Oxide coated cutting tool

OXIDE COATED CUTTING TOOL nswnn ον γιη>η

The present invention relates to a coated cutting tool for chipforming machining. The coating includes at least one alumina 5 (AI2O3) layer characterised by fine, equiaxed grains..

Cemented carbide cutting tools coated with various types of AI2O3 layers using Chemical Vapour Deposition (CVD), e.g., pure k-AI2O3, mixtures of k- and OC-AI2O3 coarse grained C1-AI2O3 and fine grained textured α-Αΐ2θ3 have been commercially available for 10 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. AI2O3 crystallises in several different phases: α, κ, γ, δ, θ 15 etc. The two most frequently occurring phases of CVD-produced wear resistant AI2O3 layers are the thermodynamically stable, a-phase and the metastable κ-phase or a mixture thereof. Generally, the k-. phase exhibits a grainsize in the range 0.5-3.0 pm . (dependent on the layer thickness) and the grains predominately grow through the 20 whole coating, forming a columnar type coating morphology..

Furthermore, the K-AI2O3 layers are free from crystallographic defects and also free from micropores and voids.

Coarsegrained (3-6 pm) 0C-AI2O3 often possesses porosity and crystallographic defects, while finegrained textured α-Αΐ2θ3 are 25 free of defects with very pronounced columnar-shaped grains.

In US 5,674,564 a method is disclosed of growing a fine grained K-AI2O3 layer by employing a low deposition temperature and a high concentration of a sulphur compound.

In US 5, 487., 625 a method is disclosed for obtaining a fine 30 grained,. (012)-textured α-Αΐ2θ3 layer consisting of columnar grains with a small cross section (about 1 pm).

In US 5,766,782 a method is disclosed for obtaining a columnar fine-grained (104)-textured a-Al2<03 layer.

The life time and the performance of a coated cutting tool are 35. closely related to the microstructure of the coating materials used. Although, coatings produced according to above mentioned prior art patents show good cutting properties there is still a strong desire further to improve coating microstructures to suit specific cutting conditions and work piece materials. A®has been mentioned above, 'all A12O3 layers produced by the. CVD technique possess a more or less columnarlike grainstructure.

An Al2O3 layer with an-equiaxed grainstructure is, however, expected to show some favourable mechanical properties, e.g. 5 resistance to crack propagation and higher cutting edge toughness, as compared to a layer with a columnar grainstructure. In addition, finegrained layers generally have smoother surfaces than coarse grained layers. During cutting less workpiece materials will adhere, onto a smooth coating surface which in turn will imply IQ lower cutting forces and less tendency for the coating to flake off. Nowadays coated cutting inserts are often brushed with SiC based brushes or blasted with finegrained AI2O3 powder In order to obtain a smooth coating surfaces, a rather costly production step.

One well-known and possible technique to produce, a fine 15 grained structure and to restrain a columnar grain growth is to deposit a so-called multilayer structure in which the columnar growth of e.g. A12O3 is periodically interrupted by the growth of a 0.05-1 pm thick layer of, a different material such as disclosed in US 4,984,940 and US 5,700,569. The latter layer should 20 preferably have a different crystal structure.or at least different lattice spacings in order to be able to initiate renucleation of the original layer.. One example of such a technique is when the Al2O3 growth periodically is interrupted by a short TiN deposition process resulting in an (Al2O3+TiN)xn 25. multilayer structure with a thickness of each individual.TiN layer, in the range 0.1-1 pm see e.g. Proceedings of the 12:th 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. 30 In Swedish patent application SE 0004272-1 a method is- disclosed for obtaining a fine grained a-Al2O3 layer consisting of essentially equiaxed grains with a grain size <1 pm. The grain . refinement is accomplished by periodically interrupting the AI2O3 process and treating the A12O3 surface with a mixture of TiC^/Pfy. 35 When.the Al2O3 process is restarted renucleation of the a-Al2O3 will take place.

It is not possible to use this method to produce finegrained K-AI2O3 since only the a-Al2O3 phase will nucleate on the TiCl4/H2 treated Al203 surface. ΙΟ - · 2Ο·: 25 30' 35 κ·ί^.2ο3 and α-Αΐ2°3 coatings used as tool materials have slightly different wear properties when cutting different materials. It is therefore also desirable to have means to produce fine grained κ—AI2D3 with a controllable grain structure.

It is the object of the present invention to provide onto a hard substrate, or preferably onto a hard substrate coated with a TiCxNyOz and/or ZrCxNyOz-layer (x+y+z-1 and x,y,z£0) at least one . .finegrained single phase K-AI2O3 layer with a microstructure which is different from the prior art columnar K-AI2O3 CVD layers mentioned above.

