US3152548A

US3152548A - Thermal insulating structure

Info

Publication number: US3152548A
Application number: US228057A
Authority: US
Inventors: Melvin M Schwartz
Original assignee: Martin Marietta Corp
Current assignee: Martin Marietta Corp
Priority date: 1962-10-03
Filing date: 1962-10-03
Publication date: 1964-10-13
Anticipated expiration: 1981-10-13

Description

Oct. 13, 1964 M. M. SCHWARTZ THERMAL INSLATING STRUCTURE Filed Oct. 3, 1962 INVENTOR. MELVIN M. SCHWARTZ BY ATTONE Y United States Patent O 3,152,548 THERMAL INSULAIING STRUCT Melvin M. Schwartz, Baltimore, Md., assigner to Martin- Maretta Corporation, Baltimore, Md., a corporation of Maryland Filed Oct. 3, 1962, Ser. No. 228,057 12 Claims. (Cl. 10292.5)

This invention relates to thermal insulating structures and more particularly to such structures which comprise an outer shell formed of a high temperature ceramic material connected to a metal inner shell.

While the structure of the present invention is useful in numerous types of heat shield applications, its advantages are most evident in the field of space fiight. When a vehicle passes through the earths atmosphere upon returning from a fiight in outer space, its nose cone is nocessarily subjected to temperatures of extreme intensities. In order to prevent burnup and subsequent disintegration of the vehicle, it has been necessary for scientists and engineers to develop new materials and structures capable of withstanding unusually high temperatures. Although considerable progress has been made in this field, a high level of effort is presently being expended by both Govemment and industry in an effort to further perfect reentry nose cone structures.

Due to the excellent heat-resistant properties of ceramic materials, they have most frequently been employed to form the outer surface of re-entry nose cone structures. Typically a high thermal resistant ceramic outer shell is joined directly to a metal inner sheil. Another method employed has been to braze open-faced honeycomb core to a backup metal inner shell, filling the honeycomb cells with a thermal insulating material and then joining thereto an outer shell formed from ceramic platelets or castable ceramic materials to complete the assembly.

Both of these prior art structures have serions limitations in that no provision is built into them to accommodate differences which result from the fact that the various materials from which they are formed do not have similar thermal expansion properties. The operating temperatures to which such structures are exposed will frequently subject the mating surface between the metal and the ceramic to such large internal stresses that the ceramic outer shell will crack resulting in a breakdown of the high thermal insulating properties of the structure. In addition, a wide choice of materials from which the structure may be fabricated is precluded by the necessity to select materials having closely matched thermal coefficients of expansion. In the first described prior art structure, the fact that no provision is made for interposing an additional insulating material between the inner and outer shells restricts the use of this type of structure to relatively low temperature applications. Although the second prior art structure described above has an insulating material imposed between its inner and outer shell, it is an extremely expensive structure since utmost care is required in its fabrication to assure that a good contact is formed between each honeycomb cell and the metal and ceramic enclosing shells.

It is therefre a primary object of the present invention to provide a thermal insulating structure having an exposed ceramic surface and which includes provisions to accommodate differences in thermal expansion between the inner and outer shells thereof. An inexpensive reentry nose cone structure is provided which includes a thermal insulating material interposed between a metal inner shell and a ceramic outer shell. The present invention provides a re-entry nose cone structure in which a ceramic outer shell is flexibly connected to a metal inner shell. These and other objects of the present invention 3,152,548 Patented Oct. 13., 1964 will become obvious as the following description of same is read in connection With the accompanying drawings in which:

FIGURE 1 is a plan view, partially cut away, illustrating a missile having a nose cone structure conforming to the principles of the present invention; and

FIG. 2 is a perspective view illustrating in detail a section of the structure of the missile nose cone shown in FIGURE 1.

The basic principle of the present invention resides in utilizing a flexible medium to connect a ceramic outer shell of a thermal insulating structure to its metal inner shell, this flexible medium occupying only a small amount of the space between the two shells. The remaining space between the ceramic outer shell and metal inner shell is filled With a pliable thermal insulating material thereby forming an extremely effective and inexpensive flexible thermal-resistant structure.

