JP4716254B2 - Surface-coated cermet cutting tool with excellent chipping resistance thanks to thick α-type aluminum oxide layer - Google Patents

Description

In the present invention, an upper layer of a hard coating layer, that is, an aluminum oxide layer (hereinafter referred to as an α-type Al ₂ O ₃ layer) having an α-type crystal structure in a state where chemical vapor deposition is formed is particularly thick. The present invention relates to a surface-coated cermet cutting tool (hereinafter referred to as a coated cermet tool) that exhibits excellent chipping resistance even when used for cutting various steels and cast iron.

Conventionally, generally on the surface of a substrate (hereinafter collectively referred to as a tool substrate) composed of a tungsten carbide (hereinafter referred to as WC) -based cemented carbide or titanium carbonitride (hereinafter referred to as TiCN) -based cermet. ,
(A) Ti carbide (hereinafter referred to as TiC) layer, nitride (hereinafter also referred to as TiN) layer, carbonitride (hereinafter referred to as TiCN) layer formed by chemical vapor deposition of the lower layers. , A carbon oxide (hereinafter referred to as TiCO) layer, and a carbonitride oxide (hereinafter referred to as TiCNO) layer, and has a total average layer thickness of 0.5 to 15 μm. Ti compound layer,
(B) an α-type Al ₂ O ₃ layer whose upper layer has an average layer thickness of 1 to 15 μm;
A coated cermet tool formed by forming a hard coating layer composed of (a) and (b) above is known, and this coated cermet tool is used for continuous cutting and intermittent cutting of various steels and cast irons, for example. It is also known that
Japanese Unexamined Patent Publication No. 6-31503

In recent years, the use of FA for cutting devices has been remarkable. On the other hand, there has been a strong demand for labor saving and energy saving and further cost reduction for cutting work, and with this purpose, especially for the purpose of further extending the service life of cutting tools. upper layer constituting the hard coating layer, i.e. excellent but the hot hardness and thickening of one step in the α-type the Al ₂ O ₃ layer having heat resistance is strongly demanded, of the α-type the Al ₂ O ₃ layer When the layer thickness exceeds 15 μm, which is the maximum average layer thickness that has been practically used in the past, the Al ₂ O ₃ crystal grains become coarser and the denseness of the layer itself is significantly reduced. since the decrease in the high-temperature strength can not be avoided, the coated cermet tool formed by depositing formed as an upper layer of such thickening α type the Al ₂ O ₃ layer a hard coating layer, the thickening α-type Al ₂ O ₃ layer due to chipping to the cutting edge (fine Chipping) is likely to occur, since it becomes very short for this result useful life, it can not be put to practical use at present.

Therefore, the present inventors focused on the α-type Al ₂ O ₃ layer having an average layer thickness of 1 to 15 μm constituting the hard coating layer of the above-described conventional coated cermet tool from the above viewpoint, Research was conducted to develop a coated cermet tool in which chipping caused by the thickened α-type Al ₂ O ₃ layer does not occur at the cutting edge even if the layer thickness is increased to an average layer thickness exceeding 15 μm. result,
(A) The TiCN layer as the lower layer constituting the hard coating layer of the conventional coated cermet tool has constituent atoms composed of Ti, carbon, and nitrogen at lattice points, as schematically shown in FIG. It has an existing NaCl type face centered cubic crystal structure (note that FIG. 1 (b) shows a state cut by the (011) plane). For example, in a normal chemical vapor deposition apparatus,
Reaction gas composition: by _{volume%, TiCl 4: 2~10%,} CH 3 CN: 0.5~3%, N 2: 10~30%, H 2: remainder,
Reaction atmosphere temperature: 800 to 900 ° C.
Reaction atmosphere pressure: 6-20 kPa,
It is formed by vapor deposition under the conditions (called normal conditions).
Reaction gas composition: by _{volume%, TiCl 4: 0.1~0.8%,} CH 3 CN: 0.05~0.3%, Ar: 10~30%, H 2: remainder,
Reaction atmosphere temperature: 930 to 1000 ° C.
Reaction atmosphere pressure: 6-20 kPa,
The reaction gas composition is relatively low in TiCl ₄ and CH ₃ CN, Ar gas is added instead of N ₂ gas, and the ambient temperature is relatively When the vapor deposition is carried out under the conditions (reaction gas composition adjustment high temperature conditions), the resulting TiCN layer (hereinafter referred to as “modified TiCN layer”) has the same crystal structure as the conventional TiCN layer, It is systematically modified to have a high temperature strength that is far superior to that of the conventional TiCN layer.

