RELATED APPLICATION DATA

This application is a § 371 National Stage Application of PCT International Application No. PCT/EP2018/073490 filed Aug. 31, 2018 claiming priority to EP 17188809.2 filed Aug. 31, 2017.

FIELD OF THE INVENTION

The present invention refers to a metal cutting tool comprising a substrate of cemented carbide, cermet, ceramic, steel or high-speed steel, and a multi-layer wear protection coating applied to the substrate. The invention further relates to a use of the metal cutting tool as well as a process for the production of such a tool.

BACKGROUND OF THE INVENTION

Cutting tools, such as those used for example for metal cutting, generally consist of a substrate (base body) made of cemented carbide (hard metal), cermet, steel or high-speed steel having a wear-resistant single-layer or multi-layer coating of metallic hard material layers deposited thereon by means of a CVD process (chemical vapor deposition) or a PVD process (physical vapor deposition). There exist various different PVD processes requiring different equipment and resulting in different coating properties, such as for example cathode sputtering, cathodic vacuum arc evaporation (Arc-PVD), ion plating, electron beam evaporation and laser ablation. Cathode sputtering such as magnetron sputtering, reactive magnetron sputtering and high power impulse magnetron sputtering (HIPIMS) and the arc evaporation belong to the PVD processes most frequently used for the coating of cutting tools.

In the cathodic vacuum arc evaporation (Arc-PVD), an arc arises between the chamber and the target, melting the target material and evaporating the same. Thereby, a large portion of the vaporized material is ionized and subsequently accelerated towards the substrate, having a negative potential (bias potential), and is deposited on the substrate surface. Depending on the used targets and the desired coating chemistry, non-reactive or reactive Arc-PVD processes are used. In the reactive Arc-PVD process a reaction gas is introduced into the PVD reactor, which then reacts with the material evaporated from the target.

Major advantages of the cathodic vacuum arc evaporation process are a high coating rate, dense layer structures due to the high degree of ionization of the vaporized material, as well as a good process stability. On the other hand, a significant disadvantage of the Arc-PVD is the process-dependent deposition of macroparticles (droplets) caused by the emission of small metal splashes, the avoidance of which is extremely complex. These droplets lead to an undesirably high surface roughness of the deposited layers.

When preparing a coated cutting tool for metal cutting, the major goals are a long service life of the tool and high cutting speeds, while providing the tool with the mechanical properties desired for the respective machining operation of the workpiece material.

WO 2014/019897 A1 describes a PVD-coated cutting tool with a wear resistant coating comprising a TiXAlyN coating layer having a multi-layer sub-structure of a plurality of periodically alternating Tix(A)Aly(A)N layers (A) with x(A)+y(A)=1 and Tix(B)Aly(B)N layers (B) with x(B)+y(B)=1, wherein the Al concentration y(B) in layers (B) is higher than the Al concentration y(A) in layers (A). The TiXAlyN coating with multi-layer sub-structure is described to exhibit a higher hardness and Young's modulus compared to conventional TiAlN coatings.

WO 2006/041367 Al describes a PVD-coated cutting tool consisting of a cemented carbide substrate and a coating comprising at least one layer of TiAlN having a thickness of 1.5 to 5 μm and residual compressive stress of a magnitude of >4 to 6 GPa. The TiAlN layer is said to adhere more effectively to the substrate compared to previously known layers.

DE 10 2014 109 942 A1 refers to PVD-coated cutting tools, the coating comprising a M1-xAlxN refractory layer with x≥68 and M being titanium, chromium or zirconium, wherein the refractory layer includes a crystalline cubic phase having a hardness of at least 25 GPa. The coating is referred to as low stress hard coating and is described to have a residual compressive stress of a magnitude of less than 2.5 GPa.

