Fu Chi Wang

Effect of Glass Phase on the Thermal Shock Resistance of ZrB2-SiC Ultra High Temperature Ceramic

Two kinds of ultra high temperature ceramics (UHTC), ZrB2-20Vol.%SiC and ZrB2-15Vol.% SiC-5Vol.% SiCn (nano-size) were prepared by spark plasma sintering (SPS). The residual strength was used to characterize thermal shock resistance of ZrB2-SiC ceramics by quenching tests in the water. The mechanism of thermal shock damage of the ceramics was examined by SEM analysis. The results showed that the formed glassy phase has significant effect on thermal shock resistance of ZrB2-SiC ceramics. When the thermal shock temperature rises, the residual strength of ZrB2-SiC ceramics after thermal shock will decrease gradually. The formed glassy phase when quenched from the temperature of 1400°C can repair or heal the cracks and induce the higher residual strength. However, when the temperature increases to 1600°C, the residual strength of ZrB2-SiC ceramics decreases significantly due to the volatilization of the glassy phase. The residual strength of ZrB2-15Vol.% SiC-5Vol.% SiCn ceramic is higher than that of ZrB2-20Vol.%SiC ceramic, because nano SiC is easy to be oxidized and to induce the formation of more glassy phases and to improve the thermal shock resistance of the sample.

Microstructure of ZrB2–SiC Composite Fabricated by Spark Plasma Sintering

ZrB2-SiC composite is a promising candidate for ultra-high temperature ceramics, which is difficult to be sintered due to strong covalent bonding of ZrB2 and SiC. ZrB2-30Vol.%SiC composite was prepared by spark plasma sintering technique (SPS) at the sintering temperature of 1850°C, sintering pressure of 50MPa, heating rate of 200°C/min and holding time of 3 minutes. The phase components and microstructure were examined by X-ray diffraction, scanning electron microscopy and transmitting electron microscopy. The results show that the product is composed of ZrB2 phase, SiC phase and ZrO2 phase. A rationalization for the presence of ZrO2 phase is based on the impurity of raw material and oxidation of ZrB2 during SPS. The consolidated product is very dense and no apparent pores exist in the microstructure. ZrO2 phase with irregular shape is found among some particles as a binder phase. It is shown that the presence of ZrO2 phase may be beneficial to the densification of ZrB2-SiC composite.

Sintering Mechanism of Ti-6Al-4V Prepared by SPS

Ti-6Al-4V compact bulk was fabricated by Spark plasma sintering (SPS) with initial pressure of 1.7 MPa, holding pressure of 50 MPa, heating rate of 100 °C/min, and holding time of 5 min at different sintering temperature. The fracture morphology of the specimen sintered at different temperatures was observed to investigate the sintering mechanism. It can be concluded that there are four stages in the SPS process: activation and rearrangement of particles, connection of particles, growth of sintering neck and bulk deformation. The high-quality bulk compact can be obtained when the above mentioned four sintering stages proceed in turn and are all fully completed. The compact bulk has the best mechanical properties when the sintering temperature was 1050°C. The relative density of the bulk Ti-6Al-4V exceeds 99.5%. The tensile strength and the elongation of Ti-6Al-4V obtained by SPS process are 901 MPa and 13.9%, respectively.

Prediction of Critical Pressure in the Preparation of Composites by Vacuum - Pressure Infiltration Process

The SiC3D/Al composite with high SiC content were prepared by vacuum– pressure infiltration process, and the prediction model of infiltration pressure threshold involving capillary pressure and viscous resistance was established. The effects of model parameters on the critical infiltration pressure were analyzed. The results show that the main factors are the permeability coefficient of SiC performs and the temperature of molten aluminum alloy. Temperature affects the melt viscosity and the wetting angle between SiC and molten alloy. The ingiltration pressure threshold of the specific infiltration system had been calculated by this model, which is in agreement with experimental results.

Full-Scale Numerical Simulation of Plasma-Sprayed Functionally Gradient Materials

To develop novel and advanced thermal barrier coatings, full-scale numerical simulation of plasma-sprayed functionally gradient materials is conducted in this paper, including the prediction of basic parameters at the nozzle exit, simulation of three dimensional simulation of the plasma jet, modeling of the interaction between the plasma jet and the particles, calculation of flight trajectories and temperature history of flying metal and ceramic particles, the interaction between the molten particles and the substrate, as well as the deposition process of the coatings. Various complex phenomena, such as turbulent effects with chemical reactions in the plasma jet, dispersion status of the particles onto the substrate, and the composition distribution of the functionally gradient materials, are fully taken into account. The numerical simulation results are found to be in good agreement with experimental evidence.

