Silicon nitride-based sintered body and method for producing same

Abstract

Disclosed is a technology for imparting a color tone difference to a silicon nitride-based sintered body. A silicon nitride-based sintered body according to the present disclosure is characterized by having a plurality of crystal grains and a grain boundary phase that bonds the crystal grains to each other, and by having a lightness difference of 5 or more in terms of the L* value. The silicon nitride-based sintered body according to the present disclosure is obtained, for example, by kneading a silicon nitride powder and a powder of an auxiliary starting material so as to obtain a granulated body, and subsequently molding the granulated body and firing the molded body under specific conditions.

Description

Silicon nitride sintered body and its manufacturing method

[0001] The present application discloses a silicon nitride sintered body and a method for producing the same.

[0002] Silicon nitride sintered bodies are used as various structural materials (Patent Documents 1 to 4). Silicon nitride sintered bodies generally have a homogeneous black or dark gray color tone. Patent Document 5 discloses a silicon nitride sintered body that facilitates foreign matter detection by having a boundary where the brightness V, as specified in JIS Z8721, is 3 or more lower on the inner side than on the surface side. The brightness boundary is set within 20% of a depth of 100 to 1000 μm from the surface of the sintered body, i.e., 80 to 1200 μm. Patent Document 6 also discloses a ceramic ball composed of a composite material of silicon nitride-based materials A and B that can be distinguished using visible or ultraviolet light, allowing optical observation of its rotation state.

[0003] Japanese Patent Application Laid-Open No. 2004-059346 Japanese Patent Application Laid-Open No. 9-227240 Japanese Patent No. 5289090 Japanese Patent Application Laid-Open No. 2003-267786 International Publication No. 2014 / 104112 Japanese Patent Application Laid-Open No. 2017-172735

[0004] In the past, techniques for homogenizing the color tone of silicon nitride sintered bodies have been disclosed, as well as techniques for imparting a certain degree of color tone difference to silicon nitride sintered bodies. However, the prior art leaves room for improvement in terms of imparting a more distinct brightness difference to silicon nitride sintered bodies. In view of the above, the present application discloses a silicon nitride sintered body having a distinct brightness difference and a method for producing the same.

[0005] The technology of the present disclosure includes the following aspects: <Aspect 1> A steel sheet having a plurality of crystal grains and a grain boundary phase that connects the crystal grains together, ＊ A silicon nitride sintered body having a lightness difference of 5 or more in terms of L value. ＊A silicon nitride sintered body of Aspect 1, having a lightness difference of 10 or more in terms of L value. <Aspect 3> A silicon nitride sintered body of Aspect 1 or 2, having the lightness difference at the surface of the sintered body. <Aspect 4> A silicon nitride sintered body of any of Aspects 1 to 3, having the lightness difference in a cross section perpendicular to the surface of the sintered body. <Aspect 5> A silicon nitride sintered body of Aspect 4, having the lightness difference in a cross section perpendicular to the surface of the sintered body, within a range of 1.0 mm or less in a direction perpendicular to the surface. <Aspect 6> A silicon nitride sintered body of Aspect 4, having the lightness difference in a cross section perpendicular to the surface of the sintered body, within a range of 0.5 mm or less in a direction perpendicular to the surface. <Aspect 7> When lightness is measured from the surface to the inside of the sintered body in a cross section of the sintered body, ＊ The silicon nitride sintered body according to any one of aspects 1 to 6, comprising a portion in which a difference in lightness in terms of a value of less than 5 and two or more uniform lightness regions each having a width of 2 mm or more are observed. ＊ The silicon nitride sintered body of any one of Aspects 1 to 7, comprising a portion having a value of 0.05. Aspect 9: The silicon nitride sintered body of any one of Aspects 1 to 8, having a total content of Al, Fe, Co, and Ni of less than 2000 ppm, and having a SiC content of less than 5000 ppm. Aspect 10: The silicon nitride sintered body of any one of Aspects 1 to 10, having an Al content of less than 1000 ppm. Aspect 11: The silicon nitride sintered body of any one of Aspects 1 to 10, wherein the grain boundary phase contains a crystalline component. Aspect 12: The grain boundary phase contains, as the crystalline component, Si ２ N ２ O, Y ２ Si ２ O ７ , Yb ２ Si ２ O ７ , Y ２ Si ３ O ３ N ４ and YbSi ３ O ３ N ４A silicon nitride sintered body according to Aspect 11, comprising at least one selected from the group consisting of: A silicon nitride sintered body according to Aspect 13, comprising kneading silicon nitride powder with powders of auxiliary materials to obtain granules, and molding and firing the granules, wherein the maximum temperature during the firing is greater than 1770°C, the atmosphere at 1700°C or higher during a temperature rise during the firing contains 99.9% by volume or more of nitrogen gas and has an absolute pressure of 0.3 MPa or more and less than 1.1 MPa, the time during the temperature rise during the firing from 1700°C to 1770°C is 40 minutes or more and 120 minutes or less, the time during the temperature rise during the firing from 1770°C to said maximum temperature is 30 minutes or more and 90 minutes or less, and the holding time at said maximum temperature during the firing is 16 hours or less.

[0006] The silicon nitride sintered body of the present disclosure has a brightness difference of at least a certain level on its surface or cross section. That is, there are relatively bright and relatively dark areas, and the brightness difference is clear enough to be visible. By imparting a brightness difference of at least a certain level to a silicon nitride sintered body, it is believed that the applications of the silicon nitride sintered body will be expanded. For example, when a silicon nitride sintered body having a brightness difference of at least a certain level is used as a wear-resistant part, the brightness of the surface of the wear-resistant part changes as wear progresses, allowing the amount of wear on the surface of the wear-resistant part to be determined visually. Furthermore, when a silicon nitride sintered body having a brightness difference of at least a certain level is used as a protective tube for various probes, the portions of the silicon nitride sintered body having a brightness difference can be used as markers to visually determine the penetration depth, etc.

[0007] FIG. 1 is a schematic diagram showing an example of a silicon nitride sintered body. FIG. 2 is a schematic diagram showing an example of a silicon nitride sintered body. FIG. 3 is a schematic diagram showing an example of a silicon nitride sintered body. FIG. 4 is a schematic diagram showing an example of a silicon nitride sintered body. FIG. 5 is a schematic diagram showing an example of a flow of a manufacturing method for a silicon nitride sintered body. FIG. 6 is a schematic diagram showing an example of surface treatment of a silicon nitride sintered body. FIG. 7 is a schematic diagram showing the shape and color tone of a silicon nitride sintered body according to Example 1. FIG. 8 shows a change in lightness in the depth direction of the silicon nitride sintered body according to Example 1. FIG. 9 is a schematic diagram showing the shape and color tone of a silicon nitride sintered body according to Example 2. FIG. 10 shows a change in lightness in the depth direction of the silicon nitride sintered body according to Example 2. FIG. 11 is a schematic diagram showing the shape and color tone of a silicon nitride sintered body according to Example 3. FIG. 12 shows a change in lightness in the depth direction of the silicon nitride sintered body according to Example 3. FIG. 13 is a schematic diagram for explaining a grinding process and a ground surface when obtaining a silicon nitride sintered body according to Example 4. FIG. 14 shows a change in lightness on the surface of the silicon nitride sintered body according to Example 4. FIG. 15 is a schematic diagram showing the shape and color tone of a silicon nitride sintered body according to Comparative Example 1. 1 shows the change in brightness in the depth direction of the silicon nitride sintered body according to Comparative Example 1. FIG. 2 is a schematic diagram showing the shape and color tone of the silicon nitride sintered body according to Comparative Example 2. FIG. 3 shows the change in brightness in the depth direction of the silicon nitride sintered body according to Comparative Example 2.

[0008] Hereinafter, a silicon nitride sintered body and a method for producing the same according to an embodiment will be described with reference to the drawings. However, the silicon nitride sintered body and the method for producing the same according to the present disclosure are not limited to the following embodiment.

[0009] 1. Silicon nitride sintered body The silicon nitride sintered body of the present disclosure has a plurality of crystal grains and a grain boundary phase that connects the crystal grains together, and ＊ It is characterized by having a brightness difference of 5 or more in terms of value.

[0010] 1.1 Crystal Grains The silicon nitride sintered body of the present disclosure is formed by bonding multiple crystal grains together via a grain boundary phase. This bonding of multiple crystal grains via a grain boundary phase can be confirmed by, for example, a microscopic image of the material structure. Each crystal grain contains silicon nitride. Each crystal grain may also contain impurities. The type of impurity contained in the crystal grains depends on the type of impurities contained in the raw materials before sintering, the type of auxiliary raw materials, the sintering atmosphere, and other factors. For example, the crystal grains constituting the silicon nitride sintered body may contain metal elements other than Si, such as Al, Fe, Co, and Ni. The crystal grains constituting the silicon nitride sintered body may also contain compounds other than silicon nitride, such as silicon carbide (SiC). The size of the crystal grains in the silicon nitride sintered body of the present disclosure is not particularly limited. The size of the crystal grains is determined by the particle size of the raw materials before sintering (e.g., silicon nitride powder) and the sintering conditions.

