Long Hao Qi

Novel Low Thermal Conductivity Ceramic Materials for Thermal Barrier Coatings

The increase of the efficiency for gas turbines leads to the increasing combustion-chambertemperatures. Rapid degradation of the conventional yttria-stabilized zirconia coatings does not fulfill therequirements at these temperatures for a reliable thermal barrier coatings (TBCs) due to the phasetransformation of zirconia and the sintering behaviour. Therefore, it is very important to develop novelceramic materials for TBCs with low thermal conductivity and long-term stability at high temperatures.In this paper, the developments of potential novel ceramic materials for TBCs with low thermalconductivity are reviewed.

Preparation and Characterisation of Gd2Zr2O7 Ceramic by Spark Plasma Sintering

Rare earth Gd2Zr2O7 ceramic was prepared by spark plasma sintering from Gd2O3 and ZrO2 powders. The powders were sintered at 1400°C for 10min. The synthesized ceramic was annealed at 800°C for 2h under air atmosphere. XRD structural and SEM microstructural characterization showed the formation of a single phase material with pyrochlore crystal structure. The relative density of Gd2Zr2O7 ceramic was measured by the Archimedes method with an immersion medium of water and the results revealed that the relative density of the ceramic was 92%. The thermal conductivity of the ceramic was tested by laser flash method from room temperature to 700°C. The result shows the thermal conductivity of Gd2Zr2O7 ceramic is lower than that of 7YSZ.

Structure and Ionic Conductivity of Ln2Zr2O7-Type Rare Earth Zirconates

Three rare earth zirconates (Sm2Zr2O7, Gd2Zr2O7 and Er2Zr2O7) were prepared by solid state reaction. The crystal structure and ionic conductivity of these zirconates were characterized by X-ray diffraction (XRD) and complex impedance spectroscopy. The results show that Sm2Zr2O7 exhibits single-phase pyrochlore structure and Er2Zr2O7 exhibits single-phase fluorite structure, while Gd2Zr2O7 has pyrochlore and fluorite structure. Among three zirconates, the ionic conductivity of Sm2Zr2O7 is highest, while that of Er2Zr2O7 is lowest.

Erosive Wear Behavior of Reaction Sintered Si3N4-SiCp Composite Ceramic in Liquid-Solid Flow

Si3N4-SiCp composites were prepared using fine Si powders as the starting materials for Si3N4, and SiCp as the aggregates. The erosive wear behavior of reaction sintered Si3N4-SiCp composite ceramic was investigated in liquid-solid flow. The results display that the composite ceramic hold a better capability of erosive wear resistance than 92 Al2O3 ceramics. The SEM pictures of the worn surface indicate that the main erosive wear mechanism of the composite ceramic is coexistence of micro-cutting and chisel-cutting.

Bending Strength of Ceramics Implanted by Titanium, Zirconium and Chromium Ions with MEVVA Source

ZTA (alumina toughened by 20 wt.% zirconia), hot-pressed silicon nitride (with totally 10 wt.% Y2O3 and Al2O3 as additives) and TZP (pressureless-sintered yttria stabilized zirconia) ceramics were implanted by various doses (5 × 1016 ions/cm2 ~ 1 × 1018 ions/cm2) of Ti, Zr, and Cr ions with a MEVVA (metal vapor vacuum arc) source implanter. The bending strength of these ceramics was investigated. It was discovered that, for different ceramics, different behaviors were presented with the same doses of implantation ions. For alumina and zirconia ceramics, the bending strength increased with increasing implantation doses of Ti and Zr ions, but decreased with high dose of Cr ions. For silicon nitride ceramics, however, the bending strength originally increased with smaller doses of metals implanted, and decreased with higher doses of metals of Ti, Zr, and Cr ions. The different behaviors are correlated to the different variations in compositions and microstructures of ceramics after ion implantation.

Spark Plasma Sintered WC-Ni Cemented Carbides with Various Contents of ZrC Nano-Powder

WC-10Ni cemented carbides with various contents (0-9 wt.%) of ZrC nanopowder were fabricated by spark plasma sintering at 1350 °C with a pressure of 50 MPa. The phase composition, microstructure and mechanical properties of the as-prepared samples were investigated. X-ray diffraction analysis revealed that excepting WC hard phase, a solid solution phase Ni (W,Zr) with different amounts of individual metal was formed with increasing ZrC nanopowder content. Scanning electron microscopy examination indicated that a proper addition content of ZrC can suppress the abnormal growth of WC grains and improve the relative density of WC-Ni cemented carbides. However, with further increase in the content of ZrC (more than 7 wt.%), the agglomeration of ZrC became more and more serious. The samples with 5 wt.% ZrC nanopowder possess a relative higher flexural strength (~1750 MPa) among all the investigated samples. When the addition content of ZrC nanopowder was 3 wt.%, the Vickers hardness of the samples reached its maximum value (~1810 HV10).

Ultrafine WC-Ni-SiCw Cemented Carbides Fabricated by Spark Plasma Sintering

Ultrafine WC-Ni cemented carbides with addition of SiC whisker (SiCw) were fabricated by spark plasma sintering. The microstructure and mechanical properties of the fabricated cemented carbides were investigated. It was found that the addition of SiC whisker had no obvious influence on the phase compositions of the cemented carbides, but the mean grain size of the cemented carbides decreased as the addition fraction of SiC whisker increased. The fabricated WC-Ni cemented carbides presented the highest hardness when 0.75 wt% SiC whisker was added. However, the addition of SiC whisker was detrimental to the flexural strength of the cemented carbides because of the formation of inhomogeneous microstructure in the WC-Ni cemented carbides.

Degassing Behavior and Solid State Reaction of Nano-Ti(CN) Base Cermets in Sintering

The chemical composition and solid state reaction of the nano-Ti(CN) base cermets in different sintering temperature were studied. The total carbon and oxygen content in compact were declined gradually with the increasing of sintering temperature, the nitrogen content in compact began to decline above 1100°C, the peak of de-gassing of N2 was formed before the emergence of liquid phase, the decomposition of N2 was arisen acutely above 1500°C. Mo2C and TaC diffused and took part in solid state reaction with Ti(CN) above 900°C, the solid state reaction was finished below 1200°C. WC diffused and took part in solid state reaction with Ti(CN) above 1100°C, it was dissolved below 1250°C, there were only two phases, Ti(CN) and Ni(Ni+Co), in the alloy.

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.

The Influence of Sintering Process on the Microstructure of Submicron Alumina Fabricated by Gelcasting

Using ultra-fine alumina powders as raw materials, the submicron transparent alumina with relative density more than 99.9% was fabricated by gelcasting and then sintering-HIP process. It was found that only the samples with relative density more than 95% after presintering could obtain fully densification by post-HIP treatment. The final grain sizes increased after post-HIP treatment and were decided by either HIPing temperature or presintering temperature, depending on which one is higher. The maximal strength can reach about 650MPa when grain size is about 1 μm.