Wear resistant PVD tool coating containing TiAlN nanolayer films

Abstract

A coated cutting tool and a process for the production thereof is provided. The coated cutting tool includes a substrate and a hard material coating, the substrate being selected from cemented carbide, cermet, ceramics, cubic boron nitride, polycrystalline diamond or high-speed steel. The hard material coating includes a (Ti,Al)N layer stack of alternately stacked (Ti,Al)N sub-layers. The layer stack has an overall atomic ratio of Ti:Al within the (Ti,Al)N layer stack within the range from 0.33:0.67 to 0.67:0.33, a total thickness of the (Ti,Al)N layer stack within the range from 1 μm to 20 μm, each of the individual (Ti,Al)N sub-layers within the (Ti,Al)N layer stack of alternately stacked (Ti,Al)N sub-layers having a thickness within the range from 0.5 nm to 50 nm, each of the individual (Ti,Al)N sub-layers within the (Ti,Al)N layer stack of alternately stacked (Ti,Al)N sub-layers being different in respect of the atomic ratio Ti:Al than an immediately adjacent (Ti,Al)N sub-layer, and other characteristics.

Claims

1. A coated cutting tool consisting of a substrate and a hard material coating, the substrate being selected from cemented carbide, cermet, ceramics, cubic boron nitride, polycrystalline diamond or high-speed steel, wherein the hard material coating comprises: a (Ti,Al)N layer stack of alternately stacked (Ti,Al)N sub-layers, the layer stack having the following characteristics: wherein an overall atomic ratio of Ti:Al within the (Ti,Al)N layer stack is within the range from 0.33:0.67 to 0.67:0.33; a total thickness of the (Ti,Al)N layer stack is within the range from 1 μm to 20 μm; each individual (Ti,Al)N sub-layers within the (Ti,Al)N layer stack of alternately stacked (Ti,Al)N sub-layers has a thickness within the range from 0.5 nm to 50 nm; each of the individual (Ti,Al)N sub-layers within the (Ti,Al)N layer stack of alternately stacked (Ti,Al)N sub-layers being different in respect of the atomic ratio Ti:Al than an immediately adjacent (Ti,Al)N sub-layer; over the thickness of the (Ti,Al)N layer stack perpendicular to the substrate surface the content of Al increases and the content of Ti decreases from an interface of the (Ti,Al)N layer stack arranged in a direction towards the substrate to the interface of the (Ti,Al)N layer stack arranged in a direction towards the outer surface of the coating; over the thickness of the (Ti,Al)N layer stack perpendicular to the substrate surface the residual stress decreases from the interface of the (Ti,Al)N layer stack arranged in the direction towards the substrate to the interface of the (Ti,Al)N layer stack arranged in the direction towards the outer surface of the coating by an amount of at least 150 MPa to at most 900 MPa, whereby the residual stress is measured by X-ray diffraction applying the sin.sup.2Ψ method based on the (2 0 0) reflection; and a residual stress within a portion of a thickness of at least 100 nm to at most 1 μm within the (Ti,Al)N layer stack from the interface of the (Ti,Al)N layer stack arranged in the direction towards the substrate is within the range of from 0 MPa to +450 MPa.

2. The coated cutting tool according to claim 1, wherein the alternately stacked TiAlN sub-layers of the (Ti,Al)N layer stack of the coating are deposited by Arc-PVD, the entire coating being deposited by Arc-PVD.

3. The coated cutting tool according to claim 1, wherein the difference of the of the atomic ratio Ti:Al of each of the individual (Ti,Al)N sub-layers within the (Ti,Al)N layer stack of alternately stacked (Ti,Al)N sub-layers over the atomic ratio Ti:Al of an immediately adjacent (Ti,Al)N sub-layer is within the range from 0.2 to 1.8.

4. The coated cutting tool according to claim 1, wherein the atomic ratio Ti:Al of the individual (Ti,Al)N sub-layers having a lower Ti content than an immediately adjacent (Ti,Al)N sub-layer is within the range from 0.2:0.8 to 0.7:0.3, and/or the atomic ratio Ti:Al of the individual (Ti,Al)N sub-layers having a higher Ti content than an immediately adjacent (Ti,Al)N sub-layer is within the range from 0.3:0.7 to 0.8:0.2.

5. The coated cutting tool according to claim 1, wherein the increase of the Al content and the decrease of the Ti content over the thickness of the (Ti,Al)N layer stack perpendicular to the substrate surface occurs step-wise or gradual.

