IL31047A - Metal bonded alumina-carbide compositions - Google Patents

Description

HETA1 BONDED ALUMINA-CARBIDE COMPOSITIONS E, IV DU Ρ0ΝΪ D2! H¾MOURS AND COMPJUSIY Background of the Invention This invention relates to metal-bonded ceramic cutting tools and more particularly is directed to dense compositions of alumina; titanium, zirconium or hafnium carbide; an iron group metal; and tungsten or molybdenum.

Aluminum oxide cutting tips are well known to the art and have been for a long time. Such tips possess advantages of extreme hardness and wear-resistance and superior hot-bending strength. They are resistant to erosion and show little welding or diffusion between the chips and the tip. They are also quite oxidation resistant. However, these advantages are largely offset by the brittleness and lack of toughness of ceramic tips and a marked tendency toward stress, cracks and eruptions.

Therefore, many attempts have been made to combine with alumina materials which would mitigate these disadvantages while not appreciably decreasing the advantages of pure alumina. An often employed practice is to bond the alumina with up to 0 or 40 per cent of a metal to increase the strength and toughness. Such compositions, generically called "cermets", demonstrate better thermal conductivity than pure alumina ceramics as well as increased strength. However the presence of the metal results in a marked decrease in overall wear-resistance of the cutting tip.

The addition of carbides to metal-alumina compositions was the next development, aimed at improving the wear-resistance and hardness which accompany the presence of the metal. Such compositions are disclosed generally in British Patent No. 841, 576 and with more particularity in German Patent No. 1, 072, 182 and British Patent No. 821, 596. However even those combinations of alumina, carbides and metal still suffer from - poor wear-resistance because of the metal and brittleness and poor thermal shock resistance because of the variety of thermal coefficients of expansion found in such mixed refractories.

We have discovered that the combination of four particular components in a narrow range of amounts results in an alumina tip of unusual properties. The combination of alumina, with titanium, zirconium or hafnium carbide or their mixtures; a metal of the iron group; and tungsten or molybdenum; within the proportional limits and with the structural characteristics set out below, produce a cutting tip with an unusual combination of hardness and strength and one which is very resistant to wear and thermal shock.

Summary of the Invention In summary, this invention is directed to dense compositions having an average grain size smaller than 10 microns and composed of two interpenetrating three-dimensional networks; one network consisting essentially of alumina, and the other network consisting essentially of zirconium, hafnium or titanium carbide or their mixtures and metal; the alumina being present in an amount of from 20 to 90 volume per cent; the titanium carbide being present in an amount of from 5 to 79 volume per cent; and the metal being present in an amount of from 1 to 20 volume per cent; said metal consisting of from 5 to 90 weight per cent iron, cobalt, nickel or their mixtures, and from 10 to 95 weight per cent tungsten, molybdenum or their mixtures; and with the limitation that the volume content of the carbide must not be less than that of the metal.

Surprisingly these compositions demonstrate excep-tional advantages over similar compositions consisting of closely related compounds and over compositions of these same compounds in different amounts. As a result of their exceptional properties the compositions of this invention are outstandingly useful in cutting and milling ferrous allcrys even at very high cutting speeds.

Brief Description of Drawing The figure is a graphical representation of the amounts of the components embraced within the compositional limits of this invention. The area A, outlined in the con-tinuous line is that area in which the compositional ratios are within the limits of this invention. The area B, outlined in the irregular broken line is that area in which the compositional ratios are within the preferred limits of this invention, and the area C, within the dash line is that area in which the compositional ratios are most preferred.

Description of the Invention Components The refractory compositions of this invention consist essentially of alumina; titanium, hafnium or zirconium carbide; an iron group metal; and tungsten or molybdenum, (a) alumina The alumina is present in the compositions of this invention in amounts ranging from 20 to 90 volume per cent.

The need for at least 20 per cent alumina is based on the desire to have the alumina present as a continuous phase.

Amounts of alumina below 20 per cent are less satisfactory because the continuity of the alumina phase is often appreciably interrupted. The presence of at least 20 volume per cent alumina insures continuity of the alumina phase under most ordinary conditions.

Conversely the amount of alumina is restricted to 90 volume per cent in that more alumina tends to prevent continuity of the electrically conductive phase of carbide and metal.

