flame spray torch 25 shown may be adapted for gravity feed of flame spray powder directly to the flame issuing from the nozzle as shown, or the powder feed may be automated by injection with a carrier gas under pressure (e.g., such as argon) from a powder feed unit.

The torch has a housing in the shape of a fivesided polygon with one leg of the polygon arranged as a handle portion 27, another leg as a base portion 28, a further leg as a feed portion 29, and another leg 30 of the polygon as the top portion of the torch. The housing 26 has coupled to it a powder feed assembly 31 and a flame assembly 32 to which is coupled nozzle 33.

The top leg portion 30 is provided with a fitting 34 adapted to receive a receptacle 35 (shown fragmentarily) for holding the flame spray powder, a metering device being employed to control powder feed comprising a feed actuator plate 36 slidably mounted in a slot 37 located in the housing top portion 30 below fitting 34. Feed plate 36 is provided with a knob 38 which protrudes upwardly above the housing and permits the sliding of feed plate 36 reciprocally toward and away from housing feed portion 29.

The agglomerates and powder mix flow by gravity unhindered through circular orifices which may range in size from 0.075 to 0.120 inch for different powders, the flow being maintained substantially constant over a mesh size range of minus 100 to plus 325 mesh.

In achieving the desired flow rate, feed plate 36 is selectively aligned with powder flow orifice 39 to control variably the flow rate of the powder from receptacle 35 through flow orifice 39 through conduit 40 and through variable spray control assembly 41. Assembly 41 has a housing 42 which holds a powder feed tube 43 and having a central core hollow cylinder 44 slidably and telescopically fitted within feed tube 43 and communicating directly with powder flow conduit 40 to deliver powder directly by gravity to feed tube 43, the powder then flowing through discharge end 45. A portion of the outer surface of feed tube 43 is provided with indexing means or grooves 46 which through latching assembly 47 enables the setting of powder feed tube 43 in order to locate discharge end 45 at the correct distance from the flame end of nozzle 33. The latching assembly comprises a holding pin 48 that is normally urged toward one of the indexing grooves 46 by spring 49, the holding pin 48 being actuated by rod 50 in making the setting. Thus, by depressing rod 50, the pin is moved out of contact with one of the indexing grooves and tube 43 set according to the desired position.

The flame assembly 32 is supported by sliding element 51 which can be lockingly moved along a track 52 located at the bottom leg 28 of housing 26, a locking pin 51A being provided as shown. Gas flow tube 53 is fixedly held by sliding element 51 and may be factory set, one end of the tube having a connector 54 for attaching to a source of oxygen and acetylene.

The powder flows down tube 43 and is discharged at 45 into the flame issuing from nozzle 43. The powder is sprayed on a metal substrate, e.g. a steel shaft, at about six to eight inches from the workpiece.

In one embodiment of the invention, TiSi.sub.2 and silicon are each separately agglomerated using a phenolic resin (e.g. phenolformaldehyde) in a solvent (ethyl alcohol) as the bonding agent to provide, following drying at 350.degree. F. (177.degree. C.), a retained amount of about 3% resin by weight in said agglomerate. Predetermined amounts of the agglomerates of average size ranging from about 30 to 140 microns are then mixed with nickel powder of average size ranging from about 30 to 140 microns, the composition of the total mix based on the metal content being about 5% TiSi.sub.2, 5% Si and 90% of Ni by weight. The powder is then flame sprayed onto a clean substrate of a 1020 steel using the oxyacetylene gravity feed torch shown in FIG. 1 to produce a strongly adherent bond coat.

As illustrative of additional embodiments of the invention, the following examples are given:

A water solution of sodium silicate was employed as a binder to agglomerate separately TiSi.sub.2 and Si. The TiSi.sub.2 particles had an average size of about 5 microns and the Si powder has a particle size ranging from about 5 to 10 microns. The TiSi.sub.2 powder was agglomerated in a Hobart mixer after adding sufficient sodium silicate solution (containing about 40% by weight of sodium silicate) to wet the powder to ultimately provide an agglomerate containing 2% by weight sodium silicate on the dry basis. The mixing was carried out at a temperature of about 150.degree. C. until the water evaporated to provide an agglomerate having an average size ranging from about 30 to 75 microns, the amount of sodium silicate in the agglomerate as stated above being about 2% by weight on the dry basis.

The Si powder was similarly agglomerated using the same silicate solution in a Hobart mixer to provide Si agglomerates having an average size ranging from about minus 200 mesh to plus 400 mesh (about 30 to 75 microns).

Predetermined amounts of the foregoing agglomerates corresponding by weight to 5% TiSi.sub.2 and 5% Si were mixed with nickel powder of average size ranging from minus 200 mesh to plus 325 mesh (about 45 to 75 microns).

The aforementioned powder mixture was flame sprayed using an oxyacetylene torch of the type illustrated in FIG. 1, the metal substrate being a 1020 steel. The average bond strength obtained after producing two separate bond coats was 5,669 psi.

TiSi.sub.2 powder of about 5 microns in size was mixed as in Example 1 with a sodium silicate solution in a Hobart mixer. The agglomerates produced were sized to provide an average size ranging from about minus 200 mesh to plus 325 mesh (about 45 to 75 microns). The TiSi.sub.2 agglomerate was mixed with nickel powder of average size ranging from about 45 to 75 microns to provide a mixture containing by weight about 5% TiSi.sub.2 and 95% nickel.

When the powder mixture was flame sprayed onto a clean 1020 steel substrate, a strongly adherent bond coat was obtained having a bond strength of about 3,917 psi.

