SiAlON composites and method of preparing the same

A dense, substantially glass free, SiAlON ceramic material and method of making such material. The material has at least an alpha-SiAlON phase and a beta-SiAlON phase and exhibits excellent high temperature oxidation resistance and strength, good room temperature toughness and hardness, and a density greater than at least 95 percent of theoretical. The method of making the ceramic material is simple and includes adding AlN in an amount sufficient to allow formation of a desired amount of alpha-SiAlON, and in addition, convert SiO.sub.2 and Al.sub.2 O.sub.3 into the beta-SiAlON phase, preferably, by adding AlN in an amount between about 1.3X+0.08Y-0.0045XY and about 1.3X+0.2Y-0.0024XY, wherein X is a weight percent of the SiO.sub.2 based on total weight of the Si.sub.3 N.sub.4 in the mixture and Y is a weight percent of alpha-SiAlON phase desired.

This invention relates generally to SiAlON composites and a method of 
preparing the same. More particularly, this invention relates to SiAlON 
materials having no greater than 3 volume percent of intergranular 
amorphous phase and further having at least an alpha-SiAlON phase and a 
beta-SiAlON phase. 
BACKGROUND OF THE INVENTION 
"SiAlON"s are phases in silicon-aluminum-oxygen-nitrogen and related 
systems, comparable in variety and diversity with mineral 
aluminosilicates. They contain one-, two-, and three-dimensional 
arrangements of silicon oxide tetrahedra in which silicon and oxygen atoms 
are partially replaced by aluminum and nitrogen. See K. H. Jack, "Sialons 
and related nitrogen ceramics," 11 J. of Materials Sci. 1135-58 (1976). 
Ceramic materials made from SiAlON typically have high toughness, and 
elevated temperature strength and oxidation resistance. These properties 
have made SiAlON ceramics a desirable candidate for many high temperature 
industrial applications. 
In an attempt to provide a ceramic SiAlON composition which is usable in 
high temperature applications, prior art methods and compositions have 
taught the combination of alpha-SiAlON with beta-SiAlON. Typically, 
alpha-SiAlON, which appears mostly as fine equiaxed grains in the 
microstructure of the material, is associated with hardness in the 
material. On the other hand, beta-SiAlON mostly appears as elongated 
fiber-like grains in the microstructure. See Hwang et al., U.S. Pat. No. 
5,227,346. Since, the beta-SiAlON material is elongated, it adds strength 
and fracture toughness to the material. Consequently, it is an advantage 
for a material made of SiAlON to incorporate both alpha-phase and 
beta-phase SiAlON. By varying starting materials in the SiAlON 
composition, it is possible to vary the alpha- to beta-SiAlON phase ratio. 
This will give rise to a series of materials where hardness and fracture 
toughness can be tailored. 
A common problem with multi-phase SiAlON sintered bodies is that one or 
more minor phases, generally intergranular amorphous (glassy) phases, are 
formed at grain boundaries between the alpha- and beta-SiAlON phases. 
These intergranular glasses are undesirable because they generally cause 
high temperature degradation and reduction of the overall strength of the 
ceramic material. The intergranular glasses also cause the bodies to have 
lower oxidation resistance, especially at high temperatures. This may lead 
to reduced mechanical reliability such as load-bearing capability of the 
sintered bodies. For a discussion of negative implications of oxidation in 
ceramics, see Tressler, "High-Temperature Stability of Non-Oxide 
Structural Ceramics", 18[9] MRS Bull. 58-63 (1993). For this reason, it 
would be advantageous to provide a multi-phase SiAlON material with 
minimal or no intergranular glasses. 
Prior art compositions and methods have attempted to rid the SiAlON 
materials of these glassy phases. For example, since oxide sintering 
additives contribute to the formation of glasses which remain at grain 
boundaries, attempts have been made to eliminate these additives from 
starting materials. However, these methods typically produce ceramic 
bodies that are difficult, if not impossible, to fully densify. 
Eliminating the additives from the starting materials also changes the 
microstructure of the resulting sintered bodies, inhibiting formation of 
elongated grains, and thus, impairing mechanical properties. 
Another method of removing the glasses from a ceramic body is a post-fire 
heat treatment. In this method, a densified ceramic body containing an 
intergranular amorphous phase is exposed to temperatures between about 
1000.degree. C. and 1600.degree. C. in order to promote crystallization of 
the glassy phase. The crystallized phase provides better resistance to 
degradation at higher temperatures than the glass. However, a problem with 
post-fire heat treatment is that complete crystallization may be inhibited 
by kinetic factors such as a large volume change upon crystallization that 
causes stress in glass residues which are constrained by surrounding 
grains. Even if complete crystallization occurs, the temperature at which 
strength degradation is observed is only raised to a temperature somewhere 
between the glass melting point (typically about 900.degree. C. to 
1000.degree. C.) to that of an eutectic temperature along the SiAlON 
grain-boundaries (typically about 1200.degree. C. to 1500.degree. C.). 
Another method of removing glass from ceramic bodies is by chemical or 
thermal leaching. Such a process is described in Clarke, "Thermodynamic 
Mechanism for Cation Diffusion Through an Intergranular Phase: Application 
to Environmental Reactions with Nitrogen Ceramics," Progress in Nitrogen 
Ceramics 421 (Martinus Nijhoff Publishers 1983). However, this type of 
method is complicated, too tedious, not efficient, and is generally 
difficult to control. 
In order to encourage reduction of the volume percent of intergranular 
amorphous phase, some methods add higher amounts of AlN to the SiAlON 
composition. See, e.g.'s, T. Ekstrom and M. Nygren, "SiAlON Ceramics", 
75[2] J. Am. Ceram. Soc. 259, 268 (1992) (addition of an "excess" 2 wt % 
AlN to the starting mixture), and T. Ekstrom, "Preparation and Properties 
of Alpha--Si--Al--O--N Ceramics," 3[2] J. of Hard Materials 109, 113-14 
(1992). However, these methods have problems similar to those described 
above and, typically, only reduce the glass to a range of between about 3 
to 6 volume percent. The methods are further complicated in multi-phase 
SiAlON systems. In multi-phase systems, due to intricate phase 
relationships, even slight changes in starting compositions produce 
numerous undesirable end products. For example, see Sun et al., 
"Subsolidus Phase Relationships in Part of the System Si, Al, Y/N, O: The 
System Si.sub.3 N.sub.4 --AlN--YN--Al.sub.2 O.sub.3 --Y.sub.2 O.sub.3," 
74[ 11] J. Am. Ceram. Soc. 2753-58 (1991). Phase relationships are further 
affected by surface oxides, inherent in nitride raw materials. In most 
cases, the surface oxides result in either intergranular amorphous phases 
or undesirable crystalline phases. Thus, adding higher amounts of AlN in 
order to significantly lower the glass content (e.g.&lt;3 volume percent 
glass) has not been applied to SiAlON ceramics having both alpha- and 
beta-SiAlON phases. 