It is preferred to have the alumina present in the compositions of this invention in amounts ranging between 0 and 75 volume per cent and most preferably in amounts ranging between 0 and 72 volume per cent, in that such amounts virtually guarantee uninterrupted phases of alumina and the electrically conductive phase.

Alumina suitable for use in the compositions of this invention can be in many forms so long as it is finely divided. Thus it can be in the form of gamma, eta or alpha alumina or mixtures of these. Alpha alumina is a preferred starting material because it does not have as high a specific surface area as gamma or eta alumina and is likely to contain less adsorbed water which can be deleterious.

The alumina to be used should be sufficiently finely divided to produce the compositions of this invention with an average grain size of less than ten microns. A suitable starting alumina is alpha alumina with a specific surface area of more than 2 m2/g. and preferably 5 to 25 m2/g.

Alumina with an ultimate crystallite size of less than 0.5 microns, as measured by X-ray line broadening techniques, is particularly preferred. Such alumina can be obtained most simply by heating anhydrous aluminum diacetate to 1200°C. for 5 or more hours.

Representative of suitable commercially available alumina is Alcoa Superground Alumina XA-16 which is characterized by X-ray examination as alpha alumina and has a specific surface area of about 13 square meters per gram which is equivalent to (b) carbides The titanium, zirconium or hafnium carbide or their mixtures are used in the compositions of this invention in amounts ranging from 5 to 79 volume per cent. At least 5 per cent carbide must be present because the amount of carbide present must not be less than the amount of metal which is present. The maximum amount of carbide which can be present is limited to 79 volume per cent because at least 20 per cent of the composition must be alumina and at least 1 per cent of metal is required.

Preferred amounts of titanium, zirconium or hafnium carbide are from 12.6 to 58 volume per cent and the most preferred amounts are 18 to 7 volume per cent. These amounts contribute most effectively to outstanding properties such as hardness and wear resistance in the compositions of this invention.

The carbides suitable for use in the compositions of this invention are titan um, zirconium or hafnium carbide or their mixtures. These carbides can be obtained commercially or can be synthesized by methods well known to the art. The carbides should have a particle size of less than 5 microns and preferably less than 2 microns. If the starting material is appreciably larger than 5 microns in particle size it can be pre-ground to reduce its size to that which is acceptable. Of course the milling of the components of this invention, which is carried out to obtain a high degree of homogeneity, will result in some comminution of the carbide and the other starting components.

Of the carbides, titanium carbide is preferred for use in the dense compositions of this invention as it is readily available, results in compositions which have an excellent balance of physical properties, and demonstrate great effectiveness when used to cut or mill ferrous alloys. ( c ) -metals The metals employed in the compositions of this invention consist of one of the iron group metals; i.e., iron, cobalt, nickel and their mixtures; and a refractory metal; i.e., molybdenum, tungsten and their mixtures.

The metals are used in amounts such that of the total metal content, 5 to 90 weight per cent is iron, cobalt, nickel or their mixtures, and 10 to 95 per cent is tungsten, molybdenum, or their mixtures. It has been found that these ratios of iron group metal to tungsten or molybdenum result in the beneficial effects growing out of the balanced thermal coefficients of expansion. Of the iron group metals, it is preferred to use nickel, and of molybdenum and tungsten, molybdenum is preferred.

The iron group metal and molybdenum or tungsten are preferably used in amounts of 40 to 80 weight per cent of iron group metal and 20 to 60 weight per cent of tungsten or molybdenum, and most preferably in amounts of 40 to 60 weight per cent of iron group metal and 40 to 60 weight per cent of tungsten or molybdenum. Such ratios contribute exceptional toughness to the compositions of this invention without softening the compositions unduly.

The amount of metal which should be present in the compositions of this invention ranges from 1 to 20 volume per cent. At least 1 volume per cent of the metal is necessary to achieve the desired toughness in the compositions of this invention and restricting the amount to 20 volume per cent helps insure necessary hardness and wear resistance.

Preferred amounts of metal are from 2 to 20 volume per cent and it is most preferred that the metal be present in amounts of from 3 to 10 volume per cent of the compositions of this invention. These preferred amounts of metal tend to insure toughness in the compositions of this invention without excessive softness or low wear-resistance.

It should be understood that within the range of 1 to 20 volume per cent metal, consisting of 5 to 90 weight per cent iron group metal and 10 to 95 weight per cent tungsten or molybdenum, there are some combinations of metal amount and metal composition which are more preferred than others. However, generally speaking it is preferred that the higher the metal content of the composition the higher the tungsten or molybdenum content of the metal.

