EP0963456A1

EP0963456A1 - Multilayered pvd coated cutting tool

Info

Publication number: EP0963456A1
Application number: EP98962794A
Authority: EP
Inventors: Torbjörn SELINDER; Mats Sjöstrand
Original assignee: Sandvik AB
Current assignee: Sandvik AB
Priority date: 1997-12-10
Filing date: 1998-12-09
Publication date: 1999-12-15
Also published as: SE9704630L; JP2001513708A; SE518151C2; SE9704630D0; IL131227A0; WO1999029920A1

Abstract

The present invention relates to a cutting tool comprising a body (1) of a sintered cemented carbide or cermet, ceramic or high speed steel on which, at least on the functioning parts of the surface of the body, a thin, adherent, hard and wear resistant coating (2) is applied. The coating comprises a laminar structure of refractory compounds in a polycrystalline, repetitive form: (MLX/Al2O3)μ (MLX/Al2O3)μ/ (MLX/Al2O3)μ/ (MLX/Al2O3)μ/... where the alternating sublayers consist of metal nitrides (or carbides) and crystalline alumina (4) of the alpha(α)- and/or the gamma(η) phase, preferably of metal nitrides and crystalline alumina of the η phase. The metal elements in the layers MLX (3) are selected from Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al. The repeat period μ (5) is essentially constant through the entire multilayered structure, and larger than 3 nm but smaller than 100 nm. The total thickness of said multilayered coating is larger than 0.5 νm but smaller than 20 νm.

Description

Multilayered PVD coated cutting tool.

The present invention relates to a cutting tool for metal machining, having a substrate of cemented carbide, cermet, ceramics or high speed steel and, on the surface of said substrate, a hard and wear resistant refractory coating is deposited by Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD) . The coating is adherently bonded to the substrate and is composed of a laminar, multilayered structure of metal nitrides or carbides in combination with alumina (Al₂0₃) with a repeat period of the individual layer thicknesses in the nanometer range (nm) , and the metal elements of the nitride or carbide are selected from Ti, Nb, Hf , V, Ta, Mo, Zr, Cr, or Al .

The process of depositing a thin refractory coating (1-20 μm) of materials like alumina, titanium carbide and/or titanium nitride onto e.g. a cemented carbide cutting tool is a well established technology and the tool life of the coated cutting tool, when used in metal machining, is considerably prolonged. The prolonged service life of the tool may under certain conditions extend up to several hundred percent greater than that of an uncoated tool. Said refractory coatings generally comprise either a single layer or a combination of layers. Modern commercial cutting tools are characterized by a plurality of layer combinations with double or multilayer structures. The total coating thickness varies between 1 and 20 micrometers (μm) and the thickness of the individual sublayers varies between a few microns and a few tenths of a micron.

The established technologies for depositing such coatings are CVD and PVD (see e.g. U.S. 4,619,866 and U.S. 4,346,123). PVD coated commercial cutting tools of cemented carbides or high speed steels usually have a single coating of TiN, TiCN or TiAlN, but combinations thereof also exist.

There exist several PVD techniques capable of producing refractory thin films on cutting tools and the most established methods are ion plating, magnetron sputtering, arc discharge evaporation and IBAD (Ion Beam Assisted Deposition) . Each method has its own merits and the intrinsic properties of the produced coating such as microstructure/grain size, hardness, state of stress, cohesion and adhesion to the underlying substrate may vary depending on the particular PVD method chosen. An improvement in the wear resistance or the edge integrity of a PVD coated cutting tool being used in a specific machining operation can thus be accomplished by optimiz- ing one or several of the above mentioned properties. Furthermore, new developments of the existing PVD techniques by i.e. introducing unbalanced magnetrons in reactive sputtering (S. Kadlec, J. Musil and .-D. Munz in J. Vac. Sci. Techn. A8(3), (1990), 1318.) or applying a steered and/or filtered arc in cathodic arc deposition (H. Curtins in Surface and Coatings Technology, 76/77, (1995) , 632 and K. Akari et al in Surface and Coatings Technology, 43/44, (1990), 312.) have resulted in a better control of the coating processes and a further im- provement of the intrinsic properties of the coating material .

With the invention of the PVD bipolar pulsed DMS technique (Dual Magnetron Sputtering) which is disclosed in DD 252 205 and DE 195 18 779, a wide range of oppor- tunities opened up for the deposition of insulating layers such as Al₂0₃. Furthermore, this method has made it possible to deposit crystalline Al₂0₃ layers at substrate temperatures in the range 500 to 800 °C.

