Electrical contact material

Abstract: A method for forming an electrical contact material comprises the steps of melting a mixture of Cu-Cr into a molten alloy, atomizing the molten alloy into fine particles to obtain alloyed particles. Cr particles in the alloyed powder disintegrate to less than 5 µ m in mean particle diameter. The alloyed powder is sintered thereafter and a mean particle diameter of chromium in the sintered article is fined in a range of 2 to 20 µ m. An electrical contact material is composed of a copper matrix and chromium particles having a mean particle diameter of 2 to 20 µ m. The chromium particles are homogeneously dispersed in the copper matrix.

Description: The present invention relates generally to an electrical contact material formed of sintered alloy powder having alloyed elements of a first metal and a second metal comprising a matrix of the first metal in which particles of the second metal are homogeneously dispersed. Furthermore, the invention relates to a method for producing such an material. A variety of such materials consisting of a matrix of a first metal having particles of a second metal dispersed therein is known from DE-OS 38 10 218. These contact materials are formed by preparing a mixture of the two metals, melting said mixture into a molten alloy, atomizing said molten alloy to obtain an alloy powder and finally sintering said alloy powder, and the particles of the second metal which are dispersed in the matrix metal have a mean particle diameter of 0,01 µm to 1 µm. The problem involved with the known electrical contact materials consists in that they are sometimes considered not to have a sufficient electrical conductivity and low contact resistance as well as sufficient arc-proof and welding-proof characteristics, which are the essential characteristics for an electrical contact material to be used for breakers or switches such as a vacuum interrupter. It is therefore the object of the invention to provide an electrical contact material having a good electrical conductivity, a low contact resistance as well as good arc-proof and welding-proof characteristics and a method for forming such an electrical contact material. This object is essentially solved in that in the electrical contact material described in the preamble of claim 1 the first material is copper and the second material is chromium and the chromium particles have a mean particle diameter of 2 to 20 µm. It has been found that an electrical contact material with an excellent electrical conductivity and a low contact resistance having good arc-proof and welding-proof characteristics can be obtained by choosing the material combination of copper and chromium and by further choosing a particle size for the chromium content in the range of 2 to 20 µm. The method for forming such an electrical conduct material is characterized in that the first metal copper is used and as the second metal chromium is used, melting of the mixture of copper and chromium is accomplished in an atmosphere of inert gas to reduce an oxygen content of said mixture to a level of less than 1000 ppm, the molten alloy of copper and chromium is atomized into an alloy powder in which the mean diameter of chromium is less than or equal to 5 µm and the alloy powder is sintered into a matrix of copper including chromium particles having a level of 2 to 20 µm, while maintaining homogenous dispersion thereof in said sintered matrix. The atomizing may be accomplished by gas atomization. The gas may be inert gas selected from the group consisting of argon and nitrogen. Alternatively, the atomizing can be accomplished by water atomization. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiments of the invention. However, the drawings are not intended to imply limitation of the invention to a specific embodiment, but are for explanation and understanding only. In the drawings: Fig. 1(a) and 1(b) are microphotographs showing metallic structure of Cu-Cr alloys of the present invention; Fig. 2 is a graph showing relationships between Cr amount and both of contact resistance ratio and weld resist current; Fig. 3 is a microphotograph showing the metallic structure of an electrical contact material formed of Cu-10wt%Cr according to the present invention; Fig. 4 is a graph showing a relationship between mean Cr particle diameter and a breaking-current of the alloys; Fig. 5 is a graph showing a relationship between mean Cr particle diameter and contact resistance of the alloys; Fig. 6 is a graph showing a relationship between mean Cr particle diameter and welding force of the alloys; Fig. 7 is a graph showing a relationship between mean Cr particle diameter and a thickness of a molten layer; Fig. 8 is a graph showing a relationship between mean Cr particle diameter and an increase rate of contact resistance after current breaking. DESCRIPTION OF THE PREFERRED EMBODIMENT According to the aspect of the present invention, an atomization technique is utilized for disintegrating mixture of alloy elements into fine alloyed powder in place of using a mechanical milling technique. Mixture of Cu-Cr is melted to obtain a molten alloy. The obtained molten alloy is disintegrated into fine particles by atomization with rapidly solidifying. Cr content included in the mixture is determined so as to be dispersed in a Cu matrix at a boundary area that the