Ceramic products and process for producing the same

Abstract: A process for producing a ceramic product comprises the steps of adding, as desired, chromium salt and magnesium salt to aluminum salt as starting materials, calcining the salts to obtain fine powder mixture of metal oxides which comprises alumina as a main component, forming the powder mixture into a desired shape of body, and sintering and hot isostatically pressing the formed body.

Description: This invention relates to ceramic products, for example, precision ceramic products such as bonding capillary, optical connector, wire guide, etc., and a process for producing the same. There have been used ceramic products, for example, as a carrier of gold wires for connecting electrodes with fingers of a lead frame on a semiconductor chip surface. Such ceramic products are obtained generally by preparing a starting powder mixture comprising primarily alumina (Al₂O₃) and additives such as chromia (Cr₂O₃), magnesia (MgO), strontia (SrO), yttria (Y₂O₅) or lanthana (La₂O₃), etc., forming and sintering the powder mixture. Processes for producing such ceramic products noted above are disclosed in Japanese Patent Application Laid-Open Print Nos. 97572/1984 (laid open June 5, 1984) [Chem. Abs. 101:154244y] and 291449/1986 (laid open Dec . 22, 1986), for example. The process disclosed in the former Print No. 97572/1984 comprises the steps of mixing 0.01 to 5.0 mol% of chromia and 0.01 to 0.5 mol% of magnesia as additives into alumina, forming the mixture and then sintering the formed body in a vacuum below 13.33 Pa (10⁻¹ torr) at a temperature ranging from 1,400 °C to 1,800 °C, to thereby facilitate control of the sintering atmosphere and reduce the amount of the additives. The process disclosed in the latter Print No. 291449/1986 comprises the steps of mixing magnesium chloride or magnesium sulfate in aqueous solution into fine powder of alumina in the proportion of 0.1 to 3.0 mol% thereof, forming the mixture after drying and granulation, and then sintering the formed body in a vacuum of 0.133 Pa (10⁻³ torr) at a temperature ranging from 1,400°C to 1,600°C (or 1,350°C to 1,550°C when the sintered body is further hot isostatically pressed), to thereby uniformly deposit spinel (MgAl₂O₄) in the grain boundary layer of alumina whereby inhibiting abnormal growth of grains, so that pores remaining in the boundaries are reduced. The products obtained by the process according to the former Print No. 97572/1984 have, however, a larger average grain size, resulting in inferior light transmissibility and hardness lower than 2000 HV, thus problems in machinability and mechanical durability would remain. On the other hand, in the process according to the latter Print No. 291449/1986, the sintering step should be performed in a vacuum, rendering the devices for performing the process large-scale. Further, in both of the Prints, there remain such drawbacks that the sintering steps are performed at high temperatures, respectively. US-A-3871891 relates to the preparation of a ceramic material by pressing an aluminium oxide powder with a binder followed by compacting and heating to a temperature of 1280°C to 1500°C. However, such a process produces products which require improvement. It is an object of the present invention to provide a ceramic product the grain sizes of which are fine and uniform, the sizes of possible pores remaining in the grain boundaries of which are restricted to small sizes, and which has a high hardness and superior light transmissibility, as well as a process for producing the same. Thus, according to one aspect, the present invention provides a ceramic product comprising: grains in a solid solution state comprising 0.7 to 3.0 weight % chromia balance alumina, or 0.7 to 3.0 weight % chromia and 0.05 weight % or less magnesia balance alumina, the sizes of said grains being no more than 4.0 µm, said grains having an average grain size of 2.0 µm or less, the sizes of the pores remaining in the interior of said product being less than 0.1 µm, and said product having a hardness of higher than 2,000 HV. In another aspect, the present invention provides a process for producing a ceramic product, comprising the steps of: preparing a uniform fine powder mixture of metal oxides comprising 0.7 to 3.0 weight % chromia and if necessary 0.05 weight % or less magnesia balance alumina by preparing salts of aluminium, chromium and, if necessary, magnesium into a suspension, spray drying said suspension to obtain a fine powder mixture of said salts and calcining said fine powder mixture; forming said powder mixture into a desired shape after adding an organic binder thereto, sintering the resultant formed body at a temperature of 1280°C to 1350°C to obtain a sintered body; and hot isostatically pressing said sintered body at a temperature of 1300°C to 1310°C under a pressure of 101.3 MPa (1,000 atm.) to 202.6 MPa (2,000 atm.). In a further aspect, the present invention provides a process for producing a ceramic product, comprising the steps of: preparing a uniform fine powder mixture of metal oxides comprising alumina only or 0.05 weight % or less magnesia balance alumina by preparing salts of aluminium and, if necessary, magnesium into a suspension, spray drying said suspension to obtain a fine powder, and calcining said fine powder, forming said fine powder into a desired shape after adding an organic binder thereto, sintering the resultant formed body at a temperature of 1280°C to 1350°C to obtain a sintered body, and hot isostatically pressing said sintered body at a temperature of 1300°C to 1310°C under a pressure of 101.3 MPa (1,000 atm.) to 202.6 MPa (2,000 atm.). Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a block diagram showing the respective steps of the production process according to the first embodiment of the present invention; Fig. 2 is a block diagram showing a modification of the step for obtaining fine powder mixture in the first embodiment; Fig. 3 is a block diagram showing the respective steps of the production process according to the second embodiment of the present invention; Fig. 4 is a block diagram showing a modification of the step for obtaining fine powder mixture in the second embodiment; and Fig. 5 is a list showing the physical properties of the ceramic products obtained in the first and the second embodiments and the conventional products. The first embodiment according to the present invention is described hereinafter with reference to Figs. 1 and 2. In this embodiment, as starting materials, there are firstly prepared aluminium salt 1, chromium salt 2 and magnesium salt 3, which change to alumina, chromia and magnesia of purities higher than 99.99 %, respectively, when spray dried and thereafter calcined. Specifically for example, ammonium alum or aluminum ammonium carbonate hydroxide (AACH:NH₄AlCO₃(OH)₂), chromium nitrate Cr(NO₃)₂ 9H₂O and magnesium nitrate Mg(NO₃)₃ 6H₂O may be used as the salts 1, 2 and 3, respectively. The salts 1, 2 and 3 are weighed and once prepared into a suspension 4 and a mono pot mill with mono balls therein, such as nylon balls. The suspension 4 is spray-dried to obtain fine powder mixture of the salts, and this powder mixture is then calcined at a temperature ranging from 500 °C to 1,050 °C, for example at 780 °C, for 2 hours, to obtain uniform fine powder mixture 5 of metal oxides, i.e., alumina, chromia and magnesia. The fine powder mixture 5 may be milled in a solvent such as ethanol to break possible agglomerates in the mixture 5, and thereafter spray dried again, if necessary. The fine powder mixture 5 contains from 0.7 weight % to 3.0 weight % chromia and from 0 (zero) to 0.05 weight % magnesia balance alumina. Magnesia is not necessarily included in the mixture 5. As shown in Fig. 2, the above noted fine powder mixture 5 of metal oxides may be also prepared by firstly spray drying and calcining the salts 1, 2 and 3 to separately obtain alumina 1a, magnesia 2a and chromia 3a, and thereafter weighing and mixing them likewise to the above. To the thus obtained fine powder mixture 5 of metal oxides is added an organic binder which is composed of thermoplastic resin as a main component, and then the mixture 5 is formed into a desired shape of body 6 by injection molding. The formed body 6 is sintered in air at a temperature ranging from 1,280 °C to 1,300 °C, in a vacuum ranging from 13.33µ Pa (10⁻⁶ torr) to 0.133 Pa (10⁻³ torr), or in one of N₂, Ar and/or H₂, after resin extraction from the body 6. When in air, the temperature is raised, for example, at the rate of 200 °C an hour, and held at 800 °C and 1,295 °C for 1 hour, respectively. The sintered body 7 is hot isostatically pressed in argon under a pressure ranging 101.3 MPa (1,000 atm.) to 202.6 MPA (2,000 atm.) at a temperature ranging from 1,300 °C to 1,310 °C for 1 hour, and thereafter precisely ground and polished by means of diamond paste, to obtain a final ceramic product 8 which is a bonding capillary, here. Referring now to Figs. 3 and 4, the second embodiment according to the present invention will be described hereinafter. In this embodiment, as starting materials, there is firstly prepared aluminium salt 11 solely, or together with magnesium salt 12, which change to alumina and magnesia of purities higher than 99.99 %, respectively, when spray-dried and thereafter calcined. Likewise to the first embodiment, as starting materials, ammonium alum or aluminum ammonium carbonate hydroxide (AACH:NH₄AlCO₃(OH)₂) and magnesium nitrate Mg(NO₃)₃ 6H₂O may be used as the salts 11 and 12, respectively. When the magnesium salt 12 is mixed with the aluminium salt 11, the former salt 12 is weighed to the amount of 0.05 weight % or less in terms of magnesia. The aluminum salt 11 is solely, or together with the magnesium salt 12, prepared into a suspension 14 in the same manner with the first embodiment. The suspension 14 is spray-dried to obtain fine