It is very difficult to determine the form in which the metals are present in the dense compositions of this invention. For example, it is known that tungsten or molybdenum can interact with carbides such as titanium or zirconium mono-carbide in such a way that some of the tungsten or molybdenum go into the carbide crystal lattice. It is also known that at high temperatures nickel will interact with aluminum oxide to form small amounts of nickel o ide-aluminum oxide spinel.

However for purposes of clarity and simplicity, references hereinafter to the metal content and to( iron, cobalt, nickel, tungsten and molybdenum will be understood to refer to the metallic form even though some of these may have interacted with other components. Thus the metal portion of the dense products of this invention is considered to consist of the iron, cobalt, nickel, tungsten and molybdenum present and the zirconium, hafnium and titanium present are considered to be in the form of monocarbides, with the exception that any-excess carbon which is present is presumed to be combined with tungsten or molybdenum. The aluminum which is present is considered to be in the form of aluminum oxide, AlaOs.

The metals suitable for use iii the compositions of this invention can be obtained as powders from commercial sources or can be prepared by known methods. The metal powders should have a particle size of less than 10 microns arid preferably less than 2 microns. (d) impurities The components 1 to be used in the compositions of this invention are preferabl quite pure. In particular it is desired to exclude impurities such as oxygen which would tend to have deleterious effects on the dense compositions of this invention.

On the other hand minor amounts of many impurities can be tolerated with no appreciable loss of properties.

Thus the metal can contain small amounts of other metals such as titanium, zirconium, tantalum or niobium as minor impurities, although low melting metals like lead should be excluded. Small amounts of carbides other than titanium, zirconium or hafnium carbide, such as several per cent of tungsten carbide, which is sometimes picked up in grinding, can be present. Even oxygen can be tolerated in small amounts such as occurs when titanium carbide has been exposed to air resulting in a few per cent of titanium oxy-carbide. However, after the powder components have been milled together and are in a highly reactive state, oxidation, particularly of the metals, occurs easily and should be avoided.

Structural characteristics In addition to characterizing the compositions of this invention on the basis of the components discussed above, the compositions can also be characterized on the basis of their structural characteristics, (a) interpenetrating three-dimensional networks The compositions of this invention are characterized as containing two interpenetrating three-dimensional networks: one of alumina; and one of metal bonded carbide.

While the effects of the presence of these two networks is not completely understood it is believed that they contribute substantially to the unusual properties of the compositions of this invention, resulting in compositions much stronger and more impact-resistant than conventional alumina ceramic cutting tools.

The presence of these co-continuous networks can be determined from analysis of the dense composition.

The continuity of the network of alumina can be ascertained by removing the carbide and metal by anodic etching in 10$ ammonium bifluoride solution. Such etching while not apparently affecting the appearance of the composition, removes the electrically conducting material frbm the outer portion of the composition nearest the surface and results in a nonconducting surface having an electrical resistivity of greater than 100,000 micro-ohm-centimeter. Proof of continuity of the alumina phase is the solid coherent surface that exists despite the obvious removal of the conducting materials.

A convenient method for removing all of the metal and carbides from the compositions of this invention and thus demonstrating the presence of a three-dimensional skeleton of alumina is to immerse small bars of the composition in a mixture of 25 cc of 12 per cent hydrofluoric acid and 5 cc of concentrated nitric acid. The bars are 0.070 inch by O. O70 inch by 1.00 inch in dimensions and they are left in the acid mixture for 4 hours during which the mixture is heated on a steam bath. The portion of the bar which remains after 24 hours is alumina and can be examined for continuity and strength by the usual means.

The compositions of this invention containing 40 or more volume per cent of alumina yield very strong alumina skeletons by the above method of analysis. Thus an alumina skeleton obtained from a composition of this invention which contains about 60 volume per cent alumina retains a transverse rupture strength of 15,000 psi. The presence of alumina in amounts of about >0 volume per cent tends to produce a fairly strong skeleton with transverse rupture strengths of about 1,400 psi. At about 20 volume per cent alumina there is usually a weak, but self-supporting structure, and below 20 volume per cent there is often little or no continuous skeleton of alumina. Removal of electrically conducting phases from the compositions containing less than 20 volume per cent alumina usually results in the recovery of alumina powder.

