Coated cutting tool

Abstract

A cutting tool comprising a substrate and a coating layer formed on the substrate, wherein the coating layer has, from a side closer to the substrate, a lower layer that contains a compound having a composition represented by (Al.sub.xTi.sub.1-x)N, and an upper layer that is formed on the lower layer and contains a compound having a composition represented by (Al.sub.yTi.sub.1-yN; the average thickness of the lower layer is 1.0 μm or more and 10.0 μm or less; the average thickness of the upper layer is 1.0 μm or more and 10.0 μm or less; and an area ratio GOS.sub.i of crystal grains having a GOS value of 1 degree or lower in the lower layer and an area ratio GOS.sub.s of crystal grains having a GOS value of 1 degree or lower in the upper layer satisfy GOS.sub.i<GOS.sub.s.

Claims

1. A cutting tool comprising a substrate and a coating layer formed on the substrate, wherein the coating layer comprises, in order from a side closer to the substrate, a lower layer that comprises a compound having a composition represented by following formula (1), and an upper layer that is formed on the lower layer and comprises a compound having a composition represented by following formula (2),
(Al.sub.xTi.sub.1-x)N  (1) in the formula (1), x represents an atomic ratio of the Al element to a total of the Al element and the Ti element, and satisfies 0.60≤x≤0.95,
(Al.sub.yTi.sub.1-y)N  (2) in the formula (2), y represents an atomic ratio of the Al element to a total of the Al element and the Ti element, and satisfies 0.50≤y≤0.85; an average thickness of the lower layer is 1.0 μm or more and 10.0 μm or less, and the average thickness of the upper layer is 1.0 μm or more and 10.0 μm or less; and an area ratio GOS.sub.i of crystal grains having a GOS value of 1 degree or lower in the lower layer and an area ratio GOS.sub.s of crystal grains having a GOS value of 1 degree or lower in the upper layer satisfy a condition represented by following formula (3),
GOS.sub.i<GOS.sub.s  (3).

2. The cutting tool according to claim 1, wherein a ratio KAM.sub.i of measurement points showing a KAM value of 1 degree or lower in the lower layer is 50% or more and 90% or less, and a ratio KAM.sub.s of measurement points showing a KAM value of 1 degree or lower in the upper layer is 50% or more and 95% or less.

3. The coated cutting tool according to claim 1, wherein the GOS.sub.s is 55% or more and 90% or less.

4. The coated cutting tool according to claim 1, wherein the GOS.sub.i is 10% or more and less than 55%.

5. The coated cutting tool according to claim 1, wherein the atomic ratio x of the Al element in the lower layer and the atomic ratio y of the Al element in the upper layer satisfy a condition represented by following formula (4):
y<x  (4).

6. The coated cutting tool according to claim 1, wherein an average thickness of the entire coating layer is 3.0 μm or more and 15.0 μm or less.

7. The coated cutting tool according to claim 1, wherein the substrate is any of a cemented carbide, a cermet, a ceramic, and a cubic boron nitride sintered body.

8. The coated cutting tool according to claim 2, wherein the GOS.sub.s is 55% or more and 90% or less.

9. The coated cutting tool according to claim 2, wherein the GOS.sub.i is 10% or more and less than 55%.

10. The coated cutting tool according to claim 3, wherein the GOS.sub.i is 10% or more and less than 55%.

11. The coated cutting tool according to claim 8, wherein the GOS.sub.i is 10% or more and less than 55%.

12. The coated cutting tool according to claim 2, wherein the atomic ratio x of the Al element in the lower layer and the atomic ratio y of the Al element in the upper layer satisfy a condition represented by following formula (4):
y<x  (4).

13. The coated cutting tool according to claim 3, wherein the atomic ratio x of the Al element in the lower layer and the atomic ratio y of the Al element in the upper layer satisfy a condition represented by following formula (4):
y<x  (4).

14. The coated cutting tool according to claim 4, wherein the atomic ratio x of the Al element in the lower layer and the atomic ratio y of the Al element in the upper layer satisfy a condition represented by following formula (4):
y<x  (4).

15. The coated cutting tool according to claim 8, wherein the atomic ratio x of the Al element in the lower layer and the atomic ratio y of the Al element in the upper layer satisfy a condition represented by following formula (4):
y<x  (4).

16. The coated cutting tool according to claim 9, wherein the atomic ratio x of the Al element in the lower layer and the atomic ratio y of the Al element in the upper layer satisfy a condition represented by following formula (4):
y<x  (4).

17. The coated cutting tool according to claim 10, wherein the atomic ratio x of the Al element in the lower layer and the atomic ratio y of the Al element in the upper layer satisfy a condition represented by following formula (4):
y<x  (4).

18. The coated cutting tool according to claim 11, wherein the atomic ratio x of the Al element in the lower layer and the atomic ratio y of the Al element in the upper layer satisfy a condition represented by following formula (4):
y<x  (4).

19. The coated cutting tool according to claim 2, wherein an average thickness of the entire coating layer is 3.0 μm or more and 15.0 μm or less.

20. The coated cutting tool according to claim 3, wherein an average thickness of the entire coating layer is 3.0 μm or more and 15.0 μm or less.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The FIGURE is a schematic cross-sectional view showing an example of a coated cutting tool according to the present invention.

DESCRIPTION OF EMBODIMENTS

(2) An embodiment for carrying out the present invention (hereinafter simply referred to as the “present embodiment”) will hereinafter be described in detail, with reference to the attached drawings as appropriate. However, the present invention is not limited to the present embodiment below. Various modifications may be made to the present invention without departing from the gist of the invention. It should be noted that, in the drawings, unless otherwise specified, positional relationships, such as vertical and horizontal relationships, are based on the positional relationships shown in the drawings. Further, the dimensional ratios of the drawings are not limited to those shown therein.

