CUTTING TOOL

Technical Field

The present invention relates to the field of coated sharp- edged cutting tools made of or comprising a sintered body embracing at least a hard material and a binder material which has been sintered under temperature and pressure to form the body. Cutting tools comprise, e.g., tools for milling (milling tools) , turning tools, indexable inserts, gear cutting tools, hobs, shank type tools, tool for threading, tapping tools.

Background of the Invention

With past and current sintering technology of powder metallurgy, cemented carbide cutting tools have been used both in uncoated and in CVD (chemical vapor deposition) and PVD (physical vapor deposition) coated conditions. CVD coating processes including MT-CVD (moderate temperature CVD or medium temperature CVD) coating processes need high temperatures, usually above 950 ⁰ C for HT-CVD (high temperature CVD) or between 800° C and 900° C for MT-CVD, and a chemically aggressive process atmosphere. This has, amongst others, well known drawbacks with reference to transverse rupture strength (TRS) and low edge strength of the cutting tools as well as to unavoidable thermal cracks of the coating.

A closer look to the drawbacks of HT-CVD should be given in the following with the coating of cemented carbides taken as an example:

a) As mentioned before, reduction of TRS of the substrate may be due to the fact that the state of the surface prior to coating is one of residual compressive stress induced by the correct grinding process, which is beneficial; this state is altered by high temperature which relieves this beneficial residual compressive stress. Therefore, independent of the coating, high temperature annealing has this effect on the carbide substrate. However, even if the substrate is not properly- ground - for instance, if it is subjected to "abusive grinding" which leaves residual tensile stress or even some surface cracks - the high temperature treatment has essentially no beneficial effect.

b) A further reduction of the TRS of the coated tool originates from the presence of thermal cracks induced by thermal expansion mismatch between the coating and substrate upon cooldown from the high CVD temperature.

The cracks run through the thickness of the coating, and thus can initiate fatigue failure under certain cutting conditions .

c) In the case of WC-Co hardmetals, it is also known that cobalt diffuses towards the surface with temperatures of about 850° C and above, which is also associated with decarburization and eta phase formation during the CVD process. Such eta phase can e.g. be formed by the decarburization of the outer region of the substrate in the initial formation of TiC or TiCN CVD first layer which is the usual underlayer for CVD AI ₂ O ₃ coating

layer. The eta phase region forms an embrittled layer of high porosity, again causing micro-cracking initiation sites as well as coating delamination tendency. At least this drawback of HT-CVD has been overcome with MT-CVD e.g. by applying a first TiCN layer at about 850° C, thereby minimizing substrate eta phase formation.

Therefore, different measures have been taken to diminish such detrimental effects. US 4,610,931 suggests to use cemented carbide bodies having a binder enrichment near the peripheral surface. In US 5,266,388 and US 5,250,367, application of a CVD coating being in a state of residual tensile stress followed by a PVD coating being in a state of residual compressive stress has been suggested for the mentioned binder enriched tools.

Despite the fact that cemented carbides have been used to illustrate the drawbacks of CVD coating processes above, the same or at least similar problems are known from other substrates having sintered bodies. Cermets also have Co, Ni (and other metals like Mo, Al, ...) binders and undergo a sintering process similar to cemented carbides. TiCN-based cermets e.g. are not as readily CVD-coated today since these substrates are more reactive with the coating gas species, causing an unwanted reaction layer at the interface. Superhard CBN tools use high-temperature high-pressure sintering techniques different from that used for carbides and cermets. However, they may also have metallic binders such as Co, Ni, ... tending to high temperature reactions during CVD coating processes. These substrates are sometimes PVD-coated with TiN, TiAlN, CrAlN or other coating systems mostly for wear indication at the cutting edges. Such coatings however can only give a limited protection against high temperature and high oxidative stress caused by high

cutting speeds applied with state of the art turning machines for example.

Ceramic tool materials based on solid AI ₂ O ₃ , Al ₂ Os-TiC; or AI2O ₃ -Si ₃ N ₄ (SiAlON) that incorporate glassy phases as binders represent another tool type: tools which are electrically insulating and therefore difficult to coat also with conventional PVD. These materials are sinter-HIPped (HIP: hot isostatic presssing) , as opposed to lower-pressure sintered carbides. Such ceramic inserts again are not CVD coated because high temperature can cause softening of the Si ₃ N ₄ substrate or cause it to loose some toughness as the amorphous glassy binder phase becomes crystalline. Uncoated materials however can allow interaction during metal cutting between their binder phases and the workpiece material and therefore are susceptible to cratering wear restricting use of such tools to limited niche applications.

