WO1991008992A1

WO1991008992A1 - Silicon carbide whisker reinforced ceramic composites and method for making the same

Info

Publication number: WO1991008992A1
Application number: PCT/US1990/007276
Authority: WO
Inventors: Aleksander J. Pyzik
Original assignee: The Dow Chemical Company
Priority date: 1989-12-13
Filing date: 1990-12-10
Publication date: 1991-06-27
Also published as: IL96661A0; AU7169791A; CN1053603A

Abstract

Use of a bimodal distribution of ''coarse'' and ''fine'' submicrometer, single crystal, silicon carbide whiskers to reinforce ceramic materials provides an increase in cutting performance over an equal amount of either coarse or fine whiskers. The coarse whiskers have a number average diameter within a range of from 0.5 to less than 1.0 micrometer and the fine whiskers have a number average diameter within a range of 0.1 to less than 0.5 micrometer. The bimodal distribution of whiskers constitutes 5 to 40 volume percent of a resultant whisker-reinforced ceramic composite material. The resultant materials are useful as cutting tools.

Description

SILICON CARBIDE WHISKER REINFORCED CERAMIC COMPOSITES AND METHOD FOR MAKING THE SAME

The present invention generally concerns ceramic composites reinforced by silicon carbide, whiskers. The composites have utility in applications requiring high temperature physical and chemical stability, abrasion wear resistance and resistance to brittle failure. One such application is the cutting or machining of metals.

Metal cutting or machining finds extensive application in manufacturing processes. Typical machining operations include shaping, planing, milling, facing, broaching, grinding, sawing, turning, boring, drilling and reaming. Some of these operations, e.g., sawing, act on both internal and external surfaces, whereas others act only on internal surfaces (reaming) or external surfaces (milling).

A common productivity measure for machining operations is stated in terms of total amount of metal removed from a work material per unit of time. Parameters specific to cutting tool performance include the material being cut, cutting speed, depth of cut, feed rate and tool life. Tipnis, in "Cutting Tool Wear", Wear Control Handbook, pages 891-893_? notes that wear is the preferred failure mode for cutting tools. Other failure modes such as fracture, chipping, softening or thermal cracking lead to catastrophic and erratic failures. Although wear is the preferred mode, Tipnis cautions that no predictive tool wear theories are available. As such, the practical approach involves generation and application of tool wear data to balance work material removal rates and economical tool life.

Tipnis summarizes basic requirements for a cutting tool as follows: "(a) it must be harder than the material being cut so as to resist forces generated during cutting; (b) it must be tough so as not to fracture under such forces; (c) it must withstand high temperatures generated at the tool-chip interface without deforming; and (d) it must not wear too rapidly." Tipnis suggests that" [t]he most important properties for a cutting tool material are: hot hardness (i.e., resistance to softening under temperatures generated at the cutting edge of the tool), toughness (i.e., resistance to fracture under impacts), chemical stability and reactivity (i.e., the resistance to dissociation and transformation under temperatures and pressures generated at the cutting edge), and tendency towards diffusion of elements (i.e., resistance to cratering at high cutting temperatures.)"

As used herein, the term "toughness" refers to resistance to premature failure, particularly during initiation of cutting, of a silicon carbide whisker reinforced, ceramic cutting tool. It does not necessarily equate to Kic measured at room temperature according to fracture mechanics definitions of toughness. Cutting performance does not correlate well with data supporting these definitions.

M. C. Shaw, in Metal Cutting Principles, page 334, (Oxford, 1984), highlights three criteria used in selecting materials for cutting tool applications. The criteria are (a) physical and chemical stability at use temperatures, (b) abrasion wear resistance and (c) resistance to brittle failure. Chemical instability at use temperatures can, for example, destroy tool materials quite rapidly through mechanisms such as melting, excessive diffusion, or a combination of welding and chipping. Shaw suggests, at pages 353-357, that three key physical properties be considered in selecting materials for use as cutting tools. One criterion is strength as measured in four-point- bend testing (Military Standard 1942b). The second criterion is hardness as measured by a Vickers indentor. The third criterion is resistance to fracture.

Fracture toughness, a commonly used indication of resistance to brittle failure, is measured by the Single Notch Beam Technique or the Chevron Notch Technique. Resistance to crack propagation, or Palmqvist Toughness is determined in conjunction with the hardness test.

