Improved Properties of Ceramic Shells by Optimizing the Surface Composition from Lanthanide-Based Composites

Abstract

The precision casting of nickel-based single-crystal superalloys imposes stringent requirements on the high-temperature stability and chemical inertness of ceramic shell face coats. To address the issue of traditional EC95 shells (95% Al₂O₃–5% SiO₂) being prone to react with the alloy melt at elevated temperatures, thereby inducing casting defects, this study proposes a lanthanide oxide-based ceramic face coat material. Three distinct powders—LaAlO₃ (LA), LaAlO₃/La₂Si₂O₇ (LAS), and LaAl₁₁O₁₈/La₂Si₂O₇/Al₂O₃ (LA11S)—are successfully prepared through solid-phase sintering of the La₂O₃-Al₂O₃-SiO₂ ternary system. Their slurry properties, shell sintering processes, and high-temperature performance are systematically investigated. The results demonstrate that optimal slurry coating effectiveness is achieved when LA powder is processed with a liquid-to-powder ratio of 3:1 and a particle size of 300 mesh. While LA shells show no cracking at 1300 °C, their face coats fail above 1400 °C due to the formation of a La₂Si₂O₇ phase. In contrast, LAS and LA11S shells suppress cracking through the La₂Si₂O₇ and LaAl₁₁O₁₈ phases, respectively, exhibiting exceptionally high-temperature stability at 1400 °C and 1500 °C. All three shells meet the high-temperature strength requirements for CMSX-4 single-crystal alloy casting. Interfacial reaction analysis and Gibbs free energy calculations reveal that Al₂O₃-forming reactions occur between the novel shells and alloy melt, accompanied by minor dissolution erosion without other chemical side reactions. This work provides a high-performance face coat material solution for investment casting of nickel-based superalloys.

1. Introduction

Nickel-based high-temperature alloys are widely used in critical areas such as aero-engine turbine blades and gas turbine hot-end components due to their excellent creep resistance, oxidation resistance and high-temperature strength [1,2]. During the casting of single-crystal alloys, traditional alumina-based ceramic shells (e.g., EC95) face two serious challenges. On the one hand, SiO₂, as the main composition, reacts with the reactive elements (Al, Hf, etc.) at the interface in the alloys to generate impurities (e.g., Al₂O₃, HfO₂), resulting in freckling defects on the surface of the castings [3,4,5]. On the other hand, the prepared shells are prone to deformation during sintering above 1500 °C, which makes it difficult to ensure the dimensional accuracy of the castings. Therefore, new surface material compositions need to be developed to form ceramic shells with excellent properties.

In recent years, rare earth oxide Y₂O₃ has been explored for high-temperature alloy casting due to its thermodynamic stability. However, its prohibitive cost significantly limits industrial-scale applications. In contrast, lanthanum oxide (La₂O₃), characterized by abundant domestic reserves, cost-effectiveness, high melting point, and elevated density, demonstrates superior feasibility for industrial implementation. At elevated temperatures, La₂O₃ reacts with Al₂O₃ and SiO₂ to form thermally stable compounds, including LaAlO₃, LaAl₁₁O₁₈, and La₂Si₂O₇. Among these derivatives, lanthanum aluminate (LaAlO₃) exhibits a Calcitonite structure, endowing it with exceptional thermal and chemical stability. As a novel rare earth particulate material, it finds extensive applications in solid oxide fuel cells, advanced ceramics, and substrate materials [6,7,8,9]. Two primary forms exist: stoichiometric La₂O₃-Al₂O₃ (1:1 molar ratio) sintering yields LaAlO₃, while a 1:11 molar ratio produces LaAl₁₁O₁₈. LaAlO₃ is used as a new type of high-temperature corrosion-resistant material in slag line bricks, which is superior to MgO-C bricks in resisting the erosion of steel slag [6]. Its excellent lattice matching with high-temperature superconductors further enables widespread use as a substrate material. Commercial production of LaAlO₃ single crystals, particularly for high-temperature superconducting thin film substrates, has shown substantial industrial prospects. Lanthanum hexaaluminate (LaAl₁₁O₁₈) is a magneto lead ferrite structure with a superior microstructure whose main properties are structural and thermochemical stability, low thermal conductivity, high coefficient of thermal expansion, low sintering rate, and low oxygen diffusion [10,11,12,13,14,15]. High-temperature processing induces rod-like morphology in LaAl₁₁O₁₈, which functions as a continuous grain boundary phase in Al₂O₃ ceramics. LaAl₁₁O₁₈ exhibits a rod-like structure at high temperatures and can be regarded as a continuous grain boundary phase. Doping in Al₂O₃ ceramics exhibits through-crystal fracture and is unfavorable for vibrational transmission of phonons due to low thermal conductivity [16,17,18,19,20]. The solid-state reaction of La₂O₃ and SiO₂ at a 1:2 molar ratio produces lanthanum disilicate (La₂Si₂O₇), a high-melting-point compound with applications spanning optical devices, electronic components, and specialty ceramics [21,22].

