Silicon nitride sintered body and cutting insert

Abstract

To provide a silicon nitride-based sintered body and a cutting insert that can enhance adhesion between a substrate and a coating layer.SOLUTION: A silicon nitride-based sintered body includes a substrate containing silicon nitride or sialon as a main component, and a coating layer covering the substrate. The coating layer includes an inner layer. In a spectrum obtained by measurement using energy dispersion type X-ray spectroscopy (EDS), with the total height of each peak of all metal elements contained in the inner layer and Si (silicon) as 100%, the total of peak height of the contained specific elements is 12.0% or more and 50.0% or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a silicon nitride sintered body and a cutting insert.

Background Art

[0002] Silicon nitride sintered bodies are excellent in mechanical strength, heat resistance, chemical stability at high temperatures, etc., and are used for cutting tools and the like. In recent years, in tools made of silicon nitride sintered bodies, in order to suppress the reaction with the workpiece, an oxidation-resistant film (coating layer) is formed on the surface of the silicon nitride sintered body (base material). And, due to the requirements for application to workpieces that are difficult to machine and for high-efficiency machining, further improvement of tool performance is demanded. For this purpose, improvement of the adhesion between the base material and the coating layer is demanded.

[0003] For example, the surface-coated silicon nitride tool of Patent Document 1 includes a base body mainly composed of silicon nitride and a coating layer that coats the surface of the base body. The coating layer has a base body-side layer made of AlON that covers the surface of the base body, an intermediate first inner layer made of TiC that covers the surface of the base body-side layer, an intermediate second inner layer made of TiCN that covers the surface of the intermediate first inner layer, and an outermost layer made of TiN that covers the surface of the intermediate second inner layer. By providing an inner layer with an intermediate coefficient of thermal expansion between the base body-side layer and the outermost layer, even when a large force is applied between adjacent layers due to a large temperature change during manufacturing or use, the residual stress is relaxed and internal strain is less likely to occur. As a result, the tool of Patent Document 1 is less likely to cause layer peeling during manufacturing or use.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] For example, there is a demand for tools that can withstand use in severe environments such as when the temperature during cutting is high, and further improvement in the adhesion between the base material and the coating layer is required. The present disclosure has been made in view of the above circumstances, and an object thereof is to provide a silicon nitride-based sintered body and a cutting insert that improve the adhesion between a base material and a coating layer. The present disclosure can be realized in the following forms.

Means for Solving the Problems

[0006] 〔1〕A silicon nitride-based sintered body including a base material mainly composed of silicon nitride or sialon, and a coating layer covering the base material, the base material contains, in total, at least one specific element selected from a specific element group consisting of Y (yttrium), La (lanthanum), Ce (cerium), Er (erbium), Dy (dysprosium), Yb (ytterbium), and Mg (magnesium) in an amount of 3.0 mass% or more and 15.0 mass% or less in terms of oxide conversion, the coating layer has an inner layer in contact with the base material and an outer layer formed outside the inner layer, the outer layer contains Al 2 O 3 or AlON, the inner layer contains Al (aluminum), Si (silicon), and the specific element contained in the base material, in the spectrum obtained by measurement using energy dispersive X-ray spectroscopy (EDS), the total height of the peaks of all the metal elements and Si (silicon) contained is set to 100%, and the total height of the peaks of the specific element contained is 12.0% or more and 50.0% or less, a silicon nitride-based sintered body.

[0007] 〔2〕The silicon nitride-based sintered body according to 〔1〕, wherein the thickness of the inner layer is 0.1 μm or more and 1.0 μm or less.

[0008] 〔3〕The silicon nitride-based sintered body according to 〔1〕 or 〔2〕, wherein the thickness of the coating layer is 1.5 μm or more and 7.0 μm or less.

[0009] 〔4〕When the thickness of the coating layer is A and the thickness of the inner layer is B, B / A is 0.05 or more and 0.35 or less. The silicon nitride sintered body according to any one of 〔1〕to 〔3〕.

[0010] 〔5〕A cutting insert composed of the silicon nitride sintered body according to any one of 〔1〕to 〔4〕.

