Alumina coated cutting tool

Abstract

A coated cutting tool insert includes a substrate of cemented carbide, cermet, ceramics, steel or cubic boron nitride having deposited thereon a coating having a total thickness of 60 ?m, including one or more layers having a wear resistant layer of ?-Al.sub.2O.sub.3 of a thickness of 1 to 45 ?m deposited by chemical vapour deposition (CVD). The ?-Al.sub.2O.sub.3 layer includes at least two portions, a first thickness portion and a second thickness portion immediately on top of the first thickness portion. The first thickness portion has an essentially columnar ?-Al.sub.2O.sub.3 grain structure, and at a transition from the first thickness portion to the second thickness portion the grain boundaries of at least 1 out of 25 neighboring grains of the ?-Al.sub.2O.sub.3 grains undergo a directional change into a direction that is essentially perpendicular, 90?45 degrees, to the grain boundaries in the first thickness portion.

Claims

1. A coated cutting tool insert comprising: a substrate comprising cemented carbide, cermet, ceramics, steel or cubic boron nitride having deposited thereon a coating of a total thickness of a maximum of 60 ?m with one or more layers including a wear resistant layer of ?-Al.sub.2O.sub.3 of a thickness of 1 to 45 ?m deposited by chemical vapour deposition (CVD), wherein when observed in a SEM microphotograph of a cross section of the ?-Al.sub.2O.sub.3 layer, the ?-Al.sub.2O.sub.3 layer including at least two portions, a first thickness portion and a second thickness portion disposed immediately on the first thickness portion, the first thickness portion having an essentially columnar ?-Al.sub.2O.sub.3 grain structure and grain boundaries, and at a transition of ?-Al.sub.2O.sub.3 crystalline grains from the first thickness portion to the second thickness portion grain boundaries of at least 1 out of 25 neighbouring grains of the transition ?-Al.sub.2O.sub.3 grains undergo a directional change into a direction that is essentially perpendicular to the grain boundaries of the first thickness portion, the directional change being 90?45 degrees.

2. The coated cutting tool insert of claim 1, wherein the ?-Al.sub.2O.sub.3 layer, at least in the first thickness portion, has a preferred growth orientation of the ?-Al.sub.2O.sub.3 grains along a <0 0 1> crystallographic direction or perpendicular to the {0 1 2} or {1 0 4} or {0 1 0} crystallographic plane.

3. A coated cutting tool insert according to claim 1, wherein at least 70% of the ?-Al.sub.2O.sub.3 grains extending to an outer surface of the ?-Al.sub.2O.sub.3 layer are terminated by facets perpendicular to an axis within 0 to 35 degrees, to a normal of the substrate surface.

4. The coated cutting tool insert of claim 3, wherein the facets terminating the ?-Al.sub.2O.sub.3 crystals are {0 0 1} crystallographic planes.

5. The coated cutting tool insert of claim 3, wherein the outer surface of the ?-Al.sub.2O.sub.3 layer has surface roughness characteristics selected from: i) a surface roughness Ra from 0.05 to 0.2 ?m, when the layer of ?-Al.sub.2O.sub.3 has a thickness of 8 ?m or more; and ii) a surface roughness Ra from 0.03 to 0.2 ?m, when the layer of ?-Al.sub.2O.sub.3 has a thickness of less than 8 ?m, whereby the surface roughness is measured on the ?-Al.sub.2O.sub.3 layer in an as-deposited state without top layers and without any post-treatment.

6. The coated cutting tool insert of claim 1, wherein an overall fiber texture of the entire ?-Al.sub.2O.sub.3 layer is characterized by a texture coefficient TC (0 0 12)>3, the TC (0 0 12) being defined as follows: $\mathrm{TC}\left(0012\right)={\frac{I\left(0012\right)}{I_0\left(0012\right)}\left[\frac{1}{n}\underset{n-1}{\overset{n}{.Math.}}\frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\right]}^{-1},$ wherein (h k l)=measured intensity of the (hkl) reflection I.sub.0(h k l)=standard intensity of the standard powder diffraction data according to JCPDF-card no. 42-1468 n=number of reflections used in the calculation, whereby the (hkl) reflections used are: (0 1 2), (1 0 4), (1 10), (1 1 3), (1 1 6), (3 0 0) and (0 0 12).

