WO2009031958A1

WO2009031958A1 - Coated drill and a method of making the same

Info

Publication number: WO2009031958A1
Application number: PCT/SE2007/050625
Authority: WO
Inventors: Maria Åstrand; Peter Müller; Thomas Heil
Original assignee: Sandvik Intellectual Property Ab
Priority date: 2007-09-05
Filing date: 2007-09-05
Publication date: 2009-03-12

Abstract

The present invention relates to a twist drill comprising a cemented carbide or high speed steel substrate and a multilayered aperiodic (Ti,Al)N-coating having a binary A/B/A/B/A/B structure, 5 where the composition of layer A is AlxTi1-xN, where x preferably is 0.40-0.75, more preferably 0.50-0.70, and most preferably 0.60- 0.68, and the composition of layer B is TiyAl1-yN, where y preferably is 0.50-1.0, more preferably 0.60-0.90, and most preferably 0.70-0.80 and the composition for the whole coating is 10 TizAl1-zN, where z preferably is 0.35-0.75, more preferably 0.40- 0.65 and where the average thickness of layer A and B is between 1-200 nm, preferably 2-50 nm. The present invention also relates to a method of making a twist drill. 15 Drills according to the present invention have an increased wear resistance and thus longer tool life.

Description

Coated drill and a method of making the same

The present invention relates to a twist drill for metal machining coated with an aperiodic (Ti, Al) N multilayered coating. The present invention also relates to a method of manufacturing a twist drill according to the present invention.

Twist drills are often coated with a hard material to increase tool life by enhancing the wear resistance. A high wear resistance is advantageous especially at the tip area of the twist drill where the cutting takes place. The trend within the cutting industry goes towards increased cutting speeds, feeds and more and more difficult materials to cut. This results in higher demands on the wear resistance. Also properties like low friction and smoothness are advantageous, mainly on other parts of the twist drill, for example to enhance chip transportation. The chip transport is particularly important when the drilled depth is relatively large compared to the nominal diameter.

A common wear mechanism in drilling is wear in the cutting edge resulting in a weaker edge followed by chipping. If chipping occurs in the middle of the cutting edge, uncontrolled chip formation can occur. If the chipping occurs closer to the periphery, corner fracture can occur. Increasing the edge toughness and wear resistance is thus of great importance to increase tool life. One possible solution to decrease the risk of thermal and/or mechanical cracking is to use a multilayered coating. The multilayered coating is expected to act as a crack inhibitor as an initial crack going down the coating may be redirected at any sublayer interface .

WO 98/48072 relates to a cutting tool comprising a body of sintered cemented carbide, ceramic or high speed steel on which at least on the functional parts of the surface of the body, a thin, adherent, hard and wear resistant coating is applied. The coating comprises a laminar, multilayered structure of refractory compounds in polycrystalline, non-repetitive form MX/NX/MX/NX ... where the alternating layers MX and NX are composed of

i metalnitrides or carbides . The sequence of ii thicknesses is essentially aperiodic throughout the entire multilayered structure.

It is an object of the present invention is to provide a twist drill with good wear resistance and thereby obtaining an increased tool life.

It has been found that thin multilayered coatings, where a layer consisting of a (Ti, Al) N with a high Al-content is alternated with a (Ti, Al) N- layer with high Ti-content, improves the tool life of the drill.

Detailed description of the invention

The present invention relates to a coated twist drill comprising a substrate body, preferably of cemented carbide or high speed steel, and an aperiodic multilayered (Ti₇Al)N coating.

By multilayered structure is herein meant a structure comprising at least 5, preferably at least 10 individual layers. However, it can comprise up to several thousand individual layers.

The drill according to the present invention is provided with an aperiodic (Ti₇Al)N multilayer comprising a binary A/B/A/B/A/B structure with thin alternating sublayers A and B being repeated throughout the entire coating.

By aperiodic is understood that the thickness of a particular individual layer in the multilayered structure does not depend on the thickness of an individual layer immediately beneath nor does it bear any relation to an individual layer above the particular individual layer. Hence, the multilayered structure does not have any repeat period in the sequence of individual layer thicknesses.

The composition of each individual layer in the multilayered structure cannot easily be measured without contribution from adjacent layers due to the small thickness. What can be measured is the average composition over the whole multilayer structure. However, the composition of each individual layer can be estimated from the composition of the targets, that is the sources used for the individual layers during deposition, but that does not give an exact composition. When thicker layers have b enough to be analysed individually, it has been found that the composition of the deposited layers can differ with a few percentages compared to the composition of the target material. Due to this fact any composition of the individual layers of the multilayered structure according to the present invention mentioned herein after are estimations from the compositions of the targets used during deposition.

