Key Engineering Materials Vol. 616

Abstract: Hafnium carbide powder was synthesized by sol-gel polycondensation of hafnium chloride with citric acid. The starting materials were dissolved in water and mixed homogeneously on a hot plate until a precomposite gel was formed. Pyrolysis of the obtained gel resulted in formation of monoclinic hafnia and amorphous carbon, which after subsequent heat treatment transformed into hafnium carbide. Materials were analyzed by means of X-ray diffraction and electron microscopy investigations. The results showed that the obtained carbide powder was composed of nearly equiaxed particles of narrow size distribution. The obtained hafnium carbide powder was densified via spark plasma sintering (SPS) at 1950 oC using molybdenun silicide as sintering additive. Microstructure and mechanical properties of the obtained hafnium carbide ceramics were investigated.

Abstract: We systematically investigated the phase transformation and grain-growth behaviors during sintering in 2 and 3 mol% Y₂O₃-stabilized tetragonal ZrO₂ (2Y and 3Y) and 8 mol% Y₂O₃-stabilized cubic ZrO₂ polycrystals (8Y). In particular, grain-boundary segregation and grain-interior distribution of Y³⁺ ions were examined by high-resolution transmission electron microscopy (HRTEM)- and scanning transmission electron microscopy (STEM)-nanoprobe X-ray energy dispersive spectroscopy (EDS) techniques. Above 1200°C, grain growth during sintering in 8Y was much faster than that in 2Y and 3Y. In the grain boundaries in these specimens, amorphous layers did not present; however, Y³⁺ ions segregated at the grain boundaries over a width of about 10 nm. The amount of segregated Y³⁺ ions in 8Y was significantly less than in 2Y and 3Y. This indicates that the amount of segregated Y³⁺ ions is related to grain growth behavior; i.e., an increase in segregated Y³⁺ ions retards grain growth. Therefore, grain-growth behavior during sintering can be reasonably explained by the solute-drag mechanism of Y³⁺ ions segregating along the grain boundary. In 2Y and 3Y, the cubic-phase regions were formed in grain interiors adjacent to the grain boundaries and/or the multiple junctions in which Y³⁺ ions segregated, which can be explained by a grain boundary segregation-induced phase transformation (GBSIPT) mechanism.

Abstract: A mechanical model is proposed to estimate internal stress during sintering of ceramic multiphase laminates. A symmetrical multi-layered laminate is assumed, and one-dimensional elastic analysis is carried out on the change in stress of each layer during sintering, based on the differences in sintering strain, thermal expansion strain and phase transformation strain between the layers. By taking a limit such that the thickness of each layer approaches infinitesimally small, the internal stress expression can be extended into the case of the materials with continuous compositional change (viz. functionally gradient materials).

Abstract: Electric current assisted sintering (ECAS) has been used for sintering of nitride ceramic powders. It was mainly used for fabrication of fine-grained silicon nitride ceramics with high plasticity at high-temperatures, because high heating rate of ECAS was effective for densification without grain growth. Recent trend of silicon nitride ceramics sintered by ECAS are for wear resistance, corrosion resistance, or high toughness. Application of silicon nitride ceramics is expanding and the ECAS is helpful for improving the properties. The ECAS is used for sintering of aluminum nitride ceramics, recently. Aluminum nitride powder could be densified without sintering additive by ECAS, but some kinds of sintering additives are effective for densification and improvement of thermal conductivity.

Abstract: By using α-and/or β-SiC powders, the effects of initial α-phase content on the microstructure and thermal properties of the SiC ceramics sintered with Y₂O₃ and Sc₂O₃ were investigated. When α-SiC powder was used, the microstructure consisted of large equiaxed grains and small equiaxed grains. The average grain size decreased with increasing α-SiC content in the starting composition. The thermal conductivity decreased with increasing α-SiC content in the starting composition. Such results suggest that the grain growth of SiC ceramics is beneficial in increasing the thermal conductivity of liquid-phase sintered SiC ceramics. The thermal conductivity of SiC ceramics processed from a 90% β-SiC-10% α-SiC powder mixture was 159 W/m∙K at room temperature.

Abstract: The CNT/B₄C composite with Al₂O₃ additive was fabricated by hot-pressing following extrusion molding of a CNT/B₄C paste, and mechanical properties of the obtained composite were investigated. Many CNTs in the composite aligned along the extrusion direction from SEM observation. 3-points bending strength of the composite was slightly lower than that of the monolithic B₄C. Elastic modulus and Vickers hardness of the composite drastically decreased with CNT addition. Fracture toughness of the composite was higher than that of the monolithic B₄C.

Abstract: Silica (SiO₂) nanolayer was coated on silicon carbide (SiC) powder by rotary chemical vapor deposition (RCVD). The SiC/SiO₂ composite were consolidated by spark plasma sintering (SPS) at 1923 K using the SiO₂ coated SiC powder. The relative density and hardness of the SiC/SiO₂ composites increased with increasing SiO₂ content, and were 97% and 17 GPa, respectively, at SiO₂ content of 22 mass%. The relative density and hardness of a composite consolidated using the mixture powders of SiC and SiO₂ (22 mass%) at 1923 K were 81% and 8 GPa, respectively.

Abstract: Polycrystalline ڂ˽SiC thick film with mm-scaled thickness was deposited on a graphite substrate using a gaseous mixture of SiCl₄ + CH₄ and H₂ at temperatures ranging from 1573 to 1823 K by chemical vapor deposition. Effect of deposition temperature (T_dep) on deposition rate, surface morphology and preferred orientation has been studied. The preferred orientation changed from <111> to <110> with increasing T_dep. The maximum deposition rate (R_dep) of 1125 ڌ̽˰̸⁻¹ has been obtained. The surface morphology has changed from six-fold pyramid to five-fold facet with increasing T_dep.

Abstract: TiB₂-TiC, TiB₂-TiN and TiB₂-TiC_xN_1-x composites were prepared by arc-melting mixtures of TiB₂, TiC and TiN powders. 28TiB₂-72TiC (mol%) composite showed a lamellar eutectic structure, while 55TiB₂-45TiN (mol%) composite with a lamellar structure consisted of TiB₂, TiN and TiB. 36TiB₂-44TiC-20TiN (mol%) was a quasi-binary eutectic composite having a rod-like structure.