Modeling of air-droplet interaction, substrate melting and coating buildup in thermal spraying

Among the many surface coating techniques now available, thermal spray is known to offer the most advantages. It can meet a wide range of technical and engineering requirements in a relatively inexpensive and easily controllable way with the capability of producing repeatable results. In the last few decades a lot of important strides have been made in the field of measurements and modelling of thermal spraying. However, due to the complex of the process and the lack of basic materials-based knowledge about the particle melting, spreading and deposition, the relationship between the process parameters and the coating properties still remains unclear. In thermal spraying, a particle is melted to form a droplet with morphology and thermal- and kinetic-energy status change by the interaction with the plasma/flame. In order to produce higher-quality coatings and expand the use of this versatile family of technologies, modelling of the particle behaviors during in-flight, spreading and deposition is essential.;Most existing theoretical studies of in-flight particle assume that the particle is in a spherical shape without voids inside. The behavior of porous particles in thermal spray has not been well understood. However, the presence of voids in the feedstock powders may have a great impact on particle in-flight behaviors such as particle acceleration, melting and oxidation because a hollowed particle is also lighter than a densed one and this will affect the particle trajectory. The particle shape also needs to be taken into account because it influences the drag force and particle feeding velocity. In this thesis, the level set method is used to study the interaction between the droplet and the surrounding air. The level set function is used to track the deformation of the free surface. The capability of this model on accurately and efficiently simulating the droplet deformation and oscillation is demonstrated. The droplet deformation during in-flight caused by the air-droplet interaction and the droplet-substrate interaction are considered here. Particles with different surface tension and morphologies are studied as well.;Droplet substrate interaction is studied to understand the substrate melting behavior. A numerical model is developed to investigate the droplet solidification, substrate melting and re-solidification. A dimensionless parameter, "temperature factor", is proposed from analysis and it can be used as an indicator to predict whether substrate melting will occur for a certain combination of the droplet and substrate. This parameter can be correlated with the maximum melting depth of the substrate. The possibility of heating up the substrate by plasma flame, and attaching a temperature-control device on the backside of the substrate to achieve substrate melting is studied. The substrate front surface temperature can be controlled at a sufficient high temperature. With additional heating from superheated molten droplets and the latent heat of droplet solidification, a thin liquid layer of the substrate can be obtained and epitaxy growth of the splats is possible. This could expand thermal spray technology to the applications of semiconductor and solar energy, both of which need epitaxy crystal with big sizes.;To better control the existing thermal spray process, it is important to develop the quantitative relationships between spray parameters and coating characteristics. Until recently, the simulation studies have been focused on two-dimensional models and prediction of the cross-section structure of deposited layers; although a few three-dimensional models are developed as well by using the statistical particle parameters as input. However, all these models failed to connect the process parameters to coating properties. The coating deposition study here focuses on the development of a computational model, which is able to build a relationship between the process parameters and the coating properties by using the particle data from LAVA 3D calculation results, and to simulate the deposition together with the porosity evolution inside the coating. We propose a set of coating build-up rules to predict the coating deposition and the pore formation, considering the influences of particle size, velocity and temperature and impact position.;This thesis investigates the connections between particle characteristics and coating properties. Momentum, heat and mass transfer phenomena related to particle in-flight, droplet impacting, spreading, and splat layering are studied. Numerical models are developed to establish the quantitative relationships between spray parameters, particle and substrate properties and deposition characteristics.