Coated cutting insert for rough turning

[0001] This application is based on and claims priority under 35 U.S.C. §119 to Swedish Application No. 0302842-0, filed Oct. 27, 2003, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a coated cemented carbide cutting tool insert particularly useful for toughness demanding machining, such as medium and rough turning of steels and also for turning of stainless steels. The disclosure preferably relates to coated inserts in which the substrate has been provided with a tough surface region in such a way that wear resistance and edge strength are obtained in the same grade.

[0003] In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicants expressly reserve the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.

[0004] Today, coated cemented carbide inserts with binder phase enriched surface zone are commonly used for turning of steel and stainless steel materials. The binder phase enriched surface zone widens the application towards tougher cutting operations.

[0005] It has been known for some time how to produce binder phase enriched surface zones on cemented carbides containing WC, binder phase and cubic carbide phase, e.g., Tobioka et al. (U.S. Pat. No. 4,277,283), Nemeth et al. (U.S. Pat. No. 4,610,931) and Yohe (U.S. Pat. No. 4,548,786).

[0006] EP-A-1 026 271 relates to a coated cemented carbide with binder phase highly alloyed with W. The insert has a binder phase enriched surface zone of a thickness <20 μm and, along a line in the direction from the edge to the centre of the insert, the binder phase content increases essentially monotonously until it reaches the bulk composition. The insert is coated with 3–12 μm columnar Ti(C,N)-layer followed by a 2–12 μm thick Al₂O₃-layer.

[0007] EP-A-1 348 779 relates to a coated cemented carbide insert with a binder phase enriched surface zone with a thickness of >20 μm and a Co-content of 4–7 wt-%. The insert is coated with a 3–15 μm thick Ti(C,N) layer followed by 3–15 μm α-Al₂O₃ and an uppermost 1–10 μm thick layer of a carbide, carbonitride or carboxynitride.

[0008] Swedish Patent Application 0201417-3 discloses a method to produce α-Al₂O₃ coatings with high wear resistance and toughness. The α-Al₂O₃ coating is formed on a bonding layer of (Ti,Al)(C,O,N) with increasing aluminum content towards the outer surface. The α-Al₂O₃ coating has a thickness ranging from 1 to 20 μm and is composed of columnar grains. The length/width ratio of the alumina grains is from 2 to 12. The coating is characterized by a strong (012) growth texture, measured using XRD, and by the almost total absence of (104), (110), (113) and (116) diffraction reflections.

[0009] The coatings for cutting tools designed for toughness demanding applications are usually composed of layers of Ti(C,N) and kappa alumina. It has been thought that the kappa phase shows better toughness properties than the alpha phase. Consequently, alpha alumina has so far only been applied for applications where wear resistance is the main concern, i.e., for grades with relatively low Co-contents.

[0010] It has surprisingly been found that a relatively thick coating including alpha alumina (α-Al₂O₃) with strong texture in combination with a substrate of relatively high cobalt content shows enhanced edge strength and toughness in medium and rough turning of steels and turning of stainless steels.

[0011] An exemplary embodiment of a cutting tool insert comprises a cemented carbide substrate and a coating. The cemented carbide substrate comprises WC, 7 to 12 wt-% Co and 5 to 11 wt-% of cubic carbides of metals from groups IVb, Vb and VIb of the periodic table, a Co-binder that is highly alloyed with W and has an S-value of 0.79 to 0.89 and a tungsten carbide phase having a mean intercept length of 0.7 to 1.4 μm. The coating comprises at least one 2 to 12 μm alumina layer including columnar α-Al₂O₃ grains and a texture coefficient TC(hkl) of (hkl) reflections (012), (104), (110), (113), (024), (116), wherein TC(012)>2.2 and TC(024)>0.6×TC(012), and wherein texture coefficient TC(hkl) is defined as

[0012] $\mathrm{TC}\left(\mathrm{hkl}\right)={\frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\left[\frac{1}{n}\sum \frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\right]}^{-1}$
where I(hkl)=measured intensity of (hkl) reflection, I₀(hkl)=standard intensity according to JCPDS card no 46-1212, and n=number of reflections used in calculation.