7. The coated cutting tool insert of claim 1, wherein the coating comprises a top coating includes a top coating having a thickness between 0.05 to 3 ?m, deposited by CVD or PVD, on top of the ?-Al.sub.2O.sub.3 layer, the top coating including one or more layers selected from the group of TiN, TiC, TiCN, ZrN, ZrCN, HfN, HfCN, VC, TiAlN, TiAlCN and AlN, or multilayers thereof, wherein the coating includes one or more refractory layers on the substrate and underneath the ?-Al.sub.2O.sub.3 layer, wherein the one or more refractory layers comprise carbide, nitride, carbonitride, oxycarbonitride or boroncarbonitride of one or more metals selected from the group consisting of Ti, Al, Zr, V and Hf, or combinations thereof, being deposited by chemical vapour deposition (CVD) or moderate temperature chemical vapour deposition (MT-CVD) and each refractory layer having a thickness of from 0.5 to 20 ?m, wherein a first refractory layer immediately on top and in contact with the substrate surface is selected from the group consisting of Ti(C,N), TiN, TiC, Ti(B,C,N), HfN, and Zr(C,N), or combinations thereof.

8. The coated cutting tool insert of claim 7, wherein the first refractory layer adjacent to the substrate surface is Ti(C,N), a refractory layer immediately underneath and in contact with ?-Al.sub.2O.sub.3 layer being cubic (Ti,Al)N, cubic (Ti,Al)(C,N,), or of a multilayer structure of alternating cubic (Ti,Al)N or cubic (Ti,Al)(C,N) layers and one or more refractory layers of carbide, nitride, carbonitride, oxycarbonitride or boroncarbonitride of one or more of Ti, Zr, V and Hf, or combinations thereof.

9. The coated cutting tool insert of claim 1, wherein the substrate includes cemented carbide of 4 to 12 wt-% Co, 0.3-10 wt-% cubic carbides, nitrides or carbonitrides of the metals from groups IVb, Vb and VIb of the periodic table, and balance WC.

10. The coated cutting tool insert of claim 1, wherein the substrate consists of cemented carbide comprising a binder phase enriched surface zone having a thickness of 5 to 30 ?m from the substrate surface, the binder phase enriched surface zone having a Co content that is at least 1.5 times higher than in a core of the substrate and having a content of cubic carbides that is less than 0.5 times the content of cubic carbides in the core of the substrate.

11. The coated cutting tool insert of claim 1, wherein the substrate includes Ti, Nb, Ta or combinations thereof.

12. A method of manufacturing a coated cutting tool insert comprising the steps of: coating a substrate comprising cemented carbide, cermet, ceramics, steel or cubic boron nitride with a coating having a total thickness of a maximum of 60 ?m including one or more layers including a wear resistant layer of ?-Al.sub.2O.sub.3 having a thickness of 1 to 45 ?m deposited by chemical vapour deposition (CVD) from a reaction gas mixture, the deposition process of the ?-Al.sub.2O.sub.3 layer including carrying out the deposition of a first thickness portion of the ?-Al.sub.2O.sub.3 layer under first process conditions to deposit an ?-Al.sub.2O.sub.3 layer having a preferred growth along a first crystallographic direction; and changing deposition conditions to carry out the deposition of a second thickness portion of the ?-Al.sub.2O.sub.3 layer under second process conditions useful to deposit an ?-Al.sub.2O.sub.3 layer with a preferred growth along a second crystallographic direction essentially perpendicular to the first crystallographic direction, whereby the deposition under the second deposition conditions terminates the deposition process of the ?-Al.sub.2O.sub.3 layer, wherein a transition of ?-Al.sub.2 O.sub.3 crystalline grain from the first thickness portion to the second thickness portion includes grain boundaries of at least 1 out of 25 neighbouring grains of the transition ?-Al.sub.2O.sub.3 grains that undergo a directional change into a direction that is essentially perpendicular to the grain boundaries of the first thickness portion, the directional change being 90?45 degrees.