(B) About the conventional TiCN layer and the modified TiCN layer of (a),
Using a field emission scanning electron microscope, as illustrated in the schematic explanatory diagrams in FIGS. 2A and 2B, each crystal grain existing within the measurement range of the surface polished surface is irradiated with an electron beam, The inclination angle formed by the normal lines of the (001) plane and the (011) plane, which are the crystal planes of the crystal grains, with respect to the normal line of the surface polished surface (FIG. 2A shows (001) of the crystal planes. When the tilt angle of the surface is 0 degree and the tilt angle of the (011) plane is 45 degrees, the tilt angle of the (001) plane is 45 degrees and the tilt angle of the (011) plane is 0 degree. However, in this case, the crystal grains have the constituent atoms composed of Ti, carbon, and nitrogen at the lattice points as described above. Each has a NaCl-type face-centered cubic crystal structure, and based on the measured tilt angle, A distribution of lattice points (constituent atom shared lattice points) in which each of the constituent atoms shares one constituent atom between the crystal grains is calculated at an interface between adjacent crystal grains, and between the constituent atomic shared lattice points. The constituent atom shared lattice point form in which N lattice points that do not share the constituent atoms (N is an even number of 2 or more on the crystal structure of the NaCl type face-centered cubic crystal) is represented by ΣN + 1, and each ΣN + 1 is the entire ΣN + 1 ( However, when a constituent atomic shared lattice point distribution graph showing the distribution ratio occupying the upper limit value is set to 28 in relation to the frequency), the highest peak exists in Σ3 in any TiCN layer, but the conventional TiCN layer is As shown in FIG. 4, the distribution ratio of Σ3 shows a relatively low constituent atom shared lattice point distribution graph of 30% or less, whereas the modified TiCN layer has the same structure as shown in FIG. 3. , Σ3 distribution This graph shows a very high distribution of atomic lattice points with a high compositional ratio of 60% or more. The distribution ratio of this high Σ3 varies depending on the TiCl ₄ and CH ₃ CN constituting the reaction gas, the Ar content, and the atmospheric reaction temperature. To do.

(C) On the surfaces of the modified TiCN layer and the conventional TiCN layer, the same conditions as the formation conditions of the conventional α-type Al ₂ O ₃ layer, that is, in a normal chemical vapor deposition apparatus,
Reaction gas composition - by _{volume%, AlCl 3: 1~5%,} CO 2: 0.5~10%, HCl: 0.3~3%, H 2 S: 0.02~0.4%, H 2 :remaining,
Reaction atmosphere temperature: 950-1100 ° C.
Reaction atmosphere pressure: 3 to 13 kPa,
When the α-type Al ₂ O ₃ layer is formed to an average layer thickness of 16 to 30 μm exceeding 15 μm, the thickened α-type Al ₂ O ₃ layer formed on the conventional TiCN layer is formed. In this case, as described above, the Al ₂ O ₃ crystal grains are greatly coarsened, and the denseness of the layer itself is remarkably lowered. Therefore, the high temperature strength is unavoidably lowered, but it is formed on the modified TiCN layer. the thickening α-type Al ₂ O ₃ layer is, the α-type the Al ₂ O ₃ layer at the time of formation, the reformed significantly influenced by the crystal array of TiCN layer, the crystal array having a said reformed TiCN layer In the thickened α-type Al ₂ O ₃ layer formed as a result, the film thickness is increased to an average layer thickness of 16 to 30 μm. Nevertheless ized, along the thickness direction, Al ₂ O ₃ grain coarsening significantly suppressed, and the layer Since also the density of the body becomes what is uniformly held, high temperature strength, comprising the equivalent high temperature strength with the layer thickness of α-type Al ₂ O ₃ layer of 1 to 15 m, but rather includes a more high-temperature strength As a result, the reduction in chipping resistance is remarkably suppressed.