[0013] Another exemplary embodiment of a cutting tool insert comprises a cemented carbide substrate and a coating. The cemented carbide substrate consists essentially of WC, 7 to 12 wt-% Co and 5 to 11 wt-% of cubic carbides of metals from groups IVb, Vb and VIb of the periodic table, a Co-binder that is highly alloyed with W and has an S-value of 0.79 to 0.89, a tungsten carbide phase having a mean intercept length of 0.7 to 1.4 μm. The coating consists essentially of at least one 2 to 12 μm alumina layer including columnar α-Al₂O₃ grains and a texture coefficient TC(hkl) of (hkl) reflections (012), (104), (110), (113), (024), (116), wherein TC(012)>2.2 and TC(024)>0.6×TC(012) and wherein texture coefficient TC(hkl) is defined as

[0014] $\mathrm{TC}\left(\mathrm{hkl}\right)={\frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\left[\frac{1}{n}\sum \frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\right]}^{-1}$
where I(hkl)=measured intensity of (hkl) reflection, I₀(hkl)=standard intensity according to JCPDS card no 46-1212, and n=number of reflections used in calculation.

[0015] A further exemplary embodiment of a cutting tool insert comprises a cemented carbide substrate and a coating. The cemented carbide substrate comprises WC, 7 to 12 wt-% Co and 5 to 11 wt-% of cubic carbides of metals from groups IVb, Vb and VIb of the periodic table, a Co-binder that is highly alloyed with W and has an S-value of 0.79 to 0.89, a tungsten carbide phase having a mean intercept length of 0.7 to 1.4 μm and wherein the cemented carbide substrate includes a binder phase enriched and an essentially cubic carbide free surface zone of a thickness of 10 to 40 μm. The coating comprises at least one 2 to 12 μm alumina layer including columnar α-Al₂O₃ grains having a length to width ratio from 2 to 10 and a texture coefficient TC(hkl) of (hkl) reflections (012), (104), (110), (113), (024), (116), and wherein TC(012)>2.2 and TC(024)>0.6×TC(012), and wherein texture coefficient TC(hkl) is defined as

[0016] $\mathrm{TC}\left(\mathrm{hkl}\right)={\frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\left[\frac{1}{n}\sum \frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\right]}^{-1}$
where I(hkl)=measured intensity of (hkl) reflection, I₀(hkl)=standard intensity according to JCPDS card no 46-1212, and n=number of reflections used in calculation. The coating further comprises a first layer adjacent the cemented carbide substrate including CVD Ti(C,N), CVD TiN, CVD TiC, MTCVD Ti(C,N), MTCVD Zr(C,N), MTCVD Ti(B,C,N), CVD HfN or combinations thereof, the first layer having a thickness of from 1 to 10 μm.

[0021] A coated cutting tool insert is provided consisting of a cemented carbide body with a composition of 7 to 12 wt-%, preferably 8 to 11 wt-%, most preferably 8.5 to 9.5 wt-% Co, 5 to 11 wt-%, preferably 6.5 to 9.5 wt-%, cubic carbides of the metals from groups IVb, Vb and VIb of the periodic table, preferably Ti, Nb and Ta, and balance WC. The ratio between the weight concentrations of Ta and Nb, e.g., Ta:Nb, is within 1.0 to 3.0, preferably 1.5 to 2.5. The ratio between the weight concentrations of Ti and Nb, e.g., Ti:Nb, is within 0.5 to 1.5, preferably 0.8 to 1.2.

[0022] The cobalt binder phase is highly alloyed with tungsten. The concentration of W in the binder phase may be expressed as the S-value=σ/16.1, where σ is the measured magnetic moment of the binder phase in μTm³kg⁻¹. The S-value depends on the content of tungsten in the binder phase and increases with a decreasing tungsten content. Thus, for pure cobalt, or a binder that is saturated with carbon, S=1 and for a binder phase that contains W in an amount that corresponds to the borderline to formation of η-phase, S=0.78.

