Thickness measurement

For each coating deposition run, a few sapphire discs covered with a silicon wafer were placed in the sample holder in order to measure the thickness of the coatings by means of Dektak (a step between a covered and uncovered part of the disc). The results are shown in Table 4.1. Small letters “a2 – j2” indicate positions of the discs in the sample holder (plan of the sample holder given in Figure 3.8, section 3.2.2). Position “a2” was located directly under the Cr and Fe50Cr targets (depending on the deposition run), whereas position “j2” was placed under the Fe30Al and Fe20Al targets (depending on the deposition run).

It can be noticed that the coatings produced in Run 2 were the thinnest, whereas in Run 3 they were the thickest.

For the coatings placed in the positions “c1”, “c3”, “e1” and “e3” (Run 2) the values are estimated. The lowest thickness of the coating deposited onto sapphire discs that Dektak could measure was ~3 µm, therefore it was established that the discs “c1” and “c3” had ~1 µm (closer to the Fe50Cr target);

and “e1”, “e3” would be thicker (~1-2 µm) due to their position in the middle of the sample holder, where two plasmas mixed together. The difficulties with measuring the thickness of those discs were probably due to a high roughness of the sapphire discs hindering the measurement of thin coatings. The estimated thicknesses of those coatings were confirmed by further analyses (such as FIB and SFEG).

Table 4.1 Thicknesses of the coatings deposited on the discs in three runs Target

After each deposition run, the coatings were analysed with ESEM/EDX in plan view. Electron images were taken in order to characterise the microstructure of the coatings, which would subsequently be compared to the surface analysis after the exposures. The EDX analysis were undertaken in order to know the exact elemental composition of the coatings and, similarly to ESEM, compare it to the composition obtained after the tests.

The compositions of the as-deposited coatings are presented in Table 4.2 - Table 4.4. They are also plotted on the graphs for their better illustration (Figure 4.14 - Figure 4.17). All characterisations were carried out in the central area of

each disc (unless indicated otherwise) of 0.02 mm² (an area analysis with a box of 200 µm x 100 µm).

Table 4.2 shows the elemental composition of the coatings deposited in Run 1 in atomic %. The amount of Cr decreases with increasing distance from the Cr target and reaches ~2 at% for sample K (placed just under Fe:Al target).

Contrary, Fe and Al content increases with increasing distance from Cr. The lowest detected amounts of Fe and Al were 0.3 and 0.2 at% respectively, whereas the highest were 58.4 and 41.5 at% for Fe and Al.

Table 4.2 Elemental composition of the “Cr + Fe30Al” (Run 1) as-deposited coatings in at% (Cr target at “A” end, Fe30Al target at “K” end) (among the same letters, for example A1 - A4 or C1 - C4) was negligible.

Figure 4.14 shows that the amount of chromium decreases along the sample holder (starting from sample A), whereas the iron and aluminium content increases reaching their highest numbers for samples J and K.

Figure 4.14 Composition of the as-deposited coatings along the sample holder for targets Cr and Fe-30wt%Al presented in atomic % (Run 1)

Table 4.3 presents the elemental composition of the coatings (in atomic %) deposited in Run 2. In this case the amount of chromium present in the coatings is much lower in comparison to the compositions from Run 1, because a Fe-50wt% Cr target was used for this deposition. The Al contents in the Run 2 coatings are also lower, because a Fe-20wt% Al target was used for sputtering (not the Fe-30wt% Al target as for Run 1) with the highest levels deposited being ~31 at%. The highest Cr content for these coatings was ~47 at% for sample B and was decreasing to reach almost 0 for the last four samples (H - K). The amount of iron in these coatings was high across the whole sample holder (between 52 – 72 at%).

Figure 4.15 is a graphic presentation of the atomic composition of the coatings from Run 2. It can be seen that the amount of Fe and Al increases along the sample holder, whereas Cr decreases at the same time. However, starting from sample G (middle of the sample holder) the content of all the constituents starts to be constant.

