Titanium diboride-based composite articles with improved fracture toughness

Composite articles, cutting tools and wear parts are prepared by densification of a mixture comprising whiskers, fibers, or particles of hard refractory transition metal carbides, nitrides or carbonitrides uniformly distributed in a titanium diboride matrix. Optionally, other dispersoids may also be incorporated. The preferred composite article or cutting tool has a fracture toughness equal to or greater than about 2.5 MN.multidot.m.sup.1/2. Methods of preparation and use are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS 
This application is related to commonly owned U.S. application Ser. Nos. 
07/158,492 and 07/158,493, filed concurrently herewith, and incorporated 
herein by reference. 
FIELD OF THE INVENTION 
This invention relates to fracture and abrasion resistant articles of 
manufacture. More particularly, it is concerned with fracture and abrasion 
resistant articles comprising transition metal carbide, nitride or 
carbonitride whishers, fibers, or particles distributed in a matrix of 
titanium diboride, as well as with methods of preparation and use. 
BACKGROUND OF THE INVENTION 
The need for materials for cutting tool applications, exhibiting improved 
toughness, good strength at elevated temperatures, and chemical inertness, 
and capable of operating at high cutting speeds has generated a widespread 
interest in ceramic materials as candidates to fulfill these requirements. 
Conventional ceramic cutting tool materials have failed to find wide 
application primarily due to their low fracture toughness. 
Therefore, many materials have been evaluated to improve ceramic 
performance, such as silicon nitride-based composites for cutting tool 
applications. Specific examples of silicon nitride-based composite cutting 
tools are discussed in U.S. Pat. No. 4,388,085 to Sarin et al. (composite 
silicon nitride cutting tools containing particles of TiC); U.S. Pat. No. 
4,425,141 to Buljan et al. (a composite modified silicon aluminum 
oxynitride cutting tool containing particulate refractory metal carbides, 
nitrides, and carbonitrides); U.S. Pat. No. 4,433,979 to Sarin et al. 
(composite silicon nitride cutting tools containing particulate hard 
refractory transition metal carbides or nitrides); U.S. Pat. No. 4,449,989 
to Sarin et al. (composite silicon nitride cutting tools coated with two 
or more adherent layers of refractory materials); and U.S. patent 
application Ser. Nos. 892,642 and 892,634 both filed Aug. 4, 1986 by 
Baldoni et al. (composite silicon nitride and silicon aluminum oxynitride 
materials, respectively, containing refractory transition metal carbide, 
nitride, or carbonitride whiskers). 
Many improvements have been made in the toughness, abrasion resistance, 
high temperature strength and chemical inertness of such materials, but 
increased demands by the cutting tool industriy require cutting tools with 
new and improved characteristics. Titanium diboride has aroused interest 
because of its hardness, but has heretofore been considered too brittle 
for use in such applications as cutting tools. 
In applications such as gray cast iron machining, ceramic tool wear has 
been found to be dominated by abrasion. Even at cutting speeds as high as 
5000 sfm, chemical reactions between tool and workpiece are negligible in 
comparison. It has been found that abrasion resistance for, for example, 
silicon nitride ceramic cutting tool materials is somewhat more dependent 
on the fracture toughness than the hardness. It may be seen, therefore, 
that further improvement in the fracture toughness of ceramic materials 
could bring about significant increases in both reliability and abrasive 
wear resistance, providing materials for cutting tools with new and 
improved characteristics. The present invention provides such new and 
improved ceramic materials. 
The wear-resistant titanium diboride-based composites according to the 
invention are also useful in wear part and structural applications, for 
example as seals, dies, parts for automotive engines, nozzles, etc, and in 
impact resistant applications, for example as ceramic armor, etc. 
SUMMARY OF THE INVENTION 
A densified, hard, abrasion resistant ceramic-based composite article of 
improved fracture toughness according to the invention includes about 5-60 
volume percent of one or more first dispersoids selected from whiskers, 
chopped fibers, and particles of refractory carbides, nitrides, and 
carbonitrides of titanium, zirconium, hafnium, vanadium, niobium, 
tantalum, chromium, molybdenum, and tungsten, and solid solutions thereof, 
uniformly distributed in a matrix of titanium diboride. 