Advantages of the Invention

[0011] In the silicon nitride sintered body of the present disclosure, since the total height of the peaks of the specific elements (at least one element selected from the group of specific elements contained in the base material and consisting of Y (yttrium), La (lanthanum), Ce (cerium), Er (erbium), Dy (dysprosium), Yb (ytterbium) and Mg (magnesium)) contained in the inner layer is 12.0% or more and 50.0% or less, the stress caused by the difference in the coefficient of thermal expansion between the base material and the coating layer is easily relaxed, and the adhesion between the base material and the coating layer can be improved. When the thickness of the inner layer is 0.1 μm or more and 1.0 μm or less, the adhesion between the base material and the coating layer is improved, and the wear resistance of the silicon nitride sintered body is improved. When the thickness of the coating layer is 1.5 μm or more and 7.0 μm or less, while improving the wear resistance of the silicon nitride sintered body, the defect resistance of the coating layer can be ensured. When the thickness of the coating layer is A and the thickness of the inner layer is B, and B / A is 0.05 or more and 0.35 or less, the adhesion between the base material and the coating layer is improved, and the wear resistance of the silicon nitride sintered body is improved. A cutting insert composed of the silicon nitride sintered body of the present disclosure can improve the adhesion between the base material and the coating layer.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0013] The following will be described in more detail. In this specification, in the description of a numerical range using "~", unless otherwise specified, the lower limit value and the upper limit value are included. For example, in the description of "10~20", both the lower limit value "10" and the upper limit value "20" are included. That is, "10~20" has the same meaning as "10 or more and 20 or less".

[0014] 1. Silicon nitride-based sintered body 1 (1) Structure of silicon nitride-based sintered body 1 FIG. 1 shows an example of the silicon nitride-based sintered body 1 of the present disclosure. As shown in FIG. 1, the silicon nitride-based sintered body 1 includes a base material 3 and a coating layer 5 that coats the base material 3. However, FIG. 1 is a diagram schematically showing a cross-sectional SEM image of the silicon nitride-based sintered body 1 obtained by SEM (Scanning Electron Microscope), and does not accurately show the actual cross-sectional SEM image.

[0015] The base material 3 is mainly composed of Si 3 N 4 or SiAlON. The base material 3 contains a specific element that is at least one element selected from a specific element group consisting of Y (yttrium), La (lanthanum), Ce (cerium), Er (erbium), Dy (dysprosium), Yb (ytterbium), and Mg (magnesium).

[0016] When the total amount of the base material 3 is 100% by mass, the base material 3 contains a total of 3.0% by mass or more and 15.0% by mass or less of specific elements in terms of oxide conversion. The specific elements contained in the base material 3 improve the adhesion between the base material 3 and the coating layer 5 by forming the inner layer 7 described below on the silicon nitride sintered body 1, and from the viewpoint of obtaining a sintered body without reducing the sinterability, the total is 3.0% by mass or more in terms of oxide conversion, preferably 5.0% by mass or more, and more preferably 7.0% by mass or more. The specific elements contained in the base material 3 are, from the viewpoint of suppressing the excessive generation of the inner layer 7 and improving the wear resistance of the silicon nitride sintered body 1, 15.0% by mass or less in total in terms of oxide conversion, preferably 13.0% by mass or less, and more preferably 11.0% by mass or less. From these viewpoints, the base material 3 preferably contains a total of 3.0% by mass or more and 15.0% by mass or less of specific elements, more preferably 5.0% by mass or more and 13.0% by mass or less, and even more preferably 7.0% by mass or more and 11.0% by mass or less in terms of oxide conversion.

[0017] The coating layer 5 has an inner layer 7 and an outer layer 9. The inner layer 7 is in contact with the base material 3. The outer layer 9 is formed outside the inner layer 7.

[0018] The thickness of the coating layer 5 is not particularly limited. From the viewpoint of improving the wear resistance of the silicon nitride sintered body 1, the thickness of the coating layer 5 is preferably 1.5 μm or more, more preferably 2.0 μm or more, and even more preferably 3.0 μm or more. From the viewpoint of ensuring the defect resistance of the coating layer 5, the thickness of the coating layer 5 is preferably 7.0 μm or less, more preferably 6.0 μm or less, and even more preferably 5.0 μm or less. From these viewpoints, the thickness of the coating layer 5 is preferably 1.5 μm or more and 7.0 μm or less, more preferably 2.0 μm or more and 6.0 μm or less, and even more preferably 3.0 μm or more and 5.0 μm or less.

