Pavol Šajgalík

SiC/Si3N4 nano/micro-composite — processing, RT and HT mechanical properties

Two SiC/Si3N4 nano/micro composites were prepared from a starting mixture of crystalline α-Si3N4, amorphous SiNC, Y2O3 and/or Al2O3. The composite material for room temperature (RT) application has high strength of 1200 MPa, Weibull modulus of 19 and moderate fracture toughness of 7 MPa m1/2. The composite for high temperature (HT) application, without Al2O3 has RT strength of 710 MPa and is able to keep 60% of its RT strength up to 1300°C. The creep resistance of composite material is approx. 1 order higher compared to relative monolith up to 1400°C.

Low-cost preparation of Si3N4-SiC micro/nano composites by in-situ carbothermal reduction of silica in silicon nitride matrix

Abstract SiC/Si 3 N 4 nano/micro composites were prepared from a mixture of α-Si 3 N 4 , amorphous carbon (carbon black), and Y 2 O 3 by carbothermal reduction of SiO 2 present at the surface of Si 3 N 4 matrix grains, or added deliberately to the starting mixture. A special heating regime allowed the outgassing of CO(g) (a product of carbothermal reduction) and reduced the residual porosity to less than 2%. The inter- and intragranular SiC inclusions containing the amorphous oxygen-rich layer at the interface between SiC and the Si 3 N 4 grains were present, originating from the reaction of free carbon with the silica melt. The reaction consumes silica in the grain boundary phase. The change of the grain boundary chemistry influences both room and high temperature properties of the nanocomposite.

Silicon carbide powder synthesis by chemical vapour deposition from silane/acetylene reaction system

Abstract Amorphous fine silicon carbide powders have been prepared via the chemical vapour deposition from reaction mixture SiH 4 –C 2 H 2 in a vertical tubular flow reactor in the temperature range 900–1250°C. Powder particles prepared at temperature 1100°C and C 2 H 2 /SiH 4 mol ratio 1.2 are equiaxial, quasispherical and agglomerated. The mean particle size of the powder is approx. 0.1–0.2 μm. The maximum agglomerate size is about 0.3 μm.

Engineering ceramics '96 : higher reliability through processing

I: Advanced Powderless and Powder Processing. Developing Short-Range Repulsive Potentials for Aqueous Processing of Reliable Ceramics F.F. Lange. Near-Net Shaping of Engineering Ceramics: Potentials and Prospects of Aqueous Injection Moulding (AIM) T. Kosmac. Non- Oxide Nanometer Powder Synthesised by CVD Method D.L. Jiang, et al. Non-Oxide and Oxide Ceramics for Preceramic Polymers for Composite Components G. Ziegler, et al. Colloidally Processed Alumina-Ceria Stabilized Zirconia Composites V.V. Srdic, L. Radonjic. The Influence of Powder Characteristics on the Properties of Alumina Ceramics Shaped by Injection Moulding from Water Based Suspensions Z.S. Rak, J. Czechowski. Synthesis, Properties and Processing of Nanosized Silicon-Carbonitride Powders A. Neumann, et al. Comparative Hot-Pressing Study of Amorphous and Crystalline Silicon Nitride Powders J. Szepvolgyi, I. Mohai. Engineering Ceramics for Polymers W. Dressler, R. Riedel. Covalent Ceramics from Organosilicon Polymers J. Bill. II: Engineering Ceramic Monoliths and Composites. In-Situ Toughening of Non Oxide Ceramics -- Opportunities and Limits M.J. Hoffmann, M. Nader. a-SiAlON and a-b SiAlON Composites Recent Research T. Ekstrom. a-SiAlON Grains with High Aspect Ratio -- Utopia or Reality? Z.J. Shen, et al. SiAlON/SiC Micro-Nano Composites Z. Lences, M. Haviar. Chemical Thermodynamics in Ceramics K. Motzfeldt. Influence of Powder Treatment Methods on Sintering, Microstructure and Properties of Si3N4-Based Materials A. Bellosi, et al. Crystallization Induced Sub-Grain Boundaries in Silicon Nitride K. Rajan, P. Sajgalik. Phase Transformation During Hot-Pressing of Si3N4-Al2O3 (P) Composite Materials F.J. Oliveira, et al. Mechanical Properties of SiC Whisker/Si3N4 Composite Prepared by an In-Situ Method S. Yamada, et al. Al2O3-NixAl Based Cermets Prepared by In-Situ Reactions Z. Panek. A Stress-Induced Phase Transformation of High Temperature Orthorhombic Phase of (R1-xLax)4Al2O9 (R=Gd,Ho) M. Shimada. III: Advancing in Mechanical, Thermal and Physical Properties through Processing. The R-Curve Response of Ceramics with Microscopic Reinforcements: Reinforcement and Additive Effects P.F. Becher, et al. Ceramics with Non-Uniform Microstructures and Anisotropic Properties K.J. Bowman, et al. Factors Influencing the Residual Stresses in Layered Silicon Nitride-Based Composites P. Sajgalik, et al. New Post-Sintering Treatments for Improved High-Temperature Performance Si3N4-based Ceramics D.P. Thompson. Edge Toughness of Brittle Materials M. Hangl, et al. Long-Term Creep Damage Development in Self-Reinforced Silicon Nitride F. Lofaj, et al. Evaluation of Creep Damage Development of Quasi-Plastic GPS Silicon Nitride by X-Ray CT H. Usami, et al. Superplastic Forming of an a Phase Rich Silicon Nitride T. Rouxel, et al. Fractography, a Tool for the Failure Characterization of Engineering Ceramics J. Dusza. Short Term Deformation and Relaxation.

