JP7592385B2 - Silicon carbide-containing articles and methods of manufacture - Google Patents

Description

This invention relates to articles made of silicon carbide-based materials that have material characteristics such as high heat resistance, high thermal conductivity, light weight and high rigidity, in particular articles manufactured by powder bed fusion, an additive manufacturing method, and to a method for manufacturing the same.

In order to produce a wide variety of metal parts in small quantities or with complex shapes, development of a 3D modeling technology using powder bed fusion is underway. This technology scans a layer of powdered modeling material with an energy beam based on slice data generated from the 3D shape data of the object to be modeled, and forms a three-dimensional object by repeating the process of locally melting and solidifying the modeling material for multiple layers. A laser beam, electron beam, etc. are used as the energy beam.

In recent years, the use of such three-dimensional modeling technology to model ceramic materials such as silicon carbide, which are difficult to process, has been studied. However, many ceramics such as carbides, borides, and nitrides have technical issues, such as the fact that when energy is suddenly applied to them, they do not melt but sublimate, or they become brittle without crystallizing when melted and solidified. Silicon carbide, which is excellent in lightness, wear resistance, thermal shock resistance, and chemical stability and is expected to be used in a wide range of fields, is a material that does not have a melting point at normal pressure and sublimates around 2545°C (the temperature value is variously believed to be 2700°C, etc.). Patent Document 1 discloses powder candidates that can be modeled using transient liquid phase sintering such as eutectic and peritectic. Examples of powder candidates for producing a modeled object made of silicon carbide include a mixture of silicon carbide, aluminum oxide, rare earth oxide, and silica, a mixture of silicon carbide, aluminum nitride, and rare earth oxide, and a mixture of silicon carbide and metallic germanium.

Special Publication No. 2016-527161

The powder described in Patent Document 1 requires silica, aluminum nitride, or metallic germanium as the essential material to be mixed with silicon carbide to form a eutectic. However, silica decomposes into silicon monoxide and oxygen at 1900°C. Aluminum nitride sublimes at 2200°C. Metallic germanium also boils at temperatures below 2400°C, so even if it is heated simultaneously with silicon carbide, which has a sublimation point of 2545°C, it is highly likely to volatilize before the silicon carbide melts. In addition, although there is no disclosure regarding the strength of the molded object, it is assumed that a molded object with low strength will be created in which the powder is partially bonded.

The object of the present disclosure, which has been made in consideration of the above problems, is to provide a molded object made primarily of silicon carbide that has sufficient mechanical strength while being manufactured using three-dimensional modeling technology. It is also to provide a method for manufacturing such a molded object.

The article according to the present disclosure is characterized in that it contains silicon carbide, a metal boride having a melting point lower than the sublimation point of silicon carbide, and silicon metal.

The method for manufacturing an article according to the present disclosure is characterized by comprising the steps of forming a powder layer using a powder mixture of a powder containing silicon carbide and a powder containing a metal boride having a melting point lower than the sublimation point of silicon carbide, and melting and solidifying the powder by scanning and irradiating the formed powder layer with an energy beam based on shape data of an object to be molded, thereby forming a molded object by repeating the steps, and further impregnating the formed molded object with metal silicon.

The above-mentioned features of the present invention make it possible to provide a shaped object made primarily of silicon carbide that has sufficient mechanical strength while being manufactured using three-dimensional shaping technology.

FIG. 2 is a schematic diagram of a three-dimensional modeling apparatus used in the manufacturing method of the present disclosure. FIG. 13 is a diagram showing the state when a shaped object is impregnated with metal silicon. FIG. 2 is a schematic diagram of a molded object created in an embodiment of the present disclosure. 1 is a SEM image of a cross section of Sample 1 produced in accordance with the present disclosure. FIG. 5 is a diagram showing a three-dimensional structure obtained by stitching together only the dark areas in the cross-sectional SEM images of FIG. 4. FIG. 5 is a diagram showing a three-dimensional structure obtained by stitching together only the light-colored regions in the cross-sectional SEM image of FIG. 4. 1 is an optical microscope image of a cross section of Sample 1 produced in accordance with the present disclosure. 1 is a SEM image of a cross section of Sample 6 produced in accordance with the present disclosure. 1 is an optical microscope image of a cross section of Sample 6 produced in accordance with the present disclosure.

The following describes an embodiment of the present disclosure with reference to the attached drawings.

First, a modeling apparatus applicable to the manufacturing method of the present disclosure will be described with reference to FIG. 1. The modeling apparatus has a chamber 101 capable of controlling the internal atmosphere by a gas introduction mechanism 113 and an exhaust mechanism 114. Inside the chamber 101, there is a modeling container 120 for modeling a three-dimensional object, and a powder layer forming mechanism 106 for spreading a powder of modeling material (hereinafter, sometimes simply referred to as "modeling material" or "powder") in the modeling container 120 to form a powder layer 111.

