Internal-oxidation method for production of electrical contact materials

Abstract: Electrical contact materials are made by the internal-oxidation of Ag ialloys containing from more than 4.5 weight % to 12 weight % Sn, from 0.05 to 0.1 wt. % of Zr and a balance of Ag, the internal oxidation being conducted at an elevated oxygen atmospheric pressure, i.e. from more than 10 to 200 atm.

Description: This invention relates to a method for making electrical contact materials by the internal-oxidation of silver-solute metals alloys, whereby the solute metals in the silver matrix are converted to metal oxides which afford the alloys a high refractoriness. And, this invention also relates to such electrical contact materials, matrices of which are silver and which are produced in accordance with this invention method. Silver-tin oxides alloys, matrices of which are silver and solute elements of which are tin which are in turn internal-oxidized to tin oxides, are widely employed today as electrical contact materials. Similarly, silver-cadmium oxides alloys are known as efficient electrical contact materials, though compared to the silver-tin oxides alloys, they are less employed, because cadmium is harmful to health and becuase of the prevention of pollution. However, there is a problem for preparing silver-tin oxides alloys by means of internal-oxidation. That is, when a silver matrix of an alloy contains more than about 4.5 weight % of tin, the entire amount of such tin can hardly be internal-oxidized by oxygen which penetrates from the outside of the silver matrix and into the matrix. In order to solve this problem, it becomes necessary to add to silver-tin alloys such auxiliary solute metals which have faster diffusion velocities and according are capable to carry and convey oxygen towards the inside of silver matrices fast. A typical example of such auxiliary solute metals is indium. In U.S. Patent No. 3,933,485, there is described that silver-tin-indium system alloys are internal-oxidized to abtain excellent electrical contact materials. In this U.S. Patent, electrical contact materials which are one of the best electrical contact materials industrially used today, are made by internal-oxidizing silver alloys containing 5-10 weight % of tin and 1.0-6 weight % of indium. However, even when indium which acts well as the above-mentioned auxiliary solute metal, is added, it is not easy to internal-oxidize tin of more than 4.5% most evenly in a silver matrix. Sometimes, tin oxides tend to segregate excessively about surface areas of the silver matrix, while they are depleted at inner areas. In addition to such drawback, indium oxides are comparatively weak. Hence, it is desireable to internal-oxidize tin in a silver matrices without employing indium, if possible. Brief Summary of the Invention In view of the above-mentioned background, the present inventor has sought for a method of the internal-oxidation of silver-tin alloys completely and evenly without employing any auxiliary solute metal element such as indium or at least with the employment of a minimum trace amount of the auxiliary solute metal element. He perceived Zr. Zr has very small solid-solubility with silver. Accordingly, when molten alloys of Ag-Sn-Zr are cooled to a solid phase, almost entire amounts of Zr contained in the alloys are precipitated dispersedly and evenly in the solid phase of Ag-Sn alloy structures. At a temperature between 500-700°C for the internal-oxidation, Zr atoms thus dispersedly precipitated remain in silver matrices as they were precipitated or as inter-metallic compounds produced with Sn at sites where they were precipitated, and act as nuclei for the oxidation and precipitation of Sn. Therefore, in this invention, as an auxiliary solute metal element which assists the internal-oxidation of Sn in Ag matrices, Zr is employed at an amount as much as small or at a nominal amount. The employment of Zr is advantageous also in respect of the fact that their oxides are not so weak, and zirconium oxides have a higher refractoriness than tin oxides. In connection with the employment of Zr for the internal-oxidation of Ag-Sn system alloys, it shall be noted that an example of the internal-oxidation of Ag-Sn 6%-Zr 0.5% are given in Japanese Preliminary Patent Publication No. 51-121795. In spite of such example, it has been found by the present inventor that when Zr is employed at a trace amount such as 0.05-0.1 weight % as employed in this invention, it is impossible to internal-oxidize Sn of an amount of more than 4.5% completely and evenly all over the silver matrices as Sn oxides which are precipitated uniformally as fine particles. Therefore, the present inventor has conducted a large number of various experiments, and discovered that even with Zr of such trace amount as 0.05-0.1 weight %, Ag-Sn alloys which contain Sn of more than 4.5% to 12%, an amount near to the upper limit of solid-solubility of Sn to Ag, can be internal-oxidized completely, if an oxygen atmosphere for the internal-oxidation is made remarkably high such as more than 10 atm. That is, in this invention, a method of the complete internal-oxidation of Ag alloys which is consisted of Sn of more than 4.5 weight % to 12 weight % and a balance of Ag, is provided in which Zr is added to the alloys at a trace amount of 0.05 weight % to 0.1 weight %, and the alloys are internal-oxidized at an oxygen atmosphere of more than 10 atm to 200 atm (1·10⁶-20·10⁶ Pa). This invention