[0019] The outer layer 9 has a "portion in contact with the inner layer 7". In FIG. 1, an example in which the outer layer 9 is a multilayer is shown. The outer layer 9 has a lower layer 9A and an upper layer 9B. The lower layer 9A is in contact with the inner layer 7. The lower layer 9A corresponds to the "portion in contact with the inner layer 7" in the outer layer 9. The lower layer 9A is Al 2 O 3Or it contains AlON. The upper layer 9B is formed outside the lower layer 9A. The upper layer 9B contains at least one selected from the group consisting of, for example, TiN, TiCN, and TiC. Note that the upper layer 9B may be omitted.

[0020] Al contained in the lower layer 9A 2 O 3 And the thermal expansion coefficients of AlON are closer to the thermal expansion coefficient of the inner layer 7 described later than the thermal expansion coefficients of TiN, TiCN, and TiC contained in the upper layer 9B. Therefore, compared with the case where the upper layer 9B is directly formed on the inner layer 7, even when the temperature of the silicon nitride sintered body 1 changes, the inner layer 7 and the outer layer 9 are less likely to peel off.

[0021] The inner layer 7 contains Al (aluminum), Si (silicon), and a specific element contained in the base material 3. In the spectrum obtained by measurement using energy dispersive X-ray spectroscopy (EDS), when the sum of the heights of all the metal elements and the peaks of Si (silicon) contained is 100%, the height of the peak of Al (aluminum) contained is preferably 6.0% or more, preferably 10.0% or more, and more preferably 14.0% or more. By doing so, the chemical affinity with the lower layer 9A containing Al 2 O 3 Or AlON is improved, and the adhesion is improved. Also, the height of the peak of Al (aluminum) is preferably 30.0% or less, preferably 26.0% or less, and more preferably 22.0% or less. By doing so, the wear resistance is improved. From these viewpoints, the height of the peak of Al (aluminum) contained in the inner layer 7 is preferably 6.0% or more and 30.0% or less, preferably 10.0% or more and 26.0% or less, and more preferably 14.0% or more and 22.0% or less. Furthermore, the inner layer 7 preferably contains oxides or oxynitrides of Si (silicon) or Al (aluminum). By doing so, the chemical affinity between the inner layer 7 and the lower layer 9A becomes high and the adhesion is improved.

[0022] As will be described later, the inner layer 7 is formed by performing a high-temperature and high-pressure treatment under specific conditions after forming a film to be the outer layer 9 on the surface of the silicon nitride-based substrate 3, and is generated by the diffusion of specific elements in the substrate 3 and Al (aluminum) contained in the film to be the outer layer 9. The inner layer 7 thus formed has a thermal expansion coefficient intermediate between that of the substrate 3 and the outer layer 9. Therefore, compared with the case where the outer layer 9 is directly formed on the substrate 3, the difference in thermal expansion coefficient between the substrate 3 and the coating layer 5 becomes smaller, so that even when the temperature of the silicon nitride sintered body 1 changes, the substrate 3 and the coating layer 5 are less likely to peel off. That is, the adhesion between the substrate 3 and the coating layer 5 is improved. Further, the inner layer 7 is uniformly formed as a layer on the surface of the substrate 3, and the compositional amounts of the specific elements and aluminum (Al) continuously change from the surface on the substrate 3 side to the surface on the outer layer 9 side. For this reason, even when the temperature of the silicon nitride sintered body 1 changes, stress is less likely to occur inside the inner layer 7, and as a result, the adhesion between the substrate 3 and the coating layer 5 is improved.

[0023] The thickness of the inner layer 7 is not particularly limited. From the viewpoint of sufficiently exerting the function of relaxing the stress generated by the difference in thermal expansion coefficient between the substrate 3 and the outer layer 9, the thickness of the inner layer 7 is preferably 0.1 μm or more, more preferably 0.2 μm or more, and still more preferably 0.3 μm or more. From the viewpoint of improving wear resistance, the thickness of the inner layer 7 is preferably 1.0 μm or less, more preferably 0.9 μm or less, and still more preferably 0.8 μm or less. From these viewpoints, the thickness of the inner layer 7 is preferably 0.1 μm or more and 1.0 μm or less, more preferably 0.2 μm or more and 0.9 μm or less, and still more preferably 0.3 μm or more and 0.8 μm or less.

