ELECTRICAL CONTACT FOR USE IN A CIRCUIT BREAKER AND A METHOD OF MANUFACTURING THE SAME
Field of the Invention This invention relates generally to electrical contacts used in electrical interruption devices such as circuit breakers and, more particularly, to an electrical contactor composed of materials having properties that provide low contact resistance, high resistance to wear and high anti-contact welding characteristics. In addition, the invention also relates to a method for the manufacture of such contact. BACKGROUND OF THE INVENTION Some circuit breakers have traditionally used electrical contacts made of silver (Ag) and cadmium oxide. Contacts made of Ag and cadmium oxide provide low contact resistance, high wear resistance and high anti-weld characteristics. However, due to environmental concerns, it is desirable to remove cadmium-based metals from electrical contacts. It is known in the art that a contact consisting of Ag and tungsten carbide (C) provides a contact having low contact resistance and high wear resistance. It is also known that graphite (Gr) can be added to contact materials as a lubricant to prevent the contacts from welding together. However, a disadvantage is that the wear resistance of the contact is negatively affected by the addition of Gr to the contact material. When the materials are mixed together in a powder form, the Gr particles tend to get smeared on the Ag, thereby allowing the formation of an interconnected network of Gr to Gr around the Ag particles when the contact materials are pressed and sintered . This interconnected network of Gr particles causes a weak synthetic bond from Ag to Ag, which causes the contact to be easily eroded during an interruption. Consequently, there is a well-defined need for an improved electrical contact made of contact materials that provide low contact resistance, high wear resistance, and high anti-weld characteristics. SUMMARY OF THE INVENTION The present invention provides an electrical contact for use in a circuit breaker that provides low contact resistance, high wear resistance, and high anti-weld characteristics. According to a preferred embodiment of the present invention, an electrical contact for a circuit breaker includes a unique material composition of 0.2-8% by weight of Gr, 10-90% by weight of W and the remaining remnant mixture in Ag. Also in accordance with this invention, the contact is manufactured using a process that yields a unique microstructure characterized by not having a Gr network interconnected around the Ag and thereby providing a strong bond from Ag to Ag. This process includes the steps of adding the appropriate amounts of Ag, W and Gr powder material together, physically mixing them together under low shear conditions to provide a homogenous powder mixture while forbidding the Gr to get on the Ag , pressing the mixture of materials to form a contact, and then sintering and coining the contact. Brief Description of the Drawing Other objects and advantages of the invention will be apparent from the following detailed description and the accompanying drawing, which is a microphotograph of an electrical contact in accordance with a preferred embodiment of the present invention. Although the invention is susceptible to various modifications and alternative forms, a specific embodiment thereof has been shown as an example in the drawing and will be described in detail. However, it should be understood that it is not intended to limit the invention to the particular form described, but on the contrary the invention will cover all equivalents, modifications and alternatives that fall within the spirit and scope of the invention, as defined by the appended claims. Detailed Description of the Preferred Embodiments For a better understanding of the present invention together with other and additional advantages, and their capabilities, reference will be made to the following disclosure and the appended claims in relation to the drawing described above. An electrical contact according to the present invention is for use in electrical interrupting devices that control electrical current, such as circuit breakers and circuit breakers. Circuit breakers are commonly used to provide automatic circuit interruption when undesirable overcurrent conditions are detected in the circuit being monitored. These overcurrent conditions include, among others, overload conditions, ground faults and short circuit conditions. Circuit breakers typically include an electrical contact on a movable blade that rotates away from a stationary contact in order to interrupt the current path. In response to an overcurrent condition, the circuit breakers generally move the blade to break the circuit path by triggering a spring-biased operating mechanism that forces the blade and its contact away from the fixed contact. The contacts used in circuit breakers typically require low contact resistance, high wear resistance and high anti-weld characteristics. Due to this requirement, the electrical contacts were composed of Ag for its properties of high electrical conductivity and cadmium oxide for its resistance to erosion and welding. However, due to concerns about how cadmium affects the environment, it was desired to eliminate its use in electrical contacts. The electrical contact according to the present invention will now be described in detail. First, a contact consisting of Ag and tungsten (), mixed in powder form, was investigated to determine if it would be satisfactory to replace a contact made with Ag and cadmium oxide. Although the Ag-W contact satisfied the requirements of low contact resistance and high wear resistance, it was found that it would be welded with an even contact during a circuit interruption. The arcing that occurs during circuit interruptions caused the Ag of both contacts to melt and collect, which would solidify them jointly upon cooling, thereby soldering the two contacts together. Next, the inventors fabricated a contact made of Ag and Gr materials. This Ag-Gr contact had high anti-weld characteristics; however, it had low wear resistance. The inventors then added graphite (Gr) to the Ag-W mixture to act as a lubricant to provide anti-weld properties. The inventors found that the Ag-W-Gr contact satisfied the requirements of low contact resistance, high wear resistance and high anti-weld characteristics. It was determined that the optimum combination of the mixture of materials was 0.2-8% by weight of Gr, 10-90% of W and the remaining mixture consisted of Ag. The Ag provides superior electrical properties, the W provides resistance properties to erosion, while the Gr provides anti-weld properties. This innovative Ag-W-Gr contact composition provides the advantage of an anti-weld contact that maintains high conductivity and high wear resistance. The process for manufacturing the contact according to the present invention will now be described. Separated powders of Ag, W and Gr materials are weighed and added to a powder mix. The powder mixture consists of between 0.2 and 8% by weight of Gr, between 10 and 90% by weight of W and between 2.0 and 89.8% by weight of Ag. A homogeneous powder mixture is established by physically mixing the powder mixture under conditions of low shear or low energy in such a manner as to prevent the Gr from smearing on the Ag. The homogeneous powder mixture is then pressed into a required form of the powder. Contact. This is followed by sintering the contact at a temperature between 500 ° C and the melting point of the Ag. The contact is then coined, or densified by re-pressing it until most of the air is forcibly ejected. The physical mixture of the material is carried out in a 16-quart liquid-solid mixer, available as model No. LB11157, from Patterson Kelly Company, of East Stroudsburg, Pennsylvania, United States. The mixer uses a mixing bar that has several pins in it to provide a smooth mixing action, which gently mixes Ag, W and Gr to produce a homogeneous powder mixture. This action of gentle physical mixing is important to prevent Gr over the Ag particles, thereby keeping the Gr as individual particles and preventing them from being in contact with each other. Because the Gr particles are not in contact with each other, the formation of an interconnected network of Gr around the Ag particles is prevented during the pressing and sintering steps of the process. The present invention provides an advantage over the prior art in that the contact materials were physically mixed in a vigorous, joint manner, causing the Gr particles to get muddy on and around the Ag particles, causing the Gr particles to touch each other. each other and inducing the formation of an interconnected network of Gr around the Ag particles. The drawing is a microphotograph showing the microstructure of the contact according to the present invention. In the photomicrograph, the white portions represent Ag, the black portions represent Gr and the gray portions represent W. As the photomicrograph shows, the contact according to the invention consists of a microstructure in which the Gr portions are not interconnected around the portions of Ag. The aforementioned manufacturing process prohibits the Gr from soaking over the Ag, thereby preventing the portions of Gr from linking to each other and forming an interconnected network of Gr around the Ag portions. As can be seen in the photomicrograph, the Ag portions are connected together, thereby forming an interconnected Ag network that provides an intense Ag to Ag bond. From the above detailed description, it can thus be seen that the present invention provides a contact that has the characteristics of low contact resistance, high wear resistance and high anti-weld characteristics to.