The present invention relates to a wear resistant coating suitable to be deposited on cutting tool inserts for chip forming metal machining. The coating comprises at least two layers with different grain size, but with essentially the same composition. The coating is deposited by Physical Vapour Deposition (PVD).

The increased productivity in modern chip forming metal machining requires tools with high reliability and excellent wear properties. It has been known since the end of the 1960s that tool life can be significantly improved by applying a suitable coating to the surface of the tool. Chemical Vapour Deposition (CVD) was the first deposition technique used for cutting tools and this method is still commonly used for deposition of TiN, Ti(C,N), and A1 ₂ 0 ₃ layers. Physical Vapour Deposition (PVD) was introduced in the 1980s and has since then been developed from deposition of stable metallic compounds like TiN or Ti(C,N) to include deposition of

multicomponent, metastable compounds like (Ti,Al)N, (Ti,Si)N, or (Al,Cr)N, by such methods as sputtering or cathodic arc evaporation. The properties of these coatings are optimised for specific applications, and thus the performance of the coatings is significantly reduced outside their respective application areas. As an example, fine grained coatings with typical grain sizes of about 5-30 nm find a typical use in end milling with very small chip thicknesses, while coarse grained coatings with typical grain sizes of about 50-500 nm are generally superior as chip thickness and temperature increase in milling and turning applications using indexable inserts.

It is an object of the present invention to provide a coating with high machining performance in a broad area of applications ranging from very small to large chip thicknesses. The present invention relates to a wear resistant coating suitable to be deposited on cutting tool inserts for chip forming metal machining. The coating according to the invention comprises at least two layers with essentially the same composition, but with different grain sizes. The coating has a wide application area ranging from fine machining using end mills to medium or rough machining with indexable inserts. The coating is deposited by Physical Vapour Deposition.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 shows fracture cross section scanning electron microscopy (SEM) images of a coating according to the invention. The coating contains two double layers (marked D in the figure), each containing one fine grained layer (marked A) and one coarse grained layer (marked B). DETAILED DESCRIPTION

The present invention relates to a wear resistant PVD coating for chip forming metal machining cutting tools. The coating comprises one or more D double layers, where each D double layer consists of one inner B layer and one outer A layer, the A layer being deposited onto the B layer without intermediate layers. Layers A and B have essentially the same chemical composition but differ from each other with respect to their average grain widths, W _A and WB, SO that WA < WB. The grain width, w, of a layer is evaluated on a fracture cross section scanning electron microscopy (SEM) image over at least 20 grains along a line perpendicular to the growth direction in the centre of the layer. With essentially the same chemical composition it is herein meant that the A and B layers are deposited from identical cathodes and that, due to differences in process conditions for A and B layer deposition, the resulting A and B layers contain the same chemical elements but the atomic content of each element, excluding nitrogen, may vary within approximately +3 at.% units, while the nitrogen content is constant within each D double layer.

Within each D double layer, preferably the A layer is fine grained with 2 < WA < 50 nm, the B layer has coarse and essentially columnar grains with 30 < WB < 500 nm, and WB/WA > 2. The thickness of the A layer is between 0.03 and 5 μιη, preferably between 0.05 and 2 μιη, and the thickness of the B layer is between 0.1 and 5 μιη, preferably between 0.2 and 2 μιη. The number of D double layers is between 1 and 100, preferably between 1 and 20, most preferably between 1 and 10. The total thickness of all D double layers is between 0.3 and 20 μιη, preferably between 0.5 and 10 μιη. The transitions between A and B layers or between D double layers are preferably either abrupt or gradual, but the coating can also comprise one or more intermediate layers between D double layers to a thickness of between 0.5 and 20 μιη.

The present invention also relates to a coating comprising one or more D double layers with a continuous decrease from coarse to fine grain size within at least one of the D double layers. The D double layer is then divided into two layer portions of fine and coarse grain size, and these layer portions are defined as layers A and B, respectively. The coating according to the invention may further comprise an inner single- and/or multilayer, as known in the art, located between the substrate and the innermost D double layer, and/or an outer single- and/or multilayer, located onto the outermost D double layer, to a total coating thickness of between 0.5 and 30 μιη, preferably between 0.5 and 15 μιη and most preferably between 0.5 and 10 μιη.

In one preferred embodiment the coating comprises one D double layer.

In another preferred embodiment the coating comprises two D double layers. In a third preferred embodiment the A and B layers have compositions according to the chemical formula (Τϊ _1-χ1-γ1 Α1 _χ1 Μ6 _γ1 )(Ν _1-α1 (¾ιΐ)ζΐ, where 0.3 < xl < 0.7, 0 < yl < 0.3, preferably 0 < yl < 0.15, most preferably yl = 0, 0.90 < zl < 1.10, preferably 0.96 < zl < 1.04, 0 < al < 0.5, preferably 0 < al < 0.3, most preferably al = 0. Me is one or more of Zr, Hf, V, Nb, Ta, Cr, Y, Sc, Ce, Mo, W, and Si, preferably one or more of Zr, Hf, V, Nb, Cr, Ce, and Si, and Q is one or more of C, B, S, and O.

