US 3006782 A
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United States Patent Ofi 3,006,782 Patented Oct. 31, 1961 ice The invention relates to oxide coated articles and provides an improvement therein and in the art 'of making them. This application is a division of my copending application Serial No. 570,602, filed March 9, 1956.
A typical flow diagram for the process is as follows:
Article to be coated Metal undercoating applied as spray, electric arc sputteriug or electroplate, etc.
Spray on retractory oxide coating One object of the invention is to provide flame resistant articles. Another object of the invention is to provide metal articles coated with refractory material which is oxidation resistant and providing further protection against oxidation for the underlying metal. Another object of the invention is to make certain articles resistant to dampness and moisture. Another object is to provide superior refractory coatings on articles which coatings are strongly adherent under diverse conditions of use. Another object is to provide composite articles which are flame resistant and which can be easily manufactured. Another object is to provide erosion resistant articles which are also resistant to oxidation.
Another object of the invention is to provide a coating on steel parts for use in aircraft engines and gas turbines giving them longer life. Another object is to provide a coating for combustion chambers and nozzles of rocket motors and the like to give them longer life. Another object is to provide superior coverings tor rockets and aircraft. Another object is to provide a coating of refractory oxide on a metal part without the use of the step of extreme roughening of the surface being coated, by severe sandblasting or gritblast-in'g to obtain maximum adhesion. Another object is to provide a coating on metal articles with improved thermal shock resistance.
Another object of the invention is to provide a superior class of coatings for such materials as plastics, carbon, and graphite. Another object is to provide a refractory coating which is also substantially impermeable for vari' ous materials such as those mentioned. Another object of the invention is to provide bearing surfaces resistant to oxidation 'at high temperatures. Another object is to provide bearings and bearing surfaces that are resistant to corrosive conditions. Another object is to provide an adherent construction for multiple and for sandwich coatings. Another object is to obtain a refractory coating on a metal without deleterious contamination of the original metal surface by particles of the blasting medium. Another object is to substitute a mild surface-cleaning operation for a severe blasting operation and yet obtain superior adhesion Without metal distortion. Another object is to roughen the surface being coated by employ- 2 ing a sprayed metal coating. Another'object is to interpose oxidation resistant metal layer between a slightly permeable refractory oxide coating and an oxidiz'able base metal. Another object is to provide a heat-resisting oxidizatio'n-r'esisting' metal layer between a base material and a refractory oxide flame-sprayed coating.
Other objects will be in part obvious or in part pointed out hereinafter.
For the formation 'of articles according to the invention, I procure a metal spray gun of the general type disclosed in patent to Erika Morf, No. 1,100,602 of June 16, 1914. Such metal spray guns are now well known and are readily available on the market so need not further be described. The metal in the form of a rod or wire is fused, atomized and sprayed by the gun and projected onto a surface the form of discrete molten particles which freeze in situ.
I further provide another spray gun, or under certain circumstances it might be the same one. This may have the characteristics disclosed in my Letters Patent No. 2,707,691 of May 3, 1955. This gun is for the purpose of coating material with refractory oxide. In general a spray gun which is suitable for fusing, atomizin'g and spraying refractory oxide can be used for fusing, atomizing and spraying metals so long as the metal is provided in the form of wire or a rod of the proper diameter for the gun and the gun has adjustments for the feed and the flame so as to adapt it to the spraying of metal. In
other cases, I may use a gun for flame spraying the refractory oxide that is adapted for the spraying 'of the oxide constituent in powder form. Also, a powder-gun may be used for the metal spraying. I
I provide an article or some material which is to be coated in accordance with the invention. This may be any of those materials indicated in the objects in solid phase provided it is reasonably rigid. This I call the base member: In many embodiments of the invention the base member is a piece of metal, either a shaped metal part or a piece of sheet metal. In other embodiments the base member may be a plastic, meaning a piece of organic material which can be molded, extruded or otherwise shaped, such as a piece of phenol-formaldehyde resin, o'f metha'crylate polymer, of styrene or many other organic materials used in the plastics industry. Hard rubber can be used for the base member and also rigid article's made 'of rubber substitute material such as bu'tadiene styrene, butadiene acrylic nitrile and chlorinated butadiene.
