JP7682660B2 - Silicon nitride sintered body and cutting insert - Google Patents

Description

This disclosure relates to silicon nitride sintered bodies and cutting inserts.

Silicon nitride sintered bodies have excellent mechanical strength, heat resistance, and chemical stability at high temperatures, and are used in cutting tools and the like. In recent years, in tools made of silicon nitride sintered bodies, an oxidation-resistant film (coating layer) has been formed on the surface of the silicon nitride sintered body (base material) to suppress reactions with the work material. Furthermore, there is a demand for tools to be applied to work materials that are difficult to machine and for highly efficient machining, so there is a demand for further improvements in tool performance. To achieve this, there is a demand for improved adhesion between the base material and the coating layer.

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 includes a base-side layer made of AlON that covers the surface of the base body, a first intermediate internal layer made of TiC that covers the surface of the base-side layer, a second intermediate internal layer made of TiCN that covers the surface of the first intermediate internal layer, and an outermost layer made of TiN that covers the surface of the second intermediate internal layer. By providing an internal layer with an intermediate thermal expansion coefficient between the base-side layer and the outermost layer, even when a large force is applied between adjacent layers due to a large temperature change during manufacture or use, the residual stress is relaxed and internal distortion is less likely to occur. As a result, the tool of Patent Document 1 is less likely to peel off during manufacture or use.

Japanese Patent Application Publication No. 10-212183

For example, there is a demand for tools that can withstand use in harsh environments, such as when the temperature during cutting is high, and there is a demand for further improvement in the adhesion between the substrate and the coating layer.
The present disclosure has been made in view of the above-mentioned circumstances, and has an object to provide a silicon nitride sintered body and a cutting insert which improve the adhesion between a substrate and a coating layer. The present disclosure can be realized in the following form.

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

[2] The silicon nitride sintered body according to [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 [1] or [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 [1] to [3], in which B/A is 0.05 or more and 0.35 or less, where A is the thickness of the coating layer and B is the thickness of the inner layer.

[5] A cutting insert made of a silicon nitride sintered body according to any one of [1] to [4].

In the silicon nitride sintered body of the present disclosure, the total peak heights of the specific element contained in the inner layer (at least one element contained in the substrate and selected from the group of specific elements consisting of Y (yttrium), La (lanthanum), Ce (cerium), Er (erbium), Dy (dysprosium), Yb (ytterbium) and Mg (magnesium)) is 12.0% or more and 50.0% or less, so that stress caused by the difference in thermal expansion coefficient between the substrate and the coating layer is easily alleviated, and adhesion between the substrate 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 substrate and the coating layer is improved, and the wear resistance of the silicon nitride sintered body is also improved.
When the thickness of the coating layer is 1.5 μm or more and 7.0 μm or less, the wear resistance of the silicon nitride sintered body can be improved while the chipping 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, if B/A is 0.05 or more and 0.35 or less, the adhesion between the substrate and the coating layer is improved and the wear resistance of the silicon nitride sintered body is improved.
A cutting insert formed from the silicon nitride sintered body of the present disclosure can improve adhesion between the substrate and the coating layer.

FIG. 2 is a schematic diagram showing a cross-sectional SEM image of a silicon nitride sintered body. FIG. 2 is a perspective view showing a cutting tool. 1 is a cross-sectional SEM image of a silicon nitride sintered body of Example 5.

The following provides a more detailed explanation. In this specification, when a numerical range is described using "to" it is intended to include both the lower limit and the upper limit unless otherwise specified. For example, "10 to 20" is intended to include both the lower limit of "10" and the upper limit of "20". In other words, "10 to 20" has the same meaning as "10 or more and 20 or less".

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

The base material 3 is mainly composed of _Si3N4 or _SiAlON . The base material 3 contains a specific element which 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).

The substrate 3 contains specific elements in a total amount of 3.0% by mass or more and 15.0% by mass or less in oxide equivalent, when the entire substrate 3 is taken as 100% by mass. The specific elements contained in the substrate 3 are 3.0% by mass or more in oxide equivalent, preferably 5.0% by mass or more, and more preferably 7.0% by mass or more, in terms of forming an inner layer 7 described later on the silicon nitride sintered body 1 to improve the adhesion between the substrate 3 and the coating layer 5, and obtaining a sintered body without reducing sinterability. The specific elements contained in the substrate 3 are 15.0% by mass or less in oxide equivalent, preferably 13.0% by mass or less, and more preferably 11.0% by mass or less, in terms of suppressing excessive generation of the inner layer 7 and improving the wear resistance of the silicon nitride sintered body 1. From these viewpoints, the base material 3 contains a total of 3.0% by mass or more and 15.0% by mass or less of the specific elements, calculated as oxides, preferably 5.0% by mass or more and 13.0% by mass or less, and more preferably 7.0% by mass or more and 11.0% by mass or less.