The presence of a continuous phase of the electrically conducting carbide and metal is apparent from the electrical conductivity of the hot-pressed compositions of this invention. The compositions of this invention preferably have a specific electrical resistivity of less than about 1 ohm-centimeter, more preferably less than about 25,000 micro-ohm-centimeter and most preferably less than 5, 000 micro-ohm-centimeter. The preferred compositions of this invention, in which carbide plus metal amount to 35 volume per cent or more, often have a specific electrical resistivity of less than, 1000 rniciO-ohm-centimeter. (b) thermal coefficients of expansion The compositions of this invention are also characterized as having two continuous interpenetrating networks with very similar thermal coefficients bf expansion. Generally the coefficient of expansion bf the alumina phase as well as -6 the carbide and metal phase will range between 4 x 10 and x 10 ^ inehes/inch/°F. at temperatures from room temperature up to 1000°F. .

As a result of the similarity of these thermal coefficients, cutting tips of the compositions of this invention are able to. undergo extreme temperature change with little or no thermal strain being generated within the composition. The compositions are Very resistant to thermal shock both as re- ,.. gards shattering and as regards surface heat cracking. (c) homogeneity and fine-grained structure The compositions of this invention are also charac-terized as having a fine grain size, smaller than 10 microns and preferably smaller than 5 microns in average grain diameter. Moreover the grain size is uniform and homogeneous throughout the composition and there is essentially no porosity in the dense compositions of this . invention. Distribution of the two co-continuous phases is also uniform and homogeneous, and generally speaking any area 100 microns square which is examined microscopically at 1000 X magnification will appear the same as any other area 100 microns square, within conventional statistical distribution limits.

The fine grain size of the compositions of this invention is of course at least partly responsible for the continuity of the interpenetrating phases. However, it also contributes along with the homogeneity and low porosity to the abrasion resistance of the compositions of this invention. Metal inclusions such as the carbide inclusions in cast iron abrade even the hardest of the metal-bonded, carbide cutting tools. Nevertheless the compositions of this invention are outstandingly abrasion resistant.

Preparation The preparation of the compositions of this invention is important, in that many of the characteristics of the compositions are achieved as a result of the manner in which they are prepared. Thus the use of fine-grained starting materials and thorough milling of the mixed components are directly related to the fine grain size and uniform homogeneity of the compositions. Other precautions observed in preparing the compositions of this invention which have important effects on the products are: (1) the prevention of excessive contamination from grinding media and moisture or oxygen in the air; (2) hot-pressing or sintering under conditions which permit the escape of volatile materials prior to densification (3) avoiding undue absorption of carbon from pressing molds by limiting their contact under absorption-promoting conditions; (4) avoiding excessive component recrystalliza- tion and resultant segregation by avoiding prolonged subjection to very high temperatures. (a) milling and powder recovery Milling of the components, to homogeneously intermix them and obtain very fine grain sizes, is carried out according to the practices common in the art. Optimum milling conditions will ordinarily involve a mill half-filled with a grinding medium such as cobalt bonded tungsten carbide balls or rods, a liquid medium such as a hydrocarbon oil, an inert atmosphere, grinding periods of from a few days to several weeks, and powder recovery also in an inert atmosphere. The recovered powder is ordinarily dried at temperatures of around 150-200*0. under vacuum, followed by screening and storage when desirable in an inert atmosphere. (b) consolidation The compositions of this invention are ordinarily consolidated to dense pore-free bodies by sintering under pressure. Consolidation is ordinarily carried out by hot-pressing the mixed powders in a graphite mold under vacuum.

When the powders are hot-pressed they are placed in the mold and inserted into the heated zone of the hot press without application of pressure thus allowing volatile impurities to escape before the composition is densified.

Pull pressure is usually applied at or near the maximum temperature.

Maximum temperatures range between l 00 and 1900°C. depending upon the amount of iron group metal present and will ordinarily be between l600 and l800eC. Maximum pressures range between 500 and 4000 psi with lower pressures being used usually in combination with lower temperatures for compositions with a high metal content, especially when the metal is rich in iron, cobalt, nickel, or their mixtures. Conversely, higher pressures and temperatures are employed for compositions low in metal and particularly when the metal is dominately molybdenum or tungsten.