EXAMPLE I I caused six pieces of stainless steel to be coated to produce articles according to my invention. The stainless steel was No. 321 stainless steel (American Iron and Steel Institute). The pieces of stainless steel were squares two inches by two inches and a sixteenth of an inch thick. These constituted the base members. They were sandblasted with Pangborn G-25 steel grit, at 40 pounds per square inch, until a uniformly roughened surface was obtained. They were then spray coated with nickel using a Metco metallizing gun with one-eighth inch wire feed. Following this, alumina was sprayed on with a gun of the type described in US. Patent No. 2,707,691. The air pressure was to pounds per square inch. In each case the flame was produced by burning acetylene with oxygen and in each case a blast of air was used for atomizing and spraying. The control of the valves for feeding gases to the spray gum is well understood by workers in this art and need not be described in detail as is also the control of the feeding mechanism and furthermore all of this is described in the aforesaid patents and in the case of metal spraying in many other patents.
The first coating of nickel upon the stainless steel was fof the base member. from discrete molten particles of metal frozen in situ on 3 .004 to .008 inch thick. The superimposed coating of alumina, which is a coating of refractory oxide stable in air at room temperature and has a melting point of over 1000 C., was .010 to .020 inch thick. Actually this alumina coating was substantiallycompletely crystalline and its melting point as determined by recent evaluations is 2015 'C. C.
The coatings and the bases constituted integral pieces. The adhesive strength between the coating of metal and thecoating of refractory oxide (alumina) was 'substantially equal to the cohesive strength of the refractory oxide coating. The individual particles of the coating of metal and the individual particles of the coating of refractory oxide were self-bonded together so that each coating constituted arigid integral structure independently The metal coating was made up the base member. The alumina coating was made up from discrete molten particles of oxide frozen in situ on the metal coating. Nickel has a melting point of 1455 C. Both the nickel and the alumina, otherwise known as aluminum oxide, were substantially pure.
The adhesion between base metal and refractory oxide coating may often be mechanical in nature. Under these conditions the degree of surface roughness of the metal becomes an important factor'toward good adhesion. Me-
chanical anchorage is aided by reentrant angles which are often present.
Under some circumstances chemical adhesion may be involved. Observations have been made on a specimen with a nickel coating on stainless steel, which had been given a final coating of zirconia. It was subjected to cyclic heating and cooling and withstood the treatment very well. Dark areas that were thought to be nickel oxide were observed in microscopic examination of a cross section, and this nickel oxide may have contributed to the superior performance by a chemical or physical mechanism. EXAMPLE H I provided more pieces of stainless steel in squares 'two inches by two inches and a sixteenth of an inch thick the other was zirconia.
4 The fourth and fifth lots were given a metal spray coating of nickel and the sixth and with alumina and some were coated with nickel-chromium alloy prior to coating with zirconia. The nickel-chromium alloy that was used was Nichrome, which is 60%-80% nickel, 11%-20% chromium and the balance iron. Just where in this range the particular metal used was, I am not sure, but all through this range the difference would be practically immaterial. I believe the wire used for spraying the metal coating was 60% Ni, 24% Fe and 16% Cr.
The following is a description of the samples, their preparation for coating, the coating operation, the thickness of coatings, and the test procedures:
Specimens.--Forty-two specimens of size 2" x 2" x 5 were sheared out of a sheet of 321 stainless steel.
Preparation.All specimens were blasted with Pangborn G-25 steel grit at 40 to pounds per square inch pressure. Grit impinged on the plates at an angle of about 90. Blasting was just sufficient to produce a uniformly rough surface. This caused specimens to be slightly convex due to compressive stress having developed in the surface because of the impact.
' Coating 0perazi0n.Coating was carried out with the equipment described in Example I. Specimens were separated into seven lots of six specimens each. Three lots were coated with refractory metal oxide directly on the blasted stainless steel surface. The coating for two of these lots was alumina (thus giving two identical lots, of which one can be considered a check test) and for seventh lots were given a metal spray coating of Nichrome. Then the fourth and sixth lots were coated with alumina and the fifth and seventh lots were coated with zirconia.
Coating thickness-Oxide coatings were 0.015: 0.005 thick. Metal undercoatings were 0.006"i0.002 thick.
