Published on September 11, 2007
MD Simulations of Hydrogen Induced Sputtering of Tungsten Carbide: MD Simulations of Hydrogen Induced Sputtering of Tungsten Carbide Niklas Juslin, Petra Träskelin, Kai Nordlund Contents: Contents Sputtering andamp; why it is of interest in ITER Tungsten and tungsten carbide in ITER A brief introduction to MD simulations of sputtering of WC Results andamp; conclusions Sputtering: Sputtering The erosion of a material surface due to particle impact Sputtering yield: the number of sputtered particles per incident particles Physical sputtering: Physical sputtering is due to momentum transfer between the incident particle and an atom in the surface. Enough energy to overcome the threshold energy must be transfered. Physical sputtering Chemical sputtering: Under bombardment amorphous structures may form on the surface. The chemical bonds in these can be broken by low energy impingement, leading to swift chemical sputtering. Chemical sputtering can occur below the threshold for physical sputtering. Chemical sputtering Sputtering in ITER: Sputtering in ITER Sputtering is of interest in the ITER fusion reactor, because energetic particles from the plasma will impinge the first wall. The erosion of the wall materials leads to: Sputtered particles affecting the plasma New properties for the wall materials Tungsten and tungsten carbide in ITER: Tungsten and tungsten carbide in ITER The divertor is designed to be mostly tungsten. Carbon layers will be present in certain parts. Thus we have tungsten carbide where W and C meet and where eroded C has drifted to W parts. A brief introduction to MD simulations of sputtering of WC: A brief introduction to MD simulations of sputtering of WC Molecular dynamics is well suited for simulations with thousands of atoms up to nanosecond time scale. Sputtering is a statistical event, so thousands of runs are needed for good results. Slide9: The interaction between the atoms are described using a potential formalism developed by Tersoff and Abell. This formalism has proven suitable for a wide range of materials and applications. We have developed parameters for W-W, W-C and W-H. For carbon and hydrogen we use well tested parameters by Brenner. The simulation box: We use a box with 1600 (W) or 1920 (WC) atoms with periodic boundaries in two directions and an open surface. For tungsten carbide we simulate with both tungsten and carbon as the outermost layer. The simulation box WC structure from side and surface: WC structure from side and surface The bombardment: The bombardment We bombard the surface with deuterium, observing the erosion and calculating the yield. Energy range of D: 10 eV to 2000 eV Temperature of the target material: 300 K Slide13: In the non-cumulative simulations we bombard a clean surface every run. In the cumulative simulations we use the surface from a previous run, simulating a surface being bombarded several times. Deuterium bombardment of tungsten: Deuterium bombardment of tungsten Deuterium bombardment of tungsten carbide: Deuterium bombardment of tungsten carbide 600 eV - CW 2 keV - WC Results: Results Non-cumulative simulations: Pure tungsten Tungsten Carbide WC CW Parameters II Parameters I Slide17: Cumulative simulations (parameters II): CW: 20 eV CW: 100 eV CW: 1000 eV WC: 20 eV Conclusions: Conclusions Chemical sputtering is important in tungsten carbide. Sputtering yields are very low for tungsten both in pure tungsten and tungsten carbide.