Preparation and Performance of Al 2 O 3 / Ti(C,N)-Added ZrO 2 Whisker and NanoCoated CaF 2 @Al(OH) 3 Powder

: The Al 2 O 3 / Ti(C,N) ceramic material added micron ZrO 2 whisker and nano coated CaF 2 @Al(OH) 3 powder was fabricated. The micron ZrO 2 whisker was for the toughening and reinforcing phase and the nano coated CaF 2 @Al(OH) 3 powder was the lubricant. For obtaining a ceramic material with optimal comprehensive mechanical properties and friction properties, the inﬂuences of di ﬀ erent compositions of the ZrO 2 whisker and nano coated CaF 2 @Al(OH) 3 powder on the microstructure and mechanical properties were analyzed, respectively. The result demonstrated that as the addition of the ZrO 2 whisker was 6 vol% and the addition of the nano coated CaF 2 @Al(OH) 3 powder was 10 vol%, the optimal self-lubricating ceramic material had optimal mechanical properties. The hardness of the ceramic material was 16.72 GPa, the ﬂexural strength was 520 MPa and the fracture toughness reached 7.16 MPa · m 1 / 2 . The formation of the intragranular structure, whisker toughening and the phase transition of ZrO 2 were the main mechanisms.


Introduction
The ceramic materials can be generally used in various fields because of excellent chemical and physical properties [1][2][3]. Aluminum oxide is an ideal material due to its inherent high hardness and excellent thermal stability [4][5][6]. Currently, aluminum oxide-based ceramic materials are broadly used in cutting tool molds, sealing rings and various high temperature engine parts [7][8][9]. However, the low-fracture toughness and the brittleness contribute to the aluminum oxide-based ceramic risk of being easily chipped in the course of processing, which seriously influences the cutting performance of the ceramic tool material [10][11][12]. The self-lubricating ceramic tool materials with excellent mechanical properties and friction properties have always been people's pursuit goal.
To acquire the ceramic material with excellent mechanical properties, many scholars have studied gradient design and layered design [13][14][15]. Yang [16] prepared the gradient composite Al-7Si-5Cu/Al 2 O 3 . The flexural strength of the material increased significantly. Katsui [17] studied the deposition of SiC layers on SiO 2 and diamond powders. The result showed that the formation of the microstructure enhanced the mechanical properties. Dang [18] prepared mullite using Al 2 O 3 powders and coated SiO 2 @SiC powders. The mechanical properties were better than without whisker. The surface modification technology can effectively enhance the mechanical properties of the ceramic materials, and the preparation is becoming more and more mature, which has become the first choice of many scientists. μm and 10-20 μm, respectively) was raw material. MgO (1 μm, purity ≥ 99.9%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was used as a sintering aid. Polyethylene glycol (PEG 4000, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was the dispersant.
The Al2O3 and MgO powders, Ti(C,N) powders were added to the absolute ethyl alcohol with polyethylene glycol, respectively. Then, the dispersions were ultrasonically dispersed and mechanically stirred for 20-30 min. Mixing the Al2O3 dispersion, the Ti(C,N) dispersion and the dispersion containing the nano coated CaF2@Al(OH)3 powder, the mixed solution was ultrasonically dispersed and mechanically stirred for 10-30 min, thereafter the dispersion was poured into ball mill tank for ball milling. After 44 h of ball milling, the ZrO2 whisker was added to the ball mill tank. The prepared multiphase suspension was dried in the vacuum drying oven (DZF-6050, Shanghai, China), and then the multiphase suspension was sieved to obtain composite powder. The composite powder was put into a graphite sleeve for cold-pressing. After the hot pressing in the vacuum hot-pressing sinter (ZR1050, Jinan, China), the Al2O3/Ti(C,N)/ZrO2/CaF2@Al(OH)3 ceramic material was fabricated. The preparation process influenced the microstructure and further influenced properties of the material [34,35]. The parameters selected in this experiment were as follows: the sintering temperature was set to 1650 °C under 30 MPa, the soaking time was selected as 20 min and the heating rate of the preparation progress was set to 20 °C/min. Figure 1 shows the process flow chart of the composite ceramic material preparation.