Resistance to wear (W) is a function of hardness and toughness. E. D. Whitney, in an article entitled "Modern Ceramic Cutting Tool Materials", Powder Metallurgy International, Volume 15, Number 4, pages 201-205 (1983), suggests that tool materials should be very hard to resist the abrasive action of the material being cut. Hardness is a necessary, but not sufficient, condition for successful cutting tool performance. -it-

Satisfactory cutting tool materials must also possess sufficient toughness and strength to withstand mechanical shocks and the like which occur during cutting operations.

Wei, in U.S. Re. 32,843, discloses ceramic composites characterized by increased toughness and resistance to fracture. Wei also discloses the preparation of such composites. The composites comprise a matrix of ceramic material having homogeneously dispersed therein 5 to 60 volume percent of silicon carbide whiskers. The whiskers have a monocrystalline structure, a diameter of about 0.6 micrometers and a length of 10 to 80 micrometers. The ceramic material may be alumina, mullite or boron carbide.

Wei notes that the use of single crystal whiskers in the ceramic composite provides a significant improvement in fracture toughness over that attainable with the ceramic material without single crystal whiskers. He suggests that the whiskers are able to absorb cracking energy.

Rhodes et al., in U.S. Patent 4,789,277, disclose a method of cutting metal with a sintered composite cutting tool having a matrix consisting essentially of alumina and 2 to 40 volume percent silicon carbide whiskers distributed throughout the matrix. The whiskers have a single crystal structure,- an average diameter on the order of 0.6 micrometers and an aspect ratio on the order of 15 to 150.

What is needed is a ceramic composite material that includes ceramic whiskers such that an improved wear resistance results, for use with cutting tools, for example. Further it is desired that the improvement be more predictable than prior art techniques that simply rely upon improving mechanical properties such as toughness.

The present invention is a densified, whisker- -reinforced ceramic composite material comprising a matrix of ceramic material having homogeneously dispersed therein 5 to 40 percent by volume of a bimodal distribution of chemically compatible single crystal whiskers, the whiskers being silicon carbide, silicon nitride, titanium carbide, mullite, titanium diboride, alumina, magnesia or boron nitride, provided, however, that the whiskers are not the same as the ceramic matrix material, the bimodal distribution being based upon relative whisker volume versus whisker width and comprising a volumetric ratio of coarse single crystal whiskers to fine single crystal whiskers of 0.1 to 1, the coarse whiskers having, prior to densification, a number average diameter of greater than or equal to 0.5 but less than 1.0 micrometer and a diameter range of from about 0.1 to 3 micrometers, the fine whiskers having, prior to densification, a number average diameter of greater than or equal to 0.1 but less than 0.5 micrometer and a diameter range of 0.01 to 1.3 micrometers, the bimodal distribution providing an increase in cutting performance over an equal amount of either coarse or fine whiskers. The whiskers are desirably single crystal silicon carbide whiskers. The silicon carbide whiskers used in the present invention are single crystals containing beta and mixed alpha and beta phases of silicon carbide. The whiskers are selected from two distinct average whisker diameter ranges to provide a volume-based, bimodal size _j- distribution. Whiskers designated as "coarse whiskers" have a number average whisker diameter within a range of from 0.5 to less than 1.0 micrometer. This represents a diameter range distribution of 0.1 to 3 micrometers. Whiskers designated as "fine whiskers" have an average

10 whisker diameter within a range of 0.1 to less than 0.5 micrometer. This represents a diameter range distribution of 0.01 to 1.3 micrometers.

The volume-based bimodal distribution of 15 whiskers, readily apparent prior to processing to convert powdered ceramic matrix material and whiskers into a densified, whisker-reinforced ceramic composite material, is, with minor variations, found in the resultant densified composite material. A graphic

20 portrayal of relative volume percentage of whiskers versus whisker width shows two gaussian curves, thereby verifying the presence of such a bimodal distribution.

The graphic portrayal is plotted using a five-point _t- smoothing program (0.1 and 0.2 micrometer channel width). In order to obtain data for such a portrayal, polished cross-sections of densified composite material, prepared using standard metallographic procedures, are subjected to scanning electron microscopy (SEM) at a

30 magnification of 5000X. SEM images are transferred to a Zeiss-Knotron Image Processing System. DMIN (fiber width) and DMAX (fiber length) are measured using standard sizing parameters. The volume, V, of each whisker or particle is calculated assuming that V=DMIN²DMAX. The data is smoothed using a five point smoothing program and processed using a RS/1 fit function with two gaussians specified. The Figure A plot of such data shows a typical bimodal distribution.