(3) The coated cutting tool of the present embodiment comprises a substrate and a coating layer formed on the substrate, wherein the coating layer has, from a side closer to the substrate, a lower layer that contains a compound having a composition represented by the following formula (1), and an upper layer that is formed on the lower layer and contains a compound having a composition represented by the following formula (2),
(Al.sub.xTi.sub.1-x)N  (1)
in the formula (1), x represents an atomic ratio of the Al element to a total of the Al element and the Ti element, and satisfies 0.60≤x≤0.95,
(Al.sub.yTi.sub.1-y)N  (2)
in the formula (2), y represents an atomic ratio of the Al element to a total of the Al element and the Ti element, and satisfies 0.50≤y≤0.85;

(4) an average thickness of the lower layer is 1.0 μm or more and 10.0 μm or less, and the average thickness of the upper layer is 1.0 μm or more and 10.0 μm or less; and

(5) an area ratio GOS.sub.i of crystal grains having a GOS value of 1 degree or lower in the lower layer and an area ratio GOS.sub.s of crystal grains having a GOS value of 1 degree or lower in the upper layer satisfy a condition represented by following formula (3),
GOS.sub.i<GOS.sub.s  (3)

(6) Because the coated cutting tool of the present embodiment is provided with the above configuration, the generation of thermal cracks is suppressed even under cutting conditions under which a load acts at high speed, so that the fracture resistance can be improved and the wear resistance can be also improved, thereby making it possible to extend the tool life. The factors that improve the wear resistance and fracture resistance of the coated cutting tool of the present embodiment are considered as follows. However, the present invention is not limited to the following factors. That is, first, in the coated cutting tool of the present embodiment, where x in the formula (1) is 0.60 or more in the lower layer that contains the compound having the composition represented by the formula (1), the wear resistance is improved due to increase in hardness caused by solid solution strengthening, and oxidation resistance is improved due to increase in Al amount. As a result, in the coated cutting tool of the present embodiment, the crater wear resistance is improved, so that the decrease in strength of the cutting edge is suppressed, thereby increasing the fracture resistance. Meanwhile, in the coated cutting tool of the present embodiment, where x in the formula (1) is 0.95 or less in the lower layer that contains the compound having the composition represented by the formula (1), the toughness is improved because Ti is contained, so that the fracture resistance is improved. Further, in the coated cutting tool of the present embodiment, where y in the formula (2) is 0.50 or more in the upper layer that contains the compound having the composition represented by the formula (2), the wear resistance is improved due to increase in hardness caused by solid solution strengthening, and oxidation resistance is improved due to increase in Al amount. As a result, in the coated cutting tool of the present embodiment, the crater wear resistance is improved, so that the decrease in strength of the cutting edge is suppressed, thereby increasing the fracture resistance. Meanwhile, in the coated cutting tool of the present embodiment, where y in the formula (2) is 0.85 or less in the upper layer that contains the compound having the composition represented by the formula (2), the toughness is improved because Ti is contained, so that the fracture resistance is improved. Further, when the GOS.sub.i in the lower layer is lower than the GOS.sub.s in the upper layer, it is possible to improve the fracture resistance and also improve the wear resistance by suppressing the generation of thermal cracks. A lower area ratio of GOS.sub.i in the lower layer means that crystal grains having high hardness are dispersed. In other words, in the lower layer, the area ratio of the crystal grains having a GOS value larger than 1 degree, that is, the crystal grains having a large strain, increases. Therefore, where the average thickness of the lower layer is 1.0 μm or more, the wear resistance of the coated cutting tool is improved. Meanwhile, where the average thickness of the lower layer is 10.0 μm or less, the adhesion between the substrate and the coating layer can be further enhanced, so that the fracture resistance of the coated cutting tool is improved. Further, a higher area ratio of GOS.sub.s in the upper layer means that the effect of suppressing the generation of thermal cracks is enhanced. Therefore, where the average thickness of the upper layer is 1.0 μm or more, the fracture resistance of the coated cutting tool is improved. Meanwhile, where the average thickness of the upper layer is 10.0 μm or less, the adhesion is improved, thereby enhancing the effect of suppressing the occurrence of peeling. Therefore, the fracture resistance of the coated cutting tool is improved. It is considered that by combining these features, the coated cutting tool of the present embodiment has improved wear resistance and fracture resistance, and as a result, the tool life can be extended.

(7) The FIGURE is a schematic sectional view showing an example of the coated cutting tool of the present embodiment. A coated cutting tool 5 comprises a substrate 1 and a coating layer 4 formed on the surface of the substrate 1. In the coating layer 4, a lower layer 2 and an upper layer 3 are stacked upward in this order from the substrate side.

(8) The coated cutting tool according to the present embodiment comprises a substrate and a coating layer formed on the surface of the substrate. Specific examples of the type of the coated cutting tool comprise an interchangeable cutting insert for milling or turning, a drill, and an end mill.

(9) The substrate used in the present embodiment is not particularly limited as long as it may be used for a coated cutting tool. Examples of such a substrate comprise a cemented carbide, a cermet, a ceramic, a cubic boron nitride sintered body, a diamond sintered body, and high-speed steel. Among them, the substrate is preferably a cemented carbide, a cermet, a ceramic or a cubic boron nitride sintered body, because more excellent wear resistance and fracture resistance can be realized, and from the same viewpoint, the substrate is more preferably a cemented carbide.

(10) The surface of the substrate may be modified. For example, where the substrate is composed of a cemented carbide, a β-free layer may be formed on the surface thereof. Further, where the substrate is made of a cermet, a hardened layer may be formed on the surface. Even if the surface of the substrate is modified as described above, the function and effect of the present invention can be obtained.

