US 3429736 A
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United States Patent 47,320/64 U.S. 'Cl. 11770 7 Claims Int. Cl. B32]: 17/06; B28b 19/00 ABSTRACT OF THE DISCLOSURE Ceramic articles have a fired coating of a eutetic mixture of metal oxides having a melting temperature below that of the ceramic and including another metal of the group of nickel, cobalt and iron. The coating does not react chemically with the ceramic and is wettable by brazing alloys to provide a hermetic seal.
This invention relates to the metallizing of ceramic articles whereby they may be brazed to metal members or to other similarly metallized ceramic parts. The invention finds particular application in the manufacture of hermetically sealed envelopes for electric discharge devices and terminal plates for other electrical apparatus.
In application Ser. No. 444,100, filed Mar. 23, 1964, and assigned to the same assignee as the instant invention, there is disclosed and claimed a ceramic article having thereon a fired coating of a mixture of metal oxides having a definite melting temperature below that of any constituent of the ceramic and not forming a chemical reaction product with the ceramic, the coating being loaded with a metallizing power such that the ceramic article may be hermetically sealed to a metal article by brazing the metal article to the loaded coating or onto a layer of another metal plated on the coating. Preferably the mixture of metal oxides is a eutectic mixture, of which several examples include 92% of Cu O, 8% SiO 50% Li O, 50% SiO 82% SiO 18% Li O; 55% Cr O 45% C20; 52% CaO, 48% B 0 36% MgO, 64% B 0 and 25% SiO 65% MnO.
The choice of metallizing powder is likewise quite wide and includes molybdenum and tungsten, nickel-chrome alloy, stainless steel, silicon carbide and molybdenum disilicide. The examples of metallizing powder given, particularly molybdenum, provide metal to ceramic seals of high strength. On the other hand molybdenum, which in common with all metallizing powders quoted in the present specification, is not wet readily by the usual brazing alloys such as copper-silver and copper-gold. The fired coating has therefore to be nickel-plated before being brazed to another article.
According to the present invention there is provided a ceramic article having a fired coating wettable by brazing alloys wherein the fired coating includes, at least on its outer surface, nickel, cobalt, or iron, or a mixture of these.
For some purposes, where a comparatively weak seal can be tolerated, the fired coating on the ceramic article may consist of a mixture of metal oxides, preferably a eutectic mixture, with metallizing powder of nickel, cobalt or iron or a mixture of these. The strength of a typical seal using iron is 8,000 lbs/inch compared to 14,500 lbs./in. for a similar seal based on molybdenum, the stresses quoted being those required to rupture a seal in a standard tensile testing machine. If the stronger seal is required, then we have found it satisfactory to prepare a coating as described in the aforementioned parent specification but, before firing, to apply a second coating containing the metal or oxide of nickel, cobalt or iron or a mixture of these and then to fire the composite coating in a reducing atmosphere.
Examples of these two methods of manufacture are given below.
EXAMPLE 1 An iron powder mixture is made up as follows:
G. Iron powder (6,) Manganese dioxide (technical) 19 Silica powder (technical) 24.5 Alumina powder 10 This mixture is ball-milled with a binder of nitrocellulose in amyl-acetate for several hours until the maximum particle size is reduced to less than 10 The resultant paint is applied to the ceramic to a thickness of 0.002 to 0.003 inch. Firing is carried out at 1200 C. in wet hydrogen for 30 minutes. The fired coating can be brazed by conventional techniques to nickel-cobalt-iron alloys without further treatment.
For providing, for example, an outer layer of an iron group metal to the molybdenum powder coating given in the example of the parent specification, the eutectic mixture given above may be used before-firing or, alternatively, the outer coating may be applied by the method given in the following example.
EXAMPLE II A molybdenum powder-loaded coating containing molybdenum powder of 99.87% or greater purity, of particle size 15;t125 grams; Mn019 grams, silica powder--24.5 grams; high purity alumina-10 grams, with a binder as described above, is applied to a ceramic article and, before firing, an outer coating of a paint made up as follows is applied:
Metal powder (iron, cobalt or nickel) 6/L-30 g. Nitrocellulose-amyl-acetate binder, 17 mls.
The molybdenum-based paint is, however, applied only to a thickness of 0.0015 of an inch and the final layer of iron group paint is 0.0005 inch thick. The ceramic article is then fired in a hydrogen furnace at 1200 C. The furnace is heated slowly and then maintained at the desired temperature above and close to the eutectic temperature for 30 minutes with a continuous flow of wet hydrogen. The articles are then cooled to about 200 C. before removal.
Instead of using metal powder for the coating, it is satisfactory to use metal oxide, the paint then consisting, by way of example, of 20 g. of metal oxide dispersed in 7 mls. of binder. The metal oxide is reduced to the metal during the hydrogen firing.
It is to be understood that the foregoing description of specific examples of this invention is made by way of example only and is not to be considered as a limitation on its scope.
1. A ceramic article having a fired coating of a eutectic mixture of metal oxides selected from the group consisting of cuprous oxide, silica, lithium oxide, chromic sesquioxide, calcium oxide, boric anhydride, magnesium oxide, manganese oxide and alumina having a definite melting temperature below that of any constituent of the ceramic and which does not chemically react therewith, the coating being fired at a temperature above and close to the eutectic temperature of between 1000 to 1300 C. for a sufiicient time for said coating to be capable of forming a hermetic seal, said coating being wettable by brazing alloys and including at least on its outer surface a metal of the group consisting of nickel, cobalt and iron.
2. A ceramic article as claimed in claim 1, wherein the coating includes a metallizing powder of said metal of the group consisting of nickel, cobalt and iron.
"3. A method of coating a ceramic article including applying a coating of a eutectic mixture of metal oxides selected from the group consisting of cuprous oxide, silica, lithium oxide, chromic sesquioxide, calcium oxide, boric anhydride, magnesium oxide, manganese oxide and alumina having a definite melting temperature below that of any constituent of the ceramic and which when fired do not chemically react with said ceramic, said coating including a metal of the group consisting of nickel, cobalt and iron and, firing said coating at a temperature close to and above said eutectic temperature of between 1000 to 1300 C. in a reducing atmosphere for a sufiicient time to obtain a coating capable of hermetic sealing to metal articles by brazing.
4. The method of claim 3 including applying a first coating including said mixture of metal oxides and a metallizing powder having molybdenum therein and applying a second coating including said metal of said group consisting of nickel, cobalt and iron before firing.
5. The article of claim 1 wherein the coating includes a second metal oxide of said metal of the group consisting of nickel, cobalt and iron.
6. The method of claim 3 wherein said metal of said group is in the form of an oxide.
7. The method of claim 3 wherein said metal of said group is in the form of a metallizing powder.
References Cited UNITED STATES PATENTS 3,051,592 8/1962 Woerner 117-123 XR 3,107,756 10/ 1963 Gallet.
3,132,044 5/ 1964 Pearsall 117123 XR 3,215,555 11/1965 Krey 117--123 XR FOREIGN PATENTS 1,134,922 8/ 1962 Germany.
ALFRED L. LEAVIIT, Primary Examiner.
20 W. F. CYRON, Assistant Examiner.
US. Cl. X.R. 11771, 123, 169