Volume 07 - Powder Metal Technologies and Applications Part 17 docx

75Cu-25W PSR 10.37 9.45- 10.00 50-79 35-60 HRB . . . . . . 414 60 C, A 70Cu-30W . . . 10.70 10.45 76 59-66 HRB . . . . . . . . . . . . A Current-carrying contacts 65Cu-35W . . . 11.06 11.40 72 63-69 HRB . . . . . . . . . . . . A Vacuum interrupter 60Cu-40W . . . 11.45 11.76 68 69-75 HRB . . . . . . . . . . . . A 50Cu-50W INF 12.30 11.90- 11.96 45-63 60-81 HRB . . . . . . . . . . . . A 44Cu-56W INF 12.87 12.76 55 79 HRB 434 63 827 120 C Oil-circuit breakers, arcing tips 40Cu-60W INF 13.29 12.80- 12.95 42-57 75-86 HRB . . . . . . . . . . . . A 35Cu-65W INF 13.85 13.35 54 83-93 HRB . . . . . . . . . . . . A 32Cu-68W INF 14.20 13.95 50 90 HRB . . . . . . 896 130 C Oil-circuit breakers, reclosing devices, arcing tips, tap change arcing tips, contactors 30Cu-70W INF 14.45 13.85- 14.18 36-51 86-96 HRB . . . . . . 1000 145 C, A 26Cu-74W INF 14.97 14.70 46 98 HRB 621 90 1034 150 . . . Circuit breaker runners, arcing tips, tap change arcing tips 25Cu-75W INF 15.11 14.50 33-48 90-100 HRB . . . . . . . . . . . . A 20Cu-80W INF 15.84 15.20 30-40 95-105 HRB 758 110 . . . . . . C 190 HV (c) . . . . . . . . . . . . 15Cu-85W PSR 16.45 16.0 20 260 HV (d) . . . . . . . . . . . . M 13.4Cu-86.6W INF 16.71 16.71 33 20 HRC 621 90 1034 150 C 10.4Cu-89.6W INF 17.22 17.22 30 30 HRC 765 111 1138 165 C Vacuum switches, arcing tips, oil-circuit breakers Tungsten-graphite-silver 48Ag-51.75W- 0.25C PSR 13.21 13.38 65 55 HRB . . . . . . 552 80 C 46Ag-53W-1C PSR 13.58 12.85 55 85 HRB . . . . . . . . . . . . C 45Ag-50W-5C PSR 11.00 10.60 37-43 45-55 HRB . . . . . . 621 90 A Circuit breakers, arcing tips Complex composite contacts 26 HRF (c) . . . . . . . . . . . . 88Ag-10Ni-2C PSR 9.63 9.37 70 64 HRF (d) . . . . . . . . . . . . C, A Sliding contacts 84 HRF (c) . . . . . . . . . . . . 25Ag-50Fe- 25Cu PSR 8.67 8.52 21 94 HRF (d) . . . . . . . . . . . . C Circuit breakers (a) PSR, press-sinter-re-press; INF, press-sinter-infiltrate; PS, press-sinter; PSE, press-sinter-extrude; IO, internal oxidation; PPSE, preoxidize-press- sinter-extrude; SF, oxidized from one direction. (b) A: Advance Metallurgy, Inc., McKeesport, PA. C: Contacts , Materials, Welds, Inc., Indianapolis, IN. E: Englehard Industries, Plainville, MA. G: Gibson Electric Inc., Delmont, PA. S: Stackpole Carbon Co., St. Marys, PA. T: Texas Instruments Inc., Attleboro, MA. M: Metz Degussa, South Plainville, NJ. W: Art Wire-Duduco, Cedar Knolls, NJ. (c) Annealed. (d) Cold worked Table 2 presents the compositions and properties of various composite contact materials. Because manufacturing methods affect the properties of materials with the same composition, the manufacturing methods are also given in Table 2. The most common methods of producing composite electrical contact materials are described in the section "Composite Manufacturing Methods" in this article. Data published by contact manufacturers usually include density, hardness, and electrical conductivity (Table 2). These data provide designers of electrical devices with the basic properties of a composite contact. Other properties, such as contact resistance, may depend on operational parameters such as force (Fig. 3). Fig. 3 Contact resistance versus force for fine silver and Ag-CdO contacts. Unarced contacts were 12.7 mm ( in.) in diameter with a 38 mm (1 in.) spherical radius. Resistance measurements were made with ac current at 50 A and 60 Hz. Characteristics that relate directly to failure modes such as arc erosion or material transfer are usually described in a qualitative manner. Very few quantitative data pertaining to these characteristics have been published because these properties depend on several test parameters. For instance, the arc erosion rate is affected by various mechanical factors: • Opening force and opening speed • Closing force and closing speed • Bouncing of the movable contact • Wiping distance • Gap between opposing contacts or electrical factors: • Current both amperage and whether ac or dc • Voltage • Power factor (inductive/capacitive) Because each variable can greatly affect the arc erosion rate of a composite, it is virtually impossible to define a universal test to evaluate erosion rate. Published data on erosion rate and welding frequency usually are collected under very specific conditions. They are valid only for qualitative description in a specific set of circumstances and cannot be extrapolated to suit other applications. The only means of learning how a composite will perform in a specific application is to test it extensively in the device in which it will be used. Examples of test data are given in Fig. 3, 4, 5, and 6. Fig. 4 Contact erosion characteristics of silver and silver- tungsten contacts. Test conditions were 115 V, 60 Hz, and 1.0 power facto r for 100,000 operations at 60 operations/min. Closing and opening speeds were 38 mm/s (1 in./s). Closing force was 980 mN (0.22 lbf) and opening force, 735 mN (0.165 lbf). Fig. 5 Contact