(11) The average thickness of the entire coating layer used in the present embodiment is preferably 3.0 μm or more and 15.0 μm or less. Where the average thickness of the entire coating layer is 3.0 μm or more, the wear resistance is improved, and where the average thickness of the coating layer is 15.0 μm or less, the adhesion and fracture resistance of the coating layer to the substrate are improved. From the same viewpoint, the average thickness of the coating layer is more preferably 4.0 μm or more and 13.0 μm or less, further preferably 5.0 μm or more and 12.0 μm or less. The average thickness of each layer and the entire coating layer in the coated cutting tool of the present embodiment can be obtained by measuring the thickness of each layer or the thickness of the entire coating layer from cross sections in three or more locations of each layer or the entire coating layer and calculating the arithmetic mean value.

(12) A calculation method for quantifying strain in a crystal grain by an electron backscatter diffraction (hereinafter also referred to as “EBSD”) method using a scanning electron microscope can be exemplified by Grain Orientation Spread (hereinafter also referred to as “GOS”) that quantifies the average misorientation in a crystal grain and Kernel Average Misorientation (hereinafter also referred to as “KAM”) that quantifies the misorientation between an arbitrary measurement point and a measurement point close thereto in a crystalline grain. The GOS value and the KAM value will be described below.

(13) GOS Value

(14) When the GOS value is small, the distribution of strain within the crystal grains becomes close to uniform. Where the area ratio of the crystal grains having a small GOS value is increased, the number of places that are the starting points of crack generation is reduced, and the generation of thermal cracks can be suppressed. Meanwhile, when the GOS value is large, the strain of crystal grains is large. Where the area ratio of the crystal grains having a large GOS value is increased, the hardness becomes high, so that the wear resistance is improved.

(15) In the coated cutting tool of the present embodiment, the area ratio GOS.sub.i of the crystal grains having a GOS value of 1 degree or lower in the lower layer and the area ratio GOS.sub.s of the crystal grains having a GOS value of 1 degree or lower in the upper layer satisfy the condition represented by the following formula (3).
GOS.sub.i<GOS.sub.s  (3)

(16) Where the GOS.sub.i in the lower layer is lower than the GOS.sub.s in the upper layer, it is possible to improve the fracture resistance and also improve the wear resistance by suppressing the generation of thermal cracks.

(17) Further, the GOS.sub.i in the lower layer is preferably 10% or more and less than 55%, more preferably 10% or more and 50% or less, further preferably 12% or more and 42% or less, and particularly preferable 14% or more and 36% or less. Where the GOS.sub.i in the lower layer is less than 55%, the GOS value is larger than 1 degree, that is, the crystal grains having a large strain and a high hardness are sufficiently dispersed, whereby the hardness is improved and the wear resistance of the coated cutting tool tends to be further improved. Meanwhile, where the GOS.sub.i in the lower layer is 10% or more, the production is facilitated.

(18) Further, the GOS.sub.s in the upper layer is preferably 50% or more and 90% or less, more preferably 53% or more and 90% or less, further preferably 55% or more and 90% or less, even more preferably 55% or more and 88% or less, and particularly preferably 55% or more and 86% or less. Where the GOS.sub.s in the upper layer is 50% or more, the number of places that are the starting points of crack generation is reduced, and the generation of thermal cracks can be further suppressed. Meanwhile, where the GOS.sub.s in the upper layer is 90% or less, the production is facilitated.

(19) In the present embodiment, the GOS value can be measured as follows. In the sample of a coated cutting tool, a cross section is exposed by polishing in a direction substantially parallel to the substrate surface at 0.5 μm from the surface of the lower layer or the upper layer toward the substrate. Using EBSD (manufactured by TexSEM Laboratories, Inc.), each measurement region of the cross section in the lower layer and the upper layer is divided into regular hexagonal measurement points (hereinafter also referred to as “pixels”). As to each of the divided pixels, the orientation of the pixel is measured by obtaining Kikuchi patterns from reflected electrons from an electron beam which has been incident on the cross section (polished surface) of the sample. The obtained orientation data is analyzed using analysis software for the EBSD, thereby calculating various parameters. The measurement conditions are as set forth below: the acceleration voltage is 15 kV, the size of the measurement region is 30 μm×50 μm, and the distance (step size) between adjacent pixels is 0.05 μm. When there is a misorientation of 5 degrees or more between adjacent pixels, it is defined as a grain boundary. Further, a region surrounded by grain boundaries is defined as one crystal grain. However, a pixel that is present independently with a misorientation of 5 degrees or more with all the adjacent pixels is not considered to be a crystal grain, and two or more pixels connected together are treated as a crystal grain.

(20) Further, the misorientation in a crystal grain between two different pixels in the same crystal grain is calculated, and the averaged value thereof is defined as the GOS value. That is, the GOS value can be expressed by following formula (5).

(21) $\begin{array}{cc}\mathrm{GOS}=\frac{\underset{i,j=1}{\overset{n}{.Math.}}{\alpha }_{\mathrm{ij}}\left(i\ne j\right)}{n\left(n-1\right)}& \left(5\right)\end{array}$
in the formula (5), n represents the number of pixels in the same crystal grain, i and j represent numbers assigned to different pixels in the crystal grain, where 1≤i and j≤n), α.sub.ij (i≠j) represents the crystal misorientation obtained from the crystal orientation at pixel i and the crystal orientation at pixel j.

(22) The measurement under the abovementioned measurement conditions and measurement range is carried out in five fields of view. Next, the total number of pixels belonging to the crystal grains (for example, cubic crystals) that constitute the lower layer or the upper layer is obtained, the GOS value is divided at intervals of 1 degree, and the number of pixels in the crystal grains in which the GOS value is included in the range of this value is aggregated and divided by the total number of pixels to create a histogram indicating the area ratio of the GOS value in the lower layer or the upper value. The area ratio GOS.sub.i of the crystal grains having a GOS value of 1 degree or lower in the lower layer and the area ratio GOS.sub.s of the crystal grains having a GOS value of 1 degree or lower in the upper layer can be obtained based on the created histogram.

