Gas Permeability for Thermal Sprayed Coatings

Abstract: Thermal spray coatings contain a complex network of cracks and pores that are often characterized cumulatively as the coating “porosity”, or total concentration of void space. In many applications coatings are used to protect the underlying material from a gaseous or aqueous environment where the presence of such pores are thought to compromise or reduce the coatings protective capability. However, the amount of gas or fluid that can flow into and through the coating depends on the quantity, dimensions, and interconnectedness of these defects not simply the total pore fraction in the coating. The ability for gas or fluids to flow through a sample is characterized by a metric known as permeability, which is expected to vary greatly with the coating’s microstructure and the given fluid medium. The helium permeability of ceramic and metallic coatings produced through various spray processes were calculated using Darcy’s law from helium leak rates measured using a commercial detector along with pressure drop across the sample. The effects of process variation and postproduction heat treatment on gas permeability further explored. Correlations to the coatings thermal and mechanical properties along with stress evolution during buildup are also given, with potential mechanisms discussed within.