Sintered ceramic bodies and ceramic metal working tools

Abstract: A sintered ceramic body which can be worked by electrical discharge machining, has high mechanical strength, and can be worked into sintered ceramic tools which have long life is disclosed. Further, sintered ceramic tools which have long life and scarcely cause flaws in work pieces are provided. The sintered ceramic body comprises ceramic particles having specific particle diameter and particle distribution.

Description: Field of the Invention The present invention relates to sintered ceramic bodies, and a process for preparing the ceramic bodies. More particularly, the invention relates to sintered ceramic bodies which can be worked by wire cutting electrical discharge machining or die-milling electrical discharge machining, without mechanical properties such as strength impaired, wherein the wire cutting and die-milling electrical discharge machinings can be stably carried out; and a process for preparing such ceramic bodies. Background of the Invention Ceramic materials are excellent in mechanical properties, especially high temperature strength, hardness, thermal shock resistance, as well as oxidation resistance, corrosion resistance, etc., but it is said that they are remarkably inferior to metallic materials in workability. Recently, high strength and high hardness materials such as cemented carbide materials have come to be shaped by electrical discharge machining, and the technique is making a rapid progress. The electrical discharge machining generally includes wire cutting electrical discharge machining and die-milling electrical discharge machining. The wire cutting electrical discharge machining is a shaping process which comprises using a thin wire of copperor brass having a diameter of 0.05 - 0.25 mm, which is being wound up under tension, as an electrode, generating discharge energy between the wire and a work piece, advancing the wire like a jigsaw and thus forming a two-dimensional shape. The die-milling electrical discharge machining is a method in which an electrode of a specific shape is used and the inverted shape is formed by electric discharge between the electrode and a work piece. Application of electrical discharge machining to ceramic materials, which are believed to be difficult to shape thereby, is lately being studied. As a result, it has been revealed that ZrO 2 and Al 2 O 3 are made shapable by electrical discharge machining by addition of a suitable amount of TiN, TiC, TiCN, TiB 2 , ZrN, NbC, etc.; and Si 3 N 4 are made shapable by addition of TiN, ZrN, etc. In electrical discharge machining, the influence of the working area effect is great. Therefore, it is necessary that minute protrusions exist dispersed and distributed all over the working area in order to easily induce electric discharge and stably conduct the electrical discharge machining. In this case, the smaller the working area, the smaller the degree of distribution, and thus electric discharge tends to occur at localized positions or points. Therefore, it is required to work with a smaller impact coefficient by reducing working speed, discharge voltage, etc. The "working area effect" herein referred to means the drop of working speed when the working area is excessively small. In the wire cutting electrical discharge machining, the working surface is a small area facing the thin wire electrode and thus the machining always suffers the influence of the working area effect. When a ceramic body containing electrically conductive ceramics is worked by wire cutting electrical discharge machining, the minute area facing the wire electrode is the particles of the conductive ceramic per se. Therefore, if the conductive ceramic particles are not uniformly distributed in a sintered ceramic body, stable wire cutting electrical discharge machining is impossible. As a measure for improving efficiency of wire cutting electrical discharge machining, it is thinkable to increase the amount of conductive ceramics to be added. It has been revealed that increase of conductive ceramics impairs the inherent mechanical properties such as strength, hardness, etc. of sintered ceramic bodies. When conductive ceramics are added, the conductive ceramics must be uniformly distributed in order to stably carry out the wire cutting electrical discharge machining. In this case, if large particle size conductive ceramics are used, the particles cause the working area effect and the wire electrode is broken unless the working speed is reduced. In die-milling electrical discharge machining, the electrode is remarkably larger in comparison with a wire and the working area is far larger in the working direction than in case of the wire electrode, and the working area effect is effectively exhibited and the areal working speed can be enhanced. In addition to the above-mentioned workability, sintered ceramic bodies involve the following problems. Generally in the shaping work using a die, in punching work for instance, work pieces slide on the surface of the die and simultaneously the die per se suffers a great stress. When this is repeated, the die surface is abraded. Microscopic observation of the abrasion reveals that it is caused by wearing of particles of the component constituting the tool, drop-off of particles per se of the component constituting the tool and simultaneous occurrence of the two phenomena. In the case of cemented carbide tools, as they are used repeatedly, work pieces come to be stained, that is, they are contaminated by the cemented carbide material. The cemented carbide tools having suffered surface abrasion can be reshaped by grinding and polishing for further use, although they are discarded when they cannot be reshaped because of their complicated shape. On the other hand, sintered ceramic tools are free from staining of work pieces and suffer less abrasion because of hardness of the particles and tool life (time period until reshaping is required) is longer than the cemented carbide tools. Although sintered ceramic tools less suffer abrasion by drop-off of particles because of strong bond between the particles in comparison with cemented carbide tools, once particle drop-off abrasion happens and especially large particles drop off, reshaping is required at this stage. But the tools are discarded if the shape is complicated and reshaping is impossible. Also when large diameter particles drop off, work pieces are susceptible to scratching and thus rejectable products are turned out. US-A-4 626 517 discloses a sintered body consisting essentially of from 40-99.5% by weight of partially stabilized ZrO 2 and from 0.5 to 60% by weight TiN. The sintered body is prepared by wet-mixing partially stabilized ZrO 2 particles with fine particles of TiN having a mean particle size of 0.1 µm, drying, powdering, press-molding and sintering the resulting mixture at 1.400 to 1.650°C. In all the specimens obtained thereby, the mean grain size was less than 3 µm. EP-A- 0 373 848 relates to an electrically conductive ZrO 2 -based sintered body comprising 50-10 vol.