Key Engineering Materials Vols. 336-338

Abstract: Using NH4HCO3 solution as precipitant, the ultrafine 3Y-ZrO2/Al2O3 precursors were prepared by the liquid-phase precipitation method. The well-dispersible ZrO2/Al2O3 composite nanopowders with particle size 10nm were obtained by calcining the precursor at 1000°C. The XRD patterns show no γ-Al2O3 and θ-Al2O3 formed during calcination. The powders have excellent sinterability and the relative density of specimen is up to 99% after sintering the green compacts at 1550°C. Microstructural analysis of the sintered bodies reveals the uniform distribution of the zirconia grains in the alumina matrix.

Abstract: Carbosilanes synthesized from high-pressure treatment of polysilane, were fractional distilled and separated. Carbosilanes, characterized by IR and 1HNMR measurement, are composed of linear and cyclic carbosilanes with some Si-H end groups. Crosslinking occurred between carbosilanes and divinyl benzene and gave gel content of 52%, and further ceramic yield of 36% upon pyrolysis at 1200°C. The pyrolyzates contain microcrystal SiC and free carbon, which was approved by XRD measurement.

Abstract: Reaction bonded silicon carbide (RB-SiC) was fabricated by phenol resin, starch, solidified agent and silicon powder through the following steps: first, carbonizing at high temperature for 7-9h, infiltrating silicon at 1450-1600oC for 0.5-2h, and then removing excessive silicon at 1700oC for 0.5h. Scanning electron microscopy and X-ray diffraction were employed to characterize and analyze the microstructures and phase composition of the preforms and the final RB-SiC products. In addition, the effect of carbonization temperature, the amount of starch and solidified agent on strength and apparent porosity of final RB-SiC were also discussed. The results showed that the carbonization process of phenol resin can be divided into three steps: at temperatures from 400oC to 600oC, the structure of polymer changes less; at temperatures from 600oC to 1000oC, the fundamental chain of polymer is destroyed, and inverts to glass-like carbon; at temperatures from 1000oC to 1200oC, with the increasing of carbonization temperature, the structure of glass-like carbon changes into the structure of disorder graphite. And the increased micro-pores could be obtained by adding starch.

Abstract: Laser surface alloying is an attractive processing to improve surface hardness, wear and corrosion resistance. In this paper, a continuous wave CO2 laser was used to irradiate commercially pure titanium surface with pre-placed active carbon powders in argon atmosphere. A compact, well-adherent, and crack-free TiC/Ti composite layer was obtained. The microstructure and phase constitution of the alloyed layers were determined and analyzed, and the micro-hardness was measured. The result of X ray diffraction (XRD) analysis shows that the alloyed layers contain TiC and Ti (martensite). The scanning electron microscopy (SEM) observation shows TiC growth morphologies have a well-developed dendrite, cellular dendrite, globular microstructure and cross-petal microstructure. The mechanism of the formation of titanium carbides is discussed. Micro-hardness of the laser surface alloyed layer was improved to 420 Hv as compared to 200 Hv of the as-received commercially pure titanium.

Abstract: Silicon carbide ceramic has many attributes including high strength and stiffness, low thermal expansion coefficient, high thermal conductivity, low density and good resistant for radiation of space particles. These attributes make SiC ceramic becoming the third generation space mirror material. This paper describes the fabrication of lightweight SiC mirror blanks. By reducing the shrinkage of green body and controlling the drying conditions, lightweight SiC mirror blank with a diameter of 612mm has been prepared by using gelcasting process. The forming green body is infiltrated with liquid silicon at the temperature of 1600°C, then a CVD SiC layer is coated on the surface of SiC mirror blank. After grinding and polishing, the mirror shows good optical properties.

Abstract: Advanced engineering ceramic technology has very remarkable reputation in the high-tech fields such as semiconductor device manufacturing. ZTA (Zirconia Toughened Alumina), ruby and alumina is applied in the manufacturing of the capillaries that demonstrate high strength, fracture toughness and long life. The advantages of the new composite material over the standard ultra-fine grain high-density material have been proven in many applications especially for packages requiring less than 50μm capillary tip diameter. In this work, micro-structural evolution of ceramic composites and its correlation with important criteria in the selection of the suitable capillary material either ZTA composites or high-density ruby ceramics for a specific package applications will be discussed.

Abstract: Current high temperature ceramics, such as ZrO2, Si3N4 and SiC, cannot be used at temperatures over 1600°C due to their low melting temperature or dissociation temperature. For ultrahigh temperature applications over 1800°C, materials with high melting points, high phase composition stability, high thermal conductivity, good thermal shock and oxidation resistance are needed. The transition metal diborides, mainly include ZrB2 and HfB2, have melting temperatures of above 3000°C, and can basically meet the above demands. However, the oxidation resistance of diboride monolithic ceramics at ultra-high temperatures need to be improved for the applications in thermal protection systems for future aerospace vehicles and jet engines. On the other hand, processing science for making high performance UHTCs is another hot topic in the UHTC field. Densification of UHTCs at mild temperatures through reactive sintering is an attracting way due to the chemically stable phase composition and microstructure as well as clean grain boundaries in the obtained materials. Moreover, the stability studies of the materials in phase composition and microstructures at ultra high application temperatures is also critical for materials manufactured at relatively low temperature. Furthermore, the oxidation resistance in simulated reentry environments instead of in static or flowing air of ambient pressure should be evaluated. Here we will report the concept, advantages and some recent progress on the reactive sintering of diboride–based composites at mild temperatures.

Abstract: In this paper Ti2AlC/TiAl composites were in-situ fabricated by spark plasma sintering (SPS) and then the effects of rapid heat-treatment on microstructure and mechanical properties of Ti2AlC/TiAl composites were investigated. After rapid heat-treatment the microstructure of TiAl matrix was significantly transformed from the near γ microstructure to duplex microstructure. Ti2AlC particles effectively refined the γ phase grains and the α2/γ lamellar colony microstructure. For the Ti2AlC/TiAl composite after rapid heat-treatment at 1200°C, the bending strength and fracture toughness reached 956.8MPa and 22.8MPa·m1/2, respectively.

Abstract: The recrystallized carbon-ceramic composites were prepared from calcined coke, coal-tar pitch and Cr by hot-press to investigate the effects of Cr on the physical properties, mechanical properties and microstructure of recrystallized graphite. Experimental results showed that Cr doped recrystallized graphites exhibited higher bending strength and compressive strength and lower electrical resistivity when the Cr content is10wt% compared with pure graphite. Microstructural analyses revealed that the degree of graphitization of recrystallized graphite increased and the layers spacing of microcrystalline decreased with increasing the Cr concentration. XRD analysis indicated that Cr added to the carbon substrate was in the form of C, Cr2C3 and Cr2O3. The catalytic mechanism of Cr dopant to accelerate the graphitization of carbon substrates can be explained by the mechanism of decomposition of the carbide.

Abstract: Silicon nitride ceramics were fabricated with Yb2O3 as the sintering additive. The effects of the amount of Yb2O3 on the microstructure and the mechanical properties such as the flexural strength and the fracture toughness were investigated. Almost fully densified Si3N4 was obtained when only 4 wt% Yb2O3 was added. Both the flexural strength and the fracture toughness increased steadily with the Yb2O3 content. The transgranular fracture mode was observed in the specimen containing 4 wt% Yb2O3, however, the intergranular fracture mode was observed in the specimen containing 10 wt% Yb2O3.