Variables de estudio

In order to perform a thorough analysis of the microstructural modifications in a material surface, it is important to work with a well-defined low-roughness base material. To do this, the materials used had a 99.98 w.t % purity and were subject to the same polishing procedure and EBSD, SEM, and X-Ray measurements in order to characterize the samples.

2.3.1

Polishing procedure

A general polishing procedure consists of two main steps:

• A first step consists of a mechanical polishing with an abrasive fine-grained silicon carbide (SiC) grinding paper. This step is usually performed using water as a lubricant; however, for samples with a tendency to oxidize, it can be performed using ethanol. The grinding steps may be followed by polishing with diamond suspensions of various sizes (3 µm, 1 µm, etc) depending on the hardness and characteristics of each sample. Mechanical polishing is performed with the aim of obtaining a flat surface with a low roughness.

• The second step consists of a chemo-mechanical process performed using an active oxide polishing suspension (OPS) which uses Coloidal Silicia (SiO2) as an abrasive.

The OPS is composed of fine silica particles (∼ 30 nm) in a alkaline solution. The polishing step with OPS is performed with a sheet of type OP-CHEM.

• The chemo-mechanical step may be replaced by an electrochemical or electropolishing procedure performed by immersing the sample in a temperature-controlled bath of electrolyte and passing a current through the sample. As a result, the metal surface is oxidized and dissolved in the electrolyte. It is performed with the objective of obtaining polished samples with a low roughness. This step was not performed for the aluminum or tungsten samples in this work due to the fact that favorable results were obtained using the chemo-mechanical procedure. Additionally, analysis performed with tungsten samples show that reliable and effective results are obtained by following the mechanical and chemo-mechanical steps (Manhard et al., 2013). Similarly, the polishing procedure recommended for pure aluminum samples consists in following a mechanical polishing procedure followed by a chemo-mechanical one (Struers, 2016). The combination of abrasion and chemical attack allows obtaining a flat, mirror-like surface free of scratches and deformations. In some cases in the course of the polishing procedure, i.e., after grinding, after polishing with diamond paste, the samples were analyzed with an optical microscope in order to check the quality of the sample. According to mea- surements made with AFM and Confocal microscopy, the surface roughness of the samples

2.3 Sample preparation and characterization 77 after polishing is approximately 10 nm.

The procedure used to polish aluminum sample with a diameter of 5 mm is shown in Table 2.1. Aluminum is a relatively soft material that quickly oxidizes, even under high vacuum conditions. This oxide layer is both adherent and impermeable. Once formed it protects the material from further oxidation (Palik, 1998b). Consequently, caution must be taken during polishing. The final polishing steps should be done with ethanol instead of water. Additionally, the samples were kept in sealed jars with ultra-pure ethanol in order to avoid oxide formation on the surface.

Steps Duration (min)

P1000 1 P1200 5 P2400 15 P4000 20 Diamond paste of 1₄µm 5 OPS 10

Table 2.1 Polishing procedure for Aluminum

Fig. 2.18 a) and b) shows the polished surface and cross-section of an aluminum sample prior to plasma exposure. The cross-section was obtained by cutting the sample using a diamond wire saw and polishing the obtained surface. Fig. 2.18 shows that the surface as well as the cross-section are almost free of defects such as voids or cracks. There are however a few small cavities or voids of a few nanometers on both the surface and the cross-section. The cross-section also presents some thin cracks whose width and height are, respectively, 10 µm and 500 nm. The small voids and cracks may be due to the cutting and polishing process or may occur during material processing. Note that similar defects, whose size do not surpass the above mentioned dimensions, were seen in other analyzed virgin samples.

Tungsten is a very hard material and hence its polishing procedure differs from that of aluminum. Longer time is devoted to each step and more steps should be included in the procedure. This should start by a rough grinding paper, such as P800 and, after mechanically polishing several smaller size grinding papers, different diamond pastes are used to ensure the sample achieves a mirror-like finish with the OPS. Table 2.2 shows the polishing procedure used for rectangular tungsten samples with a dimensions of 1.5 × 0.5 cm.

Fig. 2.18 SEM micrograph of a polished Al a) surface and b) cross section

Steps Duration (min)

P800 5 P1000 10 P1200 15 P2400 40 P4000 50 Diamond paste of 3µm 5 Diamond paste of 1µm 5 Diamond paste of 1₄µm 5 OPS 10

Table 2.2 Polishing procedure for Tungsten

2.3.2

Heat treatment

After surface polishing and cleaning in ultrasonic baths with ethanol and acetone, the specimens were subject to a heat treatment. This treatment, known as annealing, consists in heating a sample to a desired temperature, maintaining it for a specific period of time and then cooling the sample down to room temperature. Annealing is performed in order to selectively obtain samples with well-defined grains and defect structures, to reduce the number of dislocations and to relief stress. Depending on the annealing temperature, different processes may occur: recovery, recrystallization and grain growth. During recovery there is an enhanced atomic diffusion in the crystal lattice and the number of dislocation decreases. Recrystallization occurs when a new set of strain-free and equiaxed grains (i.e. having equal dimensions in all directions) is formed in the material. These grains have low dislocation densities. For metals, the recrystallization temperature is usually 0.4Tm where Tm is the

melting temperature (Calister and Rethwisch, 2010).

The recommended recrystallization temperature for aluminum is 350 K (Calister and Rethwisch, 2010). However, the sample temperature during plasma exposure at our specific

2.3 Sample preparation and characterization 79 conditions is of 620 K. In order to ensure the stability of the material microstructure during the plasma discharge, the sample is annealed at a higher temperature. Therefore, annealing for aluminum was performed under vacuum conditions (10−4_{Pa) at 673 K during 2 hours.}

Fig. 2.19 a) and b) show, respectively, an Al sample before and after annealing. It is clearly seen that after annealing grains are larger in size and dislocations are greatly reduced. Prior to annealing, this sample was completely deformed and had no defined grains.

Fig. 2.19 EBSD measurements of an aluminum sample a) before and b) after annealing

Tungsten, on the other hand, has to be annealed at much higher temperatures. The recom- mended recrystallization temperature is 1500 K. Analysis performed by Manhard (Manhard, 2011) determined that at this temperature there is a partial recrystallization of the mate- rial whereas the full recrystallization is obtained at temperatures of 1700 K. However, the maximum achievable temperature in our vacuum furnace is 1373 K. Therefore, tungsten samples were annealed at 1373 K for 1 hour at a vacuum of 10−4 _{Pa. Fig. 2.20 a) and b)}

show, respectively, a W sample before and after annealing. It is seen that prior to annealing, grains were not visible whereas after annealing grains are clearly discernible, some with sizes larger than 10 µm and some smaller ones with sizes of approximately 1 µm.

EBSD measurements were performed after annealing to verify that the samples have a small percentage of dislocations. It can be seen in Fig. 2.21 that the density of dislocations in the sample is small. In fact, it corresponds to approximately 8% of the whole surface.

Fig. 2.20 SEM micrographs of a tungsten sample a) before and b) after annealing

Fig. 2.21 Dislocations on a W sample after annealing