JP7473069B1 - Coated Cutting Tools - Google Patents

Abstract

[Problem] To provide a coated cutting tool having excellent chipping resistance, fracture resistance, and wear resistance and a long tool life. [Solution] A coated cutting tool including a substrate and a coating layer formed on the substrate surface, the coating layer including a lower layer, an intermediate layer, and an upper layer in this order, the lower layer and the upper layer including Ti and at least one selected from the group consisting of C, N, O, and B, the lower layer and the intermediate layer having an average thickness of 1.5 μm to 15.0 μm, the intermediate layer including α-type aluminum oxide, the upper layer having an average thickness of 0.5 μm to 5.0 μm, the average crack spacing X of the lower layer being 0.5 μm to less than 10.0 μm, the average crack spacing Y of the intermediate layer being 20.0 μm to 100.0 μm, and the average crack spacing Z of the upper layer being 0.5 μm to 10.0 μm. [Selected Figure] Figure 1

Description

The present invention relates to a coated cutting tool.

It has been well known that coated cutting tools, which are formed by chemical vapor deposition on the surface of a substrate made of tungsten carbide-based cemented carbide, are used for cutting metals such as steel and cast iron, and have a coating layer consisting of two or more layers of Ti compounds such as a titanium carbide (TiC) layer, a titanium nitride (TiN) layer, a titanium carbonitride (TiCN) layer, a titanium carbonate (TiCO) layer, a titanium oxynitride (TiNO) layer, and a titanium oxycarbonitride (TiCNO) layer, and an α-type aluminum oxide layer.

It is known that such coated cutting tools usually have residual tensile stress in the formed coating layer, which reduces the fracture strength of the coated cutting tool and makes it more susceptible to chipping. A technique has been proposed in which the residual tensile stress is released by introducing cracks using shot peening or the like after the coating layer is formed. This technique is described, for example, in Patent Documents 1 and 2.

Patent Document 1 describes a coated cutting tool that includes a substrate and a coating layer formed on the substrate. Patent Document 1 also describes that the coating layer of the coated cutting tool has an upper layer and a lower layer, that the average crack spacing X of the lower layer, which is composed of one or two layers of a compound composed of at least one element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, and Si, and at least one element selected from the group consisting of C, N, B, and O, is 10 to 80 μm, that the upper layer is formed on the surface of the lower layer, includes an aluminum oxide layer, that the average crack spacing Z of the aluminum oxide layer is 20 to 100 μm, and that the relationship 0<Z-X<90 is satisfied.

Patent Document 2 describes a surface-coated cutting tool in which a hard coating layer consisting of a lower layer and an upper layer is formed on the surface of a substrate made of a tungsten carbide-based cemented carbide or a titanium carbonitride-based cermet. Patent Document 2 also describes that the average thickness of at least one Ti carbonitride layer constituting the lower layer is the thickest, being 1.5 μm or more, that a crack initiation layer is formed in the Ti carbonitride layer, and that the lower layer on the substrate side from the crack initiation layer is formed with a stress relaxation layer in which cracks exist at an average density of 0.2/μm or more and less than 2/μm when measured in a direction parallel to the substrate surface. Patent Document 2 also describes that in the upper layer made of an α-type aluminum oxide layer, cracks are formed at an average density of less than 0.2/μm.

International Publication No. 2015/005364 JP 2015-188995 A

In recent years, cutting processes have become increasingly faster, with higher feed rates and deeper cuts, which has led to a tendency for tool life to be shorter than in the past. In order to improve tool life, Patent Documents 1 and 2 propose releasing the tensile residual stress in the coating layer by introducing cracks using shot peening or the like after the coating layer is formed.

Patent Document 1 describes a coated cutting tool including a coating layer having an upper layer and a lower layer. The invention described in Patent Document 1 improves the performance of the coated cutting tool to a certain extent. However, in recent years, there has been a demand for further improvement in chipping resistance. Patent Document 1 does not consider the average crack spacing of the Ti compound layer formed on the surface of the aluminum oxide layer opposite the substrate in the coating layer of the coated cutting tool. Therefore, it does not state that the average crack spacing of the Ti compound layer formed on the surface of the aluminum oxide layer opposite the substrate is 0.5 μm or more and 10.0 μm or less.

In the coating layer of the surface-coated cutting tool described in Patent Document 2, the average density of cracks in the layer consisting of an α-type aluminum oxide layer formed on the outermost surface opposite the substrate is less than 0.2 cracks/μm (more than 5 μm in terms of the average crack spacing). In the coating layer of the surface-coated cutting tool described in Patent Document 2, when an aluminum oxide layer having an average crack spacing of 20 μm or more is exposed on the outermost surface opposite the substrate, cracks are likely to occur on the surface of the coating layer during processing, so it can be said that there is room for improvement in chipping resistance and chipping resistance. In addition, when an aluminum oxide layer having an average crack spacing of more than 5 μm and less than 20 μm is exposed on the outermost surface opposite the substrate, the strength of the aluminum oxide layer and the Ti carbonitride layer formed thereunder is reduced, so it can be said that there is room for improvement in wear resistance and chipping resistance. Note that Patent Document 2 does not state that the coating layer of the coated cutting tool further has a Ti compound layer on the surface of the aluminum oxide layer opposite the substrate. Therefore, Patent Document 2 does not state that the average crack spacing in the Ti compound layer formed on the surface side of the aluminum oxide layer opposite the substrate is 0.5 μm or more and 10.0 μm or less.

The present invention has been made to solve the above problems, and aims to provide a coated cutting tool that has excellent chipping resistance, fracture resistance, and wear resistance, and has a long tool life.

To solve the problem, the present invention has the following configuration.

(Configuration 1)
Aspect 1 is a coated cutting tool including a substrate and a coating layer formed on a surface of the substrate,
the coating layer includes a lower layer, an intermediate layer, and an upper layer in this order from the substrate side;
the lower layer includes one or more Ti compound layers each comprising a Ti compound of Ti and at least one element selected from the group consisting of C, N, O, and B;
The average thickness of the lower layer is 1.5 μm or more and 15.0 μm or less,
the intermediate layer contains α-aluminum oxide,
The average thickness of the intermediate layer is 1.5 μm or more and 15.0 μm or less,
the upper layer includes one or more Ti compound layers each comprising a Ti compound of Ti and at least one element selected from the group consisting of C, N, O, and B;
The average thickness of the upper layer is 0.5 μm or more and 5.0 μm or less,
The average crack spacing X of the lower layer is 0.5 μm or more and less than 10.0 μm,
The average crack spacing Y of the intermediate layer is 20.0 μm or more and 100.0 μm or less,
The coated cutting tool has an average crack spacing Z of the upper layer of 0.5 μm or more and 10.0 μm or less.

(Configuration 2)
Configuration 2 is the coated cutting tool of configuration 1, wherein the coating layer has an average thickness of 8.0 μm or more and 30.0 μm or less.

(Configuration 3)
Configuration 3 is the coated cutting tool of configurations 1 or 2, wherein the top layer includes at least a TiCN layer.

(Configuration 4)
Configuration 4 is the coated cutting tool of configuration 3, wherein the average thickness of the TiCN layer of the upper layer is 0.5 μm or more and 5.0 μm or less.