The exhaust mechanism 114 may be equipped with a pressure adjustment mechanism such as a butterfly valve to adjust the pressure, or may be configured to adjust the atmosphere in the chamber by supplying gas and the associated pressure increase (commonly referred to as blow replacement).

The bottom of the modeling container 120 is composed of a modeling stage 107 whose position in the vertical direction can be changed by a lifting mechanism 108. The direction and amount of movement of the lifting mechanism 108 are controlled by a control unit (not shown), and the amount of movement of the modeling stage 107 is determined according to the thickness of the powder layer 111 to be formed. A structure (not shown) for installing a base plate 109 is provided on the modeling surface side of the modeling stage 107. The base plate 109 is a plate made of a meltable material such as stainless steel, and its surface is melted together with the modeling material when the first powder layer is melted and solidified, making it possible to fix the model to the base plate. Therefore, the position of the model on the base plate 109 can be maintained so that it does not shift during modeling. After modeling is completed, the base plate 109 is mechanically separated from the model.

The powder layer forming mechanism 106 has a powder storage section that stores powder material, and a supply mechanism that supplies the powder material to the modeling container 120. In addition, it may have either a squeegee or a roller, or both, for leveling the powder layer on the base plate 109 to a set thickness.

The molding apparatus further includes an energy beam source 102 for melting the molding material, scanning mirrors 103A and 103B for scanning the energy beam 112 in two axes, and an optical system 104 for focusing the energy beam on the irradiation portion. Since the energy beam 112 is irradiated from outside the chamber 101, the chamber 101 is provided with an introduction window 105 for introducing the energy beam 112 into the chamber. The power density and scanning position of the energy beam are controlled by a control unit (not shown) in accordance with the three-dimensional shape data of the object to be molded and the characteristics of the molding material acquired by the control unit. In addition, the positions of the molding container 120 and the optical system 104 are adjusted in advance so that the beam is focused near the surface of the powder layer 111 and has an appropriate beam diameter. The beam diameter on the surface affects the molding accuracy, so it is preferably 30 to 100 μm.

Next, the manufacturing method of the present disclosure will be described. First, the base plate 109 is placed on the stage 107, and the inside of the chamber 101 is replaced with an inert gas such as nitrogen or argon introduced via the gas introduction mechanism 113. When the replacement is completed, a powder layer 111 is formed on the base plate 109 by the powder layer forming mechanism 106. The powder layer 111 is formed with a thickness according to the slice pitch of the slice data generated from the three-dimensional shape data of the object to be molded, i.e., the layering pitch.

The powder used for molding in this disclosure is mainly composed of silicon carbide powder, and is a mixed powder with metal boride powder having a melting point lower than the sublimation point of silicon carbide. Powders made of compounds other than those mentioned above may be included as long as they do not significantly impair the properties of silicon carbide. The size of the silicon carbide and metal boride powder particles is preferably 3 μm to 100 μm in particle diameter, and more preferably 5 μm to 50 μm in particle diameter, because if the powder particles are too small, they will aggregate and a powder layer of uniform thickness cannot be formed, and if they are too large, high energy is required to melt them, making molding difficult. In addition, the thickness of each powder layer affects the molding accuracy, so a thickness of about 30 to 100 μm is preferable.

Here, the method for measuring the particle size of powder in this disclosure will be explained. The particle sizes contained in the powder have a distribution within a certain range, with the median and maximum particle sizes specified. SiC is measured according to the particle size evaluation method already standardized in the industry, using an electrical resistance method in accordance with JIS R 6001-2 "Particle size of abrasives for grinding wheels." Particle sizes of chromium monoboride, chromium diboride, and other particles other than SiC are measured according to JIS Z 8832 "Method for measuring particle size distribution - Electrical detection zone method."

Next, the energy beam 112 is scanned according to the slice data, and the powder in a predetermined region of the powder layer 111 is irradiated with the energy beam 112 to melt it. The energy beam source 102 is preferably capable of outputting energy of a wavelength at which the modeling material has a high absorption rate of 50% or more. In particular, to create a state in which the molten metal boride wraps around the silicon carbide during modeling, it is preferable to use an energy beam in a wavelength range in which the metal boride has a high absorption rate. When the modeling material is chromium diboride, a semiconductor fiber laser with a wavelength of 1000 to 1120 nm is suitable.