Based on existing research foundations, this study synthesized LaAlO₃ and La₂Si₂O₇ mixed powders with various stoichiometric ratios via solid-state sintering and innovatively applied them in the fabrication of investment casting shells. This study systematically investigated the rheological properties of slurries, maximum sintering temperatures, and mechanical strength of shells under different compositions. Subsequent casting trials with nickel-based superalloys (e.g., CMSX-4) were conducted to characterize interfacial reactions between the novel-facing materials and molten alloys. This work proposes a novel-facing material solution for the precision casting of single-crystal superalloys, addressing critical challenges in interfacial stability and high-temperature performance.

2. Experimental Section

2.1. Raw Materials

Since La₂O₃ was highly prone to hydrolysis, X-ray powder diffractometer (XRD) analysis was performed on it, and the results are shown in Figure 1. From Figure 1a, it could be observed that the powder predominantly exhibited diffraction peaks corresponding to La(OH)₃. Elemental surface scanning analysis (Figure 1b) confirmed that the atomic ratio of lanthanum (La) to oxygen (O) was approximately 1:3, indicating that the lanthanum oxide powder had reacted with moisture in the air to form La(OH)₃ powder (Figure 1b), which displayed a stacked distribution. The morphology of Al₂O₃ particles appeared as plate-like structures with distinct bimodal size characteristics (Figure 1c), while SiO₂ particles were rounded and spherical, showing a relatively uniform size distribution (Figure 1d).

The purpose of this study was to investigate the feasibility of replacing EC95 with rare-earth-containing compounds for investment casting-facing materials. The physical parameters of silica sol, defoamers and wetting agents required to configure the facing slurry in the experiment are listed in

Table 1. Physical property parameters of silica sol, defoamer, and wetting agent.

Materials	Liquid Color	PH	Density/g·cm−3	Viscosity/mPa·s	Manufacturer
Silica sol	Clear	6.0–8.0	2.0	30	Shanghai Bohuai Chemical Co., Ltd., China
Defoamer	White milky	6.0–8.0	0.97	26	China Wuhan Zhuoyan Co., Ltd., China
Wetting agent	Clear	5.0–7.0	0.95	98	China Wuhan Zhuoyan Co., Ltd., China

. The composition of the CMSX-4 alloy return material is shown in

Table 2. Chemical composition of CMSX-4 alloy (wt.%).

Element	Al	Co	Cr	Ta	Re	Ti	W	Mo	Hf	Ni
content	5.5	9.0	6.5	6.5	3.0	1.0	6.0	0.6	0.1	Bal.

.

2.2. Sintering Process of Ceramic Powder

Figure 2 shows the TG-DSC (Thermogravimetry-Differential Scanning Calorimetry) curve in the air atmosphere after mixing La₂O₃ and Al₂O₃. Figure 2 shows that endothermic peaks occur at about 300 °C and 500 °C, with mass losses of about 3.52% and 1.14%. This corresponded to the two-step dehydration process of La(OH)₃ (Equations (1) and (2)). At about 1300 °C, an exothermic peak and mass loss were observed, indicating that the formation of LaAlO₃ occurred at about 1300 °C, as shown in Equation (3) [23].

La(OH)₃ = LaOOH + H₂O

(1)

2LaOOH = La₂O₃ + H₂O

(2)

La₂O₃ + Al₂O₃ = 2LaAlO₃

(3)

The TG-DSC curve in Figure 2 showed that the pyrolysis process in air after the mixing of La₂O₃ and Al₂O₃ could be divided into two stages. As shown in Figure 3, the sample was first heated to 50 °C and held for 60 min. Finally, the temperature was raised to 900 °C, 1000 °C, 1100 °C, 1200 °C, 1300 °C, 1400 °C, and 1500 °C, respectively, with different colored lines used to distinguish between them. The samples were held at each temperature for 2 h, and the heating rate for sintering the samples was 2 °C/min.

2.3. Preparation of Ceramic Shell

The slurry ratios and process parameters used in the experiment are shown in

Table 3. Specific process parameters of the slurry.

