JP7577015B2 - Ceramic composition and fired ceramic body - Google Patents

Description

The present invention relates to ceramic compositions and ceramic fired bodies such as ceramic cores for use in high-temperature environments.

Super heat-resistant alloys such as Ti-based alloys, Ni-based alloys, and Co-based alloys are used in heat-resistant structural components such as turbine blades, jet engines, and turbochargers that are used in high-temperature environments (e.g., 1100°C or higher). Among these heat-resistant structural components, for example, turbine blades are provided with hollow holes that serve as air passages that are designed in a complex and highly precise manner to ensure both strength and cooling function.

Heat-resistant structural components with hollow holes can generally be manufactured by precision casting methods such as the lost wax method. Specifically, heat-resistant structural components are manufactured by using a mold with an internal space corresponding to the external structure of the component and a core with a shape corresponding to the desired hollow hole, pouring the molten superalloy (molten metal) into the cavity formed by the mold and core, allowing it to solidify, and then dissolving the core with an alkaline solution or the like to form the hollow hole. Generally, since the mold and core are required to be heat-resistant to the molten metal, refractory materials made of ceramic are used.

A typical ceramic core used in the above-mentioned precision casting method is made by pouring a ceramic clay or ceramic slurry (hereinafter referred to as clay and slurry, respectively) into a mold, molding it, and then firing it to produce a so-called near-net shape (a state close enough to the finished product that no post-processing is required). Molding methods include injection molding and casting. Conventional clays and slurries for molding ceramic cores are ceramic compositions whose main component is amorphous silica, and are mixed with other inorganic components and binders as necessary.

For example, Patent Document 1 is an example of prior art related to ceramic cores. The temperature environment during casting of heat-resistant structural components is a high-temperature environment exceeding at least 1000°C. Therefore, when casting using general amorphous silica particles, there is a risk of deformation of the core due to large shrinkage caused by changes in the crystal structure of silica or poor creep properties. In response to the above problem, Patent Document 1 describes that as a method of suppressing deformation of the ceramic core at high temperatures (improving shape stability at high temperatures), it is effective to convert part of the silica component of the ceramic core into cristobalite, which is difficult to sinter.

Special Publication No. 62-30858

In recent years, for example, with ceramic cores, the shape of the cores has become more complex and the wings have become thinner as the structure of heat-resistant structural components has become more complex. For this reason, there is a demand for ceramic compositions, which are the raw material for fired ceramic bodies such as cores, to have improved filling properties when they are manufactured into dies, but conventional amorphous silica particles and cristobalite particles have not always been able to provide the necessary filling properties.

The present invention was made in consideration of these circumstances, and aims to provide a ceramic composition and a sintered ceramic body that have excellent mold filling properties while improving the shape stability of the sintered ceramic body at high temperatures.

The ceramic composition of the present invention includes amorphous silica, cristobalite, and a binder, the binder being at least one of an organic binder and an inorganic binder, the cristobalite being entirely or at least partially spherical cristobalite, and the spherical cristobalite is contained in an amount of 0.1% by mass to 35% by mass relative to the total silica component.

Here, the above-mentioned "cristobalite" refers to crystalline silica having a cristobalite crystallization rate (ratio of cristobalite crystalline phase) of 80% or more, calculated based on the peak area (integral intensity) obtained by integrating the height of the diffraction peak appearing near 2θ = 22° in the measurement by an X-ray diffraction (XRD) device. The above-mentioned "cristobalite crystallization rate" can be quantified by analyzing the peak area of the diffraction peak by a calibration curve method or the like. In addition, the above-mentioned "silica component" may be any component consisting of _SiO2 , and may include, in addition to amorphous silica and cristobalite, tridymite, quartz, coesite, stishovite, etc.

The spherical cristobalite has a particle size of 0.5 μm to 150 μm.

Here, the "particle size" is not particularly limited in the measurement method, but is, for example, the particle size calculated by measurement using a laser scattering/diffraction particle size distribution measuring device or an image capture/analysis device, and represents the particle size calculated on a volume basis for all particles from the smallest to the largest.

The spherical cristobalite has an average aspect ratio of 1 to 3 as measured by an optical microscope.

