Silicon nitride based composite with improved fracture toughness

Composite materials, articles and cutting tools are prepared by densification to form a body comprising whiskers of hard refractory transition metal carbides, nitrides or carbonitrides uniformly distributed in a two-phase silicon nitride matrix. A first phase comprises silicon nitride grains and the second phase is an intergranular phase formed from one or more suitable densification aids. Optionally, dispersoid particles and/or polycrystalline fibers may also be incorporated. The preferred composite article or cutting tool has a fracture toughness equal to or greater than about 3.5 MPa.m.sup.1/2.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is related to U.S. patent application Ser. No. 06/892,634, 
now abandoned filed concurrently with the parent to this application, and 
entitled, "SILICON ALUMINUM OXYNITRIDE BASED COMPOSITE WITH IMPROVED 
FRACTURE TOUGHNESS"; and to U.S. patent application Ser. No. 07/158,491, 
filed Feb. 22, 1988, now U.S. Pat. No. 4,889,836 and entitled, "TITANIUM 
DIBORIDE-BASED COMPOSITE ARTICLES WITH IMPROVED FRACTURE TOUGHNESS. 
FIELD OF THE INVENTION 
This invention relates to fracture and abrasion resistant materials and to 
articles of manufacture made therefrom. More particularly, it is concerned 
with fracture and abrasion resistant materials comprising transition metal 
carbide, nitride or carbonitride whiskers distributed in a matrix 
containing silicon nitride, and with articles made therefrom. 
BACKGROUND OF THE INVENTION 
The need for materials for cutting tool applications, with 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 too 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 composite 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 carbides or nitrides of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W); 
and 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). 
Many improvements have been made in the toughness, abrasion resistance, 
high temperature strength and chemical inertness, but increased demands by 
the cutting tool industry require cutting tools with new and improved 
characteristics. In many applications, for example in gray cast iron and 
high nickel alloy machining, silicon nitride 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 silicon 
nitride ceramic cutting tool materials is directly proportional to 
K.sub.IC.sup.3/4 H.sup.1/2, where K.sub.IC is the fracture toughness and H 
is the hardness. It may be seen, therefore, that further improvement in 
the fracture toughness of silicon nitride 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 composite materials of the invention are also expected 
to find wide use in wear part and structural applications, for example in 
dies, turbines, nozzles, etc. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, there is provided a 
densified composite comprising about 5-60% by volume of whiskers of one or 
more transition metal carbides, nitrides, or carbonitrides, of titanium, 
hafnium, tantalum, niobium, or tungsten, or mixtures or solid solutions 
thereof substantially evenly distributed in a two phase matrix. The matrix 
comprises a first phase of silicon nitride and a continuous, intergranular 
phase formed from a densification aid. The whiskers are present in the 
densified composite in an amount of about 5-60% by volume. Optionally, 
particles and/or polycrystalline fibers of one or more carbides, nitrides, 
or carbonitrides, of titanium, zirconium, hafnium, vanadium, niobium, 
tantalum, chromium, molybdenum, or tungsten, or silicon carbide, titanium 
diboride, or hafnium diboride, or mixtures or solid solutions thereof may 
be admixed with the whiskers in an amount up to about 95% by volume of the 
total dispersoid mixture, but the total amount of the dispersoids should 
not exceed about 70% by volume. The densification aid preferably comprises 
about 1-25% by weight of the matrix, the balance being silicon nitride. 
The composite possesses properties of a density greater than 98% of 
theoretical, high abrasion resistance, high hardness, a fracture toughness 
greater than or equal to 3.5 MPa.m.sup.1/2, and resistance to oxidation at 
temperatures greater than 1200.degree. C. 
In accordance with other aspects of the present invention, there are 
provided composite articles of manufacture and cutting tools, coated 
composite articles and cutting tools, and a process for making the 
composite material of the present invention.

DETAILED DESCRIPTION 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 and abrasion resistant materials according to the present 
invention comprise whiskers of a hard refractory transition metal carbide, 
nitride, or carbonitride, or mixtures or solid solutions thereof dispersed 
in a two-phase 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, hafnium, 
tantalum, niobium, or tungsten 
The hard refractory whiskers incorporated into materials in accordance with 
this invention each comprise a single crystal, and preferably have an 
average diameter of about 1-5 microns and an average length of about 
10-250 microns, with a preferred aspect ratio of length to diameter of at 
least 5 to 1. 