Layers	Slurry	Sand	Viscosity /mPa·s	Dry Times/h
Transition	25% Silica sol, wetting agent, anti-foam agent	EC95, 30–60 mesh	300	4
Multipleback	25% Silica sol, wetting agent, anti-foam agent	EC95, 16–60 mesh	500	4
Sealcoat	25% Silica sol, wetting agent, anti-foam agent	None	500	5

. Among them, EC95 is an alumina sand produced by Guizhou Dazhong Seventh Grinding Wheel Co., Ltd. in Guizhou, China, containing 95% alumina, 5% silica, and a surface particle size of 60–80 mesh. The ceramic shell was prepared using a layered slurry impregnation and sand grinding process, as shown in Figure 4. After the wax mold was immersed in the pre-prepared surface slurry, the “rain drop method” was used to uniformly disperse the surface sand particles on the sample surface. Subsequently, a fan was activated to accelerate the drying of the initial surface layer. After 1 h, the aforementioned sample was immersed in the back layer slurry to apply a transition sand layer. After drying, a second transition layer was formed. The coating process for the back layer was the same as that for the transition layer, but the back layer required three coats. After the final coating dries, the sample is immersed in the back layer slurry and removed. Once the slurry no longer flows, the fan is activated to seal the slurry. After drying, the wax is removed in a dewaxing pot to obtain the shell mold.

2.4. Characterization

The pyrolytic weight loss of the raw materials was characterized using a differential scanning calorimeter (STA449C synchronous thermal analyzer NETZSCH, Selb, Germany) with a heating rate of 10 °C/min 1 from 50 °C to 1500 °C. The flexural strength was evaluated using a three-point bending test instrument (WDW300, Changchun Kexin Testing Instrument Co., Ltd., Changchun, China). Phase analysis was conducted on the raw powders, sintered powders, shell surfaces, and the bottom of the alloy after interfacial reactions using a XRD (Smartlab 9 kW, Rigaku, Tokyo, Japan). The scanning range was set from 10° to 90°, with a scanning speed of 10° per minute. A scanning electron microscope (SEM, Regulus8230HR cold field-emission SEM, Tokyo, Japan) was employed to examine the morphology of the sintered mixed powders, shell surfaces, and alloy after interfacial reactions. And the energy disperse spectroscopy (EDS, Oxford INCA, Oxford, UK) was used to characterize the element distribution in the shell. The viscosity of the slurry was measured using a digital viscometer(NDJ-S, Shenzhen DJS Co., Ltd. Shenzhen, China), with results expressed in mPa·s.

3. Results

3.1. Experimental Data Characterization

3.1.1. Characterization of LaAlO₃ Powders

The XRD patterns of the sintered products obtained by mixing La₂O₃ and Al₂O₃ in equal amounts are shown in Figure 5. As can be seen from the figure, within the temperature range of 900–1100 °C, no LaAlO₃ phase was detected in the sintered product, with the main phases being La(OH)₃, La₂O₃, and Al₂O₃. As shown in Figure 2, the reaction temperature was below 1300 °C, and Equation (3) did not occur. At this stage, the main process was the two-step dehydration of La(OH)₃ (Equations (1) and (2)). When the sintering temperature is further increased to 1200 °C, LaAlO₃ begins to appear in the sintered product, but a small amount of La₂O₃ remains, indicating that the reaction forming LaAlO₃ is incomplete. This situation changes when the sintering temperature reaches 1400 °C. When the temperature is increased to 1400 °C, the sintered powder is pure LaAlO₃. When the temperature is further increased to 1500 °C, the peak of the sintered product LaAlO₃ becomes higher and narrower, indicating better crystallinity of LaAlO₃. Therefore, the maximum sintering temperature for LaAlO₃ powder is set to 1500 °C as the most appropriate. For convenience, it is abbreviated as LA powder [23].

Further observation of the microstructural morphology of sintered products at different temperatures, combined with Energy-dispersive X-ray spectroscopy (EDS) point-scan analysis (

Table 4. Point sweep results of LA powder.

