JPH021202B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a method for sintering a porous body, and in particular, during compaction of copper-based powder, a sintering aid is added that is thermally decomposed or reduced to generate CO ₂ or H ₂ O before the sintering process. The surface of the powder particles is activated by decomposing the undecomposed substances produced during dewaxing of the lubricant with a reducing gas, and porous sintered bodies with excellent sinterability can be obtained safely and economically. This is how it was done. In general, porous sintered bodies are produced using zinc stearate (lithium,
Metal soaps such as calcium and aluminum are added as lubricants to extend the life of the mold and reduce molding pressure. This powder compact is produced when the surface of the powder particles is oxidized or covered with a stable compound.
Bonding between particles is hindered, diffusion at particle contact points becomes insufficient, and sintering takes a long time, so sintering is performed in a strongly reducing atmosphere to reduce the oxide film on the particle surface. At the same time, it prevents oxidation during sintering, resulting in a clean and stable porous sintered body. Lubricants such as the zinc stearate mentioned above also interfere with the bonding between powder particles during sintering, so a dewaxing process is used to thermally decompose the lubricant in a temperature range of 350° to 500°C and vaporize it to remove it. There is. This dewaxing step is carried out either in an oxidizing atmosphere or in a reducing or neutral atmosphere, but in either case, undecomposed lubricant is carried into the sintering step. The thermal decomposition reaction when dewaxing zinc stearate Zn(C ₁₈ H ₃₅ O ₂ ) ₂ is shown as Zn(C ₁₈ H ₃₅ O ₂ ) ₂ →ZnCO ₃ +R−CO−R at 350° to 500°C. (R is an alkyl group) → ZnO + C, and in an oxidizing atmosphere C is oxidized and removed, but ZnO remains as an undecomposed product, and in other neutral or weakly reducing and strongly reducing atmospheres
Both ZnO and C enter the sintering process as undecomposed products. The sintering process is performed in a strongly reducing atmosphere (e.g. H ₂ 75
%, _{N2 (} 25% decomposed ammonia gas),
At a sintering temperature of 700° to 1000°C, NnO is reduced to produce metallic zinc and evaporate according to the following reaction formula. ZnO+H ₂ →H ₂ O+Zn On the other hand, C causes the following reaction and becomes a reducing gas and vaporizes. H ₂ O + C → H ₂ + CO Also, when sintering is performed using CO as the atmospheric gas, ZnO and C undergo the following reactions, respectively: ZnO + CO → Zn + CO ₂ CO ₂ + C → 2 CO, and both evaporate and vaporize. However, the sintering process is carried out in a weakly reducing atmosphere, e.g.
When carried out in a gas containing 3% H ₂ and the balance N ₂ , ZnO and H ₂
The reaction with C becomes extremely slow, and not only a part is reduced but most remains, the gasification reaction of C is also very small, and these undecomposed substances are caused by contact between powder particles during sintering. This results in insufficient internal diffusion and inhibits sintering. According to the inventor's experiments, zinc stearate was heated in the air at a temperature of 500°C for 30 minutes to generate ZnO, and this ZnO was used to convert H ₂ (100%) at 800°C.
When reduced for 40 minutes in a crucible in a hydrogen furnace introduced at 17/min, the unreduced amount was 51.72% by weight.
Approximately 48% by weight is reduced, while 3% of _H2 ,
When gas with a balance of _N2 was introduced at 2/min for 40 minutes at 780°C, the amount of unreduced gas reached 78.9% by weight, which was approximately 20% by weight.