It is also the object of the present invention to provide a. high performance tool coating comprising the invented AI2O3 layer. . It is a further object of the invention to provide an alumina coated cutting tool insert with improved cutting performance in steel and nodular cast iron.

Fig. la-3a show Scanning Electron Microscope (SEM) micrographs at a high magnification in top view projections. In Fig la an AI2O3 layer according to the present invention is shown and in Fig. 2a a prior art K-AI2O3 layer. In Fig. 3a a prior art multilayer K-Al2C>3+TiN coating is shown.

Fig. lc, 2b and 3b show Scanning Electron Microscope (SEM) micrographs in so called backscatter mode at high magnification'of . polished cross-sections. In Fig. lc an AI2O3 layer with Si-rich l-ayers, A, according to the present invention is.shown, in Fig. 2 b · \ a prior art K-AI2O3 layer and in Fig. 3b a prior art multilayer k-AI2O3+TXN coating is shown. Fig. lb and in Fig. 4 show.SEM micrographs.of cross sectioned samples according to the present invention.

Surprisingly it has been found that a non-columnar, fine grained, equiaxed K-AI2O3 layer can be deposited by repeatedly introducing a controlled amount of a silicon halide, preferably SiClzj, during an Al203~deposition process

The duration·of the treatment periods as well as the silicon halide concentration are important parameters which must be optimised in order to obtain the desired result. If the silicon halide concentration is too low and/or treatment, time is too short, the renucleation of the AI2O3 layer will not be . . sufficiently dense to cover a sufficient portion of the whole coating surface. If, on the other hand, the.silicon halide concentration is too high and/or the treatment time' is too long, 40 4 149,884/2 the cohesion between the grains will be too weas resulting in a low quality coating.

The method of the present invention, thus, relates to the coating of a body with a K-AI2O3 layer during which the body is brought in contact with a hydrogen carrier gas containing one or more halides of aluminium, preferably AICI3, and a hydrolysing and/or oxidising agent, preferably CC>2, at temperature of the body between 800 and 1050 °C. During the ΆΙ2Ο3 growth a silicon halide, preferably SiCl4, is added to the reaction mixture at intervals of 1-5 minutes in a concentration of 20-50 % of the AICI3 flow. This procedure is repeatedly carried out in order to obtain a fine grained K-AI2O3 layerstructure with the desired grainsize.

In contrast to the columnar grains of prior art AI2O3 layers, the grains of the K-AI2O3 layers according to the present invention are essentially equiaxed. The resulting grainsize and' the distribution of the same are dependent on the number of S1CI4 treatments carried out. The more frequently the AI2O3 process is subjected to S1CI4 treatments, the smaller the AI2O3 grains will be. Up to 200 S1CI4 treatments are possible to be carried out. However, generally less than '100 treatments are preferred. The introduction of SXCI4 will initiate a growth of a super fine grained layer comprising Al, Si and 0 in concentrations which are dependent on the used AlCl3/SiCl4-flow ratio.. When the SiCl4-flow is turned off, renucleation of Κ-ΆΙ2Ο3 grains will take place.

One advantage of the invented method is that only one foreign element (Si) is added during the AI2O3 process in order to create the grain refinement. This is in contrast to prior art techniques which use renucleation layers consisting of TiN or (Tix,Aly)(Cz,0w,Nj) which are quite different materials from A12O3.

The resulting grainsize of the K-AI2O3 layer can be determined from a SEM micrograph at high magnification taken from cross-sectioned samples. 4a 149,884/2

Thus, More specifically according to the present invention there is now provided aA cutting tool comprising: a body of sintered cemented carbide, cermet or ceramic; and a hard and wear resistant coating applied on at least a functioning portion of a surface of the body, wherein said coating comprises a structure of one or more refractory layers of which at least one layer consists essentially of an equiaxed fine grained K-AI2O3 with a thickness of 0.5-25 μπι and with a grain size of less than 0.5 pm, and said fine grained k-A12O3 layer comprises at least one sublayer with a thickness between 0.02 and 3 pm containing Al, Si and 0 with a Si concentration between 4-34 at %, Al concentration of 0-37 at % and 0 concentration of 60-67 at %.

The invention also provides a method of coating a body with at least one fine grained k-A12O3 layer, the method comprising: contacting the body with a reaction mixture comprising a hydrogen carrier gas, one or more halides of aluminium and a hydrolysing and/or oxidising agent at 800-1050 °C; adding a sulphur agent to the reaction mixture to enhance the growth rate; and depositing a [kappa]-A12O3 layer comprising at least one silicon enriched sublayer by periodically introducing a silicon halide. 5 149,884/2

The other layers in the coating structure may be a-Al2O3z prior art coarse grained κ-Αΐ2θ3 5 (0.5-3.5 μπι), ZrO2, TiC or related carbide, nitride, carbonitride, oxycarbide and oxycarbonitride of a· metal selected from the C-roups IVB,.VB, and VI3 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- 1 q 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 pm.

