Surface free energy has been hypothesized as the cause behind the experimental evidence [Acta Mater. 62 (2014) 173–181] of a strong correlation between crystallographic orientation and surface morphology changes in helium ion-irradiated tungsten at the early stages of 'fuzz' growth in fusion plasma-facing tungsten. Here, this hypothesis is tested through self-consistent dynamical simulation based on an atomistically informed model of surface evolution in plasma-irradiated tungsten. Low-energy helium plasma irradiation of a tungsten surface, even when the ion energy is below the sputtering threshold, generates surface vacancy-adatom pairs in addition to adatoms from the flux of self-interstitial atoms to the surface resulting from the growth of high-pressure helium bubbles. Along with the stress-driven surface morphological instability as the primary driving force, preferential diffusion of these surface adatoms modulated by the associated Ehrlich–Schwoebel (ES) barriers, combined with the surface free energy anisotropy drive the anisotropic surface nanostructure growth. Simulation results for helium irradiation on W(100) show growth of pyramidal surface features, akin to experimental observations. This work confirms the hypothesis on the effect of tungsten surface crystallographic orientation on helium-ion-induced surface morphology changes, where surface mound formation is the outcome of a stress-driven surface morphological instability, while the anisotropic growth of the mounds is controlled by the ES-barrier-modulated anisotropic surface adatom diffusion and faceting of the mounds is controlled by the surface free energy anisotropy. 

Effects of Surface Anisotropy on the Surface Morphological Response of Plasma-Facing Tungsten

The austenitic 316 L stainless steel (316LSS) is an important metal for nuclear installations due to its high strength, good plastic toughness and excellent corrosion resistance, and is commonly used as the plasma-facing and structural material. In this study, 316LSS samples are irradiated with low-energy hydrogen ions (35 eV) at the temperature of 400–1300 K to investigate the surface microstructure evolution. The experimental results show that the surface of 316LSS samples remains relatively smooth at 400 K and 600 K after irradiation. Significant surface roughening including spherical protrusions is formed on the surface at 800 K and 1100 K, and step-like streaks are observed at 1300 K. However, no drastic change in the surface morphology is observed in the separate annealing experiments at 800–1300 K. It has been shown that hydrogen ion irradiation mainly causes the surface microstructure evolution of 316LSS samples. After hydrogen plasma exposure, a large number of adatoms are first formed on the surface of 316LSS because of bubble growth. The adatoms then preferentially reach the defects by diffusion at certain temperatures, and aggregate into spherical protrusions with low surface energy. The qualitative simulation results show that the irradiation temperature affects the diffusion and aggregation of adatoms on the surface, and the radius of the spherical protrusions gradually increases due to the decrease of the surface binding energy and the increase of the irradiation fluence. 

Temperature dependence of surface microstructure of 316 L stainless steel exposed to low-energy hydrogen ion irradiation

Surface morphological response of plasma-facing tungsten: Effects of surface crystallographic orientation and prediction of fuzz onset

Helium has been shown to effectively retard recrystallization and grain growth in tungsten for fusion application. However, it is unclear how hydrogen isotopes affect recrystallization, as their desorption temperature is far lower than the recrystallization temperature in tungsten, preventing them from remaining in the material and taking effect. Using rolled copper, this study investigated the effects of deuterium and helium plasma on recrystallization by leveraging the high deuterium desorption temperature in copper. Rolled copper samples exposed to deuterium or helium plasma were annealed at temperatures from 300 to 800℃. It was found that deuterium plasma exposure led to a slight retardation of the recrystallization but no impact on the grain growth. Helium plasma exposure promoted recrystallization but significantly inhibited grain growth, which is different from the effect in tungsten. This work offers preliminary insights into understanding the degradation of mechanical properties in copper-based materials with hydrogen isotopes and helium under high-temperature conditions for fusion applications. 

Effect of deuterium and helium plasma exposure on recrystallization in copper

Optimization of parameters of spray coating processes for the thermal solar absorbing film

Diffusion-assisted growth of periodic patterns on metal surfaces

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Optimization of parameters of spray coating processes for the thermal solar absorbing film

Diffusion-assisted growth of periodic patterns on metal surfaces