Table 4.3 Elemental composition of the “Fe50Cr + Fe20Al” (Run 2) as-deposited coatings in at% (Fe50Cr target at “A” end, Fe20Al at “K” end)

Coating Cr Fe Al A 43.7 52.1 4.2 B 46.7 51.6 1.7 C 40.7 55.7 3.7 D 26.2 62.9 10.9 E 9.4 71.4 19.2 F 2.3 72.0 25.7 G 0.4 70.4 29.2 H 0.2 69.9 30.0 I 0.1 69.1 30.8 J 0.1 69.2 30.8 K 0.1 70.1 29.9

Figure 4.15 Composition of the as-deposited coatings along the sample holder for targets Fe-50wt%Cr and Fe-20wt%Al presented in atomic % (Run 2)

Coatings deposited in Run 3 included detectable amounts of oxygen and nitrogen, which were probably a result of a long deposition time (32 hours) and leaving the samples in the deposition chamber overnight. Although they were kept under a vacuum of ~2×10^-7 Torr, it is assumed that during that time there must have been a leak from the laboratory atmosphere into the chamber. The

presence of N and O could not be a result of the problems with the EDX system, because the selected coatings were rechecked on the other occasion.

It is assumed that N preferably reacted to form CrN for the coatings containing high levels of Cr, whereas for the coatings with high concentration of Fe, N would dissolve in it. From a Fe-N phase diagram found in literature [123] it can be seen, that the highest amount of N that could be dissolved in iron at 550ºC is about 25 at%.

For comparison with other coating deposition runs, it was necessary to calculate the composition profiles without O and N. The coating compositions with O and N are shown in Table 4.4 and the final normalised compositions (without O and N) are presented in Table 4.5. Despite the presence of those compounds that might have been formed with O and N it was not anticipated, that there would be a significant influence on the coating behaviour during their environmental testing. The produced coatings were metallic, with no visible difference in their microstructures.

It should be noted that for Run 3, four additional coatings were produced (compared to Run 1 and 2). They were placed in the sample holder in positions as follows: d, e, f and g. These positions were chosen because coatings D – G from Run 1 performed well in their exposures, thus, having four additional discs between them allowed more compositions in this range and so the analysis of their behaviour.

Figure 4.16 depicts the graph of the elemental composition of the coatings deposited in Run 3 before being exposed, including oxygen and nitrogen. With these amounts of O and N, the highest Cr concentration is 62 at%, which decreases with increasing distance from the Cr target. Fe and Al concentrations, on the contrary, increase with decreasing distance from the Fe20Al target. The highest amount of iron is ~53 at% and 24 at% for aluminium.

It can be noticed, that the amount of nitrogen is quite high (over 20 at%) for the coatings containing high levels of Cr and it is lower for the coatings with lower Cr levels (between 12 – 14 at%).

Table 4.4 Elemental composition of the “Cr+Fe20Al” (Run 3) as-deposited coatings in atomic % including O and N (Cr target at “A” end, Fe20Al target at “K” end)

Coating Cr Fe Al O N A 61.8 0.8 0.4 15.8 21.3 B 61.8 1.3 0.8 14.6 21.5 C 59.8 2.7 1.1 12.7 23.8 D 55.5 6.7 2.6 14.6 20.6 d 48.8 9.3 3.9 15.8 22.2 E 43.6 15.2 6.3 16.9 18.0 e 36.2 21.8 9.5 17.6 14.9 F 25.9 28.1 12.8 19.5 13.7 f 17.7 35.2 15.1 19.7 12.4 G 11.0 38.2 17.4 21.8 11.7 g 6.8 43.6 19.3 18.3 12.0 H 4.7 47.0 21.4 14.0 12.9 I 1.9 49.8 22.6 12.1 13.6 J 1.0 52.6 23.9 8.8 13.7 K 0.7 50.3 23.5 13.3 12.2

Figure 4.16 Composition of the as-deposited coatings along the sample holder for targets Cr and Fe-20wt%Al presented in atomic % (Run 3)

It was decided that for coatings A – C, nitrogen and oxygen would be used to form CrN and Cr2O3 (it was not enough Al to form Al2O3). For higher levels of Fe (samples D – K), nitrogen would first dissolve in iron (up to 25%), then form CrN and then a (Cr,Al)2O3 spinel. For the last two samples (J and K) the amount of Cr was very low, therefore it was assumed that only Al2O3 could be formed. It was assumed that for the amount of Al higher than 5%, Al2O3 would preferably be formed than Fe2O3.

