Effects of Microwave Sintering Temperature and Holding Time on Mechanical Properties and Microstructure of Si3N4/n-SiC ceramics

The n-SiC (nanometer SiC) is added to be the additive in order to improve the mechanical performance of Si3N4 ceramics. A microwave sintered the ceramics at different temperature and holding times. The results shows that the Si3N4/n-SiC ceramics (85 wt% Si3N4 + 5 wt% n-SiC + 5 wt% Al2O3 + 5 wt% Y2O3) have the best mechanical properties at 1600 °C, which is beneficial to the densification and β-Si3N4 phase formation for 10 min: the density, hardness, and fracture toughness were 97.1%, 14.44 GPa, and 7.77 MPa·m1/2, which increased by 2.8%, 7.0%, and 13.1%, respectively, when compared with the ceramics (90 wt% Si3N4 + 5 wt% Al2O3 + 5 wt% Y2O3).


Introduction
Si 3 N 4 ceramic have been widely recognized as the ideal tool materials for cutting the high temperature cast iron and nickel-based alloys due to its outstanding thermal properties, wear resistance, and high temperature material properties [1][2][3][4]. Currently, high-performance densified Si 3 N 4 ceramics are usually sintered by hot pressing sintering [5,6], hot isostatic pressure sintering [7], pressure less sintering [8], microwave sintering [3,4], reaction sintering [9,10], and spark plasma sintering [11][12][13]. Among the mentioned methods, microwave sintering is a new technology, which has high-energy utilization rate, safety, and free pollution. Many researches have shown that it can not only promote the densification of sintered materials to short the sintering time, but also refine the crystalline microstructure [14][15][16][17]. Besides, it is a kind of pressure less sintering, which makes it possible to massively produce ceramic tools with complex shapes.
However, the microwave absorption of Si 3 N 4 at low temperature (less than 800 • C) is poor [18], which affects the microwave sintering process. The fracture toughness of Si 3 N 4 ceramic is usually between 5.0 MPa·m 1/2 and 7.0 MPa·m 1/2 with high hardness [3,[13][14][15][16][17], which should be improved to meet the requirements of high-performance cutting tools. When considering that n-SiC has good microwave absorption properties and high hardness at all temperature stages [18], it is added into silicon nitride ceramics. On the one hand, the uniform distributed n-SiC particles can be used as strengthening phase to improve the fracture toughness of the ceramics. On the other hand, the n-SiC particles are equivalent to many small heat sources during the sintering process, which improve the sintering properties and promote the α→β phase transformation of the Si 3 N 4 . Al 2 O 3 and Y 2 O 3 were the most common additives that were used in the conventional sintering of Si 3 N 4 ceramics according to the existing reports [19][20][21][22][23]. In the field of microwave sintering, Al 2 O 3

Experimental Procedure
The starting powders (99.9% purity, Shanghai, China) were used as: α-Si 3 N 4 (0.7 µm), n-SiC (0.1 µm), Y 2 O 3 (1 µm), and Al 2 O 3 (0.5 µm). Firstly, the starting powders that were in isopropyl alcohol were placed on a planetary ball mill (QM-3SP2, Nanjing university instrument factory, Nanjing, China) for 48 h. Subsequently, the 3 wt% polyvinyl alcohol solution was added to the mixed slurry and the ball milling was continued to ball mill for 2 h. After that, the powders were then pressurized under 180 MPa for 2 min and finally turned into a green body that has a certain shape and density, and it was sintered in a multimode high temperature vacuum microwave oven filled with N 2 . Figure 1 shows the heat preservation device for sample. Table 1 lists the compositions, sintering temperature, and holding time of Si 3 N 4 materials. T 90 wt% Si 3 N 4 + 5 wt% Al 2 O 3 + 5 wt% Y 2 O 3 is taken as a comparison to illustrate the effectiveness of n-SiC on the properties of Si 3 N 4 ceramics. a density of 99% at a sintering temperature of 1600 °C while using Al2O3 and Y2O3 as sintering additives. As a result of that these two kinds of sintering additives can contribute to the formation of the liquid phase, which can promote the densification of Si3N4 ceramics, we selected these sintering aids in our research. The study was aimed to prepare high-performance Si3N4/n-SiC ceramics, with Al2O3 and Y2O3 as sintering aids, by microwave. The effects of sintering temperature and holding time on the properties of microwave sintered Si3N4/n-SiC ceramics were analyzed. SEM microstructure analysis, X-ray diffraction (XRD) phase analysis, density, and hardness and fracture toughness.

