cutting tools

Abstract

The cutting tool includes a substrate and a coating disposed on the substrate, the coating including a first layer, the first layer being made of α-Al2O3, and having a thickness of 2 μm or more and 15 μm or less, and in a cross section taken along a normal to an interface between the substrate and the coating, the first layer includes a plurality of grain boundaries connecting an interface of the first layer close to the substrate and an interface of the first layer close to a surface of the first layer or a surface of the coating, and at least one of the grain boundaries, the first grain boundary, has a plurality of straight line segments and three or more bending points connecting two adjacent straight line segments, and in the first grain boundary, a front edge of the first layer close to the substrate is bent. a crossing angle A1 at the bend point closest to the interface and a crossing angle A2 next closest to the bend point are each 120° or more and less than 180°; at the first grain boundary, a crossing angle A4 at the bend point closest to the interface close to the surface of the first layer or the surface of the coating of the first layer and a crossing angle A3 next closest to the bend point are each 90° or more and 150° or less; at the first grain boundary, an average X1 of the crossing angles A1 and A2 and an average X2 of the crossing angles A3 and A4 satisfy the relationship of Formula 1; and a ratio N2 / N1 of the number N2 of the first grain boundaries to the number N1 of the grain boundaries is 0.2 or more.

Description

[Technical Field]

[0001] The present disclosure relates to cutting tools. [Background technology]

[0002] Conventionally, cutting tools have been used for cutting processes, which include a substrate and a coating disposed on the substrate, where the coating includes a first layer made of α-Al2O3 (Patent Document 1, Non-Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2015 / 113866 [Non-patent literature]

[0004] [Non-Patent Document 1] S. Ruppi et al.,“Influence of Process Conditions on the Growth and Texture of CVD Alpha-Alumina” Coatings 2020,10,158 Summary of the Invention

[0005] The cutting tool of the present disclosure comprises: 1. A cutting tool comprising a substrate and a coating disposed on the substrate, The coating comprises a first layer; the first layer is made of α-Al2O3, The thickness of the first layer is 2 μm or more and 15 μm or less, In a cross section taken along a normal to the interface between the substrate and the coating, the first layer includes a plurality of grain boundaries connecting an interface of the first layer close to the substrate and an interface of the first layer close to the surface of the first layer or a surface of the coating, and at least one of the grain boundaries, a first grain boundary, has a plurality of straight line segments and three or more bending points connecting two adjacent straight line segments; In the first grain boundary, a crossing angle A1 at the bending point closest to the interface close to the substrate of the first layer and a crossing angle A2 next closest to the bending point are each 120° or more and less than 180°, In the first grain boundary, a crossing angle A4 at the bending point closest to the surface of the first layer or the interface close to the surface of the coating of the first layer, and a crossing angle A3 next closest to the bending point are each 90° or more and 150° or less, In the first grain boundary, an average X1 of the crossing angles A1 and A2 and an average X2 of the crossing angles A3 and A4 satisfy the relationship of Formula 1, a ratio N2 / N1 of the number N2 of the first grain boundaries to the number N1 of the grain boundaries is 0.2 or more. X1-X2≧10° Formula 1 [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 1 is a schematic cross-sectional view illustrating one embodiment of a cutting tool according to the present disclosure. [Figure 2] FIG. 2 is an enlarged view of region II in FIG. [Figure 3] FIG. 3 is a schematic cross-sectional view of an example of a CVD (Chemical Vapor Deposition) apparatus used in manufacturing the cutting tool of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0007] [Problem to be solved by this disclosure] In recent years, there has been an increasing demand for improved tool life, particularly in face milling. Key factors for improving tool life in face milling include "wear resistance" and "chipping resistance." To improve "chipping resistance," a cutting tool has been developed that includes a substrate and a coating disposed on the substrate. The coating includes a first layer made of α-Al2O3 and having a thickness of 2 μm to 15 μm. In a cross section normal to the interface between the substrate and the coating, the first layer includes multiple grain boundaries connecting an interface of the first layer close to the substrate with a surface of the first layer or an interface of the first layer close to the surface of the coating. At least one of the grain boundaries, the first grain boundary, has multiple straight line segments and three or more bending points connecting two adjacent straight line segments. However, in such cutting tools, the nucleation of strongly (001)-oriented α-Al2O3 in the first layer tends to be unstable, making it difficult to achieve a strong orientation in the first layer. As a result, it is sometimes difficult to impart excellent "wear resistance" to the first layer. Therefore, there is a demand for cutting tools that combine excellent "wear resistance" and excellent "chipping resistance" to provide excellent tool life.

[0008] Therefore, an object of the present disclosure is to provide a cutting tool that has an excellent tool life, particularly in face milling.

[0009] [Effects of this disclosure] According to the present disclosure, it is possible to provide a cutting tool having an excellent tool life, particularly in face milling.

[0010] [Description of the embodiments of the present disclosure] First, embodiments of the present disclosure will be listed and described. (1) The cutting tool of the present disclosure is 1. A cutting tool comprising a substrate and a coating disposed on the substrate, the coating comprises a first layer; the first layer is made of α-Al2O3, The thickness of the first layer is 2 μm or more and 15 μm or less, In a cross section taken along a normal to the interface between the substrate and the coating, the first layer includes a plurality of grain boundaries connecting an interface of the first layer close to the substrate and an interface of the first layer close to a surface of the first layer or a surface of the coating, and at least one of the grain boundaries, a first grain boundary, has a plurality of straight line segments and three or more bending points connecting two adjacent straight line segments; In the first grain boundary, a crossing angle A1 at the bending point closest to the interface close to the substrate of the first layer and a crossing angle A2 next closest to the bending point are each 120° or more and less than 180°, In the first grain boundary, a crossing angle A4 at the bending point closest to the surface of the first layer or the interface close to the surface of the coating of the first layer, and a crossing angle A3 next closest to the bending point are each 90° or more and 150° or less, In the first grain boundary, an average X1 of the crossing angles A1 and A2 and an average X2 of the crossing angles A3 and A4 satisfy the relationship of Formula 1, The ratio N2 / N1 of the number N2 of the first grain boundaries to the number N1 of the grain boundaries is 0.2 or more. X1-X2≧10° Formula 1

[0011] According to the present disclosure, a cutting tool having an excellent tool life can be provided, particularly in face milling.

[0012] (2) In the above (1), the average distance D1 between two adjacent bending points in the first grain boundary along the normal to the interface between the substrate and the coating may be 0.05 μm or more and 4 μm or less, thereby providing a cutting tool with a longer tool life, particularly in face milling.