(Configuration 5)
Configuration 5 is the coated cutting tool of any one of configurations 1 to 4, wherein a difference YZ between the average crack spacing Y and the average crack spacing Z is 20.0 μm or more and 95.0 μm or less.

(Configuration 6)
A configuration 6 is the coated cutting tool of any one of configurations 1 to 5, wherein a difference Y-X between the average crack spacing Y and the average crack spacing X is 25.0 μm or more and 95.0 μm or less.

(Configuration 7)
Configuration 7 is the coated cutting tool of any of Configurations 1-6, wherein the substrate is a cemented carbide, a cermet, a ceramic, or a sintered cubic boron nitride.

The present invention provides a coated cutting tool that has excellent chipping resistance, fracture resistance, and wear resistance, and has a long tool life.

1 is a schematic cross-sectional view showing an example of a coated cutting tool according to an embodiment of the present invention. FIG.

The following describes in detail the embodiments of the present invention. Note that the following embodiments are forms for embodying the present invention and do not limit the scope of the present invention.

<Coated cutting tools>
1, the coated cutting tool of the present embodiment includes a substrate 1 and a coating layer 5 formed on the surface of the substrate 1. Specific examples of the types of coated cutting tools include indexable cutting inserts for milling or turning, drills, end mills, and the like.

<Substrate>
Examples of the substrate of the coated cutting tool of this embodiment include cemented carbide, cermet, ceramics, cubic boron nitride sintered body, diamond sintered body, and high-speed steel. The substrate of the coated cutting tool of this embodiment is preferably cemented carbide, cermet, ceramics, or cubic boron nitride sintered body. Among them, the substrate is more preferably cemented carbide or cermet, since it has excellent wear resistance and chipping resistance.

These substrates may have their surfaces modified. For example, in the case of cemented carbide, a de-β layer may be formed on the surface, and in the case of cermet, a surface-hardened layer may be formed. The effect of this embodiment is achieved even if the surface of the substrate is modified.

<Coating Layer>
1 , the coating layer 5 of this embodiment includes a lower layer 2, an intermediate layer 3, and an upper layer 4, in this order from the substrate side. The coating layer 5 of this embodiment may include layers other than the lower layer 2, the intermediate layer 3, and the upper layer 4. However, in order to obtain a coated cutting tool having excellent fracture resistance and a long tool life, the coating layer 5 of this embodiment preferably includes the lower layer 2, the intermediate layer 3, and the upper layer 4.

The average thickness of the coating layer of the coated cutting tool of this embodiment is preferably 8.0 μm or more and 30.0 μm or less, more preferably 8.4 μm or more and 29.0 μm or less, and even more preferably 12.4 μm or more and 25.3 μm or less. By having the average thickness of the coating layer be equal to or less than the upper limit, the adhesion between the substrate and the coating layer can be improved. Therefore, the chipping resistance of the coated cutting tool of this embodiment can be improved, and a coated cutting tool with excellent chipping resistance can be obtained. In addition, by having the average thickness of the coating layer be equal to or more than the lower limit, a coated cutting tool with excellent wear resistance can be obtained.

In this specification, "a lower layer on a substrate" includes both a structure in which a lower layer is disposed in contact with the surface of the substrate, and a structure in which another layer is disposed between the substrate and the lower layer. The same applies to "an intermediate layer on a lower layer" and "an upper layer on an intermediate layer." Note that "above" refers to the side farther from the substrate. For example, "an intermediate layer on a lower layer" means that the intermediate layer is on the side farther from the substrate than the lower layer.

<Lower Class>
The lower layer of the coated cutting tool of this embodiment is disposed on a substrate. The lower layer of this embodiment is preferably formed on the surface of the substrate. The lower layer of this embodiment includes one or more Ti compound layers made of a Ti compound of Ti and at least one element selected from the group consisting of C, N, B, and O.

The lower layer of the present embodiment may include one or more Ti compound layers made of a Ti compound. Specifically, the Ti compound layer included in the lower layer of the present embodiment is preferably a Ti compound layer made of Ti and at least one element selected from the group consisting of C, N, B, and O. The Ti compound layer contained in the lower layer is preferably at least one layer selected from the group consisting of a TiN layer made of TiN, a TiC layer made of TiC, a TiCN layer made of TiCN, a TiCO layer made of TiCO, a TiCNO layer made of TiCNO, and a _TiB2 layer made of _TiB2 , more preferably at least one layer selected from the group consisting of a TiN layer, a TiC layer, a TiCN layer, a TiCNO layer, and a TiCO layer, more preferably at least one layer selected from the group consisting of a TiN layer, a TiCN layer, a TiCNO layer, and a TiCO layer, and even more preferably at least one layer selected from the group consisting of a TiN layer, a TiCN layer, a TiCNO layer, and a TiCO layer, and even more preferably at least one layer selected from the group consisting of a TiN layer, a TiCN layer, and a TiCNO layer. The lower layer of this embodiment is preferably composed of two or more Ti compound layers.

In this embodiment, the lower layer preferably has a layer of TiN or TiC (also referred to as "A layer" in this specification) on the surface in contact with the substrate, and more preferably has a layer of TiN. The average thickness of the A layer is preferably 0.05 μm or more and 1 μm or less, and more preferably 0.1 μm or more and 0.5 μm or less. By providing the lower layer with a specified A layer, a coating layer with excellent adhesion can be obtained, and peeling of the coating layer is suppressed, resulting in excellent chipping resistance and further improved defect resistance.

At least one of the lower layers in this embodiment is preferably a layer made of TiCN (also referred to as "layer B" in this specification). The average thickness of layer B is preferably 1.0 μm or more and 15.0 μm or less, more preferably 1.5 μm or more and 13.5 μm or less, and even more preferably 2.0 μm or more and 13.0 μm or less. By including a TiCN layer in the lower layer, a coating layer with even more excellent wear resistance can be obtained. Layer B can be formed on the surface of layer A.

In the present embodiment, the lower layer preferably comprises a layer (also referred to as "C layer" in this specification) made of TiCNO or TiCO between the B layer and the intermediate layer, and more preferably comprises a layer made of TiCNO. The C layer is the layer that is disposed furthest from the substrate among the layers constituting the lower layer. The average thickness of the C layer is preferably 0.1 μm or more and 5.0 μm or less, and more preferably 0.2 μm or more and 2.0 μm or less. By providing the lower layer with a predetermined C layer, the adhesion between the lower layer and the intermediate layer is improved, and the chipping resistance is excellent, so that the chipping resistance is further improved. The C layer can be formed on the surface of the B layer.

In this embodiment, the lower layer preferably consists of three layers: a specified A layer, a specified TiCN layer (B layer), and a specified C layer. By having the lower layer consist of the three specified layers, a coating layer with excellent performance can be obtained, and a coated cutting tool with excellent chipping resistance, fracture resistance, and wear resistance and a long tool life can be obtained.

In this embodiment, the average thickness of the entire lower layer is 1.5 μm or more and 15.0 μm or less, preferably 1.8 μm or more and 14.7 μm or less, and more preferably 2.4 μm or more and 13.5 μm or less. When the average thickness of the lower layer is equal to or less than the upper limit value described above, the adhesion of the coating layer is improved, improving chipping resistance and providing excellent chipping resistance. When the average thickness of the lower layer is equal to or more than the lower limit value described above, providing excellent wear resistance.

