Functionally graded materials (FGMs) offer solutions to such engineering problems involving multi-layer systems with large differences in CTE, i.e. thermal barrier coatings, by allowing for a continuous change in the properties over a defined distance, thus minimizing sharp interface effects. By its nature, plasma spraying is well suited to the fabrication of FGMs. However, in order to achieve optimal performance from the material it is necessary to ensure the FGM is uniform in its compositional variation. The deposition efficiency of the particulate species as well as their trajectories will determine the degree of homogeneity of the FGM. It is therefore important that the inter-relationships between the particle size distributions, injection orientation and feed rates are determined. Towards this end, a series of investigations have been carried out to determine the effects of injection orientation on the particle segregation. Analysis of the particle segregation as a function of particle size distribution was then examined in the formation of FGMs. The results indicate that optimal deposition occurs when the various particle species trajectories converge as they approach the substrate.