[0013] (3) In the above (1) or (2), the thickness of the first layer may be less than 8 μm, and the surface of the first layer or the surface of the first layer located at the interface close to the surface of the coating may have a surface roughness Ra of 0.03 μm or more and 0.2 μm or less. This makes it possible to provide a cutting tool with a longer tool life, particularly in face milling.

[0014] (4) In the above (1) or (2), the thickness of the first layer is 8 μm or more, The surface of the first layer or the surface of the first layer located at the interface close to the surface of the coating may have a surface roughness Ra of 0.05 μm or more and 0.2 μm or less, thereby providing a cutting tool with a longer tool life, particularly in face milling.

[0015] (5) In any of the above (1) to (4), the orientation index TC(0 0 12) of the first layer may be greater than 4.5, thereby providing a cutting tool with a longer tool life, particularly in face milling.

[0016] [Details of the embodiments of the present disclosure] A specific example of a cutting tool according to one embodiment of the present disclosure (hereinafter also referred to as "the present embodiment") will be described below with reference to the drawings. In the drawings of the present disclosure, the same reference symbols represent the same or corresponding parts. Furthermore, dimensional relationships such as length, width, thickness, and depth have been changed as appropriate for clarity and simplification of the drawings, and do not necessarily represent actual dimensional relationships.

[0017] In this disclosure, the expression "A to B" means the upper and lower limits of a range (i.e., A or more and B or less), and if no unit is specified for A and a unit is specified only for B, the unit of A and the unit of B are the same.

[0018] In the present disclosure, when a compound or the like is represented by a chemical formula, unless the atomic ratio is particularly limited, it is intended to include any conventionally known atomic ratio, and should not necessarily be limited to only those within the stoichiometric range.

[0019] [Embodiment 1: Cutting tool] A cutting tool according to an embodiment of the present disclosure will be described with reference to Figures 1 and 2. Figure 1 is a schematic cross-sectional view illustrating one embodiment of the cutting tool according to the present disclosure. Figure 2 is an enlarged view of region II in Figure 1. One embodiment of the present disclosure (hereinafter also referred to as "the present embodiment") is A cutting tool 10 comprising a substrate 1 and a coating 2 disposed on the substrate 1, The coating 2 comprises a first layer 3, The first layer 3 is made of α-Al2O3, The thickness of the first layer 3 is 2 μm or more and 15 μm or less, In a cross section taken along a normal to the interface between the substrate 1 and the coating 2, the first layer 3 includes a plurality of grain boundaries GB connecting an interface I1 of the first layer 3 close to the substrate 1 and a surface S1 of the first layer 3 or an interface of the first layer 3 close to the surface of the coating 2, and at least one of the grain boundaries GB, a first grain boundary GB1, has a plurality of straight line segments and three or more bending points connecting two adjacent straight line segments; In the first grain boundary GB1, a crossing angle A1 at the bending point closest to the interface I1 close to the substrate 1 of the first layer 3 and a crossing angle A2 next closest to the bending point are each 120° or more and less than 180°, In the first grain boundary GB1, a crossing angle A4 at the bending point closest to the surface S1 of the first layer 3 or the interface close to the surface of the coating 2 of the first layer 3, and a second closest crossing angle A3 are each 90° or more and 150° or less, At the first grain boundary GB1, an average X1 of the crossing angles A1 and A2 and an average X2 of the crossing angles A3 and A4 satisfy the relationship of Formula 1, The ratio N2 / N1 of the number N2 of the first grain boundaries GB1 to the number N1 of the grain boundaries GB is 0.2 or more. X1-X2≧10° Formula 1

[0020] According to the present disclosure, it is possible to provide a cutting tool 10 that has an excellent tool life, particularly in face milling. The reason for this is presumably as follows.

[0021] In the cutting tool 10 of this embodiment, in a cross section taken along a normal to the interface between the substrate 1 and the coating 2, the first layer 3 includes a plurality of grain boundaries GB connecting an interface I1 of the first layer 3 close to the substrate 1 and a surface S1 of the first layer 3 or an interface of the first layer 3 close to the surface of the coating 2, and at least one of the grain boundaries GB, a first grain boundary GB1, has a plurality of straight line segments and three or more bending points connecting two adjacent straight line segments, and in the first grain boundary GB1, a crossing angle A1 at the bending point closest to the interface I1 of the first layer 3 close to the substrate 1 and a second closest crossing angle A2 at the bending point closest to the interface I1 of the first layer 3 close to the substrate 1 are and 2 are each 120° or more and less than 180°, and in the first grain boundary GB1, the crossing angle A4 at the bend point closest to the interface near the surface S1 of the first layer 3 or the surface of the coating 2 of the first layer 3, and the next-closest crossing angle A3 are each 90° or more and 150° or less, and in the first grain boundary GB1, the average X1 of the crossing angles A1 and A2 and the average X2 of the crossing angles A3 and A4 satisfy the relationship of the above formula 1, and the ratio N2 / N1 of the number N2 of the first grain boundaries GB1 to the number N1 of the grain boundaries GB is 0.2 or more. As a result, the crossing angle in the region of the first layer 3 close to the substrate 1 becomes sufficiently large, which makes the first layer 3 more likely to have a strong orientation, thereby improving the wear resistance of the first layer 3, especially in face milling. Furthermore, since the crossing angle in the region of the first layer 3 near the surface of the coating 2, which is subjected to a particularly large load during cutting, is sufficiently small, it becomes easier to improve the grain boundary strength of the first layer 3, thereby improving the chipping resistance of the first layer 3. Therefore, since both the wear resistance and chipping resistance of the first layer 3 can be improved, it is possible to provide a cutting tool 10 that has an excellent tool life, especially in face milling.

[0022] ≪Cutting tools≫ As shown in FIGS. 1 and 2 , a cutting tool 10 according to an embodiment of the present disclosure includes a substrate 1 and a coating 2 disposed on the substrate 1. The coating 2 preferably covers the entire surface of the substrate 1. However, it is within the scope of this embodiment if a portion of the substrate 1 is not coated with the coating 2 or if the coating 2 has a partially different configuration. In cases where a portion of the substrate 1 is not coated with the coating 2, the coating 2 is preferably disposed so as to cover at least the surface of the portion of the substrate 1 involved in cutting. In this specification, the portion of the substrate 1 involved in cutting refers to the region of the substrate 1 surrounded by the cutting edge and an imaginary surface that is, depending on the size and shape of the substrate 1, a distance from the cutting edge to the perpendicular to the tangent to the cutting edge, of, for example, 5 mm, 3 mm, 2 mm, 1 mm, or 0.5 mm.

