JP7486045B2 - 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).

To improve the cutting performance of cutting tools, there are conventional coated tools in which a hard coating layer of a Ti compound or the like is formed by vapor deposition on the surface of a tool substrate such as a tungsten carbide (hereinafter referred to as WC)-based cemented carbide. This is known to exhibit excellent wear resistance, and various proposals have been made to further improve the hard coating layer.

For example, Patent Document 1 describes a coated tool that includes a tool substrate and one or more coatings (hard coating layers) formed on the tool substrate, the coatings including at least an aluminum oxide multilayer coating, the aluminum oxide multilayer coating including two or more unit layers made of aluminum oxide containing additive elements, and has a structure in which the two or more unit layers are periodically laminated, each of the unit layers having a different type or combination of additive elements, the additive elements being at least one element selected from the group consisting of Group IVa elements, Group Va elements, Group VIa elements, Y, Ca, Mg, B, and Si of the periodic table, and the coated tool is said to have improved wear resistance.

Furthermore, for example, Patent Document 2 describes a coated tool that includes a hard coating layer consisting of a titanium-based underlayer consisting of a single layer or a multilayer consisting of two or more layers selected from titanium carbides, nitrides, carbonitrides, carbonates, nitrides and carbonitrides, and an aluminum oxide layer having an α-type crystal structure, in which titanium sulfide (Ti _x S _y (x>0, y>0)) portions protrude from the interface of the titanium-based underlayer into the aluminum oxide layer and are dispersed and distributed in the aluminum oxide layer. This coated tool is said to have excellent chipping resistance and defect resistance even when cutting gray cast iron, ductile cast iron, and the like.

JP 2008-168364 A JP 2006-205301 A

In recent years, there has been a strong demand for labor- and energy-saving in cutting processes, and as a result, the hard coating layer of coated tools is required to have even greater resistance to abnormal damage, such as chipping resistance, fracture resistance, and peeling resistance, as well as excellent durability over long periods of use.

However, although the coated tools described in Patent Documents 1 and 2 have durability under the cutting conditions described in each document, they do not have sufficient chipping resistance and fracture resistance in high-speed intermittent cutting of alloy steel, and therefore cannot be said to have satisfactory durability.
In view of the above circumstances, an object of the present invention is to provide a coated tool having sufficient durability in high-speed interrupted cutting of alloy steel.

The present inventors have conducted extensive research into improving the durability of cutting tools having a hard coating layer including two layers, a lower layer being a Ti compound layer and an upper layer being an aluminum oxide ( _hereinafter sometimes referred to as _{Al2O3 )} layer. As a result, they have found that when Ti and S coexist and segregate at the grain boundaries of the crystal grains constituting the upper _Al2O3 layer, a hard coating layer having wear resistance, fracture resistance and chipping resistance is obtained, and durability is improved in high-speed intermittent cutting of alloy steel.

The present invention is based on this finding and is as follows.
"(1) A surface-coated cutting tool having a tool substrate and a hard coating layer including a lower layer and an upper layer on the surface of the tool substrate,
the lower layer is a Ti compound layer,
The upper layer has _{an average thickness of 2.0 to 20.0 μm and includes an Al 2 O 3 layer, in which Ti and S coexist at the grain boundaries of two adjacent Al 2 O 3 crystal grains , positions} corresponding to the maximum concentrations of Ti and S are present within a region within 10 nm from the grain boundary into two crystal grains sandwiching the grain boundary, and the number of grain boundaries in which the maximum values are 2.00 times or more as compared with the maximum concentrations of Ti and S in a region within the two crystal grains sandwiching the grain boundary from more than 10 nm to 50 nm from the grain boundary is 20% or more of the total number of grain boundaries.
(2) The surface-coated cutting tool according to (1), characterized in that the Al ₂ O ₃ layer has an α-type crystal structure, has a highest peak at Σ3 in a constituent atom-sharing lattice point distribution graph of Σ3 to Σ29, and the proportion of Σ3 in the whole is 60% or more.
(3) The surface-coated cutting tool according to (1) or (2), characterized in that the ratio of Al ₂ O ₃ grain boundaries in which Ti and S coexist in the Al ₂ O ₃ layer is 30 to 80% of the total number of Al ₂ O ₃ grain boundaries.

