Patent Grant
8129040
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.
With past and current sintering technology of powder metallurgy cemented carbide cutting tools have been used both in uncoated and in CVD and PVD coated conditions. CVD as well as MT-CVD coating processes need high temperatures, usually above 950° C. for HT-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(high temperature)-CVD should be given in the following with the coating of cemented carbides taken as an example:
Therefore different measures have been taken to diminish such detrimental effects. U.S. Pat. No. 4,610,931 suggests to use cemented carbide bodies having a binder enrichment near the peripheral surface. In U.S. Pat. Nos. 5,266,388 and 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 as 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 as 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 due to high cutting speeds applied with state of the art turning machines as example.
Ceramic tool materials based on solid Al2O3, Al2O3-TiC; or Al2O3 —Si3N4 (SiAlON) that incorporate glassy phases as binders represent another tool type which are electrically insulating and therefore difficult to coat also with conventional PVD. These materials are sinter-HIPped, as opposed to lower-pressure sintered carbides. Such ceramic inserts again are not CVD coated because high temperature can cause softening of the Si3N4 substrate or cause it to lose 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 in parts 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 as e.g. Al2O3 in α- and/or γ-crystal structure, or with needed 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 well known to the man of the art all the problems as 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.
It is therefore the 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. Whereby 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 Re, a flank and a rake face and 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 Re 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 Re 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 harmfull 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, Co, Y, La. (Al1-xCrx)2O3 and Al2O3 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. As 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 from as 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 thermodynamic stable phase the corundum type structure which for example can be of the type Al2O3, (AlCr)2O3, (AlV)2O3 or more generally of the type(Me11-xMe2x)2O3, where 0.2≦x≦0.98 and Me1 and Me2 are different elements from the group Al, Cr, Fe, Li, Mg, Mn, Nb, Ti, Sb or V, will be a preferred embodiment of the oxidic layer. Detailed instructions how to perform 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 actual 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 thereby 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 elements of the wear protective layer will further comprise compounds of N, C, O, B or a mixture thereof, whereby 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.
Elements of the adhesion layer may comprise compounds of N, C, O or a mixture thereof, whereby N and O is preferred. Examples of such adhesion layers are TiN, CrN, TiAlN, CrAlN, TiCrAlN or TiON, CrON, TiAlON, CrAlON, TiCrAlON. Thickness of the adhesion layer will be preferrably between 0.1 to 1.5 μm, both. If the adhesion layer comprises a thin metalic layer situated directly on the body surface thickness of the metalic layer should be between 10 to 200 nm to give an optimized tool to coating bond. Examples of such metallic interlayers are Ti, Cr, TiAl or CrAl. Overall coating thickness will be between 2 to 30 μm, 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 e.g.
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 as an 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 Ti and Si.
All layers may be deposited up 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 of the sintered body. Low process temperatures with PVD processes and the chance to apply coatings or certain layers, especially as 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 actual 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—only by using defined fixturing systems thereby exposing certain areas of the body to a “direct” ions and/or neutrals flow—in the following called 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 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 different thickness during one PVD coating process which is completely different from CVD processes providing a uniform coating thickness on every surface independent from geometric effects due to different substrate/source positioning.
As for example using a threefold rotating spindle to fixture center holed square 13×13×5 mm inserts alternating with 8 mm spacers a ratio of the flank face thickness (dFlank) and the rake face thickness (dRake) 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 e cutting edges defining the point angle of the nose. Such inserts having a quotient QR/F=dRake/dFlank<1, where dRake is the overall coating thickness on the rake face and dFlank 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 phase. 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 QR/F should be higher than 1: QR/F=dRake/dFlank>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 as for example can be used to expose a rake face of cemented carbide made 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 up to the needs. Further on 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 especially effective.
Invented cutting tools 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 inventive tools performed as well 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° C. with comparative TiCN coatings and 550° 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 Al0.6Cr0.4 (Al0.6V0.4) arc sources. Details of such or similar applicable oxide coating processes can be found in WO 2006-099758 and other 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 MTCVD and deposition temperatures of 850° C.
Example C, experiment 14 clearly shows the detrimental influence of the CVD process to non enriched carbide grades, which is due to as 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.
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 thick MTCVD-coating from experiment 22.
It could be demonstrated by examples A to F that oxidic coatings can be benificially 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.
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