The present invention relates to a coated cutting tool insert particularly useful for short hole drilling in steel at high speed and moderate feed.

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 as 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 an insert of tough grade is used and on the periphery a more wear resistant one . The cutting inserts are brazed or mechanically clamped.

The objective of the invention is to provide a coated cutting tool insert with ability to perform at higher speeds , maintaining a good balance , regarding tool life , between periphery and center inserts .

US 5 , 786 , 069 and US 5 , 863 , 640 disclose coated cutting tool inserts with a binder phase enriched surface zone and a highly W-alloyed binder phase .

Figure 1 is a schematic drawing of a cross section of an edge of an insert gradient sintered according to the present invention where

A = binder phase enriched surface zone B = cutting edge near zone

C = a line essentially bisecting the edge .

Significant improvements with respect to resistance to plastic deformation and toughness behaviour can simultaneously

be obtained for a cemented carbide insert if a number of features are combined. The improvement in cutting performance of the cemented carbide inserts can be obtained if the cobalt binder phase is highly alloyed with W, if the essentially cubic carbide free and binder phase enriched surface zone A has a certain thickness and composition, if the cubic carbide composition near the cutting edge B is optimised and if the insert is coated with a 3-12 μm columnar Ti (C,N) -layer followed by a 2-12 μm thick Al2θ3~layer e .g . produced according to any of the patents US 5 , 766 , 782 , US 5 , 654 , 035 , US 5 , 674 , 564 or US 5 , 702 , 808 possibly with an outermost layer of 0.5-4 μm TiN. The Al ₂ θ ₃ ~layer will serve as an effective thermal barrier during cutting and thereby improve not only the resistance to plastic deformation which is a heat influenced property but also increase the crater wear resistance of the cemented carbide insert . In addition, if the coating along the cutting edge is smoothed by an appropriate technique like by brushing with a SiC-based nylon brush or by a gentle blasting with AI ₂ O ₃ grains the cutting performance can be enhanced further, in particular with respect to flaking resistance of the coating (see US 5 , 861, 210) .

According to the present invention there is now provided a coated cemented carbide insert with a <20 μm, preferably 5-15 μm, thick essentially cubic carbide free and binder phase ' enriched surface zone A (Fig. l) with an average binder phase content (by volume) of 1.2-3.0 times the bulk binder phase content . In order to obtain high resistance to plastic deformation but simultaneously avoid a brittle cutting edge the chemical composition is optimised in zone B (Fig. l) . Along line C (Fig. l) , in the direction from edge to the centre of the insert , the binder phase content increases essentially monotonously until it reaches the bulk composition. At the edge the binder phase content in vol-% is 0.65-0.75 , preferably about 0.7 times the binder phase content of the bulk. In a similar way, the cubic carbide phase content decreases along line C from about 1.3 times the content of the bulk. The depth of the binder phase depletion and cubic

carbide enrichment along line C is 100-300 μm, preferably 150- 250 μm.

The binder phase is highly W-alloyed. The content of W in the binder phase can be expressed as a CW-ratio = M ₃ / (wt-% Co • 0.0161) where

M _s is the saturation magnetisation of the cemented carbide body in hAm ² /kg and wt-% Co is the weight percentage of Co in the cemented carbide . The CW-ratio takes a value <1 and the lower the CW-ratio is the higher is the W-content in the binder phase . It has now been found according to the invention that an improved cutting performance is achieved if the CW- ratio is 0.75-0.90 , preferably 0.80-0.85.

The present invention is applicable to cemented carbides with a composition of 4-7 weight percent of binder phase consisting of Co, and 7-10 weight percent cubic carbides of the metals from groups IVa, Va or Via of the periodic table, preferably >1 wt% of each of Ti , Ta and Nb and as balance WC. The WC has an average grain size of 1.0 to 4.0 μm, preferably 2.0 to 3.0 μm. The cemented carbide body may contain small amounts , <1 volume-%, of η-phase (MgC) .

The coating comprises

- a first , innermost , layer of TiC _x NyO ₂ with x+y+z=l, pre- ferably z<0.5 , with a thickness of 0.1-2 μm, and with equiaxed or columnar grains with size <0.5 μm

- a next layer of TiC _x NyO ₂ x+y+z=l, preferably with z=0 and x>0.3 and y>0.3 , with a thickness of 4-7 μm with columnar grains and with a diameter of about <5 μm, preferably <2 μm - a next layer of TiC _x NyO ₂ , x+y+z=l with z< _. 0.5 , preferably z>0.1 , with a thickness of 0.1-2 μm and with equiaxed or needle-like grains with size < _. 0.5 μm, this layer being the same as or different from the innermost layer,

- an outer layer of a smooth, textured, fine-grained (grain size about 1 μm) a-Al ₂ U3 layer with a thickness of 3 -6 μm and a surface roughness (R _a ) of less than 0.3 mm over a measured length of 0.25 mm, and

- possibly an outermost layer of 0.5-4 μm TiN .