Therefore, PVD coatings have replaced CVD coatings partially or even completely for many operations with high demands on tool toughness or special needs on geometry. Examples for such tools are tools used for interrupted cut applications like milling or particularly sharp-edged threading and tapping tools. However, due to outstanding thermochemical resistivity and hot hardness, oxidic CVD-coatings such as, e.g. AI ₂ O ₃ in α- and/or γ-crystal structure, or thick multilayers comprising such coatings, are still in widespread use especially for rough-medium turning, parting and grooving applications in all types of materials and nearly exclusively with turning of cast iron. Such coatings could not be produced by PVD processes until recently due to principal process restrictions with electrically insulating materials and especially with oxidic coatings.

As is well known to those skilled in the art, all the problems mentioned above tend to occur and focus on the cutting edge becoming more acute with the smaller radius of the cutting edge. Therefore, to avoid edge chipping or breaking with CVD coated tools, additional geometrical limitations have to be considered for cutting edges and tool tips, with cutting edges limited to a minimum radius of 40 μm for cemented carbides for example. Additionally, further measures like applying a chamfer, a waterfall, a wiper or any other special geometry to the clearance flank, the rake face or both faces of the cutting edge are commonly used but add another often complex-to-handle production step to manufacturing of sintered tool substrates.

Summary of the Invention

It is therefore an object of the invention to provide a single or a multilayer PVD coated sharp edged cutting tool, which can at the same time exhibit satisfactory wear and thermochemical resistance as well as resistance to edge chipping. The cutting tool comprises a sintered body made of a cemented carbide, a CBN, a cermet or a ceramic material having a cutting edge with an edge radius R _e , a flank and a rake face and a single or a multilayer coating consisting of a PVD coating comprising at least one oxidic PVD layer covering at least parts of the surface of the sintered body.

In one embodiment, the edge radius R _e is smaller than 40 μm, preferably smaller than or equal to 30 μm. The covered parts of the surface comprise at least some parts of the sharp edge of the sintered body. It should be mentioned that if after sharpening of the tool there is not any posttreatment like honing, blunting or the like applied, an edge radius R _e

equal or even smaller than 20 μm can be fabricated on sintered tools. Also these tools can be coated beneficially with oxidic PVD coatings as there is not any harmful influence of the coating process, and weakening of the cutting edge does not occur.

The coating is free of thermal cracks and does not contain any halogenides or other contaminations deriving from CVD process gases. Additionally, the coating or at least the oxidic PVD layer can be free of inert elements like He, Ar, Kr and the like. This can be effected by vacuum arc deposition in a pure reactive gas atmosphere. As an example for a multilayer coating, deposition of an adhesion layer and or a hard, wear protective layer can be started in a nitrogen atmosphere followed by a process step characterized by growing oxygen flow to produce a gradient towards the oxidic coating accompanied or followed by a ramp down or shut down of the nitrogen flow. Applying a small vertical magnetic field over a surface area of the cathodic arc target may be beneficial in case of highly insulating target surfaces formed e.g. by arc processes under pure oxygen atmosphere. Detailed instructions how to perform such coating processes can be found in applications WO 2006- 099758, WO 2006-099760, WO 2006-099754, as well as in CH 1166/03 which hereby are incorporated by reference to be a part of the actual disclosure.

The oxidic layer will preferably incorporate an electrically insulating oxide comprising at least one element selected from the group of transition metals of the IV, V, VI group of the periodic system and Al, Si, Fe, Co, Ni, Y, La. (AIi- _x Cr _x )2θ ₃ and AI ₂ O ₃ are two important examples of such materials. Crystal structure of such oxides can vary and may comprise a cubic or a hexagonal lattice like an alpha (α) ,

beta (β) , gamma (γ) , delta (δ) phase or a spinel-structure. For example, oxide layers comprising films of different oxides can be applied to the tool. Despite of the fact that multilayer coatings may comprise nitrides, carbonitrides, oxinitrides, borides and the like of the mentioned elements having sharp or graded transfer zones between defined layers of different elemental or stochiometric composition, it should be mentioned that best protection against high temperature and/or high oxidative stress can be ensured only by a coating comprising at least one layer consisting of essentially pure oxides.