welding characteristics of silver and silver-tungsten contacts. Operation charact eristics are the same as for Fig. 4. Fig. 6 Results of short-circuit tests on silver-tungsten, silver-molybdenum, and silver tungsten carbide Refractory Metal and Carbide-Base Composites Refractory metals and their carbides are distinguished by high melting and boiling points and high hardness, but poor electrical and thermal conductivities and poor oxidation resistance. In pure elemental form, refractory metals perform well only under low-current conditions. Forming a composite can compensate for these drawbacks. For example, the development of composite contact materials involving silver or copper with tungsten or molybdenum or their carbides has resulted in materials that can withstand higher currents and more arcing than other contact materials, without experiencing sticking or rapid erosion. The refractory metal content can vary from 10 to 90%, although 40 to 80% usually is used in air- and oil-immersed circuit breaker devices. Refractory metals offer good mechanical wear resistance and resistance to arcing. The silver and copper provide the good electrical and thermal conductivities. Because silver and copper do not alloy with tungsten, molybdenum, or their carbides, P/M processes are required in fabrication. Depending on the composition, refractory metals containing silver or copper contact materials are made either by pressing and sintering or by the press-sinter-infiltrate method. When infiltration is used, either all refractory metal powder is compacted to shape, or a small amount of silver or copper powder is blended with the refractory metal, compacted, and sintered in a reducing atmosphere. The sintered compact is then returned to the furnace; silver or copper is added to act as the infiltrant. Most infiltrated composite contacts use silver as the infiltrant because of its excellent thermal and electrical conductivities, as well as its superb oxidation resistance. Copper infiltrant, which costs less but has very poor corrosion resistance, is used for composites that operate in noncorrosive environments such as oil, vacuum, or inert atmospheres. At temperatures above the melting point of the infiltrant, the liquid metal penetrates and fills the interconnecting voids of the pressed-and-sintered compact. Densities of 96 to 99% of theoretical can be achieved by this process. Infiltrated contact materials find use as current-carrying contacts in air- and oil-immersed circuit breakers, heavy-duty relays, automotive starters, and switches. Lower properties can be obtained by pressing and sintering. In a material made by infiltration, the function of the infiltrant (silver or copper) is twofold. First, because silver or copper does not alloy with tungsten, molybdenum, or carbides, the conductivity of the composite depends strictly on the volume percentage of infiltrant. Second, during arcing, the high temperature melts the infiltrant; consequently, the heat of fusion absorbs (quenches) a portion of the heat generated by the arc. Theoretically, the skeleton, which is made of a high-melting element, will not begin to melt until all the low-melting component evaporates. The refractory skeleton also prevents molten infiltrant from flowing by capillary action. Because of this, erosion loss of the contact is low. Properties (such as the erosion data in Fig. 4 and 6) of the contact vary with the composition of the composite. A composite with high skeletal composition has high hardness and better wear resistance, but lower current-carrying capacity. On the other hand, a high-silver composite possesses high electrical and thermal conductivities and undergoes lower temperature rise, but is softer. Compositions of Refractory Component. There is a lower limit for the composition of the skeleton material. Generally, when the amount of refractory or carbide is less than about 30 vol%, it is difficult to form a sound and uniform skeleton to accommodate the amount of silver. For practical purposes, the skeleton material should amount to a minimum of 50 wt% for tungsten and molybdenum and 35 wt% for tungsten carbide. Any composite containing lesser amounts than these limits should be made by the press-sinter-re-press method and should be considered a silver-base composite in which the function of the refractory material is to reinforce the silver matrix. For compounds with 60% or less tungsten, the classical method of mixing the powders, pressing, sintering (generally below the copper melting point), and re- pressing might also be used. Materials with 60 to 80% W are generally produced by infiltration, either of loose tungsten powder or of a pressed-and-sintered tungsten compact. Tungsten, tungsten carbide, and molybdenum powders are the most commonly used materials for making skeletons for infiltrated contacts. Composites with tungsten skeletons have the best arc-interrupting and arc-resisting characteristics and the best arc-erosion resistance. Their antiwelding