CA1256274A - Coated carbide cutting tool insert - Google Patents

Abstract

IMPROVED COATED CARBIDE
CUTTING TOOL INSERT

ABSTRACT OF THE DISCLOSURE

Cobalt cemented carbide cutting inserts are prepared for coating with a hard, wear-resistant coating by providing a cobalt enriched zone on a surface to be coated. Cobalt enrichment is effectuated by means of nitrogen gas contact with the carbide followed by a period of vacuum sintering during its sintering process of manufacture.

Description

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IMPROVED COATED CARsIDE
CUTTING TOOL INSERT
sack~round of the Invention This invention relates to an improved coated carbide cutting tool insert, and more particularly to a cobalt enriched zone in a cobalt cemented carbide inser-t substrate which supports a multiple layered coating, at least one of which layers is a thicker, hard wear-resistant carbide material.
Field of the Invention Coated cemented carbide inserts have been effecitvely utilized in many metal working operations for a number of years. Basically, they are composite materials prepared by chemical vapor depositing (CVD) processes which provide a thin layer of a hard wear ~- resistant coating, for example, titanium carbide (TiC(, on a hard metal substrate surface such as a cemented carbide (WC). In some instances, the TiC layer is preceded by an underlayer, titanium nitride (TiN) for example, and an overlayer of TiN, aluminum oxide (A12O3) and the like. Multilayer inserts have found application in a broad range of metal cutting applications, and various layers and their materials may be selected to suit the metal removal application.

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~Z~274 6OMP 2099 Description Of The Prior Art The manufacture of coated cemented carbide tools and inserts includes a number of chemical and physical requirements. The coating layers utilized must be chemically stable and phyc;ically wear resistant in various metal cutting and wearing operations. The compo-sition and thickness of these coatings are quite relevant because they must not easily spall or crack. More im-portantly, however, they must be integrally supported by and securely bonded to the insert substrate. Titanium carbide layers, titanium nitride layers, titanium carbo nitride layers, TiCN, and aluminum oxide layers, A1203 in numerous combinations, structures, and ordered layers are known in the art. However, titanium carbide, TiC, has emerged as the predominant wear surface and, accordingly, titanium carbide layers have been laid down on various substrates by a number of different processes to perform as a hard wear surface.
When there are two or more dissimilar layers, the supporting relationship between the multiple layers and a cemented carbide substrate is most important from a structural point of view, and since TiC is the important layer its relationship and bond to the cemented carbide substrate are critical. For this reason the TiC layer is - 25 usually next adjacent the substrate and some advantage is taken of the affinity of the two carbides for an inte grating structural support. Because of the noted superi-ority of TiC layers as the predominant, hard wear-resistant layer, some attention has been given to ways and mearls to use thicker TiC layers and also additional individual layers of other rnaterials which contribute to the effectiveness of the TiC layer. The result of thicker layers generally is a weakening of the structure.

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-' ' : '. ' 60r~P 2099 ~5~2 With respect to adequately supporting the hard wear resistant outer layers on cemented carbide sub-strates, and effectively supporting more and thicker , layers, recent improvements include a metallurgical gra-dation of the layers at their junctures which define transitional zones incorporating elements from each adjacent layer. In the case of a cobalt cemented carbide substrate, this gradation relates to a surface zone or region of the substrate which is enriched in cobalt in that it contains a higher average concentration of cobalt than found elsewhere in the cemented carbide. This cobalt enriched zone is used to provide improved tough-ness to the cutting edge of a coating thereon and an improved surface on which to depocit a coating such as 1; TiN and TiC. However, the processes used to accomplish this enrichment are complex, relatively expensive, sepa-rate from the carbide manufacturing process and not precise in locating the cobalt where it is most desired.

Summary of the Invention ~he present invention discloses an improved process of providing a cobalt gradation zone in a cobalt cemented carbide which is combined with a carbide manu-facturing process. The cobalt zone more effectively sup-ports thicker multilayer coatings of hard, wear-resistant materials, including coatings where TiC is not the first layer.

