HOT ISOSTATIC PRESSING OF POWDERS TO FORM HIGH DENSITY CONTACTS

Abstract: A process of hot isostatic pressing of powders to form electrical contacts is characterized by the steps: (A) mixing powders, 1 in the Drawing, from metal contain­ ing powder or metal containing powder plus carbon powder, where at least one of Ag and Cu is present, (B) thermal cleaning treatment of the powder, 2 in the Drawing, (C) granulating the thermally treated powder, 3 in the Drawing, (D) uniaxially pressing the powders without heating, 5 in the Drawing, to provide a compact, (E) placing at least one compact in a container, 6 in the Drawing, and surrounding each compact with fine particles of a separating material, (F) evacuating air from the con­ tainer, 7 in the Drawing, (G) sealing the compacts inside the container, 8 in the Drawing, (H) hot isostatic pressing, 9 in the Drawing, the compacts at a temperature from 0.5°C to 100°C below the melting point of the lower melting powder constituent, (I) gradually cooling and releasing the pressure on compacts, 10 in the Drawing, and (J) separating the compacts from the container, 11 in the Drawing.

Description: The present invention relates to improved powder metallurgy techniques which provide fully dense electrical contact members for electrical current applications. High density electrical contacts are well known. For example, Gainer, in U.S. Patent Specification No. 3,960,554, teaches mixing a minor amount of copper powder with chromium powder, pressing to form a compact, and vacuum sintering to infiltrate the chromium matrix with copper. Gainer, in U.S. Patent Specification No. 4,190,753, teaches a similar process, utilizing cold isostatic pressing, with minor amounts of chromium in copper powder. Hoyer et al., in U.S. Patent No. 4,137,076, teach a contact made from Ag, WC and Tic powders, where the mixture is compacted, and then sintered at 1,260°C in a reducing atmosphere to shrink the compact. This compact is then melt infiltrated with silver, applied in the form of a slug. All of these methods have various drawbacks in terms of providing electrical contacts having the desired properties of full density, high rupture strength, enhanced metal-metal bond, and enhanced resistance to thermal stress cracking. It is a main object of this invention to provide a process that results in electrical contacts having all of these properties. With the above object in mind, the present invention resides in a method of forming a high density electrical contact which comprises: (A) mixing: (a) powders from class 1 metals selected from Ag, Cu or mixtures thereof, with (b) powders from class 2 materials selected from CdO, W, WC, Co, Cr, Ni, C or mixtures thereof, wherein the powder particles from class 1 and 2 have particle sizes of up to 100 µm (microns) diameter; (B) heating the powders from step (A) in a reducing atmosphere at a temperature effective to provide an oxide clean surface on the powders, except CdO, and more homogeneous distribution of class 1 metals, (C) granulating the powder from step (B) to again provide powder having particle sizes of up to 100 µm (microns) diameter; (D) uniaxially pressing the powders from step (C) without heating to provide a compact that is from 65% to 95% dense; (E) placing at least one compact in a pressure-transmitting pressure-deformable container and surrounding each compact with fine particles of a separating material which aids subsequent separation of the compact and the container; (F) evacuating air from the container; (G) sealing the compacts inside the container, and then (H) hot isostatically pressing the compacts through the pressure transmitting container, to provide simultaneous hot-pressing and densification of the compacts; (I) gradually cooling and releasing the pressure on the compacts,, to provide a compact at least 98% dense; and then (J) separating the compacts from the container; characterised in that step (H) is performed at a temperature of from 0.5°C to 100°C below the melting point or decomposition point of the lowest melting powder constituent, the compacts in step (I) cool under pressure and in that the compacts are not heated before step (H). This provides oxide clean metal surfaces in combination with controlling the temperature during hot isostatic pressing, to attain high densification and eliminate the infiltration step used in the prior methods of forming electrical contacts. The term "hot isostatic pressing" is used herein to mean pressing at a temperature substantially over the generally accepted sintering temperature of the lower melting powder involved, so that fusion of the lower melting powder is almost