Coated Cutting Tool Insert with MT-CVD TiCN on TiAI(C,N)

Abstract

A coated cutting tool includes a substrate of cemented carbide, cermet, ceramics, steel or cubic boron nitride, a multi-layered wear resistant coating and at least two refractory coating layers deposited. The at least two refractory coating layers include a first coating layer and a second coating layer deposited on top of each other. The first coating layer is titanium aluminium nitride or carbonitride Ti.sub.1-uAl.sub.uC.sub.vN.sub.w, with 0.2≦u≦1.0, 0≦v≦0.25 and 0.7≦w≦1.15 deposited by CVD. The second coating layer is titanium carbonitride Ti.sub.xC.sub.yN.sub.1-y, with 0.85≦x≦1.1 and 0.4≦y≦0.85, and is deposited on top of the first coating layer by MT-CVD. The second Ti.sub.xC.sub.yN.sub.1-y coating layer has a columnar grain morphology and the overall fiber texture of the Ti.sub.xC.sub.yN.sub.1-y coating layer is characterized by a texture coefficient TC (1 1 1)>2.

Claims

1. A coated cutting comprising: a substrate of cemented carbide, cermet, ceramics, steel or cubic boron nitride; and a multi-layered wear resistant coating having a total coating thickness from 5 to 25 μm and including at least two refractory coating layers deposited by chemical vapour deposition (CVD) or moderate temperature chemical vapour deposition (MT-CVD), the at least two refractory coating layers including a first coating layer and a second coating layer being deposited on top of each other, wherein the first coating layer consists of titanium aluminium nitride or carbonitride Ti.sub.1-uAl.sub.uC.sub.vN.sub.w, with 0.2≦u≦1.0, 0≦v≦0.25 and 0.7≦w≦1.15, deposited by CVD at a reaction temperature in the range from 600° C. to 900° C., the second coating layer being titanium carbonitride Ti.sub.xC.sub.yN.sub.1-y, with 0.85≦x≦1.1 and 0.4≦y≦0.85, deposited on top of the first coating layer by MT-CVD at a reaction temperature in the range from 600° C. to 900° C., and wherein the second Ti.sub.xC.sub.yN.sub.1-y coating layer has a columnar grain morphology, an overall fiber texture of the Ti.sub.xC.sub.yN.sub.1-y coating layer being characterized by a texture coefficient TC (1 1 1)>2, the TC (1 1 1) being defined as: $\mathrm{TC}\left(111\right)={\frac{I\left(111\right)}{I_0\left(111\right)}\left[\frac{1}{n}.Math.\underset{n-1}{\overset{n}{.Math.}}.Math.\frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\right]}^{-1},$ 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. 42-1489 n=number of reflections used in the calculation, whereby the (hkl) reflections used are: (1 1 1), (2 0 0), (2 2 0) and (3 1 1).

2. The coated cutting tool of claim 1, wherein the second Ti.sub.xC.sub.yN.sub.1-y coating layer has a thickness L and an average grain diameter W, and a ratio L/W<8, or L/W<5.

3. The coated cutting tool of claim 1, wherein the grains of the second Ti.sub.xC.sub.yN.sub.1-y coating layer have an average grain diameter W of W≧0.4 μm, W≧0.7 μm, or W≧1.1 μm.

4. The coated cutting tool of claim 1, wherein the first Ti.sub.1-uAl.sub.uC.sub.vN.sub.w coating layer has a columnar grain morphology and the overall fiber texture of the first Ti.sub.1-uAl.sub.uC.sub.vN.sub.w coating layer having a maximum intensity of diffraction from the {111} crystallographic planes, as determined by X-ray diffraction (XRD) pole figure measurements or EBSD measurement, which occurs within a tilt angle from a normal to a sample substrate surface of α=±20°, α=±10°, α=±5°, or α=±1°.

5. The coated cutting tool of claim 1, wherein a length of Σ3-type grain boundaries in the second Ti.sub.xC.sub.yN.sub.1-y coating layer is less than 60%, less than 40%, or less than 30% of a total length of a sum of grain boundaries of ΣN-type with N=2n+1, 1≦n≦28, (=Σ3-49-type grain boundaries), the grain boundary character distribution being measured by EBSD.

6. The coated cutting tool of claim 1, wherein the overall fiber texture of the second Ti.sub.xC.sub.yN.sub.1-y coating layer is has a texture coefficient TC (1 1 1)>3.0, or TC (1 1 1)>3.75.

7. The coated cutting tool of claim 1, wherein crystals of the first Ti.sub.1-uAl.sub.uC.sub.vN.sub.w coating layer and the crystals of the second Ti.sub.xC.sub.yN.sub.1-y coating layer have isomorphic crystal structures or face-centered cubic (fcc) crystal structures.

8. The coated cutting tool of claim 1, wherein the multi-layered wear resistant coating comprises at least one further refractory layer between the substrate surface and the first Ti.sub.1-uAl.sub.uC.sub.vN.sub.w coating layer, the at least one further refractory layer being selected from carbides, nitrides, carbonitrides, oxycarbonitrides and borocarbonitrides of one or more of Ti, Al, Zr, V and Hf, or combinations thereof, and being deposited by chemical vapour deposition (CVD) or moderate temperature chemical vapour deposition (MT-CVD), the at least one further refractory layer including a TiN layer.

