Optimising Plasma Sprayed Tungsten Coatings

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Tungsten is the key plasma facing material for use in any future nuclear fusion device due to its high melting point, good sputter resistance and low activity. However its refractory nature leads to inherent difficulties in its processing and many traditional production routes are not available. Without tungsten plasma facing materials, there exist no viable concepts for nuclear fusion as a sustainable power supply. As such, it is one of the key areas to develop if fusion is to succeed. Much work has been put into the development of monoblock type structures, where bulk tungsten is directly joined to pipe work carrying coolant, but the behaviour is currently unacceptable due to low fracture toughness and cracking under repeated cycling. An alternative is to use tungsten coatings on a steel of copper substrate. Vacuum plasma spraying is one of the most attractive methods of producing tungsten coatings for this application, but thermal mis-match between the tungsten and substrates such as steel or copper lead to the development of complex residual stresses, which degrade the performance of the coating. Previous work has shown these stresses can causes premature failure of the coatings under thermal cycling. This project will use recently upgraded vacuum plasma spraying equipment to produce both pure and alloyed tungsten coatings on novel substrates. These substrates have been shown to have promise in reliving some of the residual stress through controlled cracking, but no full characterisation of these has been carried out. These will be characterised using state of the art microscopy and micro-mechanical testing facilities in the department of materials and finite element analysis used to understand the evolution of the stress state. Microscopy will focus on understanding the effects of processing variables on the microstructure and their eventual effect on thermal and mechanical properties. Micro mechanical testing will focus on understanding the local modulus and fracture toughness of both as sprayed and aged coatings. Micro-cantilevers will be manufactured, for the first time, in these materials using Focused Ion Beam machining (FIB). This is a recently developed method at Oxford which allows rapid testing of mechanical behaviour on small volumes of material. By testing with high temperature nanoindentation the mechanical properties (elastic modulus, failure stress and fracture toughness) will be measured not just at room temperature but also at operational temperatures. Finite element analysis will be used to model the behaviour of the coatings using the experimental data to benchmark the model. Additionally for the first time, thermal cycling will be carried out using the HIVE facility at CCFE, UK and JUDITH and FZK Julich. These tests will simulate the thermal cycles experienced in a real reactor. This data will then be fed back into the processing route for improved plasma facing coating design with longer cycles to failure.This project is funded by the EPSRC CDT in Science and Technology of Fusion Energy. This project falls within the EPSRC Energy research area.