PVD COATED CEMENTED CARBIDE CUTTING TOOL WITH IMPROVED COATING ADHESION

Abstract

A coated cutting tool includes a substrate of cemented carbide, cubic boron nitride (cBN) or cermet containing tungsten carbide hard grains and a tungsten carbide (WC) layer deposited immediately on top of the substrate surface. The tungsten carbide (WC) layer is a mixture or combination of hexagonal tungsten mono-carbide α-WC phase and cubic tungsten mono-carbide β-WC phase and unavoidable impurities.

Claims

1. A coated cutting tool comprising: substrate of cemented carbide, cermet containing tungsten carbide hard grains or cubic boron nitride; and tungsten carbide layer deposited immediately on top of the substrate surface, wherein the tungsten carbide layer consists of a mixture or combination of hexagonal tungsten mono-carbide α-WC phase and cubic tungsten mono-carbide β-WC phase and unavoidable impurities.

2. The coated cutting tool of claim 1, wherein the tungsten carbide layer has a thickness of from 1 nm to 5 μm.

3. The coated cutting tool of claim 1, wherein a single-layer or multi-layer hard material coating is deposited immediately on top of the tungsten carbide layer, wherein the hard material coating includes at least one layer of hard material selected from the group consisting of nitrides, carbides, oxides, borides and/or solid solutions thereof of one or more of the elements selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Al and Si.

4. The coated cutting tool of claim 1, wherein the substrate is a cemented carbide consisting of from 3 to 30 wt % of a binder phase of Co, Fe and/or Ni, 0 to 20 wt-% of cubic carbides, nitrides and/or carbonitrides of group IV, V and/or VI transition metals and rest tungsten carbide hard material grains.

5. The coated cutting tool of claim 1, wherein the tungsten carbide layer is deposited by a PVD method selected from HIPIMS and DMS.

6. The coated cutting tool of claim 1, wherein an amount of hexagonal tungsten mono-carbide α-WC phase within the tungsten carbide layer decreases and an amount of cubic tungsten mono-carbide β-WC phase increases from an interface at a surface of the substrate towards an outer surface of the tungsten carbide (WC) layer, whereby a change of phase amounts is gradual or stepwise.

7. The coated cutting tool of claim 1, wherein the tungsten carbide layer has a Vickers hardness HV0.015≥2500 and/or a reduced Young's modulus>450 GPa.

8. The coated cutting tool of claim 1, wherein there is a coherent transition from tungsten carbide grains exposed at a substrate surface to the tungsten carbide layer deposited immediately on top of the substrate surface, as observed by SEM.

9. A process for manufacturing a coated cutting tool according to claim 1, wherein the tungsten carbide layer immediately on top of the substrate is deposited by HIPIMS or DMS using a reaction gas composition comprising or consisting of argon and a carbon source gas, wherein the carbon source gas is provided at a partial pressure within the range from at least 4×10.sup.−5 mbar to at most 2.0×10.sup.−4 mbar, and wherein the bias voltage is within the range from 80 to 250 V.

10. The process according to claim 9, wherein the deposition of the tungsten carbide layer immediately on top of the substrate is carried out at a power density at the magnetron is from 2 to 25 W/cm.sup.2.

11. The process according to claim 9, wherein the deposition of the tungsten carbide layer immediately on top of the substrate is carried out at a pulse length of from 5 to 5000 μs.

12. The process according to claim 9, wherein the deposition of the tungsten carbide layer immediately on top of the substrate is carried out at an average pulse current of from 250 to 1000 A.

13. The process according to claim 9, wherein the deposition of the tungsten carbide layer immediately on top of the substrate is carried out at an average pulse power of from 100 kW to 2 MW.

14. The process according to claim 9, wherein the deposition of the tungsten carbide layer immediately on top of the substrate is carried out at a temperature in the range from 200 to 600° C.

15. The coated cutting tool of claim 3, wherein hard material coating includes two or more layers of hard material.

16. The coated cutting tool of claim 3, wherein the hard material is selected from the group consisting of TiN, TiC, TiAlN, TiAlC, TiAlCN, a Al.sub.2O.sub.3, γ Al.sub.2O.sub.3.