6. The coated cutting tool according to claim 1, wherein the increase of the Al content and the decrease of the Ti content over the thickness of the (Ti,Al)N layer stack perpendicular to the substrate surface is by an increase of the thicknesses of the individual (Ti,Al)N sub-layers having higher Al contents than the thicknesses of the individual (Ti,Al)N sub-layers having lower Al contents.

7. The coated cutting tool according to claim 1, wherein the (Ti,Al)N layer stack includes two or more (Ti,Al)N sub-layer stacks arranged immediately on top of each other, wherein within a same (Ti,Al)N sub-layer stack there exists a first type of individual (Ti,Al)N sub-layers each of the first type having a same composition with respect to the Ti:Al atomic ratio and a same thickness, and a second type of individual (Ti,Al)N sub-layers each of the second type having a same composition with respect to the Ti:Al atomic ratio and a same thickness, wherein the first and second types of individual (Ti,Al)N sub-layers have different Ti:Al atomic ratios.

8. The coated cutting tool according to claim 7, wherein the (Ti,Al)N layer stack consists of two (Ti,Al)N sub-layer stacks arranged immediately on top of each other.

9. The coated cutting tool according to claim 1, wherein a difference between the absolute amounts of the residual stresses a of the portion of a thickness of at least 100 nm to at most 1 μm within the (Ti,Al)N layer stack from the interface of the (Ti,Al)N layer stack arranged in the direction towards the substrate towards the interface of the (Ti,Al)N layer stack arranged in the direction towards the outer surface and of the material arranged immediately underneath, which is either the surface of the substrate or a hard material layer arranged between the substrate and the (Ti,Al)N layer stack, is ≤400 MPa.

10. The coated cutting tool according to claim 1, wherein an average grain size within the (Ti,Al)N layer stack decreases from the interface of the (Ti,Al)N layer stack arranged in the direction towards the substrate to the interface of the (Ti,Al)N layer stack arranged in the direction towards the outer surface of the coating.

11. The coated cutting tool according to claim 1, wherein the (Ti,Al)N layer stack has <5 vol-% hexagonal crystal structure, measured by XRD.

12. The coated cutting tool according to claim 1, wherein the overall residual stress σ of the (Ti,Al)N layer stack in the as-deposited state is <600 MPa.

13. The coated cutting tool according to claim 1, wherein the (Ti,Al)N layer stack has a Vickers hardness HV0.015≥2800, and/or a reduced Young's modulus >350 GPa.

14. The coated cutting tool according to claim 1, further comprising one or more further hard material layers on top of the (Ti,Al)N layer stack and/or between the substrate and the (Ti,Al)N layer stack, the one or more further hard material layers containing one or more of the elements selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Al and Si as well as one or more of N, C, 0, and B.

15. A of the coated cutting tool according to claim 1 arranged for the milling of steel.

16. A process for manufacturing the coated cutting tool of claim 1 comprising the steps of depositing a (Ti,Al)N layer stack of alternately stacked (Ti,Al)N sub-layers by means of Arc-PVD (cathodic arc deposition) using at least two different targets, each containing the metals Ti and Al, but having different contents of Ti and Al, wherein the applied arc current per target for the deposition of the (Ti,Al)N layer stack of alternately stacked (Ti,Al)N sub-layers is within the range from 50 to 180 A.

17. The process of claim 16, wherein the Arc-PVD deposition of the (Ti,Al)N layer stack is carried out at a nitrogen pressure in the range from 5 Pa to 15 Pa.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows SEM cross sections of two samples deposited at different pressures of 4 Pa and 10 Pa, respectively.

MATERIALS AND METHODS

(2) PVD Coating

(3) For PVD coatings, as described in the examples herein, a Hauzer HTC1000 (IHI Hauzer Techno Coating B.V., The Netherlands) with a chamber size of 1 m.sup.3 was used applying a circular Arc-PVD technology (CARC+) using constant magnetic field configuration during deposition.

(4) XRD (X-Ray Diffraction)

(5) XRD 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 Ni K.sub.β filter were used. The measurements were carried out over the range of 15 to 80° 2-theta with a step size of 0.03°. Grazing-incidence X-ray diffraction technique under 1° incidence angel was employed to study the crystal structure of the layers.

(6) Residual Stress

(7) The residual stresses were measured by XRD using the sin.sup.2Ψ method (c.f. M. E. Fitzpatrick, A. T. Fry, P. Holdway, F. A. Kandil, J. Shackleton and L. Suominen—A Measurement Good Practice Guide No. 52; “Determination of Residual Stresses by X-ray Diffraction—Issue 2”, 2005).

