High-Velocity Oxy-Fuel (HVOF) Suspension Spraying of Mullite Coatings

Mullite coatings (3Al(2)O(3) center dot 2SiO(2)) were deposited by suspension thermal spraying of micron-sized (D-50 = 1.8 mu m) feedstock powders, using a high-velocity oxy-fuel gun (HVOF) operated on propylene (DJ-2700) and hydrogen fuels (DJ-2600). The liquid carrier employed in this approach allows for controlled injection of much finer particles than in conventional thermal spraying, leading to coatings with low porosity and fine and homogeneous porosity distribution, making this process potentially suitable for creating thin layers with low gas permeability. In-flight particle states were measured for a number of spray conditions of varying fuel-to-oxygen ratios and standoff distances and related to the resulting microstructure, stoichiometry, phase composition (EDS, SEM, XRD), and hardness (VHN (300g.f)) of the coatings. In an attempt to retain the crystalline phase in the coatings, HVOF operating conditions were varied to limit in-flight particle melting. However, fully crystalline coatings were only obtained by gradually heating the coating during deposition to temperatures above 400 degrees C.