JP7748033B2 - surface coated cutting tools - Google Patents

Description

The present invention relates to a surface-coated cutting tool (hereinafter sometimes referred to as a coated tool). This application claims priority to Japanese Patent Application No. 2020-55011, filed on March 25, 2020. The entire contents of this Japanese patent application are incorporated herein by reference.

Conventionally, coated tools have been known in which a tool substrate made of cemented carbide or the like is coated by vapor deposition onto the surface of the tool substrate. While these coated tools have wear resistance, various proposals have been made to further improve this wear resistance, including a proposal for a coating layer containing boron.

For example, Patent Document 1 proposes a coated tool having, on the surface of a tool substrate, a lower layer made of a composite nitride of Ti, Al, and _B having an average thickness of 0.8 to 5 μm and satisfying the composition formula: (Ti1- _{(X+Z) AlXBZ} )N (wherein, in atomic ratios, X is 0.25 to 0.65 and Z is 0.01 to 0.10), an adhesive bonding layer made of a zirconium boronide layer having an average thickness of 0.1 to 0.5 μm, and an upper layer made of a zirconium boride layer having an average thickness of 0.8 to 5 μm. This coated tool is said to have excellent wear resistance even when cutting high-hardness steels and the like at high speeds.

Furthermore, for example, Patent Document 2 proposes a coated tool having a Ti-boride layer with an average thickness of 0.5 to 5 μm on the surface of a tool substrate, the layer being configured as a composite structure of crystal grains with multiple average grain sizes, the composite structure being composed of secondary crystal grains with an average grain size of 20 to 70 nm, which are made up of aggregates of primary crystal grains with an average grain size of 10 to 15 nm, and tertiary crystal grains with an average grain size of 300 to 600 nm, which are made up of aggregates of the secondary crystal grains. This coated tool is said to be able to suppress peeling of the soft coating layer due to welding during high-speed cutting of soft, difficult-to-cut materials.

Furthermore, for example, Patent Document 3 discloses a tool having a Zr boride layer with an average thickness of 0.5 to 5 μm on the surface of a tool substrate, the layer being configured as a composite structure of crystal grains having a plurality of average grain sizes,
The composite structure has an average grain size of 5 to 30 nm and is composed of an aggregate of primary crystal grains.
secondary crystal grains of 0 to 100 nm and an average grain size of 200 to 100 nm consisting of an aggregate of the secondary crystal grains
A coated tool consisting of 0 nm tertiary crystal grains has been proposed. It is said that this coating suppresses welding during high-speed cutting of hard, difficult-to-cut materials.

Japanese Patent Application Laid-Open No. 2006-1006 JP 2012-139795 A JP 2012-40615 A

While cutting equipment has become increasingly sophisticated and automated, there is a growing demand for cutting difficult-to-cut materials. This includes high-speed, interrupted cutting of materials prone to welding during cutting, such as titanium-based alloys and austenitic stainless steel.

The present invention was made in consideration of the above circumstances and proposals, and aims to provide a coated tool that exhibits excellent crack resistance and wear resistance over long periods of use, particularly when used in high-speed intermittent cutting of Ti-based alloys and austenitic stainless steels.

Here, high-speed intermittent cutting of Ti-based alloys refers to processing in which the cutting edge of a cutting tool repeatedly cuts and spins freely at cutting speeds faster than 70 m/min, and high-speed intermittent cutting of austenitic stainless steel refers to processing in which the cutting edge of a cutting tool repeatedly cuts and spins freely at cutting speeds faster than 100 m/min.

A surface-coated cutting tool according to an embodiment of the present invention includes a tool substrate and a coating layer on the tool substrate,
the coating layer is a composite boride layer of Ti and Zr having an average layer thickness of 0.5 to 5.0 μm ,
The Ti and Zr composite boride layer has an average composition in which atomic ratios x and y satisfy 0.3≦x≦0.7 and 1.5≦y≦3.0 when its composition is expressed by the composition formula: Ti _x Zr _(1−x) B _y , and further has a crystalline phase constituted by crystal grains of a hexagonal crystal structure and an amorphous phase, and the hexagonal crystal grains constituting the crystalline phase have an average grain size of 2 to 30 nm and occupy an area ratio of 50 to 95 area % of the Ti and Zr composite boride phase .

Furthermore, the surface-coated cutting tool according to the above embodiment may satisfy one or more of the following items (1) to ( 2 ).