In addition, the OC-AI2O3 layer has a preferred crystal growth orientation in either the ( 012) - , (104) - or ( HO ) - direction, preferably in the ( 012) -direction, as determined by X-ray Diffraction (XRD) measurements . A Texture Coefficient, TC, is defined as :

τc ₍ hki ₎ - ^{■ i} where I (hkl) = measured intensity of the (hkl) reflection

I _o (hkl) = standard intensity of the ASTM standard powder pattern diffraction data n = number of reflections used in the calculation, (hkl) reflections used are : ( 012) , (104) , ( 110) , (113) , (024 ) , ( 116)

According to the invention TC for the set of (012) , (104 ) or (110) crystal planes is larger than 1.3 , preferably larger than 1.5.

The invention also relates to a method of making cutting inserts comprising a cemented carbide substrate consisting of a binder phase of Co, WC and a cubic carbonitride phase with a binder phase enriched surface zone essentially free of cubic phase and a coating. The powder mixture contains 2-10 , preferably 4-7, weight percent of binder phase consisting of Co, and 4-12 , preferably 7-10 , weight percent cubic carbides of the metals from groups IVa, Va or Via of the periodic table, preferably >1 wt% of each Ti , Ta and Nb and a balance WC with an average grain size of 1.0-4.0 μm, preferably 2.0-3.0 μm.

Well-controlled amounts of nitrogen have to be added either through the powder as carbonitrides or/and added during the sintering process via the sintering gas atmosphere . The amount of added nitrogen will determine the rate of dissolution of the cubic phases during the sintering process and hence determine the overall distribution of the elements in the cemented carbide after solidification. The optimum amount of nitrogen to be added depends on the composition of the cemented carbide and

in particular on the amount of cubic phases and varies between 0.9 and 1.7% , preferably about 1.1-1.4% , of the weight of the elements from groups IVa and Va of the periodic table . The exact conditions depend to a certain extent on the design of the sintering equipment being used. It is within the purview of the skilled artisan to determine whether the requisite surface zones A and B of cemented carbide have been obtained and to modify the nitrogen addition and the sintering process in accordance with the present specification in order to obtain the desired result .

The raw materials are mixed with pressing agent and possibly W such that the desired CW-ratio is obtained and the mixture is milled and spray dried to obtain a powder material with the desired properties . Next, the powder material is compacted and sintered. Sintering is performed at a temperature of 1300-1500 ⁰ C, in a controlled atmosphere of about 5 kPa followed by cooling . After conventional post sintering treatments including edge rounding a hard, wear resistant coating according to above is applied by CVD- or MT-CVD- technique .

According to method of the invention a WC-Co-based substrate is coated with

- a first, innermost, layer of TiC _x NyO ₂ with x+y+z=l , pre- ferably z<0.5 , with a thickness of 0.1-2 μm, and with equiaxed or columnar grains with size <0.5 μm using known CVD-methods .

- a next layer of TiC _x NyO ₂ x+y+z=l , preferably with z=0 and x>0.3 and y>0.3 , with a thickness of 2 -10 μm, preferably 4-7 μm, with columnar grains and with a diameter of about <5 μm, preferably <2 μm, deposited either by MTCVD-technique

(using acetonitrile as the carbon and nitrogen source for forming the layer in the temperature range of 700-900 ⁰ C) or by high temperature CVD-technique (1000-1100 ⁰ C) , the process conditions being selected to grow layers with columnar grains , that is generally high process pressure (0.3-1 bar) . However, the exact conditions depend to a certain extent on the design of the equipment used.

- a next layer of TiC _x NyO ₂ , x+y+z=l with z^0.5 , preferably 23>0.1 , with a thickness of 0.1-2 μm and with equiaxed or needlelike grains with size _<0.5 μm, using known CVD-methods this layer being the same as or different from the innermost layer .

- an outer layer of a smooth textured (X-AI ₂ O ₃ layer with a thickness of 2-10 μm, preferably 3 -6 μm, and a surface roughness (R _a ) of less than 0.3 μm over a measured length of 0.25 mm according to US 5, 487 , 625, US 5, 851, 687 or US 5, 766, 782. When a TiC _x NyO ₂ -layer with z>0 is desired, CO ₂ and/or CO is added to the reaction gas mixture .

Example 1

A. ) Cemented carbide drilling inserts of the style CoroDrill880 , US0807P-GM, with the composition 5.5 wt% Co, 3.5 wt% TaC, 2.3 wt% NbC, 2.1 wt% TiC and 0.4 wt% TiN and balance WC with an average grain size of 2.5 μm were produced according to the invention . The nitrogen was added to the carbide powder as Ti (C, N) . Sintering was done at 1450 ⁰ C in a controlled atmosphere consisting of Ar, CO and some N2 at a total pressure of about 5 kPa .

