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In situ processed Si3N4 whiskers in the system barium aluminosilicate-Si3N4
MATERIALS SCIENCE & ENGINEERING Materials Scienceand EngineeringA195 (1995) 197-205
ELSEVIER
A
In situ processed Si3N 4 whiskers in the system barium aluminosilicate-Si3N 4 Douglas W. Freitag Loral Vought Systems, P.O. Box 650003, Dallas, TX 75265, USA
Abstract
The use of barium aluminosilicate(BAS) for transient liquid-phase sintering of Si3N4 is being explored as a low-cost processing route which can provide improved properties. The effectof processing conditionson sinterability,crystallinestructure and microstructure was examined.It was demonstrated that Si3N4 could be pressureless sintered to high densities with BAS but that high volumes of BAS are required and hexacelsian preferentiallyforms as the intergranular phase. Even though hexacelsian is structurally unstable when acting alone, when present in a BAS-SiaN4 composite,the in situ reinforcementprovided by Si3N4serves to stabilize the composite sufficientlyto permit its use in severe thermal environments.
Keywords: Silicon;Nitrogen;.Whiskers;Barium;Aluminium
1. Introduction
Advanced ceramics are being developed for a number of defense and commercial applications where operating requirements generally exceed the capabilities of more conventional materials. Examples include components in advanced turbine engines, reciprocating heat engines, waste incineration, the paper and pulp industry, chemical processing, electronic packaging, armor, cutting tools and missile radomes. In many cases, advanced ceramics are an enabling technology; in others, improvements in performance or reductions in life-cycle cost are sought through the replacement of existing materials. The market for advanced ceramics currently exceeds S 10B and is projected to grow at the annual rate of 15% through the year 2000 [1]. The ultimate growth of this market is directly related to the development of advanced ceramics which can provide the desired level of performance in a cost-effective and reliable manner. Silicon nitride (Si3N4) is a leading candidate for many of the applications being considered because of its inherent thermal stability, light weight, low thermal
1Present address: BaysideMaterials Technology,17 Rocky Glen Court, Brookeville,MD 20833, USA.
expansion, chemical stability, low lielectric constant, high hardness and high strength. Even though this unique combination of properties permits a broad range of marketing opportunities, silicon nitride with the desired level of performance and reliability has proven difficult to manufacture ~n a cost-effective manner. Although a number of processing methods have been successfully demonstrated which overcome the difficult to sinter covalent nature of silicon nitride, those based on powder processes, and in particular pressureless sintering, are the most promising for the near term. Early studies focused on the use of liquid-forming sintering aids which, when combined with silicon nitride, permit sintering to high densities at temperatures low enough to avoid SiaN4 decomposition. Although acceptable low-temperature structural properties were frequently demonstrated, elevated temperature structural properties suffered from the high volume of residual glass phase present at the grain boundaries which begin to soften at temperatures in the range of 800-1000 °C [2]. Approaches demonstrated to improve the elevated temperature properties include reducing the liquid volume [3], selection of more refractory liquid forming phases (Y203, ZrO2) [4], recrystallization of the liquid-forming phases by prolonged heat treatments [5], use of transient liquid
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phases which form solid solutions with Si3N 4 (Sialon) [6], and the addition of discontinuous reinforcements (SiCw) [7]. While generally successful in improving the elevated temperature properties, tradeoffs in processing complexity and overall performance still exist. An alternative approach proposed by Pickup and Brook [8,9] is to use additives which form a lowtemperature eutectic with SiO2 for ease of densification, but upon further heating produce a final composition which is highly refractory, does not form a solid solution with Si3N 4 and easily crystallizes upon cooling. Barium aluminosilicate (BAS) was selected as the final composition because of its highly refractory nature (melting point 1760 °C), readiness at which it crystallizes from the melt and the eutectic formed with SiO2 at 1122 °C. While the use of BAS for transient liquid-phase sintering of Si3N 4 was successful in demonstrating a reduced level of residual intergranular glass and increased elevated temperature strength, processing complexity with the use of hot pressing still exists and material performance is limited by the presence of the hexagonal form of BAS (hexacelsian) and lack of in situ formed Si3N 4 whiskers. The objectives of this work were to demonstrate the use of BAS as a pressureless transient liquid-phase sintering aid for silicon nitride, explore methods to produce the preferred monoclinic form of BAS (celsian) and identify processing conditions which provide the desired level of fl-Si3N 4 grain development. In addition to satisfying the selection criteria used by Pickup and Brook [8,9]. BAS provides an intergranular phase which exhibits high chemical stability, low thermal expansion and electrical properties superior to those provided by nitrogen-containing glasses generally found in hot-pressed and sintered Si3N 4 [10,11]. 2. Experimental methods 2.1. Sample preparation
Silicon nitride powders used in this work included Stark LC-12S and UBE-SN-E 10. Starting products for the BAS intergranular phase included stoichiometric combinations of Baikowski SM8 A1203, Nyacol A120 colloidal A1203, Atomergic SIO2, Nyacol 2034DI colloidal SiOz, Mallinckrodt AR-grade BaCO 3 and Kalichemie VL700 BaCO 3. Oxygen content provided by the Si3N 4 was accounted for through neutron activation analysis. Raw materials were batched in ethanol using a high-density polyethylene jar and Si3N 4 milling media for 12 h. Batched materials were dried, granulated in a planetary ball mill using zirconia milling jars and media and sieved to - 3 5 mesh through a nylon sieve. Billets were formed by isostatic pressing at 138
MPa and packed in Grafoil-lined graphite crucibles with a silicon nitride-based powder bed and sintered in a graphite resistance-heated furnace. Sintering temperatures ranged from 1700 to 1850 °C and sintering times from 5 to 300 min. A 0.1 Pa nitrogen atmosphere was used with heating and cooling rates ranging from 5 to 20 °C min -1. Additional samples were prepared from pre-reacted BAS starting powders, SrCO3 substituted for BaCO 3 and with extended heat treatments performed simultaneously with the cool-down portion of the sintering cycle. For pre-reacted BAS, starting products were wet blended, dried, granulated, placed in AI203 crucibles and heat treated in air for 150 h at 1500 °C. These powders were subsequently blended with Si3N 4 powders for sample preparation. 2.2. Characterization