The coated tool of the present invention has excellent durability in high-speed interrupted cutting of alloy steel.

FIG. 2 is a schematic diagram showing an example of Al ₂ O ₃ crystal grains and their grain boundaries in an Al ₂ O ₃ layer in an observation region. FIG. 1 is a schematic diagram showing an example in which segregation of Ti and S is determined to exist as a result of a line analysis of Ti and _S compositions centered on the grain boundaries of Al ₂ O ₃ crystal grains in an Al ₂ O 3 layer. 1 shows a graph of corresponding grain boundary distribution in a coated tool 5 of the present invention, which will be described later. 1 shows a corresponding grain boundary distribution graph for a comparative coated tool 3 described later.

The present invention will be described in detail below. In this specification and claims, when a numerical range is expressed as "A to B" (A and B are both numerical values), the range includes an upper limit (B) and a lower limit (A), and the upper limit (B) and the lower limit (A) have the same units.
Moreover, the compositions of the Ti compound layers such as a TiC layer, which will be described later, and the Al ₂ O ₃ layer are not limited to stoichiometric compositions, but include all compositions with conventionally known atomic ratios.

Hard Coating:
The hard coating layer has a lower layer which is a Ti compound layer including a TiC layer and an upper layer which includes an Al ₂ O ₃ layer. These layers will be described below in order.

Bottom layer:
The Ti compound layer (e.g., at least one of a TiC layer, a TiN layer, a TiCN layer, a TiCO layer, and a _TiCNO layer) contained in the lower layer is present between the tool substrate and the upper layer including the _Al2O3 layer, and may be a single layer or multiple layers, and imparts wear resistance to the hard coating layer due to its high hardness. The Ti compound layer also plays a role in imparting adhesion to both the surface of the tool substrate and the upper layer including _{the Al2O3} layer.

The average thickness of the lower layer is more preferably 3.0 to 20.0 μm. When it is 3.0 to 20.0 μm, durability is improved and chipping resistance, defect resistance, and wear resistance can be exhibited for a long period of time. The average layer thickness is even more preferably 5.0 to 15.0 μm.

Top layer:
<Average layer thickness>
The upper layer includes an Al ₂ O ₃ layer, and the average layer thickness is preferably 2.0 to 20.0 μm, because if it is less than 2.0 μm, it is too thin to ensure sufficient durability over long-term use, and if it exceeds 20.0 μm, the crystal grains become large and chipping is likely to occur.

<Segregation of Ti and S>
The proportion of grain boundaries where Ti and S segregate is confirmed, for example, as follows.

(1) Determination of Observation Area In a longitudinal section of _{the Al2O3} layer (a section perpendicular to the surface of the tool base), at least 10 locations in an area having a length of 10 μm in a direction parallel to the surface of the tool base and a length greater than the average layer thickness in the layer thickness direction of _{the Al2O3} layer are determined as observation areas.

(2) Determination of grain boundaries in each observation region Each observation region is observed by high-angle annular dark field scanning transmission microscopy (HAADF-STEM) using a transmission electron microscope (TEM). FIG. 1 is a schematic diagram showing an example of the observation result. The grain boundary in the present invention is a part of a crystal grain boundary that contacts two adjacent crystal grains, and this part is considered to be one grain boundary. In other words, one crystal grain boundary is composed of multiple grain boundaries. Incidentally, eight crystal grains A to H are illustrated in FIG. 1. For example, crystal grain B has grain boundaries with each of crystal grains A, C, E, and F, and among them, the grain boundaries with crystal grains A and F are illustrated as grain boundaries. In FIG. 1, the total number of grain boundaries in all crystal grains (total number of grain boundaries in this observation area) is 13.

