TECHNICAL FIELD The invention herein relates to ceramic bodies. More particularly it relates to ceramic bodies reinforced by single crystal or monocrystal- line silicon carbide whiskers, BACKGROUND ART The desirable properties of ceramic bodies, including strength, low porosity and heat resistance, have been known for some time to make ceramics highly valuable in many industrial applications. These pro¬ perties should serve to make the ceramics directly competitive with metals in many such applications. It has been found, however, that the ceramics' lack of "toughness" often prevents them from successfully competing with metals. Toughness, for the purposes of this invention, refers to the ability of a body to ^" resist crack propagation through the body; the tougher a materia*!, the more it is able to slow the rate of crack propagation through it. Materials which are brittle possess little toughness, and cracks which are started in such materials propagate rapidly so that such materials can fracture catastro- phically.

It has been known that the degree of tough¬ ness of a body can be significantly increased by incorporation of fibrous reinforcement into the body. if the material of the body matrix and the fibers are sufficiently bound together, the fibers will

hold the matrix pieces together.and prevent a crack from propagating beyond the location of the rein¬ forcement. Such types of reinforcements have been used in coarse-grained materials such as cements and bricks for many years.

The process of forming intricately shaped dense ceramic bodies by sintering of ceramic powders is well known. In sintering the mass of powder is molded into the desired shape and heated to an ele¬ vated temperature, usually 70-90% of the melting point of the material. The body is held at that temperature while the individual particles fuse together into a coherent unitary body. In hot pressing and hot isostatic pressing, the shaped powders are subjected to high pressure, either in a press mold or by a surrounding gas, while at tempera¬ ture. In "pressureless" sintering, the powder mass is first consolidated at near ambient temperature. The molded body is then separately heated at ambient or moderate positive pressure or' under vacuum. From a commercial and industrial point of view, the pres¬ sureless sintering operation is much more desirable since it eliminates the necessity of maintaining high temperatures and high pressures simultaneously. The operator can use common low temperature molding equipment and the heating can be conducted in open- ended continuous furnaces.

In the past it has been found possible to make reinforced ceramic composites from ceramic powders (such as alumina powder) and silicon carbide whiskers by hot pressing or hot isostatic pressing. See, for example Wei U.S. Patent 4,543,345. Such composites exhibit both high strength and high tough- ness. However, efforts to form such composites by

the more desirable pressureless sintering method have been unsuccessful. It would therefore be desirable to have a process which would permit silicon carbide whisker reinforced ceramic composites to be formed by pressureless sintering. DISCLOSURE OF THE INVENTION

We have now discovered that high strength, high toughness whisker-reinforced ceramic composites can be formed by pressureless sintering, if the whisker content is maintained in the range of 0.5-21.0% by volume of the composite and the fiber/ ceramic powder blend is first molded to a density of 55-70% of the maximum theoretical density of the final body. (All compositional percentages are volume percents unless otherwise noted. )

In its broadest form, therefore, this invention is a method for the formation of high density reinforced ceramic bodies which comprises (a) forming* a mixture containing 0.5-21.0 volume percent silicon carbide whiskers' and the balance ceramic matrix powder; (b) compacting the mixture to form a shaped body having a density of 55-70% of the theoretical maximum density of the body; and (c) heating the body at a sintering temperature of 70-90% of the melting temperature of the ceramic matrix for a period of time sufficient to sinter the powder and whisker mixture into a high-strength, high-toughness, monolithic, shaped ceramic composite having a density of at least 85% of the theoretical maximum density of the body.'

The invention also comprises the high den¬ sity, tough reinforced ceramic composite bodies formed by the method of this invention.

MODES FOR CARRYING OUT THE INVENTION

The ceramic matrix materials of the present invention can be any type of ceramic which is capable of being sintered to form coherent bodies. These may include oxides, carbides, borides and nitrides. Typi¬ cal examples include alumina, silica, aluminum sili¬ cates, silicon nitride, aluminum nitride, titanium diboride, zirconia and titanium carbide. Of these the ceramic of most interest because of its wide variety of applications is alumina. The ceramic mixture may also contain small amounts of other ceramic or doping materials to modify or enhance sin- terability or physical properties; typical of such additive materials are yttria and magnesia.

