Comparison between SEM and TEM Imaging Techniques to Determine Grain-Boundary Wetting in Ceramic Polycrystals

Two different non-oxide ceramics, Si₃N₄ and SiC, were characterized with respect to their grain-boundary structure employing both scanning and transmission electron microscopy. The latter method, which enables one to gain direct insight of the atomistic interface structure, was utilized to verify whether grain-boundary wetting occurred. SEM imaging of plasma-etched surfaces revealed a characteristic bright contrast along interfaces for both ceramics, Si₃N₄ as well as SiC, suggesting the presence of an intergranular glass film. High-resolution TEM studies of the Si₃N₄ sample confirmed that these fine bright lines along grain boundaries represent intergranular glass films separating Si₃N₄ matrix grains. However, when high-resolution TEM was employed on SiC samples, which showed a similar contrast variation across SiC grain boundaries in the SEM, the presence of residual glass films was not detected. The SiC materials showed clean grain boundaries with no indication of residual glass even at triple pockets. Chemical analysis monitored yttrium and aluminum segregation at interfaces, which creates a potential barrier (space charges) and therefore affects both the inner mean potential at the interface (Fresnel fringes) and the plasma-etching response. Although SEM imaging showed a similar interface contrast for both Si₃N₄ and SiC ceramics, HRTEM studies clearly revealed grain-boundary wetting in the former and clean interfaces in the latter material, respectively. Hence, SEM imaging and Fresnel fringe TEM imaging alone are not conclusive when characterizing interface wetting in ceramic polycrystals.     

Dissolution Kinetics of β-Si3N4 in an Mg-Si-O-N Glass

The rate of dissolution of β-Si₃N₄ into an Mg-Si-O-N glass was measured by working with a composition in the ternary system Si₃N₄-SiO₂-MgO such that Si₂N₂O rather than β-Si₃N₄ was the equilibrium phase. Dissolution was driven by the chemical reaction Si₃N₄(c)+SiO₂( l )→Si₂N₂O(c). Analysis of the kinetic data, in view of the morphology of the dissolving phase (Si₃N₄) and the precipitating phase (Si₂N₂O), led to the conclusion that the dissolution rate was controlled by reaction at the crystal/glass interface of the Si₃N₄, crystals. The process appears to have a fairly constant activation energy, equal to 621 ±40 kJ-mol⁻¹, at T=1573 to 1723 K. This large activation energy is believed to reflect the sum of two quantities: the heat of solution of β-Si₃N₄ hi the glass and the activation enthalpy for jumps of the slower-moving species across the crystal/glass interface. The data reported should be useful for interpreting creep and densification experiments with MgO-fluxed Si₃N₄.     

Silicon Nitride Joining

Hot-pressed Si₃N₄ was joined using an Mgo-A1₂O₃-SiO₂ glass composition chosen to approximate the oxide portion of the grain-boundary phase in the ceramic. After it has been heated at 1550° to 1650°, the interface of the joined ceramic is an interlocking mixture of Si₂N₂O, β-Si₃N₄, and a residual oxy-nitride glass. The kinetics of reactions between Si₃N₄ and the molten joining composition were studied by X-ray diffraction analysis of the phases present in Si₃N4 powder-glass mixtures quenched after varied heat treatments. Analytical transmission electron microscopy of the composition and micro-structure of the reaction zone in joined specimens, together with the X-ray diffraction results, suggests that the driving force for joining is the lowering of the Si₃N₄ interfacial energy when it is wet by the molten silicate, augmented by the negative Gibbs energy for the reaction SiO₂( l ) + Si₃N₄= 2Si₂N₂O.     

