This study investigated the effect of adding superhard ReB2 to atmospheric plasma sprayed (APS) coatings based on 60 wt% Al2O3 and 40 wt% ZrO2. The amorphous phases commonly present in such coatings are known to impair their performance. ReB2 was introduced as a crystallization nucleus due to its high melting point. ReB2 decomposes in the presence of moisture and oxygen into H3BO3, ReO3, HBO2, and HReO4. ReB2 was encapsulated with Al2O3 via metallothermic synthesis to improve moisture stability, yielding a powder with d90 = 15.1 μm. After milling, it was added at 20 wt% to the Al2O3-ZrO2 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 ReB2 content, the identified phases included ReB2 (2.6 wt%), Re (0.8 wt%), α-Al2O3 (30.9 wt%), η-Al2O3 (32.4 wt%), and monoclinic and tetragonal ZrO2. 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-Al2O3 (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 (H3O)(ReO4) was observed, suggesting initial moisture-induced changes. The results confirm that Al2O3-encapsulated ReB2 can enhance phase stability and crystallinity in APS coatings.
Keywords: Al2O3-ZrO2-ReB2; XRD; agglomeration; atmospheric plasma spray; chemical stability; coatings; microstructures.