(23) KAM Value

(24) The KAM value is a numerical value indicating a local misorientation which is a difference in crystal orientation between adjacent measurement points in a crystal orientation analysis based on the EBSD method. The larger the KAM value, the larger the difference in crystal orientation between adjacent measurement points, and the smaller the KAM value, the smaller the local strain in the crystal grains.

(25) In the coated cutting tool of the present embodiment, it is preferable that the ratio KAM.sub.i of the measurement points at which the KAM value is 1 degree or lower be 50% or more and 90% or less in the lower layer. Where the KAM.sub.i is 50% or more, the strain in the crystal grains is small, so that the toughness is improved and the generation of thermal cracks tends to be further suppressed. Meanwhile, it is preferable that the KAM.sub.i be 90% or less because the production is facilitated. From the same viewpoints, the KAM.sub.i is more preferably 52% or more and 85% or less, and further preferably 52% or more and 79% or less.

(26) Further, in the coated cutting tool of the present embodiment, it is preferable that the ratio KAM.sub.s of the measurement points at which the KAM value is 1 degree or lower be 50% or more and 95% or less in the upper layer. Where the KAM.sub.s is 50% or more, the strain in the crystal grains is small, so that the toughness is improved and the generation of thermal cracks tends to be further suppressed. Meanwhile, where the KAM.sub.s is 95% or less, the production is facilitated. From the same viewpoints, it is more preferable that KAM.sub.s be 52% or more and 91% or less.

(27) In the coated cutting tool of the present embodiment, when the KAM.sub.i and KAM.sub.s are in the above ranges, the strain in the crystal grains becomes small in the entire upper layer and lower layer, and the generation of thermal cracks tends to be further suppressed.

(28) In the present embodiment, the KAM value can be measured as follows. In the sample of a coated cutting tool, a cross section is exposed by polishing in a direction substantially parallel to the substrate surface at 0.5 μm from the surface of the lower layer or the upper layer toward the substrate. Using EBSD (manufactured by TexSEM Laboratories, Inc.), each measurement region of the cross section in the lower layer and the upper layer is divided into regular hexagonal measurement points (hereinafter also referred to as “pixels”). As to each of the divided pixels, the orientation of the pixel is measured by obtaining Kikuchi patterns from reflected electrons from an electron beam which has been incident on the cross section (polished surface) of the sample. The obtained orientation data is analyzed using analysis software for the EBSD, thereby calculating various parameters. The measurement conditions are as set forth below: the acceleration voltage is 15 kV, the size of the measurement region is 30 μm×50 μm, and the distance (step size) between adjacent pixels is 0.05 μm. From among the pixels adjacent to a pixel serving as a measurement center, pixels which each have a misorientation of 5 degrees or more with respect to such pixel are excluded from the calculation of KAM values as such excluded pixels are each regarded as straddling the grain boundary of the monocrystal where the pixel serving as a measurement center is located. That is, a KAM value is obtained as an average value of the misorientation of a pixel located in a crystal grain and the misorientations of the pixels which are adjacent to such pixel and which do not straddle the grain boundary of such crystal grain. That is, the KAM value can be expressed by the following formula (6).

(29) $\begin{array}{cc}\mathrm{KAM}=\frac{\underset{j=1}{\overset{n}{.Math.}}{\alpha }_{i,j}}{n}& \left(6\right)\end{array}$
in the formula (6), n represents the number of pixels j adjacent to an arbitrary pixel i in the same crystal grain, and α.sub.i,j represents the crystal misorientation obtained from the crystal orientation at the pixel i and the crystal orientation at the pixel j.

(30) Then, the KAM value at all pixels constituting the entire area of the measurement region is calculated in the lower layer or the upper layer, and the ratio of the measurement points (pixels) with the KAM value of 1 degree or lower when the total number of the measurement points (pixels) is taken as 100% is determined. The ratio of the measurement points at which the KAM value is 1 degree or lower is a numerical value obtained by averaging the ratios obtained for the measurement regions at any three locations. Further, the ratio of the measurement points where the KAM value is 1 degree or lower in the lower layer is represented as KAM.sub.i, and the ratio of the measurement points where the KAM value is 1 degree or lower in the upper layer is represented as KAM.sub.s.

(31) Lower Layer

(32) The lower layer used in the present embodiment contains a compound having a composition represented by the following formula (1).
(Al.sub.xTi.sub.1-x)N  (1)
in the formula (1), x represents an atomic ratio of the Al element to a total of the Al element and the Ti element, and satisfies 0.60≤x≤0.95.

(33) In the coated cutting tool of the present embodiment, where x in the formula (1) is 0.60 or more in the lower layer that contains the compound having the composition represented by the formula (1), solid solution strengthening increases the hardness, so that the wear resistance is improved, and since the Al amount is increased, the oxidation resistance is improved. As a result, in the coated cutting tool of the present embodiment, the crater wear resistance is improved, so that a decrease in strength of the cutting edge is suppressed, thereby improving the fracture resistance. Meanwhile, in the coated cutting tool of the present embodiment, where x in the formula (1) is 0.95 or less in the lower layer that contains the compound having the composition represented by the formula (1), since Ti is contained, the toughness is improved, and thus the fracture resistance is improved. From the same viewpoints, x in the formula (1) is preferably 0.64 or more and 0.92 or less, and more preferably 0.80 or more and 0.90 or less.

(34) The average thickness of the lower layer used in the present embodiment is 1.0 μm or more and 10.0 μm or less. Where the average thickness of the lower layer is 1.0 μm or more, the wear resistance of the coated cutting tool is improved. Meanwhile, where the average thickness of the lower layer is 10.0 μm or less, the adhesion between the substrate and the coating layer can be further enhanced, so that the fracture resistance of the coated cutting tool is improved. From the same viewpoints, the average thickness of the lower layer is more preferably 1.5 μm or more and 8.5 μm or less, and further preferably 2.0 μm or more and 7.0 μm or less.