(3) Linear analysis of each composition For all of the grain boundaries, a line analysis of the composition is carried out by energy dispersive X-ray spectroscopy (EDS) for Ti and S in a region having a measurement length of 100 nm centered on the grain boundary and spanning the grain boundary under edge-on conditions in a direction perpendicular to the grain boundaries of two adjacent crystal grains centered on the grain boundary.
Here, the edge-on condition refers to a condition in which the measurement is performed at an angle such that the electron beam incidence direction is parallel to the depth direction of the grain boundary when the sample is tilted around an axis parallel to the grain boundary (see, for example, JP 2017-5004 A).

(4) Confirmation of the presence or absence of Ti and S segregation in each observation region. When a line analysis of the composition of each grain boundary is performed and the position giving the maximum value of the detection intensity of Ti and S (corresponding to the maximum concentration of Ti and S) is within a region within 10 nm from the grain boundary into the two crystal grains, and the maximum value is 2.00 times or more compared to the maximum detection intensity of Ti and S in the region within the two crystal grains from 10 nm to 50 nm from the grain boundary (when the concentration ratio is 2.00 times or more), the grain boundary is determined to be one in which Ti and S elements coexist and are segregated to the grain boundary.
FIG. 2 is a schematic diagram showing an example of changes in the detected concentrations of Ti and S at which it is determined that Ti and S coexist and segregate. In the figure, the starting point of the measurement is set so that the grain boundary is at the 50 nm position of the measurement length on the horizontal axis.

(5) Calculation of the proportion of grain boundaries where Ti and S coexist and segregate in each observation region. The proportion (number proportion) of the number of grain boundaries where it is determined that Ti and S segregate to grain boundaries exists in the total number of grain boundaries (the total number of grain boundaries in all crystal grains in each observation region) is calculated.

(6) Derivation of the ratio of grain boundaries where Ti and S coexist and segregate. The arithmetic average of the number ratios determined for each observation region is calculated to derive the ratio of the number of grain boundaries where Ti and S coexist and segregate to the total number of grain boundaries.

When the ratio is 20% or more, the strength of the grain boundary is improved, and the toughness of the hard coating layer is improved. The reason is not clear, but it is thought that the segregation of Ti and S atoms is appropriately performed, and the residual stress in the Al ₂ O ₃ layer is relaxed without impairing the strength, and the chipping resistance is improved. There is no upper limit to this ratio as long as it is 20% or more, but a ratio of 30 to 80% is more preferable because the effect of improving the chipping resistance is more reliably exhibited when the ratio is 30 to 80%.

Moreover, the Al ₂ O ₃ -layer crystal grains have an α-type crystal structure, and in the constituent atom sharing lattice point distribution graph, the highest peak is at Σ3, and the distribution ratio of Σ3 to the whole ΣN+1 is more preferably 60% or more. If it is 60% or more, the grain boundary strength is further increased, and the wear resistance improvement effect can be improved. There is no upper limit to this ratio, but when it is 60 to 90%, the wear resistance improvement effect is more reliably exhibited.

Here, the constituent atom covalent lattice points are measured and calculated by the following method.
That is, for the _{Al2O3 -} layer crystal grains having the α-type crystal structure, the inclination angle between the normal of the (0001) plane and the (10-10) plane, which are crystal planes, and the normal of the observation polished surface is measured. The crystal grains have a corundum-type hexagonal crystal structure in which constituent atoms of Al and oxygen exist at lattice points. Based on the inclination angle obtained by measurement, the distribution of corresponding grain boundaries consisting of lattice points (constituent atom sharing lattice points) where each of the constituent atoms shares one constituent atom between the crystal grains is calculated at the interface between adjacent crystal grains, and a constituent atom sharing lattice point distribution graph is obtained, which shows the distribution ratio of each ΣN+1, which is expressed as ΣN+1, to the entire ΣN+1, in which the corresponding grain boundaries consisting of the constituent atom sharing lattice point form have N lattice points that do not share constituent atoms between the constituent atom sharing lattice points (however, N is an even number of 2 or more in the corundum-type hexagonal crystal structure, but when the upper limit of N is set to 28 from the viewpoint of distribution frequency, the even numbers 4, 8, 14, 24 and 26 do not exist).