[0023] It has now been found that improved cutting performance is achieved if the cemented carbide body has an S-value within the range 0.79 to 0.89, preferably 0.81 to 0.85.

[0024] Furthermore, the mean intercept length of the tungsten carbide phase measured on a ground and polished representative cross section is in the range 0.7 to 1.4 μm, preferably 0.9 to 1.3 μm, most preferably 1.1 to 1.3 μm. The intercept length is measured by means of image analysis on images with a magnification of 10000× and calculated as the average mean value of approximately 1000 intercept lengths.

[0025] In one embodiment, the cemented carbide is made with all three phases homogeneously distributed in the material.

[0026] In a preferred embodiment, the cemented carbide is provided with a 10 to 40 μm thick, preferably 20 to 40 μm thick, most preferably 20 to 30 μm thick, essentially cubic carbide phase free and binder phase enriched surface zone with an average binder phase content in the range 1.2 to 2.5 times the nominal binder phase content.

[0027] FIG. 1 shows a cross-sectional image of the microstructure of a tool insert according to an embodiment of the invention in approximately 1200× magnification. The tool insert 100 in the FIG. 1 image includes a substrate with a substrate interior 110 and a binder phase enriched surface zone 108. The binder phase enriched zone is also preferably essentially cubic carbide phase free. The coating depicted in FIG. 1 includes a layer of Ti(C,N) 106 applied directly to the substrate, a layer of α-Al₂O₃ 104 applied to the layer of Ti(C,N) 106, and an outermost layer of TiN 102 applied to the layer of α-Al₂O₃ 104.

[0028] A method of making cutting tool inserts as described herein is also provided. According to a preferred embodiment, a powder mixture is formed containing 7 to 12 wt-%, preferably 8 to 11 wt-%, of binder phase consisting of Co, and 5 to 11 wt-%, preferably 6.5 to 9.5 wt-%, cubic carbides of the metals from groups IVb, Vb and VIb of the periodic table, preferably Ti, Nb and Ta, and balance WC. The ratio between the weight concentrations of Ta and Nb is within 1.0 to 3.0, preferably 1.5 to 2.5. The ratio between the weight concentrations of Ti and Nb is within 0.5 to 1.5, preferably 0.8 to 1.2. Well-controlled amounts of nitrogen are added through the powder, e.g., as nitrides. The optimum amount of nitrogen to be added depends on the composition of the cemented carbide and in particular on the amount of cubic phases and is higher than 1.7%, preferably 1.8 to 5.0%, most preferably 3.0 to 4.0 wt-%, of the weight of the elements from groups IVb and Vb of the periodic table. The exact conditions depend to a certain extent on the design of the sintering equipment being used. It is within the purview of the skilled artisan to determine and to modify the nitrogen addition and the sintering process in accordance with the present specification in order to obtain the desired result.

[0029] The raw materials are mixed with a pressing agent and optionally W such that the desired S-value is obtained and the mixture is milled and spray dried to obtain a powder material with the desired properties. Next, the powder material is compacted and sintered. Sintering is performed at a temperature of 1300 to 1500° C., in a controlled atmosphere of about 50 mbar followed by cooling. After conventional post sintering treatments including edge rounding, a hard, wear resistant coating according to the below is applied by CVD- or MT-CVD-technique.

[0030] The coating comprises a first layer adjacent the body of CVD Ti(C,N), CVD TiN, CVD TiC, MTCVD Ti(C,N), MTCVD Zr(C,N), MTCVD Ti(B,C,N), CVD HfN or combinations thereof preferably of MTCVD Ti(C,N) having a thickness of from 1 to 10 μm, preferably from 3 to 8 μm, most preferably about 6 μm and α-Al₂O₃ layer adjacent said first layer having a thickness of from about 2 to 12 μm, preferably from 3 to 10 μm, most preferably about 5 μm. Preferably there is an intermediate layer of TiN between the substrate as well as in the said first layer both with a thickness of <3 μm, preferably about 0.5 μm.