properties are moderate (Fig. 5). High-energy devices usually use silver- infiltrated composites having a tungsten skeleton. Composites with tungsten carbide skeletons have better resistance to welding, better anticorrosion properties, and more stable contact resistance compared with other infiltrated composites. Devices that handle switching arcs usually use composites based on tungsten carbide skeletons. For a combination of properties, or sometimes for a special requirement, a skeleton made of a mixture of tungsten and tungsten carbide is used. The blended powder contains either the mixture of tungsten and tungsten carbide or a mixture of tungsten and graphite. In the latter case, the graphite and part of the tungsten react to form tungsten carbide during sintering. Composites with molybdenum skeletons have relatively low contact resistance and behave well in circuit- interrupting devices. For the same current-carrying capacity, a molybdenum-base composite costs less than the other two, but the antiwelding and anticorrosion properties of molybdenum-base composites are inferior to those with tungsten or tungsten carbide skeletons. Figure 6 compares the erosion characteristics of a molybdenum composite with tungsten and tungsten carbide composites. Silver-Base Composites The main advantage of a silver composite over a silver alloy is that the bulk conductivity of a silver composite depends generally on the percentage of silver by volume. An alloying element in solution greatly decreases the conductivity of silver. For instance, the volume of silver in Ag-15CdO composite is less than that in Ag-15Cd alloy, yet the electrical conductivity of the former (65% IACS) is much greater than that of the latter (35% IACS). In silver composites, the second phase forms discrete particles that are dispersed in the silver matrix. The dispersed phase improves the matrix in two ways. First, it increases the hardness of the composite material in a manner similar to dispersion hardening. Second, in the region where two mating contacts touch upon closure, the second phase particles reduce the surface area of silver-to-silver contact. This greatly reduces the tendency to stick or weld. In cases where the contacts do weld, the second-phase oxide particles (which are weaker and more brittle than silver) behave as slag inclusions and reduce the strength of the weld, allowing the device contact-separating force to pull the contacts apart. Silver-base composites can be divided into two types: type 1 uses a pure element or carbide as the dispersed phase; type 2 uses oxides as the dispersed phase. In both types, the hardness increases and the conductivities decrease as the volume fraction of dispersed phase increases, and vice versa. Silver-Base Composites with a Pure Element or Carbide. In type 1, the dispersed phase functions as a hardener and improves the mechanical properties of the silver matrix. The dispersed phase also promotes improved electrical performance such as antiwelding properties. Elements used include tungsten, tungsten carbide, molybdenum, nickel, iron, graphite, and mixtures of these materials. Silver-Tungsten and Silver-Molybdenum Composites. Silver composites (made by the press-sinter-re-press method using tungsten, tungsten carbide, and molybdenum as the dispersed phases) show electrical conductivities similar to those of infiltrated composites of the same components. However, their mechanical properties are inferior because the dispersed phases do not form a refractory skeleton. Silver-Nickel Composites. One of the elements typically combined with silver by P/M processes is nickel. Nickel is more effective as a hardening agent than copper; consequently, silver nickel is considerably harder than coin silver. At the same time, nickel does not increase contact resistance appreciably, particularly in combinations that include 15 wt% Ni or less. Silver nickel is combined in proportions ranging to about 40 wt%. Composites with nickel as the dispersed phase resist mechanical deformation or peening under impact and possess good antiwelding properties. Silver-nickel composite contacts can be used as both members of a contact pair. Sometimes, a silver-nickel composite is used as the moving contact operating against a stationary contact of a different composite such as silver-graphite. The combinations most widely used are 60Ag-40Ni and 85Ag-15Ni. These materials are very ductile and can be formed in all of the shapes in which silver contacts are used, including very thin sheets for facing large contact areas. This material is ideal for use under heavy sliding pressures. It does not gall like fine silver and coin silver, but instead takes on a smooth polish. It is therefore suitable for sliding contact purposes, as well as for make-break contacts. Silver nickel can handle much higher currents than fine silver before it begins to weld. It has a tendency to weld when operated against itself. Therefore, it is frequently used against silver