Effects of the Concentration of Inorganic Binders and Optical Absorbers on the Phase Formation and Microstructure of Flash-Lamp-Annealed Chemically Bonded Phosphate Ceramic Composites

Abstract

A chemically bonded ceramic composite is synthesized using nanosized alumina powder and aluminum dihydrogen phosphate (Al(H₂PO₄)₃, ADP) as a binder with varying solid volumetric ratios from 50% to 75%. Photonic curing of such composites by flash lamp annealing (FLA) is of interest as a new route to additive manufacturing of ceramics or rapidly producing ceramic coatings. ADP undergoes a condensation reaction with Al₂O₃ around 300°C–350°C and forms an AlPO₄ compound that is thermally stable up to 1500°C due to strong P─O─Al bonds. Herein, an FLA system that can deliver tens of kilowatts per square centimeter of radiant energy is used to rapidly transform this ADP/Al₂O₃ mixture into this AlPO₄ ceramic phase. An ADP fraction of about 55 vol% results in the lowest porosity layers having the best layer cohesion. Absorbance of the photonic energy emitted by the FLA's broadband xenon lamp (400–800 nm) is also critical to rapidly transforming these layers because optical absorbance is needed to convert the photonic energy to thermal energy. Three different optical absorbers—graphite, black iron oxide, and liquid black organic ink—are investigated as optical absorbers. Adding these absorbers is found to lower the required photonic input for ADP-to-AlPO₄ conversion from ∼350 to ∼220 J/cm², making the process even more energy efficient.