Silicon carbide sintered body and method of producing the same

Abstract: A silicon carbide sintered body containing 0.01-2% Mg, 0.2-2% B and 0.1-2% free carbon and having a density of not less than 2.80 g/cm³ is produced by adding 0.1-5% amounts of magnesium boride and 0.1-5% carbon to silicon carbide powder having an average grain size of not more than 5 µm, shaping and firing at 1,900 - 2,300°C under vacuum or in an inert gas atmosphere.

Description: This invention relates to a method of making a silicon carbide sintered body. Silicon carbide sintered bodies have excellent chemical and physical properties, so that they are noted as a material suitable for high temperature structures particularly used under severe conditions, such as gas turbine parts, high temperature exchanger and so on. On the other hand, the technique of producing such a silicon carbide sintered body is shifting from the conventional reaction sintering process or pressure sintering process to a pressureless sintering process wherein a mixed powder of silicon carbide, boron-containing additive and carbonaceous additive is sintered in an inert gas atmosphere under no pressure. In the production of the so-called B-C series silicon carbide sintered body through the above pressureless sintering process, when β-SiC is used as a starting material, since boron or a boron-containing additive (generally B₄C), effectively serving for the densification, is added, silicon carbide grains abnormally grow to about several hundred µm at a state including bubbles in the transition from β to α (abnormal grain growth) at a last sintering stage, which is different from the usual grain growth accompanying densification, so that not only is the densification suppressed but also the properties are degraded. In order to solve this problem, it is necessary that the added amount of boron is reduced, and the firing is carried out at a low temperature causing no transition from β to α, and the firing temperature is controlled with high accuracy. There have heretofore been proposed various methods for reducing the added amount of boron, for example, a method of using a specific SiC starting material which cannot necessarily be obtained easily and cheaply, such as super-high purity SiC starting material containing not more than 15 ppm of metal impurity (JP-A-60-131,863), a method of conducting a pretreatment wherein a shaped body of a starting mixture is heated under predetermined pressure and temperature and at the same time the resulting CO gas is exhausted and removed JP-B-61-3,303), a method of using a pitch requiring complicated handling as a carbon source JP-A-60-200,861, JP-A-61-242,959 and JP-A-61-256,976) and the like. Even in these methods, however, it is required to conduct the firing at a relatively low temperature causing no transition from β to α in order to suppress the abnormal grain growth at the last sintering stage. It is difficult to achieve sufficient densification at such a low firing temperature. US-A-4041117 describes a SiC sintering process at low pressure and in an inert atmosphere using a mixture of α and β grains of SiC, boron and free carbon in the range 0.1 to 1% in the form of carbonaceous organic material. It is stated that crystal grain growth control is a problem. JP-A-58-91146 describes a hot-pressing process for SiC production in which an Mg compound is employed to prevent grain growth. Boron is also present. It is an object of the invention to provide a technique for the production of silicon carbide sintered bodies which can achieve densification at a relatively low temperature without causing the conflicting drawbacks of the conventional techniques as mentioned above, such as the obstruction of the densification through grain growth and can easily produce silicon carbide sintered body having a desirable grain shape without causing the abnormal grain growth through high temperature firing. It is another object of the invention to provide a silicon carbide sintered body having high mechanical strength and improved heat resistance and oxidation resistance. The invention provides a method of producing a silicon carbide sintered body as set out in claim 1. The invention will be described with reference to the accompanying drawings, wherein: Fig. 1 is a graph showing a relation between the firing temperature and the density after the firing in the silicon carbide sintered body according to the invention as well as an increasing rate of the density through HIP treatment; Fig. 2 is an optical photomicrograph showing a grain structure of the silicon carbide sintered body according to the invention; Fig. 3 is a graph showing a relation between the amount of boron added and the density after the firing in the silicon carbide sintered body according to the invention; Fig. 4 is a graph showing a temperature dependence of the bending strength in the silicon carbide sintered body according to the invention; and Fig. 5 is a graph showing the oxidation properties of the silicon carbide sintered body according to the invention. According to the invention, silicon carbide powder having an average grain size of not more than 5 µm is used as a starting material for the silicon carbide sintered body. The reason for the limitation of average grain size to not more than 5 µm is due to the fact that when it exceeds 5 µm, a dense sintered body can not be obtained. Moreover, it is desirable that the silicon carbide powder consists of 100∼50% by weight of β-SiC powder and the remainder being substantially α-SiC or amorphous SiC powder. When the amount of β-SiC powder is less than 50% by weight, the growth of plate-like grains inherent to the case of using β-SiC powder is not sufficiently promoted and excellent mechanical properties are not obtained. The essential feature of the invention lies in the use of magnesium boride (e.g. MgB₂, MgB₄, MgB₆, MgB₁₂ or the like). The reason why the amount of magnesium boride is limited to a range of 0.1∼5% by weight of silicon carbide powder is due to the fact that when the amount of magnesium boride is