achieved and, where the pressing is from all sides at the same time, usually by a pressurized gaseous medium, as distinguished from mechanical, two-sided, uniaxial pressing. This combination of simultaneous heat and pressure results in the compact achieving near full theoretical density, predominantly by plastic flow of the lower melting temperature material. The process is further characterized in that the powders can be contacted with a brazeable metal material prior to uniaxial pressing. This process involves six basic steps: mixing, oxide cleaning, granulating, uniaxial pressing, hot isostatic pressing, and cooling under pressure. Useful powder combinations, by way of example only, include Ag + CdO, Ag + W, Ag + C; Ag + WC; Ag + WC + Co; Ag + WC + Ni; Cu + Cr; Cu + C; and Cu + WC + Co. The invention will become more readily apparent from the following description of preferred embodiments thereof shown, by way of example only, in the accompanying Drawing which shows a block diagram of the method of this invention. Referring now to the Drawing, powders selected from metal containing powder, and metal containing powder plus carbon powder, all having particles of up to 100 micrometers diameter, preferably in the range of from 0.5 micrometers to 50 micrometers diameter, are homogeneously mixed, block 1 of the Drawing. Over 100 micrometers diameter, high densities are difficult to achieve. Useful powders include two groups of powders: the first is selected from "class 1" metals, defined herein as consisting of Ag, Cu, or mixtures thereof. These are mixed with other powders from class 2 materials consisting of CdO, W, WC, Co, Cr, Ni, C, or mixtures thereof. The class 1 powders can constitute from 10 wt.% to 95 wt.% of the powder mixture. The mixed powder is then thermally treated to provide relatively clean particle surfaces, block 2 of the Drawing. This usually involves heating the powders at between approximately 450°C, for 95 wt.% Ag + 5 wt.% CdO, and 1100°C, for 10 wt.% Cu + 90 wt.% W, both for about 0.5 hour to 1.5 hours, in a reducing atmosphere, preferably hydrogen gas or dissociated ammonia. This removes oxide from the metal surfaces, yet is at a temperature low enough not to decompose any CdO present. This step has been found important to providing high densification when used in combination with hot isostatic pressing later in the process. Where minor amounts of class 1 powders are used, this step distributes such powders among the other powders, and in all cases provides a homogeneous distribution of class 1 metal powders. The treated particles, which are usually lumped together after thermal oxide cleaning, are then granulated so that the particles are again in the range of from 0.5 micrometer to 100 micrometers diameter, block 3 of the Drawing. The mixed powder is then placed in a press die. Optionally, to provide a brazeable or solderable surface for the contact, a thin strip, porous grid, or the like, of brazeable metal, such as a silver-copper alloy, or powder particles of a brazeable metal, such as silver or copper, is placed above or below the main contact powder mixture in the press die, block 4 of the Drawing. The material in the press is then uniaxially pressed in a standard fashion, without any heating or sintering, block 5 of the Drawing, at a pressure effective to provide a handleable, "green" compact, usually between 35.2 kg/cm² (500 psi) and 2,115 kg/cm² (30,000 psi). This provides a compact that has a density of from 65% to 95% of theoretical. The compact or a plurality of compacts are then placed in a pressure-transmitting, pressure-deformable, collapsible container, where each compact is surrounded by a material which aids subsequent separation of compact and container material, such as loose particles and/or a coating of ultrafine particles and/or high temperature cloth, block 6 of the Drawing. The air in the container is then evacuated, block 7 of the Drawing, and the container is sealed, usually by welding, block 8 of the Drawing. The container is usually sheet steel, and the separation material is in the form of, for example, ceramic, such as alumina or boron nitride, or graphite particles, preferably less than about 5 micrometers diameter, and/or a coating of such particles on the compact of less than about 1 micrometer diameter. The canned compacts are then placed in an isostatic press chamber, block 9 of the Drawing, where argon or other suitable gas is used as the medium to apply pressure to the container and through the container to the canned compacts. Pressure in the hot isostatic press step is between 352 kg/cm² (5,000 psi) and 2,115 kg/cm² (30,000 