Weight loss, bulk density and crystallographic phase content were evaluated for all samples after sintering. Structural property measurements, failure analysis, thermal expansion measurements and transmission electron microscopy (TEM) were performed on select samples. Bulk density was measured using the Archimedes immersion technique. Crystallographic phase content was determined by powder X-ray diffraction analysis using a G.E. Model XRD-5 X-ray diffractometer with Cu Ka radiation. Test conditions were: accelerating voltage 40 kV, current 30 mA, scanning rate 0.01 ° min -1, detector slit 0.2 ° and beam slit 1.0 °. A rotating powder sample holder was used to minimize preferred orientation effects. Quantitative analysis of a- and fl-Si3N 4 phase contents were determined by comparing the intensity ratios from the {210} planes for each phase based on the work of Gazzara and Messier [12]. Quantitative analyses of hexacelsian and celsian contents were carried out by comparing the intensity ratios from the {102} plane for hexacelsian with the {- 112} plane for celsian based on the work of Wuchina [13]. Calibration graphs for the latter analysis were developed in-house by preparing samples with various ratios of hexacelsian to celsian. Test specimens were machined from billets using standard diamond grinding techniques. Four-point bend flexure strength was measured according to MILSTD-1942A (specimen size B) [14]. Fracture toughness was measured using the chevron notched beam method [15]. Both measurements were performed at temperatures in the range 21-1200 °C in ambient air using a sificon carbide test fixture. Twenty tests were performed for each test condition. Failure analysis using scanning electron microscopy was performed on select samples to locate and characterize the source of failure. Thermal expansion measurements were performed using dual-rod, high-temperature dilatometry
D. W. Freitag /
MaterialsScience and EngineeringA195 (1995) 197-205
over the temperature range 21-1400 °C in a helium or argon atmosphere ]. Specimens for transmission electron spectroscopy were prepared from bulk samples by a combination of mechanical grinding and ion milling. Bulk materials attached to glass slides were ground to < 40/~m and subsequently transferred to slotted electron microscope grids for dimpling to a thickness of 10/~m. Final thinning was carried out by argon ion milling at 5.5 kV. Analysis was performed on a Philips 300 EM instrument operated at 80 kV2.
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Fig. 1. Effect of BAS content on the density of liquid-phase sintered Si3N4.
3.1. Sinterability of SigN4 using B A S 100m
The dominant process variable for sinterability of silicon nitride with BAS was the BAS content. Sintering time and temperature were seen to have little impact on the overall density for the temperature range 1750-1850 °C and a time duration of 60-240 min. Conversely, Pickup and Brook [8,9] showed that silicon nitride containing < 10 vol.% BAS could be sintered to near-theoretical density with hot pressing conditions of 1700 °C and 600 min. In the present study, near-theoretical density was achieved with BAS contents of 30 vol.% and pressureless sintering conditions of 1750 °C and 60 rain (Fig. 1). BAS contents < 25 vol.% could not be pressureless sintered to high densities, even for sintering temperatures of 1850 °C and a prolonged sintering time of 240 min, which implies that further densification by the solutionprecipitation process is minimal for this system, as would be expected based on the analysis of other refractory sintering aids, e.g. Y203 [16]. The dominance of the BAS content on the sinterability of silicon nitride during the particle rearrangement stage agrees with the models developed by Kingery [17] for liquidphase sintering. Above 30 vol.% BAS, pressureless sintering conditions comparable to those used by Pickup and Brook for hot pressing were shown to produce high densities and prevent oversintering (Fig. 2). Using the powder bed method to control Si3N4 decomposition during sintering, the measured weight loss was < 1 wt.% in all cases, well below values commonly reported for pressureless sintering of Si3N4 in a powder bed [18]. The kinetics of densification for liquid phase sintered Si3N4 with 30 and 40 vol.% BAS contents are shown in Fig. 2. At 1800 °C with 30 vol.% BAS, densification is complete by the time isothermal conditions ~Performed by the Universityof Texas, Arlington,TX, USA. zPerformed by Rice University,Houston, TX, USA.
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are reached in the sintering furnace. Further sintering is limited to evolving the desired fibrous microstructure. These data support the conclusion that the solutionprecipitation process contributes little to the final density for this system. At 1700 °C with 40 vol.% BAS, densification is still occurring within the first 60 min of isothermal heating. Based on the BAS-silica and BAS-alumina binary phase diagrams, the intergranular phase is expected to be a mixture of liquid and crystalline BAS at this temperature. The reduction in the glass volume fraction and subsequent increase in glass viscosity resulting from the lower sintering temperature are believed to reduce the densification rate during the particle rearrangement stage of sintering. 3.2. Celsian development Two polymorphs of BAS exist when synthetically produced, hexagonal and monoclinic. Hexacelsian is known to crystallize from the melt, is metastable below
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D. W. Freitag
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Materials Science and Engineering A195 (1995) 197-205
Monoclinic ®
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1590 °C and undergoes a reversible transformation to an orthorhombic crystal structure at 300°C [19]. Celsian is stable below 1590 °C and is preferred because of its low thermal expansion, structural stability and low dielectric properties [20]. The two crystal structures are shown in Fig. 3. The stable monoclinic form of BAS is always preceded by the formation of the metastable hexagonal form. Owing to the sluggish transformation of the hexagonal into the monoclinic form at temperatures below 1590 °C, which can easily take days using heat treatments, a number of methods have been demonstrated to promote the transformation process, e.g. mineralizers, substitution of BAS with strontium aluminosilicate (SAS), addition of celsian seeds and hydrothermal treatments [21]. A typical X-ray diffraction pattern for Si3N4 liquid phase sintered by the addition of BAS is shown in Fig.
