Effect of CeO2-Y2O3 sintering aids on the microstructure and properties of corundum-based composite ceramics

ABSTRACT Reducing the preparation cost of alumina ceramics has received significant attention, as alumina is one of the most widely used structural ceramics. In this study, corundum-based composite ceramics with a white corundum-to-zircon powder mass ratio of 7:3 were selected as the matrix. CeO2 and Y2O3 doped corundum-based composite ceramics were prepared via pressureless sintering. The total content of CeO2-Y2O3 was set at 10 wt% to study the effects of different temperatures and CeO2-Y2O3 ratios on the properties of the corundum-based composite ceramics. Physical phase and microstructural analyses of the corundum-based composite ceramics were performed using X-ray diffraction and scanning electron microscopy, respectively. The results showed that the sintering temperature of the corundum-based composite ceramics could be effectively reduced by using a solid solution of CeO2 and Y2O3. The best sintering temperature was 1400°C, and the best composition was 5 wt% CeO2-5 wt% Y2O3. The physical properties of the corundum-based composite ceramics prepared under these technological conditions include a flexural strength of 386.2 MPa, hardness of 13.5 GPa, density of 3.9128 g/cm3, linear shrinkage of 16.31% and an apparent porosity of 0.62%.


Introduction
Corundum ceramics have a wide variety of potential applications in wear-resistant materials, refractory materials, and mechanical processing owing to their high melting points, high hardness, good wear resistance, good thermal shock resistance, and high physical and chemical stability [1,2].The ionic and covalent properties of the Al-O bonds in alumina make it one of the most versatile refractory ceramic oxides.However, it is prone to anisotropy and abnormal grain growth during a long sintering process due to its high sintering temperature (above 1700°C) and low diffusion coefficient.In addition, alumina ceramics have reduced densities due to the presence of residual porosity, which is inevitable [3][4][5].Both these drawbacks illustrate that preparing low-cost, high-density alumina ceramics for commercial use in a simple and reliable manner remains a great challenge.Many researchers have studied the sintering behavior and microstructure of alumina ceramics in detail by adding sintering aids or by using various sintering techniques, such as hot-pressure [6], spark plasma [7], oscillatory pressure [8], and microwave sintering [9].Currently, the addition of sintering aids is the most effective method for decreasing the sintering temperature and increasing the density of ceramics.The temperature required to sinter alumina ceramics has been minimized by adding oxide sintering aids, which aids in forming a liquid phase.The oxides that promoted the formation of a liquid phase include CaO, MgO, SiO 2 , B 2 O 3 , TiO 2 , CuO, MnO 2 , Y 2 O 3 , La 2 O 3 , and Nb 2 O 5 [10][11][12][13][14][15][16][17][18][19].These oxides, alone or in combination, can form a glass-liquid phase, low-eutectic-point liquid phase, or lattice defects within the solid solution matrix; these are conducive to atomic diffusion, which accelerates the sintering process of alumina ceramics and is significant for sintering.After adding sintering additives, the sintering temperature of alumina ceramics was reduced to 1400°C-1450°C; however, the cooling effect was not significant [20][21][22][23][24].The addition of sintering aids also promotes the formation of anisotropic grains with a large width-to-diameter ratio, which significantly affects the final microstructure of alumina ceramics [25].
Zirconium silicate has a high dielectric constant (~15), strong corrosion resistance, high melting point (>1670°C), low thermal conductivity and expansion, and excellent chemical and phase stability.Zircon powder (ZrSiO 4 ) is generally used in refractory coatings; however, owing to a lack of resources, it has recently been compounded with white corundum to produce coatings for large castings.ZrSiO 4 decomposes into ZrO 2 and SiO 2 at high temperatures.While ZrO 2 can toughen Al 2 O 3 -based ceramics, SiO 2 can be used as a sintering aid for Al 2 O 3 .However, Y 2 O 3 can be used as both a sintering aid for Al 2 O 3 and a ZrO 2 stabilizer to prevent a phase change [26][27][28][29][30][31][32][33][34][35].Therefore, in this study, corundum-based composite ceramics were prepared by atmospheric pressure sintering method using white corundum and zircon powder as raw materials and CeO 2 -Y 2 O 3 binary system as sintering aid.The effects of CeO 2 -Y 2 O 3 sintering aids on the mechanical and sintering properties of corundumbased composite ceramics were investigated and the optimal CeO 2 -Y 2 O 3 ratio and sintering temperature were determined.1.