It would be desirable to have a method of forming a dense multi-phase 
SiAlON ceramic material having no greater than three volume percent of 
intergranular amorphous phase, but not having the above identified 
problems. 
SUMMARY OF THE INVENTION 
In a first aspect, this invention is a SiAlON ceramic material having no 
greater than 3 volume percent of intergranular amorphous phase and further 
having at least two phases comprising: 
(a) a first phase of alpha-SiAlON represented by the general formula 
M.sub.x (Si,Al).sub.12 (O,N).sub.16, wherein 0&lt;.times.&lt;2 and M is at least 
one cationic element selected from the group consisting of Li, Na, Mg, Ca, 
Sr, Ce, Y, Nd, Sm, Gd, Dy, Er, and Yb; and 
(b) a second phase of beta-SiAlON represented by the general formula 
Si.sub.6-y Al.sub.y O.sub.y N.sub.8-y, wherein 0&lt;y.ltoreq.4.3. 
The ceramic material exhibits excellent high temperature (e.g. above 
1200.degree. C.) oxidation resistance and strength and has good room 
temperature (about 23.degree. C.) toughness and hardness. In addition, 
although the intergranular amorphous phase is no greater than 3 volume 
percent based on the total volume of the material, preferably less than 1 
volume percent, density of the ceramic material is greater than 95 percent 
of theoretical, preferably greater than 99 percent. 
In a second aspect, this invention is a method for producing the SiAlON 
ceramic material of the first aspect. The method includes the steps of: 
(a) preparing a mixture of precursor materials including Si.sub.3 N.sub.4, 
AlN, optionally SiO.sub.2, optionally Al.sub.2 O.sub.3, and at least one 
oxygen or nitrogen derivative of an element selected from the group 
consisting of Li, Na, Mg, Ca, Sr, Ce, Y, Nd, Sm, Gd, Dy, Er, and Yb, 
wherein SiO.sub.2, including surface oxide from Si.sub.3 N.sub.4, is 
present in the mixture in an amount no greater than 5 weight percent of 
the total weight of the Si.sub.3 N.sub.4, 
Al.sub.2 O.sub.3, including surface oxide from AlN, is present in the 
mixture in an amount no greater than 6 weight percent based on total 
weight of the AlN, 
the Si.sub.3 N.sub.4 is present in the mixture in a range from about 70 to 
about 95 weight percent based on total weight of the mixture, and 
the AlN is present in the mixture in an amount sufficient to form a desired 
amount of alpha-SiAlON and to convert the SiO.sub.2 and Al.sub.2 O.sub.3 
into the beta-SiAlON phase, yet insufficient to form AlN polytypoids; and 
(b) subjecting the mixture to a pressure, temperature, and period of time 
sufficient to densify the mixture. 
In a preferred embodiment of this aspect of the invention, the amount of 
AlN in the mixture is a weight percent, based on the total weight of the 
mixture, between about 1.3X+0.08Y-0.0045XY and about 1.3X+0.2Y-0.0024XY, 
wherein X is the weight percent of the SiO.sub.2 based on total weight of 
the Si.sub.3 N.sub.4 and Y is between 0 and 100 and is equal to a weight 
percent of alpha-SiAlON phase desired in the ceramic material. 
This aspect of the invention discloses a simple way of preparing a 
substantially glass-free SiAlON ceramic material having at least alpha- 
and beta-SiAlON phases, as disclosed in the first aspect of this 
invention. 
DETAILED DESCRIPTION OF THE INVENTION 
Generally, this invention provides a dense, multi-phase SiAlON ceramic 
material having no greater than three volume percent of intergranular 
amorphous phase and comprising a mixture of at least an alpha-SiAlON phase 
and a beta-SiAlON phase. The ceramic material exhibits excellent high 
temperature oxidation resistance and strength and has good room 
temperature toughness and hardness. 
The alpha-SiAlON phase of this invention is represented by the general 
formula M.sub.x (Si,Al).sub.12 (O,N).sub.16. In this general formula, "M" 
is at least one cationic element selected from the group consisting of Li, 
Na, Mg, Ca, Sr, Ce, Y, Nd, Sm, Gd, Dy, Er, and Yb. "x" is a number greater 
than zero, but less than or equal to two. If the value of x exceeds 2, 
there will not be enough interstitial sites available in the lattice to 
accept the M cation, thus leading to formation of secondary phases and 
glass. Preferably, x is less than 1. Micrographs taken with a transmission 
electron microscope (TEM), or a scanning electron microscope (SEM), reveal 
that the alpha-SiAlON phase is crystalline, containing mostly fine 
equiaxed grains along with a small amount of fine elongated grains, each 
grain having a diameter less than 0.5 micrometer (.mu.m). In a preferred 
multi-phase SiAlON ceramic material, M is a multicationic mixture of Sr, 
Ca, and Y. Another preferred embodiment is when M is a multicationic 
mixture of Mg, Ca, and Y. 
The beta-SiAlON phase of this invention is represented by the general 
formula Si.sub.6-y Al.sub.y O.sub.y N.sub.8-y, wherein 0&lt;y.ltoreq.4.3. 
However, even within the above described range, if the value of y is too 
large, excessive grain growth may result. Excessive grain growth leads, in 
turn, to formation of large pores in the resulting materials. The large 
pores typically lead to a reduction in the strength and oxidation 
resistance of the ceramic material. Therefore, the value of y should, 
preferably, be larger than 0 but not more than 2, most preferably, not 
more than 1. An SEM of the beta-SiAlON phase of the preferred embodiment 
of this invention reveals large elongated grains having diameters greater 
than 0.3 .mu.m, most preferably, greater than 0.5 .mu.m. 
A significant part of this invention is the presence of no greater than 
three volume percent of intergranular amorphous phase (glassy phase) based 
on total volume of the SiAlON ceramic material. Preferably, no greater 
than one volume percent of glassy phase is present. The glassy phase, when 
present, typically comprises Si, Al, O, N, and M, wherein M is the same 
cation, or mixture of cations, that are present in the alpha-SiAlON phase. 
By eliminating or substantially reducing the glassy phase many beneficial 
properties such as excellent high temperature oxidation resistance and 
strength, as well as good room temperature toughness and hardness is 
obtained. 