( 1 ) The nanoindentation hardness of the Ti and Zr boride layer is 25 to 40 GPa.

( 2 ) With respect to the crystal grains of the hexagonal structure constituting the crystalline phase of the Ti and Zr boride layer, when the peak intensities of the 001 diffraction line, 100 diffraction line, and 101 diffraction line in X-ray diffraction are Ih(001), Ih(100), and Ih(101), respectively, the relationship 0.01≦Ih(001)/{Ih(001)+Ih(100)+Ih(101)}≦0.50 is satisfied.

According to the above, even when materials with high adhesion to coated tools, such as Ti-based alloys and austenitic stainless steels, are used in high-speed intermittent cutting, the tool exhibits excellent crack resistance and wear resistance over long periods of use.

The inventor recognized the following regarding the coated tools described in Patent Documents 1 to 3:

(1) The coated tool described in Patent Document 1 contains boron in the coating layer, enabling high-speed cutting of high-hardness steels such as alloy steels and quenched bearing steels. However, it cannot be said that the tool has sufficient cutting characteristics for high-speed intermittent cutting of materials prone to welding during cutting, such as the aforementioned Ti-based alloys and austenitic stainless steels.

(2) The coated tool described in Patent Document 2 enables high-speed cutting of soft, difficult-to-cut materials such as Al-based alloys. However, sufficient consideration has not been given to high-speed intermittent cutting of materials prone to welding during cutting, such as the aforementioned Ti-based alloys and austenitic stainless steels.

(3) The coated tool described in Patent Document 3 has a coating layer consisting of a Zr boride layer, and even when cutting hard, difficult-to-cut materials such as Ti-based alloys and high-Si-content Al-Si alloys under high-speed cutting conditions, the occurrence of welding is suppressed, improving peeling resistance and wear resistance. However, taking into account recent cutting conditions, further improvements in wear resistance are required.

Based on this understanding, the inventors conducted extensive research into Ti-boride layers and Zr-boride layers as coating layers. As a result, they discovered the novel fact that wear resistance is improved when the Ti-Zr composite boride coating layer has both a crystalline phase and an amorphous phase.

The surface-coated cutting tool according to an embodiment of the present invention will be described in detail below. Note that in this specification and claims, when a numerical range is expressed as "A to B" (where A and B are both numerical values), the range includes an upper limit (B) and a lower limit (A), and the units for the upper limit (B) and the lower limit (A) are the same. The numerical values also include tolerances.

Average thickness of coating layer:
The coating layer has a composite boride layer of Ti and Zr, and its average layer thickness is preferably 0.5 to 5.0 μm. This is because if the average layer thickness is less than 0.5 μm, it is difficult to maintain wear resistance over a long period of time, while if it exceeds 5.0 μm, chipping is likely to occur. A more preferable range of the average layer thickness is 1.0 to 2.5 μm.

The average thickness of the coating layer is measured as follows: For example, a focused ion beam (FIB) system, cross section polisher (CP), or other device is used to cut the coating layer at a longitudinal cross section (a cross section perpendicular to the tool substrate surface when the tool substrate surface is treated as a flat surface, ignoring minute irregularities on the tool substrate surface) at any desired position to prepare a sample for observation. The longitudinal cross section is then observed at multiple locations (e.g., five locations) using a scanning electron microscope (SEM), and the average thickness is calculated by arithmetically averaging the layer thicknesses.

Average composition of Ti and Zr composite boride layer:
When the composition of the Ti and Zr composite boride layer is expressed by the composition formula: Ti _x Zr _(1-x) B _y , it is preferable that the average composition of the atomic ratios x and y satisfy 0.3≦x≦0.7 and 1.5≦y≦3.0. The reason for this is as follows: When x is less than 0.3, the crystal structure is disturbed and metallic Ti is generated in the composite boride layer, which makes adhesive wear more likely to progress at the contact portion between the Ti and Zr composite boride layer and a workpiece such as a Ti-based alloy. On the other hand, when x exceeds 0.7, the crystal structure is disturbed and metallic Zr is generated in the composite boride layer, which reduces the hardness of the composite boride layer. Furthermore, if y is less than 1.5, adhesive wear is likely to occur at the contact portion between the Ti-Zr composite boride layer and the workpiece material such as a Ti-based alloy, while if y exceeds 3.0, the crystal structure becomes disordered, resulting in a decrease in hardness. The range of x is more preferably 0.4 to 0.6, and the range of y is more preferably 1.8 to 2.2.

