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HARD COATING AND ITS FORMATION METHOD, AND IIARD-COATED
TOOL
FIELD OF THE INVENTION
The present invention relates to a hard coating having excellent oxidation resistance, weai resistance, peel resistance, seizure resistance, impact lesistance, breakage resistance, etc, which covers a substrate of cemented carbide, high-speed steel, die steel, etc , a method for forming such a hard coating, and a tool having such a hard coating
BACKGROUND OF THE INVENTION
As high-speed metal cutting at a feed per one edge exceeding 0 3 mm becomes prevailing, conventional hard-coated tools have become insufficient in oxidation resistance, wear resistance, peel resistance, seizure resistance, impact resistance, breakage resistance, etc of hard coatings Accordingly, various technologies have been proposed to improve oxidation resistance, wear lesistance, peel lesistance, seizure resistance, impact resistance, bteakage lesistance, etc of the hard coalings
IP 2003-225807 A discloses a cutting tool having a hard coating layer exhibiting excellent wear resistance m high-speed cutting, the hard coating layer being formed by physically depositing a composite nitride of Ti and Y to an average thickness of 1-15 µm on a substrate of tungsten carbide-based cemented carbide or titanium caiborutride cermet, the hard coating layer having maximum-Y-component points (mimmum-Ti-component points) and Y-component-fiec points (TiN points) alternately at a predetermined interval in a layer thickness direction, the concentration of the Y component changing smoothly between the maximum-Y-component point and the Y-component-frec point, the maximum-Y-component points having a composition represented by the formula of (Ti1-Y)N, wherein x is 0 05-0 15 by atomic ratio, an interval
between the adjacent maximum-Y-component point and the Y-component-free point being 0 01-0 1 µm Japanese Patent 3,460,288 discloses a weai-resistant coated member comprising a substrate and a haid coating formed on its surface, the hard coating being formed by layers of nitrides, oxides, carbides, carbomtrides or botides of 2 or more elements selected from the group consisting of metal elements ot Groups 4a, 5a and 6a, Al and Si, such that their compositions change continuously at a period of 0 4 nm to 50 nm to a total thickness of 0 5-10 µm The hard coatings of JP 2003-225807 A and Tapanese Patent 3,460,288 are multi-layer films with tcpeatcdly changing concentrations or compositions However, because any of the above technologies uses only an arc-discharge ion plating method, edge , they do not necessarily have satisfactory seizure resistance in the cutting of steel, etc , on which seizure tends to occur
The seizure resistance of hard coatings depends on their lubrication Hard-coated tools satisfactorily usable undei seveie cutting conditions cannot be obtained without improvin g the lubrication, impact resistance and breakage resistance of hard coatings with no damage to adhesion to substrates, hardness, oxidation lesistance, wear resistance, thermal resistance, etc In addition, taking environment into consideration, demand is mounting on tools for use in dry cutting without using cutting oils containing Cl, S, P, etc , foi instance, even in the cutting of die-casting steel, called hard-to-cut materials
As a cutting tool having a hard coating with improved lubrication, JP 5-239618 A discloses a machining tool with a coating having high wear resistance and lubrication, the coating comprising at least one element selected from the group consisting of oxygen, sulfur, selenium and tellurium, and at least one element selected from the group consisting of vanadium, niobium, tantalum, chiomium, molybdenum and tungsten, and contammg molybdenum disulfide for imparting lubrication JP 11-509580 A discloses a method for forming a high-lubrication, hard coating comprising, for instance, molybdenum disulfide
and TiN on a cutting tool, using a sputtenng-ion plating system comprising a first target made of a metal sulfide (foi instance, molybdenum disulfide), and a second target made of at least one metal selected from the group consisting of titanium, vanadium, chromium, zirconium, niobium, molybdenum, tantalum, hafnium and tungsten However, these technologies do not provide hard coatings with sufficient adhesion and hardness, failing to sufficiently improve the wear resistance of cutting tools
JP 8-127863 A discloses a wear-resistant, hard laminate coating comprising as main components at least one element selected from the group consisting of elements of Groups IVa, Va and Via in the Periodic Table, Al, Si and B, and at least one element selected from the group consisting of B, C, N and O, the coating having at least 2 types of compound layers having different compositions and composition-changing layers, whose element compositions change in a thickness direction between the compound layers, the compound layers and the composition -changing layer being periodically laminated, and a crystal lattice being continuous over one period or more between the layers Japanese Patent 3,416,938 discloses a multi-layer, hard coating for cutting tools, etc , in which compound layers (for instance, TiN) and layers with element compositions changing in a thickness direction (for instance, TiAIN) are alternately laminated, a crystal lattice being continuous with strain in said composition-changing layer
JP 2001-293601 A discloses a cutting tool having a wear-resistant hard coating formed on a tool substrate, the coating comprising as a main component a nitride or carbonitride of at least one element selected from the group consisting of elements of Groups 4a, 5a and 6a in the Periodic Table and Al, said substrate being made of at least one selected from the group consisting of WC-based cemented carbide, cermet, silicon carbide, silicon nitride, alumtnum nitride, alumina, boron carbide, sintered aluminum oxide-titanium carbide, high-speed steel, die steel and stainless steel, said wear-resistant, hard coating
containing at least one type of fine, hard particles selected from the group consisting of B4C, BN, TiB2, TiB, TiC, WC, SiC, SiNx (x = 0 5-1 33) and A1203
However, any of the hard coatings described in the above references fails to meet the demand of having sufficient lubrication, peel resistance, impact resistance and breakage resistance capable of withstanding dry cutting conditions while maintaining oxidation resistance and wear resistance.