9. The coated cutting tool of claim 1, wherein the first coating layer is titanium aluminium nitride or carbonitride Ti.sub.1-uAl.sub.uC.sub.vN.sub.w, with 0.6≦u≦1.0, 0≦v≦0.1 and 0.7≦w≦1.15, or with 0.8≦u≦1.0, 0≦v≦0.05 and 0.7≦w≦1.15.

Description

DESCRIPTION OF THE INVENTION

[0022] The present invention provides a coated cutting tool consisting of a substrate of cemented carbide, cermet, ceramics, steel or cubic boron nitride and a multi-layered wear resistant coating having a total coating thickness from 5 to 25 μm and comprising at least two refractory coating layers deposited by chemical vapour deposition (CVD) or moderate temperature chemical vapour deposition (MT-CVD), the at least two refractory coating layers including a first coating layer and a second coating layer being deposited on top of each other, wherein

[0023] the first coating layer consists of titanium aluminium nitride or carbonitride Ti.sub.1-uAl.sub.uC.sub.vN.sub.w, with 0.2≦u≦1.0, 0≦v≦0.25 and 0.7≦w≦1.15, and is deposited by CVD at a reaction temperature in the range from 600° C. to 900° C.,

[0024] the second coating layer consists of titanium carbonitride Ti.sub.xC.sub.yN.sub.1-y, with 0.85≦x≦1.1 and 0.4≦y≦0.85, and is deposited on top of the first coating layer by MT-CVD at a reaction temperature in the range from 600° C. to 900° C.,

[0025] wherein the second Ti.sub.xC.sub.yN.sub.1-y coating layer has a columnar grain morphology and the overall fiber texture of the Ti.sub.xC.sub.yN.sub.1-y coating layer is characterized by a texture coefficient TC (1 1 1)>2, the TC (1 1 1) being defined as follows:

[00001]$\mathrm{TC}\left(111\right)={\frac{I\left(111\right)}{I_0\left(111\right)}\left[\frac{1}{n}.Math.\underset{n-1}{\overset{n}{.Math.}}.Math.\frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\right]}^{-1},$

[0026] wherein [0027] (h k l)=measured intensity of the (hkl) reflection [0028] l.sub.0 (h k l)=standard intensity of the standard powder diffraction data according to JCPDF-card no. 42-1489 [0029] n=number of reflections used in the calculation, whereby the (hkl) reflections used are: (1 1 1), (2 0 0), (2 2 0) and (3 1 1).

[0030] It has surprisingly found that the coated cutting tool of the present invention exhibits enhanced resistance against wear in intermittent cutting, and especially enhanced resistance against thermal cracking, compared to the prior art. The term “cutting tool”, as used herein, includes replaceable cutting tool inserts, indexable cutting tool inserts, but also solid cutting tools.

[0031] The present invention combines the novel multi-layered wear resistant coating structure, comprising a first CVD coating layer of titanium aluminium nitride or carbonitride, Ti.sub.1-uAl.sub.uC.sub.vN.sub.w, followed by a second MT-CVD coating layer of titanium carbonitride, Ti.sub.xC.sub.yN.sub.1-y, with a specifically preferred fiber texture of the second Ti.sub.xC.sub.yN.sub.1-y layer wherein the geometrically equivalent crystallographic planes {1 1 1} are found to be preferentially oriented parallel to the substrate, expressed herein as the texture coefficient TC (1 1 1).

[0032] According to the most preferred embodiment of the present invention, the first coating layer and the second coating layer are deposited immediately on top of each other, i. e. without any intermediate layer. However, the scope of the invention shall also include those embodiments comprising a thin isomorphous intermediate layer present between the first coating layer and the second coating layer, provided that the epitaxy and the remaining properties of the layer sequence are not substantially altered by such an intermediate layer. As an example, an intermediate layer could be a 5 to 30 nm thin TiN or TiC layer.

[0033] In the prior art, CVD coatings comprising both, a coating layer of titanium aluminium nitride or carbonitride as well as a coating layer of titanium carbonitride are known, even though such combinations wherein such coating layers are in direct contact with each other are not found very frequently. However, the prior art discloses only coating sequences having a coating layer of titanium carbonitride followed by a coating layer of titanium aluminium nitride or carbonitride. Since titanium aluminium nitride or carbonitride coatings are known to have superior oxidation resistance over titanium carbonitride coatings, coating sequences with the aluminium-containig layer as the outer layer are considered advantageous. There is no disclosure in the prior art of the opposite coating sequence according to the present invention, with the CVD coating layer of titanium aluminium nitride or carbonitride followed by the MT-CVD coating layer of titanium carbonitride. And, most prior art titanium carbonitride coating layers have preferred growth orientations or fiber textures, respectively, other than the second Ti.sub.xC.sub.yN.sub.1-y layer of the present invention. Thus, there is no disclosure in the prior art and it was very surprising that such a coating sequence in combination with the preferred growth orientation of the second Ti.sub.xC.sub.yN.sub.1-y layer, expressed by the TC (1 1 1), would have superior properties in respect of resistance against wear in intermittent cutting and enhanced resistance against thermal cracking.