EP 2 634 285 A1 describes a coated cutting tool comprising a substrate and a multilayered (Ti,Al)N coating. The coating comprises three zones: a first zone (A) closest to the substrate, a second zone (B) adjacent to the first zone and a third outermost zone (C), where zones (B) and (C) each comprises a multilayered aperiodic structure of alternating individual (Ti,Al)N layers X and Y, where individual layer X has a composition with a higher Ti content than individual layer Y. The multilayered structure does not have any repeat period in the sequence of at least 10 consecutive individual layers. The average composition for each zone is different from each other and the thickness of zone C is greater than the thickness of zone B. The residual compressive stress of the entire coating is described to be in the range of between−0.5 GPa and−1.5 GPa, preferably between −0.75 GPa and −1,25 GPa.

US 2014/377023 A1 refers to a coated cutting tool comprising a substrate and a comparatively thick PVD coating applied thereto, wherein one compound layer has a thickness of 10 to 30 μm and the coating has internal stress ranges from low tensile stresses, lower than 0.2 GPa, to compressive stress, lower than 3 GPa, but preferably higher than 0.8 GPa, more preferably higher than 1.3 GPa.

Object of the Invention

It is an object of the present invention to provide a coated cutting tool with improved wear resistance, a good service life, especially wherein the coating exhibits high hardness, a high Young's modulus (modulus of elasticity) and, at the same time, good adherence of the coating to the substrate.

DESCRIPTION OF THE INVENTION

The present invention is directed to a coated cutting tool consisting of a substrate and a hard material coating, the substrate being selected from cemented carbide, cermet, ceramics, cubic boron nitride (cBN), polycrystalline diamond (PCD) or high-speed steel (HSS), wherein the hard material coating comprises

a (Ti,Al)N layer stack (L) of alternately stacked (Ti,Al)N sub-layers, the layer stack (L) having the following characteristics:the overall atomic ratio of Ti:Al within the (Ti,Al)N layer stack (L) is within the range from 0.33:0.67 to 0.67:0.33;the total thickness of the (Ti,Al)N layer stack (L) is within the range from 1 μm to 20 μm;each of the individual (Ti,Al)N sub-layers within the (Ti,Al)N layer stack (L) of alternately stacked (Ti,Al)N sub-layers has a thickness within the range from 0.5 nm to 50 nm;each of the individual (Ti,Al)N sub-layers within the (Ti,Al)N layer stack (L) of alternately stacked (Ti,Al)N sub-layers being different in respect of the atomic ratio Ti:Al than an immediately adjacent (Ti,Al)N sub-layer;over the thickness of the (Ti,Al)N layer stack (L) perpendicular to the substrate surface the content of Al increases and the content of Ti decreases from the interface of the (Ti,Al)N layer stack (L) arranged in the direction towards the substrate to the interface of the (Ti,Al)N layer (L) stack arranged in the direction towards the outer surface of the coating;over the thickness of the (Ti,Al)N layer stack (L) perpendicular to the substrate surface the residual stress σ decreases from the interface of the (Ti,Al)N layer stack (L) arranged in the direction towards the substrate to the interface of the (Ti,Al)N layer stack (L) arranged in the direction towards the outer surface of the coating by an amount of at least 150 MPa to at most 900 MPa, whereby the residual stress σ is measured by X-ray diffraction applying the sin2Ψ method based on the (2 0 0) reflection;the residual stress σ within a portion of a thickness of at least 100 nm to at most 1 μm within the (Ti,Al)N layer stack (L) from the interface of the (Ti,Al)N layer stack (L) arranged in the direction towards the substrate is within the range of from 0 MPa to +450 MPa (tensile stress).

The coated cutting tool of the present invention has improved wear resistance and a good service life compared to the prior art, and the coating exhibits high hardness, a high Young's modulus (modulus of elasticity) and, at the same time, good toughness and good adherence of the coating to the substrate. These properties are advantageous in respect of wear resistance, crack resistance, flaking resistance and tool life.

Further, the coating of the coated cutting tool of the present invention exhibits a comparably smooth surface (low surface roughness), which is advantageous in most metal machining operations in respect of wear resistance, friction forces between the contacting surface of the cutting tool and the workpiece, heat development during machining and coating adhesion to the substrate.

