Abstract:
There is disclosed a surface-coated tool member of tungsten carbide based cemented carbide which has a tungsten carbide based cemented carbide substrate and a hard coating formed on the substrate. The hard coating may have one or more layers each of which is made of one material selected from the group consisting of carbide, nitride and oxide of metals in groups IV A , V A  and VI A  of the Periodic Table; solid solution of said carbide, nitride and oxide; and aluminum oxide. The cobalt content of the substrate in a surface portion at a depth of about 2 μm from a surface thereof is less than that in an interior portion at a depth of about 100 μm from said surface by at least 10%.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to surface-coated tool members of tungsten carbide (WC) based cemented carbide which have hard coatings less susceptible to separation and have superior resistance to wearing and chipping when used as cutting tools for milling or finish turning operations. 
     2. Prior Art 
     There is known a surface-coated tool member, which comprises a WC-based cemented carbide substrate and a hard coating formed thereon and comprising one or more layers each composed of one of carbides, nitrides and oxides of metals in groups IVA, VA and VIA of the Periodic Table, solid solutions of these compounds and aluminum oxide. 
     For example, Japanese Patent Application Laid-Open (18-Month Publication) No. 52-110209 describes a surface-coated WC-based cemented carbide tool member in which the hardness at a portion of the substrate near the surface thereof is reduced 2% to 20% compared with that at a interior portion of the substrate by modifying cobalt (Co) content, titanium carbide (TiC) content and grain size of WC. 
     Another surface-coated tool member disclosed in Japanese Patent Application Laid-Open No. 54-87719 comprises a soft layer which is formed near the surface of the substrate by subjecting WC-based cemented carbide containing nitrogen to sintering in a vacuum. U.S. Pat. No. 4,610,931 describes a similar tool member. 
     In each of these tool members, the cobalt content at the portion near the surface of the substrate is more than that at the interior portion thereof, and hence even though the hard coating is subjected to cracking, the cracks are prevented from propagating in the substrate by the tough surface portion containing great cobalt content. Therefore, the tool members exhibit excellent performance particularly in a rough turning operation for steel or cast iron. 
     However, although the aforesaid tool members are less susceptible to chipping due to their great toughness, the bonding strength between the hard coating and the substrate is not sufficient, and hence the hard coating is susceptible to separation, resulting in abnormal wearing. Accordingly, when a cutting tool composed of the aforesaid prior art tool member is employed in milling operation wherein a great impact is exerted on the hard coating, or in finish turning wherein shear stress is exerted on the hard coating, the tool life is reduced unduly. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a surface-coated tool member of WC-based cemented carbide which has a hard coating less susceptible to separation during milling or finish turning operations, so that it has superior resistance to wearing and chipping. 
     According to the present invention, there is provided a surface-coated tool member of WC-based cemented carbide having a WC-based cemented carbide substrate and a hard coating formed on the substrate, wherein cobalt content of the substrate at a surface portion at a depth of about 2 μm from a surface thereof is less than that at an interior portion at a depth of about 100 μm from the surface by at least 10%. 
     In the foregoing, the hard coating may comprise one or more layers each composed of one material selected from the group consisting of carbides, nitrides and oxides of metals in groups IV A , V A  and VI A  of the Periodic Table; solid solutions of the above carbides, nitrides and oxides; and aluminum oxide. In addition, the average grain size of the WC contained at the surface portion of the substrate should preferably be greater than that of the WC contained at the interior portion by at least 10%. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration showing X-ray diffraction peaks indexed by index of plane (2, 1, 1) of WC at the portion near the surface of the substrate of a tool member in accordance with the present invention; and 
     FIG. 2 is an illustration similar to FIG. 1, but showing a comparative tool member. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     After an extensive study on a surface-coated tool member of WC-based cemented carbide, the inventors have come to know that when produced by grinding a usual WC-based cemented carbide with a diamond grinding wheel, heat-treating the ground cemented carbide at a temperature no less than WC-Co eutectic temperature (no less than 1,300° C.) in a vacuum or in an inert gas atmosphere, and forming a hard coating on the cemented carbide thus heat-treated, the hard coating of the resulting tool member is less susceptible to separation during milling or finish turning operations, so that the tool member has superior resistance to wearing and chipping 
     The tool member in accordance with the present invention has been developed based on the above investigation, and is produced as follows. 
