The present invention relates to a coated cutting tool insert particularly useful for toughness demanding short hole drilling in low alloy and stainless steels . The multilayer coating greatly improves both wear resistance and resistance against plastic deformation at high feeds combined with the low speeds close to the center of the drilled hole . Drilling in metals is divided generally in two types : long hole drilling and short hole drilling. By short hole drilling is meant generally drilling to a depth of up to 3-5 times the drill diameter .

Long hole drilling puts large demands on good chip formation, lubrication, cooling and chip transport . This is achieved through specially developed drilling systems with specially designed drilling heads fastened to a drill rod and fulfilling the above mentioned demands .

In short hole drilling, the demands are not great, enabling the use of simple helix drills formed either of solid cemented carbide or as solid tool steel or of tool steel provided with a number of cutting inserts of cemented carbide placed in such a way that they together form the necessary cutting edge . In the center of the head, a tough grade of insert is sometimes used and on the periphery a more wear resistant one . The cutting inserts are brazed or mechanically clamped.

The object of the present invention is to provide a coated cutting tool insert useful for toughness demanding short hole drilling in steel . US 6, 103, 357 relates to a cutting tool comprising a body of sintered cemented carbide or cermet, ceramic or high speed steel on which at least one of the functioning 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 metal nitrides or carbides with the metal elements M and N selected from the group consisting of Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, Al and W. The sequence of individual layer thicknesses is essentially aperiodic throughout the entire multilayered structure, and layer

thicknesses are larger than 0.1 nm but smaller than 30 nm, preferably smaller than 20 nm. The total thickness of said multilayered coating is larger than 0.5 μm but smaller than 20 μm.

According to the invention there is now provided cemented carbide inserts with excellent toughness properties particularly useful for toughness demanding short hole drilling in low alloy and stainless steels consisting of WC and 8-11 wt-% Co, preferably 9.5-10.5 wt-% Co and 0.2-0.5 wt-% Cr . The WC-grains have an average grain size of 0.5-1.5 μm. The amount of W dissolved in the binder phase is controlled by adjustment of the carbon content by small additions of carbon black or pure tungsten powder. The W-content in the binder phase can be expressed as the "CW-ratio" defined as

CW-ratio = M ₃ / (wt-% Co * 0.0161) where M ₃ is the measured saturation magnetization of the sintered cemented carbide body in hAm2/kg and wt-% Co is the weight percentage of Co in the cemented carbide . The CVϋ-ratio in inserts according to the invention shall be 0.80-0.90.

The coating comprises a laminar, multilayered structure of refractory compounds in polycrystalline, non-repetitive form,

TiN+Tix- _x Al _x N+TiN+Tii- _x AlxN+TiN.... with x=0.4-0.6, preferably 0.5. In said coating the sequence of individual layer thicknesses has no repeat period but is essentially aperiodic throughout the entire multilayered structure . The individual TiN- or Tii- _X A1 _X N-Iayer thickness is larger than 1 nm but smaller than 30 nm, preferably smaller than 20 nm and varies essentially at random. The total thickness of the multilayered coating is >1 μm, preferably >2 μm but <5 μrα, preferably <4 μm.

In one embodiment there is an additional black 0.2-1 μm, preferably 0.3-0.6 μm, thick Tii- _x AlχN-layer on top of the multilayer coating.

In another embodiment there is an additional bronze-coloured homogeneous (Ti _o .84Alo.i6) N-layer with a thickness of about 0.2-0.5, preferably 0.3 μm on top of the multilayer coating. The invention also relates to a method of making coated cemented carbide inserts with excellent toughness properties particularly useful for toughness demanding short hole drilling in low alloy and stainless steels . The cemented carbide consists of WC and 8-11 wt-% Co, preferably 9.5-10.5 wt-% Co and 0.2-0.5 wt-% Cr. The WC-grains have an average grain size of 0.5-1.5 μm. The

raw materials powders are wet milled with pressing agent, small additions of carbon black or pure tungsten powder to obtain a CW- ratio in the sintered inserts of 0.80-0.90. After the wet milling the slurry is dried to a powder, compacted and sintered. After conventional post sintering treatment a coating comprising a laminar, multilayered structure of refractory compounds in polycrystalline, non-repetitive form, TiN+Tii- _x Al _x N+TiN+Tii- _x Al _x N+TiN.... with x=0.4-0.6, preferably 0.5 is deposited by cathodic arc evaporation using two pairs of arc sources consisting of pure Ti and TiAl alloy, respectively in an N2 gas atmosphere . In said coating the sequence of individual layer thicknesses has no repeat period but is essentially aperiodic throughout the entire multilayered structure . The individual TiN- or T±i_ _x A.l _x N- layer thickness is larger than 1 nm but smaller than 30 nm, preferably smaller than 20 nm and varies essentially at random. The total thickness of the multilayered coating is >1 μm, preferably >2 μm but <5 μm, preferably <4 μm.

