Surface-coated hard metal body and method of producing the same

Description

[0001] This invention relates to coated articles comprising hard metals, refractories, and especially cemented carbide substrates. More particularly, it relates to such coated hard metal or coated cemented carbide products which are adapted to receive an aluminum oxide or other oxide wear layer which is very firmly bonded to the substrate.

[0002] Such hard metal and/or cemented carbide substrates are used in tools for machining and cutting metals. Their already high wear resistance can be significantly improved by providing oxide wear layers, such as aluminum oxide wear layers, as described in U.S. 3,736,107 and 3,836,392.. However, it has become apparent that proper steps must be taken to adequately bond the oxide layer to the hard metal or cemented carbide substrate if the superior wear resistance of the oxide layer is to be realized.

[0003] In U.S. 4,018,631, it is disclosed that a selective pretreatment of cemented carbides before application of the oxide wear layer unexpectedly enhances and improves the adherence of the subsequently applied oxide wear layer. While such a procedure results in a tightly adherent surface layer of aluminum oxide or other oxide wear layer, and is therefore extremely useful, the procedure requires a high temperature for the diffusion step.

[0004] A novel coating procedure has now been discovered which provides aluminum oxide and other oxides (e.g., hafnium oxide, zirconium oxide and the like) bonded to the substrates with adherence equal to that obtained in the said '631 patent, which can be performed at normal coating temperatures. Such a procedure in its broadest aspects comprises providing a thin surface-oxidized bonding layer comprising an oxide, a carbide or oxycarbide of at least one of tantalum, niobium and vanadium, optionally aluminiz- ing the bonding layer, and finally providing an outer oxide wear layer. The present bonding layer is thin, not useful as a barrier, and possesses a composition novel in its chemical constituents. All of the foregoing patents and publications are incorporated herein by reference.

[0005] According to the invention there is provided an article of manufacture comprising:

(i) a hard metal or cemented carbide substrate having a plurality of coatings thereon;

(ii) at least one of the coatings being a surface-oxidized bonding layer comprising at least one of the carbides or oxycarbides of tantalum, niobium and vanadium; and

(iii) an oxide wear layer overlying said bonding layer.

[0006] In preferred features, the substrate is a cobalt cemented carbide; the bonding layer is 0.1 to 0.5 microns thick; aluminum is added to the bonding layer by a process to be described layer; the oxide wear layer is aluminum oxide; and the wear layer is 0.5 to 20 microns thick.

[0007] In one practice of the present invention a hard metal or cemented carbide substrate is pretreated for the reception of a wear resistant oxide coating by treating the substrate in a first atmosphere selected from carbide and oxycarbide forming atmospheres to form a bonding layer of metal selected from at least one of tantalum, niobium or vanadium on said substrate and heating the coated substrate in a second oxidizing atmosphere until at least about 50% of the surface is oxidized.

[0008] In the preferred features of the process aspect aluminum will be added to the bonding layer;' and an oxide outer wear layer, preferably an aluminum oxide wear layer, will be deposited on the bonding layer, which optionally may contain aluminum. The bonding layer may bond or form the oxide wear layer directly to the substrate or to other layers with which the substrate is previously coated: One preferred previously coated layer is TiC although broadly, this layer may comprise a carbide, nitride or carbonitride of Ti, Zr, Hf, V, Cr, W, Si, B which is an intermediate layer.

[0009] Those skilled in this art will now the general techniques used to prepared the product and carry out the process of the present invention.

[0010] One convenient way of proceeding is to provide a coating furnace held at a temperature of from about 800° C to 1300°C., and to expose a substrate in the furnace to the following sequential steps:

1. 5 to 60 minutes exposure, preferably at 1050°C., to a gaseous mixture of H₂ and 0.5 to 20 volumes percent TaCl5 or NbCl ₅, Ti Cl₄, Al Cl₃ and/or CH₄ may be optionally added during either part or all of this period.

2. 1 to 60 minutes exposure, preferably at 1100°C., to a gaseous mixture consisting of H₂ and about 1 to 50 volume percent CO₂ to oxidize and produce the pretreated substrate.

3. 5 to 60 minutes exposure, preferably at 1050°C., to a gaseous mixture of H2 and about 0.5 to 20 volume percent AlCl₃. This step for adding aluminum is optional but is preferred for best results.

4. 15 minutes' to 4 hours' exposure, preferably 1050°C., to gaseous mixture of H₂, 1 to 40 (or 60 to 95) volume percent C0₂, and 2.5 to 20 volume percent AlCl₃ to produce the aluminum oxide wear coating.

[0011] Other suitable treating atmospheres of varying proportions of constituents will occur to those skilled in the art. Likewise, other well known deposition techniques can be used such as physical vapor deposition, sputtering and pack diffusion.

