Abstract:
Dense, ceramic compositions fabricated within the Si 3  N 4  -Si 2  N 2  O-Y 2  Si 2  O 7  compatibility triangle in the Si 3  N 4  -SiO 2  -Y 2  O 3  system are extremely stable in oxidizing environments and particularly suited for use as a high temperature structural material. In addition, the hot-pressed, densified articles fabricated from compositions within the compatibility triangle exhibit improved strength and creep resistance at elevated temperatures relative to commercial Si 3  N 4 .

Description:
The invention herein described was made in the course of or under a contact or subcontact thereunder, (or grant), with the Advanced Projects Agency, Office of Naval Research, Contract No. N00014-74-C-0284. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to composite materials formed from powders, and more particularly to hot-pressed composite materials of the silicon nitride (Si 3  N 4 )-silicon dioxide (SiO 2 )-yttrium oxide (Y 2  O 3 ) system. 
     DESCRIPTION OF THE PRIOR ART 
     Silicon nitride (Si 3  N 4 ) is a ceramic material which, because of its ability to withstand high temperatures, its creep resistance and thermal shock resistance at high temperatures, has made it an ideal candidate for use in gas turbine engines such as for turbine blades and vanes. 
     Heretofore, it has been known in the production of silicon nitride bodies to add other materials thereto to enhance the strength and density thereof. Common densification aids employed in the prior art are magnesium oxide (MgO) or yttrium oxide (Y 2  O 3 ). See for example U.S. Pat. No. 3,830,652, which teaches, in part, the addition of from 1.25 to 4.45 wt. % yttrium oxide, the balance silicon nitride. In the literature: &#34;Effect of Y 2  O 3  Additions On Hot-Pressed Si 3  N 4 ,&#34; by G. E. Gazza, Bulletin of the American Ceramic Society, Volume 54, No. 9, pages 778-781 (1975). Preliminary property measurements at temperatures greater than 1300° C. indicate that Si 3  N 4  densified with Y 2  O 3  exhibits significantly improved high temperature mechanical properties relative to silicon nitride densified with the more conventional hot-pressing aid MgO. 
     While we have found these results to be accurate, we have also determined that some of the silicon nitride-yttrium oxide materials are unstable at intermediate temperatures of about 1000° C. despite their apparent stability at temperatures above 1300° C., as reported in the cited literature. We have determined that certain secondary phases are present in the silicon-yttrium-oxygen-nitrogen system which readily oxidize at intermediate temperatures of about 1000° C., having a deleterious effect on the properties of the resultant compounds. Our invention accordingly solves the problems heretofore encountered in this ceramic system by controlling the mol percentages of silicon nitride, yttrium oxide and silicon dioxide within a compatibility triangle in the ternary phase diagram whereby the harmful secondary phases are not present in the hot-pressed materials. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, it has been discovered that certain secondary phases in the Si-Y-O-N system were unstable at temperatures of above 1000° C. In accordance with our invention, we have determined that materials fabricated within a triangular area defined by the compounds Si 3  N 4  -Y 2  Si 2  O 7  -Si 2  N 2  O of the Si 3  N 4  -SiO 2  -Y 2  O 3  ternary diagram precludes the formation of the unstable compounds. Materials falling within this triangular area exhibit improved oxidation resistance in the hot-pressed, densified state as well as improved creep and high temperature strength properties. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The phase relationships between the various compounds as well as the properties and advantages of our invention will be better understood when reference is made to the drawings, in which: 
     FIG. 1 is a graph indicating the linear oxidation behavior of the secondary phase Si 3  Y 2  O 3  N 4  at 1000° C. in air; 
     FIG. 2 is the ternary phase diagram of the Si 3  N 4  -Y 2  O 3  -SiO 2  system at 1600°-1800° C.; 
     FiG. 3 is a graphical representation of the creep behavior of the materials of our invention and the silicon nitride material of the prior art; and 
