Papers by Author: Ya Ran Niu

Abstract: ZrB₂-SiC-WSi₂ (labeled as ZSW) coating and ZrB₂-SiC (labeled as ZS) coating were fabricated by vacuum plasma spray technique. The microstructure and composition of as-sprayed coating were detected through X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray energy dispersive spectrometry (EDS). Oxidation behaviors and microstructure evolution of these coatings were evaluated at 1500 °C in air and compared with ZrB₂-SiC coating. The results showed that the addition of WSi₂ improved the oxidation resistance of the ZrB₂-SiC coating. There was more liquid formed on the ZSW coating surface. Some bubbles were also observed on the coating surface, which might be resulted from gas formation. It can be concluded that the amount of WSi₂ had great influence on its function in ZrB₂-SiC system.

Abstract: ZrC-20vol.%MoSi₂ (ZM) composite coating was fabricated by vacuum plasma spray and the ablation resistance was assessed using plasma flame under low (1.94 MW/m²) and high (3.01 MW/m²) heat fluxes, respectively. Results showed that the ultimate surface temperatures of ZM coating were about 2100 °C and 2400 °C, respectively. ZM coating exhibited good ablation resistance at low heat flux, which benefited from the low evaporation of SiO₂ and the diffusion of Si derived from MoSi₂ decomposition. However, bubble-burst event took place under high heat flux. The different ablation behaviors of ZrC-MoSi₂ coating were analyzed, which might contribute to the application of ultra-high temperature ceramic coatings.

Abstract: Rare-earth silicates, especially ytterbium silicate (Yb₂SiO₅ and Yb₂Si₂O₇), have been developed for promising environmental barrier coatings (EBCs) for SiC-matrix composites. In this study, double-layer Yb₂SiO₅/Si and Yb₂Si₂O₇/Si EBC systems were deposited on C/SiC composites by air plasma spray (APS) technique, respectively. Both systems were subjected to thermal shock tests at 1400 °C. The evolution of phase composition and microstructure of those samples before and after thermal shock test were characterized. Results showed that there were penetrating microcracks in the top Yb₂SiO₅ layer and horizontal microcracks at the Yb₂SiO₅-Si interface after thermal shock test. While extremely few microcracks and no horizontal microcracks were presented in the Yb₂Si₂O₇/Si sample. The EDS analysis also showed that the Si bond layer of the Yb₂SiO₅/Si sample was oxidized more serious than that of the Yb₂Si₂O₇/Si sample. The different thermal shock behaviors of both systems were clarified based on the thermal expansion behavior, phase composition and microstructure analysis.

Abstract: In this work, TiB₂-MoSi₂ composite coatings with various contents of MoSi₂ (20 vol. % and 40 vol. %, respectively) were fabricated on SiC coated C/C substrates by low pressure plasma spray (LPPS) technique. The microstructure and phase composition of the coatings were characterized. The ablation behaviors of the composite coatings were evaluated and compared with the pure TiB₂ coating using a plasma flame of about 2200°C. The results showed that MoSi₂ was uniformly distributed in the TiB₂ matrix. All the coatings kept intact after the ablation for 60s - 180s, indicating their excellent ablation resistance. The addition of MoSi₂ had great influence on the ablation behavior of the composite coatings. The TiB₂ coating gained mass after the ablation. The mass of the TM20 coating increased firstly (60s and 120s) and then decreased at 180 s. Mass loss was observed for the TM40 coating during the whole procedure of ablation test.

Abstract: TaC-based composite coatings containing different contents of SiC (5 vol.%, 10 vol.% and 15 vol.%, respectively) were fabricated by low pressure plasma spray (LPPS) technique. The phase composition and microstructure of the coatings were characterized. The ablation property of the TaC-SiC composite coatings was evaluated by a plasma flame over 2000 ^oC. The results showed that SiC was uniformly distributed in the composite coatings. The addition of SiC was good for the ablation-resistance of the composite coatings. The mass loss of the composite coatings decreased with increasing the amount of SiC. The mass loss of the TaC-15vol.%SiC composite coating was about 0.15 % after ablation for 60 s, which exhibited the best ablation-resistance among the three kinds of coatings. It was thought that the formed Ta₂O₅-SiO₂ mixed layer on the coating surface contributed to the improved ablation resistance.