(35) Upper Layer

(36) The upper layer used in the present embodiment contains a compound having a composition represented by the following formula (2).
(Al.sub.yTi.sub.1-y)N  (2)
in the formula (2), y represents an atomic ratio of the Al element to a total of the Al element and the Ti element, and satisfies 0.50≤y≤0.85.

(37) In the coated cutting tool of the present embodiment, where y in the formula (2) is 0.50 or more in the upper layer that contains the compound having the composition represented by the formula (2), solid solution strengthening increases the hardness, so that the wear resistance is improved, and since the Al amount is increased, the oxidation resistance is improved. As a result, in the coated cutting tool of the present embodiment, the crater wear resistance is improved, so that a decrease in strength of the cutting edge is suppressed, thereby improving the fracture resistance. Meanwhile, in the coated cutting tool of the present embodiment, where y in the formula (2) is 0.85 or less in the upper layer including the compound having the composition represented by the formula (2), since Ti is contained, the toughness is improved, and thus the fracture resistance is improved. From the same viewpoints, y in the formula (2) is preferably 0.52 or more and 0.83 or less, and more preferably 0.70 or more and 0.80 or less.

(38) In the coated cutting tool of the present embodiment, it is preferable that the atomic ratio x of the Al element in the lower layer and the atomic ratio y of the Al element in the upper layer satisfy the condition represented by the following formula (4).
y<x  (4)

(39) When the atomic ratio x of the Al element in the lower layer and the atomic ratio y of the Al element in the upper layer satisfy the relation of y<x, the hardness of the upper layer is lowered and the toughness tends to be improved. As a result, the generation of thermal cracks is suppressed, and the fracture resistance of the coated cutting tool tends to be improved. Further, the Al amount can be maximized by controlling the GOS value as described above, rather than merely increasing the Ti amount in the upper layer to improve the toughness. This tends to improve the oxidation resistance of the coated cutting tool and improve the crater wear resistance. As a result, the strength of the cutting edge of the coated cutting tool is improved, so that the fracture resistance tends to be improved.

(40) The average thickness of the upper layer used in the present embodiment is preferably 1.0 μm or more and 10.0 μm or less. Where the average thickness of the upper layer is 1.0 μm or more, the fracture resistance of the coated cutting tool is improved. Meanwhile, where the average thickness of the upper layer is 10.0 μm or less, the adhesion is improved, and the effect of suppressing the occurrence of peeling is increased. Therefore, the fracture resistance of the coated cutting tool is improved. From the same viewpoints, the average thickness of the upper layer is more preferably 1.5 μm or more and 8.0 μm or less, and further preferably 2.0 μm or more and 5.0 μm or less.

(41) Method for Forming Coating Layer

(42) For example, the following methods can be used for forming each layer constituting the coating layer in the coated cutting tool of the present embodiment. However, these methods for forming the layers are not limiting.

(43) The lower layer can be formed by chemical vapor deposition at a temperature of 700° C. to 900° C. and a pressure of 3 hPa to 5 hPa by using a raw material composition of TiCl.sub.4: 0.2 mol % to 0.5 mol %, AlCl.sub.3: 0.5 mol % to 1.5 mol %, NH.sub.3: 2.0 mol % to 5.0 mol %, and H.sub.2: balance.

(44) The upper layer can be formed by chemical vapor deposition at a temperature of 700° C. to 900° C. and a pressure of 3 hPa to 5 hPa by using a raw material composition of TiCl.sub.4: 0.3 mol % to 0.8 mol %, AlCl.sub.3: 0.5 mol % to 1.0 mol %, NH.sub.3: 1.0 mol % to 5.0 mol %, and H.sub.2: balance.

(45) First, the lower layer is formed on the substrate surface. Then, the upper layer is formed on the lower layer.

(46) Further, in order to control the composition represented by the formulas (1) and (2), the raw material composition may be adjusted as appropriate. More specifically, as a method for controlling the ratio of Ti and Al, for example, where the ratio of AlCl.sub.3/(AlCl.sub.3+TiCl.sub.4) in the raw material composition is increased, the Al amount ratio tends to increase. Specifically, for example, by setting the ratio of AlCl.sub.3/(AlCl.sub.3+TiCl.sub.4) in the raw material composition to 0.6 or more and 0.85 or less, the amount ratio of Al in the formula (1) can be controlled within the above specific range. Further, for example, by setting the ratio of AlCl.sub.3/(AlCl.sub.3+TiCl.sub.4) in the raw material composition to 0.5 or more and 0.77 or less, the amount ratio of Al in the formula (2) can be controlled within the above specific range.

(47) Further, in order to increase the area ratio of the crystal grains having a GOS value of 1 degree or lower (to reduce the strain of the crystal grains), for example, the ratio of NH.sub.3 in the raw material composition can be reduced, or the forming temperature can be increased. Meanwhile, in order to reduce the area ratio of the crystal grains having a GOS value of 1 degree or lower (to increase the strain of the crystal grains), for example, the ratio of NH.sub.3 in the raw material composition can be increased, or the forming temperature can be lowered.

(48) Further, in order to increase the ratio of measurement points where the KAM value is 1 degree or lower (to reduce the strain of the crystal grains), for example, the ratio of NH.sub.3 in the raw material composition can be decreased, or the forming temperature can be increased. Meanwhile, in order to reduce the ratio of measurement points where the KAM value is 1 degree or lower (to increase the strain of the crystal grains), for example, the ratio of NH.sub.3 in the raw material composition can be increased, or the forming temperature can be lowered.

(49) Further, where the temperature at the time of forming the upper layer after forming the lower layer is set to be 50° C. or higher than the temperature at the time of forming the lower layer, the area ratio of the crystal grains having a GOS value of 1 degree or lower can be increased (the strain of the crystal grains can be reduced) and GOS.sub.i<GOS.sub.s can be satisfied.