A small number of coarse whiskers may, because of the size of such whiskers, occupy a volume equal to that occupied by a much larger number of fine whiskers. By way of illustration, a scanning electron micrograph _ of a polished cross-section of a densified composite material prepared from a mixture of alumina and 34 volume percent of SiC whiskers (14 volume percent coarse whiskers and 20 volume percent fine whiskers) shows that, of 2356 counted, only 77 have a width greater than about 1.4 micrometers. The 77 whiskers account for only 3 percent of the number of whiskers but occupy about 68 percent of whisker volume. As such, the relative volume occupied by coarse whiskers versus that occupied by fine whiskers provides a more accurate picture of whisker distribution than the mere number of such whiskers.

The use of a bimodal distribution of submicrometer, single crystal silicon carbide whiskers provides an increase in cutting performance over that attainable with the same amount of whiskers selected only from one mode of the distribution. By way of illustration, a ten percent, by volume, bimodal distribution provides a- cutting .performance at least as good as a 25 percent, by volume, content of either coarse or fine single crystal silicon carbide whiskers.

The single crystal silicon carbide whiskers which provide the bimodal distribution are suitably present in a concentration within a range of 5 to 40 percent by volume, based upon total composite volume. The concentration is desirably 10 to 34 percent by volume. A whisker content of less than five volume percent provides insufficient toughness. A whisker content in excess of forty volume percent leads to processing difficulties, particularly in hot pressing. A solution to such processing difficulties should, however, allow one to use more than forty volume percent.

The bimodal distribution suitably has a ratio of coarse whiskers to fine whiskers of 0.3 to 1.0. The ratio is desirably 0.4 to 0.8.

The silicon carbide whiskers suitably have an aspect ratio of ten or less. The aspect ratio is desirably 10. When the aspect ratio is greater than 15, cracks in a reinforced composite are believed to^* propagate across a whisker rather than along the length of a whisker. As such, there is no particular advantage to greater aspect ratios. Larger aspect ratios may, however, be used without adverse effect. Aspect ratios of less than 2.5 provide no advantage and may, in fact, have a considerable disadvantage in that they tend to increase brittleness of the resultant composite.

Single crystal silicon carbide whiskers are typically available as mixtures of whiskers with a small amount of particulate silicon carbide. Separation of particles from the whiskers without excessive, loss of whiskers is physically quite difficult. Fortunately, the presence of a small portion of silicon carbide particles does not adversely influence performance of the resultant composite. Care must be taken, however, to avoid an excess of particulate silicon carbide as brittleness, and concurrent likelihood of fracture, increases with an increase in particle loading.

Silicon carbide whiskers are believed to be particularly suitable for purposes of the present invention. Satisfactory results may, however, be attained with other whiskers such as those formed from a material selected from the group consisting of silicon nitride, titanium carbide, mullite, titanium diboride, alumina, magnesia or boron nitride. The whiskers should be selected from a chemically compatible material other than that of the matrix. As used herein, "chemically compatible" means that the whiskers and the matrix material do not react to form new phases.

The ceramic composites of the present invention are suitably prepared by hot pressing a homogeneous mixture of particulate ceramic material and the two different sizes of silicon carbide whiskers at a pressure and temperature sufficient to provide the composite with a density of greater than about 99 percent of the theoretical density of the ceramic material. The ceramic composites may, if desired, be prepared by hot isostatic pressing or sintering.

