This invention concerns a timepiece, and it relates in particular to new developments involving transparent elements visible from outside the piece, like the crystal, for example.
In putting together the components of the timepiece assembly, one of the most difficult problems to solve resides in establishing a connection between the transparent elements, usually of glass or sapphire, and other metal elements of the design. For example, to assemble the crystal on the back case of a watch case, numerous solutions have previously been proposed. That assembly can be carried out, for example, by means of mechanical locking parts, of the clamp, slide or screw type, or even by retention in a catch with or without insertion of a gasket. These designs are usually bulky and require particular arrangements in the crystal or case, which increase the difficulty of manufacture. Furthermore, that type of fastening of the crystal requires it to be thick enough.
It was also previously proposed that the crystal be attached by gluing on the case. That method of connection presents the disadvantages of a relatively low breaking stress, in the order of 5 kg/mm.sup.2 at most, and poor endurance due to degradation of the glue by external conditions (transpiration, thermal shocks, ultraviolet rays, etc.).
Still another solution is described in Swiss Pat. No. 582,909 or Japanese Pat. No. 56-96267. It consists of using for assembly of the crystal to the square an intermediate layer of solder with low melting temperature and tin-lead, gold-indium or gold-tin alloy base. Tests conducted by the applicant show that the adherence of the layer of solder to the glass is in the order of 6 kg/mm.sup.2. The choice of solder materials is limited and they tend to oxidize in the course of use. Furthermore, in order for the solder not to give an unattractive appearance to the lower part of the crystal, it is necessary to submit that zone to various metal coatings preventing diffusion of the solder to the crystal.
That is why one of the objects of this invention is to provide an externally visible transparent element of a timepiece which can be attached to another element by high-temperature soldering. Another object of the invention is to propose such a piece in which the transparent element is capable of receiving an outer decorative coating of hard material.
In that connection, it is to be noted that it is known how to make decorative or masking metal linings under crystals, those linings not being placed on an outer surface of the crystal, but being visible through it and protected by that same crystal. For the first time, the invention provides making coatings of hard material, which can, for example, but not exclusively, mask the high-temperature soldering bonds established according to the first aspect of the invention.
The advantages of such an arrangement are linked to the new esthetic effects that can thus be obtained. By placing hard decorative coatings directly on the outer surface of a crystal, it is possible, notably, to give it a very surprising appearance and a heretofore unexploited impression of depth. One can, for example, create the illusion that the crystal is surrounded by a metal rim of complex shape, etched, etc., by judiciously economizing on application of the outer lining or by successively making linings not fully overlapping, it is even possible to obtain the effects of ranges of colors on the edges of the lining. Another advantage of the use of hard outer linings is that they make it possible to mask possible surface defects on the crystal, which, on the other hand, are revealed in the case of linings applied under the crystal and visible through the latter.
These objects are accomplished according to the invention by providing on a transparent element of a timepiece an adherent lining assuring the attachment of a decorative coating and/or of an outer layer of high-temperature soldering material. Such a lining, which had heretofore not been proposed in the application described, is deposited preferably according to the chemical vapor deposit (CVD) process, which makes it possible to obtain excellent results.
In particular, the invention makes it possible to attain breaking stresses for the upper lining of 15 kg/mm.sup.2, which is comparable to the tensile strength of the sapphire itself, which ranges between 19 and 50 kg/mm.sup.2, depending on the direction of pull.
This results in new design possibilities, which also is an integral part of the invention and, notably, enables the crystal to be used as a central element of the assembly, to which the case, the band connecting parts and the decorative elements or decorative outer coatings will, for example, be attached.