Process for producing a coated hard-metal cutting body

Details Process for producing a coated hard-metal cutting body Process for producing a coated hard-metal cutting body

1993-05-07

1995-04-04

Padgett, Marianne

Coating processes

Direct application of electrical, magnetic, wave, or...

Plasma

427577, 4272551, 427228, 4273766, 4273744, 4273743, 419 14, B05D 306, B05D 302, C23C 1600, C22F 110

Patent

active

054036283

DESCRIPTION:

BRIEF SUMMARY
CROSS REFERENCE TO RELATED APPLICATIONS

This is a national phase application corresponding to PCT/DE 91/00913, filed Nov. 14, 1991 and based, in turn, upon a German application filed as P 4037480.7 on Nov. 24, 1990.


FIELD OF THE INVENTION

The present invention relates to a process for producing a cutting body from a hard metal base body which is coated with one or more thin binder-metal-free hard material layers, whereby a WC, TiC, TaC and/or NbC with Co as binder metal, containing powder, is granulated with a pressing auxiliary agent and is pressed to a pressed body, the pressed body is under vacuum until complete elimination of the pressing auxiliary agent, and thereafter the pressed body under a nitrogen free protective gas is sintered and is coated by means of a CVD (Chemical Vapor Deposition) or PVD (Plasma Vapor Deposition) process.


BACKGROUND OF THE INVENTION

In the publication "Kieffer and Benesovsky", HARDMETALS, Springer-Verlag, Vienna-New York 1965, pages 202 to 216, WC--TaC (NbC)--Co hard metals and WC--TiC--TaC (NbC)--Co hard metals are indicated to be especially suitable for the machining of steel workpieces. These alloys are composed in the broadest ranges of 35 to 80% WC, 5 to 45% TaC, 0.5 to 30% TiC and 1 to 30% binder (iron, cobalt, nickel) and have a greater ductility than pure WC--TiC--Co alloys and greater cutting resistance. Such hard metals are useful as hard metal basis bodies which are to be coated with titanium carbide, titanium nitride and/or aluminum oxide (compare DE-B-22 63 210, DE-C-22 537 45). The coatings are applied according to the state of the art, for example, by the so-called CVD (Chemical Vapor Deposition) process to increase the wear resistance of the cutting body.
Nevertheless, the known coatings have a detrimental effect upon the ductility, i.e. the brittleness of the cutting body is increased. The basis for this (increase in brittleness) are tensile stresses which develop after cooling of the hard material layer applied at about 1000.degree. C. by CVD and leading to cracks in the coating.
With higher mechanical stresses, therefore, there is the danger that cracks in the surface layer will propagate in the hard metal base body and finally give rise to breakout at the cutting edge. To avoid or to limit this disadvantageous effect, it has already been proposed to provide the hard metal base body with a ductility-increasing boundary zone. This zone close to the surface has a low content of cubic mixed carbides and carbonitrides or is free from cubic mixed carbides and carbonitrides and is enriched with such the binder metal as cobalt. As a consequence, the boundary zone has a greater ductility than the mixed carbide containing hard metal base body. The propagation of cracks from the coating into the hard metal is therefore retarded.
From DE 32 11 047 A1 and U.S. Pat. No. 4,548,786, basically two processes for producing the boundary zones enriched with cobalt and low in mixed carbides are known.
In the first process, nitrogen containing such compounds as, for example, titanium nitride or carbonitride are added to the mixture forming the hard metal in given quantities (compare DE 32 11 047 A1). With vacuum sintering of such mixtures at high temperatures between 1300.degree. and 1500.degree. C. diffusion processes take place which effect the described boundary zone modification.
In a further proposal (compare U.S. Pat. No. 4,548,786), one starts with a conventional mixture of hexagonal tungsten carbide, cubic carbides and cobalt and subjects and the mixture to the compacting sintering below the melting temperature of the binder metal at about 1250.degree. C. to a treatment in nitrogen gas. The nitriding is so carried out that sufficient nitrogen is taken up in a short time by the cubic carbides to partially convert the cubic carbides into carbonitrides. In the subsequent sintering at a higher temperature in vacuum, the desired boundary zone effect is achieved as in the previously described process, whereby the boundary zone thickness lies between 10 to 50 .mu.m.
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REFERENCES:
patent: 4018631 (1977-04-01), Hale
patent: 4374685 (1983-02-01), Suzuki et al.
patent: 4399168 (1983-08-01), Kullander et al.
patent: 4478888 (1984-10-01), Benjamin et al.
patent: 4729905 (1988-03-01), Zhed et al.
patent: 5073411 (1991-12-01), Hale
patent: 5209945 (1993-05-01), Weber
Patent abstracts of Japan, vol. 10, #345 (C-386) of JP,A, 61,-147,823 to Mitsubishi Metal Corp., Tanase Teruyoshi, "Prod. of N-containing high sintered hard Alloy", Jul. 1986.
Patent abs. of Jap., vol. 7, #155 (C-175) of JP,A, 58-067,859 to Hitachi, KK, Yuusuke Iyori, "Coated Sintered hard Alloy Prep Thereof" Apr. 1983.
Pat abs. of Jap., vol. 6, #68 (C-100) of JP,A, 57-005,860 to Nippon Tokushi Togyo, KK, Suzuki, "Prep. of Coating Tip for Cutting", Jan. 1982.

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