The energy beam 112 is preferably set to an energy intensity at a level where the powder in the area irradiated with the energy beam melts and solidifies within a few milliseconds, and the particles bond together. When powder layers are stacked, it is necessary for the modeling to melt and solidify to some extent not only the powder layer located at the outermost surface on the side irradiated with the energy beam 112, but also the powder layer directly below the powder layer irradiated with the energy beam 112. If the powder layer directly below is not melted sufficiently, the modeling will tend to peel off layer by layer, resulting in a model with low strength. When the first powder layer laid directly above the base plate 109 melts and solidifies, the irradiation conditions of the energy beam are adjusted taking into account the heat capacity, heat conductivity, etc. of the base plate so that the surface of the base plate 109 is melted at the same time.

Then, the lifting mechanism 108 lowers the modeling stage 107 by the layer pitch, and then a new powder layer is formed by spreading powder on the layer scanned with the energy beam, followed by scanning and irradiation with the energy beam 112. As described above, when the new powder layer is irradiated with the energy beam 112, a part of the layer previously scanned with the energy beam 112 (specifically, the part in contact with the new powder layer) is melted and solidified again. If the area of the new powder layer directly below the area irradiated with the energy beam 112 is already melted and solidified, the beam-irradiated area of the new powder layer mixes with the molten material of the part of the previously melted and solidified area, solidifies, and bonds with each other. By repeating these operations, a model 110 can be formed in which the areas melted and solidified by the energy beam 112 are integrated into one layer.

Since the molded object 110 is bonded to the base plate 109, it is removed from the chamber 101 together with the base plate. The base plate 109 and the molded object 110 are then cut and separated using a wire saw or a disk blade with abrasive grains such as diamond attached, and the molded object 110 can be obtained.

Next, an example of a process for impregnating a molded object with metal silicon is described with reference to Figure 2. Heat-resistant spheres 202 of uniform particle size are placed at the bottom of a crucible 201 made of a material such as graphite that does not volatilize or change quality even at the melting point of metal silicon (1414°C), so that the thickness is not more than two layers, and the molded object 110 is placed on top of the heat-resistant spheres 202. The heat-resistant spheres 202 have the effect of creating gaps so that the crucible 201 and the molded object 110 do not adhere strongly to each other due to the solidified metal silicon that seeps out of the molded object 110 during the impregnation process.

Furthermore, the porosity of the molded object 110 is derived in advance from the shape and mass, and metal silicon powder 203 is placed on the molded object in an amount greater than the amount corresponding to the voids. After that, the crucible 201 is placed in a vacuum heat treatment furnace, the inside of the furnace is replaced with argon, the pressure is appropriately reduced, and the molded object is heated from room temperature to a temperature exceeding 1414°C, which is the melting point of metal silicon, for example, 1500°C. The metal silicon that has exceeded the melting point and become liquid permeates the voids in the molded object. After that, when it is cooled to room temperature, dry air is introduced to make the pressure atmospheric, and the crucible is removed from the vacuum heat treatment furnace. During cooling, the temperature change rate is reduced to prevent distortion and stress that occur due to differences in solidification timing depending on the location at temperatures near the melting point of metal silicon. The heat-resistant spherical objects 202 attached to the surface of the molded object are removed by the solid metal silicon that has seeped out of the molded object 110, and the shape and surface are adjusted by grinding, polishing, etc., to obtain the desired product.

Powder Materials Used in the Present Disclosure
In the present disclosure, a shaping powder is prepared by mixing a powder of silicon carbide with a powder of a metal boride that forms a eutectic or hypoeutectic with silicon carbide and has a melting point lower than the sublimation point of silicon carbide. By using such a shaping powder to produce a shaped object containing a eutectic or hypoeutectic of silicon carbide and a metal boride, a shaped object with strength approaching that of silicon carbide alone is realized.

Here, we will explain eutectic/hypoeutectic.

In a mixture of material X (such as a metal) and material Y, there is a material ratio at which the melting point is lower than the melting point of each material. At that time, the material ratio at which the melting point is the lowest is called the eutectic composition, and the melting point is called the eutectic temperature.

In a eutectic composition, materials X and Y are both in the liquid phase at temperatures above the melting point, and at temperatures below the melting point, materials X and Y precipitate simultaneously. As a result, materials X and Y are composed of fine precipitate phases, forming a strong eutectic with a layered structure known as a lamellar structure.

Next, consider the case where a mixture of materials X and Y contains more material X than the eutectic composition. In this case, the mixture is in liquid phase above the melting point, but when the temperature drops below the melting point, material X solidifies first, and material X precipitates (called primary crystallization) up to the eutectic temperature. When the temperature drops to the eutectic temperature, the liquid phase excluding the precipitated crystals of material X has a eutectic composition, and when the temperature is lowered below the eutectic temperature from that state, material X and material Y precipitate simultaneously. In other words, compared to when starting from a eutectic composition, the crystals of material X start to grow larger due to the earlier precipitation of material X, resulting in a mixed structure. When there is more material Y than the eutectic composition, the crystals of material Y grow larger. These states are called hypoeutectic. Eutectic and hypoeutectic can be confirmed by observing the cross section of the molded object with a scanning electron microscope.

