Enhanced alumina layer produced by CVD

[0001] In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.

[0002] The present invention relates to a substrate of cemented carbide, cermet or ceramics onto which a hard and wear resistant coating is deposited. The coating is adherently bonded to the substrate and covers all functional parts thereof. The coating is composed of one or more refractory layers of which at least one layer is a strongly textured alpha-alumina (α-Al₂O₃).

[0003] In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.

[0004] A crucial step in deposition of different Al₂O₃ polymorphs is the nucleation step. κ-Al₂O₃ can be grown in a controlled way on {111} planes of TiN, Ti(C,N) or TiC having the face-centered cubic structure. Transmission electron microscopy (TEM) has confirmed the growth mode which is that of the close-packed (001) planes of κ-Al₂O₃ on the close-packed {111 } planes of the cubic phase with the following epitaxial orientation relationships: (001)_κ//(111)_TiX; [100]_κ//[112]_TiX. An explanation and a model for the CVD growth of metastable κ-Al₂O₃ has been proposed (Y. Yoursdshahyan, C. Ruberto, M. Halvarsson, V. Langer, S. Ruppi, U. Rolander and B. I. Lundqvist, Theoretical Structure Determination of a Complex Material: κ-Al₂O₃, J. Am. Ceram. Soc. 82(6)1365–1380 (1999)).

[0005] When properly nucleated, κ-Al₂O₃ layers can be grown to a considerable thickness (>10 μm). The growth of even thicker layers of κ-Al₂O₃ can be ensured through re-nucleation on thin layers of, for example, TiN inserted in the growing κ-Al₂O₃ layer. When nucleation is ensured the κ→α transformation can be avoided during deposition by using a relatively low deposition temperature (<1000° C.). During metal cutting high temperature conditions may be created such that the κ→α phase transformation has been confirmed to occur. In addition to phase stability there are several reasons why α-Al₂O₃ should be preferred for many metal cutting applications. (α-Al₂O₃ exhibits better wear properties in cast iron, as discussed in U.S. Pat. No. 5,137,774. Further, a layer which has been nucleated as α-Al₂O₃ does not contain any transformation cracks and stresses. Nucleated α-Al₂O₃ should be more ductile than α-Al₂O₃ formed totally or partially as a result of phase transformation, and even more ductile than κ-Al₂O₃, the plasticity of which is limited by the defect structure.

[0006] However, a stable α-Al₂O₃ phase has been found to be more difficult to be nucleated and grown at reasonable CVD temperatures than the metastable κ-Al₂O₃. It has been experimentally confirmed that α-Al₂O₃ can be nucleated, for example, on Ti₂O₃ surfaces, bonding layers of (Ti,Al)(C,O), as shown in U.S. Pat. No. 5,137,774, or by controlling the oxidation potential using CO/CO₂ mixtures, as shown in U.S. Pat. No. 5,654,035. The bottom line in all these approaches is that nucleation must not take place on the 111-surfaces of TiC, TiN, Ti(C,N) or Ti(C,O,N), otherwise κ-Al₂O₃ is obtained.

[0007] It should also be noted that in conventional prior art methods higher deposition temperatures are usually used to deposit α-Al₂O₃. When the nucleation control is not complete, as is the case in many conventional products, the resulting α-Al₂O₃ layers have, at least partly, been formed as a result of the κ-Al₂O₃→α-Al₂O₃ phase transformation. This is especially the case when thick Al₂O₃ layers are considered. These kind of α-Al₂O₃ layers are composed of larger grains with phase-transformation cracks. These coatings exhibit much lower mechanical strength and ductility than α-Al₂O₃ coatings that are composed of nucleated α-Al₂O₃.

[0008] The control of the α-Al₂O₃ polymorph on an industrial scale was achieved in the beginning of the 1990's with commercial products based on U.S. Pat. No. 5,137,774. In addition, α-Al₂O₃ has been deposited with preferred coating textures. In U.S. Pat. No. 5,654,035 an alumina layer textured in the (012) direction, and in U.S. Pat. No. 5,980,988 an alumina layer textured in the (110) direction are disclosed. In U.S. Pat. No. 5,863,640 a preferred growth either along (012), (104), or (110) directions is disclosed. U.S. Pat. No. 6,333,103 describes a modified method to control the nucleation and growth of α-Al₂O₃ along the (10(10)) direction.