Abstract: In this work, atmospheric plasma spray (APS) technology was applied to fabricate ZrC-W composite coatings. The microstructure of the composite coatings was characterized. The influence of W content on the ablation-resistant and thermal shock properties of ZrC-W composite coatings was evaluated using a plasma flame. The results show that the ZrC-W composite coatings had typically lamellar microstructure, which was mainly made up of cubic ZrC, cubic W and a small amount of tetragonal ZrO₂. The ZrC-W coatings had improved ablation resistant and thermal shock properties compared with those of the pure ZrC coating. It was supposed that the improved density, thermal conductivity and toughness of the composite coatings contributed to this phenomenon.

Abstract: Silicon coatings were prepared by vacuum plasma spraying (VPS) and air plasma spraying (APS) technologies. The samples were hydrothermally treated and then incubated in simulated body fluid (SBF) to evaluate their bioactivity and silicon wafer was used as control sample at the same time. The SBF test showed that a Ca-P layer was formed on the surface of silicon wafer and VPS-Si coating after immersion in SBF for certain time, indicating their improved bioactivity. Whereas no Ca-P layer was found on the surface of APS-Si coating. The results of X-ray photoelectron spectroscopy showed that the Si/O atomic ratio and chemical depth profiles of the silicon oxide films on the surface of silicon wafer, VPS-Si and APS-Si coatings were different. The results indicated that the bioactivity difference of silicon-based material resulted from the different composition of their surface. Hydrothermal treatment maybe a favorable method to improve the bioactivity of silicon-based material having silicon oxide of non-stoichiometric Si/O atomic ratio.

Abstract: Tungsten is one of the most promising candidates for the plasma facing materials (PFM) in fusion experiment devices. In this study, tungsten coatings were fabricated on copper alloy substrates via air plasma spraying technology. The in-flight behaviors of tungsten particles in the spraying process were investigated. The microstructure and some properties of the as-sprayed coatings, including surface roughness, bonding strength, microhardness and thermal property, were evaluated. The results showed that the temperature and velocity of the in-flight tungsten particles increased by enhancing the spraying power. Un-melted particles and microcracks were found for the coatings prepared under the low (32.7kW) and high (47.6kW) spraying powers, respectively. The highest bonding strength and microhardness were obtained for the coating prepared under a medium spraying power (42.5kW) by reason of its good in-flight behaviors. The thermal conductivity of tungsten coatings was 32.2 W٠K-1٠m-1 at room temperature, which was much lower than that of bulk tungsten.

Abstract: Silicon-based materials have been widely applied in industrial fields, such as microelectronic and solar power, for the specific electronic properties. In the present work, free-standing thick silicon coatings were fabricated by air plasma spraying (APS) and vacuum plasma spraying (VPS) technologies. The microstructure and electrical property of the coatings were characterized. It was found that the electrical resistance of APS-Si coating was higher than that of VPS-Si coating. The impedance spectroscopy results showed there was significant difference in impedance curves of APS-Si and VPS-Si coatings. The grain boundary semicircle appeared in the plot of VPS-Si coating, while did not appeared in that of APS-Si coating. It was thought that the electrical property difference of the two kind coatings was related with the oxidation and microstructure difference of the coatings.

Abstract: In this work, silicon coatings were deposited using VPS and APS technologies. Their structure and composition were characterized using XPS, SEM, and XRD. The porosity and surface-roughness of coatings were evaluated. The results showed that the VPS silicon coating possessed lower porosity compared with the APS silicon coating. The APS silicon coating surface was almost all covered by many small particles whose size is about 100 nm. The Si 2p XPS spectra indicated that silicon oxide existed in both coatings. The oxidation may have occurred during the plasma spray process. However, the VPS process could inhibit silicon from being oxidated.