Test pr0cedure.--All specimens were then put through a test to determine the relative ability of the coatings to withstand repeated heating and cooling. A laboratory electric furnace was heated to 1040 C. The six test specimens constituting a single lot were placed side by side, coatedside down, on a flat piece of stainless steel screen of about A" mesh. The screen with the speci-. mens on it was then placed on the bottom of the furnace, and the door was closed.
After 60 seconds the screen with the red hot specimens on it at a temperature of about 1000 C., were withdrawn from the furnace and supported over a vertical stream of fan-driven room temperature air moving at about 300 feet per minute. This brought the samples down to room temperature in three or four minutes. They were then examined to determine the condition of the coating, and the cycle was repeated.
Failure criteria.Lifting or flaking off of a small amount of coating constituted failure. The number of cycles to failure was recorded.
The results were as given in the following table.
Table I RESULTS OF CYCLIC HEATING AND COOLING TEST Lot Coating Max. Min. Avg. Type Failure No. Cycles Cycles Cycles 1;--. Alumina 6 1 3% Coating lifted from base metal at corners. 2.--- Alumina, 4 1 2 Same as above.
Check Test. 3---- Zirconia 11 6 7% Edges failed by slight lifting of coating. 4--.- Nickel under 10 3 8 Alumina lifted Alumina. slightly from nickel undercoat at edges. 5-.-- Nickel under Over 40. 40 40+ Slight crumbling Zirconia. at edges, no
lifting. 6-- Nlchrome under Over 40 9 34+ Edge crumbling Alumina. with slight litting of alumina coating from Nichrome undercoat at corners. 7 Nichronre under Over 40 40 40+ Slight crumbling ircoma. at some edges.
Another method of formation of the metal undercoating for articles according to the invention utilizes electric arc sputtering of the coating metal on to the base surface, in this case a metal or other conductor of electricity. The coating metal, in the form of a rod, functions as one electrode; the base material as the other electrode. A generator, such as is used in arc welding, is used to develop sufiicient voltage difference between the base material and the coating metal rod so that an arc is drawn between the two when they are brought sufliciently close together. The are causes bits of the rod to be melted and fused'onto the base material. The rod of metal to be used as coating is held in a pistol type holder on which is mounted an air vibrator that causes the holder and rod to vibrate, depositing the metal coating in discrete molten particles which freeze in situ. Then the oxide coating is applied over-the metal coating as previously described. Coating with metal by sputtering is known per se and has been described in patents and technical literature.
EXAMPLE III Four stainless steel plates, supplied for test panels by a leading manufacturer of jet engines, were of size about 6" x 2%" x $1 These were sandblasted with 6-25 crushed steel shot and then spray=metallized with 5 mils of nickel followed by refractory oxide spray coating of 25 mils thickness by the process of US. Patent No. 2,707,691. Two plates were thus coated with alumina and two with stabilized zirconia.
The heating tests were made under simulated engine conditions in the laboratory of the jet engine manufacturer, who reported that the steel plates, m'etallized with nickel, and flame sprayed with alumina and zirconia coatings, showed marked improvement over all panels previously submitted. The manufacturer also reported that the alumina panel became loose near the ends, but failure was in the coating itself, and not at the interface. The manufacturer further reported that the zirconia was even more adherent; showing no indications of failure in one test, and only slight shear failure (within the coating) near one corner of the panel in another test.
Various examples of my invention have been produced experimentally which will have many practical uses in accordance with the objects and otherwise. Among them are the following:
In place of the alumina in all of the above examples could be substituted zircon, zirconia, chromium oxide, wollastonite, spinel, etc.
EXAMPLE IV Other test samples which have been made experimentally include employment of a base member of cemented tungsten carbide in one case, and stainless steel in another. A rough layer of nickel was deposited employing the sputtering process previously described. After this, an alumina coating 10 to 20 mils thick was sprayed over the nickel. There was excellent adhesion of the alumina coating. Other metals and other hard carbides such as are used for tool bits, etc., could have been used. However, for the sputtering process, the base member must be electrically conducting.