% of TiN and 50 to 90 vol.% of TiO 2 + ZrO 2 . The sintered body is manufactured from commercially available ZrO 2 powder having an average particle size of 0.5 mm (containing 3 mole% of Y 2 O 3 ) and a TiN powder (product of 325 mesh under corresponding to a particle size of smaller than 44 µm). The present invention has been completed in order to solve the above-mentioned various problems relating to sintered ceramic tools. That is, the object of this invention is to provide sintered ceramic bodies, from which long life metal working tools, which have excellent mechanical properties, hardly cause flaws in work pieces and can be easily manufactured, can be prepared. Further, the other object of the present invention is to provide sintered ceramic bodies which are shapable by both wire cutting electrical discharge machining and die-milling electrical discharge machining and can be formed into metal working tools. Summary of the Invention The invention provides a sintered ceramic body comprising insulating ceramic particles and electrically conductive particles, wherein the mean particle diameter of the electrically conductive ceramic particles is not larger than 10 µm, and the content of the electrically conductive ceramic particles is 25-60 vol.% in 100 vol.% of the total amount of said insulating ceramic particles and said electrically conductive ceramic particles, wherein the insulating ceramic particles are at least one selected from the group consisting of zirconia and zirconia-alumina characterized in that the particle diameter of the insulating ceramic particles at the cumulative amount ratio of 90% is not larger than 10 µm. In these sintered ceramic bodies, the ratio (ε) of the mean particle diameter of the conductive ceramic particles (dsp) to the mean particle diameter of the insulating ceramic particles (dm) is preferably not larger than 9. In these sintered ceramic bodies, the mean particle diameter of the conductive ceramic particles is preferably 0.3 - 10 µm. In these sintered ceramic bodies, the mean particle diameter of the conductive ceramic particles is more preferably 0.5 - 4 µm. In these sintered ceramic bodies, the content ratio of the conductive ceramic particles is preferably 30 - 50 vol%. In these sintered ceramic bodies, the ratio (ε) is more preferably 0.01 - 9. In these sintered ceramic bodies, preferably the electric resistivity is not higher than 10 Ω·cm. In these sintered ceramic bodies, more preferably the electric resistivity is not higher than 5 Ω·cm. In these sintered ceramic bodies, most preferably the electric resistivity is not higher than 0.1 Ω·cm. In these sintered ceramic bodies, preferably the conductive ceramic particles are at least one selected from the group consisting of titanium nitride, zirconium nitride, titanium carbide, titanium carbonitride, titanium boride, tungsten carbide and niobium carbide. In these sintered ceramic bodies, more preferably the zirconia is at least one partially stabilized zirconium selected from the group consisting of zirconium oxide stabilized with 1.5 - 6 mol% of Y 2 O 3 , zirconium oxide stabilized with 8 - 10 mol% of MgO and zirconium oxide stabilized with 6 - 12 mol% of CeO 2 . The invention further provides: A process for preparing a sintered ceramic body comprising insulating ceramic particles and electrically conductive particles, wherein the insulating ceramic particles are at least one selected from the group consisting of zirconia and zirconia-alumina and the particle diameter of the insulating ceramic particles at the cumulative amount ratio of 90% is not larger than 10 µm, the mean particle diameter of the electrically conductive ceramic particles is not larger than 10 µm, and the content of the electrically conductive ceramic particles is 25 - 60 vol.% in 100 vol.% of the total amount of said insulating ceramic particles and said electrically conductive ceramic particles, comprising the steps of a) mixing insulating ceramic particles having a particle diameter at the cumulative amount ratio of 90% of not larger than 10 µm with electrically conductive ceramic particles having a mean particle diameter of not larger than 10 µm, in a ratio of 25 to 60 vol.% of said electrically conductive ceramic particles in 100 vol.% of the total amount of said insulating ceramic particles and said electrically conductive ceramic particles; b) compacting the mixture; and c) sintering said compact. In this process for preparing sintered ceramic bodies, preferably the insulating ceramic particles having a particle diameter at the cumulative amount ratio of 90% is less than 10 µm is used. The sintered ceramic body according to the invention can be used for preparing metal working tools by a process comprising shaping by electrical discharge machining said sintered ceramic body. Detailed Description of the Inventions The term "mean particle diameter" of the ceramic particles in the sintered ceramic bodies means particle diameter at the cumulative amount ratio of 50%. The term "cumulative amount ratio" can be determined by polishing a sintered ceramic body to mirror smoothness, effecting plasma etching and measuring particle size distribution in an SEM photograph by the Schwarts-Saltykov method. According to the present invention, sintered ceramic bodies containing conductive ceramic particles and insulating ceramic particles contain insulating ceramic particles whose particle diameter at the cumulative amount ratio of 90 % is not larger than 10 µm and conductive ceramic particles whose mean particle diameter (dsp) is not larger than 10 µm, preferably 0.3 - 10 µm and more preferably 0.5 - 4 µm. When the particle diameter at the cumulative amount ratio at 90 % is not larger than 10 µm, the resulting sintered ceramic bodies can be suitably worked by wire cutting