The Drawings This invention will be better understood when taken in connection with the following description and the drawings in which:

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FIG. 1 is a photomicrograph of one insert embodiment of this invention indicat:ing cobalt enrichment magnified 1500 times;
FIG. 2 is a graph indicating cobalt distribution in the enriched zone of FIG. l;
FIG. 3 is a photomicrograph of an insert of Example III magnified 1500 times; and FIG. 4 iS a graph indicating cobalt distribution in the insert of Example III
Description of the Invention In one preferred form of this invention, gaseous nitrogen is controllably injected into the sintering cycle of a cemented carbide manufacturing process in order to provide different degress of cobalt enrichment in the resulting cemented carbide substrate.
By this means, an improved cobalt enrichment zone and an improved surface are provided for subsequent deposition of hard wear resistant layers.
There are three important interrelated contributing factors to an improved cutting tool, one example of which comprises (a) a cobalt-enriched cemented carbide substrate for strength purposes, (b) an outer surface with optimum cobalt dispersion and enrichment, and (c) a multilayer coating combination of hard material ~5 layers which may include, for example, TiN, TiC and TiN.
The cemented carbide substrate of the present invention may include a number of cemented carbide substrates of different compositions but preferably is a cobalt cemented tungsten carbide substrate of the following general pro-30 portions. 2-5 wt. % of TiC, 5-10 wt. ~ TaC, 5-10 wt. %
Co. balance WC. An article is prepared by the usual powder metallurgy process, milling the powders, pressing thepowder into compact form, and sintering the compacted form at temperatures above the milling point of the cobalt phase.

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, ~l~S~;2~4 The substrate of this invention incorporates a cobalt enriched zone at or adjacent to its outer surface. Some examples of prior cobalt enrichment are found in U.s. Patents 2,612,442 - Goetzel, and 4,497,874 - Hale, assigned to the same assignee as the present invention. As one example, a cobalt cemented carbide insert is subjected to elevated temperatures at above about the melting point of the cobalt in the substrate to cause the cobalt to progress, migrate or lo diffuse to a surface region or zone. The cobalt-enriched surface zone is an important concept in the structural integrity of multilayer coated ~ inserts. The cobalt-enriched zone changes the - hardness characteristics of the interface surface between the substrate and the adjacent coating and provides a tougher surface.
A cobalt enriched zone has been achieved by various processes in the cutting tool art, involving - high-tsmperature diffusion, or higher temperature melting and migration of the cobalt to the surface.
However, not all cobalt-enriched surfaces provide the same final result for a cutting tool insert. The kind of cobalt enrichment as well as the kind of next adjacent surface are quite important. For example, a coextensive cobalt layer at the extreme outer surface of the substrate where it would be in engagement with a coating layer is undesirable and either should not be formed, or should be subsequently removed before a coating layer is deposited. Further, for some applications the content of cobalt in the enriched zone should be at least about 2 times the average amount of cobalt in the substrate.

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60~P 209~) One process for providing a cobalt enriched zone involves the addition of various compounds into the original cemented carbide po~der mixture prior to-its pressing and sintering which react to form a surface layer of tungsten carbide (WC) and cobalt (Co), and an inner hard phase region containing a portion defined as a B-1 type, solid solution hard phase, usually having a ~ face centered cubic structure of the carbides of IV-a to - VI-a group transition metals in the Periodic Table, such as (Ti,W) (C,N), in addition to the WC and Co. See U.S.
Patents 41501g5 Tobioka and ~277283 Tobioka. Forj example, Ti(CN) as a solid solution is substituted for the TiC in the cemented carbide formulation. When sintered in a vacuum such materials yield a surface zone depleted in the so called ~-1 cubic phase, and conse-quently enriched in cobalt and WC. The mechanism is believed to be a decomposition of the B-1 solid solution phase containing Ti(CN) in vacuum to form titanium which is soluDle in the liquid cobalt and is transported to the interior of the substrate.
In the improved process of the present inven-tion, the insert is treated with nitrogen during its manufacturing sintering operation so that the (W, Ti) C
which it contains is nitrided. Nitrogen gas is injected into ~he sintering furnace during the heating part of the sintering cycle, in particular during a holding period of from about 20 to about 180 minutes at a temperature of approximately 1200 to 1300 C. Higher temperature holds in nitrogen may be included subsequent to the initial 1200 C to 1300 C hold. The insert is subjected to vacuum conditions during this sintering process after nitrogen injection to promote diffusion of nitrogen out .
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~25~ 60 MP 2099 of the part, thus inducing a nitrogen gradient which sets up the cobalt enriched zone. Varying the conditior.s of nitrogen pressure, hold temperature, and hold time will affect the depth of the resulting B-1 phase depletion as well as the degree and depth of the cobalt enrichment.
Zones up to 40 microns deep and cobalt enrichment to a - level of about 15% (in a 6% nominal Co composition) have been produced. Following are specific examples of the processes of this invention.

