Key Engineering Materials Vols. 602-603

Abstract: Laminated ZrB₂-SiC-C_g ceramics were successfully prepared by tape casting - dipping and tape casting - stacking respectively. The effect of different molding process on the mechanical properties of materials was investigated. The microstructure and fracture behavior were characterized as well. The flexural strength, fracture toughness and fracture work of laminated ZrB₂-SiC-C_g ceramics prepared by tape casting-stacking were 427 MPa, 11.3 MPa·m^1/2, 415 J/m², respectively. The thickness of ZrB₂-SiC layers was about 250 μm, and the thickness of graphite layers was about 25 μm. As a comparison, Laminated ZrB₂-SiC-C_g ceramics prepared by tape casting - dipping showed non-uniform thickness of the ZrB₂-SiC layers and graphite layers. And the flexural strength, fracture toughness and fracture work of laminated ZrB₂-SiC-C_g ceramics prepared by tape casting-dipping were 252 MPa, 5.7 MPa·m^1/2, 104 J/m², respectively. The improvement on the mechanical properties of laminated ZrB₂-SiC-C_g ceramics prepared by tape casting - stacking was attributed to the uniform graphite layer and ZrB₂-SiC layer and the reducing amount of graphite infiltrated into ZrB₂-SiC matrix layer.

Abstract: ZrB₂-Cu composite is a new electrical contact materials in the integration of high conductivity, high wear resistance and good mechanical strength. In this paper, ZrB₂-Cu composites were prepared by hot-pressing sintering at 800~900 °C under a pressure of 20 MPa.The densification of ZrB₂-Cu composites was improved by the addition of nickel using an electroless metal plating technique. X-ray diffraction and scan electron microscopy were used to analyze the phase and microstructure of ZrB₂-Cu composites. The results showed that ZrB₂-Cu composites with 60 vol % Cu which was sintered at 900 °C had a higher relative density, highest flexural strength of 381 MPa and higher hardness of 2.16 GPa(HV). ZrB₂-Cu composites with 50 vol % Cu which was sintered at 900 °C had higher flexural strength of 297 MPa and the highest hardness of 2.66 GPa.

Abstract: ZrB₂, YAG and Al₂O₃ are widely applied because of some excellent performances, but ZrB₂ is easily oxidized in the high-temperature air. To make the ZrB₂ ceramics obtain better oxidation resistance, high-density ZrB₂-YAG-Al₂O₃ ceramics were prepared. The influences of coated composite powders on the densification and the oxidation resistance of ZrB₂-YAG-Al₂O₃ ceramics were investigated. The 80wt%ZrB₂-YAG-Al₂O₃ multiphase ceramic materials from different composite raw materials with the spark plasma sintering technique were successfully prepared. The densification of ZrB₂-YAG-Al₂O₃ ceramics with Al₂O₃-Y₂O₃ composite powder coated is easier than that of ZrB₂-YAG-Al₂O₃ ceramics with YAG-Al₂O₃ powder mixed. The reaction temperature is lower than the 1100¡æ for synthesizing YAG powders from Al₂O₃-Y₂O₃ composite powders. The weight gain are increased with increased the oxidation temperature. B₂O₃ is reacted with Al₂O₃ to form Al₁₈B₄O₃₃, Al₁₈B₄O₃₃ is melted and coated on the surface of ceramics to form a protective layer for the oxidation resistance of ceramics at high temperature. The oxidation weight gain of ZrB₂-YAG-Al₂O₃ ceramic with Al₂O₃-Y₂O₃ composite powder coated is lower than that of ZrB₂-YAG-Al₂O₃ ceramic with YAG-Al₂O₃ powder mixed.

Abstract: In this study, oxidation behavior of ZrB₂-MoSi₂-SiC composite was investigated in the hot-pressed 5-20 vol% SiC-containing ZrB₂-20 vol% MoSi₂-based composites which were exposed to dry air between 1100°C and 1500°C up to 10 hours. The effects of SiC additive on the oxidation behavior were assessed. Experimental results showed that the weight gain due to oxidation exposure in air increased with increasing exposure temperature and exposure time. Parabolic oxidation behavior was observed for all the compositions composites. On the other hand, the weight gain decreased with increasing amount of SiC added. The addition of SiC improved the oxidation resistance of the composites, and the improvement was enhanced with increasing amount of SiC added. In addition, X-ray diffraction was used to identify major crystalline phases present in both the as-received and the post-oxidized composites. The oxidized sample surface was characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The microstructure of the post-oxidized composites consisted of two characteristic regions: oxidized reactive region and unreactive bulk material region. Furthermore, the oxidized reactive region divided into an outermost dense silica-rich scale layer and oxidized reactive mixture layer. The improvement of the oxidation resistance due to the addition of SiC is associated with the presence of the thicker dense outermost scale layer which inhibited inward diffusion of oxygen through it.