     FIG. 4 is a graphical representation of the oxidation behavior of the materials of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Relatively pure silicon nitride (Si 3  N 4 ) powder, containing about 0.5 wt. % oxygen and 0.75 wt. % SiO 2 , requires between 15 to 25 wt. % yttrium oxide (Y 2  O 3 ) to achieve full density, &gt; 98% of theoretical density. This is achieved by hot-pressing the composite mixture at temperatures between 1600° and 1780° C. utilizing 4000 psi for about 2-4 hours. After hot-pressing such a mixture, the major phase present in the densified mass is β Si 3  N 4 . Secondary phases such as Si 3  Y 2  O 3  N 4 , Y 5  Si 3  O 12  N and an unidentified phase believed to be YSiO 2  N are also present. The proportion of each phase depends upon the SiO 2  and Y 2  O 3  content of the initial powder and the phase relations in the Si 3  N 4  -Y 2  O 3  -SiO 2  ternary system. Mechanical property measurements performed on the silicon nitride-yttrium oxide composites indicate that the material is superior to hot-pressed silicon nitride with MgO at room temperature and at 1400° C. Surprisingly, however, we have determined that severe strength degradation occurs for these materials at 1000° C. 
     In order to investigate the cause of this unusual phenomenon, the oxidation behavior at 1000° C. of silicon nitride hot-pressed with different amounts of yttrium oxide was tested. Substantial weight gains due to oxidation and severe cracking of the specimens was noted. Subsequent X-ray diffraction analysis showed that secondary phases such as Si 3  Y 2  O 3  N 4  and Y 5  Si 3  O 12  N were the cause of the oxidation at these intermediate temperatures. In order to further investigate this phenomenon, samples were fabricated solely from the secondary phase materials. 
     Si 3  Y 2  O 3  N 4  and Y 5  Si 3  O 12  N bodies were prepared by hot-pressing constituent powders containing 38.25 wt. % Si 3  N 4 , 61.75 wt. % Y 2  O 3 , and 4.76 wt. % Si 3  N 4 , 76.87 wt. % Y 2  O 3  and 18.37 wt. % SiO 2 , respectively. The bodies were hot-pressed at 1750° C. for 2 hours at a pressure of 4000 psi. Oxidation experiments were carried out at 1000° C. in air for periods of up to 140 hours. FIG. 1 illustrates the results for the Si 3  Y 2  O 3  N 4  compound. The oxidation kinetics of this material is linear, indicating that no protective surface layer forms to limit the oxidation of the secondary phase as is the case for Si 3  N 4 . 
     We have found that both secondary phases Si 3  Y 2  O 3  N 4  and Y 5  Si 3  O 12  N are unstable in oxidizing atmospheres and that both materials oxidize to yttrium silicates and SiO 2 . Thus, the cracking and strength degradation at intermediate temperatures of about 1000° C. of silicon nitride-yttrium oxide materials is caused by the oxidation of these secondary phases. We further determined that at higher temperatures, i.e. above 1300° C., SiO 2  forms over the silicon nitride and further functions to protect the unstable phases. Hence, at temperatures where the kinetics of silicon nitride oxidation are too slow, i.e. below 1300° C., the secondary phases are unprotected by the layer of SiO 2  and the secondary phases therefore oxidize to degrade the mechanical properties of the composite. 
     In order to eliminate the problem of strength degradation and oxidation at these intermediate temperatures, it is necessary to eliminate the secondary phases from the composite materials. Referring now to FIG. 2, the phase relations in the Si 3  N 4  -SiO 2  -Y 2  O 3  system at 1600°-1800° C. is depicted. We have discovered that materials fabricated in the triangular area defined by the points Si 3  N 4  (A), Si 2  N 2  O (B), and Y 2  Si 2  O 7  (C) will not contain the unstable secondary phases. Materials fabricated in any other region of this ternary phase diagram where Si 3  N 4  is an included phase contain one or more of the unstable phases and exhibit degradation at intermediate temperatures. As can be noted in FIG. 2, the Si 2  N 2  O compound is at a point where the mol ratio between Si 3  N 4  and SiO 2  is 1:1 and the Y 2  Si 2  O 7  compound is at a point where the mol ratio between Y 2  O 3  and SiO 2  is 1:2. 
     We have determined that in order to have a high strength silicon nitride-yttrium oxide composition, which will not degrade at 1000° C. intermediate temperaturess it is necessary to have silicon dioxide (SiO 2 ) in the initial mixture. We have further determined that the initial composition must fall within the compatibility triangle depicted in FIG. 2. Powder formulations having an initial composition within the compatibility triangle defined by the points ABCA of FIG. 2 were hot-pressed in a conventional non-oxidizing atmosphere at a pressure of 4000 psi at temperatures between 1750° C. and 1800° C. for periods of 1 to 3 hours. Densities of &gt; 98% of theoretical density were obtained after allowing provision in said composition for contamination by tungsten carbide. Properties of the materials formulated within the compatibility triangle were compared with the properties of commercial silicon nitride hot-pressed with MgO. The flexural strength data obtained at room temperature and at 1400° C. are shown below in Table 1. 
     
