Abstract
This study investigated the effect of adding superhard ReB(2) to atmospheric plasma sprayed (APS) coatings based on 60 wt% Al(2)O(3) and 40 wt% ZrO(2). The amorphous phases commonly present in such coatings are known to impair their performance. ReB(2) was introduced as a crystallization nucleus due to its high melting point. ReB(2) decomposes in the presence of moisture and oxygen into H(3)BO(3), ReO(3), HBO(2), and HReO(4). ReB(2) was encapsulated with Al(2)O(3) via metallothermic synthesis to improve moisture stability, yielding a powder with d(90) = 15.1 μm. After milling, it was added at 20 wt% to the Al(2)O(3)-ZrO(2) feedstock. Agglomeration parameters were optimized, and coatings were deposited under varying APS conditions onto 316L steel substrates with a NiAl bond coat. In the coating with the highest ReB(2) content, the identified phases included ReB(2) (2.6 wt%), Re (0.8 wt%), α-Al(2)O(3) (30.9 wt%), η-Al(2)O(3) (32.4 wt%), and monoclinic and tetragonal ZrO(2). The nanohardness of the coating, measured using a Vickers indenter at 96 mN and calculated via the Oliver-Pharr method, was 9.2 ± 1.0 GPa. High abrasion resistance was obtained for the coating with a higher content of η-Al(2)O(3) (48.7 wt%). The coefficient of friction, determined using a ball-on-disc test with a corundum ball, was 0.798 ± 0.03. After 15 months, the formation of (H(3)O)(ReO(4)) was observed, suggesting initial moisture-induced changes. The results confirm that Al(2)O(3)-encapsulated ReB(2) can enhance phase stability and crystallinity in APS coatings.