PVD AL2O3 COATED CUTTING TOOL

Abstract: The present invention describes a coated CBN cutting tool for metal machining. The tool consists of one or more CBN bodies with or without cemented carbide backing. The coating is composed of one or more layers of refractory compounds of which at least one layer consists of fine-grained, crystalline gamma -phase alumina, Al2O3, with a grainsize less than 0.1 mu m. The Al2O3-layer is deposited with a bipolar pulsed DMS technique (Dual Magnetron Sputtering) at substrate temperatures in the range 450 DEG C to 700 DEG C.

Description: PVD AI7O3 COATED CUTTING TOOL The present invention relates to a cutting tool for metal machining, comprising at least one body containing CBN, (polycrystalline cubic boron nitride), with or without cemented carbide backing, and on the surface of said body a hard and wear resistant refractory coating. The coating is adherently bonded to the body and covers all functional parts thereof. The coating is composed of one or more layers of refractory compounds of which at least one layer consists of fine-crystalline γ-Al2θ3, deposited by Physical Vapour Deposition (PVD). Cutting tools having cutting edges formed of a superhard abrasive such as a cubic boron nitride (CBN) based material are manufactured by powder metallurgical techniques and are mainly used for the machining of cast iron and hardened steel. Several types of CBN cutting tools are known, the majority consisting of a CBN tip that has been brazed onto a cemented carbide insert. Others have the CBN sintered directly to a cemented carbide backing of sufficient thickness to produce an insert while yet others consist of a CBN-containing body without any cemented carbide backing. Subjecting a sintered CBN body to temperatures over 1000 °C may result in unwanted structural changes in the material. Furthermore, in the case of a brazed insert the braze joint will be destroyed. Swedish patent application 9704066-1 discloses a coated cutting tool for metal machining. The coating is composed of one or more layers of refractory compounds of which at least one layer consists of fine-grained, crystalline γ-phase alumina, AI2O3, with a grainsize less than 0.1 μm. The A^Oβ-layer is deposited with a bipolar pulsed DMS technique (Dual Magnetron Sputtering) at substrate temperatures in the range 450 °C to 700 °C. WO 98/28464 discloses in the example that by applying a coating including an MTCVD-TiCN and a CVD-AI2O3 -layer to such a CBN tool material, substantial advantages in tool life can be achieved when machining hardened ball bearing steel. US 5,503,913 proposes for the improvement of the wear properties of tools with cutting edge of cubic boron nitride or polycrystalline cubic boron nitride to coat the superhard body with a 0.5-6 μm thick coating of one or more oxides of the metals Zr, Y, Mg, Ti or Al. The coating is deposited from the gas phase at temperatures up to 800 °C, preferably using a pulse plasma CVD-process. According to the present invention there is provided a cutting tool comprising at least one body containing at least 35 vol-% cubic boron nitride, CBN, provided with a hard and wear resistant refractory coating. The hard and wear resistant coating is composed of one or more layers of refractory compounds of which at least one layer, preferably the outermost layer, essentially consists of γ-Al2θ3 deposited by the DMS PVD method at substrate temperatures of 450 to 700 °C, preferably 550 to 650 °C. The inner layer(s), if any at all, between the tool body and the AI2O3 -layer, is composed of metal nitrides, carbonitrides and/or carbides with the metal elements selected from Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al. The γ-Al2θ3-layer consists of high-quality, dense, fine-grained crystalline AI2O3 with a grain size less than 0.1 μm. Furthermore, the γ-A-203 -layers are virtually free of cracks and halogen impurities and with a hardness of at least 20 GPa and a compressive stress of at least 1 GPa. In a first embodiment the PCBN material according to the invention contains more than 70 % CBN by volume. This material is particularly useful for the machining of cast iron. In a second embodiment the PCBN-material contains less than 70 % CBN by volume the remainder being other hard wear resistant constituents such as carbides, nitrides, carbonitrides, oxides or borides of the metals of groups IVa to Via of the periodic table usually TiC, TiN or Ti(C,N). An example of such a material is disclosed in US 5,639,285. This kind of PCBN-material is mainly used in cutting tools for machining of hardened steel. Often, the PCBN material also contains