Coated cutting insert

Abstract: The present invention discloses a coated cutting insert particularly useful for dry milling of grey cast iron. The insert is characterized by a WC-Co cemented carbide substrate and a coating including an innermost layer of TiC x N y O z with columnar grains and a top coating of a fine-grained α-Al 2 O 3 layer.

Description: The present invention relates to a coated cutting tool (cemented carbide insert) particularly useful for dry milling of grey cast iron. Grey cast-iron is a material which, in general, is reasonably easy to machine with cemented carbide tools. Often, long tool life can be obtained. However, the machinability of cast iron can vary considerably. The tool life may be influenced significantly by small variations in the chemical composition within the material which may be related to the casting technique used, such as the cooling conditions. Other causes for variations are the casting skin and sand inclusions, if present, or even the stability of the machine used for cutting the material. It is well-known that the machinability of a cast iron may vary from one batch to another. There is always a desire for a reliable and stable tool life in mass volume production and in particular when unmanned production is employed. When machining grey cast iron by coated milling cutters, the cutters are worn mainly by a so called adhesive wear mechanism. That is, fragments or individual grains of the layers and later also parts of the cemented carbide are successively pulled away from the cutting edge by the work piece chip formed. Such wear mechanism is further accelerated by the formation of cracks about 50 µm apart along and perpendicular to the cutting edge generally referred to as comb cracks. They are the result of thermal fatigue caused by the intermittent cutting which is typical for all milling cutting operations. Commercial cemented carbide milling inserts therefore generally contain gamma-phase, which is a solid solution of WC-TiC-TaC-NbC, in order to decrease heat influenced wear mechanism. As soon as the coating is worn out, the underlying cemented carbide will wear fast. This will lead to higher cutting forces. The comb cracks formed will, after some time, cause severe chipping of the cutting edge and the damaged tool will generate a bad surface or chipping of the machined component. Chipforming operations predominantly of adhesive wear type put high demands on both the coating and the cemented carbide. A cemented carbide that can withstand comb crack formation as long as possible and a coating that adheres well to the cemented carbide is desirable. Further, it is desired that there are low adhesion forces between the chip and the coating and that the coating has a high internal strength (cohesiveness) a property which is believed to be of significant importance. It has surprisingly been found that a straight WC-Co cemented carbide body herein referred to as the substrate essentially provided with an inner columnar grained coating structure consisting of mainly a TiC x N y O z -layer with columnar grains and a columnar like textured fine-grained α-Al 2 O 3 layer on top performs very well particularly in cast iron milling. In fact such a tool has been found to have a cutting performance far better than prior art tools. Fig 1 is a micrograph in 5000X magnification of a coated insert according to the present invention in which A - substrate B - TiC x N y O z -layer with equiaxed grains C - TiC x N y O z -layer with columnar grains D - Tic x N y O z -Fayer with equiaxed or needlelike grains E - (012)-textured α-Al 2 O 3 -Fayer with columnar like grains According to the present invention a cutting tool insert is provided of a cemented carbide substrate of a WC+Co composition containing 3-15, preferably 5-12, most preferably 5-8 wt-% Co, and with a WC grain size of 1-2 µm, preferably about 1.5 µm. In addition, the substrate may contain up to 2 wt-% cubic carbides of metals from groups IVA, VA or VIA of the periodic table such as TiC, TaC and NbC. However, a straight WC+Co composition is preferred. The coating comprises the following layers in successive order: a first innermost layer of TiC x N y O z with x+y+z=1, and z<0.5, with a thickness of 0.1-2 µm, and with equiaxed grains with size <0.5 µm a next layer of TiC x N y O z with x+y+z=1, and z=0, x>0.3 and y>0.3, with a thickness of 2-10 µm, preferably 4-7 µm, with columnar grains and with a diameter of <5 µm, preferably <2 µm a next layer of TiC x NyO z with x+y+z=1 and z≤0.5, preferably z>0.1, with a thickness of 0.1-2 µm and with equiaxed or needlelike grains with size <0.5 µm, this layer