A process according to the invention for preparing the densified, hard, 
abrasion resistant ceramic-based composite article of improved fracture 
toughness involves blending a mixture including about 95-40 volume percent 
titanium diboride powder and about 5-60 volume percent of one or more 
first dispersoids, to uniformly disperse the dispersoids in the titanium 
diboride powder. The first dispersoids are selected from whiskers, chopped 
fibers, and particles of refractory carbides, nitrides, and carbonitrides 
of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, 
molybdenum, and tungsten, and solid solutions thereof. The mixture is 
consolidated to a density of a least about 98% of theoretical density to 
form the article. 
A method according to the invention for continuous or interrupted machining 
of a steel stock involves milling, turning, or boring the stock with a 
shaped, densified, hard, abrasion resistant ceramic-based composite 
cutting tool of improved fracture toughness. The cutting tool includes a 
densified, hard, abrasion resistant ceramic-based composite article of 
improved fracture toughness including about 5-60 volume percent of one or 
more dispersoids, uniformly disttributed in a matrix of titanium diboride. 
The dispersoids are selected from whiskers, chopped fibers, and particles 
of refractory carbides, nitrides, and carbonitrides of titanium, 
zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, and 
tungsten, and solid solutions thereof. The machining speed is about 
100-1500 sfm, and the feed rate is about 0.005-0.03 in/rev.

DETAILED DESCRIPTON OF THE PREFERRED EMBODIMENTS 
For a better understanding of the present invention, together with other 
and further objects, advantages and capabilities thereof, reference is 
made to the following disclosure and appended claims. 
Fracture toughened and abrasion resistant materials according to the 
present invention comprise whiskers, chopped fibers, or particles of a 
hard refractory transition metal carbide, nitride, or carbonitride, or 
mixtures or solid solutions thereof dispersed in a titanium diboride 
matrix. By the term "transition metal carbide, nitride, or carbonitride", 
as used throughout this specification and appended claims, is meant any 
carbide, nitride, or carbonitride of titanium, zirconium, hafnium, 
vanadium, niobium, tantalum, chromium, molybdenum, or tungsten. The 
preferred dispersoid material is TiC. 
The hard refractory whiskers incorporated into materials in accordance with 
this invention each comprise a single crystal, while the fibers are 
polycrystalline. Preferably the fibers or whiskers have an average 
diameter of about 0.5-5 microns and an average length of about 6-250 
microns, with a preferred aspect ratio of length to diameter of at least 
6-200. The particles to be incorporated normally are crystalline, 
substantially equiaxed particles of about 1 to 10 microns diameter. 
Particularly advantageous composite materials may be produced by including 
whiskers, fibers, or particles which have been coated with a refractory 
material as the dispersoid in the TiB.sub.2 matrix. The preparation of 
coated fibers and particles is known. Coated whiskers and their 
preparation by CVD are describedin U.S. patent application Ser. No. 
899,835, filed Aug. 25, 1986 and commonly owned. The preferred coating 
material for the dispersoids is alumina; the preferred coating thickness 
is from a monolayer to about 1/3 of the diameter of the dispersoid. Other 
suitable coatings include zirconia, hafnia, yttria, or other refractory 
oxides with melting or decomposition points higher than 1700.degree. C., 
alone or as mixtures or solid solutions with other oxides including 
alumina; and refractory carbides, nitrides, or carbonitrides of titanium, 
zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, or 
tungsten. The coating material is different from the dispersoid material. 
Such coated dispersoids combine the bulk (e.g. mechanical) properties of 
the core material with the surface (e.g. chemical) properties of the 
coating. 
The useful life and performance of articles in accordance with this 
invention depends, in large part, on the volume taken up by the dispersed 
phase in the article. The whiskers, fibers, or particles should comprise 
about 5-60% by volume of the densified composite. The preferred range of 
refractory whiskers, fiber, or particle content is about 5-50% by volume. 
A more preferred range is about 5-30% by volume. 