Cu-Cr alloy is separated into a Cu phase and a Cr phase in the process of melting. From conventional phase diagram of Cu-Cr alloy, it is clear that if the Cr content exceed 37 wt%, the molten alloy is composed of a Cu matrix in which Cr dispersed and a Cr matrix in which Cu dispersed, particularly, if the Cr content exceeds 93 wt%, Cu dispersed in a Cr matrix. Therefore, the Cr content is determined less than or equal to 37 wt%, more preferable, determined in the range of 0.1 to 37 wt%. The mixture of Cu-Cr is prepared from Cu and Cr having low oxygen content therein to reduce oxygen content in the molten alloy. Furthermore, in order to further reduce oxygen content in the molten alloy, the mixture is deoxidized by melting in atmosphere of inert gas, such as Ar, or melting in vacuum. Thus, oxygen content in the molten alloy is reduced to less than 1000 ppm. Contamination by inevitable impurities, such as Fe or Ni, is allowable. For atomization of the molten alloy, gas atomization under high pressure using inert gas, such as Ar or N 2 , or water atomization are suitable for disintegrating the molten alloy into fine particle. EXAMPLE 1 Alloyed powder was prepared by the aforementioned gas atomization. A mixture of Cu-Cr was melted in atmosphere of argon gas or in a vacuum to obtain a molten alloy. Then, the molten alloy was atomized using argon gas under the pressure of 60 kgf/cm 2 (5.89 MPa) or 70 kgf/cm 2 (6.87 MPa). Table 1 indicates the obtained alloyed powder having various components when the Cr : Cu ratio, and melting conditions, i.e., atmosphere and temperature were varied. As shown in the Table 1 , particle sizes of the obtained Cu-Cr powder are all less than 150 µ m. Fine particles of Cr are distributed uniformly in the Cu matrix as shown in Figs. 1(a) and 1(b) . The mean particle sizes of Cr in the alloyed powder are all less than 5 µ m. Initial Cu-Cr weight ratio of the mixture is maintained in the obtained alloyed powder. Oxygen content in the powder can be reduced to less than 1000 ppm. The obtained alloyed powder was sintered to obtain an electrical contact material (the article, hereinafter) having desired characteristics. Fig. 2 shows relationships between Cr content and both of contact resistance ratio and welding resist current as compared to conventional articles. It is clear from Fig. 2 , that an adaptable range of the Cr content of the article is limited in 5 to 20 wt%. EXAMPLE 2 Cu-20wt%Cr atomized powder, having a maximum particle size of less than 150 µ m, with a mean Cr particle size of 3.5 µ m, was put into a ceramic housing having a diameter of 68 mm. Then the alloy powder was sintered at 1100 °C for 30 min. under vacuum condition. The obtained Cu-20wt%Cr article shows homogeneous Cr distribution as shown in Fig. 3, with a mean Cr particle size of 10 µ m. Cu-10wt%Cr atomized powder and Cu-5wt%Cr atomized powder were sintered similarly as the aforementioned, then articles having 55 mm of diameter were formed. Cr distribution in both articles are homogeneous. Distribution width of Cr could be narrowed, and mean Cr particle size is 10 µ m. EXAMPLE 3 Cu-20wt%Cr atomized powder, having less than 150 µ m of particle size, was canned in a metal housing having 62 mm of inner diameter. Then the alloy powder was compacted by hot isostatic pressing (HIP) at 1000 °C for 1 hour under the pressure of about 2000 kgf/cm 2 using argon gas. After compacting, the alloy was sintered. The obtained article had a 55 mm diameter. Mean particle diameter of Cr in the article is in the range of 2 to 5 µ m. Particle diameter was not significantly enlarged compared to the alloyed powder. Cu-10wt%Cr atomized powder and Cu-5wt%Cr atomized powder were compacted and sintered similarly to the aforementioned to form articles, respectively. Cr distribution in the both of articles can be also narrowed, and homogeneous Cu- Cr composition is established in both. Thus, an electrical contact material having homogeneous distribution of fine Cr particles of which mean particle diameter is less than 10 µ m can be obtained by the methods of both of EXAMPLES 2 and 3 . Figs. 4 to 8 indicate characteristics comparisons of the electrical contact material of the present invention against that of conventionally utilized material. Referring now to Fig. 4 , which shows a relationship between mean particle diameter of Cr and breaking current of Cu-5wt%Cr, Cu-10wt%Cr, and Cu-20wt%Cr, the breaking ability of an article can be raised corresponding minimization of Cr diameter. This is caused by homogeneous distribution of Cr particles allowing an arc generated by a current to be dispersed smoothly. From the results shown in Fig. 4 , 5 to 20 wt% of Cr with less than or equal to 20 µ m particle diameter is preferable. Referring now to Fig. 5 , which shows a relationship between mean Cr particle diameter and contact resistance against the same articles of Fig. 4 , contact resistance can be reduced according to minimization of Cr diameter. However, when Cr particle diameter is less than 10 µ m, hardness of the article is raised. Therefore, contact resistance tends to be increased at less than 10 µ m of Cr particle diameter. Fig. 6 shows a relationship between mean Cr particle diameter and welding force. Welding force is the force necessary for separating materials after supplying desired amount of current for desired duration under pressure