Solution, Suspension or Powder Plasma Spray Spray of Nano Structures: Different Tools for Different Jobs

Nano scale composites are potentially useful due to the well known benefits of fine grain and phase structures. It can be expected and demonstrated that multi phase nano scale microstructures resists coarsening at high temperature better than single phase materials due to the larger diffusion distances needed for coarsening.  Making such microstructures is challenging. Solution precursor plasma spray in which chemical precursors are used as the input material for thermal spray has important advantages, including the production of very fine multiphase microstructures. This method also allows the creation of through thickness cracks as is useful for thermal barrier coatings.   Suspension spray can accomplish most of the same goals except presumably the through thickness cracks, but has the drawback of requiring the production of nano powders typically from chemical precursors similar to those used in solution spray as feedstock. Multiphase nano structured coatings can also be made by powder spray if a multi phase powder is fed to the spray system.  In this presentation the general nature of deposits made by the three methods when attempting to make fine scale multiphase materials will be compared. Three example applications will be shown; 1)An application for which solution spray is the most successful method when thermal barrier coatings with segmentation cracks are needed, 2) An application in which a very fine two phase yittria magnesia microstructure is needed and suspension spray is the preferred method because it turned out that a suitable precursor was not found, and 3) An application involving a wear coating that was most successfully made by the plasma spray of a spray dried composite powder. By these examples it becomes clear that each approach - powder spray, suspension spray or solution spray, has its ideal application and that we are just beginning to discover how to choose between them.