Aluminium oxide coated tool

Abstract: According to the present invention there is provided a tool at least partly coated with at least two refractory layers of which one of the said layers is a fine grained α-Al₂O₃-layer which is the top layer along the cutting edge-line and the other a TiC x N y O z - or a ZrC x N y -layer being the top layer on the clearance face. The coated tool exhibits excellent flank and crater wear and high resistance to flaking, particularly when used for machining of low carbon steel and stainless steel. Used cutting edges can easily be identified by the naked eye.

Description: The present invention relates to an Al 2 O 3 -coated cutting tool suitable for machining of metals by turning, milling, drilling or by similar chip forming machining methods. Modem high productivity chip forming machining of metals requires reliable tools with excellent wear properties. This has so far been achieved by employing a cemented carbide tool body provided with a wear resistant coating. The cemented carbide tool body is generally in the shape of an indexable insert clamped in a tool holder. The most commonly used wear layers are TiC, TiN, TiCN and Al 2 O 3 . Both single layer and multilayer coatings are employed. CVD, PVD or similar coating techniques are used for depositing the different layers onto the cemented carbide body. During the past five to ten years coated cemented carbide tools have been improved considerably with respect to reliability and tool life. During e.g. a turning cutting operation the coated tool wears continuously on its rake face by the metal chip formed causing a crater wear. The machined work piece also slides along the clearance face of the tool causing a flank wear. During high speed cutting the tool edge may reach very high temperature at the rake face. This leads to a diffusion type crater wear. On the clearance face of the tool the temperature is significantly lower so that mainly abrasive type wear occurs. It is generally accepted that an Al 2 O 3 -layer performs best on the rake face due to its excellent ability to withstand diffusion type wear. Layers of the type MeC x N y O z , where Me is a metal selected from the group consisting of metals in the groups IVB, VB, VIB of the periodic table generally TiC x N y O z (further on denoted by TiC x N y O z ) generally performs better on the clearance face. Al 2 O 3 -layers on the other hand wear relatively fast on the clearance face and develop flank wear relatively quickly on that face. The flank wear will be particularly large for thick, >4 µm, Al 2 O 3 -layers. Flank wear influences the machined surface and may therefore limit tool life. For TiC x N y O z -type layers the situation is almost the reverse, low flank wear and faster crater wear than Al 2 O 3 . It is desirable to have a tool with high wear resistance on both the clearance face and on the rake face at the same time. Other factors influencing cutting performance of a coated tool is spalling or flaking of coatings. Flaking accelerates tool wear, in particular the flank wear. Flaking may be a result of inferior coating adhesion or it may be due to smearing or welding of work piece material onto the cutting edge and a successive withdrawal of the coating. This may occur when the adhesion strength between the chip formed and the coating material is sufficiently high. Some steels are more difficult to machine than others due to smearing and resulting flaking, for examples, stainless steel and low carbon steel. Nowadays, less machining per each component is needed. The requirements on high surface finish of the machined component only allow tools with clean smooth edge line with very little developed wear to be used. It is becoming more and more difficult for the machine operator by the naked eye to differentiate between a little used and an unused cutting edge ("edge identification"). This is particularly difficult if the top layer is Al 2 O 3 which colour is dark grey or black. By mistake, using a used tool edge e.g. during an unmanned night shift run may cause component rejection or even unwanted production stops. Edge identification can more easily be done if the insert has a top layer of TiC x N y O z or in particular if the top layer is a goldish TiN- ZrN- or HfN-layer. In U S patent 4,643,620 the coating thickness is reduced along the edge by a mechanical treatment such as brushing, lapping or barrel polishing. The object is mainly to reduce the coating thickness along the cutting edge which is claimed to improve the toughness behaviour of the cutting tool. EP-B-298 729 discloses a method of reducing edge damages during cutting by reducing the coated surface roughness by employing a mechanical polishing, lapping or brush