Abstract:
A system including a contact tip that includes an arcing surface, a base surface, and a graded structure is presented. The graded structure includes a first region comprising a first surface proximate to the arcing surface, a second region comprising a second surface proximate to the base surface, and an intermediate region disposed between the first region and the second region. A concentration of silver in the graded structure decreases from the first surface to the second surface. A method of forming a contact tip includes preparing starting materials for a first region, an intermediate region, and a second region of the contact tip. The starting materials of the first, intermediate, and second regions are sequentially added to a container to form a graded blend of starting materials. The graded blend of starting materials are compacted and heat-treated to form a contact tip having a graded structure.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to a contact arm assembly having an electrical contact in an electrical circuit breaker. 
         [0002]    Contacts and contact arm assemblies are well known in the art of circuit breakers. Contact arm assemblies having electrical contacts for making and breaking an electrical current are not only employed in electrical circuit breakers, but also in other electrical devices, such as rotary double break circuit breakers, contactors, relays, switches, and disconnects. The applications that these electrical devices are used in are vast and include, but are not limited to, the utility, industrial, commercial, residential, and automotive industries. 
         [0003]    The primary function of a contact arm assembly is to provide a carrier for an electrical contact that is capable of being actuated in order to separate the contact from a second contact and contact arm arrangement, thereby enabling the making and breaking of an electrical current in an electric circuit. Electrical contacts suitable for the noted applications typically include silver. 
         [0004]    The contact is generally bonded to the contact arm, which is typically, but not necessarily, a copper alloy, in such a manner that the assembly tolerates the thermal, electrical and mechanical stresses and will not disassemble during operation of the host device. Predominantly the contact failure occurs due to wear and tear. Factors that normally affect contact and trigger wear and tear are configuration or geometry of contact (different layer/thickness), materials choice, and processing (brazing/welding) that creates voids at the interface. Hence there is a need for improved assembly of the contacts with high interfacial quality. The system and method presented herein are directed towards addressing this need. 
       BRIEF DESCRIPTION 
       [0005]    In one embodiment, a system is presented. The system includes a contact tip that includes an arcing surface, a base surface, and a graded structure between the arcing surface and the base surface. The graded structure includes a first region comprising a first surface proximate to the arcing surface, a second region comprising a second surface proximate to the base surface, and an intermediate region disposed between the first region and the second region. Further, a concentration of silver in the graded structure decreases from the first surface to the second surface. 
         [0006]    In one embodiment, a method of forming a contact tip is presented. The method includes preparing starting materials for a first region, an intermediate region, and a second region of the contact tip. The starting materials of the first, intermediate, and second regions are sequentially added to a container to form a graded blend of starting materials. The graded blend of starting materials are compacted and heat-treated to form a contact tip having a graded structure. The graded structure has a concentration of silver decreasing from the first region to the second region. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0007]      FIG. 1  is a schematic diagram of a system including a contact tip, in accordance with one embodiment of the invention; 
           [0008]      FIG. 2  is a schematic diagram of a system including one distinctly graded structure, in accordance with one embodiment of the invention; 
           [0009]      FIG. 3  is a schematic diagram of a system including one continuously graded structure, in accordance with one embodiment of the invention; and 
           [0010]      FIG. 4  is a scanning electron micrograph of a graded structure, in accordance with one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    The systems and methods described herein include embodiments that relate to a contact arm assembly having an improved bond between contact and contact arm, thereby enabling the contact arm assembly to withstand thermal, electrical, and mechanical stresses. 
         [0012]    In the following specification and the claims that follow, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. 
         [0013]    As used herein, the term “adjacent” or “proximate” when used in context of discussion of different compositions or structure of regions or surfaces refers to “immediately next to” and it also refers to the situation wherein other components that are present between the components under discussion do not vary much with regards to the compositions or structure respectively of at least any one of the components. 
         [0014]    Referring now to  FIG. 1 , an exemplary circuit breaker system  10  is shown. The circuit breaker system  10  includes a stationary arm  20  having a fixed contact tip  22  having a fixed base surface  24  and fixed arcing surface  26 . The circuit breaker system further includes a moving arm  30  having a movable contact tip  32  having a movable base surface  34  and movable arcing surface  36 . The base surfaces  24 ,  34  of the contact tips  22 ,  32  are attached to the contact arms  20 ,  30 , and the arcing surfaces  26 ,  36  are the free surfaces. 
         [0015]    During operation, an electric arc occurs between two contact tips  22  and  32  at the arcing surfaces  26 ,  36  whenever fault current or short circuit happens. The high heat produced by the electric arc may melt both arcing surfaces  26  and  36  and a poor contact between the base  24 ,  34  and the arcing surfaces  26 ,  36  may result in transfer of contact materials from one tip to another producing uneven arcing surfaces or carbon slag on the surfaces. The carbon slag produced may adhere to the arcing surfaces  26 ,  36  and decrease electrical conductivity of the contact subjecting the arcing surfaces  26 ,  36  to mechanical and electrical degradation. Therefore, it is desired to configure the contact tips  22 ,  32  with an appropriate hardness, high wear resistance, high temperature stability, and a good bonding between the base surfaces  24 ,  34  and arcing surfaces  26 ,  36 . Further, the arcing surfaces are desired to be generally inert to oxygen and sulfur reactions. 