The ceramic matrix material is used in finely divided powdered form. The particle sizes involved will be dependent on the specific ceramic being used and the desired density of the ultimate product, as well as on the degree of reinforcement to be obtained from the Whiskers. Normally, the higher-density finished products are obtained from ceramic powders having the smaller particles sizes. Particle sizes may range from as high as approxi- mately 3 mm (6 mesh) down to approximately 0.01 urn in size. Most frequently the particle sizes will be in the range of 0.1-100 um.

The silicon carbide whiskers used as rein¬ forcement in the present invention are high strength materials which are usually formed by the high temperature reaction of silica and a carbonaceous material. The whiskers commonly have lengths in the range from about 10-100 um and average diameters on the order of 1 um or less. The crystalline structure is normally alpha or beta silicon carbide. The

fibers are 98-99% silicon carbide whisker with the impurities being a variety of metals, primarily calcium, manganese and aluminum. Silicon carbide whiskers and methods for their formation are shown in Handbook of Fillers and Reinforcements for Plastics, Chapter 25, pages 446-464 ("Whiskers", by J. V. Milewski and H. S. Katz) (Von Nostrand Reinhold Co., N.Y. 1978). Particularly preferred are the whiskers manufactured by the Advanced

Materials Group of ARCO Chemical Company, Silag, now an operating unit of Atlantic Richfield Company, a whisker source mentioned on page 447 of that reference. These silicon carbide whiskers are single crystal or monocrystalline materials manufactured from rice hulls. The silicon carbide whiskers typically have average diameters on the order 0.6 um and aspect ratios on the order of 15-150. Strengths are typically on the order of 1 million psi (70,000 ^' kg/cn_2) and tensile moduli .on the order of 60-100. million psi (4-7 million kg/cπ.2). The silicon carbide whiskers are thermally stable to 3200°F (1760°C).

Short fiber materials of the polycrystal- line type are to be distinguished from the single crystal whiskers used in this invention. The poly- crystalline filaments or chopped fibers are much larger in diameter e.g., 10 microns or larger. As taught in the Wei patent referred to above, the polycrystalline fibers

"suffer considerable degradation due to grain growth at temperatures above about 1250°C which severely limited their use in high temperature fabrication processes such as hot pressing for producing ceramic

composites of nearly theoretical density. Further, during high pressure loadings such as encountered during hot pressing, the polycrystalline fibers undergo fragmen¬ tation which detracts from the reinforcing properties of the fibers in the ceramic composite. Also, these polycrystalline fibers provided insufficient resistance to cracking of the ceramic composite since the fibers extending across the crack line or fracture plane possess insufficient tensile strength to inhibit crack growth through the composite especially after the composite has been fabricated by being exposed to elevated pressures and tempera¬ tures in hot pressing." Also see Milewski, J. V. "Short-Fiber Reinforcements: Where The Action Is", Plastics Compounding, November/ December 1979, pages 17-37. A clear distinction -is drawn between single crystal "whiskers" and polycry¬ stalline "Microfibers" on pages 17-19.

In the present invention the ceramic powder and the silicon carbide whiskers are mechanically blended to produce a thorough mixture of the fibrous and particulate components. If desired, conventional organic forming binders may be added to the mixture. There are a number of commercial mixing devices which will provide adequate blending of these compo- nents. These devices are well described in engi¬ neering handbooks and textbooks on mixing of solids. The whiskers component of the mixture will comprise 0.5-21%, preferably 2-18% of the blend. Composites containing less than 0.5% of fibers do not have a sufficient whisker content to provide

significant reinforcement and improvement of proper¬ ties over the unreinforced ceramics. Composities containing more than 21% whiskers cannot be suffi- ciently densified upon pressureless sintering to provide high strength composites.

