Oxide coated cutting tool

[0002] The present invention relates to a coated cutting tool for chipforming machining. The coating includes at least one alumina (Al₂O₃) layer characterized by fine, equiaxed grains.

[0003] In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.

[0004] Cemented carbide cutting tools can be coated with various types of Al₂O₃layers by using Chemical Vapour Deposition (CVD), e.g., pure κ-Al₂O₃, mixtures of κ- and α-Al₂O₃, coarse grained α-Al₂O₃, and fine grained textured α-Al₂O₃have been commercially available for years generally in multilayer combinations with other metal carbide and/or nitride layers, the metal being selected from transition metals of the IVB, VB and VIB groups of the Periodic Table.

[0005] Al₂O₃crystallizes in several different phases: α, κ, γ, δ, θ etc. The two most frequently occurring phases of CVD-produced wear resistant Al₂O₃layers are the thermodynamically stable α-phase and the metastable κ-phase, or a mixture thereof. Generally, the κ-phase exhibits a grainsize in the range 0.5-3.0 μm and the grains predominately grow through the whole coating forming a columnar type coating morphology. Furthermore, the κ-Al₂O₃layers are free from crystallographic defects and also free from micropores and voids.

[0006] Coarse-grained (3-6 μm) α-Al₂O₃often possesses porosity and crystallographic defects, while fine-grained textured α-Al₂O₃are free of defects with very pronounced columnar-shaped grains.

[0007] In U.S. Pat. No. 5,674,564 is disclosed a method of growing a fine-grained κ-alumina layer by employing a low deposition temperature and a high concentration of a sulphur dopant.

[0008] In U.S. Pat. No. 5,487,625 a method is disclosed for obtaining a fine grained, (012)-textured α-Al₂O₃layer consisting of columnar grains with a small cross section (about 1 μm).

[0009] In U.S. Pat. No. 5,766,782 a method is disclosed for obtaining a fine-grained (104)-textured α-Al₂O₃layer.

[0010] As mentioned above, all Al₂O₃layers produced by the CVD technique possess a more or less columnar-like grainstructure. An Al₂O₃layer with an equiaxed grainstructure is, however, expected to show some favorable mechanical properties, e.g.—resistance to crack propagation, as compared to a layer with a columnar grainstructure. One well-known and possible technique to avoid columnar grain growth is to deposit a so-called multilayer structure in which the columnar growth of Al₂O₃is periodically interrupted by the growth of a thin, 0.1-1 μm second layer such as disclosed in U.S. Pat. No. 4,984,940. The second layer should preferably have a different crystal structure or at least different lattice spacings in order to be able to initiate renucleation of the first layer. One example of such a technique is when the Al₂O₃growth periodically is interrupted by a short TiN deposition process resulting in a (Al₂O₃+TiN)xn multilayer structure with a thickness of the individual TiN layers of about 0.1-1 μm (see, e.g.—Proceedings of the 12th European CVD Conference, page pr.8-349). However such multilayer structures very often suffer from a low adherence between the two different types of layers.

[0011] It is the object of the present invention to provide onto a hard substrate, or preferably onto a hard substrate coated with a TiC_xN_yO_zlayer, at least one single phase α-Al₂O₃layer with a microstructure which is different from the prior art columnar α- or κ-Al₂O₃CVD layers mentioned above. It is also the object of the present invention to provide a high performance tool coating comprising the invented Al₂O₃layer.

[0012] It is a further object of the invention to provide an alumina coated cutting tool insert with improved cutting performance in steel, stainless steel, cast iron and in particular nodular cast iron.

[0013] According to one aspect, the present invention provides a cutting tool comprising a body of sintered cemented carbide, cermet, or ceramic superhard material, the body comprising a surface, and a hard and wear resistant coating on at least a portion of the surface, said coating comprising: one or more refractory layers of which at least one layer essentially consists of α-Al₂O₃, said α-Al₂O₃layer having equiaxed grains with an average grainsize of <1 μm and further containing striated zones containing >5 at % titanium, but no nitrogen or carbon.

[0014] According to another aspect, the present invention provides a method of coating a body with an α-alumina layer comprising: (i) bringing the body into contact with a hydrogen carrier gas containing one or more halides of aluminum and a hydrolyzing and/or oxidizing agent while the body is at a temperature of 950-1000° C.; (ii) maintaining the oxidation potential of the CVD-reactor atmosphere prior to the nucleation of Al₂O₃at a low level, using a total predetermined concentration of oxidizing species; (iii) starting Al₂O₃growth by introducing the following gases into the reaction chamber: AlCl₃, HCl and CO₂; (iv) adding a sulphur dopant after 20-60 min; (v) repeatedly stopping the CO₂, AlCl₃, HCl and the sulphur dopant for intervals of 10-50 min during which TiCl₄is allowed to enter the reactor for 1-10 min in a concentration of 1-10%; and (vi) then reintroducing AlCl₃, HCl, CO₂and the sulphur dopant, in that order.

[0015] FIG. 1ais a Scanning Electron Microscope (SEM) micrograph of an Al₂O₃layer according to the present invention.