honing. This method is according to the findings of the present inventors not sufficient to minimise smearing. The invention is illustrated by fig. 1A-1G which show the surface condition after different post treatments. It is the object of the present invention to overcome the drawbacks of prior art coated bodies and to obtain improvements with respect to the following properties: Make "used edge identification" possible. The tendency of smearing/welding of work piece material onto the cutting edge. Cutting edge flaking resistance. Simultaneous high resistance to crater-and flank wear. The inventors have made great efforts to find means to reduce the smearing of work piece material onto the cutting edge in order to improve edge flaking and flank wear resistance. It has been found by comparative cutting tests with different top layers that Al 2 O 3 is less prone to smearing than layers of the type TiC x N y O z . Particularly fine grained smooth α-Al 2 O 3 is very suitable as a coating material along the cutting edge in order to minimise smearings and thereby reduce the risk of edge line flaking. The fine-grained α-Al 2 O 3 layers may e.g. be any of the types disclosed in Swedish patent applications 9203852-0 and 9400089-0 or most likely also any other fine grained α-Al 2 O 3 -layer with other preferred growth direction. Although tools with a top layer of a fine grained α-Al 2 O 3 such as described in the above patent applications have excellent cutting properties they do not always comply with today's requirements since they e.g. suffer from the following drawbacks: "used edge identification" is difficult with the naked eye high initial flank wear is generally obtained for tools with Al 2 O 3 top layers >4 µm. As mentioned above edge identification and improved flank wear can be obtained by applying a top layer of TiC x N y O z . However, such a top layer will severely increase smearing along the edge-line when machining in the difficult materials mentioned above. The inventors have solved this problem by mechanically removing the TiC x N y O z -layer either from only the edge line or from both the rake face and the edge line. By employing this method and keeping the TiC x N y O z -layer intact on the clearance face several requirements have been fulfilled simultaneously: excellent wear resistance simultaneously on the rake face and on the clearance face excellent flaking resistance and easily identifiable used cutting edges. According to the present invention there now exists a cutting tool insert comprising a body of generally polygonal or round shape having an upper face, an opposite face and at least one clearance face intersecting said upper and lower faces to define cutting edges made of cemented carbide, titanium based carbonitride or ceramics. The insert is at least partly coated with at least two refractory layers. One is a fine grained, grain size 0.5-4.0 µm, preferably 0.5-2.0 µm, α-Al 2 O 3 -layer being the top layer along the cutting edge-line and the other is a TiC x N y O z - or a ZrC x N y -layer, preferably a TiN-, ZrN-, TiCN- and/or TiC-layer being the top layer on the clearance face. The α-Al 2 O 3 -layer has a texture in the (012)-direction or (104)-direction. A Texture Coefficient, TC, can be defined as: where I(hkl) = measured intensity of the (hkl) reflection I o (hkl) = standard intensity of the ASTM standard powder pattern diffraction data n = number of reflections used in the calculation, (hkl) reflections used are: (012), (104), (110), (113), (024), (116) According to the invention TC for the set of (012) crystal planes is larger than 1.3, preferably larger than 1.5 and for the set of (104) crystal planes TC is larger than 1.5, preferably larger than 2.5, and most preferably larger than 3.0. The α-Al 2 O 3 -layer has a thickness of 2-12 µm, preferably 4-8 µm. The other layer has a thickness of 0.1-5 µm, preferably 1-4 µ m. The total thickness of the coating including also other layers is <20 µm. According to the method of the present invention a cutting tool insert made of cemented carbide, titanium based carbonitride or ceramics is at least partly coated with at least two refractory layers of which the next outermost is a fine grained α-Al 2 O 3 -layer and the outermost is a MeC x N y O z -layer, where Me is a metal selected from the group consisting of metals in the groups IVB, VB, VIB of the periodic table, preferably Ti or Zr. This top MeC x N y O z -layer is removed along the edge line or on the edge line as well as the rake face leaving said layer essentially untouched on the clearance face. The methods used to remove the layer can be: brushing with a brush with straws containing e.g. SiC or other grinding media, polishing with diamond