Sol–gel synthesis, structure, sintering and properties of bioactive and inert nano-apatite–zirconia glass–ceramics

Abstract

We synthesized four glasses of the system 61.2SiO2–(24.3–x)CaO–4.5P2O5–10ZrO2–xK2O (x=0, 2, 4, 6mol%=Ca replaced by K) using a sol–gel route and compared their properties with a 68SiO2–27CaO–5P2O5 (mol%) Zr-free base glass. Their structure, sintering and crystallization behavior were investigated with the aim of converting the gel-glasses into dense glass–ceramics. Then, the in vitro bioactivity and mechanical properties of the optimized sintered samples were characterized. The structure of the gel-glasses was investigated by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR) and high-resolution transmission electron microscopy (HR-TEM). The sintering and crystallization kinetics of the glasses were studied by hot stage microscopy (HSM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The microstructures of the resulting glass–ceramics were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) coupled with energy dispersive spectroscopy (EDS). The apatite-forming ability of the sintered glass–ceramics in simulated body fluid (SBF) was investigated using FTIR spectroscopy and SEM. The three-point bending strength, Vickers microhardness and fracture toughness were also measured. Structural analysis by NMR and FTIR revealed that Zr acts as a glass former and K is a modifier, as expected. The K2O addition strongly improved the material׳s sinterability, e.g., 2mol% K2O decreased the optimum sintering temperature from 1300°C to 1050°C. Uniformly dispersed ZrO2 nano-crystals, with particle sizes of 25–55nm, were precipitated in the glass–ceramics. In vitro bioactivity tests confirmed that the K2O-free glass–ceramics (partially sintered at 1000°C) were bioactive and hydroxycarbonate apatite (HCA) grew on their surface after 24h in SBF. However, the >90% dense glass–ceramics with various contents of K2O exhibited low solubility and a much smaller tendency toward HCA formation. Improvement of some mechanical properties was observed for the sample containing 2mol% K2O, in which apatite and zirconia crystallized. The 3p-bending strength and fracture toughness of the 94% dense sample were approximately 140MPa and 2MPam1/2, respectively. We propose that crack deflection by the ZrO2 crystals and the presence of Zr ions in the residual glass network are prevalent for improving the materials׳ mechanical properties. Some potential applications, such as bioactive scaffolds, are suggested for these glass–ceramics.