17. The coated cutting tool of claim 4, wherein the binder phase is Co.

18. The process according to claim 9, wherein the carbon source gas is C.sub.2H.sub.2.

Description

DESCRIPTION OF THE FIGURES

[0062] FIG. 1 shows a SEM cross-section at 30.000× magnification of a cemented carbide substrate with a 93 nm thick inventive tungsten carbide (WC) layer deposited thereon (sample no. 190117005).

[0063] The line shows the interface between the substrate surface and the tungsten carbide (WC) layer. The circle marks a substrate WC grain with coherent growth of WC of the tungsten carbide layer thereon.

[0064] FIG. 2 shows a SEM cross-section at 30.000× magnification of a cemented carbide substrate with an inventive tungsten carbide (WC) layer deposited thereon (sample no. 190118001).

[0065] The line shows the interface between the substrate surface and the tungsten carbide (WC) layer and marks through the length of the line coherent growth transitions from substrate WC into the tungsten carbide layer.

[0066] FIG. 3 shows an XRD of the inventive tungsten carbide (WC) layer on cemented carbide substrate (sample no. 190117005), as shown in FIG. 1. The diffraction peaks are indexed according to JCPDS cards JCPDS 025-1047 (hexagonal α-WC) and JCPDS 020-1316 (cubic β-WC).

[0067] FIG. 4 shows an XRD of the inventive tungsten carbide (WC) layer on cemented carbide substrate (sample no. 190118001), as shown in FIG. 2. The diffraction peaks are indexed as in FIG. 3.

[0068] FIG. 5 shows a SEM cross section at 10.000× magnification of a cemented carbide substrate with a 30 nm thick inventive tungsten carbide (WC) layer (not visible at this magnification) and a multi-layer hard material coating deposited thereon. The coating sequence and the layer thicknesses are as follows:

TABLE-US-00001 1. 30 nm inventive WC layer 2. 3.2 μm TiAlN layer 3. 179 nm Al.sub.2O.sub.3 layer 4. 280 nm 2 times alternating layers of TiAlN/Al.sub.2O.sub.3 5. 245 nm TiAlN layer 6. 335 nm ZrN top layer

[0069] The deposition parameters for the inventive WC layer (1) were the same as for sample no. 190118001 described below. The TiAlN layer (2) was deposited as described below in the cutting test example 2 for the layer stack L1+L2.

[0070] For the deposition of the Al.sub.2O.sub.3 layer (3), two Al-targets (80 cm×20 cm×10 mm each) were used and a dual magnetron was applied. The bias power supply was used in a bipolar pulsed mode with 45 kHz and an off-time of 10 ms. The magnetron power supply was pulsed with 60 kHz (±2 kHz), and the pulse form was sinus shape. The cathode voltage at the stabilized stage of the process was 390 V. The Vickers hardness of the Al.sub.2O.sub.3 layer was HV3100, and the Young's modulus was 380 GPa. The further deposition parameters for the Al.sub.2O.sub.3 layer were as follows:

TABLE-US-00002 Parameter Value Ar flow [sccm] 1220 O.sub.2 flow [sccm] 101 total pressure [mPa] 1000 O.sub.2 part. press. [mPa] 10.2 Bias current [A] 35.3 Bias voltage [V] −125 Magnetron target 20 power [kW] Magnetron target 6.2 power density [W/cm.sup.2] Coil current [A] 4.5

[0071] The TiAlN in the alternating layers of TiAlN/Al.sub.2O.sub.3 (4) and in layer (5) were deposited as described for layer L2 in the cutting test example 3. The Al.sub.2O.sub.3 in the alternating layers of TiAlN/Al.sub.2O.sub.3 (4) was deposited as described before fpr layer (3). The ZrN layer (6) was deposited by arc evaporation using an arc current of 150 A per target at 4 Pa nitrogen pressure using a bias voltage of −40 V.

[0072] FIG. 6 shows a SEM cross-section at 40.000× magnification of a detail of the sample shown in FIG. 5 at the transition from the cemented carbide substrate (left side) to the first TiAlN layer (right side) with the 30 nm inventive WC layer in between. FIG. 6 illustrates how the inventive tungsten carbide (WC) layer is suitable to cover Co veins exposed at the substrate surface between WC grains to provide a barrier against binder migration or diffusion, respectively, out of the substrate.