How to create a distribution graph of constituent atom sharing lattice points in the upper layer:
For the Al ₂ O ₃ crystal grains in the upper layer of the hard coating layer, the angle between the normal of each of the crystal lattice planes of the Al ₂ O ₃ crystal grains and the normal of the observation polished surface is measured using HAADF-STEM, and the crystal orientation relationship between adjacent crystal lattices is calculated from the measurement results to obtain a corresponding grain boundary distribution graph of the Al ₂ O ₃ in the upper layer.

With each cross section of the interface region and the surface region of the upper layer as a polished surface (the means for making the polished surface can be the means used for measuring the average layer thickness described later), the coated tool is set in the lens barrel of a field emission scanning electron microscope, and the electron beam is irradiated with an accelerating voltage of 15 kV and an irradiation current of 1 nA at an incidence angle of 70 degrees on the polished surface of the cross section, to each crystal grain having a corundum-type hexagonal crystal lattice present within the measurement range of the polished surface of the cross section. More specifically, using an electron beam backscattering diffraction device, the electron beam is irradiated at intervals of 50 μm in the direction parallel to the tool base surface and at intervals of 0.1 μm/step in the region with the layer thickness of the Al ₂ O ₃ layer as the upper limit in the direction perpendicular to the tool base surface, and the normal orientation of each face of the crystal lattice constituting the crystal grain is measured at each measurement point irradiated with the electron beam.

From the measurement results, the crystal orientation relationship between the crystal lattices at adjacent measurement points was calculated. In other words, it was assumed that a grain boundary exists between adjacent measurement points where the difference in crystal orientation angle between the measurement points is 5 degrees or more, and the set of measurement points surrounded by this grain boundary was identified as one crystal grain, and the entire crystal grain was identified.
In addition, when the crystal orientation relationship between the measurement points constituting the crystal lattice interface falls within the range of an error Δθ=5° with respect to the value of the angle between the crystal grains constituting the correspondence grain boundary, it is considered that a correspondence grain boundary exists between the measurement points, and the ratio of the ΣN+1 correspondence grain boundary to the total grain boundary length is calculated.

Other layers:
It is preferable to provide an outermost layer on the upper layer, which is made of one or more Ti compound layers selected from the group consisting of a TiC layer, a TiCN layer, a TiCN layer, a TiCO layer, and a TiCNO layer, and has a total average layer thickness of 0.1 to 5.0 μm, since this provides even better adhesion resistance and wear resistance. If the total average layer thickness is less than 0.1 μm, the effect of providing the outermost layer is not fully exhibited, while if it exceeds 5.0 μm, chipping is likely to occur.

Measurement of the average layer thickness:
Here, the average thickness of each layer constituting the hard coating layer can be measured by, for example, cutting the hard coating layer at an arbitrary position using a focused ion beam system (FIB), a cross section polisher (CP), or the like to prepare a sample for observation, and then observing the longitudinal section of the sample using a scanning electron microscope (SEM), a TEM, a scanning transmission electron microscope (STEM), or an energy dispersive X-ray analysis (EDX) attached to the SEM or TEM. The measurement value can be obtained by observing multiple points (for example, five points) using a spectrometer and averaging the results.

Tool Base:
The tool substrate may be any of the substrates known in the art as the substrate of this type, provided that they do not impede the achievement of the object of the present invention. Examples include cemented carbide (WC-based cemented carbide, WC, Co, and Ti, Ta, Nb, etc.), cermet (Ti carbide, Ti nitride, Ti carbonitride, etc.), ceramics (titanium carbide, silicon carbide, silicon nitride, aluminum nitride, aluminum oxide, etc.), cBN sintered body, and diamond sintered body, and any of these is preferable.

Production method:
The hard coating layer of the coated tool of the present invention is produced by a process including three steps: (1) a step of depositing a lower layer, (2) a step of depositing an upper layer, and (3) a step of etching the upper layer.