4. While their relative contents change with processing conditions, phases present for all processing conditions examined were limited to a-Si3N4, fl-Si3N4 and hexacelsian. Unlike powders of celsian prepared for crystallographic analysis by prolonged heat treatments, post-sintering heat treatments of up to 1550 °C for 150 h were not effective in forming celsian in the presence of silicon nitride. Substitution of BAS by up to 25 tool% of SAS in combination with post-sintering heat treatments of 1400 °C for 5 h also proved ineffective. Other methods unsuccessfully explored for developing the monoclinic form of BAS include using different forms of BAS starting products and prereacted BAS for addition to the silicon nitride. Talmy and Zaykoski [22] have recently reported similar problems when liquid-phase sintering Si3N 4 at temperatures below 1590 °C using high volumes (25
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Materials Science and Engineering A195 (1995) 197-205
wt.%) of BAS eutectics rich in BaO or SiO 2. After sintering, the BAS phase formed from the melt was predominately hexacelsian. Significant increases in the celsian content were not achieved with changes to the sintering temperature, sintering time or cooling rate. Unlike the present work, celsian was, however, formed by the substitution of up to 50 wt.% BAS with SAS. A key difference between the two studies is that for the present work, the liquid phase is encouraged to react with the Si3N 4 for microstructure development and in the work of Talmy and Zaykoski, the low sintering temperatures limit reactivity between Si3N4 and the liquid phase. It can only be assumed that when allowed to react with Si3N4, the liquid phase will incorporate nitrogen, which could ultimately remain in the hexagonal crystalline structure and inhibit transformation to the monoclinic crystalline structure. However, Tredway and Risbud [23] have reported that during the crystallization of S i - B a - A I - O - N glasses, no dissolution of nitrogen in the hexacelsian crystals occurs. The nitrogen remains in the glass phase during incomplete crystallization. If a similar phenomenon occurs when using BAS to liquid-phase sinter Si3N4, the goal of complete crystallization of the intergranular phase will not be achieved and the nitrogen-rich liquid phase remaining could act to suppress nucleation of celsian. Even though hexacelsian was the only crystalline phase formed, the high silicon nitride content is seen to suppress the destructive displacive transformation observed at low temperatures for pure hexacelsian (Fig. 5). At these high Si3N 4 contents, samples could be severely thermally shocked and repeatedly thermally cycled without failure, whereas samples with silicon nitride contents below 60 vol.% frequently failed during thermal cycling. Further evidence that the presence of Si3N 4 suppresses the low-temperature transformation of hexacelsian is shown in Fig. 6. By
monitoring the thermal expansion more closely within the expected transformation temperature range, an increase in Si3N 4 c o n t e n t clearly suppresses the displacive transformation and shifts it to lower temperatures. While the benefit of forming celsian is clear from its compatibility in thermal expansion with Si3N 4 as shown in Fig. 5, these data also show that hexacelsian can be used without concern for premature failure when combined with a high Si3N 4 content.
3.3. Microstructure development The process for microstructure development during liquid-phase sintering of Si3N 4 with BAS is shown in Fig. 7. The blended powders of predominately a-Si3N * and BAS constituents are reacted at temperatures where the BAS phase becomes liquid to insure that a uniform glass composition is achieved. The liquid also serves to enhance mass transport of the Si3N 4 as it transforms from a-Si3N 4 to fl-Si3N 4 by a solutionprecipitation process. When cooled, the ideal microstructure consists of elongated grains of fl-Si3N4 in a crystalline phase of celsian. In reality, the microstructure consists of a mixture of a- and fl-Si3N 4 in a crystalline matrix of BAS separated by a thin layer of residual glass; the extent and nature of each are functions of the processing conditions. The transformation kinetics for Si3N 4 are shown in Fig. 8 for different BAS contents. The fraction of a-Si3N 4 transformed is given by Y=[fl-Si3N 4 fl-Si3N4)i,it]/a-Si3N4)init and is plotted versus log time (t). The curves are sigmoidal in shape with an initial induction period, which decreases with increasing temperature, during which negligible transformation occurs. A plateau is ultimately reached where further transformation of a- to fl-Si3N 4 is slow. For the corn-
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positions shown, little change in fl-Si3N 4 content is seen prior to the completion of densification (Fig. 2). These data agree with the prior conclusion that the solution-precipitation process is a negligible contributor to densification, as would be expected for a highly refractory sintering aid such as BAS where mass transport through a highly viscous liquid would be slow. The extent of a- to fl-Si3N 4 transformation is also seen to decrease with increasing BAS content. This trend is consistent with the literature and occurs as a result of the reduced solubility of a-Si3N 4 at the interparticle contact points and reduced supersaturation of the liquid phase with silicon and nitrogen as the liquid volume increases. However, it could simply be a result of the lower processing temperatures used for highei BAS contents to prevent oversintering. The effect of various combinations of the BAS starting products on the extent of a- to fl-Si3N 4 transforma-
Fig. 9. Effect of BAS and Si3N 4 starting powders on the extent of fl-Si3N4 development.
tion is shown in Fig. 9. Variations in the SiO 2 product provided the only significant change in the final fl-Si3N 4 content. By increasing the reactivity of the SiO 2 through its addition in colloidal form, additional time was available for the solution-precipitation stage of sintering which ultimately led to an increase in the fl-Si3N 4 content. Processing conditions which can affect the final fl-Si3N a grain structure include Si3N4 powder characteristics, which vary with the synthesis route, and sintering conditions. A transmission electron photomicrograph of a typical cross-section from a s a m ple prepared using UBE SN-E-10 Si3N4 is shown in Fig. 10. The fl-Si3N 4 grains are clearly defined by their hexagonal cross-section and exaggerated grain growth
D. W. Freitag
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Materials Science and Engineering A195 (1995) 197-205
203
Fig. 10. Transmission electron photomicrograph of a typical cross-section from a sample of Si3N4 liquid-phase sintered with 30 vol.% BAS using UBE SN-E-10 powder.