[0023] The cutting tool 10 of this embodiment can be suitably used as cutting tools 10 such as drills, end mills, indexable cutting tips for drills, indexable cutting tips for end mills, indexable cutting tips for milling (e.g., face milling), indexable cutting tips for turning, metal saws, gear cutting tools, reamers, taps, etc.

[0024] ≪Base material≫ Any conventionally known substrate 1 of this type can be used as the substrate 1. For example, cemented carbide (WC-based cemented carbide, cemented carbide containing WC and Co, cemented carbide containing carbonitrides of Ti, Ta, Nb, etc.), cermet (mainly composed of TiC, TiN, TiCN, etc.), high-speed steel, ceramics (titanium carbide, silicon carbide, silicon nitride, aluminum nitride, aluminum oxide, etc.), cubic boron nitride sintered body, or diamond sintered body is preferred.

[0025] It is particularly preferable to select a WC-based cemented carbide or a cermet (especially a TiCN-based cermet) from among these various substrates 1. These substrates 1 have an excellent balance between hardness and strength, especially at high temperatures, and therefore, when used as the substrate 1 of a cutting tool 10, they can contribute to extending the life of the cutting tool 10.

[0026] ≪Coating≫ The coating 2 includes a first layer 3. The coating 2 may consist of only the first layer 3, or may include layers other than the first layer 3 ("other layers" described below) as long as the effects of the present disclosure are not impaired. The thickness of the coating 2 may be 7 μm or more and 25 μm or less, or 10 μm or more and 15 μm or less. If the thickness of the coating 2 is less than 7 μm, the coating 2 is too thin, which tends to shorten the life of the cutting tool 10. On the other hand, if the thickness of the coating 2 exceeds 25 μm, chipping of the coating 2 is likely to occur in the early stages of cutting, which tends to shorten the life of the cutting tool 10.

[0027] The thickness of the coating 2 can be measured by observing a cross section of the coating 2 along the normal direction to the surface of the coating 2 using a scanning electron microscope (SEM). Specifically, the cross section sample is observed at a magnification of 5,000 to 10,000 times, and the observation area is 100 to 500 μm 2 The thickness width is measured at any three points in any one visual field, and the average (arithmetic mean) is taken as the "thickness." The same applies to the thickness of each layer described below unless otherwise specified.

[0028] ≪First layer≫ <Composition of the first layer> The first layer 3 is made of α-Al2O3. "Made of α-Al2O3" means that the first layer 3 may be made of only α-Al2O3, or may contain inevitable impurities in addition to α-Al2O3, as long as the effects of the present disclosure are not impaired. Examples of the inevitable impurities include chlorine atoms (Cl) and sulfur atoms (S). The total content of the inevitable impurities in the first layer 3 may be, for example, 0% by mass or more and 0.10% by mass or less, or 0.01% by mass or more and 0.05% by mass or less. In the present disclosure, the α-Al2O3 in the first layer 3 can be considered to exist as particles. The grain boundary GB described below refers to the interface between a particle and an adjacent particle.

[0029] The fact that the first layer 3 is made of α-Al2O3 is identified by X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX). The content of inevitable impurities in the first layer 3 is measured by secondary ion mass spectrometry (SIMS). It has been confirmed that there is no variation in the measurement results even if the measurement location is arbitrarily selected, as long as the measurement is performed using the same cutting tool 10.

[0030] <First layer thickness> The thickness of the first layer 3 is 2 μm or more and 15 μm or less. The thickness of the first layer 3 may be 3 μm or more and 14 μm or less, 4 μm or more and 13 μm or less, or 5 μm or more and 12 μm or less.

[0031] <Surface roughness Ra> The thickness of the first layer 3 is less than 8 μm, and the surface S1 of the first layer 3 or the surface of the first layer 3 located at the interface close to the surface of the coating 2 may have a surface roughness Ra of 0.03 μm or more and 0.2 μm or less. This makes it possible to suppress localized fracture during surface treatment of the coating 2 and facilitates the imparting of uniform residual stress, thereby providing the cutting tool 10 with a longer tool life, particularly in face milling. When the thickness of the first layer 3 is less than 8 μm, the surface roughness Ra may be 0.03 μm or more and 0.15 μm or less, or 0.03 μm or more and 0.1 μm or less.

[0032] The thickness of the first layer 3 is 8 μm or more, and the surface S1 of the first layer 3 or the surface of the first layer 3 located at the interface close to the surface of the coating 2 may have a surface roughness Ra of 0.05 μm or more and 0.2 μm or less. This can suppress localized fracture during surface treatment of the coating 2 and facilitate uniform imparting of residual stress, thereby providing the cutting tool 10 with a longer tool life, particularly in face milling. When the thickness of the first layer 3 is 8 μm or more, the surface roughness Ra may be 0.05 μm or more and 0.18 μm or less, or 0.06 μm or more and 0.15 μm or less.

[0033] The surface roughness Ra is measured in accordance with JIS B0601:2001 on a cross section of the cutting edge R taken along the normal direction to the interface between the substrate 1 and the coating 2. More specifically, an image is first obtained by capturing an image of the cross section of the cutting edge R taken along the normal direction to the interface between the substrate 1 and the coating 2 at 1000x magnification using an SEM, including the surface S1 of the first layer 3 or the surface of the first layer 3 located at the interface on the surface side of the coating 2. Next, an arbitrary rectangular observation field of 10 μm × 10 μm is identified in the image at 10,000x magnification. In the observation field, "the surface S1 of the first layer 3 or the surface of the first layer 3 located at the interface on the surface side of the coating 2" is defined as passing through any pair of opposing sides. Next, image analysis software (ImageJ, version 1.51j8: https: / / imagej.nih.gov / ij / ) is used to extract interface contour information for the observation field, thereby determining the "arithmetic mean roughness" of "the surface S1 of the first layer 3 or the surface located at the interface on the surface side of the coating 2 of the first layer 3" in the observation field. Next, the "arithmetic mean roughness" of "the surface S1 of the first layer 3 or the surface located at the interface on the surface side of the coating 2 of the first layer 3" is determined for any other four observation fields in the image. Next, the average (arithmetic mean) of the "arithmetic mean roughness" of "the surface S1 of the first layer 3 or the surface located at the interface on the surface side of the coating 2 of the first layer 3" in a total of five observation fields is calculated, thereby determining the surface roughness Ra. Note that it has been confirmed that there is no variation in the measurement results even if the measurement location is arbitrarily selected, as long as the same first layer 3 is measured.