Here, "average aspect ratio" refers to the average aspect ratio, which is the ratio of the long axis length to the short axis length (long axis length/short axis length) of each particle for a given number of particles. The average aspect ratio is a value of 1 or more.

The spherical cristobalite has an average circularity of 0.6 to 1.0 as measured by an optical microscope.

Here, the "circularity" is expressed by the relationship between the radius r, area S, and perimeter L of a particle, as shown in the following formula (1).
Circularity=4πS/ ^L2 =4π( ^πr2 )/(2πr) ² (1)
The "average circularity" refers to the average value of the circularity of each particle for a given number of particles. The circularity is a value of 1 or less.

The fired ceramic body of the present invention contains amorphous silica and cristobalite, and the cristobalite is characterized in that at least a portion of the cristobalite is spherical.

The fired ceramic body is a ceramic core for casting.

The ceramic composition of the present invention contains amorphous silica, cristobalite, and a binder, and the binder is at least one of an organic binder and an inorganic binder. Therefore, a ceramic core manufactured using the ceramic composition is less susceptible to shrinkage and creep when casting a heat-resistant structural component, and has excellent shape stability at high temperatures. In addition, all or at least a portion of the cristobalite is spherical cristobalite, and the spherical cristobalite is contained in an amount of 0.1% by mass to 35% by mass of the entire silica component, so that the collision resistance between particles constituting the silica component in the ceramic composition of the present invention is reduced, resulting in excellent fluidity. This results in excellent filling properties into a mold.

Since the particle size of the spherical cristobalite is 0.5 μm to 150 μm, the particles are less likely to aggregate and can exhibit favorable fluidity. As a result, it has excellent filling properties for the blades, etc.

Spherical cristobalite has an average aspect ratio of 1 to 3, or an average circularity of 0.6 to 1.0, as measured by an optical microscope, which further reduces the resistance to collisions between particles, making it less likely to aggregate when made into a clay or slurry-like ceramic composition, and providing better fluidity. As a result, the composition has better filling properties into molds, and can be used for ceramic cores used in the manufacture of heat-resistant structural components with more complex structures.

The ceramic sintered body of the present invention contains amorphous silica and cristobalite, at least a portion of which is spherical, so that heat-resistant structural components are less susceptible to shrinkage and creep during casting, have excellent shape stability at high temperatures, and can easily be manufactured with precision structures. Therefore, it can be suitably used for ceramic cores for casting, which have complex structures.

1 is an electron microscope photograph of the ceramic composition of Example 2 after firing. 1 is an electron microscope photograph of the ceramic composition of Comparative Example 1 after firing.

The ceramic composition of the present invention includes amorphous silica, cristobalite, and a binder. All or at least a portion of the cristobalite is spherical cristobalite. Cristobalite is difficult to sinter, so it suppresses dimensional shrinkage during firing. It also has the effect of improving creep characteristics at high temperatures, for example, when immersed in molten metal as a ceramic core. In conventional ceramic cores, when cristobalite is blended, crushed non-spherical cristobalite particles (powder) are generally used. However, in the present invention, it has been found that the use of spherical cristobalite produces various effects, which will be described later.

The ceramic composition of the present invention contains spherical cristobalite in an amount of 0.1% to 35% by mass based on the total silica component. By containing 0.1% or more by mass of cristobalite based on the total silica component, the viscosity of the clay or slurry can be reduced and the fluidity can be improved. Furthermore, since the content of spherical cristobalite in the total silica component of the composition is 35% or less by mass, the thermal shock properties are excellent, and cooling cracks, which are cracks that occur during cooling after firing, are unlikely to occur. This allows the ceramic fired body to be produced with a high yield and excellent production efficiency. The spherical cristobalite is preferably contained in an amount of 1% to 30% by mass based on the total silica component, and more preferably 2% to 25% by mass.

Because cristobalite particles are spherical, the resistance to collisions between particles is reduced, and the fluidity of the molding clay or slurry is excellent. As a result, it is suitable for producing ceramic fired bodies such as ceramic cores with complex structures and thin sections.

