Oxide coated cutting tool

Abstract

According to the present invention there is provided a body at least partially coated with one or more refractory layers of which at least one layer essentially consist of α-Al2O3. Said α-Al2O3 layer consists of essentially equiaxed grains with an average grain size of <1 μm and with a bimodal grain size distribution with coarser grains with an average grainsize in the interval 0.5-1 μm and finer grains with an average grainsize of <0.5 μm. The Al2O3 layer further contains striated zones containing titanium (>5 at %) but no nitrogen or carbon. This particular microstructure is obtained by temporarily stopping the gases needed for the growth of the Al2O3 layer and introducing TiCl4.

Description

FIELD OF THE INVENTION

[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.

DESCRIPTION OF THE RELATED ART

[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 Al2O3 layers 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-Al2O3 have 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] Al2O3 crystalizes in several different phases: α, κ, γ, δ, θ etc. The two most frequently occurring phases of CVD-produced wear resistant Al2O3 layers 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-Al2O3 layers are free from crystallographic defects and also free from micropores and voids.

[0006] Coarse-grained (3-6 μm) α-Al2O3 often possesses porosity and crystallographic defects, while fine-grained textured α-Al2O3 are 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-Al2O3 layer 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-Al2O3 layer.

[0010] As mentioned above, all Al2O3 layers produced by the CVD technique possess a more or less columnar-like grainstructure. An Al2O3 layer 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 Al2O3 is 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 Al2O3 growth 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.

SUMMARY OF THE INVENTION

[0011] It is the object of the present invention to provide onto a hard substrate, or preferably onto a hard substrate coated with a TiCxNyOz layer, at least one single phase α-Al2O3 layer with a microstructure which is different from the prior art columnar α- or κ-Al2O3 CVD layers mentioned above. It is also the object of the present invention to provide a high performance tool coating comprising the invented Al2O3 layer.

[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-Al2O3 layer 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 aluminium and a hydrolysing 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 Al2O3 at a low level, using a total predetermined concentration of oxidizing species; (iii) starting Al2O3 growth 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 TiCl4 is allowed to enter the reactor for 1-10 min in a concentration of 1-10%; and (vi) then reintroducing AlCl3, HCl, CO2 and the sulphur dopant, in that order.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0015] FIG. 1 a is a Scanning Electron Microscope (SEM) micrograph of an Al2O3 layer according to the present invention.

[0016] FIG. 1 b is a SEM micrograph at high magnification of a polished cross-section of an Al2O3 layer according to the present invention.

[0017] FIG. 2 a is a SEM micrograph prior art Al2O3 layer.

[0018] FIG. 2 b is a SEM micrograph at high magnification of a polished cross-section of an Al2O3 layer according to the prior art.

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

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

DETAILED DESCRIPTION OF THE INVENTION

[0021] Surprisingly it has been found that a non-columnar α-Al2O3 layer can be deposited by interrupting the Al2O3 growth process by obstructing the flow of the CO2, AlCl3, HCl and H2S gases to the reactor chamber and then immediately introducing TiCl4 (H2 is already present in the reactor) for a short period of time. When the reactant gases AlCl3, HCl, CO2 and H2S are allowed to reenter the reactor again in that mentioned order, renucleation of Al2O3 will take place. The duration of the TiCl4 treatment as well as the TiCl4 concentration are important parameters which must be optimized in order to obtain the desired result. If the TiCl4 concentration is too low and/or treatment time is too short, the renucleation of the Al2O3 layer will not be sufficiently dense to cover a sufficient portion of the whole coating surface. If, on the other hand, the TiCl4 concentration is too high and/or the treatment time is too long, the cohesion between the Al2O3 grains 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 aluminium and a hydrolysing 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 Al2O3 is kept at a low level keeping the total concentration of H2O, water vapor, or other oxidizing species, preferably less than 5 ppm. The Al2O3 growth is started by sequencing the following gases AlCl3, HCl and CO2 (H2 is 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 Al2O3 layer. 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) TiCl4 is allowed to enter the reactor for a period of 1-10 minutes and then again replaced by AlCl3, HCl, CO2 and the sulphur dopant in that mentioned order. This procedure is repeatedly carried out in order to obtain a striated, bimodal α-Al2O3 layer structure with the desired grainsize and texture.

[0023] In contrast to the columnar grains of prior art Al2O3 layers, the grains of the Al2O3 layers 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 TiCl4 treatments carried out. The more frequently the Al2O3 process is interrupted and the Al2O3 surface is treated with TiCl4, the smaller the Al2O3 grains will be. The large Al2O3 grains have an average grain size dc<1 μm and the small Al2O3 grains, 0.1≦df≦1/3 dc.