[0034] <Grain boundary> In a cross section taken along the normal to the interface between the substrate 1 and the coating 2, the first layer 3 includes multiple grain boundaries GB connecting the interface I1 of the first layer 3 close to the substrate 1 with the surface S1 of the first layer 3 or with an interface of the first layer 3 close to the surface of the coating 2. At least one of these grain boundaries GB, the first grain boundary GB1, has multiple straight line segments and three or more bending points connecting two adjacent straight line segments. These features improve the "chipping resistance" of the first layer 3.

[0035] In the first grain boundary GB1, the crossing angle A1 at the bending point closest to the interface I1 of the first layer 3 close to the substrate 1 and the next closest crossing angle A2 are each 120° or greater and less than 180°. In the first grain boundary GB1, the crossing angle A4 at the bending point closest to the interface close to the surface S1 of the first layer 3 or the surface of the coating 2 of the first layer 3 and the next closest crossing angle A3 are each 90° or greater and 150° or less. In the first grain boundary GB1, the average X1 of the crossing angles A1 and A2 and the average X2 of the crossing angles A3 and A4 satisfy the relationship of formula 1. X1-X2≧10° Formula 1 The upper limit of each of the crossing angles A1 and A2 may be 175° or less, or may be 170° or less.

[0036] The ratio N2 / N1, which is the number N2 of first grain boundaries GB1 to the number N1 of grain boundaries GB, is 0.2 or more. This makes it possible to improve the wear resistance and chipping resistance of the first layer 3. The upper limit of the ratio N2 / N1 may be 1.0 or less, or 0.7 or less. The ratio N2 / N1 may be 0.3 or more and 1.0 or less, 0.4 or more and 1.0 or less, or 0.5 or more and 1.0 or less.

[0037] The ratio N2 / N1 can be determined by the following method. (a1) A color map is created based on the crystal orientation of each particle (crystal grain) by performing electron backscatter diffraction image analysis on a cross section of the first layer 3 along the normal direction of the interface between the coating 2 and the substrate 1 using a field emission scanning microscope. (b1) Based on the color map created in (a1), the grain boundary GB of any one particle (crystal particle) is colored white and the other areas are colored black, thereby outputting the image to obtain a first image. (c1) Observe the pixels in each column in the first image in order from the left edge of the image, and identify the first point where the color changes from black to white as "any one grain boundary GB connecting the interface I1 close to the substrate of the first layer and the surface S1 of the first layer or an interface close to the surface of the coating of the first layer." (d1) For each column of pixels of the grain boundary GB in the first image identified in (c1), the angle of the grain boundary GB at each pixel is obtained by fitting a rectangular area of ​​15 × 15 pixels (i.e., 0.3 μm × 0.3 μm) centered on the pixel with a linear equation. (e1) If the angle calculated in (d1) for any pixel of the grain boundary GB is the maximum or minimum among the pixels of the grain boundary GB in a row 0.4 μm above and below the row of that pixel, that pixel is identified as a straight line segment. The point connecting this straight line segment and the adjacent straight line segment along the grain boundary that is closer to the substrate of the first layer is identified as an inflection point, and the value calculated by subtracting the absolute value of the difference in the angles calculated in (d1) for each straight line segment from 180° is identified as the intersection angle at this inflection point. (f1) When the grain boundary GB identified in (c1) has multiple straight line segments and three or more bending points connecting two adjacent straight line segments, the crossing angle A1 at the bending point closest to the interface I1 close to the substrate of the first layer and the crossing angle A2 closest to the next are identified in order from the substrate side of the first layer. Also, the crossing angle A4 at the bending point closest to the interface close to the surface S1 of the first layer or the surface of the coating of the first layer and the crossing angle A3 closest to the next are identified in order from the interface side close to the surface S1 of the first layer or the surface of the coating of the first layer. (g1) Calculate the average X1 by dividing the sum of the crossing angles A1 and A2 specified in (f1) by 2. Also, calculate the average X2 by dividing the sum of the crossing angles A3 and A4 specified in (f1) by 2. (h1) For "any other at least 17 grain boundary GBs connecting an interface I1 close to the substrate of the first layer and an interface close to the surface S1 of the first layer or the surface of the coating of the first layer," execute (f1) to (g1) to identify the crossing angle A1, the crossing angle A2, the crossing angle A3, the crossing angle A4, the average X1, and the average X2 for each of the other at least 17 grain boundary GBs. (i1) Of the total of 18 or more grain boundary GBs, the crossing angles A1 and A2 are each 120° or more and less than 180°, the crossing angles A4 and A3 are each 90° or more and 150° or less, and the average X1 and average X2 satisfy the relationship of Equation 1. The number of grain boundary GBs (i.e., first grain boundaries GB1) is divided by the number of grain boundary GBs (i.e., 18 or more) to determine the ``ratio N2 / N1 of the number N2 of first grain boundary GB1 to the number N1 of grain boundary GBs.''

[0038] It has been confirmed that as long as measurements are made on the same first layer 3, there is no variation in the measurement results even if the measurement location is selected arbitrarily.

[0039] <Average distance D1 between two adjacent bending points along the normal to the interface between the substrate and the coating> In the first grain boundary GB1, the average distance D1 between two adjacent bending points along the normal to the interface between the substrate 1 and the coating 2 may be 0.05 μm or more and 4 μm or less. This facilitates improving the flexural strength of the coating 2, thereby providing the cutting tool 10 with a longer tool life, particularly in face milling. The average distance D1 may be 0.06 μm or more and 3 μm or less, 0.07 μm or more and 2 μm or less, or 0.08 μm or more and 1 μm or less.

[0040] The average distance D1 can be determined by the following method. (a2) Using the same method as described in (a1) to (g1) above, the crossing angle A1, the crossing angle A2, the average X1, and the average X2 at the grain boundary GB are determined, thereby identifying any one first grain boundary GB1. (b2) For the first grain boundary GB1 identified in (a2), the absolute values ​​of the differences between the rows of two adjacent straight lines along the grain boundary at each bending point (in other words, when the position of the end of the first layer closest to the substrate is set to 0 μm, the absolute values ​​of the differences in the distance [μm] from this position along the normal direction to the interface between the coating 2 and the substrate 1) are calculated, and the average (arithmetic mean) of the absolute values ​​of the differences is calculated to identify the average distance along the normal to the interface between the substrate and the coating at two adjacent bending points. (c2) Using the same method as in (a2) to (b2) above, determine the average distance between two adjacent bending points along the normal to the interface between the substrate and the coating for each of any other four first grain boundaries GB. The average distance D1 can be determined by dividing the sum of the average distances between two adjacent bending points along the normal to the interface between the substrate and the coating for each of a total of five first grain boundaries GB1 by 5.

