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

Description:
This invention described herein was made in the course or under a contract or subcontract thereunder with the advanced products agency, Office of Naval Research, Contract No. N00014-74-C-0284. 
    
    
     FIELD OF THE INVENTION 
     Description of the Prior Art 
     Heretofore it has been known in the art to hotpress Si 3  N 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 2 ) surface layer on each Si 3  N 4  particle to form a liquid at high temperatures which aids in densifying the Si 3  N 4  particles by a solution precipitation mechanism. Upon cooling, the resultant densified mass consists of Si 3  N 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 2  can become viscous at high temperatures allowing the Si 3  N 4  grains to separate and slide under stress thus causing a degradation of the material&#39;s mechanical properties. 
     SUMMARY OF THE INVENTION 
     In accordance with our invention high purity Si 3  N 4  powder is used as a starting material and the MgO/SiO 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° C relative to commercial grade hot-pressed Si 3  N 4  and about 3 to 4 orders of magnitude decrease in creep strain rate behavior at elevated temperatures relative to the commercial grade material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to gain a better understanding of our invention reference is made to the drawings in which: 
     FIG. 1 is a graphical representation of the oxygen content of the powder wherein varying amounts of SiO 2  have been added; 
     FIG. 2 is a graphical representation of the variation in flexural strength as a function of the MgO/SiO 2  molar ratio; 
     FIG. 3 is a graphical representation of the load-deflection behavior of various materials with differing MgO contents and MgO/SiO 2  molar ratios; 
     FIG. 4 is a graphical representation comparing the creep behavior of the materials of our invention versus commercial grade silicon nitride; and 
     FIG. 5 is a graph similar to that of FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As noted above, the grain boundary phase can become viscous at high temperatures allowing the Si 3  N 4  grains to separate and slide under stress, causing a degradation of the material&#39;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 3  N 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 2  molar ratio. 
     High purity Si 3  N 4  powder was produced by nitriding Si powder with additions of 0.0, 1.0 and 3.0 wt.% SiO 2 . The phase content of the resulting powders was 83-93% α-Si 3  N 4  ; 17-7% β-Si 3  N 4  and ≦ 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 3  N 4  powders produced. 
     
                       TABLE 1______________________________________Spectrochemical Analyses of WestinghouseSi.sub.3 N.sub.4 Starting powder (wt. %)______________________________________Al                 0.08Ag                 0.001B                  0.001Ca                 0.016Cr                 0.01Fe                 0.1Mg                 0.001Mn                 0.05Mo                 0.003Ni                 0.01Pb                 0.01Sb                 0.01Sn                 0.01Ti                 0.01V                  0.005Zn                 0.01______________________________________ 
    
     FIG. 1 graphically illustrates that the oxygen content for the different batches of Si 3  N 4  powder produced is a function of the SiO 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 2  and therefore, as noted in FIG. 1, the increase in oxygen content is proportional to the SiO 2  added prior to nitriding. The molar content of the SiO 2  in the powder is ranged between 1.7 and 6.7 mole percent. 
     Si 3  N 4  powders containing different MgO/SiO 2  ratios were prepared by mixing between 2 and 8% by weight MgO with the Si 3  N 4  powders containing different SiO 2  content. In addition, the MgO/SiO 2  ratios of several powders were also varied by mixing both MgO and SiO 2  into a powder with an oxygen content of 0.4 wt.% (equivalent to 0.75 wt.% SiO 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 2  at a temperature of 1750° 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 × 0.635 × 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° C with a crosshead speed of 0.005 cm/min using a hot-pressed Si 3  N 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° C as a function of the MgO/SiO 2  ratio. The oxygen content of the Si 3  N 4  powder was used to calculate the SiO 2  content and thus the MgO/SiO 2  molar ratio. At 1400° C the mean strength increased from 170 MN/M 2  at low MgO/SiO 2  ratios to 415 MN/M 2  at an MgO/SiO 2  ratio equal to 3. Thereafter, the flexural strength decreased to 345 MN/M 2  at higher MgO/SiO 2  ratios of about 9. FIG. 2 indicates that where the MgO/SiO 2  ratio was lowered by these additions of SiO 2  to ratios of 1 and 2 produced low elevated temperature strength materials were produced. Without the additional SiO 2  the same Si 3  N 4  powders had greater MgO/SiO 2  ratios and correspondingly higher strengths at 1400° C. 
     Referring now to FIG. 3, the load-deflection curve for selected specimens with different MgO/SiO 2  ratios are depicted. As can be noted in FIG. 3, less non-elastic deformation occurs at MgO/SiO 2  ratios greater than 3. 
     The flexural strength of commercial hot-pressed Si 3  N 4  is about 25,000-35,000 psi at 1400° C compared to between 45,000 and 70,000 psi for the material of our invention with an MgO/SiO 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° C relative to the commercial Si 3  N 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 3  N 4  material is illustrated in FIG. 4 at 2550° F and in FIG. 5 at 2300° 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 2  ratio of 3 is approximately 3 to 4 orders of magnitude better than the commercial Si 3  N 4  material tested. 
     By maintaining the MgO/SiO 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° F relative to the commercial grade Si 3  N 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.