Embedding two dimensional Al₂O₃ platelets array into YSZ ceramics for high-temperature applications

Abstract

Two-dimensional (2D) materials, including graphene and MXenes, have garnered significant interest for enhancing both the mechanical and multifunctional properties of advanced structural ceramics. These materials encounter operational limitations under high temperatures, particularly due to oxidative degradation of 2D reinforcement architectures that induces composition instability and property deterioration. In this study, we developed a processing methodology for fabricating 2D Al₂O₃ platelet-reinforced yttria-stabilized zirconia (YSZ) composites, which can be stable up to 1300 ^oC in an atmosphere. Through a multi-field coupling strategy combining gravitational field modulation, high-intensity vibrational alignment, and controlled pressure densification, we achieved densified composites with parallel-aligned Al₂O₃ platelet array in YSZ matrix.‌ The composite demonstrates reduced near-infrared (NIR) transmittance below 10%, effectively blocking radiative thermal transfer. Meanwhile, The YSZ-Al₂O₃ platelet composite (YSZ-Al₂O₃-PL) simultaneously demonstrates enhanced resistance to CaO-MgO-AlO_1.5-SiO₂ (CMAS) molten salts and improved fracture toughness. The cost-effective 2D oxide/ceramic composites possess exceptional thermo-mechanical stability, promising for harsh environments.

A multi-field coupling strategy was used to develop a parallel-aligned 2D Al2O3 platelet-reinforced YSZ composite that exhibits exceptional thermo-mechanical stability, showing great promise for harsh environments.

Introduction

Structural ceramics possess exceptional potential for applications requiring elevated load-bearing capacity, precision, and resistance to corrosive environments, such as turbopumps in aero engines, land gas turbines, and orthopedic implants^1–6. The integration of two-dimensional (2D) materials, including graphene and MXenes, within ceramic matrices has garnered significant interest for enhancing the mechanical and multifunctional properties of advanced structural ceramics^2,7,8. Our previous research introduced a universal and refined strategy to uniformly incorporate and align 2D graphene arrays within the ceramic matrix, resulting in strong matrix-filler bonding facilitated by a coupling agent². This approach facilitates efficient load transfer and superior mechanical performance. The unique structure of two-dimensional-graphene-array-strengthened ceramics, where 2D graphene sheets are arranged in parallel within the 3D ceramic matrix, contributes to their excellent mechanical properties, friction properties and outstanding electromagnetic interference (EMI) shielding performance⁹.

At elevated temperatures, 2D materials like graphene and MXenes face inherent limitations due to oxidation-induced composition instability and phase transformation, resulting in accelerated property degradation^10,11. In specialized applications like thermal barrier coatings (TBCs) and refractory systems, 2D-reinforced ceramics must demonstrate enhanced infrared reflectance capabilities^12,13. A prime example is yttria-stabilized zirconia (YSZ), where elevated radiative thermal conductivity at extreme temperatures undermines its effectiveness as a thermal insulation material^14,15. YSZ also faces accelerated degradation when exposed to molten salt deposits like CaO-MgO-AlO_1.5-SiO₂ (CMAS) ash compounds at high temperatures^16,17. Although YSZ’s exceptional strain tolerance enables diverse applications, CMAS-induced corrosion near 1240 °C triggers phase transformation and strain tolerance loss during high-temperature operation¹⁷. Furthermore, thermal cycling in service environments intensifies mechanical demands—particularly regarding ductility—as repeated thermal stresses promote internal crack propagation and material degradation¹⁸. Moreover, the superior mechanical properties of YSZ primarily stem from transformation toughening and ferroelastic toughening mechanisms. As the service temperature increases, its mechanical performance, particularly fracture toughness-undergoes a significant decline. Research indicates that due to the diminished effect of transformation toughening at elevated temperatures, the fracture toughness of YSZ can decrease by approximately 50% when the temperature rises from room temperature to 1000 °C, considerably compromising its high-temperature reliability¹⁹. These requirements necessitate that 2D reinforcing materials achieve synergistic high-temperature oxidation resistance and matrix chemical compatibility to fulfill multifunctional performance in structural ceramics. This systematic evaluation identifies oxide ceramics as optimal secondary phases for 2D-enhanced ceramic composites.

Fabricating and forming ceramic composites with a specific array structure remains a challenge. Oxide ceramic composites are conventionally synthesized through ball milling, a widely employed method for fabricating oxides with complex compositions^20–22. However, this process is inadequate for fabricating ceramic composites with a specific array structure, as ball milling can easily damage the ceramic platelets. To date, there is no suitable method for introducing oxide platelets without damaging their microstructure. Our previous work introduced a universal and refined strategy to uniformly incorporate and align 2D graphene arrays in the ceramic matrix^2,9. The above method is also not suitable in the present work because the 2D ceramic sheets used here have a significantly higher density compared to graphene, such that the liquid ceramic precursor is unable to intercalate into the interlayer space of the ceramic plates. Moreover, the strength and plasticity of ceramic platelets are much weaker than that of 2D graphene, so that the integrity of ceramic platelets can hardly be maintained². As a case study, YSZ is chosen as the ceramic matrix. Al₂O₃ is widely regarded as an outstanding structural ceramic material due to its low density, high hardness, high mechanical strength, and excellent wear resistance²³. It also exhibits good thermal shock resistance and CMAS corrosion resistance, enabling it to maintain structural stability and mechanical properties under high-temperature conditions²⁴. As a result, it is commonly employed in demanding high-temperature applications such as TBCs. Furthermore, in this study, Al₂O₃ was able to coexist stably with YSZ materials over the temperature range from room temperature to the temperature at which the eutectic reaction between Al₂O₃ and ZrO₂ occurs. Combined with its relatively low thermal conductivity, high toughness, and excellent corrosion resistance, Al₂O₃ proves to be an ideal choice for reinforcing YSZ materials. We propose a unique approach to introduce two-dimensional Al₂O₃ platelets in YSZ matrices to form a specific array structure. By employing a multi-field coupling approach involving the gravitational field, strong vibration field, and pressure field, we successfully synthesized dense composite ceramic materials with a parallel array-structured Al₂O₃. We believe that this method is versatile and provides an approach for synthesizing various ceramic nanosheet-reinforced ceramic matrix composites. The infrared spectrum reflectance, CMAS resistance, and mechanical properties of two-dimensional Al₂O₃ platelets strengthened YSZ composites are comprehensively studied. Our results indicate that the near-infrared reflectance of YSZ-Al₂O₃ platelet composites (YSZ-Al₂O₃-PL) is significantly enhanced compared to YSZ, with a remaining transmittance of less than 10%. The CMAS infiltration depth of YSZ-Al₂O₃-PL is reduced to approximately 40% of that of YSZ. Furthermore, YSZ-Al₂O₃-PL exhibits higher fracture toughness than YSZ at both room temperature and elevated temperatures, thereby demonstrating superior strain tolerance. This study provides a reliable strategy for enhancing the performance of traditional high-temperature TBC ceramic materials.