In the aperiodic multilayered coating according to the present invention the composition of sublayer A is Al_xTi₁-_XN, where x is 0.40-0.75, preferably 0.50-0.70, and more preferably 0.60- 0.68. the composition of layer B is Ti_yAli__yN, where y is 0.50-1.0, preferably 0.60-0.90 and more preferably 0.70-0.80. The composition of the whole coating as measured by e.g. microprobe or EDS is Ti_zAli-_zN, where z is preferably 0.30-0.70 and more preferably 0.40-0.60. Due to the aperiodic nature of the coating, the thickness of each individual layer will vary but in average the layer thickness is within 1-200 nm, preferably 2-50 nm.

The total thickness of the coating measured on the peripheral surface is 0.5 μm to 7 μm, preferably 1 to 5 μm.

The drill preferably has a diameter of 0.5 to 35 mm, more preferably 2 to 25 mm.

In one embodiment of the present invention the coating further comprises a top layer of sufficient thickness, preferably 0.1-1 μm, of Tii-_XA1_XN where 0.5≤x≤0.70, preferably Ti_o.₃₃Alo.67N or Ti₀.₅₀Al₀.₅₀N, to give a visible, black intrinsic color.

In yet another embodiment of the present invention the coating further comprises a top layer having a bright color facilitating visual wear detection, of sufficient thickness, preferably 0.1-1 μm, preferably of Tio.₇₅Alo.₂₅N, Ti_0-84Al_0-I6N, Ti₀.90Si0.₁₀N or TiN to give a visible purple, bronze or yellow intrinsic color.

The present invention also relates to a method of making a twist drill comprising providing a cemented carbide or high speed steel substrate, and onto said substrate depositing a PVD-coating comprising a laminar, multilayered structure targets are chosen so that the composition of the layers A and B are Al_xTii-_xN and Ti_yAlx-_yN respectively, where x preferably is 0.40- 0.75, more preferably 0.50-0.70 and most preferably 0.60-0.68 and where y preferably is 0.50-1.0, more preferably 0.60-0.90, and most preferably 0.70-0.80. The composition of the whole coating, made according to the present invention, as measured by e.g. microprobe or EDS is Ti_zAli__zN, where z preferably is 0.30-0.70, more preferably 0.40-0.60. Due to the aperiodic nature of the coating, the thickness of each layer will vary but in average the layer thickness of the deposited individual layers is within 1-200 nm, preferably 2-50 nm.

The total thickness of the deposited coating as measured on the peripheral surface is 0.5 to 7 μm, preferably 1 to 5 μm.

The aperiodic structure is obtained by, during the deposition process, randomly opening and closing shutters from individual layer targets, or by randomly switching said targets on and off. Another conceivable method is by randomly rotating or moving the to-be-coated substrates, in front of said targets. This is preferably done by placing the substrates on a 3 -fold rotating substrate table arranged in order to obtain the aperiodic structure. The 3 -fold rotation can be adjusted with regard to rotating speed and rotating direction, clockwise or counter clockwise.

The coating can be deposited using any one of the common PVD techniques, such as e-beam evaporation, magnetron sputtering or cathodic arc evaporation. Preferably the coating is deposited using cathodic arc evaporation using two or three pairs of arc sources consisting of pure Ti and/or TiAl alloy(s), in an N₂ or mixed N₂+Ar gas atmosphere .

In one embodiment of the present invention a top layer is deposited having a sufficient thickness, preferably 0.1-1 μm, of Tii__xAl_xN where 0.5≤x≤0.70, preferably Tio.₃₃Alo.s7N or Ti₀.₅₀Al_0-50N to give a visible, black intrinsic color. In yet another embodiment a top layer, fc to facilitate visual wear detection, is deposited of sufficient thickness, preferably 0.1-1 um, preferably of Tio.₇₅Alo.₂₅N, Tio.₈₄Alo.₁eN, Tio.₉₀Sio.₁₀N or TiN to give a visible purple, bronze or yellow intrinsic color.

Finally, the drills are preferably post-treated.

