Note: Descriptions are shown in the official language in which they were submitted.
87~3-343 CN 1 336 1 02
SILICON NITRIDE BASED ARTICLE WITH IMPROVED FRACTURE
TOUGHNESS AND STRENGTH
This application contains subject matter related to
matter disclosed and claimed in commonly assigned co-
pending Canadian Patent Application No. 591,218-1, filed
concurrently herewith.
This invention relates to fracture and abrasion
resistant materials and to articles of manufacture made
therefrom. More particularly, it is concerned with
silicon nitride-based ceramic bodies exhibiting both
improved fracture toughness and improved strength.
The need for improved materials for cutting tool
applications, with enhanced toughness and strength at
room and elevated temperatures, and with chemical inert-
ness, 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 monolithic and composite materials for
cutting tool applications.
Silicon nitride-based ceramics are also under active
development as structural components for advanced turbine
engines. A critical factor which limits the widespread
application of these materials in such heat engines is
their tendency to catastrophic failure. Accordingly,
improvement in strength and fracture toughness of these
materials would improve their performance in the demand-
ing heat engine environment, contributing to a more rapid
development of these high performance turbine engines.
Many improvements have been made in the toughness,
abrasion resistance, high temperature strength and
A~
87-3-343 CN -2- 1 33 6 1 02
chemical inertness of silicon nitride materials, but the
stringent conditions encountered by heat engine compo-
nents and cutting tools demand even further improvement
in material characteristics. In many applications, for
example in gray cast iron and high nickel alloy machin-
ing, 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 KIC / H / ,
where KIC is the fracture toughness and H is the hard-
ness. It may be seen, therefore, that further improve-
ment 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 charac-
teristics.
Attempt has been made to increase the fracture
toughness of silicon nitride materials through the
development of a composite, in which dispersed partic-
ulate, fiber, or whisker materials are included in a
silicon nitride-based matrix. The added complexity of
composites, however, can result in improvements in
fracture toughness at the expense of strength. The
present invention provides new and improved monolithic
and composite silicon nitride-based ceramic materials
exhibiting both improved fracture toughness and improved
strength.
The wear-resistant silicon nitride-based bodies of
the invention are also useful in other wear part and
structural applications, for example in dies, nozzles,
etc.
According to one aspect of the invention, there is
provided a densified silicon nitride-based ceramic body
87-3-343 CN -3- 1 33~ 1 ~2
of improved fracture toughness and improved strength
comprising: a first phase consisting essentially of
beta-silicon nitride grains having an aspect ratio of
about 1-10 and an equivalent diameter of about 0.25-10
microns, wherein if the aspect ratio is less than about
1.5 the equivalent diameter is at least about 0.4 microns
and if the equivalent diameter is less than about 0.4
microns the aspect ratio is at least about 1.5; and an
intergranular, bonding, silica-based second phase con-
sisting essentially of silica and one or more suitableoxide densification aids; wherein the ceramic body is
formed from a starting formulation comprising silicon
nitride and about 0.5-12% by weight of the one or more
densification aids, based on the combined weight of the
one or more densification aids and the silicon nitride;
and the ceramic body has a fracture toughness of at least
5.0 MPa m~ and a modulus of rupture of at least 700 MPa.
The ceramic body according to the invention option-
ally further includes refractory whiskers or fibers
having an aspect ratio of about 3-150, uniformly distrib-
uted in the ceramic body. The equivalent diameter of the
whiskers or fibers is greater than that of the beta-
silicon nitride grains. The starting formulation com-
prises the silicon nitride, the one or more densification
aids, and about 10-50% by volume refractory whiskers or
fibers, based on the total volume of the ceramic body.
