Effects of n-Al 2 O 3 and µ -TiCN on Microstructure and Mechanical Properties of Al 2 O 3 Composite Ceramics Manufactured by Material Extrusion and Photo-Polymerization Combined Process

: Alumina (Al 2 O 3 ) composite ceramics with different composition ratio and particle-size distribution were fabricated by the material extrusion and photo-polymerization combined process (MEX-PPM) based on additive-manufacturing (AM) technology in our previous work. These particles were nanosized Al 2 O 3 (n-Al 2 O 3 ), micron-sized TiCN ( µ -TiCN) and Al 2 O 3 . Effects of n-Al 2 O 3 and µ -TiCN on Al 2 O 3 composite ceramics were investigated by characterizing the volume density, EDS spectrum, mechanical properties and microstructure of the prepared samples. It was found that n-Al 2 O 3 had a signiﬁcant effect on the hardness of Al 2 O 3 composite ceramics, µ -TiCN, with excellent performance in density, ﬂexural strength and fracture toughness. The Al 2 O 3 composite ceramics with optimum contents of 10 wt % n-Al 2 O 3 and 30 wt % µ -TiCN showed good microstructure and mechanical properties. Their porosity and volume density were at 4.073% and 4.177 g/cm 3 , respectively. Their hardness, ﬂexural strength and fracture toughness were at 16.592 GPa, 592.875 MPa and 6.308 MPa/mm 2 . The ﬂexural strength of the ceramics was signiﬁcantly higher than that of Al 2 O 3 ceramics prepared by SLA in document (178.84 ± 17.66 MPa), which had great potential in high-pressure strength structure.


Introduction
Alumina (Al 2 O 3 ) ceramic has been widely used in cutting tools [1,2], aircraft engines, integrated circuits [3] and medical human joints [4,5] due to its excellent mechanical properties, chemical stability and good economy advantages, etc. For ceramic parts with complex, multimaterial structures and high forming accuracy [6,7], the traditional forming technology is difficult to form through homogeneous composite, which has limited the performance of ceramics. In recent years, the rapid development of additive manufacturing technology can obtain ceramic parts with specific requirements through multilayer stacking [8,9], which has the advantages of free design and rapid prototyping [10,11].
At present, the additive-manufacturing-based photo-polymerization (PPM) technology and material extrusion (MEX) technology are the two most commonly used methods in ceramics fabrication due to the advantages of high precision [12,13], complex structures [14] and multimaterial distribution [15] such as stereolithography (SLA) [16], digital light processing (DLP) [17], extrusion freeforming (EFF) [18], on-demand extrusion (CODE) process [19] and direct ink writing (DIW) [20]. However, because of the limited solid content, the ceramics produce pores in degreasing and sintering, which reduce the compactness and mechanical properties [21].
The particle size and additives [22][23][24] have an impact on the properties of final ceramics, which are mainly focused on ceramics prepared by traditional technology and Crystals 2022, 12, 745 2 of 12 PPM technology. Huai et al. [25] studied the effects of nanosized and micron-sized additives (TiO 2 ) on Al 2 O 3 ceramics. The 0.15 wt % TiO 2 is conducive to the sintering properties of Al 2 O 3 ceramics under 1300-1500 • C, and nanosized TiO 2 is more conducive to the bulk density, hardness and wear properties at the same sintering temperature. Wu et al. [26] investigated the effects of particle size and the degreasing process on the densification of Al 2 O 3 ceramics. The results show that the samples containing both nanosized and micronsized particles have the highest compactness. The combination of powder with bimodal particle-size distribution and the vacuum-degreasing process can effectively realize the three-dimensional printing of ceramic products. Zhang et al. [27] studied the effects of fine particles and sintering additives on the properties of Al 2 O 3 ceramics prepared by SLA by adding fine particles and sintering additives (TiO 2 and MgO) to ceramics. The best ceramics contain 42.5 wt % coarse particles, 7.5 wt % fine particles, 1 wt % MgO and 3 wt % TiO 2 . The results showed that fine particles can improve the activity of Al 2 O 3 ceramics and reduce the sintering temperature. Sintering additives TiO 2 and MgO can improve the solid-state sintering properties of Al 2 O 3 ceramics. The final properties of ceramics are determined by the appropriate proportion of particle size and additives.