EXAMPLE V Multiple coatings can be achieved. For example, black iron specimens were used in one series of experiments. These were roughened by grit-blasting at 40 pounds per square inch pressure and a nickel coating about 6 mils thick was sprayed on using Ma nickel wire. Then a moderately soft alumina coating was sprayed on using a powder gun (model A gun of Wall Colmono'y (30., Detroit), with alpha alumina monohydrate as feed. 'Following this, a coating of hard alumina was "sprayed on using a /s" alumina rod as described in Example I. The article produced had a hard alumina erosion resistant surface coating with a laminar microstruc'ture which adapted it to withstand heating and cooling stresses without flaking off, with yieldable alumina back-up layer and nickel anchoring layer. Other oxides and combinations of oxides, as well as other metals, can be used. 7
Other combinations of multiple coatings and sandwich coatings may be employed depending on the use conditions. For example, the metal undercoating may be sprayed on with a metallizing gun and then heated on the surface to consolidate the particles and render the coating less permeable or substantially impermeable. Torching the surface is oneway to accomplish this. If the heated surface is not sufliciently rough to serve as good anchorage for the refractory oxide layer, then it may be roughened by sandblasting or by other methods such as further metal spray deposition.
Alumina over aluminum on steel;
Alumina over copper on steel; I Alumina over iron on steel, especially stainless steel; Alumina over molybdenum on steel;
Alumina over cobalt on steel;
Alumina over Monel metal on steel;
Alumina over Inconel on steel.
6 EXAMPLE VI Two specimens of black iron were grit blasted at 40 pounds per square inch with steel grit to produce a roughened surface and then spray-coated using the model A Wall Colmonoy gun feeding ti'n powder in one case and lead powder in another. They were then sprayco'ated with alumina by the process of Example I. A good adherent hard alumina coating was obtained in both cases. Care had to be taken to avoid deleterious melting of the tin undercoating, during the alumina spraying, due to its low melting point of 232 C.
The gun of patent, under suitable conditions, is capable of spraying refractory oxides over stable metals including alloys with melting points at least as high as 200 C. However, refractory metals with melting points over 1000'? C. are particularly adapted for high-temperature refractory purpose use in conjunction with refractory oxides of melting points over i000" C. Oxides and metals that are refractory to chemical and erosive action can be useful for many purposes at less-elevated temperatures, and even at room temperature or below.
A preferred metal undercoa'ting is in the range of 4 to 15 mils thickness but thinner and thicker coatings such as from 1 to 30 mils or more are usable. Likewise, a usuallypre ferred refractory oxide coating thickness is in the range from 5 to SO mils but thinner and thicker coatings such as from 2 to 200 mils or more may be desirable depending on the use conditions. There is nothing critical about any of these ranges and the most practical thicknesses 'are chosen as the best commercial balance in each case between cost and performance under the use conditions desired, such as degree of heat insulation, corrosion resistance, heat-shock resistance, erosion resistance, etc. of the product.
Rough surfaces on the metal coating are desirable as 'a foundation for the refractory oxide coating. Reentrant angles in the surface hills and valleys are helpful. The exact desired degree of roughness will depend on several factors such as the use conditions, the configur'a tion of surfaces coated, the sand-blasting medium and blasting techniques if blasting is employed, the metallizing techniques if metallizing is employed as the rougheni'ng agent to help to anchor the final refractory oxide coating in place. Cost and use requirements have to be balanced commercially. Concave surface configuration, especially inside cylindrical surfaces, heated in use on the inside such as combustion chambers and rocket nozzles are particularly satisfactory for giving good results. Sandblasting with hard tough inorganic abrasives such as fused alumina or silicon carbide avoid surface embedding with particles of metal grit that may occur when crushed steel shot is used.
Likewise the surface of the base member should be sufliciently clean and rough to give :good adhesion with the metal undercoating. My work, however, indicates in general that the surface roughness condition of the base member is of less importance as an anchoring factor for the metal coating, than is the surface condition of the metal coating as an anchoring factor for the refra'ctory oxide coating. Thus a lightly cleaned base member surface is often adequate for anchoring the metal undercoat to the base member, and the rough surface obtained directly on the metal layer coating such as with a metallizing gun or with a sputtering process, by depositing heat-plasticized metallic particles, is adequate for anchorage to the refractory oxide coating. Under these conditions, one feat ure of the step of applying a metal layer coating to a base member becomes a process of roughening the surface of application for the refractory oxide coating to promote adhesion to the base member. If the metal undercoating layer is deposited by proce sses that do not leave it with a sufficiently rough surface, then it can be roughened in other ways such as by sandblasting. Eleetroplating leaves a smooth surface.