β-Si3N4Whiskers Embedded in Oxynitride Glasses: Interfacial Microstructure

Interfacial microstructures in βP-Si₃N₄( w )-Si-Al-Y-O-N-glass systems were investigated by systematically varying the nitrogen content and the Al:Y ratio of the glass matrix. High-resolution and analytical transmission electron microscopy (HREM and AEM) studies revealed that the interfacial microstructure is a function of the glass composition. No interfacial phases were formed in glasses with low Al:Y ratios and in glasses with high Al:Y ratios and low nitrogen content, whereas epitaxial growth of an interfacial layer (100–200 μm thick) on the βP-Si₃N₄( w ) occurred in a glass matrix with high Al:Y ratio and high nitrogen content. The interfacial layer was identified to be a β'-SiAION phase. Interfaces containing the SiAION layer exhibited high debonding energy compared to Si₃N₄( w )–glass interfaces. HREM studies indicated that the lattice-mismatch strain in the SiAION layer was relieved by dislocation formation at the SiAION–Si₃N₄( w ) interface. The difference in interfacial debonding energy was, hence, attributed to the local atomic structure and bonding between the glass-β-Si₃N₄ and the glass–β'-SiAION phases. This observation was clear evidence of the strong influence of glass chemistry on the interfacial debonding behavior by altering the interfacial microstructure.     

Silicon Nitride Based Ceramic Nanocomposites

Nanocomposites (Si₃N₄/SiC) were studied by combined high-resolution transmission electron microscopy and electron energy-loss spectroscopic imaging (ESI) techniques. In ESI micrographs three types of crystalline grains were distinguished: Si₃N₄ matrix grains (0.5 μΩ), nanosized SiC particles (<100 nm) embedded in the Si₃N₄, and large SiC particles (100–200 nm) at grain boundary regions (intergranular particles). Amorphous films were found both at Si₃N₄ grain boundaries and at phase boundaries between Si₃N₄ and SiC. The Si₃N₄ grain boundary film thickness varied from 1 to 2. 5 nm. Two kinds of embedded SiC particles were observed: type A has a special orientation with respect to the matrix, and type B possesses a random orientation with respect to the matrix. The surfaces of type B particles are completely covered by an amorphous phase. The existence of the amorphous film between the matrix and the particles of type A depends on the lattice mismatch across the interface. The mechanisms of nucleation and growth of Ω-Si₃N₄ grains are discussed on the basis of these experimental results.     

Solubility of Si3N4 in Liquid SiO2

The nitrogen solubility in the SiO₂-rich liquid in the metastable binary SiO₂-Si₃N₄ system has been determined by analytical TEM to be 1%–4% of N/(O + N) at 1973–2223 K. Analysis of the near edge structure of the electron energy loss peak indicates that nitrogen is incorporated into the silicate network rather than being present as molecular N₂. A regular solution model with a positive enthalpy of mixing for the liquid was used to match the data for the metastable solubility of N in the presence of crystalline Si₃N₄ and to adjust the computed phase diagram. The solubility of Si₃N₄ in fused SiO₂ is far less than reported in liquid silicates also containing Al, Mg, and/or Y. Apparently, these cations act as modifiers that break anion bridges in the silicate network and, thereby, allow further incorporation of Si₃N₄ without prohibitive amounts of network cross-linking. Finally, indications emerged regarding the diffuse nature of the Si₃N₄-SiO₂ interface that leads to amorphous regions of higher N content.     

Interface Nanostructure of Brazed Silicon Nitride

Nanomorphologies and crystallographic orientations of the brazed interface of silicon nitride (Si₃N₄) were analyzed via high-resolution transmission electron microscopy. When Si₃N₄ was brazed using an Ag-Cu-Ti alloy, titanium nitride (TiN) nanoparticles were formed adjacent to the ceramic as reaction products, and these nanoparticles were commonly accompanied by C-phase material. The structure of Si₃N₄/TiN interface was wavy on an atomic scale, which was considered to provide anchoring points that offered high mechanical strength. The TiN nanoparticles extended along the [0001] axis of Si₃N₄. The orientation relationship between TiN and ß-Si₃N₄ was, as determined from the observed lattice images, that the [110] direction of TiN was parallel to the [0001] direction of Si₃N₄. The nature of the crystallographic relationships and interface nanomorphologies were also discussed.     

