Densified micrograin refractory metal or solid solution solution

Details Densified micrograin refractory metal or solid solution solution Densified micrograin refractory metal or solid solution solution

1995-09-22

1996-10-08

Group, Karl

Compositions: ceramic

Ceramic compositions

Carbide or oxycarbide containing

C04B 3556

Patent

active

055631070

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The invention relates generally to densified ceramic bodies prepared from refractory metal carbides, solid solution (mixed metal) carbides or mixtures thereof. The invention relates more particularly to densified bodies having an average grain size of less than 1.1 micrometers (.mu.m). It relates still more particularly to densified, polycrystalline tungsten carbide (WC) bodies prepared from WC powder having an average grain size of less than 1.1 micrometer (.mu.m).


BACKGROUND OF THE INVENTION

Early work with WC focused upon densifying WC by heating to a temperature of, for example, 2000.degree. Centigrade (.degree.C). The densified material was judged unsuitable for use in applications requiring toughness, such as in cutting tools. The unsuitability stemmed largely from the densified material's excessively brittle character.
Efforts to overcome or offset some of the brittleness led to incorporation of an amount of a metal by admixing powdered metal and WC powder to form a composite and densifying the composite at a temperature above that at which the metal melts. The metal, most frequently an iron group metal (iron, cobalt or nickel), was added to impart some of its ductility to the composite. The densified composites, also known as cemented carbides, cermets and hard metals, have been used extensively for several decades.
Cutler (U.S. Pat. No. 4,828,584) discloses ceramic bodies that are at least 98.5% by volume WC with substantially all grains having an average size of less than 15 .mu.m, preferably less than 10 .mu.m and more preferably less than 5 .mu.m. A grain size range of 0.1 to 5.0 .mu.m is reportedly quite useful. Cutler also discloses preparation of the ceramic bodies by sintering greenware made from WC particles having a diameter of less than 15 .mu.m, preferably less than 5 .mu.m. Grain sizes between 5 and 15 .mu.m provide high toughness and grain sizes between 1 and 3 .mu.m yield higher strength and lower toughness. As the grain size increases, the fracture mode changes from intergranular to transgranular fracture.
Maruyama et al. (U.S. Pat. No. 4,753,678) disclose cemented carbides based upon WC and either vanadium carbide or zirconium nitride as a hard phase and 4 to 20% by weight (wt %) of cobalt as a metal or binder phase.
Eric A. Almond et al., in "Some Characteristics of Very-Fine Grained Hardmetals", Metal Powder Report, Vol 42, No. 7/8, pages 512, 514 and 515 (July/August 1987) teach that binder-phase hard metals experience an asymptotic decrease in fracture toughness as grain size decreases.


SUMMARY OF THE INVENTION

In one aspect, the invention is a densified ceramic body consisting essentially of polycrystalline tungsten carbide having an average grain size of less than 1.1 .mu.m, the body having a density of at least 98 percent of its theoretical density and a void volume of less than 2 percent, based upon total body volume and, as grain size decreases, a concurrent increase in Vickers hardness and toughness (K.sub.IC).
In a related aspect, the present invention is a densified ceramic body consisting essentially of polycrystalline WC having an average grain size of less than 1.1 .mu.m, the body having a density of at least 98 percent of its theoretical density, a void volume of less than 2 percent, based upon total body volume, a toughness (K.sub.IC) that increases as grain size decreases and is greater than 5.0 MPa.m.sup. 1/2, a Vickers hardness that increases as grain size decreases and is greater than 2000 kg/mm.sup.2, and a fracture mode that displays an increasing percentage of transgranular fracture as grain size decreases.
Another aspect of the invention is a densified ceramic body consisting essentially of at least one ceramic material selected from refractory metal carbides and solid solution (mixed metal) carbides, the body having an average grain size within a range of from greater than 0.0 to less than 1.1 micrometers, a density of at least 98 percent of theoretical density and a void volume of less than 2 percent, based upon total body volume.



REFERENCES:
patent: 4744943 (1988-05-01), Timm
patent: 4753678 (1988-06-01), Maruyama et al.
patent: 4828584 (1989-05-01), Cutler
patent: 4945073 (1990-07-01), Dubensky et al.
patent: 5089447 (1992-02-01), Timm et al.
patent: 5215945 (1993-06-01), Dubensky et al.
A. G. Evans, "Impact Damage Mechanics: Solid Projectiles", Treatise on Materials Science and Technology, vol. 16, pp. 63-65 (1979) No Month.
J. K. Lancaster, "Abrasive Wear", Tribology in Particulate Technology, p. 329 (1987) No Month.
E. A. Almond et al., "Some Characteristics of Very-Fine-Grained Hardmetals", Metal Powder Report, Jul./Aug. 1987, vol. 42, No. 7/8.
Junichi Hojo et al., "The Sinterability of Ultrafine WC Powders Obtained by a CVD Method", Journal of Materials Science, (1980), pp. 2335-2344 No Month.

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