electrical discharge machining. The damage of work pieces caused by particles drop-off from the tools made of such sintered ceramic bodies can be held minimum. Further, when the mean particle diameter (dsp) of conductive ceramic particles is not larger than 10 µm, in addition to the condition that the particle diameter at the cumulative amount ratio of 90 % is not larger than 10 µm, the mechanical strengths, especially bending strength and fracture toughness as well as transverse rupture strength of the obtained sintered ceramic bodies are improved and they have long tool life. In the sintered ceramic bodies of the present invention, also it is important that the ratio (ε) of the mean particle diameter of the conductive ceramic particles (dsp) and the mean particle diameter of the insulating ceramic particles (dm) (ε = dsp/dm) is not larger than 9, especially 0.01 - 9. When said ratio (ε) is not larger than 9, the surface roughness (Ra) of the resulting sintered ceramic bodies is not more than 5µm, the dies made from such sintered ceramic bodies scarcely cause flaws in work pieces. It is important that the sintered ceramic bodies of the present invention comprise insulating ceramic particles whose particle diameter at the cumulative amount ratio of 90 % is not larger than 10 µm and conductive ceramic particles whose mean particle diameter is not larger than 10 µm in a ratio of 25 - 60 vol%, preferably 30 - 50 vol% of the latter with the combined amount as 100 vol%. With conductive ceramic particles of less than 25 vol%, the resulting sintered ceramic body cannot be shaped by wire cutting electrical discharge machining. The sintered ceramic bodies of the present invention have a resistivity of not higher than 10 Ω·cm, preferably not higher than 5 Ω·cm and more preferably nor higher than 0.1 Ω·cm. Sintered ceramic bodies having a resistivity of higher than 10 Ω·cm cannot be suitably worked by wire cutting electrical discharge machining. The sintered ceramic bodies of the present invention can be suitably prepared as follows. First of all, ceramic powder (hereinafter the term "powder" is used meaning particles collectively), comprising especially an insulating ceramic powder and a conductive ceramic powder, is mixed in a wet ball mill for instance, using a liquid which comprises a binder such as wax binders, acrylic binders, poval binders, etc. dispersed in a medium such as water, alcohols, ketones, etc. When sintered ceramic bodies are prepared, insulating ceramic particles whose particle diameter at the cumulative amount ratio of 90% is less than 10 µm are used because insulating ceramic powders undergo crystal growth during the sintering process. The particle diameter at the cumulative amount ratio of 90 % of the insulating ceramic particles to be used should preferably be not more than 8 µm, more preferably not more than 3 µm and most preferably not more than 1.5 µm. After the wet mixing, the medium (solvent) of the binder dispersion is removed by a suitable means such as spray drying. The obtained granulated powder may be calcined in order to adjust the particle size distribution. The calcination is carried out by heat-treating the stock powder at a temperature of 800 - 1200 °C in the atmosphere for insulating ceramic powder and in an inert atmosphere for conductive ceramic powder for 1 - 3 hours. Calcination causes crystal growth of the ceramic particles. The uncalcined granulated powder and calcined granulated powder are mixed aiming that the particle diameter at the cumulative amount ratio be not larger than 10 µm if necessary. The particle size adjustment also can be effected by mixing size-classified particles. The granulated powder whose particle size has been adjusted is shaped into a desired shape by press molding. Press molding can be effected by rubber molding, die molding, injection molding, extrusion molding, etc. The thus formed green compact is degreased, that is, the binder is removed if oxidation of the particles at high temperature in the atmosphere is feared. Degreasing or binder-removing is effected by heat-treating the compact at a temperature of 300 - 800 °C, preferably 450 - 530 °C at which the binder is decomposed, preferably in an inert atmosphere, preferably of nitrogen or argon. Thus the binder is decomposed and organic material composing the binder is removed. The binder sometimes burns due to a trace amount of oxygen which exists in an inert atmosphere or by heating in the degreasing or binder-removing. In this case, the binder is also removed. The degreased compact is sintered. The compact can be sintered as is, but it may be pre-sintered before prior to main sintering. In pre-sintering, the degreased compact is heated under pressure until 95 %, preferably 95 - 97 % of fraction to the theoretical density is attained. The temperature employed for pre-sintering is, especially when zirconia is used as an insulating ceramic, usually 1450 - 1650 °C, preferably 1480 - 1550 °C. The pressured applied is usually 1.01-10.1 bar (1 - 10 atm), preferably 2.02-5.06 bar (2 - 5 atm). In sintering, the compact as is or pre-sintered compact is heated under pressure until 99 - 100 % of fraction to the theoretical density is attained. The pressure applied in sintering is usually 506.6-2026.5 bar (500 - 2000 atm), preferably 810.6-1519.8 bar (800 - 1500 atm). The atmosphere in which pre-sintering or main sintering is carried out is an inert gas such as nitrogen, argon, etc. Thus sintered ceramic bodies of the present invention can be prepared. The thus obtained sintered ceramic bodies are dense free from minute pores in interior and in the surface. The sintered ceramic bodies can be formed into metal working tools by electrical discharge machining, especially by wire cutting electrical discharge machining. The metal working tool prepared from the sintered ceramic bodies has high hardness, higher thermal conductivity, excellent shock resistance, toughness, wear resistance, and attains dramatically increased number punching shot because of these characteristics. The metal working tool prepared from the sintered ceramic bodies of the present invention does not cause staining of work pieces when punching work is carried out with it. When lead frames are manufactured by punching for instance, conventional tools cause staining or contamination of on lead frames, which exerts undesirable influence on the final products. But the metal working tool prepared