It has been shown by XRD measurements that the (Ti,Al)N layer stack (L) of the present invention shows no or substantially no hexagonal crystal structure, even though the content of Al increases and the content of Ti decreases from the interface of the (Ti,Al)N layer stack (L) arranged in the direction towards the substrate to the interface of the (Ti,Al)N layer stack (L) arranged in the direction towards the outer surface of the coating. A high content of hexagonal (Ti,Al)N crystal structure usually impairs the hardness and the Young's modulus of the coating. Therefore, it is preferred that the (Ti,Al)N layer stack (L) has <5 vol-% hexagonal crystal structure, more preferably <2 vol-% hexagonal crystal structure, measured by XRD.

When herein referring to the residual stress (“a”) of a layer or of a substrate, in general, a distinction is made between “tensile residual stress” and “compressive residual stress”, both are herein indicated in GPa (gigapascal) or MPa (megapascal), whereby a positive value (“+”) means “tensile residual stress”, and a negative value (“−”) refers to “compressive residual stress”. Thus, when comparing residual stress levels, herein, a “higher residual stress” means that the residual stress level is more towards tensile residual stress, i.e. starting from compressive residual stress, a “higher residual stress” either means less compressive residual stress or even tensile residual stress, or starting from tensile residual stress, a “higher residual stress” means even more tensile residual stress. It will be understood that an indication of a “lower residual stress” means the opposite direction of stress level comparison.

The substrate of the coated cutting tool of the present invention is selected from cemented carbide, cermet, ceramics, cubic boron nitride (cBN), polycrystalline diamond (PCD) or high-speed steel (HSS), however, in a preferred embodiment the substrate is made of cemented carbide consisting of WC, Co binder phase, and optionally further cubic hard materials, as it is generally known in this field. Cemented carbides usually exhibit some low tensile residual stress at the substrate surface in the order of about +100 to +300 MPa. If desired, a pre-treatment of the cemented carbide substrate by blasting, shot peening and/or brushing prior to the deposition of the coating may be applied to further reduce the amount of residual stress at the substrate surface and, depending on the pre-treatment method, also in regions below the surface.

According to the present invention the residual stress σ of the (Ti,Al)N layer stack (L) is measured by X-ray diffraction applying the sin2Ψ method based on the (2 0 0) reflection. Since XRD always measures over a certain penetration depths into the layer material, it is not possible to measure the residual stress σ of the (Ti,Al)N layer (L) stack only at the very interface to the substrate or to a hard material layer immediately underneath the (Ti,Al)N layer stack (L). Therefore, in the sense of the present invention, the residual stress σ is measured within a portion of a thickness of at least 100 nm to at most 1 μm, preferably at most 750 nm, more preferably to at most 500 nm, most preferably to at most 250 nm, within the (Ti,Al)N layer stack (L) from the interface of the (Ti,Al)N layer stack (L) arranged in the direction towards the substrate. For example, if the final (Ti,Al)N layer stack (L) is intended to have a thickness of 3 to 4 μm, the residual stress σ within the first 100 nm or higher to at most 1 μm from the interface may be measured by depositing the (Ti,Al)N layer stack (L) only to the thickness required for the residual stress σ measurement, e.g. 100 nm to 1 μm, under the same conditions that are then applied for the deposition of the entire (Ti,Al)N layer stack (L). As an alternative, the entire (Ti,Al)N layer stack (L) is first deposited, and then the material is removed to a remaining thickness to be measured. In the latter case of first depositing the entire (Ti,Al)N layer stack (L) and then removing material to reduce the thickness of the (Ti,Al)N layer stack (L), care has to be taken to select and apply a method for the removal of material which does not significantly alter the residual stress within the remaining (Ti,Al)N layer stack (L) material. A suitable method for the removal of deposited coating material may be polishing, however, gentle and slow polishing using a fine-grained polishing agent should be applied. Strong polishing using a coarse grained polishing agent will rather increase the compressive residual stress, as it is known in the art. Other suitable methods for the removal of deposited coating material are ion etching and laser ablation.

