This invention relates generally to ceramic materials formed from powder, and more particularly to hot-pressed structural materials comprising silicon nitride (Si.sub.3 N.sub.4) wherein the oxygen content is controlled by maintaining the molar ratio of MgO and SiO.sub.2.

FIELD OF THE INVENTION 
Description of the Prior Art 
Heretofore it has been known in the art to hotpress Si.sub.3 N.sub.4 powder 
with the addition of small amounts of magnesium oxide (MgO) as a 
densification aid. See for example British Pat. Nos. 1,092,637 and 
1,273,145. The MgO reacts with the silicon oxide (SiO.sub.2) surface layer 
on each Si.sub.3 N.sub.4 particle to form a liquid at high temperatures 
which aids in densifying the Si.sub.3 N.sub.4 particles by a solution 
precipitation mechanism. Upon cooling, the resultant densified mass 
consists of Si.sub.3 N.sub.4 grains and a residual grain boundary phase. 
The addition of MgO therefore provides higher densities in the silicon 
nitride body than for the case where no MgO is employed. While this MgO 
addition is beneficial from the densification standpoint, we have 
determined that the residual grain boundary phase including MgO and 
SiO.sub.2 can become viscous at high temperatures allowing the Si.sub.3 
N.sub.4 grains to separate and slide under stress thus causing a 
degradation of the material's mechanical properties. 
SUMMARY OF THE INVENTION 
In accordance with our invention high purity Si.sub.3 N.sub.4 powder is 
used as a starting material and the MgO/SiO.sub.2 molar ratio is 
controlled between 3 and 5 and the MgO content is controlled at less than 
6% by weight. The hot-pressed, densified silicon nitride material of our 
invention provides a two-fold increase in strength at 1400.degree. C 
relative to commercial grade hot-pressed Si.sub.3 N.sub.4 and about 3 to 4 
orders of magnitude decrease in creep strain rate behavior at elevated 
temperatures relative to the commercial grade material.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As noted above, the grain boundary phase can become viscous at high 
temperatures allowing the Si.sub.3 N.sub.4 grains to separate and slide 
under stress, causing a degradation of the material's mechanical 
properties. The temperature where the grain boundary phase becomes viscous 
and affects the mechanical properties is determined by its chemical 
composition. We have determined that certain impurities such as CaO that 
reside within the grain boundary phase lower the temperature where the 
degradation is first observed. 
Accordingly, the CaO content must be limited to about 200 ppm maximum. In 
accordance with our observations, we have determined that the high 
temperature mechanical properties of Si.sub.3 N.sub.4 may be enhanced by 
fabricating a purer starting material. Hence, by utilizing a pure starting 
material the detrimental effects of the impurities on the grain boundary 
phase viscosity are minimized. The problem of grain boundary viscosity at 
high temperatures and under stress is still present, however, 
notwithstanding the fact that purer starting materials are utilized. This 
problem we discovered is related to the MgO content of the powder, and 
further, the MgO/SiO.sub.2 molar ratio. 
High purity Si.sub.3 N.sub.4 powder was produced by nitriding Si powder 
with additions of 0.0, 1.0 and 3.0 wt.% SiO.sub.2. The phase content of 
the resulting powders was 83-93% .alpha.-Si.sub.3 N.sub.4 ; 17-7% 
.beta.-Si.sub.3 N.sub.4 and .ltoreq. 1% Si as determined by X-ray 
diffraction analysis. The oxygen content of the representative powders was 
determined after nitriding by the inert gas fusion, thermoconductivity 
method. Table 1 below reports the impurity content of the Si.sub.3 N.sub.4 
powders produced. 
TABLE 1 
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Spectrochemical Analyses of Westinghouse 
Si.sub.3 N.sub.4 Starting powder (wt. %) 
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Al 0.08 
Ag 0.001 
B 0.001 
Ca 0.016 
Cr 0.01 
Fe 0.1 
Mg 0.001 
Mn 0.05 
Mo 0.003 
Ni 0.01 
Pb 0.01 
Sb 0.01 
Sn 0.01 
Ti 0.01 
V 0.005 
Zn 0.01 
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FIG. 1 graphically illustrates that the oxygen content for the different 
batches of Si.sub.3 N.sub.4 powder produced is a function of the SiO.sub.2 
added prior to nitriding. The oxygen content of the silicon powder 
nitrided was between 0.4 and 0.5 wt.%. It is reasonable to believe that 
the oxygen content of the Si powder is due to a surface layer of SiO.sub.2 
and therefore, as noted in FIG. 1, the increase in oxygen content is 
proportional to the SiO.sub.2 added prior to nitriding. The molar content 
of the SiO.sub.2 in the powder is ranged between 1.7 and 6.7 mole percent. 