Fig. l 1. Transmission electron photomicrograph of a typical cross-section from a sample of Si3N4 liquid-phase sintered with 30 vol.% BAS using Starck LC-12S powder.
along their crystallographic c-axis. Typical fl-Si3N4 grain dimensions were 0.1 p m in diameter and 1-2 ktm in length. The fibrous microstructure developed is consistent for microstructures developed with high glass viscosities where the reduction in local supersaturation is slow and high glass volumes provide for unconstrained growth. The apparent diameter of equiaxed a-Si3N 4 grains or freshly nucleated fl-Si3N4 grains was of the order of 0.2 ktm. Microstructures were very uniform with no apparent porosity. Using large-area energy-dispersive spectroscopy (EDS), A1 and Ba were readily detected in the desired ratio for stoichiometric BAS. However, on reducing the beam size to < 0.2 pm, BAS could not be detected in localized areas. Analysis of a large number of grains always proved to be Si3N4, which ultimately led to the conclusion that the BAS phase is uniformly distributed throughout the microstructure as a thin wetting layer on the Si3N 4 grains. High concentrations of amorphous BAS phases were not detected using microdiffraction, diffuse T E M and darkfield T E M methods. Grains were occasionally found that indicated Al-deprived barium silicate phases and sialon phases exist but not in high concentrations. T E M analysis overall proved difficult owing to the complexity of the material systems, fineness of the microstructure and lack of contrast between the phases. A transmission electron photomicrograph of a typical cross-section from a sample prepared using Starck LC-12S Si3N4 is shown in Fig. 11. The observations were similar to those discussed for the UBEprepared sample with the exception of the fl-Si3N4 grain size distribution. Samples prepared with Starck Si3N 4 contained a duplex distribution of fl-Si3N4 grains with those in the smaller size distribution being approximately one quarter of that of those seen in the UBE-prepared samples. Grains in the larger size dis-
tribution have diameters up to 2-3 p m and lengths well over 5 pm. Fewer equiaxed Si3N4 grains were detected in the Starck-prepared samples. A number of samples were also examined to determine the effects that the BAS starting powders and sintering conditions have on the microstructure. As expected, large increases in the sintering temperature or time resulted in coarsening of the microstructure but no apparent reduction in the aspect ratio as is commonly reported when oversintering [24]. Smaller variations in the sintering conditions, _+50 °C or + 60 rain from the norm, resulted in little apparent change in the microstructure. No apparent changes in the microstructures were found with changes to the BAS starting powders. The effect of changes in the microstructure on the previously described hexacelsian structural stability is shown in Fig. 12. For a fixed volume fraction of Si3N4, the structural change occurring in hexacelsian during its low-temperature crystalline phase transformation is significantly reduced through a well developed fibrous microstructure provided by the presence of fl-Si3N4. 3.4. Structural properties
The structural properties of liquid phase sintered Si3N4 with 30 vol.% BAS are summarized in Fig. 13. The room-temperature strength and fracture toughness are typical of liquid-phase sintered silicon nitride materials prepared by dry pressing powders [25]. The low structural properties are also due in part to the residual stresses generated by the large difference in thermal expansion between Si3N4 and hexacelsian which develop on cooling from processing temperatures. At elevated temperatures, excessive creep was observed. Although it could be attributed to residual glass phases common in liquid-phase sintered Si3N4, it
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Materials Science and Engineering A195 (1995) 197-205
shown to be structurally unstable when acting alone, when present in a BAS-Si3N 4 composite the in situ reinforcement provided by Si3N 4 serves to stabilize the composite sufficiently to permit its use in severe thermal environments. T h e microstructures developed were typical of that provided by a highly refractory sintering aid used in high volumes and will require further optimization of the processing conditions to provide the desired level of toughness and strength.
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This work was supported by the US A r m y Missile Command, Redstone Arsenal, A L , under contract D A S G 6 0 - 9 1 - C - 0 1 4 0 and Loral Vought Systems under I.R. & D. T h e author thanks K.K. Richardson, P.B. Aswath, D.L. Callahan, J.E Stepp, D.L. Hunn, R.C. Knight and C.D. Perry for their many contributions to this work.
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could also be a result of the fine microstructure observed. Failure analysis indicates that pullout, bridging and crack deflection at elongated fl-Si3N4 grains are contributors to the toughness and implies a chemical compatibility with BAS at their interfaces.
4.
Conclusions
It has been demonstrated that BAS could be used as a transient liquid-phase sintering aid for silicon nitride but that hexacelsian is formed on cooling independent of the processing conditions examined. To achieve densities in excess of 97% with acceptable pressureless sintering conditions, BAS contents of at least 30 vol.% are required and the starting products must be uniformly mixed and highly reactive. T h e failure to develop the preferred phase of BAS (celsian) is believed to be related to the nitrogen-rich glass which remains after incomplete crystallization of the intergranular phase• Even though hexacelsian has been
[1] B.W. Deckman, Mater. TechnoL, 8(1993) 69-73. [2] G. Ziegler, J. Heinrich and G. Wrtting, J. Mater Sci., 22 (1987) 3041-3086. [3] J. Adlerborn, M. Burstr6m, L. Hermansson and H.T. Larker, Mater Des., 8 (1987) 229-232. [4] D. Chakraborty and A.K. Mukhopadhyay, Ceram. Int., 15 (1989) 237-245. [5] D.A. Bonnel,T. Tien and M. Ruhlem, J. Am. Ceram. Soc., 70(1987) 460-465. [6] M. Mitomo, N. Kuramoto and Y. Inomata, J. Mater. Sci., 14 (1979)2309-2316. [7] S.T. Buljan, J.G. Baldoni and M.L. Huckabee, Am. Ceram. Soc. Bull., 66 (1987) 347-352. [8] H. Pickup and R.J. Brook, in W. Bunk and H. Hauser (eds.), Proc. 2nd Int. Conf. Ceramic Materials and Components for Engines, Verlag Deutsche Keramische Gesellschaft, Bad
Honnef, Germany, 1989, pp. 93-99. [9] H. Pickup and R.J. Brook, in Proc. Engineering with Ceramics 2, DKG, 1989, pp. 93-99. [10] W.W. Ho, in Proc. 5th DoD Electromagnetic Window Symposium, Vol. 1, Air Force Materials Command Report, Wright Patterson Air Force Base, OH, 1993, pp. 147-158. [11] W.W. Ho, Final Report Contract No. DAAG46-82-C-O022, Army Materials and Mechanics Research Center, Waterton, MA, 1987. [12] C.P. Gazzara and D.R. Messier, Am. Ceram. Soc. Bull., 56 (1977)777-780. [13] E.J. Wuchina, M.Sc. Thesis, University of Maryland, 1991. [14] M1L-STD-1942A, Department of the Army, Washington, DC, 1990. [15] D. Munz, G. Himsolt and J. Eschweiller, J. Am. Ceram. Soc., 63(1980) 341-342. [16] S. Hampshire, The Sintering of Nitrogen Ceramics, Parthenon Press, Carnforth, UK, 1986. [17] W.D. Kingery, J. App. Phys., 30(1959) 301-306. [18] A. Giachello, P.C. Martinengo, G. Tommasini and P. Popper, J. Mater. Sci., 14 (1979) 2825-2830.
