Silicon-Carbide-Sintered Body having Oxidation-Resistant Layer and Method of Manufacturing the Same

Abstract

Provided is a silicon-carbide-sintered body in which plural crystal grains including silicon carbide are densely formed so as to be adjacent to each other. Sc and Y elements are present in a rich phase at a triple point at which interfaces of the crystal grains forming the sintered body meet each other without solid-solution of the elements in the crystal grains. Accordingly, sintering is feasible at a temperature of 1950 C. or lower, and an EB layer including a rare-earth-Si oxide containing the Sc and Y elements is formed on a surface thereof without an EB coating process, and is also formed up to the inner region of a silicon carbide base, resulting in strong three-dimensional bonding, so that the possibility of peeling of the EB layer is reduced and a new EB layer is formed even when peeling occurs, increasing the resistance to corrosion of the silicon carbide material.

Claims

1. A silicon-carbide-sintered body comprising: a secondary-phase oxidation protective layer formed on a surface thereof when the sintered body is exposed to an oxidation atmosphere.

2. The silicon-carbide-sintered body of claim 1, wherein the secondary-phase oxidation protective layer includes a rare-earth-Si oxide.

3. The silicon-carbide-sintered body of claim 2, wherein a secondary phase is bonded to a base phase region from a surface of the sintered body to a predetermined depth in the base phase region in the sintered body.

4. The silicon-carbide-sintered body of claim 1, wherein cations of a rare earth are present in a rich phase at a triple point at which interfaces of crystal grains forming the sintered body meet each other, so that the cations of the rare earth and Si form a rare-earth-Si oxide even when the oxidation protective layer is peeled, thereby re-forming the oxidation protective layer.

5. The silicon-carbide-sintered body of claim 4, wherein the rare earth is Sc and Y.

6. The silicon-carbide-sintered body of claim 5, wherein the rare earth forms an oxidation protective layer in a form of (Sc,Y).sub.2SiO.sub.7 with cations of Sc.sub.2O.sub.3 and Y.sub.2O.sub.3.

7. The silicon-carbide-sintered body of claim 6, wherein a molar ratio of Sc.sub.2O.sub.3Y.sub.2O.sub.3 is 9:1 to 1:9.

8. The silicon-carbide-sintered body of claim 6, wherein a molar ratio of Sc.sub.2O.sub.3Y.sub.2O.sub.3 is 0.5:1 to 3.0:1.

9. The silicon-carbide-sintered body of claim 1, wherein a relative density of an SSY is 96.3% when a theoretical density of the SSY is 3.268 g/cm.sup.3.

10. A method of manufacturing a silicon-carbide-sintered body, the method comprising: mixing silicon carbide and a sintering additive containing Sc.sub.2O.sub.3Y.sub.2O.sub.3 in a solvent to form a slurry; drying the mixed slurry; sieving the dried slurry into a powder; and sintering the dried powder by pressurizing the dried powder.

11. The method of claim 10, wherein the sintering is performed in a non-oxidation atmosphere at a temperature of 1800 to 1950 C. for 0.5 to 10 hours while pressurizing the dried powder at a pressure of 10 to 50 MPa.

12. The method of claim 10, wherein the sintering further includes adding carbon in a state in which heating to 1400 C. to 1500 C. is performed without applying pressure and is maintained for a predetermined period of time before the dried powder is pressurized, so that an Si oxide on a surface of the silicon carbide is reduced to SiC to thus remove oxygen, whereby the finished sintered body and Sc.sub.2O.sub.3Y.sub.2O.sub.3 form an oxidation coat layer.

13. The method of claim 12, wherein an amount of the carbon that is added is 0.1 to 0.5 wt % based on a total amount of the powder.