[0034] In a preferred embodiment of the coated cutting tool of the present invention the second Ti.sub.xC.sub.yN.sub.1-y coating layer has a thickness L and an average grain diameter W, and the ratio L/W<8, preferably L/W<5.

[0035] It has surprisingly found that, if the ratio of layer thickness to average grain diameter, L/W, of the second Ti.sub.xC.sub.yN.sub.1-y coating layer is less than 8, the wear resistance in cutting, especially in milling operations, is significantly improved over coatings with Ti.sub.xC.sub.yN.sub.1-y coating layers of the prior art having higher a ratio of layer thickness to average grain diameter. The low L/W ratios according to this invention are obtained by applying typical growth conditions for MT-TiCN as given in the working examples below immediately on top the first Ti.sub.1-uAl.sub.uC.sub.vN.sub.w coating layer. Surprisingly, the growth direction of the grain boundaries in the second Ti.sub.xC.sub.yN.sub.1-y coating layer is altered with respect to conventional coatings, leading to a grain broadening and thus formation of grains with a smaller L/W ratio. Application of the same growth conditions on coating schemes according to the prior art leads to a grain boundary growth direction more strictly directed towards the surface normal, and thus narrower columnar grains with L/W ratios>8 are obtained.

[0036] In another preferred embodiment of the coated cutting tool of the present invention the grains of the second Ti.sub.xC.sub.yN.sub.1-y coating layer have an average grain diameter W of ≧0.4 μm, preferably ≧0.7 μm, more preferably of a ≧1.1 μm.

[0037] It has surprisingly found that, if the average grain diameter W of the grains of the second Ti.sub.xC.sub.yN.sub.1-y coating layer is 0.4 μm or more, the wear resistance in cutting, especially in milling operations, is significantly improved over coatings with Ti.sub.xC.sub.yN.sub.1-y coating layers of the prior art having grains of smaller grain diameters. This surprising effect may be related with the smaller number of grain boundaries per surface area found in the Ti.sub.xC.sub.yN.sub.1-y coating layers according to this invention compared to prior art coatings. Apparently, in the Ti.sub.xC.sub.yN.sub.1-y coating layer wear and fracture are initiated at grain boundaries during cutting operations, due to mechanical weakness and/or diffusion of elements from the work piece material into the coating. At a thickness of the second Ti.sub.xC.sub.yN.sub.1-y coating layer of more than 3.5 μm, the coating process according to this invention produces grain diameters a ≧0.7 μm. Slower deposition rates were observed to favour growth of even broader grains having an average diameter of ≧1.1 μm, which were found to have even better properties in metal cutting.

[0038] In another preferred embodiment of the coated cutting tool of the present invention the first Ti.sub.1-uAl.sub.uC.sub.vN.sub.w coating layer has a columnar grain morphology and the overall fiber texture of the first Ti.sub.1-uAl.sub.uC.sub.vN.sub.w coating layer is characterized in that the maximum intensity of diffraction from the {111} crystallographic planes, as determined by X-ray diffraction (XRD) pole figure measurements or EBSD measurement, occurs within a tilt angle from a normal to the sample substrate surface of α=±20°, preferably α=±10°, more preferably α=±5°, even more preferably α=±1°.

[0039] At tilt angles of the maximum intensity of diffraction from the {111} crystallographic planes of the first Ti.sub.1-uAl.sub.uC.sub.vN.sub.w coating layer from a normal to the sample substrate surface greater than 20°, the second Ti.sub.xC.sub.yN.sub.1-y coating layer is found to have a less pronounced columnar microstructure, and an unfavourable grain boundary orientation having high relative amounts of Σ3 boundaries. Furthermore, Ti.sub.1-uAl.sub.uC.sub.vN.sub.w coatings with {111} crystallographic texture, deposited by CVD, exhibit superiour wear resistance compared to Ti.sub.1-uAl.sub.uC.sub.vN.sub.w coatings having other texture. Taking the contribution of the wear properties of the first Ti.sub.1-uAl.sub.uC.sub.vN.sub.w coating layer to the overall performance of the cutting tool into account, the wear resistance will therefore be insufficient if the maximum intensity of diffraction from the {111} crystallographic planes occurs within a tilt angle from a normal to the surface of greater than 20°, good within a tilt angle of α=±20°, excellent within a tilt angle of α=±10°, superior within a tilt angle of α=±5°, and optimum within a tilt angle of α=±1°.

[0040] In another preferred embodiment of the coated cutting tool of the present invention the length of Σ3-type grain boundaries in the second Ti.sub.xC.sub.yN.sub.1-y coating layer is less than 60%, preferably less than 40%, more preferably less than 30% of the total length of the sum of grain boundaries of ΣN-type with N=2n+1, 1≦n≦28, (=Σ3-49-type grain boundaries), the grain boundary character distribution being measured by EBSD.