[0012] Those features of the invention which are believed to be novel are set forth with particularity in the claims appended hereto. The invention will, however, be better understood from a consideration of the preferred embodiments.

[0013] The following examples are illustrative, and the claims are not be construed as being limited thereto.

EXAMPLE 1

[0014] A commercial cemented carbide cutting tool insert of composition 85.5% WC, 6% TaC, 2.5% TiC and 6% Co was coated in the following manner:

1. Held 15 minutes at 1050°C in an atmosphere of H₂ - 5% CH₄ - 2% TiCl₄ - 5% TaCl₅ - 10% AlCl₃.

2. Held 40 minutes at 1050°C in an atmosphere of H₂ - 10% C0₂.

3. Held 10 minutes at 1050°C in an atmosphere of H₂ - 10% AlCl₃.

4. Held 60 minutes at 1050°C in an atmosphere of H₂ - 10% C0₂ - 10% AlCl₃.

[0015] This treatment resulted in a 4-micron A1₂0₃ coating which was firmly bonded to the cemented carbide substrate, through a bonding layer about 0.2 microns thick.

[0016] The coated insert was used to machine cast iron at 400 sfpm, .010 in./rev. feed rate, and the wear resistance was compared with that obtained using a commercial insert which requires a high temperature diffusion operation to make the coating.

[0017] The wear resistance of the insert coated by the above- described simplified process was found to be nerarly equal to that of the commercial insert.

EXAMPLE 2

[0018] A cemented carbide insert having the same composition as Example 1 above was coated with A1₂0₃ in the following manner:

1. Held 15 minutes at 1050°C in a gaseous mixture of H₂ - 2% TiCl₄ - 5% NbCl₅ - 10% AlCl₃, then the TiCl₄ was turned off and the insert was held an additional 10 minutes at 1050°C in the remaining mixture of H₂ - 5 % NbCl₅ - _10% AlCl_3.

2. Held 20 minutes at 1100°C in a mixture of H₂ -5% C0₂.

3. Held 20 minutes at 1050°C in a mixture of H₂ - _10% Al Cl₃ .

4. Held 45 minutes at 1050°C in a mixture of H_Z -5% CO₂ - 10% AlCl₃.

[0019] The resultant coated insert had a 3-micron Al₂0₃ coating firmly bonded to the cemented carbide substrate, through a bonding layer about 0.2 microns thick.

[0020] When used to machine cast iron (same conditions as Example 1 above), the wear resistance was found to be equivalent to the commercial insert.

EXAMPLE 3

[0021] A cemented carbide insert having the same composition as Example 1 above was pretreated then coated with Al₂0₃ in the following manner at a furnace temperature of 1050°C, and l atmosphere pressure.

1. Held 10 minutes in a gaseous mixture of H₂ -10% CH₄.

2. Held 1 minute in a gaseous mixture of H₂ - 2% TiCl₄.

3. Held 25 minutes in a mixture of H₂ and NbCl₅ (which had been heated for about 8 minutes to 320°F., held 3 mintues, and cooled with power off for 15 minutes).

4. Held 2 minutes in a gaseous mixture of H₂ -2.5% C0₂.

5. Held 10 minutes in a gaseous mixture of H₂ - 5% AlCl_3.

6. Held 60 minutes in a gaseous mixture of H₂ - 5% AlCl₃ - 9% C0₂.

[0022] The resultant coated insert had a 3-4 microns Al₂0₃ coating firmly bonded to the cemented carbide substrate, through a bonding layer about 0.2 microns thick.

[0023] When used to machine cast iron (same conditions as Example 1 above), the wear resistance is found to be equivalent to the commercial insert.

EXAMPLE 4

[0024] The procedure of Exanoke 3 was repeated, using the following conditions:

1. Held 1 minute at 1050°C.in a gaseous mixture of H₂ - 3% Ti Cl_4.

2. Held 1 minute at 1050°C. in a gaseous mixture of H₂ - 3% Ti Cl ₄ - 20% N₂.

3. Held 30 minutes in H₂ - 3% NbCl₅; + 3% TiCl₄ added for 20 seconds during middle of this period and temperature was 850°C. for first 10 minutes of this period and then increased in a linear fashion to 1050°C. by the end of the period.

4. Held 10 minutes at 1050°C. in H₂ - 11% C0₂.

5. Held 10 minutes at 1050°C. in H₂ - 7% A1C1₃.

6. Held 60 minutes in gaseous mixture of H₂ - 11% C0₂ -7% AlCl₃.

[0025] A coated insert according to this invention was obtained.

