US3197335A - Surface-mounted electrical resistance structure and method for producing same - Google Patents

Description

July 27, 1965 s, w, LEszYNsK| 3,197,335

SURFACE-MOUNTED ELECTRICAL RESISTANCE STRUCTURE AND METHOD FOR PRODUCING SAME Filed April 9, 1962 BA se' 1N VEN TOR.

@L En United States Patent O s 1a? ses spannen-Mouwen gegnerische nasisrANca steuerung AND Marston non annunciare satura Stanley W. Leszynski, '75 157th SE., Bellevue, Wash. sind apr. e, 19er, ser. No. isaerr 1s claims. (ci. 117-212) This invention relates to improvements in surfacemounted electrical resistance devices for operating in very high temperature environments or for operating in environments undergoing wide temperature changes. Applications for devices of this nature include strain gauges, resistance thermometers, electrical resistance or conductance elements for general use, and others. The novel concepts of the invention are herein illustratively described by reference to the presently preferred embodiments thereof; however, it will be recognized that certain modications and changes therein with respect to details may be made without departing from the underlying7 essentials involved.

A broad object hereof is to provide durable and reliable electrical resistance elements and the like of a type formed upon and adhered to a base surface by an insulating medium capable of withstanding extreme conditions of temperature, temperature variation, physical strain, flexure and abrasion. A related object is to achieve temperature stability and low drift in such devices.

A related object is to bond such elements with an insulating medium able to maintain a high insulation coellicient at very high operating temperatures where conventional bonding insulations tend to become excessively conductive.

A further object is to provide an insulating and bonding technique achieving the above purposes with refractory ceramic materials as the insulation.

A further object of this invention is to provide an irnproved strain gauge, resistance thermometer element, or similar device of the surface-mounted type which is readily applied by known and readily available techniques land materials, and which may be applied with any desired configuration and to surfaces of practically any shape or contour.

A further object of this invention is to provide electrical sensing devices having any selected resistance value within a convenient range, such as from a few ohms or less to 100 ohms or more, and having the capability of carrying relatively heavy current without overheating. As a result, associated electric circuit systems in which these devices are connected may be of simple, noncritical design.

In accordance with this invention as presently disclosed, components of the resistance structure are all applied by a flame-spraying technique. The term llange-spraying as used herein is intended to have a generic connotation including any of various commercially known or feasible systems by which the materials are deposited on a surface in moltenl or plastic particulate form at high temperatures and at high velocities. One suitable llamespraying method is the Thermo Spray method of Meteo, Inc.; another is the Rokide process of the Norton Co.; still another the Flame Plating process of the Linde Company. In one case, metallic material to be deposited is fed through a gun by an automatic feed mechanism into an oxyacetylene flame where it becomes softened or plasticized. A blast of compressed air atomizes the particles into a line spray and propels them out of the gun against the structure or surface to receive them. In another process, the base materials are initially powdered and may include a virtually endless variety of metals, alloys, ceramics cermets, and combinations of materials. Still another process is the so-called Plasma Jet process which uses a torch to produce a controlled high-velocity inert gas stream at extremely high temperatures (i.e., in the range between l2,000 F. and 30,000o E). The hot gases melt and accelerate to high velocity and refractory materials whether in a powder or solid form.

A particular feature of importance herein is the achievement of high strength and mechanical stability in the total structure, though ceramics are used in the insulation, by a process of lamination. According to this process, a liash coating is first llame-sprayed on the base surface whereupon one or more refractory ceramic coatings are successively llame-sprayed over the ash coating with llame-sprayed interlayers of metallic material interposed between the successive ceramic layers. Finally, the resistance strip itself is flame-sprayed on the outermost ceramic layer, which is preferably made thicker than underlying ceramic layers. By choosing a llash coating material and interlayer metal material which have a temperature expansion coefficient intermediate that of the base material and of the ceramic material, the desired strength, stability and other advantageous characteristics mentioned above are achieved in the total structure and there is minimal tendency for fracture of the refractory ceramic material or failure of its bond to the base due to dimensional changes of the latter accompanying wide variations of temperature. In a strain gauge, for example, it is possible with this improved system to achieve temperature stability and acceptable accuracy to temperatures as high as 2000 F. more or less.

Fusion of the conductive layer by heating the structure to a high temperature in a hydrogen (reducing) atmosphere imparts still greater toughness and tensile strength to the resistance structure and further minimizes drift of resistance value over a period of time.