OBJFCTS OF THE INVENTION
Accordingly, an object of the present invention is to provide a hard coaling having excellent oxidation resistance, wear resistance and lubrication, as well as excellent adhesion to a substrate, impact resistance and breakage resistance
Another object of the present invention is to provide a method for forming such a hard coating
A further object of the present invention is to provide a hard-coated tool having excellent oxidation iesistance, wear iesistance, lubrication, impact resistance and breakage resistance, and improved adhesion to the substrate, with the seizure of a work and the diffusion of work elements into the hard coating suppiesscd in high-temperaturc cutting, thereby usable foi diy cutting, high-speed cutting and high-feed cutting
DISCLOSURE OF THE INVENTION
The first hard coating or the present invention formed on a substrate surface has a multi-layer stuiclure, in which diffcicnces between average S1 and/or Mo contents S1A and/or MoA in layeis having larger Si and/or Mo contents (layers A) and average S1 and/or Mo contents SiB and/or MoB in layers having smaller Si and/oi Mo contents (layers B) arc 0 2-5 atomic % SiA and SiB are average Si contents in the layers A and B, respectively, and MoA and MoB are average Mo contents in the layers A and B, respectively
The first hard coating preferably has a composition comprising metal components represented by AlwTixMySi, wherein M is at least one transition metal element of Groups 4a, 5a and 6a in the Periodic Table, w, x, y and 7 meet the conditions of 20
The atomic ratio of the total amount of said non-metal elements (O + S + N) to the total amount of said metal elements (Al + Ti + M + Si) is preferably more than 1 0, more preferably 1 02-1 7 Said Si-O bonds are preferably in a range of 100-105 eV by ESCA
It is prefetable that a ratio (lb/la) of a peak intensity lb of a (200) plane to a peak intensity Ia of a (111) plane in its face-centered cubic structure measuicd by X-ray diffraction is 2 0 or more, and that the (200) plane has a lattice constant X of 0 4155-0 4220 nm
The second hard coating of the present invention comprises at least one metal element selected from the group consisting of transition metal elements of Groups 4a, 5a and 6a in the Periodic Table, Al, Si and B (at least one of said transition metal elements is indispensable), and at least one non-metal element selected from the group consisting of S, O, N and C (S is indispensable), a peak of S-O bonds on a surface being detected in a range of 167-170 eV by electron spectroscopy The S content is preferably 0 1-10 atomic %
The third hard coating of the present invention is formed on a substrate surface by physical vapor deposition, comprising at least one metal element selected from the group consisting of transition metal elements of Groups 4a, 5a and 6a in the Periodic Table, Al, S1 and B (at least one of said transition metal elements is indispensable), and at least one non-metal element selected from the
group consisting of S, O, N and C (S is indispensable), and having a columnar structure, in which crystal grains have a multi-layer structure having pluralities of layers having different S contents with interlayer boundary regions in which crystal lattice stripes are continuous, each layer having a thickness of 0,1-100 nm This hard coating preferably has S-0 bonds The S content is preferably 0 1-10 atomic %
A surface of the above hard coating is preferably made flat by machining
The method of the present invention for forming a multi-layer, hard coating, which comprises at least one metal element selected from the group consisting of transition metal elements of Groups 4a, 5a and 6a in the Periodic Table, Al, Si and B (at least one of said transition metal elements is indispensable), and at least one non-metal element selected from the group consisting of S, O, N and C (S is indispensable), and has a columnar structure compnsmg columnar crystal grains having pluralities of layers having diffcient S contents, on a substrate, comprises placing said substrate in a chamber comprising evaporation sources having different plasma densities and a reaction gas for physical vapor deposition, and alternately bunging said substrate closer to each evaporation source, while keeping said reaction gas in a plasma state and said evaporation sources simultaneously in an active state
Said evaporation sources are preferably an arc-discharge ion plating (AIP) target and a magnetron sputtering (MS) target, and an AIP method and an MS method are continuously and alternately conducted while keeping both evaporation sources simultaneously in an active state Columnar crystal grains having pluralities of layers having alternately and continuously changing S contents are formed by placing said substrate on a tabic, which is rotated to alternately bring said substrate closer to different targets
The tool of the present invention has the above hard coating The hard-coated tool of the present invention has, on a surface of said substrate, an
intermediate layei comprising at least one selected from the group consisting of nitrides, carbomtrides and boromtrides of Ti, TiAl alloys, Cr and W When the hard coating of the present invention is applied to wear-resistant members and heat-resistant members required to have high hardness, such as cutting tools, etc , they are provided with extiemely improved oxidation resistancc and wear resistance, and high breakage resistance because of excellent adhesion to their substrates, and the seizure of works to the tools during dry cutting are suppressed Accordingly, the hard-coated tool of the present invention can be used at high speed and feed during dry cutting The high-feed cutting means cutting at feed exceeding 0 3 mm per one edge
BRIEF DESCRIPTION OF THE DRAWINGS
Fig 1 is a schematic view showing a small vacuum chamber apparatus
comprising evaporation sources having different plasma densities for physical
vapor deposition to form the hard coating of the present invention,
Fig 2 is a transmission electron photomicrograph (magnification
50,000) showing a fractured surface of the hard coating of Example 1,
Fig 3 is a graph showing the friction coefficients of the hard coatings of
Examples 5 and 8 and Comparative Example 31,
Fig 4 is an ESC A chart of the hard coating of Example 1, Fig 5 is an electron diffraction photomicrograph showmg a
hetero-epitaxial relation in an interface of the substrate and the hard coating in
the haid coating of Example 1,
Fig 6 is a graph showing XPS analysis lesults in Example 24, Fig 7 is a graph showing XPS analysis results in Example 24, Fig 8 is a transmission electron photomicrograph (magnification
15,000) showing the structure of the hard coating of Example 24 on a fractured
surface;
Fig 9 is a transmission electron photomicrograph (magnification
20,000) showing the structure of the hard coating of Example 24 on a fractured surface
Fig 10 is a transmission electron photomiciograph (magnification 200,000) showing the structure of crystal grains in a region shown in Fig 9,
Fig 11 is a transmission electron photomicrograph (magnification 2000,000) showing black layers and gray layers in a region shown in Fig 10,
Fig 12 is a schematic view corresponding to Fig 11,
Fig 13 is an electron diffraction image of a region enclosed by a circle in Fig 12,
Fig 14 is a schematic view corresponding to Fig 13,
Fig 15 is a transmission electron photomicrograph (magnification 15,000) showing the structure of the hard coating of Comparative Example 33 on a fractured surface, and
Fig i 6 is a graph showing the friction coefficients of Examples 21 and 24 and Conventional Examples 10 and 11
BEST MODE FOR CARRYING OUT THE INVFNTION [1] Composition of hard coating
The hard coating of the present invention has a composition comprising at least one metal element selected from the group consisting of transition metal elements of Gtoups 4a, 5a and 6a in the Periodic Table, Al, Si and B (at least one of said transition metal elements JS indispensable), and at least one non-metal element selected from the group consisting of S, O, N and C (S is indispensable) O is preferably indispensable together with S as the non-metal elements
S contained in the hard coating is oxidized at a relatively low temperature of about 200°C on a surface, and the resultant oxide layer suppresses the seizure of a work as a protective layer As a result, the hard coating containing S has an extremely lowet friction coefficient than that of a
hard coating containing no S at a cutting temperature The oxtde layer prevents the diffusion of work elements into the hard coating even under heat generation by cutting, suppressing the seizure of a work and giving improved wear resistance and breakage resistance to the hard coating, thereby enabling stable cutting
The S content is pieferably 0 1-10 atomic % For instance, when S is added to a hard T1AIM0N coating of Conventional Example 3, its friction coefficient is reduced from 0.8 to 0 3-0 4 (Examples 1-15) This includes a synergistic effect with O When S is added to a hard coating containing Ti, Al, Si and/or Mo, its lubrication is kept for a long period of time However, when the S content is less than 0 1 atomic %, its lubncation is insufficient, resulting in a large fnction coefficient duung cutting under heat generation On the other hand, when the S content exceeds 10 atomic %, the crystal structure of the hard coating changes from a columnar crystal structure to an amorphous-like fine structure, resulting in the hard coating with decreased hardness, and peel resistance reduced by increase in a residual compression stress, which affects largely adhesion The S content is more preferably 0 1-7 atomic, most preferably 0 1-5 atomic %
Because B increases the hardness and lubrication of a hard coating, a tool with a hard coating containing B has a long life The improvement of hardness is obtained by a c-BN phase, and the improvement of lubncation is obtained by an h-BN phase With an optimum ratio of B to N, improved hardness and the lubrication can be given to the hard coating The ratio of the c-BN phase to the h-BN phase can be controlled by bias voltage applied duung coating