The coating layer 5 has an inner layer 7 and an outer layer 9. The inner layer 7 is in contact with the substrate 3. The outer layer 9 is formed on the outside of the inner layer 7.

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 chipping 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.

The outer layer 9 has a "portion in contact with the inner layer 7". FIG. 1 shows an example in which the outer layer 9 is a multi-layered structure. 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" of the outer layer 9. The lower layer 9A contains Al ₂ O ₃ or AlON. The upper layer 9B is formed on the outside of the lower layer 9A. The upper layer 9B contains at least one selected from the group consisting of, for example, TiN, TiCN, and TiC. The upper layer 9B may be omitted.

The thermal expansion coefficients of _Al2O3 and AlON contained in the lower layer _9A are closer to that of the inner layer 7 described below than those of TiN, TiCN and TiC contained in the upper layer 9B. Therefore, the inner layer 7 and the outer layer 9 are less likely to peel off from each other even if the temperature of the silicon nitride sintered body 1 changes, compared to the case where the upper layer 9B is directly formed on the inner layer 7.

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 peak heights of all the contained metal elements and Si (silicon) is 100%, the peak height of the contained Al (aluminum) is preferably 6.0% or more, preferably 10.0% or more, and more preferably 14.0% or more. In this way, the chemical affinity with the lower layer 9A containing Al ₂ O ₃ or AlON is improved, and therefore the adhesion is improved. In addition, the peak height of Al (aluminum) is preferably 30.0% or less, preferably 26.0% or less, and more preferably 22.0% or less. In this way, the wear resistance is improved. From these viewpoints, the peak height of Al (aluminum) contained in the inner layer 7 is preferably 6.0% to 30.0%, preferably 10.0% to 26.0%, and more preferably 14.0% to 22.0%. Furthermore, the inner layer 7 preferably contains an oxide or oxynitride of Si (silicon) or Al (aluminum). In this way, the chemical affinity between the inner layer 7 and the lower layer 9A is increased, improving adhesion.

As described later, the inner layer 7 is formed by diffusing specific elements in the substrate 3 and Al (aluminum) contained in the coating that is to become the outer layer 9 by performing high-temperature and high-pressure treatment under specific conditions after forming a coating that is to become the outer layer 9 on the surface of the substrate 3 mainly made of silicon nitride. 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 to when the outer layer 9 is formed directly on the substrate 3, the difference in thermal expansion coefficient between the substrate 3 and the coating layer 5 is smaller, so that the substrate 3 and the coating layer 5 are less likely to peel off even if the temperature of the silicon nitride sintered body 1 changes. In other words, the adhesion between the substrate 3 and the coating layer 5 is improved. In addition, the inner layer 7 is formed as a uniform layer on the surface of the substrate 3, and the composition amount of the specific elements and aluminum (Al) changes continuously from the surface on the substrate 3 side to the surface on the outer layer 9 side. Therefore, even if 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.

The thickness of the inner layer 7 is not particularly limited. From the viewpoint of fully exerting the function of relieving the stress caused 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 even more preferably 0.3 μm or more. From the viewpoint of improving abrasion resistance, the thickness of the inner layer 7 is preferably 1.0 μm or less, more preferably 0.9 μm or less, and even 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 even more preferably 0.3 μm or more and 0.8 μm or less.

When the thickness of the coating layer 5 is A and the thickness of the inner layer 7 is B, B/A is preferably 0.05 or more, more preferably 0.10 or more, and even more preferably 0.15 or more, from the viewpoint of fully exerting the function of relieving the stress caused by the difference in thermal expansion coefficient between the substrate 3 and the outer layer 9. 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 even 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 even more preferably 0.15 or more and 0.25 or less.