Metallographic investigation showed that the produced cemented carbide inserts had a cubic-carbide-free zone A with a thickness of 10 μm. Image analysis technique was used to determine the phase composition at zone B and the area along line C (Fig. l) . The measurements were done on polished cross sections of the inserts over an area of about 40 x 40 μm gradually moving along the line C. The phase composition was determined as volume fractions . The analysis showed that the cobalt content in zone B was 0.7 times the bulk cobalt content and the cubic carbide content 1.3 times the bulk cubic carbide content . The measurements of the bulk content were also done by image analysis technique . The Co-content was gradually increasing and the cubic carbide content gradually decreasing along line C in the direction from the edge to the centre of the insert .

Magnetic saturation values were recorded and used for calculating CW-values . An average CW-value of 0.84 was obtained.

The inserts were coated with a 0.5 μm equiaxed Ti (C, N) - layer followed by a 5 μm thick Ti (C,N) layer with columnar grains by using MTCVD-technique (process temperature 850 ⁰ C) . In subsequent process steps during the same coating cycle , a 1 μm thick layer with equiaxed grains of TiC _x NyO ₂ (approx. x=0.6 , y=0.2 and z=0.2 ) was deposited followed by a 4 μm thick layer of ( 012) -textured α-Al2θ3 deposited according to conditions given in US 5 , 487 , 625. XRD-measurement showed a texture coefficient TC ( 012) of 1.5. After coating the inserts were smoothed by wet blasting.

Example 2

Inserts from Example 1 were tested and compared with inserts from Sandvik commercial grade 3040 with respect to wear resistance in a short hole drilling operation. The tested inserts were mechanically clamped on the periphery of the drill head. In the center, an insert of tough grade was used according to Example 1 of Swedish Patent Application No SE 0500234-0. Tool life criteria : flank wear, crater 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 24 mm, 3XD

Insert style : CoroDrill 880 , US0807P-GM

Results . Drilled length at tool life : Inserts according to the invention >15 meters Reference inserts 5 meters

Example 3

Inserts from Example 1 were tested and compared with inserts from Sandvik commercial grade 3040 with respect to wear resistance in a short hole drilling operation. The tested inserts were mechanically clamped on the periphery of the drill head. In the center, an insert of tough grade was used according to Example 1 of Swedish Patent Application No SE 0500234-0. Tool life criteria : flank wear, crater 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: 230 m/min Feed: 0.20 mm/r

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

Results . Drilled length at tool life :

Inserts according to the invention 6 meters Reference inserts 8.4 meters

Example 4 Inserts from Example 1 were tested and compared with inserts from Sandvik commercial grade 3040 with respect to wear resistance in a short hole drilling operation. The tested inserts were mechanically clamped on the periphery of the drill head. In the center, an insert of tough grade was used according to Example 1 of Swedish Patent Application No SE 0500234-0. Tool life criteria : flank wear, crater wear or chipping >0.25 mm.

Material : Low alloy steel SS2541-03 , 330-340 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 , US0807P-GM

Results . Drilled length at tool life : Inserts according to the invention 15.4 meters Reference inserts 7 meters

Example 5

Inserts from Example 1 were tested and compared with inserts from Sandvik commercial grade 4025 with respect to wear resistance in a short hole drilling operation. The tested inserts were mechanically clamped on the periphery of the drill head. In the center, an insert of tough grade was used according to Example 1 of Swedish Patent Application No SE 0500234-0. Tool life criteria : flank wear, crater wear or chipping >0.25 mm.

Material : Low alloy steel SS2541-03 , 400 HB .

Coolant : Cooledge 5, 50 bar. Operation: Through hole, 30 mm.

Cutting speed: 300 m/min

Feed: 0.10 mm/r

Drill : Diameter 24 mm, 2XD

Insert style : CoroDrill 880 , US0807P-GM

Results . Drilled length at tool life :

Inserts according to the invention 8.5 meters

Reference inserts 5.3 meters

Example 6

Inserts from Example 1 were tested and compared with inserts from Sandvik commercial grade 3040 with respect to wear resistance in a short hole drilling operation. The tested inserts were mechanically clamped on the periphery of the drill head. In the center, an insert of tough grade was used according to Example 1 of Swedish Patent Application No SE 0500234-0. Tool life criteria : flank wear, crater wear or chipping >0.25 mm.

Material : Low alloy steel SS2541-03 , 285 HB . Emulsion : Syntilo XPS, 7% . Operation: Through hole, 40 mm. Cutting speed: 350 m/min Feed: 0.12 mm/r

Drill : Diameter 16.5 mm, 3XD Insert style : CoroDrill 880 , US0602P-GM

Results . Drilled length at tool life : Inserts invention 7.5 meters Inserts reference 3.5 meters