Plasma spray deposition on surfaces with curvature

Abstract

Plasma spraying has been widely used to apply coatings on complex engineering components due to its high deposition rate and wide choice of materials. However, the complexity of the components shape leads to the deficiency of the plasma spraying process. In addition, the coating profile continues to change, which will affect the subsequent spraying. Hitherto, the desired coating profiles need to be optimized, mostly by costly and extensive trial and error tests done by highly-trained manpower. A deposition model thus is desirable to predict the continuously changing profiles on complex surfaces. By this means, the cost and effort of the spraying test will be reduced significantly. Curved surface or even more complex component surface can be considered to comprise of multiple flat surfaces. A semi-empirical methodology to predict the deposit formation on curved substrates has been developed in this thesis. The methodology is developed by three vital steps: • Computational fluid dynamics (CFD) analysis to obtain the spatial distribution of particles and their corresponding in-flight parameters. • Droplet splatting behavior analysis to establish correlations for spread factor, aspect ratio and elongation factor with respect to the impact velocity and impact angle. • Modeling of deposit growth with time, with the data acquired in the particles parameters simulation and the correlations for splat morphologies. In the CFD analysis using FLUENT V6.03©, the spraying process is modeled as a three-dimensional steady state plasma plume by the volumetric heating of the arc gas in the torch. Solutions of the plasma flow velocity and temperature fields are firstly obtained. Particles are introduced into and interact with the plasma flow by a one-way coupling method. The heat and momentum equations are solved Abstract II to obtain the spatial distributions of the particles and their corresponding in-flight parameters, including velocity, temperature, size and mass flow rate. SprayWatch© on-line diagnostics system is used to measure the particles velocity and flight angle for the freestream case (with no substrate inclusion). The simulated results captured by a plane at the distance 80 mm agree well with the SprayWatch© measurements. The particle parameters from simulation are used in the deposition code, while the SprayWatch© measurements are used for selection of droplet parameters in the simulation of droplet splatting behavior. The effect of substrate inclusion and shape on the particle in-flight behavior is also investigated. It is found that the substrate inclusion and shape (concave or convex) significantly influence the plasma flow fields in the vicinity of the substrate The particles parameters remain relatively unaffected if their size is larger than a threshold value (10 m). The analysis of droplet splatting behavior is divided into two aspects: • A droplet impacts onto the flat substrate under normal impact. • A droplet impacts onto the curved substrate at different impact angles. Individual splats are captured by th...