14. The method of claim 10, wherein the silicon carbide includes an phase and a phase.

Description

DESCRIPTION OF DRAWINGS

[0027] FIG. 1 is a photograph showing the appearance of a specimen of a Comparative Example for comparison with an Example of the present invention, after high-temperature steam oxidation;

[0028] FIG. 2 is a photograph showing the appearance of the surface of a specimen according to the Example of the present invention, after high-temperature steam oxidation;

[0029] FIG. 3 is an XRD phase analysis graph of the surface of the specimen of FIG. 2;

[0030] FIG. 4 is an EDS analysis image of the surface of the specimen of FIG. 2;

[0031] FIG. 5 is a cross-sectional scanning electron microscope image of the EB layer formed on the surface of the specimen of FIG. 2;

[0032] FIG. 6 is a scanning electron microscope image and an EDS analysis image showing cross-sections of the EB layer followed on the surface of the specimen of FIG. 2 and a silicon carbide base phase beneath the EB layer; and

[0033] FIG. 7 is a scanning electron microscope image of the interface of the EB layer and the silicon carbide and the inner core of the silicon carbide in the specimen according to the Example of the present invention.

DETAILED DESCRIPTION

[0034] It is to be understood that the specific structure or functional description presented in the embodiments of the present invention is illustrated for the purpose of describing the embodiments according to the concept of the present invention only, and the embodiments according to the concept of the present invention may be embodied in various forms. It should also be understood that the present invention should not be construed as being limited to the embodiments described herein, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention.

[0035] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

[0036] A silicon carbide material according to an embodiment of the present invention is manufactured by using silicon carbide grains and adding Sc.sub.2O.sub.3 and Y.sub.2O.sub.3 thereto as sintering additives.

[0037] The silicon carbide is preferably and phases.

[0038] The silicon carbide material preferably includes 85 to 98 vol % of the silicon carbide and 2 to 15 vol % of Sc.sub.2O.sub.3Y.sub.2O.sub.3.

[0039] When the content of the silicon carbide is less than 85 vol %, there are disadvantages in that a liquid phase is not sufficiently formed due to the limited content of Si oxide present on the surface of the silicon carbide, and that since the viscosity of the liquid phase is increased, the diffusion rate of the silicon carbide is significantly reduced due to the liquid phase, thus reducing the sintering density to less than 96%. Further, when the content is more than 98 vol %, the liquid phase becomes insufficient, thus reducing the sintering density to less than 96%, which is undesirable.

[0040] Further, Sc.sub.2O.sub.3Y.sub.2O.sub.3 of the present invention is characterized by being present at a molar ratio of 9:1 to 1:9. Even when the molar ratio deviates from the above-described range, similar EB layers, such as Sc.sub.2Si.sub.2O.sub.7 and Y.sub.2Si.sub.2O.sub.5, are formed, in addition to (Sc,Y).sub.2SiO.sub.7, which enables a silicon carbide material coexisting with (Sc,Y).sub.2SiO.sub.7 to be manufactured. However, the Y.sub.2Si.sub.2O.sub.5 phase has a volatility possibility higher than that of the (Sc,Y).sub.2SiO.sub.7 phase. In the case of the Sc.sub.2Si.sub.2O.sub.7 phase, stability is excellent in an oxidation atmosphere at high temperatures, but when only an Sc.sub.2O.sub.3 additive is added, it is difficult to form a liquid phase due to the absence of a Y element during sintering, thus reducing the sintering density. Therefore, the inclusion of a large amount of Sc element is relatively advantageous in terms of high-temperature oxidation resistance. However, since the Y element for forming a liquid phase should be partially included, it is most preferable that a Sc.sub.2O.sub.3Y.sub.2O.sub.3 molar ratio be 0.5:1 to 2.5:1 in order to increase sinterability through sufficient generation of a liquid phase and to form a stable (Sc,Y).sub.2SiO.sub.7 phase at high temperatures. It is preferable that the content after mixing be 2 to 15 vol % based on the total composition in order to obtain a sintering density of 96% or more.

