Synthesis and processing of ultrafine powders for Si3N4 ceramics

NanoSTRUCTURED MATERIALS VOL. 1, PP. 197-202, 1992 COPYRIGHT©1992 PERGAMONPRESSLtd. ALL RIGHTSRESERVED

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SYNTHESIS AND PROCESSING OF ULTRAFINE POWDERS FOR Si3N4 CERAMICS

S. C. Danforth Department of Ceramic Engineering Rutgers, The State University Piscataway, NJ 08855-0909

Introduction Silicon nitride (Si3N4) is one of the very promising materials to be used in high temperature-high stress applications such as: tool inserts, bearing surfaces, and heat engine components etc. It was over 10 years ago when it was first proposed by Bowen (1) that new synthesis methods were required to produce "ideal" powders: Such powders have, what are by now, well known characteristics, i.e.:
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Haggerty et al.(4, 5) have developed a CO2 laser driven gas phase method to produce very high quality Si from Sill4 (for RBSN) as well as other non-oxide powders. These Si powders are fine, spherical, and loosely agglomerated (Table I). Through exceptional control over green processing, they have produced the highest strength Table I. Si3N 4 Powder Characteristics Plasma Derived Reactant Gas SiH4/NH3 Powder Si3N4 Ave. Size (nm) 260 Surface Area (m2/g) 65 Shape Equiaxed % Crystalline (X-ray) 30 % ct-Si3N4 -% 13-Si3N4 20-60 Oxygen # 2.1 wt% Carbon (wt%) 0.11 Metallic imp. 106 ppm By products $i # Strongly dependent on handling ~. & On heating to 1870K

(~Q? La~er Sill4 Si 26-30 - Spheres 100 NA NA <200 ppm -<100 ppm None

Derived Flat Flame Reactor SiH4/NH3 SiH4/NH3/N2H4 Si3N4 SiaN4__ 17 10-50 120 90-150 Spheres Equiaxed 0 0 0 100 & 0 0 0.1-6 wt% # -3 wt% 0.072 - <0.02 wt% <100 ppm None None

RBSN to date. Symons and Danforth (6, 7) have produced similarly high quality Si3N4 powders from Sill4 and NH3. These powders are ultra-fine, high purity and amorphous, Table I. Symons and Fatrez (6, 8) have demonstrated the ability to HIP densify laser synthesized S i 3 N 4 without additives. Calcote et al. (9) have developed a process to synthesize S i 3 N 4 (and other non-oxide) powders without any plasma or laser as the driving force. Instead they use reactants and reactor conditions that are self-sustaining on a flat burner. They use mixtures of Sill4 and hydrazine (N2H4) at velocities ranging from 20 to 150 cm/sec, and 100 KPa pressure. The resultant (calculated) adiabatic flame temperature ranged from 900 to 2000K. Powder was produced at rates of 10 g/min at yields close to 100%. Powder characteristics are shown in Table I. The research cited above, clearly demonstrates that there have been significant advances in the area of synthesis of ultra-fine powders for RBSN and Si3N 4 c e r a m i c s . The true value, however, of any powder, or any particular synthesis method is determined by results of very careful processing, material characterization, and property studies. The remainder of this discussion will analyze the results of several such studies. A theme has emerged throughout this analysis, namely that there appears to be a traditional way of viewing Si3N4 ceramics that may no longer be accurate. There are some new questions that we ought to be asking about S i 3 N 4 , namely, what is ultimately possible?

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Properties of High Temperature Si3_N4 S i 3 N 4 has traditionally been viewed as a material which must be made nearly fully dense (>98-99% p Th) before it can become a practical engineering material. This viewpoint is based on the ceramic communities generic experience that strength ( a ) increases with density ( p ) , with all other factors held constant. As Figure 1 clearly shows, Haggerty, Moulson, and Rice(10) have compiled a variety of experimental data that support the well known a - p relationship for S i 3 N 4 . For the moment, however, consider the possibility that the strength of S i 3 N 4 is not a single valued function of density, but rather a function of density and the p r o c e s s i n g - s p e c i f i c flaw population. H a g g e r t y ( 1 0 ) has shown that for RBSN, the traditional o - p relationship reported by Rice and Moulson no longer holds. Why? The answer lies in the detailed nature of the flaw size distribution. By taking great care over the starting Si powder characteristics, the green forming steps (i.e. colloidal dispersion and consolidation), and the nitridation, they have developed a RBSN material with an entirely different population of flaws than any previously reported material. As a result, they have made RBSN with densities in the 65-75% p Th range with average strengths between 250 (460 Max) MPa and 544 (676 max) MPa, respectively, as seen in Figure 1. More recent w o r k ( l l ) has yielded a maximum strength value of 858 MPa at p = 76 %pTh! These materials still have greater than 20% pores by volume! Their strengths are high because their flaw population is entirely different than for traditional RBSN materials. Typical Griffith flaws for the high strength materials were calculated by Haggerty to be in the range of 4 to 16~tm, while for traditional RBSN with strengths of - 3 0 0 MPa, Griffith flaws are usually in the 5 0 ~ m size range. In addition, Hg porosimetry showed most pores were in the 0.05-0.125 ~tm range. Haggerty et al.(10) have shown that such a fine pore size in conjunction with very high purity, results in an oxidation resistance that is 5 to 20 times greater than for conventional RBSN materials at 1000 and 1400oC. To take this argument one step further, I have considered the strength of S i 3 N 4 fibers reported by Legrow et a l . ( 1 2 ) T h e s e fibers were made from inorganic polymer routes. The resultant Si3N4 fiber was reported to have 20% pores by volume with a very fine scale. As a result of such a fine flaw population, the fibers had average fracture strengths of 3.1 GPa! If one attempts to account for the small sample volume of the fiber (using an estimated Weibull modulus of 20), the 3.1 GPa strength scales to - 1 G P a for a bulk material. This is an exciting result for a material that is only 80% dense. It should be noted that this 1 GPa strength is only N20% greater than the value reported above for Haggerty et a l . ( l l ) Clearly, ceramics do not have to be fully dense to have high strengths. The second area that I would like to address is the general belief that extrinsic sintering aids such A1203, Y 2 0 3 , MgO, CeO, CaO, etc. are required to achieve fully dense S i 3 N 4 . While this has been the practice for many years, it is no longer true. Recent work by Symons,(6,7) Fatrez,(8) and Tanaka et al.(13) has shown that the native SiO2 layer on S i 3 N 4 powders is sufficient to act as an "intrinsic" sintering aid during HIP'ing. Densities ranging from >95 to 99.9% p T h have been achieved at 1950oc and 200 MPa. Tanaka et al. 8 have further shown that the (Ube E-10) material exhibits constant

