Topotactically Oriented α-Si3N4 Cores in Sintered β-Si3N4

α-Si₃N₄ core structures within β-Si₃N₄ grains have been studied by transmission electron microscopy. The grains were dispersed in an oxynitride glass which was previously melted at 1600°C. The cores were topotactically related to the as-grown β-Si₃N₄ crystallites and are related to epitactical nucleation during heat treatment as the most probable mechanism.     

Quantitative Microscopy of β-Si3N4 Ceramics

The three-dimensional grain size distribution in an experimental β-Si₃N₄ material has been determined using the hexagonal prism as a model of β-Si₃N₄ grain shape. Results from quantitative microscopy of polished and etched sections were compared with computer-generated two-dimensional stereological parameters of hexagonal prisms with different aspect ratios in order to determine an average grain shape (i.e., aspect ratio) in the microstructure. Section parameter distributions for the average grain shape were obtained from the computer simulations and used in a three-dimensional reconstruction of the microstructure. The results showed that this Si₃N₄ ceramic had the postulated fibrous microstructure and a broad grain size distribution.     

Calculation of Grain-Boundary Bonding in Rare-Earth-Doped β-Si3N4

First-principles molecular orbital calculations are performed by the discrete variational Xalpha method using model clusters of rare-earth-doped β-Si₃N₄ and the interface between prismatic planes of β-Si₃N₄ and intergranular glassy films. On the basis of the total overlap population of each cluster, the rare-earth ions are implied to be stable in the grain-boundary model, while they are not stable in the bulk model. These results are consistent with experimental observations showing significant segregation of Ln³⁺ ions at the grain boundary and no solubility of Ln³⁺ into bulk β-Si₃N₄. Grain-boundary bonding is weakened with an increase of the ionic radius of the rare-earth ions, which provides a reasonable explanation for the ionic size dependence of the crack propagation behaviors as well as the growth rate of the prismatic plane in the rare-earth-doped β-Si₃N₄ during liquid-phase sintering.     

Synthesis of β-Si3N4 by Chemical Vapor Deposition

Beta-type CVD-Si₃N₄ plates (up to 1.1 mm thick) have been prepared by adding TiCl₄ vapor to the system SiCl₄-NH₃-H₂ at deposition temperatures of 1350° to 1450°C, while α-type or amorphous CVD-Si₃N₄ was obtained without TiCl₄ vapor at the same deposition temperature. Three to four wt % 777V was included in the β-type CVD-Si₃N₄ matrix. The density, preferred orientation, and lattice parameters of β-type CVD-Si₃N₄ were examined.     

Formation of β-Si3N4 Coatings by Chemical Vapor Deposition

BetaSi₃N₄ coatings were obtained by chemical vapor deposition in a fused-silica reaction tube by outside heating of the system SiCl₄-NH₃-N₂ at a deposition temperature (reaction tube temperature) of 1300°C, whereas α- and α+β-phase coatings were obtained at depositon temperatures of 1150° and 1250°C, respecively. Formation of β-phase coatings at relatively low temperatures is explained in terms of the effect of a catalytic impurity, SiO vapor from the reaction tube. The X-ray diffraction patterns and sulface morphologies of the coatings were studied.     

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.     

Insolubility of Mg in β-Si3N4 in the System Al-Mg-Si-O-N

Solid-solution formation of magnesium in β-Si₃N₄ containing AlN:Al₂O₃ was investigated. Samples were hot-pressed at 1700°C. Under the condition studied, very little or no magnesium entered the β-Si₃N₄ lattice.     

Microstructure Tailoring for High Thermal Conductivity of β-Si3N4 Ceramics

β-Si₃N₄ ceramics sintered with Yb₂O₃ and ZrO₂ were fabricated by gas-pressure sintering at 1950°C for 16 h changing the ratio of "fine" and "coarse" high-purity β-Si₃N₄ raw powders, and their microstructures were quantitatively evaluated. It was found that the amount of large grains (greater than a few tens of micrometers) could be drastically reduced by mixing a small amount of "coarse" powder with a "fine" one, while maintaining high thermal conductivity (>140 W·(m·K)⁻¹). Thus, this work demonstrates that it is possible for β-Si₃N₄ ceramics to achieve high thermal conductivity and high strength simultaneously by optimizing the particle size distribution of raw powder.     

