Toughening Mechanism of CaAl₁₂O₁₉ in Red Mud–Al₂O₃ Composite Ceramics

Abstract

The utilization of red mud in the production of ceramic products represents an efficient approach for harnessing red mud resources. Composite ceramics were prepared from Al₂O₃, red mud, and Cr₂O₃ by atmospheric pressure sintering, and the phase composition and microscopic morphology of the composite ceramics were investigated by XRD, SEM, and EDS. The flexural strength and fracture toughness of composite ceramics were measured by three-point bending and SENB methods. The results showed that the composite ceramics sintered at 1500 °C with the addition of 1.5 wt.% Cr₂O₃ had a flexural strength of 297.03 MPa, a hardness of 17.44 GPa, and a densification of 97.75% and fracture toughness of 6.57 MPa·m^1/2. The addition of Cr₂O₃ helps to improve the low strength of red mud composite ceramic samples. The CaAl₁₂O₁₉ phase can form a similar “endo-crystalline” structure with Al₂O₃ grains, which changes the fracture mode of the ceramics and thus significantly improves the fracture toughness. The wettability tests conducted on Cu and RM–Al₂O₃ composite ceramic materials revealed that the composites exhibited non-wetting behavior towards Cu at elevated temperatures, while no interfacial reactions or elemental diffusion were observed. Composites have higher surface energy than Al₂O₃ ceramic at high temperatures. The present study provides a crucial foundation for enhancing the comprehensive utilization value of red mud and the application of red mud ceramics in the field of electronic packaging.

1. Introduction

“Red mud” (RM) refers to solid waste that is discharged during the process of generating Al₂O₃. Comprised primarily of various complex mineral phases, red mud possesses a highly alkaline nature. The current situation lacks an effective solution for red mud treatment, primarily relying on the construction of multiple containment dams to store the red mud [1,2]. With prolonged stockpiling, the harmful elements in red mud can contaminate nearby water sources and cause soil alkalization. At the same time, red mud is highly adsorbent, and dams may also be at risk of breaking during the rainy season. This seriously jeopardizes the safety of people’s lives and property. Wang et al. [3] took red mud and kaolin as raw materials, changed the reaction path of mullite by adding (NH₄)₆Mo₇O₂₄, and sintered the sample at 1180 °C to obtain a sample with a flexion strength of 185.6 MPa, a bulk weight of 1.45 g/cm³, and a water absorption rate of 5.5%. He et al. [4] used red mud, clay and raw materials, and a 20% RM ceramic body was prepared by sintering at 1050 °C for 2 h. The ceramic body had a bulk weight of 1.69 g/cm³, loss on ignition of 10.32%, sintering shrinkage of 5.71%, water absorption of 19.91%, and compressive strength of 36.5 MPa. Pérez-Villarejo et al. [5] mixed red mud and clay in a 1:1 ratio and sintered at 950 °C for 1 h. The samples obtained had a linear shrinkage of 0.46%, a water absorption of 21.00%, a weight loss of 12.6% after sintering, and a compressive strength of 52.54 MPa. Triviño et al. [6] mixed red mud and granite waste in a ratio of 6:4 and sintered the sample at 1150 °C, and the sample obtained had the highest hardness (545.6 kgf/mm²) and a flexural strength of 27 MPa. The addition of red mud should be carefully controlled in order to enhance the properties of ceramic products, as red mud itself contains high levels of Na₂O and CaO, which are detrimental to the properties of cement and ceramics. Furthermore, the high content of red mud in the sample also makes it easier for the harmful elements in the red mud to leak into the environment, which limits the application of red mud ceramic products.