The bimodal distribution of silicon carbide whiskers and the ceramic powder are desirably in the form of a homogeneous admixture prior to hot pressing. The admixture may be produced by any suitable mixing technique which provides a homogeneous dispersion of the whiskers in the powder and minimizes agglomeration of the ceramic powder, whisker clumping, and whisker breakage. A particularly suitable mixing procedure, especially when the ceramic material is alumina, involves the use of an attritor mixer with alumina balls having a size of 3/16 inch (0.48 centimeter) or smaller. Care should be taken during mixing to minimize, if not eliminate, damage or destruction of whiskers. A solution of water, a dispersant and enough ammonium hydroxide to provide a solution pH of about 10.5 is admixed with alumina powder for about 30 minutes at 330 revolutions per minute (rpm) to form a uniform dispersion. Large silicon carbide whiskers wetted with water, a dispersant and a very small amount of ammonium hydroxide are added to the uniform dispersion while mixing continues. Ten minutes after completing addition of the large silicon carbide whiskers, small silicon carbide whiskers wetted with water are added to the dispersion while mixing continues. Two minutes after completing addition of the small whiskers, the attritor is stopped and its contents are dumped onto a 30 mesh (550 micrometer) screen to separate the attritor balls. After rinsing the attritor balls and equipment with deionized water, the contents are converted to a dilute slurry with additional deionized water. The dilute slurry is flocculated at a pH of 7.2 with 50 percent nitric acid. The flocculated material is dried in a 100°C. air circulating oven. The dried material is screened with a 60 mesh (250 micrometer) screen to provide a powdered admixture with a maximum agglomerate size of about 100 micrometers.

The powdered admixture is formed into a suitable configuration and hot-pressed to a density of greater than about 99 percent of the theoretical density of the ceramic material. Hot pressing may be accomplished in a suitable induction or resistance heated furnace with punches or pressing components formed of graphite or any other suitable material which is capable of withstanding the required pressures and temperatures without adversely reacting with composite constituents. In one example of a suitable procedure, the powdered admixture is poured into a ^'graphite die in a shape measuring three inches (7.6 cm) in length by 2.5 inches (6.4 cm) in width by 0.5 inch (1.3 cm) in depth. An initial pressure of 1000 psig (about 70 kg/cm²) is 10 applied to the die while the temperature is raised from ambient to about 1200°C over a period of about 30 minutes. The pressure is then increased to 5000 psig. (about 350 kg/cm²) and maintained at that level while the temperature is increased to 1725°C. over a period of

15 about 30 minutes. The die is maintained at that temperature and pressure for an additional 45 minutes. The die is then cooled over a two hour period to a temperature of 100°C. with a gradual pressure release at

20 1500°C.

The above-described hot pressing operation and the Examples below are directed to unidirectionally or uniaxially hot pressing the powdered admixture to

~r- provide composites in which the whiskers are preferentially aligned and randomly distributed in a plane or axis perpendicular to the hot pressing axis. Satisfactory results are, however, expected with other processes such as hot isostatic pressing and, perhaps,

30 rapid omnidirectional compaction.

The following examples are solely for purposes of illustration and are not to be construed as limiting the scope of the present invention. All parts and percentages are by weight unless otherwise stated. Arabic numerals represent examples within the scope of the present invention and alphabetic letters designate comparative examples.

Example 1

A series of hot-pressed materials are prepared from mixtures of 66 volume percent of alumina (AI2O3) powder (0.8 μm in size) and 34 volume percent silicon carbide (SiC) whiskers. The mixtures are prepared and hot-pressed using the mixing and hot-pressing procedures set forth hereinabove. The AI2O3 powder is grade RC-HP commercially available from Reynolds Metals Co. The SiC whiskers are nominally coarse, fine or mixtures thereof. The coarse whiskers have an average diameter of 0.94 μm, an average aspect ratio of about 10.7 (ranging from 1.1 to 77) and are commercially available from American Matrix. The fine whiskers have an average diameter of 0.22 to 0.26 μm, an average aspect ratio of about 10.6 and are commercially available from Tateho Chemical Industries Co., Ltd. The volume percentages of whiskers and the ratio of large to small whiskers, where both sizes are present, are set forth in Table I together with cutting performance of the resultant compositions. The coarse and fine whiskers are denominated in Table I respectively by letters "C" and "F".

The hot-pressed materials are diamond ground into cutting tool inserts- meeting A. .S.I, standards in the RNG 45 style. The cutting edge is chamfered at a 20° angle by 0.003 inch (0.008 cm) width. The insert is tested in a single point turning using a 30 Horsepower Le Blond 1610 Heavy Duty Lathe equipped with a variable speed (DC) drive. The cutting tool is held in place with a Kennametal holder. The cutting tools are tested on an Inconel^® 718 workpiece measuring four inches (10.2 cm) in diameter and twelve inches (30-5 cm) in length and having a hardness of 241 BHN. The workpiece is center drilled with a number 5 combined drill and countersink. The workpiece is held in the lathe by a twelve inch (30.5 cm), three-jaw, self-centering chuck, gripped on 3 inches (7.6cm) of length supported with number 4 Morse taper Nirol live center. The tool holder and quill of the tailstock containing the live center - are all adjusted for minimum overhang to insure maximum rigidity. The machine is run at a cutting speed of 750 feet per minute, a feed rate of 0.007 inches per revolution and a depth of cut of 0.100 inches. No cutting fluid is used. Successive passes are taken and the cutting edge is examined for flank wear and chipping. Testing is terminated after one minute of cutting and cutting performance in terms of uniform wear is measured. The cutting performance is also shown in

Table I.