psi), preferably between 1,056 kg/cm² (15,000 psi) and 2,115 kg/cm² (30,000 psi). Temperature in this step is from 0.5°C to 100°C below the melting point or decomposition point of the lower melting point powder constituent, preferably from 0.5°C to 20°C below such point, to provide simultaneous collapse of the container, and through its contact with the compacts, hot-pressing of the compacts, and densification of the compacts, through the pressure transmitting container, to over 98%, preferably over 99.5%, of theoretical density. Residence time in this step can be from 1 minute to 4 hours, most usually from 5 minutes to 60 minutes. Isostatic presses are well known and commercially available. As an example of this step, where a 90 wt.% Ag + 10 wt.% CdO powder mixture is used, the temperature in the isostatic press step will range from about 800°C to 899.5°C, where the decomposition point of CdO is about 900°C. Controlling the temperature during isostatic pressing is essential in providing a successful process that eliminates the infiltration steps often used in processes to form electrical contacts. The hot isostatically pressed compact is then gradually brought to room temperature and one atmosphere over an extended period of time, block 10 of the Drawing, usually 2 hours to 10 hours. This gradual cooling under pressure is very important, particularly if a brazeable layer has been bonded to the compact, as it minimizes residual tensile stress in the component layers and controls warpage due to the differences in thermal expansion characteristics. Finally, the compacts are separated from the container which has collapsed about them, block 11 of the Drawing. Contact compacts made by this method have, for example, enhanced Ag-Ag, Ag-W or Cu-Cr bonds leading to high arc erosion resistance, enhanced thermal stress cracking resistance, and can be made substantially 100% dense. In this process, there is no heating of the pressed compacts before the isostatic hot pressing step. The invention will now be illustrated with reference to the following Examples, which are not to be considered in any way limiting. EXAMPLE 1 A mixture of 90 wt.% Ag powder and 10 wt.% CdO powder, both having particle sizes below about 44 micrometers diameter, were thoroughly mixed, thermally heat cleaned of oxide at 594°C, and insured of homogeneous Ag distribution, and subsequently granulated in a mill-sieve apparatus to again have particle sizes below about 44 micrometers diameter. This powder was then placed in a die and uniaxially pressed at 352 kg/cm² (5,000 psi) to provide compacts of about 80% of theoretical density. The compacts were 2.54 cm long x 1.27 cm wide x 0.25 cm thick. Twelve of the compacts were placed in a metal can in two rows, with six compacts per row, all surrounded with ceramic particles of about 2 micrometer diameter, acting as a separation medium. Air was evacuated from the can using a vacuum pump and then the can was weld sealed. The sealed can was placed in the chamber of an isostatic press, which utilized argon gas under pressure as the medium to apply pressure on the can. Isostatic hot pressing, using a National Forge 2,112 kg/cm² (30,000 psi) press, was accomplished at a simultaneous 895°C temperature and 1,056 kg/cm² (15,000 psi) pressure for about 5 minutes. This temperature was 5°C below the decomposition temperature of CdO, the lower stable component of the powder mixture. Cooling and depressurizing was then commenced over a 6 hour period. The contacts were removed from the collapsed container and were found to be 98.5% dense, after shrinking 13% during hot-pressing. The macro structure was found to be homogeneous. EXAMPLE 2 In a similar fashion, ten contacts made from 35 wt.% Ag + 65 wt.% W powders were made, with an 0.025 cm (0.01 in.) thick Ag brazing layer, using the same pressures, but an isostatic press temperature of about 950°C, which was 11°C below the melting point of Ag, the lower stable component of the powder mixture. The contacts measured 2.54 cm long x 1.1 cm wide x 0.22 cm thick. Their properties are listed in Table 1 following, compared to standard Ag-W contacts made by liquid phase infiltration, involving mixing a little of the Ag with W, pressing, and then melting the remaining Ag over the compact to infiltrate the structure. SAMPLE 1 Standard Ag-W SAMPLE 2 Hot Isostatic Pressed Ag-W Density gram/cm³ 14.3-14.6 14.8 % Theoretical Density 96-98 99.4-99.5 Hardness R₃₀T 64-70 73-77 Macro Structure Occasional Slight Porosity Homogeneous As can be seen, results using the hot isostatic pressing process are excellent. A contact of each sample was fractured and a scanning electron micrograph