Cr₂O₃ is often used as one of the additives to improve the physical properties of alumina, Cr₂O₃ and Al₂O₃ have isostructural corundum crystal structures (hexagonal system with the same space group R-3c), and it is well known that Cr₂O₃ forms a substitutional solid solution in the Al₂O₃ lattice by ion exchange with Al³⁺ and does not form any eutectic in the whole compositional range [7]. Several studies have shown the addition of Cr₂O₃ to Al₂O₃ in different ratios to improve the mechanical properties of ceramics by means of different solidification techniques. Kafkaslıoğlu Yıldız et al. [8] investigated the effect of Cr₂O₃ additions with different volume ratios (0.5, 1, 5 vol.%) on the microstructure and mechanical properties of Al₂O₃. When the Cr₂O₃ addition was 0.5 vol.%, the flexural strength of the material was 286 MPa, which increased by 44% compared to pure Al₂O₃. This is due to the localized compressive stress caused by the grain boundary alteration of Cr³⁺ ions. The crack extension behavior changes from a predominantly along-crystal extension to mixed along-crystal and through-crystal extensions with the addition of localized compressive stresses from Cr₂O₃, leading to grain boundary strengthening. Azhar et al. [9] reported that the microstructures of ZTA composite were significantly affected by the addition of a small amount of Cr₂O₃ (∼0.6 wt.%), with the grains becoming larger and acquiring a plate-like shape. The fracture toughness was increased from 4.41 MPa·m^1/2 to 4.73 MPa·m^1/2. Manshor et al. [10] reported that with addition of 0.6 wt.% of Cr₂O₃ to the composite, an optimum density of 4.0630 g/cm³, hardness of 1681 HV, and fracture toughness of 7.15 MPa·m^1/2 were obtained. The addition of 0.6 wt.% Cr₂O₃ led to the formation of large plate-like grains, enhancing the interaction between the crack and the matrix grains. The inter-granular fracture mode caused the cracks to spread along the borders of the grains, eventually increasing the strength due to grain deflection.

The work in this paper is motivated by the exhaust gas recirculation (EGR) system utilized in diesel engines, which introduces an appropriate quantity of exhaust gas into the cylinder to participate in combustion. As a result, it effectively reduces the maximum temperature within the cylinder and subsequently mitigates NOx emissions. Similarly, adding an appropriate amount of red mud to Al₂O₃ ceramics can be used as a sintering aid for the sintering of Al₂O₃ ceramics, so as to realize the resource utilization of red mud. In this paper, the effects of different sintering temperatures and Cr₂O₃ contents on the properties and microstructures of RM–Al₂O₃ composite ceramics have been investigated by adding 10% RM and different contents of Cr₂O₃ to alumina, and the optimal sintering temperatures and addition amounts have been determined. The wetting behavior of Cu on the samples at elevated temperatures have been investigated by using the seated drop method, while the surface energy of the composite ceramics at high temperature has been estimated based on the data obtained from high-temperature contact angle measurements, so as to explore the possibility of applying RM–Al₂O₃ composite ceramics in the field of electronic packaging.

2. Materials and Methods

2.1. Raw Materials

The Al₂O₃ powder (200 nm, Al₂O₃ ≥ 99.9%) used in this study was produced by Beasley New Materials Co. (Suzhou, China). Cr₂O₃ powder (100 nm, Cr₂O₃ ≥ 99.9%) was produced by Hebei Yuanying New Materials Co. (Xingtai, China). Red mud was provided by Guangxi Huayin Aluminium Co., Ltd. (Baise, China)., and Fe₃O₄ in red mud was separated by strong magnetic separation. The red mud was dried in a drying oven at 120 °C for 24 h. The dried mass was crushed using a pulverizer. The composition of the red mud analyzed by XRF is shown in

Table 1. The chemical composition of red mud (wt.%).

Fe2O3	Al2O3	SiO2	CaO	Na2O	TiO2	Loss
29.60	18.90	16.90	15.60	11.10	5.78	2.12

. Stearic acid, polypropylene, and paraffin were produced by Shanghai McLean Biochemical Technology Co. (Shanghai, China) Cu (≥99.95%) used was produced by CMT New Materials Co. (Attleboro, MA, USA) and processed into 3 mm × 3 mm × 3 mm cubes by wire cutting.

2.2. Preparation of Composite Ceramics

The powder ingredients were proportioned in accordance with the composition presented in

Table 2. Chemical compositions (wt.%) of the samples.

