The novel technology of chemically bonded composite sol-gel (CB-CSG) coatings has been developed at UBCeram. This essentially “ceramic paint” technology operates with CSG slurries formulated to contain a sol-gel “glue”, e.g. hydrated alumina sol, and inert filler, e.g. alpha alumina or yttria stabilized zirconia (YSZ), suspended in water or other solvent. The CSG is spray-deposited on metallic surfaces, and heat-treated at about 300°C to partially dehydrate the gel-derived hydroxides. CSG film is subsequently chemically bonded (CB) through reaction of the gel-derived active alumina with metal phosphates, such as aluminum phosphate. Effectively, the microstructure of such coating includes a porous network of refractory phosphate bonded alumina and/or YSZ. The vol% of porosity can be easily controlled in 5–60% range by adjusting spray conditions. Structural integrity (e.g. hardness and “abradability”) of the coatings can be controlled through the degree of chemical bonding treatment given. The spray-heat treat operations can be repeated to build porous films in excess of 1 mm thickness. The paper reviews the basics of CB-CSG technology. This research has focused on achieving the unique, strain-tolerant columnar microstructure of the coatings, somewhat resembling that of TBC by EB-PVD. Explanation of the process parameters controlling development of the columnar microstructure of the coatings is attempted.

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