[0041] Even though it has been described in the literature that Ti.sub.xC.sub.yN.sub.1-y coatings having high relative amounts of Σ3 grain boundaries show superior wear resistance, these reports are limited to coating layer schemes with Ti.sub.xC.sub.yN.sub.1-y as a lower coating layer, and usually an upper functional layer of alumina. It has now surprisingly been found by the inventors of the present invention that within the coating architecture according to this invention with Ti.sub.xC.sub.yN.sub.1-y as a second coating layer Ti.sub.xC.sub.yN.sub.1-y with a relatively low fraction of Σ3 grain boundary length showed superior results. Even though the mechanism is not yet understood, the inventors found that the coatings produced according to this invention have less than 60% Σ3 grain boundary length of the total Σ3-49 grain boundary length. Coatings with Σ3 length fractions higher than 60% will show poor resistance against thermal cracking. Moreover, the inventors found that coatings with less than 40% Σ3 length fraction exhibit excellent wear behaviour, and coatings with less than 30% Σ3 length fraction show even less thermal cracks.

[0042] In another preferred embodiment of the coated cutting tool of the present invention the overall fiber texture of the second Ti.sub.xC.sub.yN.sub.1-y coating layer is characterized by a texture coefficient TC (1 1 1)>3.0, preferably TC (1 1 1)>3.75.

[0043] With Ti.sub.xC.sub.yN.sub.1-y coating layers having a TC (1 1 1)>3, the tools showed even less thermal cracks in milling, and in addition, also less flaking of the coating from the cutting edge. When the Ti.sub.xC.sub.yN.sub.1-y coating layer has an even higher texture coefficient TC (1 1 1)>3.75, even less flaking and thermal cracking is observed.

[0044] In another preferred embodiment of the coated cutting tool of the present invention the crystals of the first Ti.sub.1-uAl.sub.uC.sub.vN.sub.w coating layer and the crystals of the second Ti.sub.xC.sub.yN.sub.1-y coating layer have isomorphic crystal structures, preferably face-centered cubic (fcc) crystal structures.

[0045] Compared to coatings with the second Ti.sub.xC.sub.yN.sub.1-y coating layer grown on a first layer with non-isomorphic structure, such as a Ti.sub.1-uAl.sub.uC.sub.vN.sub.w composite coating layer comprising hexagonal AlN, coatings having isomorphic fcc crystal structures in both coating layers exhibit better adhesion of the second coating layer on the first coating layer.

[0046] In another preferred embodiment of the coated cutting tool of the present invention the multi-layered wear resistant coating comprises at least one further refractory layer between the substrate surface and the first Ti.sub.1-uAl.sub.uC.sub.vN.sub.w coating layer, such at least one further refractory layer being selected from carbides, nitrides, carbonitrides, oxycarbonitrides and borocarbonitrides of one or more of Ti, Al, Zr, V and Hf, or combinations thereof, and being deposited by chemical vapour deposition (CVD) or moderate temperature chemical vapour deposition (MT-CVD), preferably the at least one further refractory layer comprises or consists of a TiN layer.

[0047] It is particularly preferred to apply an CVD TiN adhesion layer of a thickness of about 0.3 to 1.5 μm immediately onto the substrate surface, followed by the first Ti.sub.1-uAl.sub.uC.sub.vN.sub.w coating layer and the second Ti.sub.xC.sub.yN.sub.1-y coating layer.

[0048] In another preferred embodiment of the coated cutting tool of the present invention the first coating layer consists of titanium aluminium nitride or carbonitride Ti.sub.1-uAl.sub.uC.sub.vN.sub.w, with 0.6≦u≦1.0, 0≦v≦0.1 and 0.7≦w≦1.15, preferably with 0.8≦u≦1.0, 0≦v≦0.05 and 0.7≦w≦1.15.

[0049] Is has been found that at aluminium contents of u≧0.6 a more pronounced (111) preferential orientation of the second Ti.sub.xC.sub.yN.sub.1-y coating layer is obtained. Titanium aluminium carbonitride coating layers with v>0 are preferred to contain carbon in amorphous state within a composite structure purely, or more preferably contain carbon as constituent of the fcc-Ti.sub.1-uAl.sub.uC.sub.vN.sub.w. At a carbon content of v>0.1 there is a risk of carbon being formed as graphite, which leads to mechanical weakening of the coating, and at a carbon content of y>0.05 carbon may not be completely incorporated into the fcc-Ti.sub.1-uAl.sub.uC.sub.vN.sub.w, but the coating may have a composite structure including amorphous carbon, which may result in a reduced toughness behaviour of the coating.

[0050] Definitions and Methods

[0051] Fiber Texture and Texture Coefficient TC

[0052] The term “fiber texture”, as used herein and as it is generally used in connection with thin films produced by vapor deposition, distinguishes the orientation of the grown grains from random orientation. Three types of textures are usually distinguished in thin films and coatings: (i) random texture, when grains have no preferred orientation; (ii) fiber texture, where the grains in the coating are oriented such that one set of geometrically equivalent crystallographic planes {h k l} is found to be preferentially oriented parallel to the substrate, while there is a rotational degree of freedom of the grains around the fiber axis which is perpendicular to this plane, and thus preferentially orientated perpendicular to the substrate; and (iii) epitaxial alignment (or in-plane texture) on single-crystal substrates, where an in-plane alignment fixes all three axes of the grain with respect to the substrate.