[0041] Further, cations of the constituent material added as the sintering additive are not solid-solved in a silicon carbide lattice but remain at the crystal grain interface and the triple point among the grains in a rich phase, causing a chemical reaction at high temperatures with the Si oxide generated on the surface during oxidation, which forms a coating layer as a (Sc,Y).sub.2Si.sub.2O.sub.7 phase. A large amount of Si oxide present on the surface of the silicon carbide powder used as a starting raw material acts as a supply source of Si and oxygen of the (Sc,Y).sub.2SiO.sub.7 phase during sintering, and accordingly, limitation of the content thereof is essential. Therefore, in order to remove Si from the surface of the silicon carbide, a process for further adding a small amount of carbon of 0.1 to 0.5 wt % to thus remove oxygen of the Si oxide and reduce the Si oxide to Si carbide during sintering is indispensable. Further, in order to prevent further oxidation, it is preferable to use ethanol rather than water as a wet solvent.

[0042] According to the embodiment of the present invention, the relative density of the silicon carbide material is 96% or more.

[0043] Further, a method of manufacturing a silicon carbide material according to another embodiment of the present invention includes mixing silicon carbide and a Sc.sub.2O.sub.3Y.sub.2O.sub.3 sintering additive in a solvent, drying a mixed slurry, and sintering a dried powder by pressurizing the dried powder.

[0044] In the present invention, first, the components included in the manufacture of the silicon carbide material are mixed in a solvent using SiC balls and polypropylene jars to thus form a slurry state. Then, the slurry may be dried, sieved, put into a graphite mold, and pressure-sintered to thus perform liquid phase sintering, thereby manufacturing the silicon carbide material.

[0045] In the sintering step, heating to 1450 C. is performed without applying pressure, and is maintained at 1450 C. for 10 minutes. This is a process for removing oxygen of the Si oxide from the surface of the silicon carbide and reducing the Si oxide to Si carbide by adding a small amount of carbon, which is essential for lowering the oxygen content. Further, this easily releases CO or CO.sub.2 gas that is additionally generated so as to prevent the formation of residual pores during the sintering. As a result of repeated experimentation, it is understood that the temperature of the process for lowering the oxygen content through the addition of carbon is optimized at about 1400 C. to 1500 C., and temperatures within ranges adjacent thereto are acceptable.

[0046] It is preferable that the sintering be performed in a non-oxidation atmosphere at a temperature of 1800 to 1950 C. for 0.5 to 10 hours while applying a pressure of 10 to 50 MPa from a temperature of 1450 C. or higher and that the non-oxidation atmosphere be a nitrogen or argon gas atmosphere.

[0047] The present invention also provides a silicon carbide material which is manufactured using the above method and in which cations of the constituent material including the silicon carbide base phase and the sintering additive added thereto are not solid-solved in a silicon carbide lattice but are present at the crystal grain interface and the triple point among the silicon carbide grains.

Example 1

[0048] -SiC (0.5 m, BF-17, H. C. Starck, Berlin, Germany), Y.sub.2O.sub.3 (99.99%, Kojundo Chemical Lab Co., Ltd., Sakado-shi, Japan), and Sc.sub.2O.sub.3 (99.99%, Kojundo Chemical Lab Co., Ltd., Sakado-shi, Japan) were used as starting materials.

[0049] 0.3 wt % of carbon was additionally mixed with a powder including 95 vol % of -SiC and 5 vol % of Sc.sub.2O.sub.3Y.sub.2O.sub.3 mixed at a molar ratio of 2:1 using SiC balls and polypropylene jars in ethanol for 24 hours to obtain a mixture. The milled slurry was dried, sieved, put into a graphite mold, and maintained at 1450 C. for 20 minutes. Sintering was then performed in a nitrogen atmosphere at 1900 C. for 1 hour while a pressure of 30 MPa was applied, thus manufacturing a silicon carbide ceramic (hereinafter, referred to as SSY).