(2) Requirements for the ratio of specific elements in the inner layer 7 In the spectrum obtained by measurement using energy dispersive X-ray spectroscopy (EDS), when the total of the peak heights of all the contained metal elements and Si (silicon) is 100%, the total of the peak heights of the contained specific elements is 12.0% or more and 50.0% or less. The total of the peak heights of the specific elements is 12.0% or more, preferably 15.0% or more, and more preferably 20.0% or more, from the viewpoint of making the thermal expansion coefficient of the inner layer 7 a value between the thermal expansion coefficient of the substrate 3 and the thermal expansion coefficient of Al ₂ O ₃ or AlON of the lower layer 9A. The total of the peak heights of the specific elements is 50.0% or less, preferably 40.0% or less, and more preferably 30.0% or less, from the viewpoint of ensuring wear resistance. From these viewpoints, the total height 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.

2. Configuration of the Cutting Tool Fig. 2 shows an example of a cutting tool (cutting insert) 10 according to 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 finishing the shape and surface thereof by at least one of cutting, grinding, and polishing. Of course, the silicon nitride sintered body 1 may be used as the cutting tool 10 as it is.

3. Method for Producing Silicon Nitride Sintered Body There is no particular limitation on the method for producing the silicon nitride sintered body. An example of the method for producing the silicon nitride sintered body is shown below.
(1) Raw material: Silicon nitride powder (α-Si ₃ N ₄ powder)
Sintering aid: ytterbium oxide powder ( _{Yb2O3 powder} )
Sintering aid: yttrium oxide powder ( _{Y2O3 powder} )
Sintering aid: lanthanum oxide powder ( _{La2O3 powder} )
Sintering aid: cerium oxide powder ( _CeO2 powder)
Sintering aid: erbium oxide powder ( _{Er2O3 powder} )
Sintering aid: dysprosium _oxide powder ( _Dy2O3 powder)
Sintering aid: manganese oxide powder (MgO powder)
Sintering aid: zinc oxide powder ( _ZrO2 powder)
Sintering aid: Aluminum _oxide powder ( _Al2O3 powder)

(2) Preparation of powder for firing The above powders are weighed out to a predetermined mixing ratio. The weighed powders are placed in a ball mill, mixed and ground with alcohol (e.g., ethanol) and grinding media. The resulting slurry is passed through a sieve, and an organic binder dissolved in ethanol is added, followed by spray drying.

(3) Press Molding The resulting mixed powder is press molded.

(4) Firing The molded body is degreased in a heating device to obtain a degreased body, which is then heated in a nitrogen atmosphere to obtain a sintered body.

(5) Polishing and Coating The obtained sintered body is processed into the shape of a cutting tool (cutting insert) to form a substrate. A coating that will eventually become the outer layer is formed on the surface of the substrate by a CVD method. In this way, a surface-coated sintered body is obtained.

(6) Inner layer formation treatment The obtained surface-coated sintered body is subjected to hot isostatic pressing (HIP) treatment as described below. The surface-coated sintered body is subjected to a two-stage heating process, a first heating process and a second heating process which follows the first heating process. In the first heating process, the body is heated in a nitrogen atmosphere and then held at the heated temperature for a certain period of time. In the second heating process, the body is heated in a nitrogen atmosphere and then held at the heated temperature for a certain period of time. In this manner, a silicon nitride sintered body is produced.

The first heating step is believed to reduce the micropores in the substrate of the resulting silicon nitride sintered body, and promote the diffusion of components derived from the sintering aid into the substrate surface. The second heating step is believed to promote the formation of an inner layer.

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 a combination of the total amount of the specific element contained in the base material and the inner layer formation treatment conditions. Specifically, the inner layer tends to be thicker and the content of the specific element increases as the total amount of the specific element in the base material increases, and the inner layer tends to be thinner and the content of the specific element decreases as the total amount of the specific element in the base material decreases. The inner layer tends to be thicker as the pressure in the first heating step increases, and tends to be thinner as the pressure in the first heating step decreases. The inner layer tends to be thicker as the holding temperature and pressure in the second heating step increases and the holding time increases.

In the following experiments, silicon nitride sintered bodies were produced as in Experimental Examples 1 to 24, and these silicon nitride sintered bodies were processed to produce cutting tools as in Experimental Examples 1 to 24. Experimental Examples 1 to 18 are working examples, and Experimental Examples 19 to 24 are comparative examples.

