Papers by Keyword: Tungsten Coating

Abstract: Chemical vapor transport deposition (CVTD) is an effective method for preparing large tungsten coatings for space thermionic reactors. In this study, a high-density, high-work-function polycrystalline tungsten coating was prepared using a WCl₆ transport agent in a concentric tube-type closed transport system. The relationship between the kinetics and the microstructures of the CVTD polycrystalline tungsten coating at the substrate temperature of 1593 K-1793 K and system pressure of 15.93 Pa-106.8 Pa was studied, which provided a basis for the preparation of high-quality tungsten coatings. At a low temperature or a low pressure, the activation energy was approximately 2 kJ/mol, the deposition rate was almost independent of the temperature changes, and the control mechanism was mass transport limited. The tungsten coating had nodules on the surface with pores in the grain boundaries and grew preferentially along <111>. At a high temperature and a high pressure, the apparent activation energy was approximately 90 kJ/mol, the value of order was approximately 1, and the control mechanism in this process range was surface limited. The tungsten coating exhibited a hexagonal pyramidal structure, and the growth direction was preferred along <110>. The average work function of the tungsten coating prepared at a temperature of 1673 K and a system pressure of 106.80 Pa was as high as 5.20 eV.

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: The W coatings were prepared by applying the vacuum plasma spraying process (VPS). The phase composition, microstructure and laser irradiation property were characterized by XRD, SEM and laser. Results showed that no tungsten oxide appears in the coatings whose phase composition is similar to the original powder. A better melting state of the coating can be obtained by using the finer powder. The deposition efficiency of the coating is higher in the cases of a higher H2 flow. There is a pitch whose size is about 100μm when the substrate was irradiated by laser. However, there is clearly no change for the W coating surface which was also irradiated under the same condition. It indicates that the laser irradiation resistance is improved significantly for stainless steel after being deposited with W coating.