Sintered ceramic composite body and method of manufacturing same

Abstract: A sintered ceramic composite body is manufactured by preparing a powdery mixture composed of a base material composed of either one of oxide ceramic such as alumina, mullite, magnesia, or the like, and nonoxide ceramic such as silicon nitride, sialon, or the like, and a reinforcement material composed of particles or platelet particles of silicon carbide which have a size ranging from 5 to 20 µm, the particles of silicon carbide being contained at a volume ratio ranging from 3 to 50 %. The platelet particles have a maximum diameter ranging from 5 to 50 µm and a thickness which is 1/3 or less of the maximum diameter. The powdery mixture is molded into a shaped product, which is then sintered in a temperature range from 1,400 to 1,900°C for the base material which is composed of oxide ceramic or in a temperature range from 1,500 to 2,000°C for the base material which is composed of nonoxide ceramic. The body has high toughness.

Description: The present invention relates to a sintered ceramic composite body with improved toughness and a method of manufacturing such a sintered ceramic composite body. Ceramic materials have long been used as refractory materials and chemical materials since they are highly resistant to corrosion and heat and high in hardness. The recent advance in the chemical technology allows highly pure materials to he refined and synthesized, and the process control technology makes it possible to produce ceramic materials having widely varied properties, which have attracted much attention in the art. Heat-resistant alloys have heretofore been employed in applications at high temperatures or in adverse environments, such as gas turbine blades. However, there has been a demand for more excellent high-temperature structural materials in view of the recent trend in the market for higher performances. Ceramic is recognized as an important material which meets the requirements in such uses, because it is much better than other materials with respect to heat resistance, acid resistance, and corrosion resistance. Ceramic materials such as silicon nitride, alumina, silicon carbide are generally brittle, and many of these materials have a fracture toughness of 5 MNm -3/2 or less. Various attempts have heretofore been made to improve the toughness of ceramic materials. One effort to toughen a ceramic material involves the addition of needle-like components such as whiskers, fibers, or the like as a reinforcement material, as disclosed in Japanese Laid-Open Patent Publication No. 59-30770. It is considered that the toughness of a ceramic material with such a reinforcement material is increased by the crack deflection effect in which cracks produced in the ceramic by whiskers or the like dispersed therein are bent, or the whisker pullout effect. However, it is difficult to disperse needle-like reinforcing elements uniformly in a ceramic material. If fibers are used as a reinforcement material, the dispersed fibers tend to be entangled together into a fiber agglomeration in the ceramic. Japanese Patent Application No. 59-25748 discloses a method of toughening an alumina ceramic material with zirconia added as a reinforcement material. According to the disclosed method, zirconia is left as a metastable tetragonal system in alumina down to room temperature, and the mechanical properties of the ceramic material at room temperature is greatly improved by residual compressive stresses which are produced due to a volume expansion by 4 % upon crystal system transformation that is caused from the tetragonal system into a monoclinic system owing to stresses at tip ends of produced cracks. Even with the above ceramic toughening method, however, if the ceramic material is left for a long time in atmosphere at a temperature higher than about 900°C which is the transformation temperature, then zirconium oxide and the base material which is a nonoxide react with each other to the extent that the properties of the base material can no longer be maintained. As a result, the above toughening effect is not achieved. Japanese Laid-Open Patent Publication No. 62-246865 discloses a sintered body of silicon nitride with a rare earth, MgO, and ZrO₂ added as sintering aids. Japanese Laid-Open Patent Publication No. 63-100067 shows a sintered body of silicon nitride which contains two or more of Y₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, and Lu₂O₃. However, silicon carbide is not contained as a reinforcement material. In efforts to increase the toughness of a ceramic material with dispersed whiskers, fibers, or the like, it is difficult to disperse whiskers, fibers, or the like uniformly. Even if these reinforcement elements can be dispersed relatively uniformly, it is impossible to produce