Characterization of microstructure and properties of TBC systems with gradient of chemical composition and porosity

The article presents results of microstructural investigation and properties characterization of gradient TBC. The study has been done on AMS 5599 type alloy with 4 different type of thermal coatings with gradients of porosity and chemical composition. In all specimens, NiCoCrAlY VPS-sprayed powder was employed as the bond coat. In addition, ZrO ₂ x 8%Y ₂O ₂ powder (applied on its own or together with NiCoCrAlY powder, to form graded coatings of varied porosity and/or chemical composition) was used for the outer layer spraying. Four variants of coatings were prepared for further investigation: variant included the spraying of bondcoat and of 5 successive YSZ and NiCoCrAlY (APS) powder layers of a given thickness, so as to obtain the composition gradient; second variant included the spraying of bondcoat type powder and of a layer consisting of YSZ and NiCoCrAlY powders mixture of a particular capacity proportion of both of them. Next two variants included the spraying of NiCoCrAlY bondcoat and YSZ ceramic layers with a different thickness. The porosity gradient was created by a suitable modification of APS spraying parameters, whereas the step and smooth chemical composition transition resulted from the consecutive spraying of zirconium oxide and NiCoCrAlY powder (step gradient), and, in the second case, the smooth proportion change of the sprayed powder from the pure 100% NiCoCrAlY to the pure 100% zirconium oxide (smooth gradient), which led to the smooth proportion change of the two powders in the transition area. First of investigated area is related to quantitative and qualitative description of TBC's ceramic top surface such as application of confocal microscopy for characterization of top coat by topographic surface maps and their 3D reconstruction. The numerical treatment of the qualitative results provided the means for quantitative characterization of the surface roughness, employing parameters such as Ra, Rz etc., and using surface and linear methods of confocal image analysis. Second area of investigations is connected to XRD characterization of TBC's systems. The purpose of this research was identification of the residual stress in the top coat of different type of thermal barrier coating with gradient of chemical composition and gradient of porosity. The research allowed the identification of qualitative and quantitative phase constitution of top coat and residual stress measurement by sin ²ψP method form surface of coatings. It was found that the dominant phase in all the top coats was tetragonal zirconia with minor addition of monoclinic type of ZrO ₂ and in the case of residual stress the tensile stress conditions was observed in the case of gradient porosity and compressive stresses in the case of chemical gradient. Another subject is related to microstructural characterization of TBC gradient systems from their quality point of view. The parameters which identify the quality of the bond coat and the ceramic coating consist: the thickness of the layers; the quality of the connection between the metal base and the bond coat; the presence of cracks and oxides; porosity; globular grains; bond coat integrity; the roughness of the bond-coat surface; the microstructure of the ceramic layer-the analysis involves the porosity assessment, randomly oriented cracking and their shape, metallic impurities and globular grains.The last area of presented investigations is related to degradations of gradient thermal barrier coatings during oxidation test. Obtained results showed that TBC with gradient of chemical compositions are not the best solutions for improvement of life-time of TBC. More effective is application of thermal insinuations with gradient of porosity.