The hard coating according to a preferred embodiment of the present invention has a composition comprising metal components represented by AlwTixMySi„ wherein M is at least one tiansition metal element of Groups 4a, 5a and 6a in the Periodic Table, and w, x, y and z lespectively meet the conditions
of20
When the metal composition contains too much Al, A12O3 is formed in a surface layer, resulting in the diffusion of Fe, etc in a work into the hard coating in an actual cutting operation, though the haid coating has an improved static thermal resistance Accordingly, w is preferably 50 or less, with w
The addition of Mo and Si to a hard TiAIN coating is effective to prevent the seizure of a work When Si is added to the hard coating in a proper amount, the migration of Al, which causes serzure, is suppressed, resulting in providing a chemically stable Al2O3 layer with improved peel resistance The addition of Mo in a proper amount can make a Ti oxide finer, thereby suppressing seizure even under heat generation by cutting
When Si is added to the hard coating covering a cutting tool, a dense Si oxide is formed by an Al oxide near surface by heat generated during cutting, thereby reducing the diffusion of Fc in a work into the hard coatmg, and thus suppressing seizure The content z of Si iS prefer ably 0 01-10 atomic % When z exceeds 10 atomic %, the structure of the hard coating on a fractured surface changes from a columnar structure to a fine grain structure, though its hardness and thermal resistance arc improved A hard coating having a fine grain structure has many crystal grain boundaries, through which oxygen in the air and Fe in a work are easily difflised by heat generated during cutting As a result, seizure occurs at cutting edges, resulting in deterioiated lubrication Accordingly, the structure of the hard coating on a fractured surface is also an important factor, and it is parttculaily important in a high-feed cutting operation that the fractiued surface has a columnai structure Further, when z exceeds 10
atomic %, the hard coating has an increased residual stress, tesulting in easy peeling occurring in an interface between the substrate and the hard coating Because seizure occurs in peeled portions, it is important to prevent peeling The lower limit of z, which is 0 01 atomic %, is a limit of easily delecting Si
When Mo is added to a haid TiAIN coating, a dense crystalline Ti oxide is formed by the oxidation of the hard coating An oxide layer having a dense crystal structure suppresses the diffusion of oxygen, which can path thiough an Si oxide and an Al oxide The dense T1 oxide crystal suppresses the peeling of an Al2O3 surface layer The content y of Mo is preferably 2-20 atomic % The addition of Mo improves the thermal stability of the hard coating, suppicssmg scizure and hardening the hard coating However, because Mo is a high-melting-point metal, when y exceeds 20 atomic %, the hardness of the hard coating rather decreases, and the structure of the physically deposited hard coating on a fractured surface changes from a columnar sttucture having excellent impact resistance to a fine grain structure, resulting in chipping and wear at an early cutting stage Further, the discharge of the evaporation sources becomes unstable, making it difficult to stably form a uniform hard coating On the other hand, when y is less than 2 atomic %, the hard coating does not have sufficiently high hardness Added Mo substantially replaces Ti or Al
With respect to the non-metal components icpresented by O,SbN100-a-b, 0 3
When the composition of the hard coating is represented by (AlwT1,MoyS1z)m(OaSbN100-a-b)n, an atomic ratio of the total amount of non-metal elements to the total amount of metal elements (n/m) is preferably more than 1 0, more preferably 1 02 or more The upper limit of n/m is preferably 1 7
[2] Structuie and properties of hard coating
A transmission electron microscopic observation leveals that the hard coating of the present invention has pluralities of layers having contrast in brightness These layers comprises layer s having larger Si and/or Mo contents (layers A), and layers having smallei Si and/or Mo contents (layeis B), the layets A and B being laminated alternately without interfaces Composition analysis by an electron probe microanalyzer (EPMA, EPM-1610 available from Shimadzu Corporation) indicates that diffeience between average Si and/oi Mo contents S1A and/or MoA in the layers A and average Si and/or Mo contents S1B and/or MOB IN the layers B is pieferably within 0 2-5 atomic % When SiA-SiB and MoA-MoB are lespcctively in a range of 0 2-5 atomic %, the hard coating has high impact resistance With difference in the contents of Si and/or Mo in the layei s A and B, it is possible to provide the hard coating with improved impact lesistance and toughness and suppiessed residual compression stiess while keeping excellent lubrication
The hard coating of the present invention also has a columnar structure, whose columnar crystal grains have pluralities of layeis having different S contents without clear interfaces, with crystal lattice stripes continuous in interlayci boundary regions The columnar structure is a crystal structure longitudinally grown in a thickness direction Though the hard coating per se is polycrystalhne, each ciystal grain has a single-crystal-like structure In addition, the columnar crystal grains have a multi-layer structure comprising pluralities of layers having different S contents in a growth direction, with ciystal lattice stripes continuous in interlayer boundary regions The continuity of the ciystal lattice stripes need not exist in all inter layer boundary regions, but there need only be mterlayer boundary regions in which crystal lattice stripes are substantially continuous, when observed by a transmission electron microscope With columnar ciystal grains having a multi-layer structure comprising pluralities of layers having diffeient S contents, the hard coating has toughness
as a whole This is because lelatively soft layers having larger S contents exhibit a cushioning effect between relatively hard layers Fuither, the hard coating containing S has high lubrication Accordingly, the multi-layer, hard coating having a columnai structure comprising pluralities of layers having diffeient S contents has high toughness and thus excellent breakage resistance, as well as high lubucalion However, the S content difference is preferably 10 atomic % at most
The hard coating of the present invention picferably has S-O bonds Particularly the existence of S-O bonds on a surface provides the hard coating with excellent lubrication, thereby suppressing severe seizure, for instance, at an early cutting stage. The S-O bonds can be confirmed by a peak in a tange of 167-174 cV in an X-ray photoelectron spectroscopy (XPS) XPS was conducted with an X-ray source of AlKα in an analysis region of 100 µm in diameter, using an electron neutralise!.
The friction coefficient of the hard coating is preferably 0 4 or less When the friction coefficient exceeds 0 4, the lubrication of the hard coating is insufficient The friction coefficient was measured at 600°C in the aii, using a ball-on-disc-type friction wear tester
Each layer in the hard coating preferably has a thickness T of 0 1-100 nm When T exceeds 100 nm, strain is genei ated in interlayer boundary regions, resulting in discontinuous lattice stripes in the crystal grains, thereby lowering the mechanical strength of the hard coating For instance, when the hatd coating is formed on a cutting tool, laminar bteakage occurs in the hard coating by cutting impact at an early cutting stage The prevention of strain in mterlayer boundary regions is effective to improve the adhesion of the hard coating to the substrate The lower limit of T is 0 1 nm, the minimum thickness for observing a layer structure by an X-ray diffraction apparatus or a transmission electron microscope Also, when a multi-layer, hard coating is formed at a lamination penod less than 0 1 nm, the resultant coating has uneven
properties To control the thickness T of each layei in the multi-layer, hard coating having columnar structure to 100 ran or less, and to make lattice stripes in each crystal grain continuous, the discharge output of an evaporation source in the MS method is preferably set at 6 5 kW or less
A hard coating obtained by discharging a metal sulfide as an evapotation source for the MS method with a relatively lower plasma density is slightly softer than a hard coating obtained by the AIP method Particularly layers having large S contents are softer With softer layers having large S contents disposed between harder layers, the hard coating has high lubrication as well as high impact resistance and toughness
In the MS method, a target of a metal sulfide such as WS, CrS, NbS, TiS, etc can be used as an evaporation souice The use of these metal sulfide targets can produce relatively soft layers having large S contents, resulting in the hard coating with excellent lubrication, impact resistance and toughness
In the AIP method with a relatively high plasma density, whose discharge energy is extremely high, it is difficult to add S to the hard coating Because it is extremely difficult to discharge a target of WS, CrS, NbS, TiS, etc in the AIP method, a usable taigct is made of (a) at least one transition metal element of Groups 4a, 5a and 6a in the Periodic Table, or (b) at least one transition metal element of Groups 4a, 5a and 6a in the Periodic Tabic, and at least one metal element selected from the group consisting of Al, Si and B
The hard coating of the present invention preferably has an average thickness (total thickness) of 0 5-10 µm To provide the hard coating with excellent lubrication and impact resistance, difference (ON-OM) between an oxygen content ON IN a region of 1-30% of the average thickness from the surface and an oxygen content OM In a region of 1-30% of the average thickness from the interface with the substrate is preferably 0 3 atomic % or more Because the addition of O increases a residual compression stress, the O content is preferably increased gradually from the start to end of coating Because the
hard coating has the largest O content near surface, a lot of metal oxides are formed to improve the lubrication of the hard coating If a large amount of O is added from an early stage of coating, a substrate surface and an inner surface of the vacuum chamber apparatus are undesirably insulated.