EXAMPLES AND METHODS

XRD (X-Ray Diffraction)

[0073] XRD measurements for phase analysis were done applying grazing incidence mode (GIXRD) on a diffractometer from Panalytical (Empyrean) using CuKα-radiation. The X-ray tube was run with line focus at 40 kV and 40 mA. The incident beam was defined by a 2 mm mask and a ⅛° divergence slit in addition to an X-ray mirror producing a parallel X-ray beam. The sideways divergence was controlled by a Soller slit with a divergence of 0.04°. For the diffracted beam path a 0.18° parallel plate collimator in conjunction with a proportional counter (OD-detector) was used. The measurement was done in grazing incidence mode (Omega=1°). The 2-theta range was about 20-80° with a step size of 0.03° and a counting time of 10 s. For the XRD-line-profile analysis a reference measurement with LaB6-powder was done under the same parameters as described above to correct for the instrumental broadening.

Hardness/Young's Modulus Measurement

[0074] The measurements of hardness and Young's modulus (=reduced Young's modulus) were performed on the flank face of the coated tools by the nanoindentation method on a Fischerscope® HM500 Picodentor (Helmut Fischer GmbH, Sindelfingen, DE) 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.

Scanning Electron Microscopy (SEM)

[0075] 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.

Substrates for Cutting Tests

[0076] For the preparation of cutting tools used in cutting tests cemented carbide cutting tool substrate bodies of the following specification were used:

Composition: 12 wt-% Co, 1.6 wt-% (Ta, Nb)C, balance WC
WC grain size: ˜1.5 μm

Geometry: ADMT160608R-F56

[0077] Vickers hardness: ˜1600 HV (unpolished surface); ˜2000 HV (polished surface)

Substrates for Analytics

[0078] For analytics of the deposited tungsten carbide layer of the present invention cemented carbide substrates of simple flat square geometry with side lengths of 15 mm with a polished surface and of the following specification were used:

Composition: 8 wt-% Co, balance WC
WC grain size: ˜1.5 μm
Vickers hardness: ˜2000 HV (on the polished surface)

PVD Coating

[0079] 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 bodies were pre-treated at 550° C.

[0080] The tungsten carbide (WC) coatings were produced by the High-Power Impulse Magnetron Sputtering (HIPIMS) process in a 6-flange PVD installation Hauzer HTC1000 (IHI Hauzer Techno Coating B.V., NL) with a chamber size of 1 m.sup.3. The substrates were rotated on rotary tables. For the HIPIMS process, a plasma generator by TRUMPF Hüttinger GmbH+Co. KG, Freiburg, DE, was used. In the PVD system one WC target of 80 cm×20 cm was used for the deposition of the tungsten carbide (WC) layer on top of the substrate surface. The depositions were run in an Ar atmosphere with the addition of C.sub.2H.sub.2. The total pressure during deposition was 0.7 Pa (7.0×10.sup.−3 mbar) corresponding to an Ar flow of ˜900 sccm. The deposition temperature was 550° C.

[0081] The C.sub.2H.sub.2 flows/partial pressures during the depositions were either zero or [0082] 10 sccm C.sub.2H.sub.2/˜0.008 Pa (8.0×10.sup.−5 mbar) C.sub.2H.sub.2 [0083] 15 sccm C.sub.2H.sub.2/˜0.013 Pa (1.3×10.sup.−4 mbar) C.sub.2H.sub.2 [0084] 30 sccm C.sub.2H.sub.2/˜0.02 Pa (2.0×10.sup.−4 mbar) C.sub.2H.sub.2

[0085] The HIPIMS average total cathode power during deposition was 10 kW corresponding to about 6.25 W/cm.sup.2 of the target. The remaining deposition parameters, “bias voltage”, “average pulse power”, “peak voltage”, “peak current”, “pulse length” and “frequency” were varied and are indicated in table 1 below. The values given are average values since the plasma conditions change constantly as the substrate table is moved.

Example 1—HIPIMS Depositions of Coatings According to the Invention and Comparative Coatings

[0086] The parameters of the HIPIMS deposition of the tungsten carbide (WC) layer are indicated in table 1, and the results are given in table 2. The HIPIMS depositions were carried out to obtain tungsten carbide (WC) layer thicknesses of about 90 to 100 μm measured on SEM cross-sections. The substrates in this example were the above-described substrates for analytics.

