Electric contact material

Abstract: An electric contact material comprising Li and/or an oxide thereof in an amount of 0.01 to 2.0% by weight, (expressed as the amount Li), at least one rare earth element and/or an oxide thereof in an amount of 0.01 to 0.2% by weight (expressed as the amount of rare earth element), optionally one or more additional components, and the balance being substantially of Ag. The material is excellent in arc resistance, wear resistance and lubricity in a low current region and is a suitable material for slide contacts and rotary slide contacts.

Description: This invention relates to an electric contact material which is excellent in arc resistance, lubricity and abrasion resistance and whose contact resistance is low and stable when it is in use, a method for manufacturing the same and an electric contact formed of the material, and more particularly to an electric contact material suitable for slide contacts mounted on electronic/electric devices such as small-sized slide switches and micro-motors driven by low currents. In the prior art, Ag-Cu alloys containing Cu at 1 to 20% by weight, Ag-Ni alloys containing Ni at 1 to 20% by weight and the like are widely used as materials for electric contacts such as make and break contacts incorporated into relays or breakers, slide contacts incorporated into slide switches and rotary slide contacts mounted on motors. However, the above materials do not have high arc resistance and abrasion resistance and sticking resistance becomes a problem. In particular, there is a problem with Ag-Cu alloy in that its contact resistance is caused to increase and become unstable due to the Cu oxide formed on its surface while it is in use. Therefore, when a slide contact formed from Ag-Cu alloy is used as an outer peripheral contact piece of the commutator of a small-sized motor, the contact resistance varies, causing an unstable speed of rotation of the motor. On the other hand, an Ag-metal oxide alloy is known as a contact material having a high sticking resistance. For example, an Ag-manganese oxide alloy (refer to Japanese Patents Nos. 51-136170 and 52-30217), Ag-indium oxide alloy (refer to Japanese Patent No. 52-9625), Ag-zinc oxide alloy (refer to Japanese Patent No. 54-149322) and an Ag-oxide alloy in which the oxide contains lithium oxide as an essential component and additionally contains more than one of aluminium oxide, calcium oxide, magnesium oxide and silicon oxide (refer to Japanese Patent No. 58-210133) are known. The above Ag-metal oxide alloys are obtained by a method of heating an alloy of a certain composition of metal elements in an oxidizing atmosphere for a predetermined time to cause internal oxidation of the added elements other than the base material or Ag and precipitate fine oxide of the added elements along the grain boundary of Ag. The Ag-metal oxide alloy formed by the above internal oxidation method provides a material whose sticking resistance or wear resistance is improved by the effect of the fine particles of the oxide of the added elements precipitated along the grain boundary of Ag when it is used as a contact material. Slide contact material is widely used for various types of printers, cameras, VTRs in the form of a slide switch for a low current region or a rotary slide contact of a micromotor. Various materials have been proposed for the above slide contact material. For example, an Ag-Cu alloy disclosed in Japanese Patent No. 58-104139, Ag-Sb alloy disclosed in Japanese Patent No. 58-104141, Ag-Zn alloy disclosed in Japanese Patent No. 58-107441 and an Ag-In alloy disclosed in Japanese Patent No. 58-107048 are known. Although not specified as a slide contact material, an Ag alloy containing Li and a rare earth element as essential components and a material obtained by subjecting the same to an internal oxidation process are known as a contact material which has good resistance and wear resistance as is disclosed in Japanese Patent No. 54-6008. The above material has been developed mainly as a make and break contact material and is effectively used in the medium current region of approx. 1 to 100 A. Recently, various types of electronic devices described above have needed to be made yet smaller and at the same time they have needed to have higher performance and higher reliability. Further, the device are used in various environments, and for example, they may sometimes be used in an organic gas atmosphere containing a small amount of ammonia or formalin or in an atmosphere of high temperature and humidity. In order to satisfy the above conditions, the electric contact incorporated into the above devices is required to have the following characteristics. First, the electric contact incorporated in the device must be made smaller as the size of the device is reduced. At the same time, the application current becomes small and the contact pressure tends to become smaller. For example, the contact is frequently used under conditions where the current is set to 50 mA to 1 A and a pressure is set less than 10 g. Thus, when the application current becomes small and the contact pressure becomes small, abnormal contact resistance tends to occur in the contact portion, so that it becomes necessary to set the contact resistance of the contact material in the contact portion low in order to solve the above problem. Further, when the contact pressure becomes small, a small arc occurs on the contact surface during the sliding operation in a slide contact, increasing wear of the material and therefore the material is required to have a higher arc resistance. Further, as the contact is made smaller, the cross sectional area of the conductor portion becomes smaller. As a result, the total resistance of the contact becomes larger and the amount of heat generated in the contact while it is used increases, and