This paper reports on the fabrication of microcrystalline β-SiAlON components by direct nitridation of polymeric green compacts. The preceramic material was obtained from a polycarbosilane modified with an aluminum alkoxide. The microstructural transformation from the polymeric bodies to the final dense ceramic components was monitored by several techniques such as BET, XRD and dilatometry. Mechanical characterization, performed by indentation at various stages of the conversion process, will also be presented.

We have previously reported the synthesis of cordierite polymer precursors directly from MgO, SiO2 and alumina.1 Removal of organics at 400°C in air and/or in oxygen gave amorphous, atomically mixed, oxide powders with surface areas of 250-400 m2/g. It is possible to convert these high surface area oxide powders to Mg-Si-Al-O-N powders by heating in a flowing NH3 atmosphere. The effects of selected processing conditions on nitrogen incorporation, in a set of well defined oxide powders, were examined. The powders are nitrided in flowing NH3, at temperatures of 700-1200°C, to obtain N contents of up to 5 atomic percent.

A procedure was developed for slip casting Si3N4-Y2O3 - SiO2 compositions. The silicon nitride was oxidized to supply the silica. The procedure resulted in 65 w/o slips with viscosities of 35 mPa s which when cast, yielded compacts which were 56 % of theoretical density. Slips with 70 w/o solids loading were also prepared having viscosities of 120 mPa.s. The properties of the slips were influenced by the starting silicon nitride powder, dispersant, and milling media. The resulting compacts could be gas pressure sintered to full density, at temperatures as low as 1850 C, using nitrogen pressures as low as 4 MPa. Strengths as high as 750 MPa were achieved and the initial results show good creep resistance at 1350 C.

Tape casting behavior of submicron silicon nitride powder(UBE SN–E1O) with Al2O3 and Y2O3 as sintering additives was studied. It was found that Hypermer KD1(polymeric dispersant) is effective dispersant for submicron silicon nitride in organic solvent system.

Optimization of the rheological properties of the slurries allows the homogeneous green tapes with green densities of 45–50% theoretical. The cast tapes could be sintered to closed porosity with densities of higher than 96% theoretical.

Si3N4 whiskers are found through SEM analysis of powder obtained by carbothermal reduction of calcined Chilean rice husks. X-ray and FTIR analysis showed that the ceramic material was mainly α-Si3N4. A stable aqueous suspension with 30 or 40 wt% Si3N4 was obtained at pH = 8.5. A maximum viscosity at pH = 4.0 and low rpm was also observed and this rheological behavior is consistent with the zeta potential data. An irregular flow behavior near pH 4.0 shows a decrease in viscosity that is related to active impurities as detected by potentiometric titrations.

These rheological studies conducted allowed production of a Si3N4 green ceramic body measuring 170 × 25 × 5 mm formed by slip casting. The optimal aqueous suspension of powder contained the following: 17.0 g Si3N4, 1.0 g Cu2O, 1.3 g bentonite, and 20.0 g water.

Silicon compacts nitrided utilizing the temperature gradient inherent to microwave heating were more fully converted to silicon nitride than was possible with similar compacts nitrided isothermally. Although nitrogen depletion prevented the reaction rate in the center from exceeding that at the surface, the temperature gradient partially counteracted the effect of nitrogen depletion. Thus the microwave-heated specimens could be nitrided fully before the reduction in porosity that accompanies the reaction eliminated the diffusion of nitrogen into the compact.

Sintered silicon nitride has many desired properties, however, for most applications these materials are too expensive to compete with metal parts. Sintered reaction-bonded silicon nitride (SRBSN) is more economical, with raw material costs <27% those of comparable high-purity materials, making it competitive with metal parts. Conventional processing of SRBSN requires long nitridation times and a two-step firing process. Microwave (MW) heating reduces the reaction times and is performed in a one-step process, thereby simplifying the operation. The flexural strength of the MW-SRBSN is equivalent to the strength of some materials made from higher-cost powders. Thus, these materials may be appropriate for a number of applications.

The microstructure of a series of microwave sintered silicon nitride based ceramics have been assessed using a combination of optical microscopy/image analysis and analytical electron microscopy. Materials were studied as-received and after post-sinter hot-isostatic pressing. The grain size of microwave sintered materials was appreciably finer than conventionally processed ceramics of similar composition, although the mechanism involved is not clear. The as-received ceramics exhibited a reverse porosity gradient (with the highest porosity level at the surface) due to heat dissipation to the cooler surroundings during sintering. This also resulted in a small increase in the β′ grain aspect ratio close to the surface arising from an increase in the glass phase viscosity as the temperature decreases. Post-sinter HIPing of microwave sintered samples resulted in the elimination of most of the bulk porosity, but not near the surface. This is due to the reverse porosity gradient previously described, which leads to a transition from closed to open porosity with decreasing density near to the surface.