     A surface of a usual WC-based cemented carbide is first ground with a diamond grinding wheel. With this procedure, a great stress is imparted to WC grains near the surface of the WC-based cemented carbide, and the WC grains are partly crushed into smaller grains. 
     The resulting cemented carbide is then heat-treated at a temperature no less than WC-Co eutectic temperature, i.e., at no less than 1,300° C., in a vacuum, in an insert gas atmosphere at the ordinary pressure, or in a pressurized inert gas atmosphere. With this procedure, the cobalt content of the substrate at a portion near its surface decreases, and the small WC grains are recrystallized into coarse grains. In addition, the portion near the surface is well crystallized so as to exhibit two diffraction peaks Kα 1  and Kα 2  indexed by index of plane (2, 1, 1) for WC in X-ray diffraction 
     In the aforesaid substrate, the cobalt content is extremely small at the surface portion of the substrate since the WC grains are recrystallized on the surface and become rich thereat. When a hard coating is formed on the surface of the substrate, inasmuch as the cobalt content at the surface portion of the substrate is less than that at the interior portion, cobalt is prevented from forming brittle η phase (W 3  Co 3  C) during coating, and from diffusing in the hard coating. Therefore, the tool member thus obtained has a very high bonding strength between the coating and the substrate. 
     On examination of the substrate after the formation of the hard coating, it has been found that the cobalt content of the substrate at a portion near its surface decreases, and the small WC grains are recrystallized into coarse grains. In addition, the portion near the surface is well crystallized so as to exhibit two diffraction peaks Kα 1  and Kα 2  indexed by index of plane (2, 1, 1) for WC in X-ray diffraction. 
     In contrast, the prior art tool member is formed by grinding a surface of WC-based cemented carbide and forming a hard coating directly on the ground surface. Hence, the cobalt content of the substrate at its surface portion is not reduced, and the WC grains at the surface portion are crushed into small ones. Therefore, cobalt forms brittle η phase easily by reacting with the crushed WC. In addition, the X-ray diffraction peaks indexed by index of plane (2, 1, 1) for WC are not separated into two peaks Kα 1  and Kα 2 . In such a prior art tool member, the bonding strength between the hard coating and the substrate is low and the tool life is short. 
     The present invention will now be illustrated by the following example: 
     EXAMPLE 1 
     There were prepared, as starting material powders, WC powder, (W, Ti)C powder (powder of solid solution consisting of 70% by weight of WC, 30% by weight of TiC), (W, Ti, Ta)C powder (powder of solid solution consisting of 50% by weight of WC, 30% by weight of TiC and 20% by weight of TaC), (W, Ti)(C, N) powder (powder of solid solution consisting of 55% by weight of WC, 25% by weight of TiC and 20% by weight of TiN), TaC powder and cobalt powder, each of which had an average particle size of 1 to 5 μm. 
     These powders were blended into the compositions set forth in Table 1, and were subjected to wet mixing in a ball mill for 72 hours and dried. Then, the mixed powders were pressed under a pressure of 1 ton/cm 2  into green compacts. The green compacts were sintered under the conditions set forth in Table 1 into WC-based cemented carbides having the same compositions as the blended compositions. Then, the WC-based cemented carbides were formed into a shape of a cutting insert in conformity with SNGN 120412 of ISO standards wit or without grinding them under the conditions set forth in Table 1. Subsequently, WC-based cemented carbide substrates A to R set forth in Table 1 were produced with or without heat-treating the aforesaid cemented carbides under the conditions set forth in Table 1, In the foregoing, the substrates A to M are obtained by carrying out heat-treatment after the grinding of the surface, while the substrates O and Q are obtained only by subjecting the cemented carbides to the surface grinding. Furthermore, the substrates N, P and R are obtained by subjecting the cemented carbides neither to the grinding nor to the heat-treatment. 