paste, controlled directed blasting with e.g. Al 2 O 3 -powders with or without masking off the clearance face. Also combinations of these methods are possible. The aim of the mechanical treatment in the present invention is as mentioned to remove the top TiC x N y O z -layer and expose the fine grained α-Al 2 O 3 layer along the edge or also the entire rake face. A reduction of coating thickness along the edge line is not desired. The used mechanical method should be so gentle that only the top TiC x N y O z -layer is removed leaving the Al 2 O 3 at the edge-line as untouched as possible. Example 1 Cemented carbide cutting inserts CNMG 120408-QM with the composition 5.5% Co, 8.6 % cubic carbides (TiC-TaC-NbC) and balance WC were coated with CVD-technique according to the following sequence: 0.7 µm TiC, 0.5 µm Ti (CO), 8.0 µm Ti(CN), 3.0 µm Al 2 O 3 and 2.8 µm TiN. The Al 2 O 3 -layer was deposited with a method that gives fine grained α-Al 2 O 3 according to Swedish Patent Application 9203853-0. The TiN-layer was deposited at 400 mbar and the other layers according to prior art techniques. The coated inserts were post treated with different methods according to below: Variant 1 A: No post treatment. Variant 1 B: Wet blasting with 150 mesh Al 2 O 3 -grits at 1.0 bar. Variant 1C: Wet blasting with 150 mesh Al 2 O 3 -grits at 1.5 bar. Variant 1D: Wet blasting with 150 mesh Al 2 O 3 -grits at 2.0 bar. Variant 1 E: Wet blasting with 325 mesh Al 2 O 3 -grits at 2.0 bar. Variant 1F: Brushing with a cylindrical nylon brush containing SiC. Variant 1 G: As 1 F but with the centre of the brush closer to the insert in order to get more efficient treatment. The different treatments resulted in different degrees of thinning and smoothness of the outer TiN-layer: Variant 1 B: A much smoother surface than 1A. The TiN-layer covering the whole surface of the insert. Variant 1C: A much smoother surface than 1A. The TiN-layer covering the whole surface of the insert. Variant 1 D: A much smoother surface than 1A. The TiN-layer was removed along the whole edge line exposing the Al 2 O 3 -layer. Variant 1E: As 1B. Variant 1 F: A much smoother surface than 1A. The TiN-layer covering the whole surface of the insert. Variant 1G: A much smoother surface than 1A. The TiN-layer was removed along the whole edge line exposing the Al 2 O 3 -layer. The surface condition of the variants is illustrated by fig. 1A-1G. Example 2 Cemented carbide cutting inserts CNMG 120408-QM with the composition 5.5 % Co, 8,6 % cubic carbides (TiC-TaC-NbC) and balance WC were coated with CVD-technique according to the following sequence: 0.6 µm TiC, 0.4 µm Ti (CO), 8.1 µm Ti(CN), 8.1 µm Al 2 O 3 and 0.9 µm TiN. The Al 2 O 3 -layer was deposited with a method that gives fine grained α-Al 2 O 3 according to Swedish Patent Application 9203853-0. The TiN-layer was deposited at 400 mbar and the other layers according to prior art techniques. The coated inserts were post treated with different methods according to below: Variant 2A: No post treatment. Variant 2B: Wet blasting with 150 mesh Al 2 O 3 -grits resulting in a smoother surface. Here the top TiN-layer was removed along the edge line as well as on the whole rake face exposing the black Al 2 O 3 -layer. Variant 2C: Brushing with a cylindrical SiC containing nylon brush. This treatment resulted in a smooth surface with only the top TiN-layer removed along the edge line exposing the Al 2 O 3 . Example 3 Cemented carbide cutting inserts CNMG 120408-QM with the composition 5.5 % Co, 8.6 % cubic carbides (TiC-TaC-NbC) and balance WC were coated with CVD-technique according to the following sequence: 1.0 µm TiC, 0.4 µm Ti(CO), 7.9 µm Ti(CN) and 5.5 µm Al 2 O 3 . The Al 2 O 3 -layer was deposited with a method that gives fine grained α-Al 2 O 3 according to Swedish Patent Application 9203853-0. The inserts were treated by wet blasting with 150 mesh Al 2 O 3 -grits (variant 3). Example 4 Cemented carbide cutting inserts CNMG 120408-QM with the composition 6.5 % Co, 8.7 % cubic carbides (TiC-TaC-NbC) and balance WC and with a 25 µm thick binder phase enriched surface zone were coated with CVD-technique according to the following sequence: 7.9 µm TiC, 4.2 µm Al 2 O 3 and 3.5 µm TiC. The Al 2 O 3 -layer was deposited with a method that gives a fine grained α-Al 2 O 3 -layer according to Swedish Patent Application 9203853-0. Variant 4A: No post treatment. Variant 4B: The inserts were brushed with a cylindrical SiC containing nylon brush, resulting in a smooth surface exposing the Al 2 O 3 -layer along the whole edge line. Example 5 Cemented carbide cutting inserts CNMG 120408-QM with the composition 6.5 % Co, 8.7 % cubic carbides (TiC-TaC-NbC) and balance WC and with a 25 µm thick binder