Study Of Microstructure And Properties Of Plasma Sprayed Nanostructured Zirconia Thermal Barrier Coatings Before And After Laser Galzing

It is gradually difficult for conventional thermal barrier coatings (TBCs) to satisfy the increasing requirements for the performance of the high temperature components with the development of modern aeronautic and space industry. So the nanometer technology and laser glazing are combined with atmospheric plasma spraying in order to prepare TBCs with better performance.In the thesis the nanostructured zirconia TBCs were manufactured by atmospheric plasma spraying using the nanostructured zirconia agglomerated powders, and the CO₂ laser beam with the continuous wave was applied to the laser glazing of nanostructured TBCs. And the microstructures, phase composition and properties (thermal shock resistance, friction and wear property and high temperature stability) of nanostructured TBCs and as-glazed TBCs were systematically investigated compared to conventional TBCs.The experimental results of microstructures and properties of the nanostructured TBCs indicated that the nanostructured TBCs exhibited a unique complex microstructure consisting of columnar grains and non-melted or partial melted nanosized particles. XRD analysis showed that the metastable tetragonal phase was the major predominant phase of the nanostructured TBCs without the presence of monoclinic phase. The nanostructured TBCs possessed better thermal shock resistance than conventional TBCs. This phenomenon was ascribed to the relatively densified coating, microcracks, nanosized grains and fine porosity which can effectively release the thermal stress. And the thermal stress was the main reason leading to the failure of nanostructured TBCs. The results of the high temperature stability of nanostuctured TBCs demonstrated that the grain size of the coatings increased with the increasing testing temperature and time, but the grain size was still lower than 100nm; and the phase composition of the coatings didn't change with the different temperature and time. So the nanostructured TBCs had better high temperature stability in terms of the change tendency of grain size and phase composition. The nanostructured TBCs can improve the wear property of the TBCs compared to the conventional coatings.The experimental results of laser glazing demonstrated that laser glazing was so sensitive to the processing parameters. The surface of the as-glazed coatings exhibited different defects such as surface color change, craters, porosity, exfoliation if the processing parameters couldn't be properly adjusted. But the defects-free as-glazed coatings can be prepared through the optimization of the laser glazing conditions. SEM observation indicated that the microstructure of the as-glazed coatings changed from single columnar grains to a combination of columnar grains in the fracture and equiaxed grains on the surface with the increase of laser energy density. The results of XRD analysis were consistent with the SEM examination. Laser glazing can notalbly enhance the life-time of TBCs with the possibility to double the number of cycles compared to the nanostructured TBCs. The two main reasons for the improvement of thermal shock resistance were to: (i) effectively decrease the permeability of the corrosion medium to the bond coat by equiaxed grain on the surface; (ii) relax the thermal stress extensively by the columnar grain and net-microcracks and increase the strain compliance of the as-glazed coatings.So it can be pointed out that the hybrid process that the nanometer technology and laser glazing are combined with atmospheric plasma spraying to prepare TBCs has been revealing a high potential for the improvement of TBCs properties.