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 should comprise about 5-60% by volume 
of the densified composite. The preferred range of refractory whisker 
content is about 5-40% by volume. A more preferred range is about 10-30% 
by volume. 
Optionally, in addition to the whisker dispersoid the composite may include 
other dispersed components. For example, particles and/or polycrystalline 
fibers may be included in an amount of about 95% by volume of the total 
dispersoid mixture. The, fibers and/or particles are of a hard carbide, 
nitride or carbonitride of titanium, zirconium, hafnium, vanadium, 
niobium, tantalum, chromium, molybdenum, or tungsten; or alumina, silicon 
nitride, silicon carbide, titanium diboride or hafnium diboride; or 
mixtures or solid solutions of these materials. The particle and/or fiber 
material may be the same or different from each other and from the whisker 
material. The total amount of the dispersoids including whiskers, fibers 
and particles, however, should not exceed about 70% and preferably is in 
the range of 5-40% by volume. 
In accordance with the principles of the present invention, the hard 
refractory dispersoids are uniformly distributed in a two-phase matrix. 
The first phase of the matrix consists essentially of silicon nitride. The 
intergranular phase or secondary phase of the matrix is formed from one or 
more densification aids. 
For optimizing the desirable properties, particularly high temperature 
strength of the composite of the present invention, it is preferable to 
maximize the high temperature stability of the secondary, intergranular 
phase and to maximize the density of the final densified composites. While 
the densification aid is essential to achieve highest densities, that is, 
densities greater than 98% of theoretical, improper composition of the 
second phase can deleteriously affect maximum attainable high temperature 
strength and creep resistance. The densification aids of the present 
invention stabilize the secondary phase to such an extent that the 
secondary phase is a refractory phase having desirable high temperature 
properties. Preferred densification aids are yttrium oxide, cerium oxide, 
zirconium oxide, hafnium oxide, silica, magnesia and mixtures thereof. The 
one or more densification aids are employed from a lower effective amount 
which permits high theoretical densities to an upper amount which does not 
unduly effect the high temperature properties. Preferably, the 
densification aids comprise about 1-25% by weight of the matrix; more 
preferably, about 1-5% by weight of the matrix. 
For optimizing the desirable high temperature properties of the composite 
of the present invention, alumina is present as a densification aid in the 
secondary phase of the matrix material in a minimal amount, i.e. less than 
about 5% by weight based on the total weight of the matrix material, and 
preferably less than about 3% by weight. 
The intergranular phase may contain additives and impurities in addition to 
the hereinbefore mentioned densification aids and additives. Such further 
additional materials may 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 weight of the secondary phase 
Impurities may be present in the starting materials used for the 
manufacture of the composite of the present invention. These impurities 
tend to become concentrated in the intergranular phase during preparation 
of the composite. Therefore, high purity starting materials are desired, 
preferably those having less than about 0.1 weight percent cation 
impurities. A typical undesirable impurity is calcium, which tends to 
deleteriously affect the secondary intergranular phase and high 
temperature properties. 
The materials described herein have a composite microstructure of 
refractory whiskers, optionally with refractory fibers and/or particulate 
refractory grains, uniformly dispersed in a matrix containing a phase of 
Si.sub.3 N.sub.4 grains, and a continuous intergranular phase formed from 
the densifying additive. Because the intergranular phase is continuous, 
its characteristics profoundly affect the high temperature properties of 
the composite material. The composite materials of the present invention 
possess high strength at temperatures in excess of 1200.degree. C., 
preferably in excess of 1500.degree. C. 
Articles formed from the densified composite material according to the 
present invention 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., and all incorporated herein 
by reference. 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, and hafnia, 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. 
In accordance with the invention, a method is provided for preparing the 
composites described above, sintering the materials to densities 
approaching theoretical density, i.e. greater than 98% of theoretical, 
while achieving optimum levels of mechanical strength and toughness at 
both room temperature and elevated temperature, making the composite 
particularly useful as cutting tools in metal removing applications. 
The hard refractory whiskers, with or without other dispersoids, are 
dispersed in the two phase matrix which is 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 Si.sub.3 N.sub.4 powder, preferably, of average particle 
size below about 3 microns. 