In an embodiment of the present invention, the (Ti,Al)N layer stack (L) is deposited immediately to the substrate surface, i.e., the (Ti,Al)N layer stack (L) is in direct contact and has an interface with the substrate surface.

In another embodiment of the present invention, the coating comprises one or more further hard material layers between the substrate and the (Ti,Al)N layer stack (L), i.e., the (Ti,Al)N layer stack (L) is in direct contact and has an interface with the outermost of the one or more further hard material layers.

It has been found that the improved properties of the present invention, including improved adhesion of the inventive coating to the substrate and improved wear resistance, are achieved if the residual stress σ of the (Ti,Al)N layer stack (L) at the interface to the substrate or to a hard material layer immediately underneath the (Ti,Al)N layer stack (L) matches to the residual stress σ at the surface of the substrate or the hard material layer. In this context, “matches” means that the difference between the respective residual stress values (Δσ) is low. According to the invention, the residual stress σ of the (Ti,Al)N layer stack (L) at the interface to the substrate or to a hard material layer immediately underneath is in the range from 0 MPa to +400 MPa, i.e., low tensile residual stress, which matches good to the low tensile residual stress in the order of about 100 to 300 MPa usually found at the surface of cemented carbide substrates.

Accordingly, in a preferred embodiment of the present invention the difference between the absolute amounts of the residual stresses σ of (i) the portion of a thickness of at least 100 nm to at most 1 μm within the (Ti,Al)N layer stack (L) from the interface of the (Ti,Al)N layer stack (L) arranged in the direction towards the substrate and of (ii) the material arranged immediately underneath, which is either the surface of the substrate or a hard material layer arranged between the substrate and the (Ti,Al)N layer stack (L), is 400 MPa, preferably 300 MPa, more preferably 200 MPa, particularly preferably 100 MPa.

In a preferred embodiment of the coated cutting tool of the present invention the alternately stacked TiAlN sub-layers of the (Ti,Al)N layer stack (L) of the coating are deposited by Arc-PVD. More preferably the entire coating is deposited by Arc-PVD.

The deposition of the inventive coating by Arc-PVD provides for a high coating rate, which has advantages for economic reasons in the production process. However, the Arc-PVD deposited inventive coating also exhibits advantageous structural features over other PVD or even CVD deposition methods, such as particularly dense layer structures resulting in the beneficial properties of the coating in metal machining. At the same time, even though the Arc-PVD process generally tends to produce droplets leading to undesirably high surface roughness of the deposited layers, in the Arc-PVD process applied according to the present invention the droplet formation is reduced, and the coatings of the present invention exhibit comparably smooth surface roughness.

In another preferred embodiment of the coated cutting tool of the present invention the difference of the of the atomic ratio Ti:Al of each of the individual (Ti,Al)N sub-layers within the (Ti,Al)N layer stack (L) of alternately stacked (Ti,Al)N sub-layers over the atomic ratio Ti:Al of an immediately adjacent (Ti,Al)N sub-layer is within the range from 0.2 to 1.8 or from 0.3 to 1.5 or from 0.4 to 1.0 or about 0.5.

In another preferred embodiment of the coated cutting tool of the present invention the decrease of the residual stress σ over the thickness of the (Ti,Al)N layer stack (L) perpendicular to the substrate surface from the interface of the (Ti,Al)N layer stack (L) arranged in the direction towards the substrate to the interface of the (Ti,Al)N layer stack (L) arranged in the direction towards the outer surface of the coating is limited to 400 MPa/μm (μm layer thickness), preferably 300 MPa/μm more preferably ≤200 MPa/μm.

In another preferred embodiment of the coated cutting tool of the present invention the atomic ratio Ti:Al of the individual (Ti,Al)N sub-layers having a lower Ti content than an immediately adjacent (Ti,Al)N sub-layer is within the range from 0.2:0.8 to 0.7:0.3, preferably from 0.3:0.7 to 0.6:0.4, and/or the atomic ratio Ti:Al of the individual (Ti,Al)N sub-layers having a higher Ti content than an immediately adjacent (Ti,Al)N sub-layer is within the range from 0.3:0.7 to 0.8:0.2, preferably from 0.4:0.6 to 0.6:0.4, most preferably about 0.5:0.5.