Atom	O/%	Al/%	La/%
1	69.07	1.51	39.42
2	57.51	19.99	22.5
3	55.40	23.58	21.02
4	56.46	21.04	22.49

), reveals that at 1200 °C and 1300 °C, La₂O₃ particles (box 1) formed by the dehydration of La(OH)₃ exhibit a layered stacking structure (highlighted by white boxes in Figure 6a,b), LaAlO₃ particles (box 2) are relatively small. As the temperature increases, the layered La₂O₃ (white boxes) diminishes and is gradually replaced by fine granular LaAlO₃ particles (yellow boxes). By 1400 °C, La₂O₃ particles completely disappear, and LaAlO₃ particles grow in size, consistent with XRD results. Analysis of particle distribution (Figure 6c,d) indicates that at lower temperatures, LaAlO₃ particles are smaller and form agglomerates with large interparticle gaps. With increasing temperature, agglomeration weakens, resulting in smaller aggregates and narrower gaps. At 1500 °C, LaAlO₃ particles no longer agglomerate, displaying a fully developed, densely packed, and nearly gap-free morphology. Comparative size analysis shows that LaAlO₃ particles are five times larger than La₂O₃ particles, demonstrating that elevated temperatures promote both the chemical conversion of La₂O₃ to LaAlO₃ and the coarsening of LaAlO₃ grains, ensuring a more complete reaction.

3.1.2. Analysis of Slurry Performance

Figure 7a displays the viscosity curves of slurries prepared by mixing sintered LA powder (sieved through 100-, 200-, and 300-mesh screens) with silica sol at varying powder-to-liquid ratios. The results reveal that as the mesh size increases (i.e., finer particle size), the slurry viscosity gradually decreases under the same powder-to-liquid ratio. At a fixed mesh size, increasing the powder-to-liquid ratio initially causes a rapid rise in viscosity; however, when the ratio exceeds 3.0, the rate of viscosity increase slows and stabilizes. Notably, for 300-mesh powder, adjusting the powder-to-liquid ratio has minimal impact on viscosity. The viscosity of the EC95 surface coating slurry currently in use (green value in Figure 7a) is 901 mPa·s, which is closest to the viscosity (1285 mPa·s) of the slurry prepared using 300-mesh powder, as shown in the green box in the figure.

Figure 7b presents concrete figures of coatings applied on wax molds using slurries prepared with LA powders of different mesh sizes (100-, 200-, and 300-mesh) at a fixed powder-to-liquid ratio of 3.0. As shown, the sample coated with 100-mesh LA powder exhibits an uneven and bumpy surface. With increasing mesh size (i.e., finer particle size), the surface roughness progressively improves, culminating in a smooth and uniformly covered surface when 300-mesh LA powder is used.

Figure 7c displays physical images of coatings prepared with 300-mesh LA powder at varying powder-to-liquid ratios. The results (Figure 7c) demonstrate that powder-to-liquid ratios of 2.8 and 3.0 yield optimal sag resistance, whereas ratios exceeding 3.0 result in uneven coating thickness and surface irregularities. This degradation occurs because excessively high ratios increase slurry viscosity, complicating the coating process. The primary reason lies in the high density of LA powder: elevated ratios promote sedimentation and reduced fluidity, causing uneven coating deposition and sagging [24].

3.1.3. Analysis of Sintering Properties of LaAlO₃ Shells

The prepared LA shells are sintered at different temperatures, and it is found that the highest withstand temperature of LA shells was 1300 °C. Figure 8a,b displays the macroscopic morphology images of LA shells sintered at 1400 °C and 1500 °C. At 1400 °C, surface cracking occurs predominantly at the edges of the shell face coat. Severe surface layer spalling and upward warping of the face coat are observed in shells sintered at 1500 °C, highlighting a significant mismatch between the face coat and transition layer. The fragmented face coat materials from both temperatures are analyzed by XRD, as shown in Figure 8c,d. At 1400 °C, the XRD pattern confirms the presence of LaAlO₃ and La₂Si₂O₇ phases, indicating a reaction between LaAlO₃ and SiO₂ in the face coat to form La₂Si₂O₇. At 1500 °C, additional Al₂O₃ peaks appear alongside LaAlO₃ and La₂Si₂O₇, revealing a secondary reaction product. The reactions can be described as follows:

2LaAlO₃ + 2SiO₂ = La₂Si₂O₇ + Al₂O₃

(4)

Figure 8e presents the micromorphology of the LA shell surface sintered at 1500 °C. The left region shows the exposed transition layer after face coat spalling, while the right retains the residual face coat. Energy dispersive X-ray spectroscopy (EDS) point analysis (

Table 5. Point sweep results of LaAlO₃ shell after high-temperature sintering.