                                           TABLE 1__________________________________________________________________________Initial Power Composition(Mol Fraction)    Measured Density                       Theoretical                              Strength** (psi)NumberSi.sub.3 N.sub.4     Y.sub.2 O.sub.3         SiO.sub.2             (gm/cc)*  Density                              25° C.                                      1400° C.__________________________________________________________________________1    0.89 0.035         0.075             3.35      3.27   86,500  48,2002     0.865     0.03         0.105             3.26      3.28   78,200  62,3003    0.84 0.055         0.105             3.43      3.29   77,000  59,5004    0.80 0.06         0.14             3.35***   3.32   73,300  46,2005    0.75 0.075         0.175             3.42      3.36   65,300  58,7006    0.50 0.10         0.40             3.26      3.19   58,500  45,600Commerical Si.sub.3 N.sub.4             3.21      3.18   95,000-110,000                                      25,000-35,000__________________________________________________________________________ *Tungsten carbide contamination causes measured density to be larger than theoretical **Average of three values at each temperature ***5% of residual porosity 
    
     As noted above, flexural strengths at 1400° C. for the materials formulated within the compatibility triangle are significantly improved relative to the commercial silicon nitride material. It is also noted in general, that the strengths decrease with decreasing silicon nitride content. 
     The excellent high temperature creep behavior of the materials of our invention is set forth in FIG. 3. Flexural creep measurements were performed at 1400° C., utilizing 15,000 psi and 30,000 psi on a material having an initial molar composition falling within the compatibility triangle of FIG. 2, viz., 0.84 Si 3  N 4 , 0.055 Y 2  O 3  and 0.105 SiO 2 . Also depicted on FIG. 4 is the creep data of a hot-pressed commercial Si 3  N 4  at 15,000 psi. As shown in FIG. 3, the creep resistance of the Si 3  N 4  -Si 2  N 2  O-Y 2  Si 2  O 7  type composition of our invention is improved by 2-3 orders of magnitude over the commercial silicon nitride, hot-pressed body. 
     Because materials hot-pressed within the Si 3  N 4  -Si 2  N 2  O-Y 2  Si 2  O 7  compatibility triangle of FIG. 2 do not contain the unstable phases, they do not exhibit rapid oxidation at intermediate temperatures of about 1000° C. FIG. 4 illustrates the oxidation behavior at 1000° C. of the material of our invention relative to the oxidation behavior of materials outside the compatibility triangle. The molar composition of the numbered materials tested in FIG. 4 are set forth in Table 2. 
     
                       TABLE 2______________________________________     Composition (mol fraction)Material Number       Si.sub.3 N.sub.4                  Y.sub.2 O.sub.3                             SiO.sub.2______________________________________1           0.87       0.13       --3           0.94       0.06       --36          0.80       0.10       0.1038          0.50       0.50       --______________________________________ 
    
     As shown in FIG. 4, the Si 3  N 4  -Si 2  N 2  O-Y 2  Si 2  O 7  type materials do not exhibit any significant oxidation at 1000° C., whereas materials outside this compatibility triangle exhibit significant oxidation due to the presence of the unstable Y-Si-N-O phases. The weight changes of the dense materials within the compatibility triangle were negligible, for example, composition 1 of FIG. 2 exhibited a weight gain of only 0.01% for 1000 hours exposure at 1000° C. in air. 
     The oxidation-resistance at 1370° C. of materials within the Si 3  N 4  -Si 2  N 2  O-Y 2  Si 2  O 7  compatibility triangle were found to be superior to any hotpressed material observed to date. Table 3 lists the oxidation behavior in air of a representative material within the compatibility triangle and compares it to materials hot-pressed with MgO. 
     
                                           TABLE 3__________________________________________________________________________                                Parabolic**       Oxidation  Oxidation                        Total Weight                                Rate ConstantMaterial    Temperature (° C.)                  Time (hrs)                        Gain (mg/cm.sup.2)                                (mg.sup.2 cm.sup.-4__________________________________________________________________________                                hr.sup.-1)Si.sub.3 N.sub.4 + Y.sub.2 O.sub.3 + SiO.sub.2 *       1370       235   0.15    9.75 × 10.sup.-5Commercial Si.sub.3 N.sub.4 ***       1370       235   2.06    1.80 × 10.sup.-2Si.sub.3 N.sub.4 + 2 w/o MgO       1370       400   1.71    7.31 × 10.sup.-3__________________________________________________________________________ *Composition, mol fraction 0.84 Si.sub.3 N.sub.4, 0.055 Y.sub.2 O.sub.3, 0.105 SiO.sub.2 **These materials exhibit parabolic oxidation kinetics: (wt gain).sup.2 = (parabolic rate constant) × (time) ***Hot-pressed with MgO. 
    
     As noted above, the material within the compatibility triangle exhibited about 1/15 the total amount of oxidation and had a parabolic rate constant which was 5 × 10 -3  less than commercial silicon nitride hot-pressed with MgO. This difference is significant for long term applications in high temperature oxidizing environments such as those present in gas turbine engines. 
     While several of the presently preferred embodiments of our invention have been described above, it will be obvious to those skilled in the art that certain modifications may be made without departing from the spirit and scope of the appended claims.