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·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.


Introduction
Calcium fluoride (CaF 2 ) crystals have good optical properties, mechanical properties, and chemical stability; therefore, calcium fluoride is widely used [1][2][3][4][5]. Calcium fluoride crystals are very important optical functional crystals, which have the advantages of wide light transmission range, high transmittance, low refractive index, low dispersion, and so on and have become an irreplaceable lens material in ultraviolet lithography objective lens systems [6][7][8][9]. At the same time, calcium fluoride has low shear strength and thermophysical and thermochemical stability at high temperature, so it is used in high-temperature solid lubrication [10][11][12][13][14]. At present, calcium fluoride has been applied in the field of self-lubricating ceramics allowing made some advances [15][16][17][18][19][20]. Wu et al. [17] prepared an Al 2 O 3 /TiC/CaF 2 multicomponent gradient self-lubricating ceramic composite by the hot-pressing method, showing that the addition of CaF 2 could improve the antifriction properties of the ceramic composite. In the work by Kong et al. [21], a ZrO 2 -MoS 2 -CaF 2 self-lubricating composite was prepared. was added, and ultrasonic dispersion and mechanical stirring were carried out for 20 min to uniformly disperse the calcium nitrate and ammonium fluoride. A calcium nitrate dispersion was slowly added into the ammonium fluoride dispersion through a constant-pressure separatory funnel, and ultrasonic stirring was continuously carried out during the process. After reacting for 30 min, the mixture was left standing for 2 h. The obtained product was centrifuged for 30 min at 4000 r/min, washed with distilled water for 3 times, and azeotropically dried to obtain a nano-calcium fluoride powder. Then, 1 g of self-made nano-CaF 2 powder was weighed, and 100 mL of absolute ethyl alcohol solution and a proper amount of dispersant polyvinylpyrrolidone (PVP) were added, followed by ultrasonic dispersion for 40 min and heating in a water bath under rapid stirring, while keeping the temperature between 35 and 45 • C. Distilled water (2.5 mL) was added to the above solution, and the pH was adjusted to 8.5 by adding an appropriate amount of ammonia water. To the above mixed solution, 1-4 mL of TEOS was slowly added dropwise, after which the mixture was continuously heated and rapidly stirred for 1 h. The obtained suspension was centrifuged at 6000 r/min for 25 min, then washed with anhydrous ethanol for 3 times. After cleaning, a wet gel was added into a 6:4 solution of n-butanol and distilled water, and after ultrasonic stirring for 30 min, the powder was azeotropically dried to obtain the CaF 2 @SiO 2 composite powder with nano-CaF 2 as core and SiO 2 as shell.

Preparation of the Self-Lubricating Ceramic Tool Materials
The purity of each raw particle was higher than 99.9%. The average particle size of each particle were as follows: Al 2 O 3 powder, 0.5 µm, TiC, 0.5 µm, MgO, 1 µm; the average size of the core-shell solid lubricant composite particles CaF 2 made in our own laboratory was 30-50 nm.
The Al 2 O 3 /TiC/CaF 2 @SiO 2 (ATCS) self-lubricating ceramic composite was prepared by the vacuum hot-pressing sintering technique. The sintering temperature was 1650 • C, the heating rate was 20 • C/min, the holding time was 20 min, and the hot-pressing pressure was 30 MPa. The volume ratio of commercially available, high-purity Al 2 O 3 to TiC was 7:3, while the volume fraction of CaF 2 @SiO 2 was 10%. For comparison, the Al 2 O 3 /TiC/CaF 2 (ATC) self-lubricating ceramic composite was prepared under the same conditions.

Performance Testing of Tool Materials
The resulting ceramic embryo body obtained was processed into a standard sample with a size of 3 mm × 4 mm × 30 mm to test the mechanical properties of the material. A bending strength test was performed by a three-point bending method with a span of 20 mm and a displacement loading speed of 0.5 mm/min. Vickers hardness was measured by a Hv-120 Vickers hardness tester with an indentation load of 196 N and dwell time of 15 s. Fracture toughness was measured by the indentation method, and fracture toughness was determined by indentation crack length [42].