The mechanical properties of the SiAlON ceramic material are readily 
measured by use of standard tests. In particular, for purposes of this 
invention, the material is evaluated for oxidation resistance, Vicker's 
Hardness, Palmqvist Toughness, fracture toughness, flexure strength 
(modulus of rupture), and flexure strength retention. These standard tests 
are described in numerous previous publications. With the exception of 
oxidation resistance and flexure strength retention, these testing methods 
are described in Hwang et al., U.S. Pat. No. 5,227,346, col. 9, line 18 
through col. 10, line 15 (incorporated herein by reference). Flexure 
strength retention, which measures the percentage of flexure strength 
retained at elevated temperature versus that at room-temperature, is 
calculated by dividing elevated temperature (e.g. 1375.degree. C.) flexure 
strength by room temperature (about 23.degree. C.) flexure strength, and 
multiplying by 100. 
Oxidation measurements are reported as mass gain per unit area at a 
specified temperature and for a given amount of time. For example, in air 
oxidation of the material of this invention is no greater than 0.25 
mg/cm.sup.2 at 1400.degree. C. for 100 hours and no greater than 1.00 
mg/cm.sup.2 at 1500.degree. C. for 50 hours. In contrast, SiAlON 
compositions having greater than three volume percent of intergranular 
amorphous phase generally have significantly higher oxidation values. See, 
e.g., Persson et al., "Oxidation Behaviour and Mechanical Properties of 
.beta.- and Mixed .alpha.-.beta.-Sialons Sintered with Additions of 
Y.sub.2 O.sub.3 and Nd.sub.2 O.sub.3," 11 J. Eur. Ceramic Soc. 363-73 
(1993). 
The Vickers Hardness test measures the resistance of a ceramic material to 
indentation. The room temperature Vickers Hardness number of the SiAlON 
ceramic material of this invention is at least about 1650 kg/mm.sup.2. 
However, generally speaking, by increasing alpha-SiAlON content (above 10 
weight percent) and decreasing glass content (below 3 volume percent), 
high hardness values (above 1700 kg/mm.sup.2) are easily obtainable with 
this invention. Preferably, the Vickers Hardness number ranges from about 
1650 kg/mm.sup.2 to about 2100 kg/mm.sup.2 at room temperature; more 
preferably, from about 1750 kg/mm.sup.2 to about 2000 kg/mm.sup.2. In 
addition, the flexure strength retention of the material is at least 55 
percent at 1375.degree. C. in air. Preferably, the flexure strength 
retention is at least 65 percent at 1375.degree. C. in air. 
Room temperature fracture toughness, which measures the resistance of a 
material to fracture under a dynamic load, is greater than about 4.0 MPa 
(m).sup.1/2, as measured by the Chevron notch technique when the loading 
displacement rate is 1 .mu.m/min. The room temperature fracture toughness 
generally ranges from about 4.0 MPa (m).sup.1/2 to about 7.5 MPa 
(m).sup.1/2. Preferably, the room temperature fracture toughness is at 
least 5.0 MPa (m).sup.1/2. 
The SiAlON bodies of this invention exhibit a room temperature Palmqvist 
toughness, which is an extension of the Vickers Hardness test, of at least 
about 25 kg/mm. Preferably, the ceramic body of this invention exhibits a 
Palmqvist toughness at room temperature within a range of from about 28 
kg/mm to about 45 kg/mm; more preferably, from about 32 kg/mm to about 40 
kg/mm. 
The alpha- and beta-SiAlON phases may be present in any amount as long as 
both phases are present in the ceramic material. Physical properties of 
the SiAlON composites can be altered by changing the ratio of the alpha 
and beta-SiAlON phases. For example, by increasing the percentage of 
alpha-phase, hardness of the material increases and toughness of the 
material decreases. Generally, a weight ratio of alpha- to beta-SiAlON, 
measured by a peak height ratio from X-ray diffraction (XRD) patterns, is 
anywhere from about 1:99 to about 90:10. Below 1:99, the material is 
typically more difficult to densify and above 90:10 strength and toughness 
diminish. Preferably, the weight ratio is between about 10:90 and about 
40:60. 
Preferably, the SiAlON ceramic material of this invention is substantially 
fully dense, having no significant porosity and a density of greater than 
95 percent of the theoretical value. More preferably, the density is 
greater than 97 percent of the theoretical value; and most preferably, 
greater than 99 percent of the theoretical value. 
The SiAlON ceramic materials of this invention are prepared by densifying a 
powder mixture of precursor materials. The precursor materials include 
Si.sub.3 N.sub.4, AlN, optionally SiO.sub.2, optionally Al.sub.2 O.sub.3, 
and at least one oxygen or nitrogen derivative of an element selected from 
the group consisting of Sr, Ca, Mg, Li, Na, Ce, Y, Nd, Sm, Gd, Dy, Er, and 
Yb. 
The Si.sub.3 N.sub.4 precursor material used in preparing the ceramic 
material of this invention is present in an amount which is suitably 
within a range of from about 70 to about 95 weight percent (wt-%) based on 
total weight of the mixture. The Si.sub.3 N.sub.4 can be any Si.sub.3 
N.sub.4 powder, including the crystalline forms of alpha-Si.sub.3 N.sub.4 
and beta-Si.sub.3 N.sub.4, or noncrystalline amorphous Si.sub.3 N.sub.4, 
or mixtures thereof. However, the preferred form of Si.sub.3 N.sub.4 
powder has a high purity and a high alpha/beta weight ratio. In addition, 
the powder may be of any size or surface area provided the ceramic 
material of this invention is obtained. Preferably, the particles have an 
average diameter within a range of from about 0.2 .mu.m to about 5.0 
.mu.m; more preferably, from about 0.2 .mu.m to about 1.0 .mu.m. The 
powder has a surface area that is desirably within a range of from about 5 
m.sup.2 /g to about 15 m.sup.2 /g, as determined by the 
Brunauer-Emmett-Teller (BET) method of measuring surface area, which is 
described in C. N. Satterfield, Heterogeneous Catalysis in Practice 102-05 
(McGraw-Hill Book Company, 1980). The range is preferably from about 8 
m.sup.2 /g to about 12 m.sup.2 /g. 