As will be apparent, at higher temperatures and pressures some of the lower melting metal components will tend to squeeze out of the compositions during densification. This tendency can be used to advantage by starting with a little more iron group metal than is desired, and operating at a high temperature and pressure. By this procedure some of the iron group metal will be squeezed out to give the desired metal content and the molten metal that is eliminated will act as a lubricant and sintering aid during pressing. By this means voids can be eliminated in spite of the highly refractory nature of the final composition.

It is important that the composition not be heated to a temperature, or for a period of time, which is in excess of that required to eliminate porosity and achieve density. Such higher temperatures or longer times result in undesirable grain growth and a resultant coarsening of the structure, and can even result in development of secondary porosity due to recrystallization, or in the formation of undesirable phases.

As will be demonstrated hereinafter, pressing temperatures in the range of 1700 to 1900°C. are usually employed for the preferred products of this invention and maximum temperature is applied for less than >0 minutes, usually no more than 10 minutes and preferably no more than 5 minutes after which the product is removed from the hot zone. By these procedures the compositions of this invention are compacted such that porosity is eliminated and maximum density attained without undue recrystallization. Such products are characterized by their fine grain size and outstanding transverse rupture strength.

The compositions of this invention, particularly those with high metal content and small particle sizes, can also be densified by cold-pressing and sintering under high vacuum provided that the above limitation on minimum sintering time at maximum temperature is followed. It is preferred to isostatically press the powder in a sealed rubber mold suspended in water in an isostatic press capable of applying high pressures (60,000 psi) hydrdically.

Utility The compositions of this invention can be employed in a variety of types of cutting tools designed for numerous use applications. They can be molded or cut into standardized disposable inserts, suitable for turning, boring or milling. Or, they can be laminated with or otherwise bonded to metal-bonded carbides or tool steels for regrindable types of tooling. They are suitable generally for metal removal of ferrous metals including machining or cutting hardened steels, alloy steels, maraging steels, cast iron, cast steel, nickel, nickel-chromium alloys, nickel based and cobalt superalloys, as well as for cutting non-metallic materials such as fiberglass-plastic laminates and ceramic compositions.

The compositions of this invention are best suited for cutting at very high speeds such metals as alloy steels (800 surface feet per minute) and cast iron (1200 surface feet per minute). This is so because of the great resistance to cratering and edge wear and retention of good hardness of the compositions of this invention at elevated temperatures. Because of their good thermal shock resistance they are particularly well suited for making repeated short cuts or other interrupted cuts in which the temperature of the cutting edge fluctuates rapidly.

The compositions of this invention can also be used in general refractory uses such as thread guides, bearings, wear-resistant mechanical parts, and as grit in resin-bonded grinding wheels and cutoff blades. In addition the composi-tions of this invention are useful in any application where their combination of refractory properties, electrical conductivity, metallophilic nature, and thermal shock resistance offer an advantage such as in making an electrically conducting ceramic-like grit for grinding wheels to be employed in electrolytic grinding.

This invention will be better understood by reference to the following illustrative examples wherein parts and percentages are by weight unless otherwise noted.

EXAMPLE 1 This is an example of a composition containing 70 volume percent of aluminum oxide, 25 volume percent of titanium carbide and 5 volume percent of metal consisting of about equal parts by weight of molybdenum and nickel.

The alumina in the form of very finely divided alpha alumina is prepared from colloidal boehmite by heating for l8 hours in air at 350°C, then increasing the heat at 100°C. per hour to a goal temperature of 1200 °C, where it is held for 2 hours. A sample of the cooled product is then treated with hydrofluoric acid and is 88$ insoluble in 2 $ aqueous hydrofluoric acid over a period of 16 hours, indicating an alpha alumina content of 88$. The specific surface area of the HF insoluble alumina is 8.6 m2/g as measured by nitrogen adsorption using the Brunauer, Emmett, Teller method. This surface area corresponds to a crystallite size of alpha alumina of about 175 millimicrons average particle diameter. Under an electron microscope the alpha alumina appears as aggregates of alumina crystals in the range from 100 to 300 millimicrons in diameter.

The titanium carbide to be used has a nominal particle size of 2 microns and a specific surface area of 3 m2/g as determined by nitrogen adsorption. An electron micrograph shows that the titanium carbide grains are approximately 2 microns in diameter and are clustered in the form of loose aggregates. The carbon content is 19.0$ and the oxygen analysis indicates a titanium dioxide content of about 2. #.