[0009] The present invention provides a new, improved alumina layer where the α-Al₂O₃ phase is nucleated α-Al₂O₃ with a strong, fully controlled (112) growth texture.

[0010] According to one aspect, the present invention provides cutting tool insert comprising a substrate having a surface at least partially coated with a coating, the coating having a total thickness of about 10–40 μm, one or more refractory layers of which at least one layer is an alumina layer, the alumina layer being composed of columnar α-Al₂O₃ grains with texture coefficients expressed as TC(hkl) of:

[0011] a) TC(012) and TC(024) both >1.8; and

[0012] b) TC(104), TC(110), TC(113) and TC(116) all <0.4;

[0013] wherein the texture coefficient TC(hkl) is defined as:

[0014] $TC\left(hkl\right)=\frac{I\left(hkl\right)}{I_0\left(hkl\right)}{\left\{\frac{1}{n}\sum \frac{I\left(hkl\right)}{I_0\left(hkl\right)}\right\}}^{-1}$

[0015] where

• • [0016] I(hkl)=measured intensity of the (hkl) reflection
  • [0017] I_O(hkl)=standard intensity according to JCPDS card no 46-1212
  • [0018] n=number of reflections used in the calculation
  • [0019] (hkl) reflections used are: (012), (104), (110), (113), (024), (116).

[0020] According to another aspect, the present invention provides a method of forming a coating, the method comprising: (i) forming a coating of (Ti,Al)(C,),N); and (ii) modifying the as-formed coating by applying alternating aluminising and oxidising treatments thereto, whereby the coating is provided with an aluminium content which increases toward the surface thereof.

[0021] It has been surprisingly found that α-Al₂O₃ layers having a strong, fully controlled (112) growth texture outperform conventional coatings which have random or other textures such as (011). The coating according the present invention is essentially free from transformation stresses, having columnar α-Al₂O₃ grains with low dislocation density and with improved cutting properties.

[0022] According to the present invention, a method has been found to control the nucleation step of α-Al₂O₃ so that a strong (012)-texture can be obtained. The method is characterized by the dominating presence of (012) texture coefficient and simultaneous absence of the other common TCs typically found in conventional coatings.

[0023] This kind of an Al₂O₃ layer is especially suited for use in toughness demanding steel cutting applications such as interrupted cutting, turning with coolant and especially intermittent turning with coolant. The other area is cast iron where the edge strength of this kind of alumina layer is superior to conventional cutting tools.

[0024] Before α-Al₂O₃ is deposited on a CVD or MTCVD-applied Ti(C,N) coating, several steps are needed. First, a modified bonding layer, such as described in U.S. Pat. No. 5,137,774 (referred to as kappa-bonding in this patent), is deposited on the Ti(C,N) layer and is characterized by the presence of an Al concentration gradient. In addition, nitrogen gas is applied during deposition of this bonding layer. The aluminium content on the surface of this layer being considerably, about i.e. —30%, higher than in the bonding layer according to U.S. Pat. No. 5,137,774, and the bonding layer of the invention also contains nitrogen. The surface of this bonding layer is subjected to an additional treatment(s) using a AlCl₃/H₂ gas mixture in order to further increase the aluminium content. Subsequently, an oxidation treatment is performed using a CO₂/H₂ gas mixture. The oxidation step is short and may be followed by a short treatment with a AlCl₃/H₂ mixture, again followed by a short oxidisation step. These pulsating or alternating (Al-treatments/oxidisation) treatments create favourable nucleation sites for α-Al₂O₃. The growth of the alumina layer onto the surface modified bonding layer is started by sequentially introducing the reactant gases in the following order: CO, AlCl₃, CO₂. The temperature shall preferably be about 1000° C.

[0025] The present invention also relates to a cutting tool insert comprising a substrate at least partially coated with a coating having a total thickness of 20–40 μm, preferably 15–25 μm. The coating composed of one or more refractory layers of which at least one layer is an alpha alumina layer. This α-Al₂O₃ layer is dense and defect-free, and is composed of columnar grains with a strong (012) texture. The columnar grains have a length/width ratio of from 2 to 12, preferably 5 to 9. The columnar grains have a width of 0.5–2.5 μm, preferably 0.5–1.0 or 1.5–2.5 μm. The total thickness of the alumina layer, or “S ”, can be 2–5 μm when the grains are 0.5–1.0 μm, and can be 5–15 μm when the grains are 1.5–2.5 μm.