Oxygen is inherently introduced into the mixture, usually as oxide or 
oxynitride coatings on surfaces of starting powders such as Si.sub.3 
N.sub.4 and AlN particles. Although it may be possible for the oxygen to 
be present in other forms, it is generally believed to exist predominantly 
in the form of Si.sub.02 and Si--O--N on the Si.sub.3 N.sub.4 and Al.sub.2 
O.sub.3 and Al--O--N on the AlN. For calculation purposes, one skilled in 
the art will be able to convert any oxygen content to an equivalent 
content of SiO.sub.2 and Al.sub.2 O.sub.3. Thus, for purposes of this 
invention, the oxygen content of the mixture is identified in terms of 
wt-% of SiO.sub.2 and Al.sub.2 O.sub.3. 
The amount of SiO.sub.2 and Al.sub.2 O.sub.3 introduced inherently from the 
Si.sub.3 N.sub.4 and AlN particles varies according to the purity of the 
starting powders and their methods of manufacture. Typically, the amount 
of SiO.sub.2, introduced naturally through the Si.sub.3 N.sub.4 powder, 
ranges from about 1.5 to about 5.0 wt-%, based on the total weight of the 
Si.sub.3 N.sub.4. Thus, for purposes of this invention, any given wt-% of 
Si.sub.3 N.sub.4 or AlN inherently includes its respective wt-% of oxide. 
Additional amounts of SiO.sub.2 may also be directly added to the mixture, 
but preferably, for purposes of this invention, the total amount of 
SiO.sub.2 introduced into the mixture (inherently and directly) is no 
greater than 5 wt-% of the total weight of the Si.sub.3 N.sub.4. The 
amount of Al.sub.2 O.sub.3 introduced naturally through the AlN powder 
typically ranges from about 1.5 to about 6.0 wt-%, based on the total 
weight of the AlN. Additional amounts of Al.sub.2 O.sub. 3 may also be 
directly added to the mixture, but preferably, for purposes of this 
invention, the total amount of Al.sub.2 O.sub.3 introduced into the 
mixture (inherently and directly) is no greater than 6.0 wt-% of the total 
weight of the AlN. 
At least one oxygen or nitrogen derivative of an element selected from the 
metal group consisting of Li, Na, Mg, Ca, Sr, Ce, Y, Nd, Sm, Gd, Dy, Er, 
and Yb is added to the mixture. The "oxygen or nitrogen derivative" 
includes any compound of oxygen, nitrogen, or both oxygen and nitrogen, 
wherein the compound has at least one element from the above identified 
metal group. The oxygen or nitrogen derivative may be added in any amount 
provided the SiAlON ceramic material of this invention is formed. 
Preferably, the oxygen or nitrogen derivative is added in an amount 
between about 0.1 wt-% and about 20.0 wt-% based on total weight of the 
mixture, wherein the wt-% is calculated as that of the oxide form of the 
derivative. One skilled in the art will be able to convert a wt-% of any 
oxygen or nitrogen derivative (e.g. CaCO.sub.3) to the equivalent wt-% in 
its oxide form (e.g. CaO). More preferably, the oxygen or nitrogen 
derivative is added in an amount between about 0.5 wt-% and about 10 wt-% 
based on total weight of the mixture. 
To obtain SiAlON ceramic materials having no greater than three volume 
percent glassy phase as measured from SEM and TEM photomicrographs, AlN 
must be introduced in an amount sufficient to allow formation of a desired 
amount of alpha-SiAlON, and in addition, sufficient to convert the oxides 
(SiO.sub.2 and Al.sub.2 O.sub.3) into the beta-SiAlON phase. AlN is 
responsible, in one role, for reacting with the Si.sub.3 N.sub.4 and the 
oxygen or nitrogen derivative(s) to form the alpha-SiAlON phase. In 
another role, in order to prevent or minimize formation of intergranular 
amorphous phases, an additional amount of AlN is required in order to 
react with the SiO.sub.2, Al.sub.2 O.sub.3, and Si.sub.3 N.sub.4 to form 
beta-SiAlON. Adding additional Si.sub.3 N.sub.4 is not necessary to form 
the beta-SiAlON in this invention since the amount of Si.sub.3 N.sub.4 
added (from about 70 to about 95 wt-%) will be sufficient to form both 
alpha- and beta-SiAlON phases. 
The amount of AlN added must, however, be less than an amount necessary to 
form AlN polytypoids. For a discussion of AlN polytypoids, see Cannard et 
al., "The Formation of Phases in the AlN-rich Corner of the Si--Al--O--N 
System," 8 J. Eur. Ceramic Soc. 375-82 (1991), and Bergman, "The 
Si--Al--O--N System at Temperatures of 1700.degree.-1775.degree. C.", 8 J. 
Eur. Ceramic Soc. 141-51 (1991). Formation of AlN polytypoids can 
adversely affect mechanical properties of the SiAlON material by lowering 
both the melting point and the oxidation resistance. The existence of AlN 
polytypoids may be measured by such methods as XRD analysis. For purposes 
of this invention, it is possible that an amount of AlN polytypoids that 
is below the detection limit of typical XRD analysis (less than about 2 
wt-%) may form without adversely affecting the material. Thus, addition of 
an amount of AlN that forms less than the XRD analysis AlN polytypoid 
detection limit is considered within the scope of this invention and "less 
than an amount necessary to form AlN polytypoids." 
Preferably, the amount of AlN in wt-% is between about 1.3X+0.08Y-0.0045XY 
and about 1.3X+0.2Y-0.0024XY, wherein "X" is the wt-% of the SiO.sub.2 
based on total weight of the Si.sub.3 N.sub.4 and "Y" is a wt-% of 
alpha-SiAlON phase desired in the ceramic material. As discussed 
previously, one skilled in the art may also convert the wt-% of SiO.sub.2 
to an equivalent oxygen content. For example, if "X'" is equal to oxygen 
content (instead of SiO.sub.2 content) based on total weight of the 
Si.sub.3 N.sub.4, the amount of AlN is between about 2.4X'+0.08Y-0.008X'Y 
and about 2.4X'+0.2Y-0.004X'Y, wherein "Y" is the wt-% of alpha-SiAlON 
phase desired in the ceramic material. 
Generally, if a molar ratio of total SiO.sub.2 to total Al.sub.2 O.sub.3 is 
at least 25 to 1, the desirable amount of AlN is closer to a lower limit 
of 1.3X+0.08Y-0.0045XY. In contrast, the desirable amount of AlN is closer 
to an upper limit of 1.3X+0.2Y-0.0024XY if a molar ratio of total 
SiO.sub.2 to total Al.sub.2 O.sub.3 is less than 25 to 1. 