[0011] High-purity silicon nitride crystals hardly absorb visible light. That is, when the crystal grains in a silicon nitride sintered body (polycrystalline body) are highly pure and do not contain impurities, the sintered body is thought to be white to off-white in color. However, actual silicon nitride sintered bodies have a homogeneous color tone of black or dark gray. This is because when metal elements such as Al, Fe, Co, and Ni are dissolved in silicon nitride crystals, the energy levels of the electrons formed by the dissolved elements become so-called color centers, resulting in a broad absorption spectrum, and the silicon nitride crystal as a whole appears dark gray to black in color (note that Al ２ O ３ Silicon nitride containing these elements in solid solution is also called sialon (SiAlON), and in this application, this type of material is also referred to as "silicon nitride." The same is true for the grain boundary phase in a sintered body (polycrystalline body), which also develops a dark color when the above elements are contained. Furthermore, as will be described later, when the grain boundary phase is amorphous, it also tends to develop a dark color. Furthermore, when a silicon nitride sintered body contains SiC, the sintered body also develops a dark color.

[0012] In this way, when silicon nitride is substituted with certain elements (for example, the above-mentioned metal elements), the color of the crystal becomes dark. Furthermore, SiC or the like mixed into the crystal also causes coloring. In the silicon nitride sintered body of the present disclosure, the crystal grains themselves may be colored as long as the above-mentioned predetermined brightness difference is ensured. However, it is better to suppress the coloring of the crystal grains themselves as much as possible, since this will result in a relatively high L. ＊ The first portion 11 (described in more detail below) has a higher L ＊ The value is secured, and the brightness difference described above tends to become large.

[0013] According to the findings of the present inventors, when the total content of Al, Fe, Co and Ni in the silicon nitride sintered body is less than 2000 ppm and the content of SiC is less than 5000 ppm, the first portion 11 has a high L ＊ The total content of Al, Fe, Co, and Ni in the silicon nitride sintered body may be 1750 ppm or less, 1500 ppm or less, 1250 ppm or less, 1000 ppm or less, 750 ppm or less, 500 ppm or less, 250 ppm or less, 100 ppm or less, or 50 ppm or less. The SiC content in the silicon nitride sintered body may be 4500 ppm or less, 4000 ppm or less, 3500 ppm or less, 3000 ppm or less, 2500 ppm or less, 2000 ppm or less, 1500 ppm or less, 1000 ppm or less, 750 ppm or less, 500 ppm or less, 250 ppm or less, 100 ppm or less, or 50 ppm or less. When the total content of Al, Fe, Co and Ni in the sintered body is 2000 ppm or more, or when the SiC content is 5000 ppm or more, the entire sintered body becomes blackened, that is, the L of the entire sintered body becomes black. ＊ The value drops significantly, and the brightness difference in the sintered body tends to become smaller.

[0014] According to the findings of the present inventors, even when the content of Al in the silicon nitride sintered body is less than 1000 ppm, the first portion 11 has a high L ＊The Al content in the silicon nitride sintered body may be 750 ppm or less, 500 ppm or less, 250 ppm or less, 100 ppm or less, or 50 ppm or less. When the Al content in the sintered body is 1000 ppm or more, the entire sintered body is colored, that is, the L of the entire sintered body is easily increased. ＊ The value tends to decrease, and the brightness difference in the sintered body tends to decrease.

[0015] Furthermore, according to the findings of the present inventors, when a silicon nitride sintered body contains a large amount of Mg, compounds with low melting points are formed, which tend to reduce the heat resistance of the silicon nitride sintered body. In this regard, the Mg content in the silicon nitride sintered body may be less than 1800 ppm.

[0016] The silicon nitride sintered body contains, for example, 80% by mass or more but less than 100% by mass, or 80% by mass or more but 97% by mass or less of silicon nitride crystal grains. The remainder may be a grain boundary phase composed of a composition other than silicon nitride. The content of silicon nitride crystal grains can be measured by X-ray diffraction using a calibration curve.

[0017] 1.2 Grain Boundary Phase The grain boundary phase may be made of any material capable of bonding crystal grains together. The grain boundary phase may be a glass phase, a crystalline phase, or a mixed phase of a glass phase and a crystalline phase. However, in the silicon nitride sintered body of the present disclosure, it is also important to control the color tone of the grain boundary phase. As described above, in addition to reducing the concentrations of impurities and SiC, it is also important to control the nitrogen content. A deficiency or excess of nitrogen atoms can also cause color centers, resulting in a broad absorption spectrum, i.e., blackening of the sintered body, and tending to reduce the brightness difference of the sintered body. Furthermore, as described above, the grain boundary phase consists of a glass phase and a crystalline phase, but the glass phase is basically non-stoichiometric and prone to color variation. In other words, as the proportion of the glass phase in the grain boundary phase increases, it becomes more difficult to control the color tone of the grain boundary phase, and the brightness difference of the sintered body tends to decrease. According to the inventor's findings, color control becomes easier when part or all of the grain boundary phase is a crystalline phase. In other words, when the grain boundary phase contains a crystalline component, the color tone of the sintered body as a whole is easily controlled, and the brightness difference described above is likely to increase.

[0018] The type of crystalline component depends on, for example, the type of auxiliary raw material (sintering aid, etc.) before sintering. As will be described later, by using appropriate auxiliary raw materials and adopting firing conditions that allow crystalline components to be generated in the grain boundary phase, the grain boundary phase will contain crystalline components. According to the findings of the present inventors, the grain boundary phase contains Si as a crystalline component. ２ N ２ O, Y ２ Si ２ O ７ , Yb ２ Si ２ O ７ , Y ２ Si ３ O ３ N ４ and YbSi ３ O ３ N ４ When the alloy contains at least one selected from the group consisting of:

[0019] 1.3 Lightness The silicon nitride sintered body of the present disclosure has a lightness of L ＊ a ＊ b ＊ (L-star, A-star, B-star) Lightness L in the color system (adopted in JIS Z 8781-4) ＊ The silicon nitride sintered body has a brightness difference of 5 or more in terms of brightness value. In this way, it is believed that by having a brightness difference of a predetermined value or more, the applications of the silicon nitride sintered body will be expanded. For example, when a silicon nitride sintered body having a brightness difference is used as a wear-resistant part, the brightness of the surface of the wear-resistant part changes as wear progresses, and the amount of wear on the surface of the wear-resistant part can be determined visually. Furthermore, when a silicon nitride sintered body having a brightness difference is used as a protective tube for various probes, the penetration depth, etc. can be determined visually using the portion of the silicon nitride sintered body having a brightness difference as a mark. Various visual determinations can be made by utilizing the brightness difference in the silicon nitride sintered body. L ＊ If the lightness difference is 5 or more, such visual judgment is easy. From the viewpoint of visibility of the lightness difference, it can be said that the larger the lightness difference in the silicon nitride sintered body, the more preferable it is. In this respect, the silicon nitride sintered body of the present disclosure has an L ＊The lightness difference may be 6 or more, 7 or more, 8 or more, 9 or more, or 10 or more. ＊ A difference in brightness of 10 or more is preferred.

[0020] When the brightness difference in the silicon nitride sintered body is used for the above-mentioned various visual judgments, it is preferable that the brightness difference be confirmed in as small an area as possible of the silicon nitride sintered body, that is, that the contrast be large. ＊ When the L value is measured, the L value is within a range of 5.0 mm, 3.0 mm, 1.0 mm, or 0.5 mm from point A to point B. ＊ It is preferable that the value change be 5 or more, and the above-mentioned difference in lightness be confirmed. ＊ a first portion having a value of ＊ and a transition portion between the first and second portions, and L in the first portion. ＊ value and L in the second part ＊ The difference between the values ​​may be 5 or more, and the length of the transition portion between the first and second portions may be 5.0 mm or less, 3.0 mm or less, 1.0 mm or less, or 0.5 mm or less. In this case, the shape and size of the first portion, the second portion, and the transition portion may be appropriately determined depending on the application of the silicon nitride sintered body. It is possible to ensure a desired lightness difference on the surface of the silicon nitride sintered body by exposing the lightness difference present in the cross section of the silicon nitride sintered body to the surface through processing. In this case, the size of the transition portion on the processed surface is determined by the size of the transition portion on the cross section of the silicon nitride sintered body and the angle of the processed surface relative to the annealed surface. The greater the angle of the processed surface relative to the annealed surface, the greater the contrast formed on the surface of the processed product. However, the processing angle can be selected by comprehensively assessing the shape, positional relationship of color tones, and strength of contrast of the desired product.