Example I
A pressed powder composite or insert composed of 83.0% WC, 6% TaC, 6% Co, and 5.0% (Wo.s Tio 5) C by ~eight was placed in a vacuum-sintering furnace on a carbon coated graphite shelf. The part was heated in the conventional manner to remove wax and then heated to 1260C. While it was being held at 1260~ C, nitrogen gas was introduced at the rate of 3 liters/
m-nute to a pressure of 600 Torr. A~ter 45 minutes of this treatment, the nitrogen was evacuated and the furn-ace temperature was raised to 1445 C for 100 minutes for sintering. Argon at a pressure of 2 Torr was injected to moderate cobalt loss whlle still allowing nitrogen dif-fusion out of the insert. The inserts were then allowed to cool at the natural cooling rate (20-30 degrees/
minute~. The micrograph of FIG. 1 of the resultlng surface region shows a 30-micron deep B-1 phase depleted layer with an increased cobalt concentration. FIG. 2 shows a plot of cobalt and titanium content versus depth below the surface as measured in a scanning electron microscope with energy-dispersive X-ray analysis. The cobalt is enriched to a peak level of 10% in the region where the titanium (~-~ phase) is depleted.

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~5~7`4 ~ample II
A pressed powder composite or insert of the same composition as Example I was placed in a vacuum--sintering furnace on a graphite shelf. The part was heated in the conventional manner to remove wax. After dewaxing, nitrogen gas was introduced at 45~ C at the rate of 3 liters/minute to a pressure of 20 Torr. The temperature was raised to 1260 C, held 45 minutes, and then raised to 1480 for 45 minutes. The nitrogen gas was then evacua~ed and then backfilled with argon to a pressure of 2 Torr. The temperature was dropped to 1445 C and held 45 minutes. The inserts were then allowed to cool at the natural cooling rate (20-30 degrees/minute).
The resulting surface structure showed a 25-micron deep B-1 phase depleted layer with an increased cobalt concen - tration having a peak level of 14.7~.

Example III
A pressed powder composite or insert composed of 64~ WC, 16.0% Wo.sTio.sC, 11.5% TaC, and ~.5% Co was placed in a vacuum-sintering furnace on a carbon-coated graphite shelf. The part was heated in the conventional manner to remove wax. Nitrogen was intro-duced to a pressure of 600 Torr at 450 C after the wax was removed, and then the part was heated to 1260 C and ~5 held at this temperature for 45 minutes. The temperature was then raised to 1480 C and held for 4~ minutes. The nitrogen was evacuated and the temperature was reduced to 1445 C. At this temperature, argon was introduced to a pressure of 2 Torr to moderate cobalt loss, and the temp-erature was held for 45 minutes. The inserts were then allowed to cool at the natural cooling rate. As seen in FIG. 3, the resulting surface region showed a 15-micron deep B-1 phase depleted cobalt enriched zone. The plot of cobalt and titanium content in FIG. 4 shows a pea~
enrichment to a level of 21.8% cGbalt at the surface.
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~æs~ 6OMP 2099 The temperature range of nitrogen injection has been varied from 1200 C to 1480 C, but the total range may extend somewhat higher and lower. It is preferable to initially introduce nitrogen below the liquidus temp-erature ~about 1300 C) to allow the infiltration of nitrogen gas before the closing off of porosity during the early stages of sintering. Injecting nitrogen only at sintering temperatures has been shown to provide shallower zones. Longer hold times would be required for equivalent nitriding. This may be necessary for treating previously sintered and ground inserts. Increasing the second nitrogen hold temperature increases the zone depth and cobalt enrichment when nitrogen is initially intro-- duced below 13~0 C. Nitrogen pressures have been uti-lized from about 6 Torr to about 600 Torr. The nitrogen treatment "hold" time had little effect on the zone depth, bu~ increased time~ up to 90 minutes, improved the cobalt enrichment. On the other hand~ the length of sintering hold time more than 45 minutes had little effect on cobalt enrichment, but increasing time increased the zone depth. The carbon content of the com-position has an e~fect on zone depth and cobalt enrich-ment reaching a maximum with increasing amounts of car-bon, and then falling off. Too much carbon (when present in levels that produce nodular carbon instead of flake carbon) may inhibit zone formation altogether.
One advantage of this invention is that the enriched zone is produced in the sintering process alone, with separate control o~er the B-1 phase depletion depth and cobalt enrichment. Also, the nitrogen treatment method can avoid the formation of a pure cobalt surface layer which interferes with adhesion of subsequently , , .