Sample	Al2O3	RM	Cr2O3
A	100	0	0
ARC0	90	10	0
ARC1	89.1	9.9	1
ARC2	88.65	9.85	1.5
ARC3	88.2	9.8	2
ARC4	87.75	9.75	2.5
ARC5	87.3	9.7	3

. In addition, 1 g of polypropylene, 1 g of paraffin, and 1 g of stearic acid were additionally added to the mixed powder (30 g total weight). A high-energy planetary ball mill was employed to grind the mixed powder for a duration of 30 h at a rotational speed of 300 revolutions per minute, utilizing anhydrous ethanol as the grinding medium. The mixed slurry was subjected to vacuum drying at 70 °C for 24 h in a drying oven. Subsequently, the dried mass was finely ground using an onyx mortar and then sieved through an 80-mesh sieve. The powder was pressed into 40 mm × 4 mm × 10 mm slabs using a hydraulic press at 15 MPa for 3 min. The samples were sintered using a MoSi₂ electric furnace (KSL-1700-2A). The temperature was ramped up to 1200 °C at a rate of 5 °C/min, followed by a further ramp-up to the target temperature at a rate of 2 °C/min. The target temperature was maintained for 2 h, and then the samples were cooled with the furnace. Additionally, each sample required a debinding process where it was held at 800 °C for 2 h.

2.3. Characterization of Composite Ceramics

The test samples underwent pre-treatment prior to conducting the tests. The ceramic samples were individually polished using 2.5 w and 1 w abrasive pastes, respectively. Subsequently, the ceramic samples with observed surface morphology were subjected to etching using an etching solution (3% HF, 5% HN₃) for a duration of 1 min in order to eliminate the glassy phase. The elemental composition of the red mud was analyzed using an X-ray fluorescence spectrometer (S8 TIGER, BRUKER, Berlin, Germany). Following this, ultrasonic cleaning was performed on the ceramic samples in anhydrous ethanol for a period of 5 min. X-ray diffraction analyses were carried out using an X-ray diffractometer (MiniFlex600-C, Rigaku Co., Ltd., Akishima, Japan) at 40 kV/15 mA, with a scanning range of 10~85 at a 2θ angle and a scanning speed of 5 °/min. A scanning electron microscope (SU8020, Hitachi High-Technologies, Tokyo, Japan) was used to observe the surface morphology and fracture morphology of the samples. The apparent density of the composite ceramics was measured using the Archimedes drainage method, and the densification was expressed as the ratio of the apparent density to the theoretical density. The phase compositions of the samples were analyzed semi-quantitatively by EDS (energy-dispersive X-ray spectroscopy). Vickers hardness tester was used to measure the hardness of the samples (loading force 5 Kg, holding pressure 15 s), and in each group of 4 specimens, each specimen was tested in 10 different positions, and then the average value was taken.

The samples were machined into 40 mm × 3 mm × 6 mm blocks by diamond wire cutting, the fracture toughness of the samples was measured using the single edge notched beam (SENB) method as shown in Figure 1a, and the loading speed was 0.5 mm/min during the test. The fracture toughness K_IC is calculated from Equation (1):

$$K_{IC}=Y{\displaystyle \frac{3PL}{2b{h}^2}}\sqrt{a}$$

(1)

where Y is the shape factor, P is the fracture load, L is the support span, a is the depth of the cut, b is the width, w is the thickness, and c is the width of the cut. When span length/thickness/width = 8:2:1, the shape factor Y is as follows:

$$Y=1.96-2.75\left({\displaystyle \frac{a}{h}}\right)+13.66({{\displaystyle \frac{a}{h}})}^2-23.98({{\displaystyle \frac{a}{h}})}^3+25.22{\left({\displaystyle \frac{a}{h}}\right)}^4$$

(2)

The flexural strength was measured by the three-point bending method as shown in Figure 1b. The size of the specimen was 40 mm × 3 mm × 12 mm, and the loading speed during the test was 0.5 mm/min. The bending strength can be calculated by Equation (3):

$$\mathsf{\sigma }={\displaystyle \frac{3PL}{2B{h}^2}}$$

(3)

where P is the fracture load, L is the support span, b is the width, and h is the thickness.