[0016] FIG. 1bis a SEM micrograph at high magnification of a polished cross-section of an Al₂O₃layer according to the present invention.

[0017] FIG. 2ais a SEM micrograph prior art Al₂O₃layer.

[0018] FIG. 2bis a SEM micrograph at high magnification of a polished cross-section of an Al₂O₃layer according to the prior art.

[0019] FIG. 3ais a SEM micrograph of a prior art multilayer Al₂O₃/TiN coating.

[0020] FIG. 3bis a SEM micrograph at high magnification of a polished cross-section of an Al₂O₃/TiN multilayer according to the prior art.

[0021] Surprisingly it has been found that a non-columnar α-Al₂O₃layer can be deposited by interrupting the Al₂O₃growth process by obstructing the flow of the CO₂, AlCl₃, HCl and H₂S gases to the reactor chamber and then immediately introducing TiCl₄(H₂is already present in the reactor) for a short period of time. When the reactant gases AlCl₃, HCl, CO₂and H₂S are allowed to reenter the reactor again in that mentioned order, renucleation of Al₂O₃will take place. The duration of the TiCl₄treatment as well as the TiCl₄concentration are important parameters which must be optimized in order to obtain the desired result. If the TiCl₄concentration is too low and/or treatment time is too short, the renucleation of the Al₂O₃layer will not be sufficiently dense to cover a sufficient portion of the whole coating surface. If, on the other hand, the TiCl₄concentration is too high and/or the treatment time is too long, the cohesion between the Al₂O₃grains will be too weak resulting in a low quality coating.

[0022] The method of the present invention thus relates to the coating of a body with an α-alumina layer during which the body is brought in contact with a hydrogen carrier gas containing one or more halides of aluminum and a hydrolyzing and/or oxidizing agent at temperature of the body between 950 and 1000° C. The oxidation potential of the CVD-reactor atmosphere prior to the nucleation of Al₂O₃is kept at a low level keeping the total concentration of H₂O, water vapor, or other oxidizing species, preferably less than 5 ppm. The Al₂O₃growth is started by sequencing the following gases AlCl₃, HCl and CO₂(H₂is already present in the reactor) into the reaction chamber in that mentioned order or by using the start-up procedures described in any of the prior art patents, U.S. Pat. No. 5,487,625 and U.S. Pat. No. 5,766,782, in order to achieve different textures of the Al₂O₃layer. After 10-60 minutes a sulphur dopant, preferably H₂S is added to the gas mixture. The flow of the CO₂, AlCl₃, HCl gases and the sulphur dopant are periodically interrupted at intervals of 10-50 minutes and 1-10% (of the hydrogen flow) TiCl₄is allowed to enter the reactor for a period of 1-10 minutes and then again replaced by AlCl₃, HCl, CO₂and the sulphur dopant in that mentioned order. This procedure is repeatedly carried out in order to obtain a striated, bimodal α-Al₂O₃layer structure with the desired grainsize and texture.

[0023] In contrast to the columnar grains of prior art Al₂O₃layers, the grains of the Al₂O₃layers according to the present invention are essentially equiaxed with a bimodal structure which is a mixture of small and large grains. The obtained grainsize and the distribution of the same are dependent on the number of TiCl₄treatments carried out. The more frequently the Al₂O₃process is interrupted and the Al₂O₃surface is treated with TiCl₄, the smaller the Al₂O₃grains will be. The large Al₂O₃grains have an average grain size d_c<1 μm and the small Al₂O₃grains, 0.1<D_f<⅓d_c.

[0024] The grainsize in the α-Al₂O₃layer can be determined from a SEM top-view micrograph at about 4000X magnification. Such a micrograph of an Al₂O₃layer surface according to the present invention is shown in FIG. 1a. In FIGS. 2aand 3a, the micrographs of prior art Al₂O₃layers are shown. The size and the shape of the grains can easily be observed. Furthermore, the striated zones in the α-Al₂O₃layer which contain titanium and oxygen are visible in a polished cross section at 4000-6000 X magnification. These striated zones which do not contain any carbon or nitrogen may also contain some aluminum. The striated zones are preferably <0.2 μm thick and the number of striated zones per μm Al₂O₃layer should be 1-10. The zones may be closely linked together but in some cases almost resembling a multilayer structure. The presence of these striated zones in the Al₂O₃structure evidently limits the Al₂O₃graingrowth and makes renucleation possible without the negative effect of fully intermediate or intervening layers.

[0025] By selecting appropriate conditions for the initial growth of the Al₂O₃layer, e.g.—according to the procedures in patents U.S. Pat. No. 5,487,625 and U.S. Pat. No. 5,766,782, Al₂O₃layers textured in the (012)-, (024)- or (104)-directions with a texture coefficient TC>1.3 can be deposited.