Performance Testing of the Ceramic Material
The ceramic material after hot pressing was cuboid with the size of 3 mm × 4 mm × 35 mm. The surface roughness Ra of the ceramic material was less than 0.1 μm. The instrument used for the hardness test of the ceramic tool material was the Hv-120 Vickers hardness tester (Hv-120, Jinan, China), which measured with the Vickers indentation method. The indentation load was set to 196 N for 15 s. The indentation of the ceramic material was observed and measured by optical microscope. Then, the length of the diagonal of the two indentations was recorded. The hardness of the ceramic material could be calculated by the function:

Performance Testing of the Ceramic Material
The ceramic material after hot pressing was cuboid with the size of 3 mm × 4 mm × 35 mm. The surface roughness Ra of the ceramic material was less than 0.1 µm. The instrument used for the hardness test of the ceramic tool material was the Hv-120 Vickers hardness tester (Hv-120, Jinan, China), which measured with the Vickers indentation method. The indentation load was set to 196 N for 15 s. The indentation of the ceramic material was observed and measured by optical microscope. Then, the length of the diagonal of the two indentations was recorded. The hardness of the ceramic material could be calculated by the function: where H V is the hardness value (GPa); P is the indentation load (N); and 2a is the arithmetic average value of the diagonal lengths of the two indentations produced. The flexural strength of the ceramic material was measured by the three-point bending method. The span value was 20 mm and the displacement loading speed was 0.5 mm/min. The flexural strength of the ceramic material could be calculated by the function: where σ f is the flexural strength (MPa); P is the maximum load (N) value loaded when the sample is broken; L is the distance (mm) between the two supports supporting the sample; and b and h are the width (mm) and height (mm) of the sample, respectively. The fracture toughness was also measured by the indentation method. The instrument used for fracture toughness test of the ceramic material was the Hv-120 Vickers hardness tester (Hv-120, Jinan, China). The fracture toughness of the ceramic material could be calculated by the function: where K IC is the fracture toughness (MPa·m 1/2 ); H V is the hardness value (GPa) measured by the Vickers indentation method; a is the half length (mm) of the diagonal lengths; and c is the half length (mm) of the crack diagonal.
Density test of the ceramic material was performed by the drainage method. The dry weight M 1 , submerged weight M 2 and wet weight M 3 of the sample were weighed by a precision electronic balance (ME105DU, Jinan, China). The density of the ceramic material could be calculated by the function: where ρ s is the density (g/cm 3 ) of the sample; ρ 0 is the density (g/cm 3 ) of the distilled water; M 1 is the weight (g) when the sample is dried; M 2 is the submerged weight (g) in the liquid; and M 3 is the wet weight (g) measured in the air after the sample is sufficiently absorbed. The relative density of the ceramic material could be calculated by the function: where ρ is the relative density of the ceramic material; ρ s is the density (g/cm 3 ) of the sample; and ρ t is the theoretical density (g/cm 3 ).
To decrease the measurement error and ensure the accuracy of the measurement, every ceramic material was tested 5 times. The arithmetic average value was the measured value of the ceramic material.

XRD Phase Composition Diagram of the Ceramic Material
The XRD (XRD is the abbreviation of X-Ray Diffraction) diffraction analysis of the ceramic material with 6 vol% ZrO 2 whisker and 10 vol% nano coated CaF 2 @Al(OH) 3 powder is shown in Figure 2.
In the ceramic material, the phase analysis indicates that the predominant phases for the composites are Ti(C,N) and Al 2 O 3 . Before the experiment, the ZrO 2 whisker exists in monoclinic phase. From Figure 2, it can be seen that ZrO 2 exists mainly in the form of t-ZrO 2 in the ceramic material. This indicates that the phase transition took place during the sintering. The ZrO 2 whisker underwent a phase change. The introduction of nano coated powder does not have a significant influence on it, which provides the fundamental condition for the phase transition toughening. The presence of the characteristic peak of CaF 2 is clearly observed. In this paper, the addition of sintering aid MgO is 0.5 vol%. The characteristic peak of MgO is not observed, because the addition amount of MgO is too small. The chemical composition of the various components constituting the composite ceramic has a good chemical compatibility, and an obvious chemical reaction does not take place.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 12 presence of the characteristic peak of CaF2 is clearly observed. In this paper, the addition of sintering aid MgO is 0.5 vol%. The characteristic peak of MgO is not observed, because the addition amount of MgO is too small. The chemical composition of the various components constituting the composite ceramic has a good chemical compatibility, and an obvious chemical reaction does not take place.

