A comprehensive one-dimensional model recently developed by the authors is used to investigate particle behaviour associated with the high velocity oxy-fuel (HVOF) spraying process. Using the spraying of WC-12%Co powder as a case study, it is shown that the Knudsen non-continuum effects can have an immense influence on the gas-solid heat and momentum transfer processes. Consistent with actual practical experience during HVOF spraying, the results of this study reveal that the low temperatures provided by the oxy-fuel combustion flame preclude complete melting of many ceramic powders possessing high melting points. However, it is shown that powders of metals and the low melting point alloys can be completely molten in the oxy-fuel flame. With WC-12%Co powder, it is found that only particles smaller than 45 μm in size can be fully molten and this is in excellent agreement with the empirically established HVOF coating practice. The results also indicate that the hypersonic flame accelerates the powder particles to very high velocities, around 700 m s-1 in some instances, and such high particle velocities are primarily responsible for the HVOF technique yielding dense and well-bonded coatings which are superior to plasma-sprayed coatings. © 1991.