Atom	O/%	Al/%	La/%	Si/%
1	59.54	37.23	3.03	0.19
2	62.74	2.30	16.71	18.25
3	60.05	36.9	2.85	0.2
4	53.80	22.23	22.72	1.24
5	52.44	41.57	5.29	0.7

) shows that the samples with similar compositions in the green boxes (positions 1, 3, and 5) correspond to Al₂O₃. Newly formed La₂Si₂O₇ phases (blue box 2) are detected near the spalling areas, consistent with XRD results, confirming the reaction between LaAlO₃ and SiO₂ in the face coat to produce La₂Si₂O₇ and Al₂O₃. At the edge of the face coat (yellow box 4), LaAlO₃ particles aggregate in a contracted configuration, indicating volumetric shrinkage during Equation (2). The presence of La₂Si₂O₇ phases in spalling zones suggests localized stress generation. Furthermore, the structural mismatch between rhombohedral LaAlO₃ crystals and monoclinic La₂Si₂O₇ reaction products drives coating delamination and cracking [25,26], as the phase transformation induces interfacial strain. Figure 8f plots the Gibbs free energy (ΔG) of Equation (2). The results show that ΔG progressively decreases with increasing temperature and remains negative across the illustrated temperature range, indicating that Equation (2) proceeds spontaneously within the 0–1600 °C interval. Previous studies report that LaAlO₃ reacts with Al₂O₃ at a 5:1 molar ratio under high temperatures to form LaAl₁₁O₁₈ (Equation (5)). However, neither XRD nor SEM analyses detect LaAl₁₁O₁₈ phases in the LA shells. This absence is attributed to insufficient Al₂O₃ content near LaAlO₃ particles in the face coat to meet the stoichiometric requirement for Equation (3). Furthermore, while lanthanum (La) can react with both Al₂O₃ and SiO₂ at elevated temperatures, thermodynamic preference drives La to preferentially respond with SiO₂ over Al₂O₃ when both coexist in the system [27].

LaAlO₃ + 5Al₂O₃ = LaAl₁₁O₁₈

(5)

3.1.4. Characterization of Mixed Powders

The research results indicate that when the temperature exceeds 1300 °C, the LaAlO₃ surface layer reacts with SiO₂ to form the La₂Si₂O₇ phase, leading to cracking and peeling. To develop a single-crystal shell surface coating material with enhanced high-temperature resistance, two methods were employed. The first method involved adding SiO₂ to La₂O₃ and Al₂O₃. To prepare a mixed powder of LaAlO₃ and La₂Si₂O₇, we added equal proportions of La₂O₃ and SiO₂ to La₂O₃ and Al₂O₃ in equal proportions and mixed and sintered them in a molar ratio of 2:1:1 [28]. The addition of La₂Si₂O₇ in the surface layer powder effectively inhibits the progression of Equation (4). The second method involves increasing the Al content based on the reactant ratio of Equation (5), as in the first method. The LaAl₁₁O₁₈ phase can replace the LaAlO₃ phase, which is prone to reacting with SiO₂, thereby preventing Equation (4) from occurring. The resulting powder exhibits excellent thermal stability.

Figure 9a presents the XRD phase diagrams of mixed powders with La: Al: Si = 2:1:1 sintered at various temperatures. The results show that La₁₀(SiO₄)₆O₃ intermediates and unreacted La₂O₃ remain in samples sintered at 1350 °C. As the temperature increases, La₁₀(SiO₄)₆O₃ and La₂O₃ progressively diminish, with complete phase transformation achieved at 1400 °C. Continue to raise the temperature to 1450 °C, and the peaks of the sintered products will be higher and narrower, which means that LaAlO₃ and La₂Si₂O₇ have better crystallinity. Therefore, the highest temperature of 1450 °C is most suitable for the sintering process of powder. The final sintered product consists of LaAlO₃ and La₂Si₂O₇ phases, meeting design specifications, and is designated as LAS powder. In Figure 9b, the blue circle shows flake-shaped particles, whose composition (point 1) is La₂Si₂O₇, as shown in

Table 6. Point sweep results of LAS powder.

Atom	O/%	Al/%	La/%	Si/%
1	67.85	-	16.46	15.69
2	59.53	18.08	22.39	-
3	73.29	-	13.05	13.66
4	64.40	33.16	2.44	-

. The yellow circle shows small particles agglomerated into spherical particles, whose composition (point 2) is LaAlO₃, as shown in Table 6. Figure 9c displays the XRD pattern of mixed powders with adjusted La: Al: Si ratios to 2:11:1, sintered at 1500 °C to eliminate LaAlO₃ formation. At this temperature, Reactions 5 and 6 predominantly occur [29,30], yielding sintered products comprising LaAl₁₁O₁₈, La₂Si₂O₇, and Al₂O₃, designated as LA11S powder. Microstructural and point-scan analyses reveal that smaller granular particles (point 3) correspond to La₂Si₂O₇, while larger polygonal crystals (point 4) represent LaAl₁₁O₁₈.