The particle size of the spherical cristobalite contained in the ceramic composition of the present invention can be freely selected, and is preferably 0.5 μm to 150 μm. By setting the minimum particle size to 0.5 μm or more, the particles do not become too fine, so they are less likely to aggregate, and a preferable flowability can be expressed. In addition, by setting the maximum particle size to 150 μm or less, excellent filling properties are obtained for blades and the like. On the other hand, the average particle size of the spherical cristobalite is preferably 3 μm to 60 μm, and more preferably 3 μm to 20 μm. Here, the average particle size represents the particle size at 50% of the cumulative value in the volume-based particle size distribution (50% volume average particle size: D50). By setting the average particle size to 60 μm or less, the particle size does not become too coarse. As a result, for example, filling defects are unlikely to occur even in thin parts of a manufacturing die for a core for a turbine blade, and the production efficiency of the core is excellent. In addition, the gaps between the particles are not too large, so the impact resistance of the core itself is also excellent.

The spherical cristobalite has a spherical shape, and the average aspect ratio measured, for example, by an optical microscope is preferably 1 to 3. The average aspect ratio is more preferably 1 to 2.5, and even more preferably 1 to 2. When the average aspect ratio is 1 to 3, the resistance to collision between particles is further reduced, aggregation is less likely to occur when the ceramic composition is made into a clay or slurry, and the ceramic composition has even better fluidity.

The shape of the spherical cristobalite can be defined by other indices instead of or in combination with the average aspect ratio described above. For example, the shape of the spherical cristobalite is preferably 0.6 to 1.0 in terms of average circularity measured by an optical microscope. The average circularity is more preferably 0.7 to 1.0, even more preferably 0.8 to 1.0, and particularly preferably 0.9 to 1.0. When the average circularity is 0.6 to 1.0, the collision resistance between particles is further reduced, aggregation is less likely to occur when the ceramic composition is made into a clay or slurry, and the ceramic composition has even better fluidity.

Cristobalite can be used in combination of spherical and non-spherical shapes. Commercially available spherical cristobalite can be used, or cristobalite of various particle sizes obtained by the method disclosed in JP-A-11-302506 can be used.

From the viewpoint of reducing shrinkage during firing, it is preferable that the amorphous silica is non-spherical. From the viewpoint of packing properties, a portion of the amorphous silica may be spherical amorphous silica. The average particle size of the amorphous silica is preferably 5 μm to 60 μm, more preferably 5 μm to 40 μm, and even more preferably 5 μm to 20 μm.

The ceramic composition may contain heat resistance adjusting particles, for example, for adjusting heat resistance when pouring molten metal into the composition. The heat resistance adjusting particles may be, for example, oxide ceramics such as alumina (Al ₂ O ₃ ), zirconia (ZrO ₂ ), magnesia (MgO), titania (TiO ₂ ), ceria (CeO ₂ ), yttria (Y ₂ O ₃ ), hafnia (HfO ₂ ), barium titanate (BaTiO ₃ ), manganese dioxide (MnO ₂ ), lime (CaO), zinc oxide (ZnO), red iron oxide (Fe ₂ O ₃ ), zircon (ZrSiO ₄ ), and mullite (Al ₆ O ₁₃ Si ₂ ), or non-oxide ceramics such as silicon nitride (Si ₃ N ₄ ), boron nitride (BN), aluminum nitride (AlN), silicon carbide (SiC), and boron carbonitride. Alternatively, they may be composite materials containing at least one of these ceramics. These ceramics can be used alone or in combination of two or more depending on the required properties of the fired body. In particular, from the viewpoint of flame retardancy, it is preferable to use alumina, zircon, hafnia, yttria, zirconia, etc. Note that the chemical formulas shown in parentheses after the names of the above substances indicate the representative compositions of the substances, and are not intended to limit the compositions of actual ceramics to those of such chemical formulas.

The binder is composed of at least one of an organic binder and an inorganic binder. The binder may contain only one of an organic binder and an inorganic binder, or may contain both. In addition, the binder can be freely selected regardless of the manufacturing method of the fired ceramic body.

When a ceramic composition is injection molded to produce a fired ceramic body, it is preferable to use an organic binder as the binder in order to obtain a viscosity suitable for injection molding. Examples of organic binders include organic binders that are solid in the room temperature range (e.g., below 45°C, typically 25±5°C), melt (soften) when heated to a predetermined temperature below 45°C, and exhibit high fluidity in a molten state at 45°C or higher. Examples of such organic binders include petroleum-based waxes such as paraffin wax, and mixtures of such waxes with water-insoluble polyoxyethylene-type nonionic surfactants.