[0031] In one embodiment the α-Al₂O₃ layer is the uppermost layer. In a preferred embodiment, there is a layer of carbide, nitride, carbonitride or carboxynitride of one or more of Ti, Zr and Hf, having a thickness of from about 0.5 to 3 μm, preferably 0.5 to 1.5 μm atop the α-Al₂O₃ layer.

[0032] The total coating thickness is preferably 7 to 15 μm, more preferably 8 to 13 μm.

[0033] The α-Al₂O₃ layer is composed of columnar grains with a strong (012) texture. The columnar grains have a length/width ratio of from 2 to 10, preferably 4 to 8, with a width of 0.5 to 3.0 μm, preferably 0.5 to 2.0, most preferably 0.5 to 1.5.

[0034] The texture coefficients (TC) for the α-Al₂O₃ layer is determined as follows:

[0035] $\begin{array}{cc}\mathrm{TC}\left(\mathrm{hkl}\right)={\frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\left[\frac{1}{n}\sum \frac{I\left(\mathrm{hkl}\right)}{I_0\left(\mathrm{hkl}\right)}\right]}^{-1}& \left(\mathrm{Eq}.1\right)\end{array}$
where

• [0036] I(hkl)=intensity of the (hkl) reflection
• [0037] I₀(hkl)=standard intensity according to JCPDS card no 46-1212
• [0038] n=number of reflections used in the calculation
• [0039] (hkl) reflections used are: (012), (104), (110), (113), (024), (116).

[0040] The texture of the alumina layer is defined as follows:

• [0041] TC(012)>2.2, preferably 2.5 to 3.5 and the related TC(024)>0.6×TC(012). In addition, in some embodiments TC(104), TC(110), TC(113), TC(116) are simultaneously each less than 0.4, preferably each less than 0.3. It is noted that the intensities of the planes 012 and 024 are related.

[0042] In an exemplary embodiment, α-Al₂O₃ is deposited on a Ti(C,N) coating, which is obtained preferably by MTCVD. Several steps can optionally be used to control nucleation as described in Swedish Patent Application 0201417-3. In an exemplary embodiment, a modified bonding layer of α-Al₂O₃ is deposited on the Ti(C,N) layer characterized by the presence of an Al concentration gradient. The controlled oxidation treatment is performed in this case using a CO₂/H₂/N₂ gas mixture, resulting in a lower O-potential than in SE 0201417-3, enhancing the (012) texture further. The oxidation step is short and may be followed by a short treatment with a AlCl₃/H₂ mixture, again followed by a short oxidation step. This kind of pulsating (Al-treatments/oxidation) treatments create favorable nucleation sites for α-Al₂O₃ and a strong (012) texture. The growth of the alumina layer onto the surface modified bonding layer is started by sequencing the reactant gases in the following order: CO, AlCl₃, CO₂. The temperature is preferably about 1000° C. For contrast, a conventional bonding layer is described in U.S. Pat. No. 5,137,774 (referred to as kappa-bonding), the entire contents of which are incorporated herein by reference.

[0043] A cemented carbide substrate in accordance with the present disclosure has a composition 9.0 wt-% Co, 3.6 wt-% TaC, 2.2 wt-% NbC, 2.9 wt-% (Ti,W)C 50/50 (H. C. Starck), 1.1 wt-% TiN and balance WC, with a binder phase alloyed with W corresponding to an S-value of 0.83. Inserts were produced by conventional milling of the raw material powders, pressing of green compacts and subsequent sintering at 1430° C. Investigation of the microstructure after sintering showed that the cemented carbide inserts had a cubic carbide free zone with a thickness of 22 μm. The cobalt concentration in the zone was 1.4 times that in the bulk of the substrate. The mean intercept length of the tungsten carbide phase was 1.2 μm.

[0044] Cemented carbide cutting inserts from Example 1 were coated with a layer of MTCVD Ti(C,N) according to step 1 (below). The thickness of the MTCVD layer was about 6 μm.