     Thereafter, hard coating layers having compositions and average thicknesses set forth in Tables 2-1 to 2-4 were formed on the substrates A to R by chemical vapor deposition method, to produce WC-based cemented carbide cutting inserts 1 to 35 of the invention and comparative WC-based cemented carbide cutting inserts 1 to 11 The cutting inserts 1 to 35 of the invention are obtained by forming hard coating layers on the substrates A to M, while the comparative cutting inserts 1 to 11 are formed by forming the hard coatings on the substrates N to R. 
     The conditions for the chemical vapor deposition method were as follows: 
     (1) TiC hard coating layer: 
     Temperature: 1,030° C. 
     Pressure: 100 Torr 
     Composition of reaction gas: 4% by volume of TiCl 4  -5% by volume of CH 4  -91% by volume of H 2   
     (2) TiN hard coating layer: 
     Temperature: 980° C. 
     Pressure: 100 Torr 
     Composition of reaction gas: 4% by volume of TiCl 4  -8% by volume of N 2  -88% by volume of H 2   
     (3) TiCN hard coating layer: 
     Temperature: 1,000° C. 
     Pressure: 100 Torr 
     Composition of reaction gas: 4% by volume of TiCl 4  -3% by volume of CH 4  -4% by volume of N 2  -89% by volume of H 2   
     (4) Al 2  O 3  hard coating layer: 
     Temperature: 1,000° C. 
     Pressure: 100 Torr 
     Composition of reaction gas: 3% by volume of AlCl 3  -5% by volume of CO 2  -92% by volume of H 2   
     For the cutting inserts 1 to 35 of the invention and the comparative cutting inserts 1 to 11, the cobalt content of a portion at a depth of 2 um from the surface of the substrate and that of an interior portion at a depth of 100 um from the surface were measured by means of EDX. The results are set forth in Tables 2-1 t 2-4. 
     Furthermore, the diffraction peaks of index of plane (2, 1, 1) for tungsten carbide were also investigated by X-ray diffraction analysis. The conditions for the analysis were as follows: 
     Target-filter: Cu-Ni 
     Voltage: 40 kV 
     Current: 40 mA 
     Recording speed: 40 mm/2θ(degree) 
     As will be seen from Tables 2-1 to 2-4, the separated to be Kα 1  and Kα 2 . 
     FIGS. 1 and 2 illustrates the diffraction patterns for both the tool member of the invention and the comparative tool member. 
     As will be seen from Table 1 and Tables 2-1 to 2-4, the tool member 25 of the invention and the comparative tool member 8 are similar to each other in that they are both produced by grinding the surface of WC-based cemented carbide containing 9% by weight of cobalt, 2% by weight of TaC and balance WC by diamond grinding wheel, and forming a hard coating composed of TiC (4 μm) and TiN (1 μm), while they differ from each other in whether the heat-treatment is conducted or not. In the tool member 25 of the invention, the diffraction peaks for index of plane (2, 1, 1) for WC are separated from each other as illustrated in FIG. 1, but in the comparative tool member 8, the strongest diffraction peaks of the first hard coating layer of TiC was strongly oriented at the index of plane (1, 1, 1). 
     The cutting inserts 1 to 35 of the invention and the comparative cutting inserts 1 to 11 were then subjected to a milling test under the following conditions: 
     (A) Milling test 
     Workpiece: Steel JIS.SNCM439 (AISI4340)(hardness HB 270) 
     Cutting speed: 180 m/min 
     Feed rate: 0.3 mm/tooth 
     Depth of cut: 3.0 mm 
     Coolant: none 
     Cutting time: 40 min 
     Then, the cutting inserts were examined for flank wear width. The results are set forth in Tables 2-1 to 2-4. In addition, the damaged state of the cutting inserts were also observed. 