[0053] The crystallographic plane of a crystal is defined by the Miller indices, h, k, l. A means to express preferred growth, i. e. that one set of geometrically equivalent crystallographic planes {h k l} is found to be preferentially oriented parallel to the substrate, is the texture coefficient TC (h k l) calculated using the Harris formula on the basis of a defined set of XRD reflections measured on the respective sample. The intensities of the XRD reflections are standardized using a JCPDF-card indicating the intensities of the XRD reflections of the same material, e. g. TiCN, but with random orientation, such as in a powder of the material. A texture coefficient TC (h k l)>1 of a layer of crystalline material is an indication that the grains of the crystalline material are oriented with their {h k l} crystallographic plane parallel to the substrate surface more frequently than in a random distribution, at least compared to the XRD reflections used in the Harris formula to determine the texture coefficient TC.

[0054] X-Ray Diffraction (XRD) Measurements

[0055] X-ray diffraction measurements were done on a XRD3003 PTS diffractometer of GE Sensing and Inspection Technologies using CuKα-radiation. The X-ray tube was run in point focus at 40 kV and 40 mA. 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 defined in such manner that a spill over of the X-ray beam over the coated face of the sample is avoided. On the secondary side a Soller slit with a divergence of 0.4° and a 25 μm thick NiKβ filter were used. Symmetrical θ-2θ scans within the angle range of 20°<2θ<100° with increments of 0.04° and 1 second counting time have been conducted. On the XRD raw data intensity corrections for thin film absorption were applied to all samples which take into account the limited thickness of the layer in contrast to the natural penetration depth in a bulk material. Furthermore an absorption correction was applied for samples where an additional layer was deposited above the MT-TiCN layer for which the TCs were calculated. See equations below:

[00002]$I_{\mathrm{corr}}^{\mathrm{TF}}=\frac{I_0}{1-\exp \left(-2.Math.\mu .Math..Math.S.Math.\text{/}.Math.\sin .Math..Math.\theta \right)}$$I_{\mathrm{corr}}^{\mathrm{Abs}}=\frac{I_0}{\exp \left(-2.Math.\mu .Math..Math.S.Math.\text{/}.Math.\sin .Math..Math.\theta \right)}$

[0056] In the equations S is the thickness of the layer in which the TCs are going to be analysed or the thickness of an absorbing top layer respectively. Finally Kα.sub.2 stripping (Rachinger method), back-ground subtraction and a parabolic peakfit with 5 measuring points were applied. For the calculation of the texture coefficients TC of the MT-TiCN layer a formalism proposed by Harris [Harris, G. B., Philosophical Magazine Series 7, 43/336, 1952, pp. 113-123] was applied. Herein the corrected net peak intensities l.sub.corr were correlated to the relative intensities l.sub.pdf taken from PDF-card 42-1489.

[00003]${\mathrm{TC}}^{\left(\mathrm{hkl}\right)}=\frac{I_{\mathrm{corr}}^{\left(\mathrm{hkl}\right)}}{I_{\mathrm{pdf}}^{\left(\mathrm{hkl}\right)}}.Math.{\left(\frac{1}{n}.Math.\underset{1}{\overset{n}{.Math.}}.Math.\frac{I_{\mathrm{corr}}^{\left(\mathrm{hkl}\right)}}{I_{\mathrm{pdf}}^{\left(\mathrm{hkl}\right)}}\right)}^{-1}$

[0057] Sample Preparation for Scanning Electron Microscopy (SEM)

[0058] Inserts were cut in cross section, mounted in a holder and then treated as follows: [0059] 1. Grinding with Struers Plano220 disc with water for 6 min [0060] 2. Polishing with 9 μm MD-Largo Diamond suspension for 3 min [0061] 3. Polishing with 3 μm MD-Dac Diamond suspension for 3:40 min [0062] 4. Polishing with 1 μm MD-Nap Diamond suspension for 2 min [0063] 5. Polishing/etching with OP-S colloidal silica suspension for at least 12 min (average grain size of the colloidal silica=0.04 μm)

[0064] The specimens were ultrasonically cleaned before SEM examination.

[0065] CVD Coatings

[0066] The CVD coatings were prepared in a radial flow reactor, type Bernex BPX 325S, having 1250 mm height and 325 mm diameter.

[0067] EBSD and E-type Grain Boundaries

[0068] Grain boundaries have a significant influence on material properties such as grain growth, creep, diffusion, electrical, optical and last but not least on mechanical properties. Important properties to be considered are e.g. the density of grain boundaries in the material, the chemical composition of the interface and the crystallographic texture, i.e. the grain boundary plane orientations and grain misorientation. Thereby, the coincidence site lattice (CSL) grain boundaries play an important role. CSL grain boundaries are characterized by the multiplicity index Σ, which is defined as the ratio between the crystal lattice site density of the two grains meeting at the grain boundaries and the density of sites that coincide when superimposing both crystal lattices. For simple structures, it is generally admitted that grain boundaries with low Σ values have a tendency for low interfacial energy and special properties. Thus, the control of the proportion of special grain boundaries and of the distribution of grain misorientations inferred from the CSL model can be considered to be important to the properties of ceramics and a way to enhance these properties.