For certain limited applications, a single refractory ceramic layer may be llame-sprayed directly upon the base surface and may comprise the sole supporting and insulating layer for the conductive strip flame-sprayed directly on such layer. However, for most applications, especially where high temperatures or wide variations in temperature are involved, a laminar insulation structure is essential to achievement of the desired characateristics of the device.

These and other features, objects and advantages of the invention will become more fully evident from the following description thereof by reference to the accompanying drawings.

FIGURE l is an edge view of an electrical resistance structure applied to a base in accordance with one embodiment of the invention.

FIGURE 2 is a similar view showing a modified embodiment.

FIGURE 3 is a similar view showing still another modified embodiment.

yFIGURE 4 is a -face view of Ia strain gauge structure applied to a base strip according to the invention.

FIGURE 5 is `an :oper-ating view 'Showing use of flame spray equipment in manufacturing resistance structures according to this invention.

In order to `satisfy the increasing requirement for strain gauges, resistance thermometers and other resistance devices capable :of :operating under extremes of temperature `and strain, the present invention utilizes la technique of depositing both the 4resistance element material and its insulation medium by `a flame-spraying technique wherein high-tempenature particles of material are projected at high velocity in Ia plastic or molten state tonto the support-ing surface. It 'is found that extraordinary temperature stability and the other attributes mentioned above are achieved when 'this technique is used :apparently because ythe material deposited has been exposed or cycled to -an extremely Vhigh temperature inthe deposit- 'layer 214 dame-sprayed on the layer i12.

ing operation itself. Enhancement .of temperature stability can be achieved Iby the further expedient of fusing the deposits in a hydrogen reduction atmosphere at high temperature `so as to :increase the ductility and amorphous vcharacter of the coating materials by removing detrithis invention rises .to the challenge of extreme environmental conditions as previously mentioned, its usefulness is not necessarily contined thereto.

Upon the base 1t) there is iirst deposited by :a flamespraying technique a flash coating or underlayer .12 of metallic substance which is to carry the remainder of the insulation structure .and the surmounted electrical resist-ance strip to be described. This dash coating is of a materia-l which exhibits a temperature expansion coefficient intermediate that of the `base l0 and of the refractory ceramic In this embodiment additional refractory ceramic iayers 16 and .IIS are flame-sprayed successively over the layer 14 and are separated by dame-sprayed interlayers of metallic material, 20 :and 22, `from the layer 114 and from each other, respectively. Preferably the outer ceramic layer d6 is materially thicker than the underlying ceramic layers yllt, and 16. Finally, the electrically conductive strip 24, of any desired configuration, is flame-sprayed on the outermost refractory ceramic layer 18.

One suitable electrical resistance strip configuration is shown in FIGURE 4 wherein the resistance element 24a compri-ses a sinuous conductor, companion to a similar conductor 2411 mounted upon the same base Olii' and insulation laminate l2-18, .as shown. These elements 24a and r24rb may represent a pair of strain gauge resistance elements mounted on a test specimen comprising the "base d. Alterna-tively, they may comprise the resistance elements of a resistance thermometer or other electrical device.

l.FIGURE l5 depicts a suitable flame-spray gun in opera- 'tion directing a stream of plasticized or molten particles against a base. A template superimposed on the base in the path of Athe stream of particles insures proper masking of the base surface surface in areas other than those to be coated. The process is a direct and :rapid one which, depending upon the choice of materials and techniques used, may be worked with practically any metal, metal alloy, ceramic, or cermet, as required for different applications of the improved electrical resistance structures.

EIn a representative case, assum-ing the base material to comprise a metal .alloy having a temperature expansion coeicient of the order of 8 or 9, the refractory ceramic material u-sed in Ithe layers t14, 116 and @18 will be aluminum oxide (A1203) while the flash coating ill-2 and the interlayer coatings y20 land 22 are of nickelchromium alloy, or molybdenum. Either of these coatings exhibi-t a temperature expansion eoeiiicient of the order of 5 or 6, which is generally intermed-iate the value of about lil lfor the base `1G and that of about for aluminum oxide.

