Influence of Direct Splat-Affecting Parameters on the Splat-Type Distribution, Porosity, and Density of Segmentation Cracks in Plasma-Sprayed YSZ Coatings

The integrity and properties of ceramic coatings produced by atmospheric plasma spraying are highly controlled by the splat morphology and splat interconnection. In this study, the influence of selected parameters (spray angle, surface velocity of the spray gun, and substrate temperature) on splat morphology and coating microstructure was investigated. A favorite set of spray gun parameters, of which their effects on splat morphology and coating microstructure have been verified by previous experiments, were used to conduct the experiments for the present work. It was found that depositing fully molten particles on a hot substrate increases the fraction of disk-like splats by about 60% at the expense of the fraction of irregular splats. Preheating the substrate also increases the pore count and level of coating porosity, while it does not influence the density of segmentation cracks. In contrast, the surface velocity of the spray gun does not affect the splat morphology while a slow speed decreases the coating porosity and plays a significant role in generating segmentation cracks. Shifting the spray angle by 15° distorts up to 20% of disk-like splats and slightly decreases the porosity level. However, changing the spray angle does not affect the generation of segmentation cracks.

Thermomechanical and Thermal Gradient Mechanical Fatigue Lifetime of Thermal Barrier Coating Systems

Detailed damage analyses of an Y2O3 stabilized ZrO2 top coat (TC)–MCrAlY bond coat (BC)–superalloy thermal barrier coating (TBC) system during thermomechanical fatigue (TMF) and thermal gradient mechanical fatigue (TGMF) tests had been performed in present work. During tests, the lifetime of TBCs was strongly dependent on the strain ranges, pre-oxidation time and the thermal gradient in TBCs. Cracks were initiated in the TGO layer, propagated along the TC/TGO or TGO/BC interface, forming the delamination cracks. When the delamination cracks connected with the segmentation cracks which were initiated and propagated in TC, the TBCs spalled. The failure mechanism and stress were analyzed, which were significantly helpful to establish the TMF/TGMF lifetime prediction model for the TBCs.

Understanding the Enhancement of Coating's Thermal Cracking Resistance by Multiple Segmentation

In this paper, the thermal shock induced cracking behavior of a segmented coating on the outer surface of a hollow cylinder has been investigated. The driving force for the propagation of multiple segmentation crack, represented by the Thermal Stress Intensity Factor (TSIF), was determined by combination of the principle of superposition and the finite element method. The maximum TSIF has been shown to occur neither at the beginning nor at the steady state of thermal transients, but at an intermediate instant. As the spacing between multiple segmentation cracks decreases, the magnitude of TSIF first plateaus, and then decreases sharply. This quantitative mechanistic result rationalizes the experimental observations that a segmented coating can exhibit much higher thermal shock resistance than an intact counterpart, if only the segmentation crack spacing is narrow enough. Some other parameters affecting TSIF, such as segmentation crack depth and convection severity, were also discussed.

Investigation of Parameters’ Effect on Segmentation Cracks Formation in Zirconia Coating

The thermal barrier coatings with segmentation cracks have been proved with excellent thermal shock resistance property, but it is unclear that how the spray parameters affect the initiation of segmentation cracks. In order to find the relationship, orthogonal design experiments were used to analyze the effect of parameters, including spray distance, transfer speed, and powder feedrate. It is found that the descending order of parameters according their effect on the formation of segmentation cracks is: spray distance, feedrate, and the transfer speed. Furthermore, the parameters for getting high segmentation crack density are: spray distance is 55mm, feedrate is 2.5RPM and transfer speed is 100mm /s.

Thermal Cycling and Hot-Corrosion Behavior of Plasma Sprayed Segmented Thermal Barrier Coatings

Modified zirconia thermal barrier coatings (TBCs) with segmentation cracks were sprayed onto a TMS 82+ single crystalline substrate. The thermal cycling lifetime of the modified TBC was improved by 10 times compared to that of the traditional non-segmented TBC. Also, the modified coating showed much better resistance to high temperature cyclic hot-corrosion.