Abstract

Surface engineering plays an important role in different industries. Enhancing the surface properties while keeping the base material properties is the ultimate goal of surface engineering. If the coating is manufactured by melting the particles with heat, it is called thermal spraying. Suspension plasma spraying (SPS) is a branch of thermal spraying where the coating material is suspended in a base liquid. The suspension is injected to a plasma that melts the particles. These molten particles strike the substrate, solidify and form the coating. One of the main challenges in SPS is clogging the suspension in the feeding line. Interaction of the particles with the tube wall may result in sedimentation (fouling) and reduction in the passing area through the tube and finally blockage of the suspension flow. In this case, the spraying process should be stopped and further actions need to be taken for cleaning the suspension line and the injector. The other challenge is the speed of the spraying. SPS is considered as a slow process compared to other conventional thermal spraying methods. Typically, suspensions have concentration of about 10-25 wt. % which means only 10-25 % of the total mass of suspension is made of the particles that actually contribute to the coating. In this study, visco-plastic lubrication is introduced to avoid clogging while increasing the concentration of the suspension up to 70 wt. %. The key element in visco-plastic lubrication is utilizing a yield stress fluid as the lubricant. Yield stress materials behave like fluids if they are submitted to a stress higher than a threshold called yield stress. If the stress is less than that, they behave like solids. In visco-plastic lubrication, a solid protective layer around the core fluid is formed that keeps the core fluid at the center and does not let it touch the tube wall. In this combined experimental and numerical study, the conditions to establish a stable core-annular flow of suspension-yield stress fluids were determined to transport high concentration suspensions.