Plasma Spraying of CaCO3 Coatings from Oyster and Mussel Shell

Abstract

The aim of this research was to establish whether the shell of green-lipped mussels and Pacific oysters could be plasma sprayed to promote and accelerate the establishment of filter feeder colonies. While CaCO3 degrades before melting under equilibrium conditions, the hypothesis considered in this work was that the high-temperature, highly nonequilibrium conditions experienced by powders during plasma spraying may be sufficient to impart enough thermal energy into the powder, at a fast enough rate, that the CaCO3 phase would melt before undergoing total thermal degradation. Sprayable mussel and oyster shell powder was manufactured by crushing and milling. A range of plasma spray parameters incorporating a wide operating window of plasma power and gas velocity were trialed. Coatings were initially formed from both powders under all plasma spray conditions used. However, the samples degraded into powdery debris with time due to the reaction of CaO with water in the air to form Ca(OH)2. No evidence of CaCO3 melting was found, indicating that it was not possible to form a coating by thermal spraying. The mechanism of inflight particle phase degradation is presented for both powders, along with proposed mechanism of initial coating formation due to melting of CaO.