6.3 Damaging and Degradation of PFM
105
Fig. 6.8 a SEM image of an individual blister on polycrystalline tungsten after plasma exposure
(38 eV/D, 3 × 10 24 D·m- 2 , 300 K) and b same surface area after sputtering a hole with Focused Ion
Beam (FIB) into the blister cap and its elastic collapse after degassing and c cross section through
the cap of the collapsed blister at the position of the hole sputtered to degas the blister. Arrows
indicate the remaining crack system of the former blister (a) and (b) tilted by 52°, (c) by −38°
(reprinted with permission from [37])
As already described, hydrogen is trapped and accumulated at grain boundaries. It
weakens the bonding between neighboring grains. In addition, H inclusion in grains
gives swelling and, as a consequence, introduces lateral stress. Since W materials
are usually manufactured by the powder metallurgy (PM) including hot rolling, their
grains tend to extend parallel to the surface. Accordingly, a particular grain parallel
to the surface is weakened by H segregated at its edge and exfoliated from the matrix
to make dome-shaped blisters [38, 41] (gas pressure mechanism) or convexity (or
ledges) [33, 35, 38] due to slipping along specific crystalline planes together with
exfoliation of grain boundaries (stress mechanism) as shown in Fig. 6.9B [38].
To clarify the blister mechanism [43, 46], it is important to know whether blister
skins are permanently deformed (plastic deformation) or elastically deformed. Even
such larger blisters than grain sizes were produced by the elastic deformation and
105
Fig. 6.8 a SEM image of an individual blister on polycrystalline tungsten after plasma exposure
(38 eV/D, 3 × 10 24 D·m- 2 , 300 K) and b same surface area after sputtering a hole with Focused Ion
Beam (FIB) into the blister cap and its elastic collapse after degassing and c cross section through
the cap of the collapsed blister at the position of the hole sputtered to degas the blister. Arrows
indicate the remaining crack system of the former blister (a) and (b) tilted by 52°, (c) by −38°
(reprinted with permission from [37])
As already described, hydrogen is trapped and accumulated at grain boundaries. It
weakens the bonding between neighboring grains. In addition, H inclusion in grains
gives swelling and, as a consequence, introduces lateral stress. Since W materials
are usually manufactured by the powder metallurgy (PM) including hot rolling, their
grains tend to extend parallel to the surface. Accordingly, a particular grain parallel
to the surface is weakened by H segregated at its edge and exfoliated from the matrix
to make dome-shaped blisters [38, 41] (gas pressure mechanism) or convexity (or
ledges) [33, 35, 38] due to slipping along specific crystalline planes together with
exfoliation of grain boundaries (stress mechanism) as shown in Fig. 6.9B [38].
To clarify the blister mechanism [43, 46], it is important to know whether blister
skins are permanently deformed (plastic deformation) or elastically deformed. Even
such larger blisters than grain sizes were produced by the elastic deformation and