The average composition of the Ti and Zr composite boride layer is measured as follows.
The Ti content (x) and the boron content (y) were both determined using an electron probe microanalyzer (EPMA) by irradiating the surface of the coating layer or five arbitrary positions on a longitudinal cross section of the coating layer with an electron beam. The characteristic X-rays obtained from each position and corresponding to the elements constituting the coating layer were analyzed to quantify the content of each element, and the results were then arithmetically averaged.

It is not clear why a Ti-Zr composite boride layer is an excellent coating layer for coated tools used when cutting highly adhesive materials such as Ti-based alloys and austenitic stainless steel, but it is believed that this is because Ti-Zr composite borides have low solid solubility in Ti-based alloys and are highly unreactive, thereby suppressing adhesive wear between the coating layer and Ti-based alloys on the abrading surface and mitigating the thermal effects on the tool substrate caused by the low thermal conductivity of Ti-based alloys.

Crystalline and amorphous phases of Ti and Zr composite boride layers:
It is preferable that the Ti-Zr composite boride layer have a crystalline phase and an amorphous phase. The reason for this preference is unclear, but is thought to be as follows: The presence of the amorphous phase produces boron oxide on the abrasive surface between the Ti-Zr composite boride layer and the Ti-based alloy workpiece during cutting, thereby imparting solid lubricity to the Ti-Zr composite nitride layer and improving the wear resistance of the Ti-Zr composite boride layer.

Furthermore, the hexagonal crystal grains (hexagonal crystals) that make up the crystalline phase are preferably fine crystal grains, with an average grain size in the range of 2 to 30 nm. The reason for this is that the fracture units in the Ti and Zr composite boride layer caused by welding are crystalline units, so if the crystal grains are fine crystals, i.e., small, the fracture units become smaller, which reduces wear of the layer that involves fracture and improves wear resistance.

The average grain size of the crystal grains that make up the crystalline phase is determined as follows: Analysis is performed using Automated Crystal Orientation Mapping (ACOM)-TEM with a Transmission Electron Microscope (TEM) to define the grain boundaries. The area enclosed by the grain boundaries is then defined as a crystal grain, and the maximum length of that area is defined as the grain size. The grain size is determined for any five crystal grains, and the arithmetic average is taken as the average grain size.

Here, the differentiation between crystalline and amorphous phases is performed as follows. Specifically, a longitudinal section is observed using a TEM, and an image is obtained at a magnification sufficient to distinguish, for example, a few nanometers in size, on the observation surface. This image is then subjected to FFT image transformation processing to select bright spots corresponding to the lattice constants (e.g., bright spots (including circular) corresponding to the (001) plane of a hexagonal crystal structure). Further, an inverse FFT transformation processing is performed, followed by binarization processing. This processing enables the crystal structure of the lattice fringes and angles with the lattice constants selected as the bright spots to be emphasized. Similar processing is performed for each lattice constant to create emphasized images corresponding to each lattice constant, such as a (001) plane emphasized image and a (100) plane emphasized image. The aforementioned emphasized images are then combined by ORing them.

Then, by performing an expansion process on the binarized image so that the lattice spacing with the largest lattice constant is filled, an image is obtained in which the lattice spacing of at least the maximum lattice width is densely filled. In this case, the filled-in parts are the crystalline phase, and the unfilled parts are the amorphous phase. Note that there are no particular limitations on the magnification, as long as the above-mentioned lattice constants can be observed.

Note that this is a process that differs from OR combining in that it calculates the logical sum of pixels in the same position in two or more images for all pixels in the images to obtain the resulting image. Specifically, if a specific pixel is a bright point in one image, it is considered a bright point, and if it is a dark point in all images, the pixel is considered a dark point.

Using the method for distinguishing between crystalline and amorphous phases described above, distinguish between crystalline and amorphous phases for each of five arbitrary fields of view. This determines the area percentage of the crystalline phase in each field of view, and the arithmetic average of these is taken as the area percentage of the crystalline phase. It is more preferable that the area percentage of the crystalline phase be 50 to 95 area percent. The reasons for this are as follows:

If the area ratio of the crystalline phase is less than 50%, the hardness will decrease due to the small amount of crystalline phase in the Ti and Zr composite boride layer, and the coating layer may not perform as well as it should. On the other hand, if the area ratio of the crystalline phase exceeds 95%, wear resistance will decrease and grain boundary fracture will become dominant, causing entire crystal grains to fall off at the grain boundaries, which may result in poor cutting performance. Here, the reason for the decrease in wear resistance is thought to be that the Ti and Zr composite boride layer makes it difficult to form boron oxide during cutting.