However, multisize particles and additives could behave differently on ceramics fabricated by MEX technology. The fine particles will reduce the fluidity and printability of ceramic raw materials [28] which limits the performance of finished products prepared by material extrusion technology with strict requirements on the rheology of ceramic raw materials. At present, the effect of multisize particles on ceramics fabricated by MEX technology has not been much studied. The effects of the multisize particles and the additives on Al 2 O 3 composite ceramics printed by the material extrusion and photopolymerization combined process (MEX-PPM) [29] have not been analyzed clearly.
In this work, nanosized Al 2 O 3 (n-Al 2 O 3 ) and microsized TiCN (µ-TiCN) were selected as additives, the Al 2 O 3 composite ceramic green body was printed by the MEX-PPM process proposed in the previous work, and a two-step vacuum-air debinding and sintering were carried out [30]. Effects of n-Al 2 O 3 and µ-TiCN on the microstructure and mechanical properties of Al 2 O 3 composite ceramic by MEX-PPM additive manufacturing were studied.
To prepare the slurry for ceramic-sample fabrication, 0.15 wt % OA with amphoteric groups were used to modify ceramic powder. A total of 1 wt % TPO was added to HDDA to prepare the prepolymer solution, then the modified powders were mixed into the prepolymer solution by ball milling for 4 h, and the mixed slurry was ultrasonically vibrated and vacuumed to obtain printable ceramic slurry. The ceramic green body was printed using the MEX-PPM additive-manufacturing process, and the tubular furnace (GSL-1700X, Hefei Kejing Material Technology Co., Ltd., Hefei, China) was used for degreasing and sintering. The process is shown in Figure 1. Our previous research results showed that ceramic slurry with viscosity greater than 8.88 Pa·s would limit the printing of MEX-PPM combined process [29]. Thus, 6 kinds of ceramic slurries with the same solid content and maximum viscosity of 8.73 Pa·s were adopted. A nozzle with a diameter of 0.5 mm was used to print ceramic samples at a speed of 5 mm/s. The samples with n-Al2O3 are represented by Sn-x, including Sn1, Sn2 and Sn3, and the samples with μ-TiCN are labeled as St-x, including St1, St2 and St3. Through preliminary experiment, the performance of Al2O3 composite ceramic sintered body with μ-TiCN less than 20 wt % is poor. The minimum content of μ-TiCN in the formula sample in this study is 20 wt %. The details of the solid contents of Al2O3 composite ceramic slurries are shown in Table 1.  Our previous research results showed that ceramic slurry with viscosity greater than 8.88 Pa·s would limit the printing of MEX-PPM combined process [29]. Thus, 6 kinds of ceramic slurries with the same solid content and maximum viscosity of 8.73 Pa·s were adopted. A nozzle with a diameter of 0.5 mm was used to print ceramic samples at a speed of 5 mm/s. The samples with n-Al 2 O 3 are represented by Sn-x, including Sn1, Sn2 and Sn3, and the samples with µ-TiCN are labeled as St-x, including St1, St2 and St3. Through preliminary experiment, the performance of Al 2 O 3 composite ceramic sintered body with µ-TiCN less than 20 wt % is poor. The minimum content of µ-TiCN in the formula sample in this study is 20 wt %. The details of the solid contents of Al 2 O 3 composite ceramic slurries are shown in Table 1. The microstructure of Al 2 O 3 composite ceramics and energy-dispersive spectroscopy (EDS) were characterized using a scanning electron microscope (Evo18, Zeiss, Oberkochen, BW, Germany). The flexural strength was measured using a three-point bending test by electronic universal testing machine (WDW-100KN, Instron Co., Boston, MA, USA); the testing bars were 30 mm × 4 mm × 3 mm (length×width×thickness) with a span of 20 mm and a loading rate of 0.5 mm/min. Hardness and fracture toughness were measured using a Micro Vickers hardness tester (HV-1000ZCM-XY, Anyi Instrument Co., Ltd., Shanghai, China); 20 indentation points were taken and held pressure at 196 N for 15 s. In addition, Crystals 2022, 12, 745 4 of 12 the density was calculated by the Archimedes method. The differences in volume density and mechanical properties of Al 2 O 3 composite ceramics under the effect of n-Al 2 O 3 and µ-TiCN were calculated by Equation (1).