The effect of oxygen content of silicon nitride powders on the properties of resulting ceramics was studied by physically and chemically treating the powder to modify its surface oxygen content. These powders were compounded with yttria and hot-pressed into dense ceramics. Strength and oxidation resistance of these ceramics were measured and correlated with the powder and ceramic compositions as well as the resulting intergranular phases. Results showed that the phases varied with slight differences in the initial powder oxygen content as predicted, and that strength could be correlated to initial oxygen concentration. Best results were obtained when the oxygen content was increased by thermal oxidation. A Taguchi Methods experimental study designed to optimize the thermal treatment resulted in silicon nitride ceramics with strength improvements of 22 and 37% at ambient temperature and 1370°C, respectively. Oxidation resistance was also improved.

While its high-temperature strength, resistance to oxidation, and other properties make silicon nitride an attractive candidate for many advanced structural applications, its propensity for brittle failure has hindered its widespread adoption. One approach to avoiding brittle failure is through incorporation of continuous fiber-reinforcement; however, conventional (powderbased) methods of silicon nitride fabrication can degrade fibers and are not amenable to the production of complex shapes. The Southwest Research Institute has developed a number of polymeric precursors to silicon nitride which are available as thermosetting liquids, and we have shown that these materials can be used in combination with near net-shape manufacturing techniques to produce fiber-reinforced silicon nitride composites. Mechanical property tests conducted at room temperature suggest that these polymer-derived composites exhibit fracture behavior comparable to those produced through conventional techniques; micromechanical investigations conducted at 800°C indicate that non-brittle failure is maintained at elevated temperature.

Current hot isostatic consolidation methodology used for the fabrication of complex-shaped Si3N4-based components requires the use of an expensive glass encapsulation technique and extended thermal exposure (in hours) of the specimen. An alternative consolidation approach involving the use of solid pressure transmitting media under high pressure, has enabled the consolidation of Si3N4 alloys without the need for glass encapsulation.

Characterization of microstructures and mechanical properties of this (MOR, fracture toughness) material has been carried out and will be presented. It has been noted that in Si 3N4/8%Y2O3-4%Al2O3 composition, consolidated using this approach, a significantly larger volume fraction of α phase has been retained compared with typically observed conversion in α⇒ β in hot isostatically pressed material or sintered material.

Key issues for addressing densification and microstructure control using this process are presented. This rapid consolidation approach appears to be a promising alternative to hot isostatic pressing for the fabrication of complex-shaped Si3N4 components.

Silicon nitride films were deposited from silane and ammonia onto single crystal silicon substrates at a total pressure of 4.7 kPa. At temperatures below 1200° C, continuous films of polycrystalline Si3N4 deposited within a narrow region of low SiH4 partial pressures but at relatively high growth rates, for example, > 10 μm/hr at 1170° C. The early stages of crystalline film growth were studied with a combination of analytical techniques: FT-IR spectroscopy, x-ray and electron diffraction and electron microscopy. During deposition faceted Si3N4 grains nucleate on a growing nanocrystalline/amorphous interlayer.

Analytical electron microscopy, X-ray diffractometry and secondary ion mass spectrometry (SIMS) have been used to characterize the intergranular microstructure of nitrided pressureless sintered (NPS) Si3N4 ceramics and the phase transformations which occur in the intergranular regions during oxidation. The phase transformations, which take place after crystallisation of the residual intergranular glass, are caused by an inward transport of oxygen and result in the development of sub-scalar phase gradients. Cubic Y2O3 stablized ZrO2 partitions from the oxynitride liquid phase sintering medium when a small amount of ZrO2(+ 3 mol% Y2O3) is added together with the Y2O3 and Al2O3 sintering additives. The latter material has a reduced oxidation resistance, presumeably due to the high oxygen conductivity of the cubic ZrO2 structure.

