微弧氧化 HfC-HfO₂ 粒子沉积烧结复合涂层的构建及其高温抗氧化性能

Niobium alloy is one of the most promising candidate materials as the critical components of hypersonic vehicles due to its excellent high-temperature performance. However, poor high-temperature oxidation resistance is the major problem that needs to be solved for industrial applications at present because of the catastrophic oxidation of niobium alloy at high temperatures. Silicide coating is considered an effective method to broaden the engineering application of niobium alloys due to the formed dense oxygen shielding layer at high temperatures. However, the thermal growth stress will continue to accumulate when the silicide coating is exposed to air at high temperatures, greatly restricting the long-term service. It is proven that a novel coating with altered chemical compositions and functionalities can be prepared by the micro arc oxidation technique with the functional particle incorporated into the electrolyte. The work aims to propose a micro arc oxidation particle deposition sintering technology by introducing HfC-HfO₂ nanoparticles, forming a multilayer ceramic protective coating on the siliconized niobium alloy. Nb521 alloy sheet with a nominal composition of Nb-5W-2Mo-1Zr was cut into 10 mm×10 mm×1 mm by wire cutting and polished with SiC sandpaper. Then, the NbSi₂ bottom layer was obtained by HAPC treatment on Nb521 alloys. The polished samples were buried in a mixture powder of Si, NaF, and Al₂O₃, with a weighted ratio of 16∶5∶9. HAPC treatment was in a vacuum atmosphere furnace with the argon shield, heated from room temperature to 1 300 ℃ at a heating rate of 5 ℃/min, and held for 8 h. HfC-HfO₂ outer layer was deposited on the NbSi₂ layer by micro arc oxidation particle deposition sintering technology, which was carried out on a double electrodes system with the AC power facility. The NbSi₂ coated sample acted as the anode, while the cathode was the stainless steel sheet. The used electrolyte consisted of Na₂SiO₃ (15 g/L), (NaPO₃)₆ (6 g/L), NaOH (1.2 g/L), HfC particles (20 g/L) and HfO₂ particles (10 g/L) with a size of 500 nm, completely dispersing in the distilled water. The electrical parameters were set to a frequency of 600 Hz, a duty cycle of 10%, a voltage of 600 V, and an optimized applied time of 11 min. All the samples were subject to isothermal oxidation tests by a muffle furnace (KSL-1700X) at 1 200 ℃ in air to evaluate the high-temperature oxidation resistance. A scanning electron microscope (SEM, Helios Nanolab600i, FEI, U.S.A.) equipped with an energy dispersive spectrometer (EDS), and a transmission electron microscope (TEM, Talos F200x, FEI, U.S.A.) were used to characterize the microstructures of the coatings. An X-ray diffractometer (XRD, Empyrean, PANalytical, Netherlands) was used to analyze the phase composition. The thickness of the HfC-HfO₂ particle deposition layer on the siliconized niobium alloy surface was approximately 45 μm. The composite coating demonstrated optimal oxidation resistance throughout the isothermal oxidation process, with a mass gain of only 6.41 mg/cm² and a parabolic rate constant of 0.367 mg²/(cm⁴·h). In contrast, a single NbSi₂ coating exhibited accelerated oxidation, with a mass gain of 13.6 mg/cm². The incorporation of HfC as an oxygen-consuming phase in the composite coating ultimately formed the HfSiO₄ skeletal structure at high temperatures, enhancing the stability of the oxide scale. Additionally, Hf-rich oxides anchored the SiO₂ oxide scale, forming a dense oxygen diffusion barrier layer, which significantly suppressed the growth of niobium oxides and crack initiation.