13. The method of claim 12, wherein said ?-Al.sub.2O.sub.3 layer is deposited by chemical vapour deposition (CVD) from a reaction gas mixture comprising H.sub.2, CO.sub.2, AlCl.sub.3, HCl and X, with X being selected from the group consisting of H.sub.2S, SF.sub.6 and SO.sub.2, or combinations thereof, and the reaction gas mixture further optionally having additions of N.sub.2, Ar, CO or combinations thereof, wherein the deposition process of the ?-Al.sub.2O.sub.3 layer further comprises at least the steps of: carrying out the deposition of the first thickness portion of the ?-Al.sub.2O.sub.3 layer under first process conditions to deposit an ?-Al.sub.2O.sub.3 layer having the preferred growth along the <0 0 1> crystallographic direction; and changing the deposition conditions to carry out the deposition of the second thickness portion of the ?-Al.sub.2O.sub.3 layer under second process conditions useful to deposit an ?-Al.sub.2O.sub.3 layer with the preferred growth along the crystallographic direction essentially perpendicular to the <0 0 1> crystallographic direction, along the <1 1 0> or the <1 0 0> crystallographic direction, whereby the deposition under the second deposition conditions terminates the deposition process of the ?-Al.sub.2O.sub.3 layer.

14. The method of any of claim 13, wherein the deposition of the ?-Al.sub.2O.sub.3 layer under the first process conditions to deposit the ?-Al.sub.2O.sub.3 layer having a preferred crystal growth along the <0 0 1> crystallographic direction is carried out for a period between 1 and 20 hours, and the deposition of the ?-Al.sub.2O.sub.3 layer under the second process conditions useful to deposit the ?-Al.sub.2O.sub.3 layer having the preferred crystal growth essentially perpendicular to the <0 0 1> crystallographic direction, along the <1 1 0> or the <1 0 0> crystallographic direction, is carried out for a period between 5 min and 3 hours; and wherein the deposition of the ?-Al.sub.2O.sub.3 layer under the first process conditions is carried out to deposit the ?-Al.sub.2O.sub.3 layer having a preferred crystal growth along the <0 0 1> crystallographic direction to a thickness from about 2 ?m to about 45 ?m, and the deposition of the ?-Al.sub.2O.sub.3 layer under the second process conditions is carried out to a thickness that is about 5 to 30% of the thickness of the ?-Al.sub.2O.sub.3 layer deposited under the first process conditions.

15. The method of claim 13, wherein the first process conditions in the CVD reaction chamber to deposit the ?-Al.sub.2O.sub.3 layer having a preferred crystal growth along the <0 0 1> crystallographic direction include a pressure in the range between 10 and 100 mbar, a temperature in the range of 800? C. to 1050? C., and reactive gas concentrations in the ranges of between 2% and 7.5% CO.sub.2, between 0.5% and 5% HCl, between 0.5% and 5% AlCl.sub.3, and between 0.2% and 1.1% X, and the second process conditions in the CVD reaction chamber useful to deposit the ?-Al.sub.2O.sub.3 layer having the preferred crystal growth essentially perpendicular to the <0 0 1> crystallographic direction, along the <1 1 0> or the <1 0 0> crystallographic direction, include a pressure in the range between 100 and 300 mbar, a temperature in the range of 800? C. to 1050? C. and reactive gas concentrations in the ranges of more than 5% CO.sub.2, between 5% and 25% HCl, between 0.5% and 3% AlCl.sub.3, and less than 0.35% X.

16. The method of claim 15, wherein in the first process conditions and/or in the second process conditions in the CVD reaction chamber to deposit the ?-Al.sub.2O.sub.3 layer the component X is a combination of H.sub.2S and SF.sub.6, whereby the volume proportion of SF.sub.6 does not exceed 15% of the volume amount of H.sub.2S.

17. The method of claim 16, wherein the first process conditions and/or the second process conditions in the CVD reaction chamber to deposit the ?-Al.sub.2O.sub.3 layer include the addition of N.sub.2, Ar, CO or combinations thereof whereby the sum of the volume proportions of N.sub.2, Ar and CO does not exceed 20% of the total volume amount of H.sub.2 in the reaction gas mixture.

18. The method of claim 12, further comprising the deposition of a top coating on top of the ?-Al.sub.2O.sub.3 layer and/or the deposition of one or more refractory layers on the substrate and underneath the ?-Al.sub.2O.sub.3 layer and/or the deposition of a first refractory layer immediately on top and in contact with the substrate surface, wherein the one or more refractory layers of carbide, nitride, carbonitride, oxycarbonitride or boroncarbonitride of one or more metals selected from the group of Ti, Al, Zr, V and Hf, or combinations thereof, being deposited by chemical vapour deposition (CVD) or moderate temperature chemical vapour deposition (MT-CVD) and each refractory layer having a thickness of from 0.5 to 20 ?m, wherein a first refractory layer immediately on top and in contact with the substrate surface is selected from the group of Ti(C,N), TiN, TiC, Ti(B,C,N), HfN, and Zr(C,N), or combinations thereof, the first refractory layer adjacent to the substrate surface being Ti(C,N), and wherein a refractory layer immediately underneath and in contact with the ?-Al.sub.2O.sub.3 layer consists of cubic (Ti,Al)N, cubic (Ti,Al)(C,N), or of a multilayer structure of alternating cubic (Ti,Al)N or cubic (Ti,Al)(C,N) layers and one or more refractory layers of carbide, nitride, carbonitride, oxycarbonitride or boroncarbonitride of one or more of Ti, Zr, V and Hf, or combinations thereof.