(8) The side-inclination method (Ψ-geometry) has been used with eight Ψ-angles, equidistant within a selected sin.sup.2Ψ range. An equidistant distribution of Φ-angles within a Φ-sector of 90° is preferred. The measurement was performed on a flank side of the tool, i.e. using an as flat surface as possible. For the calculations of the residual stress values, the Poisson's ratio=0.20 and the Young's modulus E=450 GPa have been applied. The data were evaluated using commercially available software (RayfleX Version 2.503) locating the (2 0 0) reflection by the Pseudo-Voigt-Fit function.

(9) Hardness/Young's Modulus

(10) The measurements of the hardness and the Young's modulus (reduced Young's modulus) were performed by the nanoindentation method on a Fischerscope® HM500 Picodentor (Helmut Fischer GmbH, Sindelfingen, Germany) applying the Oliver and Pharr evaluation algorithm, wherein a diamond test body according to Vickers was pressed into the layer and the force-path curve was recorded during the measurement (maximum load: 15 mN; load/unload time: 20 s; creep time: 5 s). From this curve hardness and (reduced) Young's modulus were calculated. It should be noted that the impression depth should not be more than 10% of the coating thickness, otherwise characteristics of the substrate can falsify the measurements.

(11) Scanning Electron Microscopy (SEM)

(12) The morphology of the coatings was studied by scanning electron microscopy (SEM) using a Supra 40 VP(Carl Zeiss Microscopy GmbH, Jena, Germany). Cross sections were characterized with the SE2 (Everhart-Thornley) Detector.

EXAMPLES

Example 1

Deposition of Coatings According to the Invention and Comparative Coatings

(13) In the following examples of the preparation of cutting tools according to the present invention and of comparative examples cemented carbide cutting tool substrate bodies (composition: 12 wt-% Co, 1.6 wt-% (Ta, Nb)C, balance WC; WC grain size: 1.5 μm; geometry: ADMT160608R-F56) were coated in a PVD system as indicated above. The residual stress at the surface of the substrate, measured prior to heating within the deposition chamber, was +200 MPa, i.e., a low tensile residual stress.

(14) Prior to the deposition, the substrate bodies were pretreated by ultrasonic cleaning in ethanol and plasma cleaning. The PVD reactor was evacuated to 8×10.sup.−5 mbar, and the substrate was pre-treated at 550° C.

(15) For the deposition of the (Ti,Al)N coatings, two types of targets with different atomic ratios of Ti:Al were used to produce alternately stacked (Ti,Al)N sub-layers being different in respect of the atomic ratios Ti:Al: “Ti50Al50” (Ti:Al=50:50) and “Ti33Al67” (Ti:Al=33:67). If reference is herein made to a target of a particular composition, this means that, due to the layout of the used PVD reactor, a line of four targets of the same composition were vertically arranged to allow for a homogeneous deposition throughout the height of the reactor.

(16) The targets had a diameter of 100 mm. The reactive gas for the nitride deposition was N.sub.2. Two types of (Ti,Al)N sub-layer stacks, L1 and L2, were produced. To produce the inventive coating, L1 was deposited immediately on the substrate surface, and L2 was deposited immediately on top of L1. However, to investigate the (Ti,Al)N sub-layer stacks L1 and L2 independent from each other, samples wherein only L1 was deposited immediately on the substrate surface and samples wherein only L2 was deposited immediately on the substrate surface were produced. For the deposition of L1, two targets were used: 1דTi50Al50”+1דTi33Al67”. To achieve a lower Ti content and a higher Al content in L2, for the deposition of L2 three targets were used: 1דTi50Al50”+2דTi33Al67”. The depositions were carried out at different arc currents at the targets, 100 A, 150 A or 200 A arc current per target (source), respectively. The further process parameters for the deposition of different layers are given in the following table 1.

(17) TABLE-US-00001 TABLE 1 Parameter (Ti,Al)N Targets “Ti50Al50” + “Ti33Al67” sub-layer stack Bias 40 V L1 Pressure (N.sub.2) 10 Pa Arc Current/Target 100 A, 150 A or 200 A Rotation Speed 3 rpm Temperature 550° C. (Ti,Al)N Targets “Ti33Al67” + sub-layer stack “Ti50Al50” + “Ti33Al67” L 2 Bias 40 V Arc Current/Target 100 A, 150 A or 200 A Pressure 10 Pa Rotation 3 rpm Temperature 550° C.