.It is desirable `for the usual reasons in the cas-e of a strain `gauge to maintain a composite Structure which is relatively shallow, and in which, accordingly, the individual layers of the composite or assembly are microscopical-ly thin. The term microscopically thin as used here is intended to cover thicknesses for the layers 12, 1'4, d6, 118, 20 and '22 which usually should not exceed a yfew thousand-tbs of an inch thickness individually. For example, the rliash coating may have a thickness of 0.002 of :an inch, and each of the other coating-s may have a thickness of 0.003 of yan inch, except the outermost coating. As much as possible of the total requisite insulation thickness is relegated to this outer coating in order to enhance the advantages of the lamination method. Typically this outer coating may be of the order of 0.007 of an inch in thickness. The conductive resistance strip in a typical strain gauge application may have a lthickness of the order of 0.003 of .an inch. The term shallow as used here is intended to cover composites or assemblies made up of layers -of such thicknesses, or of the order thereof, and ranging in number from that `shown in FIGURE `2 to that shown in FIG- URE` l. The latter limit is a matter of balancing the risk of leakage current flow against total thickness of the composite.

'If desired, the interlayers. could be of metals other than molybdenum or nickel-chromium alloy, although the latter is desirable in cases where temperature stability is needed. yCare should be exercised in choosing :metals which cooperatively do not exhibit a bimetal bending eifect. Thus, it is desirable to employ interlayer material and flash coating material which have approximately the same temperature expansion coefficient as that of 'the resistance `strip material so as to minimize internal strains in the gauge structure due to temperature changes.

In order to avoid leakage current flow between the metal layers 4in the laminar structure, the refractory ceramic deposits should not be much thinner than the values named above. Therefore, in order to minimize or limit the `total thickness of the composite laminate, the number of layers used must be limited. The arrangement shown in FIGURE 1 represents about the maximum, and indeed it .is usually the optimum number of layers which should be used lfor most applications considering the derivation of benefits from the laminar arrangement of insulation layers.

In FIGURE Z the electrically conductive .strip or resistance element 24C is flame-sprayed on a single refractory cera-mic layer 18e which in turn is flame-sprayed on the dash coa-ting .'12C. The latter, deposited directly on the surface of the lbase l10, :afford-s a means to prevent excessive strain in the refractory ceramic layer @8c as a result of temperature variations.

In the embodiment shown in FIGURE 3, the iiash coating is dispensed with and the resistance structure is achieved by depositing the refractory ceramic coating ld directly on the base 10 and by depositing the conductive strip 24d directly on the layer 18d. However, even though these layers are deposited by a liame spraying technique, the composite structure of FIGURE 3 is not nearly as effective as that in FIGURE 2, which, however, is not nearly as satisfactory as that in FIGURE l from the standpoint of the various objectives set forth hereinabove.

Of the various refractory ceramic materials tried, A1203 proved to be most satisfactory. ZrO, MgO, BeO and ThO represent other specific examples. However, the list of refractory ceramics which may be used with varying degrees of success runs into the hundreds. Various aluminates, silicates, nitrides and others. The considerations of high tensile strength and electrical insulation factor are of paramount importance in these instruments; however, for many applications, particularly high temperature strain gauges, the necessary degree of toughness or tensile strength, temperature stability, low drift characteristics and high insulation qualities at high temperatures are achieved, even with aluminum oxide as the refractory ceramic, only by employing the novel laminar construction employing a plurality of refractory ceramic layers separated by metallic interlayers.

The concept of name-spraying the various layers onto their respective supporting surfaces is essential to these instruments. This flame-spraying may be carried out in the atmosphere without difficulty. However, when added ductility and strength are needed, heating to high temperature in a reducing atmosphere of hydrogen is employed in order to reduce the oxides present and produce a smooth and amorphous conductive layer. Such fusion should be carried out at a temperature of the order of 2000 F., which is just the softening point of nickelchromium alloy, at which the particles run together and become thoroughly embedded in the underlying coating. lf a materially higher temperature is reached, such as of the order of 2200o F. or 2300" F., the metal tends to ball up instead of run. Other metals or metal alloys will fuse at a different temperature.

A further embodiment is the use of plasma spray equipment in forming strain gauges and other resistance elements. Since the plasma spray uses inert gas such as argon, helium, hydrogen, nitrogen or others as opposed to oxygen in the oxyacetylene spray equipment, then subsequent oxide decontamination in a hydrogen reducing atmosphere is not necessary. Also, with the extremely high temperatures available in a plasma jet, it is possible to fuse the strain gauge resistance elements during the forming of the strain gauge without subsequently subjecting the gauges to a 4fusing temperature in a hydrogen reducing atmosphere. However, care should be exercised to prevent the presence of any oxidizing atmosphere when using the plasma spray in order not to cause oxide contamination during spraying of the resistance elements. Also denser ceramic coatings are possible because of greater deposit eiciency of the plasma spray.