Experimental Procedure
The starting powders (99.9% purity, Shanghai, China) were used as: α-Si3N4 (0.7 μm), n-SiC (0.1 μm), Y2O3 (1 μm), and Al2O3 (0.5 μm). Firstly, the starting powders that were in isopropyl alcohol were placed on a planetary ball mill (QM-3SP2, Nanjing university instrument factory, Nanjing, China) for 48 h. Subsequently, the 3 wt% polyvinyl alcohol solution was added to the mixed slurry and the ball milling was continued to ball mill for 2 h. After that, the powders were then pressurized under 180 MPa for 2 min and finally turned into a green body that has a certain shape and density, and it was sintered in a multimode high temperature vacuum microwave oven filled with N2. Figure  1 shows the heat preservation device for sample. Table 1 lists the compositions, sintering temperature, and holding time of Si3N4 materials. T 90 wt% Si3N4 + 5 wt% Al2O3 + 5 wt% Y2O3 is taken as a comparison to illustrate the effectiveness of n-SiC on the properties of Si3N4 ceramics.
The density of the sintered specimens was measured by the Archimedes drainage method with distilled water. According to the method in [25], the grain size analysis is carried on. After grinding and polishing by a diamond quadrilateral indenter (HV-50, Insize, Shanghai, China) under 196.1 N load the Vickers hardness (Hv) was tested. Fracture toughness was determined while using the indentation technique in [26]. The phase composition of the sample before and after sintering was analyzed via X-ray diffractometric (D8 Advance, Bruker, Karlsruhe, Germany). The relative amounts of the α-Si3N4 and β-Si3N4 phases that were presented in the Si3N4 specimen were determined according to the peak height [27]. The microstructures of the samples were observed by Scanning electron microscopy (SEM, FEI Quanta 250F, FEI, Hillsboro, OR, USA).  The density of the sintered specimens was measured by the Archimedes drainage method with distilled water. According to the method in [25], the grain size analysis is carried on. After grinding and polishing by a diamond quadrilateral indenter (HV-50, Insize, Shanghai, China) under 196.1 N load the Vickers hardness (Hv) was tested. Fracture toughness was determined while using the indentation technique in [26]. The phase composition of the sample before and after sintering was analyzed via X-ray diffractometric (D8 Advance, Bruker, Karlsruhe, Germany). The relative amounts of the α-Si 3 N 4 and β-Si 3 N 4 phases that were presented in the Si 3 N 4 specimen were determined according to the peak height [27]. The microstructures of the samples were observed by Scanning electron microscopy (SEM, FEI Quanta 250F, FEI, Hillsboro, OR, USA). Figure 2 shows the he relative density increases first and then decreases with the increase of sintering temperature, the minimum relative density reached the minimum value 95.5%, and the maximum value 97.1% at 1400 • C and 1600 • C, respectively. As shown in Figure 3a, there are still many porosities in the sintered Si 3 N 4 /n-SiC ceramics materials at 1400 • C. This is mainly because enough liquid phases inside the sintered ceramics material cannot be formed to complete sintering at a low temperature. Figure 3b-d indicate that the porosity of the sintered Si 3 N 4 /n-SiC ceramics material was obviously reduced with as the sintering temperature increases and the material structure is more compact than that at 1400 • C. The results show that the sintering additives (5 wt% Al 2 O 3 and 5 wt% Y 2 O 3 ) were able to form a sufficient liquid phase with the increase of temperature during the sintering process. The sufficient liquid phase further promotes the particles motion (Si 3 N 4 and n-SiC) and the shrinkage of the green compact, thus the densification of sintered ceramic materials was improved. However, when the sintering temperature reaches to 1700 • C, the relative density of sintered ceramic materials slightly decrease. It can be explained that, with the increase of sintering temperature, the β-Si 3 N 4 phase gradually increases and grows, and the interlocking structure is formed, which causes the increase of the porosity and the decrease of the relative density. Another possible explanation might be the errors introduced in the process of temperature measurement. High temperature (over 1800 • C) will cause the decomposition of silicon nitride, which leads to the decrease of relative density [28]. In addition, due to the good microwave absorbing property of SiC, the temperature of the n-SiC particles is higher than the Si 3 N 4 particles' during microwave sintering; therefore, a local high temperature region is formed around the n-SiC particles. The local high temperature can promote the densification of sintered material and the α→β phase transformation of Si 3 N 4 . However, if the local temperature is above 1800 • C, the Si 3 N 4 will decompose.  