<Middle class>
The intermediate layer of the coating cutting tool of this embodiment is disposed on the lower layer. The intermediate layer of this embodiment is preferably formed on the surface of the lower layer.

The intermediate layer of the present embodiment preferably contains α-type aluminum oxide (α-Al ₂ O ₃ ).The intermediate layer of the present embodiment may be an aluminum oxide layer (α-Al ₂ O ₃ layer) made of α-type aluminum oxide.

In this embodiment, the average thickness of the intermediate layer is 1.5 μm or more and 15.0 μm or less, preferably 2.0 μm or more and 14.8 μm or less, and more preferably 2.5 μm or more and 13.5 μm or less. When the average thickness of the intermediate layer is equal to or less than the upper limit value described above, the adhesion of the coating layer is improved, so that chipping resistance is improved and chipping resistance is excellent. When the average thickness of the lower layer is equal to or more than the lower limit value described above, abrasion resistance is excellent.

<Upper layer>
The upper layer of the coating cutting tool of this embodiment is disposed on the intermediate layer. The upper layer of this embodiment is preferably formed on the surface of the intermediate layer. The upper layer of this embodiment includes one or more Ti compound layers made of a Ti compound of Ti and at least one element selected from the group consisting of C, N, O, and B.

The upper layer of this embodiment may include one or more Ti compound layers made of Ti compounds. Specifically, the Ti compound layer included in the upper layer of this embodiment is preferably a Ti compound layer composed of Ti and at least one element selected from the group consisting of C, N, B and O. The Ti compound layer included in the upper layer preferably includes at least one layer selected from the group consisting of a TiN layer made of TiN, a TiC layer made of TiC, a TiCN layer made of TiCN, a TiCO layer made of TiCO, a TiCNO layer made of TiCNO, and a _TiB2 layer made of _TiB2 , more preferably includes at least one layer selected from the group consisting of a TiN layer, a TiC layer, a TiCN layer, a TiCNO layer, and a TiCO layer, and further preferably includes at least one layer selected from the group consisting of a TiN layer, a TiCN layer, a TiCNO layer, and a TiCO layer. The upper layer of this embodiment is preferably composed of two or more Ti compound layers.

The upper layer of this embodiment may include an L layer as the layer closest to the substrate among the layers constituting the upper layer. The L layer may be a TiCNO layer, a TiCO layer, or a TiN layer. The average thickness of the L layer is preferably 0.05 μm or more and 2.0 μm or less, more preferably 0.1 μm or more and 1.0 μm or less, and even more preferably 0.2 μm or more and 0.5 μm or less. By providing the upper layer with a specified L layer, the adhesion between the intermediate layer and the upper layer is improved, and the chipping resistance is excellent, so that the chipping resistance is improved. In addition, it is preferable from the viewpoint of effectively and reliably achieving the effect of setting the average thickness of the upper layer to a specified value or more, which will be described later.

The upper layer of this embodiment preferably includes at least a TiCN layer. At least one of the upper layers of this embodiment is preferably a layer made of TiCN (also simply referred to as a "TiCN layer" in this specification). By including a TiCN layer in the upper layer, the effect of setting the average thickness of the upper layer to a predetermined value or more, described below, can be effectively and reliably achieved, and a coating layer with excellent wear resistance and chipping resistance can be obtained. When the upper layer includes the above-mentioned L layer, the TiCN layer can be formed on the surface of the L layer. Also, when the upper layer does not include an L layer, the TiCN layer can be formed on the surface of the above-mentioned intermediate layer.

The average thickness of the upper TiCN layer is preferably 0.2 μm or more and 5.0 μm or less, more preferably 0.5 μm or more and 5.0 μm or less, and even more preferably 1.0 μm or more and 4.8 μm or less. By having the average thickness of the upper TiCN layer be equal to or less than the above-mentioned upper limit value, the adhesion of the coating layer can be improved, so that the chipping resistance of the coating layer is improved and a coated cutting tool with excellent chipping resistance can be obtained. By having the average thickness of the upper TiCN layer be equal to or more than the above-mentioned lower limit value, a coating layer with excellent wear resistance can be obtained. In addition, the effect of setting the average thickness of the upper layer to a predetermined value or more, described below, can be effectively and reliably achieved, and a coating layer with excellent wear resistance and chipping resistance can be obtained.

The upper layer of this embodiment may include a U layer as the layer furthest from the substrate among the layers constituting the upper layer. The U layer is preferably a layer made of TiN (TiN layer). The average thickness of the U layer is preferably 0.05 μm or more and 2.0 μm or less, more preferably 0.1 μm or more and 1.8 μm or less, and even more preferably 0.1 μm or more and 0.5 μm or less. When the U layer is a TiN layer, it tends to be easy to identify the corners that have been used. When the average thickness of the U layer of the upper layer is equal to or less than the above-mentioned upper limit value, the adhesion of the coating layer can be improved, so that the chipping resistance of the coating layer is improved and a coated cutting tool with excellent chipping resistance can be obtained. When the average thickness of the U layer of the upper layer is equal to or more than the above-mentioned lower limit value, a coated cutting tool in which the corners that have been used are easy to identify can be obtained. The U layer can be formed on the surface of the TiCN layer of the upper layer. The upper layer may be composed of two layers, an L layer and a U layer. In this case, since the upper layer does not have a TiCN layer, the U layer can be formed on the surface of the L layer. The upper layer can also consist of only a U layer. In this case, since the upper layer does not have an L layer or a TiCN layer, the U layer can be formed on the surface of the intermediate layer.

The upper layer of this embodiment preferably consists of three layers, namely, a specified L layer, a specified TiCN layer, and a specified U layer, or two layers, namely, a specified TiCN layer and a specified U layer. By having the upper layer consist of multiple specified layers, it is possible to obtain a coating layer with excellent performance, and it is possible to obtain a coated cutting tool that has excellent chipping resistance, fracture resistance, and wear resistance and has a long tool life. Note that the upper layer of this embodiment can consist of only the specified TiCN layer.

The average thickness of the entire upper layer in this embodiment is 0.5 μm or more and 5.0 μm or less, preferably 0.6 μm or more and 4.9 μm or less, more preferably 1.0 μm or more and 4.2 μm or less, and even more preferably 1.2 μm or more and 3.6 μm or less. When the average thickness of the upper layer is equal to or less than the above-mentioned upper limit value, the adhesion of the coating layer is improved, so that the chipping resistance is improved and the chipping resistance is excellent. When the average thickness of the upper layer is equal to or more than the above-mentioned lower limit value, peeling of the coating layer, especially the upper layer, and the progression of cracks to the intermediate layer due to the blasting treatment tend to be suppressed. In addition, when the average thickness of the upper layer is equal to or more than the above-mentioned lower limit value, the effect of setting the average value of the crack interval in the upper layer to a predetermined value or less can be effectively and reliably achieved. Furthermore, it is possible to effectively and reliably set the average value of the crack interval in the upper layer to a predetermined value or more.

<Crack Spacing>
Next, the crack spacing in the coating layer of the coated cutting tool of this embodiment will be described. The lower layer, middle layer, and upper layer of the coating layer of the coated cutting tool of this embodiment have predetermined average crack spacing values as described below. In this specification, the average crack spacing value of the lower layer is referred to as "average value X." Similarly, in this specification, the average crack spacing values of the middle layer and upper layer are referred to as "average value Y" and "average value Z," respectively.

