Optimising Plasma Sprayed Tungsten Coatings

钨具有熔点高、抗溅射性能好、活性低等特点，是未来核聚变装置中主要的面向等离子体的材料。然而，它的耐火性导致其加工过程中固有的困难，许多传统的生产路线是不可用的。如果没有面向钨等离子体的材料，就不存在将核聚变作为可持续电力供应的可行概念。因此，如果核聚变要取得成功，这是需要发展的关键领域之一。人们已经投入了大量的工作来开发单块型结构，其中散装钨直接连接到输送冷却剂的管道工作，但目前这种行为是不可接受的，因为断裂韧性低，在重复循环下会开裂。另一种选择是在铜衬底的钢材上使用钨涂层。真空等离子喷涂是制备钨涂层的最有吸引力的方法之一，但钨与钢、铜等基材之间的热失配会导致产生复杂的残余应力，从而降低涂层的性能。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材上生产纯钨涂层和合金钨涂层。这些衬底已被证明有希望通过受控开裂来消除部分残余应力，但尚未对其进行完全表征。将使用材料系中最先进的显微镜和微机械测试设备以及用于了解应力状态演变的有限元分析来表征这些特性。显微镜将侧重于了解工艺变量对微观结构的影响以及它们对热和机械性能的最终影响。微观机械测试将侧重于了解喷涂层和老化涂层的局部弹性系数和断裂韧性。微悬臂梁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，不仅可以在室温下测量材料的力学性能(弹性模数、破坏应力和断裂韧性)，还可以在工作温度下测量材料的力学性能。有限元分析将被用来模拟涂层的行为，使用实验数据作为模型的基准。此外，还将首次使用位于英国CCFE、朱迪思和FZK Julich的蜂巢设施进行热循环。这些测试将模拟真实反应堆中经历的热循环。然后，这些数据将被反馈到工艺路线，以改进面向等离子的涂层设计，并延长失败周期。该项目由聚变能源科学和技术中的EPSRC 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.