graphite. The 60Ag-40Ni composite is the hardest material in the silver-nickel series. It is the most suitable for sliding contact in which pressure is high. This alloy also has the lowest rate of wear under sliding action. It is less ductile than silver-nickel materials containing less nickel, but it is still sufficiently ductile for all conventional manufacturing processes. The 85Ag-15Ni composite is the most widely used material in the silver-nickel series. Because of its ideal mechanical properties, 85Ag-15Ni is an ideal material for motor-starting contactors and is superior in this type of application to fine silver, coin silver, and copper. It is also suitable as a general-purpose contact for various types of relays and switches. The contact resistance of clean 85Ag-15Ni contacts that have not operated under load tend to be slightly lower for fine silver. However, in make-break circuits, silver tends to gradually increase contact resistance. This increase is not necessarily permanent, as contact resistance varies with the effects of arcing on the contacts. Generally, average resistance is higher than the initial resistance before the contacts operate. The contact resistance of 85Ag-15Ni is similar, except that it usually varies within a narrower range. Exhibiting nearly constant contact resistance is more important than possessing low contact resistance. 85Ag-15Ni exhibits a lower contact resistance and is also harder than coin silver. Another advantage of 85Ag-15Ni is its low flammability; that is, it makes a smaller arc than other materials. In testing of more than 40 contact materials, 85Ag-15Ni exhibited the lowest arc energy. Low arc energy is important in that the ability to break a circuit with as little flame as possible is desirable. This characteristic was primarily responsible for the adoption of 85Ag-15Ni for relays in aircraft electrical systems. Silver-Graphite Composites. Graphite is also combined with silver by P/M techniques. Graphite in silver-base composites serves as a good lubricant, reducing the damage caused by frictional forces. Silver-graphite composites are used chiefly as sliding or brush contacts. These materials have high resistance to welding and are also used as make-break contacts. In circuit breakers, they are usually paired with silver-nickel composites. The most frequently used composition is 95Ag-5C, although graphite compositions ranging from 0.25 to 90% with the remainder silver have been used. This material was developed as a circuit breaker contact material. The addition of graphite prevents welding. Frequently, 95Ag-5C is used in combination with silver-nickel or silver-tungsten contacts. It is also used in combination with pure nickel contacts and with fine silver contacts. Silver graphite is soft compared to other types of contact materials, and electrical and mechanical erosion is more rapid. 95Ag-5C has been widely used as a material for contacts in molded-case circuit breakers, sliding contacts, and contact brushes. This material is only moderately ductile and can be rolled into sheets and punched into contacts of various shapes. However, it cannot be headed to make solid rivets or bent to any great extent without cracking. It can be coined to a moderate extent. 95Ag-5C contacts can be individually molded. Depending on size, shape, and quantity, contacts of this material are either punched from rolled slabs, extruded, or individually molded from powders. Copper is combined with graphite as a substitute for silver in certain applications. A modified form of silver graphite is silver-nickel-graphite. Typical compositions are 88Ag-10Ni-2C and 77Ag-20Ni-3C. These materials are substantially harder than 95Ag-5C and exhibit superior wear resistance, but offer less protection against welding. Like 95Ag-5C, they can be manufactured from slabs or by molding individually. Composites of silver iron exhibit good antiwelding and good wear characteristics when used in creep-type thermostat devices. These materials have poor corrosion resistance. Silver-Base Composites with Dispersed Oxides. Type 2 silver-base composites use semirefractory oxides as the dispersed phase. These silver-base composites are produced by a variety of methods such as internal oxidation, preoxidation, and conventional P/M processes (see the section "Composite Manufacturing Methods" in this article). The semirefractory component of type 2 silver-base composites includes metal oxides such as CdO, SnO 2 , or ZnO. In general, the semirefractory constituents promote nonsticking qualities or provide increased resistance to wear. The Ag-CdO group of electrical contact materials is the most widely used of all the silver semirefractory contact materials. The addition of 5 to 15% CdO to silver imparts excellent nonsticking and arc quenching qualities. Because of its resistance against arc erosion and its low contact resistance, which does not increase even after switching, Ag-CdO has proved to be a universally good contact material for