CN104816526A - Surface coating cutting tool and manufacturing method thereof - Google Patents

Abstract

The present invention provides a surface-coated cutting tool exhibiting excellent chipping resistance in high-speed interrupted cutting and a method of manufacturing the same. In the surface-coated cutting tool of the present invention, the surface of the tool substrate having a solidified layer on the surface is provided with an aluminum oxide layer, the aluminum oxide layer has an average layer thickness of 0.2 to 5.0 μm, and has high smoothness. Aluminum is composed of α-type or a mixed phase of α-type and γ-type, and the proportion of titanium contained in the aluminum oxide layer is more than 0.02 atomic % and not more than 10 atomic %, and the average perimeter of the crystal grains is relative to the composition of the oxide. The ratio of the perimeters of the equal-area circles of the crystal grains of the aluminum layer is 1.8 to 3.0.

Description

Surface-coated cutting tool and manufacturing method thereof

technical field

The present invention relates to a hard coating that has excellent lubricity, chipping resistance, and wear resistance, so even when used in high-speed interrupted cutting of steel and cast iron, it can be used for a long time. Surface-coated cutting tools with excellent cutting performance.

Background technique

For a long time, it has been known to form carbides, nitrides, carbonitrides, etc. composed of at least one element selected from the IVB, VB, and VIB groups of the periodic table on the surface of the tool substrate composed of cemented carbide. The hard coating is used as a hard coating layer, thereby improving the wear resistance of cutting tools.

Moreover, in the hard film, the α-type alumina layer has excellent thermal stability, low reactivity, and high hardness. The outermost layer of the hard coating layer composed of carbides, nitrides, carbonitrides, etc. of at least one element in the group is formed by coating.

As prior art related to forming an α-type alumina layer as the hard coating layer of the outermost layer as described above, the following technology is known focusing on the grain shape: A surface-coated cutting tool, for example, in The surface evaporation of the tool base forms (a) a Ti compound layer as the lower layer, and evaporates to form (b) an α-alumina layer having the shape of a flat polygon (including a flat hexagon) as an upper layer. And the crystal grain structure of elongated shape, and contains Zr, and (c) in the crystal grains of the upper layer, the inside of the crystal grains of 60% or more in terms of area ratio is composed of at least one structure represented by Σ3 The atoms are divided by the lattice interface formed by sharing lattice points, so that the hard coating layer exhibits excellent wear resistance in high-speed heavy cutting (for example, refer to Patent Document 1).

Also, it is known that a surface-coated cutting tool in which a hard coating layer composed of titanium carbide or the like is formed on a tool base by CVD method exhibits very excellent fracture resistance and wear resistance, wherein the tool base Composed of WC-based cemented carbide, in the hard phase mainly composed of carbides, nitrides and/or their solid solutions of IVB, VB, VIB group elements, the binding phase mainly composed of iron group metals, and the rest composed of tungsten carbide The WC-based cemented carbide composed of (WC) is composed of carbides, nitrides, oxides and/or solid solutions of IVB, VB, and VIB group elements in at least a part of the grains of the hard phase. The structure of at least one compound (except the hard phase component constituting the crystal grains) is obtained, and the balance between hardness and toughness is excellent (for example, refer to Patent Document 2).

In addition, it is known that in a surface-coated cutting tool in which a hard coating layer composed of a CrN layer with an average layer thickness of 0.2 to 2.0 μm is formed by physical vapor deposition on the surface of a WC-based cemented carbide tool substrate, the The CrN layer has a height equal to the average layer thickness, and is composed of elongated flat-shaped CrN crystal grains growing in a direction vertical to the surface of the tool base, and a horizontal section at a depth of 0.1 μm from the surface of the CrN layer When the grain structure on the surface is observed, the area ratio of the long and flat CrN grains with a short side of 5 to 100nm and an aspect ratio of 3 or more occupies more than 30% of the total horizontal cross-sectional area. By setting the above structure , the hard coating exhibits excellent chipping resistance in interrupted heavy cutting (for example, refer to Patent Document 3).

In addition, as a technology focusing on the inclusion of titanium oxide in the Al ₂ O ₃ layer, it is known that when a single layer or multiple layers are coated on a tool substrate made of cemented carbide or cermet, the thickness is 0.5 to 25 μm. At least one layer has an Al ₂ O ₃ layer and a ZrO ₂ and/or HfO ₂ layer, and a third fine dispersion of titanium oxide, carbon oxide, nitrogen oxide or carbon oxynitride is introduced into the layer phase, thereby improving wear resistance (for example, refer to Patent Document 4).

As a method of forming the above-mentioned existing hard coating layer, that is, an aluminum oxide layer, the chemical vapor deposition (CVD) method and the physical vapor deposition (PVD) method are generally used. In addition, it is also known There is an aluminum oxide layer formed by a sol-gel method. The sol-gel method is a material synthesis method that can produce porous gels, organic-inorganic mixtures, glasses, ceramics, and nanocomposites from solutions. Compared with the existing melting method and sintering method, high-temperature materials can be produced at a lower temperature, and it is expected to be used in the production of products with various microstructures and various forms such as loose bodies, fibers, coatings, and particles. A relatively new coating forming method.

Patent Document 1: Japanese Patent Laid-Open No. 2009-172748

Patent Document 2: Japanese Patent Laid-Open No. 2000-38636

Patent Document 3: Japanese Patent Laid-Open No. 2011-156639

Patent Document 4: Japanese Patent Publication No. 2002-526654

In surface-coated cutting tools that are coated with an alumina layer as a hard coating layer by the CVD method, the resistance of the rake face of the coated tool during high-speed interrupted cutting of steel and cast iron can be cited. Grinding properties are improved, based inter alia on the higher thermal stability, non-reactivity of the alpha-alumina formed.

The α-alumina layer disclosed in Patent Document 1 is unsatisfactory in high temperature strength and surface properties. Therefore, when heavy cutting is performed at a higher speed, not only chipping but also thermoplastic deformation and misalignment are prone to occur. For this reason, wear resistance decreases, and there is a problem that the service life is reached in a relatively short period of time.

In addition, the titanium carbide layer disclosed in the above-mentioned Patent Document 2 has low crystallinity, poor mechanical properties and interface strength, and thus tends to be peeled off, and there is a problem that wear rapidly progresses as a result even if it is coated with an alumina layer.

Furthermore, when the CrN layer disclosed in Patent Document 3 is used in interrupted heavy cutting, the progression of cracks in the layer cannot be avoided, and this causes the problem of layer peeling and chipping.

Also, the _Al2O3 layer and the ZrO2 and/or _HfO2 layer disclosed in the above _- mentioned Patent Document ₄ , and the oxide, oxycarbide, oxynitride, or carbon oxynitride of titanium is introduced into the layer. In the composite material of the third finely dispersed phase, the problem of plastic deformation of the cutting edge due to heat generated during cutting occurs.