In forming the structure of the present invention it is necessary that a ceramic material be joined to a metal body. In a co-pending application Serial No. 14,192 entitled Method for Joining Ceramics to Other Ceramics or Metals, filed on March 11, 1960, by the present inventor, and now abandoned, an improved process is disclosed for joining ceramic materials to metal bodies, which process may be advantageously employed in forming the re-entry nose cone structure of the present invention. That application pointed out that the prior art techniques for joining a ceramic material to a metal body required that a metal bonding layer first be fired onto the ceramic surface prior to the joining operation. It was the purpose of that invention to eliminate this firing step so that, in efiect, the metal bonding layer could be securely aflixed to the ceramic material during the same operation by which the ceramic material was joined to the metal body.

In accordance with the process of the referenced copending application, the appropriate surface of. the ceramic body is first provided with a coating of metal by means of flamespraying. The flamespraying process is accomplished by utilizing a metal spray gun such as the Metco 4E Spray Gun manufactured by the Metallizing Engineering Company, Westburv, New York This metal bonding layer is normally about 1 to 5 mils in thickness. No special preparation of the ceramic body prior to fiamespraying is necessary aside from the usual cleaning operations to free the surface to be fiamesprayed of grease and dirt which otherwise may prevent intimate contact between the ceramic material and the metal bonding layer.

The metal coating deposited on the ceramic body by flamespraying is adapted to wet the ceramic substrate and to be wetetd by a brazing alloy, the application of which will be subsequently described. The bonding layer may consist of one layer of a metal such as molybdenum, molybdenumiron alloy, titanium, tuungsten or zirconium. This bonding layer may also consist of two or more layers of difierent metals which are chosen so that the layer first deposited is adapted to wet the ceramic and the layer last deposited is adapted to be Wetted by the brazing alloy. The last layers in a multilayer bonding layer are also adapted to alloy With the previously deposited layer or layers so that a strong bond exists therebetween after braz mg.

The fiamesprayed surface of the ceramic body is then arranged in close proximity to the surface of the metal body to which it is to be joined and a molten braze, adapted to wet the flamesprayed metal coating on the ceramic and the surface of the metal body, is caused to solidify therebetween so as to for-m an affective bond between the ceramic body and the metal body.

Ceramics which may be joined with metal bodies by this process include graphite and non-metallic refractory inorganic materials that are thennally stable et tempera 3 turcs in excess of about 3000 F. and which possess a a crystalline structure. Exemplary of this class of materials are alumina, zirconia, magnesia, titania, silicon carbide, tantalum carbide, tungsten carbide, tantalum nitride, zirconium nitride, molybdenum disilicide (MoSi niobium disilicide and combinations thereof. Also included, by way of example, are the materials in the following table:

Ceramic lllaerzals Carbides: Be C, Cr C CbC, MO C, ThC, TiC,

4TaC.ZrC, VC, and ZrC Nitrides: Ba N Be N CbN, AlN, TiN, VN, and S N Oxides: BeO, BaO.ZrO BaO.AI O BeO.AI O

2BeO.SiO:;, 3Al O .2SiO BaO, C602, CaO, Ca0.Zr CI'203, C210.C20g, Y203, NiO.Al O and SrO.Al O

Conventional brazing alloys may be used in the above described process and, for example, may be 99.599.9 Ag:0.50.1 Li, 92.592.7 Ag:7.07.2 Cu:0.50.1 Li, 72 Ag:28 Cu (eutectic silver base), and Braze 625 (62.5 Ag:32.5 Cu: Ni) manufactured by Handy and Harmon, New York, New York. Unalloyed silver may also serve as a braze.

