Han Xiaoyu ^{1, 2}, Liu Qi ^{1, 2}, Bo Xinwei ^{1, 2}, Wang Xiaoyu ^{1, 2}, Pan Shilong ^{1, 2}, Wu Jian ^{1, 2},

Yao Zhiyuan ^{1, 2}, He Haoran ^{1, 2}, Li Rui ^{1, 2}, Zhang Jinghui ³

(1. Chongqing Materials Research Institute Co., Ltd., Chongqing 400707, China; 2. National Engineering Research Center for Instrument Functional Materials, Chongqing 400707, China; 3. School of Integrated Circuits, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, Guangdong, China)

Extended abstract:

[Background and purposes] The rapid advancement of high-end equipment in aerospace, aviation and nuclear power has imposed stricter demands on the comprehensive performance, composite design and precision manufacturing of ultra-high-temperature ceramics. Hafnium oxide (HfO₂) is a highly promising candidate for ultra-high-temperature protective applications, owing to its ultra-high melting point (~2774 ℃), outstanding ablation resistance and excellent chemical stability. However, pure HfO₂ suffers from poor phase stability, low sintering densification and uncontrollable grain boundary structures, which severely limit its engineering utilization. Meanwhile, the exhaust temperature of the main combustion chamber in new-generation aero-engines has exceeded the thermal limit of traditional thermocouple protection tubes, creating an urgent need for high-performance materials suitable for ultra-high-temperature oxidizing environments. This work was aimed to prepare yttria-stabilized hafnia (YSH) ceramics through solid-state sintering with Y₂O₃ as the phase stabilizer and MgO/Al₂O₃ as sintering aids. The aim is to break the key bottlenecks of HfO₂ in phase stability, densification and grain boundary regulation, reveal the phase stabilization mechanism and microstructural evolution of YSH ceramics and clarify the effects of sintering aids on densification and mechanical properties, thereby providing theoretical and experimental support for the development of high-performance HfO₂-based ultra-high-temperature ceramics.

[Methods] High-purity HfO₂, Y₂O₃, MgO and Al₂O₃ powders were used as raw materials. YSH ceramics with different Y₂O₃ doping levels (x=0.03, 0.04, 0.08, 0.12; coded as 3YSH, 4YSH, 8YSH, 12YSH) were fabricated. On the basis of fully cubic-phased 8YSH, 1 wt.% MgO and 0.5 wt.% MgO+0.5 wt.% Al₂O₃ were separately added (coded as 8YSH+Mg and 8YSH+Mg+Al). The preparation route included 8 h ball milling, 80 MPa dry pressing, 1500 ℃/6 h solid-solution stabilization, grinding to 10–15 μm, 200 MPa/10 min cold isostatic pressing and 1800 ℃/2 h pressureless sintering. Phase composition was analyzed by using X-ray diffraction (XRD); microstructure, elemental distribution and second-phase formation were characterized by using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (SEM-EDS) and electron probe microanalysis (EPMA), porosity was measured by using a metallographic microscope (MM) and Vickers hardness, compressive strength and three-point bending strength (room temperature and 1500 ℃) were tested with a Vickers hardness tester and an electronic universal testing machine (UTM).

[Results] Y³⁺ substituted Hf⁴⁺ in the HfO₂ lattice, causing lattice distortion and generating oxygen vacancies for charge compensation. With increasing content of Y₂O₃, HfO₂ transformed progressively from monoclinic to cubic phase, while full cubic stabilization was achieved at ≥8 mol% Y₂O₃. In single-dopant system, porosity decreased from 9.34% (pure HfO₂) to 8.53% at 8 mol% Y₂O₃, but rebounded to 11.27% at 12 mol%, due to the segregation of excessive Y³⁺ at grain boundaries. Adding sintering agents strongly enhanced densification. 8YSH+Mg showed the lowest porosity (2.90%), while 8YSH+Mg+Al exhibited porosity of 7.87%. In 3YSH, Y³⁺ dissolved uniformly, but grains were loosely packed. In 8YSH+Mg, Mg slightly segregated at grain boundaries and formed low-melting eutectics to facilitate liquid-phase sintering and enhance densification. In 8YSH+Mg+Al, Mg and Al locally accumulated to form Mg–Al composite phases, accompanied by Y³⁺ segregation, which hindered densification. Mechanical properties were optimized at 8 mol% Y₂O₃. 8YSH+Mg possessed excellent overall properties, with Vickers hardness of 728.21 HV, compressive strength of 499.92 MPa, room-temperature bending strength of 176.74 MPa and 1500 ℃ bending strength of 133.76 MPa, corresponding to improvements of 32.4%, 6.52 times, 1.8 times and 1.98 times, as compared with those of pure HfO₂. In contrast, 8YSH+Mg+Al showed severely degraded properties, with compressive strength and bending strength dropping to 104.35 MPa and 96.21 MPa, respectively.

[Conclusions] Y₂O₃ effectively stabilized the cubic phase of HfO₂, with full cubic stabilization attained at ≥8 mol% Y2O3, which is essential for high mechanical performance. The optimal Y₂O₃ doping level is 8 mol%, while higher level doping (12 mol%) caused grain-boundary segregation, increased porosity and deteriorated properties. MgO is a highly effective sintering aid for YSH ceramics. 1 wt.% MgO in 8YSH reduced porosity to 2.90% and preserved excellent mechanical properties, making it the preferred system for high-performance YSH ceramics. The MgO–Al₂O₃ composite aid is unsuitable, because in-situ formed Mg–Al phases blocked mass transport, weakened grain boundaries, raised porosity and sharply degraded mechanical performance. The phase-stabilization mechanism of Y₂O₃ in HfO₂ was elucidated and the regulatory effects of sintering aids on the microstructure and properties of YSH ceramics were clarified, offering a technical basis for the high-performance fabrication of HfO₂-based ultra-high-temperature ceramics.

Key words: yttria-stabilized hafnia; solid-state sintering; microstructure; porosity; mechanical properties