Nanoindentation hardness of Ti and Zr composite boride layer:
The Ti and Zr composite boride layer more preferably has a nanoindentation hardness of 25 to 40 GPa. If the nanoindentation hardness is in this range, chipping resistance and wear resistance are further improved. This is presumably because, when the nanoindentation hardness is in this range, the Ti and Zr composite boride layer has a crystalline phase and an amorphous phase, which more reliably exhibits improved wear resistance.

Here, nanoindentation hardness is measured based on the nanoindentation test method (ISO 14577), in which the surface of the Ti and Zr composite boride layer is polished and a diamond Berkovich indenter is used with an indentation load of 1.96 × 10 ^-3 N (200 mgf). At this time, measurements are taken at at least 10 arbitrary points, and the arithmetic average is calculated as the hardness measurement value. In this measurement, the distance between each measurement point is set to a distance at least 20 times the indentation depth during the test.

Crystal orientation of the crystalline phase of the Ti and Zr composite boride layer:
The hexagonal 001 diffraction line, 100 diffraction line, and 101 diffraction line measured by X-ray diffraction are diffraction peaks due to the (001), (100), and (101) planes, respectively. When these peak intensities are designated as Ih(001), Ih(100), and Ih(101), respectively, it is more preferable to satisfy the relationship 0.01≦Ih(001)/{Ih(001)+Ih(100)+Ih(101)}≦0.50. Satisfying this relationship further improves wear resistance.

Here, the diffraction peak intensities of the (001), (100), and (101) planes of the hexagonal crystal can be measured using X-ray diffraction with a 2θ/θ focusing optical system using Cu-Kα radiation (wavelength λ: 0.15405 nm).

The (001) plane of a hexagonal crystal is sometimes expressed as the (0001) plane. Similarly, the (100) plane is sometimes expressed as the (10-10) plane, (1-100) plane, (01-10) plane, (-1100) plane, (-1010) plane, or (0-110) plane. Similarly, the (101) plane is sometimes expressed as the (10-11) plane, (1-101) plane, (01-11) plane, (-1101) plane, (-1011) plane, or (0-111) plane. These are plane indices that are equivalent to each other.

Other layers (bottom layer):
The coating layer including the Ti and Zr composite boride layer of this embodiment exhibits sufficiently excellent crack resistance and wear resistance over a long period of use, even when materials that have high adhesion to cutting tools, such as Ti-based alloys and austenitic stainless steels, are machined by high-speed intermittent cutting. However, when a lower layer including a Ti compound (not limited to a stoichiometric compound) layer having a total average layer thickness of 0.1 to 2.0 μm and consisting of one or more layers selected from the group consisting of a Ti carbide layer, a nitride layer, a carbonitride layer, a carbonate layer, and a carbonitride oxide layer is provided adjacent to the tool substrate in addition to the coating layer, the effect of this layer, combined with that of the lower layer, makes it possible to exhibit even more excellent chipping resistance and heat crack resistance.

Here, if the total average layer thickness of the lower layer is less than 0.1 μm, the effect of the lower layer will not be fully exerted, while if it exceeds 2.0 μm, the crystal grains in the lower layer will tend to coarsen, making chipping more likely to occur.

Tool base:
(1) Material The tool substrate may be any conventionally known substrate for this type of tool substrate, provided that it does not impede the achievement of the above-mentioned object. Examples include cemented carbide (WC-based cemented carbide, including those containing Co in addition to WC and further including carbonitrides of Ti, Ta, Nb, etc.), cermet (including those containing TiC, TiN, TiCN, etc. as the main component), ceramics (titanium carbide, silicon carbide, silicon nitride, aluminum nitride, aluminum oxide, etc.), and cBN sintered body, and any of these is preferred.

(2) Shape The shape of the tool substrate is not particularly limited as long as it is a shape that can be used as a cutting tool, and examples thereof include an insert shape and an end mill shape.

Next, we will explain some examples, but the present invention is not limited to these examples.