It is confirmed by ESCA that the hard coating of the present invention has a binding energy of S1 to oxygen in a range of 100-105 eV near surface. Due to the difference in a formation free energy between Al-0 and St-O, Si-O appears to be predominantly formed The formation of dense Si-0 increases the lubrication of the hard coating, thereby suppressing the seizure of a woik even during high-efficiency cutting
To conduct high-feed cutting, the hard coating should have high adhesion (intermolecular force) to the substrate Thus, an oriented plane ofa hard coating immediately on the substrate should be controlled such that the hard coating grows hetero-epitaxially from the substrate
With the crystal orientation controlled, the hard coating in an interface with the substrate has reduced strain In the case of a cemented carbide substrate, for instance, a hard coating should be controlled to be onented in a (200) plane, to form a hard coating having a face-centered cubic structure on a (100) plane, the predominant orientation of WC, Namely, a ratio (lb/la) of a peak intensity lb of a (200) plane to a peak intensity 1a of a (111) plane in the X-ray diffraction of the hard coating is preferably 2 or more When lb/In addition is less than 2, crystals grow with large strain in an interface with the substrate, the hard coating not only is insufficiently adhered to the substrate, but also has large internal stress, so that it easily peels off from the substrate under severe conditions such as dry cutting
Because a large residual stress reduces the adhesion of the hard coating to the substrate, the lattice constant A, of the (200) plane affecting the residual stiess is prefeiably controlled to 0 4155-0.4220. When the lattice constant X is more than 0 4220 nm, the residual compression stress of the hard coating
exceeds 8 GPa, causing the peeling of the hard coating even if there is a hetero-epitaxial relation between the hard coating and the substrate On the other hand, when the lattice constant is less than 0,4155 nm, the hard coating has too low lubrication The lattice constant X. can be controlled in a range of 0 4155-0 4220 by adjusting the amount of Al ot Si With a lot of Al or Si, the lattice constant decreases by the influence of an atom radius of an Al or S1 clement On the contrary, the reduced content of Al or a coatmg condition with a large plasma density increases the lattice constant [3] Production method of hard coatmg
To produce the multi-layer, hard coatmg of the present invention, physical vapor deposition with different plasma densities is utilized Specifically, a high-plasma-density AIP method and a low-plasma-density MS method arc simultaneously conducted in a leaction gas plasma to continuously grow crystal grains without interfaces, thereby providing crystal grains in the hard coating with large mechanical stiength. On the contrary, when the AIP method and the MS method are conducted stepwise or intermittently, clear interfaces are generated between layers in the hard coatmg, providing the hard coatmg with smaller strength
Because the AIP method generates an extremely high plasma density, ions generated in the plasma impinge on the substrate with large energy, high-quality hard coatings are formed with difficulty to suppress a residual compression stress Also, it is difficult to provide pluralities of layers with composition differences (concentration differences) by the AIP method Accordingly, the AIP method and the MS method ate prefeiably combined to provide a high-hardness coating with excellent lubrication, adhesion and wear resistance
Specifically as shown in Fig I, it is preferable to use a vacuum chamber apparatus comprising AIP targets 2 and MS targets 3, and a reaction gas suitable for both of the AIP method and the MS method the composition of each target
per se is not restrictive. The A1P target 2 may be a single alloy target or pluralities of targets of metals or alloys having different compositions When AIP method and the MS method are simultaneously conducted in the reaction gas in a plasma state while bringing the substrate alternately closer to the targets 2, 3, ions with different valences simultaneously reach the substrate When the substrate is close to the evaporation source for the high-plasma-density AIP method, a hard layer is formed And when it is close to the evaporation souice for the low-plasma-density MS method, a soft layei is formed There is a region between the hard layer and the soft layer, in which a composition changes not discontinuously but gradually (without clear interface) With the soft layer sandwiched by the hard layers via a gradually-changtng-composition region, there is a cushioning effect to provide the entire hard coating with excellent toughness and impact resistance
The hard coating obtained by this method has a columnar structure, in which each columnar crystal grain has a multi-layer structure compnsmg continuous lattice stripes without interlaces On the contrary, when the AIP method and the MS method are intermittently conducted with the AIP targets 2 and the MS targets 3 alternately discharged, the resultant multi-layer structure has interfaces between the layers with weak bonding strength due to strain generated in the interfaces
To add S to the hard coating, the evaporation source for the AIP method and the evaporation source fot the MS method are simultaneously put in an active state, to form a hard coating based on transition metal elements with or without metal elements such as Al, etc by the evapoiation source for the AIP method, and to add S from the evapoiation source for the MS method made of metal sulfides such as WS, CrS, NbS, etc Because the MS method uses a lower plasma density, S is easily added to the hard coating S evaporated from the MS target 15 ionized, to form crystal grains with other ions evaporated by the AIP method on the substrate surface. Pluralities of layers controlling crystal
grains continuously grow without inter faces, so that S is trapped in the crystal grains to an atom level To form S-O bonds in the hard coating, the reaction gas preferably contains O
When metal sulfides are used for the MS target, S can be added to the hard coating, without causing an environmental or safety problem of a chemical vapor deposition method using a reaction gas such as H2S, etc Incidentally, when WS, C1S, NbS, etc are used for the target for the high-plasma-density AIP method, a discharging phenomenon cannot easily be stabilized Also, when metal sulfides are added to the AIP target, it is difficult to add S to the hard coating Accordingly, it is preferable to add S in a relatively low plasma density state by the MS method. To include S-0 bonds in the hard coating, the reaction gas preferably contains oxygen
Even if sulfide targets such as M0S2, etc are used foi the MS method, S is dissolved in crystal grains of T1AIM0S1 compounds, etc , because the MS method is conducted simultaneously with a high-energy AIP method Accordingly, the percentage of S existing as sulfides such as M0S2, etc. in the hard coating is as small as 3% or less by area [41 Coated tool