High-temperature wettability tests were performed in a vacuum tube furnace (KJ-GSL-1800X, Hefei, China). The oxide film on the surface of the copper used for the test was polished with sandpaper and ultrasonically cleaned in anhydrous ethanol. The copper particles were placed on the ceramic samples and heated together in a vacuum tube furnace. The temperature was ramped up to the target temperature at a rate of 10 °C/min and held for ten minutes, and when the copper droplets were stabilized, a high-definition camera was used to take pictures. Finally, the contact angle was calculated using ImageJ (1.54v) software. The contact angles of Cu droplets with ceramics were measured at 1133, 1183, 1233, 1283, and 1333 °C, respectively.

3. Results and Discussion

3.1. Phase Composition

Figure 2a shows the XRD patterns of ARC2 samples sintered at different temperatures for 2 h. The main phases in the sample are corundum and CaAl₁₂O₁₉ (CA6), with small amounts of calcium silicate, quartz, pyroxene, and NaFe (SiO₃)₂. As the sintering temperature rises, most of the silicate is gradually burnt out. The XRD patterns of samples sintered at 1500 °C for 2 h, ranging from ARC1 to ARC5, are depicted in Figure 2b. The composition of the phases remains unchanged. It is worth noting that an increase in Cr₂O₃ content leads to a slight shift of the diffraction peak towards smaller angles. This can be attributed to that the formation of a complete replacement solid solution between Cr³⁺(0.64 nm) and Al³⁺ (0.57 nm), resulting in the lattice constant changing, which results in a small shift in the diffraction peak [11].

Figure 3a shows the EDS data of the corroded ARC4 sample. Based on the compositional data of the point energy spectra, it can be judged that the columnar grains in the matrix are alumina crystals, and the hexagonal flake particles are CaAl₁₂O₁₉. From the surface spectral analysis, it is observed that the Ca element is concentrated in the hexagonal flake particles, and the Cr element is uniformly distributed in the matrix. Figure 3b shows the EDS data of the uncorroded AR sample. From the results of the surface spectrum analysis, Na and Si elements are mainly concentrated at the grain boundaries, and Fe elements are uniformly distributed in the matrix, with a slightly higher concentration at the grain boundaries. From the results of the point energy spectra, the silicate phase mainly shows an amorphous pattern distributed between the alumina grains and fills the grain-to-grain voids.

3.2. Microstructural Characterization

Figure 4b–f shows SEM images of ARC2 sintered at different temperatures for 2 h. The samples sintered at 1400 °C have been incompletely sintered, with more holes inside, and the microstructure consists of fine equiaxed grains and a few hexagonal lamellar particles. The CA6 grain exhibits preferential growth along the basal plane, and the coupled diffusion of Ca²⁺ and O^2- from the Ca-rich phase to the Al-rich phase controls the formation of CA6. This anisotropy of growth rate leads to the formation and growth of CA6, preferring the orientation of the base plane which is perpendicular to the reaction front, which leads flaky CA6 being obtained after reaction and sintering [12,13]. The samples sintered at 1450 °C exhibit abnormally grown grains with surface pores. This phenomenon can be attributed to the rapid decrease in the viscosity of the glassy phase, which facilitates gas diffusion in the liquid phase and subsequently leads to pore formation on the ceramic body’s surface. A significant number of plate-like grains are observed in the samples sintered at 1500 °C. This phenomenon can be attributed to the formation of a small quantity of liquid phase within the lower-melting-point oxides present in the red mud. The uneven distribution of this liquid phase leads to the coexistence of dry and wet interfaces, resulting in variations in interfacial energy between the base surface of Al₂O₃ grains and other interfaces. Consequently, different growth rates occur on different surfaces, leading to anisotropic growth in certain grains [14].