In another preferred embodiment of the coated cutting tool of the present invention the increase of the Al content and the decrease of the Ti content over the thickness of the (Ti,Al)N layer stack (L) perpendicular to the substrate surface occurs step-wise or gradual.

In this context, a gradual increase of the Al content and decrease of the Ti content means that the atomic ratios Ti:Al within pairs of adjacent (Ti,Al)N sub-layers of lower Ti content and higher Ti content decrease essentially continuously over the thickness of the (Ti,Al)N layer stack (L). This includes that the atomic ratios Ti:Al from each pair of adjacent (Ti,Al)N sub-layers to the next pair of adjacent (Ti,Al)N sub-layers decreases. However, this also includes that the atomic ratios Ti:Al over several pairs of adjacent (Ti,Al)N sub-layers remains constant over a certain thickness portion within the (Ti,Al)N layer stack (L). “Essentially continuously” means that the decrease from one atomic ratio Ti:Al to the next occurs by a moderate amount.

A step-wise increase of the Al content and the decrease of the Ti content includes that the atomic ratio Ti:Al over several pairs of adjacent (Ti,Al)N sub-layers remains constant over a certain thickness portion within the (Ti,Al)N layer stack (L), and then the atomic ratio Ti:Al in the next several pairs of adjacent (Ti,Al)N sub-layers is lowered by a significant amount (step). In one embodiment a number of several pairs of adjacent (Ti,Al)N sub-layers with constant atomic ratio Ti:Al form a (Ti,Al)N sub-layer stack.

The increase of the Al content and the decrease of the Ti content within individual (Ti,Al)N sub-layers, either in the (Ti,Al)N sub-layers having the lower Al content, or the ones having the higher Al content, or in both, can be achieved by several ways individually or in combination. For example, an increase of the Al content and the decrease of the Ti content can be achieved by the selection of the types and number of targets containing certain amounts of Al and Ti during the progress of the deposition process. Further, the Al and Ti contents in the deposited coating layers can be varied by varying the deposition condition, such as bias and arc current.

Also, the increase of the Al content and the decrease of the Ti content over the thickness of the (Ti,Al)N layer stack (L) perpendicular to the substrate surface can be achieved by an increase of the thicknesses of the individual (Ti,Al)N sub-layers having higher Al contents than the thicknesses of the individual (Ti,Al)N sub-layers having lower Al contents.

An increase of the Al content and the decrease of the Ti content over the thickness of the (Ti,Al)N layer stack (L) perpendicular to the substrate surface may lead to different residual stresses between parts of the (Ti,Al)N layer stack (L). An increased Al content, i.e., a lower Ti:Al ratio, may give a decreased residual stress σ. Also, the residual stress σ within the (Ti,Al)N layer stack (L) can be influenced by varying the deposition condition, such as bias and arc current. Thus, by varying the Ti:Al ratio over the thickness of the (Ti,Al)N layer stack (L), and/or the deposition conditions during deposition of the (Ti,Al)N layer stack (L), a step-wise or gradual change in residual stress σ can be obtained.

As stated before, a step-wise increase of the Al content and the decrease of the Ti content can be realized by providing (Ti,Al)N sub-layer stacks of several pairs of adjacent (Ti,Al)N sub-layers with each (Ti,Al)N sub-layer stack having an essentially constant atomic ratio Ti:Al.

Therefore, in a preferred embodiment of the coated cutting tool of the present invention the (Ti,Al)N layer stack (L) comprises two or more (Ti,Al)N sub-layer stacks (L1, . . . L2, Lx) arranged immediately on top of each other, wherein within the same (Ti,Al)N sub-layer stack (L1, L2, . . . Lx) there exists a first type of individual (Ti,Al)N sub-layers each of them having the same composition with respect to the Ti:Al atomic ratio and the same thickness, and a second type of individual (Ti,Al)N sub-layers each of them having the same composition with respect to the Ti:Al atomic ratio and the same thickness, wherein the first and second types of individual (Ti,Al)N sub-layers have different Ti:Al atomic ratios. Preferably the number of (Ti,Al)N sub-layer stacks (L1, L2, . . . Lx) is from 2 to 6, more preferably from 2 to 4.