[0045] Step 1: MTCVD Coating

[0046]

	Gas mixture	TiCl4 = 4.0%
		CH3CN = 1.0%
		N2 = 20%
		Balance: H2
	Duration	150 min
	Temperature	850° C.
	Pressure	100 mbar

[0047] Onto this layer of MCTVD Ti(C,N), the following alumina layers, e.g., the layers disclosed at (a), (b) or (c) below, were deposited:

[0048] (a) 5 μm α-Al₂O₃ was deposited according to steps 2–6: (Invention).

[0049] Step 2: Bonding Layer

[0050]

	Gas mixture	TiCl4 = 2.8%
		AlCl3 = 0.8–4.2%
		CO = 5.8%
		CO2 = 2.2%
		N2 = 5–6%
		Balance: H2
	Duration	60 min
	Temperature	1000° C.
	Pressure	100 mbar

[0051] Step 3: Aluminizing Step

[0052]

	Gas mixture	AlCl3 = 0.8–4.2%
		Balance: H2
	Duration	15 min or 2 min pulsating
	Temperature	1000 C.
	Pressure	50 mbar

[0053] Step 4: Oxidizing Step

[0054]

	Gas mixture	CO2 = 0.1%
		Balance: H2 + 20% N2
	Duration	2 min or 20 s pulsating
	Temperature	1000° C.
	Pressure	100 mbar

[0055] Step 5: Nucleation Step

[0056]

	Gas mixture	AlCl3 = 3.2%
		HCL = 2.0%
		CO2 = 1.9%
		Balance H2
	Duration	60 min
	Temperature	1000° C.
	Pressure	210 mbar

[0057] Step 6: Deposition

[0058]

	Gas mixture	AlCl3 = 3.9%
		HCL = 1.5%
		CO2 = 6.2%
		H2S = 0.2%
		Balance: H2
	Duration	300 min
	Temperature	1000° C.
	Pressure	50 mbar

[0059] (b) 5 μm κ-Al₂O₃ was deposited according to the prior art, where the κ-Al₂O₃ layer was deposited without oxidation resulting in epitaxial growth of κ-Al₂O₃ on the Ti(C,N) of step 1.

[0060] (c) 5 μm α-Al₂O₃ deposited according to prior art. The nucleation control did not in this case produce 100% pure α-Al₂O₃ but instead a mixture of κ-Al₂O₃ and α-Al₂O₃ was produced. As a result, the κ-Al₂O₃ phase transformed during the deposition process to α-Al₂O₃ with a high dislocation density.

[0061] The total coating thickness of the experimental coatings was 11 μm in all cases.

[0062] The inserts from Example 2a and 2c (alpha oxides) were studied by using XRD. Coating 2c exhibited a random texture but the coating 2a according to this invention showed a clear (012) texture. Table 1 shows the obtained texture coefficients for coating 2a.

[0063]

	TABLE 1
	hkl	Coating 2a
	012	3.39
	104	0.11
	110	0.22
	113	0.15
	024	2.04
	116	0.09

[0064] Inserts according to coating 2a from Example 2 were tested against inserts according to prior art (coating 2c from Example 2) under the following conditions.

[0065]

	Work piece:	Cylindrical bar
	Insert type:	CNMG120408-M3
	Cutting speed	220 m/min
	Feed:	0.4 mm/rev
	Depth of cut:	2.5 mm
	Time in cut:	4.7 min
	Remarks:	Interrupted cut without coolant

[0066] The cutting edges are shown in FIG. 2 after 4.7 min cutting. The inserts produced according to prior art, shown in FIG. 2A, reveals severe edge chipping and crater wear. In contrast, inserts produced according to the invention, shown in FIG. 2B, do not reveal any chipping. The wear on the inserts in FIG. 2B was very uniform and the inserts could have been used much longer.

[0067] This demonstrates that the α-Al₂O₃ with a texture according to the invention has a much tougher and wear resistant behavior than the α-Al₂O₃ produced according to prior art.