Influence of ZrO2 Whisker Addition on Al2O3/Ti(C,N) Ceramic Material
Figure 3a-c represents the scanning electron microscope photographs of the fracture surfaces of the Al2O3/Ti(C,N) ceramic material with 3 vol%, 6 vol% and 9 vol% ZrO2 whisker, respectively. It can be observed that the grains are coarser in Figure 3a or Figure 3c. However, the grains in Figure 3b are finer than those in Figure 3a and the uniformity of Figure 3b is improved. The addition of the ZrO2 whisker has an influence on the grain refinement of the ceramic material, and the ZrO2 whisker can suppress the abnormal growth of the crystal grain. When the addition of the ZrO2 whisker is 9 vol%, more pores can be seen in the cross section. A possible reason for this is that under the process conditions selected by the experiment, the dispersion of the ceramic material with 9 vol% ZrO2 whisker may not achieve the desired effect, contributing to the bridging or agglomeration of the whisker. The occurrence of pores leads to a decrease in the relative density. Due to the contrast, less pores can be seen in the cross section when the additive amount ZrO2 whisker is low. The existence of pores influences the densification of the ceramic material and thus reduces the mechanical properties of the ceramic material. Whisker reunion can be found in Figure 3c mark 1, and the whisker section can also be observed by magnification, as showed in Figure 3d.

Influence of ZrO 2 Whisker Addition on Al 2 O 3 /Ti(C,N) Ceramic Material
Figure 3a-c represents the scanning electron microscope photographs of the fracture surfaces of the Al 2 O 3 /Ti(C,N) ceramic material with 3 vol%, 6 vol% and 9 vol% ZrO 2 whisker, respectively. It can be observed that the grains are coarser in Figure 3a or Figure 3c. However, the grains in Figure 3b are finer than those in Figure 3a and the uniformity of Figure 3b is improved. The addition of the ZrO 2 whisker has an influence on the grain refinement of the ceramic material, and the ZrO 2 whisker can suppress the abnormal growth of the crystal grain. When the addition of the ZrO 2 whisker is 9 vol%, more pores can be seen in the cross section. A possible reason for this is that under the process conditions selected by the experiment, the dispersion of the ceramic material with 9 vol% ZrO 2 whisker may not achieve the desired effect, contributing to the bridging or agglomeration of the whisker. The occurrence of pores leads to a decrease in the relative density. Due to the contrast, less pores can be seen in the cross section when the additive amount ZrO 2 whisker is low. The existence of pores influences the densification of the ceramic material and thus reduces the mechanical properties of the ceramic material. Whisker reunion can be found in Figure 3c mark 1, and the whisker section can also be observed by magnification, as showed in Figure 3d.
The consequences of adding 0, 3 vol%, 6 vol% and 9 vol% ZrO 2 whisker on the mechanical properties of the Al 2 O 3 /Ti(C,N) ceramic material are shown in Figure 4. The ceramic material without ZrO 2 whisker has a high hardness of 20.47 GPa. With the increasing addition of ZrO 2 whisker, the hardness is prone to decrease. When the addition of ZrO 2 whisker increases, the flexural strength of the ceramic material increases significantly from 555 to 584 MPa. Whereas when the additive amount of ZrO 2 is more than 6 vol%, the flexural strength of the ceramic material shows a decreasing trend. The ceramic material obtains maximum flexural strength when the ZrO 2 whisker addition is 6 vol%. With the increase in ZrO 2 whisker addition, the fracture toughness also shows an increasing trend. It reveals that the introduction of ZrO 2 whisker can enhance the fracture toughness of the material. The ceramic material without ZrO 2 whisker has the highest relative density. With the addition of ZrO 2 whisker increasing, the relative density decreases first and afterward increases. As can be seen from the analysis in SEM(Scanning electron microscope) morphology the presence of pores reduces the density of the material.