Optionally, in addition to the above-described dispersoid, the composite 
may include one or more other dispersed components. For example, whiskers, 
fibers, or particles of other materials may be included in an amount of 
about 5-55% by volume of the densified composite. The preferred other 
dispersoids are of hard refractory alumina, zirconia, hafnia, silicon 
nitride, tungsten carbide, or hafnium diboride; or mixtures or solid 
solutions of these materials. The total amount of all dispersoids, 
however, should not exceed about 60% and preferably is in the range of 
5-40% by volume. The hard refractory dispersoids are uniformly distributed 
in the titanium diboride matrix. 
The material of the invention may further contain additives and impurities 
in addition to the hereinbefore mentioned titanium diboride and 
dispersoids. Such further additional materials may be selected to 
contribute to the desirable final properties of the composite, and are 
preferably present in an amount less than about 5% by weight based on the 
total weight of the material. The starting materials should be selected to 
include only amounts of impurities which will not have a significant 
negative effect on the desired properties. 
The materials described herein have a composite microstructure of 
refractory whiskers, fibers, and/or particulate refractory grains, 
uniformly dispersed in a matrix containing titanium diboride grains. For 
optimizing the desirable properties, particularly the strength of the 
composite of the present invention, it is preferable to maximize the 
density of the final densified composites, that is, to densities of at 
least about 98% of theoretical. 
Articles formed from the densified composite materials described herein may 
be coated with one or more adherent layers of hard refractory materials, 
for example by known chemical vapor deposition or physical vapor 
deposition techniques. Typical chemical vapor deposition techniques are 
described in U.S. Pat. Nos. 4,406,667, 4,409,004, 4,416,670, and 
4,421,525, all to Sarin et al. The hard refractory materials suitable for 
coating articles according to the present invention include the carbides, 
nitrides, and carbonitrides of titanium, zirconium, hafnium, vanadium, 
niobium, tantalum, chromium, molybdenum, and tungsten, and mixtures and 
solid solutions thereof, and alumina, zirconia, hafnia, and yttria, and 
mixtures and solid solutions thereof. Each layer may be the same or 
different from adjacent or other layers. Such coatings are especially 
advantageous when applied to cutting tools formed from the densified 
composites of the present invention. 
A process for preparation of the composites described above involves 
consolidating or densifying, by sintering or hot pressing, the blended 
materials to densities approaching theoretical density, e.g. at least 
about 98% of theoretical, while achieving optimum levels of mechanical 
strength and fracture toughness at both room temperature and elevated 
temperature, making the composites particularly useful as cutting tools in 
metal removing applications. 
The hard refractory whiskers, fibers, or particles with or without other 
dispersoids, are thoroughly dispersed in the TiB.sub.2 matrix, for example 
by wet blending in a non-reactive medium, then drying. The mixture is then 
compacted to a high density by sintering or hot pressing techniques. A 
composition for the production of abrasion resistant materials according 
to the present invention may be made by employing TiB.sub.2 powder, 
preferably of average particle size below about 3 microns. 
In the initial compositions employed in the fabrication, the hard 
refractory transition metal carbide, nitride, or carbonitride whiskers, 
fibers, or particles comprise about 5-60% of the total volume of the 
densified article, as set out above. Optionally, as described above, other 
dispersoids may be admixed with these first dispersoids and TiB.sub.2, up 
to about 55% by volume of the dry mixture. The total volume of the 
dispersoids in the densified composite should be limited to about 60% by 
volume. In the densified composite, the balance of the composite material 
normally comprises the matrix of titanium diboride grains, although minor 
amounts of other materials may be included, as described hereinbefore. The 
starting materials may be processed to a powder compact of adequate green 
strength by thoroughly mixing the particulate or powder starting materials 
by processes such as dry milling or ball milling in a nonreactive liquid 
medium, such as toluene or methanol; admixing the whisker or fiber 
dispersoids by high shear wet blending, preferably in a nonreactive liquid 
medium; and compacting the mixture, for example by pessing, injection 
molding, extruding, or slip casting. Processing may also optionally 
include a presintering or prereacting step in which either the uncompacted 
material or the compact is heated at moderate temperatures. 
Since the strength of articles in accordance with this invention decreases 
with increasing porosity in the total compact, it is important that the 
compact be sintered or hot pressed to a density as nearly approaching 100% 
of theoretical density as possible, preferably at least about 98% of 
theoretical density. The measure of percent of theoretical density is 
obtained by a weighted average of the densities of the components of the 
compact, and is preferably at least about 2.5 MN.multidot.m.sup.3/2. 