sintered ceramic composite bodies of good properties unless specially processed in the manufacturing process. Use of whiskers, fibers, or other reinforcement materials is highly expensive. In the case where particles of zirconium oxide are dispersed for increased toughness, no toughening effect is achieved at high temperatures where no crystal system transformation progresses. If the ceramic material is left at a high temperature for a long period of time, then zirconium oxide and the base material which is a nonoxide react with each other, and the properties of the base material are no longer maintained. EP 0282879 discloses a composite sintered ceramic body having a ceramic matrix of silicon nitride, silicon oxynitride, or one of a series of inorganic oxides, containing both whiskers and particles of silicon carbide, silicon nitride, silicon oxynitride, alumina or zirconia. The whiskers have a diameter of 0.1 to 5µm and the particles have an average particle size of 5 to 60 µm. British Ceramic Proceedings, Vol. 45, 1989, pages 187 to 198 compares SiC platelet and whisker reinforcement of a SiN matrix. The platelets disclosed have a diameter of 20-70 µm and a thickness of 1-10 µm. Briefly, the present invention provides a sintered ceramic composite body which is toughened by adding silicon carbide particles of predetermined shape as a reinforcement material to a base material which is an oxide or a nonoxide, and a method of manufacturing such a sintered ceramic composite body. The present invention is defined in the appended claims. Embodiments of the present invention will now be described by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a flowchart of a method of manufacturing a sintered ceramic composite body according to the present invention; and FIGS. 2(a) and 2(b) are views showing defined dimensions of a silicon carbide particle used as a reinforcement material. A sintered ceramic composite body according to the present invention comprises, as a base material, an oxide ceramic material such as alumina, magnesia, mullite, etc., or a nonoxide ceramic material such as silicon nitride, sialon, etc., the silicon nitride containing two or more of Y₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃, or a rare earth, MgO, and ZrO₂, as a reinforcement material, particles of silicon carbide. Small amounts of other elements may be contained as impurities in the above materials. It is preferable that a sintering aid be added to the materials of the sintered ceramic composite as is the case with ordinary sintered bodies composed of oxide or nonoxide ceramic. The sintered ceramic composite according to the present invention may be shaped by any of ordinary shaping processes such as pressing, slip casting, injection molding, extrusion molding, etc. The materials of the ceramic composite are sintered in a vacuum or inert gas atmosphere by hot pressing (HP), but may also be sintered by normal-pressure sintering, sinter-HIP, or capsule HIP. The particles of silicon carbide as the reinforcement material are of a size ranging from 5 to 20 µm, and 3 to 50 % or preferably 10 to 30 % by volume of such particles of silicon carbide are added to the base material, forming a powdery mixture. Alternatively, the particles of silicon carbide as the reinforcement material are in the form of plates having a maximum diameter ranging from 5 to 50 µm, preferably, from 10 to 40 µm, and a thickness which is 1/3 or less of the maximum diameter, and 3 to 50 % or preferably 10 to 30 % of such platelet particles of silicon carbide are added to the base material, forming a powdery mixture. The powdery mixture thus obtained is molded to shape. The shaped product is then sintered in a temperature range from 1,400 to 1,900°C for the base material which is an oxide ceramic, or in a temperature range from 1,500 to 2,000°C for the base material which is a nonoxide ceramic. The sintered ceramic composite body thus produced has a high degree of toughness. Platelet particles of silicon carbide are more effective for toughening the sintered ceramic composite body than particles of silicon carbide. If the base material is a nonoxide ceramic, then silicon nitride and sialon contained in the base material are decomposed at a high rate when the sintering temperature becomes higher than 1,700°C. To avoid such accelerated decomposition, the pressure of a nitrogen atmosphere used is normally increased up to a range from 9x10⁵ to 9.9x10⁵ Nm⁻² (9 to 9.9 kg·f/cm²). If the sintering temperature were lower than the above ranges, the density of the sintered body would be low. If the sintering temperature were higher than the above ranges, the components of the base material would be decomposed. At any rate, no dense sintered body would be produced. The optimum sintering temperature varies depending on the