Forming a thermodynamically stable phase, the corundum type structure which for example can be of the type Al ₂ O ₃ , (AlCr) ₂ O ₃ , (AlV) ₂ O ₃ or more generally of the type (Meli- _x Me2 _x ) ₂ O ₃ , with 0.2 < x < 0.98 and Mel and Me2 being different elements from the group Al, Cr, Fe, Li, Mg, Mn, Nb, Ti, Sb, V, will be a preferred embodiment of the oxidic layer. Detailed instructions how to create such corundum type single or multilayered structures can be found in application CH 01614/06 which hereby is incorprated by reference .

In an embodiment of the invention, the coating comprises an adhesion layer situated directly on the body surface, and/or at least one hard wear protective layer situated between the body and the oxidic layer or between two or more consecutive oxidic layers and/or on top of the coating layers. The adhesion layer as well as the wear protective layer therein preferably comprises at least one element of the group of a transition metal from group IV, V, VI of the periodic system of the elements and of Al, Si, Fe, Ni, Co, Y, La. The compunds of the wear protective layer will further comprise N, C, 0, B or a mixture thereof, wherein N, C and CN are

preferred. Examples of such wear protective layers are TiN, TiC, CrN, CrC, TiAlN, CrAlN, TiCrAlN as well as TiCN, CrCN, TiAlCN, CrAlCN, TiCrAlCN.

Compounds of the adhesion layer may comprise N, C, 0 or a mixture thereof, wherein N and 0 is preferred. Examples of such adhesion layers are TiN, CrN, TiAlN, CrAlN, TiCrAlN or TiON, CrON, TiAlON, CrAlON, TiCrAlON. The thickness of the adhesion layer will be preferably between 0.1 and 1.5 μm. If the adhesion layer comprises a thin metallic layer situated directly on the body surface, the thickness of the metallic layer should be between 10 and 200 nm for an optimized tool- to-coating bond. Examples of such metallic interlayers are Ti, Cr, TiAl or CrAl. Overall coating thickness will be between 2 and 30 μm, and due to economy of the coating process in most cases rather between 3 to 10 μm. However, it should be mentioned that in principle, tools can be provided with even thicker coatings if there is a need for some special applications which might be high speed turning in cast iron, for example.

Another embodiment of the invention may encompass a wear protective layer comprising at least one composition- segregated film embracing a phase having a relatively high concentration of a specific element fostering phase segregation of crystal structures like Si or B for example, and a phase having a relatively low concentration of such a specific element. In one embodiment, the phase having a relatively high concentration of the specific element constitutes an amorphous or microcrystalline phase. Such films will preferably comprise a nitride or carbonitride of a combination of Cr and Si or of Ti and Si.

All layers may be deposited according to the actual needs with sharp or gradient layer-to-layer transition zones forming coatings showing a discrete or a gradient layer structure. Thickness of layers may be chosen from several micrometers down to a few nanometers if such structures should be preferable for specific applications.

Contrary to cutting tools comprising oxidic CVD layers, such PVD coated tools need no binder-enriched substrates to minimize the adverse effect of the CVD process to the TRS (transverse rupture strength) of the sintered body. Low process temperatures with PVD processes and the possibility to apply coatings or certain layers, in particular the before-mentioned wear protective layers, in a state of compressive stress proved to be useful measures against crack propagation and the risk of edge chipping. Therefore, there is no longer use for binder-enriched substrates for the majority of the actual cutting applications, which is an evident simplification for carbide tool production.

However, under certain cutting conditions, even PVD coated enriched carbide grades might be useful, for example if cutting parameters should be extended such that higher feed force is applied and an even higher TRS would be preferred.

Due to the potential higher TRS of such PVD coated hardmetal grades, not only cutting tools having a very small edge radius but also cutting tools having a smaller nose radius or point angle can be produced for special fine tooling applications. As an example, compared to conventional cemented carbide inserts having common nose radii of minimal 0.2 mm (0.008 inch) to 2.4 mm (0.094 inch), even radii like 0.15, 0.10, 0.05 and 0.01 mm could be coated and tested

under usual fine turning conditions without signs of premature tip chipping.

