Metal cutting tool with multi-layer coating

Abstract

A metal cutting tool includes a main body made of cemented carbide, cermet, ceramic, steel or high-speed steel, and a multi-layer wear protection coating. The wear protection coating includes a lower layer having an overall composition of Ti.sub.m Al.sub.(1-m) N with 0.25<m<0.55 and an overall thickness of 500 nm to 3 μm. The lower layer has 50 to 600 pairs of alternately stacked sub-layers in a sequence (A-B-A-B- . . . ) and having a composition Ti.sub.a Al.sub.(1-a) N with 0.45≤a≤0.55 and a thickness of 1 nm to 10 nm. The upper layer has 30 to 400 triples of alternately stacked sub-layers in a sequence (C-D-E-C-D-E- . . . ). The sub-layers of the upper layer have a composition Ti.sub.x Al.sub.ySi.sub.zN with x+y+z=1 and 0.20≤x≤0.45, 0.20≤y≤0.45 and 0.20≤z≤0.45 and a thickness of 1 nm to 10 nm.

Claims

1. A metal cutting tool comprising: a main body made of cemented carbide, cermet, ceramic, steel or high-speed steel; and a multi-layer wear protection coating, wherein the wear protection coating comprises: a lower layer having an overall composition Ti.sub.m Al.sub.(1-m) N with 0.25<m<0.55, and an overall thickness of the lower layer being of from 500 nm to 3 μm, wherein the lower layer includes 50 to 600 pairs of alternately stacked sub-layers (A) and (B) having a sequence (A-B-A-B- . . . ), the stacked sub-layers (A) having a composition Ti.sub.a Al.sub.(1-a) N with 0.45≤a≤0.55 and a thickness of from 1 nm to 10 nm, wherein the first sub-layer (A) in the layer stack of sub-layers (A) and (B) has a layer thickness of at least 5 nm, the stacked sub-layers (B) having a composition Ti.sub.b Al.sub.(1-b) N with 0.25≤b≤0.40 and a thickness of from 1 nm to 10 nm, and the stacked sub-layers (A) and (B) having different stoichiometrical compositions with (a-b)≥0.10; and an upper layer deposited immediately on the lower layer and having an overall composition Ti.sub.n Al.sub.oSi.sub.p N with n+o+p=1 and 0.30≤n≤0.50, 0.40≤o≤0.60 and 0.05≤p≤0.20, and an overall thickness of the upper layer being of from 500 nm to 3 μm, wherein the upper layer includes 30 to 400 triples of alternately stacked sub-layers (C), (D) and (E) having a sequence (C-D-E-C-D-E- . . . ), the sub-layers (C) of the upper layer being defined in the same way as the sub-layers (A) of the lower layer, and the sub-layers (D) of the upper layer being defined in the same way as the sub-layers (B) of the lower layer, and the sub-layers (C) and (D) of the upper layer having different stoichiometrical compositions with (a-b)≥0.10, and the sub-layers (E) of the upper layer having a composition Ti.sub.x Al.sub.ySi.sub.z N with x+y+z=1 and 0.20≤x≤0.45, 0.20≤y≤0.45 and 0.20≤z≤0.45 and a thickness of from 1 nm to 10 nm.

2. The tool according to claim 1, wherein the multi-layer wear protection coating is applied on the main body by a PVD process, more preferably by cathodic arc vapor deposition.

3. The tool according to claim 1, further comprising one or more further layers of hard material disposed on top of the upper layer and/or between the main body and the lower layer, the one or more further layers of hard material containing one or more of the elements of the groups 4a, 5a and 6a of the Periodic System, Al, Si and one or more of the non-metals N, C, O and B.

4. The tool according to claim 1, wherein the lower layer is deposited immediately on a surface of the main body.

5. The tool according to claim 1, wherein the main body is made of cemented carbide.

6. The tool according to claim 5, wherein the cemented carbide contains from 6 to 20 wt-% of Co binder, or from 8 to 16 wt-% of Co binder, or from 10 to 14 wt-% of Co binder, or from 11 to 13 wt-% of Co binder.

7. The tool according to claim 5, wherein the cemented carbide has an average WC grain size of from 0.3 to 2.0 μm, or from 0.4 to 1.5 μm, or from 0.5 to 1.2 μm.

8. The tool according to claim 1, wherein the tool is a solid hard metal rotary cutting tool.

9. A method of using a tool comprising: providing a tool according to claim 1; milling steel of the ISO-S group of work piece materials, such as heat resistant super alloys, titanium, titanium alpha-alloys, titanium beta-alloys, titanium mixed alpha+beta-alloys, and titanium mixed alpha+beta-alloys of the Ti-6A1-4V type, with the tool.