According to another aspect of the invention, there
is provided a process for producing a densified silicon
nitride-based ceramic body of improved fracture toughness
and improved strength comprising the step of: densifying
a blended powder mixture comprising silicon nitride and
about 0.5-12% by weight of one or more suitable oxide
densification aids, based on the combined weight of the
one or more densification aids and the silicon nitride,
in a nitrogen or inert atmosphere at about 1650-1850C
87-3-343 CN -4- 1 33 6 1 02
and about 3-30,000 psi, for a time sufficient to produce
a ceramic body comprising: a first phase consisting
essentially of beta-silieon nitride grains having an
aspect ratio of about 1-10 and an equivalent diameter of
about 0.25-10 mierons, wherein if the aspect ratio is
less than about 1.5 the equivalent diameter is at least
about 0.4 microns and if the equivalent diameter is less
than about 0.4 microns the aspect ratio is at least about
1.5; and an intergranular, bonding, silica-based second
phase eonsisting essentially of silica and the one or
more densification aids; and having-a fracture toughness
of at least 5.0 MPa-m~ and a strength of at least 700
MPa.
In the process according to the invention, the
blended powder mixture densified in the densifying step
optionally further includes about 10-50% by volume of
refractory whiskers or fibers having an aspect ratio of
about 3-150, based on the total volume of the ceramic
body. The equivalent diameters of the whiskers or fibers
and the silicon nitride in the blended powder mixture,
and the densification time are selected to produce the
densified ceramic body in which the equivalent diameter
of the whiskers or fibers is greater than that of the
beta-silicon nitride grains.
In accordance with other aspects of the invention,
there are provided methods according to the invention for
continuous or interrupted machining of grey cast iron
stock or nickel-based superalloy stock involve milling,
turning, or boring the stock with a cutting tool com-
prising a densified silicon nitride-based ceramic body
having a fracture toughness of at least 5.0 MPa-m~ and a
modulus of rupture of at least 700 MPa. The ceramic body
includes a first phase consisting essentially of beta-
silicon nitride grains having an aspect ratio of about
1-10 and an equivalent diameter of about 0.25-10 microns,
87-3-343 CN 1 336 t 02
and an intergranular, bonding, silica-based second phase
consisting essentially of silica and one or more suitable
oxide densification aids. In the first phase, if the
aspect ratio is less than about 1.5 the equivalent
diameter is at least about 0.4 microns, and if the
equivalent diameter is less than about 0.4 microns the
aspect ratio is at least about 1.5. The ceramic body is
formed from a starting formulation comprising silicon
nitride and about 0.5-12% by weight of the one or more
densification aids, based on the combined weight of the
one or more densification aids and the silicon nitride.
The machining speed for the grey cast iron stock is about
800-6000 sfm, and the feed rate is about 0.01-0.04
in/rev. The machining speed for the nickel-based
stock is about 200-1500 sfm and the feed rate is about
0.005-0.04 in/rev.
Some embodiments of the invention will now be
described, by way of example, with reference to the
accompanying drawing in which:
The Figure is a graphical representation of the
variation of strength with temperature for a known
material and for materials according to the invention.
The densified silicon nitride-based ceramic bodies
of the present invention comprise beta-silicon nitride
grains bonded together by an intergranular phase of a
silica-based material. The silica in the intergranular
phase is normally present in the silicon nitride compo-
nent of the starting formulation. The oxide densifica-
tion aids are also present in the intergranular phase.
The preferred densification aid is yttria, included in
the starting formulation in an amount of about 2-8% by
weight based on the combined weight of the silicon
nitride and the densification aids. The yttria may be
used alone or in combination with other suitable
87-3-343 CN -6- l 336 1 02
densification aids, for example alumina, present in the
starting formulation in an amount of about 0.5-12% by
weight. Other suitable oxide densification aids may be
included in the starting formulation with or without the
yttria and/or alumina. The densification aid, or
combination of densification aids is selected to optimize
properties desired in the ceramic body, for example high
temperature strength, chemical resistance, or oxidation
resistance. Such other suitable oxide densification aids
include, but are not limited to magnesia, ceria,
zirconia, and hafnia. The total amount of densification
aids included in the starting formulation preferably
should not exceed 12% by weight.
