Synthesis of CaF₂ Nanoparticles Coated by SiO₂ for Improved Al₂O₃/TiC Self-Lubricating Ceramic Composites

In order to reduce the influence of CaF₂ addition on the mechanical properties of self-lubricating ceramic tools, we applied a silicon dioxide (SiO₂) coating on calcium fluoride (CaF₂) nanoparticles through hydrolysis and condensation reactions using the tetraethoxysilane (TEOS) method. The powder was dried by the azeotropic method, so that it acquired a better dispersibility. The resulting composite powders were characterized using XRD (X-ray diffraction) and TEM (transmission electron microscopy), showing that the surface of CaF₂ was coated with a layer of uniform and compact SiO₂. SiO₂ shells with different thicknesses could be obtained by changing the amount of TEOS added, and the thickness of the SiO₂ shells could be controlled between 1.5 and 15 nm. At the same time, a ceramic material containing CaF₂ nanoparticles and CaF₂@SiO₂-coated nanoparticles was prepared. It had the best mechanical properties when CaF₂@SiO₂-coated nanoparticles were added; its flexural strength, fracture toughness, and hardness were 562 ± 28 MPa, 5.51 ± 0.26 MPa·m^1/2, and 15.26 ± 0.16 GPa, respectively. Compared with the ceramic tool containing CaF₂ nanoparticles, these mechanical properties were increased by 17.57%, 12.67%, and 4.88%, respectively. The addition of CaF₂@SiO₂-coated nanoparticles greatly improved the antifriction and wear resistance of the ceramic material, and the antifriction and wear resistance were balanced.