Characterization
X-ray diffraction (XRD, D8-ADVANCE, Bruker AXS Co., Karlsruhe, Germany) was used for phase identification of the CaF 2 nanoparticles and CaF 2 @SiO 2 -coated nanoparticles. X-ray diffraction was carried out using Cu Kα radiation with 40 kV and 40 mA, and the samples were analyzed at room temperature over a 2θ range from 15 • to 80 • and at a scanning rate of 10 • /min. Morphology and crystallinity of the CaF 2 nanoparticles and CaF 2 @SiO 2 -coated nanoparticles were obtained by transmission electron microscopy (TEM, JEM-1400, JEOL, Tokyo, Japan). The fracture morphology of the ceramic tools was examined and analyzed using a field-emission scanning electron microscope (SEM, Regulus8220, HITACHI, Tokyo, Japan), along with an energy-dispersive spectroscope (EDS). Figure 1 shows the XRD patterns of the as-prepared CaF 2 @SiO 2 nanoparticles and pure CaF 2 nanoparticles. Figure 1a shows the XRD patterns of CaF 2 nanoparticles. All the discernible peaks were in good agreement with the data of pure cubic CaF 2 crystals (JCPDS NO.35-0816). The diffraction peak of CaF 2 crystals was narrow and sharp, which indicated that the prepared CaF 2 had high crystallinity, and no impurity peak was detected, indicating that the prepared CaF 2 had high purity. As shown in Figure 1b, the XRD patterns of CaF 2 @SiO 2 nanoparticles were the same as the XRD patterns of pure cubic CaF 2 crystals; only at about 2θ = 20 • -25 • , the pattern for CaF 2 @SiO 2 had a low and wide peak, attributed to a silica dioxide amorphous halo [37,41].  Figure 1 shows the XRD patterns of the as-prepared CaF2@SiO2 nanoparticles and pure CaF2 nanoparticles. Figure 1a shows the XRD patterns of CaF2 nanoparticles. All the discernible peaks were in good agreement with the data of pure cubic CaF2 crystals (JCPDS NO.35-0816). The diffraction peak of CaF2 crystals was narrow and sharp, which indicated that the prepared CaF2 had high crystallinity, and no impurity peak was detected, indicating that the prepared CaF2 had high purity. As shown in Figure 1b, the XRD patterns of CaF2@SiO2 nanoparticles were the same as the XRD patterns of pure cubic CaF2 crystals; only at about 2θ = 20°-25°, the pattern for CaF2@SiO2 had a low and wide peak, attributed to a silica dioxide amorphous halo [37,41]. The TEM images of the pure CaF2 and CaF2@SiO2 nanoparticles are shown in Figure 2. Figure 2a shows a transmission electron microscope image of pure CaF2. As shown in Figure 2a, the CaF2 nanoparticles had good dispersibility and an approximately round flake structure, and the average particle size of nano CaF2 was about 30-50 nm. Figure 2b shows a transmission electron microscope image of CaF2@SiO2 nanoparticles. The TEM micrograph clearly shows that the surface of the CaF2 nanoparticles was covered by a layer of amorphous SiO2, and the smooth edge of the CaF2 was tightly covered by amorphous SiO2. The coated powder had good dispersibility, and the average thickness of the amorphous silica coating was about 3.6 nm. The thickness of the SiO2 shell in the coated CaF2@SiO2 nanoparticles can be regulated and controlled; it can be changed by varying the amount of TEOS added. Figure 3 is a high-resolution transmission electron microscopy (HRTEM) image of CaF2@SiO2-coated nanoparticles. The HRTEM image shows that the lattice fringes were about 0.319 nm, which corresponds to the (111) orientation of CaF2. Amorphous SiO2 was evenly coated on the edge of calcium fluoride, and SiO2 and the CaF2 tightly combined. This shows that SiO2 was successfully coated on the surface of CaF2 nanoparticles, providing a uniform coating and a good coating effect. The TEM images of the pure CaF 2 and CaF 2 @SiO 2 nanoparticles are shown in Figure 2. Figure 2a shows a transmission electron microscope image of pure CaF 2 . As shown in Figure 2a, the CaF 2 nanoparticles had good dispersibility and an approximately round flake structure, and the average particle size of nano CaF 2 was about 30-50 nm. Figure 2b shows a transmission electron microscope image of CaF 2 @SiO 2 nanoparticles. The TEM micrograph clearly shows that the surface of the CaF 2 nanoparticles was covered by a layer of amorphous SiO 2 , and the smooth edge of the CaF 2 was tightly covered by amorphous SiO 2 . The coated powder had good dispersibility, and the average thickness of the amorphous silica coating was about 3.6 nm. The thickness of the SiO 2 shell in the coated CaF 2 @SiO 2 nanoparticles can be regulated and controlled; it can be changed by varying the amount of TEOS added. Figure 3 is a high-resolution transmission electron microscopy (HRTEM) image of CaF 2 @SiO 2 -coated nanoparticles. The HRTEM image shows that the lattice fringes were about 0.319 nm, which corresponds to the (111) orientation of CaF 2 . Amorphous SiO 2 was evenly coated on the edge of calcium fluoride, and SiO 2 and the CaF 2 tightly combined. This shows that SiO 2 was successfully coated on the surface of CaF 2 nanoparticles, providing a uniform coating and a good coating effect.