The bond strength is determined according to ASTM designation C633-69. The determination is made by using a set of two cylindrical blocks one inch in diameter and one inch long. An end face of each block of the set is ground smooth and one face coated with the aforementioned bond coat compositions by flame spraying to a thickness of about 0.005 to 0.01 inch. A high strength overcoat is applied to the bond coat, the high strength overcoat being a nickel-base alloy known by the trade mark Inconel (7% Fe-15% Cr - balance Ni) which has a bond strength of over 10,000 psi, that is, much higher than the bond coat being tested. The thickness of the high strength overcoat is about 0.015 inch, and after depositing it, the overall coating which has a thickness ranging up to about 0.025 inch is then finished ground to about 0.015 inch. A layer of epoxy resin is applied to the overcoat layer, the epoxy layer having a bond strength of over 10,000 psi.

The other block of the set is similarly end ground and a layer of high strength epoxy resin applied to it. The two blocks of the set, one with the metal coating and the epoxy layer is clamped to the other with the epoxy faces of the blocks in abutting contact and the clamped blocks subjected to heating in an oven to 300.degree. F. (150.degree. C.) for one hour, whereby the epoxy faces strongly adhere one to the other to provide a strongly bonded joint.

The joined blocks are then pulled apart using anchoring bolts coaxially mounted on opposite ends of the joined blocks using a tensile testing machine for recording the breaking force. The bonding strength is then determined by dividing the force obtained at failure by the area of the one inch circular face of the blocks.

The bond strengths of the following flame spray powders were compared using the foregoing testing procedure:

(1) TiSi.sub.2 agglomerated+Si agglomerated+Ni.

(2) TiSi.sub.2 agglomerated+Ni.

(A) (TiSi.sub.2 Al+Ni) agglomerated.

(B) (Ni+Al) agglomerated.

Items (1) and (2) are compositions coming within the invention.

Item (A) is a composition derived from U.S. Pat. No. 4,039,318 in which TiSi.sub.2, Al and Ni powders are all agglomerated together.

In Item (B), Ni and Al powders are agglomerated together.

The composition of each of the spray powders and the ultimate bonding strength are as follows:

As will be noted, Item 1 is superior to Item A and Item 2 is superior to B. Moreover, the coatings produced in Items 1 and 2 tend to be cleaner and more ductile than the coatings of Item A or Item B.

As illustrative of the soundness of the coating produced by Item 1 as compared to A and B, reference is made to photomicrographs of FIGS. 2 to 4 taken at 160 times magnification.

As will be noted, FIG. 2 (the TiSi.sub.2 -Si-Ni system of the invention) is much cleaner and denser than the systems shown in FIG. 3 and FIG. 4. The photomicrographs of FIGS. 3 and 4 show a dispersion of oxides as dark areas throughout the cross section of the coating in addition to some voids.

As illustrative of the various flame spray compositions provided by the invention, the following examples are given:

As stated hereinbefore, the silicide powder is separately agglomerated and then mixed with the coating metal. When silicon is present, it is also added as a separately agglomerated powder to the mix, although the silicide compound can be agglomerated together with the silicon powder.

As stated hereinbefore, the agglomerates can be mixed with such coating metals as nickel-base, cobalt-base, iron-base, copper-base alloys and aluminum bronze, as well as nickel, cobalt, iron and copper per se.

The preferred alloys are those which are self-fluxing and having a melting point ranging from about 870.degree. C. to 1288.degree. C. (1600.degree. F. to 2350.degree. F.), it being understood that the alloys need not be self-fluxing. The self-fluxing alloys include those containing at least one of the metals selected from the group consisting of about 0.5% to 6% of silicon and 0.5% to 5% boron.

Examples of such alloys are given as follows:

The above alloy may be substituted in nickel content by cobalt or iron. Also, alloys of this type can consist as a matrix containing refractory carbide particles (e.g. WC) in a fine particle size to effect a further improvement in abrasion resistance. The following matrix alloy may be employed.

Again, nickel or iron may be substituted in the above formulation for a like amount of cobalt.

A particularly preferred copper-base matrix alloy which has been found useful has the following constituents in percentages by weight as indicated:

As an example of a matrix alloy within the above ranges, there may be mentioned:

The foregoing alloys are preferably employed as atomized powders. A particular nickel-base alloy is one containing about 3% Si, 2% B, 1% Cr, 0.2% Mo and the balance essentially nickel. It is preferred that the average size of the coating metal range from minus 200 mesh to plus 325 mesh.

As-sprayed coatings of substantial thickness may be produced in accordance with the invention, e.g. coating thicknesses of up to about 0.25 inch, e.g. about 0.01 to 0.125 inch. The fact that disilicides may be employed to produce the coating does not mean that only disilicides will be present. For example, the spraying of powder containing TiSi.sub.2 has resulted in a coating with TiSi, Ti.sub.5 Si.sub.3, etc., being present. Likewise, a powder containing CrSi.sub.2 has resulted in a coating containing CrSi. Generally speaking, the final coating will contain less than 15% by weight of refractory metal silicide (e.g. silicides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W) and generally less than about 10% by weight. The isolated gray areas shown in FIG. 2 are silicides.

While the sprayed metal coating of the invention has shown particular applicability to ferrous metal substrates, the sprayed metal coating is also compatible with metal substrates comprising nickel, cobalt, aluminum-base alloys and copper substrates, among other compatible metal substrates.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations thereto may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.