         [0016]    As alluded above, reliability of contact tips  22 ,  32  is desired for the increased life of the electrical switch gear. Wear and tear of contacts may be reduced by change in configuration, materials choices, and/or processing. Methods such as extrusion, die compacting, molding are commonly used for manufacturing of arcing surfaces  26 ,  36 . The arcing surfaces  26 ,  36  are normally brazed or welded on a copper base  24 ,  34  in most of the conventional electrical switch gears. Different embodiments of the present invention provide contact tips  22 ,  32  having graded structure between the base surface and arcing surface, and a new method of fabricating the contact tips  22 ,  32  without using brazing or welding and thereby eliminating voids in the contact tip  22 ,  32  structure. 
         [0017]    In one embodiment, a circuit breaker system  10  includes a graded structure  40  between the base surface and arcing surface of the fixed contact tip  22  or movable contact tip  32  as shown in  FIG. 2 . As used herein, the “graded structure between base surface and arcing surface” means that the structure between the base surface and the arcing surface has a gradient from base surface to arcing surface or vice versa. The term “gradient” as used herein means the value of a characteristic parameter of the structure changes with a change in position in the direction from base surface to arcing surface. The characteristic parameter may be composition, density, thickness, reactivity, or microstructure, for example. In one embodiment, the gradient is in the composition of the graded structure. 
         [0018]    In one embodiment, both the fixed contact tip  22  and movable contact tip  32  include the graded structure  40 . Embodiments described herein use the example of fixed contact tip  22  as having the graded structure  40 , while the movable contact tip  32  may or may not have a similar configuration. The graded structure  40  includes a multilayer architecture including a first region  50  proximate to the arcing surface, a second region  60  proximate to the base surface, and an intermediate region  70  disposed between the first region and the second region. The first region  50  includes a first surface  52  facing the arcing surface  26  and the second region  60  includes a second surface  62  facing the base surface  24 . The graded structure may optionally have further intermediate regions in between the first and second regions. 
         [0019]    In one embodiment, the graded structure  40  includes the first region  50 , second region  60 , and the intermediate region  70  in distinct, but integrated structure as shown in  FIG. 2 . In one embodiment, the first region  50 , second region  60 , and intermediate region  70  are seamless structures integrated to one another according to their layered positions as shown in  FIG. 3 , but are not distinctly separate in structure from the adjacent regions. The graded structure in this embodiment has a continuously graded structure. The interfaces of continuously graded structures may not be apparent at the macroscopic level, but may have interfaces of layers that can be identified at microscopic scale. 
         [0020]    The distinct or continuous multilayer architecture described herein is configured to be free of defects or voids and designed to be robust towards wear. This multilayer structure has superior mechanical strength, heat dissipation, and electrical performance over the current design of contacts. The graded architecture promotes reliable contact configuration, and may be formed by additive manufacturing, thereby eliminating brazing or joining of metals. 
         [0021]    Silver is considered to be an excellent contact tip  22  material because of its high thermal and electrical conductivity and considerable inertness to oxygen, nitrogen, and sulfur. However silver has a low melting point, making it prone to fusion and sticking. Further, silver is an expensive material to be used in large quantities. To overcome these challenges, in one embodiment, silver alloys or metal mixtures are used along with silver to increase hardness. 
         [0022]    In one embodiment, silver is used as the arcing surface  26 , and a concentration of silver in the graded structure  40  decreases from the first surface  52  to the second surface  62 . For example, in an embodiment in which graded structure  40  has a distinct multilayered structure, the silver may be decreased from the first surface  52  to the second surface  62  in a stepwise manner. In an embodiment where the layers are in a continuous gradation from the first surface  52  to the second surface  62 , the concentration of silver may be continuously decreased from the first surface  52  to the second surface  62 . Similarly, a concentration of copper in the graded structure  40  may decrease from the second surface  62  to the first surface  52 . In one embodiment, the arcing surface  26  includes substantially 100% silver, and the graded structure  40  may have different regions with decreasing percentage of silver from the first region  50  to the second region  60 , and the second surface  62  is substantially free of silver. In one embodiment, the base surface  24  includes substantially 100% copper. 
         [0023]    As used herein, “substantially 100%” is used to define the intended 100% composition, but may include any impurities that would not unduly degrade the arcing surface  26  or base surface  24  performance, and further would include any impurities that would have incidentally became incorporated at the surfaces during processing. In one embodiment, the concentration of silver in the arcing surface  26  is greater than about 98% and the concentration of the copper in the base surface  24  is greater than 98%. As used herein the percentages mentioned are weight percentages. 