[0024] When the thickness of the coating layer 5 is A and the thickness of the inner layer 7 is B, from the viewpoint of sufficiently exerting the function of relaxing the stress caused by the difference in the coefficient of thermal expansion between the base material 3 and the outer layer 9, B / A is preferably 0.05 or more, more preferably 0.10 or more, and still more preferably 0.15 or more. From the viewpoint of improving the wear resistance of the silicon nitride sintered body 1, B / A is preferably 0.35 or less, more preferably 0.30 or less, and still more preferably 0.25 or less. From these viewpoints, B / A is preferably 0.05 or more and 0.35 or less, more preferably 0.10 or more and 0.30 or less, and still more preferably 0.15 or more and 0.25 or less.

[0025] (2) Requirements for the ratio of specific elements in the inner layer 7 In the inner layer 7, in the spectrum obtained by measurement using energy dispersive X-ray spectroscopy (EDS), when the sum of the heights of the peaks of all the contained metal elements and Si (silicon) is 100%, the sum of the heights of the peaks of the contained specific elements is 12.0% or more and 50.0% or less. The sum of the heights of the peaks of the specific elements is 12.0% or more from the viewpoint of making the coefficient of thermal expansion of the inner layer 7 a value between the coefficient of thermal expansion of the base material 3 and that of Al 2 O 3 or AlON, and preferably 15.0% or more, and more preferably 20.0% or more. The sum of the heights of the peaks of the specific elements is 50.0% or less from the viewpoint of ensuring wear resistance, preferably 40.0% or less, and more preferably 30.0% or less. From these viewpoints, the sum of the heights of the peaks of the specific elements is 12.0% or more and 50.0% or less, preferably 15.0% or more and 40.0% or less, and more preferably 20.0% or more and 30.0% or less.

[0026] 2. Configuration of the cutting tool FIG. 2 shows an example of a cutting tool (cutting insert) 10 of the present disclosure. The shape of the cutting tool 10 is not particularly limited. For example, the silicon nitride sintered body 1 can be made into the cutting tool 10 by performing shaping and surface finishing by at least one of the processing methods of cutting, grinding, and polishing. Of course, the silicon nitride sintered body 1 may be used as the cutting tool 10 as it is.

[0027] 3. Method for manufacturing silicon nitride sintered body The method for manufacturing a silicon nitride sintered body is not particularly limited. An example of the method for manufacturing a silicon nitride sintered body is shown below. (1) Raw materials · Silicon nitride powder (α-Si 3 N 4 powder) · Sintering aid: ytterbium oxide powder (Yb 2 O 3 powder) · Sintering aid: yttrium oxide powder (Y 2 O 3 powder) · Sintering aid: lanthanum oxide powder (La 2 O 3 powder) · Sintering aid: cerium oxide powder (CeO 2 powder) · Sintering aid: erbium oxide powder (Er 2 O 3 powder) · Sintering aid: dysprosium oxide powder (Dy 2 O 3 powder) · Sintering aid: magnesium oxide powder (MgO powder) · Sintering aid: zirconium oxide powder (ZrO 2 powder) · Sintering aid: aluminum oxide powder (Al 2 O 3 powder)

[0028] (2) Preparation of powder for sintering Weigh the above powders so that they have a predetermined mixing ratio. Put the weighed powders into a ball mill and mix and grind them together with alcohol (for example, ethanol) and grinding media. Pass the obtained slurry through a sieve, add an organic binder dissolved in ethanol, and spray dry it.

[0029] (3) Press molding Press mold the obtained mixed powder.

[0030] (4) Sintering The green compact is degreased in a heating device to obtain a degreased body. The degreased body is heated in a nitrogen atmosphere to obtain a sintered body.

[0031] (5) Grinding, Coating The obtained sintered body is processed into the shape of a cutting tool (cutting insert) to serve as a base material. A coating that will finally become the outer layer is formed on the surface of the base material by the CVD method. In this way, a surface-coated sintered body is obtained.

[0032] (6) Inner layer formation treatment The obtained surface-coated sintered body is subjected to hot isostatic pressing (HIP) treatment as follows. The surface-coated sintered body is subjected to two-stage heating, namely, a first heating step and a second heating step that follows the first heating step. In the first heating step, after heating in a nitrogen atmosphere, it is held at the heated temperature for a certain period of time. In the second heating step, after heating in a nitrogen atmosphere, it is held at the heated temperature for a certain period of time. In this way, a silicon nitride-based sintered body is produced.