of 50 kgf (about 490N). From the results shown in Fig. 6 , welding force can be also reduced according to minimization of Cr diameter, as a result of reduction of the contact resistance. However, when Cr particle diameter is less than 10 µ m, the contact resistance is increased as shown in Fig. 5 , therefore, welding force can be also increased. Fig. 7 shows a relationship between mean Cr particle diameter and maximum thickness of the molten layer of the article surface after current breaking. When large mount of current is broke, the surface of the article is partially melted by the arc generated in the process of charging. The molten layer is rapidly cooled after arc annihilation, thus fine dispersion layer of Cu-Cr having rich Cr is formed on the article surface. The dispersion layer indicates good voltage withstandance, but has high resistance. Therefore, contact resistance is raised after large-current breaking. accordingly, it is preferred that the molten layer is formed thin, widely spread, and uniformly. From the results shown in Fig. 7 , the molten layer can be homogenized and thinned according to minimization of Cr diameter. Thus, increasing rate of contact resistance after current breaking can be reduced by minimization of Cr diameter. However, when Cr diameter becomes less than 10 µ m, hardness of the article increases, therefore, contact resistance is increased. Accordingly, Cr having a mean particle diameter of 2 to 20 µm which is uniformly dispersed in a Cu matrix is the most preferred composition of material for an electrical contact point. In order to obtain this composition, mean particle diameter of less than or equal to 5 µm of Cr must be selected for sintering after atomization of Cu-Cr. According to the present invention, 2 to 20 µ m of mean Cr particle diameter can be obtained because Cr particles in the alloyed powder are disintegrated to less than or equal to 5 µ m by atomizing the alloy mixture. Therefore, Cr in the obtained article can be dispersed uniformly, so breaking-current can be raised and contact resistance can be reduced, compared to electrical contact material formed by conventional powder metallurgy. Thus, the article obtained according to the method of the present invention shows excellent characteristics as electrical contact material.

Claims: An electrical contact material formed a sintered alloyed powder having alloyed elements of a first metal and a second metal comprising: a matrix of the first metal in which particles of the second metal are homogeneously dispersed, characterized in that the first metal is copper and the second metal is chromium, and the chromium particles have a mean particle diameter of 2 to 20 µm. An electrical contact material as set forth in claim 1, wherein the content of said chromium particles included in said copper matrix is in the range of 5 to 20 wt%. An electrical contact material as set forth in claim 1, wherein the content of said alloy element of chromium is in the range of 0.1 to 37 wt%. An electrical contact material as set forth in claim 1, wherein said alloy powder includes less than or equal to 5 µm of chromium homogeneously dispersed therethrough, and atomized particles having a mean particle diameter of less than or equal to 150 µm. A method for forming an electrical contact material comprising the steps of: preparing a mixture of a first metal and a second metal, melting said mixture into a molten alloy, atomizing said molten alloy to obtain an alloy powder in which the second metal is homogeneously dispersed and which the second metal is homogeneously dispersed and sintering said alloy powder, characterized in that as the first metal copper is used and as the second metal chromium is used, melting of the mixture of copper and chromium is accomplished in an atmosphere of inert gas to reduce an oxygen content of said mixture to a level of less than 1000 ppm, the molten alloy of copper and chromium is atomized into an alloy powder in which the mean diameter of chromium is less than or equal to 5 µm and the alloy powder is sintered into a matrix of copper including chromium particles having a level of 2 to 20 µm, while maintaining homogenous dispersion thereof in said sintered matrix. A method as set forth in claim 5, wherein said melting step is accomplished in an inert gas selected from the group consisting of argon and nitrogen. A method as set forth in claim 6, wherein said melting step is accomplished in a vacuum. A method as set forth in claim 6, wherein said atomizing is accomplished by gas atomizing, preferably using an inert gas selected from the group consisting of argon and nitrogen, or is accomplished by water-atomization. A method as set forth in one of the claims 6 to 8, wherein said mixture includes 0.1 to 37 wt% of chromium and / or a mean particle diameter of said alloyed powder is less than or equal to 150 µm.

Inventors: FUKAI TOSHIMASA, KOSHIRO KOICHI, NODA YASUSHI, SUZUKI NOBUTAKA, YOSHIHARA TETSUO, YOSHIOKA NOBUYUKI, FUKAI, TOSHIMASA, KOSHIRO, KOICHI, NODA, YASUSHI, SUZUKI, NOBUTAKA, YOSHIHARA, TETSUO, YOSHIOKA, NOBUYUKI, KOSHIRO, KOICHI C/O SUMITOMO METAL IND. LTD., NODA, YASUSHI CITYPAL-SHINSAWADA 402, 43, YOSHIHARA, TETSUO C/O SUMITOMO METAL IND. LTD.
Applicants: MEIDENSHA ELECTRIC MFG CO LTD, KABUSHIKI KAISHA MEIDENSHA
Application year: 1991
Main classification: H01H1
IPC codes: H01H1, H01H11