Effect of TESO Addition on Coating Thickness
TEM images of CaF2@SiO2-coated powder with different amounts of TEOS are shown in Figure  4. The amount of CaF2 was 1 g, the pH value was 8.5, and the amounts of TEOS were 1 mL, 2 mL, 3 mL, and 4 mL. The white framed images are partial enlarged views of the red selected areas. Figure  4a shows the powder obtained when 1 mL of TEOS was added; it can be seen from the figure that this TEOS amount resulted in less amorphous SiO2. At the edge of nano CaF2, the thickness of the SiO2 shell was very small, about 1.2 nm. When the amount of TEOS added was increased to 2 mL (Figure 4b), the thickness of the SiO2 shell coated on the nano-CaF2 increased to about 3.6 nm, but the coating effect was poor, and the thickness of the SiO2 shell was uneven. As shown in Figure 4c, when the amount of TEOS added was 3 mL, the thickness of the SiO2 shell increased to about 5.8 nm. At the same time, it can be seen that the amorphous SiO2 coated on CaF2 had uniform thickness and a good coating effect, but slight agglomeration occurred. When the amount of TEOS further increased to 4 mL, the thickness of the SiO2 shell reached about 13.3 nm, the coating thickness was relatively uniform, but agglomeration was more pronounced. The thickness of the SiO2 shell coated on CaF2 increased with the increase of the amount TEOS, but when the amount of TEOS added was too large, agglomeration occurred, whereas when the amount of TEOS was 3 mL, the best coating effect was obtained. SiO2 shells with different thicknesses could be obtained by changing the amount of TEOS added, and the thickness of the SiO2 shell could be controlled between 1.5 and 15 nm.