Contents of the invention

Therefore, the technical problem to be solved by the present invention, that is, the object of the present invention is to provide a surface-coated cutting tool coated with an aluminum oxide layer as a hard coating layer, which can withstand high-speed intermittent cutting of cast iron and carbon steel, etc. When used in cutting, it is difficult to cause chipping and peeling, and it exhibits excellent cutting performance for a long time.

Therefore, in order to form an alumina layer excellent in wear resistance on the surface of a tool base, the present inventors focused on a method based on a sol-gel method that has not been sufficiently studied so far as a method for forming a hard coating layer for cutting tools. The formation of the alumina layer has been intensively studied, and as a result, it has been found that when the drying and firing treatment after forming the alumina layer by the sol-gel method is performed by a predetermined method, or the alumina layer is formed by the sol-gel method, An alumina layer is formed by a sol-gel method on a Ti oxide layer previously formed by a CVD method, a PVD method, a sol-gel method, etc., using Ti oxide having an effect of promoting the crystallization of alumina, Thus, an aluminum oxide layer with a complex grain shape can be formed with a desired size and aspect ratio, and even during high-speed interrupted cutting where intermittent impact loads act on the cutting edge, the adjacent grains The grains mesh with each other along the unevenness, the adhesion between the crystal grains is improved, the chipping resistance and the peeling resistance are excellent, and the excellent cutting performance can be maintained for a long time.

That is, it has been found that the lower part of the alumina layer is preferentially crystallized by a heat treatment method capable of heating only the substrate such as the RTA (infrared heating) method, or by applying a sol-gel method of Ti oxide when forming the alumina layer. The aluminum oxide layer is arranged for the purpose of the starting point, and heat treatment is performed, so that the crystals of specific grains can be selectively grown, and the crystal growth can be formed along all the crystals in the aluminum oxide layer without hindering other grains. Crystal grains of complex shape composed of grain boundaries with high concavo-convexity grown in the direction of growth. In addition, the size and aspect ratio of the grains and the crystal structure of the alumina layer can be controlled according to the firing conditions. Through the anchoring effect of the grains, the strength of the grain boundaries is greatly improved, and because of its excellent wear resistance And lubricity, so even when performing high-speed interrupted cutting where abnormal damage such as peeling and micro chipping is likely to occur due to large impact and heat near the cutting edge, it can exhibit excellent cutting performance for a long time.

The present invention has been made in view of the above knowledge, and has the following aspects.

(1) A surface-coated cutting tool formed by coating the surface of a tool substrate composed of tungsten carbide-based cemented carbide or titanium carbonitride-based cermet to form a hard coating layer, wherein,

(a) the hard coating layer includes an aluminum oxide layer having an average layer thickness of 0.2 to 5.0 μm,

(b) the crystal grains constituting the alumina layer have an α-type crystal structure or a crystal structure of a mixed phase of α-type and γ-type,

(c) When the aspect ratio of the crystal grains is defined as the ratio of the grain diameter in the layer thickness direction to the grain diameter in the perpendicular direction to the layer thickness obtained by the electron backscatter diffraction method, the average aspect ratio of the crystal grains is The size ratio is 0.5 to 5.0, and the average value of the ratio of the perimeter length of the crystal grain to the perimeter length of a circle having an area equal to the area of the crystal grain is 1.8 to 3.0 as determined by electron backscatter diffraction method.

(2) The surface-coated cutting tool described in (1), wherein the aluminum oxide layer contains crystal grains of titanium oxide,

(a) In the alumina layer, the content ratio of Ti to all metal elements is more than 0.02 at % and not more than 10 at %,

(b) The crystal grains of the titanium oxide are titanium oxide fine particles having an average particle diameter of 0.01 to 0.10 μm, the titanium oxide fine particles are aggregated to surround the crystal grains constituting the aluminum oxide layer, and The average number of titanium oxide particles present on the perimeter of the crystal grains of the aluminum oxide layer is 5 to 50.

(3) The surface-coated cutting tool described in (1) or (2), wherein the surface-coated cutting tool is formed by coating the surface of a tool substrate made of tungsten carbide-based cemented carbide. Formed from a cladding layer, a solidified layer on the surface of the substrate with an average layer thickness of 0.5 to 3.0 μm is formed from the surface of the tool substrate along the depth direction, and the average content of Co contained in the solidified layer on the surface of the substrate as a binder phase metal is Less than 2.0% by mass.

(4) The surface-coated cutting tool described in the above (1) or (2), wherein the surface-coated cutting tool is formed by coating the surface of a tool base made of titanium carbonitride-based cermet. Formed from a coating layer, a solidified layer on the surface of the substrate with an average layer thickness of 0.5 to 3.0 μm is formed from the surface of the tool substrate in the depth direction, and the solidified layer on the surface of the substrate contains Co and Ni as binder phase metals. The total average content of is less than 2.0% by mass.

(5) A method of manufacturing a surface-coated cutting tool, which is a method of manufacturing the surface-coated cutting tool described in (1) to (4), wherein the aluminum oxide layer is obtained by a sol-gel method formed on the titanium oxide layer.

(6) The surface-coated cutting tool described in (1) or (2), wherein the hard coating layer further includes a base layer formed directly under the alumina layer.

(7) The surface-coated cutting tool described in (6) above, wherein the base layer is made of nitride or oxide containing at least one element selected from Groups IVB, VB, VIB and Si of the periodic table. things constitute.

(8) The surface-coated cutting tool described in (6) above, wherein the base layer is made of a titanium compound.

According to the surface-coated cutting tool of the present invention, the aluminum oxide layer formed by the sol-gel method is coated on the surface of the tool base, and it itself has excellent surface smoothness, lubricity, welding resistance, and resistance to corrosion. chipping, and the shape of the peripheral portion of the crystal grains constituting the alumina layer has a shape with many irregularities, so the bonding force between the crystal grains is improved. Combined with these effects, no chipping, Abnormal damage such as peeling, and exhibits excellent cutting performance in long-term use, its effect is very large.

Description of drawings

FIG. 1 shows a photograph of the structure of the longitudinal section of the aluminum oxide layer of the tool 15 of the present invention observed by TEM.

FIG. 2 shows a micrograph of the longitudinal section of the aluminum oxide layer of the tool 15 of the present invention observed by SEM.

FIG. 3 shows a photograph of the structure of the surface of the aluminum oxide layer of the tool 15 of the present invention observed by SEM.

Symbol Description

1-Ti oxide particles, 2-alumina layer (Al ₂ O ₃ layers), 3-titanium compound layer (TiO ₂ layers)

Detailed ways

Hereinafter, the present invention will be described in detail.