The raw material powders were WC powder, VC powder, TaC powder, NbC powder, _Cr3C2 powder, and Co powder, _all with average particle sizes of 1 to 3 μm. These raw material powders were blended according to the composition shown in Table 1, wet mixed in a ball mill for 72 hours, and dried. They were then pressed into a green compact at a pressure of 100 MPa. This green compact was sintered in a vacuum of 6 Pa at a temperature of 1400°C for 1 hour. After sintering, the cutting edge was honed to an R of 0.03 to produce tool substrates 1 and 2 made of WC-based cemented carbide with the insert shape specified in ISO standard CNMG120408.

The same raw material powders as above were further compounded according to the composition shown in Table 1. The mixture was wet mixed in a ball mill for 72 hours, dried, and then pressed into a green compact at a pressure of 100 MPa. This green compact was sintered in a vacuum of 6 Pa at a temperature of 1400°C for 1 hour to produce a 4 mm diameter sintered round bar for forming a cemented carbide substrate. Furthermore, the sintered round bar was ground to produce tool substrates (end mill shape) 3-4 made of WC-based cemented carbide, each with a cutting edge diameter and length of 2 mm x 4 mm and a four-flute square shape with a 40-degree helix angle.

Next, a lower layer (applied only to some of the tool substrates) and a coating layer were formed on these tool substrates 1 to 4 using the following steps (a) to (d).

(a) Each of the tool substrates 1 to 4 was ultrasonically cleaned in acetone and, in a dried state, mounted along its outer periphery at a predetermined radial distance from the central axis on a rotating table in a high-power pulse sputtering device. Meanwhile, a Ti target and sintered targets of Ti, Zr, and boron were placed in four opposing positions across the rotating table in the high-power pulse sputtering device.

(b) The inside of the device was evacuated and maintained at a vacuum of 0.1 Pa or less, while the inside of the device was heated to 500°C using a heater. A DC bias voltage of -200 V was then applied to the tool substrate, which was rotating and revolving on the rotating table. Argon (hereinafter referred to as Ar) gas was then introduced into the device as a reactive gas, creating an atmosphere of 2.0 Pa. Furthermore, a current of 40 A was passed through a tungsten filament provided in the device to excite Ar ions, and the tool substrate was subjected to Ar bombardment for 1 hour.

(c) Ar gas and nitrogen gas were introduced into the apparatus as reactive gases to create a reactive atmosphere of 0.6 Pa, and high-power pulse sputtering was performed on the Ti target under the specified pulse sputtering conditions shown in Table 2. As a result, a TiN layer having the average layer thickness shown in Table 3 was formed on the surface of the tool substrate as the lower layer of the coating layer. However, the lower layer was not formed on all tool substrates.

(d) Subsequently, the nitrogen gas introduced into the apparatus was turned off and switched to Ar gas, and the atmosphere inside the apparatus was set to 0.5 Pa. The nitrogen gas was sufficiently discharged, and the atmosphere inside the apparatus consisted of only Ar gas. Thereafter, a sintered compact target consisting of Ti, Zr, and boron was subjected to high-power pulse sputtering under the predetermined pulse sputtering conditions shown in Table 2 for a time corresponding to the layer thickness, to produce Example Coated Inserts 1 to 15 and Example Coated End Mills 16 to 30 shown in Table 3 (hereinafter, these will be collectively referred to as Examples 1 to 30 , but Examples 6 and 9 and 21 and 24 are reference examples ).

Furthermore, for comparison purposes, lower layers and coating layers were formed on these tool substrates 1 to 4 using the procedures (a) to (d) above under the conditions shown in Table 4, and comparative coated inserts 1 to 9 and comparative coated end mills 11 to 19 (10 is a missing number, hereafter referred to as Comparative Examples 1 to 9 and 11 to 19) were manufactured as comparative coated tools shown in Table 5. However, lower layers were not formed on all tool substrates.

In Table 1, "-" indicates not contained.

In Table 2, when the example is a or b and the tool base symbol ^* is α or β, this indicates that example a used tool base α and example b used tool base β (a, b, α, and β are numbers), and "-" indicates that the corresponding processing was not performed.

In Table 3, when the example is a or b and the tool base symbol ^* is α or β, this indicates that example a used tool base α and example b used tool base β (a, b, α, and β are numbers), and the intensity ratio ^** is the value of Ih{001}/{Ih{001}+Ih{100}+Ih{101}}, and "-" indicates that it does not exist.

In Table 4, when the comparative examples are a and b and the tool base symbol ^* is α and β, comparative example a indicates that tool base α was used and comparative example b indicates that tool base β was used (a, b, α, and β are numbers), and "-" indicates that the corresponding treatment was not performed.