Experimental procedure
The raw materials were weighed proportionally and placed in a polytetrafluoroethylene ball-milling tank.Al 2 O 3 balls, polyvinyl alcohol (5 wt%), and ethanol were selected as the grinding balls, binder, and ballmilling medium, respectively.The ball milling time was 4 h, and the ball-to-powder ratio was 2:1.The prepared slurry was dried in an oven at 60°C for 12 h and then passed through a 100-mesh screen.The sieved powder (25 g) was placed in a steel mold (45 mm diameter) and pressed under a uniaxial pressure of 20 MPa for 10 min to obtain a green body (φ45 × H7 mm).Subsequently, using a pressureless sintering technique, the raw billets were heated in a Muffle furnace at 2°C/min to 600°C and held for 1 h to burn out the organic additives.The temperature was then increased to the target temperature and maintained for 2 h, and the sintering temperatures were 1300°C, 1350°C, 1400°C, 1450°C, and 1500°C.Finally, the temperatures were cooled to room temperature.
The bulk density and apparent porosity of the samples were determined using Archimedes' method.The flexural strengths of the corundum-based composite ceramics were measured using an electronic universal testing machine (CMT5105; Shenzhen Xinsanji Material Testing Co., Ltd.).The samples were processed into standard specimens measuring 4 mm × 3 mm × 36 mm by diamond wire cutting, their surfaces were polished and their four corners were chamfered with sandpaper (to a depth of approximately 0.3 mm).The samples were placed on a beam with a span of 30 mm and a loading rate was 0.5 mm/min.The hardness of the samples was measured using a Wechsler hardness tester.A diamond tetragonal cone indenter with an angle of 136° on both sides was used to apply pressure to the samples, which were held at a pressure of 1 kg for 15 s.X-ray diffraction (XRD, Japan Science Smartlab SE) was used to analyze the physical phase compositions.The microstructures of the samples were analyzed using scanning electron microscopy (SEM, Zeiss GeminiSEM360 UK) and X-ray energy-dispersive spectroscopy (EDS).

Phase composition
Figure 1 shows the phase compositions of the corundum-based composite ceramics with a CeO 2 content of 5 wt% sintered at different temperatures for 2 h.At sintering temperatures of 1300°C and 1350°C, in addition to the main phases Al 2 O 3 and ZrSiO 4 , small amounts of mullite, t-ZrO 2 , Zr 0.9 Ce 0.1 O 2 , and Zr 0.8 Y 0.2 O 1.9 solid solution phases were produced in corundum-based composite ceramics.After the sintering temperature reached 1400°C, the peak of ZrSiO 4 completely disappeared from the diagram and a large number of t-ZrO 2 , mullite, Zr 0.9 Ce 0.1 O 2 , and Zr 0.8 Y 0.2 O 1.9 solid solution phases appeared.It was shown that ZrSiO 4 decomposed completely at 1400°C, decomposing into SiO 2 and ZrO 2 .At temperatures higher than    CeO2 has the fluorite structure in the cubic crystal system.Because the diameter of Ce 4+ is larger than that of O 2-, the face-centered cubic lattice is assumed to be composed of Ce 4+ , which occupy 1/2 of the octahedral voids, while O 2-occupies all four tetrahedral voids.Owing to the difference in the ionic radii and valence states, Ce 4+ and Y 3+ must replace Al 3+ to form cationic vacancies to maintain the neutrality of the corundum-based composite ceramics.The decrease in the binding energy near the vacancy results in an increase in the lattice constant and alumin lattice distortion.This lattice activation facilitates atomic diffusion and promotes the densification of ceramics.The reaction of point defects can be expressed as follows: where Ce and Y represent the positions at which Ce 4+ and Y 3+ replace Al 3+ , respectively (the upper-right mark indicates the charge remaining after substitution), and V represents the Al 3+ vacancy.According to Eq. ( 5), the equilibrium constant K of the Frankel defect reaction can be calculated as follows: From the above equations, the diffusion coefficient is determined to be proportional to the vacancy concentration(V), which is proportional to the 3/4 power of the CeO 2 concentration.As the sintering temperature increases, the diffusion coefficient increases, promoting a nonlinear increase in the sintering rate.This is an external manifestation of the internal mechanism of solid-phase reaction sintering.

