The new concept target, granular flow spallation target, is considered as one of the most promising candidate target types for ADS. The spallation target will work under extreme conditions such as high temperature and strong radiation. Besides, numerous target balls circulating in the target loop will cause strong and complex friction and wear, which affects the service life of the target balls. So far, no suitable method has been proposed to evaluate the friction and wear behavior caused by the granular collective movement.                                        

With excellent properties and low cost, the tungsten alloy balls are considered as one of the important candidates for the granular flow spallation target. By using the self-developed device, the researchers at IMP conducted the collective wear experiment of the tungsten alloy balls at 250 ℃. 

It is found that the mass loss ratio of the target balls increases firstly and then decreases, with a maximum of about 0.32wt%, and finally reaches a steady-state of about 0.28 wt.%.  

The whole process can be divided into several stages, which correspond to different friction and wear behaviors. In the initial stage, fatigue wear is dominant, and then the micro-crack network on the surface of the ball begins to sprout and develops into a crushed layer. After the crushed layer falls off, the sub-surface begins to wear. Finally, a self-assembled new layer with a structure of amorphous surrounding nanocrystallines is formed on the ball surface. As it has greater tolerance for friction defects and can inhibit the propagation of micro-cracks, this new layer is useful for improving the wear resistance of the ball surface. 

Moreover, the results also reveal the important role of wear debris. In the early stage, it contributes to the formation of top crushed layer. In the middle stage, it acts as the solid lubricant and buffer layer between the spheres, slowing down the friction and wear. And in the later stage, it participates in the formation of self-assembled layer to protect the ball. 

The results not only provide a crucial experimental method for evaluation of friction and wear behavior of the granular spallation target, but also provide evaluation data of service life of tungsten alloy balls as spallation target material from the perspective of tribology. 

Fig. 1. The wear evolution process of tungsten alloy balls during collective motion. (Image by PANG Lilong) 

Fig. 2. Schematic illustration process of wear and the formation of a new layer on the surface of the tungsten alloy ball. (Image by PANG Lilong) 

The study was published in the journal of Tribology International. 

This work was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Guangdong Province, China and Advanced Energy Science And Technology Guangdong Laboratory.