In order to obtain physical properties similar to those of silicon carbide, the inventors investigated conditions such as powder composition and particle size that would enable a eutectic or hypoeutectic state in which silicon carbide crystals are large.

The present invention will be described in more detail below using examples and comparative examples. The present invention is not limited in any way by the following examples, so long as the gist of the invention is not exceeded.

(Powder 1)
As silicon carbide, silicon carbide powder (manufactured by Pacific Random Co., Ltd., product name NC#800) with a median particle size of 14.7 μm was prepared. As chromium boride to be mixed, chromium diboride powder (manufactured by Japan New Metals Co., Ltd., product name CrB2-O, median particle size of about 5 μm) with a melting point of 2200 ° C. was prepared. These powders were mixed in a molar ratio of silicon carbide: chromium diboride = 7: 3 so as to obtain a composition powder in which a eutectic or hypoeutectic is generated, and mixed in a ball mill to obtain powder 1. The molar ratio is determined and mixed in the same manner for other powders. The median particle size here is synonymous with the median size, and means a particle size at which the cumulative frequency in the powder is 50%. The particle size distribution can be measured by a well-known laser diffraction method or scattering method.

(Powder 2)
A silicon carbide powder similar to that of Powder 1 and a vanadium diboride powder having a melting point of 2400° C. (median particle size of approximately 4 μm, manufactured by Japan New Metals Co., Ltd., product name VB2-O) were prepared and mixed in a molar ratio of silicon carbide:vanadium diboride of 1:1 to obtain Powder 2.

(Powder 3)
A silicon carbide powder similar to that of Powder 1 and a chromium monoboride powder having a melting point of 2100°C (manufactured by Japan New Metals Co., Ltd., product name CrB-O, median particle size approximately 9 µm) were mixed in a molar ratio of silicon carbide:chromium monoboride = 3:1 to obtain Powder 3.

(Powder 4)
A silicon carbide powder similar to that of Powder 1 and a titanium diboride powder having a melting point of 2920°C (trade name TiB2-N, manufactured by Japan New Metals Co., Ltd., median particle size of approximately 4 µm) were prepared and mixed in a molar ratio of silicon carbide:titanium diboride = 1:1 to obtain Powder 4.

(Powder 5)
The same silicon carbide powder as Powder 1 and zirconium diboride having a melting point of 3200°C (manufactured by Japan New Metals Co., Ltd., product name ZrB2-O, median particle size of approximately 5 μm) were prepared and mixed in a molar ratio of silicon carbide:zirconium diboride = 1:1 to obtain Powder 5.

The composition of each powder is summarized in Table 1.

[Creating objects using powder materials]
Using the above-mentioned powders 1 to 5 as materials, modeling was performed using the modeling apparatus shown in Fig. 1. Specifically, for each powder, four rectangular parallelepiped objects with a bottom area of 4 mm x 40 mm were produced on a stainless steel base plate 109. A perspective view of the four models 110 and the base plate 109 after modeling was completed is shown in Fig. 3.

A semiconductor fiber laser with a wavelength of 1070 nm was used as the energy beam source 102, and the powder layer was irradiated with a laser power of 100 W and an irradiation pitch of 50 μm. In addition, since the irradiation energy suitable for molding differs depending on the type of powder material, the scanning speed was previously set to an optimum speed between 100 mm/sec and 1000 mm/sec for each material. An attempt was made to mold 300 layers with a powder layer thickness (layer pitch) of 30 μm.

However, in the case of the model using powder 4 containing titanium diboride and the model using powder 5 containing zirconium diboride, the modeled parts began to peel off during the formation of the powder layer, making it impossible to continue the modeling, so the modeling was terminated at that point. In the models using powders 1, 2, and 3, a rectangular parallelepiped with a height of approximately 9 mm was obtained.

Next, a wire saw CS-203 (product name) manufactured by Musashino Electronics Co., Ltd. was used as a cutting device, and the model 110 was separated from the base plate 109 using a φ0.4 mm wire saw with diamond abrasive grains attached.

Here, four samples of each were obtained, with the molded object using powder 1 as the material powder as sample 1 (Comparative Example 1), the molded object using powder 2 as the material powder as sample 2 (Comparative Example 2), and the molded object using powder 3 as the material powder as sample 3 (Comparative Example 3). Note that the molded object using powder 4 as the material powder and the molded object using powder 5 as the material powder were left incomplete as mentioned above, but they were assigned numbers as sample 4 as comparative example 4 and sample 5 as comparative example 5, respectively.