The important function of the precoat or flash coat applied to the base surface, using nickel-chromium alloy, molybdenum, or other suitable metal having the desired relative temperature expansion coefficient, is to cause the flame-sprayed refractor ceramic to stick to its supporting surface. Whereas it will not stick readily to metals which have a widely different temperature expansion coeicient than the ceramic, it will stick to a metal layer which has a coefficient of intermediate value. The flash coat acts as a thermal stabilizer preventing the oxides by which the refractory ceramic is bonded to its supporting surface from popping olf due to changes of temperature.

lf desired, certain additives may be mixed with the aluminum oxide or other refractory ceramic layer during spraying in order to improve the strain characteristics of the ceramic coatings. For example, a small percentage of glass rock (silica dioxide) or glass frit tends to maintain a more stable and repeatable strain characteristic as well as to enhance the bonding quality of the coating for extremely highternperature applications for which the instrument is subjected to fusion at high temperatures in a reducing atmosphere.

Another feature worthy of note herein is the concept of not only laminating the insular structure of the gauge but of graduating the thickness of the ceramic coatings. As shown in FIGURE l, the outer refractory ceramic coating 18 is materially thicker than the inner ceramic coatings M and 16. This thickness relationship further assists in adding to the tensile strength of the composite structure and to the minimizing of coating damage due to drastically unequal coemcients of expansion of the base material and of the coating materials.

The choice of metal for the electrical resistance element depends upon the application. Nickel-chromium alloy has the characteristic, desirable for strain gauges, of substantially constant resistance throughout a wide range of temperature change while sensitively changing its resistance in response to variations in strain. Other materials would be used for temperature sensors, which would be sensitive to temperature only and relatively insensitive to strain. The alloy Nilvar is one example.

If desired the successive layers or coatings can be graduated with each other by employing two spray guns one of which is progressively throttled to decrease its output while the other is progressively opened to increase its output. Thus, there will not be a sharp line of demarcation between layers but a denite blending of metallic and ceramic substances, with cermet substance formed in the interlayer region. Such an arrangement further augments the above-mentioned desirable characteristics of these laminated gauge or resistance structures.

After completion of the layer deposits a protective ceramic overlayer may be flame-sprayed over the entire unit and beyond its edges onto the base surface in order to seal it against damage and the effects of moisture condensation and the like.

These and other aspects of the invention will be evident to those skilled in this art on the basis of the foregoing disclosure of the preferred embodiments and practices thereof.

I claim as my invention:

l. In combination, a solid metallic base for use in high temperature environments, and an assembly of superposed solid, alternately metallic and ceramic layers, between 3 and 7 in number, inclusive, adhering thereon and consisting essentially of a metallic flash coat adhering directly to the base and having a thickness of about 0.002 of an inch, an electrically insulative refractory ceramic coat adhering directly to the opposite face of the flash coat and having a thickness of about 0.003-0.007 of an inch, and an electrically conductive resistance metal strip having a thickness of about 0.003 of an inch, overlying and adhered to the opposite face of the ceramic coat, and said flash coat having a temperature expansion coefiicient intermediate those of the base metal and the refractory ceramic.

2. The combination according to claim 1 wherein the individual layers consist of heat fusible substances and have the attributes of coatings solidied in situ from a high temperature molten state.

3. The combination according to claim 2 wherein the substances are flame-sprayable and the layers have the attributes of coatings deposited by high velocity thermal spraying techniques.

4. The combination according to claim 3 wherein the layers also have the attributes of coatings heated t0 fusion temperatures in a reductive atmosphere.

5. The combination according to claim 1 wherein the resistance metal strip is directly adhered to the opposite face of the ceramic coat.

6. The combination according to claim 1 wherein the refractory ceramic is aluminum oxide, and the metal of the flash coat is selected from the group consisting of molybdenum and nickel-chromium alloy.