10. The tool according to claim 1, wherein the multi-layer wear protection coating is applied on the main body by cathodic arc vapor deposition.

11. The tool according to claim 1, wherein the tool is a milling tool.

Description

FIGURES

(1) FIG. 1 illustrates wear types and positions on a typical end milling cutter. “δa” is the length of engagement of the cutter. “KT” is the depth of crater wear. “VB1”, “VB2” and “VB3” represent different types of flank wear.

(2) FIG. 2 illustrates the different types of flank wear, “(VB1) uniform flank wear”, “(VB2) non-uniform flank wear” and “(VB3) localized flank wear”. “δa” is the length of engagement of the cutter.

(3) FIG. 3 shows a SEM of a cross-section of the inventive coating TSS3 with lower layer (LL) and upper layer (UL) at 20,000× magnification. In the upper layer (UL) the stacked sub-layers (C)-(D)-(E) can very well be seen, wherein the Si-containing sub-layers (E) appear darker than the sub-layers (C) and (D). In the lower layer (LL) a “columnar grain” like structure can be observed, whereby stacked nano sub-layers (A)-(B) are present within the individual grains. Since the sub-layers (A) and (B) both do not contain Si, such as sub-layer (E), the contrast between the sublayers (A) and (B) is very low, so that the stacked structure is difficult to see in the representation of FIG. 3.

(4) FIG. 4 shows a SEM of a cross-section of the inventive coating TSS3 after a cutting test on Ti alloy workpiece material at 1,610× magnification. (HM) designates the hard metal substrate material; (TSS3) designates the rest of the coating remained after the cutting test; (Ti) which is slightly brighter than (TSS3) in FIG. 4, is “titanium smear” from the workpiece material adhering to the tool, especially where flank wear (VB) occurred. The “titanium smear” fills up the regions where the coating was worn off by flank wear. (Pt) designates a platinum protection layer which is not part of the inventive cutting tool, but rather required for the SEM measurement.

MATERIALS AND METHODS

Electron Microprobe Microanalysis (EMPA)

(5) The chemical compositions of the coatings were determined by electron microprobe microanalysis (EMPA) using a Supra 40VP (Carl Zeiss Microscopy GmbH, Jena, Germany) equipped with an Oxford INCA EDS and at an acceleration voltage of 12 kV and a measuring time of 30 sec per spot.

X-Ray Diffraction (XRD)

(6) X-ray diffraction measurements were done on a PANalytical Empyrean X-ray diffractometer in GI (grazing incidence) mode applying an incidence angle of 1° using CuKα-radiation. The X-ray tube was run in point focus at 40 kV and 40 mA. A parallel beam optic using an X-ray mirror with a mask of 2 mm, a divergence aperture of ⅛°, and a Soller slit with a divergence of 0.04° 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 parallel plate collimator with an acceptance angle of 0.18° was used together with a proportional counting detector. For the classification of XRD reflections, the JCPDS databases were used.

Hardness/Young's Modulus

(7) 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.

Scanning Electron Microscopy (SEM)

(8) 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.

Focused Ion Beam (FIB) Milling

(9) Cross-sections of the cutting edges of worn tools were prepared using a Zeiss Crossbeam 540 (Carl Zeiss Microscopy GmbH, Jena, Germany) with FIB column. Ga ions accelerated to 30 kV were used for the milling operations.

WC Grain Size Determination in Cemented Carbide

(10) The average WC grain size of cemented carbide or cermet is determined from the value of magnetic coercivity. The relationship between coercivity and grain size of WC is described, e.g., in Roebuck et al., Measurement Good Practice No. 20, National Physical Laboratory, ISSN 1368-6550, November 1999, Revised February 2009, Section 3.4.3, pages 19-20. For the purposes of this application the WC grain size, “d”, is determined according to formula (8) on page 20 in the above-mentioned literature: K=(c.sub.1+d.sub.1W.sub.Co)+(c.sub.2+d.sub.2W.sub.Co)/d. Re-arranging one gets:
d=(c.sub.2+d.sub.2W.sub.Co)/(K−(c.sub.1+d.sub.1W.sub.Co)),
wherein d=WC grain size of the cemented carbide body, K=coercivity of the cemented carbide body in kA/m, herein measured according to standard DIN IEC 60404-7, W.sub.Co=wt % Co in the cemented carbide body, c.sub.1=1.44, c.sub.2=12.47, d.sub.1=0.04, and d.sub.2=−0.37.