smaller amounts (typically <10 wt% each) of other components, e.g. Co, Ni, WC, Al, A1N and AI2O3. In a third embodiment PCBN cutting tool is manufactured without cemented carbide backing or support. Usually such a product consists of more than 80 % CBN by volume. In a fourth embodiment the PCBN cutting tool consists of a CBN-containing material attached to a substrate preferably cemented carbide by brazing, sintering or in any other way. The cemented carbide is WC-Co with 10-20, preferably 15-17, wt-% Co. The γ-Al2θ3-layers according to the invention further give the cutting edges of the tool an extremely smooth surface finish which results in an improved surface finish also of the work piece being machined. The very smooth surface finish can be attributed to the very fine crystallinity of the coating. The "γ-Al2θ3 "-layers may also partially contain other phases from the "γ-series" like θ, δ and η. Identification of the γ- and/or θ- phases in the Al2θ3-layers according to the invention can preferably be made by X-ray diffraction. Reflexes from the (400) and (440) planes of the γ-Al2θ3 -layers occurring at the 2θ-angles 45.80 and 66.80 when using Cuj α radiation, unequivocally identify the γ-phase. Weaker reflexes from the (222), (200) and (311) planes of the γ-phase can occasionally be identified. When the θ-phase is present in the Al2θ3-layers according to the invention, said phase is identified by the reflexes from the (200, 20-2) planes. The fine-grained, crystalline γ-Al2θ3 according to the invention is strongly textured in the [440] -direction. A Texture Coefficient, TC, can be defined as: where I(hkl) = measured intensity of the (hkl) reflection I 0 (hkl) = standard intensity from the ASTM standard powder pattern diffraction data n = number of reflections used in the calculation (hkl) reflections used are: (111), (311), (222), (400) and (440) and whenever the TC(hkl)>l, there is a texture in the [hkl]-direction. The larger the value of TC(hkl), the more pronounced is the texture. According to the present invention, the TC for the set of (440) crystal planes is > 1.5. A further improvement in cutting performance can be anticipated if the edges of the γ-Al2θ3 coated cutting tools according to the invention are treated by a gentle wet- blasting process or by edge brushing with brushes based on e.g. SiC as disclosed in US 5,861,210. The total coating thickness according to the present invention varies between 0.5 and 20 μm, preferably between 1 and 15 μm with the thickness of the non-Al2θ3- layer(s) varying between 0.1 and 10 μm, preferably between 0.5 and 5 μm. The finegrained γ-Al2θ3-coating can also be deposited directly onto the CBN cutting tool and the thickness of said γ-Al2θ3 varies then between 0.5 and 15 μm preferably between 1 and 10 μm. Likewise further coatings of metal nitrides, carbonitrides and/or carbides with the metal elements selected from Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al can be deposited on top of the AI2O3 -layer, preferably TiN. The fine-grained crystalline γ-Al2θ3-layer according to the invention is deposited by a bipolar dual magnetron sputtering technique onto a moving substrate at substrate in vacuum. The substrate may be floating or pulsed biased, the exact conditions depending to a certain extent on the design of the equipment being used. The Al2θ3-layer is deposited by a reactive, pulsed magnetron sputtering in a gas mixture of argon and oxygen at a pressure of 1-5 μbar. The pulse frequency is 10 to 100 kHz, preferably 50 kHz. Deposition occurs with a rate of at least 1 nm/s with reference to a stationary arranged substrate. The magnetron target power density in time average is at least 10 W/cm.2 anr \ t ne substrate temperature is 450 to 700 °C, preferably 550 to 650 °C. Preferably the Al2θ3-layer is deposited by the sputtering of two magnetrons with Al targets that are alternatively switched as a cathode and as an anode of a magnetron sputtering apparatus. It is within the purview of the skilled artisan to determine whether the requisite grain size and phase compositions have been obtained and to modify the deposition conditions in accordance with the present specification, if desired, to affect the nano structure of the Al2θ3-layer within the frame of the invention. The layer(s) described in the present invention, comprising metal nitrides, carbides and/or carbonitrides and with the metal elements selected from Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al can be deposited by PVD-technique, CVD- and/or MTCVD- technique (Medium Temperature Chemical Vapor Deposition) particularly by pulsed magnetron sputtering.

