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%.

KEYWORDS:

• Corundum-based composite ceramics
• sintering aids
• pressureless sintering
• mechanical properties
• microstructure

1. 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 [3–5]. 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, SiO₂, B₂O₃, TiO₂, CuO, MnO₂, Y₂O₃, La₂O₃, and Nb₂O₅ [10–19]. 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 [20–24]. 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 (ZrSiO₄) 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. ZrSiO₄ decomposes into ZrO₂ and SiO₂ at high temperatures. While ZrO₂ can toughen Al₂O₃-based ceramics, SiO₂ can be used as a sintering aid for Al₂O₃. However, Y₂O₃ can be used as both a sintering aid for Al₂O₃ and a ZrO₂ stabilizer to prevent a phase change [26–35]. 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 CeO₂-Y₂O₃ binary system as sintering aid. The effects of CeO₂-Y₂O₃ sintering aids on the mechanical and sintering properties of corundum-based composite ceramics were investigated and the optimal CeO₂-Y₂O₃ ratio and sintering temperature were determined.

2. Experimental procedure

In this study, α-Al₂O₃ analytical grade powder (purity, 99.9%; particle size 5 μm; Shanghai Yunfu Nano Technology Co., Ltd., Shanghai, China), zircon powder (particle size 20 μm; composition, ZrO₂ >65 wt%, SiO₂ ≤33 wt%, TiO₂ <0.3 wt%, Al₂O₃ <1.8 wt%, P₂O₃ ≤0.2 wt%, Fe₂O₃ <0.2 wt%; Henan Yixiang Materials, Henan, China), Y₂O₃, and CeO₂ (purity, 99.0%; particle size 5 μm; Shanghai Maclin Biochemical Technology Co., Ltd., Shanghai, China)were used. Based on the results from preliminary tests, alumina, and zircon powder were mixed in a 7:3 ratio to form a composite corundum. The total content of the sintering aids (CeO₂ and Y₂O₃) was set to 10 wt%. The influence of the sintering aids on the sample properties was studied by changing the mass ratios of CeO₂ and Y₂O₃, as shown in Table 1.

Table 1. The formula compositions of samples (wt%).

Sample No.	α-Al2O3	Zircon powder	CeO2	Y2O3	CeO2:Y2O3
1	67	23	1	9	1:9
2	67	23	2	8	2:8
3	67	23	3	7	3:7
4	67	23	4	6	4:6
5	67	23	5	5	5:5
6	67	23	6	4	6:4
7	67	23	7	3	7:3
8	67	23	8	2	8:2
9	67	23	9	1	9:1

The raw materials were weighed proportionally and placed in a polytetrafluoroethylene ball-milling tank. Al₂O₃ balls, polyvinyl alcohol (5 wt%), and ethanol were selected as the grinding balls, binder, and ball-milling 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).

3. Results and discussions

3.1. Phase composition

Figure 1 shows the phase compositions of the corundum-based composite ceramics with a CeO₂ 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 Al₂O₃ and ZrSiO₄, small amounts of mullite, t-ZrO₂, Zr_0.9Ce_0.1O₂, and Zr_0.8Y_0.2O_1.9 solid solution phases were produced in corundum-based composite ceramics. After the sintering temperature reached 1400°C, the peak of ZrSiO₄ completely disappeared from the diagram and a large number of t-ZrO₂, mullite, Zr_0.9Ce_0.1O₂, and Zr_0.8Y_0.2O_1.9 solid solution phases appeared. It was shown that ZrSiO₄ decomposed completely at 1400°C, decomposing into SiO₂ and ZrO₂. At temperatures higher than 1400°C, ZrO₂ existed mainly in the crystalline form of the tetragonal phase. The formation of t-ZrO₂ could improve the mechanical properties of composites. SiO₂ with Al₂O₃ produced mullite and the added CeO₂ and Y₂O₃ sinter aids were solidly dissolved in ZrO₂ to form the Zr_0.9Ce_0.1O₂ and Zr_0.8Y_0.2O_1.9 solid solution phases. Although the actual decomposition temperature of ZrSiO₄ was 1550°C, the introduction of the CeO₂-Y₂O₃ system as a sintering aid led to a lower decomposition temperature. The reactions involved in the sintering process could be expressed by the following:

(1) $ZrSi{O}_4\stackrel{1400}{\to }Zr{O}_2+Si{O}_2$(1)

(2) $2ZrSi{O}_4+3A{l}_2{O}_3\to 2Zr{O}_2+3A{l}_2{O}_3\cdot 2Si{O}_2$(2)

(3) $3A{l}_2{O}_3+2Si{O}_2\to 3A{l}_2{O}_3\cdot 2Si{O}_2$(3)

Figure 1. XRD patterns of samples containing 5 wt% CeO₂ sintered at 1300°C-1500°C for 2 h.

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 Al₂O₃，t-ZrO₂, mullite, Zr_0.9Ce_0.1O₂, and Zr_0.8Y_0.2O_1.9. However, no CeO₂ or Y₂O₃ phases were observed in the XRD pattern, indicating that CeO₂ and Y₂O₃ were completely dissolved into the composite corundum. As shown in Figure 2(b), the increase in CeO₂ content gradually shifted the 2θ value of the (110) diffraction peak to a smaller angle; using the Bragg equation, the lattice constant of Al₂O₃ increased. The Bragg equation is expressed as follows:

(4) $2dsin\theta =n\lambda$(4)

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

CeO2 has the fluorite structure in the cubic crystal system. Because the diameter of Ce⁴⁺ is larger than that of O^2-, the face-centered cubic lattice is assumed to be composed of Ce⁴⁺, 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, Ce⁴⁺ and Y³⁺ must replace Al³⁺ 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:

(5) $3Ce{O}_2\stackrel{A{l}_2{O}_3}{\to }3C{e}_{Al}^{\bullet }+V_{Al}^{{ }^{\prime \prime \prime }}+6O_O^{\times }$(5)

(6) $Y_2{O}_3\stackrel{A{l}_2{O}_3}{\to }2Y_{Al}^{\times }+3O_O^{\times }$(6)

where Ce and Y represent the positions at which Ce⁴⁺ and Y³⁺ replace Al³⁺, respectively (the upper-right mark indicates the charge remaining after substitution), and V represents the Al³⁺ vacancy. According to Eq. (5), the equilibrium constant K of the Frankel defect reaction can be calculated as follows:

(7) $K=\frac{\left[C{e}_{Al}^{\bullet }{]}^3\right[V_{Al}^{{ }^{\prime \prime \prime }}]}{{[Ce{O}_2]}^3}=\frac{27{[V_{Al}^{{ }^{\prime \prime \prime }}]}^4}{{[Ce{O}_2]}^3}$(7)

(8) $\left[V_{Al}^{{ }^{\prime \prime \prime }}\right]=(\frac{K}{27}{)}^{\frac{1}{4}}[Ce{O}_2{]}^{\frac{3}{4}}$(8)

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 CeO₂ 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.

3.2. Physical properties

Figure 3 shows the physical properties of the corundum-based composite ceramics with a CeO₂ 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 CeO₂-Y₂O₃ system into the corundum-based composite ceramics promoted the formation of Zr_0.9Ce_0.1O₂ and Zr_0.8Y_0.2O_1.9 solid solutions, which further reduced the sintering temperature of the samples.

Figure 3. Physical properties of samples containing 5 wt% CeO₂ sintered at 1300°C-1500°C for 2 h.

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 CeO₂ concentration, whereas that of the pore size behaved in the opposite manner. When the CeO₂ 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 CeO₂ 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 CeO₂ content. The flexural strength and hardness of the sample reached their maximum values when the CeO₂ content reached 5 wt%. This is because when the CeO₂ content was less than 5 wt%, Y₂O₃ played an important role. On the one hand, Y₂O₃ can stabilize ZrO₂ 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. CeO₂ and Y₂O₃ act together when the CeO₂ content was 5 wt%, at this time density, porosity, line shrinkage and mechanical properties were highest. However, once the CeO₂ 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 CeO₂ 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.