[0026] The texture coefficients (TC) for the α-Al₂O₃ according to this invention layer is determined as follows:

[0027] $TC\left(hkl\right)=\frac{I\left(hkl\right)}{I_0\left(hkl\right)}{\left\{\frac{1}{n}\sum \frac{I\left(hkl\right)}{I_0\left(hkl\right)}\right\}}^{-1}$

[0028] where

• • [0029] I(hkl)=intensity of the (hkl) reflection
  • [0030] I_O(hkl)=standard intensity according to JCPDS card no 46-1212
  • [0031] n=number of reflections used in the calculation
  • [0032] (hkl) reflections used are: (012), (104), (110), (113), (024), (116).

[0033] The texture of the alumina layer is defined as follows:

[0034] TC(012) and TC (024) >1.8, preferably 2.5–3.5, and simultaneously TC(104), TC(110), TC(113), TC(116) <0.4, preferably <0.3.

[0035] It is noted that the intensities of the planes 012 and 024 are related.

[0036] The substrate preferably comprises a hard material such as cemented carbide, cermets, ceramics, high speed steel or a superhard material such as cubic boron nitride (CBN) or diamond, preferably cemented carbide or CBN. When CBN is used, the substrate preferably material contains at least 40 vol-% CBN. In one preferred embodiment the substrate is a cemented carbide with a binder phase enriched surface zone.

[0037] The coating comprises a first layer adjacent the substrate 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, and is preferably of Ti(C,N) having a thickness of from 1 to 20 μm, preferably from 1 to 10 μm. An α-Al₂O₃ refractory layer is provided adjacent the first layer having a thickness of from about 1 to 40 μm, preferably from 1 to 20 μm, most preferably from 1 to 10 μm.

[0038] According to one aspect, the refractory layer consists of α-Al₂O₃ According to another aspect, the refractory layer consists essentially of α-Al₂O₃, i.e.—the layer is mainly (α-Al₂O₃, but may include minor amounts of impurities and/or other phases of α-Al₂O₃. According to yet another aspect, the refractory layer comprises α-Al₂O₃.

[0039] According to one embodiment, there is also an intermediate layer of TiN between the substrate and said first layer with a thickness of <3 μm, preferably 0.5–2 μm.

[0040] In one embodiment the α-Al₂O₃ layer is the uppermost layer

[0041] In another embodiment there is another 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. This additional layer can have a thickness of from about 1 to 20 μm, preferably 2 to 8 μm.

[0042] In yet another embodiment the coating includes a layer of κ-Al₂O ₃ and/or γ-Al₂O₃, preferably atop the α-Al₂O₃, with a thickness of from 0.5 to 10 μm, preferably from 1 to 5 μm.

[0043] Specific illustrative, but non-limiting, examples of the above will now be described.

[0044] Cemented carbide cutting inserts with a composition of 5.9% Co and balance WC (hardness about 1600 HV) were coated with a layer of MTCVD Ti(C,N). The thickness of the MTCVD layer was about 8 μm. On to this layer 8 μm α-Al₂O₃ was deposited “coating a” according to the present invention as defined below, and conventional “coating b”. The inserts were studied by using XRD and the texture coefficients were determined. Table 1 shows the texture coefficients for the coating according to this invention and for the prior art coating. As can be seen a strong (012) texture is obtained according to this invention.

[0045]

TABLE 1
hkl	Invention (coating a)	Conventional (coating b)
012	2.79	0.72
104	0.05	0.64
110	0.19	1.63
113	0.07	0.87
024	3.26	1.16
116	0.15	0.97

[0046] Deposition process of “coating a” (invention) is specified as follows:

[0047]

Step 1: MTCVD coating
	Gas mixture	TiCl4 = 4.0%
		CH3CN = 1.0%
		N2 = 20%
		Balance: H2
	Duration	250 min
	Temperature	850° C.
	Pressure	100 mbar

[0048]

Step 2: Bonding layer
	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

[0049]