[0033] It is presumed that the obtained silicon nitride-based sintered body reduces the micropores of the base material by the first heating step and promotes the diffusion of components derived from the sintering aid to the surface layer portion of the base material. And it is presumed that the formation of the inner layer is promoted by the second heating step.

[0034] The composition of the inner layer is adjusted by the heating rate and pressure in the first heating step and the second heating step. The film thickness and composition of the inner layer are controlled by the total amount of specific elements contained in the base material and the combination of inner layer formation treatment conditions. Specifically, the inner layer tends to be thicker as the total amount of specific elements in the base material is larger, and the content of specific elements tends to be larger. The inner layer tends to be thinner as the total amount of specific elements in the base material is smaller, and the content of specific elements tends to be smaller. The inner layer tends to be Thickness thicker as the pressure during the first heating step is higher, and tends to be thinner as the pressure of the first heating step is lower. The inner layer tends to be thicker as the holding temperature and pressure of the second heating step are higher and the holding time is longer.

Example

[0035] In the following experiments, silicon nitride sintered compacts of Experimental Examples 1 to 24 were produced, and these silicon nitride sintered compacts were processed to obtain cutting tools of Experimental Examples 1 to 24. Experimental Examples 1 to 18 are examples, and Experimental Examples 19 to 24 are comparative examples.

[0036] 1. Production of silicon nitride sintered compact (1) Raw materials The raw material powders used for the silicon nitride sintered compacts of each experimental example shown in Table 1 were as follows. Table 1 shows the composition of the silicon nitride sintered compacts of Experimental Examples 1 to 24 (substrate type, coating layer composition, inner layer composition, etc.). · Silicon nitride powder (α-Si 3 N 4 powder), average particle size of 1.0 μm or less · Sintering aid: ytterbium oxide powder (Yb 2 O 3 powder): average particle size of 1.0 μm or less · Sintering aid: yttrium oxide powder (Y 2 O 3 powder): average particle size of 1.0 μm or less · Sintering aid: lanthanum oxide powder (La 2 O 3 powder): average particle size of 1.0 μm or less · Sintering aid: cerium oxide powder (CeO 2 powder): average particle size of 1.0 μm or less · Sintering aid: erbium oxide powder (Er 2 O 3 powder): average particle size of 1.0 μm or less · Sintering aid: dysprosium oxide powder (Dy 2 O 3 powder): average particle size of 1.0 μm or less · Sintering aid: magnesium oxide powder (MgO powder): average particle size of 1.0 μm or less · Sintering aid: zirconium oxide powder (ZrO 2 powder): average particle size of 1.0 μm or less · Sintering aid: aluminum oxide powder (Al 2 O 3 powder): average particle size of 1.0 μm or less Regarding the sintering aid, it was selected and used according to the base material types shown in Table 1 (detailed in Table 2). The "RE oxide" in Table 2 means the oxide of rare earth element RE. The element in the right parentheses in the column of "RE oxide" means the rare earth element RE contained in the base material type. For example, in the base material type A shown in Table 2, the oxide of yttrium (Y 2 O 3 ) is contained at 6.0 mass%. The "balance" in the column of "Si 3 N 4 " means the value obtained by subtracting the compounding compositions of "RE oxide", "MgO", "ZrO 2 ", and "Al 2 O 3 " from 100 mass%. For example, in the base material type A shown in Table 2, Y 2 O 3 is 6.0 mass%, Al 2 O 3 is 5.5 mass%, and Si 3 N 4 is contained at 88.5 mass%.

[0037]

Table 1

[0038]

Table 2

[0039] (2) Preparation of powder for sintering The raw material powder was put into a ball mill having an inner wall made of silicon nitride, and mixed with ethanol and grinding media. Grinding media (silicon nitride-based balls with a diameter of φ2 mm to φ10 mm) were used, and the mixture was ground and mixed for 72 hours to prepare a mixture (slurry). The slurry was passed through a sieve with a mesh opening of 250 μm, and an organic binder dissolved in ethanol was added so as to be 3.5 mass% based on 100 mass% of the mixture (slurry), and then spray-dried.

[0040] (3) Press molding The obtained mixed powder was press-molded into the shape of a tool of ISO standard SNGN120412.

[0041] (4) Sintering The green compact was degreased at 600 °C for 60 minutes in a nitrogen atmosphere of 1 atm in a heating device. The degreased body was heated at a temperature of 1700 - 1900 °C for 60 - 180 minutes in a nitrogen atmosphere of 1 - 6 atm to obtain a sintered body.