from the sintered ceramic body of the present invention is free from such staining of lead frame. Brief Description of the Attached Drawings Now the invention will be specifically described with reference to the attached drawings, of which: Fig. 1 is a schematic view of a lead frame illustrating the principle of manufacturing semi-conductor chips using the lead frame. Fig. 2 is a perspective view of an example of a punch used for punching metal plate; Fig. 3 is a perspective view of an example of a punching die. Fig. 4 is a perspective view of a ceramic body which undergoes die-milling and wire cutting electrical discharge machining for the evaluation of machinability thereof. Embodiments of the Invention Preferable metal working tools prepared from the sintered ceramic body of the invention include a punching die and a punch to be received in the opening of the punching die. Lead frames used when semi-conductor integrated circuit chips are manufactured can suitably be prepared using a punching die and a punch prepared from the sintered ceramic bodies of the present invention. As shown in Fig. 1, lead frame 1 comprises a frame proper 2 and leads 3 extending from the frame proper. In Fig. 1, a semi-conductor integrated circuit base plate is indicated by C. Fig. 1 represents only one example of the lead frames and it should be understood that there are lead frames of various shapes which can be manufactured using metal working tools prepared from the sintered ceramic bodies of the present invention. In Fig. 2, an example of the sintered ceramic body punch prepared from the sintered ceramic body of the present invention is shown. The punch 4 has an end surface 5 which corresponds to the opening provided in a punching die. Needless to say, openings to be provided in metal sheets are varied according to the designing of lead frames and, therefore, the shape of the opening of the punching die and the end surface of the punch is varied. When lead frames are manufactured, either or both of a punch prepared from the sintered ceramic body of the present invention and a punching die prepared from the sintered ceramic body of the present invention can be employed. When either of the punch and the punching die prepared from the sintered ceramic body of the present invention is used, usually the corresponding punching die or punch made of cemented carbide is used. On a punching die provided with opening or openings of predetermined shapes, a sheet of copper or a copper alloy, of which a lead frame used for packaging semi-conductor integrated circuit chips is made for instance, is placed and a punch of the corresponding shape is plunged so as to enter the opening or openings in the punching die. The punch is plunged and raised at a high speed while the metal plate is horizontally shifted and thus necessary opening or openings are formed in the metal sheet to be a lead frame. The metal working tools prepared from the sintered ceramic bodies of the present invention have high hardness, high electric resistivity, excellent shock resistance, excellent toughness, excellent wear resistance and, therefore, have dramatically increased number of punching shots because of these excellent characteristics. Reference Example 1 In this example, a combination of a punch of sintered zirconia and a die of cemented carbide for making lead frames to be used when semi-conductor integrated circuits (IC) was used as an example of the working metal tools. Punches were made as follows. A slurry was prepared by dispersing a zirconia powder partially stabilized with 3 mol% of Y 2 O 3 having a mean diameter of 0.5 µm and micro wax in ethanol in a wet ball mill. The resulting slurry was spray-dried and a granulated powder was obtained. The granulated powder was calcined at a temperature of 800 - 1200 °C for 1 hour in the atmosphere. The calcined granulated powders and uncalcined granulated powder were mixed aiming that the particle diameter at the cumulative amount ratio of 90 % was not larger than 10 µm. The mixed granulated powders were shaped into plate form. The plate shape green compacts were sintered at various temperatures in the range of 1400 - 1600 °C. Thus sintered zirconia bodies of various mean particle diameters and particle diameters at the cumulative amount ratio of 90 % as shown in Table 1 were obtained. The sintered zirconia bodies were shaped with a diamond cutter to make punches as shown as 4 in Fig. 2. Using the thus formed punches and a cemented carbide punching die as shown as 6 in Fig. 3 having a punching hole 7 which receives the punch, punching was tested. For the purpose of comparison with the punch of sintered zirconia body punching with cemented carbide punches was also carried out by punching copper sheets. The used cemented carbide was "FM10" marketed by Hitachi Tools Co. Ltd. Plates of the ceramic bodies were separately prepared as described above. The surface thereof was polished to mirror smoothness, subjected to plasma etching, and the particle size distribution was measured by SEM in accordance with the Scharts-Saltykov method. In Table 1, mean particle diameters and particle diameters at the cumulative amount ratio of 90 % of the particles used for preparation of the sintered zirconia bodies are shown together with the life of the punch, that is, the number of punching shots before the punches required reshaping. The larger the number of punching shots, the longer the punch life. In Table 1, the degree of flaws in work pieces at the millionth shot is also indicated in Table 1. In the column of "Degree of Flaws", ○ means that Rmax measured by the surface roughness measuring device is not more than 8 µm, Ra is not more than 0.8 µm and there are few or no visually observable flaws in the work pieces. Δ means that Rmax measured by the surface roughness measuring device is 8 - 15 µm, Ra is 0.8 - 1.5 µm and there are visually observable flaws in the work pieces and the degree thereof is at the same level as in the case with cemented carbide dies. X means that Rmax measured by the surface roughness measuring device is not less than 15 µm, Ra is not less than 1.5 µm and there are visually observable flaws in the work pieces and the degree thereof is larger than in the case with cemented carbide dies. Reference Example 2 In this example, a combination of a punch made of sintered alumina-zirconia bodies and a cemented carbide punching die as used in Reference Example 1 was