By using α- and β-SiC starting powders with similar particle sizes, the effects of initial α-phase content on the microstructure and the mechanical properties of the liquid-phase-sintered and subsequently annealed materials were investigated. The microstructures developed were analyzed by image analysis. When β-SiC powder was used, the grains became elongated. The average diameter decreased with increasing α-SiC content and the aspect ratio showed a maximum at 10%α-SiC and decreased with increasing α-SiC content in the starting powder. Such results suggest that microstructure can be controlled by changing α-phase content in starting powders. The strength increased with increasing α-SiC content in the starting powder while the fracture toughness decreased with increasing α-SiC content. There may be a trade-off in improving both the strength and toughness in SiC ceramics sintered with oxide additives.

Effect of Initial α‐Phase Content on Microstructure and Mechanical Properties of Sintered Silicon Carbide

A pressureless sintering process was developed for the densification of zirconium diboride ceramics containing 10-30 vol% silicon carbide particles. Initially, boron carbide was evaluated as a sintering aid. However, the formation of a borosilicate glass led to significant coarsening, which inhibited densification. Based on thermodynamic calculations, a combination of carbon and boron carbide was added, which enabled densification (relative density > 98%) by solid-state sintering at temperatures as low as 1950°C. Varying the size of the starting silicon carbide particles allowed the final silicon carbide particle morphology to be controlled from equiaxed to whisker-like. The mechanical properties of sintered ceramics were comparable with hot-pressed materials with Vickers hardness of 22 GPa, elastic modulus of 460 GPa, and fracture toughness of ∼4 4 MPa· m 1/2 . Flexure strength was ∼ 460 MPa, which is at the low end of the range reported for similar materials, due to the relatively large size (∼ 13 μm long) of the silicon carbide inclusions.

Pressureless Sintering of ZrB2–SiC Ceramics

The series of books represents a thorough treatment of ultra-high temperature materials with melting (sublimation or decomposition) points around or over 2500 °C. In the third volume are included physical (structural, thermal, electromagnetic, optical, mechanical, nuclear) and chemical (more than 1100 binary, ternary and multi-component systems, including those used for materials design, data on solid-state diffusion, wettability, interaction with chemicals, gases and aqueous solutions) properties of titanium monocarbide TiC1–x, vanadium monocarbide VC1–x and related 2D-molecular (graphene-like) carbide MXenes, discovered and developed only in the last decade. TiC1–x and VC1–x are also widely applied in the general technological and engineering practice in a wide range of temperatures as parts of highly hard materials (alloys) and special steels. This book will be of interest to researchers, engineers, postgraduate, graduate and undergraduate students alike. For the named materials, readers/users are provided with the full qualitative and quantitative assessment, which is based on the latest updates in the field of fundamental physics, chemistry, nanotechnology, materials science, design and engineering.

http://link.springer.com/content/pdf/10.1007/978-94-024-2039-5.pdf

Ultra-High Temperature Materials III: Refractory Carbides II (Ti and V Carbides)

Spark plasma sintering technique was used to in situ fabricate high dense Ti3SiC2–TiC composites. The calculated TiC volume content from X-ray diffraction (XRD) is close to the theoretical one. It is found from fracture surface observation that TiC is about 1 μm, and Ti3SiC2 is about 2–10 μm in grain size. The fracture modes consist of intergranular mainly for Ti3SiC2 and transgranular fracture mainly for TiC. With the increasing of TiC volume content, Vickers hardness increases to the maximum value of 13 GPa for Ti3SiC2-40 vol.%TiC. Fracture toughness and flexural strength of the composites are also improved compared with those of monolithic Ti3SiC2 except for Ti3SiC2-40 vol.%TiC composite. The main reasons for the sudden decrease of fracture toughness and flexural strength of Ti3SiC2-40 vol.%TiC composite can be attributed to the relatively lower density, some clusters of TiC in the composite and the transition of fracture mode from intergranular to transgranular. The thermal conductivities decreased with the addition of TiC. The minimum thermal conductivity is 22 W m °C−1 for Ti3SiC2-40 vol.%TiC composite.

Effect of TiC content on the microstructure and properties of Ti3SiC2–TiC composites in situ fabricated by spark plasma sintering

The influence of sintering temperature, holding time and pressure condition on densification and mechanical properties of bulk titanium carbide (TiC) fabricated by SPS sintering has been systematically investigated. Experimental data demonstrated that relative density and Vickers hardness ( H V ) increase with sintering temperature and holding time, but fracture toughness ( K IC ) was not significantly influenced by sintering parameters. The H V and relative density of samples consolidated by SPS technique at 1600 °C for 5 min under 50 MPa pressure (applied entire sintering cycle) reached 30.31 ± 2.23 GPa and 99.90%, respectively. H V values of ∼24–30 GPa and K IC of ∼3.7–5 MPa m 1/2 were obtained in all bulk samples with relative densities of 95.61–99.90% when fabricated under various conditions presented above, without abnormal grain growth. More pronounced effects of pressure condition on grain growth (promoted by grain-boundary diffusion) than on densification were observed. The relationship of fracture toughness and fracture mode is also discussed.