EXAMPLE. 5

[0026] A commercial cemented carbide cutting tool insert comprising 85.5% WC; 6%-TaC 2.5% TiC and 60% Co and coated with TiC of five microns thickness is subjected to the following sequence of steps in a furnace at temperature of 1050°C. and 1 atmosphere pressure:

1. 2 minutes in an atmosphere of H₂ and approximately 2% TiCl₄

2. NbCl₅ vaporizer on 8 min. to 225°F., 3 min. hold - 15 min. power off-cool.

3. 1 minute in an atmosphere of hydrogen - 3.5% C0₂ to surface oxidize.

4. 10 minutes in an atmosphere of hydrogen - 5% AlCl₃-

5. 60 minutes in an atmosphere of hydrogen - 5% AlCl₃ - 7% CO_2.

[0027] This treatment resulted in a 3-4 microns A1₂0₃ coating which was firmly bonded to the TiC coated cemented carbide substrate, through a bonding layer approximately 0.2 microns thick.

[0028] The coating adhesion of this insert was sufficient to meet the requirements of commercial A1₂0₃ - coated substrates, without a TiC layer. Direct deposit of Al₂0₃ on inserts coated with TiC fail to meet these requirements.

EXAMPLE 6

[0029] Iron was incorporated into the surface of a TiC coated cemented carbide cutting tool insert by rubbing its cutting surfaces with a piece of soft iron. The general procedure of Example 1 was then used to deposit a very thin coating of niobium carbide by the exposure of the treated surface to a mixture of H₂ and CbCl₅ gases for about 10 minutes at 1050°C. The resultant CbC coating was allowed to diffuse with the Fe (and TiC) for about 20 minutes and then this surface was lightly oxidized by exposure to a mixture of H₂ - 5% C0₂ at 1050°C. for about 15 minutes. When subsequently A1₂0₃ - coated, a very strong bond was obtained between the A1₂0₃ coating and the TiC-coated surface, noticeably better than the adhesion obtained using the same process without the Fe treatment.

[0030] The use of tantalum or niobium chloride in the steps of the. above examples is critically specific for the achievement of the desired high level of coating adherence in a single furnace operation. While titanium chloride may be used in these steps in addition to tantalum or niobium chloride, the adherence is not as good if only titanium chloride is used. Since vanadium belongs to the same group as tantalum and niobium (Group_.VB), its effectiveness is probable. The bonding layer may be treated with Al, Fe, Co, Ni, to improve the bond with the ^A1₂0₃ outer layer.

[0031] Many variations will suggest themselves to those skilled in the art in light of the above-detailed description. All obvious such variations are within the full intended scope of the invention as defined by the apppended claims.

Claims

1. An article of manufacture comprising:

(i) a hard metal or cemented carbide substrate having a plurality of coatings thereon, at least one of said coatings being a surface-oxidized bonding layer comprising at least one of the carbides or oxycarbides of tantalum, niobium and vanadium; and

(ii) an oxide wear layer overlying said bonding layer.

2. An article as claimed in claim 1 wherein the substrate is a cemented carbide, and the bonding layer is 0.1 to 0.5 microns thick.

3. An article as claimed in claim 1 or claim 2 wherein oxide wear layer (ii) is 0.5 to 20 microns thick.

4. An article as claimed in any one of claims 1 to 3 wherein bonding layer (i) includes aluminium.

5. An article as claimed in any one of the preceding claims wherein oxide wear layer (ii) comprises aluminium oxide.

6. An article as claimed in any one of claims 1 to 3 wherein said substrate has a first coating of a carbide, nitride, or carbonitride, of titanium, zirconium, hafnium, vanadium, columbium, tantalum, chromium molybdenum, tungsten, silicon or boron, to which said bonding layer is attached.

7. An article as claimed in claim 6 wherein said first coating comprises TiC.

8. An article as claimed in any one of the preceding claims where said intermediate bonding layer is aluminized.

9. A process of pretreating a hard metal or cemented carbide substrate for the reception of wear resistant oxide coatings which comprises:

(a) treating the substrate in a first atmosphere selected from carbide and oxycarbide forming atmospheres to form a bonding layer of metal selected from at least one of tantalum, niobium or vanadium on said substrate; and

(b) heating the coated substrate of (a) in a second oxidizing atmosphere until at least portions of the surface are oxidized.

10. A process as claimed in Claim 9 including the step of treating the oxidized coated substrate of (b) in a reducing atmosphere with a reducible aluminium compound to diffuse alluminium into the coating.

11. A process as claimed in claim 9 including the step of superimposing an oxide wear layer on the surface-oxidized coated substrate.

12. A process as claimed in claim 9 including the step of superimposing an oxide wear layer on the surface-oxidized, aluminium- containing coated substrate.

13. A process as claimed in claim 9 wherein said oxide wear layer is an aluminium oxide wear layer.