Morphological modification of Rh-C coatings upon low-energy Ar+ ion sputtering

In ITER, the metallic first mirrors (FMs) will undergo erosion due to their proximity to the fusion plasma and deposition of materials of the first wall, leading to mirror reflectivity's decrease. In vacuo plasma cleaning is foreseen for restoration of the FMs' optical properties by means of ion sputtering. Previously, it was shown that cyclic cleaning of polished metallic mirrors can lead to the development of pits due to low carbon amounts in the bulk mirror. The pitting formation is detrimental to the mirror's optical properties. This study aims to investigate the influence of carbon concentration on mirror morphology changes due to cyclic low-temperature plasma irradiation. Five rhodium (Rh) and carbon (C) coatings with different amounts of C were deposited on a pure Rh film on top of polished stainless steel substrates. All the samples were prepared by magnetron sputtering using a single or dual magnetron. Prior to each cycle of the plasma cleaning, a 20 nm layer of Al₂O₃ was deposited on the Rh-C samples. The plasma discharge was created with argon gas using a 60 MHz radio frequency excitation and resulted in the complete removal of the alumina layer after each cycle. The surface morphology of the mirrors was characterized by employing scanning electron microscopy (SEM) and focused ion beam (FIB). After the cyclic cleaning, the coatings containing carbon have failed, showing either partial delamination, cracking, or total delamination. Additionally, all the mirrors demonstrated the formation of mounds on the surface, while 17 at.% of carbon in the film led to the development of pits. The mechanisms of coating failure and such morphological modification are discussed in the paper.