TABLE-US-00003 TABLE 1 HIPIMS deposition parameters for WC coating layer Average Pulse Peak Voltage/ Pulse Length/ C.sub.2H.sub.2 Flow/ Bias Power Peak Current Frequency Ar flow Voltage Sample # [MW] [V]/[A] [μs]/[Hz] [sccm/sccm] [V] 181015002 0.45 1600 V/700 A 50 μs/450 Hz —/900 100 V 181017002 0.63 1600 V/700 A 100 μs/160 Hz  —/900 100 V 181023003 0.32 1200 V/550 A 100 μs/308 Hz  30/900  40 V 181023004 0.32 1200 V/550 A 100 μs/308 Hz  30/900 100 V 181025002 0.15 1000 V/230 A 500 μs/130 Hz  10/900 150 V 181025003 0.54 1700 V/845 A 50 μs/370 Hz 10/900 100 V 181026001 0.54 1700 V/845 A 50 μs/370 Hz 10/900 150 V 181026003 0.54 1700 V/845 A 50 μs/370 Hz 15/900 100 V 190117005 0.45 1600 V/800 A 50 μs/446 Hz 15/900 200 V 190118001 0.22 1200 V/270 A 500 μs/91 Hz  15/900 200 V

TABLE-US-00004 TABLE 2 Results Young's Evaluation Hardness Modulus WC Phases Mechanical Sample # [HV] [GPa] in XRD Properties 181015002 2720 515 α-WC (+)/β-WC (+)/α-W.sub.2C (+) −−− 181017002 2900 525 α-WC (+)/β-WC (+)/α-W.sub.2C (+) −−− 181023003 2200 410 α-WC (+)/β-WC (+)/α-W.sub.2C (−) −− 181023004 2215 440 α-WC (+)/β-WC (+)/α-W.sub.2C (−) −− 181025002 2720 515 α-WC (+)/β-WC (+)/α-W.sub.2C (+) −−− 181025003 2780 520 α-WC (+)/β-WC (+)/α-W.sub.2C (+) −−− 181026001 3020 545 α-WC (+)/β-WC (+)/α-W.sub.2C (+) −−− 181026003 2750 510 α-WC (+)/β-WC (+)/α-W.sub.2C (+) −−− 190117005 2700 500 α-WC (+)/β-WC (+)/α-W.sub.2C (−) +++ 190118001 2835 550 α-WC (+)/β-WC (+)/α-W.sub.2C (−) +++ “(+)” and “(−)”indicates whether or not the respective tungsten carbide phase could be detected by XRD indexed applying JCPDS cards 025-1047, 020-1313 and 035-0776, as described above. α-WC (+/−): hexagonal tungsten mono-carbide α-WC detected/not detected β-WC (+/−): cubic tungsten mono-carbide β-WC detected/not detected α-W.sub.2C (+/−): hexagonal tungsten semi-carbide α-W.sub.2C detected/not detected

[0087] In samples 181015002, 181017002, 181025002, 181025003, 181026001 and 181026003, where no or only 10 sccm C.sub.2H.sub.2 was introduced, the undesired brittle semi-carbide α-W.sub.2C was detected in XRD. Even though, hardness and Young's modulus of these samples were quite high, the mechanical properties of these samples were insufficient. The tungsten carbide layer was brittle, probably due to the presence of significant amounts of α-W.sub.2C, and adherence to the substrate was bad.

[0088] In samples 181023003 and 181023004 no semi-carbide α-W.sub.2C was detected in XRD, however, the samples exhibited low hardness and low Young's modulus, i.e. insufficient mechanical properties. The high (over-stoichiometric) C.sub.2H.sub.2 flow of 30 sccm resulted in the incorporation of graphite or amorphous carbon, respectively, into the deposited layer, which in turn led to the insufficient mechanical properties.

[0089] Samples 190117005 and 190118001 showed no semi-carbide α-W.sub.2C in XRD, and the samples exhibited high hardness, high Young's modulus, good overall mechanical properties and good adhesion to the cemented carbide substrate. These outstanding properties resulted from an optimized combination of the C.sub.2H.sub.2 flow and the applied high bias in the HIPIMS deposition process. Therefore, the tungsten carbide layers of these samples are suitable as wear resistant outer layers, but also as an intermediate layers for subsequent hard material coating layers of a cutting tool.

[0090] As can well be seen in the SEM cross-sections in FIGS. 1 and 2, samples 190117005 and 190118001 showed a high degree of coherent growth of the tungsten carbide layers on the substrate WC grain surfaces.