(D) When the TiCN layer is formed by vapor deposition, the formation conditions are set to the reaction gas composition adjusted high temperature conditions as described above, thereby improving the constitutive atomic shared lattice point distribution graph in which the distribution ratio of Σ3 is 60% or more. Therefore, in this case, according to the test results, the contents of TiCl ₄ and CH ₃ CN constituting the reaction gas and Ar are TiCl ₄ : 0.1 to 0, respectively. .8%, CH ₃ CN: 0.05 to 0.3%, Ar: 10 to 30%, or if the reaction atmosphere temperature is out of the range of 930 to 1000 ° C. The distribution ratio of Σ3 in the lattice point distribution graph becomes less than 60%, and the hysteresis effect on the thickened α-type Al ₂ O ₃ layer of the modified TiCN layer becomes insufficient, and the thickened α-type Al ₂ O ₃ The desired excellent chipping resistance cannot be ensured in the layer.
The research results shown in (a) to (d) above were obtained.

The present invention has been made based on the above research results, and as a hard coating layer on the surface of a tool base made of WC-based cemented carbide or TiCN-based cermet,
(A) Adhesive Ti compound layer formed by chemical vapor deposition, consisting of one or more of TiC layer, TiN layer, and TiCN layer, and having a total average layer thickness of 0.1 to 2 μm ,
(B) formed by chemical vapor deposition with an average layer thickness of 1-10 μm, and
Using a field emission scanning electron microscope, each crystal grain existing within the measurement range of the surface polished surface is irradiated with an electron beam, and the crystal plane of the crystal grain is normal to the surface polished surface ( The inclination angle formed by the normal lines of the (001) plane and the (011) plane is measured. In this case, the crystal grains are NaCl-type face-centered cubic crystals each having a constituent atom composed of Ti, carbon, and nitrogen at lattice points. A lattice point having a crystal structure, and each of the constituent atoms shares one constituent atom between the crystal grains at the interface between adjacent crystal grains based on the measured tilt angle obtained as a result ( The distribution of the constituent atomic shared lattice points) is calculated, and N lattice points that do not share the constituent atoms between the constituent atomic shared lattice points (N is an even number of 2 or more on the crystal structure of the NaCl type face centered cubic crystal) Existing constituent atomic shared lattice point form is ΣN + 1 In the constituent atom sharing lattice distribution graph showing the distribution ratio of each ΣN + 1 in the whole ΣN + 1 (however, the upper limit value is 28 due to the frequency), the highest peak exists in Σ3, and the Σ3 A modified TiCN layer showing a constituent atom shared lattice point distribution graph in which the distribution ratio in the entire ΣN + 1 is 60% or more,
Through the adhesive Ti compound layer and the modified TiCN layer of (a) and (b) above,
(C) a thickened α-type Al ₂ O ₃ layer having an average layer thickness of 16 to 30 μm,
A thickened α-type Al ₂ O ₃ layer formed by vapor-depositing is characterized by a coated cermet tool that exhibits excellent chipping resistance.

Next, the reason why the constituent layers of the hard coating layer of the coated cermet tool of the present invention are numerically limited as described above will be described below.
(A) Adhesive Ti compound layer The adhesive Ti compound layer adheres firmly to both the tool substrate and the modified TiCN layer, and thus has the effect of contributing to improved adhesion of the hard coating layer to the tool substrate. If the total average layer thickness is less than 0.1 μm, the desired excellent adhesion cannot be ensured. On the other hand, the total average layer thickness up to 2 μm is sufficient. Was determined to be 0.1 to 2 μm.

(B) Modified TiCN layer As described above, the modified TiCN layer in which the distribution ratio of Σ3 in the constituent atom sharing lattice distribution graph is 60% or more comes to have excellent high-temperature strength. It affects the structure of the thickened α-type Al ₂ O ₃ layer to be formed, significantly suppresses the coarsening of Al ₂ O ₃ crystal grains, and increases the thickness of the layer itself in the layer thickness direction. However, if the average layer thickness is less than 1 μm, the desired effect cannot be obtained. On the other hand, if the average layer thickness exceeds 10 μm, the thermoplastic deformation causes uneven wear. Since this causes the chipping of the thickened α-type Al ₂ O ₃ layer, the average layer thickness was set to 1 to 10 μm.