[0021] The silicon nitride sintered body of the present disclosure has an L of 45 or more, 47 or more, 50 or more, 53 or more, or 55 or more.＊ For example, the silicon nitride sintered body may contain a portion having a relatively high L ＊ a first portion having a value of ＊ and a second portion having a value, L in the first portion ＊ The value may be 45 or more, 47 or more, 50 or more, 53 or more, or 55 or more. ＊ There is no particular upper limit to the value. ＊ The value may be, for example, 75 or less or 70 or less.

[0022] The silicon nitride sintered body of the present disclosure has an L of 55 or less, 53 or less, 50 or less, 47 or less, or 45 or less. ＊ For example, the silicon nitride sintered body may contain a portion having a relatively high L ＊ a first portion having a value of ＊ and a second portion having a value, L in the second portion ＊ The value may be 55 or less, 53 or less, 50 or less, 47 or less, or 45 or less. ＊ There is no particular limitation on the lower limit of the value. ＊ The value may be, for example, 25 or more, or 30 or more.

[0023] 1.4 Specific Example Figures 1 and 2 show the L ＊ 1 and 2, the silicon nitride sintered body 10 according to one embodiment has a relatively high L ＊ a first portion 11 having a relatively low L ＊ and a second portion 12 having a value of L ＊ value and L in the second portion 12 ＊ The difference between the values ​​is 5 or more.

[0024] 1.4.1 First Form The silicon nitride sintered body according to the first form has the above-mentioned brightness difference on its surface. That is, as shown in FIG. 1 , the silicon nitride sintered body 10 according to the first form may have the above-mentioned first portion 11 and second portion 12 on its surface. In this case, the above-mentioned various visual judgments can be made by utilizing the brightness difference on the surface of the silicon nitride sintered body 10. The silicon nitride sintered body 10 may have a transition portion on its surface between the first portion 11 and the second portion 12. In this case, the length of the transition portion between the first portion 11 and the second portion 12 may be 5.0 mm or less, 3.0 mm or less, 1.0 mm or less, or 0.5 mm or less. For example, the silicon nitride sintered body 10 according to the first embodiment may have the above-mentioned brightness difference on its surface within a range of 5.0 mm or less in the direction along the surface, or may have the above-mentioned brightness difference within a range of 3.0 mm or less in the direction along the surface, or may have the above-mentioned brightness difference within a range of 1.0 mm or less in the direction along the surface, or may have the above-mentioned brightness difference within a range of 0.5 mm or less in the direction along the surface.

[0025] 1.4.2 Second Form The silicon nitride sintered body according to the second form has the above-mentioned brightness difference in its cross section. That is, as shown in FIG. 2 , the silicon nitride sintered body 10 according to the second form may have the above-mentioned first portion 11 and second portion 12 in its cross section. In this case, the above-mentioned various visual judgments can be made by utilizing the brightness difference in the cross section of the silicon nitride sintered body 10. For example, the brightness of the surface of the silicon nitride sintered body 10 changes as wear of the silicon nitride sintered body 10 progresses. The silicon nitride sintered body 10 may have a transition portion between the first portion 11 and the second portion 12 in its cross section. In this case, the length of the transition portion between the first portion 11 and the second portion 12 may be 5.0 mm or less, 3.0 mm or less, 1.0 mm or less, or 0.5 mm or less. For example, the silicon nitride sintered body 10 according to the second embodiment may have the above-mentioned brightness difference in a cross section perpendicular to its surface within a range of 5.0 mm or less in the direction perpendicular to the surface, within a range of 3.0 mm or less in the direction perpendicular to the surface, within a range of 1.0 mm or less in the direction perpendicular to the surface, or within a range of 0.5 mm or less in the direction perpendicular to the surface. In other words, the width of the transition portion in the direction perpendicular to the surface may be 5.0 mm or less, 3.0 mm or less, 1.0 mm or less, or 0.5 mm or less.

[0026] In FIG. 2, the silicon nitride sintered body 10 has a relatively low L ＊ The second portion 12 has a relatively high L ＊ However, the technology of the present disclosure is not limited to this. For example, the silicon nitride sintered body 10 may have a first portion 11 having a relatively high L ＊ The first portion 11 has a relatively low L ＊ The first portion 11 and the second portion may have a second portion 12 having a value of L ＊The brightness difference in value is less than 5, and the width in the direction from the surface of the sintered body toward the inside (particularly, the direction from the surface of the sintered body toward the centroid of the cross section) is 2 mm or more ("uniform brightness region" described below).

[0027] As shown in FIG. 3A, when the lightness of the silicon nitride sintered body according to the second embodiment is measured from the surface to the inside of the cross section of the sintered body, the lightness is ＊ The sintered body may include a portion in which two or more uniform brightness regions with a brightness difference of less than 5 in terms of the value and a width of 2 mm or more are confirmed. That is, when the brightness is measured from the surface of the sintered body toward the inside in the cross section of the sintered body, there is a "L ＊ Two or more "uniform brightness regions" having a brightness difference of less than 5 in terms of the value and a width of 2 mm or more may be confirmed. In FIG. 3A, when measuring the brightness from the surface to the inside of the sintered body, there is no particular limitation on the direction in which the brightness is measured (the direction from the surface to the inside of the sintered body). In FIG. 3A, for example, in at least one of the directions (1) to (6), "L ＊ It is sufficient to confirm two or more "uniform brightness regions" in which the brightness difference in the value is less than 5 and the width is 2 mm or more. The width of the uniform brightness region may be 2.5 mm or more, 3.0 mm or more, or 3.5 mm or more. On the other hand, there is no particular upper limit to the width of the uniform brightness region. When processing the sintered body to form a contrast on the surface of the sintered body, the processing allowance can be reduced by making the width of the uniform brightness region 8.0 mm or less.

[0028] As described above, the silicon nitride sintered body 10 may have a first portion 11 and a second portion 12 in its cross section. ＊ The value is less than 5, the width of the first portion 11 in the direction from the surface to the inside of the sintered body is 2 mm or more, and the brightness difference in the second portion 12 is L ＊ When the value of the second portion 12 is less than 5 and the width of the second portion 12 in the direction from the surface of the sintered body toward the inside is 2 mm or more, the silicon nitride sintered body 10 has a value of L ＊This includes a portion where the brightness difference in value is less than 5 and two or more uniform brightness regions with a width of 2 mm or more are confirmed.

[0029] As shown in FIG. 3B, in the cross section of the silicon nitride sintered body according to the second embodiment, when the lightness is measured from the surface of the sintered body toward the centroid of the cross section, L ＊ The sintered body may include a portion in which two or more uniform brightness regions with a brightness difference of less than 5 and a width of 2 mm or more are confirmed. That is, when the brightness is measured in a cross section of the sintered body from the surface toward the center of the interior, there is a "L ＊ In FIG. 3B, for example, two or more "uniform brightness regions in which the brightness difference in the value is less than 5 and the width is 2 mm or more" may be confirmed. ＊ It is sufficient to confirm two or more "uniform brightness regions" in which the brightness difference in the value is less than 5 and the width is 2 mm or more. The width of the uniform brightness region may be 2.5 mm or more, 3.0 mm or more, or 3.5 mm or more. On the other hand, there is no particular upper limit to the width of the uniform brightness region. When processing the sintered body to form a contrast on the surface of the sintered body, the processing allowance can be reduced by making the width of the uniform brightness region 8.0 mm or less.

[0030] As described above, the silicon nitride sintered body 10 may have a first portion 11 and a second portion 12 in its cross section. Here, the lightness difference in the first portion 11 is L ＊ The value is less than 5, the width of the first portion 11 in the direction from the surface of the sintered body toward the centroid of the cross section is 2 mm or more, and the brightness difference in the second portion 12 is L ＊ When the value of the second portion 12 is less than 5 and the width of the second portion 12 in the direction from the surface of the sintered body toward the centroid of the cross section is 2 mm or more, the silicon nitride sintered body 10 has a value of L ＊ This includes a portion where the brightness difference in value is less than 5 and two or more uniform brightness regions with a width of 2 mm or more are confirmed.

[0031] As shown in FIGS. 3A and 3B, in the cross section of the silicon nitride sintered body, "L ＊ When two or more "uniform lightness regions" are confirmed in which the lightness difference in the value is less than 5 and the width is 2 mm or more, the silicon nitride sintered body has a L ＊ The silicon nitride sintered body naturally has a lightness transition region (for example, the transition portion described above) where the lightness difference in value is 5 or more. When the one uniform lightness region and the other uniform lightness region both have a width of 2 mm or more, contrast is easily formed in the silicon nitride sintered body, and the visibility of the lightness change of the silicon nitride sintered body is thought to be further improved. For example, when the silicon nitride sintered body is used as a wear-resistant part, when the one uniform lightness region and the other uniform lightness region of the silicon nitride sintered body both have a width of 2 mm or more and there is a lightness transition region between the one uniform lightness region and the other uniform lightness region, the change in position between the uniform lightness region and the lightness transition region when the wear-resistant part wears can be more easily confirmed visually, and the amount of wear on the surface of the wear-resistant part can be more easily determined visually.

