The effect of sintering dwell time on the physicochemical properties and hardness of hydroxyapatite with insights from ab initio calculations

Using a low-compaction protocol, the optimum sintering dwell time for fabricating hydroxyapatite (HA) scaffolds was investigated. HA was synthesized through a facile direct thermal conversion approach using a calcination temperature of 900 °C at 2, 4, and 6 h dwell times. The structure and composition of the powders were determined by X-ray diffraction (XRD), scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM/EDX), and Fourier transform infrared (FTIR) analysis. In addition, porosity measurements were also conducted. Complementarily, computational simulations were conducted from first principles using density functional theory (DFT) with four different exchange-correlation functionals to calculate the structural properties and hardness of the bulk HA crystal. These calculations were done to identify the exchange-correlation functionals that closely agree with the experimental data in this study. The results revealed that with increasing sintering dwell time, the crystallite sizes of the HA powder decreased slightly from 23.62 nm to 23.37 nm, then increased to 34.15 nm. The microhardness of the samples, pre-immersion in phosphate buffer saline (PBS) for 2, 4, and 6 h were 0.38, 0.42, and 0.50 GPa, respectively, whereas post-immersion, the values of the microhardness reached 0.32, 0.48 and 0.54 GPa, respectively. The highest porosity values were 59.3% and 39.3%, observed for samples sintered at 6 h dwell times using the two methods, indicating more open pores. Conclusively, longer sintering dwell time corresponded to better mechanical properties both pre- and post-immersion in simulated body fluid. It was observed that the results from PBE-D3-BJ and optB88-vdW functionals agree better with experimental data for the lattice constants and the values are relatively close to the microhardness obtained experimentally. The reasons for a significant deviation of the microhardness values from the computational and experimental standpoint are discussed herein.