The elements contained in Samples 1 to 5 were identified by energy dispersive X-ray analysis (EDX), and the molecular structure was identified by X-ray diffraction (XRD). Sample 1 contained some oxides that were expected to result from surface oxidation, but ignoring these, it was found to be composed of the raw material powder silicon carbide and chromium diboride. Similarly, Sample 2 was found to be composed of silicon carbide and vanadium diboride.

They also used FIB-SEM to investigate how silicon carbide and metal boride are distributed in the components. FIB-SEM is a system that uses a FIB (focused ion beam) to drill into a sample, while repeatedly observing the exposed sample surface or cross section with a SEM (scanning electron microscope), and then processes the SEM images with a computer to enable three-dimensional observation of the sample structure.

Figure 4 shows an SEM image of a cross section of sample 1. The darkest part 10 is a void, the lightest area 12 is made of chromium diboride, and area 11, which is between areas 10 and 12, is made of silicon carbide. The boundary between silicon carbide area 11 and chromium diboride area 12 has a complex shape, and it could be inferred that at least one of the materials is melted. Furthermore, from the distribution of silicon carbide area 11 and chromium diboride area 12 observed in the SEM image, it was confirmed that a eutectic and hypoeutectic had been formed.

In the group of SEM images obtained for sample 1, the regions were identified by the color density, and the three-dimensional structure in which the regions 11 made of silicon carbide were connected is shown in Figure 5. Also, the three-dimensional structure in which the regions 12 made of chromium diboride were connected is shown in Figure 6.

Figures 5 and 6 show that the silicon carbide and chromium boride in sample 1 each have a three-dimensional mesh structure, with the structures intricately intertwined with each other. The area in which the three-dimensional mesh structure was confirmed using FIB-SEM was 60 μm x 45 μm x 160 μm, which greatly exceeds the median particle size of the raw material silicon carbide and chromium diboride powders, respectively, and therefore it was inferred that these materials melted and connected during the molding process.

Next, the voids contained in the molded object 110 were calculated. Since the thermal conductivity does not vary significantly except for the ends of the molded object, the joints with the plate, and the outermost surface, it was considered that the voids were present on an average basis. Therefore, the porosity was calculated by acquiring an optical microscope image of a portion having an average void in a cross section, excluding the ends of the molded object, the joints with the plate, and the outermost surface, and taking the proportion of the dark colored portion corresponding to the voids in a field of view equivalent to an area of 2.44 mm x 1.63 mm. Figure 7 shows an optical microscope image of an area having an average void in a cross section of sample 1. For each of multiple locations (10 or more locations) that are estimated to have an average void ratio, a threshold value was set for the concentration, and the portion with a concentration higher than the threshold value was determined to be a void by image analysis, and the void ratio was calculated as the area ratio to the total area. When these were averaged, the porosity was calculated to be about 30%.

[Step of incorporating metal silicon into the produced object]
Next, metallic silicon was impregnated into the molded object 110. As shown in Fig. 2, alumina spheres 202 having a diameter of 1 mm were placed on the bottom of a crucible 201 made of graphite so as not to form more than two layers, and one molded object 110 was placed on the alumina spheres.

Furthermore, metal silicon powder 203 (specific gravity 2.33, particle size 45 μm or less) was placed on the model 110 in an amount 1.5 times the volume corresponding to the porosity calculated above.

Then, the crucible 201 was placed in a vacuum heat treatment furnace (not shown), the inside of the furnace was replaced with argon, and the material was heated from room temperature to 1000°C at a temperature increase rate of 300°C/h, and held for 2 hours. After that, the pressure was reduced to an absolute pressure of 1.5 kPa in 40 minutes, and the material was heated to 1200°C at a temperature increase rate of 300°C/h, and then heated to 1500°C at a temperature increase rate of 120°C/h, and held for 2 hours.

The temperature was then lowered at 120°C/h to 1424°C, just above the melting point of silicon metal, and then slowly cooled at 6°C/h to 1400°C.

The mixture was then cooled at 300°C/h, and when it reached below 70°C, dry air was introduced to return the pressure to atmospheric pressure, and the crucible 201 was removed from the vacuum heat treatment furnace.

Using the above method, metallic silicon was added to a total of six samples: two from sample 1, two from sample 2, and two from sample 3. Sample 1 with metallic silicon added was designated sample 6 (Example 1), sample 2 with metallic silicon added was designated sample 7 (Example 2), and sample 3 with metallic silicon added was designated sample 8 (Example 3).

Furthermore, the alumina spheres that had adhered to the surface of the molded object 110 during the process of impregnating the object with metallic silicon were removed, and the shape and surface were adjusted by polishing, yielding an article containing metallic silicon measuring approximately 4 mm x 40 mm x 9 mm.

[Characteristics of the item]
Next, a three-point bending test was performed in accordance with the JIS standard for fine ceramics room temperature bending strength test method (JIS R 1601) for Samples 1, 2, 3, 6, 7, and 8. In addition, the fracture surfaces of the samples were polished, and the porosity was calculated from optical microscope images of the polished cross sections.

