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

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.