La₂O₃ + 5SiO₂ = La₂Si₂O₇

(6)

3.1.5. Comparative Analysis of Three Types of Molds

Figure 10a shows the LA shell sintered at 1300 °C. The shell surface exhibits a light tan color with smoothness and no visible cracks. Microscopic analysis reveals that LaAlO₃ particles (point 1 in Figure 10b) are primarily interconnected by a binder, accompanied by abundant pores within the face coat. These pores arise from binder decomposition at high temperatures, which increases porosity. While this enhanced porosity facilitates gas venting during casting and reduces thermal conduction, it simultaneously compromises mechanical strength. Figure 10c displays the LAS shell sintered at 1400 °C, characterized by a uniform white color, smooth surface, and excellent surface quality. Figure 10d shows the microstructure of the LAS shell, where points 2 and 3 correspond to La₂Si₂O₇ and LaAlO₃, respectively (

Table 7. EDS Point analysis results of LA and LAS shells.

Atom	O/%	Al/%	La/%	Si/%
1	64.84	16.70	18.46	-
2	65.53	-	20.39	14.08
3	70.20	15.08	14.72
4	38.66	61.34	-	-

). The particles on the shell surface are densely distributed, and compared to the LA shell, the particle spacing and pore number are significantly reduced. However, the presence of Al₂O₃ (green box) indicates that LaAlO₃ in the surface layer still reacts with SiO₂ to form Al₂O₃ and La₂Si₂O₇. Despite this reaction, the pre-existing La₂Si₂O₇ phase in the LAS shell ensures that the interparticle spacing does not drastically shrink, effectively mitigating excessive crystal structural mismatch. Consequently, the reaction at 1400 °C does not induce face coat cracking or delamination caused by poor phase compatibility.

Figure 10e shows the LA11S shell sintered at 1500 °C. The face coat appears white with slight reddish edges, exhibiting a smooth and crack-free surface devoid of wrinkles. Figure 10f reveals the microstructure of the LA11S shell. Compared to LA and LAS shells, the LA11S-derived face coat is pore-free and primarily composed of LaAl₁₁O₁₈, Al₂O₃, and La₂Si₂O₇ phases. The LaAl₁₁O₁₈ particles dominate in size (approximately 12 μm), while Al₂O₃ grains of varying dimensions cluster around them. This configuration arises because LaAl₁₁O₁₈ inhibits Al₂O₃ grain growth and mobility via a “pinning effect,” refining the Al₂O₃ grain size [31,32]. Elemental mapping confirms minimal Si content, with La₂Si₂O₇ localized between LaAl₁₁O₁₈ and Al₂O₃, acting as a binder. The LA11S shell exhibits exceptional resistance to high-temperature sintering at 1500 °C due to the outstanding thermal stability of LaAl₁₁O₁₈ in the powder. In the magnetoplumbite structure of LaAl₁₁O₁₈, La³⁺ occupies the oxygen sites within the hexagonal oxygen framework, strongly suppressing ion diffusion. Consequently, this layered compound grows slowly along directions perpendicular to its basal planes, effectively reducing the interfacial separation between the face coat and transition layer and preventing delamination of the LA11S face coat. Moreover, this structural arrangement minimizes voids within the lattice, endowing the LA11S face coat with superior thermal stability [33]. This configuration effectively inhibits oxygen diffusion, reduces oxidation rates, and enhances coating reliability and service life [34].

3.2. Properties and Applications of Ceramic Shells

In investment casting, if the residual strength of the ceramic shell is low, the shell becomes prone to breakage during post-firing handling and furnace loading. Similarly, inadequate resistance to high-temperature deformation can lead to cracking during pouring and reduced dimensional accuracy of the cast part. Therefore, residual strength and high-temperature strength are critical metrics for evaluating the mechanical performance of ceramic shells [35]. Figure 11 compares the residual and high-temperature strengths of LA, LAS, and LA11S shells sintered at 1300 °C, 1400 °C, and 1500 °C, respectively, against EC95 shells sintered under the same conditions. The results show that both residual strength and high-temperature strength of EC95 shells increase with sintering temperature. Since the back layer significantly influences mechanical properties, the strengths of all three shells also exhibit an upward trend. This is attributed to the reaction between Al₂O₃ and SiO₂ in EC95 shells during high-temperature sintering, forming mullite—a crystalline phase with high strength and thermal stability. However, the shells exhibit lower strength values than EC95 due to non-uniform interfacial bonding between their face and transition layers, which promotes crack propagation and compromises mechanical performance.

