Microstructure Evolution Of Tungsten Single-crystal Coating Serving Under Long Time At High Temperature

The{110}faces of single tungsten which is achieved by the technique of chemical vapor transport deposition on the substrate of single crystal Molybdenum alloy is one of the most promising materials which have been currently utilized for the emitter of thermionic energy converter.Emitter material required to operate at high temperature more than1500°C for long time,The coating quality of tungsten single crystal has significant effect both on the power and the worktime of the thermionic energy converter.According to currently available literatures,although various fields of tungsten single crystal abtained by CVTD have substantial development,microstructure evolution during serving at high temperature for long time has been rarely reported.Tungsten single crystal coatings are prepared by CVTD under condition of total pressure of 25.23Pa,substrate temperature of 1200?and 1300?.Tungsten single crystal coatings have obvious hexahedral morphology consist of alternating rough{110}and flat{112}.The dislocation density of tungsten single crystal coatings are of 10⁶ magnitude.After the electrochemical etching,{110}faces have paralleled stepped structure,{112}faces has regular cubic and stacked layers structure.Tungsten single crystal coatings is heated by electric and emit electrons at 1600?for 5000h.In this paper,optical microscopy?OM?,scanning electron microscopy?SEM?,conventional high-resolution X-ray diffraction?HR-XRD?technique,electron microprobe?EPMA?,etch-pit method and electron backscatter diffraction?EBSD?technique and other experimental methods are used to study the microstructure of tungsten single crystal coatings and interdiffusion layer.The main results are as follows:?1?The hardness of Mo substrate,interdiffusion layer and tungsten coating have declined after serving,the hardness of interdiffusion layer is less than that of tungsten single crystal coating.Dislocation density of tungsten single crystal coatings are of 10⁵magnitude which is based on etch-pit method,and decreased by one magnitude compared to before serving.?2?There are a large number of small grain and sub-grains on the{112}faces of tungsten single crystal coatings,while{110}faces have neither small grains nor sub-grains.Small grains have{100}preferred orientation.Because the dislocation density of{112}faces is higher than that of{110}faces,and the different nucleation work.3.The etched morphology of{112}faces almost disappeared after serving,{110}faces etched morphology retain a little,because the surface free enegy of the{110}faces is the lowest,{110}is the most stable crystal surface,so element evaporate slower.4.The thickness of interdiffusion layer between tungsten coatings and molybdenum substrate is about 12?m,Interdiffusion coefficient is of 10^-16 cm²/s magnitude calculated by Den Broeder method,With the increase of W content,interdiffusion coefficient increase firstly then decrease.5.W diffuse into Mo faster than Mo diffuse into W.So there are Kirkendall voids in Molybdenum substrate.