[0041] It has been confirmed that as long as measurements are made on the same first layer 3, there is no variation in the measurement results even if the measurement location is selected arbitrarily.

[0042] <First layer orientation index TC(0 0 12)> The orientation index TC(0 0 12) of the first layer 3 may be greater than 4.5. This further improves the strength of the particles in the first layer 3 (in other words, the α-Al2O3 particles (crystal grains)), thereby further improving the wear resistance of the first layer 3. As a result, the cutting tool 10 can be endowed with a longer tool life, particularly in face milling. The orientation index TC(0 0 12) of the first layer 3 may be greater than 4.5 and less than or equal to 8.0, greater than or equal to 5.0 and less than or equal to 7.9, or greater than or equal to 6.0 and less than or equal to 7.9.

[0043] In this specification, "the orientation index TC(0 0 12) of the first layer 3" means the orientation index TC(0 0 12) of the (0 0 12) plane in the first layer 3, among the orientation indexes TC(hkl) defined by the following formula 2.

[0044]

number

[0045] In Equation 2, I(hkl) represents the X-ray diffraction intensity of the (hkl) reflection plane, and I0(hkl) represents the standard intensity according to ICDD PDF card number 00-010-0173. Furthermore, n in Equation 2 represents the number of reflections used in the calculation, which is 8 in this embodiment. The (hkl) planes used for reflection are (012), (104), (110), (0 0 12), (113), (214), (116), and (300).

[0046] ICDD (registered trademark) is an abbreviation for International Centre for Diffraction Data, and PDF (registered trademark) is an abbreviation for Powder Diffraction File.

[0047] The orientation index TC(0 0 12) of the first layer 3 of this embodiment can be expressed by the following formula 3.

[0048]

number

[0049] Therefore, "the orientation index TC(0 0 12) of the first layer 3 is greater than 4.5" means that the value obtained by substituting TC(0 0 12) into formula 2 above to obtain formula 3 is greater than 4.5.

[0050] The above-described measurement of TC(hkl) can be performed by analysis using an X-ray diffractometer. TC(hkl) can be measured, for example, using a SmartLab (registered trademark) manufactured by Rigaku Corporation (scan speed: 21.7° / min, step: 0.01°, scan range: 15-140°) under the following conditions. In this embodiment, the measurement results of TC(hkl) using an X-ray diffractometer are referred to as "XRD results." (conditions) Characteristic X-ray: Cu-Kα Tube voltage: 45kV Tube current: 200mA Filter: Multi-layer mirror Optics: Concentration method X-ray diffraction method: θ-2θ method When using an X-ray diffraction device, X-rays are irradiated onto the rake face of a cutting tool. Since the rake face is usually uneven, while the flank face is flat, it is preferable to irradiate the flank face with X-rays to eliminate disturbance factors. In particular, X-rays are irradiated onto a portion of the flank face extending within a range of approximately 2 to 4 mm from the cutting edge ridge. This increases the reproducibility of the results. In this embodiment, the value of the orientation index TC(hkl) of the first layer 3 on the flank face of the substrate is the same as the value of TC(hkl) of the first layer 3 on the rake face of the substrate.

[0051] It was confirmed that similar results could be obtained by arbitrarily selecting multiple measurement points on the same sample and performing the above measurements on each of the measurement points.

[0052] Other Layers Examples of other layers include a base layer (not shown), an intermediate layer (not shown), and a surface layer (not shown). The base layer is a layer in contact with the substrate 1. The surface layer is a layer located on the surface of the coating 2. The intermediate layer is a layer disposed between the base layer and the first layer 3 or a layer disposed between the first layer 3 and the surface layer.

[0053] <Underlayer> The underlayer may be made of TiN or TiCN. "Made of TiN or TiCN" means that the underlayer may consist solely of TiN or TiCN, or may contain inevitable impurities in addition to TiN or TiCN. Examples of inevitable impurities include chlorine atoms (Cl), oxygen atoms (O), cobalt atoms (Co), tungsten atoms (W), nickel atoms (Ni), and boron atoms (B). The total content of inevitable impurities in the underlayer may be, for example, 0% by mass or more and 1.0% by mass or less, or 0.3% by mass or more and 0.6% by mass or less.

[0054] The underlayer is determined to be composed of TiN or TiCN by X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX). The content of unavoidable impurities in the underlayer is measured by secondary ion mass spectrometry (SIMS). It has been confirmed that, as long as measurements are taken on the same underlayer, there is no variation in the measurement results even if the measurement location is arbitrarily selected.

[0055] The thickness of the underlayer may be 0.1 μm or more and 2.0 μm or less, 0.5 μm or more and 1.5 μm or less, or 0.8 μm or more and 1.3 μm or less.

[0056] <Middle class> The intermediate layer may be made of TiCN. "Made of TiCN" means that the intermediate layer may consist only of TiCN, or may contain inevitable impurities in addition to TiCN. Examples of inevitable impurities include chlorine atoms (Cl), oxygen atoms (O), cobalt atoms (Co), tungsten atoms (W), nickel atoms (Ni), and boron atoms (B). The total content of inevitable impurities in the intermediate layer may be, for example, 0% by mass or more and 1.0% by mass or less, or 0.3% by mass or more and 0.6% by mass or less.

[0057] The fact that the intermediate layer is made of TiCN is measured by X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX). The content of unavoidable impurities in the intermediate layer is measured by secondary ion mass spectrometry (SIMS). It has been confirmed that there is no variation in the measurement results even if the measurement location is arbitrarily selected, as long as the same intermediate layer is measured.

[0058] The thickness of the intermediate layer may be 2.0 μm or more and 10 μm or less, or 4.0 μm or more and 8.0 μm or less.

[0059] <Surface layer> The surface layer may be made of TiN. "Made of TiN" means that the surface layer may consist only of TiN, or may contain inevitable impurities in addition to TiN. Examples of inevitable impurities include chlorine atoms (Cl), oxygen atoms (O), cobalt atoms (Co), tungsten atoms (W), nickel atoms (Ni), and boron atoms (B). The total content of inevitable impurities in the surface layer may be, for example, 0% by mass or more and 1.0% by mass or less, or 0.1% by mass or more and 0.4% by mass or less.