(1) Step of Depositing Lower Layer The lower layer, which is a Ti compound layer having a Ti carbonitride layer, can be deposited under known deposition conditions so that the average layer thickness is, for example, 3.0 to 20.0 μm.

(2) Upper Layer Deposition Step In this step, the upper layer including the Al ₂ O ₃ layer in which S is segregated at the grain boundaries is deposited so that the average layer thickness is 2.0 to 20.0 μm. The deposition conditions are, for example,
Reactive gas: AlCl ₃ 1.0-1.8%, CO ₂ 3.0-5.0%,
HCl 1.0-3.0%, _H2S 0.75-1.50%, _H2 remainder Reaction atmosphere temperature: 900-1000°C
Reaction atmosphere pressure: 5.0 to 10.0 kPa
can be given.

(3) Upper Layer Etching Process In this process, after the upper layer is formed, etching and heat treatment are performed using a gas containing Ti, so that Ti and S can be made to coexist and segregate at the grain boundaries of the crystal grains in _{the Al2O3} layer of the upper layer. For example, the etching conditions are as follows:
Reaction gas composition (volume %): _TiCl4 1.0-2.0%, _H2 remainder Reaction atmosphere temperature: 900-1000°C
Reaction atmosphere pressure: 5.0 to 15.0 kPa
Processing time: 30 to 120 minutes.

Next, an embodiment will be described.
Here, as an example of the coated tool of the present invention, we will describe an insert cutting tool using a WC-based cemented carbide as the tool base, but the same applies when the above-mentioned material is used as the tool base, and also when applied to drills and end mills.

As raw material powders, WC powder, TiC powder, ZrC powder, TaC powder, NbC powder, _Cr3C2 powder, TiN powder, and Co powder, all of which have an average particle size of 1 to ₃ μm, were prepared. These raw material powders were mixed to the composition shown in Table 1, wax was added, and the mixture was mixed in acetone with a ball mill for 24 hours. The mixture was dried under reduced pressure and then pressed into a green compact of a predetermined shape at a pressure of 98 MPa. The green compact was vacuum sintered in a vacuum of 5 Pa at a predetermined temperature in the range of 1370 to 1470°C for 1 hour. After sintering, tool bases A to C made of WC-based cemented carbide having an insert shape according to ISO standard CNMG120408 were produced, respectively.

Next, each of these tool substrates A to C was placed in a chemical vapor deposition apparatus, and the coated tools 1 to 10 of the present invention were produced in the following manner.
(a) First, under the conditions shown in Table 2, a Ti compound layer was formed by vapor deposition as a lower layer (the first layer, the second layer, and the third layer are closest to the tool substrate in that order) having a target total average layer thickness shown in Table 4.
(b) Next, an _Al2O3 layer was formed by chemical vapor deposition under the conditions shown in Table 3, and then an etching process was performed using a mixed gas of _TiCl4 and _H2 gas to obtain an _Al2O3 layer as an upper layer with a target _average layer thickness shown in Table ₄ .
The conditions shown in Tables 2 and 3 are given as an example of the above-mentioned production method.

For the purpose of comparison, comparative coated tools 1 to 10 shown in Table 5 were manufactured by depositing a hard coating layer under the conditions described in Tables 2 and 3 in order to form a hard coating layer that does not satisfy the requirements of the present invention.

Furthermore, when the thickness of each of the constituent layers of the hard coating layers of the coated tools 1 to 10 of the present invention and the comparative coated tools 1 to 10 was measured (measured on the longitudinal section) using a scanning electron microscope, all of them showed an average layer thickness (average value of measurements at five points) that was substantially the same as the target layer thickness. The measurement results are shown in Table 4.
In addition, the presence or absence of segregation of Ti and S to the grain boundaries of the Al ₂ O ₃ crystal grains in the Al ₂ O ₃ layer of the coated tools 1 to 10 of the present invention and the comparative coated tools 1 to 10 was confirmed by the above-mentioned method, and the ratio of the number of grain boundaries with segregation of Ti and S to the total number of grain boundaries of all crystal grains, the peak position of the corresponding grain boundary distribution graph of the α-type Al ₂ O ₃ crystal grains, and the Σ3 corresponding interface ratio (%) were obtained. The measurement results are shown in Tables 4 and 5. In addition, the corresponding grain boundary distribution graph of the coated tool 5 of the present invention is shown in Figure 3, and the corresponding grain boundary distribution graph of the comparative coated tool 3 is shown in Figure 4.