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[19] H.C. Lin and W.R. Foster, Am. MineraL, 53 (1968) 134-144. [20] E.J. Wuchina and I.G. Talmy, Naval Surface Warfare Center Report TR 89-286 (1989). [21] I.G. Talmy and D.A. Haught, in Proc. 2nd Navy Independent Research~Independent Exploratory Development Symposium, Vol. 1, Johns Hopkins University, Laurel, MD, 1989, pp. 347-355. [22] I.G. Talmy and J.A. Zaykoski, in J.D. Buckley (ed.), Proc. 17th Conf. on Metal Matrix, Carbon, and Ceramic Matrix
205
Composites, Vol. 2, NASA Conference Publication 3235, Hampton, VA, 1994, pp. 759-771. [23] W.K. Tredway and S.H. Risbud, J. Non-Cryst. Solids, 56 (1983) 135-140. [24] A.J. Pyzik, D.E Carroll and C.J. Hwang, Proc. Mater. Res. Soc. Symp., 287(1993) 411-416. [25] A.E. Pasto, J.T. Neil and C.L. Quackenbush, in L. Hench and D. Ulrich (eds.), Ultrastructure Processing of Ceramics, Glasses, and Composites, Wiley, New York, 1984, pp. 476-489.
ELSEVIER
A
In situ processed Si3N 4 whiskers in the system barium aluminosilicate-Si3N 4 Douglas W. Freitag Loral Vought Systems, P.O. Box 650003, Dallas, TX 75265, USA
Abstract
The use of barium aluminosilicate(BAS) for transient liquid-phase sintering of Si3N4 is being explored as a low-cost processing route which can provide improved properties. The effectof processing conditionson sinterability,crystallinestructure and microstructure was examined.It was demonstrated that Si3N4 could be pressureless sintered to high densities with BAS but that high volumes of BAS are required and hexacelsian preferentiallyforms as the intergranular phase. Even though hexacelsian is structurally unstable when acting alone, when present in a BAS-SiaN4 composite,the in situ reinforcementprovided by Si3N4serves to stabilize the composite sufficientlyto permit its use in severe thermal environments.
Keywords: Silicon;Nitrogen;.Whiskers;Barium;Aluminium
1. Introduction
Advanced ceramics are being developed for a number of defense and commercial applications where operating requirements generally exceed the capabilities of more conventional materials. Examples include components in advanced turbine engines, reciprocating heat engines, waste incineration, the paper and pulp industry, chemical processing, electronic packaging, armor, cutting tools and missile radomes. In many cases, advanced ceramics are an enabling technology; in others, improvements in performance or reductions in life-cycle cost are sought through the replacement of existing materials. The market for advanced ceramics currently exceeds S 10B and is projected to grow at the annual rate of 15% through the year 2000 [1]. The ultimate growth of this market is directly related to the development of advanced ceramics which can provide the desired level of performance in a cost-effective and reliable manner. Silicon nitride (Si3N4) is a leading candidate for many of the applications being considered because of its inherent thermal stability, light weight, low thermal
1Present address: BaysideMaterials Technology,17 Rocky Glen Court, Brookeville,MD 20833, USA.
expansion, chemical stability, low lielectric constant, high hardness and high strength. Even though this unique combination of properties permits a broad range of marketing opportunities, silicon nitride with the desired level of performance and reliability has proven difficult to manufacture ~n a cost-effective manner. Although a number of processing methods have been successfully demonstrated which overcome the difficult to sinter covalent nature of silicon nitride, those based on powder processes, and in particular pressureless sintering, are the most promising for the near term. Early studies focused on the use of liquid-forming sintering aids which, when combined with silicon nitride, permit sintering to high densities at temperatures low enough to avoid SiaN4 decomposition. Although acceptable low-temperature structural properties were frequently demonstrated, elevated temperature structural properties suffered from the high volume of residual glass phase present at the grain boundaries which begin to soften at temperatures in the range of 800-1000 °C [2]. Approaches demonstrated to improve the elevated temperature properties include reducing the liquid volume [3], selection of more refractory liquid forming phases (Y203, ZrO2) [4], recrystallization of the liquid-forming phases by prolonged heat treatments [5], use of transient liquid
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phases which form solid solutions with Si3N 4 (Sialon) [6], and the addition of discontinuous reinforcements (SiCw) [7]. While generally successful in improving the elevated temperature properties, tradeoffs in processing complexity and overall performance still exist. An alternative approach proposed by Pickup and Brook [8,9] is to use additives which form a lowtemperature eutectic with SiO2 for ease of densification, but upon further heating produce a final composition which is highly refractory, does not form a solid solution with Si3N 4 and easily crystallizes upon cooling. Barium aluminosilicate (BAS) was selected as the final composition because of its highly refractory nature (melting point 1760 °C), readiness at which it crystallizes from the melt and the eutectic formed with SiO2 at 1122 °C. While the use of BAS for transient liquid-phase sintering of Si3N 4 was successful in demonstrating a reduced level of residual intergranular glass and increased elevated temperature strength, processing complexity with the use of hot pressing still exists and material performance is limited by the presence of the hexagonal form of BAS (hexacelsian) and lack of in situ formed Si3N 4 whiskers. The objectives of this work were to demonstrate the use of BAS as a pressureless transient liquid-phase sintering aid for silicon nitride, explore methods to produce the preferred monoclinic form of BAS (celsian) and identify processing conditions which provide the desired level of fl-Si3N 4 grain development. In addition to satisfying the selection criteria used by Pickup and Brook [8,9]. BAS provides an intergranular phase which exhibits high chemical stability, low thermal expansion and electrical properties superior to those provided by nitrogen-containing glasses generally found in hot-pressed and sintered Si3N 4 [10,11]. 2. Experimental methods 2.1. Sample preparation