of a typical fracture surface of each contact was taken. The micrographs of the Sample 2 contact, made by the method of this invention, showed a general absence of large pore areas present in the Sample 1 contact. Also, contacts of both Sample 1 and 2 manufacture were mounted and subjected to standard short circuit testing at 600 V. and 10 KA, in a Molded Case circuit breaker. The contacts were then removed and sectioned through their thickness. Optical micrographs were then taken of each. The Sample 1 section showed surface cracks and material loss, and an infiltration serrated area. The Sample 2 contact, made by the method of this invention, showed little cracking and much less material loss. IDENTIFICATION OF REFERENCE NUMERALS USED IN THE DRAWINGS LEGEND REF. NO. FIGURE MIX POWDERS 1 1 THERMAL CLEANING 2 1 GRANULATION 3 1 UNIAXIAL PRESS 5 1 INSERT INTO CONTAINER 6 1 EVACUATING THE CONTAINER 7 1 SEALING THE CONTAINER 8 1 HOT ISOSTATIC PRESSING 9 1 COOL UNDER PRESSURE 10 1 SEPARATE 11 1 CONTACT WITH BRAZEABLE MATERIAL 4 1

Claims: A method of forming a high density electrical contact which comprises: (A) mixing: (a) powders from class 1 metals selected from Ag, Cu or mixtures thereof, with (b) powders from class 2 materials selected from CdO, W, WC, Co, Cr, Ni, C or mixtures thereof, wherein the powder particles from class 1 and 2 have particle sizes of up to 100 µm (microns) diameter; (B) heating the powders from step (A) in a reducing atmosphere at a temperature effective to provide an oxide clean surface on the powders, except CdO, and more homogeneous distribution of class 1 metals, (C) granulating the powder from step (B) to again provide powder having particle sizes of up to 100 µm (microns) diameter; (D) uniaxially pressing the powders from step (C) without heating to provide a compact that is from 65% to 95% dense; (E) placing at least one compact in a pressure-transmitting pressure-deformable container and surrounding each compact with fine particles of a separating material which aids subsequent separation of the compact and the container; (F) evacuating air from the container; (G) sealing the compacts inside the container, and then (H) hot isostatically pressing the compacts through the pressure transmitting container to provide simultaneous hot-pressing and densification of the compacts; (I) gradually cooling and releasing the pressure on the compacts, to provide a compact at least 98% dense; and then (J) separating the compacts from the container, characterised in that Step (H) is performed at a temperature of from 0.5°C to 100°C below the melting point or decomposition point of the lowest melting powder constituent, the compacts in step (I) cool under pressure and in that the compacts are not heated before step (H). The method as claimed in claim 1, characterized in that the powders are contacted with a brazeable metal material prior to step (D). The method as claimed in claim 2, characterized in that the brazeable metal material is a brazeable metal strip. The method as claimed in any of claims 1 to 3, characterized in that the powders are pressed in step (D) at from 35.2 kg/cm² to 2,115 kg/cm². The method as claimed in any of claims 1 to 4, characterized in that the hot isostatic pressing in step (H) is from 372 kg/cm² to 2,115 kg/cm², and the temperature is from 0.5°C to 20°C below the melting point or decomposition point of the lowest melting powder constituent. The method as claimed in any of claims 1 to 5, characterized in that the powder is selected from Ag + CdO; Ag + W; Ag + C; Ag + WC; Ag + WC + Co; Ag + WC + Ni; Cu + Cr; Cu + C; or Cu + WC + Co. The method as claimed in claim 6, characterized in that the powder is Ag + CdO. The method as claimed in claim 6, characterized in that the powder is Ag + W. The method as claimed in any of claims 3 to 8, characterized in that the powders have a particle size in the range of from 0.5 µm (microns) to 50 µm (microns), and they are contacted with a metal strip prior to step (D). The method as claimed in any of claims 1 to 9, characterized in that thermal treatment in step (B) is in a gas selected from hydrogen gas, or dissociated ammonia. The method as claimed in any of claims 1 to 10, characterized in that, in step (H), there is simultaneous collapse of the container and its contact with the compacts, hot-pressing, and densification of the compacts to over 99.5% of theoretical density through the pressure transmitting container.

Inventors: HOYER, NORMAN STANLEY, IYER, NATRAJ CHANDRASEKAR, MALE, ALAN THOMAS
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1989
Main classification: B22F3
IPC codes: C22C1, H01H11, B22F3, C22C32