EXAMPLE 1

(11) Substrate:

(12) The substrates used in this example 1 were solid hard metal (SHM) end mill cutters consisting of a base body of WC-12 wt-% Co with an average WC grain size of 0.5 μm and containing 1.4 wt-% Cr carbide. Two different cutter geometries were used, S1 and S2:

(13) TABLE-US-00001 Cutter Geometry S1 S2 Diameter: 16 mm 10 mm Number of cutting edges: 6 4 Length of cutting edges: 200% of 220% of the diameter the diameter Corner radius  4 mm  1 mm

(14) Coatings

(15) PVD coatings were prepared in a commercial arc evaporation system, Innova (Oerlikon Balzers) equipped with 6 cathodic arc sources. The variation in the Ti, Al and Si concentrations in the deposited coating layers was achieved by using different TiAl and TiAlSi mixed targets of different compositions in the PVD system, and the substrate was guided periodically past the different mixed targets by threefold rotation. Prior to the deposition the substrates were cleaned with an argon-ion etching process at an Ar pressure of 0.21 Pa at 170 V DC substrate bias for 30 min. The compositions of sub-layers A to E prepared in this example and the mixed target compositions used for their production were as follows:

(16) TABLE-US-00002 Sub-Layer Composition Target Composition A Ti.sub.0.50Al.sub.0.50N TiAl (50:50) B Ti.sub.0.33Al.sub.0.67N TiAl (33:67) C Ti.sub.0.50Al.sub.0.50N TiAl (50:50) D Ti.sub.0.33Al.sub.0.67N TiAl (33:67) E Ti.sub.0.33Al.sub.0.34Si.sub.0.33N TiAlSi (33:34:33)

(17) To ensure that the coating grows only in the desired fcc crystal structure a first sub-layer (A) with a thickness of approximately 30 nm was deposited immediately on the substrate surface, followed by the subsequent coating layers. The coating conditions of the sub-layers were as follows, whereby the Arc current of the first sublayer (A) was 175 A instead of 200 A for the subsequent sub-layers (A).

(18) Coating Conditions

(19) TABLE-US-00003 Specific evapora- Deposi- N.sub.2 tor Arc tion Rotation Bias Pressure flow current Temp. Speed Sub-Layer [V] [Pa] [A/cm.sup.2] [A] [° C.] [rpm] 1.sup.st A 60 4 0.9 175 550 1.5 A 60 4 1 200 550 1.5 B 60 4 0.6 120 550 1.5 C 80 4 1 200 550 1.5 D 80 4 0.6 120 550 1.5 E 80 4 0.8 160 550 1.5

(20) The following coatings according to the invention were prepared on the cutter substrates Si and S2:

(21) TABLE-US-00004 Lower Layer (LL) Upper Layer (UL) Thick- Thick- # nesses Total # nesses Total Pairs A/B Thickness Triplets C/D/E Thickness Coating (A-B) [nm] [μm] (C-D-E) [nm] [μm] TSS1 300 3/5 2.4 200 3/5/5 2.6 TSS2 300 5/3 2.4 200 5/3/5 2.6 TSS3 150 9/7 2.4 100 9/7/8 2.4

(22) The mechanical properties of the coatings (hardness and reduced Young's modulus) were measured as described above and were as follows:

(23) TABLE-US-00005 Hardness reduced Young's Tool [HV 0.015] modulus [GPa] TSS1 2700 330 TSS2 2700 330 TSS3 2700 330

(24) Comparative tools were based on the same SHM substrates (S1 and S2) as the inventive tools. The comparative tools were as follows:

(25) TABLE-US-00006 Coating COMP1 uncoated substrate COMP2 multi-layer TiAlN arc PVD coating according to EP 2 880 199 (example 1) COMP3 multi-layer TiAlN - TiSiN arc PVD coating “Ionbond Hardcut” prepared by external coater

(26) Measurement of Tool Wear

(27) The inventive tools and comparative tools were tested for tool wear in side milling tests. The tools used and the individual test parameters, as well as the results are described below for different cutting tests made.

(28) Tool wear is defined as the change in shape of the cutting part of a tool from its original shape, resulting from the progressive loss of tool material during cutting. In the present case, flank wear (VB) was measured as the specified tool-life criterion to compare inventive tools and comparative tools. Flank wear is defined as the loss of tool material from the tool flanks during cutting which results in the progressive development of a flank wear land.