many switching devices. Ag-CdO contact materials are well suited for contactors and motor starters, but are also used in circuit breakers, relays, and switches with medium to low currents. Ag-CdO material has antiwelding and antierosion properties united with constant resistance, examples of its main advantage of well-combined properties. Another favorable quality is that it has good workability. It can be fabricated by either the internal oxidation (least costly), preoxidation, or P/M methods. The Ag-CdO material can also be cold reduced or rolled quite easily. For instance, Ag-15CdO material can endure more than 70% cold reduction. Ag-SnO 2 , which is used widely in Europe as a contact material, is a class of composite materials that has the potential to replace Ag-CdO composites in many electrical contact applications. However, general comparisons of Ag-SnO 2 contacts with Ag-CdO contacts are difficult because results may depend on the specific conditions of testing. Previous concerns on the toxicity of CdO, which was one of the motivations for using Ag-SnO 2 contacts, have also been relaxed in Japan and Europe. The toxicity of CdO must be distinguished from the highly toxic nature of cadmium. Like Ag-CdO contacts, Ag-SnO 2 contacts can be produced by internal oxidation or P/M techniques. One drawback of the Ag-SnO 2 composite is that a third element (such as indium) must be added to achieve internal oxidation when the silver alloys contain more than 4% Sn. The oxidized material also does not allow a high level of cold reduction because of its brittleness. Therefore, a press-sinter-re-press method or extruded method is the most feasible way to fabricate Ag-SnO 2 although extruded products are more brittle than extruded Ag-CdO powder of similar compositions. For example, extended Ag-10SnO 2 can be subject to a maximum of 30% cold reduction compared to more than 60% for Ag-12CdO. Another drawback is the higher-temperature rise of Ag-SnO 2 contacts (as compared to Ag-CdO) after arcing. This troublesome characteristic has, however, been eliminated with Ag-SnO 2 materials made by P/M methods. Table 2 lists three grades of commercially available Ag-SnO 2 composite contact materials. Ag-SnO 2 contact materials cannot be easily brazed or welded. To be able to braze Ag-SnO 2 contacts, they are made with at least two layers, the contact layer and brazable or weldable fine silver layer. The brazing alloy can be applied separately in the shape of paste, wire, or foil, or it is already clad onto the semifinished product. Ag-ZnO is another composite material that has been tested and marketed for contact applications. Ag-ZnO, like Ag-SnO 2 composite, cannot take high cold reduction because of the brittleness of the oxidized material. When internal oxidization is used, the maximum zinc content cannot exceed 6% for good oxidation. Typical applications of a commercially available Ag-ZnO composite are listed in Table 2. Multiple-Component Composites. There is no ideal material to meet all conditions for contact applications. If required by manufacturers of switching devices, contact manufacturers can offer composite materials consisting of as many as four or five components. Most of these composites serve only special purposes. They are not universally accepted and generally cost more. Two common three-component composites are listed in Table 2. Powder Metallurgy Electrical Contact Materials Composite Manufacturing Methods The methods used to manufacture composite contact materials can be classified into three major categories: • Standard P/M processes, for producing composites from materials that cannot be conventionally alloyed • Internal oxidation processes, for producing silver-base composites with dispersed oxides • Hybrid consolidation, which is a combination of the internal oxidation and P/M consolidation processes Powder Metallurgy Methods Infiltration is used exclusively for making refractory metal and carbide-base composite contact materials. Metal powder or carbide powder is first blended to the desired composition with or without a small amount of binder to impart green strength, then is pressed and sintered into a skeleton of the required shape. Silver or copper is then infiltrated into the pores of the skeleton. This method produces the most densified composites, generally 97% or more of theoretical density. Complete densification is not possible because of the presence of some closed pores in the sintered skeleton. After infiltration, the contact is sometimes chemically or electrochemically etched so that only pure silver appears on the surface. The contact thus treated has better corrosion resistance and performs better in the early stages of use. Press-Sinter. For small refractory metal contacts (not exceeding about 25 mm, or 1 in., in diameter), a high-density material can be obtained by pressing a blended powder of exact final composition into shape and then sintering it at the melting temperature of the low-melting-point