[0024] The grainsize in the α-Al2O3 layer can be determined from a SEM top-view micrograph at about 4000× magnification. Such a micrograph of an Al2O3 layer surface according to the present invention is shown in FIG. 1a. In FIGS. 2a and 3a, the micrographs of prior art Al2O3 layers are shown. The size and the shape of the grains can easily be observed. Furthermore, the striated zones in the α-Al2O3 layer which contain titanium and oxygen are visible in a polished cross section at 4000-6000× magnification. These striated zones which do not contain any carbon or nitrogen may also contain some aluminium. The striated zones are preferably <0.2 μm thick and the number of striated zones per μm Al2O3 layer 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 Al2O3 structure evidently limits the Al2O3 graingrowth 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 Al2O3 layer, e.g.—according to the procedures in patents U.S. Pat. No. 5,487,625 and U.S. Pat. No. 5,766,782, Al2O3 layers 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: 1$\mathrm{TC}\left(\mathrm{hkl}\right)=\frac{I\left(\mathrm{hkl}\right)}{I_o\left(\mathrm{hkl}\right)}{\left\{\frac{1}{n}\sum \frac{I\left(\mathrm{hkl}\right)}{I_o\left(\mathrm{hkl}\right)}\right\}}^{-1}$

[0027] where

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

[0029] Io(hkl)=standard intensity of the ASTM standard powder pattern diffraction data

[0030] n=number of reflections used in the calculation, (hkl) reflections used are: (012), (104), (110), (113), (024), (116)

[0031] 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-Al2O3 layer according to the present invention, and said single phase α-Al2O3 layer 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.

EXAMPLE

[0032] 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, N2 and CH3CN as process gases. In subsequent process steps during the same coating cycle, a 0.5 m TiCxNyOz layer 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-Al2O3 deposited according to the invented coating process. Prior to the nucleation of the Al2O3 the 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.

[0033] Then the first Al2O3 layer step 1 was started up. The process conditions during the Al2O3 deposition were as below:

	Step
	1	2	3	4
CO2:	4%	4%	0%	4%
AlCl3:	4%	4%	0%	4%
H2S	—	0.2%	0%	0.2%
HCl	1.5%	5%	0%	5%
H2:	balance	balance	balance	balance
TiCl4	—	—	5%
Pressure:	60 mbar	60 mbar	60 mbar	60 mbar
Temperature:	1000° C.	1000° C.	1000° C.	1000° C.
Duration:	30 min	20 min	5 min	20 min

[0034] The Al2O3 layer 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/H2 altogether ten times.

[0035] XRD-analysis of the deposited α-Al2O3 showed 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.

[0036] 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.

[0037] B) The cemented carbide substrate of A) was coated with Ti(C,N) (5 μm), a 0.5 μm TiCxNyOz layer and Al2O3 (6 μm) as set forth in A) except for that the Al2O3 process 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 Al2O3 process time of 290 min. This resulted in an Al2O3 layer consisting essentially of the κ-Al2O3 phase with an average grainsize of about 2 μm, FIG. 2a.

[0038] C) The cemented carbide substrate of A) was coated with Ti(C,N) (5 μm), a 0.5 μm TiCxNyOz layer and a 6 μm of multilayered Al2O3 coating 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% H2 and a process time of 3 min. This resulted in a multilayer coating consisting of 11 layers of Al2O3 and 10 thin layers of TiN. The Al2O3 layer was determined to consist of the κ-phase.

[0039] Coated tool inserts from A), B) and C) were all wet blasted with 150 mesh Al2O3 powder in order to smooth the coating surfaces.

[0040] 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.

[0041] Cutting data:

[0042] Speed=170 m/min,

[0043] Cutting depth=2.0 mm and

[0044] Feed=0.1 mm/rev.

[0045] 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.

[0046] The results are expressed in the table below as an average value of the four inserts.

	Flaking
		Edge line	Rake face
	A) α-Al2O3	0%	only spot-wise
	single phase/striated		flaking of the
	(acc. to invention)		Al2O3 layer
	B) κ-Al2O3	90%	severe Al2O3-
	(prior art)		Flaking
	C) multilayer Al2O3/TiN	70%	Flaking between
	(prior art)		TiN and Al2O3 layers

[0047] 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.

Claims

1. 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 aluminium and a hydrolysing 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 Al2O3 at a low level, using a total predetermined concentration of oxidizing species; (iii) starting Al2O3 growth 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 TiCl4 is allowed to enter the reactor for 1-10 min in a concentration of 1-10%; and (vi) then reintroducing AlCl3, HCl, CO2 and the sulphur dopant, in that order.

2. The method of claim 1, wherein in step (ii), the concentration of the oxidizing species is below 5 ppm.

3. The method of claim 2, wherein the oxidizing species comprises H2O.

4. The method of claim 1, wherein the sulphur dopant comprises H2S.