Results and discussion

Structure characterization

We developed a strategy for preparing YSZ-Al₂O₃-PL as shown in Fig. 1. Initially, YSZ and Al₂O₃ platelets are weighed according to their volume fractions and dispersed in distilled water, with a proper amount of ammonium citrate as a dispersant. The suspension is ultrasonically stirred at 100 rpm for 30 min to ensure efficient mixing without destroying the particle morphology. This step entails the gentle initial blending of two powder materials without structural damage. Subsequently, the mixed powders are obtained through rotary evaporation, followed by drying at 100 °C for 5 h. The YSZ-Al₂O₃-PL powder undergoes further processing in a high-efficiency resonant mixer for 10 min, leveraging gravity and high-frequency vibration to create a preliminary arrangement of the platelets. The integrity of Al₂O₃ platelets is preserved unscathed throughout the process, thanks to the inherent buffering capability of YSZ powder that effectively absorbs and dissipates the mechanical forces generated during machining. Finally, dense YSZ-Al₂O₃-PL ceramics are produced by spark plasma sintering (SPS) at 1300 °C and 40 MPa for 5 min in vacuum, in which process a certain degree of orientation disorder could occur. During the sintering process, a progressive alignment of plate-like composites was observed as the applied pressure escalated in tandem with rising temperature. In this step, under the high-temperature field, the mass transfer processes between YSZ powder and Al₂O₃ platelets are accelerated. With the superimposed effects of gravitational and pressure fields, multi-field coupling conditions are formed, ultimately leading to a more ordered alignment of Al₂O₃ platelets and enhanced densification of the sample.

Fig. 1. Mechanism of the experimental process of YSZ-Al₂O₃-PL preparation.

The densified YSZ-Al₂O₃-PL composite with a parallel-aligned Al₂O₃ platelet array in the YSZ matrix, prepared via a multi-field coupling strategy combining gravitational field modulation, high-intensity vibrational alignment, and controlled pressure densification.

An array structure characterized by parallel Al₂O₃ platelets in the YSZ-Al₂O₃-PL composite is realized through the synergistic action of axial stress and elevated temperature fields, as shown in Fig. 2. The crystal structures of the dense ceramics are determined by X-ray diffraction (XRD) and are listed in Fig. 2a. YSZ shows a pure tetragonal phase (t-ZrO₂). Both samples doped with Al₂O₃ show two different phases: the main phase is t-ZrO₂, and a weak α-Al₂O₃ peak appears, indicating that Al₂O₃ is successfully composited in the ceramics and does not react with YSZ. Fig. 2b displays the fractured surface morphology of the YSZ-Al₂O₃-PL samples. The fractured surface morphology of YSZ and 10 vol% alumina particles doped YSZ (YSZ-Al₂O₃-P) samples is shown in Supplementary Fig. 1. YSZ grains in all samples are small, due to SPS sintering. Al₂O₃ appears black in scanning electron microscopy (SEM) images of YSZ-Al₂O₃-P and YSZ-Al₂O₃-PL; the particles/platelets distribute uniformly owing to ultrasonic treatment and the addition of ammonium citrate dispersant. Al₂O₃ platelets maintain their original shape and size after ultrasonic stirring, and after vibration and SPS, they form a nearly parallel structure in the YSZ matrix. All samples are fully dense. The morphology of the Al₂O₃ platelets raw powder and wider viewing images of YSZ-Al₂O₃-PL are also recorded and shown in Supplementary Fig. 2.

Fig. 2. XRD, SEM and TEM and simulation results of YSZ/Al₂O₃ grain boundary.

a XRD results of the samples. b Fractured surface of YSZ-Al₂O₃-PL by SEM. c Bright field TEM image of YSZ-Al₂O₃-PL. d HRTEM of YSZ/Al₂O₃ grain boundary corresponding to A site in (b). e Phase composition of YSZ near the YSZ/Al₂O₃ grain boundary. The inset figures show the IFFT results from different regions near the grain boundary. T denotes the tetragonal YSZ region, while T + M denotes the region containing both tetragonal and monoclinic YSZ. f Grain boundary configuration of YSZ near the YSZ/Al₂O₃ grain boundary. Large gray atoms, large green atoms, small red atoms and blue atoms stand for Al, Zr, O, and Y elements, respectively. Source data are provided as a Source Data file.

