Alumina-coated cutting tool and methods for the preparation thereof

ALUMINA-COATED CUTTING TOOL AND METHODS FOR THE PREPARATION THEREOF injjn·? niD’Bi mbxn nxmnn naixn ππ’π ίόςμ

The present invention relates to a coated cutting tool for chipforming machining.

Chemical Vapour Deposition (CVD) of alumina on cutting tools has been an industrial practice for more than 15 years. The wear properties of AI2O3 as well as of other refractory materials have been discussed extensively in the literature.

The CVD-technique has also been used to produce coatings of other metal oxides, carbides and nitrides, the metal being selected from transition metals of the IVB, VB and VIB groups of the Periodic Table. Many of these compounds have found practical applications as wear resistant or protective coatings, but few have received as much attention as TiC, TiN and AI2O3.

Cemented carbide cutting tools coated with various types AI2O3-coatings, e.g., pure K-AI2O3, mixtures of κ-and OC-AI2O3 and very coarse-grained (X-AI2O3 have been commercially available for many years. AI2O3 crystallizes in several different phases: α, κ, γ, β, Θ etc. The two most frequently occurring phases in CVD of wear resistant AI2O3-coatings are the thermodynamically stable, hexagonal α-phase and the metastable κ-phase. Generally, the κ-phase is fine-grained with a grain size in the range 0.5-2.0 pm and often exhibits a columnar coating morphology. Furthermore, K-AI2O3 coatings are free from crystallographic defects and free from micropores or voids.

The OC-AI2O3 grains are usually coarser with a grain size of 1-6 pm depending upon the deposition conditions. Porosity and crystallographic defects are in this case more common.

Often both a- and κ-phase are present in a CVD alumina coating deposited onto a cutting tool. In commercial cutting tools, AI2O3 is always applied on TiC coated carbide or ceramic substrates (see, e.g. U.S. Pat. No. 3,837,896, now Reissue U.S. Pat. No. 29,420) and therefore the interfacial chemical reactions between the TiC-surface and the alumina coating are of particular importance. In this context the TiC layer should also be understood to include layers having the formula TiCxNyOz in which the carbon in TiC is completely or partly substituted by oxygen and/or nitrogen.

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The practice of coating cemented carbide cutting tools with oxides to further increase their wear resistance is in itself well known as is evidenced in e.g. U.S. Pat. Reissue No. 29,420 and U.S. Pat. Nos. 4,399,168, 4,018,631, 4,490,191 and 4,463,033. These patents disclose oxide coated bodies and how different pretreatments e.g. of TiC-coated cemented carbide, enhance the adherence of the subsequently deposited oxide layer. Alumina coated bodies are further disclosed in U.S. Pat. No. 3,736,107, 5,071,696 and 5,137,774 wherein the AI2O3 layers comprise α, K resp a+κ combinations. US 4,619,866 describes a method for producing fast growing AI2O3 layers by utilizing a hydrolysis reaction of a metal halide under the influence of a dopant e.g. hydrogen sulphide(H2S) in the concentration range 0.01-0.2 % at a CVD deposition temperature 1000-1050°C. Under these process conditions, essentially two phases of AI2O3, the a- and the K phases, are produced. The resulting coating consists of a mixture of the smaller κ-grains and the larger α-grains. The process yields coatings with an even layer thickness distribution around the coated body.

Swedish patent application 9101953-9 discloses a method of growing a finegrained κ-alumina coating.

In Swedish Patent Application No. 9203852-0 a method is disclosed for obtaining a fine grained, (012)-textured a-AI2O3-coating. This particular AI2O3-coating applied on cemented carbide tools has been found particularly useful for machining of cast-iron.

Swedish Patent Application No. 9304283-6 discloses a body with a coating comprising one or more refractory layers of which at least one layer is a layer of OC-AI2O3 textured in the (110) direction. The alumina layer is essentially free of cooling cracks and comprises platelike grains with a length of 2-8 μπι and a length/width-ratio of 1 - 10.