The densifications profiles of ARC2 samples sintered at various temperatures for 2 h are depicted in Figure 4a. It can be observed that the densifications of the composite ceramic exhibit a gradual increase followed by a rapid decrease with increasing temperature. In the initial stage of ceramic sintering, the bonding between the particles is weak, and there are more pores in the material. As the temperature increases, sintering and solid-state diffusion begin to occur between the particles, the pores are gradually filled, the material becomes denser, and the density gradually increases. The maximum densification achieved is 97.75% for samples sintered at 1500 °C. Obvious lath-like grains can be observed from Figure 4d. This is in agreement with the findings of Cahoon et al. [15]. They observed relatively uniform-sized anisotropic growth grains in pure alumina ceramics, which appeared abruptly as the matrix approached final densification. The densities of the composite ceramics decreased rapidly when the sintering temperature exceeded 1500 °C. The density of the composite ceramics sintered at 1600 °C for 2 h was 94.85%. According to the classical equation for grain growth kinetics:

$$G^n-G_0^n=K·t$$

(4)

where G is the grain size, G₀ is the initial grain size, n is the growth index, K is a temperature-dependent constant, and t is the sintering time. This equation shows that the grain size increases with time and temperature. According to the Arrhenius equation, the grain growth rate constant K is related to the temperature T as follows:

$$K=K_0\exp \left({\displaystyle \frac{-Q}{RT}}\right)$$

(5)

where K₀ is a constant, Q is the activation energy for grain boundary migration, R is the gas constant, and T is the absolute temperature. At high temperatures, the K value increases significantly, and the grain growth rate is accelerated, especially in the late stage of high-temperature sintering, and the abnormal grain growth phenomenon is more obvious. Samples sintered at 1600 °C have very coarse grains, as shown in Figure 4f. In the case of abnormal grain growth, the larger grains occupy more volume, while the smaller grains are more difficult to move and cannot completely eliminate the inter-grain porosity. Especially, larger residual pores may form between large grains. During normal grain growth, the grain boundaries migrate uniformly and can effectively reduce the pores. However, at the interface between abnormally grown grains and small grains, the grain boundaries migrate unevenly, making it difficult to fill the pores in these regions. As a result, the densification of the material decreases and the porosity increases.

Figure 5b–f shows the surface morphology of ARC1~ARC5. Overall, the effect of Cr₂O₃ content on crystal morphology is not obvious. When the Cr₂O₃ content increased to 2.5 wt.%, obvious holes were observed in the matrix.

The densification of pure alumina specimens sintered at 1500 °C for 2 h was 92.14%. Figure 5a shows an incompletely densified structure with uneven grain size distribution and a large number of holes in the matrix. In the case of alumina, ordinary sintering at atmospheric pressure is usually above 1650 °C. The addition of red mud makes a small amount of liquid phase appear in the matrix, which accelerates the mass transfer rate during the sintering process. At the same time, under the action of capillary force generated by the liquid phase, it causes pressure and sliding between particles, which prompts the original particles to rearrange and improves the particle packing density. Based on this, this study determined that the optimum sintering temperature of ARC composite ceramics is 1500 °C, while the addition of red mud can effectively reduce the sintering temperature of Al₂O₃ ceramics.

3.3. Mechanical Properties

Figure 6a,b show the flexural strength and hardness of ARC2 sintered at different temperatures for 2 h. With the increase in sintering temperature, the flexural strength and Vickers hardness of the samples increase first and then decrease. The maximum flexural strength of the sample sintered at 1500 °C is 297.03 MPa, and the hardness value is 17.44 GPa. This is because, when sintered below 1500 °C, the sample presents an incomplete sintering state, retains more original particles, has poor interfacial binding force, and has more pores, which is mainly inter-granular fracture. When the sintering temperature is higher than 1500 °C, the grains in the matrix grow rapidly, and along with the liquid phase volatilization at a low boiling point in the grain boundary, the density of the sample decreases, resulting in a decrease in strength. The flexural strength of the sample sintered at 1600 °C is higher than that of the sample sintered at 1550 °C, because as the grain grows, it is more difficult for the crack to bypass the large grain in the process of expansion, resulting in a significantly higher proportion of trans-granular fracture (as shown in Figure 7a).