component (liquid-phase sintering). In some cases, an activating agent such as nickel, cobalt, or iron is added to improve the sintering effect on the refractory metal particles. For this process, powders of much finer particle size are required so that more bonding surface exists. However, the skeleton formed by this process is weaker than that formed by the infiltration process. Formation of the skeleton usually shrinks the apparent volume of the refractory portion of the composition, thus bleeding out the molten component onto the surface of the finished contact. Press-Sinter-Repress. The press-sinter-repress process is used for all categories of contact materials, especially those in the silver-base category. Blended powders of the correct composition are compacted to the required shape and then sintered. Afterward, the material is further densified by a second pressing (re-pressing). Sometimes the properties can be modified by a second sintering or annealing. The versatility of this process makes it applicable for contacts of any configuration and of any material. However, it is difficult to obtain material with as high a density as is obtained with other processes. Material thus produced also may have weak bonding between particles. Press-Sinter-Extrude. Blended powder of final composition is pressed into an ingot and sintered. The ingot is then extruded into wires, slabs, or other desired shapes. The extruded material may be subsequently worked by rolling, swaging, or drawing. Material made by this method is usually fully dense. The press-sinter-extrude process is used mostly for silver-base composites. Other processes used for manufacturing silver- base composite contacts are direct extrusion or direct rolling of loose powder. Although they appear to be uncommon, they are economically feasible if the equipment is properly designed and built. Internal Oxidation Silver-base composites with dispersed metal oxides can be produced by internal oxidation. In this process, a silver alloy (such as a silver-cadmium alloy) is first cast into ingots, which are rolled into strips or fabricated further into the finished product form. The silver alloy material is then heated in air or oxygen, so that the oxygen diffuses into the alloy and forms metal oxide particles (such as CdO in the case of a silver-cadmium alloy) dispersed in the silver matrix. Internal oxidation is used in the production of a substantial portion of Ag-CdO composites. The initial silver-cadmium alloy can be internally oxidized either in strip or finished product form. The silver-cadmium alloy is heated between 800 to 900 °C (1470 to 1650 °F) in a furnace with air, oxygen-enriched air, or pressurized oxygen. Under this condition, the oxygen species diffuse into the silver-cadmium alloy and oxidize the cadmium species. Upon the completion of the oxidation, the cross section of the material will display a microstructure of CdO particles embedded in a silver matrix. Contact parts are punched from the strip and then coined into required shapes. The size of the CdO particles and the uniformity of their dispersion are dependent on the temperature and the partial pressure of oxygen. Reduced temperature decreases coalescence of cadmium prior to being oxidized and thereby causes a finer dispersion of CdO. Increasing the partial pressure of oxygen in the furnace increases the diffusion rate of oxygen into the silver. This also causes a finer CdO dispersion by reducing the time available for the cadmium to coalesce. During the internal oxidation of a silver-cadmium alloy, the cadmium species become depleted in zones when the oxygen front moves into the silver-cadmium alloy. The cadmium atoms before the oxygen front immediately diffuse into the zone against the oxygen front. As the oxidation front moves from the surfaces of the strip toward the center, the concentration of the cadmium species becomes increasingly dilute as compared to the original composition. Hence, after the oxidation is completed, the cross section will display a significant oxide-deficient or oxide-depleted zone in the center of the contact body. For some applications the presence of the depletion zone is detrimental, requiring its removal or displacement from the center. There are two common methods to achieve this result. In the first, an oxidation barrier, such as ceramic glaze, is applied to one surface so that the oxidation can proceed from only one side. The second method is to laminate two silver- cadmium sheets of the same size and to form a package by welding along the four edges. After oxidation, the sheets are separated. The oxide-deficient zone will appear on one side (the inner side of the package) of each sheet. Package rolling is another technique for reducing the size of the depletion region. In this method, very thin silver- cadmium sheets are first oxidized. Then a number of sheets (for example, 16 sheets) are stacked together and hot-bond rolled into one slab. The cross section of the final