(C) Thickened α-type Al ₂ O ₃ layer As described above, due to the presence of the modified TiCN layer, it receives this organizational history, suppresses the coarsening of the crystal grains, and maintains the structural denseness. It is possible to form a thickened α-type Al ₂ O ₃ layer, and as a result, has a high-temperature strength equal to or higher than the high-temperature strength of the α-type Al ₂ O ₃ layer having an average layer thickness of 1 to 15 μm, In addition to the reduction in chipping resistance being reduced, the excellent high-temperature hardness and heat resistance of the Al ₂ O ₃ layer itself contributes to improving the wear resistance of the hard coating layer, but the average layer thickness is 16 μm. If the average layer thickness is less than 30 μm, the average layer thickness of 16-30 μm cannot be satisfactorily met. On the other hand, if the average layer thickness exceeds 30 μm, chipping is likely to occur. It was determined.

  In addition, for the purpose of identification before and after the use of the cutting tool, a TiN layer having a golden color tone may be vapor-deposited as the outermost surface layer of the hard coating layer as necessary, but the average layer thickness in this case is It may be 0.1 to 1 μm, and if the thickness is less than 0.1 μm, a sufficient discrimination effect cannot be obtained, while the discrimination effect by the TiN layer is sufficient for an average layer thickness of up to 1 μm.

In the coated cermet tool of the present invention, the modified TiCN layer as a hard coating layer has excellent high-temperature strength, and has a systematic influence on the thickened α-type Al ₂ O ₃ layer formed thereon. As a result, the thickened α-type Al ₂ O ₃ layer receives the history of the modified TiCN layer and is equivalent to the high temperature strength of the α-type Al ₂ O ₃ layer having an average layer thickness of 1 to 15 μm. In addition, since it has a high temperature strength higher than this, it has excellent wear resistance over a long period of time without occurrence of chipping even though the average layer thickness is increased to a layer thickness of 16 to 30 μm. It is something that will be exhibited.

  Next, the coated cermet tool of the present invention will be specifically described with reference to examples.

WC powder, TiC powder, ZrC powder, VC powder, TaC powder, NbC powder, Cr ₃ C ₂ powder, TiN powder, TaN powder, and Co powder all having an average particle diameter of 1 to 3 μm are prepared as raw material powders. These raw material powders are blended into the blending composition shown in Table 1, added with wax, ball milled in acetone for 36 hours, dried under reduced pressure, and pressed into a 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 within a range of 1370 to 1470 ° C. for 1 hour, and after sintering, the cutting edge portion was R: 0.06 mm honing By performing the processing, tool bases A to F made of a WC-base cemented carbide having a throwaway tip shape specified in ISO · CNMG120408 were manufactured.

Further, as raw material powders, TiCN (mass ratio, TiC / TiN = 50/50) powder, Mo ₂ C powder, ZrC powder, NbC powder, TaC powder, WC, all having an average particle diameter of 0.5 to 2 μm. Prepare powder, Co powder, and Ni powder, blend these raw material powders into the composition shown in Table 2, wet-mix for 36 hours with a ball mill, dry, and press-mold into green compact at 98 MPa pressure The green compact was sintered in a nitrogen atmosphere of 1.3 kPa at a temperature of 1540 ° C. for 1 hour, and after sintering, the cutting edge portion was subjected to a honing process of R: 0.06 mm. Tool bases a to f made of TiCN-based cermet having a chip shape conforming to ISO standards / CNMG 120212 were formed.

Next, on the surfaces of these tool bases A to F and tool bases a to f, an ordinary chemical vapor deposition apparatus is used, and under the conditions shown in Tables 3 and 4, an adhesive Ti compound layer as a hard coating layer and The modified TiCN layer was formed by vapor deposition in the combination shown in Table 5 and with the target layer thickness, and then the same thickened α-type Al ₂ O ₃ layer was also shown in Table 5 under the same conditions as shown in Table 3. The coated cermet tools 1 to 13 of the present invention were produced by vapor deposition with the combinations shown and with the target layer thickness.

  For comparison purposes, as shown in Table 6, instead of the modified TiCN layer constituting the hard coating layer of the coated cermet tools 1 to 13 of the present invention, a conventional TiCN layer is formed under the conditions shown in Table 3. Comparative coated cermet tools 1 to 13 were produced under the same conditions except for the above.