used. Punches were made as follows. A slurry was prepared by dispersing the zirconia powder partially stabilized with Y 2 O 3 used in Reference Example 1 and a high purity alumina having a mean particle diameter of 0.5 µm in the weight ratio of 80 : 20 together with micro wax as a binder in distilled water in a wet ball mill. The resulting slurry was spray-dried and a granulated powder was obtained. Plate shape sintered alumina-zirconia bodies were prepared in the same manner as in Reference Example 1 except that the sin tering temperature was 1400 - 1700 °C. The thus prepared sintered alumina-zirconia bodies were further subjected to the HIP treatment, that is, they were heat-treated at 1500 °C under the pressure of 1000 atm for 2 hours. The resulting sintered alumina-zirconia bodies were formed into punches in the same manner and the punching test and the particle size distribution measurement were carried out as in Reference Example 1. The test results were evaluated in the same manner as in Reference Example 1 and the test results and evaluation are indicated in Table 2. Example 1 In this example, a combination of a punch made of sintered zirconia-titanium nitride body of the present invention and a cemented carbide punching die as used in Reference Example 1 was used. Punches were made as follows. A slurry was prepared by dispersing zirconia powder partially stabilized with Y 2 O 3 used in Reference Example 1 and titanium nitride having a mean particle diameter of 1.2 µm in a amount ratio of 70 : 30 in distilled water together with micro wax as a binder in a wet ball mill. The resulting slurry was spray-dried and a granulated powder was obtained. The granulated powder was treated in the same manner as in Reference Example 2 and sintered zirconia-titanium nitride bodies having various particle size distributions were obtained. Punches were prepared using the sintered zirconia-titanium nitride bodies by wire-cutting electrical discharge machining instead of machining with diamond cutter. Using the resulting punches, punching tests were carried out and the results were evaluated in the same manner as in Reference Example 1. The results and evaluation are shown in Table 3. Example 2 In this example, a combination of a punch made of a sintered zirconia-alumina-titanium nitride body of the present invention and a cemented carbide punching die as used in Reference Example 1 was used. Punches were made as follows. A slurry was prepared by dispersing the zirconia powder partially stabilized with Y 2 O 3 used in Reference Example 1 and alumina powder used in Reference Example 2 and titanium nitride used in Example 1 in a amount ratio of 50 : 20 : 30 together with micro wax as a binder in distilled water in a wet ball mill. The resulting slurry was spray-dried and a granulated powder was obtained. The granulated powder was treated in the manner as in Reference Example 2 and sintered zirconia-alumina-titanium nitride bodies having various particle size distributions were obtained. Punches were prepared using the sintered zirconia-alumina-titanium nitride bodies by wire cutting electrical discharge machining instead of machining with diamond cutter. Using the resulting punches, tests for punching copper sheets were carried out and particle size distribution was measured in the same manner as in Reference Example 1. The test results were evaluated in the same manner as in Reference Example 1 and the results and evaluation are shown in Table 4. According to Examples 1 and 2, sintered ceramic bodies containing particles of conductive material such as titanium nitride attain greater number of punching shots and thus it is understood that such ceramic bodies attain longer life as punching tools in comparison with other sintered ceramic bodies containing no conductive ceramic particles. Example 3 In this example, a combination of a punching die made of sintered zirconia-titanium nitride body of the present invention and a punch made of the same cemented carbide as used in Reference Example 1 was used. Sintered zirconia-titanium nitride bodies were prepared in the same manner as in Example 1. Punching dies instead of punches were prepared in the same manner as in Example 1 and punching tests were carried in the same manner as in Reference Example 1. The test results were evaluated in the same manner as in Reference Example 1 and the results and evaluation are shown in Table 5. Example 4 In this example, a combination of a punching die made of sintered zirconia-alumina-titanium nitride bodies and a punch made of the same cemented carbide as used in Reference Example 1 was used. Sintered zirconia-alumina-titanium nitride bodies were prepared in the same manner as in Example 2. Punching dies instead of punches were prepared in the same manner as in Example 1 and punching tests were carried out in the same manner as in Reference Example 1. The test results were evaluated in the same manner as in Reference Example 1 and they are shown in Table 6. According to this example, it is understood that sintered ceramic bodies containing a conductive ceramic particles such as titanium nitride attain larger number of punching shots and provide longer life punching tools in comparison with sintered ceramic bodies containing no conductive ceramic particles. Example 5 Zirconia particles partially stabilized with 3 mol% of Y 2 O 3 having various particle size distributions and particles of TiN, TiC, TiB 2 , WC, ZrN and NbC having various particle size distribution together with micro wax as a binder were dispersed in ethanol in the ratios indicated in Table 7 in ethanol as dispersion medium in a wet ball mill for 16 hours. The obtained slurry was spray-dried and a granulated powder was obtained. The granulated powder was formed into square plates of 75 x 75 x 20 mm by applying pressure of 1.5 t/cm 2 with a die press. From each mixing composition, 101 plates were prepared. These green compacts were fired under the conditions indicated in Table 7 in a nitrogen atmosphere and thus conductive sintered ceramic bodies were obtained. One of the thus prepared conductive sintered ceramic bodies was formed into a 3 x 4 x 40 mm test piece for the three point bending test. Flexural strength was measured with the conditions of loading rate of 0.5 mm/min and span length of 30 mm at room temperature by the three point bending test based on JIS R1601. After the bending test, the surface of the sintered