Studies of the microstructure and properties of dense ceramic coatings produced by high-velocity oxygen-fuel combustion spraying

High-velocity oxygen-fuel (HVOF) spraying stands out among the various processes to improve metal and ceramic coating density and surface characteristics. This paper explores microstructure development, coating characterization and properties of HVOF sprayed alumina coatings and compares these with those produced using the conventional air plasma spray process. We report on the characterization of these coatings using small-angle neutron scattering (SANS) and X-ray computed microtomography (XMT) to explain the behavior observed for the two coating systems. Microstructure information on porosity, void orientation distribution, void mean opening dimensions and internal surface areas have been obtained using SANS. XMT (X-ray synchrotron microtomography) has been used to nondestructively image the microstructural features in 3D at a 2.7-μm spatial resolution over a 2–3 mm field of view. 3D medial axis analysis has been used for the quantitative analysis of the coarse void space in order to obtain information on the porosity, specific surface area, pore connectivity and size distribution of the larger voids in the coatings. The results reveal different pore morphologies for the two spray processes. While only globular pores are imaged in the plasma sprayed coatings due to the spatial resolution limit, highly layered porosity is imaged in the HVOF coating. When the quantitative SANS and XMT information are combined, the different thermal and mechanical properties of the two different coating types can be explained in terms of their distinctly different void microstructures.