Fig. 2c presents a bright field transmission electron microscopy (TEM) image of the YSZ-Al₂O₃-PL composite, where the bright strips correspond to Al₂O₃ platelets, while the YSZ particles occupy the remainder of the image. The elemental mapping in Supplementary Fig. 3 further substantiates this observation, revealing a strong adherence between the Al₂O₃ plates and the YSZ substrate, in line with the SEM findings. The high-resolution TEM (HRTEM) image corresponding to Site A in Fig. 2c, as displayed in Fig. 2d, clearly exhibits a heterogeneous interface, demarcated by the dark dashed line. The YSZ (002) crystal plane interfaces with the Al₂O₃ (012) plane, resulting in a non-coherent grain boundary. Lattice distortion is evident in both phases due to lattice constant mismatch and thermal expansion coefficient disparities, giving rise to the presence of dislocations. The high density of the YSZ-Al₂O₃-PL composite appears to maintain its integrity across the nanoscale to microscale dimensions, potentially contributing to enhanced thermal and mechanical properties. Fig. 2e illustrates the compositional evolution of YSZ in the vicinity of the YSZ/Al₂O₃ grain boundary, corresponding to site B in Fig. 2c. Away from the boundary, YSZ maintains a tetragonal structure; however, as it approaches, a transformation from tetragonal to a mixed tetragonal-monoclinic phase occurs. This phase transition is primarily driven by the accommodation of stress resulting from lattice mismatch and disparities in thermal expansion coefficients²⁵. Molecular dynamics simulations, as depicted in Fig. 2f, also corroborate these findings, revealing that YSZ near the grain boundary assumes a monoclinic structure, potentially minimizing overall lattice energy. In the following sections, we will demonstrate that this phase transition may play a crucial role in triggering crack deflection during crack propagation.

Thermal radiative properties

The directional-hemispherical transmittance (T) and reflectance (R) of three samples, each 1 mm thick, were measured over the wavelength range of 400 nm to 2000 nm. As depicted in Fig. 3a, transmittance is represented by solid lines. YSZ exhibits translucent behavior, particularly in the NIR waveband. At 2 μm, the transmittance of YSZ is approximately 55%, which decreases with shorter wavelengths. According to Wien’s displacement law, at a temperature of 1800 °C, the peak of thermal radiation power occurs at 1.4 μm, with the NIR waveband comprising the majority of the radiation energy. The high transmittance of YSZ at these wavelengths indicates that a significant amount of thermal radiation will penetrate it, directly heating the alloy substrate and adversely affecting the TBC system. In contrast, both samples incorporating Al₂O₃ show a marked reduction in transmittance. YSZ-Al₂O₃-PL achieves a transmittance below 10% across the 400–2000 nm range, while YSZ-Al₂O₃-P exhibits slightly higher transmittance. The dashed lines in Fig. 3a represent reflectance, showing that YSZ-Al₂O₃-P and YSZ-Al₂O₃-PL have significantly enhanced reflectance capabilities compared to pure YSZ. Al₂O₃ platelets are found to be more effective in improving reflectance than Al₂O₃ particles.

Fig. 3. Optical and thermal transport properties of YSZ-Al₂O₃ composite materials.

a Transmittance (solid lines) and reflectance (dashed lines) from 400 nm to 2000 nm of the samples. b Scattering coefficient as a function of particle size and photon wavelength obtained from Mie theory. c, d Schematic drawing of the different optical behavior between YSZ-Al₂O₃-PL and YSZ-Al₂O₃-P. e Total thermal conductivities κ (dashed lines) and approximate phonon thermal conductivities κ_p (solid lines) from room temperature to 1000 °C. f Approximate radiative thermal conductivities κ_r from room temperature to 1000 °C. Source data are provided as a Source Data file.

The reflectance of materials is directly determined by their backscattering ability. The fluctuation of the refractive index within the material significantly affects photon scattering^26,27. The refractive index of Al₂O₃ is approximately 1.77²⁸, lower than that of YSZ (n = 2.12–2.19)²⁹. Previous research has shown that the presence of a second phase with a lower refractive index enhances photon scattering, thereby increasing reflectance¹⁴. For spherical particles, when the size of the second phase approaches the wavelength of the incident radiation, the backscattering efficiency reaches its peak¹⁴. To investigate the effect of particle size on the scattering coefficient of photons of different wavelengths, we conducted Mie calculations, and the results are presented in Fig. 3b. The data show that particles within the size range of 0.4 to 3 μm exhibit a significantly higher scattering coefficient. According to SEM images, the grain size of the second phase particles in the reference YSZ-Al₂O₃-P sample is approximately 0.5-3 μm, which is located in the wavelength range where thermal radiation is most intense. Therefore, in the case of nearly spherical Al₂O₃ doping, the reflectance of the YSZ-Al₂O₃-P ceramic approaches the theoretical maximum. In contrast, the YSZ-Al₂O₃-PL ceramic exhibits even higher reflectance and lower transmittance than the YSZ-Al₂O₃-P ceramic. Considering the intentionally designed parallel distribution of the second phase, the geometric cross-section can be maximized. The backscattering efficiency factor Q of an Al₂O₃ second-phase unit is defined as Q = C/S, where C is the backscattering cross-section, and S is the geometric cross-section. The number density of the second phase (N) can be calculated from the volume fraction (V_f) of the second phase and the volume (V) of a second phase unit, by N = V_f/V, and then the backscattering coefficient (s) of YSZ-Al₂O₃ can be calculated as³⁰:

$$s=N\cdot C=\frac{V_f\cdot Q\cdot S}{V}$$ 1

Given the same V_f of 10% for both the YSZ-Al₂O₃-PL and YSZ-Al₂O₃-P samples, the variations in their specific surface areas (S/V) and Q values lead to distinct reflection behaviors. In this study, platelets with dimensions of 7 μm in diameter and 0.3 μm in thickness, as well as spherical particles with a diameter of 1.5 μm, were utilized to estimate S/V. It is calculated that the YSZ-Al₂O₃-PL sample exhibits a significantly higher S/V ratio of 3.33 times that of the YSZ-Al₂O₃-P sample. Furthermore, it has been established that at a scattering angle of θ = 90° to 180°, the flux of scattered radiation for a plate-like cylinder is greater than that for a needle-like cylinder, provided that their symmetry axes are perpendicular to the scattering plane³¹. This principle suggests that Al₂O₃ platelets possess a higher Q than Al₂O₃ spherical particles. Consequently, the YSZ-Al₂O₃-PL sample demonstrates enhanced backscattering capabilities, ensuring its high reflectance, even when compared to the nearly maximum reflectance achievable through the addition of second-phase particles. The schematic diagrams in Fig. 3c, d illustrate the aforementioned differences in optical properties between the YSZ-Al₂O₃-PL and YSZ-Al₂O₃-P samples.

The thermal conductivities of the ceramics are measured from room temperature to 1000 °C as illustrated in Fig. 3e the total thermal conductivity of YSZ increases significantly at elevated temperatures, whereas both samples embedded with Al₂O₃ exhibit continuously decreasing total thermal conductivities with temperature. At 1000 °C, the total thermal conductivity of YSZ-Al₂O₃-PL is 2.51 W/(m·K), significantly lower than that of YSZ (3.11 W/(m·K)) and also lower than the reference sample YSZ-Al₂O₃-P (2.60 W/(m·K)). The approximate phonon thermal conductivities κ_p, obtained by preventing direct transmission through the ceramics, are also plotted in Fig. 3e. Generally, κ_p decreases with temperature, except for the slight increase in YSZ after 800 °C. This anomaly is likely due to the high transmittance of YSZ, which still allows a small amount of laser penetration through the ceramic, despite the thin gold layer coating both surfaces. In other words, the phonon thermal conductivity of YSZ may be overestimated at high temperatures. At room temperature, the phonon thermal conductivities of YSZ-Al₂O₃-PL and YSZ-Al₂O₃-P are higher than that of YSZ, given that alumina has a larger intrinsic phonon thermal conductivity (~34 W/(m·K) at 25 °C. With the same volume fraction of Al₂O₃, YSZ-Al₂O₃-PL exhibits lower phonon thermal conductivity than YSZ-Al₂O₃-P over the entire temperature range. In the Debye-Klemens-Callaway model, all phonon scattering processes are considered for calculating thermal conductivity. The combined phonon relaxation time τ_c is expressed as follows³²:

$${\tau }_C^{-1}={\tau }_P^{-1}+{\tau }_D^{-1}+{\tau }_B^{-1}$$ 2

Then the phonon thermal conductivity is calculated by ref. ³²:

$$\kappa =\frac{k_B}{2{\pi }^2{v}_s}{\left(\frac{k_B}{\hslash }\right)}^3{\int }_0^{\frac{{\Theta }_D}{T}}{\tau }_C\left(x\right)\frac{x^4{e}^x}{{\left(e^x-1\right)}^2}dx$$ 3

τ_P, τ_D, τ_B are the relaxation times of Umklapp phonon-phonon scattering, defect-phonon scattering, and boundary-phonon scattering, respectively. ν_s is the mean sound velocity, Θ_D is Debye temperature, and x =$\hslash$ω/k_BT. The introduction of Al₂O₃ in YSZ could provide more boundary-phonon scattering and decrease τ_B. The main difference in phonon transport between YSZ-Al₂O₃-PL and YSZ-Al₂O₃-P stems from the density of YSZ/Al₂O₃ heterointerfaces. Compared to Al₂O₃ particles, the nearly parallel-aligned 2D Al₂O₃ sheets increase the geometric cross-section of the YSZ/Al₂O₃ heterointerfaces, thereby enhancing the probability of phonon scattering during transport. Thus, a smaller τ_c is obtained, then the phonon thermal conductivity κ_p of YSZ-Al₂O₃-PL is suppressed to a lower level.

Fig. 3f presents the approximate radiative thermal conductivity κ_r calculated by the difference between the total thermal conductivity κ and the approximate phonon thermal conductivity κ_p. The incorporation of alumina effectively diminishes the radiative thermal conductivity. This reduction is attributed to enhanced photon scattering, which leads to a greater reflection of NIR thermal radiation and a reduction in transmission. As previously discussed, the inclusion of partially parallel Al₂O₃ platelets increases the geometric cross-section and enhances the backscattering efficiency compared to Al₂O₃ particles. The radiative thermal conductivity of YSZ-Al₂O₃-PL is marginally lower than that of YSZ-Al₂O₃-P, a finding that is consistent with optical measurements. At 1000 °C, a low κ_r of 0.13 W/(m·K) is achieved, which is approximately one-fifth of that of YSZ (0.67 W/(m·K)). The aforementioned results and discussion suggest that the well-arranged Al₂O₃ platelets array significantly enhances photon scattering and phonon-boundary scattering, leading to substantial improvements in NIR reflectance and effectively suppressing radiative heat transfer.

CMAS corrosion behavior

After undergoing CMAS corrosion at 1250 °C for 5 h, the polished cross-sections of the ceramics are examined by SEM to evaluate the corrosion behavior. Before the corrosion test, the thickness of the samples was measured to determine the exact location of the upper surface in SEM images. Following the observation of morphology, the samples are ground to a thickness before corrosion to remove the residual CMAS on the upper surface. Subsequently, the phase transformation after CMAS corrosion is examined by XRD. The results are shown in Supplementary Fig. 4. For the YSZ matrix, the t-ZrO₂ phase is severely decomposed into cubic phase (c-ZrO₂) and monoclinic phase (m-ZrO₂). In the alumina-dispersed ceramics, the existence of the CaAl₂Si₂O₈ phase (anorthite) reveals that a reaction occurs between alumina and the Ca, Si elements in CMAS.