Next, with respect to the modified TiCN layer and the conventional TiCN layer constituting the hard coating layer of the above-described coated cermet tool of the present invention and the comparative coated cermet tool, the constituent atomic shared lattice point distribution graphs are respectively shown using a field emission scanning electron microscope. Created.
That is, the constituent atomic shared lattice point distribution graph is set in a lens barrel of a field emission scanning electron microscope in a state where the surfaces of the modified TiCN layer and the conventional TiCN layer are polished surfaces. An electron beam with an acceleration voltage of 15 kV at an incident angle of 70 degrees and an irradiation current of 1 nA is applied to each crystal grain existing in the measurement range of the surface polished surface, and an electron backscatter diffraction image apparatus is used to measure 30 × The inclination angle formed by the normal lines of the (001) plane and the (011) plane, which are crystal planes of the crystal grains, is measured with respect to the normal line of the surface polished surface at an interval of 0.1 μm / step in a 50 μm region. Then, based on the measured tilt angle obtained as a result, lattice points (constituent atomic shared lattices) in which each of the constituent atoms share one constituent atom between the crystal grains at the interface between adjacent crystal grains. Point) distribution and the previous A constitutive atomic shared lattice point form in which N lattice points that do not share constituent atoms between constituent atomic shared lattice points (N is an even number of 2 or more on the crystal structure of NaCl type face-centered cubic crystal) is represented by ΣN + 1. In this case, each ΣN + 1 is created by obtaining a distribution ratio of ΣN + 1 in the entire ΣN + 1 (however, the upper limit value is 28 because of the frequency).

  In each of the resulting modified TiCN layer and conventional TiCN constituent atomic point lattice distribution graphs, the distribution ratio of Σ3 in the entire ΣN + 1 (N is all even numbers in the range of 2 to 28) is shown in Table 5. , 6 respectively.

In each of the above-mentioned various constituent atom sharing lattice point distribution graphs, as shown in Tables 5 and 6, the modified TiCN layer of the coated cermet tool of the present invention has constituent atoms whose distribution ratio of Σ3 is 60% or more. In contrast to the shared lattice point distribution graph, the conventional TiCN layer of the comparative coated cermet tool showed a constituent atomic shared lattice point distribution graph with a Σ3 distribution ratio of 30% or less.
3 shows a constituent atomic shared lattice point distribution graph of the modified TiCN layer of the coated cermet tool 13 of the present invention, and FIG. 4 shows a constituent atomic shared lattice point distribution graph of the conventional TiCN layer of the comparative coated cermet tool 13. Is.

Further, for the above-described coated cermet tools 1 to 13 and comparative coated cermet tools 1 to 13, the constituent layers of the hard coating layer were observed using an electron beam microanalyzer (EPMA) and an Auger spectroscopic analyzer (layer When the longitudinal section was observed), it was confirmed that both the former and the latter were composed of an adhesive Ti compound layer and an α-type Al ₂ O ₃ layer having substantially the same composition as the target composition. Moreover, when the thickness of the constituent layer of the hard coating layer of these coated cermet tools was measured using a scanning electron microscope (same longitudinal section measurement), the average layer thickness (substantially the same as the target layer thickness) Average value of 5-point measurement) was shown.

Next, in the state where each of the various coated cermet tools is screwed to the tip of the tool steel tool with a fixing jig, the present coated cermet tools 1 to 13 and the conventional coated cermet tools 1 to 13 are as follows.
Work material: JIS / SNCM415 lengthwise equidistantly 4 round bars with flutes,
Cutting speed: 230 m / min,
Incision: 1.5mm,
Feed: 0.25mm / rev,
Cutting time: 18 minutes
Dry interrupted cutting test of alloy steel under the above conditions (cutting condition A),
Work material: JIS / S28C lengthwise equidistant round bars with 4 vertical grooves,
Cutting speed: 260 m / min,
Cutting depth: 1.2mm,
Feed: 0.2mm / rev,
Cutting time: 18 minutes
Dry intermittent cutting test of carbon steel under the conditions (cutting condition B)
Work material: JIS / FC300 round bar,
Cutting speed: 300 m / min,
Incision: 2.5mm,
Feed: 0.25mm / rev,
Cutting time: 18 minutes
The dry continuous cutting test of cast iron was performed under the above conditions (cutting condition C), and the flank wear width of the cutting edge was measured in any cutting test. The measurement results are shown in Table 7.