where ∆S i is the difference in volume density and mechanical properties; and S ij is the corresponding Al 2 O 3 composite ceramic sample. (EDS) were characterized using a scanning electron microscope (Evo18, Zeiss, Oberkochen, BW, Germany). The flexural strength was measured using a three-point bending test by electronic universal testing machine (WDW-100KN, Instron Co., Boston, MA, USA); the testing bars were 30 mm × 4 mm × 3 mm (length×width×thickness) with a span of 20 mm and a loading rate of 0.5 mm/min. Hardness and fracture toughness were measured using a Micro Vickers hardness tester (HV-1000ZCM-XY, Anyi Instrument Co., Ltd., Shanghai, China); 20 indentation points were taken and held pressure at 196 N for 15 s. In addition, the density was calculated by the Archimedes method. The differences in volume density and mechanical properties of Al2O3 composite ceramics under the effect of n-Al2O3 and μ-TiCN were calculated by Equation (1).

Results and Discussion
where ∆ is the difference in volume density and mechanical properties; and is the corresponding Al2O3 composite ceramic sample.

Effects of n-Al2O3 on the Microstructure and Mechanical Properties
n-Al2O3 with different solid contents was filled into Sn-x type Al2O3 composite ceramics; their volume density, mechanical properties and variation-testing results were shown in Figure 2.  The solid circle, pentagon, pentagram and triangle in Figure 2 correspond to volume density, hardness, flexural strength and fracture toughness, respectively. The marked colors include gray, red and blue, corresponding to samples Sn1, Sn2 and Sn3, respectively. Among Sn-x type ceramics, Sn2 with 10 wt % n-Al 2 O 3 (red marks in Figure 2) has the best performance. Its volume density is 3.891 g/cm 3 , hardness is 17.5719 GPa, fracture toughness is 5.5756 MPa/mm 2 and bending strength is 508.875 MPa. Compared with Sn1 with 5 wt % n-Al 2 O 3 (gray marks in Figure 2), the fracture toughness and bending strength of Sn2 are slightly improved, the hardness is significantly improved and the difference in hardness is 1.118 GPa. For Sn3 with 15 wt % n-Al 2 O 3 (blue mark in Figure 2), its volume density and mechanical properties were lower than Sn2 and higher than Sn1, which is caused by excessive fine-grained n-Al 2 O 3 . In addition, the differences in volume density, flexural strength and fracture toughness of Sn-x type ceramics are 0.091 g/cm 3 , 24 MPa and 0.205 MPa/mm 2 , respectively. According to the above results, n-Al 2 O 3 has a significant effect on the hardness of Al 2 O 3 composite ceramics. In order to verify that the test results of the above samples are based on the proportioning components designed in Table 1, the EDS spectra of Sn-x type composite ceramics were characterized. Since the mass percentage of other components in the Sn-x type composite ceramics is constant except µ-Al 2 O 3 and n-Al 2 O 3 , the EDS spectrum is the same, as shown in Figure 3.