7 Articles made in accordance with the invention are useful for all of the purposes described in the objects. Combustion chambers of reaction motors and the nozzles of rocket motors are more resistant to flame and will last longer when coated in accordance with this invention using oxidation resistant metal undercoating, than when coated merely in accordance with my prior patent which itself greatly advanced the art. The outstanding improvement in length of life under thermal shock made by the combination coatings of the invention is hard to explain.
Any refractory oxide having a melting point of over 1000 C. and which will form a coating as described can be used in accordance with this invention. A long list of complex oxides which can be used is given at the top of column in my aforesaid Patent No. 2,707,691 and in column 7 of this patent is a general discussion of the oxides mentioning many of the better ones.
All of the metals which are stable in air and have a melting point of over 200 C. can be used but for many practicable applications of the invention an oxidation resistant metal will be preferred. 'For the purposes of this invention an oxidation resistant metal is defined as a metalor an alloy which is at least asresistant to oxidation as is stainless steel at a temperature of 1000 C. in air. For purposes of this invention the term alloy is intended to include commercially pure metals as well as higher alloyed compositions.
The elementary metals which as such are oxidation resistant and which are components of many oxidation resistant alloys are, excluding such noble metals as platinum and gold which are most oxidation resistant but too expensive for most applications, nickel, cobalt and chromium.
In general, heat resistant oxidation resistant alloys can be classified into groups based respectively on nickel, cobalt and iron, in which the classification name corresponds to the ingredient that is present in higher numerical amount than any other ingredient of the alloy. Although chromium is an important ingredient in many of these alloys, chromium base alloys are not usually important today.
Typical heat resistant alloys are given in the following table.
Table II TYPICAL HEAT RESISTING ALLOYS Percentage by Weight Ni Or 00 Fe W Mo Ti Al Nb Nickel Base:
Nichrorne I l1 Nichrome V 20 Inconel.-- 15 M252 19 Cobalt Base:
Haynes Stellite No.31 25 55 1 8 Haynes Stellite 8-816 20 20 43 4 Iron Base:
Timken l6-256 25 16 51 18-8 Stainless SteeL. 8 18 74 Kanthal A 23 2 69 Smith No. 10 37% 55 the step of coating the metal coating with refractory metal oxide involves fusing the metaloxide to form molten metaloxide and atomizing it and projecting it upon the surface of the metal coating. In the above, fusing is synonymous with melting.
When I say that the melting point of the metal is at least as high as 200 C., I mean that 200 C. is the grinimum and this is substantially the same as stating that the melting point of the metal is above 200 C. Similarly the melting point of the oxide must be at least 1000 C. meaning at least as high as 1000 C. and this is substantially the same as stating that the melting pointv of the metal oxide should be over 1000 C. Especially in the upper range a diiference of one degree is of no consequence in temperature limits like these.
The atomizing of the metal in accordance with the preferred embodiment of this invention is achieved by the use of a blast of gas as described in many prior patents, for example the patent to Morf referred to and other patents, such as the patent to Schoop, No. 1,128,058, referred to in my prior patent. As described in my prior patent referred to herein the molten metal oxide is atomized in like manner, namely by the use of a blast of gas. Usually in both cases the blast of gas is largely air but the prodnets of combustion of the gases such as acetylene and oxygen is part of said blast.
It will thus be seen that there has been provided by this invention oxide coated metal coated articles in accordance with which the various objects hereinabove set forth together with many thoroughly practical advantages are successfully achieved. As many possible embodiments may be made of the above invention and as many changes might be made in the embodiments above set forth, it is to be understood that all matter hereinbefore set forth is to be interpreted as illustrative and not in a limiting sense.
1. Process for coating rigid materials to protect them from oxidation and for other purposes comprising fusing metal having a melting point over 200 C. thus producing molten metal, atomizing said molten metal to produce particles of molten metal, freezing the particles in situ upon the rigid piece of material to form a metal coating thereon, thereafter fusing material consisting of refractory metal oxide which has a melting point of over 1000 C. producing thereby molten metal oxide, atomizing the molten metal oxide to form discrete molten particles thereof, coincidentally projecting said molten particles of metal oxide on to the surface of said metal coating and freezing the molten particles of metal oxide in situ thereon to form a coating of said metal oxide having a melting point of over 1000 C. upon said metal coating.
