Surface coated cutting tools

Abstract

Provided is a surface-coated cutting tool that exhibits excellent wear resistance over long-term use even in high-speed intermittent milling processing 【Solution】The tool comprises a base body and a coating layer, The coating layer consists of a lower layer in contact with the base body side and an upper layer in contact with the lower layer, The lower layer has an average thickness of 0.1 to 1.0 μm and is composed of either Ti nitride or Ti carbonitride, The upper layer has an average thickness of 1.0 to 20.0 μm, and is composed of (Al x Ti 1-x )(C y N 1-y )(wherein, on average, 0.75≦x≦0.90 and 0.00≦y≦0.5 respectively), and is composed of AlTi composite nitride or AlTi composite carbonitride containing crystal grains of NaCl-type face-centered cubic structure, The average value (α) of compressive stress in the upper layer is 0.5 to 3.0 GPa, A surface-coated cutting tool wherein the crack density on the surface of the upper layer is 0.5 to 3.0 cracks / mm

Description

Technical Field

[0001] The present invention relates to a surface-coated cutting tool (hereinafter sometimes referred to as a coated tool). Background Art

[0002] Conventionally, in order to extend the service life of cutting tools, there is a coated tool in which a Ti-Al based composite single nitride layer is formed as a coating layer on the surface of a substrate such as a tungsten carbide (hereinafter sometimes referred to as WC) based cemented carbide, and this coated tool has improved wear resistance and the like. Various proposals have been made regarding the composition and structure of the coating layer in order to further improve the cutting performance of coated tools.

[0003] For example, Patent Document 1 describes a surface-coated cutting tool in which the coating layer comprises an AlTiN lower layer, an AlTiN upper layer, and a TiCN layer between the two layers, the average thickness of the three layers satisfies a predetermined relationship, and the coating layer has no cooling cracks. It is described that the surface-coated cutting tool prevents the generation of cooling cracks that serve as starting points for chipping during cutting, and exhibits excellent chipping resistance and fracture resistance even in steel milling under severe high-speed intermittent cutting conditions where an impactful high load acts on the cutting edge.

[0004] Further, for example, Patent Document 2 discloses that the coating layer is Ti 1-x Al x N layer and / or Ti 1-x Al x C layer and / or Ti 1-x Al x A surface-coated cutting tool is described in which an Al2O3 layer is disposed as an outer layer on a CN layer (x is 0.65 to 0.95), and Ti as an intermediate layer 1-x Al x N layer, Ti 1-x Al x C layer, Ti 1-x Al x Since no crack network is formed in the CN layer, it is described that the CN layer has improved crack resistance particularly in interrupted cutting and is durable. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2018-176381 [Patent Document 2] Patent No. 5863241 [Overview of the initiative] [Problems that the invention aims to solve]

[0006] This invention has been made in view of the above circumstances and proposals, and its purpose is to provide a surface-coated cutting tool that exhibits excellent wear resistance over long-term use without causing abnormal damage such as chipping or breakage, even during high-speed and intermittent milling of stainless steel. [Means for solving the problem]

[0007] A surface-coated cutting tool according to an embodiment of the present invention is It has a substrate and a coating layer, The coating layer consists of a lower layer in contact with the substrate and an upper layer in contact with the lower layer. The aforementioned lower layer has an average thickness of 0.1 to 1.0 μm and is composed of either Ti nitride or Ti carbonitride. The upper layer has an average thickness of 1.0 to 20.0 μm, and (Al x Ti 1-x )(C y N 1-y )(each with an average of 0.75≦x≦0.90 and 0.00≦y≦0.05), and composed of AlTi composite nitride or AlTi composite carbonitride containing crystal grains of the NaCl type face-centered cubic structure. The average compressive stress (α) of the upper layer is 0.5 to 3.0 GPa. The density of cracks on the surface of the upper layer is 0.5 to 3.0 cracks / mm.

[0008] Further, the surface-coated cutting tool according to the above embodiment may satisfy the following (1).