Effect of TESO Addition on Coating Thickness
TEM images of CaF2@SiO2-coated powder with different amounts of TEOS are shown in Figure  4. The amount of CaF2 was 1 g, the pH value was 8.5, and the amounts of TEOS were 1 mL, 2 mL, 3 mL, and 4 mL. The white framed images are partial enlarged views of the red selected areas. Figure  4a shows the powder obtained when 1 mL of TEOS was added; it can be seen from the figure that this TEOS amount resulted in less amorphous SiO2. At the edge of nano CaF2, the thickness of the SiO2 shell was very small, about 1.2 nm. When the amount of TEOS added was increased to 2 mL (Figure 4b), the thickness of the SiO2 shell coated on the nano-CaF2 increased to about 3.6 nm, but the coating effect was poor, and the thickness of the SiO2 shell was uneven. As shown in Figure 4c, when the amount of TEOS added was 3 mL, the thickness of the SiO2 shell increased to about 5.8 nm. At the same time, it can be seen that the amorphous SiO2 coated on CaF2 had uniform thickness and a good coating effect, but slight agglomeration occurred. When the amount of TEOS further increased to 4 mL, the thickness of the SiO2 shell reached about 13.3 nm, the coating thickness was relatively uniform, but agglomeration was more pronounced. The thickness of the SiO2 shell coated on CaF2 increased with the increase of the amount TEOS, but when the amount of TEOS added was too large, agglomeration occurred, whereas when the amount of TEOS was 3 mL, the best coating effect was obtained. SiO2 shells with different thicknesses could be obtained by changing the amount of TEOS added, and the thickness of the SiO2 shell could be controlled between 1.5 and 15 nm.

Effect of TESO Addition on Coating Thickness
TEM images of CaF 2 @SiO 2 -coated powder with different amounts of TEOS are shown in Figure 4. The amount of CaF 2 was 1 g, the pH value was 8.5, and the amounts of TEOS were 1 mL, 2 mL, 3 mL, and 4 mL. The white framed images are partial enlarged views of the red selected areas. Figure 4a shows the powder obtained when 1 mL of TEOS was added; it can be seen from the figure that this TEOS amount resulted in less amorphous SiO 2 . At the edge of nano CaF 2 , the thickness of the SiO 2 shell was very small, about 1.2 nm. When the amount of TEOS added was increased to 2 mL (Figure 4b), the thickness of the SiO 2 shell coated on the nano-CaF 2 increased to about 3.6 nm, but the coating effect was poor, and the thickness of the SiO 2 shell was uneven. As shown in Figure 4c, when the amount of TEOS added was 3 mL, the thickness of the SiO 2 shell increased to about 5.8 nm. At the same time, it can be seen that the amorphous SiO 2 coated on CaF 2 had uniform thickness and a good coating effect, but slight agglomeration occurred. When the amount of TEOS further increased to 4 mL, the thickness of the SiO 2 shell reached about 13.3 nm, the coating thickness was relatively uniform, but agglomeration was more pronounced. The thickness of the SiO 2 shell coated on CaF 2 increased with the increase of the amount TEOS, but when the amount of TEOS added was too large, agglomeration occurred, whereas when the amount of TEOS was 3 mL, the best coating effect was obtained. SiO 2 shells with different thicknesses could be obtained by changing the amount of TEOS added, and the thickness of the SiO 2 shell could be controlled between 1.5 and 15 nm. Nanomaterials 2019, 9, x FOR PEER REVIEW 6 of 11

Improvement in Mechanical Properties and Microstructurs
The scanning electron micrographs and EDS spectra of the fracture surfaces of the Al2O3/TiC/CaF2 ceramic composite are shown in Figure 5. EDS analysis results showed that the larger crystal grains in the figure 5 were alumina crystals. According to the distribution of the F element, it can be seen that a large amount of uniformly dispersed nano-calcium fluoride was distributed on the surface of the alumina crystal grains. From the SEM images of the fracture surfaces, it can be seen that fine white protrusions were uniformly distributed on the surface of the alumina grains. These white protrusions were self-made nano-calcium fluoride grains, which formed an in-crystal nanostructure. The fracture mode of the ceramic composites was mainly intergranular fracture, with a small amount of transgranular fracture.