Following blending of the components, the blended mass of particles and whiskers is densified (consolidated) in a mold or by isostatic pressing to form a shaped body having a density in the range of about 55-70%, preferably about 55-65%, and more preferably about 58-62%, of the theoretical density of the final sintered body. This densification is critical to the success of the subsequent sintering operation. If the components are not densified to this level prior to sintering the high density rein¬ forced product cannot be obtained from the subsequent sintering operation. It is preferred to densify to less than the 70% upper limit in order to avoid practical problems of binder outgasing which may be encountered at the higher levels.

The densification is performed by conven¬ tional techniques such as extrusion, injection- molding, slip casting, cold pressing or cold isostatic techniques. Mold pressures are generally in the range of from about 10-50 tons/in ² (1400-7000 kg/cm ² ), although pressures may be greater or lesser depending on the particular molding technique used and the desired shape of the end product. Generally speaking, ambient temperatures are appropriate and preferred for carrying out this initial densification. In cases where procedures such as extrusion and injec¬ tion molding are used, mild heating sufficient to soften organic binder materials, e.g. to about 300°C, can be employed. After consolidating the components

to the 55-70% of theoretical density and obtaining the "green" shape, the green body is sintered at a temperature in the range of 70-90% of the melting temperature of the matrix materials. This will normally be in the range of 1500-3200°F (800-1750°C) . More refractory materials will require sintering at higher temperatures. The green bodies are usually maintained at sintering temperature for a period of from 15 minutes to 2 hours.

The sintering is conducted in an atmosphere which will not adversely react with the component particles and whiskers. Because of the small particle and whisker size, the green bodies contain very large surface areas. Such large surface areas make oxidizable components highly reactive to oxidation in the presence of an oxidizing atmosphere at high temperatures. Consequently, the atmosphere in the sintering furnace must be of an inert or nonoxidizing - gas such as hydrogen, carbon monoxide, nitrogen or the noble gases such as argon and helium. The atmos¬ phere will normally be maintained at approximately ambient pressure. Alternatively, one can conduct the sintering under vacuum or moderate positive pressure. Vacuum sintering is normally performed at pressures on the order of 0.5 inch (12mm) of of mercury. Positive pressure sintering (distinguished from hot isostatic pressing) is normally performed at a very slight positive pressure. The pressureless sintering is carried out until the shaped body has a density which is at least about 85% of the theoretical density.

In especially advantageous practice the pressure-less sintering is carried out until the shaped body has reached closed porosity, a state

usually corresponding to a density which is at least 94% of the theoretical density. Closed porosity bodies are characterized by the substantial absence of pores or void spaces communicating with the surface - i.e., such pores or void spaces which occur in the shaped bodies do not communicate with the surface of the body but rather are contained within the interior volume of the body. It is of considerable importance that the shaped bodies which have been brought to the condi¬ tion of closed porosity as above indicated, can by containerless hot isostatic pressing procedures be brought to nearly 100% of the theoretical density thus resulting in a product which has greatly enhanced properties of strength, hardness, etc. Unless the shaped body is first brought to the closed porosity state, containerless hot isostatic pressing will not achieve the optimum benefits. ^" Thus, in an especially .advantageous prac¬ tice of the invention, appropriate amounts of the monocrystalline silicon carbide whiskers and powdered ceramic matrix material are blended to homogeneously distribute the whiskers throughout the mixture. The blended materials are cold pressed to form the green bodies which have a density which is 55-70% of the theoretical maximum. These green shapes are then subjected to pressureless sintering to the extent necessary to achieve closed porosity. This is typically achieved by raising the density of the article by sintering to 94% or higher of the theore¬ tical maximum. The closed porosity sintered articles are then subjected to containerless hot isostatic pressing to bring the density to at least about 98% of the theoretical maximum. In this way products

having outstanding properties of toughness and strength are formed while avoiding costly procedures which were heretofore deemed necessary. The hot isostatic pressing to which the closed porosity sintered composite can be subjected is preferably carried out in a gas autoclave employ¬ ing nitrogen or argon atmosphere. Since the sintered composites are characterized by closed porosity, the hot isostatic pressing is containerless - i.e., the sintered composites need not be placed in a container to accomplish the isostatic pressing. In this pres¬ sing, pressures generally ranging from about 10,000 psi up to 30,000 psi or higher are appropriate. Temperatures generally ranging from about 1500°C to about 1800°C are suitable for pressing times of about 15 minutes to about 2 hours.