Referring now to FIGURE 1 a missile 3 is shown, the nase cone 4 of which is formed of a structure 5 embodying the principles of this invention. As may be better seen in FIGURE 2, the nose cone structure 5 includes a ceramic outer shell 6, the inner surface of which is provided With a metallic coating 7. The structure 5 also includes a metal inner shell 8 which is spaced from the ceramic outer shell 6 by a plurality of wires 9 each of which is alternately ot-set in opposite directions at spaced intervals along the length thereof to form modes lit) and 11 on opposite sides thereof. While the wires 9 may take various forms to provide the desired result, a preferred form is helical in character as illustrated in FIGURE 2. Nodes 10 of the wires 9 are joined to the metal-coated surface of the ceramic outer shell and modes 11 of the wires 9 are joined to the metalinner shell 8 to connect the ceramic enter shell and the metal inner shell in spaced relationship. The plurality of wires 9 only occupy a small portion of the space between the ceramic outer shell 6 and the metal inner shell 8, the remaining space between these shells being filled with a suitable high thermal insulating material 12 such as Fiberfrax, manufactured by the Carborundum Company, 3, division et Union Carbide, or Refrasil, manufactured by the H. I. Thompson Company. It is only necessary that the insulating material 12 be pliable, have a sufficiently high thermal resistance characteristic and have breakdown temperature above the operating temperatures that may be expected to be encountered at the inner surface of the ceramic outer shell.

Typical of the metals from which the inner shell 8 may be formed are steel; refractory metals such as molyl denum, tungsten, columbium, tantalum; titanium and its alloys; zirconium and its alloys; vanadium and its alloys; noble metals such as platinum, paladium and iridium; super iron; nickel based alloys and cobalt based alloys.

The wires 9 may also be formed from any of the above mentioned metals and are preferably made of the same metal from which the inner shell 8 has been formed. Any of the above mentioned metals may be satisfactorily joined to any of the previously listed ceramics in forming the structure 5 of the present invention by the earlier described process as long as the melting temperatures of the metal inner shell 3, the wires 9 and the ceramic outer shell 6 are higher than the melting temperatures of the particular brazing alloy employed.

A preferred method of forming the re-entry nase cone structure illustrated in the drawings is to pre-form the metal inner shell 8 by conventional metal working techniques. The ceramic enter shell is preformed either in one piece, by pouring the ceramic while in a liquid state into a mold and permitting it to cool and solidify therein, or by cementing together a number of properly shaped ceramic blocks which are then cured and fired. The inner surface of the ceramic outer shell 6 is subsequenlty flamesprayed to produce the metallic coating 7 thereon. The ceramic enter shell is positioned over the metal inner shell With the wires interposed therebetween and with a brazing alloy disposed between the modes 10 and 11 of the wires 9 and the outer shell 6 and the inner shell 8, respectively. Heat is applied to bring the assembly to the brazing temperature thereby causing the ceramic outer shell 6 to be securely joined to the metal inner shell 8. The structure is completed by either pressing or blowing the insulaing material 12 into the space between the ceramic outer shell 6 and the metal inner shell 8.

The plurality of wires 9 constitute a flexible medium joining the ceramic outer shell 6 to the metal inner shell lit will be apparent that these wires 9 Will permit diffcrcnces in thermal expansion between the ceramic outer shell 6 and metal inner shell 8 Without creating interna! stresses Within the nose cone structure 5 that might otherwise cause a fracture therein and a breakdown of the thermal insulating properties thereof.

Illustrative of the type of thermal insulating structures which may be provided is the example where a ceramic body of alumina approximately /2 inch in thickness was connected to a plate of Inconel approximatcly inch in thickness by a plurality of .050 inch diameter off-set Inconel wires. lnconcl is a metal produced by the International Nickel Company, of New York, New York, The inner surface of the ceramic body was flamesprayed With a molybdenum coating approximaely 2 mils in depth and the assembly brazed at a temperature of 2350 F. in an argon atmosphere utilizing a brazing alloy of 54% paladiumz36% nickel:l0% chromium. The assembly was then subjected to external forces until the Inconel metal plate and the Inconel metal wires joined thereto were broken away from the flamesprayed alumina body. Examinaion showed that each of the modes of the Inconel wires which had been joined to the ceramic body had some or the alumina material attached thereto, proving that excellent wetting of the ceramic material had been obtained.