suppress the abnormal growth of the crystal grain. When the addition of the ZrO2 whisker is 9 vol%, more pores can be seen in the cross section. A possible reason for this is that under the process conditions selected by the experiment, the dispersion of the ceramic material with 9 vol% ZrO2 whisker may not achieve the desired effect, contributing to the bridging or agglomeration of the whisker. The occurrence of pores leads to a decrease in the relative density. Due to the contrast, less pores can be seen in the cross section when the additive amount ZrO2 whisker is low. The existence of pores influences the densification of the ceramic material and thus reduces the mechanical properties of the ceramic material. Whisker reunion can be found in Figure 3c mark 1, and the whisker section can also be observed by magnification, as showed in Figure 3d.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 12 The consequences of adding 0, 3 vol%, 6 vol% and 9 vol% ZrO2 whisker on the mechanical properties of the Al2O3/Ti(C,N) ceramic material are shown in Figure 4. The ceramic material without ZrO2 whisker has a high hardness of 20.47 GPa. With the increasing addition of ZrO2 whisker, the hardness is prone to decrease. When the addition of ZrO2 whisker increases, the flexural strength of the ceramic material increases significantly from 555 to 584 MPa. Whereas when the additive amount of ZrO2 is more than 6 vol%, the flexural strength of the ceramic material shows a decreasing trend. The ceramic material obtains maximum flexural strength when the ZrO2 whisker addition is 6 vol%. With the increase in ZrO2 whisker addition, the fracture toughness also shows an increasing trend. It reveals that the introduction of ZrO2 whisker can enhance the fracture toughness of the material. The ceramic material without ZrO2 whisker has the highest relative density. With the addition of ZrO2 whisker increasing, the relative density decreases first and afterward increases. As can be seen from the analysis in SEM(Scanning electron microscope) morphology the presence of pores reduces the density of the material.   Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 12 The consequences of adding 0, 3 vol%, 6 vol% and 9 vol% ZrO2 whisker on the mechanical properties of the Al2O3/Ti(C,N) ceramic material are shown in Figure 4. The ceramic material without ZrO2 whisker has a high hardness of 20.47 GPa. With the increasing addition of ZrO2 whisker, the hardness is prone to decrease. When the addition of ZrO2 whisker increases, the flexural strength of the ceramic material increases significantly from 555 to 584 MPa. Whereas when the additive amount of ZrO2 is more than 6 vol%, the flexural strength of the ceramic material shows a decreasing trend. The ceramic material obtains maximum flexural strength when the ZrO2 whisker addition is 6 vol%. With the increase in ZrO2 whisker addition, the fracture toughness also shows an increasing trend. It reveals that the introduction of ZrO2 whisker can enhance the fracture toughness of the material. The ceramic material without ZrO2 whisker has the highest relative density. With the addition of ZrO2 whisker increasing, the relative density decreases first and afterward increases. As can be seen from the analysis in SEM(Scanning electron microscope) morphology the presence of pores reduces the density of the material.

Influence of Nano Coated CaF2@Al(OH)3 Powder on Al2O3/Ti(C,N) Ceramic Material
The nano powder and nano coated powder have a crucial influence on the microstructure and mechanical properties of the ceramic material. The cross sections of the two ceramic materials were observed by scanning electron microscope (SEM). The results are displayed in Figure 5. Figure 5a is a SEM morphology photograph added with 10 vol% nano CaF2 powder. It can be found that a few grains have an abnormal growth and the grain distribution is not uniform. Figure 5b is a SEM morphology paragraph added with 10 vol% nano coated CaF2@Al(OH)3 powder. Compared to the material added with the same components, the nano CaF2 powder, the grain distribution is uniform

Influence of Nano Coated CaF 2 @Al(OH) 3 Powder on Al 2 O 3 /Ti(C,N) Ceramic Material
The nano powder and nano coated powder have a crucial influence on the microstructure and mechanical properties of the ceramic material. The cross sections of the two ceramic materials were observed by scanning electron microscope (SEM). The results are displayed in Figure 5. Figure 5a is a SEM morphology photograph added with 10 vol% nano CaF 2 powder. It can be found that a few grains have an abnormal growth and the grain distribution is not uniform. Figure 5b is a SEM morphology paragraph added with 10 vol% nano coated CaF 2 @Al(OH) 3 powder. Compared to the material added with the same components, the nano CaF 2 powder, the grain distribution is uniform and the density of the material is superior to the former. In Figure 5b, the material of the surface coating is nano coated CaF 2 @Al(OH) 3 powder. Moreover, the nano coated CaF 2 @Al(OH) 3 powder is well combined with the ceramic matrix. Compared with the added nano CaF 2 powder, there are many intragranular structures in the