Figure 12a–c displays cross-sectional microstructures near the bottom surfaces of recycled CMSX-4 alloy castings produced using three crucibles fabricated via the shell process. Among them, a1-a3, b1-b3, and c1-c3 are the elemental distribution maps of Ni, Al, and O, respectively. Figure 12a reveals a dark gray layer adhering to the alloy surface. The LA crucible exhibits the thinnest layer (approximately 1 μm) with sparse distribution, while the LAS and LA11S crucibles show thicker, continuous layers of 3 μm and 4 μm, respectively. Elemental mapping confirms that the continuous dark gray phase corresponds to Al₂O₃, indicating the formation of an alumina-based slag layer on the alloy surface.

Figure 13a displays the XRD spectra of slag layers on the surfaces of superalloy castings produced using LA, LAS, and LA11S crucibles. The purple and orange bars in the figure represent the presence of Al₂O₃ and NiO on the surface of the LA11S and LAS crucibles casting alloys, respectively, while the blue bar represents the presence of LaAlO₃ on the surface of the LA crucible casting alloy. The results reveal distinct compositional differences: Al₂O₃ and LaAlO₃ dominate the surface of the LA crucible-cast alloy, while Al₂O₃ and NiO are observed for the LAS crucible, and Al₂O₃, SiO₂, and NiO coexist in the LA11S crucible-cast alloy. The presence of LaAlO₃ on the LA crucible-derived surface is attributed to partial spalling of the LA crucible during molten alloy interaction caused by its inferior high-temperature strength. These dislodged LaAlO₃ particles adhere to the alloy surface upon solidification.

Figure 13b illustrates the Ellingham plots for LaAl₁₁O₁₈, LaAlO₃, La₂Si₂O₇, Al₂O₃, SiO₂ and NiO. The thermodynamic stability of these oxides under superalloy casting conditions is in the order of LaAl₁₁O₁₈ > LaAlO₃ > La₂Si₂O₇ > Al₂O₃ > SiO₂ > NiO, which is evidenced by their relative positions in the graph. This indicates that LaAl₁₁O₁₈, LaAlO₃ and La₂Si₂O₇ exhibit higher thermodynamic stability than NiO. Therefore, theoretically, the LA, LAS and LA11S material shells do not react with pure Ni melt. However, the dissolution and decomposition of silica sol (SiO₂) in the melt of these three crucible materials can introduce oxygen contamination to the Ni melt [36,37]. It is noteworthy that the addition of reactive elements such as Al to the alloys significantly increases their oxygen affinity under these conditions compared to Ni and Si. Al reacts with the oxygen released from the decomposition of SiO₂ in the crucible to form a more stabilized Al₂O₃, which accelerates the dissolution of the crucibles and exacerbates the oxygen contamination in the alloy melt, as shown in the following reaction. The schematic diagram of the interface reaction between the ceramic shell and the alloy is shown in Figure 14.

$${\mathrm{SiO}}_2\stackrel{\mathrm{i}\mathrm{n} \mathrm{m}\mathrm{e}\mathrm{l}\mathrm{t}}{\to }{\left[\mathrm{S}\mathrm{i}\right]}_{\mathrm{i}\mathrm{n} \mathrm{m}\mathrm{e}\mathrm{l}\mathrm{t}}+{\left[\mathrm{O}\right]}_{\mathrm{i}\mathrm{n} \mathrm{m}\mathrm{e}\mathrm{l}\mathrm{t}}$$

(7)

$${\left[\mathrm{A}\mathrm{l}\right]}_{\mathrm{i}\mathrm{n} \mathrm{m}\mathrm{e}\mathrm{l}\mathrm{t}}+{\left[\mathrm{O}\right]}_{\mathrm{i}\mathrm{n} \mathrm{m}\mathrm{e}\mathrm{l}\mathrm{t}}\stackrel{\mathrm{i}\mathrm{n} \mathrm{m}\mathrm{e}\mathrm{l}\mathrm{t}}{\to }\left[{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3\right]$$

(8)

