Coated cutting tool

Abstract: The present invention relates to CVD-coated cemented carbide tools used for chip forming machining. More particularly, it relates to a coating consisting of an initial layer of MetC x N 1-x adjacent to the substrate, followed by an intermediate layer of MetC x O y N z , which is further followed by a wear resistant layer of MetC x N 1-x , where the metal Met is selected from Ti,Zr,Hf,V,Nb and Ta. The intermediate layer acts as a diffusion barrier thus influencing the coating structure and coating adhesion of the following layer.

Description: The present invention relates to sintered cemented carbide bodies coated with thin, extremely wear resistant surface layers. More particularly, it relates to coated cemented carbides bodies in which a thin oxygen containing layer deposited between layers of carbides, nitrides and/or carbonitrides improves the properties of the coated body. Coated cutting tool inserts are widely used in the metal machining industry to machine iron and iron based alloys. Increasing the tool life of such tools by carbide, nitride or carbonitride and oxide coatings is a well established technology. Cutting inserts are usually coated with layers consisting of carbide, nitride and carbonitride of titanium or a related metal. The coating in many cases also includes one or several layers of alumina, a coating material which cannot conveniently be deposited directly onto a cemented carbide substrate by the conventionally used Chemical Vapour Deposition Processes (CVD). Henceforth, it is a common practise to deposit a layer of carbide, carbonitride and/or nitride of e.g. titanium prior to the deposition of Al 2 O 3 . When a cemented carbide substrate is coated with a wear resistant coating of carbide, carbonitride or nitride by a CVD process at a temperature of 800-1200°C, the constituents of the substrate e.g. tungsten, cobalt, carbon will thermally diffuse into the growing coating and influence the crystal growth direction, microstructure, coating cohesion etc. Consequently, the physical, chemical and mechanical properties of the coating will be affected by the presence of diffused species from the substrate. U.S.patent No. 5,135,801 discloses a coated cemented carbide body with an innermost (closest to the substrate) oxygen-containing layer. The object of that invention was to suppress the carbon diffusion into the coating in order to minimise the decarburization of the substrate surface by applying said oxygen-containing layer, e.g. TiCO, directly onto the cemented carbide substrate. JP-A- 61 223 181 discloses a coated cemented body with a coating comprising: (i) a first layer of TiC being 0.2-3 µm thick, (ii) an intermediate layer of TiCO or TiCNO being 0.2-3 µm and (iii) a third wear resistant layer of TiC being 1-10 µm. The coating further comprises an outermost layer being 0.5-10 µm, which can be of alumina. The present invention, describes a method that suppresses the diffusion of species from the substrate into the coating by applying a thin TiCO layer as an intermediate layer between a first TiC x N 1-x (0≤x≤1)layer applied directly onto the substrate and a further TiC x N 1-x (0≤x≤1) applied on top of the TiCO layer. The intermediate layer may also consist of successive layers. Furthermore, on top of the above mentioned combination of carbonitride, carbooxide and carbonitride layers, further layers may be deposited of for example Al 2 O 3 or again metal carbide and/or nitride layers. It has surprisingly been found that said TiCO or also TiCON intermediate layer effectively suppresses particularly the Co and W-diffusion from the substrate into the coating and thus strongly influences the microstructure and chemical properties of said coating, resulting in much enhanced wear properties of e.g. a coated cutting tool. Furthermore, the suppression of Co diffusion also improves the adhesion between adjacent layers in the sandwiched coating combination by reducing the detrimental effect of Co enrichment at layer interfaces. Fig 1 shows in 1600 X magnification a polished and etched section of a coating according to the invention in which A - cemented carbide B - first layer (TiCN) C - second layer (TiCO) D - third layer (TiCN) E - fourth layer (Al 2 O 3 ) Fig 2 shows a fracture surface in 4000X magnification of the same coating and with the same designations as in figure 1. According to the present invention there is now provided a cemented