     Moreover, the cutting inserts 1 to 35 of the invention and the comparative cutting inserts 1 to 11 were subjected to a finish turning test under the following conditions: 
     (B) Finish turning test 
     Workpiece: Steel JIS.SNCM439 (AISI4340) (hardness HB 220) 
     Cutting speed: 180 m/min 
     Feed rate: 0.2 mm/revolution 
     Depth of cut: 0.5 mm 
     Coolant: water-soluble 
     Cutting time: 40 min 
     Then, the cutting inserts were examined for width of flank wear and depth of rake surface wear. The results are set forth in Tables 2-1 to 2-4. 
     As will be seen from Tables 2-1 to 2-4, the cutting inserts 1 to 35 of the invention are less susceptible to separation as compared with any of the comparative cutting inserts 1 to 11, and have superior resistance to wearing and chipping. 
     
                                           TABLE 1__________________________________________________________________________                                Sintering Conditions   Blended Composition of Material Power (weight %)                                Temperature                                       Time                                          Atmosphere   Co     TaC        (W, Ti) C              (W, Ti, Ta) C                     (W, Ti) (C, N)                             WC (°C.)                                       (hr)                                          (Torr)__________________________________________________________________________WC -  A 6 -- --    --     --      other                                1450   1  0.05 VacuumBased B 6 1  --    --     --      other                                1450   1  0.05 VacuumCemented C 6 3  3     --     --      other                                1450   1  0.05 VacuumCarbide D 7 1  --    --     --      other                                1420   1  0.05 VacuumSubstrate E 7 -- --    5      --      other                                1420   1  0.05 Vacuum F 7 3  4     --     --      other                                1420   1  0.05 Vacuum G 8 2  --    --     --      other                                1420   1  0.05 Vacuum H 8 -- --    --     --      other                                1420   1  0.05 Vacuum I 9 2  --    --     --      other                                1400   1  0.05 Vacuum J 9 5  8     --     --      other                                1400   1  0.05 Vacuum K 10     -- --    10     --      other                                1400   1  0.05 Vacuum L 10     5  10    --     --      other                                1400   1  0.05 Vacuum M 11     5  --    --     10      other                                1400   1  0.05 Vacuum N 6 1  --    --     --      other                                1450   1  0.05 Vacuum O 6 1  --    --     --      other                                1450   1  0.05 Vacuum P 9 2  --    --     --      other                                1450   1  0.05 Vacuum Q 9 2  --    --     --      other                                1450   1  0.05 Vacuum R 6 3  --    --     3       other                                1450   1  0.05 Vacuum__________________________________________________________________________                        Grinding                              Heat-treating Conditions                        Method of                              Temperature                                     Time                        Surface                              (°C.)                                     (hr)                                        Atmosphere__________________________________________________________________________                WC -  A Diamond                              1420   1  0.01 Torr Vacuum                Based   Grinding                Cemented                      B Diamond                              1420   1  0.01 Torr Vacuum                Carbide Grinding                Substrate                      C Diamond                              1420   1  0.01 Torr Vacuum                        Grinding                      D Diamond                              1400   1  0.01 Torr Vacuum                        Grinding                      E Diamond                              1400   1  0.01 Torr Vacuum                        Grinding                      F Diamond                              1400   1  0.01 Torr Vacuum                        Grinding                      G Diamond                              1400   1  0.01 Torr Vacuum                        Grinding                      H Diamond                              1400   1  0.01 Torr Vacuum                        Grinding                      I Diamond                              1380   1  100 atm Ar                        Grinding                      J Diamond                              1380   1  100 atm Ar                        Grinding                      K Diamond                              1350   1  100 atm Ar                        Grinding                      L Diamond                              1350   1  100 atm Ar                        Grinding                      M Diamond                              1300   1  1 Torr N.sub.2 gas                        Grinding                      N --    --     -- --                      O Diamond                              --     -- --                        Grinding                      P --    --     -- --                      Q Diamond                              --     -- --                        Grinding                      R --    --     -- --__________________________________________________________________________ 
    
     
         TABLE 2  Substrate after Formation Diffraction Cutting Tests of Hard Coating Peaks for Finish WC Average (2, 1, 1) Milling Turning Co Content (wt %) Grain size (μm) Plane for Flank  Flank Crater  Composition of Hard Coating*   Reduction   Percentage WC in the Wear Damaged Wear Wear  and Average Thickness** of Surface Interior in Co Surface Interior of Coarse Surface Width State of Width Depth Substrate Each Layer (μm) Portion Portion (%) Portion Portion WC Portion (mm) Cutting (mm) (μm)   Cutting 1 A TiC(3) 3.9 6.1 36 6.0 4.9 22 Separated 0.24 Fine Chipping -- -- Inserts 2 A TiCN(3) 3.8 6.1 38 6.0 4.9 22 Separated 0.22 Fine Chipping -- -- of the 3 A TiN(3) 5.1 6.1 16 6.0 4.9 22 Separated 0.26 Fine Chipping -- -- Invention 4 B TiC(2)--TiN(1) 4.0 6.1 34 5.6 4.8 17 Separated 0.23 Fine Chipping 0.24 20  5 B TiCN(2)--TiN(1) 3.9 6.1 36 5.5 4.8 15 Separated 0.22 Fine Chipping 0.25 15  6 B TiN(2)--TiCN(1) 5.0 6.1 18 5.4 4.8 13 Separated 0.26 Fine Chipping 0.28 15  7 C TiC(2)--TiN(1) 5.1 6.0 15 5.6 4.6 22 Separated 0.27 Fine Chipping -- --  8 D TiC(3)--TiN (1) 4.3 7.1 39 4.4 3.9 13 Separated 0.20 Normal Wear -- --  9 D TiCN(3)-- TiC(1) 