ceramic material. The intragranular structures have a good effect on enhancing the mechanical properties of ceramic material [36]. In the sintering process of the material, with the growth of crystal grains, the nano powders enter the crystal with the motion of boundaries of particles. The matrix grains merge and grow, forming the intragranular structures. The nano CaF 2 can be considered as completely entering the crystal. From Figure 5b, the fracture mode is mainly intergranular fracture, and partly transgranular fracture, which can also be found. The transgranular fracture consumes quantities of fracture energy, which is conducive to enhance mechanical properties. This is also one of the main reasons for the improvement of the mechanical properties of the prepared ceramic material. Nano coated powder contributes to the dispersion of CaF 2 in ceramic matrix material. As it can be found from Figure 5b, CaF 2 is more evenly distributed in the matrix material.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 12 and the density of the material is superior to the former. In Figure 5b, the material of the surface coating is nano coated CaF2@Al(OH)3 powder. Moreover, the nano coated CaF2@Al(OH)3 powder is well combined with the ceramic matrix. Compared with the added nano CaF2 powder, there are many intragranular structures in the ceramic material. The intragranular structures have a good effect on enhancing the mechanical properties of ceramic material [36]. In the sintering process of the material, with the growth of crystal grains, the nano powders enter the crystal with the motion of boundaries of particles. The matrix grains merge and grow, forming the intragranular structures. The nano CaF2 can be considered as completely entering the crystal. From Figure 5b, the fracture mode is mainly intergranular fracture, and partly transgranular fracture, which can also be found. The transgranular fracture consumes quantities of fracture energy, which is conducive to enhance mechanical properties. This is also one of the main reasons for the improvement of the mechanical properties of the prepared ceramic material. Nano coated powder contributes to the dispersion of CaF2 in ceramic matrix material. As it can be found from Figure 5b, CaF2 is more evenly distributed in the matrix material. The results of adding 5 vol%, 10 vol% and 15 vol% nano coated CaF2@Al(OH)3 powder on the mechanical properties of the Al2O3/Ti(C,N) ceramic material are displayed in Figure 6. From Figure  6a, with the increasing addition of CaF2@Al(OH)3, the hardness is prone to decrease. The main reason is the low mechanical properties of the CaF2. The increase in lubricant addition inevitably leads to a decrease in the hardness and other properties. The flexural strength of the ceramic material first increases and afterward decreases. When the addition of nano coated CaF2@Al(OH)3 powder is 10 vol%, the flexural strength is up to 471 MPa. From Figure 6b, with the increasing addition of nano coated CaF2@Al(OH)3 powder, the fracture toughness is increased from the initial 6.50 MPa·m 1/2 to 6.60 MPa·m 1/2 . The main reason may be that the presence of massive nano coated CaF2@Al(OH)3 powder enhances the material's fracture toughness. The relative density of the ceramic material shows a trend of increasing first and afterward decreasing. When the addition of the CaF2@Al(OH)3 powder is 10 vol%, excellent comprehensive mechanical properties are obtained. The results of adding 5 vol%, 10 vol% and 15 vol% nano coated CaF 2 @Al(OH) 3 powder on the mechanical properties of the Al 2 O 3 /Ti(C,N) ceramic material are displayed in Figure 6. From Figure 6a, with the increasing addition of CaF 2 @Al(OH) 3 , the hardness is prone to decrease. The main reason is the low mechanical properties of the CaF 2 . The increase in lubricant addition inevitably leads to a decrease in the hardness and other properties. The flexural strength of the ceramic material first increases and afterward decreases. When the addition of nano coated CaF 2 @Al(OH) 3 powder is 10 vol%, the flexural strength is up to 471 MPa. From Figure 6b, with the increasing addition of nano coated CaF 2 @Al(OH) 3 powder, the fracture toughness is increased from the initial 6.50 MPa·m 1/2 to 6.60 MPa·m 1/2 . The main reason may be that the presence of massive nano coated CaF 2 @Al(OH) 3 powder enhances the material's fracture toughness. The relative density of the ceramic material shows a trend of increasing first and afterward decreasing. When the addition of the CaF 2 @Al(OH) 3 powder is 10 vol%, excellent comprehensive mechanical properties are obtained. vol%, the flexural strength is up to 471 MPa. From Figure 6b, with the increasing addition of nano coated CaF2@Al(OH)3 powder, the fracture toughness is increased from the initial 6.50 MPa·m 1/2 to 6.60 MPa·m 1/2 . The main reason may be that the presence of massive nano coated CaF2@Al(OH)3 powder enhances the material's fracture toughness. The relative density of the ceramic material shows a trend of increasing first and afterward decreasing. When the addition of the CaF2@Al(OH)3 powder is 10 vol%, excellent comprehensive mechanical properties are obtained.