[0060] The surface layer is determined to be composed of TiN using X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX). The content of unavoidable impurities in the surface layer is measured using secondary ion mass spectrometry (SIMS). It has been confirmed that, as long as measurements are taken on the same surface layer, there is no variation in the measurement results even if the measurement location is arbitrarily selected.

[0061] The thickness of the surface layer may be 0.5 μm or more and 3.0 μm or less, or 1.0 μm or more and 2.5 μm or less.

[0062] [Embodiment 2: Method for manufacturing a cutting tool] The method for manufacturing a cutting tool according to this embodiment will be described with reference to Fig. 3. Fig. 3 is a schematic cross-sectional view of an example of a CVD apparatus used in manufacturing the cutting tool according to the present disclosure.

[0063] The method for manufacturing a cutting tool of this embodiment is the same as the method for manufacturing a cutting tool as described in embodiment 1, and includes a first step of preparing a substrate 1 and a second step of forming a coating on the substrate 1, the second step including a step 2a of forming a first layer by a CVD method. Each step will be described in detail below.

[0064] ≪1st process≫ In the first step, a substrate 1 is prepared. The substrate 1 described in the first embodiment can be used.

[0065] For example, when a cemented carbide is used as the substrate 1, a commercially available substrate 1 may be used, or it may be manufactured by a general powder metallurgy method. When manufactured by a general powder metallurgy method, for example, WC powder and Co powder are mixed using a ball mill or the like to obtain a mixed powder. The mixed powder is dried and then molded into a predetermined shape to obtain a green body. The green body is then sintered to obtain a WC-Co based cemented carbide (sintered body). Next, the sintered body is subjected to a predetermined cutting edge processing such as honing, thereby manufacturing the substrate 1 made of a WC-Co based cemented carbide. Any substrate 1 other than those described above can also be prepared as long as it is a conventionally known substrate 1 of this type.

[0066] ≪Second process≫ In the second step, a coating is formed on the substrate 1 to obtain a cutting tool. The coating is formed using, for example, a CVD apparatus 50 shown in FIG. 2. The CVD apparatus 50 includes a plurality of substrate setting jigs 52 for holding the substrate 1 and a heat-resistant alloy steel reaction vessel 53 that covers the substrate setting jigs 52. A temperature control device 54 for controlling the temperature inside the reaction vessel 53 is provided around the reaction vessel 53. The reaction vessel 53 is provided with a gas introduction pipe having a gas inlet. The gas introduction pipe extends vertically in the internal space of the reaction vessel 53 in which the substrate setting jigs 52 are placed and is rotatable about an axis in the vertical direction. The gas introduction pipe is also provided with a plurality of ejection holes (through holes) for ejecting gas into the reaction vessel 53. Using this CVD apparatus 50, the first layer that constitutes the coating can be formed as follows.

[0067] The second step includes a step 2a of forming a first layer by a CVD method. When the coating includes the "other layer" described in embodiment 1, the second step can further include a step of forming the "other layer." The "other layer" can be formed by a conventionally known method.

[0068] <Step 2a: Step of forming the first layer by CVD method> In step 2a, the first layer is formed by the CVD method. More specifically, first, the substrate 1 is placed in a substrate setting jig 52, and a raw material gas for the first layer is introduced into the reaction vessel 53 from a gas inlet pipe while controlling the temperature and pressure in the reaction vessel 53 within a predetermined range. In this way, the first layer is formed on the substrate 1.

[0069] A nozzle 56 having two inlets 55, 57 is disposed in the CVD apparatus 50. The nozzle 56 is disposed so as to penetrate the area in which the substrate setting jig 52 is disposed. A plurality of injection holes (a first injection hole 61, a second injection hole 62, a third injection hole (not shown), and a fourth injection hole (not shown)) are formed in the nozzle 56 in the vicinity of the substrate setting jig 52.

[0070] The gases introduced into the nozzle 56 from the inlets 55 and 57 are not mixed in the nozzle 56 either, but are introduced into the reaction vessel 53 via different injection holes. The nozzle 56 can rotate around its own axis. An exhaust pipe 59 is also provided in the CVD apparatus 50, and the exhaust gas can be discharged to the outside from an exhaust port 60 of the exhaust pipe 59. The fixtures and the like inside the reaction vessel 53 are usually made of graphite.

[0071] The source gas used is a mixed gas of AlCl3, HCl, CO2, H2S, and H2.

[0072] First, to form a region close to the substrate of the first layer, (001) film formation and (110) film formation are alternately repeated under the following conditions (Step I): In Step I, the film formation time for the (001) film formation is 4 to 90 minutes, and the rotation speed of the nozzle for the (001) film formation is 0.2 to 2 rpm. Also in Step I, the film formation time for the (110) film formation is 4 to 90 minutes, and the rotation speed of the nozzle for the (110) film formation is 0.4 to 4 rpm. Next, to form a region close to the surface of the first layer coating, (001) film formation and (110) film formation are alternately repeated under the following conditions (Step II): In Step II, the film formation time for the (001) film formation is 5 to 180 minutes, and the rotation speed of the nozzle for the (001) film formation is 0.5 to 4 rpm. Furthermore, in step II, the deposition time for the (110) film deposition is 5 to 180 minutes, and the rotation speed of the nozzle for the (110) film deposition is 0.5 to 6 rpm. The rotation speed of the nozzle for the (001) film deposition in step I is slower by 0.1 rpm or more than the rotation speed of the nozzle for the (001) film deposition in step II. In other words, "(rotation speed of the nozzle for the (001) film deposition in step I) - (rotation speed of the nozzle for the (001) film deposition in step II)" is -0.1 rpm or less. The rotation speed of the nozzle for the (001) film deposition in step I may be slower by 0.5 rpm or more, or may be slower by 1 rpm or more than the rotation speed of the nozzle for the (001) film deposition in step II. The rotation speed of the nozzle for the (110) film deposition in step I is slower by 0.1 rpm or more than the rotation speed of the nozzle for the (110) film deposition in step II. In other words, "(the rotation speed of the nozzle for (110) film formation in step I) - (the rotation speed of the nozzle for (110) film formation in step II)" is -0.1 rpm or less. The rotation speed of the nozzle for (110) film formation in step I may be slower by 0.5 rpm or more, or may be slower by 1 rpm or more than the rotation speed of the nozzle for (110) film formation in step II. The total number of times of (001) film formation in step I and the number of times of (001) film formation in step II is 2 or more, and the total number of times of (110) film formation in step I and the number of times of (110) film formation in step II is 2 or more.This allows the formation of a first grain boundary having multiple straight line segments and three or more bending points connecting two adjacent straight line segments. The (001) film formation is performed by injecting the mixed gas from the first injection hole 61 and the second injection hole 62, and the (110) film formation is performed by injecting the mixed gas from the third injection hole (not shown) and the fourth injection hole (not shown). In each of Step I and Step II, the (001) film formation may be started first, or the (110) film formation may be started first. <(001) Film Formation Conditions> AlCl3: 0.005 to 0.02% by volume HCl: 0.01 to 0.05% by volume CO2: 1.5 to 6.0% by volume H2S: 0.3 to 1.0 volume% H2: Remaining Temperature: 950~1050℃ Pressure: 50~200hPa Flow rate: 15~60L / min <(110) Film Formation Conditions> AlCl3: 0.01 to 0.04 volume percent HCl: 0.01 to 0.05% by volume CO2: 2.5 to 10% by volume H2S: 0.02 to 0.08% by volume H2: Remaining Temperature: 950~1050℃ Pressure: 50~150hPa Flow rate: 20~40L / min