[0068] Inserts from Example 2 with coatings 2a and 2b were compared in metal cutting. Coating 2a is composed of defect free α-Al₂O₃ according to the invention and coating 2b is composed of κ-Al₂O₃ according to prior art. The test conditions were the following:

[0069]

Work piece:	Cylindrical slotted bar
Material:	SS1672-08
Insert type:	CNMG120408-M5
Cutting speed	80 m/min
Feed:	0.1, 0.125, 0.16, 0.20, 0.315, 0.4, 0.5, 0.63,
	0.8, 1.0 mm/rev gradually increased after 10 mm length
	of cut
Depth of cut:	2.0 mm
Remarks:	Interrupted machining mode test without coolant.
Tool life	Gradually increased feed until breakage. 10 edges of
criteria:	each variant were tested.

[0070] The results are presented in FIG. 3. FIG. 3 plots remaining undamaged edges after increasing feed rate as a function of feed rate (mm/rev). Both kappa-alumina (κ-Al₂O₃) results 310 and alpha-alumina (α-Al₂O₃) results 320 are shown.

[0071] The results in FIG. 3, indicate the inserts according to the invention composed of α-Al₂O₃ with a texture as disclosed herein exhibited a much better toughness than inserts with κ-Al₂O₃ produced according to prior art.

[0072] The coatings 2a and 2b from Example 2 were tested under the following conditions.

[0073]

Work piece:	Cylindrical bar
Material:	SS1672-08
Insert type:	CNMG120408-M5
Cutting speed	250 m/min
Feed:	0.4 mm/rev
Depth of cut:	2.0 mm
Time in cut:	3.6 min
Remarks:	Interrupted machining mode test without coolant; three
	edges of each variant were tested.

[0074] The prior art inserts coated with κ-Al₂O₃ had severe plastic deformation after 3.6 min cutting while those produced according to the invention exhibited very little plastic deformation. The superiority of α-Al₂O₃ to prevent plastic deformation is clear.

[0075] The following three variants were tested by interrupted turning of stainless steel.

• [0076] a. Invention Example 2a
• [0077] b. Strongly leading grade from Competitor 1 for interrupted turning of stainless steel.
• [0078] c. Strongly leading grade from Competitor 2 for interrupted turning of stainless steel.

[0079]

Work piece:	Cylindrical bar
Material:	SS2343
Insert type:	CNMG120408-M3
Cutting speed	150 m/min
Feed:	0.4 mm/rev
Depth of cut:	2.0 mm
Time in cut:	7.6 min
Remarks:	Interrupted machining mode test with coolant; three
	edges of each variant were tested.

[0080] After 7.6 minutes, the flank wear of the three variants was measured:

[0081]

	Variant	Flank wear (mm)
	a. Invention	0.20
	b. Competitor 1	0.29
	c. Competitor 2	0.26

[0082] The results show that the cemented carbide tool with a layer of α-Al₂O₃ with a texture as disclosed herein exhibits enhanced tool life as compared with competitor products.

[0083] The same variants that were tested in Examples 5 and 6 were also tested in continuous turning in ordinary carbon steel. The cutting data were:

[0084]

	Work piece:	Cylindrical bar
	Material:	SS1672-08
	Insert type:	CNMG120412-M3
	Cutting speed:	300 m/min
	Feed:	0.4 mm/rev
	Depth of cut:	2.0 mm
	Remarks:	Continuous cut with coolant
	Tool life criterion:	Flank wear >0.3 mm, three edges of
		each variant were tested.
	Results	Tool life (min)
	Invention	9.0
	Prior Art	6.0

[0085] The test results show that the cemented carbide disclosed herein, i.e., with a coating layer of α-Al₂O₃ with a texture as disclosed herein, exhibits longer tool life than prior art material with κ-Al₂O₃ in continuous cutting. Thus, the examples above show that the cemented carbide according to the invention is superior to prior art materials both regarding wear resistance and toughness.

[0086] Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without department from the spirit and scope of the invention as defined in the appended claims.