The study of the surface quality of the shells at different sintering temperatures shows that the SiO₂ in the silica sol in the surface layer of the LA crucible reacts with LaAlO₃ to form La₂Si₂O₇, which is less prone to be dissolved and decomposed in the melt, whereas there is more silica sol in the surface layer of the sintered crucible because of replacement of the surface layer material in the LAS and LA11S crucibles, which explains why The Al₂O₃ layer on the surface of the cast alloy is the thinnest in the LA crucible among the three crucibles, while the LAS and LA11S crucibles have the thickest slag floats. During the melting process, the superalloy melt interacts with the inner wall of the crucible, leading to the rapid formation of Al₂O₃ phases at the interface. These Al₂O₃ particles rise under gravity to form a slag layer. Given the intimate contact between the superalloy melt and the LAS, LA11S crucible substrates, even if interfacial reactions occur, the resulting Al₂O₃ should exhibit a continuous and uniform distribution. In comparison, Han and co-workers [30] found that the reaction layer in the CMSX-4 alloy cast using an Al₂O₃-type shell was relatively uniform, with a flat and porous interface between the reaction layer and the alloy. However, the slag thickness was approximately 25 μm, which is significantly thicker than the interface reaction layer thickness (no more than 5 μm) observed after casting with LA, LAS, and LA11S-type shells. This indicates a more intense interface reaction. Xuan and co-workers [38] investigated the interface reaction between CMSX-4 and silicon dioxide ceramics. Experimental results showed that a continuous Al₂O₃ layer and discontinuous tantalum/titanium-enriched carbide layers formed at the interface. Compared to the reaction products after casting with LA, LAS, and LA11S-type shells, the reaction products were more complex. The Al₂O₃ layer obtained from the substitution reaction between Al and SiO₂ serves as a nucleation seed for (Ta/Ti)C carbides at the CMSX-4 alloy interface, further intensifying the reaction. In summary, LA, LAS, and LA11S shell-casting nickel-based high-temperature alloys exhibit weaker interfacial reactions and generate more singular reactants, making them a promising new type of shell material with significant application prospects.

4. Conclusions

• This study investigated the sintering of LaAlO₃ powder, the properties of the prepared surface slurry, and the refractoriness of the composite shell. The refractoriness of the composite shell was improved by introducing La₂Si₂O₇ and LaAl₁₁O₁₈ to replace LaAlO₃. Three types of shells were used to cast CMSX-4 alloy, and the interface reactions were observed. The study employed scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) to assess the microstructure and phase composition of the powders, shell surfaces, and interfaces after alloy casting. The main findings and conclusions are summarized as follows: The La₂O₃-Al₂O₃ system yields high-purity LaAlO₃ powder when sintered at 1500 °C. A slurry with a powder-to-liquid ratio of 3:1 and 300-mesh particle size demonstrates optimal coating performance, forming a shell that remains crack-free at 1300 °C. However, at temperatures exceeding 1400 °C, LaAlO₃ reacts with SiO₂ to generate La₂Si₂O₇ and Al₂O₃, leading to failure of the face coat due to lattice distortion and volumetric shrinkage.
• Through optimization of the La₂O₃-Al₂O₃-SiO₂ ternary ratio, the LAS shell, containing a La₂Si₂O₇ phase in its face coat, buffers thermal stress and remains crack-free at 1400 °C. Meanwhile, the LA11S shell exhibits a layered LaAl₁₁O₁₈ structure that suppresses oxygen diffusion and grain coarsening. At 1500 °C, this shell maintains excellent surface quality, meeting the stringent requirements for single-crystal superalloy casting applications.
• After casting CMSX-4 recycled material with LA crucibles, spalled LaAlO₃ particles adhere to the alloy surface, while LAS and LA11S shells exhibit NiO formation. All three crucibles develop Al₂O₃ slag layers with thicknesses below 5 μm. Notably, no severe interfacial reactions occur between the crucibles and the alloy melt; instead, only dissolution-dominated interactions are observed.

In summary, the composite shells prepared from LA, LAS, and LA11S powders can withstand temperatures above 1300 °C and have certain mechanical properties. After casting nickel-based high-temperature alloys, the reaction phenomenon at the interface is weak, exhibiting excellent thermodynamic stability and chemical stability. They are suitable for use in the surface layer of ceramic shells to inhibit interface reactions and obtain alloys with good surface quality. Additionally, this study has not yet conducted more detailed adjustments to the introduction ratios of La₂Si₂O₇ and LaAl₁₁O₁₈. In subsequent research, reducing the introduction of La₂Si₂O₇ and increasing the proportions of LaAl₁₁O₁₈ and Al₂O₃ will facilitate further reduction in the thickness of the slag reaction layer and minimize interfacial reactions during the crucible casting of DZ22 alloy using the newly prepared powder system. Furthermore, the powder prepared in this study was only used as the surface layer in the composite shell for research purposes. Future studies could explore using the experimentally prepared slurry for coating the transition layer and back layer, thereby enhancing the mechanical properties of the shell by achieving a more compact bond between the layers.