CERAMIC COMPACT, IN PARTICULAR BASED ON SILICON NITRIDE AND CERAMIC FIBRES AND PROCESS FOR ITS PRODUCTION

Abstract: A ceramic compact is disclosed having high tem­ perature strength, toughness and reliability, which comprises a silicon nitride matrix and ceramic fibers uniformly dis­ persed in the matrix and orientated in a desired direction, said matrix and fibers being closely bonded by sintering. The compact is produced, for example, by preparing a shaped body of silicon in which ceramic fibers are uniformly dispersed and then heating and nitriding the shaped body in a nitrogen atmosphere to form a fiber-rein­ forced silicon nitride sintered compact.

Description: This invention relates to a process for the production of a ceramic shaped body containing ceramic fibers and having an excellent strength and toughness, in particular, a ceramic silicon nitride (Si₃N₄) sintered compact. Ceramics have widely been used as electrically functional materials or mechanical and structural materials because of excellent properties thereof, but the practical use of ceramics has considerably been limited when utilizing the mechanical properties since there arises a problem that a sudden breakage or a large dispersion of the strength tends to occur due to the brittleness of ceramics. Thus, various efforts have hitherto been made so as to improve the reliability or strength of ceramics. As to aluminum oxide (Al₂O₃), zirconium oxide (ZrO₂), silicon carbide (SiC) and silicon nitride (Si₃N₄), in particular, compositions and production conditions have been studied in detail. Furthermore, an incorporation of fibers, for example, metallic fibers and ceramic fibers has been studied to increase the strength and ceramics containing ceramic whiskers, for example, a silicon nitride compact containing fibrous silicon carbide dispersed have been reported as disclosed in Japanese Patent Application OPI (Kokai) No. 104069/1983. GB1335842 discloses a method of preparing a ceramic compact. In this method ceramic fibers and silicon powder are dispersed in a solvent to form a slurry which is then dried and nitrided to form a fiber reinforced silicon nitride compact. However, the studies of the compositions and production conditions have failed to realize the anticipated results and the incorporation of fibers has met the problems that the production cost is high and the uniform dispersion of fibers is very difficult. The present invention provides a process for the production of a ceramic compact, which comprises preparing a shaped body of silicon in which ceramic fibers are uniformly dispersed by dispersing silicon powder and ceramic whiskers in a solvent to form a slurry, and removing the solvent from the slurry by charging the slurry in a mold through which only the solvent is permitted to pass, thus forming the silicon body; and then heating and nitriding the shaped body in a nitrogen atmosphere to form a fiber-reinforced silicon nitride compact; characterised in that the solvent is removed by subjecting the slurry in the mold to centrifugal force, thus forming the silicon body containing the ceramic fibers orientated in parallel with the inner surface of the mold. Thus, the present invention provides a process for the production of a ceramic compact with an excellent high temperature strength, toughness and reliability. Examples of the present invention will now be described with reference to the drawing which is a cross-sectional view of one embodiment of an apparatus for centrifugal casting, suitable for practicing the process of the present invention. The inventors have made various efforts to obtain a ceramic compact excellent in high temperature strength, toughness and reliability and consequently, have found that a composite composition in which ceramic fibers are uniformly dispersed exhibits very excellent properties or performances, in particular, a combination of a matrix of silicon nitride and ceramic fibers is more excellent and a method comprising dispersing silicon and ceramic fibers and nitriding the silicon to form silicon nitride (Si₃N₄) is effective for obtaining a more compact sintered body. That is, the process of the present invention provides a ceramic compact comprising a matrix of silicon nitride (Si₃N₄) and ceramic whiskers, preferably consisting of silicon carbide whisker, uniformly distributed and dispersed therein, said matrix and fibers being closely bonded by reaction sintering. The above described ceramic compact is produced, by a process comprising obtaining a shaped body of silicon (Si) in which ceramic fibers are uniformly dispersed and then