Densification of the silicon nitride-whisker composite is aided by the 
incorporation of one or more of the densification aids listed above into 
the initial composition. In the initial compositions employed in the 
fabrication, the hard refractory whiskers 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 the whiskers, up to 
about 95% by volume of the dispersoid mixture. The total volume of the 
dispersoids in the densified composite should be limited to about 70% by 
volume. In the densified composite, the balance of the composite material 
comprises the matrix of silicon nitride grains and the intergranular phase 
formed from the densification aid. In this densified composite, the 
densification aid makes up about 1-25% by weight of the host matrix. The 
starting materials may be processed to a powder compact of adequate green 
strength by thoroughly mixing the matrix starting materials by processes 
such as dry milling or ball milling in a nonreactive liquid medium, such 
as toluene or methanol; admixing the whiskers and any other dispersoids by 
blending, preferably in a nonreactive liquid medium; and forming the 
mixture, for example by pressing, injection molding, extruding, or slip 
casting Processing may also optionally include a presintering or 
prereacting step in which either the uncompacted materials 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 greater than 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. 
To enable one skilled in the art to practice this invention, the following 
Example is provided. 
EXAMPLE 
Silicon nitride composite bodies were made from a starting powder 
formulated from 6% by weight yttria powder, 2% by weight alumina powder 
and the remainder silicon nitride powder, the mixture being dry milled 24 
hours at 140 rpm to blend the components. The starting powder was mixed 
with titanium carbide (TiC) whiskers of average diameter about 5.mu.m, 
average length about 250 .mu.m. Three batches were prepared: Batch 1 
containing 10% by volume TiC whiskers; Batch 2, 20% by volume; Batch 3, 
30% by volume. In each case, the whiskers were wet blended in methanol 
with the starting powder. 
The whisker-powder mixtures from each batch were hot pressed at a 
temperature of 1725.degree. C..+-.5.degree. C., and at a pressure of 5000 
psi for lengths of time sufficient to obtain composite bodies. A batch of 
comparative samples was also prepared in a similar manner from the same 
starting powder formulation, but without the whisker component. The 
density as percent of theoretical (% T.D.), the Knoop hardness(HKN), and 
fracture toughness (K.sub.IC) of the composite bodies of each batch are 
shown in Table 1. 
TABLE 1 
______________________________________ 
TiC Whisker 
Density, HKN, K.sub.IC 
Batch Content, v/o 
% T.D. GPa MPa .multidot. m.sup.1/2 
______________________________________ 
Comparative 
0 98.8 13.8 3.6 
1 10 100.6 13.8 4.1 
2 20 100.9 13.9 4.4 
3 30 102.2 13.1 7.1 
______________________________________ 
Relative fracture toughness values were obtained by an indentation fracture 
test utilizing a Vickers diamond pyramid indenter. In this test the length 
of cracks developed at the corners of the indentation and the indentation 
size are used to obtain fracture toughness (K.sub.IC) values by a 
relationship: 
##EQU1## 
where 
K.sub.IC =fracture toughness (MPa.multidot.m.sup.1/2) 
H=hardness (GN/m.sup.2) 
D=indentation diagonal (.mu.m) 
C.sub.L =sum of cracks (.mu.m) 
The densities shown in Table 1 for the silicon nitride-whisker composites 
tested are greater than 100 percent of theoretical. This is a result of 
chemical interactions of the constituents and modification of the 
intergranular phase which are not accounted for in the calculation of 
theoretical density. 
The above Example is not to be viewed as limiting the scope of the 
invention as claimed, but is intended only to be illustrative thereof The 
materials of 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 
silicon nitride, single crystal whiskers, and a sintering or densification 
aid are sintered to a dense, polycrystalline product. Optionally, the 
pressed green compact to be sintered may be formulated with, in an 
admixture with the whiskers, particles and/or polycrystalline fibers as 
described above. The materials may be combined before hot pressing or 
sintering by the method described in the Example, 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, and wear-resistant and 
structural parts, and will be especially useful as shaped cutting tools 
for continuous or interrupted milling, turning or boring of cast iron 
stock or high nickel (at least 50% Ni) alloy stock, e.g. Inconel.