[0069] In recent years, a scanning electron microscope (SEM)-based technique known as electron backscatter diffraction (EBSD) has emerged and has been used to study grain boundaries in ceramic materials. The EBSD technique is based on automatic analysis of Kikuchi-type diffraction patterns generated by backscattered electrons. A review of the method is provided by: D. J. Prior, A. P. Boyle, F. Brenker, M. C. Cheadle, A. Day, G. Lopez, L. Peruzzo, G. J. Potts, S. M. Reddy, R. Spiess, N. E. Timms, P. W. Trimby, J. Wheeler, L. Zetterström, The application of electron backscatter diffraction and orientation contrast imaging in the SEM to textural problems in rocks, Am. Mineral. 84 (1999) 1741-1759. For each grain of the material to be studied, the crystallographic orientation is determined after indexing of the corresponding diffraction pattern. Available commercial software makes the texture analyses as well as determination of grain boundary character distribution (GBCD) relatively uncomplicated by using EBSD. Application of EBSD to interfaces has allowed the misorientation of grain boundaries to be characterized for large sample populations of boundaries. Typically the misorientation distribution has been linked to the processing conditions of a material. The grain boundary misorientation is achieved via usual orientation parameters such as, the Euler angles, angle/axis pair, or Rodriquez vector. The CSL model is used widely as characterization tool. Over the last decade, a research area known as Grain Boundary Engineering (GBE) has emerged. GBE aims to enhance crystallography of the grain boundaries by developing improved process conditions and, in such way, to achieve better materials. EBSD has recently been used to characterize hard coatings, for reference see, H. Chien, Z. Ban, P. Prichard, Y. Liu, G. S. Rohrer, “Influence of Microstructure on Residual Thermal Stresses in TiC.sub.xN.sub.1-x and alpha-Al.sub.2O.sub.3 Coatings on WC-Co Tool Inserts,” Proceedings of the 17th Plansee Seminar 2009 (Editors: L. S. Sigl, P. Rodhammer, H. Wildner, Plansee Group, Austria) Vol. 2, HM 42/1-11.

[0070] For the preparation of the samples for EBSD measurement, the coating surfaces of the samples were polished subsequently using slurries of diamond having average grain sizes of 3 μm and 1 μm, respectively. Then, the samples were polished using colloidal silica having an average grain size of 0.04 μm. The last polishing step was done manually, and polishing time was increased stepwise until the sample quality was good enough to perform the EBSD maps, i.e. indexing of EBSD patterns would be accomplished with an average confidence index (CI)>0.2 at typical scan rates of 50-100 frames per second. The precise preparation conditions will depend on the individual sample and equipment, and can easily be determined by a person skilled in the art. The polishing removed typically between 0.5 μm and 2 μm of the outer MT-Ti.sub.xC.sub.yN.sub.1-y layer, as determined by measurement of calotte sections prior to and after the preparation, so that the remaining thickness of the second Ti.sub.xC.sub.yN.sub.1-y coating layer was between 50% and 90% of the initial layer thickness. The information depth of the electron diffraction patterns is small (on the order of a few tens of nanometers) compared to the remaining layer thickness. Care was taken to ensure that the polished surfaces were smooth and parallel to the original coating surface. Finally, the samples were ultrasonically cleaned before EBSD examination.

[0071] After cleaning the polished surfaces were analysed by SEM (Zeiss Supra 40 VP) equipped with EBSD (EDAX Digiview). The EBSD data were collected sequentially by positioning the focused electron beam on measurement points forming a hexagonal grid, using a sufficiently small step size. The normal of the sample surface was tilted 70° to the incident beam, and analysis was carried out at 15 kV. High current mode was applied together with 60 μm or 120 μm apertures. Acquisitions were made on polished surfaces with the step size of the measurement grid being chosen at least 5 times smaller than the average grain width as roughly estimated from the SEM images acquired prior to the measurement, assuring that an average of ≧25 data points per grain will be obtained. From this preliminary estimation of grain size, the surface area covered by the EBSD map was defined large enough to comprise at least 10000 grains, so that sufficient grain statistics for the evaluation of texture and misorientation is assured.

[0072] For noise reduction, a grain CI standardization with grain tolerance angle 5° and minimum grain size of 5 or 10 measurement points depending on the grain size, followed by grain dilatation, was applied as a clean-up procedure. The number of grains in the map after clean-up was well above 10000 in all cases.

[0073] For the classification of coincidence site lattice (CSL) boundaries (Σ grain boundaries), the angle tolerance Δ used corresponded to the Brandon criterion Δ=K/Σ.sup.n (K=15, n=0.5). The fraction of CSL boundaries of ΣN-type with N=2n+1, 1≦n≦28 (=Σ3-49-type grain boundaries) was thus determined.

[0074] Measurement of Ti.sub.xC.sub.yN.sub.1-y Coating Layer Thickness L and Average Grain Diameter W

[0075] For the purpose of the present invention, the layer thickness L of the Ti.sub.xC.sub.yN.sub.1-y coating layer was measured on light microscopical or electron microscopical images of a calotte section or polished cross section of the coating layer. The average grain diameter W was obtained on a planar polished sample by EBSD measurement according to the procedure and definitions given above. The remaining layer thickness after polishing was between 50% and 90% of the initial layer thickness, i.e. the average grain diameter W was measured at a height of 50 to 90% of the initial layer thickness.