When producing a fired ceramic body by casting a ceramic composition, it is preferable to use an inorganic binder as the binder in order to obtain a viscosity suitable for casting. Examples of inorganic binders include hydrolyzates of silicate, aluminosilicate, titanosilicate, etc. Specific examples of silicate that are preferable are hydrolyzates of methyl silicate, ethyl silicate, propyl silicate, butyl silicate, etc.

In addition to the amorphous silica, cristobalite, binder, and heat resistance adjusting particles, any additives can be added to the ceramic composition. Examples include coupling agents, lubricants, plasticizers, release agents, and carbon. The coupling agents are added to improve the fluidity of the clay or slurry. They may also be included as surface treatment agents to improve the dispersibility of each raw material particle. As the coupling agent, it is preferable to use a polymeric compound that has good bonding properties with the binder. Such polymeric compounds can improve the fluidity of the clay or slurry by binding with the binder.

The ceramic composition described above can be molded by various methods and then fired to produce a fired ceramic body. The ceramic composition can be molded by, for example, injection molding or casting.

First, an example of a method for manufacturing a fired ceramic body by injection molding will be described. For example, ceramic powder containing amorphous silica powder and cristobalite powder, an organic binder, and a dispersing agent are mixed to produce a clay. The ceramic powder can account for 60 to 80% by volume of the entire clay. Thermoplastic resins such as wax can be used as the organic binder. The clay is heated to 60°C to 100°C to give it fluidity, and then injected into a cooled mold, where the clay is solidified and the molded body is removed from the mold. The molded body is fired at 1000°C to 1350°C to obtain a fired ceramic body.

Next, an example of a method for producing a fired ceramic body by casting will be described. For example, a ceramic powder containing amorphous silica powder and cristobalite powder, a hydrolyzed liquid of ethyl silicate, which is an inorganic binder, and amines as a gelation promoter are mixed to produce a ceramic slurry. The slurry is poured into a metal mold and solidified by gelation of the binder. The molded body is then removed from the metal mold and fired at 850°C to 1100°C to obtain a fired ceramic body.

The fired ceramic body of the present invention contains amorphous silica and cristobalite, and at least a portion of the cristobalite is spherical. In the fired ceramic body, the content ratio of the spherical cristobalite crystal phase to the entire silica component is, for example, 0.1% by mass to 35% by mass. As shown in the examples described later, the shape of the spherical cristobalite crystal phase used as the raw material is maintained in the fired ceramic body. Therefore, it is considered that the content ratio of the spherical cristobalite crystal phase to the entire silica component in the ceramic composition is also maintained in the fired ceramic body. By containing a predetermined amount of the spherical cristobalite crystal phase, shrinkage and creep during casting of heat-resistant structural members are unlikely to occur, and the shape stability at high temperatures is excellent. In addition, since the fired ceramic body of the present invention is produced by firing the above-mentioned ceramic composition, it has excellent filling properties into a mold, and therefore ceramic cores with a precise structure can be easily manufactured.

In addition, when a molded body obtained by injection molding or casting is fired under high temperature conditions as described above, the amorphous phase in some of the amorphous silica is converted to a cristobalite crystalline phase due to the influence of trace metal elements and crystallization promoters in the composition. In the fired ceramic body, the content ratio of the entire cristobalite crystalline phase (including spherical and non-spherical) in the silica component is preferably 40 mass% or less. If the content ratio of the entire cristobalite crystalline phase is greater than 40 mass%, thermal expansion becomes large, and cooling cracks are likely to occur, which may impair the function of the fired ceramic body. Examples of elements that promote crystallization from the amorphous phase to the cristobalite crystalline phase include sodium, potassium, and magnesium.

In this specification, "cristobalite" as used in the ceramic composition refers to crystalline silica with a cristobalite crystallization rate of 80% or more, so when spherical cristobalite is contained at 35% by mass relative to the entire silica component, the ceramic composition has a cristobalite crystal phase that is substantially 28% to 35% by mass relative to the entire silica component. Therefore, the silica that is converted from amorphous silica to the cristobalite crystal phase after firing is preferably 5% to 12% by mass relative to the entire silica component.