powder of the salt 11 or fine powder mixture of the salts 11 and 12, and this powder or powder mixture is then calcined to obtain uniform fine powder mixture 15 of metal oxides, i.e., solely alumina or alumina and magnesia, again in the same manner with the first embodiment. The fine powder mixture 15 may be also prepared, as shown in Fig. 4, by firstly spray-drying and calcining the salts 11 and 12 to separately obtain fine powder 11a of alumina and fine powder 12a of magnesia, and thereafter weighing and mixing them likewise to the above. The fine powder mixture 15 may be milled by means of a solvent such as ethanol to break possible agglomerates in the mixtures 15, and thereafter spray-dried again, if necessary. Further, to the mixture 15 is added an organic binder which is composed of thermoplastic resin as a main component, and then the mixture 15 is formed into a desired shape of body 16 by injection molding. The formed body 16 is sintered after debinding, to obtain a sintered body 17, which is hot isostatically pressed, under the same conditions as the first embodiment, and thereafter precisely ground and polished by means of diamond paste, to obtain a final ceramic product 18 which is a bonding capillary, here. Referring next to Fig. 5, there are compared physical properties of test pieces P₁ to P₈ of ceramic products produced by means of one of the processes according to the first and second embodiments of the present invention with those of test pieces P₉ to P₁₂ of ceramic products produced by conventional processes. In Fig. 5, the test pieces P1 - P6 are produced by means of the process shown in the first embodiment, the test pieces P7 and P8 by means of the process shown in the second embodiment, and the test pieces P9 - P12 by the conventional processes as described below. The test pieces P9 - P11 are obtained by preparing formed bodies each comprising a uniform mixture of alumina, chromia and magnesia, and sintering the formed bodies at a temperature ranging from 1,400 °C to 2,000 °C in a vacuum lower than 13.33 Pa (10⁻¹ torr). Particularly, for the test piece 11, alumina as starting material is obtained by the Bayer methods, and a formed body is sintered at 1,575 °C for 1 hour, and then hot isostatically pressed in argon at 1,500 °C under a pressure of 101.3 MPa (1,000 atm.) for 1 hour. For the test piece 12, 99.3% alumina is used, and a formed body is sintered at 1,575 °C for 1 hour and then hot isostatically pressed in argon at 1,500 °C under a pressure of 101.3 MPa (1,000 atm.) for 1 hour. The test for durability (Dura) is performed by making a ultrasonic type bonding capillary on trial, repeating wire bonding 200,000 times and then washing the capillary with aqua regia. The surface of the tip end is observed by an electron microscope (x400 magnification), and if there is no wear in the test piece it is rated A, and if slight wear is addmitted it is rated B. The test piece which has defects and/or chipping is rated C. The test for machinability (Mach) is performed by grinding and polishing test pieces by a precise grinding and polishing machine rotating at 60 rpm under a load of 4,91 N (500gf,) as follows. Grinding: in water, with metal bond diamond of 200 # on whetstone for 10 min. Polishing: in oil, with 8 µm sized diamond paste on earthenware plate for 10 min. Polishing: in oil, with 1 µm sized diamond paste on copper plate for 20 min. Polishing: in oil, on diamond paste buff of 0.5 µm for 10 min. Where the ground and polished surface of the test piece becomes mirror-surface it is rated as A, if damages remain partially, as B, and in case that chipping is remarkable, as C. For pores of diameters of 0.1 µm or more and grain size of 4.0 µm or more, if there is no such pores in the test piece or such large sized grain, it is rated as NON, and in case that there exists such pore or large sized grain, as BE. Translucence (Tr) is checked by looking through if the letters on a newspaper placed at a distance of 1 cm apart are readable through a sintered body sliced and polished into a thickness of 0.3 mm, and test pieces are rated as Good when readable, and as Bad when not readable. Further, average grain size (Av-Sz), three point flexure strength I, Vickers hardness H, density (Dens) and thermal conductivity (Cond) of the test pieces are also shown in Figure 5, in which flexure strength I and Vickers hardness H are measured at room temperature. As is apparent from Fig. 5, the present products are improved to great extent in durability, machinability, hardness and translucence, etc. Various reasons for the improvements are given below. Because aluminum salt is adopted as the starting material, sintering step can be performed even at a lower temperature. In other words, since almost pure alumina obtained by calcining aluminum salt results in having its surface activated, sintering can be completed at lower temperature. Such sintering at low temperature inhibits abnormal growth of grains so that grain