In this study, titanium and aluminum powders mixed in different ratios were deposited on stainless steel substrates by warm spraying. Microstructure and composition of as-sprayed and heat-treated samples were characterized and the effect of adding a third element was assessed. It was found that Al content has a major influence on the thickness and porosity of heat-treated Ti-Al coatings and that adding silicon to the powder mixtures reduces the melting point of Al, causing a loss of Al-Si particles during spraying.

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.

To protect various gas turbine components against high temperature in the hot sections of power generation plants and aircraft engines, thermal barrier coatings (TBCs) have been developed and widely used. Conventional TBCs consist of a MCrAlY bond coating for oxidation resistance and a ceramic top coating for thermal insulation. High quality coatings of MCrAlYs have been produced mostly by low pressure plasma spraying but other more economical processes are also used depending on the operating conditions of the component to be coated. In this study, CoNiCrAlY powders were deposited on Inconel 718 substrate with 3 types spraying system, i.e., low pressure plasma spraying, high velocity oxy-fuel spraying, and atmosphere plasma spraying. Specimens were isothermally tested for up to 100 h in air at 1373 K. Mass gain of the coatings was measured. Microstructure of the coating cross sections and the surface oxides were observed with SEM. To identify the crystal structure of the formed oxides, the specimens were analyzed by XRD from the surface.

Type of powder feedstock greatly affects properties of zirconia coatings since they interact differently in a plasma flame and hence influence microstructure development. In this study, ZrO 2 -8wt% Y 2 O 3 thermal barrier coatings (TBCs) have been produced by atmospheric plasma spraying fused and crushed (FC) and hollow sphere (HOSP) feedstock powders. The sprayed coatings contained segmentation cracks going through the coating thickness. High substrate temperature during spraying gave rise to increased segmentation crack density (Ds). The FC powder has the capability of providing coatings with high segmentation crack density compared with the HOSP one. At lower temperature, the HOSP coating was more porous than the FC coating sprayed at similar temperature and hence exhibited a much reduced thermal diffusivity. At high plasma temperature, the HOSP particles attained higher particle surface temperatures and velocities than the FC ones. The particles temperatures and velocities for the HOSP particles were influenced more significantly by spray condition than those for FC particles.