Physical properties
Figure 3 shows the physical properties of the corundum-based composite ceramics with a CeO 2 content of 5 wt% sintered at different temperatures for 2 h.From Figure 3(a), a sintering temperature of 1300°C yields the smallest density and linear shrinkage and the highest apparent porosity.Subsequently, the density, linear shrinkage, and apparent porosity of the samples changed slightly with increasing temperature.When the sintering temperature reached 1400°C, the density and linear shrinkage of the sample reached their maximum values, and the apparent porosity reached its minimum.From Figure 3(b), the flexural strength and hardness of the corundum-based composite ceramics first increased sharply and then had small fluctuations as the sintering temperature increased, reaching their maximum values at 1400°C.Combined with the XRD pattern in Figure 1, the sample was not completely sintered at 1300°C.At temperatures above 1300°C, the activation energy required for the sintering of corundum-based composite ceramics was reached and the samples began to densify and increase in strength.This indicates that the introduction of the CeO 2 -Y 2 O 3 system into the corundum-based composite ceramics promoted the formation of Zr 0.9 Ce 0.1 O 2 and Zr 0.8 Y 0.2 O 1.9 solid solutions, which further reduced the sintering temperature of the samples.
Figure 4 shows the physical properties of the corundum-based composite ceramics with different sintering aids sintered at 1400°C for 2 h.From Figure 4(a), the density and linear shrinkage of the samples tended to increase, decrease, and then increase with increasing CeO 2 concentration, whereas that of the pore size behaved in the opposite manner.When the CeO 2 content reached 5 wt%, the density and linear shrinkage of the samples were at their largest, and the apparent porosity was at this smallest.However, when the CeO 2 content was 6 wt%, a sudden change occurred.As shown in Figure 4(b), the flexural strength and hardness of the corundum-based composite ceramics first increased and then decreased with increasing CeO 2 content.The flexural strength and hardness of the sample reached their maximum values when the CeO 2 content reached 5 wt%.This is because when the CeO 2 content was less than 5 wt%, Y 2 O 3 played an important role.On the one hand, Y 2 O 3 can stabilize ZrO 2 and prevent phase change, and on the other hand, it can refine the grains and at the same time increase the amount of liquid phase and filling the interstices of the grains, thus increasing the density of the corundum ceramics, and the strength increases.CeO 2 and Y 2 O 3 act together when the CeO 2 content was 5 wt%, at this time density, porosity, line shrinkage and mechanical properties were highest.However, once the CeO 2 content exceeded 5 wt%, the individual grain size began to increase and the increased interstitial space had a negative effect on both densification  and strength.As CeO 2 continued to increase, it could be seen in conjunction with the SEM in Figure 6 that the grains began to grow as a whole and the proportion of transgranular fracture increased, so the strength would increase again.