Figure 6c,d show the flexural strength and hardness of A~ARC5 sintered at 1500 °C. Since the pure alumina specimen was not fully sintered, its flexural strength was only 173.5 MPa, and its hardness was 8.65 GPa. Notably, the sample containing 1.5 wt.% Cr₂O₃ exhibits the highest flexural strength. In the early stage of sintering, Cr³⁺ is easily enriched on the surface of Al₂O₃ grains [16,17]. Due to the solid solution of Cr ions, the distortion energy of the Al₂O₃ grain interface is increased, and the growth rate of the chromium-rich interface is accelerated, which promotes the sintering of ceramic samples. On the other hand, the increase in grain-boundary distortion energy has a positive effect on hindering the dislocation motion, thus enhancing the flexural strength of the composite ceramics. The mechanical properties of ceramics are very sensitive to defects such as porosity and cracks. Cr₂O₃ is unstable under atmospheric pressure sintering and is easily converted to CrO₃ through a gasification–condensation mechanism, leading to a decrease in the densification of the ceramic samples. Consequently, the flexural strength and hardness of the ceramic samples increase and then decrease with the increase in Cr₂O₃ content.

The fracture morphologies of some samples are depicted in Figure 7, exhibiting a combination of through-crystal and inter-granular fractures. The growth of CA6 exhibits a strong preferred crystallographic orientation, and the reaction primarily occurs by the diffusion of Ca²⁺ into Al₂O₃. The distribution of CA6 phase in the matrix can be observed in three different forms. Firstly, they distribute at the grain boundaries and fill the voids between grains, which enhances material densification and reduces porosity and pore size, thereby improving the strength of the composite material [18]. Secondly, CA6 grows along the surface of Al₂O₃ grains, which prolong the path of crack growth and increase the energy required for crack growth during inter-granular fracture and thus improve the fracture toughness of the material [19]. However, due to the introduction of less Ca element, CA6 growing in the Al₂O₃ grain is very thin, so it can provide a limited strengthening effect, as shown in Figure 7c.

It is worth noting that in the third case, CA6 is embedded between the Al₂O₃ grains at different angles and forms an “endo-crystalline” structure with the Al₂O₃ grains [20,21], as shown in Figure 7c. CA6 and Al₂O₃ have similar coefficients of thermal expansion (8.0 × 10⁻⁶ °C⁻¹, 8.6 × 10⁻⁶ °C⁻¹, respectively) [22], and it is generally believed that CA6 and Al₂O₃ do not undergo thermal mismatch and have good compatibility in the matrix. However, this is limited to the independent existence of CA6 grains and Al₂O₃ grains, because there are gaps between grains to make up for this difference. And from the perspective of the crystal structure of CA6 and Al₂O₃, the thermal expansion behavior of the two phases is anisotropic, and the occurrence of thermal mismatch is predictable when CA6 grows at different angles within the Al₂O₃ grains. This kind of structure will produce micro-cracks at the sub-interface, weaken the effect of the main grain boundary, inhibit the dislocation movement during the fracture process, play the role of crack pinning and change the expansion direction of the main crack, and may change its fracture mode, i.e., from the original inter-granular fracture to trans-granular fracture, as shown in Figure 7d.

3.4. Wettability at High Temperature

Al₂O₃ ceramics have been extensively utilized in the fabrication of ceramic substrates and ceramic cleavers [23]. With increasing demand for lower costs in semiconductor packaging, the adoption of low-cost bonding wire becomes imperative. Consequently, copper wire will inevitably replace gold wire as the primary alternative for bonding wire in the future. By investigating the wetting behavior and interface reactions of Cu liquid on RM–Al₂O₃ composite ceramic materials at elevated temperatures, this research establishes a fundamental framework for the potential application of RM–Al₂O₃ composite ceramic materials in the field of electronic packaging.

The contact angle data of Cu and RM–Al₂O₃ composite ceramics at different temperatures are presented in Figure 8. It can be observed that the contact angle gradually decreases with increasing temperature. Moreover, all contact angles measured at temperatures above the melting point are greater than 90°, indicating the non-wetting behavior of Cu and ARC composite ceramic. The SEM images in Figure 9 illustrate the wetted interface of ARC2 with Cu droplets, revealing a clean and flat interface without any dissolution or presence of reactants. It shows that at high temperature, the elements of red mud such as iron, nickel, sodium, calcium, and silicon in RM–Al₂O₃ composite ceramics have no diffusion phenomenon, and there is no pollution of copper liquid.