It was confirmed that only % by weight was reduced. For this reason, a method of sintering in a strongly reducing atmosphere has traditionally been adopted;
Since the _H2 concentration is above the explosive limit (4%), there are safety issues such as high costs in terms of equipment and operation maintenance, and the need for skill to operate. It had the disadvantage of requiring expensive equipment. This invention was made to eliminate the above-mentioned drawbacks, and an object of the invention is to weaken the undecomposed substances of the lubricant by adding a sintering aid that decomposes it in the temperature range up to the sintering process. It is an object of the present invention to provide a method for sintering a porous body that enables sintering in a reducing atmosphere or a neutral atmosphere.It is also an object of the present invention to provide a method for sintering a porous body that can be operated safely and with reduced equipment and cost. The object of the present invention is to provide an inexpensive and economical method for sintering a porous body. In other words, this invention adds a carbonate that generates CO ₂ through self-thermal decomposition, a hydroxide that generates H 2 O, or a hydroxide that generates H ₂ O as a sintering aid to a mixture of copper-based raw material powder and a lubricant. 1 to 10% by weight of a metal oxide that generates H ₂ O and pure metal is added and compacted, and this compacted powder is heated in a weakly reducing or neutral atmosphere to dewax. The lubricant is thermally decomposed in a temperature range, and then the gas generated from the sintering aid is reacted with the products (e.g. metal oxides, carbon) during dewaxing of the lubricant in a sintering temperature range to generate the lubricant. This method is characterized by dissipating the substance as a gas, activating the powder particle surface, and sintering it. In this invention, a sintering aid is added to a copper-based powder compact, and CO ₂ or H ₂ O generated by thermal decomposition or reduction of the sintering aid is combined with decomposition products of a lubricant. It is designed to actively react during the compaction process (700°C) to generate reducing gases such as CO or H ₂ inside the powder compact. Sintering aids include substances that self-thermally decompose during the heating process up to the sintering temperature to produce CO ₂ or H ₂ O and metal oxides, as well as substances that are reduced by H ₂ and produce H ₂ O. A substance that decomposes into pure metal and pure metal can be used as appropriate. Substances that generate CO ₂ and oxides through self-decomposition include CaCO ₃ , ZnCO ₃ , MnCO ₃ , Li ₂ CO ₃ ,
Carbonates such as MgCO ₃ , FeCO ₃ , PbCO ₃ and BaCO ₃ are suitable. For example, when CaCO ₃ is thermally decomposed, CO ₂ is generated through the reaction of CaCO ₃ →CaO + CO _2. Therefore, in the sintering temperature range, the reaction is governed by an equilibrium state reaction equation in which undecomposed C and CO ₂ react. The reaction progresses to the extent that C does not affect sinterability. C+CO ₂ →2CO Therefore, undecomposed ZnO reacts with CO, becomes metallic zinc, and evaporates. ZnO + CO → Zn + CO ₂ Therefore, when such carbonates are used as sintering aids, both ZnO and C can be vaporized even in a neutral atmosphere, but in a weakly reducing atmosphere or a neutral atmosphere. It will be more effective if sintering is performed inside. Next, substances that generate H ₂ O and oxides through autothermal decomposition include Ca(OH) ₂ , Zn(OH) ₂ ,
_Al2O3・_{xH2O ,} Cd(OH) ₂ , Cr(OH) ₃ , Ba
(OH) ₂ , Mg(OH) ₂ , FeO(OH), Pb(OH) ₄ , Ni
Hydroxides such as (OH) ₂ and NiO(OH) are suitable. For example, when Ca(OH) ₂ is thermally decomposed, H ₂ O generated by the reaction Ca(OH) ₂ →CaO + H ₂ O reacts with the decomposition product C in the sintering temperature range to generate H ₂ . Generate. C+H ₂ O→H ₂ +CO Therefore, undecomposed ZnO reacts with H ₂ or CO, becomes metallic zinc, and evaporates. ZnO + H ₂ → Zn + H ₂ O ZnO + CO → Zn + CO ₂ Therefore, when such a hydroxide is used as a sintering aid, both ZnO and C, which are decomposition products, are removed even in a weakly reducing or neutral atmosphere. It becomes possible to vaporize. The metal oxides produced by thermal decomposition of the above carbonates and hydroxides are extremely stable under sintering conditions, but their volume is reduced by thermal decomposition compared to the above compounds before the reaction. Therefore, in the early stage of the sintering process, the phenomenon of rearrangement of powder particles due to surface diffusion, plastic flow, and surface tension due to the generation of a liquid phase is not hindered, and new particles generated during the sintering process are Contacts are formed to promote diffusion of the matrix powder particles and do not hinder sintering. However, such a favorable influence on the sintering mechanism is limited to the amount of carbonates and hydroxides added within the range of 1 to 10% by weight based on the compacted compact, and if it exceeds this range, This is not preferable because it inhibits the formation of new particle contact points due to particle rearrangement. Furthermore, it is reduced by _H2 in the atmosphere.