[0009] (1) The absolute value of the difference between the average compressive stress (β) on the surface of the base body and the average compressive stress (α) of the upper layer is from 0.0 to 0.3 GPa.

Effects of the Invention

[0010] The surface-coated cutting tool according to the above embodiment is excellent in chipping resistance and fracture resistance, suppresses abnormal damage, is further excellent in wear resistance, and has a long service life even in high-speed interrupted milling processing.

Brief Description of Drawings

[0011] [Figure 1] This is an example of a schematic longitudinal cross-sectional view of a surface-coated cutting tool according to an embodiment of the present invention. [Figure 2] This is a schematic diagram of measurement lines for measuring crack density.

Mode for Carrying Out the Invention

[0012] The present inventors have intensively studied measures for achieving more excellent chipping resistance in coated tools described in the aforementioned prior art documents. First, as a result of studying the coated tools described in Patent Documents 1 and 2, it has been recognized that when these coated tools are subjected to high-speed heavy interrupted processing of stainless steel, since the coating layer does not contain cracks, abnormal damage such as cracks caused by stress concentration at the cutting edge occurs due to impact during cutting, and chipping is likely to occur. That is, the inventors have obtained the finding that when cracks, which are considered to be preferably absent in Patent Documents 1 and 2, are present at a predetermined density in a cross section parallel to the base surface of an AlTi composite nitride or AlTi composite carbonitride, the coated tool exhibits excellent chipping resistance.

[0013] Hereinafter, the coated tool according to the embodiment of the present invention will be described in detail. In this specification and in the claims, when a numerical range is expressed as "L~M" (where L and M are both numerical values), it is synonymous with "greater than or equal to L and less than or equal to M," and the range includes both an upper limit (M) and a lower limit (L). If a unit is specified only for the upper limit, the unit for the lower limit is also the same.

[0014] Figure 1 schematically shows an example of a longitudinal cross-section of a coating tool according to one embodiment of the present invention (the surface of the substrate is treated as a horizontal plane, ignoring minute irregularities on the surface of the substrate, and the cross-section is perpendicular to this plane). As is clear from Figure 1, the coating tool according to this embodiment has a lower layer (2) in contact with the surface of the substrate (1), and an upper layer (3) in contact with the lower layer (2) on the tool surface side of the lower layer (2). The lower layer (2) and the upper layer (3) constitute a coating layer (4). The following explains each layer in order.

[0015] 1. Lower layer The lower layer is provided in contact with the substrate and is composed of either Ti nitride or Ti carbonitride. Their composition is not particularly restricted and is not limited to stoichiometric compositions. However, a more preferred composition is given by formula:TiC z N 1-z When expressed as such, the z-value is in the range of 0.00 ≤ z ≤ 0.70.

[0016] The average thickness of the lower layer is preferably 0.1 to 1.0 μm. The reason for this is as follows: If it is less than 0.1 μm, the effect of improving the adhesion strength between the substrate and the upper layer is not sufficiently exhibited, and if it exceeds 1.0 μm, the crystal grains of the lower layer become coarse, the toughness of the lower layer decreases, and peeling of the coating layer from the substrate becomes more likely in the early stages of machining. The average thickness of the lower layer is more preferably 0.2 to 0.7 μm.

[0017] 2. Upper layer The upper layer is provided on the tool surface side of the lower layer and is composed of AlTi composite nitride or AlTi composite carbonitride. The composition of AlTi composite nitride or AlTi composite carbonitride is (Al x Ti 1-x)(C y N 1-y (These are the average values ​​of 0.75 ≤ x ≤ 0.90 and 0.00 ≤ y ≤ 0.05, respectively.) The reason for setting the x value as described above is as follows: If the x value is less than 0.75, the hardness of the layer composed of AlTi composite nitride and AlTi composite carbonitride decreases, resulting in insufficient wear resistance. On the other hand, if the x value exceeds 0.90, the Ti content relative to Al decreases, making it easier for the layer composed of AlTi composite nitride and AlTi composite carbonitride to contain wurtzite-type hexagonal crystal grains, leading to a decrease in wear resistance. A x value of 0.77 or more and 0.85 or less is more preferable. Although carbon (C) is not required to be present, its inclusion in a predetermined amount improves adhesion between the layer composed of AlTi composite nitride or AlTi composite carbonitride and the underlying layer, and also improves lubricity, thereby mitigating impact during cutting and improving chipping resistance. However, if the y value exceeds 0.05, the hardness decreases, so a y value in the range of 0.00 ≤ y ≤ 0.05 is preferred. A y value of 0.00 ≤ y ≤ 0.03 is more preferred.