Step 3: Aluminising step
	Gas mixture	AlCl3 = 0.8–4.2%
		Balance: H2
	Duration	15 min or 2 min pulsating
	Temperature	1000 C.
	Pressure	50 mbar

[0050]

Step 4: Oxidising step
	Gas mixture	CO2 = 0.1%
		Balance: H2
	Duration	2 min or 20 s pulsating
	Temperature	1000 C.
	Pressure	100 mbar

[0051]

Step 5: Nucleation step
	Gas mixture	AlCl3 = 3.2%
		HCL = 2.0%
		CO2 = 1.9%
		Balance H2
	Duration	60 min
	Temperature	1000° C.
	Pressure	210 mbar

[0052]

Step 6: Deposition
	Gas mixture	AlCl3 = 4.2%
		HCL = 1.0%
		CO2 = 2.1%
		H2S = 0.2%
		Balance: H2
	Duration	420 min
	Temperature	1000° C.
	Pressure	50 mbar

[0053] The coatings a) and b) from the example 1 were tested with respect to edge toughness in longitudinal turning.

[0054]

	Work piece:	Cylindrical bar
	Material:	SS0130
	Insert type:	SNUN
	Cutting speed:	400 m/min
	Feed:	0.4 mm/rev
	Depth of cut:	2.5 mm
	Remarks:	dry turning

[0055] The inserts were inspected after 2 and 4 minutes of cutting. As clear from Table 2 the edge toughness of the conventional product was considerably enhanced when the coating was produced according to this invention.

[0056]

TABLE 2
	Flaking of the edge
	line (%)	Flaking of the edge
	after 2 minutes	line (%) After 6 minutes
Coating a (Invention)	0	<1
Coating b	38	62

[0057] The coating produced according to this invention was compared with a market leader, referred to here as Competitor X. This coating is composed of MTCVD Ti(C,N) and α-Al₂O₃. X-ray diffraction analysis (XRD) was used to determine the texture coefficients for these competitor coatings. Two inserts from Competitor X were randomly chosen for XRD. Table 3 shows the TCs obtained for the Competitor X. The coatings from Competitor X exhibit a (110) texture.

[0058]

TABLE 3
hkl	TC(hkl)
012	0.64	0.59
104	0.95	0.88
110	1.75	2.00
113	0.45	0.42
024	1.15	1.10
116	1.07	1.01

[0059] The inserts from the competitor X were compared with inserts produced according to the present invention with the same substrate composition and the same coating structure. Before the tests the both inserts produced according to this invention were examined by XRD. The strong (012) texture was confirmed.

[0060] Two inserts produced according to this invention were compared with two Competitor X inserts with respect to flank wear resistance in face turning of ball bearing material

[0061]

	Work piece:	Cylindrical tubes(Ball bearings)
	Material:	SS2258
	Insert type:	WNMG080416
	Cutting speed:	500 m/min
	Feed:	0.5 mm/rev
	Depth of cut:	1.0 mm
	Remarks:	Dry turning

[0062] Tool life criterion: Flank wear >0.3 μm, three edges of each variant were tested.

[0063]

	Results:	Tool life (min)
	Coating 1	22 (invention)
	Coating 2	23.5 (invention)
	Competitor 1	15.5 (conventional)
	Competitor 2	13 (conventional)

[0064] Cubic boron nitride (CBN) inserts containing about 90% of polycrystalline CBN (PCBN) were coated according to this invention and according to conventional coating techniques discussed in Example 1. The coated CBN was compared with uncoated CBN insert in cutting of steel containing ferrite. It is known that boron has a high affinity to ferrite and diffusion wear occurs at high cutting speeds.

[0065]

	Work piece:	Cylindrical bar
	Material:	SS0130
	Insert type:	SNUN
	Cutting speed:	860 m/min
	Feed:	0.4 mm/rev
	Depth of cut:	2.5 mm
	Remarks:	dry turning

[0066]

		Life time (min)
	Coated CBN (Invention)	22
	Conventional coating	14
	Uncoated CBN	11

[0067] The described embodiments of the present invention are intended to be illustrative rather than restrictive, and are not intended to represent every possible embodiment of the present invention. Various modifications can be made to the disclosed embodiments without departing from the spirit or scope of the invention as set forth in the following claims, both literally and in equivalents recognized in law.