Mechanical properties and quality of plasma sprayed, functionally graded tungsten/steel coatings after process upscaling

Abstract

The First Wall of a fusion reactor needs to withstand high heat flux as well as particle bombardment. For this, a First Wall made of steel requires a protective coating with a material that may still transfer heat for conversion to energy, such as tungsten. Its thermal expansion mismatch towards steel is overcome by vacuum plasma spraying of a functionally graded material onto the steel wall, followed by a tungsten top coat. This process was recently transferred to industry for upscaling, to develop a coating technology that can cover the large dimensions of First Wall components without deteriorating the substrate steel's properties by overheating. This work represents an instrumented indentation study of the achieved coating quality and properties, combined with microstructural analysis. Hardness profiles within coating and substrate indicate successful establishment of a linearly functionally graded material and only minor substrate overheating. The latter observation is supported by electron backscatter diffraction showing no change in the substrate's microstructure. The substrate hardness was investigated on several positions of coated plates sizing up to 500 × 250 mm2. The results indicate faster cooldown in the plate corners. Cooling channel bores that were pre-fabricated in the plates had no effect on plate hardness after coating. The elastic modulus of the coating's interlayers, determined by instrumented indentation, was found lower than predicted from bulk properties. This is attributed to the heterogeneous microstructure of the thermally sprayed coating.