Thermal Conductivity of (La0.5Sm0.5)2(Zr0.9Ce0.1)2O7 Ceramic for Thermal Barrier Coatings

Pyrochlore structure oxide (La0.5Sm0.5)2(Zr0.9Ce0.1)2O7 was prepared by mixing the appropriate amounts of ZrO2, La2O3, Sm2O3 and CeO2, followed by reacting at 1600°C for 10 h. It was shown that the thermal conductivity was much lower than that of 8YSZ, which was mainly ascribed to the oxygen vacancies and lattice strain in lattice of (La0.5Sm0.5)2(Zr0.9Ce0.1)2O7.

A Promising LaSmZr2O7 Ceramic with Pyrochlore Structure for Thermal Barrier Coatings

Based on La2Zr2O7 ceramic for thermal barrier coatings, LaSmZr2O7 ceramic doped with samarium ion was successfully prepared using solid state reaction method. The pellets were sintered at 1600°C for 10 hours in air. The phase structure, thermal conductivity and thermal expansion coefficient of LaSmZr2O7 ceramic and La2Zr2O7 ceramic were measured by XRD, laser-flash device, high-temperature dilatometry, respectively. The results show that the crystal structure of LaSmZr2O7 ceramic is not affected by the doped samarium ion in the zirconium lattice. The thermophysical results show that the thermal conductivity of the LaSmZr2O7 ceramic is lower than that of La2Zr2O7 ceramic, while the thermal expansion coefficient is higher than that of La2Zr2O7 ceramic. These results indicate that LaSmZr2O7 ceramic or Ln2Zr2O7 ceramics doped with other rare earth ions could be candidate materials for future thermal barrier coatings.

Reflectivity and Laser Ablation Behavior of La1-xSrxTiO3+δ Coating

La1-xSrxTiO3+δ (LST) has been studied in many fields due to its excellent physical and chemical properties. However, there are rare reports on its optical properties, especially reflectivity. Our previous work has prepared LST coatings by atmospheric plasma spraying technique, but the reflectivity is much lower than that of LST powder itself. In this paper, LST coating was prepared by knife coating which is a traditional convenient method. The phase structure and surface morphology of the coating were examined by X-ray diffraction and scanning electron microscopy, respectively. The optical property was characterized by UV-visible-near infrared spectroscopy and the laser ablation tests were carried out by YSL-2000 Fiber Laser (IPG Co) with the wavelength of 1070 nm and the spot size of 10 mm×10 mm. Results show that the reflectivity of the LST coatings depends on the weight ratio of LST and binder. A high-reflectivity coating can be prepared by knife coating with appropriate content of LST.

Efffect of Spark Plasma Sintering Temperature on Mechanical Properties of In Situ TiB/Ti Composites

The in-situ synthesized TiB reinforced titanium matrix composites have been prepared by spark plasma sintering technique at 950–1250°C, using mixtures of 10wt% TiB2 and 90wt% Ti powders. The effects of the sintering temperature on the mechanical properties (Vickers microhardness, yield strength and Young`s modulus) of the composites were investigated. SEM was used to analyze the reaction process and the microstructure of the compacts synthesized at different sintering temperatures. The results indicated that the in situ synthesized TiB grow rapidly with increasing sintering temperature. The composite sintered at 1250°C have the highest relative density of 99.2%. However, the composite sintered at 950°C exhibits the best Vickers microhardness of 4.64GPa and yield strength of 989MPa, respectively.

Microstructure Characteristics and Mechanical Properties of In Situ TiB/Ti Composites Prepared by Arc-Melting Technique

In this paper, in situ TiB reinforced Ti-3Al, Ti-6Al and Ti-6Al-4V matrix composites were prepared by arc-melting technique utilizing the reaction between Ti and TiB2, and then forged in the α+β phase field. Phase identification was carried out via X-ray diffraction. Microstructure of the composites was studied by optical microscopy (OM) and scanning electron microscopy (SEM). Mechanical properties of the composites after forging were measured at various temperatures by tensile experiment. The results showed that Ti-6Al-4V-2TiB composite exhibits fine equiaxed matrix microstructure with a grain size of 5-10μm. The tensile strength and elongation of the composite at room temperature reached 1069MPa and 10.0%, respectively.