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fracture strength to 1400oc (Figure 2). This strongly suggests two key points: 1) it is possible to fully densify Si3N4 without extrinsic sintering aids, and 2) Si3N4 processed with intrinsic sintering aids may have superior high temperature stability.

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If one considers the next logical step toward improving the properties of Si3N4, one quickly comes to the conclusion that a Si3N4 with no grain boundary phase is the ultimate objective. Towards this end, Symons (6,7) and Fatrez (8) have explored the possibilities of densifying Si3N4 without sintering aids and without oxygen. In this work, laser synthesized Si3N4 powder was processed either with no atmospheric exposure or with controlled exposure to 'different N 2 - H 2 0 atmospheres to yield controlled 02 levels. Figure 3 shows the HIP'ed densities of laser derived Si3N4 powder (after 1 hr at 1950oc and 200 MPa) as a function of the oxygen content, from 0.2 to 6.0 wt%. For this dry-processed 17nm powder, a high HIP'ed density has been achieved for -3 wt% oxygen. Figure 3 also shows that at 0.2 wt% oxygen, a density of 80% pTh was achieved after HIP'ing at 2150oc for 3 hrs. Samples HIP'ed with 0.2 wt% 02 showed dense regions within the microstructure. The authors feel strongly that this HIP'ed density can be increased with improved colloidal processing techniques to yield higher green densities. This indicates that achieving full density may indeed be possible at very low 02 levels, and ultimately in the absence of a glassy grain boundary phase. The last subject I wish to address is that of the role that the purity of the Si3N4 powders (and additives) plays with respect to high temperature properties. There have been numerous studies concerning the influence of extrinsic sintering aid composition on high temperature strength and creep resistance of S i 3 N 4 . ( 1 4 - 2 0 ) T o o few people have paid attention to the influence of trace impurities on the high temperature properties. Tanaka et al.(13) have shown quite convincing evidence that trace impurities such as Fe, AI and Ca (even in the range of <0.05 wt%) can play a major role in terms of the maintenance of good mechanical properties (strength, hardness, and fracture toughness) at temperatures as high as 1400oc. The exact mechanism by which these trace impurities degrade the properties is not yet fully understood. Figure 2 shows how the high temperature strength of the HIP'ed Starck LC-12 material starts to

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drop rapidly above 1000oC due to impurities. Their importance is magnified by the fact that many of these impurities will concentrate in the grain boundary phase (especially if it is a silicate glass). If this is true, and there is only 1-5 vol % glass, then the impurity concentration in the glass may be 20-100 times that indicated by bulk chemical analysis. The last point I would like to raise is whether or not, for the highest purity S i 3 N 4 ' s , with pure SiO2 as the only grain boundary phase, H 2 0 may act as an impurity. Recent work by Nilsen(21) has shown that laser synthesized Si3N4 powders oxidize at room temperature by reaction with H 2 0 not 02. It is uncertain how much of this H20 may remain after baking out the powder (prior to HIP'ing) between 600-1100°C. Figure 4 shows data from work by Weiss (22) who examined the effect of trace H 2 0 levels on the viscosity of pure SIO2. As can be seen, at temperatures between 1850 and 2 2 0 0 o c , the viscosity change with the addition of 1100 ppm H 2 0 is negligible. However, at 1 4 0 0 o c , the desired use temperature range for S i 3 N 4 , it appears that the SiO2 viscosity is reduced by approximately two orders of magnitude with the added H 2 0 . The question that arises then, is how will H 2 0 present in the grain boundary phase act to degrade the mechanical properties (creep and slow crack growth) of ultra high purity S i 3 N 4 at the highest use temperatures? As yet there are no answers to this question, 12 11

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Oxygen Content(wt%) FIG. 3. HIP'ed density vs 02 contents of laser synthesized Si3N4 powders.