Thomas Young proposed the contact angle of a liquid as the mechanical equilibrium of the drop resting on a plane solid surface at the three-phase boundary.

$${\gamma }_{lv}\cos \theta ={\gamma }_{sv}-{\gamma }_{sl}$$

(6)

The London dispersion theory is employed by Berthelot [24] to calculate interfacial tension based on the surface tensions of two phases, using the geometric mean combination rule:

$${\gamma }_{sl}={\gamma }_s+{\gamma }_l-2\sqrt{{\gamma }_s{\gamma }_l}$$

(7)

The surface tension of Cu can be calculated from the following equation [25]:

$${\gamma }_l=1.32-2.8\times {10}^{-4}(T-1358)$$

(8)

The experimental values obtained by measuring the contact angle of copper and ceramics at different temperatures are plotted as cosθ vs. γ_l. The Zismen approximation was employed to establish a linear correlation between cosθ and γ_l:

$$\cos \theta =a{\gamma }_l+b$$

(9)

The above equations are ultimately interconnected to establish the correlation between γ_s and T:

$${\gamma }_s=\mathrm{A}{T}^3+B{T}^2+CT+D$$

(10)

The fitted parameters are presented in

Table 3. Values of fitting parameters and corresponding calculation.

Sample	a	b	R2	A	B	C	D
ARC1	−2.32	2.28	0.94	−2.95 × 10−11	1.82 × 10−8	−0.0027	13.90
ARC2	−2.37	2.34	0.98	−3.08 × 10−11	2.00 × 10−8	−0.0028	14.43
ARC3	−2.56	2.55	0.92	−3.60 × 10−11	2.91 × 10−8	−0.0032	16.34
ARC4	−2.68	2.69	0.96	−3.94 × 10−11	3.47 × 10−8	−0.0035	17.68
ARC5	−2.38	2.33	0.92	−3.11 × 10−11	2.24 × 10−8	−0.0028	14.36

. The surface energy data from ARC1~ARC5 have been compared with the surface energy results for aluminum oxide reported by Eustathopoulos et al. [26] (γ_s = 20.77 − 0.007083 T), as illustrated in Figure 10. The surface energy of ARC composite ceramics decreases more slowly as the temperature decreases than that of Al₂O₃, and ARC3 and ARC4 have higher surface energy than Al₂O₃ at high temperature. As the Cr content increases, the wettability of Cu on the composite ceramic becomes worse. This can be explained in two ways. With the increase in Cr oxide, the solid solution of Cr ions in Al₂O₃ increases, leading to an increase in lattice distortion, which leads to an increase in the surface energy of the composite ceramics. Secondly, the excess addition of Cr led to an increase in the internal pores of the composite ceramics, which also led to an increase in the surface roughness of the material. According to the Wenzel model, on a hydrophobic surface, the larger the roughness is, the more hydrophobic it is.

4. Conclusions

Using red mud, Al₂O₃, and Cr₂O₃ as raw materials, high-performance red mud alumina composite ceramics have been prepared by press-free sintering. The experimental results demonstrate the following:

(1)

Red mud can significantly lower the sintering temperature of Al₂O₃ ceramics. The samples sintered at 1500 °C with the addition of 1.5 wt.% Cr₂O₃ have better properties, with a bending strength of 297.03 MPa, a fracture toughness of 6.50 MPa/m^1/2, a hardness value of 17.44 GPa and a densification of 97.75%.

(2)

There are three different distributions of CA6 in ARC composite ceramics. CA6 distributing at grain boundaries can fill the voids between grains and improve the densification of the ceramics. CA6 distributing parallel to the Al₂O₃ grains extends the crack propagation path. CA6 and alumina grains form an “endo-crystalline” structure, which can change the direction of crack propagation and transform from an inter-granular fracture to trans-granular fracture.

(3)

Cu liquid does not wet the ARC composite ceramics at high temperatures, no chemical reaction occurs at the interface, and there is no elemental diffusion into Cu liquid. Moreover, the ARC composite ceramics have a higher surface energy than Al₂O₃ at high temperatures and decrease more slowly with temperature.