As discussed previously, a preferred multiphase SiAlON ceramic material 
comprises an alpha-SiAlON phase expressed by the chemical formula M.sub.x 
(Si,Al).sub.12 (O,N).sub.16, where 0&lt;x&lt;2, and M is a multicationic mixture 
of Sr, Ca, and Y. This ceramic material preferably has an 
alpha:beta-SiAlON phase wt-% ratio of 20:80. A preferred method for 
forming this ceramic material is, first, to form a precursor mixture 
containing about 92.7 wt-% Si.sub.3 N.sub.4 having about 2.3 wt-% 
SiO.sub.2 based on total weight of the Si.sub.3 N.sub.4, about 5.5 wt-% 
AlN having about 2.1 wt-% Al.sub.2 O.sub.3 based on the total weight of 
the AlN, about 0.5 wt-% SrO, about 0.3 wt-% CaO, and about 1.0 wt-% 
Y.sub.2 O.sub.3, all wt-%'s, unless otherwise indicated, being based on 
the total weight of the precursor mixture. This mixture may then be 
densified by the method of this invention (discussed infra). 
The desirable preparation of a finely-divided powder mixture containing the 
precursor materials may be accomplished in any suitable manner with 
conventional apparatus. A preferred method of preparing the precursor 
mixture includes attrition milling, with attritor media, preferably 
zirconia balls, in a carrier medium (or solvent) for a period of time 
sufficient to form a finely-dispersed suspension. The preparation of the 
finely-dispersed suspension requires no particular order of addition of 
the precursor materials to the carrier medium. After attrition milling, 
excess carrier medium is removed by filtration or otherwise. The mixture 
is then dried and separated from the attritor media to yield a product 
having substantially the same proportions as the original ingredients. 
The carrier medium may be any inorganic or organic compound which is a 
liquid at room temperature and atmospheric pressure. Examples of suitable 
carrier media include: water; alcohols, such as methanol, ethanol and 
isopropanol; ketones, such as acetone and methyl ethyl ketone; aliphatic 
hydrocarbons, such as pentanes and hexanes; and aromatic hydrocarbons, 
such as benzene and toluene. The carrier medium is desirably aqueous. The 
function of the carrier medium is to impart a viscosity suitable for 
mixing the solid powders. Any quantity of carrier medium which achieves 
this purpose is sufficient and acceptable. Preferably, a quantity of 
carrier medium is employed such that the solids content is in a range from 
about 15 volume percent to about 50 volume percent based on a total volume 
of the suspension. Below the preferred lower limit, the viscosity of the 
solid suspension may be too low and deagglomeration mixing may be 
ineffective. Above the preferred upper limit, the viscosity may be too 
high, and deagglomeration mixing may be difficult. 
If the carrier medium is toluene, a coupling agent, such as an aluminate 
coupling agent commercially available from Kenrich Petrochemicals under 
the trade designation KEN-REACT KA 322, may be used to aid in forming a 
suspension. When using an alcohol such as methanol, a dispersant such as a 
polyethyleneimine may be used to facilitate mixing and a flocculant such 
as oleic acid may be used to ease recovery of the powder mixture. When 
using water, an amino-alcohol dispersant such as AMP98 from Angus Chemical 
Co. may be used to facilitate mixing. To limit the tendency of active 
nitrides (e.g. AlN) to hydrolyze in the presence of moisture, the active 
nitrides are generally not added into the mixture until the last 
approximately 15 minutes of milling. 
To aid in the dispersion of components of the powder mixture, one or more 
surfactants or dispersants can be added to the suspension. The choice of 
surfactant(s) or dispersant(s) can vary widely as is well-known in the 
art. Any amount of surfactant or dispersant is acceptable provided 
dispersion of powder mixture components is improved. Typically, the amount 
of surfactant is in a range of from about 0.01 to 2.0 wt-% of the powder 
mixture. 
The components of the powdered combination are added to the carrier medium 
in any manner which gives rise to a finely dispersed suspension of the 
components. Typically, the process is conducted in a large vessel at room 
temperature under air with vigorous stirring. Any common mixing means is 
suitable, such as a ball-milling device or an attrition mixer. An 
ultrasonic vibrator may be used in a supplementary manner to break down 
smaller agglomerates. The attrition mixer is preferred. 
Once mixed, the finely-dispersed suspension is ready for processing into 
greenware for eventual sintering. For example, the suspension can be 
slip-cast by techniques well-known in the art. Alternatively, the 
suspension can be dried into a powder and ground for use in a hot-pressing 
process. A typical process is disclosed in Pyzik, U.S. Pat. No. 5,120,328, 
col. 9, lines 31-65 (incorporated herein by reference). Drying may be 
accomplished by standard drying means, such as by spray-drying or oven 
drying. Preferably, drying of the admixture of the powder mixture and the 
attritor balls is accomplished in an oven under a nitrogen purge after 
removal of excess carrier medium. During the drying process, additional 
free carrier medium is removed. The temperature of the drying depends on 
the boiling point of the carrier medium employed. Typically, the drying 
process is conducted at a temperature just below the boiling point of the 
carrier medium under atmospheric pressure. After drying, the resulting 
powder is separated from the attritor media or balls and sieved through a 
screen to obtain a powder which then may be densified. 
Any method of densifying the powder will suffice provided the ceramic 
material of this invention is formed. The preferred method of densifying 
the powder mixture is by either hot-pressing, hot isostatic pressing 
(HIP), or gas pressure sintering, each of which is a well known method of 
densification in the art. Most preferably, the method is hot-pressing, 
which comprises heating the powder under pressure to obtain the densified 
ceramic body. Any standard hot-pressing equipment is acceptable, such as a 
graphite die equipped with a heating means and a hydraulic press. 
Particularly suitable results are obtained when the die is fabricated from 
a material which is substantially non-reactive with components of the 
powder mixture at hot-pressing temperatures and has a mean linear 
coefficient of expansion greater than that of the SiAlON. Hot-pressing may 
be conducted under an inert atmosphere, such as nitrogen, to prevent 
oxidation and decomposition of the Si.sub.3 N.sub.4 at high temperatures. 
The direction of pressing is uniaxial and perpendicular to the plane of 
the die plates. 
Any processing temperature and pressure will suffice providing the novel 
SiAlON ceramic material of this invention, described herein, is obtained. 
Typically, however, the temperature for hot-pressing is between about 
1550.degree. C. and 1950.degree. C. Preferably, the temperature is 
maintained within a range of from about 1750.degree. C. to about 
1950.degree. C. during pressurizing. More preferably, the range is from 
about 1825.degree. C. to about 1925.degree. C. At these temperatures for 
hot-pressing, a pressure of between about 20 MPa and about 40 MPa is 
typically applied in order to attain substantially full densification. 
