This work focuses on the high velocity oxygen-fuel (HVOF) thermal spray processing of coatings and presents a fundamental model for the process which explicitly accounts for the effect of powder size distribution. The model describes the evolution of the gas thermal and velocity fields, as well as the motion and temperature of agglomerate particles of different sizes. In addition to providing useful insight into the in-flight behavior of particles with different sizes, the model is used to make a control-relevant parametric analysis of the HVOF thermal spray process. This analysis allows us to systematically characterize the influence of controllable process variables such as combustion chamber pressure, oxygen/fuel ratio, as well as the effect of powder size distribution, on the values of particle velocity and temperature at the point of impact on substrate. Specifically, the study shows that particle velocity is primarily influenced by the combustion chamber pressure, and particle temperature is strongly dependent on the fuel/oxygen ratio. Furthermore, it shows that the particle velocity and temperature at the point of impact depend strongly on particle size. These findings are consistent with available experimental observations and set the basis for the formulation of the control problem for the HVOF process. (C) 2003 Elsevier Science Ltd. All rights reserved.