The object of the present invention is, thus, to provide onto a hard substrate or preferably onto aforementioned TiCxNyOz coating at least one single phase AI2O3 layer of the polymorph a with a desired microstructure and crystallographic

1O857.IL 3 112,206/2 texture using suitable nucleation and growth conditions such that said properties of the A12O3 layer are stabilized.

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 nodular cast iron. EP 523021 and EP 403461 disclose methods for coating cutting tools with alumina. However, neither of these documents shows a texture in accordance with that of the present invention, nor do they reveal a method for obtaining an alumina coating by controlling the sequencing of the reactant gases into the reactor. The gases are, according to the invention, introduced as follows: CO2 and CO are entering the reactor first in an N2 and/or Ar atmosphere, followed by H2 and A1C13.

Fig. 1 shows a Scanning Electron Microscope (SEM) top view micrograph at 1000 X magnification, of a typical A12O3 coating according to the invention.

The cutting tool according to the invention comprises a body of a hard alloy onto which a wear-resistant coating has been deposited. The coating comprises one or several refractory layers, of which at least one layer is a dense, fine-grained, and preferably textured A12O3 layer of the polymorph a.

Thus, according to the invention, there is provided a method of coating a cutting tool with an α-alumina coating, at which the cutting tool is brought into contact with a hydrogen carrier gas containing one or more halides of aluminium and a hydrolyzing and/or oxidizing agent at high temperature, characterized in that the oxidation potential of the CVD-reactor atmosphere prior to the nucleation of A12O3 is kept at a low level, using a total concentration of H2O or other oxidizing species, preferably below 5 ppm; that the nucleation of A12O3 is started up by controlled sequencing of the reactant gases; that CO2 and CO are entering the reactor first in an N2 and/or Ar atmosphere, followed by H2 and A1C13; that

3A 112,206/1 the temperature is preferably between 950-l,000°C during the nucleation; and that, during the growth of the AI2O3, a sulphur dopant is added, preferably H2S. A coated cutting tool according to the present invention exhibits improved wear and toughness properties compared to prior art tools, when used for machining steel or cast iron, particularly if the surface has been further smoothened by wet blasting.

More specifically, the coated tool comprises a substrate of a sintered cemented carbide body, cermet or a ceramic body, preferably of at least one metal carbide in a metal binder phase. The individual layers in the coating structure may be TiC or related carbide, nitride, carbonitride, oxycarbide and oxycarbonitride of a metal selected from the group consisting of metals in the Groups IV-B, V-B and VI-B of the Periodic Table, B, Al and Si and/or mixtures thereof. At least one of said layers is in contact with the substrate. However, at least one of the layers in the coating structure comprises a fine-grained, dense, single phase a-Al2O3 coating free of microporosity and crystallographic defects. This coating is preferentially textured with a thickness of d = 0.5-25 pm, with an average grain size (s) of 0.5 pm < s < 1 pm for 0.5 pm < d <2.5 pm, and 0.5 pm < s < 4 pm for 2.5 pm < d < 25 pm 4

The fine-grained microstructure comprises a narrow grain size distribution. Most often 80 % of the AI2O3 grains have a grain size of + 50 % of the average grain-size.

The grain-size of the AI2O3-coating is determined from a SEM top view micrograph at 5,000X magnification. Drawing three straight lines in random directions, the average distances between grain boundaries along the lines, are taken as a measure of the grain-size.

The AI2O3-layer according to the invention has a preferred crystal growth orientation in the (104) direction which is determined by X-ray Diffraction (XRD) measurements. A Texture Coefficient, TC, can be defined as : TC(hkl) = I(hkl)

Io(hkl) r 1 v i(hki) τ λη 2^0 (hkl) ·* -1 where I(hkl) - measured intensity of the (hkl) reflection

Io(hkl) - standard intensity of the ASTM standard powder pattern diffraction data n = number of reflections used in the calculation, (hkl) reflections used are: (012), (104), (110), (113), (024), (116)

According to the invention TC for the set of (104) crystal planes is larger than 1.5, preferably larger than 2.5, and most preferably larger than 3.0.

The coated body according to the invention is further characterized by a surface roughness (Ra) of the refractory coating of less than 0.3 |lm over a measured length of 0.25 mm. Preferably, the AI2O3-layer is an exposed outermost layer.