Next, the various coated tools, coated tools 1-10 of the present invention and comparative coated tools 1-10, were screwed to the tip of a tool steel bit with a fixture, and the following cutting tests were carried out: a wet high-speed, large-depth cutting test of chromium-molybdenum alloy steel (cutting condition A), and a wet high-speed intermittent cutting test of chromium-molybdenum alloy steel (cutting condition B). The flank wear width of the cutting edge was measured, and the measurement results are shown in Table 6. For tools in which peeling or chipping of the hard coating layer occurred during the cutting test, the cutting time until this occurred was measured (the occurrence of chipping at the bottom of Table 6 includes peeling of the hard coating layer).

Cutting conditions A:
Workpiece: JIS SCM440 bar material with four longitudinal grooves spaced equally apart along the length,
Cutting speed: 350 m/min
Cut: 2.0 mm
Feed: 0.25 mm/rev
Cutting time: 5 minutes (normal cutting speed, cutting depth, and feed are 200 m/min, 1.5 mm, and 0.20 mm/rev, respectively).

Cutting condition B:
Workpiece: JIS SCM440 4-slit material Cutting speed: 250 m/min
Cut: 1.5mm
Feed per revolution: 0.25 mm/rev
Cutting time: 5 minutes (normal cutting speed, cutting depth, and feed are 200 m/min, 1.0 mm, and 0.2 mm/rev, respectively).

From the results shown in Table 6, the coated tools 1 to 10 of the present invention have a hard coating layer that includes at least an Al ₂ O ₃ layer as an upper layer, and Ti and S are coexisting and segregated at the grain boundaries of the Al ₂ O ₃ crystal grains in the Al ₂ O ₃ layer, thereby exhibiting excellent cutting performance without peeling or chipping.
In contrast, the coated tools 1 to 10 of the comparative examples reached the end of their service life in a relatively short period of time due to the occurrence of peeling of the hard coating layer and chipping.

As described above, the coated tool of the present invention exhibits excellent wear resistance, peeling resistance, and chipping resistance even in high-speed intermittent cutting of alloy steel, and exhibits excellent cutting performance over long periods of use, so it is fully satisfactory in terms of improving the performance of cutting equipment, reducing the labor required for cutting, and reducing costs.

Claims (3)

A surface-coated cutting tool having a tool substrate and a hard coating layer including a lower layer and an upper layer on a surface of the tool substrate,
the lower layer is a Ti compound layer,
The upper layer has _{an average thickness of 2.0 to 20.0 μm and includes an Al 2 O 3 layer, in which Ti and S coexist at the grain boundary between two adjacent Al 2 O 3 crystal grains , positions} corresponding to the maximum concentrations of Ti and S are present within a region within 10 nm from the grain boundary into two crystal grains sandwiching the grain boundary, and the number of grain boundaries in which the maximum values are 2.00 times or more as compared with the maximum concentrations of Ti and S in a region within the two crystal grains sandwiching the grain boundary from more than 10 nm to 50 nm from the grain boundary is 20% or more of the total number of grain boundaries. _{The surface-coated cutting tool according to claim 1, characterized in that the Al2O3 layer} has an α-type crystal structure, has the highest peak at Σ3 in a constituent atom sharing lattice point distribution graph of Σ3 to Σ29, and the proportion of Σ3 in the whole is 60% or more. 3. The surface-coated cutting tool according to claim 1, wherein the ratio of Al ₂ O ₃ grain boundaries where Ti and S coexist in the Al ₂ O ₃ layer is 30 to 80% of the total number of Al ₂ O ₃ grain boundaries.

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