Hitherto, in order to give special properties such as wear resistance or corrosion resistance on the surface of a metal base material, it is often carried out to form a metal coating layer having excellent those properties on the surface of the base material.
In many cases, the metal coating is formed in the form of layer on the base material by applying a flexible metal powder molding comprising a metal powder, a binder, a plasticizer and a solvent to the metal base material and thereafter sintering it, whereby the sintered metal coating layer is metallurgically bonded to the base material. In a rare case, a metal strip for covering is prepared previously by sintering the same metal powder molding as described above, and such is bonded to the metal base material by a physical means such as screws or an adhesive, etc. to form the metal coating.
In the case that a metal powder molding is metallurgically bonded to the surface of a metal base material by sintering after adhering the molding to the surface of the above-described base material, it is necessary that the metal powder molding has good flexibility which is capable of transforming so as to fit on a curved face of the base material without causing troubles of obstructing practicability such as generation of cracks, etc. Further, in the case of obtaining a metal strip for covering which is previously sintered in a prescribed shape, it is necessary that the metal powder molding prior to sintering has flexibility which is capable of transforming by processing in the prescribed shape without causing the same troubles as described above.
In addition, this kind of metal powder molding is required to have not only good flexibility which is excellent in deformation property such as the above-described fitting deformation property or processing deformation property, but also sufficient strength for handling. Moreover, the metal powder molding is required to form a coating layer having a high density by sintering without causing contraction which is not admitted, or openings which are called cavities.
Hitherto, as this knind of moldings, compositions comprising resins such as polyvinyl ether, or methyl cellulose resin as a main component, to which a volatile plasticizer such as dibutyl phthalate, glycerine or wax, etc. is added as an agent to improve the flexibility of moldings, have been described in, for example, Japanese Patent Application (OPI) Nos. 91910/74 and 35703/81 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), Japanese Patent Publication No. 11167/70, etc.
However, the above-described plasticizers rapdily volatilze in the sintering process, because they have generally a low boiling point as compared with the abovedescribed main component. Therefore, they generate gas in an early stage of the sintering process and there is the possibility of causing numberless openings on the coating layer after sintering. Accordingly, it is necessary to reduce a temperature-raising rate during sintering as slow as possible.
Moreover, use of plasticizers have not only problems that they deteriorate strength of moldings and easily form cracks in handling, but also a problem that the coating layer after sintering is difficult to have a high density.