(a) Average layer thickness of the aluminum oxide layer constituting the hard coating:

The surface-coated cutting tool according to the embodiment of the present invention includes, as a hard coating layer, an aluminum oxide layer with an average layer thickness of 0.2 to 5.0 μm formed by a sol-gel method. If the average layer thickness of the aluminum oxide layer is If it is less than 0.2 μm, the unique effect of the present invention, which is to increase the strength of the grain boundary, by the anchoring effect between crystal grains, which is obtained by the grain boundary with high unevenness as described above, cannot be exerted. , on the other hand, when the average layer thickness exceeds 5.0 μm, peeling of the layer tends to occur, which is not preferable. Therefore, the average layer thickness of the alumina layer is set to 0.2 to 5.0 μm.

(b) Crystal structure of grains constituting the aluminum oxide layer:

The crystal forms of alumina include α, κ, γ, δ, and θ. When the sol-gel method is used to form the alumina layer, the alumina layer composed of grains with a γ-type crystal structure is mainly formed. Because the γ-type Alumina has high lubricity, so the welding resistance and the heat suppression effect during cutting are high, but the high-temperature hardness is not excellent, and it lacks wear resistance, especially in high-speed cutting where heat is large near the tool tip. It will wear out in a relatively short time, so the γ-type crystal structure alone is not enough as a hard coating for surface-coated cutting tools. In the present invention, the aluminum oxide layer is formed through prescribed drying and firing treatments, or titanium oxides such as TiO ₂ and Ti ₂ O ₃ , Ti ₃ O ₅ , Ti ₄ O ₇ are applied to the base of the aluminum oxide layer ( Hereinafter, it is also referred to as "Ti oxide") to promote crystallization, so that the crystal structure of the aluminum oxide layer can be controlled not only by firing conditions but also by the thickness of the Ti oxide base layer. In addition, in high-speed cutting where the temperature of the tool tip increases, the α-type alumina layer with excellent high-temperature hardness and heat resistance is preferred, but in the cutting method that suppresses the heat generation of the tool tip and requires welding resistance, it is different from the α-type alumina layer. The aluminum oxide preferably contains an alumina layer of gamma alumina together. Therefore, the crystal structure of crystal grains constituting the alumina layer is set to be an α type or a mixed phase of an α type and a γ type.

(c) Aspect ratio and grain shape of crystal grains constituting the alumina layer:

In the present invention, it was found that by controlling the aspect ratio of the crystal grains constituting the aluminum oxide layer to a predetermined value, and making the peripheral portion of the aluminum oxide crystal grains, that is, the shape of the grain boundaries into a shape with many irregularities, Adjacent crystal grains are meshed with each other along the unevenness, thereby imparting a high adhesive force due to the so-called anchor effect, thereby improving wear resistance and chipping resistance. In addition, regarding the shape of the peripheral portion of the crystal grain, it has been confirmed through numerous experiments that quantitative evaluation can be performed using the value of the ratio of the circumference of the crystal grain to the circumference of a circle having an area equal to the grain area of the crystal grain.

That is, for the longitudinal section of the aluminum oxide layer, using the electron backscatter diffraction method, for example, with an observation field of 8 μm x 6 μm in length and width, and a measurement step size of 50 nm, the crystallinity of each crystal within the above-mentioned observation field range is obtained for five fields of view. For particle shape, the maximum diameter in the vertical direction of the layer thickness is defined as the particle diameter in the vertical direction of the layer thickness, and the maximum diameter in the direction of the layer thickness is defined as the particle size in the layer thickness direction, and the particle diameter in the layer thickness direction relative to the layer When the average value of the grain diameter ratio in the thickness vertical direction is taken as the average aspect ratio of crystal grains in the alumina layer, wear resistance is lacking when the average aspect ratio of the crystal grains is less than 0.5, and on the other hand , and if it exceeds 5.0, it will become a coarse structure, so falling off and chipping will easily occur. Therefore, the average aspect ratio of crystal grains constituting the alumina layer is set to 0.5 to 5.0.

And, similarly to the above-mentioned average aspect ratio, by the electron backscatter diffraction method, with an observation field of 8 μm x 6 μm in length and width, and a measurement step size of 50 nm, the ratio of each crystal grain constituting the aluminum oxide layer was obtained for five fields of view. Shape, when the length of the grain boundary of each crystal grain, that is, the length of the outer periphery of each crystal grain shape, is taken as the perimeter of each crystal grain, the grain area of the crystal grain is obtained by the electron backscatter diffraction method, and the perimeter of the crystal grain is phase The average value of the ratio of the circumference of a circle having an area equal to the grain area is less than 1.8, the shape of the grain boundary has less unevenness and becomes relatively smooth, and the meshing of the grains is reduced, so the anchoring effect cannot be obtained and cannot be obtained. Fully exhibit the effect of improving the bonding force between crystal grains. On the other hand, if it exceeds 3.0, when focusing on one crystal grain, it becomes a crystal grain shape with very large unevenness, so that a fragile portion in shape such as elongated protrusions is formed, so cracks are likely to occur, and the performance is inferior. Therefore, the grain area of the crystal grains was obtained by the electron backscatter diffraction method, and the average value of the ratio of the circumference of the crystal grains to the circumference of a circle having an area equal to the grain area was set to 1.8 to 3.0.

In addition, the aluminum oxide layer can be directly formed on the tool substrate to exhibit its performance, but when the cemented carbide containing titanium carbonitride is used as the substrate, firing in a nitrogen atmosphere can make the tool substrate A large number of carbonitrides with high wear resistance of at least one of Ti, Ta, Nb, and Zr near the surface form a solidified layer on the surface of the substrate, and improve the adhesion strength between the aluminum oxide layer and the tool substrate to prolong the tool life. In addition, the hardness of the cemented carbide substrate after forming the solidified layer on the surface of the substrate is preferably 2200 to 2800 in terms of Vickers hardness (Hv). At this time, by containing a large amount of carbonitrides, the Co near the surface of the substrate is relatively reduced. For example, a scanning electron microscope (SEM) is used to observe a section of 0.5 to 3.0 μm from the surface of the tool substrate in the depth direction, and analyze it in the field of view. When the average content of Co as the binder phase metal is detected by quantitative analysis based on wavelength dispersive X-ray spectroscopy in the range of 1 μm × 1 μm, if it is less than 2.0% by mass, it will be sufficiently formed to become the main cause of surface solidification of the tool substrate. Due to the carbonitride, the wear resistance is further improved.