The solid circle, pentagon, pentagram and triangle in Figure 2 correspond to volume density, hardness, flexural strength and fracture toughness, respectively. The marked colors include gray, red and blue, corresponding to samples Sn1, Sn2 and Sn3, respectively. Among Sn-x type ceramics, Sn2 with 10 wt % n-Al2O3 (red marks in Figure 2) has the best performance. Its volume density is 3.891 g/cm 3 , hardness is 17.5719 GPa, fracture toughness is 5.5756 MPa/mm 2 and bending strength is 508.875 MPa. Compared with Sn1 with 5 wt % n-Al2O3 (gray marks in Figure 2), the fracture toughness and bending strength of Sn2 are slightly improved, the hardness is significantly improved and the difference in hardness is 1.118 GPa. For Sn3 with 15 wt % n-Al2O3 (blue mark in Figure 2), its volume density and mechanical properties were lower than Sn2 and higher than Sn1, which is caused by excessive fine-grained n-Al2O3. In addition, the differences in volume density, flexural strength and fracture toughness of Sn-x type ceramics are 0.091 g/cm 3 , 24 MPa and 0.205 MPa/mm 2 , respectively. According to the above results, n-Al2O3 has a significant effect on the hardness of Al2O3 composite ceramics. In order to verify that the test results of the above samples are based on the proportioning components designed in Table 1, the EDS spectra of Sn-x type composite ceramics were characterized. Since the mass percentage of other components in the Sn-x type composite ceramics is constant except μ-Al2O3 and n-Al2O3, the EDS spectrum is the same, as shown in Figure 3.   Table 1, and the error caused by each component element is within 2% of the error range of the EDS spectrum test. The peak corresponding to the Al element is the common content in n-Al2O3 and μ-Al2O3. To further study the influence of n-Al2O3 particles on the properties of Al2O3 composite ceramics, the microstructure of Sn-x type composite ceramics was also characterized, as shown in Figure 4.   Table 1, and the error caused by each component element is within 2% of the error range of the EDS spectrum test. The peak corresponding to the Al element is the common content in n-Al 2 O 3 and µ-Al 2 O 3 . To further study the influence of n-Al 2 O 3 particles on the properties of Al 2 O 3 composite ceramics, the microstructure of Sn-x type composite ceramics was also characterized, as shown in Figure 4.   Figure 4 shows the microstructure of Al2O3 composite ceramics Sn1, Sn2 and Sn3 at 2000× and 10000× magnification. Among them, Sn1 (Figure 4a,b) has more pores, with a porosity of 10.664%, followed by Sn3 (Figure 4e,f), with a porosity of 9.589%. Although the number of pores in Sn3 decreases, the macropores are obvious and the pore size is uneven. In comparison, Al2O3 composite ceramics Sn2 (Figure 4c,d) are more uniform and dense, with a low porosity of 8.535%. The above phenomenon is that a small amount of n-Al2O3 particles per unit volume is not enough to fill the pores between micron particles (black holes in Figure 4b), thus forming a relatively loose composite ceramic structure Sn1 (( Figure 4a). On the contrary, excessive n-Al2O3 cannot be completely filled into the pores, resulting in agglomeration, forming holes of different sizes randomly distributed in the interior of the ceramic (Figure 4e,f). Only when n-Al2O3 particles complement the pores between micron particles to obtain better ceramics. As shown in the SEM of Sn2 with 10 wt % n-Al2O3 in Figure 4c,d, the overall structure is more uniform and there are fewer holes, which is consistent with the high density of Sn2 ceramics shown in Figure 2a. In addition, the low hardness, flexural strength and fracture toughness in Figure 2a-c are also caused by inappropriate n-Al2O3 particles and a low-pressure compactness structure.   (Figure 4a,b) has more pores, with a porosity of 10.664%, followed by Sn3 (Figure 4e,f), with a porosity of 9.589%. Although the number of pores in Sn3 decreases, the macropores are obvious and the pore size is uneven. In comparison, Al 2 O 3 composite ceramics Sn2 (Figure 4c,d) are more uniform and dense, with a low porosity of 8.535%. The above phenomenon is that a small amount of n-Al 2 O 3 particles per unit volume is not enough to fill the pores between micron particles (black holes in Figure 4b), thus forming a relatively loose composite ceramic structure Sn1 (( Figure 4a). On the contrary, excessive n-Al 2 O 3 cannot be completely filled into the pores, resulting in agglomeration, forming holes of different sizes randomly distributed in the interior of the ceramic (Figure 4e,f). Only when n-Al 2 O 3 particles complement the pores between micron particles to obtain better ceramics. As shown in the SEM of Sn2 with 10 wt % n-Al 2 O 3 in Figure 4c,d, the overall structure is more uniform and there are fewer holes, which is consistent with the high density of Sn2 ceramics shown in Figure 2a. In addition, the low hardness, flexural strength and fracture toughness in Figure 2a-c are also caused by inappropriate n-Al 2 O 3 particles and a low-pressure compactness structure.