Fig. 4a, c, e display the SEM micrographs of YSZ, YSZ-Al₂O₃-P, and YSZ-Al₂O₃-PL after being attacked by molten CMAS. Firstly, the initial thickness is marked on the images by parallel horizontal lines, and then the maximum infiltration depth is measured. The YSZ ceramic is seriously corroded by molten CMAS, with an infiltration depth of ~195 μm. After 10 vol% Al₂O₃ particles addition, the infiltration depth is reduced to ~148 μm. 10 vol% Al₂O₃ platelets have even more contribution to prevent the CMAS infiltration with a depth of ~80 μm, only about 40% of the YSZ ceramic. Fig. 4b, d, f show the cross-section morphology near the bottom of the infiltration zone. For each sample, obviously different morphology appears at both sides of the infiltration zone border. Different phases are identified by energy-dispersive spectroscopy (EDS) and the above XRD results; they are marked with arrows. EDS results are shown in Supplementary Fig. 5 and Supplementary Table 1. In the infiltration zone, globular m-ZrO₂ particles distribute alternately with CMAS. The phase transformation process is constituted by dissolution and precipitation. First, YSZ grains dissolve in molten CMAS, the relatively high solubility of Y³⁺ promotes its enrichment in CMAS. Then, m-ZrO₂ with low Y content precipitates as globular particles^24,33–35. In YSZ-Al₂O₃-PL and YSZ-Al₂O₃-P, anorthite phase appears together with the CMAS-infiltrated m-ZrO₂, which has been reported to be effective for improving CMAS resistance³⁴. In YSZ-Al₂O₃-P, a dispersed granular shape of anorthite phase is observed, while in YSZ-Al₂O₃-PL, the anorthite phase shows an elongated shape and layered distribution owing to the parallel Al₂O₃ platelets. Fig. 4g shows the schematic drawing of the CMAS corrosion behavior. Parallel Al₂O₃ platelets provide a larger reaction area with molten CMAS compared with the same volume fraction of Al₂O₃ particles, thus effectively restraining the molten CMAS within a thin layer beneath the upper surface of the ceramic.

Fig. 4. CMAS resistance of YSZ, YSZ-Al₂O₃-P, and YSZ-Al₂O₃-PL samples.

a, c, e CMAS infiltration depth of YSZ, YSZ-Al₂O₃-P, and YSZ-Al₂O₃-PL, the parallel horizontal lines are the initial surfaces of the samples. b, d, f Cross-section morphology of YSZ, YSZ-Al₂O₃-P, and YSZ-Al₂O₃-PL near the bottom of the infiltration zone. The corresponding phases are marked. g Schematic drawing of the CMAS corrosion resistance mechanism of YSZ-Al₂O₃-PL, with YSZ-Al₂O₃-P and YSZ for comparison.

EDS mapping is also collected to show the distribution of the elements after the CMAS corrosion test (Fig. 5). The Ca and Si maps show the penetration of molten CMAS. In YSZ-Al₂O₃-P and YSZ-Al₂O₃-PL, a significant intensity difference appears between the infiltration zone and the ceramic, proving that the formation of anorthite phase can protect the ceramic from further attack by molten CMAS. However, in YSZ, the boundary of the infiltration zone is not clear, and there is still a considerable amount of Ca and Si penetrating beyond the depth of ~195 μm. The Zr and Y maps show a similar difference among the three samples, indicating that the anorthite phase not only inhibits CMAS penetration but also restrains the diffusion of YSZ into CMAS. Moreover, as can be seen from Fig. 4 and Fig. 5, blistering phenomena are observed on the sample surfaces after CMAS corrosion. According to literature reports³⁶, failure of ZrO₂ upon exposure to silicate molten deposits can occur due to blistering, which is primarily caused by two mechanisms: first, crystallization and phase transformation of the parent phase after partial dissolution in the melt (the t-ZrO₂→m-ZrO₂ transformation observed in this study), and second, dissolution-reprecipitation of the same phase via Ostwald ripening. Simulation studies by Kumbhar et al.³⁶ indicated that such blistering is mainly attributed to the Ostwald ripening mechanism. The experimental results of this study are consistent with this theory.

Fig. 5. EDS mapping of YSZ, YSZ-Al₂O₃-P, and YSZ-Al₂O₃-PL samples.

a–c EDS mapping of YSZ, YSZ-Al₂O₃-P, and YSZ-Al₂O₃-PL and the corresponding SEM images.

