Papers by Author: Xue Bin Zheng

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: Abstract: Calcium silicate (CS) ceramics have good biocompatibility, high bonding strength with titanium alloy substrates, and are potential candidates for implant coating materials. The osteo-integration of coated implants with host bone tissue is greatly affected by the degradation of these coating materials in biological fluid. In this paper, zirconia adopted calcium silicate (CaO-ZrO2-SiO2, CZS) ceramic powder was synthesized. Plasma spraying was used to prepare the coating with Ti-6Al-4V as substrates. Human bone marrow-derived stromal cells (hBMSCs) culture system was used to evaluate the cytocompatibility of the CS and CZS coatings. Results showed that more cells were adhered to the CZS coating with high proliferation ratio than those on CS. The dissolution of CS and hereby elevated pH value were contributed to the decreased cell adhesion and proliferation.

Abstract: Plasma sprayed hydroxyapatite (HA) coatings on titanium alloy substrates have been used extensively due to their excellent biocompatibility and osteoconductivity. However, the low bonding strength between HA and Ti substrates is still problematic in the long-term implantation lifespan. In this paper, HA/Ta composite coatings with various Ta contents (20% and 60%) were fabricated by vacuum plasma spraying (VPS). The microstructure, phase composition of the coatings was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results revealed that the coatings had a rough surface and lamellar structure including some pores. ASTM C-633 standard was used for the measurement of bonding strength, which was found increasing with the increase of Ta contents in the composite coatings. The bonding strength of the composite coating containing 60% Ta (H4T6) reached to 37.2 MPa, which is about 1.9 times that of HA coating. In vitro bioactivity evaluated in simulated body fluids (SBF) showed that bonelike apatite layer was formed on the composite coatings surface, which indicated the good bioactivity of the HA/Ta composite coatings.

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: An advanced ceramic material of perovskite structure, Sc₂O₃ (5 mol%) and Gd₂O₃ (5 mol%) co-doped SrZrO₃, was investigated as a material for thermal barrier coating (TBC) applications. Sr(Zr_0.9Sc_0.05Gd_0.05)O_2.95 (SZSG) was synthesized using ball milling prior to solid-state sintering, and had a minor second phase of Gd₂O₃. The material showed good phase stability not only from room temperature to 1400°C, but also at high temperature of 1450°C for a long period, analyzed by DSC and XRD, respectively. The thermal expansion coefficients (TECs) of the sintered bulk SZSG were recorded by a high-temperature dilatometer and revealed a positive influence on phase transitions of SrZrO₃ by co-doping Sc₂O₃ and Gd₂O₃. The thermal conductivity of SZSG had much lower values in contrast to that of SrZrO₃ and Sr(Zr_0.8Gd_0.2)O_2.9 in whole tested temperature range. The good chemical compatibility was observed for SZSG with 8YSZ or Al₂O₃ powders. All the results revealed that SZSG might be a potential material for TBC applications at higher temperatures compared with 8YSZ.

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: Biomaterials with good biocompatibility and anti-bacterial property were becoming attractive to researchers, so we used the chemical method to produce anti-bacterial vacuum plasma sprayed titanium coatings and studied In vitro bioactivity, cytotoxicity and blood compatibility of the anti-bacterial coatings in this paper. In order to evaluate the bioactivity of the treated titanium coatings, the coatings were immersed in simulated body fluid (SBF). The treated titanium coatings showed good bioactivity in this experiment. Two different methods were used to assess the cytocompatibility of the treated titanium coatings. One was extract test; the other was direct contact test. The results indicated that cells spread and adhered well on the coatings. The blood compatibility of the coatings was evaluated by haemolysis ratios. The hemolysis ratios of the coatings were below 2%, indicating of nonhemolysis for the coatings.