It is noted that the accurate measurement of high temperatures, such as 
those quoted hereinabove, is technically difficult. Some variation in the 
preferred temperature range may be observed depending on the method 
employed in measuring the temperature. The preferred temperatures of this 
invention are measured by a tungsten-rhenium thermocouple, obtained from 
and calibrated by the Omega Company. 
Generally, when densifying the mixture by HIP, a pressure of between about 
10 MPa and 200 MPa is applied at a temperature of between about 
1550.degree. C. to about 2100.degree. C. However, when densifying the 
mixture by gas pressure sintering a typical pressure of between about 1 
MPa and 10 MPa is applied at a temperature of between about 1650.degree. 
C. to about 2050.degree. C. 
The amount of time that the powder mixture is heated under pressure should 
be sufficient to bring the powder to essentially complete densification. 
Those skilled in the art will be able to determine suitable times without 
undue experimentation. For example, ram movement during hot-pressing is a 
good indicator of the extent of densification. As long as the ram 
continues to move, densification is incomplete. When the ram has stopped 
moving for at least about 15 minutes, the densification is essentially 
complete at about at least 95 percent or greater of the theoretical value. 
Preferably, the density of the material is greater than about 97 percent 
of the theoretical value when ram movement stops. Most preferably, the 
density of the material is greater than about 99 percent of the 
theoretical value when ram movement stops. Thus, the time required for 
hot-pressing is the time during ram movement plus about an additional 15 
to 30 minutes. Preferably, the time is within a range of from about 15 
minutes to about 5 hours; more preferably, from about 30 minutes to about 
90 minutes; and most preferably, about 45 minutes to about 75 minutes. 
The methods of densification, described hereinbefore, allow for the 
formation of SiAlON ceramic articles which can be used for such 
applications as cutting tools and engine components, particularly high 
wear and high temperature components. A variety of shapes can be made; one 
common shape being a flat plate. Articles such as cutting tools can be 
fabricated by slicing and grinding these plates into a variety of 
appropriate shapes. 
Illustrative Embodiments 
The following examples serve to illustrate the novel, dense, SiAlON ceramic 
materials of this invention and the method of preparing said materials. 
The examples are not intended to limit the scope of this invention. 
The Si.sub.3 N.sub.4 powder is commercially available from Ube Industries, 
Ltd. under the trade designation SN-E10. It contains: 1.2 wt-% oxygen; 
less than 100 ppm C1; less than 100 ppm Fe; less than 50 ppm Ca; and less 
than 50 ppm Al. It has a crystallinity of greater than 99.5 percent, a 
ratio of .beta./(.alpha.+.beta.) of less than 5, and a surface area of 
11.2 m.sup.2 /g. Tokuyama Soda Kabushiki Kaisha supplies AlN powder which 
has 0.9 wt-% oxygen. Moly Corp. supplies Y.sub.2 O.sub.3, and Alfa 
Products supplies MgO, SrO, Al.sub.2 O.sub.3, and SiO.sub.2. Aldrich 
Chemical Co. supplies CaO. Zirconia, in the form of zirconia balls, is 
available from Union Process.

EXAMPLE 1 
A series of precursor mixtures is prepared using the components and wt-% 
amounts identified as Mixtures (A)-(D) in Table I, infra. Each mixture is 
formulated to produce a SiAlON ceramic material having a nominal 20 wt-% 
alpha phase. "Nominal" alpha-SiAlON content refers to a calculated 
alpha-SiAlON content in a theoretical precursor mixture wherein no surface 
oxides exist in the mixture. Mixture A is not formulated to compensate for 
any SiO.sub.2 and Al.sub.2 O.sub.3 ; Mixture B is formulated to compensate 
for half of the SiO.sub.2 and Al.sub.2 O.sub.3 ; and Mixtures C and D are 
formulated, under the method of this invention, to compensate for all of 
the SiO.sub.2 and Al.sub.2 O.sub.3. 
The precursor mixtures are formed into greenware by, first, attrition 
milling each mixture separately in an attritor (Union Process batch 
attritor, Model 01HD--750 cc capacity with polytetrafluoroethylene coated 
tube and stirrer) containing zirconia balls and a carrier medium with a 
stirring rate of 300 revolutions per minute (rpm) and for a mixing time 
of 1 hour to form a slurry. Methanol is used as the carrier medium in an 
amount sufficient to provide a solids content of 15 to 30 volume percent 
solids. The slurry is poured through a 400 mesh (37 .mu.m) plastic sieve 
to remove the zirconia balls, then dried in a 65.degree. C. oven. Finally, 
the dried powder is sieved through 60 mesh (180 .mu.m) screen to form a 
finely-divided powder mixture. 
TABLE I 
______________________________________ 
Mixture Si.sub.3 N.sub.4 
Y.sub.2 O.sub.3 
SrO CaO AlN 
______________________________________ 
Mixture 95.33 1.01 0.56 0.25 2.85 
Mixture 94.06 0.97 0.54 0.25 4.19 
B 
Mixture 92.73 0.96 0.53 0.24 5.54 
C 
Mixture 91.43 0.95 0.52 0.24 6.86 
D 
______________________________________ 
About 85 grams of each powder mixture is densified by either hot pressing 
or HIP as discussed supra, and specifically identified in Tables II-V, to 
form billets having an approximate size of 1.27cm.times.3.81 cm.times.5.08 
cm for samples that are hot-pressed and 5 cm.times.2.0cm.times.5.8 cm for 
samples that are densified by HIP. These samples are identified in the 
tables as Sample Numbers 1-7. Table II identifies Mixtures (A)(C) which 
are hot pressed under flowing nitrogen at a pressure of 35 MPa. Sample 
Numbers 1-3 in Table II are hot pressed for 1 hour at 1825.degree. C. and 
Sample Numbers 4-5 are hot pressed an additional 5 hours at 1925.degree. 
C. Tables III-V identify Mixtures (A)-(D) which are densified by HIP under 
the following varying conditions: Table III densifies by HIP for 1 hour at 
1825.degree. C.; Table IV increases the amount of time at HIP to 2 hours 
at the same temperature; and Table V densifies for 1 hour, but increases 
the temperature of the HIP to 1925.degree. C. 
Each of Tables II-V report various properties for each Sample Number. The 
density of each Sample Number is measured by the water immersion method, 
as described in Richerson et al., Modern Ceramic Engineering (1982), and 
the volume percent (v/o) of intergranular amorphous phase ("glass 
content") is measured by SEM and TEM photomicrographs. Vicker's hardness, 
Palmqvist Toughness, fracture toughness, room temperature strength, 
flexure strength, flexure strength retention, and oxidation resistance are 
also measured. Results of each table are briefly discussed below each 
respective table. 