In addition, when titanium carbonitride-based cermets are used as the substrate, the atmosphere at the time of raising the temperature and holding at the highest temperature in the sintering process is set to a predetermined nitrogen atmosphere, and the pressure is reduced during the holding or when the temperature is lowered. Compared with the case where the entire sintering process is carried out in a nitrogen atmosphere at a constant pressure, the surface can be more solidified. This is because, if the process is carried out until the highest temperature is maintained in a nitrogen atmosphere at a constant pressure, carbonitrides with high hardness will be uniformly dispersed and formed inside the matrix, but if it is in the middle of heating or maintaining, the If the treatment is carried out under a relatively high nitrogen pressure, and the treatment is carried out under a nitrogen atmosphere with a further reduced pressure from the middle of holding or when the temperature is lowered, only the outermost surface of the tool substrate is denitrified, and the metals such as Ti and Nb are converted to Ni and Co. The melting of the binder phase and the diffusion from the inside to the surface of the tool base become active, and the formation of carbonitrides such as Ti and Nb is promoted on the surface, thereby forming a solidified layer on the surface of the tool base. In addition, the hardness of the cermet substrate after forming the solidified layer on the surface of the tool substrate is preferably not less than 2000 and not more than 2600 in terms of Vickers hardness (Hv). In addition, at this time, similar to the above-mentioned cemented carbide substrate, the Ni and Co near the surface of the tool substrate are relatively reduced, and if the total average content of Ni and Co as the binder phase metal is set to be less than 2.0% by mass, it will be fully formed. Carbonitrides, which are the main cause of surface solidification of the tool substrate, further improve wear resistance.

Moreover, the surface-coated cutting tool of the present invention may not directly form an aluminum oxide layer on the surface of the tool substrate, but may be formed by a physical vapor deposition (PVD) method, a chemical vapor deposition (CVD) method, or a sol-gel method. The hard film known to those skilled in the art, that is, a hard film consisting of at least one layer or more of nitrides or oxides containing at least one element selected from the group IVB, VB, VIB and Si of the periodic table. After forming the hard film, the aluminum oxide layer is coated on the surface of the hard film.

For the aluminum oxide layer constituting the hard coating layer of the surface-coated cutting tool of the present invention, as described later, as a method of forming the aluminum oxide layer by the sol-gel method, heat treatment is performed by the RTA method or the like, or by the sol-gel method. The gel method forms an aluminum oxide layer on a Ti oxide film formed by a CVD method or the like, whereby crystallization can be selectively promoted at a specific site, and the crystal grains in the aluminum oxide layer can be crystallized as Since it grows with a relatively high degree of freedom, alumina crystal grains having complex-shaped grain boundaries with high unevenness are formed.

In addition, in interrupted cutting with a large depth of cut where a large load is applied to the cutting edge, Ti oxide itself having a crystallization promoting effect is dispersed and formed in the alumina layer, thereby contributing to performance improvement. In order to disperse Ti oxide particles in the alumina layer, it is preferable to form a film of alumina on the Ti oxide substrate described above, and then decompose the Ti oxide as the substrate and diffuse and mix in the alumina layer at a temperature of 900°C or higher. temperature for firing. According to this method, Ti oxide enters into the alumina layer as fine particles and is formed around the alumina crystal grains, so the impact during interrupted cutting can be alleviated and excellent wear resistance can be exhibited for a long time.

(d) The content ratio of Ti oxide in the alumina layer:

It was found that the aluminum oxide layer of the present invention is characterized in that the shape of the peripheral portion of the crystal grains is made into a shape with many concavities and convexities through a predetermined firing treatment and using the Ti oxide of the aluminum oxide layer as a base. When used as a base, depending on firing conditions, Ti oxide particles can be introduced into the alumina layer, and wear resistance can be improved especially in interrupted cutting.

At this time, when the aluminum oxide layer was observed with a transmission electron microscope (TEM) and a scanning electron microscope (SEM), as shown in Figure 1, Figure 2, and Figure 3, it was observed that the Ti oxide crystal grains were For fine crystal grains having an average particle diameter of 0.01 to 0.10 μm, it can be seen that Ti oxide crystal grains are formed around aluminum oxide crystal grains, for example, when element mapping is performed by an energy dispersive X-ray analyzer using a TEM. Furthermore, it can be seen that the average value of the number is 5 to 50 on one perimeter of crystal grains.

In addition, regarding the content ratio of Ti in all metal elements in the aluminum oxide layer dispersed with Ti oxide, for example, in the range of 0.2 μm × 0.3 μm in the field of view of the longitudinal section, the energy dispersion type When the X-ray analyzer performs quantitative analysis within the observation field of view for five fields of view, and calculates the average value, it can be found that it exceeds 0.02 at% and is not more than 10 at%.

In addition, according to FIG. 1 , it can be observed that Ti oxide fine particles 1 (portions indicated by arrows in FIG. 1 ) are formed at grain boundaries of alumina crystal grains so as to surround the alumina crystal grains.

Here, when the average number of Ti oxide fine particles formed around the alumina crystal grains is less than 5 per circumference, the impact during cutting cannot be sufficiently alleviated. On the other hand, if it exceeds 50, isolated Alumina biscuits are not preferable because they tend to come off during cutting. Therefore, it is preferable to set the number of Ti oxide fine particles formed around the alumina crystal grains to be 5 to 50 pieces.

The aluminum oxide layer constituting the hard coating layer of the surface-coated cutting tool of the present invention can be formed by the sol-gel method described below.

Preparation of alumina sol:

First, alcohols (e.g., ethanol, 1-butanol) and water are added as solvents, and acids (e.g., hydrochloric acid, nitric acid) are added to aluminum alkoxides (e.g., aluminum sec-butoxide (ASB), aluminum propoxide) As a catalyst, after adding sodium laurate (C ₁₁ H ₂₃ COONa) or sodium dodecylbenzenesulfonate (DBSN) as a surfactant, after stirring at a temperature range of -10 to 20°C, the same as when stirring Similarly, for example, the total time of stirring and aging treatment is 12 hours or more, and the aging treatment is performed at a temperature range of -10 to 20° C. or lower to form an alumina sol. In addition, the alumina sol used in the present invention preferably uses dimethylformamide (DMF) and acetylacetone (AcAc) as a chelating agent. This is for the purpose of suppressing excessively accelerated crystallization. If a chelating agent having an effect of suppressing excessively accelerated crystallization is not used, crystallization is easily promoted, and crystallization of alumina begins to proceed in all parts of the layer, so the growth of crystals is restricted. Since the growth of other crystals is inhibited, there is a tendency to become a fine grain structure, and crystal grains having a complex shape having a desired size and aspect ratio cannot be formed. That is, by using a chelating agent to prepare to increase the crystallization start temperature, and at the same time start crystallization at a limited specific site in the aluminum oxide layer through a prescribed firing treatment and use of Ti oxide, the crystal can be formed in the The crystal growth direction can be grown with a relatively high degree of freedom, and as a result, alumina crystal grains having complex-shaped grain boundaries can be formed with a desired size and aspect ratio.