The average crack spacings X, Y, and Z of the lower, middle, and upper layers of the coating layer can be measured as follows:

First, the surface of the coating layer opposite the substrate is polished until the thickness of the upper layer, middle layer, and lower layer is 60 to 95% of their respective average thicknesses (for example, until the thickness is 70 to 80% of their respective average thicknesses), and a cross section parallel to the substrate surface is obtained. Next, the cross section thus obtained is observed with a scanning electron microscope (SEM) at a magnification of 2000 times, and multiple observation images are taken to obtain multiple cross-sectional SEM photographs. Next, the cross-sectional SEM photographs are combined to obtain an SEM photograph including a cross section in a range of 200 μm vertically and 300 μm horizontally. Next, in the obtained SEM photograph, 10 straight lines (straight lines that span the entire horizontal direction of the SEM photograph) with a length of 300 μm or more are drawn at intervals of 20 μm in the vertical direction of the SEM photograph. For each of the 10 straight lines, the intersections between the straight lines and the cracks are identified, and the distance between adjacent intersections is measured. By dividing the sum of all the distances between adjacent intersections of the 10 straight lines by the number of measured intersections, the average crack spacings X, Y, and Z for the cracks in the upper layer, middle layer, and lower layer, respectively, can be calculated.

In the coating layer of the coated cutting tool of this embodiment, the average crack spacing X of the lower layer is 0.5 μm or more and less than 10.0 μm, preferably 0.6 μm or more and 9.8 μm or less, more preferably 0.7 μm or more and 8.2 μm or less, and even more preferably 0.8 μm or more and 5.2 μm or less. When the average crack spacing X of the lower layer is equal to or less than the above-mentioned upper limit, the tensile residual stress of the lower layer is relaxed, improving toughness and chipping resistance. When the average X is equal to or more than the above-mentioned lower limit, the strength of the lower layer is improved, resulting in excellent wear resistance and chipping resistance.

In the coating layer of the coated cutting tool of this embodiment, the average crack spacing Y of the intermediate layer is 20.0 μm or more and 100.0 μm or less, preferably 26.8 μm or more and 96.0 μm or less, more preferably 31.7 μm or more and 86.4 μm or less, and even more preferably 34.8 μm or more and 85.3 μm or less. When the average crack spacing Y of the intermediate layer is equal to or less than the above-mentioned upper limit, the tensile residual stress of the intermediate layer is relaxed, improving toughness and improving chipping resistance. When the average Y is equal to or more than the above-mentioned lower limit, the strength of the intermediate layer is improved, resulting in excellent wear resistance and chipping resistance. In addition, the strength reduction of the upper layer and the lower layer is suppressed, resulting in excellent wear resistance and chipping resistance.

In the coating layer of the coated cutting tool of this embodiment, the average crack spacing Z of the upper layer is 0.5 μm or more and 10.0 μm or less, preferably 0.6 μm or more and 9.2 μm or less, more preferably 0.7 μm or more and 8.6 μm or less, and even more preferably 0.9 μm or more and 5.4 μm or less. When the average crack spacing Z of the upper layer is equal to or less than the above-mentioned upper limit, the tensile residual stress of the upper layer is relaxed, thereby improving the toughness and the chipping resistance. In addition, when the average Z is equal to or less than the above-mentioned upper limit, the occurrence of cracks during processing is suppressed, so that even if the average crack spacing in the lower layer is less than 10 μm, the chipping resistance is unlikely to decrease, and the toughness of the coating layer is further improved, resulting in excellent chipping resistance. When the average Z is equal to or more than the above-mentioned lower limit, the strength of the upper layer is improved, resulting in excellent wear resistance and chipping resistance.

In the coating layer of the coated cutting tool of this embodiment, the difference Y-Z between the average crack spacing Y of the intermediate layer and the average crack spacing Z of the upper layer is preferably 20.0 μm or more and 95.0 μm or less, more preferably 23.9 μm or more and 94.1 μm or less, and even more preferably 28.5 μm or more and 84.7 μm or less. When the difference Y-Z between the average crack spacing Y of the intermediate layer and the average crack spacing Z of the upper layer is equal to or less than the above-mentioned upper limit, the difference in residual stress between the upper layer and the intermediate layer is reduced, and adhesion is improved, resulting in improved chipping resistance and excellent chipping resistance. When the difference Y-Z is equal to or more than the above-mentioned lower limit, the strength reduction of the upper layer due to the introduction of cracks tends to be reduced, resulting in excellent wear resistance and chipping resistance.

In the coating layer of the coated cutting tool of this embodiment, the difference Y-X between the average crack spacing Y of the intermediate layer and the average crack spacing X of the lower layer is preferably 25.0 μm or more and 95.0 μm or less, more preferably 25.3 μm or more and 93.8 μm or less, and more preferably 26.0 μm or more and 85.7 μm or less. When the difference Y-X between the average crack spacing Y of the intermediate layer and the average crack spacing X of the lower layer is equal to or less than the above-mentioned upper limit, the difference in residual stress between the intermediate layer and the lower layer is reduced, and adhesion is improved, resulting in improved chipping resistance and excellent chipping resistance. When the difference Y-X is equal to or more than the above-mentioned lower limit, there is a tendency for strength reduction due to the introduction of cracks in the lower layer to be reduced, resulting in excellent wear resistance and chipping resistance.

<Method of forming coating layer>
A method for forming the coating layers (lower layer, intermediate layer and upper layer) of the coated cutting tool of this embodiment will be described.

The coating layer of the coated cutting tool of this embodiment can be formed through a lower layer forming process (step 1), a cooling process (step 2), an intermediate layer and upper layer forming process (step 3), a first blasting process (step 4), and a second blasting process (step 5). By appropriately selecting the forming conditions when forming the coating layer, it is possible to control the crack spacing to a predetermined value. In particular, by appropriately selecting the conditions for steps 2, 4, and 5, it is possible to control the average crack spacing values X, Y, and Z to within a predetermined range.

In the following description, the composition of the raw material gas for forming each layer is shown in units of "mol %."

<<Step 1: Forming Lower Layer>>
In order to form the coating layer of the coated cutting tool of this embodiment, a lower layer is first formed on the surface of the substrate. In this specification, the step of forming the lower layer may be referred to as "step 1".

The Ti compound layers included in the lower layer of the coated cutting tool of this embodiment can include a TiN layer, a TiC layer, a TiCN layer, a TiCNO layer, and a TiCO layer. When the lower layer is composed of three layers, namely, layer A, a layer made of TiCN (layer B), and layer C, three Ti compound layers can be formed on the surface of the substrate in the following order: layer A, layer made of TiCN (layer B), and layer C.

The A layer included in the lower layer is preferably a TiN layer or a TiC layer.

The TiN layer of the lower layer, layer A, can be formed by chemical vapor deposition using TiCl ₄ , N ₂ and H ₂ as source gases. The conditions for forming the TiN layer of layer A are preferably such that the source gas composition is TiCl ₄ : 5-10 mol %, N ₂ : 30.0-50.0 mol %, the remainder is H ₂ , the temperature is 800-1000°C, and the pressure is 300-400 hPa.

