Crystallinity evolution of spray pyrolyzed fluorapatite thin films by post-deposition treatment

Abstract

The stability of fluorapatite (FAP) thin films, which is required for bone ingrowth in FAP-coated bioimplants, improves with crystallinity. To investigate the crystallinity evolution of FAP thin films, they are deposited on titanium and alumina substrates through ultrasonic spray pyrolysis using different deposition arrangements. Two different solutions of pre-synthesized homemade FAP powder and chemical precursors are used for film deposition at different deposition variables. The films are next post-deposition treated in different environments and temperatures up to 1200°C. X-ray diffraction and electron microscopy are employed to study the films' morphology and crystallinity. A thermodynamic simulation software is used to investigate the stability of different phases by the construction of the Ellingham diagram, possible side reactions, and phase decomposition. The study reveals that the high-temperature post-deposition heat treatments do not enhance film crystallinity and CaHPO4 and Ca3(PO4)2 phases are detected. Treatment in argon or evacuated ampoules leads to the formation of titanium oxides due to the substrate's reaction with H2O, a decomposition product of CaHPO4. Furthermore, the oxygen partial pressure employed during the annealing process in evacuated ampoules (1.3 × 10−3 Pa) exceeds the equilibrium value of 1.3 × 10−20 Pa, which promotes substrate oxidation. The use of alumina as a substrate does not improve crystallinity, possibly due to material loss during annealing. Overall, the as-spray-deposited FAP films have better film crystallinity than that achieved with other techniques.