Development of T-ZnOw@Al2O3-incorporated low-temperature curing aluminium phosphate coating on Ti–6Al–4V alloy

Abstract In this study, a low-temperature curing aluminium phosphate coating was developed on a Ti–6Al–4V alloy by prolonged curing. The coating was composed of a phosphate binder, a T-ZnOw@Al2O3 core-shell curing agent, and an Al filler. The phosphate binder was produced by the reaction between phosphoric acid and aluminium hydroxide (P/Al = 3:1.4) and was mainly composed of Al(H2PO4)3. The T-ZnOw@Al2O3 curing agent was produced by sol-gel routes and its core-shell microstructure was verified by transmission electron microscopy. The curing mechanism of the coating in the presence of T-ZnOw@Al2O3 was investigated by obtaining its thermogravimetric differential thermal analysis curves and measuring its weight loss. The results showed that zinc oxide was released during curing. Complete curing was achieved at a relatively low temperature of 70 °C after prolonged curing for 120 min (P:Zn = 10:1). This rendered the phosphate coating suitable for practical applications. Though the peculiar tetrapod-like shape of T-ZnOw disappeared after being surrounded by Al2O3, the refining of the T-ZnOw@Al2O3 core-shell particles resulted in a decrease in the curing time and an increase in the hardness of the coating (higher than that of the ZnO@Al2O3 curing agent). The Al filler improved the surface roughness and oxidation resistance of the coating at 800 °C. A crack-free aluminium phosphate coating was achieved at the P:Zn:Al mass ratio of 10:1:0.05.