carbide body with a coating comprising three consecutive layers. Met refers to Ti, Zr, Hf, V, Nb or Ta. 1. A first MetC x N 1-x (0≤x≤1)-layer applied directly onto the cemented carbide body. It is understood that this first layer can consist of several layers MetC x N 1-x (0≤x≤1). Preferably this first layer consists of TiC. The thickness of this first layer is greater than 1 µm and up to 5 µm, preferably 1.5 - 3 µm. The minimum thickness of this first layer can be reduced to 0.5 µm if the surface binder phase present on the cemented carbide body is removed prior to the deposition. The binder phase can be removed by blasting, however, electrolytic methods are preferred e.g. according to WO 92/20841. 2. A second layer MetC x O y N z applied directly on the first TiC x N 1-x (0≤x≤1)-layer. This layer is 0.1 - 5 µm, preferably 0.2 - 1 µm thick. The composition of MetC x O y N z is 0.7≥ x+y+z ≥ 1.3 and 0.2<y<0.8, preferably 0.4<y<0.6. Preferably, Met=Ti and z=0. 3. A third layer MetC x N 1-x (0≤x≤1) applied directly on the MetC x O y N z layer. The thickness of this third wear resistant layer is 1-20 µm, preferably 5 - 12 µm. This third layer consists of TiC x N 1-x , where 0.3≤ x ≤ 0.7. Compared to prior art TiCN-layer, this TiCN-layer exhibits improved wear properties when e.g. applied on a cutting tool, which probably is due to a favourable crystal orientation of this third layer. The TiCN-layer according to the invention has a preferred crystallographic growth orientation in the (220)-direction (determined by X-ray Diffraction (XRD) measurements). 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: (111), (200), (220). According to the invention, TC for the set of (220) crystal planes is greater than 1.3, preferably greater than 1.5, most preferably greater than 1.8. On top of the above mentioned combination of carbonitride, carboxide and carbonitride layers, further layers are deposited including Al 2 O 3 . Preferably, the layers comprise an outermost layer of fine grained α-alumina according to EP-A-603 144 possibly followed by a thin TiN-layer deposited after the alumina layer has been smoothened by e.g. using a blasting method. The coating according to the present invention can be applied to cemented carbide bodies based on straight WC-Co compositions as well as WC-cubic carbides-Co-compositions where the cubic phase comprises carbides or carbonitrides of Ti, Ta and/or Nb. It can also be applied to so called gradient sintered cemented carbides which have a surface zone enriched in binder phase. e.g. according to EP-A-603 144 and WO 93/17140. The invention also relates to a method of making a body comprising a substrate of cemented carbide with a coating comprising the following 1/ a first layer or set of layers MetC x N 1-x (0≤x≤1) adjacent to the substrate where the metal Met is selected from the group of metals Ti, Zr, Hf, V, Ta, Nb and in which the substrate is contacted with a gas containing hydrogen, nitrogen and hydrocarbons and one or more halides of the above mentioned metals, forming a carbide, nitride or carbonitride at a process pressure 5 - 1050 mbar, preferably 30 - 100 mbar 2/ a second layer, directly on the first layer, of MetC x O y N z deposited in a gas containing hydrogen, nitrogen, carbon monoxide and one or more halides of the above mentioned metals forming an oxycarbide or oxynitrocarbide at a process pressure 5 - 1050 mbar, preferably 30 - 100 mbar and 3/a third layer directly on the second layer, of MetC x N 1-x (0≤x≤1) deposited in a gas containing hydrogen, nitrogen, hydrocarbon and one or more halides of the above mentioned metals forming a carbonitride at a process pressure 5 - 1050 mbar, preferably 200 - 600 mbar. Preferably an outer alumina layer is deposited by vapour deposition in a gas containing hydrogen and carbon dioxide and aluminium halides. Preferably this alumina layer is an α-alumina layer deposited by vapour deposition in a gas containing hydrogen and carbon dioxide and aluminium halides. Most preferably an outermost TiN layer is deposited on the top of the alumina layer by vapour deposition in a gas containing hydrogen, nitrogen and titanium halides. The process temperature during the CVD deposition steps, may be in the range of 800-1200°C, preferably 900-1070°C. The different process steps may be carried out in the same coating equipment or in separate equipments. Depending on the selection of the oxygen containing gas precursor during