Experimental Example 1: Measurement of Relative Density

[0050] The density of the SSY manufactured according to Example 1 was obtained using an Archimedes method, and the relative density was obtained using 3.268 g/cm.sup.3 as the theoretical density of the SSY.

[0051] The relative density of the SSY according to Example 1 was 96.3%.

Comparative Example

[0052] For comparison with Example 1, a silicon-carbide-sintered body to which 5 vol % of an Al.sub.2O.sub.3Y.sub.2O.sub.3 additive was added at a molar ratio of 1:1 was manufactured using the same manufacturing process (molding and sintering) as in the Example.

Experimental Example 2: High-Temperature Steam Oxidation

[0053] The silicon carbide materials manufactured according to Example 1 and Comparative Example 1 were exposed to a steam flow rate condition of 200 cm/s at 1700 C. for 25 hours to thus perform high-temperature steam oxidation, and photographs of the appearance and cross-section of the specimen after the high-temperature steam oxidation are shown in FIGS. 1 and 2.

[0054] The flow rate () of steam was calculated using the following Equation 1.

[00001]$\begin{array}{cc}v=\frac{\mathrm{RT}}{\mathrm{pAM}}.Math.\frac{\mathrm{dm}}{\mathrm{dt}}& \left[\mathrm{Equation}.Math..Math.1\right]\end{array}$

[0055] is the flow rate of steam, R is a gas constant, T is the temperature, p is the pressure, A is the vessel cross-sectional area, M is the molar mass of water, and dm/dt is the rate at which water at room temperature is injected into a steam generator.

[0056] Referring to FIG. 1, it can be seen that, in the case of the silicon carbide material manufactured according to the Comparative Example, the Si oxide on the outer surface due to oxidation and the liquid phase generated due to the Al.sub.2O.sub.3Y.sub.2O.sub.3 additive caused bubbling and melted inner silicon carbide grains due to the absence of a secondary phase, enabling erosion traces to appear.

[0057] Referring to FIG. 2, it can be confirmed that, due to the formation of the secondary phase covering the outer surface of the silicon carbide material manufactured according to the Example, an oxidation coat delayed additional corrosion, thereby protecting an inner base phase without internal erosion caused by oxidation.

Experimental Example 3

[0058] The phase analysis of the surface of the silicon carbide material subjected to the high-temperature steam oxidation of Experimental Example 2 was performed using an XRD with respect to Example 1, and the results are shown in FIG. 3.

[0059] Referring to FIG. 3, it was confirmed that the layer formed on the surface portion mainly included a (Sc, Y).sub.2Si.sub.2O.sub.7 phase (JCPDS No. 019-1125) and also included some cristoballite low SiO.sub.2 phases (JCPDS No. 076-0941) remaining without forming a secondary phase and a small amount of SiO.sub.2 phase (JCPDS No. 044-1394). Accordingly, the Sc and Y elements that are present at the silicon carbide grain boundary and triple point in a rich state come into contact with the SiO.sub.2 generated due to the oxidation on the surface of the silicon carbide, so that a chemical reaction occurs to thus match a stoichiometric ratio, thereby generating a (Sc,Y).sub.2Si.sub.2O.sub.7 phase. The SiO.sub.2 phase may be effectively removed to thus suppress bubbling due to the Si oxide and the generation of additional SiO.sub.2.

Experimental Example 4: Surface Microstructure and EDS (Energy Dispersive X-Ray Spectroscopy) Analysis

[0060] The surface of the specimen subjected to the high-temperature steam oxidation of Experimental Example 2 was observed with respect to Example 1, and the result of elemental distribution is shown in FIG. 4 through the EDS analysis of the observed region.

