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Ep 0698002 B1 Patentamt Europaisches ||| || 1 1| || || || || || || ||| || ||| || (19) J European Patent Office Office europeen des brevets (11) EP 0 698 002 B1 (12) EUROPEAN PATENT SPECIFICATION (45) Date of publicationation and mention (51) |nt. CI.6: C04B 35/56 of the grant of the patent: 05.11.1997 Bulletin 1997/45 (86) International application number: PCT/US94/04780 (21) Application number: 94923870.3 (87) International publication number: (22) Date of filing: 26.04.1994 WO 94/2541 2 (1 0.1 1 .1 994 Gazette 1 994/25) (54) DENSIFIED MICROGRAIN REFRACTORY METAL OR SOLID SOLUTION (MIXED METAL) CARBIDE CERAMICS VERDICHTETES FEINSTKORNIGES FEUERFESTES METALLCARBID ODER CARBIDKERAMIK AUS FESTER LOSUNG (MISCHMETALL) MATIERES CERAMIQUES DENSIFIEES A MICROGRAMS, A BASE DE CARBURE EN SOLUTION SOLIDE (METALLIQUE MIXTE) OU METALLIQUE REFRACTAIRE (84) Designated Contracting States: • DUNMEAD, Stephen, D. DE FRGBITNLSE Midland, Ml 48642 (US) • NILSSON, Robert, T. (30) Priority: 30.04.1993 US 56142 Coleman, Ml 48618 (US) (43) Date of publication of application: (74) Representative: 28.02.1996 Bulletin 1996/09 Burford, Anthony Frederick WH. Beck, Greener & Co. (73) Proprietor: 7 Stone Buildings THE DOW CHEMICAL COMPANY Lincoln's Inn Midland, Michigan 48640 (US) London WC2A 3SZ (GB) (72) Inventors: (56) References cited: • DUBENSKY, Ellen, M. EP-A- 0 360 567 US-A-I 4 945 073 Midland, Ml 48642 (US) US-A- 5 089 447 US-A-I 5 215 945 CO CM O o CO <7> Note: Within nine months from the publication of the mention of the grant of the European patent, give CO any person may notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in o a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. Q_ 99(1) European Patent Convention). LU Printed by Rank Xerox (UK) Business Services 2.14.20/3.4 EP 0 698 002 B1 Description The invention relates generally to densif ied ceramic bodies prepared from refractory metal carbides, solid solution (mixed metal) carbides or mixtures thereof. The invention relates more particularly to densif ied bodies having an aver- 5 age grain size of less than 1.1 micrometers (urn). 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 (urn). Early work with WC focused upon densifying WC by heating to a temperature of, for example, 2000° Centigrade (°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. w Efforts to overcome or offset some of the brittleness led to incorporation of an amount of a metal by admixing pow- dered 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. 15 Cutler (US-A-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 urn, preferably less than 10 urn and more preferably less than 5 urn. A grain size range of 0.1 to 5.0 urn 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 urn, preferably less than 5 urn. Grain sizes between 5 and 15 urn provide high toughness and grain sizes between 1 and 3 urn yield higher strength and lower 20 toughness. As the grain size increases, the fracture mode changes from intergranular to transgranular fracture. Maruyama et al. (US-A-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 25 decrease in fracture toughness as grain size decreases. US-A-4,945,073 and EP-A-0360567 disclose high hardness, wear resistant densified ceramic bodies formed by incomplete reaction between a carbide of a first metal, a source of a second metal and optionally carbon. The body con- tains at least one metal carbide and at least one mixed metal carbide. According to the present invention, there is provided a densified ceramic body consisting essentially of only one 30 ceramic material selected from refractory metal carbides and solid solution (mixed metal) carbides and containing less than 1 percent by weight of materials other than oxygen, 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, the body being a pressure densified body having a Palmqvist toughness of at least 14 kg/mm and a Vickers hardness of at least 1800 kg/mm2 or a sintered body having a Palmqvist toughness of at least 24 kg/mm 35 and a Vickers hardness of at least 1 700 kg/mm2. 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 urn, the body having a density of at least 98 percent of its theoretical den- sity 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|C). 40 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 urn, the body having a density of at least 98 percent of its theoretical den- sity, a void volume of less than 2 percent, based upon total body volume, a toughness (K|C) that increases as grain size decreases and is greater than 5.0 MPa • m1/2, a Vickers hardness that increases as grain size decreases and is greater than 2000 kg/mm2, and a fracture mode that displays an increasing percentage of transgranular fracture as grain size 45 decreases. Another aspect of the invention is a densified ceramic body consisting essentially of only 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. so Tungsten carbide ceramics of the invention can be tailored for use in particular applications by an appropriate choice of starting WC powder size and by controlling densif ication conditions to minimize grain growth. Desirable start- ing powder sizes fall within a range of from greater than 0.0 urn up to 1 .1 urn. The range is preferably from 0.1 to 0.6 urn, more preferably from 0.1 to 0.4 urn. Starting powder sizes of less than 0.1 urn should provide densified bodies hav- ing excellent properties. Such powders, however, may be more difficult to process than powders within a range of 0.1 55 to 1 . 1 urn. In some applications, the resultant properties may be sufficiently desirable to outweigh any such processing difficulties. Tungsten carbide powders having an average particle size of less than or equal to 1 .1 urn are commercially avail- able. One such powder, Syl-Carb™ Type SC104 (GTE Sylvania), has a nominal average particle size of 0.6 urn and includes a small amount of vanadium carbide as a grain growth inhibitor. Attriting such a powder simultaneously 2 EP 0 698 002 B1 reduces the average particle size, reduces grain size distribution, and more uniformly disperses the grain growth inhib- itor. Even in the absence of a grain growth inhibitor, attrition provides the benefits of smaller average particle size and a narrower particle size distribution. As an alternative, the WC powder may have these characteristics as synthesized. As a further alternative, powders with even larger average particle sizes may be used provided they are milled or attrited 5 under conditions sufficient to reduce the average particle size to less than or equal to 1 .1 urn. These powders neces- sarily require longer size reduction procedures and may, as a consequence, pick up additional quantities of impurities from media used to promote size reduction. WC powders need not be 100% pure. In other words, they may contain very small amounts, normally less than 1 wt% based on total powder weight, of other materials so long as the other materials do not interfere with densif ication 10 of the powder or adversely affect physical properties of resultant densified bodies. Examples of "other materials" include cobalt, iron, nickel, carbon and silicon. The other materials may, for example, be present as a result of powder synthesis procedures or as residue from milling operations. In addition to the other materials, the WC powders have an oxygen content that varies inversely with particle size.
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