Silicon nitride powders used in this work included Stark LC-12S and UBE-SN-E 10. Starting products for the BAS intergranular phase included stoichiometric combinations of Baikowski SM8 A1203, Nyacol A120 colloidal A1203, Atomergic SIO2, Nyacol 2034DI colloidal SiOz, Mallinckrodt AR-grade BaCO 3 and Kalichemie VL700 BaCO 3. Oxygen content provided by the Si3N 4 was accounted for through neutron activation analysis. Raw materials were batched in ethanol using a high-density polyethylene jar and Si3N 4 milling media for 12 h. Batched materials were dried, granulated in a planetary ball mill using zirconia milling jars and media and sieved to - 3 5 mesh through a nylon sieve. Billets were formed by isostatic pressing at 138
MPa and packed in Grafoil-lined graphite crucibles with a silicon nitride-based powder bed and sintered in a graphite resistance-heated furnace. Sintering temperatures ranged from 1700 to 1850 °C and sintering times from 5 to 300 min. A 0.1 Pa nitrogen atmosphere was used with heating and cooling rates ranging from 5 to 20 °C min -1. Additional samples were prepared from pre-reacted BAS starting powders, SrCO3 substituted for BaCO 3 and with extended heat treatments performed simultaneously with the cool-down portion of the sintering cycle. For pre-reacted BAS, starting products were wet blended, dried, granulated, placed in AI203 crucibles and heat treated in air for 150 h at 1500 °C. These powders were subsequently blended with Si3N 4 powders for sample preparation. 2.2. Characterization
Weight loss, bulk density and crystallographic phase content were evaluated for all samples after sintering. Structural property measurements, failure analysis, thermal expansion measurements and transmission electron microscopy (TEM) were performed on select samples. Bulk density was measured using the Archimedes immersion technique. Crystallographic phase content was determined by powder X-ray diffraction analysis using a G.E. Model XRD-5 X-ray diffractometer with Cu Ka radiation. Test conditions were: accelerating voltage 40 kV, current 30 mA, scanning rate 0.01 ° min -1, detector slit 0.2 ° and beam slit 1.0 °. A rotating powder sample holder was used to minimize preferred orientation effects. Quantitative analysis of a- and fl-Si3N 4 phase contents were determined by comparing the intensity ratios from the {210} planes for each phase based on the work of Gazzara and Messier [12]. Quantitative analyses of hexacelsian and celsian contents were carried out by comparing the intensity ratios from the {102} plane for hexacelsian with the {- 112} plane for celsian based on the work of Wuchina [13]. Calibration graphs for the latter analysis were developed in-house by preparing samples with various ratios of hexacelsian to celsian. Test specimens were machined from billets using standard diamond grinding techniques. Four-point bend flexure strength was measured according to MILSTD-1942A (specimen size B) [14]. Fracture toughness was measured using the chevron notched beam method [15]. Both measurements were performed at temperatures in the range 21-1200 °C in ambient air using a sificon carbide test fixture. Twenty tests were performed for each test condition. Failure analysis using scanning electron microscopy was performed on select samples to locate and characterize the source of failure. Thermal expansion measurements were performed using dual-rod, high-temperature dilatometry
D. W. Freitag /
MaterialsScience and EngineeringA195 (1995) 197-205
over the temperature range 21-1400 °C in a helium or argon atmosphere ]. Specimens for transmission electron spectroscopy were prepared from bulk samples by a combination of mechanical grinding and ion milling. Bulk materials attached to glass slides were ground to < 40/~m and subsequently transferred to slotted electron microscope grids for dimpling to a thickness of 10/~m. Final thinning was carried out by argon ion milling at 5.5 kV. Analysis was performed on a Philips 300 EM instrument operated at 80 kV2.
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Fig. 1. Effect of BAS content on the density of liquid-phase sintered Si3N4.
3.1. Sinterability of SigN4 using B A S 100m
The dominant process variable for sinterability of silicon nitride with BAS was the BAS content. Sintering time and temperature were seen to have little impact on the overall density for the temperature range 1750-1850 °C and a time duration of 60-240 min. Conversely, Pickup and Brook [8,9] showed that silicon nitride containing < 10 vol.% BAS could be sintered to near-theoretical density with hot pressing conditions of 1700 °C and 600 min. In the present study, near-theoretical density was achieved with BAS contents of 30 vol.% and pressureless sintering conditions of 1750 °C and 60 rain (Fig. 1). BAS contents < 25 vol.% could not be pressureless sintered to high densities, even for sintering temperatures of 1850 °C and a prolonged sintering time of 240 min, which implies that further densification by the solutionprecipitation process is minimal for this system, as would be expected based on the analysis of other refractory sintering aids, e.g. Y203 [16]. The dominance of the BAS content on the sinterability of silicon nitride during the particle rearrangement stage agrees with the models developed by Kingery [17] for liquidphase sintering. Above 30 vol.% BAS, pressureless sintering conditions comparable to those used by Pickup and Brook for hot pressing were shown to produce high densities and prevent oversintering (Fig. 2). Using the powder bed method to control Si3N4 decomposition during sintering, the measured weight loss was < 1 wt.% in all cases, well below values commonly reported for pressureless sintering of Si3N4 in a powder bed [18]. The kinetics of densification for liquid phase sintered Si3N4 with 30 and 40 vol.% BAS contents are shown in Fig. 2. At 1800 °C with 30 vol.% BAS, densification is complete by the time isothermal conditions ~Performed by the Universityof Texas, Arlington,TX, USA. zPerformed by Rice University,Houston, TX, USA.