[0032] 1.4.3 Third Form The silicon nitride sintered body according to the third form has the above-mentioned lightness difference both on its surface and in its cross section. That is, the silicon nitride sintered body according to the third form may be a combination of the first and second forms.

[0033] 1.5 Other Matters The silicon nitride sintered body of the present disclosure may have the above-mentioned crystal grains and grain boundary phase, and may also have the above-mentioned brightness difference. As described above, the silicon nitride sintered body may contain, for example, 80 mass % or more but less than 100 mass %, or 80 mass % or more but less than 97 mass % silicon nitride. Furthermore, the silicon nitride sintered body of the present disclosure may have various shapes depending on the application. The silicon nitride sintered body may be applied to a specific application in the shape it has immediately after sintering, or may be applied to a specific application after being subjected to surface treatment or the like after sintering. The silicon nitride sintered body of the present disclosure has a relatively high L ＊ a first portion having a value of ＊The first portion and the second portion having a value may be regularly or irregularly arranged.

[0034] 2. Method for Producing Silicon Nitride Sintered Body According to the findings of the present inventors, a silicon nitride sintered body having the above-described difference in lightness can be produced by controlling the temperature rise, particularly in the firing step. The production method shown below is one example of a method for producing the silicon nitride sintered body of the present disclosure.

[0035] As shown in Figure 4, a method for producing a silicon nitride sintered body according to one embodiment includes: kneading silicon nitride powder 1 and auxiliary material powder 2 to obtain granules 3; and molding and firing the granules 3; wherein the maximum temperature during the firing is greater than 1770°C; the atmosphere at 1700°C or higher during the temperature rise process during the firing contains 99.9% or more by volume of nitrogen gas and has an absolute pressure of 0.3 MPa or more and less than 1.1 MPa; the time required for the temperature to rise from 1700°C to 1770°C during the temperature rise process during the firing is 40 minutes or more and 120 minutes or less; the time required for the temperature to rise from 1770°C to the maximum temperature during the temperature rise process during the firing is 30 minutes or more and 90 minutes or less; and the holding time at the maximum temperature during the firing is 16 hours or less.

[0036] 2.1 Silicon nitride powder The silicon nitride powder 1 may be produced by a known method. According to the knowledge of the present inventors, the higher the purity of the silicon nitride powder 1, the higher the L in the first portion. ＊ The value tends to be higher, and the brightness difference in the sintered body tends to be larger. In this regard, the purity of the silicon nitride powder 1 may be 98.70% or more, or 98.95% or more. The size of the silicon nitride powder 1 is not particularly limited.

[0037] 2.2 Auxiliary Raw Material Powder Silicon nitride powder 1 is difficult to sinter by itself. Therefore, when producing a silicon nitride sintered body 10, the silicon nitride powder 1 is mixed with auxiliary raw material powder 2. The auxiliary raw material must be able to withstand the firing conditions described below. The auxiliary raw material can also generate the desired crystalline components in the grain boundary phase. If the melting point or softening point of the auxiliary raw material is too low, it may melt when fired under the firing conditions described below, making it difficult to produce a sintered body. Furthermore, when an auxiliary raw material with a low melting point or softening point is used, lowering the firing temperature so as not to exceed the melting point or softening point of the auxiliary raw material may prevent the maximum temperature of over 1770°C required for the present invention from being reached. Furthermore, the desired crystalline components cannot be generated in the grain boundary phase, and the sintered body as a whole is prone to blackening. In this regard, it is preferable to select an auxiliary raw material that generates the desired crystalline components in the grain boundary phase when fired under the firing conditions described below. The auxiliary raw material may contain at least one compound selected from a Si-containing compound (e.g., silicon oxide), a Y-containing compound (e.g., yttrium oxide), and a Yb-containing compound (e.g., ytterbium oxide).

[0038] 2.3 Powder Kneading and Granulation As shown in FIG. 4 , the silicon nitride powder 1 and the auxiliary material powder 2 are kneaded to obtain granules 3 of the silicon nitride powder 1 and the auxiliary material powder 2. The mixing ratio of the silicon nitride powder 1 and the auxiliary material powder 2 can be determined appropriately depending on the desired performance of the sintered body. Furthermore, the method for kneading and granulating the silicon nitride powder 1 and the auxiliary material powder 2 is not particularly limited. For example, the silicon nitride powder 1 and the auxiliary material powder 2 can be kneaded and granulated by mechanically mixing them with a binder, a dispersant, and a solvent to obtain a slurry, and then drying the slurry by spray drying or the like. In this case, the types of binder, dispersant, and solvent are not particularly limited. Furthermore, the size of the granules 3 is not particularly limited.

[0039] 2.4 Molding and sintering of granules As shown in FIG. 4, the granules 3 are molded before firing. The granules 3 may be molded by known molding means. An appropriate molding means may be selected depending on the desired shape of the sintered body. For example, the granules 3 may be filled into a mold and press-molded, or may be press-molded using an isostatic press, or a combination of these may be used. The molded body (green body) 4 is fired under the above-mentioned conditions to obtain a sintered body 10. Hereinafter, the molded body from the start of firing to the completion of firing (completion of sintering) will be referred to as the "body to be fired 5."

[0040] In the sintered body 5, nitriding progresses in the portion that will become the grain boundary phase of the silicon nitride sintered body during the sintering process. According to the findings of the present inventors, nitriding of the grain boundary phase is insufficient in the temperature range below 1700°C, while in the temperature range of 1700°C or higher in the sintering process, nitrogen penetrates into the interior of the sintered body 5, replacing atoms such as oxygen with nitrogen, making the grain boundary phase a stable composition (phase containing crystalline matter), and the color center disappears, resulting in the formation of L ＊ The L value increases inside the sintered body 5 due to this mechanism. ＊ While the value increases, excessive nitrogen is supplied near the surface, a composition with excess nitrogen is formed, and color centers may occur again. ＊ While the value increases, near the surface, L ＊ This causes a difference in brightness in the sintered body.

[0041] 2.4.1 Time from 1700°C to 1770°C in the temperature rise process According to the findings of the present inventors, in the firing process, the L of each part of the firing target body 5 is increased by the above-mentioned mechanism. ＊ Value increase and L ＊ However, in the temperature range from 1700°C to 1770°C, the L value of the entire sintered body 5 decreases. ＊ Here, if the time taken to reach 1770°C from 1700°C is too short, densification will proceed without sufficient penetration of nitrogen into the interior of the sintered body 5, and the overall L value of the sintered body finally obtained will decrease. ＊On the other hand, if the time taken to reach 1770°C from 1700°C is too long, nitrogen will penetrate into the interior of the sintered body 5 excessively, and the overall L value of the sintered body obtained will be reduced. ＊ According to the findings of the present inventors, for example, when the time taken for the temperature to rise from 1700°C to 1770°C is 40 minutes or more and 120 minutes or less, the L value of the entire sintered body 5 becomes low, making it difficult to ensure the desired lightness difference in the sintered body. ＊ The value tends to rise appropriately.

[0042] 2.4.2 Atmosphere at 1700°C or higher during the temperature rise process during sintering According to the findings of the present inventors, when the absolute pressure of nitrogen is less than 0.3 MPa in the atmosphere at 1700°C or higher during the temperature rise process during sintering, the penetration of nitrogen into the sintered body 5 becomes insufficient, and the L of the grain boundary phase is reduced. ＊ In addition, if the absolute pressure of nitrogen is 1.1 MPa or more, the nitrogen penetrates excessively deep into the sintered body 5, and the overall L value of the sintered body obtained in the end is insufficient, making it difficult to ensure the desired lightness difference in the sintered body obtained in the end. ＊ On the other hand, when the atmosphere at 1700°C or higher during the temperature rise process during firing contains 99.9% by volume or more of nitrogen gas and has an absolute pressure of 0.3 MPa or more and less than 1.1 MPa, the L value of the entire fired body 5 decreases from 1700°C to 1770°C. ＊ The value increases appropriately, and the desired lightness difference is easily ensured in the finally obtained sintered body. Note that, within the range of the present invention, the desired lightness difference is easily ensured by extending the heat treatment time under conditions of a low absolute nitrogen pressure, and conversely, by shortening the heat treatment time under conditions of a high absolute nitrogen pressure.