[0061] Next, referring to FIG. 4, it can be confirmed that the surface portion of the silicon carbide material after the high-temperature steam oxidation included Sc, Y, Si, and O elements, and that the elements were uniformly distributed therethrough. Accordingly, it can be seen that the silicon carbide was oxidized to thus form a secondary phase. Further, it can be confirmed that the secondary phase grains formed on the surface portion were sintered at high temperatures to thus be densified. Further, it can be confirmed that the surface portion was homogeneous without bubbling and large pores due to the absence of a large amount of liquid phase.

Experimental Example 5: Cross-Section Microstructure Analysis

[0062] The cross-section of the EB layer formed on the silicon carbide material subjected to the high-temperature steam oxidation of Experimental Example 2 was observed using a scanning electron microscope with respect to Example 1, and the results are shown in FIG. 5.

[0063] Referring to FIG. 5, it can be confirmed that the EB layer formed on the surface portion of the silicon carbide was homogeneous in thickness and densely formed and that the integrity was ensured without an erosion phenomenon caused by corrosion on the silicon carbide base phase. Accordingly, the EB layer formed on the outer surface of the silicon carbide may effectively prevent the formation of pore channels due to the formation of Si oxide and bubbling in a continuous oxidation atmosphere, thereby preventing corrosion and erosion from accelerating, whereby the oxidation resistance of the SiC base phase is significantly increased.

Experimental Example 6: Cross-Section Microstructure and EDS (Energy Dispersive X-Ray Spectroscopy) Analysis

[0064] The cross-sections of the EB layer formed on the silicon carbide material subjected to the high-temperature steam oxidation of Experimental Example 2 and the silicon carbide base phase were observed using a scanning microscope and were subjected to elemental distribution analysis using EDS (energy dispersive X-ray spectroscopy) with respect to Example 1, and the results are shown in FIG. 6.

[0065] Referring to FIG. 6, in the EDS mapping, a portion where a color appears is a portion where an element is present, and a black portion represents a portion where an element is not present. The elements of the EB layer formed on the outer surface layer include Sc, Y, Si, and O in the same manner as shown in FIG. 4. Sc and Y elements that are present at a triple point and the Si element of silicon carbide were present in the silicon carbide base phase, and no signs of penetration of an oxygen element were observed. This indicates that the silicon carbide base phase was completely protected from oxygen even under oxidation conditions.

Experimental Example 7: Analysis of Microstructure of Interface of EB Layer and Silicon Carbide

[0066] The cross-section of the silicon carbide material subjected to the high-temperature steam oxidation of Experimental Example 2 was plasma-etched using CF.sub.4 gas with respect to Example 1, the interface of the EB layer and the silicon carbide and the inner core of the silicon carbide were observed using a scanning microscope, and the results of observation are shown in FIG. 7.

[0067] FIG. 7 shows the microstructure of the interface of the EB layer and the silicon carbide, and it can be confirmed that strong bonding occurred at the interface of the EB layer and the silicon carbide. It can be also confirmed that a large amount of secondary phase was mixed with silicon carbide grains from the EB layer to a depth of about 20 m of the inner base phase of the silicon carbide. This shows that the EB layer is not bonded only on the surface of the silicon carbide in a two-dimensional manner but is formed up to the inner region of the silicon carbide base, so that a relatively strong three-dimensional bonding structure is ensured, thereby relatively greatly reducing the possibility of peeling compared to two-dimensional bonding using an EB coating process. It also shows that the secondary phases additionally form an EB layer on the outer surface even when the EB layer is removed under a continuous oxidation condition, thereby continuously maintaining the EB layer, which maintains the corrosion resistance of the silicon carbide material.

[0068] While the present invention has been described with reference to exemplary embodiments, it will be apparent to those skilled in the art that the invention is not limited to the disclosed exemplary embodiments and the accompanying drawings, but, on the contrary, is intended to cover various substitutions, variations, and modifications included within the technical spirit of the present invention.

[0069] All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

[0070] The use of the terms a and an and the and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms comprising, having, including, and containing are to be construed as open-ended terms (i.e., meaning including, but not limited to,) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., such as) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[0071] Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.