[0018] The average thickness of the upper layer is preferably 1.0 to 20.0 μm. This is because if the average thickness is less than 1.0 μm, the average thickness of the upper layer relative to the coating layer is insufficient, resulting in poor wear resistance. On the other hand, if the average thickness exceeds 20.0 μm, the average thickness of the upper layer becomes excessive, making chipping more likely during machining. The average thickness of the upper layer is more preferably 3.0 to 10.0 μm.

[0019] It is preferable that the AlTi composite nitride or AlTi composite carbonitride has 60 area percent or more of crystal grains having a NaCl-type face-centered cubic structure in its longitudinal section. Furthermore, in the claims and this specification, "AlTi composite nitride or AlTi composite carbonitride containing crystal grains with a NaCl-type face-centered cubic structure" means "AlTi composite nitride or AlTi composite carbonitride having 60 area percent or more of crystal grains having a NaCl-type face-centered cubic structure in its longitudinal section." Furthermore, the upper limit of the area ratio of crystal grains in the NaCl-type face-centered cubic structure may be 100%, and this area ratio of 90 area% or more is more preferable. The method for measuring the area fraction of crystal grains in a NaCl-type face-centered cubic structure will be described later.

[0020] The average compressive stress (α) of the upper layer is preferably between 0.5 and 3.0 GPa. This is because if it is less than 0.5 GPa, the application of compressive stress is insufficient, and crack propagation during machining cannot be suppressed. On the other hand, if it exceeds 3.0 GPa, the peel resistance during machining decreases. The method for measuring the average value of compressive stress will be described later.

[0021] Furthermore, it is more preferable that the absolute value of the difference between the average compressive stress of the upper layer (α) and the average compressive stress of the substrate surface (β), |α-β|, is between 0.0 and 0.3 GPa. This is because reducing the difference between the average compressive stress of the upper layer (α) and the average compressive stress of the substrate surface (β) improves the adhesion strength between the coating layer and the substrate. For this reason, a value of |α-β| of 0.3 GPa or less is preferable. Moreover, a value of 0.1 GPa or less is even more preferable.

[0022] The crack density on the surface of the upper layer is preferably 0.5 to 3.0 cracks / mm. This is because when the crack density is greater than 3.0 cracks / mm, the likelihood of fracture originating from cracks increases, leading to a decrease in chipping resistance. On the other hand, when the crack density is less than 0.5 cracks / mm, localized stress concentration when an impact is applied may cause fracture in the upper layer, also leading to a decrease in chipping resistance. The crack density is more preferably 0.8 to 1.7 cracks / mm.

[0023] Crack density is measured as follows: The upper layer is polished until its thickness is approximately halved, obtaining a cross-section parallel to the substrate surface. Then, a 1 mm square field of view is observed in this cross-section using an optical microscope (100x magnification). Next, as shown in Figure 2, ten lines with a length of 1 mm are drawn horizontally across the square observation area, at intervals that divide the vertical direction into 11 equal parts. These lines are designated as analysis lines, and the number of cracks that intersect each of the ten analysis lines is counted. The average value of these crack counts is then defined as the crack density.

[0024] 3. Other layers Other layers can be broadly categorized into two types: layers that may be intentionally created and layers that are not intentionally created (unintentional). These will be explained in order below.

