Characterization of glass–alumina functionally graded coatings obtained by plasma spraying

Glass–alumina functionally graded coatings (FGCs) were produced via plasma spraying, a deposition technique for thick (>10–20 μm) coatings production, which ensures high flexibility and good reliability. The samples were obtained by building a graded glass–alumina coating onto an alumina substrate; the coatings were designed as multi-layered systems, each layer having a mean composition slightly different from the neighbouring ones. Two different compositional gradients were considered (from 100 vol.% alumina to 100 vol.% glass and from 80–20 vol.% glass to 100 vol.% glass) and several heat treatments were performed in order to improve the substrate-coating interface and induce a controlled transformation (sintering and/or crystallization) of the glassy phase. After a preliminary screening of the as-sprayed and the heat treated samples, the most interesting ones were carefully characterized, especially from a mechanical point of view. In fact, tests such as Vickers micro-indentation allowed to appreciate the effects of the graded compositional profile and the consequences induced by thermal treatments.