Papers by Author: Qun Bo Fan

Abstract: For low thermal conductivity and high corrosion resistance, yttria stabilized zirconia (YSZ), as a top coat (TC), is widely used in thermal barrier coatings (TBCs), and the micro-structure of the TC has significant effects on it thermal shock resistance. Combining digital image processing technique with finite element mesh generation methods, finite element (EF) models based on actual microstructures of plasma sprayed YSZ thermal barrier coatings are built in this paper, so as to simulate the coating’s dynamic failure process when suffering thermal shocking loads. The cracking process is revealed by calculating both the stress and strain evolutions within the coating. Based on the proposed method, the effects of porosity and distribution are further studied. The simulation results agree well with the experimental observation, indicating that the cracks are mainly caused by pore connectivity, which promotes the growth of cracks. This work is expected to be helpful to establish the quantitative relationship between the TBCs porosity and the coating’s service performance.

Abstract: The deformation micromechanism of Zr50Cu50 bulk metallic glasses under compress loading is studied by means of molecular dynamics. The modified self part of the Van Hove function is used to evaluate the transport properties of clusters in deformation behaviors. It is found that the instable clusters are restricted by the ‘backbone’, a kind of three dimensional network structure formed by close-pack clusters. Local shear transformation zone (STZ) is initially formed in these instable clusters, merge with each other and generate eventually a whole STZ, which penetrates the glasses matrix when the backbone is destroyed with increasing loading.

Abstract: 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.

Abstract: Splat morphologies of YSZ/NiCrCoAlY TBCs by plasma spray were characterized using SEM. Based on some assumptions, plasma jet temperature field was established by numerical simulation and the effects of spray parameters are individually studied to optimize the spray process parameters. Furthermore, the interaction wetting ability was taken into account in order to investigate the interaction effect between ZrO2 and NiCrCoAlY droplets. It shows that T enhances markedly with increasing in I and decreasing in FAr; while in despite of the insignificant effect of FHe on T, the temperature of powder in plasma jet enhances with increasing in FHe. As a result of higher thermal conductivity of powder in He plasma gas. NiCrCoAlY droplet impacting solidified NiCrCoAlY splat wets well and spreads fluently while that impacts solidified YSZ splat spreads restrictedly for the poor wetting on the YSZ splat surface.

Abstract: To enhance the high-temperature stability of zirconate pyrochlore structures, one has to focus on their transformation to the disordered state, fluorite. An atomistic simulation calculation is presented in this paper to predict the propensity of rare earth zirconate pyrochlores to transform to fluorite at high temperature. By detailed calculation of defect formation energy of cation antisites and Frenkel pair, as well as their interactions, the mechanisms of disorder transformation are ascertained. The results show that the tendency of cation disorder is less than the anion’s and disorder transformation will accelerate in advanced stage. The calculation of defect energy in pyrozirconates with different cation on the A site have proved helpful in unraveling their different order-disorder transformation tendency.

Abstract: Thermal conductivity of zirconia and yttrium stabilized zirconia (YSZ) is calculated with perturbation molecular dynamics method (PMD). The results showed that thermal conductivity of YSZ is lower than that of pure zirconia and PMD is a very effective way in thermal conductivity calculation for ceramics. In higher temperature region, the calculated values show a different tendency with the experiment ones, which is because that photon conduction is not considered in PMD method. By taking photon effects into account, the calculated thermal conductivity curve is found fairly well coherent with measurements. In addition, the thermal expansion of zirconia is also presented by calculating the volumes at different temperatures. The results and methods in this paper have been proved to be very helpful in further design of new ceramic materials in the field of thermal barrier coatings (TBCs).