the contact material is required to have a small resistivity. The long service life of the contact is a factor ensuring high reliability. Therefore, the contact material is required to have sufficiently high hardness to be wear resistant. Further, in the case of a rotary slide contact incorporated into a micromotor, for example, it is necessary to keep the contact resistance with time at a low and stable level in order to suppress the fluctuation of revolutions during the operation. In particular, the contact resistance thereof must be kept constant with time even when it is used for a long time in an ammonia or organic gas atmosphere, or in a high temperature and humidity atmosphere. Therefore, the contact material is strongly required to have various corrosion resistances including oxidation resistance, sulfidization resistance, ammonia resistance and organic gas resistance. In addition, recent micromotors tend to be operated at high speed, for example, at a rotation speed of 5000 to 20000 rpm. However, in order to achieve high speed operation with high stability, the slide contact incorporated in the motor must be formed from a material having a small friction coefficient and good lubricity. It is an object of the present invention to provide a material useful for slide contacts for use in the low current region. It is a further object of the present invention to provide an electric contact material which is excellent in arc resistance, lubricity and wear resistance and whose contact resistance is low and stable when in use. The present invention, in one aspect, provides an electric contact material comprising Li and/or an oxide thereof in an amount of 0.01 to 2.0% by weight (expressed as the amount Li), at least one rare earth element and/or an oxide thereof in an amount of 0.05 to less than 0.2% by weight (expressed as the amount of rare earth element), and the balance being substantially of Ag. There is also provided a slide contact comprising such a material. The electric contact material according to the invention preferably contains an additional component comprising 0.1 to 1.0% by weight of at least one element selected from In, Sn, Zn, Mn, Pd, Sb, Cu, Mg, Pb, Cd, Cr and Bi (Zn and Mn must be set at 0.1 to 0.5% by weight) and/or 0.03 to 0.06% by weight of at least one element selected from Fe, Ni and Co. In another aspect, the invention provides an electric contact material comprising lithium oxide in an amount of 0.01 to 2.0% by weight (expressed as the amount of Li), an oxide of at least one rare earth element in an amount of 0.05 to less than 0.2% by weight (expressed as the amount of the rare earth element) and Ag or Ag alloy as the remaining portion, and an electric contact manufactured from said material. In a further aspect, the invention provides a method of manufacturing an electric contact material comprising the step of heating an alloy comprising Li in an amount of 0.01 to 2.0% by weight, at least one rare earth element in an amount of 0.05 to less than 0.2% by weight, and the balance being substantially of Ag in an oxygen atmosphere so as to effect internal oxidation of the Li and rare earth element. There is also provided a method of forming an electric contact material comprising the step of heating an alloy comprising Li in an amount of 0.01 to 2.0% by weight, at least one rare earth element in an amount of 0.05 to less than 0.2% by weight, at least one element selected from In, Sn, Zn, Mn, Pd, Sb, Cu, Mg, Pb, Cd, Cr and Bi (Zn and Mn must be set at less than 0.5% by weight) in an amount of 0.1 to 1.0 by weight and/or at least one element selected from Fe, Ni and Co in an amount of 0.03 to 0.06% by weight with the balance substantially of Ag in an oxygen atmosphere so as to effect internal oxidation of the Li and rare earth element. The electric contact material of the first aspect of this invention is an Ag alloy containing Ag as a base material and Li and at least one rare earth element. At least one of La, Ce, Pr, Nd, Sm, Sc, Y, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu may be used as the rare earth element. La and Ce among these are particularly preferable. The Li and rare earth elements enhance the hardness of a prepared Ag alloy, increasing the wear resistance thereof and decreasing the friction coefficient to enhance the lubricity and consequently enhance the arc resistance, thereby reducing the amount of wear when the alloy is used as a contact. In this case, if the amount of Li is less than 0.01% by weight and the amount of the rare earth element is less than 0.01% by weight, the above effect is not sufficient, and if the amount of Li is greater than 2.0% by weight or the amount of the rare earth element is greater than 0.2% by weight, the specific resistance of an Ag alloy obtained increases and variation in the contact resistance with time becomes larger, thereby lowering the characteristic thereof when used as a contact material, and particularly as the material of a small-sized slide contact used in a small current region. The preferred amount of Li lies in the range of 0.01 to 0.1% by weight, more preferably in the range of 0.02 to 0.1% by weight. The preferred amount of rare earth element lies in the range of 0.02 to 0.2%, more preferably 0.02 to 0.19% by weight. If at least one of In, Sn, Zn, Mn, Pd, Sb, Cu, Mg, Pb, Cd, Cr and Bi is additionally incorporated into the Ag alloy of the above composition, the lubricity and hardness of the alloy are further enhanced, thus making it possible to enhance the wear resistance. In this case, if the amount of added element or elements is less than 0.1% by weight, the above effect is insufficient; if the added amount is more than 1.0% by weight, the specific resistance of the alloy increases and variation in the contact resistance becomes large. The preferred amount lies in the range of 0.1 to 0.5% by