Densification and mechanical properties of TiC by SPS-effects of holding time, sintering temperature and pressure condition

Particle-reinforced SiC composites with the addition of TiC or TiB2 were fabricated at 1850 °C by hot-pressing. Densification was accomplished by utilizing a liquid phase formed with added Al2O3, Y2O3, and surface SiO2 on SiC. Their mechanical and electrical properties were measured as a function of TiC or TiB2 content. Adding TiC or TiB2 to the SiC matrix increased the toughness, and decreased the strength and electrical resistivity. The fracture toughnesses of SiC-50 wt% TiC and SiC-50 wt% TiB2 composites were approximately 60% and 50%, respectively, higher than that of monolithic SiC ceramics. Microstructural analysis showed that the toughening was due to crack deflection, with some possible contribution from microcracking in the vicinity of TiC or TiB2 particles.

SiC-TiC and SiC-TiB2 composites densified by liquid-phase sintering

The R-curve for Si{sub 3}N{sub 4}-40 wt% TiN composites was estimated by the indentation-strength method and compared to that of monolithic Si{sub 3}N{sub 4} with duplex microstructure. Both materials exhibited rising R-curve behavior. The Si{sub 3}N{sub 4}-TiN composites, however, displayed better damage tolerance and higher resistance to crack growth. From TEM observation, it was inferred that this superior performance of Si{sub 3}N{sub 4}-TiN composites can be attributed mainly to stress-induced microcracking at heterophase (Si{sub 3}N{sub 4}/TiN) boundaries.

R-curve behavior of silicon nitride-titanium nitride composites

A process based on liquid-phase sintering and subsequent annealing for grain growth is presented to obtain in situ-toughened SiC-30 wt% TiC composites. Its microstructures consist of uniformly distributed elongated {alpha}-SiC grains, matrixlike TiC grains, and yttrium aluminum garnet (YAG) as a grain boundary phase. The composites were fabricated from {beta}-SiC and TiC powders with the liquid forming additives of Al{sub 2}O{sub 3} and Y{sub 2}O{sub 3} by hot pressing. During the subsequent heat treatment, the {beta} {yields} {alpha} phase transformation of SiC led to the in situ growth of elongated {alpha}-SiC grains. The fracture toughness of the SiC-30 wt% TiC composites after 6-h annealing was 6.9 MPa{center_dot}m{sup 1/2}, approximately 60% higher than that of as-hot-pressed composites (4.4 MPa{center_dot}m{sup 1/2}). Bridging and crack deflection by the elongated {alpha}-SiC grains appear to account for the increased toughness of this new class of composites.

In S/Yu‐Toughened Silicon Carbide‐Titanium Carbide Composites

A process based on liquid phase sintering and subsequent annealing for grain growth is presented to obtain the in situ enhancement of toughness of SiC–30 wt%, 50 wt%, and 70 wt% TiB2 composites. Its microstructures consist of uniformly distributed elongated α-SiC grains, relatively equiaxed TiB2 grains, and yttrium aluminium garnet (YAG) as a grain boundary phase. The composites were fabricated from β-SiC and TiB2 powders with the liquid forming additives of Al2O3 and Y2O3 by hot-pressing at 1850°C and subsequent annealing at 1950°C. The annealing led to the in situ growth of elongated α-SiC grains, due to the β→α phase transformation of SiC, and the coarsening of TiB2 grains. The fracture toughness of the SiC–50 wt% TiB2 composites after 6 h annealing was 7.3 MPa m1/2, approximately 60% higher than that of as-hot-pressed composites (4.5 MPa m1/2). Bridging and crack deflection by the elongated α-SiC grains and coarse TiB2 grains appear to account for the increased toughness of the composites.

Kyeong Sik Cho

Papers

SiC-TiC and SiC-TiB2 composites densified by liquid-phase sintering

R-curve behavior of silicon nitride-titanium nitride composites

In S/Yu‐Toughened Silicon Carbide‐Titanium Carbide Composites

In situ enhancement of toughness of SiC-TiB2 composites

Microstructure and Fracture Toughness of In-situ Toughened SiC- TiC Composites