Figure 8 shows an SEM image of sample 6. When a composition analysis was performed, it was found that, among the parts occupying a certain area, the darkest area 11 was silicon carbide, the lightest area 12 was chromium diboride, and the area 13 of intermediate concentration was metal silicon. Also, the small black island-like areas 10 between the materials are voids. As with sample 1, the porosity was calculated from an optical microscope image of the cross section of sample 6. Figure 9 shows an optical microscope image of a cross section of sample 6 with an average porosity, which was used to calculate the porosity. A threshold was set for the concentration, and the parts with a concentration higher than the threshold were determined to be voids in image analysis, and the porosity was calculated as the area ratio to the total area. Also, as with sample 1, it was determined from the SEM image whether or not eutectic/hypoeutectic was formed.

The porosity of the other samples was calculated in the same manner as for samples 1 and 6, and it was determined whether or not eutectic or hypoeutectic crystals had formed. The results are shown in Table 2.

As an overall assessment, those that could not be molded into a shape of 4 mm x 40 mm x 9 mm were given a "C". Additionally, those that could be molded but had a porosity of around 30% and did not achieve sufficient bending strength were given a "B". Even these materials can be used for applications such as filters. Those that achieved a bending strength comparable to that of sintered ceramics (100 MPa or more) were given an "A" as they could be used for a variety of purposes.

Next, we will consider why Samples 1, 2, and 3 were able to be printed, and why Samples 4 and 5 could not be printed.

First, let us consider why a molded object was obtained from sample 1, which was molded from a mixed powder of silicon carbide and chromium diboride (melting point 2200°C), which has a melting point lower than the sublimation point of silicon carbide (2545°C). When a laser beam is irradiated onto the mixed powder of silicon carbide and chromium diboride and the temperature is increased, the chromium diboride reaches its melting point and melts first. It is presumed that the surface of the silicon carbide particles is then covered with the molten chromium diboride. Silicon carbide sublimes on its own, but is thought to melt at the interface between the two substances, and the melting of silicon carbide progresses from the interface between the silicon carbide and the molten chromium diboride. Even if the temperature rises to the sublimation point of silicon carbide, it is presumed that the volatilized silicon carbide dissolves into the molten chromium diboride, suppressing volatilization. Therefore, even if the temperature exceeds the sublimation point of silicon carbide due to the laser beam irradiation, the silicon carbide and chromium diboride remain in a molten state. After that, when the laser beam irradiation time ends and the temperature of the irradiated area starts to drop, it is presumed that silicon carbide and chromium diboride start to precipitate, respectively, and the two substances are mixed without any gaps, as shown in Figure 4. The same is thought to be true for sample 2, which uses a mixed powder of silicon carbide and vanadium diboride, and sample 3, which uses a mixed powder of silicon carbide and chromium monoboride.

Next, we consider why the desired object could not be obtained from sample 4, which was made from a powder mixture of silicon carbide and titanium diboride (melting point 2920°C), a metal boride with a melting point higher than the sublimation point of silicon carbide. When the temperature rises by irradiating a laser beam onto a mixture of silicon carbide and titanium diboride, the sublimation point of silicon carbide is reached before the melting point of titanium diboride. Therefore, silicon carbide begins to sublimate first, and then titanium diboride begins to melt. On the surface of the silicon carbide particles, the contact between the molten titanium diboride and the silicon carbide powder is hindered by the sublimation gas, and the contact between them is very limited. And while the titanium diboride is melting, silicon carbide also continues to sublimate, so the contact area between the two substances does not increase. In this way, the melting of silicon carbide is very limited, and it hardly precipitates even when cooled. Therefore, instead of a product with a dense intertwining of eutectic or hypoeutectic crystals like samples 1 to 3, the bond at the boundary between the silicon carbide and titanium diboride was weak, resulting in a brittle product.

Based on the above hypothesis and the experimental results mentioned above, it is believed that when molding is performed using a powder material containing silicon carbide powder and metal boride powder, which has a melting point lower than the sublimation point of silicon carbide, the eutectic or hypoeutectic crystals become intertwined with no gaps, allowing for strong bonding at the boundaries.

Next, we consider samples 1, 2, and 3, as well as samples 6, 7, and 8, which contain metallic silicon. By incorporating metallic silicon into the voids contained in the molded object 110 immediately after molding, the porosity, which was around 30%, was almost completely eliminated (to less than 1%), and it is presumed that most of the voids in the molded object 110 immediately after molding were three-dimensionally connected.