conditions of the normal-pressure sintering, the hot pressing, sinter-HIP, and the capsule HIP, and the size and amount of silicon carbide particles as the reinforcement material. If the added amount of silicon carbide particles were less than 3 % by volume, then no increase in the fracture toughness would be achieved. If the added amount of silicon carbide particles were larger than 50 %, then the density of the produced sintered ceramic composite body would be too low and hence no dense sintered ceramic composite material would be manufactured. If the size of silicon carbide particles were smaller than 5 µm, then the fracture toughness would not be improved, and if larger than 20 µm, then the density of the produced sintered ceramic composite body would be too low. The fracture toughness is measured by the SEPB (Single Edge Pre-cracked Beam) process. More specifically, a test specimen is prepared according to JIS R1601, indented by the diamond pyramid indenter of a Vickers hardness test machine, and subjected to a load for producing a pre-crack, with a pop in detected by earphones. Then, the test specimen is colored for the measurement of the pre-crack, subjected to a bending test, and thereafter measured for a fracture load. After the length of the pre-crack in the fractured test specimen is measured, a fracture toughness value is calculated according to the fracture toughness equation. FIG. 1 shows a sequence of a method according to the present invention. The base material and the reinforcement material were placed in a pot mill, and mixed with water or ethanol for 24 hours, thus forming a mixture. As described above, the reinforcement material was added at a volume ratio ranging from 10 to 30 %, either in the form of particles of silicon carbide having a size ranging from 5 to 20 or in the form of platelet particles of silicon carbide having a maximum diameter ranging from 5 to 50 µm, and a thickness which is 1/3 or less of the maximum diameter. FIGS. 2(a) and 2(b) show dimensional definitions of silicon carbide particles. As shown in FIG. 2(a), a silicon carbide particle has a minor-axis diameter (i.e., a minimum distance across the particle between two parallel lines contacting the particle) ranging from 5 to 20 µm. FIG. 2(b) illustrates a platelet silicon carbide particle having a maximum diameter ranging from 5 to 50 µm and a thickness which is 1/3 or less of the maximum diameter. The mixture produced in the pot mill was then dried at 120°C for 24 hours, and passed through a sieve having a mesh size of 149 µm, thereby forming a powder. The powder was then pressed to shape under a pressure of 200x10⁵ Nm⁻² (200 kgf/cm²), or pressed to shape under a pressure of 7x10⁸ Nm⁻² (7 tons/cm²) by a rubber press, after which the shaped product was sintered under pressure. For the base material which is an oxide ceramic, the shaped product was sintered at an HP temperature ranging from 1,400 to 1,900°C in an Ar atmosphere under 10⁵Pa (1 atm). For the base material which is a nonoxide ceramic, the shaped product was sintered at an HP temperature ranging from 1,500 to 2,000°C in an N₂ atmosphere under 10⁵Pa (1 atm). The HP pressure was 300x10⁵ Nm⁻² (300 kgf/cm²). For the base material which is composed of silicon nitride containing two or more of Y₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃, or silicon nitride containing a rare earth, MgO, and ZrO₂, and sialon, the shaped product was sintered at an HP temperature ranging from 1,500 to 2,00°C in an N₂ atmosphere under 10⁵Pa (1 atm). For the base material which is composed of a nonoxide ceramic material, the shaped product was sintered at a temperature of 1,700°C or higher in an N₂ atmosphere under 9.5x10⁵Pa (9.5 atm), for the reasons described above. Tables 1-1, 1-2, and 1-3 show the results of a fracture toughness test conducted on various inventive and comparative examples. The base materials of the examples included alumina, mullite, and magnesia as oxide ceramic materials, and silicon nitride containing two or more of Y₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃ or silicon nitride containing a rare earth, MgO, and ZrO₂, and sialon as nonoxide materials, and silicon carbide was added as a reinforcement material to the base materials under the conditions given in Tables 1-1, 1-2, and 1-3. The sintered ceramic composite body according to the present invention is high in toughness because of silicon carbide particles of suitable shape and size dispersed at a suitable volume rate to a base material which is an oxide or a nonoxide. The sintered ceramic composite body according to the present invention requires no special equipment for its manufacture, but can be manufactured by ordinary ceramic manufacturing equipment. Therefore, the sintered ceramic body material can be manufactured at a low cost.