In this embodiment, wherein the (Ti,Al)N layer stack (L) comprises two or more (Ti,Al)N sub-layer stacks (L1, L2, . . . Lx) the overall Al content within each of the (Ti,Al)N sub-layer stacks (L1, L2, . . . Lx) increases from one (Ti,Al)N sub-layer stack to the next (Ti,Al)N sub-layer stack in the direction towards the outer surface of the coating. This increase of the Al content is herein referred to as “step-wise”.

In another preferred embodiment of the coated cutting tool of the present invention, wherein the (Ti,Al)N layer stack (L) comprises (Ti,Al)N sub-layer stacks, the (Ti,Al)N layer stack (L) consists of two (Ti,Al)N sub-layer stacks (L1, L2) arranged immediately on top of each other.

In another preferred embodiment of the coated cutting tool of the present invention the average grain size within the (Ti,Al)N layer stack (L) decreases from the interface of the (Ti,Al)N layer stack (L) arranged in the direction towards the substrate to the interface of the (Ti,Al)N layer stack (L) arranged in the direction towards the outer surface of the coating. It has been shown improved wear resistance is achieved when the average grain size within the (Ti,Al)N layer stack (L) decreases, even when comparably thick coatings are applied.

In the embodiment of the invention, wherein the (Ti,Al)N layer stack (L) comprises two or more (Ti,Al)N sub-layer stacks (L1, L2, . . . Lx) the average grain size decreases from one (Ti,Al)N sub-layer stack to the next in the direction towards the outer surface of the coating.

In another preferred embodiment of the coated cutting tool of the present invention the overall residual stress σ of the (Ti,Al)N layer stack (L) in the as-deposited state is <600 MPa, preferably <300 MPa. The overall residual stress σ means the average over the entire thickness of the (Ti,Al)N layer stack.

In another preferred embodiment of the coated cutting tool of the present invention the (Ti,Al)N layer stack (L) has a Vickers hardness HV0.015≥2800, preferably ≥3100, more preferably ≥3300, and/or a reduced Young's modulus >350 GPa, preferably >400 GPa, more preferably >420 GPa.

In another preferred embodiment of the coated cutting tool of the present invention the coating comprises one or more further hard material layers on top of the (Ti,Al)N layer stack (L) and/or between the substrate and the (Ti,Al)N layer stack (L), the one or more further hard material layers containing one or more of the elements selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Al and Si as well as one or more of N, C, O, Ar and B. Preferred further hard material layers include Al2O3layers.

Also, further hard material layers can also include additional (Ti,Al)N layer stacks of the type of the invention. For example, the coating may comprise two or more of the inventive (Ti,Al)N layer stacks (L) separated by one or more hard material layers, as defined before, such as Al2O3layers.

The present invention further includes the use of the coated cutting tool of the present invention for the milling of steel, preferably for the milling of steel of the groups of work piece materials characterized as ISO-P, ISO-M, ISO-S and/or ISO-K, more preferably ISO-P and/or ISO-M, according to DIN ISO standard 513.

ISO-P steel is the largest material group in the metal cutting area, ranging from unalloyed to high-alloyed material, including steel castings and ferritic and martensitic stainless steels. The machinability is normally good, but differs a lot depending on material hardness, carbon content, etc.

ISO-M stainless steels are materials alloyed with a minimum of 12% chromium; other alloys may include nickel and molybdenum. Different conditions, such as ferritic, martensitic, austenitic and austenitic-ferritic (duplex), create a large family. A commonality among all these types is that the cutting edges are exposed to a great deal of heat, notch wear and built-up edge.