[0076] Inspection of polished cross sections of the Ti.sub.xC.sub.yN.sub.1-y coating layers according to this invention in the SEM showed a columnar microstructure. It can be assumed that basically all of the columnar grains protruding to the outer surface of the layer were nucleated on the interface between the first Ti.sub.1-uAl.sub.uC.sub.vN.sub.w coating layer and the second Ti.sub.xC.sub.yN.sub.1-y coating layer. Accordingly, the layer thickness corresponds approximately to the grain lengths of the Ti.sub.xC.sub.yN.sub.1-y coating layer.

[0077] Isomorphic Crystal Structures

[0078] For the purpose of the present invention, the term “isomorphic crystal structures” means that the crystals belong to the same space group, even though the unit cell dimensions may be different due to different sizes of the involved atoms present in the crystals of different chemical compositions. As an example according to the present invention, Ti.sub.1-uAl.sub.uC.sub.vN.sub.w crystals and Ti.sub.xC.sub.yN.sub.1-y crystals may have isomorphic crystal structures, such as face-centred cubic (fcc) crystal structures.

EXAMPLES

Example 1

Sample Preparation and Analysis

[0079] Cemented carbide cutting tool substrate bodies (composition: 90.5 wt-% WC, 1.5 wt-% TaC/NbC and 8.0 wt-% Co; geometry: SEHW1204AFN) were placed on charging trays and coated in a radial flow CVD reactor, type Bernex BPX 325S, having 1250 mm height and 325 mm diameter.

[0080] The experimental conditions for the deposition of the coatings according to the present invention (coatings 1 and 2) and for the comparative example (coating 3) are shown in table 1. All coatings according to the present invention and in the comparative example were started with a thin TiN adhesion layer. The first Ti.sub.1-uAl.sub.uN.sub.w coating layer and the second Ti.sub.xC.sub.yN.sub.1-y coating layer according to the invention were deposited directly on top of each other without any interlayers or nucleation steps.

[0081] X-Ray Diffraction (XRD) Measurements and Texture Coefficients

[0082] The outermost MT-Ti.sub.xC.sub.yN.sub.1-y layers of the coatings were analysed by XRD, and the texture coefficients of the (h k l) reflections (1 1 1), (2 0 0), (2 2 0) and (3 1 1) of TiCN were determined, as described herein. A thin film correction was applied to the XRD raw data. The results are shown in table 2.

[0083] EDS Analysis of Elemental Compositions

[0084] The elemental compositions of the Ti.sub.1-uAl.sub.uC.sub.vN.sub.w and Ti.sub.xC.sub.yN.sub.1-y layers in the coatings were determined by EDS and from the XRD peak positions by application of Vegard's law using the JCPDF cards No. 32-1383 for TIC and 38-1420 for TiN, respectively. The results are shown in table 3. The experimental error is estimated to ±3 at-%.

TABLE-US-00001 TABLE 1 Experimental conditions for coatings Layer Coating Gas concentrations Coating Layer Thickness Time Pressure Temperature [Vol.-%] No. Sequence [μm] [min] [kPa] [° C.] H.sub.2 N.sub.2 TiCl.sub.4 AlCl.sub.3 CH.sub.3CN NH.sub.3 1 TiN 0.4 90 15 850 55.0 44.0 1.03 0 0 0 (Inv.) Ti.sub.1−uAl.sub.uC.sub.vN.sub.w 8.0 150 0.35 710 98.7 0 0.03 0.23 0 1.1 MT-Ti.sub.xC.sub.yN.sub.1−y 4.5 120 7.5 800 to 835 86.9 10.9 1.77 0 0.47 0 (ramp) 2 TiN 0.4 90 15 850 55.0 44.0 1.03 0 0 0 (Inv.) Ti.sub.1−uAl.sub.uC.sub.vN.sub.w 2.5 75 0.35 675 98.7 0 0.03 0.23 0 1.1 MT-Ti.sub.xC.sub.yN.sub.1−y 2.8 90 7.5 823 to 850 86.9 10.9 1.77 0 0.47 0 (ramp) 3 TiN 0.5 55 16 900 59.8 39.1 1.1 0 0 0 (Comp.) MT-Ti.sub.xC.sub.yN.sub.1−y* 5.0 26 6 870 55.2 41.4 2.7 0 0.7 0 66 9 85.0 12.8 1.7 0 0.5 0 *deposited under 2 consecutive deposition conditions

TABLE-US-00002 TABLE 2 Texture coefficients of the outermost MT- Ti.sub.xC.sub.yN.sub.1−y layers of the coatings Texture Coefficient (TC) (h k l) reflection Coating no. 1 Coating no. 2 Coating no. 3 1 1 1 3.85 3.53 1.05 2 0 0 0.03 0.01 0.53 2 2 0 0.01 0.04 1.46 3 1 1 0.11 0.42 1.06

TABLE-US-00003 TABLE 3 Elemental compositions of Ti.sub.1−uAl.sub.uC.sub.vN.sub.w and Ti.sub.xC.sub.yN.sub.1−y layers Ti.sub.1−uAl.sub.uC.sub.vN.sub.w Ti.sub.xC.sub.yN.sub.1−y Coating no. 1 Ti.sub.0.13Al.sub.0.87C.sub.0N.sub.1.13 TiC.sub.0.56N.sub.0.44 Coating no. 2 Ti.sub.0.16Al.sub.0.84C.sub.0N.sub.1 TiC.sub.0.57N.sub.0.43 Coating no. 3 — TiC.sub.0.55N.sub.0.45

[0085] EBSD Analysis

[0086] Table 4 shows details about the EBSD measurements and data processing and results.