Impurities may be present in the starting materials
used for the manufacture of the ceramic body. The
impurities tend to become concentrated in the intergran-
ular phase during preparation of the ceramic body.
Therefore high purity starting materials are desired,
preferably those having less than about 0.1% by weight
cation impurities. A typical undesirable impurity is
calcium, which tends to deleteriously affect the second
phase and the high temperature properties.
The monolithic ceramic bodies described above have a
microstructure of beta-silicon nitride grains bonded
together by a continuous, bonding, intergranular second
phase formed from the densifying additive. Because the
intergranular second phase is continuous, its character-
istics profoundly affect the high temperature properties
of the monolithic ceramic material. The monolithic
ceramic bodies of the present invention possess high
fracture toughness and high strength at temperatures in
excess of 1200C, preferably in excess of 1500C.
In another aspect of the present invention whiskers
or fibers of hard refractory silicon carbide or a transi-
tion metal carbide, nitride, or carbonitride, or mixtures
or solid solutions thereof are dispersed in a two-phase
87-3-343 CN -7- 1 33 6 1 02
matrix. By the term "transition metal carbide, nitride,
or carbonitride", as used throughout this specification
and claims, is meant any carbide, nitride, or carboni-
tride of titanium, hafnium, tantalum, niobium, or
tungsten.
The hard refractory whiskers incorporated into
materials in accordance with this invention each comprise
a single crystal, while the fibers are polycrystalline.
The whiskers or fibers preferably have an average diame-
ter of about 1-5 microns and an average length of about
10-250 microns, with a preferred aspect ratio of length
to diameter of about 3-150.
These dispersoids may be coated if desired with a
different hard refractory material deposited as one or
more polycrystalline layers on the fiber or whisker.
Suitable coatings for the silicon carbide whiskers or
fibers include refractory oxides and nitrides. Those for
the metal carbide, nitride, or carbonitride dispersoids
include refractory oxides, nitrides, or carbides. Such
coated dispersoids may be selected to optimize bulk (e.g.
mechanical) properties and surface (e.g. chemical)
properties of the dispersoid materials in the matrix.
The useful life and performance of composite bodies
in accordance with this aspect of the invention depend,
in large part, on the volume taken up by the dispersed
phase in the article. The whiskers or fibers should
comprise about 10-50% by volume of the densified
composite.
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 grains of beta-silicon
nitride, as described above for the monolithic ceramic
body not including the whiskers or fibers. The inter-
granular phase or second phase of the matrix is formed
from one or more densification aids, as also described
- 1 3361 02
above. The degree of purity of the materials used in the
starting formulation for the composite ceramic bodies of the
invention is as described above for the monolithic bodies.
The composite ceramic bodies described herein have a
composite microstructure of refractory whiskers or fibers
uniformly dispersed in a matrix containing a first phase of
beta-Si3N4 grains and a continuous, bonding, intergranular
second 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 ceramic bodies of the
present invention possess high fracture toughness and high
strength at temperatures in excess of 1200C, preferably in
excess of 1500C.
Ceramic bodies formed from the densified monolithic
or composite materials 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. The
hard refractory materials suitable for coating monolithic or
composite ceramic bodies according to the present invention
include the refractory 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.
.~ ,,~
87-3-343 CN -9- 1 33 6 1 02
In accordance with yet another aspect of the inven-
tion, a process is provided for preparing the monolithic
or composite bodies described above, densifying 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 bodies particularly useful as cutting tools in metal
removing applications, or as structural components for
turbine engines.
The silicon nitride, the densification aid, and
optionally the hard refractory whiskers or fibers are
blended to form a starting formulation or powder mixture.