heating and nitriding the shaped body in a nitrogen atmosphere to form a fiber-reinforced silicon nitride (Si₃N₄) compact. The ceramic shaped body consists substantially of a ceramic matrix and ceramic fibers uniformly distributed and dispersed therein with a certain directional property. In the prior art process, a shaped body of silicon nitride and ceramic fibers is obtained and then subjected to compacting, while in one embodiment of the present invention, the compacting is carried out by nitriding silicon so as to prevent from shrinkage of the volume during the compacting. That is, a shaped or formed body containing fibers is hardly changed in dimension and thus hardly compacted, but this embodiment of the present invention aims to compact without changing the dimension of such a shaped body. In this embodiment, any of long fibers and short fibers could be used as the ceramic fibers but, whiskers are preferred from the standpoint of compatibility. Whiskers should be used under sufficiently dispersed state. As the ceramic fibers, there can be used fibers of alumina (Al₂O₃) , silicon nitride (Si₃N₄) , silicon carbide (SiC), carbon (C), potassium titanate (K₂TiO₃) and zirconium oxide (ZrO₂) and silicon carbide whisker is particularly preferably used because of being so compatible with a matrix of silicon nitride that best results are given. The quantity of the ceramic fibers should be 0.1 to 45 % by volume, since if it is less than 0.1 % by volume, no effect appears by the addition thereof, while if more than 45 % by volume, the fibers are too many to be uniformly dispersed. Production of the shaped body is carried out by a process comprising mixing a silicon powder and ceramic fibers in the form of a slurry and then subjecting the slurry to centrifugal casting in a mold. According to this process, the ceramic fibers can be uniformly dispersed, orientated and shaped or formed with a higher efficiency. As a solvent for mixing a silicon powder and ceramic fibers to form a slurry, there can be used, for example, water, alcohol, acetone, hexane and the like. Fig. 1 is a schematic view of one embodiment of an apparatus for centrifugal casting, used in the process of this embodiment of the present invention, in which a casting mold 3 is provided in a closed vessel 1 to be revolved by a rotation driving means 2 and the vessel 1 can be evacuated via a pipe 4 by a vacuum pump (not shown). The casting mold 3 is made of a material capable of permitting only a solvent in a slurry 6 containing a material 5 for a compact to pass therethrough, for example, gypsum, filter paper and the like. Particularly, gypsum or filter paper is preferably used because of the ease of making the casting mold and the lower price. For casting, a reduced atmosphere can be applied with better results. Referring to Fig. 1, the material 5 for a sintered compact is being formed on the inner wall of the casting mold 3 from the slurry 6. The thus obtained shaped body is sintered under the ordinary silicon nitride reaction sintering conditions, preferably at a temperature of 1300 to 1450 °C in a nitrogen gas atmosphere. In the first embodiment of the present invention, a fiber-reinforced silicon nitride compact having an excellent high temperature strength, toughness and impact strength can readily be obtained by uniformly dispersing ceramic fibers with a large strength and after preparing a shaped body, subjecting silicon to reaction sintering, thus forming a sintered compact body as Si₃N₄. In a second embodiment of the present invention, a fiber-reinforced silicon nitride (Si₃N₄) compact is produced by a process comprising mixing ceramic fibers and a silicon powder containing 0.5 to 40 % by weight of at least one member selected from the group consisting of aluminum oxide (Al₂O₃), aluminum nitride (AlN), zirconium oxide (ZrO₂), yttrium oxide (Y₂O₃), magnesium oxide (MgO), cerium oxide (CeO₂) and lanthanum oxide (La₂O₃), preparing a shaped body of the resulting mixture in such a manner that the ceramic fibers are orientated in a predetermined direction, heating and nitriding the shaped body in a nitrogen atmosphere and then further sintering and compacting the shaped body at a higher temperature in a nitrogen atmosphere. The silicon nitride (Si₃N₄) forms a sintered compact body with at least one sintering aid selected from the group consisting of Al₂O₃, AlN, ZrO₂, Y₂O₃, MgO, CeO₂ and La₂O₃. The quantity of the sintering assistant is preferably 0.5 to 40 % by weight based on the silicon nitride, since if less than 0.5 % by weight, compacting of a sintered body is insufficient, while if more