19. A coated cutting tool insert made by the method of claim 12.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows SEM microphotographs of cross sections of ?-Al.sub.2O.sub.3 layers according to a) the prior art (example 1), b) the present invention (example 2);

(2) FIG. 2 shows SEM microphotographs of the surface topographies of the ?-Al.sub.2O.sub.3 layers of FIGS. 1a) and 1b), respectively, according to a) the prior art (example 1), b) the present invention (example 2);

(3) FIG. 3 shows SEM microphotographs of cross sections of ?-Al.sub.2O.sub.3 layers according to the present invention with a multilayered top layer of alternating TiN and TiCN sub-layers deposited on a cemented carbide substrate of a cutting tool insert, whereby a) is a cross sections of the cutting edge and b) is a cross section of the rake face (example 3, coating 3.2).

(4) FIG. 4 shows a SEM microphotograph of a cross section of a ?-Al.sub.2O.sub.3 layer according to the prior art with a multilayered top layer of alternating TiN and TiCN sub-layers deposited on a cemented carbide substrate of a cutting tool insert (example 3, coating 3.1).

(5) In FIGS. 1b), 3a) and 3b) in the upper parts of the ?-Al.sub.2O.sub.3 layers the directional changes of the grain boundaries, corresponding to the abrupt changes in the growth mode from carrying out the deposition under first process conditions to second process conditions, according to the present invention, are clearly visible.

(6) Methods

(7) X-ray Diffraction (XRD) Measurements and TC Determination

(8) X-ray diffraction measurements were done on a diffractometer XRD3003PTS of GE Sensing and Inspection Technologies using Cu K.sub.?-radiation. The X-ray tube was run at 40 kV and 40 mA focused to a point. A parallel beam optic using a polycapillary collimating lens with a measuring aperture of fixed size was used on the primary side whereby the irradiated area of the sample was selected to avoid a spill over of the X-ray beam over the coated face of the sample. On the secondary side a Soller slit with a divergence of 0.4? and a 0.25 mm thick Ni K.sub.? filter were used. ?-2? scans within the angle range of 20?<2 ?<100? with increments of 0.25? have been conducted. The measurements were done on a flat face of the coated insert, preferably on the flank face. The measurements were done directly on the alumina layer as the outermost layer. Any layer present in the coating above the alumina layer to be measured, if any, is removed by a method that does not substantially influence the XRD measurement results, e. g. etching. For the calculation of the texture coefficient TC peak height intensities were used. Background subtraction and a parabolic peakfit with 5 measuring points were applied to the XRD raw data. No further corrections such as K.sub.?2 stripping or thin film correction were made.

(9) Sample Preparation for Scanning Electron Microscopy (SEM)

(10) Inserts were cut in cross section, mounted in a holder and then treated as follows: 1. Grinding with Struers Piano220 disc with water for 6 min 2. Polishing with 9 ?m MD-Largo Diamond suspension for 3 min 3. Polishing with 3 ?m MD-Dac Diamond suspension for 3:40 min 4. Polishing with 1 ?m MD-Nap Diamond suspension for 2 min 5. Polishing/etching with OP-S colloidal silica suspension for 12 min (average grain size of the colloidal silica=0.04 ?m)

(11) The specimens were ultrasonically cleaned before SEM examination.

(12) CVD Coatings

(13) The CVD coatings were prepared in a radial flow reactor, type Bernex BPX 325S, having 1250 mm height and 325 mm outer diameter. Gas streams carrying NH.sub.3 and metal chlorides, respectively, were fed into the reactor separately so that mixing occurred directly before the reaction zone. Gas flow over the charging trays was radial from a central gas tube.

(14) Roughness Measurements

(15) Roughness measurements were made according to ISO 4287, DIN 4768. A white light ConScan from CSM Instruments was used for the roughness measurements.