less than 0.1% by weight, sufficient densification is not attained and the addition effect is not recognized, while when it exceeds 5% by weight, the silicon carbide grains grow at the initial firing stage to cause insufficient densification. Preferably, the amount of magnesium boride is within a range of 0.5∼1.0% by weight. According to the invention, carbon or an organic compound producing the same amount of carbon is further added to the silicon carbide powder in an amount of 0.1∼5% by weight. The reason why the amount of carbon is limited to the above range is due to the fact that when the amount of carbon is less than 0.1% by weight, SiO₂ film is not sufficiently removed from the surface of the fired body, while when it exceeds 5% by weight, more than 2% by weight of free carbon remains in the fired body to degrade the properties. In the method of the invention, the silicon carbide powder added with predetermined amounts of magnesium boride and carbon or organic compound is fired at a temperature of 1,900∼2,300°C under vacuum or in an inert gas atmosphere after the shaping. The reason why the firing temperature is limited to the above range is due to the fact that when the firing temperature is lower than 1,900°C, sufficient densification is not achieved, while when it is higher than 2,300°C, the decomposition of silicon carbide becomes vigorous, making the surface of the resulting fired body rough and consequently a sufficiently smooth surface can not be obtained. The silicon carbide sintered body according to the invention is required to have a density of not less than 2.80 g/cm³. When the density is less than 2.80 g/cm³, it is difficult to realize excellent chemical and mechanical properties inherent to the silicon carbide sintered body according to the invention. According to the invention, by using magnesium boride as an additive, the fired body is sufficiently densified at a relatively low firing temperature without causing the abnormal grain growth found when a relatively high firing temperature is used though the amount of boron added is small, so that it is possible to easily produce the silicon carbide sintered body having a desirable grain form. Such an effect is considered to result from magnesium in the magnesium boride. Although it is not yet understood in detail how magnesium contributes to the densification and the effect of suppressing the abnormal grain growth at high temperature, it is supposed that magnesium is dissolved into silicon carbide in the low temperature region of the firing stage to promote the densification and conversion from β to α without causing the abnormal grain growth at a relatively low temperature. According to the invention, the firing can be carried out within a relatively wide temperature range, so that one need not precisely control the firing temperature as in the conventional technique. Furthermore, the amount of boron added is reduced, so that the abnormal grain growth is effectively suppressed without taking special precautions as in the conventional technique, and it is possible to provide silicon carbide sintered bodies having high strength. The silicon carbide sintered body obtained by the method of the invention has an improved oxidation resistance as compared with the conventional B-C series silicon carbide sintered body. This is due to the fact that the silicon carbide sintered body according to the invention is dense and the grain size of silicon carbide grain is small and uniform so that the oxidation reaction is uniformly promoted and further the oxide film produced on the surface through oxidation is densified by the magnesium component contained in the sintered body. Moreover, the silicon carbide sintered body according to the invention has a small grain size of silicon carbide powder compared with the conventional B-C series silicon carbide sintered body and has no open pores, so that the silicon carbide sintered body having a further increased densification degree can be obtained by subjecting to a capsule-free HIP treatment after the sintering. The invention will be described with reference to the following examples demonstrating the success of the invention. The comparison experiments were made under various conditions by using silicon carbide powder with MgB₂ and C additions or silicon carbide powder with B₄C and C additions as a starting mixture to obtain experimental results as shown in Figs. 1 to 5. Moreover, the starting silicon carbide powder used consisted of 93% by weight of β-SiC and the remainder being α-SiC and inevitable impurities and had an average grain size of 0.42 µm and a specific surface area of 20.0 m²/g and a chemical composition (% by weight) as shown in the following Table 1. The quantification of β-SiC was made according to a method described in Journal of Japanese Ceramics Associate, 87(11], pp577∼578 (1979). Total-Si 69.13 Free-SiO₂ 0.47 Free-Si 0.010 Free-C 0.51 O 0.90 Al 0.056 Fe 0.060 Ca 0.016 Mg 0.001 K <0.001 Na 0.001 Cu - Ti 0.007 N 0.27 As first, a mixture of the starting silicon carbide powder shown in Table 1 containing 1% by weight of MgB₂ and 2% by weight of C (carbon black) or a mixture containing 1% by weight of B₄C and 2% by weight of C was prepared and then added with isopropyl alcohol, which was pulverized and mixed in a wet-type ball mill. This mixture was preliminarily shaped and then pressed into a square plate of 60×60×6 mm under a hydrostatic pressure of 3 × 10⁵ kPa (3 ton/cm²). Next, the shaped body was fired up to 1,500°C under vacuum and further at a constant temperature in an argon gas atmosphere under an atmospheric pressure for 1 hour. In Fig. 1 is plotted the density of the fired body when varying the firing temperature in the firing under the argon gas atmosphere at atmospheric pressure, wherein mark ○ shows the fired body showing no abnormal grain growth and mark  shows the fired body showing abnormal grain growth. When the silicon carbide sintered body according to the invention is compared with the B-C series silicon carbide sintered body of the comparative example, it