The molybdenum powder to be used has a grain size of less than 325 mesh and a specific surface area as determined by nitrogen adsorption of 0.29 m2/g, and an average crystallite size of 35 millimicrons as determined by X-ray diffraction line- roadening. An electron micrograph shows the molybdenum powder consists of grains 1/2 to 3 microns in diameter clustered together in open aggregates. Chemical analysis of the powder reveals 0.2 percent oxygen and no other impurities in amounts over 500 pprn.

The nickel to be used is a fine powder containing 0.15$ carbon, 0.07$ oxygen, and less than 200 pprn iron.

The specific surface area of the nickel powder is 0.48 m2/g and its X-ray diffraction pattern shows only nickel, which from the line broadening has a crystallite size of 150 millimicrons. Under electron microscope, the powder appears as polycrystalllne grains 1 to 5 microns in diameter.

The powders are milled by loading 6000 grams of preconditioned cylindrical cobalt-bonded tungsten carbide inserts, 1/4 inch long and 1/4 inch in diameter, into a 1.2 liter steel rolling mill about 6 inches in diameter, also charged with 275 mis. of "Soltrol" 120 saturated parafflnic hydro- · carbon, approximate boiling point 120° centigrade. The mill is then charged with 82.6 grams of the alpha alumina, 47.0 grams of the titanium carbide powder, 7.65 grams of the molybdenum powder, and 6.68 grams of the nickel pov/der, all a3 above described.

The mill is then sealed and rotated at 90 rpm for 5 days-. The mill is then opened and the contents emptied while keeping the milling inserts inside. The mill is then rinsed out with "Soltrol" 1 0 several times until all of the milled solids are removed.

The milled powder is transferred to a vacuum evaporator, and the excess hydrocarbon 13 decanted off after the suspended material has settled. The wet residual cake is then dried under vacuum with the application of teat until the temperature within the evaporator is between 200 and 300°C, and the pressure is less than about 0.1 millimeters of mercury. Thereafter the powder is handled entirely in the absence of air.

The dry powder is passed through a 70 mesh screen in a nitrogen atmosphere, and then stored under nitrogen in sealed plastic containers. Λ consolidated billet is prepared from this powder by hot pressing the powder in a cylindrical graphite mold having a cylindrical cavity 1 inch in diameter and fitted v/ith opposing close-fitting pistons. One piston is held in place in one end of the mold cavity while 17. 5 grams of the powder is dropped into the cavity under nitrogen and evenly distributed by rotating the mold and tapping it lightly on the side. The upper piston is then put in place under hand pressure. The assembled mold and contents are then placed in a vacuum chamber of a vacuum hot press, the mold is held in a vertical position, and tho pistons extending above and below are engaged between opposing graphite rams of the press under pressure of about 100 to 200 psi. Within a period of a minute the mold is raised into the hot zone of the furnace at 1000°C. and at once the furnace temperature is increased while the positions of the rams are locked so as to prevent further movement during the heatup period. The temperature is raised from 1000 to l800°C. in 10 minutes, and the tempera-ture.of the mold is held at l800°C. for another 2 minutes to" ensure uniform heating of the sample. A pressure of 4000 psl is then applied through the pistons or four minutes.

Immediately after pressing, the mold and contents, ' still being held between the opposing rams, is moved out of the furnace into a cool zone where the mold and contents are cooled to dull red heat in about 5 minutes.

The mold and contents are then removed from the vacuum furnace and the billet is removed from the mold and sand blasted to remove any adhering carbon.

The hot pressed composition is nonporous, having no visible porosity under 1000 X magnification. Structurally the composition consists of an extremely fine co-continuous interpenetrating network of polycrystalline alpha alumina and of metal bonded titanium carbide.

The composition has a specific resistivity of about 2000 micro-ohm cm. This degree of conductivity indicates continui of the conducting components of the structure, namely, the metal and titanium carbide. Electron micrographs indicate a very fine grain structure, few grains exceeding 1 or 2 microns in size. The alumina is generally the coarsest phase.

The continuity of the alumina phase is indicated by removing the titanium carbide and metal from the composition by anodic attack for 24 hours in ammonium bifluoride solution. This leaves an electrically non-conducting porous layer on the surface, which to the eye appears to be unchanged, but under electron microscope is shown to be porous due to the removal of the electrically conducting components .