than 40 % by weight, the high temperature properties such as high temperature strength and oxidation resistance are markedly deteriorated. Better results can be obtained by adding at least two sintering assistant in combination. As the ceramic fibers, there can be used the same ceramic fibers in the same proportion as in the first embodiment. The matrix of silicon nitride (Si₃N₄) is formed by subjecting a shaped or formed body of silicon to reaction sintering in nitrogen gas at a temperature of 1300 to 1450 °C. Since a sintered compact body is hardly obtained by only the reaction sintering, however, the product is preferably subjected to further sintering in nitrogen gas at a temperature of 1700 to 1900 °C. It is preferable to compress this nitrogen gas in order to prevent Si₃N₄ from decomposition, but is not always essential. As occasion demands, the use of a hot hydrostatic press using a high pressure nitrogen gas is preferable to attain better results. A process for producing the above described ceramic compact according to this embodiment will now be illustrated. Firstly, a ceramic shaped body comprising silicon grains, ceramic grains and ceramic fibers orientated in a direction along the contour of the ceramic shaped body is obtained by a novel shaping method utilizing centrifugal force to achieve the orientation of the fiber components, which method corresponds to an improvement of the slip cast method commonly used in the production of ceramics according to the prior art. Even when a slurry is merely charged in a casting mold and cast, not only the orientation of the fiber components is insufficient, but also the distribution thereof is not uniform. On the other hand, the centrifugal casting method can give a uniform orientation and distribution of the fiber components. In the second embodiment of the present invention, therefore, a silicon powder, at least one sintering assistant selected from the group consisting of Al₂O₃, AlN, ZrO₂, Y₂O₃, MgO, CeO₂, and La₂O₃, and fibers or whiskers selected from the group consisting of those of SiC, Si₃N₄, Al₂O₃, ZrO₂, C and K₂TiO₃ are mixed with a solvent such as water, alcohol, acetone or hexane to form a slurry and then the resulting slurry is subjected to centrifugal casting to obtain a shaped body in which the fiber components are orientated along the contour of the shaped body and uniformly dispersed. Then, the thus resulting shaped body is subjected to reaction sintering in nitrogen at 1300 to 1450 °C to convert the silicon of the matrix in the shaped body into Si₃N₄ as described above, and further sintered in nitrogen at 1700 to 1900 °C to obtain a fiber-reinforced Si₃N₄ sintered compact. The nitrogen gas is preferably compressed so as to prevent Si₃N₄ from decomposition and a hot hydrostatic press using a high pressure nitrogen gas is preferably used. In an alternative method which is claimed in divisional application number a process for the production of a ceramic composite is characterized by shaping ceramic fibers into a shaped body with a desired size, impregnating the shaped body with a slurry containing a ceramic powder or silicon powder as a predominant component, drying and then sintering. In this process, it is more preferable to use silicon carbide (SiC) whiskers as the ceramic fibers, a shaped body of ceramic fibers, having a porosity of 95 to 55 %, as the shaped body, and SiC, Si₃N₄, Al₂O₃ or ZrO₂ powder as the ceramic powder. More preferably, the impregnation with a slurry is carried out by centrifugal casting, as described above, and a shaped body is impregnated with a slurry containing silicon as a predominant component, dried and then subjected to reaction sintering in nitrogen gas. In this specification, the above described porosity is defined by the following formula (1): Firstly, ceramic fibers are shaped into a preform having a porosity of 95 to 55 % as represented by the formula (1). This means that a ceramic powder to be impregnated will be dispersed in the pores of the preform, but if the porosity is more than 95 %, the preform is hard to hold a desired shape, while if less than 55, the powder is too few to achieve compacting by sintering. For example, preparation of the preform is generally carried out by a pressing method. In this process, any of continuous fibers and short fibers can be used and the same materials can be used therefore as in the embodiments of the present invention. The resulting preform is then impregnated with a ceramic powder or a powder containing predominantly silicon, for example, by a method comprising dispersing the powder in a solvent, impregnating