Table

Very Brittle with a tendency to fracture during testing

The data presented in Table I demonstrate that cutting performance, at least in terms of uniform wear, is significantly improved with a mixture of coarse and fine whiskers rather than either coarse or fine whiskers alone. The mixture of whiskers also provides more consistent results than are attainable with only one size of whisker, either coarse or fine. Similar results are expected with other compositions of the present invention.

Example 2

The procedure of Example 1 is duplicated with a different coarse whisker. The coarse whisker has a number average diameter of 0.67 micrometer and an average aspect ratio of 11.0. It is available from American Matrix. Cutting performance of cutting tool inserts prepared as in Example 1 are summarized in Table II.

Table II

* Very Brittle with a tendency to fracture during testing

The data presented in Table II demonstrate that acceptable cutting performance is attained with coarse whiskers having a smaller diameter than those of Example 1. The data also affirm that a mixture of whisker provides an improvement in cutting performance over a like amount of a single whisker size, either coarse or small. A graphic representation of relative volume percentage of whiskers versus whisker width produced, in accord with the method described above at page 6, lines 16-35; page 7, lines 1-11s a bimodal distribution.

Example 3

The procedure of Example 1 is duplicated with the composition of Sample Number 1-3 to provide a hot- pressed material. The resultant material is examined via SEM in conjunction with the Zeiss-Kontron Image Processing System according to procedure described hereinabove. During examination of a sample of the hot-pressed material, 2356 whiskers are counted in eleven fields (areas). An additional value for silicon carbide whiskers, denominated as relative volume percent, is determined by the following equation wherein X is DMIN (fiber width) :

Relative Volume Percent = 5 _5#i|75e-(x-1.004)2/1.56 _{+ 7 #}i!_7e-(x-2.401 )2/0.416_#

The relative volume percent represents a given volume percent normalized based upon total volume of whiskers.

10 A graphic portrayal representing the best fit of smoothed data produced a two gaussian curve distribution. The area under the curves provides volume percent figures as follows: about 56 percent fine ■whiskers and 44 percent coarse whiskers. The size

15 corresponding to maxima in distribution are 1.0 μm and 2.4 μm respectively for fine and coarse whiskers. The volume ratio of coarse to fine whiskers is 0.8.

The composition of Sample Number 1-3 comprises

20 14 volume percent coarse whiskers and 20 volume percent fine whiskers. Converting these numbers to relative volume percent whiskers provides the following values: coarse whiskers - about 41 percent; and fine whiskers - ~c about 59 percent.

The data show that the initial bimodal distribution of SiC whiskers remains after hot-pressing. Similar results are expected with other compositions

30 disclosed herein.

Example 4

The procedure of Example 1 is replicated except for variations in the volume percentage of silicon -17-

carbide whiskers. The ratio of coarse to fine whiskers from Sample Number 1-3 (0.7) is maintained for all samples. Cutting tools are fabricated and tested as in Example 1 for uniform wear. The test results together with the volume percentage of silicon carbide whiskers are shown in Table III.

Table III

The data presented in Table III support two observations. First, processing difficulties undoubtedly- contribute to reduced cutting performance at whisker loadings of forty volume percent and higher. Further deterioration in cutting performance is expected as whisker loading increases above 45 volume percent. Second, the mixture of whiskers provides better cutting performance, even at 10 volume percent, than 34 volume percent of either whisker. Cutting performance is expected to drop o^'ff somewhere below 10 volume percent, even with mixed whiskers. This point is readily determinable without undue experimentation. Similar results are expected with other compositions disclosed herein. Example 5 and Comparative Examples A-C

The hot-pressing procedure of Example 1 is replicated with a different ceramic material. The ceramic material is a mixture of 92 percent silicon nitride, commercially available from UBE Industries under the trade designation UBE-SN-10, 6 percent yttria and two percent of the alumina used in Example 1. All percentages are based upon mixture weight. The amount of whiskers, if any, are shown in Table IV.