Figure 2 shows the he relative density increases first and then decreases with the increase of sintering temperature, the minimum relative density reached the minimum value 95.5%, and the maximum value 97.1% at 1400 °C and 1600 °C, respectively. As shown in Figure 3a, there are still many porosities in the sintered Si3N4/n-SiC ceramics materials at 1400 °C. This is mainly because enough liquid phases inside the sintered ceramics material cannot be formed to complete sintering at a low temperature. Figure 3b-d indicate that the porosity of the sintered Si3N4/n-SiC ceramics material was obviously reduced with as the sintering temperature increases and the material structure is more compact than that at 1400 °C. The results show that the sintering additives (5 wt% Al2O3 and 5 wt% Y2O3) were able to form a sufficient liquid phase with the increase of temperature during the sintering process. The sufficient liquid phase further promotes the particles motion (Si3N4 and n-SiC) and the shrinkage of the green compact, thus the densification of sintered ceramic materials was improved. However, when the sintering temperature reaches to 1700 °C, the relative density of sintered ceramic materials slightly decrease. It can be explained that, with the increase of sintering temperature, the β-Si3N4 phase gradually increases and grows, and the interlocking structure is formed, which causes the increase of the porosity and the decrease of the relative density. Another possible explanation might be the errors introduced in the process of temperature measurement. High temperature (over 1800 °C) will cause the decomposition of silicon nitride, which leads to the decrease of relative density [28]. In addition, due to the good microwave absorbing property of SiC, the temperature of the n-SiC particles is higher than the Si3N4 particles' during microwave sintering; therefore, a local high temperature region is formed around the n-SiC particles. The local high temperature can promote the densification of sintered material and the α→β phase transformation of Si3N4. However, if the local temperature is above 1800 °C, the Si3N4will decompose.   Figure 4 shows the SEM micrographs of the sintered Si3N4/n-SiC ceramics at different temperatures. Figure 5 shows the XRD results of the sintered Si3N4/n-SiC materials. As it can be seen in Figure 4a, there is a small amount of long columnar structure and lots of powder or irregular shape, which is formed by grains of sintered material, when the temperature is 1400 °C. Figure 5a shows that the sintered samples mainly consist of α-Si3N4, β-Si3N4, SiC, and Si5AlON7 at 1400 °C. The ratios of α-Si3N4 and β-Si3N4 in the Si3N4 ceramics are 52% and 48%, respectively. This further indicates that there are not enough liquid phases, which are used to promote the sintering densification and the transformation of α-Si3N4 to β-Si3N4, at a relatively low sintering temperature. As shown in Figure  4b, most of the α-Si3N4 is transformed into a long columnar β-Si3N4 at 1500 °C, and there is only a small amount of α-Si3N4 in the sintered Si3N4/n-SiC ceramics, as shown in Figure 5. However, the crystal size of β-Si3N4 is different, there are some silicon nitride grains that grow more completely, and presenting larger grains size, and there are also many small columnar grains. As can be seen from Figure 4c, the grains of β-Si3N4 were further grown and formed a large number of big columnar grains with an average grain length of 3.25 μm at 1600 °C. As shown in Figure 4d, the surface of some β-Si3N4 grains is not smooth, and there is especially the tendency that some β-Si3N4 grains are going to turn to powder, when the temperature is 1700 °C. It is further indicated that Si3N4 has the possibility of decomposition at this temperature, which reduces the mechanical properties and densification of the sintered Si3N4/n-SiC ceramics. According to the XRD diffraction pattern ( Figure  5), the α-Si3N4 phase is completely transformed into a β-Si3N4 phase in sintered Si3N4/n-SiC ceramics when the sintering temperature is above 1600 °C.  Figure 4 shows the SEM micrographs of the sintered Si 3 N 4 /n-SiC ceramics at different temperatures. Figure 5 shows the XRD results of the sintered Si 3 N 4 /n-SiC materials. As it can be seen in Figure 4a, there is a small amount of long columnar structure and lots of powder or irregular shape, which is formed by grains of sintered material, when the temperature is 1400 • C. Figure 5a shows that the sintered samples mainly consist of α-Si 3 N 4 , β-Si 3 N 4 , SiC, and Si 5 AlON 7 at 1400 • C. The ratios of α-Si 3 N 4 and β-Si 3 N 4 in the Si 3 N 4 ceramics are 52% and 48%, respectively. This further indicates that there are not enough liquid phases, which are used to promote the sintering densification and the transformation of α-Si 3 N 4 to β-Si 3 N 4 , at