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are reached in the sintering furnace. Further sintering is limited to evolving the desired fibrous microstructure. These data support the conclusion that the solutionprecipitation process contributes little to the final density for this system. At 1700 °C with 40 vol.% BAS, densification is still occurring within the first 60 min of isothermal heating. Based on the BAS-silica and BAS-alumina binary phase diagrams, the intergranular phase is expected to be a mixture of liquid and crystalline BAS at this temperature. The reduction in the glass volume fraction and subsequent increase in glass viscosity resulting from the lower sintering temperature are believed to reduce the densification rate during the particle rearrangement stage of sintering. 3.2. Celsian development Two polymorphs of BAS exist when synthetically produced, hexagonal and monoclinic. Hexacelsian is known to crystallize from the melt, is metastable below
200
D. W. Freitag
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Materials Science and Engineering A195 (1995) 197-205
Monoclinic ®
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1590 °C and undergoes a reversible transformation to an orthorhombic crystal structure at 300°C [19]. Celsian is stable below 1590 °C and is preferred because of its low thermal expansion, structural stability and low dielectric properties [20]. The two crystal structures are shown in Fig. 3. The stable monoclinic form of BAS is always preceded by the formation of the metastable hexagonal form. Owing to the sluggish transformation of the hexagonal into the monoclinic form at temperatures below 1590 °C, which can easily take days using heat treatments, a number of methods have been demonstrated to promote the transformation process, e.g. mineralizers, substitution of BAS with strontium aluminosilicate (SAS), addition of celsian seeds and hydrothermal treatments [21]. A typical X-ray diffraction pattern for Si3N4 liquid phase sintered by the addition of BAS is shown in Fig.
4. While their relative contents change with processing conditions, phases present for all processing conditions examined were limited to a-Si3N4, fl-Si3N4 and hexacelsian. Unlike powders of celsian prepared for crystallographic analysis by prolonged heat treatments, post-sintering heat treatments of up to 1550 °C for 150 h were not effective in forming celsian in the presence of silicon nitride. Substitution of BAS by up to 25 tool% of SAS in combination with post-sintering heat treatments of 1400 °C for 5 h also proved ineffective. Other methods unsuccessfully explored for developing the monoclinic form of BAS include using different forms of BAS starting products and prereacted BAS for addition to the silicon nitride. Talmy and Zaykoski [22] have recently reported similar problems when liquid-phase sintering Si3N 4 at temperatures below 1590 °C using high volumes (25
D. W. Freitag
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Materials Science and Engineering A195 (1995) 197-205
wt.%) of BAS eutectics rich in BaO or SiO 2. After sintering, the BAS phase formed from the melt was predominately hexacelsian. Significant increases in the celsian content were not achieved with changes to the sintering temperature, sintering time or cooling rate. Unlike the present work, celsian was, however, formed by the substitution of up to 50 wt.% BAS with SAS. A key difference between the two studies is that for the present work, the liquid phase is encouraged to react with the Si3N 4 for microstructure development and in the work of Talmy and Zaykoski, the low sintering temperatures limit reactivity between Si3N4 and the liquid phase. It can only be assumed that when allowed to react with Si3N4, the liquid phase will incorporate nitrogen, which could ultimately remain in the hexagonal crystalline structure and inhibit transformation to the monoclinic crystalline structure. However, Tredway and Risbud [23] have reported that during the crystallization of S i - B a - A I - O - N glasses, no dissolution of nitrogen in the hexacelsian crystals occurs. The nitrogen remains in the glass phase during incomplete crystallization. If a similar phenomenon occurs when using BAS to liquid-phase sinter Si3N4, the goal of complete crystallization of the intergranular phase will not be achieved and the nitrogen-rich liquid phase remaining could act to suppress nucleation of celsian. Even though hexacelsian was the only crystalline phase formed, the high silicon nitride content is seen to suppress the destructive displacive transformation observed at low temperatures for pure hexacelsian (Fig. 5). At these high Si3N 4 contents, samples could be severely thermally shocked and repeatedly thermally cycled without failure, whereas samples with silicon nitride contents below 60 vol.% frequently failed during thermal cycling. Further evidence that the presence of Si3N 4 suppresses the low-temperature transformation of hexacelsian is shown in Fig. 6. By
monitoring the thermal expansion more closely within the expected transformation temperature range, an increase in Si3N 4 c o n t e n t clearly suppresses the displacive transformation and shifts it to lower temperatures. While the benefit of forming celsian is clear from its compatibility in thermal expansion with Si3N 4 as shown in Fig. 5, these data also show that hexacelsian can be used without concern for premature failure when combined with a high Si3N 4 content.
3.3. Microstructure development The process for microstructure development during liquid-phase sintering of Si3N 4 with BAS is shown in Fig. 7. The blended powders of predominately a-Si3N * and BAS constituents are reacted at temperatures where the BAS phase becomes liquid to insure that a uniform glass composition is achieved. The liquid also serves to enhance mass transport of the Si3N 4 as it transforms from a-Si3N 4 to fl-Si3N 4 by a solutionprecipitation process. When cooled, the ideal microstructure consists of elongated grains of fl-Si3N4 in a crystalline phase of celsian. In reality, the microstructure consists of a mixture of a- and fl-Si3N 4 in a crystalline matrix of BAS separated by a thin layer of residual glass; the extent and nature of each are functions of the processing conditions. The transformation kinetics for Si3N 4 are shown in Fig. 8 for different BAS contents. The fraction of a-Si3N 4 transformed is given by Y=[fl-Si3N 4 fl-Si3N4)i,it]/a-Si3N4)init and is plotted versus log time (t). The curves are sigmoidal in shape with an initial induction period, which decreases with increasing temperature, during which negligible transformation occurs. A plateau is ultimately reached where further transformation of a- to fl-Si3N 4 is slow. For the corn-
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D. W. Freitag
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BAS CONSTITUENTS
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positions shown, little change in fl-Si3N 4 content is seen prior to the completion of densification (Fig. 2). These data agree with the prior conclusion that the solution-precipitation process is a negligible contributor to densification, as would be expected for a highly refractory sintering aid such as BAS where mass transport through a highly viscous liquid would be slow. The extent of a- to fl-Si3N 4 transformation is also seen to decrease with increasing BAS content. This trend is consistent with the literature and occurs as a result of the reduced solubility of a-Si3N 4 at the interparticle contact points and reduced supersaturation of the liquid phase with silicon and nitrogen as the liquid volume increases. However, it could simply be a result of the lower processing temperatures used for highei BAS contents to prevent oversintering. The effect of various combinations of the BAS starting products on the extent of a- to fl-Si3N 4 transforma-
Fig. 9. Effect of BAS and Si3N 4 starting powders on the extent of fl-Si3N4 development.
tion is shown in Fig. 9. Variations in the SiO 2 product provided the only significant change in the final fl-Si3N 4 content. By increasing the reactivity of the SiO 2 through its addition in colloidal form, additional time was available for the solution-precipitation stage of sintering which ultimately led to an increase in the fl-Si3N 4 content. Processing conditions which can affect the final fl-Si3N a grain structure include Si3N4 powder characteristics, which vary with the synthesis route, and sintering conditions. A transmission electron photomicrograph of a typical cross-section from a s a m ple prepared using UBE SN-E-10 Si3N4 is shown in Fig. 10. The fl-Si3N 4 grains are clearly defined by their hexagonal cross-section and exaggerated grain growth
D. W. Freitag
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Materials Science and Engineering A195 (1995) 197-205
203
Fig. 10. Transmission electron photomicrograph of a typical cross-section from a sample of Si3N4 liquid-phase sintered with 30 vol.% BAS using UBE SN-E-10 powder.