The TiC layer of the lower layer, layer A, can be formed by chemical vapor deposition using TiCl ₄ , CH ₄ and H ₂ as source gases. The conditions for forming the TiC layer of layer A are preferably such that the source gas composition is TiCl ₄ : 1.0-5.0 mol %, CH ₄ : 3.0-7.0 mol %, the remainder is H ₂ , the temperature is 900-1100°C, and the pressure is 65-85 hPa.

The lower TiCN layer (B layer) can be formed by chemical vapor deposition using TiCl ₄ , CH ₃ CN and H ₂ as source gases. The conditions for forming the TiCN layer (B layer) are preferably such that the source gas composition is TiCl ₄ : 4-8 mol %, CH ₃ CN: 0.5-2.0 mol % and the remainder is H ₂ , the temperature is 750-950°C and the pressure is 60-80 hPa.

The C layer included in the lower layer is preferably a TiCNO layer or a TiCO layer.

The TiCNO layer of the lower C layer can be formed by chemical vapor deposition using TiCl ₄ , CO, N ₂ and H ₂ as source gases. The conditions for forming the TiCNO layer of the C layer are preferably a source gas composition of TiCl ₄ : 2.0-6.0 mol %, CO: 0.3-2.0 mol %, N ₂ : 20.0-50.0 mol %, and the remainder H ₂ , a temperature of 900-1100°C, and a pressure of 90-110 hPa.

The TiCO layer of the lower C layer can be formed by chemical vapor deposition using TiCl ₄ , CO and H ₂ as raw material gases. The conditions for forming the TiCO layer of the C layer are preferably such that the raw material gas has a composition of TiCl ₄ : 0.5-2.5 mol %, CO: 1.0-5.0 mol %, and the remainder is H ₂ , the temperature is 900-1100°C, and the pressure is 70-90 hPa.

<<Step 2: Cooling step>>
In the method for forming a coating layer of a coated cutting tool according to the present embodiment, a cooling step is carried out after the formation of the lower layer. In this specification, the cooling step may be referred to as “step 2”.

In the cooling step, the substrate on which the lower layer is formed is preferably cooled to a temperature of 180 to 220 ° C. and held for 50 to 70 minutes. At this time, it is preferable to hold in an _H2 gas atmosphere at a pressure of 180 to 220 hPa. By carrying out the cooling step, cracks can be introduced into the lower layer. In addition, by appropriately selecting the cooling conditions and carrying out the cooling step, a region that becomes the starting point of the cracks can be formed near the interface between the lower layer and the substrate, so that the average value X of the crack interval in the lower layer can be reduced by the first blasting step described later.

<<Step 3: Oxidation Treatment, and Step of Forming Intermediate Layer and Upper Layer>>
In the method for forming the coating layer of the coated cutting tool of this embodiment, after the cooling step, the intermediate layer and the upper layer are formed through the oxidation treatment. In this specification, the steps of the oxidation treatment and the formation of the intermediate layer and the upper layer may be referred to as "step 3".

The oxidation treatment can be performed by introducing _CO2 gas and _H2 gas at a predetermined temperature. The oxidation treatment conditions are as follows: the composition of the gas introduced is _CO2 : 0.2 to 1.0 mol% and the remainder is _H2 , the temperature is 900 to 1100°C, and the pressure is 50 to 70 hPa. The oxidation treatment time is preferably 1 to 10 minutes.

By carrying out the oxidation treatment, the surface of the lower layer is oxidized, thereby obtaining a coating layer with excellent adhesion between the lower layer and the intermediate layer, and peeling of the coating layer is suppressed, resulting in excellent chipping resistance and improved defect resistance.

In the method for forming the coating layer of the coated cutting tool of this embodiment, an intermediate layer is formed on the surface of the lower layer that has been subjected to an oxidation treatment. When forming the intermediate layer, it is preferable to first form a nucleus for the intermediate layer, and then form the intermediate layer.

The nucleation of the intermediate layer can be performed by chemical vapor deposition using AlCl ₃ , CO, CO ₂ , HCl and H ₂ as source gases. The nucleation conditions of the intermediate layer are preferably such that the source gas composition is AlCl ₃ : 1.0-5.0 mol %, CO: 0.5-2.0 mol %, CO ₂ : 0.8-3.0 mol %, HCl: 1.5-5.0 mol %, and the balance is H ₂ , the temperature is 900-1100°C, and the pressure is 60-80 hPa.

After the nucleation of the intermediate layer, the intermediate layer is formed using Al ₂ O ₃ as a material. The Al ₂ O ₃ layer of the intermediate layer is preferably an α-type Al ₂ O ₃ layer. The α-type Al ₂ O ₃ layer of the intermediate layer can be formed by chemical vapor deposition using AlCl ₃ , CO ₂ , HCl, H ₂ S and H ₂ as raw material gases. As for the film formation conditions, the composition of the raw material gas is preferably AlCl ₃ : 2.1 to 5.0 mol%, CO ₂ : 2.5 to 4.0 mol%, HCl: 2.0 to 3.0 mol%, H ₂ S: 0.1 to 0.45 mol%, and the balance is H ₂ , and the temperature is preferably 850 to 1050°C and the pressure is 60 to 80 hPa.

In this embodiment of the method for forming the coating layer of a coated cutting tool, the upper layer is formed after the intermediate layer is formed.

The Ti compound layers included in the upper layer of the coated cutting tool of this embodiment can include a TiN layer, a TiCN layer, a TiCNO layer, and a TiCO layer. When the upper layer is composed of an L layer, a TiCN layer, and a U layer, three Ti compound layers can be formed on the surface of the intermediate layer in the order of an L layer, a TiCN layer, and a U layer. The upper layer can be composed of at least one of a TiCN layer and a U layer.

Of the layers constituting the upper layer, the L layer, which is the layer closest to the substrate, can be a TiCNO layer, a TiCO layer, or a TiN layer.

The TiCNO layer of the upper L layer can be formed by chemical vapor deposition using TiCl ₄ , CO, N ₂ , and H ₂ as source gases. The conditions for forming the TiCNO layer of the L layer are preferably such that the source gas composition is TiCl ₄ : 2.0-7.0 mol %, CO: 0.4-1.5 mol %, N ₂ : 20.0-50.0 mol %, and the remainder is H ₂ , the temperature is 900-1100°C, and the pressure is 80-120 hPa.

The TiCO layer of the upper L layer can be formed by chemical vapor deposition using TiCl ₄ , CO, and H ₂ as raw material gases. The conditions for forming the TiCO layer of the L layer are preferably a raw material gas composition of TiCl ₄ : 0.8-2.5 mol %, CO: 1.5-3.5 mol %, and the remainder H ₂ , a temperature of 900-1100°C, and a pressure of 70-90 hPa.

The TiN layer of the upper L layer can be formed by chemical vapor deposition using TiCl ₄ , N ₂ , and H ₂ as source gases. The conditions for forming the TiN layer of the L layer are preferably such that the source gas composition is TiCl ₄ : 4.0-15.0 mol %, N ₂ : 20.0-60.0 mol %, and the remainder is H ₂ , the temperature is 900-1100°C, and the pressure is 300-400 hPa.

