Abstract
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·m1/2, 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.
Highlights
Calcium fluoride (CaF2) crystals have good optical properties, mechanical properties, and chemical stability; calcium fluoride is widely used [1,2,3,4,5]
These white protrusions were self-made nano-calcium fluoride grains, which formed an in-crystal nNaannoomsattreuriacltsu2r0e1.9,T9h, 1e52fr2acture mode of the ceramic composites was mainly intergranular fracture,7woift1h1 a small amount of transgranular fracture
Compared with the ceramic tool with CaF2 nanoparticles, the ceramic tool with CaF2@SiO2-coated nanoparticles had a hardness increase of 4.88%, a flexural strength increase of 17.57%, a fracture toughness increase of 12.67%
Summary
Calcium fluoride (CaF2) crystals have good optical properties, mechanical properties, and chemical stability; calcium fluoride is widely used [1,2,3,4,5]. The addition of calcium fluoride to self-lubricating ceramic materials will improve the wear resistance of the materials, while, on the other hand, it will reduce the mechanical properties of the materials and the overall reliability of the ceramic tool. Wu et al [42] prepared h-BN@Ni powders, which, compared to h-BN powders, greatly improved the mechanical properties of self-lubricating tools. In self-lubricating ceramic tools, the direct addition of calcium fluoride will significantly reduce the mechanical properties of the cutting tools, because the mechanical properties of calcium fluoride are relatively low [17,19]. Core–shell coating of calcium fluoride has been used to maintain simultaneously high mechanical properties and lubricity of the cutting tools [45]. The mechanical properties and the wear resistance of the ceramic tool were greatly improved, and the ceramic tool had antifriction properties
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