(50) The thickness of each layer in the coating layer of the coated cutting tool of the present embodiment can be measured by observing the cross-sectional structure of the coated cutting tool by using an optical microscope, a scanning electron microscope (SEM), a field emission scanning electron microscope (FE-SEM), or the like. In addition, the average thickness of each layer in the coated cutting tool of the present embodiment can be determined by measuring the thickness of each layer at three or more locations in the vicinity of a position of 50 μm from the edge line of the cutting edge toward the center of the rake surface of the coated cutting tool and determining the arithmetic mean value thereof. The composition of each layer can be measured from the cross-sectional structure of the coated cutting tool of the present embodiment by using an energy dispersive X-ray spectrometer (EDS), a wavelength dispersive X-ray spectrometer (WDS), or the like.

EXAMPLES

(51) Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

(52) As a substrate, an insert of SNGU1307ANEN-MJ (a cemented carbide having a composition of 86.5% WC-11.7% Co-1.1% NbC-0.7% Cr.sub.3C.sub.2 (% by mass)) was prepared. After round honing was performed on the ridgeline of the cutting edge of this substrate with a SiC brush, the surface of the substrate was washed.

(53) Invention Samples 1 to 13 and Comparative Samples 1 to 7

(54) After washing the surface of the substrate, a coating layer was formed by the chemical vapor deposition method. First, the substrate was loaded into a chemical vapor deposition apparatus of an external heating type, and a lower layer having the composition shown in Table 3 was formed on the surface of the substrate under the conditions of raw material composition, temperature and pressure shown in Table 1 so as to obtain the average thickness shown in Table 3. Then, the upper layer having the composition shown in Table 3 was formed on the surface of the lower layer under the conditions of the raw material composition, temperature and pressure shown in Table 2 so as to obtain the average thickness shown in Table 3. Thus, the coated cutting tools of the invention samples 1 to 13 and the comparative samples 1 to 7 were obtained.

(55) The thickness of each layer of each of the samples was obtained as set forth below. That is, using an FE-SEM, the average thickness was obtained by: measuring the thickness of each layer, from each of the cross-sectional surfaces at three locations near the position 50 μm from the edge of the coated cutting tool, toward the center of the rake surface thereof; and calculating the arithmetic mean of the resulting measurements. Using an EDS, the composition of each layer of the obtained sample was measured from the cross-sectional surface near the position 50 μm from the edge of the coated cutting tool, toward the center of the rake surface thereof.

(56) TABLE-US-00001 TABLE 1 Lower layer Temperature Pressure Raw material composition (mol %) AlCl.sub.3/ Sample No. (° C.) (hPa) TiCl.sub.4 AlCl.sub.3 NH.sub.3 H.sub.2 (AlCl.sub.3 + TiCl.sub.4) Invention sample 1 750 4 0.3 1.2 3.5 95.0 0.80 Invention sample 2 750 4 0.3 1.2 3.5 95.0 0.80 Invention sample 3 750 4 0.3 1.2 4.0 94.5 0.80 Invention sample 4 750 3 0.3 1.2 3.5 95.0 0.80 Invention sample 5 750 4 0.3 1.2 2.5 96.0 0.80 Invention sample 6 750 4 0.2 0.5 3.5 95.8 0.71 Invention sample 7 750 5 0.3 1.2 3.5 95.0 0.80 Invention sample 8 750 4 0.2 1.2 2.5 96.1 0.85 Invention sample 9 750 4 0.3 1.2 3.5 95.0 0.80 Invention sample 10 750 4 0.3 1.2 3.5 95.0 0.80 Invention sample 11 750 4 0.3 1.0 4.0 94.7 0.77 Invention sample 12 770 4 0.3 1.2 3.0 95.5 0.80 Invention sample 13 750 4 0.3 1.5 3.5 94.7 0.83 Comparative sample 1 750 4 0.3 1.2 3.0 95.5 0.80 Comparative sample 2 750 4 0.3 1.2 3.0 95.5 0.80 Comparative sample 3 750 4 0.3 1.2 3.0 95.5 0.80 Comparative sample 4 750 4 0.3 1.2 5.0 93.5 0.80 Comparative sample 5 900 4 0.3 1.2 3.0 95.5 0.80 Comparative sample 6 750 4 0.5 0.5 3.0 96.0 0.50 Comparative sample 7 750 4 0.3 1.2 4.0 94.5 0.80

(57) TABLE-US-00002 TABLE 2 Upper layer Temperature Pressure Raw material composition (mol %) AlCl.sub.3/ Sample No. (° C.) (hPa) TiCl.sub.4 AlCl.sub.3 NH.sub.3 H.sub.2 (AlCl.sub.3 + TiCl.sub.4) Invention sample 1 800 4 0.4 1.0 2.5 96.1 0.71 Invention sample 2 900 4 0.4 1.0 3.0 95.6 0.71 Invention sample 3 800 4 0.4 1.0 3.0 95.6 0.71 Invention sample 4 800 4 0.4 1.0 2.0 96.6 0.71 Invention sample 5 800 4 0.4 0.8 2.5 96.3 0.67 Invention sample 6 800 3 0.4 1.0 2.5 96.1 0.71 Invention sample 7 800 4 0.5 0.5 2.5 96.5 0.50 Invention sample 8 800 4 0.4 1.0 2.5 96.1 0.71 Invention sample 9 800 4 0.3 1.0 2.5 96.2 0.77 Invention sample 10 800 4 0.4 1.0 4.0 94.6 0.71 Invention sample 11 800 4 0.5 1.0 2.0 96.5 0.67 Invention sample 12 800 4 0.4 1.0 2.0 96.6 0.71 Invention sample 13 800 5 0.4 1.0 3.0 95.6 0.71 Comparative sample 1 800 4 0.4 1.0 4.0 94.6 0.71 Comparative sample 2 800 4 0.4 1.0 2.0 96.6 0.71 Comparative sample 3 850 4 0.4 1.0 3.0 95.6 0.71 Comparative sample 4 800 4 0.4 1.0 3.0 95.6 0.71 Comparative sample 5 700 4 0.4 1.0 3.0 95.6 0.71 Comparative sample 6 800 4 0.4 1.0 2.0 96.6 0.71 Comparative sample 7 800 5 0.8 0.5 2.0 96.7 0.38