Among the layers constituting the upper layer, the layer made of TiCN (TiCN layer) formed on the L layer or on the intermediate layer can be formed by chemical vapor deposition using TiCl ₄ , CH ₃ CN, and H ₂ as source gases. The conditions for forming the TiCN layer are preferably such that the source gas composition is TiCl ₄ : 3.0 to 10.0 mol %, CH ₃ CN: 0.5 to 2.0 mol %, and the remainder is H ₂ , the temperature is 900 to 1100°C, and the pressure is 60 to 80 hPa.

Of the layers constituting the upper layer, the U layer, which is the layer furthest from the substrate, is preferably a layer made of TiN (TiN layer). The TiN layer of the U layer can be formed under the same conditions as those for forming the TiN layer of the L layer described above.

In this manner, a coating layer including a lower layer, an intermediate layer, and an upper layer can be formed.

<<Step 4: First blasting step>>
In the method for forming a coating layer of a coated cutting tool according to the present embodiment, after the coating layer is formed as described above, it is preferable to carry out a first blasting step on the treated surface of the coating layer. In this specification, the first blasting step may be referred to as "step 4".

The first blasting process can be carried out by projecting a predetermined projection material under predetermined projection conditions onto the treated surface of the coating layer. The first blasting process is preferably a dry blasting process. Dry blasting refers to a process in which a projection material is accelerated by compressed air and projected onto the treated object.

As the shot material (media) that can be used in the first blasting process, a shot material made of a rigid material such as metal or ceramic can be used. The shot material is preferably a material that is lower in hardness and has a higher specific gravity than the material that constitutes the coating layer. The shot material is preferably a material that is at least lower in hardness than Al ₂ O _3. Specifically, it is preferable to use steel, ZrO ₂ , etc. as the shot material in the first blasting process. The shape of the shot material can be any shape. In order to generate a predetermined crack in the coating layer, the shape of the shot material is preferably spherical. The average particle size of the shot material in the first blasting process is preferably 100 to 150 μm.

The average particle size of the shot material in the first blasting process can be the particle size of 50% of the total particles (average particle size: D50). The same applies to the other average particle sizes described in this specification. The average particle size (D50) can be determined by measuring the particle size distribution using the Microtrack method (laser diffraction scattering method) and obtaining the D50 value from the results of the particle size distribution measurement. The same applies to the shot material in the second blasting process described below.

Preferred blasting conditions for the first blasting step are a projection pressure of 0.1 to 0.2 MPa, a projection angle of 70 to 90 degrees, and a projection time of 20 to 120 seconds. By using specified blasting conditions in the first blasting step, it is possible to generate specified cracks in the coating layer.

The projection angle in the first blasting process (and the second blasting process described below) refers to the angle when projected perpendicularly onto the surface of the treated surface, which is set to 90 degrees. In addition, when projecting a predetermined projection material onto the treated surface of the coating layer under predetermined projection conditions, it is preferable to carry out the projecting process while masking the surface of the coating layer other than the treated surface, so that the projection material does not collide with anything other than the treated surface. The same applies to the second blasting process.

By carrying out the first blasting process in combination with the cooling process described above, it is possible to preferentially reduce the average crack spacing in the lower layer compared to the middle and upper layers. In addition, since the material of the blasting material is a material that is less hard and has a higher specific gravity than the material that constitutes the coating layer, it is possible to suppress the occurrence of cracks that originate from the surface of the coating layer.

<<Step 5: Second blasting step>>
In the method for forming the coating layer of the coated cutting tool of this embodiment, it is preferable to carry out the second blasting step after carrying out the first blasting step on the treated surface of the coating layer as described above. In this specification, the second blasting step may be referred to as "step 5".

The second blasting process can be carried out by projecting a predetermined projection material under predetermined projection conditions onto the treated surface of the coating layer. The second blasting process is preferably a wet blasting process. Wet blasting refers to a process in which a mixture of water and projection material is accelerated by compressed air and projected onto the treated object.

The shot material (media) that can be used in the second blasting process may be a shot material made of a rigid material such as metal, ceramic, etc. The shot material preferably has a specific gravity equal to or less than that of Al ₂ O ₃ and a hardness equal to or greater than that of Al ₂ O _3. Specifically, the shot material used in the second blasting process is preferably made of Al ₂ O ₃ , SiC, cBN, etc.

The shape of the shot material used in the second blasting process can be any shape. However, as described below, it is preferable that the projection angle in the second blasting process is smaller than the projection angle in the first blasting process. The smaller the projection angle, the more likely it is that the coating layer will be removed by the blasting process. Therefore, in order to suppress removal of the coating layer in the second blasting process, it is preferable that the shape of the shot material is spherical rather than polygonal. The average particle size (D50) of the shot material in the second blasting process is preferably 30 to 50 μm.

Preferably, the blasting conditions for the second blasting process are a projection pressure of 0.1 to 0.2 MPa, a projection angle of 5 to 50 degrees, and a projection time of 10 to 260 seconds. The projection angle for the second blasting process is preferably smaller than the projection angle for the first blasting process. By using specified blasting conditions in the second blasting process, it is possible to generate specified cracks in the coating layer.

By carrying out the second blasting step, cracks can be preferentially introduced starting from the surface side of the coating layer opposite to the substrate. In addition, in the second blasting step, the smaller the projection angle is, the smaller the average crack spacing in the upper layer can be while suppressing the average crack spacing in the intermediate layer from becoming smaller. Also, by using a projection material having a hardness equal to or greater than _Al2O3 and a specific gravity equal to or less than _Al2O3 as the projection material, the average crack spacing in the upper layer can be reduced while suppressing the average crack spacing in the intermediate layer from becoming smaller.

As described above, by appropriately controlling the conditions of the cooling process (process 2), the first blasting process (process 4), and the second blasting process (process 5), the average crack spacing values X (lower layer), Y (middle layer), and Z (upper layer) can be controlled within a predetermined range.

Specifically, the average crack spacing X of the lower layer can be controlled by adjusting the projection time in the first blasting process or the average thickness of the entire lower layer. Specifically, the average crack spacing X can be reduced by lengthening the projection time in the first blasting process or reducing the average thickness of the entire lower layer.

The average crack spacing Y of the intermediate layer can be controlled by adjusting the projection angle in the second blasting process, the average thickness of the entire upper layer, and/or the average thickness of the TiCN layer if one is formed as the upper layer. Specifically, the average crack spacing Y can be reduced by reducing the projection angle in the second blasting process, increasing the average thickness of the entire upper layer, and/or increasing the average thickness of the TiCN layer of the upper layer.

The average crack spacing Z in the upper layer can be controlled by adjusting the projection time in the second blasting process. Specifically, the average crack spacing Z can be reduced by increasing the projection time in the second blasting process.

The difference Y-X between the average crack spacing Y of the intermediate layer and the average crack spacing X of the lower layer can be controlled by adjusting the average crack spacing X and the average crack spacing Y. Specifically, the difference Y-X in crack spacing can be increased by decreasing the average crack spacing X and/or increasing the average crack spacing Y.

The difference Y-Z between the average crack spacing Y of the middle layer and the average crack spacing Z of the upper layer can be controlled by adjusting the average crack spacing Z and the average crack spacing Y. Specifically, the difference Y-X in crack spacing can be increased by decreasing the average crack spacing Z and/or increasing the average crack spacing Y.

The following provides a detailed explanation of the present invention using examples, but the present invention is not limited to these.