In addition, the bending strength was improved by about 20 to 30 times, which is higher than the bending strength of metal silicon (generally said to be about 200 MPa). Since the three-dimensional structure of metal silicon itself is thought to occupy about 30% of the volume of the object, the improvement in bending strength due to the inclusion of metal silicon is estimated to be about 200 MPa x 30% = 60 MPa. However, in reality, the bending strength improved by 225 MPa from 5 MPa in the as-formed state to 230 MPa. This is presumably because the three-dimensional structures consisting of silicon carbide, chromium diboride (or vanadium diboride, chromium monoboride), and metal silicon are in contact with each other and are three-dimensionally intertwined, realizing a bending strength that is simply unpredictable. It can also be presumed that the metal silicon fills the voids, preventing the occurrence and progression of cracks, which also contributes to the improvement in strength.

[Mixing ratio of silicon carbide powder and metal boride powder]
Next, a powder obtained by mixing silicon carbide powder and chromium diboride powder was used to investigate the mixing ratio of silicon carbide and chromium diboride suitable for a molded object. The same powder as Powder 1 was used for the silicon carbide powder and the chromium diboride powder.

Powders 6 to 11 were prepared by using the silicon carbide and chromium diboride powder as a whole mixed powder of 100% and containing chromium diboride powder at molar ratios of 7.0%, 10%, 30%, 50%, 65%, and 70%, respectively. These powders were used to fabricate articles in the same manner as those using powders 1 to 5, and the resulting samples 9 to 14 were used as comparative examples 6 to 11, respectively.

Powder 6, which has a high ratio of silicon carbide, had 30 layers produced, and when the next powder layer was formed, the previous powder layer, i.e., the top layer, peeled off. Production could continue, but the same phenomenon occurred every time a new powder layer was formed, and it became impossible to continue production. On the other hand, powder 11, which has a high ratio of chromium diboride, had ball-shaped protrusions on the surface during production, and when the powder layer was formed, the roller hit the protrusions, causing the top layer to peel off, making it impossible to continue production. When the ball-shaped protrusions were later analyzed, it was found that they were mainly composed of chromium diboride. This is thought to be because the purity of the molten chromium diboride increased, causing the surface tension of the droplets formed on the surface to increase, causing them to aggregate and become larger in diameter, which then solidified.

In addition, a process of impregnating each of the samples prepared in the same manner as samples 10 to 13 with metallic silicon was carried out to produce samples 15 to 18. The process of impregnating the samples with metallic silicon was carried out without any problems. Samples 15 to 18 were designated as Examples 4 to 7, respectively. In addition, a three-point bending test and porosity calculation were carried out on each of the samples that were successfully fabricated, in the same manner as in the previous examples.

The results are shown in Table 3. The molar ratio column shows the value of (mol % of silicon carbide) / (mol % of chromium diboride).

It was found that powders with a molar ratio of silicon carbide to chromium diboride in the range of silicon carbide:chromium diboride = 90:10 to 35:65, assuming the entire mixed powder to be 100%, are suitable for molding. In other words, it was found that mixed powders with a molar ratio of silicon carbide to chromium diboride in the range of 0.54≦silicon carbide/chromium diboride≦9.00 are suitable for molding. Furthermore, it was confirmed that the bending strength of the product obtained by adding metallic silicon to the molded object was improved more than expected.

In the above examples, the study was conducted on two-component systems such as silicon carbide and chromium diboride, silicon carbide and chromium monoboride, and silicon carbide and vanadium diboride, but it does not depart from the scope of the present invention to appropriately add various boron-containing substances such as titanium boride, lanthanum boride, and boron carbide to the extent that the main characteristics are not changed. In some cases, it may be effective to lower the specific gravity or increase the strength, and they can be used as appropriate.

In addition, in the above examples, powder with a median particle size of 5 μm was used for chromium diboride and powder with a median particle size of 9 μm was used for chromium monoboride, but this is simply because powders that are commercially available were used, and does not limit the use of other particle sizes. However, the metal boride to be mixed is preferably smaller than the particle size of silicon carbide, with a particle size of 10 μm or less, so that it can be easily melted.

In the above-mentioned embodiment, the modeling was performed using a powder bed fusion method with a laser, but this is not limited to this method, and the method can be applied to other three-dimensional modeling methods that undergo a similar thermal history. For example, the method can be applied to powder bed fusion method with an electron beam, and even to directed energy deposition method, in which gas and material powder are ejected simultaneously and melted with a laser.

In the above-mentioned embodiment, in the process of incorporating metal silicon into the molded object, metal silicon powder was placed on the molded object and melted, but metal silicon wafers or metal silicon pellets may be used instead of metal silicon powder, and furthermore, a method known as the MI method may be used in which the molded object is immersed in molten metal silicon and then pulled out.

It is now possible to mold silicon carbide, which was previously difficult to do using three-dimensional modeling methods. For example, by using eutectic molded objects of silicon carbide and metal boride, it is possible to use them in heat exchangers, engine nozzles, stages, and other applications where the advantages of high heat resistance, high thermal conductivity, and high physical strength are all advantages.

