Research On Plasma Facing Tungsten-based Composites And Their Interfaces

In the fusion reactor,the first wall materials hold a very important responsibility,not only for the absorbing and transmission of the energy from fusion reaction,but be responsible for the protection of structural materials and various types of components from all kinds of radiation hazards,at the same time,avoid the fusion reaction materials,Deuterium and Tritium,from retention in the first wall materials is also very necessary.Tungsten has become the most promising first wall material due to its high melting point,high thermal conductivity,high radiation resistance and non-deuterium tritium reaction character,but the intrinsic brittleness of tungsten-based materials greatly limits its application in such hazard thermal shock conditions.Therefore,improving the toughness of tungsten-based materials is essential to accelerate the research process of fusion reactor.In this study,spark plasma sintered tungsten compoistes were investigated,tungsten fibers,tungsten foils,tantalum particles or their combinations were chosen as reinforcements.The technics of the deposition of interface coatings and their influence on tungsten composites were also studied.In the study of tungsten self-reinforced composites,tungsten fiber and tungsten films made of pure tungsten were used to reinforce the powder-sintered tungsten matrix.It was found that the pre-fabricated 200 mesh tungsten nets and rolled tungsten foils did not hinder the densification process of the tungsten composites,and both the fiber reinforced and the laminar reinforced composites show higher strength than tungsten matrix.When the ratio of tungsten foil and tungsten powder reaches 1: 2,laminar reinforced composites show the best mechanical property.The two kinds of composite strengthening mechanism of tungsten fiber and lamellae can simultaneously take effect,contributing to the mechanical properties of tungsten composites at the same time.The fiber composite has a higher efficiency in strengthening than layer composite when having the same composite phase volume.A 1 ?m TiN coating and a 200 nm TiN/Ti multilayer coating were prepared by magnetron sputtering on the surface of the tungsten fibers.It was found that the TiN/Ti multilayer coating has the highest hardness of 7.25 GPa when the TiN layer is 7nm thick and the Ti layer is 3nm thick,while the hardness of 1?m thick TiN coating is 5.99 GPa.A TiN coating was prepared by arc ion plating,the hardness of this coating was 33 GPa,which was much higher than that of magnetron sputtering coating,but its surface was polluted by large particles.The magnetron sputtered TiN/Ti multilayer coating and the TiN coating were partially destroyed during the sintering process.The melting at high temperature is the main reason for the destruction of TiN/Ti multilayer coating during the sintering process,and the high pressure is the main reason for the destruction of the TiN coating,the mechanical performance of these two samples is of the same level,both superior to that of the uncoated composites.The TiN coating processed by the arc ion plating is well maintained during the sintering process,whose composites also have the best mechanical property.In the study of tungsten-tantalum two-phase powder composites,the effects of tungsten powder size and composite phase ratio on the properties of the composites were studied by using different tungsten powders with different tantalum contents.It was found that when the size of tungsten powder was 20?m and 80?m,composites formed a "tungsten-diffusion layer-tantalum" structure,allowing the dislocations nucleates in the tantalum or transition layer and transmitted into the pure tungsten areas,making the tungsten failure mode transform from the grain boundary cracking to cleavage fracture.Composites with tungsten size of 20?m and 80?m both show the best mechanical properties when the content of tantalum is 40vt%,the former one has the best mechanical properties overall for its smaller and finer microstructure.The performance of tungsten-tantalum two-phase powder composites prepared by nano tungsten powder is generally inferior to that of pure tungsten sample.The TiN coating prepared on the surface of 80?m tungsten powder can effectively hinder the diffusion of elements between the tungsten and tantalum during the sintering process,but the coating itself is destroyed during the sintering process.The sol-gel processes for the prepaering of TiN coating lead to serious powder agglomeration problems for tungsten powder of 20?m and 300 nm,and introduce impurities at the same time,then cause the degradation of materials performance.