EXAMPLES

(16) The substrates for the test insert used in the examples herein were cemented carbide cutting tool inserts consisting of 6.0 wt % Co and balance WC with a binder phase enriched surface zone. The Vickers hardness of the substrates was measured to be about 1600 HV. The following insert geometries were used: CNMA120412 (especially for SEM and roughness measurements) and WNMG080412-NM4 (for cutting tests).
Pre-coating:

(17) The substrates for the deposition of the ?-Al.sub.2O.sub.3 coating according the examples herein were pre-coated by the following procedures.

(18) To ensure good adhesion of the coatings to the substrate, in all coating procedures the process was started by applying a 0.3 ?m thick TiN layer onto the substrate surface by CVD at a temperature of 850? C. and a pressure of 150 mbar. The react ion gas composition was 0.8 vol-% TiCl.sub.4, 44.1 vol-% N.sub.2, 55.1 vol-% H.sub.2.

(19) TiN pre-coated inserts were coated with a 5 ?m thick Ti(C,N) layer by MT-CVD using a reaction gas composition of 0.7 vol-% CH.sub.3CN, 1.9 vol-% TiCl.sub.4, 20 vol-% N.sub.2 and balance H.sub.2 at a temperature of 850-880? C.

(20) After the MT-CVD process was complete, the temperature was raised to 1000? C., and at this temperature another N-rich TiCN layer was CVD deposited for 20 min using a reaction gas composition of 2.5 vol-% TiCl.sub.4, 3.5 vol-% CH.sub.4, 30 vol-% N.sub.2 and balance H.sub.2 at a pressure of 400-500 mbar onto the MT-CVD layer to a thickness of approximately 0.3 ?m.

(21) Then, an about 0.5-1 ?m, thick (Ti,Al)(C,N,O) bonding layer was deposited on top of the MT-CVD TiCN layer using a reaction gas composition of 3 vol-% TiCl.sub.4, 0.5 vol-% AlCl.sub.3, 4.5 vol-% CO, 30 vol-% N.sub.2 and balance H.sub.2 for about 30 min at a temperature of about 1000? C. and a pressure of 80 mbar. The deposition process was followed by a purge using H.sub.2 for 10 min before starting the next step.

(22) Before the deposition of the ?-Al.sub.2O.sub.3 layer, a nucleation (oxidation) step was carried out by treating the (Ti,Al)(C,N,O) bonding layer with a gas mixture of 4 vol-% CO.sub.2, 9 vol-% CO, 25 vol-% N.sub.2 and balance H.sub.2 for 2-10 min at a temperature of about 1000 to 1020? C. and at a pressure of about 80 to 100 mbar. The nucleation step was followed by a purge using Ar for 10 min.

Example 1

Alumina Layer According to Prior Art (Coating 1Comparative Example)

(23) The alumina deposition on the pre-coated substrates, as described above, was started by introducing a reaction gas mixture of 2.5 vol-% AlCl.sub.3, 4.1 vol-% CO.sub.2 and balance H.sub.2 at a temperature of 1000? C. and a pressure of 66 mbar. The reaction g as components were introduced simultaneously. After 2 min, HCl in an amount of 2.5 vol-% was added to the reaction gas mixture flowing into the reactor. After another 8 min H.sub.2S in an amount of 0.33 vol-% was added to the reaction gas mixture flowing into the reactor.

(24) The deposition conditions were maintained for about 8 hours to obtain an about 8 ?m thick ?-Al.sub.2O.sub.3 layer. In a visual inspection the ?-Al.sub.2O.sub.3 layer appeared dark and dull. The coating was analysed by roughness measurements, cutting tests, SEM and XRD.

Example 2

Alumina Layer According to the Present Invention (Coating 2)

(25) The alumina deposition on the pre-coated substrates was carried out as described above for example 1 with the exception that the deposition time of the ?-Al.sub.2O.sub.3 layer was 6.5 hours instead of 8 hours. After 6.5 hours deposition time, the process parameters were changed to second process conditions that were known to favour a preferred crystal growth along the <1 1 0> crystallographic direction of the ?-Al.sub.2O.sub.3 layer. In the second process conditions, the composition of the reaction gas mixture was 1.5 vol-% AlCl.sub.3, 5.7 vol-% CO.sub.2, 7.5 vol-% HCl, 0.05 vol-% H.sub.2S and balance H.sub.2. The temperature was maintained at 1000? C., but the pressure was changed to 150 mbar. The deposition time under the second process conditions was 1.5 hours, and the total thickness of the thus obtained ?-Al.sub.2O.sub.3 layer was 8 ?m, as in example 1. In a visual inspection the ?-Al.sub.2O.sub.3 layer appeared dark but shiny. The coating was analysed by roughness measurements, cutting tests, SEM and XRD.