The following Examples are presented to enable those skilled in the art to 
more clearly understand and practice the present invention. These Examples 
should not be considered as a limitation upon the scope of the present 
invention but merely as being illustrative and representative thereof. 
EXAMPLES 
Titanium diboride-based composite bodies were made from a starting 
formulation of titanium diboride powder mixed with one or more 
dispersoids. Several different formulations were prepared as shown in the 
Table. In each case, the dispersoids were wet blended in a high shear 
blender in methanol with the matrix powder. The dispersoids/TiB.sub.2 
mixtures from each batch were dried at about 75.degree. C., and pressed at 
about 1750.degree. C.-1900.degree. C. and about 5000 psi for lengths of 
time sufficient to obtain composite bodies of near theoretical density, 
about 0.5-3.0 hr. The average density as percent of theoretical (%T.D.), 
hardness (Hd, GN/m.sup.2), and fracture toughness (IFT, MN/m.sup.3/2) of 
the composite bodies for each formulation are shown in the Table. Relative 
fracture toughness values were obtained by an indentation fracture test 
utilizing a Vickers diamond pyramid indenter. 
TABLE 
______________________________________ 
Dispersoids Hd, IFT, 
Ex.# v/o Matl. Form % TD GN/m.sup.2 
MN/m.sup.3/2 
______________________________________ 
1 3.7 TiC* P 100 19.0 .+-. 0.5 
2.3 .+-. 0.2 
2 3.7 TiC* P 100 18.7 .+-. 0.4 
2.1 .+-. 0.1 
3 3 WC P 99.6 18.5 .+-. 0.3 
2.1 .+-. 0.1 
4 30 NbC P 100 20.7 .+-. 0.9 
3.2 .+-. 0.1 
5 30 Mo.sub.2 C 
P 100 18.1 .+-. 0.4 
2.8 .+-. 0.1 
6 20 TaC P 99.8 18.9 .+-. 0.8 
2.9 .+-. 0.2 
7 27 (W,Ti)C P 99.6 19.4 .+-. 0.6 
2.4 .+-. 0.1 
8 30 TiC P 99.8 18.4 .+-. 0.4 
3.5 .+-. 0.1 
9 20 HfC P 97.9 18.6 .+-. 0.7 
2.8 .+-. 0.3 
10 30 TiN P 97.4 17.0 .+-. 0.6 
2.7 .+-. 0.2 
11 20 TiC W 98.5 19.0 .+-. 0.9 
3.3 .+-. 0.3 
12 0 -- -- 100 19.0 .+-. 0.5 
1.5 .+-. 0.3 
______________________________________ 
P = particles, W = whiskers, v/o = volume % 
*minor amounts of WB.sub.2 also included for grain size control. 
The materials and articles according the invention can be prepared by hot 
pressing techniques, e.g. as described above, or by hot isostatic pressing 
and sintering techniques, e.g. a technique in which pressed green compacts 
containing titanium diboride and whiskers, fibers, or particles are 
sintered to a dense, polycrystalline product. The materials may be 
combined before hot pressing or sintering by the method described in the 
Examples, or by other methods known in the art. 
Densified ceramic articles made in accordance with this invention are hard, 
tough, nonporous, abrasion resistant, and resistant to oxidation. 
Applications of these articles include, but are not limited to, cutting 
tools, mining tools, stamping and deep-drawing tools, extrusion dies, wire 
and tube drawing dies, nozzles, guides, bearings, wear-resistant and 
structural parts, and ceramic armor, and are especially useful as shaped 
cutting tools for continuous or interrupted milling, turning, or boring of 
steel stock. Such machining operations may be carried out in conventional 
equipment operated at a speed of about 100-1500 sfm, and at a feed rate of 
about 0.005-0.03 in/rev. 
While there has been shown and described what are at present considered the 
preferred embodiments of the invention, it will be obvious to those 
skilled in the art that various changes and modifications can be made 
therein without departing from the scope of the invention as defined by 
the appended claims.