Effects of µ-TiCN on the Microstructure and Mechanical Properties
To study the effects of µ-TiCN on Al 2 O 3 composite ceramics, it was necessary to characterize the volume density, mechanical properties, EDS spectrum and microstructure

Effects of μ-TiCN on the Microstructure and Mechanical Properties
To study the effects of μ-TiCN on Al2O3 composite ceramics, it was necessary to characterize the volume density, mechanical properties, EDS spectrum and microstructure of St-x type ceramics. Figure 5 shows the volume density and mechanical properties of St-x type ceramics with different content μ-TiCN. The optimum content of n-Al2O3 in St-x type composite ceramics is 15 wt %. The solid circle, pentagon, pentagram and triangle in Figure 5 correspond to volume density, hardness, flexural strength and fracture toughness, respectively. The marked colors include gray, red and blue, corresponding to samples St1, St2 and St3, respectively. The meanings of the different symbols in Figure 5 are the same as those in Figure 2. In Stx type composite ceramics, St2 has remarkable properties, with volume density of 4.177 g/cm 3 , hardness of 16.192 GPa, flexural strength of 592.875 MPa and fracture toughness of 6.3081 MPa/mm 2 (red marks in Figure 5). Compared with composite ceramic St1, the volume density, flexural strength and fracture toughness of St2 are significantly improved. For composite ceramic St3, its density and mechanical properties are only higher than St1. Nevertheless, the properties of St1 ceramics are still better than Sn2 (red hollow mark in The solid circle, pentagon, pentagram and triangle in Figure 5 correspond to volume density, hardness, flexural strength and fracture toughness, respectively. The marked colors include gray, red and blue, corresponding to samples St1, St2 and St3, respectively. The meanings of the different symbols in Figure 5 are the same as those in Figure 2. In St-x type composite ceramics, St2 has remarkable properties, with volume density of 4.177 g/cm 3 , hardness of 16.192 GPa, flexural strength of 592.875 MPa and fracture toughness of 6.3081 MPa/mm 2 (red marks in Figure 5). Compared with composite ceramic St1, the volume density, flexural strength and fracture toughness of St2 are significantly improved. For composite ceramic St3, its density and mechanical properties are only higher than St1. Nevertheless, the properties of St1 ceramics are still better than Sn2 (red hollow mark in Figure 5). Sn-x type ceramics under the effects of n-Al 2 O 3 in Figure 2. However, the difference in hardness (0.739 GPa in Figure 5b) in St-x type composite ceramics is less than that in Sn-x type ceramics (1.118 GPa in Figure 2b). This shows that µ-TiCN plays a significant role in improving the volume density, flexural strength and fracture toughness of Al 2 O 3 composite ceramics, and n-Al 2 O 3 is more beneficial to the hardness. The difference result of the above properties of St-x type ceramics is caused by the change in µ-TiCN content. To verify this conclusion, the EDS spectrum characterization of Al 2 O 3 composite ceramics St1, St2 and St3 containing µ-TiCN is carried out, as shown in Figure 6. Figure 5). Figure 5 also shows the fluctuation of volume density and mechanical properties of St-x type ceramics under the effects of μ-TiCN, in which the fluctuation of volume density, hardness, flexural strength and fracture toughness are large, with change values of 0.257 g/cm 3 , 0.739 GPa, 70.875 MPa and 0.531 MPa/mm 2 , respectively, and the differences in density, flexural strength and fracture toughness are greater than that of Sn-x type ceramics under the effects of n-Al2O3 in Figure 2. However, the difference in hardness (0.739 GPa in Figure 5b) in St-x type composite ceramics is less than that in Sn-x type ceramics (1.118 GPa in Figure 2b). This shows that μ-TiCN plays a significant role in improving the volume density, flexural strength and fracture toughness of Al2O3 composite ceramics, and n-Al2O3 is more beneficial to the hardness. The difference result of the above properties of St-x type ceramics is caused by the change in μ-TiCN content. To verify this conclusion, the EDS spectrum characterization of Al2O3 composite ceramics St1, St2 and St3 containing μ-TiCN is carried out, as shown in Figure 6.     (Figure 6a-c), and the fluctuation value is within 2% of the error range of the EDS spectrum test, which is almost close to the proportion designed in St-x type composite ceramic components.