To evaluate the effect of the blending ratio of multiple silicas with different shapes and crystal forms on the filling properties of the ceramic composition and the shape stability of the fired ceramic body produced from the composition at high temperatures, the seven types of samples shown in Table 1 were created and the following tests were performed.

Two types of amorphous silica were prepared: non-spherical amorphous silica powder and spherical amorphous silica powder. Two types of cristobalite were prepared: non-spherical cristobalite powder and spherical cristobalite powder.

The four types of silica powder used in the test are shown below.
<Non-spherical amorphous silica powder>
Shape: Non-spherical Average particle size: 20μm
<Spherical amorphous silica powder>
Shape: Spherical Average aspect ratio: 1.1
Average particle size: 6 μm
<Non-spherical cristobalite powder>
Shape: Non-spherical Average particle size: 10μm
Cristobalite crystallization rate: 90% or more <spherical cristobalite powder>
Shape: Spherical Average aspect ratio: 1.1
Average circularity: 0.95
Average particle size: 5.6μm
Cristobalite crystallization rate: 90% or more

The average aspect ratio and average circularity were measured using a Sysmex flow particle image analyzer (FPIA-3000) equipped with an optical microscope. The average particle size was measured using a laser diffraction/scattering particle size distribution analyzer (Microtrack MT3000, a particle size distribution analyzer manufactured by Nikkiso Co., Ltd.). The cristobalite crystallization rate was calculated as the ratio of the cristobalite crystalline phase among the polymorphs of SiO ₂ (quartz, cristobalite, amorphous silica, etc.) by analyzing the area of the diffraction peak appearing near 2θ = 22 ° using a calibration curve method using an X-ray diffractometer manufactured by RIGAKU Corporation.

Examples 1 to 3, Comparative Examples 1 to 3: Injection Molding Using the above powder, the ceramic compositions for injection molding of Examples 1 to 3 and Comparative Examples 1 to 3 were prepared. The ceramic compositions for injection molding were prepared by mixing amorphous silica and cristobalite in the compounding ratio shown in Table 1, and the organic binder WAX#125 was 30 volume % and the ceramic powder containing the amorphous silica, the cristobalite, and alumina was 70 volume %. The obtained ceramic composition (clay) was heated to 70°C to give it fluidity, and injected into a mold cooled to 20°C. The solidified clay (molded body) was removed from the mold, and the molded body was fired at a temperature of 1000°C to 1350°C to obtain six test pieces of 100 mm x 20 mm x 3 mm.

In the above test, non-spherical cristobalite powder and spherical cristobalite powder with a cristobalite crystallization rate of 90% or more were used. In this case, for example, in the case of the ceramic composition of Example 1, the non-spherical cristobalite has substantially 4.05% to 4.5% by mass of cristobalite crystal phase, and the spherical cristobalite has substantially 0.45% to 0.5% by mass of cristobalite crystal phase.

Example 4: Casting The above powder was used to prepare a ceramic composition for casting in Example 4. The ceramic composition for casting was prepared by mixing amorphous silica and cristobalite in the mixing ratio shown in Table 1, and further mixing alumina, a hydrolyzed liquid of ethyl silicate as an inorganic binder, and amines as a gelation promoter. The obtained ceramic composition (slurry) was poured into a mold and solidified by a gelation reaction caused by hydrolysis and dehydration condensation of ethyl silicate. The solidified molded body was then removed from the mold and fired at a temperature of 850°C to 1100°C to obtain one test piece of 100 mm x 20 mm x 3 mm.

As an alternative evaluation of the packing property of the clays (Examples 1 to 3 and Comparative Examples 1 to 3) and slurries (Example 4) prepared by the above-mentioned procedures, the viscosity (fluidity) was measured at 10 Pa using a rotational vibration type rheometer (HAAKEMARSIII). The measurement conditions are shown below. The results are shown in Table 1.
Plate: φ35mm, parallel Temperature: 70℃
Gap: 0.5 mm

To evaluate the shape stability of the sintered molded body at high temperatures, the amount of deformation was evaluated for the seven types of test pieces mentioned above when they were heat-treated at 1530°C for two hours. At this time, the test pieces were also evaluated for cracking due to cooling. The results are shown in Table 1.