a relatively low sintering temperature. As shown in Figure 4b, most of the α-Si 3 N 4 is transformed into a long columnar β-Si 3 N 4 at 1500 • C, and there is only a small amount of α-Si 3 N 4 in the sintered Si 3 N 4 /n-SiC ceramics, as shown in Figure 5. However, the crystal size of β-Si 3 N 4 is different, there are some silicon nitride grains that grow more completely, and presenting larger grains size, and there are also many small columnar grains. As can be seen from Figure 4c, the grains of β-Si 3 N 4 were further grown and formed a large number of big columnar grains with an average grain length of 3.25 µm at 1600 • C. As shown in Figure 4d, the surface of some β-Si 3 N 4 grains is not smooth, and there is especially the tendency that some β-Si 3 N 4 grains are going to turn to powder, when the temperature is 1700 • C. It is further indicated that Si 3 N 4 has the possibility of decomposition at this temperature, which reduces the mechanical properties and densification of the sintered Si 3 N 4 /n-SiC ceramics. According to the XRD diffraction pattern (Figure 5), the α-Si 3 N 4 phase is completely transformed into a β-Si 3 N 4 phase in sintered Si 3 N 4 /n-SiC ceramics when the sintering temperature is above 1600 • C.   As shown in Figure 6, the hardness and fracture toughness increase first and then decrease as the sintering temperature increased. The change trend of the Vickers hardness and fracture toughness is the same as relative density. It indicates that the high densification can significantly improve the mechanical performances of the sintered Si3N4/n-SiC ceramics. When the temperature is 1600 °C, the Vickers hardness and fracture toughness reached a maximum value of 14.44 GPa and 7.77 MPa·m 1/2 ,   As shown in Figure 6, the hardness and fracture toughness increase first and then decrease as the sintering temperature increased. The change trend of the Vickers hardness and fracture toughness is the same as relative density. It indicates that the high densification can significantly improve the mechanical performances of the sintered Si3N4/n-SiC ceramics. When the temperature is 1600 °C, the Vickers hardness and fracture toughness reached a maximum value of 14.44 GPa and 7.77 MPa·m 1/2 , As shown in Figure 6, the hardness and fracture toughness increase first and then decrease as the sintering temperature increased. The change trend of the Vickers hardness and fracture toughness is the same as relative density. It indicates that the high densification can significantly improve the mechanical performances of the sintered Si 3 N 4 /n-SiC ceramics. When the temperature is 1600 • C, the Vickers hardness and fracture toughness reached a maximum value of 14.44 GPa and 7.77 MPa·m 1/2 , respectively. As can be seen from Figures 3c and 4c, the sintered Si 3 N 4 /n-SiC ceramics were densified and formed many interlocking elongated columnar β-Si 3 N 4 grains. The main fracture mechanisms of the sintered Si 3 N 4 /n-SiC ceramics are intergranular fracture, transgranular fracture, and grain pulling out. The interlocking microstructure that formed by large elongated columnar β-Si 3 N 4 grains can greatly enhance the mechanical properties, especially the fracture toughness of silicon nitride materials. Therefore, when the temperature exceeds 1500 • C, a great quantity of elongated columnar β-Si 3 N 4 grains is formed, which significantly improves the fracture toughness (more than 7.0 MPa·m 1/2 ) of the material. respectively. As can be seen from Figure 3c and Figure 4c, the sintered Si3N4/n-SiC ceramics were densified and formed many interlocking elongated columnar β-Si3N4 grains. The main fracture mechanisms of the sintered Si3N4/n-SiC ceramics are intergranular fracture, transgranular fracture, and grain pulling out. The interlocking microstructure that formed by large elongated columnar β-Si3N4 grains can greatly enhance the mechanical properties, especially the fracture toughness of silicon nitride materials. Therefore, when the temperature exceeds 1500 °C, a great quantity of elongated columnar β-Si3N4 grains is formed, which significantly improves the fracture toughness (more than 7.0 MPa·m 1/2 ) of the material.  Table 2 shows the comparison of properties of Si3N4 with n-SiC content of 0% and 5% at 1600 °C for 10 min. When n-SiC is not added into the sintered silicon nitride materials, the density, Vickers hardness, and fracture toughness of the Si3N4 ceramics are 94.5%, 13.5 GPa, and 6.87 MPa·m 1/2 , which increased by 2.6%, 7.0%, and 13.1%, respectively. Figure 7 is SEM micrographs of the fracture surface morphology of the Si3N4 material with 0% n-SiC content. As shown in Figure 7, when the n-SiC content is 0%, there are great quantities of small porosities in the sintered Si3N4 ceramics. Additionally, the distribution and size of the β-Si3N4 grains are uneven. The results showed that the filling of SiC nanopowders evidently improved the microwave sinter ability of Si3N4 ceramics. Table 2. Comparison of properties of Si3N4 with SiC nanopowders content of 0% and 5% at 1600 °C for 10 min (wt%).   