weight, more preferably 0.1 to 0.3% by weight. When Zn or Mn among the above elements is added, the amount used should be less than 0.5% by weight. This is because the specific resistance will increase and variation in the contact resistance will become larger if it is added in more than 0.5% by weight. Furthermore, if at least one of Fe, Ni and Co is added to the Ag alloy, crystal grains in the Ag alloy obtained become smaller, thereby enhancing the wear resistance of the alloy. If the amount added is less than 0.03% by weight, the above effect cannot be sufficiently obtained, and if the amount added is greater than 0.6% by weight, segregation occurs at the time of preparation of the alloy by melting the same, increasing wear of the Ag alloy obtained due to the sliding operation thereof, and thus it is not preferred. The preferred amount lies in the range of 0.03 to 0.2% by weight, and more preferably 0.03 to 0.1% by weight. The element in the group of In, Sn, Zn, Mn, Pd, Sb, Cu, Mg, Pb, Cd, Cr, Bi and the element in the group of Fe, Ni, Co may be added separately but may also be added simultaneously. The electric contact material of this invention may be prepared by mixing a determined amount of each metal element and melting/casting the same in a high-frequency melting furnace, for example. In the case where an electric contact is manufactured from the above material, the casting of the material is subjected to a mechanical face cutting and then cold-rolled, for example, to work the same into a desired contact shape. At this time, the above contact material may be integrally formed with a base material formed of Cu or Cu alloy or Fe or Fe alloy by cladding or caulking the same in a rivet form on the entire or partial surface portion of the base material. An electric contact material comprising oxides of Li and rare earth element respectively is obtained by heating the above-described Ag alloy in an oxygen atmosphere such as atmospheric air to subject the Li and rare earth element contained therein to internal oxidation. In the above material, fine lithium oxide and oxide of rare earth element are precipitated and uniformly distributed in the base metal of Ag or Ag alloy which contains at least one element included in the group of In Sn, Zn, Mn, Pd, Sb, Cu, Mg, Pb, Cd, Cr, Bi and/or Fe, Ni, Co so that the hardness and wear resistance can be enhanced and consequently the amount of wear thereof can be reduced when it is used as a make and break contact or slide contact in comparison with an Ag alloy which is not subjected to the internal oxidation process. In this case, the amount of oxide of Li is controlled to be within the range of 0.01 to 2.0% by weight in terms of the amount of Li and the amount of oxide of rare earth element is controlled to be within the range of 0.05 to less than 0.2% by weight in terms of the amount of the rare earth element. The condition for the internal oxidation is preferably set to such a condition that Ag of the base material, the group of In and the like and the group of Fe and the like, will not cause oxidation, that is, the Li and rare earth element in the Ag alloy can be selectively oxidized. Such a condition is determined according to the amount of Li and rare earth element, the concentration of oxygen in an oxygen atmosphere, the temperature during the oxidation process, the length of time of the process and the like. For example, when the oxygen atmosphere is atmospheric air and if the amounts of Li and rare earth element are set within the above ranges, the processing temperature is preferably in the range of 200 to 800°C and the processing time is preferably in the range of 10 seconds to 2 hours, depending on the thickness of the Ag alloy. EXAMPLE Each Ag alloy of the compositions shown in the Table 1 is cast using a high-frequency melting furnace to make samples. In this case, the condition for the internal oxidation process in the table is that the atmosphere is atmospheric air, the temperature is at 400 °C and the processing time is 1 hour. For the respective samples, the area of the wearing portion and the contact resistance were measured by the fine movement frictional contact resistance test (Fretting test), the coefficient of dynamic friction was measured by using a Bowden type abrasion tester, and the contact resistance was measured before and after the samples were treated in the hot air and atmosphere under constant temperature/constant humidity conditions, as follows: Fretting test: Head : a rod formed of Ag-50% Pd and having a head portion with a radius of 1 mm Load : 5g Current : 0.1A, 1.0A Slide distance : 0.1 mm Number of times caused to slide : 200000 Sliding speed : 100 Hz. When the head had been caused to slide 200000 times, the contact resistance (mΩ) of each sample was measured by conducting currents of 0.1A and 1.0A and the area of the frictional portion thereof was measured by conducting a current of 1.0A. Coefficient of dynamic friction: Head : a rod formed of Ag-50% Pd and having a head portion with a radius of 1 mm Sliding distance : 10 mm Number of times caused to slide: 100 Sliding speed : 100 mm/min. When the head was caused to slide 100 times, the coefficient of dynamic friction (µK) was measured. Atmospheric heating test and temperature and humidity test: In the case of the air heating test, a test piece was heated in atmospheric air of 150 °C for 100 hours, and a load of 5 g and an current of 0.1 A were applied before and after the test and the contact resistance (mΩ) was measured. In the case of the temperature and humidity test, a test piece was left in an atmosphere at a temperature of 50 °C and relative humidity of 95% for 100 hours, and a load of 5g and a current of 0.1 A were applied before and after the above operation and the contact resistance (mΩ) was measured. The measurement results are shown in Table 2.