In addition, when a surfactant is used, the wettability of the sol is improved, and the uniformity of the film is improved. However, if the surfactant added to the sol is not thermally decomposed and removed from the layer in the drying process, cracks are likely to be formed, so as the amount that can be removed sufficiently, it is preferably measured in molar ratio with respect to Al alkoxide 0.1 or less.

In addition, the mechanism of the crystallization-promoting effect of alumina based on Ti oxide is not clear, but it is considered that when Ti oxide is reduced, it serves as an oxygen supply source for The starting point of oxide surface growth can be crystallized at a relatively low temperature at a limited site near Ti oxide. In addition, Ti oxide particles in alumina are arranged and formed along alumina grain boundaries during firing, but the particle size of the Ti oxide particles changes due to the atmosphere during firing. If the amount of oxygen in the atmosphere is large, , the particle size tends to become larger. If the average particle size is less than 0.01 μm, the supply of oxygen required for the oxidation of aluminum is insufficient, so it is difficult to crystallize. If the average particle size exceeds 0.10 μm, the aluminum oxide layer Contains coarse Ti oxides, which tend to be the starting point of cracking and peeling after firing. Therefore, the average particle diameter of the Ti oxide fine particles in the alumina layer is set to 0.01 to 0.10 μm.

When adopting the structure in which Ti oxide fine particles are dispersed in the alumina layer as described above, if the content ratio of Ti in all metal elements in the alumina layer is 0.02 at % or less, the stress during cutting will be eased. The amount of Ti oxide fine particles required for impact is not sufficient. On the other hand, if it exceeds 10 at%, the content ratio of titanium oxide increases too much, so that the excellent high-temperature hardness and oxidation resistance of alumina cannot be exhibited. Therefore, the content ratio of Ti in the aluminum oxide layer to all metal elements is preferably set to be more than 0.02 at % and not more than 10 at %.

Usually, when preparing alumina sol, stirring is performed at 40 to 80°C and aging treatment is performed at this stirring temperature for several hours, but in the present invention, it is preferable to carry out stirring and aging at a low temperature range of -10 to 20°C , for example, perform a long-term low-temperature treatment for a total of 12 hours or more.

Here, if the temperature during the stirring and aging treatment exceeds 20°C, the hydrolysis and polycondensation reactions will proceed rapidly, so it will be difficult to form an alumina precursor densely, and it will be difficult to form α-alumina in the subsequent firing treatment. Therefore, the upper limit of the temperature during stirring and aging treatment is set at 20°C. On the other hand, when the temperature during stirring and aging treatment is lower than -10°C, it is difficult to carry out hydrolysis and polycondensation reactions. And since crystallization is difficult in the baking process of the post process which uses Ti oxide, it is made into the low temperature range of -10-20 degreeC.

In addition, the stirring and aging time is set to be 12 hours or more in total, because the chemical reaction generated in the temperature range during the stirring and aging is sufficiently advanced to reach an equilibrium state, thereby obtaining a hydrolytic polycondensation densely formed Al and O The time required for the network to stabilize the alumina precursor sol.

Drying treatment and firing treatment:

When heat treatment is carried out by the RTA method, the alumina sol prepared as above is directly coated on the surface of the tool substrate, or coated on the surface of the tool substrate formed by chemical vapor deposition (CVD) and physical vapor deposition (PVD). The surface of the hard film as the base layer on the upper surface is then heated at a relatively slow heating rate of 5 to 20°C/sec, which is not often used in the formation of a general sol-gel film using the RTA method, In order to slow down the thermal decomposition rate of the organic components in the alumina sol, and keep it at 100-600°C, more preferably at 200-500°C for 5 minutes or more, dry and cool, then apply the alumina sol on the On the above-mentioned surface, the process of performing the drying treatment under the above-mentioned conditions is repeated until the desired film thickness is obtained, and after the dry gel of alumina is formed, the firing treatment based on the RTA method is performed at a temperature range of 600 to 1100° C. Thereby covering and forming an aluminum oxide layer. The RTA method is a method of heating by electromagnetic waves radiated from an infrared lamp, and is often used in the semiconductor industry for heat treatment of Si wafers and the like. Since non-contact and rapid temperature rise and fall are possible, and near-infrared rays are used, the ease of absorption varies not only with the transmittance of the object to be heated, but also with the surface state such as color, so localized heating is possible. In addition, in the present invention, if the rate of temperature increase is fast, the organic components in the coating film will burn and volatilize rapidly, so pores are likely to be formed in the coating film, making it difficult to form a dense film. In addition, the film thickness formed by one application is preferably 0.2 μm or less as a net film thickness after firing, and a denser film can be formed by repeating thin film formation.

And, when applying Ti oxide which is effective in promoting the crystallization of alumina, the alumina sol is applied to the surface of the base layer on which the Ti oxide layer is formed on the surface of the tool base by CVD or the like. In this case, the thickness of the film formed by one application in the same manner as above is preferably 0.2 μm or less, and a denser film can be formed by repeating thinner film formation. In addition, at this time, heat treatment such as drying and firing may not be performed by the RTA method, and a normal electric furnace is sufficient.

When an electric furnace is used, the drying treatment is repeated at least once at 100-600°C, more preferably at 200-500°C, followed by firing at a temperature range of 600-1100°C to coat and form an alumina layer.

The dried gel of alumina is formed by the drying treatment, and the α-type crystal structure or α-type gel formed of complex-shaped alumina crystal grains is formed on the surface of the hard film by the subsequent firing treatment. An alumina layer with a γ-type crystal structure and an alumina layer containing Ti oxide fine particles dispersed are formed depending on the firing conditions.

The film thickness of the aluminum oxide layer depends on the coating thickness and the number of times of coating of the alumina sol. As mentioned above, when the film thickness of the aluminum oxide layer formed by coating is less than 0.2 μm, it cannot be used as a surface in long-term use. The excellent wear resistance of the coated cutting tool, on the other hand, if the film thickness exceeds 5.0 μm, the aluminum oxide layer is likely to fall off and chip, so the film thickness of the aluminum oxide layer is set to 0.2 to 5.0 μm.

In addition, the reason why the temperature range of the drying treatment is set to 100 to 600°C, more preferably 200 to 500°C, and the temperature range of the firing treatment is set to 600 to 1100°C is that when the drying temperature is When the temperature is lower than 100°C, thermal decomposition of organic solvents and surfactants does not occur. If the temperature exceeds 600°C, the volume of the gel rapidly shrinks to cause cracks, etc., and the film is easy to peel off. Regarding the firing temperature, When the temperature is lower than 600°C, crystallization will not occur even if a prescribed firing method such as the RTA method and Ti oxide are used, so the wear resistance is insufficient. On the other hand, when firing at a temperature exceeding 1100°C , the tool matrix deteriorates and the granular structure becomes coarser, so the chipping resistance, chipping resistance, and smoothness show a downward trend.