10 The resultant hot-pressed materials are diamond ground into cutting tool inserts for comparison with a commercial cutting tool. The inserts are made according to A. .S.I, standards in the SNG 434 style with cutting

,,- edges chamfered at a 30° angle with a 0.006 inch (0.015 cm) width. The inserts are tested in a center face milling application using a 40 horsepower Cincinnati #5 single spindle, knee and saddle, vertical milling machine with a 5 horsepower variable speed table. The 0 work material is Class 30 gray cast iron measuring 4 inches (10.2 cm) wide by 12 inches (30.5_ cm) long with a measured hardness of 170 BHN. A one-tooth milling cutter having a 12 inch (30.5 cm) diameter is used with a 5° axial rake, a -5° radial rake and a 15° lead angle. 5 The machine is run at a cutting speed of 3000 surface feet per minute (metric), a 0.60 inch (0.152 cm) depth of cut, and a feed rate of 0.013 inch (0.033 cm) per revolution (or tooth). The center line of the cutter 0 and the center line of the workpiece are coaxial. No cutting fluid is used.

Successive passes are taken on the cast iron work material. The cutting edge is examined for flank wear and chipping after each pass. Testing is terminated when the flank wear or chipping exceeds 0.010 inch (0.025 cm) in depth as measured with a 30 power microscope. Cutting performance in terms of cubic inches (cubic centimeters) of material removed are shown in Table IV. Comparative Example C is an unreinforced silicon nitride cutting tool commercially available from Boride Products, Inc. under the trade designation U.S.-20.

Table IV

The data presented in Table IV show that the superiority of a mixture of coarse and fine whiskers over either coarse or fine whiskers alone remains true with ceramic materials other than alumina. The data, in conjunction with that presented in Table I, also demonstrate the suitability of the bimodal distribution whisker reinforced cutting tools in different cutting applications. Similar results are expected with other compositions disclosed herein.

Claims

CLAIMS :

1. A densified, whisker-reinforced ceramic composite material comprising a matrix of ceramic material having homogeneously dispersed therein 5 to 40 percent by volume of a bimodal distribution of chemically compatible single crystal whiskers of silicon carbide, silicon nitride, titanium carbide, mullite, titanium diboride, alumina, magnesia or boron nitride, provided, however, that the whiskers are not the- same as the ceramic matrix material, the bimodal distribution, being based upon relative whisker volume versus whisker width and consisting essentially of a volumetric ratio of coarse single crystal whiskers to fine single crystal whiskers of 0.1 to 1, the coarse whiskers having, prior to densification, a number average diameter of greater than or equal to 0.5 but less than 1.0 micrometer and a diameter range of 0.1 to 3 micrometers, the fine whiskers having, prior to densification, a number average diameter of greater than or equal to 0.1 but less than 0.5 micrometer and a diameter range of 0.01 to 1.3 micrometers, the bimodal distribution providing an increase in cutting performance over an equal amount of either coarse or fine whiskers.

2. The composite material of Claim 1 wherein the volume percentage of silicon carbide whiskers is 10 to 34. 3. The composite material of Claim 1 wherein the volumetric ratio is 0.

3 to 1.0.

4. The composite material of Claim 1 wherein the ceramic material is alumina, silicon nitride, mullite, silica, A1N, and mixtures thereof.

5. The composite material of Claim 1 wherein the whiskers have an aspect ratio of greater than about three.

6. A cutting tool fabricated from the composite material of Claim 1.

7. A densified, whisker-reinforced ceramic composite material that is a matrix of ceramic material having homogeneously dispersed therein 5 to 40 percent by volume of a bimodal distribution of single crystal silicon carbide whiskers, the bimodal distribution being based upon relative whisker volume versus whisker width and consisting of a volumetric ratio of coarse single crystal silicon carbide whiskers to fine single crystal silicon carbide whiskers of 0.1 to 1, the coarse whiskers having, prior to densification, a number average diameter of greater than or equal to 0.5 but less than 1.0 micrometer and a diameter range of 0.1 to 3 micrometers, the fine whiskers having, prior to densification, a number average diameter of greater than or equal to 0.1 but less than 0.5 micrometer and a diameter range of 0101 to 1.3 micrometers, the bimodal distribution providing an increase in cutting performance over an equal amount of either coarse or fine whiskers.