Improvement in Mechanical Properties and Microstructurs
The scanning electron micrographs and EDS spectra of the fracture surfaces of the Al 2 O 3 /TiC/CaF 2 ceramic composite are shown in Figure 5. EDS analysis results showed that the larger crystal grains in the Figure 5 were alumina crystals. According to the distribution of the F element, it can be seen that a large amount of uniformly dispersed nano-calcium fluoride was distributed on the surface of the alumina crystal grains. From the SEM images of the fracture surfaces, it can be seen that fine white protrusions were uniformly distributed on the surface of the alumina grains. These white protrusions were self-made nano-calcium fluoride grains, which formed an in-crystal nanostructure. The fracture mode of the ceramic composites was mainly intergranular fracture, with a small amount of transgranular fracture.
The scanning electron micrographs and EDS spectra of the fracture surfaces of the Al 2 O 3 /TiC/CaF 2 @SiO 2 ceramic composite are shown in Figure 6. EDS analysis results showed that Si elements were mostly distributed at the grain boundaries between the right-hand grains. From the SEM images of the fracture surfaces, it can be seen that the nano-CaF 2 protrusions on the surface of the alumina grains decreased, while the alumina grains became fine and the grain boundaries between the grains became blurred. Transgranular fracture increased, and intergranular fracture occurred in the ceramic composite. The fracture mode of the tools was mainly transgranular fracture. Transgranular fracture consumes a large amount of fracture energy, which is helpful to improve the mechanical properties of tool materials, making them compact with less defects. The addition of the coating powder plays a role in refining the crystal grains of a ceramic composite, improving the mechanical properties of the ceramic composite, enhancing the interfacial bonding force of the ceramic matrix material, changing the main fracture mode of the cutter, and improving the mechanical properties of the ceramic composite. crystal grains in the figure 5 were alumina crystals. According to the distribution of the F element, it can be seen that a large amount of uniformly dispersed nano-calcium fluoride was distributed on the surface of the alumina crystal grains. From the SEM images of the fracture surfaces, it can be seen that fine white protrusions were uniformly distributed on the surface of the alumina grains. These white protrusions were self-made nano-calcium fluoride grains, which formed an in-crystal nanostructure. The fracture mode of the ceramic composites was mainly intergranular fracture, with a small amount of transgranular fracture.    Table 1 shows the mechanical properties of ceramic tools with CaF 2 nanoparticles and CaF 2 @SiO 2 -coated nanoparticles. Compared with the ceramic tool with CaF 2 nanoparticles, the ceramic tool with CaF 2 @SiO 2 -coated nanoparticles had a hardness increase of 4.88%, a flexural strength increase of 17.57%, a fracture toughness increase of 12.67%. The flexural strength of the ceramic tool was also greatly improved, which was due to the change of the main fracture mode in the ceramic tool material; the hardness and fracture toughness of the ceramic tool were also improved. The addition of CaF 2 @SiO 2 -coated nanoparticles greatly improved the antifriction and wear resistance of the ceramic tool material; the antifriction and wear resistance of the tool material were balanced.

Conclusions
In this paper, SiO 2 was successfully coated on the surface of CaF 2 nanoparticles to prepare a nano-powder with a core-shell structure. After adding CaF 2 nanoparticles and CaF 2 @SiO 2 -coated nanoparticles into an Al 2 O 3 /TiC ceramic matrix, the mechanical properties and the micro-morphology of ceramic tools were analyzed. The effects of adding CaF 2 @SiO 2 -coated nanoparticles on the micro-morphology and mechanical properties of the ceramic tools were compared with those of adding CaF 2 nanoparticles. The following conclusions were obtained.

1.
SiO 2 shells with different thicknesses could be obtained by changing the amount of TEOS added, and the thickness of the SiO 2 shell could be controlled between 1.5 and 15 nm. However, when the TEOS amount was too large, agglomeration occurred, whereas when the TEOS amount was 3 mL, the best coating effect was obtained.

2.
The ceramic tool had the best mechanical properties when CaF 2 @SiO 2 -coated nanoparticles were added. The flexural strength, the fracture toughness, and the 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 with the CaF 2 nanoparticles, the above performances were increased by 17.57%, 12.67% and 4.88%, respectively.

3.
Compared with the ceramic tool with CaF 2 nanoparticles, the ceramic tool with CaF 2 @SiO 2 -coated nanoparticles showed a great change in its microscopic morphology. The addition of the coated powder played a role in refining the crystal grains of the ceramic tool and, at the same time, increased its transgranular fracture, improving its performance.