Optimising Plasma Sprayed Tungsten Coatings

钨因其高熔点、良好的抗溅射性和低活度而成为未来核聚变装置中关键的等离子体表面材料。但其难熔性给其加工带来了固有的困难，许多传统的生产路线无法实现。没有钨等离子体表面材料，就没有可行的核聚变作为可持续能源供应的概念。因此，如果核聚变要取得成功，它是发展的关键领域之一。许多工作已经投入到单块式结构的开发中，其中大块钨直接连接到携带冷却剂的管道工作，但由于低断裂韧性和重复循环下的开裂，目前的行为是不可接受的。另一种选择是在铜或钢基体上使用钨涂层。真空等离子喷涂是最具吸引力的钨涂层生产方法之一，但钨与基体（如钢或铜）之间的热不匹配导致复杂的残余应力的发展，从而降低了涂层的性能。以前的工作表明，这些应力会导致涂层在热循环下过早失效。该项目将使用最近升级的真空等离子喷涂设备在新型基底上生产纯钨和合金钨涂层。这些衬底已被证明有希望通过控制开裂来缓解一些残余应力，但没有对这些进行充分的表征。这些将使用最先进的显微镜和材料部门的微机械测试设备进行表征，并使用有限元分析来了解应力状态的演变。显微学将着重于了解加工变量对微观结构的影响，以及它们对热性能和机械性能的最终影响。微观力学测试将侧重于了解喷涂和老化涂层的局部模量和断裂韧性。微悬臂将首次在这些材料中使用聚焦离子束加工（FIB）制造。这是牛津大学最近开发的一种方法，可以对小体积材料的机械性能进行快速测试。通过高温纳米压痕测试，不仅可以在室温下，还可以在工作温度下测量其机械性能（弹性模量、失效应力和断裂韧性）。有限元分析将用于模拟涂层的行为，使用实验数据对模型进行基准测试。此外，热循环将首次使用英国CCFE、JUDITH和FZK Julich的HIVE设施进行。这些试验将模拟在真实反应堆中经历的热循环。然后，这些数据将反馈到加工路线中，以改进等离子体表面涂层设计，缩短失效周期。该项目由EPSRC聚变能科学与技术CDT资助。该项目属于EPSRC能源研究领域。

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.