sizes remains uniform and fine, even without addition of magnesia. The step of hot isostatic pressing gives rise to lattice strains in the body, whereby a hardness of the body can be enhanced. Particularly, since the sintering is performed at a temperature higher than 1,280 °C and the hot isostatic pressing is performed at a temperature higher than 1,300 °C under a pressure higher than 101.3 MPa (1,000 atm.), pores of 0.1 µm or more will not remain. Since the sintering is performed at a temperature lower than 1,350 °C and the hot isostatic pressing is performed at a temperature lower than 1,310 °C under a pressure lower than 202.6 MPa (2,000 atm.), sizes of grains remain no more than 4.0 µm. Particularly, when a polycrystalline artificial ruby is produced with addition of chromium, chromium is liable to be self-dispersed on material surface to form an oxide coating layers enriched in chromium on the surface of the ceramics product, to improve hardness and corrosion resistance. However, if the proportion of chromia is 0.7 weight % or less, no desired hardness and corrosion resistance can be obtained, while at a level of 3 weight % or more, pores of 0.1 µm or more will remain in the product even after hot isostatic pressing, whereby no desired toughness and strength can be expected. Thus, it is preferable to control the proportion of chromia (or chromium salt calculated in terms of chromia) in a range of 0.7 to 3.0 weight % (while treating the mixture of alumina, chromia and magnesia as 100 weight %). The radius of chromium ion is approximately equal to that of aluminum ion (with difference by about 12%), and therefore chromium ions substitute for aluminum ions during sintering, whereby lattice strains occur to improve hardness, which does not occur unless otherwise hot isostatic pressing is performed. Thus, according to the processes of the present invention, it becomes possible to obtain a ceramic product sizes of grains of which are uniform, and which has excellent translucence, excellent strength, hardness and machinability, and has dense and fine structure.

Claims: A ceramic product comprising: grains in a solid solution state comprising 0.7 to 3.0 weight % chromia balance alumina, or 0.7 to 3.0 weight % chromia and 0.05 weight % or less magnesia balance alumina, the sizes of said grains being no more than 4.0 µm, said grains having an average grain size of 2.0 µm or less, the sizes of the pores remaining in the interior of said product being less than 0.1 µm, and said product having a hardness of higher than 2,000 HV. A process for producing a ceramic product, comprising the steps of: preparing a uniform fine powder mixture of metal oxides comprising 0.7 to 3.0 weight % chromia and if necessary 0.05 weight % or less magnesia balance alumina by preparing salts of aluminium, chromium and, if necessary, magnesium into a suspension, spray drying said suspension to obtain a fine powder mixture of said salts and calcining said fine powder mixture; forming said powder mixture into a desired shape after adding an organic binder thereto, sintering the resultant formed body at a temperature of 1280°C to 1350°C to obtain a sintered body; and hot isostatically pressing said sintered body at a temperature of 1300°C to 1310°C under a pressure of 101.3 MPa (1,000 atm.) to 202.6 MPa (2,000 atm.). A process as claimed in claim 2, wherein said sintering is performed in a vacuum of 13.33 µPa (10⁻⁶ torr) to 0.133 Pa (10⁻³ torr), or in one of N₂, Ar or H₂. A ceramic product comprising grains in a solid solution state comprising alumina only, or 0.05 weight % or less magnesia balance alumina, the sizes of said grains being no more than 4.0 µm, said grains having an average grain size of 2.0 µm or less, the sizes of the pores remaining in the interior of said product being less than 1 µm, and said product having a hardness of greater than 2,000 HV and a flexural strength of 686.5 MPa (70 kg/mm²) or more. A process for producing a ceramic product, comprising the steps of: preparing a uniform fine powder mixture of metal oxides comprising alumina only or 0.05 weight % or less magnesia balance alumina by preparing salts of aluminium and, if necessary, magnesium into a suspension, spray drying said suspension to obtain a fine powder, and calcining said fine powder, forming said fine powder into a desired shape after adding an organic binder thereto, sintering the resultant formed body at a temperature of 1280°C to 1350°C to obtain a sintered body, and hot isostatically pressing said sintered body at a temperature of 1300°C to 1310°C under a pressure of 101.3 MPa (1,000 atm.) to 202.6 MPa (2,000 atm.). A process as claimed in claim 5, wherein said sintering is performed in a vacuum of 13.33 µPa (10⁻⁶ torr) to 0.133 Pa (10⁻³ torr), or in one of N₂, Ar or H₂.

Inventors: HAYASHI KOICHI, SUZUKI TAKESHI, HAYASHI, KOICHI, SUZUKI, TAKESHI
Applicants: TOTO LTD, TOTO LTD.
Application year: 1988
Main classification: C04B35
IPC codes: C04B35