Low-density HiPIMS favored ultrathick and compact chromium coatings on Zircaloy-4 alloys

Protective metallic coatings are receiving increasing attention to modify the anti-corrosion and oxidation resistance of zirconium-based fuel cladding tubes under harsh high-temperature steam/air oxidation conditions. In this work, ultrathick (& SIM;20 & mu;m) and compact Cr coatings with (001) preferential orientation were deposited on Zircaloy-4 (Zry-4) alloy by high power impulse magnetron sputtering (HiPIMS) technique. The growth characteristics of the Cr coatings were investigated as a function of the discharge plasma species, including Cr+, Cr*, Ar+, and Ar*, which were controlled by the different discharge modes during HiPIMS process. The results showed that, when the discharge feature was changed from the 'low voltage direct current magnetron sputtering (DCMS)-like' mode into a 'low density HiPIMS' mode, a rapid increase of Cr+ content in the substrate vicinity was obtained by time-integral optical emission spectra (OES), indicating a remarkably enhanced incident energy flux to the deposited Cr coating. The generalized temperature increment of the Cr coating growing surface calculated from the OES, was & SIM;100 K and was mainly affected by the incidence of Cr* atoms. A simplified collision model based on the non-penetrating ions/atoms assumption was also proposed to calculate the transferred energy from the incident energetic species and thus understand the predominant effect of HiPIMS Cr plasma on the achieved nanocrystalline Cr coating with ultra-thick and dense structure, which is still quite challenging for the DCMS deposition process.