PVD Cr/Cr2AlC/Cr (MAX phase) coated Zry-4

The Morphology of PVD MAX coating samples is shown in Fig. 58. Cracking and spallation of the coating was observed. Some coating materials spalled during sample handling for post test characterization. Dark oxides were formed after oxidation.

INITIAL 21 days 42 days 63 days

FIG. 58. Overview of the MAX phase coated Zry-4 sample during the test.

The coating appears to be smooth, although some cracks were visible. After the exposure, new morphological objects were visible as for example: grains, agglomerates. The samples exposed to 21 days were partially covered while the samples exposed to 42 and 63 days were fully covered.

FIG. 59. SEM micrographs of the MAX phase coated Zry-4 sample during the test X 1000 and X 10000.

The SEM/EDS analysis of cross section after 63 days shows an additional layer formed between the material and coating, consisting mostly of Cr (not present before). On both edges of this layer, there is a higher concentration of oxygen. The coating layer includes a small amount of Al.

42 days 63 days

FIG. 60. MAX phase coated Zry-4 cross section after 42 and 63 days (SEM).

The mass gain data are shown in Table 21. The mass change is a result of three combined processes: oxidation

TABLE 21. SURFACE MASS GAINS OF THE MAX PHASE PVD COATED ZRY-4 SAMPLE

21 days 42 days 63 days

Mass change [%] -3.555 -0.783 1.05

XRD analysis confirmed the presence of Cr as well as chromium oxide Cr2O3 – blue and green markers. Red

markers are positioned at lines place from phase Cr2AlC (database).

FIG. 61. XRD analysis of MAX phase coated Zry-4 sample during the long term corrosion test (A0 – 0 days, A1 – 21 days, A2 – 42 days, A3 – 63 days).

3.7.4. ZrSi-Cr PVD coated Zry-2

The appearance of the ZrSi-Cr coated samples during the corrosion test is shown in Fig. 62.

INITIAL 21 days 42 days 63 days

FIG. 62. Morphology of the ZrSi-Cr coated Zry-2 sample during the test.

The surface of samples became rougher with irregular structures and visible morphological objects like grains and agglomerates formed after exposure.

KIT

FIG. 63. SEM micrographs of the ZrSi-Cr coated Zry-2 sample during the test.

The samples were completed covered by oxidized products of various shapes such as fibers, needles, and plates after exposure.

The oxide layer is formed on top of the coating after the 21 days test. Cross section of the sample after 42 days exposure suggests the presence of oxides, including Cr oxide and oxide complex. Cross section of the sample after 63 days test shows a different profile of composition cross the oxides. The coating thickness is lower, but the concentration of elements is on the same level as previously. The Cross section of samples after 21 days and 42 days exposure shows the presence of oxide layers as well as coating layers.

It is shown that the coating was oxidized and protected the substrate from oxidation.

INITIAL 21 days 42 days 63 days

FIG. 64. As-prepared ZrSi-Cr coated Zry-2 cross section and after 21, 42 and 63 days (SEM).

The mass gains are summarized in Table 22. It was observed that there was no mass change after 21 days at all, and a small mass gain after 42 days but a large mass gain after 63 days. The results suggested that the coating may lose protection for the substrate after a short exposure.

TABLE 22. SURFACE MASS GAINS OF THE ZrSi-Cr PVD COATED ZRY-2 SAMPLE

21 days 42 days 63 days

Surface mass change [%] 0.0 4.733 72.625

Figure 65 shows the XRD results for the as received sample and the oxidized samples. The red markers are the hexognonal zirconium crystal structure. These peaks belong to the base material (Zry-2) spectrum. The zirconium structure was detected because the coating thickness is small and only around 2.5 µm. The Monoclinic zirconium oxide ZrO2 phase is also visible and shown as blue markers. No peak was identified for the coating material. The bulge at the position about 2𝜃 = 40 deg at A0 spectrum suggests that the coating is amorphous. For the oxidized samples, the intensity for the coating decreases and the intensity of the zirconium oxide increases, suggesting the coating may be consumed during the exposure.

The set of XRD spectra of initial and after autoclave tests of Zry-2 coated with Zr40Si24Cr36 confirmed that the oxidation process took place during the autoclave tests. Peaks characteristic for ZrO2 monoclinic phase were observed after A1 test at the point of 2𝜃 = 51^o and after A2 test at the points, 2𝜃 = 34, 42, 51 (stronger than after A1), 56 and 66^o. The peaks widening was attributed to the dispersive oxide grains.

FIG. 65. XRD analysis of ZrSi-Cr coated Zry-2 sample during the long term corrosion test (A1 – 21 days in red, A2 – 42 days in blue, A3 – 63 days).

3.7.5. AISI 348 SS

The images of AISI 348 sample showed a surface color change from grey to gold/brown after oxidation tests.

The same golden color was also observed for the oxidized Cr-coated samples.

INITIAL 21 days 42 days 63 days

FIG. 66. Overview of the AISI 348 SS sample during the test.

The surface morphology of the samples before and after autoclave tests is shown in Fig. 67. As the figures show the surface became rougher due to oxidation. Surface cracks was also observed. Various oxide forms were observed, including needle like grains, plate like grains and sphere like grains.

FIG. 67. SEM micrographs of the AISI 348 sample during the test X10000.

No difference was observed in the elemental composition of the samples before and after the autoclave tests.

The oxide layer formed at the surfaces contains all elements present in the substrate. The thickness of the formed oxide layer was about 1.0 µm.

INITIAL 21 days 42 days 63 days

FIG. 68. AISI 348 as-received cross section and after 21, 42 and 63 days (SEM).

The mass gain data are summarized in Table 23.

TABLE 23. SURFACE MASS GAINS OF THE AISI 348 SS SAMPLE

21 days 42 days 63 days

Surface mass gain [%] 0.204 1.127 0.736

The presence of regular phase in Fm-3M symmetry with the lattice parameter of a = 3.5843Å was confirmed with the red markers. The presence of iron oxide Fe2O3 was confirmed, starting from A2 spectrum with the blue markers. The peak at 2𝜃 = 44.514° may belong to the bcc phase (Im-3m) with the lattice parameter a = 2.87787 Å.

FIG. 69. XRD analysis of AISI 348 sample during the long term corrosion test test (A1 – 21 days, A2 – 42 days, A3 – 63 days).