[0025] 3-1. Layers that may be intentionally created. The outermost layer, described below, is an example of a layer that may be intentionally included.

[0026] As the outermost layer, for example, a layer made of TiN (the atomic ratio of Ti to N in TiN is not limited to stoichiometric values) may be provided. If this layer is provided, since TiN itself has a golden hue, it can be used as an identification layer to distinguish whether a coated tool is unused or used by changing its color. The average thickness of this identification layer can be, for example, 0.1 to 1.0 μm.

[0027] 3-2. Unintended Layers When switching deposition gases, the lower layer (composed of Ti nitride or Ti carbonitride) and the upper layer ((Al x Ti 1-x )(C y N 1-y (Each layer is composed of layers with an average of 0.75 ≤ x ≤ 0.90 and 0.00 ≤ y ≤ 0.05 respectively), and a very small number of layers (called unintended layers) composed of compounds different from the outermost layer (composed of TiN) may be manufactured. Even if unintended layers exist, the aforementioned problems can be solved.

[0028] 4.Base (1)Material The substrate used in this embodiment can be any conventionally known substrate material, as long as it does not hinder the achievement of the aforementioned objectives. Examples include WC-based cemented carbide (including those containing WC and Co, and further including those with added carbides or carbonitrides such as Ti, Ta, and Nb), cermets (mainly composed of TiC, TiN, TiCN, etc.), and ceramics (titanium carbide, silicon carbide, silicon nitride, aluminum nitride, aluminum oxide).

[0029] (2) Shape The shape of the base material is not particularly restricted as long as it is a shape that can be used as a cutting tool; examples include the shape of an insert for milling and the shape of a drill.

[0030] 5.Measurement method (1) Average thickness of the lower layer, upper layer, and other layers The average thickness and composition of the lower layer, upper layer, and other layers constituting the coating layer can be determined, for example, by preparing a sample for observation of the longitudinal section of the coating layer at an arbitrary location using a focused ion beam system (FIB), and then observing the longitudinal section using a scanning electron microscope (SEM) or an energy dispersive X-ray spectrometer (EDS) attached to a transmission electron microscope (TEM). The measurement is performed using line analysis at five or more locations, and the thickness of each layer is measured based on the boundaries of each layer given by the differences in composition. The composition and thickness obtained in this way are averaged to obtain the average thickness and the average composition of each layer.

[0031] Here, the surface of the substrate is determined by observing the longitudinal section and using elemental mapping to define the interface between the substrate and the lower layer. The average straight line of the roughness curve of the interface thus obtained is then arithmetically calculated and defined as the surface of the substrate.

[0032] (2) Area ratio of crystal grains having a NaCl-type face-centered cubic structure The proportion of crystal grains having a NaCl-type face-centered cubic structure is measured as follows. First, using an electron backscattered diffraction pattern (EBSD) attached to the SEM, an electron beam tilted at, for example, 0.5 to 1.0 degrees relative to the normal direction of the polished surface is irradiated in a precessional manner on the longitudinal section of the polished coating layer. The electron beam is scanned with a beam diameter of 5 nm and an interval of 0.01 μm / step, and the electron diffraction pattern is continuously acquired to analyze the crystal orientation of each measurement point. Simultaneously, the crystal structure of each measurement point is determined based on the crystal orientation and electron diffraction pattern of that measurement point.

[0033] Here, the crystal orientation is measured discretely on the measurement surface, and the region up to the midpoint between adjacent measurement points is represented by that measurement point (sometimes called a pixel), thereby determining the orientation distribution of the entire measurement surface. A regular hexagonal shape can be exemplified as a region represented by these measurement points. If there is an angular difference of 5 degrees or more in crystal orientation between adjacent pixels, or if adjacent pixels each exhibit different crystal structures, the edges of the aforementioned region where these pixels meet are defined as grain boundaries. The area enclosed by these grain boundary edges is defined as a single crystal grain. However, a single pixel that has an orientation difference of 5 degrees or more from all adjacent pixels, or that has a different crystal structure from all adjacent pixels, is not considered a crystal grain; rather, two or more pixels connected together are treated as crystal grains. By performing grain boundary determination in this way, crystal grains are identified, and the area percentage occupied by crystal grains with a NaCl-type face-centered cubic structure is determined from the crystal structure information of each measurement point.