[Example 1]

Next, the present invention will be described more specifically by way of examples.

(a1) As raw material powders, fine particle WC powder with an average particle diameter of 0.8 μm, medium-grained WC powder with an average particle diameter of 2 to 3 μm, and TiCN powder, ZrC powder, and TaC powder with an average particle diameter of 1 to 3 μm are prepared. powder, NbC powder, Cr ₃ C ₂ powder, VC powder and Co powder, these raw material powders are compounded into the prescribed compounding composition shown in Table 1, and paraffin wax is further added and ball milled in acetone for 24 hours. After drying under reduced pressure, Under the pressure of 98MPa, it is stamped into a green compact with a specified shape, and the green compact is vacuum sintered at a temperature of 1400°C for 1 hour in a vacuum of 5 Pa. After sintering, R is applied to the cutting edge. : 0.05mm edge grinding process, thereby manufacturing WC-based cemented carbide tool substrates A, B, C, D, E, E1, E2, E3, E4, with the blade shape specified in ISO·CNMG120408, E5 (referred to as tool bases A, B, C, D, E, E1, E2, E3, E4, E5).

However, for cooling to 1320° C. after holding at 1,400° C. for 1 hour, the tool base E2 was cooled in a nitrogen atmosphere of 3.3 kPa for 40 minutes, and the tool base E3 was cooled in a nitrogen atmosphere of 1 kPa for 40 minutes. The base E4 was cooled in a nitrogen atmosphere of 2 kPa for 10 minutes, and the tool base E5 was cooled in a nitrogen atmosphere of 3.3 kPa for 120 minutes, whereby the surface of the base was solidified.

(b1) Next, base layers are formed on the tool bases A to E described above. In addition, when forming the base layer, the above-mentioned tool substrates A and B were loaded into a chemical vapor deposition device, and the film formation conditions shown in Table 2 were used to form a granular layer as the base layer in advance with the film composition shown in Table 5. A TiN layer with a crystal structure, a TiCN layer with a longitudinally grown crystal structure (hereinafter referred to as 1-TiCN), and a Ti compound layer composed of TiO ₂ and Ti ₂ O ₃ . On the other hand, the above-mentioned tool bases C and D were installed in an arc ion plating apparatus as a physical vapor deposition apparatus, and an underlayer composed of a Ti _0.5 Al _0.5 N layer having a film thickness shown in Table 5 was formed in advance.

In addition, with respect to the above-mentioned tool base E, an underlayer composed of a TiO ₂ layer having a film thickness shown in Table 5 was formed in advance by a sol-gel method.

On the other hand, for the above-mentioned tool bases E1, E2, E3, E4, and E5, the base layer was not particularly formed.

(c1) Next, on the base layer, an alumina sol for coating an alumina layer by a sol-gel method was prepared as follows.

To a predetermined amount of aluminum alkoxide shown in Table 3, that is, aluminum sec-butoxide (ASB), a predetermined amount of ethanol similarly shown in Table 3 was added as a solvent, stirred at 5° C. in a thermostat, and further prepared as As a catalyst, hydrochloric acid to which a predetermined amount of water shown in Table 3 was added was dropped over 1 hour.

(d1) This was kept under constant stirring at 5°C in a constant temperature tank for 12 hours or more, and aging treatment was carried out at a low temperature of 5°C for 24 hours to prepare a predetermined amount of acetylacetone shown in Table 3. Alumina sol as a chelating agent.

The final solution composition was adjusted to become

(Aluminum sec-butoxide (ASB)):(water):(ethanol):(hydrochloric acid):(acetylacetone)=1:(50～100):20:(0.5～0.8):(0.8～1.2).

(e1) Next, for the tool substrates A to E5, the Ti compound layer formed by the chemical vapor deposition method, the physical vapor deposition method and the sol-gel method, or the surface of the substrate without special surface treatment is coated with Cloth the alumina sol.

(f1) Next, dry the coated alumina sol under the prescribed conditions shown in Table 4, repeat the coating and drying, and then perform firing under the conditions shown in Table 4 at 600 to 1100°C. The aluminum oxide layer of the present invention was formed by coating, thereby producing surface-coated cutting tools 1 to 21 of the present invention shown in Tables 5 and 6 (referred to as tools 1 to 21 of the present invention).

As a result of observing the longitudinal section of the aluminum oxide layer with a transmission electron microscope (TEM) for the tools 1 to 21 of the present invention, it was confirmed that grain boundaries with high unevenness were formed in the aluminum oxide layer. New grains of particles are formed around the grains. When confirming the crystal grain shape with high concavo-convexity, the aspect ratio of each crystal grain in the aluminum oxide layer, the perimeter length of the crystal grain relative to the area of each crystal grain, were obtained by electron backscatter diffraction method. The ratio of the circumferences of circles of equal area. In addition, when confirming the crystal structure, the crystal grains were analyzed by the selected area electron diffraction method using an X-ray diffraction device and a transmission electron microscope (TEM). As a result, a clear electron beam diffraction pattern was obtained from the crystal grains. According to the The analysis of the pattern and the X-ray diffraction pattern confirmed that alumina has an α-type or a mixed phase of α-type and γ-type. In addition, using an energy dispersive X-ray analyzer based on TEM, it was confirmed that the microparticle crystal grains were titanium oxide.

1 shows an example of a vertical cross-sectional TEM photograph of the aluminum oxide layer of the tool 15 of the present invention, and FIGS. 2 and 3 also show the vertical cross-section and surface SEM photographs of the aluminum oxide layer of the tool 15 of the present invention. In FIGS. 2 and 3 , the upper layer is an aluminum oxide layer 2 mainly composed of Al ₂ O ₃ , and the lower layer is a titanium compound layer 3 mainly composed of TiO ₂ . The white parts in Fig. 2 and Fig. 3 represent Ti oxides. In addition, the main component described above means 50% or more by mass % with respect to the total component of each layer. From FIG. 2 and FIG. 3 , it can be confirmed that the fine Ti oxide particles dispersed in the alumina layer are aggregated and formed so as to surround the alumina crystal grains.

(comparative example 1)

For comparison, a surface-coated cutting tool was manufactured by the following manufacturing method.

That is, for the tool bases A to E5 of the above (a1), the hard film was formed in the step (b1), and the alumina sol was prepared in the step (c1) (see Table 3).

Next, instead of the step of (d1), stirring was continued for 12 hours in a constant temperature tank at 40°C, and aging was performed at 40°C for 24 hours, and acetylacetone was added as a chelating agent to prepare Alumina sol.