Claims: A sintered ceramic body having a fracture toughness of at least 7,0 MNm -3/2 , consisting essentially of: a base material composed of silicon nitride containing as sintering aid one of the following combinations of compounds:- Y₂O₃, MgO and ZrO₂, Y₂O₃ and Yb₂O₃, Y₂O₃ and Er₂O₃, Yb₂O₃ and Lu₂O₃, and Y₂O₃ and Tm₂O₃; and a reinforcement material composed of particles of silicon carbide having an average particle size in the range 5 to 20 µm distributed in said base material and present in a volume ratio in the range 3 to 50%. A sintered ceramic body consisting essentially of: a base material selected from alumina, mullite, magnesia and sialon; and a reinforcement material composed of particles of silicon carbide which have an average particle size in the range 5 to 20 µm distributed in said base material and present in a volume ratio in the range 3 to 50%. A sintered ceramic body according to claim 2 wherein said base material is alumina and said body has a fracture toughness of at least 5.0 MNm -3/2 . A sintered ceramic body according to claim 2 wherein said base material is mullite and said body has a fracture toughness of at least 3.9 MNm -3/2 . A sintered ceramic body according to claim 2 wherein said base material is magnesia and said body has a fracture toughness of at least 3.6 MNm -3/2 . A sintered ceramic body according to claim 2 wherein said base material is sialon and said body has a fracture toughness of at least 5.0 MNm -3/2 . A method of manufacturing a sintered ceramic body having a fracture toughness of at least 7.0 MNm -3/2 , comprising: preparing a powder mixture of a base material composed of silicon nitride containing as sintering aid one of the following combinations of compounds:- Y₂O₃, MgO and ZrO₂, Y₂O₃ and Yb₂O₃, Y₂O₃ and Er₂O₃, Yb₂O₃ and Lu₂O₃, and Y₂O₃ and Tm₂O₃; and a reinforcement material composed of particles of silicon carbide having an average particle size in the range 5 to 20 µm, said particles of silicon carbide being present at a volume ratio in the range 3 to 50%; molding said powder mixture into a shaped product; and sintering said shaped product in the temperature range from 1,500 to 2,000°C. A method of manufacturing a sintered ceramic body, comprising: preparing a powder mixture of a base material selected from alumina, mullite, magnesia and sialon, and a reinforcement material composed of particles of silicon carbide having an average particle size in the range 5 to 20 µm, said particles of silicon carbide being present at a volume ratio in the range 3 to 50%; molding said powder mixture into a shaped product; and sintering said shaped product in the temperature range from 1,400 to 1,900°C when the base material is alumina, mullite or magnesia or in the temperature range from 1,500 to 2,000°C when the base material is sialon. A sintered ceramic body comprising: a base material composed of silicon nitride containing as sintering aid one of the following combinations of compounds:- Y₂O₃, MgO and ZrO₂, Y₂O₃ and Yb₂O₃, Y₂O₃ and Er₂O₃, Yb₂O₃ and Lu₂O₃, and Y₂O₃ and Tm₂O₃; and a reinforcement material composed of platelet particles of silicon carbide which have a maximum diameter in the range 5 to 50 µm and a thickness which is 1/3 or less of the maximum diameter, present at a volume ratio in the range 3 to 50%. A sintered ceramic body according to claim 9 having a fracture toughness of at least 8.1 MNm -3/2 . A sintered ceramic body comprising: a base material selected from alumina, mullite, magnesia and sialon; and a reinforcement material composed of platelet particles of silicon carbide which have a maximum diameter in the range 5 to 50 µm and a thickness which is 1/3 or less of the maximum diameter, present at a volume ratio in the range 3 to 50%. A sintered ceramic body according to claim 11 wherein said base material is alumina and said body has a fracture toughness of at least 6.0 MNm -3/2 . A sintered ceramic body according to claim 11 wherein said base material is sialon and said body has a fracture toughness of at least 4.6 MNm -3/2 . A method of manufacturing a sintered ceramic body, comprising: preparing a powder mixture of a base material composed of silicon nitride containing as sintering aid one of the following combinations of compounds:- Y₂O₃, MgO and ZrO₂, Y₂O₃ and Yb₂O₃, Y₂O₃ and Er₂O₃, Yb₂O₃ and Lu₂O₃, and Y₂O₃ and Tm₂O₃; and a reinforcement material composed of platelet particles of silicon carbide which have a maximum diameter in the range 5 to 50 µm and a thickness which is 1/3 or less of the maximum diameter, said platelet particles of silicon carbide being present at a volume ratio ranging from 3 to 50%; molding said powder mixture into a shaped product; and sintering said shaped product in the temperature range from 1,500 to 2,000°C. A method of manufacturing a sintered ceramic body, comprising: preparing a powder mixture of a base material selected from alumina, mullite, magnesia and sialon; and reinforcement material composed of platelet particles of silicon carbide which have a maximum diameter in the range 5 to 50 µm and a thickness which is 1/3 or less of the maximum diameter, said platelet particles of silicon carbide being present at a volume ratio ranging from 3 to 50%; molding said powder mixture into a shaped product; and sintering said shaped product in the temperature range from 1,400 to 1,900°C when the base material is alumina, mullite or magnesia or in the temperature range from 1,500 to 2,000°C when the base material is sialon.

Inventors: FURUSE YUTAKA THE TOKYO ELECTR, MATSUHIRO KEIJI, FURUSE, YUTAKA, THE TOKYO ELECTRIC POWER CO.,INC., MATSUHIRO, KEIJI
Applicants: NGK INSULATORS LTD, TOKYO ELECTRIC POWER CO, NGK INSULATORS, LTD., THE TOKYO ELECTRIC POWER CO., INC.
Application year: 1990
Main classification: C04B35
IPC codes: C04B35