is also to provide electrical contact materials obtained by this method. As described above, while in the method of this invention, the successful and complete internal-oxidation of Ag-Sn alloys of the above-mentioned specific constituents can be made, if the internal-oxidation is conducted with an oxygen atmosphere of more than 10 atm, it is not desireable in respect of costs and safety to employ a very extremely high oxygen atmosphere at an industrial scale. In this view and in order to prevent Ag from becoming liquid (at about 400 atm) in the course of internal-oxidation, 200 atm of oxygen which can commercially be obtained, is selected as a maximum atmospheric pressure employable in this invention. Preferred Embdiments This invention is explained in a further concrete manner in the following examples. (1) Ag-Sn 6 weight %-Zr 0.2 weight % In order to confirm the present inventor's above-mentioned findings that Ag alloys containing Sn of 4.5 weight % and over, can not successfully be internal-oxidized by the mere addition of Zr of a trace amount, the above constituent (1) was melted and made to an ingot of 120mm in diameter and 40mm in length. The ingot was hot-extruded into squre bars of 30mm in thickness and 50mm in width. The bars were then cut to a length of 500mm each, and their upper and lower surfaces were shaved by a thickness of 3mm each to obtain square bars of 24mm in thickness, 510mm in width, and 500mm in length. To each lower surface of the square bars, there were bounded pure silver of 2.5mm in thickness. They were rolled by pressure so that they had thickness of 1.2mm. By punching them by a punch having a cutting hole of 6mm in diameter, disk-shaped contact materials backed by the pure silver and having 6mm diameter and 1.2mm thickness were obtained. They were internal-oxidized by heating them to 620°C for 24 hours in an oxygen atmosphere of 7 atm. Vertically cut-out sections of the contacts (1) thus obtained were observed by a microscope. It was observed that subscales made from segregations of metal oxides about surfaces prevented oxygen to penetrate inwardly and, consequently prevented the oxidation of the alloys at inner deeper areas. (2) Ag-Sn 6 weight %-Zr 0.05 weight % (3) Ag-Sn 8 weight %-Zr 0.1 weight %-Ni 0.2 weight % Alloys of the above constituents (2) and (3) of this invention were made, similarly to the above (1), into disk-shaped contacts backed with pure silver, of 6mm in diameter and 1.2mm in thickness. They were internal-oxidized, similarly to the above (1), however that in this instance, the oxygen atmosphere was made to 30 atm. Microscopic observation of the contacts thus internal-oxidized, along their vertically cut sections showed that they had structures evenly dispersed with metal oxides over and through their entire areas, similarly to the undermentioned (4) contacts, and that internal oxidation was completely made. (4) Ag-Sn 0.5 height %-Cd 13 weight % For the sake of comparison, this conventional alloy (4) was prepared. Though this contains harmful Cd, it is known as it has fine and uniform internal oxidized structures, and in fact, it is one of the excellent today's electrical contact materials having good electrical properties. This alloy (4) was internal-oxidized similarly to the alloys (1), that is, in a normal oxygen atmosphere. Hardness (HRF) and electrical conductivity (IACS%) of the above internal-oxidized contact materials (2) and (3), and (4) were as follows. Welding times by anti-welding tests (conducted under electric voltage of DC 240V, initial electric current (discharge current from a condensor electric current) of 700A, contact pressure of 200g, and test cycles of 20) were as follows. (2) 0 (3) 0 (4) 5 Amounts of consumption (mg) by ASTM test method (by electric voltage of AC 200V, electric current of 50A, contact pressure of 400g, and releasing force of 600g) were as follows. (2) 12 (3) 11 (4) 20 As readily known from the above, this invention can provide electrical contact materials which are made by the internal-oxidation of Ag-Sn system alloys, provided with excellent electrical performance as electrical contacts on account of extremely fine metal oxides evenly and uniformally dispered in Ag matrices. In this invention, as seen in the above example of alloy (3), one or more elements of iron family elements (Ni, Co, and Fe) may be added to alloys, solely for account of further fining alloy structures at the expense of the balance element Ag. But, this is neither intended for the acceleration nor assistance of internal-oxidation by such elements.

Claims: A method of internal oxidation of a silver alloy to obtain an electrical contact material, in which method the alloy consists of from more than 4.5 weight % to 12 weight % Sn, from 0.05 to 0.1 weight % of Zr, and the balance Ag; and the alloy is internally oxidised in an oxygen atmosphere of from more than 10 to 200 atm (1·10⁶-20·10⁶ Pa). A method as claimed in claim 1, in which the alloy also contain one or more of the elements Ni, Co and Fe, in an amount of 0.001-1 weight % at the expense of the balance element Ag. An electrical contact material made of an internally oxidized alloy obtained by a process as claimed in claim 1 or claim 2.

Inventors: YIDA MASAHARU C O CHUGAI DENKI, YIDA, MASAHARU C/O CHUGAI DENKI KOGYO K.K.
Applicants: CHUGAI ELECTRIC IND CO LTD, CHUGAI DENKI KOGYO KABUSHIKI KAISHA
Application year: 1990
Main classification: H01H1
IPC codes: H01H11, H01H1, C22C5, C22C1, C22C32