Claims: An electric contact material comprising Li and/or an oxide thereof in an amount of 0.01 to 2.0% by weight, (expressed as the amount Li), at least one rare earth element and/or an oxide thereof in an amount of 0.05 to less than 0.2% by weight (expressed as the amount of rare earth element), and the balance being substantially of Ag. An electric contact material as claimed in claim 1, wherein the amount of Li and/or oxide is 0.01 to 0.1% by weight. An electric contact material as claimed in any one of the preceding claims, wherein said rare earth element is at least one element selected from a lanthanide group consisting of La, Ce, Pr, Nd and Sm. An electric contact material as claimed in any one of the preceding claims, which is composited with a base material of Cu, Cu alloy, Fe or Fe alloy. An electric contact material as claimed in any one of the preceding claims, which contains an additional component comprising at least one element selected from In, Sn, Zn, Mn, Pd, Sb, Cu, Mg, Pb, Cd, Cr and Bi in an amount of 0.1 to 1.0% by weight, with the proviso that the amount of Mn or Zn is 0.1 to 0.5% by weight. An electric contact material as claimed in any one of the preceding claims, which contains an additional component comprising at least one element selected from Fe, Ni and Co in an amount of 0.03 to 0.6% by weight. An electric contact material as claimed in any one of claims 1 to 4, which contains additional components comprising at least one element selected from In, Sn, Zn, Mn, Pd, Sb, Cu, Mg, Pb, Cd, Cr and Bi in an amount of 0.1 to 1.0% by weight, with the proviso that the amount of Mn or Zn is 0.1 to 0.5% by weight, and at least one element selected from Fe, Ni and Co in an amount of 0.03 to 0.6% by weight. An electric contact material as claimed in any one of the preceding claims, wherein said rare earth element is selected from La and Ce, and said one or more additional components comprise at least one element selected from In, Sn, Zn, Mn, Pd, Sb, Cu, Mg, Pd, Cd, Cr and Bi in an amount of 0.1 to 1.0% by weight with the proviso that the amount of Mn or Zn is 0.1 to 0.5% by weight, and at least one element selected from Fe, Ni and Co in an amount of 0.03 to 0.6% by weight. A slide contact comprising an electric contact material as claimed in any one of the preceding claims. A method of manufacturing an electric contact material comprising the step of heating an alloy comprising Li in an amount of 0.01 to 2.0% by weight, at least one rare earth element in an amount of 0.05 to less than 0.2% by weight, and the balance being substantially of Ag in an oxygen atmosphere so as to effect internal oxidation of the Li and rare earth element.

Inventors: MATSUDA AKIRA, SHIBATA NOBUYUKI, SHIRAKAWA RYOTOMO, SUZUKI SATOSHI, MATSUDA, AKIRA, SHIBATA, NOBUYUKI, SHIRAKAWA, RYOTOMO, SUZUKI, SATOSHI
Applicants: FURUKAWA ELECTRIC CO LTD, MABUCHI MOTOR CO, MABUCHI MOTOR KABUSHIKI KAISHA, THE FURUKAWA ELECTRIC CO., LTD.
Application year: 1990
Main classification: H01H1
IPC codes: C22C5, H01H1, H01R39, C22C1, H01H11