Effect Of Microstructure On Thermal Conductivity Of Plasma Sprayed YSZ (8wt.%) Coatings

Nowadays,yittria-stabilized zirconia?YSZ?coatings have become the world most widely used thermal barrier coatings because of its excellent mechanic and thermal performance,such as stable in chemical composition and organization structure,low thermal conductivity,adjacent coefficient of thermal expansion to metal substrate,high bending strength,good thermal shock resistance and cheap in price.Besides,plasma spraying?PS?preparing coatings with layered structure,low thermal conductivity is a relative mature production process in manufacture.As one of the most important indicators to measure thermal barrier coating's heat insulation performance,thermal conductivity is closely related to coating's microstructure.Due to the complexity of coatings'structure and restrictions of research method,most studies focused on the qualitative or quantitative analysis of single factor.By using commercial ZrO₂-8wt.%Y₂O₃ powders,the systematically correlation of process parameters and microstructure and thermal property has been studied in order to control the process to achieve desired properties and performance.The main results obtained are described as follows:1.Nine freestanding samples deposited on aluminum alloys are studied.Cross-section morphology such as pores,cracks,m-phase content,grain boundary density of the coated samples are examined by scanning electron microscopy?SEM?and electron back-scattered diffraction?EBSD?.2.Multiple linear regressions are used to develop quantitative models that describe the relationship between the particle parameters,m-phase content and features of the microstructure such as porosity,crack-porosity,and the length density of small and big angle-cracks.3.The relationship between the microstructure and thermal conductivity is investigated.Results reveal that the thermal conductivity of the coating is mainly determined by the microstructure and grain boundary density at room temperature?25°C?,and by the length density of big-angle-crack,monoclinic phase content and grain boundary density at high temperature?1200°C?.4.8 mol%yttria stabilized zirconia?8YSZ?coatings are prepared by solution precursor plasma spray?SPPS?technique under different spray conditions.Phase analysis is performed using X-ray diffraction?XRD?and Electron back scattered diffraction?EBSD?techniques.Irrespective of the processing conditions and the heat treatment temperatures,all samples displayed cubic and tetragonal zirconia phases.Vertical and horizontal cracks appeared in the microstructural analysis of the coatings.Coating prepared under spray conditions having 40 mm distance between the spray gun and the sample exhibit high hardness,both at 0 h and 10 h heat treatment holding time.The coating with 42 mm distance between the spray gun and the sample displayed lowest thermal conductivity from 400 to 1200?.