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FIG. 4. Viscosity vs temperature for two SiO2 glasses with different H20 contents.

In this paper, I have attempted to give a very brief overview of current methods of synthesizing Si3N4 powders and to describe what are the current limits in achieving high strength ceramics at high temperatures. As a result, I hope I have proposed some new ideas that will change your outlook on Si3N4. References 1. H. K. Bowen, "Physics and Chemistry of Packing Fine Powders", MIT Press, EL78-037, Cambridge MA, October 1980.

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2. Y. Kohtoku, T. Yamada, H. Miyazaki, and T, Iwai, Ceramic Materials and Components for Engines, Ed. W. Bunk et al., Deutsche Keramische Gesellschaft, Germany, 101-108, 1986. 3. O. Schulz, D. Kandziora, H. Hausner, Proc. of Third Int. Conf. Ceramic Materials and Comoonents for Engines, Ed. V. J. Tennery, Am. Cer. Soc., Westerville OH, 54-66, 1989. 4. J. S. Haggerty, G. J. Garvey, J. H. Flint, B. W. Sheldon, M. Aoki, M. Okuyama, J. E. Ritter and S. V. Nair, Ceramic Transactions, Proceedings of the First International Conference on Ceramic Powder Processing Science, Ed. G. L. Messing et al., Am. Ceram. Soc., Westerville OH, 1059-1068, 1988.. 5. J. S. Haggerty, G. Garvey, J, M. Lihrmann, and J. E. Ritter, MRS Svmoosium on Defect Prooerties and Processing of High Technology Nonm¢~alli~ Materials, Ed. Y. Chen et al., MRS, Pitt., PA, 51-62, 1986. 6. W. Symons, "The Effect of Oxygen on the Hot Isostatic Pressing Behavior of Laser Synthesized Silicon Nitride," Ph.D. Thesis, Rutgers University, October, 1989. 7. W. Symons, S. C. Danforth, Proc. of Thir~l Int. Conf. C~ramic Materials and Components for Engines, Ed. V. J. Tennery, Am. Cer. Sot., Westerville OH, 67-75, 1989. 8. Phillipe Fatrez, Masters Thesis, Rutgers University, September, 1989. "Densification of Crystalline and Amorphous Laser Synthesized Silicon Nitride". 9. H. F. Calcote, W. Felder, D. GF. Keil, and D. B. Olson, Proc. 23 rd Int. Symp. on Combustion, University of Orleans, 1739-1744, 1990. 10. J. S. Haggerty, Y. M. Chiang, Ceram. Eng. Sci. Proc., 11,7-8, 757-781, 1990. 11. A. Lightfoot, H. L. Ker, J. S. Haggerty, and J. E. Ritter, Ceram. Ene. Sci. Proc., 11, 7-8, 842-856, 1990. 12. G. E. Legrow, T. F. Lim, J. Lipowitz, and R. S. Reaoch, Bul. Am. Cer. Soc., 66, 2, 363-67, 1987. 13. I. Tanaka, G. Pezzotti, T. Okamoto, Y. Miyamoto, M. Koizumi, J. Am. Ceram. Soc., 72, [9], 1656-60, September, 1989. 14. I. P. Tuersley, G.-L. Ward, M. H. Lewis, Proc. of Third Int. Conf. Ceramic Materials and Components for Engines, Ed. V. J. Tennery, Am. Cer. Soc., Westerville OH, 856-70, 1989. 15. S. Natansohn, Proc. of Third Int. Conf. Ceramic Materials and Components for Engines, Ed. V. J. Tennery, Am. Cer. Soc., Westerville OH, 27-41, 1989. 16. M. Seltzer, Bul. Am Cer~m, Soc., 56, 4, 418-423, 1977. 17. R. R. Wills, M. C. Brockway, and G. K. Bansal, Ceramic Components for Engine, Ed. S. Somiya, et al. KTK Sci. Pub., Tokyo, 321-332, 1983. 18. A. E. Pasto, W. C. Van Schalkwyk, and F. M. Mahoney, Proc. of Third Int, Conf. Ceramic Materials and Comoonents for Engines, Ed. V. J. Tennery, Am. Cer. Sot., Westerville OH, 776-785, 1989. 19. Y. R. Xu, T. S. Yen and X. R. Ful, Proc. of Third Int. Conf. Ceramic Materials and Components for Engines, Ed. V. J. Tennery, Am. Cer. Soc., Westerville OH, 739-750, 1989. 20. R. Becker, DKG 58, [2], 93-101, February, 1981. 21. K. J. Nilsen, "The Effect of Moisture on the Surface Chemistry and Nonaqueous Dispersion Properties of Laser Synthesized Silicon Nitride," Ph.D. Thesis, Rutgers University, January, 1989. 22. W. Weiss, J. Am. Cer. Soc., 67, 3, 213, 1984.