Table 2 shows the comparison of properties of Si 3 N 4 with n-SiC content of 0% and 5% at 1600 • C for 10 min. When n-SiC is not added into the sintered silicon nitride materials, the density, Vickers hardness, and fracture toughness of the Si 3 N 4 ceramics are 94.5%, 13.5 GPa, and 6.87 MPa·m 1/2 , which increased by 2.6%, 7.0%, and 13.1%, respectively. Figure 7 is SEM micrographs of the fracture surface morphology of the Si 3 N 4 material with 0% n-SiC content. As shown in Figure 7, when the n-SiC content is 0%, there are great quantities of small porosities in the sintered Si 3 N 4 ceramics. Additionally, the distribution and size of the β-Si 3 N 4 grains are uneven. The results showed that the filling of SiC nanopowders evidently improved the microwave sinter ability of Si 3 N 4 ceramics.    Figure 8 shows the influence of holding time on the density of the sintered Si 3 N 4 /n-SiC ceramic at 1600 • C. As shown in Figure 8, the relative density increases first and then decreases as the holding time increased. The density reaches the maximum value of 97.1% when the holding time is 10 min. Figure 3c shows that the microstructure of the sintered Si 3 N 4 /n-SiC ceramic is more compact at this condition. Figure 9 presents SEM micrographs of fracture surface morphology of the Si 3 N 4 /n-SiC ceramic at 1600 • C under different holding time. As shown in Figure 9a, the microstructure of the sintered Si 3 N 4 /n-SiC ceramic is loose when the holding time is 0 min. It can be seen that the sintered Si 3 N 4 /n-SiC ceramic is not completely sintered under this condition. As shown in Figure 8b,c, there are many evenly distributed porosities in the sintered Si 3 N 4 /n-SiC ceramic when the holding time is 20 min and 30 min, respectively. The main reason is that the elongated columnar β-Si 3 N 4 grains grow with the increase of holding time, which results in a large number of porosities among the big β-Si 3 N 4 grains.  Figure 8 shows the influence of holding time on the density of the sintered Si3N4/n-SiC ceramic at 1600 °C. As shown in Figure 8, the relative density increases first and then decreases as the holding time increased. The density reaches the maximum value of 97.1% when the holding time is 10 min. Figure 3c shows that the microstructure of the sintered Si3N4/n-SiC ceramic is more compact at this condition. Figure 9 presents SEM micrographs of fracture surface morphology of the Si3N4/n-SiC ceramic at 1600 °C under different holding time. As shown in Figure 9a, the microstructure of the sintered Si3N4/n-SiC ceramic is loose when the holding time is 0 min. It can be seen that the sintered Si3N4/n-SiC ceramic is not completely sintered under this condition. As shown in Figure 8b,c, there are many evenly distributed porosities in the sintered Si3N4/n-SiC ceramic when the holding time is 20 min and 30 min, respectively. The main reason is that the elongated columnar β-Si3N4 grains grow with the increase of holding time, which results in a large number of porosities among the big β-Si3N4 grains.      Figure 11 is the XRD results of Si3N4/n-SiC ceramics at different holding time. As shown in Figure 10a, when the holding time is 0 min, most of the grains in the sintered Si3N4/n-SiC ceramics are spherical and powdery, and only a small amount of grains are columnar grains. The average grain size is 0.92 μm, which is nearly the same as that of the starting α-Si3N4 powder (0.7 μm). Figure 11a shows that the Si3N4/n-SiC ceramics material is not completely sintered due to the short holding time (0 min). As shown in Figure 4c and Figure 11b, the α-Si3N4 phase is almost completely transformed into the elongated columnar β-Si3N4 phase in the sintered Si3N4/n-SiC ceramics when the holding time is 10 min. As shown in Figure 10b,c, the distribution of β-Si3N4 crystal size is uneven when the holding time is more than 10 min, the reasonable explanations is that someβ-Si3N4 grains grow too large to occupy the growing space for other grains. Figure 10c shows that there are some larger holes or pits on the etched surface of the sintered Si3N4/n-SiC ceramics. It is a possible explanation that the liquid phase is connected to one piece for the long holding time, which causes some Si3N4 grains to fall off with the dissolution of the liquid phase during the molten NaOH corrosion process to form holes or pits.    