Design, Preparation And Evaluation Of Tungsten Coatings As Plasma Facting Materials

Tungsten has the highest melting point of all metals, the lowest vapor pressure, good thermal conductivity and high temperature strength and dimensional stability; also it does not form hybrids or co-deposits with tritium. So tungsten is choosed as main plasma facing material (PFM) for fusion reactors. CuCrZr alloy, with high heat conductivity, good machinability and weldability, is a candidate heat sink materials for fusion reactors. Plasma facing components (PFCs) comprised of W and CuCrZr alloy must withstand high temperature plasma erosion and conduct the heat flux to the coolants. Plasma spray coating technology, which attract many researchers, can fabricate the plasma facing materials and join the PFM and heat sink materials simultaneously. Because there are big differences of physical properties between the tungsten and CuCrZr alloy, high thermal stress would generate in the interface by direct join methods and/or during service conditions, which may finally lead to the delamination of coatings and then failure of the PFCs.Finite element methods (FEM) was employed to study distribution of thermal stress and W surface temperature of W/Cu functionally graded coating PFCs with different heat flux density and/or thickness of graded layers under related ITER operation conditions. The results showed that functionally graded coatings can effectively relieve thermal stress in the join interface. Surface temperature of tungsten increased linearly with increase of thickness of graded layers, and also higher heat flux density, the maximum Von Mises stress was higher. Melting of coating surface occured when transient heat flux density of off-normal event was higher than 800MW/m2.Total thickness of～4mm coatings were successfully fabricated by atmospheric plasma spray(APS) on CuCrZr substrate with Mo/Cu-Mo/W as interlayers. Coatings did not delaminate or peel off during and/or after plasma spray process. It demonstrated feasibility of fabricating thick W coatings on CuCrZr substrate with Mo interlayers by APS. No evidence of big pores and un-melting particles was founded in interface between substrate and coating or among the coating splats. Porosity of all coatings were less than 2%. Properties of coatings strongly depended on the morphology of W powder, and coatings deposited with good sphericity carbonyl W powder were superior to that of irregular crystal W powder coatings. Fracture surfaces showed typical brittle fracture feature. Oxygen content of coatings determined by energy disperse spectroscopy (EDS) were higher than 6%. There is no oxygen in columnar crystal splat of coatings, while oxygen mainly existed in porous structures. Too high oxygen content and many porous structures resulted in low thermal conductivity (less thanl2.52 W·m-1·K-1).High heat flux test of APS-W coatings were carried out under high intensity pulsed ion beam simulated ITER operation conditions. After one shot, some splats of coatings melted and number of microcrack increased. Bubbles and spalling of bubbles were observed. All coating surface remelted after ten shots. Holes were determined, which were similar to that generated after He plasma exposure. And number of microcrack increased further more.