Al₂0₃ exists in several different phases such as (alpha), κ(kappa) and χ(chi) called the "α-series" with hep (hexagonal close packing) stacking of the oxygen atoms, and in γ(gamma), θ(theta), η(eta) and δ(delta) called the "γ-series" with fee (face centered cubic) stacking of the oxygen atoms . The most often occurring Al₂0₃-phases in CVD coatings deposited on cemented carbides at conventional CVD temperatures, 1000°-1050 °C, are the stable α- and the metastable κ-phases, however, occasionally the metastable θ-phase has also been observed. According to DE 195 18 779, the DMS sputtering technique is capable of depositing and producing high- quality, well-adherent, crystalline α-Al₂0₃ thin films at substrate temperatures less than 800 °C. The "α-Al₂0₃" layers may partially also contain the gamma (γ) phase from the "γ-series" of the Al₂0₃ polymorphs . When com- pared to prior art plasma assisted deposition techniques such as PACVD as described in DE 49 09 975, the novel, pulsed DMS sputtering deposition method has the decisive, important advantage that no impurities such as halogen atoms, e.g. chlorine, are incorporated in the Al₂0₃ coating.

Conventional cutting tool material like cemented carbides consist of at least one hard metallic compound and a binder, usually cobalt (Co) , where the grain size of the hard compound, e.g. tungsten carbide (WC) , ranges in the 1-5 μm region. Recent developments have predicted improved tool properties in wear resistance, impact strength, hot hardness by applying tool materials based on ultrafine microstructures by using nanostructured WC- Co powders as raw materials (L.E. McCandlish, B.H. Kear and B.K. Kim, in Nanostructured Materials VOL. 1 pp.

119-124, 1992) . Similar predictions have been made for ceramic tool materials by for instance applying silicon- nitride/carbide-based (Si₃N₄/SiC) nanocomposite ceramics and, for Al₂0₃-based ceramics, equivalent nanocomposites based on alumina . With nanocomposite nitride/carbide and alumina hard coating materials, it is understood a multilayered coating where the thickness of each individual nitride (or carbide) and alumina layer is in the nanometer range between 3 and 100 nm, preferably between 3 and 20 nm. Since a certain periodicity or repeat period of the metal nitride/carbide and alumina layer sequence is involved, these nanoscaled, multilayer coatings have been given the generic name of "superlattice" films. With re- peat period is meant the thickness of two adjacent metalnitride/carbide and alumina layers. Several of the binary nitride superlattice coatings with the metal element selected from Ti, Nb, V and Ta, grown on both single- and polycrystalline substrates have shown an en- hanced hardness for a particular repeat period usually in the range 3-10 nm.

Fig 1 is a schematic representation of a cross-section taken through a coated body of the present invention. According to the present invention there is provided a cutting tool for metal machining such as turning (threading and parting) , milling and drilling comprising a body of a hard alloy of cemented carbide, cermet, ceramics or high speed steel, onto which a wear resistant, multilayered coating has been deposited. The shape of the cutting tool includes indexable inserts as well as shank type tools such as drills, end mills etc. More specifically, the coated tool comprises a substrate of sintered cemented carbide body or a cermet, preferably of at least one metal carbide in a metal binder phase, or a ceramic body. The substrate may also comprise a high speed steel alloy. Said substrate may also be pre- coated with a thin single- or multilayer of TiN, TiC, TiCN or TiAIN with a thickness in the micrometer range according to the prior art. The coating is applied onto the entire body or at least the functioning surfaces thereof, e.g., the cutting edge, rake face, flank face or any other surface which participates in the metal cutting process. The coated cutting tool according to the present invention exhibits improved wear resistance and toughness properties compared to prior art tools when used for machining steel or cast iron. The coating, which is adherently bonded to the substrate, comprises a laminar, multilayered structure of metal nitrides (or carbides) and crystalline alumina of the alpha (α) - and/or the gamma (γ) phase, preferably of metal nitrides and crystalline γ-Al₂0₃, has a thickness between 0.5 and 20 μm, preferably between 1 and 10 μm, most preferably between 2 and 6 μm. In the multilayered coating structure

(MLX/A1₂0₃) l (MLX/A1₂0₃) (MLX/Al₂0₃) _λ/ (MLX/A1₂0₃) _λ/ the alternating layers are MLX and Al₂0₃ (see Fig. 1) where MLX comprises a metalnitride or a metalcarbide with the metal elements M and L selected from titanium (Ti) , niobium (Nb) , hafnium (Hf) , vanadium (V) , tantalum (Ta) , molybdenum (Mo) , zirconium (Zr) , chromium (Cr) , tungsten (W) or aluminium (Al) . In said coating the repeat period λ in (MLX/Al₂0₃)χ is essentially constant throughout the entire multilayer structure. Furthermore, the repeat period is larger than 3 nm but smaller than 100 nm, preferably smaller than 50 nm, most preferably smaller than 25. The repeat period is meant the thickness of the layers MLX + Al₂0₃, i.e. two adjacent nano- layers. Preferred examples of the above described nano- multilayered coating structures are e.g. when M=L, TiN/