Abstract: Carbide-derived carbons (CDCs) are produced from carbides by removing non-carbon elements in the process of selective etching. In this paper, CDC was prepared from TiC by chlorination at the temperature range of 600~1100°C. In the chlorinating process, carbide-derived carbon with different microstructure was obtained by controlling the reaction temperature. The structures of CDC were revealed with X-ray diffraction and Raman spectroscopy. The morphologies of CDC were analyzed by scanning electron microscope. From X-ray diffraction analysis, the CDC obtained from TiC in this experiment mainly consisted of amorphous carbon. Basing on scanning electron microscopy, carbide-derived carbon from TiC maintained the shape and size of TiC particles. Keywords: Carbide-derived carbons; chlorination; TiC

Abstract: By combining ball milling processing with combustion synthesis in ultrahigh gravity field, the solidified TiC-TiB₂ composite was achieved with fine-grained and ultrafine-grained microstructure. It is considered that taking balling milling processing to mechanically activate powder blend promotes thermal explosion to occur in ultrahigh gravity field by reducing ignition temperature, increases sharply the actual temperature of full-liquid product, thereby not only accelerating liquid-liquid separation of TiC-TiB₂ liquid and Al₂O₃ droplets, but also refining the solidified microstructure by enhancing undercooling of TiC-TiB₂ liquid, finally, brings about a series of dramatic improvements in densification and mechanical properties of the ceramic.

Abstract: By combining mechanical activation and combustion synthesis in ultrahigh gravity field, the laminated composite of TiB₂ based ceramic to stainless steel was achieved in continuously-graded composition and microstructure, and within the Fe-Cr based intermediate Ti-Fe enriched carbides and fine TiB₂ platelets decreased gradually in size and volume fraction from the ceramic to stainless steel. Because of the sequent presence of thermal explosion, the dissolution of the molten stainless steel to TiC-TiB₂ liquid, the formation of diffusion-controlled concentration gradient from the ceramic liquid to the alloy liquid, the rapid sequent solidification of the ceramic and the alloy, the laminated composite is achieved in multilevel, scale-span hybrid microstructure that the different-size, different-morphology Fe-Cr alloy phases alternate with TiB₂ platelets and irregular TiC grains in size from micrometer to micro-nanometer.

Abstract: By taking combustion synthesis to prepare a series of solidified TiC-TiB₂ composites under the increased high-gravity acceleration from 500g to 2500g, then in the near-full-density composite the ultrafine-grained microstructure with the matrix of TiB₂ platelets smaller than 1 μm in thickness was achieved, and all TiB2 platelets almost shared in the contributions of bridging and pullout to toughening ceramic, thereby presenting fracture toughness of 16.5 ± 1.0 MP · m^0.5 and flexural strength of 982 ± 20 MPa.

Abstract: By taking combustion synthesis in ultrahigh gravity field to prepare solidified TiC-TiB₂ composite ceramic, laminated composite of the ceramic to Ti-6Al-4V was achieved by fusion bonding and atomic inter-diffusion, and within the joint there formed the unique microstructure of multiscale (micrometer-micro/nanometer-nanometer) and multilevel characterized by size and distribution of TiB₂ and TiB due to the presence of a series of metallurgical reaction including peritectic reaction of solidified and Ti liquid, direct growth of TiB solids from liquid Ti and subsequent eutectic reaction of TiB solids and liquid Ti. FEMSEM images of crack propagation paths at the joint showed despite the contribution from crack bridging of micro-nanometer TiB₂ and TiB platelets to crack propagation resistance, residual stress toughening and subsequent crack pinning by micro-nanometer TiB₂ ,TiB platelets and needle-like nanometer TiB grains as well as ductile phase toughening and bridging toughening by Ti metallic phases became the primary resistances to crack propagation, thereby presenting the delayed fracture behavior in the joint, so that shear fracture usually occurred at the solidified Ti alloy rather than the joint, and shear strength of 375 ± 55 MPa was achieved between the solidified ceramic and Ti alloy.

Abstract: SiC-TiB₂/B₄C composites were fabricated by hot-press sintering B₄C with silicon powder and tetrabutyl titanate (precursor of TiO₂) as sintering and reinforcement agents. The influence of additives on hot-press sintering densification, microstructure and properties of composites were studied. The results showed that TiB₂ and SiC generated by chemical reaction between additives and B₄C matrix reinforced the sintering activity of the mixed powders and accelerated significantly the hot-press sintering densification rate of B₄C from 1200 °C to 1700 °C. According to the SEM observation, the second phase of TiB₂ and SiC particles synthetized in situ sited along the grain boundaries of B₄C, meanwhile, those SiC particles of nanoscale size embedded into the B₄C grains, and thereby, intra/inter-type ceramics formed. The maximum relative density of 98.1% was obtained with 9wt.% TiO₂. The typical valus of Vickers hardness, bending strength and fracture toughness can reach 26.7 GPa, 580 MPa and 5.0 MPam^1/2, respectively.