ceramic body was polished to mirror smoothness and the polished test piece was subjected to the fracture toughness test with a load of 30 kg and a retention time of 15 sec according to the IF method (JIS R1607). The results of the bending strength test and the fracture toughness test are shown in Table 7. The polished mirror smooth surface of the test piece was subjected to plasma etching and respective particle diameters of the insulating ceramic particles and the conductive ceramic particles were measured by SEM observation. The particle size distribution was determined by the Schwarts-Saltykov method. The particle diameter at the cumulative amount ratio of 90 % of insulating ceramic particles and the mean particle diameter of the conductive ceramic particles were indicated in Table 7. The remaining 100 test plates were used for evaluation of wire cutting electrical discharge machining property. Specifically, using a die-milling electrical discharge machining apparatus with a copper electrode having a 2 mm diameter, an initial hole 8 was cut through in the plate 9, as shown in Fig. 4. Then a brass wire electrode 10 having a 0.2 mm diameter was placed in the initial hole 8 and the wire cutting electrical discharge machining was carried out. As shown in Fig. 4, the wire cutting electrical discharge machining was conducted along the indicated arrow 11. The length of the wire cutting electrical discharge machining, that is, the length of the movement of the wire, was about 500 mm for one sintered ceramic plate. The die-milling electrical discharge machinability and wire cutting electrical discharge machinability are shown in Table 7. When all the 100 plates were machined, it was judged that the sintered ceramic body is machinable by die-milling electrical discharge machining and indicated with ○ If even one plate could not be machined by die-milling electrical discharge machining, then it is judged as not machinable by die-milling electrical discharge machining and indicated with X in Table 7. Also in wire cutting electrical discharge machining, if all the 100 plates were machined by wire cutting discharge machining, it is judged as machinable by wire cutting electrical discharge machining and indicated with O in Table 7. If even one plate out of 100 plates could not be machined by wire cutting electrical machining, then it is judged that wire cutting electrical machining is impossible and indicated with X in Table 7. Thermal conductivity of the sintered ceramic bodies was measured in accordance with the laser-flash method of JIS R1611. Hardness of the sintered ceramic bodies was measured in accordance with the Vickers method of JIS R1610. The results are shown in Table 7. As apparent from the results shown in Table 7, sintered ceramic bodies comprising insulating ceramic particles whose particle diameter at the cumulative amount ratio of 90 % is not larger than 10 µm and conductive ceramic particles whose mean particle diameter is not larger than 10 µm in a content ratio of 25 - 60 vol% of conductive ceramic particles on condition that the total of the insulating ceramic particles and the conductive ceramic particles is 100 vol% are machinable by both die-milling electrical discharge machining and wire cutting electrical discharge machining and have good mechanical properties such as bending strength. Further, they are excellent in the thermal conductivity and the hardness. Further review of Table 7 reveals the following facts. When the content of conductive ceramic particles is less than 25 vol%, wire cutting electrical discharge machining is impossible. When it exceeds 60 vol%, mechanical strength, especially bending strength is poor and the sintered ceramic bodies cannot be practically used. When the particle diameter at the cumulative amount ratio of 90 % of insulating ceramic particles is in excess of 10 µm, breaking of wire electrode may occur during wire cutting electrical discharge machining. When the mean particle diameter of the conductive ceramic particles is in excess of 10 µm, the bending strength of the sintered ceramic bodies may drop. Example 6 Zirconia particles partially stabilized with 3 mol% of Y 2 O 3 having various particle size distributions and particles of high purity alumina having various particle size distributions were mixed in weight ratio of 80 : 20 and the obtained mixture and particles of TiN, TiC, TiB 2 , WC, ZrN and NbC having various particle size distributions were mixed in mixing ratios indicated in Table 8 in the same manner as in Example 5 and slurries was obtained in the same manner as in Example 5. The slurries were dried in the same manner as in Example 5 and green materials were obtained. Conductive sintered ceramic bodies were prepared from this green material in the same manner as in Example 5. In the same manner as in Example 5, bending strength and fracture strength and particle size distribution of the sintered ceramic bodies. The particle diameter at the cumulative amount ratio of 90 % of the insulating ceramic particles and the mean particle diameter of the conductive ceramic particles were determined. The thermal conductivity and the hardness were also measured. The results are indicated in Table 8. Machinability of these conductive sintered ceramic bodies in die-milling and wire cutting electrical discharge machining was evaluated in the same manner as in Example 5 and the results are shown in Table 8. As apparent from Table 8, when the content of the conductive ceramic particles is in the range of 25 - 60 vol%, wire cutting electrical discharge machining can be stably carried out. When the conductive ceramic particle content is in excess of 60 vol%, bending strength of the sintered conductive ceramic body falls. When the particle diameter of the insulating ceramic particles at the cumulative amount ratio of 90 % is in excess of 10 µm, wire electrode may be broken during wire cutting electrical discharge machining and when the mean particle diameter is in excess of 10 µm, the bending strength of the sintered body falls. Sintered ceramic bodies comprising insulating ceramic particles whose particle diameter at the cumulative amount ratio of 90 % is not larger than 10 µm and conductive ceramic particles whose mean particle diameter is not larger than 10 µm in a content ratio of 25 - 60 vol% of conductive ceramic particles on condition that the total of the insulating ceramic particles and the conductive ceramic