[0042] (5) Grinding, Coating The obtained sintered body was processed into the shape of a cutting tool of ISO standard SNGN120412 to serve as a substrate. A coating that would ultimately form the outer layer was formed on the surface of the substrate by the CVD method. When the outer layer was a multi-layer, the corresponding coatings were sequentially laminated. The components of the outer layer are as shown in Table 1. The column of "upper layer" in Table 1 shows the components constituting the upper layer and the order of lamination of each component. The component on the left in the column of "upper layer" was laminated first. For example, "TiC - TiCN - TiN" in Experimental Example 4 of Table 1 indicates that the upper layer contains TiC, TiCN, and TiN, and they are laminated on the lower layer in the order of TiC, TiCN, and TiN. That is, it indicates that TiC was laminated after the lower layer was laminated, TiCN was laminated on top of it, and further TiN was laminated on top of that. Table 3 shows the film formation conditions (raw material gas composition, film formation temperature, and gas pressure) of each component. In Table 3, for example, the raw material gas composition of "TiN" is that TiCl 4 gas is 3.4 vol%, N 2 gas is 43.1 vol%, and H 2 gas is the remaining composition ratio (53.5 vol%).

[0043]

Table 3

[0044] (6) Inner layer formation treatment The obtained surface-coated sintered body was subjected to hot isostatic pressing (HIP) treatment as follows. In the surface-coated sintered bodies of Experimental Examples 1 to 18, as the first heating step, the temperature was raised to 1200 °C at a rate of 20 °C / min in a nitrogen atmosphere of 200 to 400 atmospheres and held at that temperature for 30 minutes. Then, as the second heating step, the temperature was raised from 1400 °C to 1500 °C at a rate of 10 °C / min, and heat treatment was performed at 400 to 500 atmospheres for 60 to 120 minutes. In this way, a silicon nitride sintered body was produced. An example of the produced silicon nitride sintered body is shown in Fig. 3. Fig. 3 is a cross-sectional SEM image of the silicon nitride sintered body 11 of Example 5. A coating layer 15 (inner layer 17 and outer layer 19) is formed on the base material 13. The outer layer 19 includes a lower layer 19A and an upper layer 19B.

[0045] For example, in Experimental Example 2, base material type A was used. After the first heating step, it was held at 1200 °C, 400 atmospheres, and for 30 minutes. After the second heating step, it was held at 1500 °C, 500 atmospheres, and for 60 minutes. As a result, the thickness of the inner layer was 0.92 μm. In Experimental Example 6, base material type C was used. After the first heating step, it was held at 1200 °C, 400 atmospheres, and for 30 minutes. After the second heating step, it was held at 1500 °C, 500 atmospheres, and for 120 minutes. As a result, the thickness of the inner layer was 0.15 μm. In Experimental Example 15, base material type A was used. After the first heating step, it was held at 1200 °C, 200 atmospheres, and for 30 minutes. After the second heating step, it was held at 1400 °C, 400 atmospheres, and for 60 minutes. As a result, the thickness of the inner layer was 0.09 μm. In Experimental Example 18, base material type A was used. After the first heating step, it was held at 1200 °C, 200 atmospheres, and for 30 minutes. After the second heating step, it was held at 1500 °C, 500 atmospheres, and for 120 minutes. As a result, the thickness of the inner layer was 1.10 μm. In Experimental Example 24, as the first heating step, the temperature was raised to 1200 °C at a rate of 20 °C / min in a nitrogen atmosphere of 200 atmospheres and held for 30 minutes. Then, while maintaining the heating rate, the temperature was raised to 1500 °C, and heat treatment was performed at 300 atmospheres for 60 minutes.