Mechanism Analysis of Co-Modification of Micron ZrO 2 Whisker and Nano Coated CaF 2 @Al(OH) 3 Powder
From the analysis above, the addition of micron ZrO 2 whisker and nano coated CaF 2 @Al(OH) 3 powder have a significant influence on the ceramic material. The Al 2 O 3 /Ti(C,N) ceramic material added 6 vol% ZrO 2 whisker and 10 vol% nano coated CaF 2 @Al(OH) 3 powder was fabricated. The performance of the ceramic material was measured. The hardness of the Al 2 O 3 /Ti(C,N)/ZrO 2 /CaF 2 @Al(OH) 3 ceramic material is 16.72 GPa, the flexural strength is 520 MPa and the fracture toughness is up to 7.16 MPa·m 1/2 . For the convenience of analysis, the sintering diagram of the material is drawn in Figure 7, and the micro-morphology of the ceramic material after sintering is observed.

Mechanism Analysis of Co-Modification of Micron ZrO2 Whisker and Nano Coated CaF2@Al(OH)3 Powder
From the analysis above, the addition of micron ZrO2 whisker and nano coated CaF2@Al(OH)3 powder have a significant influence on the ceramic material. The Al2O3/Ti(C,N) ceramic material added 6 vol% ZrO2 whisker and 10 vol% nano coated CaF2@Al(OH)3 powder was fabricated. The performance of the ceramic material was measured. The hardness of the Al2O3/Ti(C,N)/ZrO2/CaF2@Al(OH)3 ceramic material is 16.72 GPa, the flexural strength is 520 MPa and the fracture toughness is up to 7.16 MPa·m 1/2 . For the convenience of analysis, the sintering diagram of the material is drawn in Figure 7, and the micro-morphology of the ceramic material after sintering is observed. In the sintering course of the Al2O3/Ti(C,N)/ZrO2/CaF2@Al(OH)3 ceramic material, with the development of temperature and soaking time, the crystal grains grow gradually. The matrix powders accommodated by ball milling will have the situation that small powders are gradually absorbed by large powders and the number of powders is continuously reduced. The growth of crystal grain rests with the motion of boundaries of particles. The boundaries in the matrix tend to migrate to the center, as indicated by mark 1 in Figure 7. Due to the shell material (Al2O3) of the lubricant being the same as the matrix powders (Al2O3) of the ceramic material, the nano powders are dispersed into the interior of the matrix material during the sintering of the ceramic material. With the intergranular nanostructure of the nano CaF2 powders dispersed inside, the surface coating design avoids powder agglomeration growth.
After hot pressing, the nano powder exists inside the matrix crystal, and the matrix and nano powders form the intragranular structure. The related studies have shown that the intragranular structure can toughen the ceramic material [37,38]. The nano powder promotes the generation of In the sintering course of the Al 2 O 3 /Ti(C,N)/ZrO 2 /CaF 2 @Al(OH) 3 ceramic material, with the development of temperature and soaking time, the crystal grains grow gradually. The matrix powders accommodated by ball milling will have the situation that small powders are gradually absorbed by large powders and the number of powders is continuously reduced. The growth of crystal grain rests with the motion of boundaries of particles. The boundaries in the matrix tend to migrate to the center, as indicated by mark 1 in Figure 7. Due to the shell material (Al 2 O 3 ) of the lubricant being the same as the matrix powders (Al 2 O 3 ) of the ceramic material, the nano powders are dispersed into the interior of the matrix material during the sintering of the ceramic material. With the intergranular nanostructure of the nano CaF 2 powders dispersed inside, the surface coating design avoids powder agglomeration growth.