particles is 100 vol% are excellent in bending strength and fracture toughness, and machinable by both die-milling electrical discharge machining and wire cutting electrical discharge machining. These sintered ceramic bodies have good thermal conductivity and high hardness. Thus metal working tools made of the sintered ceramic bodies less suffer breakage due to heat produced during a work when they are used in combination with other members, which have a thermal conductivity being different from that of the sintered ceramic bodies. Such metal working tools less suffer breakage due to mechanical contact in working. The following examples relate to sintered ceramic bodies, which comprises insulating ceramic particles whose particle diameter at the cumulative amount ratio of 90 % is not larger than 10 µm and conductive ceramic particles whose mean particle diameter is 0.3 - 10 µm, wherein the ratio of (ε) of the mean particle diameter of the conductive ceramic particles (dsp) to the mean particle diameter (dm) of the insulating ceramic particles (ε = dsp/dm) is not larger than 9 and the content of the conductive ceramic particles is 25 - 60 vol% when the total amount of the conductive ceramic particles and the insulating particles is 100 Vol%. Example 7 Zirconia powders partially stabilized with 3 mol% of Y 2 O 3 having various mean particle diameters (dm) of and titanium nitride powders having various mean particle diameters (dsp), as shown in Table 9, were prepared. The zirconia powders and titanium nitride powders are respectively mixed in ratios (ε) indicated in Table 9 and in compositions indicated in Table 10 together with micro wax as a binder in distilled water as a dispersion medium in a wet ball mill. The obtained slurries were spray-dried and granulated powders were obtained. Sintered zirconia-titanium nitride bodies were obtained in the same manner as in Reference Example 1 except that the calcining temperature was 800 - 1200 °C and the sintering temperature was 1350 - 1650 °C. The resulting sintered zirconia-titanium nitride bodies were subjected to HIP treatment, that is, they were treated at 1500 °C under the pressure of 1,013,2 bar (1000 atm) for 2 hours. Thus final zirconia-titanium nitride bodies were obtained. Specific resistivities of these sintered zirconia-titanium nitride bodies were measured by the four terminal method as described in JIS C2561. The results are shown in Table 10. In Table 10, E + 0 means 10 0 , E + 8 means 10 8 for instance and the other indications mean the same. Mean Particle Diameter of Zirconia (dm), Mean Particle Diameter of Titanium Nitride (dsp) and the Ratio(ε) (ε=dsp/ dm) dsp 0.40 1.40 2.50 3.00 dm 0.15 ε = 2.67 ε = 9.33 ε = 16.67 ε = 20.00 0.25 ε = 1.60 ε = 5.60 ε = 10.00 ε = 12.00 0.70 ε = 0.57 ε = 2.00 ε = 3.57 ε = 4.29 1.10 ε = 0.36 ε = 1.27 ε = 2.27 ε = 2.73 1.45 ε = 0.28 ε = 0.97 ε = 1.72 ε = 2.07 Electric Resistivity (Ω·cm) TiN Content (Vol%) 20 25 30 40 50 55 60 65 ε 0.28 2.5 E+8 3.4 E+0 2.3 E-3 1.1 E-3 9.2 E-4 8.4 E-4 5.4 E-4 4.9 E-4 0.36 2.4 E+8 3.4 E+0 3.5 E-3 1.2 E-3 9.2 E-4 8.4 E-4 5.4 E-4 4.9 E-4 0.57 2.4 E+8 3.6 E+0 2.6 E-3 1.1 E-3 9.2 E-4 8.5 E-4 5.4 E-4 4.9 E-4 0.97 2.5 E+8 3.5 E+0 2.5 E-3 1.3 E-3 9.3 E-4 8.4 E-4 5.5 E-4 5.0 E-4 1.27 2.5 E+8 3.4 E+0 2.5 E-3 1.2 E-3 9.4 E-4 8.6 E-4 5.5 E-4 4.9 E-4 1.60 2.6 E+8 3.5 E+0 2.7 E-3 1.2 E-3 9.3 E-4 8.6 E-4 5.5 E-4 4.9 E-4 1.72 2.7 E+8 3.7 E+0 2.7 E-3 1.3 E-3 9.4 E-4 8.6 E-4 5.6 E-4 4.9 E-4 2.00 2.7 E+8 3.6 E+0 2.8 E-3 1.3 E-3 9.4 E-4 8.6 E-4 5.6 E-4 5.0 E-4 2.07 2.6 E+8 3.6 E+0 2.8 E-3 1.4 E-3 9.4 E-4 8.7 E-4 5.8 E-4 5.1 E-4 2.27 2.8 E+8 3.8 E+0 2.9 E-3 1.4 E-3 9.4 E-4 8.6 E-4 5.6 E-4 5.1 E-4 2.67 2.7 E+8 3.7 E+0 2.8 E-3 1.5 E-3 9.5 E-4 8.7 E-4 5.6 E-4 5.0 E-4 2.73 2.7 E+8 3.8 E+0 2.8 E-3 1.5 E-3 9.4 E-4 8.6 E-4 5.7 E-4 5.1 E-4 3.57 2.8 E+8 3.8 E+0 3.0 E-3 1.5 E-3 9.4 E-4 8.6 E-4 5.7 E-4 5.1 E-4 4.29 2.9 E+8 3.9 E+0 3.0 E-3 1.4 E-3 9.5 E-4 8.7 E-4 5.7 E-4 5.0 E-4 5.60 2.9 E+8 3.8 E+0 3.0 E-3 1.4 E-3 9.4 E-4 8.7 E-4 5.8 E-4 5.1 E-4 9.33 3.5 E+8 4.4 E+0 3.9 E-3 2.0 E-3 9.6 E-4 8.7 E-4 5.7 E-4 5.1 E-4 10.00 3.9 E+8 4.7 E+0 4.4 E-3 2.1 E-3 9.7 E-4 8.7 E-4 5.8 E-4 5.2 E-4 These sintered zirconia-titanium nitride bodies were subjected to wire cutting electrical discharge machining and the surface roughness (Ra) of the worked bodies was measured with a surface roughness measurement apparatus marketed by Rank Tailor Hobson, Inc. The results are shown in Table 11. As shown in Table 11, the sintered ceramic bodies within the scope of the present invention have the surface roughness Ra of not larger than 2 µm and, therefore, they can be used as metal working tools without polishing the surface to mirror smoothness. Surface Roughness Ra (µm) TiN Content (Vol%) 20 25 30 40 50 55 60 65 ε 0.28 Electric Discharge Impossible 1.1 0.9 0.9 0.8 0.9 1.0 1.0 0.36 Electric Discharge Impossible 1.3 1.1 1.1 1.1 1.2 1.2 1.1 0.57 Electric Discharge Impossible 1.3 1.2 1.2 1.1 1.1 1.2 1.1 0.97 Electric Discharge Impossible 1.2 1.0 1.0 0.9 1.1 1.1 1.0 1.27 Electric Discharge Impossible 1.4 1.3 1.3 1.4 1.4 1.4 1.3 1.60 Electric Discharge Impossible 1.0 0.8 0.9 0.8 0.9 0.8 0.9 1.72 Electric Discharge Impossible 1.8 1.6 1.6 1.8 1.8 1.7 1.7 2.00 Electric Discharge Impossible 1.5 1.3 1.2 1.2 1.3 1.4 1.4 2.07 Electric Discharge Impossible 1.9 1.8 1.9 1.8 1.9 1.9 1.9 2.27 Electric Discharge Impossible 2.0 1.9 2.0 1.9 2.0 2.0 1.9 2.67 Electric Discharge Impossible 1.7 1.5 1.4 1.4 1.5 1.6 1.6 2.73 Electric Discharge Impossible 2.0 2.0 1.9 1.9 2.0 1.9 2.0 3.57 Electric Discharge Impossible 2.0 2.0 1.9 1.9 2.0 2.0 2.0 4.29 Electric Discharge Impossible 1.9 1.9 2.0 2.0 2.0 2.0 2.0 5.60 Electric Discharge Impossible 1.9 1.9 1.9 2.0 2.0 2.0 2.0 9.33 Electric Discharge Impossible 3.7 3.6 3.5 3.4 3.5 3.5 3.4 10.00 Electric Discharge Impossible 4.9 4.7 4.6 4.8 4.6 4.6 4.5 Example 8 Zirconia powders partially stabilized with 3 mol% of Y 2 O 3 having various mean particle diameters (d 50%) and various particle diameters at the cumulative amount ratio of 90 % (d 90%) indicated in Table 12 were mixed with titanium nitride having various mean particle diameters (d 50%) indicated in Table 12 so that the content ratio of the latter was 30 vol% together with micro wax as a binder in distilled water as a dispersion medium in a wet ball mill. From the resulting slurries, sintered zirconia-titanium nitride bodies were obtained in the same manner as in Example 7. Bending strength of these zirconia-titanium nitride