Multiscale modeling and control of thermal spray processing of nanostructured coatings

This thesis presents our work on the multiscale modeling and control of the HVOF thermal spray processing of nanostructured coatings, which is motivated by the superior qualities of nanostructured coatings and high sensitivity of the coated material to arbitrary variation in the operating environment. The central idea of the controller design is to use the models that describe the dynamic behavior of the various components of the HVOF thermal spray process to synthesize feedback controllers that can be readily implemented in practice.; We developed a comprehensive multiscale model, which includes continuum type differential equations that describe the evolution of gas and particle temperature and velocity, and a rule-based stochastic simulator that predicts the evolution of coating microstructure. Based on the process model, a detailed comprehensive parametric analysis is carried out to study the relationship between the key process parameters and the particle inflight behavior as well as the resulting coating properties. This analysis shows that the particle velocity and melting ratio play a very important role in the formation of coating microstructure, which in turn, can be almost independently adjusted by manipulating the combustion pressure and the fuel/oxygen ratio. To develop a feedback controller that can be readily implemented in practice, the control problem is formulated as the one of regulating volume-based averages of the melting ratio and velocity of the particles at the point of impact on the substrate by directly manipulating the flow rate of fuel, oxygen and air at the entrance of the HVOF gun. A multivariate control system is developed and applied to a fundamental process model. Closed-loop simulations demonstrate that the proposed control system is effective in driving the particle velocity and melting ratio at impact into set-point values and is also robust with respect to various disturbances in the operating environment.