Recombination of silica and zirconia into zircon by means of laser treatment of plasma-sprayed coatings

Abstract

Self-supported zircon (ZrSiO₄) coatings have been deposited by means of atmospheric pressure plasma spraying, a high growth rate deposition method. However, it is well known that ZrSiO₄ dissociates into ZrO₂ and SiO₂ in the high-temperature plasma torch during plasma spraying, the rapid quenching preventing reverse combination of both components into ZrSiO₄. Usually, high-temperature annealing (1,600–1,900 K) is applied to recombine SiO₂ and ZrO₂ into ZrSiO₄. In this contribution, we investigate an attractive technological alternative to recombine SiO₂ and ZrO₂ into ZrSiO₄ by laser treatment with a scanning continuous wave CO₂ laser. By carefully adjusting the CO₂ laser treatment parameters (laser power density and scanning velocity), we show that the SiO₂ and ZrO₂ phases indeed recombine into ZrSiO₄, however, with a very low recombination rate. Thus, we have investigated the addition of SiO₂-rich glassy particles to the plasma spray powders to facilitate the recombination of ZrO₂, and SiO₂ into ZrSiO₄ during the laser treatment. Furthermore, the beneficial role of the glassy particles addition to substantially lower the annealing temperature during classical heat treatments has been studied. Available evidence indicates that the glassy particles melt during heat treatment, and thus favor the mobility and availability of silica at the ZrO₂ grains, which results in a lowering of the reaction temperature and an enhancement of the reaction kinetics.