Limitaciones y problemas encontrados

Fig. 3.2 shows an example of intergrain blisters in an aluminum sample after plasma exposure. Impurity atoms, such as hydrogen, tend to segregate along the grain boundaries. In other words, hydrogen concentration tends to be greater in the grain boundaries than in the matrix. Since atoms in the grain boundaries are not bonded to the maximum number of nearest neighboring atoms, the material is mechanically more ductile in this area. As a result blistering is easier to occur than in the matrix (Calister and Rethwisch, 2010; Ren et al., 2008). Intergrain blisters result from hydrogen trapping at the grain boundaries and hence

they are always located within two or more grains. They have circular or irregular shapes with a radius ranging from 100 nm to more than 15 µm.

Another phenomenon that occurs to aluminum samples after plasma exposure is the swelling of the grains, as shown in Fig. 3.2. This swelling consists in the upward deformation of a large fraction or the whole grain. Since the samples have large grains of about 10-40 µm, grain swelling covers a more important surface area than blisters. The height of swollen blisters, as measured using the confocal optical microscope, is between 0.5 and 10 µm. Compared to other modifications, such as blistering, grain swelling presents a much larger area (i.e. the whole grain area) and a significantly lower height.

Fig. 3.2 Intergrain blisters and swollen grains on an aluminum sample exposed to H2plasma with flux of 1.6 × 1020_ions/m2_{s, fluence of1.7 × 10}24_ions/m2_{s and incident ion energy of 320 eV}

Fig. 3.3 shows an example of intragrain blisters in an aluminum sample after plasma exposure. Intragrain blisters grow solely within the grains and have mostly circular shapes. During hydrogen implantation, hydrogen is trapped in defects such as vacancies. Due to the presence of hydrogen, the vacancy-hydrogen complexes tend to organize in clusters. These clusters are further stabilized by the formation of molecular hydrogen in the hydrogen- vacancies cluster induced cavity. This process represents the initial nucleation mechanism for bubbles that may generate intragrain blisters (Condon and Schober, 1993; Ren et al., 2008).

3.1 Introduction 95

Fig. 3.3 Intragrain blisters on an aluminum sample exposed to H2plasma with flux of1.6 × 1020ions/m2s, fluence of2.2 × 1024ions/m2s and incident ion energy of 320 eV

The situation may be more complex since a secondary blister nucleation may take place. This effect corresponds to either the formation of small blisters on the top of larger ones or the formation or large blisters underneath smaller ones (see Fig. 3.4). Secondary blistering may also take place through the formation of small blisters on swollen grains (see Fig. 3.4).

Fig. 3.4 Secondary blister growth on an aluminum sample exposed to H2plasma with flux of 1.6 × 1020_ions/m2_{s, fluence of2.2 × 10}24_ions/m2_{s and incident ion energy of 320 eV}

We also observe that large blisters may form by the coalescence of smaller ones (see Fig. 3.5). This image shows several examples of blisters close enough to each other and about to coalesce. This phenomenon leads to the increase in blister size and the decrease in blister density. Therefore coalescence represents another growth mechanism besides loop punching and vacancy clustering mechanisms that were previously discussed in Chapter 1 Section 1.6.2.

Fig. 3.5 Blister growth by coalescence on an aluminum sample exposed to H₂plasma with flux of 1.6 × 1020_ions/m2_{s, fluence of2.2 × 10}24_ions/m2_{s and incident ion energy of 320 eV}

Besides blister formation and grain swelling, nanostructures of a few nm size form all over the surface after 30 min of hydrogen plasma exposure with the following plasma conditions: fluence of 3.0 × 1023 _ions/m2_{), incident ion energy of 320 eV, temperature of 618 K and a}

plasma flux of 1.6 × 1020_ions/m2_{s. Fig. 3.6 shows an example of these nanostructures.}

Fig. 3.6 Nanostructures on an Al sample exposed to H₂plasma with flux of1.6 × 1020ions/m2s, fluence of 3.0 × 1024_ions/m2_{and incident ion energy of320 eV}

3.1 Introduction 97 After 3 h of plasma exposure (fluence of 1.7 × 1024 _ions/m2_{), these nanostructres have}

a triangular shape with sharp edges and their density depends on the grain orientation, as shown in Fig. 3.7. Blister density is similar on <100> and <111> grains, with a value of 1.5 nanostructures/µm2_{. On the other hand, <110> grains have the lowest density, with a}

value of 0.7 nanostructures/µm2_{. These nanostructures were seen all over the surface of the}

sample, including on the top of blisters. This indicates that their depth is less important than that of blisters.

Fig. 3.7 SEM micrographs of nanostructures on grains whose surface normal is perpendicular to the a) <100> b) <110> and c) <111> directions on an Al sample exposed to H2plasma with flux of1.6 × 1020ions/m2s,

fluence of1.7 × 1024ions/m2and incident ion energy of320 eV. Images taken with the samples tilted 70◦ degrees with respect to the electron beam.

The formation of nanostructures was already observed in tungsten exposed to high flux (i.e. 1024_ions/m2_{s), deuterium plasma at different surface temperatures and an incident ion}

energy of 38 eV. Their formation mechanisms have not been documented yet. However, it is believed that these nanostructures form due to the fact that, at high fluxes, the kinetics of hydrogen loading at the near surface region surpasses the diffusion rate. As a result, hydrogen can accumulate and agglomerate into clusters. This allows the formation of high pressure bubbles. Due to the high pressures, aluminum atoms at the bubble wall may be pushed out of their position. This leads to the generation of dislocation loops, which results in the formation of nanostructures at the surface of the material (Xu et al., 2014).