Leaching of HVOF Composite Coatings

The use of thermal spray to produce porous coatings has been mainly concentrated in low to medium velocity techniques such as flame sprayed and plasma sprayed abradable coatings. In this work, a novel approach to the creation of porous coatings is presented. Different volume fractions of atomized powders of a ferritic (444) stainless steel and of an iron aluminide (Fe₃Al) were simply mixed in a bottle or mechanically milled in a high-energy ball mill. Each type of powder (milled and un-milled) was sprayed with similar parameters in a JP-8000 HVOF torch. Leaching of Fe₃Al from the coatings was carried out in dilute hydrochloric acid (HCl) solutions with the aid of a solartron 1286 potentiostat. After leaching, the mechanical integrity of the coatings was qualitatively assessed by means of an adhesive strip test. The effect of mixing procedures, volume fraction of each powder, and particle size distributions were correlated with the coating structures, leaching behavior, and final pore architecture. It was found that both the particle size distribution and the volume fraction of each phase must be carefully tailored to ensure good mechanical integrity after leaching.