(26) The ?-Al.sub.2O.sub.3 coatings from examples 1 and 2 were analysed by SEM. The cross-section microphotographs are shown in FIG. 1 and surface topography microphotographs are shown in FIG. 2. FIGS. 1a) and 2a) show coating 1 of example 1 (comparative example) and FIGS. 1b) and 2b) show coating 2 of example 2 (invention). From the cross-sectional images of FIGS. 1a) and 1b), as well as from the surface topography images of FIGS. 2a) and 2b), it can clearly be seen that the surface of the ?-Al.sub.2O.sub.3 coating according to the invention is much smoother due to an abrupt change in the growth of the grain boundary directions, compared to the surface of the ?-Al.sub.2O.sub.3 coating prepared according to the prior art. The coating of example 1 showed columnar grain structure of the ?-Al.sub.2O.sub.3 layer. The coating of example 2 showed a first thickness portion with columnar grains and a second thickness portion, where at the transition from the first thickness portion to the second thickness portion some of the grain boundaries undergo a directional change in relation to the direction of the grain boundary in the first thickness portion. In the coating of example 1 (prior art), no such directional change is found.

Example 3

Top Coating 1

(27) The processes of examples 1 and 2 were repeated, but, additionally, on top of the ?-Al.sub.2O.sub.3 layers a top coating was deposited. The top coating was a 1.1 ?m thick multilayer composed of 5 alternating sub-layers of TiN and TiCN terminated by TiCN.

(28) The samples are designated as coating 3.1 (=according to prior art example 1 plus top coating) and coating 3.2 (=according to inventive example 2 plus top coating).

(29) The ?-Al.sub.2O.sub.3 coatings 3.1 and 3.2 were analysed by SEM. Cross-section microphotographs of coating 3.2 on the cutting edge and on the rake face, respectively, are shown in FIGS. 3a) and 3b). The cross-sectional images of FIGS. 3a) and 3b) clearly show the extreme smoothness of the surfaces of the ?-Al.sub.2O.sub.3 layer, as well as of the top coating. A cross-section microphotograph of coating 3.1 according to the prior art is shown in FIG. 4. It clearly shows the significantly rougher surface due to the different faceting of the ?-Al.sub.2O.sub.3 grains.

Example 4

Top Coating 2

(30) The processes of examples 1 and 2 were repeated, but, additionally, on top of the ?-Al.sub.2O.sub.3 layers a different top coating than in example 3 was deposited. The top coating was a 1.5 ?m thick layer of cubic-TiAlN. The reaction gas mixture to deposit the TiAlN layer contained 0.03 vol-% TiCl.sub.4, 0.37 vol-% AlCl.sub.3 and 54.4 vol-% H.sub.2 from the chloride line and 0.25 vol-% NH.sub.3 and 44.95 vol-% H.sub.2 from the ammonia line. The deposition was carried out at a temperature of 700? C. and a pressure of 10 mbar for 40 min.

(31) The samples are designated as coating 4.1 (=according to prior art example 1 plus top coating) and coating 4.2 (=according to inventive example 2 plus top coating).

Example 5

Alumina Layer According to the Present Invention (Coating 5)

(32) The alumina deposition on the pre-coated substrates was carried out as described above for example 2 with the exception that the deposition time of the ?-Al.sub.2O.sub.3 layer under the first process conditions was 4 hours, and the deposition time under the second process conditions was 1 hour. The total thickness of the thus obtained ?-Al.sub.2O.sub.3 layer was 5 ?m

(33) Roughness (R.sub.a) measurements were performed on coatings 1, 2 and 5, and the results of an average of 5 measurements on each sample were as follows:

(34) TABLE-US-00001 Coating # R.sub.a [?m] 1 (prior art) 0.34 2 (invention) 0.13 5 (invention) 0.06

Example 6

Metal Cutting Test 1

(35) The coatings 1 (prior-art) and 2 (invention) were tested with respect to edge chipping (flaking) on the contact area in longitudinal turning in cast iron without coolant under the following conditions: Work piece: Cylindrical bar Material: SS0130 Insert type: CNMA120412 Cutting speed: 380 m/min Feed: 0.4 mm/rev Depth of cut: 2.0 mm

(36) The inserts were inspected after a cutting time of 2 minutes and 6 minute, respectively, and the results were as follows:

(37) TABLE-US-00002 Flaking after 2 min Flaking after 6 min Coating # [%] [%] 1 (prior art) 7 31 2 (invention) 0 12

(38) The results show that the edge toughness of the product according to the invention was considerably improved over the prior art product.