In Table 5, when the comparative examples are a and b and the tool base symbol ^* is α and β, comparative example a indicates that tool base α was used and comparative example b indicates that tool base β was used (a, b, α, and β are numbers), and the intensity ratio ^** is the value of Ih{001}/{Ih{001}+Ih{100}+Ih{101}}, and "-" indicates that it does not exist.

Next, the following cutting tests 1 and 2 were performed on Examples 1 to 30 and Comparative Examples 1 to 19, and the results are shown in Tables 6 and 7.

For Examples 1 to 15 and Comparative Examples 1 to 9, a dry high-speed intermittent cutting test (Cutting Test 1) was conducted under the following conditions, with each test piece screwed to the tip of a tool steel bit using a fixing jig.

Cutting test 1
Workpiece: JIS SUS316L round bar with four longitudinal grooves spaced equally apart along the length,
Cutting speed: 160m/min,
Cut: 2 mm,
Feed: 0.3 mm/rev,
Cutting time: 10 minutes

After the cutting test, the flank wear width was measured and the presence or absence of chipping was observed. However, if chipping occurred before the end of the cutting time, cutting was stopped and the time from the start of cutting was measured.
Table 6 shows the test results.

In Table 6, the cutting time (minutes) until the end of life for the comparative example indicates the cutting time (minutes) until the end of life is reached due to chipping.

Next, for Examples 16 to 30 and Comparative Examples 11 to 19, a wet high-speed intermittent cutting test (cutting test 2) was conducted using an end mill for side machining under the following conditions.

Cutting test 2
Workpiece: Block material (width 100 mm x length 250 mm) of Ti-based alloy (Ti-6% Al-4% V alloy by mass)
Cutting speed: 110m/min
Rotation speed: 17508 min ^-1
Cut: 2.0 mm
Feed: 0.07 mm/rev
End mill blade outer diameter: 2 mm

The cutting was continued until the cutting length reached 150 m (cutting time was approximately 123 minutes), and the flank wear width was measured and the occurrence of chipping was observed. However, if chipping occurred before the cutting length reached 150 m, cutting was stopped and the time from the start of cutting was measured.
Table 7 shows the test results.

In Table 7, the cutting time (minutes) until the end of life for the comparative example indicates the cutting time (minutes) until the end of life is reached due to chipping.

From the results shown in Tables 6 and 7, the examples in which the coating layer using the Ti and Zr composite boride layer has a crystalline phase and an amorphous phase exhibit excellent adhesion resistance and wear resistance in high-speed intermittent cutting of materials that have high adhesion to coated tools, such as various Ti-based alloys and austenitic stainless steel.
In contrast, in the comparative example, the cutting edge wear progresses quickly during high-speed intermittent cutting of the material with high weldability, and it is clear that the cutting edge reaches the end of its useful life in a relatively short period of time.

The above-disclosed embodiments are merely illustrative in all respects and are not restrictive. The scope of the present invention is indicated by the claims, not by the above-disclosed embodiments, and is intended to include all modifications equivalent to the claims and within their scope.

Claims (3)

1. A surface-coated cutting tool having a tool substrate and a coating layer on the tool substrate,
the coating layer is a composite boride layer of Ti and Zr having an average layer thickness of 0.5 to 5.0 μm ,
The Ti and Zr composite boride layer has an average composition in which atomic ratios x and y satisfy 0.3≦x≦0.7 and 1.5≦y≦3.0 when its composition is expressed by a composition formula: Ti _x Zr _(1-x) B _y , and further has a crystalline phase constituted by crystal grains of a hexagonal crystal structure and an amorphous phase, the hexagonal crystal grains constituting the crystalline phase have an average grain size of 2 to 30 nm, and the crystalline phase occupies an area ratio of 50 to 95 area % of the Ti and Zr composite boride phase.
A surface-coated cutting tool characterized by: 2. The surface-coated cutting tool according to claim 1, wherein the Ti-Zr composite boride layer has a nanoindentation hardness of 25 to 40 GPa. 3. The surface-coated cutting tool according to claim 1, wherein the hexagonal crystal grains of the Ti and Zr composite boride layer satisfy the relationship 0.01≦Ih(001)/{Ih(001)+Ih(100)+Ih(101)}≦0.50, where the peak intensities of the 001 diffraction line, the 100 diffraction line, and the 101 diffraction line in X-ray diffraction are Ih(001), Ih(100), and Ih(101), respectively.