[0091] In the examples shown herein the deposition parameters and conditions for a sample were kept constant throughout the deposition of the entire tungsten carbide (WC) layer thickness. However, by variation of the deposition parameters during growth of the tungsten carbide (WC) layer it was possible to change the phase distribution (amounts or ratios of tungsten mono-carbide α-WC and tungsten mono-carbide β-WC phases) and, at the same time, avoid the formation of undesired hexagonal α-W.sub.2C semi-carbide phase, as could be confirmed by TEM analysis.

[0092] For example, changing the phase distribution (amounts or ratios) from a higher to a lower ratio of hexagonal α-WC/cubic β-WC phase from the substrate surface towards the outer surface of the deposited tungsten carbide (WC) layer, was achieved by slightly lowering the C.sub.2H.sub.2 flow (partial pressure) within the optimized working window of the deposition process.

[0093] It has been found and confirmed by the examples that in the HIPIMS process of the present invention the combination of a comparably high bias voltage and of a suitably high C.sub.2H.sub.2 flow (partial pressure) is beneficial to obtain hexagonal tungsten mono-carbide α-WC and cubic tungsten monocarbide β-WC phases without detectable amounts of detrimental and thus undesired hexagonal semi-carbide α-W.sub.2C, and results in an improvement of the coherent transition from the WC grains of the substrate surface into the tungsten carbide (WC) coating layer. No C.sub.2H.sub.2 or a too low amount of C.sub.2H.sub.2 always results in the deposition of significant amounts of the undesired hexagonal semi-carbide α-W.sub.2C. A too high amount of C.sub.2H.sub.2 gives additional C phases (graphite or amorphous C, respectively). And a too low bias does not result in coherent transition from the WC grains of the substrate surface into the tungsten carbide (WC) coating layer.

Example 2—Cutting Tests

[0094] In order to assess the effect of the inventive tungsten carbide (WC) layer according to the invention coated cutting tools were produced and tested in a milling test.

[0095] For the cutting tests in this example cemented carbide substrates of the type described above for cutting tests were used. Inventive examples were coated with an about 30 nm thick tungsten carbide (WC) layer deposited immediately on top of the substrate surface as described above in example 1 for sample no. 190117005.

[0096] Subsequently, the substrates with the WC layer (invention) and without the WC layer (comparative example) were coated in the arc evaporation process with a TiAlN coating consisting of a first 2.0 μm thick layer L1 and a second 2.0 μm thick layer L2, i.e. a total thickness of 4 μm. The deposition conditions for L1 and L2 were as follows:

TABLE-US-00005 (Ti,Al)N Targets “Ti50Al50” + “Ti33Al67” sub-layer L1 Bias −40 V Pressure (N.sub.2) 10 Pa Arc Current/Target 150 A Rotation Speed 3 rpm Temperature 550° C. (Ti,Al)N Targets “Ti33Al67” + “Ti50Al50” + “Ti33Al67” sub-layer L2 Bias −40 V Pressure 10 Pa Arc Current/Target 150 A Rotation Speed 3 rpm Temperature 550° C.

[0097] The cutting tests were performed on a Fritz Werner TC630 machine under the following conditions, and the maximum wear V.sub.Bmax, i.e. the deepest crater observed on the flank face of the tool, was determined after the test.

Cutting Conditions:

[0098] Tooth Feed f.sub.z [mm/tooth]: 0.2 [0099] Feed v.sub.f [mm/min]: 120 [0100] Cutting speed v.sub.c [m/min]: 188 [0101] Cutting depth a.sub.p [mm]: 3 [0102] Workpiece material: 42CrMo4 (tensile strength Rm: 850 N/mm.sup.2) [0103] Cutting length [mm]: 5600

[0104] V.sub.Bmax of the inventive tool provided with the inventive tungsten carbide (WC) layer was 0.14 mm, whereas V.sub.Bmax of the comparative tool was 0.16 mm, i.e. the wear of the comparative tool was about 14% higher than of the inventive tool.

[0105] In addition, a cutting tool provided with a tungsten carbide layer containing a certain measurable amount of semi-carbide α-W.sub.2C was produced, as described above in example 1, and tested in the cutting test. However, no meaningful result could be obtained, since the tool quickly failed after starting the test due to brittleness of the tungsten carbide layer.

[0106] The results clearly show the advantages of a cutting tools according to the invention.