Mechanochemical-Activation-Assisted Combustion Synthesis of α-Si3N4

Combustion synthesis (CS) of α-silicon nitride (Si₃N₄) powders was accomplished at a nitrogen pressure lower than 3 MPa. The combination of mechanical activation and chemical stimulation was effective in enhancing the reactivity of Si powder reactants, which was responsible for the reduction of the minimum nitrogen pressure normally required for the CS of Si₃N₄. This breakthrough indicates that nitriding combustion of silicon in pressurized nitrogen could be promoted by activating the solid reactants instead of by increasing the nitrogen pressure. The phase content of α-Si₃N₄ in the as-synthesized product is over 90 wt%. Scanning electronic microscopy observation showed that the combustion-synthesized Si₃N₄ powders are submicron-sized particles with spherical morphologies.     

Mechanical Properties of Single-Crystal α-Si3N4

Single crystals of α-Si₃N₄ several millimeters in diameter and several millimeters long have been grown by chemical vapor deposition. Some of the microstructural and mechanical properties have been evaluated using X-ray diffracometry, optical and transmission electron microscopy, and high-temperature microhardness testing. The crystallographic growth direction determines the quality of the crystals, including the density of internal microcracks and the nature and quantity of special boundaries. The measurement of crack lengths associated with microindentations has shown that cleavage in α-Si₃N₄ is relatively isotropic. Finally, indentation fracture toughness values agree well with theoretical predictions based on recent bond-energy calculations.     

Kinetics of β-Si3N4 Grain Growth in Si3N4 Ceramics Sintered under High Nitrogen Pressure

The kinetics of anisotropic β-Si₃N₄ grain growth in silicon nitride ceramics were studied. Specimens were sintered at temperatures ranging from 1600° to 1900°C under 10 atm of nitrogen pressure for various lengths of time. The results demonstrate that the grain growth behavior of β-Si₃N₄ grains follows the empirical growth law Dⁿ– D₀ⁿ = kt , with the exponents equaling 3 and 5 for length [001] and width [210] directions, respectively. Activation energies for grain growth were 686 kJ/mol for length and 772 kJ/mol for width. These differences in growth rate constants and exponents for length and width directions are responsible for the anisotropy of β-Si₃N₄ growth during isothermal grain growth. The resultant aspect ratio of these elongated grains increases with sintering temperature and time.     

Mechanical Properties of β-Si3N4-Whisker/Si3N4-Matrix Composites

Composites containing 30 vol%β-Si₃N₄ whiskers in a Si₃N₄ matrix were fabricated by hot-pressing. The composites exhibited fracture toughness values between 7.6 and 8.6 MPa · m^1/2, compared to 4.0 MPa · m^1/2 for unreinforced polycrystalline Si₃N₄. The improvements in fracture toughness were attributed to crack wake effects, i.e., whisker bridging and pullout mechanisms.     

Thermal Conductivity of β-Si3N4: II, Effect of Lattice Oxygen

Dense β-Si₃N₄ with various Y₂O₃/SiO₂ additive ratios were fabricated by hot pressing and subsequent annealing. The thermal conductivity of the sintered bodies increased as the Y₂O₃/SiO₂ ratio increased. The oxygen contents in the β-Si₃N₄ crystal lattice of these samples were determined using hot-gas extraction and electron spin resonance techniques. A good correlation between the lattice oxygen content and the thermal resistivity was observed. The relationship between the microstructure, grain-boundary phase, lattice oxygen content, and thermal conductivity of β-Si₃N₄ that was sintered at various Y₂O₃/SiO₂ additive ratios has been clarified.     