[0073] The number of times of (001) film formation and (110) film formation is not particularly limited. The thickness of the first layer can be adjusted by appropriately adjusting the number of times of (001) film formation and (110) film formation.

[0074] <Other processes> In addition to the above steps, the second step may include a surface treatment step such as surface grinding or blasting.

[0075] <Features of the cutting tool manufacturing method according to the present embodiment> In the cutting tool manufacturing method of this embodiment, in step 2a, (001) film formation, in which the film formation time is 4 to 90 minutes and the nozzle rotation speed is 0.2 to 2 rpm, and (110) film formation, in which the film formation time is 4 to 90 minutes and the nozzle rotation speed is 0.4 to 4 rpm, are alternately performed (step I), and then (001) film formation, in which the film formation time is 5 to 180 minutes and the nozzle rotation speed is 0.5 to 4 rpm, and (110) film formation, in which the film formation time is 4 to 180 minutes and the nozzle rotation speed is 0.5 to 6 rpm, are alternately performed (step II). The (001) film formation is performed by injecting the mixed gas from the first and second injection holes, and the (110) film formation is performed by injecting the mixed gas from the third and fourth injection holes. The rotation speed of the nozzle for (001) film formation in step I is slower than the rotation speed of the nozzle for (001) film formation in step II by 0.1 rpm or more, and the rotation speed of the nozzle for (110) film formation in step I is slower than the rotation speed of the nozzle for (110) film formation in step II by 0.1 rpm or more. The total number of (001) film formations in step I and (001) film formations in step II is 2 or more, and the total number of (110) film formations in step I and (110) film formations in step II is 2 or more. As a result, the crossing angle at the grain boundary can be made relatively large in a region close to the substrate of the first layer, while the crossing angle at the grain boundary can be kept relatively small in a region close to the surface of the coating of the first layer. Therefore, the crossing angle A1, the crossing angle A2, the crossing angle A3, the crossing angle A4, the average X1 of the crossing angles A1 and A2, and the average X2 of the crossing angles A3 and A4 can each be adjusted within a desired range, and the ratio N2 / N1 of the number N2 of first grain boundaries to the number N1 of grain boundaries can be adjusted within a desired range.The present inventors have found, as a result of extensive research, that the cutting tool of the present disclosure can be realized by employing such a manufacturing method. [Example]

[0076] The present embodiment will be described in more detail with reference to examples, although the present embodiment is not limited to these examples.

[0077] <Cutting tool manufacturing> Cutting tools according to Samples 1 to 19 and 101 to 104 were produced as follows.

[0078] ≪1st process≫ As a substrate, a milling tip (shape: WGC4160R, manufactured by Sumitomo Electric Hardmetal Corp.) having the following composition was prepared. (Base material composition) Co content: 9.5% by mass Cr3C2 content: 1.0% by mass WC content: remaining

[0079] ≪Second process≫ A first layer was formed on the surface of the substrate by performing a CVD method (step 2a). First, for samples 1 to 19, 101, 102, and 104, (001) film formation was performed for the film formation time as shown in Table 1, and the conditions for the nozzle rotation speed were as shown in Table 1. Then, (110) film formation was performed for the film formation time as shown in Table 1, and the conditions for the nozzle rotation speed were as shown in Table 1. These were alternately performed the number of times shown in the "Number of repetitions [times]" column in Table 1 (step I). Next, for Samples 1 to 19, Sample 101, Sample 102, and Sample 104, (001) film formation was performed for the film formation time and under the nozzle rotation speed conditions shown in Table 2, and (110) film formation was performed for the film formation time and under the nozzle rotation speed conditions shown in Table 2, alternately the number of times shown in the "Number of repetitions [times]" column in Table 2 (Step II). For Sample 103, (001) film formation was performed once for the film formation time and under the nozzle rotation speed conditions shown in Table 1 (Step I). For Sample 103, Step II was not performed. For Samples 1 to 19 and Samples 101 to 103, the number of injection holes in the (001) film formation was "two," and the number of injection holes in the (110) film formation was "two." For sample 104, the number of injection holes in the (001) film formation was "1," and the number of injection holes in the (110) film formation was "1." In addition, each of step I and step II started with the (001) film formation. Furthermore, "(Nozzle rotation speed for (001) film formation in step I) - (Nozzle rotation speed for (001) film formation in step II)" and "(Nozzle rotation speed for (110) film formation in step I) - (Nozzle rotation speed for (110) film formation in step II)" were as shown in Table 3. For each of the (001) film formation and (110) film formation, the conditions other than the film formation time and nozzle rotation speed were as follows: <(001) Film Formation Conditions> AlCl3: 0.01% by volume HCl: 0.025% by volume CO2: 3.20% by volume H2S: 0.64% by volume H2: Remaining Temperature: 1000℃ Pressure: 100hPa Flow rate: 35L / min <(110) Film Formation Conditions> AlCl3: 0.02% by volume HCl: 0.025% by volume CO2: 5.00% by volume H2S: 0.04% by volume H2: Remaining Temperature: 1000℃ Pressure: 100hPa Flow rate: 30L / min

[0080] Next, the surface of the first layer was subjected to blasting under the following conditions. (conditions) Media Type: Ceramics Average particle size of media: 50 μm Media density: 100g / min Projection angle: 75° Projection distance: 35mm Projection pressure: As shown in Table 3 Time: 10 seconds Cutting tool rotation speed: 60 rpm

[0081] In this manner, cutting tools according to Samples 1 to 19 and 101 to 104 were produced.