Due to inherent "geometric" properties of PVD processes, a further coating feature can be given to certain sintered bodies of simple geometry - as e.g. inserts - solely by using defined fixturing systems, thereby exposing certain areas of the body to a "direct" ions and/or neutrals flow - in the following referred to as particle flow - from the arc or sputter source, whereas other areas are essentially hit by grazing or indirect incident only. In this context

"direct" means that an essential part or the makority of the particles emitted by the arc source hit the surface in an angle of about 90+15°. Therefore, layer growth on such areas is faster than growth on areas exposed to a substantially "indirect" particle flow. This effect can be used to apply coatings of varying thickness during one PVD coating process; which is completely different from CVD processes providing a uniform coating thickness on every surface independent of geometric effects due to different substrate/source positioning.

For example, using a threefold rotating spindle to fix center holed square 13x13x5 mm inserts alternating with 8 mm spacers, a ratio of the flank face thickness (d _F i _an k) and the rake face thickness (d _Ra k _e ) of about 2±0.5 could be adjusted for the inserts over the whole length of the substrate carousel of about 500 mm in a commercial Oerlikon coating unit of the RCS type, or of a length of about 900 mm in a commercially available Oerlikon BAI 1200 coating unit. Thickness measurements were made in the middle of the flank face and, for the rake face, at the bisecting line connecting two opposite noses of the insert in 2 mm distance from the cutting edges defining the point angle of the nose.

Such inserts having a quotient Q _R/F = d _Ra k _e / d _F i _a nk < 1, where d _Rake is the overall coating thickness on the rake face and d _F i _a nk is the overall coating thickness on the flank face, are particulary convenient for milling tools which due to impact stress during milling operations profit from a higher PVD coating thickness on the flank face. This effect is intensified by PVD coatings having a high residual stress which can be controlled by process parameters like substrate bias, total pressure and the like.

Contrary to milling, wear resistance of turning operations benefits from a higher coating thickness on the rake face due to the high abrasive and thermochemical wear caused by the passing chip. Therefore, in this case, quotient Q _R/F should be higher than one: Q _R/F = d _Ra ke / d _F iank > 1 • As for inserts, such a coating distribution can be produced by fixtures exposing the rake phase to direct particle flow of the arc or sputter source. Two-fold rotating magnetic fixtures, for example, can be used to expose a rake face of cemented carbide inserts directly to the source. This magnetic fixture results in additional thickness enhancement at the cutting edge which can be influenced by process parameters like substrate bias and can be utilized to improve the tool performance. For non-magnetic cutting plates, clamping or hooking fixtures can be used as required. Furthermore, for turning tools, a coating design comprising a wear protective layer made of TiN, TiC or TiCN, TiAlN or TiAlCN, AlCrN or AlCrCN situated between the body and the oxidic layer proved to be particularly effective.

Cutting tools according to the invention are applicable to a large variety of different workpiece materials as for instance all types of metals, like nonferrous metals but especially ferrous metals, cast iron and the like. Special

tools for milling or turning of such materials can be optimized as mentioned above. This makes PVD coatings a serious competitor to up-to-date CVD coatings even in until now untouched CVD fields like turning operations especially roughing and high speed finishing of steels and cast irons.

In many cutting applications, tools having an oxidic layer as the outermost layer of the coating system proved to be the best solution. This refers especially to gear cutting tools, hobs or different types of shank type tools including indexable shank type tools.

The following examples are intended to demonstrate beneficial effects of the invention with some special tools and coatings and are not intended in any way to limit the scope of the invention to such special examples. It should be mentioned that several tests have been performed in comparison to well-known applications where PVD coated tools are known to outperform CVD coatings for a long time as e.g. with threading and drilling in different types of metal materials, for dry and wet milling of non-ferrous materials, as well as for certain milling and turning applications on steel or super alloys. For such steel milling, low or medium speed up to 100 m/min but up to high feed rates from 0.2 till 0.4 mm/tooth has been applied. In most cases, tools according to the invention performed as well as or even better than well-known TiCN or TiAlN based PVD coated tools. However, one focus of the invention was to substitute CVD coatings in applications of high thermochemical and/or abrasive wear as for instance with high speed milling of iron, steel and hardened materials as well as turning of steel, iron, as e.g. cast iron, superalloys and hardened materials .