Next, similarly to (e1) above, for the tool bases A to E5, on the Ti compound layer formed by the chemical vapor deposition method, physical vapor deposition method, or sol-gel method, or on the surface without special The alumina sol is coated on the surface of the treated substrate.

Next, in the same manner as in (f1) above, the coated alumina sol is dried, and the coating and drying treatments are repeated, followed by firing treatment. However, at this time, the drying conditions and firing conditions used Under different conditions from the tool of the present invention, an aluminum oxide layer was coated on the outermost surface, thereby producing surface-coated cutting tools 1 to 21 of comparative examples shown in Tables 7 and 8 (referred to as comparative example tools 1 to 21). ).

For tools 1 to 21 of the present invention and tools 1 to 21 of comparative examples, using a transmission electron microscope with an energy dispersive X-ray analysis device, the longitudinal section of the aluminum oxide layer was observed at a field of view of 100,000 times 0.2 ×0.3μm Element mapping analysis is performed on 5 fields of view, the average value of the number of Ti oxide particles is obtained, and the result is assumed to be a plane, and the fine Ti oxide particles that will be dispersed in the alumina layer The diameter of an approximate circle when the area of is calculated as the area of a circle is measured at five points, and the average value thereof is defined as the average particle diameter of the fine Ti oxide particles.

In addition, in the range of 0.2 μm × 0.3 μm in the field of view of the longitudinal section, the energy dispersive X-ray analyzer attached to the TEM was used to perform quantitative surface analysis within the observation field of view of five fields of view, and the average value was obtained. value, thereby determining the proportion of Ti in all metal elements in the alumina layer.

In addition, the Co content on the surface of the hard substrate was quantitatively analyzed in the field of observation of the longitudinal section of the aluminum oxide layer or the hard substrate by wavelength-dispersive X-ray spectroscopy using a scanning electron microscope (SEM), and Take its average value. The Co content on the surface of the hard substrate was measured in five fields of view by surface analysis of an analysis field of view area of 1 µm×1 µm in the range of 0.5 to 3.0 µm in the depth direction from the substrate surface.

In addition, when the average layer thickness of the aluminum oxide layer was measured using a scanning electron microscope at the same time, all of them showed an average value (average value at five locations) that was substantially the same as the target layer thickness.

Tables 6 and 8 show the measurement results.

Next, for the tools 1 to 21 of the present invention and tools 1 to 21 of comparative examples, the dry high-speed interrupted cutting test of carbon steel and the wet high-speed interrupted cutting test of cast iron shown below were carried out, and the relief of the cutting edge was measured in both. Face wear width.

Cutting condition 1:

Workpiece: JIS·SCM435 round bar with four longitudinal grooves equally spaced in the length direction

Cutting speed: 360m/min

Cutting depth: 1.2mm

Feed speed: 0.2mm/rev

Cutting time: 5 minutes

(The usual cutting speed is 220m/min)

Cutting condition 2:

Workpiece: JIS·FCD450 round bar with four longitudinal grooves at equal intervals in the length direction

Cutting speed: 320m/min

Cutting depth: 1.0mm

Feed speed: 0.2mm/rev

Cutting time: 5 minutes

(The usual cutting speed is 200m/min)

These results are shown in Tables 6 and 8.

[Table 1]

[Table 2]

[table 3]

[Table 4]

(Note) Throwing means throwing a sample into a furnace kept at a predetermined temperature.

[table 5]

[Table 6]

[Table 7]

[Table 8]

(Note) Unable to judge means that the wear amount cannot be measured due to chipping.

[Example 2]

As raw material powders, TiCN (TiC/TiN=50/50 in mass ratio) powder, Mo ₂ C powder, NbC powder, TaC powder, WC powder, Co powder and Ni These powders were compounded into the specified compounding composition shown in Table 9, and were wet-mixed for 24 hours using a ball mill. After drying, they were pressed into compacts under a pressure of 98 MPa, and the compacts were placed in a nitrogen atmosphere of 1.3 kPa. Sintering is carried out at a temperature of 1540°C for 1 hour. After sintering, the cutting edge part is subjected to R: 0.07mm edge grinding, thereby manufacturing a TiCN-based metal with an ISO standard CNMG120412 blade shape. Ceramic tool bases F, G, H, I, J, J1, J2, J3, J4, J5 (referred to as tool bases F to J5). However, for the tool base J2, in a nitrogen atmosphere of 1.3 kPa, the temperature increase rate was set to 2 °C/min, and after the temperature was raised from room temperature to 1540 °C and kept for 30 minutes, it was set to a vacuum of 13 Pa and kept at 1540 °C for 30 minutes. After 10 minutes, the temperature was lowered to solidify the surface. For the tool base J3, the temperature is always raised in a vacuum of 13Pa and kept at 1540°C for 60 minutes. For the tool base J4, after the temperature is raised from room temperature to 1540°C in a nitrogen atmosphere of 1.3kPa and held for 30 minutes, it is set to a vacuum of 13Pa. , and further kept at 1540°C for 5 minutes. For the tool base J5, after raising the temperature from room temperature to 1540°C in a nitrogen atmosphere of 1.3kPa and keeping it for 30 minutes, set it to a vacuum of 13Pa, and then keep it at 1540°C for 90 minutes and then lower the temperature to harden the surface.

[Table 9]

Next, base layers are formed on the tool bases F to J described above.

In addition, when forming the base layer, the above-mentioned tool bases F and G were installed in a chemical vapor deposition apparatus, and the base layer was formed in advance with the film composition of the Ti compound in Table 10 using the film formation conditions shown in Table 2. On the other hand, the above-mentioned tool bases H and I were installed in an arc ion plating apparatus as a physical vapor deposition apparatus, and an underlayer composed of a Ti _0.5 Al _0.5 N layer having a film thickness shown in Table 10 was formed in advance.

In addition, for the above-mentioned tool base J, an underlayer composed of a TiO ₂ layer having a film thickness shown in Table 10 was formed in advance by a sol-gel method.

On the other hand, for the above-mentioned tool bases J1 to J5, the base layer was not particularly formed.

Next, Tables 3 and 4 were used in the same manner as in Example 1 for any of the above-mentioned tool bases F, G, H, I, and J on which a base layer was formed, and any of the above-mentioned tool bases J1 to J5 without a base layer. The preparation conditions, drying conditions, and firing conditions were performed to form an alumina main layer to manufacture coated tools 22-42 of the present invention shown in Tables 10 and 11 (referred to as tools 22-42 of the present invention).

For tools 22 to 42 of the present invention, the crystal structure of the aluminum oxide layer, the average aspect ratio of crystal grains, the average value of the ratio of the circumference of crystal grains to the circumference of a circle of equal area were measured and calculated in the same manner as in Example 1. The content ratio (at%) of Ti in all metal elements in the alumina layer, the average number (pieces) of Ti oxide particles existing in grain boundaries, and the average particle size of Ti oxide particles existing in grain boundaries ( μm), the total average content (at%) of Co and Ni contained in the surface of the TiCN-based cermet as a binder phase metal.