[0043] 2.4.3 Time from 1770°C to the maximum temperature during the temperature rise process during firing According to the findings of the present inventors, in the temperature range from 1770°C to the maximum temperature, nitrogen penetration and L ＊That is, by lowering the rate of temperature rise from 1770°C to the maximum temperature, or by maintaining a constant temperature from 1770°C to the maximum temperature, a sufficient amount of nitrogen is supplied to the vicinity of the surface of the sintered body 5, and the L ＊ By controlling the time, the L ＊ It is also possible to widen the size of the region with low values, and to increase the lightness difference of the finally obtained sintered body. Incidentally, a similar effect can be obtained by increasing the nitrogen gas pressure in the sintering atmosphere from 1770°C to the maximum temperature. However, if the nitrogen gas pressure is increased, nitrogen will penetrate into the interior of the sintered body 5, and the boundary between color tones will tend to become blurred. It is also possible to weaken the lightness difference of the finally obtained sintered body by increasing the heating rate from 1770°C to the maximum temperature, or by lowering the nitrogen gas pressure to limit the nitrogen supply to the sintered body 5, but this is outside the scope of the objective of the silicon nitride sintered body of the present disclosure. According to the inventor's findings, in the finally obtained sintered body, the L ＊ In order to secure a brightness difference of 5 or more and to obtain clear contrast, it is preferable to set the time required for the temperature to rise from 1770°C to the maximum temperature to 30 minutes or more and 90 minutes or less. By setting the time required for the temperature to rise from 1770°C to the maximum temperature to 30 minutes or more and 90 minutes or less, the L ＊ It is also possible to form a uniform brightness region having a brightness difference in value of less than 5 and a width of 2 mm or more. In order to achieve a width of 2.5 mm or more, the time required to reach the maximum temperature from 1770°C should be 40 minutes or more and 90 minutes or less, in order to achieve a width of 3.0 mm or more, the time required to reach the maximum temperature from 1770°C should be 50 minutes or more and 90 minutes or less, and in order to achieve a width of 3.5 mm or more, the time required to reach the maximum temperature from 1770°C should be 60 minutes or more and 90 minutes or less.

[0044] 2.4.4 Holding time at maximum temperature The holding time at the maximum temperature should be long enough to densify the entire sintered body 5, and the optimum time can vary depending on the size of the sintered body, etc. When the maximum temperature is reached, the sintered body 5 has densified to a certain extent, and an increase or decrease in the holding time in this process is unlikely to cause a change in the internal color tone. Generally, the holding time is set to a minimum of 2 hours. On the other hand, if the holding time is too long, excessive nitrogen penetration will occur, so it is desirable to set the maximum to 16 hours. Furthermore, at temperatures above 1800°C, especially above 1830°C, some of the constituent materials will decompose on the surface of the sintered body 5. The decomposition products have a high L ＊ That is, the decomposed phase generated on the surface of the sintered body 5 has a high L ＊ However, if the maximum temperature exceeds 1870°C, the sublimation decomposition of silicon nitride may reach the interior of the sintered body, resulting in a significant deterioration of mechanical properties. As mentioned above, the maximum temperature during firing may be above 1770°C, and is preferably between 1800°C and 1870°C. ＊ If you want to form a phase with a high L value, the maximum temperature should be 1830°C or higher and 1870°C or lower. ＊ If the formation of a rare earth phase is not desired, the maximum temperature may be set to more than 1770°C and less than 1830°C.

[0045] It should be noted that although a dense silicon nitride sintered body can be obtained at a low temperature by using a crystalline material with a low melting point or a glassy material with a low softening point as the material for the grain boundary phase, in this case, the overall L of the sintered body may be increased. ＊ Therefore, it is necessary to select a material with a high melting point or softening point as the material for the grain boundary phase. That is, as mentioned above, it is advisable to select a material with a high melting point or softening point as the auxiliary material.

[0046] As described above, if the absolute pressure in the furnace is less than 0.3 MPa in the temperature range of 1700°C or higher, nitrogen penetration into the sintered body 5 will be insufficient, making it difficult to ensure the desired brightness difference in the final sintered body. In the manufacturing method disclosed herein, it is also possible to control the color tone by changing the nitrogen gas pressure depending on the temperature range within the pressure range of 0.3 MPa or higher but less than 1.1 MPa described above. On the other hand, if Ar is used as the atmospheric gas, nitrogen penetration into the sintered body 5 will not occur. In other words, at temperatures above 1700°C, where nitrogen penetration is required, it is difficult to use an atmospheric gas mainly composed of Ar. On the other hand, in the temperature range of 1700°C or lower, the gas type and pressure can be selected appropriately from the perspective of economy, etc., and gases other than nitrogen can also be used.

[0047] According to the manufacturing method of the present disclosure, a L ＊ It is possible to produce a silicon nitride sintered body having a difference in brightness of 5 or more in terms of brightness value. For example, it is possible to produce the following silicon nitride sintered body (1) or (2). (1) A silicon nitride sintered body having a dual color structure in cross section. Specifically, the L ＊ The value of L in at least a part deeper than the first depth ＊ (2) A silicon nitride sintered body having a triple-layered color tone in cross section. Specifically, the L ＊ The value of L ＊ is 5 or more times greater than the value of L in at least a portion between the surface and the first depth ＊ The value of L in at least a part deeper than the first depth ＊ is 5 or more times smaller than the value.

[0048] 5, the manufacturing method of the present disclosure may include performing a surface treatment on the sintered body 10 obtained after firing. For example, by mechanically polishing or grinding the surface of the sintered body 10, at least a portion of the surface layer of the sintered body 10 may be removed to expose the interior of the sintered body 10, and a first portion 11 and a second portion 12 having a desired lightness difference may be formed on the surface of the sintered body 10 after the surface layer has been removed.

[0049] 3. Supplementary Information: The method for producing a silicon nitride sintered body according to the present disclosure is industrially applicable and allows the production of large sintered bodies with a thickness or diameter of 10 mm or more and a long side or axial length of 100 mm or more. The silicon nitride sintered body according to the present disclosure has a brightness difference that is easily discernible visually. For example, if a surface exhibiting this brightness difference is used as a wear surface for a liner material, the position of the contrast shifts due to wear. In other words, the amount of wear can be visualized by the amount of contrast shift. More specifically, a ceramic part for a wear-resistant liner has, for example, a square shape with a wear surface that is subjected to wear, with sides of 100 mm or more and a thickness of 15 mm or more. By processing the ceramic part so that a predetermined brightness difference appears within this surface with sides of 100 mm or more, the amount of wear can be visually recognized as a change in the position of the contrast. Furthermore, the silicon nitride sintered body according to the present disclosure does not soften and maintains certain mechanical properties even at high temperatures exceeding 1500°C. In other words, it can also be used for parts used at high temperatures.

[0050] The present invention will be further described below with reference to examples, but the present invention is not limited to the following examples. The present invention allows various conditions to be adopted as long as the object is achieved without departing from the gist of the present invention.

[0051] 1. Example 1 1.1 Kneading and Granulation of Raw Materials A binder and a dispersant were added to raw materials consisting of silicon oxide (3.3% by mass), ytterbium oxide (6% by mass), and the remainder being commercially available silicon nitride powder (purity: 98.84%). The mixture was mixed in a ceramic liner with a ceramic ball using pure water as a solvent. The resulting slurry was spray-dried to obtain granules.

[0052] 1.2 Molding The obtained granules were filled into a square mold of 150 mm x 150 mm and uniaxially pressed at 22 tons to obtain a uniaxially compacted product. The obtained uniaxially compacted product was sealed in a nylon pack and pressed at 1.4 t / cm ２ The mixture was subjected to isostatic pressing to prepare a green body.

[0053] 1.3 Debinding and Firing The obtained green body was debound at 350°C in an air atmosphere and then subjected to the following firing. Firing was performed by pressurizing nitrogen gas with a purity of 99.99% and controlling the absolute pressure in the furnace at 0.7 MPa. The maximum temperature during firing was 1820°C, and the time required for the temperature rise from 1700°C to 1770°C was 100 minutes, and the time required for the temperature rise from 1770°C to 1820°C was 70 minutes, with the temperature rising at a constant rate for each. After reaching the maximum temperature of 1820°C, the maximum temperature was maintained for 4 hours and then cooled, yielding a sintered body according to Example 1.

[0054] 1.4 Evaluation of Sintered Body 1.4.1 Visual Observation The obtained sintered body was a plate-shaped body having a square surface with sides of 120 mm and a thickness of 24 mm. The sintered body was cut near the center of the side to cut out a cross-sectional test piece with a width of 5 mm. When the color tone of the cross section of the sintered body was visually observed, the surface of the sintered body and a region extending up to about 0.5 mm in the direction perpendicular to the surface appeared grayish-white, with a dark gray region close to black about 5 mm thick inside, and the color from the inside to the center was grayish-white. That is, as shown in Figure 6, the color tone had a triple-layered structure with clear contrast.

[0055] 1.4.2 Color Tone Measurement The color tone of the cross section was measured. The color tone meter used for the measurement was a C50μ manufactured by Suga Test Instruments Co., Ltd., and was measured under the following conditions: illumination: 45° bidirectional, light receiving: 0°, light source: D65, measurement area: φ0.2 mm. ＊ a ＊ b ＊ The color tone was measured first at D = 0 (surface (burnt surface)), where D mm is the depth from the surface, then at D = 0.7 mm, and then measured at depths increasing by 0.5 mm each time up to the center of the thickness. The measurement results are shown in Figure 7.