As described below, coated cutting tools, Invention Products 1 to 37, were manufactured as examples of this embodiment. In addition, Comparative Products 1 to 20, coated cutting tools, were manufactured as comparative examples.

As a substrate, a cemented carbide cutting insert having a composition of 89.2 % WC-8.8 % Co-2.0 % NbC (all mass%) in the shape of CNMG120408-TM (manufactured by Tungaloy Corporation) was prepared. After performing round honing on the cutting edge of this substrate with a SiC brush, the surface of the substrate was cleaned.

Next, the substrates of the invention and comparison products were placed in an externally heated chemical vapor deposition apparatus, and a coating layer was formed on the substrate surface to have the composition and average thickness of the coating layer (lower layer, middle layer, and upper layer) shown in Tables 1 to 4 (steps 1 to 3). After that, a specified blasting treatment was performed (steps 4 and 5). The formation conditions for each layer are explained below.

<Step 1: Formation of Lower Layer>
A lower layer having the composition and average thickness shown in Tables 1 and 2 was formed on the surface of the substrate of the invention product and the comparative product by chemical vapor deposition (Step 1). As shown in Tables 1 and 2, a lower layer consisting of three layers, namely, layers A, B, and C, was formed in all the invention products and the comparative products. Table 5 shows the formation conditions of each layer for each composition in the lower layer.

<Step 2: Cooling step>
The cooling step (step 2) was carried out on the invention products 1 to 37 and the comparative products 1 to 18 and 20 in which the lower layer was formed. The cooling step was not carried out on the comparative product 19.

In the cooling step, after forming the lower layer of the invention products and the comparative products other than the comparative product 19, the chamber was evacuated to 30 hPa. Next, the temperature in the chamber was lowered to 200 ° C. and held in a _H2 gas atmosphere of 200 hPa for 60 minutes. After the cooling step of the invention products and the comparative products other than the comparative product 19 was completed, the next step 3 was performed.

<Step 3: Oxidation Treatment, and Step of Forming Intermediate Layer and Upper Layer>
The invention product and the comparative product that had been subjected to the cooling step were subjected to the oxidation treatment and the step of forming the intermediate layer and the upper layer (step 3). In the case of the comparative product 19, step 3 was performed after the lower layer was formed.

<<Oxidation treatment>>
All the invention products and the comparative products were subjected to an oxidation treatment. The oxidation treatment was carried out by introducing an oxidation treatment gas of 0.5 mol% _CO2 gas and 99.5 mol% _H2 gas, and maintaining the temperature at 1000°C and the pressure at 60 hPa for 3 minutes.

<<Formation of intermediate layer>>
An intermediate layer having the composition and average thickness shown in Tables 1 and 2 was formed by chemical vapor deposition on the surface of the lower layer of the invention and comparison products. As shown in Tables 1 and 2, an intermediate layer made of α-type _Al2O3 was formed in all invention and comparison products. Table 6 shows the conditions _for nucleation of the intermediate layer and deposition of _the α-type _Al2O3 layer.

<<Formation of upper layer>>
An upper layer having the composition and average thickness shown in Tables 3 and 4 was formed on the surface of the intermediate layer by chemical vapor deposition. As shown in Tables 3 and 4, in the inventive products 22, 23, 26, 27, and 30 to 35 and the comparative product 18, an upper layer consisting of three layers, an L layer, a TiCN layer, and a U layer, was formed. In the inventive products 18 and 20, an upper layer consisting of only a TiCN layer was formed. In the inventive product 24, an upper layer consisting of only a U layer was formed. In the other inventive products and comparative products, an upper layer consisting of two layers, a TiCN layer and a U layer, was formed. The upper layer was formed on the intermediate layer in the order of an L layer, a TiCN layer, and a U layer. Table 7 shows the conditions for forming the L layer, the TiCN layer, and the U layer of the upper layer.

<Step 4: First blasting step>
The invention product and the comparative product having the coating layers (lower layer, middle layer, and upper layer) formed as described above were subjected to the first blasting step (step 4). However, the comparative product 19 was not subjected to the first blasting step.

The blasting treatment in the first blasting step was carried out by projecting a predetermined projection material under predetermined projection conditions onto the treated surface of the coating layer. The blasting treatment in the first blasting step was carried out as a dry blasting treatment.

The shot material (media) used in the first blasting process was a spherical shot material made of steel with an average particle size of 120 μm. The blasting conditions for the first blasting process were a shot pressure of 0.15 MPa and a shot angle of 90 degrees. The shot angle refers to the angle when the shot is projected perpendicularly onto the surface of the surface to be treated. Tables 8 and 9 show the shot times for the first blasting process for each of the inventive products and the comparative products (excluding comparative product 19).

When a specific projection material is projected under specific projection conditions onto the treated surface of the coating layer, the surface of the coating layer other than the treated surface is masked to prevent the projection material from colliding with anything other than the treated surface.

<Step 5: Second blasting step>
After the first blasting step, the invention product and the comparative product having the coating layers (lower layer, middle layer and upper layer) formed as described above were subjected to the second blasting step (step 5). However, the comparative product 20 was not subjected to the second blasting step.

The blasting treatment in the second blasting step was carried out by projecting a predetermined projection material under predetermined projection conditions onto the treated surface of the coating layer. Note that the blasting treatment in the second blasting step was carried out as a wet blasting treatment.

The projection material (media) used in the second blasting step was spherical projection material made of _Al2O3 with an average particle size of 40 μm. The projection pressure was set to 0.15 MPa as the blasting condition in the second blasting step. _Tables 8 and 9 show the projection angle and projection time in the second blasting step for each of the inventive products and the comparative products (excluding comparative product 20). The projection angle means the angle when the angle when projected perpendicularly to the surface of the treated surface is set to 90 degrees.

When a specific projection material is projected under specific projection conditions onto the treated surface of the coating layer, the surface of the coating layer other than the treated surface is masked to prevent the projection material from colliding with anything other than the treated surface.

By using the above process, the inventive and comparative coated cutting tools were manufactured.

<Measurement of the average thickness of each layer>
The average thickness of each layer of the coating layer of each invention product and comparative product was measured by SEM at three points on the cross section near a position 50 μm from the cutting edge toward the center of the rake face of the coated cutting tool, and the average value of the three points was taken as the average thickness. Tables 1 to 4 show the average thickness of each layer of the coating layer of each invention product and comparative product.

<Measurement of composition of each layer>
The composition of each layer of the coating layer of each invention product and comparative product was measured by measuring the cross section of a specific layer of the coated cutting tool using an energy dispersive X-ray spectrometer (EDS). The crystal system of the intermediate layer was identified by X-ray diffraction measurement using a 2θ/θ focusing optical system. Tables 1 to 4 show the composition of each layer of the coating layer of each invention product and comparative product (type of elements contained in each layer).

<Measurement of crack spacing>

The average crack spacings X, Y, and Z of the coating layers (lower layer, middle layer, and upper layer) of the coated cutting tools of the invention and the comparative product were measured. The method for measuring and calculating the average crack spacings X, Y, and Z of the coating layers is as follows.

