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.

Governing Parameters Influencing CMAS Adhesion and Infiltration Into Environmental/Thermal Barrier Coatings in Gas Turbine Engines

Sand particulate ingestion into modern gas turbine engines for fixed wing and vertical lift aircraft is a significant challenge for both military and civilian missions. ARL as part of a DoD funded Laboratory University Collaborative Initiative (LUCI) and Vannevar Bush Fellowship at UCSD are investigating the governing parameters that primarily influences the CMAS adhesion kinetics and infiltration on the standard Yttria Stabilized Zirconia (YSZ) as part of metallic single crystal Nickel superalloys TBC and SiC/SiC CMC T/EBCs. Current research shows various parameters including CMAS viscosity, porosity, adhesion strength, contact angle (wettability factor), geological factors affecting sand formation, coating and structural substrate roughness and surface temperature, internal flow Reynolds number, temperature, pressure, Mach number, boundary layer and bleed air, coating process (columnar vs splat morphology), tortuosity factor et al affects the CMAS adhesion and infiltration. This paper is a summary of our current research to identify and study the governing parameters that affects the CMAS formation, adhesion and infiltration and the underlying interfaces between CMAS and T/EBC, bond coat and the structural substrate. This work is aligned with Army Modernization Priority Future Vertical Lift and PEO Aviation Advanced Turbine Engine (ATE) Program.

Cracking caused by cutting of plasma-sprayed hydroxyapatite coatings and its relation to the structural features of coatings deposited at different initial substrate temperatures

The present study estimated the cracking phenomenon in as-plasma-sprayed hydroxylapatite coatings (HACs) after they were being subjected to the severe cutting conditions in the direction perpendicular to the coating/substrate interface. In order to evaluate the effects of substrate preheating on the occurrence of micro-cracks, the HACs were deposited at different initial substrate temperatures (TS = 20, 100 and 200?C). The changes in phase composition and HA splat morphology with TS were observed and were correlated with the cracking occurrence. The results showed that severe cutting conditions introduced a localized cracking in the regions of HACs dominantly attributed to the brittle hydroxyl-deficient amorphous calcium phosphate (ACP) phase. This effect was particularly observable in the HACs deposited without preheating of substrate. On the other hand, the preheating of substrate reduced the presence of micro-cracks and caused insignificant changes in the average local phase composition. In HACs deposited with preheating of substrate, the HA splats (of which HACs are composed) were thinner and recrystallized HA regions seemed smaller in size and more evenly distributed. These results implied potentially important roles of the HA splat formation mechanism on the distribution of ACP and recrystallized HA regions in the as-plasma-sprayed HACs and the cracking resistance of HACs.