Structure and mechanical properties of aluminosilicate glasses

The effect of phosphate on the mechanical properties of aluminosilicate glasses so far has barely been studied. Yet, phosphate incorporation bears potential for changing the polymerisation of aluminosilicate glasses and thus their properties. This thesis presents the first detailed mechanical analysis of phospho-aluminosilicate glasses that includes elastic properties. The studied compositions comprised metaluminous glasses from the system SiO2 - Al2O3 - Na2O - P2O5 with 0 to 7.5 mol% P2O5 and 50 to 70 mol% SiO2. The glass hardness and elastic properties were assessed by several techniques of indentation and sound speed measurement and were found to decrease with increasing P2O5 content. Changes of these properties with increasing SiO2 content were less expressed and could be explained by either density changes or polymerisation. Additionally, the densification upon indentation was studied, as well as crack resistance and strain rate sensitivity. Furthermore, phosphate was found to decrease the glass transition temperature and to impede crystallisation. To tailor glass properties, the structure-property relationships need to be understood. This thesis includes a structural analysis by combined infrared and Raman spectroscopy. A far-infrared analysis of the sodium signal indicated a competition between aluminate and phosphate groups for charge-balancing sodium. Also, a correlation was found between shifts in infrared and Raman spectra and the degree of ionic bonding, represented by the theoretical optical basicity. In summary, the mechanical properties and the structure-property relationships of metaluminous phospho-aluminosilicate glasses were characterised and the analysis of the degree of ionic bonding and of the role of sodium provided new structural insights

Functionally graded tungsten/EUROFER coating for DEMO first wall: From laboratory to industrial production

The First Wall is a crucial component for the realisation of DEMO. It has to protect the tritium breeding blanket from erosion by high-energy particles while letting neutrons pass to enable breeding of tritium fuel. Furthermore, the First Wall needs to pass incoming heat in the MW/m2 range to a cooling system for conversion to electric power. These requirements sum up to one of the harshest environments imaginable for a man-made material. Structural steel components alone cannot withstand these conditions. Tungsten is a viable armour material for the First Wall because of its low sputtering yield, high melting point, low activation and good thermal con-ductivity. It is not suitable though as bulk structural material because of its brittleness. Instead, the DEMO design foresees a First Wall of reduced-activation EUROFER steel, covered with a protective layer of tungsten. Direct tungsten-steel joints suffer from failure during processing or operation because of the thermal expansion mismatch between the two materials. This is solved by application of a functionally graded material as inter-mediate layer between steel and tungsten. Such coatings made of both tungsten and EUROFER, with a compo-sitional gradient, have been produced with vacuum plasma spray technology. This technology enables manufacturing of the required millimetre-thick coatings and is suitable for upscaling. The development was supported by thermo-mechanical finite element simulations of load scenarios during processing and in-vessel service. Driven by promising results of high heat flux tests on larger, coated mock-ups the technology was transferred to industry for upscaling. Plates with a record size of 500 x 250 mm2 and cooling channels were successfully coated. This contribution presents an overview of the development process, covers the latest results of ongoing research on the coating of curved First Wall structures and addresses future requirements

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

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\u27s 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\u27s 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\u27s 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