[0087] For the classification of coincidence site lattice (CSL) boundaries (E grain boundaries), the angle tolerance Δ used corresponded to the Brandon criterion Δ=K/Σ.sup.n (K=15, n=0.5). The fraction of CSL boundaries of ΣN-type with N=2n+1, 1≦n≦28 (=Σ3-49-type grain boundaries) was thus determined.

[0088] The misorientation angle has been evaluated in the range between 5° and 62.8° being limited by the grain tolerance angle used in clean-up and the maximum possible misorientation angle for cubic symmetry, respectively. The distribution of grain boundary misorientations was evaluated by plotting the fraction of grain boundary length over misorientation angle in 50 pitches from 5° to 62.8°, i.e. increments of 1.16°, thus considering only boundaries between identified grains in the distribution. The obtained histograms of measured (correlated) misorientation distribution were compared to the uncorrelated (texture derived) distributions as calculated by the OIM analysis software. For each pitch of misorientation angle, the deviation of the correlated misorientation angle number fraction from the uncorrelated number fraction has been calculated. It has been found that for the Ti.sub.xC.sub.yN.sub.1-y layers according to the invention the deviation is by a factor smaller than 10 for all pitches. In contrast, the measured misorientation angle distribution of the layers according to the prior art shows much more pronounced spikes at 60°, the number fraction being more than 10 times higher than the uncorrelated number fraction, which correspond to a high amount of Σ3 boundaries.

TABLE-US-00004 TABLE 4 EBSD measurement and data processing Coating No. 1 2 3 Thickness of TiCN layer - as-deposited [μm] 4.5 2.8 5.0 Thickness of TiCN layer - after polishing 3.2 1.8 3 for EBSD measurement μm] EBSD mapping and clean-up parameters Map Size [μm × μm] 150 × 150 75 × 75 40 × 40 Step Size [μm] 0.075 0.05 0.05 Number of points 4621155 2600367 739662 Average CI 0.45 0.34 0.23 Minimum Grain Size used for clean-up [pixels] 10 5 5 No. of points corrected by clean-up 85193 110461 100144 fraction of points changed by clean-up 0.018 0.042 0.135 EBSD data after clean-up average grain diameter [μm] 1.16 ± 0.64 0.44 ± 0.25 0.30 ± 0.13 average grain area [μm.sup.2] 1.38 ± 1.49 0.20 ± 0.23 0.084 ± 0.08  layer thickness/average grain diameter 3.9 6.4 16.7 overall fraction of CSL boundaries (Σ3-Σ49)* 0.18 0.27 0.41 fraction of Σ3 boundaries* 0.05 0.10 0.29 ratio (number fraction Σ3/number fraction 0.27 0.39 0.70 all CSL (Σ3-Σ49) boundaries)* MD = maximum deviation of misorientation 2.8 8.1 17.1 from correlated number fraction*/** *only boundaries between identified grains **number fractions of misorientation calculated in 50 pitches from 5° to 62.8,° i.e. increments of 1.16°

[0089] Milling Tests

[0090] The cutting tools inserts with coatings nos. 1, 2 and 3 were examined in the following milling application:

[0091] Workpiece material: Grey cast iron DIN GG25

[0092] Operation: Dry milling

[0093] Feed per tooth: f.sub.Z=0.2 mm

[0094] Depth of cut: a.sub.p=3 mm

[0095] Setting angle: K=45°

[0096] Cutting speed: v.sub.c=283 m/min

[0097] The development of the maximum flank wear, V.sub.Bmax, on the main cutting edge and the number of comb cracks were observed over a milling distance of 4800 mm in 800 mm steps. In table 5, the development of V.sub.Bmax over the milling distance and the number of comb cracks at 4800 mm are shown.

[0098] In the milling test the coating according to the present invention showed a significantly higher resistance against flank wear, as well as a remarkably higher resistance against thermo mechanical shock, as shown by the non-occurrence of comb cracks.

TABLE-US-00005 TABLE 5 Milling Test Results Milling Distance Maximum Flank Wear V.sub.Bmax [mm] Coating No. 1 Coating no. 2 Coating no. 3 0 0 0 0 800 0.02 0.04 0.06 1600 0.04 0.06 0.10 2400 0.04 0.08 0.18 3200 0.06 0.10 0.22 4000 0.08 0.12 0.28 4800 0.12 0.14 0.40 Comb Cracks 0 0 5 after 4800 mm