[0046] 2. Analysis (1) Composition analysis The obtained silicon nitride sintered body was cut in a direction perpendicular to the surface layer, and the cut surface was polished to produce a mirror-polished surface including a base material, an inner layer, and an outer layer. With respect to the produced mirror-polished surface, using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (SEM-EDS), quantitative analysis of each composition in the base material, the inner layer, and the outer layer, measurement of the thickness of the inner layer, and measurement of the thickness of the coating layer were performed. The EDS measurement conditions were an acceleration voltage of 15 kV and an acceleration current of 13 μA. In the cross-sectional structure including the base material, the inner layer, and the outer layer, line analysis was performed at intervals of 0.05 μm or less in a range of 5 μm or more in a direction perpendicular to the interface between the base material and the inner layer, and quantitative analysis of the composition at each measurement point was performed. The measurement locations were selected at the position of the cutting edge, and five arbitrary fields of view in a range of 12 μm × 9 μm including the base material and the coating layer were selected. The composition ratio of the inner layer at each measurement location was calculated, and the average value of these values was calculated. The composition of the base material was identified by analyzing the amount of each element in each sintered body using well-known fluorescent X-rays and calculating the mass ratio by considering each element as a compound such as an oxide or a nitride. For example, the amount of Si was regarded as the amount of Si 3 N 4 , the amount of Mg was regarded as the amount of MgO, and the amount of Yb was regarded as the amount of Yb 2 O 3 to calculate the mass ratio. The outer layer was identified by the above-mentioned SEM-EDS and well-known X-ray diffraction analysis.

[0047] (2) Performance evaluation After chamfering the cutting edge at the position of the cutting edge to a 0.2 mm × 25° chamfered cutting edge, cutting was performed under the following conditions. Workpiece material: FC200 (block material), 200 mm × 100 mm × 100 mm Cutting speed: 1000 m / min Feed rate: 0.1 mm / rev Depth of cut: 1.0 mm Cutting conditions: Dry machining Intermittent machining was performed on the end face of a block-shaped workpiece with a cutter diameter of φ80. In a 200 mm × 100 mm surface, one-way turning with a cutting width of 50 mm for one surface was defined as one pass, and machining was carried out up to a maximum of 20 passes. Then, the presence or absence of coating layer peeling after one-pass machining and the wear amount after 20-pass machining were measured. The results are shown in Table 1.

[0048] 3. Evaluation Results The test results are also listed in Table 2 and will be examined herein. (1) Satisfaction Status and Evaluation of Each Requirement in Experimental Examples Experimental Examples 1 to 18, which are examples, satisfy the following requirements (a) to (f). Experimental Example 19, which is a comparative example, does not satisfy requirements (b) to (f). Experimental Example 20, which is a comparative example, does not satisfy requirements (b) and (f). Experimental Examples 21 to 23, which are comparative examples, do not satisfy requirement (d). Experimental Example 24, which is a comparative example, does not satisfy requirement (f). · Requirement (a): The base material contains at least one specific element selected from a specific element group consisting of Y (yttrium), La (lanthanum), Ce (cerium), Er (erbium), Dy (dysprosium), Yb (ytterbium), and Mg (magnesium). · Requirement (b): The base material contains the specific element in a total amount of 3.0 mass% or more and 15.0 mass% or less in terms of oxide conversion. · Requirement (c): The coating layer has an inner layer that contacts the base material and an outer layer formed outside the inner layer. · Requirement (d): The outer layer contains Al 2 O 3 or AlON in the portion in contact with the inner layer. · Requirement (e): The inner layer contains Al (aluminum), Si (silicon), and the specific element contained in the base material. · Requirement (f): In the spectrum obtained by measurement using energy-dispersive X-ray spectroscopy (EDS), when the total height of the peaks of all the metal elements and Si (silicon) contained is taken as 100%, the total height of the peaks of the specific element contained is 12.0% or more and 50.0% or less.

[0049] In Experimental Examples 1 to 18 which are examples, there was no peeling of the coating layer after 1 pass in the cutting test. In Experimental Examples 19 to 24 which are examples, there was peeling of the coating layer after 1 pass in the cutting test. Experimental Examples 1 to 18 satisfy requirements (a) to (f), so that the inner layer contains aluminum (Al) and contains a certain amount of the same elements as the specific elements of the base material. Therefore, it is considered that the stress caused by the difference in the coefficient of thermal expansion between the base material and the coating layer that occurs when the cutting tool becomes hot is relaxed, and the adhesion between the base material and the coating layer is improved.

[0050] Experimental Examples 1 to 14 which are examples satisfy the following requirement (g). Experimental Examples 15 to 18 which are examples do not satisfy requirement (g). · Requirement (g): The thickness of the inner layer is 0.1 μm or more and 1.0 μm or less.

[0051] In Experimental Examples 1 to 14, the wear amount after 20 passes was 0.24 mm to 0.38 mm. In Experimental Examples 15 to 18, the wear amount after 20 passes was 0.41 mm to 0.49 mm. It is considered that Experimental Examples 1 to 14 which are examples improved the wear resistance of the silicon nitride sintered body by satisfying requirement (g).