(29) Flank wear measurement is carried out parallel to the surface of the wear land and in a direction perpendicular to the original cutting edge, e.g. the distance from the original cutting edge to that limit of the wear land which intersects the original flank. Although the flank wear land on a significant portion of the flank may be of uniform size, there will be variations in its value at other portions of the flanks depending on the tool profile and edge chipping. Values of flank wear measurements shall therefore be related to the area or position along the cutting edges at which the measurement is made.

(30) Flank wear measurement distinguishes between “uniform flank wear (VB1)”, “non-uniform flank wear (VB2)” and “localized flank wear (VB3)” (see FIG. 2). In “uniform flank wear (VB1)” the wear land is normally of constant width and extends over those portions of the tool flanks adjoining the entire length of the active cutting edge. In “non-uniform flank wear (VB2)” the wear land has an irregular width and the profile generated by the intersection of the wear land and the original flank varies at each position of measurement. “Localized flank wear (VB3)” is an exaggerated and localized form of flank wear which develops at a specific part of the flank, shown in FIG. 1 at positions 1, 2 and 3. Positions 1 and 2 are on the flank of the radius (herein also referred to as “Corner”) at the end of the tool, whereas position 3 is essentially at the opposite end of the cutting edge at the depth of cut (“DOC”). Localized flank wear (VB3) at the depth of cut (position 3) is sometimes also referred to as notch wear.

(31) In the cutting tests herein, localized flank wear (VB3) was measured at the “Corner” (positions 1 and 2), as well as at the “DOC” (position 3), since flank wear was highest at these positions. “VB3.sub.average” means the average of all measured VB3 values (at the specified position) of all cutting edges of a tool (e. g.: S1=6 cutting edges; S2=4 cutting edges) and from the three cutting tests carried out with each type of tool (coating). “VB3.sub.max” is the highest VB3 value of all measured VB3 values (at the specified position) of all cutting edges of a tool and of the three cutting tests carried out with each type of tool (coating).

(32) Cutting Test 1:

(33) Inventive tools and comparative tools, each based on cutter geometry S1, were tested in side milling tests, and the localized flank wear was measured. The cutting conditions are summarized in the following table.

(34) Cutting Conditions (Cutting Test 1):

(35) TABLE-US-00007 Tooth Feed f.sub.z [mm/tooth] 0.08 Cutting speed v.sub.c [m/min] 135 Cutting width a.sub.e [mm] 1.6 (0.1 × tool diameter) Cutting depth a.sub.p [mm] 8.5 Metal Removing Rate 17.53 [cm.sup.3/min] Cooling water-in-oil emulsion with 9% oil (Blasocut B25) through internal channels and external nozzles Stop Criteria 160 passes or VB3 ≥ 0.2 mm (no. of passes) Workpiece Material Ti-6Al-4V (170 mm × 170 mm × 100 mm; tensile strength: 950 N/mm.sup.2

(36) Machining was stopped after the predefined number of passes or at an average localized flank wear of VB3≥0.2 mm at the corner

(37) The following table shows a conversion between “number of cutting cycles”, “time in cut”, “distance” and “no. of passes”:

(38) TABLE-US-00008 No of cutting cycles 5833 7583 9333 11083 Time in cut [min] 13.06 16.94 20.85 24.76 Cutting length [m] 16.80 21.84 26.88 31.92 Passes 100 130 160 190

(39) In this test the wear maximum was observed at the cutting edge radius (“Corner”; positions 1 and 2), therefore, the values measured there were taken into account. The results are shown in the following table.

(40) Results (Cutting Test 1):

(41) TABLE-US-00009 VB3.sub.average VB3.sub.max No. of Corner Corner Coating Passes [mm] [mm] TSS1 160 0.16 0.26 TSS2 160 0.18 0.37 TSS3 160 0.13 0.20 COMP1 160 0.24 0.37 COMP2 160 0.15 0.21 COMP3 160 0.18 0.24

(42) Cutting Test 2:

(43) Inventive tools and comparative tools, each based on cutter geometry S2, were tested in side milling tests, and the localized flank wear was measured. The cutting conditions are summarized in the following table.