The structures of SiC/Si3N4 interfaces and Si3N4 matrix grain boundaries in Ceramic Matrix Composites (CMC) were investigated by high resolution electron microscopy. The light element chemistry of the interfaces was analyzed by high spatial resolution (∼3 nm) position resolved EELS in a field emission TEM and by high spatial resolution EDS in a dedicated scanning transmission electron microscope (STEM). High-angle annular dark-field (HAADF) imaging (resolution < 1 nm) technique was used to determine the distribution of yttrium atoms at matrix grain boundaries and at SiC/Si3N4 interfaces. HAADF images suggest that yttrium might diffuse into Si3N4 crystals bounding the interfacial and grain boundary regions.

²-Si3N4 matrix composites reinforced with nano-meter scale SiC particles were fabricated by hot-pressing the mixture of SiC and Si3N4 fine powders. Interface microstructure and crack propagation in the composite were observed by high resolution electron microscopy. The interface between SiC and Si3N4 had good coherence, but no preferred orientation was observed. At the interface between SiC and Si3N4, the (111) face of SiC particles was flat, and sometimes a disturbed lattice structure thinner than 1 nm was observed. Near the main cracks, the microcracking at fine SiC particles/Si3N4 matrix interface was frequently observed. The cracks propagated exactly along this interface. The interface microcracking seems to contribute to the improvement of fracture toughness.

Laser synthesized, ultra-fine, amorphous, Si3N4 powders were densified via HIP'ing without any oxide sintering aids. Exposed samples were made from powder that had been exposed to the atmosphere, thereby picking up an oxide surface layer, and unexposed samples were made from powders processed entirely under glove box conditions, i.e. without oxygen contamination. TEM (and sintering) studies indicate that the exposed samples HIP'ed at temperatures in excess of the melting point of Si02, densified via a solution-reprecipitation mechanism, with a resultant intergranular glassy phase of high purity Si02. In contrast, unexposed samples had to be HIP'ed to 2050°C to achieve a density of ∼70 %p Th. These samples consisted of equiaxed β-Si3N4 grains, with localized high density regions where no inter-granular phase (crystalline or glassy) was detected to within 0.66 nm.

The static fatigue behavior of sintered Y2O3/A12O3-fluxed Si3N4 in air is controlled by slow crack growth or creep fracture. Partial devitrification of the amorphous grain boundary phase at 1000°C and 1100°C improves the static fatigue resistance with specimens surviving up to 1500 hrs. during stress rupture experiments. In this study the early stages of partial devitrification during static fatigue testing at 1000°C are investigated by conventional and analytical transmission electron microscopy with emphasis on nucleation and growth of δ-Y2Si2O7 and X1-Y2SiO5 and possible constraints from different stress states. The results show that the stress state does not affect the nature of the secondary phase assemblage. However, the amount of crystallization is higher within the tensile region of the flexural specimens than in areas which experienced compressive stresses.

The rare earth-oxide and nitride apatites were examined as grain boundary phases in silicon nitride to assess their potential for developing high toughness materials using gas-pressuresintering. Densification was dependent on the quantity of additives used with high densities achieved at equivalent oxygen contents of ∼8%. Fracture toughnesses (KIc) up to 8-10 MPa√m were obtained for some compositions. Ambient temperature flexural strengths were in the range of 400-720 MPa; however, the strengths atelevated temperatures (1200ºC) were reduced from these values.

Si3N4 ceramics prepared with refractory grain boundary phases to improve high temperature properties are difficult to densify by conventional sintering methods. Gas-pressure sintering may be used to promote densification and development of acicular grains for improved fracture toughness. The current study examined rare earth silicate sintering aids with the composition M2Si2O7, where M is a trivalent cation (Y, La, Nd). M2O3 and Si02 additions were varied to develop a number of compositions in the Si3N4—Si2N2O—M2Si2O7 ternary phase field. Pressureless sintering and gas-pressure sintering were used to densify the samples. Densification, microstructure development, oxidation resistance, and mechanical properties were evaluated and compared with respect to compositional variations and processing conditions.

The ultimate usefulness of oxynitride glasses and fibers depends upon the minimization or elimination of metallic defects that arise during processing. Despite this, the origins and chemistry of such defects in oxynitride glasses have received scant attention in the literature. The defects reduce glass transparency and cause oxynitride glass fibers to fail at relatively low stress levels. The same types of defects undoubtedly occur in the grain boundary glass phase of sintered Si3N4with unknown effects on material properties.

Exampls are shown of Si-rich metallic defects in oxynitride glasses, and their effects on glass and fiber properties are discussed. Chemical reactions that produce the defects are considered, as are chemical analysis results supporting the proposed reaction mechanisms.