product displays very thin depleted zones equal to the number of sheets. Hybrid Consolidation Various hybrid techniques use a combination of internal oxidation and P/M methods. These methods are used to produce a finer average oxide size and/or a more uniform distribution of cadmium oxides in the matrix of a Ag-CdO composite. Hybrid consolidation methods include preoxidized-press-sinter-extrude and coprecipitation. Table 3 compares Ag-CdO composites manufactured by different methods. Table 3 Comparison of Ag-CdO material made by different methods Properties Press-sinter-re- press Press-sinter- extrude Internal oxidation Preoxidize-press-sinter- extrude Performance characteristics Resistance to arc erosion 3 2 1 1 Resistance to sticking and welding 1 1 2 2 Low contact resistance and temperature rise 1 1 1 1 Arc interruption 3 2 1 1 Resistance to corrosion 1 1 1 1 Material characteristics High mechanical properties 3 2 2 1 Resistance to annealing 3 2 2 1 Electrical and thermal conductivity 2 1 1 1 Flexibility of composition 2 2 2 1 Uniform cadmium oxide distribution 1 1 3 1 Note: 1 indicates that under most conditions this is the preferred material; 2 indicates that under most conditions the material is preferable to 3, but not as good as 1; 3 indicates that the material may be acceptable, but under typical operating conditions it is not as good as 1 or 2. The preoxidized-press-sinter-extrude process combines the oxidation process and the press-sinter-extrude process. Commercially, it is called "preoxidized process." The purpose of this method is to redistribute the oxide-deficient center of Ag-CdO composites. The preoxidized process is used exclusively for making Ag-CdO material. Alloys are reduced to small particles in the shape of flakes, slugs, or shredded foil. These particles are oxidized and then consolidated with the press-sinter-extrude process. Material made by this method is more uniform than the same material made by conventional internal oxidation. Mechanical properties are superior to those of the same material made by the press-sinter-re-press method. The Ag-CdO particulates are made by one of four methods and then are pressed into ingots, sintered, and extruded according to standard metallurgical method. There are four processes to prepare the particulates: • Granulated wire: Silver- cadmium alloy is first made into wire and oxidized. The oxidized wire is then chopped into granules with a length of about 3 mm ( in.). • Low-pressure water atomization: The molten silver- cadmium alloy is atomized by water at a pressure of 100 to 200 kPa (15 to 30 psi). The approximately quarter- inch particulates are in the form of thin twisted flakes. Then the flakes are oxidized for consolidation. • High-pressure water atomization: The molten silver- cadmium alloy is atomized with high water pressure, usually higher than 2750 kPa (400 psi). The powder sizes range between 40 mesh (420 m) and 270 mesh (53 m). Then the alloy powders are oxidized before consolidation. Coprecipitation. Conventional blending or mechanical mixing of silver and CdO powders begins by dissolving the proper amounts of silver and cadmium metals in nitric acid. Compounds of silver and cadmium coprecipitate from the solution when the pH value of the solution is changed by adding either hydroxide or carbonate solutions. During subsequent calcination at about 500 °C (930 °F), the compound mixture decomposes to form a mixture of silver and CdO. Alkali metal content can be controlled in the ppm range by adequate washing. Controlled amounts of sodium, potassium, and lithium may enhance electrical life. Excessive amounts of these elements can lead to rapid erosion, restrike, and generally poor electrical life. Depending on device design, the range may be from 10 to 300 ppm. Contacts are consolidated from this mixture by conventional P/M methods. The microstructure of contacts made by this method displays a finer particle size and a more uniformly dispersed CdO phase than material made by conventional blending. Powder Metallurgy Electrical Contact Materials Availability Silver, gold, platinum, palladium, and most of their ductile alloys are available as stamped contacts. Except for material from which contact disks are produced, a variety of stock sizes are not maintained because, in general, no two applications are identical. For disks, material in strip form, varying in thickness from 0.25 mm to 1 mm (0.010 to 0.040 in.) is available. Except for tungsten and molybdenum, contact materials are ductile enough so that they can be produced in all contact forms. Tungsten and molybdenum and some of the P/M materials have lower ductility and are available in fewer forms. The commercially available forms of common electrical contact materials are listed in Table 4. Table 4 Commercially available forms of electrical contact materials Product form Manufacture method Alloy Solid rivet Wire Strip Tape Disks Attached (a) Composite weld disks (b) Clad (c) Rings Brushes Melting P/M 100 Ag X X X X X X X X X . . . X . . . 