The average crack spacing Z of the upper layer of the coating layer was measured as follows. First, the surface side of the coating layer opposite the substrate was polished until the thickness of the upper layer was 70% of the average thickness of the upper layer, and a cross section of the upper layer parallel to the substrate surface was obtained. Next, the cross section of the obtained upper layer was observed with a scanning electron microscope (SEM) at a magnification of 2000 times, and multiple observation images were taken to obtain multiple cross-sectional SEM photographs. By combining multiple cross-sectional SEM photographs, an SEM photograph including a cross section in a range of 200 μm vertically and 300 μm horizontally was obtained. In the obtained SEM photograph, 10 straight lines (straight lines across the entire horizontal direction of the SEM photograph) with a length of 300 μm or more parallel to the horizontal direction of the SEM photograph were drawn at intervals of 20 μm in the vertical direction of the SEM photograph. In each of the 10 straight lines, the intersections between the straight lines and the cracks were identified, and the distance between adjacent intersections was measured. The average crack spacing Z for the cracks in the upper layer was calculated by adding up all the distances between adjacent intersections of the 10 straight lines and dividing the total distance between all the measured intersections.

The average crack spacing Y of the intermediate layer of the coating layer was measured as follows. First, similar to the measurement of the crack spacing of the upper layer, the surface side of the coating layer opposite the substrate was polished until the thickness of the intermediate layer was 80% of the average thickness of the intermediate layer, and a cross section of the intermediate layer parallel to the substrate surface was obtained. Then, similar to the measurement of the crack spacing of the upper layer, the average crack spacing Y of the cracks in the intermediate layer was measured and calculated from an SEM photograph of the cross section of the intermediate layer.

The average crack spacing X of the lower layer of the coating layer was measured as follows. First, similar to the measurement of the crack spacing of the upper layer, the surface side of the coating layer opposite the substrate was polished until the thickness of the lower layer was 70% of the average thickness of the lower layer, and a cross section of the lower layer parallel to the substrate surface was obtained. Then, similar to the measurement of the crack spacing of the upper layer, the average crack spacing X of the cracks in the lower layer was measured and calculated from an SEM photograph of the cross section of the lower layer.

Tables 10 and 11 show the average crack spacing values X, Y, and Z of the lower, middle, and upper layers of the coated cutting tools of the invention and the comparative products measured as described above. Tables 10 and 11 also show the difference Y-Z between the average crack spacing value Y of the middle layer and the average crack spacing value Z of the upper layer, and the difference Y-X between the average crack spacing value Y of the middle layer and the average crack spacing value X of the lower layer, calculated using the average crack spacing values X, Y, and Z.

<Cutting test>
The coated cutting tools of the present invention and the comparative example were subjected to cutting tests to evaluate chipping resistance (tool life). Two types of cutting tests, Cutting Tests 1 and 2, were performed using the coated cutting tools of the present invention and the comparative example as samples.

The cutting test conditions for cutting test 1 are as follows.
[Cutting test conditions for cutting test 1]
Work material: S45C,
Shape of workpiece: Round bar with four equally spaced grooves on the outer periphery.
Cutting speed: 90 m/min,
Cutting depth: 1.0 mm,
Feed: 0.2 mm/rev.
Coolant: Yes,
Insert shape: CNMG120408-TM (manufactured by Tungaloy Corporation),
Composition of substrate: 89.2 % WC-8.8 % Co-2.0 % NbC (all by mass%)
Evaluation item: The time when the sample was chipped was regarded as the tool life, and the number of impacts until the tool life was reached was measured.

Tables 12 and 13 show the number of impacts and the evaluation until the end of the tool life in cutting test 1. In addition, the evaluation of cutting test 1 was performed as follows: when the tool life (number of impacts) was 12,000 times or more, it was evaluated as A; when it was 8,000 times or more but less than 12,000 times, it was evaluated as B; and when it was less than 8,000 times, it was evaluated as C.

The cutting test conditions for cutting test 2 are as follows.
[Cutting test conditions for cutting test 2]
Work material: SCM440,
Workpiece shape: round bar,
Cutting speed: 250 m/min,
Cutting depth: 2.0 mm,
Feed: 0.25 mm/rev.
Coolant: Yes,
Insert shape: CNMG120408-TM (manufactured by Tungaloy Corporation),
Composition of substrate: 89.2 % WC-8.8 % Co-2.0 % NbC (all by mass%)
Evaluation item: The tool life was determined as the time when the sample was chipped or the maximum flank wear width reached 0.3 mm, and the machining time until the tool life was reached was measured.

Tables 12 and 13 show the machining time and evaluation until the end of tool life in cutting test 2. In addition, the evaluation of cutting test 2 was as follows: A for tool life (machining time) of 25 minutes or more, B for tool life of 20 minutes or more but less than 25 minutes, and C for tool life of less than 20 minutes.

<<Cutting test evaluation>>
All of the inventive products were evaluated as B or higher in both cutting test 1 and cutting test 2. Therefore, it can be said that the inventive products are coated cutting tools having excellent chipping resistance and fracture resistance and a long tool life.

On the other hand, the comparative products received a rating of C in at least one of Cutting Test 1 and Cutting Test 2. Therefore, compared to the inventive products, the comparative products were found to be coated cutting tools that did not have sufficient chipping resistance and fracture resistance, or did not have sufficient wear resistance, resulting in a short tool life. Furthermore, comparative products 17 and 18 suffered chipping damage in Cutting Test 2, which led to fracture.

Reference Signs List 1 Substrate 2 Lower layer 3 Intermediate layer 4 Upper layer 5 Coating layer 6 Coated cutting tool

Claims (7)

A coated cutting tool including a substrate and a coating layer formed on a surface of the substrate,
the coating layer includes a lower layer, an intermediate layer, and an upper layer in this order from the substrate side;
the lower layer includes one or more Ti compound layers each comprising a Ti compound of Ti and at least one element selected from the group consisting of C, N, O, and B;
The average thickness of the lower layer is 1.5 μm or more and 15.0 μm or less,
the intermediate layer contains α-aluminum oxide,
The average thickness of the intermediate layer is 2.0 μm or more and 15.0 μm or less,
the upper layer includes one or more Ti compound layers each comprising a Ti compound of Ti and at least one element selected from the group consisting of C, N, O, and B;
The average thickness of the upper layer is 0.5 μm or more and 5.0 μm or less,
The average crack spacing X of the lower layer is 0.5 μm or more and less than 10.0 μm,
The average crack spacing Y of the intermediate layer is 26.8 μm or more and 100.0 μm or less,
A coated cutting tool, wherein an average crack spacing Z of the upper layer is 0.5 μm or more and 10.0 μm or less. The coated cutting tool according to claim 1, wherein the average thickness of the coating layer is 8.0 μm or more and 30.0 μm or less. The coated cutting tool according to claim 1 or 2, wherein the upper layer includes at least a TiCN layer. The coated cutting tool according to claim 3, wherein the average thickness of the TiCN layer of the upper layer is 0.5 μm or more and 5.0 μm or less. The coated cutting tool according to claim 1 or 2, wherein the difference Y-Z between the average crack spacing Y and the average crack spacing Z is 20.0 μm or more and 95.0 μm or less. The coated cutting tool according to claim 1 or 2, wherein the difference Y-X between the average crack spacing Y and the average crack spacing X is 25.0 μm or more and 95.0 μm or less. The coated cutting tool according to claim 1 or 2, wherein the substrate is a cemented carbide, a cermet, a ceramic, or a cubic boron nitride sintered body.