Substances that decompose into H ₂ O and pure metals include SnO,
SnO ₂ , PbO, CdO, ZnO, Fe ₂ O ₃ or FeO,
Metal oxides such as CuO and NiO are suitable. For example, when SnO is reduced, H ₂ O is generated through the reaction SnO + H ₂ →Sn + H ₂ O, so just like in the case of hydroxide, ZnO, which is a decomposition product of lubricant, and C can be vaporized. The pure metal Sn produced at this time forms a eutectic compound with the matrix material or diffuses into the matrix. The purpose can also be achieved by setting the amount of the metal oxide added to 1 to 10% by weight for the same reason as above. Examples of the present invention will be described below. Bronze powder (Sn10%) of 100 mesh or less is mixed with zinc stearate (0.75% by weight), and a sintering aid is added at the mixing ratio shown in Table 1 to produce 3 tons/
A molding pressure of cm ² was applied to form a molded product with a diameter of 20 mm and a length of 20 mm, which was placed in a weakly reducing atmosphere (3% H ₂ , balance N ₂ ,
1.0~2/min, 1.0Kg/cm ² ), heated at 450℃ for 20 minutes, then heated at 500℃ to ensure dewaxing.
It was heated for 20 minutes and then sintered by heating at 780°C for 60-80 minutes. This sintered body is put into a belt grinder (AA-60#).
After grinding, lap polishing (Emery 600# and
1000#), was buffed and etched with potassium dichromate solution, and the structure was observed using an optical metallurgical microscope.

【table】

[Table] Volume shrinkage rates are shown when SnO ₂ is blended at 6% by weight, and when the others are blended at 5% by weight. The quality of sintering shown in Table 1 is determined by the presence or absence of particles falling off during grinding, the surface roughness after grinding (whether it becomes a mirror surface or not), the degree of particle growth observed under an optical metallurgical microscope, the degree of bonding of particle interfaces, etc. This is the result of comprehensive judgment. As is clear from Table 1, all of the porous sintered bodies of the present invention do not have particles falling off during grinding, and when weaving is observed using a metallurgical microscope, diffusion of particles is observed, and particle growth and bonding are observed. It turned out that it works well. In addition, when a metal oxide is added as a sintering aid, the pure metal generated by thermal decomposition evaporates from the matrix, or the eutectic with the matrix is crystallized and diffused, or the metal In some cases, intermediate compounds may be formed. Next, the structures of the conventional sintered body and the sintered body of the present invention are compared using 400 times enlarged photographs taken with an optical metallurgical microscope. Figure 1 shows the structure of a conventional powder compact containing 0.8% by weight of zinc stearate, which was molded, sintered, ground, polished, and corroded under the same conditions as the examples of the present invention. Figure 1 shows the structure of a part of the examples listed in Table 1. In FIG. 1, some places where powder particles have spread can be seen, but some thin particles remain and the particles have not grown sufficiently. Particle interfaces are also clearly visible, and bonding between particles is also insufficient. Some of the particles fell off during belt grinder grinding. Figure 2a shows a case in which 5% by weight of CaCO ₃ is blended, and in the black pores traces of CaO, which is the result of part of CaCO ₃ disappearing due to thermal decomposition, are detected by an X-ray microanalyzer. Ta. CaO at the powder particle interface
Although the growth of the particles is slightly delayed due to the presence of , the particles are completely diffusion bonded at the contact points, indicating a good sintered state. No particles were observed to fall off during grinding. Figure 2b shows the case where 5% by weight of Ca(OH) ₂ is blended, and traces of CaO, which has disappeared due to thermal decomposition of Ca(OH) ₂ , are detected around the coarse pores, indicating the growth of particles. However, in other parts, particle growth is observed, and diffusion at particle contact points is also progressing.
