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 of silicon carbide which have a size of 1 µm or less and a size ranging from 5 to 20 µm, or have a size of 1 µm or less and platelet silicon carbide particles having a maximum diameter ranging from 5 to 50 µm and a thickness which is 1/3 or less of the maximum diameter, the particles of silicon carbide being contained at a volume ratio ranging from 10 to 50 %. 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 mechanical strength 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 be 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 mass 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. 61-174165 shows dispersing silicon carbide in an alumina material for an increased mechanical strength. According to the disclosed method, particles of silicon carbide having an average diameter of 3 µm or less or whiskers of silicon carbide having a diameter of 1 µm or less and a length of 20 µm or less are independently dispersed in an alumina material, applying localized residual stresses alumina grain boundaries, so that the mechanical properties of the composite material at high temperature are improved. 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 material 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-A-0400656, which claims an earlier priority date than the present application and designates some EPC states designated in the present application, discloses a matrix containing silicon carbide grains having an average diameter of not more than 1µm and further silicon carbide grains having a diameter of several nonometers to several hundred nonometers. A dispersion phase is disclosed, of silicon carbide grains of average diameter 2.50µm and/or silicon carbide whiskers having a short axis of 0.05-10µm. EP-A-0282879 discloses the addition to an alumina, mullite or zirconia matrix of silicon carbide of size 5 to 20µm and whiskers of diameter 0.5 to 1.0µm. Chemical abstracts Vol.110, No.18, page 325, abstract No.159301k, apparently published on 1 May 1989, discloses the manufacture of a composite ceramic by the moulding and hot pressing of a powder containing 20wt% silicon carbide whiskers or fibres of diameter 0.5 to 1.0µm and aspect ratio 50 and 20wt% silicon carbide particles of average size 20µm with 55wt% aluminium nitride powder and 15wt% yttria. Briefly the present invention provides a sintered ceramic composite body which is toughened and mechanically strengthened 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. In one aspect the present invention provides a sintered ceramic composite body comprising 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 of silicon carbide which have a size of 1 µm or less and plate-like particles of silicon carbide of size ranging from 5 to 50 µm and a thickness of 1/3 or less of the maximum diameter Preferably the particles of silicon carbide are present at a volume ratio ranging from 10 to 50 %. In another aspect the present invention provides a method of manufacturing a sintered ceramic composite body, as set out in claim 4 The above and other optional features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which embodiments of the present invention is shown by way of illustrative example. 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 nitrode, sialon, etc., the silicon nitride containing two or more of Y₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃, or a rare earth, MgO, and ZrO₂, and, 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 of 1 µm or smaller and are also 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 10 to 50 % or preferably 20 to 40 % of such particles and 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 and mechanical strength. 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 9 x 10⁵ to 9.9 x 10⁵ Pa (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 10 % by volume, then no increase in the fracture toughness and mechanical strength 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 body would be manufactured. No properties would be improved if silicon carbide particles having a size of 1 µm or less and platelet silicon carbide particles having a maximum diameter ranging from 5 to 50 µm and a thickness which is 1/3 or less of the maximum diameter were not simultaneously contained. Specifically, if particles having a size of 1 µm or smaller were not contained, the mechanical strength would not be improved, and if platelet particles having a maximum diameter ranging from 5 to 50 µm and a thickness which is 1/3 or less of the maximum diameter were not contained, i.e., if particles having a size of 1 µm or less and platelet particles having a maximum diameter of 5 µm or less were contained in combination, then the fracture toughness would not be improved. If particles having a size of 1 µm or less and platelet particles having a maximum diameter of 50 µm or greater were contained in combination, then the density of the produced sintered ceramic composite body would be too low. A four-point flexural strength test is carried out according to the fine ceramic flexural strength test procedure provided for in JIS R1601. 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, irrespective of whether the base material is an oxide ceramic material or a nonoxide ceramic material, the reinforcement material was added at a volume ratio ranging from 10 to 50 %, in the form of particles of silicon carbide having a size of 1 µm or less and 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 200 x 10⁵ Pa (200 kgf/cm²), or pressed to shape under a pressure of 7 x 10⁸ Pa (7 tons/cm²) by a rubber press, after which the shaped product was sintered. 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 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 10⁵ Pa (1 atm). The HP pressure was 300 x 10⁵ Pa (300 kg/cm²). For the base material which is a nonoxide ceramic material, the shaped product was sintered at a temperature of 1,700°C or higher in an N₂ atmosphere under 9.5 x 10⁵ Pa (9.5 atm), for the reasons described above. Tables 1-1 and 1-2, show the results of a fracture toughness and mechanical strength 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 a rare earth, MgO, and ZrO₂ as nonoxide materials, and silicon carbide was added as a reinforcement material to the base materials under the conditions given in Tables 1-1 and 1-2. The sintered ceramic composite body according to the present invention is high in toughness and mechanical strength 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 composite body can be manufactured at a low cost.

Claims: A sintered ceramic composite body comprising: a base material composed of either an oxide ceramic or a nonoxide ceramic; and a reinforcement material composed of particles of silicon carbide having a size of 1 µm or less and plate-like particles of silicon carbide which have a maximum diameter ranging from 5 to 50 µm and a thickness which is 1/3 or less of the maximum diameter. A sintered ceramic composite body according to claim 1, wherein said particles of silicon carbide and said platelet particles of silicon carbide are present at a volume ratio in the range of 10 to 50%. A sintered ceramic composite body according to claim 1 or claim 2, wherein said base material is selected from the oxide ceramics alumina, mullite and magnesia and the nonoxide ceramics silicon nitride and sialon. A method of manufacturing a sintered ceramic composite body, comprising: preparing a powdery mixture composed of a base material composed of an oxide ceramic or a nonoxide ceramic, and a reinforcement material composed of particles of silicon carbide having a size of 1 µm or less and platelet particles of silicon carbide which have a maximum diameter ranging from 5 to 50 µm and a thickness which is 1/3 or less of the maximum diameter, said particles of silicon carbide and said platelet particles of silicon carbide being present at a volume ratio in the range 10 to 50 %; molding said powdery 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 oxide ceramic or in the temperature range 1,500 to 2,000°C when the base material is nonoxide ceramic. A method according to claim 4, wherein said base material is selected from the oxide ceramics alumina, mullite and magnesia and the nonoxide ceramics silicon nitride and sialon.

Inventors: FURUSE YUTAKA C O THE TOKYO EL, MATSUHIRO KEIJI, FURUSE, YUTAKA, C/O THE TOKYO ELEC. 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