PVD coatings of the following examples have been deposited by a cathodic arc process; deposition temperature was between 500 ⁰ C with comparative TiCN coatings and 550 ⁰ C for oxidic coatings. For oxidic PVD coatings, substrate bias has been pulsed and a small vertical magnetic field having a vertical field component of 3 to 50 Gauss and an essentially smaller horizontal component has been applied. With experiments 25, 28, 35, 37 an additional pulse signal has been superimposed to the DC current of the Al ₀ . ₆ Cr ₀ .4 (AIo.βVo.4) arc sources. Details of such or similar applicable oxide coating processes can be found in WO 2006-099758 and the other before-mentioned documents incorporated by reference. Layer thickness of TiN and TiCN interlayers between the substrate and a top oxidic layer was between 0.5 to 1.5 μm.

Comparative CVD coatings have been deposited with MT-CVD and deposition temperatures of 850° C.

Example A) Milling of alloy steel AISI 4140 (DIN 1.7225) Tool: indexable face mill, one insert z=l Tool diameter: d = 98 mm Cutting speed: v _c = 152 m/min Feed rate: f _z = 0.25 mm/tooth Depth of cut: dc = 2.5 mm Process: down milling with coolant

Insert type: Kennametal SEHW 1204 AFTN, 12 wt% Co; chamfered sharp cutting edges for PVD coating, chamfered and honed to a very slight 40 μm radius for CVD coating.

Example B) Milling of alloy steel AISI 4140 (DIN 1.7225) Tool: indexable face mill, one insert z=l Tool diameter: d = 98 mm Cutting speed: v _c = 213 m/min Feed rate: f _z = 0.18 mm/tooth Depth of cut: dc = 2.5 mm Process: down milling, no coolant Insert type: Kennametal SEHW 1204 AFTN, 12 wt% Co; Edge preparation see example A.

Table 2)

Example C) Milling of alloy steel AISI 4140 (DIN 1.7225) Tool: indexable face mill, one insert z=l Tool diameter: d = 98 mm Cutting speed: v _c = 260 m/min Feed rate: f _z = 0.20 mm/tooth Depth of cut: dc = 3.125 mm

Process: down milling Insert type: Kennametal SEHW 1204 AFTN,

Exp.13, 15, 17,19 Co 6.0 weight% enriched carbide grade, 10.4 weight% cubic carbides.

Exp. 14,16,18,20 Co 6.0 weight% non-enr. carbide grade, 10.4 weight% cubic carbides .

Edge preparation see example A.

Example C, experiment 14 clearly shows the detrimental influence of the CVD process to non-enriched carbide grades, which is due to the mentioned process effects. On the other side, the beneficial influence of a Co-enriched surface zone shows only limited effects with PVD coatings. Advantage of PVD coatings comprising an oxidic layer is obviously as is with examples A and B.

Example D) Turning of stainless steel AISI 430F (DIN 1.4104) Cutting speed: v _c = 200 m/min Feed rate: f _z = 0.20 mm/tooth Depth of cut: dc = 1.0 mm

Process: continous turning of outer diameter

Insert type : Cermet grade, ISO VNMG 160408A11, sharp cutting edges for PVD coating, chamfered and honed to a slight 60 μm radius before CVD coating.

Additionally to the influence of the coating type and material, there can be seen a clear beneficial influence of layer thickness with oxidic PVD coatings. Nevertheless, even most thin oxidic PVD coatings show a better performance than the thick MT-CVD-coating of experiment 22.

Example E) Turning of grey cast iron Cutting speed: v _c = 550 m/min Feed rate: f _z = 0.65 mm/tooth Depth of cut: dc = 5.0 mm

Process : continous turning of outer diameter Insert type: Ceramic, Al ₂ O ₃ -TiC 20%, ISO RNGN 120400T, sharp cutting edges for PVD coating, chamfered and honed to a slight 50 μm radius before CVD coating.

Example F) Turning of forging steel _AISI 4137H (DIN 1.7225! Cutting speed: v _c = 100 m/min Feed rate: f _z = 0.80 mm/tooth Depth of cut: dc = 5 - 15 mm

Process : continous turning of outer diameter Insert type: Cemented carbide, 6% non-enriched, ISO TNMG 330924. Sharp cutting edges for PVD coating, chamfered and honed to a slight 50 μm radius before CVD coating. Table 6)

It could be demonstrated by examples A to F that oxidic coatings can be beneficially applied on sharp edged tools by PVD coating processes. A sharp edge is desirable because it leads to lower cutting forces, reduced tool-tip temperatures to a finer workpiece surface finish and to an essential improvement of tool life.