When a cutting tool is provided with the haid coating of the present invention, the seizure of a work can be prevented because of excellent lubrication, adhesion and wear resistance Particularly because the haid coating of the present invention has excellent lubrication, the adhesion and diffusion of work elements can be suppressed in a dry cutting operation generating heat to high temperatures. The cutting tools having the hard coating of the present invention are usable in dry cutting, high-speed cutting, and high-feed cutting The high-feed cutting is, for instance, cutting at a feed exceeding 0 3 mm/edge
When the hard coating surface is made flat by machining, the wear resistance is stabilized, resulting in reduced unevenness of tool life
With an inter mediate layer made of a Ti nitride, carbonitride or boronitride, TiAl alloys, Cr, W, etc formed on the substrate surface, adhesion increases between the substrate and the hard coating, theieby resulting in the hard coating with improved peel resistance and breakage resistance The hard-coated cutting tool of the present invention is suitable for dry cutting, though it is usable for wet cutting, too In any case, the existence of the intermediate layer can prevent the breakage of the hard coating, which occurs by repeated fatigue
The materials of the cutting tool, on which the hard coating of the present invention is formed, is not restrictive, but may be cemented carbide, high-speed steel, the steci, etc, The hard coating of the present invention may be formed on wear-resistant members such as dies, bearings, rolls, piston rings, shdable members, etc, and heat-resistant members such as internal combustion engine parts, etc, which require high hardness, in addition to the cutting tools
The present invention will be explained in more detail by Examples below, though it is not restricted thereto
Examples 1-15. Comparative Examples 1-27, and Conventional Examples 1-4 Using a small vacuum chamber apparatus 1 comprising AIP targets 2 and MS targets 3 as an evaporation source shown in Fig I, a hard coating was formed on a substrate of each cemented carbide insert placed on a rotating table 4 in Examples 1-15 The AIP targets 2 were made of alloys having various compositions, and the MS targets 3 were made of metal sulfides A reaction gas used was an N2 gas, a CH4 gas or an Ar/O2 mixed gas depending on targeted hard coating compositions To change the distribution of an S content in the hard coating in a lamination direction periodically and smoothly, plasma was generated at a reaction gas pressure of 3 0 Pa, simultaneously by both coating methods of AIP and MS. A substrate temperature was 400°C, and bias voltage was -40 V to -150 V In Comparative Examples 1 -27 and Conventional
Examples 1-4, on the other hand, a hard coating was formed on each insert under the same conditions as in Examples except for using physical vapor deposition with the same plasma density (AIP or MS)
The measurement of a friction coefficient of each hard coating was carried out at as high a temperature as 600°C in the an using a ball-on-disc-typc friction wear tester
The resultant hard-coated inserts were attached to the following tools to conduct cutting tests under the following conditions 1 and 2 Tn the cutting test under the condition 1, the life of each tool was expressed by cut length when cutting was made impossible by the breakage or wear of an insert edge, etc In the cutting test under the condition 2, the life of the tool was determined when the maximum wear of a flank leached 0 3 mm in a case where there was no breakage, irregulai wear oi peeling A work used in the cutting condition I had holes formed in advance at an equal interval on a surface by dulling This work surface was intermittently cut under a high-performance cutting condition to evaluate the insert life
Cutting condition 1
Tool Face mill,
Insert shape SDE53 with special shape,
Cutting method Center cutting,
Wotk shape 100 mm in width and 250 mm in length,
Work S50C (HRC30) having many dulled holes of 6 mm in
drameter,
Depth of cutting. 2 0 mm,
Cutting speed 120 m/minute.
Feed per one edge 1 0 mm/edge, and
Cutting oil not used
Cutting condition 2
Tool Face mill,
Insert shape SDE53 with special shape,
Cutting method Center cutting,
Work shape 100 mm in width and 250 mm in length,
Work S50C (WRC30),
Depth of cutting 2.0 mm,
Cutting speed 120 m/mmute,
Feed per one edge 1 0 mm/edge, and
Cutting oil not used
Tabic 1 shows the compositions and production conditions of the hard coatings of Examples, Comparative Examples and Conventional Examples, and Table 2 shows the structures and properties of the hard coalings, and the tool lives The compositions of the hard coatings were represented by (AlwTixMoySiz)m(OaSbN100-a b)n In Table 1, Al + Ti + Mo + Si = 100.0, and N = 100 0 - (O + S) n/m was an atomic ratio of the total amount of non-metal elements to the total amount of metal elements Accordingly, in Example 1, fot instance, because n/m was 1 10, the O content was 1 I x [n/(m + n) ] - 0 58 atomic %, and the S content was 1 0 x fn/(m + n)] = 0 52 atomic %, per 100 atomic % of the entire hard coating
Table I
(Table Removed)
Note (1) Maximum values measured by EPMA (electron probe
microanalyer, EPM-1610 available from Shimadzu Corporation)
Table 1 (Continued)
(Table Removed)
Note (1) Maximum value measured by EPMA
(2) The Si content measured by EPMA was 3 3 atomic %
(3) The O content measured by EPMA was 3 3 atomic %
Table 2
(Table Removed)
Note' (1) Difference (atomic %) between an Si content SiA in larger-Si-content layers A and an Si content SiB in smaller-Si-content layers B, which were measured by EPMA
(2) Difference between an oxygen content ON in a legion of I -30% of an average thickness from the surface and an oxygen content OM in a region of 1-30% of an average thickness from the interface with the substrate (measured by EPMA).
(3) The presence or absence of Si-0 bonds in a surface layer
(4) A ratio of a peak intensity lb of a (200) plane in a face-centered cubic lattice to a peak intensity la of a (111) plane, which were measured by X-iay diffraction
(5) The presence or absence of a hetero-epitaxial relation in an interface between the hard coating and the substrate
(6) Friction coefficient
(7) Lattice constant of the (200) plane
(8) Expressed by cut lenglh until the tool became unable to cut by the breakage or wear of an edge, etc
(9) Could not be detected
Table 2 (Continued)
(Table Removed)
Note (1) Difference (atomic %) between an Si content SiA in
larger-Si-contcnt layeis A and an Si content SiB in
smaller-Si-contcnt layers B, which were measured by EPMA
(2) Difference between an oxygen content ON in a region of 1-30% of an average thickness from the surface and an oxygen content OM in a region of 1-30% of an average thickness from the interface with the substrate (measured by FPMA)
(3) The presence of absence of Si-O bonds in a surface layer
(4) A ratio of a peak intensity lb of a (200) plane in a face-centered cubic lattice to a peak intensity la of a (111) plane, which were measured by X-ray diffraction
(5) The presence or absence of a hetero-epitaxial relation in an interface between the hard coating and the substrate
(6) Friction coefficient.