the CVD process step of the second layer MetC x O y N z , inclusion of other atoms than nitrogen and carbon may occur in said layer e.g. sulphur, tellurium, selenium, phosphorous and/or arsenic. The first and second layers may alternatively be deposited by Physical Vapour Deposition (PVD) such as reactive magnetron sputtering, ion plating or cathodic arc discharge deposition. Example 1 Cemented carbide cutting inserts CNMG 120408 with the composition 5.5 % Co, 8.6 % cubic carbides (TiC-TaC-NbC) and balance WC were coated with TiC for a period of 1 hr prior to the deposition of the intermediate layer TiCO (process gas composition: 4 % TiCl 4 , 7 % CO, 3 % HCl, balance H 2 , pressure 50 mbar and substrate temperature 1000°C). Following the deposition of the TiCO-layer, a wear resistant TiCN layer was deposited for a period of 4 hr at a pressure of 400 mbar followed by a further deposition of a fine grained α-Al 2 O 3 layer according to EP-A-603 144 deposited for a period of 6 hr. (Variant A). Cemented carbide cutting inserts as used for Variant A were subjected to an additional electrolytic surface cleaning operation prior to the coating process according to above. (Variant B) Cemented carbide cutting inserts as used for Variant A and B were also processed with an identical CVD process as above but excluding the second layer TiCO (Variant C). Variants A, B and C were all wet blasted with 150 mesh alumina grit in order to smoothen the coating surface and thereafter coated with a thin TiN coating. The blasting treatment in combination with the outermost TiN coating gives the inserts a bright and shiny appearance that facilitates the identification of used edges. The coated inserts were examined with respect to presence and thickness of eta-phase layer in µm, coating thickness, µm, texture coefficient (TC) for the (220) growth direction of the wear resistant TiCN-layer and mean grain size of the crystals in the Al 2 O 3 layer in µm with the following results: A B C (prior art) Etaphase traces none 0.5 TiC 1.5 0.7 1.6 TiCO 0.4 0.4 -- TiCN 7.0 7.0 7.2 Al 2 O 3 5.5 5.3 5.7 TiN 0.1 0.1 0.1 Texture Coefficient TiCN 2.0 1.9 1.2 Grain size Al 2 O 3 -layer 1.9 1.7 1.7 Example 2 (not according to the claimed invention) Cemented carbide cutting inserts CNMG 120408 with the composition 5.5 % Co, 8.6 % cubic carbides (TiC-TaC-NbC) and balance WC were coated with TiC for a period of 1.5 hr prior to the deposition of the intermediate oxygen, nitrogen-containing layer TiCON (process gas composition: 4 % TiCl 4 , 7 % CO, 3 % HCl, 25 % N 2 , balance H 2 pressure 50 mbar and substrate temperature 1000°C). Following the deposition of the TiCON layer, a wear resistant TiCN layer was deposited for a period of 4 hr at a pressure of 600 mbar followed by a TiN layer deposited for a period of 2 hr.(Variant D) Cemented carbide cutting inserts as used for Variant D were also processed with identical CVD processes but excluding the intermediate layer of TiCON.(Variant E) The coated inserts were examined with respect to presence and thickness of eta-phase layer in µm, coating thickness, µm, and texture coefficient (TC) for the (220) growth direction of the wear resistant TiCN-layer with the following results: D E (prior art) Etaphase 1.1 1.7 TiC 1.9 2.1 TiCON 0.5 -- TiCN 8.2 8.5 TiN 1.6 1.4 Texture Coefficient TiCN 1.8 1.1 Example 3 Cemented carbide cutting inserts CNMG 120408 with the composition 6.5 % Co, 8.7 % cubic carbides (TiC-TaC-NbC), balance WC and with a 25 µm thick binder phase enriched surface zone were coated with TiC for a period of 0.5 hr followed by the deposition of TiCN for a period of 0.25 hr prior to the deposition of the intermediate TiCO layer (process gas composition: 3 % TiCl 4 , 4 % CO, 4 % HCl, balance H 2 pressure 100 mbar and substrate temperature 1000°C). Following the deposition of the intermediate layer, a wear resistant TiCN layer was deposited for a period of 4 hr at a pressure of 200 mbar, followed by the deposition of a fine grained α-Al 2 O 3 layer (according to EP-A-603 144) deposited for a period of 1 hr and a top TiN layer deposited for a period of lh. (Variant F) Cemented carbide cutting inserts as used for Variant F were subjected to an additional electrolytic surface cleaning operation prior to the coating process according to above (Variant G). Cemented carbide cutting inserts as used for Variant F and G were also processed with an identical CVD process but excluding the