Claims: Claims 1. Cutting tool comprising a substrate and a coating said substrate comprising a body containing at least 35 vol-% cubic boron nitride and said coating comprising at least one thin layer of alumina, characterised in that said alumina layer es- sentially consists of gamma-alumina with a grain size less than 0.1 μm being free from halogen impurities. 2. Cutting tool according to claim 1 characterised in that the AI2O3 -layer exhibits significant X-ray diffraction reflexes from at least one of the (440) and (400) crystal planes, said at least one or more layers having a hardness of at least 20 GPa and a compressive stress of at least 1 GPa. 3. Cutting tool according to claim 1 characterised in that the Al2θ3-layer has a preferred growth orientation in the [440] -direction with a texture coefficient >1.5 defined as below: where I(hkl) = measured intensity of the (hkl) reflection I 0 (hkl) = standard intensity from the ASTM standard powder pattern diffraction data n = number of reflections used in the calculation (hkl) reflections used are: (111), (311), (222), (400) and (440). 4. Cutting tool according to any of the previous claims characterised in that the fine grained crystalline γ-Al2θ3 -layer contains portions (detectable by XRD technique) of additional alumina phases from the γ-series of the Al2θ3-polymorphs, preferably θ-phase. 5. Cutting tool according to any of the previous claims characterised in having at least one layer of thickness 0.1-10 μm, preferably 0.5-5 μm, comprising of metal nitrides, carbonitrides and/or carbides with the metal elements selected from Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al preferably TiC, TiCN, TiN or TϊAlN. 6. Cutting tool according to any of the previous claims characterised in that the outer layer is AI2O3. 7. Cutting tool according to any of the previous claims characterised in that the outer layer is TiN. 8. Cutting tool according to any of the previous claims characterised in that the PCBN material contains more than 80 % CBN by volume. 9. Cutting tool according to claims 1-7 characterised in that the PCBN material contains 35-90 % CBN by volume, the remainder being other hard wear resistant constituents such as carbides, nitrides, carbonitrides, oxides or borides of the metals of groups IVa to Via of the periodic table preferably TiC, TiN or Ti(C,N). 10. Cutting tool according to any of the previous claims characterised in that said tool consists entirely of the CBN-containing material. 11. Cutting tool according to any of claims 1-9 characterised in that said tool consists of the CBN-containing material attached to a substrate preferably cemented carbide by brazing, sintering or in any other way. 12. Cutting tool according to claim 11 characterised in that the cemented carbide is WC-Co with 10-20, preferably 15-17, wt-% Co. 13. Method of making a coated cutting tool, wherein at least one refractory layer consisting of fine-grained, crystalline γ-Al2θ3 as per claim 1, is deposited by a magnetron sputtering onto the moving substrate in a vacuum, characterised in that the Al2θ3-layer is deposited by a reactive, pulsed magnetron sputtering in a gas mixture of argon and oxygen at a pressure of 1-5 μbar, that the pulse frequency is set for 10 to 100 kHz, preferably 50 kHz, that deposition occurs with a rate of at least 1 nm/s with reference to a stationarily arranged substrate, that the magnetron target power density in time average is set for at least 10 W/cm^ and that the substrate temperature is set in the range 450 to 700 °C, preferably in the range 550 to 650 °C, depending on the material of the tool body being coated. 14. A process according to claim 13, characterised in that the AI2O3- layer is deposited by the sputtering of two magnetrons with Al targets that are alternatively switched as a cathode and as a anode of a magnetron sputtering apparatus. 15. A process according to any of the claims 13 and 14 characterised in that additional, non-Al2θ3 -layers are also deposited by a PVD process (Physical Vapor Deposition), particularly by pulsed magnetron sputtering.

Inventors: LITTECKE PETER, SOEDERBERG STAFFAN, LITTECKE, PETER, SOEDERBERG, STAFFAN
Applicants: SANDVIK AB, SANDVIK AB (PUBL)
Application year: 2000
Main classification: C23C14
IPC codes: B23B27, C04B41, C23C14, C23C30