**4. W fiber-reinforced Wf/W composites**

Since a decade, a promising method of toughening tungsten composites using tungsten fiber reinforcement has been actively pursued, namely, tungsten fiberreinforced tungsten (Wf/W) composites [64–72]. However, in the bared Wf/W, the interface is not obvious due to the chemical reaction between Wf and W matrices, so the reinforcing effect is not ideal. Therefore, in this material concept, tungsten fibers are embedded in a tungsten matrix where a proper interface has to be introduced to prevent a chemical reaction between the two components. The increase in toughness is achieved through a couple of non-plastic energy dissipation mechanisms such as matrix cracking and interfacial debonding followed by frictional sliding/pullout of the fibers, while the primary crack is bridged by mostly elastically deforming fibers [65, 73]. To maximize the energy dissipation, the interfaces need to be engineered by coating which can withstand thermal exposure during service. Du et al. [65] investigate the thermal stability of the various kinds of coated interfaces for Wf/W composites: Cu/W multilayer, Er/W multilayer, ZrOx/W bilayer, ZrOx/Zr multilayer,

### *The Tungsten-Based Plasma-Facing Materials DOI: http://dx.doi.org/10.5772/intechopen.88029*

and C/W dual layer. These coatings were selected for interface engineering with which one utilizes energy dissipation by controlled interfacial debonding and sliding achieving apparent toughness. Regarding the effect on the interfacial properties, the fracture energy of the Er/W multilayer and the ZrOx/Zr multilayer was affected by less than 10%, while it increased by 40% for the ZrOx/W bilayer. Therefore, the ZrOx/W bilayer interface is the best choice for Wf/W composite. The single direction of W fibers can enhance the fracture energy of tungsten matrix once the extrinsic toughening mechanisms of fiber pullout and plastic deformation were triggered, but they will lead to the obvious anisotropy of mechanical properties [73]. Lama et al. [74] use woven tungsten wire meshes as reinforcement in the composites (wire diameter, 127 μm; distance between wires, 1 mm), which avoided the anisotropy induced by single direction of W fibers. In addition, a stable interfacial coating zirconia film (ZrOx) was deposited on the meshes by magnetron sputtering. The zirconia coating could survive the heat loads without any notable damage or overall cracking. The coating thickness remained mostly unchanged. Therefore, under the extreme environment, the zirconia coating interfaces are stable, which is in favor of reinforcing the Wf/W composites [75, 76].