Formation of Reaction-Bonded Silicon Nitride from Silane-Derived Silicon Powders: Nucleation and Growth Mechanisms

The nucleation and growth of Si₃N₄ on silane-derived Si powders was investigated with transmission electron microscopy and FTIR spectroscopy. Thermogravimetric analysis (TGA) was also used to monitor the process through different stages of the reaction. The FTIR and TEM results provide clear evidence that the nucleation of crystalline Si₃N₄ coincides with the onset of rapid nitridation. Electron diffraction indicates that Si₃N₄ forms heteroepitaxially on the Si powder surfaces, with Si (111) || Si₃N₄(0001) and Si     || Si₃N₄     . Also, flat interfaces between the Si and Si₃N₄ (compared to the initial spherical surface of the Si powders) indicate that a significant rearrangement of the particle surface occurs during the initial stages of nitridation. The results reported here demonstrate that the rapid, low-temperature nitridation observed with silane-derived powders is possible because the Si/vapor surfaces are not covered with a continuous Si₃N₄ product layer. The measured nitridation rates are comparable to Si evaporation rates, which suggests that Si vaporization is rate limiting. This is significantly different from conventional RBSN, where nitridation is limited by solid-state diffusion through a Si₃N₄ product layer.     

Bond Energetics at Intergranular Interfaces in Alumina-Doped Silicon Nitride

In Si₃N₄ ceramics sintered with Al₂O₃, the interfacial strength between the intergranular glass and the reinforcing grains has been observed to increase with increases in the aluminum and oxygen content of the epitaxial β-Si_{6- z} Al _z O _z N_{8– z} layer that forms on the Si₃N₄ grains. This has been attributed to the formation of a network of strong bonds (cross bonds) that span the glass-crystalline interface. This proposed mechanism is considered further in light of first-principles atomic cluster calculations of the relative stabilities of bridge and threefold-bonded atomic fragments chosen to represent compositional changes at the glass/Si₃N₄ grain interface. Calculated binding energies indicate Al-N binding is favorable at the Si₃N₄ grain surface, where aluminum occupancy can promote the growth of SiAlON, further enhancing the cross-bonding mechanism of interfacial strengthening.     

Carbothermic Synthesis of Monodispersed Spherical Si3N4/SiC Nanocomposite Powder

The synthesis and structure of a monodispersed spherical Si₃N₄/SiC nanocomposite powder have been studied. The Si₃N₄/SiC nanocomposite powder was synthesized by heating under argon a spherical Si₃N₄/C powder. The spherical Si₃N₄/C powder was prepared by heating a spherical organosilica powder in a nitrogen atmosphere and was composed of a mixture of nanosized Si₃N₄ and free carbon particles. During the heat treatment at 1450°C, the Si₃N₄/C powder became a Si₃N₄/SiC composite powder and finally a SiC powder after 8 h, while retaining its spherical shape. The composition of the Si₃N₄/SiC composite powder changed with the duration of the heat treatment. The results of TEM, SEM, and selected area electron diffraction showed that the Si₃N₄/SiC composite powder was composed of homogeneously distributed nanosized Si₃N₄ and SiC particles.     

Processing, Mechanical Properties, and Oxidation Behavior of Silicon Oxynitride Ceramics

Silicon oxynitride ceramics were prepared by hot-pressing an equimolar Si₃N₄+ SiO₂ mixture with 3 mol% CeO₂. The Ce₂O₃/SiO₂ ratio of intergranular phase (liquid phase) increased as the formation of Si₂N₂O proceeded. The intergranular liquid remained as a glass on cooling until the Ce₂O₃/SiO₂ ratio exceeded a certain value, at which point the liquid crystallized. There were great differences in thermal and mechanical properties and oxideation behavior between the specimen containing intergranular glassy phase and the one containing intergranular crystalline phase (Ce₅(SiO₄)₃N–Ce_4.67(SiO₄)₃O). The specimen containing the intergranular glassy phase showed excellent hightemperature strength and oxidation resistance.     

Grain Boundary Films in Rare-Earth-Glass-Based Silicon Nitride

The thickness of the intergranular films in Si₃N₄ densified with lanthanide oxides has been systematically investigated using high-resolution transmission electron microscopy. Four lanthanide oxides—La₂O₃, Nd₂O₃, Gd₂O₃, and Yb2O3—as well as Y₂O₃ are chosen so that the results will reflect the overall trend in the effect of the lanthanide utilized. The film thicknesses increase with increasing ionic radius of the lanthanide. In addition, Si₃N₄ particles flocculated into isolated clusters in the lanthanide-based glasses are also characteristically separated by an amorphous film whose thickness is similar to that in the comparable polycrystalline ceramics, demonstrating that the film thickness is dictated entirely by the composition and not by the amount of the glass phase present.     