[0056] As shown in FIG. 7, the L ＊ The value is 47, and at the next measurement point (D = 0.7 mm), L ＊ The value decreased to 41. From there, the value increased to the position D = 5.7 mm in the depth direction. ＊ The value is in the range of 41 to 40, and further, between D = 5.7 mm and D = 6.2 mm, L ＊ The value rises to 47, and the value increases to the position of D = 12.2 mm in the center in the deeper direction. ＊ 7, the sintered body according to Example 1 had a lightness of 47 to 48 when measured from the surface of the sintered body to the inside in the cross section. ＊ The sintered body according to Example 1 had a lightness difference of less than 5 in terms of the lightness value and contained two or more uniform lightness regions each having a width of 2 mm or more. ＊ The brightness difference in terms of brightness value was less than 5, and the specimen contained two or more uniform brightness regions each having a width of 2 mm or more. The width of the uniform brightness regions was less than 0.7 mm for the outermost phase (gray-white phase), 5 mm or more and less than 6.2 mm for the phase near the surface (dark-gray phase), and 11.6 mm or more and less than 12.6 mm for the inner phase (gray-white phase).

[0057] 1.4.3 Analysis of Crystalline Phase and Density The crystal phase of the unprocessed surface of the sintered body was analyzed by X-ray diffraction, and it was found to be silicon nitride and Yb ２ Si ３ O ３ N ４ Two types of crystals were confirmed: Yb ２ Si ３ O ３ N ４ is a crystal formed by the sublimation decomposition of silicon nitride and grain boundary phase, and has high L ＊ The sintered body was cut near the center of one side to cut out a 5 mm wide evaluation test piece. The crystalline phase was confirmed by X-ray diffraction at the center of the cross section of this test piece and at a position 2 mm from the surface toward the center. At each position, silicon nitride, Si ２ N ２ O, Yb２ Si ２ O ７ , and a slight peak of SiC was confirmed. ２ Si ３ O ３ N ４ , Si ２ N ２ O and Yb ２ Si ２ O ７ constituted the grain boundary phase of the sintered body. On the other hand, no broad reflection indicating the presence of a glass phase was observed. Quantifying each crystal within the sintered body using a calibration curve measured in advance revealed that silicon nitride was 89.5 mass %, Si ２ N ２ O 2.6 mass%, Yb ２ Si ２ O ７ The calculated values ​​were 7.7 mass% of SiO2 and 0.2 mass% (2000 ppm) of SiC. The SiC was presumably mixed in during the kneading of the raw materials. The theoretical density of the crystal was calculated from the above composition ratio and was found to be 3.30 g / cm3. ３ A 1 mm thick portion was removed from the baked surface of the cross-sectional test piece by grinding, and the density was measured, resulting in 3.10 g / cm ３ The relative density of the sintered body was calculated and found to be 94%.

[0058] 1.4.4 Analysis of Impurities To evaluate the impurities contained in the sintered body, a portion of the sintered body was crushed in an agate mortar, pressure-decomposed with mixed acid, and then subjected to ICP mass spectrometry. The results were Al 841 ppm, Fe 247 ppm, Co 2 ppm, and Ni 13 ppm, totaling 1103 ppm. These impurities are thought to be mainly derived from the silicon nitride raw material.

[0059] 2. Example 2 2.1 Kneading of Raw Materials A binder and a dispersant were added to raw materials consisting of silicon oxide (6% by mass), magnesium oxide (2.5% by mass), yttrium oxide (1% by mass), and the remainder being commercially available silicon nitride powder (purity: 98.84%). The mixture was mixed in a ceramic liner with a ceramic ball using pure water as a solvent. The resulting slurry was spray-dried to obtain granules.

[0060] The obtained granules were filled into a square mold of 150 mm × 150 mm and uniaxially pressed at 22 tons to obtain a uniaxially compacted product. The obtained uniaxially compacted product was sealed in a nylon pack and pressed at 1.4 t / cm ２ The mixture was subjected to isostatic pressing to prepare a green body.

[0061] 2.3 Debinding and Firing The obtained green body was debound at 350°C in an air atmosphere and then subjected to the following firing. Firing was performed by pressurizing nitrogen gas with a purity of 99.99% and controlling the absolute pressure in the furnace at 0.5 MPa. The maximum temperature during firing was 1800°C. The temperature was increased from 1700°C to 1750°C at a constant rate over 25 minutes, held at 1750°C for 50 minutes, then increased from 1750°C to 1770°C at a constant rate over 10 minutes, and then increased from 1770°C to 1800°C at a constant rate over 50 minutes. After reaching the maximum temperature of 1800°C, the temperature was held at this maximum temperature for 4 hours and then cooled to obtain a sintered body according to Example 2.

[0062] 2.4 Evaluation of sintered body 2.4.1 Visual observation The obtained sintered body was a plate-shaped body having a square surface with a side length of 121 mm and a thickness of 25 mm. The sintered body was cut near the center of the side to cut out a cross-sectional test piece with a width of 5 mm. When the color tone of the cross section of the sintered body was visually observed, the region from the surface of the sintered body to a distance of 6 mm in the direction perpendicular to the surface appeared dark gray, and the area from the inside to the center was grayish white. That is, as shown in Figure 8, the color tone had a two-layer structure and the contrast was clear.

[0063] 2.4.2 Color Tone Measurement The color tone of the cross section was measured. The color tone was measured using a color meter C50μ manufactured by Suga Test Instruments Co., Ltd., under the conditions of illumination: 45° bidirectional, light reception: 0°, light source: D65, measurement area: φ0.2 mm. ＊ a ＊ b ＊ The color tone was measured first at D = 0 (surface (burnt surface)), where D mm is the depth from the surface, then at D = 0.7 mm, and then measured at increasing depths of 0.5 mm each time up to the center of the thickness. The measurement results are shown in Figure 9.

[0064] As shown in FIG. 9, the L ＊ The value is 42, and the depth is up to the position D = 5.7 mm. ＊ The value is in the range of 37 to 42, and further, between D = 5.7 mm and D = 6.2 mm, L ＊ The value rises to 51, and in the deeper direction, the value increases to the center position of D = 12.7 mm. ＊ 9, the sintered body according to Example 2 had a lightness of 49 to 51 when measured from the surface of the sintered body to the inside in the cross section. ＊ The sintered body according to Example 2 had a lightness difference of less than 5 in terms of the lightness value and contained two or more uniform lightness regions each having a width of 2 mm or more. ＊ The brightness difference in the value was less than 5, and the specimen contained two or more uniform brightness regions each having a width of 2 mm or more. The width of the uniform brightness region was 5.7 mm or more and less than 6.2 mm for the phase near the surface (dark gray phase), and 12.6 mm or more and less than 13.6 mm for the internal phase (grayish white phase).

[0065] 2.4.3 Analysis of Crystalline Phase and Density Analysis of the crystalline phase of the unprocessed surface of the sintered body by X-ray diffraction revealed that it was composed of silicon nitride and Si. ２ N ２ A broad reflection indicating the presence of a glass phase was confirmed along with two types of crystals, silicon nitride and Si. The sintered body was cut near the center of one side to cut out a 5 mm wide evaluation specimen. The crystalline phase was confirmed by X-ray diffraction at the center of the cross section of this specimen and at a position 2 mm from the surface toward the center. At both positions, silicon nitride and Si ２ N ２ Broad reflections were observed, indicating the presence of a glass phase, along with two types of crystals with Si. ２ N ２ O constituted the grain boundary phase of the sintered body. On the other hand, no peak of SiC was confirmed. As a result of quantifying each crystal inside the sintered body from a calibration curve measured in advance, it was found that silicon nitride was 90.5 mass %, Si ２ N ２The calculated values ​​were O 5% by mass and the remaining glass phase 4.5% by mass. A 1 mm thick portion was removed from the fired surface of the cross-sectional test piece by grinding, and the density was measured. ３ Since the sintered body was not permeated with water, the relative density was estimated to be 93% or more.

[0066] 2.4.4 Analysis of Impurities To evaluate the impurities contained in the sintered body, a portion of the sintered body was crushed in an agate mortar, pressure-decomposed with mixed acid, and then subjected to ICP mass spectrometry. The results were Al 741 ppm, Fe 304 ppm, Co 1 ppm, and Ni 10 ppm, totaling 1056 ppm. These impurities are thought to be mainly derived from the silicon nitride raw material.

[0067] 3. Example 3 3.1 Kneading of Raw Materials A binder and a dispersant were added to raw materials consisting of silicon oxide (2% by mass), yttrium oxide (3% by mass), and the remainder being commercially available high-purity silicon nitride powder (purity: 98.96%), and the mixture was mixed in a ceramic liner with a ceramic ball using pure water as a solvent. The resulting slurry was spray-dried to obtain granules.

[0068] The obtained granules were filled into a square mold of 150 mm × 150 mm and uniaxially pressed at 22 tons to obtain a uniaxially compacted product. The obtained uniaxially compacted product was sealed in a nylon pack and pressed at 1.4 t / cm ２ The mixture was subjected to isostatic pressing to prepare a green body.