[0082] [Table 1]

[0083] [Table 2]

[0084] [Table 3]

[0085] [Table 4]

[0086] <Cutting tool characteristic evaluation> <Composition of the first layer> The composition of the first layer of each sample cutting tool was determined by the method described in embodiment 1. The results are shown in the "Composition" column of the "First Layer" section of Table 4. When a component name is listed in the "Composition" column of the "First Layer" section of Table 4, it means that the first layer is composed of the component represented by that component name.

[0087] <First layer thickness> For each sample cutting tool, the thickness of the first layer was determined by the method described in embodiment 1. The results are shown in the "Thickness [μm]" column of the "First Layer" column in Table 4.

[0088] <Ratio N2 / N1> For the cutting tools of each sample, the number N1 of grain boundaries, the number N2 of first grain boundaries, and the ratio N2 / N1 were determined by the method described in embodiment 1. The results obtained are shown in the "N1 [number]" column, the "N2 [number]" column, and the "N2 / N1" column in Table 4, respectively.

[0089] <Average distance D1 between two adjacent bending points along the normal to the interface between the substrate and the coating> For each cutting tool sample, the average distance D1 between two adjacent bending points along the normal to the interface between the substrate and the coating at the first grain boundary was determined by the method described in embodiment 1. The results are shown in the "D1 [μm]" column in Table 4.

[0090] <Surface roughness Ra of the surface of the first layer or the surface located at the interface on the surface side of the first layer coating> The surface roughness Ra of the surface of the first layer or the surface located at the interface on the surface side of the coating of the first layer was determined by the method described in embodiment 1. The results obtained are shown in the "Ra [μm]" column of Table 4.

[0091] <First layer orientation index TC(0 0 12)> For each cutting tool sample, the orientation index TC(0 0 12) of the first layer was determined by the method described in embodiment 1. The results are shown in the "TC(0012)" column of the "First Layer" column in Table 4.

[0092] <Cutting test 1> Cutting was performed using the cutting tools for each sample under the following cutting conditions. The shorter of the time from the start of cutting until flank wear (Vb) exceeded 0.2 mm and the time from the start of cutting until chipping occurred was measured. The results are shown in the "Time [min]" column of "Cutting Test 1" in Table 4. (Cutting conditions) Work material: SCM435 block material Processing method: Face milling Machining speed V:250m / min Feed rate f: 0.3 mm / rev Cutting depth ap: 2.0 mm Cutting fluid: Yes The above cutting conditions correspond to the cutting conditions for face milling.

[0093] <Cutting test 2> Cutting was performed using the cutting tools for each sample under the following cutting conditions. The time from the start of cutting to when chipping occurred was measured as the cutting time. However, if no chipping occurred after 5 minutes, the cutting time was recorded as 10 minutes. The results are shown in the "Time [min]" column of "Cutting test 2" in Table 4. (Cutting conditions) Work material: S50C block material Processing method: Face milling Machining speed V:200m / min Feed rate f: 0.4 mm / rev Cutting depth ap: 2.0 mm Cutting fluid: None The above cutting conditions correspond to the cutting conditions for face milling.

[0094] If the time in Cutting Test 1 is 4.5 minutes or more and the time in Cutting Test 2 is 6.0 minutes or more, it means that the cutting tool has an excellent tool life.

[0095] The cutting tools of Samples 1 to 19 correspond to Examples. The cutting tools of Samples 101 to 104 correspond to Comparative Examples. From the results of Cutting Test 1 and Cutting Test 2 in Table 4, it was found that the cutting tools of Samples 1 to 19 had superior tool life compared to the cutting tools of Samples 101 to 104.

[0096] From the above, it was found that the cutting tools according to Samples 1 to 19 had excellent tool life.

[0097] Although the embodiments and examples of the present disclosure have been described above, it is originally intended that the configurations of the above-described embodiments and examples may be appropriately combined or modified in various ways.

[0098] The embodiments and examples disclosed herein are illustrative in all respects and should not be considered limiting. The scope of the present invention is defined by the claims, not by the embodiments and examples described above, and is intended to include meanings equivalent to the claims and all modifications within the scope of the claims. [Explanation of symbols]

[0099] 1 substrate, 2 coating, 3 first layer, 10 cutting tool, 50 CVD apparatus, 52 substrate setting jig, 53 reaction vessel, 54 temperature control device, 55, 57 inlet, 56 nozzle, 59 exhaust pipe, 60 exhaust port, 61 first injection hole, 62 second injection hole, GB grain boundary, GB1 first grain boundary, S1 surface, I1 interface.

Claims

1. A cutting tool comprising a base material and a coating disposed on the base material, The coating includes a first layer, The first layer is α-Al ２ O ３ It consists of, The thickness of the first layer is 2 μm or more and 15 μm or less. In a cross-section along the normal to the interface between the substrate and the coating, the first layer includes a plurality of grain boundaries connecting the interface of the first layer near the substrate and the interface of the first layer near the surface of the first layer or the surface of the coating of the first layer, and at least one of these grain boundaries, the first grain boundary, has a plurality of straight portions and three or more inflection points connecting two adjacent straight portions. At the first grain boundary, the intersection angle A1 at the inflection point closest to the interface near the substrate of the first layer, and the second closest intersection angle A2, are each 120° or more and less than 180°. At the first grain boundary, the intersection angle A4 at the inflection point closest to the surface of the first layer or the interface near the surface of the coating of the first layer, and the second closest intersection angle A3 are each 90° or more and 150° or less. At the first grain boundary, the average X1 of the intersection angles A1 and A2, and the average X2 of the intersection angles A3 and A4, satisfy the relationship in Equation 1. A cutting tool in which the ratio N2 / N1 of the number of first grain boundaries to the total number of grain boundaries N1 is 0.2 or more. X1-X2≧10° Formula 1

2. The cutting tool according to claim 1, wherein, at the first grain boundary, the average distance D1 of two adjacent bending points along the normal to the interface between the substrate and the coating is 0.05 μm or more and 4 μm or less.

3. The thickness of the first layer is less than 8 μm. The cutting tool according to claim 1 or claim 2, wherein the surface roughness Ra on the surface of the first layer, or on the surface located at the interface near the surface of the coating of the first layer, is 0.03 μm or more and 0.2 μm or less.

4. The thickness of the first layer is 8 μm or more. The cutting tool according to claim 1 or claim 2, wherein the surface roughness Ra on the surface of the first layer, or on the surface located at the interface near the surface of the coating of the first layer, is 0.05 μm or more and 0.2 μm or less.

5. The cutting tool according to claim 1 or claim 2, wherein the orientation index TC(0 0 12) of the first layer is greater than 4.5.

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