(7) Lattice constant of the (200) plane
(8) Expressed by cut length until the tool became unable to cut by the breakage or wear of an edge, etc
(9) Could not be detected
(10) Cutting was stopped because of peeling
(11) Crater wear occurred early
(12) Cutting was stopped because of large early peeling
(13) Immediately after peeling occurred, the hard coating was broken
(14) Cutting was stopped because of large early crater wear
(15) Cutting was stopped because of peeling
(16) Immediately after seizure occurred, the hard coating was broken
(17) Broken
(18) Small peeling occurred early
(19) Repetition of continuous composition change
The coated inserts of Examples 1-15 showed excellent cutting
performance than those of Compaiative Examples and Conventional Examples. This appears to be due to the fact that, the formation of high-hardness layers and low-hardness layeis continuously and alternately with Si contents continuously changed between these layers using both AIP and MS methods with different plasma densities provides a hard coating with excellent film sttength (breakage resistance) while maintaining wear resistance and lubrication Though methods of intermittently or continuously changing target compositions or coating conditions may alternately form layers with different Si contents, hard coatings formed by such methods have insufficient strength
In Comparative Examples 1, 8 and 10, hard coatings were formed using a single evaporation source, such that there were no composition diffeiences Because the MS method has a lower plasma density than the AIP method, the hard coating of Comparative Example 1 formed by the MS method did not have high baldness, resulting in insufficient wear resistance and thus breakage in an early cutting stage The hard coatings of Comparative Examples 8 and 10 formed by the AIP method were poor in toughness and intermittent cutting performance despite high hardness
In Comparative Examples 2, 3 and 5-7, composition differences were given to hard coatings by the AIP method, though failing to obtain targeted cutting performance Particularly in Comparative Examples 3,6 and 7, the S contents were outside the range of the present invention. Hard coatings formed only by the AIP method were poor in toughness despite high hardness, because of a high plasma density during discharging regardless of target compositions The hard coatings of Comparative Examples with composition differences are poor in breakage resistance and adhesion to the substrate because of an increased residual stress Though some of the hard coatings of Comparative Examples had hardness Hv exceeding 3500, they suffered breakage in inteimittent cutting because of small toughness, resulting in tools with short lives
The hard coatings of Comparative Examples 11 and 12 bad improved breakage resistance because of composition differences geneiated by using both AlP and MS methods However, the hard coatings did not have a columnar structuic extending continuously over pluralities oflayers because of composition differences exceeding the targeted range, icsultmg in the lamination of layers with intermittent composition differences Thus, the hard coatings were broken because of weak intcrlayet bonding, failing to obtain a targeted cutting performance
In the cutting test under the condition 2, as shown in Tabic 2, the hard coatings of Examples 1-15 had friction coefficients of 0 4 or less to steel, showing an excellent cutting performance Particularly the hard-coated inserts of Examples 3,7 and 12 showed sufficiently improved cutting lives than those of Conventional Examples 1-3 The hard coatings of Examples 3 and 12 had small fnction coefficients, thereby exhibiting excellent lubrication and decreased seizure to works at an early cutting stage Accordingly, the hard coatings of Examples 3 and 12 did not wear in a cutting distance in Comparative Examples 13-22 The coated insert of Example 12 had life 2 4 times those of Conventional Examples 2 and 3 having the longest life
The cutting lives of Examples are affected by the O contents, the existence of an Si oxide in surface layers, and the existence of hetero-cpitaxial growth The balance of the Mo and Si contents is also important Hard coatings with excellent cutting performance and long tool lives generally tend to have larger Mo contents than Si contents. Though the hard coatings of Examples have higher cutting performance than those of Conventional Examples and Comparative Examples even if the Si contents are larger than the Mo contents, higher cutting performance is obtained when Mo Si
The lubrication of the hard coatings was drastically improved by the addition of O For instance, the hard coating of Comparative Example 13 containing no O had substantially the same cutting performance as those of most
hard coatings of Conventional Examples When the O content exceeds 5 atomic %, early wear occurs despite appreciable lubrication in dynamic cutting as in Comparative Examples 15 and 16, even if the metal compositions are within the range of the present invention This appears to be due to the fact that a large O content changes the structure of the hard coating on a fractured surface from a columnar structure to a fine structure, resulting in low hardness Though there was no early breakage until a wear life in Comparative Example 15 because of good adhesion to the substrate, the hard coating of Comparative Example 16 peeled off from a flank of the insert because of low adhesion to the substrate Even within the composition range of the present invention, peeling occurred in the case of insufficient adhesion to the substrate, failing to conduct stable cutting
The hatd coating of Comparative Example 18 whose Al content was outside the range of the present invention had insufficient adhesion to the substrate In the hard coating of Comparative Example 22, the Mo content was outside the range of the present inventton When the metal composition of the hard coating is outside the range of the present invention, its fractured surface has a fine structure, resulting in rapid wear on the insert flank during cutting, and thus a short life
Fig 2 shows a fractured surface of the hard coating of Example 1 The hard coating of Example 1 had a multi-layer structure, in which layers formed by the AIP method and layers formed by the MS method were alternately and continuously laminated without interfaces It was confirmed that there were clear differences in the concentrations of Mo, Si and S in the hard coatings from surface to inside.
Fig. 3 shows the friction coefficients of hard coatings containing S and O Example 8 contained 1 atomic % of S, and Example 5 contained 1 atomic % of S and 4 8 atomic % of O Examples 5 and 8 had smaller friction coefficients than that of Comparative Example 13 containing no S and O
When the O content is 0 3 atomic % or more, the hard coating has high lubrication, suppressing the seizure of a work to the hard coating during high-performance cutting Oxygen remaining in the vacuum chamber apparatus is mixed in an amount of about 0 1 atomic % into a hard coating by physical vapoi deposition, but O on this level is insufficient to reduce the friction coefficient
It has been made clear that the method of adding O affects the cutting performance Though O was added without changing its amount from the start to end of coating in Comparative Examples 14, 17 and 20, O was added with content inclination from an interface with the substrate to a surface from the start to end of coating in Examples and Comparative Examples 15, 16, 18,19, 21 and 22 Cutting tests indicated that the hard coatings with inclined O contents had better properties
Fig 4 shows the chemical bonding state of the hard coating of Example I near surface, which was measured by ESC A It is clear from Fig 4 that the hard coating of the present invention had a binding energy of Si to oxygen in a range of l00 eV to l05 eV
As shown in Fig 5, a (100) plane of WC in the substrate is matched with a (200) plane of said hard coating of (T1AlM0S1)XOSN), resulting in a strong intermolecular force and thus high adhesion of the hard coating to the substrate
Because Comparative Examples 19, 21 and 22 had Mo and Si contents outside the ranges of the present invention, they had large residual compression stress, lesullmg in peeling at an early cutting stage It was confirmed from the above test results that the lubrication of hard coatings can be drastically improved by having metal compositions in predetermined ranges, containing O and S, and forming dense Si oxides on surface, resulting in improved tool lives
Table 2 shows a ratio of a peak intensity lb of a (200) plane in a face-centered cubic lattice to a peak intensity la of a (111) plane in X-ray