REFERENCE SIGNS LIST 101 Chamber 102 Energy beam source 103A, 103B Scanning mirror 104 Optical system 105 Introduction window 106 Powder layer forming mechanism 107 Modeling stage 108 Lifting mechanism 109 Base plate 110 Modeled object 111 Powder layer 112 Energy beam 113 Gas introduction mechanism 120 Modeling container 201 Crucible 202 Heat-resistant sphere 203 Metal silicon powder

Claims (24)

An article manufactured using three-dimensional modeling technology,
a portion including silicon carbide and a metal boride;
the portion includes a region of the metal boride in contact with the region of silicon carbide, and a region of silicon metal disposed between the region of silicon carbide and the region of the metal boride ;
The metal boride has a melting point below 2545°C . An article manufactured using three-dimensional modeling technology,
a portion including silicon carbide and a metal boride;
the portion includes a region of the metal boride in contact with the region of silicon carbide, and a region of silicon metal disposed between the region of silicon carbide and the region of the metal boride;
The article is characterized in that the metal boride is at least one selected from the group consisting of chromium diboride, vanadium diboride, and chromium monoboride. The article according to claim 2, characterized in that the metal boride is chromium diboride, and the molar ratio of the metal boride to silicon carbide is in the range of silicon carbide:chromium diboride = 90:10 to 35:65. The article according to claim 1 or 2, characterized in that the metal boride has a three-dimensional network structure. An article according to any one of claims 1 to 4, characterized in that the porosity is 1% or less. forming a powder layer using a powder including particles including silicon carbide and particles including a metal boride having a melting point lower than 2545°C;
irradiating the powder layer with an energy beam based on shape data of an object to be molded, thereby melting and solidifying the powder;
a first object is formed by repeating the steps of
A method for manufacturing an article, comprising forming a second object by adding metallic silicon to the first object. forming a powder layer using a powder containing particles including silicon carbide and particles including a metal boride selected from the group consisting of chromium diboride, vanadium diboride, and chromium monoboride;
irradiating the powder layer with an energy beam based on shape data of an object to be molded, thereby melting and solidifying the powder;
a first object is formed by repeating the steps of
A method for manufacturing an article, comprising forming a second object by adding metallic silicon to the first object. The method for manufacturing an article according to claim 6 or 7, characterized in that the metal boride is chromium diboride, and the molar ratio of the metal boride to the silicon carbide is in the range of silicon carbide:chromium diboride = 90:10 to 35:65. The method for manufacturing an article according to any one of claims 6 to 8, characterized in that in the step of melting and solidifying the powder , the metal boride is melted and solidified. The method for manufacturing an article according to any one of claims 6 to 9, wherein in the step of melting and solidifying the powder, melting and solidifying the silicon carbide is performed . The method for manufacturing an article according to any one of claims 6 to 10, wherein the first object has a void . The method for manufacturing an article according to claim 11, further comprising the step of: incorporating the metallurgical silicon in the voids of the first shaped body . The method for manufacturing an article according to any one of claims 6 to 12, wherein a particle size of the particles containing the metal boride is smaller than a particle size of the particles containing the silicon carbide. The method for manufacturing an article according to any one of claims 6 to 13, characterized in that the particles containing silicon carbide have a particle diameter of 3 µm to 100 µm. The method for manufacturing an article according to any one of claims 6 to 14, characterized in that the particles containing the metal boride have a particle diameter of 3 µm to 100 µm. The method for manufacturing an article according to any one of claims 6 to 15, characterized in that the porosity of the first object is greater than 1%. The method for manufacturing an article according to any one of claims 6 to 16, characterized in that the porosity of the second object is 1% or less. The method for manufacturing an article according to claim 11 or 12 , wherein the voids are three-dimensionally connected. 19. The method for manufacturing an article according to any one of claims 11, 12 or 18 , wherein the first shaped object has a region of silicon carbide and a region of metal boride, and the void is present between the region of silicon carbide and the region of metal boride. 20. The method for manufacturing an article according to claim 6, wherein the first shaped object has a structure in which a silicon carbide region and a metal boride region are in contact with each other . The method for manufacturing an article according to any one of claims 6 to 20, characterized in that the first object has a bending strength of less than 100 MPa, and the second object has a bending strength of 100 MPa or more. An article according to any one of claims 1 to 5, characterized in that it has a bending strength of 100 MPa or more. An article according to any one of claims 1 to 5, characterized in that the portion has a lamellar structure. The article according to any one of claims 1 to 5, characterized in that the portion has a structure in which the silicon carbide region, the metal boride region, and the metal silicon region are in contact with each other and three-dimensionally intertwined.

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