Example 1 (invention)

8 mm drills with two different geometries, Geometry 1 and 2, were made from cemented carbide with a composition of 10 wt-% Co and balance WC. These were coated according to the present invention with an aperiodic multilayered (Ti, Al) N coating, using cathodic arc evaporation. The multilayer structures were deposited from three pairs of arc targets made out of two different chemical compositions, 1 and 2, with the drills mounted on a 3 -fold rotating substrate table arranged in order to obtain the aperiodic structure. The arc evaporation was performed in an Ar+N₂- atmosphere. After depositing the coating, the drills were subjected to a wet blasting treatment.

The composition of the arc targets and the average composition of the coating are shown in Table 1.

The multilayered structure had a sequence of individual layers with an aperiodic, i.e. non-repetitive thickness. Cross section transmission electron microscopy investigation revealed that the individual nitride layer thicknesses ranged from 2 to 50 nm, and the total number of layers in each layer system exceeded 100.

Table 1

Example 2 (reference)

Drills with the same two geometries as in example 1, Geometry 1 and 2, were coated with a commercial (Ti, Al)N-coating. Example 3

Drills with Geometry 1 made according to Example 1 were compared to drills with the same geometry made according to Example 2. Three drills of each drill type were tested in a drilling operation during the following cutting conditions:

Work piece material: SS2541 (34CrNiMo6)

Operation: Drilling

V_c [m/min] 100

Feed [mm/rev] 0.15 a, depth of hole [mm] 20

Internal coolant yes

The results can be seen below. The number of drilled holes is an average of three tests. Tool life criterion was either a measurable wear criterion of v_B=0.3 mm on the clearance or 0.5 mm in the corner, or tool breakage identified by too long chip formation, very bad sound, or chipping over the whole margin.

Table 2

♦Interrupted after 4000 holes, before tool life criterion was reached.

Example 4

Drills with Geometry 2 made according to Example 1 were compared to drills with the same geometry made according to example 2. Three drills of each drill type were tested in a drilling operation during the following cutting conditions :

Work piece material: SS2541 (34CrNiMo6)

Operation: Drilling

V₀ [m/min] 100

Feed [mm/rev] 0.15 a, depth of hole [mm] 20 Internal coolant yes

Table 3

Claims

1. A twist drill comprising a cemented carbide or high speed steel substrate and a multilayered (Ti₇Al)N coating c h a r a c t e r i s e d in that the coating comprises a binary A/B/A/B/A/B structure, where the composition of layer A is Al_xTii_ _XN, where x preferably is 0.40-0.75, more preferably 0.50-0.70, and most preferably 0.60-0.68, and the composition of layer B is Ti_yAlx-_yN, where y preferably is 0.50-1.0, more preferably 0.60- 0.90, and most preferably__0.7jD.rO .-80 and the ^""composition of the whole coating is Ti_zAli__zN, where z preferably is 0.35-0.75, more preferably 0.40-0.65 and where the average thickness of the individual layers A and B preferably is between 1-200 nm, more preferably 2-50 nm.

2. A twist drill according to claim 1, c h a r a c t e r i s e d in that the aperiodic multilayered coating have a total thickness of 0.5 μm to 7 μm, preferably 1 to 5 μm.

3. A twist drill according to any of the preceding claims c h a r a c t e r i s e d in that the substrate is of cemented carbide .

4. A twist drill according to any of claims 1-3 c h a r a c t e r i s e d in that the substrate is of high speed steel.

5. Method of making a drill c h a r a c t e r i s e d in

- providing a substrate, and;

- onto said substrate deposit, by using conventional PVD techniques, an aperiodic multilayered coating having a A/B/A/B/A/B structure from targets consisting of pure Ti and/or TiAl alloy (s) chosen so the composition of layer A is Al_xTii-_xN, where x is preferably 0.40-0.75, more preferably 0.50-0.70, and most preferably 0.-60-0.68, and the composition of layer B is Ti_yAli__yN, where y preferably is 0.50-1.0, more preferably 0.60-0.90 , and most preferably 0.70-0.80 and; the composition for the whole coating d 0.35-0.75, preferably 0.40-0.65 and; where the average thickness of layer A and B is between 1-200 nm, preferably 2-50 nm.

6. A method according to claim 5, c h a r a c t e r i s e d in that the aperiodic multilayered coating have a total thickness of 0.5 μm to 7 μm, preferably 1 to 5 μm.

7. A method according to claim 5 or 6, c h a r a c t e r i s e d in that the substrate is of cemented carbide .

8. A method according to claim 5-7, c h a r a c t e r i s e d in that the substrate is of high speed steel.