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deposited coatings. In this invention there is no pool of cobalt on the outer surface or large areas of essentially cobalt. The cobalt distribution, as shown in the photomicrograph of FIG. 1, is essentially the same at and just below the outer surface. The surface is as smooth and uniform as that prepared by conventional sintering techniquas and fits well with current sintering practice.
The use of a cobalt-enriched substrate facilitates the use of certain multilayer coatings. These multilayer coated inserts include a substrate having one or more TiN, TiC, and TiN or A1~03, layered coatings thereon in various combinations or gradations. One specific improved insert is the cobalt enriched substrate of this invention coated in series with TiN, TiC, and a final TiN layer on a layer of aluminum oxide, A1203.
In such combination the most essential layer is the TiC
layer. It is the TiC layer which is the layer which does most of the work involved. It is the hardest wear-resistant layer and has been known to be the essential layer in the cutting tool insert art. It follows, therefore, that it is desirable for the TiC layer to be as thick as possible, commensurate with the structural integrity of the substrate. In this invention, a structural improvement is first achieved by the cobalt-enriched zone. The cobalt enriched zone of an insertproduced by this invention may be gainfully employed to support various multilayer coatings One example is the coatings disclosed the above-mentioned U.S. Patent 4,497,874 - Hale. In the Hale patent a first layer of TiN
is vapor-deposited on the cobalt-enriched surface for an improved correlation with the enriched zone and a subsequent layer of TiC. Because of the structural and bonding integrity of the TiN/cobalt zone relationship, a much thicker TiC working layer can be employed.

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~ 60MP 2099 Although the present invention has been des-cribed with reference to the foregoing specification, many modifications, combinations and variations of the invention will be apparent t:o those skilled in the art in light of the above teachings. The practice of the inven-tion is amenable for use with other carbide materials and other binder materials, iron and nickel as examples of other binders. It is therefore understood that changes may be made to the particular embodiments of the inven-tion, which are within the full intended scope of the invention as defined by the following claims.

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Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for providing a cobalt cemented carbide article with a cobalt-enriched zone comprising, in sequence;
(a) heating a compressed composite mixture consisting essentially of tungsten carbide, cobalt and B-1 solid solution phase forming, carbide constituents in a furnace to provide a composite heated below the sintering temperature;
(b) contacting said composite heated at a temperature in the range of 1200°C. to 1500°C. with nitrogen gas to nitride the B-1 phase; and (c) subjecting the composite having the nitrided B-1 phase to vacuum while the composite is heated at an elevated temperature below or about the sintering temperature to produce a cobalt-enriched surface zone.

2. The process of claim 1 wherein said nitrogen gas is below atmospheric pressure.

3. The process as recited in claim 2 wherein said nitrogen is introduced over an extended period of time, and thereafter the temperature of the composite is raised or lowered to its sintering temperature above about 1400°C. in a vacuum.

4. The composite made by the process of claim 3 which thereafter is coated with a hard, wear-resistant material.

5. The composite of claim 4 wherein the hard, wear-resistant material is a layer of TiN.

6. The composite as recited in claim 5 wherein a layer of tiC is deposited on the layer of TiN and a layer of Al2O3 is deposited on the layer of TiC.