phase enriched surface zone were coated with CVD-technique according to the following sequence: 7.0 µm TiC and 5.1 µm Al 2 O 3 . The Al 2 O 3 -layer was deposited with a method that gives fine grained α-Al 2 O 3 according to Swedish Patent Application 9203853-0. The inserts were treated by wet blasting with 150 mesh Al 2 O 3 -grits (variant 5). Example 6 Cemented carbide cutting inserts CNMG 120408-QM with the composition 6.5 % Co, 8.7 % cubic carbides (TiC-TaC-NbC) and balance WC and with a 25 µm thick binder phase enriched surface zone were coated with CVD-technique according to the following sequence: 5.4 µm Ti(CN), 5.3 µm Al 2 O 3 and 1.3 µm TiN. The Al 2 O 3 -layer was deposited according to prior art technique resulting in a layer of mixed α- and κ-polymorphs. The TiN-layer was deposited at 400 mbar and the other layers according to prior art techniques. Variant 6A: Not post treated. Variant 6B: Wet blasting with 150 mesh Al 2 O 3 -grits resulting in a smoother surface and the top TiN-layer removed along the edge line as well as on the whole rake face exposing the Al 2 O 3 . Variant 6C: Brushing with a cylindrical SiC containing nylon brush resulting in a smooth surface and exposing the Al 2 O 3 -layer along the whole edge line. Example 7 Tool inserts from examples 1-6 were tested with respect of edge line flaking in a facing operation in an alloyed steel (AISI 1518, W-no. 1,0580). The shape of the work piece was such that the cutting edge was intermitted three times during each revolution. Cutting data: Cutting speed 130-220 m/min, Feed 0.2 mm/rev, Depth of cut 2.0 mm. The inserts were run one cut over the work piece. The results below are expressed as percentage of the edge line in cut that obtained flaking of the coating. Variant Post treatment Al 2 O 3 exposed % Edge line flaking at edge 1A None No 63 1 B Blasted No 80 1 C Blasted No 84 1D Blasted Yes 18 1E Blasted No 70 1F Brushed No 66 1G Brushed Yes 0 2A None No 57 2B Blasted Yes 0 2C Brushed Yes 0 3 Blasted Yes 0 4A None No 87 4B Brushed Yes 0 5 Blasted Yes 0 6A None No 83 6B Blasted Yes 27 6C Brushed Yes 33 As can be seen from above the best results have been obtained when the fine grained α-Al 2 O 3 -layer has been exposed at the edge line. Post treatment resulting in smoother coating surface but not exposure of the α-Al 2 O 3 does not result in any improvement of the flaking resistance. Variants 6B and 6C with the α/κ-polymorphs exposed at the edge line do not obtain as good flaking resistance as the variants with α-Al 2 O 3 -layer exposed at the edge line. Example 8 Cutting inserts from examples 4 and 5 were run in longitudinal turning of a ball bearing steel SKF 25B. Cutting data: Cutting speed 180 m/min, Feed 0.36 mm/rev, Depth of cut 2.0 mm, coolant was used. The flank wear was measured after 2.5 min in order to study the initial wear. Variant flank wear, mm 4B 0.13 5 0.20 This example illustrates the improved flank wear resistance due to the top TiC-layer on the flank face.

Claims: A cutting tool insert made of cemented carbide, titanium based carbonitride or ceramics comprising a body of generally polygonal or round shape having an upper face, an opposite face and at least one clearance face intersecting said upper and lower faces to define cutting edges said insert being at least partly coated with at least two refractory layers of which one is a fine grained α-Al 2 O 3 -layer and the other is a TiC x N y O z - or a ZrC x N y -layer characterized in that said Al 2 O 3 -layer is the top layer along the cutting edge line and that said TiC x N y O z - or a ZrC x N y -layer is the top layer on the clearance face and that said α-Al 2 O 3 -layer has a texture in the (012)-direction or (104)-direction with a Texture Coefficient, TC being defined as: where I(hkl) = measured intensity of the (hkl) reflection I o (hkl) = standard intensity of the ASTM standard powder pattern diffraction data n = number of reflections used in the calculation, (hkl) reflections used are: (012), (104), (110), (113), (024), (116) wherein TC for the set of (012) crystal planes being larger than 1.3, preferably larger than 1.5 and for for the set of (104) crystal planes larger than 1.5, preferably larger than 2.5, and most preferably larger than 3.0. A cutting tool insert according to claim 1 characterized in that the top layer on the clearance face is TiN, ZrN, TiCN or TiC. A cutting tool insert according to any of the preceding claims characterized in that the Al 2 O 3 -layer thickness is 2-12 µm.

Inventors: LENANDER ANDERS, LJUNGBERG BJOERN, LENANDER, ANDERS, LJUNGBERG, BJOERN, Ljungberg, Björn
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 1995
Main classification: C23C16
IPC codes: C23C16, B32B19, C23C30, B23P15, B23B27