         [0024]    The graded structure  40  used herein may be composed of metals, metal alloys, metal oxides, carbides, or nitrides. In one embodiment, the graded structure  40  includes tungsten, molybdenum, nickel, carbon, or any combinations thereof. The graded structure  40  may include a metal mixture of any of these elements with silver or copper as a part of one or more regions of the graded structure  40 . A “metal mixture” as used herein is a mixture of silver or copper with a metal, non-metal, an alloy, or a compound of metal and non-metal. Thus, in one embodiment, the metal mixture may have silver-graphite (alternately silver-carbon) in a mixture form, where the silver and carbon do not generally react with each other to form a compound. In one embodiment, the silver may be in a mixture form with tungsten carbide. 
         [0025]    In one embodiment, the metal mixture includes a metal carbide, a silver-tungsten alloy, a silver-nickel alloy, silver-tungsten carbide composite, silver-molybdenum composite, or any combinations of these. In one embodiment, the graded structure  40  has an increasing gradation in the composition of the metal mixture from the surfaces to the center of the graded structure  40 . A weight averaged concentration of the metal mixture in the intermediate region  70  of the graded structure  40  may be substantially higher than the concentration of the metal mixture at the first or second regions, when compared to the concentration of silver or copper in the respective regions. 
         [0026]    In one embodiment, nickel, carbon, tungsten, molybdenum, and tungsten carbide were studied as individual metal mixtures along with silver, copper, or silver and copper. In one example, the first region  50  includes a silver-nickel metal mixture; the intermediate region  70  includes a silver-copper-nickel metal mixture; and the second region  60  is substantially copper. 
         [0027]    In another example, the first region  50  includes a silver-tungsten metal mixture with 35/65 respective weight percentage ratio, the intermediate region  70  includes a silver-copper-tungsten metal mixture with 15/20/65 respective weight percentage ratio, and the second region  60  is substantially copper. 
         [0028]    In one more example, the first region  50  includes a silver-graphite metal mixture with 95/5 respective weight percentage ratio, the intermediate region  70  includes a silver-copper-carbide metal mixture with 70/25/5 respective weight percentage ratio, and the second region  60  is substantially copper. 
         [0029]    In yet another example, the first region  50  includes a silver-tungsten carbide metal mixture with 35/65 respective weight percentage ratio, the intermediate region  70  includes a copper-tungsten carbide-tungsten metal mixture with 15/20/65 respective weight percentage ratio, and the second region  60  is substantially copper. 
         [0030]    In a further example, the first region  50  includes a silver-tungsten carbide metal mixture, the intermediate region  70  includes a silver-copper-tungsten carbide metal mixture, and the second region  60  is substantially copper. 
         [0031]    In one more example, the first region  50  includes a silver-tungsten carbide metal mixture, the intermediate region  70  includes a copper-tungsten carbide metal mixture, and the second region  60  is substantially copper. 
         [0032]    In one more example, the first region  50  includes a silver-tungsten carbide metal mixture, the intermediate region  70  includes a silver-copper-tungsten carbide metal mixture, and the second region  60  includes a copper-tungsten carbide metal mixture. 
         [0033]    In one more example, the first region  50  includes a silver-tungsten carbide metal mixture, the intermediate region  70  includes a copper-tungsten carbide-tungsten metal mixture, and the second region  60  includes a copper-tungsten carbide metal mixture. 
         [0034]      FIG. 4  depicts a scanning electron micrograph (SEM) of a graded structure  40 . The graded structure includes multiple layers of different concentrations of silver and copper. For example, the first region  50  is of 100% silver, and the second region  60  is of 100% copper. The intermediate regions  70  and  80  vary in the concentration of silver and copper in these layers. For example, layer  70  includes higher concentration of silver than copper and the region  80  includes lower concentration of silver than the amount of copper in that surface. 
         [0035]    The contact tip having graded structure  40  may be formed using specific processes that facilitate voidless joining or forming of different regions of the graded structures. For example, methods such as cold pressing, hot pressing, and hot isostatic pressing (HIP) may be used for the formation of graded structure  40 . 
         [0036]    In one embodiment, powders of different layers are individually blended, arranged in the desired layer configuration and compacted using uniaxial press. The compacted blends may be sintered in an inert atmosphere at a temperature in a range from about 650° C. to about 1000° C. Silver may be infiltrated into the pores of the compacted and sintered structure to fill the pores with silver and to deposit silver on the first surface. 
         [0037]    Alternately, a graded pore structure may be formed in the graded structure using different types and concentrations of binders during the compaction of the individual layer powders or blends. These binders during sintering evaporate and leave behind pores that can be later filled with the arcing surface material such as, for example, silver. 
         [0038]    In one embodiment, the powder blends arranged in layered configuration may be subjected to HIP or spark plasma sintering to join the different layers together, thus making an integral contact tip  22 . 
       EXAMPLES 
       [0039]    The following examples illustrate materials, methods, and results, in accordance with specific embodiments, and as such should not be construed as imposing limitations upon the claims. All components are commercially available from common suppliers. 
         [0040]    The particle size and density details of some of the powders used are as given below in Table 1. Example compositions of some parts of the graded structure along with the base surface and arcing surface are as given in Table 2. One skilled in the art will appreciate that different particle sizes and particle densities may be used to formulate the graded structure. Further, the number of graded regions and the composition and structure of base surface, arcing surface, and graded regions may be varied as a result of routine experiments to form a further improved contact tip structure. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Size 
                 Apparent Density 
               