4.3 7.1 39 4.4 3.9 13 Separated 0.20 Normal Wear -- --  10 D TiN(0.5)--TiCN(3)--TiN(0.5) 4.3 7.1 39 4.4 3.9 13 Separated 0.19 Normal Wear -- --  11 E TiC(3)--TiN(1) 4.7 7.3 36 4.1 3.7 11 Separated 0.25 Fine Chipping -- --  12 F TiC(3)--TiN(1) 5.5 7.4 26 4.5 3.7 22 Separated 0.24 Fine Chipping -- --  13 F TiCN(0.5)--TiC(3)--TiCN(0.5) 5.5 7.4 26 4.5 3.7 22 Separated 0.22 Normal Wear -- --  14 F TiN(1)--TiCN(3)--TiN(1)  5.4 7.4 27 4.4 3.7 19 Separated 0.21 Normal Wear --  --  15 G TiC(3)--Ti N(1) 4.8 7.4 35 3.8 3.4 12 Separated 0.19 Normal Wear -- --  16 G TiCN(3)--TiN(1) 4.8 7.4 35 3.8 3.4 12 Separated 0.20 Normal Wear -- -- 17 G TiCN(0.5)--TiCN(3)--TiN(0.5) 4.7 7.4 36 3.8 3.4 12 Separated 0.18 Normal Wear -- --  18 G TiC(2)--TiN(1)--TiC(1)--TiN(1) 4.9 8.0 39 3.8 3.4 12 Separated 0.18 Normal Wear -- --  19 G TiC(2)--TiCN(2)--TiN(1) 4.9 8.1 40 3.8 3.4 12 Separated 0.18 Normal Wear -- --  20 G TiC(3)--TiCN (1)--Al.sub.2 O.sub.3 (1) 5.0 8.3 40 3.8 3.4 12 Separated 0.26 Fine Chipping -- --  21 G TiC(3)--TiCN(1)--Al.sub.2 O.sub.3 (0.5)--TiN(0.5) 5.1 8.4 39 3.8 3.4 12 Separated 0.25 Fine Chipping -- --  22 H TiC(4) 5.2 8.2 37 4.0 3.4 18 Separated 0.24 Fine Chipping -- --  23 H TiCN(4) 5.1 8.2 39 3.9 3.4 15 Separated 0.23 Fine Chipping -- --  24 H TiN(5) 5.1 8.2 39 3.8 3.4 12 Separated 0.27 Fine Chipping -- --  25 I TiC(4)--Ti N(1) 5.7 9.2 38 3.5 3.0 17 Separated0.19 Normal Wear -- --  26 I TiCN(1)--TiC(3)--TiCN(1) 5.6 9.0 38 3.5 3.0 17 Separated0.19 Normal Wear -- --  27 I TiN(0.5)--TiCN(4)--TiN(0.5) 5.6 9.0 38 3.4 3.0 13 Separated0. 18 Normal Wear -- --  28 I TiC(3)--TiCN(1)--Al.sub.2 O.sub.3  (0.5)--TiN(0.5) 6.0 9.3 35 3.5 3.0 17 Separated0.24 Fine Chipping -- --  29 J TiC(2)--TiN(2) 6.2 9.0 31 2.9 2.7 7 Separated0.22 Fine Chipping -- --  30 K TiC(5) 6.7 10.1 34 2.6 2.2 18 Separated0.25 Fine Chipping -- --  31 K TiCN(6) 6.6 10.1 35 2.5 2.2 14 Separated0.27 Fine Chipping -- -- 32 K TiN(7) 6.5 10.1 35 2.5 2.2 14 Separated0.29 Fine Chipping -- --  33 K TiC(3)--TiCN(2)--TiN(1) 6.8 10.3 34 2.6 2.2 18 Separated0.27 Normal Wear -- --  34 L TiC(4)--TiN(1) 6.9 10.3 33 2.7 2.2 23 Separated0.28 Normal Wear -- --  35 M TiC(4)--TiCN(2)--TiN(1) 6.9 11.1 38 2.3 1.8 28 Separated0.29 Fine Chipping -- -- Com- 1 N TiC(2)--TiN(1) 5.7 6.1 7 5.0 4.8 4 Slightly -- Breakage 0.45 50 parative          Separated Cutting 2 N TiCN(2)--TiN(1) 5.7 6.1 7 4.9 4.8 2 Slightly -- Breakage 0.47 50 Inserts          Separated  3 N TiN(2)--TiCN(1) 5.3 6.1 5 4.9 4.8 2 Slightly -- Breakage 0.50 50           Separated  4 O TiC(2)--TiN(1) 6.1 6.1 0 4.8 4.8 0 Not 0.62 Chipping -- --           Separated  5 O TiCN(2)--TiN(1) 6.1 6.1 0 4.8 4.8 0 Not 0.61 Chipping -- -- Separated  6 O TiN(2)--TiCN(1) 6.1 6.1 0 4.8 4.8 0 Not 0.69 Chipping -- --           Separated  7 P TiC(2)--TiCN(1)--TiN(1) 8.5 9.0 6 3.2 3.0 7 Slightly 0.49 Chipping -- --           Separated  8 Q TiC(4)--TiN(1) 9.2 9.2 0 3.0 3.0 0 Not 0.45 Chipping -- --           Separated  9 R TiC(2)--TiN(1) 9.3 5.9 -58 5.6 4.6 22 Slightly 0.63 Abnormal 0.56 70  Separated  Wear  10 R TiC(2)--TiCN(1)--TiN(1) 9.3 5.9 -58 5.6 4.6 22 Slightly 0.62 Abnormal 0.56 70           Separated  Wear  11 R TiC(2)--TiCN(1)--Al.sub.2 O.sub.3 (1) 9.3 5.9 -58 5.6 4.6 22 Slightly 0.60 Abnormal 0.54 60           Separated  Wear *In the case of multiple layers, 1st layer is shown on the left **Thickness is shown in parenthesis