* EXAMPLE 1 The following example will illustrate the present invention. Several component blends composed of finely divided commercial alumina powder- (average particle size 0.4 um) and silicon carbide whiskers (80% 10-80 um length, 0.6 um average diameter) were made up containing various ratios of the two compo- nents. The components of each sample were thoroughly blended and then cold pressed to form green bodies having densities in the range of 60-70% of theoretical density. Each green sample was then sintered at 2867°F (1575°C) for 1 hour. The following Table 1 presents the results of the experiments.

TABLE 1 .

Sample Whisker Green Density Sintered Density Number Content, % Theoretical % Theoretical

Volume %

1 6.1 68 97 2 16.1 68 97 3 12.1 61 97 4 12.1 62 97 5 18.0 65 93 6 23.7 65 79 7 29.2 65 76

It will be evident from these data that the samples with the lower whisker contents could be quite successfully sintered to high density products while the samples containing higher whisker contents were not significantly improved over the green products. The other properties of the final products, including toughness and strength, correspond in improvements to the improved densities. Thus, under the conditions of this invention pressureless sintering is found to produce highly satisfactory reinforced sintered products which have the improved toughness expected of whisker reinforced bodies.

EXAMPLE 2 Composites were formed which contained

7-1/2 volume % silicon carbide whiskers in an alumina matrix using the materials described above in Example 1; also a minor amount (5%) of organic binder (poly- vinyl alcohol and Carbowax) was included. The powders were thoroughly blended and cold pressed to form various given shapes. Shape A, a cuttingtool insert, was formed by axial pressing at 20,000 psi. Shapes B and C, nozzle and plug shapes, were formed by cold isopressing for 1 minute at 12,500 psi. In this latter procedure the blends were placed in a flexible container and isopressed in liquid as indicated at ambient temperature.

The green shapes formed as above indicated were heated in air to about 550°C to effectively remove organic binder. Shape A had a density in the range 58-62% of maximum theoretical while shapes B and C had densities in the range 58-60% of maximum theoretical.

The green shape composites were subjected to pressureless sintering at ambient pressure under

a nitrogen atmosphere ^" . Sintering was accomplished by heating the green shapes to 1595°C, maintaining the temperature for 1 hour, cooling to 1250°C, maintain- ing this temperature for 1 hour and then cooling to room temperature. In each case the density of the pressureless sintered composite was in excess of 94% of the theoretical maximum and each composite was characterized by closed porosity. The closed porosity sintered composites were subjected to containerless hot isostatic pres¬ sing in a argon gas autoclave. Over a period of 4 hours the composites were heated to 1575°C and then maintained at that temperature for 1 hour, the pres- sure being 20,000 psi. Thereafter the pressed com¬ posites were cooled over a 4 hour period.

The following Table 2 shows the change in percentage of theoretical maximum density which resulted after each of the above steps.

TABLE 2 Percentage of Maximum Theoretical Density After Pressureless Sintering After HIP*

Shape A 97.96 98. .99 Shape B 96.1 98. .9

96.0 98, .7

95.8 98. .7

95.8 98, .8

I

95.6 98. .7 I— '

Shape C 96.9 98, .6 I

96.7 98, .7

97.0 98, .7

*Hot Isostatic Press

As shown by the above, the closed porosity pressureless sintered composites are brought to well in excess of 98% of the theoretical density by hot isostatic pressing thus resulting in a composite characterized by exceedingly high strength and tough¬ ness. INDUSTRIAL APPLICABILITY

The process and products of this invention have applicability in a wide variety of industrial areas. Among these are production of sintered mate¬ rials for use as high strength cutting tools and other ceramic applications where toughness of a ceramic body or coating is of importance. It will be understood that the above description is intended to be exemplary and that there will be other embodiments which are not described but which are clearly within the scope and spirit of the present invention. The scope of the invention is therefore to be limited solely by the appended claims.