               
                   
                 Material 
                 (microns) 
                 (g/cc) 
               
               
                   
                   
               
             
             
               
                   
                 Silver 
                 6.0-9.0 
                 1.7-2.2 
               
               
                   
                 Tungsten 
                 4.5-5.5 
                 2.9-3.7 
               
               
                   
                 Tungsten Carbide 
                 1.5-7.0 
                 4.0-4.7 
               
               
                   
                 Nickel 
                 4.0-7.0 
                 1.9-2.7 
               
               
                   
                 Graphite 
                 40.0-45.0 
                 1.9-2.2 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Base 
                 Example Compositions of 
                 Arcing 
               
               
                 Surface 
                 Graded Contacts 
                 Surface 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 100% Cu 
                 Ag (40-90 
                 wt %)—Ni (60-10 wt %) 
                 100% Ag 
               
               
                 100% Cu 
                 Ag (15-50 
                 wt %)—WC (85-50 wt %) 
                 100% Ag 
               
               
                 100% Cu 
                 AgC (93-99 
                 wt %)—C (7-1 wt %) 
                 100% Ag 
               
               
                 100% Cu 
                 Ag (15-50 
                 wt %)—W (85-50 wt %) 
                 100% Ag 
               
               
                   
               
             
          
         
       
     
         [0041]    Primarily three methods for the formation of the above-mentioned graded structure were explored. A press-sinter-repress (PSR) method was utilized using a uniaxial load of about 6-12 ton over a cross-sectional area of about 50-130 mm 2  to initially compact the base surface, graded structure, and arcing surface together. The compacted structure was sintered in a temperature range from about 650° C. to about 1000° C. for a time duration from about 10 minutes to about 60 minutes in an inert atmosphere of about 2-4% hydrogen in nitrogen or argon. The sintered structure was then further pressed with a pressure of about 36 to 60 ksi using cold iso-static pressing method. 
         [0042]    In another method, spark plasma sintering (SPS) method was used to join the base surface and arcing surface using a graded structure. A pressure of about 30-50 MPa and an effective sintering temperature from about 650° C. to about 775° C. was used for a hold time of about 2-10 minutes duration to compact the structure. 
         [0043]    In a hot iso-static pressing (HIP) method, the starting powders and blends were subjected to a uniaxial load of about 6-12 tons over a cross-sectional area of about 50-130 mm 2  for initial pressing, and then further pressed at a temperature range from about 650° C. to about 750° C. at a pressure range from about 20 ksi to about 30 ksi for about 1-3 hours&#39; time duration. 
         [0044]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.