This work focuses on the modeling and feedback control of the high velocity oxygen fuel (HVOF) thermal spray processing of WC-Co coatings. A first-principles-based process model is proposed to explore the fluid dynamics and particle inflight behavior in the HVOF flow field. Rule-based stochastic simulation, which encapsulates the main features involved in the deposition process, is used to capture the evolution of coating microstructure. The multiscale modeling of HVOF thermal spray process reveals that the velocity and melting degree of particles at the point of impact on the substrate play a very important role in the formation of coating microstructure, which in turn, can be almost independently controlled by pressure and fuel/oxygen ratio, respectively. Based on model predictions and available experimental observations, the control problem is formulated as the one of regulating volume-based averages of particle velocity and melting degree at impact on the substrate by manipulating the feed rate of air, oxygen and fuel at the entrance of the thermal spray gun, through which the pressure and oxygen/fuel ratio can be independently adjusted. A feedback control system is then developed and applied to a detailed mathematical model of the process. Closed-loop simulations show that the feedback controller is effective in driving the controlled outputs to the desired set-point values and also robust with respect to various kinds of disturbances in the operating environment. (c) 2005 Elsevier B.V. All rights reserved.