the preform with the resulting slurry and then evaporating the solvent. Useful examples of the ceramic powder are SiC, Si₃N₄, Al₂O₃ and ZrO₂. Furthermore, a sintering aid such as Y₂O₃ can be added to the ceramic powder or silicon powder. As the solvent, for example, there are used water, alcohol and the like and in the case of the ceramic powders, alcohol is preferable in order to prevent the powders from oxidation. A preform having a porosity of 95 to 55 %, i.e. having a fiber content of 5 to 45 % by volume is impregnated with a ceramic powder or silicon powder, thus obtaining a shaped body having a powder content of 5 to 75 % by volume and a total quantity of the fiber and powder of 50 to 80 % by volume. The impregnation is preferably carried out by a centrifugal casting method using a centrifugal force whereby the preform is uniformly impregnated with a slurry and the removal of the solvent can rapidly be carried out. The centrifugal casting method is carried out, for example, by an apparatus as shown in Fig. 1, but in this embodiment, the casting mold 3 is formed so as to be adapted to the contour of a preform 5 and made of a material capable of permitting only a solvent in a slurry 6 containing a ceramic powder or silicon powder to pass therethrough, for example, gypsum, filter paper and the like. Particularly, gypsum or filter paper is preferably used because of the ease of making the casting mold and the lower price. As shown in Fig. 1, the preform 5 is set in the mold 3, the slurry 6 is poured in the preform 5 and the preform 5 is impregnated with the powder while subjecting to centrifugal force. During the casting, a reduced atmosphere is preferably used with good results. The resulting shaped body is dried in known manner and then sintered. In the case of using Si₃N₄ as the matrix, in particular, a shaped body impregnated with a silicon powder can be obtained and subjected to reaction sintering in an atmosphere of nitrogen to form Si₃N₄. When sintering a ceramic powder in ceramic fibers, the volume is so hard to shrink that compacting is difficult, but when a silicon powder is subjected to reaction sintering, a sintered compact body can easily be obtained without necessity of the volume shrinkage. As illustrated above, the feature of the alternative process consists in obtaining a composite of a ceramics and ceramic fibers by previously impregnating a preform of the ceramic fibers with a slurry of a ceramic powder or silicon powder, drying to remove the solvent and sintering. The the alternative process provides for the uniform dispersion of ceramics and ceramic fibers, which has hitherto been considered difficult. Furthermore, when using a centrifugal casting method utilizing a centrifugal force in order to fill a preform with a ceramic powder or silicon powder, a uniform shaped body can be given. Therefore, using both the present invention and the alternative process there is provided fiber-reinforced silicon nitride ceramics excellent in strength as well as toughness by the use of a ceramic shaped body consisting essentially of a ceramic matrix and ceramic fibers, characterized in that the ceramic fibers are orientated. This ceramic compact is very effective as structural materials at high temperature, for example, structural materials for gas turbines, because of having a higher strength at a high temperature and a large toughness. This is significant in view of that the ceramics of the prior art are lacking in the high temperature strength and toughness and thus have not been put to practical use. Moreover, the process of embodiments of the present invention has the advantage in addition to the provision of the above described fiber-reinforced silicon nitride compact that it enables the use of a centrifugal casting method, thus reducing the production cost and providing the compact economically. The following examples are given in order to illustrate the instant invention in greater detail without limiting the same. Example 1 15% by weight of SiC whiskers with a mean grain size of 0.3 µm and a length of 50 µm were added to a Si powder with a mean grain size of 0.5 µm and mixed with water as a solvent for 8 hours to form a slurry containing Si and SiC. The resulting slurry was subjected to centrifugal casting using a gypsum mold to obtain a shaped body with an outer diameter of 30 mm, inner diameter of 20 mm and length of 60 mm, which was then dried at 100 °C for 3 days and further subjected to reaction sintering in a nitrogen stream at 1350 °C for 8 hours, thus obtaining a sintered Si₃N₄ compact with a density of 2.90. The compact