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.

Deformation and solidification process of molten metal droplets was directly observed by high speed video camera system. These processes depend on the droplet property, and affect the microstructure of splats. Relationship between formation process and microstructure of splats was discussed. Abstract only; no full-text paper available.

This study investigates the relationship between splat morphology and adhesion strength in plasma sprayed coatings. The shape and size of the splats are examined and found to differ, in some cases significantly, depending on whether the splats are in the center or on the periphery of the spray pattern. Splats in the center of the pattern are formed by particles with higher temperature and velocity than those on the periphery and they tend to produce higher quality layers. In an experiment in which alumina layers are sprayed through a graphite slit filter, coating adhesion is shown to be superior, as expected. Paper includes a German-language abstract.

New method for characterization of coating microstructures and for evaluation of coating property by means of surface morphology has been proposed. In this paper, the distribution of shape and dimensions of splat was examined using quantitative analysis of scanning electron microscope images from the surface of spray pattern as well as the surface of coating. Results obtained in this study indicate that it is necessary to analyze the spray pattern as well as the surface morphology in order to estimate the coating property by means of the distribution of splat which composes the coating. Moreover, the splats, which are in the interface between the substrate and the coating, should have the same morphology as those of the coating surface. Therefore, the analysis of the surface morphology is important even for the evaluation of coating adhesion behavior.

Effects of spray parameters, such as spray distance, SD, and substrate temperature, Ts, and post heat treatment on the structure and properties of plasma-sprayed zircon coatings were investigated. Zircon was totally decomposed by plasma spray; the coatings were composed of tetragonal zirconia (t-ZrO2) and amorphous silica (a-SiO2), because of the rapid cooling of molten particle right after the impingement to the substrate. Porosity of the as-sprayed coatings was highly affected by both of substrate temperature and spray distance. In all range of the spray distance which had been tried in this study, higher substrate temperature resulted in lower porosity of the coatings; the coating with porosity of 2% was obtained at Ts = 1573K with SD = 95mm. Porosity also decreased with decrease of spray distance. By the heat treatment at 1473K, t-ZrO2 transformed to monoclinic zirconia (m-ZrO2) and a-SiO2 crystallized to cristobalite, respectively. Cracks in the coating disappeared, and open porosity decreased. This can be attributed to sintering of SiO2 and phase transformation of ZrO2. After the heat treatment at 1673K, the coating was composed of ZrSiO4 with dispersed fine m-ZrO2 particle. Open porosity of all the coatings increased up to 10% at this temperature. This is because of volume shrinkage during the formation of zircon.

A novel method for characterization of microstructure of coatings is presented. Properties of plasma spray coatings are affected by their microstructure, which depends on the spraying conditions. Therefore, a detailed knowledge of microstructure is very important in order to know the coating formation mechanism and the properties of the coatings. There are many studies to characterize the microstructure of coatings. In most of those studies, the microstructures are characterized from the polished cross-section of the coatings, and the results strongly depend on the preparation methods. In this study, a new method for the characterization of coating microstructures by means of surface morphology is proposed. The distribution of shape and dimensions of splats were examined using quantitative analysis of scanning electron microscope images from the surface of the coatings. The results indicate that the surface morphology strongly depends on the spraying conditions. Keywords: characterization, coating property, splat morphology, equivalent diameter, shape factor

A high-speed video recording system was developed to measure the velocity and temperature of thermal spray particles in flight. The system consists of three image sensors and a cubic beam-split prism that directs light, based on color or wavelength, to three imaging planes. In this study, one of sensors is used to measure particle velocity while the other two measure particle temperature. The prospect of simultaneously measuring the temperature and velocity of individual particles in flight is discussed along with the insight it provides on the formation of spray coatings.

Plasma sprayed ZrO 2 -CeO 2 and ZrO 2 -CeO 2 -Y 2 O 3 coatings were investigated to develop advanced thermal barrier coating (TBC) with improved thermal and mechanical properties. The addition of a large amount of CeO 2 to ZrO 2 decreased the thermal conductivity, but it also reduced the mechanical property like hardness. Addition of Y 2 O 3 to ZrO 2 -CeO 2 was effective to improve the hardness. Double layered coating composed of a low thermal conductive top layer, such as ZrO 2 -CeO 2 -Y2O 3 and a high strength base layer like yttria stabilized zirconia was fabricated. It exhibited an excellent thermal shock resistance and high thermal barrier capability.