TABLE II 
__________________________________________________________________________ 
Prepared By Hot Press 
Sample Number 1* 2* 3 4* 5 
Mixture A B C A C 
__________________________________________________________________________ 
Hot Press Condition 
1825/1 
1825/1 
1825/1 1825/1 + 
1825/1 + 
(.degree.C./hrs.) 1925/5 
1925/5 
Density gm/cc 3.217 3.216 2.50 3.218 3.212 
(% Theoretical Density) 
(99.9%) 
(99.9%) 
(77.5%) 
(99.9%) 
(99.6%) 
Glass Content, v/o 
6.3 4.5 &lt;1 6.8 &lt;1 
Vicker's Hardness (14 Kg 
1694 1811 &lt;1000 1694 1901 
load), Kg/mm.sup.2 
Palmqvist Toughness, Kg/mm 
45.7 38.3 -- 38.8 32.3 
Fracture Toughness (MPa .multidot. m1/2 
6.2 6.1 -- 7.6 5.8 
Room Temperature Strength, 
902 837 -- 885 723 
MPa 
Flexure Strenth Retention at 
55 60 -- 46 76 
1375.degree. C. (%) 
Oxidation, Weight Gain 
(mg/cm.sup.2) 
1400.degree. C. - 100h 
0.93 0.70 -- 0.98 0.21 
1500.degree. C. - 50h 
5.01 4.18 -- 5.33 0.60 
Oxidized Products 
cristobalite 
cristobalite 
-- cristobalite 
cristobalite 
Y--Al- 
Y-silicate Y-- Al- 
silicate silicate 
__________________________________________________________________________ 
*This Sample is not an embodiment of the invention. 
Sample Numbers 1, 2, and 4 have too much glass (&gt;3 v/o) and have less 
oxidation resistance and strength retention than Sample Number 5, which 
was prepared from a precursor material mixture formulated and processed in 
accordance with the present invention. Sample Number 3 is the same 
precursor material mixture as Sample Number 5, having a glass content less 
than 1 volume percent, but the density is low (77.5%). Increasing the 
temperature and time at hot press on Sample Number 3 produces Sample 
Number 5, a substantially fully dense (99.6%) material which still has a 
glass content less than 1 volume percent. 
TABLE III 
__________________________________________________________________________ 
Prepared By HIP 
Sample Number 6* 7* 8 9 
__________________________________________________________________________ 
Mixture A B C D 
Hot Isostatic Press 
1825/40/1 
1825/40/1 
1825/40/1 
1825/40/1 
Condition (.degree.C./MPa/hrs) 
Density gm/cc 3.220 3.216 3.216 3.219 
(% Theoretical Density) 
(100%) (99.9%) 
(99.9%) 
(99.7%) 
Glass Content, v/o 
6.1 4.6 &lt;1 &lt;1 
Vicker's Hardness (14 Kg 
1727 1889 1931 1914 
load), Kg/mm.sup.2 
Palmqvist Toughness, Kg/mm 
45.2 38.1 35.0 26.8 
Fracture Toughness (MPa .multidot. m1/2) 
6.5 6.3 5.6 5.1 
Flexure Strenth Retention at 
53 58 75 80 
1375.degree. C. (%) 
Oxidation, Weight Gain 
506 4.5 0.63 0.57 
mg/cm.sup.2 1500.degree. C. for 50 hrs 
Oxidation Products 
cristobalite 
cristobalite 
cristobalite 
cristobalite 
Y--Al-silicate 
Y--Al-silicate 
__________________________________________________________________________ 
*This Sample is not an embodiment of the invention. 
Sample Numbers 6 & 7 have too much glass (&gt;3 v/o) and have less oxidation 
resistance and strength retention than Sample Numbers 8 & 9, which have 
been prepared from precursor material mixtures formulated and processed in 
accordance with the present invention. 
TABLE IV 
__________________________________________________________________________ 
Prepared by HIP 
Sample Number 10* 11* 12 13 
__________________________________________________________________________ 
Mixture A B C D 
Hot Isostatic Press 
1825/40/2 
1825/40/2 
1825/40/2 
1825/40/2 
Condition (.degree.C./MPa/hrs) 
Density gm/cc 3.210 3.209 3.220 3.213 
(% Theoretical Density) 
(100%) (99.7%) 
(99.8%) 
(99.5%) 
Glass Content, v/o 
5.9 4.3 &lt;1 &lt;1 
Vicker's Hardness (14 Kg 
1694 1811 1901 1889 
load), Kg/mm.sup.2 
Palmqvist Toughness, Kg/mm 
36.2 39.5 35.5 26.3 
Flexure Strenth Retention at 
46 56 73 80 
1375.degree. C. (%) 
Oxidation, Weight Gain 
5.2 4.4 0.60 0.59 
mg/cm.sup.2 1500.degree. C. for 50 hrs 
Oxidation Products 
cristobalite 
cristobalite 
cristobalite 
cristobalite 
Y--Al-silicate 
Y--Al-silicate 
__________________________________________________________________________ 
*This Sample is not an embodiment of the invention. 
Materials 10 & 11 have too much glass (&gt;3 v/o) and have less oxidation 
resistance and strength retention than Sample Numbers 12 & 13, which have 
been prepared from precursor material mixtures formulated and processed in 
accordance with the present invention. 
TABLE V 
__________________________________________________________________________ 
Prepared By HIP 
Sample Number 14* 15* 16 17 
__________________________________________________________________________ 
Mixture A B C D 
Hot Isostatic Press 
1925/40/1 
1925/40/1 
1925/40/1 
1925/40/1 
Condition (.degree.C./MPa/hrs) 
Density gm/cc 3.218 3.216 3.216 3.215 
(% Theoretical Density) 
(99.9%) 
(99.9%) 
(99.7%) 
(99.5%) 
Glass Content, v/o 
6.2 4.5 &lt;1 &lt;1 
Vicker's Hardness (14 Kg 
1634 1654 1833 1890 
load), Kg/mm.sup.2 
Palmqvist Toughness, Kg/mm 
35.9 41.7 37.7 31.4 
Oxidation, Weight Gain 
5.5 4.6 0.63 0.58 
mg/cm.sup.2 1500.degree. C. for 50 hrs 
Oxidation Products 
cristobalite 
cristobalite 
cristobalite 
cristobalite 
Y--Al-silicate 
Y--Al-silicate 
__________________________________________________________________________ 
*This Sample is not an embodiment of the invention. 