[0069] 3.3 Debinding and Firing The obtained green body was debound at 350°C in an air atmosphere and then subjected to the following firing. Firing was performed by pressurizing nitrogen gas with a purity of 99.99% and controlling the absolute pressure in the furnace at 0.4 MPa. The maximum temperature during firing was 1830°C, and the time required for the temperature rise from 1700°C to 1770°C was 80 minutes, and the time required for the temperature rise from 1770°C to 1830°C was 65 minutes, with the temperature rising at a constant rate for each. After reaching the maximum temperature of 1830°C, the maximum temperature was maintained for 4 hours and then cooled, yielding a sintered body according to Example 3.

[0070] 3.4 Evaluation of the sintered body 3.4.1 Visual observation The obtained sintered body was a plate-shaped body with a square surface measuring 120.5 mm on a side and a thickness of 26.5 mm. The sintered body was cut near the center of the side to obtain a cross-sectional test piece with a width of 5 mm. When the color tone of the cross section of the sintered body was visually observed, the surface of the sintered body and the area extending up to about 0.9 mm in the direction perpendicular to the surface appeared almost white, with a dark gray area about 4 mm thick inside that, and a grayish-white color from the inside to the center. That is, as shown in Figure 10, the color tone had a triple-layered structure with clear contrast.

[0071] 3.4.2 Color Tone Measurement The color tone of the cross section was measured. The color tone meter used for the measurement was a C50μ manufactured by Suga Test Instruments Co., Ltd., and was measured under the following conditions: illumination: 45° bidirectional, light receiving: 0°, light source: D65, measurement area: φ0.2 mm. ＊ a ＊ b ＊ The color tone was measured first at D = 0 (surface (burnt surface)), where D mm is the depth from the surface, then at D = 0.7 mm, and then measured at depths increasing by 0.5 mm each time up to the center of the thickness. The measurement results are shown in Figure 11.

[0072] As shown in FIG. 11, the L ＊ The value was 69, and at the next measurement point (D = 0.7 mm), ＊ The value was as high as 68. Between D = 0.7 mm and D = 1.2 mm, L ＊ The value drops to 52, and from there to the position D=4.2 mm in the depth direction, L ＊ The value is in the range of 49 to 52, and further, between D = 4.2 mm and D = 4.7 mm, L ＊ The value rises to 66, and in the deeper direction, the value increases to the position of D = 13.2 mm at the center. ＊ 11, the sintered body according to Example 3 had a lightness of 64 to 67 when measured from the surface of the sintered body to the inside in the cross section. ＊The sintered body according to Example 3 had a lightness difference of less than 5 in terms of the lightness value and contained two or more uniform lightness regions each having a width of 2 mm or more. ＊ The brightness difference in terms of brightness value was less than 5, and the specimen contained two or more uniform brightness regions each having a width of 2 mm or more. The width of the uniform brightness regions was 0.7 mm or more and less than 1.2 mm for the outermost phase (white phase), 4 mm or more and less than 5 mm for the phase near the surface (dark gray phase), and 17.1 mm or more and less than 18.1 mm for the inner phase (grayish white phase).

[0073] 3.4.3 Analysis of Crystalline Phase and Density The crystalline phase of the unprocessed surface of the sintered body was analyzed by X-ray diffraction, and it was found to be composed of silicon nitride and Y. ２ Si ３ O ３ N ４ Two types of crystals were confirmed. ２ Si ３ O ３ N ４ is a crystal formed by the sublimation decomposition of silicon nitride and grain boundary phase, and has high L ＊ The crystalline phase was confirmed by X-ray diffraction at the center of the cross section of the cross-sectional test piece and at a position 2 mm from the surface toward the center. ２ N ２ O and Y ２ Si ２ O ７ Three types of crystals were confirmed. ２ Si ３ O ３ N ４ , Si ２ N ２ O and Y ２ Si ２ O ７ constituted the grain boundary phase of the sintered body. On the other hand, no SiC peak or broad reflection indicating the presence of a glass phase was observed. Quantifying each crystal within the sintered body using a calibration curve measured in advance revealed that silicon nitride was 94.7 mass %, Si ２ N ２ O is 0.8 mass %, Y ２ Si２ O ７ The theoretical density of the crystal was calculated from this composition to be 3.21 g / cm ３ A 2 mm thick portion was removed from the baked surface of the cross-sectional test piece by grinding, and the density was measured, resulting in 3.11 g / cm ３ The relative density of the sintered body was calculated and found to be 97%.

[0074] 3.4.4 Analysis of Impurities To evaluate the impurities contained in the sintered body, a portion of the sintered body was crushed in an agate mortar, pressure-decomposed with mixed acid, and then subjected to ICP mass spectrometry. The results were Al 10 ppm, Fe 25 ppm, Co less than 0.1 ppm, and Ni 2 ppm, totaling approximately 37 ppm.

[0075] 4. Example 4 4.1 Processing of sintered body After taking a 5 mm cross-sectional test piece in Example 2, a sintered body of 121 mm × 57 mm × 25 mm thick was processed to form an area with a difference in brightness of color tone on the processed surface. The processing method is as follows.

[0076] Grinding was performed on a 121 mm x 57 mm surface. As shown in Figure 12, grinding was performed so that the normal to the machined surface upon completion of grinding would be inclined at 10° in a direction perpendicular to the longitudinal direction relative to the original surface. In other words, the amount of grinding was greater on one 121 mm side and less on the other side. Using this processing method, processing was terminated when the entire 121 mm x 57 mm sintered surface of the sintered body had been ground. The amount of grinding was 10.1 mm on one of the 121 mm sides and 0.1 mm on the other side. The dimensions of the completed ground surface were 121 mm x 58 mm.

[0077] 4.2 Visual Observation When the color tone of the resulting ground surface was visually observed, the area from the side with the least amount of grinding to the side with the most amount of grinding appeared dark gray up to about 35 mm, and from there toward the side with the most amount of grinding, a central area about 109 mm wide was grayish white. As shown in Figure 12, the color tone had a two-layer structure with clear contrast. The boundary between the two layers of color tone was parallel or perpendicular to the 121 mm side, and the dark gray area was distributed in a U-shape.

[0078] 4.3 Color Tone Measurement The color tone of the cross section was measured. The color tone meter used for the measurement was a C50μ manufactured by Suga Test Instruments Co., Ltd., and was measured under the following conditions: illumination: 45° bidirectional, light receiving: 0°, light source: D65, measurement area: φ0.2 mm. ＊ a ＊ b ＊ The color tone was measured at D = 28 mm, where D mm is the distance from the center of the 121 mm side on the side with the lesser amount of grinding, and then measured perpendicular to the 121 mm side, increasing the distance by 0.5 mm at a time, up to D = 40 mm, passing through the point where the color tone changes. The measurement results are shown in Figure 13.

[0079] As shown in FIG. 13, the L ＊ The value is in the range of 39 to 42, and the L ＊ The value was in the range of 49 to 51. The measurement point of D=33.5 mm was in the transition region of the color tone, and the L ＊ The value was 44. The width of the transition region was less than 1 mm.

[0080] 5. Comparative Example 1 5.1 Kneading of Raw Materials A binder and a dispersant were added to raw materials consisting of silicon oxide (1% by mass), aluminum oxide (12% by mass), aluminum nitride (5% by mass), and the remainder being commercially available silicon nitride powder (purity: 98.13%), and the mixture was mixed in a ceramic liner with a ceramic ball using pure water as a solvent. The resulting slurry was spray-dried to obtain granules.

[0081] The obtained granules were filled into a square mold of 150 mm × 150 mm and uniaxially pressed at 22 tons to obtain a uniaxially compacted product. The obtained uniaxially compacted product was sealed in a nylon pack and pressed at 1.4 t / cm ２ The mixture was subjected to isostatic pressing to prepare a green body.

[0082] 5.3 Degreasing and Firing The obtained green body was degreased at 350°C in an air atmosphere and then fired as follows. Firing was performed by pressurizing nitrogen gas with a purity of 99.99% and controlling the absolute pressure inside the furnace at 0.7 MPa. The maximum temperature during firing was 1800°C, and the time required for the temperature rise from 1700°C to 1770°C was 100 minutes, and the time required for the temperature rise from 1770°C to 1800°C was 50 minutes, with the temperature rising at a constant rate for each. After reaching the maximum temperature of 1800°C, the maximum temperature was maintained for 4 hours and then cooled, yielding a sintered body according to Comparative Example 1.

[0083] 5.4 Evaluation of sintered body 5.4.1 Visual observation The obtained sintered body was a plate-shaped body having a square surface with a side length of 121 mm and a thickness of 25 mm. The sintered body was cut near the center of the side to obtain a cross-sectional test piece with a width of 5 mm. When the color tone of the cross section of the sintered body was visually observed, it was found to be a nearly uniform dark gray color from the surface to the interior of the sintered body. In other words, as shown in Figure 14, no clear contrast or color differences depending on the region were observed.

