Abstract
Tungsten is the prime candidate material for plasma facing components of future fusion devices. Plasma spraying, with its ability to coat large areas, including non-planar surfaces, with a significant thickness, is a prospective fabrication technology for components subject to moderate heat loads, e.g., the first wall of the Demonstration Reactor (DEMO). The functionality of such coatings is critically dependent on their adhesion to the underlying material. This in turn, is influenced by a variety of processing-related factors, chief among them being the state of the interface. In this study, the effects of two factors—surface roughness and the presence of thin interlayers—were investigated. Two different levels of roughness of steel substrates were induced by grit blasting, and two thin interlayers—titanium (Ti) and tungsten (W)—were applied by physical vapor deposition prior to plasma spraying of W by a Water Stabilized Plasma (WSP) torch. Coating adhesion was determined by a shear adhesion test. The structures of the coatings and the interfaces, as well as the characteristics of the fractured surfaces, were observed by SEM.
Highlights
Nuclear fusion is a prospective energy source for the future, which brings the promise of a safe, relatively clean and large-scale energy source
This paper focuses on the adhesion of plasma sprayed tungsten on steel substrates
The detailed characterization has shed some light on the phenomena taking place at the interface and allowed us to discuss the effect of interlayers
Summary
Nuclear fusion is a prospective energy source for the future, which brings the promise of a safe, relatively clean and large-scale energy source. A transition temperature was found, above which contiguous, disk-shaped splats typically form, and below which they tend to splash and fragment [17] This has been attributed to adsorbates and condensates on the substrate surface [18], which evaporate under the hot splat and form a gaseous barrier between the two surfaces or are trapped under the splat in the form of bubbles [19,20], in both cases increasing the thermal resistance. In [25], tungsten as plasma facing material was deposited by vacuum plasma spraying (VPS) on graphite with SiC and Ti interlayer; only coatings with Ti exhibited sufficient adhesion strength in thermal exposure tests. The shear adhesion test is complemented by detailed characterization of single splats, interfaces and the fracture surfaces
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