In one embodiment a final black 0.2-1 μm, preferably 0.3-0.6 μm, thick Tii- _x Al _x N-layer in the multilayer coating is deposited by cathodic arc evaporation using one pair of arc sources consisting of a TiAl-alloy in an N2 gas atmosphere .

In another embodiment a final bronze-coloured homogeneous (Ti _o . ₈₄ Alo.i ₆ ) N-layer with a thickness of about 0.2-0.5, preferably 0.3 μm is deposited on top of the multilayer coating using arc deposition from an arc source consisting of a Tio.84Alo.i6~alloy in an atmosphere of Ar=400 seem and N2=800 seem.

Example 1

Aperiodic multilayers were deposited by cathodic arc evaporation on drilling inserts made of cemented carbide with composition WC + 10wt-% Co and average WC grain size of 1.0 μm and a CW-ratio of 0.86. The coating was deposited from two pairs of arc sources consisting of pure Ti and TiAl alloy, respectively. The arc evaporation was performed in an N2 gas atmosphere . The resulting total coating thickness was 3.0 μm, and consisted of a TiN+Tio. ₅ Al _o . ₅ N multilayer having a sequence of individual lamellae 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 15 nm, and the total number of layers was about 400.

Half of the inserts were coated with an additional black Tio. ₅ Al _o . ₅ N-layer of about 0.3-0.6 μm thickness .

The other half of the inserts was coated with an additional homogeneous (Tio. ₈ 4Alo. ₁ e) N-layer with a thickness of about 0.3 μm. This layer was deposited using arc evaporation from an arc source consisting of a Tio. ₈₄ Alo.i ₆ ~alloy in an atmosphere of Ar=400 seem and N2=800 seem. In this way a stable bronze colour, on all inserts and also from batch to batch, was obtained.

Example 2

Bronze coloured inserts from example 1 were tested and compared with inserts from Sandvik commercial grade 1020 with respect to toughness in a short hole drilling operation . The tested inserts were mechanically clamped on the center of the drill head. In the periphery, inserts according to Swedish Patent Application No SE 0500235-7 were used. Tool life criteria: crater wear, plastic deformation, flank wear, or chipping >0.25 mm.

Material : Low alloy steel SS2541-03, 285 HB . Emulsion: Blasocut BC25, 7% .

Operation : Through hole, ^" 48 mm.

Cutting speed: 260 m/min

Feed: 0.10 mm/r

Drill : Diameter 23 mm, 3XD Insert style : CoroDrill 880, US0802C-GM

Results . A surprisingly significant difference in tool life, regarding crater wear resistance, was seen . The inserts according to the invention showed a much improved crater wear resistance compared to the inserts reference . Drilled length at tool life :

Inserts invention >40 meters

Inserts reference tool failure after 30 meters

Example 3 Black inserts from Example 1 were tested and compared with inserts from Sandvik commercial grade 1020 with respect to toughness in a short hole drilling operation. The tested inserts were mechanically clamped on the periphery of the drill head. In the center, bronze coloured inserts from Example 1 were used. Tool

life criteria : crater wear, plastic deformation, flank wear, or chipping >0.25 mm.

Material : Low alloy steel SS2541-03, 270-285 HB . Emulsion : Blasocut BC25, 7% .

Operation: Through hole, 50 mm. Cutting speed : 200 m/min Feed: 0.15 mm/r

Drill : Diameter 24 mm, 3XD Insert style : CoroDrill 880 , US0807P-GM

Results . Drilled length at tool life :

Inserts invention >20 meters

Inserts reference 13.3 meters

Example 4

Bronze coloured inserts from Example 1 were tested and compared with inserts from Sandvik commercial grade 1020 with respect to toughness in a short hole drilling operation. The tested inserts were mechanically clamped on the center of the drill head. In the periphery, black inserts from Example 1 were used. Tool life criteria : crater wear, plastic deformation, flank wear, or chipping >0.25 mm.

Material : Low alloy steel SS2541-03, 300 HB.

Emulsion : Syntilo XPS , 6.5% , 10 bar .

Operation : Through hole, 40 mm.

Cutting speed: 150 m/min

Feed: 0.20 mm/r Drill : Diameter 24 mm, 3XD

Insert style : CoroDrill 880, US0802C-GM

Results : At high feeds combined with the low speeds near the center of the hole the inserts invention showed a much improved wear resistance and resistance against plastic deformation compared to the inserts reference . Drilled length at tool life : Inserts invention 13.5 meters Inserts reference 2.3 meters