Chemical analysis shows, in addition to the alumina, titanium carbide, molybdenum and nickel, the presence of about of iron, presumably attrition from the mill, and 4$ by weight of tungsten presumably present as tungsten carbide and about 0. 5 of cobalt, both probably picked up from attrition of the milling inserts.

The billet, which i3 1 inch in diameter and about 0. ^0 inch in thickness, is cut so that a piece slightly larger than one half inch square is removed from the center. Strips 0.070 inch in .thickness are cut from the material remaining to each side of this center piece and are further cut in 0,tfr O inch x 0.70 inch square bars for testing transverse rupture strength. Other portion's of the billet are used for indentation hardness tests and for other product characterization.

The transverse rupture strength as measured by bending the 0.070 inch x 0.070 inch test bars on a 9/l6 inch span is about 150, 000 psl. The hardness is 94.0 on the Rockwell A scale.

The square centerpiece is finished as a cutting tip to exact dimensions, 1/2 x 1/2 x 3/l6 inch and the corners are finished with a l/32nd inch radius, a style known in the industry as SNG-432. This tip is employed as a single tooth in a 4 inch diameter milling cutter to face mill dry and on center class 30 (170 BHN) grey cast iron bars 2 inches wide at a surface speed of 1000 feet per minute and a feed rate of 0.0060 inch per tooth with a depth of cut ranging irregularly between 0.050 and O.15O inch and including the scale and skin from the casting proces . Milling Is continued under these conditions for 156 inches of bar length without wearing out. Upon examination the cutting tip shows only 0.010 inch of uniform flank wear and 0.015 inch of local flank wear, with no cratering on the face of the tool and no breaking or chipping of the edge.

Under these same cutting .conditions, commercially available carbide tools wear out after cutting less than 100 inches and commercially available ceramic cutting inserts break immediately.

The same Insert is also employed for single tooth face milling of 2 inch wide bars of AISI 240 steel having a hardness of 3 0 Brinnell. The milling is carried out dry and on center with a 4 inch diameter head at 1000 surface feet per minute and 0.006 inch feed per tooth, with a 0.050 inch depth of cut. Under these conditions the single tooth tool life is 100 inches of bar length.

Under these same conditions commercially available alumina cuttin tools will not cut at all and commercially available carbide tools will cut less than 40-45 inches of bar length per tooth before complete, failure.

EXAMPLE 2 The procedure of Example 1 is repeated except that the components are used in amounts to give a hot-pressed composition containing 60 volume percent alumina, ¾ volume percent titanium carbide, and 5 volume percent of metal consisting of 50 weight percent nickel and 50 weight percent molybdenum.

A cutting tip prepared as in Example 1 from this hot-pressed composition performs exceptionally well as a milling tip for metal cutting in tests similar to those of Example 1.

EXAMPLES 3-16 The following examples were carried out using the raw materials and procedures described in Example 1 except as otherwise noted. The raw materials used in the following examples, other than titanium carbide, molybdenum and nickel, are characterized as follows: alumina - Alcoa Superground Alumina XA-I6, characterized by X-ray examination as alpha alumina and has a: specific surface area of 13 square meters per gram, cobalt - Welded Carbide Tool Co., "Cobalt P", powdered, cubic form cobalt, 99*9$ pure with a surface area of 1.6 square meters per gram. hafnium carbide - Materials for Industry, fine hafnium carbide powder with a specific surface area 0.5 square meters per gram. iron - Baker and Adamson, purified, reduced, iron ball milled for three days, has a specific surface area 1.5 square meters per gram and contains 0.8$ oxygen. tungsten - General Electric Co., fine tungsten powder with a specific surface area of 2 square meters per gram and contains 0.19$ oxygen, zirconium carbide - Materials for Industry, fine zirconium carbide powder with a specific surface area of 0.5 square meters per gram and an oxygen content of 0.1& The milling conditions designated A, B and C in Table 1 correspond to the general conditions of Example 1 with the following provisions: A. 4000 grams of cobalt-bonded tungsten carbide inserts are used in a 1 . 5 liter steel mill with 575 cc of Soltrol oil.

B. 14, 000 grams of cobalt-bonded tungsten carbide inserts are used in a one gallon steel mill with 8l4 cc of Soltrol oil.

C. 6000 grams of cobalt-bonded tungsten carbide inserts are used in a 1 .5 liter steel mill with 575 cc of Soltrol oil.

The pressing cycle designated I, II and III in Table 1 corresponds to the general conditions of Example 1 with the following provisions.