Al₂0₃/TiN/Al₂0₃/TiN/ Al₂0₃/TiN/ or when L≠M, TiAlN/

A1₂0₃/ TiAlN/ A1₂0₃/ TiAlN/ Al₂0₃/TiAlN/

Referring to Fig.l there is shown a substrate 1 coated with a laminar, multilayered nitride/carbide and alumina coating 2 with the individual metal nitride (or carbide) layers being MLX 3 and the individual alumina layers 4 and the repeat period λ 5, the thickness of the metalnitride/carbide layer and the alumina layer is essentially constant throughout the entire multilayer coating.

The laminar coatings above exhibit a columnar growth mode with no or very little porosity at the grain boundaries. The coatings also possess a substantial waviness in the sublayers which originates from the sub- strate surface roughness.

For a cutting tool used in metal machining, several advantages are provided by the present invention with nanostructured lamellae coatings deposited on substrates of hard, refractory materials such as cemented carbides, cermets and ceramics . In a lamellae coating of

(MLX/A1₂0₃) χl (MLX/Al₂0₃)_λ/ .... on cemented carbides, the hardness of the coating is usually enhanced over individual single layers of MLX and Al₂0₃ with a layer thickness on a μm scale simultaneously as the intrinsic stress is smaller. The first observation, enhanced hardness in the coating, results in an increased abrasive wear resistance of the cutting edge while the second observation of less intrinsic stress in the coating, provides an increased capability of absorbing stresses exerted on the cutting edge during a machining operation. Furthermore, the invented coating gives the cutting edges of the tool an extremely smooth surface finish which, compared to prior art coated tools, results in an improved surface finish also of the workpiece being machined.

The laminar, nanostructured coatings according to the present invention can be deposited on a carbide, cermet, ceramic or high speed steel substrate either by CVD or PVD techniques, preferably by the PVD bipolar pulsed dual magnetron sputtering (DMS) technique, by successively forming individual sublayers on the tool substrate at a substrate temperature of 450°-700 °C, preferably 550-650 °C, by switching on and off separate magnetron systems .

Claims

1. Cutting tool comprising a body of sintered cemented carbide or cermet, ceramics or high speed steel and on which at least on the functioning parts of the surface of the body, a thin, adherent, hard and wear resistant coating is applied, said coating charact eri zed in comprising a laminar, multi- layered structure of refractory compounds in polycrys- talline, repetitive form, (MLX/A1₂0₃) ╧çl (MLX/A1₂0₃) _╬╗/ (MLX/Al₂0₃) _╬╗/ (MLX/ Al₂0₃) _╬╗/ where the alternating layers are MLX and Al₂0₃, and the MLX sublayers comprise a metal nitride or a metal carbide with the metal elements M and L selected from Ti, Nb, Hf , V, Ta, Mo, Zr, Cr, W and Al , and the Al₂0₃ sub- layers consist of crystalline Al₂0₃ of the alpha (╬▒) - and/or gamma (╬│) phase, and in said coating the repeat period ╬╗ is essentially constant throughout the entire multilayered structure, and where the said repeat period ╬╗ is larger than 3 nm but smaller than 100 nm and that the total thickness of said multilayered coating is larger than 0.5 ╬╝m but smaller than 20 ╬╝m.

2. Cutting tool according to claim 1 charac t eri zed in that the crystalline A1₂0₃ sublayers consist of the gamma (╬│) alumina phase

3. Cutting tool according to claim 2 charac t eri zed in that the MLX sublayers are composed of metal nitrides .

4. Cutting tool according to claim 3 charac t eri zed in that the sublayers of the metal nitrides consists of TiAlN and TiN, preferably TiAlN.

5. Cutting tool according to any of the preceding claims charac t eri zed in that the repeat period ╬╗ ranges from 3 to 50 nm, preferably from 3 to 25 nm.

6. Cutting tool according to any of the preceding claims characte ri zed in that said coating has a total thickness of 1 to lO╬╝m, preferably from 2 to 6 ╬╝m.

7. Cutting tool according to any of the preceding claims charact eri zed in that said tool body is a cemented carbide or a cermet .