Aluminium, silicon and oxygen may exist together in some minerals like andalusite, silliminite, kayanite'and mullite 15 (AlgSi40i3). A method to deposit mullite coatings by'CVD is disclosed in US Patent 5,763,008, a gas mixture of AICI3, S1CI4, CO2 and H2 is used. The present invention uses the same chemical compounds but a sulphur compound is added to form and control the growth of the mentioned sublayer containing silicon. Under the 20 invented process conditions no mullite has been observed.

Example A) Cemented carbide cutting inserts in style CNMG 120408-PM with the composition 7.5 weight-% Co, 1.8 wt% TiC, 0.5 wt% TiN, 3 25 wt% TaC, 0.4 wt% NbC and balance WC were coated with a 1 μπι thick layer of TiN using conventional CVD-technique followed by a 6 μια TiCN layer employing the MTCVD-technique using T1CI4, H2' ^2 and CH3CN as process gasses. In subsequent process steps during the. same coating cycle, a 0.5 pm TiCxNyOz layer with an approximate 30 composition corresponding to x=0.5, y=0.3 and z=0.2 was deposited followed by a 6 pm thick layer of K-AI2O3 deposited according to the invented coating process. Prior to the nucleation of the AI2O3 the oxidation potential of the carrier gas H2 (only gas present in the reactor) i.e. the water vapour concentration, was explicitly 35 set forth to a low level, less than 5 ppm. Then the Al203~process was started up. The process conditions during the AI2O3 deposition were as below: 6 1 2 3 4 ·..·' COg: 4% 4% 4% 4% . AICI3: . 4% .4% . 4% 4% . h2s . - .0.15 ' 0.2% 0.2% HCl .. 1.5% 5% 5% •5% >2 balance balance balance balance SiCl4 - 2% ;. Pressure :- 60 mbar 60 mbar 60 mbar' . 60 mbar Temperature: 1000°C 1000°C 1000°C 1000°C Duration: 30 min . 20 min 22 min . 2 min The AI2O3 layer was· deposited by proceeding through step 1, and 4 and then looping between step 4 and step 3 eight times. : 15 Hence, the .Al203-process was treated with SiCl4 altogether nine . times . " XRD-arialysis of the deposited AI2O3 layer showed that it consisted essentially of the kappa phase. No diffraction peaks , from the mullite-phase (AlgSi20l3) could be detected. .20 From the SEM-micrographs ,-taken from a polished cross sectioned sample, similar to Fig lc, it was possible to observe nine AI2Q3- ' . layers with a thickness of about 0.5 pm and nine extremely fine grained'.layers corresponding to the SiCl4-treatment steps. 4 with a . thickness· of about 0.15 pm. The grainsize was estimated from a 25 fractured sample at. 70000 times .magnification to .be 0.5 pn for the·

Al2O3“layers and less than 0.1 pm for the Si containing, layers. The inserts appeared multicoloured due to light interference in the. transparent layer structure.' The coating surfaces were very smooth. 39 A chemical analysis of the thin sublayers was carried out on a cross sectioned sample in a Hitachi S-4300 FEG-SEM equipped with ' an EDS-system with a Link Si(Li)-detector. The analysis resulted . in the following estimated concentrations Al= 32 at%, Si'= 8 at% and 0=60 at%. . 35 B) Cemented carbide cutting inserts in style CNMG 120408-PM with the same composition as in A) were coated.with a 1 pm thick layer of TiN using conventional CVD-technique followed by a 6 pm TiCN layer employing the MTCVD-technique using T1CI4, Hg, Ng and 4q CH3CN as· process gasses. In subsequent process steps during the same dating cycle, a 0.5 μπι TiCxNyOz layer with an approximate composition corresponding to x—0.5, y=0.3 and z=0.2 was deposited followed by a 6 μιη thick layer of K-AI2O3 deposited according to the. invented coating- process. Prior to the nucleation of the AI2O3. the oxidation potential of the carrier gas H2 (only gas present in the reactor) i.e. the water vapour concentration, was explicitly set forth to a low level, less than 5 ppm.

Then the first AI2O3 layer step 1 was started up.. The process conditions during, the AI2.O3 deposition were as below: . 10 .15 20

Step 1 2 ' 3 CO2 :. 4% 4% . ' 4% AICI3: 4% 4% 4% h2s . - - 0.2% 0.2% HCl 1.5% 5% 5% h2 balance balance balance SiCl4 - - - . 2% Pressure: 60 mbar 60 mbar 60 mbar Temperature: . 1QQO°C . 1000°C 1000°C Duration: 30 min 5 min 1.5 min 25 30 35

The AI2O3 layer was deposited by proceeding through step 1, 2 and 3 and then looping between step 3 and step . 2 altogether 35 times. Hence, the Al203~process was treated with SiCl4 altogether 36 times. XRD-analysis of the deposited AI2O3 layer showed that it consisted essentially of the kappa phase. No diffraction peaks from the mullite-phase (AlgSi20i3) could be.detected. .