Grain-Boundary Microstructure and Chemistry of a Hot Isostatically Pressed High-Purity Silicon Nitride

Two high-purity Si₃N₄ materials were fabricated by hot isostatic pressing without the presence of sintering additives, using an amorphous laser-derived Si₃N₄ powder with different oxygen contents. High-resolution transmission electron microscopy and electron energy-loss spectroscopy (EELS) analysis of the Si₃N₄ materials showed the presence of an amorphous SiO₂ grain-boundary phase in the three-grain junctions. Spatially resolved EELS analysis indicated the presence of a chemistry similar to silicon oxynitride at the two-grain junctions, which may be due to partial dissolution of nitrogen in the grain-boundary film. The chemical composition of the grain-boundary film was SiN_xO_y, (x ∼ 0.53 and y ∼ 1.23), and the triple pocket corresponded to the amorphous SiO₂ containing ∼2 wt% nitrogen. The equilibrium grain-boundary-film thickness was measured and found to be smaller for the material with the lower oxygen content. This difference in thickness has been explained by the presence of the relatively larger calcium concentration in the material with the lower amount of SiO₂ grain-boundary phase, because the concentration of foreign ions has been shown to affect the grain-boundary thickness.     

Controlled Crystallization in Self-Reinforced Silicon Nitride with Y2O3, SrO, and CaO: Crystallization Behavior

The controlled crystallization of the amorphous grain boundary phase has been examined in a series of self-reinforced Si₃N₄ materials with added Y₂O₃, SrO, and CaO. The effects of time, temperature, atmosphere, glass content, glass chemistry, and matrix Si₃N₄ on the crystallization have been investigated. The stability of the crystallized product, the crystallization kinetics ( T-T-T curve), and crystallization mechanisms have also been examined. Crystallization produced an oxynitroapatite containing Y, Sr, and Ca over a broad range of heat-treatment conditions and glass compositions. The oxynitroapatite was compatible with Si₃N₄ and remained stable up to 1600°C. At low temperatures (<1350°C), the rate-limiting crystallization mechanism was oxygen diffuson in the glass, and at higher temperatures (>1350°C) the rate-limiting crystallization step changed to either the formation of new Si₃N₄ grains or solute diffusion in the glass.     

High-Temperature Chemistry and Oxidation of ZrB2 Ceramics Containing SiC, Si3N4, Ta5Si3, and TaSi2

The effect of Si₃N₄, Ta₅Si₃, and TaSi₂ additions on the oxidation behavior of ZrB₂ was characterized at 1200°–1500°C and compared with both ZrB₂ and ZrB₂/SiC. Significantly improved oxidation resistance of all Si-containing compositions relative to ZrB₂ was a result of the formation of a protective layer of borosilicate glass during exposure to the oxidizing environment. Oxidation resistance of the Si₃N₄-modified ceramics increased with increasing Si₃N₄ content and was further improved by the addition of Cr and Ta diborides. Chromium and tantalum oxides induced phase separation in the borosilicate glass, which lead to an increase in liquidus temperature and viscosity and to a decrease in oxygen diffusivity and of boria evaporation from the glass. All tantalum silicide-containing compositions demonstrated phase separation in the borosilicate glass and higher oxidation resistance than pure ZrB₂, with the effect increasing with temperature. The most oxidation-resistant ceramics contained 15 vol% Ta₅Si₃, 30 vol% TaSi₂, 35 vol% Si₃N₄, or 20 vol% Si₃N₄ with 10 mol% CrB₂. These materials exceeded the oxidation resistance of the ZrB₂/SiC ceramics below 1300°–1400°C. However, the ZrB₂/SiC ceramics showed slightly superior oxidation resistance at 1500°C.     

Enhanced Crystallization of a Glassy Phase in Silicon Nitride by the Addition of Scandia

A Sc₂O₃-MgO-Al₂O₃-SiO₂ melt reacted with bulk pieces of hot-pressed Si₃N₄, dissolving Si₃N₄ at the surface and penetrating into the interior via the grain boundaries. During subsequent cooling, the glass phase in the Si₃N₄ easily crystallized and fine crystallites were formed, but no phase separation was observed. This behavior contrasts strongly with that observed for a similar melt reaction without Sc₂O₃.     