The flexural strength (three-point bending strength) of the seven types of test pieces described above was also measured in accordance with JIS R 1601.

In addition, using the seven types of test pieces mentioned above, the porosity was calculated from the amount of pores measured based on the Archimedes method or mercury intrusion method.

The cristobalite crystallization rate of the fired ceramic composition (test piece) was measured in the same manner as for the ceramic composition.

In Examples 1 to 4, which contain a predetermined amount of spherical cristobalite, the viscosity of both the clay and the slurry was lower (0.5 Pa·s to 8 Pa·s) than in Comparative Example 1, which does not contain spherical silica components (viscosity 12 Pa·s). As a result, it can be said that the ceramic compositions of Examples 1 to 4 have excellent fluidity and good filling properties into a mold. On the other hand, Comparative Example 2, which uses a ceramic composition to which no cristobalite is added, has a low viscosity because it contains a predetermined amount of spherical amorphous silica, but the creep characteristics of the test piece were reduced. In addition, in Comparative Example 3, which uses a ceramic composition containing 40 mass% of spherical cristobalite relative to the entire silica component, cooling cracks occurred in the test piece. In contrast, Examples 1 to 4, which use a ceramic composition containing 0.5 mass% to 35 mass% of spherical cristobalite relative to the entire silica component, have excellent creep characteristics of the test piece and no cooling cracks occurred.

The test pieces of Examples 1 to 4 had a flexural strength of 10 MPa to 20 MPa and a porosity of 25% to 35% (not shown in Table 1). These results show that the test pieces have good properties that can be used in products.

As mentioned above, after firing, some of the amorphous silica is converted to the cristobalite crystal phase. In fact, in the case of Comparative Example 2, in which cristobalite was not added, the total content of the cristobalite crystal phase in the test piece was about 3 mass% relative to the total silica component. In addition, the total content of the cristobalite crystal phase in the test pieces of Examples 1 to 4 was 40 mass% or less relative to the total silica component. Therefore, it is believed that the ceramic cores obtained by firing the ceramic compositions of Examples 1 to 4 have small thermal expansion, are less likely to crack when cooled, have good high-temperature creep properties, and have properties that can withstand casting.

Figures 1 and 2 show electron microscope photographs of the test pieces of Example 2 (Figure 1) and Comparative Example 1 (Figure 2), respectively. A spherical crystal phase (spherical cristobalite 1) can be seen in Figure 1, but no spherical crystal phase can be seen in Figure 2. From these results, it can be seen that the spherical cristobalite mixed as a raw material maintains its spherical shape even after firing, and that no change in shape occurs due to sintering.

Although an example of a ceramic composition and a fired ceramic body has been described above, the ceramic composition and fired ceramic body of the present invention are not limited to the above-mentioned configuration.

The ceramic composition of the present invention has excellent filling properties into molds, so it can be used to manufacture ceramic sintered bodies such as ceramic cores with precise structures. In addition, the ceramic sintered body of the present invention is less susceptible to shrinkage and creep during casting of heat-resistant structural components and has excellent shape stability at high temperatures, so it can be suitably used to manufacture heat-resistant structural components with complex structures.

1. Spherical cristobalite

Claims (5)

A ceramic composition comprising amorphous silica, cristobalite, and a binder,
The binder is at least one of an organic binder and an inorganic binder,
The amorphous silica is non-spherical amorphous silica,
The cristobalite is entirely or at least partially spherical cristobalite, and the spherical cristobalite has an average circularity of 0.8 to 1.0 as measured by an optical microscope;
The ceramic composition is characterized in that the spherical cristobalite is contained in an amount of 0.1 mass % to 35 mass % based on the total silica component. 2. The ceramic composition according to claim 1, characterized in that the ceramic composition contains heat resistance adjusting particles, and the heat resistance adjusting particles are at least one selected from the group consisting of alumina, zircon, hafnia, yttria, and zirconia. 3. The ceramic composition according to claim 1, wherein the particle size of the spherical cristobalite is 0.5 μm to 150 μm. A fired ceramic body, obtained by firing the ceramic composition according to any one of claims 1 to 3. 5. The fired ceramic body according to claim 4 , which is a ceramic core for casting.