being the same as or different from the innermost layer, and an outer layer of a smooth, textured, fine-grained (grain size about 1 µm) α-Al 2 O 3 layer with a thickness of 2-10 µm, preferably 3-6 µm, and a surface roughness (R a ) of less than 0.3 mm over a measured length of 0.25 mm. Preferably, this Al 2 O 3 -Fayer is the outermost layer but it may also be followed by further layers such as a thin (0.1 - 1 µm) decorative layer of e.g.TiN as is known in the art. In addition, the α-Al 2 O 3 layer has a crystal growth orientation in either the (012)-, (104)- or (110)-direction, preferably in the (012)-direction, as determined by X-ray Diffraction (XRD) measurements. A Texture Coefficient, TC, is 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), (104) or (110) crystal planes is larger than 1.3, preferably larger than 1.5. According to the method of the invention a WC-Co-based substrate consisting of WC, 3-15 weight-% Co and ≤ 2 weight-% carbides of metals from groups IVA, VA or VIIA of the periodic table is coated with the following layers in successive order: a first innermost layer of TiC x N y O z with x+y+z=1 and z<0.5, with a thickness of 0.1-2 µm, and with equiaxed grains with size <0.5 µm using known CVD-methods. a next layer of TiC x N y O z with x+y+z=1 and z=0, x>0.3 and y>0.3, with a thickness of 2-10 µm, preferably 4-7 µm, with columnar grains and with a diameter of <5 µm, preferably <2 µm, deposited either by MTCVD-technique, using acetonitrile as the carbon and nitrogen source for forming the layer in the temperature range of 700-900 °C or by high temperature CVD-technique in the temperature range of 1000-1100 °C, the process conditions being selected to grow layers with columnar grains, that is generally high process pressure (0.3-1 bar). However, the exact conditions depend to a certain extent on the design of the equipment used. a next layer of TiC x N y O z with x+y+z=1 and z≤<0.5, preferably z>0.1, with a thickness of 0.1-2 µm and with equiaxed or needlelike grains with size ≤0.5 µm, using known CVD-methods this layer being the same as or different from the innermost layer. an outer layer of a smooth textured α-Al 2 O 3 layer with a thickness of 2-10 µm, preferably 3-6 µm, and a surface roughness (R a ) of less than 0.3 µm over a measured length of 0.25 mm according to known CVD technique, e.g. Swedish patent 501 527 (EP-A-0 603 144). When a TiC x N y O z -layer with z>0 is desired, CO 2 and/or CO is added to the reaction gas mixture. Example 1 A) Cemented carbide milling inserts of style TNEF1204AN-65 and of style SEKN1204AZ with the composition 6 % Co and balance WC were coated with a 0.5 µm equiaxed TiCN-layer followed by a 5 µm thick TiCN layer with columnar grains by using MTCVD-technique (process temperature 850 °C). In subsequent process steps during the same coating cycle, a 1 µm thick layer with equiaxed grains of TiC x N y O z (approx. x=0.6, y=0.2 and z=0.2) was deposited followed by a 4 µm thick layer of (012)-textured α-Al 2 O 3 deposited according to conditions given in Swedish patent 501 527. XRD-measurement showed a texture coefficient TC(012) of 1.5. After coating the inserts were smoothed by wet blasting. B) Cemented carbide milling inserts of style TNEF1204AN-65 as in A) were coated with a 1 µm layer with equiaxed grains of TiCN followed by 5 µm thick layer of columnar grains of a TiCN by high temperature CVD-technique (1020 °C, process pressure 400 mbar). In subsequent process steps during the same coating cycle, a 1 µm thick layer with equiaxed grains of TiC x N y O z (approx. x=0.6, y=0.2 and z=0.2) was deposited followed by a 4 µm thick layer of (012)-textured α-Al 2 O 3 deposited according to conditions given in Swedish patent 501 527. XRD-measurement showed a texture coefficient TC(012) of 1.6. After coating the inserts were smoothed by wet blasting. C) Cemented carbide milling cutting inserts of style TNEF 1204AN-65 with the composition 5.5 % Co and 8.5 % cubic carbides and balance WC were coated under the procedure given in A). After coating the inserts were smoothed by wet blasting. D) Milling inserts from the same cemented carbide batch as in C) were coated with equiaxed 5 µm TiCN-coating and 4 µm Al 2 O 3 carried out according to prior art technique resulting in a mixture of coarse α- and fine κ-Al 2 O 3 grains in the layer. The inserts were smoothed by wet blasting. Example 2 Inserts from A) were tested as below: Operation: Face milling with SANDVIK AUTO cutter, dia: 250 mm Workpiece: Specially designed component. Material: SS0125 (grey cast iron HB=205) Cutting speed: 129 m/min Feed rate: 0.24 mm/tooth Depth of cut: 2.0 mm