had a structure such that the SiC whiskers were laminarly orientated, a bending strength at room temperature of 60 kg/mm² and a bending strength at 1200 °C of 45 kg/mm², which was considered as a more excellent high temperature strength. In addition, the compact exhibited a Charpy impact value of 0.5 kg/cm² being at least 5 times as large as that of an Si₃N₄ compact containing no whisker. In comparison with an ordinary Si₃N₄ compact with a density of 3.15, bending strength at room temperature of 75 kg/mm² and bending strength at 1200 °C of 35 kg/mm², it will clearly be understood that the composite ceramics obtained according to the present invention are more excellent in properties, in particular, high temperature strength. Example 2 75 % by weight of Si powder with a mean grain diameter of 0.7 µm, 5 % by weight of Al₂O₃, 5 % by weight of Y₂O₃ and 15 % by weight of SiC whiskers with a diameter of 0.3 µm and length of 60 µm were mixed and dispersed in ethanol to form a slurry. The resulting slurry was then subjected to centrifugal casting to obtain a shaped body in which the whiskers are orientated in parallel with the inner wall of the casting mold and which had an outer diameter of 30 mm, inner diameter of 20 mm and length of 60 mm. This shaped body was then subjected to reaction sintering in a nitrogen stream at 1380 °C and further sintered in nitrogen at 1750 °C and 5 atm for 2 hours. The thus obtained compact had a relative density of 95 %, bending strength at room temperature of 70 kg/mm², bending strength at 1200 °C of 70 kg/mm² and K IC (stress intensity factor) of 9.5 MN/m 3/2 . Example 3 Sintering assistants and ceramic fibers as shown in Table 1 were mixed with Si powder and subjected to production of ceramic compacts in an analogous manner to Example 2. The compacts had properties shown in Table 1. It is apparent from these results that if the quantity of a sintering assistant is less than 0.5 % by weight or more than 40 % by weight, the properties of the compact are largely degraded and the quantity of the sintering assistant ranging from 0.5 % by weight to 40 % by weight is optimum for the present invention. Sintering Assistant and Quantity Thereof(wt %) Ceramic Fibers Properties of Compacts Density Bending Strength at Room Temp Bending Strength at 1200 °C Remarks Al₂O₃ 0.2 SiC Whisker 83.0 20 20 Comparison Y₂O₃ 0.2 Al₂O₃ 0.3 -do- 92.0 55 60 Present Invention Y₂O₃ Al₂O₃ 10 -do- 96.0 65 65 -do- Y₂O₃ 10 Al₂O₃ 21 -do- 97.0 65 30 Comparison Y₂O₃ 21 Al₂O₃ 5 SiC Long Fiber 92.0 75 70 Present Invention MgO 5 Al₂O₃<-> 5 ·2AlN SiC Whisker 96.0 70 70 -do- Y₂O₃ 3 Al₂O₃ 5 -do- 96.0 70 70 -do- ZrO₂ 5 Example 4 15 % by weight of β-type Si₃N₄ whiskers with a mean grain diameter of 1.0 µm and a length of 30 µm were added to a Si powder with a mean grain diameter of 0.5 µm, shaped and sintered in an analogous manner to Example 1. The thus obtained compact had a composite structure of the β-type Si₃N₄ whiskers and β-type Si₃N₄ grains and a bending strength at room temperature of 65 kg/mm² and a bending strength at 1200 °C of 50 kg/mm², which properties were considered excellent.

Claims: A process for the production of a ceramic compact, which comprises preparing a shaped body of silicon in which ceramic fibers are uniformly dispersed by dispersing silicon powder and ceramic whiskers in a solvent to form a slurry, and removing the solvent from the slurry by charging the slurry in a mold through which only the solvent is permitted to pass, thus forming the silicon body; characterised in that the solvent is removed by subjecting the slurry in the mold to centrifugal force, thus forming the silicon body containing the ceramic fibers orientated in parallel with the inner surface of the mold. The process of Claim 1, where the ceramic fibers are selected from the group consisting of aluminium oxide, silicon nitride, carbon, potassium titanate, silicon carbide and zirconium oxide and mixtures thereof. The process of Claim 1, wherein the ceramic fibers are present in a proportion of 0.1 to 45% by volume. The process of Claim 1, wherein the slurry further contains at least one member selected from the group consisting of aluminium oxide, aluminium nitride, zirconium oxide, yttrium oxide, magnesium oxide, cerium oxide and lanthanum oxide and mixtures thereof of a proportion of 01.5 to 40% by weight. A process of Claim 1, wherein the heating and nitriding is carried out at a temperature of 1300 to 1450°C.

Inventors: DOI, YOSHIHIKO, MIYAKE, MASAYA, YAMAKAWA, AKIRA
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1987
Main classification: C04B35
IPC codes: C04B35