The powder mixture is then densified or compacted to a
high density, for example by sintering, hot pressing, or
hot isostatic pressing techniques. A star~ing composi-
tion for the production of the strong, tough, abrasion
resistant materials according to the present invention
may be made by employing Si3N4 powder, normally predomi-
nantly alpha-Si3N4, and preferably of average particle
size below about 3 microns.
Densification of the silicon nitride-based mono-
lithic material or the silicon nitride/whisker composite
material is aided by the incorporation of one or more of
the above-described densification aids into the initial
composition. In the starting formulations employed in
the fabrication, hard refractory whiskers or fibers
optionally comprise about 10-50% of the total volume of
the densified article, as described above, while the
densification aid comprises about 0.5-12~ by weight,
based on the combined weight of the densification aid and
the silicon nitride in the starting composition. In the
starting formulation, the balance of the mixture normally
comprises the silicon nitride powder.
The starting materials may be processed to a powder
compact of adequate green strength by thoroughly mixing
87-3-343 CN -10- 1 33 6 1 02
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 or
fibers, if included, by blending, preferably in a non-
reactive 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 moder-
ate temperatures.
Since the strength of monolithic or composite
articles in accordance with this invention decreases with
increasing porosity in the total compact, it is important
that the compact be densified 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.
The microstructural tailoring of the ceramic mate-
rials described herein is critical to providing mono-
lithic and composite bodies exhibiting both improved
fracture toughness and improved strength. This micro-
structural tailoring involves careful control of the
silicon nitride grain size and aspect ratio. In the
composite materials it also involves careful control of
the dispersoid content and its size relative to the
matrix grains. These sizes are expressed as the equiva-
lent diameters of the silicon nitride grains and the
whiskers or fibers. By the term "equivalent diameter",
as used throughout this specification and claims, is
meant the average diameter of an equiaxed particle of the
same volume as the particle, grain, whisker, or fiber.
The terms "equivalent diameter", "grain size", "aspect
ratio", and the like, as used throughout this
~7-3-343 CN -11- 1 336 1 02
specification and claims, refer to the average values of
these measurements within the ceramic body.
An increase in both fracture toughness and strength
of silicon nitride monolithic and composite materials
with an increase in grain size is unexpected, since
ceramics in general are expected to exhibit lower
strength with increased grain size (Kingery, Introduction
to Ceramics, John Wiley & Sons Inc., NY, London, 624
(1960); Evans, J. Am. Cer. Soc., 65, 127-137 (1982)). It
has been further observed that silicon nitride ceramics
containing alumina and yttria sintering aids exhibit this
behavior (G. Watting et al., Sci. of Ceramics, Proc. Non
Oxide Tech. and Eng. Ceramics, Limerick, Ireland (1986)).
It has been found, however, that an increase in
silicon nitride grain size and control of the silicon
nitride aspect ratio, through the densification process
according to the invention, can achieve an increase in
fracture toughness with an unexpected concomitant in-
crease in the strength of the silicon nitride body.
Further, when reinforcing whiskers or fibers are
included in the material, the whiskers or fibers being of
a prescribed aspect ratio and relative size, a further
increase in resistance to fracture may be achieved, again
with a concomitant increase in strength. This too is
unexpected in light of the teachings of U.S. Patent No.
4,543,345, which states that additions of silicon carbide
whiskers to silicon nitride bodies do not produce in-
creased toughness.
To achieve a monolithic or composite ceramic body
according to the invention, the powder mixture described
above is compacted and densified in nitrogen or an inert
atmosphere, e.g., argon, at a pressure of about 3-30,000
psi and a temperature of about 1650-1850C, and held at
the maximum temperature for a prolonged time, normally
about 2-12 hours. The time at maximum temperature is
sufficient to achieve grain growth in the beta-silicon
87-3-343 CN -12- l 3 3 6 1 G 2
nitride component of the ceramic body and the microstruc-
ture described above. The improved properties of the
resultant body are unexpected, in light of statements
found in the prior art that extended times at high
temperatures result in a decrease of fracture strength as
well as fracture toughness (Ziegler, Heinrich, and
~otting, J. Mater. Sci, 22, 3041-3086 (1987)).