Insert-type: TNEF1204AN-65 Note: The operation was run without coolant (dry). RESULTS: Tool-life, min Grade A (according to invention) 40 Tool-life criterion was chipping on the exit side of the workpiece. Example 3 Inserts from A) were tested in another milling operation as below: Operation: Face milling with SANDVIK 145-cutter, dia: 63 mm. Workpiece: Hydraulic engine component Material: SS0125 (grey cast iron HB=220) Cutting speed: 178 m/min Feed rate: 0.17 mm/tooth Depth of cut: 2-3 mm Insert-type: SEKN1204AZ Note: The operation was run without coolant (dry). RESULTS: Tool-life, hours Grade A (invention) 24 Tool-life criterion was surface finish. Example 4 Inserts from A), C) and D) were tested against each other in a milling operation Operation: Face milling with SANDVIK AUTO cutter, diameter 250 mm. Workpiece: Engine block Material: SS0125 (GG26Cr) grey cast iron Cutting speed: 120 m/min Feed rate: 0.28 mm/tooth Depth of cut: 4-5 mm Insert-type: TNEF1204AN-65 Note: The operation was run without coolant (dry). The inserts were all run 868 passes and compared with respect to edge chipping. Results: Variant Degree of edge chipping A (invention) none C (only improved coating) minor D (prior art) severe The results show the benefit of the coating according to the invention as well as combining this with the WC/Co-carbide substrate in order to achieve maximum performance. Example 5 Inserts from A), B) and D) were tested against each other in a milling operation. Operation: Face milling with SANDVIK AUTO cutter, diameter 250 mm. Workpiece: Special designed test component Material: SS0125 grey cast iron, HB= 205 Cutting speed: 129 m/min Feed rate: 0.30 mm/tooth Depth of cut: 2 mm Insert-type: TNEF1204AN-65 Note: The operation was run without coolant (dry). The inserts were all run 48 passes and compared with respect to edge chipping. Results: Variant Relation width of chipped area A (invention) 0.3 B (invention) 0.4 D (prior art) 1.0 The results show in practise equal performance of A) and B). The only difference is the coating temperature used when depositing the TiCN-layers. The prior art inserts show highest wear.

Claims: Cutting insert for milling of grey cast iron comprising a substrate and a coating c h a r a c t e r i s e d in that said substrate consists of WC, 3-15 weight-% Co and ≤2 weight-% carbides of metals from groups IVA, VA or VIA of the periodic table and in that said coating comprises the following layers in successive order: a first innermost layer of TiC x N y O z with x+y+z=1 and z<0.5 with a thickness of 0.1-2 µm, and with equiaxed grains with size ≤0.5 µm a next layer of TiC x N y O z with x+y+z=1 and x>0.3, y>0.3 and z=0 with a thickness of 2-10 µm with columnar grains with a diameter of <5 µm a next layer of TiC x N y O z with x+y+z=1 and z≤0.5 with a thickness of 0.1-2 µm and with equiaxed or needlelike grains with size ≤0.5 µm an outer layer of a smooth, textured, fine-grained α-Al 2 O 3 layer with a thickness of 2-10 µm and with a texture in the (012)-direction or (104)-direction or (110)-direction with a texture coefficient TC larger than 1.3 where TC is 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). Cutting insert according to the preceding claim characterised in an outermost coating of a thin 0.1-1 µm TiN-layer. Method of making a cutting insert comprising a substrate consisting of WC, 3-15 weight-% Co and ≤2 weight-% carbides of metals from groups IVA, VA or VIA of the periodic table and a coating wherein said WC-Co-based substrate is coated with the following layers in successive order: a first innermost layer of TiC x N y O z with x+y+z=1 and z<0.5 with a thickness of 0.1-2 µm, and with equiaxed grains with size <0.5 µm using known CVD methods a next layer of TiC x N y O z with x+y+z=1 and x>0.3, y>0.3 and z=0 with a thickness of 2-10 µm with columnar grains with a diameter of <5 µm deposited either by moderable temperature CVD-technique, using acetonitrile as the carbon and nitrogen source for forming the layer in the temperature range of 700-900 °C, or by high temperature CVD-technique in the temperature range of 1000-1100°C a next layer of TiC x N y O z with x+y+z=1 and z≤0.5 with a thickness of 0.1-2 µm and with equiaxed or needlelike grains with size ≤0.5 µm using known CVD methods an outer layer of a smooth textured α-Al 2 O 3 layer with a thickness of 2-10 µm according to known CVD-technique.

Inventors: LJUNGBERG BJOERN, OLSSON BJOERN, LJUNGBERG, BJOERN, OLSSON, BJOERN, Ljungberg, Björn, OLSSON, Björn
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 1996
Main classification: C23C30
IPC codes: C23C16, B32B9, B23C5, C23C30, B23B27, B23P15