Manuel E. Brito

Hot Isostatic Pressing to Increase Thermal Conductivity of Si 3 N 4 Ceramics

Highly anisotropic Si 3 N 4 ceramics were successfully fabricated by tape-casting of raw α–Si 3 N 4 powders with β–Si 3 N 4 single-crystal particles as seed particles and Y 2 O 3 as an effective sintering aid, followed by hot isostatic pressing at a temperature of 2773 K for 2 h under a nitrogen gas pressure of 200 MPa. The microstructure consists of very large elongated grains (diameter ~10 μm; length of ~200 μm), highly oriented in the tape-casting direction. The thermal conductivity along this direction reaches 155 W m -1 K -1 at room temperature, but varies significantly between room temperature and 1273 K. This thermal conductivity is closely related to (1) formation of extremely large elongated β–Si 3 N 4 grains with a reduced amount of crystal defects due to the high-temperature firing and to (2) orientation of β–Si 3 N 4 grains due to addition of seed particles and to tape-casting.

Microstructure designing of silicon nitride

A new concept of materials design that allows simultaneous control of the morphologies and distribution of the structural elements at plural scale levels to create a new family of advanced ceramics was proposed. The validity of the concept was experimentally demonstrated using silicon nitride ceramics as model materials. Controlling anisotropic grain growth by seeding of small amounts of morphologically regulated, β-silicon nitride single crystals (micro-scale level control), combined with alignment of the seed particles by tape casting followed by stacking of laminates (macro-scale level control) allows compatibility of high strength and high fracture toughness in this material, with a high degree of reliability for the mechanical strength.

Multifunctional Si3N4/(β-SiAlON+TiN) layered composites

Abstract Layered multifunctional ceramic composites on the base of Si 3 N 4 and TiN have been prepared by tape casting. The reaction conditions for in situ preparation of β-SiAlON + TiN composite were optimised and dense Si 3 N 4 /(β-SiAlON + TiN) layered materials were prepared by hot pressing. The bending strength and fracture toughness of layered materials measured in the direction perpendicular to the layer alignment were remarkably higher (1184 MPa and 9.75 MPa m 1/2 ) in comparison to the “monolithic” β-SiAlON + TiN composite (647 MPa and 4.71 MPa m 1/2 ). High anisotropy was achieved for the electrical resistance of the layered materials in parallel (6.10 −2 Ω cm) and perpendicular (5×10 11 Ω cm) direction to the layer alignment.

Intragranular crack deflection and crystallographic slip in Si3N4/SiC nano-composites

Abstract Crack deflection is a potential mechanism for toughening essentially brittle materials. Deflection generally occurs along grain boundaries and is enhanced by particles with high aspect ratios. Si 3 N 4 /SiC ceramic/ceramic composites contain elongated β Si 3 N 4 grains as a major phase and provide a good illustration for this mechanism. As a result, such materials demonstrate a relatively good fracture toughness. It is shown in this study, using transmission electron microscopy (TEM), that transgranular cracks, which develop at low temperatures and/or high deformation rates (present case), may also be deflected by intragranular SiC inclusions. Cracks use the 100 planes and propagate along the 〈100〉 directions of the silicon nitride hexagonal lattice. Further, after large plastic deformations at high temperature, slip defects are observed in silicon nitride grains, with many step-like deviations (‘cross’ slip), and the slip system is found to be [001] {100}.

Improved creep resistance in anisotropic silicon nitride

Tensile creep behavior of silicon nitride with aligned rodlike grains (anisotropic silicon nitride), fabricated by superplastic forging, was investigated at elevated temperatures. Creep rate of the anisotropic silicon nitride was about 1 order of magnitude lower than that of the isotropic one (without forging). The stress sensitivities for the isotropic and anisotropic specimens at 1200 °C were 2.1 and 2.6, respectively, and that for the anisotropic specimen at 1250 °C was 3.6. The grain alignment should cause a remarkable improvement in the creep resistance when a tensile stress is applied along the alignment direction.

Novel microstructures in microwave sintered silicon nitride

Preliminary results on microwave sintering of seeded silicon nitride show that a well defined bi-modal grain size distribution is attainable in Si{sub 3}N{sub 4}-Y{sub 2}O{sub 3}-Al{sub 2}O{sub 3}-MgO sintered bodies by microwave sintering at 28 GHz of materials needed with {beta}-Si{sub 3}N{sub 4} particles (2 vol. %). A positive effect on the mechanical performance is anticipated for these microstructurally controlled silicon nitride ceramics.

Tailoring the Intergranular Phases in Silicon Nitride for Improved Toughness

Intergranular glass phases can have a significant influence on fracture resistance (R-curve behavior) of Si nitride ceramics and appears to be related to debonding of the {beta}-Si{sub 3}N{sub 4}/oxynitride-glass interfaces. Applying the results from {beta}- Si{sub 3}N{sub 4}-whisker/oxynitride-glass model systems, self- reinforced Si nitrides with different sintering additive ratios were investigated. Si nitrides sintered with a lower Al{sub 2}O{sub 3}: Y{sub 2}O{sub 3} additive ratio exhibited higher stead-state fracture toughness together with a steeply rising R-curve. Analytical electron microscopy suggested that the different fracture behavior is related to the Al content in the SiAlON growth band on the elongated grains, which could result in differences in interfacial bonding structures between the grains and the intergranular glass.

Microstructural features in sialon/SiCpl composites observed by TEM

Microstructural characterization by TEM of hot-pressed sintered bodies in the Si3N4-Er2O3-AlN-SiCpl system was performed. Particular importance was given to determine the role that SiC platelets with smooth and crystallographically well defined surfaces play in the microstructural evolution of these composites.

Microstructure control of silicon nitride by seeding rod-like β-Si3N4 particles

ABSTRACT Rod-like β-Si3N4 single crystal particles with a diameter of about 1μm and length of 5μm were grown from a mixture of β-Si3N4 and oxides, followed by acid rinse treatment to remove residual glassy phase. These β-Si3N4 particles were used as seed crystals to control microstructure of sintered silicon nitride under a low nitrogen gas pressure. The effect of seeding on the microstructure development and mechanical properties of silicon nitride were investigated.