bodies was measured in accordance with JIS R1601. The results are shown in Table 12. These zirconia-titanium nitride bodies were subjected to wire cutting electrical discharge machining and machinability thereby was evaluated in the same matter as in Example 5. The results are shown in Table 12. Punches as shown in Fig. 2 were made of the zirconia-titanium nitride bodies by wire cutting electrical machining. Using these punches and cemented carbide punching dies punching tests were carried out in the same manner as in Reference Example 1. The results are shown in Fig. 12. Example 9 The procedures of Example 8 were repeated except that the titanium nitride content was 50 vol% instead of 30 vol% and punches and punching dies were prepared. Bending strength, machinability in wire cutting and die-milling electrical discharged machining were measured. Also tool life was measured by punching test. The results are shown in Table 13. From the results shown in Table 12 and 13, it is apparent that with the titanium nitride content of less than 25 vol%, zirconia-titanium nitride sintered bodies cannot be worked by wire cutting electrical discharge machining; when the mean particle diameter of the conductive ceramic particles is in the range of 0.3 - 10 µm and the particle diameter of the insulating ceramic particles at the cumulative amount ratio of 90 µm is 10 µm at the largest, sintered ceramic bodies can be prepared, which have good bending strength, machinability in wire cutting and die-milling electrical discharge machining, from which long life punching tools can be manufactured. The metal working tool made of the sintered ceramic bodies within the scope of the present invention does not cause staining of work pieces. That is, lead frames are not contaminated and, therefore, semi-conductor chips does not suffer any undesirable influence. Effect of the Invention According to the present invention, sintered ceramic bodies which can be worked by both wire cutting and die-milling electrical discharge machinings and have excellent mechanical strengths are provided. According to the present invention, sintered ceramic bodies which have high mechanical strength and can be worked into ceramic tools which have long life and scarcely cause flaws in work pieces as well as a process for preparing such sintered ceramic bodies are provided.

Claims: Sintered ceramic body comprising insulating ceramic particles and electrically conductive particles, wherein the mean particle diameter of the electrically conductive ceramic particles is not larger than 10 µm, and the content of the electrically conductive ceramic particles is 25-60 vol.% in 100 vol.% of the total amount of said insulating ceramic particles and said electrically conductive ceramic particles, the insulating ceramic particles are at least one selected from the group consisting of zirconia and zirconia-alumina and characterized in that the particle diameter of the insulating ceramic particles at the cumulative amount ratio of 90% is not larger than 10 µm. The sintered ceramic body as claimed in claim 1, wherein the ratio (ε) of the mean particle diameter of the conductive ceramic particles (dsp) to the mean particle diameter of the insulating ceramic particles (dm) is not larger than 9. The sintered ceramic body as claimed in claim 1, wherein the mean particle diameter of the conductive ceramic particles is 0.3 - 10 µm. The sintered ceramic body as claimed in claim 3, wherein the mean particle diameter of the conductive ceramic particles is 0.5 - 4 µm. The sintered ceramic body as claimed in claim 1, wherein the content ratio of the conductive ceramic particles is 30 - 50 vol.%. The sintered ceramic body as claimed in claim 2, wherein the ratio is 0.01 - 9. The sintered ceramic body as claimed in claim 1, whose electric resistivity is not higher than 10 Ω·cm. The sintered ceramic body as claimed in claim 7, whose electric resistivity is not higher than 5 Ω·cm. The sintered ceramic body as claimed in claim 8, whose electric resistivity is not higher than 0.1 Ω·cm. The sintered ceramic body as claimed in claim 1, wherein the conductive ceramic particles are at least one selected from the group consisting of titanium nitride, zirconium nitride, titanium carbide, titanium carbonitride, titanium boride, tungsten carbide and niobium carbide. The sintered ceramic body as claimed in claim 10, wherein the zirconia is at least one partially stabilized zirconium selected from the group consisting of zirconium oxide stabilized with 1.5 - 6 mol% of Y 2 O 3 , zirconium oxide stabilized with 8 - 10 mol% of MgO and zirconium oxide stabilized with 6 - 12 mol% of CeO 2 . A process for preparing a sintered ceramic body comprising insulating ceramic particles and electrically conductive particles, wherein the insulating ceramic particles are at least one selected from the group consisting of zirconia and zirconia-alumina and the particle diameter of the insulating ceramic particles at the cumulative amount ratio of 90% is not larger than 10µm, the mean particle diameter of the electrically conductive ceramic particles is not larger than 10 µm, and the content of the electrically conductive ceramic particles is 25 - 60 vol.% in 100 vol.% of the total amount of said insulating ceramic particles and said electrically conductive ceramic particles, comprising the steps of a) mixing insulating ceramic particles having a particle diameter at the cumulative amount ratio of 90% of not larger than 10 µm with electrically conductive ceramic particles having a mean particle diameter of not larger than 10 µm, in a ratio of 25 to 60 vol.% of said electrically conductive ceramic particles in 100 vol.% of the total amount of said insulating ceramic particles and said electrically conductive ceramic particles; b) compacting the mixture; and c) sintering said compact. The process for preparing a sintered ceramic body as claimed in claim 12, wherein the insulating ceramic particles having a particle diameter at the cumulative amount ratio of 90% is less than 10 µm is used.

Inventors: MORISHITA TAKAYOSHI, SUZUKI KEIJI, YOGO TETSUJI, MORISHITA, TAKAYOSHI, SUZUKI, KEIJI, YOGO, TETSUJI, Morishita, Takayoshi, NGK Spark Plug Company Ltd., Suzuki, Keiji, NGK Spark Plug Company Ltd., Yogo, Tetsuji, NGK Spark Plug Company Ltd.
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD.
Application year: 1996
Main classification: C04B35
IPC codes: C04B35, B23H9, H01B1