Figure 11 is the XRD results of Si 3 N 4 /n-SiC ceramics at different holding time. As shown in Figure 10a, when the holding time is 0 min, most of the grains in the sintered Si 3 N 4 /n-SiC ceramics are spherical and powdery, and only a small amount of grains are columnar grains. The average grain size is 0.92 µm, which is nearly the same as that of the starting α-Si 3 N 4 powder (0.7 µm). Figure 11a shows that the Si 3 N 4 /n-SiC ceramics material is not completely sintered due to the short holding time (0 min). As shown in Figures 4c and 11b, the α-Si 3 N 4 phase is almost completely transformed into the elongated columnar β-Si 3 N 4 phase in the sintered Si 3 N 4 /n-SiC ceramics when the holding time is 10 min. As shown in Figure 10b,c, the distribution of β-Si 3 N 4 crystal size is uneven when the holding time is more than 10 min, the reasonable explanations is that someβ-Si 3 N 4 grains grow too large to occupy the growing space for other grains. Figure 10c shows that there are some larger holes or pits on the etched surface of the sintered Si 3 N 4 /n-SiC ceramics. It is a possible explanation that the liquid phase is connected to one piece for the long holding time, which causes some Si 3 N 4 grains to fall off with the dissolution of the liquid phase during the molten NaOH corrosion process to form holes or pits.

Influence of Holding Time on the Properties of Si3N4/n-SiC Ceramics
holding time is more than 10 min, the reasonable explanations is that someβ-Si3N4 grains grow too large to occupy the growing space for other grains. Figure 10c shows that there are some larger holes or pits on the etched surface of the sintered Si3N4/n-SiC ceramics. It is a possible explanation that the liquid phase is connected to one piece for the long holding time, which causes some Si3N4 grains to fall off with the dissolution of the liquid phase during the molten NaOH corrosion process to form holes or pits.   Figure 12 shows the influence of different holding time on the Vickers hardness and fracture toughness of Si3N4/n-SiC ceramics at a constant temperature of 1600 °C. As can be seen from it, the hardness and fracture toughness are both gradually rising first and then falling over time, and it is clear that they both reach their maximum value, 14.44 GPa and 7.77 MPa·m 1/2 , respectively, after the ceramics having been sintered for 10 min. It turns out that the very first ten minutes of the thermal insulation stage plays a key role in improving the microstructure and mechanical performances of the microwave sintered Si3N4/n-SiC ceramics, while a longer holding time, which is more than 10 min, will result in abnormal grain growth, uneven grain size and pores, and finally lead to a degradation in material properties, according to the experiments.   Figure 12 shows the influence of different holding time on the Vickers hardness and fracture toughness of Si3N4/n-SiC ceramics at a constant temperature of 1600 °C. As can be seen from it, the hardness and fracture toughness are both gradually rising first and then falling over time, and it is clear that they both reach their maximum value, 14.44 GPa and 7.77 MPa·m 1/2 , respectively, after the ceramics having been sintered for 10 min. It turns out that the very first ten minutes of the thermal insulation stage plays a key role in improving the microstructure and mechanical performances of the microwave sintered Si3N4/n-SiC ceramics, while a longer holding time, which is more than 10 min, will result in abnormal grain growth, uneven grain size and pores, and finally lead to a degradation in material properties, according to the experiments.  Figure 12 shows the influence of different holding time on the Vickers hardness and fracture toughness of Si 3 N 4 /n-SiC ceramics at a constant temperature of 1600 • C. As can be seen from it, the hardness and fracture toughness are both gradually rising first and then falling over time, and it is clear that they both reach their maximum value, 14.44 GPa and 7.77 MPa·m 1/2 , respectively, after the ceramics having been sintered for 10 min. It turns out that the very first ten minutes of the thermal insulation stage plays a key role in improving the microstructure and mechanical performances of the microwave sintered Si 3 N 4 /n-SiC ceramics, while a longer holding time, which is more than 10 min, will result in abnormal grain growth, uneven grain size and pores, and finally lead to a degradation in material properties, according to the experiments.  Table 3 lists the relative density and mechanical performances of other Si3N4 materials with different sintering aids and sintering methods. It can be seen that the results vary in not only the different sintering methods based sintering processes, but also the processes using the same sintering method. When compared with the listed results in Table 3, the fracture toughness of newly proposed microwave sintered Si3N4/n-SiC ceramics have obtained an increase of 12.8-27.4%. It is safe to say that preparing high-performance ceramic materials is of great significance with the rapid development of microwave sintering technology.