No particles were observed to fall off during grinding. Figure 2c shows the case where 3.4% by weight of SnO is blended. Particle growth and internal diffusion have progressed considerably, pores between particles have disappeared everywhere, and the sintering progress has reached the middle stage. There is. No particles were observed to fall off during grinding. FIG. 2d shows the case where 6.3% by weight of SnO ₂ is blended, and at the surface part, SnO ₂ is reduced and a liquid phase sintered structure of Sn and bronze powder is observed. The core has a structure similar to that shown in Figure 2c, but the undissolved Sn in the bronze α phase, which is the matrix, forms a Sn-rich δ-type compound at the particle interface, which acts as a binder for the matrix particles and forms a matrix. The particles became spherical, indicating that sintering was progressing. No particles were observed to fall off during grinding. Next, when considering the blending ratio of the sintering aid of this invention, the amount of lubricant, which is closely related to the amount of sintering aid, determines the degree of damage to the mold as a single sintered powder, and the degree of damage to the molding die. Considering the ease of sintering, the effect on the performance of the finished sintering device, and economic efficiency, it is usually added in an amount of about 0.5 to 1.0%, so if the amount of sintering aid is less than 1%, it will cause problems during dewaxing of the lubricant. Dissipation of decomposition products is insufficient, and if the amount exceeds 10%, oxides or undecomposed substances will increase as auxiliary decomposition products, which will lead to poor rearrangement and diffusion of powder particles during sintering. The amount of the sintering aid is preferably within the range of 1 to 10%. As described above, in sintering a copper-based porous body, self-thermal decomposition occurs into CO ₂ or H ₂ O and oxides before the sintering temperature is reached in a weakly reducing atmosphere or a neutral atmosphere. or a sintering aid that is reduced with H ₂ and decomposed into H ₂ O and pure metal.
The composition is such that 10% by weight is added, compacted, dewaxed, and sintered. Therefore, according to the present invention, even if dewaxing and sintering of a powder compact is carried out under a weakly reducing atmosphere or a neutral atmosphere depending on the type of sintering aid,
Metal oxides and carbon, which are decomposition products from the lubricant during dewaxing, release _CO2 or carbon in the sintering temperature range.
A reducing gas is generated inside the powder compact by reacting with H ₂ O, and this gas can vaporize the remaining decomposition products, and these decomposition products form bonds between powder particles. It is prevented from interfering with the sintering process. In addition, these sintering aids have a compounding ratio of 1 to 1.
By appropriately adding within the range of 10% by weight, oxides or pure metals generated by thermal decomposition or reduction do not hinder the rearrangement of powder particles during sintering, allowing diffusion at particle interfaces or contact points. Since it is vaporized or vaporized, it cannot inhibit sintering. Furthermore, since this invention can perform dewaxing and sintering using a weakly reducing or neutral atmosphere, it not only enables safe operation without the risk of explosion, but also enables decomposed ammonia gas generation. This eliminates the need for expensive equipment such as a device, and provides the effect that porous bodies can be sintered economically at low cost.

[Brief explanation of drawings]

FIG. 1 is a micrograph showing the structure of a conventional porous sintered body, and FIGS. 2a to 2d are micrographs showing the structure of the porous sintered body of the present invention.

Claims (1)

[Claims] 1 Mix metal soap as a lubricant with the raw material powder of the copper-based compacted compact, and add to this mixture as a sintering aid a carbonate that generates CO ₂ by autothermal decomposition,
or hydroxides that generate H ₂ O, or metal oxides that are reduced to generate H ₂ O and pure metals,
The lubricant is added in an amount of 1 to 10% by weight and pressure molded, and the green compact is heated in a weakly reducing or neutral atmosphere to thermally decompose the lubricant in the dewaxing temperature range, and then in the sintering temperature range. The method is characterized in that the gas generated from the sintering aid is reacted with a product produced during dewaxing of the lubricant, and the product is made into a gaseous body and dissipated, thereby activating the powder particle surface and sintering. A method for sintering a porous body.