(44) Cutting Conditions (Cutting Test 2):

(45) TABLE-US-00010 Tooth Feed f.sub.z [mm/tooth] 0.04 Cutting speed v.sub.c [m/min] 100 Cutting width a.sub.e [mm] 2 (0.2 × tool diameter) Cutting depth a.sub.p [mm] 2.5 Cooling water-in-oil emulsion with 9% oil (Blasocut B25) through internal channels and external nozzles Stop Criteria Test was stopped when the second best tool had (no. of passes) VB3 ≥ 0.2 mm Workpiece Material Ti-6Al-4V (175 mm × 175 mm × 50 mm; tensile strength: 950 N/mm.sup.2

(46) The following table shows a conversion between “number of cutting cycles”, “time in cut”, “distance” and “no. of passes”:

(47) TABLE-US-00011 Time in cut [min] 34.36 48.11 54.98 79.03 Cutting length [m] 17.5 24.5 28 40.25 Passes 100 140 160 230

(48) Results (Cutting Test 2):

(49) TABLE-US-00012 VB3.sub.average VB3.sub.max VB3.sub.average VB3.sub.max No. of Corner Corner DOC DOC Coating Passes [mm] [mm] [mm] [mm] TSS1 160 0.020 0.027 0.032 0.035 COMP2 160 0.051 0.086 0.228 0.295 TSS1 230 0.052 0.059 0.051 0.060 COMP2 230 --- --- --- --- “---”: stop criterion was reached

(50) It could be seen that the wear of the tool coated with the coating according to the invention shows a very even wear at the corner (positions 1 and 2) and at the DOC (position 3), so the test was run until the tool coated with TSS1 was the only remaining tool in the test. The comparative tool showed a much higher wear at the DOC (position 3) than at the corner (positions 1 and 2) at 160 passes, and 230 passes were not reached (stop criterion reached).

(51) Cutting Test 3:

(52) The inventive tools and comparative tools, each based on cutter geometry S2, were tested in side milling tests, and the localized flank wear was measured. The cutting conditions are summarized in the following table.

(53) Cutting Conditions (Cutting Test 3):

(54) TABLE-US-00013 Tooth Feed f.sub.z [mm/tooth] 0.11 Cutting speed v.sub.c [m/min] 130 Cutting width a.sub.e [mm] 1 (0.1 × tool diameter) Cutting depth a.sub.p [mm] 2.5 Cooling water-in-oil emulsion with 9 % oil (Blasocut B25) through internal channels and external nozzles Stop Criteria (no. of passes) 120 passes or VB3 0.2 mm Workpiece Material Ti-6A1-4V (175mm × 175mm × 50mm; tensile strength: 950 N/mm.sup.2

(55) Results (Cutting Test 3):

(56) TABLE-US-00014 VB3.sub.average VB3.sub.max VB3.sub.average VB3.sub.max No. of Corner Corner DOC DOC Coating Passes [mm] [mm] [mm] [mm] TSS1 120 0.057 0.100 0.143 0.193 COMP2 120 0.056 0.144 0.162 0.307

(57) The machining conditions in cutting test 3 are rather demanding, therefore, the tools were worn after a comparatively low number of passes.

(58) Cutting Test 4:

(59) The inventive tools and comparative tools, each based on cutter geometry S2, were tested in side milling tests, and the localized flank wear was measured. The cutting conditions are summarized in the following table.

(60) Cutting Conditions (Cutting Test 4):

(61) TABLE-US-00015 Tooth Feed f.sub.z [mm/tooth] 0.04 Cutting speed v.sub.c [m/min] 100 Cutting width a.sub.e [mm] 2 (0.2 × tool diameter) Cutting depth a.sub.p [mm] 2.5 Cooling water-in-oil emulsion with 9% oil (Blasocut B25) through internal channels and external nozzles Stop Criteria 200 passes or VB3 ≥ 0.2 mm (no. of passes) Workpiece Material Ti-6Al-4V (175 mm × 175 mm × 50 mm; tensile strength: 950 N/mm.sup.2

(62) Results (Cutting Test 4):

(63) TABLE-US-00016 VB3.sub.average VB3.sub.max VB3.sub.average VB3.sub.max No. of Corner Corner DOC DOC Coating Passes [mm] [mm] [mm] [mm] TSS1 120 0.026 0.030 0.033 0.038 COMP1 120 0.030 0.049 0.031 0.039 COMP2 120 0.032 0.042 0.038 0.053 TSS1 200 0.063 0.114 0.060 0.114 COMP1 200 0.052 0.108 0.057 0.165 COMP2 200 0.076 0.195 0.066 0.178

(64) In this test uncoated tools (COMP1) were also tested, since uncoated tools are still in use in this field, because in some applications in titanium machining no benefits due to coatings have been observed, and in the field of end mills the reconditioning of tools is much easier and faster when the tools are used uncoated. The test was stopped before end of tool life.