(58) TABLE-US-00003 TABLE 3 Coating layer Lower layer (Al.sub.xTi.sub.1−x)N Upper layer (Al.sub.yTi.sub.1−y)N Atomic ratio of Average Atomic ratio of Average Total Sample No. Al element, x thickness (μm) Al element, y thickness (μm) thickness (μm) Invention sample 1 0.90 5.0 0.80 2.0 7.0 Invention sample 2 0.90 2.0 0.80 10.0 12.0 Invention sample 3 0.90 10.0 0.80 2.0 12.0 Invention sample 4 0.90 5.0 0.80 1.0 6.0 Invention sample 5 0.90 1.0 0.80 5.0 6.0 Invention sample 6 0.64 5.0 0.80 2.0 7.0 Invention sample 7 0.90 5.0 0.52 2.0 7.0 Invention sample 8 0.95 5.0 0.80 2.0 7.0 Invention sample 9 0.90 5.0 0.85 2.0 7.0 Invention sample 10 0.90 5.0 0.80 2.0 7.0 Invention sample 11 0.80 7.0 0.70 5.0 12.0 Invention sample 12 0.90 5.0 0.80 2.0 7.0 Invention sample 13 0.90 3.0 0.80 2.0 5.0 Comparative sample 1 0.90 5.5 0.80 0.5 6.0 Comparative sample 2 0.90 0.5 0.80 5.5 6.0 Comparative sample 3 0.90 2.0 0.80 14.0 16.0 Comparative sample 4 0.90 14.0 0.80 2.0 16.0 Comparative sample 5 0.90 5.0 0.80 2.0 7.0 Comparative sample 6 0.45 5.0 0.80 2.0 7.0 Comparative sample 7 0.90 5.0 0.35 2.0 7.0

(59) Measurement of GOS Value

(60) In the sample of the coated cutting tool, a cross section was exposed by polishing in a direction substantially parallel to the substrate surface at 0.5 μm from the surface of the lower layer or the upper layer toward the substrate. Using EBSD (manufactured by TexSEM Laboratories, Inc.), each measurement region of the cross section in the lower layer and the upper layer was divided into regular hexagonal measurement points (hereinafter also referred to as “pixels”). As to each of the divided pixels, the orientation of the pixel was measured by obtaining Kikuchi patterns from reflected electrons from an electron beam which had been incident on the cross section (polished surface) of the sample. The obtained orientation data was analyzed using analysis software for the EBSD, thereby calculating various parameters. The measurement conditions were as set forth below: the acceleration voltage was 15 kV, the size of the measurement region was 30 μm×50 μm, and the distance (step size) between adjacent pixels was 0.05 μm. When there was a misorientation of 5 degrees or more between adjacent pixels, it was defined as a grain boundary. Further, a region surrounded by grain boundaries was defined as one crystal grain. However, a pixel that was present independently with a misorientation of 5 degrees or more with all the adjacent pixels was not considered as a crystal grain, and two or more pixels connected together were treated as a crystal grain. Further, the misorientation in a crystal grain between two different pixels in the same crystal grain was calculated, and the averaged value thereof was defined as the GOS value. That is, the GOS value was calculated by the following formula (5).

(61) $\begin{array}{cc}\mathrm{GOS}=\frac{\underset{i,j=1}{\overset{n}{.Math.}}{\alpha }_{\mathrm{ij}\left(i\ne j\right)}}{n\left(n-1\right)}& \left(5\right)\end{array}$
in the formula (5), n represents the number of pixels in the same crystal grain, i and j represent numbers assigned to different pixels in the crystal grain (where 1≤i and j≤n), α.sub.ij (i≠j) represents the crystal misorientation obtained from the crystal orientation at pixel i and the crystal orientation at pixel j.

(62) The measurement under the abovementioned measurement conditions and measurement range was carried out in five fields of view. Next, the total number of pixels belonging to the crystal grains that constitute the lower layer or the upper layer was obtained, the GOS value was divided at intervals of 1 degree, and the number of pixels in the crystal grains in which the GOS value was included in the range of this value was aggregated and divided by the total number of pixels to create a histogram indicating the area ratio of the GOS value in the lower layer or the upper value. The area ratio GOS.sub.i of the crystal grains having a GOS value of 1 degree or lower in the lower layer and the area ratio GOS.sub.s of the crystal grains having a GOS value of 1 degree or lower in the upper layer were obtained based on the created histogram. The results are shown in Table 4.

(63) Measurement of KAM Value

(64) The KAM value in the lower layer and the upper layer was measured as follows. In the sample of the coated cutting tool, a cross section was exposed by polishing in a direction substantially parallel to the substrate surface at 0.5 μm from the surface of the lower layer or the upper layer toward the substrate. Using EBSD (manufactured by TexSEM Laboratories, Inc.), each measurement region of the cross section in the lower layer and the upper layer was divided into regular hexagonal measurement points (hereinafter also referred to as “pixels”). As to each of the divided pixels, the orientation of the pixel was measured by obtaining Kikuchi patterns from reflected electrons from an electron beam which had been incident on the cross section (polished surface) of the sample. The obtained orientation data was analyzed using analysis software for the EBSD, thereby calculating various parameters. The measurement conditions were as set forth below: the acceleration voltage was 15 kV, the size of the measurement region was 30 μm×50 μm, and the distance (step size) between adjacent pixels was 0.05 μm. From among the pixels adjacent to a pixel serving as a measurement center, pixels which each had a misorientation of 5 degrees or more with respect to such pixel were excluded from the calculation of KAM values as such excluded pixels were each regarded as straddling the grain boundary of the monocrystal where the pixel serving as a measurement center was located. That is, a KAM value was obtained as an average value of the misorientation of a pixel located in a crystal grain and the misorientations of the pixels which were adjacent to such pixel and which did not straddle the grain boundary of such crystal grain. That is, the KAM value was calculated by the following formula (6).