Example 7

Metal Cutting Test 2

(39) The coatings 3.1, 3.2, 4.1 and 4.2 of examples 3 and 4 were tested with respect to flank wear in a turning operation without coolant under the following conditions: Work piece: Cylindrical bar Material: 56NiCrMoV7 Insert type: WNMG080412-NM4 Cutting speed: 125 m/min Feed: 0.32 mm/rev Depth of cut: 2.5 mm

(40) The inserts were inspected for maximum flank wear on the cutting edge [VBmax] after 6 minutes of cutting, and the results were as follows:

(41) TABLE-US-00003 VB max after 6 min Coating # [mm] 3.1 (prior art) 0.35 3.2 (invention) 0.21 4.1 (prior art) 0.15 4.2 (invention) 0.08

Example 8

EBSD Analysis

(42) EBSD analysis of coating 2 was performed in a Zeiss SUPRA40VP scanning electron microscope (SEM) with a field emission cathode using a 60 ?m aperture and 15 kV acceleration voltage working in high current mode with a 70? incident angle of the electron beam to the polished samples surface at about 12 mm working distance. The EBSD system was EDAX (Digiview camera), and the TSL OIM Data Collection 6 and TSL OIM Analysis 6.2 software packages were used for data collection and analysis, respectively.

(43) An insert of WNMG0804012-NM4 geometry coated with coating 2 was subjected to the EBSD measurement in the as-coated state, i.e. without any polishing or other surface preparation, on a planar part of the rake face. A 4?4 binning and a background subtraction was performed on the camera picture. The exposure time corresponded to 30 frames per second. The map size was 60?60 ?m with 0.15 ?m step size and a hexagonal grid of measurement points. Indexing of the diffraction pattern was done by Hough transformation. The 185031 data points thus recorded were indexed with an average confidence index (CI) of 0.27. The fraction of points with CI>0.1 was 76.7%, the fraction of CI>0.3 was 44.6%. The CI was calculated by the TSL OIM Analysis 6.2 software during automated indexing of the diffraction pattern. For a given diffraction pattern several possible orientations may be found which satisfy the diffraction bands detected by the image analysis routines. The software ranks these orientations (or solutions) using a voting scheme. The confidence index is based on the voting scheme and is given as CI=(V.sub.1?V.sub.2)/V.sub.IDEAL where V.sub.1 and V.sub.2 are the number of votes for the first and second solutions and V.sub.IDEAL is the total possible number of votes from the detected bands. The confidence index ranges from 0 to 1. Even though there are cases when a pattern may still be correctly indexed even at a confidence index of 0, the CI can be regarded as statistical a measure for the pattern quality, which is highly dependent on the surface roughness. Samples with rough surfaces have to be polished to an extremely low roughness in order to get satisfactory pattern quality and indexing for EBSD. A CI value greater than 0.3 corresponds to 99% accuracy of the automated pattern indexing, general patterns indexed with a CI>0.1 are considered to be correct. For correct indexing, i.e. to obtain EBSD maps with average CI greater than 0.1, prolonged polishing with abrasive/etching agents of typically 0.05 ?m grain size, thus to R.sub.a values well below 0.05 ?m is necessary [M. M. Nowell et al., Microscopy Today 2005, 44-48].

(44) From the image and pattern quality obtained on coating 2 in the as-deposited state, it can thus be concluded that a large fraction, >75% of the surface area, is composed of grains with flat crystal facets nearly parallel to the sample surface and also to the substrate surface. In contrast, no EBSD patterns of sufficient quality could be obtained from the prior art surface coating 1 in the as-deposited state, since surface morphology is dominated by grains with facets intersecting at higher angles, yielding a profile with grain-to grain depth on the order of 1 ?m. An EBSD map of coating 1 was indexed with an average CI of 0.04 only.