Direct Bonding of Ceramic Parts by a Silver Alloy Solder

The present invention relates to the use of a silver alloy as the solder when directly bonding ceramic parts to each other or ceramic to metal parts.

Ceramics alone or ceramics combined with metals allow making components combining the advantages of ceramics and of metals. However, the applicability of such components largely depends on suitable bonding methods.

In industry, ceramics are joined to other ceramics or to metals mostly by soldering metallized ceramics. For that purpose, a layer of manganese-molybdenum suspension is deposited on the solder zone of the ceramic and is baked into it at high temperature under a hydrogen atmosphere. The oxides produced at the boundary layer form spinels with the ceramic and thereby provide good adhesion. To improve solder wetting, typically a nickel layer will be added. The ceramic so metallized can be soldered using conventional hard-solders, for instance, silver-copper hard solders. This procedure however entails many steps and therefore is cumbersome.

A more economical bonding technique between ceramics and metals is provided by active soldering. Using so-called active solders, ceramic and metal parts can be bonded together in one operational step in the absence of prior ceramic metallization. Mainly silver, or silver-copper alloys with low proportions of titanium or zirconium are used as active solders. Typically, the titanium contents are between 0.5 and 5% by weight. At low titanium contents, these solders are ductile and can be easily processed into solder shapes. However significant degradation in forming takes place as the titanium content increases.

Binary silver-titanium alloys illustratively containing 1% titanium are known (French patent document No. 1,174,074). When bonding zirconium oxide to steel CK 45, such solders will offer shear strengths of 130 to 150 MPa. However, they incur the drawback of a comparatively high solidus temperature and hence of soldering temperature. The solidus temperature may be lowered by adding copper and therefore mainly silver-copper-titanium solders are used for active soldering ceramics, such solders illustratively containing 3% titanium and a silver-copper eutectic (see for instance METALL, 1973, pp. 259-65). However such solders exhibit less ductility than the silver-titanium solders. Furthermore solder alloys are known which contain 60 to 65% silver, 24 to 25% copper, 5 to 10% indium and about 5% titanium (French patent document No. 1,174,330). However, these solders entail the drawback that the addition of indium renders the alloys more brittle.

Accordingly, it is an object of the present invention to discover a silver alloy to be used as solder in the direct bonding of ceramic parts to each other or to metal parts (steel, e.g. mild steel, CK 45, Fe-Cr steel) and exhibiting high ductility and offering high solder connection strength while simultaneously having a low soldering temperature.

In attaining the above and other objects, one feature of the invention resides in using an alloy of 70 to 99% by weight silver, 0.5 to 10% by weight tin or 0.5 to 15% by weight indium and 0.5 to 15% by weight titanium, zirconium and mixtures thereof.

Ceramic articles may include oxidic as well as non-oxidic, e.g. Al.sub.2 O.sub.3, ZrO.sub.2, SiC, B.sub.4 C, Si.sub.3 N.sub.4. Metal parts include steels, Ni-based alloys, Fe-Ni alloys, among others.

Preferably alloys with 92 to 99% by weight silver, 0.5 to 4% by weight indium and 0.5 to 4% by weight titanium or 92 to 99% by weight silver, 0.5 to 10% by weight tin and 0.5 to 4% by weight titanium shall be used.

Surprisingly, it was found that indium can be added in amounts up to 15% by weight and tin in amounts up to 10% by weight to silver titanium alloys without thereby significantly degrading the ductility of such alloys. Contrary to the case of the cuprous silver-indium-titanium alloys, no brittle phase is formed, whereby even for titanium contents up to 15% by weight the alloys retain good forming properties. These alloys can be processed by cold-rolling with appropriate intermediate annealing into solder sheets. In addition to their high ductility, these solders also exhibit very high mechanical strength of ceramic-metal bonding and comparatively low operational temperatures.

The alloys below were used successfully to solder ceramic parts of test pieces 20.times.20.times.6 mm.sup.3 :

All alloys exhibit good ductility and work ability.

Further variations and modifications of the foregoing will become apparent to those skilled in the art and are intended to be encompassed by the claims appended hereto.