For Ti, N and C elements from µ-TiCN, the atomic mass and percentage from St1 to St3 measured by the EDS spectrum gradually increase, in which the atomic mass and percentage of Ti elements are (19. that the compactness and mechanical properties of Al 2 O 3 composite ceramics are affected by the quality of µ-TiCN. Therefore, the Al 2 O 3 composite ceramic St2 with 30 wt % µ-TiCN has good properties.
In addition, the SEM morphology of St-x type ceramics was characterized, and the effects of µ-TiCN on microstructure of Al 2 O 3 composite ceramics was studied, as shown in Figure 7.
is within 2% of the error range of the EDS spectrum test, which is almost close to the proportion designed in St-x type composite ceramic components.
In addition, the SEM morphology of St-x type ceramics was characterized, and the effects of μ-TiCN on microstructure of Al2O3 composite ceramics was studied, as shown in Figure 7.  Figure 7 shows the microstructure of a St-x type Al2O3 composite ceramic sintered body under low magnification and high magnification. At low magnification, the compactness of St-x type composite ceramics is better than Sn-x type ceramics.  Figure 7 shows the microstructure of a St-x type Al 2 O 3 composite ceramic sintered body under low magnification and high magnification. At low magnification, the compactness of St-x type composite ceramics is better than Sn-x type ceramics.
For St-x type composite ceramics, St1 (Figure 7a,b) has the highest porosity of 8.909%, followed by St3 (Figure 7e,f) with a porosity of 7.184%, and St2 (Figure 7c,d) has a lower porosity of 4.073%. At high magnification, it can also be observed that the significance and randomness of St3 ceramic pores are higher than St1 and St2. In comparison, St2 has a more uniform and dense microstructure (Figure 7c,d), which is consistent with the best volume density of St2 in Figure 5a. In addition, St1 with the lowest volume density (3.92 g/cm 3

) in
St-x type ceramics is also higher than Sn2 with the best volume density (3.891 g/cm 3 ) in Sn-x type ceramics.
The above phenomena are mainly caused by the following reasons: (1) µ-TiCN volume density (5.08 g/cm 3 ) is higher than that of Al 2 O 3 . In the same unit volume, µ-TiCN content increases, µ-Al 2 O 3 particles decrease and the overall density increases; (2) When the content of µ-TiCN exceeds the optimal value, the different van der Waals forces between particles lead to the aggregation of n-Al 2 O 3 around the same type of µ-Al 2 O 3 to form agglomeration, which produces pores of different sizes between these agglomeration and µ-TiCN (black hole in Figure 7f). Compared with Al 2 O 3 with high hardness, µ-TiCN has lower internal stress and high toughness, which is also the key factor for the best performance of Al 2 O 3 composite ceramic St2 with 10 wt % n-Al 2 O 3 and 30 wt % µ-TiCN. This shows that the appropriate amount of n-Al 2 O 3 and µ-TiCN is more conducive to the properties of Al 2 O 3 composite ceramics.

Conclusions
In this study, Sn-x and St-x type Al 2 O 3 composite ceramics were successfully prepared by the MEX-PPM process-based additive manufacturing with mixed micron-sized and nanosized particles as raw materials. The effects of n-Al 2 O 3 and µ-TiCN on the mechanical properties and microstructure of Al 2 O 3 composite ceramics were studied by degreasing, sintering and performance test. The following conclusions were listed as follows: (1) n-Al 2 O 3 has a significant effect on the hardness of Al 2 O 3 composite ceramics. The change in hardness of Sn-x type ceramics effected with n-Al 2 O 3 was 1.118 GPa, which was higher than the change in St-x type ceramics effected with µ-TiCN (0.739 GPa). The volume density and porosity of 10 wt % n-Al 2 O 3 ceramics are improved, which is better than that of the other Sn-x ceramics. Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: Data are available upon request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.