ISO-S heat-resistant super alloys include a great number of high-alloyed iron, nickel, cobalt and titanium based materials. They are sticky, create built-up edge, harden during working (work hardening), and generate heat. They are very similar to the ISO-M area but are much more difficult to cut, and reduce the tool life of the insert edges.

ISO-K cast iron is, contrary to steel, a short-chipping type of material. Grey cast irons (GCI) and malleable cast irons (MCI) are quite easy to machine, while nodular cast irons (NCI), compact cast irons (CGI) and austempered cast irons (ADI) are more difficult. All cast irons contain SiC, which is very abrasive to the cutting edge.

ISO-P and ISO-M steels put high demands on fatigue resistance of the tool, and the coated cutting tools of the present invention have shown to exhibit high fatigue resistance and, at the same time, high hardness, a high Young's modulus, good toughness and good adherence.

Furthermore, the coated cutting tools of the present invention are also suitable for work piece materials requiring high abrasive wear, such as ISO-K materials. Due to the improved adhesion properties of the coatings of the present invention, the coatings can be prepared at high coating thicknesses, which results in improved resistance against abrasive wear.

The present invention further includes a process for manufacturing the coated cutting tool of the present invention comprising the steps of depositing a (Ti,Al)N layer stack (L) of alternately stacked (Ti,Al)N sub-layers by means of Arc-PVD (cathodic arc deposition) using at least two different targets each of them containing the metals Ti and Al, but having different contents of Ti and Al, wherein the applied arc current per target for the deposition of the (Ti,Al)N layer stack (L) of alternately stacked (Ti,Al)N sub-layers is within the range from 50 to 180 A, preferably from 100 to 150 A at a target diameter of 100 mm.

It has been shown that this process enables the production of the tools as described above. In particular, using this process coatings are obtainable having no or a minimum hexagonal structure within the (Ti,Al)N layer stacks, and a particularly low surface roughness is achieved.

In a preferred embodiment the process of the present invention comprises mechanical post-treatment of the coated tool by blasting, shot-peening or brushing, wherein the conditions of the post-treatment are selected to reduce the residual stress of the overall coating by ≥100 MPa, preferably ≥150 MPa. The mechanical post-treatment generally reliefs tensile residual stress and/or introduces compressive residual stress in the coating, sometimes also down into the substrate depending on the mechanical post-treatment conditions (applied energy) and the coating thickness and type.

The preferred mechanical post-treatment method is dry or wet blasting applying a blasting pressure in the range of 1 to 10 bar, a blasting time in the range of 2 to 60 seconds, a blasting angle of 90° relative to the blasted surface, and using a blasting medium selected from Al2O3, ZrO2or other hard materials generally used in this technology.

In another embodiment the process of the present invention comprises mechanical pre-treatment of the uncoated substrate prior to the deposition of the coating by blasting, shot-peening or brushing to reduce the residual stress of the substrate surface, wherein the conditions of the pre-treatment are preferably selected to reduce the residual stress of the substrate surface to a value within the range of from −300 MPa to +500 MPa, preferably from −100 MPa to +400 MPa, more preferably from 0 MPa to +300 MPa. The mechanical pre-treatment is useful to reduce the residual stress of the substrate surface to better match with the residual stress of the coating layers applied immediately on the substrate surface. Further, the mechanical pre-treatment is useful to clean the substrate surface prior to the Arc-PVD coating. It may be carried out by the same means as the post-treatment, however, ultrasound or plasma treatment may be applied in addition to or instead of blasting, shot-peening or brushing.

In another embodiment the process of the present invention the Arc-PVD deposition of the (Ti,Al)N layer stack (L) is carried out at a nitrogen pressure in the range from 5 Pa to 15 Pa, preferably from 8 Pa to 12 Pa. If the nitrogen pressure is too low, such as 4 Pa or lower, as it is used in many prior art processes, it has turned out that the roughness of the coating becomes undesirably high due to increased droplet formation, as can be seen from the examples below andFIG. 1.

Further features, embodiments, and advantages of the present invention will become apparent from the following description of examples of the present invention. However, the examples are not intended to limit the invention in any way.