[0084] 5.4.2 Color Tone Measurement The color tone of the cross section was measured. The color tone meter used for the measurement was a C50μ manufactured by Suga Test Instruments Co., Ltd., and was measured under the following conditions: illumination: 45° bidirectional, light receiving: 0°, light source: D65, measurement area: φ0.2 mm. ＊ a ＊ b ＊ The color tone was measured first at D = 0 (surface (burnt surface)), where D mm is the depth from the surface, then at D = 0.7 mm, and then measured at increasing depths of 0.5 mm each time up to the center of the thickness. The measurement results are shown in Figure 15.

[0085] As shown in FIG. 15, from the surface (burned surface of the sintered body) to the inside, ＊ The value is in the range of 32 to 35, and the L ＊ Although there was a slight tendency for the values ​​to increase, the brightness was uniform and low.

[0086] 5.4.3 Analysis of Crystalline Phase and Density When the crystalline phase of the unprocessed surface of the sintered body was analyzed by X-ray diffraction, broad peaks indicating the presence of silicon nitride and a small amount of glass phase were confirmed. No SiC peaks were observed. Furthermore, the crystalline phase was confirmed by X-ray diffraction at the center of the cross section of the cross-sectional test piece and at a position 2 mm from the surface toward the center. Only broad reflections indicating the presence of silicon nitride and a small amount of glass phase were confirmed at both positions. The crystals inside the sintered body were quantified using a calibration curve measured in advance, and the calculations showed that silicon nitride was 98.5% by mass and the glass phase was 1.5% by mass. The density of the sintered body was measured and found to be 3.00 g / cm. ３ Since the water absorption rate of the sintered body was zero, it was estimated that the relative density reached 94% or more.

[0087] 5.4.4 Analysis of Impurities To evaluate the impurities contained in the sintered body, a portion of the sintered body was crushed in an agate mortar, pressure-decomposed with mixed acid, and then subjected to ICP mass spectrometry. The results were Al 6614 ppm, Fe 367 ppm, Co less than 2 ppm, and Ni 15 ppm, totaling approximately 6998 ppm. Most of the Al was derived from the auxiliary raw materials, and the other elements are thought to be derived from impurities in the silicon nitride raw material.

[0088] 6. Comparative Example 2 6.1 Kneading of Raw Materials A binder and a dispersant were added to raw materials consisting of silicon oxide (5% by mass), magnesium oxide (3% by mass), yttrium oxide (1% by mass), and the remainder being commercially available high-purity silicon nitride powder (purity: 98.96%), and the mixture was mixed in a ceramic liner with a ceramic ball using pure water as a solvent. The resulting slurry was spray-dried to obtain granules.

[0089] The obtained granules were filled into a square mold of 150 mm × 150 mm and uniaxially pressed at 22 tons to obtain a uniaxially compacted product. The obtained uniaxially compacted product was sealed in a nylon pack and pressed at 1.4 t / cm ２ The mixture was subjected to isostatic pressing to prepare a green body.

[0090] 6.3 Degreasing and sintering The obtained green body was degreased at 350°C in an air atmosphere, and then sintered as follows. The sintering was carried out at atmospheric pressure without pressurizing the furnace, with nitrogen gas of 99.99% purity flowing through the furnace. The maximum temperature during sintering was 1650°C, and the sintered body was held at this maximum temperature for 4 hours and then cooled to obtain a sintered body according to Comparative Example 2.

[0091] 6.4 Evaluation of Sintered Body 6.4.1 Visual Observation The obtained sintered body was a plate-shaped body having a square surface with sides of 120.5 mm and a thickness of 25 mm. The sintered body was cut near the center of the side to cut out a cross-sectional test piece with a width of 5 mm. When the color tone of the cross section of the sintered body was visually observed, it was found to be dark gray overall from the surface to the interior of the sintered body. That is, as shown in Figure 16, no clear contrast or color differences depending on the region were observed. However, although there was no clear contrast, a gradual gradation was observed near the surface, with the color becoming darker toward the surface.

[0092] 6.4.2 Color Tone Measurement The color tone of the cross section was measured. The color tone meter used for the measurement was a C50μ manufactured by Suga Test Instruments Co., Ltd., and was measured under the following conditions: illumination: 45° bidirectional, light reception: 0°, light source: D65, measurement area: φ0.2 mm. ＊ a ＊ b ＊ The color tone was measured first at D = 0 (surface (burnt surface)), where D mm is the depth from the surface, then at D = 0.7 mm, and then measured at increasing depths of 0.5 mm each time up to the center of the thickness. The measurement results are shown in Figure 17.

[0093] As shown in FIG. 17, the L ＊ The L* value gradually increased from 42 to 46. After that, the L* value was in the range of 45 to 46 up to the center. ＊ The values ​​were in the range of 42 to 46, and no sudden change in color tone was observed.

[0094] 6.4.3 Analysis of Crystalline Phase and Density When the crystalline phase of the unprocessed surface of the sintered body was analyzed by X-ray diffraction, broad peaks indicating the presence of silicon nitride and glass phases were confirmed. No SiC peaks were observed. In addition, the crystalline phase was confirmed by X-ray diffraction at the center of the cross section of the cross-sectional test piece and at a position 2 mm from the surface toward the center. At both positions, only broad reflections indicating the presence of silicon nitride and glass phases were confirmed.

[0095] 6.4.4 Analysis of Impurities To evaluate the impurities contained in the sintered body, a portion of the sintered body was crushed in an agate mortar, pressure-decomposed with mixed acid, and then subjected to ICP mass spectrometry. The results were Al 189 ppm, Fe 167 ppm, Co 1 ppm, and Ni 11 ppm, totaling 368 ppm.

[0096] 7. Summary The results of Examples 1 to 3 and Comparative Examples 1 and 2 are summarized in Table 1 below.

[0097]

[0098] REFERENCE SIGNS LIST 1 silicon nitride powder 2 powder of auxiliary raw material 3 granulated body 4 molded body 5 body to be sintered 10 silicon nitride sintered body 11 first part 12 second part

Claims

1. A steel sheet having a plurality of crystal grains and a grain boundary phase that bonds the crystal grains together, ＊ A silicon nitride sintered body having a brightness difference of 5 or more.

2. L ＊ 2. The silicon nitride sintered body according to claim 1, having a brightness difference of 10 or more in terms of brightness value.

3. A silicon nitride sintered body according to claim 1 or 2, wherein the brightness difference is present on the surface of the sintered body.

4. A silicon nitride sintered body according to any one of claims 1 to 3, which has the above-mentioned brightness difference in a cross section perpendicular to the surface of the sintered body.

5. The silicon nitride sintered body according to claim 4, wherein in a cross section perpendicular to the surface of the sintered body, the brightness difference is within a range of 1.0 mm or less in a direction perpendicular to the surface.

6. The silicon nitride sintered body according to claim 4, wherein in a cross section perpendicular to the surface of the sintered body, the brightness difference is within a range of 0.5 mm or less in a direction perpendicular to the surface.

7. When measuring the brightness from the surface of the sintered body to the inside of the cross section of the sintered body, ＊ 7. The silicon nitride sintered body according to claim 1, comprising a portion in which a brightness difference in value is less than 5 and two or more uniform brightness regions each having a width of 2 mm or more are observed.

8. L 55 or more ＊ The silicon nitride sintered body according to any one of claims 1 to 7, comprising a portion having a value.

9. The silicon nitride sintered body according to any one of claims 1 to 8, wherein the total content of Al, Fe, Co and Ni is less than 2000 ppm, and the content of SiC is less than 5000 ppm.

10. The silicon nitride sintered body according to any one of claims 1 to 9, wherein the Al content is less than 1000 ppm.

11. The silicon nitride sintered body according to any one of claims 1 to 10, wherein the grain boundary phase contains a crystalline component.

12. The grain boundary phase contains, as the crystalline component, Si ２ N ２ O, Y ２ S ２ O ７ , Yb ２ S ２ O ７ , Y ２ S ３ O ３ N ４ and YbSi ３ O ３ N ４ The silicon nitride sintered body according to claim 11, comprising at least one selected from the group consisting of:

13. A method for producing a silicon nitride sintered body according to any one of claims 1 to 12, comprising: kneading silicon nitride powder with powder of an auxiliary raw material to obtain granules; and molding and firing the granules, wherein the maximum temperature during the firing is above 1770°C; the atmosphere at 1700°C or higher during the heating process during the firing contains 99.9 volume % or more of nitrogen gas and has an absolute pressure of 0.3 MPa or more and less than 1.1 MPa; the time during the heating process during the firing from 1700°C to 1770°C is 40 minutes or more and 120 minutes or less; the time during the heating process during the firing from 1770°C to said maximum temperature is 30 minutes or more and 90 minutes or less; and the holding time at said maximum temperature during the firing is 16 hours or less.

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