[0034] The conditions for acquiring the electron diffraction pattern used in this measurement include irradiating the polished surface with an electron beam accelerating at 15kV at an incident angle of 70 degrees and an irradiation current of 1nA onto each individual crystal grain within the measurement range of the polished surface, and acquiring the pattern at intervals of 0.01μm / step for the hard coating layer over a length of 50μm horizontally to the substrate surface and less than the thickness of the composite nitride layer in the normal direction.

[0035] (3) Compressive residual stress Using an X-ray diffraction (XRD) apparatus, for example, under conditions of 45kV 40mA, the flat surface of the rake face of the coated tool is irradiated with X-rays using Cu-Kα rays as the source. For the upper layer, the peak at the (111) plane of the AlTi composite nitride or AlTi composite carbonitride is obtained, and for the substrate, the peak at the WC(211) plane is obtained, and sin 2 Measurement will be performed using the φ method.

[0036] 6. Manufacturing method An example of a manufacturing method according to an embodiment of the present invention will be described. In this example, after the lower layer and upper layer are formed, a blast treatment is performed. The following will be described in order.

[0037] (1) Lower layer For example, the following film deposition conditions 1) or 2) can be used as examples. 1) A layer composed of Ti nitride; Reaction gas composition (volume %) TiCl4: 4.0~6.0%, N2: 25.0~35.0%, H2: remainder Reaction atmosphere pressure: 10.0~15.0 kPa Reaction atmosphere temperature: 800~900℃

[0038] 2) A layer composed of Ti carbonitride; Reaction gas composition (volume %) TiCl4:3.0~6.0%, N2:20.0~30.0% CH4:0.0~20.0%, CH3CN:0.0~1.5%, H2:Remaining Reaction atmosphere pressure: 7.0~12.0 kPa Reaction atmosphere temperature: 800~950℃

[0039] (2) Upper layer An example of film deposition conditions for the upper layer using the CVD method is shown. Reaction gas composition (volume %) Gas group A: TiCl4: 0.01-0.03%, AlCl3: 0.03-0.09% N2:0.0~10.0%, C2H4:0.0~0.5%, H2:Remaining Gas group B: NH3: 0.2-0.8%, H2: 25.0-35.0% Reaction atmosphere pressure: 4.0~5.0 kPa, Reaction atmosphere temperature: 700~850℃ Supply cycle: 1.0~5.0 seconds, Gas supply time per cycle: 0.15~0.25 seconds, Phase difference between the supply of gas group A and the supply of gas group B: 0.10~0.20 seconds

[0040] 3) Blast treatment For blast treatment, an example can be used where ZrO2 abrasive grains with a central particle size of 125 to 425 μm are used, the blasting pressure is 0.2 to 0.4 MPa, and the blasting time is 4 to 30 seconds. For the media used, an example can be a 200 mm diameter SUS sieve in accordance with ISO 3310-1 with an appropriate mesh size, and the particle size is adjusted by the dry media sieving method in accordance with ISO 2591-1. [Examples]

[0041] Next, we will describe some examples. Here, as an example, we describe its application to a milling cutter using a WC-based cemented carbide as the base material. However, the base material composition may be the one described above, and it may also be applied to inserts, drills, etc.

[0042] As raw material powders, WC powder, TiC powder, TaC powder, NbC powder, Cr3C2 powder, and Co powder were prepared and blended as shown in Table 1. After adding wax and mixing in acetone with a ball mill for 24 hours, the mixture was dried under reduced pressure and then press-molded into a compact of a predetermined shape at a pressure of 98 MPa. This compact was then vacuum-sintered in a vacuum of 5 Pa at a predetermined temperature within the range of 1420°C for 1 hour to produce three substrates 1 to 3 made of WC-based cemented carbide having a shape for a milling cutter with the shape of Mitsubishi Materials' ASX445R12506E.