Figure 4. Physical properties of different samples sintered at 1400°C for 2 h.

3.3. Microstructure

Figure 5 shows the SEM images of sections of the corundum-based composite ceramics with a CeO₂ 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 5. SEM of the sample containing 5 wt % CeO₂ sintered at 1300°C-1500°C for 2 h: (a) 1300°C; (b) 1350°C; (c) 1400°C; (d) 1450°C; (e) 1500°C.

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 CeO₂ 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 Y₂O₃ can inhibit abnormal grain growth. This CeO₂ concentration corresponded to the beginning of decreasing intensity, as shown in Figure 4(b). When the CeO₂ 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 CeO₂ started to play a role. As the CeO₂ content continued to increase, CeO₂ 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 6. SEM of different samples after sintering at 1400°C for 2 h: (a) 1 wt% CeO₂; (b) 2 wt% CeO₂; (c) 3 wt% CeO₂; (d) 4 wt% CeO₂₃; (e) 5 wt% CeO₂; (f) 6 wt% CeO₂; (g) 7 wt% CeO₂; (h) 8 wt% CeO₂; (i) 9 wt% CeO₂.

Figure 7 shows the EDS diagram of a section of corundum-based composite ceramics with the section of the 5 wt% CeO₂-5 wt% Y₂O₃ sintered at 1400°C for 2 h. As can be seen from the figure, SiO₂ combined with part of Al₂O₃ to produce a small amount of mullite, and there is still part of ZrSiO₄ not completely decomposed. Y₂O₃, CeO₂ combined with ZrO₂, and according to the XRD pattern, it can be analyzed that most of Y₂O₃ and CeO₂ were soluted into ZrO₂, forming a solid solution and reducing the sintering temperature.

Figure 7. EDS of the sample containing 5 wt % CeO₂ sintered at 1400°C for 2 h.

4. Conclusion

Corundum-based composite ceramics doped with 10 wt% CeO₂ and Y₂O₃ were prepared. The effects of different temperatures and ratio of CeO₂-Y₂O₃ 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% CeO₂-5 wt% Y₂O₃ 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 CeO₂-Y₂O₃ sintering aids effectively reduced the sintering temperature.

2. The effects of different ratios of CeO₂-Y₂O₃ on the properties of corundum-based composite ceramics were studied at 1400°C. With increasing CeO₂ content, the grains’ fracture mode was mainly transgranular pulling out. The best performance was obtained with the 5 wt% CeO₂-5 wt% Y₂O₃ system. The flexural strength was 386.2 MPa, the hardness was 13.5 GPa, the density was 3.9128 g/cm³, the linear shrinkage was 16.31%, and the apparent porosity was 0.62%.

3. During sintering with the binary sintering aids, CeO₂ and Y₂O₃ solid solutions were added to the corundum-based composite ceramics, forming Zr_0.9Ce_0.1O₂ and Zr_0.8Y_0.2O_1.9 solid solutions, which promoted the decomposition of ZrSiO₄ and the sintering of the corundum-based composite ceramics.

Acknowledgments

Authors appreciate the financial supported by the National Natural Science Foundation of China (Grant No. 51872118), the Key Research and Development Program of Shandong Province (Grant No. 2020JMRH0401), the Shandong Provincial Natural Science Foundation (Grant No. ZR2023ME127, ZR2019MEM055). The project supported by State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology). This work was financially supported by the Taishan Scholars Program, and the Case-by-Case Project for Top Outstanding Talents of Jinan.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the Key Technology Research and Development Program of Shandong Province [2020JMRH0401]; National Natural Science Foundation of China [51872118]; Natural Science Foundation of Shandong Province [ZR2023ME127]; Natural Science Foundation of Shandong Province [ZR2019MEM055].