VACUUM BRAZING OF METAL TO CERAMICS

The question of vacuum brazing of metal to ceramics has already been raised in the pages of this journal in an abstract devoted to this subject¹. The method described there consists of the following. The ceramic surface is coated with a paint made from molybdenum powder, fired in a reducing atmosphere at a temperature of 1300°C, then coated again with a paint made this time from nickel powder, and fired in a hydrogen atmosphere up to 1000°C. The surface treated in this way can be brazed with special alloys by means of special solders.

Bondley² reports another method, using titanium hydride. In this case beryllia-magnesia-silicate ceramics are fired and coated, just as indicated above, with a special paint or enamel made from a mixture of titanium hydride with nitro lacquer. The paint is applied with a brush or by means of a spray gun. After this the ceramic surface is immediately brazed to the metal. In this process refractory solders are used—silver, copper-silver solder, or other metals with a melting temperature in the range 900–1000°C.

Brazing is carried out in vacuum or in hydrogen at temperatures on the order of 1000°C. In the very first stages of heating, the titanium hydride dissociates and pure titanium remains on the ceramic. The hydrogen liberated in this process is chemically very active, since it is in the atomic state. Its presence is essential for improving the quality of cleaning of the brazed surfaces, because it serves as a reducing agent with respect to the impurities and contaminants present on the surfaces of the ceramic and the metal. Clean surfaces are more easily brazed.

When such a temperature is reached that the main mass of hydrogen has been evolved from the titanium hydride, the silver solder melts, alloys with the titanium, and this titanium-silver alloy is firmly fixed on the ceramic. Tensile tests show that this joint proves stronger than the ceramic itself.

The fact that brazing metal to ceramics can take place in vacuum makes it possible to obtain good degassing of the brazed parts. Oxides do not form on the surfaces of metals and ceramics; their presence is necessary when glass is fused to metal in order to ensure wetting of the metal by the molten glass. This removes the problem of combating oxidation of the parts of the metal adjacent to the brazed joint, which presents a considerable difficulty in glass-to-metal seals.

The high temperature used in brazing of this kind generally exceeds the possible temperatures at which the…

…the brazed joint, and therefore guarantees the absence of gas evolution during operation.

To eliminate stresses arising as a result of differences in the coefficients of thermal expansion of the parts being brazed, the author brazes parts made of chromium steel (FeCl), containing from 14 to 30% chromium, to magnesium-silicate ceramics. In this case their coefficients of thermal expansion are almost equal to one another. Annealing after brazing of such parts is not required. The heating and cooling processes proceed very rapidly. Thus, the author notes that heating from 0° to 900° C is usually carried out in the course of 3 minutes.

From all the foregoing it is clear that this method makes it possible to obtain vacuum-tight, mechanically strong brazed joints of metal with ceramics, using a single operation. In the course of this operation the surface of the ceramic is metallized and, with the aid of refractory brazing alloys, is brazed to the surface of the metal. In the method discussed earlier¹, this process is extended over 3 operations.

V. V. Fëdorov

References Cited

1. “Vacuum Brazing of Metal to Ceramics,” UFN 33, issue 2 (1947).
2. Bondley, Electronics 20, 6 (1947).