The results are shown in Table 11.

[Table 10]

[Table 11]

(comparative example 2)

Using the same tool bases F to J5 as the tool bases used in Example 2, the sol-gel method was used in the same manner as in Example 2, using the sol preparation conditions shown in Table 3 and the drying/gelling conditions shown in Table 4. The firing conditions were to form the alumina main layer until the predetermined target layer thickness shown in Table 13 was obtained, and the coated tools 22 to 42 of the comparative examples shown in Tables 12 and 13 (referred to as comparative tool 22 ~42).

The crystal structure of the aluminum oxide layer, the average aspect ratio of the crystal grains, the average value of the ratio of the circumference of the crystal grains to the circumference of a circle of equal area, and the content ratio of Ti in all metal elements in the aluminum oxide layer (at %), the average number (pieces) of Ti oxide particles present at grain boundaries, the average particle diameter (μm) of Ti oxide particles present at grain boundaries, Co Table 13 shows the total average content (at%) of Ni and Ni.

[Table 12]

[Table 13]

With regard to the tools 22 to 42 of the present invention and the tools 22 to 42 of the comparative examples described above, a dry high-speed interrupted cutting test was performed under the following conditions.

Cutting condition 1:

Workpiece: JIS·SCM435 round bar with four longitudinal grooves equally spaced in the length direction

Cutting speed: 280m/min

Cutting depth: 1.4mm

Feed speed: 0.25mm/rev

Cutting time: 5 minutes

(The usual cutting speed is 220m/min)

Cutting condition 2:

Workpiece: JIS·FCD450 round bar with four longitudinal grooves at equal intervals in the length direction,

Cutting speed: 340m/min

Cutting depth: 0.8mm

Feed speed: 0.4mm/rev

Cutting time: 5 minutes

(The usual cutting speed is 200m/min),

These results are shown in Tables 11 and 13.

From the results shown in Tables 6 and 11 and Tables 8 and 13, it is clear that in the tools 1 to 42 of the present invention, the surface of the tool substrate is coated with an aluminum oxide layer by the sol-gel method, and the aluminum oxide layer The layer has excellent surface smoothness, welding resistance, and adhesion between crystal grains, so even when used in high-speed interrupted cutting of steel, cast iron, etc., abnormal damage such as chipping and peeling will not occur, In addition, it exhibits excellent wear resistance and chip discharge performance in long-term use.

On the other hand, in comparative example tools 1 to 42 in which crystal grains of complicated shapes were not formed on the alumina layer on the surface, although the surface smoothness and welding resistance were excellent, the adhesion between the crystal grains was insufficient, and the resistance to welding was not sufficient. Due to the large impact of intermittent cutting, micro chipping near the sharpening part is especially prone to occur, so it is easy to cause abnormal damage, cannot maintain excellent wear resistance, crater wear progresses, and thus reaches the service life in a short time , which is obvious.

In addition, in the foregoing examples, the performance of the hard coating was evaluated using an insert-shaped tool, but it goes without saying that the same results can be obtained with a drill, an end mill, or the like.

Industrial availability

According to the surface-coated cutting tool of the present invention, an aluminum oxide layer is formed on the surface by coating with a sol-gel method, and the aluminum oxide layer has excellent surface smoothness, welding resistance, and adhesion between crystal grains, Therefore, even when used in high-speed interrupted cutting of steel, cast iron, etc., abnormal damage such as chipping and peeling will not occur, and excellent cutting performance will be exhibited in long-term use, thereby achieving a longer tool life. In fact, the practical effect is very large.

Claims (8)

1. A surface-coated cutting tool is formed by coating the surface of a tool substrate made of tungsten carbide-based cemented carbide or titanium carbonitride-based cermet to form a hard coating layer, wherein, (a) the hard coating layer includes an aluminum oxide layer having an average layer thickness of 0.2 to 5.0 μm, (b) the crystal grains constituting the alumina layer have an α-type crystal structure or a crystal structure of a mixed phase of α-type and γ-type, (c) When the aspect ratio of the crystal grains is defined as the ratio of the grain diameter in the layer thickness direction to the grain diameter in the perpendicular direction to the layer thickness obtained by the electron backscatter diffraction method, the average aspect ratio of the crystal grains is The size ratio is 0.5 to 5.0, and the average value of the ratio of the circumference of crystal grains to the circumference of a circle having an area equal to that of the crystal grains is 1.8 to 3.0. 2. The surface-coated cutting tool according to claim 1, wherein, the aluminum oxide layer contains grains of titanium oxide, (a) In the alumina layer, the content ratio of Ti to all metal elements is more than 0.02 at % and not more than 10 at %, (b) The crystal grains of the titanium oxide are titanium oxide fine particles having an average particle diameter of 0.01 to 0.10 μm, the titanium oxide fine particles are aggregated to surround the crystal grains constituting the aluminum oxide layer, and The average number of titanium oxide particles present on the perimeter of the crystal grains of the aluminum oxide layer is 5 to 50. 3. The surface-coated cutting tool according to claim 1 or 2, wherein, The surface-coated cutting tool is formed by coating the surface of a tool substrate composed of tungsten carbide-based cemented carbide to form a hard coating layer, A base surface solidified layer having an average layer thickness of 0.5 to 3.0 μm is formed in a depth direction from the surface of the tool base, and the average content of Co as a binder phase metal contained in the base surface solidified layer is less than 2.0% by mass. 4. The surface-coated cutting tool according to claim 1 or 2, wherein, The surface-coated cutting tool is formed by coating the surface of a tool substrate composed of titanium carbonitride-based cermets to form a hard coating layer, A solidified layer on the surface of the substrate having an average layer thickness of 0.5 to 3.0 μm is formed along the depth direction from the surface of the tool substrate, and the total average content of Co and Ni as a binder phase metal contained in the solidified layer on the surface of the substrate is less than 2.0 mass %. 5. A method of manufacturing a surface-coated cutting tool, which is a method of manufacturing the surface-coated cutting tool according to any one of claims 1 to 4, wherein, The aluminum oxide layer is formed on the titanium oxide layer by a sol-gel method. 6. The surface-coated cutting tool according to claim 1 or 2, wherein, The hard coating layer further includes a base layer formed directly under the alumina layer. 7. The surface-clad cutting tool according to claim 6, wherein, The base layer is composed of a nitride or oxide containing at least one element selected from Groups IVB, VB, VIB and Si of the periodic table. 8. The surface-clad cutting tool of claim 6, wherein: The base layer is composed of a titanium compound.