High-Resolution Electron Microscopy of Chemically Vapor-Deposited β-Si3N4-TiN Composites

Microstructures of Si₃N₄-TiN composites prepared by chemical vapor deposition (CVD) were investigated by the multibeam imaging technique using a 1 MV electron microscope. High-resolution images showed a number of fibrous TIN crystallites dispersed in the matrix of CVD β-Si₃N₄. Crystallographic orientation relations between β-Si₃N₄ and TiN were determined directly from the observed images in the subcell scale. The fibrous axis of TiN is parallel to the (110) direction of the NaCl structure and lies along the c axis of the hexagonal β-Si₃N₄ crystal. Domain boundaries, planar faults, nonplanar faults, and dislocations were found in the CVD β-Si₃N₄ matrix near the TiN crystallites. The origin of the structure defects is briefly discussed in connection with the formation of TiN crystallites in the matrix.     

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.     

Nucleation and Growth of α'-SiAlON on α-Si3N4

Morphology, composition, and growth defects of α'-SiAION have been studied in a fine-grained material with an overall composition Y_0.33Si₁₀Al₂O₁N₁₅ prepared from α-Si₃N₄, AlN, Al₂O₃, and Y₂O₃ powders. TEM analysis has shown that fully grown α'-SiAloN grains always contain an α-Si₃N₄ core, implicating heterogeneous nucleation operating in the present system. The growth mode is epitaxial, despite the composition and lattice parameter difference between α-Si₃N₄ and α'-SiAlON. The inversion boundary that separates two domains in the seed crystal is seen to continue in the grown α'-SiAION. Lacking a special growth habit, the growth typically proceeds from more than one site on the seed crystal, and the different growth fronts impinge on each other to give an equiaxed appearance of α'-SiAlON. Misfit dislocations on the α/α'interface are identified as [0001] type ( b = 5.62 Å) and 1/3 [1 2 10] type ( b = 7.75 Å). These nucleation and growth characteristics dictate that microstructural control of α'-SiAlON must rest on the size distribution of the starting α-Si₃N₄ powder.     

Fabrication of Hard and Strong α/β-Si3N4 Composites With MgSiN2 as Addivitives

α/β-Si₃N₄ composites with various α/β phase ratios were prepared by hot pressing at 1600°–1650°C with MgSiN₂ as sintering additives. An excellent combination of mechanical properties (Vickers indentation hardness of 23.1 GPa, fracture strength of about 1000MPa, and toughness of 6.3 MPa·m^1/2) could be obtained. Compared with conventional Si₃N₄-based ceramics, this new material has obvious advantages. It is as hard as typical in-situ-reinforced α-Sialon, but much stronger than the latter (700 MPa). It has comparable fracture strength and toughness, but is much harder than β-Si₃N₄ ceramics (16 GPa). The microstructures and mechanical properties can be tailored by choosing the additive and controlling the heating schedule.     

Microstructure and Mechanical Properties of the in situβ-Si3N4/α'-SiAlON Composite

The in situ β-Si₃N₄/α'-SiAlON composite was studied along the Si₃N₄–Y₂O₃: 9 AlN composition line. This two phase composite was fully densified at 1780°C by hot pressing Densification curves and phase developments of the β-Si₃N₄/α'-SiAlON composite were found to vary with composition. Because of the cooperative formation of α'-Si AlON and β-Si₃N₄ during its phase development, this composite had equiaxed α'-SiAlON (∼0.2 μm) and elongated β-Si₃N₄ fine grains. The optimum mechanical properties of this two-phase composite were in the sample with 30–40%α', which had a flexural strength of 1100 MPa at 25°C 800 MPa at 1400°C in air, and a fracture toughness 6 Mpa·m^1/2. α'-SiAlON grains were equiaxed under a sintering condition at 1780°C or lower temperatures. Morphologies of the α°-SiAlON grains were affected by the sintering conditions.