[0052] Experimental Examples 4 and 17 which are examples satisfy the following requirement (h). Experimental Examples 14 and 18 which are examples do not satisfy requirement (h). · Requirement (h): The thickness of the coating layer is 1.5 μm or more and 7.0 μm or less.

[0053] In Experimental Example 4, the wear amount after 20 passes was 0.29 mm. In Experimental Example 14, the wear amount after 20 passes was 0.38 mm. It is considered that Experimental Example 4 improved the wear resistance of the silicon nitride sintered body by satisfying requirement (h) compared to Experimental Example 14 which does not satisfy requirement (h).

[0054] In Experimental Example 17, which is an example, the wear amount after 20 passes was 0.42 mm. In Experimental Example 18, the wear amount after 20 passes was 0.49 mm. It is considered that Experimental Example 17, by satisfying requirement (h), had improved wear resistance of the silicon nitride sintered body compared to Example 18 that did not satisfy requirement (h).

[0055] Experimental Examples 12 and 16, which are examples, satisfy the following requirement (i). Experimental Examples 13 and 17 do not satisfy requirement (i). · Requirement (i): When the thickness of the coating layer is A and the thickness of the inner layer is B, B / A is 0.05 or more and 0.35 or less.

[0056] In Experimental Example 12, the wear amount after 20 passes was 0.29 mm. In Experimental Example 13, the wear amount after 20 passes was 0.35 mm. It is considered that Experimental Example 12, by satisfying requirement (i), had improved wear resistance compared to Example 13 that did not satisfy requirement (i).

[0057] In Experimental Example 16, which is an example, the wear amount after 20 passes was 0.41 mm. In Experimental Example 17, which is an example, the wear amount after 20 passes was 0.46 mm. It is considered that Experimental Example 16, by satisfying requirement (i), had improved wear resistance compared to Example 17 that did not satisfy requirement (i).

[0058] 4. Effects of the Examples In the silicon nitride sintered body of this example, the stress caused by the difference in thermal expansion coefficient between the base material and the coating layer was relaxed, and the adhesion between the base material and the coating layer could be improved. As a result, it was found that the tool life could be extended by suppressing the peeling of the coating layer even in cutting processes where the cutting edge becomes high temperature, more so than before.

[0059] This disclosure is not limited to the embodiments detailed above, and various modifications or changes are possible.

Explanation of Reference Numerals

[0060] 1, 11: Silicon nitride sintered body 3, 13: Base material 5, 15: Coating layer 7, 17: Inner layer 9, 19: Outer layer 9A, 19A: Lower layer 9B, 19B: Upper layer 10: Cutting tool (cutting insert)

Claims

1. A silicon nitride sintered body comprising a substrate mainly composed of silicon nitride or sialon, and a coating layer covering the substrate, wherein: the substrate contains, in total, 3.0% by mass or more and 15.0% by mass or less in terms of oxide of at least one specific element selected from a specific element group consisting of Y (yttrium), La (lanthanum), Ce (cerium), Er (erbium), Dy (dysprosium), Yb (ytterbium), and Mg (magnesium); the coating layer has an inner layer in contact with the substrate and an outer layer formed outside the inner layer; The outer layer has a portion in contact with the inner layer made of Al 2 O 3 or AlON, substantially free of Si and the specific element, the inner layer contains Al (aluminum), Si (silicon), and the specific element contained in the substrate; in the spectrum obtained by measurement using energy dispersive X-ray spectroscopy (EDS), when the total height of the peaks of all metal elements and Si (silicon) contained is taken as 100%, the total height of the peaks of the specific element contained is 12.0% or more and 50.0% or less, a silicon nitride sintered body.

2. The silicon nitride sintered body according to claim 1, wherein the thickness of the inner layer is 0.1 μm or more and 1.0 μm or less.

3. The silicon nitride sintered body according to claim 1 or claim 2, wherein the thickness of the coating layer is 1.5 μm or more and 7.0 μm or less.

4. The silicon nitride sintered body according to any one of claims 1 to 3, wherein when the thickness of the coating layer is A and the thickness of the inner layer is B, B / A is 0.05 or more and 0.35 or less.

5. A cutting insert composed of the silicon nitride sintered body according to any one of claims 1 to 4.

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