1. Preparation of silicon nitride sintered body (1) Raw materials The following raw material powders were used for the silicon nitride sintered body of each experimental example shown in Table 1. Table 1 shows the configurations (substrate type, coating layer configuration, inner layer configuration, etc.) of the silicon nitride sintered body of experimental examples 1 to 24.
Silicon nitride powder (α-Si ₃ N ₄ powder), average particle size 1.0 μm or less Sintering aid: ytterbium oxide powder (Yb ₂ O ₃ powder), average particle size 1.0 μm or less Sintering aid: yttrium oxide powder (Y ₂ O ₃ powder), average particle size 1.0 μm or less Sintering aid: lanthanum oxide powder (La ₂ O ₃ powder), average particle size 1.0 μm or less Sintering aid: cerium oxide powder (CeO ₂ powder), average particle size 1.0 μm or less Sintering aid: erbium oxide powder (Er ₂ O ₃ powder), average particle size 1.0 μm or less Sintering aid: dysprosium oxide powder (Dy ₂ O ₃ powder), average particle size 1.0 μm or less Sintering aid: manganese oxide powder (MgO powder), average particle size 1.0 μm or less Sintering aid: zinc oxide powder (ZrO ₂ powder): average particle size 1.0 μm or less Sintering aid: aluminum oxide powder (Al ₂ O ₃ powder): average particle size 1.0 μm or less The sintering aid was selected and used according to the substrate type shown in Table 1 (details are given in Table 2). "RE oxide" in Table 2 means the oxide of the rare earth element RE. The element in parentheses on the right side of the "RE oxide" column means the rare earth element RE contained in the substrate type. For example, substrate type A shown in Table 2 contains 6.0 mass% of yttrium (Y) oxide (Y ₂ O ₃ ). "Remainder" in the "Si ₃ N ₄ " column means the value obtained by subtracting the compounding composition of "RE oxide", "MgO", "ZrO ₂ " and "Al ₂ O ₃ " from 100 mass%. For example, the base material type A shown in Table 2 contains 6.0 mass % of Y ₂ O ₃ , 5.5 mass % of Al ₂ O ₃ , and 88.5 mass % of Si ₃ N ₄ .

(2) Preparation of powder for firing The raw powder was placed in a ball mill with an inner wall made of silicon nitride and mixed with ethanol and grinding media. The raw powder was ground and mixed for 72 hours using grinding media (silicon nitride-based balls with a diameter of 2 mm to 10 mm) to prepare a mixture (slurry). The slurry was passed through a sieve with a mesh size of 250 μm, and an organic binder dissolved in ethanol was added to the mixture (slurry) at a ratio of 3.5% by mass relative to 100% by mass, followed by spray drying.

(3) Press Molding The obtained mixed powder was press molded into a tool shape conforming to ISO standard SNGN120412.

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

(5) Polishing, Coating The obtained sintered body was processed into the shape of a cutting tool of ISO standard tool shape SNGN120412 to be used as a substrate. A coating that would eventually become the outer layer was formed on the substrate surface by CVD. When the outer layer was a multi-layered layer, the corresponding coatings were laminated in sequence. The components of the outer layer are as shown in Table 1. The "upper layer" column in Table 1 indicates the components that make up the upper layer and the order of lamination of each component. The component on the left of the "upper layer" column is laminated first. For example, "TiC-TiCN-TiN" in Experimental Example 4 in Table 1 indicates that the upper layer contains TiC, TiCN, and TiN, and that TiC, TiCN, and TiN are laminated on the lower layer in this order. In other words, it indicates that after the lower layer is laminated, TiC is laminated, TiCN is laminated on top of that, and TiN is further laminated on top of that. Table 3 shows the film formation conditions (source gas composition, film formation temperature, and gas pressure) for each component. In Table 3, for example, the source gas composition of "TiN" is 3.4 vol% _TiCl4 gas, 43.1 vol% _N2 gas, and the remaining composition ratio (53.5 vol%) is _H2 gas.

(6) Inner Layer Formation Treatment The obtained surface-coated sintered body was subjected to hot isostatic pressing (HIP) treatment as described below. In the surface-coated sintered bodies of Experimental Examples 1 to 18, the first temperature-raising step was performed by raising the temperature to 1200°C at 20°C/min under a nitrogen atmosphere of 200 to 400 atm and maintaining the temperature for 30 minutes. Then, in the second temperature-raising step, the temperature was raised from 1400°C to 1500°C at 10°C/min and heat treatment was performed at 400 to 500 atm for 60 to 120 minutes. In this manner, 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 a substrate 13. The outer layer 19 includes a lower layer 19A and an upper layer 19B.