is apparent that sufficient densification is achieved even at a low firing temperature in the invention. Furthermore, it is clear that the silicon carbide sintered body having a required density and no abnormal grain growth is obtained over a wide firing temperature range according to the invention. In other words, the precise control of the firing temperature is not required in the invention. Moreover, an arrow shown in Fig. 1 indicates an amount of density increase when the sintered body is subjected to an HIP (hot isostatic pressing) treatment at 1,900°C in an argon gas atmosphere under a pressure of 2,000 times atmospheric pressure, from which it is apparent that more densification of the sintered body according to the invention can easily be achieved by the HIP treatment as compared with the B-C series silicon carbide sintered body. Each MgB₂-C series silicon carbide sintered body and B-C series silicon carbide sintered body produced at the firing temperature of 2,100°C as shown in Fig. 1 was subjected to polishing and etching, and thereafter the grain structure on the etched surface was observed by an optical microscope. Fig. 2 shows microphotographs of the surfaces of these sintered bodies. As seen from Fig. 2, the MgB₂-C series silicon carbide sintered body according to the invention shows no abnormal grain growth and consists of plate-like silicon carbide grains having a uniform grain size. On the other hand, in the B-C series silicon carbide sintered body, the silicon carbide grains enter bubbles to grow abnormally up to about several hundred microns. Moreover, the amount of α-SiC in the fired body was about 60% in both the silicon carbide sintered bodies. Then, the starting silicon carbide powder as mentioned above was added with 2% by weight of carbon and a variable amount of boron, which was subjected to the same procedure as mentioned above and fired at a temperature of 2,100°C in an argon gas atmosphere under an atmospheric pressure for 1 hour. The density of the resulting fired body is plotted in Fig. 3 as a function of the amount of boron together with the results for MgB₂-C SiC bodies according to the invention. As seen from Fig. 3, according to the invention, the required densification can be achieved without taking special precautions as in the conventional technique though the amount of boron added is small. Fig. 4 shows results of four-point bending strength measured according to JIS R1601 (test method of bending strength in fine ceramics) on the silicon carbide sintered bodies obtained by firing the shaped body prepared in the same manner as in Fig. 1 at 2,100°C in an argon gas atmosphere under atmospheric pressure. As seen from Fig. 4, according to the invention, the higher strengths can be achieved as compared with the B-C series silicon carbide sintered body of the comparative example. In Fig. 5 is plotted a change of oxidation increment with a lapse of time when the silicon carbide sintered body obtained in the same manner as in Fig. 4 is left to stand in air at a temperature of 1,500°C, which indicates the oxidation property of the sintered body. As seen from Fig. 5, the silicon carbide sintered body according to the invention has superior oxidation resistance to the B-C series silicon carbide sintered body of the comparative example. Finally, Table 2 shows analytical values of magnesium, boron and free carbon measured on the specimens of magnesium boride-C series sintered body and B-C series sintered body after firing at 2,100°C for 1 hour as well as the measured value of the density of the fired body and the observation result of abnormal grain growth. As seen from Table 2, the density of the fired body is low when the amount of magnesium boride added is less than 0.15% by weight (Comparative Example 1), while the density lowers when the amount is more than 5% by weight (Comparative Example 2). On the other hand, the density of the fired body is low when carbon is not added (Comparative Example 3), while when the amount of carbon added exceeds 5% by weight, not only is the density of the fired body lower but also the amount of free carbon degrading the properties of the sintered body becomes larger (Comparative Example 4). Furthermore, in the B-C series silicon carbide sintered body (Comparative Example 5), the high density sintered body having a density of 3.08 g/cm³ is obtained, but the abnormal grain growth is observed. In contrast, when the amount of magnesium boride added is within a range of 0.1∼5% by weight and the amount of carbon added is within a range of 0.1∼5% by weight, silicon carbide sintered bodies containing 0.01∼2% by weight of magnesium, 0.02∼2% by weight of boron and 0.1∼2% by weight of free carbon and having a density of not less than 2.80 g/cm³ are obtained as seen from Examples 1 to 8. As mentioned above, according to the invention, the silicon carbide powder can be fired over a wide firing temperature range without causing the various problems as in the conventional techniques to produce dense silicon carbide sintered bodies having desired grain form without causing abnormal grain growth, and high strength and oxidation resistance.

Claims: A method of making a SiC sintered body having a density of not less than 2.80 g/cm³, comprising (a) forming a shaped body from a powder of silicon carbide having an average grain size of not more than 5 µm and further containing (i) 0.1-5% by weight of magnesium boride selected from MgB₂, MgB₄, MgB₆ and MgB₁₂ and (ii) 0.1-5% by weight of carbon or an organic compound producing 0.1-5% by weight of carbon, and (b) firing the shaped body at a temperature in the range 1900-2300°C in a vacuum or an inert gas atmosphere so as to form said sintered body. A method according to claim 1, wherein the powder of silicon carbide contains by weight at least 50% β-SiC with the remaining SiC if any being α-SiC or amorphous SiC.

Inventors: KAWASAKI SHINJI, TAKAHASHI TOMONORI, KAWASAKI, SHINJI, TAKAHASHI, TOMONORI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1988
Main classification: C04B35
IPC codes: C04B35, C01B31