Example 2

Residual Stress, Hardness and Young's Modulus

(18) For the (Ti,Al)N sub-layer stacks L1 and L2, deposited according to example 1, residual stress, hardness and Young's modulus were measured, as described above. Since the parameters during the deposition of an individual (Ti,Al)N sub-layer stacks L1 or L2, respectively, were held constant, the residual stress within an individual (Ti,Al)N sub-layer stack was constant, but differing from one individual layer stack to another one due to the different compositions and depending on the applied arc current. As described above, each of the (Ti,Al)N sub-layer stacks L1 and L2, respectively, was deposited immediately on the surface of the substrate. The thicknesses of the layer stacks was about 2-4 μm. The results are shown in the following table 2.

(19) TABLE-US-00002 TABLE 2 (Ti,Al)N Arc Cur- Pres- Resid- sub-layer rent/ sure ual Vickers Young's stacks target (N.sub.2) Stress Hardness Modulus Sample [L1, L2] [A] [Pa] [MPa] [HV] [GPa] S1 L1 100 10 +286 3234 472 S2 L1 150 10 +348 3407 467 S3 L1 200 10 −182 3533 480 S4 L2 100 10 +76 3332 512 S5 L2 150 10 −274 3221 438 S6 L2 200 10 −504 3397 411

(20) The results show that the applied arc current has an influence on the residual stress of the deposited (Ti,Al)N sub-layer stack, which further depends on the composition. Higher arc current produces layer stacks exhibiting less tensile residual stress or more compressive residual stress, respectively. The same tendency applies for higher overall Al contents, as in L2 compared to L1.

(21) The experiments further show that the (Ti,Al)N sub-layer stack L1, which is the one deposited first on the substrate surface, exhibits low tensile residual stress according to the present invention, when deposited at an arc current of 100 A or 150 A (S1 and S2), whereas an arc current of 200 A (S3) results in low compressive stress, which is outside the present invention. The match of (Ti,Al)N sub-layer stacks L1 of samples S1 and S2 to the substrate surface with respect to the residual stress level improves the adhesion of the coating to the substrate and allows for high coating thicknesses without the disadvantage of flaking. This is of special interest for sharp cutting edges (drills, milling tools).

(22) Furthermore, the coatings of the samples of this example were analyzed by XRD, as described above. In the coatings deposited at an arc current of 150 A or 100 A (S1, S2, S4, S5) no hexagonal phase was found. In the coatings deposited at an arc current of 200 A (S3 and S6) small amounts of hexagonal phase were observed in the (Ti,Al)N sub-layer stacks L1 and L2. Further, the deposition rates were lower at lower arc current, 100 A<150 A<200 A.

Example 3

Surface Roughness

(23) To compare the influence of the nitrogen pressure in the deposition process, two (Ti,Al)N sub-layer stacks of the type L2 were deposited at 4 Pa and 10 Pa, respectively, and surface roughness measurements were carried out. The results are shown in the following table 2. SEM cross sections of the samples are shown in FIG. 1. It can clearly be seen that the sample deposited at 4 Pa(FIG. 1A), shows larger droplets than the sample deposited at 10 Pa(FIG. 1B). Accordingly, the sample deposited at 10 Pa exhibits a much smoother surface roughness. It has been observed that, generally, smoother surfaces with less droplets and defects are obtained if higher pressure is applied during the deposition process.

(24) TABLE-US-00003 TABLE 3 (Ti,Al)N Arc Surface Surface sub-layer Current/ Pressure Roughness Roughness stacks Target (N.sub.2) Sa Sq Sample [L1, L2] [A] [Pa] [nm] [nm] S7 L2 150 4 123 181 S8 L2 150 10 64 125

Example 4

Cutting Tests

(25) In order to assess the effect of the coating according to the invention, compared to conventional coatings, with respect to cutting properties, multi-layer coated cutting tools were produced and tested in a milling test. The inventive cutting tool in this example is referred to as sample “HC318”, whereas comparative cutting tool is herein referred to as sample “HC359”. It is to be mentioned that the coating of the comparative cutting tool “HC359” is a commercially very successful conventional multi-layer coating. The difference between the coating of the inventive sample “HC318” and the comparative sample “HC359” is only in the innermost layer immediately on top of the substrate surface, whereby even the innermost layers in both samples were TiAlN layers. All of the remaining layers of the multi-layer coating were the same in both samples.