For example, in Experimental Example 2, the substrate type A was used, and after the first heating step, the substrate was held at 1200°C, 400 atm, and 30 minutes, and after the second heating step, the substrate was held at 1500°C, 500 atm, and 60 minutes. As a result, the thickness of the inner layer was 0.92 μm. In Experimental Example 6, the substrate type C was used, and after the first heating step, the substrate was held at 1200°C, 400 atm, and 30 minutes, and after the second heating step, the substrate was held at 1500°C, 500 atm, and 120 minutes. As a result, the thickness of the inner layer was 0.15 μm. In Experimental Example 15, the substrate type A was used, and after the first heating step, the substrate was held at 1200°C, 200 atm, and 30 minutes, and after the second heating step, the substrate was held at 1400°C, 400 atm, and 60 minutes. As a result, the thickness of the inner layer was 0.09 μm. In Experimental Example 18, substrate type A was used, and after the first heating step, the substrate was held at 1200°C, 200 atm, and 30 minutes, and after the second heating step, the substrate was held at 1500°C, 500 atm, and 120 minutes, resulting in an inner layer thickness of 1.10 μm. In Experimental Example 24, the substrate was heated to 1200°C at 20°C/min in a nitrogen atmosphere at 200 atm and held for 30 minutes in the first heating step. The substrate was then heated to 1500°C while maintaining the heating rate, and heat-treated at 300 atm for 60 minutes.

2. Analysis (1) Composition Analysis The silicon nitride sintered body obtained was cut in a direction perpendicular to the surface, and the cut surface was polished to prepare a mirror-polished surface including the substrate, inner layer, and outer layer. A scanning electron microscope (SEM-EDS) equipped with an energy dispersive X-ray spectrometer was used to quantitatively analyze the compositions of the substrate, inner layer, and outer layer, and to measure the thickness of the inner layer and the thickness of the coating layer. 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 substrate, inner layer, and outer layer, line analysis was performed in a range of 5 μm or more perpendicular to the interface between the substrate and the inner layer with a measurement interval of 0.05 μm or less, and a quantitative analysis of the composition at each measurement point was performed. As the measurement points, 5 arbitrary visual fields in a range of 12 μm x 9 μm including the substrate and coating layer were selected at the position of the cutting edge, the composition ratio of the inner layer at each measurement point was calculated, and the average value of these values was calculated. The composition of the substrate was identified by analyzing the amount of each element in each sintered body by known fluorescent X-ray, and calculating the mass ratio by regarding each element as a compound such as an oxide or nitride. For example, the mass ratio was calculated by taking the amount of Si as the amount of Si ₃ N ₄ , the amount of Mg as the amount of MgO, and the amount of Yb as the amount of Yb ₂ O _3. The outer layer was identified by the above-mentioned SEM-EDS and known X-ray diffraction analysis.

(2) Performance Evaluation After a 0.2 mm x 25° chamfering was performed on the cutting edge, cutting was performed under the following conditions.
Material: FC200 (block material), 200mm x 100mm x 100mm
Cutting speed: 1000m/min Feed speed: 0.1mm/rev
Cutting depth: 1.0 mm
Cutting conditions: dry machining The end face of a block-shaped workpiece was intermittently machined with a cutter diameter of φ80. On a 200 mm x 100 mm surface, tooling was performed with a cutting width of 50 mm, with one pass being a reciprocating motion on one surface, up to an upper limit of 20 passes. Then, the presence or absence of peeling of the coating layer after one pass machining and the amount of wear after 20 passes machining were measured. The results are shown in Table 1.

3. Evaluation results The test results are shown in Table 2 and will be discussed.
(1) Satisfaction status and evaluation of each requirement of the experimental examples Experimental examples 1 to 18, which are working 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 a specific element which is at least one element selected from the group consisting of Y (yttrium), La (lanthanum), Ce (cerium), Er (erbium), Dy (dysprosium), Yb (ytterbium), and Mg (magnesium).
Requirement (b): The base material contains specific elements in a total amount of 3.0% by mass or more and 15.0% by mass or less in terms of oxides.
Requirement (c): The coating layer has an inner layer in contact with the substrate, and an outer layer formed on the outside of 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 a spectrum obtained by measurement using energy dispersive X-ray spectroscopy (EDS), when the total height of the peaks of all contained metal elements and Si (silicon) is taken as 100%, the total height of the peaks of the contained specific elements is 12.0% or more and 50.0% or less.