Materials 14 & 15 have too much glass (&gt;3 v/o) and have less oxidation 
resistance than Sample Numbers 16 & 17, which have been prepared from 
precursor material mixtures formulated and processed in accordance with 
the present invention. 
EXAMPLE 2 
A series of precursor mixtures is prepared using the methods described in 
Example 1, with the exception that each mixture is formulated to have a 
different alpha-SiAlON content. These mixtures are identified as Mixtures 
(E)-(H) in Table VI, infra. Mixtures F and H are formulated, under the 
method of this invention, to compensate for the SiO.sub.2 and Al.sub.2 
O.sub.3. 
TABLE VI 
______________________________________ 
Mixture Si.sub.3 N.sub.4 
Y.sub.2 O.sub.3 
SrO CaO AlN 
______________________________________ 
E 97.99 0.32 0.17 0.25 1.27 
F 95.15 0.31 0.17 0.24 4.13 
G 87.71 3.00 1.66 .025 
7.38 
H 85.43 2.92 1.61 0.24 9.79 
______________________________________ 
Mixtures (E)-(H) are each hot pressed under flowing nitrogen at 35 MPa 
under conditions set out in Table VII to form billets of approximate size 
1.27 cm.times.3.81 cm.times.5.08 cm, identified as Sample Numbers 18-21. 
In this table "nominal" alpha-SiAlON content refers to the alpha-SiAlON 
content present in a theoretical composition wherein no surface oxides 
exist in starting materials. "Measured" refers to actual alpha-SiAlON 
content which is present in the composition as measured by XRD. Table VII 
reports various properties for each Sample Number. Results of the table 
are briefly discussed below the table. 
TABLE VII 
______________________________________ 
Prepared By Hot Press 
Sample Number 
18* 19 20* 21 
______________________________________ 
Composition E F G H 
Hot Press Condition 
1825/1 1825/1 + 1825/1 1825/1 
(.degree.C./hrs) 1925/1 
Density, gm/cc 
3.205 3.083 3.242 3.247 
(% Theoretical 
(100%) (96.2%) (99.8%) 
(99.9%) 
Density) 
Glass Content, v/o 
4.8 &lt;1 9.5 2.8 
.alpha.-SiAlON content, w/o 
nominal 10 10 50 50 
measured 0 12 70 47 
Vicker's Hardness 
1737 1873 1931 2110 
(14 Kg load), 
Kg/mm.sup.2 
Palmqvist Toughness, 
28.1 33.5 29.5 26.9 
Kg/mm 
Oxidation Weight 
6.0 0.51 6.7 0.93 
Gain, mg/cm.sup.2 
1500.degree. C. for 50 hrs 
______________________________________ 
*This Sample is not an embodiment of the invention. 
By adjusting the amount of AlN to compensate for surface oxides (calculated 
as SiO.sub.2 and Al.sub.2 O.sub.3) by the method of this invention in 
Sample Numbers 19 and 21, the amount of glass is less than in Sample 
Numbers 18 and 20, and the actual alpha-SiAlON content is close to the 
nominal alpha-SiAlON content. One may also note that Sample Number 19 has 
been densified under hot press conditions for an additional one hour at 
1925.degree. C. However, as demonstrated in Table II, additional time and 
temperature at hot press conditions has minimal effect on glass content in 
the materials of this invention, but is advantageous for increasing the 
density. 
EXAMPLE 3 
A series of precursor mixtures is prepared using the methods described in 
Example 2. These mixtures are identified as Mixtures (I)-(L) in Table 
VIII, infra. Mixtures J and L are formulated, under the method of this 
invention, to compensate for the SiO.sub.2 and Al.sub.2 O.sub.3. 
TABLE VIII 
______________________________________ 
Mixture Si.sub.3 N.sub.4 
Y.sub.2 O.sub.3 
MgO CaO AlN 
______________________________________ 
I 96.27 0.60 0.33 0.25 2.54 
J 93.55 0.59 0.32 0.24 5.30 
K 94.89 1.94 0.00 0.00 3.17 
L 92.25 1.88 0.00 0.00 5.87 
______________________________________ 
Mixtures (I)-(L) are each hot pressed under flowing nitrogen at 35 MPa 
under conditions set out in Table IX to form billets of approximate size 
1.27 cm.times. 3.81 cm.times.5.08 cm, identified as Sample Numbers 22-25. 
Table IX reports various properties for each Sample Number. Results of the 
table are briefly discussed below the table. 
TABLE IX 
______________________________________ 
Prepared By Hot Press 
Sample Number 
22* 23 24* 25 
______________________________________ 
Mixture I J K L 
Hot Press Condition 
1825/1 1825/1 + 1825/1 1825/1 + 
(.degree.C./hrs) 1925/1 1925/1 
Density, gm/cc 
3.216 3.205 3.225 3.210 
(% Theoretical 
(100%) (99.7%) (100%) (99.5%) 
Density) 
Glass Content, v/o 
5.3 &lt;1 5.2 &lt;1 
.alpha.-SiAlON content, w/o 
nominal 20 20 30 30 
measured 0 23 5 36 
Vicker's Hardness 
1655 1815 1849 1989 
(14 Kg load), 
Kg/mm.sup.2 
Palmqvist Toughness, 
32.5 34.1 36.8 31.2 
Kg/mm 
Oxidation Weight 
5.8 &lt;1 4.9 &lt;1 
Gain, mg/cm.sup.2 
1500.degree. C. for 50 hrs 
______________________________________ 
*This Sample is not an embodiment of the invention. 
This example demonstrates that the invention is also applicable to both 
multi-cation (Y--Mg--Ca)-SiAlON and single-cation (e.g., Y-SiAlON) 
systems. Sample Numbers 23 and 25, formulated under the method of this 
invention, both have glass contents less than 1 volume percent. 
Therefore, from the results of Examples 1-3, as shown in Tables I-IX, it 
can be seen that the invention provides a ceramic material which exhibits 
excellent oxidation resistance and strength retention at elevated 
temperatures, and good room temperature toughness and hardness. The 
present material may have these superior properties due to the reduction 
of intergranular amorphous phase in SiAlON having both alpha- and 
beta-SiAlON phases. This reduction of glassy phase seems to yield a 
material which has superior properties over prior art materials. 
While the invention has been described in terms of specific embodiments, it 
will be appreciated that other embodiments could readily be adapted by one 
skilled in the art. Accordingly, the scope of the invention is to be 
limited only by the following claims.