[0026] The texture Coefficient, TC, is defined as: $\mathrm{TC}\left(\mathrm{hkl}\right)=\frac{I\left(\mathrm{hkl}\right)}{I_o\left(\mathrm{hkl}\right)}\left\{\frac{1}{n}\sum \frac{I\left(\mathrm{hkl}\right)}{I_o\left(\mathrm{hkl}\right)}\right\}-1$

[0027] where

[0028] I(hkl)=measured intensity of the (hkl) reflection

[0029] I_o(hkl)=standard intensity of the ASTM standard powder

[0030] pattern diffraction data

[0031] n=number of reflections used in the calculation, (hkl)

[0032] reflections used are: (012), (104), (110), (113), (024), (116)

[0033] The coated body may comprise a cutting tool with a substrate of cemented carbide, cermet or a ceramic superhard material and a coating consisting of a hard wear resistant material and in said coating at least one layer is a single phase α-Al₂O₃layer according to the present invention, and said single phase (α-Al₂O₃layer having a thickness in the range 0.5-25 μm. The other layers in the coating structure may be TiC or related carbide, nitride, carbonitride, oxycarbide and oxycarbonitride of a metal selected from the Groups IVB, VB, and VIB of the Periodic Table, the elements B, Al and Si and/or mixtures thereof. Such other layers may be deposited by CVD, PACVD (Plasma CVD), PVD (Physical Vapour Deposition) or MT-CVD (Moderate Temperature CVD). At least one of such other layers is in contact with the substrate. The total thickness of the coating of the cutting tool can vary between 1 and 30 μm.

[0034] A) Cemented carbide cutting inserts in style CNMG 120412-KM with the composition 6 weight-% Co and balance WC were coated with a 5 μm thick layer of TI(C,N) using the MTCVD-technique with TiCl₄, H₂, N₂and CH₃CN as process gases. In subsequent process steps during the same coating cycle, a 0.5 μm TiC_xN_yO_zlayer with an approximate composition corresponding to x=0.5, y=0.3 and z=0.2 was deposited followed by a 6 μm thick layer of α-Al₂O₃deposited according to the invented coating process. Prior to the nucleation of the Al₂O₃the oxidation potential of the carrier gas H₂(only gas present in the reactor) i.e. the water vapor concentration, was explicitly set to a low level, i.e.—less than 5 ppm.

[0035] Then the first Al₂O₃layer step I was started up. The process conditions during the Al₂O₃deposition were as below:

[0036] The Al₂O₃layer was deposited by proceeding through step 1, 2 and 3 and then looping between step 3 and step 2 nine times and finishing the process by step 4. Hence, the Al₂O₃-process was interrupted and treated with TiCl₄/H₂altogether ten times.

[0037] XRD-analysis of the deposited α-Al₂O₃showed a strongly textured structure with a texture coefficient TC(012) of 1.7 of the (012) planes and TC(024) of 1.5 of the (024) planes.

[0038] From the SEM-micrographs taken from the top surface, similar to FIG. 1a, the grainsize was determined. The coarse grains had an average grainsize of 0.9 μm and the fine grains had an average grainsize of 0.3 μm.

[0039] B) The cemented carbide substrate of A) was coated with Ti(C,N) (5 μm), a 0.5 μm TiC_xN_yO_zlayer and Al₂O₃(6 μm) as set forth in A) except for that the Al₂O₃process was carried out according to prior art technique, i.e.—the same process as described under A.) except for that the TiCl₄/H₂-treatments were excluded and an Al₂O₃process time of 290 min. This resulted in an Al₂O₃layer consisting essentially of the κ-Al₂O₃phase with an average grainsize of about 2 μm, FIG. 2a.

[0040] C) The cemented carbide substrate of A) was coated with Ti(C,N) (5 μm), a 0.5 μm TiC_xN_yO_zlayer and a 6 μm of multilayered Al₂O₃coating on top as set forth in A) except for that step 3 was substituted by a prior art TiN-process step. The process parameters for this TiN-step were as follow: 2% TiCl₄, 40% N₂, 58% H₂and a process time of 3 min. This resulted in a multilayer coating consisting of 11 layers of Al₂O₃and 10 thin layers of TiN. The Al₂O₃layer was determined to consist of the κ-phase.

[0041] Coated tool inserts from A), B) and C) were all wet blasted with 150 mesh Al₂O₃powder in order to smooth the coating surfaces.

[0042] The cutting inserts were then tested with respect to edge line and rake face flaking in a facing operation in nodular cast iron. The shape of the machined work piece was such that the cutting edge is intermitted twice during each revolution.

[0043] Cutting data:

[0044] Speed=170 m/min,

[0045] Cutting depth=2.0 mm and

[0046] Feed=0.1 mm/rev.

[0047] The inserts were run one cut over the face of the work piece. This test is very decisive and demanding when cutting nodular cast iron. The percentage of the edge line in cut that obtained flaking into the carbide substrate was recorded for each insert tested as well as to what extent flaking occurred on the rake phase of the cutting insert.

[0048] The results are expressed in the table below as an average value of the four inserts.

[0049] While the present invention has been described by reference to the above-mentioned embodiments, certain modifications and variations will be evident to those of ordinary skill in the art. Therefore, the present invention is to limited only by the scope and spirit of the appended claims.