Si.sub.3 N.sub.4 powders containing different MgO/SiO.sub.2 ratios were 
prepared by mixing between 2 and 8% by weight MgO with the Si.sub.3 
N.sub.4 powders containing different SiO.sub.2 content. In addition, the 
MgO/SiO.sub.2 ratios of several powders were also varied by mixing both 
MgO and SiO.sub.2 into a powder with an oxygen content of 0.4 wt.% 
(equivalent to 0.75 wt.% SiO.sub.2). Mixing and particle size reduction 
was performed by milling the powder with methanol in polyethylene bottles 
using tungsten carbide cylindrical grinding media. Oxygen analysis before 
and after milling showed no changes that could not be accounted for by the 
MgO addition. 
After stir-drying, the milled composite powders were hot-pressed in a 
nitrogen atmosphere in graphite dies with a stress of 28 MN/m.sup.2 at a 
temperature of 1750.degree. C between 1-4 hours to produce 5 cm diameter 
by 0.75 cm discs. Graphite dies with appropriate coatings were used in 
accordance with standard hot-pressing techniques. Densities were 
determined by water immersion. The densities of the hot-pressed composite 
silicon nitride particles were between 3.20 grams per cc and 3.29 grams 
per cc. Bar specimens 0.317 .times. 0.635 .times. 3.17 cm were sectioned 
and ground. Room temperature flexural strength measurements were made at a 
crosshead speed of 0.05 cm/min using a metal fixture (0.635 cm inner and 
1.905 cm outer loading spans). Elevated temperature measurements were 
performed in air at 1400.degree. C with a crosshead speed of 0.005 cm/min 
using a hot-pressed Si.sub.3 N.sub.4 fixture (0.950 cm inner and 2.222 cm 
outer loading spans). 
FIG. 2 is a graphical representation of the flexural strength data at room 
temperature and at 1400.degree. C as a function of the MgO/SiO.sub.2 
ratio. The oxygen content of the Si.sub.3 N.sub.4 powder was used to 
calculate the SiO.sub.2 content and thus the MgO/SiO.sub.2 molar ratio. At 
1400.degree. C the mean strength increased from 170 MN/M.sup.2 at low 
MgO/SiO.sub.2 ratios to 415 MN/M.sup.2 at an MgO/SiO.sub.2 ratio equal to 
3. Thereafter, the flexural strength decreased to 345 MN/M.sup.2 at higher 
MgO/SiO.sub.2 ratios of about 9. FIG. 2 indicates that where the 
MgO/SiO.sub.2 ratio was lowered by these additions of SiO.sub.2 to ratios 
of 1 and 2 produced low elevated temperature strength materials were 
produced. Without the additional SiO.sub.2 the same Si.sub.3 N.sub.4 
powders had greater MgO/SiO.sub.2 ratios and correspondingly higher 
strengths at 1400.degree. C. 
Referring now to FIG. 3, the load-deflection curve for selected specimens 
with different MgO/SiO.sub.2 ratios are depicted. As can be noted in FIG. 
3, less non-elastic deformation occurs at MgO/SiO.sub.2 ratios greater 
than 3. 
The flexural strength of commercial hot-pressed Si.sub.3 N.sub.4 is about 
25,000-35,000 psi at 1400.degree. C compared to between 45,000 and 70,000 
psi for the material of our invention with an MgO/SiO.sub.2 molar ratio of 
between 3 and 4. It is noted therefore that the material of our invention 
provides about a two-fold increase in flexural strength at 1400.degree. C 
relative to the commercial Si.sub.3 N.sub.4. Room temperature strengths 
are similar for both materials. 
In addition to the improved flexural strength of our materials, they also 
exhibit enhanced resistance to creep at elevated temperature. The creep 
behavior of the materials of our invention and that of the commercial 
Si.sub.3 N.sub.4 material is illustrated in FIG. 4 at 2550.degree. F and 
in FIG. 5 at 2300.degree. F. The material tested in FIG. 4 was under a 
stress of 15,000 psi while the material of FIG. 5 was under a stress of 
30,000 psi. As shown in the drawings, the creep resistance of the 
materials of this invention with an MgO/SiO.sub.2 ratio of 3 is 
approximately 3 to 4 orders of magnitude better than the commercial 
Si.sub.3 N.sub.4 material tested. 
By maintaining the MgO/SiO.sub.2 molar ratio between 3-5 and more 
preferably between 3 and 4 and by maintaining the MgO content below 6 wt.% 
the mechanical property degradation of the material is increased by about 
350.degree. F relative to the commercial grade Si.sub.3 N.sub.4 materials. 
This increase in operating temperature is significant for high temperature 
structural materials such as those employed in gas turbine applications. 
The materials of this invention therefore are particularly suited for such 
components, for example, turbine blades and vanes.