Hardness and fracture toughness at various temperatures

Table 1 reveals that the YSZ-Al₂O₃-PL composite exhibits higher toughness and hardness compared to pure YSZ. The enhanced hardness of YSZ-Al₂O₃-PL can be attributed to the high hardness of Al₂O₃. Moreover, the literature indicates that both the hardness and toughness of YSZ diminish as temperature increases^19,37. This phenomenon can be attributed to the reduced strength of chemical bonds within YSZ and the suppression of phase transformation toughening, thus rendering YSZ-based materials susceptible to failure as mid-to-high temperature structural ceramics. We further characterized the high-temperature (600 °C and 800 °C) trends in toughness and hardness of YSZ and YSZ-Al₂O₃-PL using indentation methods. The measured toughness and hardness values are also presented in Table 1. It is evident that the toughness of both materials decreases with rising temperature. As previously mentioned, this behavior arises from the weakened chemical bonding strength and inhibited phase transformation toughening. Furthermore, according to the Griffith theory for brittle materials, the decline in fracture toughness is primarily attributed to the reduction in elastic modulus and surface energy. At elevated temperatures, the ions in YSZ and Al₂O₃ exhibit pronounced anharmonic vibrations, causing them to deviate gradually from their equilibrium positions and increasing the lattice energy. This, in turn, lowers the elastic modulus and surface energy, weakens the chemical bond strength, and ultimately leads to bond fracture under lower applied loads. Consequently, the high-temperature toughness of these ceramic materials gradually decreases with increasing temperature. However, the YSZ-Al₂O₃-PL exhibited superior hardness and toughness, demonstrating that Al₂O₃ platelets can effectively enhance the mechanical properties of YSZ at elevated temperatures. YSZ is renowned for its high toughness, which is primarily attributed to the phase transformation toughening and ferroelastic domain switching mechanisms^38,39. Thus, YSZ exhibits higher fracture toughness compared to Al₂O₃ ceramics. However, the introduction of Al₂O₃-PL into YSZ may reduce the overall toughness of the material to some extent. Therefore, further clarification of the mechanism behind its toughness evolution is required. YSZ is renowned for its high fracture toughness, mainly originating from phase transformation toughening and ferroelastic domain switching mechanisms^38,39. Consequently, YSZ generally exhibits higher toughness than Al₂O₃ ceramics. Intuitively, incorporating Al₂O₃-PL into YSZ would be expected to reduce the overall toughness of the composite due to the intrinsically brittle nature of Al₂O₃. However, experimental results reveal an opposite trend-an enhancement in fracture toughness-implying the existence of additional toughening mechanisms that merit further investigation.

Table 1.

Hardness and toughness of YSZ and YSZ-Al₂O₃-PL

Samples	25 °C	600 °C	800 °C
YSZ Hardness (GPa)	13.39	4.92	4.17
YSZ-Al2O3-PL Hardness (GPa)	14.86	5.74	4.74
Toughness (MPam1/2)	3.26	1.50	1.13
YSZ-Al2O3-PL Toughness (MPam1/2)	3.76	1.70	1.40

Fig. 6 presents the molecular dynamics (MD) results for two YSZ-Al₂O₃-PL models, including the crack path obtained from indentation methods. The strain-stress curves and model configurations at three key stages are depicted in panels Fig. 6a–g. The development and transformation of two models are visually depicted in the accompanying supplementary movies (Supplementary Movie 1 and Supplementary Movie 2)⁴⁰. Initially, YSZ-Al₂O₃-PL displays ductile fracture behavior. At point 1, a phase transition from tetragonal to monoclinic structure occurs when the crack is parallel to the YSZ-Al₂O₃ interface, as shown in Fig. 6b. As the load increases, the monoclinic phase expands until point 2, evident in both Fig. 6a, c. The YSZ| |Al₂O₃-(0001) interface, known for its relatively high interface energy as verified by the TEM image shown in Fig. 2 that YSZ transform from tetragonal to monoclinic phase, facilitates easy nucleation of cracks under tensile strain, resulting in crack bridging. In the model with a pre-crack perpendicular to the Al₂O₃ plate, similar behavior is observed, with differences attributed to the Al₂O₃ plate’s defects (yellow area in Fig. 6e, which has also presented in Supplementary Fig. 6). New cracks form within the plate before the main crack propagates, causing strain field interactions that lead to crack bridging and branching (Fig. 6f, g). The mechanism can be summarized as follows: phase transition initiates before crack propagation, followed by tetragonal phase transformation near the crack tip, and crack bridging and branching accompany the crack’s progression.

Fig. 6. Molecular dynamics simulation and crack propagation in YSZ-Al₂O₃-PL.

a The strain-stress curve illustrates the mechanical behavior, with the red (pre-crack parallel to the Al₂O₃ plate) and black (pre-crack perpendicular to the Al₂O₃ plate) lines. b–d Snapshots of the YSZ-Al₂O₃-PL model correspond to key points 1-3 along the red curve. e–g Snapshots of the YSZ-Al₂O₃-PL model correspond to key points 1-3 along the black curve. The color bar in the figure represents the values of the atomic coordination number. SEM images of the crack paths are provided for both the h YSZ and i, j YSZ-Al₂O₃-PL samples. Source data are provided as a Source Data file.

Figure 6h–j illustrates the crack path derived from indentation, revealing significant crack deflection in the YSZ-Al₂O₃-PL sample, particularly when the crack propagates parallel to the Al₂O₃-PL interface. This observation aligns well with the simulation results. To further substantiate the phase transition in both YSZ and YSZ-Al₂O₃-PL, Raman spectra from various positions in these materials are presented in Supplementary Fig. 7, echoing the findings from molecular dynamics simulations. Supplementary Fig. 8 illustrates the indentation morphologies and crack propagation paths at 600 °C and 800 °C. Notably, the toughening effect of Al₂O₃ platelets remains uncompromised, as significant crack deflection was observed during propagation, which is consistent with both room-temperature experimental results and computational simulations. These findings suggest that the Al₂O₃ platelets-reinforced YSZ material maintains excellent mechanical performance across a wide temperature range, from room temperature to mid-high service temperatures.