100 Pd X X X X X X X X X . . . X . . . 100 Au X X X X X . . . . . . X . . . . . . X . . . 100 Ru . . . . . . X . . . X X . . . . . . . . . . . . X X 100 Ir . . . X X . . . X X . . . . . . . . . . . . X X 100 Pt X X X . . . X X X X . . . X X X [...]... 92.5Ag-7.5Cu 90Ag-10Cu 75Ag-24.5Cu0.5Ni 72Ag-28Cu 99Ag-1Pd 97Ag-3Pd 90Ag-10Pd 90Ag-10Au 97Ag-3Pt 85Ag-15Cd 95Ag-5CdO 90Ag-10CdO 85Ag-15CdO 90Ag-10Fe 90Ag-10W 50Ag-50WC 65Ag-35WC 75Ag-25Zn 85Ag-15Ni 70Ag-30Ni 70Ag-30Mo 97Ag-3 graphite 95Ag-5 graphite 60Pd-40Ag 60Pd-40Cu 95Pd-5Ru 75Au-25Ag 90Au10Cu(Coin) 95Pt-5Ru 90Pt-10Ru 90Pt-10Rh 90Pt-10Ir 85Pt-15Ir 80Pt-20Ir 75Pt-25Ir 65Pt-35Os 69Au-25Ag6Pt 60Ru-35Ir-5Pt... 1980 18 R German, Particle Packing Characteristics, Metal Powder Industries Federation, 1989, p 21 9-2 52 19 L Albano-Muller, Filter Elements of Highly Porous Sintered Metals, Powder Metall Int., Vol 14, 1982, p 7 3-7 9 20 F Lenel, Powder Metallurgy Principles and Applications, Metal Powder Industries Federation, 1980, p 14 3-1 54 21 V Tracey, The Roll-Compaction of Metal Powders, Powder Metall., Vol 12 (No... Vol 65 (No 2), 1966, p 8 0-8 3 81 J Snyder, P/M Porous Parts, Powder Metallurgy, Vol 7, ASM Handbook, American Society for Metals, 1984, p 69 6-7 00 82 W Mossner, "Applications and Properties of Controlled Porosity P/M Parts," SSI Sintered Specialties, 1986 83 F Lenel, Powder Metallurgy Principles and Applications, Metal Powder Industries Federation, 1980, p 35 9-3 80 84 W Johnson and M Shenuski, Controlling... Materials: Current and Future Trends, Int J Powder Metall Powder Technol., Vol 12 (No 1), 1976, p 2 5-3 5 8 Porous Metal Products for OEM Applications, Mott Technical Handbook, Mott Corporation, 1996, Sections 100 0-9 000 22 N Williams and V Tracey, Porous Nickel for Alkaline Battery and Fuel Cell Electrodes: Production by Roll-Compaction, Int J Powder Metall., Vol 4 (No 2), 1968, p 4 7-6 2 77 C Moreland and B Williams,... and Size Distributions," American Filtration Society Meeting on the Pore, 1991, p 1-1 2 51 "Permeable Sintered Metal Materials Determination of Fluid Permeability," Standard 4022, International Standards Organization, 1987 52 R German, Powder Metallurgy Science, 2nd ed., Metal Powder Industries Federation, 1994, p 38 0-3 86 53 F Lenel, Chapter 15, Powder Metallurgy Principles and Applications, Metal Powder. .. Steel P/M Filters, Int J Powder Metall., Vol 12 (No 4), 1976, p 37 1-3 86 14 R German, Powder Metallurgy Science, 2nd ed., Metal Powder Industries Federation, 1994, p 2 8-8 1 15 M Phillips and J Porter, Comp., Advances in Powder Metallurgy and Particulate Materials, Part 1, Metal Powder Industries Federation, 1995 Porous Powder Metallurgy Technology Mark Eisenmann, Mott Corporation Processing Methods After... Materials, Poroshk Metall., 1989, p 66 8-6 73 61 "Determination of Properties of Sintered Bronze P/M Filter Powders," Standard 39, Metal Powder Industries Federation, 1983 62 "Tension Test Specimens for Pressed and Sintered Metal Powders," Standard 10, Metal Powder Industries Federation, 1963 63 J Davis, Ed., ASM Specialty Handbook: Stainless Steels, ASM International, 1994, p 21 1-2 12 64 D Ro and E Klar, Corrosion... green strength and require careful handling; they often have to be manually picked off the press and placed in a sintering tray Higher-green-density parts in the 50 to 80% range have sufficient green strength to allow the press feed shoe to eject the part off the die table and into a container For low-density parts made from coarse powders, press rates can be less than 5 parts/min and part handling is... steel and nickel-base alloy powders are produced by water or gas atomization (refer to Sections on production of stainless steel and high-alloy powders and production of nickel-base powders in this Volume) Water atomization produces powders with rounded, irregular shapes that can be processed by compaction and sintering, as shown in Fig 3 and 4 These particles interlock when compacted to form a part. .. Current and Future Trends, Int J Powder Metall Powder Technol., Vol 12 (No 1), 1976, p 2 5-3 5 3 V Tracey and N Williams, The Production and Properties of Porous Nickel for Alkaline Battery and Fuel Cell Electrodes, Electrochem Technol., Vol 3 (No 1-2 ), 1965, p 1 7-2 5 4 M Eisenmann, A Fischer, H Leismann, and R Sicken, P/M Composite Structures for Porous Applications, Proc 1988 Int P/M Conf., Metal Powder . breakers (a) PSR, press-sinter-re-press; INF, press-sinter-infiltrate; PS, press-sinter; PSE, press-sinter-extrude; IO, internal oxidation; PPSE, preoxidize-press- sinter-extrude; SF, oxidized. 37 1-3 86 14. R. German, Powder Metallurgy Science, 2nd ed., Metal Powder Industries Federation, 1994, p 2 8-8 1 15. M. Phillips and J. Porter, Comp., Advances in Powder Metallurgy and Particulate. certain applications. A modified form of silver graphite is silver-nickel-graphite. Typical compositions are 88Ag-10Ni-2C and 77Ag-20Ni-3C. These materials are substantially harder than 95Ag-5C and