intermediate TiCO layer. (Variant G) The coated inserts were examined with respect to presence and thickness of eta-phase layer in µm, coating thickness and texture coefficient (TC) for the (220) growth direction of the wear resistant TiCN-layer: F G H (prior art) Etaphase 0.7 traces 1.0 TiC 1.2 0.8 1.4 TiCN 0.3 0.2 0.4 TiCO 0.4 0.4 -- TiCN 3.6 3.7 3.9 Al 2 O 3 2.1 1.9 2.2 TiN 0.2 0.2 0.2 Texture Coefficient TiCN 1.7 1.6 1.1 Example 4 The coated cutting inserts Variant A to H were tested with respect to flaking of the coating when caused by edge deformation, in longitudinal turning of an alloyed steel (AISI 4340) at a Feed of 0.7 mm/rev, Cutting Depth of 2.0 mm and varying the Cutting Speed in order to provoke different degrees of plastic deformation of the cutting edge. The edges were then examined with respect to plastic deformation measured as an edge depression and the concomitant flaking of the coating was estimated with the following results. Inserts A, B, F, G according to the invention and insert D did not exhibit any flaking at all at an edge depression of at least 80 µm. Inserts C showed excessive loss of the coating on the clearance face at an edge depression already of 40 µm. Inserts E showed excessive loss of the coating on the clearance face at an edge depression already of 35 µm. Inserts H showed excessive loss of the coating on the clearance face at an edge depression already of 45 µm. Example 5 The coated cutting inserts from Example 1 were then tested with respect to rake face flaking in a facing operation in grey cast iron (AISI No35B).The shape of the machined work piece was such that the cut was interrupted twice during each revolution. Cutting data: Speed = 500 m/min Feed = 0.5 mm/rev. Cutting depth = 3.0 mm The inserts were then examined with respect to flaking of the alumina coating on the rake face of the cutting insert. The results are expressed as rake face area subjected to flaking in relation to total rake face area in contact with the work piece chip. Results Flaking Inserts A (present invention) 0 % Inserts B (present invention) 0 % Inserts C (prior art) 19 % Example 6 The coated cutting inserts from Example 3 were then tested with respect to rake face flaking in a facing operation in grey cast iron (AISI No35B).The shape of the machined work piece was such that the cut was interrupted twice during each revolution. Cutting data: Speed = 400 m/min Feed = 0.3 mm/rev. Cutting depth = 3.0 mm The inserts were then examined with respect to flaking of the alumina coating on the rake face of the cutting insert. The results are expressed as rake face area subjected to flaking in relation to total rake face area in contact with the work piece chip. Results Flaking Inserts F (present invention) 0 % Inserts G (present invention) 0 % Inserts H (prior art) 26 %

Claims: Coated cemented carbide cutting tool comprising a substrate and a coating in which said coating comprises a first layer or a set of layers MetC x N 1-x (0≤x≤1) adjacent to the substrate followed by a second layer consisting of MetC x O y N z , where 0.7≥ x+y+z ≥ 1.3 and 0.2<y<0.8, preferably 0.4<y<0.6, which is further followed by a third wear resistant layer of MetC x N 1-x (0≤x≤1) where the metal Met is selected from the group of metals Ti, Zr, Hf, V, Nb and Ta, the second layer being 0.1 - 5 µm thick, preferably 0.2 - 1.0 µm thick and the thickness of the first layer or set of layers MetC x N 1-x where(0≤x≤1) adjacent to the substrate being greater than 1µm and up to 5µm or alternatively 0.5-5 µm when the substrate is essentially free from surface cobalt and the thickness of the wear resistant third layer MetC x N 1-x (0≤x≤1) being 1-20 µm, characterized in, that said wear resistant third layer consists of TiC x N 1-x where 0.3≤ x ≤ 0.7, said layer having a preferred crystallographic growth orientation in the (220)-direction with a texture coefficient TC greater than 1.3, 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: (111), (200), (220) said coating comprising an outer alumina layer consisting of a single phase α-alumina having a grain size s of 0.5 µm < s < 3 µm. Coated tool according to claim 1 characterized in, that said coating comprises a TiN outermost layer being 0.01 - 1 µm thick.

Inventors: AKESSON LEIF, LENANDER ANDERS, AKESSON, LEIF, LENANDER, ANDERS
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 1995
Main classification: C23C16
IPC codes: C23C16, B23B27, C23C30, B32B9, B23P15