In the working examples, Experimental Examples 1 to 18, there was no peeling of the coating layer after one pass in the cutting test. In the working examples, Experimental Examples 19 to 24, there was peeling of the coating layer after one pass in the cutting test. In Experimental Examples 1 to 18, by satisfying requirements (a) to (f), the inner layer contains aluminum (Al) and a certain amount of the same element as the specific element of the substrate. Therefore, it is believed that the stress caused by the difference in thermal expansion coefficient between the substrate and the coating layer that occurs when the cutting tool becomes hot is alleviated, and the adhesion between the substrate and the coating layer is improved.

Examples 1 to 14, which are working examples, satisfy the following requirement (g). Examples 15 to 18, which are working 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.

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 believed that Experimental Examples 1 to 14, which are examples, improved the wear resistance of the silicon nitride sintered body by satisfying requirement (g).

The experimental examples 4 and 17, which are examples of the present invention, satisfy the following requirement (h). The experimental examples 14 and 18, which are examples of the present invention, do not satisfy the requirement (h).
Requirement (h): The thickness of the coating layer is 1.5 μm or more and 7.0 μm or less.

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 believed that Experimental Example 4, which satisfies requirement (h), has improved wear resistance of the silicon nitride sintered body compared to Example 14, which does not satisfy requirement (h).

In the working example, Experimental Example 17, 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 believed that Experimental Example 17, which satisfies requirement (h), has improved wear resistance of the silicon nitride sintered body compared to Example 18, which does not satisfy requirement (h).

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

In Experimental Example 12, the amount of wear after 20 passes was 0.29 mm. In Experimental Example 13, the amount of wear after 20 passes was 0.35 mm. It is believed that Experimental Example 12, which satisfies requirement (i), has improved wear resistance compared to Example 13, which does not satisfy requirement (i).

In the working example, Experimental Example 16, the wear amount after 20 passes was 0.41 mm. In the working example, Experimental Example 17, the wear amount after 20 passes was 0.46 mm. It is believed that the working example, Experimental Example 16, which satisfies requirement (i), has improved wear resistance compared to Example 17, which does not satisfy requirement (i).

4. Effects of the Example In the silicon nitride sintered body of the present example, the stress caused by the difference in thermal expansion coefficient between the substrate and the coating layer was alleviated, and the adhesion between the substrate and the coating layer was improved. As a result, it was found that the tool life can be extended by suppressing peeling of the coating layer even in cutting processing where the cutting edge becomes hotter than before.

This disclosure is not limited to the embodiments detailed above, and various modifications and variations are possible.

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 (5)

A silicon nitride sintered body comprising a substrate mainly composed of silicon nitride or sialon and a coating layer coating the substrate,
The base material contains a specific element, which is at least one element selected from the group consisting of Y (yttrium), La (lanthanum), Ce (cerium), Er (erbium), Dy (dysprosium), Yb (ytterbium), and Mg (magnesium), in a total amount of 3.0% by mass or more and 15.0% by mass or less in terms of oxide,
The coating layer has an inner layer in contact with the base material and an outer layer formed on the outside of the inner layer,
the outer layer has a portion in contact with the inner layer that is made of _{Al2O3 or} AlON and is substantially free of Si and the specific element ;
The inner layer contains Al (aluminum), Si (silicon), and the specific element contained in the base material,
The inner layer is a silicon nitride sintered body, in which, when the sum of the heights of the peaks of all contained metal elements and Si (silicon) is taken as 100%, the sum of the heights of the peaks of the contained specific elements is 12.0% or more and 50.0% or less in a spectrum obtained by measurement using energy dispersive X-ray spectroscopy (EDS). 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. The silicon nitride sintered body according to claim 1 or 2, wherein the thickness of the coating layer is 1.5 μm or more and 7.0 μm or less. The silicon nitride sintered body according to any one of claims 1 to 3, wherein B/A is 0.05 or more and 0.35 or less, where A is the thickness of the coating layer and B is the thickness of the inner layer. A cutting insert made of a silicon nitride sintered body according to any one of claims 1 to 4.

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