Table 4.5 presents the normalised elemental composition of the coatings (excluding oxygen and nitrogen). It can be seen that the level of Cr is between 1 and 96 at%, 2.6 to ~76 at% for Fe and 1 to 25 at% for Al. The same as for Run 2, Fe-20wt% Al target was used, therefore the aluminium level is lower in comparison to coatings from Run 1, where it was ~41 at%.

Table 4.5 Normalised elemental composition of the “Cr + Fe20Al” (Run 3) as-deposited coatings in at% (Cr target at “A” end, Fe20Al target at “K” end)

coating Cr Fe Al

Figure 4.17 is a graph of the normalised coating compositions (using data from Table 4.5) from “Cr + Fe20Al” deposition run (Run 3). Similarly for other targets, the amount of chromium decreases with increasing distance from the Cr target;

iron content increases with getting closer to the Fe:Al target and stays almost at

the same level for the last four coatings (H – K). Aluminium content, similarly to iron, increases when the distance from the Fe:Al target is smaller.

Figure 4.17 Normalised composition of the as-deposited coatings along the sample holder for targets Cr and Fe-20wt%Al presented in atomic % (Run 3)

Another way of illustrating coating compositions is a Fe-Cr-Al ternary diagram, which shows the relationship between those three elements (in wt%). Each dot on the diagram represents a different coating composition (Figures 4.18 - 4.20), whereas the red line illustrates the composition of the targets used for sputtering in each run. Also, it guides what ideal coating compositions should be deposited with these particular targets. As can be seen, all the coatings cluster well close to the red line (especially in Figure 4.18 and Figure 4.19). Coatings from Run 3 (“Cr + Fe20Al”) are located a bit lower in comparison to the coatings from Run 1 and 2, nevertheless their compositions are acceptable.

Figure 4.18 Fe-Cr-Al ternary diagram with the as-deposited coating compositions obtained in Run 1 (Cr and Fe30Al targets)

Figure 4.19 Fe-Cr-Al ternary diagram with the as-deposited coating compositions obtained in Run 2 (Fe50Cr and Fe20Al targets)

Figure 4.20 Fe-Cr-Al ternary diagram with the as-deposited coating compositions obtained in Run 2 (Fe50Cr and Fe20Al targets)

The GSE images in Figures 4.21 - 4.23 show the microstructures of the as-deposited coatings (before the exposures). Because their microstructures looked similar for all the coatings, it was decided to present only examples for each deposition run. All of the selected coatings have number ‘2’, because they were placed in the second row in the sample holder. It is important to indicate, that, despite the fact that they have the same position labels (A2, D2, H2, K2), their compositions are different, because these particular coatings were deposited in three different runs when different targets were sputtered.

Figure 4.21 shows the examples of the microstructures of the coatings deposited in Run 1 (“Cr + Fe30Al”), Figure 4.22 in Run 2 (“Fe50Cr + Fe20Al”) and Figure 4.23 in Run 3 (“Cr + Fe20Al”). It can be seen that all the coatings presented were homogeneous in their structure, with some cracks and/or voids, although more cracks/voids are visible for the coatings from Run 3. This was probably caused by a long deposition process (32 hours). The impact of cracks before and after the tests was not included in the surface area calculations. The thicknesses of these coatings varied between 10.5 (for discs located at the beginning of the sample holder) to 16 µm for discs placed near the Fe:Al target and it is about 2-3 times greater in comparison to coatings from Run 1. It can be

seen that the microstructure of the coatings A2 and D2 from Figure 4.22 (Run 2) differs from the other coatings. It is possibly caused by their different composition and thickness; they were located close to the Fe50Cr target (which was difficult to sputter) and are much thinner in comparison to the ones from Run 1 or 3.

Figure 4.21 Examples of the microstructures of unexposed coatings from Run 1 (“Cr + Fe30Al”)

Figure 4.22 Examples of the microstructures of unexposed coatings from Run 2 (“Fe50Cr + Fe20Al”)

Figure 4.23 Examples of the microstructures of unexposed coatings from Run 3 (“Cr + Fe20Al”)