[0043] Next, a lower layer was formed on these substrates 1 to 3 using the film formation conditions shown in Table 2, and an upper layer was formed using the film formation conditions shown in Table 3. Then, blast treatment was performed using ZrO2 abrasive grains under the conditions shown in Table 4 to produce Examples 1 to 8 shown in Table 5. In none of the examples was an outermost layer provided.

[0044] Furthermore, for comparative purposes, a lower layer was deposited on substrates 1-3 using the deposition conditions shown in Table 2, and an upper layer was deposited using the deposition conditions shown in Table 3. Blasting treatment was then performed using the conditions shown in Table 4 to produce comparative examples 1-8 shown in Table 5. No outermost layer was added to any of the comparisons.

[0045] In Table 5, the average thickness and composition of the lower and upper layers, the area ratio of crystal grains having a NaCl-type face-centered cubic structure, the compressive residual stress of the upper layer and substrate surface, and the crack density were measured using the method described above.

[0046] [Table 1]

[0047] [Table 2]

[0048] [Table 3]

[0049] [Table 4]

[0050] [Table 5]

[0051] Table 5 indicates that "-" does not exist.

[0052] Next, with Examples 1-8 and Comparative Examples 1-8 clamped to the tip of a tool steel cutter using a fixing jig, the following dry intermittent cutting tests of stainless steel were performed, and the flank wear width of the cutting edge was evaluated after the test. The results are shown in Table 6. Note that flank wear width (mm) refers to the flank wear width after the cutting time has elapsed, and cutting time (minutes) refers to the cutting time (minutes) until it was determined that chipping had occurred based on the detection of abnormal noise.

[0053] ≪Cutting conditions≫ Cutting tests: Dry face milling, center cut cutting Work material: JIS / SUS304L Block material measuring 100mm in width and 400mm in length Rotation speed: 1613 min -1 , Cutting speed: 400m / min. Cut: 2.0mm, Feed rate per tooth: 0.25 mm / tooth Cutting time: 7 minutes

[0054] [Table 6]

[0055] As is clear from Table 6, none of the examples reached the end of their lifespan during the cutting time of the cutting test. However, the comparative examples that did not satisfy even one of the requirements specified for the coated tool of the present invention reached the end of their lifespan in less than half the cutting time of the cutting test. Thus, the tool of the present invention exhibits excellent chipping resistance and fracture resistance even in high-speed and intermittent milling operations, and furthermore, exhibits a long lifespan due to its excellent wear resistance. [Explanation of symbols]

[0056] 1 Base 2 Lower layer 3 Upper layer 4 Covering layer

Claims

1. A surface-coated cutting tool having a substrate and a coating layer, The coating layer consists of a lower layer in contact with the substrate and an upper layer in contact with the lower layer. The aforementioned lower layer has an average thickness of 0.1 to 1.0 μm and is composed of either Ti nitride or Ti carbonitride. The upper layer has an average thickness of 1.0 to 20.0 μm, (Al ｘ Ti １－ｘ ) (C ｙ N １－ｙ ) (each with an average of 0.75 ≤ x ≤ 0.90 and 0.00 ≤ y ≤ 0.05), and composed of AlTi composite nitride or AlTi composite carbonitride containing crystal grains of the NaCl type face-centered cubic structure. The average value (α) of the compressive stress in the upper layer is 0.5 to 3.0 GPa. The density of cracks on the surface of the upper layer is 0.5 to 3.0 cracks / mm. A surface-coated cutting tool characterized by the following features.

2. The surface-coated cutting tool according to claim 1, characterized in that the absolute value of the difference between the average compressive stress (β) of the substrate surface and the average compressive stress (α) of the upper layer is 0.0 to 0.3 GPa.

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