In the present work, Al₂O₃ platelet array is embedded in YSZ through ultrasonic mixing and a high-efficiency resonant mixing process. The YSZ-Al₂O₃-PL ceramic is prepared by subsequent SPS. Pure YSZ and 10 vol% alumina particles doped YSZ ceramics are also prepared as reference samples. Microstructure characterization reveals that the alumina platelets form a nearly parallel structure in the YSZ matrix, and alumina platelets adhere well to the YSZ matrix. Compared with YSZ, the NIR reflectance of YSZ-Al₂O₃-PL is significantly enhanced, and only less than 10% transmittance remains. The presence of alumina second phase with a lower refractive index promotes the backscattering of photons. More importantly, compared with alumina particles, the parallel distribution of the platelets maximizes the geometric scattering cross-section and increases the backscattering efficiency. Due to the lower transmittance, the radiative thermal conductivity of YSZ-Al₂O₃-PL is reduced to 0.13 W/(m·K) at 1000 °C, approximately one-fifth of YSZ. The phonon thermal conductivity of YSZ-Al₂O₃-PL is also reduced because of the higher boundary-phonon scattering. The alumina addition also improves the CMAS resistance of YSZ. The CMAS infiltration depth of YSZ-Al₂O₃-PL is reduced to ~40% of YSZ, owing to the good protection of the anorthite phase and the large contact area of the parallel platelets with molten CMAS. A parallel alumina platelet array is expected to shield the increasing radiative heat transfer at elevated temperatures and retard the material failure induced by CMAS attack, thus making YSZ more suitable to work at high temperatures. Meanwhile, YSZ-Al₂O₃-PL exhibits enhanced fracture toughness in comparison to YSZ, making it more strain-tolerant. The present work provides a versatile and viable approach for developing two-dimensional oxide-strengthened structural ceramics for high-temperature applications.

Methods

Preparation of the ceramics and CMAS powder

YSZ powder (8 wt% yttria), α-alumina platelets (Ronaflair White Sapphire, Merck, Germany), and α-alumina powder (particle size: 0.5–3 μm) are used. Three different compositions, pure YSZ, YSZ-Al₂O₃-PL, and YSZ-Al₂O₃-P, are designed and prepared. Raw powders of YSZ-Al₂O₃-PL and YSZ-Al₂O₃-P are prepared by mixing YSZ and alumina powder with ultrasonic stirring. Firstly, YSZ and Al₂O₃ powders are weighed according to the volume fraction, and dispersed into 150 mL of distilled water. 1 wt% ammonium citrate is added as a dispersant. Then, the suspension is stirred at 100 rpm for 30 min in an ultrasonic machine. The well-mixed powders are obtained by rotary evaporation and dried for 5 h at 100 °C. Before sintering, the powders are sieved through a 200-mesh screen, while the YSZ-Al₂O₃-PL powder is further processed by a high-efficiency resonant mixer (LABSM−1, CASC, China) for 10 min to form a preliminary arrangement of the platelets. Pure YSZ powder is sintered by SPS (LABOX-225, Japan) at 1250 °C for 5 min in vacuum with a pressure of 40 MPa to form a dense sample. Both YSZ-Al₂O₃ fully dense ceramics are prepared by SPS at 1300 °C, 40 MPa for 5 min in vacuum. For YSZ-Al₂O₃-PL, we expect that the pressure sintering could form a parallel alumina platelet array in the ceramic. After sintering, all samples are calcined at 1100 °C for 2 h to remove the remaining graphite. The samples are then machined into Φ10 mm, the upper and lower surfaces are polished to be parallel with each other, and their thickness is around 1 mm.

CMAS powder is prepared with CaO, MgO, Al₂O₃, and SiO₂ powders, with a molar ratio of 33Ca-9Mg−13Al-45Si. Before mixing, the oxides are calcined at 800 °C for 2 h to remove the absorbed moisture. Then the mixed powders are ball-milled with ethanol for 5 h at 250 rpm. The slurry is dried by rotary evaporation to obtain CMAS powder, which is then ground and sieved through a 200-mesh screen.

Structures and properties characterization

The phase structures of the ceramics are examined by XRD (D/max-RB, Rigaku, Japan, Cu/Kα), with a scanning speed of 8°/min between 2θ = 10° to 90°. The grain morphology and the arrangement of alumina in YSZ are observed on the fractured surface of the samples through SEM (Merlin, Zeiss, Germany). TEM (JEOL JEM-2100F, Japan) was employed to investigate the interface between YSZ and Al₂O₃ in the YSZ-Al₂O₃-PL composite sample. The inverse Fourier transform of TEM images is typically performed using DigitalMicrograph (DM) software. The sample, approximately 40 nm in thickness, was prepared using a focused ion beam (FIB, Crossbeam 340, Carl Zeiss, Germany) technique. After selecting a specific surface area, a protective Pt layer was deposited prior to employing a Ga ion beam for processing, which involved a cutting stage at 30 kV and 30 nA (gradually reduced to 1 nA) followed by a thinning stage where parameters were sequentially reduced from 30 kV/30 pA to 5 kV/10 pA until achieving electron transparency, and concluded with a final 2 kV/10 pA polish to remove the amorphous surface layer. Raman spectroscopy measurements were conducted within the wavenumber range of 100 to 1000 cm⁻¹ using a LabRAM HR Evolution spectrometer (Horiba, France) at room temperature, employing an argon-ion laser (532 nm) for excitation.

The directional-hemispherical reflectance and transmittance of the samples were measured using a UV-Vis-NIR spectrophotometer (Lambda 950, PerkinElmer, USA) equipped with an integrating sphere. The wavelength range was set to 400–2000 nm. The thermal diffusivities (α) of the samples were determined via the laser flash method (LFA467, NETZSCH, Germany). Each sample was measured twice. For the first measurement, the samples were coated with a thin graphite layer, and total thermal diffusivities were measured from 25 °C to 1200 °C. In the second measurement, the samples were first coated with a thin gold layer before graphite coating, a common practice to prevent direct laser transmission through translucent samples^41,42. Subsequently, the approximate phonon thermal diffusivities (α_p) were derived. Through two sets of measurements, the approximate radiative thermal diffusivities can be expressed as α_r = α–α_p. The corresponding thermal conductivities (κ) are calculated by:

$$\kappa =\rho C_P\alpha$$ 4

C_p is mass-specific heat capacity, which is calculated as the weighted average of the constituents’ values (Al₂O₃, ZrO₂, and Y₂O₃, obtained from Ref⁴³.) according to the Neumann-Kopp law. The sample density (ρ) is measured by the Archimedes method.