I. The sample and mold are inserted into the hot zone at a temperature of 1500°C.

II. The sample and mold are inserted into the hot zone at a temperature of 1175°C.

III. The sample and mold are inserted into the hot zone at a temperature of 1000°C.

The .metal cutting tests designated 1, 2 and 5 in Table 1 correspond to the general conditions of the cutting tests in Example 1 with the following provisions: 1. High Speed Turning Test on AISI 1045 steel (Brinell Hardness Number of 185) . The speed is 900 surface feet per minute (SFM); the feed is 0.005 inches per revolution (IPR) the depth of cut is 0.050 inches; and there is negative rake. Uniform and local flank wear is measured after 10 minutes of Single Tooth Pace Milling Test on AISI J O steel (Rockwell C Hardness of 3β ) . A inch milling head is used; the work is on center; dry; speed is 535 SFM; the feed is 0.0053 IPR; the depth of cut is 0. 100, the width of cut 2 inches; and there is negative rake. Tool life is measured in length of cut in inches. High Speed Turning Test on cast iron (Brinell Hardness of 170) . The speed is 1250 SFM; the feed is 0.005 IPR, the depth of cut is 0.050 inches; and there is negative rake. Uniform and local flank wear is measured after 10 minutes of dry turning.

TABLE 1 TABLE 1

Claims (15)

CLAIMS :

1. Dense compositions having an average grain size smaller than 10 microns and composed of two interpenetrating three-dimensional networks, one network of alumina and the other network of metal and a carbide selected from the group consisting of zirconium carbide, hafnium carbide, titanium carbide and their mixtures, the composition consisting essentially of 20 to 90 volume percent alumina, 5 to 79 volume percent carbide and 1 to 20 volume percent metal, said metal consisting essentially of 5 to 90 weight percent of a metal selected from the group consisting of iron, cobalt, nickel and their mixtures and 10 to 95 weight percent of a metal selected from the group consisting of tungsten, molybdenum and their mixtures, with the limitation that the volume percent of carbide must not be less than that of the metal.

2. A dense composition of Claim 1 in which the alumina is present in an amount ranging from 40 to 75 volume percent, the carbide is present in an amount ranging from 12.6 to 58 volume percent and the metal is present in an amount ranging from 2 to 20 volume percent.

3. A dense composition of Claim 1 in which the carbide is titanium carbide.

4. A dense composition of Claim 1 in which the metal consists essentially of nickel and molybdenum.

5. A dense composition of Claim 1 in which the average grain size is smaller than 5 microns.

6. A dense composition of Claim 1 in which the metal consists essentially of 40 to 60 weight percent of a metal selected from the group consisting of iron, cobalt, nickel and their mixtures and 40 to 60 weight percent of a metal selected from the group consisting of tungsten, molybdenum and their mixtures.

7. · A hot pressed composition having an average grain size smaller than 10 microns and composed of two interpenetrating three-dimensional networks, one network of alumina, and the other network of titanium carbide and metal, the composition consisting essentially of 40 to 75 volume percent alumina, 12.6 to 8 volume percent titanium carbide and 2 to 20 volume percent of metal consisting essentially of from 5 to 90 weight percent nickel and 10 to 95 weight percent molybdenum with the limitation that the volume percent of titanium carbide must not be less than that of the metal.

8. A hot pressed composition of Claim 7 in which the alumina is present in an amount ranging from 0 to 72 volume percent .

9. · A hot pressed composition of Claim 7 in which the titanium carbide is present in an amount ranging from 18 to 4 volume percent.

10. A hot pressed composition of Claim 7 in which the metal is present in an amount ranging from 5 to 10 volume percent.

11. A hot pressed composition of Claim 7 in which the metal consists essentially of from 40 to 60 weight percent nickel and 40 to 60 weight percent molybdenum.

12. Λ hot pressed composition of Claim 7 in which the average grain size is smaller than 5 microns.

13. · A hot pressed composition of Claim 7 consisting essentially of 50 to 72 volume percent alumina, l8 to 7 volume percent titanium carbide, and 3 to 10 volume percent of metal.

14. A hot pressed composition of Claim 13 in which the metal consists essentially of from 40 to 60 weight percent nickel and from 40 to 60 weight percent molybdenum.

15. A hot pressed composition of Claim 13 in which the average grain size is smaller than 5 microns.