(26) The cemented carbide substrates were the same as described above in example 1. In each case the multi-layer coating structures consisted of a total of eleven alternatively arranged TiAlN and Al.sub.2O.sub.3 layers.

(27) In the inventive example, “HC318”, the innermost layer deposited immediately on the substrate surface was a 4.3 μm thick (Ti,Al)N coating according to the present invention, consisting of a first 2.3 μm thick (Ti,Al)N sub-layer stack L1 and a second 2.0 μm (Ti,Al)N sub-layer stack L2, prepared as described in example 1 at arc currents of 150A and at 10 Pa for each of L1 and L2.

(28) In the comparative example “HC359” the innermost layer deposited immediately on the substrate surface was a 4.2 μm thick TiAlN layer deposited in a conventional Arc-PVD process, as follows:

(29) Innermost TiAlN Layer of the Comparative Example:

(30) TABLE-US-00004 Parameter TiAlN Target “Ti33Al67” layer Bias 40 V Pressure (N.sub.2) 10 Pa Arc Current/Target 150 A Rotation Speed 3 rpm Temperature 550° C.

(31) The subsequent coating layers on top of the innermost layer of each of the samples “HC318” and “HC359”, respectively, were deposited under the same conditions and with the same thicknesses for each sample, and they consisted of a sequence of four about 0.5-0.6 μm thick Al.sub.2O.sub.3 layers and four about 0.5-0.6 μm thick TiAlN layers, alternatively deposited on top of each other, starting with the Al.sub.2O.sub.3 layer immediately on top of the distinguishing innermost layer. The 0.5-0.6 μm thick TiAlN layers were deposited under the same conditions as the innermost TiAlN layer of the comparative example “HC359” (see table above), and the Al.sub.2O.sub.3 layers were deposited by dual magnetron sputtering at a 20 kW, a total gas pressure of 0.45 Pa, an Ar flow of 500 sccm, an 02 flow of about 125 sccm, at a bias voltage of 125 V, pulsed with 40 kHz and 10 μs off time and 22 A bias current as well as 480 V cathode voltage after hysteresis (at the operating point).

(32) The metal cutting performance of the cutting tool samples “HC318” and “HC359” was tested in a face milling operation using a face milling cutter type F2010.UB.127.Z08.02R681M (according to DIN4000-88) from Walter A G, Tubingen, Germany, on a Heller FH 120-2 machine under the following conditions.

(33) Cutting Conditions: Tooth Feed f.sub.z [mm/tooth]: 0.2 Feed v.sub.f [mm/min]: 120 Spindle speed 600 rpm Cutting speed v.sub.c [m/min]: 235 Cutting depth a.sub.p [mm]: 3 Workpiece material: 42CrMo4; tensile strength Rm: 820 N/mm.sup.2

(34) The following table 4 shows the results of the cutting tests, wherein V.sub.B is the minimum wear at the flank faces of the tool, V.sub.Bmax is the maximum wear, i.e. the deepest crater observed on the flank face of a tool, and V.sub.R is the wear at the cutting edge radius.

(35) TABLE-US-00005 TABLE 4 Cutting V.sub.b V.sub.bmax V.sub.R length Sample ID [mm] [mm] [mm] [mm] “HC318” 0.03 0.10 0.12 4800 “HC359” 0.07 0.25 0.30 4800

(36) The results clearly show that the incorporation of the inventive layer according to the invention into the coating structure shows a significant reduction of wear, both at the flank face and the cutting edge radius, in comparison to the very similar layer sequence of the comparative sample being distinguished only in respect of the innermost coating layer.

Example 5

Influence of Post Treatment Operations

(37) The sequence of (Ti,Al)N sub-layer stacks L1+L2 was produced as in the inventive example, “HC318” of example 4 and post treated by shot peening. The residual stress at the coating surface, the Vickers hardness and the Young's modulus were measured, as described above, and were compared to the as-deposited coating. The results are shown in table 5 below.

(38) Shot Peening Parameters:

(39) TABLE-US-00006 Blasting pressure 5.3 bar Blasting Angle 90° Blasting Distance 10 cm Blasting Material ZrO.sub.2 balls (diameter 70-120 μm) Blasting time 10 sec

(40) TABLE-US-00007 TABLE 5 Vickers Young's Residual Stress Hardness Modulus Sample [MPa] [HV] [GPa] post treated −350 3334 432 as deposited −150 3238 414

(41) The post treatment process increases the compressive residual stress at the coating surface. Furthermore, a slight increase in hardness and Young's modulus was measured on the post-treated sample.