From a SEM-micrograph taken from across sectioned sample at a magnification of 70 000 times the grainsize was estimated to be 0.13 pm for the AI2O3 layers and less than 0.04 pm for the layers corresponding to the' SiCl4 treatment steps 3. The inserts appeared violet/green in colour and had extremely smooth coating surfaces. C) The cemented carbide substrate of A) was coated with TiCN (5 pm), a 0.5 pm TiCxNyOz layer and 6 pm AI2O3 as set forth in A) except for that the Al2°3 process was carried out according to prior art technique that is, only step 1 and 2 as described under A) and the process time of step 2 was 290 min. This resulted in an AI2O3 layer consisting essentially of the. K-AI2O3 phase with an 40 8 averaijt^grainsize of about 2 pm with a grain structure similar to what is shown in Fig 2a.

D) The cemented carbide substrate of A) was coated with TiCN (5 pm) , a 0.5 pm TiCxNyOz layer and a 6 pm of multilayered 5 Al203/TiN-coating as below:

Step 1 2 3 4 co2 4% 4% ' .0.% . ’ 4%. AICI3 . 4% ,· 4% 0%. 4% h2s - 0.2% . 0% . 0.2% HCl . 1.5% 5% ... 0% 5% «2 balance balance ' balance balance TiCl4 . · . - 2% n2 • 40% Pressure: 60 mbar· 60 mbar 60 mbar. 60 mbar Temperature: 1000°C 1000°C 1000°C 1000°C Duration: 30 min 20 min : . 3 min 20 min 20 The AI2O3 layer, was deposited by proceeding through step 1,· 2 3.and 4 and then looping between step 4 and step 3 nine. times. Hence, the Al203~process was interrupted ten times by a .TiN-process.

This resulted in a. multilayer coating consisting of 11 layers 25 ’of AI2O3 and 10 thin layers of TiN. The A12O3 layer was determined to consist of essentially the kappa phase. .

Some of the inserts from A), Β), C) and D) were brushed with a . cylindrical nylon brush containing SiC grains. .. 30 .. Brushed and unbrushed cutting inserts from A) B) C) and D) were then tested with respect to edge line and rake face flaking in two types of work piece materials under different cutting conditions. 35 Cutting operation 1: A facing operation in nodular cast iron (AISI 60-40-18, DIN CGC40). The shape of the machined work piece was such that the cutting edge was out of cut twice per revolution of the workpiece. .40 4< 9, (Siting data:

Speed = 160 m/min,

Feed - 0.1 mm/rev

Depth of cut = 2.0 mm 5

The inserts were run one cut over the face of the work piece.. This test is very decisive arid demanding when cutting nodular cast iron. " ·

The percentage of the edge line in cut that obtained flaking . was recorded for each insert tested as well as to what extent flaking occurred on the rake face of the cutting insert. The results are expressed in table 1 below as an average value of the. four inserts (four edges). 15 Cutting operation 2: A facing operatiori in an alloyed steel (AISI 1519,W-no 1.0580). The shape of the workpiece was such that . the cutting edge was out of cut three times per revolution.

Cutting data: 20 . Speed: 130-220 m/min

Feed: ' . ' 0.2 mm/rev . Depth of cut: 2.0 mm

Five inserts (edges) were run one cut over the workpiece. The; 25 results in table 2 are expressed as percentage of the edge-line in cut that obtained flaking of the coating.·

Table;1 Cutting operation 1.

Variant Edge line flaking A unbrushed (acc. to invention) 5% A brushed (acc. to invention) 0% B unbrushed (acc. to invention) '0% B brushed(acc. to invention) 0% C unbrushed (prior art) 65% C brushed (prior art) 5% D unbrushed (prior art) 30% D brushed (prior art) 0% T^frle 2 Cutting operation 2 10

Variant Edge line flaking Rake face Flaking' A brushed (acc. to invention) 5% Only spot wise AI2O3 Flaking B brushed (acc. to invention) 3% Only spot wise AI2O3 Flaking . C brushed (prior art) 90% Severe AI2O3 flaking D brushed (prior art) 70% Flaking of AlgOj and step wise between TiN and AI7O3 layers

The results from table 1 show that unbrushed coatings .. according to the present invention work equally well 'as the 5 brushed inserts in alloyed steel and results in table 2 show that the inserts according to invention also work surprisingly well in nodular cast iron and far better than prior art inserts.

--'W