Conclusions
In this paper, the microwave sintering process of the Si3N4/n-SiC ceramics (85 wt% Si3N4 + 5 wt% n-SiC + 5 wt% Al2O3 + 5 wt% Y2O3) was studied, focusing on the effects of a different temperature and holding time on the properties and microstructure. The major conclusions that are derived from this study are as follows: • A certain sintering temperature around 1600 °C is necessary for the densification and β-Si3N4 phase formation of the Si3N4/n-SiC ceramics. Held at a temperature below 1500 °C, the Si3N4/n-SiC ceramics cannot be completely sintered. However, a higher temperature over 1700 °C, can cause the decomposition of Si3N4 or Si3N4's reacting with sintering aids (Al2O3 and Y2O3), which will lead to the decrease of density, Vickers hardness, and fracture toughness of the Si3N4/n-SiC.

•
The density, hardness, and fracture toughness will all gradually degrade if the Si3N4/n-SiC ceramics are sintered with microwave exceeding 10 min. Although the long holding time makes some β-Si3N4 grains grow larger, it also causes the uneven size of the β-Si3N4 grain, and pores appear between the big grains at the same time. Accordingly, it can be concluded that the  Table 3 lists the relative density and mechanical performances of other Si 3 N 4 materials with different sintering aids and sintering methods. It can be seen that the results vary in not only the different sintering methods based sintering processes, but also the processes using the same sintering method. When compared with the listed results in Table 3, the fracture toughness of newly proposed microwave sintered Si 3 N 4 /n-SiC ceramics have obtained an increase of 12.8-27.4%. It is safe to say that preparing high-performance ceramic materials is of great significance with the rapid development of microwave sintering technology.

Conclusions
In this paper, the microwave sintering process of the Si 3 N 4 /n-SiC ceramics (85 wt% Si 3 N 4 + 5 wt% n-SiC + 5 wt% Al 2 O 3 + 5 wt% Y 2 O 3 ) was studied, focusing on the effects of a different temperature and holding time on the properties and microstructure. The major conclusions that are derived from this study are as follows: • A certain sintering temperature around 1600 • C is necessary for the densification and β-Si 3 N 4 phase formation of the Si 3 N 4 /n-SiC ceramics. Held at a temperature below 1500 • C, the Si 3 N 4 /n-SiC ceramics cannot be completely sintered. However, a higher temperature over 1700 • C, can cause the decomposition of Si 3 N 4 or Si 3 N 4 's reacting with sintering aids (Al 2 O 3 and Y 2 O 3 ), which will lead to the decrease of density, Vickers hardness, and fracture toughness of the Si 3 N 4 /n-SiC.

•
The density, hardness, and fracture toughness will all gradually degrade if the Si 3 N 4 /n-SiC ceramics are sintered with microwave exceeding 10 min. Although the long holding time makes some β-Si 3 N 4 grains grow larger, it also causes the uneven size of the β-Si 3 N 4 grain, and pores appear between the big grains at the same time. Accordingly, it can be concluded that the microwave sintered Si 3 N 4 /n-SiC ceramics should not be held for too long. In this paper, the optimum holding time of microwave sintered Si 3 N 4 /n-SiC ceramics is presented to be 10 min at 1600 • C.