The following Examples are presented to enable those
skilled in the art to more clearly understand and prac-
tice the present invention. These Examples should not beconsidered as a limitation upon the scope of the present
invention, but merely as being illustrative and represen-
tative thereof.
EXAMPLE 1
A powder mixture of silicon nitride, 1.5% by weight
alumina, and 6% by weight yttria was dry ball milled for
24 hours using silicon nitride milling media. The powder
was processed in a graphite die coated with boron
nitride, and hot pressed at 3500 psi and 1725C in
nitrogen for 90 minutes for Example la, or 400 minutes
for Example lb. The properties and grain sizes of the
resulting densified ceramic bodies at room temperature
and elevated temperatures are shown in Table 1 below and
in Figure 1. These results show an increase in both
fracture toughness and modulus of rupture (i.e. strength)
at room and elevated temperature with an increase in the
equivalent grain diameter of the beta-silicon nitride
grains resulting from the increase in hot pressing time.
E~AMPLE 2
A powder mixture of the composition described above
for Example 1, but with 30% by volume of the powder
mixture substituted by silicon carbide whiskers having an
equivalent diameter of 1.95 microns and an aspect ratio
of 33, was wet blended in methanol in a high shear
-
1 33 6 1 02
87-3-343 CN -13-
blender to disperse the whiskers throughout the mixture.
The blended mixture was then hot pressed as described
above for Example 1 for 400 minutes. The properties of
the resulting composite ceramic body are shown in Table
1, while the elevated temperature properties of the
composite are compared in Figure 1 to those for the
bodies of Example 1. These composite bodies exhibit
increases in both fracture toughness and strength over
both the conventional bodies and the improved monolithic
bodies of Example 1.
TABLE 1
H.P.Time, Aspect Equiv. KIC, MOR @ 25c
Ex # min. % T.D. Ratio Dia, microns MPa m~ MPa
la 90 99.3 1.8(G) 0.37 (G) 4.7 773
lb 400 99.1 1.8(G) 0.59 (G) 5.4 886
2 400 99.1 1.8(G) 0.84 (G) 6.4 975
12(W) 1.95 (W)
20 (G) = Si3N4 grains; (W) = SiC whiskers
EXAMPLE 3
A cutting tool (3b) of the composite material
according to the invention was compared with a standard
silicon nitride cutting tool (3a) in a machining applica-
tion. A turning operation was performed on an Inconel
workpiece (Inconel 718 ( Rc45)) at a cutting speed of 800
sfm and a feed rate of 0.006 in/rev. The depth of cut
was 0.040 in. The averagé notch wear of the two types of
cutting tool after 1 minute is shown below in Table 2.
The cutting tool according to the invention exhibited
significantly reduced notch wear during machining, as
compared to the standard silicon nitride cutting tool.
87-3-343 CN -14- 1 33 6 1 0 2
TABLE 2
H.P. Time, Ave.Notch Wear,
Ex # Mat'l. min in
3a Si3N4 90
1.5% A12O3
6% Y2O3
10 3b 3 4 0.017
1.5% Al2O3
6% Y2O3
30% vol SiC W
The densified monolithic and composite ceramic
bodies according to the invention are hard, non-porous,
and exhibit room and elevated temperature strength and
fracture toughness higher than that of conventional
silicon nitride materials. These bodies are useful for
ceramic articles including, but not limited to cutting
tools, extrusion dies, nozzles, dies, bearings, and wear
resistant structural parts. These bodies are especially
useful as ceramic components for heat engines and as
shaped cutting tools for continuous or interrupted
milling, turning, or boring of grey cast iron stock or
high nickel (at least 50% nickel) superalloy stock, e.g.
Inconel.
While there has been shown and described what are at
present considered the preferred embodiments o the
invention, it will be apparent 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 claims.
