Twin wire arc spray process modification for production of porous metallic coatings

• Arc spray process modification to produce porous metallic coatings
• Application of Response Surface Method to optimize the process
• A maximum of 20% pores in the alloy 625 coating was achieved via modified arc spray system

One approach to protecting surfaces exposed to high temperature gas flows is to form a boundary layer of cooler gas along the surface, typically by allowing cool gas to flow from internal passages through a pattern of holes drilled in the surface of the component. The production of this pattern of fine holes is time consuming and expensive. In this study we have explored the feasibility of creating high fractions of open porosity extending through the outer face of a metallic foam core sandwich structure heat shield by controlling the porosity content of the thermally sprayed faces. The faces were fabricated by twin wire arc spraying of alloy 625 on the filled surface of the metallic foam core. Modification of the spray process was required to obtain the desired level of open porosity in the coatings, since manipulation of the twin wire arc gun operating parameters alone was not sufficient. An external nozzle was mounted in line with the gun axis to spray polyester powder particles, as a pore generating agent, onto the substrate surface during coating deposition. The effects of twin wire arc gun operating parameters and the polyester powder spray conditions were studied using statistical design of experiments to increase the porosity of the deposits. It was found that the feeding rate of the polyester powder is the most significant factor affecting the coating porosity content, while the gun operating parameters have a relatively small influence. Embedding polyester particles in the coating by spraying through the external nozzle resulted in a pore content of up to approximately 20%.