Metal founding – Process – Shaping liquid metal against a forming surface
Patent
1986-11-03
1988-01-19
Godici, Nicholas P.
Metal founding
Process
Shaping liquid metal against a forming surface
164463, 427319, 427328, B22D 1100
Patent
active
047199628
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for selectively forming at least one coating strip consisting of a metal or alloy on a substrate consisting of another metal.
There are several methods for coating a metal substrate with another metal. In particular, there is electroplasting, chemical vapour deposition (CVD) and physical vapour deposition (PVD), hot roll-bonding methods and finally coating methods which use molten metal, either by dipping the substrate in a liquid-metal bath, as in the case of zinc coating, or by depositing the liquid metal on the substrate.
The electrolytic and the CVD and PVD methods are expensive because of the time they require, and roll bonding proves to be a delicate operation when layers in particular of the order of 10 to 40 .mu.m are required. For their part, the coating methods which use a molten metal or alloy have the drawback that it is difficult to control the structure of the coating. In fact, when a hot substrate is coated with molten metal, there must be proper wetting of the substrate metal by the molten coating metal. This wetting action is dependent on the contact time and the temperature at which this contact occurs. During this wetting process, diffusion of the substrate metal into the coating metal occurs. This diffusive process is interrupted by the formation of an intermetallic compound between the substrate and the coating metal and by solidification of the coating. In certain applications, in particular electrical applications where the resistivity of the coating is a factor of primary importance, the presence of an alloy formed as a result of dissolution of the substrate metal is not acceptable, since most of the coating must consist of metal or alloy with a purity preferably higher than 99%. In view of the fact that, during wetting of the substrate, a more or less large amount of the substrate metal diffuses into the coating metal or alloy, thereby ensuring good adhesion of the coating, it has not been possible to use the numerous deposition methods which employ molten metal or alloy, for a number of electrical and electronic applications in particular, e.g. for integrated circuit connection bases or for electrical contact elements. The use of such methods would result in a considerable increase in the productivity of the coating operations used for these substrates.
In No. FR-A-1,584,626 it has already been proposed to coat a steel strip with a layer of aluminium or aluminium alloy by passing this strip vertically from the bottom upwards through a vertical slit of a supply nozzle connected to a crucible containing molten aluminium. As it passes into the liquid aluminium contained inside the vertical slit, the strip, which moves from the bottom upwards, creates capillary forces which counterbalance the gravitational force exerted on the liquid and is covered with metal at the outlet of this slit. After solidification, a steel strip coated with aluminium is obtained. This is a method, therefore, by means of which it is possible to obtain layers of 20 to 100 .mu.m thickness, a layer of intermetallic compound with a thickness no greater than 2 .mu.m being formed at the interface.
Using such a method, selective coatings in the form of strips cannot be obtained on a substrate; the latter can only be coated. It is not possible, using this method, to obtain coatings with thicknesses as small as 4 .mu.m. However, there are numerous applications, particularly in electronics, where it is necessary to provide coatings in the form of strips, whose thickness is less than 10 .mu.m. As the thickness of the coating decreases, the throughput of metal is reduced by a proportionate amount. Consequently, in the case of the method described in the abovementioned document, in which the liquid metal is kept in equilibrium by the capillary forces of the strip which moves from the bottom upwards, if the throughput of metal is reduced, the dwell time of the molten metal inside the nozzle increases, thereby giving rise to the danger that this molten metal may be gradually contami
REFERENCES:
patent: 2959829 (1960-11-01), Brennan
patent: 3201275 (1965-08-01), Alplaus
patent: 4033398 (1977-07-01), Laithwaite
patent: 4596207 (1986-06-01), Witt et al.
Boswell Peter
Haour Georges
Richter Dag F.
Wagnieres Willy
Battelle (Memorial Institute)
Godici Nicholas P.
Seidel Richard K.
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