Hybrid aerosol deposition (HAD) is a new coating method to deposit homogeneous nano-structured ceramic coatings. An accurate evaluation of the fabricated coating properties is required. In this study, α-Al 2 O 3 fine powder was sprayed by HAD. The obtained coatings were dense and uniform with a nanocrystalline structure. An X-ray diffraction measurement revealed that the fabricated HAD Al 2 O 3 coatings mainly consisted of α-Al 2 O 3 phase. The hardness and Young's modulus of the HAD Al 2 O 3 coatings were evaluated by a micro-Vickers method and a nanoindentation method using the Weibull distribution. The hardness of HAD Al 2 O 3 coatings measured by micro-Vickers was ~1400 HV (~15 GPa). The variation of mechanical properties of HAD coatings measured by the nanoindentation method was extremely small compared to those of plasma-sprayed coatings, which also indicates that HAD coatings contain less pores and cracks than plasma-sprayed coatings.

Deposition of protective dense environmental barrier layers is a promising solution to improve the reliability and environmental durability of the next-generation turbines and other industrial applications. In this context, spraying of fine particles could enhance the formation of fine dense coating microstructures with improved properties. In AIST we are focusing on the spraying of the fine particles via different spraying technologies including suspension plasma spraying, as well as deposition of the fine solid particles directly by aerosol deposition (AD) and plasma-assisted aerosol deposition (so-called Hybrid Aerosol Deposition; HAD. The HAD is a new coating window to spray the fine ceramic particles via the implementation of a low-power rf-plasma source to assist the aerosol deposition at room temperature. This study introduced the feasibility of utilization of HAD as an outstanding technology for deposition of dense ceramic coatings on different substrate materials and 3D deposition capability. Highly dense and well-adhered Al 2 O 3 coatings without obvious observable cracks and bulk-like properties were successfully fabricated on different substrate materials of SUS 304, Aluminium, Al 2 O 3 and glass, via HAD of fine particles. The substrate material and its hardness significantly influenced the first deposition step, which determined the coating adhesion and properties. Furthermore, homogeneously uniform, dense, and crack-free coating with a strong adhesion has been fabricated successfully on cylindrical substrates with 6.3 mm diameter. During HAD spraying the plasma activated the surface of the particles without reaching to the molten state, then the activated particles impact and stuck with the substrate by room temperature impact consolidation mechanism. Therefore, the fabricated coatings had the same crystal structure as the starting feedstock powder, and the activated surface act as glue and improved the deposition efficiency and 3D capabilities. Herein, the deposition phenomena of HAD makes it as a promising candidate technology for development of environmental and sealing layers of highly dense microstructure, with the targeted crystalline phase structure, without stoichiometric composition nor phase transformation and improved deposition efficiency on multi-shape components in different fields such as environmental, thermal barrier coatings (TBCs), environmental barrier coatings (EBCs) and gas turbine applications.

The effects of powder loading on temperature distributions of yttria-stabilized zirconia particles during flight, which can be used as a melting status indicator, were investigated in atmospheric dc plasma spraying. Commercially available diagnostic systems were utilized to measure the state of in-flight particle parameters. As the powder feed rate was increased, the intensity of a peak related to the latent heat increased, suggesting the increase of semi molten particles. Interesting findings are that the deposition efficiency of the coating actually increased in some conditions at higher powder feed rates. This implies that higher molten degrees of particles do not always give higher deposition efficiencies. The loading effects also affected the result of diagnostics, which requires special care when the diagnostic condition is different from the actual spray conditions.

Spreading and rapid solidification behavior of millimeter-scale molten drops of 8 wt.% yttria-stabilized zirconia (YSZ) has been experimentally studied utilizing a novel splittable aerodynamic levitator (ADL). The focus was especially on the effect of initial undercooling on the splat formation. An YSZ sphere approximately 2.3 mm in diameter was levitated in a splittable ADL nozzle and melted by a carbon dioxide laser. The molten drop was dropped by splitting the ADL nozzle and impacted on a substrate 15 cm below at a speed of 1.7 m/s. The spreading and solidification behavior of the impacting drop was observed with a high-speed digital video camera. The undercooling of YSZ drops reached to more than 500 K at a containerless state, and the solidification rate was on the order of 1 m/s at this state. When drops were dropped at superheated states, the drop solidified after flattening completed. Meanwhile, when impacted at large undercooling, the drop spreading was suppressed by the solidification. Drastic difference was observed when a drop was impacted on a substrate covered with acetone liquid. The drop was splashing, recoiling, and rebounding despite fact that splashing would not occur at this impact condition.

Understanding the impacting phenomena of yttria-stabilized zirconia (YSZ) particles and following coating formation in plasma spraying process is of importance to control and design the microstructure of coatings such as thermal barrier coatings. To this aim, recently, the authors have developed a novel in situ monitoring system for particle impacts under atmospheric dc plasma spraying conditions. This system utilized a high-speed video camera coupled with a long-distance microscope and was capable of capturing the particle-impinging phenomena at one million frames per second. To understand the coating formation mechanism, two approaches were attempted, that is, observation of the single splat formation and the following coating formation as the integration of splats. In the former case, the deformation and cooling processes of YSZ droplets impinging on substrates were captured successfully. In the latter case, multiple-droplet-impacting phenomena were observed as an ensemble treatment. Representing coating process, the tower formation (1- dimensional) and bead formation (2-dimentional) were observed under typical plasma spray conditions for thermal barrier coatings. By using a triggering system coupled with the motion of a robot, impact events were recorded for every pass. The obtained images clearly showed the coating formation resulted by the integration of single splats.

Recently, the thermal spray community has focused considerable attention on Cold Spray and Warm Spray techniques, in which the temperatures of sprayed particles are kept under the melting points and adhesion occurs based on the impact phenomenon between a solid particle and a substrate. The mechanisms of adhesion are still unclear but the degree of the mechanical deformation at the interface is considered to be one key factor. However, it is very difficult to directly measure the strain at the interface. Instead, in this work, the strain fields on a substrate around an impacted particle were measured by applying Electron Moiré method, and corresponded to the spray conditions by a warm spray deposition.

It has been said that plasma-sprayed ceramics particles are often supercooled before the impact on substrate. Some numerical models of the droplet impact actually included the supercooling effects. However, there is no report that has experimentally confirmed the effects on splat morphology. Therefore, in this research, we have mainly investigated the supercooling effects on splat morphology as well as splat microstructure. To achieve this, we developed an in-situ measurement technique utilizing radiation from a melt particle to monitor the impact of single particle successively under plasma spraying. The system was able to identify each single particle, which enabled us to correlate the splat morphology with impact velocity and thermal history of each particle during the impact. Yttria-stabilized zirconia powders were sprayed onto quartz glass substrate by the argon-hydrogen dc-rf hybrid plasma under atmospheric pressure. Waveforms of emissions and thermal history obtained during the impact were precisely analyzed. Especially, we closely examined thermal history during particle spreading to find the recalescence. In addition, splat morphologies were examined statistically in relation to their thermal histories. Based on the measurement, we also evaluated the viscosity of zirconia, cooling rate, and thermal contact resistance experimentally.