Fig. l 1. Transmission electron photomicrograph of a typical cross-section from a sample of Si3N4 liquid-phase sintered with 30 vol.% BAS using Starck LC-12S powder.
along their crystallographic c-axis. Typical fl-Si3N4 grain dimensions were 0.1 p m in diameter and 1-2 ktm in length. The fibrous microstructure developed is consistent for microstructures developed with high glass viscosities where the reduction in local supersaturation is slow and high glass volumes provide for unconstrained growth. The apparent diameter of equiaxed a-Si3N 4 grains or freshly nucleated fl-Si3N4 grains was of the order of 0.2 ktm. Microstructures were very uniform with no apparent porosity. Using large-area energy-dispersive spectroscopy (EDS), A1 and Ba were readily detected in the desired ratio for stoichiometric BAS. However, on reducing the beam size to < 0.2 pm, BAS could not be detected in localized areas. Analysis of a large number of grains always proved to be Si3N4, which ultimately led to the conclusion that the BAS phase is uniformly distributed throughout the microstructure as a thin wetting layer on the Si3N 4 grains. High concentrations of amorphous BAS phases were not detected using microdiffraction, diffuse T E M and darkfield T E M methods. Grains were occasionally found that indicated Al-deprived barium silicate phases and sialon phases exist but not in high concentrations. T E M analysis overall proved difficult owing to the complexity of the material systems, fineness of the microstructure and lack of contrast between the phases. A transmission electron photomicrograph of a typical cross-section from a sample prepared using Starck LC-12S Si3N4 is shown in Fig. 11. The observations were similar to those discussed for the UBEprepared sample with the exception of the fl-Si3N4 grain size distribution. Samples prepared with Starck Si3N 4 contained a duplex distribution of fl-Si3N4 grains with those in the smaller size distribution being approximately one quarter of that of those seen in the UBE-prepared samples. Grains in the larger size dis-
tribution have diameters up to 2-3 p m and lengths well over 5 pm. Fewer equiaxed Si3N4 grains were detected in the Starck-prepared samples. A number of samples were also examined to determine the effects that the BAS starting powders and sintering conditions have on the microstructure. As expected, large increases in the sintering temperature or time resulted in coarsening of the microstructure but no apparent reduction in the aspect ratio as is commonly reported when oversintering [24]. Smaller variations in the sintering conditions, _+50 °C or + 60 rain from the norm, resulted in little apparent change in the microstructure. No apparent changes in the microstructures were found with changes to the BAS starting powders. The effect of changes in the microstructure on the previously described hexacelsian structural stability is shown in Fig. 12. For a fixed volume fraction of Si3N4, the structural change occurring in hexacelsian during its low-temperature crystalline phase transformation is significantly reduced through a well developed fibrous microstructure provided by the presence of fl-Si3N4. 3.4. Structural properties
The structural properties of liquid phase sintered Si3N4 with 30 vol.% BAS are summarized in Fig. 13. The room-temperature strength and fracture toughness are typical of liquid-phase sintered silicon nitride materials prepared by dry pressing powders [25]. The low structural properties are also due in part to the residual stresses generated by the large difference in thermal expansion between Si3N4 and hexacelsian which develop on cooling from processing temperatures. At elevated temperatures, excessive creep was observed. Although it could be attributed to residual glass phases common in liquid-phase sintered Si3N4, it
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shown to be structurally unstable when acting alone, when present in a BAS-Si3N 4 composite the in situ reinforcement provided by Si3N 4 serves to stabilize the composite sufficiently to permit its use in severe thermal environments. T h e microstructures developed were typical of that provided by a highly refractory sintering aid used in high volumes and will require further optimization of the processing conditions to provide the desired level of toughness and strength.
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Fig. 12. Effect of microstructure development due to changes in sintering time on the low-temperature transformation of hexacelsian for Si3N4 liquid-phase sintered with 30 vol.% BAS.
~.
m 600
This work was supported by the US A r m y Missile Command, Redstone Arsenal, A L , under contract D A S G 6 0 - 9 1 - C - 0 1 4 0 and Loral Vought Systems under I.R. & D. T h e author thanks K.K. Richardson, P.B. Aswath, D.L. Callahan, J.E Stepp, D.L. Hunn, R.C. Knight and C.D. Perry for their many contributions to this work.
6
500
References
4
_~
300
3
200
2
"u
LJ.
• 100
,~-
0
'''1
0
....
200
i ....
400
I ....
600
I ....
800
I ....
1000
0
1200
Temperature, °C
Fig. 13. Typical structural properties for liquid-phase sintered Si3Na liquid-phase sintered with 30 vol.% BAS.
could also be a result of the fine microstructure observed. Failure analysis indicates that pullout, bridging and crack deflection at elongated fl-Si3N4 grains are contributors to the toughness and implies a chemical compatibility with BAS at their interfaces.
4.
Conclusions
It has been demonstrated that BAS could be used as a transient liquid-phase sintering aid for silicon nitride but that hexacelsian is formed on cooling independent of the processing conditions examined. To achieve densities in excess of 97% with acceptable pressureless sintering conditions, BAS contents of at least 30 vol.% are required and the starting products must be uniformly mixed and highly reactive. T h e failure to develop the preferred phase of BAS (celsian) is believed to be related to the nitrogen-rich glass which remains after incomplete crystallization of the intergranular phase• Even though hexacelsian has been
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