This invention may be embodied in other ways without departing from the spirit or essential character thereof. The embodiment of the invention described herein is therefore iliustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come Within the meaning and range of equivalency of the claims are intended to be embraced therein.

The invention claimed is:

1. An improved thermal insulang structure comprising:

(a) a metal inner shell;

(b) a ceramic outer shell disposed over said metal inner shell in spaced relationship thereto; and

(c) flexible metal means disposed between said metal inner shell and said ceramic outer shell to connect said metal inner shell to said ceramic outer shell in said spaced relationship while permitting said metal inner shell and said ceramic outer shell to expand upon exposure to high temperntures Without adversely afecting the strength of the connection formed therebetween.

2. The structure of claim 1 wherein said flexible metal means includes at least one wire.

3. The structure of claim 1 wherein said flexible metal means and said metal inner shell are formed of the same material.

4. The structure of claim 1 wherein said flexible metal means occupies only a small portion of the space between said metal inner shell and said ceramic outer shell.

5. The structure of claim 4 wherein the remaining portion of said space between said metal inner shell and said cenamic outer shell is filled with a pliable thermal insulating mater-lai.

6. An improved re-entry nose cone structure comprismg:

(a) a metal inner shell substanfially conforming to the contour of said nose cone;

(b) a ceramic enter shell disposed over said metal inner shell in spaced relationship thereto, the inner surface of said ceramic outer shell being provided With a metallic coafing; and

(c) flexible metal means disposed between said metal inner shell and said ceramic outer shell to connect said metal inner shell to said ceramic outer shell in said spaced relationship while permitting said metal inner shell and said cenamic outer shell to expand upon exposure to high temperatures without adversely affecting the strength of the connection formed therebetween.

7. The structure of claim 6 wherein said flexible metal means occupies only a small portion of the space between said metal inner shell and said ceramic outer shell.

8. The structure of claim 7 Wherein the remaining portion of said space between said metal inner shell and said ceramic enter shell is filled with a pliable thermal insulating material.

9. An improVed reentry nose cone structure comprising:

(a) a metal inner shell substantially conforming to the contour of said nose cone;

(b) a ceramic outer shell disposed over said metal inner shell in spaced relationship thereto, the inner surface of said ceramic outer shell being provided with a metallic coating; and (c) a plurality of wires altemately off-set in opposite directions along the lengths thereof to form alternate modes on either side thereof, said plurality of Wires being disposed between said metal inner shell and said ceramic outer shell such that a first set of said alternate modes of said wires is in contact With said metal inner shell and the second set of said alternate nodes of said wires is in contact With said ceramic outer shell, said first set of alternate modes being brazed to said metal inner shell and said second set of alternate modes being brazed to said ceramic outer shell to fiexibly maintain said ceramic outer shell in spaced relationship to said metal inner shell. 10. The structure of claim 9 wherein the remaning space between said metal inner shell and said ceramic outer shell is filled with a pliable thermal insulating matenal.

11. The structure of claim 9 wherein each of said wires is helical in form.

12. The structure of claim 9 wherein said metal inner shell and said plurality of wires are formed of the same material.

References Cited in the file of this patent UNITED STATES PATENTS Henson July 4, 1961 Dobson Nov. 20, 1962 OTHER REFERENCES

Claims

1. AN IMPROVED THERMAL INSULATING STRUCTURE COMPRISING: (A) A METAL INNER SHELL; (B) A CERAMIC OUTER SHELL DISPOSED OVER SAID METTAL INNER SHELL IN SPACED RELATIONSHIP THERETO; AND (C) FLEXIBLE METAL MEANS DISPOSED BETWEEN SAID METAL INNER SHELL AND SAID CERAMIC OUTER SHELL TO CONNECT SAID METAL INNER SHELL TO SAID CERAMIC OUTER SHELL IN SAID SPACED RELATIONSHIP WHILE PERMITTING SAID METAL INNER SHELL AND SAID CERAMIC OUTER SHELL TO EXPAND UPON