Impurity Phases in Hot-Pressed Si3N4

Impurity phases in commercial hot-pressed Si₃N₄ were investigated using transmission electron microscopy. In addition to the dominant, β-Si₃N₄ phase, small amounts of Si₂N₂O, SiC, and WC were found. Significantly, a continuous grain-boundary phase was observed in the ∼ 25 high-angle boundaries examined. This film is ∼ 10 Å thick between, β-Si₃N₄ grains and ∼ 30 Å thick between Si₂N₂O and β-Si₃N₄ grains.     

The Role of TiN in the Intergranular Phase-Forming Process in TiN-Dispersed Si3N4 Nanocomposites

Internal interfaces in two ceramic systems, monolithic Si₃N₄ (SN) and TiN-dispersed Si₃N₄ nanocomposite (STN), were characterized by analytical transmission electron microscopy (TEM). In monolithic SN both MgO and Y₂O₃ dopants are preferentially hosted by the vitreous intergranular phase in pockets at triple grain junctions (TJ), whereas in STN composites the highest dopant concentrations were observed in grain and phase boundaries. The width of grain boundary films, as revealed by high-resolution TEM imaging, varied between ≈0.8 nm in monolithic SN and ≈1.0–1.2 nm in STN. Intergranular films with increased width ≈1.8 nm were detected in SN–TiN phase boundaries. Although no enrichment of Ti could be detected in the intergranular phase, it appears that the presence of TiN dispersants indirectly contributes to the intergranular phase formation. It is assumed that TiO₂ impurities sitting on TiN particle surfaces react with the matrix phase, resulting in a more oxidic nature of intergranular films due to increased SiO₂ supply in intergranular regions. Phase-specific Si-L_2,3 energy-loss near edge structure features, which could serve as fingerprints for phase identification, were observed in spatial-difference electron energy-loss spectra from grain boundary films and TJ pockets.     

Bonding Mechanisms in Silicon Nitride Brazing

We investigated the wetting behavior and reactions of different metals on Si₃N₄ using sessile drop measurements, analysis of reaction layers, and measurements of strengths of joined bars. Active metals, such as Al and Ti, and alloys that contain them react with Si₃N₄ and cause wetting and spreading at the interface. Al-Si₃N₄ reaction at 900°C produced a thin layer of Al₂O₃ at the interface. Reaction between Si₃N₄ and Ag-Cu-To braze alloys at 900°C resulted in a complex microstructure in the reaction zone that contained TiN and titanium silicides. Breaking strengths of Si₃N₄ bars joined with the Ag-Cu-Ti braze alloys were higher than those for Si₃N₄ joined with Al, primarily because of the better wetting by the Ag-Cu-Ti alloys. Nonreactive metals and alloys such as Sn, In, Ag-Cu, and Ag-Cu-Sn neither wet, spread, nor adhere to Si₃N₄ substrates.     

Reactions Which Form Both Silicide and Nitride Layers

Reactions of Ti which form both a silicide and a nitride after rapid thermal annealing have been characterized using Auger depth profiling, X-ray diffraction, and transmission electron microscopy. Reactions of Ti with Si annealed in a nitrogen ambient at temperatures above 700°C form a thin TiN layer on TiSi₂ (C54). A similar structure is formed by annealing Ti deposited on thin Si₃N₄ films on Si in Ar at 1000°C. The reaction of nitrogen directly with TiSi₂ begins at temperatures above 950°C, but at temperatures greater than 1050°C, TiSi₂ completely reacts to form TiN. The reaction of Ti with thick Si₃N₄ films was studied in greater detail. At lower temperatures in Ar, mostly Ti₅Si₃ is formed at the interface, with some contaminant oxygen and nitrogen released from the reaction, uniformly dissolved throughout the unreacted Ti. Also, a very thin TiN layer exists at the Ti₅Si₃/Si₃N₄ interface. At higher temperatures, a three-layered structure, TiN/TiSi _x /TiN/(unreacted Si₃N₄) develops, allowing a conducting layer to be formed on an insulator. A mechanism explaining the formation of the three-layered structure is discussed. The important reactions are the fast dissolution of nitrogen into the unreacted Ti and the thermal instability of the Ti silicide/Si₃N₄ interface.