diffraction, the lattice constant X of the (200) plane, and cutting test results Because the Tb/Ia and the lattice constant were outside the ranges of the present invention despite the addition of S and O in Comparative Examples 1-10 and 23-27, crater wear and peeling occuried easily The hard coatings of Compaiativc Examples 2, 6-9 and 23-27 having lattice constants of the (200) plane exceeding 0 4230 nm had extremely short cutting lives regardless of the metal compositions and the S and O contents The lattice constant affects the internal stress of the hard coating With large breakage oi peeling occurs easily in the hard coating because of a laige residual compiession stress in an interface between the substiatc and the hard coating, even if the hard coating giows hetero-epitaxially from the substrate, resulting in a short tool life Accordingly, to provide a hard coating with a stable cutting performance, it is important to properly control crystal orientation as well as the metal composition, the S and O content, and the O-adding method
Examples 16-29, Comparative Examples 28-41, and Conventional Examples 5-11
Using a small vacuum chamber apparatus 1 shown in Fig I comprising A1P targets 2 and MS targets 3 as evaporation souiccs, a hard coating was formed on each insert substrate of cemented carbide placed on a lotating table 4 The A1P targets 2 were made of alloys of various compositions, and the MS taigets 3 were made of metal sulfides A reaction gas was an N2 gas, a CH4 gas ot an A1/O2 mixed gas depending on targeted hard coating compositions To change an S content distribution in the hard coating peiiodically and smoothly in a lamination direction, the pressure of the icaction gas was set at 3 0 Pa to geneiate plasma for both film-forming methods of AlP and MS The substiate temperature was set at 400°C, and bias voltage of -40 V to -150 V was applied
The resultant hard-coated inserts were subjected to cutting tests under the following condition 3 Works vvcie made of SKD61 steel (hardness HRC
45) for die-casting molds SKD61 is easily adhered to an insert edge at an early cutting stage, thereby seveiely damaging a hard coating at the edge A work surface was cut under high-performancc cutting conditions to evaluate cuttable length until the hard coating was broken by seizure, wear or heat cracking Table 3 shows the compositions of targets used for ptoducing the hard coatings, and S-0 bonds and S contents in the hard coatings, and Table 4 shows the structures of the hard coatings and cutting test results
Cutting condition 3
Tool Face mill,
Inseit shape SDB53 with special shape,
Cutting method Center cutting,
Work shape Width 100 mm x length 250 mm,
Work SKD61 with hardncss HRC45,
Depth of cutting 1 5 mm,
Cultmg speed 100 m/minute,
Feed per one edge 0 6 mm/edge, and
Cutting oil Not used
Table 3
(Table Removed)
Note (1) The presence or absence of Si-0 bonds in a sm face layer
(2) Maximum values measured by EPMA
(3) The S content was less than a detection sensitivity of 0 1 atomic %
Table 4
(Table Removed)
Note (1) Broken
(2) Because of large scizure cutting was stopped
(V) Because of large seizure at an early cutting stage, spai k occurred
(4) Because oflarge wear at an early cutting stage, the hard coating was broken
(5) Because the hard coating peeled, evaluation was stopped
(6) Because spark occuried, evaluation was stopped
(7) Despite good cutting performance at an early stage, the hard coating +wore rapidly and was broken
(8) After early peeling of MoS2, the hard coating was broken
(9) After early peeling of MoS2, the hard coating was broken
(10) Because of early peeling of WS2, evaluation was stopped
(11) Because of early peeling of MoS2, the hard coating was bioken
(12) Though evaluation was continued alter the peeling of M0S7, spark occuried
(13) Peeling occurred by seizure at an eaily cutting stage
(14) Interface
* Could not be detected
It is clear from Table 4 that the hard coatings ol Examples 16-29 had a columnar structure Each columnar crystal grain had a multi-layer structure comprising pluralities of layers having different S contents, and crystal lattice stripes are continuous in inter layer boundary regions It is clear that inserts having hard coatings compusing layers each having a thickness T in a range of 0 1-100 run have excellent cutting performance It was confirmed that the existence of S-0 bonds and the S content in a surface layer of the hard coating affect the cutting perfoimance of a coated insert The inserts having the hard coatings of Examples 16-29 could cut works whose cutting has conventionally been difficult
The hard coating of Example 24 containing S by using a target of NbS showed the best cutting peiformancc As shown in Fig 6, the hard coating of
Example 24 bad S-O bonds in a lange of 167-] 74 eV in XPS It is considered that the existence of S-O bonds suppressed seizure at an early cutting stage In addition to the S-O bonds, the existence of Nb-0 bonds at 200-21 5 cV (Fig 7) and metal sulfides at 161-164 cV (Fig 6) was confiimed Because sulfides and oxides having excellent lubrication weic formed in a surface layer of the hard coating of Example 24, the seizuie of a woik metal was remarkably suppressed In addition, because an MS target of NbS was used at a dischaige output of 6 5 kW, the entire hard coating had an S content of 4 8 atomic %, within the range of the present invention As shown in Fig 8, It was confirmed by the observation ot the structure of a fractuted surface that the hard coaling had a columnar structure It was confiimed that the hard-coated inserts having such compositions and structure had excellent mechanical strength in a shear direction in cutting operations with severe impact, such as high-feed working, etc
It is clear from Fig 9, a transmission election photomierograph (magnification 20,000) of a fractured surfacc of the hard coating of Example 24, that each crystal grain in the hard coating having a columnar structure had a multi-layer structure It is also clear from Fig 10, a transmission election photomicrograph (magnification 200,000) of part of the crystal giain shown in Fig 9, that the crystal grain had a multi-layer stiuctuie comprising pluralities of alternately laminated clcai-contrast layers (black layers and gray layers) It was confiimed by election diffraction that each ciystal grain grew in substantially the same direction perpendicular to the substrate surface It is cleat from the stripe pattern shown in Fig 10 that each layer was as thick as about 3-4 nm Incidentally, the numbers of stripes are not equal between Figs 9 and 10 because of difference in magnification
Part of the field of Fig 10 was further observed by a magnification of 2,000,000 and the result is shown in Fig 11 The observation region of Fig 11 was magnified while confinning the positions of black layers and gray layers
in Fig 10, so that the black layers and the gray layers in Fig 1 1 correspond to those in Fig 10 Two lines depicted in Fig 1 1 separate regions corresponding to the black layers and the gray layers Fig 12 is a schematic figure corresponding to the photograph of Fig 11 It should be noted that the interval oi lattice stupes is expanded for explanation It is clear from Fig 11 that crystal lattice stupes were continuous in inter layer boundary regions in the multi-layer structure The crystal lattice stripes need not have continuity in all boundary regions, but there need only be regions having continuity in lattice stripes in a transmission electron photomicrograph Though there is a black legion on the left side of Fig 11, this has nothing to do with the black layer shown in Fig 10
Fig 13 shows an election diffraction image of a region surrounded by a circle in Fig 12, and Fig 14 is n schematic view oi Fig 13 As is clear from Ftgs 13 and 14, the electron diffiaction images of the black layers indicated by stars are substantially aligned with those of the gray layers indicated by circles, suggesting that the lattice stripes were continuous by an epitaxial relation in the boundary regions of the black layers and the gray layers It is thus cleai that the columnar crystal grains having a multi-layer structure are like a single crystal
As the compositions of black layers and gray layers in the multi-layer columnar crystal grains in Example 24, the compositions at a point P (black layer ) and a point Q (gray layer) in Fig 11 were measured by an energy dispersive X-ray analyzer (EDX) attached to a transmission electron microscope Table 5 shows the compositions of black layers and gray layers Because the S content difference exceeding 10 atomic % makes the crystal structurc fine, the S content difference should be controlled within 10 atomic % In Example 24, because of the NbS discharge output of 6 5 kW the S content difference was 4 0 atomic %