The textured AI2O3-coating according to the invention is obtained by careful control of the oxidation potential of the CVD-reactor atmosphere prior to the nucleation of AI2O3. The total concentration level of H2O or other oxidizing species should preferably be below 5 ppm. However, the nucleation of AI2O3 is initiated by a controlled sequencing of the reactant gases as follows: CO2 and CO are first entering the reactor in a H2 free atmosphere e g in the presence of N2 or/and Ar then H2 and AICI3 are allowed into the reactor. The temperature

10857.IL shall be 850-1100 °C, preferably 950-1000°C, during the nucleation. However, the exact conditions depend to a certain extent on the design of the equipment used. It is within the purview of the skilled artisan to determine whether the requisite texture and coating morphology have been obtained and to modify the nucleation and the deposition conditions in accordance with the present specification, if desired, to effect the amount of texture and coating morphology.

Example 1 A) Cemented carbide cutting inserts with the composition 6.5 % Co, 8.5 % cubic carbides and balance WC were coated with a 5.5 gm thick layer of TiCN. In subsequent process steps during the same coating cycle, a 6 gm thick layer of α-Α^Οβ was deposited. Prior to the nucleation, the oxidation potential of the hydrogen carrier gas, i.e. the water vapour concentration, was explicitly set forth to a low level, less than 5 ppm (See also U.S. Pat. No. 5,071,696). A hydrogen free reaction gas mixture comprising N2, CO2 and CO was first introduced into the CVD-reactor. The reaction gases were sequentially added in the given order. After a preset time H2 and AICI3 were allowed into the reactor. During the deposition of AI2O3, H2S was used as a dopant.

The gas mixtures and other process conditions during the AI2O3 deposition steps comprised:

Step 1. 2 CO2 4% 4% AICI3 4% 4% CO 2% - h2s - 0.2% HCI 1% 4% H2 balance balance Pressure 55 mbar 100 mbar Temperature 1000°C 1000°C Duration 1 hr 7.5 hr

10857.IL XRD-analysis showed a texture coefficient, TC(104), of 3.2 of the (104) planes in the single a phase of the AI2O3 coating. SEM-studies showed a fine grained, 6 |im thick AI2O3-coating with an average grain size of = 2.1 |lm. B) The cemented carbide substrate of A) was coated with TiCN (5.5 Jim) and AI2O3 (6 |Xm) as set forth in A) except that the AI2O3 process was carried out according to prior art technique resulting in a mixture of coarse a- and fine K-AI2O3 grains in the coating.

Coated tool inserts from A), and B) were all wet blasted with 150 mesh AI2O3 powder in order to smoothen the coating surface.

The cutting inserts were then tested with respect to edge line and rake face flaking in a facing operation in nodular cast iron (AISI 60-40-18, DIN GGG40). The shape of the machined work piece was such that the cutting edge is intermitted twice during each revolution.

Cutting data:

Speed - 150 m/min,

Cutting Depth = 2.0 mm and

Feed = 0.1 mm/rev.

The inserts were run one cut over the face of the work piece.

The results are expressed in the table below as percentage of the edge line in cut that obtained flaking as well as the rake face area subjected to flaking in relation to total contact area between the rake face and the work piece chip. A) single phase/textured a-AI2O3 Β) α+κ AI2O3

Flaking Edge line 90 (%)

Rake face 6 (acc. to the invention) 86

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Example 2

The cutting inserts from A) and B) were also tested with respect to edge line flaking in a facing operation in an alloyed steel (AISI 1518, W-no. 1,0580). The shape of the 5 machined work piece was such that the cutting edge is intermitted three times during each revolution. 10

Cutting data:

Speed = 130-220 m/min, Cutting Depth = 2 mm and Feed = 0.2 mm/rev.

The inserts were run one cut over the face of the work piece.

The result below is expressed as percentage of the edgeline in cut that obtained flaking. 15 20 D) single phase/textured OC-AI2O3 E) α+κ AI2O3

Flaking (%) Edge line 28 (according to the invention)

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