Researchers have made significant advances in the fabrication of hydroxyapatite/zirconia (HAP/ZrO2) nanocomposites by introducing a novel low-temperature mineralization sintering process (LMSP) at just 130 °C. This innovation aims to bolster the mechanical properties of hydroxyapatite, typically limited by its poor strength when used alone, and expands its applicability, particularly within the biomedical field.
Hydroxyapatite, the main inorganic component of human bones and teeth, possesses excellent biocompatibility and bioactivity; yet, its mechanical properties have restricted its use primarily to non-load-bearing applications such as implant coatings. The introduction of zirconium dioxide (ZrO2) is promising due to its durability and mechanical integrity. Traditionally, the challenge has been the requirement of high sintering temperatures for ZrO2, which can lead to detrimental phase changes within HAP.
The research team, funded by initiatives from Japan, executed experiments using LMSP, which allows ceramics to be densified at much lower temperatures than conventional methods. This process operates under the principle of using simulated body fluid (SBF) to encourage biomineralization, leading to the formation of apatite-like crystals at the particle boundaries, thereby significantly enhancing composite strength.
Results indicated exceptional mechanical properties, with samples containing 10 vol% ZrO2 reaching relative densities of 88.3 ± 1.1%. Key metrics such as Vickers hardness, fracture toughness, and Young’s modulus were reported at impressive levels: 3.68 ± 0.18 GPa for hardness, 1.11 ± 0.10 MPa·m1/2 for fracture toughness, and Young’s modulus of 83.91 ± 1.93 GPa, substantially higher than those of pure HAP.
The low sintering temperatures employed not only reduced energy consumption but also prevented the phase decomposition of HAP—a common issue when exposed to high temperatures. According to the researchers, "The low sintering temperature can help prevent the formation of unexpected phases due to phase decomposition observed in the high-temperature sintering process." This attribute is especially significant for biomedical implants where maintaining material integrity is imperative.
While enhancing mechanical properties was successfully achieved at 10 vol% ZrO2, the study noted degradation of these properties at higher concentrations, attributed to rising porosity and insufficient interaction between ZrO2 and SBF solutions. This suggests careful consideration is needed when optimizing the proportions of zirconia.
Overall, the study presents LMSP as not merely beneficial but necessary for advancing the capabilities of hydroxyapatite/zirconia composites. With mechanical properties achieved at low sintering temperatures, researchers are optimistic about the future applications of these composites, especially for dental and orthopedic implants, marking significant progress toward addressing the challenges posed by conventional high-temperature sintering methods.