From the results shown in Tables 5 to 7, each of the coated cermet tools 1 to 13 of the present invention has a constituent atom shared lattice point distribution graph in which one of the hard coating layers has a distribution ratio of Σ3 of 60% or more. The thickened α-type Al ₂ O ₃ layer composed of the modified TiCN layer and deposited on the TiCN layer is strongly affected by the history of the modified TiCN layer and is systematically affected. Although it is a thick film, it has a high temperature strength equal to or higher than that of an α-type Al ₂ O ₃ layer having an average layer thickness of 1 to 15 μm, so that the chipping of the cutting edge portion is performed. While the generation is remarkably suppressed and excellent wear resistance is exhibited over a long period of time, the hard coating layer exhibits a constituent atom shared lattice point distribution graph in which the distribution ratio of Σ3 is 30% or less. the thickening α type the Al ₂ O ₃ layer was deposited formed thereon In comparison coated cermet tools 1 to 13 comprising both a high temperature strength insufficient cause of the thickening α type the Al ₂ O ₃ layer, chipping occurs in the cutting edge, leading to a relatively short time using life It is clear.

As described above, the coated cermet tool according to the present invention can be applied to various steels and cast irons even if the α-type Al ₂ O ₃ layer constituting the hard coating layer is thickened to an average layer thickness of 16 to 30 μm. It shows excellent chipping resistance in cutting processes, etc., exhibits excellent wear resistance over a long period of time, and can extend the service life. It can be used satisfactorily for labor saving, energy saving, and cost reduction.

It is a schematic diagram which shows the crystal structure of the NaCl type face centered cubic crystal which the modified TiCN layer which comprises a hard coating layer, and the conventional TiCN layer have. It is a schematic explanatory drawing which shows the measurement aspect of the inclination angle of the (001) plane of a crystal grain and the (011) plane in various TiCN layers which comprise a hard coating layer. 4 is a constituent atomic shared lattice point distribution graph of a modified TiCN layer constituting a hard coating layer of the coated cermet tool 13 of the present invention. 6 is a constituent atomic shared lattice point distribution graph of a conventional TiCN layer constituting a hard coating layer of a comparative coated cermet tool 13.

Claims (1)

As a hard coating layer on the surface of the tool base composed of tungsten carbide based cemented carbide or titanium carbonitride based cermet,
(A) All are formed of one or more of Ti carbide layer, nitride layer, and carbonitride layer formed by chemical vapor deposition, and have a total average layer thickness of 0.1 to 2 μm. Adhesive Ti compound layer,
(B) formed by chemical vapor deposition with an average layer thickness of 1-10 μm, and
Using a field emission scanning electron microscope, each crystal grain existing within the measurement range of the surface polished surface is irradiated with an electron beam, and the crystal plane of the crystal grain is normal to the surface polished surface ( The inclination angle formed by the normal lines of the (001) plane and the (011) plane is measured. In this case, the crystal grains are NaCl-type face-centered cubic crystals each having a constituent atom composed of Ti, carbon, and nitrogen at lattice points. A lattice point having a crystal structure, and each of the constituent atoms shares one constituent atom between the crystal grains at the interface between adjacent crystal grains based on the measured tilt angle obtained as a result ( The distribution of the constituent atomic shared lattice points) is calculated, and N lattice points that do not share the constituent atoms between the constituent atomic shared lattice points (N is an even number of 2 or more on the crystal structure of the NaCl type face centered cubic crystal) Existing constituent atomic shared lattice point form is ΣN + 1 In the constituent atom sharing lattice distribution graph showing the distribution ratio of each ΣN + 1 in the whole ΣN + 1 (however, the upper limit value is 28 due to the frequency), the highest peak exists in Σ3, and the Σ3 A modified titanium carbonitride layer showing a constituent atom shared lattice distribution graph in which the distribution ratio in the entire ΣN + 1 is 60% or more,
Through the adhesive Ti compound layer and modified titanium carbonitride layer of (a) and (b) above,
(C) a thickened α-type aluminum oxide layer having an average layer thickness of 16 to 30 μm and having an α-type crystal structure in a state of chemical vapor deposition;
A surface-coated cermet cutting tool that exhibits excellent chipping resistance with a thickened α-type aluminum oxide layer.

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