2. Process according to claim 1 in which, after the particles of molten metal are frozen in situ upon the rigid piece of material to form a metal coating thereon, the metal coating is roughened to promote adherence of the coating of metal oxide thereon.
3. Process according to claim 1 in which the molten metal is atomized with a blast of gas to produce the particles of molten metal.
4. Process according to claim 1 in which a second coating of metal oxide which is only oxide and of different characteristics from said coating of metal oxide is superimposed upon the first coating of metal oxide by the steps of fusing and atomizing the metal oxide for the second coating, projecting it upon the first coating and freezing it insitu thereon.
5. Process according to claim 1 in which the coating of refractory metal oxide is produced from a sintered rod of said oxide.
n 6. Coating process for the protection of oxidizable rigid materials comprising applying a coating of metal having a melting point over 200 C. to a piece of said material, then fusing, atomizing and spraying material consisting of refractory metal oxide which has a melting point of over 1000 C. on to said coating of metal.
7. Process according to claim 6 in which the surface of the metal coating is roughened prior to the fusing, atomizing and spraying of the molten metal oxide.
8. Process according to claim 6 in which the coating of refractory metal oxide is produced from a sintered rod of said oxide.
9. Process for coating metals comprising providing a metal base member to be coated, applying a coating of metal having a melting point over 200 C. to said metal base member, then fusing a rod of material consisting of refractory metal oxide and atomizing and spraying said fused material which has a melting point of over 1000 C. on to said coating of metal.
10. Process according to claim 9 in which the metal base member is coated with metal by means of fusing, atomizing and spraying metal having a melting point over 200 C.
11. Process according to claim 9 in which said rod is formed of sintered refractory metal oxide material.
12. A process for coating rigid materials to protect them from oxidation and for other purposes comprising fusing a refractory metal having a melting point over 1000 C. thus producing molten metal, atomizing said molten metal, to produce particles of molten metal, freezing the particles in situ upon the rigid piece of material to form a metal coating thereon, thereafter fusing material consisting of refractory metal oxide which has a melting point of over 1000 C. producing thereby molten metal oxide, atomizing the molten metal oxide to form discrete molten particles thereof, coincidentally projecting said molten particles of metal oxide on to the surface of said metal coating and freezing the molten particles of metal oxide in situ thereon to form a coating of said metal oxide having a melting point of over 1000 C. upon said metal coating.
13. A process for coating rigid materials to protect them from oxidation for other purposes comprising fusing a refractory metal having a melting point over 1000 C.
thus producing molten metal, atomizing said molten metal, to produce particles of molten metal, freezing the particles in situ upon the rigid piece of material to form a metal coating thereon, thereafter progressively feeding and fusing a rod of material consisting of a sintered rod of refractory metal oxide which has a melting point of over 1000 C. producing thereby molten metal oxide, atomizing the molten metal oxide to form discrete molten particles thereof, coincidentally projecting said molten particles of metal oxide on to the surface of said metal coating and freezing the molten particles of metal oxide in situ thereon to form a coating of said metal oxide having a melting point of over 1000 C. upon said metal coating.
14. A coating process for the protection of oxidizable rigid materials comprising applying a coating of refractory metal having a melting point over 1000 C. to a piece of said material, then spraying fused atomized material, consisting of refractory metal oxide which has a melting point of over 1000' C. on to said coating of metal.
15. A coating process for the protection of oxidizable rigid materials comprising applying a coating of refractory metal having a melting point over 1000 C. to a piece of said material, then fusing, atomizing, and spraying material consisting of refractory metal oxide which has a melting point of over 1000 C. on to said coating of metal.
16. Process according to claim 15 in which the coating of refractory metal oxide is produced from a sintered rod of said oxide.
References Cited in the file of this patent UNITED STATES PATENTS 2,707,691 Wheildon May 3, 1955 2,730,458 Schulze Jan. 10, 1956 2,839,292 Bellamy June 17, 1958 2,930,106 Wrotnowski Mar. 29, 1960