Table 5
(Table Removed)
Fig 16 shows the friction coefficients of Examples 2 land 24 and Conventional Examples 10 and 11, which were measuied as in Examples 1-15 Fig 16 shows that the hard coatings of Fxamples 21 and 24 containing S had friction coefficients ol 0 4 or less, exhibiting excellent lubrication
To exhibit excellent cutting performance, NbS appears to be suitable for the target Thus, the tools having the hard coatings of the piesent invention having excellent lubucation serzure provided satisfactory results even in working metals causing severe adhesion
Using the coated insert ol Example 24, which was best in the cutting test, intermittent cutting was conducted to a work with many drilled holes of 6 mm in diametei as in a the mold As a result, stable cutting could be conducted without breakage even by severe impact This appcais to be due to the fact that the multi-layer structure comprising layers having proper thickness provides the hard coating with extremely improved toughness
In Comparative Examples 28, 29, 3 1-33, 37-39 and 41, the S content was as much as more than 10 atomic % Among them, the hard coating of Comparative Example 33 having an S content of 14 atomic % has an amorphous structute as shown in Fig 15, with as low hardness as about 26 GPa, failing to exhibit a satisfactory cutting pei formance To obtain mechanical strength withstanding severe use, it is important to control the S content properly
The hard coating of Comparative Example 30 had S-O bonds in a surface layer with an S content of 9 1 atomic % , with in the range of the present invention However, because each layer in the hard coating formed by
simultaneous discharge of AIP and MS (discharge output of an MS target was 6 6 kW) has a thickness more than 100 nm, the crystal lattice stupes of each layer had strain with discontinuity in intet layer regions, and thus the hard coating had a fine crystal structure Accordingly, the coated insert of Comparativc Example 30 wore early
The hard coating of Comparative Example 36 had no S-O bonds despite the inclusion of S, because O was not added to a reaction gas This indicates that though the addition of S improves cutting performance, it is insufficient to suppress serzure which tends to occur at an eaily cutting stage Because the discharge output of WS, and NbS was as low as ) kW in Comparative Examples 34 and 35, the one-layer thickness in their multi-Iayei sttuctuies was 0 2 nm and 0 8 nm, respectively, and their S contents were as small as undetectable by XPS analysis Therefore thev suffered severe serzure at an early cutting stage Particulaily in Compaiative Example 35, spaik occulted, stopping the cutting evaluation This indicates the importance of controlling the content and bonding state of S in the hard coating, and one-layer thickness conesponding to a lamination period
As described above, the hard coating ot the present invention has excellent oxidation resistance wear resistance, lubncation, adhesion to a substrate, impact lesistance and brcakage resistance Accordingly, a cutting tool having the hard coating of the present invention can stably conduct not only high-efficiency dry cutting but also intermittent cutting to die-casting mold steel causing severe seizuie, enjoying a long life
We claim:
1. A hard coating formed on a substrate surface, said hard coating comprising at least one metal element selected from the group consisting of transition metal elements of Groups 4a, 5a and 6a in the Periodic Table, Al, Si and B (at least one of said transition metal elements is indispensable), and at least one non-metal element selected from the group consisting of S, O, N and C (S is indispensable), and having a columnar structure, in which crystal grains have a multi—layer structure having pluralities of layers having different S contents, wherein differences between average Si contents SiA in layers having larger Si contents and average Si contents SiB in layers having smaller Si contents are 0.2-5 atomic %.
2. The hard coating as claimed in claim 1, having a composition comprising metal components represented by AlwTixMySiz, wherein M is at least one transition metal element of Groups 4a, 5a and 6a in the Periodic Table, and w, x, y and z meet the conditions of 20≤ w≤50, 25≤x≤75, 2≤y≤20, 0.01≤z≤10,w+x+y+z-100, and w≤x+y+z, by atomic %), and non-metal components represented by OaSbN100-a-b, wherein a and b meet the conditions of 0.3
3 The hard coating as claimed in claim 2, wherein M is Mo.
4. The hard coating as claimed in claim 2 or 3, wherein an atomic ratio of the total amount of said non-metal elements to the total amount of said metal elements is more than 1.0.
5. The hard coating as claimed in claim 4, wherein an atomic ratio of the total amount of non-metal elements to the total amount of metal elements is 1 .02-1 .7.
6. The hard coating as claimed in any one of claims 2-5, wherein said Si-0 bonds are in a range of 100-105 eV by ESCA.
7. The hard coating as claimed in any one of claims 1-6, which has a ratio Ib/la of 2.0 or more, wherein Ia and Ib are peak intensities of (111) and (200) planes, respectively, of a face-centered
cubic structure measured by X-ray diffraction, said (200) plane having a lattice constant: of 0.4155-0.4220 nm.
8. A hard coating comprising at least one metal element selected from the group consisting of transition metal elements of Groups 4a, 5a and 6a in the Periodic Table Al, Si and B (at least one of said transition metal elements is indispensable), and at least one non-metal element selected from the group consisting of S, Q, N and C (S is indispensable), and having a columnar structure, in which crystal grains have a multi—layer structure having pluralities of layers having different S contents, wherein S-0 bonds exist on a surface of the crystal grains, and a peak of the S-0 bonds on the surface is detected in a range of 167-170 eV by electron spectroscopy.
9. The hard coating as claimed in claim 8, having an S content of 0.1 -10 atomic %.
10. A hard coating formed on a substrate surface by physical vapor deposition, which comprises at least one metal element selected from the group consisting of transition metal elements of Groups 4a, 5a and 6a in the Periodic Table, Al, Si and B (at least one of said transition metal elements is indispensable); and at least one non-metal element selected from the group consisting of S, 0, N and C (S is indispensable), having a columnar structure, in which crystal grains have a multi—layer structure having pluralities of layers having different S contents with interlayer boundary regions through which crystal lattice stripes are continuous, each layer having a thickness of 0.1 -100 nm.
11. The hard coating as claimed in claim 10, having S-0 bonds.
12. The hard coating as claimed in claim 10 or 11, having an S content of 0.1-10 atomic %.
13. The hard coating as claimed in any one of claims 1-12, whose surface is made flat by machining.
14. A method for forming a hard coating on a substrate, said hard coating comprising at least one metal element selected from the group consisting of transition metal elements of Groups
4a, 5a and 6a in the Periodic Table, Al, Si and B (at least one of said transition metal elements is indispensable), and at least one non-metal element selected from the group consisting of S, O, N and C (S is indispensable), and having a columnar structure, in which crystal grains have a multi—layer structure having pluralities of layers having different S contents, said method comprising placing said substrate in a chamber comprising evaporation sources having different plasma densities and a reaction gas for physical vapor deposition, and alternately bringing said substrate closer to each evaporation source, while keeping said reaction gas in a plasma state and said evaporation sources simultaneously in an active state.
15. The method for forming a hard coating claimed in claim 14, wherein said evaporation sources are an arc-discharge ion-plating target and a magnetron sputtering target, both targets being simultaneously placed in an active state to continuously and alternately conduct arc-discharge ion plating and magnetron sputtering.
16. The method for forming a hard coating as claimed in claim 15, wherein said substrate is placed on a table, which is rotated to alternately bring said substrate closer to targets having different plasma densities for physical vapor deposition.
17. A tool having the hard coating as claimed in any of claims 1-13, wherein an intermediate layer comprising at least one selected from the group consisting of nitrides, carbonitrides and boronitrides of Ti, TiAl alloys, Cr and W is formed on a surface of said substrate |