Multifunctional Al<inf>2</inf>O<inf>3</inf>:Eu²⁺/Sm^2+,3+ + Y<inf>2</inf>O<inf>3</inf>:Eu³⁺ Coatings Created by the Plasma Electrolytic Oxidation

Abstract

The industrial applications of luminescent coatings for temperature sensing and high-security QR codes have multiple inherent problems in their application (speed, cost, industrial feasibility) or properties (poor adhesion, chemical, thermal stability, or thermal contact). Luminescent coatings created by the electrochemical Plasma Electrolytic Oxidation (PEO) process have superior properties and can be grown directly within minutes, but are limited to the number of possible luminescent dopants. Having the unlimited possibility for doping on the PEO coatings would provide for superior coatings in all industrially appealing features. Herein, it is shown that adding any luminescent powder results in its partial dissociation and incorporation of its constituents, while it also gets incorporated as a whole. In this manner, alumina coatings are grown with various combinations and concentrations of divalent samarium and europium, and yttrium oxide with trivalent europium ions. These polycrystalline coatings whose microhardness is next to that of the diamond produce intense luminescence in the blue and red regions. Al2O3:Eu2+/Sm2+/3+ + Y2O3:Eu3+ coatings show an excellent temperature sensing performance from the cryogenic up to the high temperatures. Ultimately, the proof-of-concept of the luminescent QR code by the electrochemical process is realized creating a highly secure code with unprecedented environmental stability.