(65) $\begin{array}{cc}\mathrm{KAM}=\frac{\underset{j=1}{\overset{n}{.Math.}}{\alpha }_{i,j}}{n}& \left(6\right)\end{array}$
in the formula (6), n represents the number of pixels j adjacent to an arbitrary pixel i in the same crystal grain, and α.sub.i,j represents the crystal misorientation obtained from the crystal orientation at the pixel i and the crystal orientation at the pixel j.

(66) Then, the KAM value at all pixels constituting the entire area of the measurement region was calculated in the lower layer or the upper layer, and the ratio of the measurement points (pixels) with the KAM value of 1 degree or lower when the total number of the measurement points (pixels) was taken as 100% was determined. The ratio of the measurement points at which the KAM value was 1 degree or lower was a numerical value obtained by averaging the ratios obtained for the measurement regions at any three locations. Further, the ratio of the measurement points where the KAM value was 1 degree or lower in the lower layer was represented as KAM.sub.i, and the ratio of the measurement points where the KAM value was 1 degree or lower in the upper layer was represented as KAM.sub.s. The measurement results are shown in Table 4.

(67) TABLE-US-00004 TABLE 4 Lower layer Upper layer Lower layer Upper layer Relation between Sample No. KAM.sub.i(%) KAM.sub.s(%) GOS.sub.i(%) GOS.sub.s(%) GOS values Invention sample 1 73 87 22 74 GOS.sub.i < GOS.sub.s Invention sample 2 53 91 20 86 GOS.sub.i < GOS.sub.s Invention sample 3 50 82 14 68 GOS.sub.i < GOS.sub.s Invention sample 4 52 68 25 62 GOS.sub.i < GOS.sub.s Invention sample 5 70 79 36 78 GOS.sub.i < GOS.sub.s Invention sample 6 55 80 26 72 GOS.sub.i < GOS.sub.s Invention sample 7 63 77 24 70 GOS.sub.i < GOS.sub.s Invention sample 8 78 80 32 75 GOS.sub.i < GOS.sub.s Invention sample 9 64 69 20 72 GOS.sub.i < GOS.sub.s Invention sample 10 52 52 18 55 GOS.sub.i < GOS.sub.s Invention sample 11 68 85 16 82 GOS.sub.i < GOS.sub.s Invention sample 12 79 80 54 85 GOS.sub.i < GOS.sub.s Invention sample 13 69 73 20 68 GOS.sub.i < GOS.sub.s Comparative sample 1 52 53 25 54 GOS.sub.i < GOS.sub.s Comparative sample 2 62 72 44 62 GOS.sub.i < GOS.sub.s Comparative sample 3 47 92 22 84 GOS.sub.i < GOS.sub.s Comparative sample 4 26 81 14 68 GOS.sub.i < GOS.sub.s Comparative sample 5 88 31 62 20 GOS.sub.s < GOS.sub.i Comparative sample 6 48 70 35 64 GOS.sub.i < GOS.sub.s Comparative sample 7 37 67 26 62 GOS.sub.i < GOS.sub.s

(68) Using the obtained Invention Samples 1 to 13 and Comparative Samples 1 to 7, a cutting test was conducted under the following conditions.

(69) Cutting Test 1 Workpiece material: AISI 4140, Cutting speed: 300 m/min, Feed rate per blade: 0.20 mm/t, Depth of cut: 2.0 mm, Cutting width: 76 mm, Coolant: none, Evaluation items: the tool life was assumed to be reached when the sample was fractured or the maximum flank wear width reached 0.3 mm, and the machining length up to the tool life was measured. Further, when the machining length was 3.0 m, the number of thermal cracks generated in the coated cutting tool was measured. The measurement results are shown in Table 5.

(70) TABLE-US-00005 TABLE 5 Cutting test Number of thermal cracks generated at machining Machining length of 3.0 m length (m) Invention sample 1 0 17 Invention sample 2 0 19 Invention sample 3 1 13 Invention sample 4 1 12 Invention sample 5 0 11 Invention sample 6 0 13 Invention sample 7 0 15 Invention sample 8 0 17 Invention sample 9 0 16 Invention sample 10 1 10 Invention sample 11 0 15 Invention sample 12 0 11 Invention sample 13 0 13 Comparative sample 1 3 5 Comparative sample 2 0 8 Comparative sample 3 3 3 Comparative sample 4 — 1 Comparative sample 5 2 3 Comparative sample 6 0 5 Comparative sample 7 0 7 * Since the Comparative Sample 4 was fractured at a machining length of 1 m, the number of thermal cracks was not measured.

(71) The results shown in Table 5 indicate that in all invention samples, when the working length was 3.0 m, the number of thermal cracks generated in the coated cutting tool was 1 or less, and the machining length up to the tool life was 10 m or more. Meanwhile, all the comparative samples had the machining length up to the tool life of 7 m or less, and the comparative samples 1, 3 and 5 had two or more thermal cracks in the coated cutting tool. Therefore, it can be seen that the wear resistance and fracture resistance of the invention samples are generally superior to those of the comparison samples.

(72) From the above results, it was found that the invention samples have a long tool life as a result of being excellent in wear resistance and fracture resistance.

INDUSTRIAL APPLICABILITY

(73) Since the coated cutting tool of the present invention has excellent wear resistance and fracture resistance, the tool life can be extended as compared with the prior art, and from such a viewpoint, the coated cutting tool of the present invention has industrial applicability.

REFERENCE SIGNS LIST

(74) 1: Substrate, 2: Lower layer, 3: Upper layer, 4: Coating layer, 5: Coated cutting tool.