'7. A combination according to claim 1 wherein the resistance metal strip is adhered to the opposite face of the ceramic coat through the medium of a subassembly of superposed solid, alternately metallic and ceramic interlayers, between 2 and 4 in number, inclusive, interposed between the ceramic coat and the strip, the underrnost of which interlayers is metallic and the outermost of which is ceramic, and each metallic of which interlayers has a thickness of about 0.002 of an inch and each ceramic of which interlayers has a thickness of about 0.003-0.007 of an inch, the metallic interlayer having a temperature expansion coeiiicient intermediate those of the base metal and the refractory ceramic.

d. The combination according to claim 7 wherein the ceramic interlayers are increasingly thicker in the direction transverse the subasselnbly relatively from the ceramic coat to the resistance metal strip.

9. T he combination according to claim 7 wherein the metallic interlayers and the flash coat have approximately the same temperature expansion coeicient as that of the resistance metal strip.

10. A method of forming an insulated high temperature electrical resistance element on a solid metallic base structure, comprising forming an assembly of superposed solid, alternately metallic and ceramic coatings, between 3 and 7 in number, inclusive, on the base structure, by employing flame sprayable metallic and ceramic substances for the coatings and depositing the coatings in a high temperature molten state by a high velocity thermal spraying technique, the coatings being applied and solidiled in situ from such state, firstly, as a metallic llash coat adhering directly to the base structure and having a thickness of about 0.002 of an inch, secondly, as an electrically insulative refractory ceramic coat adhering directly to the opposite face of the flash coat and having a thickness of about 0.003-0.007 of an inch, and thirdly, as an electrically conductive resistance metal strip having a thickness ot about 0.003 of an inch, overlying and adhered to the opposite face of the ceramic coat, the metal of the flash coat being selected to have a temperature expansion coefficient intermediate those of the base metal and the refractory ceramic.

11. The method according to claim 10 wherein the resistance metal strip is adhered to the opposite face of the ceramic coat through the medium of a subassernbly of superposed solid, alternately metallic and ceramic intercoatings, between 2 and 4 in number, inclusive, interposed between the ceramic coat and the strip, and formed by employing flame sprayable metallic and ceramic substances for the intercoatings and depositing the intercoatings in a high temperature molten state by a high velocity thermal spraying technique, the underrnost of the intercoatings being metallic and the outermost being ceramic, the metallic of the intercoatings having a thickness of about 0.002 of an inch and the ceramic of the intercoatings having a thickness of about 0.003-0.007 of an inch, and the metallic intercoating having a temperature expansion coefficient intermediate those of the base metal and the refractory ceramic.

12. The method according to claim 10 wherein the resistance metal strip is directly adhered to the opposite face of the Ceramic coat.

13. The method according to claim 10 wherein the assembly is subsequently heated to fusion temperatures in a rcductive atmosphere.

References Cited by the Examiner UNITED STATES PATENTS 2,252,464 8/41 Kearns et al 117--212 X 2,556,132 6/51 Ball 117--227 2,715,666 8/55 Stincheld 117-227 X 2,837,619 6/58 Stein 338--308 X 2,939,807 6/60 Needham 117-212 K2,962,393 11/60 Ruckelshaus 117-217 X 2,996,696 8/61 Harman. 3,023,390 2/62 Moratis et al 117--217 X WILLIAM D. MARTIN, Primary Examiner.

MURRAY KATZ, Examiner.

Claims (1)

1. IN COMBINATION, A SOLID METALLIC BASE FOR USE IN HIGH TEMPERATURE ENVIRONMENTS, AND AN ASSEMBLY OF SUPERPOSED SOLID, ALTERNATELY METALLIC AND CERAMIC LAYERS, BETWEEN 3 AND 7 IN NUMBER, INCLUSIVE, ADHERING THEREON AND CONSISTING ESSENTIALLY OF A METALLIC FLASH COAT ADHERING DIRECTLY TO THE BASE AND HAVING A THICKNESS OF ABOUT 0.002 OF AN INCH, AN ELECTRICALLY INSULATIVE REFRACTORY CERAMIC COAT ADHERING DIRECTLY TO THE OPPOSITE FACE OF THE FLASH COAT AND HAVING A THICKNESS OF ABOUT 0.003-0.007 OF AN INCH, AND AN ELECTRICALLY CONDUCTIVE RESISTIVE METAL STRIP HAVING A THICKNESS OF ABOUT 0.003 OF AN INCH, OVERLYING AND ADHERED TO THE OPPOSITE FACE OF THE CERAMIC COAT, AND SAID FLASH COAT HAVING A TEMPERATURE EXPANSION COEFFICIENT INTERMEDIATE THOSE OF THE BASE METAL AND THE REFRACTORY CERAMIC.

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