After hot pressing, the nano powder exists inside the matrix crystal, and the matrix and nano powders form the intragranular structure. The related studies have shown that the intragranular structure can toughen the ceramic material [37,38]. The nano powder promotes the generation of more intragranular structures, which enhances the mechanical properties of the ceramic material. As shown in Figure 7, the nano powders form intragranular structures. The appearance of this intragranular structure is one of the main reasons for enhancing the mechanical properties of the ceramic material prepared in this paper. The ZrO 2 whiskers have a unique phase transition effect. In the sintering process, when the temperature reaches 1170 • C, the m-ZrO 2 is completely converted into the t-ZrO 2 . In the cooling process, when the temperature is less than 950 • C, the matrix materials have the binding effect on the t-ZrO 2 , which hinders the conversion of the t-ZrO 2 into the m-ZrO 2 and enables the t-ZrO 2 to be preserved at room temperature. The toughening effect of the phase transition generated by the ZrO 2 is mainly attributed to the crack growth being inhibited by the phase transition of t-ZrO 2 . The existence of the t-ZrO 2 in the material is the necessary condition for phase transformation toughening. This makes up for the natural defects caused by the azimuth angle of the dispersion whisker reinforced the ceramic materials. This synergistic effect makes the ZrO 2 whisker-toughened ceramic material obviously different from the ceramic material toughened by adding the dispersion whisker alone. As indicated by the mark 2 partially ZrO 2 whiskers after hot pressing are distributed in intercrystalline, which provides the conditions for whisker toughening in the process of crack propagation [39]. The toughening mechanisms of the ZrO 2 whisker in the ceramic material are the whisker bridging and the crack deflection. During the whisker bridging and the crack deflection, the energy consumption occurs [40], which is conducive to enhancing the fracture toughness of the ceramic material.

Conclusions
The ZrO 2 whisker and nano coated CaF 2 @Al(OH) 3 powder were added to the Al 2 O 3 /Ti(C,N) self-lubricating ceramic material simultaneously.
The modification of the Al 2 O 3 /Ti(C,N)/ZrO 2 /CaF 2 @Al(OH) 3 ceramic tool material was completed by using two different toughening mechanisms.
(1). The ZrO 2 whisker with 0, 3 vol%, 6 vol% and 9 vol% were added to the Al 2 O 3 /Ti(C,N) ceramic material. The result revealed that when the additive amount of the ZrO 2 whisker was 6 vol%, the hardness, the flexural strength and the fracture toughness values were 19.1 GPa, 584 MPa and 6.61 MPa·m 1/2 , respectively. Whisker toughening and the phase transition toughening of the ZrO 2 enhanced the mechanical properties of the ceramic material. (2). The addition of nano coated CaF 2 @Al(OH) 3 powder also had a significant influence on the mechanical properties of the ceramic material. The intragranular structures played a good role in enhancing the mechanical properties of the ceramic material. When the nano coated CaF 2 @Al(OH) 3 powder addition was 10 vol%, the mechanical properties of the ceramic material was the best. The hardness, flexural strength and fracture toughness values of the prepared ceramic material were 18.58 GPa, 471 MPa and 6.50 MPa·m 1/2 , respectively. (3). The ZrO 2 whisker of 6 vol% and nano coated CaF 2 @Al(OH) 3 powder of 10 vol% were simultaneously added to the Al 2 O 3 /Ti(C,N) ceramic material. The hardness of the Al 2 O 3 /Ti(C,N)/ZrO 2 /CaF 2 @Al(OH) 3 ceramic material was 16.72 GPa, the flexural strength was 520 MPa, and the fracture toughness reached 7.16 MPa·m 1/2 . The fracture toughness of the Al 2 O 3 /Ti(C,N)/ZrO 2 /CaF 2 @Al(OH) 3 self-lubricating ceramic tool material was better than the added ZrO 2 whisker or added nano coated CaF 2 @Al(OH) 3 powder of the Al 2 O 3 /Ti(C,N) ceramic material, separately. The formation of intragranular structure, whisker toughening and phase transition of ZrO 2 were the main mechanisms.