In fourth preferred embodiment the A and B layers have compositions according to the chemical formula (Ti ₁ _ _X 2- _y 2Si _X 2Me _y 2)(N ₁ _ _a 2Q _a 2) _Z 2, where 0.02 < x2 < 0.25, 0 < y2 < 0.3, preferably 0 < y2 < 0.15, most preferably y2 = 0, 0.90 < z2 < 1.10, preferably 0.96 < z2 < 1.04, 0 < a2 < 0.5, preferably 0 < a2 < 0.3, most preferably a2 = 0. Me is one or more of Zr, Hf, V, Nb, Ta, Cr, Y, Sc, Ce, Mo, W, and Al, preferably one or more of Zr, Hf, V, Nb, Cr, Ce, and Al, and Q is one or more of C, B, S, and O. In a fifth preferred embodiment the A and B layers have compositions according to the chemical formula (Cr ₁ _ _X 3_y3Al _X 3Me _y 3)(N ₁ - _a 3Qa3)z3 , where 0.3 < x3 < 0.75, 0 < y3 < 0.3, preferably 0 < y3 < 0.15, most preferably y3 = 0, 0.90 < z3 < 1.10, preferably 0.96 < z3 < 1.04, 0 < a3 < 0.5, preferably 0 < a3 < 0.3, most preferably a3 = 0. Me is one or more of Zr, Hf, V, Nb, Ta, Cr, Y, Sc, Ce, Mo, W, and Ti, preferably one or more of Zr, Hf, V, Nb, Cr, Ce, and Ti, and Q is one or more of C, B, S, and O.

The A and B layers according to the invention are deposited by PVD, preferably by cathodic arc evaporation. The variation in grain size can be achieved by several means, for example by 1) changing the magnetic field at the cathode, 2) changing the deposition temperature, and/or 3) changing the evaporation current. It is within the purview of the skilled artisan to determine by experiments the appropriate process conditions. Example 1

A (Ti,Al)N coating according to the invention was deposited by cathodic arc evaporation onto cemented carbide inserts with main composition 90 wt% WC + 10 wt% Co.

Before deposition, the inserts were cleaned in ultrasonic baths of an alkali solution and alcohol. The deposition chamber was evacuated to a base pressure of less than 2.0x10 ^" Pa, after which the inserts were sputter cleaned with Ar ions. The coating was deposited from TiAl composite cathodes with composition Ti:Al = 34:66 in 99.995% pure N ₂ atmosphere at a total pressure of 4 Pa, using a bias voltage of -80 V and an evaporation current of 90 A at 450 °C. The magnetic field in front of the cathode surface was adjusted between two levels, M _str0 ng and M _wea k, to yield A and B layers, respectively, where M _strong is mainly perpendicular to the cathode surface and has a field strength varying over the cathode surface in the range 3-20 mT, and M _wea k is also mainly perpendicular to the cathode surface with a field strength in the range 0.5-2.5 mT. First, a B layer was deposited at M _wea k for 20% of the total deposition time, then an A layer at M _strong for 30%, and then the same sequence was repeated once. The coating was studied with scanning electron microscopy (SEM). Figure 1 shows SEM images of a fracture cross section where the two D double layers, each consisting of one A and one B layer, are clearly seen. The average grain width, w, was evaluated along lines as indicated in figure lb. Both A layers had fine, equiaxed grains with w ~ 19 nm and both B layers had coarser, columnar grains with w ~ 61 nm. The total layer thickness was about 2 μιη.

Example 2 A (Ti,Al)N coating according to the invention was deposited by cathodic arc evaporation onto cemented carbide inserts with main composition 90 wt% WC + 10 wt% Co. The deposition conditions were the same as for example 1, but first a B layer was deposited at M _wea k for 70% of the deposition time and then an A layer at Mstrong for 30%. The average grain widths, w, were evaluated to w ~ 70 nm for layer B and w ~ 18 nm for layer A. The total thickness was about 2 μιη.

Example 3

The coatings from examples 1 (here labelled Invl) and 2 (Inv2) were tested in a milling operation with the following data:

Geometry: XOEX120408R-M07

Application: Square shoulder milling

Work piece material: AISI 316L

Cutting speed: 160 m/min

Feed: 0.15 mm/tooth

Depth of cut: 2 mm

Width of cut: 13 mm (26%)

Tool life criteria: Flank wear (vb) > 0.3 mm

As references, two commercially available (Ti,Al)N coatings of similar composition and thickness as the inventive coating was used, Ref 1 and Ref2. Ref 1 is current state- of-the-art for this specific milling application and has columnar and coarse grains with w ~ 100 nm. Ref2 is fine grained with w ~ 15 nm. Coating Tool life

Invl 15 min

Inv2 17 min

Refl 15 min

Ref2 5 min

The table shows that the attained tool lives for the inventive coatings were found to be on the same level or even higher than that of Refl and significantly higher than that of Ref2.

Example 4

The coatings from examples 1 (here labelled Invl) and 2 (Inv2) were tested in fine machining with coated end mills using the following data:

Geometry: 10 mm square shoulder cemented carbide end mill

Application: Square shoulder milling

Work piece material: Ck45W

Cutting speed: 120 m/min (3800 rpm)

Feed: 0.05 mm/tooth (380 mm/min)

Depth of cut: 13 mm

Width of cut: 5 mm

Tool life criteria: Cutting edge condition (swarf deterioration)

As references, the same commercially available (Ti,Al)N coatings as in example 3 were used, Refl and Ref2. Ref2 is current state-of-the-art for this specific application. Coating Cutting length

Invl 60 m

Inv2 70 m

Refl 10 m

Ref2 60 m

The table shows that the attained tool lives for the inventive coatings were found to be similar to or higher than that of Ref2 and significantly higher than that of Refl .