Reinforcement of silicon nitride ceramics by β-Si3N4 Whiskers

Abstract The silicon nitride based ceramics are reinforced by β-Si3N4 whiskers. β-Si3N4 whiskers do not influence the sinterability of the composite up to an addition of 10 wt%. The improvement of fracture toughness at room temperature of silicon nitride ceramics reinforced by β-Si3N4 whiskers is observed.

Polysilazane-derived micro/nano Si3N4/SiC composites

The pyrolised polysilazanes poly(hydridomethyl)silazane NCP 200 and poly(urea)silazane CERASET derived Si–C–N amorphous powders were used for preparation of micro/nano Si3N4/SiC composites by hot pressing. Y2O3–Al2O3 and Y2O3–Yb2O3 were used, as sintering aids. The resulting ceramic composites of all compositions were dense and polycrystalline with fine microstructure of average grain size <1 μm of both Si3N4 and SiC phases. The fine SiC nano-inclusions were identified within the Si3N4 micrograins. Phase composition of both composites consist of α, β modifications of Si3N4 and SiC. High weight loss was observed during the hot pressing cycle, 12 and 19 wt.% for NCP 200 and CERASET precursors, respectively. The fracture toughness of both nanocomposites (NCP 2000 and CERASET derived) was not different. Indentation method measured values are from 5 to 6 MPa m1/2, with respect to the sintering additive system. Fracture toughness is slightly sensitive to the SiC content of the nanocomposite. Hardness increases with the content of SiC in the nanocomposite. The highest hardness was achieved for pyrolysed CERASET precursor with 2 wt.% Y2O3 and 6 wt.% Yb2O3, HV ≅23 GPa. This is a consequence of the highest SiC content as well as the chemical composition of additives.

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.

Layered Si3N4 composites with enhanced room temperature properties

Four silicon nitride multilayer composites were prepared by hot pressing. Internal stress distribution along the layer boundary dictates the anisotropy of mechanical properties of the composite. The room-temperature bending strength and fracture toughness of all layered composites was higher than the bending strength of related monoliths. Layered ceramic materials exhibited higher tolerance to flaws in comparison to the monolithic ceramic.

THERMAL ANALYSIS STUDY OF POLYMER-TO-CERAMIC CONVERSION OF ORGANOSILICON PRECURSORS

The organosilicon precursors attract significant attention as substances, which upon heating in inert or reactive atmosphere convert directly to oxide or non-oxide ceramics, like nitrides, carbides, carbonitrides, boroncarbonitrides, oxycarbides, alons, etc. In characterisation, and in study of conversion of these polymers to ceramics thermal analysis plays an important role. The degree of cross-linking of the polymer vital for achievement of high ceramic yield is estimated with the use of thermal mechanical analysis (TMA). Decomposition of polymers and their conversion to ceramics is studied by the combination of differential thermal analysis (DTA), differential scanning calorimetry (DSC) thermogravimetry(TG), and mass spectrometry (MS). The use of these methods in study of the polymer-to-ceramic conversion is illustrated by case studies of a commercially available poly(allyl)carbosilane as the precursor of SiC, and a poly(hydridomethyl)silazane as the precursor of SiCN.

Carbon reduction reaction in the Y2O3–SiO2 glass system at high temperature

Abstract In order to assess the role of carbon with respect to the grain boundary chemistry of Si3N4-based ceramics model experiments were performed. Y2O3–SiO2 glass systems with various amount of carbon (from 1 to 30 wt.%) were prepared by high-temperature treatment in a graphite furnace. High carbon activity of the furnace atmosphere was observed. EDX analysis proved the formation of SiC by the carbothermal reduction of SiO2 either in the melt or in the solid state. The melting temperature of the Y2O3–SiO2 system is strongly dependent on the amount of reduced SiO2. XRD analysis of the products documented the presence of Y2Si2O7, Y2SiO5 and Y2O3 crystalline phases in that order with an increasing amount of free C in the starting mixture. The reduction of Y2O3 was not confirmed.