In the CMAS corrosion tests, the upper surfaces of the samples are uniformly covered with 30 mg/cm² of CMAS powder, followed by calcination at 1250 °C for 5 h. The cross sections through the center of the samples are prepared using a diamond wire cutting machine and then polished. SEM is employed to observe the reaction between molten CMAS and the samples. EDS on SEM is utilized to detect the distribution of elements and distinguish between different phases.

The mechanical properties of YSZ-Al₂O₃-PL were evaluated using indentation techniques, specifically a micro-indentation tester, the Omnimet MHT, from Buehler, USA. Vicker’s hardness and fracture toughness were measured under a load of 10 N and a dwell time of 10 s. The hardness, expressed as Vicker’s Hardness (H_V), and fracture toughness (K_IC) were calculated using the following established equations⁴⁴:

$$H_V=0.464\frac{P}{d^2}$$ 5

$$K_{\mathrm{I}C}=0.035\times {\left(\frac{H_V}{E}\right)}^{-\frac{5}{2}}{H}_{V}a{c}^{-\frac{1}{2}}{\phi }^{-\frac{3}{5}}\left(\mathrm{c}/\mathrm{a}<2.5\right)$$ 6

Where, P and d represent the load and length of the diagonal of the indentation, respectively. a and c denote half the indentation diagonal length and the crack length from the indentation center, respectively. φ is the constraint factor with a value of 3. The elastic modulus of YSZ and Al₂O₃ are chosen as 210 GPa and 416 GPa, respectively^14,45. The elastic modulus of YSZ-Al₂O₃-PL is calculated using the mixing rule, which yields 230 GPa.

The high temperature microhardness and fracture toughness assessments were performed using a micro-indentation tester with a load of 20 N and a dwell time of 10 s. The Vicker’s hardness (H_V) and Vicker’s toughness (K_VIF) values were calculated using established relationships⁴⁶:

$$K_{VIF}=0.16\cdot H_V\cdot a^{1/2}\cdot {\left(c/a\right)}^{-3/2}\left(c/a>2.5\right)$$ 7

Modelling and calculating details

In the initial stage, a tetragonal zirconia model with 12416 atoms was constructed, featuring dimensions of 66.28570 Å × 10.19780 Å × 186.46919 ų. A corresponding Al₂O₃ model, with a size of 64.97000 Å × 9.52120 Å × 49.44000 ų, was also generated. To create a ZrO₂-Al₂O₃ composite model, the two structures were merged along their ZrO₂-(001) and Al₂O₃-(0001) surfaces, as depicted in Supplementary Fig. 9. A specific number of Y³⁺ ions replaced Zr⁴⁺ ions in the ZrO₂-Al₂O₃ model in order to create the YSZ-Al₂O₃ model. Charge neutrality was maintained by introducing an appropriate amount of oxygen vacancies in the ZrO₂ component by randomly removing O^2- ions.

Two additional composite models were generated from the YSZ-Al₂O₃ starting point by expanding the original model in the [001] and [100] crystallographic directions. All model visualizations were produced using the OVITO software for clarity and consistency^47,48.

MD simulations were conducted using the LAMMPS software (LAMMPS (19 Nov 2024))⁴⁸, employing a combination of a short-range Born-Mayer-Buckingham potential and a long-range Coulombic potential (Eq. (8)) to model the interactions between ions. This approach, known for its predictive accuracy, has been effectively utilized in studies of ZrO₂ phase transitions⁴⁹.

$$V\left(r_{ij}\right)=A_{ij}{e}^{-r_{ij}/{\rho }_{ij}}-\frac{C_{ij}}{{r_{ij}}^6}+\frac{1}{4\pi {\epsilon }_0}\frac{q_i{q}_j}{r_{ij}}$$ 8

In Eq. (8), A_ij, C_ij and ρ_ij are interaction parameters obtained from references⁵⁰, r_ij is the distance between ion i and j, q_i and q_j are the ionic charges of ion i and j, respectively.

A pre-crack with a length of about 6 nm is inserted in the two models (parallel to [100] and [001]) in order to perform the crack propagation simulation. The models were first relaxed at 300 K for 30 ps to eliminate the internal stress and followed by a deformation in the x-direction with a strain rate of 1%/ps and a maximum strain of 30%.

The scattering coefficient as a function of wavelength and the radius of the Al₂O₃ particle is estimated by the Mie theory via the MiePlot v4.6.13 software⁵¹. In the calculations, a monochromatic unpolarized plane wave was selected as the light source, with an irradiance of 1 W/m².

Author contributions

Z.Y. and C.W. initiated the concepts. Z.Y. designed the experiments. Z.Y., X.Z., J.J., Y.Y., and X.L. conducted the experiments. X.L. performed the MD simulations. Y.Y. performed the scattering coefficient simulations. Z.Y., X.Z., J.J., and Y.Y., X.L., W.P. and C.W. analyzed the data. Z.Y., X.L. and C.W. wrote the manuscript. All the authors contributed to manuscript preparation.

Peer review

Peer review information

Nature Communications thanks Qiaomu Liu, Xinghua Su, and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

The data generated and analyzed during this study are available from the corresponding author upon request. Source data are provided with this paper. All calculation results have been uploaded to Figshare (10.6084/m9.figshare.30945296)⁴⁰. Source data are provided with this paper.

Code availability

The code generated during this study is available from the corresponding author upon request.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-026-69355-7.