Microstructure
Figure 5 shows the SEM images of sections of the corundum-based composite ceramics with a CeO 2 content of 5 wt% sintered for 2 h at different temperatures.Because the sample was not fully sintered at 1300°C (Figure 5(a)), the structure was loose, resulting in charge accumulation and fuzzy imaging.As the temperature increased, the corundum-based composite ceramics began to sinter, the grains gradually grew, the pores were filled and the structure became denser.Figure 5(b) shows that the porosity has been significantly reduced.When the temperature reached 1400°C, a large number of dimples appear in corundum-based composite ceramics.The crystal broke mainly by transgranular fracture and grain stripping, which is also the main reason for the increase in strength.However, when the temperature exceeded 1400, small particles tended to dissolve in the matrix while large particles tended to grow.Excessive grain growth would lead to abnormal grain growth and its fracture would be dominated by along-crystal fracture, leading to a reduction in strength.
Figure 6 shows the SEM images of cross-sections of corundum-based composite ceramics with different sintering aids sintered at 1400°C for 2 h.When the CeO 2 content increased to 3 wt%, the percentage of grain pullout gradually decreased, and the percentage of abnormal grains started to increase (Figure 5 (a-c)), indicating that Y 2 O 3 can inhibit abnormal grain growth.This CeO 2 concentration corresponded to the beginning of decreasing intensity, as shown in Figure 4(b).When the CeO 2 content was increased to 5 wt%, the grains started to decrease in size and the percentage of grain pullout increased (Figure 5(d,e)), indicating that CeO 2 started to play a role.As the CeO 2 content continued to increase, CeO 2 dominated and the proportion of transgranular fractures began to decrease (Figure 5(f-i)).This is also the main reason for the consequent increase in intensity (Figure 4(b)).
Figure 7 shows the EDS diagram of a section of corundum-based composite ceramics with the section of the 5 wt% CeO 2 -5 wt% Y 2 O 3 sintered at 1400°C for 2 h.As can be seen from the figure, SiO 2 combined with part of Al 2 O 3 to produce a small amount of mullite, and there is still part of ZrSiO 4 not completely decomposed.Y 2 O 3 , CeO 2 combined with ZrO 2 , and according to the XRD pattern, it can be analyzed that most of Y 2 O 3 and CeO 2 were soluted into ZrO 2 , forming a solid solution and reducing the sintering temperature.

Conclusion
Corundum-based composite ceramics doped with 10 wt% CeO 2 and Y 2 O 3 were prepared.The effects of different temperatures and ratio of CeO 2 -Y 2 O 3 on the sintering properties, physical properties, and microstructure were investigated.The main conclusions are as follows: (1) Corundum-based composite ceramics were prepared using a 5 wt% CeO 2 -5 wt% Y 2 O 3 binary sintering aid at sintering temperatures of 1300°-C-1500°C.A combination of XRD, SEM, and physical property analyses shows that the samples were not completely sintered at 1300°C.With an increase in temperature, the sample began to sinter, the grains grew gradually, and its mechanical performance initially increased and then decreased.When the temperature reached 1400°C, the performance was at its best.Binary

Figure 2 Figure 1 .
Figure 2 shows the phase composition of the corundum-based composite ceramics with different sintering aids sintered for 2 h at 1400°C.As shown in Figure 2(a), the primary crystalline phases in the corundum-based composite ceramics were Al 2 O 3 , t-

Figure 2 .
Figure 2. XRD patterns of different samples sintered at 1400°C for 2 h.

Figure 4 .
Figure 4. Physical properties of different samples sintered at 1400°C for 2 h.
CeO 2 -Y 2 O 3 sintering aids effectively reduced the sintering temperature.(2) The effects of different ratios of CeO 2 -Y 2 O 3 on the properties of corundum-based composite ceramics were studied at 1400°C.With increasing CeO 2 content, the grains' fracture mode was mainly transgranular pulling out.The best performance was obtained with the 5 wt% CeO 2 -5 wt% Y 2 O 3 system.The flexural strength was 386.2 MPa, the hardness was 13.5 GPa, the density was 3.9128 g/cm 3 , the linear shrinkage was 16.31%, and the apparent porosity was 0.62%.(3) During sintering with the binary sintering aids, CeO 2 and Y 2 O 3 solid solutions were added to the corundum-based composite ceramics, forming Zr 0.9 Ce 0.1 O 2 and Zr 0.8 Y 0.2 O 1.9 solid solutions, which promoted the decomposition of ZrSiO 4 and the sintering of the corundum-based composite ceramics.

Figure 7 .
Figure 7. EDS of the sample containing 5 wt % CeO 2 sintered at 1400°C for 2 h.
ZrO 2 , mullite, Zr 0.9 Ce 0.1 O 2 , and Zr 0.8 Y 0.2 O 1.9 .However, no CeO 2 or Y 2 O 3 phases were observed in the XRD pattern, indicating that CeO 2 and Y 2 O 3 were completely dissolved into the composite corundum.As shown in Figure2(b), the increase in CeO 2 content gradually shifted the 2θ value of the (110) diffraction peak to a smaller angle; using the Bragg equation, the lattice constant of Al 2 O 3 increased.The Bragg equation is expressed as follows: